Evolution Pathway from Iron Precursors to Fe–N ₄ Single-Atom Catalysts via High-Temperature Cyanide Coordination Chemistry

The rational synthesis of atomically dispersed iron–nitrogen–carbon (Fe–N–C) catalysts is hindered by the opacity of the high-temperature pyrolysis, in which the specific chemical drivers of active site formation remain elusive. Here, we employ a spatially separated chemical vapor pyrolysis strategy to decouple this process and probe the role of vapor-phase nitrogen-containing species in the evolution from iron oxide aggregates to isolated Fe–N 4 sites. In this model system, HCN is found to be more effective than NH 3 in promoting the disintegration of FeO x aggregates through the transient formation of Fe─C≡N intermediates. We further identify a synergistic process in which HCN facilitates FeO x aggregate dispersion, while Fe species reciprocally catalyze the dissociation of HCN, thereby accelerating the construction of nitrogen-rich support that stabilizes the dispersed Fe atoms. Spectroscopic analysis and ab initio molecular dynamics (AIMD) simulations collectively support this ligand-mediated atomization pathway and pinpoint the temperature onset of Fe–N 4 formation at 800 °C. This method effectively deconvolutes the Fe–N 4 formation from supremely complex pyrolysis processes, illuminating pathways for the controllable design of M–N–C catalysts.