Keto-anthraquinone covalent organic framework for H2O2 photosynthesis with oxygen and alkaline water

doi:10.21203/rs.3.rs-3678746/v1

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Keto-anthraquinone covalent organic framework for H2O2 photosynthesis with oxygen and alkaline water

https://doi.org/10.21203/rs.3.rs-3678746/v1

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Hydrogen peroxide (H₂O₂) photosynthesis is an attractive carbon-neutral process for decentralized applications, but suffers from insufficient activity of catalysts due to the high energy barrier of hydrogen extraction from H₂O without sacrificial reagent. Herein, we report that mechanochemically synthesized keto-form anthraquinone covalent organic framework (Kf-AQ) is able to directly synthesize H₂O₂ with molecular oxygen and alkaline water (pH = 13) in the absence of any sacrificial reagents, with a superior production rate of 4784 µmol h^− 1 g^− 1 under visible light irradiation (λ > 400 nm) and an impressive apparent quantum yield (AQY) of 15.8% at 400 nm. Characterization results revealed that the strong alkalinity resulted in the formation of OH⁻(H₂O)_n clusters in water, which were first adsorbed on keto moieties of Kf-AQ and then more easily dissociated into molecular oxygen and active hydrogen with the injection of photoelectrons, because the energy barrier of hydrogen extraction from OH⁻(H₂O)_n was largely lowered by weakening the H-bonded networks of H₂O molecules owing to the excessive electrons in OH⁻. The produced active hydrogen quickly diffused to react with anthraquinone to generate anthrahydroquinone, which was subsequently oxidized by molecular oxygen to selectively produce H₂O₂. This study provides a novel efficient H₂O₂ photosynthesis material, and also sheds light on the importance of hydrogen extraction from H₂O for photocatalytic H₂O₂ synthesis.

Physical sciences/Materials science/Materials for energy and catalysis/Photocatalysis

Physical sciences/Chemistry/Catalysis/Photocatalysis

Hydrogen peroxide (H₂O₂), a chemical with increasing market share, finds extensive applications in biomedicine, disinfection, bleaching, organic synthesis, and water treatment^1–4. The well-known industrial production of H₂O₂ is the anthraquinone (AQ) process, which suffers from intensive energy consumption and waste discharge⁵. As a green and carbon-neutral alternative, solar driven oxygen reduction strategy of H₂O₂ synthesis from molecular oxygen and water attracts more and more attention^6–9. Although many photocatalysts are effective for the H₂O₂ synthesis, high dosages of organic sacrificial reagents such as isopropanol are always used to scavenge photogenerated holes and offer hydrogen for the H₂O₂ formation, which obviously bring in undesired impurity and also increase the cost of H₂O₂ synthesis¹⁰. In comparison with organic sacrificial reagents, water is more inexpensive and convenient hydrogen source, but an intrinsically poor hydrogen donor, because water molecules have a high O-H bond dissociation energy (BDE, 492 kJ mol^− 1 for homolytic cleavage) ^11,12. Thus, highly efficient H₂O₂ photosynthesis only with molecular oxygen and water is of great significance, but remains a giant challenge.

It is well known that the hydrogen-bond (H-bond) in adsorbed water clusters plays critical role on water dissociation during photocatalysis^13,14. At a “pseudodissociated” state¹⁴, the intermolecular H-bond facilitates the cleavage of water O-H bond at < 1 monolayer coverage¹⁵, and interface H-bond also promotes photogenerated hole transfer and water oxidation by the strongly coupling of H-bond with holes¹⁶. Unfortunately, strong hydrogen bond network among water clusters inhibits the water dissociation¹⁷. Thus, an accurate control of H-bond network and adsorbed water monolayers over photocatalysts is vital for water dissociation¹³. Recently, scientists found that excess electrons of OH⁻ anions in alkaline water could induce reorganization of hydrogen bond in water clusters, thus further diminishing the overall energy barrier of alkaline hydrogen evolution reaction (HER)¹⁸. However, it is still unknown whether this alkaline based H-bond network manipulation strategy is feasible for the H₂O₂ photosynthesis.

