## 3 Results and Discussion
### Optical Emission Spectroscopy  
To better elucidate the reaction mechanism of ammonia synthesis when N\({}_{2}\) is used as the discharge gas, we investigated the OES of the WF-DBD reactor under various conditions. Figure 6 shows the OES of sealed N\({}_{2}\) and flowing N\({}_{2}\) (50-300 SCCM) as discharge gases. As seen in Figure 6a, the wavelength range between 300 and 450 nm is dominated by the N\({}_{2}\) second positive band system (SPS) at 311-380 nm, which corresponds to the N\({}_{2}\)  
Figure 4: (**a**) Concentrations of NH\({}_{4}\)\({}^{+}\), NO\({}_{3}\)\({}^{-}\), and NO\({}_{2}\)\({}^{-}\) produced in aqueous solutions under different N\({}_{2}\) flow rates, and (**b**) the energy consumption and the selectivity of NH\({}_{4}\)\({}^{+}\). Figure 5: (**a**) Concentrations of NH\({}_{4}\)\({}^{+}\), NO\({}_{3}\)\({}^{-}\), and NO\({}_{2}\)\({}^{-}\) produced in aqueous solutions under different peak–peak voltages, and (**b**) the energy consumption and the selectivity of NH\({}_{4}\)\({}^{+}\). \(\rightarrow\) B\({}^{3}\)T\({}_{B}\)) transition [30]. This is followed by the N\({}_{2}\) first negative band system (FNS) at 390-440 nm, corresponding to the N\({}_{2}\)\({}^{+}\) (B\({}^{2}\)\(\sum_{u}\)\({}^{+}\)\(\rightarrow\) X\({}^{2}\)\(\sum_{g}\)\({}^{+}\)) transition [31]. This indicates the existence of electronically excited nitrogen molecules (R2) and ionization of nitrogen molecules (R3) during the synthesis of ammonia [19]. The OH peak was not observed at 309 nm, which suggests that the OH generated is not in the gas phase and may be rapidly dissolved in the liquid phase after generation. \[\mathrm{N}_{2}+\mathrm{e} \rightarrow\mathrm{N}_{2}^{*}+\mathrm{e} \tag{2}\] \[\mathrm{N}_{2}+\mathrm{e} \rightarrow\mathrm{N}_{2}^{+}+\mathrm{e} \tag{3}\]  
Vibrationally excited N\({}_{2}\)(v) has been recognized as an important activator in ammonia or NO\({}_{x}\) synthesis [32; 33]. At lower flow rates of nitrogen, NH\({}_{4}\)\({}^{+}\) predominated in the products, indicating more conversion of the active species to ammonia; at higher flow rates, NO\({}_{3}\)\({}^{-}\) predominated in the products, indicating more conversion of the active species to NO\({}_{3}\)\({}^{-}\) (Figure 4). The intensity variations of the SPS and FNS bands in OES can be characterized by the changes in the characteristic peak intensities. The values of the characteristic peaks of the SPS and FNS bands at different gas flow rate states are normalized as shown in Figure 6b. It can be seen that ammonia synthesis at lower flow rates corresponds to the intensity of the SPS bands of OES, while at higher flow rates it corresponds to NO\({}_{3}\)\({}^{-}\) synthesis. The above spectral evidence suggests that excited N\({}_{2}\)\({}^{*}\) is associated with plasma-activated ammonia synthesis processes at low flow rates of N\({}_{2}\). Therefore, the ammonia synthesis process at low flow rates of N\({}_{2}\) can be qualitatively characterized using SPS band intensities. Figure 6c,d shows the trends of the characteristic peak intensities of the SPS and FNS bands with time for different gas flow rate states, and it can be seen that the intensities of both of them decrease with the Increase in the discharge time. This may be attributed to the fact that part of the energy during the reaction process is used for the decomposition of water at the liquid surface to produce hydrogen, and the  
Figure 6: **(a)** OES of the WF-DBD reactor under sealed N\({}_{2}\) and flowing N\({}_{2}\) conditions. **(b)** Normalized relative intensity of the N\({}_{2}\) (C \(\rightarrow\) B) and the N\({}_{2}\)\({}^{+}\) (B \(\rightarrow\) X). **(c,d)** The temporal evolution of the emission intensity of the N\({}_{2}\) (C \(\rightarrow\) B) and the N\({}_{2}\)\({}^{+}\) (B \(\rightarrow\) X). other part of the energy is lost in the form of heat, resulting in a decrease in the intensity of the gas electric plasma emission spectra.