Preparation method of iron-cobalt nanometer alloy electrocatalyst

Abstract

The present application relates to the field of metal nanomaterial preparation, in particular to a preparation method of iron-cobalt nanometer alloy electrocatalyst, which solves the problems of high reaction condition requirement, inability to synthesize uniform small particle size and low electrocatalytic performance in the existing preparation method of iron-cobalt alloy. Different proportions of iron source and cobalt source are used as raw materials, which are uniformly dispersed in a water-base-alcohol-organic acid system, then heated to a set temperature in a hydrothermal reactor and reacted for a period of time, after cooling, washing and drying, a cobalt-doped iron oxide precursor is obtained; the oxide precursor is heat treated in a tube furnace in a reducing atmosphere, and after cooling, an iron-cobalt nanometer alloy electrocatalyst (particle size 10-20 nm) is obtained. The nanometer alloy particles obtained by the present application have small particle size, uniform composition, high yield, and can realize efficient electrocatalytic reduction of nitrate to prepare ammonia in a water environment with wide potential range, wide electrolyte concentration and wide nitrate concentration, and have the potential for large-scale application.

Description

TECHNICAL FIELD

[0001] The application relates to the field of metal nanomaterial preparation, and particularly relates to a preparation method of an iron-cobalt nanometer alloy electrocatalyst. BACKGROUND

[0002] Nitrate (NO3 - ) pollutants are common byproducts of industrial and agricultural activities and have become one of the most important environmental pollution and disease risk factors worldwide. Ammonia (NH3) is a key raw material in the fields of agriculture and chemical industry and is also one of the ideal "zero-carbon" energy carriers in future sustainable energy and one of the most important chemicals in the world, which has a very high economic added value. Therefore, how to electrochemically reduce nitrate pollutants in water bodies into value-added ammonia is of great importance to the sustainable development of modern agriculture and industry. Nitrate reduction to ammonia is a multi-electron process, which requires a very high overpotential, and designing a new, efficient and inexpensive non-noble metal catalyst is the key to realizing the high-efficiency nitrate reduction to ammonia process.

[0003] In recent years, iron-based and cobalt-based inorganic metal nanomaterials have shown excellent electrochemical catalytic activity in the electrocatalytic reduction of nitrate to ammonia (NO3 - RR) reaction, however, there are few reports on the use of iron-cobalt alloys for the electrocatalytic NO3 - RR. Since the atomic sizes of iron and cobalt are close, iron-cobalt alloys can form infinite solid solutions, thereby exhibiting multiple forms, such as Co3Fe7, CoFe, Co7Fe3, Co 0.72 Fe 0.28 , etc. If the types and preparation conditions of iron-cobalt precursors are not effectively controlled, it is usually difficult to obtain iron-cobalt alloys with uniform size distribution, which is not conducive to the study of the physicochemical properties and catalytic mechanism of iron-cobalt alloys. In addition, the coupling of two different materials to form a heterojunction can fully exert their respective strengths and produce excellent synergistic effects, especially the atomic-level heterojunction interface is a concentrated distribution area of high reactivity. While forming an infinite solid solution, iron-cobalt alloys will form a large number of defects, thereby producing strong grain boundary and interface effects. However, large-sized iron-cobalt alloys result in low specific surface area of the material and greatly reduce the number of atomic-level interface active sites exposed. In summary, the construction of iron-cobalt alloys with uniform, small size and adjustable composition has important research and application value for obtaining high-efficiency NO3 - RR catalysts.

[0004] For the existing preparation method of Co3Fe7nanometer alloy: the method described in the Chinese patent with publication number CN109215921A requires the use of electrospinning / high temperature reduction method (500℃), which requires high equipment and takes a long time. The prepared Co3Fe7particles have a large particle size (50-100 nm). The hydrothermal / high temperature pyrolysis method (800℃) described in the Chinese patent with publication number CN116815235A can only obtain Co3Fe7particles with large particle size and uneven size distribution (1-10 μm). Similarly, literature 1 [Chemical Engineering Journal, 2022, 429, 132174] uses sol-gel / high temperature reduction method (850℃), which produces Co3Fe7particles with large particle size (20-100 nm) and uneven size. Literature 2 [Advanced Science, 2020, 7, 2000747] uses foam adsorption / high temperature reduction method (900℃), which obtains Co3Fe7particles with an average size of about 100 nm. Literature 3 [NanoEnergy, 2021, 86, 106111] describes the dopamine polymer coating / high temperature reduction method (800℃), which can obtain smaller Co3Fe7particles (10-20 nm), but the preparation process is complicated, the energy consumption is high, and the yield is low.

