A cobalt-based catalyst derived from a zifs structure, and a preparation method and application thereof

Abstract

The application belongs to the technical field of electrocatalytic materials, and particularly relates to a ZIFs structure derived cobalt-based catalyst and a preparation method and application thereof. The preparation method comprises the following steps: uniformly mixing a methanol solution of a cobalt salt and a methanol solution of 2-methyl imidazole to obtain a mixed solution; adding a methanol solution of a zinc salt and a copper salt into the mixed solution to perform a reaction, and sequentially performing centrifugation, washing and drying treatment on a reaction product to prepare a precursor; and performing a pyrolysis reaction on the precursor under a protective atmosphere to obtain the ZIFs structure derived cobalt-based catalyst. Through rational structure design and precise regulation of multiple active sites, a synergistic catalytic system capable of efficiently adsorbing and activating NO3 ‑ and rapidly hydrogenating reaction intermediates is constructed, which is a key to break through the performance bottleneck of NO3 ‑ RR and realize efficient and green electrochemical synthesis of ammonia.

Description

Technical Field

[0001] This invention belongs to the field of electrocatalytic materials technology, specifically relating to a cobalt-based catalyst derived from ZIFs structure, its preparation method, and its application. Background Technology

[0002] Ammonia (NH3) is both a cornerstone of modern agriculture and a promising carbon-free energy carrier. Currently, industrial ammonia synthesis mainly relies on the energy-intensive and high-emission Haber-Bosch process, which contradicts green and low-carbon strategies. Meanwhile, water nitrates (NO3)... - With pollution worsening, traditional treatment technologies often struggle to balance treatment efficiency with resource recovery. Electrocatalytic reduction of nitrates to ammonia (NO3) is a promising approach. - The RR (Rapid Regeneration) technology can directly convert water pollutants into high-value chemicals at ambient temperature and pressure, achieving the dual goals of environmental remediation and green synthesis. However, in practical applications, the complex octet / nine-proton transfer process makes the reaction pathway difficult to control, easily generating byproducts such as nitrites. Furthermore, the intense hydrogen evolution competition reaction (HER) in the aqueous system severely depletes electrons, making it difficult to improve the selectivity and Faraday efficiency (FE) of ammonia synthesis.

[0003] Developing high-performance and low-cost electrocatalysts is crucial for the electrocatalytic reduction of nitrates to ammonia. Although metal-organic framework (MOF) derivatives (such as ZIFs) have attracted considerable attention due to their high specific surface area and nitrogen-doped structures, single-component materials have significant limitations. For example, cobalt-based materials derived from ZIF-67 have limited effectiveness against NO3-. - While ZIF-8 exhibits strong conductivity, it lacks active sites. Although abundant and inexpensive transition metals (Fe, Co, Ni, Cu, etc.) are considered ideal substitutes for noble metals due to their unique d-band electronic structure, single metal sites are insufficient to synergistically complete the NO3- activation process. - The two key steps are "adsorption activation" and "intermediate hydrogenation". For example, although Cu is good at adsorption, it is not good at hydrogenation, and although Co is good at hydrogenation, its activation efficiency is low. This intrinsic contradiction makes it difficult for a single catalyst to achieve both high activity and high selectivity. Summary of the Invention

[0004] The purpose of this invention is to provide a cobalt-based catalyst derived from a ZIF structure, its preparation method, and its application, thereby overcoming the shortcomings of existing technologies. By constructing a ZIF-67@ZIF-8 core-shell structure and doping it with copper, a cobalt-based catalyst with abundant active sites and good conductivity is formed after high-temperature pyrolysis. This catalyst is used in the electrocatalytic nitrate reduction reaction, exhibiting excellent ammonia yield and Faradaic efficiency, achieving efficient and green electrochemical ammonia synthesis under mild conditions.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows: In a first aspect, the present invention provides a method for preparing a cobalt-based catalyst derived from a ZIFs structure, comprising the following steps: mixing a methanol solution of a cobalt salt and a methanol solution of 2-methylimidazole uniformly to obtain a mixed solution; A methanol solution of zinc salt and copper salt was added to a mixed solution to carry out the reaction. The reaction product was then centrifuged, washed and dried sequentially to obtain the precursor. The precursor was subjected to pyrolysis under a protective atmosphere to obtain a cobalt-based catalyst derived from the ZIF structure. The molar ratio of the cobalt salt, zinc salt and copper salt is (5-6):(5-6):1; The molar ratio of the cobalt salt to 2-methylimidazole is 1:(3-4).

