Analysis of HO2 and OH formation mechanisms using FM and UV spectroscopy in dimethyl ether oxidation.
Analysis of HO2 and OH formation mechanisms using FM and UV spectroscopy in dimethyl ether oxidation.
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DOI：
10.1021/jp067646j
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发表时间：
2007-04
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影响因子：
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通讯作者：
K. Suzaki;Kentaro Tsuchiya;M. Koshi;A. Tezaki
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文献类型：
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作者：
K. Suzaki;Kentaro Tsuchiya;M. Koshi;A. Tezaki
通过使用近红外调频光谱检测 HO2 和 OH，以及使用紫外吸收光谱检测 CH3OCH2O2，研究了 CH3OCH3 光解引发氧化中的产物形成途径作为温度 (298-600 K) 和压力 (20-90 Torr) 的函数。该反应通过 Cl2、O2 和 CH3OCH3 混合物的脉冲光解引发。 HO2 和 OH 产率是通过与已建立的参考混合物（包括 CH3OH）进行比较而获得的。 CH3OCH2O2 产率还可通过根据 UV 吸收估计 CH3OCH2O2/HO2 比率的程序获得。一个值得注意的发现是，OH 产率比已知的 C2 和 C3 烷烃产率高 1 个数量级，随着温度的升高从 10% 增加到 40%。 HO2 产率逐渐增加直至 500 K，并在 500 K 后急剧上升至 40%。CH3OCH2O2 曲线迅速上升，随后逐渐衰减，其时间常数与缓慢的 HO2 形成一致。为了预测物种分布和产量，基于现有模型和本研究得出的新反应途径，构建了低压条件下二甲醚的简单氯引发氧化模型。为了模拟 OH 的快速形成，需要从 CH3OCH2 + O2 直接形成 OH。我们还建议考虑通过 QOOH 异构化为 HOQO 物质和 OH + CH3OCH2O2 --> HO2 + CH3OCH2O 的新 HCO 形成途径，以解释快速和缓慢的 HO2 形成以及总产率。包含这些新途径的构建模型已成功预测了所研究的整个温度和压力范围内的实验结果。结果表明，HO2的形成机制在500 K时发生变化，即通过HCHO + OH形成HCO + O2，并且上述提出的直接HCO形成在500 K以上占主导地位，而CH3OCH2O2自反应和OH + CH3OCH2O2反应之后的一系列反应在500 K以下主要起作用。CH3OCH2热分解反应的压力依赖速率常数由于其对HO2形成具有较大的负敏感性，因此已单独测量对于消除高温下 CH3OCH2 + O2 机理的模糊性至关重要。
Product formation pathways in the photolytically initiated oxidation of CH3OCH3 have been investigated as a function of temperature (298-600 K) and pressure (20-90 Torr) through the detection of HO2 and OH using Near-infrared frequency modulation spectroscopy, as well as the detection of CH3OCH2O2 using UV absorption spectroscopy. The reaction was initiated by pulsed photolysis with a mixture of Cl2, O2, and CH3OCH3. The HO2 and OH yield is obtained by comparison with an established reference mixture, including CH3OH. The CH3OCH2O2 yield is also obtained through the procedure of estimating the CH3OCH2O2/HO2 ratio from their UV absorption. A notable finding is that the OH yield is 1 order of magnitude larger than those known in C2 and C3 alkanes, increasing from 10% to 40% with increasing temperature. The HO2 yield increases gradually until 500 K and sharply up to 40% over 500 K. The CH3OCH2O2 profile has a prompt rise, followed by a gradual decay whose time constant is consistent with slow HO2 formation. To predict species profiles and yields, simple chlorine-initiated oxidation model of DME under low-pressure condition was constructed based on the existing model and the new reaction pathways, which were derived from this study. To model rapid OH formation, OH direct formation from CH3OCH2 + O2 was required. We have also proposed that a new HCO formation pathway via QOOH isomerization to HOQO species and OH + CH3OCH2O2 --> HO2 + CH3OCH2O are to be considered, to account for the fast and slow HO2 formations, as well as the total yield. The constructed model including these new pathways has successfully predicted experimental results throughout the entire temperature and pressure ranges investigated. It was revealed that the HO2 formation mechanism changes at 500 K, i.e., HCO + O2 via HCHO + OH and the above proposed direct HCO formation dominates over 500 K, while a series of reactions following CH3OCH2O2 self-reaction and OH + CH3OCH2O2 reaction mainly contribute below 500 K. The pressure dependent rate constant of the CH3OCH2 thermal decomposition reaction has been separately measured since it has large negative sensitivity for HO2 formation and is essential to eliminate the ambiguity in the CH3OCH2 + O2 mechanism at higher temperature.