Preparation method and application of cobalt metal catalyst with two-dimensional layered nanosheet structure

Abstract

The invention discloses a preparation method and application of a cobalt metal catalyst with a two-dimensional layered nanosheet structure, and belongs to a metal organic framework (MOFs) material. The method comprises the following steps: mixing cobalt chloride and 2, 5-dicarboxylic acid pyrazine in an N, N-dimethylformamide solution, and carrying out solvothermal reaction in a stainless steel hydrothermal reaction kettle. And centrifuging the obtained reaction liquid, drying in vacuum, and grinding to obtain the catalyst which is yellow solid powder in appearance. The catalyst is prepared into ink, and the ink is coated on hydrophilic carbon paper to carry out electrosynthesis ammonia performance test. The catalyst has a two-dimensional layered nanosheet structure, so that the specific surface area of the material is increased, and more catalytic active sites can be exposed. Meanwhile, the nitrogen element doped in the catalyst adjusts the electronic structure on the surface of cobalt atoms, the activity of catalytic sites is effectively improved, and the transfer rate of reaction charges is increased. The catalyst has efficient performance on electrosynthesis ammonia reaction, and has good commercial value.

Description

Technical Field

[0001] The present invention belongs to electrocatalytic nanomaterials, and specifically relates to a preparation method and application of a cobalt metal catalyst used for electrocatalytic nitrate reduction to synthesize ammonia, and particularly relates to a metal organic framework (MOFs) material with a two-dimensional layered nanosheet structure. Background Art

[0002] The nitrogen cycle is a basic element cycle in nature. In recent years, due to the interference of human activities, nitrates have accumulated on the surface in large quantities, 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 technology is a green conversion pathway with both environmental and economic benefits. This technology can not only effectively remove nitrate pollutants in water bodies, but also convert them into ammonia products with important industrial value. In the practical application of electrocatalytic nitrate reduction to ammonia synthesis technology, the development of high-performance and low-cost electrocatalysts is the key to its industrial application. Existing catalysts still face difficulties such as low catalytic efficiency, poor selectivity, low yield and low energy utilization. The catalysts that have been put into practical application are often some precious metal catalysts (ruthenium, iridium, platinum), but due to the high price and low reserves of precious metals, they cannot be put into large-scale application. Therefore, people are committed to the development of non-precious metal catalysts, and cobalt shows great potential in electrocatalytic nitrate reduction to ammonia among non-precious metals. Combining organic ligands with metal atoms to form metal-organic framework materials is a common material modification method. Among the common cobalt metal organic framework materials, Co-TPA, which is composed of terephthalic acid organic ligands and cobalt metal atoms, has been widely used in various catalytic fields. However, the effect of applying Co-TPA in NITRR is not ideal. There are problems such as poor selectivity, poor stability, and low Faraday efficiency, which seriously limit its practical application. Summary of the invention

[0003] In order to solve the problems existing in the prior art, the purpose of the present invention is to provide a method for preparing a cobalt metal catalyst for efficient electrocatalytic nitrate reduction to produce ammonia, aiming to solve the problems of low selectivity, low yield and low ammonia yield in the prior art.

[0004] In order to achieve the above object, the technical solution adopted by the present invention is as follows: A method for preparing a cobalt metal catalyst having a two-dimensional layered nanosheet structure comprises the following steps: (S1) dissolving cobalt chloride hexahydrate and 2,5-dicarboxylic acid pyrazine in a molar ratio of 1:0.6-1:1.2 in N , N-dimethylformamide solvent, stirring to obtain a uniform green mixed solution; wherein the concentration of cobalt chloride is 0.02-0.03 mol / L, and the concentration of 2,5-dicarboxylic acid pyrazine is 0.01-0.02 mol / L; (S2) transferring the mixed solution obtained in step (S1) to a hydrothermal reactor; performing a solvent thermal reaction to obtain a yellow suspension; (S3) transferring the suspension obtained in step (S2) to a centrifuge tube, centrifuging, and discarding the supernatant; washing, drying, and grinding to obtain a catalyst.

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

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

[0007] A cobalt metal catalyst with a two-dimensional layered nanosheet structure is prepared by the above method.

[0008] An application of a cobalt metal catalyst having a two-dimensional layered nanosheet structure, wherein the catalyst is applied to electrocatalytic nitrate reduction to produce ammonia. Furthermore, when electrocatalytic nitrate reduction to produce ammonia, 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).

