23 October 2008

A nano view could aid fuel cell capabilities

It is truly amazing to see a reaction take place, especially at the nano level. By viewing nanoscale catalysts in action, it could now lead to improved pollution control and fuel cell technologies.

These catalysts restructured themselves in response to various gases swirling around them, like a chameleon changing its color to match its surroundings, said scientists from the U.S. Department of Energy's Lawrence Berkeley National Laboratory.

Using a spectroscopy system at Berkeley Lab's Advanced Light Source, researchers watched as nanoparticles composed of two catalytic metals changed their composition in the presence of different reactants. Until now, scientists relied on snapshots of catalysts taken before and after a reaction, never during.

This new view could give scientists the ability to develop cheaper and smarter catalysts fine-tuned to drive the chemistry of everyday life, such as reactions that sweep toxins from pollutants, feed hydrogen fuel cells, and drive fuel refinement techniques.

Fuel cell
Rhodium-palladium nanoparticles (left) and platinum-palladium nanoparticles (right), as revealed by transmission electron microscopy, morph as their surroundings change. Tapping into this change could help streamline and guide catalytic reactions.

It could also expedite the development of catalysts that mop up all the substances in a reaction except the desired product, the hallmark of "green chemistry."

"Now we can dream," said Gabor Somorjai, a surface science and catalysis expert who holds joint appointments with Berkeley Lab's Materials Sciences Division and UC Berkeley's department of chemistry. He conducted the research with Miquel Salmeron, a spectroscopy leader that holds joint appointments with Berkeley Lab's Materials Sciences Division and UC Berkeley's department of materials sciences and engineering.

"By watching catalysts change in real time, we can possibly design smart catalysts that optimally change as a reaction evolves," Somorjai said.

Catalysts, which are substances that speed up chemical reactions, are as ubiquitous as they are valuable. They are essential to the production of industrially important chemicals. They also play a large role in environmental chemistry, most famously exemplified by catalytic converters, which reduce toxic emissions from vehicle tailpipes.

Because of their importance, researchers are working to better understand how catalysts work and how to improve them. Until now, however, researchers could only view nanoscale catalysts before and after a reaction. The crucial segment, how a catalyst morphs during a reaction, remained a guessing game.

And that is a huge obstacle. As Somorjai said, it is like trying to understand someone's life by observing the person as a newborn baby, then fast-forwarding to old age. What transpires in between is incredibly important but also incredibly difficult to decipher by observing two widely disparate stages.

"It's difficult to tune a catalyst to do exactly what you want unless you know how it adapts during a reaction," Salmeron said. "With our work, we can for the first time see what the catalyst is doing during the reaction, not before and after."

Using techniques developed in his lab, Somorjai synthesized nanoscale particles composed of common catalytic metals. Some particles consisted of rhodium and palladium, while others had platinum and palladium.

Next, to see how these bimetallic catalysts change in the presence of reactants, they turned to one of the few spectroscopy instruments in the world that enables scientists to study catalytic and biological phenomena in their natural environment, that is, at almost normal pressures and in the presence of different chemicals. Salmeron developed the instrument.

Like all spectroscopy systems, the instrument identifies elements by detecting their unique spectral signals. But unlike most, the ambient pressure photoelectron spectroscopy system works under similar pressures and environments faced by everyday phenomena, instead of requiring a carefully controlled vacuum.

Using this system, the scientists watched, in real time, as the bimetallic nanoparticles restructured themselves when exposed to different gases, such as nitrogen oxide, carbon monoxide, and hydrogen. In the presence of some reactants, rhodium rose to a particle's surface. While in the presence of other reactants, palladium rose to the surface.

"From one gas to another, we observed different metals segregating to a catalyst's surface, which is the part of the catalyst that drives chemical reactions," Somorjai said. "And this makes all the difference in establishing how the catalyst participates in the chemistry."

With this information, scientists can develop nanoparticle catalysts and reactants tailored to most efficiently yield a product, whether it is gasoline or cleaner emissions. For example, researchers can engineer bimetallic nanoparticle catalysts in which one metal rises to the surface during an initial stage of a reaction and a different metal rises to the surface in a latter stage. The goal is to ensure the most active metal is on the catalyst's surface precisely when it is needed most. In this way, researchers can develop the final product as quickly and cheaply as possible.

For more information, go to www.isa.org/manufacturing_automation.