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 VV species, likely to be in tetrahedral coordination. In addition, VIV dispersed in the cassiterite matrix led to the formation of a VxSn1 –xO2 solid solution. When the V content is 10 atom%, additional formation of bulk V2O5 has been observed. The VIV 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 VV oxide is necessary in order to achieve good selectivity, since the VIV-doped SnO2 was not selective to phthalic anhydride. In contrast, the activity was proportional to the overall vanadium content.