Preparation method and application of a molybdenum monatomic photocatalyst for nitrogen reduction to produce ammonia

Abstract

The application discloses a preparation method of a molybdenum monatomic photocatalyst for nitrogen reduction to produce ammonia and application thereof, and belongs to the technical field of photocatalytic material preparation. 2 In the c-COF, the covalent organic framework photocatalyst is obtained; the covalent organic framework photocatalyst finely controls the local coordination environment of the covalent organic framework by anchoring molybdenum monatomic; the unique electronic structure and catalytic activity of the molybdenum monatomic can effectively optimize the electronic cloud distribution of the COF, enhance the adsorption and activation capacity of nitrogen molecules, effectively inhibit the recombination of photo-generated electron-hole pairs and increase the photo-generated electron lifetime, so that the covalent organic framework photocatalyst has excellent ammonia production performance, and the ammonia production activity can reach 372.6 micromole*gram ‑1 ·h ‑1 per gram per hour under visible light and pure water conditions.

Description

Technical Field

[0001] This invention belongs to the field of photocatalytic material preparation technology, specifically relating to a method for preparing a molybdenum single-atom photocatalyst for nitrogen reduction to ammonia and its application. Background Technology

[0002] Ammonia (NH3) is an essential chemical substance in industrial and agricultural production, with wide applications in agricultural fertilizers, chemical synthesis, pharmaceutical preparation, and as a clean energy carrier. Currently, global ammonia synthesis mainly relies on the Haber-Bosch process, which synthesizes ammonia through the catalytic reaction of nitrogen and hydrogen under high temperature and pressure. Although this technology is mature, it emits large amounts of carbon dioxide and consumes a significant amount of energy, seriously conflicting with current low-carbon and sustainable development goals. With the advancement of science and technology and the needs of social development, seeking new, low-energy-consumption, and environmentally friendly methods for the artificial synthesis of ammonia has become a development trend.

[0003] Natural nitrogen fixation methods cannot produce enough ammonia to meet the needs of industrial and agricultural development, necessitating artificial ammonia synthesis. Photocatalytic nitrogen fixation is an energy-saving and environmentally friendly technology that utilizes renewable solar energy to reduce nitrogen under mild conditions. It boasts significant advantages such as environmental friendliness, simple operation, and low cost, and is considered an ideal alternative to traditional processes, demonstrating great potential for sustainable ammonia production and environmental protection. However, the core challenge lies in the design of the photocatalyst. Traditional photocatalysts lack sufficient active sites, making effective adsorption and activation of nitrogen difficult. Furthermore, the easy recombination of photogenerated electrons and holes significantly reduces the energy conversion efficiency during photocatalysis, resulting in low quantum efficiency and hindering the overall improvement of photocatalytic nitrogen fixation efficiency. Specifically, the extremely high N≡N bond dissociation energy of nitrogen molecules requires catalysts with strong electron-donating capabilities to promote activation, while effectively separating photogenerated electron-hole pairs to extend carrier lifetime. Achieving these dual objectives through material design is key to improving the efficiency of photocatalytic nitrogen fixation.

[0004] Covalent organic frameworks (COFs) are a class of crystalline organic porous polymers formed by extending building units through covalent bonds based on the principles of framework chemistry. They possess advantages such as structural diversity, low density, high thermal stability, and permanent porosity. In recent years, COF materials have become a research hotspot in the field of photocatalytic nitrogen fixation due to their unique advantages, including abundant porous structures, excellent light absorption properties, and rich active sites. Their porous channels provide pathways and sites for the diffusion and adsorption of nitrogen molecules, and their unique light absorption properties can effectively utilize photoexcited electronic transitions, thereby effectively promoting the activation and reduction of nitrogen molecules.

