A cobalt single-atom catalyst with spin state gradient regulation, its preparation method and application

Abstract

The present invention belongs to the technical field of water treatment materials, and specifically relates to a spin state gradient-regulated cobalt single-atom catalyst and its preparation method and application. The preparation method includes: loading a cobalt salt on a metal-organic framework material ZIF-8, performing temperature-raising pyrolysis in an inert atmosphere to form a Co-N4 precursor; introducing a sulfur-containing gas and then performing secondary temperature-raising pyrolysis to gradually replace nitrogen coordination with sulfur atoms; sequentially performing acid washing and alkali washing on the pyrolysis product to obtain a spin state gradient-regulated cobalt single-atom catalyst Co-S x N 4‑x , where x = 1 to 3. The present invention uses the Co@ZIF-8 composite as a template for the uniform dispersion of Co single atoms, so that Co atoms are uniformly dispersed in the pores and nodes of ZIF-8, avoiding metal agglomeration during the high-temperature carbonization process. And the spin state of cobalt single atoms is regulated by sulfur doping gradient to achieve a synergistic improvement of catalytic activity and stability, so as to achieve the purpose of efficiently degrading and removing organic pollutants in water.

Description

Technical Field

[0001] The present invention belongs to the technical field of water treatment materials, and specifically, relates to a cobalt single-atom catalyst with spin state gradient regulation, a preparation method thereof, and an application thereof. Background Art

[0002] The widespread use and recalcitrant characteristics of persistent emerging organic pollutants (ECs) such as antibiotics and phenols have posed a severe challenge to the water environment. The increasing prevalence of ECs has exacerbated the contradiction between water quality deterioration and the escalating demand for clean water resources.

[0003] The heterogeneous advanced oxidation technology based on persulfate can produce strongly oxidizing reactive species and has excellent effects on removing ECs in water. However, the correlation between catalytic activity and the complex microstructure of the catalyst is not yet clear, and the catalytic mechanism is uncontrollable. Therefore, designing and regulating the microstructure of the catalytic center, deeply understanding the relationship between the catalytic performance of persulfate and the microstructure of the catalyst, and realizing the regulation of reaction activity and mechanism have become the key to the development of this technology.

[0004] Single-atom catalysts (SACs) have unique metal center chemical environments, controllable electronic structures, and high metal utilization rates, and have attracted much attention in the field of heterogeneous catalysis research in recent years. Both the electron transfer and orbital interaction between the catalyst and the reactant / intermediate exhibit spin-dependent characteristics, and the reaction kinetics / thermodynamics are sensitive to the spin configuration. The type of spin state is determined by the number of d-orbital electrons, the electron pairing energy, and the crystal field splitting energy, and it can be adjusted by the chemical environment around the metal center. The spin state (low spin, medium spin, high spin) of the metal center has a decisive influence on catalytic activity. However, existing methods (such as ligand modification) are difficult to achieve continuous gradient regulation of the spin state and lack in-depth analysis of the "chemical environment - spin state - reaction path" correlation.

[0005] Although single-atom catalysts (SACs) have shown potential in environmental catalysis, existing research has mainly focused on the regulation of coordination environments and coordination numbers, while neglecting the dominant role of spin states in reaction pathways. For example, Chinese Patent Application Publication No. CN116637653A discloses an iron single-atom catalyst with controllable sulfur content and coordination mode and its application. The preparation method includes using nitrogen-allylthiourea and trithiocyanuric acid as sulfur-containing organic ligands and sulfur-containing supramolecular precursors respectively, chelating with iron ions, and using the supramolecular self-assembly process to controllably prepare an iron single-atom catalyst with controllable sulfur content and coordination mode, which improves the activation efficiency of persulfate and the selectivity of reactive oxygen species. However, this invention relies on supramolecular self-assembly and high-temperature pyrolysis processes, which are cumbersome and difficult to scale up. Another example is Chinese Patent Application Publication No. CN117861708A, which discloses a high-coordination iron single-atom catalyst for electro-Fenton to highly selectively generate singlet oxygen, its preparation method and application. The preparation method includes using graphitic carbon nitride as the carrier, where the iron single atoms are coordinated with six nitrogen atoms, making the iron single atoms in a high-spin state, and only singlet oxygen is generated during the electro-Fenton process. However, this invention does not establish a spin state gradient regulation and cannot achieve the synergistic optimization of "coordination-spin-mechanism". Summary of the Invention

[0006] The object of the present invention is to provide a cobalt single-atom catalyst with spin state gradient regulation, its preparation method and application in the treatment of water by activating persulfate. By sulfur doping gradient regulation of the coordination environment and spin state of cobalt single atoms, combined with a two-stage pyrolysis process and a post-treatment method, a high-performance catalyst Co-S x N 4-x .

[0007] To achieve the above object of the invention, the technical solution provided by the present invention is as follows:

[0008] A preparation method of a cobalt single-atom catalyst with spin state gradient regulation, comprising the following steps:

[0009] (1) Loading a cobalt salt on a ZIF-8 metal-organic framework to form a Co@ZIF-8 composite;

[0010] (2) Thermally decomposing the Co@ZIF-8 composite in an inert atmosphere to form a Co-N4 precursor;

[0011] (3) After introducing a sulfur-containing gas, performing secondary thermal decomposition to gradually replace nitrogen coordination with sulfur atoms;

[0012] (4) Sequentially performing acid washing and alkali washing on the pyrolysis product to obtain a cobalt single-atom catalyst with spin state gradient regulation Co-S x N 4-x , where x = 1 to 3.

