An electrochemical ammonia synthesis catalyst, its preparation method and application

Abstract

This invention provides a catalyst for electrochemical ammonia synthesis, its preparation method, and its application. The catalyst comprises carbon-coated magnetic nano-Fe3O4 and γ-aminopropyltriethoxysilane APTES grafted onto its surface. The mass percentage of APTES in the catalyst is 6-8%, and the mass percentage of Fe3O4 is 79-85%. The preparation method includes obtaining the precursor Fe3O4@polymer via a solvothermal method and a one-pot method, followed by carbonization to obtain Fe3O4@C, and then performing a simple grafting treatment on the surface to obtain grafted Fe3O4@C-APTES. The synthesis method of this invention is simple, and the unique grafted structure of the catalyst is beneficial for the adsorption of the reaction substrate in nitrate reduction and for inhibiting the competitive hydrogen evolution reaction, and forms a synergistic effect with the active phase to improve the overall catalytic activity. The catalyst has a rich pore structure, which is beneficial for the dispersion of active sites and the mass transfer process of nitrate. Furthermore, carbon coating can improve the stability and conductivity of the catalyst. The ammonia production rate of this invention is as high as 4.5 mg / cm³. ‑2 h ‑1 Furthermore, its performance remains above 90% of its original level even after 12 hours, demonstrating excellent stability.

Description

Technical Field

[0001] This invention belongs to the field of electrocatalytic reduction of nitrate to produce ammonia, and relates to an electrochemical ammonia synthesis catalyst, its preparation method and application. Background Technology

[0002] Ammonia, as one of the world's largest-produced chemical industrial products, plays an indispensable role in agricultural fertilizers, chemical raw materials, and clean energy carriers. Although the traditional Haber-Bosch process has been used for over a century, its harsh conditions of high temperature and high pressure result in energy consumption accounting for 1%–2% of global total energy consumption and contributing approximately 1.5% of global greenhouse gas emissions, severely hindering the realization of sustainable development. Therefore, developing green, low-energy-consumption ammonia synthesis technologies has become an urgent priority. Electrocatalytic nitrate reduction ammonia synthesis technology is an emerging green method for ammonia synthesis. Powered by clean energy sources such as solar and wind power, and using water as a proton source, it converts nitrates into ammonia at ambient temperature and pressure. This not only significantly reduces carbon emissions but also provides a new approach to "turning waste into treasure" for wastewater treatment. Nitrates in industrial wastewater and agricultural runoff pose a threat to aquatic ecosystems and human health, while electrocatalytic nitrate reduction ammonia (NO3RR) technology can efficiently convert them into high-value-added ammonia, achieving both environmental and economic benefits. The reduction of nitrate to ammonia involves an eight-electron transfer process. Its complexity stems from the planar resonance structure of the nitrate ion and its multiple intermediate transformation pathways, leading to slow reaction kinetics and a predisposition to side reactions. The performance of the catalyst directly determines the reaction activity, ammonia selectivity, and stability. A good electrode catalyst must possess advantages such as strong substrate adsorption capacity, good selectivity, and high stability. In the research of electrocatalytic reduction of nitrate to ammonia, iron-based catalysts have attracted widespread attention due to their high selectivity for the target product, ammonia. However, in practical applications, these catalysts still suffer from insufficient intrinsic activity and poor long-term operational stability, which significantly restricts their further development. Therefore, developing iron-based electrocatalysts that combine high activity and excellent stability is of great significance for advancing the practical application of nitrate reduction to ammonia technology. Summary of the Invention

[0003] To address the above technical problems, this invention provides an electrochemical ammonia synthesis catalyst, its preparation method, and its application. The precursor Fe3O4@polymer is obtained at room temperature via solvothermal and one-pot methods, followed by carbonization to obtain Fe3O4@C. A simple surface grafting treatment is then performed to obtain the Fe3O4@C-APTES catalyst. The catalyst is then used for electrocatalytic reduction of nitrate to ammonia, exhibiting high efficiency, high activity, farad efficiency, and excellent stability.

