A ternary heterojunction composite material and a preparation method and application thereof

Abstract

The application discloses a ternary heterojunction composite material and a preparation method and application thereof. The composite material comprises a transition metal oxide, a defective titanium dioxide and carbon nitride; and the defective titanium dioxide and the carbon nitride form a type II heterojunction. The in-situ synthesis method is adopted to ensure that a close interface is formed between the heterojunctions, to promote charge migration in a space charge layer, to overcome the serious carrier recombination problem of the single semiconductor of the carbon nitride or the titanium dioxide, and to greatly improve the activity and stability of the photocatalytic carbon dioxide reduction due to the enhanced light absorption performance, the improved charge separation efficiency and the increased active sites.

Description

Technical Field

[0001] This invention belongs to the field of photocatalysis technology, specifically relating to a ternary heterojunction composite material for photocatalytic carbon dioxide, its preparation method, and its application. Background Technology

[0002] In recent years, the large-scale use of fossil fuels has triggered a series of serious problems such as the energy crisis and global warming. Utilizing photocatalysis technology to convert CO2 produced from fossil fuel combustion into valuable carbon-containing chemicals is an important means of reducing carbon emissions and alleviating the energy crisis, and is one of the key pathways to achieving my country's carbon neutrality goal. Photocatalysis technology utilizes photogenerated electrons and holes generated by semiconductors under light to achieve redox reactions, effectively reducing carbon dioxide to high-value-added products such as HCOOH, CO, HCHO, CH3OH, and CH4. With the continuous deepening of photocatalysis research, various semiconductor materials such as TiO2, CdS, Fe2O3, Cu2O, and C3N4 have been successively used for photocatalytic carbon dioxide reduction. Compared to single-type semiconductors, constructing heterojunction structures using two or more semiconductor materials can effectively regulate the light absorption, charge separation, and surface reaction characteristics of the materials, which is one of the key means to improve photocatalytic efficiency. However, existing heterojunction systems suffer from structural instability, limited interfacial contact, and complex synthesis methods, requiring appropriate methods for regulation. Summary of the Invention

[0003] To overcome the problems of existing technologies, this invention provides a ternary heterojunction composite material comprising transition metal oxides, defective titanium dioxide, and carbon nitride; the defective titanium dioxide and carbon nitride constitute a type II heterojunction. Utilizing oxygen vacancies can effectively increase the free carrier density in titanium dioxide, promote photogenerated charge migration, extend carrier lifetime, and simultaneously provide more active sites for surface reactions, promoting the activation of carbon dioxide molecules. The transition metal oxides are primarily grown as co-catalysts on the semiconductor titanium dioxide and carbon nitride.

[0004] According to one embodiment of the present invention, the mass fraction of the transition metal oxide is 0.5% to 10%, the mass fraction of the defective titanium dioxide is 30% to 80%, and the mass fraction of the carbon nitride is 30% to 60%.

[0005] According to one embodiment of the present invention, the transition metal is one or more of copper, nickel, iron, zinc, and cobalt.

[0006] Another object of the present invention is to provide a method for preparing the above-mentioned composite material, comprising the following steps:

[0007] S1. Weigh the transition metal salt precursor and titanium-containing precursor, mix them with the solvent, and then centrifuge and wash the resulting sample after a solvothermal reaction.

[0008] S2. Weigh out the carbon and nitrogen-containing organic compound, mix it thoroughly with the sample obtained in step S1, and then freeze-dry it.

[0009] S3. The product obtained in step S2 is calcined and cooled to room temperature to obtain the composite material.

[0010] The technical solution of the present invention uses titanate ester as titanium source precursor, which has a higher CO2 reduction rate compared with other titanium source precursors, such as TiCl4.

[0011] According to one embodiment of the present invention, in step S1, the titanium-containing precursor is a titanate, preferably at least one of isopropyl titanate and tetrabutyl titanate.

[0012] According to one embodiment of the present invention, in step S1, the solvent is an alcohol, preferably one or more of ethanol, glycerol, and isopropanol; more preferably a mixture of ethanol and glycerol.

[0013] The solution system is an alcohol solution, which can effectively control the alcoholysis rate of titanium-containing precursors, making their growth more uniform.

[0014] According to one embodiment of the present invention, in step S1, the solvothermal reaction temperature is 150°C-200°C and the time is 24 hours-72 hours.

[0015] According to one embodiment of the present invention, in step S1, the transition metal salt is at least one of the following: hydrochloride, nitrate, acetate, and sulfate of the corresponding metal.

[0016] According to one embodiment of the present invention, in step S2, the carbon-nitrogen-containing organic compound is at least one of melamine, dicyandiamide, and urea.

[0017] According to one embodiment of the present invention, in step S3, the calcination temperature is 520℃-600℃, and the calcination time is 2 hours-4 hours.