[0006] This invention utilizes the controlled amounts of cobalt salt and 2-methylimidazole to facilitate the initial formation of the ZIF-67 core. Then, the addition of cobalt and copper enables the uniform growth of a Cu-doped ZIF-8 shell on the surface of the ZIF-67 core. Furthermore, a rich porous structure is constructed through a pyrolysis process, utilizing the volatilization of zinc to create pores. The introduction of copper optimizes the electronic state density, thereby achieving high exposure of active sites, enhanced anti-sintering ability, and excellent catalytic stability in the derived cobalt-based catalyst.

[0007] Secondly, the present invention provides a method for preparing a cobalt-based catalyst derived from a ZIFs structure, which yields a cobalt-based catalyst derived from a ZIFs structure.

[0008] Thirdly, this invention provides the application of cobalt-based catalysts derived from ZIFs structures in the electrocatalytic reduction of nitrates to ammonia. The core of this invention lies in constructing an adsorption-hydrogenation tandem catalytic mechanism through the introduction of Cu and a unique core-shell pyrolysis structure. The nitrogen-doped porous carbon formed after the pyrolysis of the ZIF-8 shell provides a porous structure with a high specific surface area, which helps expose active sites and optimize mass transfer; after Cu doping, it forms an adsorption-hydrogenation tandem mechanism with Co: Cu adsorbs and activates NO3. - The Co site is used for relay hydrogenation, which avoids the accumulation of intermediates and improves catalyst efficiency.

[0009] Fourthly, the present invention provides a method for electrocatalytic reduction of nitrate to synthesize ammonia. In a three-electrode system, ammonia is obtained by electrolyzing an electrolyte using a constant voltage method. In the three-electrode system, a glassy carbon electrode coated with a cobalt-based catalyst derived from a ZIF structure is used as the working electrode, an Hg / HgO electrode is used as the reference electrode, a platinum mesh is used as the counter electrode, and the electrolyte is a 0.5-1.5 mol / L KOH and a 0.1-0.5 mol / L KNO3 solution.

[0010] The beneficial effects of this invention are: (1) This invention utilizes Cu-doped core-shell structure ZIF-67@ZIF-8 as a precursor to prepare a catalytically active metal / nitrogen-doped carbon composite material through carbonization, synthesizing a higher-performance electrocatalyst for the reduction of nitrate to ammonia. Experimental comparisons show that it has a higher current density (LSV). During high-temperature pyrolysis, the evaporation of zinc leads to the formation of a porous structure in the material, providing a porous structure with a high specific surface area, which helps to expose active sites and optimize mass transfer, thereby improving the catalytic activity of the catalyst.

[0011] (2) This invention innovatively utilizes the functional division of labor between Cu and Co sites to construct an adsorption-hydrogenation tandem catalytic mechanism. Cu sites preferentially adsorb and convert NO3. - The Co site then completes the hydrogenation step. This synergistic mechanism overcomes the bottleneck of single metal sites being unable to simultaneously handle substrate adsorption and intermediate hydrogenation, effectively suppressing NO2. - The accumulation of intermediates significantly improved the yield and Faraday efficiency of NH3.

[0012] (3) The preparation process of this invention is relatively simple, the raw material cost is low, and it is suitable for large-scale preparation, providing an efficient and stable non-precious metal catalyst for electrocatalytic ammonia synthesis. Attached Figure Description

[0013] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0014] Figure 1 The image shows the X-ray diffraction (XRD) pattern of the ZIFs-derived cobalt-based core-shell catalyst prepared in Example 1.

[0015] Figure 2 EDS image of the ZIF-67@ZIF-8-Cu core-shell structure precursor prepared in Example 1.

[0016] Figure 3 Transmission electron microscopy (TEM) image of the ZIFs-derived cobalt-based core-shell catalyst prepared in Example 1.

[0017] Figure 4 The image shows a comparison of the linear sweep voltammetry (LSV) curves of the catalysts in Example 1 and Comparative Examples 1-3 in the electrocatalytic nitrate reduction reaction.

