id
stringlengths 36
40
| text
stringlengths 50
3.89M
| source
stringclasses 1
value | added
stringdate 2025-04-07 03:56:57
2025-04-07 04:04:00
| created
stringlengths 0
16
| metadata
dict |
|---|---|---|---|---|---|
004acd65-0f8a-4342-8761-8da04dfc73fe.5 | **Native Colombian Fruits and Their by-Products: Phenolic Profile, Antioxidant Activity and Hypoglycaemic Potential**
**Monica Rosa Loizzo 1, Paolo Lucci 2,\*, Oscar Núñez 3, Rosa Tundis 1, Michele Balzano 4, Natale Giuseppe Frega 4, Lanfranco Conte 2, Sabrina Moret 2, Daria Filatova 3, Encarnación Moyano <sup>3</sup> and Deborah Pacetti <sup>4</sup>**
Received: 25 January 2019; Accepted: 27 February 2019; Published: 3 March 2019
**Abstract:** The phenols and fatty acids profile and in vitro antioxidant and hypoglycaemic activity of seed, peel, pulp or pulp plus seeds of Colombian fruits from Solanaceae and Passifloraceae families were investigated. Ultra-High Performance Liquid Chromatography (UHPLC)-High Resolution Mass Spectrometry (HRMS) revealed the presence of chlorogenic acid as dominant phenolic compound in Solanaceae samples. Based on the Relative Antioxidant Score (RACI) and Global Antioxidant Score (GAS) values, *Solanum quitoense* peel showed the highest antioxidant potential among Solanaceae samples while *Passiflora tripartita* fruits exhibited the highest antioxidant effects among Passifloraceae samples. *P. ligularis* seeds were the most active as hypoglycaemic agent with IC50 values of 22.6 and 24.8 μg/mL against α-amylase and α-glucosidase, respectively. Considering that some of the most promising results were obtained by the processing waste portion, its use as functional ingredients should be considered for the development of nutraceutical products intended for patients with disturbance of glucose metabolism.
**Keywords:** Passifloraceae; Solanaceae; liquid chromatography; hypoglycaemic; α-amylase
#### **1. Introduction**
Colombia is a country with high levels of biodiversity and is home to a wide variety of exotic fruits. Traditionally, tropical fruit are consumed locally; nowadays, increasing production and more efficient transportation and refrigeration systems have led to increased global consumption with considerable quantities of tropical fruits that are now exported on a global scale. Tropical fruits not only present an attractive and characteristic exotic taste and aroma, but also represent a valuable source of bioactive compounds beneficial for humans. However, given the diversity of Colombian native and exotic species, the health properties and the chemical composition of some of these fruits have not been sufficiently studied.
It is well known that fruit phytochemicals play an important role against many ailments, including heart disease, diabetes, high blood pressure, cancer, etc. Among them, diabetes mellitus (DM) is
projected to reach pandemic proportion in the next 25 years [1]. The persistent hyperglycaemia status that characterises both forms of DM causes an increase in the production of reactive oxygen species (ROS) both of cytosolic and mitochondrial origin. ROS could determine the long-term deterioration of pancreatic islet β-cell by affecting mitochondrial Adenosine triphosphate (ATP) production that is necessary for insulin secretion. The consequent mitochondrial dysfunction influences insulin sensitivity within muscle, liver and adipose tissue [2]. One of the most common approaches to reduce the intake of sugar is by the reduction of their intestinal absorption using carbohydrates-hydrolysing enzymes inhibitors. In our previous research articles, the potential role of natural products as α-amylase and α-glucosidase inhibitors and antioxidant compounds was demonstrated [3–5].
In this context, to enhance scientific knowledge for the composition and health benefits of Colombian fruits and their by-products, we selected *Solanum quitoense*, *Physalis peruviana*, and *Cyphomandra betacea* (synonymous *Solanum betaceum* Cav) belonging to Solanaceae family, and *Passiflora pinnatistipula*, *Passiflora tripartita* and *Passiflora ligularis* belonging to Passifloraceae family.
*S. quitoense* has a characteristic *Citrus* flavour, sometimes described as a combination of rhubarb and lime [6]. Fruits are used to prepare juice or a drink called lulada. The fruit of *P. peruviana* is a smooth berry largely used to prepare snacks, pies or jams. It is used also in salads combined with avocado [7]. *Cyphomandra betacea* is one of the most popular fruit in South America where it is used to make juice. Fruits of *P. tripartita* are generally consumed raw or as ingredient of ice creams, fruit salads, pies, jellies or to make drinks [7]. *P. ligularis* fruit is also employed for the preparation of processed products like marmalade and jelly [3], while *P. pinnastipula* is consumed as drinks, ice-cream or marmalades [7].
After the selection, we have screened the fatty acids and the phenolics profile, as well as the antioxidant and hypoglycaemic potential of extracts from peel, pulp, seed or pulp plus seed of selected Colombian fruits in order to investigate the functional potential of each fruits portion, and to identify a potential use of the processing waste of these extracts as functional ingredients.
#### **2. Materials and Methods**
#### *2.1. Chemicals and Reagents*
All chemicals and reagents used in this study were purchased from Sigma-Aldrich Chemical Co. Ltd. (Milan, Italy) and VWR International (Milan, Italy) and, unless specified otherwise, were analytical grade or higher.
#### *2.2. Plant Material and Extraction Procedure*
Fruits were purchased at the local market in Bogotá (Colombia). At least 10 fruits were combined for each of the three replicated samples (one per month). Samples were examined and cleaned by using distilled water. Pulp, peel and seed were manually separated wherever it is possible in order to submit each portion to extraction procedure. The freeze-dried vegetable material (2.5 g of clean seeds or 5 g of peel or 5 g of pulp or seed + pulp) were finely ground with an Ultraturrax device (IKA®-Werke GmbH & Co. KG, Königswinter, Germany) and undergone to cold extraction with absolute ethanol (40 mL) for 24 h at room temperature (20 ◦C). The top phase was filtered through Whatman filter paper #4 (9.0 cm diameter) and the residue was re-extracted with 30 mL of absolute ethanol. The filtered top phases were combined and dried in a rotary evaporator (Buchi R-300, Milan, Italy) with operating temperature of 35 ◦C. Extraction yield ranged from 7.2 to 36.5 g ethanolic extracts on 100 g dry product in Solanaceae family and from 2.2 to 49.1 g/100 g dry product in Passifloraceae family. Extracts were then stored at −20 ◦C until further analysis.
#### *2.3. Ultra-High Performance Liquid Chromatography (UHPLC)-High Resolution Mass Spectrometry (HRMS). Conditions and Analysis of Phenolic Compounds in Colombian Fruits*
UHPLC-HRMS analysis was carried out using an Accela UHPLC system (Thermo Fisher Scientific, San Jose, CA, USA) equipped with a quaternary pump. For the chromatographic separation, an Ascentis Express C18 (150 × 2.1 mm, 2.7 μm, Supelco) column was employed. The mobile phase was a mixture of solvent A (0.1% formic acid in water) and solvent B (0.1% formic acid in acetonitrile) and the flow rate was 300 μL/min. Gradient elution program was run as stated: from 0–1 min, 10% B; 1–20 min, linear gradient from 10 to 95% B; 20–23 min, isocratic step at 95% B; 23–25 min, back to initial conditions at 10% B, and 25–30 min, isocratic step at 10% B for column re-equilibration. The UHPLC system was coupled to a Q-Exactive quadrupole-Orbitrap (Thermo Fisher Scientific) mass spectrometer equipped with a heated electrospray probe ionization source (H-ESI II). HESI-II was operated in negative ionisation mode. Nitrogen was used as a sheath gas, sweep gas and auxiliary gas at flow rates of 60, 0 and 10 a.u. (arbitrary units), respectively. Heater temperature was set at 350 ◦C. Capillary temperature was set at 320 ◦C and electrospray voltage at −2.5 kV. The HRMS instrument was operated in full MS scan with a m/z range from 100 to 1500, and the mass resolution tuned into 70,000 full width half maximum (FWHM) at m/z 200, with an automatic gain control (AGC) target (the number of ions to fill C-Trap) of 5.0e5 with a maximum injection time (IT) of 200 ms. Selected analytes belonging to different phenolic classes, namely gallic acid, (+)-catechin hydrate, p-coumaric acid, chlorogenic acid, (−)-epicatechin, ferulic acid, homogentisic acid, polydatin, syringaldehyde, taxifolin, umbelliferon, sinapic acid, kaempferol and vanillic acid, were monitored.
#### *2.4. Fatty Acids Profile*
An aliquot (20 mg) of ethanol extract from peel, seed and pulp of each fruit underwent alkaline transmethylation [8]. The gas-chromatography (GC) analysis of fatty acid methyl esters (FAME) was carried out using GC-430 apparatus (Varian, Palo Alto, CA, USA) equipped with a Flame Ionization Detector (FID) and a CPSil88 fused silica capillary column (100 m, 0.25 mm internal diameter, film thickness 0.2 μm, Chrompack, Middelburg, Netherlands). The carrier gas was helium at a flow rate of 1.6 mL/min; the oven temperature program started from 160 ◦C, rose to 240 ◦C at a rate of 4 ◦C/min and then remained at 240 ◦C for 10 min. The injector temperature was 260 ◦C. The sample was injected into a split/splitless system. Peaks were identified by comparison with known standards.
#### *2.5. ABTS and DPPH Radical Scavenging Assays*
The radical scavenging potential was investigated by using two different spectrophotometric methods: 2,2 -azino-bis (3-ethylbenzothiazoline-6-sulphonic acid (ABTS) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) assays [9]. The radicals (ABTS or DPPH) scavenging ability was calculated as follows:
$$\text{scAverging activity} = [(\text{A}\_0 - \text{A})/\text{A}\_0] \times 100$$
where A0 is the absorbance of the control reaction and A is the absorbance in the presence of the extract. Ascorbic acid was used as positive control.
#### *2.6. β-Carotene Bleaching Test*
The protection of extract on lipid peroxidation was measured as previously described [9]. Briefly, β-carotene solution was added to linoleic acid and 100% Tween 20. The absorbance of the samples, standard and control was measured at 470 nm against a blank at t = 0 and successively at 30 and 60 min. Propyl gallate was used as positive control.
#### *2.7. FRAP (Ferric Reducing Ability Power) Assay*
The FRAP assay was applied following the procedure previously described [7]. The FRAP value represents the ratio between the slope of the linear plot for reducing Fe3+-TPTZ reagent by different Colombian fruits extract compared to the slope of the plot for FeSO4. Butylated hydroxytoluene (BHT) was used as positive control.
#### *2.8. Relative Antioxidant Capacity Index (RACI) Calculation*
Relative antioxidant capacity index (RACI) is a statistical application to integrate the antioxidant capacity values generated from different in vitro methods [10]. Thus, data obtained from ABTS, DPPH, β-carotene bleaching tests and FRAP tests were used to calculate RACI value for samples. The standard score is calculated by using the following Equation:
$$(\mathbf{x} - \mu)/\sigma$$
where x is the raw data, μ is the mean and σ is the standard deviation.
#### *2.9. Global Antioxidant Score (GAS)*
For each sample, the average of T-scores was used to calculate the GAS value. T-score is calculated by the following Equation:
$$\text{T-score} = (\text{\textdegree } - \text{min}) / (\text{max} - \text{min})$$
where min and max, respectively, represent the smallest and largest values of variable X among the investigated extract [10].
#### *2.10. a-Amylase and a-Glucosidase Inhibitory Assays*
The α-amylase inhibitory test was performed as previously described [11]. Concisely, α-amylase solution, starch solution and colorimetric reagent were prepared. Control and samples at different concentrations were added to starch solution and left to react with the enzyme at room temperature for 5 min. The generation of maltose was quantified at 540 nm by the reduction of 3,5-dinitrosalicylic acid to 3-amino-5-nitrosalicylic acid.
The α-glucosidase inhibition was measured as previously described [11]. Both control and samples (at concentrations in the range 0.01–1 mg/mL) were added to maltose solution and left to equilibrate at 37 ◦C. The reaction was started by adding the enzyme and left to incubate at 37 ◦C for 30 min. A perchloric acid solution was used to stop the reaction. The supernatant was collected and mixed with peroxidase/glucose oxidase and *o*-dianisidine and left to incubate for 30 min at 37 ◦C. The absorbance was measured at 500 nm. Acarbose was the positive control in both tests.
#### *2.11. Statistical Analysis*
The inhibitory concentration 50% (IC50) was calculated by non-linear with the use of Prism Graphpad Prism version 4.0 for Windows, GraphPad Software, San Diego, CA, USA. Differences within and between groups were evaluated by one-way ANOVA followed by a multicomparison Dunnett's test (α = 0.05): \*\*\*\* *p* < 0.0001, \*\*\* *p* < 0.001, \*\* *p* < 0.05, compared with the positive controls.
The concentration-response curve was obtained by plotting the percentage of inhibition versus the concentrations.
#### **3. Results and Discussion**
#### *3.1. UHPLC-ESI-HRMS Phenolic Profile*
The phenolic profile of the ethanol extracts of Colombian fruits was outlined by UHPLC-ESI-HRMS analysis (Table 1). A total of 16 phenolic compounds were detected. Concerning the Solanaceae family, *C. betacea* showed the highest phenol concentrations. In all fruits, the peel extracts presented higher phenol amounts than those derived from pulp and seed. Despite a different content, chlorogenic acid resulted in the most abundant compound in all extracts. An appreciable amount of rutin hydrate was found in the peel extract from *S. quitoense*. This finding was in agreement
with data reported by Gancel et al. [12], who found that the main chlorogenic acid and dihydrocaffeoyl spermidine were located in all parts of *S. quitoense*, whereas the flavonol glycosides were exclusively present in the peel. *trans*-Cinnamic acid was recorded only in the *C. betacea* pulp extract. Moreover, although at low concentrations, ferulic acid and sinapic acid were revealed exclusively in peel and pulp extracts from *C. betacea*. A different phenolic profile was reported in *C. betacea* pulp (yellow variety) by Mertz et al. [13]. Besides chlorogenic acids, they found glycoside ester of caffeic and ferulic acids. These differences could be related to the different solvent used for phenolic extraction. In fact, acetone was used as extraction solvent [13]. Markedly different phenolic composition occurred in extracts from fruits belonging to Passifloraceae family. *P. pinnatistipula* peel extract showed *trans*-cinnamic acid as main component, followed by sinapic and gallic acids. In pulp extract, only two compounds were identified such as polydatin and rutin hydrate. The most abundant compound identified in seed extract was (−)-epicatechin. Differently, the peel extract of *P. tripartita* showed ferulic acid and (+)-catechin as main compounds. (+)-Catechin was more abundant in pulp and seed extracts, respectively. Moreover, the pulp was characterised by a high content of sinapic acid (211.4 ppm). The other identified compounds were in the range 0.2–11.9 ppm. Regarding *P. ligularis* fruit, the peel extract was mainly formed by (+)-catechin, whereas the pulp+seed extract was characterised by low total phenolic amounts (<5 ppm) with polydatin and vanillin as the main compounds. Unfortunately, no data about phenolic profile in the literature were found for the investigated Passifloraceae fruits.
#### *3.2. Fatty Acid Composition*
The average fatty acid composition of extracts from peel, pulp and seed of the investigated fruits is reported in Table 2. The fatty acid composition of the extracts changed according to the family fruit and to the part of the fruit. Generally, all extracts were mainly formed by essential fatty acids, such as α-linoleic (C18:2 n-6) and α-linolenic acid (C18:3 n-3) acids. In detail, linoleic acid represented the most abundant fatty acid in all Passifloraceae fruit extracts, except in the pulp extract from the *P. tripartita* fruit. This extract did not contain monounsaturated fatty acids. It presented four fatty acids: palmitic (C16:0), stearic (C18:0), linoleic and linolenic acids. The latter was the most abundant.
Concerning the Solanaceae family, α-linolenic acid (C18:3 n-6) was exclusively found in extracts from *P. peruviana*. Moreover, all extracts from *S. quitoense* and *P. peruviana* presented α-linolenic acid as major fatty acid. Differently, peel and pulp extracts from *C. betacea* presented the oleic acid (C18:1 n-9) as the predominant fatty acid, whereas the major fatty acid in seed extract was α-linoleic acid. In a previously study, Ramakrishnan et al. [14] investigated seed oil of *C. betacea* and they found that the most abundant fatty acids were linoleic (70.47%) and oleic (14.93%) acids.
#### *3.3. Antioxidant Activity*
Different tests were applied to screen the antioxidant activity of Colombian fruits. A concentration-effects relationship was found for all tested extracts in all tests except for FRAP assay (Table 3). DPPH and ABTS test were used to screen the radical scavenging potential. Among the Solanaceae family, the *S. quitoense* peel showed the highest DPPH radical scavenging potential with an IC50 of 38.8 μg/mL followed by *C. betacea* seed (IC50 of 57.9 μg/mL). Higher IC50 values were found when an ABTS assay was used. A promising protection of lipid peroxidation was observed with *S. quitoense* pulp+seed (IC50 of 6.9 μg/mL) followed by *P. peruviana* peel (IC50 of 10.2 μg/mL).
SFA, saturated fatty acid; MUFA,
monounsaturated
fatty acid; PUFA,
polyunsaturated
fatty acid; n.d.; not detectable. Results represents means ±
standard deviation (S.D.) (*<sup>n</sup>* = 3).
*Foods* **2019** , *8*, 89
Cation2,2-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS+), β-Carotene bleaching test, Ferric Reducing Ability Power (FRAP); Relative antioxidant capacity Index (RACI); Global Antioxidant Score (GAS); a Propyl gallate, ascorbic acid and butylated hydroxytoluene (BHT) were used as positive control in antioxidant test, while acarbose was used in carbohydrate hydrolysing enzyme inhibition assays. Differences within and between groups were evaluated by one-way ANOVA followed by a multicomparison Dunnett's test (=0.05): \*\*\*\* *p* < 0.0001, \*\*\* *p* < 0.001, \*\* *p* < 0.05 compared with the positive controls.
*C. betacea* peel exhibited a FRAP value similar to those reported for the positive control BHT (63.9 versus 63.2 μM Fe(II)/g). The antioxidant potential of *C. betacea* non-edible portion in terms of radicals scavenging ability and ferric reducing agent was previously described [15]. Several works evidenced also the antioxidant potential of *P. peruviana* whole fruit [9].
Considering Passifloraceae family, *P. tripartita* extracts showed the most promising DPPH radical scavenging activity with IC50 values of 3.2, 3.8, and 3.9 μg/mL for seed, pulp and peel, respectively. Additionally, peels exhibited an interesting ABTS+· radical scavenging potential (IC50 value of 50.1 μg/mL) and the best protection of lipid peroxidation (IC50 value of 3.8 μg/mL). Moreover, *P. tripartita* showed the highest ferric reducing ability and in particular seed exhibited a 1.4-time higher FRAP value than those reported for BHT. Analysis of the chemical profile revealed a high content of sinapic acid, (−)-epicatechin and ferulic acid in the pulp, seed and peel, respectively.
To get a ranking of Colombian fruits antioxidant capacity, relative antioxidant capacity index (RACI) and Global Antioxidant Score (GAS) were calculated. Both statistical applications were generated from the perspective of statistics by integrating the antioxidant capacity values generated from different in vitro methods (Table 3). Based on RACI and GAS value among fruits derived from Solanaceae family, *S. quitoense* peel had the highest antioxidant potential, while *P. tripartita* pulp exhibited the highest antioxidant capacity among investigated Passifloraceae fruits. Pearson's correlations coefficient evidenced a positive correlation between the content of identified phenol and FRAP value for both Solanaceae and Passifloraceae (*r* = 0.79 and 0.81, respectively). Phenolic compounds showed redox properties acting as reducing agents, hydrogen donators and singlet oxygen quenchers [16]. Selected Colombian fruits have been shown to be a good source of phenols that are known to possess a great antioxidant potential. In particular, these phytochemicals are able to act as free radical scavengers, metal chelator and to protect lipids from peroxidation. Moreover, the antioxidant activity exerted by phenols is the results of influence on cell signalling pathways and on gene expression. The antioxidant activity results from a complex interaction between different compounds in phytocomplex, which produce synergistic effect.
#### *3.4. Carbohydrate Hydrolysing Enzymes Inhibition by Colombian Fruits Extracts*
The inhibition of carbohydrates-hydrolysing enzymes α-amylase and α-glucosidase was investigated and results are herein reported. All investigated samples inhibited both enzymes in a concentration-dependent manner, however, generally, α-glucosidase showed to be more sensible (Table 3). An analysis of the data evidenced that *S. quitoense* peel extract had the best activity with IC50 values of 27.9 and 31.8 μg/mL for α-glucosidase and α-amylase, respectively. Additionally, a promising hypoglycaemic activity was observed with *P. peruviana* peel that showed IC50 values of 34.1 and 37.6 μg/mL against α-amylase and α-glucosidase, respectively. A lower IC50 value was found with *C. betacea* peel against α-glucosidase (IC50 of 32.9 μg/mL). Among Passiflorareae fruits, *P. ligularis* pulp + seed extract showed the highest activity with IC50 values of IC50 values of 22.6 and 24.8 μg/mL against α-amylase and α-glucosidase, respectively, followed by *P. pinnatistipula* (IC50 values of 46.4 and 37.7 μg/mL against α-amylase and α-glucosidase, respectively). Several samples showed a lower IC50 value than those reported for the largely prescribed drugs acarbose herein used as positive control.
Among the identified phytochemicals, flavonoids were mainly involved in the management of Type 2 Diabetes Mellitus (T2DM). These compounds were able to (a) inhibit carbohydrates-hydrolysing enzymes; (b) inhibit sodium-dependent glucose transporter 1 (SGLT1); (c) stimulate insulin secretion; (d) reduce hepatic glucose output; (e) enhance insulin-dependent glucose uptake. In particular, rutin, which was particularly abundant in *S. quitoense* peel extract, inhibited both -amylase and α-glucosidase with IC50 values of 0.043 and 0.037 μM, respectively [17]. A similar consideration could be done for chlorogenic acid that inhibited both α-amylase and α-glucosidase with IC50 values of 25 and 26.1 μM, respectively [18]. Different hypoglycaemic mechanisms were reported in the literature for this phytochemical. It stimulates glucose uptake in skeletal muscle and suppression of hepatic glucose production by the activation of AMPK. In addition, it has been found that it could modulate glucose in
both genetically and healthy metabolic related disorders including DM [19]. Moreover, *S. quitoense* peel extract was rich in linoleic acid. This acid showed a more potent α-glucosidase inhibitory activity than acarbose, whereas a weaker anti α-amylase activity was observed [20].
#### **4. Conclusions**
In the present study, we investigated the phenols and fatty acids profile and in vitro antioxidant and hypoglycaemic activity of edible and non-edible parts of Colombian fruits from Solanaceae and Passifloraceae families. The phenolic profile of the ethanol extracts of selected exotic fruits was reported herein for the first time. A total of 16 phenolic compounds were detected, with peel extracts showing the highest amount of phenolics compared to pulp and seed samples. Generally, chlorogenic acid and rutin hydrate were the most representative compounds. Considering only the fruit species in which it was possible to separate peel, pulp and seed, the results showed how the omega-3 fatty acids preferentially accumulated in the pulp and in the peel rather than in the seeds. Furthermore, it was found that *S. quitoense*, which contains considerable amount of rutin hydrate, had the highest antioxidant potential among the Solanaceae samples, while *P. tripartita* fruits exhibited the highest antioxidant capacity among the investigated Passifloraceae samples. Moreover, carbohydrate-hydrolysing enzyme inhibitory activity studies highlighted a promising hypoglycaemic activity of *P. ligularis* seed and *S. quitoense* peel samples. The latter demonstrated that carbohydrates-hydrolysing enzymes inhibition was higher than the commercial drug acarbose. Although some identified flavonoids possess hypoglycaemic activity, it is not possible to attribute these interesting properties to these compounds, but rather to the phytocomplex in which a synergism of action can occur.
Overall, the present results supported the possibility of using peels or seeds, which are normally considered as food by-products of juice production, as potential sources of bioactive ingredients in functional beverages, nutraceutical or dietary supplement formulations for the treatment and/or prevention of several diseases associated with oxidative stress such as type 2 diabetes.
**Author Contributions:** Conceptualization, P.L.; Data curation, N.G.F.; Formal analysis, R.T.; Investigation, M.R.L., O.N., R.T., M.B., D.F. and D.P.; Project administration, P.L.; Supervision, M.R.L., P.L., D.P. and O.N.; Visualization, M.B. and D.P.; Writing—original draft, M.R.L. and D.P.; Writing—review & editing, L.C., S.M. and E.M.
**Funding:** This research received no external funding
**Acknowledgments:** The authors are grateful to the Spanish Ministry of Economy and Competitiveness under the project CTQ2015-63968-C2-1-P. Daria Filatova is grateful to the European Commission for the category A Fellowship for Erasmus Mundus master program.
**Conflicts of Interest:** The authors declare that they have no conflict of interest.
#### **References**
© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
| doab | 2025-04-07T03:56:58.788471 | 1-5-2021 17:44 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004acd65-0f8a-4342-8761-8da04dfc73fe",
"url": "https://mdpi.com/books/pdfview/book/2994",
"author": "",
"title": "Application of Liquid Chromatography in Food Analysis",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783039433629",
"section_idx": 5
} |
004acd65-0f8a-4342-8761-8da04dfc73fe.6 | **Effect of Artificial LED Light and Far Infrared Irradiation on Phenolic Compound, Isoflavones and Antioxidant Capacity in Soybean (***Glycine max* **L.) Sprout**
#### **Md Obyedul Kalam Azad †, Won Woo Kim †, Cheol Ho Park and Dong Ha Cho \***
College of Biomedical Science, Kangwon National University, Chuncheon 24341, Korea;
[email protected] (M.O.K.A.); [email protected] (W.W.K.); [email protected] (C.H.P.)
**\*** Correspondence: [email protected]; Tel.: +82-33-250-6475
† Both authors contributed equally.
Received: 1 October 2018; Accepted: 19 October 2018; Published: 22 October 2018
**Abstract:** The effect of light emitting diode (LED) light and far infrared irradiation (FIR) on total phenol, isoflavones and antioxidant activity were investigated in soybean (*Glycine max* L.) sprout. Artificial blue (470 nm), green (530 nm) LED and florescent light (control) were applied on soybean sprout, from three to seven days after sowing (DAS) in growth chamber. The photosynthetic photon flux density (PPFD) and photoperiod was 150 ± <sup>5</sup> <sup>μ</sup>mol m<sup>−</sup>2s−<sup>1</sup> and 16 h, respectively. The FIR was applied for 30, 60 and 120 min at 90, 110 and 130 ◦C on harvested sprout. Total phenolic content (TP) (59.81 mg/g), antioxidant capacity (AA: 75%, Ferric Reduction Antioxidant Power (FRAP): 1357 μM Fe2+) and total isoflavones content (TIC) (51.1 mg/g) were higher in blue LED compared to control (38.02 mg/g, 58%, 632 μM Fe2+ and 30.24 mg/g, respectively). On the other hand, TP (64.23 mg/g), AA (87%), FRAP (1568 μM Fe2+) and TIC (58.98 mg/g) were significantly increased by FIR at 110 ◦C for 120 min among the treatments. Result suggests that blue LED is the most suitable light to steady accumulation of secondary metabolites (SM) in growing soybean sprout. On the other hand, FIR at 110 ◦C for 120 min is the best ailment to induce SM in proceed soybean sprout.
**Keywords:** controlled environment; far infrared irradiation (FIR); light emitting diode (LED) light; flavonoid; soybean sprouts
#### **1. Introduction**
Soybean sprouts are one of the most favorable healthy food by consumers in many countries—especially Korea, China and Japan. Sprouting soybean (*Glycine max* L.) are high in quality protein and dietary fiber and containing a lot of functional materials, including isoflavones. Consumption of isoflavones are associated with human health benefits, such as decreased risk of heart disease, menopausal symptoms, cardiovascular disease, as well as breast prostate cancers [1–3]. Isoflavones are categorized chemically according to their functional group, such as glycosides (daidzin, glycitin, genistin) and agylcones (daidzein, glycitein, genistein) (Figure 1).
**Figure 1.** Chemical structures of isoflavones.
Artificial light emitting diode (LED) light are being extensively used in controlled production system in order to improve the plant food quality. Light quality directly influences plant growth and chemical composition; therefore, it can be used as an external stimuli to obtain vegetal material with tailored composition [4]. The effects of LED illumination in sprout cultivation has been investigated in several species, such as *Brassica* spp. [5], pea, broccoli, mustard, borage, amaranth, kale, beet, parsley [6], and buckwheat [7].
Artificial blue LED light enhance secondary metabolites, such as ascorbate, total phenolic, anthocyanin, flavonoid contents, and antioxidant activity in basil [6]. Blue light is more efficiently absorbed by photosynthetic pigments than other spectral regions. Sun et al. [7] found that blue light drive CO2 fixation primarily in the upper palisade mesophyll while green light penetrates deeper and drives CO2 fixation in the lower palisade and upper spongy mesophyll. Green light is not directly involved in photosynthesis—however, it may affect plant growth and the synthesis of endogenous substances [8]. Swatz et al. [9] suggested that the effects of green light on plant growth and development are similar to those of blue light. Similar positive effects of blue and green light on plant growth, such as photosynthetic capacity and phytochemical production, have been reported on various plants [10,11].
Several reports have shown that the far infrared (FIR) enhanced nutritional quality of the plant foods viz. Chinese herbs, peanut, citrus cakes [12]. Plant secondary metabolites are present as a strong intermolecular covalently bound form with long chain of polymer [13]. The high penetration power of FIR helps the exudation of chemical components in the plant cells and thereby altering biological activity [14]. It is well documented that FIR liberate and activate low-molecular-weighted natural antioxidants in plants [15,16]. Previous researchers studied that FIR significantly increased free radical scavenging capacity in citrus press-cakes and total phenolic content in buckwheat sprouts [17,18].
However, there is not many data available on artificial LED light and FIR effect on phenolic content and antioxidant capacity of soybean sprouts either growing or processing stage, respectively. Therefore, the objectives of this study was to evaluate the effect of LED light and FIR on the accumulation and induce of bioactive compounds in soybean sprouts during growing and processing, respectively.
#### **2. Materials and Methods**
#### *2.1. Plant Growth Conditions and LED Light Application*
Soybean (*Glycin max* L. var. Seoritae) seeds were ringed with cold water and soaked for 24 h. Then the cleaned seeds were put into a planter having small holes in the bottom. The planter was put in the dark growth chamber. Artificial blue (450–495 nm), green (510–550 nm) LED (Green Power LED Production module. Philips, Poland) and florescent lamps (as a control) were turned on to soybean
sprout from two DAS till eight DAS. The photosynthetic photon flux density (PPFD), day/night temperature, relative humidity, CO2 and a photoperiod of the growth chamber were maintained at <sup>150</sup> ± <sup>5</sup> <sup>μ</sup>mol m−2s−1, 20 ± 0.5 ◦C, 75 ± 5%, 160 ± 10 ppm and 16 h, respectively. The samples were harvested at 3th, 4th, 5th, 6th and 7th DAS.
#### *2.2. Extraction of Soybean Sprout Grown under LED Lights*
The harvested samples were dried using oven at 50 ◦C and prepared powder using a grinder. Soybean dried powder 1 g were suspended in 100 mL of 80% ethanol and kept over-night in a shaker at room temperature. The extracts were filtered through Advantech 5B filter paper (Tokyo Roshi Kaisha Ltd., Saitama, Japan) and dried using a vacuum rotatory evaporator (EYLA N-1000, Tokyo, Japan) in 40 ◦C water bath to get crude extract. The crude extract was freeze dried to get moisture content 8–10%. Dried crude extract was diluted using 80% ethanol to prepare 1000 mg/L stock solution and kept at 4 ◦C for further analysis.
#### *2.3. Application of FIR Irradiation and Extraction Method of Soybean Sprout*
An independent experiment was conducted for the application of FIR irradiation on the soybean sprout. In this study, commercially available florescent lamp was used during sprout growing stage. Other climatic factors, photoperiod and duration of the light was same as described above.
Seven days old soybean sprouts were harvested and dried in oven at 50 ◦C and prepared powder using a grinder. Two grams of powder were mixed with 4 mL of water in a glass petri dish and exposed to FIR dryer (HKD-10; Korea Energy Technology, Seoul, Korea) at 90 ◦C, 110 ◦C, 130 ◦C temperatures, for 30 min, 60 min, 90 min. The treated samples were freeze dried to obtain moisture content 8–10%.
A control sample was collected without FIR application. The FIR treated dried powder and control samples of 1 g were suspended in 100 mL of 80% ethanol and kept over-night in a shaker at room temperature. The extract was filtered and produced crude extract by rotary evaporator as described above. Freeze dried crude extract was diluted using 80% ethanol to prepare 1000 mg/L stock solution.
#### *2.4. Estimation of Total Phenolic Content*
Total phenolic content (TP) was determined according to Folin-Ciocalteu assay [19]. In brief, a sample aliquot of 1 mL of stock solution was added to a test tube containing 0.2 mL of phenol reagent (1 M). The volume was increased by adding 1.8 mL of deionized water and the solution was vortexed and left for 3 min for reaction. Furthermore, 0.4 mL of Na2CO3 (10% in water, *v*/*v*) was added and the final volume (4 mL) was adjusted by adding 0.6 mL of deionized water. The absorbance was measured at 725 nm by spectrophotometer after incubation for 1 h at room temperature. The TP was calculated from a calibration curve using gallic acid standard and expressed as mg of gallic acid equivalent (GAE) per g dry weight basis (dwb).
#### *2.5. Estimation of Isoflavones Content*
Total six isoflavones, such as daidzin, glycitin, genistin, daidzein, glycitein and genistein, were analyzed. For the isoflvones estimation, 5 mL of stock solution were centrifuged at 3500 rpm for 10 min. The supernatant was filtered through 0.2 μm pore size syringe filter, type GV (Millipore, Bedford, MA, USA) prior to high performance liquid chromatography injection (HPLC) (Agilent, Stevens Creek Blvd Santa Clara, Santa Clara, CA, USA). Chromatographic separation was performed using a hypersil GOLD RP-18 column (250 mm × 4.00 mm, 5 μm) equipped with a photo diode array detector (Dionex Ultimate PDA-3000, (Dionex Softron GmbH, Dornierstrasse, Germany) at a flow rate of 1 mL min−<sup>1</sup> at 30 ◦C with an injection volume of 30 μL. Eluent A contained methanol: Acetonitrile (95:5, *v*/*v*) and eluent B contained water-acetic acid (94:6 mL/mL). Linear gradient was used starting with 5% B in A to reach 40% B in A in 25 min. Standard curve and retention times were calibrated using pure
standards of soybean isoflavones (Sigma Aldrich Co., St. Louis, MO, USA). All samples were analyzed in triplicate and results were expressed as milligrams per gram (mg/g).
#### *2.6. DPPH Free Radical Scavenging Capacity*
The antioxidant capacity was determined on the basis of the scavenging capacity of the stable 2, 2-diphenyl-1 picryl hydrazyl (DPPH) free radical according to methods described by Braca et al. [20] with slight modifications. One mL of stock solution was added to 3 mL of DPPH. The mixture was shaken vigorously and left to stand at room temperature in the dark for 30 min. The absorbance was measured at 517 nm using a spectrophotometer (UV-1800 240 V, Shimadzu Corporation, Kyoto, Japan). The percent inhibition activity were calculated against a blank sample using the following equation: inhibition (%) = (blank sample-extract sample/blank sample) × 100.
#### *2.7. Ferric Reduction Antioxidative Power (FRAP Assay)*
The FRAP was determined according to methods described by Benzie and Strain [21]. The FRAP reagent contained 20 mL of a 10 mM TPTZ (2,4,6-tripyridyl-s-triazine) solution in 40 mM HCl, 20 mL of 20 mM iron (III) chloride hexahydrate and 200 mL of 0.3 M acetate buffer at pH 3.6. The FRAP reagent was incubated at 37 ◦C in a water bath. Aliquots from stock solution of 5 mL were mixed with 45 μL of FRAP reagent. The absorbance was measured at 593 nm using spectrophotometer. Ferrous sulfate heptahydrate was used as a standard for the calibration curve and the results were expressed as FRAP value (μM Fe2+).
#### *2.8. Statistical Analysis*
Data are reported as mean ± standard deviation from triplicate analysis. Analysis of variance (ANOVA) accompanied with LSD and Tukey tests (SPSS, Version 15, IBM, New York, NY, USA)) were conducted to identify the significant differences among the samples (*p* < 0.05).
#### **3. Results and Discussion**
#### *3.1. Effect of Artificial LED Light on the Accumulation of Total Phenol, Isoflavones Content and Antioxidant Capacity*
Light is the important environmental que to improve the bioactive compounds in plant materials [22]. Light stimulate the enzyme activation and regulate the enzyme synthesize pathways, such as PAL (phenylalanine ammonia-lyase) activity in phenyl-propanoid pathways, which promote the bioactive compound accumulation in plant [23]. The isoflvones and total phenol content of the soybean sprouts grown under artificial LED light were shown in Table 1 and Figure 2. It is clearly depicted that isoflavones and total phenol were accumulated higher in the soybean sprout grown under blue LED light compared to the green and florescent light. The isoflavones, such as daidzin, glycitin, genistin, daidzein, glycerin and genistein, were significantly (*p* < 0.05) increased at the five and six DAS. A reduction of the isoflavones contents were observed with increasing DAS. Studies showed that blue light is the most effective lighting source to synthesis flavonoid compounds by stimulating PAL, CHS (chalcone synthesis) and DFR (dihydroflavonol-4-reductage) gene expression [24]. Chi et al. [25], showed that soybean sprouts grown under artificial LED light condition contained higher isoflavones content than those grown under florescent light. Cevallos and Cisneros [26] found higher phenolic content of soybean sprout at seven days of germination. Additionally, Troszy ´nska et al. [27] reported the highest concentration of polyphenols in lentils at seven days of germination. Conversely, phenolic content in mung bean decreased with germination days [28].
**Table 1.** Isoflavones contents in soybean sprout grown under different light treatments. (mg/g, dwb).
**Figure 2.** Total phenolic content in soybean sprouts grown under different light. Different lowercase letters within the row indicates significant differences (*p* < 0.05) according to ANOVA. LED: light emitting diode.
The DPPH inhibition activity provide the overall dietary antioxidant ability of the soybean sprout. This measurement is based on the reducing ability of antioxidants towards DPPH radical.
From the Figure 3 it is clearly shown that higher free radical activity (DPPH and FRAP) is demonstrated in blue LED light (75%) compared to green (69%) and control (58%). The FRAP assay showed a large difference in antioxidant reduction profile of soybean sprout in different LED treatments. The FRAP capacity is more than 2 times higher in soybean sprout when grown under blue LED light compared to control. Antioxidant properties of the sprouted vegetables were greatly enhanced by the blue light treatment [29]. Wu et al. [30] noticed that blue light have positive significant effect to improve antioxidant capacity of the sprouted pea seed.
**Figure 3.** Antioxidant capacity (DPPH and FRAP) of soybean sprout under different light treatment. The values are mean ± SE. (*n* = 3). DPPH, 2-diphenyl-1 picryl hydrazyl; FRAP, Ferric Reduction Antioxidant Power; SE, standard deviation. Different lowercase letters within the row indicates significant differences (*p* < 0.05) according to ANOVA.
The variation of flavonoids during sprouting can be due to catalytic activity of enzymes, such as hydrolases and polyphenoloxidases, which are reported to be activated in sprouting process [29]. Additionally, during the sprouting process, enzyme synthesis occur, which can result in the enhancement of intrinsic secondary compounds.
#### *3.2. Effect of FIR on the Accumulation of Total Phenol, Isoflavones Content and Antioxidant Capacity*
Total phenol and isoflavones content of FIR treated soybean sprouts are presented in Table 2 and Figure 3. It is clearly observed that FIR temperature and treatment duration has tremendous effect on TP and isoflavones accumulation in soybean sprout. All the isoflavones, such as daidzinm glycitin, genistin, daidzein, glycitein and genistein, were significantly increased (*p* < 0.05) at 110 ◦C among the FIR temperature. A significant increase of total isoflavones (58.98 mg/g) were observed in FIR of 110 ◦C at 120 min, which is nearly 2.3 times higher than the control (25.64 mg/g).
In this study, an application of FIR as a thermal treatment on soybean sprout caused an increase in isoflavones content at 110 ◦C with exposure time 120 min. Further increase in the FIR temperature decreased the isoflavones content. Increased isoflavones content is due to the transformation of high molecular compounds to low molecular compounds resulting from the breakage of covalent bonds of polymerized polyphenols [12]. A similar trend of increase in phenolic compound, due to different thermal treatment was also observed in different types of fruit and vegetables like sweet corn, ginseng, garlic, tomato, grapes, and onion [14]. The optimum temperature and treatment time of the FIR is depending on the plant materials. Ghimeray et al. [18] observed a highest quantity of the total phenol, total flavonoid and quercetin content of the buckwheat at 120 ◦C for 60 min of the FIR treatment. It is shown that, the isoflavone content reduced by FIR when exposed for a long time at a high temperature. Similar results were also reported by previous researcher [18].
It is observed in Figure 4 that FIR 110 ◦C/120 min treatment had the highest FRAP value (1568 μM Fe2+) among the FIR treatments. Comparing our results with the previous results by Ghimeary et al. [18] demonstrated that FIR treatment enhanced DPPH inhibition of the buckwheat. This result is also in agreement with the finding of Randhir er al. [26] where they showed that thermal treatment increased DPPH activity in buckwheat sprout. Our findings are concordant with previous results of total phenol content (Figure 2), showing the phenolics that reacts in Folin–Ciocalteu reaction has a similar interaction in FRAP reaction. Previous study showed higher phenolic and flavonoid content in buckwheat, which was directly correlated with their higher reduction powder [18].
**Figure 4.** Antioxidant capacity (DPPH and FRAP) of soybean sprout under FIR treatment. The values are mean ± SE. (*n* = 3). FIR: far infrared irradiation.
#### **4. Conclusions**
Growing soybean sprout under artificial blue LED light has shown the highest total phenol, total isoflavones content and antioxidant capacity (DPPH, FRAP). Results also highlighted that FIR thermal treatment increased TP and TIC content in a temperature and exposure time dependent manner. The highest TP and TIC were achieved at 110 ◦C with exposure time 120 min of FIR. Likewise, scavenging capacity on DPPH and FRAP also increased. The FIR irradiation enhanced aglycone production in soybean sprout, due to the breakage of glycoside bonds of isoflavones in a temperature dependent manner. The greatest increase in bioactive compounds of soybean sprouts was achieved at five to six DAS. The bioactive compounds of soybean sprouts may be optimized by tuning the artificial LED light condition and FIR application strategy during growing and processing, respectively.
**Author Contributions:** M.O.K.A., W.W.K. performed the experiment, M.O.K.A. wrote the article, C.H.P. revised and improved the manuscript and D.H.C. designed and supervised of the study.
**Funding:** This research received no external funding.
**Acknowledgments:** This study was supported by 2014 research grant (No. C1010686-01-01) and Institute of Bioscience and Biotechnology from Kangwon National University, Korea.
**Conflicts of Interest:** The authors declare no conflict of interest.
#### **References**
© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
| doab | 2025-04-07T03:56:58.790143 | 1-5-2021 17:44 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004acd65-0f8a-4342-8761-8da04dfc73fe",
"url": "https://mdpi.com/books/pdfview/book/2994",
"author": "",
"title": "Application of Liquid Chromatography in Food Analysis",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783039433629",
"section_idx": 6
} |
004acd65-0f8a-4342-8761-8da04dfc73fe.7 | **Characterization of Sparkling Wines According to Polyphenolic Profiles Obtained by HPLC-UV/Vis and Principal Component Analysis**
#### **Anaïs Izquierdo-Llopart \* and Javier Saurina \***
Department of Chemical Engineering and Analytical Chemistry, University of Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Spain
**\*** Correspondence: [email protected] (A.I.-L.); [email protected] (J.S.); Tel.: +34-934021277 (J.S.)
Received: 13 December 2018; Accepted: 8 January 2019; Published: 10 January 2019
**Abstract:** Cava is a sparkling wine obtained by a secondary fermentation in its own bottle. Grape skin contains several compounds, such as polyphenols, which act like natural protectors and provide flavor and color to the wines. In this paper, a previously optimized method based on reversed phase high performance liquid chromatography (HPLC) with ultraviolet/visible (UV/Vis) detection has been applied to determine polyphenols in cava wines. Compounds have been separated in a C18 core-shell column using 0.1% formic acid aqueous solution and methanol as the components of the mobile phase. Chromatograms have been recorded at 280, 310 and 370 nm to gain information on the composition of benzoic acids, hidroxycinnamic acids and flavonoids, respectively. HPLC-UV/vis data consisting of compositional profiles of relevant analytes has been exploited to characterize cava wines produced from different base wine blends using chemometrics. Other oenological variables, such as vintage, aging or malolatic fermentation, have been fixed over all the samples to avoid their influence on the description. Principal component analysis and other statistic methods have been used to extract of the underlying information, providing an excellent discrimination of samples according to grape varieties and coupages.
**Keywords:** liquid chromatography; polyphenols; protected designation of origin; coupages; sparkling wine (cava); characterization; chemometrics
#### **1. Introduction**
Cava is a sparkling wine of high quality with Protected Designation of Origin (PDO) produced by the Champenoise method based on the second fermentation and aging period in its own bottle [1,2]. Cava is highly popular in our society: it is currently the most exported Spanish wine. For this reason, control strategies are needed to guarantee the standards of quality and the maintenance of organoleptic characteristics among winemaking batches. Part of this evaluation is carried out by sensory analysis with a group of expert panelists. Alternatively, analytical methods can be implemented to satisfy the increasing demand of controls, to reduce costs and to achieve reliable results [3]. The study of polyphenols in wines is a hot topic in the field of food analysis, because of the implications on organoleptic and descriptive issues [3,4]. For instance, the role of anthocyanins as principal pigments of wine or the influence of tannins on bitterness and astringency are well-known, while, apparently, they have a little direct contribution to odor [5].
Red wines from black skin grapes are much richer in polyphenols than rosé or white wines due to the simple fact that these compounds are extracted from the skin grape during the maceration process. Furthermore, cavas produced from the Chardonnay variety have a high content in polyphenolic substances; however, they differ in composition from those found in the varieties of black grapes [6]. Moreover, polyphenols are the substrate for some enzymatic changes of must and the main responsible for non-enzymatic auto-oxidation processes of the wines. Polyphenols have also shown a great interest as nutraceuticals because of the wide range of beneficial effects attributed, including antioxidant, antimicrobial and anti-inflammatory activities [7,8].
It has been pointed out elsewhere that naturally occurring components of food product may serve as efficient descriptors of some food features such as origin, varietal constituents, manufacturing practices, etc. [3,9]. Among the different food matrices, wines have been studied extensively on the basis of contents of chemical species, such as volatile compounds [10], inorganic elements [11,12], organic acids [13], amino acids and biogenic amines [14], polyphenols [15,16], etc. The corresponding compositional profiles and instrumental fingerprints have been used as the source of chemical information [17] to tackle characterization, classification and authentication issues, often with the assistance of chemometric methods.
In the field of cava and wines, polyphenolic data can be correlated with factors (e.g., geographic origin, climate and terrain, grape varieties, time of harvest and winemaking procedures) with a high impact on quality and economic issues, such as the qualification with the PDO or the final price [6]. Some applications have been reported on the role of polyphenols and related compounds as descriptors of cava features. It should be remarked that, in some of the cases mentioned below, principal component analysis (PCA) and related chemometric methods (cluster analysis, discriminant analysis, etc.) were used to facilitate the extraction of relevant information. A pioneering publication evaluated the descriptive ability of main components of base white wines from Macabeu, Xarel·lo and Parellada grapes to infer on their discrimination according to varieties, vintages or wineries; in the study, authors pointed out the possibilities of species such as cinnamic acids as potential varietal markers [18]. In another case, Martinez-Lapuente et al. studied the effect of alternative grape varieties on the polyphenolic composition and sensorial features of sparkling wines [19]. Patters on color and taste attributes and overall quality depending on varieties were drawn. It was concluded that Alvaring and Verdejo wines resulted in highly attractive alternatives. In a recent paper, the effect of some oenological practices on the composition of phytochemicals of rosé sparkling wines was evaluated; the influence of some additives to preserve the levels of the most abundant polyphenols was assessed as well [20]. Stefenon and coworkers investigated the influence of aging on lees on the polyphenolic content and antioxidant activity of sparkling wines obtained under Champenoise and Charmat winemaking protocols [21]. The authors found out changes in the evolution of the contents of species, such as tirosol, gallic acid, resveratrol or piceid, which varied differently depending on the vinification method. Another study compared some parameters of Champenoise, Charmat and Asti methods on the aromatic composition of Moscato Giallo sparkling wines. The authors concluded that the total polyphenolic content and the in vitro antioxidant activity of samples did not show significant differences among the three vinifications [22]. An investigation on biological aging and storage of cava wines revealed differences in the composition of browning components related to phenolic and furfural species as a function of the process time; the concentration of hydroxylmethylfurfural could be proposed as an index of cava quality [23]. A similar research also demonstrated the influence of oenological and aging processes on color changes and phenolic composition of rosé sparkling wines; in this case, color features could be associated to the transformation of polymeric species into absorbing anthocyanins as a consequence of aging [24]. Finally, Bosch-Fusté et al. investigated the viability of the total index of phenols defined as the absorbance at 280 nm as a quality marker of cava wines subjected to an accelerated aging process. The study of two sets of samples showed an increase in the phenolic index after seven weeks of accelerated breeding [25].
In this paper, relevant polyphenols were determined in different cava samples by an HPLC-UV/vis method previously established [26]. Compounds were separated chromatographically on a C18 column under an elution program based on increasing the percentage of methanol. In order to focus on varietal and blending issues more specifically, samples under study were chosen homogeneously regarding some winemaking factors such as vintage, aging in cellars and application
of malolactic fermentation. As a result, descriptors of wine blending could be investigated without the influence of other disturbances. PCA was applied to compositional data to carry out an exploratory study of the coupages made with different grape varieties. Despite the lack of selective descriptors, results showed that various polyphenols were more abundant in some coupages. As a result, we concluded that the proposed method resulted in a simple, fast and suitable approach towards the characterization and classification of cavas according to base wine varieties and blends using polyphenolic concentrations as the source of information.
#### **2. Materials and Methods**
#### *2.1. Chemicals and Solutions*
The mobile phase was prepared with formic acid (>96%, Sigma-Aldrich, St. Louis, MO, USA), methanol (UHPLC-Supergradient, Panreac, Barcelona, Spain) and water (Elix3, Millipore, Bedford, MA, USA). Polyphenols of analytical quality were purchased from Sigma-Aldrich (St. Louis, MO, USA): gallic, homogentisic, protocatechuic, caftaric, gentisic, vanillic, caffeic, syringic, ferulic, *p*- coumaric acids, (+)-catechin, (−)-epicatechin, ethyl gallate, resveratrol, rutin, myricetin and quercetin. These compounds were selected according to their abundance and relevance in white and rosé cavas. Stock standard solutions of 1000 mg L−<sup>1</sup> of each polyphenol were prepared in MeOH. Working standard solutions consisting of a mixture of analytes at concentrations ranging from 20 to 0.05 mg L−<sup>1</sup> were prepared in MeOH:water (1:1, *v:v*).
#### *2.2. Samples*
Cava samples under analysis were kindly provided by the winery Codorníu Raventós S.A. Cavas were produced from base wines of Penedès and Costers del Segre regions (both from Catalonia, Spain). Samples consisted of white and rosé cavas all of them of 2015 vintage and aged for a period of 18 months, elaborated from various coupages of base wines of Chardonnay, Macabeu, Xarel·lo, Parellada, Pinot Noir, black Garnacha and Trepat varieties as follows: Coupage C (58 samples) corresponded to the three classical varieties: Macabeu, Xarel·lo and Parellada; Coupage E (six samples) was based on the classical combination with a small percentage of Chardonnay; Coupage A (15 samples) consisted of Chardonnay (70%) plus Macabeu, Xarel·lo and Parellada blending (30%); Coupage G (three samples) was Chardonnay 100%; Coupage S (two samples) was Chardonnay (50%) plus Macabeu and Xarel·lo (50%); Coupage T (four samples) corresponded to a rosé cava from Chardonnay (30%) and Pinot Noir (70%); Coupage V (nine samples) corresponded to a rosé cava from Pinot Noir, black Garnacha and Trepat. For each coupage, samples were independently produced (i.e., from different wine batches) and bottled. From the point of view of sugar content, the set comprised 11 brut nature (<3 g L−<sup>1</sup> sugar), 61 brut (3–12 g L<sup>−</sup>1), 13 dry (17–32 g L−1) and 12 semi-dry (32–50 g L−1).
Samples were degasified and filtered through a nylon membrane (0.45 μm pore size) prior HPLC-UV/vis analysis. A quality control solution consisting of mixture of 50 μL of each cava sample was prepared to evaluate the reproducibility of the chromatographic method and the significance of the descriptive PCA models. Cava samples were analyzed randomly and the quality control was injected every 10 samples.
#### *2.3. Chromatographic Method*
The chromatographic system consisted of an Agilent Series 1100 HPLC Chromatograph (Agilent Technologies, Palo Alto, CA, USA) equipped with a quaternary pump (G1311A), a degasser (G1322A), an automatic injection system (G1392A) and a diode array detector (G1315B). An Agilent ChemStation for LC 3D (Rev. A. 10.02) software was used for instrument control and data processing.
The chromatographic method was optimized and validated elsewhere [26]. Briefly, the separation was carried out in a Kinetex C18 column (Phenomenex, Torrance, CA, 100 mm × 4.6 mm internal diameter with 2.6 μm particle size). 0.1% of formic acid in water (*v/v*) (solvent A) and methanol (solvent B) were used to create the following elution gradient and the flow rate was 1 mL min<sup>−</sup>1: from 0 to 20 min, B(%) 15–60 lineal increase; from 20 to 22 min, B(%) 60–90 lineal increase; from 22 to 27 min, B(%) 90 isocratic range, cleaning step; from 27 to 27.5 min, B(%) 90–15; from 27.5 to 30, B(%) 15 isocratic range, conditioning step. The injection volume was 10 μL. Chromatograms were acquired at 280, 310 and 370 nm.
#### *2.4. Data Analysis*
Preliminary statistics on polyphenolic contents in cava samples were gained from box and radial plots obtained with Excel. Exploratory studies by Principal Component Analysis (PCA) were carried out using the PLS-Toolbox working with MATLAB. The data matrix to be treated consisted of concentration values of quantified polyphenols in the cava samples. Data was autoscaled to achieve a similar weight to all the polyphenols regardless differences in amplitude and magnitude.
Sample patterns related to the different grape varieties and blends and other oenological practices were deduced form the interpretation of the scatter plot of scores of the first principal components (PCs). The distribution of variables on the space of the PC, shown in the plot of loadings, provided information on polyphenol correlations. Compounds up- or down expressed, which could be considered as tentative markers, were assessed from the simultaneous interpretation of scores and loadings graphs.
#### **3. Results and Discussion**
As indicated above, an HPLC-UV/vis method previously developed for the determination of relevant polyphenols in white wines [26] was here extended to cava (sparkling) wines. Samples under study were similar with respect to various oenological features such as aging, vintage, malolactic fermentation or Champenoise vinification, thus, the variability was associated to the varietal origin of grapes and the combination of base wines. The three main cava classes corresponded to the so-called classical blend (Macabeu, Xarel.lo and Parellada), the rosé blend (Pinot Noir, black Garnacha and Trepat) and the Chardonnay monovarietal cava. The other cava types evaluated consisted of combinations of classical, rosé and Chardonnay wines in different percentages. Figure 1 shows the chromatograms of representative rosé, classical and Chardonnay samples recorded at 280 nm. The chromatogram of a standard mixture of polyphenol at 5 mg L−<sup>1</sup> was also included for identification purposes. It can be seen that most of the selected compounds were detected although some differences in the concentration levels were encountered among coupages. Analytes were quantified by linear regression models stablish using 10 standard mixtures with concentrations ranging from 0.05 to 20 mg L<sup>−</sup>1. Differences in compositional profiles were the basis of further sample discrimination as a function of base wine varieties and coupages.
The analysis of the 97 samples under study revealed that, in general, hydroxycinnamic acids and their esters with tartaric acid were predominant species in this kind of winemaking process; benzoic acids were also abundant, while flavonoids were scarcer. As detailed in Table 1, wine samples were rich in gentisic and caftaric acids, with mean concentrations of 32.2 and 11.2 mg L−1, respectively. Gallic, homogentisic and caffeic acids, and catechin occurred at moderate levels, ranging from 1 to 5.5 mg L<sup>−</sup>1. Other minor components such as epicatechin, protocatechuic acid, p-coumaric acid vanillic acid and syringic acid were found at mean concentrations below 1 mg L−1. Resveratrol and flavonoids, such as rutin and myricetin, were scarce (not detected in all the samples). The descriptive ability of each polyphenol (i.e., the information contained in each variable) was accounted from the relative standard deviation of the concentration in the series of cavas. The variability ranges given in Table 1 indicated that concentrations of gallic and gentisic acids were quite homogeneous in all the samples regardless of grape varieties or coupages, with overall relative standard deviation (RSD) values below 10%. This finding suggested that the apparent discriminant capacity of these polyphenols to classify cava wines was certainly limited. In contrast, contents of compounds such as epicatechin, protocatechuic acid, vanillic acid, ferulic acid and syringic acid were more heterogeneous, with RSD (%) values higher
than 30%. As a result, we guessed that they could result in biomarker candidates to distinguish the different cava classes.
**Figure 1.** Chromatograms recorded at 280 nm using the proposed HPLC-UV/vis method. (**A**) Standard solution at 5 mg L−1; (**B**) Rosé wine composed of Pinot Noir, Trepat and black Garnacha varieties (**C**) Classical grape varieties coupage composed of Macabeu, Xarel·lo and Parellada and (**D**) White cava composed of Chardonnay variety. Peaks assignment: (1) gallic acid, (2) homogentisic acid, (3) protocatechuic acid, (4) caftaric acid, (5) gentisic acid, (6) catechin, (7) vanillic acid, (8) caffeic acid, (9) syringic acid, (10) ethyl gallate, (11) epicatechin, (12) *p*-coumaric acid, (13) ferulic acid, (14) resveratrol, (15) rutin and (16) myricetin.
**Table 1.** Average concentration values of polyphenols in the set of samples under study.
Standard deviation (SD) and relative standard deviation (RSD) indicated the variability of concentrations as a measure of discriminating capacity among samples.
Boxplots and radial diagrams were used to explore the performance of polyphenols to distinguish cava as a function of coupages. Figure 2 depicts the overall analyte content and mean concentration values of analytes in the white and rosé cava classes.
**Figure 2.** Boxplots with whiskers with the polyphenolic composition of the sets of (**A**) white and (**B**) rosé cava samples under study. Error bars indicated the variability in the concentration values.
In general, rosé (Pinot Noir, Trepat and black Garnacha combinations) and Chardonnay cavas showed the richest overall polyphenol contents, while the classical bends of Macabeu, Xarel·lo and Parellada cavas contained lower polyphenol levels (Figure 3A). In accordance with our previous results, gentisic acid was homogenously distributed among classes (Figure 3B); concentrations of the vast majority of samples ranged between 25 and 35 mg L−1. This compound was not up- or down-expressed in any particular variety, thus it resulted in a poor descriptor. Additionally, gallic acid was widespread, although its concentration was slightly increased in rosé samples with Pinot Noir and black Garnacha varieties. Syringic acid was important in rosé cavas while it was quite residual in classical and Chardonnay-based cavas (Figure 3C); a similar pattern was observed for protocatechuic acid and epicatechin. Catechin (Figure 3D) and homogentisic acid were more abundant in Chardonnay and occurred at low concentrations in the coupages rich in classical varieties; as a result, catechin contents were highly dependent on the percentages of this variety in the samples. Another interesting pattern corresponded to vanillic acid which was present at low concentrations in rosé cavas (ca. 0.2 mg L<sup>−</sup>1) and reached 5- to 10-fold higher concentrations in the other classes (Figure 3E). All the hydroxycinnamic acids (caffeic, ferulic, coumaric, etc.) behaved in a similar way, being more abundant in Chardonnay and rosé samples. Other compounds were less relevant for descriptive purposes as they displayed a less systematic behavior.
**Figure 3.** Radial plots of polyphenolic concentrations in the different coupages. (**A**) Overall content of analytes; (**B**) gentisic acid; (**C**) syringic acid; (**D**) catechin; (**E**) vanillic acid. Solid line indicates the mean value; dotted lines indicated the ± standard deviation values. Variety assignation: Ma, Macabeu; Xa, Xarel·lo.
#### *Principal Component Analysis*
Compositional profiles were used as the data to be treated by PCA for a more comprehensive study focused on identifying potential markers of the different classes of wine varieties and blends. The dataset consisted of concentration values of each polyphenol in the cava samples and the quality controls (QCs). As remarked in the experimental section, QCs were included to assess the soundness of the model. A first exploration carried out on the whole data matrix indicated that samples were reasonably distributed according to the coupages. Some minor components, such as resveratrol, rutin and myricetin, with concentrations close to the detection limits of the method, displayed high variability (RSD values in the QC replicates higher than 20%); thus, they were excluded from the dataset to obtain a more robust and accurate description of sample behavior. The rest of the variables, which exhibited RSD values lower that 6%, were taken for analysis.
The PCA model working with the refined set indicated that circa 75% of the information was captured with three principal components (PCs). In particular, PC1 retained 41.3% of variance, PC2 23.3% and PC3 9.9%. Scatter plots of scores of the relevant PCs revealed interesting patterns on sample distribution. In the case of PC1 versus PC2 (Figure 4A), QCs were located in a compact group in the central area of the graph, thus proving that the overall model was reliable and robust.
Interestingly, PC1 modeled the overall polyphenolic content with the richest cavas on the right and the poorest on the left. PC2 explained differences among rosé, classical and Chardonnay (top, center and bottom, respectively). More specifically, three main trends in the sample distribution were encountered, namely: (i) cavas in the left side were mainly elaborated with classical grape varieties of Macabeu, Xarel·lo and Parellada; (ii) samples in the bottom-right corner corresponded to monovarietal Chardonnay; (iii) rosé cavas prepared with black grapes appeared in the top-right corner. Intermediate situations of coupages elaborated with variable percentages of Chardonnay (e.g., classical varieties combined with 30, 50 or 70% of Chardonnay or rosé varieties combined with 30% Chardonnay) were distributed accordingly (see Figure 4A). Finally, although not shown here, PC3 mainly modeled the variations of Chardonnay proportions in the coupages, with larger scores for those samples containing higher percentages.
The scatter plot of loadings of PC1 versus PC2 (Figure 4B) described the behavior of the polyphenolic variables. It was first deduced that the levels of some compounds were reasonably correlated, e.g., syringic and gallic acids, and caffeic and coumaric acids, with correlation coefficients higher than 0.6. Other species were more negatively correlated (e.g., ethyl gallate and caftaric acid). The simultaneous study of scores and loadings indicated that classical cava coupages contained higher amounts of gentisic acid and ethyl gallate as they occupied the left side of the graphs. Syringic and gallic acids were representative of rosé cavas elaborated with red grapes varieties of Pinot Noir, Trepat and red Garnacha. Finally, catechin, homogentisic, caftaric, caffeic and coumaric acids were more abundant in Chardonnay cavas and coupages created with high percentages of this variety.
**Figure 4.** Principal component analysis of the dataset consisting of polyphenol concentrations of each cava sample. (**A**) Plot of scores of PC1 versus PC2; (**B**) plot of loadings of PC1 versus PC2. Acronyms: C Chardonnay; Ma Macabeu; Pa Parellada; Xa Xarel·lo; QC Quality control; R rosé. Symbols: Star = classical coupage (Ma + Xa + Pa); Triangle (vertex up) = rosé cava; Triangle (vertex down) = Chardonnay cava; circle = QC.
Apart from the relationship between phenolic matter and grape varieties, the sugar content was another variable of the set of samples under study. The liquor of expedition after bottle disgorging provided different concentrations of sugar, ranging from 50 g L−<sup>1</sup> for semi-dry to less than3gL−<sup>1</sup> for brut nature. In the current case, PCA plots showed that the sugar content was uncorrelated with polyphenols and had no influence on the sample distribution. Regarding geographical factors and agricultural practices that may affect substantially the phenolic levels, their effect on the set of samples under study was buffered, since enologists combined appropriate proportions of base wines from different regions to obtain products with more constant physicochemical and sensorial attributes, according to commercial parameters of quality and the costumer's preferences. Finally, as indicated above, other variables such as vintage and aging, which may notably influence on the polyphenolic composition, were controlled for; thus, patterns observed mainly depended on the combinations of base wines.
#### **4. Conclusions**
Sparkling wines belonging to the protected designation of origin Cava were analyzed with a chromatographic method for the determination of polyphenols. Wines from different coupages were considered including monovarietal Chardonnay samples, rosé samples composed of Pinot Noir, Trepat and black Garnacha, the classical blends of Macabeu, Xarel·lo and Parellada and other mixtures containing variable amounts of Chardonnay. Other oenological variables, such as vintage, aging and malolactic fermentation, were fixed for all the samples so that they did not affect the description. In general, overall polyphenolic contents were higher in Chardonnay wines, followed by rosé, while the classical coupages were circa 20% poorer in these components. Patterns deduced from boxplots and radial diagrams were confirmed from a comprehensive data exploration by principal component analysis. Although no molecule was found to be a specific descriptor of cava classes, syringic acid was more abundant in rosé wines, the hydroxycinnamic acids were up-expressed in Chardonnay samples and vanillic acid was almost residual in red grape products. Summarizing, polyphenolic profiling combined with a chemometric data analysis resulted in an excellent approach to deal with the characterization of cava wines according to the coupages. This strategy could be extended to further classification and authentication purposes.
**Author Contributions:** A.I.-L. performed all the experiments and wrote the paper. J.S. conceived, supervised the work and revised the paper.
**Funding:** This research was funded by the Spanish Ministry of Economy and Competitiveness (project CTQ2014-56324-C2-1-P) and by Generalitat de Catalunya (project 2017 SGR-171).
**Acknowledgments:** The authors gratefully acknowledge the support received from and Codorníu Raventós winery providing the samples.
**Conflicts of Interest:** The authors declare no conflict of interest.
#### **References**
© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
#### *Article*
| doab | 2025-04-07T03:56:58.791260 | 1-5-2021 17:44 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004acd65-0f8a-4342-8761-8da04dfc73fe",
"url": "https://mdpi.com/books/pdfview/book/2994",
"author": "",
"title": "Application of Liquid Chromatography in Food Analysis",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783039433629",
"section_idx": 7
} |
004acd65-0f8a-4342-8761-8da04dfc73fe.8 | **Characterization and Determination of Interesterification Markers (Triacylglycerol Regioisomers) in Confectionery Oils by Liquid Chromatography-Mass Spectrometry**
**Valentina Santoro 1,\*, Federica Dal Bello 1, Riccardo Aigotti 1, Daniela Gastaldi 1, Francesco Romaniello 1,2, Emanuele Forte 2, Martina Magni 2, Claudio Baiocchi <sup>1</sup> and Claudio Medana <sup>1</sup>**
Received: 15 January 2018; Accepted: 12 February 2018; Published: 16 February 2018
**Abstract:** Interesterification is an industrial transformation process aiming to change the physico-chemical properties of vegetable oils by redistributing fatty acid position within the original constituent of the triglycerides. In the confectionery industry, controlling formation degree of positional isomers is important in order to obtain fats with the desired properties. Silver ion HPLC (High Performance Liquid Chromatography) is the analytical technique usually adopted to separate triglycerides (TAGs) having different unsaturation degrees. However, separation of TAG positional isomers is a challenge when the number of double bonds is the same and the only difference is in their position within the triglyceride molecule. The TAG positional isomers involved in the present work have a structural specificity that require a separation method tailored to the needs of confectionery industry. The aim of this work was to obtain a chromatographic resolution that might allow reliable qualitative and quantitative evaluation of TAG positional isomers within reasonably rapid retention times and robust in respect of repeatability and reproducibility. The resulting analytical procedure was applied both to confectionery raw materials and final products.
**Keywords:** confectionery fats; positional isomers; silver-ion HPLC; triglycerides
#### **1. Introduction**
There is a great demand from food industry of fats with different degrees of solidity. However, most of the fats coming from natural sources are oils so they need to be suitably transformed in order to modulate their hardness. Interesterification, by changing the position of fatty acids within the original triglyceride structure, is the process more commonly used to modify consistency of oils [1]. Such result may be obtained either chemically using a catalyst or by the use of enzymes. In both cases, positional isomers (regioisomers) from original triglycerides are formed and the amount of their formation modulates the final physical properties of the fat.
TAGs (triglycerides) composition of a fat is perceived by the consumer since it is associated with the spreading properties, hardness and palatability of the product.
Small variations in TAG structure may have an impact on crystallization and polymorphic properties of fats. Thermal processes applied for technological reasons (e.g., deodorization) may produce positional isomers, inducing changes in the physical characteristics of fats. In fact,
deodorization at high temperatures induces formation of asymmetric TAGs influencing crystallization behavior. The above changes must be monitored by analytical methods and kept under control in the course of the industrial process in relation to quality control and research and development of new TAGs composition that can improve product quality.
Among natural fats, cocoa butter has a particular relevance because of its unusual and highly valued physical properties and its peculiar triglycerides' content. They are symmetrical monounsaturated TAGs containing three major fatty acids: palmitic (P), stearic (S) and oleic (O) and three main glycerides, palmitoyl-oleoyl-palmitoyl glycerol (POP), palmitoyl-oleoyl-stearoyl glycerol (POS) and stearoyl-oleoyl-stearoyl glycerol (SOS), with the unsaturated fatty acid located in the central position as it is typical in vegetable oils.
If thermal process for deodorization is applied to cocoa butter, positional isomers may be produced with a change in the physical characteristics of the product. As already said, deodorization at high temperatures induces a significant formation of asymmetric TAGs influencing the crystallization behavior.
The main analytical technique for regiospecific analysis of TAGs is silver-ion high performance liquid chromatography (HPLC) coupled via an atmospheric pressure chemical ionization (APCI) source to a mass (MS) detector. Silver ion chromatography is based on the interaction of silver ions, bounded to a cation-exchange stationary phase, with π electrons of double bonds of unsaturated TAGs [2,3]. However, even if silver-ion HPLC remains the unique separation technique of unsaturated fats, its intrinsically difficult use in terms of repeatability and reproducibility must be underlined. Generally, the equilibration times are long and separations in gradient conditions are not easily reproducible.
In addition, conditioning times between successive analyses are time-consuming due to the necessary transition between two mobile phase compositions having different strength.
Furthermore, separation in isocratic conditions does require daily, before starting a sequence, long conditioning times and blank chromatographic runs to attain reproducibility. Separation of TAGs containing fatty acids having different unsaturation degrees by silver-ion HPLC has been the object of several publications [4–18].The aim of many of them was to obtain a detailed qualitative profile of complex mixtures of animal and vegetable fats. However, baseline separation of variously unsaturated TAGs, necessary for reliable quantitative analyses, was not always achieved because of the matrix complexity. In many cases, when three columns in series were used, retention times were inevitably long [15,16].
More recently, the use of traditional reversed phase column (polymeric bonded phase) was reported. However, the separation of critical regioisomer couples not always was enough to assure accurate quantitative evaluation of each component and their retention times were also long [19].
An interesting recent work [20] presents a rapid separation of TAG regioisomers in animal fats by Differential Mobility Spectrometry (DMS). However, the optimization of various experimental parameters such as the separation and compensation voltages applied to DMS electrodes, the type and flow rate of chemical modifier and the dwell time of analyte ions in the DMS cell is required. In addition, to be expensive, the technique is sophisticated and scarcely suitable to the analytical skill generally present in industry internal quality control laboratory.
The aim of the present paper is to describe a separation method of individuated typical TAGs of cocoa butter, palm and other tropical oils mainly used in confectionery industry. The main couples of triacylglycerol regioisomers that had to be separated, identified and quantified were, respectively, POP/PPO (palmitoyl-palmitoyl-oleoyl glycerol), POS/PSO (palmitoyl-stearoyl-oleoyl glycerol, SOS/SSO (stearoyl-stearoyl-oleoyl glycerol) and POO/OPO (palmitoyl-oleoyl-oleoyl glycerol/oleoylpalmitoyl-oleoyl glycerol), SOO/OSO (stearoyl-oleoyl-oleoyl glycerol/oleoyl-stearoyl-oleoyl glycerol).
The chromatographic problem we had to face up to was challenging because we needed to obtain retention times as low as possible and selective enough to allow a reliable peak integration. A task not easy to obtain because we had two classes of compounds with very different retention times due to a different number of double bonds (e.g., POP in respect to POO, etc.) and with a very subtle structural difference within the single class (different position of an oleic acid residue within the triglyceride structure). Separation of the regioisomer couples before cited, reported in literature, was characterized by very long retention times (40–60 min) [14–16] not compatible with the analysis times useful for a confectionery industry internal control laboratory.
As a matter of fact, the high chromatographic selectivity necessary to separate the regioisomers belonging to the same class is conflicting with the quite lower selectivity required to separate the constituents of the two classes. In other words, higher selectivity conditions (elution phase of low force) necessary to separate the couples POP/PPO, POS/PSO and SOS/SSO would cause the species POO/OPO and SOO/OSO to have very different and longer retention times. As a consequence, we will try to find a compromise and, differently from the works reported in literature, we used cation exchange columns silver modified by us in order to achieve a better control on retention mechanism by varying, if necessary, the degree of silver modification of the stationary phase.
Due to the small number of TAG isomers to be separated belonging to two classes of molecules differing by the number of double bonds, we adopted two different isocratic steps and a rapid transition between them as a more controllable separation mode.
#### **2. Materials and Methods**
#### *2.1. Reagents*
Isopropanol, acetonitrile, *n*-heptane and ethyl acetate LC-MS (Liquid Chromatography-Mass Spectrometry)-grade solvents, POP, PPO, POS, PSO, SOS, SSO, OPO, POO, SOO, and OSO TAG pure standards and anhydrous Na2SO4 were purchased from Sigma-Aldrich (St. Louis, MO, USA).
#### *2.2. Sample Preparation*
Samples obtained from confectionery industry were previously heated to complete fusion and homogenized by mechanical stirring. Successively were dehydrated with Na2SO4 and filtered through Buchner. A sample aliquot (10.0 mg) was added to of *n*-heptane (10 mL) and analyzed after suitable dilution.
#### *2.3. Calibration Curve Preparation*
A stock solution of all standards mixed together was prepared in *n*-heptane at a concentration of 1000 ppm each. To draw a calibration curve, the stock solution was diluted to contain 1.0, 2.0, 3.0, 5.0, 8.0, 10.0 ppm, respectively.
#### *2.4. Instrumentation*
The separation system was a binary solvent HPLC Ultimate 3000 (Thermo Fisher Scientific, Milan, Italy) interfaced through an APCI ionization source to a linear ion trap coupled to a high resolution mass analyzer (LTQ-Orbitrap Thermo Fisher Scientific, Milan, Italy). APCI ionization source heated at 450 ◦C was used in positive ions mode in the mass range 240–1000 Th.
Capillary temperature was 250 ◦C, flow rate of sheath gas and auxiliary gas were set at 35.0 and 15.0 arbitrary units, capillary voltage was 25.0 V, source voltage 6.0 kV, and tube lens 110.0 V. Mass resolution was set at 30,000.
#### *2.5. Chromatography*
A cation exchange column (Luna SCX, Phenomenex, 150 × 2.0 mm, 5 μm, 100 Å) silver-modified by us was used. A gradient separation made up by two isocratic steps at different mobile phase compositions separated by a rapid transition time of five minutes between them was used. The two mobile phase compositions (*n*-heptane:ethylacetate 93:7 and 90:10, respectively) were chosen according to elution times and peak resolution. Flow rate was 0.300 mL/min. The injection volume was 10.0 μL.
#### *2.6. Silver Modification of SCX Columns*
After the inversion of the flow direction, we rinsed the column with methanol/acetonitrile (CH3OH/CH3CN) 1:1 for about 45–60 min (flow rate 1.0 mL/min). As the concentration of AgNO3 solution and the number and volume of injections determine the retention characteristic of the modified SCX stationary phase, we tested three concentrations of AgNO3 solution (0.57 M, 1.18 M and 1.86 M). During the rinsing procedure, ten 50.0 μL injections of a 1.18 M AgNO3 solution in CH3CN allowed for obtaining a chromatographic result suited to our requirements. The column was successively inverted to the original position and rinsed with dichloromethane for an hour at a flow rate of 0.20 mL/min.
#### *2.7. Statistical Evaluation of Final Data and Validation*
The analytical procedure, the object of the present work, is destined to be applied to raw material and to final confectionery products (interesterified or not), so real samples of whatever concentration may be at one's disposal. However, validation parameters like Limit of Detection (LOD) or Limit of Quantitation (LOQ) were anyway evaluated. Very important for our purposes are parameters like linearity, measure uncertainty, repeatability and reproducibility. Their definition is assuming particular significance in the present analytical context because taking under control the mobile and stationary phase conditions ruling the chromatographic performances was a particularly demanding task as described before.
In addition, as already stated, a determination of the amount of positional isomers, as accurate as possible, is functional to establish a reliable relationship with the interesterification conditions.
A calibration curve was realized in six replicates and each curve was made by six points spanning from 1.0 to 10.0 ppm. The straight line was calculated by a linear regression method and each calibration point was the mean of the six replicates. The *R*<sup>2</sup> value was used as a first linearity evaluator. The successive statistical calculations were based on the evaluation of the mean calibration curve error. Hubaux-Vos formulae were applied [21].
In Table 1, all data concerning linearity, measure uncertainty, repeatability, reproducibility, LOD and LOQ for the positional isomers object of this study are reported along with the mean calibration curves for the standards.
**Table 1.** Validation data: calibration curve parameters, repeatability, reproducibility, uncertainty measure, limit of detection (LOQ) and limit of quantification (LOQ).
**Table 1.** *Cont.*
<sup>1</sup> Repeatibility; mean of three replicates; <sup>2</sup> reproducibility; mean of six replicates.
#### **3. Results and Discussion**
In literature, there are no papers devoted explicitly to specific regioisomers separation. They are concerning characterization of triacylglycerol composition in animal fats [13] or complex natural triacylglycerol mixture [15]. General works regarding the effect of temperature and mobile phase composition [12] on regioisomer separation involved triacylglycerol positional isomers very different from those envisaged in the present work. Anyway, when couples like POP/PPO or SOS/SSO, important in our case, were occasionally present in some of the reported separations in literature, they showed retention times very long (between 40 and 60 min) and the obtained chromatographic selectivity was due to the use of three columns in series. Such separation conditions are incompatible with the typical exigencies of industrial analytical laboratories. Thus, we had to reformulate the separation in a manner more tailored to the molecular characteristics of the specific TAG regioisomers of interest in confectionery industry and to the performance of their internal laboratories.
As a matter of fact, the choice of chromatographic separation conditions characterized by two isocratic steps reported in the Materials and Methods section was the result of a fine-tuning of the mobile phase strength. The mobile phase composition must be strong enough to assure reasonably short retention times and, at the same time, not too strong to level out the chromatographic resolution based on different stereochemistry of TAG positional isomers.
At first, we checked the difference in selectivity between two elution strength modifiers like acetonitrile and ethyl acetate, which have a very different elution force. Mixtures of *n*-hexane or *n*-heptane and a very small percent of acetonitrile (0.8–0.11%) provide good separations of unsaturated TAGs [3–5]. In these cases, acetonitrile concentration must be very low due to its high elution force and its poor miscibility with *n*-hexane or *n*-heptane. Such a mixture is unstable and tends to give chromatographic results not easily reproducible. The HPLC binary pump was unsatisfactory in the delivery of the mixed mobile phase because of its low mixing equilibrium; consequently, pre-mixing was necessary. Sometimes, in order to overcome this problem, isopropanol is used to intermediate the two immiscible solvents [11–18]. The equilibrium of the mixture was
always precarious. Subtle differences in mixture preparation, temperature oscillations and different rates of unavoidable evaporation from the HPLC reservoirs of the three mobile phase components were all factors that often had a substantial influence on retention times. Maintaining time elution reproducibility was a challenge, and day-by-day chromatographic variations were recorded, making conditioning times progressively longer. All of these troubles are perfectly managed in academic studies where careful attention to difficult experimental conditions can be paid. On the contrary, in the case of internal laboratories of confectionery industry, a more easily applicable protocol would be at one's disposal.
Alternatively, the use of ethyl acetate as a modifier of *n*-heptane allowed for obtaining slightly better reproducible chromatography, conditioning times shorter and by far better results' reproducibility.
Different elution conditions were also tested and eventually a satisfactory chromatographic resolution was obtained with two successive isocratic segments involving *n*-heptane:ethyl acetate compositions of 93:7 and 90:10, respectively, with five minutes to change from the initial to final composition. In addition, only one column was used to obtain a satisfactory separation.
In Figure 1, the separation of standard samples of regioisomeric couples POP/PPO, POS/PSO, SOS/SSO, POO/OPO and SOO/OSO, at the same concentration (10.0 ppm), monitored by a linear ion trap is shown.
**Figure 1.** Chromatographic separation of standard TAG (triglyceride) regioisomers visualized at the extracted precursor ion m/z values.
As it can be seen, the chain length of TAG fatty acids has extremely limited influence on the retention times. The different spatial position of the double bond of oleic acid in sn-2 with respect to sn-1/sn-3 positions is enough to produce separation of regioisomers with the same number of double bonds whereas the presence of two double bonds change drastically the retention times.
The MS analyser provided structural information already coming from the thermal lability of triglycerides in the hot (450 ◦C) APCI ionization source. Triglycerides tend to release their constituent fatty acids in a typical way involving primarily the ones in the external position and to a lesser extent the one in the central position [22]. This behavior allows for distinguishing the isobaric couples of positional isomers.
An example of full mass spectra of standard TAG positional isomers POP/PPO is shown in Figure 2a,b. The qualitative pattern of thermal product ions is very similar but with different signal intensity as a consequence of the selective fragmentation pathway privileging the loss of fatty acids in external positions.
**Figure 2.** Full mass spectra of isobaric positional isomers POP (palmitoyl-oleoyl-palmitoyl glycerol) (**a**) and PPO (palmitoyl-palmitoyl-oleoyl glycerol) (**b**) showing the specificity of thermal fragmentation mechanism.
