A Ti3C2 / g-C3N4 composite photocatalyst and its preparation method and application

Abstract

The present invention belongs to the field of photocatalysis technology and discloses a Ti3C2 / g-C3N4 composite photocatalyst, its preparation method and application. The composite photocatalyst is based on a single layer or a few layers of Ti3C2 nanosheets, and g-C3N4 is grown on the Ti3C2 nanosheets to form a 2D / 2D heterostructure. (1) Ti3C2 powder and a nitrogen-rich precursor are dispersed in water and uniformly dispersed by ultrasonication under the protection of an inert gas; (2) the mixed solution obtained in step (1) is freeze-dried and then ground, and then calcined at 450-600°C for 1-4 hours to obtain a Ti3C2 / g-C3N4 composite photocatalyst; wherein the calcination gas atmosphere is a nitrogen-hydrogen mixture or an argon-hydrogen mixture. The Ti3C2 / g-C3N4 composite photocatalyst is used in the photocatalytic selective oxidation of 5-hydroxymethylfurfural to prepare 2,5-diformylfuran and the synergistic hydrogen production. The catalyst of the present invention has a stable structure and high photocatalytic activity, can be used as a bifunctional photocatalyst to simultaneously achieve hydrogen production and organic conversion, and can realize automatic separation of liquid products and gaseous products, which is conducive to practical production applications.

Description

Technical Field

[0001] The present invention belongs to the technical field of photocatalysis, and specifically relates to a Ti3C2 / g-C3N4 composite photocatalyst and a preparation method and application thereof. Background Art

[0002] Hydrogen (H2), as a clean, renewable energy source, plays a vital role in sustainable development and is one of the most promising candidates to replace fossil fuels. Because the slow kinetics of the water oxidation half-reaction severely inhibit the overall water splitting efficiency, various hole sacrificial agents (such as triethanolamine, ascorbic acid, and lactic acid) are often added to enhance photocatalytic hydrogen evolution activity. Unfortunately, this approach wastes hole energy and produces oxidation products that are uneconomical and even environmentally unfriendly.

[0003] Bioalcohols, as natural and renewable carbon resources, are a promising approach to achieving carbon neutrality through catalytic conversion of these compounds into high-value-added chemicals. 5-Hydroxymethylfurfural (HMF), a renewable C6 biomass platform molecule, contains two distinct functional groups (aldehyde and hydroxyl groups) that readily oxidize to form a variety of value-added derivatives, such as 2,5-diformylfuran (DFF), 2,5-difurandicarboxylic acid (FDCA), 5-formyl-furancarboxylic acid (FFCA), and 5-hydroxymethyl-2-furancarboxylic acid (HMFCA). This has attracted extensive research attention worldwide in recent years. DFF is considered an important intermediate monomer for the synthesis of furan-based biopolymers, pharmaceuticals, and fungicides. Traditional oxidation methods for DFF require not only precious metal catalysts (Pd, Ru, Pt, etc.), but also high reaction temperatures (220°C) and pressures (2 MPa of oxygen), resulting in significant energy consumption and complex, expensive production equipment. Therefore, the selective oxidation of HMF to DFF using photocatalytic methods is of great significance. Currently, photoexcited holes and / or reactive oxygen species (e.g., •OH, •O 2- , 1 O2) has been reported as an oxidant that promotes alcohol oxidation, but reactive oxygen species can further oxidize or even mineralize the products, thereby reducing product selectivity and yield. Therefore, it is highly desirable to develop an efficient bifunctional photocatalyst that can fully utilize photogenerated electrons and holes to simultaneously produce clean hydrogen fuel and high-value-added organic chemicals. Summary of the Invention

[0004] In order to solve the waste of hole sacrificial agent in photocatalytic hydrogen evolution and the harsh reaction conditions in traditional catalytic HMF conversion to prepare DFF, the purpose of the present invention is to provide a Ti3C2 / g-C3N4 composite photocatalyst and its preparation method and application.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A Ti3C2 / g-C3N4 composite photocatalyst, wherein the composite photocatalyst is based on a single layer or a few layers (2-3 layers) of Ti3C2 nanosheets, and g-C3N4 is grown on the Ti3C2 nanosheets to form a 2D / 2D heterostructure; wherein the mass proportion of Ti3C2 in the composite photocatalyst is 1.5-9.0%, more preferably 5.0%.

