Nitrogen-doped carbon nanotubes enhanced Fenton chemistry: Role of near-free iron(III) for sustainable iron(III)/iron(II) cycles

•NCNT can elevate the oxidation potential of Fe(III) by increasing Fe-O bond length of FeOH2+.•Hydrogen peroxide was used as green electron donors to enhance Fenton chain reactions.•Hydroxyl radical can be sustainably produced via NCNT-mediated sustainable Fenton oxidation.•It is a common property of nitrogen doped carbon materials to enhance the reactivity of Fe(III). The sluggish kinetics of Fe(II) recovery strongly impedes the scientific progress of Fenton reaction (Fe(II)/H2O2) towards practical application. Here, we propose a novel mechanism that metal-free nitrogen-doped carbon nanotubes (NCNT) can enhance Fenton chemistry with H2O2 as electron donors by elevating the oxidation potential of Fe(III). NCNT remarkably promotes the circulation of Fe(III)/Fe(II) to produce hydroxyl radical (•OH) with excellent stability for multiple usages (more than 10 cycles) in the NCNT/Fe(III)/H2O2 system. Although carbonyl on NCNT can act as the electron supplier for Fe(III) reduction, the behavior of NCNT is distinct from common reductants such as hydroxylamine and boron. Electrochemical analysis and density functional theory calculation unveil that nitrogen sites of NCNT can weakly bind with Fe(III) to elevate the oxidation potential of Fe(III) (named near-free Fe(III), primarily FeOH2+) at pH ranging from 2.0 to 4.0. Without inputs of external stimulations or electron sacrificers, near-free Fe(III) can promote H2O2 induced reduction of Fe(III) to initiate Fenton chain reactions for long-lasting generation of •OH. To our delight, it is a common property of N-doped carbon materials (e.g., graphene, carbon nanofibers, and acetylene black), our research thus provides a novel, sustainable, and green strategy for promoting Fenton chemistry. [Display omitted]