A single-atom catalyst with asymmetric nitrogen-phosphorus coordination, a preparation method thereof, and a water treatment method and use thereof

Abstract

The present application relates to a kind of single-atom catalysts of nitrogen and phosphorus asymmetric coordination, its preparation method and water treatment method and use.The second heteroatom phosphorus is introduced into single-atom first shell in the present application, the symmetry of typical single-atom M-N4 structure is broken, and the single-atom M-N3P1 site asymmetric coordination single-atom catalyst with is obtained, catalytic ozone activity is effectively improved, and can be used for catalytic ozone oxidation and sewage advanced treatment.The catalyst preparation method of the present application is simple, safe, low in cost, and is conducive to practical application.

Description

Technical Field

[0001] This invention relates to the fields of catalytic materials and water treatment, and particularly to a single-atom catalyst with asymmetric coordination of nitrogen and phosphorus, its preparation method, an ozone catalytic oxidation water treatment method based on the catalyst, and the use of the catalyst for the degradation of organic matter. Background Technology

[0002] Heterogeneous ozone catalytic oxidation is an effective and promising technology for advanced wastewater treatment. It primarily utilizes the decomposition of ozone on the surface of a solid catalyst to generate oxidizing reactive oxygen species, thereby degrading organic pollutants in the water. The design and development of highly efficient and stable ozone catalysts are crucial for both the generation of reactive oxygen species and the degradation of organic pollutants. In recent years, single-atom catalysts have been extensively studied in the field of catalytic oxidation for water treatment due to their maximum atom utilization efficiency and precisely controllable active centers.

[0003] Most currently developed single-atom catalysts are nitrogen-coordinated single atoms, and the optimized active sites are typically planar symmetric configurations of four nitrogen-coordinated metal single atoms. Patent document 1 (CN117816216A) discloses a Fe single-atom catalyst, Fe SA / CN, which uses an imidazole-based metal-organic framework ZIF-8 as a carbon-based precursor support to anchor the Fe component. Low-boiling-point zinc is evaporated at temperatures above 800°C, and the Fe nodes are reduced in situ through carbonization of the organic linker, synthesizing a Fe single-atom / nitrogen-doped porous carbon (Fe SA / CN) catalyst with an Fe-N coordination configuration. The application of this catalyst in the efficient activation of peroxymonosulfate degradation of p-nitrophenol was also investigated.

[0004] Although single-atom catalysts exhibit high catalytic activity, there is still room for improvement in their catalytic activity. Patent document 2 (CN117334928A) discloses a supported nitrogen-phosphorus co-doped iron single-atom bifunctional electrocatalyst, Fe1 / NCP catalyst. This catalyst uses ZIF-8 as a precursor, introduces an iron source, coats it with a polymer, and then anchors Fe single atoms in N and P co-doped porous carbon through pyrolysis and phosphating. The first shell has a FeN4 coordination structure, and P is introduced into the second shell at the active Fe site, forming a unique Fe-NC-P coordination structure. This nitrogen-phosphorus co-doped iron single-atom bifunctional electrocatalyst exhibits excellent electrocatalytic activity under alkaline conditions. Patent document 2 does not investigate the catalytic activity of the structure with the first shell being FeN4 and the second shell being Fe-NC-P in water treatment, and this coordination structure also has room for further improvement. Summary of the Invention

[0005] The problem the invention aims to solve

[0006] Although patent document 2 introduces P into the catalyst, P is introduced into the second shell of the Fe sites; the first shell remains a planar symmetric configuration of four nitrogen-coordinated metal single atoms. To effectively enhance the catalyst's ozone catalytic activity for ozone catalytic oxidation and wastewater treatment, it is necessary to alter the single-atom coordination environment and break the symmetry of the single-atom M-N4 sites.

[0007] Solution for solving the problem

[0008] To address the aforementioned issues, this invention provides an asymmetrically coordinated single-atom catalyst by introducing a second heteroatom P to regulate the coordination environment of the first shell of the single atom. A single-atom M-N3P1 site is designed, breaking the symmetry of the typical single-atom M-N4 structure and effectively enhancing the catalytic ozone activity.

[0009] Specifically, the present invention provides a method for preparing a single-atom catalyst with asymmetric nitrogen-phosphorus coordination, which includes the following steps:

[0010] Step S1: Dissolve the monatomic metal salt, zinc salt, and phosphorus source in an alcohol solvent to obtain solution A; dissolve 2-methylimidazole in an alcohol solvent to obtain solution B; pour solution A into solution B; stir, let stand, separate, and dry at room temperature and pressure to obtain the catalyst precursor MP@ZIF-8; and

[0011] Step S2: The precursor MP@ZIF-8 is calcined under inert gas protection to obtain the nitrogen-phosphorus asymmetric coordinated single-atom catalyst, namely M-NPC.

