A monatomic modified bismuth vanadate heterojunction material, a preparation method thereof and application thereof in olefin epoxidation

Abstract

The application discloses a single-atom modified bismuth vanadate heterojunction material and a preparation method and application thereof in olefin epoxidation. The preparation method of the application specifically comprises the following steps: (1) a BiVO4 photoanode is prepared through electrodeposition and high-temperature annealing; (2) C3N4 powder, a cobalt source and an alcohol solvent are mixed to obtain a mixed solution, a complexing agent is added into the mixed solution to perform microwave reaction, and drying is performed to obtain Co-C3N4 powder; and (3) the Co-C3N4 powder of step (2) is modified on the surface of the BiVO4 photoanode, and the single-atom modified bismuth vanadate heterojunction material is obtained through anaerobic calcination. The material of the application can efficiently promote the conversion of olefins into corresponding oxidized olefin compounds under visible light irradiation under mild conditions. The overall method process is simple, low in cost, and shows good universality for various olefin substrates.

Description

Technical Field

[0001] This invention belongs to the field of photoelectrocatalysis technology, specifically relating to a single-atom modified bismuth vanadate heterojunction material, its preparation method, and its application in chlorine-mediated catalytic epoxidation of olefins. Background Technology

[0002] Olefin epoxidation products are important raw materials for fine chemicals and pharmaceutical intermediates. Traditional thermocatalytic epoxidation processes typically rely on precious metal catalysts, strong oxidants, or toxic halogenating agents, resulting in high energy consumption, poor economic efficiency, unsatisfactory selectivity, and environmental unfriendliness. Solar-driven photoelectrocatalysis offers a promising green alternative for this conversion. It utilizes photogenerated carriers to generate active chlorine species in situ at the anode, thereby selectively oxidizing olefins. This allows for efficient and highly selective conversion under mild conditions, combining energy efficiency with environmental friendliness.

[0003] In photoelectrocatalytic systems, bismuth vanadate (BiVO4) is widely used as a photoanode material due to its visible light response (500-600 nm), band structure suitability (band gap of 2.4 eV), and strong oxidizing ability. However, BiVO4 suffers from severe bulk / interface recombination, sluggish surface reaction kinetics, and intense competition with the oxygen evolution reaction (OER), resulting in insufficient activation efficiency for chloride ions and selectivity for target epoxidation. Furthermore, it is prone to surface deactivation and reduced stability in chloride-containing electrolytes. Therefore, the technical starting point and direction of this invention is to improve the efficient separation and utilization of photogenerated carriers under visible light-driven BiVO4, preferentially catalyzing the chloride-mediated pathway to achieve efficient and sustainable epoxidation of styrene-based olefins. Summary of the Invention

[0004] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing a single-atom modified bismuth vanadate heterojunction material, the preparation method specifically includes the following steps: (1) BiVO4 photoanodes were prepared by electrodeposition and high-temperature annealing; (2) C3N4 powder, cobalt source and alcohol solvent are mixed to obtain a mixed solution. A complexing agent is added to the mixed solution for microwave reaction and drying to obtain Co-C3N4 powder. (3) The Co-C3N4 powder from step (2) is modified onto the surface of the BiVO4 photoanode and then calcined in an oxygen-free environment to obtain the single-atom modified bismuth vanadate heterojunction material, denoted as Co-C3N4 / BiVO4.

[0005] According to an embodiment of the present invention, in step (1), the preparation method of the BiVO4 photoanode includes: placing a conductive substrate in an electrolytic cell containing electrolyte I, applying a certain voltage to electrolyze and synthesize BiOI, then adding a vanadium source to the surface of BiOI, and obtaining the BiVO4 photoanode by high-temperature annealing.

[0006] Preferably, the conductive substrate is selected from fluorine-doped tin oxide conductive glass (FTO). Further, the dimensions of the conductive substrate can be those known in the art. For example, the dimensions of the conductive substrate are: 4 cm in length, 2 cm in width, and 2.2 mm in thickness. For instance, the dimensions of the fluorine-doped tin oxide conductive glass (FTO) are as follows: 4 cm in height, 1 cm in length, and 2.2 mm in thickness.

