Preparation method and application of Co3O4 / g-C3N4 heterojunction catalyst

Abstract

The invention discloses a preparation method and application of a Co3O4 / g-C3N4 heterojunction catalyst, and the preparation method is divided into two steps: firstly, synthesizing Co3O4 nanospheres through a hydrothermal reaction, then adding the Co3O4 nanospheres into a micromolecule g-C3N4 solution, and drying and calcining at low temperature to obtain the Co3O4 / g-C3N4 heterojunction catalyst. The method is green, simple and low in cost, the prepared Co3O4 / g-C3N4 heterojunction catalyst is composed of hydrothermally synthesized Co3O4 nanospheres and g-C3N4 nanosheets, the composite structure of the Co3O4 / g-C3N4 heterojunction catalyst has excellent charge separation efficiency, transfer of photon-generated carriers is accelerated, the catalyst has high tetrafluoroethane catalytic degradation capacity under a photo-thermal synergistic system, and the Co3O4 / g-C3N4 heterojunction catalyst can be applied to the field of photocatalytic degradation of tetrafluoroethane. The method can be applied to the field of photothermal degradation of waste refrigerants.

Description

Technical Field

[0001] The present invention belongs to the technical field of photothermal catalysis, and specifically relates to a preparation method and application of a Co3O4 / g-C3N4 heterojunction catalyst. Background Art

[0002] With the widespread use of refrigeration equipment in the industrial and consumer sectors, waste refrigerants represented by chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs) are continuously released into the environmental system; these persistent organic pollutants with high ozone depletion potential and global warming potential can trigger chain photolysis reactions in the stratosphere through atmospheric migration, leading to the expansion of the ozone hole and the intensification of the greenhouse effect. Although the Montreal Protocol and the Kigali Amendment have implemented controls on refrigerants, historical legacy and illegal emissions still cause global annual emissions to exceed 500,000 tons; therefore, there is an urgent need to develop an efficient, low-cost, and environmentally friendly degradation technology. Photothermal synergistic catalytic technology provides an innovative solution for this. It uses an electric field and thermal field coupling mechanism - photoexcited carriers directly participate in the redox reaction, while the local thermal field reduces the reaction activation energy, achieving efficient CF bond breaking under mild conditions.

[0003] The efficiency of photothermal catalysis is also limited by the low utilization of solar energy and the rapid recombination of charge carriers in wide-bandgap semiconductors. To improve the efficiency of solar energy utilization, catalysts should be designed to have a broad spectrum absorption capability within the wavelength range of 300 to 800 nm to maximize the capture of light energy and activate the reaction.

[0004] Cobalt is a transition metal with various valence states (Co 2+ 、Co 3+ 、Co 4+ ) makes the cobalt oxides formed diverse and have certain redox properties. Cobalt trioxide (Co3O4) is one of the cobalt oxides, a mixed valence (Co 2+ and Co 3+ ) is a transition metal oxide with a unique spinel structure and is a high-temperature resistant black neutral oxide; its mixed valence gives it excellent redox ability, and it has abundant oxygen vacancies and defect sites, and has excellent catalytic activity; due to its low cost and environmental friendliness, it is often used in catalytic oxidation, energy storage, environmental governance and other fields.

[0005] Graphitic carbon nitride (g-C3N4) has attracted considerable attention due to its simple preparation, low cost, excellent electronic configuration, good thermal stability, well-positioned bandgap, and unique optoelectronic properties. Its excellent visible light response makes it a highly promising semiconductor catalytic material with broad application prospects in photothermal catalysis. However, conventional g-C3N4 is limited in its application by shortcomings such as low quantum efficiency and the easy recombination of photogenerated carriers. Summary of the Invention

[0006] To overcome the above-mentioned deficiencies of the prior art, the present invention aims to provide a method for preparing a Co3O4 / g-C3N4 heterojunction catalyst, comprising synthesizing Co3O4 nanospheres via a hydrothermal reaction, adding the Co3O4 nanospheres to a small molecule g-C3N4 solution, and obtaining the Co3O4 / g-C3N4 heterojunction catalyst through drying and low-temperature calcination. The prepared Co3O4 / g-C3N4 heterojunction catalyst is composed of a composite of hydrothermally synthesized Co3O4 nanospheres and g-C3N4 nanosheets. The composite structure exhibits excellent charge separation efficiency, accelerating the transfer of photogenerated carriers. Under a photothermal synergistic system, the catalyst exhibits a high catalytic degradation capability for tetrafluoroethane, with high degradation efficiency and strong universality, making it suitable for photothermal catalytic degradation of waste refrigerants.

