Transition metal single atom carbon-based catalysts and methods for making the same

Abstract

The present application provides a transition metal single atom carbon-based catalyst, characterized by comprising a transition metal single atom, a nitrogen-doped carbon substrate and an oxygen-doped carbon substrate, and the carbon-based catalyst comprises a 4-coordination structure unit M-N4 structure and a 5-coordination structure unit M-N2O3 structure of the transition metal single atom in the structure of the carbon-based catalyst. The carbon-based catalyst also has high conductivity, and the existence of the two coordination structures can synergistically catalyze the oxidative dehydrogenation of organic matter and the efficient reduction of hydrogen peroxide and oxygen to generate hydroxyl radicals and superoxide radical active species. The synergistic oxidation of the two free radicals on the organic matter in water can significantly improve the mineralization degree of the organic pollutants, thereby efficiently catalyzing the degradation of the organic pollutants in water. The present application also provides a method for preparing the carbon-based catalyst.

Description

Technical Field

[0001] This invention relates to the field of materials structure, specifically to a carbon-based catalyst based on a single transition metal atom and its preparation method. Background Technology

[0002] In recent years, shrinking metal particles with catalytically active centers to the atomic scale has become an effective method to improve catalytic activity and selectivity, known as single-atom catalysts. A single-atom catalyst refers to an isolated single atom dispersed on a support, with no interaction between the atoms. Single-atom catalysts were first reported in 2011 (B. Qiao, A. Wang, X. Yang, et al. Nat. Chem. 2011, 3, 634.), which consist of metal particles immobilized on FeO. x The isolated Pt atoms on the surface exhibited high activity and stability for the oxidation and preferential oxidation of CO in H2. In recent years, single-atom catalysts have attracted much attention due to their high atomic efficiency, catalytic activity, and selectivity.

[0003] In the preparation of single-atom catalysts, the synthesis process, precursor, and substrate are three key factors affecting their performance. Regarding substrate selection, single nanostructures with high specific surface area or their heteroatom-doped, defect-derived materials are generally preferred, as they can better immobilize metal atoms on the support, forming active sites. Abundant carbon-based materials, due to their high specific surface area, excellent conductivity, and tunable surface properties, are often used as substrates for supporting metal single atoms, enabling coordination bonding. It is noteworthy that the structural regulation of the active site is often crucial for controlling catalytic performance; synthesizing a specific structural unit can promote the activation reaction. For example, the synthesis of pyrrole-type FeN4 sites enhances catalytic degradation by promoting oxygen transfer and activating PMS (Y.Xiong, H.Li, C.Liu, et al. Adv. Mater. 2022, 2110653). However, the pyrrole-type FeN4 sites prepared by Zhang et al. exhibit significant intrinsic activity, good O2 adsorption energy, and four-electron reaction selectivity, resulting in excellent ORR activity and stability of the catalyst (N. Zhang, T. Zhou, M. Chen, et al. Energy & Environmental Science. 2020, 13, 111). Furthermore, the simultaneous coordination of a single Mn atom with four N atoms to form an active MnN4 site can catalyze the ozone reaction to generate ·OH under acidic conditions (Z. Guo, Y. Xie, J. Xiao, et al. J. Am. Chem. Soc. 2019, 141, 12005). Ji et al. obtained the FeN3P active center through precise coordination of P and N; this structural unit can rapidly dissociate H2O2 (S. Ji, B. Jiang, H. Hao, et al. Nature Catalysis. 2021, 4, 407). In addition to the four-coordinate structural unit, the five-coordinate structure of Fe-N4O was synthesized and used as a catalytic active site for CO2 reduction (X. Wang, Y. Pan, H. Ning, et al. Applied Catalysis B: Environmental. 2020, 266, 118630). As can be seen from the above, the target reaction can be effectively catalyzed and activated by constructing specific metal ion coordination structures. However, catalysts containing only a single structural unit have not yet been reported to co-construct active units with multiple different coordination structures on a single carbon substrate.

