The sulfidation of gamma-alumina and titania supported (cobalt)molybdenum oxide catalysts monitored by EXAFS

The sulfidation of gamma-alumina- and titania-supported (cobalt)molybdenun oxide catalysts has been studied with X-ray absorption spectroscopy and temperature programmed sulfidation (TPS). The catalysts were stepwise sulfided at temperatures between 298 and 673 K and their structure was determined with EXAFS spectroscopy. On alumina oxygen-sulfur exchange starts at a temperature just above room temperature resulting in the formation of monomeric and dimeric molybdenun sulfide species containing disulfide ligands. Between 448 and 523 K the disulfide ligands are reduced with hydrogen and only dimeric molybdenun sulfide species with a Mo-Mo distance of 2.77 Angstrom remain. Above 523 K these dimers aggregate to larger MoS2 particles. In contrast, on titania molybdenum oxide is sulfided to yield isolated molybdenum sulfide monomers at 523 K, which similarly aggregate to MoS2 particles between 523 and 623 K. No molybdenum sulfide dimers are observed as intermediates. Both on alumina and titania all intermediates are anchored to the support via Mo-O linkages with a bond distance of 2.0 Angstrom. The addition of Co to Mo/Al2O3 accelerates the rate of sulfidation, demonstrated by the TPS patterns. However, on titania a lowering of the sulfidation rate is observed next to the presence of molybdenun sulfide dimers, that are not present in the unpromoted catalyst. These observations can be ascribed to the presence of CoMoO4 in the oxidic catalyst. (C) 1997 Academic Press.