Structural characteristics and reactivity properties of highly dispersed Al2O3/SiO2 and V2O5/Al2O3/SiO2 catalysts

Highly dispersed Al2O3/SiO2 and V2O5/Al2O3/SiO2 catalysts under various con ditions (e.g., hydration, dehydration, and methanol chemisorption) were inv estigated by in situ Raman and UV-vis-NIR diffuse reflectance spectroscopie s. Temperature-programmed reduction and methanol oxidation were employed as chemical probe reactions to examine the reducibility and reactivity proper ties of these catalysts. The spectroscopic results revealed that the surfac e vanadium oxide species on the Al2O3/SiO2 supports are predominantly isola ted VO4 units [O=V(O-Support)(3)] in the dehydrated state. The surface vana dium oxide species preferentially interact with the aluminum oxide species on the silica surface. Consequently, the reduction behavior of the surface vanadium oxide species is closer to that of V2O5/Al2O3 at higher alumina lo ading. Furthermore, the turnover frequency of the surface VO4 species on Al 2O3/SiO2 for methanol oxidation to redox products (formaldehyde, methyl for mate, and dimethoxy methane) increases by an order of magnitude relative to the V2O5/SiO2 catalysts and is comparable to that of the V2O5/Al2O3 cataly sts. It is concluded that the substitution of the Si(IV)-O- by the less ele ctronegative Al(III)-O(-)ligands for the isolated VO4 units is responsible for the enhanced reactivity of the surface V cations. (C) 2000 Academic Pre ss.