[0004] To achieve the above objectives, the present invention adopts the following technical solution: An electrochemical ammonia synthesis catalyst comprises carbon-coated magnetic nano-Fe3O4, denoted as Fe3O4@C, and γ-aminopropyltriethoxysilane APTES grafted onto the surface of Fe3O4@C, the catalyst being denoted as Fe3O4@C-APTES, wherein the mass percentage of APTES in the catalyst is 6-8% and the mass percentage of Fe3O4 is 79-85%.

[0005] The magnetic nano-iron oxide has a particle size of 150~230nm.

[0006] This invention also provides a method for preparing an electrochemical ammonia synthesis catalyst, comprising the following steps: S1 mixes and polymerizes magnetic nano-Fe3O4, phenol, aldehyde, amine and ammonia in a solvent to obtain Fe3O4@polymer precursor; S2 The Fe3O4@polymer precursor is carbonized, washed, and dried to obtain Fe3O4@C; S3 grafted Fe3O4@C with γ-aminopropyltriethoxysilane to obtain grafted Fe3O4@C-APTES.

[0007] In step S1, magnetic nano Fe3O4 is mixed with a solvent, and then phenol, amine, ammonia and aldehyde are added in sequence for mixing and polymerization.

[0008] The phenols include one or more of phenol, resorcinol, phloroglucinol, and bisphenol A; the aldehydes include one or more of benzaldehyde, glyoxal, butyraldehyde, glutaraldehyde, and formaldehyde; and the amines include one or more of ethylamine, ethylenediamine, propylamine, hexylamine, and hexamethylenediamine.

[0009] The molar ratio of the magnetic nano-Fe3O4, phenol, aldehyde, amine and ammonia is (1~1.8):4:(7~9):1:(20~30).

[0010] The polymerization conditions described in step S1 are: reaction at room temperature for 0.5-1 h, followed by aging at 75-85°C for 3.5-4 h, with a mechanical stirring speed of 200-300 rpm.

[0011] Step S1 also includes cleaning the solid product and performing magnetic separation after polymerization, and drying to obtain the Fe3O4@polymer precursor.

[0012] The carbonization temperature in step S2 is 400℃~900℃, and the carbonization time is 2~3 hours. Preferably, the carbonization temperature is 550℃~600℃.

[0013] Step S2 involves carbonization under an inert atmosphere, with the temperature increased to the required carbonization temperature at a rate of 2–5 °C / min.

[0014] When the carbonization process is complete and the inert atmosphere protection is stopped in step S2, ethanol is added to the ceramic boat in the carbonization container to isolate it from air.

[0015] In step S3, the solvent for grafting is toluene, xylene, or water, the volume ratio of γ-aminopropyltriethoxysilane to solvent is (2~3):50, the reaction time is 3~6h, and the mechanical stirring speed is 200~300rpm.

[0016] Step S3 also includes washing the solid product and performing magnetic separation after grafting treatment.

[0017] The method for preparing magnetic nano-Fe3O4 includes mixing iron salt, alkali source and stabilizer in a solvent at room temperature, transferring to a hydrothermal reactor for reaction, washing the solid product and magnetically separating it to obtain magnetic nano-Fe3O4.

[0018] The iron salt is anhydrous ferric chloride; the alkali source is sodium acetate; the stabilizer is sodium citrate; and the solvent is ethylene glycol, which also acts as a reducing agent.

[0019] The mass ratio of the iron salt, alkali source, stabilizer and solvent is 16 : (6~18) : (30~60) : 555.

[0020] The hydrothermal conditions are a reaction at 180~220℃ for 7~8 hours.

[0021] The solid product cleaning method involves washing the solid product with water and ethanol alternately 3 to 5 times.

[0022] This invention also provides the application of Fe3O4@C-APTES catalyst in the electrocatalytic reduction of nitrate to ammonia.

[0023] The beneficial effects of this invention are: (1) This invention innovatively prepares a grafted carbon-coated Fe3O4 catalyst, whose unique grafting structure can facilitate the reaction of NO3- in the diffusion layer. - The diffusion rate into the Helmholtz layer inside the electrode is increased, the surface charge of the catalyst is optimized, and the adsorption of the reaction substrate is facilitated.

