## 3 Results and Discussion
### Effect of CH\({}_{4}\) and NH\({}_{3}\) on the Product Selectivity of the DBD-SCR Hybrid System  
To explore the mechanisms involved, the products observed by FTIR in DBD-SCR are shown in Figure 5. Figure 5a shows the effect of SIE on NO abatement with 12% O\({}_{2}\) in the DBD-NH\({}_{3}\)-SCR process. The NO concentration decreased gradually, however, the concentrations of N\({}_{2}\)O and NO\({}_{2}\) increased obviously with increasing SIE. In addition, as shown in Figure 5b, the peak intensities of NO, NO\({}_{2}\), and N\({}_{2}\)O, with peaks at 1900 cm\({}^{-1}\), 1600 cm\({}^{-1}\), and 2200 cm\({}^{-1}\), respectively, increased with increasing O\({}_{2}\) concentration. Products detected in the DBD-NH\({}_{3}\)-SCR process with the assistant agent CH\({}_{4}\) at different SIE and O\({}_{2}\) concentrations are shown in Figure 5c,d. These results show that there was less N\({}_{2}\)O and NO\({}_{2}\) in the process compared with the results shown in Figure 5a,b under excess O\({}_{2}\). This result indicates that DBD-NH\({}_{3}\)-SCR with CH\({}_{4}\) as an assistant reduction agent achieved a preferable performance and product selectivity for NO\({}_{X}\) abatement. To further verify and explore the above experimental results, the final products for the two DBD-cataly systems with only NH\({}_{3}\)/O\({}_{2}\) or N\({}_{2}\)/O\({}_{2}\) in the feed are shown in Figure 6a,b, respectively. According to Figure 6a, N\({}_{2}\)O (16.76 ppm) and NO\({}_{2}\) (10.97 ppm) were observed in the final products when the supplied gases only included NH\({}_{3}\), O\({}_{2}\), and N\({}_{2}\) balance gas. According to a previous research report [47], O\({}_{2}\) not only competitively shares the input power but also contributes to the oxidation of NO. NO, N\({}_{2}\), and NH\({}_{3}\) were dissociated by electron impact dissociation reactions in the discharge area, namely, e + NO \(\rightarrow\) e + NO\({}^{*}\), e+N\({}_{2}\)\(\rightarrow\)e+2N and e+NH\({}_{3}\)\(\rightarrow\)e+NH\({}_{2}\)+\(\cdot\)H. Excited N, NO\({}^{*}\), NH\({}_{3}\), and \(\cdot\)NH\({}_{2}\) would recombine with O and O\({}_{2}\) to produce new NO\({}_{X}\). However, according to Figure 6b, there was only O\({}_{3}\) (98.16 ppm) in the reaction system of N\({}_{2}\)/O\({}_{2}\). This indicates that the electrons generated from the DBD-SCR reactors could not decompose N\({}_{2}\) in this reaction system, similar conclusions are studied in another paper [54]. Figure 4: (**a**) N\({}_{2}\)O concentration for DBD–SCR at SIE ranging from 128 J/L to 512 J/L. (**b**) NO\({}_{2}\) concentration for DBD–SCR at SIE ranging from 128 J/L to 512 J/L. Experimental conditions: 12% O\({}_{2}\), and other conditions are the same as those in Figure 2.  
Figure 5: The product concentration and Fourier transform infrared spectroscopy (FTIR) spectra observed in different systems over V\({}_{2}\)O\({}_{5}\)–WO\({}_{3}\)/TiO\({}_{2}\) with different O\({}_{2}\) and SIE. (**a,b**): DBD–NH\({}_{3}\)–SCR, (**c,d**): DBD–NH\({}_{3}\)–CH\({}_{4}\)–SCR. Feed gas composition: 400 ppm NO, balance N\({}_{2}\) and 90 ppm NH\({}_{3}\) in DBD–NH\({}_{3}\)–SCR, 45 ppm CH\({}_{4}\) and 45 ppm NH\({}_{3}\) in DBD–NH\({}_{3}\)–CH\({}_{4}\)–SCR. Figure 6: FTIR spectra of final products for the DBD-cataly system with NH\({}_{3}\)/O\({}_{2}\) and N\({}_{2}\)/O\({}_{2}\) (reaction conditions: (**a**) 90 ppm NH\({}_{3}\), 12% O\({}_{2}\) and balance N\({}_{2}\); (**b**) 12% O\({}_{2}\) and 88% N\({}_{2}\)). Because NH\({}_{3}\) could recombine with excited O to produce new NOx, it is an inevitable fact that O\({}_{2}\) has a negative effect on NO removal in DBD-NH\({}_{3}\)-SCR, which is consistent with the results shown in Figure 3. On the contrary, the recombination of NH\({}_{3}\) and O\({}_{2}\) could be controlled in DBD-NH\({}_{3}\)-CH\({}_{4}\)-SCR. During plasma discharge, hydrocarbon could be decomposed and further generate useful intermediates, such as methyl (-CH\({}_{3}\)) and methyldioxy (CH\({}_{3}\)O\({}_{2}\)), which could react with NO molecules [55; 56].