Hydrogen, sulphur and chlorine coadsorption on Pd(111): a theoretical study of poisoning and promotion

First principles calculations for the coadsorption of hydrogen with sulphur and chlorine on Pd(111) are presented. Individually, both sulphur and chlorine poison hydrogen adsorption, sulphur being the more efficient poison. The observed sulphur poisoning is a structural effect. The adsorption energy decreases and the diffusion barrier increases for hydrogen atoms in the vicinity of sulphur adatoms. A sulphur coverage of 0.33 ML is sufficient to completely poison hydrogen adsorption on Pd(111). The presence of chlorine adatoms on the sulphur-poisoned surface is found to stabilise localised hydrogen adsorption. The possible promotional effects of chlorine on sulphur-poisoned catalysts are discussed.