[0012] According to the preparation method described above, the metal salt is one or more of the following: metal acetylacetonate, acetate, dicerocene, and citrate, and the phosphorus source is triphenylphosphine.

[0013] In step S1, the molar ratio of the single-atom metal salt to the zinc salt is (0.002-0.6):1, the molar ratio of the zinc salt to 2-methylimidazole is 1:(2-10), and the amount of alcohol solvent added relative to the zinc salt is (5-50) mL / mmol.

[0014] In step S2, the calcination temperature is 800℃~1500℃ and the time is 1h~6h.

[0015] This invention also provides a nitrogen-phosphorus asymmetric coordinated single-atom catalyst obtained by the preparation method described above, wherein the single-atom catalyst is based on a carbon framework, and metal atoms M and N, P are dispersed on the surface of the carbon framework. The single-atom catalyst is represented by M-NPC.

[0016] The metal atoms in the single-atom catalyst are positively charged, and the coordination environment of the first shell is three N atoms and one P atom, existing in a single-atom M-N3P1 configuration.

[0017] According to the single-atom catalyst described above, the metal atom is selected from one or two of Fe, Co, Mn, Cu, and Ni; preferably, the metal atom is Fe.

[0018] According to the single-atom catalyst described above, the loading of metal atoms in the single-atom catalyst is 0.2–2.0 wt%; the specific surface area of ​​the single-atom catalyst is 500–1000 m². 2 / g.

[0019] The present invention also provides a method for water treatment using a single-atom catalyst obtained by the preparation method described above, or using a single-atom catalyst described above.

[0020] The method utilizes the single-atom catalyst to decompose ozone and generate reactive oxygen species to degrade organic matter in water.

[0021] According to the method described above, the active oxygen species is selected from one or more of surface hydroxyl radicals, surface adsorbed atomic oxygen, and singlet oxygen.

[0022] According to the method described above, the dosage of the single-atom catalyst is 0.02-5 g / L, the concentration of ozone is 2-20 mg / L, and the flow rate of ozone is 0.1-2 L / min.

[0023] According to the method described above, the water includes drinking water, domestic sewage or industrial wastewater, and the industrial wastewater further includes one or more of the following: pharmaceutical wastewater, coal gasification wastewater, coal olefins wastewater, landfill leachate, petrochemical wastewater, printing and dyeing wastewater, and papermaking wastewater.

[0024] This invention further provides the use of a single-atom catalyst obtained by the preparation method described above, or the use of the single-atom catalyst described above, for the degradation of organic matter, wherein the single-atom catalyst is used to decompose ozone to generate reactive oxygen species to degrade organic matter.

[0025] The active oxygen species are selected from one or more of surface hydroxyl radicals, surface adsorbed atomic oxygen, and singlet oxygen.

[0026] The effects of the invention

[0027] The above-described technical solution of the present invention has the following beneficial effects:

[0028] (1) This invention breaks the typical single-atom M-N4 coordination structure and obtains a single-atom catalyst with nitrogen-phosphorus asymmetric coordination by introducing P atoms into the first single-atom shell.

[0029] (2) Compared with catalysts with M-N4 and M-N4P1 structures, the single-atom catalyst with M-N3P1 configuration of the present invention has a more negative adsorption energy for small molecules, which is more conducive to the adsorption and diffusion of ozone and pollutants. The asymmetrically coordinated single-atom M-N3P1 site has higher catalytic ozone activity.

[0030] (3) The preparation method of the nitrogen-phosphorus asymmetric coordination single-atom catalyst of the present invention is simple, safe and low cost, which is beneficial to practical application. Attached Figure Description

[0031] Figure 1 A flowchart illustrating the preparation process of the nitrogen-phosphorus coordinated single-atom iron ozone catalyst of this invention is shown.

[0032] Figure 2 Scanning electron microscope (SEM) images of the catalyst precursor and catalyst obtained in the embodiments and comparative examples of the present invention are shown.

[0033] Figure 3 The TEM-EDS characterization results of the Fe-NPC catalyst obtained in Example 1 are shown: (a) TEM image; (b) TEM dark field image and EDS image.

[0034] Figure 4 The following are spherical aberration electron micrographs of the Fe-NPC catalyst obtained in Example 1: (a) HAADF-STEM image; (b) Three-dimensional Gaussian function fitting plot of the red square region.

[0035] Figure 5 The XAFS spectra of the Fe-NPC catalyst obtained in Example 1 and the control are shown: (a) XANES spectrum; (b) Fe valence state analysis; (c) k-space EXAFS spectrum; (d) R-space FT-EXAFS spectrum.

[0036] Figure 6 The following are WT-EXAFS contour plots of Fe foil and Fe-NPC catalyst obtained in Example 1: (a) Fe foil; (b) Fe-NPC catalyst.

