Preparation method of monatomic catalyst as well as product and application of monatomic catalyst

Abstract

The invention relates to the technical field of monatomic catalysts, in particular to a preparation method of a monatomic catalyst and a product and application of the monatomic catalyst, the preparation method comprises the following steps: S1, performing first carbonization treatment, acid pickling and water washing on a carbon precursor to obtain a carbon shell substrate; s2, carrying out mixed reaction on the carbon shell substrate obtained in the step S1 and a nitrogen precursor, and then carrying out secondary carbonization treatment to obtain a nitrogen-doped carbon shell substrate; and S3, mixing the nitrogen-doped carbon shell substrate obtained in the step S2 with an acidic metal salt, and carrying out a hydrothermal reaction at 50-100 DEG C to obtain the monatomic catalyst. According to the preparation method, the formation of nano-particle impurities is reduced from multiple aspects, the problems of subsequent repeated high-temperature calcination and acid pickling are avoided, the steps are simple and convenient, the conditions are mild and easy to control, efficient preparation can be achieved, and the obtained monatomic catalyst is more excellent in performance.

Description

Technical Field

[0001] The present invention relates to the technical field of single-atom catalysts, and in particular to a preparation method of a single-atom catalyst and a product and application thereof. Background Art

[0002] With the rapid development of human society in modern times, the amount of CO in the atmosphere has 2 As the main greenhouse gas, it is very harmful to human body and ecological environment. Energy crisis and environmental problems are becoming more and more prominent. 2 The conversion and utilization of CO is a hot topic in scientific research all over the world. 2 Molecular production of fuel (CO 2 RR), achieving clean energy conversion and CO 2 The dual path of emission reduction is a requirement for developing a low-carbon circular economy. 2 It is difficult to achieve efficient electrocatalytic reduction, so the preparation of highly active electrocatalytic materials is CO 2 The pain points of catalysis need to be solved urgently. The application of efficient single-atom catalysts is undoubtedly the key to achieve electrocatalytic reduction of CO 2 The key to the application.

[0003] Single-atom catalysts are highly efficient catalysts with a single active center that can achieve precise control of reactions. They are widely used in chemical reactions in various fields. 2 The application of single atom catalysts in CO 2 It is widely used in catalytic hydrogenation, electrochemical reduction, photocatalytic reduction, etc. 2 The application of single-atom catalysts with noble metal atoms (Ag, Pt) as catalytic active sites in electrochemical reduction has shown its potential in CO 2 Excellent catalytic performance in RR, such as the solutions disclosed in patents CN202310365475.X and CN202211485485.9, are catalysts with precious metal atoms as catalytic active sites.

[0004] However, due to the high cost of precious metal catalysts, in recent years, the emergence of heteroatom (nitrogen, sulfur, etc.) doped carbon-loaded transition metal (iron, cobalt, nickel, etc.) catalysts with metal oxides, metal carbides or metal nitride particles dispersed on the surface of carbon-based materials as catalytic active sites is expected to replace precious metal catalysts with the advantages of higher economic benefits and environmental protection. Therefore, this type of catalyst has also been widely used in CO 2 In RR.

[0005] CO 2The preparation method of the RR catalyst directly affects its performance and cost. Therefore, developing a suitable preparation method is of great significance for the development of single-atom catalysts. Currently, the common preparation methods mainly include high-temperature reduction method, sol-gel method, and traditional hydrothermal method: 1) High-temperature reduction method, as disclosed in CN202310571188.4, which is simple to prepare and suitable for large-scale production, but requires a large amount of energy and has a high cost; 2) Sol-gel method, as disclosed in CN202210964899.3, which uses simple equipment and the obtained product has good chemical uniformity, but there are problems such as complex gel preparation steps and easy agglomeration; 3) Traditional hydrothermal method, as disclosed in CN202310647813.9, this method has a low cost, but has strict requirements for temperature control. Excessive temperature is likely to cause single atoms to agglomerate and form nanoparticle impurities, affecting the purity of the catalyst and CO 2 RR performance, and the target product cannot be generated in one step. Finally, repeated high-temperature calcination and pickling are required, making the preparation conditions harsh and the obtained product doped with more nanoparticle impurities, greatly reducing the preparation efficiency and performance of the prepared single-atom catalyst.

[0006] In view of this, it is indeed necessary to provide a technical solution to solve the above problems. Summary of the Invention

[0007] One of the purposes of the present invention is to provide a preparation method of a single-atom catalyst in view of the deficiencies of the prior art, so as to solve the problems that the product obtained by the current hydrothermal method is doped with more nanoparticle impurities and requires repeated high-temperature calcination and pickling.

