非共轭连接三维共价有机框架维持高还原电位用于光催化氧化反应

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

摘要/Abstract

摘要：

在全球探索“双碳”战略与绿色化工的宏观背景下, 通过太阳能驱动的人工光合成技术, 将氧气分子通过两电子氧还原路径(2e^- ORR)原位转化为过氧化氢(H₂O₂)已成为替代传统高能耗、高污染工业蒽醌法的理想清洁途径. 在众多新兴的半导体光催化材料中, 共价有机框架(COFs)因其高度可调的π共轭骨架、明确的晶体结构以及规整的开放式孔道, 为调控激子行为与分子传质提供了各向同性的理想平台. 尽管过去的研究普遍强调通过增强骨架的π共轭延展性来提升电荷输运效率, 但学术界对于共轭度如何具体影响多相界面催化活性动力学的核心机制, 依然缺乏微观维度的本质理解. 共轭程度的增强伴随着带隙变窄导致还原电位降低和激子无辐射复合加剧. 因此, 如何通过连接键化学实现电子结构的精细平衡, 已然成为构筑下一代高效光催化产H₂O₂材料的核心科学问题.
针对如何平衡电子共轭度与光催化氧化还原能力这一核心热力学难题, 本文从分子层面的连接键工程出发, 设计并构筑了一系列具有高度结构可比性的骨架模型. 以具有强发光特性的供体-受体型(D-A)分子—2,3,5,6-四(3,6-二(4-甲酰基苯基)-9H-咔唑-9-基)对苯二甲腈(TB)为核心构筑单元, 通过精细调控缩合反应的连接键类型, 成功构筑了三种拓扑结构相近, 但连接键共轭程度存在显著差异的结晶性多孔COFs. 这三种材料分别包括具有高度π共轭亚胺键连接的TB-DA COF、引入弱共轭酰腙键拓扑骨架的TB-TDA COF, 以及在连接位点非π共轭、具有孤立脲键(尿素键)结构的TB-PDU COF. 光催化性能测试呈现出颠覆传统认知的反常共轭效应. 在不添加任何牺牲剂的纯水与氧气饱和体系中, 材料的光催化产H₂O₂活性并未如预期般随着共轭程度的增强而提升, 反而表现出完全相反的依赖关系. 具有非共轭脲键连接特征的TB-PDU COF展现出最出色的光催化活性, 其H₂O₂生成速率为1580 μmol g^-1 h^-1, 明显优于弱共轭酰腙键连接的TB-TDA COF(1288 μmol g^-1 h^-1, 更是共轭亚胺键TB-DA COF (523 μmol g^-1 h^-1)的三倍. 该催化活性在连续循环测试中保持了优异的化学与结构稳定性. 为了深挖这一反常共轭效应背后的微观物理机制, 我们系统结合了原位红外光谱表征与密度泛函理论计算证实其2e^- ORR反应机理. 从反应热力学角度来看, 脲键的引入削弱了过度的电子离域, 成功使TB-PDU COF的导带位置保持在更具负电性的电位上, 为氧气分子的单电子生成超氧自由基(•O₂^-)以及后续的2e^- ORR提供了大得多的热力学驱动力. 此外, 脲键高度极性的官能团特征赋予了TB-PDU COF极为优异的亲水界面, 接触角测试证实其亲水性.
综上, 本工作不仅阐明了连接键化学对激发态电荷行为、多相界面传质过程与氧化还原热力学的精妙协同调控作用, 更突破了盲目追求长程共轭的传统思维定势, 为今后理性设计高选择性、高活性的2e^- ORR光催化剂提供了崭新的微观构筑依据.

关键词: 三维共价有机框架, 连接基团工程, 激子动力学, 过氧化氢生产, 光催化

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

文忠梁, 邓茂君, 冯潇, Pascal Van der Voort. 非共轭连接三维共价有机框架维持高还原电位用于光催化氧化反应[J]. 催化学报, 2026, 91: 386-395.

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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链接本文: https://www.cjcatal.com/CN/10.1016/S1872-2067(26)65209-6

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

图/表 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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