A rough carbon shell coated with Fe 3 O 4 Catalyst preparation methods and their application in the electrocatalytic reduction of nitrate to ammonia

Abstract

This invention provides a rough carbon shell coated with Fe 3 O 4 The preparation method of the catalyst and its application in the electrocatalytic reduction of nitrate to ammonia. The preparation method includes the electrocatalytic reduction of Fe at room temperature. 3 O 4 Nanospheres or their dispersions, phenol, aldehyde solutions, fatty amines, and ammonia are added to a solvent and stirred to induce a reaction. After the reaction, the mixture is heated for aging. Following magnetic separation, washing, drying, and carbonization, a rough carbon shell coated with Fe is obtained. 3 O 4 Catalyst; Phenol: Fe 3 O 4 The molar ratio of nanospheres is 1:(0.5–1.5); the concentration of the aldehyde solution is 10–85 mM, and the addition rate is 0.5–2 mL / min. This invention rapidly prepares rough carbon shell-coated Fe using a simple solution synthesis method. 3 O 4 The catalyst, the conductivity of the carbon material shell and its ability to adsorb and enrich substrates enhance the Fe core. 3 O 4 The catalytic activity of nanoparticles; due to Fe 3 O 4 The synergistic effect of nanoparticles and a rough carbon shell coating enabled the prepared sample to effectively electrocatalytically reduce nitrate to ammonia.

Description

A method for preparing a rough carbon shell-coated Fe3O4 catalyst and its application in the electrocatalytic reduction of nitrate to ammonia. Technical Field

[0001] This invention relates to the field of electrocatalytic reduction of nitrate to ammonia technology, and more specifically, to a method for preparing a rough carbon shell coated Fe3O4 catalyst and its application in electrocatalytic reduction of nitrate to ammonia. Background Technology

[0002] NO3 - Nitrate is one of the main nitrogen pollutants in groundwater. The accumulation of nitrate in the environment leads to an increase in the concentration of toxic nitrite, which is converted from nitrate, thus causing health problems for organisms. Electrochemical nitrate reduction technology, as an emerging denitrification method, shows great promise for application. This technology can not only remove nitrate from water and solve the problem of nitrate pollution in water bodies, but also selectively generate ammonia, which can serve as an important industrial nitrogen source and energy carrier. Therefore, the development of efficient electrocatalytic nitrate reduction to ammonia synthesis technology is of great significance.

[0003] Currently reported NO3 - Electrocatalytic reduction catalysts are mainly classified into three categories: non-metallic carbon catalysts, transition metal catalysts, and noble metal catalysts. Among these, non-metallic carbon catalysts have relatively low ammonia production efficiency, while noble metal catalysts, although having higher electrocatalytic ammonia synthesis efficiency, are limited in their large-scale industrial application due to their high price and scarcity of resources. Transition metals are inexpensive, and their d-orbital structure and electronic states are favorable for NO3 production. - The adsorption and activation of Fe catalysts. Fe-based catalysts have been widely used as highly efficient catalysts for oxygen reduction reactions under alkaline conditions. It has been reported that iron catalysts with multiple mixed valence states (such as Fe) 0 Fe 2+ and Fe 3+ Fe can selectively reduce nitrates to nitrites, ammonia, or nitrogen. For example, under alkaline conditions, Fe(OH)₂ can selectively reduce nitrates to ammonia. Soils and sediments containing Fe(II)–Fe(III) hydroxides or ferrous sulfate can reduce nitrites to nitrous oxide and ammonia. Under lower pH conditions, nano-zero-valent iron can achieve a chemical conversion rate of 95–100% for nitrates. These studies indicate that Fe-based electrocatalysts have the potential to replace noble metal catalysts in the electrochemical reduction of nitrates to ammonia. However, the d-orbital electrons of transition metals also favor the formation of metal-H bonds, causing a severe competitive hydrogen evolution reaction (HER), thus leading to poor selectivity in the electrocatalytic nitrogen fixation ammonia reaction. The electrocatalytic reduction of nitrates to ammonia is a heterogeneous process occurring at the electrode surface; therefore, electrode materials, structures, or their interface modifications and controls can all affect catalytic performance and selectivity. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a method for preparing a coarse carbon shell-coated Fe3O4 catalyst and its application in the electrocatalytic reduction of nitrate to ammonia. A coarse carbon shell-coated Fe3O4 nanocomposite material is rapidly prepared using a simple solution synthesis method. The conductivity and substrate adsorption and enrichment capacity of the carbon shell enhance the catalytic activity of the core Fe3O4 nanoparticles. Due to the synergistic effect of the Fe3O4 nanoparticles and the coarse carbon shell coating, the prepared sample can effectively electrocatalytically reduce nitrate to ammonia.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A method for preparing a rough carbon shell coated Fe3O4 catalyst includes the following steps:

