Surface structures, reduction pattern and oxygen chemisorption of V2O5/SiO2catalysts

Abstract

The influence of V₂O₅ loading (2–50 wt.%) on surface structures and the reduction pattern of V₂O₅/SiO₂ (VPS) catalysts has been systematically investigated by diffuse reflectance UV–VIS spectroscopy and temperature-programmed reduction (TPR) measurements in the temperature range 200–1100°C. Both TPR and UV–VIS spectra of VPS catalysts indicate that at loadings lower than 10% vanadia forms isolated surface VO₄^3- species, while two-dimensional structures and V₂O₅ crystallites become the prevailing species in highly loaded (>10% V₂O₅) systems. A mathematical modelling approach to the TPR spectra has allowed the influence of the loading on the concentration of the above surface V⁵⁺ species to be highlighted. The easier reduction of isolated VO₄^3- species has been ascribed to the tetrahedral coordination geometry (T_d) of the V ions therein. By contrast, less reducible V₂O₅ crystallites, with V ions in an octahedral coordination (O_h), give rise to several reduction peaks in the range 600–1000°C which monitor a step-wise reduction pattern. The chemisorption properties of VPS catalysts and bulk V₂O₅ have been comparatively probed by high- (HTOC) and low-temperature oxygen chemisorption (LTOC) measurements. The effect of loading on the dispersion of V₂O₅/SiO₂ catalysts has been thoroughly addressed.