[0009] An electrode is prepared by using the above catalyst.

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

[0011] The electrode is used in electrocatalytic nitrate reduction to produce ammonia. Furthermore, 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 having a two-dimensional layered nanosheet structure comprises the following steps: (S1) dissolving cobalt chloride hexahydrate and 2,5-dicarboxylic acid pyrazine in a molar ratio of 1:0.6-1:1.2 in N , N-dimethylformamide solvent, under the conditions of 40±2°C and 1000±500 rpm, magnetic stirring was performed for 30±5 min to obtain a uniform 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) transferring the mixed solution obtained in step (S1) into a 50 mL polytetrafluoroethylene-lined stainless steel hydrothermal reactor; placing the reactor in a programmable temperature-controlled oven, heating it to 120±2°C at a heating rate of 2-5°C / min, and maintaining it for 24±0.5h for 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 N , N - Wash with dimethylformamide, anhydrous ethanol and deionized water 3-5 times respectively, centrifuge under the same conditions after each washing to finally obtain a pure yellow solid product; (S4) placing the yellow solid obtained in step (S3) in a vacuum drying oven and drying at 60°C and -0.1 MPa for 12 h; transferring the dried block solid to a mortar and grinding it three times to obtain a yellow powder catalyst.

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

[0014] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention adopts 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 the electrolyte; the pore surface is rich in coordinatively unsaturated metal sites, which is conducive to the adsorption and activation of reactants. Metal organic framework is a three-dimensional porous structure material composed of central metal atoms and organic ligands, with 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 elements, the electronic structure of cobalt atoms is adjusted, and the activity of the catalytic sites is effectively enhanced.

[0015] (2) The cobalt metal catalyst prepared by the present invention presents a two-dimensional layered nanosheet structure with a large specific surface area. There is an interlayer spacing of 3-5 nm between the layered structures, which is conducive to the diffusion of reactants and products. The edges of the nanosheets are rich in active sites, which can significantly improve 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 the present invention has excellent catalytic activity.

[0016] (3) In the catalyst material prepared by the present invention, the coordination effect between the cobalt metal center and the nitrogen and oxygen atoms can adjust the central 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 reduced charge transfer resistance; the cobalt-nitrogen coordination structure is stable and can maintain structural integrity within the potential range of -0.1 to -0.8 V (vs. RHE).

[0017] The present invention uses 2,5-dicarboxylic acid pyrazine ligands to replace terephthalic acid ligands and form a new metal organic framework material Co-PZ with Co. Compared with terephthalic acid, the pyrazine ligand replaces the two carbon atoms on the benzene ring with nitrogen atoms on the basis of having a large π bond. When the cobalt atom forms a ligand with it, it can coordinate with the oxygen on the carboxyl group and the nitrogen on the pyrazine ring at the same time, thereby effectively improving the stability of the catalyst. At the same time, the introduction of nitrogen elements adjusts the electronic structure of the surface of the cobalt metal atom, effectively enhancing its catalytic performance in the field of electrosynthesis of ammonia. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The scanning and transmission electron microscope images and element distribution diagrams of the present invention are shown.

[0019] Figure 2 It is the X-ray diffraction spectrum of the present invention.

[0020] Figure 3 The X-ray photoelectron spectrum of the present invention is Figure 4 This is the linear voltammetric test diagram of the present invention. Figure 5 This is the double-layer capacitance test diagram of the present invention Figure 6 The constant potential electrolysis and UV test curves of the present invention and its control group are shown in FIG. Figure 7 It is a test graph of ammonia yield, Faraday efficiency and selectivity of the present invention and its control group. DETAILED DESCRIPTION

[0021] The present invention is further described below in conjunction with the accompanying drawings and examples. The embodiments of the present invention include but are not limited to the following examples.

