Chloro-axial coordination copper monatomic catalyst, preparation method and application

Abstract

The application discloses a chlorine axial coordination copper monatomic catalyst, a preparation method and application, and belongs to the technical field of catalytic materials. In the preparation, dicyandiamide is first calcined and treated by thermal stripping for several times to prepare a layered carbon carrier rich in nitrogen defects and anchoring sites; then, the carrier is mixed with copper salt, lithium salt and potassium salt, and a molten salt thermal reaction is carried out under an inert atmosphere; with the high-temperature fluidity and space confinement effect of the molten salt, copper atoms are stably anchored in a monodispersed manner, and a Cu-N x -Cl chlorine axial coordination active center is formed, so that the target catalyst is prepared. When the catalyst is used in a CO2 electrocatalytic reduction reaction, the methane faradaic efficiency reaches 73% under an industrial-grade current density, and the catalyst has good catalytic stability and structural durability in a wide current density range. ‑2 The application provides a new way for controllable preparation of a high-performance monatomic catalyst and utilization of CO2 resources.

Description

Technical Field

[0001] This invention relates to the field of catalytic materials technology, specifically to a chlorine-axially coordinated copper single-atom catalyst prepared by thermal stripping-molten salt method, and discloses the specific preparation process of the catalyst and its application in the electrochemical reduction of CO2 to methane. Background Technology

[0002] With the increasingly severe global climate change problem, carbon emission reduction and carbon resource utilization have become key development needs. Electrocatalytic CO2 reduction technology can convert greenhouse gas CO2 into high-value-added fuels and chemicals such as methane, providing important technical support for building a carbon cycle system and achieving carbon neutrality. Among various catalytic systems, copper-based catalysts have become the core research object in this field due to their unique advantages in mediating multi-electron reduction of CO2 and the directed generation of hydrocarbons. However, traditional copper-based catalysts (such as copper nanoparticles and copper oxides) generally suffer from poor product selectivity, high reaction overpotentials, and easy deactivation and degradation at industrial-grade current densities, which seriously restricts their practical application and industrialization.

[0003] Single-atom catalysts offer new avenues for improving catalytic performance due to their near 100% atomic utilization and uniform active site structure. However, traditional copper single-atom catalysts often feature planar Cu-N4 as the main active structure. Their highly symmetrical coordination environment makes it difficult to precisely control the adsorption energy of key intermediates in the CO2 reduction to methane process. This not only leads to low methane Faradaic efficiency but also makes them prone to metal atom migration, aggregation, and structural reconstruction at high current densities, compromising catalytic stability.

[0004] To address the aforementioned issues, enhancing the CO2-to-CH4 conversion performance through precise control of the local coordination environment of copper single atoms has become an important research direction in the field. For example, the existing study "Manipulating local coordination of copper single atom catalyst enables efficient CO2-to-CH4 conversion, DOI:10.1038 / s41467-023-39048-6" directly demonstrated the effectiveness of local coordination control strategies in optimizing catalytic performance by introducing boron atoms into the first coordination layer of the Cu-N4 moiety and constructing Cu-N2B2 active sites. However, such strategies, which focus solely on modifying the first coordination layer, still struggle to simultaneously achieve both active site stability and long-term catalytic performance at high current densities.

[0005] Against this backdrop, axial coordination engineering has emerged as a key approach to precisely control the electronic structure of single-atom metal centers and synergistically optimize catalytic selectivity and stability. This strategy introduces additional ligands along the axial direction of the planar coordination structure, breaking the geometric and electronic distribution symmetry of the active sites, and regulating the d-band electronic structure of the central metal, thereby achieving directional optimization of the reaction pathway and intermediate adsorption strength. Notably, among the many selectable axial ligands, chlorine atoms, due to their unique physicochemical properties, are theoretically one of the optimal choices for constructing axial coordination structures for highly efficient copper single-atom catalysts: chlorine atoms have moderate electronegativity, which allows for precise control of the d-band center position of the copper center through electronic induction effects, optimizing its adsorption and desorption energy barriers for key intermediates in the CO2 reduction to methane production, while also forming stable coordination bonds with copper atoms; simultaneously, the chlorine atom radius is moderate, preventing excessive steric hindrance from disrupting the planar Cu-N structure. x Compared to other elements such as fluorine (excessive electronegativity, which easily leads to the breakage of coordination bonds), iodine (excessive atomic radius, which easily causes steric hindrance), and sulfur (insufficient coordination stability), chlorine axial coordination is more likely to achieve a synergistic improvement in the uniformity of active site structure and catalytic performance. It is an ideal axial ligand for the CO2 electroreduction to methane reaction.

[0006] Although the chlorine axial coordination strategy has significant theoretical advantages and application potential in optimizing the electronic structure of copper single-atom catalysts and improving the performance of CO2 electroreduction to methane, achieving Cu-N coordination in copper single-atom catalysts remains a challenge. x The controllable, efficient, and stable preparation of -Cl active centers remains a significant challenge. Existing technologies often rely on complex precursor designs, vapor deposition, or multi-step wet chemical processes, which generally suffer from drawbacks such as cumbersome preparation procedures, harsh reaction conditions, heterogeneous coordination environments, low active site loading, and poor structural stability. These limitations hinder large-scale preparation and industrial applications, and also restrict the practical implementation and promotion of chlorine-axis coordination strategies.

