On the Nature of Three-Atom Metal Cluster Catalysis for N2 Reduction to Ammonia

Catalytic N2 activation and reduction for ammonia synthesis has been subject of intense research interest. Cluster-modified catalysts have been proposed as promising candidates for nitrogen activation due to the featured active sites and maximized synergistic effect. However, the nature of metal clusters itself has not been fully unveiled. Herein, we report a systematic investigation of N2 activation and reduction on three-atom metal clusters (M3) of all the 20 transition metals in the third and fourth periods of elements. We evaluate the catalysis of these M3 clusters by taking into consideration three critical processes, namely, N2 dissociation, hydrogenation, and NH3 desorption. The TMI series of the M3 clusters (Group 3B–5B metals) are found to support N2 dissociation spontaneously, in contrast to the TMII and TMIII clusters (i.e., Groups 6B–8B and 1B–2B). Based on the three criteria, Y3, Sc3, Zr3, and Nb3 are identified as eligible candidates for ammonia synthesis. These clusters show preferable hollow-site N2 adsorption and strong orbital hybridization, with electronic backdonation from the metal d orbitals to both π* and π/σ orbitals of N2. Further studies on ammonia synthesis have been conducted by applying Y3 and Nb3 clusters supported on graphene (Y3/G and Nb3/G), illustrating superior activity and potential application of such M3 clusters. This work validates the three-atom cluster catalysis and guides the design of efficient catalysts for N2 fixation.