Method for preparing cobalt metal catalyst with two-dimensional layered nanosheet structure and application thereof

Abstract

Preparation and application of cobalt metal catalysts with two-dimensional layered nanosheet structures, belonging to the metal-organic framework (MOF) materials. The method involves mixing cobalt chloride with pyrazine 2,5-dicarboxylate in… N , N A dimethylformamide solution was subjected to a solvothermal reaction in a stainless steel hydrothermal reactor. The resulting reaction solution was then centrifuged, vacuum dried, and ground to obtain the catalyst, which appeared as a yellow solid powder. The catalyst was formulated into an ink and coated onto hydrophilic carbon paper for electro-ammonia synthesis performance testing. The catalyst possesses a two-dimensional layered nanosheet structure, which is beneficial for increasing the specific surface area of ​​the material and exposing more catalytic active sites. Simultaneously, the nitrogen doping in the catalyst modulates the electronic structure of the cobalt atom surface, effectively enhancing the activity of the catalytic sites and increasing the charge transfer rate. This catalyst exhibits high efficiency in the electro-ammonia synthesis reaction and possesses significant commercial value.

Description

Technical Field

[0001] This invention pertains to electrocatalytic nanomaterials, specifically relating to a method for preparing and applying a cobalt metal catalyst for the electrocatalytic reduction of nitrates to ammonia, and particularly to a metal-organic framework (MOF) material with a two-dimensional layered nanosheet structure. Background Technology

[0002] The nitrogen cycle is a fundamental element cycle in nature. In recent years, due to human activities, nitrates have accumulated in large quantities on the Earth's surface, causing an imbalance in the global nitrogen cycle and posing a serious threat to the environment and human health. Electrocatalytic nitrate reduction to ammonia synthesis is a green conversion pathway that combines environmental and economic benefits. This technology can not only effectively remove nitrate pollutants from water bodies but also convert them into ammonia, a product with significant industrial value. In the practical application of electrocatalytic nitrate reduction to ammonia synthesis, developing high-performance, low-cost electrocatalysts is key to its industrial application. Existing catalysts still face difficulties such as low catalytic efficiency, poor selectivity, low yield, and low energy utilization. Currently, catalysts used in practical applications are often noble metal catalysts (ruthenium, iridium, platinum), but their high price and low reserves prevent large-scale application. Therefore, researchers are committed to developing non-noble metal catalysts, and cobalt has shown great potential among non-noble metals for electrocatalytic nitrate reduction to ammonia. Combining organic ligands with metal atoms to form metal-organic framework materials is a common material modification method. Currently, among common cobalt metal-organic framework materials, Co-TPA, composed of terephthalic acid organic ligands and cobalt metal atoms, has been widely used in various catalytic fields. However, the application of Co-TPA in NITRR has not yielded ideal results, exhibiting problems such as poor selectivity, poor stability, and low Faraday efficiency, which severely limits its practical application. Summary of the Invention

[0003] To address the problems existing in the prior art, the present invention aims to provide a method for preparing a cobalt metal catalyst for efficient electrocatalytic reduction of nitrate to ammonia, thereby solving the problems of low selectivity, low yield, and low ammonia yield in the prior art.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for preparing a cobalt metal catalyst with a two-dimensional layered nanosheet structure includes the following steps: (S1) Dissolve cobalt chloride hexahydrate and pyrazine 2,5-dicarboxylate in a molar ratio of 1:0.6-1:1.2 in... N , NIn a dimethylformamide solvent, the mixture was stirred to obtain a homogeneous green mixed solution; wherein the concentration of cobalt chloride was 0.02-0.03 mol / L and the concentration of 2,5-dicarboxylic acid pyrazine was 0.01-0.02 mol / L. (S2) Transfer the mixed solution obtained in step (S1) to a hydrothermal reactor; carry out a solvothermal reaction to obtain a yellow suspension; (S3) Transfer the suspension obtained in step (S2) to a centrifuge tube, centrifuge, discard the supernatant; wash, dry, and grind to obtain the catalyst.

[0005] The conditions for the solvothermal reaction are: heating to 120±5℃ at a heating rate of 2-5℃ / min and holding for 24±0.5 h.