[0018] According to one embodiment of the present invention, in step S3, the calcination atmosphere is an inert gas atmosphere, preferably argon.

[0019] Calcination in an inert gas atmosphere can preserve the oxygen defects in the catalyst, further improving the CO generation rate and selectivity in photocatalytic CO2 reduction.

[0020] Another object of the present invention is to provide the application of the above-described composite material or the composite material prepared by the above-described preparation method in photocatalytic carbon dioxide reduction.

[0021] According to one embodiment of the present invention, the step of photocatalytic CO2 reduction includes: adding the photocatalyst to a photocatalytic reaction system containing carbon dioxide and water.

[0022] Beneficial effects:

[0023] Compared with the prior art, the advantages of the present invention are as follows:

[0024] 1. Through a hydrothermal reaction between a divalent transition metal and a tetravalent titanium-containing precursor, the formation of bulk oxygen vacancies in titanium dioxide is induced by utilizing the charge difference and subsequent calcination in an inert gas atmosphere. Simultaneously, surface oxygen vacancies can be generated in situ via photoreduction during the photocatalytic reaction. Utilizing these oxygen vacancies can effectively increase the free carrier density in titanium dioxide, promote photogenerated charge migration, prolong carrier lifetime, and provide more active sites for surface reactions, thus promoting the activation of carbon dioxide molecules.

[0025] 2. The introduction of transition metals in this invention has two advantages: first, it utilizes transition metal atoms to induce the formation of oxygen vacancies during the hydrothermal process; second, it utilizes transition metals as active sites to activate water molecules to generate protons, promote the hydrogenation of carbon dioxide, and improve reaction efficiency.

[0026] 3. The present invention uses an in-situ synthesis method to ensure the formation of a tight interface between heterojunctions and promote charge migration within the space charge layer. This can overcome the problem of severe carrier recombination in carbon nitride or titanium dioxide single semiconductors. At the same time, due to the enhanced light absorption performance, improved charge separation efficiency, and increased active sites, the activity and stability of photocatalytic carbon dioxide reduction can be greatly improved.

[0027] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0028] Appendix Figure 1 This is a transmission electron microscope (TEM) image of the ternary heterojunction material prepared in Example 1.

[0029] Appendix Figure 2 This is the electron paramagnetic resonance (EPR) spectrum of the ternary heterojunction material prepared in Example 1.

[0030] Appendix Figure 3 This is the UV-Vis absorption spectrum (UV-visDRS) of the ternary heterojunction material prepared in Example 1.

[0031] Appendix Figure 4 This is a test diagram of the carbon dioxide photocatalytic reduction performance of ternary heterojunction materials. Detailed Implementation

[0032] The technical solution of the present invention will be further described below with reference to specific embodiments. The scope of protection of the present invention is not limited to the following embodiments; these embodiments are listed for illustrative purposes only and do not limit the present invention in any way.

[0033] In the context of this specification, except where expressly stated, any matters or issues not mentioned herein are directly applicable to matters known in the art without modification. Furthermore, any embodiments described herein can be freely combined with one or more other embodiments described herein, and any resulting technical solutions or concepts shall be considered part of the original disclosure or original record of this invention, and should not be regarded as new content not disclosed or anticipated herein, unless those skilled in the art consider such combination to be clearly unreasonable.

[0034] Photocatalytic carbon dioxide reduction applications

[0035] The obtained catalyst was used in a simulated photocatalytic reduction of CO2 under sunlight. Activity testing was conducted using an online vacuum photocatalytic reactor with a 300W xenon lamp as the light source. The reaction conditions were: 0.1g catalyst, ml water, and high-purity CO2 gas introduced after three vacuum cycles to achieve a pressure of 0.08MPa inside the reactor. Samples were taken every hour after the reaction began, and the products were analyzed using a gas chromatograph equipped with a flame ionization detector and a thermal conductivity detector. Quantification was performed using a pre-determined standard curve. The reaction produced three products: CH4, CO, and a trace amount of H2 as a byproduct.

[0036] Example 1:

[0037] 2g of tetrabutyl titanate precursor, 0.2g of nickel nitrate, 50mL of ethanol, and 30mL of glycerol were weighed and thoroughly mixed, then transferred to a polytetrafluoroethylene-lined hydrothermal reactor. The mixture was hydrothermally reacted at 200℃ for 24 hours. After cooling in the furnace, the resulting product was thoroughly washed with water and ethanol to obtain the titanium hydroxyl precursor. 2g of melamine was weighed and thoroughly mixed with the titanium hydroxyl precursor, freeze-dried at -30℃, calcined at 550℃ for 4 hours in an argon atmosphere, and cooled to room temperature to obtain the ternary heterojunction material. (Appendix) Figure 1 TEM images of the sample show that titanium dioxide and carbon nitride are in close contact, forming a heterogeneous interface. (Attached) Figure 2 The EPR spectrum of the sample is shown below; the signal at g = 2.003 confirms the presence of oxygen vacancies. (Attached) Figure 3 The corresponding UV-Vis absorption spectrum shows that the ternary heterojunction material exhibits excellent light absorption performance. (See attached image.) Figure 4 As shown, the sample in Example 1 exhibited excellent photocatalytic carbon dioxide reduction activity, with corresponding CO and CH4 generation rates of 500.6 μmol g⁻¹. -1 h -1and 69.4 μmol g -1 h -1 .

