MECHANISM OF POISONING THE CATALYTIC ACTIVITY OF PD(100) BY A SULFUR ADLAYER

The modification of the potential-energy surface (PES) of H-2 dissocia tion over Pd(100) as induced by the presence of a (2 x 2) S adlayer is investigated by density-functional theory and the linear augmented pl ane wave method. It is shown that the poisoning effect of S originates from the formation of energy barriers hindering the dissociation of H -2. The barriers are in the entrance channel of the PES and their magn itude strongly depends on the lateral distance of the H-2 molecule fro m the S adatoms.