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.