[0007] (1) Fe3O4 nanospheres or their dispersion, phenol, aldehyde solution, fatty amine and ammonia water are added to a solvent at room temperature and stirred to react. After the reaction, the temperature is raised to age the nanospheres. After magnetic separation, washing and drying, rough polymer-coated Fe3O4 nanocomposite microspheres are obtained. The molar ratio of phenol to Fe3O4 nanospheres is 1:(0.5 to 1.5). The concentration of the aldehyde solution is 10 to 85 mM and the addition rate is 0.5 to 2 mL / min.

[0008] (2) The Fe3O4 nanocomposite microspheres coated with polymer were carbonized to obtain a rough carbon shell coated Fe3O4 catalyst.

[0009] The Fe3O4 nanospheres have a particle size of 200–500 nm.

[0010] The molar ratio of phenol: aldehyde: fatty amine is 1:2:(0.1~1).

[0011] The preparation method of the Fe3O4 nanospheres includes dissolving ferric chloride hexahydrate, sodium citrate dihydrate, and sodium acetate in ethylene glycol, stirring thoroughly until a clear solution is obtained, and keeping the solution at 180-220℃ for 8-12 hours in a high-pressure reactor. After magnetic separation and washing, Fe3O4 nanospheres are obtained. The molar ratio of ferric chloride hexahydrate: sodium citrate dihydrate: sodium acetate: ethylene glycol is 1:(0.17-0.35):(36-70):89.5.

[0012] The carbonization temperature is 500–800℃, and the carbonization time is 2–3 hours. The specific carbonization method is as follows: under the protection of an inert gas, the temperature is increased from room temperature to 150℃ at a rate of 1–3℃ / min, and held at this temperature for 60 minutes to volatilize small molecule components such as water; then the temperature is increased to the final carbonization temperature of 500–800℃ at a rate of 1–5℃ / min, and held at this temperature for 2–3 hours.

[0013] The solvent is one or more of water, methanol, ethanol, and ethylene glycol.

[0014] The fatty amine is any one or more of ethylamine, ethylenediamine, propylamine, hexylamine, and hexamethylenediamine.

[0015] The phenol is one or more of phenol, resorcinol, phloroglucinol, and bisphenol A.

[0016] The aldehyde is one or more of benzaldehyde, glyoxal, butyraldehyde, glutaraldehyde, and 37 wt.% formaldehyde.

[0017] The reaction time is 2 to 4 hours.

[0018] The aging temperature is 60–90℃, and the aging time is 4–20 hours.

[0019] The present invention also provides the application of the aforementioned rough carbon shell coated Fe3O4 catalyst in the electrocatalytic reduction of nitrate to ammonia.

[0020] The electrocatalytic reduction of nitrate is carried out in an H-type two-chamber reaction cell, with a proton exchange membrane separating the cathode chamber and the anode chamber. In the cathode chamber, a hydrophobic carbon paper coated with the catalyst is used as the working electrode, and a KCl-saturated Hg / HgO electrode is used as the reference electrode. In the anode chamber, a platinum sheet is used as the counter electrode. A constant voltage is applied to cause nitrate in the cathode chamber solution to be reduced to ammonia through an electrochemical reduction reaction.

