Rate Coefficient Measurements of the Reaction CH 3 + O 2 = CH 3 O + O

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               3
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<ul><li><p>Rate Coefficient Measurements of the Reaction CH3 + O2 ) CH3O + O</p><p>S. M. Hwang, Si-Ok Ryu, and K. J. De WittDepartment of Chemical Engineering, The UniVersity of Toledo, Toledo, Ohio 43606</p><p>M. J. Rabinowitz*National Aeronautics and Space Administration, Glenn Research Center at Lewis Field, CleVeland, Ohio 44135</p><p>ReceiVed: March 23, 1999; In Final Form: June 4, 1999</p><p>Rate coefficients for the reaction CH3 + O2 ) CH3O + O were measured behind reflected shock waves ina series of lean CH4-O2-Ar mixtures using hydroxyl and methyl radical diagnostics. The rate coefficientsare well represented by an Arrhenius expression given ask ) (1.60-0.47</p><p>+0.67) 1013 exp(-15813( 587 K/T)cm3 mol-1 s-1. This expression, which is valid in the temperature range 1575-1822 K, supports the downwardtrend in the rate coefficients that has been reported in recent determinations. All measurements to date, includingthe present study, have been to some extent affected by secondary reactions. The complications due to secondaryreactions, choice of thermochemical data, and shock-boundary layer interactions that affect the determinationof the rate coefficients are examined.</p><p>Introduction</p><p>Methyl radical is formed rapidly during the preignition phasein the combustion of nearly all hydrocarbons. It is relativelyunreactive and is consumed only slowly until a large populationof reactive radicals, such as O atoms or hydroxyl radicals, isencountered. In a flame this necessitates transport through spaceto either a hotter or leaner region, and in a shock tube it requires,in essence, transport through time. The radical-radical reactionsthat then ensue are rapid, exothermic, and chain-propagating.An exothermic and chain-propagating reaction is also possiblebetween methyl radical and molecular oxygen. Such a reaction,and indeed any methyl radical-molecular oxygen reaction,would effectively control the flow of reaction during the radical-starved preignition phase. Important characteristics of flamepropagation and ignition behavior are governed by the flow ofreaction in the preignition phase, and so both the rate andproducts of such a reaction would be attractive subjects forspeculation and experimental evaluation. And so they have been.</p><p>Early studies of hydrocarbon oxidation, concerned mainlywith the overall structure of the reaction mechanism, wereperformed using methane as a representative and experimentallyamenable fuel.1-11 The accepted oxidation pathway for methylradical during the preignition phase was the chain-propagatingreaction CH3 + O2 ) CH2O + OH. This is an exothermicreaction (rH298 ) 286.6 kJ mol-1) that must proceed via afour-center multiple bond rearrangement and so should beexpected to proceed slowly. However, the reported rate coef-ficient values were unexpectedly high (and much higher thanare currently accepted). There were good reasons for this. Theearly researchers did not yet recognize the sweeping complexityof the methane oxidation mechanism; their mechanisms con-tained only 16-23 reaction steps. This led them to believe thatthey had achieved chemical isolation, i.e., a system in which</p><p>the target reaction alone controls the measured characteristics.Also, and of equal importance, rapid growth of chain carrierswas attained in the early mechanisms through the chain-branching reaction H+ O2 ) OH + O, which, at the time, wasbelieved to be very fast. That belief was challenged by Schott,12</p><p>who in 1973, established a rate coefficient about half the thenaccepted value. It soon became clear that another reaction wouldbe needed to provide sufficient chain branching in methaneoxidation. An obvious source of chain branching is the reactionchannel CH3 + O2 ) CH3O + O. These two chain-branchingreactions are analogous; both are endothermic radical-moleculereactions that increase the chain carrier number and increasethe free valence number. The dominance of the chain-branchingchannel was demonstrated in the 1974 study of Brabbs andBrokaw13 that examined the growth of the radical pool inmethane oxidation. They proposed the title reaction as the onlypossible explanation of their measurements. Since that time thereaction rate and products have been the subject of variousstudies. In general the dominance of the chain-branching channelat combustion temperatures was upheld,8,14-24 although somestudies25,26 have indicated that the chain-propagating channelwas dominant. Various levels of theoretical studies have beenperformed26-28 with nearly all supporting the dominance of thechain-branching channel.