## 3 Results and Discussion
### Plasma-Catalytic Conversion of Biogas
In contrast, catalysts with lower acidity, especially with a higher proportion of weak acid sites, tend to exhibit better coke resistance. The NH\({}_{3}\)-TPD analyses revealed that the 5 Ni-5 Fe/Al\({}_{2}\)O\({}_{3}\) catalyst exhibited a lower density of acidic sites compared to the monometallic counterparts. This reduction in surface acidity likely contributed to the suppression of coke formation, which is often associated with acid-catalyzed reactions. These results strongly suggest the considerable potential of plasma-mediated catalytic processes for the efficient conversion of biogas, a mixture of CH\({}_{4}\) and CO\({}_{2}\), into valuable liquid products, thereby offering a sustainable route for resource utilization. To further elucidate the composition of the liquid products obtained, the selectivity of various oxygenated compounds generated using the 5 Ni-5 Fe/Al\({}_{2}\)O\({}_{3}\) catalyst under plasma discharge was meticulously analyzed and presented in Table 3. A detailed examination of the data, as presented in Table 4, revealed that acetic acid exhibited the highest selectivity, registering 23.9%, while formaldehyde displayed the lowest, at 10.5%. Additionally, other oxygenates, including methanol, propanoic acid, ethanol, and isopropyl alcohol, were detected with selectivity of 19.1%, 16.3%, 14.5%, and 15.7%, respectively. These findings provide valuable insights into the product distribution and selectivity of the plasma-catalytic process, highlighting its potential for the production of a diverse range of oxygenated chemicals. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Run** & **H\({}_{2}\) Yield (\%)** & **CO Yield (\%)** & **C\({}_{x}\)H\({}_{y}\) Yield (\%)** & **Liquid Yield (\%)** & **Coke (\%)** \\ \hline Catalyst only & 0 & 0 & 0 & 0 & 0 \\ Plasma only & 48.9 & 29.2 & 26.7 & 0 & 44.1 \\
10 Fe/Al\({}_{2}\)O\({}_{3}\) & 44.2 & 28.4 & 25.9 & 30.9 & 14.8 \\
10 Ni/Al\({}_{2}\)O\({}_{3}\) & 43.3 & 27.8 & 25.1 & 34.0 & 13.1 \\
2.5 Ni-7.5 Fe/Al\({}_{2}\)O\({}_{3}\) & 42.4 & 27.5 & 24.6 & 35.6 & 12.3 \\
7.5 Ni-2.5 Fe/Al\({}_{2}\)O\({}_{3}\) & 41.7 & 27.3 & 24.3 & 36.7 & 11.7 \\
5 Ni-5 Fe/Al\({}_{2}\)O\({}_{3}\) & 40.1 & 27.1 & 23.9 & 38.6 & 10.4 \\ \hline \end{tabular}
\end{table} 
Table 3: Products analysis in plasma-catalytic experiments for biogas conversion. When biogas is converted into liquid chemicals in a plasma environment, it likely happens through a radical chain reaction. In this process, radicals like CO, CH\({}_{3}\), and OH are key players in converting CH\({}_{4}\) and CO\({}_{2}\)[10; 20]. In a plasma setting, CO\({}_{2}\) becomes excited through electron collisions, breaking it down into CO and O radicals. Similarly, methane breaks apart to form H and CH\({}_{x}\) radicals (where x can be 1 to 3), with CH\({}_{3}\) being the most common. These CH\({}_{x}\) radicals can come together to form C\({}_{2}\)-C\({}_{4}\) hydrocarbons, as seen in the experiments [21]. High-energy electrons can also break down the CH\({}_{x}\) radicals further, causing carbon to accumulate on the catalyst surface. OH radicals form when H and O radicals combine, and H radicals from methane can interact with excited CO\({}_{2}\) to create more OH and CO radicals [10]. When CH\({}_{3}\) and OH radicals react, they form methanol, while C\({}_{2}\)H\({}_{5}\) radicals can combine with OH radicals to make ethanol. The reaction between O/OH radicals and CH\({}_{x}\) may lead to formaldehyde (HCHO). CO radicals also react with CH\({}_{3}\) to form acetyl radicals, which then react with CH\({}_{3}\) to produce acetone. Using a catalyst in the DBD plasma system changes the plasma discharge, shifting it from gas microdischarges to surface discharges [22]. This shift can help prevent the breakdown of CH\({}_{x}\) into carbon byproducts. Plus, the catalyst's porous structure provides a larger surface area, which helps increase the production of liquid products [22]. Among the various liquid products formed during plasma-assisted biogas conversion, acetic acid consistently exhibited the highest selectivity.