[0038] Example 2:

[0039] 2g of tetrabutyl titanate precursor, 0.3g of cobalt nitrate, 50mL of ethanol, and 30mL of glycerol were weighed and thoroughly mixed, then transferred to a polytetrafluoroethylene-lined hydrothermal reactor. The mixture was hydrothermally reacted at 200°C for 24 hours. After cooling in the furnace, the resulting product was thoroughly washed with water and ethanol to obtain the titanium hydroxyl precursor. 4g of melamine was weighed and thoroughly mixed with the titanium hydroxyl precursor, freeze-dried at -30°C, calcined at 550°C for 4 hours in an argon atmosphere, and cooled to room temperature to obtain the ternary heterojunction material. Photocatalytic activity testing showed that the CO and CH4 generation rates in this example were 897.1 μmol g⁻¹. -1 h -1 and 120.6 μmol g -1 h -1 .

[0040] Example 3:

[0041] 2g of tetrabutyl titanate precursor, 0.2g of copper nitrate, 50mL of ethanol, and 30mL of glycerol were weighed and thoroughly mixed, then transferred to a polytetrafluoroethylene-lined hydrothermal reactor. The mixture was hydrothermally reacted at 200℃ for 24 hours. After cooling with the furnace, the resulting product was thoroughly washed with water and ethanol to obtain the titanium hydroxyl precursor. 6g of melamine was weighed and thoroughly mixed with the titanium hydroxyl precursor, freeze-dried at -30℃, calcined at 550℃ for 4 hours in an argon atmosphere, and cooled to room temperature to obtain the ternary heterojunction material. Photocatalytic activity testing showed that the CO and CH4 generation rates in this example were 1024.5 μmol g⁻¹. -1 h -1 and 199.4 μmol g -1 h -1 .

[0042] Example 4:

[0043] 2g of isopropyl titanate precursor, 0.2g of ferric nitrate, 30mL of ethanol, and 40mL of glycerol were weighed and thoroughly mixed, then transferred to a polytetrafluoroethylene-lined hydrothermal reactor. The mixture was hydrothermally reacted at 200°C for 24 hours. After cooling with the furnace, the resulting product was thoroughly washed with water and ethanol to obtain the titanium hydroxyl precursor. 4g of melamine was weighed and thoroughly mixed with the titanium hydroxyl precursor, freeze-dried at -30°C, calcined at 600°C for 4 hours in an argon atmosphere, and cooled to room temperature to obtain the ternary heterojunction material. Photocatalytic activity testing showed that the CO and CH4 generation rates in this example were 1346.2 μmol g⁻¹. -1 h -1 and 241.8 μmol g-1 h -1 .

[0044] Example 5:

[0045] 2g of isopropyl titanate precursor, 0.4g of copper acetate, 50mL of ethanol, and 30mL of glycerol were weighed and thoroughly mixed, then transferred to a polytetrafluoroethylene-lined hydrothermal reactor. The mixture was hydrothermally reacted at 180°C for 24 hours. After cooling in the furnace, the resulting product was thoroughly washed with water and ethanol to obtain the hydroxyl titanium precursor. 6g of melamine was weighed and thoroughly mixed with the hydroxyl titanium precursor, freeze-dried at -30°C, calcined at 550°C for 4 hours in an argon atmosphere, and cooled to room temperature to obtain the ternary heterojunction material. Photocatalytic activity testing showed that the CO and CH4 generation rates in this example were 1293.0 μmol g⁻¹. -1 h -1 and 225.3 μmol g -1 h -1 .

[0046] Example 6:

[0047] 2g of tetrabutyl titanate precursor, 0.2g of nickel acetate, 60mL of ethanol, and 20mL of glycerol were weighed and thoroughly mixed, then transferred to a polytetrafluoroethylene-lined hydrothermal reactor. The mixture was hydrothermally reacted at 180°C for 48 hours. After cooling with the furnace, the resulting product was thoroughly washed with water and ethanol to obtain the titanium hydroxyl precursor. 2g of melamine was weighed and thoroughly mixed with the titanium hydroxyl precursor, freeze-dried at -30°C, calcined at 550°C for 4 hours in an argon atmosphere, and cooled to room temperature to obtain the ternary heterojunction material. Photocatalytic activity testing showed that the CO and CH4 generation rates in this example were 612.3 μmol g⁻¹. -1 h -1 and 94.7 μmol g -1 h -1 .

