A high-loading manganese-based monatomic catalyst, a preparation method and application thereof

Abstract

The application discloses a preparation method of a high-loading manganese-based monatomic catalyst, which comprises the following steps: a, adding a silica sol into a water solution of nitrogen-containing organic matter to obtain a first liquid mixture, and evaporating to obtain a first solid mixture; b, performing first calcination treatment on the first solid mixture under an inert atmosphere; c, adding an acid or alkali solution into the product after the calcination in the step b to remove the silica sol, so as to prepare a nitrogen-doped carbon material; d, dissolving a manganese salt, a nitrogen-containing organic ligand and a nitrogen-containing organic matter in an organic solvent to obtain a second liquid mixture; e, adding the nitrogen-doped carbon material into the second liquid mixture, and evaporating to obtain a second solid mixture; and f, performing second calcination treatment on the second solid mixture under an inert atmosphere, so as to prepare the manganese-based monatomic catalyst. The method disclosed by the application effectively improves the loading amount of Mn elements in the monatomic catalyst, and the catalytic performance is excellent.

Description

Technical Field

[0001] This invention belongs to the field of single-atom catalyst technology, specifically relating to a high-load manganese-based single-atom catalyst and its preparation method, and further relating to the application of the manganese-based single-atom catalyst. Background Technology

[0002] Energy and the environment play a crucial role in human societal development, and most energy- and environment-related chemical processes involve catalysis to accelerate reaction rates and improve product selectivity. Catalysis processes are generally classified into two main categories: homogeneous catalysis and heterogeneous catalysis. Homogeneous catalysts have flexible active site structures, exhibiting extremely high catalytic activity and unique reaction selectivity; however, they dissolve in the reaction system, making recycling difficult. Heterogeneous catalysts are typically prepared by loading nanoparticles of active components onto specific supports. Heterogeneous catalysis is a surface reaction process; only a small number of surface atoms in the active component nanoparticles participate in the catalytic process, while the majority of the active component remains unutilized and may even trigger side reactions. Single-atom catalysts are heterogeneous catalysts with atomically dispersed metal centers as active sites, achieving maximum metal atom dispersion and possessing a highly unsaturated coordination environment and uniform active centers. Single-atom catalysts simultaneously overcome the shortcomings of both homogeneous and heterogeneous catalysts, exhibiting extremely superior catalytic performance in energy, environmental remediation, and other catalytic reaction processes.

[0003] After nearly 10 years of development, most transition metal elements in the periodic table (and even some non-metal elements such as B, I, Si, and P) can now be used to prepare single-atom catalysts using appropriate methods. Among the many single-atom-based catalysts, Mn-based single-atom catalysts have attracted widespread attention due to their advantages such as low cost, high intrinsic activity, excellent selectivity, and good stability.

[0004] Nitrogen-doped carbon materials are important supports for the preparation of Mn-based single-atom catalysts. The N atoms doped in the carbon material coordinate with Mn atoms, stabilizing Mn atoms and inhibiting their aggregation. Mn-based single-atom catalysts supported on nitrogen-doped carbon materials typically require high-temperature calcination at temperatures above 900°C under an inert atmosphere to enhance the graphitization of the carbon support and improve the conductivity and stability of the carbon material. However, the high-temperature calcination process easily leads to the loss and aggregation of Mn, reducing the Mn content in the catalyst. Typically, the Mn content in single-atom catalysts is less than 1 wt%, which undoubtedly reduces the reaction efficiency per unit volume or unit area of ​​the reactor.

[0005] Therefore, it is necessary to conduct in-depth research on manganese-based single-atom catalysts to increase the loading of Mn element, thereby improving the performance of the catalyst. Summary of the Invention

[0006] This invention is based on the inventors' discoveries and understanding of the following facts and problems: Currently, the preparation of Mn-based single-atom catalysts is mainly based on the spatially confined high-temperature pyrolysis method using metal-organic frameworks (MOFs). However, the Mn content in the Mn-based single-atom catalysts obtained by this method is only 0.68 wt%, far below the requirements of actual catalytic processes (Nature Catalysis, 2018, 1, 935-945). Wu et al. proposed a two-step calcination method to increase the single-atom loading (Nature Nanotechnology, 2022, 17, 174-181), namely, firstly, achieving metal-support coordination through low-temperature calcination (300℃), and then obtaining the single-atom catalyst through a second high-temperature calcination (550℃). This method can increase the loading of Mn-based single-atom catalysts to 10 wt%. However, the graphitization transition temperature of carbon materials is typically above 600℃, and increases with increasing calcination temperature. The 550℃ calcination temperature in this method is insufficient to achieve the graphitization transition of the carbon support, resulting in poor performance of the obtained Mn-based single-atom catalyst. However, further increasing the calcination temperature leads to Mn atom aggregation, thereby losing catalytic activity. Therefore, preparing high-load Mn-based single-atom catalysts capable of withstanding calcination at temperatures above 900℃ is of significant practical importance for promoting the industrial application of single-atom catalysts.

[0007] This invention aims to at least partially solve one of the technical problems in related technologies. To this end, embodiments of this invention propose a high-load manganese-based single-atom catalyst and its preparation method. The prepared single-atom catalyst can withstand high-temperature calcination above 900℃ without agglomeration, effectively increasing the loading of Mn. Furthermore, this catalyst has low preparation cost, wide applicability, and can be mass-produced, showing broad application prospects.

[0008] A method for preparing a high-load manganese-based single-atom catalyst according to an embodiment of the present invention includes:

[0009] a. Add silica sol to an aqueous solution containing nitrogenous organic matter to obtain a first liquid mixture, and evaporate to obtain a first solid mixture;

[0010] b. The first solid mixture from step a is subjected to a first calcination treatment under an inert atmosphere;

[0011] c. Add an acid or alkaline solution to the product after calcination in step b to remove the silica sol in the product and prepare nitrogen-doped carbon material.

[0012] d. Dissolve manganese salt, nitrogen-containing organic ligand, and nitrogen-containing organic compound in an organic solvent to obtain a second liquid mixture;

[0013] e. Add the nitrogen-doped carbon material obtained in step c to the second liquid mixture obtained in step d, and evaporate to obtain a second solid mixture;

[0014] f. The second solid mixture obtained in step e is subjected to a second calcination treatment under an inert atmosphere to obtain a manganese-based single-atom catalyst.

