Efficient green synthesis of ammonia: from mechanistic understanding to reactor design for potential production

Ammonia (NH₃), one of the world's most vital chemicals and energy carriers, has attracted wide attention. Currently, NH₃ is mainly produced using the traditional, energy-intensive Haber-Bosch (H-B) technology, which has a large impact on the environment. Therefore, developing a low-cost, high-efficiency, and eco-friendly way to produce NH₃ is highly desirable. Photo-, electro-, photoelectro-, and alkali-metal-mediated catalytic reactions powered by renewable and clean energy under ambient conditions offer alternatives to the H-B process and have recently gained significant interest. However, efficient nitrogen reduction is a key requirement, limiting the selectivity and activity for the green synthesis of NH₃ because the N₂ activation process in a green catalytic system is difficult to complete due to its thermodynamic instability and chemical inertness. Compared to the reduction of N₂, the catalytic reduction of some soluble and harmful high-valent sources (e.g., NO, NO₂^-, and NO₃^-) is considered an effective method for increasing NH₃ synthesis efficiency. This review article focuses on the important features of the green catalytic conversion of multiple nitrogen resources into NH₃ by summarizing the fundamental mechanistic understanding, catalytic descriptors, and current advances, along with the various catalysts used for these conversion strategies and their structure-activity relationships. Meanwhile, opportunities and prospects for reactor design and construction for potential NH₃ production at high current densities are also discussed, focusing on achieving a high yield rate, Faraday efficiency, and energy efficiency. This will provide valuable guidance for constructing catalysts and optimizing reaction systems that can meet the needs of practical applications.