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A sulfonic acid-functionalized donor–acceptor conjugated organic polymer for enhanced photocatalytic hydrogen peroxide production†

Qingxia Zhu^a, Haoxi Wang^a, Xiaobo Luo^a, Wuzi Zhao^a, Danfeng Wang^a, Shiyuan Zhou*^a, Lingyun Xu*^b, Guangfeng Liu*^a and Peiyang Gu*^a
aJiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou, 213164, PR China. E-mail: zhoushiyuan@cczu.edu.cn; guangfeng.liu@cczu.edu.cn; gupeiyang0714@cczu.edu.cn
^bAnalysis and Testing Center, Soochow University, Suzhou, 215123, China. E-mail: lyxu@suda.edu.cn

First published on 10th July 2025

In recent years, research on H₂O₂ synthesis using solar light, oxygen, and water based on photocatalytic technology has garnered widespread attention.⁷ To date, a variety of photocatalysts have been developed, including graphitic carbon nitride (g-C₃N₄),⁸ hyper-crosslinked polymers (HCPs),⁹ porous aromatic frameworks (PAFs),¹⁰ polymers of intrinsic microporosity (PIMs),¹¹ and porous organic polymers (POPs).¹² Among these, conjugated organic polymers (COPs) have emerged as a research hotspot within the category of POPs due to their unique conjugated structures and excellent optoelectronic properties.¹³ COPs exhibit highly tunable molecular structures, enabling precise optimization of light absorption, band structures, and charge separation to boost photocatalytic activity. Key research strategies include electron donor–acceptor (D–A) design, molecular structure control, linkage optimization, and elemental doping.^14,15 Notably, D–A construction stands out as particularly promising for effectively modulating COP band structures and enhancing charge carrier separation/transfer efficiency.

Porphyrins, with their nitrogen-rich structure and extended π-conjugation, demonstrate superior visible-light absorption and stability, making them ideal COP building blocks. However, COPs connected via covalent bonds typically exhibit insufficient structural stability. To address these issues, this study innovatively introduces pyrimidine as an electron-acceptor unit to construct D–A structures. This molecular-level design strategy enables directional control of charge separation via the D–A structure, simultaneously enhancing light absorption capacity, optimizing band structures for catalytic requirements, creating efficient ORR active sites, and boosting interfacial electron density.

In this work, we synthesized a D–A structured porphyrin–pyrimidine COP (BMO) via Sonogashira coupling, exhibiting broad light absorption. To boost H₂O₂ production, –SO₃H groups were introduced via post-modification (Fig. 1a), yielding the high-performance photocatalyst BMS with triple optimization: (1) enhanced charge separation/transfer, (2) tailored electronic structures, and (3) improved active-site microenvironment. The strong electron-withdrawing –SO₃H groups simultaneously boosted hydrophilicity and O₂ adsorption, significantly accelerating ORR kinetics and elevating the H₂O₂ yield.

	Fig. 1 (a) The synthetic procedure for BMS. The (b) ss-13C NMR spectra, (c) FT-IR spectra, and (d) full XPS spectra of BMO and BMS; (e) the O 1s and S 2p high-resolution XPS spectra of BMS.

BMO and BMS structures were confirmed by ss-¹³C NMR and FT-IR. In Fig. 1b, peaks at 109 ppm (pyrrole carbons) and 120–142 ppm (aromatic/porphyrin carbons) verify the molecular frameworks.¹⁶ The peaks of –CC– and carbons in pyrimidine rings are observed at δ = 95 ppm and δ = 155 ppm,¹⁷ confirming the successful construction of BMO. In the FT-IR spectra (Fig. 1c), the stretching vibration of the N–H bond in the porphyrin unit appears around 3000 cm⁻¹. The peak at 2362 cm⁻¹ corresponds to the alkyne bond (–CC–), confirming the successful polymerization between the porphyrin and pyrimidine.¹⁸ The peak at 1640 cm⁻¹ is associated with the stretching vibration of –CC–. Additionally, the stretching vibration peaks of OSO are observed at 1115 cm⁻¹ and 1011 cm⁻¹,¹⁹ indicating the successful incorporation of sulfonic acid groups. Moreover, the XPS spectrum of BMS reveals the presence of additional peaks corresponding to S elements, in addition to C and N (Fig. 1d and Fig. S3a–d, ESI†). In the high-resolution S 2p XPS spectrum (Fig. 1e), two peaks centered at 165.5 and 164.5 eV are observed, which are assigned to S 2p1/2 and S 2p3/2 in the OSO bond.^19,20 This confirms the presence of sulfonate groups and indicates their successful integration into the structure. The relevant descriptions of Fig. S4–S10 are provided in the ESI.†

