Sn monatomic / atomic cluster electrocatalyst with imitated nano-enzyme structure as well as preparation method and application of Sn monatomic / atomic cluster electrocatalyst

Abstract

The invention discloses a Sn monatomic / atomic cluster electrocatalyst with an imitated nano-enzyme structure and a preparation method and application of the Sn monatomic / atomic cluster electrocatalyst. The Sn monatomic / atomic cluster electrocatalyst is prepared by the following steps: dispersing tin salt and a nanofiber precursor in an organic solvent to form an electrostatic spinning solution; and sequentially carrying out pre-oxidation treatment, high-temperature treatment and high-temperature reduction treatment on the nanofiber membrane obtained by spinning. The catalyst is composed of the Sn SAs and the SnACs, the Sn SAs serves as a metal center, the Sn SAs and the adjacent SnACs are synergistically enhanced, the organic ligand function of enzyme active sites is simulated, and the catalytic performance of hydroxylamine prepared through nitrite reduction is remarkably improved. The prepared catalyst shows excellent performance in nitrite reduction reaction, hydroxylamine Faraday efficiency reaches 84.51%, the catalyst has good long-term stability, and an efficient and environment-friendly solution is provided for the field of green chemistry.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalytic materials, and relates to a Sn single atom / atomic cluster electrocatalyst with a nanozyme-like structure, a preparation method thereof and an application thereof. The catalyst can be used for the reaction of electrocatalytic synthesis of hydroxylamine from nitrite. Background Art

[0002] Hydroxylamine (NH2OH) is an important nitrogen-containing intermediate and has wide applications in agriculture, textile, pharmaceutical, semiconductor and nuclear energy industries. It is a key precursor for the synthesis of high-value chemicals such as nylon-6, and the global annual demand exceeds 100,000 tons. The harsh reaction conditions (high temperature and high pressure) required by the traditional Raschig process limit its industrial applicability, which prompts people to search for more sustainable NH2OH synthesis routes. Due to its mild operating conditions, clean energy utilization rate and high selectivity, electrocatalytic technology provides a promising alternative to traditional methods. The method of electrocatalytic conversion of nitrate / nitrite to NH2OH has dual advantages in the synthesis of hydroxylamine, that is, it can not only produce high-value chemicals, but also reduce the nitrate / nitrite pollution in wastewater. However, since the nitrate / nitrite reduction reaction (NO3 / NO2RR) involves complex multi-electron transfer and proton-coupling steps, multiple products will be generated, and NH2OH is usually a reactive intermediate and is easily over-reduced to ammonia. Compared with NH3, this over-reduction significantly reduces the Faraday efficiency and current density of NH2OH production. Therefore, the direct synthesis of NH2OH from nitrogen and hydrogen remains a major challenge, and the development of highly selective and efficient electrocatalysts that can stabilize the NH2OH intermediate and inhibit over-reduction will be a major breakthrough in green chemistry. Summary of the Invention

[0003] Aiming at the above problems, the purpose of the present invention is to provide a Sn single atom / atomic cluster electrocatalyst with a nanozyme-like structure, a preparation method thereof and an application thereof, so as to improve the selectivity and catalytic efficiency of nitrite reduction to hydroxylamine, and solve the problems such as low Faraday efficiency and current density existing in the electrocatalytic reduction of nitrate / nitrite to synthesize NH2OH by using the existing methods.

[0004] Aiming at the above purpose, the present invention first provides a highly efficient and stable Sn single atom / atomic cluster electrocatalytic material and a preparation method thereof. By simulating the active site structure of biological reductase, the catalytic material uses Sn single atoms (SAs) as metal centers and cooperates with adjacent Sn atomic clusters (ACs), thereby significantly improving the selectivity and catalytic efficiency of nitrite reduction to hydroxylamine.