[0013] Further, in step (1), the cobalt salt is one of cobalt acetylacetonate, cobalt nitrate, cobalt acetate, cobalt chloride and their hydrates.

[0014] Further, in step (2), the inert atmosphere is one of N2 and Ar, the heating rate is 5 - 10 °C / min, the temperature is 800 - 900 °C, and the heat preservation time is 1 - 3 h.

[0015] Further, in step (3), the secondary heating pyrolysis is carried out at a heating rate of 5 - 10 °C / min to 900 - 1100 °C, and then the heat preservation time is 2 - 4 h.

[0016] Further, the sulfur-containing gas is formed by mixing sulfur vapor and a carrier gas (Ar / N2), and the volume concentration of sulfur vapor in the sulfur-containing gas is 2 - 5%; the flow rate of the sulfur-containing gas is 10 - 50 sccm.

[0017] Among them, the sulfur vapor is derived from the sulfur vapor generation and transportation system. High-purity sublimed sulfur powder (purity ≥ 99.9%, particle size < 100 μm) is used as the sulfur source, placed in a quartz boat, and then placed in the low-temperature section of an independent temperature zone reaction furnace; the sulfur powder is heated to 200 - 300 °C by a precise temperature control device to sublimate and generate sulfur vapor. Then, an inert gas (Ar or N2) is used as the carrier gas, and its flow rate is adjusted to 10 - 50 sccm by a mass flow meter, carrying the sulfur vapor into the main reaction zone to react with the Co-N4 precursor by displacement, and the volume concentration of sulfur vapor in the sulfur-containing gas is accurately controlled to 2 - 5 vol% by a mass flow meter. The volume concentration C s of sulfur vapor is calculated as follows:

[0018]

[0019] In the formula, R S is the sulfur sublimation rate (g / min), M S = 32.06 g / mol, ρ S is the density of the carrier gas (g / L), and v is the flow rate.

[0020] Further, in step (4), for pickling, one of sulfuric acid, nitric acid, and hydrochloric acid with a concentration of 0.5 - 3 M is used to wash away metal particles from the product;

[0021] For the alkali wash, one of sodium hydroxide, potassium hydroxide, sodium carbonate, and sodium bicarbonate with a concentration of 0.5 - 2 M is used to wash away unstable sulfur from the product.

[0022] The second object of the present invention is to provide a cobalt single-atom catalyst Co-S x N 4-x with spin state gradient regulation, which is prepared by the above preparation method.

[0023] Further, for the cobalt single-atom catalyst Co-S x N 4-x in which, the central effective magnetic moment of the cobalt single atom is 2.259 - 3.147 μ B , corresponding to the continuous regulation of the spin state from the medium-spin Co-S1N3 to the high-spin Co-S3N1;

[0024] wherein the sulfur doping ratio is regulated by the gradient of the temperature rise pyrolysis temperature and the sulfur vapor concentration, and satisfies the S / N atomic ratio of 1:3 - 3:1 respectively.

[0025] Further, for the cobalt single-atom catalyst Co-S x N 4-x has a specific surface area ≥ 800 m² / g, and the loading mass of the cobalt single atom is 0.5 - 1.5%.

[0026] The third object of the present invention is to provide the application of the above catalyst Co-S x N 4-x for catalytic degradation of organic pollutants in wastewater.

[0027] Further, first add the catalyst Co-S x N 4-x to the wastewater for ultrasonic dispersion, and then add persulfate for catalytic degradation reaction to remove organic pollutants in the sewage.

[0028] Further, based on the volume of the wastewater, the addition amount of the catalyst Co-S x N 4-x is 50 - 500 mg / L, and the addition amount of the peroxide is 20 - 300 mg / L;

[0029] and / or, the pH of the reaction solution for the catalytic degradation reaction is 3 - 11, the reaction temperature is 20 - 50 °C, and the time is 10 - 60 min.

[0030] Further, the persulfate is any one of potassium monopersulfate, sodium monopersulfate, potassium peroxydisulfate, and sodium peroxydisulfate;

[0031] The organic pollutants include tetracycline (TC), sulfadiazine (SDZ), sulfamethoxazole (SMX), sulfisoxazole (SIZ), bisphenol A (BPA), or phenol (PhOH).

[0032] The present invention has the following beneficial effects:

[0033] (1) The present invention utilizes the sulfur doping gradient to regulate the spin state of cobalt single atoms and optimize their electronic structure and reaction path. Specifically:

[0034] Continuous regulation of spin state: Gradient substitution of sulfur atoms for nitrogen coordination (Co-S x N 4-x , x = 1 - 3). By using S with a weak ligand field strength to gradually replace N with a strong ligand field strength, the crystal field splitting energy is reduced. Compared with the intermediate spin Co-S1N3, the spin state of the cobalt center continuously changes towards the high spin Co-S3N1. And due to sulfur doping, the occupancy of electrons in the t 2g orbital becomes less, and the occupancy of electrons in the e g orbital increases, thus enhancing the hybridization efficiency between the Co 3d orbital and the O 2p orbital in persulfate.