[0005] For the preparation method of other iron-cobalt nanometer alloys with different element ratios:

[0006] 1、CoFe alloy: The method described in the Chinese patent with publication number CN116885218A uses polymerization to load Co and Fe metal precursor salts onto g-C3N4, followed by high temperature calcination (900℃). The prepared CoFe alloy particles have a large size and uneven distribution (20-200 nm). Literature 4 [Chemical Engineering Journal, 2021, 420, 130516] obtains CoFe nanometer alloy loaded on a carbon matrix by high temperature pyrolysis (1000℃) of CoFe-MOFs (metal organic framework) precursor, which has a large particle size (100-300 nm).

[0007] 2. Co7Fe3 alloy: The method described in Chinese patent CN109926083A uses sodium borohydride to reduce soluble Fe and Co salts at room temperature under inert gas, and Co7Fe3 alloy particles with a particle size of 50-200 nm are prepared. Literature 5 [Nano Research, 2023, 16, 2519-2527] obtained nitrogen-doped carbon-coated Co7Fe3 nanometer alloy by high-temperature pyrolysis (900℃) of ZnCoFe-ZIF (zeolite imidazole framework) precursor, with a particle size of 5-50 nm, a large size distribution difference and a low yield. The above methods are complex, high in equipment energy consumption, not conducive to large-scale production, and cannot prepare uniform, small size (<10 nm) and metal element ratio adjustable iron-cobalt nanometer alloy particles.

[0008] So far, there is no literature or published patent on the preparation method of iron-cobalt nanometer alloy with uniform size, small particle size, and adjustable metal element ratio for neutral electrolyte electrocatalytic reduction of nitrate. SUMMARY

[0009] The purpose of the present application is to provide a preparation method of iron-cobalt nanometer alloy electrocatalyst, which solves the problems of high reaction condition requirement, inability to synthesize uniform small particle size and low electrocatalytic performance of existing iron-cobalt alloy preparation methods.

[0010] The technical solution of the present application is:

[0011] A preparation method of iron-cobalt nanometer alloy electrocatalyst, the specific steps are as follows:

[0012] (1) Disperse different proportions of iron salt and cobalt salt precursors into water to form a suspension A, at the same time disperse the alkali in the mixed solution of alcohol and organic acid to form a soap B, pour the suspension A into the soap B and stir to mix until the soap B is fully dissolved to form a suspension C;

[0013] The iron salt used is one or more of acetylacetone iron, iron chloride, iron nitrate, and iron sulfate containing trivalent iron ions, and the required amount of iron salt is in the molar concentration range of 0.01-0.50 mol / L; the cobalt salt used is one or more of acetylacetone cobalt, cobalt chloride, cobalt nitrate, and cobalt sulfate containing trivalent cobalt ions, and the required amount of cobalt salt is in the molar concentration range of 0.01-0.50 mol / L;

[0014] (2) Pour the suspension C into the inner container of the solvothermal reaction kettle and seal, then keep it at a set temperature for a certain time to form iron-cobalt oxide, and after cooling, washing and drying, obtain cobalt-doped iron oxide precursor nanoparticles;

[0015] (3) placing the cobalt-doped iron oxide precursor nanoparticles in a tube furnace in a reducing atmosphere, and keeping at a set temperature for a certain time, and obtaining the target iron-cobalt nanometer alloy electrocatalyst after cooling, with a particle size range of 5-500 nm; the iron-cobalt nanometer alloy electrocatalyst comprises the following components in atomic percentage: 0.01%-99.99% of cobalt, and the balance of iron.

[0016] In the preparation method of the iron-cobalt nanometer alloy electrocatalyst, in step (1), the iron salt is preferably acetylacetone iron, and the molar concentration thereof is preferably 0.06 mol / L; the cobalt salt is preferably acetylacetone cobalt, and the molar concentration thereof is preferably 0.04 mol / L.

[0017] In the preparation method of the iron-cobalt nanometer alloy electrocatalyst, in step (1), in order to obtain an electrocatalyst with uniform size and controllable morphology, the volume of water in the raw materials is 20-80 mL, the mass of the alkali in the raw materials is 0.5-4.0 g, the volume of the alcohol and organic acid mixed solution in the raw materials is 20-80 mL, and the mixed volume ratio of the alcohol and the organic acid is 0.2-5.0.