[0021] The beneficial effects of this invention are as follows:

[0022] (1) The catalyst prepared by this invention is a carbon composite non-precious metal catalyst. It utilizes clean electrical energy for electrocatalytic nitrate reduction under ambient temperature and pressure conditions, exhibiting excellent Faradaic efficiency and ammonia yield. Compared with previously reported precious metal catalysts, this method helps reduce production costs and offers greater economic benefits. Furthermore, it boasts the advantages of being green, environmentally friendly, clean, and sustainable.

[0023] (2) By controlling the rate at which polymer monomers are added to the reaction solution, thereby controlling the polymerization rate, a rough polymer-coated Fe3O4 composite material was successfully prepared. By adjusting the monomer concentration, the polymer layer thickness can be controlled, and Fe3O4@sC materials with partial Fe3O4 coating can be prepared.

[0024] (3) The ammonia production Faraday efficiency of the present invention reaches 99.7%, and the Faraday efficiency can be maintained at more than 95% in long-term stable operation experiments, which can be adapted to a variety of wastewater environment systems.

[0025] (4) The improvement in catalytic performance mainly stems from the synergistic effect of Fe3O4 and the carbon coating layer. The carbon layer alternates with the exposed active sites on the Fe3O4 surface to form a network structure, which effectively promotes charge transport. The carbon layer also enhances the catalytic effect by inhibiting the competitive hydrogen evolution reaction and helping to improve the selectivity of ammonia production. Attached Figure Description

[0026] Figure 1 is a SEM image of Fe3O4@-Polymer prepared in Comparative Example 2;

[0027] Figure 2 is a SEM image of the Fe3O4@sC-1 nanospheres prepared in Example 1;

[0028] Figure 3 is a TEM image of the Fe3O4@sC-1 nanospheres prepared in Example 1;

[0029] Figure 4 shows the electrocatalytic ammonia production rate of catalysts with different carbon shell thicknesses in Example 7;

[0030] Figure 5 shows the electrocatalytic ammonia production cycle stability test of the catalyst in Example 9. Detailed Implementation

[0031] To facilitate understanding of the present invention, a more comprehensive description will be provided below. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the present invention.

[0032] The sources of raw materials used in the nanopolymer-coated Fe3O4 and carbon-coated Fe3O4 nanospheres of this invention are shown in Table 1.

[0033] Table 1. Sources of raw materials and equipment used in this invention

[0034]

[0035]

[0036] Comparative Example 1

[0037] 3.25 g of ferric chloride hexahydrate and 1.2 g of sodium citrate dihydrate were added to 60 mL of ethylene glycol and stirred for 2 hours to obtain a clear solution. Then, 6 g of anhydrous sodium acetate was dissolved in 40 mL of ethylene glycol. The ethylene glycol solution containing sodium acetate was then added dropwise to the ethylene glycol dispersion of ferric chloride hexahydrate and sodium citrate, followed by vigorous stirring for 40 minutes. The solution was transferred to a 200 mL polytetrafluoroethylene-sealed autoclave and solvated at 200 °C for 10 hours. After cooling to room temperature, the obtained black product was collected using an external magnet and washed three times alternately with water and ethanol to obtain Fe3O4 nanospheres with a particle size of 220 nm.

[0038] The Fe3O4 nanospheres used in the subsequent comparative examples and examples were all obtained by the above method.