[0037] Figure 7 EXAFS fitting diagrams of Fe foil and the Fe-NPC catalyst obtained in Example 1 in k-space and R-space are shown: (ab) Fe foil; (cd) Fe-NPC catalyst.

[0038] Figure 8The formation energies of nitrogen-phosphorus coordinated single-atom iron sites and controls are shown: (a) N4 site; (b) N3P1 site; (c) Fe-N4 site; (d) Fe-N3P1 site.

[0039] Figure 9 The results of the control catalytic degradation of p-hydroxybenzoic acid by the Fe-NPC catalyst obtained in Example 1 and the catalyst obtained in the comparative example are shown: (a) degradation curve; (b) kinetic fitting curve; (c) kinetic constant.

[0040] Figure 10 The results of the control catalytic ozone deep treatment of coal chemical wastewater with the Fe-NPC catalyst obtained in Example 1 and the catalyst obtained in the comparative example are shown: (a) COD change; (b) TOC removal rate; (c) reuse experiment; (d) iron ion concentration in influent and effluent.

[0041] Figure 11 The adsorption energies of nitrogen-phosphorus coordinated single-atom iron sites and control sites for small molecules are shown. Detailed Implementation

[0042] The following describes embodiments of the present invention, but the invention is not limited thereto. The present invention is not limited to the configurations described below; various modifications can be made within the scope of the claims. Embodiments and examples obtained by appropriately combining the disclosed technical means in different embodiments and examples are also included within the technical scope of the present invention. Furthermore, all documents described in this specification are incorporated herein by reference.

[0043] Unless otherwise defined, the technical and scientific terms used in this specification have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0044] In the context of describing this specification (especially in the context of the appended claims), the terms “a,” “an,” and “the (described)” and similar language will be interpreted to cover both the singular and plural, unless otherwise indicated herein or clearly contradicted by the context.

[0045] In this specification, the range of values ​​referred to as “value A ~ value B” or “value A - value B” refers to the range including the endpoint values ​​A and B.

[0046] In this specification, the word "may" has both the meaning of performing a certain process and the meaning of not performing a certain process. In this specification, "optional" or "optionally" means that the event or situation described below may or may not occur, and the description includes both the case where the event occurs and the case where the event does not occur.

[0047] In this specification, the terms "some specific / preferred embodiments," "other specific / preferred embodiments," "some specific / preferred technical solutions," and "other specific / preferred technical solutions" refer to specific elements (e.g., features, structures, properties, and / or characteristics) related to the described embodiment that are included in at least one of the embodiments described herein, and may or may not exist in other embodiments. Furthermore, it should be understood that these elements can be combined in any suitable manner in various embodiments.

[0048] The term "comprising" and any variations thereof in the specification and claims of this invention are intended to cover a non-exclusive inclusion. For example, a process, method, system, product, or apparatus that comprises a series of steps or units is not limited to the steps or units listed, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to such processes, methods, products, or apparatus.

[0049] First aspect

[0050] A first aspect of the present invention provides a single-atom catalyst with asymmetric nitrogen-phosphorus coordination. The single-atom catalyst of the present invention has a carbon framework as a substrate, with metal atoms M and N, P dispersed on the surface of the carbon framework. The single-atom catalyst is represented by metal atoms M-NPC.

[0051] The metal atoms in the single-atom catalyst of the present invention are positively charged, and the coordination environment of the first shell is three N atoms and one P atom, existing in the form of a metal atom M-N3P1 configuration. Therefore, the coordination configuration of the metal atoms in the single-atom catalyst of the present invention is different from the planar symmetric configuration (metal atom M-N4) of the usual four nitrogen-coordinated metal single atoms.

[0052] The metal atom in the single-atom catalyst used in this invention may be selected from one or two of Fe, Co, Mn, Cu, and Ni. Preferably, the metal atom may be Fe.

[0053] In the single-atom catalyst of the present invention, the loading of metal atoms can be 0.2–1.0 wt%, for example, 0.3 wt%, 0.5 wt%, 0.7 wt%, 0.9 wt%, etc. The specific surface area of ​​the single-atom catalyst of the present invention can be 500–1000 m². 2 / g, for example, can be 600m 2 / g、700m 2 / g、800m 2 / g、900m 2 / g, etc. The average pore size can be 0.3 to 1.0 nm, for example, 0.4 nm, 0.6 nm, 0.8 nm, etc.

[0054] Second aspect

[0055] The second aspect of the present invention relates to a method for preparing asymmetric coordinated single-atom catalysts. In Patent Document 2, ZIF-8 is first prepared, then an iron source is introduced into ZIF-8 for polymer coating, followed by pyrolysis and phosphating to anchor Fe single atoms in N and P co-doped porous carbon, wherein P is introduced into the second shell of the active center Fe site.