[0008] To achieve the above purpose, the present invention adopts the following technical solutions:

[0009] A preparation method of a single-atom catalyst includes the following steps:

[0010] S1. Perform the first carbonization treatment on the carbon precursor, followed by pickling and washing with water to obtain a carbon shell substrate;

[0011] S2. Mix and react the carbon shell substrate obtained in step S1 with a nitrogen precursor, and then perform the second carbonization treatment to obtain a nitrogen-doped carbon shell substrate;

[0012] S3. Mix the nitrogen-doped carbon shell substrate obtained in step S2 with an acidic metal salt, and then carry out a hydrothermal reaction at 50-100 °C to obtain a single-atom catalyst.

[0013] Preferably, the mass content ratio of the metal element in the obtained single-atom catalyst is 0.3% - 1.2%.

[0014] Preferably, the carbon precursor is sodium citrate and / or biochar, the nitrogen precursor is a saturated urea solution, and the acidic metal salt is at least one of divalent iron salts, cobalt salts, and nickel salts.

[0015] Preferably, in step S1, the heating rate of the first carbonization treatment is 5-10 °C / min, the first carbonization temperature is 600-1000 °C, and the first carbonization time is 0.5-2 h.

[0016] Preferably, in step S1, before the first carbonization treatment, the carbon precursor is subjected to dehydration pretreatment, and the dehydration pretreatment is: drying the carbon precursor at 120-180 °C for 10-14 h to obtain the dehydrated carbon precursor.

[0017] Preferably, in step S2, the mixing reaction step of the carbon shell substrate and the nitrogen precursor is: first adding the carbon shell substrate to the nitrogen precursor for mixing, performing ultrasonic oscillation, then stirring at a constant temperature of 50-80 °C until the solution is completely evaporated and crystallized, and finally performing a second carbonization treatment on the crystal particles in an inert gas atmosphere.

[0018] Preferably, in step S2, the time of the ultrasonic oscillation is 5 to 20 minutes, the inert gas is at least one of nitrogen, helium, neon, krypton, or xenon; the heating rate of the second carbonization treatment is 5-10 °C / min, the second carbonization temperature is 400-800 °C, and the second carbonization time is 0.5-2 h.

[0019] Preferably, in step S3, the nitrogen-doped carbon shell substrate and the acidic metal salt are mixed in deionized water, and after mixing evenly, the mixed solution is transferred to an oxygen-free environment for hydrothermal reaction for 2-7 h. After the reaction, it is cooled, washed, and dried to obtain a single-atom catalyst; wherein, the mass ratio of the nitrogen-doped carbon shell substrate to the acidic metal salt is (2-3):1.

[0020] The second object of the present invention is to provide a single-atom catalyst prepared by the above-mentioned preparation method of the single-atom catalyst.

[0021] The third object of the present invention is to provide an application of the above-mentioned single-atom catalyst in CO 2 reduction.

[0022] The beneficial effects of the present invention are as follows: The preparation method of the present invention provides a new hydrothermal preparation idea. First, carbonization treatment is carried out, and then nitrogen doping is carried out. After treatment, the nitrogen-doped carbon shell-based nanoparticles have few impurities, laying a foundation for the yield of subsequent hydrothermal loading of metal atoms. Then, a low-temperature acidic hydrothermal method without adding a reducing agent is used to fix metal atoms on the surface of N-C in one step. Compared with the relatively high hydrothermal reaction temperature of 120-180 °C, the low-temperature reaction of 50-100 °C in the present invention can avoid the migration and aggregation of metal atoms at high temperatures to form nanoparticles, ensuring the uniform loading of metal atoms. Thereby, the effective active sites of the catalyst are increased, and the obtained single-atom catalyst-doped nanoparticles have few impurities. Therefore, there is no problem of subsequent repeated high-temperature calcination and pickling in one-step generation. The preparation method has simple steps, mild and easy-to-control conditions, can be efficiently prepared, and the obtained single-atom catalyst has excellent performance, especially in the 2 electrocatalytic reduction of CO shows more excellent catalytic performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 SEM image of the catalyst of Example 1 of the present invention.

[0024] Figure 2 is Figure 1 the enlarged SEM image.

[0025] Figure 3 EDS analysis diagram of the catalyst of Example 1 of the present invention.

[0026] Figure 4 Element mapping diagrams of C, N, and Fe of the catalyst of Example 1 of the present invention.

