The origin of the support effect in supported metal oxide catalysts: in situ infrared and kinetic studies during methanol oxidation

The strong influence of the oxide support upon the turn-over frequency (TOF) of methanol oxidation over supported metal oxide catalysts has been well documented in recent years. However, the mechanistic origins (adsorption equilibrium of methanol to methoxy species, rate-determining methoxy surface decomposition, or product desorption equilibrium) of this interesting phenomenon are not completely understood. In order to obtain additional insight, the steady-state surface concentrations of adsorbed methoxy intermediates on monolayer catalysts (similar to 8 V atoms nm(-2)) of V(2)O(5)/(TiO(2), CeO(2), Al(2)O(3) and ZrO(2)) have been quantified with in situ transmission infrared spectroscopy/mass spectrometry. Calculations of the adsorption equilibrium constant, K(ads), show a similar to six times increase for vanadia on oxide supports of Al < Ti < Zr < Ce, whereas the methoxy surface decomposition rate constant, k(rds) (rds = rate-determining step), shows a similar to 22 times increase in value over these same catalysts. Thus, changes in both the adsorption equilibrium and the methoxy decomposition properties of supported metal oxide catalysts appear to be responsible for the support effect, although the methoxy decomposition is clearly the reaction step that is more sensitive to the specific metal oxide support. More fundamentally, the support effect appears to correlate with the electronegativity of the support cation, which is proposed to have influence upon the rate of methoxy decomposition through hydride abstraction and on the steady-state equilibrium adsorption capacity of methanol to surface methoxy intermediates through the basicity of the bridging V-O-support bond (the adsorption site). (C) 1999 Elsevier Science B.V. All rights reserved.