Preparation method and application of molybdenum monatomic photocatalyst for ammonia production through nitrogen reduction

Abstract

The invention discloses a preparation method and application of a molybdenum monatomic photocatalyst for producing ammonia through nitrogen reduction, and belongs to the technical field of photocatalytic material preparation. According to the preparation method, molybdenum hexacarbonyl is taken as a metal precursor, and molybdenum single atoms are introduced into a covalent organic framework material sp2c-COF by adopting a solvothermal synthesis method, so that the covalent organic framework photocatalyst is obtained; according to the covalent organic framework photocatalyst, molybdenum single atoms are anchored, the local coordination environment of a covalent organic framework is finely controlled, the unique electronic structure and catalytic activity of the molybdenum single atoms can effectively optimize electron cloud distribution of COFs, the adsorption and activation capacity on nitrogen molecules is enhanced, and the covalent organic framework photocatalyst has the advantages that the covalent organic framework photocatalyst is high in catalytic activity and high in catalytic activity; the covalent organic framework photocatalyst can effectively inhibit the compounding of photo-induced electron-hole pairs so as to prolong the service life of photo-induced electrons, so that the covalent organic framework photocatalyst has excellent ammonia production performance, and the ammonia production activity can reach 372.6 [mu] mol.g <-1 >. H <-1 > under the conditions of visible light and pure water.

Description

Technical Field

[0001] The present invention belongs to the technical field of the preparation of photocatalytic materials, and particularly relates to a preparation method and application of a molybdenum single-atom photocatalyst for nitrogen reduction to produce ammonia. Background Art

[0002] Ammonia (NH3) is an essential chemical substance in industrial and agricultural production and has extensive applications in fields such as agricultural fertilizers, chemical synthesis, pharmaceutical preparation, and clean energy carriers. Currently, global ammonia synthesis production mainly relies on the Haber-Bosch process, which realizes synthesis by catalyzing the reaction of nitrogen and hydrogen under high temperature and high pressure conditions. Although this process is technically mature, it emits a large amount of carbon dioxide while consuming a large amount of energy, which seriously conflicts with the current low-carbon sustainable development goal. With the progress of science and technology and the needs of social development, seeking a new method for artificial ammonia synthesis with low energy consumption and environmental friendliness has become a development trend.

[0003] The ammonia production from the nitrogen fixation methods existing in nature cannot meet the demands of industrial and agricultural development. Therefore, it is necessary to synthesize ammonia through artificial methods. Photocatalytic nitrogen fixation technology is an energy-saving and environmental-friendly nitrogen fixation technology that uses renewable solar light to reduce nitrogen under mild conditions. It has significant advantages such as environmental friendliness, simple operation, and low cost, and is regarded as an ideal way to replace traditional processes, showing great potential in sustainable ammonia production and environmental protection issues. However, the core challenge of this technology lies in the design of photocatalysts. On the one hand, traditional photocatalysts lack sufficient active sites and are difficult to effectively adsorb and activate nitrogen. On the other hand, photo-generated electrons and holes are extremely easy to recombine, greatly reducing the energy conversion efficiency in the photocatalytic process, resulting in low photo-quantum efficiency and difficult to improve the overall efficiency of photocatalytic nitrogen fixation. Specifically, the extremely high N≡N bond dissociation energy of nitrogen molecules requires the catalyst to have a strong electron supply ability to promote its activation, and at the same time, it is necessary to effectively separate photo-generated electron-hole pairs to extend the carrier lifetime. How to achieve the above two goals through material design is the key to improving the efficiency of photocatalytic nitrogen fixation.

[0004] Covalent Organic Frameworks (COFs) are a class of crystalline organic porous polymers formed by covalently connecting building units according to the principle of framework chemistry, and have the advantages of structural diversity, low density, high thermal stability, and permanent porosity. In recent years, due to their unique advantages such as rich porous structures, excellent light absorption characteristics, and abundant active sites, covalent organic framework materials (COFs) have become a research hotspot in the field of photocatalytic nitrogen fixation. Their porous channels can provide channels and sites for the diffusion and adsorption of nitrogen molecules, and their unique light absorption characteristics can effectively utilize photo-excited electron transitions, thus effectively promoting the activation and reduction of nitrogen molecules.