[0018] In the preparation method of the iron-cobalt nanometer alloy electrocatalyst, in step (1), the volume of water in the raw materials is preferably 40 mL, the mass of the alkali in the raw materials is preferably 2.0 g, the volume of the alcohol and organic acid mixed solution in the raw materials is preferably 40 mL, and the mixed volume ratio of the alcohol and the organic acid is preferably 1.0.

[0019] In the preparation method of the iron-cobalt nanometer alloy electrocatalyst, in step (1), the alkali used is one or two or more of sodium hydroxide, potassium hydroxide and ammonia water, the alcohol used is one or two or more of methanol, ethanol and ethylene glycol, and the organic acid used is one or two or more of oleic acid, linoleic acid and stearic acid.

[0020] In the preparation method of the iron-cobalt nanometer alloy electrocatalyst, in step (1), the alkali used is preferably sodium hydroxide, the alcohol used is preferably ethanol, and the organic acid used is preferably oleic acid.

[0021] In the preparation method of the iron-cobalt nanometer alloy electrocatalyst, in step (2), the iron-cobalt oxide precursor is prepared by a solvothermal method, the material of the reaction kettle used in the solvothermal method is one of stainless steel, aluminum alloy, copper and tantalum, the inner liner of the reaction kettle is one of polytetrafluoroethylene (PTFE) and high-density polyethylene (HDPE), the capacity of the inner liner is 50 mL-200 mL, the temperature range of the solvothermal method is 140-220℃, and the time range of the reaction is 2-24 h.

[0022] In the preparation method of the iron-cobalt nanometer alloy electrocatalyst, in step (2), the temperature of the solvothermal method is preferably 180℃, and the time of the reaction is preferably 10 h.

[0023] The preparation method of the iron-cobalt nanometer alloy electrocatalyst, in step (3), the target iron-cobalt nanometer alloy electrocatalyst is prepared by a low-temperature thermal reduction method, wherein: the heating temperature rising speed of the tube furnace is 0.5-10 ℃ / min, the holding temperature is 50-1100 ℃, and the holding time is 0.5-24 h; the reducing atmosphere of the tube furnace is one of the following: ammonia / argon mixed gas, the volume percentage of ammonia is 5%-100%; ammonia / nitrogen mixed gas, the volume percentage of ammonia is 5%-100%; hydrogen / argon mixed gas, the volume percentage of hydrogen is 5%-100%; and hydrogen / nitrogen mixed gas, the volume percentage of hydrogen is 5%-100%.

[0024] The preparation method of the iron-cobalt nanometer alloy electrocatalyst, in step (3), the heating temperature rising speed of the tube furnace is preferably 5 ℃ / min, the holding temperature is preferably 350 ℃, and the holding time is preferably 2 h.

[0025] The design idea of the present application is as follows:

[0026] Based on the special requirements of electrocatalytic reactions on active sites of catalysts, charge and mass transfer, combined with the process and cost optimization objectives to meet the scale application, the iron-cobalt nanometer alloy electrocatalyst is obtained by using abundant, cheap and easily available non-noble metal raw materials through a simple synthesis operation method.

[0027] The preparation process and features of the material in the present application are as follows: (1) different proportions of iron source and cobalt source are used as reaction raw materials, which are uniformly dispersed in a water-base-alcohol-organic acid system, then poured into a hydrothermal reactor, heated to a set temperature for a period of time, and then cooled, washed and dried to obtain a cobalt-doped iron oxide precursor; (2) the oxide precursor is heat-treated in a tube furnace in a reducing atmosphere, and then cooled to obtain an iron-cobalt nanometer alloy electrocatalyst (particle size 10-20 nm).

[0028] Compared with the prior art, the present application adopts a general solvothermal method and a low-temperature thermal reduction method, and by adjusting the morphology of the cobalt-doped iron oxide precursor and the solvothermal and thermal reduction temperature, an iron-cobalt nanometer alloy electrocatalyst with uniform size and adjustable metal element ratio is synthesized. The iron-cobalt nanometer alloy electrocatalyst synthesized by this method not only realizes efficient electrocatalytic reduction of nitrate to prepare ammonia in a wide potential range, a wide electrolyte concentration and a wide nitrate concentration water environment, but also has the performance advantages of high activity, high selectivity and high faradaic efficiency in electrocatalytic reduction of nitrate to prepare ammonia.

