A photocatalyst for the photocatalytic reduction of CO2 to methane, its preparation method and application

Abstract

This invention belongs to the field of photocatalytic energy conversion technology, specifically relating to a photocatalyst for the photocatalytic reduction of CO2 to methane, its preparation method, and its application. The preparation method of the photocatalyst includes the following steps: adding a 2,2'-bipyridine solution to a copper salt solution in steps and mixing well; then adding an oxalic acid solution in steps and stirring to react, obtaining a copper dimer, wherein the molar ratio of copper salt, 2,2'-bipyridine, and oxalic acid is 0.5-1:0.5-1:0.25-0.5; adding the copper dimer dispersion in batches to a mesoporous graphitic carbon nitride mpg dispersion, wherein the mass ratio of mpg to copper dimer is 250:15-30; stirring and reacting at 20-40℃ for 10-30 h, and collecting the solid product; and then, in an air atmosphere, keeping the solid product at 230-270℃ for 5-15 h to obtain the final product. By introducing stable micro- and nano-bubbles and regulating the reaction interface, the reaction gas is mild and safe, and the reaction system is simple and easy to operate. This can effectively improve mass transfer efficiency and quantum efficiency, thereby significantly increasing the CH4 yield, reducing overpotential, and suppressing side reactions.

Description

Technical Field

[0001] This invention belongs to the field of photocatalytic energy conversion technology, specifically relating to a photocatalyst for the photocatalytic reduction of CO2 to methane, its preparation method, and its application. Background Technology

[0002] The statements herein provide only background information in relation to this invention and do not necessarily constitute prior art.

[0003] Photocatalytic CO2 reduction to methane involves a photocatalyst absorbing photon energy under light irradiation, exciting electron-hole pairs, and driving a redox reaction between carbon dioxide and water molecules to produce methane and oxygen. Methane is a high-quality fuel, and this method can achieve carbon cycling and energy storage.

[0004] Many existing photocatalysts (such as TiO2) exhibit low methane yields and purity in the CO2 reduction to methane process, and side reactions are difficult to suppress. Electrons and holes readily recombine before migrating to the catalyst surface or reaction interface, releasing heat or light energy instead of participating in the redox reaction, thus reducing the number of effective charge carriers. These problems severely hinder the transformation of photocatalytic CO2 reduction to methane into efficient, large-scale applications. Summary of the Invention

[0005] To address the technical problems existing in the prior art, the present invention provides a catalyst for photocatalytic CO2 reduction to methane, its preparation method and application.

[0006] To address the above technical problems, in a first aspect, the present invention provides a method for preparing a photocatalyst for the photocatalytic reduction of CO2 to methane, comprising the following steps:

[0007] The 2,2'-bipyridine solution was added to the copper salt solution in steps and mixed well; then the oxalic acid solution was added to it in steps and the reaction was stirred to obtain a copper dimer. The molar ratio of copper salt, 2,2'-bipyridine and oxalic acid was 0.5-1: 0.5-1: 0.25-0.5.

[0008] A copper dimer dispersion was added in batches to a mesoporous graphitic carbon nitride (mpg) dispersion, with a mass ratio of mpg to copper dimer of 250:15-30. The mixture was stirred at 20-40°C for 10-30 hours, and the solid product was collected.

[0009] The solid product is obtained by heating it at 230-270°C for 5-15 hours in an air atmosphere.

[0010] Secondly, the present invention provides a photocatalyst for the photocatalytic reduction of CO2 to methane, which is prepared by the aforementioned preparation method.

[0011] Thirdly, the present invention provides the application of the photocatalyst in the photocatalytic reduction of CO2 to methane.

