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

Abstract

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)

Active Sites for Propane Oxidation: Some Generic Considerations

“…It is noted that this long time of equilibration may be due to the low temperature and the reduced total pressure of about 0.25 mbar. For experimental details see [56,57] Top Catal (2011) 54:627-638 629…”

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

“…19 Thus, a clear distinction between monomeric and oligomeric species solely based on vibrational spectroscopy appears to be difficult. However, recent experiments on silica SBA-15 supported vanadium oxide catalysts employing X-ray absorption spectroscopy in the soft (NEXAFS) 20 and hard X-ray range (EXAFS) 21 as well as IR spectroscopy (using NO as probe molecule) 23 have provided clear evidence for the presence of oligomeric species (besides monomeric species). A mixture of monomeric and non-monomeric species even at low loadings had been proposed earlier on the basis of a detailed UV-Vis, Raman and IR spectroscopic study.…”

“…[18][19][20][21][22][23][24][25] In the following, important aspects of these findings will be briefly discussed. While it is now generally accepted that dehydrated vanadium oxide at low vanadium density (typically at o2 atoms nm…”

Restructuring of silica supported vanadia during propane oxidative dehydrogenation studied by combined synchrotron radiation based in situ soft X-ray absorption and photoemission

“…Oxygen K-edge excitation spectra have been recorded in the Auger electron yield mode by operating the electron spectrometer with a pass energy of 100 eV as a X-ray absorption spectroscopy (XAS) detector to minimize contributions from the gas phase to the spectra. O K-edge spectra of the sample surface have been corrected for the remaining effects of O 2 gas phase absorption following a procedure developed in our group [18]. In order to increase the signal to noise ratio a data reduction by a factor 2 has been applied to the raw spectra (containing about 1000 points per scan) by averaging adjacent points.…”

“…Absolute energy calibration has been achieved by setting the π* resonance of the O 2 gas phase signal to 530.9 eV and the spectral resolution was about 150 meV. Further details of the metrology and data treatment are presented elsewhere [18].…”