[0029] The present application has the following advantages and beneficial effects:

[0030] 1. The iron-cobalt nanoscale alloy electrocatalyst synthesized by the method has excellent nitrate reduction activity in a neutral electrolyte environment, and can achieve high nitrate removal rate, ammonia synthesis selectivity and Faraday efficiency at a lower overpotential.

[0031] 2. The iron-cobalt nanoscale alloy electrocatalyst synthesized by the method has wide applicability and can be effectively catalyzed in electrolytes with wide potential, wide NO3 - concentration and wide electrolyte concentration.

[0032] 3. The method uses a solvothermal method in a water-base-alcohol-organic acid system to synthesize a cobalt-doped iron oxide precursor, and then a low-temperature thermal reduction treatment is performed to obtain the iron-cobalt nanoscale alloy electrocatalyst. The method can be performed under general hydrothermal conditions and low-temperature thermal reduction conditions, and has the production advantages of simple operation, simple process, short preparation period, low cost and low energy consumption, and has the potential for large-scale application. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 : XRD patterns of iron-based nanoscale metal particles (Fe), different proportion of cobalt-doped iron-cobalt nanoscale alloy particles (Co x Fe 1-x , x represents the addition of cobalt raw materials in different molar ratios, and the total molar amount of metal raw materials is 4.0 mmol), and cobalt-based nanoscale metal particles (Co). In the figure, the horizontal coordinate 2θ represents the diffraction angle (°), and the vertical coordinate Intensity represents the intensity (a.u.).

[0034] Figure 2 : (a) iron-based nanoscale metal particles (Fe), (b)-(j) different proportion of cobalt-doped iron-cobalt nanoscale alloy particles (Co x Fe 1-x ), and (k) cobalt-based nanoscale metal particles (Co) under a scanning electron microscope.

[0035] Figure 3 : Structure (a), (b) and energy spectrum analysis (c) of the optimal iron-cobalt nanoscale alloy particles (Co3Fe7) under an atomic resolution spherical aberration electron microscope.

[0036] Figure 4 : Polarization curves (a) and Tafel slope comparison (b) of iron-based nanoscale metal particles (Fe), optimal iron-cobalt nanoscale alloy particles (Co3Fe7), and cobalt-based nanoscale metal particles (Co) for nitrate reduction to ammonia electrocatalysis. In the (a) figure, the horizontal coordinate Potential vs. RHE represents the potential (V), and the vertical coordinate current density represents the current density (mA / cm 2 ). In the (b) figure, the horizontal coordinate Log j represents the current density (mAcm-2 ) vs. RHE (V).

[0037] Figure 5 : Co-doped Fe-Co nanalloy particles with different Co doping amounts x Fe 1-x at 500 ppm NO3 - NO3 - removal efficiency and NH3 selectivity. In the figure, the horizontal axis Doping amount of Co represents the Co doping amount (%), the left vertical axis NO3 - removal efficiency represents the NO3 - removal efficiency (%), corresponding to the left value of each group, the right vertical axis Selectivity for NH3 represents the NH3 selectivity (%), corresponding to the right value of each group.

[0038] Figure 6 : Co3Fe7 alloy with optimal performance at 500 ppm NO3 - NO3 - removal efficiency, Faradaic efficiency and NH3 selectivity. In the figure, the horizontal axis Potential vs. RHE represents the potential (V), the left vertical axis NH3 yield rate represents the NH3 yield rate (μg h -1 cm -2 ), and the right vertical axis Percentage represents the Faradaic efficiency or NH3 selectivity (%); each group from left to right is: yield rate represents the NH3 yield rate, Faradaic efficiency represents the Faradaic efficiency (FE), and Selectivity represents the NH3 selectivity.

[0039] Figure 7 : Co3Fe7 alloy with optimal performance at 500 ppm NO3 - NO3 - removal efficiency and NH3 selectivity. In the figure, the horizontal axis Concentration of Na2SO4 represents the molar concentration of Na2SO4 (M), the left vertical axis NO3 - removal efficiency represents the NO3 -Removal rate (%) corresponding to left value of each group, right ordinate Selectivity for NH3 represents the synthesis ammonia selectivity (%) corresponding to right value of each group.