By applying the same experimental conditions to interesterified samples obtained from the confectionery industry, shea oil or palm olein, it is possible to obtain evidence of their interesterification degree, as shown in the chromatographic profiles of Figure 3a,b. As expected, the signal intensity of the different couples of regioisomers in the two oils reflects their TAG composition, in particular the low presence of palmitic acid in shea oil TAG constituents.
**Figure 3.** Chromatographic separation of TAG regioisomers typical of interesterified Shea oil (**a**) and Palm oil (**b**) visualized at the extracted precursor ion m/z values.
Figure 4a–c shows LC-MS chromatograms of a cocoa butter samples pre and after deodorization at two different temperatures (220 ◦C and 260 ◦C) corresponding to the increasing presence of thermally formed positional isomers, respectively (as an example, only the couple POP, PPO is reported).
**Figure 4.** LC-MS (Liquid chromatography-mass spectrometry) chromatograms of cocoa butter samples pre deodorization (**a**) and deodorized at two different temperatures 220 ◦C (**b**) and 260 ◦C (**c**), corresponding to the absence and to the presence of thermally formed positional isomers, respectively.
In addition to confectionery industry raw materials, samples of final products were analyzed. In Tables 2–4, meaningful results are reported about a commercial biscuit filling cream and two spreading creams. As it can be seen, there is evidence of TAG regioisomer formation in two final products with respect to the original raw materials used.
**Table 2.** Comparison of TAG (triglyceride) percent composition between original raw materials and confectionery final products (biscuit filling cream).
**Table 3.** Comparison of TAG percent composition between original raw materials and confectionery final products (spreading cream A).
<sup>1</sup> Coming from vaccine butter fraction present in milk powder.
**Table 4.** Comparison of TAG percent composition between original raw materials and confectionery final products (spreading cream B).
Thus, it was possible to establish a reliable relationship between the extent of positional isomers formation and the experimental conditions to which industry samples can undergo. In this way, on the one hand, undesirable regioisomer formation may be avoided and, on the other, a regioisomer formation may be guided until the achievement of the desired physico-chemical properties.
#### **4. Conclusions**
Interesterification process of vegetable oils is of paramount importance in the confectionery industry because positional isomer formation allows for modulating the fat property. On the other hand, unwanted interesterification events may occur during thermal treatment of fats as in the deodorizing process. Therefore, either in the case of intentionally producing an interesterification process or checking its unwanted occurrence, it is necessary to have at one's disposal an analytical method helping to establish a sound relationship between the extent of positional isomer presence and the experimental conditions causing their formation.
In this work, we developed an analytical protocol based on LC-MS analysis able to identify and quantify the positional isomers typical of confectionery fats undergoing the interesterification process or thermal treatment. The time required for the global analysis was relatively short, the chromatographic resolution and efficiency were satisfactory and the mass detection allowed for identifying the isobaric components of each positional isomer couple.
In conclusion, the method described may well be considered a good diagnostic tool of interesterification consequences that are strictly connected to confectionery product quality.
**Acknowledgments:** The authors greatfully acknowledge Soremartec Italy srl for providing the raw materials for the experiments.
**Author Contributions:** V.S., F.D.B. and F.R. designed and performed the chemical analysis; V.S. and C.B. wrote the paper; D.G. and R.A. analyzed the data; E.F. and M.M. proposed the topic and contributed reagents/materials/analysis tools; C.M. performed a scientific supervision and manuscript revising.
**Conflicts of Interest:** The authors declare no conflict of interest.
#### **References**
© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
| doab | 2025-04-07T03:56:58.793349 | 1-5-2021 17:44 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004acd65-0f8a-4342-8761-8da04dfc73fe",
"url": "https://mdpi.com/books/pdfview/book/2994",
"author": "",
"title": "Application of Liquid Chromatography in Food Analysis",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783039433629",
"section_idx": 8
} |
004acd65-0f8a-4342-8761-8da04dfc73fe.9 | **Authentication and Quantitation of Fraud in Extra Virgin Olive Oils Based on HPLC-UV Fingerprinting and Multivariate Calibration**
#### **Núria Carranco 1, Mireia Farrés-Cebrián 1, Javier Saurina 1,2 and Oscar Núñez 1,2,3,\***
Received: 30 January 2018; Accepted: 20 March 2018; Published: 21 March 2018
**Abstract:** High performance liquid chromatography method with ultra-violet detection (HPLC-UV) fingerprinting was applied for the analysis and characterization of olive oils, and was performed using a Zorbax Eclipse XDB-C8 reversed-phase column under gradient elution, employing 0.1% formic acid aqueous solution and methanol as mobile phase. More than 130 edible oils, including monovarietal extra-virgin olive oils (EVOOs) and other vegetable oils, were analyzed. Principal component analysis results showed a noticeable discrimination between olive oils and other vegetable oils using raw HPLC-UV chromatographic profiles as data descriptors. However, selected HPLC-UV chromatographic time-window segments were necessary to achieve discrimination among monovarietal EVOOs. Partial least square (PLS) regression was employed to tackle olive oil authentication of Arbequina EVOO adulterated with Picual EVOO, a refined olive oil, and sunflower oil. Highly satisfactory results were obtained after PLS analysis, with overall errors in the quantitation of adulteration in the Arbequina EVOO (minimum 2.5% adulterant) below 2.9%.
**Keywords:** high performance liquid chromatography; UV detection; multivariate calibration; food authentication; olive oils; fraud quantitation
#### **1. Introduction**
Olive oil is one of the main ingredients in the Mediterranean diet, and over the past few years its consumption has also spread outside the Mediterranean basin. This is due to both its particular taste and remarkable nutritional properties. In fact, virgin olive oil (VOO) is the primary fat source in the Mediterranean diet, and it is obtained either by mechanical or direct pressing of the pulp of the olive fruit (*Olea europaea* L.). The olives, after being crushed to form pomace, are homogenized and pressed. VOO is not subjected to any treatment other than washing, decantation, centrifugation, and filtration [1]. Extra-virgin olive oil (EVOO) is considered to be the highest quality olive oil, and is characterized by high levels of beneficial constituents [2]. Apart from the natural unrefined state, olive oils of lower quality known as refined olive oils (ROOs) are also produced. For example, an extracted oil with poor sensory characteristics, high free fatty acid content, or peroxide values exceeding the established limits, is unfit for human consumption. This oil is then classified as lampante oil and needs to be refined. Additionally, pomace oils are solvent-extracted from olive pomace and also undergo refining [1,3]. Triglycerides are the major constituents of EVOOs, representing more than 98% of the total weight. The fatty acid composition of these triglycerides involves saturated fatty acids (11%), around 80% monounsaturated fatty acids (oleic acid being the most important feature of olive oils in comparison to other vegetable oils), and polyunsaturated fatty acids (9%). Furthermore, EVOO also contains antioxidant and anti-inflammatory compounds such as squalene, phytosterols, and highly bioavailable polyphenols and phenolic acids belonging to different families [2,4–9]. These compounds provide important beneficial health effects.
The beneficial characteristics of EVOOs and their high quality with respect to other edible oils make them an expensive product, susceptible to being adulterated with oils produced from cheaper fruits or seeds [10], or even with other lower quality olive oils. Moreover, lately there have been suspicions among consumers and administrations that in some cases producers have labeled virgin olive oils or even refined ones as EVOOs in order to increase profits. A comprehensive academic food fraud study spanning 30 years in the Journal of Food Science showed that olive oil was the single most commonly referenced adulterated food of any type in scholarly articles from 1980 to 2010 [11]. This is a relevant problem not only because of the financial issues affecting olive oil-producing countries such as Spain or Italy, but also in terms of health and consumer protection concerns. However, identification of some fraud is especially difficult, or can be time- and solvent-consuming when using traditional methodologies for the identification of adulteration, e.g., when the cheaper oil is added to EVOO at a level lower than a certain percentage. Therefore, the reduction of the current levels of fraud in olive oils requires the constant development of new analytical techniques for the detection of possible adulteration. In this regard, the study of compositional differences among olive oils and other edible vegetable oils which could be used as adulterants (e.g., because of their low cost) has been the basis for authentication approaches.
The types of polyphenols and phenolic acids and their concentration levels in olive oils depend on multiple parameters such as climate conditions, water resources, growing area, cultivation techniques, soil management, and the degree of maturation of the olives. Moreover, the oil production method (malaxation and extraction) can also affect the final quantities of these compounds [12–15]. As a result, polyphenol and phenolic acid content can be exploited as a source of analytical data to establish vegetable oil product classifications and to achieve product authentication [16,17], thus preventing fraud by detecting and quantifying product adulteration.
Several analytical methodologies have been employed in the determination of polyphenols in olive oils [18,19]. One of the most critical points is the extraction procedure, which is frequently addressed by liquid–liquid extraction (LLE) [20–24] and solid phase extraction (SPE), mainly using diol-bonded SPE cartridges [8,22,24–28]. Capillary electrophoresis (CE), liquid chromatography (LC), and gas chromatography (GC) are among the separation techniques usually employed for the determination of polyphenols [18,19,29]. However, the necessity for a derivatization step prior to GC separation [30] and the low sensitivity of CE methods when on-column UV detection is employed [20,24,31] make LC the technique of choice nowadays for the determination of polyphenols and phenolic compounds in olive oils. With respect to detection, electrochemical detection (ECD) [26], fluorescence [32], ultra-violet (UV) detection [8,24,32–34], mass spectrometry (LC-MS) [35] or tandem mass spectrometry (LC-MS/MS) [24,27,28,36] are currently used. Recently, due to the complexity of edible oil samples and the high structural variability of polyphenols, liquid chromatography coupled to high resolution mass spectrometry (LC-HRMS) appears to be the best technique for the identification and determination of phenolic compounds in vegetable oils [22,25], although these techniques are still expensive for many food control laboratories. Lately, interest in the classification and characterization of olive oils with respect to olive varieties, as well as in the identification and quantitation of fraud to guarantee product authentication, has been increasing, and several studies devoted to these topics can be found in the literature [21,37–40].
The aim of the present work was to develop a simple, cheap, and reliable high performance liquid chromatography method with UV detection (HPLC-UV) for the generation of polyphenolic fingerprints in the classification and authentication of olive oils. Data corresponding to the HPLC-UV polyphenolic fingerprints recorded at different wavelengths were considered as a source of potential descriptors to be exploited for the characterization and classification of edible oils (olive, sunflower, soy, and corn oils) and olive oils from different olive cultivars (Arbequina, Picual, Hojiblanca, and Cornicabra) by exploratory principal component analysis (PCA). Finally, Arbequina EVOO was adulterated with different amounts (2–85%) of Picual EVOO, a commercial low quality ROO, and a sunflower oil. The HPLC-UV data was evaluated by means of partial least squares (PLS) regression for authentication purposes as well as for the quantification of adulterant content.
#### **2. Materials and Methods**
#### *2.1. Chemicals and Standard Solutions*
Unless specified, analytical grade reagents were used. Methanol (Chromosolv™ for HPLC, ≥99.9%), hexane, and formic acid (≥98%) were obtained from Sigma-Aldrich (St. Louis, MO, USA), and ethanol (absolute) from VWR International Eurolab S.L. (Barcelona, Spain).
Water was purified using an Elix 3 coupled to a Milli-Q system (Millipore, Bedford, MA, USA) and was filtered through a 0.22-μm nylon membrane integrated into the Milli-Q system.
#### *2.2. Instrumentation*
An Agilent 1100 Series HPLC instrument equipped with a G1311A quaternary pump, a G1379A degasser, a G1392A autosampler, a G1315B diode-array detector, and a computer with the Agilent Chemstation software (Rev. A 10.02), all from Agilent Technologies (Waldbronn, Germany), was employed to obtain the HPLC-UV chromatographic fingerprints for the PCA and PLS studies. Chromatographic separation was carried out in reversed-phase mode by using a Zorbax Eclipse XDB-C8 column (150 × 4.6 mm i.d., 5 μm particle size) also provided by Agilent Technologies. Formic acid (0.1%, *v*/*v*) aqueous solution (solvent A) and methanol (solvent B) were used as mobile phase to stablish the gradient elution as follows: 0–2 min at 10% B (initial conditions); 2–4.5 min linear gradient from 10% B to 25% B; 4.5–7 min at 25% B; 7–22 min linear gradient from 25% B to 90% B; 22–24 min at 90% B; 24–25 min back to initial conditions at 10% B; and 25–30 min at 10% B for column equilibration. A mobile phase flow-rate of 1 mL min−<sup>1</sup> and an injection volume of 10 μL were employed. Photodiode array (PDA) acquisition from 190 to 600 nm was performed to register UV spectra and to guarantee peak purity when necessary. HPLC-UV fingerprints for PCA and PLS analysis were obtained by direct UV absorption detection at 257, 280, and 316 nm.
#### *2.3. Samples and Sample Treatment*
Two sets of vegetable oil samples belonging to different trademarks and purchased from markets in Barcelona (Spain) were used in this work. The first set of samples consisted of 72 vegetable oils distributed as follows: 47 olive oils, 16 sunflower oils, 2 corn oils, 2 soy oils, and 5 vegetable oils produced from mixtures of seeds (3 sunflower/corn oils and 2 sunflower/soy oils). Among the 47 olive oils belonging to this first set of samples, 10 were obtained from Arbequina olives and 5 from Picual olives. No information regarding the olive cultivar of other 32 olive oil samples was available. The second set of samples consisted of 66 EVOO samples as follows: 23 from Arbequina olives, 19 from Picual olives, 12 from Hojiblanca olives, and 12 from Cornicabra olives. Additionally, several refined olive oil samples were also employed for the adulteration studies.
Sample treatment was carried out following a previously described method with some modifications [28]. For that purpose, 2.00 g of oil were weighed into a 15 mL polytetrafluoroethylene (PTFE) tube (Serviquimia, Barcelona, Spain), and extracted with 2 mL of an ethanol:water (70:30, *v*/*v*) solution by shaking vigorously for 2 min using a vortex (Stuart, Stone, UK). The extract was then centrifuged for 5 min at 3500 rpm (Rotanta 460 RS centrifuge, Hettich, Tuttlingen, Germany). In order to facilitate the quantitative recovery of the extract solution, the PTFE tube was then frozen for 24 h at −18 ◦C, and the corresponding extract transferred into another 15 mL PTFE tube for further clean-up. Defatting was performed with 2 mL of hexane, shaking vigorously for 2 min in a vortex, and centrifuging for 5 min at 3500 rpm. Finally, the aqueous ethanolic extracts were transferred into 2 mL injection vials to be analyzed with the proposed HPLC-UV method.
Besides, a quality control (QC) consisting of a mixture of 50 μL of each ethanolic aqueous sample extract was prepared to evaluate the repeatability of the method and the robustness of the chemometric results.
For authentication studies by PLS regression, three cases were considered in which an Arbequina EVOO sample was adulterated with different amounts (from 2.5 to 85%) of a Picual EVOO sample, a ROO sample, or a sunflower oil sample, respectively. Hence, apart from those pure extracts (three samples each), mixtures of the Arbequina EVOO sample and the adulterant oil were as follows: 85% adulterant (3 samples), 80% adulterant (3 samples), 60% adulterant (3 samples), 55% adulterant (3 samples), 50% adulterant (8 samples), 40% adulterant (3 samples), 30% adulterant (3 samples), 20% adulterant (3 samples), 12% adulterant (3 samples), 10% adulterant (3 samples), 7% adulterant (3 samples), 5% adulterant (3 samples), 3% adulterant (3 samples), and 2.5% adulterant (3 samples), for each adulterant oil employed.
#### *2.4. Data Analysis*
SOLO chemometric software from Eigenvector Research was used for calculations with PCA and PLS regression [41]. A detailed description of the theoretical background of these methods is given elsewhere [42].
Data matrices to be treated by PCA consisted of the HPLC-UV chromatographic fingerprints obtained at different acquisition wavelengths (257, 280, and 316 nm). HPLC-UV chromatograms were pretreated to improve the data quality while minimizing solvent and matrix interferences, peak shifting, and baseline drifts (for additional details see [43]). In some cases, segmented HPLC-UV fingerprints were employed to improve PCA classification. Scatter plots of scores and loadings of the principal components (PCs) were used to investigate the structure of maps of samples and variables, respectively.
The percentage of the oil used for adulteration (Picual EVOO, ROO, or sunflower oil) in the adulterated Arbequina EVOO samples analyzed was quantified using PLS. Samples available were distributed among training and test sets as follows. Training set: 100% adulterant (3 samples), 80% adulterant (3 samples), 60% adulterant (3 samples), 50% adulterant (8 samples), 40% adulterant (3 samples), 20% adulterant (3 samples), 10% adulterant (3 samples), 5% adulterant (3 samples), 2.5% adulterant (3 samples), and 100% Arbequina EVVO (3 samples). The remaining samples considered as unknown (85% adulterant, 55% adulterant, 30% adulterant, 12% adulterant, 7% adulterant and 3% adulterant, 3 samples each) were used for validation and prediction purposes. For both training and test steps, X-data matrices consisted of the HPLC-UV chromatographic fingerprints and the Y-data matrices contained the oil adulteration percentages.
#### **3. Results and Discussion**
#### *3.1. Exploratory Studies by Principal Component Analysis*
As a first study, the characterization and classification of olive oils with respect to other edible vegetable oils (sunflower, corn, and soy oils, as well as mixtures of them) was attempted by using raw HPLC-UV chromatographic profiles (i.e., absorbance over time) as analytical data for PCA. Reversed-phase HPLC-UV chromatographic fingerprints were obtained by employing a previously developed method in a Zorbax Eclipse XDB-C8 column under gradient elution using methanol and 0.1% aqueous formic acid solutions as mobile phase [21]. HPLC-UV chromatographic fingerprints were studied at three acquisition wavelengths: 257, 280, and 316 nm. The reproducibility of the extraction step and its influence on the PCA results was studied elsewhere [21]. Relative standard deviation (RSD%) values of polyphenol features ranged from 0.3 to 8.3% depending on the levels of components. Replicate samples have similar PCA scores, thus indicating that extraction variabilities did not significantly affect the descriptive and quantitative performance of chemometric models. The variability of chromatographic runs was evaluated from the repetitive injections of the QC (every 10 sample injections) throughout the series. In this way, the influence of peak shifting and/or baseline drift on the PCA models was deduced from trends observed in the scores plot. When QCs were not clustered randomly due to systematic deviations, corrective data pretreatments such as peak alignment and baseline subtraction were applied [43].
A first set of samples consisting of 72 commercially available vegetable oils was employed. The ethanolic extracts as well as the QCs (see Experimental section) were randomly analyzed with the proposed method, and the obtained HPLC-UV fingerprints subjected to PCA. At first, the performance of the resulting models was quite limited due to experimental variability of chromatograms, displaying peak shifting and baseline drifts that affected the distribution of samples. For this reason, chromatogram pre-processing was recommendable to improve the analytical performance of PCA models. Thus, data matrices were autoscaled and then subjected to PCA. Figure 1 shows the score plots of PC1 vs. PC2 when processing HPLC-UV chromatographic fingerprint data registered at (a) 257 nm; (b) 280 nm; and (c) 316 nm.
As can be seen, in general QCs appeared in compact groups, demonstrating a good repeatability and robustness of results, with the exception of the dataset at 316 nm (Figure 1c) where a relatively higher dispersion on QCs was observed. Regarding sample characterization and classification, a good differentiation between olive oils against the other oil samples was obtained. In all cases, olive oils tend to be distributed to the right part of the plots across PC1, a component that seems to be related to the polyphenolic content. All other fruit or seed oils apart from olive ones were located to the left part of the PC1 vs. PC2 plot, and a certain discrimination among samples was observed although this was not enough to achieve reasonable clustering according to the fruit/seed of origin (sunflower, soy, or corn). Nevertheless, the results obtained after this exploratory study with PCA were very promising with regard to the authentication of olive oil samples. Regarding the set of 47 EVOO samples, Arbequina and Picual oils were mainly distributed in predominant areas. Oils of non-declared cultivars were basically located in intermediate positions in agreement with their expected blended nature. The HPLC-UV chromatographic fingerprint data was refined by considering only the information of several chromatographic time segments as a way to improve the differentiation and classification of oil samples. For that reason, chromatographic data, at the three wavelengths evaluated, was autoscaled and the following time windows were selected: (1) a 5–8 min segment; (2) an 11–15 min segment; and (3) an 11–20 min segment, as well as the data matrices combining two time windows: (1) 5–8 min + 11–15 min segments; and (2) 5–8 min + 11–20 min segments, and were subjected to PCA. From the direct observation of the HPLC-UV chromatographic profiles obtained from the analyzed samples (see as example Figure S1 in the Supplementary Materials depicting the HPLC-UV fingerprints at 280 nm of two olive oils and a sunflower oil) the studied time segments seem to have, a priori, a certain differentiation, and PCA results showed discrimination between olive oils and other vegetable oils (see two selected examples in Figure S2 in the Supplementary Materials). None of the models evaluated allowed a better classification than the one obtained after considering the full HPLC-UV chromatographic fingerprints (Figure 1).
**Figure 1.** Scatter score plots (PC1 vs. PC2) when processing raw HPLC-UV (High performance liquid chromatography method with ultra-violet detection) chromatographic fingerprints of 47 olive oils and 25 other fruit seed oils (sunflower, corn, soy, and some mixtures of them) registered at (**a**) 257 nm; (**b**) 280 nm; and (**c**) 316 nm. Ellipse is grouping the quality controls.
In a second study, the characterization and classification of EVOOs with respect to the olive cultivar of origin (Arbequina, Picual, Hojiblanca, and Cornicabra) was attempted by using the raw HPLC-UV chromatographic profiles and PCA. For that purpose, a new set of samples was analyzed, consisting of 66 commercially available EVOOs distributed as follows: 23 from Arbequina olives, 19 from Picual olives, 12 from Hojiblanca olives, and 12 from Cornicabra olives. As an example, Figure 2 shows the HPLC-UV chromatographic fingerprints registered at 257 nm from (a) Arbequina; (b) Picual; (c) Hojiblanca; and (d) Cornicabra monovarietal EVOOs. In general, the HPLC-UV chromatographic profiles are different depending on the cultivar, as expected, although some similarities can be observed. These profiles can be divided into three time-window segments. The first one, from 4 to 13 min, is characterized for the presence of very few extracted compounds with low signal intensities. In the
second one, from 13 to 21 min, most of the extracted compounds when using ethanol:water (70:30, *v*/*v*) solution are eluted. It is characterized for richer profiles with slight differences depending on the olive cultivar of origin. For instance, if we consider some selected time-window sections within this segment, it is possible to see that from 13 to 16 min, a similar signal profile was obtained for the Arbequina, Hojiblanca, and Cornicabra cultivars (with only slight differences in signal intensity) in comparison to the Picual cultivar. From 16 to 17 min it seems that three different signal profiles can be observed: one obtained for Hojiblanca and Cornicabra, and the characteristic ones for Arbequina and Picual, respectively. In contrast, the time-window section from 17 to 21 min clearly provides a very different UV chromatographic fingerprint for Arbequina in comparison to the other three cultivars, which seem to differ only in signal intensity. Finally, a last time-window segment from 23 to 26 min can be considered, the Hojiblanca cultivar being the one showing a characteristic profile in comparison to the other three olive cultivars evaluated. The differences observed in both the chromatographic patterns and the signal relative abundances seem to be representative of each olive variety so that they can be exploited as potential chemical descriptors to achieve classification and authentication of oils by chemometric methods.
Thus, raw HPLC-UV chromatograms of the 66 EVOOs were evaluated as the first PCA model, and the obtained score plots (PC1 vs. PC2) at the three evaluated wavelengths are depicted in Figure S3 (Supplementary Materials). Discrimination of EVOOs regarding the olive cultivar was not achieved successfully under these conditions. Most of the Arbequina EVOOs seem to be clustered together and separated from the other EVOOs at the bottom (data registered at 257 and 280 nm) and at the top (data registered at 316 nm) areas of the score plots, in agreement with the fact that Arbequina EVOOs clearly show the most different raw HPLC-UV chromatographic fingerprints (Figure 2a) in comparison to the other three olive cultivars.
Then, with the aim of improving the classification of EVOOs, several PCA models were evaluated by considering several time-window segments as above. The best results were obtained when HPLC-UV chromatographic profile segments of 5–8 min, 13–21 min, and 23–26 min (registered at 280 nm) were combined. The score plot (PC1 vs. PC2) obtained under these conditions is shown in Figure 3a. As can be seen, by employing only some specific HPLC-UV chromatographic segments, acceptable discrimination and classification among the analyzed EVOOs regarding the olive cultivar of origin was achieved. Quality control samples were clustered together and located at the middle of the model (close to the center area) showing again the good repeatability and robustness of the chromatographic and chemometric results. Discrimination along both PC axes was observed. Most of the Arbequina EVOO samples are located at the bottom-left of the plot, being clearly differentiated through the PC1 and PC2 axes against the other cultivars. Most of the Picual EVOO samples appear grouped at the top of the plot. Hojiblanca and Cornicabra EVOOs are located between the other two olive cultivars, but are still well grouped and differentiated. Obviously, complete discrimination among the analyzed samples is not achieved, as expected. For example, two Picual EVVO samples are clearly located close to where Arbequina EVOOs are clustered, while three Arbequina EVOOs are separated from their group and distributed along the areas assigned to the other three cultivars (see Figure 3a). These samples cannot be considered outliers because this behavior can be expected in this kind of study. It should be taken into account that the total amounts of extracted compounds (mainly polyphenols and phenolic acids), and their distribution in olive oils do not only depend on the olive fruit cultivar of origin but also on many other parameters such as growing area, cultivation techniques, water resources, soil management, degree of olive maturation, climate, and even the oil production method [12–15]. As a result, EVOO samples cultivated or produced under similar conditions might display some similar features and, thus, may be located in close areas. Finally, a last unsupervised PCA was performed by simultaneously combining the HPLC-UV chromatographic fingerprint profiles from 13 to 21 min obtained at the three studied wavelengths (257, 280, and 316 nm). With this model, a data matrix with a dimension of 66 samples × 4053 signals was built and subjected to PCA, and the obtained scatter plot of scores (PC1 vs. PC2) is shown in Figure 3b. The combination
of these richer time-window segments obtained simultaneously at different acquisition wavelengths also allowed us to achieve an acceptable classification of the analyzed EVOO with respect to the olive cultivar of origin thanks to the discrimination among both the PC1 and PC2 axes. In this case, quality controls appeared well clustered and close to the center of the plot. Arbequina EVOOs are clustered at the left area of the plot, Picual EVOOs at the top-right section, and Hojiblanca and Cornicabra EVOOs are in the bottom area of the plot and are differentiated through the PC1 axis.
**Figure 2.** HPLC-UV chromatographic fingerprint registered at 257 nm for four extra-virgin olive oil (EVOOs) obtained from (**a**) Arbequina; (**b**) Picual; (**c**) Hojiblanca; and (**d**) Cornicabra monovarietal olive cultivars.
**Figure 3.** Principal component analysis (PCA) results (scatter score plot of PC1 vs. PC2) in the analysis of 66 EVOOs obtained from monovarietal olive cultivars by employing as data: (**a**) HPLC-UV chromatographic profile segments of 5–8 min, 13–21 min, and 23–26 min simultaneously (registered at 280 nm); and (**b**) a combination of the HPLC-UV chromatographic profile segments from 13 to 32 min obtained at 257, 280, and 316 nm, simultaneously.
The acceptable results obtained from the exploratory PCA of the analyzed olive oils as compared to other fruit seed oils, and the analysis of EVOOs with respect to the olive cultivar of origin, suggested that the proposed HPLC-UV chromatographic fingerprints can be used as potential descriptors to achieve sample authentication and to prevent adulteration fraud in the commercialization of olive oils.
#### *3.2. Authentication and Adulteration Studies by Partial Least Square Regression*
The main objective of the present work is to develop a simple and reliable method to confirm the authenticity of EVOOs and to identify and quantify fraud. In the production of EVOOs, product quality is one of the main parameters guaranteeing the olive cultivar of origin, especially when product designations of origin (PDOs) are involved. However, in order to reduce production costs, EVOOs can be adulterated with cheaper fruit seed oils (sunflower, corn, etc.) or even with lower quality olive oils such as refined ones. To study this issue, a specific commercially available Arbequina
monovarietal EVOO was adulterated with: (1) a commercially available Picual monovarietal EVOO, in order to evaluate whether the proposed method can guarantee PDOs of monovarietal EVOOs; (2) a commercially available ROO; and (3) a commercially available sunflower oil. In the latter two cases, the focus was on demonstrating the capacity of the proposed method to identify adulteration fraud of EVOOs with cheaper and lower quality oils. Because the difficulty of identifying and quantifying frauds increases when the adulterant is present at low concentrations, adulteration percentages from 2.5 to 85% were studied. The number of samples employed for both calibration and validation sets in the PLS studies is indicated in the experimental section. Thus, adulteration mixtures were prepared and were then analyzed with the proposed HPLC-UV method. The chromatographic fingerprints obtained at the three evaluated wavelengths were subjected to PLS. In all cases, the number of latent variables (LVs) to be used for the assessment of the models was estimated by cross validation based on both Venetian blinds with 5 splits and random blocks (4 blocks and 3 repeats). Results using the two approaches were similar.
For each one of the three adulteration cases studied, the number of LVs was estimated by depicting the latent variable number versus root-mean-square errors in cross validation (RMSECV). Then, the PLS model was built and its performance was evaluated from the scatter plot of actual versus predicted adulteration percentages. Finally, the obtained errors for both calibration and prediction steps were calculated in order to assess the overall quality of the PLS model.
The first PLS model was built from raw HPLC-UV chromatographic fingerprint data without removing any signal at the three UV acquisition wavelengths. Figure 4 shows (a) the estimation of LVs; (b) the validation results in the calibration step; and (c) the validation results in the prediction step obtained in the three adulteration cases studied for HPLC-UV data registered at 257 nm.
As can be seen for the first adulteration study (with a Picual EVOO as adulterant), the lowest prediction error was attained with five LVs, so this number was chosen to quantify the Picual monovarietal percentage with respect to the adulteration of the Arbequina monovariatel EVOO sample. The agreement between the actual adulteration percentages and the predicted ones was highly satisfactory (Figure 4), with calibration errors and prediction errors (summarized in Table 1) of 0.27% and 0.25%, respectively. Similar results were achieved at the other two acquisition wavelengths (Table 1), with overall calibration and prediction errors below 0.67% and 0.41%, respectively. In the second adulteration evaluated (with ROO as the adulterant), the lowest prediction error was also attained with five LVs. Similar to the first case, the agreement between the actual ROO adulteration percentages and the predicted ones was very satisfactory (Figure 4), with calibration and prediction errors (Table 1) of 0.28% and 0.20%, respectively. These errors increased in the less favorable situation only up to 1.54% and 1.42% for the calibration and the prediction models, respectively, when the other two acquisition wavelengths were used. Finally, regarding the third adulteration study (sunflower oil as an adulterant), very similar results as compared to those previously commented were observed, with overall calibration and prediction errors below 0.77% (Table 1) for the three acquisition wavelengths.
Although the results obtained up to this point were very satisfactory, a second PLS model was built by removing from the data matrix the less discriminant HPLC chromatographic segments among the analyzed samples: the death volume segment (from 0 to 3 min) and the chromatographic cleaning and column conditioning segment (from 26 to 30 min). Thus, only the HPLC-UV chromatographic fingerprint from 3 to 26 min (including all the time-window segments discussed in the PCA section) was considered for each of the three adulterations studied. In this case, the lowest prediction error was attained with four LVs, so this number was chosen to quantify adulterant percentages. The corresponding calibration and prediction errors for all the adulteration cases studied are summarized in Table S1 (Supplementary Materials). Again, a very good agreement between the actual adulteration percentages and the predicted values was achieved, with overall errors below 2.88%.
**Figure 4.** Partial least square (PLS) regression results for the adulteration of an Arbequina monovarietal EVOO using a Picual monovarietal EVOO, a ROO, and a sunflower oil as adulterants. Data set: raw HPLC-UV chromatographic fingerprints registered at 257 nm. (**a**) Estimation of the optimum number of latent variables; (**b**) validation results in the calibration step; and (**c**) validation results in the prediction step. RMSECV: root-mean-square errors in cross validation.
**Table 1.** Partial least square (PLS) calibration and prediction errors in the identification and quantitation of adulterants (Picual EVOO (extra-virgin olive oil), refined olive oil (ROO), and sunflower oil) in an Arbequina EVOO when using raw HPLC-UV (High performance liquid chromatography method with ultra-violet detection) chromatographic fingerprints as sample descriptors.
Considering the results depicted in Table 1 and Table S1 (Supplementary Materials) we can see that, overall, data acquired at 316 nm produced higher errors (although always quite low and within the accepted parameters) in comparison to the other two acquisition wavelengths. This can be explained by the fact that data at this wavelength seems to be less informative in comparison to the other two, as was previously observed with the QCs employed in the PCA studies (see for instance Figure 1).
After the results obtained in this study, the first PLS model (employing the raw HPLC-UV chromatographic profile) with registered data either at 257 nm or at 280 nm can be proposed to carry out the identification and quantitation of fraud in EVOO samples.
#### **4. Conclusions**
A simple, cheap, and reliable method based on HPLC-UV has been developed to characterize and distinguish olive oils (EVOO, VOO, and ROO) from other seed-fruit oils (sunflower, corn, soy), Monovarietal EVOOs were also characterized with respect to the olive cultivar of origin, using chromatographic fingerprints and chemometrics.
Exploratory PCA showed a noticeable discrimination between olive oils and other fruit seed oils (although only slight differentiation within the latter group was achieved) when raw HPLC-UV chromatographic fingerprints (registered at any of the three acquisition wavelengths evaluated) were employed as data descriptors. In contrast, in order to achieve differentiation among monovarietal EVOOs with respect to the olive cultivar of origin (Arbequina, Picual, Hojiblanca, and Cornicabra) the selection of some discriminant time-window segments within the HPLC-UV chromatographic profile was necessary. HPLC-UV chromatographic profile segments of 5–8 min, 13–21 min, and 23–26 min (registered at 280 nm) provided acceptable discrimination among olive cultivars. Similar results were observed when simultaneously combining the HPLC-UV chromatographic fingerprint profiles from 13 to 21 min obtained at the three acquisition wavelengths.
Finally, a PLS study of the authenticity of a specific Arbequina monovarietal EVOO was performed to achieve the identification and quantitation of fraud. The sample was adulterated from 2.5 to 85% by employing three oils as adulterants: (1) Picual monovarietal EVOO; (2) a ROO; and (3) a sunflower oil. A first PLS model employing raw HPLC-UV chromatographic fingerprints without removing any signal allowed the quantification of the studied adulteration frauds with overall quantitation and prediction errors below 1.54%. A second PLS model using only HPLC chromatographic fingerprints within the time segment from 3 to 26 min provided overall quantitation and prediction errors below 2.88%.
Certainly, some adulteration issues can be easily addressed by tracking specific markers or other families of oil components. In any case, the results obtained in this work allow us to propose HPLC-UV fingerprinting and chemometrics as a simple and efficient method for the characterization, classification, and authentication of olive oils. The method is able to guarantee olive oil monovarietal PDOs, and can also detect and quantify EVOO adulteration (minimum 2.5% adulterant) with other less expensive fruit seed oils or cheaper and lower quality olive oils such as refined ones.
**Supplementary Materials:** The following are available online at http://www.mdpi.com/2304-8158/7/4/44/s1. Table S1: PLS calibration and prediction errors in the identification and quantitation of adulterants (Picual EVOO, ROO, and sunflower oil) in an Arbequina EVOO when using raw HPLC-UV chromatographic fingerprints from 3 to 26 min as sample descriptors. Figure S1: HPLC-UV chromatographic fingerprints registered at 280 nm for olive oils (a,b) and sunflower oil (c). Figure S2: (a) PCA results (scores plot of PC1 vs. PC3) employing as analytical data the HPLC-UV chromatographic fingerprints obtained for time segment 14–20 min at 280 nm; (b) PCA results (score plot of PC1 vs. PC2) employing as analytical data the HPLC-UV chromatographic fingerprints obtained combining time segments of 5–8 min and 11–20 min at 280 nm. Figure S3: PCA results (score plots of PC1 vs. PC2) for the analysis of 66 EVOOs when using raw HPLC-UV chromatographic fingerprints registered at (a) 257 nm; (b) 280 nm; and (c) 316 nm as sample descriptors.
**Acknowledgments:** The authors gratefully acknowledge the financial support received from Spanish Ministry of Economy and Competitiveness under the projects CTQ2014-56324-C2-1-P and CTQ2015-63968-C2-1-P, and from the Agency for Administration of University and Research Grants (Generalitat de Catalunya, Spain) under the projects 2017 SGR-171 and 2017 SGR-310.
**Author Contributions:** M.F.-C. and N.C. performed all the experiments. J.S. and O.N. conceived and designed the experiments, supervised the work. M.F.-C. and N.C. wrote the paper.
**Conflicts of Interest:** The authors declare no conflict of interest.
#### **References**
© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
MDPI St. Alban-Anlage 66 4052 Basel Switzerland Tel. +41 61 683 77 34 Fax +41 61 302 89 18 www.mdpi.com
*Foods* Editorial Office E-mail: [email protected] www.mdpi.com/journal/foods
MDPI St. Alban-Anlage 66 4052 Basel Switzerland
Tel: +41 61 683 77 34 Fax: +41 61 302 89 18 | doab | 2025-04-07T03:56:58.794933 | 1-5-2021 17:44 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004acd65-0f8a-4342-8761-8da04dfc73fe",
"url": "https://mdpi.com/books/pdfview/book/2994",
"author": "",
"title": "Application of Liquid Chromatography in Food Analysis",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783039433629",
"section_idx": 9
} |
004db1df-dc76-42af-952f-1f5e82b7822b.0 | # **Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)**
Edited by Roberto Fiorenza Printed Edition of the Special Issue Published in *Catalysts*
www.mdpi.com/journal/catalysts
| doab | 2025-04-07T03:56:58.804196 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 0
} |
004db1df-dc76-42af-952f-1f5e82b7822b.1 | **Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)**
| doab | 2025-04-07T03:56:58.804294 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 1
} |
004db1df-dc76-42af-952f-1f5e82b7822b.2 | **Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)**
Editor
**Roberto Fiorenza**
MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade • Manchester • Tokyo • Cluj • Tianjin
*Editor* Roberto Fiorenza Dip. Scienze Chimiche Universita di Catania ` Catania Italy
*Editorial Office* MDPI St. Alban-Anlage 66 4052 Basel, Switzerland
This is a reprint of articles from the Special Issue published online in the open access journal *Catalysts* (ISSN 2073-4344) (available at: www.mdpi.com/journal/catalysts/special issues/aop environ).
For citation purposes, cite each article independently as indicated on the article page online and as indicated below:
LastName, A.A.; LastName, B.B.; LastName, C.C. Article Title. *Journal Name* **Year**, *Volume Number*, Page Range.
**ISBN 978-3-0365-3566-1 (Hbk) ISBN 978-3-0365-3565-4 (PDF)**
© 2022 by the authors. Articles in this book are Open Access and distributed under the Creative Commons Attribution (CC BY) license, which allows users to download, copy and build upon published articles, as long as the author and publisher are properly credited, which ensures maximum dissemination and a wider impact of our publications.
The book as a whole is distributed by MDPI under the terms and conditions of the Creative Commons license CC BY-NC-ND.
| doab | 2025-04-07T03:56:58.804324 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 2
} |
004db1df-dc76-42af-952f-1f5e82b7822b.3 | **Contents**
Reprinted from: *Catalysts* **2021**, *11*, 664, doi:10.3390/catal11060664 . . . . . . . . . . . . . . . . . . **157**
| doab | 2025-04-07T03:56:58.804415 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 3
} |
004db1df-dc76-42af-952f-1f5e82b7822b.5 | **Roberto Fiorenza**
Roberto Fiorenza (PhD) is a researcher in industrial chemistry at the department of Chemical Sciences of the University of Catania (Italy). He works on the synthesis and characterization of new materials (catalysts) for environmental and energy applications (purification of air, water, production and hydrogen, CO<sup>2</sup> valorization, etc.), focusing his research on semiconductors and on TiO2-based materials. He is the author and co-author of various papers in international peer review journals and a reviewer of some international journals dealing with catalysis and material science. He teaches the courses Sustainable Industrial Chemistry (to Master's degree students in chemical science) and Sustainable Industrial Chemistry and Materials for the Environmental Protection (to PhD students).
In June 2019, he received the Adolfo Parmaliana 2019 Award from the catalysis group of the Italian chemical society for the best doctoral thesis on the theme catalysis for a sustainable development; the same work also earned him an award from the Gioenia Academy of Catania in the October 2018. In 2017, he received the Young Student Award at the international EMRS conference (European Materials Research Society) Spring Meeting 2017 in Strasbourg.
He carried out parts of his research work at the CNR-IMM of Catania (ESF SENTI project), at the University of Palermo with the research group of Prof. Leonardo Palmisano, at the University of Namur (Belgium) with the research group of Prof. Bao-Lian Su, at the Plataforma Solar de Almeria (Spain) with the research group of Prof. Sixto Malato and at the University of Duisburg-Essen (Germany) in the group of Prof. S. Reichenberger.
In February 2022, he achieved the National Scientific qualification as an associate professor in the Italian higher education system, for the disciplinary field of 03/C2 Industrial and Applied Chemistry.
| doab | 2025-04-07T03:56:58.804445 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 5
} |
004db1df-dc76-42af-952f-1f5e82b7822b.6 | **Preface to "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)"**
The quality of air and water is a key contemporary problem. The globalization economy and the current pandemic situation have given rise to new problems related to environmental protection. Catalysis science has always given smart, green, and scale-up eco-friendly solutions. In the last year, together with the traditional and efficient catalytic thermal treatments, new and emerging techniques such as photothermal treatments or advanced oxidation processes have provided good results both for air and water purification.
Based on the above considerations, this Special Issue on "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)"highlights the state of research on VOC oxidation (catalytic oxidation, photocatalytic oxidation or photothermal catalytic oxidation), air purification, and wastewater treatments (adsorption, membrane filtration, AOP, photocatalysis, Fenton and PhotoFenton, ozonation, etc.), as well as on the development of new catalysts for environmental protection.
From the high-quality papers presented in this book, researchers interested in these topics can find interesting and fascinating information, with contributions from some of the most important University and research centers in the world.
> **Roberto Fiorenza** *Editor*
| doab | 2025-04-07T03:56:58.804611 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 6
} |
004db1df-dc76-42af-952f-1f5e82b7822b.7 | *Editorial* **Heterogeneous Catalysis and Advanced Oxidation Processes (AOPs) for Environmental Protection (VOC Oxidation, Air and Water Purification)**
**Roberto Fiorenza**
Department of Chemical Sciences, University of Catania, Viale A. Doria 6, 95125 Catania, Italy; [email protected]; Tel.: +39-0957385012
The quality of air and water is a crucial and critical contemporary problem. The more globalized economy, and the rapid growth of new economic powers, have given rise to new problems related to environmental protection. The science of catalysis has always given smart, green, and scalable, eco-friendly solutions. In the last year, together with the traditional and efficient catalytic thermal treatments, new and emerging techniques such as photothermal treatments or advanced oxidation processes (AOPs such as photocatalysis, Fenton and photo-Fenton, ozonation, etc.) have provided good results both in air and water purification [1,2]. All these technologies can also drive the transition from "classical" industrial chemistry to sustainable industrial chemistry, where not only are the processes involved in environmental purification versatile, green and environmentally friendly, but the employed catalysts are as well. This approach can help to overcome some relevant contemporary issues, some of which have been amplified by the COVID-19 pandemic situation. This has highlighted the necessity to ensure a high quality of air in both indoor and outdoor environments, or has drastically drawn attention to water polluted by the plastic waste (used face masks, for example). Plastics, together with other emerging water contaminants (such as pesticides, pharmaceutics, and antibiotics), are serious problems for water purity; conventional treatments do not work efficiently on these dangerous compounds.
All these aspects are well discussed and investigated in this Special Issue.
In particular, for air purification (including volatile organic compound (VOC) removal), the reviews of *Fiorenza* [3] and *Zhou and Yun* [4] examined two different aspects. The advantages and drawbacks on the utilization of bimetallic-based catalysts were analyzed in the first review, with particular attention on the bimetallic-gold-based samples, and considering both catalytic and the photocatalytic approaches [3]. In the second review, the attention focused on harmful pharmaceutical VOCs emitted in China. The developments in catalytic combustion, photocatalytic oxidation, non-thermal plasma, and electron beam treatments were discussed, together with the development of catalysts used in these processes [4].
The developments of eco-friendly catalysts and unconventional photocatalysts, not based on TiO2, which can also represent possible solutions to the crisis of the raw material exportation [5] was also explored in two other papers of this Special Issue [6,7]; the good catalytic, photocatalytic, and phothermo-catalytic properties of MnOx-ZrO<sup>2</sup> are presented, making these composites a promising future choice, as an example of an economical, notcritical, and high-performing catalyst applied for the removal of some dangerous VOCs such as toluene.
The innovative aspect that joins the water remediation from emerging contaminants with new materials was the core of this collection of papers. The degradation of fluoroquinolone antibiotics [8] and dyes [9], the treatment of cosmetic wastewaters [10], and the removal of bisphenol A [11] were originally investigated, employing Au@ZnO-rGOgC3N<sup>4</sup> [8], ZnO thin film [9], magnetite, hematite, and zero-valent iron [10], and alkaline active materials [11], respectively. These studies demonstrated synergisms between the
**Citation:** Fiorenza, R. Heterogeneous Catalysis and Advanced Oxidation Processes (AOPs) for Environmental Protection (VOC Oxidation, Air and Water Purification). *Catalysts* **2022**, *12*, 317. https://doi.org/10.3390/ catal12030317
Received: 5 March 2022 Accepted: 8 March 2022 Published: 10 March 2022
**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.
**Copyright:** © 2022 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).
photocatalytic properties of the new compounds, or that combinations of different AOPs can be the most attractive and valuable strategies to remove these recalcitrant pollutants from water. In this context, the use of aqueous ozone and UV photolysis represents a sustainable solution for the bleaching of fabrics with a low environmental impact [12].
Finally, the Fenton and photo-Fenton-like processes were proposed to remove other water pollutants such as pesticides [13] and rhodamine B dye [14], using alternative catalysts such as reduced CeO<sup>2</sup> [13] and the chalcopyrite (CuFeS2) [14].
In conclusion, as the Guest Editor of this Special Issue, I would like to extend my appreciation to all the authors for their high-level articles, and I thank all the reviewers for their comments on the manuscripts. I hope that readers will find the results in the articles on this topic interesting and useful for their research. Thanks also to the editorial staff of *Catalysts*, for their help and ensuring the success of this Special Issue.
**Funding:** This research received no external funding.
**Conflicts of Interest:** The author declares no conflict of interest.
| doab | 2025-04-07T03:56:58.804726 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 7
} |
004db1df-dc76-42af-952f-1f5e82b7822b.9 | **Development of Pharmaceutical VOCs Elimination by Catalytic Processes in China**
### **Lilong Zhou 1,\*, Chen Ma <sup>1</sup> , Jonathan Horlyck <sup>2</sup> , Runjing Liu <sup>1</sup> and Jimmy Yun 1,2,3,\***
Received: 26 May 2020; Accepted: 9 June 2020; Published: 13 June 2020
**Abstract:** As a byproduct of emerging as one of the world's key producers of pharmaceuticals, China is now challenged by the emission of harmful pharmaceutical VOCs. In this review, the catalogue and volume of VOCs emitted by the pharmaceutical industry in China was introduced. The commonly used VOC removal processes and technologies was recommended by some typical examples. The progress of catalytic combustion, photocatalytic oxidation, non-thermal plasma, and electron beam treatment were presented, especially the development of catalysts. The advantages and shortages of these technologies in recent years were discussed and analyzed. Lastly, the development of VOCs elimination technologies and the most promising technology were discussed.
**Keywords:** VOCs; catalytic combustion; China; elimination technology; pharmaceutical industry
| doab | 2025-04-07T03:56:58.805452 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 9
} |
004db1df-dc76-42af-952f-1f5e82b7822b.10 | **1. Introduction**
Recently, the Chinese public has become increasingly concerned about the levels of chemical air pollution present in the form of haze. A main contributing factor to this pollution is the release of volatile organic compounds (VOCs) from the industry. VOCs are organic compounds with boiling points in the range of 50–260 ◦C at atmospheric pressure or with a Reid vapor pressure of over 10.3 Pa at room temperature (293.15 K) and atmosphere pressure (101.325 kPa) [1,2]. The pharmaceutical industry is a major source of these VOCs [3,4], which can have serious ramifications, such as toxicity, carcinogenesis, mutagenesis, photochemical pollution, haze, and fog [5–9]. The pharmaceutical industry has developed greatly in China, as the production of bulk drug intermediates and Chinese patent drugs increased from 205,070 to 340,830 kilotons, and 112,890 to 374,600 kilotons in the past 10 years (2007–2016), respectively [10]. Meanwhile, the amounts of released VOCs increased from about 174.8 to 393.2 kt in the pharmaceutical industry [11,12]. The guiding emission standards of VOCs went into effect on 1 January, 2018 in China, which led to new imputes to upgrade VOCs elimination equipment and technologies in factories across China [13].
| doab | 2025-04-07T03:56:58.805540 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 10
} |
004db1df-dc76-42af-952f-1f5e82b7822b.11 | **2. Catalogue and Emission Amounts of VOCs in the China Pharmaceutical Industry**
| doab | 2025-04-07T03:56:58.805643 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 11
} |
004db1df-dc76-42af-952f-1f5e82b7822b.12 | *2.1. Catalogue of Pharmaceutical VOCs in China*
Medicines in China are mainly produced in six ways, biological fermentation, chemical synthesis, extraction, coagulation preparation, bioengineering, and treatment of traditional Chinese medicine [4,14]. The different methods produce varying levels of pollution, with the order of emitted VOCs being biological fermentation > chemical synthesis > extraction > bioengineering > treatment of traditional Chinese medicine > coagulation preparation [15]. All these processes can be plagued by the need for high volumes of solvents, the consumption of large quantities of complex organic precursors, or the production of volatile byproducts. Some of the volatile organic compounds used or produced in these processes may be released into the atmosphere, which causes air pollution. The released VOCs primarily include alkanes, alcohols, ketones, aromatic hydrocarbons, halohydrocarbons, amines, esters, ether, aldehyde, carboxylic acid, and sulfur containing organic compounds [4,11,12,14,15].
Biological fermentation is often used to produce antibiotics, vitamins, and amino acids, via processes which include fermentation, separation, purification, and refinement [16,17]. The primary source of emitted VOCs are the solvents used in these processes, especially in the separation and purification steps. Additionally, H2S is also produced as a byproduct in some fermentation processes.
The chemical synthesis technology is usually used for the production of medicine which can be used for prevention, cure, and diagnosis of the disease [18,19]. It contains the units to synthesize intermediates from raw materials, modify the structure of intermediates, purify the products, and dry the final products. The VOCs from this process comprise solvents and unreacted intermediates, which are more complex and often harder to be eliminated than those produced from the biological fermentation. These VOCs include heptane, toluene, xylene, methanol, n-propyl alcohol, isopropanol, phenol, aminomethane, dimethylamine, aminobenzene, cyclohexylamine, triethylamine, butyraldehyde, acetone, chloroform, chlorobenzene, etc.
Extraction involves the use of physical, chemical, and biological methods to separate a substance from a mixture. In the case of the pharmaceutical industry, extraction is carried out to separate organic compounds of interest from liquid solvents [16,20]. As a result, extraction processes require a large supply of solvents, including toluene, naphtha, methanol, ethanol, isopropanol, phenol, acetone, ether, acetic ether, diethylamine, dichloromethane, dichloloethane, chloroform, etc. VOCs are released during extraction mainly from the organic solvents used within the process.
The bioengineering method is a new way to produce some new medicines, via processes such as cloning antibodies, genetic engineering drugs, and genetic engineering vaccines [21]. The VOCs released from this process are similar to those of biological fermentation. These VOCs come from the solvents used within the process and as byproducts of certain reactions. They include n-hexane, methanol, ethanol, formaldehyde, acetaldehyde, formic acid, propanediol, acetone, aminoehtyl alcohol, acetonitrile, acetic acid, acetocaustin, N, N-dimethylformamide, phenol, butanone, 4-methyl-2-pentone, n-propyl alcohol, isopropanol, n-pentanol, isopropyl ether, isobutyl aldehyde, etc.
Another process of pharmaceutical production is the manufacture of traditional Chinese medicines or certain Chinese patent drugs [22]. As this is a traditional process which involves the use of only limited organic solvents to treat the natural animals and plants, the production of VOCs is limited. Hence, this process only emits a spot of VOCs, SO2, and smoke.
Coagulation preparation involves the formation of larger particle agglomerates from fine particle suspensions. In the synthesis of pharmaceuticals, it is the process of mixing the active ingredients with helper constituents (often called coagulants) to produce a drug which has the desired particle size. The physical nature of the coagulation process means that the waste produced via this method is primarily a solid particle with little notable VOC production [4,14–16,22].
| doab | 2025-04-07T03:56:58.805673 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 12
} |
004db1df-dc76-42af-952f-1f5e82b7822b.13 | *2.2. The Guiding Emission Standards of VOCs in China*
The guiding emission standards of VOCs include six kinds of VOCs on the boundary of factories (Table 1) and four pollutant classifications with a total of 16 compounds in the areas surrounding workshops and installations (Table 2) [13]. These parameters are determined in 1 h by 3~4 samples for the average value. They are detected by the portable instruments or GC. These target VOCs are carcinogenic, odorous, and harmful organic compounds, which are mainly used as solvents and raw materials.
**Table 1.** The extreme emission value of volatile organic compounds (VOCs) on the boundary of factories and the highest concentration of selected VOCs allowed in 1 h for employees [13].
a. They will be put into effect after the publication of national standards.
**Table 2.** The limiting value of specific pollutants in workshops and installations [13].
a. They will be put into effect after the publication of national standards.
| doab | 2025-04-07T03:56:58.805946 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 13
} |
004db1df-dc76-42af-952f-1f5e82b7822b.14 | *2.3. VOCs Emissions in the Chinese Pharmaceutical Industry*
China is the second largest producer of pharmaceutical products, only behind the United States of America. There are more than 1300 kinds of drug intermediates, 30 types of medicaments, and over 4500 pharmaceutical products made in China [23]. According to previous studies, the total VOCs emissions scaled linearly with the amount of final pharmaceutical products. [24]. The results showed that about 0.55 kg of VOCs were discharged to the atmosphere for the production of each 1 kg of final drug products.
The total VOCs emissions from the pharmaceutical industry increased by over 120% from about 174.8 kt in 2007 to 393.2 kt in 2016 (Figure 1) [12]. Although the VOCs emitted from the pharmaceutical industry only account for approximately 1.1% of China's total VOCs emissions, the absolute emission amount is very large. VOCs emitted from the pharmaceutical industry are potentially more harmful to human beings and ecosystems than the VOCs emitted from other sources. Compared with other emission sources, such as decoration, oil extraction and refining, catering, shoemaking and furniture manufacturing, the VOCs from pharmaceutical industries are more diverse with higher local concentrations and are harder to be eliminated.
more diverse with higher local concentrations and are harder to be eliminated.
The total VOCs emissions from the pharmaceutical industry increased by over 120% from about 174.8 kt in 2007 to 393.2 kt in 2016 (Figure 1) [12]. Although the VOCs emitted from the pharmaceutical industry only account for approximately 1.1% of China's total VOCs emissions, the absolute emission amount is very large. VOCs emitted from the pharmaceutical industry are potentially more harmful to human beings and ecosystems than the VOCs emitted from other
**Figure 1.** The production of bulk drugs and Chinese patent drugs and VOCs emissions from 2007 to 2016. **Figure 1.** The production of bulk drugs and Chinese patent drugs and VOCs emissions from 2007 to 2016.
### **3. The Developing Technologies to Dispel VOCs 3. The Developing Technologies to Dispel VOCs**
temperatures [29,30].
and washing apparatuses.
## *VOCs Elimination Technologies used in China's Pharmaceutical Industry VOCs Elimination Technologies used in China's Pharmaceutical Industry*
VOCs elimination technologies used in China were originally developed with the aim of recycling organic compounds to reduce cost, but recently the use and implementation of these technologies are targeted at minimizing the environmental impacts of VOCs. Technologies applied in the elimination of VOCs in China include condensation, absorption, adsorption, membrane purification, incineration, catalytic combustion, and the non‐thermal plasma process (Figure 2) [15,25–27]. These methods are applied according to different working conditions, such as temperature and pressure, depending upon the VOCs targeted for removal. They also have distinct advantages and disadvantages. The development of these VOCs elimination technologies will be briefly introduced and discussed in this review. It is important to note that a wide range of VOCs which are often produced in a single waste stream and the various treatment technologies have different efficacies for the removal of certain VOCs. In fact, multiple technologies are often combined to eliminate the VOC mixtures from the waste streams. VOCs elimination technologies used in China were originally developed with the aim of recycling organic compounds to reduce cost, but recently the use and implementation of these technologies are targeted at minimizing the environmental impacts of VOCs. Technologies applied in the elimination of VOCs in China include condensation, absorption, adsorption, membrane purification, incineration, catalytic combustion, and the non-thermal plasma process (Figure 2) [15,25–27]. These methods are applied according to different working conditions, such as temperature and pressure, depending upon the VOCs targeted for removal. They also have distinct advantages and disadvantages. The development of these VOCs elimination technologies will be briefly introduced and discussed in this review. It is important to note that a wide range of VOCs which are often produced in a single waste stream and the various treatment technologies have different efficacies for the removal of certain VOCs. In fact, multiple technologies are often combined to eliminate the VOC mixtures from the waste streams. *Catalysts* **2020**, *10*, 668 5 of 49
**Figure 2.** Classification of VOCs control techniques. Reprinted with permission from [26], 2000, Elsevier Ltd. **Figure 2.** Classification of VOCs control techniques. Reprinted with permission from [26], 2000, Elsevier Ltd.
Condensation is the conversion of gas phase VOC mixtures with different vapor pressures to liquid via a decrease in temperature [28]. This technology is often used to recycle the solvents used Condensation is the conversion of gas phase VOC mixtures with different vapor pressures to liquid via a decrease in temperature [28]. This technology is often used to recycle the solvents used in
in pharmaceutical production, with the key advantages that equipment requirements and operation
workshop before incineration and absorption to reduce the load on more complex and expensive downstream technologies. Water and air are the most commonly used cooling mediums for the condensation process, but ice, cold salt solutions and organic mediums have been used where cooling temperatures below 10 °C are required, such as the CaCl2 solution, NaCl solution, and ethylene glycol aqueous solution [4]. The condensation efficiency is sensitive to the temperature and pressure and is suitable for the removal of high concentration VOCs which exist as liquefied at moderate
The removal of VOCs via adsorption involves the use of porous materials, while absorption utilizes solvents. The porous materials used in adsorption need a high absorption capacity, large surface area, good pore structure, stable chemical properties, high physical strength, and tolerance of acidic/basic conditions. The adsorption materials commonly used in this technology include activated carbon, porous silica, zeolite, and porous resin [31–35]. The solvents commonly used in the absorption of VOCs are water, acid solution, alkali solution, and other organic compounds [36]. The components used in the adsorption process include spray columns, filled towers, columns of trays,
Both the adsorption and absorption methods have a high VOC removal efficiency and can almost completely remove VOCs from waste gas with low energy consumption (Figure 3). They can be used to recycle organic solvents and valuable compounds while remaining economically feasible. This technology is often used for the treatment of a large flow of waste gas with low VOC concentration in processes such as fix bed adsorption, moving bed adsorption, fluid‐bed adsorption, and pressure swing adsorption [37–39]. Disadvantages of the adsorption and absorption process are also noteworthy, such as huge equipment requirements, complex procedures, and the need for desorption and regeneration of saturated absorbents. Due to its high overall VOC removal efficiency,
this technology is commonly used in many pharmaceutical factories.
pharmaceutical production, with the key advantages that equipment requirements and operation of the condensation process are simple. It makes the cost of condensation lower than other technologies. Another advantage is that the gas produced from condensation is pure. Hence, condensation is often used as the first procedure to treat the waste gas from a pharmaceutical workshop before incineration and absorption to reduce the load on more complex and expensive downstream technologies. Water and air are the most commonly used cooling mediums for the condensation process, but ice, cold salt solutions and organic mediums have been used where cooling temperatures below 10 ◦C are required, such as the CaCl<sup>2</sup> solution, NaCl solution, and ethylene glycol aqueous solution [4]. The condensation efficiency is sensitive to the temperature and pressure and is suitable for the removal of high concentration VOCs which exist as liquefied at moderate temperatures [29,30].
The removal of VOCs via adsorption involves the use of porous materials, while absorption utilizes solvents. The porous materials used in adsorption need a high absorption capacity, large surface area, good pore structure, stable chemical properties, high physical strength, and tolerance of acidic/basic conditions. The adsorption materials commonly used in this technology include activated carbon, porous silica, zeolite, and porous resin [31–35]. The solvents commonly used in the absorption of VOCs are water, acid solution, alkali solution, and other organic compounds [36]. The components used in the adsorption process include spray columns, filled towers, columns of trays, and washing apparatuses.
Both the adsorption and absorption methods have a high VOC removal efficiency and can almost completely remove VOCs from waste gas with low energy consumption (Figure 3). They can be used to recycle organic solvents and valuable compounds while remaining economically feasible. This technology is often used for the treatment of a large flow of waste gas with low VOC concentration in processes such as fix bed adsorption, moving bed adsorption, fluid-bed adsorption, and pressure swing adsorption [37–39]. Disadvantages of the adsorption and absorption process are also noteworthy, such as huge equipment requirements, complex procedures, and the need for desorption and regeneration of saturated absorbents. Due to its high overall VOC removal efficiency, this technology is commonly used in many pharmaceutical factories. *Catalysts* **2020**, *10*, 668 6 of 49
**Figure 3.** A typical activated carbon VOC removal (solvent recovery) plant. Reprinted with permission from [26], 2000, Elsevier Ltd. **Figure 3.** A typical activated carbon VOC removal (solvent recovery) plant. Reprinted with permission from [26], 2000, Elsevier Ltd.
Incineration is another widely used technology in pharmaceutical factories to eliminate VOCs (Figure 4). If VOC recycling is not technologically or financially feasible, incineration is a suitable method to completely eliminate VOCs [40]. Incineration is carried out by burning VOCs in a stove or kiln. Ideally, incineration results in the conversion of VOCs into CO2 and H2O in an efficient, simple, and safe manner. However, it has multiple shortcomings. If the VOCs concentration is too low to support the incineration, additional fuel is needed, which increases the running cost. Additionally, Incineration is another widely used technology in pharmaceutical factories to eliminate VOCs (Figure 4). If VOC recycling is not technologically or financially feasible, incineration is a suitable method to completely eliminate VOCs [40]. Incineration is carried out by burning VOCs in a stove or kiln. Ideally, incineration results in the conversion of VOCs into CO<sup>2</sup> and H2O in an efficient, simple, and safe manner. However, it has multiple shortcomings. If the VOCs concentration is too low to support the incineration, additional fuel is needed, which increases the running cost. Additionally,
**Figure 4.** Schemes of thermal oxidation. (a) Regenerative thermal oxidation; (b) recuperative thermal
To overcome the shortages of the incineration process for VOC destruction, the catalytic combustion technology was developed. In this process, VOCs are decomposed over catalysts at a low temperature (lower than 500 °C, Figures 5 and 6) [1,2,41–44]. The key factor which governs the catalytic combustion process is the activity of catalysts. Various kinds of catalysts have been used for the catalytic combustion of VOCs, such as noble metal catalysts, transition metal oxides, perovskite catalysts, and concentrated oxidation catalysts. The advantages for this technology are low operation temperature, decreased energy input requirements, high VOC removal efficiency, and minimal generation of toxic byproducts. Catalytic combustion is suitable for the treatment of waste streams containing VOCs across a wide range of concentrations. The main disadvantages of catalytic combustion are high investment requirements for equipment, short catalyst lifetime, and the need
oxidation. Reprinted with permission from [26], 2000, Elsevier Ltd.
some VOCs are less suitable for incineration, because the incomplete combustion of halogenated and other harmful VOCs can result in the release of toxic chemicals such as dioxin, NOx, and CO [40].
permission from [26], 2000, Elsevier Ltd.
some VOCs are less suitable for incineration, because the incomplete combustion of halogenated and other harmful VOCs can result in the release of toxic chemicals such as dioxin, NOx, and CO [40]. and safe manner. However, it has multiple shortcomings. If the VOCs concentration is too low to support the incineration, additional fuel is needed, which increases the running cost. Additionally, some VOCs are less suitable for incineration, because the incomplete combustion of halogenated
kiln. Ideally, incineration results in the conversion of VOCs into CO<sup>2</sup> and H2O in an efficient, simple,
**Figure 3.** A typical activated carbon VOC removal (solvent recovery) plant. Reprinted with
Incineration is another widely used technology in pharmaceutical factories to eliminate VOCs (Figure 4). If VOC recycling is not technologically or financially feasible, incineration is a suitable
*Catalysts* **2020**, *10*, 668 6 of 49
**Figure 4.** Schemes of thermal oxidation. (a) Regenerative thermal oxidation; (b) recuperative thermal oxidation. Reprinted with permission from [26], 2000, Elsevier Ltd. **Figure 4.** Schemes of thermal oxidation. (**a**) Regenerative thermal oxidation; (**b**) recuperative thermal oxidation. Reprinted with permission from [26], 2000, Elsevier Ltd.
To overcome the shortages of the incineration process for VOC destruction, the catalytic combustion technology was developed. In this process, VOCs are decomposed over catalysts at a low temperature (lower than 500 °C, Figures 5 and 6) [1,2,41–44]. The key factor which governs the catalytic combustion process is the activity of catalysts. Various kinds of catalysts have been used for the catalytic combustion of VOCs, such as noble metal catalysts, transition metal oxides, perovskite catalysts, and concentrated oxidation catalysts. The advantages for this technology are low operation temperature, decreased energy input requirements, high VOC removal efficiency, and minimal generation of toxic byproducts. Catalytic combustion is suitable for the treatment of waste streams containing VOCs across a wide range of concentrations. The main disadvantages of catalytic combustion are high investment requirements for equipment, short catalyst lifetime, and the need To overcome the shortages of the incineration process for VOC destruction, the catalytic combustion technology was developed. In this process, VOCs are decomposed over catalysts at a low temperature (lower than 500 ◦C, Figures 5 and 6) [1,2,41–44]. The key factor which governs the catalytic combustion process is the activity of catalysts. Various kinds of catalysts have been used for the catalytic combustion of VOCs, such as noble metal catalysts, transition metal oxides, perovskite catalysts, and concentrated oxidation catalysts. The advantages for this technology are low operation temperature, decreased energy input requirements, high VOC removal efficiency, and minimal generation of toxic byproducts. Catalytic combustion is suitable for the treatment of waste streams containing VOCs across a wide range of concentrations. The main disadvantages of catalytic combustion are high investment requirements for equipment, short catalyst lifetime, and the need for process-specific designs, which are tailored to the waste stream. Nonetheless, the development of viable materials for the catalytic combustion process is still a hotspot for catalysis science. *Catalysts* **2020**, *10*, 668 7 of 49 for process‐specific designs, which are tailored to the waste stream. Nonetheless, the development of viable materials for the catalytic combustion process is still a hotspot for catalysis science. The mechanism of catalytic combustion is considered by three types, with these being the Mars‐ Van Krevelen (MVK) model, Langmuir‐Hinshelwood (LH) model, and Eley‐Rideal (ER) model. In the Mars‐Van Krevelen model, the VOCs molecules are initially adsorbed on the active sites, upon which they react with the oxygen species within the catalyst and are decomposed. Then, the reduced catalyst is re‐oxidized by the supply of oxygen to the reactor. In the Langmuir‐Hinshelwood model, the adsorbed VOCs molecules react directly with the adsorbed oxygen molecules, all occurring on the catalyst surface. In the Elay‐Rideal mechanism, the adsorbed oxygen reacts with the VOCs molecules in the gas phase. The reaction pathway which follows depends on both the catalyst
**Figure 5.** Scheme of catalytic oxidation. Reprinted with permission from [26], 2000, Elsevier Ltd. **Figure 5.** Scheme of catalytic oxidation. Reprinted with permission from [26], 2000, Elsevier Ltd.
The mechanism of catalytic combustion is considered by three types, with these being the Mars-Van Krevelen (MVK) model, Langmuir-Hinshelwood (LH) model, and Eley -Rideal (ER) model. In the Mars-Van Krevelen model, the VOCs molecules are initially adsorbed on the active sites, upon which they react with the oxygen species within the catalyst and are decomposed. Then, the reduced catalyst is re-oxidized by the supply of oxygen to the reactor. In the Langmuir-Hinshelwood model, the adsorbed VOCs molecules react directly with the adsorbed oxygen molecules, all occurring on the
**Figure 6.** Schematic diagram of a reverse flow reactor. Reprinted with permission from [26], 2000,
Biodegradation is a widely used process for the treatment of pharmaceutical wastewater (Figure 7) [45,46]. It also can be applied to the treatment VOCs in the gas phase, especially for low concentration VOCs which are suitable for the growth of microorganisms. This process works with using the VOCs as a feedstock for the microorganisms, where they are converted to cytoplasm, CO2, and H2O. The sulfur and nitrogen elements in VOCs can be transformed to H2S, nitrate, or N2 at moderate temperatures. However, VOCs emitted from the pharmaceutical process often contain aromatics or halogens, which would poison the microorganism, rendering this method largely
Elsevier Ltd.
unviable in the abatement of pharmaceutical VOCs.
materials and the target VOCs in individual systems.
catalyst surface. In the Elay-Rideal mechanism, the adsorbed oxygen reacts with the VOCs molecules in the gas phase. The reaction pathway which follows depends on both the catalyst materials and the target VOCs in individual systems. **Figure 5.** Scheme of catalytic oxidation. Reprinted with permission from [26], 2000, Elsevier Ltd.
*Catalysts* **2020**, *10*, 668 7 of 49
for process‐specific designs, which are tailored to the waste stream. Nonetheless, the development of
The mechanism of catalytic combustion is considered by three types, with these being the Mars‐ Van Krevelen (MVK) model, Langmuir‐Hinshelwood (LH) model, and Eley‐Rideal (ER) model. In the Mars‐Van Krevelen model, the VOCs molecules are initially adsorbed on the active sites, upon which they react with the oxygen species within the catalyst and are decomposed. Then, the reduced catalyst is re‐oxidized by the supply of oxygen to the reactor. In the Langmuir‐Hinshelwood model, the adsorbed VOCs molecules react directly with the adsorbed oxygen molecules, all occurring on the catalyst surface. In the Elay‐Rideal mechanism, the adsorbed oxygen reacts with the VOCs molecules in the gas phase. The reaction pathway which follows depends on both the catalyst
viable materials for the catalytic combustion process is still a hotspot for catalysis science.
**Figure 6.** Schematic diagram of a reverse flow reactor. Reprinted with permission from [26], 2000, Elsevier Ltd. **Figure 6.** Schematic diagram of a reverse flow reactor. Reprinted with permission from [26], 2000, Elsevier Ltd.
Biodegradation is a widely used process for the treatment of pharmaceutical wastewater (Figure 7) [45,46]. It also can be applied to the treatment VOCs in the gas phase, especially for low concentration VOCs which are suitable for the growth of microorganisms. This process works with using the VOCs as a feedstock for the microorganisms, where they are converted to cytoplasm, CO2, and H2O. The sulfur and nitrogen elements in VOCs can be transformed to H2S, nitrate, or N2 at moderate temperatures. However, VOCs emitted from the pharmaceutical process often contain aromatics or halogens, which would poison the microorganism, rendering this method largely unviable in the abatement of pharmaceutical VOCs. Biodegradation is a widely used process for the treatment of pharmaceutical wastewater (Figure 7) [45,46]. It also can be applied to the treatment VOCs in the gas phase, especially for low concentration VOCs which are suitable for the growth of microorganisms. This process works with using the VOCs as a feedstock for the microorganisms, where they are converted to cytoplasm, CO2, and H2O. The sulfur and nitrogen elements in VOCs can be transformed to H2S, nitrate, or N<sup>2</sup> at moderate temperatures. However, VOCs emitted from the pharmaceutical process often contain aromatics or halogens, which would poison the microorganism, rendering this method largely unviable in the abatement of pharmaceutical VOCs. *Catalysts* **2020**, *10*, 668 8 of 49
**Figure 7.** A simple schematic sketch of a bio‐filtration system. Reprinted with permission from [26], 2000, Elsevier Ltd. **Figure 7.** A simple schematic sketch of a bio-filtration system. Reprinted with permission from [26], 2000, Elsevier Ltd.
Non‐thermal plasma is also a commonly used technology for the elimination of VOCs [47–49]. Free electrons and radicals formed during the plasma process react with VOCs and lead to the degradation of VOCs to CO2 and H2O. There are some advantages of the non‐thermal plasma technology, such as a low press drop across the reactor, compact size, and simple equipment structure. The process can be started immediately without warm up and can treat VOCs with solid Non-thermal plasma is also a commonly used technology for the elimination of VOCs [47–49]. Free electrons and radicals formed during the plasma process react with VOCs and lead to the degradation of VOCs to CO<sup>2</sup> and H2O. There are some advantages of the non-thermal plasma technology, such as a low press drop across the reactor, compact size, and simple equipment structure. The process can be started immediately without warm up and can treat VOCs with solid particles and liquid drops.
funds available for installation and running costs. Jie Hao et al. summarized the scope of application for different VOCs elimination technologies (Table 3) [50]. Condensation and adsorption recycling are suitable for recycling VOCs. Catalytic combustion and incineration can remove the VOCs with a moderate concentration (3000–1/4 LEL) at high temperatures, while biodegradation and non‐thermal plasma are suitable for abatement of low concentration VOCs at moderate temperatures. Hence, when selecting the efficient and economic VOCs abatement technologies, the scope of application for
**Table 3.** VOCs elimination techniques and their operating conditions [50].
**Discharge rate**
**(m3∙h−1) Temperature (°C)**
**(mg∙m−3)**
Adsorption recycling 100–1.5×104 <6 × 104 <45
combustion 3000–1/4 LEL \* <4 <sup>×</sup> <sup>104</sup> <500
combustion 1000–1/4 LEL <4 <sup>×</sup> <sup>104</sup> <500
incineration 1000–1/4 LEL <4 <sup>×</sup> <sup>104</sup> >700
Biodegradation <1000 <1.2 × 104 <45
Condensation 104–105 <104 <150
Preheated incineration 3000–1/4 LEL <4 × 104 >700
Adsorption concentration <1500 104–1.2 × 105 <45
combustion to achieve a superior abatement of pharmaceutical VOCs.
different technologies needs to be considered.
Preheated catalytic
Thermal storage catalytic
Thermal storage
**Abatement technologies VOC Concentration**
particles and liquid drops. The non‐thermal plasma technology can be combined with catalytic
The non-thermal plasma technology can be combined with catalytic combustion to achieve a superior abatement of pharmaceutical VOCs.
The application of different VOCs elimination technologies depends on the factors such as the range and concentration of VOCs present in the waste stream, the volume of waste stream, and the funds available for installation and running costs. Jie Hao et al. summarized the scope of application for different VOCs elimination technologies (Table 3) [50]. Condensation and adsorption recycling are suitable for recycling VOCs. Catalytic combustion and incineration can remove the VOCs with a moderate concentration (3000–1/4 LEL) at high temperatures, while biodegradation and non-thermal plasma are suitable for abatement of low concentration VOCs at moderate temperatures. Hence, when selecting the efficient and economic VOCs abatement technologies, the scope of application for different technologies needs to be considered.
**Table 3.** VOCs elimination techniques and their operating conditions [50].
\* Lower explosive limit (LEL).
In a survey of 771 industrial applications of VOCs elimination processes, including 330 cases in China and 441 cases in various other countries (Figure 8), Jinying Xi et al. examined how often the various technologies were used [51]. The data showed that the most commonly used technology in China was adsorption (38%), followed by catalytic combustion (22%) and biodegradation (15%), while in other countries, the most often utilized were biodegradation (29%) and catalytic combustion (29%), followed by adsorption (16%). Due to its simple operation, low capital cost, and ability to recycle VOCs across a wide range of concentrations, adsorption was the most widely used technology in China. However, in some instances, the adsorption equipment was not well maintained and used correctly for recycling of VOCs. Adsorption, membrane purification, and condensation were the most effective methods, therefore most commonly applied, in instances where VOCs were present in concentrations above 10,000 mg·m−<sup>3</sup> . Catalytic combustion and incineration were used for the destruction of VOCs in the concentration range of 2000~10,000 mg·m−<sup>3</sup> , where recycling is not financially viable. Biodegradation and non-thermal plasma were applied for the treatment of VOCs in a lower concentration than 2000 mg·m−<sup>3</sup> . Some examples are introduced in the following section to illustrate the application of these technologies for the elimination of pharmaceutical VOCs in China [3,4,14,51,52].
In the production of cefuroxime axetil, cefuroxime sodium, and cefotaxime sodium, the emitted VOCs include methanol, acetone, dichloromethane, DMF, acetic ether, and cyclohexane. One reported setup for the removal of these VOCs via a combination of condensation and adsorption technologies is described in Figure 9. Firstly, a portion of the various solvents was removed across a three-stage condensing unit comprised of a single stage of recycled water condensation, followed by two stages of 7 ◦C water condensation. The VOCs which were unable to be removed via the condensation process were removed by a two-stage activated carbon adsorption tower. The VOC-rich waste gas feedstock had a total VOC concentration of 2400 mg·m−<sup>3</sup> , of which 800 mg·m−<sup>3</sup> was attributed to methanol. It entered the first condensation unit at a rate of 2000 m<sup>3</sup> ·h −1 . More than 95% of the total VOC content was removed after the combined condensation and adsorption process. As a result, the emitted concentration and discharge rate of methanol was reduced to 18.9–29.3 mg·m−<sup>3</sup> and 0.08 kg·h −1 , respectively, while the concentration and discharge rate of the other VOCs present was 55–63 mg·m−<sup>3</sup> and 0.16 kg·h −1 , respectively. Both of these levels were in accordance with the emission standards of Hebei province where the factory was located. effective methods, therefore most commonly applied, in instances where VOCs were present in concentrations above 10,000 mg∙m−3. Catalytic combustion and incineration were used for the destruction of VOCs in the concentration range of 2000~10,000 mg∙m−3, whererecycling is not financially viable. Biodegradation and non‐thermal plasma were applied for the treatment of VOCs in a lower concentration than 2000 mg∙m−3. Some examples are introduced in the following section to illustrate the application of these technologies for the elimination of pharmaceutical VOCs in China [3,4,14,51,52].
correctly forrecycling of VOCs. Adsorption, membrane purification, and condensation were the most
*Catalysts* **2020**, *10*, 668 9 of 49
Non‐thermal <500 <3 × 104 <80 \* Lower explosive limit (LEL).
In a survey of 771 industrial applications of VOCs elimination processes, including 330 cases in China and 441 cases in various other countries (Figure 8), Jinying Xi et al. examined how often the various technologies were used [51]. The data showed that the most commonly used technology in China was adsorption (38%), followed by catalytic combustion (22%) and biodegradation (15%), while in other countries, the most often utilized were biodegradation (29%) and catalytic combustion (29%), followed by adsorption (16%). Due to its simple operation, low capital cost, and ability to
**Figure 8.** The ratios of VOCs elimination technologies used in China (**a**) and other countries (**b**) (the numbers in the brackets are the numbers of companies using the related technologies). Reprinted with **Figure 8.** The ratios of VOCs elimination technologies used in China (**a**) and other countries (**b**) (the numbers in the brackets are the numbers of companies using the related technologies). Reprinted with permission from [51], copyright 2012, CNKI. *Catalysts* **2020**, *10*, 668 10 of 49 was 55–63 mg∙m−<sup>3</sup> and 0.16 kg∙h−1, respectively. Both of these levels were in accordance with the emission standards of Hebei province where the factory was located.
condensation process were removed by a two‐stage activated carbon adsorption tower. The VOC‐ rich waste gas feedstock had a total VOC concentration of 2400 mg∙m−3, of which 800 mg.m−<sup>3</sup> was **Figure 9.** Representation of the VOC abatement process in a factory which produced cefuroxime axetil, cefuroxime sodium, and cefotaxime sodium. Reprinted with permission from [4], 2016, CNKI. **Figure 9.** Representation of the VOC abatement process in a factory which produced cefuroxime axetil, cefuroxime sodium, and cefotaxime sodium. Reprinted with permission from [4], 2016, CNKI.
attributed to methanol. It entered the first condensation unit at a rate of 2000 m3∙h−1. More than 95% of the total VOC content was removed after the combined condensation and adsorption process. As a result, the emitted concentration and discharge rate of methanol was reduced to 18.9–29.3 mg∙m−<sup>3</sup> and 0.08 kg∙h−1, respectively, while the concentration and discharge rate of the other VOCs present Yan Li has investigated the VOC abatement process in four separate companies in Taizhou city, Zhejiang province [4]. The main products in these four companies were clindamycin, clindamycin phosphate; losartan potassium, valsartan, nevirapine; meropenem, imipenem; and ciprofloxacin, spirolactone, respectively. There were three different procedures used in these four companies. The various procedures included adsorption, catalytic combustion, and non‐thermal plasma technologies. The author assessed the efficiencies of the processes by measuring the concentration of a range of VOCs including benzene, toluene, xylene, methanol, formaldehyde, dichloromethane, chloroform, acetic ether, butylene oxide, acetonitrile, dimethylformamide, dysodia, and isopropanol before and after treatment. The results showed that after these elimination processes, the concentrations of VOCs in emitted waste gas were lowerthan the required concentration in standards for emissions of atmospheric pollutants from the pharmaceutical industry in Zhejiang province [53]. Yan Li has investigated the VOC abatement process in four separate companies in Taizhou city, Zhejiang province [4]. The main products in these four companies were clindamycin, clindamycin phosphate; losartan potassium, valsartan, nevirapine; meropenem, imipenem; and ciprofloxacin, spirolactone, respectively. There were three different procedures used in these four companies. The various procedures included adsorption, catalytic combustion, and non-thermal plasma technologies. The author assessed the efficiencies of the processes by measuring the concentration of a range of VOCs including benzene, toluene, xylene, methanol, formaldehyde, dichloromethane, chloroform, acetic ether, butylene oxide, acetonitrile, dimethylformamide, dysodia, and isopropanol before and after treatment. The results showed that after these elimination processes, the concentrations of VOCs in emitted waste gas were lower than the required concentration in standards for emissions of atmospheric pollutants from the pharmaceutical industry in Zhejiang province [53].
