## 3 Results and Discussion
### Nitrogen Fixation Process  
#### 3.2.1 Gas Phase Products  
The NF pathways toward nitrogen oxides formation were already comprehensively investigated elsewhere [50; 51; 52]. This work therefore only intends to give a brief overview of the NF kinetics, considering the species generated in the plasma. The reactions discussed within this study together with their reaction rate constants are summarized in the SI, Table S1. In the gaseous phase of an air plasma, NO radicals are preliminary species formed via the so-called Zeldovich mechanism [53; 54]. Under nonequilibrium plasma conditions, the main driving force in this mechanism is molecular nitrogen excited states, including electronically and vibrationally excited nitrogen molecules. Simplified reactions are shown in the reactions below:  
Reaction (1):  
\[\rm N_{2}+O\rightarrow\rm NO+N\]  
Reaction (2):  
\[\rm N+O_{2}\rightarrow\rm NO+O\]  
However, in the presence of water, this mechanism can be extended by a third reaction involving OH radicals, which are one of the main products resulting from water dissociation:  
Reaction (3):  
\[\rm N+OH\rightarrow\rm NO+H\]Reactions (1)-(3) are also known as the extended Zeldovich mechanism, where Reaction (3) has a reaction rate constant of approximately one order of magnitude higher than Reaction (2). In turn, OH radicals are mainly generated through reactions with electronically excited oxygen atoms, molecular nitrogen, and via direct electron dissociation, as presented in Table S1. This means that the energetic species generated by plasma can be spent differently depending on the gas composition, and it is difficult to predict which mechanism is more efficient without solving (at least) a 0-D computer model. In a next step, the produced NO can be oxidized to form NO\({}_{2}\) as follows [55; 56]:  
Reaction (4):  
\[\text{NO}+\text{O}_{3}\rightarrow\text{O}_{2}+\text{NO}_{2}\]  
However, Reaction (4) requires the presence of ozone which is not stable at high temperatures, as is the case in this work [57]. Under this condition, other pathways resulting in NO\({}_{2}\) generation can be as follows:  
Reaction (5):  
\[\text{NO}+\text{O}\rightarrow\text{NO}_{2}\]  
Reaction (6):  
\[2\text{NO}+\text{O}_{2}\rightarrow\text{2 NO}_{2}\]  
Considering these mechanisms, the plasma outlet gas was monitored for the presence of NO and NO\({}_{2}\) species. The results are shown in Figure 6 for both Me-Me and Me-L configurations as a function of SEI for gas flow rates of 4, 8, and 12 L min\({}^{-1}\) (the results for all flow rates under investigation are shown in the SI, Figure S7). According to the series of graphs illustrated in Figure 6, both NO and NO\({}_{2}\) concentrations increase with increasing SEI. Indeed, a higher power leads to an increase in SEI, and, thus, a higher number of energetic electrons. These electrons collide with carrier gas molecules (N\({}_{2}\) and O\({}_{2}\)), forming energetic species, e.g., electronically or vibrationally excited states, which contribute to NO and NO\({}_{2}\) formation [28]. Interestingly, the curves show an identical behavior across all experimental gas flow rates. This observation suggests that the mechanisms through which NO\({}_{\text{x}}\) species are being generated are independent of the used gas flow rates. It is also worth mentioning that the NO\({}_{\text{x}}\) formation in both systems for all investigated gas flow rates more rapidly increases for SEI values higher than 1000 J/L compared to its increase at lower SEI values. This can be attributed to the more often plasma streamer appearance and the increase in plasma-treated gas volume at higher SEI values, indicating a shift in the occurring plasma-based nitrogen fixation chemical kinetics. At high SEI values, the high plasma frequency promotes the formation of reactive species that can trigger reverse processes, causing a decrease in NO\({}_{\text{x}}\) formation, as was already shown elsewhere [51].