[0022] A method for preparing a cobalt metal catalyst having a two-dimensional layered nanosheet structure comprises the following steps: (S1) dissolving cobalt chloride hexahydrate and 2,5-dicarboxylic acid pyrazine in a molar ratio of 1:0.6-1:1.2 in N,N-dimethylformamide (DMF) solvent, stirring at 40±2°C and 1000 rpm for 30±5 min to obtain a uniform green mixed solution; wherein the concentration of cobalt chloride is 0.025 mol / L, and the concentration of 2,5-dicarboxylic acid pyrazine is 0.015 mol / L; (S2) transferring the mixed solution obtained in step (S1) into a 50 mL polytetrafluoroethylene-lined stainless steel hydrothermal reactor with a filling degree of 60-80%; placing the reactor in a programmable temperature-controlled oven, heating it to 120±2°C at a heating rate of 2-5°C / min, and maintaining it for 24±0.5 h for a solvothermal reaction to obtain a yellow suspension; (S3) transferring the reaction product obtained in step (S2) into a 50 mL centrifuge tube, centrifuging at 10000±500 rpm for 5±0.5 min, and discarding the supernatant; washing with DMF, anhydrous ethanol and deionized water for 3-5 times respectively, centrifuging under the same conditions after each washing, and finally obtaining a pure yellow solid product; (S4) The yellow solid obtained in step (S3) is placed in a vacuum drying oven and dried at 60±2°C and -0.1 MPa for 12±0.5 h. The dried block solid is transferred to an agate mortar and ground carefully three times to obtain a yellow powder catalyst.

[0023] The specific implementation cases are as follows: Example 1

[0024] A method for preparing a cobalt metal catalyst having a two-dimensional layered nanosheet structure comprises the following steps: (S1) 0.2 g of cobalt chloride hexahydrate and 0.15 g of 2,5-dicarboxylic acid pyrazine were dissolved in 30 mL of N,N-dimethylformamide (DMF) solvent and stirred at 40 °C and 1000 rpm for 30 min to obtain a uniform green mixed solution; (S2) transferring the mixed solution obtained in step (S1) into a 50 mL polytetrafluoroethylene-lined stainless steel hydrothermal reactor with a filling degree of 60%; placing the reactor in a programmable temperature-controlled oven, heating it to 120°C at a heating rate of 5°C / min, and maintaining it for 24 h for a solvothermal reaction to obtain a yellow suspension; (S3) The reaction product obtained in step (S2) was transferred to a 50 mL centrifuge tube, centrifuged at 10,000 rpm for 5 min, and the supernatant was discarded; DMF, anhydrous ethanol and deionized water were used to wash three times each, and centrifuged under the same conditions after each washing to finally obtain a pure yellow solid product; (S4) The yellow solid obtained in step (S3) is placed in a vacuum drying oven and dried at 60°C and -0.1 MPa for 12 h. The dried block solid is transferred to an agate mortar and ground carefully three times to obtain a yellow powder catalyst Co-PZ.

[0025] Comparative Example 1 The preparation method in Example 1 was used to generate Co-MOF-74, except that 2,5-dihydroxyterephthalic acid was used to replace the 2,5-dicarboxylic acid pyrazine ligand.

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

[0027] Example 2

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

[0029] Example 3

[0030] Figure 1 This is a scanning electron microscope image of the catalyst Co-PZ loaded on a carbon paper substrate in Example 1. It can be seen from the figure that the catalyst Co-PZ presents a two-dimensional layered nanosheet structure, with multiple nanosheets superimposed on each other, wherein a single 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 to the mass transfer of reactants and the exposure of active sites. At the same time, the scanning electron microscope image of the catalyst loaded on the carbon paper base shows that the layered nanosheet catalyst is tightly and evenly dispersed on the carbon paper. The transmission electron microscope image once again confirms the nanosheet structure of Co-PZ. At the same time, it can be seen from the element distribution diagram that each element is evenly distributed, which verifies the orderly combination of metal cobalt and pyrazine ligands and the successful synthesis of the material.

[0031] Figure 2 The X-ray diffraction characterization (XRD) results of the present invention are shown in Table 1. By comparing the crystal data structure of the catalyst with the information in the crystal database, it can be inferred that the metal organic framework material CO-PZ was successfully synthesized. Figure 3The results of X-ray photoelectron spectroscopy (XPS) characterization of the catalyst are shown. In the XPS graph, the fitting results of the cobalt 2p3 / 2 and 2p1 / 2 characteristic peaks at 780.5 eV and 796.2 eV indicate that part of the cobalt atoms in the catalyst exist in the form of Co³⁺. This valence state distribution is conducive to the catalytic process and the formation of highly active species CoOOH, which is beneficial to the nitrate reduction reaction.