[0015] The advantages and technical effects of the preparation method of the high-loading manganese-based single-atom catalyst in this invention are as follows: 1. In this invention, silica sol is introduced into nitrogen-containing organic matter to form a first liquid mixture. The silica sol contains silica microspheres. These silica microspheres can act as hard templates during the high-temperature pyrolysis of organic matter and metal precursors. After the first calcination treatment, the silica microspheres are dissolved and removed by acid or alkali, which can generate a large number of mesoporous channels in situ, thereby preparing a nitrogen-doped carbon material with high specific surface area and abundant mesoporous channels; 2. In this invention, the large number of mesoporous channels in the nitrogen-doped carbon material facilitates the diffusion of manganese salt and nitrogen-containing organic matter in the channels in subsequent steps. On the other hand, it can expose the nitrogen-containing functional groups inside the nitrogen-doped carbon material, thereby significantly increasing the number of usable nitrogen-containing functional groups. The nitrogen-containing functional groups in the nitrogen-doped carbon material provide rivet sites for subsequent adsorption and stabilization of manganese salt; 3. In this invention, silica sol is introduced into the second liquid mixture to form a first liquid mixture. Nitrogen organic ligands and nitrogen-containing organic compounds are used in this invention. The nitrogen atoms in the nitrogen-containing organic ligands coordinate with Mn ions to form Mn complexes, which stabilize Mn ions and inhibit Mn ion aggregation during the subsequent high-temperature pyrolysis process of the second calcination. The nitrogen-containing organic compounds act as a link between the Mn complexes and the nitrogen-doped carbon material during the second calcination process. After high-temperature calcination, they, along with the Mn complexes, become part of the nitrogen-doped carbon material. Furthermore, the addition of nitrogen-containing organic compounds can increase the nitrogen content of the nitrogen-doped carbon material, thereby increasing the bonding sites for single-atom Mn. 4. In this embodiment of the invention, the second calcination treatment allows manganese to form stable Mn-N bonds with the nitrogen-containing functional groups in the nitrogen-doped carbon material. The nitrogen-doped carbon material can withstand high-temperature calcination above 900℃ without aggregation, effectively inhibiting Mn aggregation and loss, thereby significantly increasing the Mn content in the catalyst. 5. The method of this invention has low preparation cost, wide applicability, and can achieve large-scale preparation, showing broad application prospects.

[0016] In some embodiments, step a further includes adding a manganese salt solution to the first liquid mixture of step a, stirring, and then evaporating to obtain a first solid mixture; preferably, the manganese salt includes at least one of manganese chloride, manganese sulfate, manganese nitrate, manganese acetate, and manganese acetylacetone.

[0017] In some embodiments, in step a, the nitrogen-containing organic compound includes at least one of PVP, chitosan, dicyandiamine, cyanamide, melamine, 2,6-diaminopyridine, and 2-methylimidazole.

[0018] In some embodiments, in step a, the silica sol comprises water-soluble silica microspheres with a diameter of 5-100 nm, and preferably, the content of silica microspheres in the silica sol is 10 wt%-70 wt%.

[0019] In some embodiments, in step a, the mass ratio of silica microspheres and nitrogen-containing organic matter in the first liquid mixture is (100-200):(20-100), preferably (100-200):(40-100).

[0020] In some embodiments, in step b, the inert atmosphere is at least one of nitrogen, argon, and helium; and / or, the first calcination temperature is 600-1200°C, and the calcination time is 0.5-12h.

[0021] In some embodiments, in step c, an acid or alkali solution is added to the product after calcination in step b, and the mixture is heated to 20-100°C and digested for 0.5-24 hours to remove the silica sol.

[0022] Preferably, the acid includes at least one of hydrochloric acid, sulfuric acid, nitric acid, and hydrofluoric acid; more preferably, the mass concentration of the acid is 1wt%-50wt%.

[0023] Preferably, the alkali includes at least one of NaOH solution and KOH solution; more preferably, the mass concentration of the alkali is 1wt%-50wt%.

[0024] In some embodiments, in step d, the nitrogen-containing organic ligand includes at least one of bipyridine, cyanamide, and 1,10-phenanthroline; and / or, the nitrogen-containing organic compound includes at least one of PVP, chitosan, dicyandiamide, cyanamide, melamine, 2,6-diaminopyridine, and 2-methylimidazole; and / or, the manganese salt includes at least one of manganese chloride, manganese sulfate, manganese nitrate, manganese acetate, and manganese acetylacetone; and / or, the organic solvent includes at least one of methanol, ethanol, isopropanol, acetonitrile, and N,N-dimethylformamide.

[0025] Preferably, in the second liquid mixture, the molar ratio of the manganese salt, the nitrogen-containing organic ligand, and the nitrogen-containing organic compound is 1:(0.5-20):(1-100).

[0026] In some embodiments, in step e, the mass ratio of manganese salt to nitrogen-doped carbon material in the second liquid mixture is 1:(0.5-10).

[0027] In some embodiments, in step f, the inert atmosphere is at least one of nitrogen, argon, and helium; and / or, the second calcination temperature is 600-1200°C, and the calcination time is 0.5-12h.

[0028] This invention also provides a high-load manganese-based single-atom catalyst, which is prepared using the method described in this invention.

[0029] The advantages and technical effects of the high-load manganese-based single-atom catalyst in this invention are achieved by using the prepared nitrogen-doped carbon material as a support, which has abundant mesoporous channels and a high specific surface area. The large number of mesoporous channels not only facilitates the diffusion of manganese salts and nitrogen-containing organic matter in the channels, but also effectively exposes the nitrogen-containing functional groups in the nitrogen-doped carbon material. During the high-temperature calcination process, manganese elements form stable Mn-N bonds with nitrogen-containing functional groups, which can effectively inhibit the aggregation and loss of Mn elements, thereby significantly increasing the loading of Mn single-atom sites in the catalyst and improving the catalytic performance of the catalyst.

