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.
