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 N b2O5 were prepared by the incipient-wetness impregnation method employ ing 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 w ere determined by X-ray absorption near-edge spectroscopy (XANES) at t he Mo L(3)-edge. Under ambient conditions, the structures of the hydra ted 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 c onditions, the structures of the surface molybdenum oxide species depe nd on both the specific oxide support and surface coverage. At low sur face coverages of MoO3 on Al2O3 and TiO2, the primary species is isola ted and tetrahedral coordinated. At high surface coverages of MoO3, fo r TiO2 the primary species is polymerized and octahedral coordinated, but for Al2O3 there is a mixture of tetrahedral and octahedral coordin ated species. The MoO3/ZrO2 system appears to be similar to the MoO3/A l2O3 system, and the MoO3/Nb2O5 system appears to be similar to the Mo O3/TiO2 system. The surface molybdenum oxide species on SiO2 is isolat ed and appears to possess a coordination that is in between tetrahedra l and octahedral. Monolayer coverage was achieved at the same surface density of molybdenum oxide on the different oxide supports with the e xception of SiO2. Only low loadings of molybdenum oxide can be dispers ed on SiO2 due to the low concentration and reactivity of the surface OH groups.