STRUCTURAL DETERMINATION OF SUPPORTED V2O5-WO3/TIO2 CATALYSTS BY INSITU RAMAN-SPECTROSCOPY AND X-RAY PHOTOELECTRON-SPECTROSCOPY

A series of supported V2O5/TiO2, WO3/TiO2 and V2O5-WO3/TiO2 samples have been characterized by means of Raman spectroscopy under ambient as well as in situ dehydrated conditions and by X-ray photoelectron spectroscopy. Under ambient conditions two different hydrated surface vanadia species and crystalline V2O5 have been identified in V2O5/TiO2 as a function of surface coverage. Under dehydrated conditions two types of surface vanadium oxide species are found in V2O5/TiO2 samples: a highly distorted vanadium oxide species and a moderately distorted vanadium oxide species, and their relative concentration is a function of the vanadia surface coverage. Under ambient conditions tetrahedrally coordinated surface tungsten oxide species, octahedrally coordinated surface polytungstate species, and crystalline WO3 are observed in WO3/TiO2 samples as a function of surface coverage. Dehydration converts all the two-dimensional tungsten oxide species into a highly distorted octahedrally coordinated structure. The molecular structures of the V2O5-WO3/TiO2 mixed samples are not influenced by the sequence of impregnation of the starting materials. Under ambient conditions and low vanadia coverage, the hydrated surface vanadium oxide species undergo a structural change due to the acidic nature of tungsten oxide species. Crystalline V2O5 and crystalline WO3 are not influenced by the presence of the other metal oxide. Under dehydrated conditions, both the highly distorted surface vanadium oxide species and the octahedrally coordinated surface tungsten oxide species do not appear to be influenced by each other at all loadings. The only influence observed in this study is that the moderately distorted vanadium oxide species becomes more abundant in the presence of tungsten oxide.