Sulphur poisoning of water-gas shift catalysts: site blocking and electronic structure modification

Abstract

Density-functional theory calculations were used to study sulphur poisoning of low Miller index surfaces of Cu(111), (100) and (110). The adsorption properties of S on the studied facets were investigated in different adsorption sites and at coverages between 0.25 and 1 ML. The comparison of adsorption energies among different facets showed that S adsorbs most strongly to the most open surface (110) and most weakly to the most close-packed surface (111). We found that the adsorption energy of S generally gets weaker with increasing coverage, and we related the coverage dependence to an adsorbate-induced surface electronic structure modification which broadens the surface d-band and which causes a consequent reduction in the average energy of the d-band centre due to conservation of d-band filling. The combination of site blocking and surface electronic structure modification leads to a reduction in the surface reactivity of the catalyst at higher coverages of S.