[0030] This invention also provides an application of a high-load manganese-based single-atom catalyst in wastewater treatment. Attached Figure Description

[0031] Figure 1 These are the X-ray diffraction patterns of the Mn-NCF and Mn-NCS samples in Example 3;

[0032] Figure 2 These are aberration-corrected electron micrographs of the Mn-NCF and Mn-NCS samples from Example 3;

[0033] Figure 3 This is an evaluation diagram of the degradation effect of Rhodamine B by PMS activated by Mn-NCF and Mn-NCS samples in Example 3. Detailed Implementation

[0034] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0035] This invention provides a method for preparing a high-load manganese-based single-atom catalyst, comprising:

[0036] a. Add silica sol to an aqueous solution containing nitrogenous organic matter to obtain a first liquid mixture, and evaporate to obtain a first solid mixture;

[0037] b. The first solid mixture from step a is subjected to a first calcination treatment under an inert atmosphere;

[0038] c. Add an acid or alkaline solution to the product after calcination in step b to remove the silica sol in the product and prepare nitrogen-doped carbon material.

[0039] d. Dissolve manganese salt, nitrogen-containing organic ligand, and nitrogen-containing organic compound in an organic solvent to obtain a second liquid mixture;

[0040] e. Add the nitrogen-doped carbon material obtained in step c to the second liquid mixture obtained in step d, and evaporate to obtain a second solid mixture;

[0041] f. The second solid mixture obtained in step e is subjected to a second calcination treatment under an inert atmosphere to obtain a manganese-based single-atom catalyst.

[0042] In the preparation method of the high-loading manganese-based single-atom catalyst of this invention, silica sol is introduced into nitrogen-containing organic matter to form a first liquid mixture. The silica sol contains silica microspheres, which can act as hard templates during the high-temperature pyrolysis of organic matter and metal precursors. After the first calcination treatment, the silica microspheres are dissolved and removed by acid or alkali, which can generate a large number of mesoporous channels in situ, thereby preparing a nitrogen-doped carbon material with high specific surface area and abundant mesoporous channels. In this embodiment of the invention, the large number of mesoporous channels in the nitrogen-doped carbon material is beneficial to the diffusion of manganese salt and nitrogen-containing organic matter in the channels in subsequent steps. On the other hand, it can expose the nitrogen-containing functional groups inside the nitrogen-doped carbon material, thereby significantly increasing the number of usable nitrogen-containing functional groups. The nitrogen-containing functional groups in the nitrogen-doped carbon material provide anchoring sites for subsequent adsorption and stabilization of manganese salt. In this embodiment of the invention, nitrogen-containing organic ligands and nitrogen-containing organic compounds are introduced into the second liquid mixture. In this invention, nitrogen atoms in the nitrogen-containing organic ligands coordinate with Mn ions to form Mn complexes, which stabilize Mn ions and inhibit Mn ion aggregation during the subsequent high-temperature pyrolysis process of the second calcination. The nitrogen-containing organic matter also acts as a linker between the Mn complexes and the nitrogen-doped carbon material during the second calcination. After high-temperature calcination, it transforms into part of the nitrogen-doped carbon material along with the Mn complexes. Furthermore, the addition of the nitrogen-containing organic matter increases the nitrogen content of the nitrogen-doped carbon material, thereby increasing the bonding sites for single-atom Mn. In this embodiment, the second calcination treatment allows manganese to form stable Mn-N bonds with the nitrogen-containing functional groups in the nitrogen-doped carbon material. The nitrogen-doped carbon material can withstand high-temperature calcination above 900°C without aggregation, effectively inhibiting Mn aggregation and loss, thus significantly increasing the Mn content in the catalyst. The method of this invention has low preparation cost, wide applicability, and can achieve large-scale preparation, showing broad application prospects.

[0043] In some embodiments, step a further includes adding a manganese salt solution to the first liquid mixture of step a, stirring, and then evaporating to obtain a first solid mixture; preferably, the manganese salt includes at least one of manganese chloride, manganese sulfate, manganese nitrate, manganese acetate, and manganese acetylacetone. In this embodiment of the invention, the introduction of a manganese salt solution into the first liquid mixture allows Mn elements to be anchored to the nitrogen-doped carbon material through Mn-N bonds during the first calcination process, forming Mn-based single-atom sites. Although this process can load a relatively low amount of Mn atoms, the addition of Mn salt during calcination increases the graphitization degree of the nitrogen-doped carbon support, resulting in a more structurally stable nitrogen-doped carbon support. This provides more and more stable nitrogen-containing "anchoring" sites for subsequent impregnation and second calcination processes, which is beneficial for the Mn salt to be firmly "anchored" to the nitrogen-doped carbon material during the second calcination process, significantly increasing the loading of the Mn-based single-atom catalyst.

[0044] In some embodiments, in step a, the nitrogen-containing organic compound includes at least one of PVP, chitosan, dicyandiamine, cyanamide, melamine, 2,6-diaminopyridine, and 2-methylimidazole.

[0045] In some embodiments, in step a, the silica sol comprises water-soluble silica microspheres with a diameter of 5-100 nm, and preferably, the content of silica microspheres in the silica sol is 10 wt%-70 wt%.

[0046] In some embodiments, in step a, the mass ratio of silica microspheres to nitrogen-containing organic matter in the first liquid mixture is (100-200):(20-100), preferably (100-200):(40-100); further, if manganese salt is added to the first liquid mixture, the mass ratio of silica microspheres, nitrogen-containing organic matter, and manganese salt in the silica sol is (100-200):(20-100):1, preferably (100-200):(40-100):1. In these embodiments, the preferred ratio of silica microspheres to nitrogen-containing organic matter in the first liquid mixture is beneficial for increasing the number of nitrogen-containing functional groups in the nitrogen-doped carbon material and for promoting the formation of mesoporous channels, thereby further improving the activity of the catalyst. Excessive use of nitrogen-containing organic matter will prevent some of it from coating the silica sol, hindering the formation of mesoporous channels in the resulting nitrogen-doped carbon material. Insufficient use of nitrogen-containing organic matter will reduce the nitrogen doping level and the number of nitrogen-containing functional groups, which is detrimental to increasing the Mn loading in the catalyst. Excessive use of silica microspheres will reduce the yield of nitrogen-doped carbon material; insufficient use of silica microspheres will reduce the number of mesoporous channels, hindering the diffusion of Mn and nitrogen-containing organic matter during the subsequent second calcination process, thus negatively impacting the Mn loading in the catalyst.

