Structure and reactivity of a model catalyst alloy under realistic conditions

Using a combined experimental and theoretical approach, we show that a thin RhO₂ oxide film forms on a Pt₂₅Rh₇₅(100) surface at elevated oxygen pressures and temperatures prior to the bulk oxidation. By the use of in situ surface x-ray diffraction under realistic CO oxidation reaction conditions, we show that the onset of the growth of thin RhO₂ oxide film coincides with an increase in CO₂ production. During the reaction, the consumed oxide film is continuously re-grown by oxygen in the gas phase. Our theoretical results strongly suggest that the CO adsorbs on the metallic substrate but reacts with the O in the RhO₂ oxide film at the border between the RhO₂ oxide film and the metallic substrate. This scenario could explain the experimental observations of oxidation reactions on other late transition metal surfaces as well as on their corresponding nanoparticles under realistic conditions.