TEMPERATURE-PROGRAMMED SULFIDING OF VANADIUM-OXIDES AND ALUMINA-SUPPORTED VANADIUM-OXIDE CATALYSTS

Sulfiding of bulk and alumina-supported vanadium oxides has been studi ed using temperature-programmed sulfiding and reduction techniques. Bu lk compounds (V2O5, V2O3) and V/Al2O3 catalysts are sulfided via a sim ilar mechanism. For bulk V2O5, two major sulfiding steps have been ide ntified. At temperatures up to 673 K, V2O5 is reduced to V2O3 by O-S e xchange and subsequent rupture of V-S bonds where H2S acts as reducing agent. Sulfiding to V2S3 takes place above 673 K. The catalysts are s ulfided more easily than the bulk oxides due to the higher dispersion of the vanadium species. In catalysts sulfided at 673 K which are stil l partially oxidic, four types of sulfur have been observed, viz, adso rbed H2S, stoichiometric sulfur, S-H groups, and nonstoichiometric (ex cess) sulfur (S-x). There are indications that (isothermal) room tempe rature H2S adsorption can be used to determine the dispersion of the s upported microcrystallites at higher vanadium loadings. From the prese nt results it is inferred that alumina-supported vanadium-based cataly sts, when sulfided at temperatures commonly applied in hydrotreating o perations, essentially consist of an oxide, the outer surface of which is sulfided. (C) 1995 Academic Press, Inc.