[0007] The molten salt method, with its excellent fluidity at high temperatures, prominent spatial confinement effect, and good reaction compatibility, has been gradually applied to the synthesis of various nanocatalytic materials, providing a new technical approach to solving the problem of atomic aggregation in the preparation of single-atom catalysts. However, research on directly applying the molten salt method to the construction of chlorine-axially coordinated copper single-atom catalysts is still relatively scarce; more importantly, conventional single molten salt systems cannot simultaneously achieve efficient construction of nitrogen-rich carbon supports, highly monodispersed copper atoms, and precise in-situ introduction of chlorine-axial ligands. The key challenge lies in how to organically combine thermal exfoliation technology for efficient preparation of nitrogen-rich carbon supports with the molten salt confinement effect to precisely construct homogeneous and stable Cu-N catalysts in a one-step process. x The -Cl active center remains a core technological gap that has yet to be solved in this field.

[0008] In summary, existing CO2 electroreduction to methane catalytic systems still face several insurmountable technical barriers: In terms of catalytic performance, traditional copper-based and ordinary copper single-atom catalysts exhibit low methane selectivity and insufficient catalytic stability at industrial-grade current densities; in terms of active site structure, precise control of the copper center coordination environment and electronic structure is difficult to achieve, failing to meet the reaction requirements for deep CO2 reduction to methane; and in terms of preparation processes, existing methods generally suffer from complex processes, high raw material costs, difficulty in scaling up production, and difficulty in ensuring the uniformity and stability of active sites, severely hindering the industrialization of this technology.

[0009] To address all the aforementioned technical challenges, a method has been proposed that is simple in process, uses inexpensive raw materials, can be mass-produced, and can controllably prepare chlorine-axially coordinated copper single-atom catalysts by constructing a "thermal stripping-molten salt confinement" synergistic system. This has become an urgent need in the field and a key measure to solve existing technical problems and promote technological progress in this field. Summary of the Invention

[0010] The purpose of this invention is to address the problems existing in the prior art and provide a chlorine-axially coordinated copper single-atom catalyst based on a thermal exfoliation-molten salt method, its preparation method, and its application. In the preparation process, dicyandiamine is first calcined and thermally exfoliated in a static air atmosphere to obtain a layered carbon support rich in nitrogen defects and anchoring sites. This carbon support is then thoroughly ground and mixed with a copper salt and a lithium-potassium salt molten salt mixture. Under an inert atmosphere, heat treatment is performed utilizing the high-temperature fluidity and spatial confinement effect of the molten salt system to ultimately achieve highly monodisperse and stably anchored copper atoms, and to construct a Cu-N catalyst with a chlorine-axially coordinated structure in situ. x -Cl active center. The prepared catalyst exhibits excellent catalytic performance in the electrocatalytic reduction of CO2 to methane, especially at 400 mA cm⁻¹. -2 At industrial-grade current densities, the methane Faradaic efficiency reaches over 73%, and it can operate stably continuously for over 10 hours, demonstrating excellent catalytic stability and structural durability. This process offers significant advantages such as readily available and inexpensive raw materials, simple and efficient steps, mild and controllable reaction conditions, and ease of large-scale production. It not only provides an efficient strategy for the controllable preparation of high-performance axially coordinated copper single-atom catalysts for chlorine, but also offers a new technological pathway for the industrial development of CO2 resource utilization and carbon cycle technologies.

[0011] To achieve the above-mentioned technical objectives, the present invention is implemented through the following technical solution: a method for preparing a chlorine axially coordinated copper single-atom catalyst, comprising the following steps: 1) Dicyandiamine was calcined to obtain a nitrogen-rich layered carbon matrix precursor, which was then cooled and ground into a uniform powder. 2) The nitrogen-rich layered carbon matrix precursor powder was subjected to several thermal exfoliation treatments to obtain nitrogen-rich layered carbon matrix powder. 3) After uniformly mixing the nitrogen-rich layered carbon matrix powder, copper salt, and lithium-potassium salt mixed molten salt, grind it into a fine mixed powder without obvious particles; at least one of the copper salt, lithium salt, and potassium salt is a chloride salt to provide the chlorine source required for axial coordination; the mixed powder is subjected to molten salt thermal reaction under an inert atmosphere to obtain a blocky catalyst precursor solid product; 4) The catalyst precursor solid product is ground, washed and dried to obtain a chlorine axially coordinated copper single-atom catalyst.

[0012] The preparation principle of this chlorine-axially coordinated copper single-atom catalyst is as follows: First, dicyandiamine is calcined and then thermally exfoliated under static air atmosphere to construct a nitrogen-deficient layered carbon matrix as a catalyst support. Then, utilizing the high fluidity and polarity of the molten salt system under a high-temperature inert atmosphere, the stability of the Cu-Cl bond in the copper salt is disrupted, causing the copper species to dissociate into individual copper atoms. Furthermore, the resulting spatial confinement effect effectively inhibits the migration and aggregation of copper atoms. Finally, the released copper atoms are precisely captured by the nitrogen-deficient sites on the carbon support surface, forming a stable planar Cu-N bond. x The coordination center, along with some chloride ions acting as axial ligands to form coordination bonds with the copper center, thus precisely constructing Cu-N x -Cl active site.

[0013] Preferably, this invention employs a mixture of lithium and potassium salts to form a eutectic molten salt system. Compared to using only lithium or only potassium salts, this eutectic molten salt system significantly lowers the melting point, enhances high-temperature fluidity and spatial confinement effect, achieves high monodispersion of copper atoms under milder reaction conditions, and stably anchors copper atoms to form axially coordinated Cu-N. x The -Cl active center helps the catalyst maintain excellent methane selectivity and structural stability even at industrial-grade high current densities.