[0035] Dynamic evolution of the d-band center: The low electronegativity of sulfur (χ = 2.58) increases the electron density of cobalt, shifting the d-band center (Ed) towards the Fermi level. The upward shift of the d-band center optimizes the adsorption-activation equilibrium between Co and PMS: A moderate Ed (such as that of the intermediate spin state Co-S1N3) partially fills the antibonding orbital (σ * or π * ), which not only enhances the adsorption of persulfate but also avoids the increase in the desorption energy barrier of the intermediate caused by overly strong adsorption.

[0036] (2) The present invention uses sulfur doping gradient to regulate the spin state of cobalt single atoms, realizing the directional enhancement of the non-radical electron transfer path (ETP). Specifically:

[0037] Orbital hybridization and electron transfer: Sulfur doping enhances the Co 3d - O 2p orbital hybridization, promotes the adsorption of persulfate, forms a stable persulfate surface complex intermediate, and promotes the electron transfer from pollutants to the persulfate surface complex. Appropriate sulfur doping improves the ability of the Co active center to accept and contribute electrons, enhances the interfacial charge transfer kinetics, and is conducive to the strong interaction between the persulfate surface complex and organic pollutants.

[0038] Path switching mechanism: Sulfur doping reduces the valence state of Co, weakens the oxidation ability of Co, inhibits the generation of traditional free radicals or high-valent cobalt oxide species (Co(IV)=O), and instead directly oxidizes pollutants through ETP, enhancing the degradation ability of organic pollutants in water.

[0039] (3) The present invention uses sulfur doping gradient to regulate the spin state of cobalt single atoms, realizing the synergistic improvement of catalytic activity and stability, and achieving the purpose of degrading and removing organic pollutants in water. Specifically:

[0040] Volcanic-type activity trend: The synergistic regulation of the spin state and d-band center of cobalt single atoms results in a volcanic curve of catalytic activity. The reaction activity of (Co-S1N3) with an intermediate spin state reaches the peak, balancing the adsorption energy and electron transfer efficiency, becoming the peak point of performance, and superior to traditional Co-N4 and most cobalt-based catalysts reported in the literature (such as Co3O4, etc.).

[0041] Anti-interference and universality: The non-radical ETP mechanism is hardly inhibited by humic acid (HA) and most ions and shows high catalytic activity in the pH range of 3 - 11, breaking through the acidic dependence of the traditional Fenton system.

[0042] Ultra-low metal leaching and cycle stability: The acid / alkali double washing process removes unstable metals and sulfur species, with low cobalt leaching amount, strong recyclability, and stable structure.

[0043] (4) The present invention utilizes sulfur-doped gradient to regulate the spin state of cobalt single atoms, achieving scientific value and technological innovation, specifically as follows:

[0044] Spin state-activity quantitative model: Establish a three-dimensional correlation model of sulfur-doped gradient, spin state, and reaction path, providing a theoretical framework for the rational design of single-atom catalysts.

[0045] Dynamic mechanism analysis: Reveal the dynamic process of Co 3d orbital reconstruction induced by sulfur doping, clarify the "adsorption - electron transfer - desorption" synergistic mechanism, and promote the in-depth study of the mechanism of advanced oxidation technology.

[0046] (5) The present invention uses the Co@ZIF-8 composite as a template for the dispersion of Co single atoms, enabling Co atoms to be uniformly dispersed in the pores and nodes of ZIF-8, avoiding metal agglomeration during the high-temperature carbonization process. The imidazole ligand of ZIF-8 contains nitrogen atoms, which can form stable Co-N coordination bonds with Co²⁺. This coordination environment is retained during the carbonization process to form active sites, directly affecting the electronic structure of the catalyst. In addition, during the sulfur doping process, the microporous structure of ZIF-8 adsorbs sulfur molecules, ensuring the uniform distribution of sulfur atoms. Some sulfur atoms replace nitrogen in the Co-N coordination to form Co-S x N 4-x mixed coordination structure, further optimizing the electronic structure.

[0047] (6) The present invention realizes the gradient doping of sulfur atoms, the reconstruction of the coordination structure, and the continuous regulation of the spin state through secondary temperature-programmed pyrolysis. High temperature promotes the graphitization of the carbon matrix, enhancing the carrier stability. At the same time, sulfur atoms form strong bonds (Co-S bonds) with cobalt, preventing the sintering or loss of active sites.

[0048] (7) By introducing sulfur vapor, the present invention realizes precise regulation of the coordination environment of cobalt in the gas phase, avoiding uneven doping caused by doping with solid sulfur sources.

[0049] (8) The Co-S prepared by the present invention x N 4-x needs to be pickled to wash away metal particles and then alkali-washed to wash away unstable sulfur, thereby improving the purity, activity site uniformity and stability of the final product.

