Monatomic photocatalyst Cu-OmBpy-COF as well as preparation method and application thereof

Abstract

The invention relates to the technical field of photocatalysis, in particular to a monatomic photocatalyst Cu-OmBpy-COF as well as a preparation method and application thereof. The novel OmBpy-COF which is bonded in two connection modes of an imino group and an oxazole ring is synthesized through a one-pot method, and copper metal is anchored into a covalent organic framework through an impregnation method to obtain the monatomic catalyst Cu-OmBpy-COF. Compared with pure OmBpy-COF, the Cu-OmBpy-COF has the advantages that the carrier separation efficiency can be improved, the yield of reducing carbon dioxide into CO through photocatalysis is remarkably enhanced, and the yield of CO is 3815.94 mu mol.g <-1 > under irradiation of a xenon lamp for 5 hours and is 5.4 times that of the pure OmBpy-COF. The anchoring of the metal copper ions increases the adsorption sites of carbon dioxide, improves the reduction activity of carbon dioxide and the selectivity of CO production, greatly improves the yield of carbon monoxide, and has excellent photocatalytic stability.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of photocatalysis, in particular to a single-atom photocatalyst Cu-OmBpy-COF and a preparation method and application thereof. BACKGROUND

[0002] At present, the continuous rise of global carbon emissions pushes the climate crisis into a more severe situation, and efficient capture and conversion of excess CO2 in the atmosphere has become a core challenge to break the environmental dilemma. Under this background, the solar-driven photocatalytic CO2 reduction reaction (CO2RR) has become a key technology connecting carbon emission reduction and renewable energy utilization due to its unique advantages - it can use solar energy as power to convert greenhouse gas CO2 into high-value CO, providing an important path to alleviate carbon emission pressure and break the "energy consumption-environmental degradation" cycle. Therefore, in view of the demand for photocatalytic CO2 reduction reaction, it is crucial to precisely design and prepare excellent adaptive photocatalysts for photocatalytic reduction of carbon dioxide.

[0003] As a new type of crystalline porous material, covalent organic frameworks (COFs) can be used as a new type of photocatalyst due to their excellent crystallinity, controllable chemical composition and efficient light absorption performance, and have great application potential in the field of photocatalytic CO2 reduction. However, the separation efficiency of photo-generated electron-hole pairs of pure COFs is low, and the excited carriers are prone to rapid recombination, and the activation ability of CO2 molecules is weak, which reduces the number of effective charges participating in the CO2 reduction reaction. Still limits its application in the field of photocatalytic reduction of carbon dioxide. SUMMARY

[0004] In order to solve the above problems, the application provides a single-atom photocatalyst Cu-OmBpy-COF and a preparation method and application thereof. The single-atom photocatalyst Cu-OmBpy-COF provided by the application can photocatalytically reduce CO2 to CO, showing high catalytic activity.

[0005] In order to achieve the above purpose, the application provides the following technical scheme: The application provides a preparation method of a single-atom photocatalyst Cu-OmBpy-COF, comprising the following steps: 1) dispersing 2,4,5-triamino-6-hydroxy pyrimidine sulfate, [2,2'-bipyridine]-5,5-dimethyl formaldehyde and acetic acid solution in an organic solvent to obtain a mixed solution; 2) sequentially performing liquid nitrogen freezing, vacuumizing and thawing on the mixed solution in step 1), and then performing reaction to obtain a crude product; 3) sequentially performing washing, Soxhlet extraction and vacuum drying on the crude product in step 2) to obtain a covalent organic framework OmBpy-COF material; 4) ultrasonic dispersion of the covalent organic framework OmBpy-COF and copper acetate anhydrous in acetonitrile, after stirring, suction filtration and drying to obtain a single-atom photocatalyst Cu-OmBpy-COF.

[0006] Preferably, the mass of the 2,4,5-triamino-6-hydroxy pyrimidine sulfate salt, the mass of [2,2'-bipyridine]-5,5-diformyl, the volume of the acetic acid solution and the volume of the organic solvent in step 1) are 47.8 mg:63.6 mg:0.4 mL:2 mL.