[0014] Furthermore, in step 1), the calcination process is carried out in a static air atmosphere, the calcination heating rate is 2-10℃ / min, preferably 3℃ / min, the calcination temperature is 450-650℃, preferably 550℃, and the calcination time is 2-5 h, preferably 3 h.

[0015] Further, in step 2), the thermal stripping treatment is carried out in a static air atmosphere, the thermal stripping heating rate is 5-15℃ / min, preferably 5℃ / min, the thermal stripping temperature is 400-600℃, preferably 500℃, the single thermal stripping time is 3-6 h, preferably 5 h, and the thermal stripping process is repeated 2-5 times, preferably 3 times.

[0016] Further, in step 3), the copper salt is selected from at least one of copper chloride, cuprous chloride, copper nitrate, and copper acetate, preferably copper chloride; the lithium salt is selected from at least one of lithium chloride and lithium nitrate, preferably lithium chloride; and the potassium salt is selected from at least one of potassium chloride and potassium nitrate, preferably potassium chloride.

[0017] Furthermore, in step 3), the mass ratio of nitrogen-rich layered carbon matrix powder, copper salt, lithium salt, and potassium salt is 1:0.1~1:2~8:2~8.

[0018] Further, in step 3), the inert atmosphere is argon or nitrogen, the heating rate of the molten salt thermal reaction is 2-8℃ / min, preferably 5℃ / min, the reaction temperature is 300-800℃, preferably 500℃, and the reaction time is 1-5 h, preferably 2 h.

[0019] Furthermore, in step 4), the drying temperature is 60-100℃ and the drying time is 6-24 h.

[0020] This application also claims protection for a chlorine-axially coordinated copper single-atom catalyst prepared by the above method, wherein the catalyst uses a nitrogen-rich layered carbon matrix as a support, and copper is uniformly dispersed on the surface of the support in the form of isolated single atoms, with each copper atom forming a Cu-N configuration with nitrogen atoms in the plane of the support. x The coordination center consists of a chlorine atom that forms a coordinate bond with the copper center axially, thus forming a stable Cu-N complex. x -Cl active center structure, where x is 2, 3 or 4.

[0021] The aforementioned chlorine-axially coordinated copper single-atom catalyst can be applied to the electrocatalytic reduction of CO2 to methane, using an alkaline electrolyte and a three-electrode flow electrolysis cell system; at 400 mA cm⁻¹ -2 At industrial-grade current densities, the peak methane faradaic efficiency can reach over 73%, and during 10 hours of continuous stable operation, the methane faradaic efficiency can be stably maintained at over 65%, demonstrating excellent potential for industrial applications.

[0022] The chlorine-axially coordinated copper single-atom catalyst provided in this application exhibits high catalytic performance due to its unique Cu-N composition. x The -Cl active center structure. This structure precisely modulates the electronic structure and coordination microenvironment of the central copper atom through the axial coordination of the chlorine atom. Specifically, the highly electronegative chlorine ligand not only breaks the traditional planar Cu-N structure... xThe symmetry of the sites also induces electron transfer from the copper center to the chlorine atom, effectively optimizing the adsorption / desorption energy barrier of the copper sites for key reaction intermediates in the CO2 electroreduction to methane process, thereby significantly improving the selectivity of the CO2 methanation pathway. Simultaneously, this axial coordination bond enhances the stability of the active sites, effectively suppressing the migration or aggregation of copper atoms under high current density operating conditions, ensuring the long-term durability of the catalyst at industrial-grade current densities. Therefore, this catalyst achieves a balance of high activity, high selectivity, and high stability, meeting the requirements for large-scale industrial applications of CO2 electroreduction to methane.