[0039] Comparative Example 2

[0040] Weigh 132 mg of resorcinol and place it in a 250 mL round-bottom flask. Add 200 mL of a mixed solution of deionized water and ethanol (water / ethanol = 1 1 / 9) and dissolve thoroughly. Take a dispersion containing 100 mg of Fe3O4 nanospheres with a particle size of 220 nm and add it to the above round-bottom flask. Continue stirring and dissolving for 10 min. Add 300 μL of n-propylamine aqueous solution (1.0 M) and 30 μL of ammonia water (commercially available) catalyst. Then add 176 μL of formaldehyde aqueous solution (37 wt.%) to the reaction system and stir for 1 h to obtain a white colloidal solution. Heat the obtained white colloidal solution to 80 °C and stir for 4 h. After the reaction is complete, separate the product by magnetic attraction, wash with deionized water, and dry the product to obtain monodisperse polymer-coated Fe3O4 nanospheres. The polymer-coated Fe3O4 nanospheres obtained in this embodiment have a diameter of 270±20 nm and a polymer layer thickness of 50±20 nm, denoted as Fe3O4@-Polymer. Their scanning electron microscope image is shown in Figure 1. The polymer-coated Fe3O4 nanospheres were carbonized in a carbonization furnace under argon protection, with the temperature increased from room temperature to 150 °C at a rate of 3 °C / min and held for 60 min. Then, the temperature was increased from 150 °C to the final carbonization temperature of 600 °C at a rate of 5 °C / min and held for 120 min, yielding carbon-coated Fe3O4 nanospheres, denoted as Fe3O4@C.

[0041] Example 1

[0042] Weigh 66 mg of resorcinol and add it to a 250 mL round-bottom flask, then add 190 mL of deionized water to dissolve it completely. Add a dispersion containing 200 mg of Fe3O4 nanospheres with a particle size of 220 nm to the same flask and stir to dissolve for 10 min. Add 150 μL of 1.0 M n-propylamine aqueous solution and 30 μL of commercially available ammonia catalyst. Dilute 88 μL of 37 wt.% formaldehyde aqueous solution to 5 mL and add it dropwise (1 mL / min) to the reaction system, stirring for 1 h to obtain a white colloidal solution. Heat the white colloidal solution to 80 °C and stir for 4 h. After the reaction is complete, separate the product using a magnet, wash with deionized water, and dry the product to obtain monodisperse, rough polymer-coated Fe3O4 nanospheres. The polymer-coated Fe3O4 nanospheres obtained in this example have a diameter of 240 ± 5 nm and a polymer layer thickness of 10 ± 5 nm. It is denoted as Fe3O4@s-Polymer-1.

[0043] Example 2

[0044] The nanopolymer prepared in Example 1 was carbonized in a carbonization furnace. Under argon protection, the temperature was increased from room temperature to 150°C at a rate of 3°C / min and held for 60 min. Then, the temperature was increased from 150°C to the final carbonization temperature of 600°C at a rate of 5°C / min and held for 120 min, resulting in Fe3O4 nanospheres coated with a rough carbon layer that retained their morphology. The diameter of the rough carbon-coated Fe3O4 nanospheres obtained in this example was 230±5 nm, and the carbon layer thickness was 7±4 nm. This was denoted as Fe3O4@sC-1. Its scanning electron microscope image is shown in Figure 2, and its TEM image is shown in Figure 3.

[0045] Example 3

[0046] Weigh 132 mg of resorcinol and place it in a 250 mL round-bottom flask, then add 190 mL of deionized water to dissolve it completely. Add a dispersion containing 100 mg of Fe3O4 nanospheres with a particle size of 220 nm to the same flask and stir for 10 min to dissolve. After the resorcinol is fully dissolved, add 300 μL of 1.0 M n-propylamine aqueous solution and 30 μL of commercially available ammonia catalyst. Dilute 176 μL of 37 wt.% formaldehyde aqueous solution to 5 mL and add it dropwise to the reaction system (1 mL / min). Stir for 1 h to obtain a white colloidal solution. Heat the white colloidal solution to 80 °C and stir for 4 h. After the reaction is complete, separate the product using a magnet, wash with deionized water, and dry the product to obtain monodisperse, rough polymer-coated Fe3O4 nanospheres. The polymer-coated Fe3O4 nanospheres obtained in this embodiment have a diameter of 250±10 nm and a polymer layer thickness of 25±5 nm, and are designated Fe3O4@s-Polyme-2. After carbonization according to the steps in Example 2, they are denoted as Fe3O4@sC-2.

