Process for the preparation of a copper single-atom catalyst enriched in pyridinic nitrogen types

Abstract

The application discloses a preparation method of a copper monatomic catalyst rich in pyridine nitrogen and application of the catalyst to electrocatalytic reduction of nitrate to ammonia. The catalyst exhibits good catalytic performance for reduction of nitrate to ammonia in an alkaline medium. Theoretical and experimental results prove that the copper monatomic catalyst rich in copper-pyridine nitrogen-carbon active sites improves adsorption energy of NO3- and corresponding intermediates and reduces an energy barrier of a rate control step, thereby promoting a process of reduction of nitrate to ammonia.

Description

Technical Field

[0001] The present invention relates to the technical field of electrocatalytic material preparation, and in particular to a method for preparing a pyridine nitrogen-rich copper single-atom catalyst and application thereof in nitrate reduction to produce ammonia. Background Art

[0002] The 14th Five-Year Plan and the Outline of the Long-Term Goals for 2035 propose promoting green development and a circular economy, facilitating a green transformation of my country's development model, achieving efficient and circular resource utilization, and contributing to the achievement of dual carbon goals. In March 2022, the National Development and Reform Commission and the National Energy Administration jointly issued the "14th Five-Year Plan New Energy Storage Development Implementation Plan," which emphasized expanding the application of hydrogen and ammonia energy storage, focusing on pilot demonstrations of long-cycle energy storage technologies such as renewable energy ammonia production. Ammonia (NH3) is an important chemical raw material and is also considered a renewable energy carrier. Its main advantages include high energy density (22.5 MJ / kg), green and carbon-free production, and high transportation efficiency. Currently, the main method for industrial ammonia synthesis is the Haber-Bosch process. However, the Haber-Bosch process requires the combustion of large amounts of fossil fuels to achieve the required high temperatures and pressures, and the initial hydrogen production process through methane reforming also releases large amounts of greenhouse gases, causing serious environmental pollution. Therefore, there is an urgent need to explore green and sustainable ammonia synthesis technologies to decentralize the traditional energy-intensive and environmentally polluting ammonia synthesis industry.

[0003] In recent years, nitrate pollution has become a serious global environmental problem. Its main sources are fuel and wastewater discharge from food production industries such as agriculture, animal husbandry, and forestry, as well as industrial sectors. Therefore, converting harmful nitrate into more valuable ammonia can not only solve the environmental problems caused by nitrate pollution, but also has the potential to replace traditional ammonia synthesis processes and achieve resource recycling. However, the electrochemical nitric acid reduction reaction to ammonia (NO3RR) involves the transfer of 8 electrons and the coupling of 9 protons. The complex electron / proton transfer is bound to reduce the reaction kinetics and is accompanied by numerous intermediates and by-products (such as nitrite and hydrogen), thereby affecting the Faradaic efficiency (FE) and yield (YR) of NH3. Therefore, designing electrocatalysts with the goal of addressing the above challenges of NO3RR is the optimal approach to achieve efficient ammonia production.

[0004] Carbon materials have the advantages of good conductivity, large specific surface area, high crustal abundance, stable and adjustable structure, and have become one of the most important catalysts or catalyst supports. In carbon-based metal catalysts obtained by introducing metals into carbon materials, there is usually a strong interaction between the metal and the carbon support, which can be used to adjust the electronic structure of the active metal center to achieve efficient catalysis. Previous literature has reported a large number of carbon-based metal catalysts. The size range of these carbon-based non-metal catalysts includes nanometers, clusters and atoms. Compared with nanocatalysts, single-atom catalysts (SACs) have atomically distributed active centers, maximum atomic utilization efficiency, unsaturated coordination environment and tunable electronic structure, which can achieve the combined advantages of homogeneous and heterogeneous catalysts. At the same time, the isolated sites in single-atom catalysts can produce highly selective adsorption of specific substances, which shows great catalytic potential for the NO3RR process with complex intermediates.

