DIRECT RATE-CONSTANT MEASUREMENTS FOR H+CH4-]CH3+H-2, 897-1729-K, USING THE FLASH-PHOTOLYSIS SHOCK-TUBE TECHNIQUE

A kinetic study of the reaction of hydrogen atoms with methane was performed using the Brookhaven flash photolysis-shock tube (FP-ST) facility. Experiments were conducted in the reflected shock regime over the temperature range 897-1729 K under pseudo-first-order conditions ([CH4]/[H] greater-than-or-equal-to 200). Hydrogen atoms were monitored by using atomic resonance absorption and rate constants were derived directly from decays of absorbance signals. Separate experiments, in which the initial H-atom and radical concentrations were varied, demonstrated the absence of kinetic complications due to secondary reactions. This observations was further verified by the results of a computer modeling study. The rate constant values obtained in this research may be fitted with equal uncertainty to either a two-parameter expression, k1(T) = (1.78 +/- 0.12) x 10(-10) exp[-(6440 +/- 80) K/T)], or three-parameter expression, k1(T) = 1.6 x 10(-19)T2.57 +/- 0.76 exp[-(3340 +/- 920) K/T]. Units are cm3 molecule-1 s-1 with uncertainties quoted at the one standard deviation level. The preexponential term for the three-parameter expression and its uncertainty are given in logarithmic form: In (A) = -43.3 +/- 6.2. The mean deviation of the data from either expression is about +/- 10% over the entire temperature range. Rate constants for the reverse reaction, k-1(T), were computed from our measured k1(T) values, using the equilibrium constant for reaction 1, at each experimental temperature. These data were also fitted to both two-parameter and three-parameter expressions with equal uncertainty. Combining the present FP-ST data with those obtained in flow tube experiments (by Kurylo, Hollinder, and Timmons), expressions for k1(T) and k1(T) are evaluated for the temperature span 400 K less-than-or-equal-to T less-than-or-equal-to 1800 K: k1(T) = 6.4 x 10(-18)T2.11 +/- 0.18 exp[-(3900 +/- 140) K/T] cm3 molecule-1 s-1 and k-1(T) = 6.6 x 10(-20)T2.24 +/- .18 exp[-(3220 +/- 140) K/T] cm3 molecule-1 s-1. The data of this study are compared with previous experimental and theoretical results. For example, good agreement is found between the data reported here for k1(T) and the calculated values reported by Truhlar and co-workers; but there is a discrepancy between the present results and the values recommended by Warnatz