SURFACE-STRUCTURES OF SUPPORTED MOLYBDENUM OXIDE CATALYSTS - CHARACTERIZATION BY RAMAN AND MO L(3)-EDGE XANES

Supported molybdenum oxide catalysts on TiO2, Al2O3, ZrO2, SiO2, and Nb2O5 were prepared by the incipient-wetness impregnation method employing aqueous solutions of ammonium heptamolybdate ((NH4)(6)Mo7O24.4H2O) The molecular structures of the surface molybdenum oxide species were investigated by Raman spectroscopy, and their local site symmetries were determined by X-ray absorption near-edge spectroscopy (XANES) at the Mo L(3)-edge. Under ambient conditions, the structures of the hydrated surface molybdenum oxide species are controlled by the net surface pH at the point of zero charge (PZC) and are the; same as observed in aqueous solutions: MoO42-, Mo7O246-, and Mo8O264-. Under dehydrated conditions, the structures of the surface molybdenum oxide species depend on both the specific oxide support and surface coverage. At low surface coverages of MoO3 on Al2O3 and TiO2, the primary species is isolated and tetrahedral coordinated. At high surface coverages of MoO3, for TiO2 the primary species is polymerized and octahedral coordinated, but for Al2O3 there is a mixture of tetrahedral and octahedral coordinated species. The MoO3/ZrO2 system appears to be similar to the MoO3/Al2O3 system, and the MoO3/Nb2O5 system appears to be similar to the MoO3/TiO2 system. The surface molybdenum oxide species on SiO2 is isolated and appears to possess a coordination that is in between tetrahedral and octahedral. Monolayer coverage was achieved at the same surface density of molybdenum oxide on the different oxide supports with the exception of SiO2. Only low loadings of molybdenum oxide can be dispersed on SiO2 due to the low concentration and reactivity of the surface OH groups.