SHOCK-TUBE KINETIC-STUDY OF METHANE DISSOCIATION - 1726-K LESS-THAN-OR-EQUAL-TO-T-LESS-THAN-OR-EQUAL-TO 2134-K

In this kinetic study of the thermal dissociation of methane in argon, primary product H atoms were monitored directly using the sensitive ARAS detection method. Absolute H atom concentration was determined in separate calibration experiments using N2O/H2 mixtures diluted in argon. Rate constants were obtained under or near low-pressure limit conditions in shock tube experiments performed in the reflected regime. The CH4 mole fraction was varied between 100 and 1300 ppm, and conversions of CH4 were typically 0.2% or less. The temperature ranged from 1726 to 2134 K. Rate data were analyzed at T less-than-or-equal-to 1900 K using a steady-state approximation, and at higher temperatures, rate constants were derived from simulations of [H](t) profiles. The results of 37 experimental determinations were fit to the following Arrhenius expression: k-degrees-diss(T) = 6.8 x 10(-8) exp(-44900K/T) cm3 molecule-1 S-1. The overall uncertainty of values derived from this expression is estimated to be about +/- 50%. RRKM and master equation calculations were performed to evaluate the fall-off behavior of the unimolecular rate constant and to allow comparison of the present results with those of previous studies of methane dissociation. A large discrepancy is noted, and some reasons for this disagreement are discussed.