[0047] In some embodiments, in step b, the inert atmosphere is at least one of nitrogen, argon, and helium; and / or, the first calcination temperature is 600-1200℃, and the calcination time is 0.5-12h. In this embodiment of the invention, a first solid mixture formed from silica sol and nitrogen-containing organic matter is subjected to a first calcination treatment. During the calcination process, the silica sol dehydrates and transforms into silicon dioxide, and the nitrogen-containing organic matter decomposes and polymerizes to form nitrogen-doped carbon material. If the calcination temperature is too low, the graphitization degree of the carbon material will be low, reducing the stability of the material; if the calcination temperature is too high, nitrogen will volatilize and be lost, reducing the nitrogen content of the nitrogen-doped carbon material, leading to a reduction in the bonding sites of subsequent single-atom Mn, which in turn leads to the aggregation of Mn elements, reducing the number of single-atom Mn active sites and lowering the performance of the catalyst.

[0048] In some embodiments, in step c, an acid or alkali solution is added to the product after calcination in step b, and the mixture is heated to 20-100°C for 0.5-24 hours to remove the silica sol. Preferably, the acid includes at least one of hydrochloric acid, sulfuric acid, nitric acid, and hydrofluoric acid; more preferably, the mass concentration of the acid is 1wt%-50wt%. Preferably, the alkali includes at least one of NaOH solution and KOH solution; more preferably, the mass concentration of the alkali is 1wt%-50wt%. In these embodiments, the use of strong acid or strong alkali is beneficial for the effective digestion of silica sol, forming mesoporous channels and improving catalytic activity. Furthermore, if manganese salt is introduced into the first liquid mixture, some Mn elements cannot form Mn-N bonds with N elements during the first calcination process, leading to Mn element agglomeration and growth. The acid or alkali solution can wash away these agglomerated, unstable Mn species, improving the stability of the catalyst.

[0049] In some embodiments, in step d, the nitrogen-containing organic ligand includes at least one of bipyridine, cyanamide, and 1,10-phenanthroline; the nitrogen-containing organic compound includes at least one of PVP, chitosan, dicyandiamide, cyanamide, melamine, 2,6-diaminopyridine, and 2-methylimidazole; the manganese salt includes at least one of manganese chloride, manganese sulfate, manganese nitrate, manganese acetate, and manganese acetylacetone; and / or, the organic solvent includes at least one of methanol, ethanol, isopropanol, acetonitrile, and N,N-dimethylformamide. Preferably, in the second liquid mixture, the molar ratio of the manganese salt, the nitrogen-containing organic ligand, and the nitrogen-containing organic compound is 1:(0.5-20):(1-100). In the embodiments of the present invention, the preferred amounts of each substance in the second liquid mixture are beneficial to further increase the loading of Mn element in the catalyst and improve catalytic performance. Introducing nitrogen-containing organic ligands into the second liquid mixture allows the nitrogen atoms of these ligands to coordinate with Mn ions, forming stable complexes. This results in stable Mn-N bonds during high-temperature calcination, inhibiting the aggregation and growth of Mn. Simultaneously, the nitrogen-containing organic ligands undergo pyrolysis and polymerization during calcination, transforming into nitrogen-doped carbon material with single-atom Mn active sites along with the nitrogen-containing organic matter and Mn salt. Introducing nitrogen-containing organic matter into the second liquid mixture further increases the nitrogen content of the nitrogen-doped carbon material, providing more stable single-atom Mn active sites. During calcination, the nitrogen-containing organic matter, along with the Mn complexes, transforms into nitrogen-doped carbon material adhering to the surface of the nitrogen-doped carbon material obtained in step c. Excessive nitrogen-containing organic ligand content increases catalyst preparation costs, while insufficient content leads to Mn aggregation and growth during high-temperature calcination. Furthermore, excessive nitrogen-containing organic matter content can clog mesopores, hindering the efficient utilization of single-atom Mn active sites, while insufficient content reduces nitrogen doping and the number of nitrogen-containing functional groups, negatively impacting the Mn loading in the catalyst.

[0050] In some embodiments, in step e, the mass ratio of manganese salt to nitrogen-doped carbon material in the second liquid mixture is 1:(0.5-10). In this embodiment of the invention, a preferred mass ratio of nitrogen-doped material to manganese salt is beneficial for obtaining a high-load, stable manganese-based single-atom catalyst. If the mass of the Mn salt used is too low, the goal of increasing the single-atom Mn loading cannot be achieved; if the mass of the Mn salt used is too high, Mn salt agglomeration during high-temperature calcination will result in the absence of single-atom active sites.

[0051] In some embodiments, in step f, the inert atmosphere is at least one of nitrogen, argon, and helium; the second calcination temperature is 600-1200℃, preferably 700-1200℃, more preferably 900-1200℃, and the calcination time is 0.5-12h. In this embodiment of the invention, the second calcination temperature is further preferred, which is beneficial to the graphitization of nitrogen-doped carbon materials. Furthermore, since the nitrogen-doped carbon materials prepared in this embodiment of the invention can withstand high-temperature calcination, the Mn element in the prepared catalyst will not agglomerate, thereby increasing the Mn element loading in the catalyst.

[0052] This invention also provides a high-load manganese-based single-atom catalyst, prepared using the method described in this invention.

[0053] The high-load manganese-based single-atom catalyst of this invention uses a nitrogen-doped carbon material as a support, which has abundant mesoporous channels and a high specific surface area. The numerous mesoporous channels not only facilitate the diffusion of manganese salts and nitrogen-containing organic matter within the channels, but also effectively expose the nitrogen-containing functional groups in the nitrogen-doped carbon material. Manganese forms stable Mn-N bonds with the nitrogen-containing functional groups, effectively inhibiting the aggregation and loss of Mn, thereby significantly increasing the Mn content in the catalyst and improving the catalyst's catalytic performance.

