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Molecular Structure of a Supported VO 4 Cluster and Its Interfacial Geometry as a Function of the SiO 2 , Nb 2 O 5 , and ZrO 2 Support

2006; American Chemical Society; Volume: 110; Issue: 29 Linguagem: Inglês

10.1021/jp060749h

1520-6106

Daphne E. Keller, D.C. Koningsberger, Bert M. Weckhuysen,

Transition Metal Oxide Nanomaterials

The influence of the support oxide on the molecular structure of a VO4 cluster and its interfacial geometry has been determined for SiO2, Nb2O5, and ZrO2 as supports. Raman, IR, UV−vis−NIR diffuse reflectance, electron spin resonance, and extended X-ray absorption fine structure (EXAFS) spectroscopies were used to characterize the supported vanadium oxide clusters after dehydration. It has been found that for all supports under investigation the vanadium ion is tetrahedral coordinated and consists of one VO and three V−O bonds. For a VO4/SiO2 catalyst it has been established that only one O neighbor is shared with the SiO2 support via a V−Ob−Sisupport bond with an angle of ∼101° (±0.5°) and a V···Si distance of 2.61 Å. The absence of a second vanadium atom in the vicinity of the vanadium oxide cluster further subverts the classical assignment of the 920 cm-1 Raman band to a V−O−V related vibration. The EXAFS results combined with structural modeling using Cerius2 software lead to structural constraints, which imply a similar V−Ob−Msupport interaction for Nb2O5 and ZrO2 as well. The V−Ob and the V···Msupport distances depend on the geometry of each support surface. The results show that the classical model with three V−Ob−Msupport bonds could not be experimentally observed with EXAFS under the applied measuring conditions. Additional experiments with IR and Raman spectroscopy under experimental conditions mimicking those of the EXAFS measurements reveal the presence of V−OH groups, giving further support for the presence of a OV(OH)2−Ob−M moiety at the support surface.