Structural characteristics and reactivity/reducibility properties of dispersed and bilayered V2O5/TiO2/SiO2 catalysts

Dispersed and bilayered V2O5/TiO2/SiO2 catalysts were successfully synthesized by the incipient wetness impregnation method, and their surface structures under various conditions were extensively investigated by combined in situ Raman, UV-vis-NLR diffuse reflectance and X-ray absorption near-edge spectroscopies, as well as X-ray photoelectron spectroscopy. Temperature-programmed reduction and methanol oxidation were employed as chemical probe reactions to examine the reducibility and reactivity/selectivity of these catalysts. The spectroscopic results revealed that both vanadium oxide and titanium oxide on SiO2 are dispersed as two-dimensional metal oxide overlayers. The surface vanadium oxide species on the dispersed TiO2/SiO2 are predominantly isolated VO4 units [O=V(O-support)(3)] in the dehydrated state and become polymerized VO5/VO6 units upon hydration. The surface vanadium oxide species preferentially interact with the surface titanium oxide species on silica, and the V(V) cations possess oxygenated ligands of Si(IV)-O- and Ti(IV)-O- in the O=V(O-support)(3) unit with varying ratios from 3:0 to 0:3, which depends on the vanadia and titania loadings. The varying ratio of the Si(IV)-O- and Ti(IV)-O- oxygenated ligands significantly affects the chemical properties of the isolated VO4 units. Consequently, the reducibility of the surface vanadium oxide species is altered and the reduction occurs over a wider temperature range. In addition, the methanol oxidation turn-over frequency of the surface VO4 species on TiO2/SiO2 increases an order of magnitude relative to V2O5/SiO2 Thus, the oxygenated ligands around the V cations play a critical role in determining the reactivity of the surface vanadium oxide species on the oxide supports.