[0005] The single-atom structure increases the surface free energy of the material. Although it improves the activity of the material, it also increases the tendency of single atoms to aggregate to form clusters or nanoparticles, which in turn leads to the deactivation of active sites. Therefore, it is of great significance to select a suitable substrate material to enhance the interaction between metal atoms and the substrate. Gel materials generally have a three-dimensional multi-level network structure. Their porous structure with high specific surface area can not only provide more active sites for anchoring metal atoms, but also limit the aggregation of metal atoms. By high-temperature treatment of gel materials with carbon-carbon covalent bonds (CC) or carbon-nitrogen covalent bonds (CN), porous graphitic carbon with high specific surface area, abundant exposed edges, high conductivity and high electrochemical stability can be obtained. It can be seen that gel-derived carbon materials are excellent supports for efficiently anchoring metal atoms. Therefore, designing carbon-based single-atom catalysts based on gel materials is a very novel and worthy of exploration synthetic method. At present, copper-based catalysts show excellent activity in NO3RR. However, due to the lack of feasible preparation strategies, these copper single-atom catalysts have low metal loadings, and the selectivity and yield for NO3RR are ultimately unsatisfactory. In addition, directly regulating the microenvironment of single-atom sites by changing the cross-linker of the precursor remains a major challenge. Summary of the Invention

[0006] The present invention provides a preparation method of a pyridine nitrogen-rich copper single-atom catalyst and its application in electrocatalytic nitrate reduction to produce ammonia, which solves the problems of low catalytic efficiency and poor selectivity of existing catalysts.

[0007] In order to solve this technical problem, the present invention provides the following technical solutions:

[0008] The preparation method of the pyridine nitrogen-rich copper single-atom catalyst comprises the following steps:

[0009] S1: Pyrrole monomer (pyrrole), polyethyleneimine (PEI) and sodium dodecylbenzenesulfonate (SDBS) are mixed and dissolved in HCl solution, marked as solution A;

[0010] S2 dissolves ammonium persulfate (APS) in HCl solution and is labeled as solution B;

[0011] S3: placing solution A and solution B in a refrigerator to cool, quickly mixing solution A and solution B after cooling, and placing the mixture in a refrigerator to stand to fully polymerize into a hydrogel;

[0012] S4: The polymerized hydrogel is washed and purified alternately with ethanol and deionized water. The purified hydrogel is dispersed in a CuCl2 solution and stirred overnight. The hydrogel with adsorbed copper atoms is freeze-dried in a freeze dryer. The dried sample is then ground into a uniform powder.

[0013] S5: calcining the sample powder under an argon atmosphere. After carbonization, the powder is added to an HCl solution and allowed to stand to remove metallic impurities.

[0014] S6: The acid-washed powder was washed with deionized water until neutral and then dried in a vacuum oven. Finally, the obtained sample was carbonized a second time under the same conditions to obtain a pyridinium-rich nitrogen-type copper single-atom catalyst, denoted as PR-CuNC.

[0015] S7 dispersed PR-CuNC into a solution consisting of deionized water, isopropyl alcohol, and Nafion binder.

[0016] Ultrasonic treatment was performed. The dispersion was then applied to the surface of carbon paper and allowed to dry naturally.

[0017] Preferably, in step S1, the volume of the pyrrole monomer is 416 μL, the mass of polyethyleneimine is 0.101 g, the mass of sodium dodecylbenzenesulfonate is 0.697 g, the concentration of HCl is 1.0 M, and the volume is 10 mL.

[0018] Preferably, the mass of ammonium persulfate in step S2 is 1.397 g, the concentration of HCl is 1.0 M, and the volume is 10 mL.

[0019] Preferably, the cooling temperature in step S3 is 2° C. and the standing time is 12 hours.

[0020] Preferably, the concentration of the CuCl2 solution in step S4 is 0.2 M, the stirring time is 12 hours, and the freezing temperature of the freeze dryer is about -60°C.

