A universal preparation method for single-atom catalysts based on programmed Joule thermal flash technology

Abstract

This invention discloses a universal preparation method for single-atom catalysts based on programmed Joule thermal flash technology. The invention aims to address the technical problems of existing single-atom catalyst preparation methods, such as long preparation cycles, high costs, and applicability only to specific metal systems. This invention utilizes Joule heating to generate instantaneous ultra-high temperature pulses, achieving in-situ decomposition and atomic-level dispersion of metal precursors on carbon nitride supports in an extremely short time. This effectively inhibits metal atom migration and aggregation, improves single-atom loading and stability, and eliminates the need for prolonged high-temperature pyrolysis, offering advantages of low energy consumption and high efficiency. This technology exhibits good universality for different metals and can rapidly prepare a series of single-atom catalysts in batches, providing a new technical pathway for the large-scale production and widespread application of single-atom catalysts. The prepared single-atom catalysts demonstrate excellent catalytic activity and stability in reactions such as the catalytic degradation of organic pollutants.

Description

Technical Field

[0001] This invention relates to a method for preparing a single-atom catalyst. Background Technology

[0002] Single-atom catalysts are a new type of catalytic material in which isolated metal atoms are stably anchored on the surface of a support, achieving a high degree of dispersion of active metal atoms. Due to their advantages such as high metal atom utilization, uniform active sites, and tunable electronic structure and coordination environment, single-atom catalysts show broad application prospects in energy conversion, organic synthesis, and environmental remediation. However, the preparation of single-atom catalysts still faces two challenges: first, there is a lack of universal preparation methods applicable to multiple metal elements, with most existing technologies only suitable for specific metal systems; second, traditional preparation processes typically suffer from long reaction times, high energy consumption, complex processes, and difficulties in large-scale production.

[0003] Currently, single-atom catalysts are mainly prepared through methods such as high-temperature pyrolysis, defect anchoring, atomic layer deposition, and electrochemical deposition. Although these methods can obtain atomically dispersed active sites, they generally suffer from drawbacks such as long preparation cycles, high costs, and limited applicability, making it difficult to meet the demands for efficient, low-cost, and large-scale preparation. Furthermore, existing methods are mostly based on a customized approach of "one method to prepare one metal," which cannot achieve rapid, parallel, and large-scale preparation of multiple single-atom catalysts. Therefore, developing a simple, rapid, and universally applicable new method for synthesizing single-atom catalysts is of great significance for advancing basic research and industrial applications in single-atom catalysis. Summary of the Invention

[0004] The present invention aims to address the technical problems of existing methods for preparing single-atom catalysts, such as long preparation cycles, high costs, and applicability only to specific metal systems, and to provide a universal preparation method for single-atom catalysts based on programmed Joule thermal flash technology.

[0005] The universal preparation method of the single-atom catalyst based on programmed Joule thermal flash technology of the present invention is carried out according to the following steps:

[0006] 1. Melamine and a metal salt are added to a hydrochloric acid solution and ultrasonically dispersed until completely dissolved. Then, the mixture is placed in a rotary evaporator for rotary evaporation and then dried in a vacuum drying oven to obtain a uniformly dispersed solid precursor powder. The obtained precursor powder is placed in a muffle furnace and annealed in an air atmosphere. It is then cooled to room temperature in the furnace to obtain monometallic doped carbon nitride Mg-C3N4. The metal in the metal salt is M.

[0007] 2. ① Dissolve tris(hydroxymethyl)aminomethane in ultrapure water, and adjust the pH of the solution to 8.5-9.0 with hydrochloric acid under stirring to obtain a Tris buffer solution; then add Mg-C3N4 prepared in step 1 to the Tris buffer solution, stir and disperse, and then add L-DOPA and polyethyleneimine (PEI) in sequence, and continue stirring the reaction at room temperature; after the reaction is completed, filter, collect the solid product and place it in a vacuum drying oven to dry; grind the dried sample thoroughly, and then transfer it to an ultra-fast high-temperature furnace and introduce a protective atmosphere;

[0008] ② The temperature is raised from room temperature to 550℃~560℃ within 1 second and held for 60s~65s, then raised from 550℃~560℃ to 1000℃~1050℃ within 2 seconds and held for 40s~45s; then cooled from 1000℃~1050℃ to 350℃~360℃ within 3 seconds and held for 10s~15s;

[0009] ③ Raise the temperature from 350℃~360℃ to 1000℃~1050℃ within 5s and hold for 40s~45s, then lower the temperature from 1000℃~1050℃ to 350℃~360℃ within 3s and hold for 10s~15s;

[0010] ④ Repeat step ③ four times. Finally, cool the furnace to room temperature, add the product to the hydrochloric acid solution, stir and wash, filter, and then dry in a vacuum drying oven to obtain a single-atom catalyst, denoted as M-SAC.

