High-temperature shock tube study of the reactions CH3 + OH → products and CH3OH + Ar → products

Shock Tube Measurement of the High-Temperature Rate Constant for OH + CH3 → Products

The reaction between hydroxyl (OH) and methyl radicals (CH3) is critical to hydrocarbon oxidation. Motivated by the sparseness of its high-temperature rate constant data and the large uncertainties in the existing literature values, the current study has remeasured the overall rate constant of the OH + CH3 reaction and extended the measurement temperature range to 1214-1933 K, using simultaneous laser absorption diagnostics for OH and CH3 radicals behind incident and reflected shock waves. tert-Butyl hydroperoxide and azomethane were used as pyrolytic sources for the OH and CH3 radicals, respectively. The current study bridged the temperature ranges of existing experimental data, and good agreement is seen between the current measurement and some previous experimental and theoretical high-temperature studies. A recommendation for the rate constant expression of the title reaction, based on the weighted average of the high-temperature data from selected studies, is given by k1 = 4.19 × 10(1)(T/K)(3.15) exp(5270 K/T) cm(3) mol(-1) s(-1) ±30%, which is valid over 1000-2500 K.

High-Temperature Measurements of the Rate Constants for Reactions of OH with a Series of Large Normal Alkanes: n-Pentane, n-Heptane, and n-Nonane

Rate constants for the overall reactions of OH with n-pentane, n-heptane, and n-nonane were measured in shock tube experiments behind reflected shock waves. Narrow-linewidth laser absorption by OH at 306.7 nm was used in pseudo first-order experiments with temperatures between 869 to 1364 K. tert-Butyl hydroperoxide (TBHP) was used as the OH precursor. Experiments were also performed to study the kinetics of the TBHP decomposition and resulting product chemistry, and an accurate mechanism describing OH precursor chemistry effects was developed to model OH concentration time-history in the n-alkane + OH experiments. The experimental results for the n-alkane + OH rate constant measurements can be expressed as rate constants in Arrhenius form as

k n-pentane + OH = 2.10 × 10-10 exp(-2038/T[K]) (869–1364 K),

k n-heptane + OH = 2.43 × 10-10 exp(-1804/T[K]) (869–1364 K),

k n-nonane + OH = 3.17 × 10-10 exp(-1801/T[K]) (884–1352 K),

each in units of cm3 molecule-1 s-1. The present rate constants measured for OH with n-pentane and n-heptane show agreement within 20% with recent work by Sivaramakrishnan and Michael [J. Phys. Chem. A, 113 (2009) 5047]. The measurements of the rate constant for n-nonane + OH presented here represent the first in the literature to depict the temperature dependence of the rate constant above 800 K. The measurements of each n-alkane + OH rate constant studied were compared with two models in the literature used to estimate the rate constants of n-alkane + OH reactions. The Structure-Activity Relationship of Kwok and Atkinson [Atmos. Environ., 29 (1995) 1685] shows the best agreement with the current data for all three n-alkanes over the entire temperature range studied, demonstrating that this model is capable of predicting the overall rate constants for reactions of OH with n-pentane, n-heptane, and n-nonane for temperatures up to 1364 K.