An example of a catalytic VOC treatment process is shown in Figure 10 below, which uses a regenerative catalytic oxidizer. The waste gas from the workshop was collected and combined with the waste gas from the sewage station, storehouse, and solid waste pile. The gas mixture was first washed with a water and alkali solution, then dehydrated and defogged. The dry gas was filtered, An example of a catalytic VOC treatment process is shown in Figure 10 below, which uses a regenerative catalytic oxidizer. The waste gas from the workshop was collected and combined with the waste gas from the sewage station, storehouse, and solid waste pile. The gas mixture was first washed with a water and alkali solution, then dehydrated and defogged. The dry gas was filtered, then heated
**Figure 10.** VOCs abatement process in the A factory. Reprinted with permission from [4], 2016, CNKI
Two companies used a process which combines a regenerative thermal oxidizer (RTO) with three condensation stages connected in a series (Figure 11). The waste gas was pretreated in the workshop below determining the lower explosion limit (LEL) of the mixture. Then, fresh air was
.
running fee of the whole VOCs elimination system was about RMB 1 million per year.
then heated with a preheater with an attached regenerative heat transfer. The preheated gas mixture
with a preheater with an attached regenerative heat transfer. The preheated gas mixture entered the catalytic reactor and was combusted. After catalytic combustion, the gas was washed again with a water and alkali solution and emitted to high altitude atmosphere by a fan. The removal efficiencies of dysodia and non-methane hydrocarbons were 68.07% and 94.33%, respectively. The running fee of the whole VOCs elimination system was about RMB 1 million per year. washed with a water and alkali solution, then dehydrated and defogged. The dry gas was filtered, then heated with a preheater with an attached regenerative heat transfer. The preheated gas mixture entered the catalytic reactor and was combusted. After catalytic combustion, the gas was washed again with a water and alkali solution and emitted to high altitude atmosphere by a fan. The removal efficiencies of dysodia and non‐methane hydrocarbons were 68.07% and 94.33%, respectively. The running fee of the whole VOCs elimination system was about RMB 1 million per year.
the waste gas from the sewage station, storehouse, and solid waste pile. The gas mixture was first
*Catalysts* **2020**, *10*, 668 10 of 49
was 55–63 mg∙m−<sup>3</sup> and 0.16 kg∙h−1, respectively. Both of these levels were in accordance with the
**Figure 9.** Representation of the VOC abatement process in a factory which produced cefuroxime axetil, cefuroxime sodium, and cefotaxime sodium. Reprinted with permission from [4], 2016, CNKI.
Yan Li has investigated the VOC abatement process in four separate companies in Taizhou city, Zhejiang province [4]. The main products in these four companies were clindamycin, clindamycin phosphate; losartan potassium, valsartan, nevirapine; meropenem, imipenem; and ciprofloxacin, spirolactone, respectively. There were three different procedures used in these four companies. The various procedures included adsorption, catalytic combustion, and non‐thermal plasma technologies. The author assessed the efficiencies of the processes by measuring the concentration of a range of VOCs including benzene, toluene, xylene, methanol, formaldehyde, dichloromethane, chloroform, acetic ether, butylene oxide, acetonitrile, dimethylformamide, dysodia, and isopropanol before and after treatment. The results showed that after these elimination processes, the concentrations of VOCs in emitted waste gas were lowerthan the required concentration in standards for emissions of atmospheric pollutants from the pharmaceutical industry in Zhejiang province [53]. An example of a catalytic VOC treatment process is shown in Figure 10 below, which uses a
emission standards of Hebei province where the factory was located.
**Figure 10.** VOCs abatement process in the A factory. Reprinted with permission from [4], 2016, CNKI **Figure 10.** VOCs abatement process in the A factory. Reprinted with permission from [4], 2016, CNKI.
. Two companies used a process which combines a regenerative thermal oxidizer (RTO) with three condensation stages connected in a series (Figure 11). The waste gas was pretreated in the workshop below determining the lower explosion limit (LEL) of the mixture. Then, fresh air was Two companies used a process which combines a regenerative thermal oxidizer (RTO) with three condensation stages connected in a series (Figure 11). The waste gas was pretreated in the workshop below determining the lower explosion limit (LEL) of the mixture. Then, fresh air was added in accordance with the determined LEL, such that the proper fuel, oxygen ratio, would be present in the RTO. After that, the gas mixture was oxidized in the RTO at a temperature of 850 ◦C with 98% of the thermal energy recycled. The high temperature gas exiting the RTO was cooled in a cooling tower, then further cooled across a three-stage condensation setup before it was emitted to the atmosphere. The dysodia concentration in the total vent was 300 mg·m−<sup>3</sup> , while the concentration of non-methane hydrocarbons was lower than 85 mg·m−<sup>3</sup> . These concentrations correspond to removal rates of 89% and 92% for dysodia and non-methane hydrocarbons, respectively. The main drawback of this procedure was that the condensation step was not effective for dichloromethane recycling and a lot of HCl generated from the incineration of chlorinated organic compounds, which in turn led to the corrosion of the equipment.
A process which combines the non-thermal plasma and catalytic oxidation techniques (oxidation of VOCs by H2O<sup>2</sup> in a low pH) to treat VOCs in waste gas is outlined in Figure 12. The waste gas, with a high VOC concentration, was pretreated, combined with exhaust gas, and washed with an alkali solution. Then, the washed gas entered the catalytic combustion/oxidation tower, where a portion of the VOCs content was oxidized to CO<sup>2</sup> and H2O. The oxidized waste gas was washed with water and entered a dehydrator to remove humidity. After that, the waste gas from the sewage station was added to the treated gas, upon which the gas mixture was treated with a non-thermal plasma, to remove additional VOCs. The gas was dehydrated again and entered into the second catalytic oxidation tower. The gas was washed by the alkali solution again and emitted to the atmosphere. The dysodia concentration was reduced by 84% to no more than 250 mg·m−<sup>3</sup> . The removal efficiency of non-methane hydrocarbons content was approximately 92%, with a concentration of less than 85 mg·m−<sup>3</sup> in the emitted gas.
These procedures (Figures 9–12 Figure 9 Figure 10 Figure 11 Figure 12) are representative of the range of VOCs elimination technologies currently in use in China. Almost all kinds of VOCs abatement technologies have been applied in the treatment of waste from the Chinese pharmaceutical industry. While the application of these technologies has successfully decreased the emission of VOCs, the
high cost of investment and low efficiency are still the main factors which hinder the application of these technologies in the industry. The adsorption, absorption, and biodegradation technologies face challenges concerning the production of secondary pollutants, the desorption of adsorbed VOCs, and the production of waste water and sludge. The incineration process is effective in removing the issue of secondary waste production, but it requires a large energy input and has safety risks posed by the high temperature and use of a flame within a factory. The catalytic combustion and non-thermal plasma technologies partially circumvent the issues of waste and high temperature, but they are currently costly techniques with a short equipment lifetime. Thus, the continuing improvement of these technologies and development of new technologies is needed. With the new emission standards/law coming into effect, VOC elimination processes need to be upgraded in many Chinese pharmaceutical factories. As such, there is a desire for novel, effective, and energy efficient technologies in the near future. *Catalysts* **2020**, *10*, 668 11 of 49 added in accordance with the determined LEL, such that the proper fuel, oxygen ratio, would be present in the RTO. After that, the gas mixture was oxidized in the RTO at a temperature of 850 °C with 98% of the thermal energy recycled. The high temperature gas exiting the RTO was cooled in a cooling tower, then further cooled across a three‐stage condensation setup before it was emitted to the atmosphere. The dysodia concentration in the total vent was 300 mg∙m−3, while the concentration of non‐methane hydrocarbons was lower than 85 mg∙m−3. These concentrations correspond to removal rates of 89% and 92% for dysodia and non‐methane hydrocarbons, respectively. The main drawback of this procedure was that the condensation step was not effective for dichloromethane recycling and a lot of HCl generated from the incineration of chlorinated organic compounds, which
**Figure 11.** VOCs abatement process in the B and C factory, utilizing a regenerative thermal oxidizer (RTO) and condensation. Reprinted with permission from [4], 2016, CNKI . **Figure 11.** VOCs abatement process in the B and C factory, utilizing a regenerative thermal oxidizer (RTO) and condensation. Reprinted with permission from [4], 2016, CNKI. *Catalysts* **2020**, *10*, 668 12 of 49
**Figure 12.** VOCs abatement process in the D factory. Reprinted with permission from [4], 2016, CNKI. **Figure 12.** VOCs abatement process in the D factory. Reprinted with permission from [4], 2016, CNKI.
### These procedures (Figures 9–12) are representative of the range of VOCs elimination **4. The Developing Technologies for VOCs Elimination**
**4. The Developing Technologies for VOCs Elimination**
future.
in turn led to the corrosion of the equipment.
technologies currently in use in China. Almost all kinds of VOCs abatement technologies have been applied in the treatment of waste from the Chinese pharmaceutical industry. While the application of these technologies has successfully decreased the emission of VOCs, the high cost of investment and low efficiency are still the main factors which hinder the application of these technologies in the industry. The adsorption, absorption, and biodegradation technologies face challenges concerning the production of secondary pollutants, the desorption of adsorbed VOCs, and the production of waste water and sludge. The incineration process is effective in removing the issue of secondary waste production, but it requires a large energy input and has safety risks posed by the high temperature and use of a flame within a factory. The catalytic combustion and non‐thermal plasma technologies partially circumvent the issues of waste and high temperature, but they are currently costly techniques with a short equipment lifetime. Thus, the continuing improvement of these As outlined in the previous section, the VOC abatement technologies currently in use in China are hindered by some key limitations, such as high construction and running costs, low removal efficiencies for complex VOCs, and high temperature or energy requirements. To address these shortcomings, significant research is being carried out to improve the traditional methods of VOC removal, such as adsorption, catalytic combustion, and non-thermal plasma [1,2,33,41,43,44,47–49,54]. In addition, new technologies which can avoid the disadvantages of the traditional technologies have emerged, such as photocatalytic oxidation, condensation-oxidation, and electron beam treatment [2,42,48,55]. A summary of the various catalysts and conditions that have been investigated as active materials for the catalytic oxidation of VOCs is contained in Table 4 below.
As outlined in the previous section, the VOC abatement technologies currently in use in China are hindered by some key limitations, such as high construction and running costs, low removal efficiencies for complex VOCs, and high temperature or energy requirements. To address these shortcomings, significant research is being carried out to improve the traditional methods of VOC removal, such as adsorption, catalytic combustion, and non‐thermal plasma [1,2,33,41,43,44,47– 49,54]. In addition, new technologies which can avoid the disadvantages of the traditional technologies have emerged, such as photocatalytic oxidation, condensation‐oxidation, and electron beam treatment [2,42,48,55]. A summary of the various catalysts and conditions that have been investigated as active materials for the catalytic oxidation of VOCs is contained in Table 4 below.
technologies and development of new technologies is needed. With the new emission standards/law
[65] Pt/Al2O3
| doab | 2025-04-07T03:56:58.806028 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 14
} |
004db1df-dc76-42af-952f-1f5e82b7822b.15 | *4.1. Catalytic Combustion*
Catalytic combustion has received attention recently, as it shows great potential to address the shortcomings of the incineration method. Catalytic combustion is suitable for waste gas streams with low VOC concentrations and a moderate flow rate. Compared with the incineration method, catalytic combustion has been shown to efficiently remove VOCs from waste streams with a wide range of VOC concentrations. In addition, it operates at a lower temperature (293 versus 673 K) than incineration and resistant to the production of undesirable byproducts, such as dioxins and NO<sup>x</sup> [105]. The diversity of VOC species necessitates the development of different kinds of catalysts for the combustion method, with commonly used catalysts including noble metals, non-noble metal oxides concentrated oxidation catalysts [1,2,41–44].
Previous reviews covering certain aspects of the catalytic combustion of VOCs have been published. For example, K. Everaert et al. reviewed, analyzed, and discussed the reaction kinetics, reactors, and reaction conditions of catalytic combustion research prior to 2004 [41]. Muhammad Shahzad Kamal et al. and Zhixiang Zhang et al. covered the recent progress in the development of combustion catalysts [2,44]. L.F. Liotta and W.B. Li et al. reviewed the mechanism of VOCs catalytic combustion over noble metal catalysts and non-noble metal catalysts, respectively [1,42]. In this section, we will briefly introduce the more recent development of new catalysts materials which have been used in this process, analyze the advantages and disadvantages they provide, and strategies for their successful implementation.
| doab | 2025-04-07T03:56:58.807957 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 15
} |
004db1df-dc76-42af-952f-1f5e82b7822b.16 | 4.1.1. Noble Metal Catalysts
The general consensus of previous studies is that noble metal catalysts show the best catalytic performance in the combustion of non-halogenated VOCs. The noble catalysts which have been investigated include platinum, palladium, ruthenium, iridium, gold, and silver (Table 5) [2,41–44]. Due to their size-dependence catalytic properties and high price, noble metal catalysts are often supported on porous supports, such as γ-Al2O3, SiO2, zeolite, and other non-metal oxides to increase the dispersion of noble metal nanoparticles and surface area, which can improve the catalytic efficiency of noble metal catalysts [106–111]. Catalysts are often supported on the substrate, which is in the form of a monolith or honeycomb material, such as cordierite, aluminum, and stainless steel [112]. Noble catalysts display high efficiencies for VOC removal at a lower temperature than other kinds of catalysts. Pt shows the best catalytic combustion of VOCs, exhibiting equal removal efficiencies at operating temperatures of up to 100 K lower than those used for other noble metals [42,44]. Two key factors were found to influence the catalytic performance of alumina supported Pt, dispersion and loading amount, with an increase in either of these properties associated with an obvious improvement of catalytic activity [106,113,114]. The metal particle size is also an important factor that influences the catalytic activity. Changes to the active metal particle size can also enhance catalytic performance. For instance, when the crystalline size of alumina supported Pt increased from 1.0 to 15.5 nm, the oxidation rate was found to increase by a factor of 10 [115]. The same phenomenon was also found in the oxidation of propylene over alumina supported Pt and Pd catalysts.
An important factor in the performance of noble metal catalysts for VOC combustion is specificity; in other words, the target VOCs molecules. For instance, P. Papaefthimiou et al. found that Pt and Pd supported on alumina showed good performance for the oxidation of benzene and butanol, but not ethyl acetate, with Pd generally outperforming Pt [57]. M. J. Patterson et al. discovered that alumina supported rhodium is the most active noble metal for 1-hexene, but not for aromatics, while benzene can be decomposed the most easily on platinum, and palladium showed the best catalytic performance in abatement of toluene [116]. The impact of certain non-VOC species in the waste stream can also impact performance, where these species can potentially 'poison' the catalyst or cause side reactions to occur. One example is the presence of CO, which was found to have little effect on the performance of Pd catalysts, but significantly inhibit the activity over Pt [117].
Transition metal oxides can be utilized as both supports and promoters for noble metal catalysts. Pt, Pd, Ru, Au supported on MgO, SnO2, Co3O4, NiO, TiO2, CeO2, La2O3, ZrO<sup>2</sup> or PrO<sup>2</sup> have been explored for the oxidation of toluene, benzene, xylene, propene, light alkane, ethanol, propanol, butanol, formaldehyde, acetone, and acetic acid, respectively [42]. The role of the transition metal oxides is not only to supply a large surface area to disperse the noble metal particles, but in some cases, it can also improve the catalytic performance of noble metal particles by enhancing the mobility of lattice oxygen species. Previous studies showed that active oxygen species formed on cobalt oxide spinel-type crystallites can enhance the catalytic oxidation over PdO supported on alumina [118,119].
The reduction properties also influence the oxidation ability of supported noble metal catalysts. T. Mitsui et al. prepared SnO2, CeO2, and ZrO<sup>2</sup> supported Pt and Pd catalysts for the abatement of acetaldehyde [120]. These prepared catalysts were treated in an H2/N<sup>2</sup> flow and calcined in the atmosphere. The results showed that the SnO<sup>2</sup> supported Pt and Pd showed the best catalytic performance among the calcined catalysts in the atmosphere, while after treatment in an H2/N<sup>2</sup> flow, the catalytic activity of SnO<sup>2</sup> supported Pt and Pd decreased due to the formation of inactive inter-metallic phases (PtSn and Pd3Sn2). In contrast, CeO<sup>2</sup> and ZrO<sup>2</sup> supported catalysts showed the improved catalytic activity after reduction. In the elimination of formaldehyde over a TiO2-supported catalyst, Pt/TiO<sup>2</sup> showed a superior catalytic performance to Rh/TiO2, Pd/TiO2, Au/TiO2, and neat TiO<sup>2</sup> [121]. Other research showed that a series of supported Pt, Pd, and Au catalysts can even partially eliminate formaldehyde at room temperature [122].
Recently, the use of MnO<sup>x</sup> based materials as catalyst supports has gained attention. In one instance, supporting Ag on NiO-doped MnO<sup>2</sup> showed a high activity towards the combustion of o-xylene [89]. The improvement in xylene oxidation was attributed to the enhanced oxygen activation and mobility afforded to the catalyst support via the addition of NiO and Ag. Wenbo Pei et al. explored the use of ordered, mesoporous Mn2O<sup>3</sup> supports with embedded Pt particles for the catalytic combustion of toluene [66]. They found that the strong interaction between Pt and Mn2O<sup>3</sup> in the ordered structure improves the activity and stability of the catalyst.
The single noble metal catalysts cannot satisfy the requirements of VOCs combustion. Therefore, some mixed noble metal catalysts have been developed to combine the advantages of different noble metal catalysts, such as Pt-Au, Cu-Au, and Pd-Au. T. Tabakova et al. found that the Pd deposition on the deposited gold showed the best catalytic performance for benzene combustion, which was totally eliminated at 200 ◦C. It also showed good stability [57]. M. Hosseini et al. showed that the deposition of palladium on aurum supported on TiO<sup>2</sup> (Pd(shell)-Au(core)/TiO2) can significantly improve the catalytic activity for oxidation of toluene and propylene [67]. Der Shing Lee et al. deposited Au-Pd bimetallic nanoparticles on CeO<sup>2</sup> for toluene degradation, which showed a much better catalytic performance than Au/CeO<sup>2</sup> and Pd/CeO<sup>2</sup> catalysts due to the synergistic effect of gold and palladium [68]. The addition of non-noble metals also can improve the catalytic activity of noble metal catalysts. Roberto Fiorenza et al. prepared Au-Ag/CeO<sup>2</sup> and Au-Cu/CeO<sup>2</sup> bimetallic catalysts for alcohol oxidation and CO oxidation. These two catalysts showed higher selectivity for intermediate products higher CO conversion at a low temperature (100 ◦C) than Au/CeO<sup>2</sup> [93]. The addition of Au also can improve the performance of Al2O<sup>3</sup> supported Cu-Pt catalysts in DMDS oxidation [81].
Noble metal catalysts showed a high catalytic activity and remarkable thermal stability in catalytic elimination of VOCs. However, the use of noble metal catalysts is also associated with distinct disadvantages. Firstly, the high cost of noble metal limits their application in the industrial abatement of VOCs. Secondly, the presence of chlorine, sulfur, CO, and water can suppress the catalytic performance significantly [42,123–125]. The regeneration and recycling of noble metal catalysts poisoned by Cl and S is difficult, so they are not suited to the treatment chlorine and sulfur-containing VOCs. In fact, the release of chlorine and sulfur containing VOCs is particularly common in pharmaceutical production processes, so chlorine- and sulfur-containing VOCs need to be removed prior to the treatment by noble metal catalysts, which will further increase the cost of waste gas purification.
| doab | 2025-04-07T03:56:58.808210 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 16
} |
004db1df-dc76-42af-952f-1f5e82b7822b.17 | 4.1.2. Non-Noble Metal Catalysts
To address the cost of noble metals, non-noble metal oxides catalysts were developed for the abatement of VOCs. The materials which have been studied as non-noble metals include the derivatives of transition metals and rare earth elements, such as Ti, Cu, Mn, Al, Ce, Co, Fe, Cr, and V (Table 6) [1,126–129]. Although transition metal oxides catalysts generally showed a lower catalytic activity than noble metal catalysts for the oxidation of VOCs, they have many advantages, such as a resistance to chlorine and sulfur poisoning, tunable material properties, low cost, long on-stream lifetime, easy regeneration, and low environmental impact. The non-noble metal oxides catalysts applied and studied in VOCs abatement include CuOx, MnO2, FeOx, NiOx, CrOx, and CoOx.
The non-noble metal oxides catalyst systems which we will discuss include ones in which the metal oxide is both supported or unsupported. Due to the presence of mobile oxide species in lattice, Co2O<sup>3</sup> displays excellent reduction and oxidation abilities. As such, studies have shown Co2O<sup>3</sup> to be one of the best catalysts used for the combustion of benzene, toluene, propane, 1,2-dichloroethane, and 1,2-dichlorobenzene [130–133]. The catalytic activity of Co2O<sup>3</sup> is determined by the method of preparation, treatment conditions, and surface area.
MnO<sup>2</sup> is another commonly used metal oxide catalyst, which has been applied and studied in the abatement of n-hexane, acetone, benzene, ethanol, toluene, propane, trichloroethene, ethyl acetate, and NO<sup>x</sup> [133–139]. The catalytic activity can be tuned by the preparation method and depends on the structure, surface area, support materials, and oxidation states of catalysts. In the catalytic combustion of ethyl acetate and hexane, MnO<sup>2</sup> even achieved a better activity than Pt/TiO<sup>2</sup> [140]. Yonghui Wei et al. removed the La atoms from the LaMnO<sup>3</sup> perovskite to prepare MnO<sup>2</sup> with a high surface area (>150 m<sup>2</sup> /g), upon which it showed excellent catalytic activity in the oxidation of toluene [96]. Zhang Kai et al. synthesized the nano-cubic MnO<sup>x</sup> which has a large specific surface area, many oxygen vacancies, and good low temperature reducibility. The conversion of toluene via combustion was more than 90% at 350 ◦C [95]. Xueqin Yang et al. found that the acid treatment did not change the morphology of the catalyst, but could improve the oxidation ability of the catalyst by increasing the number of Mn4<sup>+</sup> species and structural defects on the surface of the catalyst [69]. A common theme throughout the implementation of Mn-based metal oxide catalysts for VOC oxidation is the availability and mobility of oxygen within the MnOx, which is attributed to the oxidation and reduction ability of the Mn afforded by the multiple oxidation states in which it can exist.
Copper oxides are another kind of efficient catalysts used in total oxidation of methane, methanol, ethanol, and acetaldehyde [141,142]. The main factors which influence the catalytic activity are the Cu oxidation state and the availability of lattice oxygen. The addition of other metal oxides, such as CeO2, can enhance the catalytic ability noticeably [133].
Chromium oxides are also promising oxidation catalysts, especially for the combustion of halogenated VOCs [124,133,143]. For chromium oxides, highly crystalline samples showed a better catalytic activity than amorphous ones [144]. Rotter et al.'s research showed that, when using TiO<sup>2</sup> as a support material, chromium oxides achieved higher catalytic oxidation of trichloroethylene than manganese oxide, cobalt oxide, and iron oxide [82]. Chromium has also been successfully supported on silica, alumina, porous carbon, and clay to eliminate pollutants such as carbon tetrachloride, chloromethane, trichloroethylene, ethyl chloride, chlorobenzene, and perchloroethylene [142]. However, chromium oxides also suffer deactivation due to the reaction between chromium and chlorine to form Cr2Cl<sup>2</sup> [145,146].
CeO<sup>2</sup> is a widely used catalyst in oxidation reactions due to its strong interactions with other metals, high oxygen storage capacity, and ready shuttling between the Ce3<sup>+</sup> and Ce4<sup>+</sup> states [147–149]. Dai et al. compared the removal of chlorinated alkanes and alkylenes over CeO<sup>2</sup> [83]. The results showed that CeO<sup>2</sup> is more efficient when it comes to oxidizing chlorinated alkanes than chlorinated alkylenes. CeO<sup>2</sup> also faced the deactivation problem due to the absorption of Cl<sup>2</sup> and HCl on the surface [150], so the design of chlorine resistance metal oxide catalysts is still a challenge which must be overcome.
Vanadium oxides were also developed to decompose chlorinated VOCs, such as polychlorinated pollutants and dichlorobenzene due to its tolerance of chlorine and sulfur compounds [151]. The presence of water can enhance and suppress the catalytic activity of V2O<sup>5</sup> via the removal of surface absorbed chlorine and reduction of active sites, respectively [152]. Other non-noble metal oxides were also investigated for abatement of VOCs, such as NiO and FeOx, which require further improvement of catalytic efficiency [153].
The above discussion suggests that the use of a single metal oxide as VOC oxidation catalysts is too often plagued by either a low catalytic activity or catalyst poisoning. Thus, focus has shifted to the development of mixed metal oxide catalysts such as Mn-Ce, Mn-Cu, Co-Ce, Sn-Ce, Mn-Co, and Ce-Cu oxides [119,154–157]. The logic here is that combining two metal oxides with different materials and catalytic properties allows for a synergistic enhancement in performance. The previous studies showed that the rate determining step of VOC catalytic combustion was the oxygen removal from the catalysts lattice [119], so the goal of mix-metal oxides catalysts design was the enhancement of the lattice oxygen species availability.
The addition of copper into CeO<sup>2</sup> can promote the catalytic efficiency due to a synergistic effect, leading to a highly efficient decomposition of ethyl acetate, ethanol, propane, benzene, and toluene [158–160]. MnOx-CeO<sup>2</sup> has been applied for the destruction of ethanol, formaldehyde, hexane, phenol, ethyl acetate, and toluene [155,161–164]. Mn-Co oxides catalysts also showed improved catalytic activity relative to either MnO<sup>x</sup> or Co2O<sup>3</sup> in the combustion of ethyl acetate and n-hexane [100]. CeO2-CrO<sup>x</sup> showed excellent catalytic activity for the decomposition of chlorinated VOCs [165], while the removal of chlorobenzene over MnOx-TiO<sup>2</sup> and MnOx-TiO2-SnO<sup>x</sup> showed much better catalytic performance than not only the individual oxides, but also achieved removal efficiencies on par with noble metal catalysts [166]. A three-dimensional ordered mesoporous material of mixed cerium-manganese oxide was prepared for the efficient catalytic combustion of chlorine-containing VOCs due to its large specific surface, enriched Ce3<sup>+</sup> content, oxygen vacancies, active oxygen species, and acidic sites. It showed good water resistance and high airspeed applicability. However, catalyst deactivation caused by inorganic chlorine adsorption still occurred [90]. Layered copper manganese oxide has been prepared for the catalytic combustion of CO and VOCs, which showed efficient activity due to the interfacial structure of mixed phases and the formation of the Cu2+-O2−-Mn4<sup>+</sup> entity [58]. Acidic sites can be provided by support to prevent the decrease of catalytic activity. CeO2@SiO<sup>2</sup> was prepared to catalyze the combustion of 1,2-dichloroethane. SiO<sup>2</sup> can provide weak acid sites, as well as promote the adsorption and activation of 1,2-dichloroethane and the desorption of generated HCl [85].
A vast number of preparation methods exist for the synthesis of mixed metal oxide catalysts, including thermal decomposition, impregnation, co-precipitation, and the sol-gel method [167–169]. The selection of preparation methods depends on the properties of catalysts and the application situation. Furthermore, as discussed above, complex ordered microporous, multilayer or core-shell structures have recently been applied to catalytic oxidation processes to access properties which come from having a highly controlled particle composition and morphology. The chance to alter not only the metal centers present within mixed metal oxide systems, but also the relative metal ratios.
**Table 6.** Non-noble metal catalysts for catalytic combustion of VOCs.
| doab | 2025-04-07T03:56:58.808798 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 17
} |
004db1df-dc76-42af-952f-1f5e82b7822b.18 | 4.1.3. Perovskite Catalysts
Perovskite-type oxides are a kind of composite oxides which have a similar structure with CaTiO3, and can be expressed by ABO3. The common way to modify the perovskite catalysts is replacement of the cation B by B' to tune the redox ability or enhance the stability [1]. With the replacement of the cation B, the crystal lattice would be distortion which leads to the enhancement of redox ability and improvement of stability. The most commonly used perovskite for catalytic combustion of VOCs is LaBO3, in which B can be Co, Fe, Ni, Mn, and Sr (Table 7) [60,97,170,171]. Huang et al. used Sr partially replaced La in LaCoO<sup>3</sup> for catalytic combustion of propyl alcohol, toluene, and cyclohexane [70]. The results showed that the doped LaCoO<sup>3</sup> showed a better catalytic performance than the undoped one, and the modified catalysts were stable in the reaction. R. Spinicci et al. compared the catalytic activity of LaMnO<sup>3</sup> and LaCoO<sup>3</sup> for catalytic combustion of acetone, isopropanol, and benzene [60]. They suggested that LaMnO<sup>3</sup> showed a better performance than LaCoO3. In oxidation of isopropanol, acetone was the intermediate product. The surface oxygen species played a key role in this process. The increase of oxygen pressure is positive for the catalytic combustion of VOCs over these perovskite catalysts. G. Sinquin et al. applied LaMnO<sup>3</sup> and LaCoO<sup>3</sup> for the catalytic combustion of chlorinated VOCs, such as CH2Cl<sup>2</sup> and CCl4. LaMnO<sup>3</sup> showed a better chlorine resistance than LaCoO<sup>3</sup> [97]. Mihai Alifanti et al. supported LaCoO<sup>3</sup> on cerium-zirconium oxides (Ce1-xZrxO2, x = 0–0.3) for the catalytic combustion of benzene and toluene [172]. The results showed that all the supported catalysts showed a better performance than Ce1-xZrxO<sup>2</sup> and 20% loaded LaCoO<sup>3</sup> showed about 10 times higher catalytic activity than LaCoO<sup>3</sup> for toluene oxidation due to its large surface area and good oxygen mobility. S. I. Suárez-Vázquez et al. synthesized SrTi1-xBxO<sup>3</sup> (B = Mn, Cu) for toluene destruction [71]. Mn could replace Ti and enter the perovskite structure, while Cu could not. The Mn doped catalysts showed the highest catalytic activity and can completely decompose toluene to CO<sup>2</sup> at a temperature lower than 350 ◦C. Perovskite also can be prepared from solid waste such as the obsoleting lithium battery. Mingming Guo et al. prepared manganese-based perovskite catalyst from the waste lithium battery for catalytic combustion of toluene, which showed a better catalytic activity than pure manganese perovskite catalyst due to more Mn4<sup>+</sup> ions and lattice oxygen species, as well as high specific surface area [72]. In order to increase the amounts of active sites, Junxuan Yao et al. removed the La ions from LaCoO<sup>3</sup> to obtain the disordered Co3O4. It showed a better catalytic activity for propane combustion than the one prepared by other methods [94]. To further improve the catalytic activity, γ-MnO<sup>2</sup> was calcined on the surface of SmMnO<sup>3</sup> which had a large specific surface area, high Mn4+/Mn3<sup>+</sup> and Olatt/Oads. Compared with SmMnO3, it showed a better catalytic activity and stability (10 vol% water) in the process of catalytic reaction [61]. Jingsi Yang et al. assembled the LaMnO<sup>3</sup> perovskite in MnO<sup>2</sup> and adjusted La/Mn to 15. The redox ability of the catalyst was improved by enhancing the interaction between the active phase and the support [73]. The ratio of citric acid and metal ion (La3+Mn2+) was also tested to find out the best composite of perovskite catalysts. Zakaria Sihaib et al. prepared LaMnO<sup>3</sup> with different ratios. The results show that the catalyst with the ratio of 0.5 to 1.5 has the best catalytic performance and the amount of citric acid affects the specific surface area of perovskite catalyst [74]. Li Wang et al. added Sr into the LaMnO<sup>3</sup> to prepare La0.5Sr0.5MnO3. The amount of Mn4<sup>+</sup> and the oxidation ability of vinyl chloride has been improved after HCl modification [104]. The perovskite catalysts showed a good catalytic activity for combustion of VOCs at low temperature due to their tunable redox property by replacing the B atom. However, they also have some disadvantages, such as low thermal stability. The catalytic activity and stability of perovskite catalysts need to be further improved.
chlorination.
4.1.4. Concentrated Oxidation Catalysts
| doab | 2025-04-07T03:56:58.809387 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 18
} |
004db1df-dc76-42af-952f-1f5e82b7822b.19 | 4.1.4. Concentrated Oxidation Catalysts decomposed at 250 °C overthe NaX zeolite. The addition of Pt on the zeolite can significantly increase the catalytic activity. The main obstacle for the application of zeolite was the coke that formed during
The low concentration of VOCs can limit the catalytic efficiency of catalysts. Some porous materials were investigated to the concentration and decomposition of VOCs, such as zeolite, γ-Al2O<sup>3</sup> (Table 8, Figure 13) [84,101,173–175]. R. Beauchet et al. tried to decompose the isopropanol and o-xylene mixture over the CsX, NaX, and HY zeolite [173]. O-xylene and isopropanol were totally decomposed at 250 ◦C over the NaX zeolite. The addition of Pt on the zeolite can significantly increase the catalytic activity. The main obstacle for the application of zeolite was the coke that formed during the reaction, which led to the short lifetime. Amir Ikhlaq et al. studied the mechanism and kinetics of decomposition of chlorinated VOCs by ozonation over the γ-Al2O<sup>3</sup> and ZSM-5 zeolite [174]. The results suggest that ozone reacts with the absorbed VOCs on the surface of catalysts. However, the ozonation process will increase the cost in most of the industrial processes. Yuexin Peng et al. supported MnO<sup>2</sup> on the Al-rich <sup>β</sup>-zeolite to degrade toluene [75]. The T<sup>90</sup> is 285 ◦C, which is much lower than the MnO<sup>2</sup> supported on γ-Al2O<sup>3</sup> due to the lattice oxygen species in MnO<sup>2</sup> and absorbed oxygen species on the zeolite. Cu and Co were also used to modify the β-zeolite for the destruction of toluene and trichloroethylene [76]. The modified zeolite showed good stability in the reaction. The catalytic activity is mainly from the supported metal oxides. The suitable acidity and strong oxidation stability can improve the CO<sup>2</sup> selectivity. Dedong He et al. modified the HZSM-5 zeolite with a series of rare earth elements, including La, Ce, Pr, Nd, Sm, Y, and Er, for the catalytic decomposition of CH3SH [91]. Cu was also used to modify ZSM-5 for the combustion of acrylonitrile. The isolated Cu is the active center. The SiO2/Al2O<sup>3</sup> ratio can affect the ion exchange capacity and the catalytic performance of the catalyst. When the ratio is 26, the catalyst shows the best catalytic activity [92]. SBA-15 was used to support MnO<sup>x</sup> for the combustion of toluene and showed good catalytic activity [176]. The La modified HZSM-5 zeolite showed much better activity and stability than HZSM-5 due to the tunable acidity, which can promote the adsorption and activation of the CH3SH molecule and inhibit the formation of coke deposit. The previous researches showed the potential application in the industry. However, to fulfill the requirements of the industry, the concentration-catalysis process to remove VOCs needs more research on lifetime, catalytic efficiency. the reaction, which led to the short lifetime. Amir Ikhlaq et al. studied the mechanism and kinetics of decomposition of chlorinated VOCs by ozonation over the γ‐Al2O3 and ZSM‐5 zeolite [174]. The results suggest that ozone reacts with the absorbed VOCs on the surface of catalysts. However, the ozonation process will increase the cost in most of the industrial processes. Yuexin Peng et al. supported MnO2 on the Al‐rich β‐zeolite to degrade toluene [75]. The T90 is 285 °C, which is much lower than the MnO2 supported on γ‐Al2O3 due to the lattice oxygen species in MnO2 and absorbed oxygen species on the zeolite. Cu and Co were also used to modify the β‐zeolite for the destruction of toluene and trichloroethylene [76]. The modified zeolite showed good stability in the reaction. The catalytic activity is mainly from the supported metal oxides. The suitable acidity and strong oxidation stability can improve the CO2 selectivity. Dedong He et al. modified the HZSM‐5 zeolite with a series of rare earth elements, including La, Ce, Pr, Nd, Sm, Y, and Er, for the catalytic decomposition of CH3SH [91]. Cu was also used to modify ZSM‐5 for the combustion of acrylonitrile. The isolated Cu is the active center. The SiO2/Al2O3 ratio can affect the ion exchange capacity and the catalytic performance of the catalyst. When the ratio is 26, the catalyst shows the best catalytic activity [92]. SBA‐15 was used to support MnOx for the combustion of toluene and showed good catalytic activity [176]. The La modified HZSM‐5 zeolite showed much better activity and stability than HZSM‐5 due to the tunable acidity, which can promote the adsorption and activation of the CH3SH molecule and inhibit the formation of coke deposit. The previous researches showed the potential application in the industry. However, to fulfill the requirements of the industry, the concentration‐catalysis process to remove VOCs needs more research on lifetime, catalytic efficiency.
*Catalysts* **2020**, *10*, 668 32 of 49
materials were investigated to the concentration and decomposition of VOCs, such as zeolite, γ‐Al2O3
The low concentration of VOCs can limit the catalytic efficiency of catalysts. Some porous
**Figure 13.** Scheme of adsorption/oxidation apparatus. Reprinted with permission from [1], 2009, Elsevier Ltd. **Figure 13.** Scheme of adsorption/oxidation apparatus. Reprinted with permission from [1], 2009, Elsevier Ltd.
**Table 8.** Concentrated oxidation catalysts for catalytic combustion of VOCs.
| doab | 2025-04-07T03:56:58.809701 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 19
} |
004db1df-dc76-42af-952f-1f5e82b7822b.20 | 4.1.5. The Influence Factors on Catalytic Performance
In the real industrial process, a lot of factors influence the catalytic efficiency. Firstly, the kind of VOCs determines the selection of catalysts. For example, noble metal catalysts show the best VOCs eliminating efficiency, but they are not suitable for the destruction of chlorine and sulfur containing VOCs due to the poison of the catalysts. Secondly, the surface area of the catalysts is the main factor that influences the catalytic activity. Research shows that MnO<sup>2</sup> with a higher surface area showed much better catalytic performance than the one with a lower surface area [133]. Thirdly, the crystal type of catalysts also influences the catalytic performance of the catalysts with the same content. For instance, the catalytic performance of TiO<sup>2</sup> with different crystal types, namely rutile and anatase, showed a different catalytic activity [121,167]. The humidity is the common content in the industrial waste gas. In most of the reports, water molecules can suppress the catalytic activity due to the complete adsorption on active sites and destruction of catalysts [177]. In other researches, humidity plays a positive role in the oxidation. In the catalytic combustion of chlorobenzene over VOx/TiO2, VOx-WOx/TiO2, and VOx-MoOx/TiO2, water can remove the adsorbed Cl<sup>−</sup> from the catalysts surface and react with chlorine to produce HCl [134]. The water vapor can show a different effect on the same VOCs over different catalysts. Kullavanijayam et al. reported that water enhanced the catalytic oxidation of cyclohexene over the ceria–alumina supported Pt and Rh, but it had a negative effect on the oxidation of cyclohexane over the ceria–alumina supported Pd catalyst [178]. CO is another poison for precious metal catalysts, so during the catalytic combustion, enough air is needed to avoid the generation of CO. The life of catalysts is also important for catalytic combustion. The main obstacle for the development of catalysts for the catalytic combustion of halogen and sulfur containing VOCs is the short life of common catalysts. Although V oxides show less catalytic efficiency than other catalysts, it is still used in the elimination of halogen and sulfur containing VOCs due to its long catalytic life [152]. Since after the installation of catalytic combustion equipment, it will be in operation for a long time, therefore, the life of the catalysts determines the cost of catalytic combustion.
The catalytic combustion process has been well developed in recent years. The diversity of the catalysts has been investigated. However, there are still some obstacles on the way to the industrial application, such as short lifetime, high cost, and no universality to different kinds of VOCs. More work should be done on these problems.
| doab | 2025-04-07T03:56:58.809975 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 20
} |
004db1df-dc76-42af-952f-1f5e82b7822b.21 | *4.2. Photocatalytic Oxidation*
Photocatalytic reactions have drawn a lot of attention and have been well developed in recent years since Fujishima found the splitting water to H<sup>2</sup> and O<sup>2</sup> over TiO<sup>2</sup> [55,179,180]. Different kinds of photocatalysts have been developed to treat VOCs containing waste water, such as TiO2, WO3, ZnO2, CdS, g-C3N4, and BiOBr (Table 9) [56,180–185]. The mechanism of photocatalysis is that when the light with a suitable wavelength radio on catalysts (semiconductors), the electrons and holes were separated and generated on the surface of catalysts, then the radicals of •OH and O2<sup>−</sup> was formed on the surface of catalysts, the VOCs reacted with these radicals and decomposed to CO<sup>2</sup> and H2O at last [186,187]. In the water solution, water can react with the catalysts and form •OH, which is positive for the decomposition of VOCs in water. The photocatalytic elimination of VOCs in the gas phase follows a similar mechanism, but the radicals are main O2- due to the shortage of humidity. In this section, the progress in the elimination of VOCs in the gas phase by photocatalytic methods was mainly discussed.
The most studied photocatalyst for the elimination of VOCs is TiO<sup>2</sup> [44]. Wilson F. Jariam et al. degraded 17 kinds of VOCs with the concentration range of 400–600 ppmv over TiO<sup>2</sup> under the radiation of ultraviolet light [98]. The results showed that trichloroethylene (99.9%), isooctane (98.9%), acetone (98.5%), methanol (97.9%), methyl ethyl ketone (97.1%), t-butyl methyl ether (96.1%), dimethoxymethane (93.9%), methylene chloride (90.4%), methyl isopropyl ketone (88.5%), isopropanol (79.7%), chloroform (69.5%), and tetrachloroethylene (66.6%) were decomposed efficiently over TiO2. The photodegradation of isopropylbenlene (30.3%), methyl chloroform (20.5%), and pyridine (15.8%)
on TiO<sup>2</sup> was not as efficient as other VOCs. The catalytic lifetime was also tested by toluene. The conversion of toluene decreased to 20.9% after a 150 min test, but the deactivated catalysts can be easily regenerated by washing with H2O<sup>2</sup> and illumination. F. B. Li et al. prepared La ion doped TiO<sup>2</sup> by the sol-gel method for photodegradation of benzene, toluene, ethylbenzene, and o-xylene in the gas phase [188]. The results showed that the La ion doped TiO<sup>2</sup> performed much better than the pure TiO2. This was due to the improved adsorption ability and the enhanced electron–hole pairs separation by the presence of Ti3<sup>+</sup> and the electron transfer between the conduction band/defect level and lanthanide crystal field state. Tânia M. Fujimoto et al. supported the palladium on TiO<sup>2</sup> for the photocatalytic decomposition of octane, isooctane, n-hexane, and cyclohexane in a low concentration (100~120 ppmv) [99]. The modified catalysts showed excellent catalytic activity in the decomposition of VOCs rapidly. V. Héquet et al. used a closed-loop reactor to study the mixture effect over the P<sup>25</sup> TiO2/SiO<sup>2</sup> mixture (Figure 14) [189]. They have developed the accurate analytical methods to identify and quantify the majority of the potential formed intermediates, which provide an efficient way to study the reaction mechanism. Yajie Shu et al. used Mn doped TiO<sup>2</sup> to degrade benzene by O<sup>3</sup> under vacuum ultraviolet (VUV) irradiation [62]. The doped TiO<sup>2</sup> showed better performance than the undoped one and P<sup>25</sup> due to the formation of highly reactive oxidizing species. Jian Ji et al. showed that compared with the one without UV radiation, the UV radiation can improve the removal efficiency of benzene by about 10% (Figure 15) [63]. Marta Stucchi et al. developed a simultaneous photodegradation system for the VOC mixture elimination by TiO<sup>2</sup> powders, which showed a good efficiency [86]. Huiling Huang et al. developed Mn modified ZSM-5 as catalysts for VUV photolysis combined with ozone-assisted catalytic oxidation and studied the mechanism [102]. The catalysts showed good efficiency. Although most of the catalysts showed excellent activity in the elimination of VOCs under UV light, the light utilization efficiency was still low due to the low percent of the UV light in nature light. Xufang Qian et al. designed mesoporous TiO<sup>2</sup> films coated with carbon foam for photodegradation of acetone and toluene, which can converse more than 90% of VOCs to CO<sup>2</sup> under visible light due to the plausible carbon doping and the strong interaction between the TiO<sup>2</sup> precursor and the hydro-carbon foams [100]. The graphite-SiO2-TiO<sup>2</sup> composite and BiOBr@SiO<sup>2</sup> flower-like nanospheres were also used for photodegradation of VOCs under visible light and showed good catalytic activity [64]. The low solar utilization ratio was the main obstacle to improve photocatalytic efficiency. In order to improve the efficiency, materials with a good light adsorption ability was applied. Yun-En Lee et al. prepared black-TiO<sup>2</sup> and LFO/black-TiO<sup>2</sup> and they showed excellent photo catalytic activity for the removal of toluene and IPA due to their good light adsorption ability [77]. The modification of g-C3N<sup>4</sup> by hydroxyl groups can enhance visible light-driven photocatalytic properties of g-C3N<sup>4</sup> obviously which can improve the adsorption energy of g-C3N<sup>4</sup> for water and phenol [88]. The structure of catalysts was also carefully tuned to improve the catalytic activity [190]. Bettini S et al. insulated a layer of SiO<sup>2</sup> between zinc oxide and nano silver and controlled the thickness of the insulated layer, which enhanced the photocatalytic oxidation ability of the catalyst significantly [78]. The photocatalytic activity of ZnO can also be improved by doping CuO [191]. The photocatalytic elimination of VOCs is one of the most promising methods. However, the industrial application of this method is still a problem.
*Catalysts* **2020**, *10*, 668 36 of 49
*Catalysts* **2020**, *10*, 668 36 of 49
**Figure 14.** A 420‐L of continuous closed‐loop photocatalytic reactor: (**1**) Photocatalytic unit containing the TiO2 photocatalytic medium and the ultraviolet (UV) lamps, (**2**) fan, (**3**) air input and sampling points for analysis, (**4**) air tranquilization chamber, (**5**) flow rate measurement. Reprinted with permission from [176], 2018, Elsevier Ltd.. **Figure 14.** A 420-L of continuous closed-loop photocatalytic reactor: (**1**) Photocatalytic unit containing the TiO<sup>2</sup> photocatalytic medium and the ultraviolet (UV) lamps, (**2**) fan, (**3**) air input and sampling points for analysis, (**4**) air tranquilization chamber, (**5**) flow rate measurement. Reprinted with permission from [176], 2018, Elsevier Ltd.. **Figure 14.** A 420‐L of continuous closed‐loop photocatalytic reactor: (**1**) Photocatalytic unit containing the TiO2 photocatalytic medium and the ultraviolet (UV) lamps, (**2**) fan, (**3**) air input and sampling points for analysis, (**4**) air tranquilization chamber, (**5**) flow rate measurement. Reprinted with permission from [176], 2018, Elsevier Ltd..
**Figure 15.** The schematic diagram of the VUV‐PCO system. Reprinted with permission from [63], 2017, Elsevier Ltd. **Figure 15.** The schematic diagram of the VUV‐PCO system. Reprinted with permission from [63], 2017, Elsevier Ltd. **Figure 15.** The schematic diagram of the VUV-PCO system. Reprinted with permission from [63], 2017, Elsevier Ltd.
There is a large amount of reports on the photocatalytic elimination of VOCs. The advantages of this process are green, energy saving, and clean. However, the reported catalysts are not efficient enough for an industrial application at the present stage. Firstly, the reaction needs more time than other methods. Secondly, most of the reported catalysts could not use visible light as energy to start the reactions, which limited the improvement of quantum efficiency, so it still needs to develop efficient photocatalysts and design effective reaction systems. There is a large amount of reports on the photocatalytic elimination of VOCs. The advantages of this process are green, energy saving, and clean. However, the reported catalysts are not efficient enough for an industrial application at the present stage. Firstly, the reaction needs more time than other methods. Secondly, most of the reported catalysts could not use visible light as energy to start the reactions, which limited the improvement of quantum efficiency, so it still needs to develop efficient photocatalysts and design effective reaction systems. There is a large amount of reports on the photocatalytic elimination of VOCs. The advantages of this process are green, energy saving, and clean. However, the reported catalysts are not efficient enough for an industrial application at the present stage. Firstly, the reaction needs more time than other methods. Secondly, most of the reported catalysts could not use visible light as energy to start the reactions, which limited the improvement of quantum efficiency, so it still needs to develop efficient photocatalysts and design effective reaction systems.
**Table 9.** *Cont*.
### *4.3. Non-Thermal Plasma Process 4.3. Non‐Thermal Plasma Process*
In the non-thermal plasma process, electrons and their surroundings are not in a thermal equilibrium, so the electrons are heated by electric discharges instead of the gas itself, and produce the electrons with high energy, active radicals and ions which can promote numerous chemical reactions in the ionized zones. It can be used to treat the high flow for both low (<100 ppmv) and high (>1000 ppmv) concentrations of VOCs, including toluene, benzene, acetone, trichloroethylene, etc. (Table 10) [47]. The main bottlenecks for the commercialization of the technology are the formation of poison byproducts and high energy consumption. The discharge methods are important for the VOCs removal efficiency, which includes corona discharge, surface discharge, microwave discharge, dielectric barrier discharge, and packed bed dielectric barrier discharge [190–194]. Among these discharge types, the packed bed dielectric barrier discharge shows the most potential in the industry application [195]. Savita K. P. Veerapandian et al. reviewed the packed bed DBD [47]. The influences of dielectric constant, packing materials size, shape, surface properties, and the byproducts formation were discussed. In the non‐thermal plasma process, electrons and their surroundings are not in a thermal equilibrium, so the electrons are heated by electric discharges instead of the gas itself, and produce the electrons with high energy, active radicals and ions which can promote numerous chemical reactions in the ionized zones. It can be used to treat the high flow for both low (<100 ppmv) and high (>1000 ppmv) concentrations of VOCs, including toluene, benzene, acetone, trichloroethylene, etc. (Table 10) [47]. The main bottlenecks for the commercialization of the technology are the formation of poison byproducts and high energy consumption. The discharge methods are important for the VOCs removal efficiency, which includes corona discharge, surface discharge, microwave discharge, dielectric barrier discharge, and packed bed dielectric barrier discharge [190–194]. Among these discharge types, the packed bed dielectric barrier discharge shows the most potential in the industry application [195]. Savita K. P. Veerapandian et al. reviewed the packed bed DBD [47]. The influences of dielectric constant, packing materials size, shape, surface properties, and the byproducts formation were discussed.
Using porous and catalytic materials as the packed bed can increase the resistant time of VOCs and decrease the unwanted byproducts, such as O3, NOx, and CO (Figure 16) [196]. The packed bed materials can be non-catalytic porous materials, such as activated carbon, porous Al2O3, glass, zeolite, graphene oxide, and the catalysts, such as metal oxides, noble metal loaded metal oxides, and catalytic porous materials [197]. For example, using Al2O<sup>3</sup> as the packed bed can concentrate VOCs molecules on its surface and weaken the bond energy of VOCs, which can enhance the dissociation when these adsorbed molecules encounter the active species in the plasma and increase the collision probability and deep oxidation of VOCs [198]. Lee et al. used porous γ-Al2O<sup>3</sup> as the packed bed to oxidize toluene to CO2. It showed a 100% conversion of toluene and high CO<sup>2</sup> selectivity [199]. Gandhi's research results showed that a large surface area and pore volume of alumina can not only increase the conversion of ethylene to CO<sup>2</sup> or CO, but also decrease the selectivity of unwanted byproducts, such as acetaldehyde, acetylene, methane, N2O, and O<sup>3</sup> [200]. Other researches also showed the same phenomenon in the abatement of benzene, acetone, formaldehyde, TCE, chlorobenzene in non-thermal plasma with the packed bed which has a large surface area and pore volume, such as porous alumina, TiO2, zeolite, and porous metal oxides [199,201–203]. Due to the large surface area, the retention time and concentration of VOCs molecules increased which led to the increase of collisions between VOCs and active species [200], the adsorption effect can weaken the chemical bond of VOCs [204], and form micro-discharges in the micro-pores in addition to the micro-discharges in the gas phase. Using porous and catalytic materials as the packed bed can increase the resistant time of VOCs and decrease the unwanted byproducts, such as O3, NOx, and CO (Figure 16) [196]. The packed bed materials can be non‐catalytic porous materials, such as activated carbon, porous Al2O3, glass, zeolite, graphene oxide, and the catalysts, such as metal oxides, noble metal loaded metal oxides, and catalytic porous materials [197]. For example, using Al2O3 as the packed bed can concentrate VOCs molecules on its surface and weaken the bond energy of VOCs, which can enhance the dissociation when these adsorbed molecules encounter the active species in the plasma and increase the collision probability and deep oxidation of VOCs [198]. Lee et al. used porous γ‐Al2O3 as the packed bed to oxidize toluene to CO2. It showed a 100% conversion of toluene and high CO2 selectivity [199]. Gandhi's research results showed that a large surface area and pore volume of alumina can not only increase the conversion of ethylene to CO2 or CO, but also decrease the selectivity of unwanted byproducts, such as acetaldehyde, acetylene, methane, N2O, and O3 [200]. Other researches also showed the same phenomenon in the abatement of benzene, acetone, formaldehyde, TCE, chlorobenzene in non‐thermal plasma with the packed bed which has a large surface area and pore volume, such as porous alumina, TiO2, zeolite, and porous metal oxides [199,201–203]. Due to the large surface area, the retention time and concentration of VOCs molecules increased which led to the increase of collisions between VOCs and active species [200], the adsorption effect can weaken the chemical bond of VOCs [204], and form micro‐discharges in the micro‐pores in addition to the micro‐discharges in the gas phase.
**Figure 16.** Schematic diagram of the plasma‐driven catalyst (PDC) reactor. Reprinted with permission from [196], 2007, Elsevier Ltd. **Figure 16.** Schematic diagram of the plasma-driven catalyst (PDC) reactor. Reprinted with permission from [196], 2007, Elsevier Ltd.
The addition of catalysts in the non‐thermal plasma process as the packed bed can enhance the VOCs removal efficiency significantly due to the plasma assisted reactions on the surface of the The addition of catalysts in the non-thermal plasma process as the packed bed can enhance the VOCs removal efficiency significantly due to the plasma assisted reactions on the surface of the catalyst.
catalyst. These plasma‐assisted reactions can generate some active radical species, such as
These plasma-assisted reactions can generate some active radical species, such as intermediates with electrons, O·, OH·, N2·, NO·, N2O· in the gas phase and O·, OH· on the surface of catalysts, which can improve the VOCs removal efficiency and increase the CO<sup>x</sup> selectivity [80]. The catalysts include metal oxides, such as CuO, MnO2, CeO2, NiO, CoO2, and Fe2O3, and noble metals, such as Pt, Au, and Ag [65,196,203,205–209]. The different kinds of oxygen species in catalysts, namely lattice oxygen, surface oxygen, and absorbed oxygen, are very helpful for the oxidation of VOCs, which can activate the oxygen molecules in the gas phase. P.J. Asilevi et al. established a laboratory scale DBD reactor for the removal of toluene by ·O and ·OH which were generated from the reaction between O<sup>2</sup> and H2O and the removal efficiency of VOCs can be improved by increasing the oxygen concentration and relative humidity [210]. Lu et al. used FeOx/SBA-15 as the packed bed to eliminate toluene in the non-thermal plasma process. The results showed that the presence of Fe2<sup>+</sup> can increase the toluene removal efficiency and CO<sup>x</sup> selectivity obviously and reduce the formation of unwanted and toxic byproducts [80]. Zhu et al. prepared 5 wt% CuO/-Al2O<sup>3</sup> as the packed bed to the abatement acetone. It showed better performance than the one without CuO due to the better reducibility and abundant active oxygen species [87]. Li et al. prepared Pt/Al2O<sup>3</sup> and it showed higher decomposition efficiency than the unloaded Al2O<sup>3</sup> due to the presence of Pt, which increases the number of active sites and reduces the activation energy of the decomposition reaction and suppresses harmful NO<sup>x</sup> formation [65]. The research by Hua Song et al. showed that CoMnOx/TiO<sup>2</sup> can obviously improve the plasma removal of VOCs [103].
There are other factors that influence the VOCs removal efficiency, such as humidity and the plasma structure. In industrial conditions, the humidity in the VOCs stream is the factor which must be considered. The humidity has both a positive and negative effect on the removal of VOCs in the non-thermal plasma process. The water molecules can modify the surface of the packed bed, especially the catalytic packed bed, quench the free electrons and active species, and produce OH radicals during the discharge process [103,202,207,211,212]. A lot of studies showed that the presence of humidity can reduce the VOCs removal efficiency by quenching the high energy electrons and reactive species, and modify the catalysts surface (BaTiO3) in the decomposition of toluene, benzene, xylene, and C2F<sup>2</sup> [103,207,212]. On the other hand, the humidity can suppress the formation of toxic byproducts, such as CO and NOx by deactivation of some oxygen species in the catalysts [213,214]. The optimization of the plasma structure also can enhance the VOCs decomposition efficiency. Muhammad Farooq Mustafa et al. designed a double dielectric barrier discharge reactor, in which the conversion of tetrachloroethylene, toluene, trichloroethylene, benzene, ethyl acetate, and carbon disulfide can be 100% with BaTiO<sup>3</sup> and HZSM-5 as catalysts, respectively [215].
There are also some disadvantages in the non-thermal plasma process, such as high energy consumption, low VOCs conversion (mostly about 30% to 70%), low CO<sup>x</sup> selectivity, and produced unwanted byproducts. The other barrier for the application of non-thermal plasma in the industry is the high cost and the high energy consumption. The way to solve these problems is the development of a highly efficient catalytic packed bed and the optimization of an electrode structure.
**Table 10.** Catalysts for VOCs elimination through the non-thermal plasma process.
| doab | 2025-04-07T03:56:58.810299 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 21
} |
004db1df-dc76-42af-952f-1f5e82b7822b.22 | *4.4. Electron Beam Treatment*
In the electron beam technology, an electron beam generated from an EB accelerator and absorbed by the background gas to ionize the air and form some active oxidizing radicals, such as ·OH and ·HO<sup>2</sup> and excited ions as O (3P), which is critical for the destruction of VOCs [216–218]. The oxidizing radicals reacted with VOCs and decomposed them to inorganic compounds. There are three types of accelerators used for VOCs decomposition, including transformer accelerator, ultrahigh frequency accelerator, and linear microwave accelerator [217]. This technology has been used for the elimination of acetaldehyde, benzene, dodecane, ethylbenzene, hexadecane, pentane, styrene, tetradecane, toluene, trimethylamine, turpentine, xylene, dimethyl sulfide, dimethyl disulfide, methyl mercaptan, and chlorinated VOCs in a lab or pilot scale [218–228]. The electron beam can treat the waste gas with a low concentration of VOCs (10 to hundreds ppm). The initial electrons from an electron beam can hardly decompose the VOCs molecules, while it can react with background gases and form active radicals. The way to improve the abatement rate of aromatic VOCs includes the addition of humidity (formation of OH·), ammonia, chlorinated VOCs, ozone, and using an absorbed dose [220,229]. The reaction temperature, VOCs molecular structure, and background gas also influence the decomposition efficiency. However, there are also some drawbacks of the electron beam technology, the generation of active oxidizing radicals, e.g., ·OH and ·HO<sup>2</sup> can lead to the creation of toxic intermediates, especially in the case of the VOCs emission with an unknown composition, such as the production of toxic byproducts (aerosol, ozone, CO, and other trace organic compounds). The equipment is also too complex. It still needs more research for the industrial application.
| doab | 2025-04-07T03:56:58.811352 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 22
} |
004db1df-dc76-42af-952f-1f5e82b7822b.23 | **5. Outlook of the Di**ff**erent Kinds of Technologies**
Different kinds of VOCs were emitted from different parts of pharmaceutical production processes. With the new VOCs discharge standards coming into force, it is critical for the pharmaceutical companies to eliminate VOCs exhaustively in China. According to the production processes and conditions, the suitable VOCs elimination technologies must be selected with the highest efficiency and at the lowest cost. Adsorption, absorption, incineration, catalytic combustion, biodegradation, and non-thermal plasma technologies have been applied in factories for the abatement of VOCs. Adsorption and absorption are commonly the first used methods to recycle useful VOCs. Then, the rest of the VOCs were decomposed by other technologies, such as incineration, catalytic combustion, non-thermal plasma, biodegradation, and non-thermal plasma. The key point for the improvement of catalytic combustion and non-thermal plasma is the development of efficient catalysts or packed bed materials. Some new technologies are emerging for the abatement of VOCs, such as photocatalysis and electron beam radiation. These new technologies have shown good potential for the elimination of VOCs with a high efficiency and low energy cost. However, they still have some engineering problems for the industrial application. It is important to develop VOCs elimination technologies to decrease the amounts of VOCs in the discharged waste gas. The final solution for the VOCs is avoiding the emission of VOCs from the whole pharmaceutical production processes by developing new chemical and engineering technologies for the production of medicine.
**Author Contributions:** L.Z. has written the main content of this paper. C.M. has added some new research papers of this topic and summarized the catalysts used in different researches. J.H. has improved the English writing of the whole paper and written parts of photocatalysis parts. R.L. has written the parts of VOCs elimination policies in China and improve the structure of the paper. J.Y. has designed the whole structure of the paper and improved the manuscript a lot. All authors have read and agreed to the published version of the manuscript.
**Funding:** This research was funded by the Scientific Technology Research Program for the University in Hebei Province Youth Project (No. QN2020126).
**Acknowledgments:** The authors gratefully acknowledge the help of Jing Wang and Ping Chen from Tianjushi Engineering&Technology Group Co., Ltd. who gave us a lot of suggestions about the VOCs treatment technologies and policies in China. The authors gratefully acknowledge the help of Rose Amal for the help of improvement the parts of catalytic processes for the treatment of VOCs.
**Conflicts of Interest:** The authors declare no conflict of interest.
| doab | 2025-04-07T03:56:58.811513 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 23
} |
004db1df-dc76-42af-952f-1f5e82b7822b.25 | *Review* **Bimetallic Catalysts for Volatile Organic Compound Oxidation**
| doab | 2025-04-07T03:56:58.811743 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 25
} |
004db1df-dc76-42af-952f-1f5e82b7822b.26 | **Roberto Fiorenza**
Department of Chemical Sciences, University of Catania, Viale A. Doria 6, 95125 Catania, Italy; [email protected]; Tel.: +39-393-6586864
Received: 25 May 2020; Accepted: 9 June 2020; Published: 12 June 2020
**Abstract:** In recent years, the impending necessity to improve the quality of outdoor and indoor air has produced a constant increase of investigations in the methodologies to remove and/or to decrease the emission of volatile organic compounds (VOCs). Among the various strategies for VOC elimination, catalytic oxidation and recently photocatalytic oxidation are regarded as some of the most promising technologies for VOC total oxidation from urban and industrial waste streams. This work is focused on bimetallic supported catalysts, investigating systematically the progress and developments in the design of these materials. In particular, we highlight their advantages compared to those of their monometallic counterparts in terms of catalytic performance and physicochemical properties (catalytic stability and reusability). The formation of a synergistic effect between the two metals is the key feature of these particular catalysts. This review examines the state-of-the-art of a peculiar sector (the bimetallic systems) belonging to a wide area (i.e., the several catalysts used for VOC removal) with the aim to contribute to further increase the knowledge of the catalytic materials for VOC removal, stressing the promising potential applications of the bimetallic catalysts in the air purification.
**Keywords:** VOCs; bimetallic catalysts; air purification
| doab | 2025-04-07T03:56:58.811772 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 26
} |
004db1df-dc76-42af-952f-1f5e82b7822b.27 | **1. Introduction**
Volatile organic compounds (VOCs) are a wide group of organic compounds characterized to boiling points less than 250 ◦C at room temperature and at atmospheric pressure [1]. Due to their carcinogenic and toxic nature, most VOCs are considered major causes of air pollution. Indeed, their emission in the environment leads to the formation of secondary dangerous compounds, due to the occurrence of chemical reactions with other airborne pollutants such as NO<sup>x</sup> and SOx, which results in the formation of tropospheric ozone and photochemical smog [2,3]. Long exposure to these pollutants leads to serious problems for human health [4,5]. Global economic and industrial development over the years has caused an exponential increase of anthropogenic VOC emission [6]. VOC discharges include outdoor sources, such as transport, industrial and petrochemical processes, etc., and indoor sources, such as household products, solvents, office materials, cleaning products, domestic cooking, etc. [7]. The emitted VOCs encompass alkanes, paraffins, olefins, aromatics, alcohols, ketones, aldehydes, esters, sulfur/nitrogen-containing VOCs, and halogenated VOCs. Among them, the most common and toxic are benzene, phenol, toluene, styrene, formaldehyde, propylene, and acetone [8], whereas Cl-VOCs and in general halogenated VOCs, due to their inherent stability and toxicity, are also very dangerous [9].
Different technologies have been developed for VOC treatment, and they can be divided into nondestructive and destructive VOC removal. The former include adsorption, membrane separation, and condensation [10–12]. Among these, the adsorption process is considered one of the most efficient treatments, owing to a low energy consumption, relatively low operation cost, and simple
operations for the adsorption/regeneration of the adsorbent [12,13]. With adsorption, it is possible to remove, without the generation of dangerous byproducts, a low/medium concentration of VOCs (<1000 mg/m<sup>3</sup> ) [1]. Regarding destructive (i.e., oxidative) processes, the most commonly used ones are thermal (not-catalytic) combustion and catalytic oxidation, both of which can be applied to treat a medium/high concentration (>5000 mg/m<sup>3</sup> ) of VOCs [14,15]. In particular, catalytic conversion has some advantages compared to thermal incineration; indeed, this process has become more popular than noncatalytic treatments. Catalytic oxidation allows converting VOCs into less toxic substances, such as carbon dioxide and water, in a temperature range much lower than thermo-oxidation [16,17]. Specifically, with catalytic conversion, the operating temperature range is 200–500 ◦C or even lower, whereas in thermal incineration, temperatures are higher (800–1200 ◦C). Lower temperatures permit reducing the production of dioxins and NOx. Furthermore, catalytic oxidation is more versatile and cheaper, especially when it comes to processing low concentrations of organic compounds [18]. In recent years, new technologies have been applied for the elimination of VOCs at low concentrations, namely, the advanced oxidation process (AOP), e.g., photocatalytic degradation, ozone treatment, Fenton oxidation methods [19,20], biodegradation [21], and phytoremediation [22].
Due to the economic and technological advantages of catalytic oxidation, widespread efforts have been committed to the selection of high-performing catalysts for this process. However, considering the large number of organic molecules and the problematic nature of VOCs mixtures, the design and optimization of catalytic materials are challenging tasks. Both noble and transition metals have been widely used as catalysts for either nonhalogenated or halogenated VOCs [23–25]. Notwithstanding their high costs, the supported noble-metal catalysts are widely applied due to their intrinsic features, such as resistance to deactivation, ease of regeneration, and highly catalytic performance [26–28]. These features strictly depend on the synthetic procedure adopted for the preparation of the supported metal catalyst, as well as the type of metal salt precursor, the metal loading, the kind of support, and the particle size [29–31]. Furthermore, VOCs and air/oxygen content, total gas stream rate, and employed reactor (membrane reactor, fixed-bed reactor, etc.) are key parameters that can affect overall catalytic activity [32–34].
In the literature, there are many studies that deeply analyze single or various parameters that influence the final results of catalytic oxidation applied to VOC treatment, including the catalysts used [16,35,36], the nature of VOCs [2,9,37], the combination of different technologies [1,38], the type of reactor [39], or the performances in practical applications [40].
One of the less explored strategies to enhance the catalytic activity of supported noble/transition metal catalysts is the addition of a second metal (noble and/or transition) to the first one.
This work analyzes a little aspect of the VOC catalytic treatment topic: the advantages of using supported bimetallic catalysts with respect to monometallic counterparts, focusing on the morphological, chemicophysical, and textural properties of these peculiar materials, and how these features can influence the catalytic activity.
This review aims to enlarge the scientific panorama about VOC removal through catalytic oxidation, focusing on the bimetallic catalysts, an aspect not yet systematically examined in the literature.
| doab | 2025-04-07T03:56:58.811908 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 27
} |
004db1df-dc76-42af-952f-1f5e82b7822b.28 | **2. Bimetallic Catalysts for VOC Oxidation**
Bimetallic nanoparticles (NPs) are a kind of materials formed by two different metals and characterized with peculiar features [41,42]. Specifically, they can show new properties resulting from the combination of features arising from the monometallic counterparts. Usually, the obtained physicochemical properties of bimetallic systems give a holistic result, i.e., the final properties are not the simple additive features of the monometallic analogs, but in many cases, it is possible to exploit a great improvement with new properties due to the presence of synergistic effects [43,44]. Since their application in the field of petrochemistry [45], bimetallic systems are being widely applied in heterogeneous catalysis in various reactions, such as hydrogenation [46,47], reforming [48,49], H<sup>2</sup> production and purification [50–53], and oxidation [54–56]. Recently, they have also been applied in the biomedical field [57].
American Chemical Society.
On the basis of the morphology of the bimetallic system, it is possible to classify the main arrangements into three typologies: core–shell or multishell structures, heterostructures, and random or homogeneous alloys (Figure 1) [43,44,58]. On the basis of the morphology of the bimetallic system, it is possible to classify the main arrangements into three typologies: core–shell or multishell structures, heterostructures, and random or homogeneous alloys (Figure 1) [43,44,58].
*Catalysts* **2020**, *10*, x FOR PEER REVIEW 3 of 25
**Figure 1.** Possible morphologies of bimetallic nanoparticles (one metal in red, the other in dark yellow): (**a**), (**b**) core–shell system; (**c**) multishell system; (**d**) subcluster segregated systems; and ordered (**e**) and random (**f**) homogeneous alloys. Reprinted (adapted) from [58], Copyright 2008, **Figure 1.** Possible morphologies of bimetallic nanoparticles (one metal in red, the other in dark yellow): (**a**,**b**) core–shell system; (**c**) multishell system; (**d**) subcluster segregated systems; and ordered (**e**) and random (**f**) homogeneous alloys. Reprinted (adapted) from [58], Copyright 2008, American Chemical Society.
Various factors influence the final morphology of bimetallic systems: (1) the intrinsic strength of the bonds between the two metals in comparisons to the strength of the bonds between the two monometallic constituents (in particular, if the resulting alloy bond strength is greater, reciprocal mixing is preferred, and, in the opposite case, the segregation of the two metals is favored); (2) the balance in the surface energy of the metals, where the monometallic element that owns the lower surface energy will move to the surface, establishing a shell structure, while the metal with the smaller atomic sizes will collocate to the core; (3) the mixing is favored when an electron/charge transfer between the metallic counterparts is verified; and (4) magnetic effects can influence the final Various factors influence the final morphology of bimetallic systems: (1) the intrinsic strength of the bonds between the two metals in comparisons to the strength of the bonds between the two monometallic constituents (in particular, if the resulting alloy bond strength is greater, reciprocal mixing is preferred, and, in the opposite case, the segregation of the two metals is favored); (2) the balance in the surface energy of the metals, where the monometallic element that owns the lower surface energy will move to the surface, establishing a shell structure, while the metal with the smaller atomic sizes will collocate to the core; (3) the mixing is favored when an electron/charge transfer between the metallic counterparts is verified; and (4) magnetic effects can influence the final structure between the two metals [44,58,59].
structure between the two metals [44,58,59]. The morphologies are also strictly affected by the preparation method adopted. In general, the synthesis of the bimetallic systems can be carried out with the solid-state or with the solution methods [60–62]. The latter are preferred because solid-state techniques require high temperatures and long times for annealing procedures, thus decreasing the surface area of the bimetallic catalyst and affecting catalytic performance in a crucial way. By contrast, with solution methods, it is possible to The morphologies are also strictly affected by the preparation method adopted. In general, the synthesis of the bimetallic systems can be carried out with the solid-state or with the solution methods [60–62]. The latter are preferred because solid-state techniques require high temperatures and long times for annealing procedures, thus decreasing the surface area of the bimetallic catalyst and affecting catalytic performance in a crucial way. By contrast, with solution methods, it is possible to control the nucleation and growth processes by modifying the reaction parameters.
control the nucleation and growth processes by modifying the reaction parameters. In the main catalytic reactions, the bimetallic system is supported on a specific support [63]. The commonly used techniques for the preparation of supported bimetallic catalysts are: impregnation [64,65], co-precipitation [66,67], deposition–precipitation [50,68], thermal decomposition [69], and chemical reduction [70]. The choice of a proper support is another key feature that strongly influences the catalytic activity of the bimetallic system. In particular, for VOC oxidation, and for oxidation reactions in general, the utilization of an active support (i.e., having redox properties and lattice oxygen mobility), for example, CeO2 or Fe2O3, can contribute to the formation of a synergistic effect involving both the metals and the same support [68,71], whereas other supports, with high surface areas, such as zeolites, silica, alumina, etc., favor the high dispersion of the metal species and, In the main catalytic reactions, the bimetallic system is supported on a specific support [63]. The commonly used techniques for the preparation of supported bimetallic catalysts are: impregnation [64,65], co-precipitation [66,67], deposition–precipitation [50,68], thermal decomposition [69], and chemical reduction [70]. The choice of a proper support is another key feature that strongly influences the catalytic activity of the bimetallic system. In particular, for VOC oxidation, and for oxidation reactions in general, the utilization of an active support (i.e., having redox properties and lattice oxygen mobility), for example, CeO<sup>2</sup> or Fe2O3, can contribute to the formation of a synergistic effect involving both the metals and the same support [68,71], whereas other supports, with high surface areas, such as zeolites, silica, alumina, etc., favor the high dispersion of the metal species and, therefore, the interaction between the metals and the same support [16,72].
Since the first works of Haruta and co-workers [73], gold-based catalysts have generated many investigations focused on the catalytic properties of this peculiar metal. With respect to the platinum-
therefore, the interaction between the metals and the same support [16,72].
Since the first works of Haruta and co-workers [73], gold-based catalysts have generated many investigations focused on the catalytic properties of this peculiar metal. With respect to the platinum-group catalysts, gold's properties are in some cases superior; consequently, the supported gold-based materials have found many applications in catalytic reactions dealing with environmental protection and energy production [50,72]. In the specific field of VOC catalytic oxidation, gold-based catalysts play a key role. For these reasons, bimetallic alloys with gold are firstly examined. *Catalysts* **2020**, *10*, x FOR PEER REVIEW 4 of 25 group catalysts, gold's properties are in some cases superior; consequently, the supported gold-based materials have found many applications in catalytic reactions dealing with environmental protection and energy production [50,72]. In the specific field of VOC catalytic oxidation, gold-based catalysts play a key role. For these reasons, bimetallic alloys with gold are firstly examined.
### *2.1. Gold-Based Bimetallic Catalysts 2.1. Gold-Based Bimetallic Catalysts*
The important role of gold-based catalysts in heterogeneous catalysis was disclosed through the high activity of this metal in CO oxidation and selective oxidations [74,75]. After these studies, many works dealing with this catalyst have emerged, widely expanding the use of gold catalysts [72,76]. With respect to the other commonly used noble metals, namely, Pt and Pd, gold catalysts show peculiar features such as resistance to O<sup>2</sup> poisoning and high selectivity. Another key parameter that strictly affects the catalytic activity of Au catalysts is size-dependence: To be active, gold particles must be, in general, smaller than 5 nm, which balances the proportion of the low coordination surface active sites (edges and corners) [77]. The important role of gold-based catalysts in heterogeneous catalysis was disclosed through the high activity of this metal in CO oxidation and selective oxidations [74,75]. After these studies, many works dealing with this catalyst have emerged, widely expanding the use of gold catalysts [72,76]. With respect to the other commonly used noble metals, namely, Pt and Pd, gold catalysts show peculiar features such as resistance to O2 poisoning and high selectivity. Another key parameter that strictly affects the catalytic activity of Au catalysts is size-dependence: To be active, gold particles must be, in general, smaller than 5 nm, which balances the proportion of the low coordination surface active sites (edges and corners) [77].
For other applications, as well as for VOC oxidation, the catalytic activity of gold-supported samples is affected by many factors: (a) gold–support interaction; (b) gold loading; (c) the valence state of gold; (d) the adopted synthesis and pre-treatment conditions utilized; and (e) the concentration and nature of the chosen VOC target [78,79]. Among the monometallic gold catalysts employed for VOC oxidation, the Au/CeO<sup>2</sup> sample showed a great performance in the oxidation of oxygenated molecules, i.e., aldehydes, ketones, esters, and alcohols [72]. It was reported that the high activity was related to the enhancement in the surface oxygen reducibility/mobility of the CeO<sup>2</sup> support. The oxidation mechanism followed a Mars–Van Krevelen (MvK) mechanism, where the ceria lattice oxygens were actively involved in the oxidation pathway [68,80] (Figure 2). For other applications, as well as for VOC oxidation, the catalytic activity of gold-supported samples is affected by many factors: (a) gold–support interaction; (b) gold loading; (c) the valence state of gold; (d) the adopted synthesis and pre-treatment conditions utilized; and (e) the concentration and nature of the chosen VOC target [78,79]. Among the monometallic gold catalysts employed for VOC oxidation, the Au/CeO2 sample showed a great performance in the oxidation of oxygenated molecules, i.e., aldehydes, ketones, esters, and alcohols [72]. It was reported that the high activity was related to the enhancement in the surface oxygen reducibility/mobility of the CeO2 support. The oxidation mechanism followed a Mars–Van Krevelen (MvK) mechanism, where the ceria lattice oxygens were actively involved in the oxidation pathway [68,80] (Figure 2).
**Figure 2.** Lattice ceria oxygens of Au/CeO2 catalyst involved in catalytic oxidation of formaldehyde. Reprinted (adapted) from [80], Copyright 2014, American Chemical Society. **Figure 2.** Lattice ceria oxygens of Au/CeO<sup>2</sup> catalyst involved in catalytic oxidation of formaldehyde. Reprinted (adapted) from [80], Copyright 2014, American Chemical Society.
Nevertheless, the application of gold catalysts to industrial level is limited due to certain drawbacks, such as the decrease of activity in the presence of high concentration of moisture and the Nevertheless, the application of gold catalysts to industrial level is limited due to certain drawbacks, such as the decrease of activity in the presence of high concentration of moisture and the aggregation of gold nanoparticles at high temperatures [81,82].
aggregation of gold nanoparticles at high temperatures [81,82]. Gold-based bimetallic systems are considered a useful solution to overcome the cited limitations, due to the combination of properties from the gold and the second metal. Among the various metals employed to be joined with gold, it has been found that Pd, Ag, and Cu show a high miscibility, and Gold-based bimetallic systems are considered a useful solution to overcome the cited limitations, due to the combination of properties from the gold and the second metal. Among the various metals employed to be joined with gold, it has been found that Pd, Ag, and Cu show a high miscibility, and these systems can be prepared with a wide range of methodologies [83,84], whereas Pt, Ru, Co, Fe, and
these systems can be prepared with a wide range of methodologies [83,84], whereas Pt, Ru, Co, Fe, and Ni are only partially or not at all miscible with gold [84]. Consequently, Au-Pd followed by Au-
Ag and Au-Cu are the most utilized gold-based bimetallic catalysts for VOC oxidations.
2.1.1. Au-Pd Catalysts
Ni are only partially or not at all miscible with gold [84]. Consequently, Au-Pd followed by Au-Ag and Au-Cu are the most utilized gold-based bimetallic catalysts for VOC oxidations.
| doab | 2025-04-07T03:56:58.812397 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 28
} |
004db1df-dc76-42af-952f-1f5e82b7822b.29 | 2.1.1. Au-Pd Catalysts
Gold is miscible with palladium in all compositions; consequently, while the formation of gold–palladium alloys is favored, the segregation of single metals was, in fact, avoided [76,83]. In Table 1 are reported some of the experimental results of the application of the Au-Pd systems in catalytic oxidation of various VOCs (T<sup>90</sup> = temperature at which the 90% of conversion was achieved).
In particular, Hosseini et al. [85] synthetized Au-Pd catalysts supported on mesoporous TiO<sup>2</sup> for the removal of toluene, propene, and a gaseous mixture of both. Interestingly, they found that catalytic activity is influenced by the morphology of the core–shell structure with the best performance shown with the Au-core/Pd-shell. By contrast, with the reverse morphology (Au-shell/Pd-core) the catalytic activity was lower, due to the lower affinity of gold for oxygen adsorption (in this case, the rate determining step of the reaction that followed a Langmuir–Hinshelwood mechanism) caused by the poor ability of gold to polarize oxygen molecules. In the same context, Barakat et al. [86] investigated the catalytic stability of bimetallic Au-Pd/doped TiO<sup>2</sup> samples under severe testing conditions (exposing the catalyst to 110 h of a gaseous toluene/air stream). The bimetallic catalyst maintained a good activity even after a long time. The interaction between the Nb-doped TiO<sup>2</sup> support and the Au-Pd system allowed obtaining a cycle-like activity of the catalyst. This oscillatory behavior was related to the existence of carbonaceous compounds adsorbed on the surface of the spent catalyst that, together with the formed OH radicals, favored the reduction of palladium. The redox process of palladium was linked to the cyclic-like activity of the bimetallic sample (Figure 3). *Catalysts* **2020**, *10*, x FOR PEER REVIEW 5 of 25 Gold is miscible with palladium in all compositions; consequently, while the formation of gold– palladium alloys is favored, the segregation of single metals was, in fact, avoided [76,83]. In Table 1 are reported some of the experimental results of the application of the Au-Pd systems in catalytic oxidation of various VOCs (T90 = temperature at which the 90% of conversion was achieved). In particular, Hosseini et al. [85] synthetized Au-Pd catalysts supported on mesoporous TiO2 for the removal of toluene, propene, and a gaseous mixture of both. Interestingly, they found that catalytic activity is influenced by the morphology of the core–shell structure with the best performance shown with the Au-core/Pd-shell. By contrast, with the reverse morphology (Aushell/Pd-core) the catalytic activity was lower, due to the lower affinity of gold for oxygen adsorption
<sup>1</sup> Nominal concentration, weight percentage (wt%).
**Figure 3.** Catalytic oxidation of toluene under ageing condition over Au-Pd/Nb-TiO2 catalyst. Figure from [86]. **Figure 3.** Catalytic oxidation of toluene under ageing condition over Au-Pd/Nb-TiO<sup>2</sup> catalyst. Figure from [86].