[0012] The beneficial effects achieved by one or more embodiments of the present invention described above are as follows:

[0013] The photocatalyst material Cu2@mpg of this invention exhibits excellent photocatalytic performance and effectively improves methane yield. A key component of this material, mpg-C3N4 (mesoporous g-C3N4), possesses a honeycomb porous structure and a high specific surface area (up to 373 m²). 2 The Cu2@mpg catalyst exhibits excellent catalytic performance, superior to traditional bulk (bulk g-C3N4) and ordinary g-C3N4. The methane yield of the prepared Cu2@mpg catalyst is also significantly higher than that of Cu2@C3N4 and Cu2@bulk.

[0014] The copper dimer interacts with the π-conjugated system of mpg-C3N4 to form a heterojunction, promoting the separation of photogenerated electron-hole pairs and enhancing catalytic activity. The combination of micro / nano bubble water and Cu2@mpg effectively improves the selectivity and formation efficiency of the reaction product methane.

[0015] This invention requires only a basic light source, catalyst suspension, and reaction vessel to achieve a highly efficient energy conversion process. It is simple to operate and possesses excellent scalability, allowing for flexible adjustment of the reactor size to meet specific production needs.

[0016] This invention replaces the air in the reaction system with N2 or other gases. The system formed by this three-phase method can be used for photocatalytic nitrogen fixation to synthesize ammonia or other gas-liquid catalytic reactions. Attached Figure Description

[0017] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0018] Figure 1 This is a photograph of the apparatus for photocatalytic CO2 reduction to methane in Example 1 of this invention;

[0019] Figure 2 This is the curve showing the relationship between the input knob reading of the micro / nano generator and the micro / nano bubble yield in Embodiment 1 of the present invention;

[0020] Figure 3 This is a comparison chart of methane yield in photocatalytic carbon dioxide to methane production with and without micro-nano bubble water assistance in Example 1 of the present invention;

[0021] Figure 4This is a comparison chart of the catalytic performance of the three materials in Example 1, Comparative Example 1, and Comparative Example 2 of the present invention;

[0022] Figure 5 This is a comparison chart of methane yield between Example 1 and Comparative Example 3 of the present invention;

[0023] Figure 6 This is a comparison chart of the catalytic performance of catalysts with different ratios prepared in Examples 1-4 of this invention;

[0024] Figure 7 This is a physical image of Cu2@mpg prepared in Example 1 of this invention. Detailed Implementation

[0025] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0026] To address the technical problems mentioned in the background section, this invention provides a method for preparing a photocatalyst for the photocatalytic reduction of CO2 to methane, comprising the following steps:

[0027] The 2,2'-bipyridine solution was added to the copper salt solution in steps and mixed well; then the oxalic acid solution was added to it in steps and the reaction was stirred to obtain a copper dimer. The molar ratio of copper salt, 2,2'-bipyridine and oxalic acid was 0.5-1: 0.5-1: 0.25-0.5.

[0028] A copper dimer dispersion was added in batches to a mesoporous graphitic carbon nitride (mpg) dispersion, with a mass ratio of mpg to copper dimer of 250:15-30. The mixture was stirred at 20-40°C for 10-30 hours, and the solid product was collected.

[0029] The solid product is obtained by heating it at 230-270°C for 5-15 hours in an air atmosphere.

[0030] 2,2'-Bipyridine, as a bidentate ligand, interacts with Cu via the N atom. 2+ Coordination bonds are formed; oxalic acid acts as a bridging ligand, connecting the two Cu atoms. 2+ At the center, a stable binuclear structure (copper dimer) is formed; the stirring reaction promotes the full binding of ligands and metal ions, generating a copper dimer with a uniform structure.

[0031] mpg (mesoporous graphitic carbon nitride) has a honeycomb porous structure and a π-conjugated system, providing a large specific surface area and adsorption sites. Copper dimers are uniformly attached to the mesoporous surface / pores of mpg through π-π conjugation (the benzene ring of bipyridine matches the conjugated structure of mpg) or physical adsorption. By controlling the ratio of the two, the aggregation of copper dimers can be effectively avoided, ensuring that the active sites are fully exposed.

