Towards green and efficient chemical looping ammonia synthesis: design principles and advanced redox catalysts

Ammonia plays an essential role in agriculture and next-generation energy systems but is currently synthesized industrially through the Haber–Bosh (HB) process under harsh conditions with high CO₂ emissions. Chemical looping ammonia synthesis (CLAS) is an attractive alternative to the traditional HB process as it can break the Brønsted–Evans–Polanyi (BEP) scaling relationship and circumvent the competitive adsorption of N₂ and H₂ on metal catalysts by decoupling ammonia production into multiple reaction steps. The realization of highly efficient CLAS relies on developing redox catalysts with high activity at low temperatures. This review describes recent theoretical and experimental progresses in CLAS. The rational design of redox catalysts underlines the advantages of combined numerical and experimental approaches for the development of efficient redox catalysts towards green and efficient CLAS processes. Redox catalysts and external field-assisted technologies for lowering the reaction temperature and accelerating the reaction kinetics of CLAS are spotlighted, and relevant reaction mechanisms are discussed. The feasibility of the CLAS process based on the techno-economic analysis is reviewed. The challenges and opportunities of redox catalysts and reaction systems for CLAS are further discussed.