[0006] The washing method is N , N Wash with dimethylformamide, anhydrous ethanol, and deionized water.

[0007] A cobalt metal catalyst having a two-dimensional layered nanosheet structure, the catalyst being prepared by the method described above.

[0008] An application of a cobalt metal catalyst with a two-dimensional layered nanosheet structure is disclosed, wherein the catalyst is used in the electrocatalytic reduction of nitrate to ammonia. Further, in the electrocatalytic reduction of nitrate to ammonia, the electrolyte is 0.1 M KNO3 and 1 M KOH, and the operating potential range is -0.1 V to -0.8 V (vs. RHE).

[0009] An electrode prepared using the above-described catalyst.

[0010] An electrode preparation method is as follows: the catalyst is prepared into a catalyst ink with a concentration of 5 mg / mL, and loaded onto the surface of a carbon paper electrode by drop coating, with a loading amount of 0.5-1.5 mg / cm².

[0011] The electrode is used in the electrocatalytic reduction of nitrate to produce ammonia. Further, the electrolyte is 0.1 M KNO3 and 1 M KOH, and the working potential range is -0.1 V to -0.8 V (vs. RHE).

[0012] Specifically, a method for preparing a cobalt metal catalyst with a two-dimensional layered nanosheet structure includes the following steps: (S1) Dissolve cobalt chloride hexahydrate and pyrazine 2,5-dicarboxylate in a molar ratio of 1:0.6-1:1.2 in... N , NIn dimethylformamide solvent, the mixture was magnetically stirred at 40±2℃ and 1000±500 rpm for 30±5 min to obtain a homogeneous green mixed solution; wherein the concentration of cobalt chloride was 0.025±0.005 mol / L and the concentration of 2,5-dicarboxylic acid pyrazine was 0.015±0.005 mol / L. (S2) Transfer the mixed solution obtained in step (S1) to a 50 mL stainless steel hydrothermal reactor lined with polytetrafluoroethylene; place the reactor in a programmable temperature controlled oven and heat it to 120±2℃ at a heating rate of 2-5℃ / min, and maintain it for 24±0.5h to carry out a solvothermal reaction to obtain a yellow suspension. (S3) Transfer the reaction product obtained in step (S2) to a 50 mL centrifuge tube, centrifuge at 10000±500 rpm for 5 min, and discard the supernatant; use sequentially... N , N Wash with dimethylformamide, anhydrous ethanol and deionized water 3-5 times each, and centrifuge under the same conditions after each wash to finally obtain a pure yellow solid product. (S4) The yellow solid obtained in step (S3) was placed in a vacuum drying oven and dried at 60°C and -0.1 MPa for 12 h. The dried block solid was transferred to a mortar and ground 3 times to obtain a yellow powdered catalyst.

[0013] The cobalt metal catalyst with a two-dimensional layered nanosheet structure is used in the electrocatalytic reduction of nitrate to ammonia.

[0014] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention uses a metal-organic framework material composed of cobalt metal and organic ligands, which has a three-dimensional porous structure, which is conducive to the full penetration and mass transfer of electrolyte; the surface of the pores is rich in coordination unsaturated metal sites, which is conducive to the adsorption and activation of reactants. The metal-organic framework is a three-dimensional porous structure material composed of a central metal atom and organic ligands, which has a high specific surface area and adjustable pore structure. The cobalt metal catalyst with a two-dimensional layered nanosheet structure synthesized by solvothermal method belongs to the metal-organic framework material, which has a large specific surface area, and the special structure effectively increases the electrochemical active sites of the catalyst. At the same time, due to the introduction of nitrogen, the electronic structure of cobalt atoms is adjusted, which effectively enhances the activity of catalytic sites.

[0015] (2) The cobalt metal catalyst prepared in this invention exhibits a two-dimensional layered nanosheet structure with a large specific surface area. The interlayer spacing of 3-5 nm between the layers facilitates the diffusion of reactants and products. The edges of the nanosheets are rich in active sites, which significantly enhances the catalytic activity. At a potential of -0.6 V (vs. RHE), the Faraday efficiency reaches 93±1.5%, and the ammonia yield is 34.9±0.8 mg h⁻¹ mgcat⁻¹, indicating that this invention possesses excellent catalytic activity.

