SIMULATION STUDIES OF ADSORPTION-INDUCED SURFACE SEGREGATION IN BIMETALLIC Pd–Cu NANOSTRUCTURES

We use the Monte-Carlo (MC) simulation technique to study the role of various coverages of (CO) adsorption on the spatial distribution of atoms on the surface of fcc cubo-octahedral Pd–Cu nanoparticles. The coordination-dependent pair interaction energy, required for the MC simulation, has been computed by an empirical formula obtained from the experimental values of dimer energy, (100) surface energy and the energy for monovacancy creation for constituent metals. It has been found that for clean particles of all sizes the surface is enriched in Cu. However, with increase in (CO) adsorption the extent of Cu segregation gradually reduces and at higher (CO) coverage there is segregation reversal leading to Pd segregation. At low coverage, usually the six- and seven-coordinated sites are occupied by Cu atoms. But at higher coverage (θ>0.25) they are also gradually occupied by Pd atoms along with the fcc(100)- and fcc(111)-like face sites. For a full monolayer coverage surface sites of all coordinates are occupied by Pd atoms. The significance of these results in the context of catalytic activity of alloys is discussed.