Size sensitivity of supported Ru catalysts for ammonia synthesis: From nanoparticles to subnanometric clusters and atomic clusters

Ammonia synthesis is structure sensitive, and a minute change in the catalyst structure would cause a dramatic change in activity. To date, none of the studies reveal the metal size effect at a subnanometer scale on NH3 synthesis, and such investigation remains a challenge. Here, we report the synthesis of Ru catalysts with sizes ranging from single atoms, atomic clusters (ACCs), sub-nanometric clusters, to nanoparticles (NPs) by adjusting precursor and/or loading of Ru. Sub-nanometric Ru catalysts not only exhibit performance different from that of NPs but also follow a different route for N-2 activation. The strong intra-cluster interaction of Ru atomic clusters enables the formation of strong interactions of Ru d-orbitals with the sigma and pi orbitals of N-2 molecules, resulting in N-2 activation over Ru ACCs to occur more easily than that over Ru NPs. Consequently, Ru ACCs display an unprecedentedly high NH3 synthesis rate and large turnover frequency at mild conditions.