Zhao Qian, Zhang Yingjia, Sun Wuchuan, Deng Fuquan, Yang Feiyu, Huang Zuohua
State Key Laboratory of Multiphase Flow in Power Engineering , Xi'an Jiaotong University , Xi'an 710049 , China.
J Phys Chem A. 2019 Feb 7;123(5):971-982. doi: 10.1021/acs.jpca.8b09074. Epub 2019 Jan 25.
As a renewable source of energy, ethanol has been widely used in internal combustion engines as either a gasoline alternative fuel or a fuel additive. However, as the chemical source term of the computational fluid dynamics simulation of combustors, there remains a disagreement in understanding the chemical kinetic mechanism of ethanol. The reaction mechanism of ethanol + HȮ is a well-known crucial reaction class in terms of predicting the reactivity of ethanol as well as ethylene formation under engine-relevant conditions. However, the kinetic parameters of the reactions are basically extrapolated by analogy to the n-butanol + HȮ system calculated by Zhou et al. (Zhou et al. Int. J. Chem. Kinet. 2012, 44 (3), 155-164). The reliability of such an analogy remains to be seen because no direct theoretical or experimental evidence is available in the literature to date. In this study, thermal rate coefficients of H-atom abstraction reactions for the ethanol + HȮ system were determined by using both conventional transition-state theory and canonical variational transition-state theory, with the potential energy surface evaluated at the CCSD(T)/cc-pVTZ//M06-2x/def-TZVP level. The quantum-mechanical effects were corrected by the zero-curvature tunneling method at low temperatures (<750 K), and difference schemes of two Eckart functions were fitted to optimize the minimum energy path curves. Torsional modes of the -CH and -OH groups were treated by using the hindered-internal-rotation approximation. Furthermore, the rate coefficients of the title reaction were calculated at both the CCSD(T)/cc-pVTZ//M06-2x/def-TZVP and CCSD(T)/CBS//M06-2x/def-TZVP levels of theory with uncertainty of a factor of 3. Similar to the n-butanol + HȮ system, the title system is dominated by α-site H-atom abstraction, but the rate coefficients of the three channels are slightly slower than that of the n-butanol + HȮ system. In general, the new calculations show only a limited effect on the ethanol reactivity at low pressures and high temperatures (>1300 K), but they prevent the kinetic mechanisms from overpredicting ignition delay times under engine-relevant conditions.
作为一种可再生能源，乙醇已被广泛用作内燃机的汽油替代燃料或燃料添加剂。然而，作为燃烧器计算流体动力学模拟的化学源项，在理解乙醇的化学动力学机制方面仍存在分歧。乙醇 + HȮ 的反应机制是预测发动机相关条件下乙醇反应性以及乙烯形成方面一个众所周知的关键反应类别。然而，这些反应的动力学参数基本上是通过类比 Zhou 等人计算的正丁醇 + HȮ 系统外推得到的（Zhou 等人，《国际化学动力学杂志》，2012 年，44(3)，155 - 164）。由于迄今为止文献中没有直接的理论或实验证据，这种类比的可靠性还有待观察。在本研究中，使用传统过渡态理论和正则变分过渡态理论确定了乙醇 + HȮ 系统氢原子提取反应的热速率系数，势能面在 CCSD(T)/cc - pVTZ//M06 - 2x/def - TZVP 水平进行评估。在低温（<750 K）下，通过零曲率隧穿方法校正量子力学效应，并拟合两个 Eckart 函数的差分方案以优化最小能量路径曲线。 -CH 和 -OH 基团的扭转模式采用受阻内旋转近似处理。此外，在 CCSD(T)/cc - pVTZ//M06 - 2x/def - TZVP 和 CCSD(T)/CBS//M06 - 2x/def - TZVP 理论水平上计算了标题反应的速率系数，不确定性为 3 倍。与正丁醇 + HȮ 系统类似，标题系统以 α 位氢原子提取为主，但三个通道的速率系数略低于正丁醇 + HȮ 系统。总体而言，新的计算结果表明，在低压和高温（>1300 K）下对乙醇反应性的影响有限，但它们可防止动力学机制在发动机相关条件下过度预测点火延迟时间。
