Analysis of silica-supported vanadia by X-ray absorption spectroscopy: Combined theoretical and experimental studies

Abstract

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.

Active Sites for Propane Oxidation: Some Generic Considerations

“…The grafted vanadia species also did not form any AA in our reaction systems being in line with dimer sites [121] as predominant active species. Larger oligomers [122], mimicking the active site in M1 seem not to be frequently present on these systems.…”

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

“…Figure 2 presents photon energy range of 8 and 12V are compared with the spectrum of V 2 O 5 in Figure 2(B). As deduced in a recent work by comparison of experimental with theoretical spectra obtained by ab initio DFT calculations the shape of the O K-NEXAFS of the dehydrated catalysts shown in Figure 2(B) with a V loading of 8 wt-% and below on SBA15 suggests the presence of both monomeric and polymeric species [26]. The O K-edge spectrum of 12 and 14V show a more pronounced shoulder at 529.5 eV and about 531.6 eV (Figure 2(B)) as the lower loading catalysts.…”

“…The O K-edge feature in the energy range between 528 and 534 eV are determined by peculiarities of the V-O bonding while resonances at photon energy above approximately 534 eV reflect mainly the O 1s excitations of Si-O-Si bonds, i.e. of the support material SBA-15 as explained in great detail in the literature [26]. The varying intensity ratio of the vanadium NEXAFS relative to the oxygen related NEXAFS in Figure 2(A) reflects directly the different V loading of the catalysts.…”

“…Hence it can be concluded that the geometric arrangement of V species in highly loaded catalysts is not the same as in V 2 O 5 but possesses only some specific features of the V 2 O 5 bulk structure. Based on this observation it seems reasonable to assign some three-dimensional character to V species with a metal loading equal and above 12V while at V loadings of 8 wt-% and below this character is missing although V-O-V bonds (2-dimensional vanadium oxide polymers) are present [10,26].…”

Site-specific bonding of copper adatoms to pyridine end groups mediating the formation of two-dimensional coordination networks on metal surfaces

“…At the core-excited carbon or nitrogen centers IGLO-III type ([7s6p2d]) basis sets [29] are used in order to adequately describe inner-shell relaxation effects while the other carbon/nitrogen centers in the clusters are accounted for by effective core potentials (ECPs) for the 1s shell and [3s3p1d] valence basis sets to avoid 1s core-orbital mixing [30,31] (see also Ref. [32]). …”