Iron-group single-atom catalysts for ammonia synthesis and decomposition

Ammonia (NH3) plays a pivotal role in the global economy other than being considered one of the most promising C-free liquid hydrogen carrier for addressing a sustainable energy transition. The current Haber-Bosch process for ammonia production accounts for similar to 1.3% of global CO2 emissions, hence making urgent the development of sustainable alternative synthetic pathways. Green NH3 production from molecular N-2, H2O and renewable energy represents a valuable choice. In this review, recent achievements in N-2 photo- and (photo)electrochemical reduction to NH3 promoted by iron-group single-atom catalysts (SACs) are discussed. The influence of SACs on the optical, electrochemical and catalytic properties of the photoactive materials for the green ammonia production by photocatalytic and photo-electrocatalytic technologies is reported. As far as the electrochemical N-2 reduction is concerned, the structure-reactivity relationship and the influence of the metal coordination environment on materials ultimate activity will be discussed. Despite ammonia productivity values are still far from that necessary for industrial requirements, many seminal outcomes appeared in the literature clearly highlight the potentiality of Fe SACs to reach remarkable N-2 reduction efficiency and NH3 selectivity. Compared to SACs for ammonia synthesis, those for the decomposition to produce hydrogen are much more limited both in number and active metals. Ru-SACs have mainly been developed and are featured with relevant activity around 400 degrees C, which is a major achievement considering the poor activity of traditional Ru catalysts at low temperatures. Only very recently, Co and Ni SACs have been reported showing promising performances, even comparable to some noble metal catalysts, also in photo-driven applications.