Abstract

Nanostructured Fe–N–C materials represent a new type of "platinum-like" non-noble-metal catalyst for various electrochemical reactions and organic transformations. However, no consensus has been reached on the active sites of the Fe–N–C catalysts because of their heterogeneity in particle size and composition. In this contribution, we have successfully prepared atomically dispersed Fe–N–C catalyst, which exhibited high activity and excellent reusability for the selective oxidation of the C–H bond. A wide scope of substrates, including aromatic, heterocyclic, and aliphatic alkanes, were smoothly oxidized at room temperature, and the selectivity of corresponding products reached as high as 99%. By using sub-ångström-resolution HAADF-STEM in combination with XPS, XAS, ESR, and Mössbauer spectroscopy, we have provided solid evidence that Fe is exclusively dispersed as single atoms via forming FeNx (x = 4–6) and that the relative concentration of each FeNx species is critically dependent on the pyrolysis temperature. Among them, the medium-spin FeIIIN5 affords the highest turnover frequency (6455 h–1), which is at least 1 order of magnitude more active than the high-spin and low-spin FeIIIN6 structures and 3 times more active than the FeIIN4 structure, although its relative concentration in the catalysts is much lower than that of the FeIIIN6 structures.