[0032] The mass ratio of mpg to copper dimer can be 250:15, 250:16, 250:17, 250:18, 250:19, 250:20, 250:21, 250:22, 250:23, 250:24, 250:25, 250:26, 250:27, 250:28, 250:29, or 250:30; preferably, the mass ratio of mpg to copper dimer is 250:17-25, and more preferably 250:20-25.

[0033] High-temperature insulation causes partial decomposition of the organic ligands in the copper dimer, transforming them into stable copper-based active sites. The interaction between these copper-based active sites and mpg forms heterojunctions, promoting the separation of photogenerated electron-hole pairs. This method preserves the mesoporous structure and high specific surface area of ​​mpg, maintaining its ability to adsorb CO2.

[0034] In the prepared photocatalyst Cu2@mpg, mpg provides a mesoporous structure and a conjugated framework, which is responsible for adsorbing CO2 and transporting electrons. The active copper species transformed from copper dimers serve as catalytic centers to drive the CO2 reduction reaction. The heterojunction structure inhibits electron-hole recombination and improves photocatalytic efficiency.

[0035] In some embodiments, the copper salt is CuCl2 or Cu(NO3)2.

[0036] 2,2'-Bipyridine as a bidentate ligand with Cu 2+ Coordination bonds are formed, with oxalic acid acting as a bridging ligand connecting the two Cu atoms. 2+ At the center, this ratio range can guarantee each Cu 2+ Both can bind to sufficient ligands to form a stable binuclear structure, avoiding mononuclear copper complexes or random aggregations caused by insufficient ligands.

[0037] If the ligand (bipyridine / oxalic acid) is in excess, the excess ligand may adsorb onto the mpg surface or block mesopores, affecting the subsequent recombination of copper dimers with mpg; if the ligand is insufficient, uncoordinated Cu 2+ It easily forms inactive copper salt precipitates, reducing the density of active sites on the catalyst.

[0038] This ratio ensures a uniform copper dimer structure, allowing for even dispersion when subsequently loaded onto mpg, forming an effective heterojunction structure, promoting the separation of photogenerated electron-hole pairs, and enhancing catalytic performance.

[0039] In some embodiments, the stirring reaction time is 0.5-2 hours when preparing copper dimers.

[0040] In some embodiments, after mixing mpg and copper dimer, the reaction is stirred for 20-30 hours and the reaction temperature is 25-35°C.

[0041] When MPG and copper dimer are mixed and stirred, an interfacial recombination reaction occurs between the copper dimer and MPG. Specifically, the copper dimer is uniformly loaded onto the mesoporous surface / pores of MPG through π-π conjugation or physical adsorption. Room temperature recombination avoids the decomposition of organic ligands (2,2'-bipyridine, oxalic acid) in the copper dimer, maintaining its binuclear structural integrity; at the same time, it protects the honeycomb mesoporous structure of MPG from high temperature damage, preserving its high specific surface area and adsorption sites. The moderate reaction rate at room temperature, combined with a stirring time of 20-30 hours, allows the copper dimer to slowly and uniformly attach to the surface / pores of MPG, avoiding excessively high local concentrations that could lead to copper dimer aggregation and ensuring sufficient exposure of active sites.

[0042] Room temperature reactions do not require additional heating equipment, reducing energy consumption and operational complexity, while ensuring the stability of the reaction system and reducing the occurrence of side reactions.

[0043] In some embodiments, the obtained solid is kept in air at 240-260°C for 7-12 hours to obtain Cu2@mpg.

[0044] Secondly, the present invention provides a photocatalyst for the photocatalytic reduction of CO2 to methane, which is prepared by the aforementioned preparation method.

[0045] Thirdly, the present invention provides the application of the photocatalyst in the photocatalytic reduction of CO2 to methane.