[0011] This invention achieves atomic-level dispersion and stable anchoring of metal elements on the surface of a support through a two-step programmed annealing process of low-temperature selective anchoring and high-temperature pulsed cyclic heating, thereby obtaining a single-atom catalyst with uniform structure and highly dispersed metal sites.

[0012] The method of this invention generates instantaneous ultra-high temperature pulses through programmed Joule heating, enabling in-situ decomposition and atomic-level dispersion of metal precursors on carbon nitride supports within an extremely short time. This effectively inhibits metal atom migration and aggregation, improves single-atom loading and stability, and eliminates the need for prolonged high-temperature pyrolysis, offering advantages such as low energy consumption and high efficiency. This technology exhibits good versatility across different metals, enabling the rapid batch preparation of a series of single-atom catalysts. It provides a novel technical pathway for the large-scale production and widespread application of single-atom catalysts. The prepared single-atom catalysts demonstrate excellent catalytic activity and stability in reactions such as the catalytic degradation of organic pollutants. It effectively solves the problems of poor versatility and high energy consumption associated with traditional methods, and provides a new pathway for the large-scale preparation of single-atom catalysts, representing a highly promising alternative. Attached Figure Description

[0013] Figure 1This is a temperature diagram of the Joule heating process in step two of Experiment 1.

[0014] Figure 2 X-ray diffraction pattern of Fe-SAC prepared in Experiment 1;

[0015] Figure 3 Transmission electron microscope image of Fe-SAC and elemental distribution by EDS-Mapping;

[0016] Figure 4 The images show aberration-corrected electron microscopy images and EDS-mapping elemental distribution maps of Fe-SAC.

[0017] Figure 5 XAFS spectrum of Fe element in Fe-SAC prepared for Experiment 1;

[0018] Figure 6 X-ray diffraction pattern of Co-SAC prepared for Experiment 2;

[0019] Figure 7 Transmission electron microscope (TEM) image and EDS-Mapping elemental distribution map of Co-SAC;

[0020] Figure 8 The images show the spherical aberration electron microscope images (AC-HAADF-STEM) and EDS-Mapping elemental distribution maps of Co-SAC.

[0021] Figure 9 XAFS spectra of Co element in Co-SAC;

[0022] Figure 10 The X-ray diffraction pattern of Ni-SAC prepared in Experiment 3;

[0023] Figure 11 Transmission electron microscope image and EDS-Mapping elemental distribution map of Ni-SAC;

[0024] Figure 12 The images show aberration-corrected electron microscopy images and EDS-mapping elemental distribution maps of Ni-SAC.

[0025] Figure 13 XAFS spectrum of Ni element in Ni-SAC;

[0026] Figure 14 X-ray diffraction pattern of Mn-SAC prepared for Experiment 4;

[0027] Figure 15 Transmission electron microscope image and EDS-Mapping elemental distribution map of Mn-SAC;

[0028] Figure 16 Aberration-corrected electron microscopy image of Mn-SAC and elemental distribution map using EDS-mapping;

[0029] Figure 17 XAFS spectra of Mn element in Mn-SAC;

[0030] Figure 18 This is a comparison chart of the performance of M-SAC-activated PMS in degrading phenol in Experiment 5;

[0031] Figure 19 The figure shows the effect of different anions on the degradation performance of phenol by Fe-SAC activated PMS in Experiment 6. Detailed Implementation

[0032] Specific Implementation Method 1: This implementation method is a universal preparation method for a single-atom catalyst based on programmed Joule thermal flash technology, specifically carried out according to the following steps:

[0033] 1. Melamine and a metal salt are added to a hydrochloric acid solution and ultrasonically dispersed until completely dissolved. Then, the mixture is placed in a rotary evaporator for rotary evaporation and then dried in a vacuum drying oven to obtain a uniformly dispersed solid precursor powder. The obtained precursor powder is placed in a muffle furnace and annealed in an air atmosphere. It is then cooled to room temperature in the furnace to obtain monometallic doped carbon nitride Mg-C3N4. The metal in the metal salt is M.