[0027] Figure 5 For the catalyst of Example 1 of the present invention in N 2 and CO 2 saturated 0.5 mol·L -1 linear voltammogram in potassium bicarbonate solution.

[0028] Figure 6 SEM image of the catalyst of Example 5 of the present invention.

[0029] Figure 7 is Figure 6 the enlarged SEM image.

[0030] Figure 8 EDS analysis diagram of the catalyst of Example 5 of the present invention.

[0031] Figure 9 Element mapping diagrams of C, N, and Ni of the catalyst of Example 5 of the present invention.

[0032] Figure 10Variation diagram of current density and carbon monoxide Faraday efficiency of the catalyst in Example 5 of the present invention at -0.6V potential.

[0033] Figure 11 Carbon monoxide Faraday efficiency diagram of the catalyst in Example 5 of the present invention. Detailed implementation manners

[0034] To make the technical solutions and advantages of the present invention clearer, the present invention and its beneficial effects will be further described in detail below, but the implementation manners of the present invention are not limited thereto.

[0035] The single-atom catalyst prepared by the present invention is mainly applied in CO 2 reduction, such as CO 2 catalytic hydrogenation, electrochemical reduction, photocatalytic reduction. Of course, it can also be used in other fields of material production, which will not be elaborated here too much.

[0036] The preparation method of the single-atom catalyst includes the following steps:

[0037] S1. Perform the first carbonization treatment on the carbon precursor, followed by pickling and washing with water to obtain a carbon shell substrate;

[0038] S2. Mix and react the carbon shell substrate obtained in step S1 with a nitrogen precursor, and then perform the second carbonization treatment to obtain a nitrogen-doped carbon shell substrate;

[0039] S3. Mix the nitrogen-doped carbon shell substrate obtained in step S2 with an acidic metal salt, and then carry out a hydrothermal reaction at 50 - 100 °C to obtain a single-atom catalyst.

[0040] In step S1, the carbon precursor is first subjected to a separate carbonization treatment to form a carbon shell framework, and then acid-soaked and washed to preferentially remove the generated nanoparticle impurities, ensuring the purity of the carbon shell substrate participating in the subsequent reaction. The preparation method of the present invention does not mix and calcine the metal salt with the carbon precursor in the early stage, effectively reducing the generation of nanoparticle impurities. Especially in step S1, the nanoparticle impurities are removed in advance, ensuring the generation and yield of the subsequent single-atom catalyst.

[0041] In step S2, nitrogen is first doped into the carbon shell substrate. Similarly, no metal salt is added for mixing and calcining together, which allows nitrogen to be uniformly doped into the carbon shell substrate, with N-C evenly distributed, further reducing the generation of nanoparticle impurities and laying a foundation for the subsequent loading of metal atoms.

[0042] Then, it is mixed with an acidic metal salt and subjected to a hydrothermal reaction at a low temperature. At the above hydrothermal reaction temperature, the reaction rate can be adaptively regulated. Metal atoms are not easily migrated and agglomerated to form nanoparticles, and are uniformly fixed on the surface of N-C, thereby effectively increasing the active sites of the single-atom catalyst. At the same time, due to the previous carbonization and nitrogen doping treatment, the obtained nitrogen-doped carbon shell substrate has a large number of pores and strong structural loading capacity, and Fe atoms can be fixed on the surface of N-C at a relatively low hydrothermal temperature of 50-100 °C. After the above treatment, the produced single-atom catalyst has a high yield and few nanoparticle impurities, and no complicated processes such as calcination, pickling and calcination are required subsequently.

[0043] Through experimental verification, if nitrogen and metal atoms are mixed with a carbon precursor for preparation at the beginning, if the hydrothermal reaction temperature is relatively low, the catalytic performance is poor after subsequent calcination, and if it is relatively high, reaching above 120 °C, there are serious problems of metal atom agglomeration, large amounts of nanoparticle impurities generated, and low product purity. Finally, repeated high-temperature calcination and pickling are required to remove impurities, greatly reducing the preparation efficiency and performance of the prepared single-atom catalyst.

[0044] Specifically, the hydrothermal reaction temperature in the present invention can be 50-60 °C, 60-70 °C, 70-80 °C, 80-90 °C or 90-100 °C. Preferably, the hydrothermal reaction temperature is 60-80 °C. The hydrothermal reaction time can be 2-7 h, specifically including but not limited to 2 h, 3 h, 4 h, 5 h, 6 h or 7 h.

