Shock tube measurements of methyl radical+molecular oxygen kinetics and the heat of formation of the hydroxyl radical.

摘要

The reaction of methyl radicals (CH3) with molecular oxygen (O2), a rate-controlling process in the ignition of natural gas under radical-poor conditions, has been investigated in high-temperature shock tube experiments. The overall rate coefficient, k1 = k1a + k1b, and individual rate coefficients for the two high-temperature product channels, (1a) producing CH3O+O and (1b) producing CH 2O+OH, were determined using ultra-lean mixtures of CH3I and O2 in Ar/He. Narrow-linewidth UV laser absorption at 306.7 nm was used to measure OH concentrations, for which the normalized rise time is sensitive to the overall rate coefficient k1 but relatively insensitive to the branching ratio and to secondary reactions. Atomic resonance absorption spectroscopy measurements of O-atoms were used for a direct measurement of channel (1a). Through the combination of two different diagnostics, rate coefficient expressions for both channels were determined. Over the temperature range 1590 to 2430 K, k1a = 6.08 x 107T 1.54exp(-14005/T) cm3mol-1s -1 and k1b = 68.6 T2.86exp(-4916/T) cm3mol-1s-1. The overall rate coefficient is in close agreement with a recent ab initio calculation and one other shock tube study, while comparison of k1a and k1b to these and other experimental studies yields mixed results.The standard heat of formation of OH at 298 K, Deltaf H°298(OH), a fundamental thermochemical parameter influencing the equilibrium constants of many combustion and atmospheric chemical reactions, has been determined from shock tube measurements spanning the temperature range 1964 to 2718 K and at pressures of 1 to 2.4 atm. Low-concentration, lean and stoichiometric mixtures of H2 and O2 in Ar produced well-controlled levels of OH in a "partial equilibrium" state, in which the OH concentration was dependent only on the thermochemical parameters of the reacting species. Laser absorption was used to measure OH concentrations with sufficient accuracy (2--4%) to clearly determine the value of the heat of formation. Over the range of experimental conditions, the average determination is DeltafH° 298(OH) = 8.92 +/- 0.16 kcal/mol with a standard deviation sigma = 0.04 kcal/mol. This value is 0.40 to 0.48 kcal/mol below the previously accepted values, and agrees with recent theoretical calculations, experimental studies using the positive-ion cycle, and calculations using thermochemical cycles.