[0034] 2. ① Dissolve tris(hydroxymethyl)aminomethane in ultrapure water, and adjust the pH of the solution to 8.5-9 with hydrochloric acid under stirring to obtain a Tris buffer solution; then add Mg-C3N4 prepared in step 1 to the Tris buffer solution, stir and disperse, and then add levodopa and polyethyleneimine in sequence, and continue stirring the reaction at room temperature; after the reaction is completed, filter, collect the solid product and place it in a vacuum drying oven to dry; grind the dried sample thoroughly, and then transfer it to an ultra-fast high-temperature furnace and introduce a protective atmosphere;

[0035] ② The temperature is raised from room temperature to 550℃~560℃ within 1 second and held for 60s~65s, then raised from 550℃~560℃ to 1000℃~1050℃ within 2 seconds and held for 40s~45s; then cooled from 1000℃~1050℃ to 350℃~360℃ within 3 seconds and held for 10s~15s;

[0036] ③ Raise the temperature from 350℃~360℃ to 1000℃~1050℃ within 5s and hold for 40s~45s, then lower the temperature from 1000℃~1050℃ to 350℃~360℃ within 3s and hold for 10s~15s;

[0037] ④ Repeat step ③ four times. Finally, cool the furnace to room temperature, add the product to the hydrochloric acid solution, stir and wash, filter, and then dry in a vacuum drying oven to obtain a single-atom catalyst, denoted as M-SAC.

[0038] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the metal salt mentioned in step one is a chloride hydrate, sulfate, or acetate. Everything else is the same as in Specific Implementation Method One.

[0039] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method One or Two in that the metal M mentioned in step one is Fe, Co, Ni, or Mn. Everything else is the same as in Specific Implementation Method One or Two.

[0040] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in that the mass ratio of melamine to metal salt in step one is 100:1. Everything else is the same as in Specific Implementation Methods One to Three.

[0041] Specific Implementation Method Five: This implementation method differs from Specific Implementation Method Four in that the rotary evaporation process conditions in step one are: stirring at 90°C for 6 hours at a rotation speed of 60 rpm. Everything else is the same as in Specific Implementation Method Four.

[0042] Specific Implementation Method Six: This implementation method differs from Specific Implementation Method Five in that the temperature of the vacuum drying oven mentioned in step one is 60°C. Everything else is the same as in Specific Implementation Method Five.

[0043] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Method Six in that the annealing conditions described in step one are: heating from room temperature to 550°C at a rate of 15°C / min and annealing for 2 hours. Everything else is the same as in Specific Implementation Method Six.

[0044] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Method Seven in that the mass ratio of tris(hydroxymethyl)aminomethane to levodopa in step two is (150~160):1. Everything else is the same as in Specific Implementation Method Seven.

[0045] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Method Eight in that: the mass ratio of tris(hydroxymethyl)aminomethane to Mg-C3N4 in step two is (80~100):1; the average molecular weight of polyethyleneimine in step two is 10000, and the mass ratio of tris(hydroxymethyl)aminomethane to polyethyleneimine is (200~210):1. Everything else is the same as in Specific Implementation Method Eight.

[0046] Specific Implementation Method 10: This implementation method differs from Specific Implementation Method 9 in that: in step 2, ② the temperature is raised from room temperature to 550°C within 1 second and held for 60 seconds, then raised from 550°C to 1000°C within 2 seconds and held for 40 seconds; then the temperature is lowered from 1000°C to 350°C within 3 seconds and held for 10 seconds.

[0047] ③ Increase the temperature from 350℃ to 1000℃ within 5 seconds and hold for 40 seconds, then decrease the temperature from 1000℃ to 350℃ within 3 seconds and hold for 10 seconds. The rest is the same as in specific implementation method nine.

[0048] The invention was verified using the following experiments:

[0049] Experiment 1: This experiment demonstrates a universal preparation method for Fe single-atom catalysts based on programmed Joule thermal flash technology, specifically carried out according to the following steps:

[0050] 1. Add 10g of melamine and FeCl3·6H2O together to 100mL of hydrochloric acid solution, ultrasonically disperse for 30min until completely dissolved, then place in a rotary evaporator for rotary evaporation, and then place in a vacuum drying oven (60°C) to dry, to obtain a uniformly dispersed solid precursor powder; place the obtained precursor powder in a muffle furnace, anneal in air atmosphere, and cool with the furnace to room temperature to obtain monometallic doped carbon nitride Fe-g-C3N4;

[0051] The hydrochloric acid solution contains 0.1M HCl;

[0052] The mass ratio of melamine to FeCl3·6H2O is 100:1;

[0053] The process conditions for rotary evaporation are: stirring at 90°C for 6 hours and rotating at 60 rpm;

[0054] The annealing conditions are as follows: the temperature is increased from room temperature to 550°C at a rate of 15°C / min and annealed for 2 hours;