[0018] Figure 5 To test the ammonia yield (mmol·g) of the catalysts in Example 1 and Comparative Examples 1-3 at different potentials. cat -1 ·h -1 A comparison chart of Faraday efficiency (FE) and Faraday efficiency (FE). Detailed Implementation

[0019] Those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be construed as limiting the scope of the invention. Specific conditions not specified in the embodiments are performed under conventional conditions or conditions recommended by the manufacturer. Components whose manufacturers are not specified are all commercially available conventional products.

[0020] Terminology Explanation: ZIFs (zeolite imidazolium ester framework materials): composed of transition metal ions (such as Co) 2+ Zn 2+ Metal-organic frameworks (MOFs) are a class of materials that self-assemble with imidazole ligands, exhibiting high porosity and good thermal stability.

[0021] Core-shell structure: refers to a structure formed by nanoparticles of one material being encapsulated by another material. In this patent, ZIF-67 is the core and ZIF-8 is the shell.

[0022] Electrocatalytic reduction of nitrate to ammonia (NO3) - RR: refers to the process of using a catalyst as a medium to transfer NO3 under the influence of an electric field. - The electrochemical process of reduction to NH3.

[0023] To address the problems of weak nitrate adsorption, slow hydrogenation kinetics, easy accumulation of intermediates, and severe hydrogen evolution competition in existing catalysts, this invention constructs a catalyst that can efficiently adsorb and activate NO3 through rational structural design and precise control of multiple active sites. - This synergistic catalytic system, capable of rapidly hydrogenating reaction intermediates, is based on the introduction of Cu and a unique core-shell pyrolysis structure, which constructs an adsorption-hydrogenation tandem catalytic mechanism. Specifically, the nitrogen-doped porous carbon formed after the pyrolysis of the ZIF-8 shell provides a porous structure with a high specific surface area, which helps expose active sites and optimize mass transfer; after Cu doping, it forms an adsorption-hydrogenation tandem mechanism with Co: Cu adsorbs and activates NO3. - The Co site is used for relay hydrogenation, which avoids the accumulation of intermediates and improves catalyst efficiency.

[0024] In a first aspect, the present invention provides a method for preparing a cobalt-based catalyst derived from a ZIFs structure, comprising the following steps: mixing a methanol solution of a cobalt salt and a methanol solution of 2-methylimidazole uniformly to obtain a mixed solution; A methanol solution of zinc salt and copper salt was added to a mixed solution to carry out the reaction. The reaction product was then centrifuged, washed and dried sequentially to obtain the precursor. The precursor was subjected to pyrolysis under a protective atmosphere to obtain a cobalt-based catalyst derived from the ZIF structure. The molar ratio of the cobalt salt, zinc salt and copper salt is (5-6):(5-6):1; The molar ratio of the cobalt salt to 2-methylimidazole is 1:(3-4).

[0025] The inventors discovered during their research that constructing ZIF-67@ZIF-8 with a clear core-shell structure is beneficial for fully leveraging the synergistic catalytic effect between Cu and Co. If Zn 2+ If the amount of Zn is too small, the shell cannot completely cover the ZIF-67 core, leading to the aggregation and growth of cobalt particles during subsequent pyrolysis; if the amount of Zn is too small... 2+ Too much heat and an excessively thick shell are detrimental to electron transport and mass transfer between the core and shell.

[0026] When the copper doping level is too low, there are insufficient Cu sites, resulting in insufficient nitrate adsorption and activation, a high reaction initiation potential, and a decrease in ammonia yield. When the copper doping level is too high, it will destroy the crystal structure of the zinc-based zeolite imidazole ester framework material, making it impossible to obtain a catalyst with a complete structure.

[0027] If the total amount of 2-methylimidazole is too low, after the first step of ZIF-67 formation, the remaining ligands will not be sufficient to completely coordinate Zn. 2+ This can lead to incomplete shell growth, poor crystallinity, or even failure to form a core-shell structure. If the total amount of 2-methylimidazole is too high, the excessively high ligand concentration during the first mixing step can cause ZIF-67 to nucleate too quickly, resulting in crystals that are too small or have irregular morphology. Furthermore, excessive free ligands may react with Zn in subsequent steps. 2+ / Cu 2+ Homogeneous nucleation (individual nucleation in solution) occurs, interfering with the controlled growth of the core-shell structure.