[0007] The preparation method of the Ti3C2 / g-C3N4 composite photocatalyst comprises the following steps:

[0008] (1) Disperse Ti3C2 powder and nitrogen-rich precursor in water and disperse them evenly under ultrasonication under inert gas protection; wherein, Ti3C2 powder: nitrogen-rich precursor: water = (0-200) mg: 30 g: (15-50) mL, and the amount of Ti3C2 powder is not 0;

[0009] (2) The mixed solution obtained in step (1) is freeze-dried and then ground, and then calcined at 450-600°C for 1-4 hours to obtain a Ti3C2 / g-C3N4 composite photocatalyst; wherein the calcination gas atmosphere is a nitrogen-hydrogen mixture or an argon-hydrogen mixture.

[0010] Preferably, in step (1), the inert gas is nitrogen or argon.

[0011] Preferably, in step (1), ultrasonic dispersion is performed for 10 to 60 minutes.

[0012] Preferably, in step (2), the volume proportion of hydrogen in the nitrogen-hydrogen mixed gas or the argon-hydrogen mixed gas is 10-20%.

[0013] Preferably, in step (2), the temperature is raised to the calcination temperature at a heating rate of 1-20°C / min.

[0014] Preferably, in step (1), the nitrogen-rich precursor is one or more of urea, melamine, dicyandiamide, monocyanamide, and thiourea.

[0015] Preferably, in step (1), Ti3C2 powder is prepared by the following method:

[0016] (a) Add commercial Ti3AlC2 to an acid solution and continuously stir in a water bath at 25-60°C for 24-48 hours for acid etching; wherein Ti3AlC2: acid solution = (0.1-2) g: (5-30) mL; the acid solution is HF or a HCl-LiF mixed solution, the HF concentration is 35-60 wt%, and the HCl-LiF mixed solution is prepared by dissolving LiF in a 6-12 M HCl solution, and the LiF concentration in the mixed solution is 2-10 M;

[0017] (b) The suspension obtained after the acid etching treatment in step (a) is centrifuged, washed, and dried to obtain Ti3C2 powder.

[0018] The Ti3C2 / g-C3N4 composite photocatalyst is used in the photocatalytic selective oxidation of 5-hydroxymethylfurfural (HMF) to prepare 2,5-diformylfuran (DFF) and synergistic hydrogen production.

[0019] Preferably, the Ti3C2 / g-C3N4 composite photocatalyst and 5-hydroxymethylfurfural are uniformly dispersed in a solvent, and a xenon lamp is used as a light source to initiate a photocatalytic reaction in an oxygen-free atmosphere; wherein the Ti3C2 / g-C3N4 composite photocatalyst: 5-hydroxymethylfurfural: solvent = (5-25) mg: (0.1-2) mmol: 100 mL; and the solvent is deionized water, acetonitrile, tetrahydrofuran, n-hexane, or toluene.

[0020] Preferably, the oxygen-free atmosphere is a vacuum atmosphere or an inert atmosphere.

[0021] Beneficial effects:

[0022] (1) The present invention uses a simple and rapid method called thermal polymerization to prepare an efficient and stable Ti3C2 / g-C3N4 composite system. This system effectively improves the efficiency of the photocatalyst in the photocatalytic HMF selective oxidation and hydrogen evolution reaction, providing a new solution for the early realization of industrial production of new energy;

[0023] (2) The present invention constructs an efficient Ti3C2 / g-C3N4 composite photocatalyst for rapid separation and transfer of photogenerated carriers by precisely controlling the reactant components (the type of nitrogen-rich precursor and the ratio of the nitrogen-rich precursor to Ti3C2) and reaction conditions (calcination atmosphere, calcination temperature, and calcination time), so that photogenerated electrons and holes can be effectively utilized at the same time to obtain hydrogen energy and high-value-added organic chemicals. Among them, g-C3N4 has good optical properties and defect controllability, and is resistant to acid, alkali, and light corrosion. Therefore, the present invention is a very meaningful technology;

[0024] (3) In the preparation method of the present invention, a simple calcination treatment is directly performed on the mixture of Ti3C2 and nitrogen-rich precursor to form a Ti3C2 / g-C3N4 catalyst, wherein Ti3C2 serves as a substrate for the growth of g-C3N4. The ammonia formed during the formation of g-C3N4 can peel off the layered Ti3C2 into a single layer or a few layers. In addition, the type and number of defects in g-C3N4 can be regulated by simply changing the reaction atmosphere (nitrogen-hydrogen mixture or argon-hydrogen mixture), thereby promoting the adsorption and activation of reactants in the catalytic reaction. The operation process is simple and has low energy consumption, and does not cause pollution to the environment, meets environmentally friendly requirements, and is suitable for large-scale production.

