Motivated by a recent detailed experimental study [Y. Ma et al., Phys. Chem. Chem. Phys., 2011, 13, 10741], the structure and local reactivity of PdAg/Pd(111) surface alloys were studied using periodic density functional theory calculations. As a probe of the local reactivity, CO adsorption energies were evaluated as a function of concentration and configuration of silver and palladium atoms and the CO coverage and related to the underlying electronic structure. According to the calculations, the formation of PdAg/Pd(111) surface alloys is found to be energetically stable. We find in accordance with the experiment that the adsorption on the surface alloy is dominated by ensemble effects, whereas electronic ligand and strain effects effectively cancel each other out. Furthermore, we elucidate the mechanism of CO adsorption on small Pd ensembles upon higher exposure.
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