Methodology for the structural characterisation of V_xO_y species supported on silica under reaction conditions by means of in situ O K-edge X-ray absorption spectroscopy

A methodology and its application to silica supported vanadia are presented that allows for a detailed analysis of structural peculiarities of the vanadia species on the surface. We introduce in situ oxygen K-edge X-ray absorption spectroscopy (XAS) based on the Auger electron yield (AEY) technique as a characterisation tool to assist vibrational spectroscopy that has been applied extensively in earlier work on this system. The analysis of the O K-near edge X-ray absorption fine structure (NEXAFS) allows a clear distinction between separate vanadia, silica and interface contribution in contrast to vibrational spectroscopy with strongly overlapping contributions due to vibrational coupling. Differently coordinated oxygen can be identified in the O K-NEXAFS spectrum by comparison with theoretical spectra obtained by state of the art density-functional theory (DFT) calculation. A study of catalysts with different V loadings (0 wt% V, 2.7 wt% V, and 10.8 wt% V) shows that the contributions of silica to the NEXAFS appear in an energy region well separated from the spectral signature of oxygen bound to vanadium. Dehydrated catalysts with high (10.8 wt% V) and low (2.7 wt% V) vanadium loading on silica SBA-15 show identical NEXAFS under in situ conditions. This finding indicates that independent of the loading a distribution of vanadia species with a very similar molecular structure, including non-monomeric as well as possibly monomeric configurations and different from crystalline V2O5, is present on this model catalyst