[0050] Therefore, the present invention regulates the spin state and electronic structure of cobalt single atoms through sulfur doping gradient, breaks through the activity-stability trade-off of traditional catalysts, realizes efficient and green water treatment through non-free radical pathways, and provides an optimal solution for the atomic-level design of environmental functional materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 TEM images of Co-N4 prepared in Comparative Example 1 and catalysts Co-S prepared in Examples 1-3 x N 4-x where a is the TEM image of Co-N4 in Comparative Example 1, b is the TEM image of Co-S1N3 in Example 1, c is the TEM image of Co-S2N2 in Example 2, and d is the TEM image of Co-S3N1 in Example 3;

[0052] Figure 2 X-ray absorption fine structure (XAFS) test results of Co-N4 prepared in Comparative Example 1 and catalysts Co-S prepared in Examples 1-3 x N 4-x where a is the Co K-edge X-ray absorption near-edge structure (XANES) spectra of Co-N4, catalyst Co-S x N 4-x and the reference sample, and b is the Fourier transform extended X-ray absorption fine structure (FT-EXAFS) spectra;

[0053] Figure 3 BET diagrams of Co-N4 prepared in Comparative Example 1 and catalysts Co-S prepared in Examples 1-3 x N 4-x ;

[0054] Figure 4 Magnetic susceptibility curve test diagrams of Co-N4 prepared in Comparative Example 1 and catalysts Co-S prepared in Examples 1-3 x N 4-x ;

[0055] Figure 5 BET diagrams of Co-N4 prepared in Comparative Example 1 and catalysts Co-S prepared in Examples 1-3 x N4-x Degradation effect diagram of sulfadiazine by activated potassium peroxymonosulfate;

[0056] Figure 6 For Co-N4 prepared in Comparative Example 1 and the catalysts Co-S prepared in Examples 1-3 x N 4-x Contribution diagram of active species for the degradation of sulfadiazine by activated potassium peroxymonosulfate of Detailed implementation manners

[0057] The present invention will be further described below in conjunction with specific embodiments, but the substantial content of the present invention is not limited to what is described in the following embodiments. The methods are all conventional methods unless otherwise specified, and the materials can all be obtained from public commercial channels unless otherwise specified. Those skilled in the art should know that any simple transformation or substitution based on the substantial content of the present invention belongs to the protection scope required by the present invention.

[0058] In this application, the metal-organic framework material ZIF-8 is zinc 2-methylimidazole MOF, which is obtained by mixing a zinc salt and an organic ligand in a suitable solvent and heating and reacting. Its commonly used organic solvents include N,N-dimethylformamide (DMF), ethylene glycol, methanol, etc.

[0059] The preparation method of the metal-organic framework material ZIF-8 is known. The specific preparation methods of the metal-organic framework material ZIF-8 in each embodiment of this application are as follows:

[0060] Dissolve 2.14 g of zinc nitrate hexahydrate in 50 mL of methanol solution, dissolve 2.32 g of dimethylimidazole in 50 mL of methanol solution, mix the two solutions, stir at room temperature for 12 h, and after centrifugal washing, dry in vacuum at 60 °C for 10 h to obtain the metal-organic framework material ZIF-8.

[0061] Example 1:

[0062] (1) Add 1.2 g of ZIF-8 and 0.05 g of cobalt acetylacetonate to 50 mL of methanol solution, stir at room temperature for 12 h, and then centrifuge and dry to obtain the Co@ZIF-8 composite;

[0063] (2) Heat pyrolyze the Co@ZIF-8 composite in Ar at a heating rate of 5 °C / min to 800 °C, and then keep the temperature for 3 h to form a Co-N4 precursor;

[0064] (3) Then introduce sulfur vapor (where the volume concentration of sulfur vapor in the sulfur-containing gas is 2% and the flow rate is 20 sccm), heat pyrolyze at a heating rate of 5 °C / min to 900 °C for secondary heating pyrolysis, and then keep the temperature for 3 h to gradually replace nitrogen coordination with sulfur atoms;

[0065] (4) The pyrolysis product was pickled in 1 M HNO3 solution for 12 h, centrifuged and washed, and then vacuum dried at 60 °C for 10 h; then pickled in 1 M NaOH solution for 2 h, centrifuged and washed, and vacuum dried at 60 °C for 10 h. The prepared catalyst was denoted as Co-S1N3.

[0066] Comparative Example 1:

[0067] (1) 1.2 g of ZIF-8 and 0.05 g of cobalt acetylacetonate were added to 50 mL of methanol solution, stirred at room temperature for 12 h, and then centrifuged and dried to obtain Co@ZIF-8 composite;

[0068] (2) The Co@ZIF-8 composite was heated to 800 °C at a rate of 5 °C / min under Ar atmosphere, held for 3 h, and then naturally cooled to form Co-N4 precursor;

[0069] (3) Pickled in 1 M HNO3 solution for 12 h, then centrifuged and washed, and vacuum dried at 60 °C for 10 h. The prepared catalyst was denoted as Co-N4.

[0070] Example 2:

[0071] The same as Example 1, except that: the temperature of the secondary heating pyrolysis in step (3) was 1000 °C. The prepared catalyst was denoted as Co-S2N2.

[0072] Example 3:

[0073] The same as Example 1, except that: the temperature of the secondary heating pyrolysis in step (3) was 1100 °C. The prepared catalyst was denoted as Co-S3N1.

[0074] Detection Example 1:

[0075] The catalysts Co-N4, Co-S1N3, Co-S2N2, and Co-S3N1 prepared from the catalysts in Comparative Example 1 and Examples 1-3 were subjected to relevant tests, and the test results were as follows:

[0076] (1) Figure 1 In which a is the TEM image of Co-N4, b is the TEM image of Co-S1N3, c is the TEM image of Co-S2N2, and d is the TEM image of Co-S3N1. The results show that the catalysts Co-S x N 4-x prepared in the present invention have a similar polyhedral morphology and no nanoclusters or particles are observed. The doping of S and the increase in temperature cause the Co-N part to break at high temperature, so the catalyst becomes rougher and more porous. This structural change significantly improves the specific surface area, active site exposure rate, and mass transfer efficiency of the catalyst.

