Modeling alloy catalyst surfaces and bimetallic catalyst particles

Alloy and bimetallic catalyst surfaces have been modelled using two different techniques. Surfaces of a planar, semi-infinite nature have been modelled using a classical thermodynamic approach while surfaces of small (\u3c100 A) bimetallic catalyst particles have been modelled using the Monte Carlo simulation technique;The multilayer surface segregation model which was developed to predict compositions in the surface region of a material is capable of handling more than two components, systems which exhibit non-ideal solution behavior, and systems comprised of elements (or molecules) of differing sizes;This model was applied to the Ag-Au, Cu-Ni, and Au-Ni binary systems as well as the Cu-Ni-Pt ternary system. Silver was found to segregate to the outermost atomic layer in the Ag-Au binary and surface layers were found to be alternately enriched or depleted (relative to the bulk region) in silver. In the Cu-Ni binary, copper was found to segregate to the first atomic layer and decrease in concentration in a monotonic fashion until the bulk region was reached. Both regular solution and Margules solution models were used in this system with the Margules model giving slightly better agreement with experimental data. Gold was found to segregate to the surface in the Au-Ni binary. Greater segregation of gold was predicted when the size difference between the elements was accounted for. In the Cu-Ni-Pt ternary system, copper was found to segregate to the surface causing a relative depletion of both nickel and platinum in the outermost atomic layer;The Monte Carlo simulation technique was used with the surface modified pair potential model in order to study bimetallic catalyst particles with dispersions on the order of 30 to 60%. Systems modelled were the Pt-Ib (Ib = Cu, Ag, Au), Ag-Ru, and Pt-Rh bimetallics. In general, it was found that the Ib metals underwent surface segregation in the group VIII-Ib systems and also tended to occupy the low-coordinated sites on the crystal surface first. With the Pt-Rh bimetallic particles, platinum was found to undergo net surface segregation, although in contrast to the behavior observed in the group VIII-Ib systems, the metal which was segregated was found to be relatively depleted at the low-coordinated corner and edge sites of the crystallite