Rate constants for the reaction of ground state atomic oxygen with methanol

Abstract

The reaction of O(3 P) with methanol has been studied using the complementary discharge flow and flash photolysis techniques. In both cases, resonance fluorescence detection of atomic oxygen was employed. The discharge flow (DF) apparatus was used in a temperature range of 298–998 K while the flash photolysis (FP) apparatus was used in the overlapping range of 329–527 K. The apparent bimolecular rate constants for the O‐atom/methanol reaction obtained from DF experiments at low temperatures (T?450 K) were independent of both the initial O‐atom concentration and the mode of O‐atom production. In addition, large excesses of O2 were added to the flow to intercept the primary reaction product (CH2OH), but had no apparent effect on the measuredrate constant. Results from the two methods were in good agreement within this limited temperature range (∼300–500 K). At temperatures above ∼450 K, the apparent rate constants obtained from DF experiments were increasingly sensitive to the O2 concentration, with the rate constants being smaller when determined in the presence of large [O2]. Since the initial O‐atom concentrations were on the order of 1011 or less, a simple stoichiometry effect can be ruled out. However, the results of the present kinetic experiments indicated that heterogeneous pyrolysis of CH3OH may have occurred in the flow system. This observation is consistent with studies of the adsorption of methanol on silica surfaces. This problem was apparently overcome by adding small amounts of O2 and the rate constants obtained in this way were seen to agree well with values extrapolated from the lower temperature DF and FP experiments. The rate data from DF and FP experiments were thus combined to obtain the following Arrhenius expression (298–998 K): k 1 (T) = (2.70±0.50)×10−11 exp(−5030±160/R T cm3 molecule−1 s−1. This result is compared with those of previous studies, and kinetic complications (stoichiometry) and heterogeneous effects are further discussed.