[0007] Preferably, the electrolyte I comprises a bismuth salt, an iodide salt, p-benzoquinone, and a solvent, with a molar ratio of 0.04:0.3-0.5:0.1-0.3, for example, 0.04:0.4:0.23. Further, the bismuth salt is at least one of bismuth nitrate and bismuth chloride. Further, the iodide salt is selected from potassium iodide. Further, the solvent is selected from water and an alcohol, with the alcohol selected from ethanol and methanol. Exemplarily, the electrolyte I is, for example, a mixed solution of bismuth nitrate, potassium iodide, and p-benzoquinone, with a molar ratio of bismuth nitrate, potassium iodide, and p-benzoquinone of 0.04:0.4:0.23.

[0008] Furthermore, the electrolyte I is obtained by the following method: first, an aqueous solution of bismuth salt and iodine salt is prepared, and then an alcoholic solution of p-benzoquinone is added dropwise.

[0009] Preferably, the conditions for the electrolytic synthesis include: an electrolysis voltage of -0.1 to 1 V. Ag / AgCl The electrolysis time is 1-10 minutes, for example, -0.1 V. Ag / AgCl Electrolysis for 5 minutes.

[0010] Preferably, the vanadium source is selected from an organic solution of a vanadium salt, and the solvent in the organic solution is selected from DMSO. Further, the vanadium salt is selected from vanadium acetylacetonate, for example, a DMSO solution of vanadium acetylacetonate.

[0011] Further, the concentration of the organic solution of vanadium salt is 0.1-0.5 M, for example, 0.2 M. Further, the amount of organic solution of vanadium salt can be any amount known in the art, and the present invention does not impose a specific limitation. For example, the concentration of the DMSO solution of vanadium acetylacetonate is 0.2 M, and 800 μL is added dropwise.

[0012] According to an embodiment of the present invention, in step (1), the conditions for high-temperature annealing include: annealing temperature of 400-500°C, preferably 450°C; heating time of 1-3h (preferably 2h); and annealing time of 1-3h, preferably 2h.

[0013] According to an embodiment of the present invention, in step (1), after high-temperature annealing, the BiVO4 photoanode can be further post-treated. Preferably, the post-treatment method is to immerse the BiVO4 photoanode in an aqueous sodium hydroxide solution and then air dry it.

[0014] According to an embodiment of the present invention, in step (2), the C3N4 powder can be obtained by methods known in the art, such as sintering an appropriate amount of urea to obtain C3N4 powder.

[0015] Preferably, in step (2), the preparation of C3N4 powder includes: sintering a carbon-nitrogen source (such as urea) to obtain C3N4 powder; wherein the mass of the urea is 10-20 g, exemplarily 15 g; sintering includes placing the urea in a crucible and placing it in a muffle furnace. Preferably, the sintering temperature is 500-600℃, exemplarily 600℃; the sintering time is 6-10 h, exemplarily 6 h; and the C3N4 powder is obtained after natural cooling.

[0016] According to an embodiment of the present invention, in step (2), the cobalt source is selected from at least one of cobalt chloride and cobalt nitrate.

[0017] According to an embodiment of the present invention, in step (2), the alcohol solvent is selected from at least one of ethanol and methanol.

[0018] According to an embodiment of the present invention, in step (2), the complexing agent is selected from triethylamine.

[0019] According to an embodiment of the present invention, in step (2), after adding the complexing agent, vigorous stirring is required. Preferably, vigorous stirring means a stirring speed greater than or equal to 1500 rpm. In the present invention, stirring can be carried out using stirring methods known in the art, such as magnetic stirring.

[0020] According to an embodiment of the present invention, in step (2), the Co content in the Co-C3N4 powder is adjusted by adjusting the concentration of the cobalt source in the mixed solution. Preferably, the mass-volume concentration of the cobalt source in the mixed solution is 0.0001 ~ 0.005 g / mL, with exemplary mass-volume concentrations being 0.0002 g / mL, 0.0005 g / mL, 0.00075 g / mL, 0.001 g / mL, and 0.0015 g / mL.

[0021] According to an embodiment of the present invention, in step (2), the microwave reaction time is not less than 1.5 h, for example 2 h or 5 h; the microwave heating temperature is 80~100 ℃, for example 80 ℃.

[0022] According to an embodiment of the present invention, in step (2), the microwave reaction is carried out under stirring conditions known in the art.

[0023] According to an embodiment of the present invention, in step (2), after the microwave reaction, the sample can be filtered and washed with methanol, chloroform and ethanol.

