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Sulfiding is a challenge Typical CoMo and NiMo catalysts need to be sulfided to achieve an active state. This operation in fact corresponds to a fantastic reorganization of the molecular structure at the surface of the support.	Click to enlarge
The oxide form, whether on a fresh or regenerated catalyst, contains polymolybdates anions well dispersed on the alumina support. When sulfiding in the presence of hydrogen and a sulfur compound (which decomposes into hydrogen sulfide), the atoms migrate on the surface to form a totally different structure. The slabs of molybdenum sulfide look like a sort of sandwich of molybdenum atoms between two layers of sulfur atoms.
	This transformation goes step-by-step from the oxidic form to the sulfide through various oxysulfide intermediates to ensure that the operating conditions are ideal to obtain the best possible active phase. The technique of Transmission Electron Microscopy can assess the dispersion (click here to see TEM photographs).
The magic of the CoMoS / NiMoS phase Neither Mo sulfide alone nor cobalt sulfide is catalytically very active for HDS reactions. Why is only the mixture of cobalt and molybdenum active with a maximum activity for a ratio of around 1:3 of cobalt atoms versus the total ? This can be understood when looking at the structure of the CoMoS phase. The slabs of MoS₂ are surrounded by cobalt atoms, which only find a good fit at this edge position. The active site is assumed to be created in the vicinity of some of these cobalt atoms. The magic ratio Co / ( Co + Mo ) can now be understood, considering the average size of the slabs and the relative number of edge sites versus total metallic atoms. >> Click here to see the CoMoS structure