H₂S/Cu(111): A model study of sulfur poisoning of water-gas shift catalysts

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 (sqrt(7) x sqrt(7)) structure (θ_s=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 (1-2.6θ_s). 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.