Recent advances in ammonia synthesis over ruthenium single-atom-embedded catalysts: A focused review

In order to achieve the net-zero carbon emission target in 2050 by reducing the use of fossil fuels and their environmental impact, ammonia is increasingly considered as a key energy carrier for the future energy transition. Industrial-scale NH3 synthesis via the Haber-Bosch process revolutionized global agriculture and industry. However, this process runs under harsh reaction conditions and is therefore very energy-intensive and releases vast quantities of CO2; this leaves ample room for improvement by developing highly efficient catalysts that would allow the process to take place under mild reaction conditions. Since the end of the 20th century, ammonia synthesis has been extensively investigated by employing Ruthenium (Ru)-based catalysts (as secondgeneration catalysts in this process) because of their high activity and outstanding thermal stability. However, their practical applications are hindered by the extremely low abundance and high cost of the precious Ru metal and the poor understanding of the nature of the Ru catalytic mechanism. Single-atom (SA) systems have emerged as a promising class of catalysts that could enable the highly active and selective conversion of N2 to NH3. Notably, reducing the Ru particle size to the sub-nanometer and SA scale could dramatically enhance the catalytic efficiency in NH3 synthesis by increasing the number of active sites over the surface of catalysts and adjusting the structure sensitivity. This review briefly introduces the fundamental mechanism of NH3 synthesis over Ru-based catalysts and the general parameters for assessing their catalytic activity. Moreover, we discuss indepth mechanistic studies and recent advances in Ru SA catalysts as an efficient platform for ammonia synthesis by thermocatalysis, photocatalysis, and electrocatalysis. We also review density functional theory calculations investigating the NH3 yield of the reported catalysts. Finally, we discuss economical aspects and future perspectives that may provide solutions for improving NH3 production. The present review aims to provide a comprehensive understanding of the advancement in the catalytic ammonia synthesis field and serves as a guide for the future design of highly efficient SA-embedded catalysts.