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

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Abstract
Vanadium oxide (1 wt %) supported on γ-Al2O3 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 VO4 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 VO4 entity (1 VO 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 Al2O3 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 VO4 entity could be clarified, leading to a new molecular structure of the interface between VO4 and Al2O3. It was found that VO4 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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