[0005] The first object of the present invention is to provide a preparation method of a Sn single atom / atomic cluster electrocatalyst with a nanozyme-like structure, comprising the following steps:

[0006] (1) Disperse a tin salt and a nanofiber precursor in an organic solvent to prepare a uniform electrospinning solution;

[0007] (2) Use electrospinning to spin the electrospinning solution prepared in step (1) to obtain a nanofiber membrane;

[0008] (3) Place the nanofiber membrane prepared in step (2) in an air atmosphere and calcine it for pre-oxidation treatment to obtain a pre-oxidized fiber membrane;

[0009] (4) Under an inert gas atmosphere, heat the pre-oxidized fiber membrane prepared in step (3) to 600 - 800 °C at a rate of 1 - 10 °C / min and keep it at a constant temperature for 3 - 5 h for high-temperature treatment. After the high-temperature treatment, under a mixed atmosphere of inert gas and NH3, heat it to 700 - 1200 °C at a rate of 1 - 10 °C / min and keep it at a constant temperature for 1 - 5 h for high-temperature reduction. After the reduction, cool it to room temperature to prepare a Sn single atom / atomic cluster electrocatalyst in-situ loaded on carbon nanofibers.

[0010] In one embodiment of the present invention, the nanofiber precursor in step (1) includes one or more of polyacrylonitrile, polyvinylpyrrolidone, and polyvinyl alcohol.

[0011] In one embodiment of the present invention, the tin salt in step (1) includes one or two of stannous chloride and tin acetate, preferably stannous chloride.

[0012] In one embodiment of the present invention, in the electrospinning solution of step (1), the mass fraction of the nanofiber precursor is 5 - 20%, preferably 8 - 18%, and more preferably 10 - 16%.

[0013] In one embodiment of the present invention, in step (1), the mass ratio of the nanofiber precursor to the tin salt is 3 - 20:1, preferably 3 - 8:1.

[0014] In one embodiment of the present invention, the organic solvent in step (1) includes one or several of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, ethanol, water, and acetone, preferably N,N-dimethylformamide.

[0015] In one embodiment of the present invention, the organic solvent in step (1) is a mixture of N,N-dimethylformamide and ethanol, and the mass ratio of N,N-dimethylformamide to ethanol is 1 - 4:1.

[0016] In one embodiment of the present invention, the parameters of the electrospinning in step (2) are as follows: the spinning voltage is 12 - 25 kV, the distance from the receiving device to the spinning needle is 10 - 20 cm, and the solution flow rate is 0.02 - 0.04 mL / h.

[0017] In one embodiment of the present invention, the heating rate during the calcination in step (3) is 5 - 10 °C / h. During the calcination process, the temperature needs to be raised to 180 - 230 °C and maintained for 2 - 5 h for pre-oxidation treatment.

[0018] In one embodiment of the present invention, the inert gas in step (4) is one of argon and nitrogen.

[0019] In one embodiment of the present invention, the flow rate ratio of the inert gas to NH3 in step (4) is 8 - 20:1.

[0020] In one embodiment of the present invention, the flow rate ratio of the inert gas to NH3 in step (4) is 10 - 20:1, preferably 12 - 18:1, further preferably 14 - 16:1, and even more preferably 15:1.

[0021] In one embodiment of the present invention, the heating rates during the high-temperature treatment and high-temperature reduction processes in step (4) are both 1 - 10 °C / min, preferably 1 °C / min, 5 °C / min, and 10 °C / min.

[0022] In one embodiment of the present invention, the heating rate during the high-temperature treatment and high-temperature reduction processes in step (4) is 5 °C / min.

[0023] In one embodiment of the present invention, during the high-temperature treatment in step (4), after the temperature is raised to 600 °C, it is kept at a constant temperature for 3 h.

[0024] In one embodiment of the present invention, during the high-temperature reduction in step (4), after the temperature is raised to 1000 °C, it is kept at a constant temperature for 3 h.

[0025] In one embodiment of the present invention, the loading amount of the Sn single-atom / atomic cluster electrocatalyst in step (4) is 1 - 5%.

[0026] In one embodiment of the present invention, the size of the Sn atomic clusters in step (4) is 1 - 2 nm.

[0027] In one embodiment of the present invention, the diameter size of the carbon nanofibers in step (4) is 100 - 500 nm.

[0028] The second object of the present invention is to utilize the Sn single-atom / atomic cluster electrocatalyst Sn ACSAs / NCNFs with a nanozyme-like structure prepared by the above preparation method.

[0029] The third object of the present invention is to provide an application of the Sn single-atom / atomic cluster electrocatalyst SnACSAs / NCNFs with the above-mentioned nanozyme-like structure in the field of electrocatalysis.