[0024] (2) The catalyst of the present invention has a rich microporous structure, which can significantly promote the reaction of nitrate ions (NO3-) in heterogeneous electrocatalytic systems. - The mass transfer process of the reaction is improved, thereby enhancing the overall efficiency of the reaction. At the same time, its multi-level pore structure helps to achieve high dispersion and full exposure of active components, thereby improving the utilization rate of active sites and having good economic benefits.

[0025] (3) The hydrophobic properties of APTES are beneficial to suppressing competitive hydrogen evolution reactions, and can also form a synergistic effect with the active phase to improve the overall catalytic activity.

[0026] (4) The ammonia production rate of this invention is as high as 4.5 mg / cm³. -2 h -1 The Faraday efficiency is close to 100%, and even after long-term and cyclic reactions, the Faraday efficiency can still be maintained at more than 90% of the original efficiency, demonstrating excellent stability and adaptability to various wastewater environment systems. Attached Figure Description

[0027] Figure 1 This is a SEM image of Fe3O4@C-550.

[0028] Figure 2 The image shows the XRD pattern of Fe3O4@C-550.

[0029] Figure 3 This is a nitrogen adsorption diagram of Fe3O4@C-APTES.

[0030] Figure 4 This is an HRTEM image of Fe3O4@C-APTES.

[0031] Figure 5 Infrared spectra of Fe3O4@C-APTES, Fe3O4@C-550 and APTES reagents.

[0032] Figure 6 Raman for Fe3O4@C-APTES and Fe3O4@C-550.

[0033] Figure 7 The graph shows the ammonia production rate and Faraday efficiency of Fe3O4@C materials at different pyrolysis temperatures.

[0034] Figure 8 The graph shows the ammonia production rate and Faraday efficiency for Fe3O4@C-APTES at different reduction potentials.

[0035] Figure 9 The graph shows the cyclic stability test results for Fe3O4@C-APTES.

[0036] Figure 10 This is a graph showing the long-term potentiostatic stability of Fe3O4@C-APTES.

[0037] Figure 11 This is a performance comparison chart among Fe3O4, Fe3O4@C-550, and Fe3O4@C-APTES materials. Detailed Implementation

[0038] The present invention will be further described in detail below with reference to specific examples, but the present invention is not limited to the specific embodiments.

[0039] Example 1 (1) Preparation of Solution A: Sodium acetate (1.2 g) was dissolved in ethylene glycol (10 ml) and stirred continuously at 50 °C for 30 min. Preparation of Solution B: Anhydrous ferric chloride (0.64 g) and trisodium citrate (0.24 g) were stirred continuously at 50 °C for 30 min, and then dissolved in ethylene glycol (10 ml). Solution A was injected into Solution B at a controlled rate of 2 mL / min, and then reacted at 50 °C for 30 min. The mixed solution was then transferred to a 50 ml polytetrafluoroethylene liner, which was placed in a hydrothermal reactor and reacted at 200 °C for 8 hours. After cooling to room temperature, the mixture was washed three times with ethanol / water to remove residual organic matter, and magnetic separation was performed to obtain magnetic Fe3O4 nanoparticles with a particle size of 150~230 nm.

[0040] (2) 50 mg of magnetic Fe3O4 nanoparticles were uniformly dispersed in 180 ml of aqueous solution and mechanically stirred (200 rpm). Then, 66 mg of resorcinol was added, followed by 150 µl (0.83 mmol / L) of n-propylamine after 10 min, and 3 ml (1.5 mol / L) of ammonia solution was introduced. After 15 min, 10 ml (0.1 mmol / L) of formaldehyde was injected at a controlled rate of 2 mL / min, and the reaction was carried out at 24°C for 30 min. The temperature was then raised to 80°C and aged for 4 h to complete the polymerization process, generating the Fe3O4@Polymer composite material. After cooling to room temperature, the material was washed three times with ethanol / water to remove residual organic matter and then magnetically separated to obtain the magnetic nano-precursor Fe3O4@Polymer.

