Solid solutions of Ni and Co molybdates in silica-dispersed and bulk catalysts prepared by sol-gel and citrate methods

Silica-dispersed catalysts based on cobalt, nickel and mixed Ni-Co molybdates of various compositions are prepared by a sol-gel procedure from silicon alkoxides, metal nitrates and ammonium heptamolybdate. For comparative purposes, the corresponding bulk molybdates are obtained by the citrate route. The polymorphism and the textural properties of these catalysts are investigated by means of X-ray diffraction, UV-visible diffuse reflectance spectroscopy and Raman spectroscopy. Their surface and textural characteristics are investigated by X-ray photoelectron spectroscopy and nitrogen or krypton physisorption. In the sol-gel prepared catalysts, the β-phase of Ni_1−xCo_xMoO₄, which is the most active phase in the oxidative dehydrogenation of light alkanes, is stabilized over the whole composition range. In the analogous bulk catalysts prepared by the citrate method, this phase is stabilized only at high Co content (x?0.55). Dispersion of the active phase in silica is shown to improve the propene productivity in comparison with bulk catalysts obtained by citrate or coprecipitation methods.