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Atomic transition metal–nitrogen–carbon electrocatalysts exhibit outstanding activity in various electrocatalytic reactions. The challenge lies in predicting the structure of the active center, which may undergo changes under applied potential and interact with reactants or intermediates. Advanced characterization techniques, particularly in-situ X-ray absorption spectroscopy (XAS), provide crucial insights into the structural evolution of the metal active center during the reaction. In this study, nitrate reduction to ammonia (NO3RR) was selected as a model reaction, and we introduced in-situ XAS to reveal the structural evolution during the catalytic process. A novel single atom catalyst of iron loaded on three-dimensional nitrogen–carbon nanonetwork (designated as Fe SAC/NC) was successfully synthesized. We unraveled the structural transformations occurring as pyrrole-N4-Fe transitions to pyrrole-N3-Fe throughout the NO3RR process. Notably, the Fe SAC/NC catalyst exhibited excellent catalytic activity, achieving a Faradaic efficiency of 98.2% and an ammonia generation rate of 22,515 μg·h−1·mgcat−1 at −0.8 V versus reversible hydrogen electrode. Theoretical calculations combined with in-situ spectroscopic characterization showed that pyrrole-N3-Fe reduced the energy barrier from *NO to *NHO and improved the selectivity of ammonia. This provides a robust reference for the design of efficient nitrate-to-ammonia synthesis catalysts.
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