In Situ Spectroscopic Investigation of Molecular Structures of Highly Dispersed Vanadium Oxide on Silica under Various Conditions

Abstract

The molecularly dispersed V2O5/SiO2 supported oxides were prepared by the incipient wetness impregnation of 2-propanol solutions of V-isopropoxide. The experimental maximum dispersion of surface vanadium oxide species on SiO2 was achieved at ~12 wt % V2O5 ( ~2.6 V atoms/nm^2 ). The surface structures of the molecularly dispersed V2O5/SiO2 samples under various conditions were extensively investigated by in situ Raman, UV- vis-NIR DRS and XANES spectroscopies. The combined characterization techniques revealed that in the dehydrated state only isolated VO4 species are present on the silica surface up to monolayer coverage. Interestingly, the three-member siloxane rings on the silica surface appear to be the most favorable sites 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 species is dependent on the degree of hydration. The molecular structure of the fully hydrated vanadium oxide species closely resembles V2O5·nH2O gels, rather than V2O5 crystallites. The fully hydrated surface vanadium oxide species are proposed to be chain and/or two-dimensional polymers with highly distorted square-pyramidal VO5 connected by V-OH-V bridges, which are stabilized on the silica surface by the sixth neighbor of Si-OH hydroxyls via Si-OH···V hydrogen bonds. In analogy to the hydration process, alcoholysis occurs during methanol chemisorption, and similar molecular structures are proposed to interpret the interaction between methanol molecules and the surface vanadium oxide species on silica.