A nickel-zinc alloy catalyst doped with a metal oxide promoter, and a preparation method and application thereof

Abstract

The application aims to provide a nickel-zinc alloy catalyst doped with a metal oxide additive, a preparation method and application thereof, and belongs to the technical field of waste gas treatment and environmental protection catalysis. x Zn@yMO n -Al2O3, wherein x represents the molar ratio of nickel and zinc in feeding, y represents the mass fraction of the additive, and MO n represents the oxide of different alkaline earth metals or rare earth metals; the catalyst has good catalytic performance in a methane dry reforming reaction, can achieve CH4 conversion rate of about 46%, CO2 conversion rate of about 70% and product synthesis gas hydrogen-carbon ratio of about 1 under harsh conditions of methane enrichment, and no carbon deposition is generated in 100 h stability test.

Description

Technical Field

[0001] This invention belongs to the field of waste gas treatment and environmental catalysis technology, specifically relating to a nickel-zinc alloy catalyst doped with metal oxide additives, its preparation method, and its application. Background Technology

[0002] With the concept of a circular economy gaining widespread acceptance, the conversion and utilization of biomass has attracted considerable attention. Some biomass gases can be used to replace fossil fuels in the production of syngas. Biogas is an important renewable biomass energy source, primarily composed of approximately 60% methane (CH4), 30% carbon dioxide (CO2), 0-5% nitrogen (N2), and less than 1% hydrogen (H2). Direct emissions of biogas, with its large amounts of CH4 and CO2 as major greenhouse gases, would have a significant impact on the environment, climate change, and ecosystem balance. However, utilizing the abundant CH4 and CO2 in biogas as important carbon resources through the dry methane reforming (DRM) route allows for the resource utilization of these two gases. The syngas produced can then be further converted into downstream high-value-added products through Fischer-Tropsch synthesis and other processes. Therefore, by leveraging the dry methane reforming reaction route, biogas, coke oven gas, and off-gas rich in methane and carbon dioxide can be fully recovered and utilized, resulting in significant economic benefits and effectively extending the biomass chemical and coal chemical product processing chain.

[0003] Methane dry reforming is a strongly endothermic reaction with a volume increase, and its critical temperature is 640℃. It requires relatively high temperatures to occur, thus catalysts are often added to lower the reaction energy barrier. However, when the temperature is between 557℃ and 700℃, side reactions such as methane cracking and CO disproportionation occur, producing C as a byproduct. α It can be reduced by CO2, H2O, and H2, but at the same time, C α It can also form C through aggregation and rearrangement. β Species, C β Low activity and poor conversion make it prone to forming encapsulated graphite carbon or filamentous carbon, which clogs the active metal sites of the catalyst, especially in gases such as biogas. When methane is in excess, the carbon deposition caused by methane cracking at high temperatures is more severe, leading to a faster catalyst deactivation rate. Meanwhile, due to the presence of the counter-current gasification side reaction, the consumption of H2 and the generation of CO2 result in a product syngas (H2 / CO) ratio of less than 1, which is not conducive to its direct use as a chemical feedstock. Appropriately increasing the methane ratio can control the hydrogen-to-carbon ratio; therefore, improving the catalyst's resistance to carbon deposition in methane-rich dry reforming reactions is crucial.

[0004] In catalysts used for the DRM reaction, noble metals such as Ru, Rh, Pd, and Pt, as active metal components, exhibit excellent performance but are expensive. Non-noble metal catalysts, often based on Ni, possess activity rates comparable to noble metals, but are prone to carbon deposition, embedding, and high-temperature sintering, leading to catalyst deactivation. Introducing transition metals into alloys with Ni to improve catalyst activity and stability is one effective strategy. Nickel-zinc alloys, by expanding the interstitial radius of cubic Ni octahedrons, promote C cycling. Furthermore, different supports and promoters can also affect catalytic activity; enhancing the basicity of the support is beneficial for CO2 adsorption, thereby accelerating C cycling. α The conversion process reduces carbon buildup. By introducing small amounts of alkaline earth metal oxides or rare earth metal oxides as promoters, the acidity / alkalinity of the catalyst, the interaction forces between the active component and the support, and the electron density of metal atoms can be controlled, thereby improving the catalyst's performance and resistance to carbon buildup. Summary of the Invention

[0005] The purpose of this invention is to provide a nickel-zinc alloy catalyst doped with metal oxide additives, its preparation method and application. The catalyst prepared by this method achieves stable conversion of CO2 and CH4 under methane-rich conditions, exhibits good resistance to carbon deposition, and achieves a synthesis gas ratio (H2 / CO2) of 1, which is beneficial to downstream industrial processes.

