Mechanistic Insights into NOx Production in Microwave Plasma at Intermediate Pressure

Microwave plasma has emerged as one of the most energy-efficient approaches for nitrogen fixation. To elucidate the underlying mechanisms at intermediate pressure, a quasi-1.5D physicochemical multitemperature model is developed under varying N2-O2 compositions. The plasma shape and radial gas temperature profile, derived from the emission intensity distribution and the Doppler broadening of the 777 nm O(5S ←5P) atomic oxygen triplet, serve as key model inputs for determining the power density profile and turbulent viscosity, respectively. The model captures the coupled interplay among vibrational, chemical, and electron kinetics in microwave plasma NOx synthesis, with particular emphasis on the role of vibrational excitation at 80 mbar. The energy costs predicted by the model show good agreement with the experimental results measured using Fourier-transform infrared spectroscopy. Nonthermal behavior within the plasma core is found to strongly promote NO formation. Radial diffusion emerges as a key mechanism for sustaining chemical nonequilibrium, and improving overall NO yield. Key reactions involved in NO formation and destruction under different initial gas mixtures are discussed. Finally, it is suggested that the energy cost can be improved by optimizing the plasma shape. This work offers fundamental insights into the underlying plasma-chemical mechanisms and establishes a predictive framework to guide the future design and optimization of energy-efficient microwave plasma technologies for nitrogen fixation.