[0032] like Figure 4 As shown, the electrochemical performance of the catalyst was tested using linear sweep voltammetry (LSV) on the electrochemical workstation CHI660e. The left figure is a comparison of the linear voltammetric curves of the catalyst Co-PZ in the presence and absence of nitrate electrolyte. It can be seen that at the same potential, the test current density of the catalyst Co-PZ containing nitrate electrolyte is significantly higher than the current density of the catalyst without nitrate electrolyte, indicating that the catalyst has obvious catalytic activity for nitrate. The right figure is a comparison of the linear voltammetric curves of the catalyst Co-PZ of the present invention and the catalyst of the control group. It can be seen that at the same potential, the current density of the catalyst Co-PZ is significantly higher than that of the control group, especially at a potential of -0.6V relative to the reversible hydrogen electrode, the current density reaches 170 mAcm -2 .

[0033] Figure 5 From the double-layer capacitance test of the catalyst, it can be clearly seen that the double-layer capacitance of the Co-PZ catalyst is significantly higher than that of the catalysts Co-MOF-74 and Co(OH)2 in Comparative Example 1.

[0034] Figure 6 The constant potential electrolysis of the catalyst Co-PZ of the present invention and the catalyst of the control group and the ultraviolet absorption test results of ammonia measured by the indigo blue method are shown in the figure. It can be seen that the current density of Co-PZ during the constant potential electrolysis process is significantly higher than that of the catalyst Co-MOF-74 and Co(OH)2 in comparative example 1. The peak value of the ultraviolet absorption curve at 656nm is significantly higher than that of the other two groups.

[0035] Figure 7 The ammonia yield, Faraday efficiency and selectivity test diagram of the catalyst. It can be clearly seen that the ammonia yield, Faraday efficiency and selectivity of the catalyst Co-PZ of the present invention are far superior to those of the catalyst Co-MOF-74 and Co(OH)2 in Comparative Example 1, especially 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⁻¹mgcat⁻¹, indicating that the present invention has excellent electrocatalytic nitrate reduction to ammonia activity (specific values ​​can be seen in the attached table).

[0036]

[0037]

[0038]

[0039] The above embodiments are only preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any changes made by adopting the design principles of the present invention and performing non-creative work on this basis should fall within the protection scope of the present invention.

Claims

1. A method for preparing a cobalt metal catalyst having a two-dimensional layered nanosheet structure, characterized in that: The steps include: (S1) dissolving cobalt chloride hexahydrate and 2,5-dicarboxylic acid pyrazine in a molar ratio of 1:0.6-1:1.2 in N , N -dimethylformamide solvent, stirring to obtain a uniform green mixed solution; wherein the concentration of cobalt chloride is 0.02-0.03 mol / L, and the concentration of 2,5-dicarboxylic acid pyrazine is 0.01-0.02 mol / L; (S2) transferring the mixed solution obtained in step (S1) to a hydrothermal reaction vessel; performing a solvothermal reaction to obtain a yellow suspension; (S3) transferring the suspension obtained in step (S2) into a centrifuge tube, centrifuging, discarding the supernatant; washing, drying, and grinding to obtain a catalyst.

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

3. The method for preparing a cobalt metal catalyst having a two-dimensional layered nanosheet structure according to claim 1, characterized in that: The washing method N , N -washed 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 to 3.

5. The use of a cobalt metal catalyst having a two-dimensional layered nanosheet structure as claimed in claim 4, characterized in that: The catalyst is used in electrocatalytic nitrate reduction to produce ammonia.

6. The use of a cobalt metal catalyst having a two-dimensional layered nanosheet structure according to claim 5, characterized in that: Electrocatalytic nitrate reduction to ammonia was performed with an electrolyte of 0.1 M KNO3 and 1 M KOH over a working potential range of -0.1 V to -0.8 V (vs. RHE).

7. An electrode, characterized in that: The electrode is made 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 on the surface of a carbon paper electrode by a drop coating method, with a loading amount of 0.5-1.5 mg / cm².

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

10. Use of an electrode according to claim 9, characterized in that: Electrocatalytic nitrate reduction to ammonia was performed with an electrolyte of 0.1 M KNO3 and 1 M KOH over a working potential range of -0.1 V to -0.8 V (vs. RHE).

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