One-step direct ammonia synthesis by pulse-modulated microwave plasma at atmospheric pressure

In this study, we explored a one-step direct synthesis of NH₃ under mild experimental conditions utilizing pulse-modulated microwave plasma technology at atmospheric pressure. At a substantial gas flow rate, a microwave plasma jet was formed and the microwave-assisted ammonia synthesis can be realized. Impacts of various parameters including the gas flow rate, gas component, microwave absorbed power, pulse modulation frequency, and pulse duty cycle on ammonia synthesis were systematically investigated. To indicate the reaction path of ammonia synthesis, the distributions of both the gas temperature and active species were also studied using optical emission spectra technology. It is found that a considerable amount of ammonia was directly synthesized without involvement of any catalysts, the highest ammonia production rate and energy efficiency (EE), up to 2.93 μmol·min⁻¹ and 6.64×10⁻² g·(kW·h) ⁻¹, respectively, were achieved under low microwave power of 84.42 W. The duty cycle has obvious influences on the synthesis efficiency, compared to a duty cycle of 80%, the ammonia synthesis rate, EE and nitrogen conversion decreased by about 22% at a duty cycle of 100%. This finding underscores the significance of incorporating pulse modulation in the microwave discharge process for ammonia synthesis. Furthermore, it was found that vibrational excitation of microwave plasma has a significant driving effect on ammonia synthesis.