[0007] Preferably, the concentration of the acetic acid solution in step 1) is 6 mol / L. The organic solvent is N,N-dimethylacetamide and o-dichlorobenzene, and the volume ratio of the N,N-dimethylacetamide and o-dichlorobenzene is 1:2.

[0008] Preferably, the number of cycles of liquid nitrogen freezing, vacuumizing and thawing in step 2) is 3. The reaction conditions include a temperature of 120℃ and a time of 72 h.

[0009] Preferably, the crude product in step 3) is washed with acetone, tetrahydrofuran and deionized water in sequence. The solvent used in the Soxhlet extraction is tetrahydrofuran, and the Soxhlet extraction time is 12 h. The vacuum drying temperature is 80℃, and the time is 12 h.

[0010] Preferably, the mass of the covalent organic framework OmBpy-COF, the mass of copper acetate anhydrous and the volume of acetonitrile in step 4) are 30 mg:30 mg:30 mL.

[0011] Preferably, the ultrasonic dispersion time in step 4) is 30 min. The stirring time is 10 h. The drying conditions include a temperature of 80℃ and a time of 12 h.

[0012] The application also provides a single-atom photocatalyst Cu-OmBpy-COF prepared by the preparation method.

[0013] The application also provides an application of the single-atom photocatalyst Cu-OmBpy-COF in photocatalytic reduction of carbon dioxide to generate carbon monoxide.

[0014] Preferably, the application comprises the following steps: dispersing the monatomic photocatalyst Cu-OmBpy-COF in a solvent, and then performing a reaction under irradiation of a 300W xenon lamp after carbon dioxide is introduced.

[0015] Advantages of the present application: (1) The present application selects 2,4,5-triamino-6-hydroxy pyrimidine sulfate and [2,2'-bipyridine]-5,5-dimethyl formaldehyde as monomers, and a new covalent organic framework OmBpy-COF with two kinds of connections of imine and oxazole ring is synthesized for the first time by one-pot method. Based on the irreversibility of part of the imine bond, a stable oxazole ring is formed, and compared with the COF connected by traditional imine bond, the material has higher stability in water environment.

[0016] (2) The monatomic photocatalyst Cu-OmBpy-COF is prepared by anchoring copper ions to the covalent organic framework (OmBpy-COF), which increases the active site and effectively promotes the charge separation and transfer in the framework, thereby significantly improving the photocatalytic reduction yield. After irradiation of a xenon lamp (λ>420nm) for 5h, the carbon monoxide yield reaches 3815.94 μmol·g -1 , and the monatomic photocatalyst Cu-OmBpy-COF prepared by the present application has excellent photocatalytic stability.

[0017] (3) The raw materials used in the present application are low in price, the preparation method is simple, the synthesis conditions are mild, the reaction process has low energy consumption, and the like, and the present application has good economic benefits and application prospect for large-scale production and application. BRIEF DESCRIPTION OF DRAWINGS

[0018] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiments will be briefly introduced as follows.

[0019] Figure 1 XRD spectra of OmBpy-COF and Cu-OmBpy-COF; Figure 2 Infrared spectra of OmBpy-COF and Cu-OmBpy-COF; Figure 3 UV-Vis diffuse reflectance spectra (UV-Vis) of OmBpy-COF and Cu-OmBpy-COF; wherein a is an absorption spectrum, b is a band gap energy, and c is an energy level structure; Figure 4 is a scanning electron microscope image of OmBpy-COF and Cu-OmBpy-COF; Figure 5Synchronous radiation spectra of Cu atoms in Cu-OmBpy-COF monatomic photocatalyst; wherein (a) is the normalized Cu k-edge X-ray near-edge structure (XANES) spectrum of Cu atoms, (b) is the experimental k 2 -Weighted Cu K-edge X-ray absorption fine structure spectrum (EXAFS) curve; Fig. 6 is an electrochemical impedance spectrogram (EIS) of OmBpy-COF and Cu-OmBpy-COF; Fig. 7 is a performance graph of OmBpy-COF and Cu-OmBpy-COF photocatalyst photocatalytic CO2 reduction to generate CO; Fig. 8 is a cycle stability experiment graph of Cu-OmBpy-COF photocatalyst; Fig. 9 is an in-situ infrared spectrogram of Cu-OmBpy-COF photocatalytic CO2 RR under different illumination times. DETAILED DESCRIPTION

