Cobalt sulfide phase structure catalysts

Structure of Cobalt Sulfide Phase in Carbon-Supported Co and Co—Mo Sulfide Catalysts... 

An X-Ray absorption spectroscopy study has been carried out on a carbon-supported Co and Co-Mo sulfide catalyst, the latter consisting of a fully sulfided Co-Mo-S (type II) phase. Detailed information on the structure of the cobalt sulfide phase is obtained by comparing the EXAFS and XANES spectra of the catalysts with those of CogS8 and CoS2 reference compounds. It is shown that the cobalt atoms in the Co-Mo-S phase have an octahedral-like sulfur coordination while the cobalt atoms in the sulfided Co/C catalyst have a larger fraction of octahedral cobalt than Co Sg. On the basis of these results, the high HDS activity of a sulfided Co/C catalyst can be understood since it appears that the structure of the cobalt sulfide phase in Co/C is in agreement with that in Co-Mo/C. In the Co-Mo-S phase, approximately one cobalt atom is in contact with one molybdenum atom at a distance of 2.85 A. 

A different model was postulated by Duchet et al. (9) and by Vissers et al. (10) who observed a high activity for pure cobalt sulfide supported on activated carbon and hence explained the activity of a sulfided Co-Mo/C catalyst completely by the very high activity of the cobalt sites. In their theory the authors implicitly assumed that a cobalt site in sulfided Co/C has the same activity as a cobalt site in sulfided Co-Mo/C. However, this does not have to be the case since the structure of cobalt sulfide in Co/C can be very different from that in Co-Mo/C. In order to elucidate the structure of the cobalt sulfide phase in a sulfided Co/C and a Co-Mo/C catalyst we applied in this study the EXAFS and XANES (X-Ray Absorption Near Edge Structure) techniques at the Co K-edge. 

These tetrahedral distorted cobalt atoms can be observed by NMR as a pure phase on carbon supports in the absence of molybdenum and are thus stable these probably correspond to the Co sites observed by Topspe s group using Mossbauer spectroscopy because Craje et al. (93) found a similar Mossbauer doublet for both cobalt in CoMo catalysts and pure cobalt sulfide on carbon support. They are also active for HDS and confirm the findings of Prins and co-workers (94) and Ledoux (96). These different structures are in full agreement with the XANES experiments performed by Prins and co-workers (95) and Ledoux (96). These structures also led Ledoux et al. to an incorrect interpretation of the synergy effect (64). On poorly dispersed catalysts supported on silica or in bulk form, their presence and activity are large enough to explain the increase in activity when cobalt is added to molybdenum, but on well-dispersed catalysts i.e., on alumina or carbon support this interpretation is shown to be incorrect if the activity is carefully measured. 

Co-Mo/C catalyst in both catalysts a high sulfur coordination of the cobalt atoms is present. This is an interesting observation with regard to the activity per cobalt atom in both catalyst systems. Vissers et al. (10) stated that the intrinsic activity of a cobalt site in sulfided Co/C can be close to that in sulfided Co-Mo/C. Hence, our results of the structural resemblance between sulfided Co/C and Co-Mo/C, support the theory of Vissers et al. (10) that the cobalt phase in sulfided Co-Mo catalysts can be the actual active phase. Moreover, on the basis of these results the high HDS activity of a sulfided Co/C catalyst can be understood. 

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