Driving force for surface segregation in bimetallic catalysts

Abstract

The catalytic properties of bimetallic clusters are influenced by the detailed structure of the cluster surface. We briefly describe two methods developed to treat this problem, from the computationally demanding corrected effective medium (CEM) theory (a non-self-consistent density functional based method) to the computationally simple method of surface modified pair potentials (SMPP). Parametrization of the latter is accomplished using the former. Comparisons of theoretical predictions with experimental data are made for the heat of formation in Rh x Pt 1− x alloys and for surface segregation behavior in Rh 0.9Pt 0.1(111). Results on the shape, site composition and surface micromixing are shown for 201 atom clusters (dispersion of 0.6) and Rh x Pt 1− x (111). A fundamental point follows from the fact that the CEM calculated cohesive energy of Rh is slightly smaller than that of Pt, but the surface energy of Rh is significantly larger: the driving force for surface segregation is the relative surface energies not the relative cohesive energies. In general, one can predict surface energy differences by properly accounting for the variation of the bond energy with coordination.