Alloy surface segregation in reactive environments: First-principles atomistic thermodynamics study ofAg3Pd(111)in oxygen atmospheres

We present a first-principles atomistic thermodynamics framework to describe the structure, composition, and segregation profile of an alloy surface in contact with a (reactive) environment. The method is illustrated with the application to a ${\mathrm{Ag}}_{3}\mathrm{Pd}(111)$ surface in an oxygen atmosphere, and we analyze trends in segregation, adsorption, and surface free energies. We observe a wide range of oxygen adsorption energies on the various alloy surface configurations, including binding that is stronger than on a $\mathrm{Pd}(111)$ surface and weaker than that on a $\mathrm{Ag}(111)$ surface. This and the consideration of even small amounts of nonstoichiometries in the ordered bulk alloy are found to be crucial to accurately model the Pd surface segregation occurring in increasingly O-rich gas phases.