[0028] In some other embodiments, the cobalt salt is selected from one or more of cobalt nitrate, cobalt chloride, and cobalt sulfate; The zinc salt is selected from one or more of zinc nitrate, zinc chloride, and zinc sulfate; The copper salt is selected from one or more of copper nitrate, copper chloride, and copper sulfate.

[0029] In some other embodiments, the cobalt salt is cobalt nitrate, the zinc salt is zinc nitrate, and the copper salt is copper nitrate. Easily decomposable nitrates (cobalt nitrate, zinc nitrate, and copper nitrate) are selected as metal sources to ensure that the precursor can utilize the complete volatilization of zinc during pyrolysis to construct a multi-level porous structure with ultra-high specific surface area, inducing copper atoms to achieve atomic-level uniform dispersion in the cobalt matrix. At the same time, the nitrate ions in the nitrates are beneficial for subsequent nitrogen doping.

[0030] The molar ratio of cobalt nitrate, zinc nitrate, and copper nitrate is 5.5:5.5:1. Nearly equal amounts of cobalt and zinc, plus a small amount of copper, balance the structural and electronic effects. Zinc volatilizes to create pores, cobalt acts as the active center, and copper modulates the electronic structure.

[0031] The molar ratio of the cobalt salt to 2-methylimidazole is 1:4. A slight excess of ligand ensures complete coordination and avoids unreacted metal ions affecting the structure.

[0032] For example, the precursor preparation process is as follows: First, a methanol solution of Co(NO3)2·6H2O and a methanol solution of 2-methylimidazole are dissolved by sonication for 5 min each, slowly mixed for about 2 min, and sonicated for another 2 min. Then, a methanol mixture containing Zn(NO3)2·6H2O and Cu(NO3)2·3H2O is slowly added (2 min), and sonication is continued for 10 min. The entire reaction is carried out at room temperature. The resulting product is washed several times by centrifugation with methanol and dried in a vacuum drying oven at 40°C.

[0033] In some other embodiments, the temperature of the pyrolysis reaction is 900-1000°C, and the heating rate is 2-10°C·min. -1 The heat preservation time is 1-3 hours; The protective atmosphere is one or more of nitrogen, argon, or helium. Preferably, the temperature of the pyrolysis reaction is 950°C, and the heating rate is 5°C·min. -1 The heat preservation time is 2 hours, and the protective atmosphere is nitrogen.

[0034] Secondly, the present invention provides a method for preparing cobalt-based catalysts derived from ZIFs structures, and the resulting cobalt-based catalysts derived from ZIFs structures.

[0035] In some other embodiments, the cobalt-based catalyst derived from the ZIFs structure has a core-shell structure, chemically designated ZIF-67@ZIF-8-Cu, comprising a ZIF-67 core and a ZIF-8-Cu shell covering its surface.

[0036] In some other embodiments, the cobalt-based catalyst derived from the ZIFs structure is a cobalt-copper / nitrogen-doped carbon composite material with a dodecahedral framework structure; copper-doped cobalt nanoparticles are uniformly dispersed inside the dodecahedral framework structure, with copper distributed in atomic form on the framework; the exterior of the dodecahedral framework structure is connected to carbon nanotubes.

[0037] Thirdly, this invention provides the application of ZIFs-derived cobalt-based catalysts in the electrocatalytic reduction of nitrate to ammonia. The ZIFs-derived cobalt-based catalysts exhibit excellent performance in the reduction of nitrate to ammonia, thanks to their unique core-shell derived structure and the synergistic catalytic effect between Cu and Co.

[0038] Fourthly, the present invention provides a method for electrocatalytic reduction of nitrate to synthesize ammonia. In a three-electrode system, ammonia is obtained by electrolyzing an electrolyte using a constant voltage method. In the three-electrode system, a glassy carbon electrode coated with a cobalt-based catalyst derived from a ZIF structure is used as the working electrode, an Hg / HgO electrode is used as the reference electrode, a platinum mesh is used as the counter electrode, and the electrolyte is a 0.5-1.5 mol / L KOH and a 0.1-0.5 mol / L KNO3 solution.