[0041] (3) 50 mg Fe3O4@Polymer was transferred to a 5 ml ceramic boat and carbonized at 550 °C for 2 h under argon (99.8 sccm) protection. After carbonization, when the temperature dropped to room temperature, the inert gas was turned off and ethanol was added to the ceramic boat to isolate the material from air. Subsequently, magnetron sputtering was performed to obtain Fe3O4@C-550.

[0042] Figure 1 This is a SEM image of Fe3O4@C-550. The image shows that the material has a uniform particle size and a wrinkled carbon shell on the surface.

[0043] Figure 2 The image shows the XRD pattern of Fe3O4@C-550. The figure indicates that iron in the material mainly exists as Fe3O4.

[0044] By changing the carbonization temperature to 400°C, 500°C, 550°C, 600°C, and 700°C, the corresponding Fe3O4@C-400, Fe3O4@C-500, Fe3O4@C-550, Fe3O4@C-600, and Fe3O4@C-700 were obtained.

[0045] Example 2 100 mg of Fe3O4@C-550 was diffused into 18.6 ml of toluene using ultrasound. The mixture was then continuously mechanically stirred at 25 °C (300 rpm) to form a suspension. 1.4 ml of APTES was injected into the suspension and reacted for 4 h to covalently graft onto the carbon surface. The mixture was then washed three times sequentially with toluene, ethanol, and deionized water, followed by magnetron sputtering to remove unbound silane molecules, yielding Fe3O4@C-APTES, with an APTES mass percentage of 7% and a Fe3O4 mass percentage of 82%.

[0046] Figure 3 This is a nitrogen adsorption diagram of Fe3O4@C-APTES. The diagram shows that the material possesses a rich microporous structure.

[0047] Figure 4 The image shows the HRTEM image of Fe3O4@C-APTES. It can be seen from the image that the material has a clear crystal structure with lattice spacings of 0.21 nm, 0.33 nm and 0.24 nm, which correspond to the (100) crystal plane, (022) crystal plane of carbon material and the (311) crystal plane of Fe3O4, respectively.

[0048] Figure 5 The images show the infrared spectra of Fe3O4@C-APTES, Fe3O4@C-550, and APTES reagent. By comparing the peak positions of the catalyst before and after grafting with those of pure APTES, it can be concluded that APTES was successfully grafted onto the catalyst surface.

[0049] Figure 6 The figures show the Raman spectra of Fe3O4@C-APTES and Fe3O4@C-550. The figures reveal the Ig of carbon on the catalyst surface before and after grafting. D / I G =2.5~3, the carbon on the material surface exists in two forms: graphitic carbon and disordered carbon.

[0050] Example 3 Electrocatalytic nitrate reduction to ammonia production test (1) The performance test of electrocatalytic nitrate reduction to ammonia was conducted using a standard three-electrode system in an H-type electrolytic cell with a Dutch Lvium electrochemical workstation. The working electrode was hydrophobic carbon paper (Toray TGPH060F, Japan) loaded with Fe3O4@C-APTES, the reference electrode was Ag / AgCl, and the counter electrode was platinum foil (area 0.7 × 0.7 cm²). 50 mL of 0.1 M KOH solution containing 0.1 M NaNO3 was injected into both the cathode and anode chambers. The two chambers were separated by a pretreated Nafion 117 proton exchange membrane (DuPont). Before use, the membrane was sequentially cleaned and activated with 5 wt% H2O2 solution (80 ℃, 1 h), deionized water (0.5 h), 5 wt% H2SO4 solution (80 ℃, 1 h), and deionized water (0.5 h). Before testing, linear scanning voltammetry was performed at a scan rate of 5 mV / s until the polarization curve reached a stable state. Subsequently, a 30-minute constant potential test was conducted at different potentials. Stability evaluation was performed at the optimal potential, using the same working electrode for eight consecutive cycles, with a new electrolyte replaced in each cycle to maintain consistent reaction conditions. A 12-hour long-term stability test was then conducted, with the electrolyte replaced every 3 hours to maintain consistent reaction conditions.

