A method for preparing a cobalt tetroxide / copper oxide-cuprous oxide composite catalyst for the electroreduction of nitrates to ammonia.

Abstract

This invention discloses a method for preparing a cobalt tetroxide / copper oxide-cuprous oxide composite catalyst for electro-reduction of nitrate to ammonia, comprising the following steps: (1) adding sodium hydroxide and ammonium persulfate to water to obtain solution A; (2) immersing pretreated copper foam in solution A to obtain copper foam with copper hydroxide on its surface; removing the copper foam from solution A, washing and drying it; (3) adding cobalt salt and urea to water to obtain solution B; (4) placing the copper foam with copper hydroxide on its surface from step (2) into solution B and performing a hydrothermal reaction at high temperature; washing and drying the solid after the reaction; (5) calcining the dried product at 320~360℃ for 1.5~2h to obtain the cobalt tetroxide / copper oxide-cuprous oxide composite catalyst. The composite catalyst prepared by the method of this invention has good catalytic activity and can achieve excellent ammonia Faradaic efficiency and high ammonia yield when electrocatalyzing nitrate reduction reaction under a specific window voltage.

Description

Technical Field

[0001] This invention relates to a method for preparing a cobalt tetroxide / copper oxide-cuprous oxide composite catalyst for the electroreduction of nitrates to ammonia. Background Technology

[0002] Due to the crucial role of ammonia in fertilizer production and its potential as a carbon-free energy carrier, global demand for ammonia continues to grow, driving research into sustainable alternatives to the energy-intensive Haber-Bosch process. Although nitrogen is a traditional feedstock, its low solubility in aqueous solution and the high breaking energy of the N≡N bond significantly limit reaction kinetics. In contrast, alternative nitrogen sources such as nitrates and nitrous oxide offer greater advantages for ammonia formation kinetics due to their higher solubility in water and the lower breaking energy of the N≡O bond in nitrate. Therefore, the electrocatalytic nitrate reduction reaction (NO₃RR) is considered a potential pathway for ammonia synthesis under environmental conditions, while also potentially mitigating nitrate pollution in water bodies.

[0003] Existing catalysts reported for electrocatalytic nitrate reduction reactions include single cobalt-based catalysts. Although their unique electronic structure holds great potential for electrocatalytic nitrate reduction reactions, their practical application is still limited by undesirable reaction pathways: firstly, their strong adsorption of reaction intermediates can easily lead to active site poisoning, thereby reducing catalytic stability; secondly, at high reduction potentials, competition from the hydrogen evolution reaction significantly weakens the ammonia Faraday efficiency; and finally, cobalt's insufficient ability to hydrogenate nitrite, a key intermediate in the deoxygenation step, leads to the easy accumulation of byproducts, thereby inhibiting the selectivity of ammonia formation and reducing the yield. Summary of the Invention

[0004] Purpose of the invention: The purpose of this invention is to provide a method for preparing a cobalt tetroxide / copper oxide-cuprous oxide composite catalyst. The composite catalyst prepared by this method has good catalytic activity and can achieve excellent ammonia Faradaic efficiency and high ammonia yield when electrocatalyzing nitrate reduction reaction under a specific window voltage.

[0005] Technical solution: The preparation method of the cobalt tetroxide / copper oxide-cuprous oxide composite catalyst of the present invention includes the following steps:

[0006] (1) Add sodium hydroxide and ammonium persulfate to water (ammonium persulfate acts as an oxidant in the system, and its extremely strong oxidizing driving force can oxidize metallic copper to Cu under mild conditions). 2+ ), to obtain solution A;

[0007] (2) Immerse the pretreated copper foam in solution A to obtain copper foam with copper hydroxide on the surface; remove the copper foam from solution A, clean and dry it;

[0008] (3) Add cobalt salt and urea to water to obtain solution B;

[0009] (4) Place the copper foam with copper hydroxide generated on the surface in step (2) into solution B and perform a hydrothermal reaction at high temperature; clean and dry the solid after the reaction.

[0010] (5) The dried product was calcined at 320~360℃ for 1.5~2h to obtain the cobalt tetroxide / copper oxide-cuprous oxide composite catalyst.

[0011] In step (1), the mass ratio of sodium hydroxide to ammonium persulfate is 4~5:1.375~1.425.

[0012] In step (2), the soaking time is 5 to 10 minutes; after the copper foam is taken out of solution A, it is rinsed 2 to 3 times with anhydrous ethanol and ultrapure water, and then dried in a vacuum oven at 60 to 65°C for 6 to 9 hours.

[0013] The pretreatment of the foamed copper is specifically as follows: the foamed copper is sequentially immersed in a mixture of acetone and ethanol with a volume ratio of 1:1, 1.0 mol / L hydrochloric acid, and ultrapure water, and ultrasonically treated for 8 to 15 minutes each time to remove the oily substances and oxide layer on the surface.