[0005] Meanwhile, single-atom catalysts (SACs), by isolating and dispersing metal atoms on a support surface, can maximize the utilization of metal atoms. Their unique electronic structure and coordination environment can significantly enhance substrate adsorption and activation capabilities, exhibiting excellent performance in a series of important catalytic reactions such as small molecule activation and conversion, organocatalysis, and electrocatalysis. Transition metals (such as Fe, Mo, and Ru) have shown great potential in single-atom catalytic systems due to their d-orbital electronic properties, and therefore, the loading of transition metal elements has been extensively studied. The introduction of transition metals can change the electron cloud distribution of photocatalysts, enhance the adsorption and activation capabilities of nitrogen molecules, and thus improve photocatalytic nitrogen fixation performance. Among them, the unique electronic structure and catalytic activity of molybdenum single atoms can effectively optimize the electron cloud distribution of COFs, enhance the adsorption and activation capabilities of nitrogen molecules, effectively suppress the recombination of photogenerated electron-hole pairs, and increase the lifetime of photogenerated electrons, resulting in excellent ammonia production performance of the catalyst.

[0006] For example, Chinese patent CN113445074A, filed on June 29, 2021, discloses a molybdenum single-atom catalyst, its preparation method, and its application. The method involves adding a molybdenum source to an electrospinning solution to form a precursor mixture; electrospinning the precursor mixture to form an electrospun film; drying the electrospun film; and then pre-oxidizing and carbonizing the dried film to anchor molybdenum single atoms onto multichannel carbon fibers co-doped with nitrogen (N) and sulfur (S) heteroatoms, thus obtaining a molybdenum atom catalyst. However, this invention has a relatively cumbersome operation process, requiring the precursor mixture to be electrospun first to form an electrospun film. A Chinese patent with publication number CN110571442A and application date of September 19, 2019, discloses a molybdenum single-atom catalyst, its preparation method, and its application. It uses a porous carbon framework as a support, which has abundant pore structure and a large surface area. This provides a good site for anchoring molybdenum single atoms and also provides convenient channels for reactant molecules O2 and product molecules to enter and exit. Simultaneously, due to the excellent conductivity of porous carbon, it can also promote the electrocatalytic oxygen reduction reaction. However, after pyrolysis in a nitrogen and / or rare gas atmosphere, this invention requires etching with hydrochloric acid solution to obtain carbon-supported MoO3.

[0007] Current research has attempted to combine metal single atoms with COFs to enhance catalytic performance, but precisely controlling the anchoring sites of metal atoms and optimizing their local coordination environment remains a technical challenge. Furthermore, existing methods often require complex synthesis steps or high-temperature treatments, leading to high costs and difficulty in scaling up. To address these issues, there is an urgent need to design a novel preparation strategy for photocatalysts that introduce molybdenum single atoms into the COF framework, leveraging the synergistic effect of its porous structure and molybdenum single atoms to significantly enhance nitrogen adsorption and photogenerated electron utilization efficiency. Summary of the Invention

[0008] To address the problems existing in the prior art, this invention discloses a method for preparing a molybdenum single-atom photocatalyst for nitrogen reduction to produce ammonia and its application. The covalent organic framework photocatalyst, by anchoring molybdenum single atoms, precisely controls the local coordination environment of the covalent organic framework, enhances the adsorption and activation ability of nitrogen molecules, effectively inhibits the recombination of photogenerated electron-hole pairs, and thus increases the lifetime of photogenerated electrons, giving it excellent ammonia production performance.

[0009] The technical solution of the present invention is as follows:

[0010] One objective of this invention is to provide a method for preparing a molybdenum single-atom photocatalyst for nitrogen reduction to ammonia production. The method involves using 1,3,6,8-tetrakis(4-formylphenyl)pyrene and 2,2′-([2,2′-bipyridine]-5,5′-diyl)diacetonitrile as raw materials, and reacting them with 1,4-dioxane and KOH aqueous solution to obtain a covalent organic framework material sp. 2 c-COF, and then using molybdenum hexacarbonyl as a metal precursor, molybdenum single atoms were introduced into the covalent organic framework material sp using a solvothermal synthesis method. 2 In c-COF, a covalent organic framework photocatalyst Mo / sp containing a molybdenum single atom was obtained. 2 c-COF.