[0045] In some embodiments, the mass content ratio of the metal element in the obtained single-atom catalyst is 0.3%-1.2%.

[0046] In some embodiments, the carbon precursor is sodium citrate and / or biochar, and the nitrogen precursor is a saturated urea solution. The preparation method of the present invention can use cheap raw materials as precursors, greatly reducing the production cost and being beneficial to industrial applications.

[0047] In some embodiments, the acidic metal salt is at least one of divalent iron salts, cobalt salts, and nickel salts. Specifically, the divalent iron salt can be ferrous sulfate, the cobalt salt can be cobalt sulfate, and the nickel salt can be nickel sulfate.

[0048] In some embodiments, in step S1, the heating rate of the first carbonization treatment is 5-10 °C / min, the first carbonization temperature is 600-1000 °C, and the first carbonization time is 0.5-2 h. Controlling the first carbonization treatment conditions within the above range, there is sufficient temperature and time for carbonization, which can ensure a high carbon content of the carbon shell substrate. Specifically, the first carbonization temperature can be 600-700 °C, 700-800 °C, 800-900 °C or 900-1000 °C.

[0049] In some embodiments, in step S1, before the first carbonization treatment, the carbon precursor is dehydrated and pretreated. The dehydration pretreatment is as follows: the carbon precursor is dried at 120-180 °C for 10-14 h to obtain the dehydrated carbon precursor. The carbon precursor preferably used in the present invention is sodium citrate or biochar. Performing dehydration treatment in advance helps the morphology of the carbon shell substrate after subsequent carbonization, so as to support the loading of metal atoms and thus obtain a single-atom catalyst with more excellent performance.

[0050] In some embodiments, in step S2, the mixing reaction step of the carbon shell substrate and the nitrogen precursor is as follows: first, the carbon shell substrate is added to the nitrogen precursor for mixing, and ultrasonic oscillation is performed. Then, it is stirred at a constant temperature of 50-80 °C until the solution is completely evaporated and crystallized. Finally, the crystallized particles are subjected to a second carbonization treatment in an inert gas atmosphere. To ensure uniform doping of nitrogen into the carbon shell substrate, the carbon shell substrate is slowly added to the nitrogen precursor, and the evaporation and crystallization process is also slow stirring. First, evaporate and crystallize to fix the non-volatile nitrogen and the carbon shell substrate, and then perform the second carbonization to enable uniform doping of nitrogen into the carbon shell substrate. If the carbon shell substrate and the nitrogen precursor are directly calcined together, since part of the nitrogen gas will volatilize when the nitrogen precursor is heated, not only the nitrogen doping rate is reduced, but also the uniformity of its doping will be affected.

[0051] In some embodiments, in step S2, the time of the ultrasonic oscillation is 5 minutes to 20 minutes, and the inert gas is at least one of nitrogen, helium, neon, krypton or xenon; the heating rate of the second carbonization treatment is 5-10 °C / min, the second carbonization temperature is 400-800 °C, and the second carbonization time is 0.5-2 h. Selecting the second carbonization temperature lower than the first carbonization temperature is beneficial to doping nitrogen into the carbon shell substrate while ensuring the morphology of the carbon shell substrate, so as to obtain a nitrogen-doped carbon shell substrate. Specifically, the second carbonization temperature can be selected according to the first carbonization temperature.

[0052] In some embodiments, in step S3, the nitrogen-doped carbon shell substrate and the acidic metal salt are mixed in deionized water. After mixing evenly, the mixed solution is transferred to an oxygen-free environment for hydrothermal reaction for 2-7 h. After the reaction, it is cooled, washed and dried to obtain a single-atom catalyst; wherein, the mass ratio of the nitrogen-doped carbon shell substrate to the acidic metal salt is (2-3):1.

[0053] The oxygen-free environment can be achieved by filling an inert gas in a closed container. The protection of the inert gas can prevent the product from being oxidized and further ensure the production efficiency of the product. For the hydrothermal reaction of the present invention, no reducing agent needs to be added, and the Fe atoms can be fixed on the N-C surface in one step by the low-temperature acidic solvothermal method, with high catalyst production rate and high purity.

[0054] Next, the present invention and its beneficial effects will be further described in detail in conjunction with specific embodiments and the accompanying drawings of the specification, but the implementation manners of the present invention are not limited thereto.