Regarding toluene degradation, Xie et al. [88] studied the catalytic performance of the bimetallic gold–palladium system supported on three-dimensionally ordered macroporous (3DOM) Co3O4. The bimetallic sample showed a much higher activity compared to its nonmetallic counterparts, with a T<sup>90</sup> of about 100 and 30 ◦C lower compared to monometallic gold and palladium, respectively. The peculiar features of the 3DOM supports (such as higher porosity and ordered pore channels) were also exploit by the same authors [91], who utilized an Au-Pd bimetallic sample prepared via chemical reduction, but employing the Mn2O<sup>3</sup> as support. The bimetallic catalysts confirmed the higher activity compared to monometallic gold and palladium in the oxidation of different VOCs, such as methane and o-xylene. To further boost catalytic activity, doping with Fe of the Au-Pd/3DOM-Mn2O<sup>3</sup> catalyst allowed modifying the structural properties of the alloy NPs. With this modification, oxygen activation and the methane adsorption ability were increased, enhancing, as final result, the overall catalytic activity.
Catalytic performance in various mixtures of VOCs (toluene/m-xylene, ethyl acetate/m-xylene acetone/m-xylene, and acetone/ethyl acetate) was examined by Xia et al. [87] on Au-Pd prepared via chemical-reduction-supported αMnO<sup>2</sup> nanotubes. In this case, the authors focused on an MvK-like mechanism with the mutual interaction of the Au-Pd nanoparticles with the α-MnO<sup>2</sup> that improved the mobility/reactivity of the surface lattice oxygen of the support. In catalytic oxidation of VOC mixture, the rate-determining step is the competitive adsorption between the various VOCs on the surface of the catalyst. With the bimetallic catalyst, the authors measured the total oxidation of a single component and of the VOC mixture at T < 300 ◦C. The same bimetallic sample also showed a high catalytic stability in the long time (50 h) on stream experiments.
Tabakova et al. [89] focused their work on the removal of benzene utilizing an Au-Pd bimetallic system synthetized by deposition–precipitation and supported on Fe-doped CeO2. The superior performance of the bimetallic catalyst with respect to the monometallic ones was highlighted through comparison of the T90, which was <sup>≈</sup><sup>95</sup> ◦C for the bimetallic sample, <sup>≈</sup><sup>180</sup> ◦C for the Pd/Fe-CeO2, and <sup>≈</sup><sup>190</sup> ◦C for the Au/Fe-CeO2. In addition, in this case, the synergistic interaction between the alloy nanoparticles and the support enhanced the mobility of the ceria surface lattice oxygen, further boosted by doping with iron. Moreover, this strong interaction facilitated the nucleation of the noble metal particles on the surface of cerium oxide.
Benzyl alcohol oxidation on Au-Pd bimetallic catalysts was extensively studied by the research group of Prati et al., and an exhaustive comparison of the performance, in this reaction, of the bimetallic Au-Pd systems present in the literature, together with a deep analysis of the various adopted preparation methods are reported in [83]. The same research group [76,83] studied the catalytic behavior of different bimetallic systems composed of Au and various other metals supported on activated carbon prepared using the sol immobilization methodology. Interestingly, they found a structural correlation depending on the second metal utilized. Specifically, an alloy structure was obtained using Pd and Pt, whereas a core–shell morphology was attained with Ru, while with Cu, a phase segregation of this metal, instead of gold, was favored. In the benzyl alcohol tests, the bimetallic synergistic effect was exploited only with copper and palladium.
The strong interaction between gold and palladium, beneficial for catalytic oxidation of benzyl alcohol, was also examined by Li et al. [90] using CeO<sup>2</sup> with different morphology as support. The different morphology of cerium oxide (rod, cube, and polyhedrons), where gold/palladium nanoparticles were deposited–precipitated (Figure 4), affected the catalytic performance. Specifically, the Au-Pd supported on ceria rod showed a higher benzyl alcohol conversion with respect to the samples supported on CeO<sup>2</sup> cubes and CeO<sup>2</sup> polyhedrons and was thus related to the smaller particle size of ceria rod compared to the other CeO<sup>2</sup> supports; moreover, this particular morphology also favored a higher concentration of ceria oxygen defects, enhancing the mobility/reducibility of the ceria surface oxygens. By contrast, the sample supported on the CeO<sup>2</sup> cube exhibited the highest selectivity in benzaldehyde.
Kucherov et al. [92] investigated the performance of mono- and bimetallic gold-based catalysts for the removal of dimethyldisulfide (DMDS), an S-VOC. They corroborate the performance of Au-Pd catalysts also for this type of VOC. The bimetallic sample supported on TiO<sup>2</sup> demonstrated a stable selectivity in benzaldehyde.
performance and assisted with the removal of DMDS at T <sup>&</sup>lt; <sup>155</sup> ◦C with the formation of SO<sup>2</sup> and elemental S. performance and assisted with the removal of DMDS at T < 155 °C with the formation of SO2 and elemental S.
Pd catalysts also for this type of VOC. The bimetallic sample supported on TiO2 demonstrated a stable
Kucherov et al. [92] investigated the performance of mono- and bimetallic gold-based catalysts
*Catalysts* **2020**, *10*, x FOR PEER REVIEW 7 of 25
different morphology of cerium oxide (rod, cube, and polyhedrons), where gold/palladium nanoparticles were deposited–precipitated (Figure 4), affected the catalytic performance. Specifically, the Au-Pd supported on ceria rod showed a higher benzyl alcohol conversion with respect to the samples supported on CeO2 cubes and CeO2 polyhedrons and was thus related to the smaller particle size of ceria rod compared to the other CeO2 supports; moreover, this particular morphology also favored a higher concentration of ceria oxygen defects, enhancing the mobility/reducibility of the ceria surface oxygens. By contrast, the sample supported on the CeO2 cube exhibited the highest
**Figure 4.** TEM (Transmission electron microscope) and HRTEM (High-resolution transmission electron microscope), images of: Au-Pd/CeO2-rod (**a, b**); Au-Pd/CeO2-polyhedron (**c**, **d**); and Au-Pd/CeO2-cube (**e, f**) catalysts. Figure from Ref. [90], Copyright 2020, Elsevier. **Figure 4.** TEM (Transmission electron microscope) and HRTEM (High-resolution transmission electron microscope), images of: Au-Pd/CeO<sup>2</sup> -rod (**a,b**); Au-Pd/CeO<sup>2</sup> -polyhedron (**c**,**d**); and Au-Pd/CeO<sup>2</sup> -cube (**e,f**) catalysts. Figure from Ref. [90], Copyright 2020, Elsevier.
| doab | 2025-04-07T03:56:58.813368 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 29
} |
004db1df-dc76-42af-952f-1f5e82b7822b.30 | 2.1.2. Au-Ag and Au-Cu Catalysts
The establishment of a strong interaction between gold and silver with the formation of an alloy or of bimetallic clusters was investigated by our research group both in VOC oxidation and in H<sup>2</sup> purification towards the preferential oxidation of CO (PROX reaction) [50,53,68]. In particular, we evaluated the catalytic activity of Au-Ag and Au-Cu bimetallic samples supported on CeO<sup>2</sup> toward the degradation of 2-propanol and ethanol. A higher activity was found of the gold–silver sample with respect to Au-Cu and the monometallic counterparts. The higher activity of the gold–silver system was correlated to a higher mobility/reactivity of ceria surface oxygens, due to a strong synergistic interaction between the gold–silver nanoparticles and the cerium oxide. A linear correlation was stated
2.1.2. Au-Ag and Au-Cu Catalysts
from [68], Copyright 2015, Springer Nature.
considering the T<sup>50</sup> of alcohol oxidation and the TPR (temperature-programmed reduction) initial temperature, i.e., the temperature at which the reduction of ceria surface oxygens started considering the analyzed samples (Figure 5). The Au-Ag/CeO<sup>2</sup> catalyst displayed the lowest reduction temperature and T50. correlation was stated considering the T50 of alcohol oxidation and the TPR (temperatureprogrammed reduction) initial temperature, i.e., the temperature at which the reduction of ceria surface oxygens started considering the analyzed samples (Figure 5). The Au-Ag/CeO2 catalyst displayed the lowest reduction temperature and T50.
synergistic interaction between the gold–silver nanoparticles and the cerium oxide. A linear
*Catalysts* **2020**, *10*, x FOR PEER REVIEW 8 of 25
The establishment of a strong interaction between gold and silver with the formation of an alloy or of bimetallic clusters was investigated by our research group both in VOC oxidation and in H2 purification towards the preferential oxidation of CO (PROX reaction) [50,53,68]. In particular, we evaluated the catalytic activity of Au-Ag and Au-Cu bimetallic samples supported on CeO2 toward the degradation of 2-propanol and ethanol. A higher activity was found of the gold–silver sample
**Figure 5.** Temperature at which the 50% of conversion of ethanol and 2-propanol was achieved (T50) versus TPR (temperature-programmed reduction) initial temperature: (filled diamond) Au-Ag/CeO2; (filled circle) Au-Cu/CeO2; (filled square) Au/CeO2; (filled triangle) Ag/CeO2; (times) Cu/CeO2. Figure **Figure 5.** Temperature at which the 50% of conversion of ethanol and 2-propanol was achieved (T50) versus TPR (temperature-programmed reduction) initial temperature: (filled diamond) Au-Ag/CeO<sup>2</sup> ; (filled circle) Au-Cu/CeO<sup>2</sup> ; (filled square) Au/CeO<sup>2</sup> ; (filled triangle) Ag/CeO<sup>2</sup> ; (times) Cu/CeO<sup>2</sup> . Figure from [68], Copyright 2015, Springer Nature.
Nagy et al. [93] studied the performance of Au-Ag nanoparticles supported on SiO2 synthetized from the adsorption of bimetallic colloids in the oxidation of benzyl alcohol. The authors focused their research on the crucial importance of the molar ratio between the two metals. In particular, a synergistic effect was verified that reflects a higher activity at a low Ag/Au molar ratio (best result Ag/Au = 23/77). For the authors, the synergy is activated by the optimal concentration of the two metals, which increased the activation of both oxygens from gas-phase and from the support. In the same context, a correlation between catalytic activity and the concentration of gold and silver was measured by our research group in the PROX reaction [50] with a higher concentration of gold or silver with respect to the second metal that was detrimental for the overall catalytic performance, whereas the best results were obtained with an approximately equal concentration of gold and silver Nagy et al. [93] studied the performance of Au-Ag nanoparticles supported on SiO<sup>2</sup> synthetized from the adsorption of bimetallic colloids in the oxidation of benzyl alcohol. The authors focused their research on the crucial importance of the molar ratio between the two metals. In particular, a synergistic effect was verified that reflects a higher activity at a low Ag/Au molar ratio (best result Ag/Au = 23/77). For the authors, the synergy is activated by the optimal concentration of the two metals, which increased the activation of both oxygens from gas-phase and from the support. In the same context, a correlation between catalytic activity and the concentration of gold and silver was measured by our research group in the PROX reaction [50] with a higher concentration of gold or silver with respect to the second metal that was detrimental for the overall catalytic performance, whereas the best results were obtained with an approximately equal concentration of gold and silver (1% wt–1% wt).
(1% wt-1% wt). The crucial importance of the molar ratio between gold and the other metal was also stated in the review of Bracey et al. [94], focused on the Au-Cu system. Specifically, in one of the analyzed works, the following order of reaction in catalytic oxidation of propene is reported: AuCu (1:3 molar ratio)/TiO2 > AuCu (1:1 molar ratio)/TiO2 > AuCu/TiO2 (3:1 molar ratio) > Au/TiO2. The content of copper, in fact, strongly influenced the dispersion of the metal nanoparticles, with a high amount of copper in the alloy that caused a decrease in the size of the metal particles, thus contributing to enhancint the activity and selectivity into propene oxide [95]. In the same review, it was illustrated that, when investigating another reaction, such as selective oxidation of benzyl alcohol to benzaldehyde, the more active bimetallic catalyst was the sample with the higher concentration of gold (the catalyst AuCu/SiO2 with a molar ratio of 4:1). Similarly to the previous examples, the The crucial importance of the molar ratio between gold and the other metal was also stated in the review of Bracey et al. [94], focused on the Au-Cu system. Specifically, in one of the analyzed works, the following order of reaction in catalytic oxidation of propene is reported: AuCu (1:3 molar ratio)/TiO<sup>2</sup> > AuCu (1:1 molar ratio)/TiO<sup>2</sup> > AuCu/TiO<sup>2</sup> (3:1 molar ratio) > Au/TiO2. The content of copper, in fact, strongly influenced the dispersion of the metal nanoparticles, with a high amount of copper in the alloy that caused a decrease in the size of the metal particles, thus contributing to enhancint the activity and selectivity into propene oxide [95]. In the same review, it was illustrated that, when investigating another reaction, such as selective oxidation of benzyl alcohol to benzaldehyde, the more active bimetallic catalyst was the sample with the higher concentration of gold (the catalyst AuCu/SiO<sup>2</sup> with a molar ratio of 4:1). Similarly to the previous examples, the bimetallic catalyst was prepared by impregnation, but in this case, a higher concentration of gold is fundamental to achieve a high selectivity (98%) to benzaldehyde.
The above-discussed literature data on AuCu bimetallic catalysts were mainly focused on the selective oxidation of VOCs, whereas the work of Nevanperä et al. [96] dealt with catalytic oxidation of DMDS with bimetallic gold-based catalysts (Au-Cu and Au-Pt) supported on γ-Al2O3, CeO2, and CeO2-Al2O<sup>3</sup> prepared by surface redox reduction. Among the examined supports, the alumina gave the best results, whereas the addition of gold enhanced the catalytic activity of both monometallic copper and platinum samples, Au-Cu catalysts being the most active system. Interestingly, the authors noted that the same Au-Cu catalyst led to the formation of dangerous byproducts, such as carbon monoxide and formaldehyde. This was attributed to the high concentration of reactive surface oxygens favored by the presence of copper oxide and to the dissociation of the oxygen that started at a lower temperature with respect to the monometallic samples, with the consequent modification of the surface acid and basic sites of the bimetallic catalyst. By contrast, selectivity towards CO<sup>2</sup> and H2O was higher in the Au-Pt sample.
| doab | 2025-04-07T03:56:58.813893 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 30
} |
004db1df-dc76-42af-952f-1f5e82b7822b.31 | 2.1.3. Other Au-Based Bimetallic Catalysts
As discussed in the last examined work, among the other Au-based bimetallic catalysts, the Au-Pt system exhibited promising performance in VOC oxidation [96–98].
Kim et al. [97] investigated catalytic oxidation of toluene employing the Au-Pt/ZnO-Al2O<sup>3</sup> catalyst prepared by impregnation in air or H2. They found that the bimetallic sample prepared in air led to an increase of the gold particle size and a decrease of the Pt with respect to the same particles synthetized in H<sup>2</sup> stream, where an inverse correlation was verified (the gold size decreased, and the platinum size increased). Due to the crucial importance of the gold nanoparticles that facilitated the total oxidation of toluene and that increased the reduction of the surface oxygen of the mixed oxide support, the catalytic performance was higher with the bimetallic sample synthetized in H<sup>2</sup> stream and calcined at 400 ◦<sup>C</sup> (Pt and Au mean size of about 5 nm). In another study [98], the same authors correlated the catalytic activity of the same bimetallic samples, even in the total oxidation of toluene, to the molar ratio of gold and platinum, finding the following order of activity: Pt75Au25 > Pt67Au33 > Pt100Au0 > Pt50Au50 > Pt33Au67 > Pt25Au75 > Pt0Au100. The small amount of gold promotes the total oxidation of toluene due to the formation of a strong metal–metal interaction.
The good affinity of gold with noble metals was also confirmed via the catalytic performance in VOC removal of the Au-Ru system [99,100]. Sreethawong et al. [99] investigated catalytic oxidation of methanol over gold–ruthenium samples prepared through impregnation and supported on SiO2. The characterization measurements (TPR, SEM, and XRD) suggested the occurrence of an interaction between the two metals exploited with a particular composition (3.32 wt%Ru–0.61 wt% Au), which led to obtaining a good catalytic activity, notwithstanding the fact that the two metals were not miscible in their bulk phase. Interestingly, if alumina was used as support, the formation of byproducts (methyl formate, formic acid, dimethyl ether, and formaldehyde) other to CO<sup>2</sup> was detected, with an increase of methanol conversion. Catalytic oxidation of methanol was also analyzed by Calzada et al. [100] with Au-Ru/TiO<sup>2</sup> catalysts prepared by deposition–precipitation with urea. The authors highlighted that the synergistic effect between the two metals was activated at a low conversion temperature (from room temperature to T = 50 ◦C), with a dependence on the bimetallic atomic ratio (the best-performing one was Ru:Au 0.75:1). Interestingly, the DRIFT (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) CO spectra (Figure 6) illustrated as the interaction between the two metals decreased CO adsorption in the Ru surface sites in the Ru-Au 0.75:1 sample, an indication of the modification of the surface gold sites.
*Catalysts* **2020**, *10*, x FOR PEER REVIEW 10 of 25
**Figure 6.** DRIFT (Diffuse Reflectance Infrared Fourier Transform Spectroscopy)-CO spectra of the mono- and bimetallic Ru-Au/TiO2 samples at room temperature. Figure from [100]. Copyright 2017, **Figure 6.** DRIFT (Diffuse Reflectance Infrared Fourier Transform Spectroscopy)-CO spectra of the monoand bimetallic Ru-Au/TiO<sup>2</sup> samples at room temperature. Figure from [100]. Copyright 2017, Elsevier.
Elsevier. The modifications of the bimetallic surface sites due to interaction between the two metals strongly occurring with the Ru:Au 0.75:1 ratio were the reason behind the higher catalytic activity of this bimetallic sample compared to those of the monometallic catalysts and the Ru:Au 1:1 sample. In The modifications of the bimetallic surface sites due to interaction between the two metals strongly occurring with the Ru:Au 0.75:1 ratio were the reason behind the higher catalytic activity of this bimetallic sample compared to those of the monometallic catalysts and the Ru:Au 1:1 sample. In addition, in this case, the formation of formates as intermediates of the oxidation reaction was verified.
addition, in this case, the formation of formates as intermediates of the oxidation reaction was verified. Catalytic activity in the total oxidation of toluene of the Au-Ir bimetallic catalyst supported on TiO2 was studied by Torrente-Murciano et al. [101]. Similarly to the previous case, the synergistic interaction between the two metals allowed them to sensibly decrease the T90 that was ≈230 °C for the bimetallic sample, ≈250 °C for the monometallic gold, and ≈270 °C for Ir/TiO2. The key factors that deeply influenced catalytic activity were the strong metals–support interaction exploited with the Catalytic activity in the total oxidation of toluene of the Au-Ir bimetallic catalyst supported on TiO<sup>2</sup> was studied by Torrente-Murciano et al. [101]. Similarly to the previous case, the synergistic interaction between the two metals allowed them to sensibly decrease the T<sup>90</sup> that was <sup>≈</sup><sup>230</sup> ◦C for the bimetallic sample, <sup>≈</sup><sup>250</sup> ◦C for the monometallic gold, and <sup>≈</sup><sup>270</sup> ◦C for Ir/TiO2. The key factors that deeply influenced catalytic activity were the strong metals–support interaction exploited with the bimetallic system, which also permitted diminishing the loss of activity due to the metals sintering at high temperatures. Furthermore, the intimate contact between iridium and gold modified the bimetallic surface-active sites enhancing oxygen activation.
bimetallic system, which also permitted diminishing the loss of activity due to the metals sintering at high temperatures. Furthermore, the intimate contact between iridium and gold modified the bimetallic surface-active sites enhancing oxygen activation. In the work cited in Section 2.2, Kucherov et al. [92] demonstrated a good activity of Au-Rh In the work cited in Section 2.2, Kucherov et al. [92] demonstrated a good activity of Au-Rh supported on HZSM-5 zeolite into the oxidation of the DMDS in SO<sup>2</sup> at 290 ◦C. In this case, the zeolite support, owing to a high surface area, favored a high dispersion of the metals, a feature that is beneficial for catalytic activity.
supported on HZSM-5 zeolite into the oxidation of the DMDS in SO2 at 290 °C. In this case, the zeolite support, owing to a high surface area, favored a high dispersion of the metals, a feature that is beneficial for catalytic activity. Regarding gold–copper bimetallic systems, there are certain studies with other transition metals, Regarding gold–copper bimetallic systems, there are certain studies with other transition metals, utilized together with gold for VOC oxidation. Au-Co and Au-Fe interaction were principally investigated. In the examined works, the gold atoms interacted with the second metal present as a doping agent of the support [102–104].
utilized together with gold for VOC oxidation. Au-Co and Au-Fe interaction were principally investigated. In the examined works, the gold atoms interacted with the second metal present as a doping agent of the support [102–104]. Solsona et al. [102] synthetized gold nanoparticles anchored on cobalt containing mesoporous silica (UVM-7). The interaction between gold and cobalt permitted increasing catalytic activity in the oxidation of toluene and propane with respect to Au/UVM-7 and Co/UVM-7 catalysts. The presence of gold enhanced the reducibility of cobalt, present as Co3O4 at the Au-Co interface, thus facilitating the redox cycle of cobalt, with an MvK-like mechanism, which boosted catalytic oxidation of VOCs. In the same context, Albonetti et al. [103,104] deeply investigated the catalytic behavior of gold catalysts supported on mesoporous silica (SBA-15) via an iron oxide layer obtaining the Solsona et al. [102] synthetized gold nanoparticles anchored on cobalt containing mesoporous silica (UVM-7). The interaction between gold and cobalt permitted increasing catalytic activity in the oxidation of toluene and propane with respect to Au/UVM-7 and Co/UVM-7 catalysts. The presence of gold enhanced the reducibility of cobalt, present as Co3O<sup>4</sup> at the Au-Co interface, thus facilitating the redox cycle of cobalt, with an MvK-like mechanism, which boosted catalytic oxidation of VOCs. In the same context, Albonetti et al. [103,104] deeply investigated the catalytic behavior of gold catalysts supported on mesoporous silica (SBA-15) via an iron oxide layer obtaining the Au/FeOx/SBA-15 composite. The good dispersion of nanosized gold favored the incidence of a strong synergism between gold and iron that led to an optimal activity in the combustion of methanol (T<sup>90</sup> <sup>≈</sup> <sup>140</sup> ◦C).
Au/FeOx/SBA-15 composite. The good dispersion of nanosized gold favored the incidence of a strong synergism between gold and iron that led to an optimal activity in the combustion of methanol (T90 ≈ 140 °C). As a conclusion of this generic overview of gold-based bimetallic catalysts applied at VOC As a conclusion of this generic overview of gold-based bimetallic catalysts applied at VOC oxidation, it is possible to recognize some fundamental features of these peculiar catalysts: (a) the essential action of the nanosized gold that is able to establish a metal–metal surface interaction with a wide range of both noble and transition metals; (b) the occurrence of a synergism between the two
oxidation, it is possible to recognize some fundamental features of these peculiar catalysts: (a) the essential action of the nanosized gold that is able to establish a metal–metal surface interaction with metals that allows sensibly decreasing the light-off temperatures of VOC oxidation; (c) the synergistic effect, which is not simply the addition of the single characteristics of the corresponding monometallic samples but leads to exploring new physicochemical properties; (d) the mutual interaction between the two metals which also strongly influences metals–support interaction (in particular, if the support is a reducible oxide (CeO2, MnOx, CoOx, etc.), the gold-based bimetallic cluster increases the mobility of the surface oxygen of the support, enhancing, in this way, catalytic oxidation towards an MvK mechanism). If the support is a nonreducible or hardly reducible oxide (TiO2, SiO2, and zeolites), the high dispersion of the bimetallic alloy and the modifications of the metals surface active sites allow enhancing oxygen adsorption, improving, as a final result, the overall catalytic performance. The most employed preparation methods of the supported gold-based bimetallic catalysts are impregnation (wet or wetness), deposition–precipitation, and chemical reduction.
| doab | 2025-04-07T03:56:58.814386 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 31
} |
004db1df-dc76-42af-952f-1f5e82b7822b.32 | *2.2. Other Bimetallic Catalysts*
Among the other noble metals, the most employed catalysts for catalytic oxidation of VOCs are platinum-based materials [16], and similarly to gold, platinum has shown a good affinity with palladium [105–108]. In general, as can be seen from Table 2, the use of noble-metals-based bimetallic catalysts has allowed obtaining a good performance in the removal of VOCs, whereas the utilization of transition-metals-based materials has led to shift at high temperatures in the total conversion of VOCs. However, especially in recent years, the necessity to reduce the amount of expensive noble metals has led to exploring a new synergism between noble and transition metals.
<sup>1</sup> Nominal concentration, weight percentage (wt%).
Fu et al. [105] prepared, for the hydrothermal method, a Pt-Pd bimetallic sample supported on mesoporous silica. Comparably to the catalytic behavior of gold-based samples, the synergism between the two metals allowed obtaining a superior performance in the removal of toluene with respect to the monometallic samples, with an improvement in the reducibility of palladium, involved in the redox cycle PdO→Pd<sup>0</sup> , and in oxygen adsorption capability.
The good catalytic activity of the Pt-Pd system was also confirmed by Kim et al. [106] in the degradation of benzene. Catalysts were synthetized via wetness impregnation on γ-Al2O3. Metal–metal interaction was favored by the formation of small and uniform particles and, as stated in the previous paragraphs, a specific amount ratio between the two metals (the optimum in the cited work is 0.3 wt% Pt–2%wt Pd). A higher concentration of platinum led to a remarkable decrease in activity due to the blockage of the active sites. The same authors in another paper [107] confirmed with a deep XPS (X-ray photoelectron spectroscopy) analysis the crucial role of the ratio between the metals to avoid the obstruction of the catalyst surface sites. The removal of methanol, acetone, and methylene chloride was instead studied by Sharma et al. [108], utilizing ceramic Raschig rings coated with Pt and Pd on fluorinated carbon. The authors measured a higher activity of the bimetallic catalyst with respect to monometallic ones. Furthermore, the hydrophobic nature of this particular bimetallic catalyst allowed obtaining a 90% of degradation of methanol and acetone at about 150 and 300 ◦C respectively, whereas
60% of degradation was achieved at 400 ◦C for the methylene chloride. In this case, a good correlation was established with a semi-empirical Langmuir–Hinshelwood model, which is able to predict the oxidation rate of each VOC in a gas mixture (methanol, acetone, and methylene chloride). *Catalysts* **2020**, *10*, x FOR PEER REVIEW 12 of 25 model, which is able to predict the oxidation rate of each VOC in a gas mixture (methanol, acetone,
Ethanol adsorption and oxidation were investigated by the research group of Wittayakun et al. [114] with a Pt-M (M = Co, Cu, Mn) sample supported on silica MCM-41. Among the transition metals, cobalt gave the best results, and in particular, the bimetallic 0.5 wt% Pt–15 wt% Co exhibited the best ethanol adsorption and CO<sup>2</sup> desorption. Interestingly, the authors identified two different reaction mechanisms considering the platinum monometallic sample and the bimetallic platinum–cobalt one (Figure 7). and methylene chloride). Ethanol adsorption and oxidation were investigated by the research group of Wittayakun et al. [114] with a Pt-M (M = Co, Cu, Mn) sample supported on silica MCM-41. Among the transition metals, cobalt gave the best results, and in particular, the bimetallic 0.5 wt% Pt–15 wt% Co exhibited the best ethanol adsorption and CO2 desorption. Interestingly, the authors identified two different reaction mechanisms considering the platinum monometallic sample and the bimetallic platinum– cobalt one (Figure 7).
**Figure 7.** (**a**) Ethanol oxidation mechanism on 0.5 wt% Pt/MCM-41; and (**b**) ethanol oxidation mechanism on 0.5 wt% Pt–15 wt% Co/MCM-41. Figure modified from [114]. Copyright 2012, Elsevier. **Figure 7.** (**a**) Ethanol oxidation mechanism on 0.5 wt% Pt/MCM-41; and (**b**) ethanol oxidation mechanism on 0.5 wt% Pt–15 wt% Co/MCM-41. Figure modified from [114]. Copyright 2012, Elsevier.
Specifically, in the monometallic sample after the adsorption of ethanol, a formation was verified of a parallel adsorbed acetaldehyde, further converted into monodentate acetate and at end, dissociated and desorbed as carbon dioxide, methane, and water (Figure 7a). In the bimetallic catalyst, by contrast, the ethoxy species reacted with the adsorbed oxygen to give a bidentate acetate species that was transformed into carbon dioxide (Figure 7b). The modification of the ethanol adsorption led to a higher ethanol conversion with the monometallic platinum sample in comparison with the bimetallic Pt-Co sample that, conversely, showed a higher catalytic stability. Even with the bimetallic platinum-based catalysts, Chantaravitoon et al. [115] examined the performance of a Pt-Sn/γ-Al2O3 catalyst prepared with impregnation, for the oxidation of methanol. The authors noted from the temperature-programmed desorption (TPD) measurements of methanol oxidation that on the bimetallic catalyst, methanol decomposed as H2 and CO and the desorption peaks shifted at higher temperatures, increasing the amount of Sn. In addition, in this case, the monometallic Pt catalyst exhibited a better performance compared to the bimetallic one; however, the addition of a Specifically, in the monometallic sample after the adsorption of ethanol, a formation was verified of a parallel adsorbed acetaldehyde, further converted into monodentate acetate and at end, dissociated and desorbed as carbon dioxide, methane, and water (Figure 7a). In the bimetallic catalyst, by contrast, the ethoxy species reacted with the adsorbed oxygen to give a bidentate acetate species that was transformed into carbon dioxide (Figure 7b). The modification of the ethanol adsorption led to a higher ethanol conversion with the monometallic platinum sample in comparison with the bimetallic Pt-Co sample that, conversely, showed a higher catalytic stability. Even with the bimetallic platinum-based catalysts, Chantaravitoon et al. [115] examined the performance of a Pt-Sn/γ-Al2O<sup>3</sup> catalyst prepared with impregnation, for the oxidation of methanol. The authors noted from the temperature-programmed desorption (TPD) measurements of methanol oxidation that on the bimetallic catalyst, methanol decomposed as H<sup>2</sup> and CO and the desorption peaks shifted at higher temperatures, increasing the amount of Sn. In addition, in this case, the monometallic Pt catalyst exhibited a better performance compared to the bimetallic one; however, the addition of a small amount of Sn (<5 wt%) reduced the deactivation of the catalyst in the long-time tests.
small amount of Sn (<5 wt%) reduced the deactivation of the catalyst in the long-time tests. In addition, Ru-based bimetallic compounds were discreetly studied for VOC oxidation [109,110,116]. Liu et al. [109] prepared Ru-M (M = Co, Mn, Ce, Fe, Cu) samples supported on TiO2, In addition, Ru-based bimetallic compounds were discreetly studied for VOC oxidation [109,110,116]. Liu et al. [109] prepared Ru-M (M = Co, Mn, Ce, Fe, Cu) samples supported on TiO2, evaluating catalytic performance in the degradation of benzene. Among the various metals,
evaluating catalytic performance in the degradation of benzene. Among the various metals, 1% wt
1% wt Ru–5% wt Co showed the best activity; the presence of ruthenium, in fact, increased the reducibility of Co3O4. The authors stated also that the presence of water vapor inhibited benzene oxidation at T = 210 ◦C.
The total oxidation of propene was examined on Ru-Re/γ-Al2O<sup>3</sup> by Baranowska et al. [116]. As discussed before, this nonconventional combination between these two metals also allowed increasing catalytic stability in the consecutive tests instead of the overall catalytic activity, which remained superior with the monometallic Ru sample. The addition of Re (the best composition being 5% wt Ru–3% wt Re) hampered the formation of RuO<sup>2</sup> agglomerates. In this way, the dispersion of ruthenium is favored, allowing a higher stability compared to monometallic ruthenium catalysts. Ye et al. [110] performed a catalytic test regarding chlorobenzene removal with Ru-Ce/TiO<sup>2</sup> samples prepared via impregnation. Interestingly, on the basis of the crystalline phase of titanium dioxide, the catalytic activity changed. At 280 ◦C, the bimetallic sample showed a conversion of 91% and 86% if supported on TiO<sup>2</sup> rutile and TiO<sup>2</sup> P25 (80% anatase, 20% rutile), respectively. The mixed crystalline phase of P25 was the best support for the monometallic ruthenium catalyst. By contrast, with respect to the work of Baranowska et al. [116] for this reaction and with the titanium dioxide support, dispersion was not the major parameter that affected catalytic activity; indeed, on 1%wt Ru–5%wt Ce/TiO<sup>2</sup> (rutile), the abundant RuO<sup>2</sup> clusters favored both catalytic activity and stability.
Interaction with Ce/CeO<sup>2</sup> was also investigated by Yue et al. [117] but utilizing palladium. The performance of the bimetallic catalyst Pd-Ce/ZMS-5 synthetized through impregnation was evaluated on the degradation of methyl ethyl ketone (MEK). The presence of cerium oxide considerably increased the acid sites of palladium, enhancing at the same time the re-oxidation of Pd and boosting, in the end, the overall MEK degradation rate through an MvK mechanism.
Another bimetallic catalyst with palladium was prepared by Arias et al. [111]. In this work, the synergistic effect between palladium and manganese was explored utilizing alumina as support. The authors followed the oxidation of a VOC mixture (formaldehyde/methanol), concluding also in this case that an MvK-like mechanism was the reaction pathway, with the interaction between palladium and manganese favoring the oxidation of VOCs due to the activation of the reactive lattice oxygen of PdO and MnOx.
Various bimetallic samples were tested for catalytic oxidation of VOCs, studying both the physicochemical properties and the catalytic activity of silver-containing samples [113,118–120]. In particular, Jodaei et al. [113,118] tested different Ag-M bimetallic samples supported on ZMS-5 zeolite obtained via ionic exchange. The authors investigated the catalytic combustion of ethyl acetate, finding this order of activity and stability: Fe-Ag/ZSM-5 > Co-Ag/ZSM-5 > Mn-Ag/ZSM-5 > Ag/ZSM-5. The high dispersion of silver was favored by an optimal amount of iron (1.3 wt% Fe–1.75 wt% Ag), thus activating a synergistic effect between the two metals. In the same context, Izadkhah et al. [119] made a theoretical model for the removal of ethyl acetate. In particular, considering the preparation condition, the formulation, and loading of the promoter of silver, with their algorithm, it was possible to identify the optimal catalyst for this reaction. Among the first transition metal series, the bimetallic catalyst that exhibited superior performance compared to the monometallic silver was Fe-Ag/ZSM-5, thus confirming the experimental results of Jodaei et al. [113,118], Ni-Ag/ZSM-5 and V-Ag/ZSM-5.
Complete oxidation of formaldehyde at T < 90 ◦C was obtained by Qu et al. [120] with Ag-Co/MCM-41 silica. The key feature able to sensibly increase catalytic performance with respect to the monometallic silver was electron transfer between silver and cobalt that enhanced the reducibility of cobalt oxide, increasing, at the same time, the activation of surface oxygen on the bimetallic catalyst. Furthermore, the high metal–metal support interaction (SMMI) at the optimal Ag/Co mass ratio (3:1) favored a faster adsorption–dissociation of formaldehyde on the Ag species with respect to the Co3<sup>+</sup> sites (Figure 8), thus decreasing the light-off temperature of VOC oxidation.
activity.
*Catalysts* **2020**, *10*, x FOR PEER REVIEW 14 of 25
**Figure 8.** Formaldehyde oxidation on monometallic Ag/MCM-41, Co/MCM-41 and bimetallic Ag-Co/MCM-41. Figure from [120]. Copyright 2014, Elsevier. **Figure 8.** Formaldehyde oxidation on monometallic Ag/MCM-41, Co/MCM-41 and bimetallic Ag-Co/MCM-41. Figure from [120]. Copyright 2014, Elsevier.
On the same support (MCM-41 silica), Pârvulescu et al. [121] synthetized with the hydrothermal method various Co-based bimetallic mesostructures (Co-V, Co-La, Co-Nb) characterized vy a high surface area and narrow pore size distribution. The oxidation of styrene and benzene was deeply influenced by the addition of a second metal component. Indeed, although the addition of La did not result in any synergistic effect, the addition of vanadium favored the oxidation of benzene, whereas the addition of niobium facilitated the removal of styrene, demonstrating that the presence of the second metal changed the surface-active sites of cobalt. On the same support (MCM-41 silica), Pârvulescu et al. [121] synthetized with the hydrothermal method various Co-based bimetallic mesostructures (Co-V, Co-La, Co-Nb) characterized vy a high surface area and narrow pore size distribution. The oxidation of styrene and benzene was deeply influenced by the addition of a second metal component. Indeed, although the addition of La did not result in any synergistic effect, the addition of vanadium favored the oxidation of benzene, whereas the addition of niobium facilitated the removal of styrene, demonstrating that the presence of the second metal changed the surface-active sites of cobalt.
Similarly to cobalt, copper-based bimetallic catalysts also showed a good activity in the removal of VOCs [112,122,123]. Kim et al. [112] found an optimal interaction between Mn and Cu for the total oxidation of toluene. The order of activity considering other transition metals as a second component was: 5% wt Cu-15%wt Mn/γ-Al2O3 > 5% wt Co-15% wt Mn/γ-Al2O3 > 5% wt Ni-15% wt Mn/γ-Al2O3 > 15% wt Mn/γ-Al2O3 > 5% wt Fe-15% wt Mn/γ-Al2O3. The interaction between Mn and Cu favored a high dispersion of manganese, increasing, at the same time, the mobility/reducibility of manganese oxide. The oxidation of toluene was studied recently by Djinović et al. [122], who had examined the performance of monometallic CuO and bimetallic Cu-FeOx composites supported on KIL-2 silica. The utilization of two reducible oxides allowed increasing the amount and reactivity of oxygen species, which included adsorbed (O− and O2−) and lattice (O2−) oxygens at the Cu-FeOx interface, providing a substantial decrease of T90 that was ≈350 °C for the bimetallic cluster instead of ≈450 °C of monometallic copper oxide, whereas the FeOx/KIL-2 silica reached only 30% of toluene conversion at 450 °C. Similarly to cobalt, copper-based bimetallic catalysts also showed a good activity in the removal of VOCs [112,122,123]. Kim et al. [112] found an optimal interaction between Mn and Cu for the total oxidation of toluene. The order of activity considering other transition metals as a second component was: 5% wt Cu–15% wt Mn/γ-Al2O<sup>3</sup> > 5% wt Co–15% wt Mn/γ-Al2O<sup>3</sup> > 5% wt Ni–15% wt Mn/γ-Al2O<sup>3</sup> > 15% wt Mn/γ-Al2O<sup>3</sup> > 5% wt Fe–15% wt Mn/γ-Al2O3. The interaction between Mn and Cu favored a high dispersion of manganese, increasing, at the same time, the mobility/reducibility of manganese oxide. The oxidation of toluene was studied recently by Djinovi´c et al. [122], who had examined the performance of monometallic CuO and bimetallic Cu-FeO<sup>x</sup> composites supported on KIL-2 silica. The utilization of two reducible oxides allowed increasing the amount and reactivity of oxygen species, which included adsorbed (O<sup>−</sup> and O<sup>2</sup> <sup>−</sup>) and lattice (O2−) oxygens at the Cu-FeO<sup>x</sup> interface, providing a substantial decrease of T<sup>90</sup> that was <sup>≈</sup><sup>350</sup> ◦C for the bimetallic cluster instead of <sup>≈</sup>450 ◦C of monometallic copper oxide, whereas the FeOx/KIL-2 silica reached only 30% of toluene conversion at 450 ◦C.
Abdullah et al. [123] investigated the oxidation of a Cl-VOC mixture (dichloromethane (DCM), trichloroethylene (TCE) and trichloromethane (TCM)) with Cu-Cr/ZMS-5. Interestingly, in this case, the presence of water vapor in the gas feed enhanced the total oxidation to CO2. The presence of water vapor favored the formation of reactive carbocations. Furthermore, H2O was beneficial in blocking chlorine-transfer reactions. Indeed, an important deactivation effect was found with the bimetallic catalyst at a higher Cl/H gas feed ratio, and chlorination led to a decrease in metals' reducibility that resulted in a low degradation efficiency. The reaction was driven by an MvK mechanism. Abdullah et al. [123] investigated the oxidation of a Cl-VOC mixture (dichloromethane (DCM), trichloroethylene (TCE) and trichloromethane (TCM)) with Cu-Cr/ZMS-5. Interestingly, in this case, the presence of water vapor in the gas feed enhanced the total oxidation to CO2. The presence of water vapor favored the formation of reactive carbocations. Furthermore, H2O was beneficial in blocking chlorine-transfer reactions. Indeed, an important deactivation effect was found with the bimetallic catalyst at a higher Cl/H gas feed ratio, and chlorination led to a decrease in metals' reducibility that resulted in a low degradation efficiency. The reaction was driven by an MvK mechanism.
At this point, it is possible to highlight some differences through comparison of the catalytic performance of gold-based bimetallic samples with the others reported above. For the nongoldcontaining samples category, supports with a high surface area or with a tunable pore size distribution (silica, zeolite, alumina, etc.) were preferred to favor the dispersion of the active metals. Platinum-based samples gave the best results, with a second metal that in many cases enhances catalytic stability rather than overall VOC conversion. Although noble-metals–bimetallic catalysts showed the best performance, in recent years, in order to reduce the high cost of these catalysts, the addition or replacement of at least one of the noble metals with a cheaper transition metal is an interesting approach to reduce the total material cost while maintaining an acceptable catalytic At this point, it is possible to highlight some differences through comparison of the catalytic performance of gold-based bimetallic samples with the others reported above. For the nongold-containing samples category, supports with a high surface area or with a tunable pore size distribution (silica, zeolite, alumina, etc.) were preferred to favor the dispersion of the active metals. Platinum-based samples gave the best results, with a second metal that in many cases enhances catalytic stability rather than overall VOC conversion. Although noble-metals–bimetallic catalysts showed the best performance, in recent years, in order to reduce the high cost of these catalysts, the addition or replacement of at least one of the noble metals with a cheaper transition metal is an interesting approach to reduce the total material cost while maintaining an acceptable catalytic activity.
| doab | 2025-04-07T03:56:58.814991 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 32
} |
004db1df-dc76-42af-952f-1f5e82b7822b.33 | **3. Bimetallic Catalysts for the Photocatalytic Oxidation of VOCs**
The urgent request for a "greener" and sustainable industrial chemistry has driven a huge field of research towards alternative ways to treat VOCs instead of catalytic combustion. Among the various AOPs (see the Introduction section), photocatalytic oxidation is the most applied process. With respect to catalytic thermal oxidation, this technique allows working at room temperature, exploiting the chemicophysical processes activated by an appropriate light radiation interacting with the surface of a semiconductor photocatalyst [40,124–126]. Specifically, dangerous organic compounds are oxidized by hydroxyl, and super oxide radicals are generated by the interaction between the photoelectrons and photoholes of the photocatalyst with water and oxygen. These photoelectrons and photoholes are formed when an adequate wavelength (λ ≤ of the band gap, Eg, of the semiconductor) irradiates the photocatalyst [127,128]. The most used photocatalysts are metal oxides or sulfides such as TiO2, ZnO, WO3, ZrO2, CeO2, Fe2O3, ZnS, and CdS, and among them, TiO<sup>2</sup> and ZnO are the most used [129,130]. Due to its properties, such as its nontoxicity, relatively low cost, and high activity, especially under UV irradiation, titanium dioxide was deeply investigated both in academic and industrial research [124,126]. With a band gap varying from 3.0 to 3.2 eV depending on the crystalline form, TiO<sup>2</sup> is able to exploit only 5% of solar radiation, thus limiting its practical applications. Photothermocatalytic oxidation is a multicatalytic approach that accepts the contemporaneous utilization of a light source to activate the photocatalyst, and thermal heat to boost the conversion of organic molecules and to increase the yield to CO2. A proper structural and/or chemical modification of titanium dioxide together with this multicatalytic approach can be considered a suitable solution to decrease the total energy consumption, maintaining the high conversion values typical of thermocatalytic oxidation of VOCs [131–133]. Another connected strategy to increase the photocatalytic performance of titanium dioxide under solar/visible light irradiation is doping with metal or nonmetal elements [134–136], and a not yet largely explored strategy is the combination of a bimetallic alloy with TiO<sup>2</sup> [137,138]. The same metals typically employed for catalytic oxidation of VOCs, such as Au, Ag, Pt, Pd, and Cu in the nanoparticle size, if irradiated with (usually) a visible light irradiation, allow exploiting the localized surface plasmon resonance (LSPR) through collective oscillations of the electrons in the surface of the metal nanoparticles. This effect, combined with the photocatalytic properties of TiO2, is a performance solution to obtain a visible-light-driven photocatalyst [124,139,140]. In addition, for this particular application, bimetallic compounds can help to overcome some of the drawbacks of single metals. For example, the LSPR of some noble metals such as Pd, Pt, and Rh is not efficiently activated by solar irradiation, and a possible combination with the most effective plasmonic metals, such as Au, Ag or Cu, leads to takeing advantages of both the LSPR effect and of the reactive catalytic behavior of the other noble metals [137].
For the above considerations, in VOC photo-oxidation, the most investigated system was the Au-Pd bimetallic compound joined with a semiconductor photocatalyst [141–145]. In these materials, the good affinity of gold and palladium, already discussed in terms of thermocatalytic performance (Section 2.1.1.), is in this case utilized to increase the photocatalytic performances of titanium dioxide or of another semiconductor oxide.
Colmenares et al. [141] synthetized an Au-Pd/TiO<sup>2</sup> photocatalyst with the original technique of sonophotodeposition (Figure 9). The bimetallic sample exhibited high activity (83%) and good selectivity (70%) in the partial oxidation of methanol to methyl formate after 120 min of UV irradiation (125 W mercury lamp λmax = 365 nm). Although the bimetallic catalyst showed a low selectivity to CO<sup>2</sup> (≈30%), demonstrating that this approach is better suitable for selective oxidation than the total oxidation of VOCs, the reported material synthesis and adopted reaction conditions are a fascinating way to obtain results with an energy-efficient procedure and a selective photocatalyst in a short time and under mild conditions.
*Catalysts* **2020**, *10*, x FOR PEER REVIEW 16 of 25
**Figure 9.** Sonophotodeposition setup before (**a**) and during the deposition procedure (**b**): (1) batch photoreactor; (2) argon line; (3) switched off 6 W UV lamp; (4) ultrasonic bath; (5) switched on 6W UV lamp; (6) reflux condenser; and (7) lamp cooling system (20 °C). Figure from [141], Copyright 2015, John Wiley and Sons **Figure 9.** Sonophotodeposition setup before (**a**) and during the deposition procedure (**b**): (1) batch photoreactor; (2) argon line; (3) switched off 6 W UV lamp; (4) ultrasonic bath; (5) switched on 6W UV lamp; (6) reflux condenser; and (7) lamp cooling system (20 ◦C). Figure from [141], Copyright 2015, John Wiley and Sons.
In a further study, the same research group [142] developed a density functional methodology to analyze the reaction mechanism of the selective photo-oxidation of methanol on the bimetallic Au-Pd/TiO2 sample. The theoretical investigation showed, as with the formation of a synergistic interaction between gold and palladium, a superior photoelectron–hole separation, was verified in comparison with the monometallic samples. Furthermore, it was shown that to favor total photooxidation to CO2, the dissociation of molecular oxygen should be driven preferentially on Pd to favor the formation of PdO sites, where complete oxidation (no methyl formate formation) to carbon dioxide occurred. In a further study, the same research group [142] developed a density functional methodology to analyze the reaction mechanism of the selective photo-oxidation of methanol on the bimetallic Au-Pd/TiO<sup>2</sup> sample. The theoretical investigation showed, as with the formation of a synergistic interaction between gold and palladium, a superior photoelectron–hole separation, was verified in comparison with the monometallic samples. Furthermore, it was shown that to favor total photo-oxidation to CO2, the dissociation of molecular oxygen should be driven preferentially on Pd to favor the formation of PdO sites, where complete oxidation (no methyl formate formation) to carbon dioxide occurred.
Cybula et al. [143] investigated the performance of an Au-Pd bimetallic sample supported on rutile TiO2 synthetized with a water in oil microemulsion methodology, in the photocatalytic oxidation of toluene and phenol under visible light irradiation (25 LEDs (ߣmax = 415 nm)). In particular, the authors focused on the effect of calcination temperature on materials' preparation. The bimetallic sample calcined at 350 °C achieved 65% of toluene degradation and 22% of phenol conversion after 60 min of visible light irradiation. The performances were inferior compared to the photoactivity of monometallic palladium (79% in the toluene degradation and 24% in the phenol removal); however, the synergistic effect combined with a strong metals–support interaction was better exploited in the UV-vis tests, where the intrinsic photoactivity of rutile TiO2 also made a substantial contribution in removal efficiency. In fact, with the bimetallic sample, 100% of phenol degradation was achieved after 60 min of irradiation instead of the 56% of Pd/TiO2. Cybula et al. [143] investigated the performance of an Au-Pd bimetallic sample supported on rutile TiO<sup>2</sup> synthetized with a water in oil microemulsion methodology, in the photocatalytic oxidation of toluene and phenol under visible light irradiation (25 LEDs (λmax = 415 nm)). In particular, the authors focused on the effect of calcination temperature on materials' preparation. The bimetallic sample calcined at 350 ◦C achieved 65% of toluene degradation and 22% of phenol conversion after 60 min of visible light irradiation. The performances were inferior compared to the photoactivity of monometallic palladium (79% in the toluene degradation and 24% in the phenol removal); however, the synergistic effect combined with a strong metals–support interaction was better exploited in the UV-vis tests, where the intrinsic photoactivity of rutile TiO<sup>2</sup> also made a substantial contribution in removal efficiency. In fact, with the bimetallic sample, 100% of phenol degradation was achieved after60 min of irradiation instead of the 56% of Pd/TiO2.
The interaction of the gold–palladium compound with other semiconductors was examined by the research group of Zhang et al. [144,145]. The photocatalytic oxidation of the trichloroethylene was studied on Au-Pd/BiPO4 nanorods and on Au-Pd/MoO3 nanowires. Interestingly, with the deposition of the Au-Pd alloy on the surface of the BiPO4 nanorod, the photocatalytic degradation rate increased quickly, being about 25 times higher compared to that achieved with bare BiPO4. The authors proposed, on the basis of the characterization measurements, the reaction mechanism illustrated in Figure 10. Under visible light irradiation (solar simulator with a 440 nm cut-off filter), Au-Pd/BiPO4 were excited due to the LSPR of the Au-Pd alloy. An effective charge carrier separation was achieved due to electron transfer from the conduction band (CB) of BiPO4 to the Au-Pd surface interface, whereas the photoholes remained confined in the valence band (VB) of BiPO4. Subsequently, the same photoelectrons present in the surface of the Au-Pd alloy reacted with the oxygens in the gas-phase that were successively reduced into superoxide radicals. These radicals together with the holes in the VB of BiPO4 oxidize the trichloroethylene in water and carbon dioxide. The interaction of the gold–palladium compound with other semiconductors was examined bythe research group of Zhang et al. [144,145]. The photocatalytic oxidation of the trichloroethylene was studied on Au-Pd/BiPO<sup>4</sup> nanorods and on Au-Pd/MoO<sup>3</sup> nanowires. Interestingly, with the depositionof the Au-Pd alloy on the surface of the BiPO<sup>4</sup> nanorod, the photocatalytic degradation rate increased quickly, being about 25 times higher compared to that achieved with bare BiPO4. The authors proposed, on the basis of the characterization measurements, the reaction mechanism illustrated in Figure 10. Under visible light irradiation (solar simulator with a 440 nm cut-off filter), Au-Pd/BiPO<sup>4</sup> were excited due to the LSPR of the Au-Pd alloy. An effective charge carrier separation was achieved due to electron transfer from the conduction band (CB) of BiPO<sup>4</sup> to the Au-Pd surface interface, whereas the photoholesremained confined in the valence band (VB) of BiPO4. Subsequently, the same photoelectrons present in the surface of the Au-Pd alloy reacted with the oxygens in the gas-phase that were successivelyreduced into superoxide radicals. These radicals together with the holes in the VB of BiPO<sup>4</sup> oxidize the trichloroethylene in water and carbon dioxide.
*Catalysts* **2020**, *10*, x FOR PEER REVIEW 17 of 25
**Figure 10.** Reaction mechanism of the trichloroethylene oxidation with the Au-Pd/BiPO4 photocatalyst. Figure from [144]. Copyright 2017, Elsevier. **Figure 10.** Reaction mechanism of the trichloroethylene oxidation with the Au-Pd/BiPO<sup>4</sup> photocatalyst. Figure from [144]. Copyright 2017, Elsevier.
The synergistic effect between gold and palladium nanoparticles that permitted increasing charge carrier separation, enhancing in this way photocatalytic activity, was verified, also employing MoO3 nanowires as a semiconductor photocatalyst [145]. The synergistic effect between gold and palladium nanoparticles that permitted increasing charge carrier separation, enhancing in this way photocatalytic activity, was verified, also employing MoO<sup>3</sup> nanowires as a semiconductor photocatalyst [145].
The exploration of the synergistic effect with a bimetallic alloy in the photocatalytic oxidation of VOCs has not yet been largely investigated in the literature, and only fa ew examples are present considering other compounds instead of the Au-Pd system [146–149]. The possibility of favoring a selective oxidation route due to the high selectivity of gold–silver nanoparticles was examined by Han et al. [146] even in partial oxidation of methanol to methyl formate. In this case, the Au-Ag/TiO2 powders prepared via chemical reduction showed good results with a methanol conversion of 90% and a selectivity to methyl formate of about 85% under UV irradiation (500 W high pressure mercury lamp, λmax = 365 nm). Similarly to the Au-Pd systems, the bimetallic alloy enhanced the photoelectron–photohole separation with electron transfer from the conduction band of TiO2 (excited by the UV irradiation) to the gold–silver surface interface. The exploration of the synergistic effect with a bimetallic alloy in the photocatalytic oxidation of VOCs has not yet been largely investigated in the literature, and only fa ew examples are present considering other compounds instead of the Au-Pd system [146–149]. The possibility of favoring a selective oxidation route due to the high selectivity of gold–silver nanoparticles was examined by Han et al. [146] even in partial oxidation of methanol to methyl formate. In this case, the Au-Ag/TiO<sup>2</sup> powders prepared via chemical reduction showed good results with a methanol conversion of 90% and a selectivity to methyl formate of about 85% under UV irradiation (500 W high pressure mercury lamp, λmax = 365 nm). Similarly to the Au-Pd systems, the bimetallic alloy enhanced the photoelectron–photohole separation with electron transfer from the conduction band of TiO<sup>2</sup> (excited by the UV irradiation) to the gold–silver surface interface.
Another alloy with silver, i.e., Ag-Pt, was studied by Zieli'nska-Jurek et al. [147] in the photooxidation of toluene under visible light irradiation (LEDs, λmax = 415 nm). The authors found an interesting correlation regarding the order of deposition of the Ag-Pt/TiO2 photocatalysts prepared by sol–gel. In particular, the best sample (best photoactivity) was the material where the silver precursor was added before the platinum one. It was fundamental, in fact, to obtain particles with a definite size and dispersion (Ag-Pt size between 6–12 nm). In this way, it was possible to increase the toluene degradation rate with respect to the monometallic samples. By contrast, the bimetallic sample prepared with a simultaneous addition of metals precursors on TiO2 gave a lower photoactivity and different metal size and distribution. The authors concluded that platinum size had a greater influence than silver in determining overall photocatalytic activity. Recently, the same research group evaluated photocatalytic performance in both toluene and acetaldehyde degradation and of *Penicillium chrysogenum*, a dangerous fungus present in the indoor environment with Ag-Pt/TiO2 and Cu-Pt/TiO2 samples [148]. Both bimetallic samples showed a higher fungicidal activity under visible light irradiation than bare TiO2, whereas in VOC degradation, the Ag-Pt system was betterperforming compared to Cu-Pt. The peculiar activity of both bimetallic samples was ascribed to the interfacial charge transfer process between the two metals and the TiO2 confirmed by the quenching of fluorescence (i.e., intensity diminution of the TiO2 photolumiscence bands) due to the presence of Another alloy with silver, i.e., Ag-Pt, was studied by Zieli'nska-Jurek et al. [147] in the photo-oxidation of toluene under visible light irradiation (LEDs, λmax = 415 nm). The authors found an interesting correlation regarding the order of deposition of the Ag-Pt/TiO<sup>2</sup> photocatalysts prepared by sol–gel. In particular, the best sample (best photoactivity) was the material where the silver precursor was added before the platinum one. It was fundamental, in fact, to obtain particles with a definite size and dispersion (Ag-Pt size between 6–12 nm). In this way, it was possible to increase the toluene degradation rate with respect to the monometallic samples. By contrast, the bimetallic sample prepared with a simultaneous addition of metals precursors on TiO<sup>2</sup> gave a lower photoactivity and different metal size and distribution. The authors concluded that platinum size had a greater influence than silver in determining overall photocatalytic activity. Recently, the same research group evaluated photocatalytic performance in both toluene and acetaldehyde degradation and of *Penicillium chrysogenum*, a dangerous fungus present in the indoor environment with Ag-Pt/TiO<sup>2</sup> and Cu-Pt/TiO<sup>2</sup> samples [148]. Both bimetallic samples showed a higher fungicidal activity under visible light irradiation than bare TiO2, whereas in VOC degradation, the Ag-Pt system was better-performing compared to Cu-Pt. The peculiar activity of both bimetallic samples was ascribed to the interfacial charge transfer process between the two metals and the TiO<sup>2</sup> confirmed by the quenching of fluorescence (i.e., intensity diminution of the TiO<sup>2</sup> photolumiscence bands) due to the presence of the metal alloy.
the metal alloy. Wolski et al. [149] studied the mechanism of methanol photo-oxidation on bimetallic Au-Cu catalyst supported on Nb2O5 with an in operando IR methodology under both UV and visible light irradiation. Interestingly, they found that photocatalytic activity is strictly related to the light sources used and to the number of Brønsted/Lewis acid sites present on the surface of the catalysts. Wolski et al. [149] studied the mechanism of methanol photo-oxidation on bimetallic Au-Cu catalyst supported on Nb2O<sup>5</sup> with an in operando IR methodology under both UV and visible light irradiation. Interestingly, they found that photocatalytic activity is strictly related to the light sources used and to the number of Brønsted/Lewis acid sites present on the surface of the catalysts. Specifically, under visible light irradiation, the synergism between gold and copper led to an increase in the amount
Specifically, under visible light irradiation, the synergism between gold and copper led to an increase in the amount of Brønsted/Lewis acid sites on the niobia, with a consequent higher activity of of Brønsted/Lewis acid sites on the niobia, with a consequent higher activity of bimetallic samples compared to that of monometallic and pure Nb2O<sup>5</sup> samples. Furthermore, the total oxidation to CO<sup>2</sup> was favored. By contrast, with UV light irradiation, the major activation of niobia (E<sup>g</sup> ≈ 3.2 eV) favored selective oxidation into dimethoxymethane, formaldehyde, and methyl formate.
In this short chapter, the state-of-the-art of the application of bimetallic structures as chemical modifiers of conventional and unconventional semiconductor photocatalysts was examined. This approach is relatively new, and the effects of alloy synergism on the photocatalytic process are currently under investigation. The promising results, especially obtained by combining the LSPR effect of both noble and transition metals with semiconductor photoactivity, together with a possible multicatalytic strategy (i.e., a photothermo atalytic approach employing a bimetallic/semiconductor catalyst and a solar/visible light source) could in the future be a fascinating strategy to develop a greener and sustainable technology applied to the removal of volatile organic compounds.
| doab | 2025-04-07T03:56:58.816061 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 33
} |
004db1df-dc76-42af-952f-1f5e82b7822b.34 | **4. Conclusions**
In this review, the application of bimetallic catalysts for VOC oxidation was examined in terms of catalytic activity and physicochemical properties. Among the various systems, gold-based bimetallic catalysts exhibited a good performance in the degradation of a wide range of VOCs. The presence of nanosized gold was essential to decreasing the light-off temperature of VOC oxidation, whereas interaction with the second metal allowed increasing the reactivity of the employed support or enhancing oxygen activation. Although platinum-based bimetallic samples usually did not overcome the degradation yields achieved with the monometallic platinum catalysts, they showed a substantial improvement of catalytic stability due to the synergistic effect between platinum and the second noble or transition metal. Finally, the application of the already stated synergisms in catalytic thermo-oxidation, for example, strong Au-Pd interaction, can be successfully transferred to new technologies for VOC abatement, such as photocatalytic oxidation, with the exploitation of new mutual effects such as surface plasmon resonance combined with the high reactivity of noble/transition metals. This can be a promising strategy to achieve significant progress in the technologies applied to the improvement of air quality.
**Funding:** R.F. thanks the PON project "AIM" founded by the European Social Found (ESF) CUP: E66C18001220005 for the financial support.
**Acknowledgments:** This review was written during the Italian quarantine due to COVID-19. I want to dedicate this work to all the people who could not stay at home because they fought against the virus. R.F. thanks S. Scirè and C. Crisafulli who have positively transferred the knowledge and the passion for the research in the field of heterogeneous catalysis focused on environmental applications.
**Conflicts of Interest:** The author declares no conflict of interest.
| doab | 2025-04-07T03:56:58.817081 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 34
} |
004db1df-dc76-42af-952f-1f5e82b7822b.36 | *Article* **Optimized Synthesis Routes of MnOx-ZrO<sup>2</sup> Hybrid Catalysts for Improved Toluene Combustion**
**Xin Huang <sup>1</sup> , Luming Li 2,3,\*, Rong Liu 4,\*, Hongmei Li <sup>2</sup> , Li Lan <sup>5</sup> and Weiqi Zhou <sup>1</sup>**
**Abstract:** In this contribution, the three Mn-Zr catalysts with MnxZr1−xO<sup>2</sup> hybrid phase were synthesized by two-step precipitation route (TP), conventional coprecipitation method (CP) and ball milling process (MP). The components, textural and redox properties of the Mn-Zr hybrid catalysts were studied via XRD, BET, XPS, HR-TEM, H<sup>2</sup> -TPR. Regarding the variation of synthesis routes, the TP and CP routes offer a more obvious advantage in the adjustment of the concentration of MnxZr1−xO<sup>2</sup> solid solution compared to the MP process, which directly commands the content of Mn4+ and oxygen vacancy and lattice oxygen, and thereby leads to the enhanced mobility of reactive oxygen species and catalytic activity for toluene combustion. Moreover, the TP-Mn2Zr3 catalyst with the enriched exposure content of 51.4% for the defective (111) lattice plane of MnxZr1−xO<sup>2</sup> exhibited higher catalytic activity and thermal stability for toluene oxidation than that of the CP-Mn2Zr3 sample with a value of 49.3%. This new observation will provide a new perspective on the design of bimetal catalysts with a higher VOCs combustion abatement.
**Keywords:** Mn-Zr solid solution; toluene; active oxygen; combustion
| doab | 2025-04-07T03:56:58.817255 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 36
} |
004db1df-dc76-42af-952f-1f5e82b7822b.37 | **1. Introduction**
Volatile organic compounds (VOCs) have been considered important harmful pollutants and can be transferred into secondary aerogel and ozone via complex photochemical reactions in the atmosphere, which are threatening the ecological environment and human health [1–4]. It is urgent to adopt effective routes to control VOCs emissions. In particular, the toluene with toxicity and carcinogenicity is one of the most common VOCs, be discharged from the petrochemical industry, and be normally selected as the target VOCs to test the activity of catalyst.
Up to now, several techniques, such as adsorption and absorption, thermal incineration, plasma, membrane separation, biological treatment, and catalytic oxidation, have been widely reported for the degradation of VOCs [5]. Among them, catalytic oxidation has been considered one of the most promising technologies because it can completely convert VOCs under relatively low temperatures (<400 ◦C) into harmless CO<sup>2</sup> and H2O with low energy consumption, which has gained a lot of attention either in scientific and industrial fields. The key issue of catalytic oxidation is to design catalysts with high catalytic activity and low cost [6,7].
Given the active components of the VOCs catalysts, catalysts can be divided into noble metal catalysts and non-noble metal catalysts. There is no doubt that the supported noble metal catalysts have higher activity at lower temperatures. However, the disadvantages of
**Citation:** Huang, X.; Li, L.; Liu, R.; Li, H.; Lan, L.; Zhou, W. Optimized Synthesis Routes of MnOx-ZrO<sup>2</sup> Hybrid Catalysts for Improved Toluene Combustion. *Catalysts* **2021**, *11*, 1037. https://doi.org/10.3390/ catal11091037
Academic Editor: Roberto Fiorenza
Received: 2 August 2021 Accepted: 25 August 2021 Published: 27 August 2021
**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.
**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).
noble metal catalysts also need to be taken into consideration, such as high price, scarce resources, poor thermal stability, and sulfur resistance, etc., which fades their industrial application [8–10]. Therefore, it is necessary to develop alternative catalysts with high catalytic activity. The transition metal oxides have many advantages, such as relatively low price, good activity, abundant content in the earth, and environmental friendliness, which have attracted great interest [11]. For example, manganese-based oxides have been confirmed as one of the most potential alternates to noble metal catalysts and exhibited high catalytic activity due to the high mobility of lattice oxygen and the existence of transformation of unstable valence states, such as Mn4+ <sup>↔</sup> Mn3+ <sup>↔</sup> Mn2+ [12–15]. In addition, the physical and chemical characteristics of the catalysts, such as morphology [8,16], specific surface area [17], oxygen vacancy [18], reducibility, and reactivity of lattice oxygen [19], oxygen migration rate [14], also determine the efficiency of the catalytic oxidation of VOCs. However, it is difficult to efficiently remove certain VOCs for a single phase of MnO<sup>x</sup> due to the poor thermal stability [12,20]. Some studies have reported that the synthesized Mn-M (M = Co [21], Cu [22], Ce [15,23], Zr [24,25]) composite metal oxides can improve catalytic activity towards VOCs abatement through the strong interaction between the parent material and the modifier. Among them, ZrO2-based materials show excellent corrosion resistance, high stability, and ionic conductivity, and the ZrO2-based solid solutions keep high catalytic activity [26]. For instance, MnOx-ZrO<sup>2</sup> bimetal oxides catalysts present a good catalytic activity due to their excellent redox property and thermal stability [27,28].
Choudhary et al. [29] reported that the reactivity of the lattice oxygen of Mn-doped ZrO<sup>2</sup> catalysts depended on the Mn/Zr ratio and calcination temperature; the Mn-doped ZrO<sup>2</sup> (cubic) with the Mn/Zr ratio of 0.25 prepared at a calcination temperature of 600 ◦C processed the best methane combustion activity. Gutie'rrez-Ortiz et al. [24] reported that Mn-Zr mixed oxides exhibited better catalytic activity for 1,2-dichloroethane (DCE) and trichloroethylene (TCE) than that of pure zirconia and manganese oxide, which was attributed to the modification effects of surface acid sites coupled with the readily accessible active oxygen. Zeng et al. [30] successfully manufactured an MnZrO<sup>x</sup> catalyst with a three-dimensional microporous (3DM) structure, which hold a better reducibility and oxygen mobility, accompanied by higher Mn4+/Mn3+ and Oβ/Oα, and the special structure significantly promoted the adsorption of reactant molecules and the mobility of lattice oxygen and propane oxidation. Compared with the B-Mn0.6Zr0.4 catalyst prepared by traditional sol–gel methods, the activation energy of propane combustion on 3DM MnZrO<sup>x</sup> decreased from 156.2 kJ·mol−<sup>1</sup> to 105.0 kJ·mol−<sup>1</sup> . Moreover, Yang et al. synthesized a series of impurity MnxZr1−xO<sup>2</sup> catalysts, and it was found that low-valent manganese (Mn2+) can enter zirconium lattice by substituting Zr4+ and inducing the formation of oxygen vacancy, and its concentration hinges on the doping content, which is conducive to the activation of oxygen on the surface of Mn-doped c-ZrO2(111) crystal plane, thereby improving the catalytic performance for toluene oxidation [31].
Recently, we found that preparation process would play a critical role in adjusting the structural properties of nanocatalysts in oxidation reaction [32,33]. Particularly, the precipitation sequence determined the structural, redox properties, and textural stability of ZrO2-based catalysts [34]. Inspired by those points, we constructed Mn-Zr bimetal catalysts based on the controllable catalytic active centers of MnxZr1−xO<sup>2</sup> solid solution via a modulated precipitation sequence and further evaluated the catalytic capacity of the combustion of toluene model molecules, and the synergistic effect between ZrO<sup>2</sup> and manganese oxide was also analyzed.
| doab | 2025-04-07T03:56:58.817393 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 37
} |
004db1df-dc76-42af-952f-1f5e82b7822b.40 | 2.1.1. XRD Analysis
The XRD patterns of the as-prepared Mn-Zr catalysts with different Mn/Zr ratios are shown in Figure 1. Obviously, the intensity and position of the characteristic peak varies with the Mn/Zr ratio and synthesized routes. The intensity of characteristic peaks
of MnxZr1−xO<sup>2</sup> solid solution was increased as the Mn/Zr ratio decreased, in which the diffraction peaks located at 30.42◦ , 35.28◦ , 50.75◦ , and 60.33◦ correspond to the (111), (200), (220), and (311) crystal planes of MnxZr1−xO<sup>2</sup> solid solution (JCPDS PDF#77-2157), respectively (Figure 1a). On the contrary, the low Mn/Zr ratio resulted in the mixture phase formation, such as ZrO<sup>2</sup> (JCPDS PDF# 78-0048), or a small amount of solid solution, which is ascribed to the difference in ion radius. The ion radius of Mnn+ cations (Mn2+(0.83 Å), Mn3+(0.64 Å) and Mn4+(0.53 Å)) is smaller than Zr4+(0.84 Å). Therefore, it can be speculated that Mn cations readily partially replace Zr cations in the host lattice, whereas it is very difficult [24]. Additionally, the incorporation of Mn dramatically changes the crystal phase of zirconia (Figure 1b). Moreover, the preparation route played an important role in regulating the intensity of characteristic peaks of the MnxZr1−xO<sup>2</sup> solid solution. The two-step precipitation strategy is easier to obtain a high content of active phase solid solution as well as trace amounts of manganese oxide. Besides, it can be found that TP-Mn2Zr3 has more obvious (111) crystal plane characteristic peaks compared to CP-Mn2Zr3.; the content of (111) crystal plane of TP-Mn2Zr3 sample accounted for 51.4%, which is considered the active center [31], while the CP-Mn2Zr3 is ca. 49.3%. This is the reason why the TP-Mn2Zr3 sample has a better catalytic performance. The sample (MP-Mn2Zr3) prepared by mechanical ball mills was as a reference. The result showed that the separated mixture of ZrO<sup>2</sup> and Mn2O<sup>3</sup> oxides caused no synergistic effect between MnO<sup>x</sup> and ZrO2, and a poor catalytic activity for toluene oxidation under the ball milling conditions. (220), and (311) crystal planes of MnxZr1−xO<sup>2</sup> solid solution (JCPDS PDF#77-2157), respectively (Figure 1a). On the contrary, the low Mn/Zr ratio resulted in the mixture phase formation, such as ZrO<sup>2</sup> (JCPDS PDF# 78-0048), or a small amount of solid solution, which is ascribed to the difference in ion radius. The ion radius of Mnn+ cations (Mn2+(0.83 Å ), Mn3+(0.64 Å ) and Mn4+(0.53 Å )) is smaller than Zr4+(0.84 Å ). Therefore, it can be speculated that Mn cations readily partially replace Zr cations in the host lattice, whereas it is very difficult [24]. Additionally, the incorporation of Mn dramatically changes the crystal phase of zirconia (Figure 1b). Moreover, the preparation route played an important role in regulating the intensity of characteristic peaks of the MnxZr1−xO<sup>2</sup> solid solution. The two-step precipitation strategy is easier to obtain a high content of active phase solid solution as well as trace amounts of manganese oxide. Besides, it can be found that TP-Mn2Zr3 has more obvious (111) crystal plane characteristic peaks compared to CP-Mn2Zr3.; the content of (111) crystal plane of TP-Mn2Zr3 sample accounted for 51.4%, which is considered the active center [31], while the CP-Mn2Zr3 is ca. 49.3%. This is the reason why the TP-Mn2Zr3 sample has a better catalytic performance. The sample (MP-Mn2Zr3) prepared by mechanical ball mills was as a reference. The result showed that the separated mixture of ZrO<sup>2</sup> and Mn2O<sup>3</sup> oxides caused no synergistic effect between MnO<sup>x</sup> and ZrO2, and a poor catalytic activity for toluene oxidation under the ball milling conditions.
The XRD patterns of the as-prepared Mn-Zr catalysts with different Mn/Zr ratios are shown in Figure 1. Obviously, the intensity and position of the characteristic peak varies with the Mn/Zr ratio and synthesized routes. The intensity of characteristic peaks of MnxZr1−xO<sup>2</sup> solid solution was increased as the Mn/Zr ratio decreased, in which the diffraction peaks located at 30.42°, 35.28°, 50.75°, and 60.33°correspond to the (111), (200),
*Catalysts* **2021**, *11*, x FOR PEER REVIEW 3 of 15
**2. Results and Discussion** *2.1. Material Characterization*
2.1.1. XRD Analysis
**Figure 1.** XRD patterns of catalysts with different molar ratios of Mn/Zr (**a**) and different preparation routes (**b**) (two-step precipitation (TP); conventional coprecipitation (CP); ball milling process (MP)).
Moreover, the lattice parameters and grain size of CP-Mn2Zr3, TP-Mn2Zr3, MP-Mn2Zr3, ZrO2, and α-Mn2O<sup>3</sup> samples were summarized in Table 1. Compared with MP-Mn2Zr3 and CP-Mn2Zr3, the TP-Mn2Zr3 catalyst has the lowest lattice parameters (5.04 Å) and the smallest grain size (8.4 nm). Indeed, Mn cations with different valence exhibit a different ionic radius. The more Mnn+ cations (Mn2+, Mn3+, and Mn4+) with smaller radii (0.83, 0.64, and 0.53 Å, respectively) incorporated into c-zirconia to replace the Zr4+ (0.84 Å) can reduce the lattice parameters [28,35,36]. Based on this, we can infer that
the improved preparation process is more beneficial to the synthesis of the MnxZr1−XO<sup>2</sup> solid solution and decreases the grain size.
**Table 1.** Data obtained from XRD analyses of two-step precipitation (TP), conventional coprecipitation (CP), ball milling process (MP)-Mn2Zr3, ZrO<sup>2</sup> , and α-Mn2O<sup>3</sup> samples.
<sup>a</sup> The average crystal size was calculated by the Scherrer equation from the XRD data.
| doab | 2025-04-07T03:56:58.817919 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 40
} |
004db1df-dc76-42af-952f-1f5e82b7822b.41 | 2.1.2. BET Analysis
It was accepted that the catalytic activity of nanocatalysts is closely related to their surface texture, such as specific surface area, average pore size, and pore volume. Figure 2 shows the nitrogen adsorption-desorption curves of as-obtained catalysts (TP-Mn2Zr3, CP-Mn2Zr3, and MP-Mn2Zr3), and it can be observed that these catalysts have similar adsorption isotherms, which can be classified as typical type IV adsorption isotherms based on the IUPAC classification [37]. Hysteresis loops demonstrated relative pressure in the range of 0.4–1 in all three samples, and there was no adsorption saturation platform in the range of higher relative pressure, suggesting the catalysts have an irregular pore structure that may be caused by the slit-shaped pores. The type IV isotherm is usually the basis for judging whether there are mesopores in the catalyst materials [8,38]. Considering the presence of type IV adsorption isotherm and H3 hysteresis loop in the catalyst, it can be determined that irregular mesopores were formed in all the catalysts [39]. Figure 2b shows the pore size distribution of the catalyst, which further confirmed the existence of the mesoporous structure in the catalysts. The pore size mainly distributed in the range of 2–40 nm and centered around 10 nm. Specifically, the value of the specific surface area, average pore diameter, and pore volume of the catalyst were listed in Table 2. It was observed that the catalyst prepared by mechanical stirring showed the lowest specific surface area, only 23.7 m2/g, whereas the catalyst doped with Zr obviously demonstrated an improved specific surface area. The specific surface areas of TP-Mn2Zr3 and CP-Mn2Zr3 catalysts were 99.7 and 139.5 m2/g, respectively, which may promote the exposure of active sites on the surface of the catalysts [31] and contribute to the improved catalytic performance. Among these oxides, the average pore sizes of TP-Mn2Zr3, CP-Mn2Zr3, and MP-Mn2Zr3 catalysts are 10.8, 5.9, and 13.5 nm, respectively. It was found that the TP-Mn2Zr3 catalyst has the largest pore volume of 0.27 cm3/g, followed by CP-Mn2Zr3 (0.20 cm3/g) and MP-Mn2Zr3 (0.08 cm3/g) catalysts. *Catalysts* **2021**, *11*, x FOR PEER REVIEW 5 of 15 **Table 2.** The surface area, average pore diameter, and pore volume of TP-Mn2Zr3, CP-Mn2Zr3, and MP-Mn2Zr3 catalysts. **Samples SBET (m<sup>2</sup> /g) Average Pore Diameter (nm) <sup>a</sup> Pore Volume (cm<sup>3</sup> /g) <sup>b</sup>** TP-Mn/Zr = 2/3 99.7 10.8 0.27 CP-Mn/Zr = 2/3 139.5 5.9 0.20 MP-Mn/Zr = 2/3 23.7 13.5 0.08 <sup>a</sup> Bases on the total adsorption average pore width (4V/A by BET, A = SBET). <sup>b</sup> Based on the BJH Adsorption cumulative volume.
**Figure 2.** Nitrogen adsorption-desorption isotherms (**a**) and BJH pore-size distributions (**b**) of TP-Mn2Zr3, CP-Mn2Zr3, and MP-Mn2Zr3 catalysts. **Figure 2.** Nitrogen adsorption-desorption isotherms (**a**) and BJH pore-size distributions (**b**) of TP-Mn2Zr3, CP-Mn2Zr3, and MP-Mn2Zr3 catalysts.
oclinic zirconium dioxide into cubic zirconium dioxide [26].
In order to further study the morphology of the catalysts, TEM experiments were carried out on the CP-Mn2Zr3 and TP-Mn2Zr3 catalysts, as shown in Figure 3. The results suggested that both CP-Mn2Zr3 and TP-Mn2Zr3 exhibit morphology of nanoparticles (Figure 3a,c) and the lattice spacings of ca. 2.54 and 2.94 Å , well attributed to the (111) crystal plane and (200) crystal plane of the MnxZr1−xO<sup>2</sup> solid solution, respectively (Figure 3b,d). This is in line with the XRD results. Therefore, it can be inferred that Mn ions are readily embedded in the lattice of ZrO<sup>2</sup> and form Mn-Zr solid solution via co-precipitation or optimized two-step precipitation route and lead to the phase transformation from mon-
2.1.3. HRTEM Analysis
3g-1, STP )
TP**–**Mn2Zr3 CP**–**Mn2Zr3 MP**–**Mn2Zr3
**(a)**
and MP-Mn2Zr3 catalysts.
**Table 2.** The surface area, average pore diameter, and pore volume of TP-Mn2Zr3, CP-Mn2Zr3, and MP-Mn2Zr3 catalysts.
**Table 2.** The surface area, average pore diameter, and pore volume of TP-Mn2Zr3, CP-Mn2Zr3, and
TP-Mn/Zr = 2/3 99.7 10.8 0.27 CP-Mn/Zr = 2/3 139.5 5.9 0.20 MP-Mn/Zr = 2/3 23.7 13.5 0.08 <sup>a</sup> Bases on the total adsorption average pore width (4V/A by BET, A = SBET). <sup>b</sup> Based on the BJH
**(b)**
**/g) Average Pore Diameter (nm) <sup>a</sup> Pore Volume (cm<sup>3</sup>**
**/g) <sup>b</sup>**
TP**–**Mn2Zr3 CP**–**Mn2Zr3 MP**–**Mn2Zr3
*Catalysts* **2021**, *11*, x FOR PEER REVIEW 5 of 15
dV/dW (cm
3g-1 nm-1
)
<sup>a</sup> Bases on the total adsorption average pore width (4V/A by BET, A = SBET). <sup>b</sup> Based on the BJH Adsorption cumulative volume.
### 2.1.3. HRTEM Analysis 2.1.3. HRTEM Analysis In order to further study the morphology of the catalysts, TEM experiments were
MP-Mn2Zr3 catalysts.
**Samples SBET (m<sup>2</sup>**
Adsorption cumulative volume.
In order to further study the morphology of the catalysts, TEM experiments were carried out on the CP-Mn2Zr3 and TP-Mn2Zr3 catalysts, as shown in Figure 3. The results suggested that both CP-Mn2Zr3 and TP-Mn2Zr3 exhibit morphology of nanoparticles (Figure 3a,c) and the lattice spacings of ca. 2.54 and 2.94 Å, well attributed to the (111) crystal plane and (200) crystal plane of the MnxZr1−xO<sup>2</sup> solid solution, respectively (Figure 3b,d). This is in line with the XRD results. Therefore, it can be inferred that Mn ions are readily embedded in the lattice of ZrO<sup>2</sup> and form Mn-Zr solid solution via coprecipitation or optimized two-step precipitation route and lead to the phase transformation from monoclinic zirconium dioxide into cubic zirconium dioxide [26]. carried out on the CP-Mn2Zr3 and TP-Mn2Zr3 catalysts, as shown in Figure 3. The results suggested that both CP-Mn2Zr3 and TP-Mn2Zr3 exhibit morphology of nanoparticles (Figure 3a,c) and the lattice spacings of ca. 2.54 and 2.94 Å , well attributed to the (111) crystal plane and (200) crystal plane of the MnxZr1−xO<sup>2</sup> solid solution, respectively (Figure 3b,d). This is in line with the XRD results. Therefore, it can be inferred that Mn ions are readily embedded in the lattice of ZrO<sup>2</sup> and form Mn-Zr solid solution via co-precipitation or optimized two-step precipitation route and lead to the phase transformation from monoclinic zirconium dioxide into cubic zirconium dioxide [26].
**Figure 3.** High resolution transmission electron microscope (HRTEM) images of CP-Mn2Zr3 (**a**,**b**) and TP-Mn2Zr3 (**c**,**d**). **Figure 3.** High resolution transmission electron microscope (HRTEM) images of CP-Mn2Zr3 (**a**,**b**) and TP-Mn2Zr3 (**c**,**d**).
### 2.1.4. XPS Analysis 2.1.4. XPS Analysis
**(a)**
Mn2Zr3.
Intensity(a.u.)
