A method for synthesizing ammonia by electrocatalytic reduction of nitrate

Abstract

This invention discloses a method for synthesizing ammonia by electrocatalytic reduction of nitrate. In an electrochemical catalytic system, a zinc-doped copper catalyst is used as a working electrode. Nitrate is used as a nitrogen source, and ammonia is synthesized under the action of the zinc-doped copper catalyst and an electrolyte. The catalyst of this invention can simultaneously achieve high Faradaic efficiency and yield at high currents, providing a potential for CO2-free ammonia synthesis and promoting the efficient development of the global nitrogen cycle.

Description

Technical Field

[0001] The invention belongs to the field of chemistry and chemical engineering, and particularly relates to a method for synthesizing ammonia by electrocatalytic reduction of nitrate. Background Art

[0002] Ammonia, as a fundamental chemical, is widely used in agriculture and the chemical industry to produce value-added chemicals such as fertilizers, pharmaceuticals, dyes, fuels, and explosives. Ammonia has a hydrogen storage density of up to 17.8% and can be liquefied for storage and transportation at room temperature and relatively low pressure, making it a promising hydrogen storage material. Currently, industrial ammonia synthesis primarily utilizes the Haber-Bosch process, considered one of the greatest inventions of the 20th century and responsible for 90% of the world's annual ammonia production. However, due to the slow kinetics of the nitrogen and hydrogen reaction, this process requires high temperatures (300-600°C) and high pressures (15-40 MPa). The entire process not only accounts for 1-2% of global energy consumption but also releases approximately 400 million tons of carbon dioxide annually. Given the significant energy and environmental costs, green and sustainable strategies are needed to achieve efficient ammonia synthesis under ambient conditions. In addition to traditional chemical methods for ammonia synthesis, electrochemical ammonia synthesis has recently attracted widespread attention. This is an ideal, resource-efficient and environmentally friendly method that can be carried out in a simple reactor under ambient conditions using renewable energy sources such as solar and wind power and emits no carbon dioxide. At the same time, electrochemical methods also provide a way to directly convert renewable electricity into chemicals or chemical energy.

[0003] Nitrates are abundant in the biosphere. Due to human activities, such as the combustion of nitrogen-containing fossil fuels, the discharge of industrial wastewater, and the bacterial decomposition of fertilizers, the concentration of nitrates in the environment gradually increases over time. Nitrate accumulation can lead to serious diseases and threaten human health. Nitrates, on the other hand, are much easier to activate than nitrogen gas, making them ideal feedstocks for the electrocatalytic synthesis of ammonia. The electrocatalytic reduction of nitrates to ammonia is a valuable pathway for the synthesis of ammonia, not only for the synthesis of basic chemicals but also for the global nitrogen cycle.

[0004] The electrocatalytic synthesis of ammonia from nitrate is a complex process involving eight electron transfer. To improve the activity of the catalyst, various methods have been used to synthesize the catalyst in recent years. The Au / C reported in 2020 had a Faradaic efficiency of 26%; the Fe0 / Fe3O4 reported in 2020 had a Faradaic efficiency of 32% and a yield of 4.6 μmol h -1 cm -2 ; PTCDA / O-Cu reported in 2020, with a Faradaic efficiency of 77% and a yield of 436±85 μg h -1 cm -2The Ti foil reported in 2020 has a Faradaic efficiency of 82% and an ammonia current density of only 22 mA cm -2 ; NbO reported in 2022 x The yield was only 55.0 μg h -1 mg -1 The yield of Co-SACs reported in 2022 was 433.3 μg h -1 cm -2 Furthermore, other researchers have reported that using materials such as Cu nanosheets, Cu / Ni, ZVT sheets, and Co-CNPs as catalysts for the electrocatalytic reduction of nitrate to ammonia also struggles to achieve both high Faradaic efficiency and yield at high current densities. Therefore, the current electrocatalytic reduction of nitrate to ammonia requires optimized material design and improved catalytic systems to achieve high performance at high current densities. Therefore, designing new, efficient catalysts to promote the rapid development of electrocatalytic reduction of nitrate to ammonia is an effective approach to advancing the green nitrogen cycle. Summary of the Invention

[0005] One of the objects of the present invention is to provide a catalyst.