[0077] (2) Figure 2For catalysts Co-N4 and Co-S x N 4-x XAFS test results. In the figure, a represents the catalysts Co-N4 and Co-S x N 4-x and the comparison samples (in the figure, "CoFoil" is cobalt foil, "CoO" is cobalt oxide, "Co3O4" is cobalt tetroxide, the ordinate "Normalized xμ(E)" represents the normalized X-ray absorption intensity, and "Energy" represents the energy of the X-ray (unit: electron volt)) of the Co K-edge XANES spectrum. The results show that with the doping of S, the valence state of Co-S x N 4-x gradually decreases.

[0078] In the figure, b is the FT-EXAFS spectrum. The Co-N scattering path of the Co-N4 sample is observed. After S doping, the Co-S scattering path is observed. With the doping of S, the intensity of the Co-N path decreases, and the intensity of Co-S increases. Co-S x N 4-x is confirmed to have a single-atom structure of Co because no Co-Co scattering path is observed. After the coordination environment fitting and quantification, the catalyst structures in Comparative Example 1 and Examples 1 to 3 are determined to be Co-N4, Co-S1N3, Co-S2N2, and Co-S3N1 respectively. Further confirmation of the gradient regulation change of the Co single-atom coordination environment.

[0079] (3) Figure 3 For the catalysts Co-N4 and Co-S prepared in Comparative Example 1 and Examples 1 to 3 x N 4-x BET diagrams. The results show that the BET surface areas of Co-N4, Co-S1N3, Co-S2N2, and Co-S3N1 are 755.2, 899.5, 930.2, and 950.9 m 2 / g respectively. With the increase in temperature, the surface area increases, and the main reason is that the etching effect at high temperature increases the total pore volume.

[0080] (4)Inductively coupled plasma optical emission spectrometry (ICP-OES) determines that the loaded masses of Co in Co-N4, Co-S1N3, Co-S2N2, and Co-S3N1 are 0.86%, 0.88%, 0.90%, and 0.84% respectively.

[0081] (5) Figure 4 For the Co-N4 and Co-S x N 4-x catalysts prepared in Comparative Example 1 and Examples 1 to 3, the magnetization curve test diagram (measured by a superconducting quantum interference device under H = 1 KOe). The results show that Co-Sx N 4-x The central effective magnetic moments (μ eff ) of cobalt single atoms in the catalyst are 1.630 μ B (Co-N4), 2.259 μ B (Co-S1N3), 3.045 μ B (Co-S2N2), 3.147 μ B (Co-S3N1), respectively. The calculated unpaired electron numbers are 0.912, 1.470, 2.205, and 2.302, respectively. Therefore, the spin states of Co follow the order of Co-N4 < Co-S1N3 < Co-S2N2 < Co-S3N1. With the doping of S, the ligand field strength weakens, the crystal field splitting energy decreases, and the spin state increases. The feasibility of the method for regulating the coordination environment and spin state of cobalt single atoms by sulfur doping gradient is confirmed.

[0082] Detection Example 2:

[0083] Test for activating peroxymonosulfate to degrade sulfadiazine in wastewater:

[0084] Weigh 8 mg each of the catalysts Co-N4, Co-S1N3, Co-S2N2, and Co-S3N1 prepared in Comparative Example 1 and Examples 1-3, and add them to 100 mL of water samples containing 20 mg / L of sulfadiazine (denoted as C0), respectively. Then add 30 mg of potassium peroxymonosulfate to each, adjust the pH value to 7, and carry out the reaction at 25 °C. After shaking well at 180 rpm in a shaker, samples are taken at certain time intervals (such as taking water samples at 0, 2, 5, 10, 20, and 30 minutes), and after filtration, the sulfadiazine content (denoted as C) in the water samples is detected by high performance liquid chromatography, as specifically Figure 5 shown. It can be seen from the figure that after 30 minutes of reaction, the removal rates of Co-N4, Co-S1N3, Co-S2N2, and Co-S3N1 for activating PMS to remove sulfadiazine are 65.3%, 100%, 94.6%, and 91.2%, respectively, and the reaction rate values (kobs) follow the order of Co-S1N3 (0.178 min -1 ) > Co-S2N2 (0.090 min -1 ) > Co-S3N1 (0.074 min -1 ) > Co-N4 (0.029 min -1 ).

[0085] Detection Example 3:

[0086] Benzoic acid, nitrobenzene, p-chlorobenzoic acid, furfuryl alcohol, and methyl phenyl sulfoxide are used as quantitative detection of hydroxyl radicals (·OH) and sulfate radicals (SO4 ·-), superoxide radical (O2 ·- ), singlet oxygen ( 1 O2) and high-valent cobalt oxide (Co(Ⅳ)=O) steady-state concentration probes. The contributions of different reactive species in Example 2 were quantitatively detected by competitive kinetics. The results are as Figure 6 shown. The contribution of Co(Ⅳ)=O in the Co-N4 / PMS system was 69.6%, dominating the degradation of sulfadiazine. However, with the gradual substitution of S for N, the reaction pathway changed, and the contributions of electron transfer (ETP) in the Co-S1N3 / PMS, Co-S2N2 / PMS, and Co-S3N1 / PMS systems were 84.9%, 50.2%, and 41.1% respectively.