[0016] (3) In the catalyst material prepared by the present invention, the coordination between the cobalt metal center and the nitrogen and oxygen atoms can adjust the center position of the metal d band and optimize the adsorption energy of the reactants; the nitrogen-carbon skeleton of the pyrazine ligand has good electronic conductivity and the charge transfer resistance is reduced; the cobalt-nitrogen coordination structure is stable and can maintain structural integrity in the potential range of -0.1 to -0.8 V (vs. RHE).

[0017] This invention employs a 2,5-dicarboxylic acid pyrazine ligand to replace the terephthalic acid ligand, forming a novel metal-organic framework material, Co-PZ, with Co. Compared to terephthalic acid, the pyrazine ligand, while possessing a large π-bond, replaces two carbon atoms on the benzene ring with nitrogen atoms. When forming a ligand with pyrazine, the cobalt atom can simultaneously coordinate with both the oxygen on the carboxyl group and the nitrogen on the pyrazine ring, thereby effectively improving the catalyst's stability. Simultaneously, the introduction of nitrogen modulates the electronic structure of the cobalt metal atom surface, effectively enhancing its catalytic performance in the electrosynthesis of ammonia. Attached Figure Description

[0018] Figure 1 These are scanning and transmission electron microscope images and elemental distribution maps of the present invention.

[0019] Figure 2 This is the X-ray diffraction pattern of the present invention.

[0020] Figure 3 The X-ray photoelectron spectrum of this invention Figure 4 The linear voltammetry test diagram of the present invention Figure 5 This is a test diagram of the double-layer capacitance of the present invention. Figure 6 The graphs show the constant potential electrolysis and ultraviolet test curves of the present invention and its control group. Figure 7 The graph shows the ammonia yield, Faraday efficiency, and selectivity of the present invention and its control group. Detailed Implementation

[0021] The present invention will be further described below with reference to the accompanying drawings and embodiments. The embodiments of the present invention include, but are not limited to, the following embodiments.

[0022] A method for preparing a cobalt metal catalyst with a two-dimensional layered nanosheet structure includes the following steps: (S1) Cobalt chloride hexahydrate and pyrazine 2,5-dicarboxylate were dissolved in N,N-dimethylformamide (DMF) solvent at a molar ratio of 1:0.6-1:1.2 and stirred at 40±2℃ and 1000 rpm for 30±5 min to obtain a homogeneous green mixed solution; wherein the concentration of cobalt chloride was 0.025 mol / L and the concentration of pyrazine 2,5-dicarboxylate was 0.015 mol / L. (S2) Transfer the mixed solution obtained in step (S1) to a 50 mL stainless steel hydrothermal reactor lined with polytetrafluoroethylene, with a filling degree of 60-80%; place the reactor in a programmable temperature controlled oven and heat it to 120±2℃ at a heating rate of 2-5℃ / min, and maintain it for 24±0.5 h to carry out a solvothermal reaction to obtain a yellow suspension. (S3) Transfer the reaction product obtained in step (S2) to a 50 mL centrifuge tube, centrifuge at 10000±500 rpm for 5±0.5 min, and discard the supernatant; wash with DMF, anhydrous ethanol and deionized water 3-5 times each, centrifuge under the same conditions after each wash, and finally obtain a pure yellow solid product. (S4) The yellow solid obtained in step (S3) was placed in a vacuum drying oven and dried at 60±2℃ and -0.1 MPa for 12±0.5 h; the dried block solid was transferred to an agate mortar and ground carefully 3 times to obtain a yellow powder catalyst.