[0006] The present invention adopts the following technical solution:

[0007] A nickel-zinc alloy catalyst doped with a metal oxide additive, comprising metallic Ni, metallic Zn, metal oxide Al2O3, and an alkaline earth metal oxide or rare earth metal oxide MO as the additive. n Composition; the molecular formula of the catalyst is Ni x Zn@yMO n -Al2O3, where: x represents the molar ratio of nickel to zinc, y represents the mass fraction of the doped additive, and MO n This refers to oxides of different alkaline earth metals or rare earth metals.

[0008] Furthermore, 1 ≤ x ≤ 3.

[0009] Furthermore, the adjuvant MO n The mass fraction y is 1 wt%-5 wt%.

[0010] Furthermore, the content of the metallic Ni is 34 wt%-62 wt%.

[0011] Furthermore, the alkaline earth metal oxide or rare earth metal oxide MO n It includes at least one of MgO, CaO, CeO2, and La2O3.

[0012] A method for preparing a nickel-zinc alloy catalyst doped with a metal oxide additive includes the following steps:

[0013] The first step is to prepare an alkaline solution: dissolve the soluble carbonate completely to obtain solution A, and then uniformly disperse the zinc source ZIF-8 in solution A;

[0014] The second step is to prepare a metal ion solution: fully dissolve the nickel salt solution, aluminum salt solution, and soluble salt containing metal element M to obtain solution B;

[0015] The third step is to synthesize the metal oxide precursor: add solution B to solution A, add a precipitant to adjust the pH to 8-12, stir evenly and let stand, centrifuge and wash until the solution is neutral, dry and then calcine to obtain the metal oxide precursor.

[0016] Step 4, reduction: The metal oxide precursor is reduced in an H2 atmosphere to obtain the nickel-zinc alloy catalyst with the doped additive.

[0017] Furthermore, the soluble carbonate mentioned in the first step includes sodium carbonate or potassium carbonate, and the concentration of solution A is 1-2 mol / L.

[0018] Furthermore, the nickel salt mentioned in the second step includes one or a mixture of nickel nitrate, nickel sulfate, and nickel acetate, and the concentration of the nickel salt solution is 0.5-2 mol / L; the aluminum salt includes one or a mixture of aluminum nitrate, aluminum sulfate, aluminum acetate, and boehmite, and the concentration of the aluminum salt solution is 0.5-2 mol / L.

[0019] Furthermore, the molar ratio of the nickel salt solution to the aluminum salt solution in the second step is 3-1:1.

[0020] Furthermore, the soluble salt containing metal element M mentioned in the second step includes any one or a mixture of several of nitrates, sulfates and acetates, and the concentration of the soluble salt solution containing metal element M is 0.5-2 mol / L.

[0021] Furthermore, the precipitant mentioned in the third step includes any one of urea, ammonia, sodium hydroxide, potassium hydroxide, and a mixed precipitant of EDTA and sodium hydroxide.

[0022] Furthermore, the roasting temperature in the third step is 400-600℃, and the roasting time is 2-5 hours.

[0023] Furthermore, the reduction temperature in step four is 400-600℃, the reduction time is 2-5h, and the concentration of hydrogen in the reduction environment is 10-100%.

[0024] A nickel-zinc alloy catalyst doped with metal oxide additives is applied to the dry reforming reaction of methane-rich substances.

[0025] The preparation method of this nickel-zinc alloy catalyst doped with metal oxide additives effectively improves the catalyst's stability and resistance to carbon deposition, enabling stable conversion of methane and carbon dioxide even under harsh methane-rich conditions, with a syngas (H2 / CO) ratio of 1. Furthermore, this catalyst employs a co-precipitation synthesis method, avoiding the drawbacks of hydrothermal synthesis, which involves high temperature, high pressure, and long cycles, resulting in a simpler and lower-cost operation. The doping additives effectively improve the interaction between the metal and the support, ensuring uniform dispersion of metal particles. Simultaneously, they modify the acidic sites on the alumina support surface, increasing the catalyst's surface basicity and enhancing its adsorption and activation capacity for CO2 molecules. This allows for the timely conversion of C atoms generated from methane cracking, preventing the formation of C-C bonds that could lead to catalyst deactivation.

