Structural Characteristics and Catalytic Properties of Highly Dispersed ZrO2/SiO2 and V2O5/ZrO2/SiO2 Catalysts

Abstract

Highly dispersed ZrO2/SiO2 and V2O5/ZrO2/SiO2 catalysts were successfully synthesized by the incipient wetness impregnation method. The surface structures of these catalysts in hydrated and dehydrated states were characterized by in situ Raman and UV−vis−near-infrared diffuse reflectance spectroscopies. Temperature-programmed reduction and methanol oxidation were employed as chemical probe reactions to examine the reducibility and reactivity/selectivity properties of these catalysts. These characterization techniques demonstrate that both zirconium oxide and vanadium oxide species are highly dispersed as two-dimensional metal oxide overlayers on the silica support. The spectroscopic results revealed that the surface vanadium oxide species on the highly dispersed ZrO2/SiO2 supports are predominantly isolated VO4 units [OV(O−support)3] in the dehydrated state and become polymerized vanadium oxide species upon hydration. The surface vanadium oxide species preferentially interact with the zirconium oxide species on the silica surface. The substitution of SiIV−O- by ZrIV−O- ligands significantly affects the chemical properties of the isolated VO4 units: the reducibility of the surface vanadium oxide species increases, and the methanol oxidation turnover frequency (TOF) increases by 1−2 orders of magnitude relative to V2O5/SiO2. The present study demonstrates that the support effect, variation in the reactivity of supported metal oxide catalysts due to different oxide supports, essentially originates from the difference in oxygenated ligands around the active metal cations.