[0004] This invention utilizes different functional groups on a carbon substrate to enable nitrogen-doped conjugated carbon to form a four-coordinate complex structure with a divalent metal. Simultaneously, the dangling bonds of the carbon nanosheets coordinate with metal single atoms in both the transverse and longitudinal directions to form a five-coordinate structure. This provides a carbon-based catalyst containing both four-coordinate and five-coordinate transition metal single atoms and offers applications of carbon-based catalysts in the field of photocatalytic degradation. Summary of the Invention

[0005] To achieve the objectives of this invention, the following technical solution is adopted:

[0006] In a first aspect, the present invention provides a carbon-based catalyst of a transition metal single atom, comprising a transition metal single atom, a nitrogen-doped carbon substrate, and an oxygen-doped carbon substrate, wherein the structure of the carbon-based catalyst includes a 4-coordination structural unit M-N4 structure and a 5-coordination structural unit M-N2O3 structure of the transition metal single atom.

[0007] Furthermore, the nitrogen-doped carbon substrate structure contains conjugated C=N bonds, which form a 4-coordination unit M-N4 structure with transition metal single atoms. As a specific embodiment of the present invention, the 4-coordination unit M-N4 structure is as follows: Fig. 1-2 As shown.

[0008] Furthermore, the nitrogen-doped carbon substrate structure also contains dangling cyano groups (-C≡N) and conjugated C=N at its structural edge, and the oxygen-doped carbon substrate structure contains dangling carbon-oxygen double bonds (-C=O) and hydroxyl groups (-C-OH) at its structural edge. The dangling cyano groups (-C≡N), the conjugated C=N at the structural edge, the dangling carbon-oxygen double bonds (-C=O), and the hydroxyl groups (-C-OH) form a 5-coordination structural unit M-N₂O₃ structure with the transition metal single atom. As a specific embodiment of the present invention, the 5-coordination structural unit M-N₂O₃ structure is as follows... Fig. 1-2 As shown.

[0009] Furthermore, the transition metal single atom can be selected from all common transition metals, preferably one or more of Fe, Co, Ni, Cu, Zn, and Mn;

[0010] Furthermore, the nitrogen-doped carbon substrate is obtained by calcining a nitrogen-rich organic compound. The nitrogen-rich organic compound is a precursor to the nitrogen-doped carbon substrate; after high-temperature calcination, the nitrogen-rich organic compound is transformed into a nitrogen-doped carbon substrate. Specifically, in the preparation method of this invention, the nitrogen-rich organic compound, the nitrogen-doped carbon substrate, and a metal salt solution are mixed to allow the nitrogen-rich organic compound to complex with metal atoms. After high-temperature calcination, the complex is transformed into a nitrogen-doped substrate and a metal atom 4-coordination structural unit M-N4 structure. The nitrogen-rich organic compound is selected from one or more of urea, dicyandiamide, dimethylformamide (DMF), and melamine, preferably melamine. When melamine is selected, the melamine structure can form nitrogen-rich carbon nanosheets after high-temperature calcination.

[0011] Furthermore, the oxygen-doped carbon substrate is selected from one or more of oxygen-doped carbon quantum dots, oxygen-doped carbon nanotubes, graphene oxide, graphite oxide, and fullerene oxide, with oxygen-doped carbon quantum dots and graphene oxide being preferred. When graphene oxide is selected, the hydroxyl groups -C-OH in the graphene oxide are perpendicular to the surface of the graphene oxide.

[0012] Furthermore, to enhance charge transfer between active sites, the carbon-based catalyst exhibits high electrical conductivity. By adjusting the proportions of transition metal single atoms, nitrogen-doped carbon substrate, oxygen-doped carbon substrate, and nitrogen doping content, a highly conductive carbon-based catalyst was obtained. The internal resistance of this material was measured to be 50Ω-800Ω using electrochemical impedance spectroscopy.

[0013] In a preferred embodiment, the carbon-based catalyst satisfies one or more of the following conditions:

[0014] (1) The carbon-based catalyst includes transition metal single atoms, nitrogen-rich carbon nanosheets, oxygen-doped carbon quantum dots, and graphene oxide;

[0015] (2) The mass percentage of the transition metal single atom in the carbon-based catalyst is 1.0-30%;

[0016] (3) The proportion of nitrogen atoms in the carbon-based catalyst is 1.0-50%;

[0017] (4) 50%-70% of the nitrogen atoms in the carbon-based catalyst participate in the formation of the nitrogen-doped carbon conjugated structure sp2 C=N in the nitrogen-rich carbon nanosheets;

