Screening a Suitable Mo Form Supported on Graphdiyne for Effectively Electrocatalytic N2 Reduction Reaction: From Atomic Catalyst to Cluster Catalyst.

Single-atom catalysts (SACs) stand out from the atomically dispersed catalysts due to their high specific activity and 100% atomic utilization ratio. However, besides inheriting most of the advantages of SACs, multiple-atom centered site catalysts not only boost higher metal atom loading but also provide more flexible active sites. In this work, by using spin-polarized density functional theory calculations, we systematically investigated the electrochemical nitrogen reduction reaction (eNRR) performance catalyzed by Mox (x = 1-4) supported on graphdiyne (GDY). Our results showed that N2 was favorably adsorbed on the substrates via a well-known "acceptance-donation" mechanism, which can be deeply understood by good multiple linear regressions between the adsorption Gibbs free energy of N2 and lengths or integrated crystal orbital Hamilton populations of Mo-N and N-N bonds. According to the designed screening criteria, Mo3@GDY was found to be most active toward NRR with high selectivity and stability. The predicted onset potential was only -0.32 V. The activity originates from a moderate N adsorption energy, which can balance the thermodynamics of the two potential potential-determining steps, i.e., N2 + H+ + e- = *N2H and *NH2 + H+ + e- = NH3. Moreover, the GDY serves as an electron reservoir during the whole NRR process, where it can provide electrons or accept electrons arbitrarily depending on the need of each elementary step, suggesting that the GDY sheet is a very suitable platform for electrocatalysis applications. The superior electrocatalytic performance of the triple-atom catalyst compared to that of the SACs, double-atom catalyst, and the quadruple-atom cluster catalyst toward NRR offers a huge opportunity for the exploration of a new generation of electrochemical catalysts, where the metal clusters should be highlighted.