[0056] The preparation method of this invention differs from that in Patent Document 2. In the preparation method of the nitrogen-phosphorus coordinated single-atom ozone catalyst of this invention, a zinc-based zeolite imidazole framework (ZIF-8) is used as the main nanocage, and its cavity diameter is approximately... The maximum aperture is approximately Using the acetylacetonate of a metal as the metal source guest molecule, its molecular diameter is approximately The guest molecule with triphenylphosphine as the phosphorus source has a molecular diameter of approximately A single acetylacetonate molecule or triphenylphosphine molecule can be encapsulated within a single ZIF-8 unit cell. During the preparation process, acetylacetonate (metal source) and triphenylphosphine (phosphine source) are simultaneously introduced, and a nitrogen-phosphine coordinated single-atom iron site is successfully synthesized through a spatially confined dual encapsulation strategy.

[0057] Specifically, the preparation method of the nitrogen-phosphorus asymmetric coordinated single-atom catalyst of the present invention includes the following steps:

[0058] Step S1: Dissolve the monatomic metal salt, zinc salt, and phosphorus source in an alcohol solvent to obtain solution A; dissolve 2-methylimidazole in an alcohol solvent to obtain solution B; pour solution A into solution B; stir, let stand, separate, and dry at room temperature and pressure to obtain the catalyst precursor MP@ZIF-8; and

[0059] Step S2: The precursor MP@ZIF-8 is calcined under inert gas protection to obtain the nitrogen-phosphorus asymmetric coordinated single-atom catalyst.

[0060] The following provides a detailed explanation of each step.

[0061] Step S1

[0062] In step S1, the monatomic metal salt, zinc salt and phosphorus source are first dissolved in an alcohol solvent to obtain solution A, and 2-methylimidazole is dissolved in an alcohol solvent to obtain solution B. Then, solution A is quickly poured into solution B, and the mixture is stirred, allowed to stand, separated and dried at room temperature and pressure to obtain the catalyst precursor MP@ZIF-8.

[0063] In this invention, the monatomic metal salt can be one or more of the following: acetylacetonate, acetate, dicerocene, and citrate, preferably acetylacetonate; the phosphorus source is triphenylphosphine. The zinc salt can be zinc nitrate, zinc chloride, etc. The alcohol solvent can be methanol, ethanol, propanol, etc. The alcohol solvents used for solution A and solution B can be the same or different, preferably the same.

[0064] In this invention, the molar ratio of the monatomic metal salt to the zinc salt can be (0.002–0.6):1, more preferably (0.02–0.5):1. If the amount of zinc salt is too large, its dispersing effect on the transition metal will be insufficient, and the transition metal particles in the catalyst will tend to agglomerate. The molar ratio of zinc salt to 2-methylimidazole is 1:(2–10), more preferably 1:(5–9); the amount of solvent added relative to the zinc salt is (5–50) mL / mmol, more preferably (10–30) mL / mmol. When the amount of solvent is too low, porous crystals are difficult to synthesize; when the amount of solvent is too high, the synthesized catalyst tends to agglomerate.

[0065] After pouring solution A into solution B, the stirring time can be 0.5–2 h, and the settling time can be 12–36 h. After settling, the precipitate can be recovered by centrifugation, then washed three times with an alcohol solvent, and dried in an oven at 50–70 °C for 10–15 h to obtain the precursor MP@ZIF-8.

[0066] The preparation of the single-atom catalyst of the present invention does not require additional heating and pressurization. It can be prepared by dissolving the single-atom metal salt, zinc salt, and 2-methylimidazole in a solvent and stirring the reaction at room temperature and pressure. It has good safety, low energy consumption, and is more convenient to operate.

[0067] Step S2

[0068] In step S2, the catalyst precursor MP@ZIF-8 is calcined and pyrolyzed under inert gas protection to obtain the single-atom catalyst M-NPC of the present invention.

[0069] The inert gas is preferably nitrogen or argon, the calcination temperature is preferably 800–1200℃, more preferably 900–1100℃, and the pyrolysis time is 2–5 h, more preferably 3–4 h. Pyrolysis is preferably carried out in a tubular furnace.

[0070] The method for preparing the single-atom catalyst of the present invention is simple in operation, safe, low in cost, and has good repeatability, and the catalyst prepared has good stability.

[0071] Third aspect

[0072] A third aspect of the present invention provides a method for water treatment using the asymmetrically coordinated single-atom catalyst described in the first aspect or using the asymmetrically coordinated single-atom catalyst obtained by the preparation method described in the second aspect, wherein the asymmetrically coordinated single-atom catalyst is used to decompose ozone to generate reactive oxygen species to degrade organic matter in water.