[0048] Furthermore, comparative experiments demonstrated that the synergistic effect of titanium dioxide and carbon nitride plays an important role in improving its reactivity, and also confirmed that the presence of oxygen vacancies is one of the keys to achieving efficient photocatalytic carbon dioxide reduction.

[0049] Comparative Example 1:

[0050] 2g of tetrabutyl titanate, 0.2g of nickel acetate, 60mL of ethanol, and 20mL of glycerol were weighed and thoroughly mixed, then transferred to a polytetrafluoroethylene-lined hydrothermal reactor. The mixture was hydrothermally reacted at 200℃ for 24 hours. After cooling with the furnace, the resulting product was thoroughly washed with water and ethanol to obtain the hydroxyl titanium precursor. After drying, the sample was calcined at 550℃ for 4 hours in an argon atmosphere. After cooling to room temperature, Comparative Example 1 was obtained. Photocatalytic activity testing showed that the corresponding CO and CH4 formation rates for this comparative example were 210.4 μmol g⁻¹. -1 h -1 and 13.1 μmol g -1 h -1 .

[0051] Comparative Example 2:

[0052] 2g of tetrabutyl titanate, 0.2g of copper acetate, 60mL of ethanol, and 20mL of glycerol were weighed and thoroughly mixed, then transferred to a polytetrafluoroethylene-lined hydrothermal reactor. The mixture was hydrothermally reacted at 200℃ for 24 hours. After cooling with the furnace, the resulting product was thoroughly washed with water and ethanol to obtain a hydroxyl titanium precursor. 2g of melamine was weighed and thoroughly mixed with the hydroxyl titanium precursor, freeze-dried at -30℃, and calcined at 550℃ in air for 4 hours (to remove oxygen defects) at a heating rate of 2℃ / min. After cooling to room temperature, the ternary heterojunction material was obtained. Photocatalytic activity testing showed that the corresponding CO and CH4 generation rates in this comparative example were 277.7 μmol / g. -1 h -1 and 39.9 μmol g -1 h -1 .

[0053] Comparative Example 3:

[0054] Other conditions were the same as in Example 1, except that the solvent was replaced with water. Photocatalytic activity tests showed that the corresponding CO and CH4 generation rates in this comparative example were 123.8 μmol g⁻¹. -1 h -1 and 41.0 μmol g -1 h -1 .

[0055] Comparative Example 4:

[0056] Other conditions were the same as in Example 1, except that the titanium source was replaced with TiCl4. Photocatalytic activity tests showed that the corresponding CO and CH4 generation rates in this comparative example were 141.9 μmol g⁻¹. -1 h -1 and 41.5 μmol g -1 h -1 .

[0057] Table 1 Summary of Results

[0058]

[0059] As can be seen from the data in Table 1, the present invention uses ternary heterojunction materials, calcines under an inert atmosphere to retain oxygen defects, and uses alcohol solvent and titanate ester as titanium source, which can effectively improve the generation rate of CO and CH4 in the photocatalytic CO2 reduction process.

Claims

1. The application of a ternary heterojunction composite material in the photocatalytic reduction of carbon dioxide to CO and CH4, characterized in that, The composite material comprises transition metal oxides, defective titanium dioxide, and carbon nitride; the defective titanium dioxide and carbon nitride form a type II heterojunction. The preparation method of the ternary heterojunction composite material specifically includes the following steps: S1. Weigh the transition metal salt precursor and titanium-containing precursor, mix them with the solvent, and then centrifuge and wash the resulting sample after a solvothermal reaction. S2. Weigh out the carbon and nitrogen-containing organic compound, mix it thoroughly with the sample obtained in step S1, and then freeze-dry it. S3. The product obtained in step S2 is subjected to calcination in an inert gas atmosphere, and the composite material is obtained after cooling to room temperature. The titanium-containing precursor is at least one of isopropyl titanate and tetrabutyl titanate; the solvent is a mixture of ethanol and glycerol.

2. The application according to claim 1, characterized in that, The transition metal is one or more of copper, nickel, iron, zinc, and cobalt.

3. The application according to claim 1, characterized in that, In step S1, the solvothermal reaction temperature is 150℃-200℃, and the time is 24 hours-72 hours.

4. The application according to claim 1, characterized in that, In step S2, the carbon-nitrogen organic compound is at least one of melamine, dicyandiamide, and urea.

5. The application according to claim 1, characterized in that, In step S3, the calcination temperature is 520℃-600℃, and the calcination time is 2 hours-4 hours.

6. The application according to claim 1, characterized in that, The inert gas is argon.

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