[0018] (5) The carbon-based catalyst contains 35-85% carbon atoms;

[0019] (6) Carbon-based catalysts: 1.0%-20% of carbon atoms participate in the formation of cyano dangling bonds -C≡N at the edge of nitrogen-rich carbon nanosheets;

[0020] (7) The proportion of oxygen atoms in the carbon-based catalyst is 5.0-20%;

[0021] (8) In carbon-based catalysts, 1.0% to 50% of oxygen atoms participate in the formation of oxygen-containing functional groups -C=O at the edge of oxygen-doped carbon quantum dots;

[0022] (9) Carbon-based catalysts have 1.0% to 50% oxygen atoms participating in the formation of hydroxyl groups -C-OH perpendicular to the plane of graphene oxide;

[0023] The highly conductive carbon substrate includes nitrogen-rich carbon nanosheets, oxygen-doped carbon quantum dots, and graphene oxide. The nitrogen-rich carbon nanosheets are obtained by calcining nitrogen-rich organic compounds such as melamine.

[0024] In a second aspect, the present invention also provides a method for preparing the carbon-based catalyst, the method comprising the following steps: mixing a portion of an oxygen-doped carbon substrate with a nitrogen-rich organic compound, then adding a transition metal salt solution and the remaining oxygen-doped carbon substrate, mixing evenly and drying, and then performing calcination and post-treatment steps to obtain the carbon-based catalyst; preferably, mixing oxygen-doped carbon quantum dots with melamine, then adding transition metal single atoms and graphene oxide, mixing evenly and drying, and then performing calcination and post-treatment steps to obtain the carbon-based catalyst.

[0025] In a preferred embodiment, the method includes the following steps:

[0026] (1) Prepare oxygen-doped carbon quantum dots, melamine and graphene oxide dispersions with concentrations of 0.01-10 mg / mL and metal salt solutions with concentrations of 1-5 mg / mL respectively;

[0027] (2) Mix a certain volume of oxygen-doped carbon quantum dots with melamine dispersion in a certain proportion for 2-12 hours at a temperature of 50-200℃.

[0028] (3) Mix a certain volume of transition metal salt solution with the sample obtained in step (2), then add the graphene oxide solution in step (1), mix evenly and then dry at a temperature of 50-100℃.

[0029] (4) Calcine the solid obtained in step (3) in a nitrogen or argon protective atmosphere, with a gas flow rate of 20-100 mL / min, a temperature of 400-800℃, a speed of 0.1-10℃ / min, and a duration of 1-6 h.

[0030] (5) The solid obtained in step (4) is acid washed in hydrochloric acid with a concentration of 0.5-10 mol / L at 10-60°C for 6-48 hours, then washed with water until neutral and dried to obtain the carbon-based catalyst.

[0031] Furthermore, in step (2) or step (3), the mixing is carried out by means including hydrothermal, stirring or shaking; in step (3), the drying process is drying by blower or vacuum oven, heating and evaporation or rotary evaporation.

[0032] In a third aspect, the present invention also provides the application of carbon-based catalysts in the field of photocatalytic degradation.

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0034] 1. The carbon-based catalyst of the present invention contains both four-coordinate and five-coordinate metal single-atom sites: by utilizing different functional groups (-C-OH, -C=O, -C≡N, NC conjugated units) of nano-carbon materials to achieve coordination bonding with metal single atoms, the coordination environment is changed, and reactions different from those of ordinary four-coordinate sites can occur.

[0035] 2. In the carbon-based catalyst of the present invention, the four-coordinate and five-coordinate metal single-atom sites can produce synergistic catalytic effects: the four-coordinate complex structure can reduce oxygen in the system to form superoxide radicals, while the five-coordinate structure can oxidize organic matter in the system while reducing hydrogen peroxide; the carbon-based catalyst with the coexistence of four-coordinate and five-coordinate metal single-atom sites can simultaneously realize the half-reaction of rapid dehydrogenation and electron loss of organic matter, as well as the half-reaction of oxygen gaining electrons to generate superoxide radicals and hydrogen peroxide gaining electrons to generate active hydroxyl radicals.