[0020] The application provides a preparation method of a monatomic photocatalyst Cu-OmBpy-COF, comprising the following steps: 1) dispersing 2,4,5-triamino-6-hydroxy pyrimidine sulfate, [2,2'-bipyridine]-5,5-dimethyl formaldehyde and acetic acid solution in an organic solvent to obtain a mixed solution; 2) sequentially performing liquid nitrogen freezing, vacuumizing and thawing on the mixed solution in step 1), and then performing reaction to obtain a crude product; 3) sequentially performing washing, Soxhlet extraction and vacuum drying on the crude product in step 2) to obtain a covalent organic framework OmBpy-COF material; 4) ultrasonic dispersing the covalent organic framework OmBpy-COF in step 3) and anhydrous copper acetate in acetonitrile, stirring, and then performing suction filtration and drying to obtain a monatomic photocatalyst Cu-OmBpy-COF.

[0021] The application disperses 2,4,5-triamino-6-hydroxy pyrimidine sulfate, [2,2'-bipyridine]-5,5-dimethyl formaldehyde and acetic acid solution in an organic solvent to obtain a mixed solution. In the application, the mass of 2,4,5-triamino-6-hydroxy pyrimidine sulfate, the mass of [2,2'-bipyridine]-5,5-dimethyl formaldehyde, the volume of acetic acid solution and the volume of organic solvent are preferably 47.8 mg:63.6 mg:0.4 mL:2 mL. In the application, the concentration of the acetic acid solution is preferably 6 mol / L. In the application, the organic solvent is preferably N,N-dimethylacetamide and o-dichlorobenzene, and the volume ratio of N,N-dimethylacetamide to o-dichlorobenzene is preferably 1:2.

[0022] In this invention, the mixed solution is sequentially subjected to liquid nitrogen freezing, vacuuming, and thawing, followed by a reaction to obtain a crude product. Preferably, the cycle of liquid nitrogen freezing for 1 minute, vacuuming, and thawing at room temperature is repeated three times. Preferably, the reaction conditions include a temperature of 120°C and a reaction time of 72 hours.

[0023] This invention involves sequentially washing, Soxhlet extraction, and vacuum drying of the crude product to obtain the covalent organic framework OmBpy-COF material. In this invention, the crude product is preferably washed sequentially with acetone, tetrahydrofuran, and deionized water. In this invention, the solvent used for the Soxhlet extraction is preferably tetrahydrofuran, and the Soxhlet extraction temperature is 100°C for 12 hours. In this invention, the vacuum drying temperature is preferably 80°C for 12 hours.

[0024] This invention involves ultrasonically dispersing the covalent organic framework OmBpy-COF and anhydrous copper acetate in acetonitrile, followed by stirring, filtration, and drying to obtain the single-atom photocatalyst Cu-OmBpy-COF. In this invention, the preferred mass ratio of the covalent organic framework OmBpy-COF, the anhydrous copper acetate, and the acetonitrile is 30 mg:30 mg:30 mL. The preferred ultrasonic dispersion time is 30 min. The preferred stirring time is 10 h. The preferred drying conditions include a temperature of 80°C and a time of 12 h.

[0025] The present invention also provides a single-atom photocatalyst Cu-OmBpy-COF prepared by the preparation method described in the above technical solution.

[0026] This invention also provides the application of the single-atom photocatalyst Cu-OmBpy-COF described in the above-mentioned technical solution in the photocatalytic reduction of carbon dioxide to carbon monoxide. In this invention, the application preferably includes the following steps: dispersing the single-atom photocatalyst Cu-OmBpy-COF in a solvent, introducing carbon dioxide, and then reacting under 300W xenon lamp irradiation.