[0030] In one embodiment of the present invention, the application includes using the Sn single-atom / atomic cluster electrocatalyst SnACSAs / NCNFs as a catalyst to electrocatalytically reduce nitrite to synthesize hydroxylamine.

[0031] In one embodiment of the present invention, the method for electrocatalytically reducing nitrite to synthesize hydroxylamine includes the following steps:

[0032] Using a mixed solution of 0.5 M potassium bicarbonate (KHCO3) and 0.5 M potassium nitrite (KNO2) as the electrolyte, the carbon paper electrode loaded with the catalyst as the working electrode, a platinum wire or a platinum sheet as the counter electrode, and a silver / silver chloride electrode (Ag / AgCl) as the reference electrode, and performing an electrocatalytic reaction in a three-electrode system.

[0033] In one embodiment of the present invention, the amount of the catalyst loaded on the carbon paper electrode is 0.5 - 2 mg / cm 2 , the applied voltage range is -0.61 to -1.21 V (vs RHE), the reaction is carried out at room temperature (20 - 30 °C) and under magnetic stirring conditions of 200 - 500 rpm, and the constant voltage time is 0.5 - 1 hour.

[0034] Beneficial effects:

[0035] (1) The present invention synthesizes SnACSAs / NCNFs by using the electrospinning technique in combination with the high-temperature calcination method. By optimizing the ratio of the inert gas and NH3 mixed atmosphere and the calcination temperature during the high-temperature calcination process, the gradual transformation from SnNPs to SnACs and then to SnSAs is realized, and a highly dispersed structure is formed within the NCNFs matrix. The synergistic effect between Sn SAs and SnACs effectively enhances the electrocatalytic performance. The preparation method is simple and controllable and is applicable to large-scale production.

[0036] (2) Compared with traditional multi-metal catalysts, the single-atom design in the present invention makes full use of the activity of each Sn atom, significantly reduces the preparation cost of the catalyst, and conforms to the "atom economy" principle of single-atom catalysis. In addition, the preparation process of the SnACSAs / NCNFs catalytic material is simple and easy to scale up production, further reducing the threshold of industrial application.

[0037] (3) The catalyst of the present invention efficiently converts nitrite to hydroxylamine under mild conditions, not only reducing the requirement for high temperature and high pressure in the traditional Raschig process but also significantly reducing the energy consumption during the reaction. In addition, the reduction and conversion of nitrite effectively alleviate the nitrogen pollution problem in the environment, meeting the requirements of green chemistry.

[0038] (4) Through electrospinning combined with high-temperature calcination, the present invention regulates the calcination temperature and the proportion of NH3 atmosphere, realizing the gradual transformation of Sn from nanoparticles (Sn NPs) to atomic clusters (SnACs) and then to single atoms (Sn SAs), and forming a highly dispersed structure within the nitrogen-doped carbon nanofiber (NCNFs) substrate. The prepared tin single atom and atomic cluster / nitrogen-doped carbon nanofiber composite (SnACSAs / NCNFs) catalytic material exhibits excellent performance in the nitrite reduction reaction, with a hydroxylamine Faraday efficiency of 84.51% and good long-term stability, providing an efficient and environmentally friendly solution for the field of green chemistry. Description of the Drawings

[0039] Figure 1 It is the microscopic morphology diagram of the catalyst SnNPs / CNFs obtained in Comparative Example 1; among them, (a) is the scanning electron microscopy (SEM) image of Sn NPs / CNFs, (b)-(c) are the transmission electron microscopy (TEM) images of SnNPs / CNFs, and (d) is the mapping spectrum of Sn NPs / CNFs.

[0040] Figure 2 It is the microscopic morphology diagram of the catalyst Sn ACSAs / NCNFs obtained in Example 1; among them, (a) is the transmission electron microscopy (TEM) image of SnACSAs / NCNFs, (b) is the aberration-corrected scanning transmission electron microscopy (AC-STEM) of SnACSAs / NCNFs, and (c) is the mapping spectrum of SnACSAs / NCNFs.

[0041] Figure 3 It is the microscopic morphology diagram of the catalyst Sn SAs / NCNFs obtained in Comparative Example 2; among them, (a) is the aberration-corrected scanning transmission electron microscopy (AC-STEM) of Sn SAs / NCNFs, and (b) is the mapping spectrum of Sn SAs / NCNFs.