</p><p>The relative importance of the two product channels stillremains at issue. Published experimental rate coefficient expres-sions for the chain-propagating channel, evaluated at 1600 K,span a factor of 19, while the expressions for the chain-branchingchannel span a factor of 6 at that temperature. The reportedvalues of the chain-branching reaction at 1600 K have declinedwith the passage of time in a manner that is more or lesssinuosoidal. This trend has not been decisive, and the largeuncertainty is unacceptable for such an important reaction. Thegeneral decline is, in part, a result of the increasing sophisticationevident in kinetic modeling arising from a growing appreciationof the role of secondary reactions. It is our belief that secondaryreaction chemistry plays a significant role in the discrepancy</p><p>* To whom correspondence should be addressed. E-mail marty@grc.nasa.gov.</p><p> Permanent address: School of Chemical Engineering and Technology,Yeungnam University, Kyongsan, Korea.</p><p>5949J. Phys. Chem. A1999,103,5949-5958</p><p>10.1021/jp990998o CCC: $18.00 1999 American Chemical SocietyPublished on Web 07/09/1999</p></li><li><p>between the various rate coefficient determinations that hasmotivated us to reinvestigate the title reaction.</p><p>Experimentalists have exploited a plethora of diagnostics forstudies of the title reaction covering a broad range of mixturesand conditions.13,15-25,29 As always, a compromise has beenmade between experimental convenience and chemical isolation.We have also made such a compromise in our study, permitting,over a somewhat narrow temperature range, fruitful applicationof our experimental apparatus and analytical techniques. Allexperiments were performed behind reflected shock waves inlean CH4-O2-Ar mixtures using two different optical diag-nostics. The rate coefficients were derived by matching char-acteristic times obtained from absorption profiles of methylradical at ca. 214 nm and hydroxyl radical at ca. 310 nm.</p><p>Experimental Section</p><p>A rolled-square stainless steel shock tube with a crosssectional diameter of 63.5 mm was used for all experiments.Before each run the test section was routinely pumped to 3Torrusing a Varian V60 turbopump. All shocks were initiated within1 min of admitting the test gas mixture. Gas mixtures wereprepared manometrically and allowed to stand at least 48 hbefore use. Maximum uncertainty in mixture composition wasless than 0.5% of the nominal mole fraction for each component.The stated purities of the gases were the following: CH4,99.999%; O2, 99.999%; Ar, 99.9999%. All gases were usedwithout further purification.</p><p>Optical access was provided via two 25.4 mm S1-UVAwindows flush-mounted with the interior walls of the shock tube.Shock velocities were measured using a series of flush-mounted113A21 PCB Piezotronics pressure transducers coupled toPhillips PM6666 programmable counter-timers. One pressuretransducer was axially coincident with the center of theobservation windows, 12.7 mm distant from the end wall.Reflected shock conditions were derived from incident shockvelocities in a three-step process. First, velocities were fitted toa second-order polynomial in distance. Next, the velocity,extrapolated to the end wall, was used to solve the standardshock relationships, assuming full vibrational relaxation and nochemical reaction at the shock front. Finally, the computed shockproperties were corrected for boundary layer interaction effectsin a fashion similar to the method of Michael and Sutherland.30</p><p>The postshock pressure, needed for the boundary layer correc-tion, was measured using the pressure transducer centered abovethe windows. Pressure transducers were calibrated, in theirmounting assemblies, at the NASA Glenn Research CenterCalibration Laboratory.</p><p>Two separate optical diagnostics were employed. Light atca. 214 nm, absorbed mainly by methyl radical and molecularoxygen, was produced using a BHK, Inc. Premier zinc lamprun in dc mode. Light at ca. 310 nm, P1(5) line of the (0,0)band of the OH A2+-X2 transition, was produced using aCW laser pumped doubled ring dye-laser apparatus, which, inour case, was a Coherent Innova 200 CW argon ion laserpumping a Coherent 899-21 ring dye-laser running Kiton Red620 dye with intracavity frequency doubling achieved using anangle-tuned LiIO3 crystal. The usual paraphernalia were em-ployed for both light sources. The 214 nm light was passedthrough a collimating lens before entering the shock tube. A 2mm slit was placed over the exit window to both enhance timeresolution and reduce emission from the shock-heated gases(plausibly CO fourth positive band). The detector was aninterference filter-PMT-buffer combination comprised of a214 nm interference filter (10 nm fwhm), a Thorn EMI 9924QB</p><p>PMT, and a custom high-speed buffer-amplifier circuit usedto isolate the anode from the signal cables. The distance fromthe shock tube to the detector was 1.0 m to minimize emissionsfrom within the shock tube falling onto the detector. At distancesbelow 0.5 m, emission from the shock-heated gases becameobservable. At distances below 0.25 m, the emission sufficientlymodified the signal to corrupt the interpretation of the signal.When placed adjacent to the shock tube window, the maximumemission intensity was significantly greater than the lampintensity for all of our mixtures. The broad emission signal wasfound to peak approximately at the same time as the maximumpressure rise, which occurred slightly after the maximum 214nm absorption. This strong emission modified the experimentaltrace by shortening the apparent time to peak absorption andso shortening the ignition characteristics measured on the basisof peak absorption (see Results below). Rate coefficients reducedfrom such corrupted data would manifest spuriously high values.