Physicochemical properties of MoO 3TiO 2 prepared by an equilibrium adsorption method

Abstract

The adsorption phenomena of molybdena species onto titania surfaces and the surface properties of the catalysts have been studied by using an equilibrium adsorption method. 95Mo NMR and UV spectroscopic studies show that the aqueous molybdena species vary as a function of the pH of the impregnating solution. For acidic pH values, polymeric species, Mo 7O 24 6 ions, are present, while in the basic solutions it is the monomeric MoO 4 2− ions that are present. The adsorbed amounts of molybdate anion are strongly dependent on the pH of the impregnating solution and increase as an inverse function of the pH. XRD, Raman, and XPS data of the calcined samples show that monolayer coverage of molybdenum oxide is established at pH 3.98 (6.6 wt%). The Raman studies reveal that the molybdenum oxide monolayer is composed of distorted octahedra. At more acidic pH regions, pH < 3.98, crystalline MoO 3 is formed above monolayer coverage. The results of catalytic oxidation of methanol show that the catalysts up to monolayer coverage of surface molybdate species possess higher turnover numbers than the catalysts possessing more than monolayer coverage (presence of crystalline MoO 3. The primary methanol oxidation product is dimethoxymethane at low conversions; methyl formate is next in abundance. The selectivity for dimethyl ether, which occurred as a side reaction on the acidic sites of catalysts, increases as the Mo loading increases.