[0046] In some embodiments, the method for photocatalytic reduction of CO2 to methane using the aforementioned photocatalyst includes the following steps:

[0047] A mixed system was obtained by mixing a photocatalyst, triethanolamine, and KHCO3 micro / nanobubbles in water. The micro / nanobubbles were prepared from oxygen-containing gas. In the mixed system, the mass percentage of the photocatalyst was 0.02%-0.07%, the concentration of KHCO3 was 0.3-0.7M, and the mass percentage of triethanolamine was 10%-25%.

[0048] CO2 is introduced into the mixed system to remove air, and then CO2 is continuously introduced to produce methane through photocatalysis under light irradiation.

[0049] This invention introduces a stable micro / nano bubble to regulate the reaction interface and uses a three-phase method of micro / nano bubble-water-catalyst for photocatalytic CO2 to methane production. The micro / nano bubble water carries a negative charge, and the formation of this negative charge is related to the free electrons released when the bubble collapses. The negative charge inhibits electron recombination through Coulomb repulsion, which can prolong the carrier lifetime. This characteristic can improve the charge separation efficiency on the catalyst surface in the photocatalytic reaction.

[0050] Micro- and nano-bubbles are made of air or oxygen. Oxygen has a strong electron affinity and can accept photoexcited electrons (O2+ e-). - →O2 - •) Enhance O2 - The generation of ·, O2 - • It is a strong oxidizing free radical, and can participate in subsequent reactions as an intermediate in photocatalytic CO2 reduction, O2 - • It reacts with water molecules or protons to generate hydroxyl radicals (·OH) that participate in the oxidation half-reaction. At the same time, this reduces the recombination of e- / h+, providing more electrons for the generation of CH4, thereby improving the production efficiency of CH4.

[0051] Micro- and nano-bubbles oscillate left and right in water due to the movement of water molecules. They enhance the mass transfer rate in the photocatalytic system through compression, promote the adsorption and diffusion of CO2 on the catalyst surface, and accelerate the desorption of methane, thereby further improving the production efficiency of CH4.

[0052] The micro-nano bubbles and the photocatalyst of this invention synergistically regulate the reaction interface, effectively improving methane yield and selectivity and suppressing side reactions; at the same time, they reduce the reaction overpotential, enhance the catalytic activity of the photocatalyst, and achieve efficient CO2 reduction to methane.

[0053] Preferably, the volume ratio of triethanolamine to KHCO3 nanobubble water is 10:35-45, and more preferably 10:40.

[0054] Preferably, the particle size of the photocatalyst is 100-500 nm.

[0055] Preferably, during the photocatalytic CO2 to methane production process, the photocatalyst is continuously stirred to keep it in a suspended and dispersed state.

[0056] Photocatalysts need to absorb photon energy to excite electron-hole pairs. A suspended, dispersed state ensures that each catalyst particle is uniformly in contact with the light source, preventing sedimentation that could lead to insufficient light exposure at the bottom particles and improving light utilization efficiency. The suspended state significantly increases the contact area between the catalyst and reactants (CO2, micro / nano bubble water, etc.), promoting the adsorption and diffusion of CO2 on the catalyst surface, while simultaneously accelerating the desorption of the reaction product methane, preventing product accumulation and blockage of active sites.

[0057] Stirring can inhibit the agglomeration of catalyst particles due to van der Waals forces and other effects, maintain the full exposure of active sites, and preserve high catalytic activity. Suspension and dispersion combined with stirring can enhance the mass transfer rate in the reaction system, enabling reactants to quickly reach the catalyst surface and products to leave in a timely manner, thereby further increasing the reaction rate.

[0058] Preferably, the diameter of the micro / nano bubbles is 0.1-50 μm, and the dispersion density of the micro / nano bubbles in water is 1-9 × 10⁻⁶. 16 per mL.

[0059] In some embodiments, the wavelength of the illumination is λ≥420 nm, and the irradiance is 1-100 mW / cm². 2 .