[0051] (2) NH3 concentration detection: After color development with Nessler's reagent, the ammonia concentration in the electrolyte after the electrocatalytic nitrate reduction reaction was quantitatively analyzed using a UV spectrophotometer. First, a standard curve was plotted: using ammonium chloride as the standard ammonia source, a series of standard solutions with concentrations ranging from 0.5 to 5 μg / mL were prepared; 5 mL of each standard solution was taken, and 0.1 mL of potassium sodium tartrate solution and 0.1 mL of Nessler's reagent were added sequentially, mixed, and allowed to stand for 20 min. A UV-Vis spectrophotometer was used to scan within the wavelength range of 400–500 nm, and the absorbance was measured at 420 nm. A standard curve was established with ammonia nitrogen concentration as the abscissa and absorbance as the ordinate. When analyzing the sample, electrolyte was taken from the cathode chamber, appropriately diluted to 5 mL, and 0.1 mL of potassium sodium tartrate solution and 0.1 mL of Nessler's reagent were added simultaneously. After color development under the same conditions, the absorbance at 420 nm was measured, and the ammonia concentration was calculated based on the standard curve.

[0052] Example 4 (1) The electrocatalytic performance of the material obtained in Example 1 for the reduction of nitrate to ammonia was determined according to the test method in Example 3. The working potential was -0.31 to -0.41 V (vs. RHE), and the reaction time was 30 min. The performance comparison graph is shown below. Figure 7 As shown, the carbonization temperature affects the degree of graphitization of the carbon shell on the final catalyst surface, thus affecting the catalytic performance.

[0053] (2) The electrocatalytic performance of the material obtained in Example 2 for the reduction of nitrate to ammonia was determined according to the test method in Example 3. The working potential was -0.17 to -0.57 V (vs. RHE), and the reaction time was 30 min. The performance graphs at different potentials are shown below. Figure 8 As shown, the catalytic performance exhibits a volcanic pattern with respect to potential, with the optimal potential being -0.37 V (vs. RHE). Subsequently, eight cycles were performed at the optimal potential to measure its stability. Figure 9 As shown, the ammonia production rate and Faraday efficiency of the catalyst remained above 90% of their initial performance after eight cycles. Long-term stability tests were then conducted on the cycled catalyst. Figure 10 As shown in the figure. The test conditions were -0.37V (vs. RHE) for 12 h, with the electrolyte replaced every 3 h. The results in the graphs indicate that the catalyst maintains good reaction performance even under long-term reaction conditions. Combined cyclic stability tests and long-term stability tests demonstrate that Fe3O4@C-APTES exhibits good stability.

[0054] (3) The electrocatalytic performance of Fe3O4 and Fe3O4@C-550 obtained in Example 1 and Fe3O4@C-APTES obtained in Example 2 was determined according to the test method in Example 3. The optimal potential performance of each was selected for comparison. The results are as follows: Figure 11 Comparing Fe3O4 with Fe3O4@C-550, it can be found that the introduction of carbon is beneficial to the improvement of reaction activity. This is because the introduction of carbon shell can improve the overall conductivity of the catalyst, thereby increasing the electron transport rate. The performance of the grafted catalyst is nearly 60% higher than that of Fe3O4@C, indicating that surface grafting treatment is beneficial to NO3RR. Furthermore, the Faraday efficiency of the grafted catalyst is close to 100%, indicating that the moderate hydrophobicity brought by the grafted structure inhibits the hydrogen evolution reaction and improves electron utilization.

[0055] In summary, the advantages of this material are: 1. The presence of a carbon shell can improve the stability and conductivity of the catalyst; 2. The unique APTES grafting treatment reduces the negative charge on the catalyst surface, thereby weakening the negatively charged cathode electrode's effect on the reactant NO3. - The repulsive effect of the grafted structure is beneficial to the adsorption of the reaction substrate; 3. The hydrophobicity brought by the grafted structure reduces the water content at the electrode surface interface, inhibits the competitive hydrogen evolution reaction, and thus improves the electron utilization rate of NO3RR. The grafted catalyst effectively solves the problems of intense side reaction competition and slow reaction rate. Therefore, the electrocatalytic performance of this catalyst is superior to that of most reported iron-based catalysts. Among them, the Fe3O4@C-APTES material obtained in Example 2 has a yield as high as 4.5 mg cm⁻¹. -2 h-1 Faraday efficiency is close to 100%.