[0023] The beneficial effects of this invention are as follows: 1. This application innovatively constructs a "thermal exfoliation-molten salt confinement" synergistic preparation system. By calcining dicyandiamine and performing multiple thermal exfoliations under a static air atmosphere, a layered carbon matrix carrier rich in nitrogen defects and anchoring points is precisely constructed. Furthermore, relying on the high fluidity, strong polarity, and spatial confinement effect of the mixed molten salt at high temperatures, copper atoms are dispersed in situ at high temperatures as single atoms, while chlorine atoms are introduced in situ as axial ligands, thus efficiently constructing a Cu-N matrix with a uniform structure and stable coordination environment in one step. x -Cl active centers fundamentally solve the technical problems of traditional preparation methods in constructing axial coordination structures, such as cumbersome steps, uneven coordination environment, and easy instability of active structures. 2. The preparation process provided in this application has the advantages of simplicity and scalability. It only includes two key steps: thermal stripping and molten salt thermal reaction. It does not require the use of complex templates, highly corrosive gases or precious metal additives. The process route is simple and efficient. The raw materials used, such as dicyandiamine, copper salt and lithium-potassium mixed molten salt, are widely available and inexpensive. The reaction conditions are mild and controllable and the operation is simple. The mixed molten salt system has multiple functions as a reaction medium and a metal dispersant, which can efficiently achieve the dispersion and anchoring of copper atoms. The chlorine source required for axial coordination comes from at least one chloride salt among copper salt, lithium salt and potassium salt. After the reaction, only simple washing is required to remove residual salt. The overall process is green and environmentally friendly, easy to scale up for production, and has good potential for industrial application. 3. The catalyst prepared in this application has a well-defined and highly efficient active center. Copper is highly dispersed in single-atom form and stably anchored on a nitrogen-doped layered carbon support, forming a planar Cu-N catalyst. x Cu-N coordination with axial Cl coordination x-Cl active site; the introduction of axial chloride ligands can precisely regulate the d-band electronic structure of the copper center through electronic regulation, optimize the adsorption energy and reaction pathway of the catalyst for key intermediates of CO2 reduction, efficiently promote CO bond breaking and CH bond formation, thereby significantly improving the selectivity of CO2 electroreduction to methane conversion; at the same time, this rigid coordination structure can greatly improve the structural stability of the active center in the reaction system. 4. The chlorine-axially coordinated copper single-atom catalyst prepared in this application exhibits excellent catalytic performance in the electrocatalytic reduction of CO2 to methane, at 400 mA cm⁻¹. -2 At industrial-grade current densities, the methane Faraday efficiency can reach 73%, significantly outperforming conventional copper-based catalysts and chlorine-free axially coordinated copper single-atom catalysts; at 200-600 mA cm⁻¹... -2 It can maintain high methane selectivity over a wide current density range, demonstrating excellent adaptability to operating conditions and promising prospects for industrial applications. 5. The catalyst prepared in this application possesses both excellent structural durability and electrochemical stability at 400 mAcm⁻¹. -2 Even after continuous operation at high industrial current densities for over 10 hours, it can still stably maintain high methane selectivity with no significant decay in electrode potential; this is attributed to the strong anchoring effect of molten salt confinement on copper atoms, and the Cu-N... x The inherent high chemical stability of the -Cl coordination structure can effectively suppress the migration, aggregation and dissolution of copper species during the reaction process, successfully breaking through the stability bottleneck of high-activity catalysts being prone to rapid deactivation under high current density. Attached Figure Description

[0024] Figure 1 The image shows a scanning electron microscope (SEM) image of the chlorine-axially coordinated copper single-atom catalyst prepared in Example 1. Figure 2 Aberration-corrected transmission electron microscope (AC-TEM) image of the chlorine axially coordinated copper single-atom catalyst prepared in Example 1; Figure 3 The elemental distribution surface scan diagram of the chlorine axially coordinated copper single-atom catalyst prepared in Example 1; Figure 4 X-ray diffraction (XRD) patterns of the chlorine-axially coordinated copper single-atom catalyst prepared in Example 1 and the chlorine-free axially coordinated copper single-atom catalyst prepared in Comparative Example 1. Figure 5 The X-ray photoelectron spectroscopy (XPS) Cl 2p spectra of chlorine in the chlorine-axially coordinated copper single-atom catalyst prepared in Example 1 and the chlorine-free coordinated copper single-atom catalyst prepared in Comparative Example 1 are shown. Figure 6The X-ray absorption near-edge structure (XANES) spectra of copper element for the chlorine-axially coordinated copper single-atom catalyst prepared in Example 1 and the chlorine-free coordinated copper single-atom catalyst prepared in Comparative Example 1 are shown. Figure 7 Fourier transform extended X-ray absorption fine structure (FT-EXAFS) spectra of copper element for the chlorine-axially coordinated copper single-atom catalyst prepared in Example 1 and the chlorine-free coordinated copper single-atom catalyst prepared in Comparative Example 1. Figure 8 Nitrogen adsorption-desorption (BET) curves of the chlorine axially coordinated copper single-atom catalysts prepared in Example 1 and Comparative Example 4 are shown. Figure 9 The chlorine-axially coordinated copper single-atom catalyst prepared in Example 1 operates at 100–600 mA cm⁻¹ -2 The graph shows the test results of the product Faraday efficiency distribution during the electrochemical reduction of CO2 within the current density range. Figure 10 The catalysts prepared for Examples 1-6 and Comparative Examples 1-4 were tested at 400 mA cm⁻¹. -2 Comparison of the Faraday efficiency performance of methane electrochemical reduction of CO2 at different current densities; Figure 11 The chlorine-axially coordinated copper single-atom catalyst prepared in Example 1 was tested at 400 mA cm⁻¹. -2 Figure showing the stability test results under current density. Detailed Implementation

[0025] The following embodiments further illustrate the content of the present invention, but should not be construed as limiting the present invention. Any modifications and substitutions made to the methods, steps, or conditions of the present invention without departing from the essence of the invention are within the scope of the present invention.

[0026] Example 1

[0027] This embodiment discloses a chlorine-axially coordinated copper single-atom catalyst, the specific preparation process of which is as follows:

[0028] (1) Preparation of nitrogen-rich layered carbon matrix precursor: Weigh 10.0 g of dicyandiamine and place it in a clean crucible. Place the crucible in a muffle furnace and heat it to 550°C at a rate of 3°C / min under static air atmosphere. Maintain the temperature for 3 h. After the reaction is completed, let the crucible cool naturally to room temperature with the furnace. Take out the cooled product and grind it into a uniform powder in an agate mortar for later use.

[0029] (2) Thermal stripping treatment: The ground powder is put back into the muffle furnace and heated to 500°C at a rate of 5°C / min under static air atmosphere. The temperature is kept constant for 5 h to complete one thermal stripping operation. The above thermal stripping process is repeated twice (a total of 3 thermal stripping operations are completed) to finally obtain a nitrogen-rich layered carbon matrix yellow powder, which is collected for later use.

