Preserving high reduction potential in 3D COFs through non-conjugated linkages for enhanced solar-driven O2 reduction

doi:10.1016/S1872-2067(26)65209-6

Abstract

Abstract:

Photocatalytic production of hydrogen peroxide (H₂O₂) via the two-electron oxygen reduction reaction (ORR) represents a sustainable alternative to the energy-intensive anthraquinone process. Covalent organic frameworks (COFs), with tunable π-conjugated structures and ordered porosity, have emerged as one of the most promising photocatalysts. However, the role of π-conjugation in governing photocatalytic activity remains poorly understood. Herein, we report a linkage-engineering strategy to systematically investigate the impact of conjugation on H₂O₂ production using a donor-acceptor (D-A) building unit, 2,3,5,6-tetrakis(3,6-bis(4-formylphenyl)-9H-carbazol- 9-yl)terephthalonitrile (TB). Three structurally analogous COFs with distinct linkage chemistries were constructed, including a conjugated imine-linked TB-DA COF, a weakly conjugated hydrazone-linked TB-TDA COF, and a non-conjugated urea-linked TB-PDU COF. Unexpectedly, photocatalytic evaluation under aqueous conditions reveals a reversed activity trend, with TB-PDU COF (1580 μmol g^-1 h^-1) outperforming TB-TDA COF (1288 μmol g^-1 h^-1) and TB-DA COF (523 μmol g^-1 h^-1). Mechanistic studies demonstrate that the non-conjugated urea linkage suppresses charge recombination, enhances hydrophilicity to facilitate O₂ accessibility, and preserves favorable reduction potential by avoiding excessive electronic delocalization. These findings deviate from the conventional conjugation-driven design paradigm and highlight the critical role of linkage chemistry in balancing charge dynamics, interfacial properties, and reaction thermodynamics, providing new insights into the rational design of COF-based photocatalysts for efficient H₂O₂ production.

Key words: 3D covalent organic framework, Linkage engineering, Exciton dynamics, Hydrogen peroxide, Photocatalysis

Zhongliang Wen, Maojun Deng, Xiao Feng, Pascal Van Der Voort. Preserving high reduction potential in 3D COFs through non-conjugated linkages for enhanced solar-driven O₂ reduction[J]. Chinese Journal of Catalysis, 2026, 91: 386-395.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(26)65209-6

https://www.cjcatal.com/EN/Y2026/V91/I12/386

Figures/Tables 6

Scheme 1. The synthetic routes and chemical structures of the monomers and 3D COFs.

Fig. 1. PXRD patterns and Pawley refinements of TB-DA COF (a), TB-TDA COF (b), and TB-PDU COF (c). 13C Solid-state NMR spectra of TB-DA COF (d), TB-TDA COF (e), and TB-PDU COF (f).

Fig. 2. N2 adsorption isotherms (a), pore size distributions (b), FT-IR spectra (c), and XPS N 1s spectra (d) of TB-DA COF, TB-TDA COF, and TB-PDU COF.

Fig. 3. (a) UV-vis spectra of TB-DA COF, TB-TDA COF, and TB-PDU COF. Band alignment (b), EIS (c), and photocurrent responses (d) of TB-DA COF, TB-TDA COF, and TB-PDU COF.

Fig. 4. (a) H2O2 production by the three COFs during 5 h of reaction. (b) Different atmospheres. (c) Photocatalytic H2O2 production of TB-DA COF, TB-TDA COF, and TB-PDU COF under different conditions. (d) Recycling of TB-PDU COF for 5 cycles.

Fig. 5. (a) Local spectrum of in-situ DRIFTS. The spectra of these time intervals were differentially processed (subtracting the infrared spectra measured at 0 min from all the infrared spectra measured at that time to highlight the changes in the infrared peaks). (b) Calculated Gibbs free energy diagram of 2e- ORR. (c) Proposed mechanism of photocatalytic H2O2 production by TB-based COFs through 2e- ORR.

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Preserving high reduction potential in 3D COFs through non-conjugated linkages for enhanced solar-driven O₂ reduction

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