THE MOLECULAR-STRUCTURES AND REACTIVITY OF V2O5/TIO2/SIO2 CATALYSTS

A series of V2O5/TiO2/SiO2 catalysts were structurally investigated by in situ Raman spectroscopy and chemically probed by methanol oxidation in order to determine the molecular structure-reactivity relationships of the V2O5/TiO2/SiO2 catalysts. Only surface TiOx species are present on the 3% TiO2/SiO2 catalysts, and the surface TiOx species as well as bulk TiO2 (anatase) particles coexist on the 40% TiO2/SiO2 catalyst. The deposition of 1-3% vanadium oxide onto 3% TiO2/SiO2 and 4% vanadium oxide onto 40% TiO2/SiO2 forms only a surface vanadium oxide phase. In situ Raman studies reveal that the surface vanadium oxide species preferentially exist on the titania sites of the TiO2/SiO2 system. The interaction between the surface vanadia and the surface titania overlayer on SiO2 increases the methanol oxidation reactivity by two orders of magnitude relative to V2O5/SiO2. In the presence of bulk TiO2 (anatase) particles on the SiO2 support, the reactivity of the surface vanadia further increases by an order magnitude relative to the catalysts containing only surface titania, and is close to that of surface vanadia on bulk TiO2. This suggests that the surface VOx-TiO2 (bulk) interactions results in a more active site than the surface VOx-TiOx-SiO2 interactions. In addition, the V2O5/TiO2/SiO2 catalysts exhibit high selectivity towards HCHO because redox sites are predominant on the surface of these catalysts with essentially no acid site present.