Effect of annealing in oxygen on alloy structures of Pd–Au bimetallic model catalysts†
It has been reported that Pd–Au bimetallic catalysts display improved catalytic performance after adequate calcination. In this study, a model catalyst study was conducted to investigate the effects of annealing in oxygen on the surface structures of Pd–Au alloys by comparing the physicochemical properties of Pd/Au(111) surfaces that were annealed in ultrahigh vacuum (UHV) versus in an oxygen ambient. Auger electron spectroscopy (AES) and Basin hopping simulations reveal that the presence of oxygen can inhibit the diffusion of surface Pd atoms into the subsurface of the Au(111) sample. Reflection–absorption infrared spectroscopy using CO as a probe molecule (CO-RAIRS) and King–Wells measurements of O2 uptake suggest that surfaces annealed in an oxygen ambient possess more contiguous Pd sites than surfaces annealed under UHV conditions. The oxygen-annealed Pd/Au(111) surface exhibited a higher activity for CO oxidation in reactive molecular beam scattering (RMBS) experiments. This enhanced activity likely results from the higher oxygen uptake and relatively facile dissociation of oxygen admolecules due to stronger adsorbate–surface interactions as suggested by temperature-programmed desorption (TPD) measurements. These observations provide fundamental insights into the surface phenomena of Pd–Au alloys, which may prove beneficial in the design of future Pd–Au catalysts.