Theoretical insight into the oxidation mechanism of NO2 and SO2 on TiO2 surface: The role of H2O, NH3 and SO42-
Graphical abstract
Introduction
Sulfate (SO42−) is a critical part in secondary aerosol, resulting in a serious environmental problem (Liu et al., 2017a). Atmospheric sulfate aerosol particles play a directly important role in poor air quality and the earth's climate (Xiao et al., 2013; Yun et al., 2019). The addition of sulfate aerosol can alter the microphysical properties of clouds by acting as cloud condensation nuclei (CCN) then exhibit indirect effect on climate (Miyamoto et al., 2020). And, sulfate particles contribute to the haze produced, which is prone to respiratory diseases and lung problems in humans (Sun et al., 2016). Therefore, it is particularly important to explore the source of sulfate in the atmosphere. Most of the sulfate content, in the atmosphere, is derived from the oxidation of SO2 (Hung and Hoffmann, 2015). The common ways of SO2 oxidation are gas-phase oxidation and in-cloud oxidation through atmospheric oxidants that include hydroxyl radical (OH•) (Sitha et al., 2011; Huang et al., 2014; Long et al., 2017), hydroperoxyl (HO2•) (Chen et al., 2014), dissolved ozone (O3) (Yun et al., 2018) and hydrogen peroxide (H2O2) (Zuo and Hoigné, 1993; Ding et al., 2014).
When the haze happens, the aerosol content continues to increase, resulting in the significant decline in ultraviolet radiation. Therefore, the concentration of photochemical oxidants decreases, preventing SO2 from being oxidized in these common ways (Zheng et al., 2015a; Gao et al., 2016a). In addition, experiments found that tiny water particles in the air can trap NO2 to approach the ground layer resulting in a higher concentration of NO2 than sunny days, due to the high hygroscopicity of aerosol particles (Cheng et al., 2016; Gao et al., 2016b). Considerable studies have demonstrated the content of sulfate continues to increase when the content of aerosol particle and NO2 increase (Sun et al., 2014). Therefore, new oxidation pathways need to be explored to explain the formation of sulfate under severe haze in the presence of a higher concentration of NO2 (Wang et al., 2014; Zheng et al., 2015b). Some researchers have considered that NO2 could effectively react with SO2 as a key missing pathway to form sulfate (Ma et al., 2008; Liu et al., 2012; Zhang et al., 2015a; Zhang et al., 2015b). Wang et al. (Wang et al., 2016) also have proposed the following reactions of SO2 and NO2 on fine aerosols or under cloudy conditions based on experimental studies:the reaction (1) indicates that the final products primarily include SO42− and HONO. Among them, HONO is the main source of hydroxyl radicals, increasing the oxidative properties of the atmosphere(Gómez Alvarez et al., 2013). In our previous study, the reaction mechanism of SO2 and NO2 in the gas-phase is investigated and the result indicated that SO2 is difficult to produce sulfate, and H2O and NH3 molecules may exhibit nearly no effect on the formation of sulfate(Wang et al., 2019).
The catalytic effect of mineral dust on the oxidation of SO2 by NO2 has been recognized as an important formation pathway of sulfate in haze episode (He et al., 2014; Wang et al., 2020). Chang et al. (Liu et al., 2012) and Ma et al. (Ma et al., 2008) detected that surface-N2O4 as the crucial oxidant for the oxidation of tetravalent sulfur species S(IV), when investigated the heterogeneous reactions of SO2 and NO2 on different particulate matter surface by in situ DRIFTS. However, there are a few problems to be solved in these researches. The specific influence of N2O4 formation in the heterogeneous oxidation of SO2 is unclear. And, the detailed mechanisms for the total reactions (SO2 and N2O4) at the molecular-level are still unclear and quantitative thermodynamic and kinetic data for elementary reactions have not been elucidated. Titanium dioxide (TiO2), one of the important atmospheric particulate matter, although the content accounts for a relatively small portion(Reid et al., 2003; Chen et al., 2012). However, titanium dioxide (TiO2) has a large number of stationary sources in direct contact with the atmosphere. Some studies have demonstrated that titanium dioxide (TiO2) nanoparticles, as material additive, are widely applied to the surface of glass, wall covering, pavement, concrete and so on (Salthammer and Fuhrmann, 2007; Wang et al., 2007) to remove atmospheric pollutants, such as VOCs, CO2, and NO2 (Liu et al., 2017b). Due to large surface area to volume ratios and the high photocatalytic activity, it may potentially play a significant role in atmospheric chemistry. In summary, TiO2 particulate surface can be used as an important influence factor to study the reaction of SO2 and NO2 to sulfate formation. In addition, the effect of other atmospheric components (including water molecules, ammonia, and generated sulfate) contained on the particulate surface on the reaction of SO2 and NO2 should also be discussed.
In this paper, we research the detailed information for the total reaction mechanisms at molecular-level using the density functional theory (DFT) method. And, the effects of particulate surface (TiO2) and other atmospheric components adsorbed on the particulate surface on the reaction of SO2 with NO2 also are confirmed.
Section snippets
Computational models and methods
As is well-known, TiO2 has three kinds of crystal forms, including rutile, anatase and brookite(Fujishima et al., 2008). The brookite is easily transformed and therefore is rarely used in the study of surface reaction. With regard to rutile, its photo-catalytic performance is inferior to anatase, as well as the major constituent of TiO2 nanoparticles is commonly the anatase(Lazzeri et al., 2001; Labat et al., 2008). The most stable surface among the many surfaces of TiO2-anatase is the (101)
The adsorption configuration of NO2
In order to consider the effect of the particulate surface on the reaction, NO2 and SO2 may be accommodated on TiO2 surface successively. As shown in Fig. S1, anatase (101) surface contains two kinds of adsorption sites: the coordinative unsaturated 5c-Ti and 2c-O, as well as the saturated 6c-Ti and 3c-O. According to some studies, researchers verify that 5c-Ti and 2c-O are more active than the others(Miller et al., 2011). In addition, when NO2 adsorbed in these adsorption sites, it has three
Conclusions
Density functional theory method is employed to study the reaction mechanism of SO2 and NO2 to form bisulfate and HONO under the effect of particulate surface (TiO2) and other atmospheric components (H2O, NH3, and SO42−). In addition, the important role of intermediate N2O4 formation in the reaction is also studied. The most stable adsorption of the NO2 is found to reside at the bridge site (II-2 and II-3) on particulate surface (TiO2). After that, the dimer of II-2 (trans-ONONO2 and cis-ONONO2
Funding
This work was supported by National Natural Science Foundation of China (21976109, 21607011); Natural Science Foundation of Shandong Province (ZR2018MB043); Fundamental Research Fund of Shandong University (2018JC027); Shandong Province Key Research and Development Program (2019GSF109037).
CRediT authorship contribution statement
Zehua Wang: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Visualization, Writing - review & editing. Chenxi Zhang: Conceptualization, Methodology. Guochun Lv: Conceptualization, Formal analysis, Writing - review & editing. Xiaomin Sun: Resources, Supervision, Funding acquisition, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work was supported by National Natural Science Foundation of China (21976109 and 21607011), Natural Science Foundation of Shandong Province (ZR2018MB043), Fundamental Research Fund of Shandong University (2018JC027), Shandong Province Key Research and Development Program (2019GSF109037).
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