COMPARISON OF GRAFTED VANADIA SPECIES ON ZRO2, TIO2, SIO2 AND TIO2/SIO2 MIXED OXIDES

Vanadia layers have been deposited onto several zirconia, titania, silica and titania-silica mixed-oxide supports, using the selective reaction of vanadyl triisopropoxide with surface hydroxy groups of the carrier material. The influence of the support, its crystallinity and its crystallographic modification, on the structure of the immobilized vanadia layers were investigated. Surface vanadia species formed upon single and multiple graftings were characterized by Raman, diffuse reflectance FTIR (DRIFT) and UV-VIS diffuse reflectance spectroscopy. On ZrO2, the rapid formation of multilayer structures was observed, as a consequence of the weak interaction between this support and the dispersed vanadia. Comparing the vanadia grafted on low-surface area crystalline ZrO2 with the one grafted on high-surface-area amorphous ZrO2, we noted that the crystallinity and surface area, which are of lesser importance at low coverage, exert an increasing influence on the vanadia structure at higher loadings. On TiO2, small clusters, two-dimensional layer structures and multilayers are identified. The high-surface-area silica support favours a high dispersion of vanadia in the form of clusters and ribbons of limited lateral extent; the influence of the surface hydration state and of thermally induced structural changes of the vanadia species are investigated. On the TiO2/SiO2 mixed-oxide gel, vanadia species are anchored on TiO2 at low loadings and spread out over the SiO2 fractions of the surface at higher coverages.