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The sulfidation of γ -alumina-and titania-supported (cobalt)molybdenum 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 molybdenum sulfide species containing disulfide ligands. Between 448 and 523 K the disulfide ligands are reduced with hydrogen and only dimeric molybdenum sulfide species with a Mo-Mo distance of 2.77Å remain. Above 523 K these dimers aggregate to larger MoS 2 particles. In contrast, on titania molybdenum oxide is sulfided to yield isolated molybdenum sulfide monomers at 523 K, which similarly aggregate to MoS 2 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 2.0Å. The addition of Co to Mo/Al 2 O 3 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 molybdenum sulfide dimers, that are not present in the unpromoted catalyst. These observations can be ascribed to the presence of CoMoO 4 in the oxidic catalyst.

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