[0026] The beneficial effects of this invention are as follows:

[0027] The above-described technical solution of the present invention can synthesize a nickel-zinc alloy catalyst doped with metal oxide additives. On the one hand, this catalyst solves the problem of catalyst deactivation due to carbon deposition caused by methane cracking under methane-rich conditions, effectively improving the catalyst's stability. On the other hand, this catalyst exhibits good CH4 and CO2 activity in methane-rich dry reforming reactions, and can control the product syngas (H2 / CO) ratio to 1, meeting the needs of the high-value-added chemical product manufacturing field. Attached Figure Description

[0028] Figure 1 The image shows the XRD pattern of the precursor before calcination in Example 1.

[0029] Figure 2 This is a SEM image of the precursor before calcination in Example 1.

[0030] Figure 3 The image shows the XRD pattern of the metal oxide precursor in Example 1.

[0031] Figure 4 This is a SEM image of the metal oxide precursor in Example 1.

[0032] Figure 5 This is the BET diagram of the metal oxide precursor in Example 1.

[0033] Figure 6 The image shows the XRD pattern of the metal alloy catalyst in Example 1.

[0034] Figure 7 This is a SEM image of the metal alloy catalyst in Example 1.

[0035] Figure 8 This is a graph showing the catalyst stability test results for experimental group 2.

[0036] Figure 9 The image shows the XRD pattern of the catalyst after stability testing in experimental group 2. Detailed Implementation

[0037] Example 1

[0038] A method for preparing a nickel-zinc alloy catalyst doped with a metal oxide additive includes:

[0039] Step S1: Dissolve 7.95 g of anhydrous sodium carbonate in 100 mL of deionized water, stir well, then add 2.15 g of ZIF-8 to the solution and stir for 30 min to obtain a milky white solution.

[0040] Step S2: Dissolve 8.72g Ni(NO3)2·6H2O, 3.75g Al(NO3)3·9H2O, and 0.47g Ca(NO3)2·4H2O in deionized water and stir for 30 min to ensure complete dissolution of the salt solution. Add the salt solution dropwise to the milky white solution, add NaOH as a precipitant, adjust the pH to 10, and stir until homogeneous. Transfer the solution to a centrifuge cup and centrifuge at 4500 r / min for 3 min to obtain the product. Centrifuge the product again with distilled water, wash until neutral, and then dry in an oven to obtain a green solid powder. Finally, calcine the powder in a muffle furnace at a heating rate of 2℃ / min at 500℃ for 3 h to obtain the metal oxide precursor NiO-ZnO@2.5CaO-Al2O3.

[0041] Step S3: The metal oxide precursor NiO-ZnO@2.5CaO-Al2O3 obtained in step S2 is reduced at 550℃ under a hydrogen atmosphere for 2 h to obtain the metal alloy catalyst Ni3Zn@2.5CaO-Al2O3.

[0042] Example 2

[0043] A method for preparing a nickel-zinc alloy catalyst doped with a metal oxide additive includes:

[0044] Step S1: Dissolve 7.95 g of anhydrous sodium carbonate in 100 mL of deionized water, stir well, then add 2.15 g of ZIF-8 to the solution and stir for 30 min to obtain a milky white solution.

[0045] Step S2: Dissolve 8.72g Ni(NO3)2·6H2O, 3.75g Al(NO3)3·9H2O, and 0.94g Ca(NO3)2·4H2O in deionized water and stir for 30 min to ensure complete dissolution of the salt solution. Add the salt solution dropwise to the milky white solution, add NaOH as a precipitant, adjust the pH to 10, and stir until homogeneous. Transfer the solution to a centrifuge cup and centrifuge at 4500 r / min for 3 min to obtain the product. Centrifuge the product again with distilled water, wash until neutral, and then dry in an oven to obtain a green solid powder. Finally, calcine the powder in a muffle furnace at a heating rate of 2℃ / min at 500℃ for 3 h to obtain the metal oxide precursor NiO-ZnO@5.0CaO-Al2O3.

[0046] Step S3: The metal oxide precursor NiO-ZnO@5.0CaO-Al2O3 obtained in step S2 is reduced at 550℃ under a hydrogen atmosphere for 2 h to obtain the metal alloy catalyst Ni3Zn@5.0CaO-Al2O3.