[0021] Preferably, the concentration of HCl in step S5 is 0.5 M, the soaking time is 4 hours, and the soaking temperature is 80° C.

[0022] Preferably, the temperature of the vacuum drying oven in step S6 is 80°C.

[0023] Preferably, step S7 specifically comprises: dispersing 3 mg of PR-CuNC into a solution consisting of 360 μL of distilled water, 120 μL of isopropanol and 20 μL of Nafion binder, ultrasonically treating for 15 minutes, and then dropping 20 μL of the dispersion onto the carbon paper surface.

[0024] This solution also provides an electrode prepared with a pyridine nitrogen-rich copper single atom catalyst, which is prepared by the above-mentioned preparation method.

[0025] This solution also provides the application of the above-mentioned electrode in electrocatalytic nitrate reduction to produce ammonia.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] This scheme provides a preparation strategy for regulating the coordination environment of single-atom catalyst sites. A pyridine-rich copper single-atom catalyst derived from polypyrrole / polyethyleneimine three-dimensional semi-interpenetrating hydrogel was prepared. The catalyst has significant NO3RR activity in an alkaline environment.

[0028] This scheme highlights the regulation strategy for the type of coordinated nitrogen atoms in copper single-atom catalysts. By directly introducing polyethyleneimine, a pyridine nitrogen-rich single-atom catalyst was synthesized. The coordination environment around the copper atom in the catalytic center was directly regulated at the molecular level, effectively adjusting the electronic structure of the catalyst and improving the intermediate reaction path of the catalytic process. This is beneficial to improving the activity and selectivity of various electrocatalytic reactions. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings:

[0030] Figure 1 The preparation process of pyridine-nitrogen-rich copper single-atom catalyst and the schematic diagram of NO3RR catalytic process on Cu site;

[0031] Figure 2 For morphology characterization; among them, (a) is the SEM image of PR-CuNC; (b) is the HAADF-STEM image of PR-CuNC; (c) is the EDS mapping image of PR-CuNC;

[0032] Figure 3 is the XRD of PR-CuNC;

[0033] Figure 4For structural characterization; (a) is the XPS full spectrum of PR-CuNC; (b) is the XPS N1s fine spectrum of PR-CuNC; (c) is the Cu 2p spectrum of PR-CuNC;

[0034] Figure 5 XAFS characterization of PR-CuNC; (a) XANES of Cu foil, Cu2O, CuO, and PR-CuNC; (b) FT-EXAFS data of Cu foil, Cu2O, CuO, and PR-CuNC; (c) WT spectrum of PR-CuNC;

[0035] Figure 6 Electrochemical tests: (a) LSV test with and without 0.1 M NO3ˉ in 0.1 M KOH electrolyte; (b) It curves of constant potential test at different applied potentials.

[0036] Figure 7 Figure 1 is the standard curve for UV spectrophotometry test. (a) is the UV absorption spectrum of the standard ammonia concentration; (b) is the fitted standard curve; (c) is the UV absorption spectrum of the standard nitrite concentration; (d) is the stability test of the fitted standard curve PR-CuNC at -0.5 V vs. RHE.

[0037] Figure 8 (a) Faradaic efficiency of ammonia; (b) ammonia yield; (c) normalized ammonia yield; (d) Faradaic efficiency of ammonia, nitrite and hydrogen;

[0038] Figure 9 (a) Constant potential test curve of PR-CuNC; (b) Faradaic efficiency and yield of ammonia corresponding to the stability test;

[0039] Figure 10 (a) H NMR spectrum; (b) Ammonia produced under UV absorption spectrum and H NMR spectrum;

[0040] Figure 11 is the charge difference density of the two Cu-N4 models;

[0041] Figure 12 is the Cu d and N p orbital partial density of states (PDOS) of the two Cu-N4 models;

[0042] Figure 13 It is the intermediate structure of nitrate reduction to ammonia;

[0043] Figure 14 Gibbs free energy diagram for the nitrate reduction reaction to produce ammonia.

