Temperature Programmed Sulfiding of CoO/MoO3/γ‐Al2O3 Catalysts

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Abstract
The sulfiding of CoO/MoO3/γ‐Al2O3 catalysts was studied by a technique called Temperature Programmed Sulfiding (TPS). Cobalt loading, calcination temperature and pretreatment were varied.CoO/MoO3/γ‐Al2O3 catalysts sulfide at lower temperatures than model compounds such as CoO, MoO3 and CoMoO4 because of the higher dispersion in the catalysts. Sulfiding of Mo in CoO/MoO3/γ‐Al2O3 catalysts is dominated by O‐S exchange, reduction of Mo is due to Mo‐S bond rupture followed by reduction of elemental sulfur, in the same way as occurs in MoO3/γ‐Al2O3 catalysts.For Co in CoO/MoO3/γ‐Al2O3 catalysts the sulfiding pattern depends strongly on the calcination temperature: low calcination temperatures (785 K) lead to catalysts which show sulfiding of Co species in the same temperature range as Mo, and an increase in the reduction rate of elemental sulfur formed by reduction of Mo oxysulfides.Sulfiding of CoO/MoO3/γ‐Al2O3 leads to Co species which are stabilized and more disperse than those in sulfided CoO/MoO/γ‐Al2O3 catalysts. Calcination temperatures higher than 785 K cause a decrease of the amount of easily sulfidable Co species, and an increase in the amount of Co which sulfides at high temperature (1000 K).As was found for MoO3/γ‐Al2O3 and CoO/γ‐Al2O3 catalysts the “water content” of CoO/MoO/γ‐Al2O3 catalysts has an influence on the sulfiding patterns: “wet” catalysts (containing up to 6% water) with low Co loading sulfide at lower temperature than dried catalysts. Sulfiding (in TPS) proceeds at a higher rate than reduction by H2 (in TPR).