[0055] Example 1

[0056] Prepare an Fe-N-C single-atom catalyst, and its preparation method includes the following steps:

[0057] 1) Take sodium citrate (Na 3 C 6 H 5 O 7 ·2H 2 O) and dry it in an oven at 155 °C for 12 h for dehydration pretreatment to remove crystal water, and then grind it into powder; subsequently, spread the powder evenly in a crucible and place it in a tube furnace filled with N 2 for the first carbonization treatment. The heating rate of the first carbonization treatment is 5 °C / min, the first carbonization temperature is 800 °C, and the first carbonization time is 1 h; after the carbonization treatment is completed, cool it to room temperature, soak and wash it with excessive hydrochloric acid to remove impurities, and then rinse it repeatedly with deionized water until the filtrate is neutral, and finally dry it at 60 °C for 12 h to obtain a carbon shell substrate;

[0058] 2) Slowly add the obtained carbon shell substrate (500 mg) to an excessive saturated urea solution, first ultrasonically vibrate for 10 minutes, and then slowly stir the solution at a constant temperature of 60 °C until the solution completely evaporates and crystallizes; then place the crystal particles in a tube furnace filled with N 2 for the second carbonization treatment. The heating rate of the second carbonization treatment is 5 °C / min, the second carbonization temperature is 600 °C, and the second carbonization time is 2 h to obtain a nitrogen-doped carbon shell substrate;

[0059] 3) Take 50 mg of the nitrogen-doped carbon shell substrate and 20 mg of ferrous sulfate and mix them into 50 mL of deionized water, ultrasonically vibrate for 10 min, stir for 20 min, then transfer the mixed solution to a deoxygenated Teflon autoclave, carry out a hydrothermal reaction at 70 °C for 5 h, and finally cool it to room temperature, filter, wash, and dry to obtain an Fe-N-C single-atom catalyst.

[0060] The characterization data graph of the Fe-N-C single-atom catalyst is as Figures 1-4 shown. It can be seen from Figures 1-2 that a well-distributed CS carbon shell structure is formed in the catalyst, and thus a hollow structure with a diameter of about 20 nm is formed. In addition, the aggregation of CS also forms a large number of macropores with a diameter of about 140 nm. It can be seen from Figures 3-4 that Fe single atoms are formed on the carbon shell of CS@N-Fe-600, and the Fe-N-C single-atom catalyst is successfully prepared.

[0061] Example 2

[0062] Prepare an Fe-N-C single-atom catalyst, and its preparation method includes the following steps:

[0063] 1) Take sodium citrate (Na 3 C 6 H 5 O 7 ·2H 2 O) and dry it in an oven at 155 °C for 12 h for dehydration pretreatment to remove crystal water, and then grind it into powder; subsequently, spread the powder evenly in a crucible and place it in a tubular furnace filled with N 2 for the first carbonization treatment. The heating rate of the first carbonization treatment is 10 °C / min, the first carbonization temperature is 1000 °C, and the first carbonization time is 0.5 h; after the carbonization treatment is completed, cool it to room temperature, soak and wash it with excessive hydrochloric acid to remove impurities, and then rinse it repeatedly with deionized water until the filtrate is neutral, and finally dry it at 60 °C for 12 h to obtain a carbon shell substrate;

[0064] 2) Slowly add the obtained carbon shell substrate (600 mg) to an excessive saturated urea solution, first ultrasonically vibrate for 20 minutes, and then slowly stir the solution at a constant temperature of 70 °C until the solution completely evaporates and crystallizes; then place the crystal particles in a tubular furnace filled with N 2 for the second carbonization treatment. The heating rate of the second carbonization treatment is 10 °C / min, the second carbonization temperature is 800 °C, and the second carbonization time is 1 h to obtain a nitrogen-doped carbon shell substrate;

[0065] 3) Take 60 mg of the nitrogen-doped carbon shell substrate and 20 mg of ferrous sulfate and mix them into 50 mL of deionized water, ultrasonically vibrate for 20 min, stir for 30 min, and then transfer the mixed solution to a deoxygenated Teflon autoclave for hydrothermal reaction at 100 °C for 2 h. Finally, cool it to room temperature, filter, wash, and dry to obtain the Fe-N-C single-atom catalyst.

[0066] Example 3

[0067] Prepare an Fe-N-C single-atom catalyst, and its preparation method includes the following steps:

[0068] 1) Take sodium citrate (Na 3 C 6 H 5 O 7 ·2H 2 O) and dry it in an oven at 155 °C for 12 h for dehydration pretreatment to remove crystal water, and then grind it into powder; subsequently, spread the powder evenly in a crucible and place it in a tubular furnace filled with N 2The first carbonization treatment is carried out in a tube furnace. The heating rate of the first carbonization treatment is 7 °C / min, the first carbonization temperature is 600 °C, and the first carbonization time is 2 h. After the carbonization treatment is completed, it is cooled to room temperature, soaked and cleaned with excessive hydrochloric acid to remove impurities, then repeatedly rinsed with deionized water until the filtrate is neutral, and finally dried at 60 °C for 12 h to obtain a carbon shell substrate.