[0011] Furthermore, it includes the following steps:

[0012] S1. Weigh 1,3,6,8-tetra(4-formylphenyl)pyrene and 2,2′-([2,2′-bipyridine]-5,5′-diyl)diacetonitrile and place them in a thick-walled solvent storage bottle. Add 1,4-dioxane and 4M KOH aqueous solution to the bottle.

[0013] S2. The reaction mixture of S1 is degassed, vacuum-sealed, and heated by a three-stage liquid nitrogen freezing-pump-thawing cycle;

[0014] S3. Cool the reaction mixture in S2 to room temperature, centrifuge, wash, and vacuum dry to obtain the covalent organic framework sp. 2 c-COF;

[0015] S4, Weigh out the covalent organic framework sp 2 c-COF and molybdenum hexacarbonyl were placed in a thick-walled pressure-resistant bottle, and toluene was added. The mixture was then ultrasonically stirred to ensure homogeneity.

[0016] S5. Argon gas is continuously bubbled into the homogeneous catalytic reaction system of S4 at a constant rate to carry out the reaction;

[0017] S6. After the reaction is complete and cooled to room temperature, the solid precipitate is collected and washed repeatedly with methanol and water until the filtrate is colorless. After vacuum drying, a covalent organic framework photocatalyst containing molybdenum single atoms is obtained.

[0018] Further, in S1, the ratio of 1,3,6,8-tetrakis(4-formylphenyl)pyrene, 2,2′-([2,2′-bipyridine]-5,5′-diyl)acetonitrile, 1,4-dioxane and 4M KOH aqueous solution is 20-150 mg: 15-100 mg: 0.5-5 mL: 0.05-1 mL.

[0019] Furthermore, S1 is a 25mL thick-walled solvent storage bottle.

[0020] Furthermore, the heating conditions in S2 are heating at 100-160℃ for 70-80 hours.

[0021] Further, in step S3, the reaction mixture is washed by centrifugation with water, THF, methanol, and acetone in sequence until the supernatant is clear; the vacuum drying time is 10-30 hours.

[0022] Furthermore, the S4 covalent organic framework sp 2 The dosage ratio of c-COF, molybdenum hexacarbonyl and toluene is 20-50 mg: 2-25 mg; 10-60 mL; and the sonication time is 0.5-2 h.

[0023] Furthermore, S4 is a 15mL thick-walled pressure-resistant bottle.

[0024] Furthermore, the argon gas injection rate in S5 is 50-250 mL / min. -1 The catalytic reaction is carried out at 100-160℃ for 5-6 hours.

[0025] Further, the solid precipitate washed in S6 is vacuum dried at 80-100℃ for 10-30 hours.

[0026] The second objective of this invention is to provide a molybdenum single-atom photocatalyst for nitrogen reduction to produce ammonia.

[0027] The third objective of this invention is to provide an application of a molybdenum single-atom photocatalyst for nitrogen reduction to ammonia at room temperature and pressure without sacrificial agents in the photocatalytic reduction of nitrogen to ammonia.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] 1. This invention innovatively anchors molybdenum single atoms to a novel sp... 2In covalent organic framework (COF) materials, a novel Mo / sp covalent organic framework photocatalyst based on transition metal single-atom supported covalent organic framework was developed through a two-step method for precise and controllable synthesis. 2 c-COF. This invention first constructs sp-COF with unique conjugated structure and chemical bonding properties. 2 The c-COF substrate, with its fully conjugated C=C bonds (para-diamine or hydrazone bonds), significantly extends π-electron delocalization, reduces the band gap, and improves carrier mobility. Subsequently, using hexacarbonylmolybdenum as a precursor, uniform loading of molybdenum single atoms is achieved via a toluene solvothermal reaction, avoiding the loss of active sites caused by high-temperature carbonization in traditional carbon materials. Simultaneously, sp... 2 The pre-defined bipyridine coordinating group in the c-COF framework enables in-situ anchoring of the molybdenum precursor, improving single-atom dispersion. This method overcomes the limitations of traditional COF functionalization, achieving for the first time the coordination coupling of molybdenum single atoms with the COF framework, providing a new strategy for the rational design of novel single-atom catalysts.

