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September 2013

Understanding Surface Chemistry in Catalysts Atom-by-Atom

Movies lead to surprising conclusions about palladium on iron oxide catalyst

Kjell Schroder

Scanning tunneling microscopy allows researchers to watch the movement of individual palladium atoms (green/yellow) on an iron oxide support (blue) and determine the nucleation and growth mechanisms of palladium clusters from carbon-oxygen-palladium molecules (white).

Everyone is on the hunt for a bargain, and scientists and engineers are no different. Catalysts are the chemist's way of getting more product while paying less energy. Chemicals, referred to as reactants, are adsorbed from a gas or liquid onto an active catalyst that is coated on a support structure. The electronic properties and physical spacing of the catalyst's materials provide a lower energy pathway for converting adsorbed chemicals into another species. Catalysts play a huge role in commercial energy technologies, from cracking petroleum into gasoline, to reducing carbon monoxide and nitrous oxide pollution coming out your car's tail pipe via a catalytic converter.

Catalysis is poised to provide significant innovations for the new energy economy as well. Fuel cells, biofuels, and carbon capture are some of the areas of technology where improved and economical catalysts could be game changers. However, the reasons why the stability and structure of many catalysts change over time remain a mystery. One major problem with catalysts comes from the coarsening of the active material, which limits the ability of the catalyst to do its job. Coarsening is when the active material aggregates or clusters together, lowering the surface area where catalysis can take place and changing the arrangement of the active material on the support, which may put an end to the catalyst.

Researchers at the Center for Atomic-Level Catalyst Design (CALCD) investigated palladium on iron oxide because it catalyzes reactions including oxygen reduction, and methane oxidation, making it a good model system. The CALCD team, including Gareth S. Parkinson, studied the adsorption of common molecules on the system. Using scanning tunneling microscopy (STM), they watched the movement of individual palladium atoms. They found that carbon monoxide strongly accelerated aggregation, while surface hydroxyl groups, oxygen and hydrogen bound together, kept palladium atoms stationary. Computer simulations showed how a carbon monoxide molecule lifts the palladium atom out from the surface, allowing it to move around more freely.

From their STM movies (some frames of which are shown in the figure) the researchers reached two surprising conclusions. First, at least three carbon-oxygen-palladium molecules had to bump into each other for a cluster to form; collisions between two palladium atoms were not enough. Second, the clusters themselves were mobile and further aggregated into stable ~15-atom clusters. This behavior is different than what researchers usually suspect, where a cluster forms at a surface defect site and only grows when free atoms diffuse to it.

These studies shed light on the role of surface chemistry and the mechanisms by which coarsening takes place in the palladium iron oxide model system. Understanding these properties helps explain why catalysts become less effective over time and how they may be stabilized for longer life.

More Information

Parkinson GS, Z Novotny, G Argentero, M Schmid, J Pavelec, R Kosak, P Blaha, and U Diebold. 2013. "Carbon Monoxide-Induced Adatom Sintering in a Pd-Fe3O4 Model Catalyst." Nature Materials 12:724-728. DOI: 10.1038/nmat3667

Acknowledgments

This work was supported by the Center for Atomic-Level Catalyst Design, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. G.S.P. acknowledges support from the Austrian Science Fund project. R.K. and J.P. acknowledge stipends from the TU Vienna doctoral college CATMAT.

About the author(s):

  • Kjell Schroder is a Ph.D. candidate in Materials Science and Engineering at the University of Texas at Austin advised by Keith Stevenson and Lauren Webb. He is a member of EFRC:CST for Understanding Charge Separation and Transfer at Interfaces in Energy Materials and holds a B.A. in Physics from Lewis and Clark College in Portland, Oregon.

Why Catalysts Cluster and Fail

Atomic view shows three-atom collision needed

Researchers "filmed" the movement of individual palladium atoms (green/yellow) on an iron oxide support (blue) and determined why the catalyst clusters and fails.

If you've been to the auto mechanic and heard the phrase catalytic converter you know that a failed converter is expensive to replace. Scientists want to understand why the palladium catalyst in this system, and in others, fails. Using lab experiments and calculations, they examined a process called coarsening or clustering. They wanted to understand the phenomena at an atomic level. They found that three carbon-oxygen-palladium molecules need to bump together to form a cluster. After the clusters form, they are mobile and further combine. This behavior is different than the expectation that a cluster forms at a surface defect and grows when free atoms diffuse in. Understanding the atom's behavior helps explain why catalysts fade and how to prevent it. Scientists at the Center for Atomic-Level Catalyst Design, led by Louisiana State University, performed the research.

More Information

Parkinson GS, Z Novotny, G Argentero, M Schmid, J Pavelec, R Kosak, P Blaha, and U Diebold. 2013. "Carbon Monoxide-Induced Adatom Sintering in a Pd-Fe3O4 Model Catalyst." Nature Materials 12:724-728. DOI: 10.1038/nmat3667

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.