[0006] The catalyst provided by the present invention is a metal-doped copper catalyst, wherein the metal is at least one of zinc, tin, cerium, nickel, iron, vanadium, indium, cadmium, and cobalt, preferably zinc, and the mass ratio of the metal to copper is 1:20-1:60, preferably 1:44;

[0007] The metal-doped copper catalyst is prepared by a method comprising the following steps:

[0008] 1) dissolving a copper compound in water, placing it in an ice bath, adding an alkaline aqueous solution, refrigerating, and transferring the solution into an autoclave for a hydrothermal reaction;

[0009] 2) After the hydrothermal treatment, the resulting material is washed, centrifuged, and dried to obtain copper oxide powder;

[0010] 3) dissolving the obtained copper oxide powder in an organic solvent, adding a metal compound aqueous solution, stirring, washing, centrifuging, and drying to obtain a black powder;

[0011] 4) calcining the obtained black powder, washing and drying;

[0012] 5) The dried powder is subjected to in-situ electrochemical reduction in an electrolyte to obtain the product.

[0013] In step 1) of the above method, the copper compound can be selected from copper sulfate (CuSO4), copper trifluoromethanesulfonate (Cu(CF3SO3)2), copper fluoride (CuF2), copper chloride (CuCl2), copper bromide (CuBr2), copper iodide (CuI2), copper nitrate (Cu(NO3)2), copper acetate (Cu(CH3COO)2), disodium copper ethylenediaminetetraacetate (C 10 H 20 CuN2Na2O 12 ), copper acetylacetonate (Cu(acac)2) and at least one of its hydrates, specifically copper sulfate pentahydrate;

[0014] The alkaline aqueous solution can be selected from at least one of a NaOH aqueous solution, a KOH aqueous solution, a Na2CO3 aqueous solution, a K2CO3 aqueous solution, a NaHCO3 aqueous solution, and a KHCO3 aqueous solution, and can specifically be a NaOH aqueous solution;

[0015] The ice bath should immerse the copper compound aqueous solution;

[0016] The ice bath time may be 0.1-2 hours, specifically 0.25 hours;

[0017] The refrigerated temperature may be -10-10°C, specifically 3°C;

[0018] The refrigeration time can be 10-48 hours, specifically 24 hours;

[0019] The temperature of the hydrothermal reaction may be 60-200°C, specifically 130°C;

[0020] The hydrothermal reaction time may be 5-30 hours, specifically 18 hours;

[0021] In the step 2), the centrifugation step has a rotation speed of 3000-11000 rpm, specifically 9000 rpm; and a time of 5-50 min, specifically 6 min;

[0022] The washing solvent is selected from at least one of deionized water, ethanol, acetone, methanol and mixed solutions thereof, and specifically deionized water and ethanol;

[0023] The drying can be vacuum drying, the vacuum drying temperature can be 60-130°C, specifically 60°C, and the time can be 5-20h, specifically 12h;

[0024] In the step 3), the organic solvent may be selected from at least one of ethanol (C2H5OH), methanol (CH3OH), acetone ((CH3)2CO), dimethyl sulfoxide (DMSO), and N,N-dimethylformamide (DMF), and may specifically be ethanol;

[0025] The metal compound may be selected from at least one of zinc chloride (ZnCl2), tin chloride (SnCl4), stannous chloride (SnCl), cerium chloride (CeCl3), nickel chloride (NiCl2), ferrous chloride (FeCl2), ferric chloride (FeCl3), vanadium chloride (VCl3), indium chloride (InCl3), cadmium chloride (CrCl2), and cobalt chloride (CoCl2);

[0026] The molar ratio of copper in the copper compound to the metal in the metal compound is 500:1-1:500, specifically 10:1-1:1, 3-5:1, 4.1-4.5:1, and more specifically 4.24:1;

[0027] The stirring temperature may be 10-100°C, specifically 25°C;

[0028] The stirring time may be 1-50h, specifically 2.5h;

[0029] In step 4), the calcination is carried out in an inert atmosphere, which may be argon or nitrogen. The calcination temperature may be 300-700° C., and may be 400° C.; the calcination time may be 0.5-10 h, and may be 3 h.

