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 III N 5 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 III N 6 structures and 3 times more active than the Fe II N 4 structure, although its relative concentration in the catalysts is much lower than that of the Fe III N 6 structures.