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

Laser Raman spectroscopy (LRS) and ultraviolet-visible diffuse reflectance spectroscopy (UVDRS) were used to characterize silica-supported, hydrated molybdenum oxide prepared from (NH4)6Mo7O24.4H2O, H2(MoO3C2O4).2H2O, MoCl5, Mo(eta-3,C3H5)4, and Mo2(eta-3-C3H5)4. Molybdenum loadings of 0.1-10 wt % were studied. LRS was also used to characterize dehydrated molybdenum oxide, under in situ conditions, prepared from Mo2(eta-3-C3H5)4 at a loading of 6.4 wt % Mo. Fumed silica (Cab-O-Sil EH-5) and silica gels (Davison 952, Rhone-Poulenc X400LS, and Nishio Kogyo) were used as supports. Raman spectra were recorded either in ambient air following calcination or in an in situ cell under oxygen purge. For the hydrated, ambient samples, all precursors were found, by LRS, to form octahedrally coordinated surface polymolybdate species at Mo loadings above 0.4 wt %. No Raman evidence was found for the existence of isolated, surface molybdenum oxide tetrahedra. In situ Raman showed that the octahedrally coordinated, hydrated polymolybdate species could be reversibly transformed into an isolated structure upon dehydration. Four crystalline compounds were also observed: MoO3, CaMoO4, Na2Mo2O7, and Na2MoO4. The formation of these crystalline compounds depended upon the Mo loading, the method of preparation, and the presence of impurities on the support. UVDRS, without supplementary evidence, is shown to be ineffective in differentiating tetrahedrally and octahedrally coordinated molybdenum for the systems studied. The formation of the hydrated, octahedrally coordinated surface polymolybdate species is shown to be related to the aqueous chemistry of Mo6+.