[0073] The reactive oxygen species are selected from surface hydroxyl radicals and surface adsorbed atomic oxygen (*O). ad ) and singlet oxygen ( 1 One or more of the following: O2.

[0074] Ozone (O3), as a powerful oxidant, can effectively remove pollutants such as phenols, cyanides, sulfides, pesticides, and petroleum hydrocarbons from wastewater. In ozone catalysis systems using asymmetrically coordinated single-atom catalysts, the catalyst can decompose ozone to produce the most superoxide anion radicals (O2). ·- O2 ·- It is a key intermediate in the ozone catalytic oxidation chain reaction, and it can evolve into other reactive oxygen species. O2 ·- It possesses both oxidizing and reducing properties, and its self-disproportionation process can generate singlet oxygen. 1 O2). Non-free radical species. 1 The indirect oxidation pathway of O2 plays an important role in the degradation of organic pollutants.

[0075] In addition, surface-adsorbed atomic oxygen (*O) was also identified in the ozone decomposition products. ad Reactive oxygen species. *O ad It has strong oxidizing properties, with an oxidation-reduction potential of about 2.43V. It can oxidize and degrade organic pollutants adsorbed on the catalyst surface through surface reaction processes.

[0076] In a specific implementation of the water treatment method, an asymmetrically coordinated single-atom catalyst is packed into a reactor, and ozone and water to be treated are introduced to degrade organic matter in the water. To balance degradation efficiency and cost, the dosage of the asymmetrically coordinated catalyst is preferably 0.02–5 g / L, more preferably 0.04–2 g / L, and even more preferably 0.05–0.5 g / L; the ozone concentration is 2–20 mg / L, more preferably 3–20 mg / L; and the ozone flow rate is 0.1–2 L / min, more preferably 0.2–1 L / min.

[0077] The water to be treated in this invention includes drinking water, domestic sewage, or industrial wastewater. More specifically, industrial wastewater includes one or more of the following: pharmaceutical wastewater, coal gasification wastewater, coal-to-olefins wastewater, landfill leachate, petrochemical wastewater, printing and dyeing wastewater, and papermaking wastewater. This invention can effectively degrade and remove recalcitrant organic matter from the water under normal temperature and pressure, achieving high organic matter removal efficiency without requiring additional heating and pressurization of the reactor. This presents a potential advantage in the advanced treatment of complex and practical wastewater.

[0078] Fourth aspect

[0079] A fourth aspect of the present invention provides the use of the asymmetrically coordinated single-atom catalyst described above or the asymmetrically coordinated single-atom catalyst obtained by the preparation method described above for the degradation of organic matter, wherein the asymmetrically coordinated single-atom catalyst is used to decompose ozone to generate reactive oxygen species to degrade organic matter, wherein the reactive oxygen species are selected from surface hydroxyl radicals, surface-adsorbed atomic oxygen (*O) ad ) and singlet oxygen ( 1 One or more of O2.

[0080] The organic matter mentioned here mainly refers to recalcitrant organic matter, which generally includes polycyclic aromatic hydrocarbons, heterocyclic compounds, chlorinated aromatic compounds, organic cyanides, drugs, pesticides, etc.

[0081] In some preferred embodiments of the present invention, for actual advanced wastewater treatment, the asymmetrically coordinated single-atom catalyst ozone system of the present invention achieves a removal rate of organic matter that is more than twice that of a single ozone system. Therefore, the asymmetrically coordinated single-atom catalyst of the present invention exhibits superior reactivity in the ozone catalytic oxidation process for advanced water purification.

[0082] Example

[0083] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0084] Example 1

[0085] 2.96 g of zinc nitrate hexahydrate, 4 mmol of ferric acetylacetone, and 6 mmol of triphenylphosphine were weighed and dissolved in 100 mL of methanol using ultrasound to obtain solution A. 6.56 g of 2-methylimidazole was weighed and dissolved in 100 mL of methanol by stirring at room temperature to obtain solution B. Solution A was rapidly poured into solution B at room temperature and pressure, and stirring was continued for 1 h, followed by standing for 24 h. The precipitate was collected by centrifugation, washed three times with methanol, and dried to obtain the FeP@ZIF-8 precursor.

[0086] The precursor was placed in a tube furnace and heated to 1000℃ at a rate of 5℃ / min under an argon atmosphere, and held at that temperature for 3 hours. During the pyrolysis process, metallic zinc species evaporated. After naturally cooling to room temperature, the catalyst was removed to obtain the Fe-NPC catalyst, in which the monatomic iron existed in the Fe-N3P1 configuration.