[0023] Specific implementation examples are as follows: Example 1

[0024] A method for preparing a cobalt metal catalyst with a two-dimensional layered nanosheet structure includes the following steps: (S1) Dissolve 0.2 g of cobalt chloride hexahydrate and 0.15 g of pyrazine 2,5-dicarboxylic acid in 30 mL of N,N-dimethylformamide (DMF) solvent, and stir at 40 °C and 1000 rpm for 30 min to obtain a uniform green mixed solution. (S2) The mixed solution obtained in step (S1) is transferred to a 50 mL stainless steel hydrothermal reactor lined with polytetrafluoroethylene, with a filling degree of 60%; the reactor is placed in a programmable temperature controlled oven and heated to 120°C at a heating rate of 5°C / min, and kept for 24 h to carry out a solvothermal reaction to obtain a yellow suspension. (S3) Transfer the reaction product obtained in step (S2) to a 50 mL centrifuge tube, centrifuge at 10000 rpm for 5 min, and discard the supernatant; wash with DMF, anhydrous ethanol and deionized water three times each, centrifuge under the same conditions after each wash, and finally obtain a pure yellow solid product. (S4) The yellow solid obtained in step (S3) was placed in a vacuum drying oven and dried at 60°C and -0.1 MPa for 12 h. The dried block solid was transferred to an agate mortar and ground carefully 3 times to obtain a yellow powdered catalyst Co-PZ.

[0025] Comparative Example 1 Using the preparation method described in Example 1 above, Co-MOF-74 was generated by replacing the 2,5-dicarboxylic acid pyrazine ligand with only 2,5-dihydroxyterephthalic acid.

[0026] Cobalt hydroxide crystals Co(OH)2 were synthesized separately as a comparison for subsequent experiments.

[0027] Example 2

[0028] The catalyst was prepared into a catalyst ink with a concentration of 5 mg / mL and loaded onto the surface of a carbon paper electrode using a drop-coating method with a loading of 0.5 mg / cm². Electrocatalytic nitrate reduction reaction was tested in 0.1 M KNO3 and 1 M KOH electrolytes, with an operating potential range of -0.1 V to -0.8 V (vs. RHE).

[0029] Example 3

[0030] Figure 1 This is a scanning electron microscope (SEM) image of the Co-PZ catalyst supported on a carbon paper substrate in Example 1. The image shows that the Co-PZ catalyst exhibits a two-dimensional layered nanosheet structure, with multiple nanosheets stacked on top of each other. Each individual nanosheet has a thickness of 5-15 nm and a lateral dimension of 200-500 nm. This unique structural feature increases the specific surface area of ​​the catalyst, which is beneficial for mass transfer of reactants and exposure of active sites. The SEM image of the catalyst supported on the carbon paper substrate also shows that the layered nanosheet catalyst is tightly and uniformly dispersed on the carbon paper. The transmission electron microscope (TEM) image further confirms the nanosheet structure of Co-PZ. Furthermore, the elemental distribution diagram shows that all elements are uniformly distributed, verifying the orderly binding of metallic cobalt and the pyrazine ligand, and the successful synthesis of the material.

[0031] Figure 2 The X-ray diffraction (XRD) characterization results of this invention are shown. By comparing the crystal structure data of the catalyst with information in a crystal database, the successful synthesis of the metal-organic framework material CO-PZ can be inferred. Figure 3The X-ray photoelectron spectroscopy (XPS) characterization results of the catalyst are shown. In the XPS image, the fitting results of the characteristic peaks of cobalt 2p3 / 2 and 2p1 / 2 at 780.5 eV and 796.2 eV indicate that some cobalt atoms in the catalyst exist in the form of Co³⁺. This valence state distribution is beneficial to the catalytic process and the formation of the highly active species CoOOH, which is conducive to the nitrate reduction reaction.

[0032] like Figure 4 The electrochemical performance of the catalyst was tested using linear sweep voltammetry (LSV) on an electrochemical workstation CHI660e. The left figure compares the linear voltammetry curves of the Co-PZ catalyst with and without nitrate electrolyte. It can be seen that at the same potential, the current density of the Co-PZ catalyst with nitrate electrolyte is significantly higher than that without nitrate electrolyte, demonstrating significant catalytic activity against nitrates. The right figure compares the linear voltammetry curves of the Co-PZ catalyst of this invention with the control group catalyst. It can be seen that at the same potential, the current density of the Co-PZ catalyst is significantly higher than that of the control group, especially at a potential of -0.6V relative to the reversible hydrogen electrode, where the current density reaches 170 mAcm⁻¹. -2 .

