Nitrogen fixation by atmospheric microwave plasma is a promising green technology. However, the energy consumption (EC) of nitrogen fixation driven by microwave plasma is still too high in existing reports, and there is a lack of effective methods to reduce it. In this paper, a method of local electric field enhancement (LEFE) in atmospheric microwave plasma is reported, which is achieved in an optimized metal array structure that can increase the electric field intensity by 385 times. The experimental results show that the reduction of EC reaches up to 20% and the nitrogen oxide production increases by up to 26% with the LEFE. The achieved EC is as low as 1.78 MJ mol⁻¹ with a corresponding nitrogen oxide production of 3.53%. To the best of our knowledge, this is the first study to explore the role of LEFE on nitrogen fixation, and the EC in this study is the lowest among the existing studies of nitrogen fixation by atmospheric pressure microwave air plasma.

Microwave plasma-driven nitrogen fixation can occur at atmospheric pressure without complex processing conditions. However, this method still faces the challenge of high energy consumption and low production. Combined plasma–catalyst systems are widely used to increase production and reduce energy consumption in nitrogen fixation. However, the efficacy of currently used catalysts remains limited. In this paper, the metal–organic framework materials (MOFs) copper benzene-1,3,5-tricarboxylate (Cu-BTC) and zeolitic imidazolate framework-8 (ZIF-8) are combined with atmospheric microwave plasma for nitrogen fixation. The experimental results show that they have a better catalytic effect than the ordinary catalyst zeolite socony mobil-5 (ZSM-5). The maximum nitrogen oxide concentration reaches 33,400 ppm, and the lowest energy consumption is 2.05 MJ/mol. Compared to no catalyst, the production of nitrogen oxides (NO_x) can be increased by 17.1%, and the energy consumption can be reduced by 14.6%. The stability test carried out these catalysts demonstrates that they have a stable performance within one hour. To the knowledge of the authors, this is the first effort to study the synergistic effects of atmospheric microwave plasma and MOFs on nitrogen fixation. This study also introduces a potentially eco-friendly approach to nitrogen fixation, characterized by its low energy consumption and emissions.

Conventional atmospheric microwave plasma sources face challenges in achieving plasma generation at the milliwatt power level, often requiring a reduction in operating pressure to generate plasma at lower power levels. This article presents a novel atmospheric microwave plasma source based on the coaxial structure to generate plasma at the milliwatt power level at atmospheric pressure. First, numerical simulations based on the finite element method (FEM) and equivalent transmission line model (ETLM) have been done to design the structure. Simulation results show that at the optimal structure, the microwave reflection ratio $\vert \textit{S}_{\text{11}}\vert$ at 2.45 GHz is equal to $-$ 19.58 dB before plasma excitation and the electric field strength reaches 6.03 $\times$ 10 $^{\text{5}}$ V/m when the input power is 1 W. Second, a practical experimental system is built according to the simulations. Experimental results show that the $\vert \textit{S}_{\text{11}}\vert$ at its resonant frequency is $-$ 19.2 dB before plasma excitation, which agrees well with the simulations. Moreover, the minimum input microwave power values for generating argon, CO $_{\textit{2}}$ , and air plasmas are 100 mW, 5 W, and 10 W, respectively. Finally, this novel atmospheric microwave plasma source has been used to decompose and convert CO $_{\text{2}}$ , the highest conversion rate and energy efficiency reach 24.1% and 49.2%, respectively, which are comparable to low-pressure microwave plasmas. Thus, the proposed microwave plasma source in this article has great potentials for industrial applications to reduce carbon emissions.

The rectangular tapered waveguide is the most widely used atmospheric microwave plasma source because of its simple structure, reliable performance, and low cost. However, due to the small generated plasma volume and short residence time, this plasma source has low energy efficiency in gas conversion applications such as nitrogen fixation and methane reforming. This paper proposes a novel atmospheric-pressure microwave plasma source based on a circular waveguide to enlarge the plasma volume. Firstly, the microwave working mode in the circular waveguide is analyzed based on the electromagnetic theory. By optimizing the waveguide dimension and loading double metal ridges into the waveguide, the electric field will work in a superposition mode of TE₁₁₁ and TM₀₁₀ modes, focus on the discharge area, and reach its maximum intensity there. Secondly, this device is manufactured and used to generate plasma in different conditions. Compared with the rectangular tapered waveguide, this device can generate larger plasmas under the same conditions. Finally, an atmospheric microwave plasma experimental system is built for nitrogen fixation. Measured results show that the proposed device has higher nitrogen fixation production and lower energy cost compared to the rectangular tapered waveguide. Moreover, the proposed device is easy to cascade, thereby it has great potential for promotion in industrial applications.

Microwave plasma-driven nitrogen fixation can occur at atmospheric pressure without greenhouse gas emissions. However, this method still faces the challenge of high energy consumption and low production. Plasma-catalyst combined systems are widely used to increase production and reduce energy consumption in nitrogen fixation. However, the efficacy of current catalysts remains limited. In this paper, the metal-organic framework materials CuBTC and ZIF-8 are combined with the atmospheric microwave plasma for nitrogen fixation. The experimental results show their better catalytic effect than the ordinary catalyst ZSM-5. The maximum nitrogen oxide concentration reaches 33400 ppm, and the lowest energy consumption is 2.05 MJ/mol. Compared to no catalyst, the nitrogen oxide production can be increased by 17.1%, and the energy consumption can be reduced by 14.6%. The transmission electron microscopy (TEM) images of the catalysts reveal obvious differences in the catalyst particle sizes. Notably, catalysts with smaller particle sizes attain higher NOx concentrations compared to their larger counterparts. To the knowledge of the authors, this is the first effort to study the synergistic effects of atmospheric microwave plasma and metal-organic framework materials on nitrogen fixation.