[0055] 2. ① Dissolve 60.55g of tris(hydroxymethyl)aminomethane in 50mL of ultrapure water. Adjust the pH of the solution to 8.5 with hydrochloric acid under stirring to obtain a Tris buffer solution. Then add 0.7g of Fe-g-C3N4 prepared in step 1 to the Tris buffer solution and stir to disperse for 30min. Then add 0.4g of levodopa and 0.3g of polyethyleneimine in sequence. Stir and react continuously at room temperature for 48h. After the reaction is completed, filter the product through a 0.22μm filter membrane, collect the solid product and dry it in a vacuum drying oven for 24h. Grind the dried sample thoroughly and then weigh 0.2g and transfer it to an ultra-fast high-temperature furnace, and purge with argon gas.

[0056] The average molecular weight of the polyethyleneimine is 10,000;

[0057] ② The temperature is raised from room temperature to 550℃ in 1 second and held for 60 seconds, then raised from 550℃ to 1000℃ in 2 seconds and held for 40 seconds; then the temperature is lowered from 1000℃ to 350℃ in 3 seconds and held for 10 seconds.

[0058] ③ Increase the temperature from 350℃ to 1000℃ within 5 seconds and hold for 40 seconds, then decrease the temperature from 1000℃ to 350℃ within 3 seconds and hold for 10 seconds;

[0059] ④ Repeat step ③ 4 times (e.g.) Figure 1 As shown in the figure, the product was finally cooled to room temperature in the furnace, added to 100 mL of 2 mol / L hydrochloric acid solution, stirred and acid washed for 24 h, filtered with a 0.22 μm filter membrane, and then dried in a vacuum drying oven (60°C) to obtain a single-atom catalyst, denoted as Fe-SAC.

[0060] Figure 2 The X-ray diffraction pattern of the Fe-SAC prepared in Experiment 1 shows only broad, diffuse diffraction peaks of the carbon support; no characteristic diffraction peaks of Fe metal, iron oxides, or other iron-containing crystalline phases were detected. This indicates that after a two-step annealing process combining low-temperature selective anchoring (heating from room temperature to 550℃ within 1 second) and high-temperature pulsed cyclic heating, the Fe species did not form detectable crystalline nanoparticles or clusters, but rather existed stably on the support surface in a highly dispersed form. This result preliminarily proves that the obtained Fe-SAC has single-atom dispersion characteristics.

[0061] Figure 3 The images show transmission electron microscopy (TEM) images and EDS-mapped elemental distributions of Fe-SAC. Figure 4 The images show aberration-corrected electron microscopy (AEM) image and EDS-mapped elemental distribution of Fe-SAC. Further analysis was performed using transmission electron microscopy (TEM). Figure 3 ) and spherical aberration corrected high-angle annular dark-field scanning transmission electron microscope (AC-HAADF-STEM, such as Figure 4The microstructure of Fe-SAC was characterized. No obvious metal nanoparticles or aggregates were observed in the TEM images, indicating a high degree of dispersion of Fe species. Isolated bright spots uniformly distributed on the carbon substrate surface were observed in the AC-HAADF-STEM images, with significantly higher brightness than the surrounding support, belonging to atomically dispersed Fe species, proving that Fe atoms are stably anchored to the carbon support surface in single-atom form. Furthermore, EDS elemental surface scanning results showed that Fe, C, and N elements were uniformly distributed throughout the support surface, with no obvious elemental enrichment regions found, further confirming that Fe species do not form nanoparticle or cluster structures, but rather exist in Fe-SAC in a highly dispersed single-atom form.

[0062] To further verify the single-atom structure of Fe-SAC, the sample was characterized by synchrotron X-ray absorption fine structure (XAFS). Figure 5 XAFS spectra of Fe element in Fe-SAC, such as Figure 5 As shown in Figure a, the K-edge XANES spectrum of Fe is compared with standard samples Fe foil, Fe3O4, and iron phthalocyanine (FePc). The results show that the absorption edge of Fe-SAC lies between that of Fe foil and Fe3O4, indicating that the oxidation state of Fe is between 0 and +3, confirming that Fe exists in a positive valence state. Simultaneously, the near-edge spectrum of Fe-SAC (Figure b) highly matches the spectral characteristics of the FePc standard sample, suggesting that the coordination environment of Fe and N is similar to the Fe-N4 structure in FePc. Further Fourier transform and quantitative fitting analysis of the EXAFS spectrum (Figure c) shows that Fe-SAC exhibits only a main peak at approximately 1.5 Å in R-space, attributed to the Fe-N coordination shell. The fitting results show that the coordination number of the first shell of the central Fe atom is approximately 4. No signal belonging to Fe-Fe coordination was detected throughout the fitting range, ruling out the possibility of Fe nanoparticles or clusters. Based on the XANES and EXAFS analysis results above, it can be confirmed that the Fe atoms in Fe-SAC are dispersed on the support in the form of isolated single atoms, with each Fe atom coordinated to four N atoms, forming a Fe-N4 coordination structure.

