Kinetics and Reaction Pathways of Methanol Oxidation on Platinum

Article abstract of DOI:10.1021/j100398a043

Methanol oxidation kinetics were measured on Pt wires in a flow reactor at pressures between 30 and 130 Pa.The kinetics were measured as a function of oxygen-to-methanol equialence ratio Φ and wire temperature.In methanol-lean feeds (Φ < 1), H2CO, CO2, and H2O were the only products; in methanol-rich feeds (Φ > 1) CO, H2, H2CO, CO2, and H2O were observed.Experiments with 18O2 showed that the principal mathanol oxidation pathway does not involve C-O bond dissociation.However, the 18O2 experiments, together with other features of the methanol oxidation data, also provided evidence for a minor oxidation pathway (accounting for less than 1 percent of the product CO2) which proceeds through a carbon intermediate.Interpretation of the CH3OH oxidation data was aided by comparison with data obtained previously for H2CO and CO oxidation (McCabe, R.W.; McCready, D.F.Chem.Phys.Lett. 1984. 111. 89).In particular, similarities between CH3OH oxidation and H2CO and CO oxidation indicate that the principal oxidation pathways for CH3OH, H2CO, and CO are kinetically homologous , i.e. they share the common feature of an adsorbed CO intermediate.A mathematical model is presented which describes the principal CH3OH oxidation pathway as a series reaction involving adsorbed H2CO and CO intermediates.Formaldehyde may not be the only adsorbed H2CO intermediate.Consistent with experimental results, the model predicts that inhibition by adsorbed CO should be weaker for CH3OH and H2CO oxidation than for CO oxidation.The model leads to a simplified, generalized expression for the rates of CH3OH, H2CO, and CO oxidation to CO2, thus reflecting the homologous nature of these reactions.The main effect of the minor pathway is to decrease the rate of the major pathway at Φ > 0.1.Additionally, the minor pathway accounts for certain features of methanol oxidation, including rate instabilities at certain values of Φ and deactivation of Pt wires in pure methanol, observed both in the present study and in preious studies in other laboratories.