Hydrogen on PdAu surfaces: Diffusion and segregation

Abstract

The possibility for hydrogen to dissociate on palladium surfaces and to diffuse subsurface makes palladium interesting for hydrogen storage and membrane applications. The properties of palladium may be modified and tuned by metal alloying. It is clear that the mixing of the metals in the surface region depend on the gas phase and the operating conditions. In this thesis, Density Functional Theory (DFT) calculations have been used to investigate hydrogen adsorption on Pd3Au surfaces and by studying the fcc(111) surface composition as a function of coverage how hydrogen may change the alloy composition in the surface layer. In addition, the effect of alloying on hydrogen subsurface diffusion have been investigated. The surface layer is composed entirely of Au in the absence of hydrogen. Increasing the hydrogen coverage favors Pd segregation to the first and the second surface layer. The segregation is induced by the strong hydrogen-palladium bond. Alloying is found to have marked effects on the diffusion of hydrogen into the bulk of the metal. The most important changes with respect to the bare Pd metal is a reduction of the surface to subsurface barrier and a slightly higher barrier for diffusion in the interior of the alloy.