Theoretical Study On The Reaction Mechanism Of The Hydrolysis Reaction Of Sulfur Dioxide Catalyzed By Atmospheric Molecules And Clusters

Environmental pollution has become the focus of common concern worldwide.Since China maintains the coal based energy structure, the smoke type pollution induced by sulfur dioxide(SO2) has existed for a long time. Meanwhile, the rapid increase of vehicles has led to regional compound air pollution in some cities. Air pollution has become much more serious especially in heating season in some northern cities in China and it has resulted in frequent haze weather. SO2 directly leads to acid rains in the atmosphere. In addition, the oxidation and conversion of SO2 in cloud and liquid droplets can lead to sulfuric acid and sulfate particles which are harmful to the health of human beings. Hydrolysis in cloud droplets, fogs and atmospheric aerosols is one of the most important methods of SO2 deposition in the environment and it has great significance to understand the reaction mechanism of hydrolysis reaction of SO2 in the atmosphere. Previous studies have showed that the hydrolysis reaction is unfavorable kinetically and thermodynamically when there are only few water molecules, although some molecules or clusters in the atmosphere may have catalytic effect in the hydrolysis reaction of SO2. Therefore, in order to have a more profound understanding on the reaction mechanism of hydrolysis reaction of SO2 in the atmosphere, the density functional theory and ab initio calculations were used to study the hydrolysis reaction of SO2 catalyzed by some atmospheric molecules and clusters systematically. The details are as follows:(1) The hydrolysis reaction of SO2 catalyzed by water and hydrated ammonia clustersAs the widespread molecules in atmosphere, H2 O and NH3(or the hydrated complexes) can catalyze some atmospheric reactions. In this part, the reaction mechanism of SO2 in the clusters of SO2-(H2O)n(n=1-5) and SO2-(H2O)n-NH3(n=1-3), has been studied at the MP2/aug-cc-p VDZ level of theory. Single-point energies were refined at CCSD(T)/ aug-cc-p VDZ level of theory. We conducted molecular dynamics(MD) simulations to search for the possible geometries of the more complicated clusters SO2-(H2O)n(n=4-5). The reaction in pure water clusters is thermodynamically unfavorable. The additional water in the clusters reduces the energy barrier for the reaction and the effect of each water decreases with the increasing number of water molecules in the clusters. There is a considerable energy barrier for reaction in SO2-(H2O)5, 5.7 kcal/mol. With ammonia included in the cluster,SO2-(H2O)n-NH3, the energy barrier is reduced more dramatically, to 1.9 kcal/mol with n=3, and the corresponding product of hydrated ammonium bisulfate NH4HSO3-(H2O)2 is also stabilized thermodynamically. The present study shows that ammonia has larger kinetic and thermodynamic effects than water in promoting the hydrolysis reaction of SO2 in small clusters favorable in the atmosphere.(2) The hydrolysis reaction of SO2 catalyzed by sulfuric acid clustersAs a dominant precursor of atmospheric aerosols, sulfuric acid has been proved to exhibit remarkable catalytic effects in many hydrogen transfer and hydrolysis reaction processes. In the second part, the hydrolysis reaction of SO2 in small hydrated sulfuric acid clusters SO2-(H2SO4)n-(H2O)m(m=1,2; n=1,2) has been studied at the B3LYP/cc-p V(T+d)Z level of theory. Single-point energies were refined using MP2.5method and considering the extrapolation to the complete basis set limit(CBS). We conducted molecular dynamics(MD) simulations to search for the possible geometries of SO2-H2SO4-(H2O)2 and SO2-(H2SO4)2-H2 O. The study indicates that sulfuric acid exhibits a dramatic catalytic effect on the hydrolysis reaction of SO2 as it lowers the energy barrier by over 20 kcal/mol. The reaction with monohydrated sulfuric acid(SO2 + H2 O + H2SO4-H2O) has the lowest energy barrier of 3.83kcal/mol. The energy barriers for the three hydrolysis reactions are in the order SO2 +H2SO4-H2 O > SO2 +(H2SO4)2-H2 O > SO2 + H2SO4-(H2O)2. Furthermore, sulfurous acid is more strongly bonded to the hydrated sulfuric acid(or dimer) clusters than the corresponding reactant(mono-hydrated SO2). In other words, the hydrolysis of SO2 is thermodynamically favorable with the presence of hydrated sulfuric acid(or dimer).The rate constants of the three types of reaction at the temperature range of 200-320 K were calculated in terms of the transition state theory(TST). The results of the kinetic calculations indicate that H2SO4-H2 O is responsible for the atmospheric deposition of SO2 in the stratosphere where the hydrated sulfuric acid keeps in high concentration.Consequently, sulfuric acid promotes the hydrolysis of SO2 both kinetically and thermodynamically.(3) The hydrolysis reaction of SO2 catalyzed by sulfurous acidThe hydrolysis reaction of SO2 with the sulfurous acid as an auto-catalyst were investigated at the CCSD(T)/CBS//B3 LYP /cc-p V(T+d)Z level of theory. The results indicate that the hydrolysis reaction of SO2 to form sulfurous acid involving additional H2SO3 was investigated using high-level computational methods. With H2SO3, the reaction takes place via a double proton transfer process with a cage-like structure, which is different from the planar ring structure involved in a corresponding process with an additional water molecule. Our results show that H2SO3 is a better catalyst than water, as the barrier height for the H2SO3-catalyzed reaction is only 5.5kcal/mol, compared to over 25.0 and 15.0 kcal/mol for the reaction without a catalyst and the H2O-catalyzed reaction, respectively. In addition, the sulfurous acid dimer from the H2SO3-catalyzed reaction is more stable than hydrated H2SO3 from the H2O-catalyzed reaction. According to the spatial geometries and NBO analysis, the remarkable catalytic effect of H2SO3 may arise from the strengthened intermolecular interactions causing by the appearance of sulfurous acid. The rate constants of the SO2+ 2H2 O and SO2 + H2 O + H2SO3 reactions at the temperature range of 200-320 K were calculated in terms of the transition state theory(TST). Considering the existence of sulfurous acid in the aqueous phase and acidic aerosols, as well as the importance of SO2 and H2 O in the atmosphere, our results will have potentially significant implications on the homogeneous and heterogeneous nucleation processes.