[0054] This invention also provides an application of a high-load manganese-based single-atom catalyst in wastewater treatment. The high-load manganese-based single-atom catalyst of this invention can achieve efficient activation of persulfate and can be used for the efficient treatment of organic wastewater.

[0055] The present invention will now be described in detail with reference to the embodiments and accompanying drawings.

[0056] Example 1

[0057] 4 g of 2,6-diaminopyridine and 100 mL of water were added to a 200 mL beaker and stirred for 90 min until the 2,6-diaminopyridine was completely dissolved to form a transparent solution. 22 g of silica sol containing 50 wt% water-soluble silica microspheres with a diameter of 50 nm was added and stirred for 120 min; this is labeled solution A. 60 mg of MnSO4 was added to 50 mL of water and stirred until dissolved; this is labeled solution B. Solution B was added dropwise to solution A and stirred for 24 h. The mass ratio of silica microspheres, 2,6-diaminopyridine, and MnSO4 in the silica sol was 183:66:1. The solution was heated to 100 °C to evaporate the water, then placed in a 100 °C oven and dried overnight. The resulting solid was ground into a fine powder and placed in a tube furnace under a N2 atmosphere at 15 °C for 1 min. -1The temperature was increased to 700℃ at a rising rate, and calcined for 6 hours to obtain a black powder. The black powder was ground, and 15 mL of water was added, followed by 2 mL of 5 wt% HF solution. The mixture was sonicated for 4 hours to remove silica. After centrifugation, the sample was washed three times with water and dried under vacuum at 80℃ to obtain the Mn-NCF sample.

[0058] 0.8 g Mn(NO3)2, 0.8 g bipyridine, and 1.2 g PVP were dissolved in 20 mL of isopropanol to form solution C, i.e., the molar ratio of manganese salt Mn(NO3)2, nitrogen-containing organic ligand bipyridine, and nitrogen-containing organic compound PVP was 1:1.06:2.08. 1.2 g of Mn-NCF sample was weighed and added to solution C (the mass ratio of manganese salt Mn(NO3)2 to nitrogen-doped carbon material Mn-NCF was 1:1.5) with stirring. Stirring was continued for 1 h, followed by standing for 24 h. The solvent was evaporated at 120 °C, and the mixture was dried in an oven at 100 °C. The resulting solid was ground into a fine powder and placed in a tube furnace under an Ar atmosphere at 20 °C for [time missing]. -1 The heating rate was increased to 1000℃, and the mixture was calcined for 6 hours to obtain a black powder, which is the high-load Mn-based single-atom catalyst, denoted as Mn-NCS.

[0059] Example 2

[0060] 3.4 g of dicyandiamine and 200 mL of water were added to a 500 mL beaker and stirred for 180 min until the dicyandiamine was completely dissolved to form a transparent solution. 20 g of silica sol containing 50 wt% water-soluble silica microspheres with a diameter of 50 nm was added and stirred for 20 min; this is labeled solution A. 90 mg of Mn(OAc)₂ was added to 20 mL of water and stirred until dissolved; this is labeled solution B. Solution B was added dropwise to solution A and stirred for 2 h. The mass ratio of silica microspheres, dicyandiamine, and Mn(OAc)₂ in the silica sol was 111:38:1. The solution was heated to 70 °C to evaporate the moisture, then placed in a 90 °C oven and dried overnight. The resulting solid was ground into a fine powder and placed in a tube furnace under an Ar atmosphere at 20 °C for [time missing]. -1 The temperature was increased to 600℃ at a rising rate, and calcined for 0.5 h to obtain a black powder. The black powder was ground, and 9 mL of water was added, followed by 2 mL of 30 wt% KOH solution. The mixture was sonicated for 2 h to remove silica. After centrifugation, the sample was washed twice with water and dried under vacuum at 90℃ to obtain the Mn-NCF sample.

[0061] 0.6 g Mn(acac)2, 1.6 g cyanamide, and 2.9 g melamine were dissolved in 90 mL of ethanol to form solution C, i.e., the molar ratio of manganese salt Mn(acac)2, nitrogen-containing organic ligand cyanamide, and nitrogen-containing organic compound melamine was 1:16.17:9.78. 0.9 g of Mn-NCF sample was weighed and added to solution C under stirring, i.e., the mass ratio of manganese salt Mn(acac)2 to nitrogen-doped carbon material Mn-NCF was 1:1.5. The mixture was stirred for 6 h and allowed to stand for 12 h. The solvent was evaporated at 80 °C and dried in an oven at 60 °C. The resulting solid was ground into a fine powder and placed in a tube furnace. Under a nitrogen atmosphere, the temperature was increased to 1200 °C at a heating rate of 2 °C min⁻¹ and calcined for 4 h to obtain a black powder, which is the high-load Mn-based single-atom catalyst, denoted as Mn-NCS.

[0062] Example 3

[0063] Add 3g of chitosan and 60mL of water to a 100mL beaker and stir for 30min until the chitosan is completely dissolved to form a transparent solution. Add 16g of silica sol containing 50wt% water-soluble silica microspheres with a diameter of 50nm and stir for 20min, which is labeled as solution A. Add 50mg of Mn(acac)2 to 30mL of water and stir to dissolve, which is labeled as solution B. Add solution B dropwise to solution A and stir for 4h. The mass ratio of silica microspheres, chitosan, and Mn(acac)2 in the silica sol is 160:60:1. Heat at 90℃ to evaporate the moisture and place in an 80℃ oven to dry overnight. Grind the resulting solid into a fine powder and place it in a tube furnace under a He atmosphere at 25℃ for 1min. -1 The temperature was increased to 600℃ at a rising rate, and calcined for 12 hours to obtain a black powder. The black powder was ground, and 2 mL of water and 2 mL of 20 wt% NaOH solution were added. The mixture was sonicated for 1 hour to remove silica. After centrifugation, the sample was washed 6 times with water and dried under vacuum at 70℃ to obtain the Mn-NCF sample.

