Nitrogen reduction reaction on small iron clusters supported by N-doped graphene: A theoretical study of the atomically precise active-site mechanism

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.