Abstract
Supported vanadium oxide catalysts have been the subject of detailed investigations for many decades and a relatively large amount of information is available on their structure, however, the exact molecular structure and the way these surface species are anchored on the support oxide has not yet been unambiguously determined. It
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has been attempted in this work to determine the exact molecular structure of supported vanadium oxides on several support oxides under various experimental conditions by using a wide variety of characterization techniques, although the main emphasis has been on the application of X-ray absorption spectroscopy. Furthermore, we have explored the potential of X-ray absorption spectroscopy to probe metal oxide-support effects in a quantitative manner. It is found that the support ionicity can be related to the catalytic performances of supported vanadium oxide catalysts. The molecular structure of low-loaded vanadium oxide catalysts has been investigated on four different supports, i.e. Al2O3, Nb2O5, SiO2 and ZrO2. Raman and UV-VIS diffuse reflectance spectroscopy in combination with EXAFS and structural models showed that the majority of the vanadium oxide species are present as a monomeric VO4 cluster. Under the measurement conditions applied the VO4 cluster was found to have only one V-O-Msupport bond. It has to be stressed that the overall vanadium oxide structure is the same on all supports under investigation: one V=O bond, one V-O-Msupport bond and two V-O bonds. The nature of the V-O groups has been investigated for a series of silica-supported vanadium oxide catalysts varying in their vanadium oxide loading. No evidence was found for the presence of a peroxo species or V-O-V bridges on the silica surface for low-loaded vanadium oxide catalysts. It was found that trace amounts of water resulted in the presence of V-OH groups on the vanadium oxide cluster. Total hydration of low-loaded supported vanadium oxide catalysts resulted in the formation of larger vanadium oxide clusters. The exact size and geometric structure of these clusters depend on the PZC of the support material. A novel characterization tool to study the influence of the support oxide material on the structural, electronic and catalytic properties of supported vanadium oxide species has been explored. It has been shown that Atomic XAFS (AXAFS) spectroscopy is able to probe variations in the vanadium valence orbitals as a function of nearest and next nearest neighbours. It has been illustrated that, although the overall molecular structure of the supported vanadium oxide is the same on all supports, the AXAFS intensity is a function of the support oxide. The support effect is a next nearest neighbour (Msupport) effect that acts through the V-Ob-Msupport bond. The trend in AXAFS intensity showed a linear relation with the catalytic performance of the supported vanadium oxide catalysts in both selective oxidation and dehydrogenation reactions.
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