[0030] In the step 5), the electrolyte may be at least one of a NaOH aqueous solution, a KOH aqueous solution, a Na2CO3 aqueous solution, a K2CO3 aqueous solution, a NaHCO3 aqueous solution, a KHCO3 aqueous solution, a K2SO4 aqueous solution, a Na2SO4 aqueous solution, and a KNO3 aqueous solution, and may specifically be a mixed aqueous solution of K2SO4 and KNO3;

[0031] The potential applied for the electrochemical reduction is 0 to -2 V vs. RHE, specifically -0.8 V;

[0032] The electrochemical reduction time may be 1 min-30 min, specifically 5 min.

[0033] The in-situ electrochemical reduction adopts an H-type electrolytic cell, specifically, a platinum sheet as a working electrode, an Ag / AgCl electrode as a reference electrode, and a platinum mesh as a counter electrode.

[0034] The invention also provides an electrode material.

[0035] The electrode material provided by the present invention consists of the above-mentioned metal-doped copper-based catalyst and carbon paper.

[0036] The electrode material is prepared by a method comprising the following steps: dispersing a metal-doped copper catalyst and a Nafion D-521 dispersion into an organic solvent, and applying the obtained dispersion dropwise onto commercial carbon paper to obtain:

[0037] Wherein, the organic solvent is selected from at least one of the following: acetone, ethanol, isopropanol, methanol;

[0038] The mass fraction of the Nafion D-521 dispersion is 5-20wt%;

[0039] The ratio of the Nafion D-521 dispersion to the catalyst is 5-100 μL: 10 mg;

[0040] The amount of the catalyst used can be 0.1-100 mg·cm -2 , specifically 0.14 mg cm -2 .

[0041] The present invention also provides an electrochemical catalytic system.

[0042] The electrochemical catalytic system uses the above electrode material as a working electrode and also includes a reaction electrolyte and a reaction device.

[0043] The reaction electrolyte is selected from at least one of the following: NaOH aqueous solution, KOH aqueous solution, Na2CO3 aqueous solution, K2CO3 aqueous solution, NaHCO3 aqueous solution, KHCO3 aqueous solution, K2SO4 aqueous solution, and Na2SO4 aqueous solution;

[0044] The concentration of the reaction electrolyte is 0.1-10M;

[0045] The reaction device is an H-type electrolytic cell or a flow-type electrolytic cell, and specifically can be an H-type electrolytic cell.

[0046] The application of the above-mentioned metal-doped copper catalyst, electrode material and electrochemical catalytic system in the electrocatalytic reduction of nitrate to synthesize ammonia also falls within the scope of protection of the present invention.

[0047] The present invention also provides a method for synthesizing ammonia by electrochemical catalytic reduction of nitrate.

[0048] The method for synthesizing ammonia by electrochemical catalytic reduction of nitrate provided by the present invention comprises: in the above-mentioned electrochemical catalytic system, using nitrate as a nitrogen source, reacting to synthesize ammonia under the action of a metal-doped copper catalyst and an electrolyte.

[0049] The electrolyte may be a mixed solution of 0.5M K2SO4 and 0.1M KNO3;

[0050] The above electrode materials were used as working electrodes, Ag / AgCl electrodes as reference electrodes, and platinum mesh as counter electrodes;

[0051] The reaction potential is -0.1 to -1.5 V vs. RHE (-0.7 to -2.1 V vs. Ag / AgCl), specifically -0.35 to -0.85 V vs. RHE; the reaction time can be 0.2-100 h, specifically 0.5 h.

[0052] The present invention proposes a method for efficiently synthesizing ammonia under electrochemical reduction using nitrate as a nitrogen source, metal-doped copper as a catalyst, and a mixed solution of potassium sulfate and potassium nitrate as an electrolyte. The catalyst of the present invention can simultaneously achieve high Faradaic efficiency and yield at high currents. The present invention lays an excellent foundation for the industrial development of synthetic ammonia, providing a possibility for achieving carbon dioxide-free ammonia synthesis and promoting the efficient development of the global nitrogen cycle. In addition, the present invention uses nitrate, which is commonly found in wastewater, as a nitrogen source, which promotes environmental optimization and has important commercial value, and is of great significance to the establishment of a resource-saving and environmentally friendly human society. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of Zn / Cu;

