Bond breaking at surfaces: Electrons or phonons?

The surface segregation in Pd based alloys has been investigated by density-functional band-structure calculations. Twelve different metals were substituted in Pd(1 1 1) slabs at a level of 5%: Ag, Au, Cd, Cu, Fe, Mn, Ni, Pb, Pt, Rh, Ru, and Sn. The segregation energy (the difference in calculated total free energy between surface sites and bulk-like sites) was calculated for each alloy, and the results were in very good agreement with experimental data where available. Particularly, we predict an oscillatory depth profile for Cu and Ni, similar to what has been found experimentally for the PdNi(1 0 0) surface. There are more or less pronounced correlations between the segregation energy and the relaxed position at the surface of the substituted atom, the metal radius of the substituted atom, and the experimental surface energy of the metal. It is proposed that the segregation energy is an indirect measure of the stability of Pd based hydrogen permeable membranes.