Insights of Oxygen Reduction Selectivity and Fe Leaching from Fe-N-C Nanozyme in Ascorbate Oxidation

Ascorbic acid (H2A) is a well-known antioxidant to protect cellular components from free radical damage, meanwhile it is also emerged as pro-oxidant in cancer therapy. However, such contradictory mechanisms underlying H2A oxidation are not well understood. Here, we report the discovery of Fe leaching during catalytic H2A oxidation using Fe-N-C nanozyme as a ferritin mimic and its influence in selectivity of oxygen reduction reaction (ORR). Owing to the heterogeneity, Fe-Nx sites in Fe-N-C primarily catalyzed the H2A oxidation and 4e ORR via an iron-oxo intermediate; meanwhile marginal N-C sites catalyzed the 2e ORR via an O2 intermediate with H2O2 production, although which was less favorable in kinetics and hardly observable in the early stage. Nonetheless, trace O2 accumulated and attacked Fe-Nx sites, leading to a linear leakage of unstable Fe ions up to 420 ppb when the concentration of H2A increased. As a result, a substantial fraction (~40%) of N-C sites on Fe-N-C were activated, and a new path for Fenton-type H2A oxidation was finally enabled. After Fe ions diffused into the bulk solution, the ORR at the N-C sites stopped at H2O2 production, which was the origin for the pro-oxidant effect by H2A. This work highlights the Fe-leakage occurring on Fe-N-C nanozymes and uncovers the multifaceted insights of ORR selectivity in H2A oxidation under realistic conditions.