[0027] To further illustrate the present invention, the following detailed description is provided in conjunction with embodiments, but these should not be construed as limiting the scope of protection of the present invention.

[0028] Example 1 1. Preparation of OmBpy-COF photocatalyst (1) Accurately weigh 47.8 mg of 2,4,5-triamino-6-hydroxypyrimidine sulfate, 63.6 mg of [2,2'-bipyridine]-5,5-diformaldehyde into a glass synthesis tube, add 0.65 mL of N,N-dimethylacetamide and 1.35 mL of o-dichlorobenzene mesitylene, ultrasonic dispersion after the solvent, add 0.4 mL of 6M aqueous acetic acid, mix again to obtain a mixed solution.

[0029] (2) After the above mixed solution is subjected to liquid nitrogen freezing-vacuumizing-thawing cycle operation 3 times, it is vacuum sealed and placed in a constant temperature oven, kept at 120°C for 72h to obtain a crude product.

[0030] (3) The above crude product is solid-liquid separated, and the solid is washed with acetone, tetrahydrofuran and deionized water 3 times, respectively. After the solid is separated by suction filtration, the obtained solid is vacuum dried, wrapped with filter paper and subjected to Soxhlet extraction with tetrahydrofuran. The Soxhlet extraction temperature is 100°C, and the time is 12h. Subsequently, the solid after Soxhlet extraction is placed in a vacuum drying oven and vacuum dried at 80°C for 12h to obtain a COF red-brown solid powder, which is counted as OmBpy-COF.

[0031] Example 2 2. Preparation of Cu-OmBpy-COF photocatalyst (1) Weigh 30 mg of OmBpy-COF and 30 mg of anhydrous copper acetate obtained into a 100 mL beaker, add 30 mL of acetonitrile solvent, ultrasonic dispersion for 30 min, and magnetic stirring for 10h; (2) After stirring, the mixed solution after stirring is subjected to solid-liquid separation, and the solid product after solid-liquid separation is washed with acetonitrile. The solid product is placed in a constant temperature oven and kept at 80°C for 12h to obtain a monatomic photocatalyst Cu-OmBpy-COF.

[0032] 3. Photocatalytic performance test: In order to evaluate the catalytic performance of the monatomic photocatalyst Cu-OmBpy-COF, its photocatalytic CO2RR effect was tested under visible light illumination (300W xenon lamp, λ>420 nm). The system is used for photocatalytic CO2RR experiment without using additional cocatalyst, sacrificial agent and photosensitizer. The photocatalytic CO2 reduction was carried out in a 100 mL quartz reactor, and a water bath was used to control the temperature. 10 mg of monatomic photocatalyst Cu-OmBpy-COF was dispersed in 30 mL of mixed solvent (DMF / H2O, v=1:1) in the reactor and stirred with a magnetic stirrer at 500 r min -1The quartz reactor was sealed with a quartz glass top, purged with CO2(99.999%) for 30 min, and the temperature of the reaction solution was maintained at 25°C. After xenon lamp irradiation, CO was detected by gas chromatography-flame ionization detector (GC-FID) using a headspace sampler, and sampling was performed every 1 h.

[0033] 4. Result analysis: (1) The PXRD patterns of the obtained OmBpy-COF and Cu-OmBpy-COF materials are shown in Figure 1 From the figure, it can be seen that the COF has a strong diffraction peak at 4.98 0 , corresponding to the (100) section, indicating that the COF material has good crystallinity; the spectrum of OmBpy-COF after anchoring metal Cu ions has basically not changed, which indicates that anchoring copper ions does not affect its crystal structure.