MP-Mn2Zr3
CP-Mn2Zr3
TP-Mn2Zr3
Intensity(a.u.)
XPS experiments were performed to analyze the components and valence states of elements, as shown in Figure 4 and Table 3. It was observed that the surface elements of the catalyst are mainly composed of O (ca. 530 eV), Zr (ca. 183), and Mn (ca. 642 eV) (Figure 4a) [30,40]. Figure 4b shows the XPS spectra of Mn 2p; it was integrated into two char-XPS experiments were performed to analyze the components and valence states of elements, as shown in Figure 4 and Table 3. It was observed that the surface elements of the catalyst are mainly composed of O (ca. 530 eV), Zr (ca. 183 eV), and Mn (ca. 642 eV) (Figure 4a) [30,40]. Figure 4b shows the XPS spectra of Mn 2p; it was integrated into
acteristic peaks by deconvolution. The characteristic peaks located at 641.7 eV and 643.4
**(b)**
**(d)**
TP-Mn2Zr3
CP-Mn2Zr3
MP-Mn2Zr3
**Figure 4.** XPS spectra of (**a**) full spectra, (**b**) Mn 2p, (**c**) O 1s, (**d**) Zr 3d over TP, CP, and MP-
Intensity(a.u.)
Binding energy(ev) <sup>190</sup> <sup>188</sup> <sup>186</sup> <sup>184</sup> <sup>182</sup> <sup>180</sup> <sup>178</sup> <sup>176</sup>
Zr3s
1000 800 600 400 200 0
(529.7)
Binding energy(ev)
**(c)** O1s <sup>O</sup><sup>α</sup>
O<sup>β</sup> (531.0)
540 538 536 534 532 530 528 526
Zr3pC1s Zr3d O1s Mn2p O KLL
Mn Aguer peak
eV can be attributed to Mn3+ and Mn4+ species, respectively [1,17,41].
TP-Mn2Zr3 CP-Mn2Zr3 MP-Mn2Zr3
Mn3s
Mn3p
Intensity(a.u.)
645 640
Zr4+(184.3)
Mn4+(643.3) Mn3+(641.7)
Binding Energy(ev)
Binding energy(ev)
Mn 2p
TP-Mn2Zr3
CP-Mn2Zr3
MP-Mn2Zr3
Zr4+(181.9) Zr 3d
two characteristic peaks by deconvolution. The characteristic peaks located at 641.7 eV and 643.4 eV can be attributed to Mn3+ and Mn4+ species, respectively [1,17,41]. acteristic peaks by deconvolution. The characteristic peaks located at 641.7 eV and 643.4 eV can be attributed to Mn3+ and Mn4+ species, respectively [1,17,41].
XPS experiments were performed to analyze the components and valence states of elements, as shown in Figure 4 and Table 3. It was observed that the surface elements of the catalyst are mainly composed of O (ca. 530 eV), Zr (ca. 183), and Mn (ca. 642 eV) (Figure 4a) [30,40]. Figure 4b shows the XPS spectra of Mn 2p; it was integrated into two char-
**Figure 3.** High resolution transmission electron microscope (HRTEM) images of CP-Mn2Zr3 (**a**,**b**)
*Catalysts* **2021**, *11*, x FOR PEER REVIEW 6 of 15
and TP-Mn2Zr3 (**c**,**d**).
2.1.4. XPS Analysis
**Figure 4.** XPS spectra of (**a**) full spectra, (**b**) Mn 2p, (**c**) O 1s, (**d**) Zr 3d over TP, CP, and MP-**Figure 4.** XPS spectra of (**a**) full spectra, (**b**) Mn 2p, (**c**) O 1s, (**d**) Zr 3d over TP, CP, and MP-Mn2Zr3.
Mn2Zr3. **Table 3.** Analysis of the surface composition and content of samples based on XPS results.
Furthermore, the ratio of Mn4+/Mn3+ of the catalyst calculated based on the relative area was closely related to the preparation routes. The TP-Mn2Zr3 catalyst prepared by an improved precipitation method had a higher Mn4+/Mn3+ value, which reached 0.68, which followed the MP-Mn2Zr3 (0.53) and CP-Mn2Zr3 catalysts (0.50), respectively. The strong interaction between Mn and Zr in Mn-Zr solid solution can promote the occurrence of the charge transfer process and produce more Mn4+ [30], which plays a key role in the catalytic degradation of toluene. In other words, the stronger interaction between Mn and Zr in the catalyst of TP-Mn2Zr3 can be conducive to higher catalytic oxidation of toluene. Meanwhile, it is widely accepted that the oxygen mobility of manganese-based oxides is closely related to the transformation ability of manganese species between different valence states, and the increase of the concentration of high valence metal cations promotes the chemical potential and mobility of oxygen [42].
O 1s XPS spectra of the three catalysts were exhibited in Figure 4c, which can be split into two peaks by deconvolution, the bind-energy located at 529.0−530.0 eV and 531.0−532.0 eV can be ascribed to the surface lattice oxygen (Oα) and chemisorbed oxygen and/or defect oxides (O2−, O<sup>2</sup> <sup>2</sup>−, or O−) (Oβ) [28], respectively. It was noted that the content of O<sup>α</sup> is more than 60% for the three samples, suggesting the O<sup>α</sup> is dominated. Furthermore, the decreasing trend of
Oβ/O<sup>α</sup> value is as follows: TP-Mn2Zr3(0.65) > MP-Mn2Zr3(0.63) > CP-Mn2Zr3 (0.59), which is positively correlated with the value of Mn4+/Mn3+(TP-Mn2Zr3 (0.68) > MP-Mn2Zr3(0.53) > CP-Mn2Zr3 (0.50)). It was reported that both lattice oxygen and chemisorbed oxygen and/or defect oxides are active oxygen species that can participate in the oxidation of toluene, which improves the oxidation performance of the catalyst [19,35]. Herein, the optimized coprecipitation catalyst displays better catalytic activity due to the contribution of lattice oxygen and absorbed oxygen.
The spin-splitting peaks of the Zr 3d orbit can be observed in Figure 4d, centered at 181.9 eV and 184.3 eV and belong to 3d5/2 and 3d3/2 orbit, respectively, suggested that Zr cations exist in the catalyst in the form of tetravalent [30]. Meanwhile, it was found that there was hardly a change in the position of the Zr 3D orbital splitting peak in TP-Mn2Zr3, CP-Mn2Zr3, and MP-Mn2Zr3 samples. It was worth noting that the XPS signal peak of Zr in TP-Mn2Zr3 and CP-Mn2Zr3 samples originated from the Mn-Zr solid solution, while the signal peak of Zr in the MP-Mn2Zr3 sample was derived from pure zirconia. This indicates that the Zr4+ ion is very stable [43], though the Mnn+ ions were entered the framework of ZrO2. Moreover, the proportion of Mn, Zr, and O in the catalyst is related to the preparation routes (Table 3). The order of the proportion of Mn elements follows: MP-Mn2Zr3 > CP-Mn2Zr3 > TP-Mn2Zr3, while that of Zr and O is the opposite; this suggested that the concentration of Mn-Zr solid solution various with the content of Mn ions entered the framework of ZrO2.
| doab | 2025-04-07T03:56:58.818198 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 41
} |
004db1df-dc76-42af-952f-1f5e82b7822b.42 | 2.1.5. H2-TPR Results Analysis
The H2-TPR experiments were employed to investigate the reducibility of the catalysts. The H2-TPR cures of TP-Mn2Zr3, CP-Mn2Zr3, and MP-Mn2Zr3 samples in the range of 100–700 ◦C are shown in Figure 5. There exist three obvious characteristic peaks of hydrogen consumption. It was reported that about 15 wt.% of MnO is soluble in ZrO<sup>2</sup> due to the solidified eutectic temperature [44]. Herein, the three characteristic peaks of catalysts were mainly attributed to the reduction of MnOx, in which Peak 1 located below 250 ◦C shows the reduction of amorphous MnOx, dispersed on the surface of Mn-Zr solid solution, and peak 2 (250–380 ◦C) should be linked to the reduction of Mn2O<sup>3</sup> to Mn3O4, while peak 3 (380–480 ◦C) corresponds to the reduction of Mn3O<sup>4</sup> to MnO, respectively [26,31,45]. Moreover, the TP-Mn2Zr3 and CP-Mn2Zr3 catalysts behave an excellent low-temperature reduction performance compared to that of MP-Mn2Zr3. Especially for the TP-Mn2Zr3 sample, the locations of reduction peaks are separately reduced to 224, 303, and 400 ◦C, suggesting it possessed a stronger mobility of reactive oxygen species and leads to a better catalytic activity for VOCs abatement [31]. *Catalysts* **2021**, *11*, x FOR PEER REVIEW 8 of 15 °C shows the reduction of amorphous MnOx, dispersed on the surface of Mn-Zr solid solution, and peak 2 (250–380 °C) should be linked to the reduction of Mn2O<sup>3</sup> to Mn3O4, while peak 3 (380–480 °C) corresponds to the reduction of Mn3O<sup>4</sup> to MnO, respectively [26,31,45]. Moreover, the TP-Mn2Zr3 and CP-Mn2Zr3 catalysts behave an excellent lowtemperature reduction performance compared to that of MP-Mn2Zr3. Especially for the TP-Mn2Zr3 sample, the locations of reduction peaks are separately reduced to 224, 303, and 400 °C, suggesting it possessed a stronger mobility of reactive oxygen species and leads to a better catalytic activity for VOCs abatement [31].
**Figure 5.** H2-TPR profiles of TP-Mn2Zr3, CP-Mn2Zr3, and MP-Mn2Zr3 catalysts. **Figure 5.** H<sup>2</sup> -TPR profiles of TP-Mn2Zr3, CP-Mn2Zr3, and MP-Mn2Zr3 catalysts.
It was found that the reduction temperature of pure zirconia usually appears above 600 °C, and the incorporation of Mn into the framework of zirconia to form a solid solution of MnxZr1−xO<sup>2</sup> would contribute to the decreasing of reduction temperature [26,29]. In this It was found that the reduction temperature of pure zirconia usually appears above 600 ◦C, and the incorporation of Mn into the framework of zirconia to form a solid solution of MnxZr1−xO<sup>2</sup> would contribute to the decreasing of reduction temperature [26,29]. In
contribution, the reduction peak of zirconia was not observed on the MP-Mn2Zr3 catalyst,
Mn2Zr3 and CP-Mn2Zr3 catalysts, which may be the result of the synergistic interaction between the surface MnO<sup>x</sup> and ZrO2 alloyed to form Mn-Zr solid solution [31,46], which
The catalytic oxidation performance of the synthesized catalysts for toluene abatement was assessed. The functional cures between the conversion of toluene on the catalysts and reaction temperature are depicted in Figure 6. All the catalysts can achieve complete catalytic oxidation for toluene below 300 °C. As shown in Figure 6a, the CP-Mn2Zr3 catalyst exhibits better catalytic oxidation activity for toluene. The values of T<sup>50</sup> (the 50% conversion of 1000 ppm toluene) and T<sup>90</sup> (the 90% conversion of 1000 ppm toluene) of the CP-Mn2Zr3 catalyst are 270 °C and 278 °C, respectively. In addition, the order of catalytic ability of catalysts is depicted as CP-Mn2Zr3 > CP-Mn3Zr1 > CP-Mn3Zr2 > CP-Mn1Zr1 > CP-Mn1Zr3. It is evident that the catalytic activity was closely linked with the Mn/Zr ratio, which controlled the component of the Mn-Zr solid solution. Thereby, it can be inferred that the Mn-Zr solid solution is in the active phase and greatly influences the catalytic
well corresponds to the XRD results.
*2.2. Evaluation of Catalytic Activity*
activity for toluene.
this contribution, the reduction peak of zirconia was not observed on the MP-Mn2Zr3 catalyst, indicating that MnxZr1−xO<sup>2</sup> species cannot be formed by the ball milling process. In contrast, the shouldered temperature reduction peak (ca. 400 ◦C) was displayed in both TP-Mn2Zr3 and CP-Mn2Zr3 catalysts, which may be the result of the synergistic interaction between the surface MnO<sup>x</sup> and ZrO<sup>2</sup> alloyed to form Mn-Zr solid solution [31,46], which well corresponds to the XRD results.
| doab | 2025-04-07T03:56:58.818792 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 42
} |
004db1df-dc76-42af-952f-1f5e82b7822b.43 | *2.2. Evaluation of Catalytic Activity*
The catalytic oxidation performance of the synthesized catalysts for toluene abatement was assessed. The functional cures between the conversion of toluene on the catalysts and reaction temperature are depicted in Figure 6. All the catalysts can achieve complete catalytic oxidation for toluene below 300 ◦C. As shown in Figure 6a, the CP-Mn2Zr3 catalyst exhibits better catalytic oxidation activity for toluene. The values of T<sup>50</sup> (the 50% conversion of 1000 ppm toluene) and T<sup>90</sup> (the 90% conversion of 1000 ppm toluene) of the CP-Mn2Zr3 catalyst are 270 ◦C and 278 ◦C, respectively. In addition, the order of catalytic ability of catalysts is depicted as CP-Mn2Zr3 > CP-Mn3Zr1 > CP-Mn3Zr2 > CP-Mn1Zr1 > CP-Mn1Zr3. It is evident that the catalytic activity was closely linked with the Mn/Zr ratio, which controlled the component of the Mn-Zr solid solution. Thereby, it can be inferred that the Mn-Zr solid solution is in the active phase and greatly influences the catalytic activity for toluene. *Catalysts* **2021**, *11*, x FOR PEER REVIEW 9 of 15
**Figure 6.** Activity test of catalysts with different molar ratios of Mn/Zr (**a**) and prepared by different strategies (**b**). **Figure 6.** Activity test of catalysts with different molar ratios of Mn/Zr (**a**) and prepared by different strategies (**b**).
Moreover, the catalytic activity assessing experiments were also carried out on CP-Mn2Zr3, TP-Mn2Zr3, and MP-Mn2Zr3 catalysts, and the results are depicted in Figure 6b. Obviously, the TP-Mn2Zr3 catalyst exhibits a better catalytic performance with T<sup>50</sup> and T<sup>90</sup> values of 263 °C and 269 °C, respectively, followed by CP-Mn2Zr3. The performance of the catalyst with MP-Mn2Zr3 is the lowest, with T<sup>50</sup> and T<sup>90</sup> values of 275 °C and 287 °C, which is much lower than the TP-Mn2Zr3 and CP-Mn2Zr3 catalysts. The results clearly show that the TP-Mn2Zr3 and CP-Mn2Zr3 catalysts with the existence of the Mn-Zr solid solution exhibit higher catalytic activity for toluene abatement than that of the MP-Mn2Zr3 catalyst without the formation of the Mn-Zr solution. The outstanding catalytic performance of the TP-Mn2Zr3 catalyst for toluene should be attributed to the more ex-Moreover, the catalytic activity assessing experiments were also carried out on CP-Mn2Zr3, TP-Mn2Zr3, and MP-Mn2Zr3 catalysts, and the results are depicted in Figure 6b. Obviously, the TP-Mn2Zr3 catalyst exhibits a better catalytic performance with T<sup>50</sup> and T<sup>90</sup> values of 263 ◦C and 269 ◦C, respectively, followed by CP-Mn2Zr3. The performance of the catalyst with MP-Mn2Zr3 is the lowest, with T<sup>50</sup> and T<sup>90</sup> values of 275 ◦C and 287 ◦C, which is much lower than the TP-Mn2Zr3 and CP-Mn2Zr3 catalysts. The results clearly show that the TP-Mn2Zr3 and CP-Mn2Zr3 catalysts with the existence of the Mn-Zr solid solution exhibit higher catalytic activity for toluene abatement than that of the MP-Mn2Zr3 catalyst without the formation of the Mn-Zr solution. The outstanding catalytic performance of the TP-Mn2Zr3 catalyst for toluene should be attributed to the more exposed defect (111) crystal plane of MnxZr1−xO<sup>2</sup> and the improved capacity of mobility of active oxygen.
posed defect (111) crystal plane of MnxZr1−xO<sup>2</sup> and the improved capacity of mobility of active oxygen. The stability of the texture for the Mn-Zr catalysts will determine their potential application to some extent. In this work, five consecutive catalytic light-off cycle tests were performed on the TP-Mn2Zr3 and CP-Mn2Zr3 catalysts, and the results are shown in Figure 7. The value of the T<sup>90</sup> value of the cycle 2 experiment for the TP-Mn2Zr3 catalyst is 259 °C, which is lower than that of the cycle 1 experiment (269 °C), and runs 3, 4, and 5 on the TP-Mn2Zr3 catalyst maintain a similar value of T<sup>90</sup> (259 °C ± 1). On the contrary, the The stability of the texture for the Mn-Zr catalysts will determine their potential application to some extent. In this work, five consecutive catalytic light-off cycle tests were performed on the TP-Mn2Zr3 and CP-Mn2Zr3 catalysts, and the results are shown in Figure 7. The value of the T<sup>90</sup> value of the cycle 2 experiment for the TP-Mn2Zr3 catalyst is 259 ◦C, which is lower than that of the cycle 1 experiment (269 ◦C), and runs 3, 4, and 5 on the TP-Mn2Zr3 catalyst maintain a similar value of T<sup>90</sup> (259 ◦C ± 1). On the contrary, the CP-Mn2Zr3 catalyst performed inferior cyclic stability, in which the value of T<sup>90</sup> for cycles 1, 2, 3, 4, and 5 was 278, 269, 264, 264, and 269 ◦C, respectively. The enhanced
CP-Mn2Zr3 catalyst performed inferior cyclic stability, in which the value of T<sup>90</sup> for cycles
the activation effect at 300 °C [8]. To view the cyclic stability directly, the function curves between cycle times and toluene conversion at 265 °C are depicted as the inset of Figure 7a,b, which imply that the TP-Mn2Zr3 catalyst exhibited better cycle stability. Conversely, the order of cyclic stability of CP-Mn2Zr3 obeys the volcanic model, which implies the
inferior stability of CP-Mn2Zr3.
100
**(a)**
oxidation performance for both catalysts after running the first experiment should be attributed to the activation effect at 300 ◦C [8]. To view the cyclic stability directly, the function curves between cycle times and toluene conversion at 265 ◦C are depicted as the inset of Figure 7a,b, which imply that the TP-Mn2Zr3 catalyst exhibited better cycle stability. Conversely, the order of cyclic stability of CP-Mn2Zr3 obeys the volcanic model, which implies the inferior stability of CP-Mn2Zr3. *Catalysts* **2021**, *11*, x FOR PEER REVIEW 10 of 15 *Catalysts* **2021**, *11*, x FOR PEER REVIEW 10 of 15 100 99.1885 99.42363 **(b)**
**Figure 7.** Activity cycle test of the TP-Mn2Zr3 catalyst (**a**) and the CP-Mn2Zr3 catalyst (**b**). **Figure 7.** Activity cycle test of the TP-Mn2Zr3 catalyst (**a**) and the CP-Mn2Zr3 catalyst (**b**). Based on studies reports, the bimetal oxide catalysts involve two reaction mechanisms in the oxidation for toluene, namely the Langmuir–Hinshelwood (L-H) and Mars–
Based on studies reports, the bimetal oxide catalysts involve two reaction mechanisms in the oxidation for toluene, namely the Langmuir–Hinshelwood (L-H) and Mars– van Krevelen (Mv-K) mechanisms. Increasing the temperature, the L-H mechanism is gradually weakened, and the Mv-K mechanism gradually occupies a dominant position. The adsorbed oxygen directly oxidizes the adsorbed organic molecules, which is accorded to the L-H mechanism; on the other hand, lattice oxygen is activated at a higher temperature, and the consumed lattice oxygen can be readily supplemented from gaseous oxygen, Based on studies reports, the bimetal oxide catalysts involve two reaction mechanisms in the oxidation for toluene, namely the Langmuir–Hinshelwood (L-H) and Mars–van Krevelen (Mv-K) mechanisms. Increasing the temperature, the L-H mechanism is gradually weakened, and the Mv-K mechanism gradually occupies a dominant position. The adsorbed oxygen directly oxidizes the adsorbed organic molecules, which is accorded to the L-H mechanism; on the other hand, lattice oxygen is activated at a higher temperature, and the consumed lattice oxygen can be readily supplemented from gaseous oxygen, which obeys the Mv-K mechanism [42,47]. van Krevelen (Mv-K) mechanisms. Increasing the temperature, the L-H mechanism is gradually weakened, and the Mv-K mechanism gradually occupies a dominant position. The adsorbed oxygen directly oxidizes the adsorbed organic molecules, which is accorded to the L-H mechanism; on the other hand, lattice oxygen is activated at a higher temperature, and the consumed lattice oxygen can be readily supplemented from gaseous oxygen, which obeys the Mv-K mechanism [42,47]. Figure 8a reveals the effect of different WHSVs on the activity of the TP-Mn2Zr3 catalyst. Obviously, the value of T<sup>90</sup> is positively correlated with WHSV; in other words, a
which obeys the Mv-K mechanism [42,47]. Figure 8a reveals the effect of different WHSVs on the activity of the TP-Mn2Zr3 catalyst. Obviously, the value of T<sup>90</sup> is positively correlated with WHSV; in other words, a longer contact time is beneficial to improve the catalytic performance. The longevity experiments for 3000 min were performed on TP-Mn2Zr3 and CP-Mn2Zr3 catalysts, as shown in Figure 8b. It was worth noting that the TP-Mn2Zr3 catalysts maintained a splendid catalytic activity (>90% toluene conversion). While the CP-Mn2Zr3 catalyst declined a lot after 1000 min and the value of toluene degradation was sharply decreased to about Figure 8a reveals the effect of different WHSVs on the activity of the TP-Mn2Zr3 catalyst. Obviously, the value of T<sup>90</sup> is positively correlated with WHSV; in other words, a longer contact time is beneficial to improve the catalytic performance. The longevity experiments for 3000 min were performed on TP-Mn2Zr3 and CP-Mn2Zr3 catalysts, as shown in Figure 8b. It was worth noting that the TP-Mn2Zr3 catalysts maintained a splendid catalytic activity (>90% toluene conversion). While the CP-Mn2Zr3 catalyst declined a lot after 1000 min and the value of toluene degradation was sharply decreased to about 50%, the conversion of toluene was only kept at ca. 33% after the 3000 min combustion reaction. longer contact time is beneficial to improve the catalytic performance. The longevity experiments for 3000 min were performed on TP-Mn2Zr3 and CP-Mn2Zr3 catalysts, as shown in Figure 8b. It was worth noting that the TP-Mn2Zr3 catalysts maintained a splendid catalytic activity (>90% toluene conversion). While the CP-Mn2Zr3 catalyst declined a lot after 1000 min and the value of toluene degradation was sharply decreased to about 50%, the conversion of toluene was only kept at ca. 33% after the 3000 min combustion reaction.
160 180 200 220 240 260 280 Temperature(℃) 0 1000 2000 3000 0 Reaction(min) **Figure 8.** The effect of different WHSVs on the activity of the TP-Mn2Zr3 catalyst (**a**); Longevity test over TP-Mn2Zr3 and CP-Mn2Zr3 in 1000 ppm toluene (**b**). **Figure 8.** The effect of different WHSVs on the activity of the TP-Mn2Zr3 catalyst (**a**); Longevity test over TP-Mn2Zr3 and CP-Mn2Zr3 in 1000 ppm toluene (**b**).
CP-Mn2Zr3 in 1000 ppm toluene (**b**).
0
| doab | 2025-04-07T03:56:58.819036 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 43
} |
004db1df-dc76-42af-952f-1f5e82b7822b.44 | **3. Discussion**
It was widely accepted that the physical and chemical properties, such as component of active phase, specific surface area, Mn valence, concentration of adsorbed oxygen/lattice oxygen and reduction ability, can command the activity of the catalyst [48]. For the component factor of the active phase, we found that the ratio of Mn/Zr and their synthesis routes play a vital role in adjusting the content of active centers, and a low molar ratio (<2/3) of Mn/Zr would lead to the formation of a solid solution of MnxZr1−xO2. As the molar ratio goes up, a second phase, such as Mn2O<sup>3</sup> and Mn3O4, will be formed, as depicted in Figure 1a. This is mainly attributed to the solubility limit of manganese in the zirconia lattice [44]. However, a more complex mixed components does not necessarily result in a better catalytic activity, as the CP-Mn2Zr3 catalyst presented a better catalytic performance among the samples prepared by the co-precipitation process (Figure 6a). Moreover, the improved co-precipitation route can further promote the exposure of the active centers and enhance the catalytic performance. Therefore, it can be inferred that the optimized catalytic activity is linked to components of the active phase and mainly hinges on the ratio of Mn/Zr and their synthesis routes. On the other hand, co-precipitation and improved co-precipitation routes are ready to prepare catalysts with a large specific surface area and enhance their catalytic activities.
In addition, the concentrations of Mn4+ and O<sup>α</sup> species of oxidizing catalyst were crucial for the oxidative degradation of VOCs [43,49]. The order of the Oβ/O<sup>α</sup> follows in descending order as Mn4+/Mn3+, which was depicted as TP-Mn2Zr3 > MP-Mn2Zr3 > CP-Mn2Zr3, indicating the concentration of Mn4+ was positively correlated with the content of oxygen vacancy [40]. In combination with the results of catalytic activity and components of active phase, the improved catalytic activity should be attributed to the promoted effects of the solid solution of MnxZr1−xO2. Moreover, the shouldered shape of the reduction peak for TP-Mn2Zr3 and CP-Mn2Zr3 catalysts can also account for the formation of MnxZr1−xO<sup>2</sup> active species, which corresponds well with the results of XRD.
It was reported that the catalytic mechanism of toluene on Mn-based catalysts involves the Mv-K and L-H mechanism. Both lattice oxygen and adsorbed oxygen species can participate in the activation-oxidation process of VOCs [42,47,50]. In this paper, the enhanced catalytic activity of Mn-Zr catalysts may be ascribed to the collective effects of manganese zirconium bimetal because the formation of the solid solution of MnxZr1−xO<sup>2</sup> would be conducive to decrease the formation energy of oxygen vacancy; the oxygen molecule was absorbed and activated to active oxygen to enhance catalytic activity. Furthermore, the oxygen molecule is easily adsorbed on the defective (111) surface of the Mn-Zr catalyst with oxygen vacancy, and the distance of O–O bond was elongated, which probably suggested that the absorbed O<sup>2</sup> is easily activated and broken, which is in favor of the quick replenishment of consumed oxygen molecular during VOCs combustion [31]. This is in good agreement with the H2-TPR results. Therefore, it is supposed that the enriched exposure of the defective (111) surface of the MnxZr1−xO<sup>2</sup> active species would lead to better catalytic activity for toluene combustion, and this can account for the outstanding performance of TP-Mn2Zr3.
| doab | 2025-04-07T03:56:58.819500 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 44
} |
004db1df-dc76-42af-952f-1f5e82b7822b.46 | *4.1. Materials*
Zirconium (IV) oxynitrate hydrate (ZrO(NO3)2·H2O) and Manganese nitrate (Mn(NO3)2) solution 50 wt.% were purchased from Chengdu Hua Xia Chemical Reagent Co., Ltd. (Chengdu, China). Ammonia (NH3·H2O) (25~28%) reagent was purchased from Chengdu Ke Long chemicals Co., Ltd (Chengdu, China). All the reagents are an analytical reagent (AR), with no need for further treatment.
| doab | 2025-04-07T03:56:58.819756 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 46
} |
004db1df-dc76-42af-952f-1f5e82b7822b.47 | *4.2. Catalyst Preparation*
Firstly, Mn-Zr hybrids catalysts with different ratios were prepared by conventional optimized co-precipitation routes [24,25]. In a typical preparation, manganese nitrate
solution (50 wt.%) (3.58, 7.16, 7.16, 10.74, and 10.74 g, respectively) and zirconium (IV) oxynitrate hydrate (7.48, 7.48, 4.99, 4.99, and 2.49 g, respectively) were added into 100 mL deionized water and vigorously stirred for 2 h to form homogeneous solution, respectively. Then ammonia was added dropwise until PH = 9 (Scheme 1a). The obtained precipitates are filtrated and washed until pH = 7. After that, the precipitate was dried in an oven at 90 ◦C for 5 h, followed by calcination at 550 ◦C for 5 h under an air atmosphere. Herein the ratios of Mn/Zr were listed as 1/3, 2/3, 1/1, 3/2, and 3/1, marked as CP = Mn1Zr3, CP = Mn2Zr3, CP = Mn1Zr1, CP = Mn3Zr2, and CP = Mn3Zr1, respectively. lution (50 wt.%) (3.58, 7.16, 7.16, 10.74, and 10.74 g, respectively) and zirconium (IV) oxynitrate hydrate (7.48, 7.48, 4.99, 4.99, and 2.49 g, respectively) were added into 100 mL deionized water and vigorously stirred for 2 h to form homogeneous solution, respectively. Then ammonia was added dropwise until PH = 9 (Scheme 1a). The obtained precipitates are filtrated and washed until pH = 7. After that, the precipitate was dried in an oven at 90 °C for 5 h, followed by calcination at 550 °C for 5 h under an air atmosphere. Herein the ratios of Mn/Zr were listed as 1/3, 2/3, 1/1, 3/2, and 3/1, marked as CP = Mn1Zr3, CP = Mn2Zr3, CP = Mn1Zr1, CP = Mn3Zr2, and CP = Mn3Zr1, respectively.
*Catalysts* **2021**, *11*, x FOR PEER REVIEW 12 of 15
Firstly, Mn-Zr hybrids catalysts with different ratios were prepared by conventional optimized co-precipitation routes [24,25]. In a typical preparation, manganese nitrate so-
*4.2. Catalyst Preparation*
**Scheme 1.** The synthesis of catalyst by coprecipitation (**a**), and optimized coprecipitation (**b**). **Scheme 1.** The synthesis of catalyst by coprecipitation (**a**), and optimized coprecipitation (**b**).
The TP-Mn2Zr3 catalysts were prepared by two-step precipitation strategies. Specifically, manganese nitrate solution (50 wt.%) (7.16 g) and zirconium (IV) oxynitrate hydrate (7.48 g) were added into 50 mL deionized water and vigorously stirred for 2 h to form a homogeneous solution, respectively. Then ammonia was added dropwise into the two solutions until pH = 9 (Scheme 1b). We mixed the two solutions under vigorous stirring for 2 h. The washing, filtering, drying and calcining processes were the same procedures for CP-Mn2Zr3. Additionally, the final product is recorded as TP-Mn2Zr3. For comparison, MnO<sup>x</sup> and ZrO<sup>2</sup> catalysts were prepared by the above routes. Besides, the MP-Mn2Zr3 sample was prepared by mechanical ball mill of a mixture of MnO<sup>x</sup> and ZrO2. The ball milling experiment was carried out on a planetary ball mill (XQM-0.4). Actually, the stainless-steel balls with different diameters (15, 12, 10, 8, and 5 mm) were mixed and added into a ball milling tank (50 mL) and then stirred for 2 h at 400 r/min. In addition, the total mass of stainless-steel balls was 150 g, and the mass of Mn2O<sup>3</sup> and ZrO<sup>2</sup> was 1.84 g and 2.16 g, respectively, to keep the molar ratio of Mn/Zr at 2/3. The TP-Mn2Zr3 catalysts were prepared by two-step precipitation strategies. Specifically, manganese nitrate solution (50 wt.%) (7.16 g) and zirconium (IV) oxynitrate hydrate (7.48 g) were added into 50 mL deionized water and vigorously stirred for 2 h to form a homogeneous solution, respectively. Then ammonia was added dropwise into the two solutions until pH = 9 (Scheme 1b). We mixed the two solutions under vigorous stirring for 2 h. The washing, filtering, drying and calcining processes were the same procedures for CP-Mn2Zr3. Additionally, the final product is recorded as TP-Mn2Zr3. For comparison, MnO<sup>x</sup> and ZrO<sup>2</sup> catalysts were prepared by the above routes. Besides, the MP-Mn2Zr3 sample was prepared by mechanical ball mill of a mixture of MnO<sup>x</sup> and ZrO2. The ball milling experiment was carried out on a planetary ball mill (XQM-0.4). Actually, the stainless-steel balls with different diameters (15, 12, 10, 8, and 5 mm) were mixed and added into a ball milling tank (50 mL) and then stirred for 2 h at 400 r/min. In addition, the total mass of stainless-steel balls was 150 g, and the mass of Mn2O<sup>3</sup> and ZrO<sup>2</sup> was 1.84 g and 2.16 g, respectively, to keep the molar ratio of Mn/Zr at 2/3.
### *4.3. Catalyst Activity Evaluation 4.3. Catalyst Activity Evaluation*
The catalytic degradation of toluene was assessed in a fixed-bed stainless reactor (id = 7 mm). Prior to the activity test, the catalyst (0.20 g, 40–60 mesh) and quartz sands (0.30 g, 40–60 mesh) were well mixed and then loaded into the center of the reactor and activated at 300 °C for 1 h with a 60 mL/min air flow rate. After that, the temperature was dropped to 150 °C, and 1000 ppm toluene gas was continuously fed into the fixed-bed reactor; the total flow is controlled at 50 mL/min to correspond to a weight hourly space velocity (WHSV) of 15,000 mL gcat−<sup>1</sup> h−<sup>1</sup> . In addition, the catalytic activity of TP-Mn2Zr3 was tested under different WHSVs. The exhausted gas from a fixed-bed reactor was analyzed using a gas chromatography device (SP-7890 PLUS, Rui Hong Co, Ltd., Tengzhou, China) with a flame ionization detector (FID) and equipped polyethylene glycol capillary column and a thermal conductivity detector (TCD). The activity of the catalyst was measured by the toluene conversion (), which can be calculated as follow: The catalytic degradation of toluene was assessed in a fixed-bed stainless reactor (id = 7 mm). Prior to the activity test, the catalyst (0.20 g, 40–60 mesh) and quartz sands (0.30 g, 40–60 mesh) were well mixed and then loaded into the center of the reactor and activated at 300 ◦C for 1 h with a 60 mL/min air flow rate. After that, the temperature was dropped to 150 ◦C, and 1000 ppm toluene gas was continuously fed into the fixedbed reactor; the total flow is controlled at 50 mL/min to correspond to a weight hourly space velocity (WHSV) of 15,000 mL gcat <sup>−</sup><sup>1</sup> h −1 . In addition, the catalytic activity of TP-Mn2Zr3 was tested under different WHSVs. The exhausted gas from a fixed-bed reactor was analyzed using a gas chromatography device (SP-7890 PLUS, Rui Hong Co, Ltd., Tengzhou, China) with a flame ionization detector (FID) and equipped polyethylene glycol capillary column and a thermal conductivity detector (TCD). The activity of the catalyst was measured by the toluene conversion (*Xcon*), which can be calculated as follow:
$$X\_{\rm out} = \frac{\mathbb{C}\_{\rm in} - \mathbb{C}\_{\rm out}}{\mathbb{C}\_{\rm in}} \tag{1}$$
(1)
where *Cin* stand for the inlet toluene concentration, and *Cout* represent the outlet toluene concentration after 30 min reaction.
| doab | 2025-04-07T03:56:58.819804 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 47
} |
004db1df-dc76-42af-952f-1f5e82b7822b.48 | *4.4. Catalyst Characterization*
The investigation of the phase composition of the catalysts was carried out by the Rigaku DX-2700 (Rigaku, Tokyo, Japan) diffractometer equipment with Cu-Kα (λ = 0.154 nm) as the radiation source. The scan started from 10 to 80◦ with a scanning rate of 0.06◦/s. The results of the specific surface area and pore size distribution of the catalysts were obtained by nitrogen adsorption and desorption experiments on V-SorbX800 (Jin Aipu, Beijing, China) equipment at −196 ◦C. The microstructure of the fresh catalysts was investigated by an FEI Tecnai G2 F20 (GCEMarket, Blackwood, NJ, America) Transmission electron microscopy (TEM) equipped with a HAADF detector. The TEM experiment was carried out in an accelerated voltage environment of 2000 volts. The X-ray photoelectron spectroscopy was obtained by the PHI 5000 (Ulvac-PHI, Inc., Kanagawa, Japan) spectrometer with Al Kα as the radiation. The obtained binding energy was calibrated with C1s (284.8 eV) as the internal reference standard. AutoChem2920 (Micromeritics Instrument Corp, Norcross, GA, USA) chemisorption analyzer was used to analyze the reduction performance of the fresh catalysts. Before the reduction process, 30 mg prepared catalyst was pretreated at 300 ◦C for 1 h in an N<sup>2</sup> atmosphere (30 mL/min) to remove oxygen.
| doab | 2025-04-07T03:56:58.820106 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 48
} |
004db1df-dc76-42af-952f-1f5e82b7822b.49 | **5. Conclusions**
In this paper, a series of Mn-Zr catalysts with different Mn/Zr ratios were successfully prepared via co-precipitation and improved co-precipitation routes, and their catalytic performance for toluene combustion was evaluated. It was found that the TP-Mn2Zr3 catalyst possesses the lowest T<sup>50</sup> and T<sup>90</sup> temperature at 263 and 269 ◦C and exhibits practical cycle stability. Moreover, the relationship between the catalyst performance and texture was deeply investigated via some characterization techniques, including XRD, HRTEM, BET, H2-TPR, and XPS. The results showed that the doping of Mn enables the crystal structure of ZrO<sup>2</sup> to transform from the monoclinic to the cubic phase, exposes the stable c-ZrO<sup>2</sup> (111) phase, and increases the specific surface area of Mn-Zr bimetal catalysts via the co-precipitation strategy. In particular, the optimized co-precipitation process enriches more exposure of the defective (111) surface of the MnxZr1-xO<sup>2</sup> solid solution to active oxygen molecular and more oxygen vacancy as well as the higher concentration of Mn4+, which is conducive to the mobility of oxygen to improve their catalytic activity for toluene combustion. This new observation will provide a promising strategy to design excellent catalysts for VOCs abatement.
**Author Contributions:** X.H.: completing the experiments and writing—original manuscript. L.L. (Luming Li): project administration, providing the experiment ideas, and writing—review. R.L., H.L., L.L. (Li Lan) and W.Z.: analysis and agree to take responsibility for the accuracy and authenticity of the research work. All authors have read and agreed to the published version of the manuscript.
**Funding:** This research was supported by the Open Fund of the Key Laboratory of Coarse Cereal Processing, Ministry of Agriculture and Rural Affairs, Sichuan Engineering and Technology Research Center of Coarse Cereal Industrialization (2020CC020).
**Data Availability Statement:** The data of this research is available within the manuscript.
**Acknowledgments:** We are grateful to the experimental platform provided by the Institute of Environmental Catalysis and Dust Treatment of Chengdu University. Thanks to the TEM characterization provided from Xiamen University and the XPS analyzed provided by Institute for Advanced Study of Chengdu University.
**Conflicts of Interest:** The authors declare no conflict of interest.
| doab | 2025-04-07T03:56:58.820229 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 49
} |
004db1df-dc76-42af-952f-1f5e82b7822b.51 | *Article* **VOCs Photothermo-Catalytic Removal on MnOx-ZrO<sup>2</sup> Catalysts**
**Roberto Fiorenza \* , Roberta Agata Farina, Enrica Maria Malannata, Francesca Lo Presti and Stefano Andrea Balsamo**
> Dipartimento di Scienze Chimiche, Università di Catania, Viale A. Doria 6, 95125 Catania, Italy; [email protected] (R.A.F.); [email protected] (E.M.M.); [email protected] (F.L.P.); [email protected] (S.A.B.)
**\*** Correspondence: [email protected]; Tel.: +39-0957385012
**Abstract:** Solar photothermo-catalysis is a fascinating multi-catalytic approach for volatile organic compounds (VOCs) removal. In this work, we have explored the performance and the chemicophysical features of non-critical, noble, metal-free MnOx-ZrO<sup>2</sup> mixed oxides. The structural, morphological, and optical characterizations of these materials pointed to as a low amount of ZrO<sup>2</sup> favoured a good interaction and the ionic exchange between the Mn and the Zr ions. This favoured the redox properties of MnOx increasing the mobility of its oxygens that can participate in the VOCs oxidation through a Mars-van Krevelen mechanism. The further application of solar irradiation sped up the oxidation reactions promoting the VOCs total oxidation to CO<sup>2</sup> . The MnOx-5 wt.%ZrO<sup>2</sup> sample showed, in the photothermo-catalytic tests, a toluene T<sup>90</sup> (temperature of 90% of conversion) of 180 ◦C and an ethanol T<sup>90</sup> conversion to CO<sup>2</sup> of 156 ◦C, 36 ◦C, and 205 ◦C lower compared to the thermocatalytic tests, respectively. Finally, the same sample exhibited 84% toluene conversion and the best selectivity to CO<sup>2</sup> in the ethanol removal after 5 h of solar irradiation at room temperature, a photoactivity similar to the most employed TiO<sup>2</sup> -based materials. The as-synthetized mixed oxide is promising for an improved sustainability in both catalyst design and environmental applications.
**Keywords:** VOC; photothermo catalysis; toluene; ethanol; manganese oxide; zirconium oxide
| doab | 2025-04-07T03:56:58.820422 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 51
} |
004db1df-dc76-42af-952f-1f5e82b7822b.52 | **1. Introduction**
Nowadays, the quality of air, both in indoor and outdoor environments, is an extremely important concern. Furthermore, the COVID-19 emergency has pointed to the necessity of clean air to discourage virus infection. Among the air pollutants, volatile organic compounds (VOCs) include many of the most dangerous substances for both human health and the environment. Different strategies were employed to remove VOCs from the air, and an innovative and sustainable solution is represented by solar photocatalytic or photothermo-catalytic oxidation [1,2]. Compared to the most used catalytic or non-catalytic VOCs combustion, the photocatalytic process allows one to exploit solar irradiation with green advantages to work at milder conditions using renewable energy [2]. However, the performance of the photocatalysts is much lower compared to the catalysts employed for the thermocatalytic removal of VOCs [3], and for these reasons, the multi-catalytic approach of the photothermo-catalysis is a fascinating way to obtain high VOCs removal values of thermocatalysis but at lower temperatures, increasing, at the same time, the energy efficiency of the process. To design performing photothermo catalysts, different properties are required [4,5]. Analogously to photocatalysis, it is necessary to have a semiconductor material that, after solar irradiation, is able to generate photoelectrons and photoholes in its conduction (CB) and valence (VB) bands, respectively. It should have redox properties activated with the temperature; in this way, the superficial/mobile oxygens of the catalyst or of the support can participate in the oxidation of VOCs increasing the overall activity [3,6]. Finally, the photothermo catalysts should be resistant to both long-time irradiation and heating. The preparation of mixed oxides or composites is the
**Citation:** Fiorenza, R.; Farina, R.A.; Malannata, E.M.; Lo Presti, F.; Balsamo, S.A. VOCs Photothermo-Catalytic Removal on MnOx-ZrO<sup>2</sup> Catalysts. *Catalysts* **2022**, *12*, 85. https://doi.org/10.3390/catal 12010085
Academic Editor: Xintong Zhang
Received: 27 December 2021 Accepted: 12 January 2022 Published: 13 January 2022
**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.
**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).
best and easiest way to combine all of these features. Indeed, with the formation of a suitable heterojunction, it is possible to exploit solar irradiation, decreasing the bandgap (Eg) of the main semiconductor oxide, profiting from both the photocatalytic activity of the principal oxides and the thermocatalytic activity of the hosted oxide. Moreover, the introduction of host ions in the lattice of the main oxide allows one to create defects and oxygen vacancies that favour VOCs oxidation [7,8].
The TiO2-CeO<sup>2</sup> composites showed promising performance in the photo-thermal approach for both VOCs removal and CO<sup>2</sup> reduction [3,9]; however, one of the side effects of the current pandemic situation is the crisis of raw materials exportation, and as a consequence, in 2020, titanium featured in the EU critical raw materials list [10]. Considering that up until now, TiO<sup>2</sup> is the most studied and applied semiconductor, both in academia and in industrial research focused on photocatalytic applications, the exploration of unconventional non-critical (photo)catalysts is highly required.
In this work, we have investigated the photothermo-catalytic properties of MnOx-ZrO<sup>2</sup> mixed oxides, with the aim of finding new and sustainable alternatives to the most common TiO2-based photocatalysts, and without the addition of noble metal co-catalysts, usually used in the catalytic and photocatalytic removal of VOCs [11], to obtain even more environmentally friendly catalysts, in the end.
Manganese oxide exists in four stable forms (MnO, MnO2, Mn3O<sup>4</sup> and Mn2O3), and all of them own a semiconductor electronic structure characterized by the partially filled d orbitals which permit the electronic d-d transitions under UV or visible light irradiation [12]. Based on the preparation method, it is common to obtain a non-stoichiometric oxide or a mixture of different MnO<sup>x</sup> oxides with the +II, +III and/or +IV oxidation states. The high mobility/reducibility of manganese oxide lattice oxygens is particularly useful for VOCs removal [13,14], whereas the redox properties of MnO<sup>x</sup> can be particularly advantageous for the photothermo-catalytic oxidation of VOCs, as well as its low bandgap (in the range 2.0–3.5 eV depending of the crystalline structure [12,15]) that can allow a more efficient use of solar radiation.
Zirconium oxide (zirconia) was largely used as a support of several noble metalbased catalysts used for the thermocatalytic oxidation of VOCs, due to its high stability, thermal resistance, and ionic conductivity [16,17]. Furthermore, it is a large bandgap semiconductor (E<sup>g</sup> of about 5.0 eV [18] or lower depending to the zirconia synthesis). Therefore, its coupling with a lower bandgap semiconductor (as MnOx) can be a performing and fascinating strategy to reduce the odds of charge recombination (a common reason for photocatalysts deactivation) and to synergistically exploit both the thermal stability and the redox properties of MnO<sup>x</sup> and ZrO<sup>2</sup> [19,20] together with their photocatalytic features.
We have also determined the chemico-physical and the photocatalytic, thermocatalytic and photothermo-catalytic activities of MnOx-ZrO<sup>2</sup> oxides in the oxidation of toluene and ethanol, chosen as VOCs models, due to the high toxicity nature of toluene and to the wide use of ethanol as a solvent in many industrial processes and as an octane booster in combustion engines, whose incomplete oxidation can give the emission of dangerous compounds, as acetaldehyde, in the environment [21].
| doab | 2025-04-07T03:56:58.820862 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 52
} |
004db1df-dc76-42af-952f-1f5e82b7822b.54 | *2.1. Structural, Morphological, Textural and Optical Properties of the Samples*
The XRD patterns of the analysed samples are shown in Figure 1. The precipitation of manganese chloride (II) with NaOH and the employed calcination temperature (600 ◦C for 2 h) allowed to obtain the Mn3O4.The signals at 2θ = 18.1◦ , 28.9◦ , 31.0◦ , 32.4◦ , 36.0◦ , 38.1◦ , 44.3◦ and 50.8◦ are, indeed, in accordance with the PDF card. No.: 00-080-0382 of pure Mn3O<sup>4</sup> (Hausmannite). Bare ZrO<sup>2</sup> was obtained with the ammonia-driven precipitation of zirconyl nitrate. The signals fitted with the PDF card No. 00-079-1771 of zirconium oxide, with the typical diffraction peaks at 2θ = 30.2◦ , 35.2◦ and 50.3◦ . Interestingly, the co-precipitation with NaOH of both metals salt precursors created substantial changes in the crystalline structure of manganese oxide. The addition of 5 wt.% of zirconium oxide
led to a mixed Mn2O3/Mn3O<sup>4</sup> phase being present the diffraction peak at 2θ = 32.9◦ ; that is, the typical fingerprint of Mn2O<sup>3</sup> (PDF card No. 00-071-0636, [12,22]), together with the signals at 2θ = 38.2◦ (overlapped with the signal of Mn3O4) and 55.1◦ that are also ascribed to manganese (III) oxide [12,22]. The increase of ZrO<sup>2</sup> content (MnOx-10%ZrO<sup>2</sup> sample) restored the main presence of Mn3O<sup>4</sup> with a trace of Mn2O3. In both the mixed oxides, the signals related to ZrO<sup>2</sup> are absent, probably due to the low amount of hosted oxide and/or to the good dispersion of zirconium oxide on manganese oxide. substantial changes in the crystalline structure of manganese oxide. The addition of 5 wt.% of zirconium oxide led to a mixed Mn2O3/Mn3O4 phase being present the diffraction peak at 2θ = 32.9°; that is, the typical fingerprint of Mn2O3 (PDF card No. 00-071-0636, [12,22]), together with the signals at 2θ = 38.2° (overlapped with the signal of Mn3O4) and 55.1° that are also ascribed to manganese (III) oxide [12,22]. The increase of ZrO2 content (MnOx-10%ZrO2 sample) restored the main presence of Mn3O4 with a trace of Mn2O3. In both the mixed oxides, the signals related to ZrO2 are absent, probably due to the low amount of hosted oxide and/or to the good dispersion of zirconium oxide on manganese oxide.
precipitation of zirconyl nitrate. The signals fitted with the PDF card No. 00-079-1771 of zirconium oxide, with the typical diffraction peaks at 2θ = 30.2°, 35.2° and 50.3°. Interestingly, the co-precipitation with NaOH of both metals salt precursors created
*Catalysts* **2022**, *12*, x FOR PEER REVIEW 3 of 17
**Figure 1.** X-ray Diffraction (XRD) patterns of the examined samples. **Figure 1.** X-ray Diffraction (XRD) patterns of the examined samples.
The ion radius of Zr4+ (0.84 Å) is similar of Mn2+(0.83 Å), and this can favour the ionic exchange between these cations [20,23]. On the contrary, the smaller radius of Mn3+ (0.64 Å) makes the Zr4+/Mn3+ exchange more difficult. Probably, when the amount of ZrO2 is low, the Zrions partially replace the Mn2+ promoting, in this way, the main presence of Mn3+, whereas a higher amount of ZrO2 led to a preferential surface covering of the MnOx instead of a lattice incorporation of the zirconium ions in MnOx [20,23]. Thus, it can explain the major presence of Mn (III) on MnOx-5%ZrO2 sample, and the coexistence of Mn II and III in the MnOx-10%ZrO2. The main crystalline size of the samples (Table 1) was determined by applying the Scherrer formula on the principal diffraction peaks of the oxides (2θ =36.0° for Mn3O4, 30.2° for ZrO2, 32.9° for MnOx-5% ZrO2, whereas for MnOx-10% ZrO2, the value was mediated considering both the 2θ = 32.4° and 36.0° signals). The addition of zirconium oxide led to a slight increase of the crystalline size of manganese oxide, whereas the bare ZrO2 showed the lowest crystalline size (8 nm). However, this latter oxide, in accordance with the surface area values reported in the literature [18], showed the lowest surface area (Table 1, 26.2 m2/g), whereas the bare Mn3O4 exhibited the highest BET surface area (99.6 m2/g). Compared to the bare Mn3O4, the slight increase of the crystallite size of the mixed oxides determined a decrease of their surface area, which were about 85–86 m2/g for both the MnOx-ZrO2 samples (Table 1). The ion radius of Zr4+ (0.84 Å) is similar of Mn2+(0.83 Å), and this can favour the ionic exchange between these cations [20,23]. On the contrary, the smaller radius of Mn3+ (0.64 Å) makes the Zr4+/Mn3+ exchange more difficult. Probably, when the amount of ZrO<sup>2</sup> is low, the Zr ions partially replace the Mn2+ promoting, in this way, the main presence of Mn3+, whereas a higher amount of ZrO<sup>2</sup> led to a preferential surface covering of the MnO<sup>x</sup> instead of a lattice incorporation of the zirconium ions in MnO<sup>x</sup> [20,23]. Thus, it can explain the major presence of Mn (III) on MnOx-5%ZrO<sup>2</sup> sample, and the coexistence of Mn II and III in the MnOx-10%ZrO2. The main crystalline size of the samples (Table 1) was determined by applying the Scherrer formula on the principal diffraction peaks of the oxides (2θ = 36.0◦ for Mn3O4, 30.2◦ for ZrO2, 32.9◦ for MnOx-5% ZrO2, whereas for MnOx-10% ZrO2, the value was mediated considering both the 2θ = 32.4◦ and 36.0◦ signals). The addition of zirconium oxide led to a slight increase of the crystalline size of manganese oxide, whereas the bare ZrO<sup>2</sup> showed the lowest crystalline size (8 nm). However, this latter oxide, in accordance with the surface area values reported in the literature [18], showed the lowest surface area (Table 1, 26.2 m2/g), whereas the bare Mn3O<sup>4</sup> exhibited the highest BET surface area (99.6 m2/g). Compared to the bare Mn3O4, the slight increase of the crystallite size of the mixed oxides determined a decrease of their surface area, which were about 85–86 m2/g for both the MnOx-ZrO<sup>2</sup> samples (Table 1).
**Table 1.** Structural, textural and optical properties of the examined samples.
**Table 1.** Structural, textural and optical properties of the examined samples. **Sample Crystallite Size (nm) a BET Surface Area**
*Catalysts* **2022**, *12*, x FOR PEER REVIEW 4 of 17
<sup>a</sup> Estimated by XRD.
The SEM-EDX measurements (Figure 2) were performed to evaluate the presence of zirconium oxide on MnOx. The adopted precipitation methods led to, indifferently to the investigated samples, a non-homogenous morphology with spherical particles (Figures 2a and S1). From the EDX elemental analysis (Figure 2b,c, Table 2), it is possible to note that a little surface segregation of zirconium in the MnOx-10%ZrO<sup>2</sup> sample was detected, whose zirconium wt.% was 3.7 times higher (instead of twice as expected considering the nominal concentration) compared to the MnOx-5%ZrO2. In accordance with the XRD data, the increase of the amount of ZrO<sup>2</sup> led to an enrichment of zirconium oxide on the surface of MnOx, whereas in the MnOx-5%ZrO<sup>2</sup> mixed oxide, the ZrO<sup>2</sup> was mainly embedded in the lattice of MnOx. The SEM-EDX measurements (Figure 2) were performed to evaluate the presence of zirconium oxide on MnOx. The adopted precipitation methods led to, indifferently to the investigated samples, a non-homogenous morphology with spherical particles (Figures 2a and S1). From the EDX elemental analysis (Figure 2b,c, Table 2), it is possible to note that a little surface segregation of zirconium in the MnOx-10%ZrO2 sample was detected, whose zirconium wt.% was 3.7 times higher (instead of twice as expected considering the nominal concentration) compared to the MnOx-5%ZrO2. In accordance with the XRD data, the increase of the amount of ZrO2 led to an enrichment of zirconium oxide on the surface of MnOx, whereas in the MnOx-5%ZrO2 mixed oxide, the ZrO2 was mainly embedded in the lattice of MnOx.
**Figure 2.** (**a**) SEM image of the MnOx-5%ZrO2 as representative sample; Energy Dispersive X-ray (EDX) spectra of MnOx-10%ZrO2 (**b**) and MnOx-5%ZrO2 (**c**). The EDX spectra were mediated considering four different zones of the samples. **Figure 2.** (**a**) SEM image of the MnOx-5%ZrO<sup>2</sup> as representative sample; Energy Dispersive X-ray (EDX) spectra of MnOx-10%ZrO<sup>2</sup> (**b**) and MnOx-5%ZrO<sup>2</sup> (**c**). The EDX spectra were mediated considering four different zones of the samples.
**Table 2.** EDX elemental analysis of the examined samples. The presence of carbon was due to the carbon tape used to perform the measurements.
The surface valence state of the components of the catalysts were analysed through X-ray photoelectron spectroscopy (XPS) (Table 3). Interestingly, on the surface of MnOxbased samples, the ratio of the Mn3+/Mn2+ ions obtained considering the area of the deconvoluted spectra (see Figure S2, spectra of MnOx-5%ZrO<sup>2</sup> as representative sample) was the highest for the MnOx-5%ZrO<sup>2</sup> mixed oxide. As pointed to also by the structural properties determined by XRD, in this sample, a strong ionic interaction between the Mn2+ and the Zn4+ was particularly favoured, leading to an increase in the amount of Mn3+ ions. Moreover, in this sample, the ratio between the surface lattice oxygen (Oα) located at about 530 eV and the chemisorbed/defective oxygen (Oβ) at 532 eV was also the highest (Table 3, Figure S2), suggesting that the ionic exchange between the zirconium and the manganese ions also promoted a higher concentration of the manganese oxide surface oxygens. These, as reported, can participate in VOCs oxidation, improving the catalytic activity of the catalysts [24]. Finally, the binding energy of the Zr 3d5/2 at about 182.0 eV is the typical fingerprint of ZrO<sup>2</sup> [25]. The surface atomic percentage of Zr was 3.5 higher (2.77%) on the MnOx-10%ZrO<sup>2</sup> compared to MnOx-5%ZrO<sup>2</sup> (0.73%) confirming, as too stated by the EDX analysis, the surface covering of zirconia on manganese oxide, verified increasing the amount of ZrO2.
**Table 3.** XPS analysis and binding energy (BE in eV) of the components of the investigated samples.
The UV-DRS of the samples were reported in the Figure 3. The MnOx-based samples (Figure 3a) showed a remarkable lower reflectance compared to the bare ZrO<sup>2</sup> (Figure 3b), and thus can be highlighted considering also the colours of the as-synthesized powders (dark brown for the MnOx-based materials and white for the zirconium oxide). The optical bandgaps of the semiconductor oxides were estimated plotting the modified Kubelka– Munk function versus hν, as reported in the literature ([26], inset Figure 3b as representative sample). Interestingly, as established by XRD, the good crystallinity of ZrO<sup>2</sup> and its nanosize (8 nm, Table 1) allowed us to obtain a ZrO<sup>2</sup> with a lower bandgap (3.02 eV) compared to the other E<sup>g</sup> reported in the literature for this oxide (about 5.0 eV that, however, can be narrowed down to 2–1.5 eV on the basis of the adopted preparation method [18,27]). No substantial variations were observed comparing the unmodified Mn3O<sup>4</sup> and the MnOx-ZrO<sup>2</sup> based-oxides, with an E<sup>g</sup> of 3.26–3.29 eV (Table 1). Probably, the low amount and the good dispersion of ZrO<sup>2</sup> on MnO<sup>x</sup> did not alter the bandgap of the manganese oxide. All the manganese oxide-based samples exhibited a similar E<sup>g</sup> to TiO<sup>2</sup> (3.0–3.2 eV on the basis of the crystalline form [28]); thus, they can exploit the UV-A portion of solar irradiation.
portion of solar irradiation.
**Figure 3.** (**a**) UV-DRS (Diffuse Reflectance Spectroscopy) of the MnOx-based samples; (**b**) UV-DRS spectra of bare ZrO2. In the inset; the estimation of the optical bandgap through the modified Kubelka–Munk function. **Figure 3.** (**a**) UV-DRS (Diffuse Reflectance Spectroscopy) of the MnOx-based samples; (**b**) UV-DRS spectra of bare ZrO<sup>2</sup> . In the inset; the estimation of the optical bandgap through the modified Kubelka–Munk function.
TiO2 (3.0–3.2 eV on the basis of the crystalline form [28]); thus, they can exploit the UV-A
### *2.2. Photocatalytic, Thermocatalytic and Photothermo-Catalytic Removal of Toluene in Gas 2.2. Photocatalytic, Thermocatalytic and Photothermo-Catalytic Removal of Toluene in Gas Phase*
*Phase* Figure 4a shows the solar photocatalytic activity in the oxidation of toluene at room temperature after 5 h of irradiation. The highest conversion value was obtained with the MnOx-5% ZrO2 (84%) followed by the MnOx-10% ZrO2 and the bare Mn3O4 (51% and 47%, respectively), whereas pure ZrO2 exhibited the lowest conversion value (33%). In accordance with the literature [3,29], in our experimental condition, the only detected by-products were carbon dioxide and water with traces of benzaldehyde (selectivity in the range 1–3%). Although a real comparison with the other reported data for this reaction is very difficult, due to the various experimental conditions adopted by the other research groups (Table 4), the performance of MnOx-5% ZrO2 mixed oxide is very promising, being similar to (considering the initial concentration of 1000 ppm of toluene) or slightly lower than the most used TiO2-based photocatalysts, or to other unconventional semiconductors (Table 4). Figure 4a shows the solar photocatalytic activity in the oxidation of toluene at room temperature after 5 h of irradiation. The highest conversion value was obtained with the MnOx-5% ZrO<sup>2</sup> (84%) followed by the MnOx-10% ZrO<sup>2</sup> and the bare Mn3O<sup>4</sup> (51% and 47%, respectively), whereas pure ZrO<sup>2</sup> exhibited the lowest conversion value (33%). In accordance with the literature [3,29], in our experimental condition, the only detected by-products were carbon dioxide and water with traces of benzaldehyde (selectivity in the range 1–3%). Although a real comparison with the other reported data for this reaction is very difficult, due to the various experimental conditions adopted by the other research groups (Table 4), the performance of MnOx-5% ZrO<sup>2</sup> mixed oxide is very promising, being similar to (considering the initial concentration of 1000 ppm of toluene) or slightly lower than the most used TiO2-based photocatalysts, or to other unconventional semiconductors (Table 4).
**Table 4.** Data comparison of the photocatalytic oxidation of toluene.
**Figure 4.** (**a**) Solar photocatalytic oxidation of toluene after 5h of irradiation; (**b**) effect of the wt.% of ZrO2 on MnOx in the solar photocatalytic toluene conversion; (**c**) thermocatalytic oxidation of toluene; (**d**) photothermo-catalytic oxidation of toluene on the investigated materials. **Figure 4.** (**a**) Solar photocatalytic oxidation of toluene after 5h of irradiation; (**b**) effect of the wt.% of ZrO<sup>2</sup> on MnO<sup>x</sup> in the solar photocatalytic toluene conversion; (**c**) thermocatalytic oxidation of toluene; (**d**) photothermo-catalytic oxidation of toluene on the investigated materials.
**Table 4.** Data comparison of the photocatalytic oxidation of toluene. **Catalysts Experimental Conditions Toluene Conversion Ref.** MnOx-5%ZrO2 1000 ppm Toluene, 5 h irradiation solar lamp (300 W, 10.7 mW/cm2), room T, 150 mg catalyst 84% this work Brookite TiO2-5% CeO2 1000 ppm Toluene, 2 h irradiation solar lamp (300 25% [3] It is worth noting that 5 wt.% of zirconia was the best amount to obtain a synergistic positive effect on the MnOx. Indeed, the samples prepared with the same procedures reported in the par. 4.1 but adding the 3 wt.% and 15 wt.% of zirconium oxide caused a decrease of activity (66% of toluene conversion for MnOx-3% ZrO<sup>2</sup> and 48 % for MnOx-15% ZrO2, i.e., the same conversion value of the bare manganese oxide). The results pointed to a photocatalytic "volcano" trend (Figure 4b). The positive effects of the addition of ZrO<sup>2</sup> on MnO<sup>x</sup> reached the maximum with 5% of zirconia, following a progressive decrease at higher amounts. This can be reasonably due, as confirmed by XRD, SEM-EDX and XPS measurements, to a progressive surface coverage of MnOx, due to the presence of a large amount of ZrO2. This caused a decrease in the photoactivity considering also the lower photocatalytic performance of bare ZrO<sup>2</sup> compared to manganese oxide (Figure 4a). The
detection of a specific amount of the hosted oxide on the main oxide is a typical trend of the mixed oxide-based semiconductors. A large amount of the second component (hosted oxide) can cover the surface active sites of the main oxide, decreasing the overall photocatalytic activity of the photocatalyst [35,36].
Thermal catalytic combustion is the most used process to increase the removal efficiency of toluene. The thermocatalytic activity of the investigated samples is reported in the Figure 4c. Moreover, for this catalytic approach, MnOx-5% ZrO<sup>2</sup> gave the best results. The T<sup>90</sup> (the temperature at which the 90% of toluene conversion was reached) values were 216 ◦C, 231 ◦C, 240 ◦C and 383 ◦C for MnOx-5% ZrO2, MnOx-10% ZrO2, Mn3O4, and the bare ZrO<sup>2</sup> respectively, confirming the order of activity measured in the photocatalytic tests at room temperature.
To further decrease toluene T90, the solar photothermo-catalytic tests were employed on the same investigated samples (Figure 4d). The solar-assisted thermo catalytic approach allowed one to decrease the T<sup>90</sup> of 36 ◦C (180 ◦C) with respect to thermocatalytic tests on the best mixed oxide, the MnOx-5% ZrO<sup>2</sup> catalyst, and in general, a decrease of T<sup>90</sup> was verified for all the catalysts, with even the same order of activity: MnOx-5% ZrO<sup>2</sup> > MnOx-10% ZrO<sup>2</sup> (T<sup>90</sup> = 217 ◦C) > Mn3O<sup>4</sup> (T<sup>90</sup> = 226 ◦C) > ZnO<sup>2</sup> (T<sup>90</sup> = 245 ◦C). Interestingly, the highest T<sup>90</sup> decrease was verified with the bare zirconium oxide (138 ◦C lower compared to the thermocatalytic toluene T90) where the activation of the zirconia photocatalytic properties was fundamental to promote the toluene total oxidation.
The positive synergistic effect due to the addition of a small amount of zirconia on the MnO<sup>x</sup> and the solar multi-catalytic approach led to obtaining a low toluene T90, considering the absence of noble metals co-catalysts. The obtained value of T<sup>90</sup> with the MnOx-5% ZrO<sup>2</sup> sample (180 ◦C) is comparable or lower with respect to the other MnOxbased catalysts reported in the literature (in the range 200 ◦C–270 ◦C considering an initial toluene concentration of 1000 ppm [20,37]).
The influence of the gas hourly space velocity (GSHV) was reported in the Figure S3a considering the best sample (MnOx-5% ZrO2). We have chosen, for all the tests, a GSHV of 8 <sup>×</sup> <sup>10</sup><sup>4</sup> mL/gcat·h, indeed, as expected, and as reported in the literature [38], with a high flow rate; the conversion rate of toluene to CO<sup>2</sup> and water (the only by-products detected also in all the thermo and photothermo-catalytic tests) was slower, whereas a GSHV < 8 <sup>×</sup> <sup>10</sup><sup>4</sup> mL/gcat·h did not substantially modify the conversion rate.
| doab | 2025-04-07T03:56:58.821416 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 54
} |
004db1df-dc76-42af-952f-1f5e82b7822b.55 | *2.3. Photocatalytic, Thermocatalytic and Photothermo-Catalytic Removal of Ethanol in Gas Phase*
The ethanol being an alcohol was more reactive than the aromatic toluene, but its oxidation can give various by-products; the most common in the gas phase oxidation was acetaldehyde [3,31,39], which is also the main by-product detected in all the investigated catalytic approaches here discussed, whereas a very low selectivity (<2%) was detected in CO, formic acid, and acetic acid. In the solar photocatalytic tests, MnOx-5% ZrO<sup>2</sup> confirmed its highest activity compared to the other samples (Figure 5) with an ethanol conversion of 98% and the highest selectivity to CO<sup>2</sup> (43%), the most important feature for the VOCs removal. The mixed oxide with the 10 wt.% of ZrO<sup>2</sup> showed a little decrease of photoactivity (ethanol conversion of 86%) and a higher selectivity to acetaldehyde (60%) with respect to CO<sup>2</sup> (36%). These data, in line with the photo-oxidation of toluene (Figure 4a), pointed to, in our experimental conditions, the 5 wt.% being the optimal amount of zirconia to have a synergistic effect with MnOx. Among the bare oxides, the manganese oxide showed a higher ethanol conversion, with a higher selectivity to CO<sup>2</sup> compared to ZrO2. This latter oxide promoted the partial oxidation to acetaldehyde (selectivity of 74%) and consequently exhibited the lowest selectivity to CO<sup>2</sup> (22%).
**Figure 5. Figure 5.** Solar photocatalytic oxidation of ethanol after 5 h of irradiation. Solar photocatalytic oxidation of ethanol after 5 h of irradiation.
In Table 5 and Figure S4, the data of the thermocatalytic oxidation of ethanol are reported. MnOx-5%ZrO<sup>2</sup> showed, also for this VOC, the best performance, with the lowest T<sup>90</sup> (189 ◦C) and a maximum conversion to acetaldehyde of 66% at 176 ◦C. Moreover, MnOx-10%ZrO<sup>2</sup> and Mn3O<sup>4</sup> showed the same maximum conversion to acetaldehyde, but at a higher temperature (200 ◦C for MnOx-10%ZrO<sup>2</sup> and 226 ◦C for bare manganese oxide). Consequently, MnOx-5%ZrO<sup>2</sup> also exhibited the lowest T<sup>90</sup> related to the conversion to CO<sup>2</sup> (361 ◦C). It is verified also for this approach, a little negative effect of the increased amount of zirconium oxide on the MnOx, with higher T<sup>90</sup> of MnOx-10%ZrO<sup>2</sup> compared to MnOx-5%ZrO2. The highest conversion to acetaldehyde was obtained with the bare zirconia (maximum conversion of 98% at 409 ◦C) confirming, as also detected in the photocatalytic tests at room temperature, the tendency of this catalyst to promote the partial oxidation of ethanol instead of the total combustion.
the highest T90 decrease was verified with the bare zirconium oxide (138 °C lower compared to the thermocatalytic toluene T90) where the activation of the zirconia photocata-
The positive synergistic effect due to the addition of a small amount of zirconia on the MnOx and the solar multi-catalytic approach led to obtaining a low toluene T90, considering the absence of noble metals co-catalysts. The obtained value of T90 with the MnOx-5% ZrO2 sample (180 °C) is comparable or lower with respect to the other MnOx-based catalysts reported in the literature (in the range 200 °C–270 °C considering an initial tolu-
The influence of the gas hourly space velocity (GSHV) was reported in the Figure S3a considering the best sample (MnOx-5% ZrO2). We have chosen, for all the tests, a GSHV of 8·104 mL/gcat·h, indeed, as expected, and as reported in the literature [38], with a high flow rate; the conversion rate of toluene to CO2 and water (the only by-products detected also in all the thermo and photothermo-catalytic tests) was slower, whereas a GSHV <
The ethanol being an alcohol was more reactive than the aromatic toluene, but its oxidation can give various by-products; the most common in the gas phase oxidation was acetaldehyde [3,31,39], which is also the main by-product detected in all the investigated catalytic approaches here discussed, whereas a very low selectivity (<2%) was detected in CO, formic acid, and acetic acid. In the solar photocatalytic tests, MnOx-5% ZrO2 confirmed its highest activity compared to the other samples (Figure 5) with an ethanol conversion of 98% and the highest selectivity to CO2 (43%), the most important feature for the VOCs removal. The mixed oxide with the 10 wt.% of ZrO2 showed a little decrease of photoactivity (ethanol conversion of 86%) and a higher selectivity to acetaldehyde (60%) with respect to CO2 (36%). These data, in line with the photo-oxidation of toluene (Figure 4a), pointed to, in our experimental conditions, the 5 wt.% being the optimal amount of zirconia to have a synergistic effect with MnOx. Among the bare oxides, the manganese oxide showed a higher ethanol conversion, with a higher selectivity to CO2 compared to ZrO2. This latter oxide promoted the partial oxidation to acetaldehyde (selectivity of 74%)
*2.3. Photocatalytic, Thermocatalytic and Photothermo-Catalytic Removal of Ethanol in Gas*
lytic properties was fundamental to promote the toluene total oxidation.
8·104 mL/gcat·h did not substantially modify the conversion rate.
and consequently exhibited the lowest selectivity to CO2 (22%).
ene concentration of 1000 ppm [20,37]).
*Phase*
**Table 5.** Data of the thermocatalytic oxidation of ethanol on the investigated samples.
Interestingly also for the removal of ethanol, the multi-catalytic reaction (i.e., the photothermo-catalysis) allowed us to improve the performance related to ethanol oxidation (Table 6, Figure S5). With MnOx-5%ZrO2, the T<sup>90</sup> of ethanol conversion was lowered to 34 ◦C (154 ◦C), a value that is comparable or lower, considering an initial ethanol concentration of 1000 ppm, with respect to the other MnOx-based materials reported in the literature (in the range 127 ◦C (initial ethanol concentration of 300 ppm) −200 ◦C (initial ethanol concentration of 600–1945 ppm) [37,40]). The total oxidation to CO<sup>2</sup> was favoured on this sample, and for this reason, the maximum conversion to acetaldehyde was low (35% at 118 ◦C), with a decrease of 205 ◦C of the T<sup>90</sup> related to the conversion to CO<sup>2</sup> compared to the thermocatalytic tests.
**Table 6.** Data of the photothermo-catalytic oxidation of ethanol on the investigated samples.
A further decrease of the maximum conversion to acetaldehyde was verified with MnOx-10%ZrO<sup>2</sup> (24%), but at a higher temperature (127 ◦C) compared to the MnOx-5%ZrO2, confirming that, with these mixed oxides, the combustion of ethanol was favoured with respect to its partial oxidation. For all the tested samples, similar to the photothermal oxidation of toluene, there was a positive effect of the solar light irradiation, with a contextual decrease of conversion temperatures compared to the thermocatalytic tests (comparison between Tables 5 and 6). The unmodified zirconia remained the less active catalyst, however, the solar-assisted reaction decreased T<sup>90</sup> of ethanol conversion of 13 ◦C compared to the tests without irradiation.
| doab | 2025-04-07T03:56:58.822446 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 55
} |
004db1df-dc76-42af-952f-1f5e82b7822b.56 | **3. Discussion**
The mixed oxides MnOx-ZrO<sup>2</sup> here investigated showed promising performance in the removal of VOCs in the gas phase, considering the absence of noble metals co-catalysts and an initial VOCs concentration of 1000 ppm. The amount of zirconium oxide added on manganese oxide is a key parameter to improve the catalytic and the photocatalytic performance. The as-synthesized samples showed a comparable optical bandgap, in the range 3.0–3.3 eV (Table 1), similar to the TiO2, and able to exploit the UV-A portion of the solar light. The addition of zirconia on manganese oxide led to a slight decrease of the surface area (Table 1) that, however, did not comprise the catalytic activity of the mixed oxides.
The presence of a small amount of zirconium oxide on MnO<sup>x</sup> allowed, as stated by XRD and SEM-EDX, an ionic exchange between Zr4+ and Mn2+; this favoured the formation of a synergistic effect between the two oxides, with structural changes in the bulk of MnOx. These modifications led to increasing the (photo)catalytic activity compared to the bare oxides. Indeed, when reducible oxides, i.e., which own mobile/reducible oxygens, were used for the oxidation of VOCs, the total oxidation to CO<sup>2</sup> is favoured, because these oxygens can participate in the reaction with a Mars–Van Krevelen (MvK) mechanism [41,42]. The oxygen vacancies on the surface of the oxide will be subsequently filled by the O<sup>2</sup> present in the gas phase.
This mechanism was boosted up with the photothermo-catalytic approach because the photocatalytic mechanism generated the superoxide (O<sup>2</sup> •−) and hydroxyl (OH• ) radicals [43,44], that being more reactive of the molecular O2, increased the rate of the total oxidation of VOCs (reactions a–i, Figure 6) and the re-filling of the oxygen vacancies, being the mobile oxygens of MnO<sup>x</sup> activated by the heating [13,45]. For these reasons, the conversion temperatures of both toluene and ethanol oxidation were sensibly lower compared to the thermocatalysis, especially with the MnOx-5%ZrO<sup>2</sup> sample. In this way, it was possible to exploit a double positive effect: (i) the solar irradiation effect: that allowed the formation of more reactive species, (ii) the thermal effect: that activated the redox mobility of the manganese oxide oxygens [13,20,45]. *Catalysts* **2022**, *12*, x FOR PEER REVIEW 12 of 17
*Photothermo-catalytic mechanism: Photothermo-catalytic mechanism:*
- (a) Charge carriers formation: MnOx-5%ZrO2 + hν(solar) → MnOx-5%ZrO2 (hVB++ e−CB) (a) Charge carriers formation: MnOx-5%ZrO<sup>2</sup> + hν(solar) → MnOx-5%ZrO<sup>2</sup> (hVB <sup>+</sup>+ e<sup>−</sup> CB)
•−
+O2 •−
•−→ CO2 + H2O
• → MnOx-
- (d) Oxygen from the mixed oxide: MnOx-5%ZrO2 → MnOx-5%ZrO2 (Vo) + 1/2 O2(g)from oxide (d) Oxygen from the mixed oxide: MnOx-5%ZrO<sup>2</sup> → MnOx-5%ZrO<sup>2</sup> (Vo) + 1/2 O2(g)from oxide
- heat (e) VOC oxidation: VOC + O<sup>2</sup> (g) + O2(g)from oxide heat → CO<sup>2</sup> + H2O
- (e) VOC oxidation: VOC + O2 (g)+ O2(g)from oxide ሱሮ CO2 + H2O (f) Oxygen restoring : MnOx-5%ZrO2 (Vo) + 1/2 O2(g) → MnOx-5%ZrO2 (f) Oxygen restoring: MnOx-5%ZrO<sup>2</sup> (Vo) + 1/2 O2(g) → MnOx-5%ZrO<sup>2</sup>
- *(iii) Solar photothermal effect* heat (g) MnOx-5%ZrO<sup>2</sup> + hν heat → MnOx-5%ZrO<sup>2</sup> (Vo) + 1/2 O2(g)from oxide + OH• + O<sup>2</sup> •−
- (g) MnOx-5%ZrO2 + hν ሱሮ MnOx-5%ZrO2 (Vo) + 1/2 O2(g)from oxide+ OH• (h) Improved VOC oxidation: VOC + O2 (g)+ O2(g)from oxide + OH• + O2 (h) Improved VOC oxidation: VOC + O<sup>2</sup> (g) + O2(g)from oxide + OH• + O<sup>2</sup> •− → CO<sup>2</sup> + H2O
- (i) Oxygen speeded up restoring: MnOx-5%ZrO2 (Vo) + 1/2 O2(g) + O2 5%ZrO2 (i) Oxygen speeded up restoring: MnOx-5%ZrO<sup>2</sup> (Vo) + 1/2 O2(g) + O<sup>2</sup> • → MnOx-5%ZrO<sup>2</sup>
It is worth noting that the reactions (a–c) and (d–f) are also involved in the solar photocatalysis at room temperature and in the bare thermocatalytic tests, respectively. The multi-catalytic effect (reactions g–i) allowed one to increase the performance and to favour the total oxidation of the employed VOCs to CO2. It is worth noting that the reactions (a–c) and (d–f) are also involved in the solar photocatalysis at room temperature and in the bare thermocatalytic tests, respectively. The multi-catalytic effect (reactions g–i) allowed one to increase the performance and to favour the total oxidation of the employed VOCs to CO2.
Another confirmation of the proposed MvK mechanism was reported in the Figure S3b. In the phothermo-catalytic oxidation of toluene with the MnOx-5%ZrO2 sample, the air (more interesting from a practical point of view) was replaced in the gas mixture with the pure oxygen. It is possible to note that the presence of oxygen led to a beneficial effect for the toluene conversion to CO2, being the T90 lower of 25 °C (155 °C) compared to the test with air (180 °C). This can be reasonably ascribed to the easier oxygen restoring on Another confirmation of the proposed MvK mechanism was reported in the Figure S3b. In the phothermo-catalytic oxidation of toluene with the MnOx-5%ZrO<sup>2</sup> sample, the air (more interesting from a practical point of view) was replaced in the gas mixture with the pure oxygen. It is possible to note that the presence of oxygen led to a beneficial effect for the toluene conversion to CO2, being the T<sup>90</sup> lower of 25 ◦C (155 ◦C) compared to the test with air (180 ◦C). This can be reasonably ascribed to the easier oxygen restoring on
the catalyst surface (reaction i), in an oxygen-rich environment, favouring, in this way, the
and zirconium oxide (especially at low amount of ZrO2) improved the photothermo-catalytic mechanism with the redox process on MnOx that was favoured by the ionic exchange between the zirconium and the manganese ions. On the contrary, an increased amount of zirconium oxide led to a progressive deposition of the hosted oxide on the surface of MnOx covering, in this way, the surface-active sites of manganese oxide [35,36]. For these reasons, the optimal performance was obtained with 5 wt.% of ZrO2. In this contest, the mobility of the surface oxygens of the MnOx-5%ZrO2 sample was favoured by the MnOx redox properties, and consequently, it is strictly related to its reducibility. Furthermore, the
MvK route.
the catalyst surface (reaction i), in an oxygen-rich environment, favouring, in this way, the MvK route.
As stated by the characterization data, the good interaction between the manganese and zirconium oxide (especially at low amount of ZrO2) improved the photothermocatalytic mechanism with the redox process on MnO<sup>x</sup> that was favoured by the ionic exchange between the zirconium and the manganese ions. On the contrary, an increased amount of zirconium oxide led to a progressive deposition of the hosted oxide on the surface of MnO<sup>x</sup> covering, in this way, the surface-active sites of manganese oxide [35,36]. For these reasons, the optimal performance was obtained with 5 wt.% of ZrO2. In this contest, the mobility of the surface oxygens of the MnOx-5%ZrO<sup>2</sup> sample was favoured by the MnO<sup>x</sup> redox properties, and consequently, it is strictly related to its reducibility. Furthermore, the amount of the surface oxygens on MnOx-5%ZrO<sup>2</sup> was higher compared to the other samples, as detected by XPS. To have a further confirmation of the high reducibility/mobility of the surface oxygens of MnOx-5%ZrO2, the H2-temperature-programmed reduction (TPR) measurements were carried out, and the sample profiles were reported in Figure 7. In accordance with the literature data [20,46], the TPR profiles of the MnOx-based samples were characterized to broad reduction peaks, due to the occurrence of several reduction processes of the Mn ions. As expected, the MnOx-5%ZrO<sup>2</sup> sample showed the lowest reduction feature (201 ◦C) attributed to the reduction of Mn2O<sup>3</sup> to Mn3O<sup>4</sup> [46], 111 ◦C and 117 ◦C lower compared to the same reduction feature of Mn3O<sup>4</sup> and MnOx-10%ZrO2, respectively. This reduction peak was also more intense for the MnOx-5%ZrO<sup>2</sup> compared to the other MnOx-based samples confirming, as detected by XRD and XPS, the major presence of Mn3+ ions on MnOx-5%ZrO2. The higher temperature reduction signals in the range 300–480◦ were ascribed to the further reduction of Mn3O<sup>4</sup> to MnO [46]. Moreover, in this case, the sample with 5 wt.% of ZrO<sup>2</sup> showed the highest reducibility (i.e., the lowest peak temperature). This is connected to the highest mobility/reducibility of the surface oxygens of MnOx-5%ZrO2, which favours the MvK mechanism, and therefore a better VOCs abatement. The reduction temperature of bare ZrO<sup>2</sup> started at a temperature above 500 ◦C [47], and for this reason, in our analysis (in the range 50–550◦C), its reduction peak was not complete. *Catalysts* **2022**, *12*, x FOR PEER REVIEW 13 of 17 amount of the surface oxygens on MnOx-5%ZrO2 was higher compared to the other samples, as detected by XPS. To have a further confirmation of the high reducibility/mobility of the surface oxygens of MnOx-5%ZrO2, the H2-temperature-programmed reduction (TPR) measurements were carried out, and the sample profiles were reported in Figure 7. In accordance with the literature data [20,46], the TPR profiles of the MnOx-based samples were characterized to broad reduction peaks, due to the occurrence of several reduction processes of the Mn ions. As expected, the MnOx-5%ZrO2 sample showed the lowest reduction feature (201 °C) attributed to the reduction of Mn2O3 to Mn3O4 [46], 111 °C and 117 °C lower compared to the same reduction feature of Mn3O4 and MnOx-10%ZrO2, respectively. This reduction peak was also more intense for the MnOx-5%ZrO2 compared to the other MnOx-based samples confirming, as detected by XRD and XPS, the major presence of Mn3+ ions on MnOx-5%ZrO2. The higher temperature reduction signals in the range 300–480° were ascribed to the further reduction of Mn3O4 to MnO [46]. Moreover, in this case, the sample with 5 wt.% of ZrO2 showed the highest reducibility (i.e., the lowest peak temperature). This is connected to the highest mobility/reducibility of the surface oxygens of MnOx-5%ZrO2, which favours the MvK mechanism, and therefore a better VOCs abatement. The reduction temperature of bare ZrO2 started at a temperature above 500 °C [47], and for this reason, in our analysis (in the range 50–550°C), its reduction peak was not complete.