[0087] Example 4:

[0088] (1) 1.0 g of ZIF-8 and 0.08 g of anhydrous cobalt chloride were added to 100 mL of methanol solution, stirred at room temperature for 12 h, and then centrifuged and dried to obtain the Co@ZIF-8 composite;

[0089] (2) The Co@ZIF-8 composite was thermally decomposed in Ar at a heating rate of 7 °C / min to 850 °C, then held for 2 h to form the Co-N4 precursor;

[0090] (3) Then, sulfur vapor (the volume concentration of sulfur vapor in the sulfur-containing gas was 3% and the flow rate was 30 sccm) was introduced, and the temperature was raised to 900 °C at a heating rate of 7 °C / min for secondary thermal decomposition and held for 4 h to gradually replace the nitrogen coordination with sulfur atoms;

[0091] (4) The pyrolysis product was pickled in 0.5 M H2SO4 solution for 24 h, centrifuged and washed, and then vacuum dried at 60 °C for 10 h; then it was alkali washed in 1 M KOH solution for 2.5 h, centrifuged and washed, and vacuum dried at 60 °C for 10 h. The prepared catalyst was denoted as Co-S1N3.

[0092] Comparative Example 2:

[0093] The same as Comparative Example 1, the difference is only that: 1.0 g of ZIF-8 and 0.08 g of anhydrous cobalt chloride were added to 100 mL of methanol solution, stirred at room temperature for 12 h, and then centrifuged and dried to obtain the Co@ZIF-8 composite.

[0094] The finally prepared catalyst was denoted as Co-N4'.

[0095] Comparative Example 3:

[0096] The same as Example 4, the difference is only that: the flow rate of sulfur vapor was 0 sccm, and the obtained catalyst was denoted as Co-NC-900.

[0097] Comparative Example 4:

[0098] Same as Example 4, except that: the temperature of the secondary temperature-raising pyrolysis is 800 °C, and the obtained catalyst is denoted as Co-SNC-800.

[0099] Comparative Example 5:

[0100] Same as Example 4, except that: the temperature of the secondary temperature-raising pyrolysis is 1500 °C, and the obtained catalyst is denoted as Co-SNC-1500.

[0101] Comparative Example 6:

[0102] (1) The preparation of the Co@ZIF-8 composite is the same as in Example 4;

[0103] (2) Pass in a sulfur-containing vapor (where the volume concentration of sulfur vapor in the sulfur-containing gas is 5% and the flow rate is 30 sccm), and heat the Co@ZIF-8 composite in Ar at a heating rate of 10 °C / min to 900 °C for temperature-raising pyrolysis, and then keep it at this temperature for 2 h to form a Co-SNC' precursor;

[0104] (3) Pickle in a 0.5 M H2SO4 solution for 24 h, centrifuge and wash, then dry in vacuum at 60 °C for 10 h; then pickle in a 1 M KOH solution for 2.5 h, centrifuge and wash, and dry in vacuum at 60 °C for 10 h. The obtained catalyst is denoted as Co-SNC'.

[0105] Comparative Example 7:

[0106] (1) Add 1.0 g of glucose, 0.5 g of melamine and 0.08 g of anhydrous cobalt chloride to 100 mL of methanol solution, stir at room temperature for 12 h, and then centrifuge and dry to obtain a Co-NC composite;

[0107] (2) Pass in a sulfur-containing vapor (where the volume concentration of sulfur vapor in the sulfur-containing gas is 5% and the flow rate is 30 sccm), and heat the Co-NC composite in Ar at a heating rate of 10 °C / min to 900 °C for temperature-raising pyrolysis, and then keep it at this temperature for 2 h to form Co-SNC;

[0108] (3) Pickle in a 0.5 M H2SO4 solution for 24 h, centrifuge and wash, then dry in vacuum at 60 °C for 10 h; then pickle in a 1 M KOH solution for 2.5 h, centrifuge and wash, and dry in vacuum at 60 °C for 10 h. The obtained catalyst is denoted as Co-SNC".

[0109] Detection Example 4:

[0110] As in Test Example 2, 5 mg of the catalysts prepared in Experimental Example 4 and Comparative Examples 2-7 were added to 100 mL of bisphenol A (content 15 mg / L) water sample, and then 2 mg of sodium peroxymonosulfate was added to adjust the pH value to 3. The mixture was shaken at 200 rpm in a shaker at 50°C. After 60 minutes, samples were taken to detect the concentration of bisphenol A in the water samples. The results are shown in Table 1.

[0111] Test example 5:

[0112] The same as in Detection Example 3, the contribution rate of the active species that plays a key role in the reaction process in Detection Example 4 was quantitatively detected. The results are shown in Table 1.

[0113] Table 1:

[0114]

[0115] Compared with Co-S1N3 prepared in Example 4, the catalyst Co-N4' prepared in Comparative Example 2 has no sulfur doping regulation, the spin state of the Co atom is low, and the ability to bind with persulfate is poor, which makes it easier to generate Co(IV)=O during the catalytic process and the catalytic effect is poor.

[0116] The Co-NC-900 prepared in Comparative Example 3 is sulfur-free, has a reduced nitrogen content and an increased carbon content compared to Comparative Example 2, and a lower steady-state concentration of Co(IV)=O.