[0063] Experiment 2: Preparation of Co single-atom catalyst (Co-SAC)

[0064] The difference from Experiment 1 is that FeCl3·6H2O in step 1 is replaced with an equal mass of CoCl2·6H2O, while everything else is the same as in Experiment 1, and a Co single-atom catalyst (Co-SAC) is finally obtained.

[0065] Figure 6 The X-ray diffraction pattern of Co-SAC prepared in Experiment 2 is shown below. Figure 6As shown, the spectrum only exhibits broad, diffuse diffraction peaks of the carbon support, with no characteristic diffraction peaks of Co metal, cobalt oxide, or other cobalt-containing crystalline phases detected. This indicates that after a two-step annealing process combining low-temperature selective anchoring and high-temperature pulsed cyclic heating, the Co species did not form detectable crystalline nanoparticles or clusters, but rather existed stably on the support surface in a highly dispersed form. This result preliminarily suggests that the obtained Co-SAC possesses single-atom dispersion characteristics.

[0066] Figure 7 The images show transmission electron microscopy (TEM) images and EDS-Mapping elemental distribution maps of Co-SAC. Figure 8 The images show aberration-corrected electron microscopy (AC-HAADF-STEM) and EDS-mapping elemental distribution maps of the Co-SAC. No obvious metal nanoparticles or aggregates were observed in the TEM images, indicating a high degree of dispersion of Co species. The AC-HAADF-STEM images reveal isolated bright spots uniformly distributed on the carbon substrate surface, with significantly higher brightness than the surrounding support. These correspond to atomically dispersed Co species, indicating that Co atoms are stably anchored to the carbon support surface in single-atom form. Furthermore, EDS elemental surface scanning results show that Co, C, and N elements are uniformly distributed across the entire support surface, with no obvious enrichment regions, further demonstrating that Co species do not form nanoparticles or clusters but exist in the Co-SAC in a highly dispersed single-atom form.

[0067] Figure 9 The image shows the XAFS spectrum of Co in Co-SAC. To further verify the single-atom structure and local coordination environment of Co-SAC, XAFS characterization was performed on the sample. Figure 9As shown in Figure a, Co-SAC is compared with standard samples of Co foil, Co₂O₃, and cobalt phthalocyanine (CoPc) in the Co K-edge XANES spectrum. The results show that the absorption edge of Co-SAC is located between Co foil and Co₂O₃, indicating that the Co species exists in a positive valence state. Meanwhile, the near-edge structural features of Co-SAC (Figure b) are highly similar to those of the CoPc standard sample, indicating the existence of a local coordination environment similar to Co-N₄ around Co. Further Fourier transform and quantitative fitting analysis of the EXAFS spectrum (Figure c) shows that Co-SAC exhibits a distinct main peak only at approximately 1.4 Å to 1.6 Å in R-space, corresponding to the first coordination shell of Co-N, and no characteristic peaks belonging to Co-Co coordination were detected. The fitting results indicate that the first coordination layer of the central Co atom is mainly composed of four N atoms, with a coordination number close to 4, forming a stable Co-N₄ structure. Combining the results of XRD, TEM, AC-HAADF-STEM and XAFS analyses, it can be confirmed that the Co atoms in Co-SAC are uniformly dispersed on the surface of the carbon support in the form of isolated single atoms, and coordinate with four N atoms to form Co-N4 active centers.

[0068] Experiment 3: Preparation of Ni single-atom catalyst (Ni-SAC)

[0069] The difference from Experiment 1 is that FeCl3·6H2O in step 1 was replaced with an equal mass of NiCl2·6H2O, while everything else was the same as in Experiment 1, and Ni single-atom catalyst (Ni-SAC) was finally obtained.

[0070] Figure 10 The X-ray diffraction pattern of Ni-SAC prepared in Experiment 3 is shown below. Figure 10 As shown, the spectrum only exhibits broad, diffuse diffraction peaks of the carbon support, and no characteristic diffraction peaks of Ni metal, nickel oxide, or other nickel-containing crystalline phases were detected. This indicates that after the two-step annealing process combining low-temperature selective anchoring and high-temperature pulsed cyclic heating, the Ni species did not form detectable crystalline nanoparticles or clusters, but rather existed stably on the support surface in a highly dispersed form. This result preliminarily suggests that the obtained Ni-SAC has single-atom dispersion characteristics.

