Molecularly Engineered Covalent Organic Frameworks for Hydrogen Peroxide Photosynthesis

Abstract Synthesizing H2O2 from water and air via a photocatalytic approach is ideal for efficient production of this chemical at small‐scale. However, the poor activity and selectivity of the 2 e− water oxidation reaction (WOR) greatly restricts the efficiency of photocatalytic H2O2 production. Herein we prepare a bipyridine‐based covalent organic framework photocatalyst (denoted as COF‐TfpBpy) for H2O2 production from water and air. The solar‐to‐chemical conversion (SCC) efficiency at 298 K and 333 K is 0.57 % and 1.08 %, respectively, which are higher than the current reported highest value. The resulting H2O2 solution is capable of degrading pollutants. A mechanistic study revealed that the excellent photocatalytic activity of COF‐TfpBpy is due to the protonation of bipyridine monomer, which promotes the rate‐determining reaction (2 e− WOR) and then enhances Yeager‐type oxygen adsorption to accelerate 2 e− one‐step oxygen reduction. This work demonstrates, for the first time, the COF‐catalyzed photosynthesis of H2O2 from water and air; and paves the way for wastewater treatment using photocatalytic H2O2 solution.


Part 1 Experimental Section
Materials.

Synthesis of AP-
were put into a 10 mL glass ampoule. Then add a mixed solvent of mesitylene and dioxane (1:1), and ultrasonic treatment to make it evenly dispersed. Then quickly add S-4 0.5 mL of 3 M acetic acid (AcOH) in water and sonicate again. After degassing, react at 120°C for 3 days. Then a large amount of acetone was used to wash off the solid and dried under vacuum. [3] Synthesis of COF-TfpBpy-Mo. COF-TfpBpy (20 mg) and Mo(CO)6 (11 mg) were dispersed in 20 ml toluene solution and reacted at 110 °C for 6 h under the protection of N2 atmosphere. After cooling to room temperature, it was washed with methanol and water, and vacuum dried at 80 °C for 12 h.  Useing UV-2600 (Shanghai Tianmei Scientific Instrument Co., Ltd.) to measure absorbance at 552 nm. Calibrate the H2O2 concentration by diluting a 30% H2O2 stock solution. [5] RhB degradation. 10 mg Rhodamine B (RhB) and 278 mg FeSO4 ·7H2O were dissolved in a 1 L volumetric flask. Taking out 3.5 ml RhB solution and put it in a 10 ml centrifuge tube, and then 0.5 ml photocatalytic H2O2 solution was gradually added.

Synthesis
The color change of RhB was recorded by video. Modifing Gaussian path and smearing width is 0.2 eV.

Sterilization of
Geometries were optimized until the energy and the force were converged to 1.0×10 −5 eV/atom and 0.05 eV/Å, respectively. An energy cutoff was set as 400 eV for the plane-wave expansion of the electronic wave function. A vacuum region of 15 Å was introduced to avoid the interactions between the periodic slabs. Besides, k-point grid is set to 3 * 2 * 1.
The adsorption energy (Eads) of O2 or H2O molecule on the surface is calculated as follow: where Etotal represents the energy of surface with adsorbed O2 or H2O molecule, Esurface and EO2 or EH2O represent the energies of isolated surface and O2 or H2O, respectively. S-8

Figure S1
The XRD (a), IR (b) and solid state 13 C NMR (c, d) spectra of COF-TfpBpy and AP-TfpBpy.  Note S1: The powder X-ray diffraction (PXRD) pattern of COF-TfpBby (red line) shows that a strong peak corresponding to the (100) plane reflection is displayed at 2θ=3.68°,

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indicating that there is an open channel. The broad peak at 2θ=26° is mainly due to the π-π accumulation between the COF layers corresponding to the (002) plane. The relatively broad peaks can be attributed to strain defects and grain size effects in the crystal lattice. However, AP-TfpBpy (green line) has no obvious diffraction peak at 2θ=3.68°, indicating that its structure has no crystal form ( Figure S1a). Based on the experimental and simulated XRD pattern, COF-TfpBpy should be a COF materials with AA stacking mode rather than conjugated porous polymer.
According to FT-IR (Figure S1b), strong peaks corresponding to the stretching frequency of the ketone form are observed at 1608 cm -1 (C=O) and 1579 cm -1 (C=C) ( Figure S1b). It is the same as the COF-TfpBpy reported in the previous literature.
AP-TfpBpy also has two corresponding strong peaks at 1607 cm -1 and 1579 cm -1 , indicating that C=O and C=C also exist, and the other parts may be different in infrared due to crystallinity.
As shown in Figure S1c, However, it can be seen from the SEM image that COF-TfpBpy has a tendency to form a spherical structure, but AP-TpBpy does not.
The adsorption and desorption curves and BET of COF-TfpBby and AP-TfpBpy under N2 are shown in Figure S4. It is known that the specific surface areas of COF-TfpBby and AP-TfpBpy are 939.44 m 2 g -1 and 865.45 m 2 g -1 , respectively.

