H 2S/Cu(111): A model study of sulfur poisoning of water-gas shift catalysts

Abstract

The interaction of H2S with Cu(111) has been studied with XPS, AES, ISS and LEED; and the effects of sulfur adatoms upon the kinetics of the medium-pressure water-gas shift reaction over Cu(111) have been measured. At 120 K, H2S adsorption is partially molecular. By 200 K, some of the H2S desorbs and the rest dissociates to produce sulfur adatoms bonded on top of the Cu(111) surface. The surface saturates at room temperature in a (7x7) structure (θs=0.43) characteristic of a nearly close-packed sulfur overlayer, very similar to layers in (111)-oriented Cu2S. Sulfur adatoms are quite stable to heating in vacuum or H2, with a reaction probability for removal per H2 collision with the surface of ≤10−9 at 636 K. The rate of the forward water-gas shift reaction (H2O+CO→H2+CO2) decreases linearly with sulfur coverage as (1–2.6θs). This poisoning is attributed to steric blocking by the sulfur adatoms of the sites required for dissociative water adsorption. The site ensemble required for this step is small (1–2 Cu atoms). The well-known sensitivity of shift catalysts to sulfur poisoning is discussed in the light of these results.