Influence of the precursor (nature and amount) on the morphology of MoO3 crystallites supported on silica

The influence of the nature of the precursor and the amount used is investigated in the preparation of MoO3 catalysts supported on a mesoporous silica by a wet impregnation method. Four precursors are compared: ammonium heptamolybdate, peroxopolymolybdate, a citrate complex of MO and an oxalate complex of Mo. The MoO3 loading is adjusted between 0 and 2 theoretical monolayers of MoO3. Characterization of the samples was performed by N-2 physisorption, X-ray diffraction and scanning electron microscopy with as main objective the evaluation of the dispersion of MoO3 on the silica and of the morphology of MoO3 crystallites eventually obtained. At low loadings, MoO3 appears as amorphous material remaining at the outer surface of the silica. At high loadings, MoO3 appears as molybdite crystals. The loading above which MoO3 crystals are obtained depends on the precursor used. The morphology of MoO3 crystals, when obtained, also depends on the precursor used. Isotropic crystals are obtained with Mo oxalate and peroxopolymolybdate while Mo citrate leads to crystals with largely developed basal faces. Ammonium heptamolybdate leads to crystals with an intermediate morphology. Our results thus show that changing the nature and the amount of precursor used to impregnate MoO3 on silica offers a promising possibility to adjust the morphology and, to some extent, the dispersion of MoO3 crystals at the surface of the support.