Surface segregation phenomena in Pt-Pd nanoparticles: dependence on nanocluster size

Classical molecular dynamics (MD) simulations are used to investigate the effect of the nanocluster size on surface segregation phenomena of Pt alloys containing 10, 30, 50, 70 and 90% Pd. Atomic distribution is examined in graphite-supported nanoclusters with approximate diameters of 2 and 4 nm, using a simulated annealing procedure with temperatures varying from 1200 down to 353 K. Following this annealing route, it is found that at concentrations of Pd below a certain threshold, Pt segregates to the surface, whereas Pd segregates to the surface when the overall concentration of Pd is above that threshold. Moreover, the threshold concentration depends on the size, being approximately 50% for the 2 nm nanocluster and in the order of 60% for the 4 ran nanocluster. It is also found that the percent of the surface enriched either in Pt or Pd at a given overall concentration, as well as the nature of the exposed crystallographic faces, depend significantly on the cluster size. Our studies suggest that surface segregation behavior in Pt-Pd supported nanoclusters is influenced by: differences in surface energies, interaction of the clusters with the substrate, and probably most importantly by the fabrication protocol. The implications of these issues on catalytic processes are discussed.