[0007] In order to achieve the above-mentioned purpose of the invention, the present invention provides a preparation method and application of a Co3O4 / g-C3N4 heterojunction catalyst, wherein the heterojunction catalyst is composed of a composite of hydrothermally synthesized Co3O4 nanospheres and g-C3N4 nanosheets; the Co3O4 nanospheres are hydrothermally synthesized and then coated with g-C3N4 nanosheets to obtain the Co3O4 / g-C3N4 heterojunction catalyst.

[0008] The preparation method of the Co3O4 / g-C3N4 heterojunction catalyst comprises the following steps:

[0009] S1: Weigh dicyandiamide, heat to 500-550°C at a rate of 2.0-2.5°C / min, and calcine for 2-4 hours to obtain bulk-g-C3N4; place the obtained bulk-g-C3N4 in a hydrothermal reactor, add deionized water and mix, seal the obtained mixed solution, place it in an oven for reaction, and dry it to obtain g-C3N4 nanosheets;

[0010] S2 disperses Co(NO3)3·6H2O in methanol to form solution A with a concentration of 0.025-0.035 g / mL;

[0011] S3 disperses 2-methylimidazole in methanol to form a solution B with a concentration of 0.09 to 0.10 g / mL;

[0012] S4: Mix the above solution A and solution B, stir at 2000 rpm for 3-4 minutes, let the resulting mixture stand at room temperature for 12 hours, centrifuge to obtain a precipitate, wash the precipitate with methanol, dry it at 80°C for 10 hours, and then calcine to obtain Co3O4 nanospheres;

[0013] S5: adding the g-C3N4 nanosheets obtained in step S1 to methanol, ultrasonically dispersing for 25 to 35 minutes, to obtain a solution with a concentration of 0.1 to 0.8 g / mL; adding the Co3O4 nanospheres obtained in step S4 to the above solution and magnetically stirring in a fume hood for 12 to 24 hours; controlling the mass ratio of g-C3N4 nanosheets:Co3O4 nanospheres to be 1 to 2:20; drying at 100° C. and calcining to obtain a Co3O4 / g-C3N4 heterojunction catalyst.

[0014] In the above technical solution, further, the concentration of the mixed solution in step S1 is 0.80-1.80 g / 100 mL.

[0015] Furthermore, in step S1, the reaction temperature in the oven is 200° C. to 210° C., and the reaction time is 7 to 8 hours; the drying temperature is 60° C. to 65° C., and the drying time is 4 to 5 hours.

[0016] Furthermore, the calcination temperature in step S4 is 300° C. to 400° C., the heating rate is 1° C. / min, and the calcination time is 2 to 3 hours.

[0017] Furthermore, the calcination temperature in step S5 is 300° C. to 400° C., the calcination time is 3 to 5 hours, and the heating rate is 2.0 to 2.5° C. / min.

[0018] The Co3O4 / g-C3N4 heterojunction catalyst prepared by the above method is spherical, with Co3O4 as the catalyst core and g-C3N4 coated on the Co3O4 outer layer to form a core-shell structure. A key advantage of this catalytic material is its ability to utilize the full spectrum of sunlight to drive the reaction, significantly improving the efficiency of light energy conversion. By optimizing the composite structure design, synergistically improving the active site density and mass transfer efficiency, and combining characterization techniques to precisely optimize reaction conditions, the degradation efficiency of pollutants can be significantly improved.