[0014] In step (3), the cobalt salt is cobalt nitrate hexahydrate; the mass ratio of cobalt nitrate hexahydrate to urea is 1:1~1.1; the cobalt salt and urea are added to water and stirred at 30~40℃ for 60~80 minutes to obtain solution B (stirring at slightly above room temperature accelerates dissolution and degassing, and the stirring time is long. Because the solid added quickly is prone to forming local high-concentration clumps, long-term stirring ensures uniform mixing and dispersion, and avoids uneven mixing leading to local nucleation in subsequent hydrothermal reactions, resulting in uneven particles).

[0015] In step (4), the hydrothermal reaction temperature is 100~120℃ and the reaction time is 10~12 hours. The solid is first ultrasonically cleaned three times with anhydrous ethanol and ultrapure water respectively, and then dried in a vacuum oven at 60~65℃ for 15~20 hours.

[0016] In step (5), the heating rate is 2~2.5℃ / min; the cobalt tetroxide / copper oxide-cuprous oxide composite catalyst exhibits a layered nanoflower-like structure, the formation of which is the result of multi-stage synergistic growth. The urea decomposition rate at 100℃ in the hydrothermal reaction is related to the Co... 2+The release rate is matched to achieve slow anisotropic growth; excessively high temperatures lead to rapid isotropic deposition, forming a dense coating layer rather than an open petal structure; simultaneously, moderate temperatures during calcination cause dehydration shrinkage, resulting in nanoscale roughness (nanocrystalline protrusions) on the petal surface, further increasing the specific surface area of ​​the material without destroying the flower-like structure; if the calcination temperature is too high, the petals will sinter and pulverize, while if the temperature is too low, structural water will remain, resulting in poor thermal stability. The layered nanoflower structure has an ultra-high specific surface area, hierarchical pore structure, exposed high-index crystal planes and heterojunction interfaces, giving it multiple active sites, fast mass transfer, and high intrinsic activity when used as a catalyst for electrocatalytic nitrate reduction, thus achieving good ammonia Faradaic efficiency and ammonia yield. In the system of this invention, the coexistence of CuO and Cu2O is mainly achieved by controlling the calcination temperature (350℃). This temperature is at the initiation window of CuO thermal decomposition, where surface oxygen is easily removed to generate Cu2O, while bulk lattice oxygen diffusion is hindered, retaining CuO, forming a gradient structure rich in Cu2O on the surface and CuO in the bulk.

[0017] The cobalt tetroxide / copper oxide-cuprous oxide composite catalyst prepared by the above method is used as a catalyst in the electrocatalytic reduction of nitrate to ammonia reaction. The specific application process is as follows: In an H-type battery, the electrocatalytic system adopts a three-electrode system, with a platinum sheet electrode as the counter electrode, a silver chloride electrode as the reference electrode, and the catalyst as the working electrode; an electrolyte containing potassium nitrate and potassium hydroxide is added to both the cathode and anode reaction cells. The concentration of potassium nitrate in the electrolyte is 0.1~0.12 mol / L, and the concentration of potassium hydroxide is 1.0~1.2 mol / L; the three-electrode system is placed in the above electrolyte, and the electrocatalytic reduction reaction of nitrate is carried out under argon saturation conditions based on linear sweep voltammetry; wherein the potential range of the linear sweep voltammetry is -1.5~0.5V, preferably -1.1~-0.6V, and more preferably -1.0V.

[0018] This invention introduces copper to regulate the electronic state and reaction pathway of cobalt: copper sites enhance nitrate capture and deoxygenation efficiency, and cobalt sites enhance water dissociation / hydrolysis to generate and enrich the required *H, thereby providing a stable proton flux for subsequent hydrogenation of nitrogen intermediates, ultimately improving ammonia generation efficiency and increasing the selectivity of the target product.

[0019] Beneficial Effects: Compared with the prior art, the present invention has the following significant advantages: The layered nanoflower-like cobalt tetroxide / copper oxide-cuprous oxide composite catalyst prepared by the present invention has good electrocatalytic activity. Based on the layered nanoflower morphology, abundant oxygen vacancies, and modulated electronic structure caused by strong Co-Cu interfacial interactions, the composite catalyst of the present invention can effectively accelerate the key reactions of *NO2 and *NH4+. xThe conversion of intermediates provides more protons for ammonia formation; the cobalt tetroxide / copper oxide-cuprous oxide composite catalyst of this invention can achieve an ammonia Faradaic efficiency of over 98% and 25.89 mg / h / cm under -1.0 V vs. RHE conditions. 2 ammonia yield. Attached Figure Description

[0020] Figure 1 This is a scanning electron microscope image of the cobalt tetroxide / copper oxide-cuprous oxide composite catalyst of Example 1;

[0021] Figure 2 The X-ray diffraction pattern of the cobalt tetroxide / copper oxide-cuprous oxide composite catalyst of Example 1 is shown below.

