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Title: Experimental and theoretical rate constants for CH{sub 4} + O{sub 2} {yields} CH{sub 3} + HO{sub 2}

Abstract

In this study, rate constants for the primary initiation process in low to moderate temperature CH{sub 4} oxidation CH{sub 4} + O{sub 2} {yields} CH{sub 3} + HO{sub 2} have been measured in a reflected shock tube apparatus between 1655 and 1822 K using multipass absorption spectrometric detection of OH radicals at 308 nm. After rapid dissociation of HO{sub 2} yielding H atoms, which are instantaneously converted to OH by H + O{sub 2} {yields} OH + O, the temporal concentration of OH radicals was observed as the final product from the rate-controlling title reaction. The present work utilizes 18 optical passes corresponding to a total path length of 1.6 m. This configuration gives a signal to noise ratio of unity at {proportional_to}3 x 10{sup 12} radicals cm{sup -3}. Hence, kinetics experiments could be performed at conditions of low [CH{sub 4}]{sub 0} (60-70 ppm), thereby substantially reducing secondary chemistry. Possible implications of CH{sub 4} dissociation contributing to the OH formation rates were considered. The present experimental results agree with a priori variational transition state theoretical (VTST) calculations, k{sub th}=3.37 x 10{sup -19}T{sup 2.745} exp (-26,041K/T)cm{sup 3}molecule{sup -1} s{sup -1}, clearly showing overlap of experiment and theory, within experimental error. Themore » new rate constant values obtained in this study are 8-10 times higher than the values used in the popular mechanisms GRI-Mech 3.0 and Leeds Methane Mechanism, version 1.5. (author)« less

Authors:
; ; ;  [1]
  1. Chemistry Division, Argonne National Laboratory, Argonne, IL 60439 (United States)
Publication Date:
OSTI Identifier:
20880642
Resource Type:
Journal Article
Resource Relation:
Journal Name: Combustion and Flame; Journal Volume: 149; Journal Issue: 1-2; Other Information: Elsevier Ltd. All rights reserved
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; METHANE; METHYL RADICALS; HYDROXYL RADICALS; CHEMICAL REACTION KINETICS; VARIATIONAL METHODS; SIGNAL-TO-NOISE RATIO; DISSOCIATION; OXIDATION; ABSORPTION; TEMPERATURE RANGE 1000-4000 K; OXYGEN