[0036] 3. The carbon-based catalyst of the present invention has high conductivity: With the support of the high conductivity of the carbon substrate, spontaneous and efficient charge transfer between active sites can be achieved, and visible light can be used to accelerate charge transfer efficiently; under this action, the continuously generated superoxide radicals and hydroxyl radicals couple with each other to rapidly oxidize and degrade organic wastewater.

[0037] 4. The carbon-based catalyst of the present invention also has the advantages of simple process operation, abundant reserves, low cost and easy industrial production. Attached Figure Description

[0038] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0039] Fig. 1 This is a side view of the structural schematic diagram of the carbon-based catalyst of the present invention;

[0040] Fig. 2 This is a top view of the structural schematic diagram of the carbon-based catalyst of the present invention;

[0041] Fig. 3 Aberration-corrected electron micrograph of the carbon-based catalyst prepared in Example 1 of this invention;

[0042] Fig. 4 XPS spectra of the carbon-based catalysts prepared in Example 1, Comparative Example 1, and Comparative Example 2 of this invention;

[0043] Fig. 5 Synchrotron radiation spectra of the carbon-based catalysts prepared in Example 1 and Comparative Example 1 of this invention;

[0044] Fig. 6 The H2O2- cyclic voltammetry curves and structural comparison diagrams for Embodiment 1, Comparative Embodiment 1, and Comparative Embodiment 2 of the present invention are shown below.

[0045] Fig. 7 The O2-cyclic voltammetry curves and structural comparison diagrams for Embodiment 1, Comparative Embodiment 1, and Comparative Embodiment 2 of the present invention are shown below.

[0046] Fig. 8 The diagram shows the methylene blue (MB) cyclic voltammetry curves and structural comparisons of Example 1, Comparative Example 1, and Comparative Example 2 of this invention.

[0047] Fig. 9 The graph shows a comparison of the degradation performance of the carbon-based catalysts prepared in Example 1, Comparative Example 1, and Comparative Example 2 of the present invention, as well as a graph showing the cycle stability of Example 1.

[0048] Fig. 10 This is a comparison diagram of the degradation of the carbon-based catalyst prepared in Example 1 of the present invention under different catalytic conditions;

[0049] Fig. 11 The results show the characterization of free radicals in the photocatalytic degradation system of the carbon-based catalyst prepared in Example 1 of this invention. Detailed Implementation

[0050] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0051] Example 1

[0052] First, oxygen-doped carbon quantum dots (0.015 mg / mL), melamine (10 mg / mL), and graphene oxide dispersion (0.015 mg / mL) were provided, along with a ferric chloride solution (3 mg / mL). First, the oxygen-doped carbon quantum dots and melamine dispersion were hydrothermally mixed (200°C, 12 h), then added to the ferric chloride solution. After stirring until homogeneous, the graphene oxide dispersion was added, and the mixture was heated to dryness at 90°C. Subsequently, the resulting precursor was heated to 550°C at a rate of 2.3°C / min under a nitrogen atmosphere (flow rate 50 mL / min) and held for 4 h. After grinding, the resulting solid was acid-washed with 2 mol / L hydrochloric acid at 20°C for 24 h to obtain the carbon-based catalyst of Example 1.

[0053] Example 2

[0054] First, oxygen-doped carbon quantum dots (0.1 mg / mL), melamine (8 mg / mL), and graphene oxide dispersion (0.01 mg / mL) were provided, along with a nickel nitrate solution (5 mg / mL). The oxygen-doped carbon quantum dot dispersion and melamine dispersion were first stirred and mixed thoroughly (80°C, 2 h), then added to the nickel nitrate solution and stirred continuously (80°C, 2 h). After thorough mixing, the graphene oxide dispersion was added, and the mixture was evaporated to dryness at 90°C. The resulting precursor was then heated to 500°C at a rate of 5°C / min under an argon atmosphere (flow rate 100 mL / min) and held for 4 h. After grinding, the resulting solid was acid-washed with 0.5 mol / L hydrochloric acid at 50°C for 48 h to obtain the carbon-based catalyst of Example 2.