Different from traditional metal oxide photocatalysts of poor interfacial H-bond modulation capacity, covalent organic frameworks (COFs), famous metal-free molecular photocatalysts possessing huge potential in H₂O₂ photosynthesis, are very powerful to regulate H-bond at molecular levels because of their variable and designable organic units^19,20. Among various organic units, AQ moieties is believed to be the best redox center for the H₂O₂ synthesis, as the oxidation of the hydrogenated AQ (anthrahydroquinone, HAQ) by molecular oxygen can selectively produce H₂O₂, which is thermodynamically spontaneous and commercially used⁵. Recently, several AQ-containing COFs (such as TPE-AQ, TpAQ, AQTEE-COP, and AQTT-COP) were designed to promote photogenerated charge separation and facilitate WOR for efficient H₂O₂ photosynthesis with pure water upon visible light irradiation (> 400 nm), and their best activity reached 3221 µmol g^− 1 h^− 1 without manipulating H-bond network^21–24.

As a typical AQ-containing COFs, TpAQ synthesized by β-ketoenamines links of 2,6-diaminoanthraquinone (AQ) and 2, 4, 6-triformylphloroglucinol (Tp), is often a mixture of keto- and enol- forms due to the formation of tautomerism during the polymerization (Fig. S1). Different from the unstable enol-form that mainly form weak H-bond with oxygen in H₂O²⁵, keto-form AQ COFs (Kf-AQ) is a more favorable proton acceptor to combine with hydrogen in H₂O via strong H-bond. Generally, traditional solvothermal method with acetic acid catalysis tends to produce enol-form dominant COFs. Although alkaline solution (such as OH⁻) induces the transformation of enol-form into keto-form²⁶, the NaOH addition disfavored the solvothermal synthesis of Kf-AQ, because the excessive solvents would consume NaOH to form carboxylates. Thus, the controlled synthesis of Kf-AQ is crucial for superior H₂O₂ photosynthesis, but never reported previously.

Herein we demonstrate the first mechanochemical synthesis of keto-form anthraquinone covalent organic framework (Kf-AQ) for direct H₂O₂ photosynthesis with molecular oxygen and alkaline water (pH = 13), and this Kf-AQ could deliver a record H₂O₂ production rate of 4784 µmol h^− 1 g^− 1 in the absence of any sacrificial reagents under visible light irradiation (λ > 400 nm). The critical roles of hydroxide anions and keto-form AQ moieties for efficient H₂O₂ production are carefully clarified via in-situ characterization and theory calculations.

Synthesis and structure characterization. Kf-AQ was mechanochemically synthesized by a Schiff-base condensation reaction of Tp and AQ with CH₃COONa (NaAc) as the catalysts (Fig. 1a). Fourier-transformed infrared spectra (FT-IR) spectra clearly revealed a new C-N stretching band at 1260 cm^− 1 and a disappeared N-H stretching band at 3459 ~ 3151 cm^− 1 for NH₂ groups in AQ (Fig. S2)²⁷. The as-prepared Kf-AQ powder displays a red-black color, corresponding to its outstanding optical absorption with the edge extended to 900 nm (Fig. S3), which is obviously red-shifted as compared to the absorption edge at 780 nm of TpAQ prepared by a traditional solvothermal method²⁸. The simulated powder X-ray diffraction (PXRD) pattern of Kf-AQ with eclipsed AA stacking mode agreed well with the experimental data (Fig. 1b), suggesting the validity of such structure in Kf-AQ. Particularly, the broad peak at 26.54^o was caused by the strong π-π stacking construction arisen from the existence of a multilayered COF structure with an interlayer distance of 3.48 Å. TEM and SEM images also displayed that Kf-AQ had a lamellar stacking structure and excellent crystallinity (Fig. 1c, d and Fig. S4)²⁸. Moreover, Kf-AQ had a specific surface area of 129 m² g^− 1 and a pore size of 2.12 nm (Fig. S5), which was well matched with the simulated value (2.18 nm) in Fig. 1a.