[0042] Figure 4 It is the X-ray diffraction pattern (XRD) of the catalysts Sn NPs / CNFs, Sn ACSAs / NCNFs, and SnSAs / NCNFs obtained in Example 1 and Comparative Examples 1-2.

[0043] Figure 5X-ray photoelectron spectroscopy (XPS) spectra of the catalysts Sn NPs / CNFs, Sn ACSAs / NCNFs, and SnSAs / NCNFs obtained in Example 1 and Comparative Examples 1-2; among them, (a) is the Sn 3d spectrum and (b) is the N1s spectrum.

[0044] Figure 6 X-ray absorption fine structure (XAFS) characterization of the catalysts Sn ACSAs / NCNFs and Sn SAs / NCNFs obtained in Example 1 and Comparative Example 2; among them, (a) is the near-edge absorption structure spectrum and (b) is the extended-edge structure spectrum.

[0045] Figure 7 LSV curves of the electrocatalytic nitrite reduction tests of the catalysts Sn NPs / CNFs, Sn ACSAs / NCNFs, and SnSAs / NCNFs obtained in Example 1 and Comparative Examples 1-2 in a mixed electrolyte of 0.1 M KHCO3 + 0.1 M KNO2.

[0046] Figure 8 Electrocatalytic performance of the catalysts Sn NPs / CNFs, Sn ACSAs / NCNFs, and SnSAs / NCNFs obtained in Example 1 and Comparative Examples 1-2 for the reduction of nitrite to hydroxylamine in a mixed electrolyte of 0.1 M KHCO3 + 0.1 M KNO2; among them, (a) is the Faraday efficiency and yield of Sn ACSAs / NCNFs at different voltages, (b) is the Faraday efficiency and yield of Sn SAs / NCNFs at different voltages, and (c) is the Faraday efficiency and yield of Sn NPs / CNFs at different voltages.

[0047] Figure 9 Comparison of the Faraday efficiencies of the catalysts Sn ACSAs / NCNFs, Sn ACSAs / CNFs, Sn ACSAs / NCNFs-800, and Sn ACSAs / NCNFs-1200 obtained in Example 1 and Comparative Examples 3-5 for electrocatalytic nitrite reduction in a mixed electrolyte of 0.1 M KHCO3 + 0.1 M KNO2.

[0048] Figure 10 Stability test of the electrocatalytic reduction of nitrite to hydroxylamine of the catalyst Sn ACSAs / NCNFs obtained in Example 1 at -0.91 V vs. RHE in a mixed electrolyte of 0.1 M KHCO3 + 0.1 M KNO2. Detailed implementation

[0049] To better understand the present invention, the content of the present invention will be further clarified below in conjunction with examples, but the content of the present invention is not limited to the embodiments given below.

[0050] Example 1

[0051] A preparation method of an Sn single atom / atomic cluster electrocatalyst with a nanozyme-like structure includes the following steps:

[0052] (1) Take 0.5 mmol of stannous chloride dihydrate and add it to a solution of 15 g of polyvinylpyrrolidone with a mass fraction of 15 wt% in DMF / ethanol (mass ratio 2:1). Use magnetic stirring to obtain a homogeneous and transparent solution. Electrospinning is used to spin the above solution, controlling the electrospinning positive electrode voltage to be 15 kV, the distance between the positive electrode and the receiver to be set at 20 cm, and the solution pushing rate to be 0.2 mL h-1, thus obtaining Sn metal salt / PVP precursor nanofibers;

[0053] (2) Clamp 0.8 g of the prepared nanofiber membrane between graphite sheets, place it in a tubular furnace, and heat it to 230 °C at a heating rate of 10 °C / h in air and hold for 3 h, so that the fiber membrane can form a stable structure and effectively prevent the destruction of the fibers during carbonization. Then, heat it to 800 °C at a heating rate of 5 °C / min in an argon atmosphere and hold for 3 h. After the constant temperature at 800 °C ends, continue to heat it to 1000 °C at a heating rate of 5 °C / min in an Ar / NH3 flow ratio of 15:1 atmosphere and hold for 3 h. After the constant temperature ends and it cools down to room temperature, the catalytic material Sn ACSAs / NCNFs is prepared. High-temperature thermal reduction and ammonia promote the breaking of Sn-Sn bonds, enabling Sn atoms to stably exist in the form of single atoms and atomic clusters in NCNFs. After the constant temperature ends and it cools down to room temperature, the catalytic material Sn ACSAs / NCNFs is prepared.