**Figure 7.** H2-TPR (Temperature programmed reduction) profiles of the investigated samples. **Figure 7.** H<sup>2</sup> -TPR (Temperature programmed reduction) profiles of the investigated samples.
Between the photocatalytic, the thermocatalytic and the photothermo-catalytic removal of VOCs, although the solar photocatalytic reaction has the advantages of work at room temperature and that with the MnOx-5%ZrO2, it reached a similar activity of the most used TiO2-based materials (Table 4); to have a complete VOCs removal, it is neces-Between the photocatalytic, the thermocatalytic and the photothermo-catalytic removal of VOCs, although the solar photocatalytic reaction has the advantages of work at room temperature and that with the MnOx-5%ZrO2, it reached a similar activity of the most used TiO2-based materials (Table 4); to have a complete VOCs removal, it is necessary
sary to have contextual heating. For this purpose, the solar photothermo-catalysis can be an optimal solution to obtain the good performance of the thermocatalysis, but with an
the best sample (MnOx-5%ZrO2) tested in our experimental conditions showed a decrease of 36 °C and 34 °C of the toluene and ethanol T90 conversion compared to the thermocatalytic tests favouring in both the reactions; the total oxidation to CO2 (the T90 of ethanol
Finally, the stability in the time-on steam toluene removal of MnOx-5%ZrO2 was good (Figure 8, toluene solar photothermo-oxidation) and pointed to the MnOx-ZrO2 catalyst being a promising versatile material for application in thermocatalysis, photocatalysis,
conversion to CO2 was lowered of 205 °C, Tables 5 and 6).
and photothermo-catalysis.
to have contextual heating. For this purpose, the solar photothermo-catalysis can be an optimal solution to obtain the good performance of the thermocatalysis, but with an energy saving, due to the lower temperature required for the VOCs conversion. Indeed, the best sample (MnOx-5%ZrO2) tested in our experimental conditions showed a decrease of 36 ◦C and 34 ◦C of the toluene and ethanol T<sup>90</sup> conversion compared to the thermocatalytic tests favouring in both the reactions; the total oxidation to CO<sup>2</sup> (the T<sup>90</sup> of ethanol conversion to CO<sup>2</sup> was lowered of 205 ◦C, Tables 5 and 6).
Finally, the stability in the time-on steam toluene removal of MnOx-5%ZrO<sup>2</sup> was good (Figure 8, toluene solar photothermo-oxidation) and pointed to the MnOx-ZrO<sup>2</sup> catalyst being a promising versatile material for application in thermocatalysis, photocatalysis, and photothermo-catalysis. *Catalysts* **2022**, *12*, x FOR PEER REVIEW 14 of 17
**Figure 8.** Stability test of MnOx-5%ZrO2 catalyst in the toluene solar photothermo-catalytic oxida-**Figure 8.** Stability test of MnOx-5%ZrO<sup>2</sup> catalyst in the toluene solar photothermo-catalytic oxidation.
## tion. **4. Materials and Methods**
### **4. Materials and Methods** *4.1. Catalysts Synthesis*
*4.1. Catalysts Synthesis* Bare manganese oxide was prepared by chemical precipitation with NaOH (1 M) (Panreac Química SLU, Castellar del Vallès (Barcelona), Spain). In particular, a certain amount of manganese (II) chloride tetrahydrate (Sigma-Aldrich, Buchs, Switzerland) was dissolved in demineralized water and heated at 70 °C. After the NaOH was added dropwise until the pH = 10. Successively, the solution was stirred and kept at 70 °C for 2 h. Bare manganese oxide was prepared by chemical precipitation with NaOH (1 M) (Panreac Química SLU, Castellar del Vallès (Barcelona), Spain). In particular, a certain amount of manganese (II) chloride tetrahydrate (Sigma-Aldrich, Buchs, Switzerland) was dissolved in demineralized water and heated at 70 ◦C. After the NaOH was added dropwise until the pH = 10. Successively, the solution was stirred and kept at 70 ◦C for 2 h. After digestion for 24 h, the slurry was filtered and dried at 120 ◦C overnight. Finally, the resultant powders were calcined in air at 600 ◦C for 2 h.
After digestion for 24 h, the slurry was filtered and dried at 120 °C overnight. Finally, the resultant powders were calcined in air at 600 °C for 2 h. A similar procedure was followed for the bare ZrO2. In this case, the zirconyl nitrate hydrate (Fluka, Buchs, Switzerland) and ammonia (as precipitant agent, 25–28%, Sigma-A similar procedure was followed for the bare ZrO2. In this case, the zirconyl nitrate hydrate (Fluka, Buchs, Switzerland) and ammonia (as precipitant agent, 25–28%, Sigma-Aldrich, Buchs, Switzerland) were used, following the same procedures reported above, and the same thermal treatments (drying at 120 ◦C, and calcination at 600 ◦C for 2 h).
Aldrich, Buchs, Switzerland) were used, following the same procedures reported above, and the same thermal treatments (drying at 120 °C, and calcination at 600 °C for 2 h). For the MnOx-ZrO2 mixed oxides, the NaOH-driven precipitation was employed using the required stoichiometric amount of zirconyl nitrate hydrate to obtain the chosen nominal concentration in weight percentage (wt.%) of ZrO2. Moreover, in this case, the For the MnOx-ZrO<sup>2</sup> mixed oxides, the NaOH-driven precipitation was employed using the required stoichiometric amount of zirconyl nitrate hydrate to obtain the chosen nominal concentration in weight percentage (wt.%) of ZrO2. Moreover, in this case, the samples were dried at 120 ◦C and calcined in air at 600 ◦C for 2 h.
### samples were dried at 120 °C and calcined in air at 600 °C for 2 h. *4.2. Catalysts Characterization*
(FWHM) of the Kα of Mn.
*4.2. Catalysts Characterization* The sample structures were determined through the X-ray powder diffraction (XRD) The sample structures were determined through the X-ray powder diffraction (XRD) using a Smartlab Rigaku diffractometer (Rigaku Europe SE, Hugenottenallee 167 Neu-Isenburg 63263, Germany) in Bragg–Brentano mode, equipped with a rotating anode of Cu
using a Smartlab Rigaku diffractometer (Rigaku Europe SE, Hugenottenallee 167 Neu-Isenburg 63263, Germany) in Bragg–Brentano mode, equipped with a rotating anode of
with field emission scanning electron microscopy (FE-SEM) using a ZEISS SUPRA 55 VP (Carl Zeiss QEC Gmb, Garching b. München,Germany). The composition of the powders was carried out by the energy dispersive X-ray (EDX) analysis using an INCA-Oxford (Oxford Instruments plc, Tubney Woods, Abingdon, Oxfordshire, United Kingdom) windowless detector, and a resolution of 127 eV determined using the half-height amplitude
The BET surface area values were determined by N2 adsorption–desorption measurements with a Sorptomatic 1990 instrument (Thermo Quest, Milano, Italy). Before the
measurements, the catalysts were outgassed overnight at 200 °C.
Kα radiation operating at 45 kV and 200 mA. The surface morphology was examined with field emission scanning electron microscopy (FE-SEM) using a ZEISS SUPRA 55 VP (Carl Zeiss QEC Gmb, Garching b. München, Germany). The composition of the powders was carried out by the energy dispersive X-ray (EDX) analysis using an INCA-Oxford (Oxford Instruments plc, Tubney Woods, Abingdon, Oxfordshire, United Kingdom) windowless detector, and a resolution of 127 eV determined using the half-height amplitude (FWHM) of the Kα of Mn.
The BET surface area values were determined by N<sup>2</sup> adsorption–desorption measurements with a Sorptomatic 1990 instrument (Thermo Quest, Milano, Italy). Before the measurements, the catalysts were outgassed overnight at 200 ◦C.
The UV-vis Diffuse Reflectance spectra (UV-Vis DRS, Diffuse Reflectance Spectroscopy) measurements were performed with a Jasco V- 670 spectrometer (Jasco Europe S.R.L., Cremella, Italy) provided with an integration sphere and using barium sulphate (Fluka, Buchs, Switzerland) as standard. The estimation of the optical band gap of the samples was determined using the Kubelka–Munch function [26].
The X-ray photoelectron spectroscopy (XPS) was performed with a K-alpha X-ray photoelectron instrument (Thermo Fisher Scientific, Waltham, MA, USA), employing the C 1s peak at 284.9 eV (of adventitious carbon) as reference.
The H2-TPR (Temperature programmed reduction) profiles of the samples were obtained using a home-made flow equipment (gas-mixture 5 vol.% H<sup>2</sup> in Ar) and a TCD detector, following the procedures reported in ref. [48].
| doab | 2025-04-07T03:56:58.822873 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 56
} |
004db1df-dc76-42af-952f-1f5e82b7822b.57 | *4.3. Photo, Thermo and Photothermo-Catalytic Oxidation of VOCs*
The thermocatalytic removal of VOCs in gas phase and atmospheric pressure was carried out in a fixed bed flow reactor packed with the powder catalysts (0.15 g, 80–140 mesh), using the same experimental conditions described in the ref. [3]. A heating ramp of 10 ◦C was used in all the tests from room temperature to 500 ◦C. To assure a steady-state before the catalytic measurements, the gas mixture (1000 ppm VOCs; 10 vol.% air, rest He) was flowed on the catalyst for 30 min. No substantial contribution due to the adsorption process was detected. The reaction products were analysed by a gas chromatography (Smart-IQ+ Thermo Onix, Thermo Fisher Scientific 168 Third Avenue, Waltham, MA, USA 0245) utilizing a packed column with 10% FFAP on Chromosorb W (from Merck KGaA, Darmstadt, Germany) with a FID (Flame Ionization Detector), coupled with a quadrupole mass spectrometer (VG quadropoles, Fergutec B.V. Dragonder 13 C, 5554 GM Valkenswaard, The Netherlands).
The photocatalytic and the photothermo-catalytic tests were performed with the same instruments described above. The simultaneous irradiation in the phothermo-catalytic tests was made with an artificial solar lamp (OSRAM Vitalux 300 W, 10.7 mW/cm<sup>2</sup> , OSRAM Opto Semiconductors GmbH, Leibniz, Regensburg Germany). In the photocatalytic tests, a fan located near the reactor allowed us to maintain a constant temperature, avoiding the overheating effects due to lamp emission.
| doab | 2025-04-07T03:56:58.823825 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 57
} |
004db1df-dc76-42af-952f-1f5e82b7822b.58 | **5. Conclusions**
The MnOx-ZrO<sup>2</sup> mixed oxides exhibited promising performance in the removal of toluene and ethanol in the gas phase, especially in the multi-catalytic solar photothermal approach. The ionic interaction between the manganese and the zirconium ions exploited with the addition of a low amount of zirconium oxide allowed us to boost up the Mars–van Krevelen mechanism of the VOCs oxidation favouring the total oxidation of VOCs to CO2. Furthermore, with the photothermo-catalysis, a decrease of the conversion temperatures compared to the thermocatalysis was verified, and MnOx-5 wt.%ZrO<sup>2</sup> also showed good stability. Finally, with the same catalyst in the solar photocatalytic tests at room temperature, a similar activity of the most used TiO2-based materials was obtained, pointing to the fact that this sample can be promising for the VOCs remediation technologies, being cheaper, not critical, and performing considering the absence of noble metal co-catalysts.
**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/catal12010085/s1, Figure S1: SEM images of the other investigated samples; Figure S2: XPS characterization of the MnOx-5% ZrO<sup>2</sup> sample; Figure S3: Photothermocatalytic oxidation of toluene: influence of different parameters on MnOx-5% ZrO<sup>2</sup> sample; Figure S4: thermocatalytic oxidation of ethanol; Figure S5: Photothermo-catalytic oxidation of ethanol.
**Author Contributions:** Conceptualization, R.F.; investigation, R.A.F., R.F., F.L.P., E.M.M., S.A.B.; writing—original draft preparation, R.F.; writing—review and editing, R.F.; supervision, R.F. All authors have read and agreed to the published version of the manuscript.
**Funding:** This research received no external funding.
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.
**Acknowledgments:** The authors thank S. Scirè (University of Catania) for the use of the laboratory facilities and the Bio-nanotech Research and Innovation Tower (BRIT) laboratory of the University of Catania for the Smartlab diffractometer facility. R.F. thanks the PON "AIM" of the European Social Found for the support.
**Conflicts of Interest:** The authors declare no conflict of interest.
| doab | 2025-04-07T03:56:58.823967 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 58
} |
004db1df-dc76-42af-952f-1f5e82b7822b.60 | *Article* **Photocatalytic Degradation of Fluoroquinolone Antibiotics in Solution by Au@ZnO-rGO-gC3N<sup>4</sup> Composites**
**Abniel Machín 1,\* , Kenneth Fontánez <sup>2</sup> , José Duconge <sup>3</sup> , María C. Cotto <sup>3</sup> , Florian I. Petrescu <sup>4</sup> , Carmen Morant <sup>5</sup> and Francisco Márquez 3,\***
**Abstract:** The photocatalytic degradation of two quinolone-type antibiotics (ciprofloxacin and levofloxacin) in aqueous solution was studied, using catalysts based on ZnO nanoparticles, which were synthesized by a thermal procedure. The efficiency of ZnO was subsequently optimized by incorporating different co-catalysts of gC3N<sup>4</sup> , reduced graphene oxide, and nanoparticles of gold. The catalysts were fully characterized by electron microscopy (TEM and SEM), XPS, XRD, Raman, and BET surface area. The most efficient catalyst was 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> , obtaining degradations of both pollutants above 96%. This catalyst has the largest specific area, and its activity was related to a synergistic effect, involving factors such as the surface of the material and the ability to absorb radiation in the visible region, mainly produced by the incorporation of rGO and gC3N<sup>4</sup> in the semiconductor. The use of different scavengers during the catalytic process, was used to establish the possible photodegradation mechanism of both antibiotics.
**Keywords:** ciprofloxacin; levofloxacin; ZnO; gC3N<sup>4</sup> ; rGO; Au nanoparticles
| doab | 2025-04-07T03:56:58.824147 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 60
} |
004db1df-dc76-42af-952f-1f5e82b7822b.61 | **1. Introduction**
Antibiotics have become emerging pollutants due to their widespread use and persistence in the environment [1–3]. The origin is very varied, although they come mainly from medical treatments, agricultural, livestock, and industrial production [4–7]. The presence of antibiotics in the natural environment presents a serious health risk, since they can lead to the development of antibiotic-resistant bacteria and, in general, promote the destabilization of the natural environment [8,9]. Fluoroquinolone-based antibiotics, particularly ciprofloxacin and levofloxacin, are widely used in human and veterinary medicine [1–3]. They are used to fight bacterial infections like pneumonia, kidney or prostate infections, and even skin infections. In fact, this family of antibiotics is by far the most widely used for medical and veterinary treatments. Due to the chemical structure, after consumption by humans or animals, only between 15 and 20% is metabolized, so the non-metabolized antibiotic is eliminated into the environment through urine or feces [10]. Once released into the natural environment, these antibiotics are highly recalcitrant to degradation, which is why they have been detected in very worrying quantities in wastewater, surface water, ground water, and even in drinking water [11]. In general, some antibiotics can be eliminated from water bodies by different techniques such as adsorption, nanofiltration, coagulation, electrolysis,
**Citation:** Machín, A.; Fontánez, K.; Duconge, J.; Cotto, M.C.; Petrescu, F.I.; Morant, C.; Márquez, F. Photocatalytic Degradation of Fluoroquinolone Antibiotics in Solution by Au@ZnO-rGO-gC3N<sup>4</sup> Composites. *Catalysts* **2022**, *12*, 166. https://doi.org/10.3390/ catal12020166
Academic Editor: Roberto Fiorenza
Received: 8 January 2022 Accepted: 22 January 2022 Published: 28 January 2022
**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.
**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).
or even biodegradation [12,13]. These decontamination techniques have been implemented in wastewater processing plants, especially in developed countries, although these methods are very expensive, require large spaces and are not very efficient. In fact, already treated waters continue to show high levels of these pollutants [14]. As an alternative procedure to those already mentioned above, it is worth highlighting the chemical transformation through advanced oxidation processes (AOPs), and especially those involving semiconductor photocatalysts [15]. These processes are based on the catalytic photodegradation of organic pollutants in aqueous solution, being much more efficient than conventional methods, as well as being friendly with the environment [15]. The AOPs produce highly oxidized species such as radicals and other reactive species, which degrade, through a chain reaction, organic pollutants in solution allowing complete mineralization [16,17].
Among the most widely used semiconductor materials, it is worth mentioning titanium oxide (TiO2) and zinc oxide (ZnO), due to their low cost, non-toxicity, large specific surface area of some of their forms, and high catalytic activity [18,19]. The main limitation of both semiconductors is the wide bandgap (ca. 3.2 eV in TiO<sup>2</sup> anatase and 3.0 eV in TiO<sup>2</sup> rutile, and 3.37 eV in ZnO wurtzite), which requires irradiation with UV light for the photoactivation process [20]. UV radiation from the solar spectrum is limited to about 5% of the total radiation reaching the earth's surface, so these catalysts are clearly inefficient [21]. However, alternatives such as doping these semiconductors with metallic nanoparticles and other additives deposited on the surface have made it possible to reduce bandgap and substantially promote electron-hole separation, avoiding rapid recombination processes [22]. The incorporation of Au nanoparticles on the surface of semiconductors enables the appearance of surface plasmon resonance (SPR), greatly improving the storage capacity and charge separation which increases the photocatalytic activity of the heterostructure [23,24]. Nevertheless, there are many other factors that decisively affect the photocatalytic behavior of the material, especially morphology, particle size, crystallinity and specific area, controlled during the synthesis procedure [25]. Graphene and especially graphene oxide (GO) provides scaffolds to anchor other components, due to their two-dimensional structure and large surface area [26]. There are many examples of nanohybrids prepared with GO with extraordinary optical, electrical, and thermal properties that have a direct effect on the catalytic activity of the material [27,28]. On the other hand, the incorporation of GO to ZnO has been shown to increase the activity, reducing the photocorrosion of the semiconductor, facilitating the separation of charges and the inhibition of the electron-hole recombination processes [29]. Additionally, the two-dimensional structure of GO greatly improves the interaction with organic pollutants, accelerating the subsequent photocatalytic degradation [30]. Graphitic carbon nitride (gC3N4) [31], is an allotropic material of carbon nitride, rich in nitrogen, and with truly extraordinary properties that allow its use in different applications such as catalysis, photodegradation of organic pollutants, CO<sup>2</sup> fixation, catalysis and even in energy storage systems [32–34]. Among the most relevant properties, it is worth highlighting its great thermal and chemical stability, its two-dimensional structure capable of facilitating interaction with other materials, and its simple synthesis method [34]. The purpose of doping with noble metal nanoparticles and the manufacture of complex hybrid composites has been to develop more efficient catalytic heterostructures, capable of showing reduced bandgap, altered electronic properties that allow efficient generation of hole-electron pairs, and inhibition of recombination processes [35]. Over the last few years, complex systems have been developed, with improved and increasingly outstanding properties, which have been applied to many processes, and especially to the degradation of highly persistent organic pollutants. In this sense, it is worth highlighting the catalysts based on metallic nanoparticles, dispersed on semiconductors such as TiO2, ZnO, Fe2O3, ZnS, or CdS [36]. Other catalysts, based on heterostructures formed by rare earth metal oxides, combined with graphitic carbon nitride have also been designed, showing good catalytic behavior [37].
Taking into account all the previous research, the objective of this work has been to obtain catalysts, based on the incorporation of gold nanoparticles on the surface of heterostructures formed by ZnO nanoparticles, to which reduced GO (rGO) and graphitic carbon nitride have been incorporated. These catalysts were used for the photodegradation of ciprofloxacin (CFX) and levofloxacin (LFX) in aqueous solution. The structural properties and morphology of the most active catalyst has been investigated using different spectroscopic and analytical techniques, such as XRD, XPS, Raman, DRS, SEM, TEM, and BET analysis. Finally, based on the results obtained, a possible photodegradation mechanism of the two antibiotics studied is proposed.
| doab | 2025-04-07T03:56:58.824266 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 61
} |
004db1df-dc76-42af-952f-1f5e82b7822b.63 | *2.1. Characterization of Catalysts*
Three types of catalysts were synthesized, based on Au@ZnONPs, Au@ZnONPs-3%rGO, and Au@ZnONPs-3%rGO-3%gC3N4. In these catalysts, the percentage of rGO and gC3N<sup>4</sup> was always maintained at 3%, although percentages of Au nanoparticles of 1%, 5% and 10% were used, thus, a total of nine catalysts were obtained. All these catalysts were used for the photodegradation of CFX and LFX, and the most efficient catalyst (10%Au@ZnONPs-3%rGO-3%gC3N4) was the one that was fully characterized by different techniques.
The BET surface area of the catalysts was analyzed (see Table 1). ZnO nanoparticles showed a relatively large surface area of 24 m2g −1 . Incorporation of Au on the surface increased the specific area of the material to 58 m2/g in the case of 1%Au@ZnONPs, and continued to increase, as a function of Au loading, up to a maximum value of 78 m2/g in 10%Au@ZnONPs. This behavior, which has been described previously with different metallic nanoparticles, is expected to contribute to the catalytic activity of the material. The incorporation of 3%rGO, which already has a very large surface area, considerably increases the surface area of the material, reaching values as high as 196 m2/g in the case of 10%Au@ZnONPs-3%rGO. The incorporation of gC3N<sup>4</sup> also increased the area of the material, although not as drastically as in the case of rGO, reaching area values of 229 m2/g in the case of 10%Au@ZnONPs-3%rGO-3%gC3N4. This last catalyst, with the highest specific area, is the most active in the processes studied, as will be seen later.
**Table 1.** BET surface area of the different as-synthesized materials.
The dispersion of 10%Au on the support of ZnONPs was followed by elemental mapping (see Figure 1). Figure 1b,c show the distribution of Zn and Au, respectively, corresponding to the SEM image of Figure 1a. As can be seen, Au presents some aggregates, although, in general, and considering the high percentage of the metal, a good dispersion is observed. Using this material as the starting catalyst, 3% rGO and 3% gC3N<sup>4</sup> were incorporated. Figure 2 shows the TEM and HR-TEM images obtained for the most active catalyst in the photodegradation reactions (10%Au@ZnONPs-3%rGO-3%gC3N4). Figure 2a shows the TEM image of the catalyst, in which the heterostructure can be observed, together with the highly dispersed gold nanoparticles on the surface. The inset image presents the selected area electron diffraction (SAED) pattern of the photocatalyst, demonstrating the crystalline nature of the sample. Figure 2b shows the HRTEM image of a ZnO nanoparticle showing the distinct lattice fringes with interplanar spacing of 0.28 nm, indexed to the (100) crystal plane of ZnO with a hexagonal wurtzite structure [38]. Figure 2c shows a
region of the catalyst gC3N4. A crystalline structure is observed, whose lattice distance is approximately 0.33 nm, corresponding to the (002) plane of gC3N<sup>4</sup> [39,40]. Figure 2d shows the distribution of gold nanoparticles on the heterostructure, together with the area of the sample that has been identified as rGO. One of these nanoparticles is the one that has been magnified in Figure 2e, whose interplanar spacing of ca. 0.23 nm has been indexed to (111) crystal plane of Au [41]. Figure 2f presents the ultra-high resolution detail of a gold nanoparticle just over 1 nm in diameter. This particle has been further enlarged to appreciate the details of the icosahedral structure. The white lines delineate the boundaries between five different crystal domains on the nanoparticle. One of the faces shows the interplanar spacing of 0.23 nm, assigned to Au (111).
**Figure 1.** (**a**) SEM image of 10%Au@ZnONPs, (**b**) the corresponding elemental mapping of Zn and (**c**) Au. HV (20 keV), magnification 2500×, and WD (15 mm).
**Figure 2.** (**a**) TEM and (**b**–**f**) HR-TEM images of 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> . (**a**) Low magnification and SAED pattern of the heterostructure; (**b**) magnified image of a ZnO nanoparticle; (**c**) crystalline structure of gC3N<sup>4</sup> ; and (**d**–**f**) images at different magnification of AuNPs.
10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> was also characterized by X-ray diffraction (XRD). Figure 3 shows the diffraction pattern of the catalyst, along with that of rGO, gC3N4, and ZnONPs for comparison purposes. There is a broad peak at ca. 23.8◦ for rGO, assigned to the (002) crystal plane, indicating that most of the oxygen functional groups, characteristic of GO, have been removed from the surface [42]. Additionally, rGO shows a second peak at about 43◦ assigned to the (100) plane of the hexagonal carbon structure. The XRD pattern of gC3N<sup>4</sup> is shown in Figure 3b. Two broad peaks are observed at ca. 13◦ and 27.2◦ , which have been assigned to crystalline planes (100) and (002), respectively [39,40]. The peak shown at 27.2◦ , and which is also the most intense, corresponds to an interplanar spacing of 0.33 nm, the crystallographic planes which were observed in Figure 1c. The diffraction peaks of ZnONPs (see Figure 3c) can be unambiguously indexed to the ZnO phase of
hexagonal wurtzite [43], whose peaks are the dominant ones in the 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> catalyst, as observed in Figure 3d. Neither rGO nor gC3N<sup>4</sup> are detected in the XRD of the catalyst, possibly because the proportion of these components in the heterostructure is too small. Additionally, a low intensity peak is observed at ca. 38.1◦ , which has been assigned to Au (111) [41], and whose crystallographic planes were observed in Figure 2e.
**Figure 3.** (**a**) XRD patterns of rGO; (**b**) gC3N<sup>4</sup> ; (**c**) ZnONPs; and (**d**) 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> .
Figure 4 shows the Raman spectrum of 10%Au@ZnONPs-3%rGO-3%gC3N4, along with those of rGO, gC3N4, and ZnONPs. As can be seen, rGO is characterized by two broad bands at 1357 and 1600 cm−<sup>1</sup> (Figure 4a) that have been assigned to bands D (A1g mode), and G (E2g mode of sp<sup>2</sup> carbon atoms), respectively. The intensity ratio of these bands (IG/ID) is ca. 0.95, indicating that the GO reduction is not too high [43]. gC3N<sup>4</sup> (Figure 4b) shows two typical bands, similar to those seen in rGO, although slightly displaced. Band D is associated with the possible presence of sp<sup>3</sup> carbon, justified by structural defects and disarrangements, while band G is associated, in the same way as in rGO, with the presence of sp<sup>2</sup> carbon [44]. ZnONPs is characterized by showing two peaks at at ca. 566 cm−<sup>1</sup> and 1143 cm−<sup>1</sup> (see Figure 4c), which have been assigned to A1-LO and E1-LO vibration modes of ZnO, respectively [45,46]. Both peaks are indicative that ZnONPs has a wurtzite-like crystal structure, as established by XRD. The Raman spectrum of 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> is characterized by showing a main peak at 1042 cm−<sup>1</sup> , which could come from the peak observed at 1143 cm−<sup>1</sup> in ZnONPs. Around ca. 630 cm−<sup>1</sup> an undefined peak is observed, which could also have its origin in the peak observed at 566 cm−<sup>1</sup> in ZnONPs. The displacements observed in the peaks of the catalyst support the fact that this material is an integrated heterostructure separate from its different components. Additionally, two small peaks are observed at ca. 1301 and 1588 cm−<sup>1</sup> that must necessarily have their origin in the contribution of rGO and gC3N4.
**Figure 4.** (**a**) Raman spectra of rGO; (**b**) gC3N<sup>4</sup> ; (**c**) ZnONPs; and (**d**) 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> .
The absorption of radiation by catalysts is crucial for their catalytic activity, so the different systems were analyzed using Tauc plots. As shown in Figure 5a, ZnONPs shows a bandgap in the UV region (3.23 eV). The incorporation of Au nanoparticles on the surface of the semiconductor (Figure 5b) produces a slight shift at 3.21 eV, still in the UV region. gC3N<sup>4</sup> (Figure 5c) and rGO (Figure 5d) show bandgaps at 2.79 and 2.33 eV, respectively, already in the visible range. The addition of these components will be able to displace the bandgap of the catalyst towards the visible region. In fact, Figure 5e shows how the bandgap moves towards 2.77 eV in the 10%Au-ZnONPs-3%rGO composite. The addition of gC3N<sup>4</sup> (Figure 5f) still causes a greater displacement, in this case up to 2.73 eV, falling squarely in the visible region. The results obtained justify the activity of the 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> heterostructure under irradiation with visible light, as will be described in the section corresponding to catalytic results.
10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> was also characterized by XPS. As shown in Figure 6a, two peaks at 1020.6 eV and 1044.1 eV have been assigned to the binding energies of Zn2p3/2 and Zn2p1/2, respectively, indicating the presence of Zn2+ [47]. Furthermore, the spin-orbit splitting of these two peaks at 23.5 eV also confirmed the presence of ZnO [48]. The transition corresponding to O1s (see Figure 6b) showed a major peak at ca. 530.2 eV, which was assigned to O2<sup>−</sup> species in the ZnO network, and a shoulder at ca. 532.1 eV, assigned to O2<sup>−</sup> in oxygen-deficient regions, respectively [49]. The Au4f peak (Figure 6c) was fitted to two peaks at 83.3 and 86.9 eV, attributed to Au4f7/2 and Au4f5/2 double peaks, respectively, in metallic gold (Au<sup>0</sup> ) [50]. The C1s transition (Figure 6d) showed a peak at 287.6 eV, which was assigned to C-N-C bonds, and a less intense one at 284.8 eV attributed to the aromatic C atom in the s-triazine ring, respectively [51,52]. Nitrogen from graphitic carbon nitride was evidenced by the N1s transition (see Figure 6e). This clearly asymmetric peak could be deconvolved into two components, showing an intense peak at 398.7 eV, which was assigned to C=N-C, indicating the presence of triazine rings, and a less intense peak at 400.1 eV that was assigned to the presence of tertiary N atoms (N-(C)3) [53,54]. As previously shown, the XPS analysis further confirmed the association of the different components (Au, ZnO, rGO, and gC3N4) in the heterostructure.
**Figure 5.** (**a**) Tauc plots of (αhν) <sup>2</sup> versus energy (eV), and determination of the bandgap energy of ZnONPs; (**b**) 10%Au@ZnONPs; (**c**) gC3N<sup>4</sup> ; (**d**) 10%Au@ZnONPs-3%rGO; (**e**) 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> and (**f**) rGO.
**Figure 6.** (**a**) XPS core level spectra for 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> : Zn2p; (**b**) O1s; (**c**) Au4f; (**d**) C1s; and (**e**) N1s.
| doab | 2025-04-07T03:56:58.824888 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 63
} |
004db1df-dc76-42af-952f-1f5e82b7822b.64 | *2.2. Photocatalytic Degradation*
Before proceeding to the CFX and LFX photodegradation, several preliminary studies were carried out to establish the optimal reaction conditions. To do this, a study of the optimal pH was initially carried out, and it was established that the most appropriate pH for both antibiotics was to use solutions at pH = 7, which guaranteed the maximum solubility of both contaminants in water. Another parameter that was evaluated was the initial antibiotic concentration. For this, the three catalytic systems studied were used, using, in all cases, the highest concentration of Au (10%). The results for CFX and LFX are shown in Figure S1. As can be seen, there is a clear relationship between the percentage of degradation and the initial concentration of the antibiotic. In the case of CFX (Figure S1a), the highest activity in the three catalytic systems studied was obtained at a concentration of 20 mM. In the case of LFX (see Figure S1b), the optimal concentration was half (10 mM), possibly because LFX is slightly less soluble in water than CFX. Another of the preliminary studies that were carried out established the optimal amount of catalyst in the reaction medium. As can be seen in Figure S2, there is a clear correlation between the amount of catalyst and the percentage of degradation. For both antibiotics, a sustained increase in the efficiency of the process was observed, until reaching a load of 1.1 gL−<sup>1</sup> in CFX (Figure S2a), and 1 gL−<sup>1</sup> in LFX (Figure S2b). From these values, a drop in efficiency was observed, which has been justified on the basis of possible radiation scattering processes that potentially occur from a certain catalyst load. Taking into account the above considerations, the optimal reaction conditions for each antibiotic were established. In the case of CFX, the optimized conditions were: pH = 7, [CFX]<sup>i</sup> = 20 mM, and catalyst loading of 1.1 gL−<sup>1</sup> , while for LFX, the reaction conditions were: pH = 7, [CFX]<sup>i</sup> = 10 mM, and catalyst loading of 1 gL−<sup>1</sup> .
Figures 7 and 8 show the results of photodegradation of CFX and LFX, respectively, for each of the catalysts studied. The study was carried out for a total time of 180 min and, for both contaminants, the behavior was very different depending on the catalyst. In the case of CFX (Figure 7), it was observed that after 3 h, the degradation ranged between 73% and 99%. The highest efficiency was obtained with 10%Au@ZnONPs-3%rGO-3%gC3N4, while the lowest degradation was obtained with 1%Au@ZnONPs. In this last catalyst, the percentage of Au on the surface clearly affects the reaction, facilitating absorption and formation of electron-hole pairs, showing greater efficiency with 5%Au@ZnONPs, and a maximum efficiency with 10%Au@ZnONPs. In the case of LFX, the behavior is similar, showing the highest degradation with 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> (96%), and the lowest with 1%Au@ZnONPs (51%). As seen with CFX, the addition of a higher gold load greatly increased the efficiency of the process. For both CFX and LFX, the incorporation of rGO considerably improved the efficiency of photodegradation, which was maximized with the addition of gC3N4, especially in catalysts with 10%Au. Another observation to consider when comparing the degradation profiles of CFX and LFX is the different rate at which the process is carried out. The degradation of CFX is certainly faster than that of LFX, indicating that CFX is more recalcitrant to degradation. The greater degradation observed when incorporating rGO and, additionally, gC3N<sup>4</sup> is due to a synergistic effect of improvement of the electrical and conductive properties of the material, and to an increase in absorption in the visible region, as evidenced by the determination of the bandgap (see Figure 5).
In order to investigate the kinetic behavior of the photodegradation of CFX and LFX, the pseudo-first-order kinetics were studied by representing −ln (C/C0) vs. the irradiation time (see Figure S3). The results obtained are shown in Table S1, indicating that the apparent rate constant of the catalysts increases with an increasing amount of Au on the surface, and when incorporating rGO and gC3N4. The correlation coefficients R<sup>2</sup> (see Table S1) are higher for the linear fits of LFX than CFX although, as derived from the results obtained, for both contaminants the degradation clearly follows pseudo-first-order kinetics. This behavior can also be correlated with the BET surface area of these catalysts. Thus, as shown in Table 1, the materials that show the highest BET area values are those that have better catalytic behavior, with higher apparent rate constants.
**Figure 7.** Photodegradation rate of CFX as a function of time, using the different catalysts evaluated.
**Figure 8.** Photodegradation rate of LFX as a function of time, using the different catalysts evaluated.
To further study the photocatalytic activity of 10%Au@ZnONPs-3%rGO-3%gC3N4, control experiments and recyclability tests were carried out. The control experiments for both contaminants were similar (Figure S4). After 3 h of reaction, it was found that CFX and LFX were highly recalcitrant. Under photolysis conditions, that is, in the absence of the catalyst with the rest of the components present, a very slight reduction in the concentration of both CFX and LFX was observed, which indicates that radiation absorption is the only pathway responsible for this change and, as seen in Figure S4, it is clearly inefficient. In the absence of oxygen, a greater degradation of CFX than LFX was observed but, in both cases, the percentage of degradation after 3 h of reaction is not relevant. From control experiments, photocatalysis is shown to be the primary degradation route, indicating that catalysis, or even photolysis, is totally insufficient to degrade antibiotics. The results of recyclability of 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> for the degradation of CFX and LFX during five cycles are shown in Figure S5. The catalyst was recovered after each cycle by centrifugation (3000 rpm for 20 min). After the supernatant was decanted, the catalyst was washed three times with deionized water and once with ethanol (using centrifugation–sonication cycles), and dried for at least 5 h at 60 ◦C. The parameters used for the degradation of each antibiotic were set to optimal values, as previously described. In CFX (Figure S5a) it was found that the degradation remained practically constant after each cycle, going from 99% after the first cycle to 97% after the fifth cycle. In the case of LFX (Figure S5b), the recyclability was not as good, going from a degradation of 97% in the first cycle to 90% after the fifth cycle. In both cases (CFX and LFX) the photocatalyst recovered after the fifth cycle was analyzed by XRD (not shown) showing a diffraction pattern without notable differences with respect to the XRD of the catalyst before use.
| doab | 2025-04-07T03:56:58.825553 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 64
} |
004db1df-dc76-42af-952f-1f5e82b7822b.65 | *2.3. Proposed Photodegradation Mechanism for CFX and LFX*
As previously shown, gold nanoparticles improve catalytic efficiency, which increases appreciably when rGO and gC3N<sup>4</sup> are incorporated. The most efficient catalyst for the photodegradation of CFX and LFX (10%Au@ZnONPs-3%rGO-3%gC3N4) incorporates different characteristics that act synergistically on its behavior, that is, increased surface area and smaller bandgap. The considerable increase in the specific area of the catalyst can provide more active sites during the photocatalytic reaction, thus producing more photogenerated electrons, which can also lead to less recombination of photogenerated charge carriers. This decrease in recombination is also favored by the presence of Au nanoparticles, which act as electron sinks. The displacement towards the visible region of the bandgap, with respect to other catalysts studied, is another of the decisive factors for the observed behavior.
In this context, and to evaluate the photodegradation mechanism, some scavengers were added to the reaction. In this photocatalytic study, benzoquinone (BQ), ethylenediaminetetraacetic acid disodium salt (EDTA), and methanol (MetOH), were employed to capture superoxide radicals (·O2−), holes (h<sup>+</sup> ), and hydroxyl radicals (·OH), respectively [55,56], determining the percentage of degradation of CFX and LFX after 3 h of reaction (see Figure S6). As can be seen, BQ hindered photoactivity noticeably, suggesting the main role of the O2<sup>−</sup> reactive species in the photodegradation process. EDTA and MetOH hindered the reaction to a lesser extent, which supports the fact that h<sup>+</sup> and ·OH do not play a prominent role in the degradation process. This effect is similar for both antibiotics (see Figure S6a,b), although in the case of LFX, the effects of all scavengers were certainly greater.
Taking into account these results, together with the determination of the bandgaps (see Figure 5), a photodegradation mechanism of CFX and LFX using 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> (Figure 9) has been proposed. For this, the Mulliken electronegativity theory [57,58] has been used, which allows establishing the band edge position of the
different components of the catalyst and, in this way, determining the migration direction of the photogenerated charge carriers in the composite (Equations (1) and (2)).
$$\rm{E\_{CB}} = \rm{X} - \rm{E\_C} - 0.5 \rm{E\_g} \tag{1}$$
$$\mathbf{E\_{VB}} = \mathbf{E\_{CB}} + \mathbf{E\_{g}} \tag{2}$$
where ECB and EVB are the edge potentials of the valence band (VB) and conduction band (CB), respectively, X is the absolute electronegativity, E<sup>C</sup> is the energy of free electrons on the hydrogen scale (4.50 eV) [59,60], and E<sup>g</sup> is the bandgap. X values for ZnO and gC3N<sup>4</sup> are 5.75 [61] and 4.73 eV [62], respectively. The calculated ECB and EVB edge positions for Au@ZnONPs are −0.355 and 2.855 eV, respectively, while for gC3N<sup>4</sup> the calculated values were −1.165 and 1.625 eV, respectively, being in agreement with values previously determined in other investigations [62,63].
**Figure 9.** Schematic diagram of the proposed photodegradation mechanism of CFX and LFX under visible radiation.
Under visible radiation (Figure 9), the electrons of the VB of gC3N<sup>4</sup> are excited towards the conduction band (CB), giving rise to h<sup>+</sup> in the VB. Due to the potential difference with respect to ZnONPs, the electrons of the conduction band of gC3N<sup>4</sup> move towards the CB of Au@ZnONPs. Au acts as a sink for electrons [64,65], which is why they move towards Au and it is there that they react with the adsorbed molecular oxygen to generate superoxide anions, which in turn can react with water molecules to form hydroxyl radicals. According to studies carried out with different scavengers, the O2<sup>−</sup> anion is the one that preferentially participates in the photodegradation process of antibiotics, giving rise to small molecules as a by-product of the reaction, CO<sup>2</sup> and water. The holes formed in the VB of gC3N<sup>4</sup> and, to a lesser extent in ZnO, will promote the oxidation of CFX and LFX also leading to degradation. However, this second pathway, as evidenced above, is not the most favored pathway. AuNPs, in addition to acting as electron sinks and thus reducing recombination processes, also contribute to the system through the surface plasmon resonance mechanism, reacting with molecular oxygen and generating superoxide anions. Photogenerated electrons are also transferred to rGO that, as in the case of gold, acts as an electron acceptor and transport medium in the photocatalytic system, suppressing the recombination of e−-h<sup>+</sup> pairs. Photodegradation reactions therefore also occur on rGO sheets, increasing the specific surface area and active reaction sites [65].
| doab | 2025-04-07T03:56:58.826011 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 65
} |
004db1df-dc76-42af-952f-1f5e82b7822b.67 | *3.1. Reagents and Materials*
All the reagents were used as received, without further purification. All the solutions were prepared using deionized water (Milli-Q water, 18.2 MΩcm−<sup>1</sup> at 25 ◦C). Zn(CH3COO)<sup>2</sup> (99.99%, trace metal basis), HAuCl4•3H2O (ACS Reagent, 49.0+% Au basis), Ethanol (200 proof, anhydrous, ≥99.5%), NaBH<sup>4</sup> (99.99% trace metals basis), melamine (2,4,6-Triamino-1,3,5-triazine, 99%), reduced graphene oxide (rGO), ethylenediaminetetraacetic acid, disodium salt dihydrate (EDTA, ACS Reagent, 99.9+%), P-benzoquinone (Spect. Grade, 99.5+%), and methanol (HPLC Plus, 99.9+%) were provided by Sigma– Aldrich (Darmstadt, Germany). NaOH (98+%) and HCl (36% *w*/*w* aq. soln) were acquired from Alfa Aesar. Whatman® Puradisc 13 disposable syringe filters (Clifton, NJ, USA) were used for the aliquot filtration process.
| doab | 2025-04-07T03:56:58.826359 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 67
} |
004db1df-dc76-42af-952f-1f5e82b7822b.68 | *3.2. Synthesis of Nanomaterials*
Zinc oxide nanoparticles (ZnONPs) were synthesized by thermal decomposition of anhydrous zinc acetate. In a standard synthesis, 5 g of zinc acetate was introduced into an alumina crucible which was covered by a perforated alumina lid, and subsequently placed in a tube furnace. Next, the sample was subjected to heat treatment in air flow (300 mL/min) using a temperature ramp of 3 ◦C/min until reaching 450 ◦C. The sample was kept at this temperature for 2 h then proceeded to cool, maintaining an air flow of 100 mL/min and a ramp of −10 ◦C/min. The sample was recovered the next day. Graphitic carbon nitride was synthesized using the procedure described by Mo et al. [66]. For this, 2 g of melamine were introduced into a crucible, which was subsequently treated at 650 ◦C (heating rate of 2 ◦C/min) in flowing nitrogen (200 mL/min), for 4 h. After treatment, the synthesized material was subjected to exfoliation. For this, 200 mg of gC3N<sup>4</sup> were put in contact with 20 mL of concentrated H2SO<sup>4</sup> (with stirring for 6 h). The solution was then treated with ultrasound for 2 h. The mixture was decanted and centrifuged, washed with water several times until neutral, and finally rinsed with ethanol to facilitate drying. The gC3N<sup>4</sup> crystals obtained after the described process showed a characteristic white color.
The deposition of Au NPs has been described elsewhere [49]. In a typical synthesis, 1g of ZnONPs was dispersed in 100 mL of H2O and the mixture was sonicated for 30 min. After that, the desired amount of the gold precursor (HAuCl4·3H2O) was added to the reaction mixture and kept stirring for 30 min. Finally, a solution of NaBH<sup>4</sup> (10 mg in 10 mL of H2O) was added dropwise while maintaining stirring. After the reagent was added, the solution was kept stirring for an additional 30 min. The reaction product was separated by centrifugation, washed 4 times with deionized water, and dried overnight at 80 ◦C. The different compounds of Au@ZnONPs were identified as 1%Au@ZnONPs, 5%Au@ZnONPs and 10%Au@ZnONPs, where the numbers correspond to the percentage by weight of gold nanoparticles deposited in each sample. The material obtained was used for the following stages of preparation of the catalysts. Thus, 200 mg of ZnONPs containing the gold nanoparticles were dispersed in a solution containing 10 mL of ethanol and 40 mL of deionized water, and the mixture was vigorously stirred for 30 min. Subsequently, the reduced graphene oxide (rGO) was added, and the suspension was kept under stirring for 1 h. Subsequently, the product was separated from the solution by centrifugation, washed 4 times with deionized water and dried overnight at 80 ◦C. Next, and by same procedure described for rGO, the incorporation of graphitic carbon nitride (gC3N4) was carried out. Finally, the product was collected, sealed, and stored at room temperature. The different catalysts prepared, based on Au@ZnONPs, Au@ZnONPs-rGO, and Au@ZnONPs-rGOgC3N<sup>4</sup> were identified indicating the percentage of gold incorporated on the surface. In all cases, the amount of rGO and gC3N<sup>4</sup> used was 3% by weight.
| doab | 2025-04-07T03:56:58.826429 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 68
} |
004db1df-dc76-42af-952f-1f5e82b7822b.69 | *3.3. Characterization of the Catalysts*
The morphology of the catalysts was characterized by Scanning Electron Microscopy (SEM), using a Hitachi S-3000N instrument (Westford, MA, USA), equipped with a EDX Quantax EDS X-Flash 6I30 Analyzer, and by High Resolution Transmission Electron Microscopy (HRTEM), using a JEOL 3000F (Peabody, MA, USA). XPS measurements were performed on an ESCALAB 220i-XL spectrometer (East Grinstead, United Kingdom), using the non-monochromated Mg Kα (1253.6 eV) radiation of a twin-anode, operating at 20 mA and 12 kV in the constant analyzer energy mode, with a PE of 40 eV. Brunauer Emmett Teller (BET) specific areas were measured using a Micromeritics ASAP 2020, according to N<sup>2</sup> adsorption isotherms at 77 K. Raman spectra were collected using a DXR Raman Microscope (Thermo Scientific, Waltham, MA, USA), employing a 532 nm laser source at 5 mW power and a nominal resolution of 5 cm−<sup>1</sup> . Diffuse reflectance measurements were carried out on a Perkin Elmer Lambda 365 UV-Vis spectrophotometer (Perkin Elmer, Waltham, MA, USA), equipped with an integrating sphere. The bandgap value was obtained from the plot of the Kubelka–Munk function versus the energy of the absorbed light [67]. X-ray powder diffraction patterns (XRD) were collected using a Bruker D8 Discover X-ray Diffractometer (Madison, WI, USA), in Bragg–Brentano goniometer configuration. The X-ray radiation source was a ceramic X-ray diffraction Cu anode tube type Empyrean of 2.2 kW. XRD diffractograms were recorded in angular range of 30–75◦ at 1◦ min−<sup>1</sup> , operating at 40 kV and 40 mA.
| doab | 2025-04-07T03:56:58.826758 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 69
} |
004db1df-dc76-42af-952f-1f5e82b7822b.70 | *3.4. Photocatalytic Experiments*
To test the activity of the synthesized catalysts in the photodegradation process of ciprofloxacin and levofloxacin, a solar simulator composed of three white annular bulbs with a total irradiation power of 90 watts was used. In the case of ciprofloxacin, <sup>a</sup> <sup>2</sup> <sup>×</sup> <sup>10</sup>−<sup>5</sup> <sup>M</sup> solution was prepared, while for levofloxacin the concentration used was 10−<sup>5</sup> M. Next, the desired catalyst was added, the amount of which also depended on the antibiotic used. After adding the catalyst and adjusting the pH to 7, the system was kept in the dark for 30 min to allow the system to reach adsorption–desorption equilibrium. Then, a small amount of H2O<sup>2</sup> (2 mL, 0.005%) was added, and additional oxygen was provided to the system by constantly bubbling air into the reaction mixture. The white bulbs were then turned on and the photocatalytic system was maintained with constant agitation and irradiation. The reaction was monitored for a period of 180 min and photodegradation was followed by taking aliquots every 15 min. After filtering the aliquots with Whatman® Puradisc 13 (Clifton, NJ, USA) disposable syringe filters, the samples were immediately analyzed at room temperature with a Perkin Elmer Lambda 35 UV-vis spectrophotometer (San José, CA, USA).
| doab | 2025-04-07T03:56:58.826896 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 70
} |
004db1df-dc76-42af-952f-1f5e82b7822b.71 | **4. Conclusions**
The photocatalytic degradation of two quinolone-type antibiotics (CFX and LFX) in aqueous solution was studied, using catalysts based on ZnONPs, which were synthesized by means of a thermal procedure. Subsequently, the efficiency of ZnONPs was optimized by incorporating different cocatalysts of gold, rGO, and gC3N4, obtaining a total of nine different catalysts that were used in the photodegradation reaction of CFX and LFX. The most efficient catalyst was 10%Au@ZnONPs-3%rGO-3%gC3N4, allowing degradations of both pollutants above 96%. This catalyst has the largest specific area, and its activity has been related to a synergistic effect, involving factors as relevant as the surface of the material and the ability to absorb radiation in the visible region, mainly produced by the incorporation of rGO and gC3N<sup>4</sup> to the semiconductor. The use of different scavengers during the catalytic process, together with the determination of bandgaps of the different components of the photocatalyst, has made it possible to establish a possible photodegradation mechanism of CFX and LFX in which superoxide radicals (·O2−) are the main reactive species involved in the process. The results obtained are certainly relevant because, in less than 3 h, almost complete photodegradation of CFX and LFX occurs, with conversions above 96%. Some catalysts based on TiO<sup>2</sup> doped with boron have shown high degradation percentages (ca. 88%) [68]. In other cases, copper tungstate (CuWO4) catalysts doped with graphene have made it possible to obtain CFX degradations of ca. 97% [69]. Other
catalysts based on ZnO doped with silver allowed degradation to 99% of CFX [49], although preliminary studies showed many difficulties in photodegrading LFX. Considering the results published so far, the catalyst developed in this research is cost effective, easy to synthesize, and highly effective to for the degradation of both CFX and LFX, opening up a wide field of possibilities in environmental decontamination processes. In addition, the developed catalyst could have relevant uses for facing environmental problems generated by other pollutants, from an applied and global point of view.
**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/catal12020166/s1, Figure S1: Evaluation of the initial concentration of (a) CFX and (b) LFX on the catalytic efficiency of 10%Au@ZnONPs, 10%Au@ZnONPs-3%rGO, and 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> in the photodegradation reaction; Figure S2: Evaluation of the initial concentration of 10%Au@ZnONPs, 10%Au@ZnONPs-3%rGO, and 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> on the efficiency of the photodegradation reaction of (a) CFX and (b) LFX; Figure S3: Pseudo-first order kinetics of photodegradation of (a) CFX and (b) LFX using different catalysts; Figure S4: Control experiments for 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> with (a) CFX and (b) LFX, under visible radiation; Figure S5: Recyclability of 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> after five consecutive catalytic cycles of photodegradation of (a) CFX and (b) LFX under visible radiation; Figure S6: Photocatalytic activity of 10%Au@ZnONPs-3%rGO-3%gC3N<sup>4</sup> on the degradation of (a) CFX and (b) LFX in the presence of various scavengers under visible radiation; Table S1: The pseudo-first-order kinetics constants for the photodegradation of CFX and LFX.
**Author Contributions:** Conceptualization, A.M., F.M.; methodology, F.M., A.M.; formal analysis, F.M., M.C.C., J.D.; investigation, A.M., K.F., C.M.; resources, F.M., C.M., F.I.P.; writing—original draft preparation, F.M.; writing—review and editing, F.M., A.M., K.F., C.M., M.C.C., J.D.; supervision, A.M., F.M.; project administration, A.M., F.M.; funding acquisition, A.M., F.M., M.C.C., J.D., C.M., F.I.P. All authors have read and agreed to the published version of the manuscript.
**Funding:** Financial support from the NSF Center for the Advancement of Wearable Technologies-CAWT (Grant 1849243), and from the framework of the UE M-ERA.NET 2018 program under the StressLIC Project (Grant PCI2019-103594) are gratefully acknowledged.
**Data Availability Statement:** The data is contained in the article and is available from the corresponding authors on reasonable request.
**Acknowledgments:** The authors thank Raúl S. García for his arduous technical help in the procedures for the synthesis and characterization of materials. Technical assistance of I. Poveda from "Servicio Interdepartamental de Investigacion, SIdI" at Autonomous University of Madrid (Spain), is gratefully acknowledged. The facilities provided by the National Center for Electron Microscopy at Complutense University of Madrid (Spain) is gratefully acknowledged. K.F. thanks PR NASA Space Grant Consortium for a graduate fellowship (#80NSSC20M0052).
**Conflicts of Interest:** The authors declare no conflict of interest.
| doab | 2025-04-07T03:56:58.827011 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 71
} |
004db1df-dc76-42af-952f-1f5e82b7822b.73 | **Comparative Study of ZnO Thin Films Doped with Transition Metals (Cu and Co) for Methylene Blue**
**William Vallejo 1,\* , Alvaro Cantillo <sup>1</sup> , Briggitte Salazar <sup>1</sup> , Carlos Diaz-Uribe <sup>1</sup> , Wilkendry Ramos <sup>1</sup> , Eduard Romero <sup>2</sup> and Mikel Hurtado 3,4**
**Photodegradation under Visible Irradiation**
Received: 14 March 2020; Accepted: 6 May 2020; Published: 11 May 2020
**Abstract:** We synthesized and characterized both Co-doped ZnO (ZnO:Co) and Cu-doped ZnO (ZnO:Cu) thin films. The catalysts' synthesis was carried out by the sol–gel method while the doctor blade technique was used for thin film deposition. The physicochemical characterization of the catalysts was carried out by Raman spectroscopy, scanning electron microscopy (SEM), X-ray diffraction, and diffuse reflectance measurements. The photocatalytic activity was studied under visible irradiation in aqueous solution, and kinetic parameters were determined by pseudo-first-order fitting. The Raman spectra results evinced the doping process and suggested the formation of heterojunctions for both dopants. The structural diffraction patterns indicated that the catalysts were polycrystalline and demonstrated the presence of a ZnO wurtzite crystalline phase. The SEM analysis showed that the morphological properties changed significantly, the micro-aggregates disappeared, and agglomeration was reduced after modification of ZnO. The ZnO optical bandgap (3.22 eV) reduced after the doping process, these being ZnO:Co (2.39 eV) and ZnO:Co (3.01 eV). Finally, the kinetic results of methylene blue photodegradation reached 62.6% for ZnO:Co thin films and 42.5% for ZnO:Cu thin films.
**Keywords:** thin films; ZnO; doping; heterogeneous photocatalysis
| doab | 2025-04-07T03:56:58.827325 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 73
} |
004db1df-dc76-42af-952f-1f5e82b7822b.74 | **1. Introduction**
Synthetic dyes are commonly used by various industries, especially textile ones. These physically and chemically stable compounds are harmful to the environment. Synthetic dyes are recalcitrant compounds that exhibit high solubility in water and accumulate in both wastewater and industrial effluents [1,2]. Currently, water pollution is one of the major challenges of the modern world, and the recovery of wastewater by conventional methods is not suitable for emergent pollutants [3,4]. Heterogeneous photocatalysis can be satisfactorily applied for the decontamination of natural samples through the photocatalytic degradation of toxic pollutants from complex matrices, such as river water and wastewater [5]. All renewable technologies have become a promising alternative for both energy generation and wastewater treatments. Solar photocatalysis is a suitable option to degrade recalcitrant
pollutants from water [6,7]. Different catalysts have been reported to exhibit photocatalytic activity (e.g., TiO<sup>2</sup> [8], Fe2O<sup>3</sup> [9], ZnO [10], CuO [11], CdS [12], WO3, and SnO<sup>2</sup> [13]). ZnO has been used as a photocatalyst, but its high bandgap value (~3.3 eV) is one of its major drawbacks. ZnO is not photocatalytically active at longer wavelengths of the electromagnetic spectrum. As a consequence, using solar irradiation as the main energy source to develop practical applications is a challenge for ZnO [14]. Some alternative strategies to extend ZnO photoresponse in the visible light region are: (a) ZnO doping [15–18], (b) co-doping [19,20], (c) coupling with lower band gap semiconductors [21,22], (d) surface plasmon resonance [23–26], (e) quantum dots [27], and (f) sensitization with natural and synthetic dyes [28–31]. With the doping process, optical and catalytic properties can be tuned by doping [32], and the ZnO bandgap generates intragap electronic states inside the semiconductor [33]. Some transition metals (e.g., Co2+, Ag+, Cu2+, Mn2+) have been used to enhance the properties of ZnO [34–37]. Among these, copper is an economical option. Because the ionic size of Cu<sup>2</sup> is close to Zn2+, Cu2<sup>+</sup> ions can replace Zn2<sup>+</sup> ions to modify the absorption spectrum [38]. Kuriakose et al. reported that Cu-doped ZnO nanostructures photodegraded organic dyes. Their analysis associated the enhanced photocatalytic activity to the combined effects of: (i) the separation of charge carriers and (ii) the optimal Cu doping load [39]. Another transition element is cobalt, as the ionic size of the Co2<sup>+</sup> ion is close to that of Zn2+. Co2<sup>+</sup> ions can replace Zn2<sup>+</sup> and generate minimal distortion in the crystalline lattice [35,40]. Yongchun et al. reported the synthesis of cobalt-doped ZnO nanorods and reported an improvement in the performance of alizarin red photocatalytic degradation [41]. In addition, Kuriakose et al. synthesized Co-doped ZnO nanodisks and nanorods, and reported photocatalytic activity improvement as the by-effect of the doping process due to: (i) the charge separation efficiency and (ii) the surface area [42]. Poornaprakash et al. reported 66.5% efficiency in photocatalytic degradation of Rhodamine B under artificial solar light illumination on Co-doped ZnO nanorods [43]. The incorporation of these kinds of metals inside the ZnO structure can modify optical properties by extending the photodegradation ability towards the largest wavelength of the electromagnetic spectrum [44,45]. In this work, we report a facile wet chemical method for the synthesis of Cu and Co-doped ZnO thin films with highly enhanced photocatalytic activity. The metal doping process leads to highly efficient visible light photocatalytic degradation of methylene blue.
| doab | 2025-04-07T03:56:58.827456 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 74
} |
004db1df-dc76-42af-952f-1f5e82b7822b.76 | *2.1. Structural Study*
Figure 1 shows the XRD pattern for the catalysts synthesized in this study. The hexagonal wurtzite phase (JCPDS No. 36−1451) is identified as a crystalline structure for ZnO thin films, with the signals of the diffraction pattern corresponding to those reported by other authors [46]. The doping process did not affect the main signals in the diffraction patterns, as the XRD patterns for ZnO:Cu and ZnO:Co showed signals of a wurtzite ZnO structure. However, the XRD patterns showed a change in the intensity of the signals, suggesting that metal ions could substitute Zn2<sup>+</sup> ions after the doping process. Lima et al. suggested that the change in the intensities could be associated with changes in both (i) grain size due to network defects and (ii) oxygen vacancies [47,48].
We used the Debye–Scherrer equation to calculate the crystalline domain size of the catalysts, using the full width at half maximum (FWHM) for the highest peak (101), with θ being the Scherrer diffraction angle [49]. Although there is no clear tendency between the intensity of the signal and metal doping load (see Figure 1b,c), all samples reduced the grain size of catalysis after the doping process (see Table 1). This could be explained by the incorporation of Co2<sup>+</sup> and Cu2<sup>+</sup> ions as dopants into the ZnO after the doping process [48,49]. Finally, the structural results suggest that ZnO films incorporated metallic ions. This observation was verified by Raman spectroscopy and diffuse reflectance, as described in the next sections.
*Catalysts* **2020**, *10*, x FOR PEER REVIEW 3 of 13
**Figure 1.** (**a**) X-ray diffraction patterns for the catalysts synthesized in this study. (**b**) Comparison of the highest peak (101) for ZnO:Cu. (**c**) Comparison of the highest peak (101) for ZnO:Co. **Figure 1.** (**a**) X-ray diffraction patterns for the catalysts synthesized in this study. (**b**) Comparison of the highest peak (101) for ZnO:Cu. (**c**) Comparison of the highest peak (101) for ZnO:Co.
**Table 1.** Crystallographic results for the X-ray characterization of both undoped ZnO and metaldoped ZnO thin films. **Table 1.** Crystallographic results for the X-ray characterization of both undoped ZnO and metal-doped ZnO thin films.
ZnO:Cu 1% 0.2601 11,649 32.1 \* FWHM: full width at half maximum.
ZnO:Cu 3% 0.2803 9202 29.8 ZnO:Cu 5% 0.3075 5300 27.2
| doab | 2025-04-07T03:56:58.827985 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 76
} |
004db1df-dc76-42af-952f-1f5e82b7822b.77 | *2.2. Raman Study*
*2.2. Raman Study*
\*FWHM: full width at half maximum. The Raman spectra of the catalysts are shown in Figure 2. All the peaks correspond with wurtzite–ZnO (C<sup>4</sup> 6v): (i) 97.4 cm−<sup>1</sup> (vibrational mode E2L), (ii) 340 cm (E2H–E2L), and (iii) 437.0 cm−<sup>1</sup> and 581 cm−<sup>1</sup> (A<sup>1</sup> vibrational mode) [50,51]. Figure 2a shows the Raman spectra for Cu-doped ZnO thin films. Signals E2L (~99 cm−<sup>1</sup> ) and E2H (~437 cm−<sup>1</sup> ) widen and decrease after the doping process—a behavior that can be explained by the incorporation of Cu2<sup>+</sup> into the ZnO lattice. Additionally, this behavior has been associated with reduction of ZnO crystallinity by the formation of nanocomposites [52]. For greater Cu loads, Figure 2a shows two new signals, the first one located at 298 cm−<sup>1</sup> and a second weak one at 614 cm−<sup>1</sup> . These two signals can be attributed to modes A1g and B2g for CuO, respectively, and this result suggests the formation of a ZnO–CuO heterojunction during the synthesis process. The hydrodynamic stability of the suspension is affected by the concentration of reagents; so for obtaining greater Cu loads, the CuO generation is feasible. This result is in line with previous reports [39]. 581 cm−1 (A1 vibrational mode) [50,51]. Figure 2a shows the Raman spectra for Cu-doped ZnO thin films. Signals E2L (~99 cm−1) and E2H (~437 cm−1) widen and decrease after the doping process**—**a behavior that can be explained by the incorporation of Cu2+ into the ZnO lattice. Additionally, this behavior has been associated with reduction of ZnO crystallinity by the formation of nanocomposites [52]. For greater Cu loads, Figure 2a shows two new signals, the first one located at 298 cm−1 and a second weak one at 614 cm−1. These two signals can be attributed to modes A1g and B2g for CuO, respectively, and this result suggests the formation of a ZnO**–**CuO heterojunction during the synthesis process. The hydrodynamic stability of the suspension is affected by the concentration of reagents; so for obtaining greater Cu loads, the CuO generation is feasible. This result is in line with previous reports [39].
*Catalysts* **2020**, *10*, x FOR PEER REVIEW 4 of 13
wurtzite**–**ZnO (C46v): (i) 97.4 cm−1 (vibrational mode E2L), (ii) 340 cm (E2H**–**E2L), and (iii) 437.0 cm−1 and
**Figure 2.** Raman spectra for: (**a**) ZnO:Cu and (**b**) ZnO:Co thin films. Inside the figures are the Raman vibration modes, where (\*) corresponds to defects inside the ZnO structure. **Figure 2.** Raman spectra for: (**a**) ZnO:Cu and (**b**) ZnO:Co thin films. Inside the figures are the Raman vibration modes, where (\*) corresponds to defects inside the ZnO structure.
Figure 2b shows the Raman spectrum for ZnO:Co; the intensity of A1LO, E2L, and E2H modes decreases for these films. The ZnO:Co thin films (doping load 5%) show four new signals at 490 cm−1, 526 cm−1, 626 cm−1, and 725 cm−1. These signals could be attributed to the possible presence of Co3O4, and these results confirm both the doping process and the formation of a heterojunction of ZnO**–** Co3O4 [53,54]. Figure 2b shows the Raman spectrum for ZnO:Co; the intensity of A1LO, E2L, and E2H modes decreases for these films. The ZnO:Co thin films (doping load 5%) show four new signals at 490 cm−<sup>1</sup> , 526 cm−<sup>1</sup> , 626 cm−<sup>1</sup> , and 725 cm−<sup>1</sup> . These signals could be attributed to the possible presence of Co3O4, and these results confirm both the doping process and the formation of a heterojunction of ZnO–Co3O<sup>4</sup> [53,54].
### *2.3. Morphological Study 2.3. Morphological Study*
Figure 3 shows SEM images for the catalysts. Figure 3a shows that the ZnO films formed microaggregates (~220 nm) composed of quasi-spherical ZnO nanoparticles (around 40 nm in diameter), and this is a typical result for this material sensitized by the sol**–**gel method. Figure 3b,c shows that the morphological properties changed significantly after the doping process. Regarding the ZnO:Cu thin films, Figure 3b shows the formation of nanorods. Meanwhile, Figure 3c shows the formation of nanosized elongated particles of various shapes (~100 nm) from the ZnO:Co thin films. Likewise, Figure 3c shows that the agglomeration on the catalyst surface reduced and the microaggregates disappeared. Different nanostructures have been reported for ZnO (e.g., nanorods, nanotubes, nanobelts, nanosprings, nanospirals, nanorings) [55]. It is known that ZnO's morphological properties rely on synthesis conditions, and in our case, it is clear that the metal ions used during synthesis reduced the agglomeration and changed the thin films' morphology [56]. Figure 3 shows SEM images for the catalysts. Figure 3a shows that the ZnO films formed microaggregates (~220 nm) composed of quasi-spherical ZnO nanoparticles (around 40 nm in diameter), and this is a typical result for this material sensitized by the sol–gel method. Figure 3b,c shows that the morphological properties changed significantly after the doping process. Regarding the ZnO:Cu thin films, Figure 3b shows the formation of nanorods. Meanwhile, Figure 3c shows the formation of nanosized elongated particles of various shapes (~100 nm) from the ZnO:Co thin films. Likewise, Figure 3c shows that the agglomeration on the catalyst surface reduced and the microaggregates disappeared. Different nanostructures have been reported for ZnO (e.g., nanorods, nanotubes, nanobelts, nanosprings, nanospirals, nanorings) [55]. It is known that ZnO's morphological properties rely on synthesis conditions, and in our case, it is clear that the metal ions used during synthesis reduced the agglomeration and changed the thin films' morphology [56].
*Catalysts* **2020**, *10*, x FOR PEER REVIEW 5 of 13
*Catalysts* **2020**, *10*, x FOR PEER REVIEW 5 of 13
**Figure 3.** SEM images: (**a**) ZnO, (**b**) ZnO:Cu 5% and (**c**) ZnO:Co 5% thin films. **Figure 3.** SEM images: (**a**) ZnO, (**b**) ZnO:Cu 5% and (**c**) ZnO:Co 5% thin films.
### *2.4. Optical Study 2.4. Optical Study* **Figure 3.** SEM images: (**a**) ZnO, (**b**) ZnO:Cu 5% and (**c**) ZnO:Co 5% thin films.
The diffuse reflectance spectra for the catalysts are shown in Figure 4. We used the Kubelka**–** Munk (KM) remission function for determining the bandgap energy value of the catalysts [57]. The use of the KM remission function makes it possible to obtain an analog to Tauc plots [58,59]: The diffuse reflectance spectra for the catalysts are shown in Figure 4. We used the Kubelka–Munk (KM) remission function for determining the bandgap energy value of the catalysts [57]. The use of the KM remission function makes it possible to obtain an analog to Tauc plots [58,59]: *2.4. Optical Study* The diffuse reflectance spectra for the catalysts are shown in Figure 4. We used the Kubelka**–** Munk (KM) remission function for determining the bandgap energy value of the catalysts [57]. The
((∝ሺሺ
$$(F(\mathbb{R}\_{\approx})hv)^{\frac{1}{2}} = A(hv - E\_{\mathfrak{J}}) \tag{1}$$
**Figure 4.** Reflectance diffuse spectra for both catalysts. **Figure 4.** Reflectance diffuse spectra for both catalysts. **Figure 4.** Reflectance diffuse spectra for both catalysts.
Figure 5 shows plots for ሺܨሺܴ∝)݄ݒ( <sup>ଵ</sup> ଶൗ versus *(hv)* and table 2 lists the optical properties of the catalysts. Figure 5 shows that ZnO had a bandgap value (Eg) of 3.22 eV, a value that corresponds with that reported by Srikant el al. (3.1 eV and 3.2 eV) [60,61]. For doped ZnO catalysts, the Eg value was lower, and this behavior is associated with the reduction of the Fermi level of ZnO by the generation of intragap states. For ZnO:Cu, the modification of the bandgap can be attributed to the induction of 3d states of Cu located inside the bandgap of ZnO [37]. Additionally, the visible light absorption observed for doped ZnO can be attributed to intragap transitions between Cu 3d and Zn 4s states. Furthermore, the ZnO:Co 5% catalyst has a lower bandgap value compared to other catalysts. This reduction is attributed to s-d and p-d exchange interactions between ZnO and Co2+ ions [62]. The 3d levels of Co2+ are located within the bandgap of ZnO, which can create new bands at larger wavelengths [63]. Some photoluminescence studies of the transition metal doping ZnO nanoparticles suggest that this important reduction in Eg value is due to oxygen deficiency [64]. Finally, the Figure 5 shows plots for ሺܨሺܴ∝)݄ݒ( <sup>ଵ</sup> ଶൗ versus *(hv)* and table 2 lists the optical properties of the catalysts. Figure 5 shows that ZnO had a bandgap value (Eg) of 3.22 eV, a value that corresponds with that reported by Srikant el al. (3.1 eV and 3.2 eV) [60,61]. For doped ZnO catalysts, the Eg value was lower, and this behavior is associated with the reduction of the Fermi level of ZnO by the generation of intragap states. For ZnO:Cu, the modification of the bandgap can be attributed to the induction of 3d states of Cu located inside the bandgap of ZnO [37]. Additionally, the visible light absorption observed for doped ZnO can be attributed to intragap transitions between Cu 3d and Zn 4s states. Furthermore, the ZnO:Co 5% catalyst has a lower bandgap value compared to other catalysts. This reduction is attributed to s-d and p-d exchange interactions between ZnO and Co2+ ions [62]. The 3d levels of Co2+ are located within the bandgap of ZnO, which can create new bands at larger wavelengths [63]. Some photoluminescence studies of the transition metal doping ZnO nanoparticles Figure 5 shows plots for (*F*(*R*∝)*hv*) 1 <sup>2</sup> versus (*hv*) and Table 2 lists the optical properties of the catalysts. Figure 5 shows that ZnO had a bandgap value (*Eg*) of 3.22 eV, a value that corresponds with that reported by Srikant el al. (3.1 eV and 3.2 eV) [60,61]. For doped ZnO catalysts, the *E<sup>g</sup>* value was lower, and this behavior is associated with the reduction of the Fermi level of ZnO by the generation of intragap states. For ZnO:Cu, the modification of the bandgap can be attributed to the induction of 3d states of Cu located inside the bandgap of ZnO [37]. Additionally, the visible light absorption observed for doped ZnO can be attributed to intragap transitions between Cu 3d and Zn 4s states. Furthermore, the ZnO:Co 5% catalyst has a lower bandgap value compared to other catalysts. This reduction is attributed to s-d and p-d exchange interactions between ZnO and Co2<sup>+</sup> ions [62]. The 3d levels of Co2<sup>+</sup> are located within the bandgap of ZnO, which can create new bands at larger wavelengths [63]. Some photoluminescence studies of the transition metal doping ZnO nanoparticles suggest that this important reduction in *E<sup>g</sup>* value is due to oxygen deficiency [64]. Finally,
formation of nanoheterojunctions in the catalyst surface leads to an enhanced separation of charge
suggest that this important reduction in Eg value is due to oxygen deficiency [64]. Finally, the
the formation of nanoheterojunctions in the catalyst surface leads to an enhanced separation of charge carriers, increasing photocatalytic efficiency in addition to the doping process. The generation of these heterostructures has been reported for photocatalytic applications [65]. *Catalysts* **2020**, *10*, x FOR PEER REVIEW 6 of 13 carriers, increasing photocatalytic efficiency in addition to the doping process. The generation of
these heterostructures has been reported for photocatalytic applications [65].
**Figure 5.** Kubelka**–**Munk (KM) fitting for the catalysts synthesized in this study. **Figure 5.** Kubelka–Munk (KM) fitting for the catalysts synthesized in this study.
**Table 2.** Band gap and results of pseudo-first-order model fitting.
### the Co-doped ZnO films. Compared to the Co-doped ZnO films, the bandgap values of this catalyst did not change; however, the best photodegradation result for ZnO:Cu was 42.5%, a value greater *2.5. Photocatalytic Study*
catalysts.
than that obtained for the ZnO thin films. The combined effect of the doping process and the heterojunction can explain this behavior. The photodegradation kinetics of MB on catalysts were studied by using the pseudo-first-order model [66]: ݒሾெሿ ൌ ሾܤܯሿ݁ିೌ௧ (2) Figure 6 shows the decrease of MB as a function of time for all tests performed under visible irradiation. The MB concentration did not change after 140 min under visible irradiation, verifying the stability of MB dye. Furthermore, ZnO films did not show photocatalytic activity under visible irradiation (<3%). This result is in accordance with the ZnO bandgap energy value, and this photocatalyst is active only under UV irradiation. The ZnO:Co 5% catalyst reported the highest photocatalytic activity. This result can be explained by the lower bandgap value of the ZnO:Co 5% catalyst compared to other catalysts. The ZnO:Cu catalysts showed less photocatalytic activity than the Co-doped ZnO films. Compared to the Co-doped ZnO films, the bandgap values of this catalyst did not change; however, the best photodegradation result for ZnO:Cu was 42.5%, a value greater than that obtained for the ZnO thin films. The combined effect of the doping process and the heterojunction can explain this behavior. The photodegradation kinetics of MB on catalysts were studied by using the pseudo-first-order model [66]:
$$w\_{[MB]} = [MB]\_o e^{-k\_{ap}t} \tag{2}$$
*2.5. Photocatalytic Study*
carriers, increasing photocatalytic efficiency in addition to the doping process. The generation of
**Figure 5.** Kubelka**–**Munk (KM) fitting for the catalysts synthesized in this study.
Figure 6 shows the decrease of MB as a function of time for all tests performed under visible irradiation. The MB concentration did not change after 140 min under visible irradiation, verifying the stability of MB dye. Furthermore, ZnO films did not show photocatalytic activity under visible irradiation (<3%). This result is in accordance with the ZnO bandgap energy value, and this photocatalyst is active only under UV irradiation. The ZnO:Co 5% catalyst reported the highest photocatalytic activity. This result can be explained by the lower bandgap value of the ZnO:Co 5% catalyst compared to other catalysts. The ZnO:Cu catalysts showed less photocatalytic activity than the Co-doped ZnO films. Compared to the Co-doped ZnO films, the bandgap values of this catalyst did not change; however, the best photodegradation result for ZnO:Cu was 42.5%, a value greater than that obtained for the ZnO thin films. The combined effect of the doping process and the heterojunction can explain this behavior. The photodegradation kinetics of MB on catalysts were
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4
Photon energy (eV)
ݒሾெሿ ൌ ሾܤܯሿ݁ିೌ௧ (2)
these heterostructures has been reported for photocatalytic applications [65].
ZnO ZnO:Co 1% ZnO:Co 3% ZnO:Co 5% ZnO:Cu 1% ZnO:Cu 3% ZnO:Co 5%
0
5
10
15
20
**(**F(R)hv**)**1/2
25
30
35
40
**Figure 6.** Methylene blue (MB) concentration vs. time of visible irradiation on the synthesized **Figure 6.** Methylene blue (MB) concentration vs. time of visible irradiation on the synthesized catalysts.
catalysts. Time (*t*) is expressed in minutes and *<sup>k</sup>app* is the apparent reaction rate constant (min−<sup>1</sup> ). Table 2 lists the kinetic parameters for the studied catalysts. Among all the catalysts, the ZnO thin films (*kapp* <sup>=</sup> 0.2 <sup>×</sup> <sup>10</sup>−<sup>4</sup> min−<sup>1</sup> ) showed the lowest *kap* value, while the best results were obtained for ZnO:Co 5% (*kapp* <sup>=</sup> 7.2 <sup>×</sup> <sup>10</sup>−<sup>3</sup> min−<sup>1</sup> ) and ZnO:Cu 5% (*kapp* <sup>=</sup> <sup>4</sup> <sup>×</sup> <sup>10</sup>−<sup>3</sup> min−<sup>1</sup> ). In the best case, the kinetic rate constant was 36 times higher than the ZnO thin films. A combined effect could be present: (i) Cu doping in ZnO and (ii) the formation of a nanoheterojunction (ZnO–CuO). This synergic effect could be a reason for the increase in photocatalytic yields. The heterostructure generation for the methyl orange photodegradation under visible light irradiation has been reported before [67]. Table 3 lists other reports for the use of doped ZnO with different metals as catalysts. Our results indicate that the catalysts produced in this study are suitable options for solar photocatalytic applications.
**Table 3.** *kapp* values for different catalysts (ZnO doped with different metals) under visible irradiation.
The ZnO films did not show photocatalytic activity under visible irradiation. However, two different processes can contribute to photocatalytic degradation under visible irradiation: (i) the intraband transitions as dopants allow the doped ZnO thin films to absorb visible light, generating charge pairs; (ii) CuO and Co3O<sup>4</sup> can absorb visible light after the formation of ZnO–CuO and ZnO–Co3O<sup>4</sup> heterojunctions, generating charge pairs. In this case, the electron can be transferred to the conduction band of ZnO. After electrons are located at the conduction band of ZnO, different reactive oxygen species (ROS) can be generated (e.g., *O*<sup>−</sup> 2 ; *OH*), starting the MB photodegradation. Scheme 1 shows the general scheme of energetic levels for doped ZnO thin films and the ROS generation.
**Scheme 1.** Hypothetical scheme of energetic levels for metal-doped ZnO thin films: (**a**) Metal doping process. (**b**) Metal doping process and the generation of a heterojunction [64,70,71]*.* After charge pairs **Scheme 1.** Hypothetical scheme of energetic levels for metal-doped ZnO thin films: (**a**) Metal doping process. (**b**) Metal doping process and the generation of a heterojunction [64,70,71]. After charge pairs generation, the ROS can be yielded on the catalyst surface and MB degradation starts.
generation, the ROS can be yielded on the catalyst surface and MB degradation starts.
| doab | 2025-04-07T03:56:58.828135 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 77
} |
004db1df-dc76-42af-952f-1f5e82b7822b.78 | **3. Materials and Methods**
## **3. Materials and Methods** *3.1. Synthesis and Characterization*
*3.1. Synthesis and Characterization* The ZnO synthesis was carried out according to a previous report [69]: Twenty five mL of ammonium hydroxide (NH4OH) (25%**–**35% *w/w*) reactive grade (Merck) was placed in a 250 mL glass beaker, then 0.500 M (Zn(CH3COO)2.2H2O) (Merck) was added dropwise at a rate of 1.7 mL.min−1 for 1 hour, at a temperature of 85 °C under constant agitation at 300 rpm. After that, the The ZnO synthesis was carried out according to a previous report [69]: Twenty five mL of ammonium hydroxide (NH4OH) (25–35% *w*/*w*) reactive grade (Merck) was placed in a 250 mL glass beaker, then 0.500 M (Zn(CH3COO)2·2H2O) (Merck) was added dropwise at a rate of 1.7 mL.min−<sup>1</sup> for 1 h, at a temperature of 85 ◦C under constant agitation at 300 rpm. After that, the suspension stood for three days at room temperature then the solid was filtered and dried for 5 h at 100 ◦C [72,73].
suspension stood for three days at room temperature then the solid was filtered and dried for 5 h at 100 °C [72,73]. For the ZnO doping process, we used a similar procedure as previously described. While adding Zn2+ ions, we also added salts of the doping metals (CuSO4.5H2O) (Merck) for copper doping and (Co(CH3COO)2.4H2O) (JT Baker) for the cobalt doping processes. The synthesis of doped ZnO powder was performed at 1.0%, 3.0%, and 5.0%. The thin films were deposited using the doctor blade method, and the suspension was placed in a glass measuring 2 cm high and 2 cm wide. The thin films were heated for 30 min at 90 °C to evaporate the solvent, and finally, the sintering process was performed at 500 °C for 2 h [73,74]. Using this procedure, we obtained thin films (6 μm thickness). The film thickness was measured through a Veeco Dektak 150 profilometer. The physical chemistry properties of the films were studied by X-ray diffraction, diffuse reflectance spectrophotometry, and Raman spectroscopy assay. X-ray diffraction patterns were obtained using a Shimadzu 6000 diffractometer using Cu Kα radiation (λ = 0.15406 nm) as an X-ray source with a diffraction angle in the 2θ range (20**–**80°). Diffuse reflectance spectra were obtained with a Lambda 4 Perkin**–**Elmer spectrophotometer equipped with an integration sphere. The compositional properties of the materials were studied by Raman spectroscopy in a DXR device equipped with a 780 nm laser. The morphological properties were studied by scanning electron microscopy, under an excitation energy For the ZnO doping process, we used a similar procedure as previously described. While adding Zn2<sup>+</sup> ions, we also added salts of the doping metals (CuSO4·5H2O) (Merck) for copper doping and (Co(CH3COO)2·4H2O) (JT Baker) for the cobalt doping processes. The synthesis of doped ZnO powder was performed at 1.0%, 3.0%, and 5.0%. The thin films were deposited using the doctor blade method, and the suspension was placed in a glass measuring 2 cm high and 2 cm wide. The thin films were heated for 30 min at 90 ◦C to evaporate the solvent, and finally, the sintering process was performed at 500 ◦C for 2 h [73,74]. Using this procedure, we obtained thin films (6 µm thickness). The film thickness was measured through a Veeco Dektak 150 profilometer. The physical chemistry properties of the films were studied by X-ray diffraction, diffuse reflectance spectrophotometry, and Raman spectroscopy assay. X-ray diffraction patterns were obtained using a Shimadzu 6000 diffractometer using Cu Kα radiation (λ = 0.15406 nm) as an X-ray source with a diffraction angle in the 2θ range (20–80◦ ). Diffuse reflectance spectra were obtained with a Lambda 4 Perkin–Elmer spectrophotometer equipped with an integration sphere. The compositional properties of the materials were studied by Raman spectroscopy in a DXR device equipped with a 780 nm laser. The morphological properties were studied by scanning electron microscopy, under an excitation energy of 5 and 1 kV. The metallic content of the films was determined by plasma emission spectroscopy using the SM 3120 B technique, EPA 3015A modified for solids (see Supporting Information).
