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 timedependent 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 V2p 3/2 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.

In this study we combine density-functional theory (DFT) calculations on oxygen core excitations in vanadia-silica model clusters with experimental in situ X-ray absorption fine structure (NEXAFS) near the oxygen K edge of vanadia model catalysts supported by SBA-15 silica in order to identify structural details of the vanadia species. The silica support is found to contribute to the NEXAFS spectrum in an energy range well above that of the vanadium oxide units allowing a clear separation between the corresponding contributions. Further, differently coordinated oxygen which is characteristic for particular vanadia species, monomeric or polymeric, can be clearly identified in the theoretical spectra in agreement with the oxygen K-edge NEXAFS measurements. The comparison of the theoretical and experimental NEXAFS spectra provides clear evidence that under in situ conditions different molecular vanadia species, in particular polymeric VO x , exist at the catalyst surface and the exclusive presence of monomeric vanadia groups can be clearly ruled out. The present analysis goes beyond earlier work applying vibrational spectroscopy to the present systems where, as a result of extended vibrational coupling, a separation between vanadia, silica, and interface contributions is less successful.

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. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Silica (SBA-15) supported vanadium oxide was used for a kinetic study of the oxidative dehydrogenation of propane in a fixed bed reactor. Prior to this study, spectroscopic characterization using a variety of techniques such as FTIR spectroscopy, Raman spectroscopy, DR UV-Vis spectroscopy and X-ray Phototelectron Spectroscopy revealed the absence of bulk vanadia and a high dispersion of active surface sites for the investigated catalyst. The kinetic data evaluation was based on a formal kinetic approach. Calorimetric measurements were used to determine the heat of adsorption of propane on the catalyst. The data indicate that the primary combustion of propane is negligible. Reaction orders of one for the propane dehydrogenation and propene combustion indicate the participation of these species in the respective rate determining step. The zero reaction order determined for the catalyst reoxidation reveals a participation of lattice oxygen in this reaction step. Higher activation energies of propane dehydrogenation as compared to the propene combustion indicate the participation of the weaker allylic C-H bond of propene in the rate determining step of the propene combustion. This results in higher propene selectivites at elevated temperatures. Kinetic parameters, including apparent and real activation energies and the equilibrium constant of the propane adsorption allowed for a comparison with theoretical predictions and show a good agreement.

The local structure of vanadium oxide supported on nanostructured SiO2 (VxOy/SBA-15) was investigated by in situ X-ray absorption spectroscopy (XAS). Because the number of potential parameters in XAS data analysis often exceeds the number of "independent" parameters, evaluating the reliability and significance of a particular fitting procedure is mandatory. The number of independent parameters (Nyquist) may not be sufficient. Hence, in addition to the number of independent parameters, a novel approach to evaluate the significance of structural fitting parameters in XAS data analysis is introduced. Three samples with different V loadings (i.e. 2.7 wt %, 5.4 wt %, and 10.8 wt %) were employed. Thermal treatment in air at 623 K resulted in characteristic structural changes of the V oxide species. Independent of the V loading, the local structure around V centers in dehydrated VxOy/SBA-15 corresponded to an ordered arrangement of adjacent V2O7 units. Moreover, the V2O7 units were found to persist under selective oxidation reaction conditions.