[0054] Comparative Example 1

[0055] The difference between Comparative Example 1 and Example 1 is that in step (2), the Ar / NH3 flow ratio in step (2) is 16:0, and the catalyst SnNPs / CNFs is obtained.

[0056] Comparative Example 2

[0057] The difference between Comparative Example 2 and Example 1 is that in step (2), the Ar / NH3 flow ratio is 10:1, and the catalyst SnSAs / NCNFs is obtained.

[0058] By increasing the proportion of NH3, the Sn-Sn bonds in the Sn atomic clusters are further broken, enabling Sn atoms to stably exist in the form of single atoms in NCNFs.

[0059] Comparative Example 3

[0060] The difference between Comparative Example 3 and Example 1 is that in step (2), it is heated to 800 °C at a heating rate of 5 °C / min under an argon atmosphere, held at a constant temperature for 3 h. After the constant temperature at 800 °C ends, it is continuously heated to 1200 °C at a heating rate of 5 °C / min under an atmosphere with an Ar / NH3 flow ratio of 15:1, and held at a constant temperature for 3 h. After the constant temperature ends and it is cooled to room temperature, the catalytic material Sn ACSAs / NCNFs-1200 is prepared.

[0061] Comparative Example 4

[0062] The difference between Comparative Example 4 and Example 1 is that in step (2), it is heated to 800 °C at a heating rate of 5 °C / min under an argon atmosphere, held at a constant temperature for 3 h. After the constant temperature at 800 °C ends, it is held at a constant temperature for 3 h under an atmosphere with an Ar / NH3 flow ratio of 15:1. After the constant temperature ends and it is cooled to room temperature, the catalytic material Sn ACSAs / NCNFs-800 is prepared.

[0063] Comparative Example 5

[0064] The difference between Comparative Example 5 and Example 1 is that in step (2), it is continuously heated to 1000 °C at a heating rate of 5 °C / min under an atmosphere with an Ar / H2 flow ratio of 15:1, and held at a constant temperature for 3 h. After it is cooled to room temperature, the catalytic material Sn AC / CNFs is prepared.

[0065] Electrocatalytic nitrite reduction experiment:

[0066] The electrocatalytic reduction of nitrite (NO2 - ) is carried out using the catalyst to synthesize hydroxylamine (NH2OH). The specific process is as follows: The experiment is carried out in an H-type electrolytic cell. The two chambers of the electrolytic cell are separated by a proton exchange membrane (Nafion 117). The electrolyte in the cathode chamber is a mixed solution of 0.5 M potassium bicarbonate (KHCO3) and 0.5 M potassium nitrite (KNO2), and the electrolyte in the anode chamber is a 0.5 M KHCO3 solution. The volume of the electrolyte in both chambers is 50 mL, and the pH is controlled at 6.8 - 7.2. A glassy carbon electrode is used as the working electrode (coated with the catalyst, the loading amount is 0.8 mg / cm 2 ), a platinum wire is used as the counter electrode, and a saturated calomel electrode (SCE) is used as the reference electrode. After assembly, the electrolytic cell is connected to an electrochemical workstation. In the experiment, a voltage of -1.0 V (vs SCE) is applied, and a constant voltage reduction reaction is carried out under the conditions of room temperature (25 °C) and magnetic stirring at 500 rpm for 2 hours. After the reaction ends, the electrolyte is collected from the cathode chamber, and the product is analyzed.

[0067] Quantitative analysis of hydroxylamine was carried out by the acidic iron ion method: 2 mL of the electrolyte was mixed with 2 mL of 0.1 M FeCl3 (dissolved in 0.1 M HCl), and the absorbance was measured at a wavelength of 450 nm by a UV-visible spectrophotometer. The hydroxylamine concentration was calculated according to the standard curve (0–50 μM NH2OH).

