A RAMAN AND ULTRAVIOLET DIFFUSE REFLECTANCE SPECTROSCOPIC INVESTIGATION OF ALUMINA-SUPPORTED MOLYBDENUM OXIDE

Laser Raman spectroscopy and ultraviolet-visible diffuse reflectance spectroscopy were used to characterize alumina-supported molybdenum oxide prepared from Mo2(eta-3-C3H5)4, H2(MoO3C2O4).2H2O, and (NH4)6Mo7O24.4H2O at loadings ranging from 0.67 to 13.3 wt % Mo. The structure of the calcined Mo6+/Al2O3, under ambient conditions, was found to be independent of the molybdenum precursor and the preparation pH. The hydrated surface molybdate structure was found to be governed by the inherent acid-base properties of the molybdena/alumina system and the molybdenum weight loading. At low Mo loadings (< 1 Mo atom/nm2), isolated MoO4 tetrahedra dominate although a low degree of polymerization was observed. At 2-2.5 Mo atoms/nm2, the majority of the molybdenum was incorporated into octahedrally coordinated molybdenum polyanions. Crystalline MoO3 was detected above monolayer coverage (5-6 Mo atoms/nm2).