AB-INITIO CALCULATION OF THE POTENTIAL-ENERGY SURFACE FOR THE DISSOCIATION OF H-2 ON THE SULFUR-COVERED PD(100) SURFACE

The presence of sulfur atoms on the Pd(100) surface is known to hinder the dissociative adsorption of hydrogen. Using density-functional the ory and the full-potential linear augmented plane-wave method, we inve stigate the potential energy surface (PES) of the dissociative adsorpt ion of H-2 On the sulfur covered Pd(100) surface. The PES is changed s ignificantly compared to the dissociation on the clean Pd(100) surface , particularly for hydrogen close to the S atoms. While the hydrogen d issociation at the clean Pd(100) surface is nonactivated, for the (2 x 2) 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 co rrugated PES. The largest barriers are in the vicinity of the sulfur a toms due to the strong repulsion between sulfur and hydrogen. Still th e hydrogen dissociation on the (2x2) sulfur covered Pd(100) surface is exothermic. Thus the poisoning effect of sulfur adatoms for H-2 disso ciation at low sulfur coverage (Theta(S) less than or equal to 0.25) i s mainly governed by the formation of energy barriers, not by blocking of the adsorption sites. For the c(2 x 2) sulfur adlayer (Theta(S)= 0 .5), the PES for hydrogen dissociation is purely repulsive. This is du e to the fact that for all different possible adsorption geometries th e hydrogen molecules come too close.to the sulfur adatoms before the d issociation is completed.