[0047] Example 3

[0048] A method for preparing a nickel-zinc alloy catalyst doped with a metal oxide additive includes:

[0049] Step S1: Dissolve 7.95 g of anhydrous sodium carbonate in 100 mL of deionized water, stir well, then add 2.15 g of ZIF-8 to the solution and stir for 30 min to obtain a milky white solution.

[0050] Step S2: Dissolve 8.72g Ni(NO3)2·6H2O, 3.75g Al(NO3)3·9H2O, and 0.26g Ce(NO3)3·6H2O in deionized water and stir for 30 min to ensure complete dissolution of the salt solution. Add the salt solution dropwise to the milky white solution, add ammonia as a precipitant, adjust the pH to 9.5, and stir until homogeneous. Transfer the solution to a centrifuge cup and centrifuge at 4500 r / min for 3 min to obtain the product. Centrifuge the product again with distilled water, wash until neutral, and then dry in an oven to obtain a green solid powder. Finally, calcine the powder in a muffle furnace at a heating rate of 2℃ / min at 500℃ for 3 h to obtain the metal oxide precursor NiO-ZnO@2.5CeO2-Al2O3.

[0051] Step S3: The metal oxide precursor NiO-ZnO@2.5CeO2-Al2O3 obtained in step S2 is reduced at 600℃ under a hydrogen atmosphere for 2 h to obtain the metal alloy catalyst Ni3Zn@2.5CeO2-Al2O3.

[0052] Example 4

[0053] A method for preparing a nickel-zinc alloy catalyst doped with a metal oxide additive includes:

[0054] Step S1: Dissolve 7.95 g of anhydrous sodium carbonate in 100 mL of deionized water, stir well, then add 2.15 g of ZIF-8 to the solution and stir for 30 min to obtain a milky white solution.

[0055] Step S2: Dissolve 8.72g Ni(NO3)2·6H2O, 3.75g Al(NO3)3·9H2O, and 0.60g Ce(NO3)3·6H2O in deionized water and stir for 30 min to ensure complete dissolution of the salt solution. Add the salt solution dropwise to the milky white solution, add ammonia as a precipitant, adjust the pH to 9.5, and stir until homogeneous. Transfer the solution to a centrifuge cup and centrifuge at 4500 r / min for 3 min to obtain the product. Centrifuge the product again with distilled water, wash until neutral, and then dry in an oven to obtain a green solid powder. Finally, calcine the powder in a muffle furnace at a heating rate of 2℃ / min at 500℃ for 3 h to obtain the metal oxide precursor NiO-ZnO@5.0CeO2-Al2O3.

[0056] Step S3: The metal oxide precursor NiO-ZnO@5.0CeO2-Al2O3 obtained in step S2 is reduced at 600 °C under a hydrogen atmosphere for 2 h to obtain the metal alloy catalyst Ni3Zn@5.0CeO2-Al2O3.

[0057] The performance of the catalyst prepared in Example 1 was tested.

[0058] Experimental group 1

[0059] The performance of the catalyst prepared in Example 1 was investigated using a fixed-bed reactor. The fixed-bed reactor had an outer diameter of 12 mm, an inner diameter of 8 mm, and a length of 485 mm. Reactants and products were detected by a gas chromatograph equipped with a thermal conductivity detector (TCD) and a soap bubble flow meter.

[0060] Reaction conditions: 0.05 g catalyst mixed with 0.5 g quartz sand was loaded into the isothermal zone of the fixed-bed reactor and heated to 600℃ under Ar atmosphere. The mixture was then switched to a CH4 / CO2 / Ar mixture of 1.2 / 1 / 7.8 at a flow rate of 60 mL / min. The reaction pressure was atmospheric pressure, which refers to one atmosphere, or 1 bar.

[0061] Experimental groups 2-5

[0062] The difference from experimental group 1 is that the volumetric flow rate of CH4 in the mixed gas was 1.4, 1.6, 1.8 and 2.0, respectively.

[0063] Test Example 1

[0064] The precursor green solid powder before calcination in step S2 of Example 1 was characterized.

[0065] Figure 1 This is the XRD pattern of the green solid powder precursor in Example 1. From... Figure 1 As can be seen, characteristic diffraction peaks of hydrotalcite appeared at 2θ of 11.6°, 23.5°, 34.2°, 38.8°, and 60.6°. These peaks are attributed to the (003), (006), (009), (015), and (113) crystal planes of LDHs, respectively, proving that the catalyst precursor has an LDHs structure. Characteristic diffraction peaks at 2θ of 31.8°, 34.4°, 36.3°, 47.5°, 56.6°, 62.9°, 66.4°, and 67.9° are all attributed to ZnO.

