Kinetics of methanol oxidation over platinum wire catalysts

Abstract

The oxidation of methanol at atmospheric pressure over platinum wire catalysts has been studied over a wide range of experimental conditions (T= 310–660 K, P_O2/P_CH3OH= 2–100). Two experimental techniques were used. In the conventional flow-reactor experiments kinetic parameters were obtained from measurement of gas-phase concentrations, while in the microcalorimeter experiments kinetic parameters were derived from the rates of heat generation at the catalyst surface.

At 373 K, for P_O2/P_CH3OH= 25, a point within the experimental range common to both techniques, the specific rates of oxidation determined by each method were 120 nmol s^–1 cm^–2(conventional flow reactor) and 100 nmol s^–1 cm^–2(microcalorimeter). The use of the microcalorimeter enabled kinetic measurements to be made over the full experimental range. It was found that the observed kinetics were dependent on both temperature and oxygen : methanol ratio. Thus in the general rate expression: R=kP^m_CH3OHPⁿ_O2 the values of m and n were found to vary between 0 and ½(P_O2/P_CH3OH⩽ 5, T < 380 K), through 1 and ½(P_O2/P_CH3OH= 8–15, T= 325–390 K), to 1 and O (T > 405 K, all ratios).

Non-steady-state kinetics were also observed for P_O2/P_CH3OH= 5 at temperatures between 380 and 405 K.

A mechanism, which is consistent with the observations, is proposed involving the interaction of gaseous methanol with adsorbed atomic oxygen to produce adsorbed methoxy species. These species react with further oxygen to produce formaldehyde or carbon dioxide.