[0005] Meanwhile, by isolating and dispersing metal atoms on the surface of the support, single-atom catalysts (SACs) can maximize the utilization rate of metal atoms. Their unique electronic structure and coordination environment can significantly enhance the substrate adsorption and activation ability, showing excellent performance in a series of important catalytic reactions such as small molecule activation and transformation, organic catalysis, and electrocatalysis. Transition metals (such as Fe, Mo, Ru, etc.) show great potential in the single-atom catalysis system due to their d-orbital electron characteristics, so the loading of transition metal elements has also been widely studied. The introduction of transition metals can change the electron cloud distribution of the photocatalyst and enhance the adsorption and activation ability of nitrogen molecules, thus improving the 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 ability of nitrogen molecules, effectively inhibit the recombination of photo-generated electron-hole pairs, increase the lifetime of photo-generated electrons, and make the catalyst have excellent ammonia production performance.

[0006] For example, Chinese Patent No. CN113445074A, with an application date of June 29, 2021, discloses a molybdenum single-atom catalyst, its preparation method and application. A molybdenum source is added to an electrospinning solution to form a precursor mixed solution; the precursor mixed solution is electrospun to form an electrospun film; the electrospun film is dried, and the dried electrospun film is subjected to pre-oxidation treatment and carbonization treatment to anchor molybdenum single atoms on multi-channel carbon fibers co-doped with nitrogen (N) and sulfur (S) heteroatoms to obtain a molybdenum atom catalyst. However, the operation steps of this invention are relatively cumbersome and require electrospinning the precursor mixed solution to form an electrospun film first; Chinese Patent No. CN110571442A, with an application date of September 19, 2019, discloses a molybdenum single-atom catalyst, its preparation method and application. Using a porous carbon skeleton as the support, which has a rich pore structure and a large surface area, on the one hand, it provides a good site for anchoring molybdenum single atoms, and on the other hand, it also provides a convenient access channel for reactant molecules O2 and product molecules; at the same time, due to the excellent conductivity of porous carbon, it can also promote the occurrence of electrocatalytic oxygen reduction reaction. However, after pyrolysis in a nitrogen and / or noble gas atmosphere, this invention still needs to be etched with hydrochloric acid solution to obtain carbon-supported MoO3.

[0007] At present, some studies have attempted to combine metal single atoms with COFs to improve the catalytic performance. However, how to accurately control the anchoring sites of metal atoms and optimize their local coordination environment remains a technical difficulty. In addition, existing methods often require complex synthesis steps or high-temperature treatment, resulting in high costs and difficulty in large-scale production. To address the above problems, there is an urgent need to design a new preparation strategy for a photocatalyst that introduces molybdenum single atoms into the COF framework, and utilize the synergistic effect of its porous structure and molybdenum single atoms to significantly enhance nitrogen adsorption and the utilization efficiency of photo-generated electrons. Summary of the Invention

[0008] To solve the problems existing in the prior art, the present invention discloses a preparation method and application of a molybdenum single-atom photocatalyst for nitrogen reduction to produce ammonia. By anchoring molybdenum single atoms, the local coordination environment of the covalent organic framework is finely controlled, enhancing the adsorption and activation ability of nitrogen molecules, effectively inhibiting the recombination of photo-generated electron-hole pairs, thereby increasing the lifetime of photo-generated electrons, and endowing it with excellent ammonia production performance.

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

[0010] One of the objectives of the present invention is a preparation method of a molybdenum single-atom photocatalyst for nitrogen reduction to produce ammonia. Using 1,3,6,8-tetrakis(4-formylphenyl)pyrene and 2,2′-([2,2′-bipyridine]-5,5′-diyl)diacetonitrile as raw materials, adding 1,4-dioxane and an aqueous KOH solution for a mixed reaction to obtain the covalent organic framework material sp 2 c-COF, and then using molybdenum hexacarbonyl as a metal precursor, introducing molybdenum single atoms into the covalent organic framework material sp 2 c-COF by a solvothermal synthesis method to obtain a covalent organic framework photocatalyst Mo / sp 2 c-COF containing molybdenum single atoms.

