SnO2–V2O5-based catalysts. Nature of surface species and their activity in o-xylene oxidation

Abstract

V–Sn–O mixed oxides have been prepared by reaction between tin oxohydrate and vanadium pentoxide at 700 °C. The samples were characterized by means of XPS and UV–VIS diffuse reflectance spectroscopy (DRS). It has been found that different vanadium species formed, the relative amounts of which were a function of the vanadia loading. At a vanadium loading of 4.3 atom%, the SnO₂ surface was covered with a dispersed V^V species, likely to be in tetrahedral coordination. In addition, V^IV dispersed in the cassiterite matrix led to the formation of a V_xSn_{1 –x}O₂ solid solution. When the V content is 10 atom%, additional formation of bulk V₂O₅ has been observed. The V^IV in solid solution modified the electronic properties of the SnO₂. The calcination temperature also affected the distribution of the vanadium species. Temperatures higher than 700 °C favoured the segregation of SnO₂ and V₂O₅. The samples were tested in the oxidation of o-xylene to phthalic anhydride. It has been found that bulk V^V oxide is necessary in order to achieve good selectivity, since the V^IV-doped SnO₂ was not selective to phthalic anhydride. In contrast, the activity was proportional to the overall vanadium content.