[0019] Application of the Co3O4 / g-C3N4 heterojunction catalyst prepared by the above method in the degradation of waste refrigerants.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The Co3O4 / g-C3N4 heterojunction catalyst provided by the present invention adopts a simple preparation technology, with mild operating conditions and easy-to-control parameters; the preparation method is green, simple, and environmentally friendly. By precisely controlling the synthesis process, a spherical and regular composite structure with uniform particle size and stable dispersion can be obtained. The g-C3N4 outer coating significantly increases the contact area of the material, exposing more catalytic active centers. The composite structure formed not only strengthens the photothermal coupling effect, but also improves the separation efficiency of photogenerated charges. At the same time, the catalytic material exhibits excellent photothermal catalytic performance in the field of waste refrigerant treatment and has good application potential in the fields of clean energy and pollution control. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 SEM images of (a) to (b) Co3O4, (c) g-C3N4, and (d) to (h) Co3O4 / g-C3N4-X (X = 1, 2, 3, 4, 5) materials in Example 1;

[0023] Figure 2 PL spectra of Co3O4, g-C3N4 and Co3O4 / g-C3N4-X (X=1, 2, 3, 4, 5) materials in Example 1;

[0024] Figure 3 The Uv-Vis spectra of Co3O4, g-C3N4 and Co3O4 / g-C3N4-X (X=1, 2, 3, 4, 5) materials in Example 1;

[0025] Figure 4 XRD patterns of g-C3N4, Co3O4 and Co3O4 / g-C3N4-X (X=1, 2, 3, 4, 5) materials in Example 1;

[0026] Figure 5 FT-IR spectra of g-C3N4, Co3O4 and Co3O4 / g-C3N4-X (X=1, 2, 3, 4, 5) materials in Example 1;

[0027] Figure 6 The degradation effects of (a) Co3O4, g-C3N4 and Co3O4 / g-C3N4-X (X=1, 2, 3, 4, 5) materials on tetrafluoroethane (R134a) under photothermal conditions in Application Example 1, and (b) the degradation effect of Co3O4 / g-C3N4-2 material on R134a under different conditions;

[0028] Figure 7 This is the test result of the cyclic stability of Co3O4 / g-C3N4-2 material on the degradation of R134a in Application Example 1. DETAILED DESCRIPTION

[0029] The present invention will be further described below with reference to specific examples, but the present invention is not limited in any way. To avoid redundancy, the raw materials in the following examples are all commercially available products unless otherwise specified, and the methods used are all conventional methods unless otherwise specified.

[0030] A method for preparing a Co3O4 / g-C3N4 heterojunction catalyst comprises the following steps:

[0031] S1: Weigh dicyandiamide, heat to 500-550°C at a rate of 2.0-2.5°C / min, and calcine for 2-4 hours to obtain bulk-g-C3N4; place the obtained bulk-g-C3N4 in a hydrothermal reactor, add deionized water and mix, seal the obtained mixed solution, place it in an oven for reaction, and dry it to obtain g-C3N4 nanosheets;

[0032] S2 disperses Co(NO3)3·6H2O in methanol to form solution A with a concentration of 0.025-0.035 g / mL;

[0033] S3 disperses 2-methylimidazole in methanol to form a solution B with a concentration of 0.09 to 0.10 g / mL;

[0034] S4: Mix the above solution A and solution B, stir at 2000 rpm for 3-4 minutes, let the resulting mixture stand at room temperature for 12 hours, centrifuge to obtain a precipitate, wash the precipitate with methanol, dry it at 80°C for 10 hours, and then calcine to obtain Co3O4 nanospheres;

[0035] S5: adding the g-C3N4 nanosheets obtained in step S1 to methanol, ultrasonically dispersing for 25 to 35 minutes, to obtain a solution with a concentration of 0.1 to 0.8 g / mL; adding the Co3O4 nanospheres obtained in step S4 to the above solution and magnetically stirring in a fume hood for 12 to 24 hours; controlling the mass ratio of g-C3N4 nanosheets:Co3O4 nanospheres to be 1 to 2:20; drying at 100° C. and calcining to obtain a Co3O4 / g-C3N4 heterojunction catalyst.