[0030] 2. The sp constructed in this invention 2 The c-COF substrate possesses periodically arranged nanopores and a rigid framework, maintaining structural stability even in strong acids or bases. This resists structural deformation during catalytic processes, overcoming the drawback of traditional carbon supports that are prone to pore collapse during prolonged reactions, leading to decreased activity. Simultaneously, the sp... 2 The periodic pores and bipyridine domains of the c-COF substrate provide uniformly distributed and defined anchoring points for Mo atoms, preventing agglomeration and avoiding the uneven single-atom loading that is often caused by the random distribution of heteroatom doping sites in traditional carbon materials.

[0031] 3. The Mo / sp described in this invention 2 c-COF photocatalysts, through the regulation of local coordination environment, endow materials with unique electronic structures and catalytic activities. 2 c-COF integrates atomic-level electronic structure design with nanoscale confinement effects, utilizing the directional electron cloud of a fully conjugated rigid framework to allow molybdenum atoms to interact with sp atoms through unoccupied d orbitals. 2 The conjugated π system of c-COF forms a strong coupling, inducing a redistribution of local electron cloud density. 2 The rigid channels of the c-COF substrate oriented N molecules through spatial confinement, combined with electronic regulation of molybdenum sites, transform the catalytic process from "random collisions" to "orbital regulation," enhancing the chemisorption and activation capabilities of nitrogen molecules while effectively suppressing the recombination of photogenerated electron-hole pairs and extending the lifetime of photogenerated electrons. This characteristic enables it to exhibit excellent ammonia synthesis performance under visible light at room temperature and pressure. Spectrophotometric analysis using Nassler reagent showed an ammonia yield of 372.6 μmol·g⁻¹ after 1 hour of illumination, providing a highly efficient solution for photocatalytic nitrogen fixation under mild conditions.

[0032] 4. The preparation process described in this invention has significant efficiency, scalability, and techno-economic advantages. It not only achieves precise control of the crystallinity and porosity of COFs through the coupling of liquid nitrogen freezing-pump-thawing degassing and solvothermal reaction, but also uses a mild toluene solvent system for the molybdenum loading step, avoiding harsh conditions of high temperature and high pressure. At the same time, the entire process does not require complex post-processing, which is significantly better than the traditional metal-doped COFs preparation process and has the potential for industrial application. Attached Figure Description

[0033] Figure 1 The covalent organic framework sp described in this invention 2 c-COF and the covalent organic framework photocatalyst Mo / sp containing molybdenum single atoms 2 X-ray powder diffraction pattern of c-COF;

[0034] Figure 2 The covalent organic framework sp described in this invention 2 c-COF and the covalent organic framework photocatalyst Mo / sp containing molybdenum single atoms 2 Fourier transform infrared spectrum of c-COF;

[0035] Figure 3 The covalent organic framework sp described in this invention 2 c-COF and the covalent organic framework photocatalyst Mo / sp containing molybdenum single atoms 2 Aberration-corrected transmission electron microscopy image of c-COF;

[0036] Figure 4 The covalent organic framework sp described in the performance test of this invention 2 c-COF and the covalent organic framework photocatalyst Mo / sp containing molybdenum single atoms 2 Comparison of photocatalytic ammonia production by c-COF. Detailed Implementation

[0037] The present invention will be further described below with reference to preferred embodiments. The embodiments given are only for illustrating the present invention and are not intended to limit the scope of the present invention.

[0038] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0039] In the quantitative experiments in the following examples, three independent replicate experiments were set up, and the average value of the results was taken.

[0040] Unless otherwise specified, the experimental methods described in the following examples are conventional methods.

[0041] Example 1

[0042] This embodiment provides a method for preparing a molybdenum single-atom photocatalyst for nitrogen reduction to produce ammonia, comprising the following steps:

[0043] S1. Weigh 20 mg of 1,3,6,8-tetra(4-formylphenyl)pyrene and 15 mg of 2,2′-([2,2′-bipyridine]-5,5′-diyl)diacetonitrile and place them in a 25 mL thick-walled solvent storage bottle. Add 0.5 mL of 1,4-dioxane and 0.05 mL of 4MKOH aqueous solution to the bottle.

