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

V-Sn-O mixed oxides have been prepared by reaction between tin oxohydrate and vanadium pentoxide at 700 degrees 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 VxSn1-xO2 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 degrees 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.