Nature of dispersed vanadium oxide: influence of the silica support structure and synthesis methods

Dispersed vanadium oxide samples were prepared on the basis of two differently structured high surface area silica materials (Aerosil 300, SBA-15). For each support material incipient wetness impregnation and a grafting/ion exchange procedure were applied to prepare catalyst samples with comparable vanadium density. The influence of the silica support material and preparation method on the vanadium oxide structure and dispersion has been studied using diffuse reflectance UV-Vis spectroscopy, visible Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). By independent spectroscopic characterization a fully consistent picture regarding the relation of the structure (UV-Vis, Raman) and dispersion (XPS) is developed. Based on the present structural data and recent findings the dispersed vanadium oxide is proposed to consist of monomers and oligomers with a distorted tetrahedral (VO4) structure containing one short V=O bond. A different degree of hydroxylation of vanadium gives rise to two V=O stretching bands at 1027 and 1040 cm(-1). The structure and dispersion of vanadium oxide are more strongly influenced by the support material than by the synthesis method. In this regard the Aerosil 300 samples show a higher degree of oligomerization, i.e. less dispersion of the surface vanadium oxide species, than the SBA-15 samples. Likewise incipient wetness impregnation leads to more oligomerized species than grafting/ion-exchange.