First-principles quantum chemical studies on reaction of hydrogen superoxide with water and sulfur dioxide at supercritical conditions

Abstract

As people increasingly focus on air pollution, it is essential to study the chemical reaction mechanisms in atmospheric reactions. Among the element cycles in the atmosphere, hydrogen-oxygen species such as OH, HO2, H2O2, H2O2 and H2O are particularly important, and their reaction mechanisms require further investigation, especially the HO2 radical, which reacts with almost all pollutants. Additionally, reactions under supercritical conditions, such as high pressure and temperature, which are vital in the aerospace industry, have not been fully researched due to the high cost of accurate calculations.

Previous studies have already constructed the potential energy surface (PES) at different levels of theory including MRCI and CCSD requiring a large number of stationary calculations for obtaining continuous PES. In this study, we use both DFT and ab-initio electronic structure calculations with perturbation theory combined with machine learning methods to construct continuous PES, resulting in more reasonable reaction mechanisms.

To better understand atmospheric reaction mechanisms, we investigated the HO2 radical's reaction with the most abundant greenhouse gas, water vapor, and the SO2 molecule. We conducted full-dimensional PES calculations with ab initio electronic structure calculations. For the formation of the HO2 system, we used machine learning methods to fit the PES with minimal error, and molecular dynamic simulations were performed to obtain effective trajectories. Traditional TST and CVT methods, as well as quasi-classical trajectory-based fitting PES, were used to obtain accurate rate constants.

For larger systems such as the HO2 self-reaction, HO2 reacting with water and SO2, we employed TST and CVT methods to elucidate the possible reaction mechanisms. In Chapter 5, we first calculated and fitted the PES of the HO2 self-reaction at a relatively high precision level. Additionally, we calculated the energy change resulting from the torsion of the dihedral angle in the TS2 structure. By calculating the reaction rate coefficients and comparing them with experimental data, we observed that the self-reaction rate of HO2 reaches its maximum at 800K, followed by a decrease in the reaction equilibrium rate as the temperature decreases. Furthermore, we optimized the structure along the Intrinsic Reaction Coordinate (IRC) path using both B2PLYP/cc-pVTZ and B3LYP/cc-pVTZ methods for the reaction between the HO2 radical and a single H2O molecule. Rate coefficients were calculated using various methods. Analysis of the calculated reaction equilibrium constants revealed a rapid decrease, up to three orders of magnitude, in the equilibrium constant as the temperature increases, which is in excellent agreement with experimental results that the formation of the HO2·H2O complex dominates at lower temperatures.

In Chapter 6, we investigated the possible reaction pathway and rate coefficients for the reaction between SO2 and the HO2 radical in the presence of water molecules. Analysis of the results of the SO2 + HO2·H2O and H2O reactions revealed that water vapor promotes the formation of (HSO4-)·H2O radical, acting as a weak positive catalyst. Regarding the formation of H2SO3·HO2, the HO2 radical acts as a catalyst rather than a reactant, dominating the reaction channel for the SO2 + HO2·H2O reaction.