[0039] In some other embodiments, the loading of the ZIFs-derived cobalt-based catalyst on the working electrode is 0.3-0.4 mg·cm³. -2 .

[0040] For example, the electrolyte is a 1.0 mol / L KOH and 0.1 mol / L KNO3 solution. The loading of the ZIFs-derived cobalt-based catalyst on the working electrode is 0.34 mg·cm⁻¹. -2 .

[0041] The following is a further explanation with reference to specific embodiments and comparative examples: Example 1 1. Synthesis of the ZIF-67@ZIF-8-Cu core-shell structure precursor, as detailed below: First, weigh 0.0094 mol of Co(NO3)2·6H2O using a 0.0017 mol balance and dissolve it in 37.5 mL of methanol; this solution is labeled A. Then, weigh 0.038 mol of 2-methylimidazole and dissolve it in 75 mL of methanol; this solution is labeled B. Additionally, weigh 0.0094 mol of Zn(NO3)2·6H2O and 0.0017 mol of Cu(NO3)2·3H2O, and dissolve them together in 37.5 mL of methanol; this solution is labeled C. All solutions (solutions A through C) were ultrasonicated at room temperature for 5 min to ensure complete dissolution and homogeneity.

[0042] After the solution clarified, solution A was slowly added dropwise to solution B using a dropper (addition time approximately 2 min), followed by sonication for another 2 min to allow the ZIF-67 core to initially form. Next, solution C was slowly added dropwise to the above mixture using a dropper (approximately 2 min). After the addition was complete, sonication was continued for 10 min to allow the Cu-doped ZIF-8 shell to grow uniformly on the surface of the ZIF-67 core. After the reaction was complete, the resulting purple product was washed several times by centrifugation with methanol, and the product was collected and dried overnight in a vacuum oven at 40°C.

[0043] 2. Preparation of cobalt-based catalysts derived from ZIF structures, the calcination steps are as follows: The dried ZIF-67@ZIF-8-Cu core-shell precursor powder was spread evenly in a ceramic boat and placed in a tube furnace. Under a nitrogen atmosphere, the mixture was heated at 5°C·min. -1 The temperature was increased from room temperature to 950°C and held at 950°C for 2 hours. After pyrolysis, the mixture was allowed to cool naturally to room temperature, and the black powder was collected, which was the target cobalt-based catalyst.

[0044] The samples prepared in Example 1 were subjected to XRD, EDS, and transmission electron microscopy tests, and the results are as follows: Depend on Figure 1 The XRD pattern clearly shows the characteristic diffraction peaks of metallic Co (44.2°, 51.5°, 75.8°), proving the successful formation of metallic cobalt and a carbon matrix after pyrolysis. A broad graphitic carbon peak (26.4°) is also present. No characteristic diffraction peaks of metallic Cu or copper compounds were observed in the pattern, indicating that Cu did not form large-sized grains in the catalyst but existed in a highly dispersed state. This is beneficial for synergistic catalysis between Cu and Co sites.

[0045] Depend on Figure 2 As shown in the corresponding EDS image of the precursor (before calcination), Co is concentrated within the dodecahedral framework, while Zn is uniformly distributed on the framework, clearly exhibiting a core-shell structure of ZIF-67@ZIF-8-Cu (ZIF-67 as the core and ZIF-8 as the shell). This structure proves that the present invention successfully synthesized the expected core-shell precursor, which is a key foundation for obtaining excellent catalytic performance in subsequent pyrolysis.

[0046] Depend on Figure 3 TEM images of the ZIFs-derived cobalt-based core-shell catalyst show that the pyrolyzed material perfectly retains the dodecahedral framework structure of the precursor. Uniformly distributed copper-doped cobalt nanoparticles are observed within this framework; simultaneously, a large number of carbon nanotubes are generated outside the dodecahedral framework. This multi-level structure, with embedded cobalt particles and externally connected carbon nanotubes, is beneficial for the exposure of active sites and rapid electron transport.

