Abstract
Highly dispersed Al2O3/SiO2 and V2O5/Al2O3/SiO2 catalysts under various conditions (e.g., hydration, dehydration, and methanol chemisorption) were investigated by in situ Raman and UV–vis-NIR diffuse reflectance spectroscopies. Temperature-programmed reduction and methanol oxidation were employed as chemical probe reactions to examine the reducibility and reactivity properties of these catalysts. The spectroscopic results revealed that the surface vanadium oxide species on the Al2O3/SiO2 supports are predominantly isolated VO4 units [O=V(O–Support)3] in the dehydrated state. The surface vanadium 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 loading. Furthermore, the turnover frequency of the surface VO4 species on Al2O3/SiO2 for methanol oxidation to redox products (formaldehyde, methyl formate, and dimethoxy methane) increases by an order of magnitude relative to the V2O5/SiO2 catalysts and is comparable to that of the V2O5/Al2O3 catalysts. It is concluded that the substitution of the Si(IV)–O− by the less electronegative Al(III)–O− ligands for the isolated VO4 units is responsible for the enhanced reactivity of the surface V cations.
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