Solid-state NMR spectra revealed that Kf-AQ had an almost exclusive keto-form structure (Fig. 1e). The chemical shifts at 184 ppm, 144 ppm and 109 ppm in ¹³C NMR spectra were all indexed to the keto-form structure of Kf-AQ^25,27,29, while the chemical shift at 4.4 ppm for the enol-form (c, C-OH) was absent in the ¹H NMR spectra (Fig. S6)³⁰. Only C, O and N elements were present in the XPS survey spectrum of Kf-AQ, without any residual Na (Fig. S7). More evidences of keto-form structure in Kf-AQ could be found in high resolution XPS spectra (Fig. S8). The content of C = O in Kf-AQ was approximately 60%, about three times that of C-OH (22%) (Fig. 1f). The content of C-N-H (56%), corresponding to the deconvolution peak at the binding energy of 400.4 eV in N 1s spectra, was obviously higher than that of C = N (31%) (Fig. S9a)³¹, while C 1s XPS spectra also illustrated more C-C (52%) and less C-O (38%) in Kf-AQ (Fig. S9b)³². All these above results supported the successful synthesis of an exclusive keto-form AQ COF.

The formation of Kf-AQ might be attributed to a NaAc-catalyzed Schiff-base condensation process as follows. Upon the heat generated from the collision of balls, the carbonyl oxygen on Tp monomer undergoes a nucleophilic addition with Na⁺ to form aldehyde salts^33,34, resulting in the neighbor carbon acquiring a positive charge to fulfill another nucleophilic addition with nitrogen atoms in AQ. The generated α-hydroxyl undergoes further dehydration with adjacent amino hydrogen to form an enol-form COF. Subsequently, Ac⁻ anions as the Lewis base tend to bind with the hydrogen of hydroxyl group in enol moieties and then induce electron transfer from oxygen to alkene group, thus enabling the enol-form transformation into the thermodynamically more stable keto-form moiety (Fig. S10). Such a transformation cannot be driven in the traditional solvothermal synthesis, but might partially occur in alkaline water to produce a keto-form dominated AQ COF^25,35.

Efficient H ₂ O ₂ photosynthesis. The H₂O₂ photosynthesis performance of Kf-AQ was evaluated by dispersing the powder in water at neutral and alkaline solutions (pH = 9, 11, 13, 14) with continuous O₂ purging. Upon visible light (λ > 400 nm) irradiation, the rate of H₂O₂ production at pH 13 reached as high as 4784 µmol h^− 1 g^− 1 (Fig. 2a), a new record in H₂O₂ photosynthesis of AQ containing COFs with water (Fig. 2b and Table S1). Upon a prolonged irradiation for 5 h, the H₂O₂ production was steadily growing (Fig. S11), and kept constant during five cycles of reaction (Fig. 2c). The crystal structure and surface functional groups of the reacted Kf-AQ did not change (Fig. S12), demonstrating its excellent stability for the H₂O₂ photosynthesis. The contribution of Na⁺ to the enhanced H₂O₂ production was ruled out by the replacement of NaOH with NaCl and KOH (Fig. S13), confirming the crucial promoting effect of hydroxide anions on the H₂O₂ photosynthesis of Kf-AQ.

The kinetic of H₂O₂ production was analyzed by fitting the time-dependent H₂O₂ production curves (Text S2). Kf-AQ exhibited the highest H₂O₂ formation rate constant (k_f, 31.39 µM min^− 1), but a medium decomposition rate constant (k_d) (0.031 µM min^− 1) at pH 13 (Fig. 2d and Table S2). Thus, the superior H₂O₂ photosynthesis performance of Kf-AQ with alkaline water was mainly attributed to its better H₂O₂ formation ability. The apparent quantum efficiencies (AQY) of Kf-AQ at different wavelengths were well matched with its absorption spectrum, and the highest value appeared at 400 nm and reached 15.8% (Fig. 2e and Table S3). To the best of our knowledge, the AQY of Kf-AQ is higher than those of most reported H₂O₂ synthesis photocatalysts^36,37. The solar-to-chemical conversion (SCC) efficiency of Kf-AQ was estimated to be 0.70% at pH 13 (Fig. S14 and Table S4), which was almost seven times of the average solar-to-biomass conversion (SBC) efficiency in nature²³.

