Theoretical insight into the oxidation mechanism of NO2 and SO2 on TiO2 surface: The role of H2O, NH3 and SO42-

Abstract

In this paper, density-functional theory (DFT) was employed to investigate the reaction mechanism of SO2 and NO2 and the important role of intermediate N2O4 formation. In addition, the effect of pure particulate surface (TiO2) and particulate surface containing other atmospheric components (H2O, NH3 and SO42−) on the conversion of SO2 and NO2 to sulfate also was analyzed. The detailed information that intermediate N2O4 acts as the oxidant was demonstrated in this oxidation reaction of SO2 and NO2 (as a rate-determining step). The pure particulate surface (TiO2) has hardly effect on the oxidation process of SO2 and N2O4. Whereas, different amounts of H2O and NH3 molecules as well as SO42−, contained on the particulate surface, can effectively reduce the activation energy of oxidation step. And, the optimal process is that SO2 is oxidized by cis-ONONO2 with the energy barrier of 4.54 kcal/mol when one NH3 molecule and one H2O molecule are contained on the TiO2(101) surface. When more H2O molecules are contained on particulate surface, SO2 tends to form HSO3− first. However, HSO3− is more difficult to be further oxidized by asy-ONONO2 than SO2. This study gains more insight into the contribution of SO2 and NO2 to haze and the potential impact of atmospheric constituents (including H2O/NH3/TiO2 and SO42−/TiO2) on the formation sulfate.