[0030] (3) Preparation of catalyst precursor by molten salt thermal reaction: Accurately weigh 0.2 g of the above yellow powder, 0.1 g of copper chloride, 1.35 g of lithium chloride and 1.5 g of potassium chloride. Place the four materials together in an agate mortar and grind them thoroughly for 30 min until a uniform and fine mixed powder without obvious particles is obtained. Transfer the mixed powder to a clean crucible and place the crucible in a tube furnace. First, continuously introduce nitrogen into the tube furnace to replace the air in the furnace. The nitrogen gas introduction time should not be less than 30 min to ensure that the air in the furnace is completely replaced. After the air replacement is completed, the temperature is increased to 500℃ at a heating rate of 5℃ / min and kept constant for 2 h for molten salt thermal reaction. After the reaction is completed, close the tube furnace, stop the nitrogen supply, and allow the crucible to cool naturally to room temperature with the furnace to obtain a blocky solid product.

[0031] (4) Post-treatment and preparation of catalyst: Take out the cooled block solid product, grind it into powder in an agate mortar, wash the powder repeatedly with deionized water for no less than 3 times, and then wash it 3 times with anhydrous ethanol until the filtrate is neutral to completely remove residual impurities and soluble salts; after washing, transfer the solid powder to a vacuum drying oven and dry it in a vacuum at 80℃ for 10 h to obtain chlorine axially coordinated copper single-atom catalyst.

[0032] Example 2

[0033] The only difference between this embodiment and Example 1 is that the preparation conditions of the nitrogen-rich layered carbon matrix precursor are adjusted. In step (1) of Example 1, "heating to 550°C at a rate of 3°C / min under static air atmosphere and holding at a constant temperature for 3h" is replaced with "heating to 650°C at a rate of 5°C / min under static air atmosphere and holding at a constant temperature for 2h". The remaining steps and process parameters are the same as in Example 1, and finally, a chlorine axially coordinated copper single-atom catalyst is obtained.

[0034] Example 3

[0035] The only difference between this embodiment and Example 1 is that the cumulative number of thermal stripping treatments is adjusted: "Repeat the above thermal stripping process twice (to complete a total of 3 thermal strippings)" in step (2) of Example 1 is replaced with "Repeat the above thermal stripping process once (to complete a total of 2 thermal strippings)"; the remaining steps and process parameters are completely consistent with Example 1, and finally a chlorine axially coordinated copper single-atom catalyst is obtained.

[0036] Example 4

[0037] The only difference between this embodiment and Example 1 is that the amount of materials used to prepare the catalyst precursor by molten salt thermal reaction is adjusted: in step (3) of Example 1, "accurately weigh 0.2 g of the above yellow powder, 0.1 g of copper chloride, 1.35 g of lithium chloride, and 1.5 g of potassium chloride" is replaced with "accurately weigh 0.5 g of the above yellow powder, 0.1 g of copper chloride, 2.5 g of lithium chloride, and 2.5 g of potassium chloride"; the remaining steps and process parameters are completely consistent with Example 1, and finally, a chlorine axially coordinated copper single-atom catalyst is obtained.

[0038] Example 5

[0039] The only difference between this embodiment and Example 1 is that the amount of materials used to prepare the catalyst precursor by molten salt thermal reaction is adjusted: in step (3) of Example 1, "accurately weigh 0.2 g of the above yellow powder, 0.1 g of copper chloride, 1.35 g of lithium chloride, and 1.5 g of potassium chloride" is replaced with "accurately weigh 0.2 g of the above yellow powder, 0.2 g of copper chloride, 1.5 g of lithium chloride, and 1.5 g of potassium chloride"; the remaining steps and process parameters are completely consistent with Example 1, and finally, a chlorine axially coordinated copper single-atom catalyst is obtained.

[0040] Example 6

[0041] The only difference between this embodiment and Example 1 is that the drying conditions in the catalyst post-treatment process are adjusted: in step (4) of Example 1, "transfer the solid powder to a vacuum drying oven and dry it at 80°C for 10 h" is replaced with "transfer the solid powder to a forced-air drying oven and dry it at 60°C for 24 h"; the remaining steps and process parameters are completely consistent with Example 1, and finally, a chlorine axially coordinated copper single-atom catalyst is obtained.

[0042] Comparative Example 1

[0043] The only difference between this comparative example and Example 1 is that the types of materials and the feeding method in the catalyst precursor preparation stage of the molten salt thermal reaction are adjusted: the "accurately weigh 0.2 g of the above yellow powder, 0.1 g of copper chloride, 1.35 g of lithium chloride and 1.5 g of potassium chloride in step (3) of Example 1, put the four materials together in an agate mortar, mix and grind them thoroughly for 30 min until a uniform and fine mixed powder without obvious particles is obtained" is replaced with "accurately weigh 0.2 g of the above yellow powder and 0.1 g of copper nitrate, stir them in 50 mL of anhydrous ethanol at room temperature until the ethanol is completely evaporated, collect the obtained powder and grind it thoroughly in an agate mortar for 30 min until it is uniformly mixed"; the remaining steps and process parameters are completely consistent with Example 1, and finally a chlorine-free coordinated copper single-atom catalyst is obtained.

[0044] This comparative example uses anhydrous ethanol to assist dispersion before grinding in order to ensure that the uniformity of mixing copper nitrate and yellow powder is no less than the dry grinding effect of the molten salt system in Example 1.