Citation Formats

Srinivasan, N.K., Michael, J.V., Harding, L.B., and Klippenstein, S.J. Experimental and theoretical rate constants for CH{sub 4} + O{sub 2} {yields} CH{sub 3} + HO{sub 2}. United States: N. p., 2007. Web. doi:10.1016/J.COMBUSTFLAME.2006.12.010.
Srinivasan, N.K., Michael, J.V., Harding, L.B., & Klippenstein, S.J. Experimental and theoretical rate constants for CH{sub 4} + O{sub 2} {yields} CH{sub 3} + HO{sub 2}. United States. doi:10.1016/J.COMBUSTFLAME.2006.12.010.
Srinivasan, N.K., Michael, J.V., Harding, L.B., and Klippenstein, S.J. 2007. "Experimental and theoretical rate constants for CH{sub 4} + O{sub 2} {yields} CH{sub 3} + HO{sub 2}". United States. doi:10.1016/J.COMBUSTFLAME.2006.12.010.
@article{osti_20880642,
title = {Experimental and theoretical rate constants for CH{sub 4} + O{sub 2} {yields} CH{sub 3} + HO{sub 2}},
author = {Srinivasan, N.K. and Michael, J.V. and Harding, L.B. and Klippenstein, S.J.},
abstractNote = {In this study, rate constants for the primary initiation process in low to moderate temperature CH{sub 4} oxidation CH{sub 4} + O{sub 2} {yields} CH{sub 3} + HO{sub 2} have been measured in a reflected shock tube apparatus between 1655 and 1822 K using multipass absorption spectrometric detection of OH radicals at 308 nm. After rapid dissociation of HO{sub 2} yielding H atoms, which are instantaneously converted to OH by H + O{sub 2} {yields} OH + O, the temporal concentration of OH radicals was observed as the final product from the rate-controlling title reaction. The present work utilizes 18 optical passes corresponding to a total path length of 1.6 m. This configuration gives a signal to noise ratio of unity at {proportional_to}3 x 10{sup 12} radicals cm{sup -3}. Hence, kinetics experiments could be performed at conditions of low [CH{sub 4}]{sub 0} (60-70 ppm), thereby substantially reducing secondary chemistry. Possible implications of CH{sub 4} dissociation contributing to the OH formation rates were considered. The present experimental results agree with a priori variational transition state theoretical (VTST) calculations, k{sub th}=3.37 x 10{sup -19}T{sup 2.745} exp (-26,041K/T)cm{sup 3}molecule{sup -1} s{sup -1}, clearly showing overlap of experiment and theory, within experimental error. The new rate constant values obtained in this study are 8-10 times higher than the values used in the popular mechanisms GRI-Mech 3.0 and Leeds Methane Mechanism, version 1.5. (author)},
doi = {10.1016/J.COMBUSTFLAME.2006.12.010},
journal = {Combustion and Flame},
number = 1-2,
volume = 149,
place = {United States},
year = 2007,
month = 4
}
  • No abstract prepared.
  • The reflected shock tube technique with multipass absorption spectrometric detection of OH radicals at 308 nm, using either 36 or 60 optical passes corresponding to total path lengths of 3.25 or 5.25 m, respectively, has been used to study the bimolecular reactions, OH + CF3H CF3 + H2O (1) and CF3 + H2O OH + CF3H (-1), between 995 and 1663 K. During the course of the study, estimates of rate constants for CF3 + OH products (2) could also be determined. Experiments on reaction -1 were transformed through equilibrium constants to k1, giving the Arrhenius expression k1 = (9.7more » {+-} 2.1) x 10{sup -12} exp(-4398 {+-} 275K/T) cm3 molecule-1 s-1. Over the temperature range, 1318-1663 K, the results for reaction 2 were constant at k2 = (1.5 {+-} 0.4) x 10{sup -11} cm3 molecule-1 s-1. Reactions 1 and -1 were also studied with variational transition state theory (VTST) employing QCISD(T) properties for the transition state. These a priori VTST predictions were in good agreement with the present experimental results but were too low at the lower temperatures of earlier experiments, suggesting that either the barrier height was overestimated by about 1.3 kcal/mol or that the effect of tunneling was greatly underestimated. The present experimental results have been combined with the most accurate earlier studies to derive an evaluation over the extended temperature range of 252-1663 K. The three parameter expression k1 = 2.08 x 10{sup -17} T1.5513 exp(-1848 K/T) cm3 molecule-1 s-1 describes the rate behavior over this temperature range. Alternatively, the expression k1,th = 1.78 x 10{sup -23} T3.406 exp(-837 K/T) cm3 molecule-1 s-1 obtained from empirically adjusted VTST calculations over the 250-2250 K range agrees with the experimental evaluation to within a factor of 1.6. Reaction 2 was also studied with direct CASPT2 variable reaction coordinate transition state theory. The resulting predictions for the capture rate are found to be in good agreement with the mean of the experimental results and can be represented by the expression k2,th = 2.42 x 10{sup -11} T-0.0650 exp(134 K/T) cm3 molecule-1 s-1 over the 200-2500 K temperature range. The products of this reaction are predicted to be CF2O + HF.« less
  • Overall rate constants for the reactions O + C{sub 2}H{sub 2} {yields} products and O + C{sub 2}D{sub 2} {yields} products have been measured between {approximately}850 and 1,950 K by the flash photolysis-shock tube (FP-ST) technique. The branching ratio of the process O + C{sub 2}H{sub 2} {yields} HCCO + H has been estimated by comparison experiments to the reaction O + H {yields} OH + H, under identical conditions of first-order O-atom depletion, photolyte concentration, and flash energy.
  • The flow of a three-component gas mixture over a plane surface is investigated within the framework of laminar boundary-layer theory. Homogeneous reactions are frozen; a heterogeneous recombination reaction takes place on the surface of the plate, and the catalytic properties of the surface suffer a discontinuity. An approximate analytical solution of the problem is obtained, making it possible to determine the flow parameters after the point of discontinuity of the catalytic properties when the flow parameters at the outer boundary of the boundary layer and the distribution of the heat flux density on the inset, which can be determined experimentally,more » are known.« less