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

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 CO₂ concentrations. Here, we investigate potential energy surfaces of the four elementary reactions in the CH₃ + O₂ reactive system in the presence of one CO₂ molecule. In the case of reaction CH₃ + O₂ → CH₂O + 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 CO₂ may form a six-membered ring transition state and reduce the activation barrier by ∼0.6 kcal/mol. In case of reaction CH₃ + O₂ → CH₃O + O (R2 channel), covalent participation of CO₂ 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 CH₃ + O₂ → CHO + H₂O (R3 channel) with covalent participation of CO₂, the activation barrier is lowered by 0.5 kcal/mol. The reaction CH₂O + OH → CHO + H₂O (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 CO₂. These new findings are expected to improve the kinetic reaction mechanism describing combustion processes in supercritical CO₂ medium.