[0054] Figure 2 This is the high-resolution TEM (HRTEM) image of Zn / Cu;

[0055] Figure 3 Element distribution map of Zn / Cu (EDS Mapping);

[0056] Figure 4 is the X-ray diffraction analysis (XRD) pattern of Zn / Cu;

[0057] Figure 5 This is the ammonia partial current density result diagram of Zn / Cu;

[0058] Figure 6 is the Faraday efficiency diagram of Zn / Cu;

[0059] Figure 7 This is the Zn / Cu yield result diagram. DETAILED DESCRIPTION

[0060] The present invention will be further described in detail below in conjunction with specific embodiments. The examples provided are only for illustrating the present invention and are not intended to limit the scope of the present invention. The examples provided below can serve as a guide for further improvements by those skilled in the art and are not intended to limit the present invention in any way.

[0061] Unless otherwise specified, the experimental methods in the following examples are conventional methods and were performed according to the techniques or conditions described in the literature in the field or according to the product instructions. The materials and reagents used in the following examples, unless otherwise specified, were all commercially available.

[0062] Example 1. Preparation and characterization of catalyst

[0063] Take the preparation of a Zn / Cu catalyst as an example. First, 24.96g of CuSO4·5H2O was dissolved in 100mL of deionized water and stirred continuously in an ice-water bath to form a homogeneous blue solution. 200mL of 1.2M NaOH solution was slowly added dropwise to the solution with continuous stirring for 0.25h. The solution was refrigerated at 3°C for 24h, then transferred to a Teflon-lined autoclave, sealed, and hydrothermaled at 130°C for 18h before naturally cooling to room temperature. The solution was then separated by centrifugation, washed multiple times with a deionized water-ethanol solution, and dried under vacuum at 60°C for 12h. 200mg of the resulting powder was dissolved in 5mL of ethanol, and 1mL of a 0.51M ZnCl2 aqueous solution was slowly added dropwise with continuous stirring for 2.5h. The solution was separated by centrifugation, washed multiple times with a deionized water-ethanol solution, and dried under vacuum at 60°C. The resulting black powder was calcined at 400°C for 3h. In situ electrochemical reduction was carried out at a potential of -0.8V vs. RHE (H-type electrolytic cell, with platinum sheet as working electrode, Ag / AgCl electrode as reference electrode, and platinum mesh as counter electrode) for 5 minutes. After the above steps, the Zn / Cu catalyst was obtained. By adjusting the concentration of the added ZnCl2 to 0.59M, 0.66M and no ZnCl2, Zn / Cu-2, Zn / Cu-3 and Zn-free Cu catalysts were obtained. Based on the above method, we also prepared Sn / Cu, Ce / Cu, Ni / Cu, Fe / Cu, V / Cu, In / Cu, Cd / Cu and Co / Cu catalysts by adding 0.59mM SnCl4, CeCl3, NiCl2, FeCl3, VCl3, InCl3, CdCl2 and CoCl2 aqueous solutions, respectively.

[0064] We systematically characterized the Zn / Cu catalyst. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed that the Zn / Cu catalyst had a uniform and irregular flake morphology. The spacing of the lattice fringes in high-resolution TEM (HRTEM) was 0.21 nm, which can be attributed to the (111) crystal plane of Cu ( Figure 2 ). Element distribution map (EDS Mapping) research shows that Cu and Zn elements are evenly distributed on the catalyst surface ( Figure 3 X-ray diffraction analysis (XRD) showed that there was only one Cu crystal phase in the Zn / Cu catalyst ( Figure 4 ).

[0065] Example 2: Electrochemical Catalysis of Nitrate to Ammonia

[0066] To prepare the working electrode, 10 mg of the Zn / Cu catalyst prepared in Example 1 and 5 μL of Nafion D-520 dispersion (5 wt%) were uniformly dispersed in 1 mL of ethanol. 36 μL of the dispersion was evenly dropped onto a surface area of 1 × 0.5 cm -2 The hydrophobic carbon paper surface was dried at room temperature, and the actual loading of each electrode catalyst was 0.14 mg cm -2 (Calculated based on the difference between the weight of carbon paper before adding the catalyst and the weight after adding the catalyst).