Subsequently, the photochemical and electrochemical properties were measured. The ultraviolet–visible diffuse reflectance spectroscopy (UV-vis DRS) reveals that both polymers exhibit efficient light absorption in the range of 200–1000 nm (Fig. 2a). The optical band gaps (E_g) were measured to be 1.76 eV for BMO and 1.56 eV for BMS from the Tauc plots. Based on the Mott–Schottky measurements (Fig. 2b), the flat-band potentials of BMO and BMS were determined to be −0.74 and −0.71 V, respectively. For n-type semiconductors, the conduction band (CB) potential is typically ∼0.2 V higher than the flat-band potential.²¹ Therefore, the corresponding CB positions were calculated to be −0.54 V (BMO) and −0.51 V (BMS), and the corresponding valence band (VB) values were calculated as 1.22 and 1.05 V according to the equation VB = E_g + CB. Thus, the band structures are illustrated in Fig. S11 (ESI†), indicating the feasibility of triggering both ORR and WOR processes for efficient H₂O₂ production.²²

	Fig. 2 (a) The UV-vis diffuse reflectance spectra of BMO and BMS; inset: Tauc plots; the (b) Mott–Schottky plots, (c) photocurrent response, and (d) EIS Nyquist plots.

The photocurrent response (Fig. 2c) and electrochemical impedance spectroscopy (EIS, Fig. 2d) reveal that BMS possesses exceptional photogenerated charge separation efficiency and charge transfer capability. Additionally, the Koutecky–Levich plot (Fig. S12, ESI†) derived from the rotating disk electrode voltammetry measurement reveals that the electron transfer numbers (n) for BMO and BMS are 1.37 and 1.89, respectively. This indicates that BMS is more preferrable to undergo a 2e⁻ ORR pathway during photocatalytic H₂O₂ production. Furthermore, as shown in Fig. 3, Kelvin probe force microscopy (KPFM) under both dark and light conditions for the two photocatalysts was conducted.²³ Under visible light, BMS shows significant surface potential changes (ΔCPD = 41 mV) under dark conditions, while BMO exhibits minimal variation (ΔCPD = 3 mV). This confirms that –SO₃H modification establishes a stronger built-in electric field in BMS, enhancing charge separation and photocatalytic H₂O₂ production efficiency.

	Fig. 3 The surface potential images, surface potential changes in the dark and under visible light irradiation, and the corresponding CPD profiles of (a) BMO and (b) BMS.

H₂O₂ production increased with photocatalyst loading (Fig. S13 and S14, ESI†), with 5 mg (0.25 mg mL⁻¹) being optimal based on rate and concentration. Next, the effects of the atmosphere (O₂, air, and N₂) were tested (Fig. 4a), revealing the following trend of H₂O₂ yield: O₂ > air > N₂. Compared to air, O₂ enhanced H₂O₂ production by only 18% (BMO) and 16% (BMS), so air was chosen for practicality. Notably, BMS achieved 2.59 mmol g⁻¹ h⁻¹ in air, outperforming many reported photocatalysts (Fig. S16 and Table S1, ESI†). Furthermore, the addition of h⁺ scavengers significantly boosted the H₂O₂ production (Fig. S17, ESI†). This is primarily due to the inhibition of e⁻/h⁺ pair recombination derived from the addition of h⁺ scavengers, thus promoting the ORR process triggered by e⁻ for improved performance.

	Fig. 4 (a) The photocatalytic H2O2 production performance of BMO and BMS under different atmospheres; the H2O2 production of BMS in the presence of (b) different scavengers in air and (c) different scavengers in N2; and (d) time-resolved in situ DRIFT spectra of BMS.

To elucidate the key reactive oxygen species (ROS) during the photocatalytic H₂O₂ production process, tert-butanol (t-BA), 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), β-carotene, and AgNO₃ were employed as specific scavengers for ˙OH, ˙O₂⁻, ¹O₂ and e⁻, respectively. As shown in Fig. 4b, an anomalous increase in H₂O₂ production was observed upon the addition of t-BA, this may be attributed to the fact that, in addition to ˙OH, t-BA can also serve as a h⁺ scavenger.²⁴ The introduction of β-carotene has minimal effect on H₂O₂ production, indicating that ¹O₂ is not a primary ROS. In contrast, the addition of TEMPO and AgNO₃ both resulted in a significant reduction, confirming the dominant roles of ˙O₂⁻ and e⁻. This indicates that the indirect 2e⁻ ORR mediated by ˙O₂⁻ may be the dominant pathway. To further validate the reaction pathway, quenching experiments with TEMPO and AgNO₃ were conducted under an N₂ atmosphere (Fig. 4c). The addition of AgNO₃ still significantly inhibited the H₂O₂ production, and a rate of only 0.16 mmol g⁻¹ h⁻¹ was observed. This demonstrates that the predominant pathway for the WOR is the 4e⁻ WOR (H₂O + 4h⁺ → O₂ + 4e⁻ + 4H⁺) to produce O₂, e⁻, and H⁺ to trigger the ORR, rather than the 2e⁻ WOR (H₂O + 2h⁺ → H₂O₂ + 2H⁺), to directly produce H₂O₂. The addition of TEMPO also inhibited the H₂O₂ production under an N₂ atmosphere, further confirming that the 4e⁻ WOR triggered the indirect ORR. In the isotopic labeling experiment using H₂¹⁸O and N₂ as the reaction solvent and atmosphere, the peak at m/z = 36 belonging to ¹⁸O₂ was directly detected in the headspace of BMS after being irradiated for 1 h (Fig. S18, ESI†). This further demonstrated the key role of the 4e⁻ pathway in the WOR process. After 5 cycles (Fig. S20, ESI†), BMS retained a 79.5% H₂O₂ yield and maintained its chemical structure, as proved by FT-IR and XPS (Fig. S21 and S22, ESI†), confirming outstanding recyclability. To detect the generated ROS, electron paramagnetic resonance (ESR) spectroscopy was conducted under both dark and illuminated conditions. The detailed analysis is shown in Fig. S23 (ESI†). Furthermore, to monitor the intermediates during the photocatalytic H₂O₂ production process, the in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was conducted taking BMS as an example. As shown in Fig. 4d, with prolonged irradiation time, the signals corresponding to C–OO (1040 cm⁻¹), ˙O₂⁻ (1118 cm⁻¹), ˙OOH* (1280 cm⁻¹), and HOOH* (1364 cm⁻¹) gradually increased.^25–27 This confirms the feasibility of an indirect 2e⁻ ORR pathway mediated by ˙O₂⁻ to effectively produce H₂O₂.