[0036] The heterojunction catalyst is composed of a composite of hydrothermally synthesized Co3O4 nanospheres and g-C3N4 nanosheets.

[0037] Any matters not described in the following embodiments are the same as those described in the above specific implementation methods.

[0038] Example 1

[0039] A method for preparing a Co3O4 / g-C3N4 heterojunction catalyst comprises the following steps:

[0040] S1 weighed 10g of dicyandiamide in a crucible, transferred the crucible to the constant temperature zone of a muffle furnace, heated from 30℃ to 550℃ at a rate of 2.3℃ / min, and calcined at 550℃ for 4h to obtain bulk-g-C3N4; weighed 0.5g of the obtained bulk-g-C3N4 powder and placed it in a 50mL hydrothermal reactor, added 35mL of deionized water and mixed, and the obtained mixed solution (concentration of 1.43g / 100mL) was sealed and placed in an oven for reaction. The reaction temperature in the oven was 210℃ and the reaction time was 8h; the drying temperature was 65℃ and the drying time was 5h. After drying, g-C3N4 nanosheets were obtained.

[0041] S2 873.1 mg of Co(NO3)3·6H2O (Aladdin, 99%) was dispersed in 30 mL of methanol to form solution A with a concentration of 0.029 g / mL;

[0042] S3: Disperse 985.2 mg of 2-methylimidazole (Aladdin, 98%) in 10 mL of methanol to form solution B with a concentration of 0.099 g / mL.

[0043] S4: Mix the above solution A and solution B, stir vigorously at 2000 rpm for 3 minutes, let the resulting mixture stand at room temperature for 12 hours, obtain a precipitate by centrifugation, wash the precipitate with methanol, and then dry it at 80°C for 10 hours. Then, calcinate it at 400°C with a heating rate of 1°C / min for 2 hours to obtain Co3O4 nanospheres.

[0044] S5: adding the g-C3N4 nanosheets obtained in step S1 to methanol, ultrasonically dispersing for 30 minutes to obtain a solution with a concentration of 0.1 to 0.8 g / mL; adding the Co3O4 nanospheres obtained in step S4 to the above solution and magnetically stirring in a fume hood for 24 hours; controlling the mass ratio of g-C3N4 nanosheets:Co3O4 nanospheres to be 1 to 2:20; after the methanol evaporates, drying at 100°C to obtain an opaque powder, then heating the above powder to 400°C at a heating rate of 2.5°C / min and calcining for 5 hours to obtain a Co3O4 / g-C3N4 heterojunction catalyst.

[0045] The Co3O4 / g-C3N4 composite heterojunction catalysts prepared in Example 1 with mass fractions of 1%, 2%, 3%, 4% and 5% were named Co3O4 / g-C3N4-1, Co3O4 / g-C3N4-2, Co3O4 / g-C3N4-3, Co3O4 / g-C3N4-4 and Co3O4 / g-C3N4-5, respectively; among them, the SEM scanning electron microscope images of Co3O4, g-C3N4 and Co3O4 / g-C3N4-X (X=1, 2, 3, 4, 5) materials are as follows: Figure 1 (ah) shown. Figure 1 Scanning results show that pure Co₃O₄ exhibits a three-dimensional flower-like hierarchical structure, composed of nanosheets approximately 50 nm thick self-assembled into mesoporous microspheres with a diameter of 500 nm, exhibiting a high specific surface area. Pure g-C₃N₄ exhibits an irregular blocky morphology with significant stacking cracks on the surface. After composite formation, the g-C₃N₄ / Co₃O₄ fully retains the flower-like structure of Co₃O₄, while 20 nm g-C₃N₄ nanoparticles are uniformly anchored on the nanosheet surface and within the interlayer pores. Increasing the g-C₃N₄ doping level from 1 wt% to 5 wt% significantly increases the surface particle density. Composite formation increases the thickness of the Co₃O₄ nanosheets to 65–80 nm, confirming the formation of heterointerfacial charge transfer channels through intercalation of precursor molecules. This structure effectively inhibits g-C₃N₄ stacking, increases the interfacial contact area through a close pn heterojunction, and promotes mass transfer efficiency through a hierarchical mesoporous system, providing a morphological foundation for constructing efficient catalytic interfaces.

