CO Adsorption-Driven Surface Segregation of Pd on Au/Pd Bimetallic Surfaces: Role of Defects and Effect on CO Oxidation

We use density functional theory (DFT) to study CO-adsorption-induced Pd surface segregation in Au/Pd bimetallic surfaces, dynamics of Pd-Au swapping, effect of defects on the swapping rate, CO-induced Pd clustering, and the reaction mechanism of CO oxidation. The strong CO-philic nature of Pd atoms supplies a driving force for the preferential surface segregation of Pd atoms and Pd cluster formation. Surface vacancies are found to dramatically accelerate the rate of Pd-Au swapping. We find that Pd clusters consisting of at least four Pd atoms prefer to bind O-2 rather than CO. These clusters facilitate the rapid dissociation of O-2 and supply reactive oxygen species for CO oxidation. Our findings suggest that geometric, electronic, and dynamic effects should be considered in the function of bimetallic alloys or nanoparticles whose components asymmetrically interact with reacting molecules.