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AP-TfpBpy's Brunauer-Emmett-Teller (BET) is significantly reduced, indicating that the crystallinity of COFs directly affects the size of its active contact surface. [6] The pore size distribution graph of COF-TfpBpy and AP-TfpBpy prepared by solvothermal method has a peak at 1.89 nm, indicating that the material is a mesoporous material.
S-12  Note S2: Figure S5a shows that the XRD spectrum of g-C3N4 has two peaks at 12.7° and 28°, which are consistent with the typical graphite phase carbon nitride diffraction peaks, corresponding to the (100) peak of the in-plane repeating unit and stacked between the aromatic ring layer (002) peak, which means that g-C3N4 is obtained.
The FTIR spectrum of g-C3N4 is shown in Figure S5b. The characteristic peak at 2900-3400 cm -1 corresponds to the stretching vibration absorption peak of -NH2 or -OH, the peak in the range of 1200-1650 cm -1 corresponds to the stretching vibration of the CN heterocyclic ring, and the peak at 810 cm -1 is the characteristic peak of the triazine ring structure. [7] The g-C3N4 prepared by thermal polymerization has a small specific surface area, low catalytic efficiency, and lack of active sites. As shown in Figure S5. heating in air. [8] As shown in Figure S7, it can be seen from the scanning electron microscope (SEM) that g-C3N4 is a large layered structure, and the transmission electron microscope (TEM) image shows that its thickness is relatively thin.
S-15  The experimental method is: adjust the pH of the sample, and then add DPD and POD solutions. If there is hydrogen peroxide in the water, the hydrogen peroxide will oxidize POD, and the oxidation product of POD will then oxidize DPD into positive ion radical DPD· ＋ . DPD· ＋ is a pink compound, it has two absorption peaks, 510 mm and 551 mm. Because this method can only detect low-concentration hydrogen peroxide in water, and our photocatalytic production of hydrogen peroxide has a higher concentration, it must be diluted before testing. [9] In order to detect the influence of different electronic sacrificial reagents on the detection method, as shown in Figure S9,

Note S5:
COF-TfpDaaq shows a strong diffraction peak of 3.5° and a broad peak of 27°, corresponding to the (100), and (001) reflection surfaces of the structure, respectively ( Figure S15a). Fourier transform infrared ( Figure S15b) and 13 C crosspolarization magic angle spinning (CP-MAS) solid-state NMR spectroscopy (Figure S15c, d) further confirms COF-TfpDaaq structures. The appearance of a new C-N stretching vibration peak at 1250 cm -1 is characteristic of β-ketoenamine C-N bond in COF-TfpDaaq. [20] The preferential formation of the keto form rather than the enol tautomer is further confirmed by the C=O stretching vibration at 1615 cm -1 and the absence of O-H bond resonance in its FT-IR spectrum. As the spectra of COF-TfpDaaq exhibit resonances at 145 ppm that are assigned to the enamine carbon (=CNH) and α-enamine carbon at 115 ppm. In addition, the resonance at ~180 ppm corresponds to the ketone resonance. XPS spectra show the presence of C, N, and O in COF-TfpDaaq (Figure S16). High-resolution X-ray photoelectron spectroscopy also differentiates between C, N and O atoms in the different bonding environments. [21] The morphology of as-prepared catalysts was characterized by Scanning Electron Microscope (SEM) and Transmission electron microscopy (TEM) S-26 ( Figure S17). According to SEM, it can be seen that COF-TfpDaaq is an interconnected porous network structure, and it also shows a structure similar to folds or wrinkles.
According to the COF-TfpBby N2 adsorption and desorption curve and BET ( Figure S18), it is known that its specific surface area is 653.39 m 2 g -1 . and the pore size distribution of COF-DAAQ has a peak at 2.14 nm, indicating that the material is a mesoporous material.
In addition, the energy band structure of COF-TfpDaaq and the positions of CB and VB are calculated by DRS spectra, Kubelka-Munk converted reflectance spectra and Mott-Schottky diagrams. It is found that the values of VB, CB and optical band gap correspond to 2.34, 0.32, 2.02 V ( Figure S19).