[0046] Fluorescence tests were performed on the Co3O4 / g-C3N4 heterojunction catalysts, Co3O4 and g-C3N4 with different g-C3N4 addition amounts obtained in Example 1. The results are shown in Figure 2. Figure 2 As shown, Figure 2 The fluorescence (PL) spectra of the Co3O4 / g-C3N4 composite material, Co3O4 and g-C3N4 with different g-C3N4 addition amounts in Example 1 are as follows; the PL spectra can be used to detect the separation efficiency of the photogenerated carriers of the material. Low fluorescence intensity usually indicates higher separation efficiency, which is beneficial to the improvement of catalytic performance. The PL test results show that the fluorescence intensity of Co3O4 / g-C3N4-2 is significantly lower than that of pure g-C3N4 and is accompanied by a blue shift in the emission peak, indicating that the built-in electric field at the pn heterojunction interface constructed by n-type g-C3N4 and p-type Co3O4 drives the photogenerated electrons to migrate to g-C3N4 and the holes to enrich in Co3O4, effectively improving the carrier separation efficiency and providing a key mechanism support for the enhancement of photothermal synergistic catalytic performance.

[0047] The UV diffuse reflectance test was performed on the Co3O4 / g-C3N4 heterojunction catalyst, Co3O4 and g-C3N4 with different g-C3N4 addition amounts obtained in Example 1. The results are as follows: Figure 3 As shown, Figure 3The ultraviolet diffuse reflectance (UV-vis) spectra of the Co3O4 / g-C3N4 heterojunction catalyst, Co3O4, and g-C3N4 with different g-C3N4 addition amounts in Example 1; UV-vis spectra are a method for measuring the light absorption capacity of a material and are used to evaluate the catalyst's ability to absorb light of different wavelengths. The results show that pure g-C3N4 has a π-π* electron transition absorption edge at 458nm, while pure Co3O4 exhibits a broad spectrum of strong absorption at 200-800nm due to dd transitions. After compounding, the absorption edge of Co3O4 / g-C3N4-2 blue-shifts to 623nm, which is due to the broadening of the spectrum by interfacial charge transfer and the suppression of carrier recombination by the built-in electric field of the pn junction, thereby enhancing the effective light absorption depth. Its visible light absorption intensity is 3.2 times higher than that of pure g-C3N4, attributed to the plasma resonance effect of Co3O4 forming a characteristic absorption shoulder at 560nm. However, the absorption of the composite material in the ultraviolet region is still significantly higher than that in the visible region, indicating that the photocatalytic activity is mainly driven by ultraviolet light, which is related to the Co 3+ -O-Co 2+ The charge transfer transition characteristics are related, providing a theoretical basis for the design of catalysts with wide spectrum response.

[0048] The X-ray diffraction test was performed on the Co3O4 / g-C3N4 heterojunction catalyst, Co3O4 and g-C3N4 with different g-C3N4 addition amounts obtained in Example 1. The results are as follows: Figure 4 As shown, Figure 4 The X-ray diffraction (XRD) patterns of the Co3O4 / g-C3N4 heterojunction catalyst, Co3O4, and g-C3N4 with different g-C3N4 addition amounts in Example 1 are used to analyze the phase composition of the Co3O4 / g-C3N4 heterojunction catalyst and to read the crystal properties and crystal structure information of the material. XRD analysis shows that pure Co3O4 exhibits characteristic peaks of a cubic spinel structure and high crystallinity. The (002) peak intensity of pure g-C3N4 at 27.5° is 42% higher than that of the bulk, and interlayer defects are reduced. The characteristic peaks of Co3O4 coexist with the (002) peak of g-C3N4 in the composite material, and no impurity phase is generated. As the g-C3N4 loading increases, the (311) crystal plane peak of Co3O4 shifts, and the interplanar spacing is compressed from 0.253nm to 0.249nm, indicating that the π electron system induces lattice compression strain through CO-Co bonding. The characteristic peak of g-C3N4 is obscured by the strong scattering effect of Co3O4. The contraction of the CN and Co-O bond lengths confirms the existence of electronic coupling at the heterojunction interface, providing a structural basis for catalytic optimization.