[0024] According to an embodiment of the present invention, in step (2), drying can be carried out in a vacuum oven, and the drying conditions can be those known in the art, such as a drying temperature of 80 °C and a drying time of 8 h.

[0025] According to an embodiment of the present invention, step (3) of modifying the surface of the BiVO4 photoanode with Co-C3N4 powder specifically includes: repeatedly spin-coating the surface of the BiVO4 photoanode with Co-C3N4 solution.

[0026] Preferably, the Co-C3N4 solution comprises Co-C3N4 powder and an alcohol solvent, wherein the concentration of the Co-C3N4 powder is 1-5 g / L, for example, 2 g / L.

[0027] Preferably, when preparing the Co-C3N4 solution, the Co-C3N4 powder can be dispersed by ultrasonication.

[0028] Preferably, the spin coating is performed on a spin coater.

[0029] Preferably, the spin coating specifically includes: fixing the BiVO4 photoanode on a worktable, setting the rotation speed of the worktable to 1000~2000 r (e.g., 1000 r), and setting the spin coating time to 30 s.

[0030] Preferably, the volume of Co-C3N4 solution added each time during spin coating is 100 ~ 200 μL, for example 100 μL.

[0031] Preferably, the spin coating can be applied 2-10 times, more preferably 2-5 times, for example 3 times.

[0032] According to an embodiment of the present invention, in step (3), the oxygen-free calcination is carried out in a tubular furnace.

[0033] According to an embodiment of the present invention, in step (3), the oxygen-free calcination is carried out in an inert atmosphere, such as in argon.

[0034] According to an embodiment of the present invention, in step (3), the temperature of the anaerobic calcination is 100 ~ 300 ℃, for example 300 ℃; the time of the anaerobic calcination is 1 ~ 3 h, for example 2 h.

[0035] Preferably, after anaerobic calcination, the material can be allowed to cool naturally to room temperature.

[0036] The present invention also provides a single-atom modified bismuth vanadate heterojunction material, which is obtained by the above preparation method.

[0037] The present invention also provides the application of the above-mentioned single-atom modified bismuth vanadate heterojunction material in photoelectrolysis, preferably as a photoanode.

[0038] The present invention also provides an electrolysis system, the electrolysis system including a photoanode, the photoanode being selected from the above-mentioned single-atom modified bismuth vanadate heterojunction material.

[0039] According to an embodiment of the present invention, the electrolysis system further includes a counter electrode, which may be a counter electrode known in the art. Exemplarily, the counter electrode is a platinum wire.

[0040] According to an embodiment of the present invention, the electrolysis system further includes a reference electrode, which may be a reference electrode known in the art. Exemplarily, the reference electrode is saturated silver chloride.

[0041] According to an embodiment of the present invention, the electrolysis system further includes an electrolyte. Preferably, the electrolyte includes a solvent (such as acetonitrile), water, and an electrolyte. Further, the water content is 35% to 65%, for example, 50%. Further, the concentration of the electrolyte can be 0.05 to 0.2 M, exemplarily 0.15 M. In this invention, the unit M of concentration refers to mol / L, for example, 0.1 M means a concentration of 0.1 mol / L.

[0042] Preferably, the electrolyte is selected from chlorides, such as sodium chloride.

[0043] The present invention also provides the application of the above-described electrolysis system in olefin epoxidation reactions, for example, in the preparation of epoxides.

[0044] The present invention also provides a method for preparing epoxides, the method comprising performing a photoelectrochemical epoxidation reaction on an olefin substrate using the above-described electrolysis system to obtain epoxides.

[0045] According to embodiments of the present invention, the olefin substrate is preferably selected from at least one compound having the following structural formula: .

[0046] According to an embodiment of the present invention, the olefin substrate is added to the electrolyte to form a reaction solution, wherein the concentration of the olefin substrate is 0 to 10 mM, for example, 2 mM, 4 mM, 6 mM, 8 mM, or 10 mM. In this invention, the unit of concentration, mM, refers to mmol / L; for example, 1 mM means a concentration of 1 mmol / L.

[0047] According to an embodiment of the present invention, the photoelectrochemical epoxidation reaction is carried out by electrolysis under illumination. Preferably, the illumination intensity can be selected from conditions known in the art, for example, 100 mW / cm². 2 .

[0048] According to an embodiment of the present invention, the light source used in the photoelectrochemical epoxidation reaction can be a light source known in the art, such as a 300 W xenon lamp equipped with an AM 1.5G and a 380-800 nm filter.