[0040] Figure 8 : wide NO3 - NO3 - Removal rate, Faradaic efficiency and synthesis ammonia selectivity. In the figure, the horizontal coordinate NO3 - - Concentration represents the NO3 - - Concentration (ppm), left ordinate NH3 yield rate represents the ammonia yield rate (μg h -1 cm -2 ), right ordinate Percentage represents the Faradaic efficiency or synthesis ammonia selectivity (%); each group from left to right in turn: yield rate represents the ammonia yield rate, Faradaic efficiency represents the Faradaic efficiency (FE), selectivity represents the synthesis ammonia selectivity. DETAILED DESCRIPTION

[0041] In the specific implementation process, iron-based nanometer metal particles (referred to as Fe), different concentration cobalt-doped iron-cobalt nanometer alloy particles (referred to as Co x Fe 1-x ), cobalt-based nanometer metal particles (referred to as Co) are taken as comparative objects. For the preparation of iron-cobalt nanometer metal particles Co 0.4 Fe 0.6 with optimal performance (crystal phase structure is Co3Fe7, iron-cobalt feeding molar ratio is 0.6:0.4, x=0.4, total molar amount of metal raw materials is 4.0 mmol): 20 mL of ethanol and 20 mL of oleic acid are added into a polytetrafluoroethylene reaction kettle inner container with a capacity of 100 mL, and then stirred, followed by adding 2 g of NaOH, and then reacting and coagulating into a soap-like substance. 848 mg (2.4 mmol) of iron acetylacetone and 570 mg (1.6 mmol) of cobalt acetylacetone are dispersed in 40 mL of deionized water, and then poured into the above-mentioned reaction kettle inner container. A glass rod is used for stirring until the soap-like substance is initially dissolved, and then continuously stirred by a stirrer for 10 min until it is fully dissolved, and then the stirrer is removed. The above-mentioned polytetrafluoroethylene reaction kettle inner container is sealed and packaged into a stainless steel shell, and then placed in an oven for heating to 180℃ for 10 h. After cooling to room temperature, the upper emulsion is poured off, and the bottom precipitate is collected in a centrifuge tube and centrifuged at a speed of 12000 r / min for 5 min. The same centrifugation condition is used to sequentially use deionized water / ethanol for cleaning, and the cleaning is repeated twice, and then the sample is vacuum dried at 60℃ to obtain the precursor Cox Fe 3-x O4 nanoparticles. The precursor was ground into powder in an agate mortar and placed in a ceramic boat. The mixture was heated to 350°C at a rate of 5°C / min and held for 2 hours in a tube furnace under a reducing atmosphere (a hydrogen / argon mixture, with hydrogen comprising 5% by volume). After cooling to room temperature, the sample was removed and ground again in an agate mortar to obtain the iron-cobalt nanoalloy catalyst. For pure iron and pure cobalt metals, only iron acetylacetone or cobalt acetylacetone was used as a substitute, while other conditions remained unchanged. Other iron-cobalt alloys with different iron-cobalt metal element ratios were prepared using different molar ratios of iron acetylacetone and cobalt acetylacetone.

[0042] The present invention will now be further described in detail with reference to embodiments and accompanying drawings.

[0043] Example 1

[0044] Iron-based nano-metal particles (Fe) and cobalt-doped iron-cobalt nano-alloy particles of varying concentrations (Co) x Fe 1-x Cobalt-based nanoparticles (Co) were used as a comparison object to study the phase composition of the samples prepared according to this method.

[0045] X-ray testing equipment and conditions: Rigaku D / max 2500 X-ray diffractometer, Cu Ka-ray. (e.g.) Figure 1 As shown in the figure, Fe and Co are... 0.1 Fe 0.9 Co 0.2 Fe 0.8 Co 0.3 Fe 0.7 Co 0.4 Fe 0.6 (crystal structure is Co3Fe7), Co 0.5 Fe 0.5 Co 0.6 Fe 0.4 Co 0.7 Fe 0.3 Co 0.8 Fe 0.2 Co 0.9 Fe 0.1 X-ray diffraction patterns of Co and Co. Among these, the Co3Fe7 card shows a striking resemblance to Co in a series of previously reported X-ray diffraction PDF cards related to cobalt-iron alloys, not only in the diffraction peak positions but also in the X-ray diffraction patterns of Co. 04 Fe 06 The most suitable match, and also the closest in molar ratio of the chemical formula, therefore Co is considered to be the most suitable. 0.4 Fe 0.6The crystal phase structure is Co3Fe7. The spectrum shows that pure iron particles are obtained without cobalt doping, cobalt-doped iron-cobalt nano-alloy particles with different concentrations of cobalt are obtained according to the doping concentration, and cobalt metal particles are obtained only by adding cobalt.