[0011] Furthermore, it includes the following steps:

[0012] S1. Weigh 1,3,6,8-tetrakis(4-formylphenyl)pyrene and 2,2′-([2,2′-bipyridine]-5,5′-diyl)diacetonitrile and put them into a thick-walled solvent storage bottle, and add 1,4-dioxane and a 4M aqueous KOH solution thereto;

[0013] S2. Subject the reaction mixture in S1 to three cycles of liquid nitrogen freezing - pumping - thawing for degassing, vacuum seal it, and heat it;

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

[0015] S4. Weigh the covalent organic framework sp 2 c-COF and molybdenum hexacarbonyl into a thick-walled pressure-resistant bottle, add toluene, and mix them evenly by ultrasonic treatment;

[0016] S5. Continuously bubble argon into the uniformly mixed catalytic reaction system in S4 at a constant speed for reaction;

[0017] S6. After the reaction is completed and cooled to room temperature, collect the solid precipitate and wash it with methanol and water multiple times until the filtrate is colorless. After vacuum drying, a molybdenum single-atom-containing covalent organic framework photocatalyst is obtained.

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

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

[0020] Further, in S2, the heating condition is heating at 100 - 160 °C for 70 - 80 h.

[0021] Further, in S3, water, THF, methanol, and acetone reagents are used in sequence to centrifuge and wash the reaction mixture until the supernatant is clear; the vacuum drying time is 10 - 30 h.

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

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

[0024] Further, in S5, the bubbling rate of argon is 50 - 250 mL·min -1 , and the catalytic reaction is carried out at 100 - 160 °C for 5 - 6 h.

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

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

[0027] The third object of the present invention is to provide an application of a molybdenum single-atom photocatalyst for nitrogen reduction to produce ammonia in photocatalytic reduction of nitrogen to ammonia without a sacrificial agent under normal temperature and pressure.

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

[0029] 1. The present invention innovatively anchors molybdenum single atoms on a novel sp 2in covalent organic framework (COF) materials, and achieve precise and controllable synthesis through a two-step method, developing a new type of covalent organic framework photocatalyst Mo / sp based on single-atom loading of transition metals 2 c-COF. The present invention first constructs an sp 2 c-COF substrate with unique conjugated structures and chemical bond characteristics. The fully conjugated characteristics of the C═C bond (compared with imine bonds or hydrazone bonds) significantly expand the π-electron delocalization, reduce the bandgap, and improve the carrier mobility. Subsequently, using molybdenum hexacarbonyl as a precursor, uniform loading of molybdenum single atoms is achieved through a toluene solvothermal reaction, avoiding the loss of active sites caused by high-temperature carbonization of traditional carbon materials. At the same time, the preset bipyridine coordination groups in the sp 2 c-COF skeleton enable in-situ anchoring of the molybdenum precursor, improving the single-atom dispersion. This method breaks through the limitations of traditional COF functionalization and realizes the coordination coupling of molybdenum single atoms with the COF skeleton for the first time, providing a new strategy for the rational design of novel single-atom catalysts.

[0030] 2. The sp 2 c-COF substrate constructed by the present invention has periodically arranged nanopores and a rigid skeleton, and remains structurally stable in strong acids or strong bases, resisting structural deformation during the catalytic process and overcoming the defect that traditional carbon carriers are prone to pore collapse during long-term reactions, resulting in a decrease in activity. At the same time, the periodic pores and bipyridine domains of the sp 2 c-COF substrate provide uniformly distributed and defined anchoring sites for the anchoring of Mo atoms, preventing aggregation and avoiding the uneven loading of single atoms caused by the usually random distribution of heteroatom doping sites in traditional carbon materials.

[0031] 3. The Mo / sp 2 c-COF photocatalyst of the present invention endows the material with unique electronic structures and catalytic activities through local coordination environment regulation. The sp 2 c-COF combines atomic-level electronic structure design and nanoconfinement effects. Using the directional electron cloud of the fully conjugated rigid skeleton, the molybdenum atoms form strong coupling with the conjugated π system of the sp 2 c-COF through unoccupied d orbitals, inducing a redistribution of the local electron cloud density. The rigid pores of the sp 2 c-COF substrate orient N molecules through the space confinement effect. Combining the electronic regulation of the molybdenum sites, the catalytic process is transformed from "random collision" to "orbital regulation", enhancing the chemisorption and activation ability of nitrogen molecules, effectively suppressing the recombination of photo-generated electron-hole pairs, and prolonging the lifetime of photo-generated electrons. This property enables it to exhibit excellent ammonia synthesis performance under normal temperature, normal pressure, and visible light. Measured by Nessler reagent spectrophotometry, the ammonia production rate reaches 372.6 μmol·g-1 after 1 hour of illumination, providing an efficient solution for photocatalytic nitrogen fixation under mild conditions.