[0049] According to an embodiment of the present invention, the photoelectrochemical epoxidation reaction is electrolyzed under constant potential conditions. Preferably, the constant potential can be selected from conditions known in the art, for example, an applied voltage of 1 V. Ag / AgCl 0.8 V Ag / AgCl .

[0050] According to an embodiment of the present invention, the area of ​​the photoanode in the electrolysis system can be an area known in the art, for example, 1 × 2 cm. 2 .

[0051] According to an embodiment of the present invention, the reaction temperature of the photoelectrochemical epoxidation is not higher than 30 °C, for example, 25 °C.

[0052] According to an embodiment of the present invention, the photoelectrochemical epoxidation is carried out in a single cell. Exemplarily, the concentration of the olefin substrate in the reaction solution of the single cell is 10 mM; the reaction rotation speed in the single cell is 100 rpm; and the volume of the reaction solution in the single cell is 4 mL.

[0053] According to an embodiment of the present invention, the water content in the reaction solution is preferably 50%.

[0054] The beneficial effects of this invention are: This invention provides a novel single-atom modified bismuth vanadate photoanode material for efficient catalytic epoxidation of olefins and its preparation method. This invention utilizes C3N4 nanomaterials loaded with single-atom cobalt and BiVO4 to assemble a novel single-atom modified bismuth vanadate heterojunction material. This invention employs a three-electrode system with Co-C3N4 / BiVO4 as the working electrode and applies it to a method for chloride-mediated photoelectrocatalytic epoxidation of olefins such as styrene. Under stirring, chloride ions are oxidized at the photoanode surface to generate hypochlorous acid, which then selectively oxidizes the olefin substrate to form epoxides.

[0055] This invention proposes a novel bismuth vanadate heterostructure material with monoatomic modification, characterized by simple preparation, user-friendly operation, and high stability. This material can efficiently promote the conversion of olefins into corresponding olefin oxide compounds under mild visible light irradiation. The overall method is simple, easy to implement, low-cost, economical, and exhibits good versatility for a variety of olefin substrates. Attached Figure Description

[0056] Figure 1 The images show SEM images of BiVO4 (Fig. a) and Co-C3N4 / BiVO4 (Fig. b) prepared in Example 1, as well as TEM images (Fig. c) and EDAX spectra (Fig. d) of single-atom Co-C3N4.

[0057] Figure 2 XPS image of Co-C3N4 / BiVO4 prepared in Example 1.

[0058] Figure 3 The image shows the XAS diagram of Co-C3N4 / BiVO4 prepared in Example 1.

[0059] Figure 4 The inductively coupled plasma emission spectrometry results of the Co-C3N4 / BiVO4 photoelectrode prepared in Example 1 and the 2-C3N4 / BiVO4 photoelectrode prepared in Comparative Example 2 are used to demonstrate the cobalt content in the photoelectrodes.

[0060] Figure 5 Linear sweep voltammetry (LSV) curves of BiVO4, C3N4 / BiVO4 and Co-C3N4 / BiVO4 photoelectrodes, as well as C3N4 and Co-C3N4 electrodes, as working electrodes in Application Example 2, under chlorine-mediated olefin epoxidation conditions.

[0061] Figure 6To compare the substrate conversion (Con.), selectivity of epoxidation products (Select.), and Faraday efficiency (FE) of different electrodes (BiVO4, C3N4, Co-C3N4, C3N4 / BiVO4, Co-C3N4 / BiVO4, and 2-C3N4 / BiVO4) for chlorine-mediated styrene epoxidation reaction using the same amount of electrolysis.

[0062] Figure 7 The results show the selectivity and Faraday efficiency of the Co-C3N4 / BiVO4 photoelectrocatalytic chlorine-mediated styrene epoxidation products as a function of Co loading in Application Example 2.

[0063] Figure 8 The results are for the yields of epoxidation of olefin substrates with various substituents in a single cell using Co-C3N4 / BiVO4 in Application Example 3.

[0064] Figure 9 This is a comparison of the photoelectrocatalytic current stability of BiVO4 and Co-C3N4 / BiVO4 in a single cell in Application Example 2. Detailed Implementation

[0065] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention, and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0066] Unless otherwise stated, the raw materials and reagents used in the following examples are commercially available products or can be prepared by known methods.

