Discriminating Catalytically Active FeNx Species of Atomically Dispersed Fe-N-C Catalyst for Selective Oxidation of the C-H Bond

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-angstrom-resolution HAADF-STEM in combination with XPS, XAS, ESR, and Mossbauer 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 (FeN5)-N-III 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 (FeN6)-N-III structures and 3 times More active than the (FeN4)-N-II structure, although its relative concentration in the catalysts is much lower than that of the (FeN6)-N-III structures.