[0034] (2) The Fourier transform infrared spectra of the monomer and OmBpy-COF and Cu-OmBpy-COF are shown in Figure 2 After the synthesis of OmBpy-COF, the stretching peaks of the amine group monomer at 3080-3350 cm -1 and the aldehyde group monomer at 1680 cm -1 all disappeared, but a C=N bond stretching signal appeared at 1590 cm -1 , the fingerprint peak of the oxazole ring corresponding to the C-O-C conjugate stretching vibration characteristic peak appeared at 1100-1020 cm -1 , and at the same time, the oxazole ring was conjugated with the pyrimidine ring, and the ring skeleton C=C conjugate stretching vibration peak appeared at 1520 cm -1 , further confirming the successful preparation of OmBpy-COF.

[0035] (3) The UV-visible light absorption spectrum of the new covalent organic material OmBpy-COF loaded with single-dispersed metal copper catalyst in this embodiment is shown in Figure 3 (a), it can be seen that the two COFs have strong absorption in the visible light region, especially in the range of 400-500 nm. The optical band gap results calculated by Tauc diagram are shown in Figure 3 (b), the band gap energy of Cu-OmBpy-COF is 2.18 eV, which is narrower than that of OmBpy-COF (2.43 eV). The energy level structure diagram of the material is shown in Figure 3In (c), both OmBpy-COF and Cu-OmBpy-COF meet the CO2 / CO reduction potential (-0.53 V vs NHE), which meets the thermodynamic requirement for the reduction of CO2 to CO. The as-synthesized monatomic photocatalyst Cu-OmBpy-COF can absorb visible light more effectively than pure OmBpy-COF.

[0036] (4) The scanning electron micrographs of OmBpy-COF and Cu-OmBpy-COF are shown in Figure 4 It can be observed that the COF has a sheet structure, and the morphology does not change after the introduction of Cu ions, indicating that the anchoring of metal Cu ions does not change the morphology of the COF.

[0037] (5) The normalized Cu k-edge XANES spectrum of Cu-OmBpy-COF monatomic photocatalyst and the experimental k 2 The weighted Cu K-edge EXAFS curve is shown in Figure 5 , which shows that Cu-OmBpy-COF is a Cu-O / N with only one shell, and the valence state of Cu is slightly lower than +2, which is a monatomic photocatalyst.

[0038] (6) The electrochemical impedance spectroscopy (EIS) of OmBpy-COF and Cu-OmBpy-COF is shown in Figure 6 Compared with OmBpy-COF, the curvature radius of Cu-OmBpy-COF is smaller, indicating that the resistance of charge transfer of Cu-OmBpy-COF is small, and the transfer rate of charge is fast, thereby improving the photocatalytic performance.

[0039] (7) The CO evolution rate of OmBpy-COF and Cu-OmBpy-COF is shown in Figure 7 After continuous irradiation for 5 hours, the original OmBpy-COF can photocatalyze CO2 to generate CO, with a yield of 711.07 µmol g -1 After anchoring metal copper ions, the CO yield of Cu-OmBpy-COF is significantly improved to 3815.94 µmol g -1 , which is 5.4 times that of pure COF, indicating that metal copper ions as catalytically active centers significantly enhance the CO2 adsorption and activation ability and improve the photocatalytic activity.

[0040] (8) The results of the cycle test of Cu-OmBpy-COF are shown in Figure 8 After 4 cycles in the mixed solution, Cu-OmBpy-COF still maintains good reduction performance.

[0041] (9) The in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis of Cu-OmBpy-COF photocatalytic CO2 RR under different illumination times is shown inFigure 9 As shown, after irradiation, multiple peaks gradually appeared in the range of 1300 ~ 2100 cm -1 , respectively corresponding to the formation of bidentate carbonate (b-CO3 2- ), monodentate carbonate (m-CO3 2- ) and *COOH intermediate. This indicates that the path of photocatalytic CO2RR to generate CO on Cu-OmBpy-COF follows CO2→*COOH→*CO→CO. Compared with OmBpy-COF, these new peaks appear obviously and gradually increase in intensity with the increase of irradiation time, indicating that the anchoring of single Cu ion can significantly promote the generation of C1 intermediate, thereby improving the photocatalytic activity.