[0044] In order to make the objects, technical solutions and advantages of the present application clearer, below, combined with examples, the present application is further described in detail. The illustrative embodiments of the present application and the description thereof are only used to explain the present application and not as a limitation of the present application. DETAILED DESCRIPTION

[0045] Example 1

[0046] (1) Preparation of PR-CuNC

[0047] First, 416 μL of pyrrole monomer, 0.101 g of polyethyleneimine (PEI) and 0.697 g of sodium dodecylbenzenesulfonate (SDBS) were dissolved in 10 mL of 1.0 M aqueous HCl solution, labeled as solution A. 1.397 g of ammonium persulfate (APS) was added to 10 mL of 1.0 M aqueous HCl solution, labeled as solution B. After cooling solution A and solution B to 2℃, the above two solutions were quickly mixed, and the mixture was placed in a refrigerator at 2℃ for 12 hours to ensure sufficient polymerization to form a hydrogel. Finally, the polymerized hydrogel was washed with deionized water and anhydrous ethanol alternately for several times to remove unreacted residual impurities and by-products. The purified hydrogel was freeze-dried at low temperature (about -60℃) and low pressure (about 1.0 Pa). After that, the dried sample was ground into a uniform powder and transferred to a porcelain boat. The sample powder was heated to 800℃ at a rate of 10℃ min -1 -1 under argon atmosphere, and then kept at 800℃ for 2 hours. After carbonization, the powder was added to 0.5 M HCl solution and kept at 80℃ for 4 hours to remove metal impurities. The acid-washed powder was washed with deionized water to neutral (pH = 7) and then dried in a vacuum drying oven at a temperature of 80℃. Finally, the obtained sample was subjected to a second carbonization under the same conditions. The catalyst prepared by this method is named PR-CuNC.

[0048] Example 2

[0049] The electrochemical performance test was carried out at room temperature by H-type electrolytic cell connected to Donghua DH7003 electrochemical workstation. The H-type electrolytic cell was separated by Nafion 115 ion exchange membrane with a size of 3.5 cm x 3.5 cm. Before the experiment, the Nafion membrane was soaked in 5wt% hydrogen peroxide solution for 60 minutes, and the water bath heating temperature was 80℃. Then it was water bath heated at 80℃ for 60 minutes in deionized water. The working electrode was selected from the carbon paper of PR-CuNC catalyst ink in Example 1. The geometric area of the carbon paper was 0.5 x 3 cm 2 , and the active area was 0.5 x 0.5 cm 2Hydrophobic carbon paper was soaked in ethanol and deionized water for 15 minutes and ultrasonically treated to remove surface impurities. 3 mg of catalyst powder and 20 μL of Nafion solution (5 wt%, Dupont, USA) were added to a mixture of deionized water (360 μL) and isopropanol (120 μL) and ultrasonically treated for 30 minutes to prepare catalyst ink. 20 μL of homogeneous catalyst ink was dropped onto an active area of ​​0.25 cm 2 , mass loading is 0.48 mg cm -2 on a hydrophobic carbon paper. The other side of the carbon paper without the catalyst was sealed with insulating hot melt adhesive to make the final working electrode. A three-electrode working system was used for electrochemical experiments, with a saturated calomel electrode (SCE, 1M KCl) and a graphite rod as the reference electrode and counter electrode, respectively. The counter electrode was placed in the anode cell, and the reference electrode and working electrode were placed in the cathode cell. 30mL of 0.1M KOH and 0.1MKNO3 solution was used as the cathode electrode solution, and the anode was 0.1M KOH solution. All potentials of the reference reversible hydrogen electrode (RHE) were obtained by the following calibration formula: E(RHE) = E(SCE) + 0.059×pH + 0.253V. Before the electrochemical measurement, a pure argon gas flow was injected into the cathode electrolyte for 30 minutes to remove oxygen and nitrogen. All electrodes were cyclic voltammetry (CV) in the potential range of +0.5V to -0.9V vs. RHE at 100mV s -1 The electrode was activated for 10 cycles at a scan rate of 10 mV s -1 Linear sweep voltammograms (LSVs) were obtained at a scan rate of +0.5 to -0.9 V relative to RHE. Current-time (It) curves were recorded for 30 min at different potentials using chronoamperometry, and the corresponding electrolytes were collected for calculation of NH3 yield and Faradaic efficiency.

