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Physicochemical studies on a series of cobalt-molybdenum supported on silica catalysts containing the same amount of oxide phase with an atomic ratio r = Co/(Co + Mo) ranging from 0 to 1 and the phases obtained after reduction by hydrogen allow us a better understanding of the structure of these solids. X-ray diffraction, DRS, and XPS measurements of the oxide phase, the study of the reduction by hydrogen by use of gravimetric measurements, and ESR and DRS measurements carried out on the reduced phase show that weak interaction of the molybdenum with the silica (as compared with those on alumina) strongly modify the nature of the species presented on the support. It has been shown that cobalt and molybdenum almost stoichiometrically form C0M0O4 (a and b phases). Furthermore when Mo03 and Co304 are in excess they cover the CoMo04 crystals forming a geod structure. In conclusion, it is proposed that, when the interactions of the molybdenum with the support are weak, the CoMo "bilayer" precursor of the active sites is not easily formed. Consequently the strong interaction between cobalt and molybdenum leads to the formation of bulk CoMo04, compound which does not form, after activation, the active phase in HDS catalysts.

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