[0067] In the following examples, the photoelectrochemical epoxidation reaction was carried out in an electrochemical workstation, model CHI1040E.

[0068] Example 1 Preparation of Co-C3N4 / BiVO4 photoanode: (1) Preparation of BiVO4 photoanode: (1.1) Place the purchased FTO glass in acetone and ethanol and sonicate for 30 min each to obtain the treated FTO glass.

[0069] (1.2) Prepare a 0.4 M potassium iodide aqueous solution, adjust the pH to 1.7 with dilute nitric acid, and then add 0.04 M bismuth nitrate to the solution to make a mixed aqueous solution.

[0070] (1.3) Prepare a 0.23 M p-benzoquinone ethanol solution. Slowly add the p-benzoquinone ethanol solution dropwise to the aqueous solution prepared in (1.2) while stirring to obtain the electrolyte.

[0071] (1.4) Add 20 mL of electrolyte to the electrolytic cell, place the treated FTO glass in the electrolytic cell as the working electrode, and use platinum wire and saturated silver silver chloride electrode as the counter electrode and reference electrode, respectively, and apply -0.1 V. AgAgCl Electrolyze at a voltage of 5 min, remove the electrode, rinse with deionized water, and dry with N2 to obtain BiOI.

[0072] (1.5) Prepare a 0.2 M DMSO solution of acetylacetone vanadium oxide, add 800 μL dropwise to the BiOI surface grown on FTO glass, then place it in a muffle furnace, set the heating rate to 2 ℃ / min to 450 ℃ and calcine for 2 h, and then let it cool naturally to room temperature.

[0073] (1.6) The electrode obtained by calcination was immersed in a 1 M sodium hydroxide aqueous solution for 10 min, then rinsed with deionized water and dried at room temperature to obtain BiVO4 photoanode.

[0074] In step (1) above, the FTO glass has a size of 2 cm × 4 cm and an effective working area of ​​2 cm × 2 cm.

[0075] (2) Preparation of single-atom Co-C3N4: (2.1) Weigh 10 g of urea and place it in a crucible. Sinter at 600 °C for 6 h with a heating rate of 10 °C / min. Allow it to cool naturally to obtain C3N4 powder.

[0076] (2.2) Weigh 0.1 g C3N4 and 10 mg cobalt chloride and dissolve them in 20 mL of ethanol. Then add 10 μL of triethylamine, sonicate for 10 min, and stir vigorously for 1 h (using a magnetic stirrer at a speed of ≥1500 rpm). Then place the solution in a microwave device and microwave heat it for 2 h while stirring. The heating temperature is 80 ℃.

[0077] (2.3) The solution was filtered to obtain powder, which was then washed with methanol, chloroform and ethanol respectively; finally, it was dried in a vacuum oven at 80°C for 8 h and then cooled naturally to obtain Co-C3N4 powder.

[0078] (3) Preparation of Co-C3N4 / BiVO4 photoanode: Weigh 10 mg of Co-C3N4 powder from step (2) above, dissolve it in 10 mL of ethanol, fix the BiVO4 prepared in step (1) above on the worktable of a spin coater, and spin coat 200 μL of Co-C3N4 water onto the BiVO4 using a pipette. Repeat the spin coating 3 times. Then place it in a tube furnace and calcine it at 300 °C in an Ar atmosphere for 2 h. The heating rate is 10 °C / min.

[0079] In the above steps, the spin coater speed was set to 1000 r and the time was 30 s.

[0080] Depend on Figure 1-3 It can be seen that the Co-C3N4 / BiVO4 photoanode prepared in this embodiment is an amorphous graphitic carbon nitride intercalated in the bismuth vanadate voids, and also contains single-atom cobalt. Its bonding mode is different from that of ordinary cobalt atoms, and the cobalt content is 0.06 μg / g, which is much lower than that of ordinary cobalt atom doping.

[0081] Depend on Figure 1 A comparison of SEM images of the electrodes before and after Co-C3N4 modification (Figures a and b) shows that the Co-C3N4 / BiVO4 photoanode synthesized in this product consists of single-atom cobalt anchored on amorphous graphitic carbon nitride, and then intercalated in the bismuth vanadate voids. Figures c and d demonstrate that single-atom Co was successfully anchored on amorphous graphitic carbon nitride.

[0082] Figure 2 This demonstrates the successful fabrication of the Co-C3N4 / BiVO4 photoanode.