[0087] The flowchart for the preparation of the nitrogen-phosphorus coordinated single-atom iron ozone catalyst in this embodiment is shown below. Figure 1 As shown. Figure 1 As shown, in Zn 2+ During the formation of ZIF-8 with 2-methylimidazole, iron acetylacetone and triphenylphosphine are introduced, which self-assemble into a FeP@ZIF-8 precursor with a rhombic dodecahedral shape. During pyrolysis, zinc evaporates, and the nitrogen and phosphorus species in the precursor coordinate with iron to form an Fe-NPC catalyst. The Fe-NPC catalyst catalyzes the decomposition of O3, generating reactive oxygen species.

[0088] Comparative Example 1

[0089] Except for the absence of acetylacetone iron and triphenylphosphine, the NC catalyst used as a control was prepared in the same manner as in Example 1, with ZIF-8 as its precursor.

[0090] Comparative Example 2

[0091] Except for the absence of an iron source, the NPC catalyst used as a control was prepared using the same method as in Example 1, with its precursor represented by P@ZIF-8.

[0092] Comparative Example 3

[0093] Except for the absence of a phosphorus source, the Fe-NC catalyst used as a control was prepared using the same method as in Example 1. The precursor of Fe-NC is represented by Fe@ZIF-8.

[0094] Characterization of catalysts

[0095] Surface morphology: SEM images of the catalyst were obtained using a scanning electron microscope (SEM, Zessi Gemini SEM 500). TEM, high-resolution TEM (HRTEM), and EDS images were obtained using a transmission electron microscope (TEM, JEOL-2010F) equipped with an energy dispersive spectroscopy (EDS, INCA-IET 200). High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) images were obtained using a FEI Titan 80-300 instrument.

[0096] Physicochemical properties: The coordination structure of the catalyst was analyzed using XAFS, FT-EXAFS, and WT-EXAFS. The metal loading in the catalyst was analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES, Thermo Fisher iCAP7400). The specific surface area of ​​the catalyst was analyzed using a nitrogen adsorption-desorption analyzer (Quantachrome).

[0097] SEM images of the catalyst precursors and catalysts obtained in the above embodiments and comparative examples are shown below. Figure 2 In the middle. For example Figure 2 As shown, encapsulating iron acetylacetonate or triphenylphosphine does not significantly affect the morphology of ZIF-8, which still maintains its rhombic dodecahedral shape. However, in the presence of iron acetylacetonate, the ZIF-8 particles grow significantly and exhibit good dispersibility. After high-temperature annealing, the low-boiling-point zinc species volatilize, and the nitrogen and phosphorus species in the precursor coordinate with iron to form Fe-NPC catalysts. Due to the small size of the ZIF-8 and P@ZIF-8 precursor nanoparticles, calcined NC and NPC exhibit significant agglomeration. However, when an iron source is present, the Fe@ZIF-8 and FeP@ZIF-8 precursor particles are well dispersed, and the pyrolyzed Fe-NC and Fe-NPC still maintain their rhombic dodecahedral morphology, but their surfaces become rougher.

[0098] The TEM-EDS characterization results of the Fe-NPC catalyst in Example 1 are shown in... Figure 3 In the middle. For example Figure 3 As shown, the TEM-EDS characterization results of Fe-NPC show that C, N, P, and Fe elements are uniformly distributed and highly overlapping. Similarly, no iron agglomerates in the form of nanoparticles were observed in the TEM image, indicating that iron species may exist in a single-atom form. Further analysis of the existence of iron species in Fe-NPC at a smaller scale was conducted using HAADF-STEM (see [link to HAADF-EDS]). Figure 4 ).like Figure 4As shown, highly dispersed bright spots (highlighted by yellow circles) are observed on the carbon framework substrate. By fitting the red square region with a three-dimensional Gaussian function, a distinct peak of a single iron atom can be seen, indicating that the iron species do indeed exist in a single-atom form.

[0099] The content, valence state, and coordination environment of iron species in Fe-NPC from Example 1 were further precisely quantitatively characterized. The iron species loading in Fe-NPC was determined to be 0.62 wt% by ICP-OES. The specific surface area was determined to be 823 m² / g by nitrogen adsorption-desorption. 2 / g.

