Elucidating the Mechanistic Origin of a Spin State-Dependent FeNx–C Catalyst toward Organic Contaminant Oxidation via Peroxymonosulfate Activation

Atomically dispersed metals on nitrogen-doped carbon matrices have attracted extensive interest in the removal of refractory organic pollutants. However, a thorough exploration of the particular structure for each active site and specific effects of these sites still remains elusive. Herein, an Fe-pyridinic N4 structure in a single-atom catalyst (FeNx–C) was constructed using a facile pyrolysis strategy, and it exhibited superior catalytic activity in peroxymonosulfate (PMS) activation toward organic contaminant oxidation. The various Fe species and relative amounts of each Fe site in the FeNx–C catalyst were validated using X-ray absorption spectroscopy and 57Fe Mössbauer spectroscopy, which showed critical dependencies on the precursor ratio and calcination temperature. The positive correlations between relative content of high-spin state species (FeII and FeIII) and catalytic performance were found to determine the reactive species generation and electron transfer pathway in the FeNx–C/PMS system. Moreover, catalytic performance and theoretical calculation results revealed that FeII–N4 in the high-spin state (S = 2) tends to activate PMS to form sulfate and hydroxyl radicals via a one-electron transfer process, while the FeIII–N4 moiety (S = 5/2) is prone to high-valent iron species generation with lower free energy. Benefiting from finely tuned active sites, a single-atom FeNx–C catalyst achieved favorable applicability in actual wastewater treatment with efficient resistance of the common water matrix. The present work advances the mechanistic understanding of spin state-dependent persulfate activation in single-atom catalysts and provides guidance to design a superior catalyst based on spin state descriptions.