Ordering and oxygen adsorption in Au-Pt/Pt(111) surface alloys

Bimetallic surface alloys are considered a promising type of catalyst for improved activity and selectivity. Using first-principles density functional calculations and the cluster expansion (CE) method, we have studied the surface structure and ordering of a mixed Au-Pt surface layer on Pt(111). Even though the Au-Pt system is well-known to phase-separate in the bulk, we predict a series of T = 0 K energetically stable, laterally ordered striped structures in the Au-Pt(111) surface. The formation of such ordered structures is the result of a competition between the strain relaxations (favoring short-period stripe ordering) and the unfavorable Au-Pt bonds at stripe interfaces (favoring long-period phase separation). We have also investigated the oxygen adsorption on these ordered structures and a disordered Au-Pt(111) surface alloy. The disordered surface is modeled by a two-dimensional special quasirandom structure (SQS) that mimics the pair and triplet correlation functions of a disordered binary distribution. The oxygen binding energy is highly correlated with the number and type of the nearest neighbor metal surface atoms to the adsorbed oxygen.