[0047] Example 4: Electrocatalytic nitrate reduction for ammonia production test procedures

[0048] (1) Electrode preparation: In the electrolysis and cyclic voltammetry experiments, electrodes were prepared on carbon paper (Toray TGPH060F hydrophobic carbon paper from Japan) using a drop-feed method. In a 3 mL sample tube, 6 mg of the samples obtained in Comparative Examples 1-2 and Examples 1-3 were dispersed in a certain volume of a mixture of 5 wt% Nafion, deionized water, and anhydrous ethanol, and sonicated for 20 min to form a catalyst slurry. The slurry was then carefully and evenly dripped onto a 1.0 × 1.0 cm plate using a micropipette. 2 The carbon paper was placed on a carbon paper substrate. After adding 100 μL, the carbon paper was dried in an oven at 50 °C for 1 hour. The blank electrode consisted only of carbon paper and was prepared according to the same procedure.

[0049] (2) Electrochemical reduction of NO3 - Setup: Electrochemical performance testing was conducted in an H-type battery using a three-electrode configuration on an Ivium electrochemical workstation in the Netherlands. The electrode from step (1) above was used as the working electrode, and the mercury oxide electrode was used as the reference electrode. Platinum foil (1.0 × 1.0 cm) was used. 2 ) as the counter electrode. Add 50 mL of 0.1 M KOH (containing 0.1 M NO3) to the cathode chamber. -) solution, add another 50 mL of 0.1 MkOH solution to the anode chamber. A pretreated Nafion 117 proton exchange membrane (DuPont) was used as the separator. Before electrochemical testing, Nafion 117 was heated in 5% H₂O₂ aqueous solution at 80 °C for one hour, then soaked in deionized water for half an hour; then heated in 5% wtH₂SO₄ at 80 °C for one hour, then soaked in deionized water for half an hour. Before testing, linear sweep voltammetry (LSV) was performed at a scan rate of 5 mV / s until the polarization curve stabilized. Then, potentiostatic tests were performed at different potentials for 0.5 hours. For stability testing, six cycles were performed using the same working electrode at the optimized potential. A new electrolyte was used for each cycle, while other parameters remained constant.

[0050] (3) NH3 Concentration Detection: Nessler's reagent method was used to detect ammonia nitrogen. First, a certain amount of electrolyte was taken from the reactor and diluted to 5 mL. Next, 0.1 mL of sodium tartrate solution and 0.1 mL of Nessler's reagent were added to the above solution. After standing for 20 min, the absorbance at 420 nm was detected by ultraviolet-visible spectrophotometry. Standard Curve Plotting: Ammonium chloride was used as the ammonia source, and a standard curve was plotted by preparing a series of standard solutions of different concentrations.

[0051] Example 5

[0052] The electrocatalytic reduction of NO3 by Fe3O4 nanospheres in Comparative Example 1 and the materials in Comparative Examples 2, 2, and 3 were determined according to the test method of Example 4. - The performance of the reaction was tested at a potential of -1.365 V vs. HgO, and the reaction performance is shown in Figure 4. The activity of the Fe3O4 nanoparticles themselves is not high, with a Faradaic efficiency of only 71.41% for ammonia. This is because although Fe3O4 nanoparticles can be directly exposed, they lack the conductivity and substrate adsorption and enrichment capabilities of carbon materials, resulting in low electrocatalytic activity. In contrast, Fe3O4@sC-1 coated with a thin layer of rough carbon shell exhibits the highest ammonia production rate, with a Faradaic efficiency reaching an astonishing 99.17%. Conversely, the thicker the carbon shell on the outside of Fe3O4, the lower the ammonia production efficiency. Testing of Fe3O4@C revealed a decreased ammonia production rate and a Faradaic efficiency of 70.98%. Increased carbon shell thickness leads to the covering of the catalytically active Fe3O4 sites, resulting in reduced reaction activity.