[0025] (4) The photocatalytic HMF oxidation in the present invention does not require precious metal catalysts, nor does it require harsh oxidation reaction conditions such as high temperature and high pressure. The required equipment cost is low, and the reaction process does not cause pollution and is environmentally friendly. In addition, clean energy hydrogen can be obtained at the same time, which can provide a reliable technical means for the efficient conversion of biomass resources.

[0026] (5) The catalyst of the present invention has a stable structure and high photocatalytic activity. It can be used as a bifunctional photocatalyst to simultaneously achieve hydrogen production and organic conversion, and realize automatic separation of liquid products and gas products, which is conducive to practical production applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 : Scanning electron microscope image of Ti3C2 prepared in Example 1.

[0028] Figure 2 : Scanning electron microscope image of g-C3N4 prepared in Control Example 1.

[0029] Figure 3 : X-ray photoelectron spectrum of the product prepared in Example 3.

[0030] Figure 4 : Thermogravimetric analysis of the product prepared in Example 1.

[0031] Figure 5: Scanning electron microscopy (ae) and transmission electron microscopy (f, g) of the products Ti3C2(x%) / g-C3N4 prepared in Examples 1 to 5, wherein (a) is a scanning electron microscopy of Ti3C2(1.5%) / g-C3N4, (b) is a scanning electron microscopy of Ti3C2(2.5%) / g-C3N4, (c) is a scanning electron microscopy of Ti3C2(5.0%) / g-C3N4, (d) is a scanning electron microscopy of Ti3C2(7.5%) / g-C3N4, (e) is a scanning electron microscopy of Ti3C2(9.0%) / g-C3N4, (f, g) are transmission electron microscopy images of Ti3C2(5.0%) / g-C3N4, and (g1, g2) are enlarged views of the areas shown in Figure 1 and 2 in Figure g.

[0032] Figure 6 :The product Ti3C2( x %) / g-C3N4 X-ray diffraction (XRD) pattern.

[0033] Figure 7 : Cyclic stability characterization diagram of the product Ti3C2(5.0%) / g-C3N4 prepared in Example 3. DETAILED DESCRIPTION

[0034] To make the present invention clearer and more specific, the present invention is further described in detail below. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0035] Example 1

[0036] A method for preparing a Ti3C2 / g-C3N4 composite photocatalyst comprises the following steps:

[0037] (1) Add 0.5 g of commercial Ti3AlC2 to 15 mL of hydrofluoric acid (concentration 40 wt%) and stir continuously in a 30 °C water bath for 48 h for acid etching;

[0038] (2) The suspension obtained after the acid etching treatment in step (1) is centrifuged, washed with deionized water until the pH is neutral, and then vacuum dried to obtain a layered Ti3C2 powder;

[0039] (3) Disperse 15 mg of Ti3C2 powder obtained in step (2) and 30 g of urea in 20 mL of deionized water and ultrasonically treat for 30 min under nitrogen protection;

[0040] (4) The mixed solution obtained in step (3) was freeze-dried for 24 h and then ground. It was then heated to 550 °C in a H2 / N2 mixed gas (hydrogen volume accounted for 10%) at a heating rate of 10 °C / min and calcined for 2 h to obtain the product.

[0041] Example 2

[0042] A method for preparing a Ti3C2 / g-C3N4 composite photocatalyst comprises the following steps:

[0043] Steps (1) and (2) are the same as in Example 1;

[0044] (3) Disperse 20 mg of Ti3C2 powder obtained in step (2) and 30 g of urea in 20 mL of deionized water and ultrasonically treat for 30 min under nitrogen gas protection;

[0045] (4) The mixed solution obtained in step (3) was freeze-dried for 24 h and then ground. It was then heated to 550 °C in a H2 / N2 mixed gas (hydrogen volume accounted for 10%) at a heating rate of 10 °C / min and calcined for 2 h to obtain the product.

[0046] Example 3

[0047] A method for preparing a Ti3C2 / g-C3N4 composite photocatalyst comprises the following steps:

[0048] Steps (1) and (2) are the same as in Example 1;

[0049] (3) Disperse 40 mg of Ti3C2 powder obtained in step (2) and 30 g of urea in 20 mL of deionized water and ultrasonically treat for 30 min under nitrogen gas protection;

[0050] (4) The mixed solution obtained in step (3) was freeze-dried for 24 h and then ground. It was then heated to 550 °C in a H2 / N2 mixed gas (hydrogen volume accounted for 10%) at a heating rate of 10 °C / min and calcined for 2 h to obtain the product.

