Atomically dispersed metal catalysts towards nitrogen reduction for Ammonia: From homogeneous to heterogeneous

Ammonia (NH3) is an essential chemical in agriculture and industry, which plays a crucial role in human life and sustainable development. To date, ammonia production mainly relies on the energy-intensive Haber-Bosch process, resulting in significant energy consumption and CO2 emissions. Electrocatalytic nitrogen reduction reaction (NRR) that can produce NH3 under ambient conditions is considered as a potential alternative to Haber-Bosch. However, electrocatalytic NRR has various limitations, including low ammonia yield, inferior selectivity due to competing hydrogen evolution reactions, and sluggish reaction kinetics. By virtue of maximum metal atom utilization and tunable electronic structure, atomically dispersed metal catalysts (ADMCs) offer great opportunities for NRR performance optimization, and thus being the most promising NRR catalysts. In this review, we summarize the developmental evolution of ADMCs from homogeneous to heterogeneous phases. We start with a fundamental understanding of the reaction mechanism, detection methods, and key evaluation parameters of NRR. Following this, atomically dispersed homogeneous metal catalysts, including noble metal-based, transition metal-based, and titanium chloride-based, are presented with their advantages and disadvantages analyzed in detail. To conquer the shortcomings of homogenous catalysts, heterogenous catalysts with various supports are more appealing and their recent progress is documented as well. Finally, we present the challenges and perspectives of the ADMCs catalysts for NRR, aiming to provide guidance for advanced NRR catalyst design.