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 diss ociative adsorption of hydrogen. Using density functional theory we st udied the adsorption of hydrogen on clean and sulphur precovered Pd(10 0) surfaces. The results show that the poisoning effect of sulphur is not caused by a strict blocking of hydrogen adsorption sites in the vi cinity of S adatoms. For a (2x2) sulphur adlayer (coverage theta(s)=1/ 4) hydrogen adsorption remains an exothermic process for all surface h ollow sites not occupied by sulphur. The blocking of hydrogen adsorpti on happens only for higher sulphur coverages. We conclude that the poi soning of Pd(100) is a combined effect of the formation of energy barr iers hampering the H-2 dissociation and a modest decrease of the adsor ption energy in the vicinity of the sulphur adatoms.