[0066] Figure 2 This is a SEM image of the green solid powder before calcination in Example 1. This shows the morphological characteristics of the catalyst before calcination.

[0067] Test Example 2

[0068] The metal oxide precursor after calcination in step S2 of Example 1 was characterized.

[0069] Figure 3 The image shows the XRD pattern of the metal oxide NiO-ZnO@2.5CaO-Al2O3 in Example 1. Figure 3 As can be seen, characteristic diffraction peaks belonging to the NiO (111), (200), and (220) crystal planes appeared at 2θ values ​​of 37.2°, 43.3°, and 62.9°. Similarly, characteristic diffraction peaks belonging to ZnO were observed at 2θ values ​​of 31.8°, 34.4°, 36.3°, 47.5°, 56.6°, 62.9°, 66.4°, and 67.9°. No characteristic diffraction peaks of CaO were obtained because the CaO content was low and the particle dispersion was high.

[0070] Figure 4 The image shows a SEM image of the metal oxide NiO-ZnO@2.5CaO-Al2O3 in Example 1, illustrating the morphology and structure of the NiO-ZnO@2.5CaO-Al2O3 catalyst after calcination.

[0071] Figure 5 The BET diagram for the metal oxide NiO-ZnO@2.5CaO-Al2O3 in Example 1 is shown below. Figure 5As can be seen, its adsorption-desorption isotherm is type IV, and an H3 type hysteresis loop appears, indicating that there is a mesoporous structure in the catalyst.

[0072] Test Example 3

[0073] The metal alloy catalyst reduced in step S3 of Example 1 was characterized.

[0074] Figure 6 The image shows the XRD pattern of the metal alloy catalyst Ni3Zn@2.5CaO-Al2O3 in Example 1. Figure 6 As can be seen, characteristic diffraction peaks belonging to the Ni3Zn alloy appear at 2θ of 43.7°, 50.9°, and 75.3°, indicating that the Ni3Zn alloy was successfully formed during the hydrogen reduction process of the metal oxide NiO-ZnO@2.5CaO-Al2O3. The absence of characteristic diffraction peaks for Al2O3 is due to the low crystallinity of this oxide, making it undetectable by XRD.

[0075] Figure 7 This is a SEM image of the Ni3Zn@2.5CaO-Al2O3 metal alloy catalyst in Example 1. The image illustrates the morphology and structure of the Ni3Zn@2.5CaO-Al2O3 catalyst.

[0076] Test Example 4

[0077] Figure 8 The figure shows the catalyst stability test results for experimental group 2. The catalyst can stably achieve a CH4 conversion of about 46% and a CO2 conversion of about 70% in the methane-rich dry reforming reaction, and no carbon deposition is produced, which indicates that the catalyst has good catalytic performance.

[0078] Table 1 is compiled based on the test data of experimental groups 1-5.

[0079] Table 1. Effect of different methane concentrations on the Ni3Zn@Al2O3-2.5CaO catalyst in methane-rich dry reforming reaction.

[0080]

[0081] As shown in Table 1, the catalyst synthesized in this invention maintains near-equilibrium CH4 conversion and high CO2 conversion under methane-rich atmosphere conditions. The CH4 conversion and CO2 conversion reach 46% and 70% respectively when the CH4:CO2 ratio is 1.4, and the H2 / CO ratio is approximately 1. This methane-rich dry reforming reaction can achieve different H2 / CO ratios by adjusting the methane concentration.

[0082] Test Example 5

[0083] The catalyst in Experimental Group 2 was characterized after stability testing.

[0084] Figure 9 The image shows the XRD pattern of the catalyst after stability testing in experimental group 2. Figure 9 It is evident that after 100 h of reaction, the peaks of the Ni3Zn alloy in the Ni3Zn@2.5CaO-Al2O3 catalyst shifted. Specifically, the (111) crystal plane shifted from 43.7° to 42.8°, the (200) crystal plane from 50.9° to 49.8°, and the (220) crystal plane from 75.3° to 73.1°, indicating that it transformed into an interstitial carbide catalyst. This transformation occurred because, during the methane dry reforming reaction, carbon atoms generated from methane cracking entered the Ni3Zn octahedral interstitial lattice, forming interstitial carbon atoms, causing the lattice to expand and the diffraction peaks to shift to lower angles. The formation of these interstitial carbon atoms can prevent the formation of C-C bonds during the reaction, thus avoiding carbon deposition and enhancing the stability of the catalyst.