[0033] Figure 5 The double-layer capacitance test of the catalyst clearly shows that the double-layer capacitance of Co-PZ catalyst is significantly higher than that of Co-MOF-74 and Co(OH)2 in Comparative Example 1.

[0034] Figure 6 The images show the UV absorption results of the constant-potential electrolysis of the Co-PZ catalyst and the control catalyst, as well as the determination of ammonia using the indophenol blue method. It can be seen that the current density of Co-PZ during constant-potential electrolysis is significantly higher than that of the catalysts Co-MOF-74 and Co(OH)2 in Comparative Example 1. The peak value of the UV absorption curve at 656 nm is significantly higher than that of the other two groups.

[0035] Figure 7 The figures show the ammonia yield, Faradaic efficiency, and selectivity of the catalyst. It is evident that the ammonia yield, Faradaic efficiency, and selectivity of the Co-PZ catalyst of this invention far exceed those of the catalysts Co-MOF-74 and Co(OH)2 in Comparative Example 1. In particular, at a potential of -0.6 V (vs. RHE), the Faradaic efficiency reaches 93±1.5%, and the ammonia yield is 34.9±0.8 mg h⁻¹mgcat⁻¹, indicating that this invention possesses excellent electrocatalytic activity for the reduction of nitrate to ammonia (specific values ​​are shown in the attached table).

[0036]

[0037]

[0038]

[0039] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any changes made based on the design principles of the present invention, or any non-creative modifications made thereon, shall fall within the scope of protection of the present invention.

Claims

1. A method for preparing a cobalt metal catalyst with a two-dimensional layered nanosheet structure, characterized in that, Includes the following steps: (S1) Dissolve 0.2 g of cobalt chloride hexahydrate and 0.15 g of pyrazine 2,5-dicarboxylate in 30 mL of N,N-dimethylformamide solvent and stir to obtain a uniform green mixed solution; (S2) Transfer the mixed solution obtained in step (S1) to a hydrothermal reactor; carry out a solvothermal reaction to obtain a yellow suspension; (S3) Transfer the suspension obtained in step (S2) to a centrifuge tube, centrifuge, discard the supernatant; wash, dry, and grind to obtain the catalyst.

2. The method for preparing a cobalt metal catalyst with a two-dimensional layered nanosheet structure according to claim 1, characterized in that, The conditions for the solvothermal reaction are: heating to 120±5℃ at a heating rate of 2-5℃ / min and holding for 24±0.5 h.

3. The method for preparing a cobalt metal catalyst with a two-dimensional layered nanosheet structure according to claim 1, characterized in that, The washing method is N , N Wash with dimethylformamide, anhydrous ethanol, and deionized water.

4. A cobalt metal catalyst having a two-dimensional layered nanosheet structure, characterized in that: The catalyst is prepared by the method described in any one of claims 1-3.

5. The application of the cobalt metal catalyst with a two-dimensional layered nanosheet structure as described in claim 4, characterized in that: The catalyst is used in the electrocatalytic reduction of nitrates to produce ammonia.

6. The application of the cobalt metal catalyst with a two-dimensional layered nanosheet structure according to claim 5, characterized in that: Electrocatalytic reduction of nitrate to ammonia was performed using 0.1 M KNO3 and 1 M KOH as the electrolyte, with an operating potential range of -0.1 V to -0.8 V relative to RHE.

7. An electrode, characterized in that: The electrode is prepared using the catalyst described in claim 4.

8. The method for preparing an electrode according to claim 7, characterized in that, The preparation method is as follows: the catalyst is prepared into a catalyst ink with a concentration of 5 mg / mL, and loaded onto the surface of a carbon paper electrode by drop coating, with a loading amount of 0.5-1.5 mg / cm².

9. The application of the electrode according to claim 7, characterized in that: The electrode is used in the electrocatalytic reduction of nitrate to produce ammonia.

10. The application of an electrode according to claim 9, characterized in that: Electrocatalytic reduction of nitrate to ammonia was performed using 0.1 M KNO3 and 1 M KOH as the electrolyte, with an operating potential range of -0.1 V to -0.8 V relative to RHE.

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