Hydrogen absorption and hydrogen-induced reverse segregation in palladium-silver surface

Hydrogen absorption from surface to second subsurface layer of ordered and substitutionally disordered Pd3Ag(110) was investigated by performing density functional theory-based calculations. Ag segregation to topmost layer was found to be energetically favored for clean surface structure. The absorption of H in the alloy surface, for both ordered and substitutionally disordered structures, is generally characterized by lower activation barrier as compared to pure Pd surface. When H is present in the surface or subsurface regions, our results strongly suggest the tendency of subsurface Pd atoms to segregate to the surface or near surface region. The reverse segregation of Pd, along with the large lattice constant and the modified electronic structure of Pd atoms in Pd3Ag alloy could help in understanding the experimentally determined high hydrogen permeability of PdAg membranes.