[0067] All electrochemical experiments were performed using an electrochemical workstation (CHI 660E, Shanghai Chenhua Instrument Co., Ltd.). Electrolysis experiments were conducted at 25°C in a 50-ml H-type electrolytic cell using a three-electrode system. The three electrodes consisted of a working electrode, an Ag / AgCl reference electrode, and a platinum mesh counter electrode. Saturated potassium chloride solution was added to the reference electrode. Before the experiment, the reference electrode was calibrated according to the literature. A Nafion-117 proton exchange membrane separated the cathode and anode. The anode and cathode electrolytes were each 37 ml of a mixed aqueous solution of 0.5 M KSO and 0.1 M KNO. Argon was bubbled through the electrolyte for 30 minutes to purge oxygen. Gaseous products were collected using a gas bag and analyzed using a gas chromatograph (HP 4890D). Liquid products were detected using an ultraviolet spectrophotometer (Perkin Elmer Lambda 1050+).

[0068] The experimental results of electrocatalytic nitrate reduction with different metal doping and zinc doping levels are shown in Tables 1 and 2. As can be seen from the table, the catalytic system of the present invention has excellent selectivity for ammonia. Compared with other materials, Zn / Cu-2 has the highest catalytic activity. The Faradaic efficiency of ammonia can reach 98.40% after 0.5 h of reaction at a potential of -0.55 V vs. RHE, and the partial current density of ammonia can reach 134.70 mA cm -2 The yield reached 76831.32 μg h -1 mg cat. -1 , while achieving high Faradaic efficiency and high yield at high current density. Through screening of different materials and doping levels, it was found that a moderate zinc doping level would promote the conversion of nitrate to nitrite, thereby promoting the production of ammonia.

[0069] Table 1:

[0070] sample <![CDATA[j NH3 / mA cm -2 ]]> <![CDATA[FE NH3 / %]]> <![CDATA[Yield / μg h -1 mg cat. -1 > Zn / Cu-2 134.70 98.40 76831.32 Sn / Cu 58.89 65.38 21961.83 Ce / Cu 13.10 73.40 4320.72 Ni / Cu 18.94 75.51 6423.61 Fe / Cu 15.90 71.50 5107.45 V / Cu 35.33 30.14 4785.155 Cd / Cu 40.44 24.87 5469.585 Co / Cu 46.75 30.10 5224.481

[0071] Table 1 was conducted in an H cell, with an Ag / AgCl reference electrode, a Pt mesh counter electrode, and a catalyst loading of 0.14 mg / cm -2 The electrolytes for both cathode and anode were 0.5M K2SO4+0.1M KNO3, the applied potential was -0.55V vs.RHE (after 80% IR compensation), and the reaction time was 0.5h.

[0072] Table 2

[0073]

[0074] Table 2 was conducted in an H cell, with an Ag / AgCl reference electrode, a Pt mesh counter electrode, and a catalyst loading of 0.14 mg / cm -2 The electrolytes for both cathode and anode are 0.5MK2SO4+0.1MKNO3, the applied potential is -0.35V~-0.85Vvs.RHE (after 80% IR compensation), and the reaction time is 0.5h.

[0075] The present invention has been described in detail above. For those skilled in the art, without departing from the purpose and scope of the present invention, and without the need to carry out unnecessary experimental conditions, the present invention can be implemented in a wide range under equivalent parameters, concentrations and conditions. Although the present invention provides specific embodiments, it should be understood that further improvements can be made to the present invention. In short, according to the principles of the present invention, this application is intended to include any changes, uses or improvements to the present invention, including changes that depart from the disclosed scope in this application and are made using conventional techniques known in the art.

Claims

1. A method for preparing a metal-doped copper catalyst, wherein: The metal is at least one of zinc, tin, nickel and cadmium; The method comprises the following steps: 1) dissolving a copper compound in water, placing it in an ice bath, adding an alkaline aqueous solution thereto, refrigerating it, transferring the solution to an autoclave, and performing a hydrothermal reaction; 2) After the hydrothermal treatment, the resulting material is washed, centrifuged, and dried to obtain copper oxide powder; 3) dissolving the obtained copper oxide powder in an organic solvent, adding a metal compound aqueous solution, stirring, washing, centrifuging, and drying to obtain a black powder; 4) calcining, washing and drying the obtained black powder; 5) The dried powder is electrochemically reduced in situ in an electrolyte to obtain; The metal compound is a compound of at least one metal selected from the group consisting of zinc, tin, nickel, and cadmium.

