Chemisorption properties of thin Pd overlayers on Nb(110) and Ag(111)

Abstract

A simple model of hydrogen adsorption on a Pd covered Nb(110) substrate is presented. In agreement with recent experiments, it is found that for the commensurate bcc (110) phase, dissociative H 2 adsorption is not favoured in contrast to the incommensurate fcc (111) phase. The major reason for the dramatic reduction in surface reactivity for the bcc (110) phase, is due to the increase in Pd-Pd distances which results in a narrowing of the d band and also a lowering of the antibonding resonance made up of the H 1s orbital and the substrate d states, to an energy position where it is now occupied. A similar behaviour is expected for H adsorbed on a Pd/Ag(111) composite substrate, where at low Pd coverages (less than one monolayer) the expanded Pd layer is expected to act against the H 2 dissociation. Calculations of the one-electron energies due to the metal d states for H on Pd and Nb surfaces, indicate a larger chemisorption energy for Nb than Pd. The H-surface bond is thus more easily broken for a Pd covered Nb surface, than for a clean Nb surface, thereby resulting in a faster H penetration into the bulk of the composite substrate.