Mechanism investigation. We first checked the basic semiconductor properties of Kf-AQ to understand its superior performance in H₂O₂ photosynthesis. The Tauc plot showed that the band gap of Kf-AQ was 1.55 eV (Fig. S15a), and ultraviolet photoelectron spectroscopy (UPS) determined its valence band potential (E_VB) as 1.90 V (Fig. S15b), suggesting that the conduction band potential (E_CB) of Kf-AQ was accordingly calculated as 0.35 V. Therefore, both 2e⁻ ORR (0.68 V vs. RHE) to produce H₂O₂ and 4e⁻ WOR (1.23 V vs. RHE) to evolute O₂ were thermodynamically feasible for Kf-AQ photocatalysis (Fig. 3a)³⁸. We further conducted density functional theory (DFT) calculations to elucidate the exciton dissociation in photocatalysis by using a dimer model of Kf-AQ. As depicted in Fig. 3b, the highest occupied molecular orbital (HOMO) of the dimer uniformly dispersed in the whole structure, while the lowest unoccupied molecular orbital (LUMO) localized over AQ units. Thus, the HOMO-LUMO transition under excitation can redistribute electron density from the Tp moieties to the adjacent AQ units, thus resulting in effective intramolecular charge transfer in Kf-AQ.

We then explored the sources of H and O for the H₂O₂ production by various control experiments and isotopic labeling analysis. In comparison to oxygen atmosphere, either air or N₂ purging resulted in poor H₂O₂ production (Fig. 3c), and the O₂ concentration in an airtight oxygen saturated suspension decreased obviously during photocatalysis (Fig. S16), suggesting the dominated contribution of ORR to the H₂O₂ production. AgNO₃ was added as the electron scavenger in N₂ atmosphere to evaluate the contribution of water oxidation. The negligible amount of H₂O₂ generated in the Kf-AQ suspension ruled out the direct contribution of WOR to the H₂O₂ production. However, H₂O₂ was obviously produced in case of N₂ purging and absence of AgNO₃, suggesting that photocatalytically produced O₂ via 4e⁻ WOR (Fig. 3c) enabled the consequent ORR to produce H₂O₂ (Fig. S17). Significant H₂O₂ was only detected in the mixed solution of H₂O and acetonitrile (v/v = 1:1) other than pure acetonitrile (Fig. 3d), confirming that water was the exclusive hydrogen source for the H₂O₂ photosynthesis.

We conducted the isotopic photoreaction experiments by purging H₂¹⁶O suspensions with ¹⁸O₂ gas during the H₂O₂ photosynthesis, and then used MnO₂ to catalytically decompose the as-synthesized H₂O₂ into oxygen. After 8 h of photoreaction, strong ¹⁸O₂ (m/z = 36, 93.7%) and very weak ¹⁶O₂ (m/z = 32, 6.3%) signals appeared in the gas chromatography-mass spectra (GC-MS) of collected gas (Fig. 3e), demonstrating that H₂¹⁸O₂ was the dominated product and mainly came from the reduction of ¹⁸O₂. Gradually, the signal of ¹⁸O₂ peak decreased (80.5%), accompanying with an increased ¹⁶O₂ signal (19.5%) at 24 h of reaction, because the photocatalytic oxidation of H₂¹⁶O produced ¹⁶O₂ to increase the proportion of H₂¹⁶O₂ in the products. The electrons transfer number (n) of ORR was further measured to be about 2.06 ~ 2.09 by the RDE method (Fig. 3f and Fig. S18)²⁴. Thus, we suppose that both 2e⁻ ORR and 4e⁻ WOR take place during H₂O₂ photosynthesis over Kf-AQ at pH 13.

To probe the active sites of Kf-AQ for the H₂O₂ photosynthesis, we synthesized two control COFs by respectively replacing the monomers of Tp and AQ with 1,3,5-trimethylbenzaldehyde (LZU) and 2,6-diaminoanthracene (DA), namely LZUAQ and TpDA (Fig. S19 and S20). Their H₂O₂ photosynthesis performance was much worse than that of Kf-AQ (Fig. S21), suggesting that anthraquinone groups were the indispensable active sites for ORR, and the keto and AQ conjugated configuration accounted for the efficient WOR over Kf-AQ.