</p><p>A standard three-path configuration was used for the 310 nmlaser light: a signal beam passed through the shock tube andthen onto a detector, an intensity reference beam passed directlyonto a detector, and a wavelength reference beam passed througha burner-stabilized methane-air flame (used to determine theline center of the hydroxyl absorption feature). The detectorswere similar to that used in the 214 nm experiment with theobvious change in the interference filter, now centered at 310nm (10 nm fwhm). A less obvious but equally important changewas the replacement of the full 11 dynode configuration in thePMT with a custom configuration using only five dynodes toensure full optical response and linearity for the higher lightflux. Use of the standard dynode chain and the recommendedinterdynode potentials, required for proper electron optics, wouldhave led, at our high light intensity levels, to a situation wherethe anode current would have vastly exceeded 10% of thedynode current. Above that level it is generally accepted thatPMT performance is degraded, resulting in a loss of linearityand optical response, i.e., a lower apparent absorption.31</p><p>Verification of the linearity and optical response of our detectorswas achieved via calibration using a set of Melles Griot neutraldensity filters that, at 310 nm, covered the range 0.337-2.12in optical density (OD). The overall electronic time constantdetermined for the entire PMT-buffer-cable system for bothsets of experiments was less than 0.5s.</p><p>Selection of Experimental Conditions and ComputerSimulation</p><p>Simulation of the experimental profiles was performed usingthree different computer codes as was appropriate. ChemkinII 32 and LSENS33 are widely available chemical kinetics codes,and the third is a custom code based on the LSODE34 integrator,which has built-in features beneficial for optimization. Indis-tinguishable results were obtained using the three codes for aseries of calculations with GRI_MECH, version 2.11,35 andeither NASA thermodynamic data36 or GRI_thermodynamicdata, version 2.11.35</p><p>Extensive sensitivity analyses were performed to determineoptimum mixtures and conditions for our diagnostics. Whenchemical isolation cannot be achieved, as has so far been thecase for the study of the title reaction, the rate coefficients ofthe trial reaction mechanism used in the sensitivity analysis mustbe critically examined. Yu et al.23 (YWF) published a sensitivitystudy for the title reaction that demonstrated dominant sensitivityfor the OH profile at 1600 K in a lean CH4-O2-Ar mixture.However, their measured value for the rate coefficient at thattemperature was 1.5 times smaller than the value they used in</p><p>5950 J. Phys. Chem. A, Vol. 103, No. 30, 1999 Hwang et al.</p></li><li><p>their sensitivity study. Subsequently, Braun-Unkhoff et al.24</p><p>(BNF) measured the rate coefficients for the title reaction andreported an expression that at 1600 K was about half the YWFvalue. Therefore, on the basis of the experimental results ofBNF and our observation that the rate coefficient has beengenerally diminishing over time (see Introduction above), wereduced the rate coefficient expression used by YWF in theirsensitivity study by a factor of 3 in our sensitivity analysis. Thisresulted in a restriction of the experimentally useful temperaturerange as explained below. It is always worthwhile to rememberthat sensitivity analysis is a procedure that results in the localmeasurement of the response gradient at an arbitrarily definedlocation in the parameter space no matter how adeptly enacted.We are constrained in our choice of location in parameter spaceto those that are currently held to be physically realistic. Allother reactions in the GRI_MECH, version 2.11, were usedunchanged.</p><p>For the selection of mixture composition, sensitivities tocharacteristic times (defined in Results below) for the twoavailable diagnostics, 310 nm OH absorbance and 214 nmmethyl radical absorption, were computed. An experimentallyfeasible lean methane mixture, 1.0% CH4-4.0% O2-95.0% Ar, ) 0.5, atT ) 1650 K andF ) 33.0mol cm-3, suffers fromlarge sensitivities for pyrolytic reactions such as CH3 + H (+M)) CH4 (+M), CH4 + H ) CH3 + H2, CH3 + CH3 (+M) )C2H6 (+M), and CH3 + CH3 ) C2H5 + H. The sensitivities topyrolytic reactions can be reduced by reducing the equivalenceratio, by reducing the mixture strength, or by increasing therate of the title reaction. Reducing both the equivalence ratioand the mixture strength results in acceptable sensitivity valuesfor the title reaction with respect to the secondary reactions,not true chemical isolation but a situation not entirely dissimilar.Thus, very lean mixtures were chosen f...</p></li></ul>

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