[0060] The present invention will be further described below with reference to the embodiments.

[0061] Example 1

[0062] The photoreactor used in the photoreaction experiment, such as Figure 1 As shown.

[0063] A method for preparing a photocatalyst for the photocatalytic reduction of CO2 to methane includes the following steps:

[0064] (1) mpg is a special form of C3N4, and its production process is as follows:

[0065] ① Weigh 5g of cyanamide powder and 12.5g of LUDOX HS-40 colloidal silica into a beaker, and shake the beaker until the white cyanamide powder is completely dissolved;

[0066] ② Place the above mixture in an oil bath and heat and stir at 100°C for 12 hours to obtain a white mixture of cyanamide and SiO2 solids;

[0067] ③ Grind the solid mixture into powder in a mortar until there is no obvious grainy texture;

[0068] ④ The powder was placed in a covered crucible and placed in a muffle furnace for thermal oxidation etching. The heating program was set to a heating rate of 2.3℃ / min, and the temperature was raised to 500℃ and held for 4 hours to obtain a brownish-yellow powder.

[0069] ⑤ Transfer the brownish-yellow powder to a plastic centrifuge tube, add 4M NH4HF2 solution until it is completely submerged, place in an ice bath and stir for 48 hours (the entire reaction should be carried out in a fume hood, and the centrifuge tube should be in a semi-closed state to prevent the generated NH3 from overflowing in time and causing an explosion).

[0070] ⑥ The product was washed three times each by centrifugation with deionized water and anhydrous ethanol. The product was then dried overnight in a vacuum drying oven at 60°C to obtain mpg.

[0071] (2) The preparation method of copper dimer is as follows:

[0072] ① Disperse 1.6 mmol CuCl2·2H2O in 20 mL of deionized water by ultrasonication to obtain solution a;

[0073] ② Disperse 1.6 mmol of 2,2'-bipyridine in 10 mL of methanol using ultrasonication to obtain solution b;

[0074] ③ Disperse 0.8 mmol of oxalic acid in 10 mL of deionized water using ultrasound to obtain solution c;

[0075] ④ Then add solutions b and c dropwise to solution a, and continue stirring for 1 hour;

[0076] ⑤ Finally, by centrifugation, washing with water and methanol three times, and vacuum drying, a light blue solid was obtained, which is the copper dimer.

[0077] (3) 0.5 g of mesoporous graphitic carbon nitride mpg was ultrasonically dispersed in 50 mL of methanol solution to obtain dispersion A;

[0078] 42 mg of copper dimer was ultrasonically dispersed in 5 mL of methanol solution to obtain dispersion B;

[0079] Dispersion B was added dropwise to dispersion A and stirred at 25°C for 24 hours. The resulting solid was then placed in a muffle furnace and calcined (in air atmosphere) at a heating rate of 2°C / min, from 50°C to 250°C, and held at that temperature for 10 hours. The final product, Cu2@mpg, was obtained. Figure 7 As shown.

[0080] Experimental steps for photocatalytic carbon dioxide to methane production:

[0081] Catalyst: Cu2@mpg (50mg);

[0082] The first group consists of: adding catalyst (Cu2@mpg), KHCO3 and 10 mL TEOA to 40 L of water, with the concentration of KHCO3 being 0.5 M, which is Cu2@mpg-KHCO3;

[0083] The second group consisted of adding the catalyst (Cu2@mpg) and 10 mL of TEOA to 40 mL of 0.5 M KHCO3 micro / nanobubble water. The diameter of the air micro / nanobubbles ranged from 0.1 to 50 μm, and the dispersion density of the air micro / nanobubbles in the water was 1 × 10⁻⁶. 16 The concentration of KHCO3 is 0.5 M, which is Cu2@mpg-KHCO3-air bubble water.