We then employed in-situ FTIR and Raman spectra to further understand the critical role of water adsorption and dehydrogenation in the superior H₂O₂ photosynthesis of Kf-AQ. Upon irradiation, three obvious O-H stretching vibration bands appeared in the in-situ FTIR spectra of Kf-AQ (Fig. 4a), corresponding to the water clusters including Na⁺(H₂O)₃ or OH⁻(H₂O)₃ (3540 cm^− 1), OH⁻(H₂O)₄ (3410 cm^− 1), and OH⁻(H₂O)₅ (3292 cm^− 1)^39,40. These adsorbed water clusters were the proton precursors for H₂O₂ photosynthesis, which can be further checked by in-situ Raman spectra. The notable O-H stretching bands in Raman spectra at around 3000–3700 cm^− 1 can be deconvoluted into three bands, corresponding to the four-coordinated hydrogen bonded water network (V₁, 3254 cm^− 1), the two-coordinated single donor hydrogen bonded water clusters (V₂, 3420 cm^− 1) and the Na⁺ ion hydrated water (Na⋅H₂O) clusters (V₃, 3553 cm^− 1), respectively (Fig. 4b)^41–43. The intensity of these bands for Kf-AQ was significantly higher than those for TpAQ, suggesting the formation of stronger hydrogen bond between keto moiety (-C = O) and OH⁻(H₂O)_n clusters⁴³, possibly because the vibrational dipole moment (the direction of O-H bonds) in the clusters (such as Na⋅H₂O) is parallel to the direction of the interfacial electric field, thus favoring the combination of hydrogen in the clusters with carbonyl groups of Kf-AQ. Simultaneously, V2 and V3 were the dominant forms in the Kf-AQ Raman spectrum, and generally had relatively weaker hydrogen bond network than V1, the dominant form in the TpAQ Raman spectrum. These differences can be attributed to the strong interaction between water clusters and carbonyl groups of Kf-AQ, resulting in the disorder and stretching of H-bonds in the arrangement of water molecules⁴⁴, and the strong dipole-dipole force between Na⁺ and H₂O molecules in the Na⁺ solvation structures further destroy the water-water interactions to form small water clusters of weak H-bonding environment⁴⁴, thus favoring the photocatalytic dissociation of water and release of hydrogen. Therefore, the unique exclusive keto-form of Kf-AQ enhanced the adsorption and dissociation of water, thereby promoting hydrogen abstraction from water for the H₂O₂ photosynthesis.

We compared water adsorption over Kf-AQ and TpAQ by DFT calculations. In case of one water molecule adsorption, the adsorption energy of Kf-AQ was − 0.26 eV, much lower than that of TpAQ (-0.18 eV) (Fig. S22). Increasing water cluster sizes to (H₂O)₃, the adsorption energy of Kf-AQ decreased to -0.35 eV, and further decreased to -0.44 eV for the OH⁻(H₂O)₂ clusters, which was the dominant form of adsorbed water in alkaline water (Fig. S23), suggesting the superior water adsorption capability of Kf-AQ. Moreover, the bond energy of terminal H-O in OH⁻(H₂O)₂ form was 4.3 eV, much lower than that of (H₂O)₃ (5.9 eV) (Fig. 4c), suggesting the easier hydrogen dissociation from the terminal water, and thus favoring the subsequently combination with the neighboring H₂O to form hydronium ion (H₃O⁺)¹⁸.

We detected the intermediates of H^*_ads and OH_ads species by the cyclic voltammogram (CV) (Fig. 4d). The H^*_ads species generated in the reduction stage by reducing hydronium ion (H₃O⁺) were oxidized, corresponding to an oxidative peak at about 0.25 V vs. RHE^45,46. The oxidative peaks in the CV curves were more distinct with the increase of hydroxide anion concentrations, suggesting the increase of H^*_ads dosages at strong alkaline conditions¹⁸. Simultaneously, the reduction peak at 0.77 V was attributed to the reversible adsorbed OH_ads species, which were produced via the loss of electrons in OH⁻ (ref. ⁴⁷). The OH_ads species would be stabilized by forming hydroxyl-water-alkali metal cation cluster (OH_ads-Na⁺-(H₂O)_n), thus accordingly preventing its depletion by H₃O⁺. Therefore, we propose that the dissociation of H₂O into H^*_ads and OH_ads species takes place in the 2e⁻ ORR and 4e⁻ WOR pathways.

