Supported molybdenum oxide catalyst. Structure and chemistry of oxidized, reduced, and sulfided surfaces.

Abstract

Highly dispersed molybdena-titania catalyst can be prepared by an equilibrium adsorption method. In this method, molybdate anions adsorb onto positively charged titania surfaces via electrostatic attraction by controlling pH of the impregnating solution. The amounts of absorbed molybdate anion are strongly dependent on pH of the impregnating solution and they increase as an inverse function of pH. XRD, Raman, and XPS data of the calcined samples show that monolayer coverage of molybdenum oxide over-layer possesses a highly distorted MoO6 group with a molecular geometry resembling a distorted square pyramid. The Mo species are fixed by an oxygen bridge with Ti, as Mo-O-Ti, or with neighboring Mo, as Mo-O-Mo.The catalytic oxidation of methanol over the surface molybdate species on titania possess higher turnover numbers and higher selectivities of partial oxidation products than do the catalysts exceeding monolayer coverage which possess the crystalline MoO3 phase. Changes of the surface properties either, after reduction and sulfiding treatment over monolayer catalyst, have also been investigated. The NO chemisorption and XPS studies show that two types of active sites appeared after reduction treatment: one site is active for hydrogenation of 1, 3-butadiene and the other site is active for metathesis of propene. A higher degree of coordinative unsaturations of Mo is required for hydrogenation than for metathesis. After subjecting sulfiding treatments to the catalyst, hydrogenolysis of thiophene requires the ensemble of doubly or triply coordinative unsaturations.