 It is conventionally accepted that the energetic threshold separating the binding energy of "physisorption" from that of "chemisorption" is about 0.5 eV per adsorbed species.Due to specificity, the nature of chemisorption can greatly differ, depending on the chemical identity and the surface structural properties.The bond between the adsorbate and adsorbent in chemisorption is either ionic or covalent.