[0022] Figure 3 The diagram shows the ammonia Faraday efficiency of the cobalt tetroxide / copper oxide-cuprous oxide composite catalyst and the copper oxide-cuprous oxide composite catalyst in Example 1 at various potentials under argon conditions in the electrocatalytic reduction of nitrate to ammonia reaction. Detailed Implementation

[0023] Example 1

[0024] The preparation method of the cobalt tetroxide / copper oxide-cuprous oxide composite catalyst of the present invention includes the following steps:

[0025] (1) Pretreatment of copper foam: The copper foam was immersed in a mixture of acetone and ethanol with a volume ratio of 1:1, hydrochloric acid with a volume ratio of 1.0 mol / L, and ultrapure water in sequence, and ultrasonic treatment was performed for 10 minutes each time to remove the oily substances and oxide layer on the surface.

[0026] (2) Add 5.0 g of sodium hydroxide and 1.425 g of ammonium persulfate to 50 mL of ultrapure water and mix for 10 minutes under magnetic stirring to obtain solution A;

[0027] (3) Immerse the pretreated copper foam in solution A for 10 minutes. The solution turns light blue and a layer of blue copper hydroxide can be seen on the surface of the copper foam. Take the copper foam with copper hydroxide on the surface out of solution A, rinse it three times each with anhydrous ethanol and ultrapure water, and dry it in a vacuum oven at 60°C for 8 hours.

[0028] (4) Add 3.49 g of cobalt nitrate hexahydrate and 3.60 g of urea to 56 mL of ultrapure water, heat the solution to 30 °C and stir for 60 minutes to obtain solution B;

[0029] (5) Load solution B into a Teflon stainless steel reactor, place the foamed copper from step (3) into solution B, heat in an oven at 100°C for 12 hours to carry out hydrothermal reaction, and then cool naturally to room temperature after the reaction.

[0030] (6) The product after hydrothermal reaction was ultrasonically washed three times with anhydrous ethanol and ultrapure water respectively, and dried in a vacuum oven at 60°C for 15 hours.

[0031] (7) The product dried in step (7) was calcined in a tube furnace at 350°C for 2 hours with a heating rate of 2°C / min to obtain a layered nanoflower-like cobalt tetroxide / copper oxide-cuprous oxide composite.

[0032] pass Figure 1 It can be seen that the cobalt tetroxide / copper oxide-cuprous oxide composite obtained by this invention exhibits a layered nanoflower-like structure. Through... Figure 2 It can be seen that the composite obtained in Example 1 is composed of cobalt tetroxide, copper oxide and cuprous oxide.

[0033] The application of copper oxide-cuprous oxide and the cobalt tetroxide / copper oxide-cuprous oxide composite prepared in Example 1 as catalysts in the electrocatalytic reduction of nitrate to ammonia reaction is as follows:

[0034] The products were analyzed using colorimetric methods: Ammonia was quantitatively analyzed using indoblue ultraviolet-visible absorption spectroscopy. Standard curves were established using ammonia standard solutions with mass concentrations of 0, 0.5, 1.0, 2.0, 3.0, 4.0, and 5.0 ppm. A certain volume of the test reaction solution or its dilution was added to a cuvette, and the absorbance was measured at a preset wavelength of 655 nm. The concentration of ammonia in the test sample was calculated from the standard curve. Nitrite was quantitatively analyzed using Grise ultraviolet-visible absorption spectroscopy. Standard curves were established using nitrite standard solutions with mass concentrations of 0, 0.2, 0.4, 0.8, 1.0, and 2.0 ppm. A certain volume of the test reaction solution or its dilution was added to a cuvette, and the absorbance was measured at a preset wavelength of 540 nm. The concentration of nitrite in the test sample was calculated from the standard curve.

[0035] A proton exchange membrane separates the cathode and anode of the H-type battery to prevent cross-contamination of products. The electrocatalytic system employs a traditional three-electrode system: a platinum sheet electrode as the counter electrode, a silver chloride electrode as the reference electrode, and a catalyst as the working electrode. Before the electrocatalytic experiment, the cathode cell was aerated with argon gas at a flow rate of 50 sccm for 30 minutes, and the flow rate was maintained at 30 sccm during the experiment (the argon gas flow rate during the electrocatalytic experiment was 30 sccm). 60 mL of electrolyte containing potassium nitrate and potassium hydroxide was added to both the cathode and anode cells; the concentration of potassium nitrate in the electrolyte was 0.1 mol / L, and the concentration of potassium hydroxide was 1.0 mol / L.