[0044] S2. The reaction mixture of S1 was degassed and vacuum-sealed by three cycles of liquid nitrogen freezing-pumping-thawing, and then heated at 160°C for 70 hours.

[0045] S3. Cool the reaction mixture in S2 to room temperature, wash five times successively with water, THF, methanol, and acetone until the supernatant is clear, and then vacuum dry for 10 hours to obtain the covalent organic framework sp. 2 c-COF;

[0046] S4. Weigh out 20mg of covalent organic framework sp. 2 c-COF and 2 mg of molybdenum hexacarbonyl were placed in a 15 mL thick-walled pressure-resistant bottle, 10 mL of toluene was added, and the mixture was sonicated for 0.5 min to make it homogeneous.

[0047] S5. Propel argon gas at a rate of 50 mL / min. -1 The mixture was continuously bubbled into the homogeneous S4 catalytic reaction system and catalyzed at 100°C for 5 hours.

[0048] S6. After the reaction is complete and cooled to room temperature, the solid precipitate is collected and washed repeatedly with methanol and water. It is then vacuum dried at 80°C for 10 hours to obtain a covalent organic framework photocatalyst containing molybdenum single atoms.

[0049] Example 2

[0050] This embodiment provides a molybdenum single-atom photocatalyst for nitrogen reduction to produce ammonia, the preparation method of which includes the following steps:

[0051] S1. Weigh 85 mg of 1,3,6,8-tetra(4-formylphenyl)pyrene and 58 mg of 2,2′-([2,2′-bipyridine]-5,5′-diyl)diacetonitrile and place them in a 25 mL thick-walled solvent storage bottle. Add 2.7 mL of 1,4-dioxane and 0.52 mL of 4MKOH aqueous solution to the bottle.

[0052] S2. The reaction mixture of S1 was degassed and vacuum-sealed by three cycles of liquid nitrogen freezing-pumping-thawing, and then heated at 130°C for 75 hours.

[0053] S3. Cool the reaction mixture in S2 to room temperature, wash five times successively with water, THF, methanol, and acetone until the supernatant is clear, and then vacuum dry for 20 h to obtain the covalent organic framework sp. 2 c-COF;

[0054] S4, Weigh out 35mg of covalent organic framework sp 2 c-COF and 14 mg of molybdenum hexacarbonyl were placed in a 15 mL thick-walled pressure-resistant bottle, and 35 mL of toluene was added. The mixture was then sonicated for 75 min to ensure homogeneity.

[0055] S5. Propel argon gas at a rate of 150 mL / min. -1 The mixture was continuously bubbled into the homogeneous S4 catalytic reaction system and catalyzed at 130°C for 5.5 hours.

[0056] S6. After the reaction is complete and cooled to room temperature, the solid precipitate is collected and washed repeatedly with methanol and water. It is then dried under vacuum at 90°C for 20 hours to obtain a covalent organic framework photocatalyst containing molybdenum single atoms.

[0057] Example 3

[0058] This embodiment provides a method for preparing a molybdenum single-atom photocatalyst for nitrogen reduction to produce ammonia, comprising the following steps:

[0059] S1. Weigh 150 mg of 1,3,6,8-tetra(4-formylphenyl)pyrene and 100 mg of 2,2′-([2,2′-bipyridine]-5,5′-diyl)diacetonitrile and place them in a 25 mL thick-walled solvent storage bottle. Add 5 mL of 1,4-dioxane and 1 mL of 4 M KOH aqueous solution to the bottle.

[0060] S2. The reaction mixture of S1 was degassed and vacuum-sealed by three cycles of liquid nitrogen freezing-pumping-thawing, and then heated at 100°C for 80 hours.

