Temperature-Programmed Reduction of Oxidic and Sulfidic Alumina-Supported NiO, WO3, and NiO-WO3 Catalysts

Abstract

The reduction behaviour of oxidic and sulfidic alumina-supported Ni, W, and NiW catalysts has been studied using temperature-programmed reduction (TPR). Besides giving structural information on the oxidic and sulfidic state, the TPR patterns gave detailed information on the sulfidability of the various oxidic species. From the oxidic species only dispersed NiO was completely sulfidable at 673 K, whereas the other species were partly sulfidable (mixed Ni-W phase, dispersed WO3, and Ni in the surface alumina layer) or not sulfidable (Ni in subsurface alumina layers and NiWO4). The sulfided catalysts contained at least four types of sulfur, viz. stoichiometric sulfur behaving as S in a bulk analogue (WS2, Ni3S2, NiS1+x) or in NiWS (NiWS is not directly observed under TPR conditions due to sintering into Ni3S2 and WS2), a reactive type of sulfur indicated as Sx, S-H groups, and adsorbed H2S. Except NiS1+x and Sx, no sulfided phase reduces under HDS conditions. Although hydrogenation of sulfur might well be an elementary step in the desulfurisation reaction, only a weak correlation was observed between the HDS activity and the reduction rate of the Sx species.