[0069] 2) Slowly add the obtained carbon shell substrate (400 mg) into an excessive saturated urea solution, ultrasonically vibrate for 5 minutes first, and then keep the solution stirring slowly at a constant temperature of 50 °C until the solution is completely evaporated and crystallized. Then place the crystal particles in a tube furnace filled with N 2 for the second carbonization treatment. The heating rate of the second carbonization treatment is 7 °C / min, the second carbonization temperature is 400 °C, and the second carbonization time is 2 h to obtain a nitrogen-doped carbon shell substrate.

[0070] 3) Take 40 mg of the nitrogen-doped carbon shell substrate and 20 mg of ferrous sulfate and mix them into 50 mL of deionized water, ultrasonically vibrate for 5 min, stir for 20 min, then transfer the mixed solution to a deoxygenated Teflon autoclave, carry out a hydrothermal reaction at 50 °C for 7 h, finally cool to room temperature, filter, wash and dry to obtain the Fe-N-C single-atom catalyst.

[0071] Example 4

[0072] To prepare a Co-N-C single-atom catalyst, the preparation method includes the following steps:

[0073] 1) Take sodium citrate (Na 3 C 6 H 5 O 7 ·2H 2 O) and dry it in an oven at 155 °C for 12 h for dehydration pretreatment to remove crystal water, and then grind it into powder. Subsequently, spread the powder evenly in a crucible and place it in a tube furnace filled with N 2 for the first carbonization treatment. The heating rate of the first carbonization treatment is 6 °C / min, the first carbonization temperature is 900 °C, and the first carbonization time is 1 h. After the carbonization treatment is completed, it is cooled to room temperature, soaked and cleaned with excessive hydrochloric acid to remove impurities, then repeatedly rinsed with deionized water until the filtrate is neutral, and finally dried at 60 °C for 12 h to obtain a carbon shell substrate.

[0074] 2) Slowly add the obtained carbon shell substrate (500 mg) into an excessive saturated urea solution, ultrasonically vibrate for 10 minutes first, and then keep the solution stirring slowly at a constant temperature of 60 °C until the solution is completely evaporated and crystallized. Then place the crystal particles in a tube furnace filled with N 2The second carbonization treatment is carried out in a tubular furnace. The heating rate of the second carbonization treatment is 6 °C / min, the second carbonization temperature is 700 °C, and the second carbonization time is 2 h to obtain a nitrogen-doped carbon shell substrate;

[0075] 3) Take 50 mg of the nitrogen-doped carbon shell substrate and 20 mg of cobalt sulfate and mix them in 50 mL of deionized water. Ultrasonically vibrate for 15 min and stir for 15 min. Then transfer the mixed solution to a deoxygenated Teflon autoclave and carry out a hydrothermal reaction at 80 °C for 4 h. Finally, cool to room temperature, filter, wash, and dry to obtain the Co-N-C single-atom catalyst.

[0076] Example 5

[0077] To prepare the Ni-N-C single-atom catalyst, the preparation method includes the following steps:

[0078] 1) Take sodium citrate (Na 3 C 6 H 5 O 7 ·2H 2 O) and dry it in an oven at 155 °C for 12 h for dehydration pretreatment to remove crystal water, and then grind it into powder; Subsequently, spread the powder evenly in a crucible and place it in a tubular furnace filled with N 2 to carry out the first carbonization treatment. The heating rate of the first carbonization treatment is 8 °C / min, the first carbonization temperature is 700 °C, and the first carbonization time is 1 h; After the carbonization treatment is completed, cool to room temperature, soak and clean with excessive hydrochloric acid to remove impurities, then rinse repeatedly with deionized water until the filtrate is neutral, and finally dry at 60 °C for 12 h to obtain a carbon shell substrate;

[0079] 2) Slowly add the obtained carbon shell substrate (500 mg) to an excessive saturated urea solution. First, ultrasonically vibrate for 10 minutes, and then keep the solution stirring slowly at a constant temperature of 60 °C until the solution completely evaporates and crystallizes; Then place the crystal particles in a tubular furnace filled with N 2 to carry out the second carbonization treatment. The heating rate of the second carbonization treatment is 8 °C / min, the second carbonization temperature is 500 °C, and the second carbonization time is 2 h to obtain a nitrogen-doped carbon shell substrate;