[0055] Example 3

[0056] Synthesis process: First, oxygen-doped carbon quantum dots (5 mg / mL), melamine (0.5 mg / mL), and graphene oxide dispersion (0.5 mg / mL) and copper sulfate solution (1 mg / mL) were provided. The oxygen-doped carbon quantum dot dispersion and melamine dispersion were first mixed thoroughly by shaking (50°C, 5 h), then added to the copper sulfate solution and stirred continuously (50°C, 2 h). After thorough mixing, the graphene oxide dispersion was added, and the mixture was rotary evaporated at 90°C. The resulting precursor was then heated to 600°C at a rate of 10°C / min under an argon atmosphere (flow rate 20 mL / min) and held for 4 h. After grinding, the resulting solid was acid-washed with 5 mol / L hydrochloric acid at 30°C for 12 h to obtain the carbon-based catalyst of Example 3.

[0057] Comparative Example 1

[0058] First, oxygen-doped carbon quantum dots (0.015 mg / mL), melamine dispersion (10 mg / mL), and ferric chloride solution (3 mg / mL) were provided. The oxygen-doped carbon quantum dots and melamine dispersion were first hydrothermally mixed (200 °C, 12 h), then added to the ferric chloride solution. After stirring until homogeneous, the mixture was heated to dryness at 90 °C. Subsequently, the resulting precursor was heated to 550 °C at a rate of 2.3 °C / min under a nitrogen atmosphere (flow rate 50 mL / min) and held for 4 h. After grinding, the resulting solid was acid-washed with 2 mol / L hydrochloric acid at 20 °C for 24 h to obtain the carbon-based catalyst of Comparative Example 1.

[0059] Comparative Example 2

[0060] Synthesis process: First, melamine with a concentration of 10 mg / mL and ferric chloride solution with a concentration of 3 mg / mL were provided. The melamine dispersion was first added to the ferric chloride solution, stirred until homogeneous, and then heated to dryness at 90 °C. Subsequently, the resulting precursor was heated to 550 °C at a rate of 2.3 °C / min under a nitrogen atmosphere (flow rate 50 mL / min) and held for 4 h. After grinding, the resulting solid was acid-washed with 2 mol / L hydrochloric acid at 20 °C for 24 h to obtain the carbon-based catalyst of Comparative Example 2.

[0061] Test case

[0062] The carbon-based catalyst of Example 1 was scanned using aberration-corrected electron microscopy, and the results are as follows: Fig. 3 As shown. From Fig. 3 As can be seen, the transition metals are uniformly distributed in the carbon-based catalyst in the form of single atoms.

[0063] X-ray photoelectron spectroscopy was used to qualitatively and semi-quantitatively analyze the changes in elemental composition, valence state, and content on the surface of the materials in Example 1 and Comparative Examples 1-2. The results are as follows: Fig. 4As shown in the figure, sp2 C=N is present in both Example 1 and Comparative Examples 1 and 2, and sp2 C=N originates from the CN structure after calcination of melamine. -C≡N is present in both Example 1 and Comparative Examples 1 and 2, and -C≡N originates from the dangling bonds generated at the edges of the CN structure after the addition of the transition metal. Oxygen-doped carbon quantum dots are present in both Example 1 and Comparative Example 1, while no oxygen-doped carbon quantum dots are added in Comparative Example 2. The proportions of -C=O in O1s in Example 1, Comparative Example 1, and Comparative Example 2 are 29.4%, 32.4%, and 6.5%, respectively. Therefore, the oxygen-containing functional group -C=O from oxygen-doped carbon quantum dots is present in Example 1 and Comparative Example 1. Graphene oxide is present in Example 1, while no graphene oxide is added in Comparative Examples 1 and 2. The proportions of -CO in O1s in Example 1, Comparative Example 1, and Comparative Example 2 are 28.5%, 6.7%, and 7.1%, respectively. Therefore, in Example 1, there is an oxygen-containing functional group -C-OH from rGO.

[0064] Fe K-edge data were collected using the BL14W1 beam of the Shanghai Synchrotron Radiation Facility (SSRF) to analyze the oxidation state, local atomic structure, and coordination environment of transition metal single atoms in a carbon catalyst. The test results of Example 1 and Comparative Example 1 are as follows: Fig. 5 As shown in the results, both Example 1 and Comparative Example 1 involve mixed coordination of Fe-N and Fe-O. The coordination number of Comparative Example 1 is the same as that of FePc, i.e., four-coordinate, while the coordination number of Example 1 is clearly between FePc and Fe2O3, i.e., between four-coordinate and six-coordinate. Therefore, in Example 1, the inclusion of graphene oxide introduces -C-OH, increasing the overall coordination number of Example 1; that is, -C-OH induces the formation of a five-coordinate structure.

