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

Authors: 
Wachs, I., Lehigh University
Moskowitz, B., Lehigh University
Kim, T., Stony Brook University
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 (Kads, Î?Hads, and Aads) and surface reaction kinetic (krds and Eact) 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 (Kads) involving O-H bond breaking. For the supported VOx/SiO2 catalyst, the TOF was affected by an increase in Ea for C-H bond scission of the surface methoxy as well as a decrease in Kads. The TOF, krds and Kads 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.