[0050] Figure 1 The preparation process of Example 1 is shown. Specifically, PEI is introduced into pyrrole polymerization to form PPy-PEI hydrogel, in which PPy chains and PEI chains are uniformly interpenetrated. Thanks to the rich primary and secondary amine groups of PEI, PPy-PEI hydrogel has a stronger ability to anchor Cu ions than simple PPy. Pyrrole and PEI cross-linking effectively improves the scarce nitrogen active sites and provides a diverse coordination environment. In addition, PPy-PEI hydrogel has a unique three-dimensional semi-interconnected nanostructure with rich N sites and a large specific surface area, which is conducive to the adsorption of Cu ions and also prevents the aggregation of Cu particles during the subsequent carbonization process. Finally, the copper-adsorbed PPy-PEI hydrogel is subjected to secondary carbonization to obtain PR-CuNC, in which highly dispersed Cu atoms are anchored on the hydrogel-derived carbon substrate.

[0051] Figure 2 In (a), the scanning electron microscope image of Example 1 shows a porous particle structure composed of interconnected nanoparticles. Figure 2 As shown in (b), aberration correction was performed using a high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) to determine the atomic distribution. The HAADF-STEM image of Example 1 shows light spots scattered along the edge and surface of the carbon substrate. Since the atomic mass of the Cu element in the catalyst is much greater than that of other elements (C, N, O), it can be determined that the light spots represent isolated Cu atoms. Figure 2 In (c), energy dispersive X-ray spectroscopy (EDS mapping) analysis further confirmed the uniform dispersion of Cu, N, and C elements in Example 1, indicating that Cu atoms are uniformly distributed on the nitrogen-doped carbon substrate.

[0052] exist Figure 3 The XRD spectrum of Example 1 shows a broad peak at 25.6°, corresponding to the (002) plane of graphitic carbon, while no diffraction peaks of the Cu-based crystalline phase are observed. Therefore, there are no aggregated Cu-based compounds in PR-CuNC, and the Cu species are mainly amorphous.

[0053] The elemental composition and valence state of the catalyst were determined by X-ray photoelectron spectroscopy (XPS). Based on the XPS spectrum, the elemental composition and atomic ratio of Example 1 were determined ( Figure 4 a) The atomic ratio of nitrogen in PR-CuNC is 8.21%, which can be attributed to the presence of PEI with abundant amino groups. Figure 4 As shown in (b), the high-resolution N1s XPS spectrum of Example 1 can be divided into four peaks: pyridinic-N (398.2 eV), pyrrolic-N (399.3 eV), graphitic-N (400.8 eV), and oxidized-N (401.7 eV). The absolute content of each nitrogen structure was calculated, with the absolute content of pyridinic-N being 3.20% and the absolute content of pyrrolic-N being 0.76%. This indicates that the introduction of PEI successfully synthesized a catalyst richer in pyridinic nitrogen structures. Figure 4 (c) Cu 2p XPS spectrum of Example 1, showing that Cu 2p 3 / 2 and Cu 2p 1 / 2 The XPS characteristic peaks at 934.55 and 954.40 eV correspond to Cu(II), and the XPS characteristic peaks at 932.40 and 952.14 eV correspond to Cu(I) / Cu(O). The content of Cu in Example 1 was more accurately determined to be 6.01 wt% by inductively coupled plasma optical emission spectroscopy (ICP-OES).

