In situ spectroscopic investigation of molecular structures of highly dispersed vanadium oxide on silica under various conditions

The molecularly dispersed V2O5/SiO2 supported oxides were prepared by the i ncipient wetness impregnation of 2-propanol solutions of V-isopropoxide. Th e experimental maximum dispersion of surface vanadium oxide species on SiO2 was achieved at similar to 12 wt % V2O5 (similar to 2.6 V atoms/nm(2)). Th e surface structures of the molecularly dispersed V2O5/SiO2 samples under v arious conditions were extensively investigated by in situ Raman, UV-vis-NI R DRS and XANES spectroscopies. The combined characterization techniques re vealed that in the dehydrated state only isolated VO4 species are present o n the silica surface up to monolayer coverage. Interestingly, the three-mem ber siloxane rings on the silica surface appear to be the most favorable si tes for anchoring the isolated, three-legged (SiO)(3)V=O species. Hydration dramatically changes the molecular structure of the surface vanadium oxide species. The specific structure of the hydrated surface vanadium oxide spe cies is dependent on the degree of hydration. The molecular structure of th e fully hydrated vanadium oxide species closely resembles V2O5. nH(2)O gels , rather than V2O5 crystallites. The fully hydrated surface vanadium oxide species are proposed to be chain and/or two-dimensional polymers with highl y distorted square-pyramidal VO5 connected by V-OH-V bridges, which are sta bilized on the silica surface by the sixth neighbor of Si-OH hydroxyls via Si-OH ... V hydrogen bonds. In analogy to the hydration process, alcoholysi s occurs during methanol chemisorption, and similar molecular structures ar e proposed to interpret the interaction between methanol molecules and the surface vanadium oxide species on silica.