Fe(III)-mediated self-sustaining photo-Fenton system on metal-free pyridine-COF: Interfacial electron transfer for water purification

doi:10.1016/S1872-2067(25)64890-X

Abstract

Abstract:

Sluggish Fe(III)/Fe(II) cycling and inefficient two-electron H₂O₂ production severely limit the practicality of conventional photocatalytic Fenton systems. In this study, we present a metal-free pyridine-based covalent organic framework (TpBpy-COF) that enables a highly efficient and self-sustaining photo-Fenton process. The system is designed to integrate dual-path H₂O₂ production—through oxygen reduction reaction (ORR) and water oxidation reaction (WOR), along with interfacial Fe(III) reduction. Electron-rich dual-pyridinic nitrogen (μ-N,N) bridging sites facilitate Fe(III)/Fe(II) redox cycling and direct 2e^- ORR, while β-ketoenamine-linked benzene units promote 2e^- WOR, working synergistically to enable continuous in situ H₂O₂ generation. This cooperative mechanism leads to outstanding water purification performance, including rapid degradation of pharmaceuticals (e.g., caffeine), complete microbial inactivation, and robust stability across diverse real water matrices. In-situ spectroscopy and density functional theory calculations elucidate the atomic-scale synergy: pyridinic-N sites selectively reduce Fe(III) and activate O₂, while β-ketoenamine-linked benzene units oxidize water via spatially decoupled charge transfer, collectively enabling autonomous Fenton cycles. This work pioneers a self-sustained Fenton paradigm through a metal-free COF architecture that synergizes dual-path H₂O₂ generation and autonomous Fe(III)/Fe(II) cycling, offering a solar-driven platform for eco-adaptive water purification with negligible reliance on exogenous reagents or energy inputs.

Key words: Metal-free Fenton catalysis, Covalent organic frameworks, Self-sustaining redox cycling, Advanced oxidation processes, In-situ H₂O₂ production, Water purification

Rumeng Zhang, Muke Lin, Yimu Jiao, Cheng Chen, Mengling Hu, Hao Zhou, Dehua Xia. Fe(III)-mediated self-sustaining photo-Fenton system on metal-free pyridine-COF: Interfacial electron transfer for water purification[J]. Chinese Journal of Catalysis, 2026, 82: 225-237.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(25)64890-X

https://www.cjcatal.com/EN/Y2026/V82/I3/225

Figures/Tables 5

Fig. 1. Synthesis and comprehensive characterization of TpBpy-COF. (a) Schematic illustration of the synthesis of TpBpy-COF. (b) Atomic structure model of TpBpy-COF. (c) Experimental PXRD pattern with simulated AA-stacking configuration. (d) SEM image of TpBpy-COF. (e) TEM image. (f) HRTEM image. (g) HAADF-STEM image and corresponding elemental maps. (h) N2 adsorption-desorption isotherms and corresponding pore size distribution. (i,j) FTIR and Raman spectrum of TpBpy-COF. (k) High-resolution XPS spectra of C 1s and N 1s. (l) UV-vis DRS with Tauc plot inset (bandgap determination). (m) Mott-Schottky plot (flat-band potential analysis). (n) Transient photocurrent of TpBpy-COF (inset: KPFM potential mapping). (o) Surface potential from KPFM image of TpBpy-COF (inset: average surface potential selection area).

Fig. 2. Degradation performance and mechanistic study of TpBpy-COF/Fe(III) photo-self-Fenton system. (a b) CAF removal efficiency under different control conditions within 60 min. Pseudo-first-order kinetics of CAF degradation as a function of TpBpy-COF dosage (c) and Fe(III) concentration (95% CI) (d). (e) Light-intensity-dependent rate constant (k) for CAF degradation. (f) High-valent iron species probed by PMSO oxidation to PMSO2 (30 μmol L-1). (g) EPR spectra of ·OH, O2?-, and 1O2 in the system. (h) ROS contribution to CAF degradation and the impact of different atmospheres (O2, N2). (i) CAF removal efficiency in diverse real water matrices. Error bars represent standard deviation from triplicate test.

Fig. 3. Dual-pathway mechanism for photocatalytic in situ H2O2 generation. (a) H2O2 production performance of TpBpy-COF under visible light irradiation (inset: comparative yields under light and dark conditions). (b) Quenching effects on H2O2 yield with EDTA-2Na (20 mmol L-1) and KBrO3 (0.1 mol L-1). (c) Quenching effects with CHCl3 (20 m mol L-1) and TBA (10%). (d) In-situ ATR-SEIRAS spectra for the photocatalytic system of TpBpy-COF. (e) Calculated free-energy diagram for ORR pathways with different sites. (f) Gibbs free-energy diagram for 2e- WOR-H2O2 and 4e- WOR-H2O2 pathway. (g,h) Optimized O2 adsorption configurations at two distinct active sites. (i) H2O adsorption configuration. (j) Dual-pathway (ORR/WOR) H2O2 generation mechanism via O2 and H2O activation on TpBpy-COF (isosurface ±0.0013 e ?-3, yellow/blue: electron depletion/accumulation).

Fig. 4. Fe(III)/Fe(II) cycling promotion and interfacial reaction mechanisms. Fe species in TpBpy-COF/Fe(III) system at different initial concentrations: 40 μmol L-1 (a) and 80 μmol L-1 (b). XPS Fe 2p spectra of used TpBpy-COF catalyst: after 5 min (c) and 60 min (d) reaction. (e) CV curves under different conditions. (f) Open-circuit potential profiles with sequential additions of reagents. ([Na2SO4]0 = 0.5 mol L-1, [Fe(III)]add = 1.0 mmol L-1, [CAF]add = 10.0 mg L-1). (g) Comparative activation barriers for Fe(III) reduction at different sites. (h) Projected density of states (PDOS) of Fe(III)-adsorbed TpBpy-COF systems. (i) Fe(III) activation mechanism showing charge transfer, N-Fe coordination, and 2D ELF (yellow/blue: electron depletion/accumulation regions, isosurface = ±0.0016 e ?-3).

Fig. 5. Multifunctionality and stability of TpBpy-COF/Fe(III) photo-self-Fenton system. (a) Temporal evolution of pollutant concentrations (CAF, SMX, PCM, NPX, CBZ). (b) Pseudo-first-order kinetics of pollutant degradation with observed rate constants (kobs). (c) Recyclability test for CAF degradation over six cycles. (d) Antimicrobial activity against E. coli (inset: colony-forming unit comparisons pre-/post-treatment). (e) Photocatalytic self-Fenton flow reactor and schematic diagram. (f) Underlying mechanisms governing the elevated oxidative capability in the TpBpy-COF/Fe(III) system.

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