[0032] 4. The preparation process of the present invention has remarkable high efficiency, scalability and technical economy. It not only realizes the precise regulation of the crystallinity and porosity of COFs through the coupling of liquid nitrogen freezing - pumping - thawing degassing and solvothermal reaction, but also adopts a mild toluene solvent system in the molybdenum loading step to avoid harsh conditions of high temperature and high pressure. At the same time, the whole process does not require complex post - treatment, which is significantly superior to the traditional preparation process of metal - doped COFs and has the potential for industrial application. Description of the Drawings

[0033] Figure 1 For the covalent organic framework sp 2 c - COF and the covalent organic framework photocatalyst Mo / sp 2 X - ray powder diffraction pattern of c - COF;

[0034] Figure 2 For the covalent organic framework sp 2 c - COF and the covalent organic framework photocatalyst Mo / sp 2 Fourier transform infrared spectrum of c - COF;

[0035] Figure 3 For the covalent organic framework sp 2 c - COF and the covalent organic framework photocatalyst Mo / sp 2 Aberration - corrected transmission electron microscopy image of c - COF;

[0036] Figure 4 For the covalent organic framework sp 2 c - COF and the covalent organic framework photocatalyst Mo / sp 2 Photocatalytic ammonia production comparison chart of c - COF. Detailed Embodiments

[0037] The following further describes the present invention with reference to preferred embodiments. The provided embodiments are only for clarifying the present invention and not for limiting the scope of the present invention.

[0038] The materials, reagents, etc. used in the following embodiments can be obtained from commercial sources without special instructions;

[0039] In the following quantitative tests, three independent repeated experiments are set, and the results are averaged;

[0040] The experimental methods in the following embodiments are conventional methods without special instructions;

[0041] Example 1

[0042] This embodiment provides a preparation method of 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 4 M KOH aqueous solution thereto;

[0044] S2. Subject the reaction mixture in S1 to three cycles of liquid nitrogen freezing - pumping - thawing for degassing and vacuum sealing, and heat it at 160 °C for 70 h;

[0045] S3. Cool the reaction mixture in S2 to room temperature, and centrifuge and wash it successively with water, THF, methanol, and acetone 5 times until the supernatant is clear. After vacuum drying for 10 h, obtain the covalent organic framework sp 2 c-COF;

[0046] S4. Weigh 20 mg of the covalent organic framework sp 2 c-COF and 2 mg of molybdenum hexacarbonyl in a 15 mL thick-walled pressure-resistant bottle, add 10 mL of toluene, and ultrasonicate for 0.5 min to mix them evenly;

[0047] S5. Continuously bubble argon into the catalytic reaction system in S4 that is evenly mixed at a rate of 50 mL·min -1 and catalyze the reaction at 100 °C for 5 h;

[0048] S6. After the reaction ends and cools to room temperature, collect the solid precipitate and wash it repeatedly with methanol and water, and vacuum dry it at 80 °C for 10 h 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, and its preparation method comprises 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 4 M KOH aqueous solution thereto;

[0052] S2. Subject the reaction mixture in S1 to three cycles of liquid nitrogen freezing - pumping - thawing for degassing and vacuum sealing, and heat it at 130 °C for 75 h;

[0053] S3. Cool the reaction mixture in S2 to room temperature, and centrifuge and wash it successively with water, THF, methanol, and acetone 5 times until the supernatant is clear. After vacuum drying for 20 h, covalent organic framework sp 2 c-COF is obtained;

[0054] S4. Weigh 35 mg of covalent organic framework sp 2 c-COF and 14 mg of molybdenum hexacarbonyl into a 15 mL thick-walled pressure-resistant bottle, add 35 mL of toluene, and ultrasonically mix for 75 min to make it evenly mixed;

[0055] S5. Continuously bubble argon into the catalytic reaction system evenly mixed in S4 at a speed of 150 mL·min -1 and catalyze the reaction at 130 °C for 5.5 h;

[0056] S6. After the reaction is completed and cooled to room temperature, collect the solid precipitate and wash it repeatedly with methanol and water, and vacuum dry it at 90 °C for 20 h to obtain a covalent organic framework photocatalyst containing single-atom molybdenum.

