Preferential segregation to the step edges on Pt–Re catalyst particles

Monte Carlo simulations combined with the “macroscopic atom” model are used to investigate surface segregation in platinum–rhenium reforming catalysts. Two irregularly shaped particles of different size are implemented as an approximation to the active grains in commercial catalysts. Disordered configurations with bulk concentrations between 20 and 80 at.% Pt are studied. For all configurations segregation of Pt, the catalytically active element, is observed, up to 100 at.% Pt for the Pt-richer particles. The extent of segregation is in full agreement with the considerably lower surface tension of Pt and the negative enthalpy of solution in the Pt–Re system. It is furthermore less pronounced at higher temperatures, as it should be for exothermic segregation in disordered alloys. For the lowest concentrations the bulk is depleted upon segregation and this of course is more pronounced for the smaller particle. The interesting phenomenon that is observed is a strongly preferential segregation to the lattice sites with lowest coordination at the step edges. By systematically increasing the Pt-content of the particle the surface sites are sequentially filled in order of increasing coordination, in full agreement with thermodynamical considerations. This observation possibly points at steps and kinks as the active catalyst sites and is also in agreement with experimental evidence on Pt-surfaces [Surf. Sci. 128 (1983) 176] that showed a systematical increase in activity for rougher surfaces.