[0054] Considering the limitations of XPS in characterizing precise structures, we further analyzed the fine structure of Example 1 by X-ray absorption fine structure (XAFS) spectroscopy to obtain more accurate coordination structure information at the atomic scale. Cu foil, Cu2O, and CuO were selected as standard control calibration samples for the test. Figure 5 (a) shows the normalized Cu K-edge X-ray absorption near-edge structure (XANES) spectra of PR-CuNC, Cu foil, Cu2O, and CuO. Compared with Cu2O and Cu foil, the adsorption edge of PR-CuNC is located in the high-energy region near CuO, indicating that the Cu atoms are in an oxidized state. The first-order derivative of PR-CuNC is between that of Cu2O and CuO ( Figure 5 a inset), which confirms that the Cu valence state of PR-CuNC is between +1 and +2, which is consistent with the Cu 2p XPS observation of PR-CuNC. Fourier transform X-ray absorption fine structure (FT-EXAFS) spectrum was obtained to verify the precise structure of Cu atoms in PR-CuNC, such as Figure 5 (b) Compared with copper foil, Cu2O and CuO, PR-CuNC only shows a The single peak centered at is attributed to the Cu-N bond. Figure 5 (c) is the wavelet transform (WT) diagram of Example 1. The only maximum intensity of PR-CuNC is at The positions are assigned to Cu-N bonds, indicating that there is a strong interaction between Cu atoms and the carbon substrate, and the copper atoms and nitrogen atoms coordinate with each other to form uniformly dispersed copper single atoms.

[0055] In order to investigate the effect of different catalyst sizes on NO3RR activity, the electrocatalytic performance of PR-CuNC in 0.1M KOH and 0.1M KNO3 was evaluated. Figure 6 As shown in (a), first, linear sweep voltammetry (LSV) tests were performed in 0.1M KOH solutions with and without 0.1M KNO3. In the electrolyte without nitrate, the current was mainly generated by the hydrogen evolution side reaction. After adding 0.1M KNO3, the sweep current increased significantly, indicating that nitrate reduction occurred. The current began to change significantly at -0.3V vs. RHE, and the main reason for the current increase was due to nitrate reduction. PR-CuNC began to reduce nitrate at +0.1V, and the current density reached 90mA cm at -0.9V. -2 , showing excellent electrocatalytic nitrate reduction activity.

[0056] To further evaluate the NO3RR performance, the current density-time (It) curves of PR-CuNC in the range of -0.1 to -0.6 V vs. RHE were recorded ( Figure 6b), and Faraday efficiencies (FE) and yields (YR) were determined by UV-Vis spectrophotometry. Standard curves of ammonia and main by-product nitrite are shown in Figure 7 (a-d). PR-CuNCs reached a maximum FE of 94.61% at -0.5 V vs. RHE Figure 8 (a-d). PR-CuNCs reached a maximum FE of 94.61% at -0.5 V vs. RHE NH3 h -1 cm -2 ( Figure 8 b). To avoid the influence of Cu content, the active sites were normalized to copper, as shown in Figure 8 (c). The YR of PR-CuNCs at -0.5 V vs. RHE Cu for 130.71 mg NH3 mg Cu -1 h -1 , showing excellent activity. The nitrate reduction reaction involves complex intermediates and by-products, as shown in Figure 8 (d). Faraday efficiencies of main by-product nitrite (NO2 - ) and hydrogen (H2) were determined. NO2 - was the main product of PR-CuNCs at low overpotential. As more negative potentials were applied, the FE of NO2 - decreased gradually. In addition, the competitive HER affected NH3 selectivity at -0.4 V, and the highest Faraday efficiency was 6.92% at -0.6 V on PR-CuNCs.

