Surface composition of AgPd single-atom alloy catalyst in an oxidative environment

Single-atom alloys (SAAs) have recently gained considerable attention in the field of heterogeneous catalysis research due to their potential for novel catalytic properties. While SAAs are often examined in reactions of reductive atmospheres, such as hydrogenation reactions, in the present work, we change the focus to AgPd SAAs in oxidative environments since Pd has the highest catalytic activity of all metals for oxidative reactions. Here, we examine how the chemical reactivity of AgPd SAAs differs from its constituent Pd in an oxidative atmosphere. For this purpose, electronic structure changes in an Ag_0.98Pd_0.02 SAA foil in 1 mbar of O₂ were studied by in situ x-ray photoemission spectroscopy and compared with the electronic structure of a Pd foil under the same conditions. When heated in an oxidative atmosphere, Pd in Ag_0.98Pd_0.02 partly oxidizes and forms a metastable PdO_x surface oxide. By using a peak area modeling procedure, we conclude that PdO_x on Ag_0.98Pd_0.02 is present as thin, possibly monolayer thick, PdO_x islands on the surface. In comparison to the PdO formed on the Pd foil, the PdO_x formed on AgPd is substantially less thermodynamically stable, decomposing at temperatures about 270 °C lower than the native oxide on Pd. Such behavior is an interesting property of oxides formed on dilute alloys, which could be potentially utilized in catalytic oxidative reactions such as methane oxidation.