[0071] Figure 11 The images show transmission electron microscope (TEM) images and EDS-Mapping elemental distribution maps of Ni-SAC. Figure 12The images show aberration-corrected electron microscopy (AEM) image and EDS-mapping elemental distribution map of Ni-SAC. No obvious metal nanoparticles were observed in the TEM images, indicating a high degree of Ni species dispersion. The AC-HAADF-STEM images reveal isolated bright spots uniformly distributed on the carbon substrate surface, with significantly higher brightness than the surrounding support, corresponding to atomically dispersed Ni species. This indicates that Ni atoms are stably anchored to the carbon support surface in single-atom form. Furthermore, EDS elemental surface scanning results show that Ni, C, and N elements are uniformly distributed across the entire support surface, with no obvious elemental enrichment regions found. This further proves that Ni species do not form nanoparticles or clusters, but rather exist in Ni-SAC in a highly dispersed single-atom form.

[0072] Figure 13 The image shows the XAFS spectrum of Ni element in Ni-SAC. To further verify the single-atom structure and local coordination environment of Ni-SAC, synchrotron X-ray absorption fine structure (XAFS) characterization was performed on the sample. Figure 13 As shown in Figure a, the Ni-SAC is compared with standard samples of Ni foil and nickel phthalocyanine (NiPc) in the Ni K-edge XANES spectrum. The results show that the absorption edge of Ni-SAC is located between Ni foil and NiPc, indicating that Ni species exist in a positive valence state. Meanwhile, the near-edge structural features of Ni-SAC (Figure b) are highly similar to those of the NiPc standard sample, indicating the existence of a Ni-N4-like local coordination environment around Ni. Further Fourier transform and quantitative fitting analysis of the EXAFS spectrum (Figure c) shows that Ni-SAC exhibits a distinct main peak only at approximately 1.4 Å to 1.6 Å in R-space, corresponding to the first coordination shell of Ni-N, while no characteristic peak at approximately 2.1 Å to 2.3 Å at the Ni-Ni coordination shell is detected. The fitting results indicate that the first coordination layer of the central Ni atom is mainly composed of four N atoms, with a coordination number close to 4, forming a stable Ni-N4 structure. No Ni-Ni scattering pathways were observed throughout the fitting range, indicating the absence of obvious nickel nanoparticles or clusters in the sample. Combined with XRD, TEM, AC-HAADF-STEM, and XAFS analyses, it can be confirmed that Ni atoms in Ni-SAC are uniformly dispersed on the carbon support surface in the form of isolated single atoms, and coordinate with four N atoms to form Ni-N4 active centers.

[0073] Experiment 4: Preparation of Mn single-atom catalyst (Mn-SAC)

[0074] The difference from Experiment 1 is that FeCl3·6H2O in step 1 was replaced with an equal mass of MnCl2·4H2O, while everything else was the same as in Experiment 1, and Ni single-atom catalyst (Mn-SAC) was finally obtained.

[0075] Figure 14 The X-ray diffraction pattern of Mn-SAC prepared for Experiment 4 is shown below. Figure 14 As shown, the spectrum only exhibits broad, diffuse diffraction peaks of the carbon support, and no characteristic diffraction peaks of Mn metal, manganese oxide, or other manganese-containing crystalline phases were detected. This indicates that after the two-step annealing process combining low-temperature selective anchoring and high-temperature pulsed cyclic heating, the Mn species did not form detectable crystalline nanoparticles or clusters, but rather existed stably on the support surface in a highly dispersed form. This result preliminarily suggests that the obtained Mn-SAC has single-atom dispersion characteristics.

[0076] Figure 15 The images show transmission electron microscopy (TEM) images and EDS-Mapping elemental distribution maps of Mn-SAC. Figure 16 These are aberration-corrected electron microscopy (AEM) images and EDS-mapping elemental distribution maps of Mn-SAC. No obvious metallic nanoparticles were observed in the TEM images, indicating a high degree of Mn species dispersion. The AC-HAADF-STEM images show isolated bright spots uniformly distributed on the carbon substrate surface, with significantly higher brightness than the surrounding support, corresponding to atomically dispersed Mn species. This indicates that Mn atoms are stably anchored to the carbon support surface in single-atom form. Furthermore, EDS elemental surface scanning results show that Mn, C, and N elements are uniformly distributed across the entire support surface, with no obvious elemental enrichment regions found. This further proves that Mn species do not form nanoparticles or clusters, but rather exist in Mn-SAC in a highly dispersed single-atom form.