The density functional theory (DFT) calculations provided atomic-level insights into the photocatalytic mechanism. The HOMO and LUMO distributions of BMO/BMS are presented in Fig. S24 (ESI†). Electrostatic surface potential (ESP) analysis (Fig. 5a) identified negative charges (red) concentrated on –SO₃H and pyrimidine N sites. BMS shows a much larger dipole moment (16.63 D) compared to BMO (6.27 D), confirming that –SO₃H enhances charge asymmetry and promotes transfer. Secondly, DFT calculations identified three adsorption sites on BMO (porphyrin unit, alkyne bond, and pyrimidine carbon) and four on BMS (same as BMO and a carbon adjacent to –SO₃H), as shown in Fig. 5b. DFT calculations reveal that the carbon adjacent to –SO₃H (site 4) shows the strongest O₂ adsorption (−0.99 eV, Fig. 5c) due to its electron-withdrawing effect. Gibbs free energy analysis (Fig. 5d) identifies ˙O₂⁻ formation as the rate-limiting step, with a lower energy barrier for BMS (1.34 eV), confirming its more favorable 2e⁻ ORR pathway (O₂ → ˙O₂⁻ → *OOH → H₂O₂) and superior catalytic activity.²⁸ Based on the experimental and theoretical analyses, a potential mechanism of photocatalytic H₂O₂ synthesis has been proposed for BMS (Fig. 5e). In the ORR pathway, the introduction of a –SO₃H group significantly enhances the O₂ adsorption of BMS, and O₂ is adsorbed on the carbon atom adjacent to the –SO₃H group via a Pauling-type configuration. The adsorbed O₂ molecules are initially reduced by a photogenerated e⁻ to form ˙O₂⁻ (step I), which can partially react with photogenerated h⁺ to generate ¹O₂. Subsequently, two key intermediates, *OOH (˙O₂⁻ + H⁺ → *OOH) and *HOOH (*OOH + H⁺ + e⁻ → *HOOH), are sequentially formed (steps II and III), ultimately yielding H₂O₂ (step IV). During the WOR pathway, the porous framework of the BMS material, due to its enhanced hydrophilicity, facilitates the adsorption of H₂O molecules (step V). The adsorbed H₂O molecules undergo a 4e⁻ WOR pathway with the assistance of photogenerated h⁺ to produce O₂ (H₂O + 4h⁺ → O₂ + 4e⁻ + 4H⁺) and simultaneously provide the necessary H⁺ and O₂ for the ORR process (step VI). Finally, the generated H₂O₂ desorbs from the catalyst surface (step VII).

	Fig. 5 (a) The optimized geometries, electrostatic surface potentials, dipole moments, and (b) potential O2 adsorption sites of BMO (left) and BMS (right); (c) the O2 adsorption energies at different sites; (d) free energy diagram for the ORR processes; and (e) the illustration of key steps in the ORR and WOR pathways for BMS.

In summary, we developed a donor–acceptor covalent organic polymer (BMS) via Sonogashira coupling of a porphyrin and pyrimidine, followed by –SO₃H modification. The –SO₃H groups enhanced hydrophilicity, O₂ adsorption, and charge transfer, achieving a remarkable H₂O₂ production rate of 2.59 mmol g⁻¹ h⁻¹ (3.4× improvement). Experimental and theoretical analyses revealed a dual-pathway mechanism involving a 2e⁻ ORR and a 4e⁻ WOR, providing a novel COP design strategy for efficient H₂O₂ synthesis with broad application potential.

This work was supported by the Changzhou Introduction Program of Innovative Leading Talents (CQ20220111) and the Changzhou applied basic research program (CJ20240046). G. Liu thanks the Jiangsu Specially Appointed Professor Foundation. We also thank the Analysis and Testing Center, NERC Biomass of Changzhou University for the assistance with NMR analysis.

Conflicts of interest

Data availability

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