These above results strongly suggest a synergism of keto and anthraquinone moieties in Kf-AQ for superior H₂O₂ photosynthesis from water and oxygen, as depicted in Fig. 5. Initially, OH⁻(H₂O)_n clusters preferentially adsorbs onto the keto-form moieties in Kf-AQ, thus weakening the H-O bond of the terminal H₂O via forming the H-OH(H₂O)_n−1OH⁻ clusters and facilitating the dehydrogenation in water molecules. The detached protons then combine with the neighboring H₂O to form H₃O⁺. Upon visible light irradiation, surface H₃O⁺ on Kf-AQ can be reduced by interfacial electrons (e⁻) to release H^*_ads species, which preferentially bind with the quinone groups (-C = O) in AQ and subsequently hydrogenate AQ to yield anthrahydroquinone (H₂AQ). Afterwards, the parahydrogen atoms of H₂AQ are abstracted to produce radicals, which react with O₂ to form 1,4-endoperoxide species, a well-known intermediate for the formation of H₂O₂, which was confirmed by the new Raman peak at 891 cm^− 1 (Fig. S24). Then, 1,4-endoperoxide species couples the adjacent hydrogen in the hydroxyl group of H₂AQ to release H₂O₂. Meanwhile, another dissociation product, OH_ads intermediate, would not be dissociated as OH⁻ within the interface layer, but form an adsorbed hydroxyl-water-alkali metal cation cluster (OH_ads-Na⁺-(H₂O)_n)⁴⁷. Upon visible light irradiation, the photogenerated holes (h⁺) oxidizes this OH_ads to produce O₂ in a 4e⁻ WOR pathway. Therefore, the formation of OH_ads-Na⁺-(H₂O)_n and H₃O⁺ intermediates over Kf-AQ at high pH conditions facilitates water oxidation and hydrogen extraction from H₂O molecules, resulting in its superior photocatalytic H₂O₂ production.

We have demonstrated the synthesis of a keto-form anthraquinone containing COF via a mechanochemical process and its superior H₂O₂ photosynthesis in alkaline water, with a record H₂O₂ production rate of 4784 µmol h^− 1 g^− 1 under visible light irradiation in the absence of sacrificial reagents. The keto-form structure in Kf-AQ can promote the water adsorption through the formation of OH⁻(H₂O)_n clusters with weakened hydrogen bonds, which accordingly enhances the dehydrogenation of water and promotes efficient H₂O₂ photosynthesis. The manipulating H-bond network of adsorbed water clusters represents a novel strategy to break the rate-limiting step of hydrogen extraction from water, and bring insights for the design of highly active photocatalysts to realize efficient H₂O₂ photosynthesis from only water and oxygen.

Synthesis of Kf-AQ. Mechanochemical synthesis was conducted to produce Kf-AQ by use of a planetary ball mill (SFM-1, Hefei Kejing Material Technology Co., Ltd). Typically, 2,4,6-triformylphloroglucinol (Tp, 126 mg, 0.20 mmol), 2,6-diaminoanthraquinone (AQ, 213 mg, 0.30 mmol), and CH₃COONa (5 mg) were placed in a 50 mL agate grinding jar, with fifteen 7 mm diameter and ten 5 mm diameter agate balls. Then, the mixture was ground at room temperature with a rotation speed of 400 rpm for 6 h. After that, the obtained precursors were washed with N, N-dimethylformamide and acetone, and then dried in a vacuum oven at 120 ^oC for 12 h. The obtained photocatalyst was denoted as Kf-AQ.

Synthesis of TpAQ. TpAQ was synthesized by Schiff-base condensation of Tp and AQ according to a modified previous method²⁸. In a 10 mL Schlenk tube, 2,4,6-triformylphloroglucinol (71.4 mg, 0.20 mmol) and 2,6-diaminoanthraquinone (42.1 mg 0.30 mmol) were charged. Then, N, N-dimethylacetamide (2.0 mL) was added as the solvent, and the suspensions were sonicated for 10 min. Subsequently, 0.3 mL glacial acetic acid was added. After that, the ampoule was degassed by freeze-pump-thaw three times and then sealed on. The Schlenk tube was put into an oven and heated at 120 ^oC for 72 h. The obtained powder was washed with N, N-dimethylformamide and acetone, and then dried in a vacuum oven at 120 ^oC for 12 h.

