Influence of the geometric structure on the V L3 near edge X-ray absorption fine structure from vanadium phosphorus oxide catalysts

Abstract

We present the V L3 near edge X-ray absorption fine structure (NEXAFS) of a vanadium phosphorus oxide (VPO) catalyst. The spe ctrum is related to the V3d-O2p hybridised unoccupied states. The overall peak position at the V L3-absorption edge is determined by the formal oxidation state of the absorbing vanadium atom. Details of the absorption fine structure are influenced by the geometric structure of the compound. Empirically we found a linear relationship between the energy position of several absorption resonances and the V -O bond length of the participating atoms. This allows to identify the contribution of specific V-O bonds to the near edge X-ray absorption fine structure. The bond length / resonance position relationship will be discussed under consideration of relations between geometric structure and NEXAFS features observed in X-ray absorption experiments and theory.

Selective oxidation of methanol to form dimethoxymethane and methyl formate over a monolayer V2O5/TiO2 catalyst

“…The L 3 -edge spectrum of the fully oxidized vanadia obtained in oxygen atmosphere shows the well-defined fine structure with at least four distinct resonances. The line shape is typical for V 2 O 5 [70,71]. The position of the main resonance is 518.9 eV is also very close to the value of 519.1 eV reported for bulk V 2 O 5 [70].…”

Dynamic surface behaviour of VPO catalysts under reactive and non-reactive gas compositions: an in situ XAS study

“…This fact is used to conclude from the resonance energy position upon the valence of the metal ion. A linear shift to lower energies with decreasing oxidation state is observed [30,25] as described before in connection with the discussion of Fig. 5.…”

“…We chose a semi-empirical approach to get this valuable information about the V-O bond character. We showed in a previous work that the NEXAFS of V 2 O 5 and VPO exhibit certain similarities [25]. This is probably due to their resembling local geometric structure.…”

“…Although the details of the spectrum depend on the normalisation prior to the subtraction, it becomes clear that the spectral weight of the difference spectrum (a) is shifted to higher photon energies compared to the spectrum of the VPO catalyst before the heating in oxygen (e). This is typical for V 5+ species [25,27]. The An unambiguous identification of the species is difficult but it becomes evident that a V 5+ species was formed.…”

“…It is not well defined at which photon energy the states with mainly e g character start to dominate the NEXAFS. Following the reasoning in [25] based on an empirical bond length / resonance position relationship we assumed that all intensity up to resonance V6 is dominated by t 2g character. This seems to be justified because V7 is a broad resonance, which is typical for overlapping e g states in contrast to the well separated, narrow t 2g states (FWHM V7=1.6 eV, FWHM V6=0.9 eV).…”