[0064] 0.75 g Mn(NO3)2, 1.8 g bipyridine, and 6 g dicyandiamine were dissolved in 80 mL of water to form solution C, i.e., the molar ratio of manganese salt Mn(NO3)2, nitrogen-containing organic ligand bipyridine, and nitrogen-containing organic compound dicyandiamine was 1:2.56:15.86. 0.7 g of Mn-NCF sample was weighed and added to solution C (the mass ratio of manganese salt Mn(NO3)2 to nitrogen-doped carbon material Mn-NCF was 1:0.93) with stirring. Stirring was continued for 12 h, followed by standing for 2 h. The solvent was evaporated at 90 °C, and the mixture was dried in an oven at 80 °C. The resulting solid was ground into a fine powder and placed in a tube furnace under a He atmosphere at 1 °C / min. -1 The heating rate was increased to 1050℃, and the product was calcined for 4 hours to obtain a black powder, which is the high-load Mn-based single-atom catalyst, denoted as Mn-NCS.

[0065] Figure 1 The figures show the X-ray diffraction patterns of the Mn-NCF and Mn-NCS catalysts prepared in this embodiment. As can be seen from the figures, only diffraction peaks of the nitrogen-doped carbon support were detected in the Mn-NCF and Mn-NCS catalysts; no diffraction peaks of Mn-containing species (including manganese oxide, manganese nitride, manganese carbide, metallic manganese, etc.) were detected, indicating that the Mn species achieved atomic-level dispersion on the nitrogen-containing carbon support. Compared with Mn-NCF, the diffraction peaks of the carbon support in the X-ray diffraction pattern of the Mn-NCS catalyst shifted to a higher number, and the intensity of the diffraction peaks increased, indicating that the graphitization degree of the nitrogen-containing carbon support was improved after two-step high-temperature calcination treatment, which is beneficial to enhancing the stability of the Mn-NCS catalyst.

[0066] Figure 2 The images show aberration-corrected scanning transmission electron microscopy (STEM) images of the Mn-NCF and Mn-NCS catalysts prepared in this embodiment. No nanoparticles or nanoclusters are observed; only atomically dispersed Mn atoms are visible, confirming that both Mn-NCF and Mn-NCS are single-atom catalysts. Compared to Mn-NCF, the number of atomically dispersed bright spots in the aberration-corrected STEM images of the Mn-NCS catalyst is significantly increased, directly demonstrating an increase in the single-atom Mn loading. This indicates that a high-loading Mn-based single-atom catalyst can be synthesized via a two-step method.

[0067] Example 4

[0068] 8 g of 2,6-diaminopyridine and 50 mL of water were added to a 100 mL beaker and stirred for 20 min until the 2,6-diaminopyridine was completely dissolved to form a transparent solution. 4 g of silica sol containing 50 wt% water-soluble silica microspheres with a diameter of 50 nm was added and stirred for 10 min; this is labeled solution A. 10 mg of Mn(OAc)₂ was added to 30 mL of ethanol and stirred until dissolved; this is labeled solution B. Solution B was added dropwise to solution A and stirred for 2 h. The mass ratio of silica microspheres, 2,6-diaminopyridine, and Mn(OAc)₂ in the silica sol was 200:80:1. The solution was heated to 80 °C to evaporate the moisture, then placed in a 60 °C oven and dried overnight. The resulting solid was ground into a fine powder and placed in a tube furnace under a N₂ atmosphere at 10 °C for 1 min. -1 The heating rate was increased to 950℃, and calcination was carried out for 2 hours to obtain a black powder. The black powder was ground, and 15 mL of water was added, followed by 10 mL of a mixed solution of 30 wt% NaOH and 30 wt% KOH. The mixture was sonicated for 1 hour to remove silica. After centrifugation, the sample was washed five times with water and dried under vacuum at 90℃ to obtain the Mn-NCF sample.

[0069] 0.9 g Mn(NO3)2, 1.5 g bipyridine, and 9 g melamine were dissolved in 50 mL of methanol to form solution C, i.e., the molar ratio of manganese salt Mn(NO3)2, nitrogen-containing organic ligand bipyridine, and nitrogen-containing organic compound melamine was 1:1.61:13.22. 1.5 g of Mn-NCF sample was weighed and added to solution C (the mass ratio of manganese salt Mn(NO3)2 to nitrogen-doped carbon material Mn-NCF was 1:10) while stirring. Stirring was continued for 12 h, followed by standing for 6 h. The solvent was evaporated at 85 °C, and the mixture was dried in an oven at 95 °C. The resulting solid was ground into a fine powder and placed in a tube furnace under a N2 atmosphere at 15 °C for [time missing]. -1 The temperature was increased to 1200℃ at a heating rate and calcined for 6 hours to obtain a black powder, which is the high-load Mn-based single-atom catalyst, denoted as Mn-NCS.

[0070] Example 5

[0071] Add 4g of melamine and 60mL of water to a 200mL beaker and stir for 180min until the melamine is completely dissolved to form a transparent solution. Add 40g of silica sol containing 50wt% water-soluble silica microspheres with a diameter of 50nm and stir for 30min, denoted as solution A. Add 120mg of MnSO4 to 20mL of water and stir to dissolve, denoted as solution B. Add solution B dropwise to solution A and stir for 8h. The mass ratio of silica microspheres, melamine, and MnSO4 in the silica sol is 167:33:1. Heat to 90℃ to evaporate the moisture, place in a 60℃ oven and dry overnight. Grind the resulting solid into a fine powder and place in a tube furnace under an Ar atmosphere at 5℃ for 1 minute. -1 The temperature was raised to 650℃ and calcined for 6 hours to obtain a black powder. The black powder was ground, 20 mL of water was added, followed by 5 mL of 1 wt% HF solution, and the mixture was sonicated for 3 hours to remove silica. The sample was then centrifuged, washed with water 9 times, and vacuum dried at 50℃ to obtain the Mn-NCF sample.

[0072] 0.9 g MnSO4, 1.5 g cyanamide, and 2.4 g PVP were dissolved in 40 mL of ethanol to form solution C, where the molar ratio of manganese salt MnSO4, nitrogen-containing organic ligand cyanamide, and nitrogen-containing organic compound PVP was 1:6.02:3.52. 1.8 g of Mn-NCF sample was weighed and added to solution C (where the mass ratio of manganese salt MnSO4 to nitrogen-doped carbon material Mn-NCF was 1:2) with stirring. Stirring was continued for 8 h, followed by standing for 12 h. The solvent was evaporated at 95 °C, and the mixture was dried in an oven at 75 °C. The resulting solid was ground into a fine powder and placed in a tube furnace under an Ar atmosphere at 10 °C for [time missing]. -1 The heating rate was increased to 1150℃, and the mixture was calcined for 6 hours to obtain a black powder, which is the high-load Mn-based single-atom catalyst, denoted as Mn-NCS.