[0053] Example 6

[0054] The electrocatalytic reduction of NO3 in the material from Example 2 was determined according to the test method of Example 4. -To assess the cycling stability, Fe3O4@sC-1 was continuously tested under constant current at -1.365V vs. HgO. The performance graph is shown in Figure 5. The test conditions remained consistent with previous tests, with six consecutive cycles of 0.5 hours each. The ammonia production efficiency showed almost no decline. - The ammonia production rate remained high even after six consecutive recovery cycles, with an NH3 selectivity of 97.21%.

[0055] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0056] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for preparing a Fe3O4 catalyst coated with a rough carbon shell, characterized in that: The process includes the following steps: (1) Fe3O4 nanospheres or their dispersion, phenol, aldehyde solution, fatty amine and ammonia are added to a solvent at room temperature and stirred to react. After the reaction, the temperature is raised to age the nanospheres. After magnetic separation, washing and drying, a rough polymer-coated Fe3O4 nanocomposite microspheres are obtained. The molar ratio of phenol to Fe3O4 nanospheres is 1:(0.5 ~ 1.5). The concentration of the aldehyde solution is 10 ~ 85 mM and the addition rate is 0.5 ~ 2 mL / min. (2) The rough polymer-coated Fe3O4 nanocomposite microspheres are carbonized to obtain a rough carbon shell-coated Fe3O4 catalyst. The molar ratio of phenol to aldehyde to fatty amine is 1:2:(0.1 ~ 1).

2. The method for preparing a rough carbon shell coated Fe3O4 catalyst as described in claim 1, characterized in that: The Fe3O4 nanospheres have a particle size of 200 ~ 500 nm.

3. The method for preparing a rough carbon shell coated Fe3O4 catalyst as described in claim 1 or 2, characterized in that: The preparation method of the Fe3O4 nanospheres includes dissolving ferric chloride hexahydrate, sodium citrate dihydrate, and sodium acetate in ethylene glycol, stirring thoroughly until a clear solution is obtained, and keeping the solution in a high-pressure reactor at 180-220 °C for 8-12 h. After magnetic separation and washing, Fe3O4 nanospheres are obtained. The molar ratio of ferric chloride hexahydrate: sodium citrate dihydrate: sodium acetate: ethylene glycol is 1:(0.17-0.35):(36-70):89.

5.

4. The method for preparing a rough carbon shell coated Fe3O4 catalyst as described in claim 1, characterized in that: The carbonization temperature is 500~800 ℃, and the carbonization time is 2~3 h.

5. The method for preparing a rough carbon shell coated Fe3O4 catalyst as described in claim 1, characterized in that: The solvent is one or more of water, methanol, ethanol, and ethylene glycol.

6. The method for preparing a rough carbon shell coated Fe3O4 catalyst as described in claim 1, characterized in that: The phenol is one or more of phenol, resorcinol, phloroglucinol, or bisphenol A; the aldehyde is one or more of benzaldehyde, glyoxal, butyraldehyde, glutaraldehyde, and 37 wt.% formaldehyde; the fatty amine is any one or more of ethylamine, ethylenediamine, propylamine, hexylamine, and hexamethylenediamine.

7. The method for preparing a rough carbon shell coated Fe3O4 catalyst as described in claim 1, characterized in that: The reaction time is 2 to 4 hours.

8. The method for preparing a rough carbon shell coated Fe3O4 catalyst as described in claim 1, characterized in that: The aging temperature is 60 ~ 90 ℃, and the aging time is 4 ~ 20 h.

9. The application of the rough carbon shell coated Fe3O4 catalyst according to claim 1 in the electrocatalytic reduction of nitrate to ammonia.

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