Insight in plasma-catalysis nitrogen fixation: underlying mechanism of NO and NO₂ formation in a microwave plasma coupled with heterogeneous catalysts

Nitrogen fixation (NF) is a process that converts molecular nitrogen into valuable compounds, such as NH₃ or NO_x, which are used to produce various industrial products¹. Commercial NF processes consume a significant amount of energy and have environmental drawbacks. As an alternative, a plasma-catalysis process can be applied to improve energy efficiency and reduce byproducts of the NF process. However, the underlying mechanisms of the plasma-catalyst interactions remain unclear, which hinders further advancements in both NO_x production rates and reduced energy consumption². Therefore, understanding the plasma-catalyst interactions for NO_x synthesis, specifically in the form of NO and NO₂, is crucial for sustainable artificial N-fixation. In this work, we present the results of post-plasma catalysis NO_x formation in a microwave discharge. Heterogeneous catalysts made of transition metal oxides (Mo and Co) supported on γ-alumina were prepared. To explain the underlying mechanism, the performance of NO_x formation is discussed in relation to the characteristics of the catalysts and plasma properties, collectively. The characterization of catalysts and in-situ plasma diagnostics indicate that NO formation likely occurs in the plasma phase. On the other hand, NO₂ is formed in the plasma-catalysis phase due to the adsorbed oxidized nitrogen species originating from the N₂-O₂ plasma. However, considering the catalyst position in our experiments and the lifetime of the reactive species in the system, the catalysts have a limited effect on the enhancement of NO_x. Our studies demonstrate that NO_x formation can be significantly influenced and optimized by experimental conditions or by tuning the plasma-catalyst interaction.