[0061] S3. Cool the reaction mixture in S2 to room temperature, wash five times successively with water, THF, methanol, and acetone until the supernatant is clear, and dry under vacuum for 30 h to obtain the covalent organic framework sp. 2 c-COF;

[0062] S4. Weigh out 50mg of covalent organic framework sp. 2 c-COF and 25 mg of molybdenum hexacarbonyl were placed in a 15 mL thick-walled pressure-resistant bottle, and 60 mL of toluene was added. The mixture was then sonicated for 20 min to ensure homogeneity.

[0063] S5. Propel argon gas at a rate of 250 mL / min. -1 The mixture was continuously bubbled into the homogeneous S4 catalytic reaction system and catalyzed at 160°C for 6 hours.

[0064] S6. After the reaction is complete and cooled to room temperature, the solid precipitate is collected and washed repeatedly with methanol and water. It is then dried under vacuum at 100°C for 30 hours to obtain a covalent organic framework photocatalyst containing molybdenum single atoms.

[0065] Performance testing

[0066] 1. X-ray powder diffraction (XRD) pattern analysis

[0067] sp by XRD analysis 2 c-COF and Mo / sp 2 The crystal structure of c-COF is shown in the following results. Figure 1 shown. Mo / sp 2 c-COF and maternal sp 2 The consistent characteristic diffraction peak positions and intensities of c-COF and p-COF indicate that the anchoring of molybdenum single atoms did not alter the crystal structure of the parent material. Furthermore, the intensity and position of the interlayer packing peaks did not shift significantly, further confirming that the metal anchoring process did not cause lattice distortion and successfully preserved sp... 2 c-COF exhibits high crystallinity and long-range order.

[0068] 2. Fourier Transform Infrared (FT-IR) Spectroscopy Analysis

[0069] Comparison of sp through FT-IR testing 2 c-COF and Mo / sp 2 The chemical bond characteristics of c-COF are as follows: Figure 2 As shown. Mo / sp in the FT-IR spectrum. 2 c-COF retains sp 2 c-COF exhibits characteristic peaks such as C=N (1625 cm⁻¹) and C=C (1580 cm⁻¹), with no new functional group peaks observed. Mo / sp 2 The c-COF exhibits characteristic absorption peaks consistent with the parent material, indicating that molybdenum is bound to the COF framework in single-atom form via coordinate bonds, and the anchoring metal does not alter the framework structure of the parent material.

[0070] 3. Characterization by aberration-corrected transmission electron microscopy (AC-TEM)

[0071] Mo / sp were displayed using AC-TEM high-resolution imaging. 2 Atomically dispersed bright spots exist on the c-COF surface, as shown in the following results. Figure 3 As shown. The AC-TEM image clearly shows Mo / sp. 2 The uniformly distributed atomically dispersed bright spots on the c-COF surface match the size of a single molybdenum atom, and the absence of nanoparticles or clusters indicates that molybdenum metal single points are uniformly loaded on the surface of the parent material.

[0072] 4. Photocatalytic nitrogen fixation performance test

[0073] Using a 300W xenon lamp as the light source and visible light (λ≥420nm) as the incident light, the concentration of 5mg of sp in a 25mL ultrapure water reaction system was measured. 2 c-COF and Mo / sp 2 The photocatalytic ammonia production performance of c-COF is shown in the following results. Figure 4 As shown. (Through Mo / sp) 2 c-COF and sp 2 A comparison of the c-COF activity profiles shows that, compared to the parent sp... 2 c-COF sample, Mo / sp, a visible-light photocatalyst supported on a covalent organic framework by a molybdenum single atom. 2 The performance of ammonia produced by c-COF is greatly improved. The corresponding NH3 / NH4 ratio in the sample was determined by Nassler's reagent spectrophotometry. + The concentration of ammonia produced after 1 hour of light exposure can reach 372.6 μmol·g. -1 This indicates that the single-atom sites of molybdenum effectively promote nitrogen adsorption activation and photogenerated carrier separation efficiency, thus contributing to the Mo / sp... 2 c-COF exhibits excellent photocatalytic stability, enabling efficient room-temperature and room-pressure photocatalytic ammonia synthesis.

