POISONING OF PD(100) FOR THE DISSOCIATION OF H-2 - A THEORETICAL-STUDY OF COADSORPTION OF HYDROGEN AND SULFUR

The presence of sulphur adatoms on Pd(100) is known to hinder the dissociative adsorption of hydrogen. Using density functional theory we studied the adsorption of hydrogen on clean and sulphur precovered Pd(100) surfaces. The results show that the poisoning effect of sulphur is not caused by a strict blocking of hydrogen adsorption sites in the vicinity of S adatoms. For a (2x2) sulphur adlayer (coverage theta(s)=1/4) hydrogen adsorption remains an exothermic process for all surface hollow sites not occupied by sulphur. The blocking of hydrogen adsorption happens only for higher sulphur coverages. We conclude that the poisoning of Pd(100) is a combined effect of the formation of energy barriers hampering the H-2 dissociation and a modest decrease of the adsorption energy in the vicinity of the sulphur adatoms.