Nonprecious metal catalysts are known of significance for electrochemical N₂ reduction reaction (NRR) of which the mechanism has been illustrated by ongoing investigations of single atom catalysis. However, it remains challenging to fully understand the size-dependent synergistic effect of active sites inherited in substantial nanocatalysts. In this work, four types of small iron clusters Fe_n (n = 1–4) supported on nitrogen-doped graphene sheets are constructed to figure out the size dependence and synergistic effect of active sites for NRR catalytic activities. It is revealed that Fe₃ and Fe₄ clusters on N₄G supports exhibit higher NRR activity than single-iron atom and iron dimer clusters, showing lowered limiting potential and restricted hydrogen evolution reaction (HER) which is a competitive reaction channel. In particular, the Fe₄-N₄G displays outstanding NRR performance for “side-on” adsorption of N₂ with a small limiting potential (−0.45 V). Besides the specific structure and strong interface interaction within the Fe₄-N₄G itself, the high NRR activity is associated with the unique bonding/antibonding orbital interactions of N-N and N-Fe for the adsorptive N₂ and NNH intermediates, as well as relatively large charge transfer between N₂ and the cluster Fe₄-N₄G.

已知非贵金属催化剂对于电化学N ₂还原反应（NRR）具有重要意义，其机理已通过对单原子催化的持续研究得到了阐明。然而，充分理解在大量纳米催化剂中遗传的活性位点的尺寸依赖性协同效应仍然具有挑战性。在这项工作中，构建了氮掺杂石墨烯片上负载的四种类型的小铁簇Fe _n（n = 1-4），以计算尺寸依赖性和活性位点对NRR催化活性的协同效应。结果表明，Fe ₃和Fe ₄聚集在N _4上G载体表现出比单铁原子和铁二聚体簇更高的NRR活性，显示出较低的极限电势和受限制的析氢反应（HER），这是竞争性反应通道。尤其是，Fe ₄ -N ₄ G对N _2的“侧向”吸附具有很小的极限电势（-0.45 V）表现出出色的NRR性能。除了Fe ₄ -N ₄ G本身的特定结构和强大的界面相互作用外，高NRR活性还与NN和N-Fe对吸附性N ₂和NNH中间体以及独特的键合/反键轨道相互作用有关N ₂之间的电荷转移相对较大和簇Fe ₄ -N ₄ G.