[0068] The by-product ammonia (NH3) was detected using the indophenol blue method: 2 mL of the electrolyte was reacted with 3 mL of the indophenol blue reagent, and the absorbance was measured at a wavelength of 720 nm. The Faraday efficiency (FE) and yield (mg / L·h) of hydroxylamine were calculated based on the experimental data. By optimizing the voltage, catalyst loading, and electrolyte conditions, the selectivity and yield of hydroxylamine were effectively improved, while the formation of by-products was reduced.

[0069] The performance tests of electrocatalytic nitrite reduction mainly included linear sweep voltammetry (LSV) and constant voltage (I-t) tests. The LSV test was used to evaluate the electrochemical activity of the catalyst. It was carried out in the potential range of 0 V to -2 V vs. RHE at a scan rate of 5 mV / s, and the experiment was completed at room temperature (25 °C). The I-t test was carried out under constant voltage conditions of -0.61 V, -0.71 V, -0.81 V, -0.91 V, -1.01 V, -1.11 V, -1.21 V vs. RHE for 1 hour. At the same time, the electrolyte after the reaction was collected, and the concentration of hydroxylamine (NH2OH) was measured by the acidic iron ion method to evaluate the electrochemical reduction performance of the catalyst and the yield of hydroxylamine.

[0070] Faraday efficiency (FE) formula

[0071]

[0072] where z = 4, F = 96485 C / mol, n NH2OH is the number of moles of hydroxylamine generated, Q is the total charge recorded by the electrochemical workstation, I is the current intensity (in amperes, A), and t is the electrolysis time (in seconds, s).

[0073] Yield formula

[0074]

[0075] where n NH2OH is the number of moles of hydroxylamine generated, V is the volume of the electrolyte (L), and t is the reaction time (h).

[0076] Figure 1 (a) and Figure 1(b) are the scanning electron microscope image and transmission electron microscope image of the Sn NPs / CNFs prepared in Comparative Example 1. It can be seen from these two figures that the diameter of the carbon nanofibers (CNFs) is 10 - 20 nm, and the CNFs are randomly distributed to form a three-dimensional network structure. The Sn nanoparticles can be evenly distributed on the carbon nanofibers (CNFs), and the sizes are relatively uniform, which is beneficial to the progress of the catalytic reaction. Figure 1 (d) is the high-resolution transmission electron microscope image of Sn NPs / CNFs, which shows that the interplanar spacing of the (200) crystal plane of the Sn nanoparticles is indicating the successful preparation of Sn NPs / CNFs. In addition, Figure 1 (d) is the STEM-EDS mapping image of Sn NPs / CNFs. It can be found that the Sn element shows a uniform distribution on the particles, further confirming the successful preparation of the Sn NPs / CNFs nanoparticles.

[0077] Figure 2 (a) is the transmission electron microscope image of the SnACSAs / NCNFs prepared in Example 1. It can be seen from this figure that during the calcination process, the introduction of NH3 gas inhibits the formation of observable Sn NPs on the CNFs. The surface of Sn ACSAs / NCNFs is smooth and there are no nanoparticles. Figure 2 (b) is the atomic-resolution high-angle annular dark field image of SnACSAs / NCNFs. It can be seen from this figure that Sn ACs exist on the CNFs substrate (marked by the blue dashed circle), along with a large number of Sn SAs (highlighted by the pink dashed circle). The average diameter of the Sn ACs is 1.59 ± 0.10 nm. The steady increase in the nitrogen doping level creates a nitrogen-rich environment inside the CNFs, promoting the formation of multiple Sn-N bonds, thereby enhancing the dispersion and stability of Sn. This nitrogen doping promotes the breakage of Sn-Sn bonds, resulting in the stabilization of Sn atoms as single atoms and atomic clusters within the NCNFs. Figure 2 (c) is the STEM-EDS mapping image of Sn ACSAs / NCNFs. It can be seen from this figure that the Sn and N elements are evenly distributed, further confirming the spatial dispersion of Sn ACs and Sn SA within the NCNFs and the successful preparation.