### of 5 and 1 kV. The metallic content of the films was determined by plasma emission spectroscopy using the SM 3120 B technique, EPA 3015A modified for solids (see Supporting Information). *3.2. Photocatalytic Test*
*3.2. Photocatalytic Test* Methylene blue (MB) was chosen as the pollutant model in this study. The experiments were Methylene blue (MB) was chosen as the pollutant model in this study. The experiments were carried out in a batch reactor using an LED tape as a source of visible radiation (cold white light 17 watts), and the incident photon flow per unit volume I<sup>o</sup> was 5.8 <sup>×</sup> 10−<sup>7</sup> Einstein\*L−<sup>1</sup> s −1 . Before irradiation,
carried out in a batch reactor using an LED tape as a source of visible radiation (cold white light 17 watts), and the incident photon flow per unit volume Io was 5.8 × 10−7 Einstein\*L−1s−1. Before the MB solution was kept in the dark for 90 min at 250 rpm to reach adsorption–desorption equilibrium on the catalysts' surface. Photodegradation was carried out using 50 ± 0.025 mL of an MB solution (10 mg·L −1 ) saturated with oxygen at pH 7.0. The concentration of dye was determined through the spectrophotometric method (Thermo Scientific–Genesys 10S) using 665 nm as a fixed wavelength, with a calibration curve (correlation coefficient R = 0.997) for the use of the Lambert–Beer equation.
| doab | 2025-04-07T03:56:58.828919 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 78
} |
004db1df-dc76-42af-952f-1f5e82b7822b.79 | **4. Conclusions**
We synthesized and characterized ZnO thin films doped with Co and Cu. Raman results corroborated the doping process, which suggested the generation of a heterostructure. For the doped ZnO catalysts, the results show a reduction in the *E<sup>g</sup>* values (from 3.22 to 2.42 eV for the best catalyst). This behavior is associated with a reduction of the Fermi level of ZnO by the presence of intragap states. The photocatalytic test indicated that doped catalysts had greater photocatalytic activity than unmodified catalysts, which could be attributed to (i) the generation of intraband states for the insertion of Co and Cu into ZnO, and (ii) the generation of ZnO–CuO and ZnO–Co3O<sup>4</sup> heterojunctions. Furthermore, the rate of the photodegradation process (ZnO:Co 5%) was 36 times greater than the rate for unmodified ZnO, and the k*app* values for the catalysts synthesized in this study had a suitable photocatalytic activity compared to other reports.
**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/10/5/528/s1. Figure S1: ICP calibration curve for: (a) Co and (b) Cu content. Table S1: Results ICP for catalysts. Fitting curves of kinetic model.
**Author Contributions:** Conceptualization, W.V.; methodology, W.V., C.D.-U., B.S., A.C., W.R., E.R. and M.H.; validation, W.V., C.D.-U., B.S., A.C.; formal analysis, W.V., C.D.-U., B.S., A.C.; investigation, W.V., C.D.-U., B.S., A.C.; resources, W.V., C.D.-U., B.S., A.C., W.R., E.R. and M.H.; data curation, W.V., C.D.-U., B.S., A.C.; writing—original draft preparation, W.V., C.D.-U., B.S., A.C., writing—review and editing, W.V., C.D.-U., B.S., A.C., W.R., E.R., M.H.; visualization, W.V., C.D.-U.; supervision, W.V., C.D.-U.; project administration, W.V., C.D.-U.; funding acquisition, W.V., C.D.-U. All authors have read and agreed to the published version of the manuscript.
**Funding:** This research was funded by Universidad del Atlántico.
**Acknowledgments:** W.V., C.D.-U., B.S., and A.C. would like to thank Universidad del Atlántico.
**Conflicts of Interest:** The authors declare that there is no conflict of interest.
| doab | 2025-04-07T03:56:58.829188 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 79
} |
004db1df-dc76-42af-952f-1f5e82b7822b.81 | *Article* **Magnetite, Hematite and Zero-Valent Iron as Co-Catalysts in Advanced Oxidation Processes Application for Cosmetic Wastewater Treatment**
**Jan Bogacki \* , Piotr Marcinowski , Dominika Bury , Monika Krupa, Dominika Scie ´ zy ´nska and ˙ Prasanth Prabhu**
> Faculty of Building Services, Hydro and Environmental Engineering, Warsaw University of Technology, 00-653 Warsaw, Poland; [email protected] (P.M.); [email protected] (D.B.); [email protected] (M.K.); [email protected] (D.S.); [email protected] (P.P.) ´ **\*** Correspondence: [email protected]
**Abstract:** Background: There is a need for more effective methods of industrial wastewater treatment. Methods: Cosmetic wastewater was collected and subjected to H2O2/Fe3O4/Fe2O3/Fe<sup>0</sup> and UV/H2O2/Fe3O4/Fe2O3/Fe<sup>0</sup> process treatment. Results: Total organic carbon (TOC) was decreased from an initial 306.3 to 134.1 mg/L, 56.2% TOC removal, after 120 min of treatment for 1:1 H2O2/COD mass ratio and 500/500/1000 mg/L Fe3O4/Fe2O3/Fe<sup>0</sup> catalyst doses. The application chromatographic analysis allowed for the detection and identification of pollutants present in the wastewater. Identified pollutants were removed during the treatment processes. Processes carried out at a pH greater than 3.0 were ineffective. The UV process was more effective than the lightless process. Conclusions: The applied processes are effective methods for wastewater treatment. Chromatographic results confirmed the effectiveness of the treatment method. The kinetics of the process were described by the modified second-order model. On the basis of ANOVA results, the hypothesis regarding the accuracy and reproducibility of the research was confirmed.
**Keywords:** industrial wastewater; advanced oxidation processes; zero valent iron; magnetite; hematite
| doab | 2025-04-07T03:56:58.829354 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 81
} |
004db1df-dc76-42af-952f-1f5e82b7822b.82 | **1. Introduction**
The cosmetics market is booming and it is one of the fastest growing consumer markets. It globally generated EUR 474.2 billion in 2019. The coronavirus pandemic resulted in a decrease in industry revenues in 2020 by only 1.2%, to EUR 468.3 billion [1].
The constantly increasing production of cosmetics is accompanied by the side effect of producing increasing amounts of waste and wastewater. Cosmetic wastewater (CW) is created by washing production lines with water with surfactants and disinfectants, so CW contains the same compounds as those that are present in cosmetics. A typical industrial-scale CW treatment method is coagulation coupled with dissolved air flotation (C/DAF) followed by biological treatment [2,3]. This method is highly effective, but not enough [4] to remove micropollutants considered to be particularly harmful, such as polycyclic musk, UV filters, heavy metals, and microplastics [5–11]. Fragrances and UV filters are contaminants of emerging concern (CEC) [5]. The most commonly used and thus detected in the polycyclic musk environment are galaxolide (1,3,4,6,7,8-hexahydro-4,6,6,7,8,8,-hexamethyl-cyclopenta[g]benzopyran, HHCB) and tonalide (6-acetyl-1,1,2,4,4,7 hexamethyltetraline, AHTN), while the most important UV filters are benzophenone-3 (2-hydroxy-4-methoxyphenyl)-phenylmethanone, BP-3) and 4-MBC (4-methylbenzylidene camphor). These compounds often have the potential for bioaccumulation and also show estrogenic activity [6]. Heavy metals such as Zn, Cu, and Fe are typically used in cosmetics as physical UV filters, dyes, or enzyme components. However, even metals such as silver or bismuth are used as bactericides or mask ingredients [7]. Plastics are usually chemically
**Citation:** Bogacki, J.; Marcinowski, P.; Bury, D.; Krupa, M.; Scie ´ zy ´nska, D.; ˙ Prabhu, P. Magnetite, Hematite and Zero-Valent Iron as Co-Catalysts in Advanced Oxidation Processes Application for Cosmetic Wastewater Treatment. *Catalysts* **2021**, *11*, 9. https://dx.doi.org/10.3390/catal110 10009
Received: 20 November 2020 Accepted: 22 December 2020 Published: 24 December 2020
**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.
**Copyright:** © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/ licenses/by/4.0/).
inert, but under environmental conditions, they are broken down into microscopic grains that penetrate the body even at the cellular level. Their content in organisms increases as they move up the food chain [8]. Due to their persistence to decomposition, they form layers or even islands floating on the water or they accumulate in the soil or bottom sediments, depending on their density [9]. There is a need to develop a policy for dealing with substances that are components of cosmetics [10]. Cosmetic micropollutants during treatment in a biological treatment plant do not decompose but pass into the sludge phase [6]. Their presence is detected in globally collected environmental samples, in concentrations usually below 100 µg/L or 100 µg/kg, depending on sample type [11].
In order to increase the effectiveness of CW treatment, the possibilities of improving classically used coagulation and DAF processes were investigated [12–15]. Advanced oxidation processes (AOPs) [15–19] and the improvement of biological treatment [20–23] were also tested. Attempts were also made to improve the entire treatment, including both chemical and biological methods [24–26].
Many alternatives to classical coagulation and DAF for CW treatment technologies are being developed. Promising ones are AOPs, consisting of the effective generation of strong oxidants, namely radicals. In the case of AOPs in which the production of radicals is catalyzed by the presence of Fe2+ ions (Fenton's process and its modifications), a major problem [27] is to ensure the appropriate quantity and availability of Fe2+ ions. The amount of Fe2+ ions in a solution is influenced by many factors, including pH, the efficiency of Fe2+ ion recovery from Fe3+, and the rate of Fe2+ ion release from the carrier. This problem is solved in two ways: by controlling the Fe2+/Fe3+ ions ratio or by the controlled continuous introduction of Fe2+ ions into the solution. Both strategies pose numerous technical difficulties when applied in practice; therefore, iron-based heterogeneous cocatalysts are gaining interest. Among them are Fe<sup>0</sup> (metallic iron, zero-valent iron, ZVI), Fe2O<sup>3</sup> (hematite), and Fe3O<sup>4</sup> (magnetite) [28–31]. Oxides act through coordinating surface sites of Fe2+ that form complexes with contaminants and reduce them [28].
The aim of this study is to determine the effectiveness of the joint use of Fe<sup>0</sup> , Fe2O3, and Fe3O<sup>4</sup> as mutually supportive catalysts using synergy effects in the AOP treatment of industrial wastewater. This is the first article where Fe2O3, Fe3O4, and Fe<sup>0</sup> were mutually used in one process as co-catalysts supporting modified Fenton processes to treat cosmetic wastewater.
| doab | 2025-04-07T03:56:58.829498 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 82
} |
004db1df-dc76-42af-952f-1f5e82b7822b.84 | *2.1. Raw Wastewater*
CW parameters used in the experiments are shown in Table 1. Low values of parameters indicating the content of organic compounds (total organic carbon, TOC and chemical oxygen demand, COD), and the almost complete absence of suspended solids (TSS) and nitrogen compounds (total Kjeldahl nitrogen, TNK), indicated the effective operation of the preliminary treatment (C/DAF) at the production plant. The very high value of the electrolytic conductivity indicated significant wastewater salinity that cannot be derived from only aluminum coagulants used in wastewater treatment in the factory or reagents for pH correction. Despite the high five day biochemical oxygen demand (BOD5) value and theoretically high potential susceptibility to biological treatment (described as BOD5/COD ratio, 0.382), even a small amount of raw cosmetic wastewater has a negative effect on biological wastewater treatment plants, hence the need for further treatment.
**Table 1.** Cosmetic wastewater parameters.
| doab | 2025-04-07T03:56:58.829891 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 84
} |
004db1df-dc76-42af-952f-1f5e82b7822b.85 | *2.2. Kinetics Matching*
In the case of the classical Fenton process, it involves catalytic radical oxidation and final coagulation combined with coprecipitation. The applied modification of the process causes the concentration of iron (II) ions to change due to the dissolution of metallic iron— Fe (II)—amount constantly increasing. In addition, as a result of UV irradiation, there is an increased reduction in iron (III) during the Fenton reaction to iron (II), which results in at least a theoretical decrease in homogeneous catalyst demand. The use of magnetite and hematite as cocatalysts, and metallic iron, leads to the appearance of the surface of solid catalyst heterogeneous processes, including sorption or ion exchange. All these processes are at least partially independent and sometimes even antagonistic. Therefore, describing the kinetics of the treatment process is not easy. Four equations were used to describe the kinetics:
• First-order reaction with respect to the TOC value:
$$\text{TOC} = \text{TOC}\_0 \times \text{e}^{-\text{kt}} \tag{1}$$
• Second-order reaction with respect to the TOC value:
$$\text{TOC} = \left(\text{kt} + 1/\text{TOC}\_0\right)^{-1} \tag{2}$$
• Modified first-order reaction with respect to the TOC value:
$$\text{TOC} = (\text{TOC}\_0 - \text{b}) \times \text{e}^{-\text{kt}} + \text{b} \tag{3}$$
• Modified second-order reaction with respect to the TOC value:
$$\text{TOC} = (\text{kt} + (\text{TOC}\_0 - \text{b})^{-1})^{-1} + \text{b} \tag{4}$$
Equations (1) and (2) are the descriptions of the usual first- and second-order kinetics. Typically, however, the kinetics of a specific chemical reaction is described with clearly defined substrates and products. In the case of the description of wastewater, it is a complex mixture of many chemical compounds present in various concentrations. From a practical point of view, it is not possible to determine the concentrations of all chemical compounds and, most importantly, to predict all chemical reactions taking place. Therefore, collective parameters such as BOD5, COD, and TOC are described. In the case of treatment processes where hydrogen peroxide is used, it may remain after the process. While we ran the process to ensure that it was decomposed (and iodometrically checked), the decomposition of hydrogen peroxide takes time. Hydrogen peroxide is a well-known disruptor in COD measurement. During radical oxidation, wastewater is at least partially sterilized, which also affects BOD determination. Although both disturbing factors (in the determination of BOD and COD) can be eliminated, from a practical point of view, it is easier to use TOC notation, which is considered more reliable and unambiguous in its interpretation. Therefore, it was decided to describe all kinetics in relation to one collective TOC parameter. The idea behind first- and second-order kinetics is that the reaction can be
completed, i.e., until the substrate is completely used. Under the conditions of our experiment, this means zeroing the TOC value and complete decomposition of the pollutants. However, it is not possible to obtain complete TOC elimination. There is always a certain amount left, hence the idea to modify the description of kinetics. As such, there was a certain number of compounds that could be removed in our process, but some would be persistent to decomposition. The amounts of these substances can be described as possible and impossible to remove TOC. Value "b" Equations (3) and (4), represents the content of this persistent, so-called "hard" TOC. The remaining amount of nonpersistent TOC can still be decomposed, and the description is related to first- or second-order kinetics. An example of the application of four kinetic models is presented in Figure 1. The best match was obtained for the modified second-order kinetics model. be completed, i.e., until the substrate is completely used. Under the conditions of our experiment, this means zeroing the TOC value and complete decomposition of the pollutants. However, it is not possible to obtain complete TOC elimination. There is always a certain amount left, hence the idea to modify the description of kinetics. As such, there was a certain number of compounds that could be removed in our process, but some would be persistent to decomposition. The amounts of these substances can be described as possible and impossible to remove TOC. Value "b" Equations (3) and (4), represents the content of this persistent, so-called "hard" TOC. The remaining amount of nonpersistent TOC can still be decomposed, and the description is related to first- or second-order kinetics. An example of the application of four kinetic models is presented in Figure 1. The best match was obtained for the modified second-order kinetics model.
**Figure 1.** Example kinetic model results: 1500/1500/1000 Fe3O4/Fe2O3/Fe0 doses (mg/L) H2O2/COD mass ratio 1:1, UV irradiation, pH = 3.0. **Figure 1.** Example kinetic model results: 1500/1500/1000 Fe3O4/Fe2O3/Fe<sup>0</sup> doses (mg/L) H2O2/COD mass ratio 1:1, UV irradiation, pH = 3.0.
### *2.3. Treatment Processes 2.3. Treatment Processes*
Detailed doses and proportions of the reagents used during the research on CW catalytic treatment are presented in Table S1, while treatment results are shown in Figures Detailed doses and proportions of the reagents used during the research on CW catalytic treatment are presented in Table S1, while treatment results are shown in Figures 2–5.
2–5. Treatment is more effective as the process takes longer to run. The use of UV light increases the effectiveness of the treatment compared to a non-light-assisted process with the same doses of reagents.
In each of the non-light-assisted experiments, the most intensive TOC removal, around 50 mg/L, was obtained in the first 15 min. Such a situation could be observed, for, e.g., 2:1 H2O2/COD ratio and 250/250/1500 mg/L Fe3O4/Fe2O3/Fe<sup>0</sup> catalyst doses (Figure 3). In the mentioned sample, after 30 min of the process, TOC was 221.7 mg/L; for 15 min from the first measurement, it was decreased by 27 mg/L. During subsequent measurements made at 15-, 30-, and 60-min time differences, TOC decreased more slowly. A better treatment effect was obtained by using a lower ratio of 1:1 H2O2/COD. The lowest TOC, 182.0 mg/L, was obtained for 1:1 H2O2/COD ratio and 500/500/3000 mg/L Fe3O4/Fe2O3/Fe<sup>0</sup> catalyst doses after a 120 min process time (Figure 2). The second-lowest TOC, 198.4 mg/L, was for 1:1 H2O2/COD ratio and 500/500/1000 mg/L Fe3O4/Fe2O3/Fe<sup>0</sup> catalyst doses. The highest TOC 218.0 mg/L, only 28.8% TOC removal after a 120-min process time, was obtained for 375/375/250 mg/L Fe3O4/Fe2O3/Fe<sup>0</sup> catalyst doses and 2:1 H2O2/COD ratio. On the basis of the presented data, for non-light-assisted processes, with regard to catalyst doses at constant values of hydrogen peroxide, lower TOC values were achieved for higher doses of the catalysts: 4000 mg/L, slightly lower for 2000 mg/L,
and the lowest for 1000 mg/L. In most cases, lower values of TOC were recorded for the lower 1:1 H2O2/COD ratio. The TOC values determined after the treatment process with the 2:1 H2O2/COD ratio were higher than the value for 1:1 H2O2/COD ratio. The exception was the process involving 1000 mg/L of hematite, 1000 mg/L of magnetite, and 2000 mg/L of metallic iron, in which the difference was 40 mg/L. *Catalysts* **2021**, *11*, x FOR PEER REVIEW 5 of 14 *Catalysts* **2021**, *11*, x FOR PEER REVIEW 5 of 14
**Figure 2.** Cosmetic wastewater (CW) treatment results with different Fe3O4/Fe2O3/Fe0 doses (mg/L) H2O2/COD mass ratio 1:1, without UV irradiation, pH = 3.0. **Figure 2.** Cosmetic wastewater (CW) treatment results with different Fe3O4/Fe2O3/Fe<sup>0</sup> doses (mg/L) H2O2/COD mass ratio 1:1, without UV irradiation, pH = 3.0. **Figure 2.** Cosmetic wastewater (CW) treatment results with different Fe3O4/Fe2O3/Fe0 doses (mg/L) H2O2/COD mass ratio 1:1, without UV irradiation, pH = 3.0.
2:1, without UV irradiation, pH = 3.0. **Figure 3.** CW treatment results with different Fe3O4/Fe2O3/Fe0 doses (mg/L) H2O2/COD mass ratio 2:1, without UV irradiation, pH = 3.0. **Figure 3.** CW treatment results with different Fe3O4/Fe2O3/Fe<sup>0</sup> doses (mg/L) H2O2/COD mass ratio 2:1, without UV irradiation, pH = 3.0.
*Catalysts* **2021**, *11*, x FOR PEER REVIEW 6 of 14
**Figure 4.** CW treatment results with different Fe3O4/Fe2O3/Fe0 doses (mg/L) H2O2/COD mass ratio 1:1, UV irradiation, pH = 3.0. **Figure 4.** CW treatment results with different Fe3O4/Fe2O3/Fe<sup>0</sup> doses (mg/L) H2O2/COD mass ratio 1:1, UV irradiation, pH = 3.0. **Figure 4.** CW treatment results with different Fe3O4/Fe2O3/Fe0 doses (mg/L) H2O2/COD mass ratio 1:1, UV irradiation, pH = 3.0.
**Figure 5.** CW treatment results with different Fe3O4/Fe2O3/Fe0 doses (mg/L) H2O2/COD mass ratio 2:1, UV irradiation, pH = 3.0. **Figure 5.** CW treatment results with different Fe3O4/Fe2O3/Fe0 doses (mg/L) H2O2/COD mass ratio 2:1, UV irradiation, pH = 3.0. **Figure 5.** CW treatment results with different Fe3O4/Fe2O3/Fe<sup>0</sup> doses (mg/L) H2O2/COD mass ratio 2:1, UV irradiation, pH = 3.0.
Treatment is more effective as the process takes longer to run. The use of UV light increases the effectiveness of the treatment compared to a non-light-assisted process with the same doses of reagents. In each of the non-light-assisted experiments, the most intensive TOC removal, around 50 mg/L, was obtained in the first 15 min. Such a situation could be observed, for, e.g., 2:1 H2O2/COD ratio and 250/250/1500 mg/L Fe3O4/Fe2O3/Fe0 catalyst doses (Figure 3). In the mentioned sample, after 30 min of the process, TOC was 221.7 mg/L; for 15 min from the first measurement, it was decreased by 27 mg/L. During subsequent measurements made at 15-, 30-, and 60-min time differences, TOC decreased more slowly. A better treatment effect was obtained by using a lower ratio of 1:1 H2O2/COD. The lowest TOC, 182.0 mg/L, was obtained for 1:1 H2O2/COD ratio and 500/500/3000 mg/L Fe3O4/Fe2O3/Fe0 Treatment is more effective as the process takes longer to run. The use of UV light increases the effectiveness of the treatment compared to a non-light-assisted process with the same doses of reagents. In each of the non-light-assisted experiments, the most intensive TOC removal, around 50 mg/L, was obtained in the first 15 min. Such a situation could be observed, for, e.g., 2:1 H2O2/COD ratio and 250/250/1500 mg/L Fe3O4/Fe2O3/Fe0 catalyst doses (Figure 3). In the mentioned sample, after 30 min of the process, TOC was 221.7 mg/L; for 15 min from the first measurement, it was decreased by 27 mg/L. During subsequent measurements made at 15-, 30-, and 60-min time differences, TOC decreased more slowly. A better treatment effect was obtained by using a lower ratio of 1:1 H2O2/COD. The lowest TOC, 182.0 mg/L, was obtained for 1:1 H2O2/COD ratio and 500/500/3000 mg/L Fe3O4/Fe2O3/Fe0 In the light-assisted process, the lowest TOC, 134.1 mg/L (56.2% TOC removal), was obtained after 120 min for 1:1 H2O2/COD ratio and 500/500/1000 mg/L Fe3O4/Fe2O3/ Fe<sup>0</sup> catalyst doses (Figure 5). In the initial phase of the process, a slower decrease in TOC value was visible for the 2:1 H2O2/COD ratio, to 260–270 mg/L. However, a faster decrease was observed in the mixture of iron compounds mass equal to 4000 mg/L. This may indicate that the weight of the catalysts was optimal for the higher dose of hydrogen peroxide and accelerated the process. For smaller total catalyst concentrations, 2:1 H2O2/COD ratio led to the inhibition of the reaction. The decrease in TOC value for 4000 mg/L of catalyst 2:1 H2O2/COD ratio was comparable with that in samples with a lower oxidant concentration for up to 30 min. After this time, for the 1:1 H2O2/COD ratio, reactions slowed down significantly. An exception may be the sample of 500/500/1000 mg/L Fe3O4/Fe2O3/Fe<sup>0</sup>
catalyst doses, where TOC decreased throughout the experiment. In this case, the heterocatalytic reaction could have contributed to the steady decline. The optimal selection of reagent doses ensured the decomposition of the organic pollutants on the surface of the catalysts. For a higher concentration of the oxidant, the given doses of the catalysts did not give an outstanding result, and at a lower concentration of H2O2, it had a greater effect on TOC decomposition. In the experiments where the concentration of iron compounds was 4000 mg/L, there was no visible difference in the rate of the processes, resulting from the concentration of hydrogen peroxide. For the experiments carried out at the concentration of iron compounds of 2000 and 1000 mg/L, however, there was a difference according to the dose of hydrogen peroxide. Lower concentrations of the oxidant resulted in faster TOC removal in the first few minutes of the experiment. At higher concentrations, removal was slower, and more time was required for the reaction to come to a halt. Even though the reaction took longer, in most cases, efficiency for TOC removal at a higher concentration of H2O<sup>2</sup> did not exceed the effectiveness for the 1:1 H2O2/COD ratio. The exceptions were the concentrations of 1000/1000/3000 mg/L Fe3O4/Fe2O3/Fe<sup>0</sup> , in which efficiency was higher at a higher oxidant concentration.
Additionally, an experiment was performed that demonstrated the influence of pH on the efficiency of the pollutant oxidation process (Figure 6). The process was the most effective at pH 2 and 3. The processes carried out at a pH greater than 3 were ineffective, and the decrease in TOC value from 15 min until the end of the experiment was not significant. At pH 2 and 3, TOC decrease was visible throughout the process. At pH 2, sediment in the sample was swollen and occupied the largest volume compared to in the other samples. Sediment after the process carried out at pH 4 was a reddish color and had a volume comparable to that in the processes at pH 3. Obtained sediment during the experiment at pH 5 was brown, and its amount was the smallest in comparison to that formed during the process at other pH values. In the experiment carried out at pH 6, the sediment after the process was reddish, and its structure was comparable to that of the sediment at pH 3 and 4. The red may have indicated the presence of iron (III) hydroxide, which is formed at a high solution pH. *Catalysts* **2021**, *11*, x FOR PEER REVIEW 8 of 14
**Figure 6.** CW treatment results at different pH, 500/500/1000 Fe3O4/Fe2O3/Fe0 doses (mg/L) H2O2/COD mass ratio 1:1, UV irradiation. **Figure 6.** CW treatment results at different pH, 500/500/1000 Fe3O4/Fe2O3/Fe<sup>0</sup> doses (mg/L) H2O2/COD mass ratio 1:1, UV irradiation.
H2O2 is a weak acid. Its stability increases with a decrease in pH value, which may impede its catalytic decomposition into a hydroxyl ion and radical. However, these effects were not strongly observed during the experiment. Hydrogen peroxide under alkaline H2O<sup>2</sup> is a weak acid. Its stability increases with a decrease in pH value, which may impede its catalytic decomposition into a hydroxyl ion and radical. However, these effects
conditions decomposes rapidly with the evolution of oxygen. For this reason, no attempt
served in all experiments at the termination stage of the Fenton reaction. In the case of iron (II) and (III), its source in the process was twofold: the surface of stable iron oxides magnetite and hematite, and the dissolution of metallic iron. The solubility (corrosion) of metallic iron occurs quickly under strongly acidic conditions; under neutral conditions, the process is very slow. Therefore, in neutral conditions, contact between the reactants is difficult to achieve during the entire duration of the process. In an acidic environment, due to the constant increase in the content of dissolved iron, its availability, and thus the intensity of the Fenton reaction, increases steadily with time. Additionally, the form of iron is strongly pH-dependent. At pH 5, iron hydroxides with small solubility begin to form, and the coagulation process begins. For iron hydroxide, due to minimal solubility, the optimal value for carrying out the coagulation process was around 6.0 and above 8.5. This was another reason for abandoning the experiment in alkaline conditions, as radicals were terminated on the sludge flocs, which resulted in rapidly decreasing process effi-
The statistical analysis was described on the basis of Miller [32]. ANOVA was used to determine the magnitude of variability in the average concentrations of TOC and to check whether differences in the average test results for TOC for individual process conditions (for different doses of catalysts) may have been caused by random errors (Tables
Variance is estimated using two methods: the method determining the variability within a given sample, and variability between samples. The difference in the performed tests was the different durations and reagent doses of the process. The above statement is
If the hypothesis were true, then there would be no large difference between calculated values. If the hypothesis were not true, then the between-group estimate would be greater than the intergroup estimate. This is due to the high variability between samples.
ciency.
S2 and S3, Figures S4 and S5).
a null hypothesis.
were not strongly observed during the experiment. Hydrogen peroxide under alkaline conditions decomposes rapidly with the evolution of oxygen. For this reason, no attempt was made to operate the process under alkaline conditions. However, the presence of the formation of oxygen bubbles hindering the sedimentation of the formed sludge was observed in all experiments at the termination stage of the Fenton reaction. In the case of iron (II) and (III), its source in the process was twofold: the surface of stable iron oxides magnetite and hematite, and the dissolution of metallic iron. The solubility (corrosion) of metallic iron occurs quickly under strongly acidic conditions; under neutral conditions, the process is very slow. Therefore, in neutral conditions, contact between the reactants is difficult to achieve during the entire duration of the process. In an acidic environment, due to the constant increase in the content of dissolved iron, its availability, and thus the intensity of the Fenton reaction, increases steadily with time. Additionally, the form of iron is strongly pH-dependent. At pH 5, iron hydroxides with small solubility begin to form, and the coagulation process begins. For iron hydroxide, due to minimal solubility, the optimal value for carrying out the coagulation process was around 6.0 and above 8.5. This was another reason for abandoning the experiment in alkaline conditions, as radicals were terminated on the sludge flocs, which resulted in rapidly decreasing process efficiency.
The statistical analysis was described on the basis of Miller [32]. ANOVA was used to determine the magnitude of variability in the average concentrations of TOC and to check whether differences in the average test results for TOC for individual process conditions (for different doses of catalysts) may have been caused by random errors (Tables S2 and S3, Figures S4 and S5).
Variance is estimated using two methods: the method determining the variability within a given sample, and variability between samples. The difference in the performed tests was the different durations and reagent doses of the process. The above statement is a null hypothesis.
If the hypothesis were true, then there would be no large difference between calculated values. If the hypothesis were not true, then the between-group estimate would be greater than the intergroup estimate. This is due to the high variability between samples. To check if the difference was significant, Snedecor's one-sided F test with α = 0.05 was performed.
The following null hypothesis was made for ANOVA: the tests were performed accurately, and reproducibility was achieved in TOC results. The value of the F parameter was lower than that of the critical F, which means that the hypothesis is true. The mean values in the samples were similar, and similar conclusions were found during TOC analysis. Even the optimal value did not significantly differ from the other values. From the perspective of the performed tests, this is a favorable phenomenon, as it proved the accuracy and repeatability of the performed tests. However, the process itself was not effective, and from the perspective of the conducted process, its effectiveness was not favorable.
| doab | 2025-04-07T03:56:58.829969 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 85
} |
004db1df-dc76-42af-952f-1f5e82b7822b.86 | *2.4. HS-SPME–GC–MS Analysis*
Head space-solid phase micro extraction–gas chromatography–mass spectrometry (HS-SPME–GC–MS) analysis results are shown in Table 2. GC-MS chromatograms, for raw and treated samples (the sample with the lowest TOC after the process was selected), are shown in Figures S1 and S2.
The identified compounds were mainly cosmetic bases (e.g., decamethyltetrasiloxane or decamethylcyclopentasiloxane) and fragrances (e.g., 1,3,4,6,7,8-hexahydro-4,6,6, 7,8,8-hexamethyl-cyclopenta(*g*)-2-benzopyran (galaxolide) or 4-isopropenyl-1-methyl-1 cyclohexene (limonene)).
All compounds detected with HS-SPME-GC-MS were removed during the treatment process. No new compound was detected during the process. HS-SPME-GC-MS is a useful tool that can be used to confirm the high efficiency of the treatment process.
**Table 2.** Raw wastewater head space-solid phase micro extraction–gas chromatography–mass spectrometry (HS-SPME–GC– MS) analysis results.
| doab | 2025-04-07T03:56:58.831021 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 86
} |
004db1df-dc76-42af-952f-1f5e82b7822b.87 | **3. Discussion**
An innovative solution was applied that has not yet been used as a cosmetic wastewater treatment technique, nor has it appeared in other industries. After separate analysis of the effectiveness of each catalyst, metallic iron with hematite and metallic iron with magnetite [33], the compounds were combined to create a unique mixture in order to check its properties and effectiveness during the treatment of cosmetic wastewater. The highest efficiency of the process was achieved when using the catalyst proportion in which there was a significant advantage of metallic iron and a comparable lower dose of magnetite and hematite (quantitative ratio of the compounds was 250/250/1500 mg/L). The lowest efficiency of the process was obtained when the catalyst was used, in which a significantly lower dose of metallic iron of 250 mg/L, and slightly higher concentrations of hematite and magnetite were used, 375 mg/L in all cases. When using a total concentration of 2000 mg/L of the mixed catalyst, the process was the most effective. The lowest concentrations were obtained with the use of a lower dose of 1000 mg/L reagent. The lower dose was insufficient to efficiently perform oxidation. When a higher dose of hydrogen peroxide was used, the process was also not as effective as when a higher dose of the mixed catalysts was used.
Analyzing the obtained results, they complied with those of earlier research [33], indicating that the excess of hydrogen peroxide adversely affects the performed process, decreasing its effectiveness.
The use of a higher dose of the oxidant caused a lower efficiency in TOC removal from CW. Excess hydrogen peroxide is an inhibitor of the reaction. Then, a process may take place (reaction Equation (5)) where a hydroperoxide radical is formed (oxidationreduction potential 1.7 V) which is much less reactive than the hydroxyl radical (2.8 V). Hydroperoxide radicals react with hydroxyl radicals (reaction Equation (6)) to form a water molecule and an oxygen molecule. Reactive molecules merge with each other to form substrates that are not strong oxidants.
$$\text{HO}^{\bullet} + \text{H}\_2\text{O}\_2 \rightarrow \text{HO}\_2^{\bullet} + \text{H}\_2\text{O} \tag{5}$$
$$\rm{HO}\_2\rm{}^{\bullet} + \rm{HO}^{\bullet} \rightarrow \rm{H}\_2\rm{O} + \rm{O}\_2\tag{6}$$
$$\text{Fe}^{2+} + \text{H}\_2\text{O}\_2 \rightarrow \text{FeO}^{2+} + \text{H}\_2\text{O} \tag{7}$$
$$\text{FeO}^{2+} + \text{H}\_2\text{O}\_2 \rightarrow \text{Fe}^{2+} + \text{O}\_2 + \text{H}\_2\text{O} \tag{8}$$
$$\text{FeO}^{2+} + \text{RH} \rightarrow \text{Fe}^{2+} + \text{ROH} \tag{9}$$
$$\text{[Fe(C}\_2\text{O}\_4)] + \text{H}\_2\text{O}\_2 \rightarrow \text{[Fe(C}\_2\text{O}\_4)]^+ + \text{OH}^- + \text{OH}^\bullet \tag{10}$$
$$\text{[Fe(C}\_2\text{O}\_4)\_3]^{3-} + \text{hv} \rightarrow [\text{Fe(C}\_2\text{O}\_4)\_2]^{2-} + \text{C}\_2\text{O}\_4 \text{°{}^{\bullet}\$} \tag{11}$$
$$\text{Fe}^{3+} + \text{C}\_2\text{O}\_4^{\bullet-} \rightarrow \text{Fe}^{3+} + 2\text{CO}\_2 \tag{12}$$
$$\text{Fe(RCOO)}^{2+} + \text{hv} \rightarrow \text{Fe}^{2+} + \text{R}^{\bullet} + \text{CO}\_2 \tag{13}$$
Intermediate compounds are formed in the catalytic cycle. One is oxoiron FeIVO [34], which is formed as a result of the reaction initiating the Fenton process, described by reaction Equation (7).
With an excess of hydrogen peroxide, oxoion reacts with it and forms Fe2+, oxygen, and water (reaction (8)). This is a reaction that stops the process. Oxoiron is also involved in the oxidation of organics (reaction (9)), which is much slower than the one with the hydroxyl radical.
Factors accelerating the formation of radicals may be ligands present in CW, which form complexes and chelates with iron. The oxalate ligand reacts with hydrogen peroxide to form a hydroxyl radical according to reaction Equation (10). Oxalates (diethyl, dimethyl, diisopropyl, diisobutyl, sodium) are present in cosmetics and thereby also in CW. In acidic conditions, at a low pH of 3.0, acid hydrolysis of oxalates esters could take place.
The process was supported by UV radiation (full emission spectrum is shown in Figure S3), so its influence on reaction kinetics in the presence of oxalates should be considered. Absorbed radiation causes the decarboxylation of the ligand with the release of CO<sup>2</sup> and reduction in Fe3+ to Fe2+ (reactions (11) and (12)). The oxidation of organic compounds occurs at high speed in the presence of oxalates. Complex compounds with carboxylic acids under UV radiation reduce the iron, and alkyl radical and carbon dioxide are produced (reaction (13)). These reactions lead to the fast mineralization of organic compounds in wastewater.
Improperly selected doses of catalysts (too small an amount of iron) have a negative effect on the high-efficiency treatment process. Regardless of the amount of catalyst and hydrogen peroxide, the effectiveness of the treatment processes increases with time.
The longest process time, 120 min, was the most effective. The 15 min process time was the least effective, as this was not enough time to carry out the treatment process. However, the duration and higher costs of the process should be considered, and the optimal time should be selected so as to maintain high efficiency with an appropriate cost of treatment. For lightless processes and 120-min process time, the process is no longer profitable, and an equally high efficiency was achieved after 60 min of treatment. Such an observation was not made in the case of light-assisted processes. For them, extending the process time to 120 min is still profitable. The process showed the greatest efficiency in relation to its duration during the first 15 min, and TOC was mostly decreased in a short period of time. Then, sequential TOC measurements showed lower speed of the treatment process.
Most of the studies that used hematite or magnetite were carried out on pollutants present in components. For example, hematite was used as a catalyst by Araujo et al. [35]. In total, 20 g of hematite and a dose of 800 mg/L of hydrogen peroxide allowed for achieving 99% treatment efficiency of the process after 120 min. Our research was carried out for much lower catalyst doses and a different type of contaminant with a much more complex matrix (wastewater from the cosmetic industry).
In samples taken after 15- and 30-min process time, a large amount of evolving gas bubbles was visible because of the decomposition of unreacted hydrogen peroxide. The color of the precipitate depended on the ratio of iron to magnetite and hematite. Dependence was visible; the greater the amount of metallic iron in the sample was, the more orange the sediment was. No greenish sediment was observed, which would indicate the presence of Fe2+ ions. On this basis, it was concluded that the oxidant doses were correctly selected.
There was a separation in the sediment phases into an upper rusty one (oxidized iron) and lower black one (magnetite, metallic iron). The exact mechanism of the separation process is unknown, but this could be due to the difference between densities, as iron hydroxide has a lower density than that of magnetite and metallic iron.
| doab | 2025-04-07T03:56:58.831103 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 87
} |
004db1df-dc76-42af-952f-1f5e82b7822b.89 | *4.1. Wastewater*
Samples of real cosmetic wastewater, pre-treated by coagulation coupled with dissolved air flotation, were taken for the tests. The samples were taken from an industrial plant located in Poland.
| doab | 2025-04-07T03:56:58.831569 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 89
} |
004db1df-dc76-42af-952f-1f5e82b7822b.90 | *4.2. Treatment Process*
Zero-valent iron (Ferox Target, 325 mesh) was supplied by Hepure (Hillsborough, NJ, USA). Hematite (10 µm) was supplied by Kremer (Aichstetten, Germany); magnetite (10 µm) was supplied by Kremer (Aichstetten, Germany); 30% H2O<sup>2</sup> solution was supplied by Stanlab (Lublin, Poland).
Doses of H2O2, Fe<sup>0</sup> , magnetite, and hematite were selected in preliminary tests. Treatment processes were carried out in a 1.5 L reactor filled with 1 L of the sample, stirred at 300 rpm (Heidolph MR3000, Schwabach, Germany). pH in the treatment processes was set to 3.0, unless otherwise stated. The experiment on the influence of pH on the efficiency of the process was carried out at pH 2.0, 3.0, 4.0, 5.0 and 6.0. Samples were taken after 15, 30, 60 and 120 min of the process. The process was stopped as a result of alkalization to pH 9.0 with 3M NaOH (POCh, Gliwice, Poland).
The source of radiation was medium-pressure Fe/Co 400W lamp type HPA 400/30 SDC with 94W UVA power (Philips, Amsterdam, The Netherlands). The lamp spectrum is shown in Figure S3. Details of the experiment setup are shown in Table S1.
| doab | 2025-04-07T03:56:58.831611 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 90
} |
004db1df-dc76-42af-952f-1f5e82b7822b.91 | *4.3. Analytical Methods*
Total organic carbon (TOC), five-day biochemical oxygen demand (BOD5), chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), total suspended solids (TSS), ammonia, surfactants, pH, and conductivity were determined according to the standard methods.
TOC was determined using a TOC-L analyzer (Shimadzu, Kyoto, Japan) with an OCT-L8-port sampler (Shimadzu, Kyoto, Japan). Chromatographic analysis was conducted with an Agilent 7890A (Santa Clara, CA, USA) gas chromatograph coupled with a Leco TruTOF (St. Joseph, MI, USA) mass spectrometer. The detailed methodology is described elsewhere [33].
| doab | 2025-04-07T03:56:58.831710 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 91
} |
004db1df-dc76-42af-952f-1f5e82b7822b.92 | **5. Conclusions**
Due to the increasing consumption of cosmetics, an effective and inexpensive method of CW treatment is needed. The effective treatment of CW in accordance with applicable legal standards is difficult. The results of this research confirmed the effectiveness of the pretreatment of wastewater currently used by most industrial plants with the use of C/DAF.
CW can be effectively treated with both the UV/H2O2/Fe3O4/Fe2O3/Fe<sup>0</sup> and the H2O2/Fe3O4/Fe2O3/Fe<sup>0</sup> process. The condition for the application of an effective CW treatment by catalytic oxidation is the use of an appropriate dose of hydrogen peroxide as a catalyst.
The best match of the results to kinetic models was obtained by second-order equations with a modification, taking into account the amount of undegraded compounds during the treatment process.
On the basis of the ANOVA results, the hypothesis regarding the accuracy and reproducibility of the research was confirmed.
**Supplementary Materials:** The following are available online at https://www.mdpi.com/2073-4 344/11/1/9/s1, Table S1: Experimental setup, Table S2: One-way analysis of variance for mean values of TOC for 1:1 H2O2/COD ratio, Table S3: One-way analysis of variance for mean values of TOC for 2:1 H2O2/COD ratio. Figure S1: GC-MS chromatogram: Raw wastewater, Figure S2: GC-MS chromatogram: Treated wastewater, 500/500/1000 mg/L Fe3O4/Fe2O3/Fe<sup>0</sup> catalyst doses, 1:1 H2O2/COD ratio, pH = 3.0, 120 min, Figure S3: The emission spectrum of the lamp, Figure S4: Anova plots of the UV/H2O2/Fe3O4/Fe2O3/Fe<sup>0</sup> process, Figure S5: Anova plots of the H2O2/Fe3O4/Fe2O3/Fe<sup>0</sup> process.
**Author Contributions:** Conceptualization, methodology, writing—original draft preparation, review and editing, supervision: J.B. and P.M.; laboratory work and data processing: J.B., P.M., D.B., M.K., D.S. and P.P. All authors have read and agreed to the published version of the manuscript. ´
**Funding:** This research received no external funding.
**Acknowledgments:** The authors thank Hepure for providing ZVI Ferox Target samples as research material.
**Conflicts of Interest:** The authors declare no conflict of interest.
| doab | 2025-04-07T03:56:58.831765 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 92
} |
004db1df-dc76-42af-952f-1f5e82b7822b.94 | *Article* **Alkali-Activated Materials as Catalysts for Water Purification**
**Anne Heponiemi \* , Janne Pesonen , Tao Hu and Ulla Lassi**
Research Unit of Sustainable Chemistry, University of Oulu, P.O. Box 4300, FI-90014 Oulu, Finland; [email protected] (J.P.); [email protected] (T.H.); [email protected] (U.L.) **\*** Correspondence: [email protected]
**Abstract:** In this study, novel and cost-effective alkali-activated materials (AAMs) for catalytic applications were developed by using an industrial side stream, i.e., blast furnace slag (BFS). AAMs can be prepared from aluminosilicate precursors under mild conditions (room temperature using non-hazardous chemicals). AAMs were synthesized by mixing BFS and a 50 wt % sodium hydroxide (NaOH) solution at different BFS/NaOH ratios. The pastes were poured into molds, followed by consolidation at 20 or 60 ◦C. As the active metal, Fe was impregnated into the prepared AAMs by ion exchange. The prepared materials were examined as catalysts for the catalytic wet peroxide oxidation (CWPO) of a bisphenol A (BPA) aqueous solution. As-prepared AAMs exhibited a moderate surface area and mesoporous structure, and they exhibited moderate activity for the CWPO of BPA, while the iron ion-exchanged, BFS-based catalyst (Fe/BFS30-60) exhibited the maximum removal of BPA (50%) during 3 h of oxidation at pH 3.5 at 70 ◦C. Therefore, these new, inexpensive, AAM-based catalysts could be interesting alternatives for catalytic wastewater treatment applications.
**Keywords:** alkali-activated material; geopolymer; blast furnace slag; catalytic wet peroxide oxidation; Fe-catalyst; bisphenol A
| doab | 2025-04-07T03:56:58.831922 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 94
} |
004db1df-dc76-42af-952f-1f5e82b7822b.95 | **1. Introduction**
Alkali-activated materials (AAMs) are inorganic, amorphous compounds that contain [SiO4] <sup>4</sup><sup>−</sup> and [AlO4] <sup>5</sup>−, which can be prepared by using aluminosilicates in addition to hydroxides, carbonates, or silicates of alkali and alkaline earth metals. The calcium content affects the AAM structure; therefore, materials with a low Ca content comprise a three-dimensional, highly interconnected aluminosilicate framework (also known as a geopolymer) [1], while those with a high Ca content comprise a cross-linked and non-cross linked structure that resembles that of tobermorite [2]. The use of AAMs, and especially geopolymers, has been investigated in the building industry as a more environmentfriendly alternative to Portland cement, due to their chemical and physical stability [3–5], as well as in more advanced applications, such as adsorbents for the removal of impurities from wastewater [6–9] and composite materials [4]. Moreover, owing to the fact that the structure of AAMs is similar to that of zeolites, their use as catalytic materials can also be exploited. Compared with the synthesis of zeolites, that of AAMs can be performed at ambient pressure and room temperature, using cost-effective raw materials (kaolin clay or industrial waste, such as fly ash), making AAMs fascinating, environment-friendly alternatives to commercial zeolite. Only a few years ago, Sazama et al. reported the synthesis of AAM-based catalysts [10] by the modification of metakaolin geopolymers for the catalytic reduction of nitrogen oxides by ammonia, as well as the total oxidation of volatile hydrocarbons. Furthermore, metakaolin-based geopolymers and steel slagcontaining AAMs also have been examined for photocatalytic applications [11,12] and biodiesel production [13]. Therefore, AAMs are interesting alternatives as catalysts, as well as for water-phase applications like catalytic wet peroxide oxidation (CWPO).
Bisphenol A (BPA) is an estrogenic compound commonly used in the production of polycarbonate plastic and epoxy resin, which are further utilized in several daily-use
**Citation:** Heponiemi, A.; Pesonen, J.; Hu, T.; Lassi, U. Alkali-Activated Materials as Catalysts for Water Purification. *Catalysts* **2021**, *11*, 664. https://doi.org/10.3390/catal 11060664
Academic Editor: Roberto Fiorenza
Received: 7 May 2021 Accepted: 22 May 2021 Published: 23 May 2021
**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.
**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).
plastic products, such as drinking bottles, containers, thermal paper, etc. [14]. BPA can be spread into water bodies during the manufacturing process, and also from daily-use plastic products. Due to its endocrine disrupting character for humans and environment [15], the removal of it from wastewaters is essential. Several techniques, such as activated sludge treatment [16], membrane bioreactors [17], and sorption [18,19] have been used for the removal of BPA from wastewaters. In addition of these, advanced oxidation processes (AOPs) have been effectively used for the oxidation of BPA in wastewaters [20]. Techniques like photolysis [21] and ozonation [22], as well as hybrid processes like UV/H2O<sup>2</sup> [23], UV/O<sup>3</sup> [24], UV/TiO<sup>2</sup> [25], and O2/H2O<sup>2</sup> [26], have been successfully used for the oxidation of BPA. In this study, one of AOPs, CWPO, is studied for the removal of BPA from water. In CWPO, hydrogen peroxide is used as an oxidizing agent to decompose organic compounds from wastewater. Transition metals, typically Fe and Cu, are used as catalysts in the reaction to decompose hydrogen peroxide to active hydroxyl radicals (Equation (1)) [27].
$$\rm M^{n+} + H\_2O\_2 \rightarrow M^{(n+1)+} + HO^- + HO^\bullet \tag{1}$$
The formed hydroxyl radicals can further oxidize organic compounds, according to Equation (2):
$$\text{RH} + \text{HO}^{\bullet}\text{-}2 \rightarrow \text{R}^{\bullet} + \text{H}\_{2}\text{O}\_{2} \tag{2}$$
To obtain the reduced form of the active metals, they must be dispersed on suitable supports [28,29]. Various materials have been used as supports in CWPO. Carbon-based materials, such as activated carbon [30], graphite, carbon black [31], carbon nanotubes [32], and biomass-based carbons [33], have been successfully used for the degradation of organics with CWPO. Moreover, zeolites [34,35] and clay materials [36,37] also have been applied as supports for Fe and Cu; therefore, AAMs with a chemical composition similar to those of zeolites and clay minerals are interesting alternatives as carriers for use in CWPO.
However, for the use of industrial side streams as raw materials in a catalyst, the prepared material must exhibit stability, especially when the prepared material is used for water treatment applications. Typically, catalyst stability for photocatalytic experiments has been evaluated in consecutive tests [38] and by characterization of the used materials after experiments, e.g., by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) [39,40].
In this study, the industrial side stream from the steel industry, i.e., blast furnace slag (BFS), was applied as a raw material to produce cost-effective catalytic materials for water purification. Catalysts were prepared by mixing different ratios of BFS and NaOH, followed by their consolidation at 20 ◦C and 60 ◦C. Moreover, iron was impregnated as the active metal in the AAMs via ion exchange. The as-prepared materials were characterized, e.g., by XRD, and their surface area and catalytic activity were examined for the CWPO of a bisphenol A (BPA) aqueous solution. Particular attention has been focused on the stability of materials; therefore, the possible leaching of the main elements (such as Ca, Si, Al, Mg, and Na) has been investigated before the CWPO of BPA under 2 MPa and 150 ◦C. Furthermore, the concentrations of the main elements of the as-prepared materials was analyzed after oxidation experiments from water samples, as well as those from the used catalysts.
| doab | 2025-04-07T03:56:58.832151 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 95
} |
004db1df-dc76-42af-952f-1f5e82b7822b.96 | **2. Results and Discussion**
In this section, the stability and characteristics, such as phase composition and specific surface area, of the prepared materials are discussed. In addition, the activity of AAMs for the CWPO of BPA is evaluated. The prepared materials were named according to their NaOH concentration and consolidation temperature (Section 3.1).
| doab | 2025-04-07T03:56:58.832565 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 96
} |
004db1df-dc76-42af-952f-1f5e82b7822b.97 | *2.1. Stability of Alkali-Activated Materials*
Table 1 lists the conductivity values of aqueous solutions after 4 h at 150 ◦C under an N<sup>2</sup> atmosphere of 2 MPa and an AAM concentration of 4 g/dm<sup>3</sup> .
**Table 1.** Conductivity of aqueous solutions after 4 h at 150 ◦C under an N<sup>2</sup> atmosphere of 2 MPa and AAM concentration of 4 g/dm<sup>3</sup> .
The conductivity values of aqueous solutions after 4 h of experiments were 200–300 µS/cm; according to these values, the alkali activation of BFS stabilized the material. With the increase in the amount of NaOH in the sample, the conductivity of aqueous solutions decreased slightly. In addition, the curing temperature affected the conductivity, i.e., samples that were first cured at 60 ◦C for 24 h exhibited lower conductivity than those cured at room temperature.
Table 2 lists the concentrations of Ca, Si, and Al in aqueous solutions after 4 h at 150 ◦C, under an N<sup>2</sup> atmosphere of 2 MPa and an AAM concentration of 4 g/dm<sup>3</sup> .
**Table 2.** Ca, Si, and Al concentrations of aqueous solutions after 4 h at 150 ◦C, under an N<sup>2</sup> atmosphere of 2 MPa and an AAM concentration of 4 g/dm<sup>3</sup> .
In addition to those of Ca, Si, and Al, Mg and Na concentrations also were analyzed from water samples by inductively coupled plasma–optical emission spectroscopy (ICP-OES). However, the magnesium concentration was less than the detection limit (≤0.1 mg/dm<sup>3</sup> ), and the maximum sodium concentration was 1 mg/dm<sup>3</sup> after 4 h at 150 ◦C and an N<sup>2</sup> atmosphere of 2 MPa. All of the samples exhibited almost the same Ca and Si concentrations. However, with the increase in the amount of NaOH in the samples, the leaching of aluminum increased. Clearly, alkali activation immobilized Al in the inorganic matrix, but basicity enhanced its dissolution [41]. Furthermore, curing at room temperature led to the enhanced dissolution of Ca and Al from AAMs. A curing temperature of 60 ◦C has been found to be favorable for geopolymer preparation. Muñiz-Villarreal et al. [42] have reported that the optimum dissolution and formation of hydroxy species and oligomers, as well as further polymerization or condensation, occur at 60 ◦C. Therefore, with the increase in the curing temperature of the BFS-based AAMs, the leaching of Ca, Si, and Al decreased. Thus, based on these stability tests, AAMs that are first cured at 60 ◦C for 24 h are further characterized by XRD, diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS), field emission scanning electron microscope with energydispersive X-ray spectroscopy (FESEM-EDS), ICP-OES, and surface area techniques, as well as being examined for the CWPO of a BPA aqueous solution.
| doab | 2025-04-07T03:56:58.832615 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 97
} |
004db1df-dc76-42af-952f-1f5e82b7822b.98 | *2.2. Characterization of AAMs*
Table 3 lists the results of the Brunauer–Emmett–Teller (BET) surface areas of the prepared AAMs.
**Table 3.** BET-specific surface areas (SSA) and pore volumes (PV) of AAMs.
The specific surface area of BFS was negligible, while alkali activation led to the increased surface area of all samples (Table 3). Samples prepared by using the highest amount of NaOH exhibited the highest specific surface area, as well as the highest pore volume. Clearly, a low Si/Na ratio favored the formation of a porous structure in the samples. Sindhunata et al. [43] have reported the highest pore volume for fly-ash-based geopolymers at a SiO2/Na2O ratio < 1. Moreover, the samples cured at 60 ◦C for 24 h exhibited a slightly higher specific surface area, and hence a higher pore volume, than those prepared at room temperature. The higher curing temperature promoted the removal of excess water from the material structure, which in turn increased the porosity of samples further. Furthermore, higher curing temperatures (>50 ◦C) have been reported to particularly increase the amount of mesopores in the material [43].
As can be observed from the surface area results, no significant differences between the AAMs were observed. Therefore, to examine the effect of the Na concentration of samples on the catalytic behavior, samples with the lowest and highest Na concentration (BFS17.5-60 and BFS30-60) were selected as support materials for Fe catalysts. Surprisingly, the surface areas of the Fe catalysts were greater than those of the BFS17.5-60 and BFS30-60 pure supports (Table 3). This result was related to the calcination performed for Fe catalysts. During heat treatment, excess water and carbon dioxide of the support material, and well as traces of Fe salt, evaporated from the AAM structure, enabling the increase in the specific surface area [44]. In addition, the calcination of Fe catalysts led to the decomposition of hydrotalcite (Figure 1, XRD results), which also affected the surface area of materials [44]. Furthermore, as-prepared AAMs mainly exhibited a mesoporous structure (i.e., pore diameter between 2 and 50 nm), with ~10% of pores exhibiting a diameter of less than 2 nm. However, by the addition of Fe to BFS17.5-60 and BFS30-60 via ion exchange, the number of mesopores decreased to 80%, while micropores accounted for only a small percentage of the total pore volume. Moreover, macropores accounted for only ~15% of the total pore volume in Fe/BFS17.5-60 and Fe/BFS30-60, while before Fe ion exchange, pores greater than 50 nm were not detected (i.e., heat treatment enhanced the formation of large pores).
**Figure 1.** X-ray diffractograms of BFS raw material, supports, and Fe catalysts. (#) ICDD file 00- 022-0700 (Mg6Al2CO3(OH)16·4H2O, hydrotalcite); (¤) ICDD file 01-083-4609 (CaCO3); (\*) ICDD file 04-015-7029 (Fe2O3); (+) ICDD file 04-008-8146 (Fe3O4). **Figure 1.** X-ray diffractograms of BFS raw material, supports, and Fe catalysts. (#) ICDD file 00- 022-0700 (Mg6Al2CO<sup>3</sup> (OH)16·4H2O, hydrotalcite); (¤) ICDD file 01-083-4609 (CaCO<sup>3</sup> ); (\*) ICDD file 04-015-7029 (Fe2O<sup>3</sup> ); (+) ICDD file 04-008-8146 (Fe3O<sup>4</sup> ).
Figure 1 shows the X-ray diffractograms of the BFS raw material; BFS17.5-60, BFS20- 60, BFS25-60, and BFS30-60 supports; and Fe/BFS17.5-60 and Fe/BFS30-60 catalysts. In the X-ray diffractogram of BFS, crystal peaks were not observed, but only one wide halo at 2θ between 22° and 40° was observed, which is characteristic of an amorphous material. After the alkali activation of BFS with NaOH, peaks were observed at 2θ values of 11.3°, 22.8°, 34.5°, and 38.6° (denoted with #), corresponding to hydrotalcite (Mg6Al2CO3(OH)16·4H2O (ICDD file 00-022-0700)), and the high-intensity peak at ~29.4° corresponded to CaCO3 (ICDD file 01-083-4609). However, the broad "hump" observed at 2θ of 28–35° was still present in the X-ray diffractograms of all supports, indicative of a partly amorphous structure. After the ion exchange of BFS17.5-60 and BFS30-60 with the Fe solution, the peaks observed at 2θ values of 24.1°, 33.2°, 35.6°, 40.9°, 49.5°, and 54.1° (denoted by \*) and at 35.7°, 43.4°, 57.4°, and 63.0° (denoted by +) revealed the presence of Fe2O3 (ICDD file 04-015-7029) and Fe3O4 (ICDD file 04-008-8146) phases, respectively. Owing to the heat treatment of Fe catalysts, hydrotalcite was decomposed [44], and peaks corresponding to hydrotalcite were not observed in the X-ray diffractograms of Figure 1 shows the X-ray diffractograms of the BFS raw material; BFS17.5-60, BFS20-60, BFS25-60, and BFS30-60 supports; and Fe/BFS17.5-60 and Fe/BFS30-60 catalysts. In the X-ray diffractogram of BFS, crystal peaks were not observed, but only one wide halo at 2θ between 22◦ and 40◦ was observed, which is characteristic of an amorphous material. After the alkali activation of BFS with NaOH, peaks were observed at 2θ values of 11.3◦ , 22.8◦ , 34.5◦ , and 38.6◦ (denoted with #), corresponding to hydrotalcite (Mg6Al2CO3(OH)16·4H2O (ICDD file 00-022-0700)), and the high-intensity peak at ~29.4◦ corresponded to CaCO<sup>3</sup> (ICDD file 01-083-4609). However, the broad "hump" observed at 2θ of 28–35◦ was still present in the X-ray diffractograms of all supports, indicative of a partly amorphous structure. After the ion exchange of BFS17.5-60 and BFS30-60 with the Fe solution, the peaks observed at 2θ values of 24.1◦ , 33.2◦ , 35.6◦ , 40.9◦ , 49.5◦ , and 54.1◦ (denoted by \*) and at 35.7◦ , 43.4◦ , 57.4◦ , and 63.0◦ (denoted by +) revealed the presence of Fe2O<sup>3</sup> (ICDD file 04-015-7029) and Fe3O<sup>4</sup> (ICDD file 04-008-8146) phases, respectively. Owing to the heat treatment of Fe catalysts, hydrotalcite was decomposed [44], and peaks corresponding to hydrotalcite were not observed in the X-ray diffractograms of Fe/BFS17.5-60 and Fe/BFS30-60.
Fe/BFS17.5-60 and Fe/BFS30-60. Figure 2 shows the DRIFT spectra of BFS, BFS17.5-60, BFS30-60, Fe/BFS17.5-60, and Fe/BFS30-60. In the DRIFT spectrum of the BFS raw material, only a few peaks were observed. The band at ~1420 cm−1 corresponded to Na2CO3 [45], and the strong peak at ~1110 Figure 2 shows the DRIFT spectra of BFS, BFS17.5-60, BFS30-60, Fe/BFS17.5-60, and Fe/BFS30-60. In the DRIFT spectrum of the BFS raw material, only a few peaks were observed. The band at ~1420 cm−<sup>1</sup> corresponded to Na2CO<sup>3</sup> [45], and the strong peak at ~1110 cm−<sup>1</sup> corresponded to pure silica [46].
cm−1 corresponded to pure silica [46].
AAMs [47].
**Figure 2.** DRIFT spectra of BFS, BFS17.5-60, BFS30-60, Fe/BFS17.5-60, and Fe/BFS30-60 samples. **Figure 2.** DRIFT spectra of BFS, BFS17.5-60, BFS30-60, Fe/BFS17.5-60, and Fe/BFS30-60 samples.
Alkali-activated samples exhibited several peaks in the analyzed region. The peak at 3730 cm−1 for BFS17.5-60 corresponded to silanol groups, which interact with other atoms—for example, in silanol nests [47]—and the absorption peak at ~3700 cm−1 revealed the presence of four coordinated Al [48]. Moreover, the band at 3610 cm−1 for BFS17.5-60 corresponded to the bridging hydroxyl groups [49]. In the DRIFT spectra of Fe/BFS17.5- 60 and Fe/BFS30-60, the peak centers were shifted to higher wavenumbers than those for the samples without iron, probably due to calcination, and the absorption band corre-Alkali-activated samples exhibited several peaks in the analyzed region. The peak at 3730 cm−<sup>1</sup> for BFS17.5-60 corresponded to silanol groups, which interact with other atoms—for example, in silanol nests [47]—and the absorption peak at ~3700 cm−<sup>1</sup> revealed the presence of four coordinated Al [48]. Moreover, the band at 3610 cm−<sup>1</sup> for BFS17.5-60 corresponded to the bridging hydroxyl groups [49]. In the DRIFT spectra of Fe/BFS17.5-60 and Fe/BFS30-60, the peak centers were shifted to higher wavenumbers than those for the samples without iron, probably due to calcination, and the absorption band corresponding to the silanol groups (3730 cm−<sup>1</sup> ) disappeared by the introduction of iron into AAMs [47].
sponding to the silanol groups (3730 cm−1) disappeared by the introduction of iron into The absorbance bands for BFS17.5-60 and BFS30-60 were observed at 715, 840, 1373, and 1790 cm−1, corresponding to CO32−-containing compounds [46]. Bands at 840 and 1790 cm−1 connected to Na2CO3, and the band at 715 cm−1 corresponded to CaCO3 [46], while that observed at 1373 cm−1 corresponded to hydrotalcite [50], which was also detected in the X-ray diffractograms of these samples (Figure 1). In the DRIFT spectra of Fe/BFS17.5- 60 and Fe/BFS30-60, these peaks were slightly shifted to higher wavenumbers, especially for the band corresponding to hydrotalcite, indicative of its decomposition as a result of heat treatment. Furthermore, the peak at ~1650 cm−1 observed in all samples corresponded The absorbance bands for BFS17.5-60 and BFS30-60 were observed at 715, 840, 1373, and 1790 cm−<sup>1</sup> , corresponding to CO<sup>3</sup> <sup>2</sup>−-containing compounds [46]. Bands at 840 and 1790 cm−<sup>1</sup> connected to Na2CO3, and the band at 715 cm−<sup>1</sup> corresponded to CaCO<sup>3</sup> [46], while that observed at 1373 cm−<sup>1</sup> corresponded to hydrotalcite [50], which was also detected in the X-ray diffractograms of these samples (Figure 1). In the DRIFT spectra of Fe/BFS17.5-60 and Fe/BFS30-60, these peaks were slightly shifted to higher wavenumbers, especially for the band corresponding to hydrotalcite, indicative of its decomposition as a result of heat treatment. Furthermore, the peak at ~1650 cm−<sup>1</sup> observed in all samples corresponded to the H–OH stretching vibrations characteristic of absorbed water [51], the intensity of which slightly decreased due to the heat treatment of iron-containing samples.
to the H–OH stretching vibrations characteristic of absorbed water [51], the intensity of which slightly decreased due to the heat treatment of iron-containing samples. All AAMs exhibited several bands corresponding to the Al and Si bonds. The bands at 435–483 cm−1 corresponded to the Si–O–Si and O–Si–O bending vibrations [52], while All AAMs exhibited several bands corresponding to the Al and Si bonds. The bands at 435–483 cm−<sup>1</sup> corresponded to the Si–O–Si and O–Si–O bending vibrations [52], while the absorption peak at ~600 cm−<sup>1</sup> revealed the presence of Si–O–Si and Al–O–Si symmetric stretching vibrations [53]. The band at ~900 cm−<sup>1</sup> in the spectra of BFS17.5-60 and BFS30-60
corresponded to the Si–O stretching and Si–OH bending modes [53]. Moreover, the band at ~1170 cm−<sup>1</sup> corresponded to the Si–O–Si and Al–O–Si asymmetric stretching vibrations [53], and this band was broadened in the spectra of Fe/BFS17.5-60 and Fe/BFS30-60, due to the calcination of these samples [45]. According to [54–56], Fe2O<sup>3</sup> and Fe3O<sup>4</sup> species should exhibit IR vibrations at 550 and 780 cm−<sup>1</sup> and 571 and 590 cm−<sup>1</sup> , respectively. However, owing to the overlap of the Si and Al vibrations in this wavenumber region, peaks corresponding to Fe cannot be observed in the DRIFT spectra of the prepared samples. the absorption peak at ~600 cm−1 revealed the presence of Si–O–Si and Al–O–Si symmetric stretching vibrations [53]. The band at ~900 cm−1 in the spectra of BFS17.5-60 and BFS30- 60 corresponded to the Si–O stretching and Si–OH bending modes [53]. Moreover, the band at ~1170 cm−1 corresponded to the Si–O–Si and Al–O–Si asymmetric stretching vibrations [53], and this band was broadened in the spectra of Fe/BFS17.5-60 and Fe/BFS30- 60, due to the calcination of these samples [45]. According to [54–56], Fe2O3 and Fe3O4 species should exhibit IR vibrations at 550 and 780 cm−1 and 571 and 590 cm−1, respectively. How-
Table 4 lists the concentrations (as wt %) of Ca, Si, Al, Mg, Fe, and Na of BFS17.5-60, BFS30-60, Fe/BFS17.5-60, and Fe/BFS30-60, as determined by ICP-OES analysis. ever, owing to the overlap of the Si and Al vibrations in this wavenumber region, peaks corresponding to Fe cannot be observed in the DRIFT spectra of the prepared samples. Table 4 lists the concentrations (as wt %) of Ca, Si, Al, Mg, Fe, and Na of BFS17.5-60,
**Table 4.** Metal concentrations (as wt %) of selected samples, as determined by ICP-OES analysis. BFS30-60, Fe/BFS17.5-60, and Fe/BFS30-60, as determined by ICP-OES analysis.
The Ca concentrations of the prepared samples were several times lower than those in BFS, while the Si, Al, Mg, and Na concentrations were about the same as those in the raw material (Table 5, experimental). The leaching of Ca probably occurred during the washing of the AAMs using deionized water. BFS contained ~0.5 wt % iron, and ion exchange led to the increase in the iron concentration to 5–7 wt % for Fe/BFS30-60 and Fe/BFS17.5-60, respectively. The theoretical amount of iron by the employed impregnation method was 5.3 wt %, indicating that ion exchange between BFS17.5-60 and the iron salt is slightly better than that between BFS30-60 and the iron salt. The Ca concentrations of the prepared samples were several times lower than those in BFS, while the Si, Al, Mg, and Na concentrations were about the same as those in the raw material (Table 5, experimental). The leaching of Ca probably occurred during the washing of the AAMs using deionized water. BFS contained ~0.5 wt % iron, and ion exchange led to the increase in the iron concentration to 5–7 wt % for Fe/BFS30-60 and Fe/BFS17.5-60, respectively. The theoretical amount of iron by the employed impregnation method was 5.3 wt %, indicating that ion exchange between BFS17.5-60 and the iron salt is slightly better than that between BFS30-60 and the iron salt.
**Table 5.** Elemental composition of the blast furnace slag as determined by ICP-OES analysis <sup>1</sup> . **Table 5.** Elemental composition of the blast furnace slag as determined by ICP-OES analysis. 1
<sup>1</sup> Elements with wt % > 0.01 were reported. 1 Elements with wt % > 0.01 were reported.
Figure 3 shows the FESEM images of BFS, Fe/BFS17.5-60, and Fe/BFS30-60. AAMs clearly exhibited an irregular, non-crystalline shape (Figure 3b,c). According to EDS analysis, the Al and Mg concentrations were ~5 wt %, while on the Fe catalyst surface, the Si and Ca concentrations were a few percent less than those in the bulk, as determined by ICP-OES (Table 4). Figure 3 shows the FESEM images of BFS, Fe/BFS17.5-60, and Fe/BFS30-60. AAMs clearly exhibited an irregular, non-crystalline shape (Figure 3b,c). According to EDS analysis, the Al and Mg concentrations were ~5 wt %, while on the Fe catalyst surface, the Si and Ca concentrations were a few percent less than those in the bulk, as determined by ICP-OES (Table 4).
**Figure 3.** FESEM micrographs of BFS (**a**), Fe/BFS17.5-60 (**b**), and Fe/BFS30-60 (**c**). Dimensions in figures: 1 µm (**a**) and 2 µm (**b**,**c**). **Figure 3.** FESEM micrographs of BFS (**a**), Fe/BFS17.5-60 (**b**), and Fe/BFS30-60 (**c**). Dimensions in figures: 1 µm (**a**) and 2 µm (**b**,**c**).
### *2.3. Oxidation Experiments with AAMs 2.3. Oxidation Experiments with AAMs*
The prepared AAMs, namely BFS17.5-60, BFS20-60, BFS25-60, and BFS30-60, which were first cured at 60 ◦C for 24 h, were examined for the CWPO of a BPA aqueous solution. Figure 4 shows the results of these experiments. The prepared AAMs, namely BFS17.5-60, BFS20-60, BFS25-60, and BFS30-60, which were first cured at 60 °C for 24 h, were examined for the CWPO of a BPA aqueous solution. Figure 4 shows the results of these experiments.
**Figure 4.** Removal of bisphenol A over AAMs as a function of the reaction time. Reaction conditions: concentration [*c*], *c*[BPA] = 60 mg/dm3, *c*[H2O2] = 1.5 g/dm3, *c*[catalyst] = 4 g/dm3, tempera-**Figure 4.** Removal of bisphenol A over AAMs as a function of the reaction time. Reaction conditions: concentration [*c*], *c*[BPA] = 60 mg/dm<sup>3</sup> , *c*[H2O<sup>2</sup> ] = 1.5 g/dm<sup>3</sup> , *c*[catalyst] = 4 g/dm<sup>3</sup> , temperature [*T*] = 50 ◦C, initial pH (6–7).
ture [*T*] = 50 °C, initial pH (6–7).
Oxidation reactions were performed at 50 °C at an initial pH of 6–7, a catalyst concentration of 4 g/dm3, and a H2O2 concentration of 1.5 g/dm3. In the absence of a catalyst (not shown), only ~10% of BPA removal was observed, while in the presence of AAMs, BPA removal of 35–39% after 180 min oxidation was observed. Oxidation proceeded during 2.5 h for all samples and stabilized for 3 h. The oxidant H2O2 was added in batches; hence, the final addition was performed at 2 h sampling. The total organic carbon (TOC) was measured from the initial and final samples, and 27–31% of organics were removed. The dissolved oxygen (DO) concentration of the BPA samples changed from ~9.5 mg O2/dm3 to 8.1 mg O2/dm3 during 180 min oxidation, revealing that at the end of the run, oxygen is still present in the samples. Probably, the used reaction temperature (50 °C) was not sufficiently high for the effective decomposition of H2O2 to form active ·OH radicals. In several studies, a higher reaction temperature has been reported to enhance the degra-Oxidation reactions were performed at 50 ◦C at an initial pH of 6–7, a catalyst concentration of 4 g/dm<sup>3</sup> , and a H2O<sup>2</sup> concentration of 1.5 g/dm<sup>3</sup> . In the absence of a catalyst (not shown), only ~10% of BPA removal was observed, while in the presence of AAMs, BPA removal of 35–39% after 180 min oxidation was observed. Oxidation proceeded during 2.5 h for all samples and stabilized for 3 h. The oxidant H2O<sup>2</sup> was added in batches; hence, the final addition was performed at 2 h sampling. The total organic carbon (TOC) was measured from the initial and final samples, and 27–31% of organics were removed. The dissolved oxygen (DO) concentration of the BPA samples changed from ~9.5 mg O2/dm<sup>3</sup> to 8.1 mg O2/dm<sup>3</sup> during 180 min oxidation, revealing that at the end of the run, oxygen is still present in the samples. Probably, the used reaction temperature (50 ◦C) was not sufficiently high for the effective decomposition of H2O<sup>2</sup> to form active ·OH radicals. In several studies, a higher reaction temperature has been reported to enhance the degradation of H2O2, thereby enhancing pollutant removal [57–59].
dation of H2O2, thereby enhancing pollutant removal [57–59]. As all of the AAMs exhibited similar activities for the removal of BPA, samples with the lowest and highest NaOH concentration were selected for further research. Iron was impregnated onto BFS17.5-60 and BFS30-60 samples by ion exchange (Section 3.1), and As all of the AAMs exhibited similar activities for the removal of BPA, samples with the lowest and highest NaOH concentration were selected for further research. Iron was impregnated onto BFS17.5-60 and BFS30-60 samples by ion exchange (Section 3.1), and the prepared Fe catalysts were examined under different reaction conditions.
the prepared Fe catalysts were examined under different reaction conditions. First, the effect of the addition of the active metal on BFS17.5-60 and BFS30-60 was examined at 50 °C at the initial pH, and a catalyst loading of 4 g/dm3. After 3 h oxidation, BPA removal of 42% and 45% for Fe/BFS17.5-60 and Fe/BFS30-60 were observed, respectively (Figure 5). Using the comparison of BPA removal over AAMs without the active First, the effect of the addition of the active metal on BFS17.5-60 and BFS30-60 was examined at 50 ◦C at the initial pH, and a catalyst loading of 4 g/dm<sup>3</sup> . After 3 h oxidation, BPA removal of 42% and 45% for Fe/BFS17.5-60 and Fe/BFS30-60 were observed, respectively (Figure 5). Using the comparison of BPA removal over AAMs without the active metal (Figure 4), the addition of Fe led to the increased activity of both catalysts, namely
BPA removal of 6% and 10% for BFS17.5-60 and BFS30-60, respectively. TOC removal after 3 h oxidation was at the same level for both catalysts compared to that over the pure supports (30% and 33% for Fe/BFS17.5-60 and Fe/BFS30-60, respectively). During oxidation, the DO concentration decreased slightly from ~9 mg/O<sup>2</sup> dm<sup>3</sup> to 6.2–6.6 mg/O<sup>2</sup> dm<sup>3</sup> , indicating that hydrogen peroxide is not consumed completely in the runs. Therefore, Fe/BFS17.5-60 and Fe/BFS30-60 were further examined at higher reaction temperatures. BPA removal of 6% and 10% for BFS17.5-60 and BFS30-60, respectively. TOC removal after 3 h oxidation was at the same level for both catalysts compared to that over the pure supports (30% and 33% for Fe/BFS17.5-60 and Fe/BFS30-60, respectively). During oxidation, the DO concentration decreased slightly from ~9 mg/O2 dm3 to 6.2–6.6 mg/O2 dm3, indicating that hydrogen peroxide is not consumed completely in the runs. Therefore, Fe/BFS17.5-60 and Fe/BFS30-60 were further examined at higher reaction temperatures.
metal (Figure 4), the addition of Fe led to the increased activity of both catalysts, namely
*Catalysts* **2021**, *11*, 664 9 of 19
**Figure 5.** Bisphenol A removal at reaction temperatures of 50 °C, 70 °C, and 100 °C with the Fe/BFS17.5-60 and Fe/BFS30-60 catalysts, at an initial pH of 3.5, a reaction time of 3 h, [BPA] = 60 **Figure 5.** Bisphenol A removal at reaction temperatures of 50 ◦C, 70 ◦C, and 100 ◦C with the Fe/BFS17.5-60 and Fe/BFS30-60 catalysts, at an initial pH of 3.5, a reaction time of 3 h, [BPA] = 60 mg/dm<sup>3</sup> , *c*[H2O<sup>2</sup> ] = 1.5 g/dm<sup>3</sup> , and *c*[catalyst] = 4 g/dm<sup>3</sup> .
mg/dm3, *c*[H2O2] = 1.5 g/dm3, and *c*[catalyst] = 4 g/dm3.
To investigate the effect of temperature on the CWPO of BPA over Fe/BFS17.5-60 and Fe/BFS30-60, oxidation experiments were performed at 70 °C and 100 °C at the initial pH of the BPA aqueous solution. Typically, with the increase in the reaction temperature, the oxidation rate increases. Furthermore, the decomposition rate of H2O2 to active hydroxyl radicals also increases. A higher reaction temperature led to the improved degradation of BPA during 3 h oxidation, with the maximum of 5% over Fe/BFS17.5-60 at 70 °C (Figure 5). The increase in the reaction temperature to 100 °C did not affect BPA removal. During oxidation, the DO concentration decreased from 8.0 mg/O2 dm3 to 5.7 mg/O2 dm3 and from ~10.0 mg/O2 dm3 to 4.2 mg/O2 dm3 at 70 °C and 100 °C, respectively, revealing that hydrogen peroxide is consumed in the reaction. However, owing to the low degradation level of BPA at 100 °C, hydrogen peroxide was probably decomposed directly to H2O without To investigate the effect of temperature on the CWPO of BPA over Fe/BFS17.5-60 and Fe/BFS30-60, oxidation experiments were performed at 70 ◦C and 100 ◦C at the initial pH of the BPA aqueous solution. Typically, with the increase in the reaction temperature, the oxidation rate increases. Furthermore, the decomposition rate of H2O<sup>2</sup> to active hydroxyl radicals also increases. A higher reaction temperature led to the improved degradation of BPA during 3 h oxidation, with the maximum of 5% over Fe/BFS17.5-60 at 70 ◦C (Figure 5). The increase in the reaction temperature to 100 ◦C did not affect BPA removal. During oxidation, the DO concentration decreased from 8.0 mg/O<sup>2</sup> dm<sup>3</sup> to 5.7 mg/O<sup>2</sup> dm<sup>3</sup> and from ~10.0 mg/O<sup>2</sup> dm<sup>3</sup> to 4.2 mg/O<sup>2</sup> dm<sup>3</sup> at 70 ◦C and 100 ◦C, respectively, revealing that hydrogen peroxide is consumed in the reaction. However, owing to the low degradation level of BPA at 100 ◦C, hydrogen peroxide was probably decomposed directly to H2O without the formation of hydroxyl radicals.
the formation of hydroxyl radicals. Typically, homogeneous iron catalysts for CWPO (Fenton process) are used at a pH of ~3, which is known to be optimum for the decomposition of organic compounds [60]. The effect of pH on the degradation level of BPA was investigated at pH 3.5, in addition to the initial pH (6–7) by using Fe/BFS17.5-60 and Fe/BFS30-60 catalysts. The effect of pH was examined at 50 °C, 70 °C, and 100 °C. The pH of the BPA solution was adjusted to 3.5 Typically, homogeneous iron catalysts for CWPO (Fenton process) are used at a pH of ~3, which is known to be optimum for the decomposition of organic compounds [60]. The effect of pH on the degradation level of BPA was investigated at pH 3.5, in addition to the initial pH (6–7) by using Fe/BFS17.5-60 and Fe/BFS30-60 catalysts. The effect of pH was examined at 50 ◦C, 70 ◦C, and 100 ◦C. The pH of the BPA solution was adjusted to 3.5 using 2.0 M HNO<sup>3</sup> before oxidation. At 50 ◦C and pH 3.5, BPA removal increased by 5% over Fe/BFS17.5-60, while over Fe/BFS30-60, it was almost the same after 3 h
oxidation compared to experiments performed at the initial pH of BPA (Figure 5). The DO concentration of the liquid samples was considerably higher (at the end of the run for Fe/BFS17.5-60, 14 mg/O<sup>2</sup> dm<sup>3</sup> ) than that after oxidation at the initial pH. Therefore, acidic pH promotes the formation of hydroxyl radicals during the reaction. However, Fe catalysts did not exhibit considerably higher activity for BPA removal than that at the initial pH, probably due to the basic surfaces of Fe/BFS17.5-60 and Fe/BFS30-60.
