Selective Oxidation of the Intermetallic Compound PtAl₂ from Ab Initio Thermodynamics

The atomic structures of pristine and oxygen-covered PtAl2(100) and PtAl2(110) surfaces are studied by density functional theory modeling. Surface phase diagrams for oxygen adsorption on the two low-index PtAl2 surfaces are constructed. We show that the composition for the pristine surfaces is determined by the bulk composition, where the most stable surface configurations are the bulk terminations over the dominant range of the chemical potential of Al examined. By contrast, oxygen adsorption induces surface segregation of Al and leads to selective oxidation of Al upon increasing the oxygen chemical potential. An atomic and dynamic perspective of the oxygen adsorption-induced surface segregation is presented. The aluminum oxide formation results in Pt enrichment in the subsurface region that hinders inward oxygen embedment and outward Al diffusion, thereby serving as a protective layer against further oxidation-induced alloy degradation.