Orientation-Dependent Hindrance to the Oxidation of Pd-Au Alloy Surfaces

Oxidation of monometallic Pd and bimetallic Pd₃Au alloy surfaces are observed by in situ ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) at an elevated pressure (100 mTorr O₂ ambient). It is directly evidenced that the alloying with Au hinders the surface oxidation of Pd₃Au surfaces compared with monometallic Pd surfaces. Remarkably, the oxidation behavior is clearly different between Pd₃Au(111) and (100) surfaces. The (100) surface has a relatively Pd-rich surface composition, and the surface oxide layer is formed, whereas the (111) surface has a Au-rich composition, and the surface oxidation is quite limited. A combined approach of experimental and theoretical techniques reveals that Pd/Au surface composition and atomic arrangement are key factors determining the oxidation behavior.