[0046] Example 2

[0047] Iron-based nano-metal particles (referred to as Fe), iron-cobalt nano-alloy particles doped with different concentrations of cobalt (referred to as Co x Fe 1-x ), and cobalt-based nano-metal particles (referred to as Co) are used as comparative objects to study the morphology of samples prepared according to the method.

[0048] Morphology characterization equipment: FEI Nova620 type scanning electron microscope. As shown in Figure 2 Figures (a), (b), (c), (d), (e), (f), (g), (h), (i), (j), and (k) are micro-morphology diagrams of Fe, Co 0.1 Fe 0.9 , Co 0.2 Fe 0.8 , Co 0.3 Fe 0.7 , Co 0.4 Fe 0.6 , Co 0.5 Fe 0.5 , Co 0.6 Fe 0.4 , Co 0.7 Fe 0.3 , Co 0.8 Fe 0.2 , Co 0.9 Fe 0.1 , and Co, respectively. As can be seen from Figure (a), the synthesized Fe is a homogeneous substance with a particle size of 20-40 nm. As can be seen from Figures (b) to (j), the synthesized iron-cobalt nano-alloy particles have uniform size distribution, and the addition of Co within a proper range (0% to 30 at%) can significantly reduce the catalyst particle size (a-d, 10-40 nm), while excessive addition (50 at% to 90 at%) can significantly increase the particle size (g-j, 20-100 nm). As can be seen from Figure (k), the synthesized Co has a particle size of about 20 nm.

[0049] Example 3

[0050] The cobalt-doped iron-cobalt nano-alloy particles Co 0.4 Fe 0.6 with the optimal electrocatalytic performance (crystal phase structure Co3Fe7) are used as research objects to study the crystal structure prepared according to the method.

[0051] Morphology characterization equipment: ARM200F spherical aberration correction transmission electron microscope. As shown in Figure 3 Fig. (a), (b) are atomic resolution images of Co3Fe7, the upper left corner is the corresponding diffraction pattern, and the lower right corner is the corresponding crystal model. As can be seen from Fig. (a) to Fig. (b), the clear high-resolution pattern and single diffraction spot type clearly prove that the particle is a single crystal, and completely coincides with the crystal model, which confirms the effectiveness of the preparation method of the synthesized cobalt-doped iron-cobalt nano-alloy particles (Co3Fe7). Fig. (c) is an energy spectrum image of Co3Fe7, which clearly shows the uniform distribution of Fe and Co elements and a part of the surface adsorbed oxygen.

[0052] Example 4

[0053] Taking iron-based nano-metallic particles (abbreviated as Fe), the optimal cobalt-doped iron-cobalt nano-alloy particles Co 0.4 Fe 0.6 (Co3Fe7 crystal structure), cobalt-based nano-metallic particles (abbreviated as Co) as the research object, the effect of cobalt doping on the initial potential, current density and Tafel slope of nitrate reduction in the iron-cobalt nano-metallic particles in the neutral electrolyte electrocatalysis was studied.

[0054] Neutral electrolyte electrocatalytic nitrate reduction test: (1) Preparation of catalyst ink: 10 mg of sample and 2 mg of conductive carbon black were dispersed in 600 μL of deionized water and 350 μL of isopropyl alcohol mixed solution, 50 μL of 5% Nafion solution was added, and ultrasonic treatment was performed for 30 min to form a uniform catalyst ink. (2) Preparation of working electrode: 200 μL of catalyst ink was drop-casted on a 1.5 cm x 1.5 cm hydrophilic carbon paper (TORAY, TGP-H-060) in two times, and baked at 60°C under an infrared baking lamp. (3) Configuration of H-type electrolytic cell: A proton exchange membrane (DUPONT, N211) was clamped at the connection of the H-type electrolytic cell with a single cell volume of 70 mL, and 50 mL of 0.1 M sodium sulfate solution containing 500 ppm NO3 — N was poured into the two sides, respectively, and platinum sheet electrode was inserted as the counter electrode and silver-silver chloride electrode as the reference electrode. (4) LSV curve test: EC-Lab VSP 300 electrochemical workstation was used for testing, the scanning speed was 5 mV / s, and the scanning range was 0V- -0.9V (vs. RHE). The test results are as follows Figure 4As shown in the figures, Figures (a), (b) are the electrochemical polarization curves and corresponding Tafel slope curves of Fe, Co3Fe7, Co, respectively. As can be seen from Figure (a), the initial potential of Co3Fe7 is significantly improved relative to Fe; and the current density of Co3Fe7 is significantly improved relative to Co. As can be seen from Figure (b), the Tafel slope of Co3Fe7 is the lowest, representing that it has faster electrocatalytic kinetics process. Both figures confirm that Co3Fe7 has the best performance of electrocatalytic reduction of nitrate to ammonia.

