Quantum Chemical Study of CH₃ + O₂ Combustion Reaction System: Catalytic Effects of Additional CO₂ Molecule

Abstract

The supercritical carbon dioxide diluent is used to control the temperature and to increase the efficiency in oxycombustion fossil fuel energy technology. It may affect the rates of combustion by altering mechanisms of chemical reactions, compared to the ones at low CO concentrations. Here, we investigate potential energy surfaces of the four elementary reactions in the CH + O reactive system in the presence of one CO molecule. In the case of reaction CH + O → CHO + OH (R1 channel), van der Waals (vdW) complex formation stabilizes the transition state and reduces the activation barrier by ∼2.2 kcal/mol. Alternatively, covalently bonded CO may form a six-membered ring transition state and reduce the activation barrier by ∼0.6 kcal/mol. In case of reaction CH + O → CHO + O (R2 channel), covalent participation of CO lowers the barrier for the rate limiting step by 3.9 kcal/mol. This is expected to accelerate the R2 process, important for the branching step of the radical chain reaction mechanism. For the reaction CH + O → CHO + HO (R3 channel) with covalent participation of CO, the activation barrier is lowered by 0.5 kcal/mol. The reaction CHO + OH → CHO + HO (R4 channel) involves hydrogen abstraction from formaldehyde by OH radical. Its barrier is reduced from 7.1 to 0.8 kcal/mol by formation of vdW complex with spectator CO. These new findings are expected to improve the kinetic reaction mechanism describing combustion processes in supercritical CO medium.

Quantum chemical and master equation study of OH + CH2O → H2O + CHO reaction rates in supercritical CO2 environment

“…In this work, we used an alternative approach, where prereactive vdW complex is assumed to be in equilibrium with reactants and will show this to be a viable approximation. First, we calculated accurate activation energies for R1 and R2, based on MEP described previously at DFT + vdW theory level 11 and confirmed that the additional CO 2 molecule reduces the activation barrier by stabilizing the transition state more than a prereactive complex (PRC). Next, we predicted reaction rate constants for R1 and R2 at a wide range of temperatures and pressures using master equation formalism.…”

“…Previously, we found two possible mechanisms for the R2 reaction: One included several steps with the extra CO 2 molecule covalently bound in the transition states and intermediates, and the other one involved a hydrogen transfer from CH 2 O to OH to form water, with transition state stabilized by vdW complex formation with extra CO 2 molecule. The activation barrier for the former mechanism was found to be 30.2 kcal/mol at M11D3 + ZPE, whereas the barrier for the latter was found to be 0.8 kcal/mol.…”

“…In the first paper of this series, we investigated the effect of CO 2 on the CO + OH reactive system and found a lower activation energy pathway in which CO 2 participated covalently . In the following papers, we reported lowering of the activation barrier of seven other combustion reactions via van der Waals (vdW) interactions with CO 2 . To obtain a better description of these vdW interactions, we used a higher theory level (CBS‐QM11) and refined the reaction pathways .…”

“…9 In the following papers, we reported lowering of the activation barrier of seven other combustion reactions via van der Waals (vdW) interactions with CO 2 . 10,11 To obtain a better description of these vdW interactions, we used a higher theory level (CBS-QM11) and refined the reaction pathways. 9,12 Next, we applied the master equation solver to obtain rate constants for the OH + CO → H + CO 2 reaction with and without an additional CO 2 molecule.…”

“…More recently, Masunov et al also found critical points on the potential energy surface for R1 and one competing reaction at M11/6-311G**+GD3 theory level with zero-point energy (ZPE) correction. 11 They calculated activation barrier to be 4.3 kcal/mol. In this work, we will refine that result using higher theory level CBS-QM11, introduced earlier.…”