Discriminating Catalytically Active FeN_<i>x</i> Species of Atomically Dispersed Fe–N–C Catalyst for Selective Oxidation of the C–H Bond

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 FeN_x (x = 4-6) and that the relative concentration of each FeN_x species is critically dependent on the pyrolysis temperature. Among them, the medium-spin Fe^IIIN₅ 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 Fe^IIIN₆ structures and 3 times more active than the Fe^IIN₄ structure, although its relative concentration in the catalysts is much lower than that of the Fe^IIIN₆ structures.