[0084] The mixtures from the first and second groups were placed in separate reactors. CO2 was introduced into the mixtures at a flow rate of 15 mL / min for 30 min to purge air. Then, CO2 was continuously introduced, and photocatalytic CO2 to methane production was carried out under illumination. Magnetic stirring was used during the reaction at a rate of 300 rad / min, and irradiation was performed for 6 hours using a 365 nm LED lamp with a 420 nm filter, resulting in a radiation illuminance of 100 mW / cm². 2 .

[0085] Experiments were conducted using KHCO3 micro-nano bubble water prepared when the pressure knob of the micro-nano bubble generator was adjustable to approximately 20 (air was introduced at a flow rate of 200 mL / min). Figure 2 As shown.

[0086] The experimental results of the first and second groups are as follows: Figure 3 As shown, the presence of air micro-nano bubbles can effectively improve the methane yield when using Cu2@mpg as a photocatalyst for photocatalytic carbon dioxide to methane production.

[0087] Comparative Example 1

[0088] The difference from Example 1 is that mpg is replaced with ordinary g-C3N4, and Cu2@C3N4 composite photocatalyst is prepared using the method of Example 1.

[0089] Comparative Example 2

[0090] The difference from Example 1 is that mpg is replaced with bulk, and Cu2@bulk composite photocatalyst is prepared using the method of Example 1.

[0091] Comparative Example 3

[0092] The difference from Example 1 is that CO2 is introduced during the preparation of micro-nano bubble water to obtain CO2 bubble water. Everything else is the same as the second group of Example 1.

[0093] Other materials were also used for comparison in the experiment. In Comparative Example 1 and Comparative Example 2, mpg was replaced with ordinary g-C3N4 and bulk (mpg, ordinary g-C3N4 and bulk are all different states of graphitic carbon nitride. bulk belongs to blocky graphitic carbon nitride, ordinary g-C3N4 belongs to sheet graphitic carbon nitride, and mpg belongs to mesoporous graphitic carbon nitride), and Cu2@C3N4 and Cu2@bulk composite photocatalysts were prepared respectively.

[0094] The catalytic experiment conditions were the same as those in the first group of Example 1 (without air micro / nano bubbles in the system), demonstrating that Cu2@mpg is the material with the best catalytic effect. Figure 4 As shown.

[0095] Figure 5 This is a comparison chart of the experimental results of the first and second groups in Example 1 and Comparative Example 3, as shown below. Figure 5 It is known that adding air micro-nano bubbles to the catalytic reaction system can effectively increase the methane yield, while adding carbon dioxide micro-nano bubbles will reduce the methane yield to a certain extent. Therefore, air is chosen as the gas for preparing micro-nano bubbles.

[0096] Example 2

[0097] A method for preparing a photocatalyst for the photocatalytic reduction of CO2 to methane includes the following steps:

[0098] The preparation of mesoporous graphitic carbon nitride mpg and copper dimers is the same as in Example 1.

[0099] 0.5 g of mesoporous graphitic carbon nitride (mpg) was ultrasonically dispersed in 50 mL of methanol solution to obtain dispersion A;

[0100] 32 mg of copper dimer was ultrasonically dispersed in 5 mL of methanol solution to obtain dispersion B;

[0101] Dispersion B was added dropwise to dispersion A and stirred at 30°C for 30 h. The resulting solid was then placed in a muffle furnace and calcined at 2°C / min from 50°C to 260°C and held at that temperature for 10 h (in air atmosphere) to obtain the final product, Cu2@mpg.

[0102] Example 3

[0103] A method for preparing a photocatalyst for the photocatalytic reduction of CO2 to methane includes the following steps:

[0104] The preparation of mesoporous graphitic carbon nitride mpg and copper dimers is the same as in Example 1.

