Reverse surface segregation in Cu-Pd bimetallic catalysts at low concentrations of Cu

This work presents atomic-level simulations that study the surface segregat ion profiles in Cu-Pd catalysts at various concentrations of Cu. It is foun d that Cu normally enriches on the surface of 201-atom Cu Pd clusters but d epletes from the surface at very low Cu concentrations. This phenomenon was observed in experiments previously and call be reproduced by the molecular dynamics/Monte Carlo-collectcd effective medium theory if the latest modif ications, to fit the alloy heats of mixing, are included in the interaction potential. This study also reveals different surface segregation behaviour s for clusters having different shapes. It appears that most of the thermal ly accessible shapes of the cluster exhibit reverse surface segregation. Ho wever, these shapes are metastable. The lowest-energy configuration tested, having the cube-octahedral shape, does not show reverse surface segregatio n; instead, it shows normal surface segregation with Cu enrichment on the s urface at all concentrations. Thus, this work suggests that the experimenta lly observed clusters that demonstrate reverse surface segregation are meta stable.