Sulfides XANES-EXAFS study

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. 

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