Abstract
The sulfidation of γ-alumina- and titania-supported(cobalt)molybdenum oxide catalysts has been studied with X-rayabsorption spectroscopy and temperature programmed sulfidation (TPS).The catalysts were stepwise sulfided at temperatures between 298 and673 K and their structure was determined with EXAFS spectroscopy. Onalumina oxygen–sulfur exchange starts at a temperature just aboveroom temperature resulting in the formation of monomeric and dimericmolybdenum sulfide species containing disulfide ligands. Between 448and 523 K the disulfide ligands are reduced with hydrogen and onlydimeric molybdenum sulfide species with a Mo–Mo distance of 2.77 Åremain. Above 523 K these dimers aggregate to larger MoS2particles.In contrast, on titania molybdenum oxide is sulfided to yieldisolated molybdenum sulfide monomers at 523 K, which similarlyaggregate to MoS2particles between 523 and 623 K. Nomolybdenum sulfide dimers are observed as intermediates. Both onalumina and titania all intermediates are anchored to the support viaMo–O linkages with a bond distance of 2.0 Å. The addition of Co toMo/Al2O3accelerates 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 CoMoO4in the oxidic catalyst.
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