Submitted for publication in Physical Chemistry Chemical Physics: 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 (Al2O3,Nb2O5, SiO2 and ZrO2) on the molecular structure of hydrated vanadium oxide specieshas been investigated with EXAFS spectroscopy for low-loaded vanadium oxidecatalysts. It was found that the degree of clustering (i.e., the V---V coordinationnumber) and the V---V distance increase with decreasing PZC of the support oxide;i.e., Al2O3 (8.7) < ZrO2 (7) < Nb2O5 (3.3) < SiO2 (2). Upon hydration the silica-supported vanadium oxide exhibited a clear alteration in the position of the oxygenatoms surrounding the central vanadium atom and thenumber of oxygen atoms aroundvanadium increased to five. In contrast, only minor changes in the molecular structurewere detected for the alumina-, niobiaand zirconia-supported vanadium oxidecatalysts. Based on a detailed analysis of the EXAFS data a semi-quantitativedistribution of vanadium oxide species present on the surface of the different supportoxides can be obtained, which is in good agreement with earlier characterizationstudies making primarily use of Raman spectroscopy.