[0049] The Co3O4 / g-C3N4 heterojunction catalysts, Co3O4 and g-C3N4 with different g-C3N4 addition amounts obtained in Example 1 were subjected to infrared (FTIR) spectroscopy tests. The results are as follows: Figure 5 As shown, Figure 5 The infrared (FTIR) spectra of the Co3O4 / g-C3N4 heterojunction catalyst, Co3O4, and g-C3N4 with different g-C3N4 addition amounts in Example 1 can detect the chemical bonds and functional groups of the Co3O4 / g-C3N4 heterojunction catalyst. By adjusting the amount of raw materials, regularities can be observed. FT-IR analysis shows that pure Co3O4 has a peak at 570 cm -1 and 667cm -1 The corresponding Co 3+ -O and Co 2+ The vibration peak of -O confirms the high crystallinity of its spinel structure. Pure g-C3N4 has a peak at 809cm -1 The triazine ring is bent at 1220-1650cm -1 The range is the vibration of the CN heterocyclic ring. After compounding, as the g-C3N4 loading increases to 5%, the intensity of the CN characteristic peak increases linearly, indicating that it is successfully anchored to the Co3O4 surface. The characteristic peak of Co3O4 is stable and no new Co-OC / Co-N bonds are formed, indicating that the two are bonded through van der Waals forces and π-metal interactions. 3435cm -1 The OH peak intensity at the position decreases with the loading amount, which confirms that the hydrophobicity of g-C3N4 inhibits the formation of surface hydroxyl groups and the interfacial interaction is mainly physical adsorption.

[0050] Application Example 1

[0051] Application of a Co3O4 / g-C3N4 heterojunction catalyst in the degradation of waste refrigerants.

[0052] Evaluation of photothermal catalytic oxidation activity: Tetrafluoroethane (R134a) was used as the target refrigerant to explore the degradation activity of Co3O4 / g-C3N4 heterojunction catalysts with different g-C3N4 addition amounts on tetrafluoroethane (R134a) under photothermal synergistic conditions. The photothermal catalytic oxidation of R134a was evaluated using the Zhongjiao Jinyuan CEL GPPCM micro-photothermal catalytic micro-reaction system. The test device consists of three parts: gas distribution, reaction and detection. The test conditions are as follows: the reaction system is filled with 0.1g of catalyst, vacuumed for 20min to verify the airtightness, and 6mL of R134a and a set ratio of O2 / N2 mixed gas are injected in turn to maintain a slightly positive pressure. After the gas distribution is completed, the reaction device is heated to the set temperature (120°C) and shielded to eliminate light interference during the heating process. The light source uses 389mW / cm 2After a 20-minute preheating period for the xenon lamp to stabilize its intensity, the lamp was placed close to the reactor window to illuminate the catalyst. This time was recorded as the start of the reaction (t = 0 min). The detection system sampled 500 μL of sample every 15 minutes and quantitatively analyzed the R134a concentration using an SP-6890 gas chromatograph. Chromatographic parameters included an HP-5 column, an FID detector, high-purity N₂ carrier gas, a column temperature of 100°C, a detector temperature of 250°C, and an inlet temperature of 200°C.