[0047] Comparative Example 1 The preparation of ZIF-67-Cu pyrolysis samples is as follows: First, using the same total molar amounts as in Example 1, methanol solutions of Co(NO3)2·6H2O (0.0094 mol / 37.5 mL methanol), Cu(NO3)2·3H2O (0.0017 mol / 37.5 mL methanol), and 2-methylimidazole (0.038 mol / 75 mL methanol) were prepared. The Co(NO3)2·6H2O and 2-methylimidazole solutions were mixed and sonicated for 2 min. Then, the Cu(NO3)2·3H2O solution was slowly added dropwise to the mixture, and sonication was continued for 10 min. Subsequent centrifugation, washing, drying, and pyrolysis conditions (temperature raised to 950°C) were the same as in Example 1. This comparative example aims to illustrate that direct mixing and pyrolysis cannot form an effective core-shell spatial separation structure, resulting in the ineffective synergy of Cu and Co sites.

[0048] Comparative Example 2 The preparation of ZIF-67@ZIF-8 crystal pyrolysis samples (without Cu doping) is detailed below: Unlike Example 1, Cu(NO3)2·3H2O was not added to solution C; instead, 0.0094 mol Zn(NO3)2·6H2O was dissolved in 37.5 mL of methanol. All other synthesis, washing, drying, and pyrolysis conditions (950°C) were the same as in Example 1. This comparative example was used to verify the indispensable role of Cu sites in constructing the adsorption-hydrogenation tandem mechanism.

[0049] Comparative Example 3 The preparation of pyrolysis samples of pure ZIF-67 crystals is as follows: 0.0094 mol Co(NO3)2·6H2O and 0.038 mol 2-methylimidazole were dissolved in 75 mL of methanol, respectively, and sonicated for 5 min. The solutions were then mixed and sonicated for another 10 min. The products were centrifuged, washed, dried, and then pyrolyzed at 950°C under a N2 atmosphere for 2 h. This comparative example was used to compare the performance of catalysts with a single Co site.

[0050] Comparative Example 4 Copper additives were loaded onto the ZIF-67@ZIF-8 crystal (without Cu doping) sample prepared in Comparative Example 2 using an equal-volume impregnation method, as detailed below: 0.0017 mol Cu(NO3)2·3H2O was dissolved in 37.5 mL of methanol, and 2 g of ZIF-67@ZIF-8 precursor was impregnated with the same volume. The mixture was then dried at 100 °C to obtain copper-impregnated ZIF-67@ZIF-8 precursor. The dried powder was spread evenly in a porcelain boat and placed in a tube furnace. The furnace was heated at 5 °C / min under a nitrogen atmosphere. -1The temperature was increased from room temperature to 950℃ and held at 950℃ for 2 hours. After pyrolysis, the mixture was naturally cooled to room temperature, and the black powder was collected to obtain Cu-ZIF-67@ZIF-8.

[0051] Studies have found that copper salts are physically adsorbed onto the outer surface and pores of the precursor, lacking framework coordination anchors. During high-temperature pyrolysis, these copper ions readily migrate and aggregate into larger copper nanoparticles, mainly distributed on the outer surface of the dodecahedron, making it difficult for them to effectively penetrate the framework and form partial CoCu alloys or atomic-scale tight interfacial contacts with cobalt nanoparticles. Simultaneously, copper agglomerates may block the porous channels derived from ZIF-8, affecting mass transfer. These structural differences lead to the following performance defects: firstly, the agglomerated copper particles expose fewer active sites, significantly reducing their adsorption capacity for nitrate; secondly, the distance between copper and cobalt sites is too great and their distribution is uneven, failing to form an effective adsorption-hydrogenation cascade, hindering the adsorption of reaction intermediates (such as NO2). - Once activated at the copper site, it is difficult to quickly transfer to the adjacent cobalt site for subsequent deoxygenation and hydrogenation; instead, it desorbs into the electrolyte, thus significantly reducing the ammonia generation rate and Faraday efficiency.

