Ab initio calculation of the potential energy surface for the dissociation of H_2 on the sulfur covered Pd(100) surface

Abstract

The presence of sulfur atoms on the Pd(100) surface is known to hinder the dissociative adsorption of hydrogen. Using density-functional theory and the full-potential linear augmented plane-wave method, we investigate the potential energy surface (PES) of the dissociative adsorption of H_2 on the sulfur covered Pd(100) surface. The PES is changed significantly compared to the dissociation on the clean Pd(100) surface, in particular for hydrogen close to the S atoms. While the hydrogen dissociation at the clean Pd(100) surface is non-activated, for the (2x2) sulfur adlayer (coverage Theta_S= 0.25) the dissociation of H_2 is inhibited by energy barriers. Their heights strongly depend on the distance between the hydrogen and sulfur atoms leading to a highly corrugated PES. The largest barriers are in the vicinity of the sulfur atoms due to the strong repulsion between sulfur and hydrogen. Still the hydrogen dissociation on the (2x2) sulfur covered Pd(100) surface is exothermic. Thus the poisoning effect of sulfur adatoms for H_2 dissociation at low sulfur coverage (Theta_S <= 0.25) is mainly governed by the formation of energy barriers, not by blocking of the adsorption sites. For the c(2x2) sulfur adlayer (Theta_S= 0.5), the PES for hydrogen dissociation is purely repulsive. This is due to the fact that for all different possible adsorption geometries the hydrogen molecules come too close to the sulfur adatoms before the dissociation is completed.