The activity of a heterogeneous catalyst is dependent on the structure and composition of its surface — both of which can change in response to variations in the environment. These differences in reactivity can be extreme in nanoparticle catalysts because of their size-to-volume ratio. Now a team led by Gabor Somorjai and Miquel Salmeron from the Lawrence Berkeley National Laboratory have used a model catalytic system to show1 just how dramatic the structural and chemical changes can be.

Using X-ray photoelectron spectroscopy (XPS), Rh0.5Pd0.5 nanoparticles were shown to consist of a predominantly rhodium shell and a central core region made up mostly of palladium atoms. The researchers obtained in situ structural and compositional information following exposure of the nanoparticles to oxidizing and catalytic gases. This was only possible by taking advantage of recent breakthroughs in ambient-pressure XPS — previous XPS studies have been restricted to high-vacuum conditions.

In an environment of nitric oxide, the rhodium shell is almost completely oxidized to RhOy. On the addition of carbon monoxide — to create catalytic conditions — the structure of the nanoparticles changes. Palladium atoms migrate to the surface and rhodium migrates in the opposite direction to the core, and is reduced to Rh0. This extreme change in both nanoparticle structure and composition is solely in response to changes in the gaseous environment. The changes are reversible, so removing carbon monoxide from the reactant feed causes the nanoparticles to restructure back to a palladium core and a RhOy shell.