(688c) Origin of the Support Effect for Methanol Oxidation for Supported Vanadium Oxide Catalysts

The selective oxidation of methanol was investigated over monolayer supported vanadium oxide catalysts as a function of the specific oxide support (CeO₂, ZrO₂, TiO₂, Al₂O₃, and SiO₂). The preferential formation of HCHO reflects the redox nature of the catalytic surface vanadia sites. The methanol oxidation reaction rate was found to vary by a factor of 10³ as the support was varied (CeO₂ > ZrO₂ > TiO₂ > Al₂O₃ > SiO₂). The combination of CH₃OH-Temperature Programmed Surface Reaction (TPSR) spectroscopy and steady-state methanol oxidation studies allowed for quantitative determination of the adsorption thermodynamic (K_ads, Î?H_ads, and A_ads) and surface reaction kinetic (k_rds and E_act) parameters to better understand the origin of the support effect. For the non-silica catalysts, the TOF was found to be primarily dependent on the CH₃OH equilibrium adsorption constant (K_ads) involving O-H bond breaking. For the supported VO_x/SiO₂ catalyst, the TOF was affected by an increase in E_a for C-H bond scission of the surface methoxy as well as a decrease in K_ads. The TOF, k_rds and K_ads methanol oxidation parameters generally decreased with increasing electronegativity of the oxide support cation. These findings demonstrate that, with the exception of the SiO₂ support, that it is the adsorption step and not the surface reaction step that is responsible for the orders of magnitude in the reaction rate variation of methanol oxidation reaction rate by the different oxide support ligands.