[0045] Comparative Example 2

[0046] The only difference between this comparative example and Example 1 is that the molten salt system and the metal source are replaced with a chlorine-free nitrate system: in step (3) of Example 1, "accurately weigh 0.2 g of the above yellow powder, 0.1 g of copper chloride, 1.35 g of lithium chloride, and 1.5 g of potassium chloride" is replaced with "accurately weigh 0.2 g of the above yellow powder, 0.1 g of copper nitrate, 1.35 g of lithium nitrate, and 1.5 g of potassium nitrate"; the remaining steps and process parameters are completely consistent with Example 1, and finally a chlorine-free axially coordinated copper single-atom catalyst is obtained.

[0047] Comparative Example 3

[0048] The only difference between this comparative example and Example 1 is that the mixed molten salt of lithium chloride and potassium chloride is replaced with a single molten salt of lithium chloride. In step (3) of Example 1, "accurately weigh 0.2 g of the above yellow powder, 0.1 g of copper chloride, 1.35 g of lithium chloride and 1.5 g of potassium chloride" is replaced with "accurately weigh 0.2 g of the above yellow powder, 0.1 g of copper nitrate and 2.85 g of lithium chloride". The remaining steps and process parameters are completely consistent with those of Example 1, and finally, a chlorine axially coordinated copper single-atom catalyst is obtained.

[0049] Because lithium chloride alone has a high melting point, its melting degree, fluidity, and spatial confinement effect at 500℃ are significantly inferior to those of lithium chloride-potassium chloride eutectic mixed molten salt. Therefore, the copper atom dispersion is poor, and optimal catalytic performance cannot be achieved.

[0050] Comparative Example 4

[0051] The only difference between this comparative example and Example 1 is that the thermal stripping process is not performed: the product of the first calcination of dicyandiamine is directly used as the carbon-nitrogen matrix, the thermal stripping process of step (2) in Example 1 is omitted, and steps (3) and (4) are directly performed after step (1) to finally obtain the chlorine axially coordinated copper single-atom catalyst.

[0052] Related performance tests

[0053] 1. The chlorine-axially coordinated copper single-atom catalyst prepared in Example 1 exhibits a rough, aggregate-like macroscopic morphology. Figure 1 This structure facilitates the exposure of more active sites and optimizes the mass transfer efficiency of reactants / products. To accurately characterize its microstructure at the atomic scale, aberration-corrected transmission electron microscopy was used to analyze the microscopic morphology of the catalyst. The results are as follows: Figure 2As shown, metallic Cu did not form obvious nanoparticles or large-sized aggregates, but was uniformly dispersed on the nitrogen-doped carbon support in the form of isolated single atoms (marked by red circles in the figure), confirming the successful construction of copper single-atom active centers.

[0054] 2. Figure 3 The image shows a transmission electron microscope (TEM) scan of the elemental distribution of the chlorine-axially coordinated copper single-atom catalyst prepared in Example 1. The image clearly shows that Cu has been successfully loaded into the catalyst and is uniformly dispersed synchronously with N in the support. At the same time, Cl is also uniformly distributed inside the support without obvious local enrichment or agglomeration. This further confirms that Cu, N and Cl are all highly dispersed in the catalyst.

[0055] 3. X-ray diffraction (XRD) pattern Figure 4 The results show that the chlorine-axially coordinated copper single-atom catalyst prepared in Example 1 and the chlorine-free axially coordinated copper single-atom catalyst prepared in Comparative Example 1 both exhibited characteristic diffraction peaks corresponding to the carbon support, and no characteristic diffraction signals of copper metal or copper oxide were detected. This indicates that Cu species are highly dispersed on the carbon support in an atomic-level manner in both catalysts, and no obvious crystalline copper particles are formed. X-ray photoelectron spectroscopy (XPS) Cl 2p spectrum (…) Figure 4 As you can see, the chlorine-axially coordinated copper single-atom catalyst prepared in Example 1 exhibits significant Cl2p values ​​near approximately 198 eV and approximately 200 eV, respectively. 3 / 2 and 2p 1 / 2 Characteristic peaks were observed in this region, while the chlorine-free coordinated copper single-atom catalyst prepared in Comparative Example 1 only showed weak noise at the baseline level, with no obvious Cl signal observed. This confirms that Cl was successfully introduced and stably present in the catalyst system of Example 1, forming a Cu-N catalyst. x The formation of -Cl axial coordination active centers provides direct spectroscopic evidence.

[0056] 4. Figure 6 The figures show the X-ray absorption near-edge structure (XANES) spectra of copper for the catalysts prepared in Example 1 and Comparative Example 1. As can be seen from the figures, the absorption lines of Example 1 are located at the standard cuprous oxide (Cu)... + ) and standard copper oxide (Cu) 2+ The spectral lines of the sample directly indicate that the valence state of Cu in this catalyst is between +1 and +2; while the absorption lines of the comparative example 1 are closer to those of standard copper oxide, indicating that the valence state of Cu is mainly +2. This difference further confirms that the introduction of axial coordination of chlorine effectively regulates the electronic structure and valence state distribution of the Cu center, providing a key electronic regulation basis for optimizing the electroreduction performance of CO2.

