Masunov Artëm E, Wait Elizabeth E, Atlanov Arseniy A, Vasu Subith S
National Research Nuclear University MEPhI , Kashirskoye shosse 31, Moscow 115409, Russia.
South Ural State University , Lenin pr. 76, Chelyabinsk 454080, Russia.
J Phys Chem A. 2017 May 18;121(19):3728-3735. doi: 10.1021/acs.jpca.7b02638. Epub 2017 May 4.
In oxy-fuel combustion, the pure oxygen (O), diluted with CO is used as oxidant instead air. Hence, the combustion products (CO and HO) are free from pollution by nitrogen oxides. Moreover, high pressures result in the near-liquid density of CO at supercritical state (sCO). Unfortunately, the effects of sCO on the combustion kinetics are far from being understood. To assist in this understanding, in this work we are using quantum chemistry methods. Here we investigate potential energy surfaces of important combustion reactions in the presence of the carbon dioxide molecule. All transition states and reactant and product complexes are reported for three reactions: HCO + HO → HCO + HO (R1), 2HO → HO + O (R2), and CO + OH → CO + H (R3). In reaction R3, covalent binding of CO to the OH radical and then the CO molecule opens a new pathway, including hydrogen transfer from oxygen to carbon atoms followed by CH bond dissociation. Compared to the bimolecular OH + CO mechanism, this pathway reduces the activation barrier by 5 kcal/mol and is expected to accelerate the reaction. In the case of hydroperoxyl self-reaction 2HO → HO + O the intermediates, containing covalent bonds to CO are found not to be competitive. However, the spectator CO molecule can stabilize the cyclic transition state and lower the barrier by 3 kcal/mol. Formation of covalent intermediates is also discovered in the HCO + HO → HCO + HO reaction, but these species lead to substantially higher activation barriers, which makes them unlikely to play a role in hydrogen transfer kinetics. The van der Waals complexation with carbon dioxide also stabilizes the transition state and reduces the reaction barrier. These results indicate that the CO environment is likely to have a catalytic effect on combustion reactions, which needs to be included in kinetic combustion mechanisms in supercritical CO.
在氧燃料燃烧中，用与二氧化碳混合的纯氧（O₂）作为氧化剂来替代空气。因此，燃烧产物（CO₂和H₂O）不会受到氮氧化物的污染。此外，高压会导致处于超临界状态的二氧化碳（scCO₂）具有接近液体的密度。不幸的是，scCO₂对燃烧动力学的影响远未得到理解。为了有助于理解这一点，在这项工作中我们使用量子化学方法。在这里，我们研究了在二氧化碳分子存在下重要燃烧反应的势能面。报道了三个反应的所有过渡态以及反应物和产物络合物：HCO + HO₂ → H₂CO + HO₂（R1）、2HO₂ → H₂O₂ + O₂（R2）和CO + OH → CO₂ + H（R3）。在反应R3中，CO与OH自由基的共价结合，然后CO分子开启了一条新途径，包括氢从氧转移到碳原子，随后是C-H键断裂。与双分子OH + CO机制相比，这条途径将活化能垒降低了5千卡/摩尔，预计会加速反应。在氢过氧自由基自反应2HO₂ → H₂O₂ + O₂的情况下，发现含有与CO共价键的中间体没有竞争力。然而，旁观的CO分子可以稳定环状过渡态并将能垒降低3千卡/摩尔。在HCO + HO₂ → H₂CO + HO₂反应中也发现了共价中间体的形成，但这些物种导致活化能垒大幅升高，这使得它们不太可能在氢转移动力学中发挥作用。与二氧化碳的范德华络合也稳定了过渡态并降低了反应能垒。这些结果表明，CO环境可能对燃烧反应具有催化作用，这需要纳入超临界CO₂中的燃烧动力学机制。