[0077] Figure 17 The XAFS spectrum of Mn in Mn-SAC is shown in Figure 1. In the Mn K-edge XANES spectrum (Figure 1a), Mn-SAC is compared with standard samples of Mn foil, MnO2, and manganese phthalocyanine (MnPc). The results show that the absorption edge position of Mn-SAC is significantly higher than that of Mn foil but lower than that of MnO2, indicating that the Mn species exists in a positive valence state. Simultaneously, the near-edge structural features of Mn-SAC (Figure 2b) are highly similar to those of the MnPc standard sample, indicating the existence of a local coordination environment similar to Mn-N4 around Mn. Further Fourier transform and quantitative fitting analysis of the EXAFS spectrum (Figure 2c) shows that Mn-SAC exhibits a distinct main peak only at approximately 1.4–1.6 Å in R-space, corresponding to the first coordination shell of Mn-N. The fitting results indicate that the first coordination shell of the central Mn atom is mainly composed of four N atoms, with a coordination number close to 4, forming a stable Mn-N4 structure. Combining the results of XRD, TEM, AC-HAADF-STEM and XAFS analyses, it can be confirmed that Mn atoms in Mn-SAC are uniformly dispersed on the surface of the carbon support in the form of isolated single atoms, and coordinate with four N atoms to form Mn-N4 active centers.

[0078] Experiment 5: This experiment evaluated the performance of the single-atom catalysts prepared in Experiments 1-4 in activating persulfate (PMS) to degrade organic pollutants. Phenol was used as the target pollutant in the catalytic oxidation experiment. Specific test steps and results are as follows:

[0079] Two mg of each single-atom catalyst (Fe-SAC, Co-SAC, Ni-SAC, and Mn-SAC) was weighed and added to 200 mL of a 0.1 mM phenol solution. The catalyst was magnetically stirred at 600 rpm to ensure thorough dispersion. PMS was then added to bring the PMS concentration in the system to 0.2 mM, and timing was initiated. Samples were taken at 1, 2, 4, 7, 10, 15, 20, 28, 45, and 60 min during the reaction. The phenol concentration in the solution was determined using ultra-high performance liquid chromatography (UPLC), and the phenol removal rate and reaction kinetic parameters were calculated.

[0080] Figure 18 The figure shows a comparison of the performance of M-SAC in activating PMS to degrade phenol in Experiment 5. As shown, all single-atom catalysts can effectively activate PMS, but their catalytic activities differ, in the order of Co-SAC > Fe-SAC > Mn-SAC > Ni-SAC. Co-SAC exhibits the highest activity, rapidly removing phenol within 10 minutes; Fe-SAC is second, achieving complete degradation in approximately 20 minutes. Pseudo-first-order kinetic fitting results indicate that all systems conform to the pseudo-first-order kinetic model, with the apparent rate constant order consistent with the activity. Co-SAC has the largest rate constant, demonstrating the strongest activation ability for PMS, followed by Fe-SAC and Mn-SAC. This indicates that different metal centers significantly affect the PMS activation efficiency, with Co single-atom sites being most conducive to promoting the oxidative degradation of phenol. In summary, the single-atom catalysts prepared in this invention all possess the ability to activate and degrade phenol using PMS.

[0081] Experiment 6: To investigate the effect of coexisting anions on the degradation performance of phenol by Fe-SAC-activated PMS, solutions containing 10 mM Cl were prepared. - SO4 2- NO3 - H2PO3 - A phenol solution with an initial phenol concentration of 0.1 mM was prepared. 2 mg of Fe-SAC was added to 200 mL of each solution and dispersed by magnetic stirring at 600 rpm. PMS was then added to achieve a PMS concentration of 0.25 mM, and the reaction was started. Samples were taken at 1, 2, 4, 7, 10, 15, 20, 28, 45, and 60 min during the reaction. The phenol concentration in the solution was determined using ultra-high performance liquid chromatography (UPLC), and the phenol removal rate was calculated to evaluate the effect of different coexisting anions on the phenol degradation performance of the Fe-SAC / PMS system.

[0082] Figure 19 The figure shows the effect of different anions on the performance of Fe-SAC activated PMS in degrading phenol in Experiment 6. The control group did not have any additional anions added, as shown in the figure. - SO4 2- NO3 - and H2PO3 - Under the given conditions, the performance of Fe-SAC-activated PMS in degrading phenol is minimally affected, and phenol can still be rapidly removed. Even with the interference of coexisting anions and natural organic matter, the Fe-SAC / PMS system maintains a high phenol degradation efficiency, indicating that the prepared Fe single-atom catalyst has good environmental adaptability and anti-interference ability, and can maintain stable catalytic oxidation performance under complex aquatic conditions, demonstrating good potential for practical application.