[0057] The same electrode was used for 10 consecutive electrolysis experiments at -0.5 V vs. RHE and I-t curves were recorded Figure 9 (a). After 10 cycles, the FE was always higher than 85% and the YR was stable at 120 mg NH3 mg Cu -1 h -1 above Figure 9 (b). In addition, 15 N isotope-labeled proton nuclear magnetic resource 1 (H NMR) verified the accuracy of NH3 measurement and eliminated the possible ammonia contamination from the environment. The electrochemical experiments were performed with 14 NO3 - and 15 NO3 - as N source, and maleic acid was used as an internal standard for quantifying the produced NH3. As shown in Figure 10 (a), the NMR spectra measured from the products with 14 NO3 - as N source exhibited the characteristic peaks belonging to 14NH4 + There are three peaks with an interval of 52Hz. 15 NO3 - The NMR spectrum measured for the N source showed 15 NH4 + It is a characteristic double peak with an interval of 72Hz. Figure 10 bIt can be seen that 15 NH4 + The yield (17.04 μmol) and 14 NH4 + The yield of NH3 was very close to that of NH3 (16.92 μmol), indicating that the measured NH3 originated solely from the electrocatalytic nitric acid process. The ammonia content determined by 1HNMR (16.92 μmol) was comparable to that determined by UV-Vis spectrophotometry (16.96 μmol), confirming the reliability of the UV spectrophotometric results.

[0058] In order to further understand the reaction mechanism of the excellent nitrate selectivity of pyridinium-rich N-type Cu single-atom catalyst, density functional theory (DFT) calculations were used to study its structure-activity relationship. A Cu-N4 catalyst model was constructed, in which the Cu single atom was coordinated with four pyridinic-N (Cu-pyridinic-N4) and four pyridinic-N (Cu-pyrrolic-N4). The charge difference density of the above two Cu-N4 models is shown as follows: Figure 11 As shown in Figure 3, they depict distinctly different charge distributions. Compared to Cu-pyridinic-N4, Cu-pyrrolic-N4 exhibits electron depletion around the Cu atoms and stronger electron accumulation around the C atoms bonded to N. The partial density of states (PDOS) of Cu-pyridinic-N4 and Cu-pyrrolic-N4 are shown in Figure 3. Figure 12 Compared with Cu-pyridinic-N4, the Cu d orbital of Cu-pyrrolic-N4 has obvious energy level splitting and negative shift of the active center near the Fermi level. Due to the obvious hybridization between Cu d and N p orbitals and the higher electron density overlap area on the bonding orbitals, Cu-pyridinic-N4 is more conducive to electron transfer and has a better effect on the adsorbed NO3 - The different distributions of the density of states indicate that the electronic structure of Cu-N4 is regulated by the type of coordinated N.

[0059] The intermediates of NO3RR on the two model surfaces were further analyzed in detail by Gibbs free energy calculations. Figure 13 ) and reaction pathways ( Figure 14Specifically, Cu-pyridinic-N4 has a better NO3ˉ adsorption capacity in the first step of NO3RR than Cu-pyrrolic-N4. Then, * NO3 is gradually produced by the decomposition of NO * NO2 and * In the subsequent hydrogenation process from *NO to *NHO, a large energy barrier needs to be overcome. The energy barrier of Cu-pyridinic-N4 is +0.07eV, which is lower than the +0.51eV of Cu-pyrrolic-N4, indicating that Cu-pyridinic-N4 has a stronger protonation activity for *NO. * NO formation * NHO, * NH2O and * NH2OH. Finally, * NH2OH undergoes another NO cracking and hydrogenation process to generate * NH3, * The final NH3 molecules are obtained by desorption of NH3 from the catalyst surface. In general, except for the *NHO generation step which is considered to be the rate-limiting reaction, the NO3RR reactions on both Cu-N4 models are exothermic. - Both the adsorption capacity and the lower energy barrier indicate that the Cu-pyridinic-N4 structure is more favorable for NO3 - Synthetic ammonia.

