Abstract
The catalytic performance of V2O5-WO3 catalysts in the oxidation of toluene was investigated and correlated with features shown by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and chemical analysis of vanadium valence states. At low vanadium concentrations high selectivity and activity was obtained and these catalysts consisted of highly dispersed V2O5 supported on WO3. V4+ formation upon use was highest in this region. Increased vanadium concentrations resulted in decreased dispersion with formation of less active and less reducible crystallites, and both activity and selectivity decreased towards values for V2O5. Dissolution of W6+, increasing with WO3 concentration, affects crystallization of V2O5. The (010) to (101) ratio, from XRD data, increases at the lowest addition levels, but decreases strongly at higher levels. Activity and selectivity are not simply correlated with this ratio, but overall are inversely correlated with it. Activity and selectivity correlate with the increase in the V4+-to-ΣV ratio with use in the reaction, indicating the importance of reduced phases and surface defects.
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