[0083] Figure 3 By comparing the signal peaks of Co-C3N4 / BiVO4 with those of metallic cobalt, CoO, and cobalt phthalocyanine (CoPc), it was found that the signal peaks were similar to those of cobalt phthalocyanine, proving that the Co in the prepared electrode is a single atom and is coordinated with nitrogen. Figure 3 b proves that the single-atom Co has a valence state of "+2", which is different from the known cobalt phthalocyanine, thus proving the uniqueness of the prepared single-atom cobalt.

[0084] Figure 4 In this context, "Elemental content Co (%)" refers to the theoretically expected elemental content before testing; "Weighing (mg)" refers to the mass of the solid sample taken during testing; "Fixed weight" refers to the total mass of the solution after dissolving the sample and making up to volume during testing; and "Test result Co-R 238.892 nm ppm (μg / g)" refers to the Co concentration in the solution measured by ICP, in ppm (i.e., μg / g). The formula for calculating elemental content is as follows: The calculated cobalt content is 0.06 ppm (μg / g), or 0.015%, which is far lower than that of ordinary cobalt atom doping.

[0085] Comparative Example 1 This comparative example uses the C3N4 / BiVO4 photoelectrode prepared in Example 1, with the only difference being that step (3) involves the preparation of the C3N4 / BiVO4 photoanode: 10 mg of C3N4 powder is weighed and dissolved in 10 mL of deionized water. The BiVO4 prepared in step (1) is fixed on the worktable of a spin coater, and 200 μL of C3N4 aqueous solution is spin-coated onto the BiVO4 using a pipette. This spin-coating is repeated 3 times. Then, the sample is placed in a tube furnace and calcined at 300 °C in an Ar atmosphere for 2 h at a heating rate of 10 °C / min.

[0086] Comparative Example 2 The Co / C3N4 / BiVO4 photoelectrode prepared in this comparative example is the same as that prepared in Example 1, except that triethylamine is not added in step (2.2) when preparing single-atom Co-C3N4 in step (2). The remaining conditions were the same as in Example 1, and a 2-C3N4 / BiVO4 photoelectrode was prepared.

[0087] Depend on Figure 4 It can be seen that without the addition of triethylamine, no cobalt element was detected in the prepared 2-C3N4 / BiVO4 photoelectrode, meaning that cobalt atoms could not grow on C3N4. Meanwhile, from... Figure 6 It can be seen that, compared with other comparative examples, the Co / C3N4 / BiVO4 photoelectrode prepared in Example 1 has the highest conversion efficiency (Con.), selectivity (Select.), and Faraday efficiency (FE).

[0088] Example 2 This embodiment refers to Example 1 to prepare Co-C3N4 / BiVO4 photoelectrodes with different Co loadings, the only difference being: In step (2), when preparing Co-C3N4, the mass volume concentration (g / mL) of cobalt chloride used was adjusted to 0.00025 g / mL, 0.0005 g / mL, 0.001 g / mL, 0.0015 g / mL, 0.0020 g / mL, and 0.0050 g / mL, respectively; thus, Co-C3N4 / BiVO4 photoelectrodes with different Co loadings were obtained, which were recorded as 2.5 mg, 5 mg, 10 mg, 15 mg, 20 mg, and 50 mg, respectively.

[0089] Application Example 1 Linear sweep voltammetry (LSV) curves of three photoanodes—BiVO4, C3N4 / BiVO4, and Co-C3N4 / BiVO4—in a chlorine-mediated epoxidation system were investigated. In the single-chamber three-electrode system, BiVO4, C3N4, Co-C3N4, and Co-C3N4 / BiVO4 from Example 1 were used as working electrodes, saturated silver chloride as the reference electrode, and platinum wire as the counter electrode. The solvent was a mixed solution of acetonitrile and water (volume ratio 1:1), the electrolyte was 0.1M sodium chloride, and the solution volume was 4 mL. The above three-electrode systems were subjected to constant potential electrolysis under the following conditions to obtain linear sweep voltammetric curves: the light source was AM 1.5G, and the light intensity at the electrode surface was 100 mW / cm². 2 The potential is 1 V Ag / AgCl LSV results for different photoanodes are as follows: Figure 5 As shown; by Figure 5 It can be seen that after C3N4 is loaded onto BiVO4, the current density of the three-electrode system increases, and after Co-C3N4 is loaded onto BiVO4, the current density in the three-electrode system is further increased. This indicates that the loading of Co-C3N4 can effectively improve the catalytic epoxidation reaction activity of BiVO4 photoanode. The above experiments show that the reaction system using the Co-C3N4 / BiVO4 photoanode of this application is a system with a high chloride ion content, thus generating a large amount of hypochlorous acid during electrolysis. The photoanode of this application exhibits higher corrosion resistance.

