No abstract

Silica is a relatively inert support in comparison with alumina. For the same amount of active phase, it has been shown that the nature of the compound present on these two supports is very different in the range of composition from 0 to 1 cobalt:molybdenum atomic ratio. We have determined the influence of the amount of oxide phase present on the support on the structure of Mo/Si02 and CoMo/Si02 catalysts by using X-ray diffraction, DRS measurements, and a gravimetry study of the reduction by hydrogen. In the first series of catalyst, the molybdenum oxide content ranges from 2.8 to 20.6 wt %. In the second one, the amount of active phase (Co304 + Mo03) ranges from 3.7 to 27.7 wt %. In the latter case the atomic ratio Co/(Co + Mo) is constant and equal to 0.36. When only molybdenum is supported on silica both attached Mo(VI) interacting with Si02 and bulk Mo03 are present in proportion which depends on the amount of molybdenum oxide phase. The addition of cobalt on Mo/Si02 catalyst brings about a detachment of molybdenum oxide in interaction with Si02 and formation of CoMo04. Furthermore, there is no evidence of cobalt oxide formation.

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