[0036] The three-electrode system was placed in the above-mentioned electrolyte. Under argon saturation, the electrocatalytic reduction of nitrate to ammonia was carried out using linear sweep voltammetry (the potential range of linear sweep voltammetry is -1.1 to -0.6 V). Electrolysis was performed for 1 hour at each potential. The electrolyte was then removed, and the concentrations of nitrite and ammonia in the electrolyte after the reaction at each electrolysis potential were obtained by using the colorimetric method mentioned above. The ammonia Faradaic efficiency of each catalyst for the electrocatalytic reduction of nitrate was further calculated. The specific results are shown in Table 1.

[0037] Table 1

[0038]

[0039] pass Figure 3 Copper oxide-cuprous oxide and cobalt tetroxide / copper oxide-cuprous oxide produce NH3 and the byproduct NO2. - A comparison of the Faraday efficiency shows that the cobalt tetroxide / copper oxide-cuprous oxide (Co3O4 / Cu) prepared in this invention has the best Faraday efficiency. x O) While efficiently generating NH3, it also effectively inhibits NO2. - The accumulation of these technologies has demonstrated their excellent selective regulatory capabilities.

Claims

1. A method for preparing a cobalt tetroxide / copper oxide-cuprous oxide composite catalyst for the electroreduction of nitrate to ammonia, characterized in that, Includes the following steps: (1) Sodium hydroxide and ammonium persulfate are added to water to obtain solution A; (2) Immerse the pretreated copper foam in solution A to obtain copper foam with copper hydroxide on the surface; remove the copper foam from solution A, clean and dry it; (3) Add cobalt salt and urea to water to obtain solution B; (4) Place the copper foam with copper hydroxide generated on the surface in step (2) into solution B and perform a hydrothermal reaction at high temperature; clean and dry the solid after the reaction. (5) The dried product was calcined at 320~360℃ for 1.5~2h to obtain the cobalt tetroxide / copper oxide-cuprous oxide composite catalyst.

2. The preparation method of the cobalt tetroxide / copper oxide-cuprous oxide composite catalyst according to claim 1, characterized in that: In step (1), the mass ratio of sodium hydroxide to ammonium persulfate is 4~5:1.375~1.

43.

3. The preparation method of the cobalt tetroxide / copper oxide-cuprous oxide composite catalyst according to claim 1, characterized in that: In step (2), the soaking time is 5 to 10 minutes; the drying temperature is 60 to 65°C and the drying time is 6 to 9 hours.

4. The preparation method of the cobalt tetroxide / copper oxide-cuprous oxide composite catalyst according to claim 1, characterized in that: In step (3), the cobalt salt is cobalt nitrate hexahydrate; the mass ratio of the mixed nitric acid hexahydrate and urea is 1:1~1.

2.

5. The preparation method of the cobalt tetroxide / copper oxide-cuprous oxide composite catalyst according to claim 1, characterized in that: In step (3), cobalt salt and urea are added to water and stirred at 30-40°C for 50-80 minutes to obtain solution B.

6. The preparation method of the cobalt tetroxide / copper oxide-cuprous oxide composite catalyst according to claim 1, characterized in that: In step (4), the temperature of the hydrothermal reaction is 100~120℃ and the reaction time is 10~15 hours.

7. The method for preparing the cobalt tetroxide / copper oxide-cuprous oxide composite catalyst according to claim 1, characterized in that: In step (4), the drying temperature is 60~65℃ and the drying time is 15~20 hours.

8. The preparation method of the cobalt tetroxide / copper oxide-cuprous oxide composite catalyst according to claim 1, characterized in that: In step (5), the heating rate is 2~2.5℃ / min.

9. The preparation method of the cobalt tetroxide / copper oxide-cuprous oxide composite catalyst according to claim 1, characterized in that: In step (5), the cobalt tetroxide / copper oxide-cuprous oxide composite catalyst is in the form of layered nanoflowers.

10. The application of the cobalt tetroxide / copper oxide-cuprous oxide composite catalyst prepared by the method of claim 1 as a catalyst in the electrocatalytic reduction of nitrate to ammonia reaction, characterized in that, The specific application process is as follows: In the H-type battery, the electrocatalytic system adopts a three-electrode system, with a platinum sheet electrode as the counter electrode, a silver chloride electrode as the reference electrode, and a catalyst as the working electrode; an electrolyte containing potassium nitrate and potassium hydroxide is added to both the cathode and anode reaction cells, with the concentration of potassium nitrate being 0.1~0.12 mol / L and the concentration of potassium hydroxide being 1.0~1.2 mol / L; the three-electrode system is placed in the above electrolyte, and the electrocatalytic reduction reaction of nitrate is carried out under argon saturation conditions based on linear sweep voltammetry; wherein the potential range of linear sweep voltammetry is -1.5~0.5V.

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