Structural chemistry of Co-Mo-alumnina catalysts

Abstract

The structural chemistry of Co-Mo-alumina hydrodesulfurization catalysts has been critically reviewed in this article. The location and nature of cobalt in the sulfided catalysts has been discussed. Small MoS₂, crystallites (10 to 30 Å) that occur on the surface of a sulfided sample are unstable due to the high, net negative charge on the sulfur ions on their edges. They incorporate cations like Co²⁺ at the edges of the MoS₂ sheet (not intercalated between the sheets) and attain stability. It is shown that for the range of crystallito sizes of Mo_S, in these systems, a maximum of one Co ion can be incorporated per two Mo ions in this manner. Cobalt thus lends structural stability to the MoS₂, crystallites and suppresses the excessive formation of anion vacancies at crystallite edges. Molybdenum sulfide crystallites on sulfided Mo-A1₂O₃, lacking this stabilizing influence of cobalt, lose excess sulfur, leading to the creation of a larger concentration of multiple anion vacancies at their edges. The presence of these strongly acidic multiple vacancies, however, leads to a lower structural stability of these crystallites. Moreover, these strongly acidic sites are also deactivated faster in the presence of unsaturated molecules (like diolefins, olefins, etc.), accounting for the fact that the superiority of Co-Mo-Al₂O₃ over Mo-Al₂O₃ manifests itself mainly in the steady-state activity [61-63]. The advantages of this model over those currently in vogue are the following: (1) It offers an explanation for the rather high cobalt concentration [Co/(Co + Mo)=0.31 necessary for optimum catalytic activity, (2) it accounts for the established accessibility of cobalt in the Co-MoS₂ phase to probe moledules [40], and (3) the stabilization of the small MoS₂ crystallites in sulfided Co-Mo-Al₂O₃ compared to Mo-Al₂O₃ with the consequent lower loss of sulfur ions from the former in flowing hydrogen (Table 3 and Ref. 28). The migration and sintering of MoS₂ crystallites in sulfided Mo-₂O₃ but not co-Mo-Al₂O₃, observed by Okamoto et al. [21], is also in accord with the above picture of stabilization of small MoS₂ crstallites by cobalt.