[0051] Example 4

[0052] A method for preparing a Ti3C2 / g-C3N4 composite photocatalyst comprises the following steps:

[0053] Steps (1) and (2) are the same as in Example 1;

[0054] (3) Disperse 50 mg of Ti3C2 powder obtained in step (2) and 30 g of urea in 20 mL of deionized water and ultrasonically treat for 30 min under nitrogen gas protection;

[0055] (4) The mixed solution obtained in step (3) was freeze-dried for 24 h and then ground. It was then heated to 550 °C in a H2 / N2 mixed gas (hydrogen volume accounted for 10%) at a heating rate of 10 °C / min and calcined for 2 h to obtain the product.

[0056] Example 5

[0057] A method for preparing a Ti3C2 / g-C3N4 composite photocatalyst comprises the following steps:

[0058] Steps (1) and (2) are the same as in Example 1;

[0059] (3) Disperse 60 mg of Ti3C2 powder obtained in step (2) and 30 g of urea in 20 mL of deionized water and ultrasonically treat for 30 min under nitrogen gas protection;

[0060] (4) The mixed solution obtained in step (3) was freeze-dried for 24 h and then ground. It was then heated to 550 °C in a H2 / N2 mixed gas (hydrogen volume accounted for 10%) at a heating rate of 10 °C / min and calcined for 2 h to obtain the product.

[0061] Comparative Example 1

[0062] g-C3N4 catalyst: 10 g of urea was added to a magnetic boat with a lid, which was then placed in a tubular furnace. The mixture was then heated to 550°C in a H2 / N2 mixture (hydrogen accounted for 10% by volume) at a heating rate of 10°C / min and calcined for 2 h to obtain g-C3N4.

[0063] Product Characterization

[0064] Figure 1 This is a scanning electron microscope image of Ti3C2 prepared in Example 1. As can be seen from the figure: Ti3C2 presents a loose "accordion"-like multilayer nanostructure with a lateral size of several microns.

[0065] Figure 2 This is a scanning electron microscope image of g-C3N4 prepared in Control Example 1. As can be seen from the figure, the original g-C3N4 exhibits typical two-dimensional thin layer characteristics and has some visible porous structures.

[0066] Figure 3 This is the X-ray photoelectron spectrum of the product prepared in Example 3. The figure shows the characteristic peaks of g-C3N4 (NC=N and C-NH) and Ti3C2 (C-Ti) in the C 1s spectrum. Combined with the N 1s and Ti 2p spectra, this indicates that the prepared product is a Ti3C2 / g-C3N4 composite material.

[0067] In order to test the specific content of Ti3C2 in the product, two methods are used: (1) Weighing method: After the product is prepared, it is weighed and marked as W. Because the amount of Ti3C2 remains unchanged before and after calcination, the mass content of Ti3C2 in the product is x %=Ti3C2 dosage / W, taking the product of Example 1 as an example, W=1000mg, x %=15 / 1000=1.5%; (2) Thermogravimetric curve method: The product prepared in Example 1 was subjected to thermogravimetric analysis at 50~800℃. The results are as follows: Figure 4 As shown in the figure, it can be seen that when the temperature rises to 700℃, g-C3N4 is burned out, and only 2.1wt% TiO2 remains. Ti3C2 is completely oxidized to form TiO2. Since the molar amount of Ti atoms remains unchanged during the entire thermogravimetric analysis, it is assumed that the mass fraction of Ti3C2 is x %, is calculated using the following formula:

[0068] ;

[0069] It is known that the mass fraction of Ti3C2 in the product of Example 1 is 1.5%, which is consistent with the mass content calculated by the above-mentioned product weighing method. Therefore, the product of Example 1 is marked as Ti3C2 (1.5%) / g-C3N4.

[0070] Subsequently, the same weighing method was used to obtain the following results: the mass content of Ti3C2 in the product of Example 2 was 2.5%, and the corresponding product was marked as Ti3C2 (2.5%) / g-C3N4; the mass content of Ti3C2 in the product of Example 3 was 5.0%, and the corresponding product was marked as Ti3C2 (5.0%) / g-C3N4; the mass content of Ti3C2 in the product of Example 4 was 7.5%, and the corresponding product was marked as Ti3C2 (7.5%) / g-C3N4; the mass content of Ti3C2 in the product of Example 5 was 9.0%, and the corresponding product was marked as Ti3C2 (9.0%) / g-C3N4.

