Molecular Dynamics Study of Combustion Reactions in a Supercritical Environment. Part 2: Boxed MD Study of CO + OH → CO2 + H Reaction Kinetics

Molecular Dynamics of Combustion Reactions in Supercritical Carbon Dioxide. 6. Computational Kinetics of Reactions between Hydrogen Atom and Oxygen Molecule H + O2 ⇌ HO + O and H + O2 ⇌ HO2

Reactions of the hydrogen atom and the oxygen molecule are among the most important ones in the hydrogen and hydrocarbon oxidation mechanisms, including combustion in a supercritical CO₂ (sCO₂) environment, known as oxy-combustion or the Allam cycle. Development of these energy technologies requires understanding of chemical kinetics of H + O₂ ⇌ HO + O and H + O₂ ⇌ HO₂ in high pressures and concentrations of CO₂. Here, we combine quantum treatment of the reaction system by the transition state theory with classical molecular dynamics simulation and the multistate empirical valence bonding method to treat environmental effects. Potential of mean force in the sCO₂ solvent at various temperatures 1000-2000 K and pressures 100-400 atm was obtained. The reaction rate for H + O₂ ⇌ HO + O was found to be pressure-independent and described by the extended Arrhenius equation 4.23 × 10^-7 T^-0.73 exp(-21 855.2 cal/mol/RT) cm³/molecule/s, while the reaction rate H + O₂ ⇌ HO₂ is pressure-dependent and can be expressed as 5.22 × 10^-2 T^-2.86 exp(-7247.4 cal/mol/RT) cm³/molecule/s at 300 atm.

Molecular Dynamics of Combustion Reactions in Supercritical Carbon Dioxide. Part 5: Computational Study of Ethane Dissociation and Recombination Reactions C2H6 ⇌ CH3 + CH3

Fossil fuel oxy-combustion is an emerging technology where the habitual nitrogen diluent is replaced by high-pressure supercritical CO₂ (sCO₂), which increases the efficiency of energy conversion. In this study, the chemical kinetics of the combustion reaction C₂H₆ ⇌ CH₃ + CH₃ in the sCO₂ environment is predicted at 30-1000 atm and 1000-2000 K. We adopt a multiscale approach, where the reactive complex is treated quantum mechanically in rigid rotor/harmonic oscillator approximation, while environment effects at different densities are taken into account by the potential of mean force, produced with classical molecular dynamics (MD). Here, we used boxed MD, where enhanced sampling of infrequent events of barrier crossing is accomplished without application of the bias potential. The multistate empirical valence bond model is applied to describe free radical formation accurately at the cost of the classical force field. Predicted rates at low densities agree well with the literature data. Rate constants at 300 atm are 2.41 × 10¹⁴ T^-0.20 exp(-77.03 kcal/mol/ RT) 1/s for ethane dissociation and 8.44 × 10^-19 T^1.42 exp(19.89 kcal/mol/ RT) cm³/molecule/s for methyl-methyl recombination.

Kinetic Study on the Self-Assembly of Au(I)-Thiolate Lamellar Sheets: Preassembled Precursor vs Molecular Precursor

Molecular self-assembly has played an important role in nanofabrication. Due to the weak driving forces of noncovalent bonds, developing molecular nanoassemblies that have both robust preparation conditions and stable structure is a challenge. In our previous work, we have developed a reversible self-assembly system of Au(I)-thiolate coordination polymer (ATCP) to form colloidal lamellar sheets and demonstrated the high tailorability and stability of their structures, as well as their promising applications in gold nanocluster/nanoparticle fabrication and UV light shielding. Here, we first reported our progress in exploring a robust and green assembly protocol toward ATCP colloidal lamellar sheets in water by allowing the molecular precursors of HAuCl₄ and the thiol ligand to form ATCP preassembled intermediates. In this way, colloidal ATCP lamellar sheets can be prepared in a wide range of synthetic concentrations ([Au]₀ ≥ 2 × 10^-4 M) and at broad assembly temperatures (80-100 °C) with similar high yields (>80%). The assembly kinetics at different conditions are also studied in detail to help understand the robust assembly process. The robust and green synthetic protocols will pave a way for their real applications.

Molecular Dynamics Study of Combustion Reactions in Supercritical Environment. Part 3: Boxed MD Study of CH3 + HO2 → CH3O + OH Reaction Kinetics

The kinetics of reaction CH₃ + HO₂ → CH₃O + OH in supercritical carbon dioxide media at pressures from 0.3 to 1000 atm in the temperature range (600-1600) K was studied using boxed molecular dynamics simulations at QM/MM theory level with periodical boundary conditions. The mechanism of this process includes two consecutive steps: formation and decomposition of CH₃OOH intermediate. We calculated the activation free energies and rate constants of each step, then used Bodenstein's quasistationary concentrations approximation to estimate the rate constants of the reaction. On the basis of the temperature dependence of the rate constants, parameters in the extended Arrhenius equation were determined. We found that reaction rate of each step, as well as overall reaction, increases with increasing CO₂ pressure in the system. The most effective zone for the process is T = 1000-1200 K, and the CO₂ pressure is about 100 atm.