2. The method according to claim 1, wherein: The copper compound is selected from at least one of copper sulfate, copper trifluoromethanesulfonate, copper fluoride, copper chloride, copper bromide, copper iodide, copper nitrate, copper acetate, disodium copper ethylenediaminetetraacetate, copper acetylacetonate and hydrates thereof; The alkaline aqueous solution is selected from at least one of a NaOH aqueous solution, a KOH aqueous solution, a Na2CO3 aqueous solution, a K2CO3 aqueous solution, a NaHCO3 aqueous solution, and a KHCO3 aqueous solution; The ice bath time is 0.1-2h; The refrigerated temperature is -10-10 o C, the refrigeration time is 10-48h; The temperature of the hydrothermal reaction is 60-200 o C, the hydrothermal reaction time is 5-30h; In step 3), the organic solvent is selected from at least one of ethanol, methanol, acetone, dimethyl sulfoxide, and N,N-dimethylformamide; The metal compound is selected from at least one of zinc chloride, stannic chloride, stannous chloride, nickel chloride, and cadmium chloride; The mass ratio of the copper compound to the metal compound is 500:1-1:500; In the step 4), the calcination is carried out in an inert atmosphere at a temperature of 300-700 o C, the calcination time is 0.5-10 h; In the step 5), the electrolyte is at least one of a NaOH aqueous solution, a KOH aqueous solution, a Na2CO3 aqueous solution, a K2CO3 aqueous solution, a NaHCO3 aqueous solution, a KHCO3 aqueous solution, a K2SO4 aqueous solution, a Na2SO4 aqueous solution, and a KNO3 aqueous solution; The potential applied for the electrochemical reduction is 0 to -2 V vs. RHE; The electrochemical reduction time is 1 min-30 min.

3. The metal-doped copper catalyst prepared by the method of claim 1 or 2, wherein: The metal is at least one of zinc, tin, nickel and cadmium.

4. An electrode material, comprising the metal-doped copper-based catalyst according to claim 3 and carbon paper.

5. A method for preparing the electrode material according to claim 4, comprising: dispersing a metal-doped copper catalyst and a Nafion D-521 dispersion in an organic solvent, and drop-coating the resulting dispersion onto commercial carbon paper to obtain the electrode material. in, The organic solvent is selected from at least one of the following: acetone, ethanol, isopropanol, and methanol; The mass fraction of the Nafion D-521 dispersion is 5-20wt%; The ratio of the Nafion D-521 dispersion to the catalyst is 5-100 μL:10 mg; The catalyst dosage is 0.1-100 mg·cm -2 .

6. An electrochemical catalytic system, comprising the electrode material of claim 4 as a working electrode, and further comprising a reaction electrolyte and a reaction device.

7. The electrochemical catalytic system according to claim 6, characterized in that: The reaction electrolyte is selected from at least one of the following: NaOH aqueous solution, KOH aqueous solution, Na2CO3 aqueous solution, K2CO3 aqueous solution, NaHCO3 aqueous solution, KHCO3 aqueous solution, K2SO4 aqueous solution, and Na2SO4 aqueous solution; The concentration of the reaction electrolyte is 0.1-10 M; The reaction device is an H-type electrolytic cell or a flow-type electrolytic cell.

8. Use of the metal-doped copper catalyst according to claim 3, the electrode material according to claim 4, or the electrochemical catalytic system according to claim 6 in the electrocatalytic reduction of nitrate to synthesize ammonia.

9. A method for synthesizing ammonia by electrochemical catalytic reduction of nitrate, comprising: using nitrate as a nitrogen source in the electrochemical catalytic system of claim 6, and reacting to synthesize ammonia under the action of a metal-doped copper catalyst and an electrolyte.

10. The method according to claim 9, wherein: The electrolyte is a mixed solution of 0.5M K2SO4 and 0.1M KNO3; The reaction potential is -0.1~-1.5V vs. RHE; and the reaction time is 0.2-100 h.

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