Surface segregation in bimetallic clusters: Predictions using a molecular dynamics/Monte Carlo corrected effective medium theory

Abstract Computer simulations were performed to study the surface composition of Rh x Pd 1 − x , Rh x Ni 1 − x and Ni x Pd 1 − x clusters where 0.1 ⩽ x ⩽ 0.9. Clusters containing 201 and 1289 atoms were simulated at 200, 600 and 1000 K. The simulation consists of the Monte Carlo (MC) exchange of the positions of unlike atoms and the molecular-dynamics (MD) evolution of all atoms between exchanges. A corrected effective medium (CEM) theory based interatomic potential, with acronym MD/MC-CEM, was used to generate the forces. Our simulations indicate that the metal with lower surface energy segregates first to the catalytically very important edge-corner sites, then to other surface sites. Increasing the temperature decreases the degree of segregation at edge-corner sites more strongly than on the entire surface. Interestingly, NiPd exhibits stronger surface segregation but weaker edge-corner segregation than RhNi. Results for other 201- and 1289-atom 50%-50% bimetallic clusters at 600 K, selected from Rh, Ni, Pd, Cu and Ag, are also presented. In particular, the large lattice-size mismatch in NiAg and CuAg invalidates the simple geometrical constraints on surface segregation that hold for other bimetallic clusters.