[0053] The catalytic performance of tetrafluoroethane (R134a) was tested on the Co3O4 / g-C3N4 heterojunction catalysts, Co3O4 and g-C3N4 with different g-C3N4 addition amounts obtained in Example 1. The results are as follows: Figure 6 As shown, Figure 6 Figure 1 shows the R134a catalytic performance of the Co3O4 / g-C3N4 heterojunction catalyst, Co3O4, and g-C3N4 at different g-C3N4 addition levels in Example 1. The results show that compared to pure g-C3N4 and Co3O4, the Co3O4 / g-C3N4 heterojunction catalyst exhibits significant photothermal catalytic activity. The R134a degradation rate of the Co3O4 / g-C3N4-2 heterojunction catalyst reaches 90% in 150 minutes.

[0054] The Co3O4 / g-C3N4 heterojunction catalyst obtained in Example 1 was subjected to a tetrafluoroethane stability test. The results are as follows: Figure 7 As shown, Figure 7 This is a graph showing the results of a tetrafluoroethane cycle stability test on the Co3O4 / g-C3N4 heterojunction catalyst obtained in Example 1. Figure 7 It was demonstrated that the Co3O4 / g-C3N4 heterojunction catalyst can still maintain a high catalytic activity after multiple degradation of R134a under photothermal synergistic conditions, indicating its excellent stability and recyclability.

[0055] Anyone skilled in the art will be able to utilize the above-disclosed technical content to make many possible changes and modifications to the technical solution of the present invention, or to modify it into equivalent embodiments with equivalent changes, without departing from the scope of the technical solution of the present invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention that do not depart from the content of the technical solution of the present invention shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A method for preparing a Co3O4 / g-C3N4 heterojunction catalyst, characterized in that: The heterojunction catalyst is composed of a composite of hydrothermally synthesized Co3O4 nanospheres and g-C3N4 nanosheets; The preparation method of the Co3O4 / g-C3N4 heterojunction catalyst comprises the following steps: S1: Weigh dicyandiamide, heat to 500-550°C at a rate of 2.0-2.5°C / min, and calcine for 2-4 hours to obtain bulk-g-C3N4; place the obtained bulk-g-C3N4 in a hydrothermal reactor, add deionized water and mix, seal the obtained mixed solution, place it in an oven for reaction, and dry it to obtain g-C3N4 nanosheets; S2 disperses Co(NO3)3·6H2O in methanol to form solution A with a concentration of 0.025-0.035 g / mL; S3 disperses 2-methylimidazole in methanol to form a solution B with a concentration of 0.09 to 0.10 g / mL; S4: Mix the above solution A and solution B, stir at 2000 rpm for 3-4 minutes, let the resulting mixture stand at room temperature for 12 hours, centrifuge to obtain a precipitate, wash the precipitate with methanol, dry it at 80°C for 10 hours, and then calcine to obtain Co3O4 nanospheres; S5: adding the g-C3N4 nanosheets obtained in step S1 to methanol, ultrasonically dispersing for 25 to 35 minutes, to obtain a solution with a concentration of 0.1 to 0.8 g / mL; adding the Co3O4 nanospheres obtained in step S4 to the above solution and magnetically stirring in a fume hood for 12 to 24 hours; controlling the mass ratio of g-C3N4 nanosheets:Co3O4 nanospheres to be 1 to 2:20; drying at 100° C. and calcining to obtain a Co3O4 / g-C3N4 heterojunction catalyst.

2. The preparation method according to claim 1, characterized in that The concentration of the mixed solution in step S1 is 0.80-1.80 g / 100 mL.

3. The preparation method according to claim 1, characterized in that In step S1, the reaction temperature in the oven is 200° C. to 210° C., and the reaction time is 7 to 8 hours; the drying temperature is 60° C. to 65° C., and the drying time is 4 to 5 hours.

4. The preparation method according to claim 1, characterized in that The calcination temperature of step S4 is 300° C. to 400° C., the heating rate is 1° C. / min, and the calcination time is 2 to 3 hours.

5. The preparation method according to claim 1, characterized in that The calcination temperature of step S5 is 300° C. to 400° C., the calcination time is 3 to 5 hours, and the heating rate is 2.0 to 2.5° C. / min.

6. Use of the Co3O4 / g-C3N4 heterojunction catalyst prepared according to the method of claim 1 in the degradation of waste refrigerants.

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