AB-INITIO CALCULATIONS OF HYDROGEN ADSORPTION ON (100) SURFACES OF PALLADIUM AND RHODIUM

We report an all-electron density-functional-theory study of the inter action of hydrogen with Pd(100) and Rh(100) surfaces. We use the local -density approximation for the exchange-correlation energy functional and the full-potential linear-muffin-tin-orbital method. Various cover ages, between THETA = 0.25 and THETA = 2 are considered. In particular , we discuss the adsorption energies, stable adsorption sites, adsorpt ion-induced surface relaxations, and the work-function changes. The re sults show that for coverages THETA less-than-or-equal-to 1 at Pd(100) the fourfold hollow site is energetically favorable. For higher cover ages it is predicted that the additional hydrogen goes subsurface. The work function is found to increase with coverage up to THETA = 1 and for higher coverages (i.e., when the subsurface sites get occupied) it remains roughly at the THETA = 1 level. Hydrogen adsorption at Rh(100 ) is found to be very similar to Pd(100) up to THETA = 1. The results are compared to available experimental data as well as to other calcul ations.