  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.
