A multicomponent, multilayer model of surface segregation in alloy catalysts

Abstract

A model for the quantitative prediction of surface segregation in alloy systems is presented. A chemical thermodynamic approach is used in the development of the model, and a multilayer surface region is assumed. Features of this model include multicomponent capability (the components may be different sizes) and the ability to use virtually any mixing model. Surface free energy data are incorporated and no adjustable parameters are needed. The modeling method is discussed in detail with examples given for the AgAu, CuNi, and AuNi binary systems and the CuNiPt ternary system. Both regular and Margules solution models were used, but the Margules model agreed more closely with experimental data for the CuNi system. The segregating element in the AuNi binary was found to be gold. Greater segregation of gold and better agreement with experimental data resulted when the size differences between the elements were accounted for. In the CuNiPt system, copper was predicted to segregate to the surface, causing a relative depletion of both nickel and platinum in the first atomic layer. The second layer was enriched in platinum and depleted in both nickel and copper.