[0057] 5. Figure 7The Fourier transform extended X-ray absorption fine structure (FT-EXAFS) spectra of the catalysts prepared in Example 1 and Comparative Example 1 are shown. The figure shows that the characteristic peaks of Example 1 are dominated by Cu-N coordination signals, accompanied by a weak Cu-Cl coordination signal, and no obvious Cu-Cu metallic bond characteristic peaks are observed. This result directly confirms that Cu atoms exist in an atomically dispersed state on the support surface, without metal agglomeration or nanoparticle formation. Further fitting results (Table 1) indicate that Cu atoms in Example 1 exist in a Cu-N2-Cl coordination form, while Cu atoms in Comparative Example 1 exist in a Cu-N3 coordination form, with no Cu-Cl signal.

[0058] Table 1. EXAFS fitting parameters of Cu atoms in the catalysts prepared in Example 1 and Comparative Example 1

[0059]

[0060] 6. Figure 8 The figures show the nitrogen adsorption-desorption isotherms of the catalysts prepared in Example 1 and Comparative Example 4. As can be seen from the figures, the nitrogen adsorption volume of the catalyst prepared in Example 1 is significantly higher than that in Comparative Example 4, directly reflecting its more developed pore structure. Combined with the pore structure parameters shown in Table 2, it can be seen that compared to Comparative Example 4, the catalyst prepared in Example 1 has significantly improved specific surface area and pore volume, while the average pore size is significantly reduced, forming a finer and more interconnected porous network. This optimized pore structure not only effectively increases the number of exposed active sites but also significantly promotes mass transport during the reaction process, providing important structural support for improving the catalytic efficiency of the material.

[0061] Table 2. Pore structure parameters of the catalysts prepared in Example 1 and Comparative Example 4

[0062]

[0063] Combining macroscopic morphology and microstructure characterization results, it is shown that the efficient preparation of chlorine-axially coordinated copper single-atom catalysts was successfully achieved through a thermal exfoliation-molten salt method, and its atomic-level dispersion characteristics are similar to those of Cu-N x The -Cl coordination structure provides a solid structural foundation for the subsequent excellent CO2 electroreduction catalytic performance.

[0064] Application examples

[0065] The catalysts prepared in Examples 1-6 and Comparative Examples 1-4 were respectively used as working electrodes and assembled into a three-electrode flow electrolysis cell system for electrocatalytic performance testing. The electrolysis cell used an anion exchange membrane to separate the anode and cathode flow chambers. The electrolyte for both the cathode and anode was 1.0 M KOH solution. The counter electrode was a foam electrode, and the reference electrode was a silver / silver chloride (Ag / AgCl) electrode. The CO2 gas flow rate was controlled at 20 mL / min during the reaction.

[0066] Cyclic voltammetry (CV) activation: The electrode was activated using a Shanghai Chenhua CHI 760E electrochemical workstation with a potential range of -0.5 to -1.9 V (vs. RHE) and a scan rate of -50 mV·s. -1 Cyclic scanning was performed for 30 cycles until the electrochemical performance of the electrode tended to stabilize.

[0067] Faraday efficiency (FE) test: After the electrode is activated and stabilized, switch to constant current-time test mode; the gaseous products generated by the cathode are quantitatively detected online by Fuli 9790II gas chromatograph (GC), and the Faraday efficiency of each product is accurately calculated accordingly.

[0068] Figure 9 The catalyst prepared in Example 1 was used in the range of 100–600 mA cm⁻¹. -2 The graph shows the Faradaic efficiency distribution of CO2 electroreduction products within a current density range. As can be seen from the graph, as the current density increases from 100 mA cm⁻¹... -2 Gradually increase to 400 mA cm -2 The Faraday efficiency of methane showed a continuous upward trend, reaching a peak of 73%; when the current density was further increased to 600 mA cm⁻¹, the efficiency remained high. -2 While the Faraday efficiency of methane decreased slightly, it remained stable above 60%. Meanwhile, the Faraday efficiencies of byproducts such as hydrogen, carbon monoxide, and ethylene remained consistently low, demonstrating the excellent selectivity of this catalyst for the electroreduction of CO2 to methane.

[0069] The catalysts prepared in Examples 1-6 and Comparative Examples 1-4 were subjected to a temperature of 400 mA cm⁻¹. -2 A comparative test was conducted on the performance of CO2 electroreduction for methanogenesis under different current densities, and the results are as follows: Figure 10As shown, the methane Faradaic efficiency of Comparative Examples 1 and 2 is significantly lower than that of the Examples. This result indicates that the axial coordination structure of chlorine is the core key to improving the selectivity of the catalyst in the electroreduction of CO2 to methane. The methane Faradaic efficiency of Comparative Example 3 is significantly lower than that of Example 1, indicating that the use of a eutectic mixed molten salt system for preparation is an important prerequisite for achieving optimal catalytic performance. The methane Faradaic efficiency of Comparative Example 4 is also significantly lower than that of Example 1. This result indicates that the nitrogen-rich carbon matrix prepared through multiple thermal exfoliation processes is a good support for anchoring active centers and ensuring efficient catalysis. Although the methane Faraday efficiency of Example 5 was higher than that of Comparative Examples 1 and 2, it was slightly lower than that of Comparative Examples 3 and 4 and other examples. This is mainly because when the amount of copper chloride added was excessive, the loading of copper species exceeded the saturation anchoring capacity of the nitrogen-rich layered carbon matrix. Excess copper ions could not form stable coordination bonds with nitrogen atoms in the support, and thus migrated and agglomerated during the molten salt thermal reaction, forming copper group clusters or nanoparticles, which destroyed the single-atom dispersion state of the catalyst. At the same time, the copper agglomeration sites significantly aggravated the hydrogen evolution side reaction and reduced the selectivity of CO2 electroreduction to methane. In addition, changes in the amount of lithium chloride added also slightly affected the molten salt reaction environment, which was unfavorable to Cu-N x The stable construction of the -Cl axially coordinated active center ultimately resulted in the catalytic performance of Example 5 being significantly inferior to that of Example 1. However, the catalytic performance of Example 5 was still significantly better than that of Comparative Examples 1 and 2, which had no chlorine axial coordination, further demonstrating that chlorine axial coordination remains the core factor in improving catalyst selectivity.