Claims

1. A universal preparation method for a single-atom catalyst based on programmed Joule thermal flash technology, characterized in that... The method is performed according to the following steps:

1. Melamine and a metal salt are added to a hydrochloric acid solution and ultrasonically dispersed until completely dissolved. Then, the mixture is placed in a rotary evaporator for rotary evaporation and then dried in a vacuum drying oven to obtain a uniformly dispersed solid precursor powder. The obtained precursor powder is placed in a muffle furnace and annealed in an air atmosphere. It is then cooled to room temperature in the furnace to obtain monometallic doped carbon nitride Mg-C3N4. The metal in the metal salt is M.

2. ① Dissolve tris(hydroxymethyl)aminomethane in ultrapure water, and adjust the pH of the solution to 8.5-9 with hydrochloric acid under stirring to obtain a Tris buffer solution; then add Mg-C3N4 prepared in step 1 to the Tris buffer solution, stir and disperse, and then add levodopa and polyethyleneimine in sequence, and continue stirring the reaction at room temperature; after the reaction is completed, filter, collect the solid product and place it in a vacuum drying oven to dry; grind the dried sample thoroughly, and then transfer it to an ultra-fast high-temperature furnace and introduce a protective atmosphere; ② The temperature is raised from room temperature to 550℃~560℃ within 1 second and held for 60s~65s, then raised from 550℃~560℃ to 1000℃~1050℃ within 2 seconds and held for 40s~45s; then cooled from 1000℃~1050℃ to 350℃~360℃ within 3 seconds and held for 10s~15s; ③ Raise the temperature from 350℃~360℃ to 1000℃~1050℃ within 5s and hold for 40s~45s, then lower the temperature from 1000℃~1050℃ to 350℃~360℃ within 3s and hold for 10s~15s; ④ Repeat step ③ four times. Finally, cool the furnace to room temperature, add the product to the hydrochloric acid solution, stir and wash, filter, and then dry in a vacuum drying oven to obtain a single-atom catalyst, denoted as M-SAC.

2. The universal preparation method of a single-atom catalyst based on programmed Joule thermal flash technology according to claim 1, characterized in that... The metal salt mentioned in step one is a chloride hydrate, sulfate, or acetate.

3. The universal preparation method of a single-atom catalyst based on programmed Joule thermal flash technology according to claim 2, characterized in that... The metal M mentioned in step one is Fe, Co, Ni or Mn.

4. A universal preparation method for a single-atom catalyst based on programmed Joule thermal flash technology according to claim 3, characterized in that... The mass ratio of melamine to metal salt mentioned in step one is 100:

1.

5. A universal preparation method for a single-atom catalyst based on programmed Joule thermal flash technology according to claim 1, characterized in that... The process conditions for rotary evaporation in step one are: stirring at 90°C for 6 hours and rotating at 60 rpm.

6. A universal preparation method for a single-atom catalyst based on programmed Joule thermal flash technology according to claim 1, characterized in that... The temperature of the vacuum drying oven mentioned in step one is 60°C.

7. A universal preparation method for a single-atom catalyst based on programmed Joule thermal flash technology according to claim 1, characterized in that... The annealing conditions described in step one are: heating from room temperature to 550°C at a rate of 15°C / min and annealing for 2 hours.

8. A universal preparation method for a single-atom catalyst based on programmed Joule thermal flash technology according to claim 1, characterized in that... The mass ratio of trihydroxymethylaminomethane to levodopa in step two is (150~160):

1.

9. A universal preparation method for a single-atom catalyst based on programmed Joule thermal flash technology according to claim 8, characterized in that... The mass ratio of tris(hydroxymethyl)aminomethane to Mg-C3N4 in step two is (80~100):1; the average molecular weight of polyethyleneimine in step two is 10000, and the mass ratio of tris(hydroxymethyl)aminomethane to polyethyleneimine is (200~210):

1.

10. A universal preparation method for a single-atom catalyst based on programmed Joule thermal flash technology according to claim 1, characterized in that... In step two, ② the temperature is raised from room temperature to 550℃ in 1 second and held for 60 seconds, then raised from 550℃ to 1000℃ in 2 seconds and held for 40 seconds; then the temperature is lowered from 1000℃ to 350℃ in 3 seconds and held for 10 seconds. ③ Increase the temperature from 350℃ to 1000℃ within 5 seconds and hold for 40 seconds, then decrease the temperature from 1000℃ to 350℃ within 3 seconds and hold for 10 seconds.

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