[0060] By adjusting the structure of the coordinated N atoms, a hydrogel-derived pyridinic N-rich Cu single-atom catalyst was synthesized. By forming a semi-interpenetrating PPy-PEI hydrogel with abundant N sites, the absolute content of pyridinic-N reached 3.20%. Under low overpotential conditions, PR-CuNC achieved NO3 - Efficient reduction to NH3, with a maximum Faradaic efficiency of 94.61% and a yield of 130.71 mg NH3 mg Cu -1 h -1 By NO3 - Reduction experiments and DFT calculations revealed that different types of coordinating nitrogen atoms effectively modulate the electronic structure between Cu and N atoms. The high NO₃RR activity and selectivity exhibited by PR-CuNC stem from the coordination of single-atom Cu sites with pyridinic-N atoms. This project proposed a synthetic strategy for directly manipulating the local microenvironment of single-atom catalysts at the molecular scale.

[0061] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a pyridine-nitrogen-rich copper single-atom catalyst, characterized in that: S1. Dissolve pyrrole monomer, polyethyleneimine, and sodium dodecylbenzenesulfonate in a mixture of HCl (solution A). The volume of the pyrrole monomer is 416 μL, the mass of polyethyleneimine is 0.101 g, the mass of sodium dodecylbenzenesulfonate is 0.697 g, the concentration of HCl is 1.0 M, and the volume is 10 mL. S2. Dissolve ammonium persulfate in HCl solution (labeled as Solution B); the mass of ammonium persulfate is 1.397 g, the concentration of HCl is 1.0 M, and the volume is 10 mL. S3, placing solution A and solution B in a refrigerator to cool, quickly mixing solution A and solution B after cooling, and placing the mixture in a refrigerator to stand to fully polymerize into a hydrogel; S4. The polymerized hydrogel was washed and purified alternately with ethanol and deionized water. The purified hydrogel was dispersed in a CuCl2 solution and stirred overnight. The hydrogel with adsorbed copper atoms was freeze-dried in a freeze dryer. The dried sample was ground into a uniform powder. The concentration of the CuCl2 solution was 0.2 M. S5. calcining the sample powder under an argon atmosphere. After carbonization, the powder is added to an HCl solution and allowed to stand to remove metal impurities. The calcination temperature is 800°C. S6. The acid-washed powder was washed with deionized water until neutral, and then dried in a vacuum drying oven. The obtained sample was carbonized for the second time under the same conditions to obtain a pyridinium-rich nitrogen-type copper single-atom catalyst, which was recorded as PR-CuNC. S7 dispersed PR-CuNC into a solution consisting of deionized water, isopropyl alcohol and Nafion binder, and ultrasonicated. The dispersion was then applied to the surface of carbon paper and allowed to dry naturally.

2. The method for preparing a pyridine-nitrogen-rich copper single-atom catalyst according to claim 1, wherein: In step S3, the cooling temperature is 2° C. and the standing time is 12 hours.

3. The method for preparing the pyridine-nitrogen-rich copper single-atom catalyst according to claim 1, wherein: The stirring time in step S4 is 12 hours, and the freezing temperature of the freeze dryer is -60°C.

4. The method for preparing a pyridine-nitrogen-rich copper single-atom catalyst according to claim 1, wherein: In step S5, the concentration of HCl is 0.5 M, the soaking time is 4 hours, and the soaking temperature is 80° C.

5. The method for preparing the pyridine-nitrogen-rich copper single-atom catalyst according to claim 1, wherein: The temperature of the vacuum drying oven in step S6 is 80°C.

6. The method for preparing the pyridine-nitrogen-rich copper single-atom catalyst according to claim 1, wherein: The step S7 specifically comprises: dispersing 3 mg of PR-CuNC into a solution consisting of 360 μL of distilled water, 120 μL of isopropanol, and 20 μL of Nafion binder, ultrasonically treating for 15 minutes, and then dropping 20 μL of the dispersion onto the carbon paper surface.

7. An electrode loaded with a pyridinic nitrogen-rich copper single-atom catalyst, characterized in that: It is prepared by the preparation method according to any one of claims 1 to 6.

8. Use of the electrode according to claim 7 in the electrocatalytic reduction of nitrate to produce ammonia.

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