Rate Constants for CH₃ + O₂ → CH₃O + O at High Temperature and Evidence for H₂CO + O₂ → HCO + HO₂

Abstract

The reaction of CH 3 with O 2 has been studied in a reflected shock tube apparatus between 1600 and 2100 K. CH 3 was prepared from the fast thermal decomposition of CH 3 I, and O atom atomic resonance absorption spectrometry (ARAS) was used to observe absolute [O] t . [CH 3 I] 0 was sufficiently low so that most secondary reactions were negligible, allowing for unambiguous determination of the rate constant for CH 3 + O 2 f CH 3 O + O. The rate constant expression for this reaction derived to match the experimental data is k ) (3.90 ( 0.40) × 10 -11 exp(-16858 ( 1127 K/T) cm 3 molecule -1 s -1 . To explain O atom concentration buildup at longer times, a fast reaction between H 2 CO and O 2 was postulated. Rate constants for this process were derived by fitting the long-time O atom profiles. Last, the four-center reaction CH 3 + O 2 f CH 2 O + OH was found to be of negligible importance over the temperature range of the study.

High-temperature measurements of the rates of the reactions CH2O + Ar → Products and CH2O + O2→ Products

“…The current rate coefficient data agree very well with the recent evaluation of Baulch et al [15]. The Baulch et al preferred expression is an optimum fit to the low-temperature data of Baldwin et al [11] and the high-temperature data of Michael and co-workers [12,14]. At temperatures lower than $2000 K, there is reasonable agreement with the direct measurements of Michael et al [14].…”

“…A shock tube study by Hidaka et al [10] suggests an activation energy that is substantially higher than a linear extrapolation of lower temperature measurements [11] of reaction (2). Michael et al [12] invoked a large rate coefficient for reaction (2), 2.5-5 times greater than GRI-Mech 3.0, to describe their O-atom ARAS profiles in studies to measure the rate coefficient of the reaction CH 3 +O 2 fi Products. In more recent work on the CH 3 + O 2 reaction system, Michael and co-workers [13] indirectly inferred rate data for CH 2 O + O 2 by fitting OH and O-atom measurements to detailed model simulationsthe data reduction was complicated by the presence of competing reaction sensitivities, in particular from the reaction between CH 3 and O 2 .…”

Solid-state formation of CO2via the H2CO + O reaction

“…are endothermic 2 and present high (>1600 K/k b ) activation barriers (Michael et al 1999;Braslavsky & Heicklen 1976). Moreover, the inefficiency of reactions (1) and (2) is confirmed by the absence of newly formed species.…”

Flow reactor studies of methyl radical oxidation reactions in methane-perturbed moist carbon monoxide oxidation at high pressure with model sensitivity analysis

“…Nevertheless, in recent studies by Michael et al [16] and Hwang et al [17], which were primarily concerned with the determination of reaction (19), there is still disagreement about the rate constant of reaction (3). Both groups agree with Yu et al about the dominance of reaction (19) at high temperatures but conflict in that [16] finds the best agreement when the rate constant of reaction (3) is lowered to zero, while [17] finds that a finite rate for this reaction is necessary. At temperatures closer to 1000 K, the data are sparse, with Yu et al identifying the Baldwin and Golden study and the Grela et al study as the only direct measurements available [13].…”