[0117] In the Co-SNC-800 prepared in Comparative Example 4, the temperature of the secondary pyrolysis was 800°C, resulting in limited substitution of sulfur for nitrogen, and the spin state of Co atoms was slightly improved compared with Co-N4. However, the catalytic effect was reduced compared with that of Example 4.

[0118] In the Co-SNC-1500 prepared in Comparative Example 5, the temperature of the secondary pyrolysis was 1500°C. The high temperature resulted in the aggregation of cobalt clusters and the formation of cobalt sulfide, which was removed during the acid washing process. The Co content in the catalyst was reduced and nanoparticles were present. SO4 ·- Dominates the degradation of bisphenol A.

[0119] In Comparative Example 6, Co-SNC' was prepared by a single direct pyrolysis method. ZIF-8 carbonization and sulfur doping were carried out simultaneously. The sulfur atoms may directly compete with the incompletely formed Co-N bonds, resulting in uneven sulfur doping and removal in the alkaline washing step. Sulfur and nitrogen atoms randomly occupy coordination sites at high temperatures, making it difficult to accurately control the sulfur doping gradient; and the spin states of Co atoms are randomly distributed, making it difficult to optimize dp hybridization.

[0120] Comparative Example 7: Co-SNC prepared by using Co-NC composite (without ZIF-8 template), in which Co atoms are partially agglomerated and nitrogen is unevenly distributed, the specific surface area is reduced, and SO4·- Dominant degradation of bisphenol A.

[0121] Example 5:

[0122] (1) Add 1.0 g of ZIF-8 and 0.10 g of cobalt acetate into 100 mL of methanol solution, stir at room temperature for 12 h, and then centrifuge and dry to obtain the Co@ZIF-8 composite;

[0123] (2) First, place the Co@ZIF-8 composite in Ar, heat it at a heating rate of 5 °C / min to 800 °C for pyrolysis, and then keep it at this temperature for 2 h to form the Co-N4 precursor;

[0124] (3) Introduce sulfur vapor (the volume concentration of sulfur vapor in the sulfur-containing gas is 5% and the flow rate is 25 sccm), and then heat it at a heating rate of 5 °C / min to 900 °C, keep it at this temperature for 3 h for secondary pyrolysis to gradually replace the nitrogen coordination with sulfur atoms;

[0125] (4) Pickle the pyrolysis product in 0.5 M H2SO4 solution for 12 h, centrifuge and wash, and then dry it in vacuum at 60 °C for 10 h; then pickle it in 1 M KOH solution for 2.5 h, centrifuge and wash, and dry it in vacuum at 60 °C for 10 h. The prepared catalyst is denoted as Co-S1N3.

[0126] Example 6:

[0127] (1) Add 1.0 g of ZIF-8 and 0.10 g of cobalt acetate into 100 mL of methanol solution, stir at room temperature for 12 h, and then centrifuge and dry to obtain the Co@ZIF-8 composite;

[0128] (2) First, place the Co@ZIF-8 composite in N2, heat it at a heating rate of 10 °C / min to 900 °C for pyrolysis, and then keep it at this temperature for 1 h to form the Co-N4 precursor;

[0129] (3) Introduce sulfur-containing vapor (the volume concentration of sulfur vapor in the sulfur-containing gas is 5% and the flow rate is 50 sccm), heat it at a heating rate of 10 °C / min to 1000 °C and keep it at this temperature for 2 h for secondary pyrolysis to gradually replace the nitrogen coordination with sulfur atoms;

[0130] (4) Pickle the pyrolysis product in 0.5 M H2SO4 solution for 12 h, centrifuge and wash, and then dry it in vacuum at 60 °C for 10 h; then pickle it in 1 M KOH solution for 4 h, centrifuge and wash, and dry it in vacuum at 60 °C for 10 h. The prepared catalyst is denoted as Co-S2N2.

[0131] Comparative Example 8:

[0132] Weigh 100 mg of cobalt nitrate hexahydrate, 200 mg of gallic acid, and 4 g of thiocyanate powder, mix them, add them to 20 mL of ethanol, stir, and dry at 80 °C;

[0133] Place the mixture in a tubular furnace, heat it from room temperature to 800 °C at a rate of 5 °C / min, and keep it at this temperature for 2 h. During the calcination process, pass nitrogen for protection;

[0134] After the calcination is completed, let it cool naturally to room temperature. Immerse the obtained black powder in 0.5 M dilute sulfuric acid for 12 h, then wash it with water until it is neutral, and dry it under vacuum. The prepared catalyst is denoted as Co-SNC.

[0135] Detection Example 6:

[0136] Same as Detection Example 2. Add 50 mg each of the catalysts Co-S1N3, Co-S2N2, and Co-SNC prepared in the above Experimental Examples 5 and 6 and Comparative Example 8 to 100 mL of a phenol (20 mg / L) water sample, then add 30 mg of potassium persulfate, adjust the pH to 11, and shake it at 200 rpm in a shaker at 20 °C. After 30 minutes, take samples and detect the concentration of phenol in the water samples respectively. The degradation and removal rates are 100.0%, 91.2%, and 78.8% respectively.

[0137] Detection Example 7:

[0138] Same as Detection Example 3, quantitatively detect the contribution rates of the active species that play a key role in the reaction process in Detection Example 6. The contribution rates of ETP in the Co-S1N3 / PMS, Co-S2N2 / PMS, and Co-SN / PMS systems are 91.8%, 80.4%, and 66.4% respectively.