[0080] 3) Take 50 mg of the nitrogen-doped carbon shell substrate and 20 mg of nickel sulfate and mix them in 50 mL of deionized water. Ultrasonically vibrate for 20 min and stir for 25 min. Then transfer the mixed solution to a deoxygenated Teflon autoclave and carry out a hydrothermal reaction at 90 °C for 6 h. Finally, cool to room temperature, filter, wash, and dry to obtain the Ni-N-C single-atom catalyst. The characterization data graph is as Figures 6-9 shown. The catalyst also forms a hollow structure and aggregates a large number of macroporous structures. CombiningFigures 8-9 The element mapping diagram also shows that the Ni-N-C single-atom catalyst was successfully prepared.

[0081] Comparative Example 1

[0082] Refer to Patent CN117448862A to prepare the Fe-N5 single-atom catalyst. The specific preparation method includes the following steps:

[0083] 1) Dissolve 3.39 g of Zn(NO 3 ) 2 ·6H 2 O and 50 mg of Fe(NO 3 ) 3 ·9H 2 O in 38 mL of methanol solution to obtain a zinc nitrate / iron nitrate solution; dissolve 3.39 g of 2-methylimidazole in 38 mL of methanol solution to obtain a 2-methylimidazole solution;

[0084] 2) Mix and stir the zinc nitrate / iron nitrate solution and the 2-methylimidazole solution to obtain a mixed solution. Then transfer the mixed solution to a 100 mL stainless steel autoclave with a polytetrafluoroethylene lining. Under the condition of a temperature of 60 °C, the mixed solution is hydrothermally reacted for 24 h to obtain a hydrothermal product;

[0085] 3) Centrifuge, wash, and dry the hydrothermal product in sequence to obtain the dried powder;

[0086] 4) Place the dried powder in a tubular furnace. Under a high-purity nitrogen atmosphere, heat the temperature to 1000 °C at a heating rate of 2 °C / min, and then calcine for 3 h under a high-purity nitrogen atmosphere and at a temperature of 1000 °C to obtain the Fe-N5 single-atom catalyst.

[0087] Comparative Example 2

[0088] Different from Example 1, the hydrothermal reaction temperature in this comparative example is 180 °C.

[0089] The rest is the same as in Example 1 and will not be elaborated here.

[0090] Characterize the single-atom catalysts obtained in the above Examples 1-5 and Comparative Examples 1-2 and test the catalytic performance of the carbon dioxide electroreduction reaction.

[0091] The test method for the catalytic performance is as follows: Use a standard three-electrode system to test the carbon dioxide electroreduction performance. Use the carbon paper loaded with the single-atom catalyst of the present invention as the working electrode, the saturated calomel electrode as the reference electrode, and the carbon rod as the counter electrode. Use 0.5 mol·L -1A potassium bicarbonate solution was used as the electrolyte. Before the potentiostatic test, carbon dioxide gas was introduced for half an hour to saturate the solution. The potential range during the test was -0.5 to -1.0 V (vs. RHE). The gaseous products generated by the reaction were detected by gas chromatography, and the product selectivity was calculated to obtain the current density change diagram and the Faraday efficiency diagram of CO.

[0092] The test results are shown in Table 1 below and Figures 1-11 as follows.

[0093] Table 1 Maximum value of Faraday efficiency

[0094] Mass ratio of metal element Maximum value of Faraday efficiency of CO Example 1 0.4％ 95.5％ Example 2 0.3％ 94.2％ Example 3 0.6％ 98.9％ Example 4 0.8％ 93.5％ Example 5 1.2％ 98.4％ Comparative Example 1 0.4％ 84.1％ Comparative Example 2 0.4％ 82.6％

[0095] From Table 1 and Figures 1-11 the results, it can be seen that for the single-atom catalyst prepared by the preparation method of the present invention, due to the formation of a well-distributed carbon shell structure before the hydrothermal reaction and combined with the one-step low-temperature acidic hydrothermal preparation, the finally prepared product contains a large number of hollow structures and has good carbon skeleton supportability. For example, Figures 1-2 as shown in the Fe-N-C single-atom catalyst, a large number of macropores with a diameter of about 140 nm are formed in its structure. When these single-atom catalysts are applied to the CO 2 reduction, the catalytic activity of current inexpensive transition metals used for reducing CO 2 molecules to produce fuels is greatly improved, the production cost is effectively reduced, and it is more suitable for industrial production.