[0105] 0.5 g of mesoporous graphitic carbon nitride (mpg) was ultrasonically dispersed in 50 mL of methanol solution to obtain dispersion A;

[0106] 52 mg of copper dimer was ultrasonically dispersed in 5 mL of methanol solution to obtain dispersion B;

[0107] Dispersion B was added dropwise to dispersion A and stirred at 30°C for 30 h. The resulting solid was then placed in a muffle furnace and calcined at 2°C / min from 40°C to 240°C and held at that temperature for 12 h (in air atmosphere) to obtain the final product, Cu2@mpg.

[0108] Example 4

[0109] A method for preparing a photocatalyst for the photocatalytic reduction of CO2 to methane includes the following steps:

[0110] The preparation of mesoporous graphitic carbon nitride mpg and copper dimers is the same as in Example 1.

[0111] 0.5 g of mesoporous graphitic carbon nitride (mpg) was ultrasonically dispersed in 50 mL of methanol solution to obtain dispersion A;

[0112] 60 mg of copper dimer was ultrasonically dispersed in 5 mL of methanol solution to obtain dispersion B;

[0113] Dispersion B was added dropwise to dispersion A and stirred at 30°C for 30 h. The resulting solid was then placed in a muffle furnace and calcined at a heating rate of 2°C / min from 40°C to 270°C and held for 12 h in air atmosphere to obtain the final product, Cu2@mpg.

[0114] The catalytic performance of catalysts prepared in different ratios in Examples 1, 2, 3, and 4 was compared. The catalytic experiments were the same as the first group in Example 1, demonstrating that the ratio of mpg to copper dimer in Example 1 had the best catalytic effect. Figure 6 As shown.

[0115] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. Application of photocatalysts in photocatalytic CO2 reduction to methane The method for photocatalytic CO2 reduction to methane includes the following steps: mixing the photocatalyst, triethanolamine, and KHCO3 micro-nano bubble water to obtain a mixed system, wherein the gas used to prepare the micro-nano bubbles is air; introducing CO2 into the mixed system to remove the air, and then continuously introducing CO2 to produce methane under light irradiation; wherein Micro-nano bubble water carries a negative charge; The preparation method of the photocatalyst includes the following steps: adding a 2,2'-bipyridine solution to a copper salt solution in steps and mixing well; then adding an oxalic acid solution in steps and stirring to react, obtaining a copper dimer, wherein the molar ratio of copper salt, 2,2'-bipyridine and oxalic acid is 0.5-1:0.5-1:0.25-0.5; adding the copper dimer dispersion in batches to a mesoporous graphitic carbon nitride mpg dispersion, wherein the mass ratio of mpg to copper dimer is 250:15-30, stirring and reacting at 20-40℃ for 10-30h, and collecting the solid product; and holding the solid product at 230-270℃ for 5-15h in an air atmosphere to obtain the final product.

2. Use according to claim 1, characterized in that: The copper salt is CuCl2 or Cu(NO3)2.

3. Use according to claim 1, characterized in that: When preparing copper dimers, the stirring reaction time is 0.5-2 hours.

4. The application according to claim 1, characterized in that: The mass ratio of mpg to copper dimer is 250:17-25. After mixing mpg and copper dimer, the reaction is stirred for 20-30 hours at a temperature of 25-35℃.

5. The application according to claim 1, characterized in that: In the mixed system, the mass percentage of photocatalyst is 0.02%-0.07%, the concentration of KHCO3 is 0.3-0.7M, and the mass percentage of triethanolamine is 10%-25%.

6. The application according to claim 1, characterized in that: The volume ratio of triethanolamine to KHCO3 micro-nano bubble water is 10:35-45.

7. The application according to claim 1, characterized in that: The particle size of the photocatalyst is 100-500 nm; Alternatively, the diameter of the micro-nano bubbles is 0.1-50 μm.

8. The application according to claim 1, characterized in that: During the photocatalytic CO2 to methane production process, the photocatalyst is continuously stirred to keep it in a suspended and dispersed state. or the wavelength of the light is λ > 420 nm and the radiant exposure is 1 - 100 mW / cm 2 .

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