[0085] In summary, the nickel-zinc alloy catalyst with doped additives prepared in this invention maintains high conversion rates and stability of CH4 and CO2 under the harsh conditions of methane-rich dry reforming, and the product syngas (H2 / CO) ratio can be controlled. At a CH4:CO2 ratio of 1.4, the following results can be achieved: CH4 conversion of approximately 46% and CO2 conversion of approximately 70%, with an H2 / CO ratio of approximately 1. The Ni3ZnC formed during the reaction... 0.7 Interstitial carbides effectively prevent the formation of C-C bonds, exhibiting excellent anti-carbon deposition properties. Furthermore, this catalyst is produced using a co-precipitation method, which is simple, low-cost, and easily industrial-scaled.

Claims

1. The application of a nickel-zinc alloy catalyst doped with metal oxide additives in the dry reforming reaction of methane-rich reactors, characterized in that: The catalyst is composed of metallic Ni, metallic Zn, and metal oxide Al2O3, with doped auxiliary agents of alkaline earth metal oxides or rare earth metal oxides MO. n Composition; the molecular formula of the catalyst is Ni x Zn@yMO n -Al2O3, where: x represents the molar ratio of nickel to zinc, y represents the mass fraction of the doped additive, and MO n Represents oxides of different alkaline earth metals or rare earth metals; The molar ratio of nickel to zinc is: 1 ≤ x ≤ 3; The adjuvant MO n The mass fraction y is 1 wt%-5 wt%; The content of the metallic Ni is 34 wt%-62 wt%; The alkaline earth metal oxide is CaO, and the rare earth metal oxide is CeO2; In the methane-rich dry reforming reaction, the volume ratio of methane to carbon dioxide is 1.2, 1.4, 1.6, 1.8, or 2.

0.

2. The application according to claim 1, characterized in that: The preparation method of the nickel-zinc alloy catalyst doped with metal oxide additives includes the following steps: The first step is to prepare an alkaline solution: dissolve the soluble carbonate completely to obtain solution A, and then uniformly disperse the zinc source ZIF-8 in solution A; The second step is to prepare a metal ion solution: fully dissolve the nickel salt solution, aluminum salt solution, and soluble salt containing metal element M to obtain solution B; The third step is to synthesize the metal oxide precursor: add solution B to solution A, add a precipitant to adjust the pH to 8-12, stir evenly and let stand, centrifuge and wash until the solution is neutral, dry and then calcine to obtain the metal oxide precursor. Step 4, reduction: The metal oxide precursor is reduced in an H2 atmosphere to obtain the nickel-zinc alloy catalyst with the doped additive.

3. The application according to claim 2, characterized in that: The soluble carbonates mentioned in the first step include sodium carbonate or potassium carbonate, and the concentration of solution A is 1-2 mol / L.

4. The application according to claim 2, characterized in that: The nickel salt mentioned in the second step includes one or a mixture of nickel nitrate, nickel sulfate, and nickel acetate, and the concentration of the nickel salt solution is 0.5-2 mol / L; the aluminum salt includes one or a mixture of aluminum nitrate, aluminum sulfate, and aluminum acetate, and the concentration of the aluminum salt solution is 0.5-2 mol / L. The molar ratio of the nickel salt solution to the aluminum salt solution is 3-1:

1.

5. The application according to claim 2, characterized in that: The soluble salt containing metal element M mentioned in the second step includes any one or a mixture of several of nitrates, sulfates and acetates, and the concentration of the soluble salt solution containing metal element M is 0.5-2 mol / L.

6. The application according to claim 2, characterized in that: The precipitant mentioned in the third step includes any one of urea, ammonia, sodium hydroxide, potassium hydroxide, and a mixed precipitant of EDTA and sodium hydroxide; The roasting temperature in the third step is 400-600℃, and the roasting time is 2-5 hours.

7. The application according to claim 2, characterized in that: The reduction temperature in step four is 400-600℃, the reduction time is 2-5h, and the concentration of hydrogen in the reduction environment is 10-100%.

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