[0057] Example 3

[0058] This example provides a preparation method of a molybdenum single-atom photocatalyst for nitrogen reduction to produce ammonia, including the following steps:

[0059] S1. Weigh 150 mg of 1,3,6,8-tetrakis(4-formylphenyl)pyrene and 100 mg of 2,2′-([2,2′-bipyridine]-5,5′-diyl)diacetonitrile and put them into a 25 mL thick-walled solvent storage bottle, and add 5 mL of 1,4-dioxane and 1 mL of 4 M KOH aqueous solution thereto;

[0060] S2. Degas and vacuum-seal the reaction mixture in S1 through three cycles of liquid nitrogen freezing-pump-thawing, and heat it at 100 °C for 80 h;

[0061] S3. Cool the reaction mixture in S2 to room temperature, and centrifuge and wash it successively with water, THF, methanol, and acetone 5 times until the supernatant is clear. After vacuum drying for 30 h, covalent organic framework sp 2 c-COF is obtained;

[0062] S4. Weigh 50 mg of covalent organic framework sp 2 c-COF and 25 mg of molybdenum hexacarbonyl into a 15 mL thick-walled pressure-resistant bottle, add 60 mL of toluene, and ultrasonically mix for 20 min to make it evenly mixed;

[0063] S5. Continuously bubble argon into the catalytic reaction system evenly mixed in S4 at a speed of 250 mL·min -1 and catalyze the reaction at 160 °C for 6 h;

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

[0065] Performance Test

[0066] 1. X-ray Powder Diffraction (XRD) Pattern Analysis

[0067] The crystal structures of sp 2 c-COF and Mo / sp 2 c-COF are analyzed by XRD, and the results are as Figure 1 shown. The characteristic diffraction peak positions and intensities of Mo / sp 2 c-COF and the parent sp 2 c-COF are consistent, indicating that the anchoring of single-atom molybdenum does not change the crystal structure of the parent material, and the intensity and position of the interlayer stacking peaks do not show significant shifts, further confirming that the metal anchoring process does not cause lattice distortion and successfully preserves the high crystallinity and long-range order of sp 2 c-COF.

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

[0069] The chemical bond characteristics of sp 2 c-COF and Mo / sp 2 c-COF are compared by FT-IR test, and the results are as Figure 2 shown. In the FT-IR spectrum, Mo / sp 2 c-COF retains the characteristic peaks of sp 2 c-COF such as C=N (1625 cm-1) and C=C (1580 cm-1), and no new functional group peaks appear. The characteristic absorption peaks of Mo / sp 2 c-COF showing the same as the parent material indicate that molybdenum is bonded to the COF framework in the form of single atoms through coordination bonds, and the anchored metal does not change the framework structure of the parent material.

[0070] 3. Aberration-Corrected Transmission Electron Microscopy (AC-TEM) Characterization

[0071] High-resolution imaging by AC-TEM shows that there are atomically dispersed bright spots on the surface of Mo / sp 2 c-COF, and the results are as Figure 3 shown. The AC-TEM image clearly shows that atomically dispersed bright spots are evenly distributed on the surface of Mo / sp 2 c-COF, which is consistent with the size of single-atom molybdenum, and there are no nanoparticles or clusters, indicating that single-point molybdenum metal is evenly loaded on the surface of the parent material.

[0072] 4. Photocatalytic nitrogen fixation performance test

[0073] Using a 300 W xenon lamp as the light source, the photocatalytic ammonia production performance of 5 mg sp 2 c-COF and Mo / sp 2 c-COF was tested in a 25 mL ultrapure water reaction system under visible light (λ≥420 nm). The results are as follows Figure 4 shown. By comparing the activity diagrams of Mo / sp 2 c-COF and sp 2 c-COF, it can be seen that compared with the parent sp 2 c-COF sample, the ammonia production performance of the covalent organic framework visible light photocatalyst Mo / sp 2 c-COF loaded with single molybdenum atoms has been greatly improved. The corresponding NH3 / NH4 + concentration in the sample was determined by the Nessler reagent spectrophotometry method. The ammonia production amount after 1 h of illumination can reach 372.6 μmol·g -1 . It shows that the single molybdenum atom sites effectively promote the nitrogen adsorption and activation and the separation efficiency of photogenerated carriers, endowing Mo / sp 2 c-COF with excellent photocatalytic stability to achieve efficient photocatalytic ammonia synthesis at room temperature and atmospheric pressure.

