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Abstract

The influence of water on the dispersion and structure of silica SBA-15 supported vanadia model catalysts has been studied using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, which were combined within one experimental setup, as well as UV-Vis diffuse reflectance spectroscopy. By performing time-dependent XPS experiments the influence of UHV/X-ray radiation could be eliminated by extrapolation of the observed temporal changes to t = 0. XPS characterization reveals that the V2p3/2 emission consists of two contributions, which are assigned to vanadia with distinctly different cluster size. Dehydration by treatment in oxygen flow at elevated temperatures leads to a significant increase in total intensity and a substantial redistribution of spectral weight to higher binding energies as a result of an increase in the vanadia dispersion. The V/Si XPS intensity ratio of the dehydrated samples closely follows the corresponding bulk ratio over the whole range of vanadium loadings (0-22 wt% V) studied. This observation renders possible a correlation of XPS and Raman results allowing for quantification of Raman features such as the relative cross sections of the vanadyl surface species. It is shown that the observed changes in vanadia dispersion are directly associated with the changes in the molecular structure of the surface vanadia species.