[0070] The catalyst prepared in Example 1 was assembled in a flow electrolyzer and subjected to a flow electrolysis at 400 mA cm⁻¹. -2 Long-term continuous electrocatalytic stability testing was conducted at industrial-grade current densities, and the results are as follows: Figure 11 As shown, the methane Faradaic efficiency did not show significant decline throughout the entire test period, remaining consistently above 65%, and the product selectivity remained excellent and stable. These results indicate that the chlorine-axially coordinated copper single-atom catalyst prepared in Example 1 can achieve high efficiency at 400 mA cm⁻¹. -2 It achieved long-term stable operation for 10 hours under high industrial current density, while maintaining high methane selectivity, which strongly verifies its potential and feasibility for large-scale industrial application in the field of CO2 electrochemical reduction to methane.

[0071] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. However, the above description is merely a specific embodiment of the present invention, and the technical features of the present invention are not limited thereto. Any other embodiments derived by those skilled in the art without departing from the technical solution of the present invention should be covered within the scope of the present invention.

Claims

1. A method for preparing a chlorine-axially coordinated copper single-atom catalyst, characterized in that, Includes the following steps: 1) Dicyandiamine was calcined to obtain a nitrogen-rich layered carbon matrix precursor, which was then cooled and ground into powder. 2) The nitrogen-rich layered carbon matrix precursor powder was subjected to several thermal exfoliation treatments to obtain nitrogen-rich layered carbon matrix powder. 3) The nitrogen-rich layered carbon matrix powder, copper salt and lithium-potassium salt mixed molten salt are mixed and ground evenly to obtain a mixed powder. At least one of the copper salt, lithium salt and potassium salt is a chloride salt to provide the chlorine source required for axial coordination. The mixed powder is subjected to molten salt thermal reaction under an inert atmosphere to obtain a blocky catalyst precursor solid product. 4) The catalyst precursor solid product is ground, washed and dried to obtain a chlorine axially coordinated copper single-atom catalyst.

2. The method for preparing a chlorine-axially coordinated copper single-atom catalyst as described in claim 1, characterized in that, In step 1), the calcination process is carried out in a static air atmosphere, with a calcination heating rate of 2-10℃ / min, a calcination temperature of 450-650℃, and a calcination time of 2-5 h.

3. The method for preparing a chlorine-axially coordinated copper single-atom catalyst as described in claim 1, characterized in that, In step 2), the thermal stripping treatment is carried out in a static air atmosphere. The thermal stripping heating rate is 5-15℃ / min, the thermal stripping temperature is 400-600℃, the single thermal stripping time is 3-6 h, and the thermal stripping process is repeated 2-5 times.

4. The method for preparing a chlorine-axially coordinated copper single-atom catalyst as described in claim 1, characterized in that, In step 3), the copper salt is selected from at least one of copper chloride, cuprous chloride, copper nitrate, and copper acetate; the lithium salt is selected from at least one of lithium chloride and lithium nitrate; and the potassium salt is selected from at least one of potassium chloride and potassium nitrate.

5. The method for preparing a chlorine-axially coordinated copper single-atom catalyst as described in claim 1, characterized in that, In step 3), the mass ratio of nitrogen-rich layered carbon matrix powder, copper salt, lithium salt, and potassium salt is 1:0.1~1:2~8:2~8.

6. The method for preparing a chlorine-axially coordinated copper single-atom catalyst as described in claim 1, characterized in that, In step 3), the inert atmosphere is argon or nitrogen, the heating rate of the molten salt thermal reaction is 2-8℃ / min, the reaction temperature is 300-800℃, and the reaction time is 1-5 h.

7. The method for preparing a chlorine-axially coordinated copper single-atom catalyst as described in claim 1, characterized in that, In step 4), the drying temperature is 60-100℃ and the drying time is 6-24 h.

8. A chlorine-axially coordinated copper single-atom catalyst, characterized in that, It is prepared according to the preparation method of any one of claims 1-7, wherein the catalyst uses a nitrogen-rich layered carbon matrix as a support, and copper is dispersed on the surface of the support in the form of single atoms, with each copper atom and nitrogen atoms in the plane of the support forming Cu-N x The coordination center is formed by a chlorine atom forming a coordinate bond with the copper center axially, thus forming Cu-N. x -Cl active center structure, where x is 2, 3 or 4.

9. The application of the chlorine-axially coordinated copper single-atom catalyst as described in claim 8 in the electrocatalytic reduction of CO2 to methane.

10. The application as described in claim 9, characterized in that, The application employs an alkaline electrolyte and a three-electrode flow electrolysis cell system; at 400 mA cm⁻¹ -2 At industrial-grade current densities, the peak methane faradaic efficiency reaches over 73%, and during 10 hours of continuous stable operation, the methane faradaic efficiency remains stable at over 65%.

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