[0055] Example 5

[0056] Fe-based nanometallic particles (referred to as Fe), different proportion of cobalt-doped nanometer cobalt-iron solid solution particles (referred to as Co x Fe 1-x ), cobalt-based nanometallic particles (referred to as Co) as a comparative object, the effect of cobalt doping on the removal rate and selectivity of neutral electrocatalytic nitrate reduction reaction in iron-based nanometallic particles was studied.

[0057] Neutral electrolyte electrocatalytic nitrate reduction test: (1) The pre-test work is shown in Example 4. (2) i-t curve test: The time-current curve test was carried out at a constant potential of-0.69V (vs. RHE), and every 3h was stopped and a small amount of electrolyte was taken for dilution, and continued to the 9th hour. (3) Spectroscopic specific detection of NO3 - , NO2 - , NH4 + : 1, configure a series of concentration gradient of NO3 - , NO2 - , NH4 + standard solution and test to obtain the corresponding absorbance-concentration standard baseline. 2, by measuring the absorption spectrum of the electrolyte reacted for different times, the concentration of NO3 - , NO2 - , NH4 + in the electrolyte at different reaction times was calculated by bringing the absorbance-concentration standard baseline, and the results are shown in Figure 5 As can be seen from Figure 5 , it is found that the removal rate of NO3 - and the selectivity of synthetic ammonia both show a volcano-shaped distribution with the doping ratio, indicating that the electrocatalytic reduction of nitrate to ammonia has the best performance when Co is added (i.e. Co3Fe7), which can obtain a NO3 - removal rate of 97.8% and a synthetic ammonia selectivity of 99.8%.

[0058] Examples 6-8

[0059] Co 0.4 Fe 0.6As the research object, test its NO3 - removal rate, faradic efficiency and ammonia synthesis selectivity in a wide range of reduction potentials, electrolytes and nitrate concentrations.

[0060] Neutral electrolyte electrocatalytic nitrate reduction test: (1) Test pre-work as in Example 4. (2) i-t curve test and spectral specificity detection conditions: see Example 5. Test results are as shown in Figures 6-8 Figure 6 , Figure 7 , Figure 8 respectively represent Co 04 Fe 06 In a wide range of reduction potentials (-0.49 to -0.89 V), a wide range of electrolytes (sodium sulfate molar concentration 0.02 to 0.2 M), and a wide range of nitrate concentrations (NO3 — N concentration in electrolyte 50 to 1000 ppm), all have excellent nitrate reduction performance.

[0061] The results of the examples show that the iron-cobalt nanometer alloy particle preparation method provided by the application can effectively improve the neutral nitrate reduction performance of iron-based nanometer metal particles by doping cobalt. The obtained optimal Co 0.4 Fe 0.6 Catalyst can achieve excellent NO3 - removal rate, faradic efficiency and ammonia synthesis selectivity in a wide range of reduction potentials, electrolyte concentrations and nitrate concentrations. Compared with the reported iron and cobalt-based neutral nitrate reduction electrocatalysts, it has the advantages of better performance, more efficient reaction, more energy-saving process and simpler synthesis method, and has potential value for scale promotion. In addition, this method also has important reference value for the preparation of more types of transition metal-based nanometer alloys with certain morphology in the future.​