[0073] Example 6

[0074] The method is the same as in Example 3, except that no manganese salt is added during the preparation of the nitrogen-doped carbon material Mn-NCF sample. Specifically:

[0075] Add 3g of chitosan and 60mL of water to a 100mL beaker and stir for 30min until the chitosan is completely dissolved to form a transparent solution. Add 16g of silica sol containing 50wt% water-soluble silica microspheres with a diameter of 50nm and stir for 20min. This solution is labeled A. The mass ratio of silica microspheres to chitosan in the silica sol is 160:60. Heat at 90℃ to evaporate the water in solution A, and place it in an 80℃ oven to dry overnight. Grind the resulting solid into a fine powder and place it in a tube furnace under a He atmosphere at 25℃ for 1 minute. -1 The temperature was increased to 600℃ at a rising rate, and calcined for 12 hours to obtain a black powder. The black powder was then ground, and 2 mL of water and 2 mL of 20 wt% NaOH solution were added. The mixture was sonicated for 1 hour to remove silica. The sample was then centrifuged, washed with water 6 times, and vacuum dried at 70℃ to obtain a nitrogen-doped carbon support sample.

[0076] 0.75 g Mn(NO3)2, 1.8 g bipyridine, and 6 g dicyandiamine were dissolved in 80 mL of water to form solution C, i.e., the molar ratio of manganese salt Mn(NO3)2, nitrogen-containing organic ligand bipyridine, and nitrogen-containing organic compound dicyandiamine was 1:2.56:15.86. 0.7 g of nitrogen-doped carbon support sample was weighed and added to solution C (the mass ratio of manganese salt Mn(NO3)2 to nitrogen-doped carbon material was 1:0.93) with stirring. Stirring was continued for 12 h, followed by standing for 2 h. The solvent was evaporated at 90 °C, and the sample was dried in an oven at 80 °C. The resulting solid was ground into a fine powder and placed in a tube furnace under a He atmosphere at 1 °C / min. -1 The heating rate was increased to 1050℃, and the product was calcined for 4 hours to obtain a black powder, which is the Mn-based single-atom catalyst, denoted as Mn-NCS.

[0077] Comparative Example 1

[0078] The method is the same as in Example 3, except that no silica sol is added during the preparation of Mn-NCF, and NaOH solution is not used for digestion after calcination, and subsequent water washing, ethanol and vacuum drying are omitted. The remaining steps are the same as in Example 3.

[0079] Comparative Example 2

[0080] The method is the same as in Example 3, except that bipyridine is not added, and only Mn(NO3)2 and dicyandiamine are used to form solution C.

[0081] Comparative Example 3

[0082] The method is the same as in Example 3, except that dicyandiamine is not added, and only Mn(NO3)2 and bipyridine are used to form solution C.

[0083] Comparative Example 4

[0084] The method is the same as in Example 3, except that the nitrogen-containing organic compound used in solution C is urea.

[0085] Because the urea was too alkaline, the Mn salt in solution C formed a flocculent precipitate, so no further experiments were conducted.

[0086] The Mn element in the Mn-NCF and Mn-NCS samples prepared in Examples 1-6 and Comparative Examples 1-3 was determined by ICP-MS, and the results are shown in Table 1.

[0087] Table 1

[0088]

[0089] The catalysts prepared in Examples 1-6 and Comparative Examples 1-3 were subjected to application tests.

[0090] The Mn-NCS catalysts prepared in each example and comparative example, as well as the Mn-NCF sample prepared in Example 3, were used to study the activation of persulfate (PMS) to degrade Rhodamine B (RhB). The experimental procedure was as follows: 5 mg of sample was added to a 250 mL Erlenmeyer flask, and 200 mL of Rhodamine B stock solution (20 mg / L) was added. -1 Stir the mixture in the dark for 60 min until adsorption equilibrium is reached; add persulfate (PMS) to the above mixture and start the reaction; take 1 mL of the reaction solution every 2 min, filter to remove catalyst powder, quench the active oxide species with sodium thiosulfate solution, and then separate and detect the conversion rate of Rhodamine B by high performance liquid chromatography. The results are shown in Table 2 and 3. Figure 3 The stability of the Mn-NCS catalyst prepared in Example 3 was tested after five repeated uses, and the test results are shown in Table 3.

[0091] Table 2

[0092] 10-minute conversion rate Example 1 90.1％ Example 2 93.6％ Example 3 96.2％ Example 4 91.7％ Example 5 94.3％ Example 6 83.8％ Comparative Example 1 32.3％ Comparative Example 2 35.9％ Comparative Example 3 33.7％

[0093] As shown in Table 1, the high-loading catalysts prepared in the embodiments of the present invention can achieve a conversion rate of Rhodamine B of over 80% within 10 minutes. In particular, Examples 1-5 can achieve a conversion rate of over 90% within 10 minutes, and Example 3 even reaches over 96%. In the methods of Examples 1-5, manganese salt is introduced during the formation of nitrogen-doped carbon material. Although this step only allows the nitrogen-doped carbon material to be loaded with less than 0.09% Mn atoms, the addition of Mn salt during the first calcination process increases the graphitization degree of the nitrogen-doped carbon support, resulting in a more stable nitrogen-doped carbon support. This provides more and more stable nitrogen-containing "riveting" sites for subsequent impregnation and second calcination processes, allowing the Mn salt to be more firmly "riveted" to the nitrogen-doped carbon material during the second calcination process, significantly increasing the loading of the Mn-based single-atom catalyst. Compared with Example 6, Example 3 introduced a trace amount of manganese salt during the formation of nitrogen-doped carbon material, and the Mn loading in the resulting catalyst can be as high as 3.98%, while that in Example 6 is only 2.87%. It can be seen that introducing manganese salt during the preparation of nitrogen-doped carbon material can significantly increase the Mn loading of the catalyst.

