(302d) Microwave Catalytic Synthesis of Ammonia for Energy Storage and Transformation

This paper presents an innovative approach of producing energy-dense, carbon-neutral liquid ammonia as a means of energy carrier. The approach synergistically integrates microwave reaction chemistry with novel heterogeneous catalysis that decouples N₂ activation from high temperature and high pressure reaction, altering reaction pathways and lowering activation energy. Results have shown that ammonia synthesis can be carried out at 280 ℃ and ambient pressure to achieve ~1 mmol NH₃/g cat. /hour over supported Ru catalyst systems. Adding promoters of K, Ce and Ba has significantly improved the ammonia production rate over Ru-based catalysts that could be attributing to enhanced electromagnetic sensitivity of the catalysts under microwave irradiation. We have observed that under microwave catalytic conditions, ammonia yield higher than corresponding thermal equilibrium can be obtained using either Fe or Ru based catalysts. This study also illustrates the advantages of an innovative variable-frequency microwave reactor for ambient pressure ammonia synthesis. It is the first reporting of variable frequency microwave catalytic reaction that can be utilized to selectively activate stable molecules, such as N₂, for ammonia synthesis. Overall, microwave catalytic ammonia synthesis is fundamentally different from commercial Haber-Bosch process. It can be tolerant to intermittent renewable energy supply, therefore effectively operated at variable rates of production.