[0052] Comparative Example 5 ZIF-8@ZIF-67-Cu was prepared using ZIF-67 as the outer shell and ZIF-8 as the core. The specific preparation method is as follows: The synthesis of the ZIF-8@ZIF-67-Cu core-shell structure precursor is detailed below: First, weigh 0.0094 mol Zn(NO3)2·6H2O using a 0.0017 mol balance and dissolve it in 37.5 mL of methanol; this solution is denoted as solution A. Then, weigh 0.038 mol 2-methylimidazole and dissolve it in 75 mL of methanol; this solution is denoted as solution B. Additionally, weigh 0.0094 mol Co(NO3)2·6H2O and 0.0017 mol Cu(NO3)2·3H2O, and dissolve them together in 37.5 mL of methanol; this solution is denoted as solution C. All solutions (solution A to solution C) are ultrasonicated at room temperature for 5 min to ensure complete dissolution and homogeneity.

[0053] After the solution clarified, solution A was slowly added dropwise to solution B using a dropper (addition time approximately 2 min), followed by sonication for another 2 min to allow the ZIF-8 core to initially form. Next, solution C was slowly added dropwise to the above mixture using a dropper (approximately 2 min). After the addition was complete, sonication was continued for 10 min to allow the Cu-doped ZIF-67 shell to grow uniformly on the surface of the ZIF-8 core. After the reaction was complete, the resulting purple product was washed several times by centrifugation with methanol, and the product was collected and dried overnight in a vacuum drying oven at 40°C.

[0054] The calcination steps are the same as in Example 1.

[0055] The study found that although the design of Comparative Example 5 was "ZIF-8 core first, then ZIF-67-Cu shell", the actual synthesis of Co... 2+ The nucleation rate of ZIF-67 formation with 2-methylimidazole is generally higher than that of Zn. 2+ ZIF-8 formation was faster. In Comparative Example 5, ZIF-8 was only pre-formed after: 2 min of dropwise addition; 2 min of sonication, followed by the addition of Co. 2+ / Cu 2+ This timeframe may not be sufficient to form a fully mature, stable, and uniformly sized ZIF-8 core; a large number of free ligands will still remain in the system. Adding Co... 2+ Afterwards, Co 2+ It tends to rapidly self-nucleate in solution. After pyrolysis, due to the lack of spatial confinement of an ordered core-shell structure, the distribution of Co / Cu species is disordered, and the metal particles are prone to sintering and agglomeration; at the same time, the carbon skeleton loses effective support after zinc volatilization, resulting in the collapse of the dodecahedral framework and the inability to form carbon nanotubes. Ultimately, the catalyst has poor conductivity and mass transfer capacity, and its electrocatalytic nitrate reduction performance is significantly worse than that of Example 1.

[0056] Application Example 1 The performance test of electrocatalytic nitrate reduction to ammonia synthesis is as follows: The catalysts prepared in Example 1 and Comparative Examples 1-3 were used for electrochemical tests. A typical three-electrode system was employed on a Chenhua electrochemical workstation. A glassy carbon electrode coated with the catalyst was used as the working electrode (catalyst loading: 0.34 mg·cm⁻¹). -2 The Hg / HgO electrode was used as the reference electrode, and a platinum mesh was used as the counter electrode. The electrolyte was 1 M KOH + 0.1 MKNO3. All potentials were converted to potentials relative to the reversible hydrogen electrode (RHE) using the Nernst equation.

[0057] Electrochemical test results as follows Figure 4 and Figure 5 As shown. Figure 4 The linear sweep voltammetry (LSV) curves show that, at the same potential, the nitrate reduction current density generated by the catalyst in Example 1 is significantly higher than that in Comparative Examples 1-3, indicating that the catalyst prepared in Example 1 has faster reaction kinetics and stronger electrocatalytic nitrate reduction activity. The LSV curve comparison directly demonstrates that the synergistic effect of the core-shell structure and Cu doping in Example 1 can effectively reduce the reaction overpotential and improve the current response. Figure 5 Performance comparison data shows that, At a potential of 0.2 V vs RHE, the ammonia yield in Example 1 reached 1176.4 mmol·g. cat-1 ·h -1 The Faraday efficiency was as high as 99.9%, which was higher than that of Comparative Example 1 (797.1 mmol·g). cat -1 ·h -1 98.3% FE), Comparative Example 2 (606.9 mmol·g) cat -1 ·h -1 97.7% FE) and Comparative Example 3 (569.3 mmol·g) cat -1 ·h -1 (97.5% FE). This result quantitatively confirms the dual advantages of the catalyst of this invention in terms of ammonia production rate and selectivity, demonstrating the effectiveness of the adsorption-hydrogenation tandem catalytic mechanism brought about by the Cu-doped core-shell structure. The results above indicate that the cobalt-based catalyst derived from the ZIF structure prepared in Example 1 has excellent performance in the reduction of nitrate to ammonia, which is attributed to its unique core-shell derived structure and the synergistic catalytic effect between Cu and Co.