Claims

1. A method for preparing an iron-cobalt nanalloy electrocatalyst, characterized in that, The specific steps are as follows: (1) different proportions of iron and cobalt salt precursors are dispersed into water to form a suspension A, while the base is dispersed in a mixed solution of alcohol and organic acid to form a soap B, and the suspension A is poured into the soap B to form a suspension C after stirring and mixing until the soap B is fully dissolved; The iron salt used is one or more of acetylacetone iron, iron chloride, iron nitrate and iron sulfate containing trivalent iron ions, and the required amount of iron salt is in the range of 0.01-0.50 mol / L; the cobalt salt used is one or more of acetylacetone cobalt, cobalt chloride, cobalt nitrate and cobalt sulfate containing trivalent cobalt ions, and the required amount of cobalt salt is in the range of 0.01-0.50 mol / L; In step (1), to obtain an electrocatalyst with uniform size and controllable morphology, the volume of water in the raw materials is in the range of 20-80 mL, the mass of the base in the raw materials is in the range of 0.5-4.0 g, the volume of the mixed solution of alcohol and organic acid in the raw materials is in the range of 20-80 mL, and the mixed volume ratio of alcohol and organic acid is in the range of 0.2-5.0; (2) the suspension C is poured into the inner container of the solvothermal reactor, then sealed, and kept at a set temperature for a certain period of time to form iron-cobalt oxide, which is cooled, washed and dried to obtain cobalt-doped iron oxide precursor nanoparticles; (3) the cobalt-doped iron oxide precursor nanoparticles are placed in a tube furnace in a reducing atmosphere, and kept at a set temperature for a certain period of time, and then cooled to obtain the target iron-cobalt nano-alloy electrocatalyst, which has a particle size in the range of 5-500 nm; the iron-cobalt nano-alloy electrocatalyst includes the following components in atomic percentage, cobalt is 0.01%-99.99%, and the balance is iron.

2. The method of claim 1, wherein the iron-cobalt nanalloy electrocatalyst is prepared by the steps of: In step (1), the iron salt is acetylacetone iron, and the molar concentration is 0.06 mol / L; the cobalt salt is acetylacetone cobalt, and the molar concentration is 0.04 mol / L.

3. The method for preparing the iron-cobalt nano-alloy electrocatalyst according to claim 1, characterized in that, In step (1), the volume of water in the raw materials is 40 mL, the mass of the base in the raw materials is 2.0 g, the volume of the mixed solution of alcohol and organic acid in the raw materials is 40 mL, and the mixed volume ratio of alcohol and organic acid is 1.

0.

4. The method of claim 1, wherein the iron-cobalt nanalloy electrocatalyst is prepared by the steps of: In step (1), the base used is one or more of sodium hydroxide, potassium hydroxide and ammonia water, the alcohol used is one or more of methanol, ethanol and ethylene glycol, and the organic acid used is one or more of oleic acid, linoleic acid and stearic acid.

5. The method of claim 1, wherein the iron-cobalt nanalloy electrocatalyst is prepared by the steps of: In step (2), the iron-cobalt oxide precursor is prepared by solvothermal method, the material of the reactor used for solvothermal reaction is one of stainless steel, aluminum alloy, copper and tantalum, and the inner container of the reactor is one of polytetrafluoroethylene and high-density polyethylene; the capacity of the inner container is in the range of 50 mL-200 mL, the temperature of the solvothermal reaction is in the range of 140-220 ℃, and the reaction time is in the range of 2-24 h.

6. The method for preparing the iron-cobalt nanalloy electrocatalyst according to claim 5, characterized by, In step (2), the temperature of the solvothermal reaction is 180 ℃, and the reaction time is 10 h.

7. The method of claim 1, wherein the iron-cobalt nanalloy electrocatalyst is prepared by the steps of: In step (3), the target iron-cobalt nanometer alloy electrocatalyst is prepared by low-temperature thermal reduction method, wherein: the heating rate of the tubular furnace is 0.5-10 ℃ / min, the holding temperature is 50-1100 ℃, and the holding time is 0.5-24 h; the reducing atmosphere of the tubular furnace is one of the following: ammonia / argon mixed gas, the volume percentage of ammonia is 5%-100%; ammonia / nitrogen mixed gas, the volume percentage of ammonia is 5%-100%; hydrogen / argon mixed gas, the volume percentage of hydrogen is 5%-100%; hydrogen / nitrogen mixed gas, the volume percentage of hydrogen is 5%-100%.

8. The method of claim 7, wherein the iron-cobalt nanalloy electrocatalyst is prepared by the steps of: In step (3), the heating rate of the tubular furnace is 5 ℃ / min, the holding temperature is 350 ℃, and the holding time is 2 h.

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