Vanadium oxide monolayer catalysts. 3. A Raman spectroscopic and temperature-programmed reduction study of monolayer and crystal-type vanadia on various supports

Vanadium(V) oxide supported on 7-A1203, GO2, Cr2O3, Si02, Ti02, and Zr02 was studied by X-ray fluorescence, by X-ray diffraction, and especially by the combination of Raman spectroscopy and temperature-programmed reduction (TPR) for qualitative and quantitative structural analysis, respectively. Catalysts were prepared via ion-exchange and wet-impregnation methods. The V contents ranged from - 1 to 40 wt % V. At low surface concentrations only surface vanadate phases of two-dimensional character are observed for all carriers. According to Raman and TPR data the structure of these surface vanadate species is independent of the preparation technique. At medium and high surface concentrations, the webimpregnated samples already contain crystalline V20b At equal surface concentrations the ion-exchanged catalysts contain no V205 crystallites. An exception is Si02 on which also crystalline V206 is formed in both preparation techniques. Monolayer stability toward thermal treatment decreases in the order AZO>3 Ti02> Ce02,w hereas on heating ion-exchanged V/Si02 the crystalline V206 spreads out over the silica surface. The reducibilities of the ion-exchanged catalyats, as measured by TPR, can be used as a measure for the contact interaction between vanadia and the carrier oxides. At temperatures of 500-800 K, this interaction ranges from strong with titania to weak with silica as a carrier.