The molecular structures and reactivity of V₂O₅/TiO₂/SiO₂ catalysts

Abstract

A series of V₂O₅/TiO₂/SiO₂ catalysts were structurally investigated byin 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 TiO _x species are present on the 3% TiO₂/SiO₂ catalysts, and the surface TiO _x 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 SiO₇ 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 VO _x -TiO₂ (bulk) interactions results in a more active site than the surface VO _x -TiO _x -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.