[0096] Among them, from the comparison between Example 1 and Comparative Example 1, it can be seen that even if a hydrothermal reaction at a lower temperature is used, since high-temperature carbonization is still required to obtain a single-atom catalyst subsequently, the finally exhibited CO selectivity is not high. On the contrary, if the method of Patent CN116752174A is used, that is, "high-temperature hydrothermal first, then calcination and pickling", the obtained single-atom catalyst has relatively good CO selectivity, but it also brings harsh preparation conditions and more impurities in the product. And if the preparation method in Comparative Example 2 is used, that is, "carbonization and doping first, then high-temperature hydrothermal", a single-atom catalyst with high selectivity for CO cannot be obtained either. The preparation method provided by the present invention, that is, "carbonization and doping first, then low-temperature hydrothermal", uses a nitrogen-doped carbon shell substrate as the raw material for the hydrothermal reaction. The steps synergistically affect each other, and there is no need for repeated high-temperature calcination and pickling subsequently. The prepared product has more excellent catalytic performance.

[0097] Based on the disclosure of the above specification, those skilled in the art to which the present invention pertains are also able to make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the above specific embodiments, and any obvious improvements, substitutions, or variations made by those skilled in the art based on the present invention fall within the protection scope of the present invention. In addition, although some specific terms are used in this specification, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims

1. A method for preparing a single atom catalyst, characterized in that: The following steps are involved: S1, subjecting the carbon precursor to a first carbonization treatment, acid washing and water washing to obtain a carbon shell substrate; S2, mixing the carbon shell substrate obtained in step S1 with a nitrogen precursor for reaction, and then performing a second carbonization treatment after mixing evenly to obtain a nitrogen-doped carbon shell substrate; S3, adding water to the nitrogen-doped carbon shell substrate obtained in step S2 and the acidic metal salt to mix, and then placing at 50-100° C. for hydrothermal reaction to obtain a single atom catalyst.

2. The method for preparing a single atom catalyst according to claim 1, characterized in that: The mass content of metal elements in the obtained single-atom catalyst is 0.3% to 1.2%.

3. The method for preparing a single atom catalyst according to claim 1, characterized in that: The carbon precursor is sodium citrate and / or biochar, the nitrogen precursor is a saturated urea solution, and the acidic metal salt is at least one of a divalent iron salt, a cobalt salt, and a nickel salt.

4. The method for preparing a single atom catalyst according to claim 1, characterized in that: In step S1, the heating rate of the first carbonization treatment is 5-10°C / min, the first carbonization temperature is 600-1000°C, and the first carbonization time is 0.5-2h.

5. The method for preparing a single atom catalyst according to claim 1 or 4, characterized in that: In step S1, the carbon precursor is pre-treated by dehydration before the first carbonization treatment. The dehydration pre-treatment includes drying the carbon precursor at 120-180° C. for 10-14 hours to obtain a dehydrated carbon precursor.

6. The method for preparing a single atom catalyst according to claim 1, characterized in that: In step S2, the steps of mixing the carbon shell substrate and the nitrogen precursor are as follows: first, the carbon shell substrate is added to the nitrogen precursor and mixed, ultrasonically shaken, and then stirred at a constant temperature of 50 to 80° C. until the solution is completely evaporated and crystallized, and finally the crystal particles are subjected to a second carbonization treatment under an inert gas atmosphere.

7. The method for preparing a single atom catalyst according to claim 6, characterized in that: In step S2, the ultrasonic oscillation time is 5 minutes to 20 minutes, and the inert gas is at least one of nitrogen, helium, neon, krypton or xenon; the heating rate of the second carbonization treatment is 5-10°C / min, the second carbonization temperature is 400-800°C, and the second carbonization time is 0.5-2h.

8. The method for preparing a single atom catalyst according to claim 1, characterized in that: In step S3, the nitrogen-doped carbon shell substrate and the acidic metal salt are mixed in deionized water and ultrasonically shaken. After mixing evenly, the mixed solution is transferred to a deoxygenated environment for hydrothermal reaction for 2 to 7 hours. After the reaction, it is cooled, washed and dried to obtain a single atom catalyst; wherein the mass ratio of the nitrogen-doped carbon shell substrate to the acidic metal salt is (2 to 3):

1.

9. A single atom catalyst, characterized in that The catalyst is prepared by the method for preparing the single atom catalyst according to any one of claims 1 to 8.

10. Use of the single atom catalyst according to claim 9 in CO2 reduction.

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