[0065] Cyclic voltammetry (CV), i.e., current-potential curves, was characterized in an electrochemical workstation. The current collector of the working electrode was hydrophilic carbon cloth. The test conditions included CV-H2O2, CV-O2, and CV-MB, respectively, by adding 100 μL of H2O2 to the electrolyte and purging oxygen with N2, purging O2 (10 mL min), etc. -1 This is achieved by adding 100 μL of MB and purging with N2 to remove oxygen. Example 1, and the test results of comparative Examples 1-2 are as follows: Fig. 6-8 As shown. From Fig. 6As can be seen, two reduction peaks appear in the CV when H2O2 is present. The reduction peak at 350 mV is closely related to the content of -C=O. All examples show a reduction peak near 0 mV, and the peak intensity increases with the presence of Fe, which is closely related to the -C≡N generated after the addition of the transition metal. Therefore, the H2O2 activation center is a coordination structure containing both -C=O and -C≡N, and this is present in both Comparative Example 1 and Example 2, indicating a four-coordinate structure. Fig. 7 As can be seen, a reduction peak appears in the CV when O2 is present, and the reduction peak at 510 mV is closely related to the content of sp2 C=N. Therefore, the O2 activation center is a coordination structure with sp2 C=N, which is present in Comparative Example 1, Comparative Example 2, and Example 1, i.e., a four-coordinate structure M-N4. Fig. 8 As can be seen, an oxidation peak appears on the CV when MB is present. This oxidation peak at 730 mV is closely related to the content of -C-OH, i.e., the presence of rGO. Therefore, the activation center of MB is the five-coordinate structure M-N2O3, which is an additional -C-OH introduced after the addition of rGO. This structure was present in Example 1 but not in Comparative Examples 1 and 2. Fig. 6-8 It can be seen that in Example 1, there are both tetracoordinate M-N4 and pentacoordinate M-N2O3 active centers that activate H2O2, O2 and organic matter.

[0066] Conventional photocatalytic degradation experiment: The experiment was conducted at 25±2℃ under xenon lamp illumination to simulate visible light (300W Xe lamp, CEL-HXF 300, 420nm filter). 100 mL of 200 mg / L methylene blue solution was placed in the photocatalytic reactor, and 1 mol / L... -1 Adjust the pH of the solution with NaOH and HCl solution, and add the prescribed amount of catalyst powder (0.25 g / L). -1 The system was continuously stirred in the dark for 60 minutes to reach adsorption-desorption equilibrium. Then, 2 mL of H₂O₂ was added simultaneously with the introduction of a light source to initiate the photocatalytic degradation reaction. Samples were taken at specific reaction times, centrifuged, and the supernatant was analyzed to calculate the corresponding concentration, thus determining the degradation efficiency.

[0067] Catalytic Cycle Testing: The degradation rate of organic pollutants detected by repeated experiments is an important criterion for judging the stability of the catalyst. The catalyst dosage and other reaction conditions are kept consistent in the repeated experiments. The amount of catalyst collected from multiple degradation experiments ensures that a sufficient amount of catalyst is added after each cycle. The specific experimental procedure is as follows: After the catalytic experiment, solid-liquid separation is performed by centrifugation, and the catalyst is washed multiple times with deionized water and alcohol. After vacuum drying, it is collected directly and then reused in the degradation experiment. The specific catalytic experimental steps are the degradation experimental steps described above.

[0068] Mineralization capacity test: Mineralization degree is one of the important indicators for judging whether a catalyst can completely degrade pollutants in water, that is, the ability of the catalyst to completely degrade pollutants in water to produce CO2 and H2O. The concentration of residual organic carbon in the reaction solution after the photocatalytic degradation experiment was tested by a total organic carbon analyzer, so as to calculate the removal rate of organic carbon in water by the catalyst. The calculation process is as shown in equation (1):

[0069]

[0070] Where: Mineralization: degree of mineralization, %

[0071] TOC: Concentration of organic carbon in the reaction solution at a specific reaction time, in mg·L⁻¹ -1

[0072] TOCO: Concentration of organic carbon in the initial reaction solution, mg·L⁻¹ -1 .