[0071] Figure 5Scanning electron microscopy (ae) and transmission electron microscopy (f, g) of the products Ti3C2(x%) / g-C3N4 prepared in Examples 1 to 5, wherein (a) is a scanning electron microscopy of Ti3C2(1.5%) / g-C3N4, (b) is a scanning electron microscopy of Ti3C2(2.5%) / g-C3N4, (c) is a scanning electron microscopy of Ti3C2(5.0%) / g-C3N4, (d) is a scanning electron microscopy of Ti3C2(7.5%) / g-C3N4, (e) is a scanning electron microscopy of Ti3C2(9.0%) / g-C3N4, (f, g) are transmission electron microscopy images of Ti3C2(5.0%) / g-C3N4, and (g1, g2) are enlarged views of the areas shown in 1 and 2 in Figure g; It can be seen from the figure that: Ti3C2( x %) / g-C3N4 both showed a typical 2D / 2D stacked flake morphology, and the surface and edge of Ti3C2 were covered with an amorphous g-C3N4 layer with a thickness of several nanometers, indicating the formation of a two-dimensional heterostructure.

[0072] Figure 6 The product Ti3C2( x %) / g-C3N4. As can be seen from the figure: the two peaks at 12.9° and 27.3° correspond to the (100) and (002) crystal planes of g-C3N4, respectively; the characteristic peak of Ti3C2 is difficult to see in the composite material, and only a weak characteristic peak of Ti3C2 (002) crystal plane appears in the XRD spectrum of Ti3C2 (9.0%) / g-C3N4. This may be due to the low content of Ti3C2 and its good dispersion in the composite material, which further confirms the successful synthesis of the composite material.

[0073] Application Example 1

[0074] 25 mg of Ti3C2 (prepared in Examples 1-5) x %) / g-C3N4 composite photocatalyst, g-C3N4 prepared in Control Example 1, and 0.2mmol HMF were added to 100mL deionized water and ultrasonically dispersed for 10min. The mixture was poured into a 250mL Beijing Perfect Photocatalytic Reactor. After vacuuming to remove the air, the reaction solution was irradiated with a full-spectrum light source provided by a 300W xenon lamp for 0.5h. The liquid and gas samples were sampled at regular intervals and quantitatively analyzed by high performance liquid chromatography and gas chromatography, respectively.

[0075] The test results are shown in Table 1. It can be seen that after 4 hours of light exposure, the conversion rate of HMF of g-C3N4 alone is only 11.4%. Under the same conditions, the conversion rate of HMF of Ti3C2( xThe photocatalytic hydrogen production and HMF oxidation activities of the 3.5% / g-C3N4 composite material gradually increased with the increase of Ti3C2 content. When the Ti3C2 content was 5.0%, the HMF conversion rate could reach 93.3% and the hydrogen production rate was 1.83 mmol·g -1 •h -1 ; When the Ti3C2 content increases further, the reaction activity decreases instead, which may be due to the covering effect of Ti3C2 affecting the light absorption ability of g-C3N4.

[0076]

[0077] Cyclic stability test: Taking the catalyst Ti3C2 (5.0%) / g-C3N4 prepared in Example 3 as an example, the catalyst Ti3C2 (5.0%) / g-C3N4 after the reaction in the above Application Example 1 was centrifuged with deionized water, washed several times and then dried. Then, the second cycle catalytic reaction was carried out according to the reaction conditions of Application Example 1. Five cycles were carried out continuously according to the same operating steps, each cycle lasting 5 hours, for a total of 25 hours.

[0078] Figure 7 This is the cyclic stability diagram of Ti3C2(5.0%) / g-C3N4 prepared in Example 3. As can be seen from the figure: the catalytic activity of the catalyst remains basically unchanged during five consecutive cycles, that is, the catalyst has no obvious deactivation, confirming that the catalyst has good cyclic stability.