[0074] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for preparing a molybdenum single-atom photocatalyst for nitrogen reduction to ammonia production, characterized in that, Using 1,3,6,8-tetra(4-formylphenyl)pyrene and 2,2′-([2,2′-bipyridine]-5,5′-diyl)diacetonitrile as raw materials, a covalent organic framework material sp was prepared by reacting 1,4-dioxane with an aqueous solution of KOH. 2 c-COF, and then using molybdenum hexacarbonyl as a metal precursor, a solvothermal synthesis method was used to introduce molybdenum single atoms into the covalent organic framework material sp. 2 In c-COF, a covalent organic framework photocatalyst Mo / sp containing a molybdenum single atom was obtained. 2 c-COF; Includes the following steps: S1. Weigh 1,3,6,8-tetrakis(4-formylphenyl)pyrene and 2,2′-([2,2′-bipyridine]-5,5′-diyl)diacetonitrile and place them in a thick-walled solvent storage bottle. Add 1,4-dioxane and 4M KOH aqueous solution to the bottle. S2. The reaction mixture of S1 is degassed, vacuum-sealed, and heated by a three-stage liquid nitrogen freezing-pump-thawing cycle; S3. Cool the reaction mixture in S2 to room temperature, centrifuge, wash, and vacuum dry to obtain the covalent organic framework sp. 2 c-COF; S4, Weigh out the covalent organic framework sp 2 c-COF and molybdenum hexacarbonyl were placed in a thick-walled pressure-resistant bottle, and toluene was added. The mixture was then ultrasonically stirred to ensure homogeneity. S5. Argon gas is continuously bubbled into the homogeneous catalytic reaction system of S4 at a constant rate to carry out the reaction; S6. After the reaction is complete and cooled to room temperature, the solid precipitate is collected and washed repeatedly with methanol and water until the filtrate is colorless. After vacuum drying, a covalent organic framework photocatalyst containing molybdenum single atoms is obtained. The S4 covalent organic framework sp 2 The dosage ratio of c-COF, molybdenum hexacarbonyl and toluene is 20-50 mg: 2-25 mg; 10-60 mL; and the sonication time is 0.5-2 h.

2. The method for preparing a molybdenum single-atom photocatalyst for nitrogen reduction to ammonia production according to claim 1, characterized in that, The ratio of 1,3,6,8-tetrakis(4-formylphenyl)pyrene, 2,2′-([2,2′-bipyridine]-5,5′-diyl)acetonitrile, 1,4-dioxane and 4M KOH aqueous solution in S1 is 20-150 mg: 15-100 mg: 0.5-5 mL: 0.05-1 mL.

3. The method for preparing a molybdenum single-atom photocatalyst for nitrogen reduction to ammonia production according to claim 1, characterized in that, The heating condition in S2 is heating at 100-160℃ for 70-80 h.

4. The method for preparing a molybdenum single-atom photocatalyst for nitrogen reduction to ammonia production according to claim 1, characterized in that, In step S3, the reaction mixture is washed sequentially with water, THF, methanol, and acetone by centrifugation until the supernatant is clear; the vacuum drying time is 10-30 h.

5. The method for preparing a molybdenum single-atom photocatalyst for nitrogen reduction to ammonia production according to claim 1, characterized in that, The argon gas is introduced at a rate of 50-250 mL / min in S5. -1 The catalytic reaction is carried out at 100-160℃ for 5-6 hours.

6. The method for preparing a molybdenum single-atom photocatalyst for nitrogen reduction to ammonia production according to claim 1, characterized in that, The solid precipitate washed in step S6 is vacuum dried at 80-100℃ for 10-30 h.

7. A molybdenum single-atom photocatalyst for nitrogen reduction to produce ammonia, prepared by the method according to any one of claims 1-6.

8. The application of the molybdenum single-atom photocatalyst for nitrogen reduction to ammonia as described in claim 7 in the photocatalytic reduction of nitrogen to ammonia without sacrificial agents at room temperature and pressure.

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