Surface structures of supported molybdenum oxide catalysts under ambient conditions

Abstract

Two sets of supported molybdenum oxide catalysts, wet (dried at room temperature) and calcined (calcined at 773 K) samples, were prepared by an equilibrium adsorption method at different pH values of the impregnating solution. The adsorbed amounts of molybdenum oxide species onto the oxide support are strongly dependent on the pH of the impregnating solution and increase with decreasing pH. The Raman spectroscopic studies reveal that the surface molybdenum oxide species under ambient conditions, wet and calcined, are hydrated and essentially in an aqueous medium. Furthermore, the surface structures of molybdenum oxide species on the oxide support were found to depend on the net surface pH at point of zero charge (PZC) under ambient conditions. The net surface pH at PZC under ambient conditions is determined by the specific oxide support and surface molybdenum oxide coverage. The surface molybdenum oxide structures in the wet, uncalcined, samples are not only dependent on the net surface pH at PZC but also on the number of NH 4 + cations which coordinate to the surface molybdenum oxide species for compensation of net charge: Mo 7O 24 6− species in NH 4 +-rich concentrations (high pH region) favor formation of (NH 4) 6Mo 7O 24 · 4H 2O. Upon calcination, the NH 4 + ions are removed and the surface molybdenum oxide species become rehydrated upon exposure to air by adsorbing moisture. Consequently, the structures of surface molybdenum oxide species in the calcined samples which have been exposed to ambient are also dependent on the net surface pH at PZC.