[0073] Synergistic Comparative Catalytic Degradation Experiment: This experiment aimed to verify the synergistic effect of oxygen and hydrogen peroxide in the degradation system of Example 1. The procedure was the same as the conventional photocatalytic degradation experiment, except for the addition of an external atmosphere, such as nitrogen or oxygen. For the photocatalytic degradation experiment under hydrogen peroxide alone, nitrogen was introduced into the reaction solution 2 hours before the reaction began and continued throughout the reaction to eliminate oxygen and ensure that hydrogen peroxide was the only oxidant. For the photocatalytic degradation experiment under oxygen alone, oxygen was introduced into the reaction solution 2 hours before the reaction began and continued throughout the reaction without adding hydrogen peroxide, ensuring that oxygen was the only oxidant. The photocatalytic degradation experiment with the combined effect of hydrogen peroxide and oxygen was based on the oxygen-only degradation experiment, with hydrogen peroxide added simultaneously with illumination, resulting in the presence of both oxygen and hydrogen peroxide as oxidants in the system.

[0074] Electron paramagnetic resonance (ESR) characterization: Reactive oxygen species (ROSs) that may be generated in the degradation system of Example 1 in this patent were detected using an A200 electron paramagnetic resonance spectrometer (ESR) manufactured by Bruker GmbH, Germany. 5,5-Dimethyl-1-pyrrolline-N-oxide (DMPO) was used as a ·OH and ·O2 molecule. - Spin trapping agents were used to detect the formation of spin adducts DMPO-·OH and DMPO-·O2. - To determine whether hydroxyl radicals and superoxide radicals are generated in the system.

[0075] Example 1, and the conventional photocatalytic degradation results compared with Examples 1-2 are as follows: Fig. 9As shown in the figure, since Example 1 contains both tetracoordinate M-N4 and pentacoordinate M-N2O3 active centers that activate H2O2, O2, and organic matter, its degradation activity is significantly enhanced compared to Comparative Example 1 and Comparative Example 2. Furthermore, the cycling performance and mineralization capacity of Example 1 were tested, and Example 1 demonstrated excellent cycling and mineralization capabilities, fully illustrating its practical application potential in the field of water pollution control.

[0076] The results of the synergistic comparative catalytic degradation experiment in Example 1 are as follows: Fig. 10 As shown. When oxygen and hydrogen peroxide are present in the system, the photocatalytic degradation effect of Example 1 is significantly higher than that of the systems with oxygen or hydrogen peroxide alone. This result indicates that oxygen and hydrogen peroxide have a synergistic effect in the photocatalytic degradation process of Example 1, and also verifies that the structure of Example 1 contains active centers that activate both H2O2 and O2. The active free radicals generated in the photocatalytic degradation system of Example 1 are as follows: Fig. 11 As shown. ESR characterization revealed the simultaneous detection of DMPO-·OH and DMPO-·O2 in the system. - The characteristic peaks indicate that hydroxyl radicals and superoxide radicals are generated simultaneously during the photocatalytic degradation process in Example 1. This result shows that, in the presence of multiple active sites, the simultaneous activation of hydrogen peroxide and oxygen in Example 1 generates hydroxyl radicals and superoxide radicals that can be degraded.

[0077] In addition, the concentration of metal ions in the catalyst of Examples 1-3 and Comparative Examples 1-2 was detected by ICP testing; and the XPS spectra, degradation efficiency and mineralization of Examples 2-3 were tested. The test results are shown in Table 1.

[0078] Table 1. Analysis of the structure and degradation results of each embodiment.

[0079]

[0080] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A carbon-based catalyst with a single transition metal atom, characterized in that, The carbon-based catalyst includes transition metal single atoms, nitrogen-doped carbon substrates, and oxygen-doped carbon substrates. The structure of the carbon-based catalyst includes a 4-coordination unit M-N4 structure and a 5-coordination unit M-N2O3 structure of the transition metal single atom. The nitrogen-doped carbon substrate structure contains a conjugated bond C=N, which forms a 4-coordination structural unit M-N4 structure with a transition metal single atom; the nitrogen-doped carbon substrate structure also contains a dangling cyano group -C≡N at its structural edge and a conjugated bond C=N at its structural edge; the oxygen-doped carbon substrate structure contains a dangling carbon-oxygen double bond -C=O and a hydroxyl group -C-OH at its structural edge; the dangling cyano group -C≡N, the conjugated bond C=N at the structural edge, the dangling carbon-oxygen double bond -C=O, and the hydroxyl group -C-OH form a 5-coordination structural unit M-N2O3 structure with a transition metal single atom; The oxygen-doped carbon substrate is composed of oxygen-doped carbon quantum dots and oxygen-doped graphene. The nitrogen-doped carbon substrate is a nitrogen-rich carbon nanosheet formed by calcining melamine at high temperature.