[0100] XANES was used to determine the precise valence state and specific coordination structure of iron species in nitrogen-phosphorus coordinated single-atom iron. The XANES diagram (see [link to diagram]) is shown below. Figure 5 As can be seen in the graph, the Fe-NPC curve lies between the Fe foil and Fe2O3, and is closer to the Fe3O4 position, indicating that the iron species carries a positive charge. Further linear fitting of the Fe-K edge energy to the iron valence state in the standard product yields an estimated charge number of approximately +2.56 for the iron species in Fe-NPC. Further Fourier transform and wavelet transform were performed on the EXAFS plots of Fe-NPC and the standard product. (See FT-EXAFS and WT-EXAFS plots). Figure 6 Only Fe-N peaks were recorded in the data, with no Fe-Fe peaks observed. This further indicates that the iron species in Fe-NPC exist in the form of monatomic iron. Further fitting analysis of the k-space and R-space of EXAFS yielded the following results: Figure 7 As shown, the single-atom iron in the Fe-NPC catalyst has a structure coordinated by three nitrogen atoms and one phosphorus atom, which is the Fe-N3P1 configuration. The formation energies of the N4, N3P1, Fe-N4, and Fe-N3P1 configurations were calculated using DFT. The formation energy of the Fe-N3P1 configuration is -13.93 eV, indicating that it can be spontaneously generated during heat treatment (see...). Figure 8 In summary, the single-atom iron in the Fe-NPC catalyst of the present invention carries a positive charge, the first shell coordination environment is three N atoms and one P atom, and the existing form is Fe-N3P1 configuration.

[0101] Catalytic performance test

[0102] Chemical oxygen demand (COD): Detected using a multi-parameter water quality analyzer (Lianhua 5B-3BV8), the measurement method was potassium dichromate rapid digestion spectrophotometry.

[0103] p-Hydroxybenzoic acid: High performance liquid chromatography (Agilent 1200) was used with a C18 reversed-phase column. Mobile phase A was pure methanol, and mobile phase B was 0.1% phosphoric acid aqueous solution. The flow rate was 1.0 mL / min. The column oven temperature was 30℃, and a DAD detector with a wavelength of 280 nm was used.

[0104] TOC removal efficiency (removal rate) = (change in target concentration before and after treatment (C-C0)) / (original concentration of target before treatment (C0)) × 100%.

[0105] Deep processing efficiency = Difference in COD before and after processing / Original COD value before processing × 100%.

[0106] The ozone catalytic reaction was conducted in a powder fluidized bed experiment using a sequencing batch or semi-sequential batch operation mode. A certain amount of single-atom catalyst was added to the wastewater in the column reactor. At the same time, the ozone generator (Longevity EXT120) used pure oxygen as the source to generate ozone, which was then introduced into the reaction solution through the aeration head via an ozone concentration detector (Tonglin 3S-J5000) to start the catalytic reaction. Samples were taken and filtered at regular intervals to measure the organic matter in the wastewater.

[0107] Catalyst performance evaluation for simulated p-hydroxybenzoic acid wastewater: The simulated p-hydroxybenzoic acid wastewater concentration was 50 mg / L, with no pH adjustment and an initial pH of approximately 4.3. The target wastewater volume for a single treatment was 250 mL, the ozone concentration was 3 mg / L, the gas flow rate was 0.2 L / min, and the reaction time was 60 min.

[0108] Degradation of p-hydroxybenzoic acid, a model pollutant:

[0109] The nitrogen-phosphorus coordinated single-atom iron catalyst (Fe-NPC) prepared in Example 1 can efficiently catalyze the ozone oxidation to remove p-hydroxybenzoic acid, a type of organic pollutant in water. For example... Figure 9 As shown, iron-free NC and NPC have almost no adsorption capacity for p-hydroxybenzoic acid, while Fe-NC and Fe-NPC can effectively adsorb p-hydroxybenzoic acid, with adsorption efficiencies of 33% and 51%, respectively, after 60 min. The nitrogen-phosphorus co-coordinated single-atom iron exhibits better adsorption performance for the model pollutant, which is beneficial for the diffusion mass transfer and catalytic degradation process of the model pollutant. The catalytic oxidative degradation process of p-hydroxybenzoic acid conforms to a pseudo-first-order kinetic model. Since p-hydroxybenzoic acid is an electron-rich phenolic pollutant, ozone, which has a certain oxidizing ability, can react with it, with a reaction rate constant of 0.057 min. -1 Due to the low catalytic activity of NC and NPC, the NC-NPC catalytic ozone system did not significantly improve the degradation effect of p-hydroxybenzoic acid compared to ozone alone; the degradation kinetic constants of the two systems were only 0.063 and 0.064 min, respectively. -1When iron species were introduced, the rate constant for the removal of p-hydroxybenzoic acid by Fe-NC / O3 was significantly increased to 0.088 min. -1 Fe-NPC exhibits optimal reactivity, and the rate constant of the Fe-NPC-catalyzed ozone system can be further increased to 0.123 min. -1 This is approximately 2.2 times that of a standalone ozone system. Therefore, the Fe-NPC catalyst containing the asymmetric single-atom iron site Fe-N3P1 group exhibits better reaction characteristics in the ozone catalytic oxidation of water treatment.

