A STUDY OF THE STRUCTURE OF VANADIUM-OXIDE DISPERSED ON ZIRCONIA

Raman and UV-vis diffuse reflectance spectroscopy were used to charact erize the structure of vanadia dispersed on high surface area zirconiu m oxide. Two-dimensional vanadia species with tetrahedral coordination appear on the surface of the ZrO2 and expand in size with increasing V loading. Crystalline V2O5 appears when the vanadia loading exceeds a n apparent surface density of 7.0 V atoms/nm(2), and ZrV2O7 is formed as a consequence of zirconia migration into the V2O5 crystallites. A m odel for the structure of two-dimensional vanadia overlayer is propose d based on the experimental data and information taken from the litera ture. Vanadia is found to absorb the light scattered by the support, a nd this gives rise to a reduction in the intensity of the Raman bands for zirconia as the surface loading of vanadia increases. Partial redu ction of the dispersed vanadia increases the absorbance of the vanadia and alters the profile of absorbance versus frequency in such a manne r as to increase the intensity of the Raman bands for zirconia relativ e to those for vanadia. Examination of Raman spectra taken after repea ted reduction-oxidation cycles suggests that reduction occurs via remo val of oxygen from the vanadia monolayer without agglomeration or reor ganization of the surface species.