[0079] Application control example 1

[0080] 25 mg of the Ti3C2 (5.0%) / g-C3N4 catalyst prepared in Example 3 and 0.2 mmol of HMF were added to 100 mL of deionized water, and after ultrasonic dispersion for 10 min, the mixture was poured into a 250 mL Beijing Perfil photocatalytic reactor. The reaction solution was irradiated with a 300 W xenon lamp as a full-spectrum light source for 0.5 h in an air atmosphere. Liquid and gas samples were sampled at regular intervals and quantitatively analyzed by high performance liquid chromatography and gas chromatography, respectively. The result showed that the photocatalytic hydrogen evolution rate was 1.65 mmol·g -1 •h -1 After 4 hours, the conversion rate of HMF was 90.7%, and the selectivity of DFF was 15.2%. The hydrogen evolution rate, the conversion rate of HMF, and the selectivity of DFF were significantly lower than those of the photocatalytic reaction under vacuum atmosphere in Application Example 1. The reason for this is that the presence of oxygen in the air atmosphere will generate active oxygen free radicals with stronger oxidizing ability, causing further oxidation of DFF and significantly reducing the selectivity of DFF.

Claims

1. An application of a Ti3C2 / g-C3N4 composite photocatalyst, characterized by: Used for photocatalytic selective oxidation of 5-hydroxymethylfurfural to produce 2,5-diformylfuran and synergistic hydrogen production; The Ti3C2 / g-C3N4 composite photocatalyst uses a single layer or a few layers of Ti3C2 nanosheets as a substrate, and g-C3N4 grows on the Ti3C2 nanosheets to form a 2D / 2D heterostructure. The mass proportion of Ti3C2 in the composite photocatalyst is 1.5-9.0%. The preparation steps of the Ti3C2 / g-C3N4 composite photocatalyst are as follows: (1) Disperse Ti3C2 powder and nitrogen-rich precursor in water and disperse them evenly under ultrasonication under inert gas protection; wherein, Ti3C2 powder: nitrogen-rich precursor: water = (0-200) mg: 30 g: (15-50) mL, and the amount of Ti3C2 powder is not 0; (2) The mixed solution obtained in step (1) is freeze-dried and then ground, and then calcined at 450-600°C for 1-4 hours to obtain a Ti3C2 / g-C3N4 composite photocatalyst; wherein the calcination gas atmosphere is a nitrogen-hydrogen mixture or an argon-hydrogen mixture.

2. The use of the Ti3C2 / g-C3N4 composite photocatalyst according to claim 1, characterized in that: The Ti3C2 / g-C3N4 composite photocatalyst and 5-hydroxymethylfurfural were uniformly dispersed in a solvent, and a xenon lamp was used as a light source to initiate the photocatalytic reaction in an oxygen-free atmosphere. Wherein, the Ti3C2 / g-C3N4 composite photocatalyst: 5-hydroxymethylfurfural: solvent = (5-25) mg: (0.1-2) mmol: 100 mL; the solvent is deionized water, acetonitrile, tetrahydrofuran, n-hexane or toluene.

3. The use of the Ti3C2 / g-C3N4 composite photocatalyst according to claim 1, characterized in that: In step (1), the inert gas is nitrogen or argon.

4. The use of the Ti3C2 / g-C3N4 composite photocatalyst according to claim 1, characterized in that: In step (1), ultrasonic dispersion is performed for 10 to 60 minutes.

5. The use of the Ti3C2 / g-C3N4 composite photocatalyst as claimed in claim 1, characterized in that: In step (2), the volume proportion of hydrogen in the nitrogen-hydrogen mixed gas or the argon-hydrogen mixed gas is 10-20%.

6. The use of the Ti3C2 / g-C3N4 composite photocatalyst according to claim 1, characterized in that: In step (2), the temperature is raised to the calcination temperature at a heating rate of 1-20°C / min.

7. The use of the Ti3C2 / g-C3N4 composite photocatalyst according to claim 1, characterized in that: In step (1), the nitrogen-rich precursor is one or more of urea, melamine, dicyandiamide, monocyanamide, and thiourea.

8. The use of the Ti3C2 / g-C3N4 composite photocatalyst according to claim 1, characterized in that: In step (1), Ti3C2 powder is prepared as follows: (a) Add commercial Ti3AlC2 to an acid solution and continuously stir in a water bath at 25-60 °C for 24-48 h for acid etching. Wherein, Ti3AlC2: acid solution = (0.1-2) g: (5-30) mL; the acid solution is HF or HCl-LiF mixed solution, the HF concentration is 35-60 wt%, and the HCl-LiF mixed solution is prepared by dissolving LiF in 6-12 M HCl solution, and the LiF concentration in the mixed solution is 2-10 M; (b) The suspension obtained after the acid etching treatment in step (a) is centrifuged, washed, and dried to obtain Ti3C2 powder.

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