[0078] Figure 3 (a) is the atomic-resolution high-angle annular dark field image of the Sn SAs / NCNFs prepared in Comparative Example 2. It can be seen from this figure that during the calcination process, with the increase in the NH3 flow rate, the formation of atomically dispersed Sn atoms (represented by the pink dashed circle) in the NCNFs is confirmed, indicating that the Sn ACs are completely converted into isolated Sn SAs. Figure 3(b) is the STEM-EDS mapping image of Sn SAs / NCNFs. It can be seen from this figure that Sn is uniformly distributed in NCNFs and no Sn aggregation is detected.

[0079] Figure 4 XRD patterns of SnNPs / CNFs, Sn ACSAs / NCNFs, and Sn SAs / NCNFs are shown. It can be seen from these figures that as the Sn particle size decreases, the intensity of its XRD diffraction peaks gradually weakens: Sn NPs / CNFs exhibit typical diffraction peaks of tetragonal Sn crystal planes; the diffraction peaks of Sn ACSAs / NCNFs are significantly weakened, indicating a reduction in particle size; while Sn SAs / NCNFs only show the characteristic peaks of graphitic carbon and no obvious Sn diffraction peaks are observed. These results indicate that the particle size of Sn gradually decreases from the nanoscale to the atomic scale, providing a theoretical basis for the development of catalysts with a highly dispersed structure.

[0080] Figure 5 (a) shows the Sn 3d spectra of XPS for SnNPs / CNFs, Sn ACSAs / NCNFs, and Sn SAs / NCNFs. It can be seen from this figure that as the Sn particle size decreases, Sn is gradually oxidized and finally completely converted to high-valent Sn in Sn SAs / NCNFs. 4+ 。 Figure 5 (b) shows the N1s XPS spectra of Sn NPs / CNFs, Sn ACSAs / NCNFs, and Sn SAs / NCNFs. NH3 treatment promotes the formation of Sn-N bonds, and the Sn-N bonding ratios in Sn ACSAs / NCNFs and Sn SAs / NCNFs reach 11.2% and 13.9% respectively. This Sn-N interaction effectively stabilizes the Sn atoms in the NCNF matrix, providing an important basis for the development of highly efficient and stable catalysts.

[0081] Figure 6 (a) shows the X-ray absorption near-edge structure (XANES) spectra of Sn ACSAs / NCNFs and Sn SAs / NCNFs. It can be seen from this figure that the Sn oxidation state in Sn SAs / NCNFs is higher, which is Figure 5 consistent with the Figure 6 (b) shows the extended X-ray absorption fine structure (EXAFS) spectra of Sn ACSAs / NCNFs and Sn SAs / NCNFs. It can be seen from this figure that Sn atoms in Sn SAs / NCNFs exist in an atomically dispersed form and there is no Sn-Sn bond. These results clearly illustrate the transformation process of Sn from nanoparticles to single-atom structures, verifying the key role of NH3 treatment in regulating the dispersion state of Sn.

[0082] Figure 7 LSV curves for the electrocatalytic nitrite reduction tests of Sn NPs / CNFs, Sn ACSAs / NCNFs, and Sn SAs / NCNFs in a mixed electrolyte of 0.1 M KHCO3 + 0.1 M KNO2. It can be seen from this figure that, compared with Sn NPs / CNFs and Sn SAs / NCNFs, Sn ACSAs / NCNFs can significantly enhance the NO2RR activity.

[0083] Figure 8 Performance of electrocatalytic nitrate reduction to hydroxylamine of Sn NPs / CNFs, Sn ACSAs / NCNFs, and Sn SAs / NCNFs in a mixed electrolyte of 0.1 M KHCO3 + 0.1 M KNO2. It can be seen from this figure that Sn ACSAs / NCNFs achieved a Faradaic efficiency of 84.51% for NH2OH at -0.91 V vs. RHE and a yield of 1.93 mmol·h - 1 mg cat -1 of NH2OH, both significantly superior to Sn NPs / CNFs and Sn SAs / NCNFs. This illustrates the importance of the synergistic interaction between Sn SAs and Sn ACs in guiding the reduction of nitrite to NH2OH.