H ₂ O ₂ photosynthesis. H₂O₂ photosynthesis was conducted in a homemade quartz cuvette reactor. Generally, 5 mg of the photocatalysts were ultrasonically dispersed into 30 mL water whose initial pH was adjusted by 0.1 M NaOH solution. Then, the suspension was stirred for 15 min in the dark with continuously O₂ purging. After that, the reactor was illuminated by a 300 W Xe lamp (PLS-SXE300, Beijing Perfectlight) with a cut-off filter (λ > 400 nm). During the reaction, 1.5 mL of reaction mixture was withdrawn at every 15 min interval, and then filtrated through a 0.45 µm polyether sulfone (PES) filter for H₂O₂ detection.

The H₂O₂ concentration was measured by the N, N-diethylp-phenylenediamine (DPD)-horseradish peroxidase (POD) colorimetry method⁴⁸. Typically, 3.0 mL of phosphate buffer (0.2 M, pH = 6) was added into a 15.0 mL colorimetric tube, and then 1.0 mL sample, 50 µL DPD, 50 µL POD were all added into the mixture. Then, ultrapure water was added to set the volume to 10 mL. Finally, the absorbance was measured on a UV-vis spectrophotometer (TU-1810) at 551 nm to determine H₂O₂ concentration according to the predetermined standard curve.

In-situ FTIR measurements. In-situ FTIR spectra were obtained by using a Thermo Scientific Nicolet Is50, equipped with a commercial chamber from Harrick Scientific. Typically, 5 mg of Kf-AQ was dispersed into 30 mL H₂O at pH = 13. The formed uniform dispersion was bubbled with O₂ for 15 min in the dark, and then the background spectrum was collected. After that, the reaction chamber was irradiated by visible light (λ > 400 nm), and then the spectrum was collected at a one min interval.

Isotopic experiments. Specifically, 5 mg Kf-AQ was added into H₂¹⁶O (30 mL) within a glass tube (50 mL). The formed dispersion was sonicated for 10 min and bubbled with Ar for 30 min. Then, the reaction tube was sealed with rubber septum cap and vacuum. ¹⁸O₂ gas was introduced to the tube by a syringe. The reaction tube was illuminated by a 300 W Xe lamp with a cut-off filter (λ > 400 nm). After photoreaction for 8 h and 24 h, the reaction solution was purged by Ar for 5 min to remove the residual ¹⁸O₂ gas. The dispersion was filtered and injected into another clean tube, which was saturated with Ar and contained 200 mg MnO₂ powder. The generated gas was collected by an Aluminum foil air pocket (5 mL) and detected on a Shimadzu GC-MS system (Agilent 7890 A/ 5975C).

DFT calculations. The DFT calculation used the method in previous references⁴⁹. Briefly, geometry optimizations without symmetry restriction are performed by using the DFT/B3LYP/6-31G(d, p) basis sets and scrf-smd solvent model. All calculations were performed on Gaussian 09.

Data availability

Source data are provided with this paper. The data that support the findings of this study are available from the corresponding author upon reasonable request.

Competing interests

The authors declare no competing interests.

Additional Information

Supplementary Information is available online or from the author.

Author contributions

X. Z., M. L., B. Z., and L. Z. contributed to design of this study, X. Z., M. L., and L. Z. wrote the manuscript. X. Z., J. M., C. C., and S. C. conducted experiments and performed data analysis. X. W. provided DFT calculations and analysis.

Acknowledgement

Financial supports from the National Natural Science Foundation of China (Nos. 22376138, 22206125, 52070128) and National Key R&D Program of China (2022YFA1205602) are gratefully acknowledged.

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Keto-anthraquinone covalent organic framework for H2O2 photosynthesis with oxygen and alkaline water

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Keto-anthraquinone covalent organic framework for H2O2 photosynthesis with oxygen and alkaline water

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