2. The carbon-based catalyst according to claim 1, characterized in that, The transition metal single atom is selected from one or more of Fe, Co, Ni, Cu, Zn, and Mn.

3. The carbon-based catalyst according to claim 1 or 2, characterized in that, The carbon-based catalyst comprises transition metal single atoms, nitrogen-rich carbon nanosheets, oxygen-doped carbon quantum dots, and graphene oxide, and the carbon-based catalyst satisfies one or more of the following conditions: (1) The mass percentage of the transition metal single atom in the carbon-based catalyst is 1.0-30%; (2) The proportion of nitrogen atoms in the carbon-based catalyst is 1.0-50%; (3) 50%-70% of the nitrogen atoms in the carbon-based catalyst participate in the formation of the nitrogen-doped carbon conjugated structure sp2C=N in the nitrogen-rich carbon nanosheets; (4) The carbon-based catalyst contains 35-85% carbon atoms; (5) Carbon-based catalysts: 1.0%-20% of carbon atoms participate in the formation of cyano dangling bonds -C≡N at the edge of nitrogen-rich carbon nanosheets; (6) The proportion of oxygen atoms in the carbon-based catalyst is 5.0-20%; (7) 1.0% to 50% of the oxygen atoms in the carbon-based catalyst participate in the formation of oxygen-containing functional groups -C=O at the edge of the oxygen-doped carbon quantum dots; (8) Carbon-based catalysts: 1.0% to 50% of oxygen atoms participate in the formation of hydroxyl groups -C-OH perpendicular to the plane of graphene oxide.

4. A method for preparing the carbon-based catalyst according to any one of claims 1-3, characterized in that, The process includes the following steps: mixing a portion of the oxygen-doped carbon substrate with melamine, then adding a transition metal salt solution and the remaining oxygen-doped carbon substrate, mixing thoroughly, drying, and then calcining and post-processing steps to obtain the carbon-based catalyst.

5. The method according to claim 4, characterized in that, Oxygen-doped carbon quantum dots were mixed with melamine, then a transition metal salt solution and graphene oxide were added. After mixing evenly and drying, the carbon-based catalyst was obtained through calcination and post-treatment steps.

6. The method according to claim 5, characterized in that, Includes the following steps: (1) Prepare oxygen-doped carbon quantum dots, melamine and graphene oxide dispersions with concentrations of 0.01-10 mg / mL and transition metal salt solutions with concentrations of 1-5 mg / mL respectively. (2) Mix a certain volume of oxygen-doped carbon quantum dots with melamine dispersion in a certain proportion for 2-12 hours at a temperature of 50-200℃. (3) Mix a certain volume of transition metal salt solution with the sample obtained in step (2), then add the graphene oxide solution in step (1), mix evenly and then dry at a temperature of 50-100℃. (4) Calcine the solid obtained in step (3) in a nitrogen or argon protective atmosphere, with a gas flow rate of 20-100 mL / min, a temperature of 400-800℃, a speed of 0.1-10℃ / min, and a duration of 1-6 h. (5) The solid obtained in step (4) is acid washed in hydrochloric acid with a concentration of 0.5-10 mol / L at 10-60°C for 6-48 hours, then washed with water until neutral and dried to obtain the carbon-based catalyst.

7. The method according to claim 6, characterized in that, In step (2) or step (3), the mixing is carried out by means including hydrothermal, stirring or shaking; in step (3), the drying process is drying by blowing or vacuum oven, heating and evaporation or rotary drying.

8. The application of the carbon-based catalyst according to any one of claims 1-3, or the carbon-based catalyst prepared by the method according to any one of claims 4-7, in the field of photocatalytic degradation.

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