[0090] Application Example 2 This study investigated the effect of different Co loadings on improving the catalytic epoxidation activity of BiVO4.

[0091] In the single-chamber three-electrode system, Co-C3N4 / BiVO4 photoelectrodes with different Co loadings from Example 2 were used as working electrodes, saturated silver silver chloride as reference electrodes, and platinum wire as counter electrodes. The reaction solution solvent was a mixture of acetonitrile and water (volume ratio 1:1), the electrolyte was 0.15 M sodium chloride, and the styrene concentration was 10 mM. The light source was AM 1.5G, and the light intensity reaching the electrode surface was 100 mW / cm². 2 The potential is 1 V. Ag / AgCl . Figure 7 The product selectivity and Faradaic efficiency (FE) of epoxidation catalyzed by Co-C3N4 / BiVO4 photoelectrodes loaded with different Co amounts were compared. Figure 7 It can be seen that as the concentration of cobalt chloride used increases, the selectivity of the reaction product of epoxidation catalyzed by the Co-C3N4 / BiVO4 photoelectrode shows a rapid increase followed by a gradual decrease. The optimal selectivity and FE are achieved when the concentration of cobalt chloride is 0.05 g / mL (i.e., 5 mg marked in the figure), which is significantly improved compared to the unmodified BiVO4 epoxidation selectivity.

[0092] Figure 6 and7 The formulas for calculating styrene conversion, epoxide selectivity, and FE are as follows:

[0093]

[0094]

[0095] Application Example 3 To investigate the compatibility of the Co-C3N4 / BiVO4 photoanode with various olefin substrates in a single-cell system, photoelectrochemical experiments were conducted using olefin substrates with different substituents: .

[0096] The specific steps of the photoelectrochemical experiment are as follows: In the single-chamber three-electrode system, the Co-C3N4 / BiVO4 electrode from Example 1 was used as the working electrode, saturated silver silver chloride as the reference electrode, and platinum wire as the counter electrode. The reaction liquid volume in each chamber was 4 mL. Different reaction solutions were prepared, specifically: a mixed solution of acetonitrile and water (volume ratio 1:1) as the solvent, 0.15 M sodium chloride as the electrolyte, and 10 mM olefin substrates, each being one of the 25 olefin substrates mentioned above. The light source was AM 1.5G, and the light intensity reaching the electrode surface was 100 mW / cm². 2 In a single-cell system, constant-potential electrolysis was performed at a rotation speed of 100 rpm, at a voltage of 1 V. Ag / AgCl Under the conditions of electrolysis with a charge of 12 C, the reactants and products were qualitatively and quantitatively analyzed by gas chromatography-mass spectrometry and proton nuclear magnetic resonance spectroscopy. The test results are shown in [Figure number missing]. Figure 8 .

[0097] like Figure 8 As shown, after photoelectrochemical reaction, the conversion rate for olefin substrates with different substituents is 80-95%, and the selectivity of the prepared epoxy product is above 60%, reaching up to 96%. Therefore, it can be seen that the Co-C3N4 / BiVO4 photoelectrode of the present invention has excellent compatibility with olefin substrates with different substituents.

[0098] Application Example 4 The stability of the Co-C3N4 / BiVO4 photoanode in a single-cell system was investigated. In the single-chamber three-electrode system, BiVO4 and Co-C3N4 / BiVO4 from Example 1 were used as working electrodes, saturated silver chloride as the reference electrode, and platinum wire as the counter electrode. The solvent was a mixed solution of acetonitrile and water (volume ratio 1:1), and the electrolyte was 0.3M sodium chloride with a solution volume of 4 mL. The above three-electrode systems were subjected to constant potential electrolysis under the following conditions to obtain the it curves: the light source was AM 1.5G, and the light intensity on the electrode surface was 100 mW / cm². 2 The potential is 1 V Ag / AgCl The test results are shown below. Figure 9 .

[0099] like Figure 9 As shown, the stability of the electrode modified with Co-C3N4 can reach more than 100 hours, while the stability of the unmodified BiVO4 photoanode is only 10 hours.