[0139] In this application, sulfur doping is carried out by introducing sulfur vapor, while sulfur powder is added in Comparative Example 8. Sulfur powder is prone to cause local over-doping, making the distribution of active sites of the prepared catalyst uncontrollable, resulting in too high a spin state of the Co center, too strong adsorption with persulfate, an increase in the desorption energy barrier of the intermediate, and too many unpaired electrons being prone to transfer to the peroxo bond of persulfate to break it, thus limiting the catalytic performance.

[0140] From the above, it can be concluded that sulfur doping in this application regulates the continuous evolution of the Co spin state, and sulfur doping promotes the increase of the spin state by reducing the crystal field splitting energy. In addition, the low electronegativity of sulfur increases the electron density of cobalt, and the d-band center moves towards the Fermi level, optimizing the adsorption and activation balance of PMS. And the catalytic activity (kobs) shows a volcano-shaped curve with the sulfur doping amount (x). The Co 3d-O 2p orbital hybridization efficiency of the medium spin state Co-S1N3 is the highest, promoting ETP and further improving the removal of pollutants, and its removal rate reaches more than 90%. Through the three-dimensional design of sulfur doping gradient, coordination number, and spin state, the present invention realizes the directional optimization of "chemical environment - electronic structure - reaction path".

[0141] Finally, it should be noted that the above embodiments are only preferred embodiments of the present invention, which are only used to explain the present invention and do not limit the present invention. For those skilled in the art, they can still modify the technical solutions recorded in the foregoing embodiments or equivalently replace some of the technical features. Any changes, replacements, and modifications made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. Application of a cobalt single-atom catalyst Co-S with spin state gradient regulation x N 4-x , characterized in that: The catalyst Co-S x N 4-x is used for catalytic degradation of organic pollutants in wastewater; the catalyst Co-S x N 4-x The preparation method thereof comprises the following steps: (1) Load a cobalt salt onto the metal-organic framework material ZIF-8 to form a Co@ZIF-8 composite; (2) Thermally decompose the Co@ZIF-8 composite by heating in an inert atmosphere to form a Co-N4 precursor; (3) After introducing a sulfur-containing gas, perform secondary thermal decomposition by heating to gradually replace the nitrogen coordination with sulfur atoms; (4) The pyrolysis products are successively subjected to acid washing and alkali washing to obtain a cobalt single-atom catalyst Co-S with spin state gradient regulation x N 4-x , where x = 1 to 3; Among them, the catalytic degradation of organic pollutants in wastewater is to first add the catalyst Co-S x N 4-x to the wastewater and disperse it by ultrasonic waves, and then add persulfate for catalytic degradation reaction to remove organic pollutants in the sewage; the persulfate is any one of potassium monopersulfate, sodium monopersulfate, potassium peroxydisulfate, and sodium peroxydisulfate; the organic pollutants include tetracycline, sulfadiazine, sulfamethoxazole, sulfisoxazole, bisphenol A or phenol.

2. The application according to claim 1, wherein: In step (1), the cobalt salt is one of cobalt acetylacetonate, cobalt nitrate, cobalt acetate, cobalt chloride, and their hydrates.

3. The application according to claim 1, wherein: In step (2), the inert atmosphere is N2 or Ar. The thermal decomposition by heating is carried out at a heating rate of 5 - 10 °C / min, heated to 800 - 900 °C, and then held for 1 - 3 h.

4. The application according to claim 1, wherein: In step (3), the secondary thermal decomposition by heating is carried out at a heating rate of 5 - 10 °C / min, heated to 900 - 1100 °C, and then held for 2 - 4 h.

5. The application according to claim 1, wherein: The sulfur-containing gas is a mixture of sulfur vapor and a carrier gas. The carrier gas is Ar or N2, and the volume concentration of sulfur vapor in the sulfur-containing gas is 2 - 5%; the flow rate of the sulfur-containing gas is 10 - 50 sccm.

6. The application according to claim 1, characterized in that: In step (4), pickling is carried out using one of sulfuric acid, nitric acid, and hydrochloric acid with a concentration of 0.5 - 3 M to wash away metal particles from the product; The alkali washing is carried out using one of sodium hydroxide, potassium hydroxide, sodium carbonate, and sodium bicarbonate with a concentration of 0.5 - 2 M to wash away unstable sulfur from the product.

7. The application according to claim 1, wherein: The catalyst Co-S x N 4-x The effective magnetic moment of the cobalt single atom in it is 2.259-3.147 μ B , corresponding to the continuous regulation from the medium-spin Co-S1N3 to the high-spin Co-S3N1; Among them, the sulfur doping ratio is regulated by the gradients of the thermal decomposition temperature and the sulfur vapor concentration, and satisfies an S / N atomic ratio of 1:3 to 3:

1.

8. The application according to claim 1, characterized in that: The catalyst Co-S x N 4-x has a specific surface area of ≥800 m² / g, and the loading mass of cobalt single atoms is 0.5-1.5%.

9. The application according to claim 1, characterized in that: Based on the volume of the wastewater, the addition amount of the catalyst Co-S x N 4-x is 50 - 500 mg / L, and the addition amount of the peroxide is 20 - 300 mg / L; And / or, the pH value of the reaction solution for the catalytic degradation reaction is 3 - 11, the reaction temperature is 20 - 50 °C, and the reaction time is 10 - 60 min.

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