KINETICS OF DEPOSITION OF THE MO-OXO SPECIES ON THE SURFACE OF GAMMA-ALUMINA

The kinetics of deposition (adsorption, surface reaction) of molybdenu m-oxo species an the surface of gamma-alumina was studied at;various p H's, impregnation temperatures, solute concentrations, and ionic stren gths. The kinetic study confirmed the findings of the thermodynamic st udy performed previously concerning the influence of the aforementione d parameters on the extent of the Mo-(vi) deposition and the mechanism of that deposition as well. It was found that the decrease in pH and ionic strength and the increase in the impregnation temperature increa se the time required in order for the surface of the support to be sat urated by the molybdenum-oxo species. Moreover, the kinetic study sugg ested that, at low pH's where adsorption predominates, the polymeric o xo species are exclusively adsorbed in the first 20 min and the deposi tion proceeds through the adsorption of the monomeric oxo species. Thi s is expected to be very useful in designing the preparation of the mo lybdenum-supported gamma-alumina catalysts. The simple model developed allowed the interpretation of the kinetic curves achieved at various pH values and the determination of the kinetic constants. These consta nts may be used to model the development of the Mo profiles on gamma-a lumina extrudates, to interpret the influence of pH on the characteris tics of the kinetic curves achieved at various pH values, and to under stand better the deposition mechanism.