Surface Alloy Composition Controlled O2 Activation on Pd-Au Bimetallic Model Catalysts

Oxygen is an important reactant in several catalytic conversions and partial oxidation reactions on Pd-Au alloy surfaces; however, adsorption and dissociation are not fully understood, especially as a function of the surface alloy composition. In this study, we probe the influence of the atomic makeup of the surface of Pd-Au catalysts regarding control of the catalytic activity toward O-2 dissociation and the reactivity of the resulting oxygen adatoms. To experimentally investigate this, we prepared various bimetallic surfaces under ultrahigh vacuum via evaporation of Pd onto a Au(111) surface. Hydrogen molecules were used to characterize the composition of the Pd-Au surfaces, which we simplistically group into two categories: (i) Pd-Au interface sites and (ii) Pd(111)-like island sites. When the Pd coverage is 1.0 ML, which predominantly indicates Pd-Au interface sites, no dissociative adsorption of O-2 at 300 K is observed, but dissociation begins to be measurable on the surfaces with larger Pd loadings (greater than 1.5 ML), which we believe leads to Pd(111)-like islands on the surface. We also find that adsorbed oxygen atoms are very reactive at the Pd-Au interface sites via measurements of the CO oxidation reaction at relatively low temperatures (<200 K); however, CO oxidation can also take place at higher temperatures (similar to 400 K) and in this case is very dependent on Pd coverage, being strongly related to the number of Pd(111)-like islands, which bind O-a relatively strongly. From our experimental results, we estimate the barrier to dissociation of O-2 and also the CO oxidation reaction barrier, which is an indirect measure of the reactivity of the adsorbed atomic oxygen. From our analysis, we find that, upon increasing Pd coverage, the dissociation barrier for O-2 steadily decreases and, further, the reaction barrier for CO oxidation continuously increases. Finally, oxygen molecularly adsorbs on the Pd-Au bimetallic surface and is a precursor to dissociative O-2 chemisorption, just as with pure Pd surfaces, and additionally, the enhanced reactivity of adsorbed atomic oxygen originates at the interfaces between Pd and Au domains.