At pH 3.5 and 70 ◦C (Figure 5), the DO concentration was the same during tests compared to that in experiments at the initial pH, and the pH change of the BPA solution led to an increase in BPA removal by only 3% and 2% over Fe/BFS17.5-60 and Fe/BFS30-60, respectively. At 100 ◦C and pH 3.5, BPA removal after 3 h was around the same for both Fe catalysts compared with that observed at 100◦C and at the initial pH. However, notably, owing to the basicity of Fe catalysts, the pH of the BPA solution changed to basic during runs in all experiments. The decomposition of H2O<sup>2</sup> to ·OH radicals is the key step in CWPO. However, under a basic reaction pH, the generation of hydroxyl radicals was restricted, thereby further decreasing the degradation of BPA [61]. Thus, the change in pH marginally affects BPA removal.
The adsorption capacity of the Fe catalysts was examined under the severest reaction conditions in this study, i.e., pH of 3.5, a reaction temperature of 100 ◦C, in the absence of the oxidant, and a catalyst concentration of 4 g/dm<sup>3</sup> . For Fe/BFS17.5-60 and Fe/BFS30-60, during the 3 h experiment, 12% and 17% of BPA was adsorbed, respectively, revealing that Fe/BFS17.5-60 is catalytically more active than Fe/BFS30-60. The higher adsorption capacity of Fe/BFS30-60 was related to the higher specific surface area of this sample (Table 3).
| doab | 2025-04-07T03:56:58.832764 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 98
} |
004db1df-dc76-42af-952f-1f5e82b7822b.99 | *2.4. Stability of the Used Catalysts*
The possible leaching of the elements from the prepared AAMs was examined by ICP-OES in detail, in addition to the leaching tests (Section 2.1) after oxidation. The oxidized water samples were immediately filtered after 3 h CWPO using a 0.45 µm cellulose nitrate filter to remove the solid catalysts. The Al, Si, and Ca concentrations were determined from the oxidized water samples catalyzed by BFS17.5-60, BFS20-60, BFS25-60, and BFS30-60, and in addition to these elements, Fe was analyzed from the filtered samples catalyzed by Fe/BFS17.5-60 and Fe/BFS30-60. According to the results, the leaching of Ca and Si was observed under all of the utilized reaction conditions with all catalysts. In all of the oxidized water samples, the Ca concentration was 25–50 mg/dm<sup>3</sup> , and the Si concentration was 11–19 mg/dm<sup>3</sup> . However, notably, the Ca concentration was slightly lower in the water samples catalyzed by AAMs without iron. Therefore, the heat treatment of Fe catalysts (Section 3.1) led to the increased dissolution of Ca in the water phase during oxidation treatment. The leaching of Ca was around the same level as that detected in stability tests (Section 2.1) with BFS17.5-60, BFS20-60, BFS25-60, and BFS30-60, revealing that these samples also can be used at reaction temperatures > 100 ◦C and pressures ≥ 2 MPa.
The Al concentration of aqueous BPA samples oxidized at the initial pH was 1.0–1.4 mg/dm<sup>3</sup> . The dissolution of Al was slightly higher at 150 ◦C and 2.0 MPa (Table 1), i.e., under conditions of the stability test, than that under CWPO reaction conditions. However, in the case of oxidation experiments performed at a pH of 3.5, and at temperatures 50 ◦C, 70 ◦C, and 100 ◦C over that of Fe/BFS17.5-60 and Fe/BFS30-60, 0.6–1.2 mg/dm<sup>3</sup> of Al was leached from the catalysts in the obtained effluents. Therefore, the dissolution of Al from the prepared AAMs is more dominant in the CWPO of BPA, which is conducted at the initial pH. Onisei et al. [41] have investigated the leaching behavior of several elements (e.g., Si, Pb, Ca, Zn, Al) from fly ash-based geopolymers. The study was performed in the pH range of 6–13. According to their results, the leaching of Al increased slightly in the pH range of 10.5–13.0. In the CWPO of BPA, the initial pH of the BPA solution was 6–7. However, at the end of the run, the effluent pH was ~11, due to the basic character of the Fe catalysts. Moreover, in CWPO experiments, which were started at a pH of 3.5, the pH of the BPA solution was ~10 in the oxidized water sample. Therefore, the adjustment of
the pH at the start of the CWPO of BPA did not considerably affect the removal of BPA, but it decreased the leaching of Al from the Fe catalysts. the pH at the start of the CWPO of BPA did not considerably affect the removal of BPA, but it decreased the leaching of Al from the Fe catalysts.
of the BPA solution was ~10 in the oxidized water sample. Therefore, the adjustment of
*Catalysts* **2021**, *11*, 664 11 of 19
However, the leaching of Ca, Si, and Al was not related to the removal of BPA, because those elements were not active in CWPO. The stability of the material is a key characteristic of the catalyst; therefore, the preparation method of AAM-based catalysts should be carefully considered. Moreover, the leaching of iron was rather negligible (maximum of 0.2 mg/dm<sup>3</sup> at 70 ◦C and at the initial pH) using Fe/BFS17.5-60 and Fe/BFS30-60 at the employed reaction temperatures and pH. Therefore, the CWPO of BPA with these catalysts proceeded via a heterogeneous reaction. However, the leaching of Ca, Si, and Al was not related to the removal of BPA, because those elements were not active in CWPO. The stability of the material is a key characteristic of the catalyst; therefore, the preparation method of AAM-based catalysts should be carefully considered. Moreover, the leaching of iron was rather negligible (maximum of 0.2 mg/dm3 at 70 °C and at the initial pH) using Fe/BFS17.5-60 and Fe/BFS30-60 at the employed reaction temperatures and pH. Therefore, the CWPO of BPA with these catalysts proceeded via a heterogeneous reaction.
The activity and durability in consecutive tests and the effect of heat treatment as a regeneration method were examined using Fe/BFS7.5-60 at 50 ◦C, at the initial pH, and at H2O<sup>2</sup> and catalyst concentrations of 1.5 g/dm<sup>3</sup> and 4.0 g/dm<sup>3</sup> , respectively. To have sufficient material for consecutive tests and regeneration, 12 runs were performed in total, and the catalysts used in these experiments were collected and combined. Between consecutive experiments, the used catalyst was filtered from the effluent and dried at 105 ◦C for the subsequent runs. The activity and durability in consecutive tests and the effect of heat treatment as a regeneration method were examined using Fe/BFS7.5-60 at 50 °C, at the initial pH, and at H2O2 and catalyst concentrations of 1.5 g/dm3 and 4.0 g/dm3, respectively. To have sufficient material for consecutive tests and regeneration, 12 runs were performed in total, and the catalysts used in these experiments were collected and combined. Between consecutive experiments, the used catalyst was filtered from the effluent and dried at 105 °C for the subsequent runs.
After the first oxidation reaction, BPA removal of 41% was observed, which decreased to ~6% after the second run using the same catalyst (Figure 6). Furthermore, BPA removal after the third experiment, which used Fe/BFS17.5-60 twice, was practically the same (34%) as that observed in the second run, indicative of the catalyst's stability for multiple cycles in the CWPO of BPA. However, the removal of BPA after three cycles using Fe/BFS17.5-60 was around the same as that using BFS17.5-60 for one cycle, revealing that the addition of Fe does not significantly affect catalytic activity due to the basic reaction pH. Moreover, TOC results confirmed the reusability of Fe/BFS17.5-60, while BPA removal was the same during the three consecutive tests. After the first oxidation reaction, BPA removal of 41% was observed, which decreased to ~6% after the second run using the same catalyst (Figure 6). Furthermore, BPA removal after the third experiment, which used Fe/BFS17.5-60 twice, was practically the same (34%) as that observed in the second run, indicative of the catalyst's stability for multiple cycles in the CWPO of BPA. However, the removal of BPA after three cycles using Fe/BFS17.5-60 was around the same as that using BFS17.5-60 for one cycle, revealing that the addition of Fe does not significantly affect catalytic activity due to the basic reaction pH. Moreover, TOC results confirmed the reusability of Fe/BFS17.5-60, while BPA removal was the same during the three consecutive tests.
**Figure 6.** Consecutive tests using Fe/BFS17.5-60 for the CWPO of BPA. Reaction conditions: *c*[BPA] = 60 mg/dm3, *c*[H2O2] = 1.5 g/dm3, *c*[catalyst] = 4 g/dm3, *T* = 50 °C, initial pH (6–7). **Figure 6.** Consecutive tests using Fe/BFS17.5-60 for the CWPO of BPA. Reaction conditions: *c*[BPA] = 60 mg/dm<sup>3</sup> , *c*[H2O<sup>2</sup> ] = 1.5 g/dm<sup>3</sup> , *c*[catalyst] = 4 g/dm<sup>3</sup> , *T* = 50 ◦C, initial pH (6–7).
The regeneration of once-used Fe/BFS17.5-60 was examined by heat treatment at 250 ◦C and 500 ◦C. The procedure was performed by increasing the temperature at a rate of 1 ◦C/min to the reaction temperature, at which the catalyst was kept for 2 h. After oxidation, 34% and 32% BPA removal was observed at 250 ◦C and 500 ◦C, respectively, using Fe/BFS17.5-60. Therefore, the regeneration procedure is not effective at returning the activity of the catalysts to the original level. In addition, carbon deposition is confirmed to not be responsible for the activity decrease of Fe/BFS17.5-60, because heat treatment at elevated temperatures is a typical regeneration procedure for catalysts with carbon deactivation [62]. The regeneration of once-used Fe/BFS17.5-60 was examined by heat treatment at 250 °C and 500 °C. The procedure was performed by increasing the temperature at a rate of 1 °C/min to the reaction temperature, at which the catalyst was kept for 2 h. After oxidation, 34% and 32% BPA removal was observed at 250 °C and 500 °C, respectively, using Fe/BFS17.5-60. Therefore, the regeneration procedure is not effective at returning the activity of the catalysts to the original level. In addition, carbon deposition is confirmed to not be responsible for the activity decrease of Fe/BFS17.5-60, because heat treatment at elevated temperatures is a typical regeneration procedure for catalysts with carbon deactivation [62].
### *2.5. Characterization of the Used Catalysts 2.5. Characterization of the Used Catalysts*
Figure 7 shows the X-ray diffractograms of BFS17.5-60, BFS30-60, Fe/BFS17.5-60, and Fe/BFS30-60 after oxidation at the initial pH and at a reaction temperature of 50 ◦C. According to XRD analysis, the hydrotalcite phase (denoted by #, ICDD file 00-022-0700) was still present in BFS17.5-60 and BFS30-60, and the CaCO<sup>3</sup> phase (¤, ICDD file 01-083- 4609) was observed in all samples. Moreover, in the X-ray diffractograms of Fe/BFS17.5-60 and Fe/BFS30-60, the Fe3O<sup>4</sup> and Fe2O<sup>3</sup> iron phases (denoted by +: ICDD file 04-008-8146 and \*: ICDD file 04-015-7029, respectively) were still present, but the high Ca concentration of samples led to the overlap of the CaCO<sup>3</sup> peaks with those of Fe3O<sup>4</sup> and Fe2O<sup>3</sup> at 2θ of 36◦ and Fe3O<sup>4</sup> at 2θ of 43◦ . Figure 7 shows the X-ray diffractograms of BFS17.5-60, BFS30-60, Fe/BFS17.5-60, and Fe/BFS30-60 after oxidation at the initial pH and at a reaction temperature of 50 °C. According to XRD analysis, the hydrotalcite phase (denoted by #, ICDD file 00-022-0700) was still present in BFS17.5-60 and BFS30-60, and the CaCO3 phase (¤, ICDD file 01-083-4609) was observed in all samples. Moreover, in the X-ray diffractograms of Fe/BFS17.5-60 and Fe/BFS30-60, the Fe3O4 and Fe2O3 iron phases (denoted by +: ICDD file 04-008-8146 and \*: ICDD file 04-015-7029, respectively) were still present, but the high Ca concentration of samples led to the overlap of the CaCO3 peaks with those of Fe3O4 and Fe2O3 at 2θ of 36° and Fe3O4 at 2θ of 43°.
**Figure 7.** X-ray diffractograms of used BFS17.5-60, BFS30-60, Fe/BFS17.5-60, and Fe/BFS30-60. (#) ICDD file 00-022-0700 (Mg6Al2CO3(OH)16·4H2O, hydrotalcite); (¤) ICDD file 01-083-4609 (CaCO3); (\*) ICDD file 04-015-7029 (Fe2O3); (+) ICDD file 04-008-8146 (Fe3O4). **Figure 7.** X-ray diffractograms of used BFS17.5-60, BFS30-60, Fe/BFS17.5-60, and Fe/BFS30-60. (#) ICDD file 00-022-0700 (Mg6Al2CO<sup>3</sup> (OH)16·4H2O, hydrotalcite); (¤) ICDD file 01-083-4609 (CaCO<sup>3</sup> ); (\*) ICDD file 04-015-7029 (Fe2O<sup>3</sup> ); (+) ICDD file 04-008-8146 (Fe3O<sup>4</sup> ).
Furthermore, acidic pH and a higher reaction temperature did not affect the phase structure, and hydrotalcite was still observed in the X-ray diffractograms of BFS30-60 and BFS17.5-60 (results not shown).
The specific surface area of samples was analyzed after the oxidation of BPA at the initial pH and at reaction temperatures of 50 ◦C for BFS17.5-60 and BFS30-60, as well as reaction temperatures of 50 ◦C and 70 ◦C for Fe/BFS17.5-60 and Fe/BFS30-60. The BET results for used BFS17.5-60 and BFS30-60 revealed that the specific surface areas were 10.4 and 18.5 m2/g, respectively, revealing that the surface area of BFS30-60 decreases to ~30%, while a rather negligible change in the surface area of BFS17.5-60 was observed (Table 3). For Fe catalysts, the specific surface areas increased after oxidation. In case of Fe/BFS17.5- 60, the surface area was ~45% higher, and in case of Fe/BFS30-60, it doubled compared to that of the fresh catalyst (Table 3). Clearly, during oxidation, the Fe catalyst surface is refined by H2O2. For example, Han et al. [63] and Liu et al. [64] have used hydrogen peroxide to modify surface properties, i.e., to increase the surface area and porosity of materials. However, the larger specific surface area did not improve the removal of BPA in consecutive tests using Fe/BFS17.5-60 (Figure 6); therefore, the CWPO of BPA is not a surface area-specific reaction, as is the case for the catalytic wet air oxidation of BPA [65].
| doab | 2025-04-07T03:56:58.833988 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 99
} |
004db1df-dc76-42af-952f-1f5e82b7822b.100 | **3. Materials and Methods**
In this section, the preparation method and characterization techniques of catalysts are described. In addition, reaction conditions for the CWPO of the bisphenol A aqueous solution are presented.
| doab | 2025-04-07T03:56:58.834926 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 100
} |
004db1df-dc76-42af-952f-1f5e82b7822b.101 | *3.1. Preparation of Alkali-Activated Materials and Fe Catalysts*
AAMs were synthesized using powdered blast furnace slag (BFS) obtained from the Finnish steel industry. Table 5 lists the elemental composition of the slag, as determined by ICP-OES analysis.
AAMs were prepared by mixing 40 g of BFS with 17.5, 20.0, 25.0, and 30.0 g of 50 wt % sodium hydroxide (NaOH ≥ 97%, Merck, Darmsdtadt, Germany). The formed pastes were poured into molds, followed by consolidation in plastic bags at room temperature for 168 h. For comparison, consolidation was first performed at 60 ◦C for 24 h and then for 144 h at room temperature. The prepared samples were named according to their NaOH concentration and consolidation temperature (Table 6). Before use, the materials were crushed using a jaw crusher, sieved to a particle size of 0.5–2.0 mm, and washed with de-ionized water. As the active metal, Fe was impregnated on the AAMs by ion exchange. First, 5 g of AAM and 0.5 dm<sup>3</sup> of 0.01 M (NH4)2Fe(SO4)<sup>2</sup> (99.0–101.5%, Merck, Darmsdtadt, Germany) were mixed in a sand bath and left overnight at 80 ◦C. The solids were collected by filtration, washed with deionized water, and dried overnight at 120 ◦C. Finally, the prepared Fe catalysts were subjected to calcination at 500 ◦C for 2 h, increasing the temperature from room temperature to the target temperature at a rate of 1 ◦C/min.
**Table 6.** Abbreviations of the prepared AAMs and Fe catalysts. Samples were named according to their NaOH concentrations and curing temperatures.
| doab | 2025-04-07T03:56:58.834977 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 101
} |
004db1df-dc76-42af-952f-1f5e82b7822b.102 | *3.2. Stability of AAMs*
The stability of as-prepared AAMs was examined by measuring the possible leaching of the main elements of samples (i.e., Al, Ca, Mg, Na, and Si) to the water phase. Before the test, samples were crushed using a jaw crusher, sieved to a particle size of 1–2 mm, washed with deionized water, and dried. Stability tests were performed in a pressurized reactor at 150 ◦C and under a nitrogen atmosphere of 2 MPa. The crushed AAMs at a concentration of 4 g/dm<sup>3</sup> were continuously stirred for 4 h with 0.16 dm<sup>3</sup> of deionized water. The ready leaching of elements was detected by conductivity measurement during and after the test, while the possible dissolution of Al, Ca, Mg, Na, and Si was analyzed by inductively coupled plasma–optical emission spectroscopy (ICP-OES, Thermo Electron iCAP 6500 Duo, Thermo Fisher Scientific, Waltham, MA, USA).
| doab | 2025-04-07T03:56:58.835119 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 102
} |
004db1df-dc76-42af-952f-1f5e82b7822b.103 | *3.3. Characterization of Samples*
The surface morphology and chemical composition of the prepared AAMs were analyzed by field emission scanning electron microscopy (FESEM; Carl Zeiss Microscopy GMbH, Jena, Germany) combined with energy-dispersive X-ray spectroscopy (EDS; analyzer at the Centre for Material Analysis, University of Oulu, Finland). The phase composition of AAMs was determined by powder X-ray diffraction (XRD) with a PANalytical X 0Pert Pro X-ray diffractometer (Malvern PANalytical, Almelo, The Netherlands). XRD analysis was performed by scanning two theta values between 10◦ and 70◦ with monochromatic Cu Kα1 (λ = 1.5406 Å) at 45 kV and 40 mA at a scan speed of 0.021 ◦/s. Crystalline phases were identified by HighScore Plus software using the Powder Diffraction File standards from the International Centre for Diffraction DATA ICDD (PDF-4+ 2020 RDB). Diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) was employed to investigate the degree of polymerization of the prepared samples. DRIFT spectra were recorded on a Bruker PMA 50 Vertex 80 V (Bruker, Billerica, MA, USA), equipped with a Harrick Praying Mantis diffuse reflection accessory and a high-temperature reaction chamber, by baseline measurement using KBr. Before analysis, the sample chamber was flushed with nitrogen (100 cm3/min), heated at a rate of 10 ◦C/min to the target temperature of 120 ◦C, and maintained at that temperature for 30 min. Measurements were conducted at 400–4000 cm−<sup>1</sup> with a resolution of 4 cm−<sup>1</sup> and 500 scans per minute. The specific surface areas and porosity were obtained from nitrogen adsorption–desorption isotherms at the liquid nitrogen temperature (–196 ◦C) by the Brunauer–Emmett–Teller (BET) method on a Micromeritics ASAP 2020 system (Micromeritics Instrument Corporation, Norcross, GA, USA). The pore size distribution was calculated by density functional theory (DFT) [66]. Furthermore, the main elements of the prepared samples (Al, Ca, Mg, Na, and Si) and the active metal Fe were analyzed by ICP-OES analysis (Thermo Electron iCAP 6500 Duo, Thermo Fisher Scientific, Waltham, MA, USA).
| doab | 2025-04-07T03:56:58.835198 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 103
} |
004db1df-dc76-42af-952f-1f5e82b7822b.104 | *3.4. Catalytic Wet Peroxide Oxidation Experiments*
Oxidation experiments with a BPA aqueous solution (60 mg/L) were performed in a three-necked flask equipped with a reflux condenser. BFS30-60, BFS25-60, BFS20-60, BFS17.5-60, Fe/BFS30-60, and Fe/BFS17.5-60 were examined at a reaction temperature of 50 ◦C, a H2O<sup>2</sup> concentration of 1.5 g/dm<sup>3</sup> (stoichiometric amount to total oxidation of BPA), and a catalyst loading of 4 g/dm<sup>3</sup> , with a reaction volume of 0.16 dm<sup>3</sup> . Oxidation was started while the reaction temperature was reached by the addition of H2O2, which was added in batches to maintain a stable oxidation agent concentration during the 3 h test. Water samples were taken as a function of reaction time, which were filtered using a 0.45 µm filter paper. The pH and dissolved oxygen (DO) were measured from water samples during the experiment. Furthermore, the effects of pH and temperature on oxidation were examined using Fe/BFS30-60 and Fe/BFS17.5-60. For evaluating the stability and reusability of the prepared materials, Fe/BFS17.5-60 was examined in three consecutive oxidation reactions. In addition, the regeneration of used Fe/BFS17.5-60 was
performed by heating the catalyst for 2 h at two temperatures (i.e., 250 ◦C and 500 ◦C) to examine the effect of heat treatment on the activity of the used catalyst.
| doab | 2025-04-07T03:56:58.835366 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 104
} |
004db1df-dc76-42af-952f-1f5e82b7822b.105 | *3.5. Water Sample Analysis*
The BPA concentration of the water samples was determined by high-pressure liquid chromatography (HPLC) equipped with a Waters 996 photodiode array (PDA) detector (Waters Corp., Milford, MA, USA) at a wavelength of 226 nm. A mixture of 0.1% trifluoracetic acid (TFA) in methanol and 0.1% TFA in water at a flow rate of 0.4 cm3/min was used as the eluent mixture to separate compounds on a SunFireTM C18 5-m 2.1 × 100 mm column (Waters Corp., Milford, MA, USA) operated at 30 ◦C. The total organic carbon (TOC) concentration of water samples was determined from the initial and final samples on a Skalar FormacsHT Total Organic Carbon/total nitrogen analyzer (Breda, The Netherlands). Possible leaching of Al, Ca, Mg, Na, and Si was analyzed from the final samples after oxidation by ICP-OES analysis (Thermo Electron iCAP 6500 Duo, Thermo Fisher Scientific, Waltham, MA, USA).
| doab | 2025-04-07T03:56:58.835474 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 105
} |
004db1df-dc76-42af-952f-1f5e82b7822b.106 | **4. Conclusions**
In this study, novel, eco-efficient, BFS-based alkali-activated materials were prepared and examined as catalysts for the CWPO of a BPA aqueous solution. AAMs consolidated at 60 ◦C were selected for catalytic studies, as they were more stable in the aqueous phase, and the phase structure was more porous than that of the samples cured at room temperature. BFSXX-60 samples exhibited moderate activity for the CWPO of BPA at 50◦C and at the initial pH. The catalytic activities of Fe/BFS17.5-60 and Fe/BFS30-60 were examined at reaction temperatures of 50 ◦C, 70 ◦C, and 100 ◦C, and at the initial pH and a pH of 3.5. The addition of iron to the BFS-based materials led to the increased removal of BPA, with the highest BPA removal (50%) achieved using Fe/BFS30-60 at a pH of 3.5 at 70 ◦C. Furthermore, Fe/BFS17.5-60 exhibited moderate activity, even after three consecutive tests, and no change in the phase structure of the AAMS after the oxidation reaction was observed. Although prepared AAMs are interesting alternatives for catalytic water-phase applications, dissolution of Ca and Si, as well as small amounts of Al, was observed from AAMs during oxidation. In addition, the basic character of the material prevented higher removal of BPA. Therefore, additional attention should be focused on the stability and surface pH (e.g., pretreatment with acid) of AAMs in our future studies.
**Author Contributions:** Methodology, A.H. and J.P.; software, A.H; investigation, A.H. and T.H.; data curation, A.H.; writing—original draft preparation, A.H.; writing—review and editing, all authors; visualization, A.H.; project administration, U.L.; funding acquisition, U.L. and A.H. All authors have read and agreed to the published version of the manuscript.
**Funding:** This work was partly funded by the Renlund Foundation, within the project "New catalyst materials for wastewater treatment from industrial side streams".
**Data Availability Statement:** The data presented in this study are available within the article (tables and figures). The data presented in this study are available on request from the corresponding author.
**Acknowledgments:** Financial support from Renlund Foundation is gratefully acknowledged. Henrik Romar, Katariina Hautamäki, Tiina Leskelä, Ilkka Vesavaara, Jere Taipalus, Eemeli Koskela, Santeri Impiö, and Carlos Gonzales are acknowledged.
**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
| doab | 2025-04-07T03:56:58.835633 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 106
} |
004db1df-dc76-42af-952f-1f5e82b7822b.109 | **Eco-Friendly Cotton**/**Linen Fabric Treatment Using Aqueous Ozone and Ultraviolet Photolysis**
### **Kengo Hamada 1,\* , Tsuyoshi Ochiai 1,2 , Yasuyuki Tsuchida <sup>3</sup> , Kyohei Miyano <sup>4</sup> , Yosuke Ishikawa <sup>4</sup> , Toshinari Nagura <sup>4</sup> and Noritaka Kimura <sup>3</sup>**
Received: 30 September 2020; Accepted: 28 October 2020; Published: 2 November 2020
**Abstract:** Chemicals for the scouring and bleaching of fabrics have a high environmental load. In addition, in recent years, the high consumption of these products has become a problem in the manufacture of natural fabric products. Therefore, environmentally friendly, low-waste processes for fabric treatment are required. In this paper, we discuss the bleaching of fabrics using advanced oxidation processes (AOP). These processes use electrochemically generated aqueous ozone and ultraviolet (UV) irradiation to achieve bleaching. However, colour reversion often occurs. In this study, we suppressed unwanted colour reversion by treatment with rongalite. After treatment, changes in fabric colour were determined by measuring the colour difference and reflectance spectra. The best bleaching effect was obtained when ozone and UV irradiation treatments were combined, achieving results similar to those of a conventional bleaching method after 60 min of UV irradiation. In addition, the AOP treatment resulted in the simultaneous scouring of the fabric, as shown by the increased hydrophilicity of the fabric after AOP treatment. Thus, this AOP process represents a new fabric bleaching process that has an extremely low environmental impact.
**Keywords:** advanced oxidation processes; ozone; ultraviolet; bleaching; fabrics
| doab | 2025-04-07T03:56:58.835853 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 109
} |
004db1df-dc76-42af-952f-1f5e82b7822b.110 | **1. Introduction**
In the manufacture of natural fabric products, such as cotton and linen, alkaline chemicals and surfactants are used to remove contaminants (i.e., scouring). In addition, to bleach coloured components derived from natural products, an aqueous solution of sodium hypochlorite or heat treatment is used (i.e., bleaching) [1]. Therefore, the manufacture of natural fibre products is energy intensive, and the environmental load is high, especially because of the need for subsequent wastewater treatment [2–4]. An alternative to conventional bleaching with chemicals is advanced oxidation processes (AOP), which combine the treatment of hydrogen peroxide or aqueous ozone generated by electrolysis [5–9] with irradiation by ultraviolet (UV) light [10–12] and oxidation treatment using enzymes or microwave heating [13,14]. These methods do not use environmentally persistent agents and have low energy costs. In addition, oxidative bleaching with AOP allows the omission of the refining step used in conventional methods, again resulting in a more environmentally friendly method. However, after oxidative bleaching by AOP, the whiteness of the fabric deteriorates (colour reversion),
especially during heating (ironing) after bleaching. Therefore, oxidative bleaching by AOP requires further optimisation. coloured components derived from natural products [15,16]; a chemical probe method [17,18] was used to estimate the amount of OH radicals generated from ozone by UV irradiation to understand the bleaching mechanism. Moreover, we proposed a reduction treatment with rongalite, a common
*Catalysts* **2020**, *10*, x FOR PEER REVIEW 2 of 12
fabric deteriorates (colour reversion), especially during heating (ironing) after bleaching. Therefore,
In this study, we developed a bleaching technology with a low environmental load that combines aqueous electrochemically generated ozone and UV irradiation for the combined bleaching
In this study, we developed a bleaching technology with a low environmental load that combines aqueous electrochemically generated ozone and UV irradiation for the combined bleaching and scouring of cotton and linen. Changes in fabric colour were observed by measuring the colour difference and reflectance spectroscopy. The OH radicals generated by UV irradiation decompose the coloured components derived from natural products [15,16]; a chemical probe method [17,18] was used to estimate the amount of OH radicals generated from ozone by UV irradiation to understand the bleaching mechanism. Moreover, we proposed a reduction treatment with rongalite, a common reducing agent used in the textile industry, to suppress colour reversion after bleaching, which succeeded in minimising the colour reversion after ironing. Our method has excellent bleaching properties and extremely low environmental impact; thus, this method could replace conventional scouring and bleaching methods. reducing agent used in the textile industry, to suppress colour reversion after bleaching, which succeeded in minimising the colour reversion after ironing. Our method has excellent bleaching properties and extremely low environmental impact; thus, this method could replace conventional scouring and bleaching methods. **2. Results** *2.1. Fabric Bleaching Using Aqueous Ozone under UV Irradiation* Figure 1a shows the relationship between processing time and the colour of cotton samples. For comparison, three types of treatment were performed: aqueous ozone + UV, aqueous ozone, and
### **2. Results** water + UV. The colour difference values decreased over time in all treatments. The treatment combining aqueous ozone and UV showed the greatest change, followed by those with aqueous
### *2.1. Fabric Bleaching Using Aqueous Ozone under UV Irradiation* ozone and water + UV. In addition, the change in colour difference was approximated as a first-order
oxidative bleaching by AOP requires further optimisation.
Figure 1a shows the relationship between processing time and the colour of cotton samples. For comparison, three types of treatment were performed: aqueous ozone + UV, aqueous ozone, and water + UV. The colour difference values decreased over time in all treatments. The treatment combining aqueous ozone and UV showed the greatest change, followed by those with aqueous ozone and water + UV. In addition, the change in colour difference was approximated as a first-order reaction, and the reaction rate constant was obtained (Figure 1b). The reaction rate constants for aqueous ozone + UV, aqueous ozone, and water + UV were 0.029, 0.015, and 0.009, respectively. reaction, and the reaction rate constant was obtained (Figure 1b). The reaction rate constants for aqueous ozone + UV, aqueous ozone, and water + UV were 0.029, 0.015, and 0.009, respectively. The changes in the OH radical concentration were determined using a chemical probe fluorescence method. Figure 1b shows the relationship between the treatment time and OH radical concentration. No change in the OH radical concentration was observed in the aqueous ozone treatment without UV irradiation. In contrast, in the aqueous ozone treatment, the concentration of OH radicals increased with UV irradiation and became constant 10 min after the start of irradiation.
**Figure 1.** (**a**) Relationship between treatment time and colour difference (Δ*E*\**ab* values are with respect to the colour of cotton bleached using a conventional chemical method) for each bleaching method (red: aqueous ozone + ultraviolet (UV), blue: aqueous ozone, green: water + UV). (**b**) Relationship between treatment time and OH radical concentration (red: aqueous ozone with UV, blue: aqueous ozone without UV). **Figure 1.** (**a**) Relationship between treatment time and colour difference (∆*E*\**ab* values are with respect to the colour of cotton bleached using a conventional chemical method) for each bleaching method (red: aqueous ozone + ultraviolet (UV), blue: aqueous ozone, green: water + UV). (**b**) Relationship between treatment time and OH radical concentration (red: aqueous ozone with UV, blue: aqueous ozone without UV).
The changes in fabric colour were quantified using reflectance spectroscopy measurements. Figure 2 shows the reflectance spectra of cotton samples treated by AOP and conventional chemical bleaching for different periods. In the AOP-treated samples, the reflectance increased in the region The changes in the OH radical concentration were determined using a chemical probe fluorescence method. Figure 1b shows the relationship between the treatment time and OH radical concentration. No change in the OH radical concentration was observed in the aqueous ozone treatment without UV irradiation. In contrast, in the aqueous ozone treatment, the concentration of OH radicals increased with UV irradiation and became constant 10 min after the start of irradiation.
The changes in fabric colour were quantified using reflectance spectroscopy measurements. Figure 2 shows the reflectance spectra of cotton samples treated by AOP and conventional chemical bleaching for different periods. In the AOP-treated samples, the reflectance increased in the region
below 550 nm with time, and a spectrum equivalent to that of the cotton fabrics bleached by the conventional method was achieved 60 min after the start of UV irradiation. Figure 3 shows photographs of the fabrics before bleaching, after AOP treatment for 60 min, and after conventional bleaching. The cotton fabrics before bleaching had a dull yellow colour. The cotton fabrics treated by AOP had a similar whiteness to that of the cotton bleached by the conventional method. below 550 nm with time, and a spectrum equivalent to that of the cotton fabrics bleached by the conventional method was achieved 60 min after the start of UV irradiation. Figure 3 shows photographs of the fabrics before bleaching, after AOP treatment for 60 min, and after conventional bleaching. The cotton fabrics before bleaching had a dull yellow colour. The cotton fabrics treated by AOP had a similar whiteness to that of the cotton bleached by the conventional method. below 550 nm with time, and a spectrum equivalent to that of the cotton fabrics bleached by the conventional method was achieved 60 min after the start of UV irradiation. Figure 3 shows photographs of the fabrics before bleaching, after AOP treatment for 60 min, and after conventional bleaching. The cotton fabrics before bleaching had a dull yellow colour. The cotton fabrics treated by AOP had a similar whiteness to that of the cotton bleached by the conventional method.
*Catalysts* **2020**, *10*, x FOR PEER REVIEW 3 of 12
*Catalysts* **2020**, *10*, x FOR PEER REVIEW 3 of 12
**Figure 2.** Reflectance spectra of cotton samples treated by advanced oxidation processes (AOP) for different periods (black dots, blue, green, yellow, and red indicate 0, 15, 30, 45, and 60 min, respectively) and conventional chemical bleaching (black). **Figure 2.** Reflectance spectra of cotton samples treated by advanced oxidation processes (AOP) for different periods (black dots, blue, green, yellow, and red indicate 0, 15, 30, 45, and 60 min, respectively) and conventional chemical bleaching (black). **Figure 2.** Reflectance spectra of cotton samples treated by advanced oxidation processes (AOP) for different periods (black dots, blue, green, yellow, and red indicate 0, 15, 30, 45, and 60 min, respectively) and conventional chemical bleaching (black).
**Figure 3.** Photographs of the cotton samples before bleaching, after advanced oxidation processes (AOP) treatment for 60 min, and after conventional bleaching. **Figure 3.** Photographs of the cotton samples before bleaching, after advanced oxidation processes (AOP) treatment for 60 min, and after conventional bleaching. **Figure 3.** Photographs of the cotton samples before bleaching, after advanced oxidation processes (AOP) treatment for 60 min, and after conventional bleaching.
Figure 4 shows the reflectance spectra of linen samples treated using each bleaching treatment, as well as conventional chemical bleaching. The treatment with the combination of aqueous ozone and UV resulted in the largest increase in reflectance, followed by those with aqueous ozone and water + UV. Similar to that of the cotton samples, the percentage reflectance of the linen samples after AOP treatment increased. Notably, the samples treated by AOP for 120 min had improved reflectance over the conventional fabric. Figure 4 shows the reflectance spectra of linen samples treated using each bleaching treatment, as well as conventional chemical bleaching. The treatment with the combination of aqueous ozone and UV resulted in the largest increase in reflectance, followed by those with aqueous ozone and water + UV. Similar to that of the cotton samples, the percentage reflectance of the linen samples after AOP treatment increased. Notably, the samples treated by AOP for 120 min had improved reflectance over the conventional fabric. Figure 4 shows the reflectance spectra of linen samples treated using each bleaching treatment, as well as conventional chemical bleaching. The treatment with the combination of aqueous ozone and UV resulted in the largest increase in reflectance, followed by those with aqueous ozone and water + UV. Similar to that of the cotton samples, the percentage reflectance of the linen samples after AOP treatment increased. Notably, the samples treated by AOP for 120 min had improved reflectance over the conventional fabric.
Figure 5 shows photographs of the linen samples after bleaching treatment. The fabric before bleaching was grey. The bleaching process occurred in the centre, where UV irradiated aqueous ozone was in direct contact with the fabric. The linen fabrics treated with AOP for 120 min showed whiteness similar to that of the conventional method in the central spot. Figure 5 shows photographs of the linen samples after bleaching treatment. The fabric before bleaching was grey. The bleaching process occurred in the centre, where UV irradiated aqueous ozone was in direct contact with the fabric. The linen fabrics treated with AOP for 120 min showed whiteness similar to that of the conventional method in the central spot. Figure 5 shows photographs of the linen samples after bleaching treatment. The fabric before bleaching was grey. The bleaching process occurred in the centre, where UV irradiated aqueous ozone was in direct contact with the fabric. The linen fabrics treated with AOP for 120 min showed whiteness similar to that of the conventional method in the central spot.
**Figure 4.** Reflectance spectra of linen samples treated by conventional chemical bleaching (black), no treatment (black dot), water + UV treatment for 60 min (blue), ozone water treatment for 60 min (light blue), AOP treatment for 60 min (green), and AOP treatment for 120 min (red). **Figure 4.** Reflectance spectra of linen samples treated by conventional chemical bleaching (black),no treatment (black dot), water <sup>+</sup> UV treatment for 60 min (blue), ozone water treatment for 60 min (light blue), AOP treatment for 60 min (green), and AOP treatment for 120 min (red). **Figure 4.** Reflectance spectra of linen samples treated by conventional chemical bleaching (black), no treatment (black dot), water + UV treatment for 60 min (blue), ozone water treatment for 60 min (light blue), AOP treatment for 60 min (green), and AOP treatment for 120 min (red).
**Figure 5.** Photographs of the linen samples before bleaching, after water + ultraviolet (UV) treatment for 60 min, after aqueous ozone treatment for 60 min, after advanced oxidation processes (AOP) treatment for 60 min, after AOP treatment for 120 min, and after conventional bleaching. **Figure 5.** Photographs of the linen samples before bleaching, after water + ultraviolet (UV) treatment for 60 min, after aqueous ozone treatment for 60 min, after advanced oxidation processes (AOP) treatment for 60 min, after AOP treatment for 120 min, and after conventional bleaching. **Figure 5.** Photographs of the linen samples before bleaching, after water + ultraviolet (UV) treatment for 60 min, after aqueous ozone treatment for 60 min, after advanced oxidation processes (AOP) treatment for 60 min, after AOP treatment for 120 min, and after conventional bleaching.
Figure 6 shows the effect of AOP treatment on the fabric samples. The percentage reflectance increased in the region below 550 nm with and without scouring. The change in the reflectance spectrum of the cotton cloth was constant regardless of whether it was scoured. In addition, Figure 7 (left) shows a photograph of a drop of water on a cotton sample before scouring. Figure 7 (right) shows a photograph of a wetted fabric sample after bleaching by AOP treatment for 30 min. Before scouring and bleaching, the fabric was water repellent. After AOP treatment for 30 min, the cotton lost its water repellence, and the water drop penetrated the fabric. Figure 6 shows the effect of AOP treatment on the fabric samples. The percentage reflectance increased in the region below 550 nm with and without scouring. The change in the reflectance spectrum of the cotton cloth was constant regardless of whether it was scoured. In addition, Figure 7 (left) shows a photograph of a drop of water on a cotton sample before scouring. Figure 7 (right) shows a photograph of a wetted fabric sample after bleaching by AOP treatment for 30 min. Before scouring and bleaching, the fabric was water repellent. After AOP treatment for 30 min, the cotton lost its water repellence, and the water drop penetrated the fabric. Figure 6 shows the effect of AOP treatment on the fabric samples. The percentage reflectance increased in the region below 550 nm with and without scouring. The change in the reflectance spectrum of the cotton cloth was constant regardless of whether it was scoured. In addition, Figure 7 (left) shows a photograph of a drop of water on a cotton sample before scouring. Figure 7 (right) shows a photograph of a wetted fabric sample after bleaching by AOP treatment for 30 min. Before scouring and bleaching, the fabric was water repellent. After AOP treatment for 30 min, the cotton lost its water repellence, and the water drop penetrated the fabric.
*Catalysts* **2020**, *10*, x FOR PEER REVIEW 5 of 12
*Catalysts* **2020**, *10*, x FOR PEER REVIEW 5 of 12
**Figure 6.** Reflectance spectra of the cotton samples after advanced oxidation processes (AOP) treatment for different periods (black, green, and red indicate 0, 30, and 60 min AOP treatment, respectively) with (dots) and without (solid line) scouring. **Figure 6.** Reflectance spectra of the cotton samples after advanced oxidation processes (AOP) treatment for different periods (black, green, and red indicate 0, 30, and 60 min AOP treatment, respectively) with (dots) and without (solid line) scouring. **Figure 6.** Reflectance spectra of the cotton samples after advanced oxidation processes (AOP) treatment for different periods (black, green, and red indicate 0, 30, and 60 min AOP treatment, respectively) with (dots) and without (solid line) scouring.
**Figure 7.** Photographs of cotton samples before scouring and bleaching and after advanced oxidation processes (AOP) treatment for 30 min treated with a drop of water. **Figure 7.** Photographs of cotton samples before scouring and bleaching and after advanced oxidation processes (AOP) treatment for 30 min treated with a drop of water. **Figure 7.** Photographs of cotton samples before scouring and bleaching and after advanced oxidation processes (AOP) treatment for 30 min treated with a drop of water.
Figure 8 shows the reflectance spectra of AOP-treated samples (0 and 60 min) before and after ironing at 200 °C. When the cotton was dried after AOP treatment, the reflectance decreased below 500 nm. When the ironed fabric was again subjected to AOP treatment, the spectrum was restored to the pre-ironing state. Ironing and AOP treatment were repeated five times, but only the results of the second ironing/AOP cycle are shown in Figure 8. The colour of the fabric changed from yellowish white to white and back after each of the five ironing/AOP cycles. Figure 8 shows the reflectance spectra of AOP-treated samples (0 and 60 min) before and after ironing at 200 °C. When the cotton was dried after AOP treatment, the reflectance decreased below 500 nm. When the ironed fabric was again subjected to AOP treatment, the spectrum was restored to the pre-ironing state. Ironing and AOP treatment were repeated five times, but only the results of the second ironing/AOP cycle are shown in Figure 8. The colour of the fabric changed from yellowish white to white and back after each of the five ironing/AOP cycles. Figure 8 shows the reflectance spectra of AOP-treated samples (0 and 60 min) before and after ironing at 200 ◦C. When the cotton was dried after AOP treatment, the reflectance decreased below500 nm. When the ironed fabric was again subjected to AOP treatment, the spectrum was restored to the pre-ironing state. Ironing and AOP treatment were repeated five times, but only the results of thesecond ironing/AOP cycle are shown in Figure 8. The colour of the fabric changed from yellowish white to white and back after each of the five ironing/AOP cycles.
| doab | 2025-04-07T03:56:58.835970 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 110
} |
004db1df-dc76-42af-952f-1f5e82b7822b.111 | *2.2. Suppression of Colour Reversion Using Rongalite Treatment*
second AOP treatment 15 min after ironing (red).
second AOP treatment 15 min after ironing (red).
Figure 9 shows the reflectance spectra of cotton samples with and without rongalite treatment. The reflectance spectrum of the cotton fabric subjected to AOP treatment for 60 min increased with respect to that of the untreated sample. The reflectance spectrum of the cotton fabric ironed after AOP treatment decreased below 550 nm. On the other hand, the cotton fabric ironed after rongalite treatment did not show a decrease in the reflectance spectrum below 550 nm, and the reflectance spectra were maintained after AOP treatment. Figure 10 shows photographs of the cotton fabrics after AOP for 60 min, after ironing at 200 ◦C, and after ironing at 200 ◦C following rongalite treatment. The cotton sample after AOP was white. The cotton sample after ironing was not as white as that treated by AOP alone. The cotton sample after rongalite treatment and ironing was as white as that after AOP alone.
**Figure 8.** Reflectance spectra of cotton samples before (black dots) and after (black line) advanced oxidation processes (AOP) for 60 min, colour reversion after ironing at 200 °C (green), and after a
**Figure 8.** Reflectance spectra of cotton samples before (black dots) and after (black line) advanced oxidation processes (AOP) for 60 min, colour reversion after ironing at 200 °C (green), and after a
**Figure 6.** Reflectance spectra of the cotton samples after advanced oxidation processes (AOP) treatment for different periods (black, green, and red indicate 0, 30, and 60 min AOP treatment,
**Figure 7.** Photographs of cotton samples before scouring and bleaching and after advanced oxidation
Figure 8 shows the reflectance spectra of AOP-treated samples (0 and 60 min) before and after ironing at 200 °C. When the cotton was dried after AOP treatment, the reflectance decreased below 500 nm. When the ironed fabric was again subjected to AOP treatment, the spectrum was restored to the pre-ironing state. Ironing and AOP treatment were repeated five times, but only the results of the
respectively) with (dots) and without (solid line) scouring.
processes (AOP) treatment for 30 min treated with a drop of water.
**Figure 8.** Reflectance spectra of cotton samples before (black dots) and after (black line) advanced oxidation processes (AOP) for 60 min, colour reversion after ironing at 200 °C (green), and after a second AOP treatment 15 min after ironing (red). **Figure 8.** Reflectance spectra of cotton samples before (black dots) and after (black line) advanced oxidation processes (AOP) for 60 min, colour reversion after ironing at 200 ◦C (green), and after a second AOP treatment 15 min after ironing (red). cotton sample after AOP was white. The cotton sample after ironing was not as white as that treated by AOP alone. The cotton sample after rongalite treatment and ironing was as white as that after AOP alone. cotton sample after AOP was white. The cotton sample after ironing was not as white as that treated by AOP alone. The cotton sample after rongalite treatment and ironing was as white as that after AOP alone.
**Figure 9.** Reflectance spectra of cotton samples before (black dots) and after (black line) advanced oxidation processes (AOP) for 60 min and those of samples ironed at 200 °C with (red) and without (green) rongalite treatment. **Figure 9.** Reflectance spectra of cotton samples before (black dots) and after (black line) advanced oxidation processes (AOP) for 60 min and those of samples ironed at 200 ◦C with (red) and without (green) rongalite treatment. **Figure 9.** Reflectance spectra of cotton samples before (black dots) and after (black line) advanced oxidation processes (AOP) for 60 min and those of samples ironed at 200 °C with (red) and without (green) rongalite treatment.
**Figure 10.** Photographs of cotton samples after advanced oxidation processes (AOP) for 60 min, after ironing at 200 °C, and after ironing at 200 °C following rongalite treatment. **Figure 10.** Photographs of cotton samples after advanced oxidation processes (AOP) for 60 min, after ironing at 200 °C, and after ironing at 200 °C following rongalite treatment. **Figure 10.** Photographs of cotton samples after advanced oxidation processes (AOP) for 60 min, after ironing at 200 ◦C, and after ironing at 200 ◦C following rongalite treatment.
Figure 11 shows the reflectance spectra of linen samples with and without rongalite treatment. The percentage reflectance of the linen fabrics subjected to AOP treatment for 120 min increased compared with that of the untreated sample. The reflectance spectrum of the linen fabric ironed after AOP treatment decreased at all wavelengths. In contrast, the cotton fabric ironed after rongalite treatment did not show a decrease, and the reflectance spectra were maintained after AOP treatment. Figure 11 shows the reflectance spectra of linen samples with and without rongalite treatment. The percentage reflectance of the linen fabrics subjected to AOP treatment for 120 min increased compared with that of the untreated sample. The reflectance spectrum of the linen fabric ironed after AOP treatment decreased at all wavelengths. In contrast, the cotton fabric ironed after rongalite treatment did not show a decrease, and the reflectance spectra were maintained after AOP treatment. Figure 11 shows the reflectance spectra of linen samples with and without rongalite treatment. The percentage reflectance of the linen fabrics subjected to AOP treatment for 120 min increased compared with that of the untreated sample. The reflectance spectrum of the linen fabric ironed after AOP treatment decreased at all wavelengths. In contrast, the cotton fabric ironed after rongalite treatment did not show a decrease, and the reflectance spectra were maintained after AOP treatment.
sample after AOP alone.
sample after AOP alone.
Figure 12 shows photographs of the linen fabrics after AOP for 120 min and after ironing at 200 °C,
Figure 12 shows photographs of the linen fabrics after AOP for 120 min and after ironing at 200 °C,
Figure 12 shows photographs of the linen fabrics after AOP for 120 min and after ironing at 200 ◦C, with and without rongalite treatment. The linen sample after AOP was white, but, after ironing, some whiteness was lost. After rongalite treatment and ironing, the sample remained as white as the sample after AOP alone. *Catalysts* **2020**, *10*, x FOR PEER REVIEW 7 of 12 *Catalysts* **2020**, *10*, x FOR PEER REVIEW 7 of 12
**Figure 11.** Reflectance spectra of linen samples without advanced oxidation processes (AOP; black dots) and after AOP for 120 min (black line) and of AOP-treated samples after ironing at 200 °C without (green) and with (red) rongalite treatment. **Figure 11.** Reflectance spectra of linen samples without advanced oxidation processes (AOP; black dots) and after AOP for 120 min (black line) and of AOP-treated samples after ironing at 200 ◦C without (green) and with (red) rongalite treatment. **Figure 11.** Reflectance spectra of linen samples without advanced oxidation processes (AOP; black dots) and after AOP for 120 min (black line) and of AOP-treated samples after ironing at 200 °C without (green) and with (red) rongalite treatment.
**Figure 12.** Photographs of the linen samples after advanced oxidation processes (AOP) for 120 min, after AOP and ironing at 200 °C, and after AOP and ironing at 200 °C following rongalite treatment. **Figure 12.** Photographs of the linen samples after advanced oxidation processes (AOP) for 120 min, after AOP and ironing at 200 °C, and after AOP and ironing at 200 °C following rongalite treatment. **Figure 12.** Photographs of the linen samples after advanced oxidation processes (AOP) for 120 min,after AOP and ironing at 200 ◦C, and after AOP and ironing at 200 ◦C following rongalite treatment.
### **3. Discussion 3. Discussion 3. Discussion**
Figure 1a shows that the combination of aqueous ozone and UV light was most suitable for bleaching, although a bleaching effect was also obtained when using aqueous ozone alone and water + UV. Moreover, the reaction rate constant of the aqueous ozone + UV system was not a simple sum of those of the aqueous ozone and water + UV systems. Therefore, it appears that the cotton was bleached by the synergistic effect of ozone and UV irradiation. Figure 1b shows that OH radicals were generated by UV irradiation of the aqueous ozone. It is known that OH radicals decompose coloured components in fabrics during AOP [19,20], and the generation of OH radicals is likely responsible for the high reaction constant shown in Figure 1a. Specifically, excited singlet oxygen is produced by the UV irradiation of ozone at 310 nm, whereas excited triplet oxygen is produced by the visible light irradiation of ozone (> 460 nm). The bleaching OH radicals are generated from excited singlet oxygen [21]. Disodium terephthalate (NaTA) can be used as an indicator of the generated OH radicals, forming 2-terephthalic acid in the process [17]; approximately 0.35 ppb OH radicals were produced by UV irradiation. OH radicals are extremely reactive [22]; thus, their lifetime is very short. For these reasons, the OH radical concentration in Figure 1b is low, compared with the ozone concentration. Figure 1a shows that the combination of aqueous ozone and UV light was most suitable for bleaching, although a bleaching effect was also obtained when using aqueous ozone alone and water + UV. Moreover, the reaction rate constant of the aqueous ozone + UV system was not a simple sum of those of the aqueous ozone and water + UV systems. Therefore, it appears that the cotton was bleached by the synergistic effect of ozone and UV irradiation. Figure 1b shows that OH radicals were generated by UV irradiation of the aqueous ozone. It is known that OH radicals decompose coloured components in fabrics during AOP [19,20], and the generation of OH radicals is likely responsible for the high reaction constant shown in Figure 1a. Specifically, excited singlet oxygen is produced by the UV irradiation of ozone at 310 nm, whereas excited triplet oxygen is produced by the visible light irradiation of ozone (> 460 nm). The bleaching OH radicals are generated from excited singlet oxygen [21]. Disodium terephthalate (NaTA) can be used as an indicator of the generated OH radicals, forming 2-terephthalic acid in the process [17]; approximately 0.35 ppb OH radicals were produced by UV irradiation. OH radicals are extremely reactive [22]; thus, their lifetime is very short. For these reasons, the OH radical concentration in Figure 1b is low, compared with the ozone concentration. Figure 1a shows that the combination of aqueous ozone and UV light was most suitable for bleaching, although a bleaching effect was also obtained when using aqueous ozone alone and water + UV. Moreover, the reaction rate constant of the aqueous ozone + UV system was not a simple sum of those of the aqueous ozone and water + UV systems. Therefore, it appears that the cotton was bleached by the synergistic effect of ozone and UV irradiation. Figure 1b shows that OH radicals were generated by UV irradiation of the aqueous ozone. It is known that OH radicals decompose coloured components in fabrics during AOP [19,20], and the generation of OH radicals is likely responsible for the high reaction constant shown in Figure 1a. Specifically, excited singlet oxygen is produced by the UV irradiation of ozone at 310 nm, whereas excited triplet oxygen is produced by the visible light irradiation of ozone (> 460 nm). The bleaching OH radicals are generated from excited singlet oxygen [21]. Disodium terephthalate (NaTA) can be used as an indicator of the generated OH radicals, forming 2-terephthalic acid in the process [17]; approximately 0.35 ppb OH radicals were produced by UV irradiation. OH radicals are extremely reactive [22]; thus, their lifetime is very short. For these reasons, the OH radical concentration in Figure 1b is low, compared with the ozone concentration.
Figures 2 and 3 show that the AOP treatment that combined aqueous ozone and UV (60 min) had the same bleaching effect as the conventional method. As unbleached cotton contains coloured Figures 2 and 3 show that the AOP treatment that combined aqueous ozone and UV (60 min) had the same bleaching effect as the conventional method. As unbleached cotton contains coloured Figures 2 and 3 show that the AOP treatment that combined aqueous ozone and UV (60 min) had the same bleaching effect as the conventional method. As unbleached cotton contains coloured
components, it had a low reflectance spectrum below 550 nm. Cotton contains approximately 90%
components, it had a low reflectance spectrum below 550 nm. Cotton contains approximately 90%
coloured components, like cotton, and the low reflectance spectrum of the unbleached linen shown in Figure 4 is due to these components. As shown in Figures 4 and 5, in the case of linen, it took 120
coloured components, like cotton, and the low reflectance spectrum of the unbleached linen shown in Figure 4 is due to these components. As shown in Figures 4 and 5, in the case of linen, it took 120 components, it had a low reflectance spectrum below 550 nm. Cotton contains approximately 90% cellulose and approximately 10% non-cellulosic matter, comprising proteins, waxes, pectin, and ash [1]. Therefore, on the basis of the results, these components were decomposed by AOP treatment to the same extent as in the conventional bleaching method. The linen before bleaching also contained coloured components, like cotton, and the low reflectance spectrum of the unbleached linen shown in Figure 4 is due to these components. As shown in Figures 4 and 5, in the case of linen, it took 120 min of AOP to bleach to the same extent as the conventional method. Thus, bleaching time is dependent on the amount of coloured components contained in the linen. Unlike cotton, linen contains only ~70% cellulose and a high proportion of non-cellulosic matter, which complicates the bleaching process. In addition, non-cellulosic matter contains a small amount of dark-coloured lignin [23], which suggests that linen will require a longer treatment time. As shown in Figure 6, AOP-treated cotton fabrics without scouring were bleached to the same degree as those with scouring and AOP treatment. In addition, Figure 7 shows that AOP treatment eliminated the water repellence of the cotton fabrics observed before scouring, resulting in hydrophilicity. As previously mentioned, before scouring, cotton contains wax [1], which yields water repellence. Therefore, the results in Figures 6 and 7 suggest that AOP treatment removes both coloured compounds and residual components (wax and oil) simultaneously. Thus, unlike conventional processes, our AOP process achieves scouring and bleaching at the same time.
In Figure 8, the colour return after the AOP process is shown by the reflectance spectra. In particular, the reflectance of the ironed sample was reduced, compared with that of the sample after AOP treatment. However, the reflectance recovered again after AOP treatment following ironing. However, repeated ironing resulted in the appearance of colour. The ironing and AOP treatment were repeated five times, but only the second cycle is shown in Figure 8. These results suggest that the colour reversion after ironing and AOP treatment is a reversible reaction. Cellulose, the main component of cotton, can be oxidised, and the oxidised form shows absorption in the near-UV to visible light region [24]. Even after AOP treatment, the cellulose in cotton may be oxidised, resulting in colour reversion. Therefore, in this study, rongalite, a reducing agent, was used to suppress colour reversion after AOP treatment and ironing. As shown in Figures 9 and 10, without rongalite treatment, reflectance decreased after ironing, whereas rongalite-treated fabrics did not show a decrease in reflectance in the wavelength region below 500 nm, even after ironing. In addition, even in the case of linen, rongalite-treated fabrics did not show a decrease in reflectance after ironing; the reflectance spectra after AOP treatment were similar to those before treatment (Figures 11 and 12).
Figure 13 shows the proposed mechanisms of colour reversion and suppression of colour reversion after rongalite treatment of fabrics. AOP treatment could damage the fabric [12,25], and previous studies on the cause of colour reversion after oxidative bleaching have reported that some of the OH groups of cellulose (particularly those at the 2nd and 3rd positions) are oxidised, resulting in the formation of C=O groups and some conjugated double bonds [24]. Thus, colour reversion could be suppressed by the reduction of the C=O groups generated in the cellulose fibres of OH groups after bleaching. Therefore, before ironing, the AOP-treated fabric was treated with a strongly reducing rongalite solution to reduce the C=O groups. Consequently, subsequent heat treatment (ironing) did not result in the formation of conjugated double bonds.
achieves scouring and bleaching at the same time.
(ironing) did not result in the formation of conjugated double bonds.
min of AOP to bleach to the same extent as the conventional method. Thus, bleaching time is dependent on the amount of coloured components contained in the linen. Unlike cotton, linen contains only ~70% cellulose and a high proportion of non-cellulosic matter, which complicates the bleaching process. In addition, non-cellulosic matter contains a small amount of dark-coloured lignin [23], which suggests that linen will require a longer treatment time. As shown in Figure 6, AOPtreated cotton fabrics without scouring were bleached to the same degree as those with scouring and AOP treatment. In addition, Figure 7 shows that AOP treatment eliminated the water repellence of the cotton fabrics observed before scouring, resulting in hydrophilicity. As previously mentioned, before scouring, cotton contains wax [1], which yields water repellence. Therefore, the results in Figures 6 and 7 suggest that AOP treatment removes both coloured compounds and residual components (wax and oil) simultaneously. Thus, unlike conventional processes, our AOP process
In Figure 8, the colour return after the AOP process is shown by the reflectance spectra. In particular, the reflectance of the ironed sample was reduced, compared with that of the sample after AOP treatment. However, the reflectance recovered again after AOP treatment following ironing. However, repeated ironing resulted in the appearance of colour. The ironing and AOP treatment were repeated five times, but only the second cycle is shown in Figure 8. These results suggest that the colour reversion after ironing and AOP treatment is a reversible reaction. Cellulose, the main component of cotton, can be oxidised, and the oxidised form shows absorption in the near-UV to visible light region [24]. Even after AOP treatment, the cellulose in cotton may be oxidised, resulting in colour reversion. Therefore, in this study, rongalite, a reducing agent, was used to suppress colour reversion after AOP treatment and ironing. As shown in Figures 9 and 10, without rongalite treatment, reflectance decreased after ironing, whereas rongalite-treated fabrics did not show a decrease in reflectance in the wavelength region below 500 nm, even after ironing. In addition, even in the case of linen, rongalite-treated fabrics did not show a decrease in reflectance after ironing; the reflectance spectra after AOP treatment were similar to those before treatment (Figures 11 and 12). Figure 13 shows the proposed mechanisms of colour reversion and suppression of colour reversion after rongalite treatment of fabrics. AOP treatment could damage the fabric [12,25], and previous studies on the cause of colour reversion after oxidative bleaching have reported that some of the OH groups of cellulose (particularly those at the 2nd and 3rd positions) are oxidised, resulting in the formation of C=O groups and some conjugated double bonds [24]. Thus, colour reversion could be suppressed by the reduction of the C=O groups generated in the cellulose fibres of OH groups after bleaching. Therefore, before ironing, the AOP-treated fabric was treated with a strongly
**Figure 13.** Proposed mechanisms of the suppression of colour reversion in fabrics and the effect of rongalite treatment. **Figure 13.** Proposed mechanisms of the suppression of colour reversion in fabrics and the effect of rongalite treatment. *Catalysts* **2020**, *10*, x FOR PEER REVIEW 9 of 12
### **4. Materials and Methods 4. Materials and Methods**
radicals.
Cotton and linen cloths were provided by Nisshinbo Textrail Co., Ltd. (Tokyo Japan). The colour differences of the fabrics were measured using a colour reader (CR-10, Konica Minolta Japan, Inc., Tokyo, Japan) and expressed as *L*\**a*\**b*\* and ∆*E*\**ab* values [26,27]. Here, *L*\* represents brightness, equalling zero for a black diffuser and 100 for a perfectly reflecting one; *a*\* represents colour on the red–green axis, being positive for red and negative for green; and *b*\* represents colour on the blue–yellow axis, being positive for yellow and negative for blue. The colour difference was calculated as ∆*E*\**ab* = ((∆*L*\*)<sup>2</sup> + (∆*a*\*)<sup>2</sup> + (∆*b*\*)<sup>2</sup> ) 1/2 , where ∆*L*\* is the difference in brightness between two vivid surfaces and ∆*a*\* and ∆*b*\* are the differences in the colour coordinates *a*\* and *b*\*, respectively. Figure 14 shows the bleaching experiment in which aqueous ozone and UV irradiation were combined. UV light is emitted above a cloth sample placed on a glass plate. As a UV light source, an LA-310UV manufactured by Hayashi Clock Industry Co., Ltd. (Tokyo, Japan) was used. In addition, the electrolytically generated aqueous ozone was produced by an aqueous ozone generator (Quick O<sup>3</sup> Pico, AOD-TH2 manufactured by Aidenshi Co., Ltd., Nasushiobara, Japan) and dropped onto the fabric at the same position as the UV spot. Cotton and linen cloths were provided by Nisshinbo Textrail Co., Ltd. (Tokyo Japan). The colour differences of the fabrics were measured using a colour reader (CR-10, Konica Minolta Japan, Inc., Tokyo, Japan) and expressed as *L*\**a*\**b*\* and ∆*E*\**ab* values [26,27]. Here, *L*\* represents brightness, equalling zero for a black diffuser and 100 for a perfectly reflecting one; *a*\* represents colour on the red–green axis, being positive for red and negative for green; and *b*\* represents colour on the blue– yellow axis, being positive for yellow and negative for blue. The colour difference was calculated as ∆*E*\**ab* = ((∆*L*\*)2 + (∆*a*\*)2 + (∆*b*\*)2) 1/2, where ∆*L*\* is the difference in brightness between two vivid surfaces and ∆*a*\* and ∆*b*\* are the differences in the colour coordinates *a*\* and *b*\*, respectively. Figure 14 shows the bleaching experiment in which aqueous ozone and UV irradiation were combined. UV light is emitted above a cloth sample placed on a glass plate. As a UV light source, an LA-310UV manufactured by Hayashi Clock Industry Co., Ltd. (Tokyo, Japan) was used. In addition, the electrolytically generated aqueous ozone was produced by an aqueous ozone generator (Quick O3 Pico, AOD-TH2 manufactured by Aidenshi Co., Ltd., Nasushiobara, Japan) and dropped onto the fabric at the same position as the UV spot.
**Figure 14.** Photograph of a bleaching experiment. The fabric sample was set on a glass plate. Water containing electrochemically generated ozone was dropped onto the fabric, and the sample was irradiated with ultraviolet (UV) light. **Figure 14.** Photograph of a bleaching experiment. The fabric sample was set on a glass plate. Water containing electrochemically generated ozone was dropped onto the fabric, and the sample was irradiated with ultraviolet (UV) light.
The rongalite treatment was carried out by the dipping method. The sample after AOP treatment was immersed in a 0.5 wt% rongalite aqueous solution for 1 h, then drained and dried on a hot plate The rongalite treatment was carried out by the dipping method. The sample after AOP treatment was immersed in a 0.5 wt% rongalite aqueous solution for 1 h, then drained and dried on a hot plate at
at 200 °C for 1 min. Figure 15 shows photographs and a schematic of the measurement of the amount
disodium terephthalate chemical probe [17,18]. As shown in Figure 15, aqueous ozone irradiated with UV was collected and then mixed with disodium terephthalate (NaTA, TCI Co., Ltd., Tokyo, Japan) until completely dissolved. Then, while irradiating the solution with UV light, the generated fluorescence was detected. An SEC-2000-UV/VIS Spectrometer manufactured by ALS Co., Ltd. (Tokyo, Japan) was used for UV light generation and fluorescence detection. Disodium terephthalate, used as an indicator, reacts with OH radicals in aqueous solution to produce 2-hydroxyterephthalic acid, which fluoresces at 425 nm under UV irradiation, allowing quantification of the generated OH 200 ◦C for 1 min. Figure 15 shows photographs and a schematic of the measurement of the amount of OH radicals in the aqueous ozone under UV irradiation. It is well known that OH radicals are generated by AOP treatment with aqueous ozone and UV irradiation [15,16,19,20]. In this study, the OH radical concentration in the ozone solution during AOP treatment was measured using a disodium terephthalate chemical probe [17,18]. As shown in Figure 15, aqueous ozone irradiated with UV was collected and then mixed with disodium terephthalate (NaTA, TCI Co., Ltd., Tokyo, Japan) until completely dissolved. Then, while irradiating the solution with UV light, the generated fluorescence was detected. An SEC-2000-UV/VIS Spectrometer manufactured by ALS Co., Ltd. (Tokyo, Japan) was used for UV light generation and fluorescence detection. Disodium terephthalate, used as an indicator, reacts with OH radicals in aqueous solution to produce 2-hydroxyterephthalic acid, which fluoresces at 425 nm under UV irradiation, allowing quantification of the generated OH radicals. *Catalysts* **2020**, *10*, x FOR PEER REVIEW 10 of 12
**Figure 15.** Photographs and schematic of OH radical quantification by fluorescence spectroscopy. **Figure 15.** Photographs and schematic of OH radical quantification by fluorescence spectroscopy.
### **5. Conclusions 5. Conclusions**
**References**
In this paper, we reported a two-step fabric treatment method based on AOP treatment using aqueous ozone and UV irradiation with rongalite treatment. The AOP process removed coloured components in fabric via decomposition with active oxygen species (ozone and the OH radicals generated in aqueous solution). The OH radical concentration in the aqueous solution was measured by a chemical probe, which can easily and quickly measure OH radicals on the spot, and it is applicable to industrial-scale process control. The reflectance spectra of the fabric samples revealed an increase in bleaching with increasing treatment time, and, after 60 min for cotton and 120 min for linen, bleaching comparable to that of the conventional method was achieved. In addition, we confirmed that AOP treatment is suitable for decomposing waxes and oils contained in the fabric before scouring. Therefore, this AOP process can replace conventional scouring and bleaching processes. Furthermore, we succeeded in preventing colour reversion by reducing cellulose oxidised by the AOP treatment with inexpensive rongalite. The AOP + rongalite treatment for fabrics is attractive because it produces little waste and consumes little energy compared with the conventional method. Thus, this method is a promising alternative to conventional bleaching in natural fibre In this paper, we reported a two-step fabric treatment method based on AOP treatment usingaqueous ozone and UV irradiation with rongalite treatment. The AOP process removed coloured components in fabric via decomposition with active oxygen species (ozone and the OH radicalsgenerated in aqueous solution). The OH radical concentration in the aqueous solution was measured by a chemical probe, which can easily and quickly measure OH radicals on the spot, and it is applicableto industrial-scale process control. The reflectance spectra of the fabric samples revealed an increase in bleaching with increasing treatment time, and, after 60 min for cotton and 120 min for linen, bleaching comparable to that of the conventional method was achieved. In addition, we confirmed that AOP treatment is suitable for decomposing waxes and oils contained in the fabric before scouring. Therefore, this AOP process can replace conventional scouring and bleaching processes. Furthermore, we succeeded in preventing colour reversion by reducing cellulose oxidised by the AOP treatmentwith inexpensive rongalite. The AOP <sup>+</sup> rongalite treatment for fabrics is attractive because it produces little waste and consumes little energy compared with the conventional method. Thus, this method isa promising alternative to conventional bleaching in natural fibre processing.
processing. **Author Contributions:** K.H., T.O., K.M., Y.I., T.N., and N.K. participated in the study design and conducted the study; K.H., T.O., and Y.T. collected and analysed the data. K.H wrote the manuscript. All authors have read **Author Contributions:** K.H., T.O., K.M., Y.I., T.N. and N.K. participated in the study design and conducted the study; K.H., T.O. and Y.T. collected and analysed the data. K.H. wrote the manuscript. All authors have read and agreed to the published version of the manuscript.
and agreed to the published version of the manuscript. **Funding:** This research received no external funding.
321, doi:10.1016/j.carbpol.2012.05.044.
doi:10.1016/j.carbpol.2009.07.019.
**Conflicts of Interest:** The authors declare no conflicts of interest.
1. Abdel-Halim, E.S. An effective redox system for bleaching cotton cellulose. *Carbohydr. Polym.* **2012**, *90*, 316–
2. Hebeish, A.; Hashem, M.; Shaker, N.; Ramadan, M.; El-Sadek, B.; Hady, M.A. New development for combined bioscouring and bleaching of cotton-based fabrics. *Carbohydr. Polym.* **2009**, *78*, 961–972,
insightful discussion. We would like to thank Editage (www.editage.com) for English language editing.
**Funding:** This research received no external funding.
**Acknowledgments:** We are grateful to Nisshinbo Textile Inc. for preparing the fabrics. We are also grateful to Ayato Koibuchi and Kosuke Miura (Nagaoka University of Technology) for help with the experiments and for insightful discussion. We would like to thank Editage (www.editage.com) for English language editing.
**Conflicts of Interest:** The authors declare no conflict of interest.
| doab | 2025-04-07T03:56:58.836551 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 111
} |
004db1df-dc76-42af-952f-1f5e82b7822b.113 | **CeO<sup>2</sup> for Water Remediation: Comparison of Various Advanced Oxidation Processes**
**Roberto Fiorenza <sup>1</sup> , Stefano Andrea Balsamo <sup>1</sup> , Luisa D'Urso <sup>1</sup> , Salvatore Sciré 1 , Maria Violetta Brundo <sup>2</sup> , Roberta Pecoraro <sup>2</sup> , Elena Maria Scalisi <sup>2</sup> , Vittorio Privitera <sup>3</sup> and Giuliana Impellizzeri 3,\***
Received: 30 March 2020; Accepted: 20 April 2020; Published: 21 April 2020
**Abstract:** Three different Advanced Oxidation Processes (AOPs) have been investigated for the degradation of the imidacloprid pesticide in water: photocatalysis, Fenton and photo-Fenton reactions. For these tests, we have compared the performance of two types of CeO2, employed as a non-conventional photocatalyst/Fenton-like material. The first one has been prepared by chemical precipitation with KOH, while the second one has been obtained by exposing the as-synthetized CeO<sup>2</sup> to solar irradiation in H<sup>2</sup> stream. This latter treatment led to obtain a more defective CeO<sup>2</sup> (coded as "grey CeO2") with the formation of Ce3<sup>+</sup> sites on the surface of CeO2, as determined by Raman and X-ray Photoelectron Spectroscopy (XPS) characterizations. This peculiar feature has been demonstrated as beneficial for the solar photo–Fenton reaction, with the best performance exhibited by the grey CeO2. On the contrary, the bare CeO<sup>2</sup> showed a photocatalytic activity higher with respect to the grey CeO2, due to the higher exposed surface area and the lower band-gap. The easy synthetic procedures of CeO<sup>2</sup> reported here, allows to tune and modify the physico-chemical properties of CeO2, allowing a choice of different CeO<sup>2</sup> samples on the basis of the specific AOPs for water remediation. Furthermore, neither of the samples have shown any critical toxicity.
**Keywords:** ceria; pesticide; photocatalysis; photo-Fenton; AOPs
| doab | 2025-04-07T03:56:58.837813 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 113
} |
004db1df-dc76-42af-952f-1f5e82b7822b.114 | **1. Introduction**
Among the environmental questions of the present, water pollution by emergent contaminants, such as pharmaceuticals and pesticides, is a serious problem, making their removal a challenging task [1]. In particular, the use of pesticides has increased over the years to improve the production of agricultural goods and to satisfy the contextual growth of world population. Pesticides are a wide group of chemical compounds classified as persistent hazardous pollutants owing to a very high time of retention in water and giving rise to accumulation in sediment and in water effluents. They are also easily transferred over a long distance [2]. Their presence in the environment, especially in water, even at low concentrations, is a serious problem for both living organisms and human health.
Advanced Oxidation Processes (AOPs) are among the new, green and performing solutions for the removal of pesticides from water [3,4]. In these processes, the oxidation of the hazardous contaminants is obtained through the production of highly reactive radical species, such as •O<sup>2</sup> −, •O<sup>3</sup> <sup>−</sup>, or OH• . Different AOPs can be simultaneously utilized to avoid the generation of by-products
in treated water [5]. In this context, the photocatalysis and the Fenton process are two of the most promising AOPs [6]. The degradation of pesticides in water by means of photocatalysis allows to efficiently remove these pollutants with a moderate formation of secondary products and the selectivity of the process can be enhanced if peculiar materials (such as molecularly imprinted photocatalysts) are employed [7–10]. The hydroxyl radicals are formed in this process after the irradiation of a semiconductor photocatalyst with UV or a solar/visible light source with the consequent formation of photoelectrons in the conduction band and photoholes in the valence band of the photocatalyst [11]. The Fenton process involves the reaction between Fe2<sup>+</sup> and hydrogen peroxide to give the hydroxyl radicals (reaction 1):
$$\mathrm{Fe}^{2+} + \mathrm{H}\_{2}\mathrm{O}\_{2} \rightarrow \mathrm{Fe}^{3+} + \mathrm{OH}^{\bullet} + \mathrm{OH}^{-} \tag{1}$$
The further reaction of the ferric ions with the excess of H2O<sup>2</sup> re-generates the ferrous ions with the formation of the hydroperoxyl radicals (HOO• ) (reaction 2):
$$\text{Fe}^{3+} + \text{H}\_2\text{O}\_2 \rightarrow \text{Fe}^{2+} + \text{HOO}^\* + \text{H}^+ \tag{2}$$
The regeneration of ferrous ions can be accelerated, enhancing also the efficiency of the overall degradation process, in the presence of visible or near ultraviolet irradiation (i.e., the photo-Fenton process, reactions 3-5), with the consequent formation of further hydroxyl radicals [12,13]. Furthermore, some Fe(III)–carboxylate complexes originated from the coordination of Fe3+, and organic intermediates can adsorb in the UV–vis region, and other Fe2<sup>+</sup> species can be formed through the ligand-to-metal charge transfer (LMCT) (reaction 4). Finally, also the zero-valent iron species can be considered a source of Fe2<sup>+</sup> (reaction 5) [14].
$$\text{Fe}^{3+} + \text{hv} + \text{H}\_2\text{O} \rightarrow \text{Fe}^{2+} + \text{OH}^\bullet + \text{H}^+ \tag{3}$$
$$[\text{Fe}^{3+}\text{ (RCO}\_2\text{)}]^{2+} + \text{hv} \rightarrow \text{Fe}^{2+} + \text{CO}\_2 + \text{R}^{\bullet} \tag{4}$$
$$\text{Fe}^{0} + \text{hv} \rightarrow \text{Fe}^{2+} + 2\text{e}^{-} \text{ (}\lambda < 400 \text{ nm)}\tag{5}$$
The photo-Fenton process was successfully applied in the degradation of various pesticides and pharmaceuticals under solar light irradiation [15,16].
Among the various semiconductors used for the photocatalytic applications, recently, cerium oxide (CeO2, commonly called *ceria*), a largely used catalyst in many thermo-catalytic reactions [17,18], was examined as an alternative to the most used metal oxide photocatalysts (such as TiO<sup>2</sup> and ZnO [19–22]). The most attractive properties of CeO<sup>2</sup> are: the lower band-gap (around 2.7–2.8 eV) compared to TiO<sup>2</sup> and ZnO, making the material sensitive to visible light; the presence of empty 4f energy levels that facilitate the electron transfers; the high stability in the reaction medium; the high oxygen mobility related to the reversible Ce4+/Ce3<sup>+</sup> transformation, and the ability to form nonstoichiometric oxygen-deficient CeO2-x oxide [23]. The presence of defect centres in the CeO2, together with the high oxygen mobility and the consequential redox properties can be exploited in the Fenton-like reactions, that in this case, are different to the radical-attacking mechanism of conventional iron-based Fenton process, are driven by the interaction between the hydrogen peroxide and the surface defective Ce3<sup>+</sup> centres (reactions 6–8, [24,25]):
$$\rm{Ca^{3+}} + \rm{H\_2O\_2} \rightarrow \rm{Ce^{4+}} + \rm{OH^{\bullet}} + \rm{OH^{-}} \tag{6}$$
$$\mathrm{H\_2O\_2 + OH^\* \to H\_2O + HOO^\*}\tag{7}$$
$$\rm{Ce^{4+}} + \rm{HOO^{\bullet}} \rightarrow \rm{Ce^{3+}} + \rm{H^{+}} + \rm{O\_{2}} \tag{8}$$
The conventional iron species catalysts in the Fenton process require a strict operating pH range (between 3 and 4), thus increasing the overall process cost, whereas with CeO<sup>2</sup> it is possible to work at neutral pH [26].
On the basis of the above considerations, in this work we have studied the degradation of a largely used insecticide, i.e., the imidacloprid (C9H10ClN5O2), by the comparison between three different AOPs: solar photocatalysis, Fenton and solar photo-Fenton, taking advantage of the wide versatility of CeO<sup>2</sup> that can be used both as a photocatalyst and as a Fenton-like reagent. *Catalysts* **2020**, *10*, x FOR PEER REVIEW 3 of 16
The imidacloprid (hereafter called "IMI") is a neonicotinoid pesticide, which acts similarly to the natural insecticide nicotine [27]. Although IMI is not directly used in water, it is commonly transferred to water channels, and it presents a high leachability [28,29]. For its high toxicity, solubility, and stability, the presence of IMI in water even at low concentrations is a serious environmental concern. The imidacloprid (hereafter called "IMI") is a neonicotinoid pesticide, which acts similarly to the natural insecticide nicotine [27]. Although IMI is not directly used in water, it is commonly transferred to water channels, and it presents a high leachability [28,29]. For its high toxicity, solubility, and stability, the presence of IMI in water even at low concentrations is a serious
The Fenton-like process through ceria is activated by the presence of non-stoichiometric Ce3<sup>+</sup> centres on the surface of CeO<sup>2</sup> (reaction 6, [30,31]). One of the simpler methods to induce these defects is the exposure of CeO<sup>2</sup> to sunlight [32]. Indeed, the interaction of CeO<sup>2</sup> with the efficient UV solar photons (i.e., the photons with an energy higher than the band-gap of CeO2) led to a release of the labile ceria surface oxygens with the formation of CeO2-x defects. After the loss of oxygen, the cerium atoms adopted the most stable configuration, i.e., the Ce3<sup>+</sup> oxidation state. environmental concern. The Fenton-like process through ceria is activated by the presence of non-stoichiometric Ce3+ centres on the surface of CeO<sup>2</sup> (reaction 6, [30,31]). One of the simpler methods to induce these defects is the exposure of CeO2 to sunlight [32]. Indeed, the interaction of CeO<sup>2</sup> with the efficient UV solar photons (i.e., the photons with an energy higher than the band-gap of CeO2) led to a release of the labile ceria surface oxygens with the formation of CeO2-x defects. After the loss of oxygen, the cerium
In this context, we have synthetized two different types of CeO2: the first through one of the most employed preparation procedures for this oxide, as the precipitation with KOH from a cerium nitrate solution [23,33]; the second type using the same synthesis but irradiating the samples just after calcination with a solar lamp and in the presence of a H<sup>2</sup> flow, with the aim to further increase the surface defects on CeO2. Interestingly, with this original modified strategy we have obtained a "grey CeO2" instead of the typical yellow coloured ceria. The possibility to generate further defects on CeO<sup>2</sup> with solar exposure in a reducing atmosphere was especially exploited, due to a synergistic mechanism able to in situ provide the Ce3<sup>+</sup> species, mainly in the solar photo-Fenton tests. atoms adopted the most stable configuration, i.e., the Ce3+ oxidation state. In this context, we have synthetized two different types of CeO2: the first through one of the most employed preparation procedures for this oxide, as the precipitation with KOH from a cerium nitrate solution [23,33]; the second type using the same synthesis but irradiating the samples just after calcination with a solar lamp and in the presence of a H<sup>2</sup> flow, with the aim to further increase the surface defects on CeO2. Interestingly, with this original modified strategy we have obtained a "grey CeO2" instead of the typical yellow coloured ceria. The possibility to generate further defects on CeO<sup>2</sup> with solar exposure in a reducing atmosphere was especially exploited, due to a synergistic mechanism able to in situ provide the Ce3+ species, mainly in the solar photo-Fenton tests.
| doab | 2025-04-07T03:56:58.837982 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 114
} |
004db1df-dc76-42af-952f-1f5e82b7822b.115 | **2. Results and Discussion 2. Results and Discussion**
| doab | 2025-04-07T03:56:58.838655 | 6-5-2022 13:29 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "004db1df-dc76-42af-952f-1f5e82b7822b",
"url": "https://mdpi.com/books/pdfview/book/5163",
"author": "",
"title": "Heterogeneous Catalysis and Advanced Oxidation Processes (AOP) for Environmental Protection (VOCs Oxidation, Air and Water Purification)",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036535661",
"section_idx": 115
} |
Subsets and Splits
No saved queries yet
Save your SQL queries to embed, download, and access them later. Queries will appear here once saved.