[0058] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a cobalt-based catalyst derived from a ZIF structure, characterized in that, Includes the following steps: A methanol solution of cobalt salt and a methanol solution of 2-methylimidazole were mixed thoroughly to obtain a mixed solution. A methanol solution of zinc salt and copper salt was added to a mixed solution to carry out the reaction. The reaction product was then centrifuged, washed and dried sequentially to obtain the precursor. The precursor was subjected to pyrolysis under a protective atmosphere to obtain a cobalt-based catalyst derived from the ZIF structure. The molar ratio of the cobalt salt, zinc salt and copper salt is (5-6):(5-6):1; The molar ratio of the cobalt salt to 2-methylimidazole is 1:(3-4).

2. The method for preparing the cobalt-based catalyst derived from the ZIFs structure as described in claim 1, characterized in that, The cobalt salt is selected from one or more of cobalt nitrate, cobalt chloride, and cobalt sulfate; The zinc salt is selected from one or more of zinc nitrate, zinc chloride, and zinc sulfate; The copper salt is selected from one or more of copper nitrate, copper chloride, and copper sulfate.

3. The method for preparing the cobalt-based catalyst derived from the ZIFs structure as described in claim 1, characterized in that, The cobalt salt is cobalt nitrate, the zinc salt is zinc nitrate, and the copper salt is copper nitrate; The molar ratio of cobalt nitrate, zinc nitrate, and copper nitrate is 5.5:5.5:

1. The molar ratio of the cobalt salt to 2-methylimidazole is 1:

4.

4. The method for preparing the cobalt-based catalyst derived from the ZIFs structure as described in claim 1, characterized in that, The pyrolysis reaction is carried out at a temperature of 900-1000℃, with a heating rate of 2-10℃·min. -1 The heat preservation time is 1-3 hours; The protective atmosphere is one or more of nitrogen, argon, or helium. Preferably, the temperature of the pyrolysis reaction is 950°C, and the heating rate is 5°C·min. -1 The heat preservation time is 2 hours, and the protective atmosphere is nitrogen.

5. A cobalt-based catalyst derived from a ZIFs structure, prepared by the method described in any one of claims 1-4.

6. The cobalt-based catalyst derived from the ZIFs structure as described in claim 5, characterized in that, The cobalt-based catalyst derived from the ZIFs structure has a core-shell structure, chemically labeled ZIF-67@ZIF-8-Cu, comprising a ZIF-67 core and a ZIF-8-Cu shell covering its surface.

7. The cobalt-based catalyst derived from the ZIFs structure as described in claim 5, characterized in that, The cobalt-based catalyst derived from the ZIFs structure is a cobalt-copper / nitrogen-doped carbon composite material with a dodecahedral framework structure. Copper-doped cobalt nanoparticles are uniformly dispersed inside the dodecahedral framework structure, with copper distributed in atomic form on the framework. The exterior of the dodecahedral framework structure is connected to carbon nanotubes.

8. The application of a cobalt-based catalyst derived from the ZIFs structure according to any one of claims 5-7 in the electrocatalytic reduction of nitrate to ammonia.

9. A method for electrocatalytic reduction of nitrate to synthesize ammonia, characterized in that, In a three-electrode system, ammonia is produced by electrolyzing the electrolyte using a constant voltage method. In this three-electrode system, a glassy carbon electrode coated with a cobalt-based catalyst derived from the ZIFs structure as described in any one of claims 5-7 is used as the working electrode, an Hg / HgO electrode is used as the reference electrode, a platinum mesh is used as the counter electrode, and the electrolyte is a solution of 0.5-1.5 mol / L KOH and 0.1-0.5 mol / L KNO3.

10. The method for electrocatalytic reduction of nitrate to synthesize ammonia as described in claim 9, characterized in that, The loading of the ZIFs-derived cobalt-based catalyst on the working electrode is 0.3-0.4 mg·cm³. -2 .

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