[0074] The above are only the embodiments of the present invention, and thus do not limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made by using the content of the specification of the present invention, or directly or indirectly applied in other related technical fields, shall be similarly included in the patent protection scope of the present invention.

Claims

1. A preparation method of a molybdenum single-atom photocatalyst for nitrogen reduction to ammonia, characterized in that, Using 1,3,6,8-tetrakis(4-formylphenyl)pyrene and 2,2′-([2,2′-bipyridine]-5,5′-diyl)dinitrile as raw materials, 1,4-dioxane and an aqueous KOH solution were added and mixed for reaction to prepare the covalent organic framework material sp 2 c-COF. Then, using molybdenum hexacarbonyl as a metal precursor, molybdenum single atoms were introduced into the covalent organic framework material sp 2 c-COF by solvothermal synthesis to obtain the molybdenum single atom-containing covalent organic framework photocatalyst Mo / sp 2 c-COF.

2. The preparation method of a molybdenum single-atom photocatalyst for nitrogen reduction to ammonia according to claim 1, wherein It 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 put them into a thick-walled solvent storage bottle, and add 1,4-dioxane and 4M KOH aqueous solution thereto; S2. Degas and vacuum-seal the reaction mixture in S1 through three cycles of liquid nitrogen freezing - pumping - thawing and then heat it; S3. Cool the reaction mixture in S2 to room temperature, centrifuge and wash it, and obtain covalent organic framework sp after vacuum drying 2 c-COF; S4. Weigh the covalent organic framework sp 2 c-COF and molybdenum hexacarbonyl into a thick-walled pressure-resistant bottle, add toluene, and mix them evenly by ultrasonic treatment; S5. Continuously bubble argon into the catalytic reaction system which is well-mixed in S4 at a constant speed for reaction; S6. After the reaction is completed and cooled to room temperature, collect the solid precipitate and wash it with methanol and water for several times until the filtrate is colorless, and obtain the molybdenum single-atom covalent organic framework photocatalyst after vacuum drying.

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

4. The preparation method of a molybdenum single-atom photocatalyst for nitrogen reduction to ammonia according to claim 2, characterized in that, The heating condition in S2 is heating at 100 - 160 °C for 70 - 80 h.

5. The preparation method of a molybdenum single-atom photocatalyst for nitrogen reduction to produce ammonia according to claim 2, characterized in that, In S3, successively use water, THF, methanol, and acetone reagents to centrifuge and wash the reaction mixture until the supernatant is clear; the vacuum drying time is 10 - 30 h.

6. The preparation method of a molybdenum single-atom photocatalyst for nitrogen reduction to ammonia according to claim 2, wherein, The dosage ratio of the covalent organic framework sp 2 c-COF, molybdenum hexacarbonyl and toluene in S4 is 20 - 50 mg: 2 - 25 mg: 10 - 60 mL; the ultrasonic time is 0.5 - 2 h.

7. The preparation method of a molybdenum single-atom photocatalyst for nitrogen reduction to ammonia according to claim 2, wherein, The bubbling rate of argon in S5 is 50 - 250 mL·min -1 , and the catalytic reaction is carried out at 100 - 160 °C for 5 - 6 h.

8. The preparation method of a molybdenum single-atom photocatalyst for nitrogen reduction to ammonia according to claim 2, characterized in that, Vacuum-dry the solid precipitate after washing in S6 at 80 - 100 °C for 10 - 30 h.

9. A molybdenum single-atom photocatalyst for nitrogen reduction to ammonia prepared by the method according to any one of claims 1 - 8.

10. An application of the molybdenum single-atom photocatalyst for nitrogen reduction to ammonia according to claim 9 in photocatalytic reduction of nitrogen to ammonia without a sacrificial agent at normal temperature and pressure.

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