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

摘要

This work presents atomic-level simulations that study the surface segregation profiles in Cu-Pd catalysts at various concentrations of Cu. It is found that Cu normally enriches on the surface of 201-atom Cu Pd clusters but depletes 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 modifications, to fit the alloy heats of mixing, are included in the interaction potential. This study also reveals different surface segregation behaviours for clusters having different shapes. It appears that most of the thermally accessible shapes of the cluster exhibit reverse surface segregation. However, these shapes are metastable. The lowest-energy configuration tested, having the cube-octahedral shape, does not show reverse surface segregation; instead, it shows normal surface segregation with Cu enrichment on the surface at all concentrations. Thus, this work suggests that the experimentally observed clusters that demonstrate reverse surface segregation are metastable. [References: 36]