Single-atom and cluster catalysts for thermocatalytic ammonia synthesis at mild conditions

Ammonia (NH₃) is closely related to the fields of food and energy that humans depend on. The exploitation of advanced catalysts for NH₃ synthesis has been a research hotspot for more than one hundred years. Previous studies have shown that the Ru B₅ sites (step sites on the Ru (0001) surface uniquely arranged with five Ru atoms) and Fe C₇ sites (iron atoms with seven nearest neighbors) over nanoparticle catalysts are highly reactive for N₂-to-NH₃ conversion. In recent years, single-atom and cluster catalysts, where the B₅ sites and C₇ sites are absent, have emerged as promising catalysts for efficient NH₃ synthesis. In this review, we focus on the recent advances in single-atom and cluster catalysts, including single-atom catalysts (SACs), single-cluster catalysts (SCCs), and bimetallic-cluster catalysts (BCCs), for thermocatalytic NH₃ synthesis at mild conditions. In addition, we discussed and summarized the unique structural properties and reaction performance as well as reaction mechanisms over single-atom and cluster catalysts in comparison with traditional nanoparticle catalysts. Finally, the challenges and prospects in the rational design of efficient single-atom and cluster catalysts for NH₃ synthesis were provided.