## 4 Reaction Mechanism  
For NH\({}_{3}\)-SCR, the reaction of NH\({}_{3}\) with NO occupied the main position in the reaction system. NH\({}_{3}\) could convert NO to N\({}_{2}\) by the reaction, \(4\text{NH}_{3}+4\text{NO}+\text{O}_{2}\to 4\text{N}_{2}+6\text{H}_{2}\text{O}\)[57]. However, the NO removal efficiency was unsatisfactory in DBD-NH\({}_{3}\)-SCR with excess O\({}_{2}\), because high-energy electrons generated from DBD could excite NH\({}_{3}\), NO, and O\({}_{2}\) in the feed gas and produced high-energy particles, namely, \(\text{e}+\text{NO}\to\text{e}+\text{NO}^{*}\), \(\text{e}+\text{O}_{2}\to\text{e}+2\text{O}\), \(\text{e}+\text{NH}_{3}\to\text{e}+\cdot\text{NH}_{2}+\cdot\text{H}\). Furthermore, excited NO\({}^{*}\), NH\({}_{3}\), and \(\cdot\text{NH}_{2}\) recombined with O\({}_{2}\) as well as these high-energy particles on the catalyst surface to generate new NO\({}_{\text{X}}\) through the reactions [54; 57; 58; 59]:  
\[\text{NO}^{*}+\text{O}\to\text{NO}_{2};\]  
\[\text{NO}_{2}+\text{O}\to\text{NO}_{3};\]  
\[2\text{NH}_{3}+2\text{O}_{2}\to\text{N}_{2}\text{O}+3\text{H}_{2}\text{O}\;;\]  
\[4\text{NH}_{3}+4\text{NO}+3\text{O}_{2}\to 4\text{N}_{2}\text{O}+6\text{H}_{2} \text{O};\]  
\[4\text{NH}_{3}+5\text{O}_{2}\to 4\text{NO}+6\text{H}_{2}\text{O};\]  
\[2\cdot\text{NH}_{2}+\text{O}_{2}\to 2\text{NO}+2\text{H}_{2};\]  
\[\cdot\text{NH}_{2}+2\text{O}\to\text{NO}+\text{H}_{2}\text{O}.\]  
In DBD-NH\({}_{3}\)-CH\({}_{4}\)-SCR, except for the reaction of \(4\text{NH}_{3}+4\text{NO}+\text{O}_{2}\to 4\text{N}_{2}+6\text{H}_{2}\text{O}\), \(\text{CH}_{4}\), as the assistant reduction agent, could react with NO and O\({}_{2}\) to convert NO to N\({}_{2}\), namely, \(\text{CH}_{4}+2\text{NO}+\text{O}_{2}\to\text{N}_{2}+2\text{H}_{2}\text{O}+ \text{CO}_{2}\). Meanwhile, CH\({}_{4}\) was converted to \(\cdot\text{CH}_{3}\), namely, \(\text{e}+\text{CH}_{4}\to\text{e}+\text{CH}_{3}+\cdot\text{H}\)[60], which further produced CH\({}_{3}\)O\({}_{2}\) as well as NO\({}_{2}\) and CO\({}_{2}\)[61]. Furthermore, NO\({}_{2}\), generated from NO oxidation by O and CH\({}_{3}\)O\({}_{2}\) could react with NO as well as NH\({}_{3}\) to form N\({}_{2}\) and H\({}_{2}\)O through the reaction: \(\text{NO}+\text{NO}_{2}+2\text{NH}_{3}\to 2\text{N}_{2}+3\text{H}_{2}\text{O}\). This is the fast-NH\({}_{3}\)-SCR reaction, which has been verified to achieve effective reduction of NO\({}_{\text{X}}\) to N\({}_{2}\)[62; 63; 64]. In addition, NO\({}_{2}\) could be oxidized to NO\({}_{3}\) in the DBD-system with excess O\({}_{2}\) as the reaction of NO\({}_{2}+\text{O}\to\text{NO}_{3}\). Another byproduct of N\({}_{2}\)O could react with \(\cdot\text{CH}_{3}\), O and NO, such as the following reactions [65; 66; 67]:  
\[\cdot\text{CH}_{3}+\text{N}_{2}\text{O}\to\text{CH}_{3}\text{O}+\text{N}_{2};\]  
\[\text{O}+\text{N}_{2}\text{O}\to\text{O}_{2}+\text{N}_{2};\]  
\[\text{O}+\text{N}_{2}\text{O}\to 2\text{NO};\]  
\[\text{NO}+\text{N}_{2}\text{O}\to\text{NO}_{2}+\text{N}_{2}.\]  
Taking all above-mentioned facts into account, the presence of NH\({}_{3}\)/CH\({}_{4}\) in the feed gases significantly enhances the reduction of NO\({}_{\text{X}}\) to N\({}_{2}\) and the product selectivity of NO abatement in DBD-NH\({}_{3}\)-CH\({}_{4}\)-SCR.  
A schematic diagram of the main chemical reaction mechanism for DBD-SCR using CH\({}_{4}\) as the assistant agent is shown in Figure 8.