[0094] In Comparative Example 1, the absence of silica sol hindered the formation of mesopores in the nitrogen-doped carbon material, limiting the exposure of nitrogen-containing functional groups and preventing an increase in Mn loading in the catalyst, thus leading to a decrease in catalytic performance. In Comparative Example 2, the lack of the nitrogen-containing organic ligand bipyridine in solution C hindered the formation of Mn complexes, resulting in Mn ion aggregation; the conversion rate of Rhodamine B was only 35.9% after 10 min. In Comparative Example 3, the absence of nitrogen-containing organic matter in solution C resulted in too few bonding sites for single-atom Mn, leading to a low Mn loading in the catalyst and consequently a decrease in catalytic performance.

[0095] Table 3

[0096] Example 3 10-minute conversion rate 1st time 96.0％ 2nd time 94.9％ 3rd time 93.3％ 4th 92.7％ 5th 92.1％

[0097] As can be seen from Table 3, the catalyst prepared in the embodiments of the present invention can stably load manganese on the support, and can still achieve a conversion rate of over 92% for Rhodamine B after 5 cycles, demonstrating excellent stability.

[0098] Figure 3 The experimental results are for the Mn-NCF and Mn-NCS catalysts prepared in Example 3. From... Figure 3It can be seen that when Mn-NCF is used as a catalyst, the conversion rate of Rhodamine B is 60% after 10 min of reaction; when high-load Mn-NCS is used as a catalyst, the conversion rate of Rhodamine B exceeds 96% after 10 min of reaction. The Rhodamine B degradation experiment results show that the method of this embodiment can prepare high-load Mn-NCS. Furthermore, with the increase in the number of single-atom Mn active sites, the Mn-NCS catalyst can efficiently activate PMS to achieve rapid degradation of Rhodamine B dye.

[0099] In this invention, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0100] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a high-load manganese-based single-atom catalyst for activating PMS to degrade Rhodamine B, characterized in that, include: a. Add silica sol to an aqueous solution of nitrogen-containing organic matter to obtain a first liquid mixture. Add a manganese salt solution to the first liquid mixture, stir, and then evaporate to obtain a first solid mixture. The mass ratio of silica microspheres, nitrogen-containing organic matter, and manganese salt in the silica sol is (100-200):(20-100):

1. The nitrogen-containing organic matter includes at least one of PVP, chitosan, dicyandiamine, cyanamide, melamine, 2,6-diaminopyridine, and 2-methylimidazole. b. The first solid mixture from step a is subjected to a first calcination treatment under an inert atmosphere; c. Add an acid or alkaline solution to the product after calcination in step b to remove the silica sol in the product and prepare nitrogen-doped carbon material. d. Dissolve manganese salt, nitrogen-containing organic ligand, and nitrogen-containing organic compound in an organic solvent to obtain a second liquid mixture, wherein the nitrogen-containing organic ligand includes at least one of bipyridine and 1,10-o-phenanthroline; and the nitrogen-containing organic compound includes at least one of PVP, chitosan, dicyandiamine, cyanamide, melamine, 2,6-diaminopyridine, and 2-methylimidazole. e. Add the nitrogen-doped carbon material obtained in step c to the second liquid mixture obtained in step d, and evaporate to obtain a second solid mixture; f. The second solid mixture obtained in step e is subjected to a second calcination treatment under an inert atmosphere to obtain a manganese-based single-atom catalyst.

2. The preparation method according to claim 1, characterized in that, In step a, the manganese salt includes at least one of manganese chloride, manganese sulfate, manganese nitrate, manganese acetate, and manganese acetylacetone.

3. The preparation method according to claim 1 or 2, characterized in that, In step a, the silica sol comprises water-soluble silica microspheres with a diameter of 5-100 nm; And / or, in the first liquid mixture: the mass ratio of silica microspheres, nitrogen-containing organic matter and manganese salt in the silica sol is (100-200):(40-100):

1.

4. The preparation method according to claim 3, characterized in that, The silica sol contains 10wt%-70wt% silica microspheres.

5. The preparation method according to claim 1 or 2, characterized in that, In step b, the inert atmosphere is at least one of nitrogen, argon, and helium; and / or, the first calcination temperature is 600-1200℃, and the calcination time is 0.5-12h.

6. The preparation method according to claim 1 or 2, characterized in that, In step c, an acid or alkali solution is added to the product after calcination in step b, and the mixture is heated to 20-100 °C and digested for 0.5-24 h to remove the silica sol.

7. The preparation method according to claim 6, characterized in that, The acid includes at least one of hydrochloric acid, sulfuric acid, nitric acid, and hydrofluoric acid, and the mass concentration of the acid is 1 wt%-50 wt%.

8. The preparation method according to claim 6, characterized in that, The alkali includes at least one of NaOH solution and KOH solution, and the mass concentration of the alkali is 1 wt%-50 wt%.

9. The preparation method according to claim 1 or 2, characterized in that, In step d, the manganese salt includes at least one of manganese chloride, manganese sulfate, manganese nitrate, manganese acetate, and manganese acetylacetone; and / or, the organic solvent includes at least one of methanol, ethanol, isopropanol, acetonitrile, and N,N-dimethylformamide.

10. The preparation method according to claim 1 or 2, characterized in that, In step d, the molar ratio of the manganese salt, nitrogen-containing organic ligand, and nitrogen-containing organic compound in the second liquid mixture is 1:(0.5-20):(1-100).

11. The preparation method according to claim 1 or 2, characterized in that, In step e, the mass ratio of manganese salt to nitrogen-doped carbon material in the second liquid mixture is 1:(0.5-10).

12. The preparation method according to claim 1 or 2, characterized in that, In step f, the inert atmosphere is at least one of nitrogen, argon, and helium; and / or, the second calcination temperature is 600-1200℃, and the calcination time is 0.5-12 h.

13. A high-load manganese-based single-atom catalyst, characterized in that, It is prepared by any one of claims 1-12.

14. The application of the high-load manganese-based single-atom catalyst of claim 13 in the activation of PMS for the degradation of Rhodamine B.

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