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 ∼ 1.3% of global CO2 emissions, hence making urgent the development of sustainable alternative synthetic pathways. Green NH3 production from molecular N2, H2O and renewable energy represents a valuable choice. In this review, recent achievements in N2 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 N2 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 N2 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 °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. Graphic abstract: (Figure presented.)