Thermal decomposition of oxoperoxooxalato complexes of vanadium /V/

Joniaková, D.; Schwendt, Peter

doi:10.1016/0040-6031(85)85974-8

Cited by 7 publications

(5 citation statements)

References 4 publications

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“…The assignment to the O−O mode was initially proposed by Gijzeman et al and later elaborated by a DFT calculation where the two oxygen atoms were described as a chemisorbed oxygen molecule to the V center. However, the broadband at around 920 cm −1 is clearly observed for our high-temperature (above 673 K) dehydrated VO x /SiO 2 samples and so is not likely due to O−O vibration because solid-phase vanadium peroxo compounds are thermally unstable and dissociate between 473 and 573 K . So this assignment can be also discarded for a highly dehydrated VO x system.…”

Section: Discussionmentioning

confidence: 82%

Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts

2009

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The molecular structure of silica-supported vanadium oxide (VO x ) catalysts over wide range of surface VO x density (0.0002−8 V/nm2) has been investigated in detail under dehydrated conditions by in situ multiwavelength Raman spectroscopy (laser excitations at 244, 325, 442, 532, and 633 nm) and in situ UV−vis diffuse reflectance spectroscopy. Resonance Raman scattering is clearly observed using 244 and 325 nm excitations, whereas normal Raman scattering occurs using excitation at the three visible wavelengths. The observation of strong fundamentals, overtones, and combinational bands due to selective resonance enhancement effect helps clarify assignments of some of the VO x Raman bands (920, 1032, and 1060 cm−1) whose assignments have been controversial. The resonance Raman spectra of dehydrated VO x /SiO2 show a VO band at a smaller Raman shift than that in visible Raman spectra, an indication of the presence of two different surface VO x species on dehydrated SiO2 even at submonolayer VO x loading. Quantitative estimation shows that the two different monomeric VO x species coexist on silica surface from very low VO x loadings and transform to crystalline V2O5 at VO x loadings above the monolayer. It is postulated that one of the two monomeric VO x species has pyramidal structure and the other is in the partially hydroxylated pyramidal mode. The two VO x species show similar reduction−oxidation behavior and may both participate in redox reactions catalyzed by VO x /SiO2 catalysts. This study demonstrates the advantages of multiwavelength Raman spectroscopy over conventional single-wavelength Raman spectroscopy in structural characterization of supported metal-oxide catalysts.

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Section: Discussionmentioning

confidence: 82%

Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts

2009

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“…9. While there are indications from experiment [52] that vanadium peroxo compounds may be thermally unstable under equilibrium conditions they could well exist under constant oxygen pressure at 400° C and contribute additional intensity to the high energy tail of the experimental peak. The VSi 7 O 15 H 11 cluster simulating the hydroxyl variant (SiO)-V=O(OH) 2 of the umbrella structure, see Fig.…”

Section: Resultsmentioning

confidence: 99%

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

Cavalleri

Hermann

Knop‐Gericke

et al. 2009

Journal of Catalysis

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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.

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“…An argument pro this assignment is that vibrations of vanadium peroxo species in solutions and solids are generally known to be located in the 900 cm -1 region, as is shown in the spectrum of VO(O-O) + (Figure 12a). However, one has to be aware that vanadium peroxo species are thermally unstable and crystalline vanadium peroxo species dissociate between 473 and 573 K. 63 To verify this optional assignment, the stability of an impregnated vanadium peroxo species on Al 2 O 3 was checked and the typical peroxo vibration (ν(O-O)) at 875 cm -1 disappeared after drying at 120°C for one night. One could argue, however, that such peroxo species would be still present, although with altered vibrational frequencies due to the anchoring to the support.…”

Section: A V-(o-o) Vibrationmentioning

confidence: 99%

ΛO₄ Upside Down: A New Molecular Structure for Supported VO₄ Catalysts

et al. 2005

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Vanadium oxide (1 wt %) supported on γ-Al 2 O 3 was used to investigate the interface between the catalytically active species and the support oxide. Raman, UV-vis-NIR DRS, ESR, XANES, and EXAFS were used to characterize the sample in great detail. All techniques showed that an isolated VO 4 species was present at the catalyst surface, which implies that no V-O-V moiety is present. Surprisingly, a Raman band was present at 900 cm -1 , which is commonly assigned to a V-O-V vibration. This observation contradicts the current literature assignment. To further elucidate on potential other Raman assignments, the exact molecular structure of the VO 4 entity (1 VdO bond of 1.58 Å and 3 V-O bonds of 1.72 Å) together with its position relative to the support O anions and Al cation of the Al 2 O 3 support has been investigated with EXAFS. In combination with a structural model of the alumina surface, the arrangement of the support atoms in the proximity of the VO 4 entity could be clarified, leading to a new molecular structure of the interface between VO 4 and Al 2 O 3 . It was found that VO 4 is anchored to the support oxide surface, with only one V-O support bond instead of three, which is commonly accepted in the literature. The structural model suggested in this paper leaves three possible assignments for the 900 cm -1 band: a V-O-Al vibration, a V-O-H vibration, and a V-(O-O) vibration. The pros and cons of these different options will be discussed.

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Thermal decomposition of oxoperoxooxalato complexes of vanadium /V/

Cited by 7 publications

References 4 publications

Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts

Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts

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

ΛO₄ Upside Down: A New Molecular Structure for Supported VO₄ Catalysts

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Thermal decomposition of oxoperoxooxalato complexes of vanadium /V/

Cited by 7 publications

References 4 publications

Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts

Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts

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

ΛO4 Upside Down: A New Molecular Structure for Supported VO4 Catalysts

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ΛO₄ Upside Down: A New Molecular Structure for Supported VO₄ Catalysts