Microwave heating-assisted chemical looping ammonia synthesis over Mn-Fe and Mn-Fe-BaH2 nitrogen carriers

Ammonia, a promising hydrogen carrier, holds significant potential for advancing the storage and transportation of renewable energy and facilitating the decarbonization of industry, transportation, and households. Conventional ammonia synthesis technologies, like Harber-Bausch technology, often operate under harsh operating conditions and/or with low production rates or encounter substantial thermodynamic limitations, impeding their scalability. In this study, we developed a new ammonia synthesis technology based on microwave heatingassisted chemical looping to minimize these challenges. We applied synthetic nitrogen carriers, i.e., Mn-Fe and Mn-Fe-BaH2 synthesized by mechanical and/or coprecipitation approaches, to transport lattice nitrogen from a nitridation step (at atmospheric pressure and 400 degrees C) to a hydrogenation step (at atmospheric pressure and 150-350 degrees C), where ammonia was produced. The simultaneous application of microwave heating and chemical looping resulted in a lower gas-phase temperature compared to the solid phase within a gas-microwave absorber solid system. With this strategy, we minimized undesirable secondary gas-phase reactions, circumvented thermodynamic limitations associated with ammonia production, and achieved a significantly higher ammonia production rate compared to conventional heating methods. The Mn-Fe-BaH2 nitrogen carrier demonstrated a higher ammonia production rate, i.e., around 36,000 mu mol/g.h under MW heating, surpassing previously reported values in literature and the Mn-Fe nitrogen carriers synthesized in this work.