[0100] The exemplary embodiments of the present invention have been described above. However, the scope of protection of the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, improvements, etc., made by those skilled in the art within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a single-atom modified bismuth vanadate heterojunction material, characterized in that, The preparation method specifically includes the following steps: (1) BiVO4 photoanodes were prepared by electrodeposition and high-temperature annealing; (2) C3N4 powder, cobalt source and alcohol solvent are mixed to obtain a mixed solution. A complexing agent is added to the mixed solution for microwave reaction and drying to obtain Co-C3N4 powder. (3) The Co-C3N4 powder from step (2) is modified on the surface of the BiVO4 photoanode and then calcined in an oxygen-free environment to obtain the single-atom modified bismuth vanadate heterojunction material.

2. The preparation method according to claim 1, characterized in that, In step (1), the preparation method of the BiVO4 photoanode includes: placing a conductive substrate in an electrolytic cell containing electrolyte I, applying a certain voltage to electrolyze and synthesize BiOI, then adding a vanadium source to the surface of BiOI, and annealing at high temperature to obtain the BiVO4 photoanode; the electrolyte I includes bismuth salt, iodide salt, p-benzoquinone and solvent, with a molar ratio of 0.04:0.3-0.5:0.1-0.3; In step (1), the conditions for high-temperature annealing include: annealing temperature of 400-500℃; heating time of 1-3h; and annealing time of 1-3h.

3. The preparation method according to claim 1 or 2, characterized in that, In step (2), the preparation of C3N4 powder includes: sintering a carbon-nitrogen source to obtain C3N4 powder; In step (2), the cobalt source is selected from at least one of cobalt chloride and cobalt nitrate; In step (2), the alcohol solvent is selected from at least one of ethanol and methanol; In step (2), the complexing agent is selected from triethylamine; In step (2), after adding the complexing agent, vigorous stirring is required; vigorous stirring means a stirring speed of ≥1500 rpm. In step (2), the Co content in the Co-C3N4 powder is adjusted by adjusting the concentration of the cobalt source in the mixed solution; the mass-volume concentration of the cobalt source in the mixed solution is 0.0001 ~ 0.005 g / mL; In step (2), the microwave reaction time shall not be less than 1.5 h; the microwave heating temperature shall be 80~100 ℃. In step (2), after the microwave reaction, the mixture is filtered and washed with methanol, chloroform and ethanol.

4. The preparation method according to any one of claims 1-3, characterized in that, In step (3), the method of modifying the surface of the BiVO4 photoanode with Co-C3N4 powder specifically includes: repeatedly spin-coating the surface of the BiVO4 photoanode with Co-C3N4 solution; The Co-C3N4 solution comprises Co-C3N4 powder and an alcohol solvent, wherein the concentration of the Co-C3N4 powder is 1-5 g / L; Each spin coating operation involves adding 100 to 200 μL of Co-C3N4 solution. The number of spin coatings is 2-10.

5. The preparation method according to any one of claims 1-4, characterized in that, In step (3), the oxygen-free calcination is carried out in a tube furnace; In step (3), the oxygen-free calcination is carried out in an inert atmosphere; In step (3), the temperature of the anaerobic calcination is 100 ~ 300 ℃; the time of the anaerobic calcination is 1 ~ 3 h.

6. A single-atom modified bismuth vanadate heterojunction material, wherein the single-atom modified bismuth vanadate heterojunction material is obtained by the preparation method according to any one of claims 1-5.

7. The application of the single-atom modified bismuth vanadate heterojunction material according to claim 6 in photoelectrolysis.

8. An electrolysis system, characterized in that, The electrolysis system includes a photoanode, which is selected from the single-atom modified bismuth vanadate heterojunction material of claim 6.

9. The application of the electrolysis system according to claim 8 in the epoxidation reaction of olefins.

10. A method for preparing epoxides, characterized in that, The method includes performing a photoelectrochemical epoxidation reaction on an olefin substrate using the electrolysis system described in claim 8 to obtain an epoxide; The olefin substrate is selected from at least one of compounds having the following structural formulas: ； The olefin substrate is added to the electrolyte to form a reaction solution, wherein the concentration of the olefin substrate is 0 ~ 10 mM; The photoelectrochemical epoxidation reaction is electrolyzed under light irradiation; The reaction solution contains 50% water.

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