Claims

1. An electrochemical ammonia synthesis catalyst, characterized in that: The catalyst includes carbon-coated magnetic nano-Fe3O4, denoted as Fe3O4@C, and γ-aminopropyltriethoxysilane APTES grafted onto the surface of Fe3O4@C, denoted as Fe3O4@C-APTES. The mass percentage of APTES in the catalyst is 6-8%, and the mass percentage of Fe3O4 is 79-85%.

2. The electrochemical ammonia synthesis catalyst as described in claim 1, characterized in that: The magnetic nano-Fe3O4 has a particle size of 150~230µm.

3. A method for preparing the electrochemical ammonia synthesis catalyst according to claim 1, characterized in that: Includes the following steps: S1 mixes and polymerizes magnetic nano-Fe3O4, phenol, aldehyde, amine and ammonia in a solvent to obtain Fe3O4@polymer precursor; S2 The Fe3O4@polymer precursor is carbonized, washed, and dried to obtain Fe3O4@C; S3 grafted Fe3O4@C with γ-aminopropyltriethoxysilane to obtain grafted Fe3O4@C-APTES.

4. The preparation method of the electrochemical ammonia synthesis catalyst according to claim 3, characterized in that: In step S1, magnetic nano-Fe3O4 is mixed with a solvent, and then phenol, amine, ammonia, and aldehyde are added sequentially for mixing and polymerization; and / or, The phenol includes one or more of phenol, resorcinol, phloroglucinol, and bisphenol A; the aldehyde includes one or more of benzaldehyde, glyoxal, butyraldehyde, glutaraldehyde, and formaldehyde; the amine includes one or more of ethylamine, ethylenediamine, propylamine, hexylamine, and hexamethylenediamine; and / or, The molar ratio of the magnetic nano-Fe3O4, phenol, aldehyde, amine and ammonia is (1~1.8):4:(7~9):1:(20~30).

5. The preparation method of the electrochemical ammonia synthesis catalyst according to claim 3, characterized in that: The polymerization conditions described in step S1 are: reaction at room temperature for 0.5-1 h, followed by aging at 75-85°C for 3.5-4 h, with mechanical stirring at 200-300 rpm; and / or, Step S1 also includes cleaning the solid product and performing magnetic separation after polymerization, and drying to obtain the Fe3O4@polymer precursor.

6. The method for preparing the electrochemical ammonia synthesis catalyst as described in claim 3, characterized in that: The carbonization temperature in step S2 is 400℃~900℃, and the carbonization time is 2~3 hours; and / or, When the carbonization process is complete and the inert atmosphere protection is stopped in step S2, ethanol is added to the ceramic boat in the carbonization container to isolate it from air.

7. The method for preparing the electrochemical ammonia synthesis catalyst as described in claim 3, characterized in that: In step S3, the grafting treatment uses toluene, xylene, or water as the solvent, with a volume ratio of γ-aminopropyltriethoxysilane to solvent of (2-3):50, a reaction time of 3-6 hours, and a mechanical stirring speed of 200-300 rpm; and / or, Step S3 also includes washing the solid product and performing magnetic separation after grafting treatment.

8. The method for preparing the electrochemical ammonia synthesis catalyst as described in claim 3, characterized in that: The method for preparing the magnetic nano-Fe3O4 includes mixing iron salt, alkali source, and stabilizer in a solvent at room temperature, transferring the mixture to a hydrothermal reactor for reaction, washing the solid product, and then magnetically separating it to obtain the magnetic nano-Fe3O4. 4。 9. The method for preparing the electrochemical ammonia synthesis catalyst as described in claim 8, characterized in that: The mass ratio of the iron salt, alkali source, stabilizer, and solvent is 16 : (6~18) : (30~60) : 555; and / or, The hydrothermal conditions are a reaction at 180~220℃ for 7~8 hours.

10. The application of the electrochemical ammonia synthesis catalyst according to any one of claims 1 and 2 in the electrocatalytic reduction of nitrate to produce ammonia.

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