[0084] Figure 9 Faradaic comparison chart for the electrocatalytic nitrite reduction to hydroxylamine of Sn ACSAs / NCNFs, Sn ACSAs / CNFs, Sn ACSAs / NCNFs-800, and Sn ACSAs / NCNFs-1200 in a 0.1 M KHCO3 + 0.1 M KNO2 mixed electrolyte. According to the results in the figure, it can be seen that Sn ACSAs / NCNFs reached a Faradaic efficiency of 84.51% for NH2OH at -0.91 V vs. RHE, significantly superior to Sn ACSAs / CNFs, Sn ACSAs / NCNFs-800, and Sn ACSAs / NCNFs-1200. This indicates that 1000 °C is the optimal treatment temperature and the Ar / H2 mixed atmosphere (flow ratio of 15:1) is the best choice.

[0085] Figure 10 Curve graph of the stability test of Sn ACSAs / NCNFs. It can be seen from this figure that Sn ACSAs / NCNFs exhibited significant stability after continuous cyclic testing for 88 h at -0.91 V vs. RHE, and the Faradaic efficiency of NH2OH always remained at about 80%, showing good catalytic stability.

[0086] The embodiments provided above are not intended to limit the scope covered by the present invention, nor are the described steps intended to limit the order of their execution. Obvious improvements made by those skilled in the art in combination with the existing well-known general knowledge also fall within the protection scope defined by the claims of the present invention.

Claims

1. A preparation method of an Sn single atom / atomic cluster electrocatalyst with a nanozyme-like structure, characterized in that, It includes the following steps: (1) Disperse a tin salt and a nanofiber precursor in an organic solvent to prepare a uniform electrospinning solution; (2) Use electrospinning to spin the electrospinning solution prepared in step (1) to obtain a nanofiber membrane; (3) Place the nanofiber membrane prepared in step (2) in an air atmosphere for calcination for pre-oxidation treatment to obtain a pre-oxidized fiber membrane; (4) Under an inert gas atmosphere, heat the pre-oxidized fiber membrane prepared in step (3) at a rate of 1-10 °C / min to 600-800 °C and keep it at a constant temperature for 3-5 h for high-temperature treatment. After the high-temperature treatment, under a mixed atmosphere of inert gas and NH3, heat it at a rate of 1-10 °C / min to 700-1200 °C and keep it at a constant temperature for 1-5 h for high-temperature reduction. After the reduction, cool it to room temperature to prepare a Sn single atom / atomic cluster electrocatalyst in-situ loaded on carbon nanofibers.

2. The preparation method according to claim 1, characterized in that, The nanofiber precursor in step (1) includes one or more of polyacrylonitrile, polyvinylpyrrolidone, and polyvinyl alcohol. The tin salt includes one or two of stannous chloride and tin acetate, preferably stannous chloride.

3. The preparation method according to claim 1, wherein In the electrospinning solution of step (1), the mass fraction of the nanofiber precursor is 5-20%, the mass ratio of the nanofiber precursor to the tin salt is 3-20:1, and the organic solvent includes one or several of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, ethanol, water, and acetone.

4. The preparation method according to claim 1, characterized in that, The organic solvent in step (1) is a mixture of N,N-dimethylformamide and ethanol, and the mass ratio of N,N-dimethylformamide to ethanol is 1-4:

1.

5. The preparation method according to claim 1, characterized in that, The parameters of the electrospinning in step (2) are: the electrospinning voltage is 12-25 kV, the distance from the receiving device to the electrospinning needle is 10-20 cm, and the solution flow rate is 0.02-0.04 mL / h.

6. The preparation method according to claim 1, characterized in that, The heating rate during the calcination in step (3) is 5-10 °C / h. During the calcination process, the temperature needs to be raised to 180-230 °C and kept for 2-5 h for pre-oxidation treatment.

7. The preparation method according to claim 1, characterized in that, The inert gas in step (4) is one of argon and nitrogen, and the flow rate ratio of the inert gas to NH3 is 8-20:

1.

8. The preparation method according to claim 1, characterized in that, In the catalyst of step (4), the loading amount of Sn single atoms / atomic clusters is 1-5%, the size of the Sn atomic clusters is 1-2 nm, and the diameter size of the carbon nanofibers is 100-500 nm.

9. The Sn single atom / atomic cluster electrocatalyst Sn ACSAs / NCNFs with a nanozyme-like structure prepared by the preparation method according to any one of claims 1-8.

10. Application of the Sn single atom / atomic cluster electrocatalyst SnACSAs / NCNFs with a nanozyme-like structure according to claim 9 in the field of electrocatalysis.

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