Elucidation of the molecular structure of hydrated vanadium oxide species by X-ray absorption spectroscopy: correlation between the V...V coordination number and distance and the point of zero charge of the support oxide

The effect of the point of zero charge (PZC) of the support oxide (Al(2)O(3), Nb(2)O(5), SiO(2) and ZrO(2)) on the molecular structure of hydrated vanadium oxide species has been investigated with EXAFS spectroscopy for low-loaded vanadium oxide catalysts. It was found that the degree of clustering (i.e., the V[dot dot dot]V coordination number) and the V...V distance increase with decreasing PZC of the support oxide; i.e., Al(2)O(3) (8.7) < ZrO(2) (7) < Nb(2)O(5) (3.3) < SiO(2) (2). Upon hydration the silica-supported vanadium oxide exhibited a clear alteration in the position of the oxygen atoms surrounding the central vanadium atom and the number of oxygen atoms around vanadium increased to five. In contrast, only minor changes in the molecular structure were detected for the alumina-, niobia- and zirconia-supported vanadium oxide catalysts. Based on a detailed analysis of the EXAFS data a semi-quantitative distribution of vanadium oxide species present on the surface of the different support oxides can be obtained, which is in good agreement with earlier characterization studies primarily making use of Raman spectroscopy.     

Molecular structure of a supported VO4 cluster and its interfacial geometry as a function of the SiO2, Nb2O5, and ZrO2 support

The influence of the support oxide on the molecular structure of a VO(4) cluster and its interfacial geometry has been determined for SiO(2), Nb(2)O(5), and ZrO(2) 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 V=O and three V-O bonds. For a VO(4)/SiO(2) catalyst it has been established that only one O neighbor is shared with the SiO(2) support via a V-O(b)-Si(support) bond with an angle of approximately 101 degrees (+/-0.5 degrees ) and a V...Si distance of 2.61 A. 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 Cerius(2) software lead to structural constraints, which imply a similar V-O(b)-M(support) interaction for Nb(2)O(5) and ZrO(2) as well. The V-O(b) and the V...M(support) distances depend on the geometry of each support surface. The results show that the classical model with three V-O(b)-M(support) 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 O=V(OH)(2)-O(b)-M moiety at the support surface.