[0110] Deep treatment of coal chemical wastewater:

[0111] The advanced treatment process of recalcitrant organic wastewater from coal chemical industry further confirmed that Fe-NPC from Example 1 has greater advantages in actual wastewater treatment (see [link]). Figure 10 ).from Figure 10 (a) It can be seen that the COD of the biochemical effluent from the coal chemical wastewater used is approximately 95 mg / L, requiring advanced treatment; the results of the comparative experiments show that after advanced treatment using separate ozone, NC / O3, NPC / O3, Fe-NC / O3, and Fe-NPC / O3 systems, the effluent CODs are 78 mg / L, 73 mg / L, 71 mg / L, 56 mg / L, and 48 mg / L, respectively. From Figure 10 (b) It can be seen that the Fe-NPC catalytic ozone system has the best deep treatment efficiency, with COD and TOC removal efficiencies of 48% and 47%, respectively. Figure 10 As shown in (c), the results of five reuse experiments of Fe-NPC showed that the COD of the water was below 50 mg / L, indicating that the nitrogen-phosphorus coordinated single-atom iron catalyst has stable catalytic activity. The iron ion concentration in the influent and the effluent after five reuses was also measured. It can be seen that almost no iron ions were leached during the reuse of the Fe-NPC catalyst, indicating that the nitrogen-phosphorus coordinated single-atom iron is very stable (see [reference]). Figure 10 (d)).

[0112] Adsorption energy comparison:

[0113] The adsorption energies of the Fe-N3P1 configuration and its control for ozone molecules and the pollutant p-hydroxybenzoic acid were compared. Figure 11 The calculation results show that, compared with Fe-N4 and Fe-N4P1 structures, the asymmetric coordination single-atom Fe-N3P1 configuration has a more negative adsorption energy for small molecules, which is more conducive to the adsorption and diffusion processes of ozone and pollutants, thereby improving ozone catalytic performance and pollutant removal efficiency. DFT static calculations indicate that the asymmetric single-atom Fe-N3P1 site has higher catalytic ozone activity.

[0114] Industrial availability

[0115] In this invention, by introducing the second heteroatom P into the first shell of a single atom, the symmetry of the typical single-atom M-N4 structure is broken, and a single-atom catalyst with asymmetric coordination of the single-atom M-N3P1 site is obtained, which effectively improves the catalytic activity of ozone.

Claims

1. A method for water treatment using a nitrogen-phosphorus asymmetric coordinated single-atom catalyst, characterized in that, The method utilizes the single-atom catalyst to decompose ozone and generate reactive oxygen species to degrade organic matter in water. The preparation method of the nitrogen-phosphorus asymmetric coordinated single-atom catalyst includes the following steps: Step S1: Dissolve the monatomic metal salt, zinc salt, and phosphorus source in an alcohol solvent to obtain solution A; dissolve 2-methylimidazole in an alcohol solvent to obtain solution B; pour solution A into solution B; stir, let stand, separate, and dry at room temperature and pressure to obtain the catalyst precursor MP@ZIF-8; and Step S2: The precursor MP@ZIF-8 is calcined under an inert gas atmosphere to obtain the nitrogen-phosphorus asymmetric coordinated single-atom catalyst, namely M-NPC. The metal atoms in the single-atom catalyst are positively charged, and the coordination environment of the first shell consists of three N atoms and one P atom, existing in a single-atom M-N3P1 configuration. The single-atom metal salt is a metal acetylacetone salt, the phosphorus source is triphenylphosphine, and the metal atom in the single-atom catalyst is Fe.

2. The method according to claim 1, wherein in step S1, the molar ratio of the single-atom metal salt to the zinc salt is (0.002~0.6):1, the molar ratio of the zinc salt to 2-methylimidazole is 1:(2~10), and the amount of alcohol solvent added relative to the zinc salt is (5~50) mL / mmol; In step S2, the calcination temperature is 800℃~1500℃ and the time is 1h~6h.

3. The method according to claim 1 or 2, wherein the single-atom catalyst is based on a carbon framework, and metal atoms M and N, P are dispersed on the surface of the carbon framework.

4. The method according to claim 3, wherein the loading of metal atoms in the single-atom catalyst is 0.2~2.0 wt%; and the specific surface area of ​​the single-atom catalyst is 500~1000 m². 2 / g.

5. The method according to claim 1 or 2, wherein the active oxygen species is selected from one or more of surface hydroxyl radicals, surface-adsorbed atomic oxygen, and singlet oxygen.

6. The method according to claim 1 or 2, wherein the dosage of the single-atom catalyst is 0.02~5 g / L, the concentration of ozone is 2~20 mg / L, and the flow rate of ozone is 0.1~2 L / min.

7. The method according to claim 1 or 2, wherein the water includes drinking water, domestic sewage or industrial wastewater.

8. The method according to claim 7, wherein the industrial wastewater includes one or more of the following: pharmaceutical wastewater, coal gasification wastewater, coal olefins wastewater, petrochemical wastewater, printing and dyeing wastewater, and papermaking wastewater.

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