[0042] In summary, the present application first synthesizes a new covalent organic framework OmBpy-COF connected by imine group and oxazole ring; on this basis, a copper ion-anchored covalent organic framework single-atom photocatalyst Cu-OmBpy-COF is further prepared. Under the condition of not adding sacrificial agent and photosensitizer, after continuous irradiation for 5h under visible light, Cu-OmBpy-COF shows the optimal catalytic activity, and the CO production is as high as 3815.94 µmol g -1 , and the CO selectivity is 100%. The photocatalyst not only has excellent photocatalytic stability, but also provides a new research idea and direction for the structural design and performance optimization of high-efficiency COF-based photocatalysts for reducing carbon dioxide.

[0043] Although the above embodiment has made a detailed description of the present application, it is only a part of the embodiments of the present application, not all the embodiments, and people can also obtain other embodiments according to the present embodiment without creativity, which all belong to the protection scope of the present application.

Claims

1. A method for preparing a single-atom photocatalyst Cu-OmBpy-COF, characterized in that, Includes the following steps: 1) Disperse 2,4,5-triamino-6-hydroxypyrimidine sulfate, [2,2'-bipyridine]-5,5-dicarboxaldehyde and acetic acid in an organic solvent to obtain a mixed solution; 2) The mixed solution described in step 1) is sequentially subjected to liquid nitrogen freezing, vacuuming, and thawing, followed by reaction to obtain the crude product; 3) The crude product described in step 2) is washed, Soxhlet extracted and vacuum dried sequentially to obtain the covalent organic framework OmBpy-COF material; 4) The covalent organic framework OmBpy-COF and anhydrous copper acetate obtained in step 3) are ultrasonically dispersed in acetonitrile, stirred, filtered and dried to obtain the single-atom photocatalyst Cu-OmBpy-COF.

2. The preparation method according to claim 1, characterized in that, In step 1), the mass ratio of 2,4,5-triamino-6-hydroxypyrimidine sulfate, the mass of [2,2'-bipyridine]-5,5-dicarboxaldehyde, the volume of acetic acid solution, and the volume of organic solvent is 47.8 mg: 63.6 mg: 0.4 mL: 2 mL.

3. The preparation method according to claim 1, characterized in that, Step 1) The concentration of the acetic acid solution is 6 mol / L; The organic solvent is N,N-dimethylacetamide and o-dichlorobenzene, and the volume ratio of N,N-dimethylacetamide and o-dichlorobenzene is 1:

2.

4. The preparation method according to claim 1, characterized in that, Step 2) involves three cycles of liquid nitrogen freezing, vacuuming, and thawing. The reaction conditions include a temperature of 120°C and a time of 72 h.

5. The preparation method according to claim 1, characterized in that, Step 3) The crude product is washed sequentially with acetone, tetrahydrofuran, and deionized water; The solvent used in the Soxhlet extraction was tetrahydrofuran, and the Soxhlet extraction time was 12 h. The vacuum drying temperature is 80℃ and the time is 12 h.

6. The preparation method according to claim 1, characterized in that, Step 4) The mass ratio of the covalent organic framework OmBpy-COF, the mass of anhydrous copper acetate, and the volume of acetonitrile is 30 mg:30 mg:30 mL.

7. The preparation method according to claim 1, characterized in that, Step 4) The ultrasonic dispersion time is 30 minutes; The stirring time is 10 h; The drying conditions include a temperature of 80°C and a time of 12 hours.

8. A single-atom photocatalyst Cu-OmBpy-COF prepared by the preparation method according to any one of claims 1 to 7.

9. The application of the single-atom photocatalyst Cu-OmBpy-COF according to claim 8 in the photocatalytic reduction of carbon dioxide to carbon monoxide.

10. The application according to claim 9, characterized in that, The application includes the following steps: dispersing the single-atom photocatalyst Cu-OmBpy-COF in a solvent, introducing carbon dioxide, and then reacting under 300W xenon lamp irradiation.

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