Figure S20
The XRD (a) and IR (b) spectra of COF-TfpBd.

Note S6:
COF-TfpBD shows a strong diffraction peak of 3.4°, corresponding to the (100), reflection surfaces of the structure (Figure S20a). The FT-IR spectrum of COF-TfpBD ( Figure S20b) shows a typical broad and strong band, which corresponds to the characteristic C=C stretch of aromatic hydrocarbons in the 1596 cm -1 region and the CN stretch of amines close to 1257 cm -1 . There is also a weak band close to 1618 cm -1 , which corresponds to the C=O bond of the ketone form in the tautomeric keto-enamine form. [22] N2 adsorption isotherm at 77 K to check the structural stiffness and permanent porosity of COF-TfpBD ( Figure S21). The BET surface area of COF-TfpBD is 596.6 m 2 g -1 .The pore size distribution of COF-TfpBD was found to be between 2.15 nm.
In addition, the energy band structure of COF-TfpDaaq and the positions of CB and VB are calculated by DRS spectra, Kubelka-Munk converted reflectance spectra and Mott-Schottky diagrams. It is found that the values of VB, CB and optical band gap correspond to 2.13, -0.07, 2.20 V (Figure S22).

Figure S23
The XRD (a) and IR (b) spectra of COF-TfpPa.

Note S7:
The powder X-ray diffraction (PXRD) pattern of COF-TfpPa shows a strong peak at 4.5°, corresponding to the reflection from the (100) plane ( Figure S23a).
There are also small peaks at 2θ=8.3°, 11.9° and 26.8° of COF-TfpPa, which are attributed to the (200), (210) and (001) reflection planes. [23] Interestingly, the FT-IR spectrum does not show the characteristic stretch bands of hydroxyl (-OH) or imine (C=N) functional groups ( Figure S23b). If the compound exists in the form of enol, it should be there. On the contrary, they present a strong peak under the peak at 1578 cm -1 in the ketone form, indicating that it exists in the ketone form. However, because of the peak broadening in the extended structure, the C=O peaks of COF-TfpPa at 1616 cm −1 are merged with the C-C stretching band at 1578 cm −1 and appeared as a shoulder. And the BET surface area of COF-TfpPa is 614.74 m 2 g -1 . The pore size distribution of COF-TfpBD was found to be between 2.14 nm (Figure S24).

Note S8:
According to the XRD patterns of COF-TfpBpy and COF-TfpBpy-Mo ( Figure   S1a and Figure S26a), it can be seen that there are no extra peaks, indicating that the S-33 solid COF frame structure is still retained after the cobalt modification.
The FTIR spectrum shows that strong peaks corresponding to the stretching frequency of the ketone form are observed at 1608 cm -1 (C=O) and 1579 cm -1 (C=C) ( Figure S26b). COF-TfpBpy-Mo still has the same characteristic functional groups as COF-TfpBpy, but some red shifts and obvious broadening of the C-N peak have occurred, which means that Mo and the bipyridine N atom are coordinated in the COF skeleton.
According to the transmission electron microscope (TEM) image (Figure S27a),

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Note S9: The XRD pattern of AP-TfbBpy has a main diffraction peak at 2θ=24.5°, corresponding to the (001) plane; AP-TfbBpy has no obvious diffraction peak at 2θ=3.7°, indicating that its structure has no crystal form ( Figure S30a).
The FT-IR spectrum of AP-TpbBpy shows a strong C=N stretch at 1618 cm -1 (Figure S30b), indicating the formation of an imine bond. The characteristic peak at 1271 cm -1 is attributed to the strong stretching of C-N. The new peak formed at 1584 cm-1 is designated as the C=C stretch, but the peak of AP-TfbBpy at this point is weakened to be negligible, indicating that TfbBpy does not exist in the ketone form.
In addition, the energy band structure of COF-TfbBpy and the positions of CB and VB are calculated by DRS spectra, Kubelka-Munk converted reflectance spectra and Mott-Schottky diagrams. It is found that the values of VB, CB and optical band gap correspond to 2.18, 0.13, 2.05 V ( Figure S31).

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Part 14 The influence of specific surface area and porosity Figure S32 The amount of H2O2 generated per unit surface area of COF-TfpBpy, AP-TfpBpy, COF-TfpBd, COF-TfpPa and COF-TfpDaaq.