A Preparation Method of Palladium Nanoplate Electrode Catalyst Supported on Titanium Carbide

Abstract

The present invention discloses a preparation method of a palladium nanosheet electrode catalyst supported on titanium carbide in the field of electrocatalytic technology, aiming to solve problems such as the aggregation and stacking of two-dimensional palladium nanostructures and the hindrance of the synergistic effect between the substrate and palladium metal in the prior art. It includes: preparing a reaction precursor of titanium carbide nanosheets; adding the reaction precursor of titanium carbide nanosheets into a 1-methyl-2-pyrrolidone solution, and performing ultrasonic dispersion to obtain a mixed solution I; adding polyvinylpyrrolidone and hexacarbonylmolybdenum into the mixed solution I, and performing ultrasonic mixing to obtain a uniform mixed solution II; adding palladium acetylacetonate into the mixed solution II, uniformly mixing and then adding acetic acid, and performing an oil bath reaction to obtain a black precipitate, followed by centrifugal washing and freeze-drying to obtain a palladium nanosheet / titanium carbide catalyst. The palladium nanosheet electrode catalyst supported on titanium carbide prepared by the present invention has a large specific surface area and a high electron mobility, and can effectively improve the electrocatalytic activity.

Description

Technical Field

[0001] The present invention relates to a preparation method of a palladium nanosheet electrode catalyst supported on titanium carbide, belonging to the technical field of electrocatalysis. Background Art

[0002] The environmental pollution caused by the extensive use of fossil fuels and the increasing global demand for energy have forced people to explore and develop eco-friendly renewable energy to replace non-renewable fossil fuels. Direct methanol fuel cells (DMFCs) have gradually attracted wide attention due to their own advantages, such as wide raw material sources, high energy conversion efficiency, and broad operating temperature ranges. However, problems such as the slow kinetics of the methanol oxidation reaction and the easy generation of poisoning by-products need to be solved urgently. Therefore, the preparation of a reasonable anode catalyst is crucial for the development of DMFCs.

[0003] Catalysts are the core components of fuel cells and greatly affect the performance of the cells. Currently, the commonly used anode catalysts are platinum and platinum-based materials. However, due to the low reserves and high costs of platinum and platinum-based materials, they cannot be commercialized on a large scale. Through research, it has been found that palladium materials, which belong to the same family as platinum, have similar catalytic properties, but their reserves are more abundant and can be effective substitutes for platinum. However, the activity and stability of palladium-based catalysts limit their applications. Therefore, many studies have adjusted the performance of palladium-based catalysts by optimizing the morphology of palladium, creating many nanostructures, including 0D nanostructures (nanoparticles), 1D nanostructures (nanorods, nanotubes), 2D nanostructures (nanosheets), and 3D nanostructures (nanoflowers, nanorings). Among them, 2D palladium nanostructures have advantages such as a large specific surface area, high electron mobility, and a large number of unsaturated active palladium atoms, thus showing great catalytic advantages. However, most 2D nanostructures exhibit disordered stacking and aggregation, which hinders the generation of active sites and thus the electrooxidation process of methanol. To solve this problem, further reasonable adjustment of two-dimensional palladium nanostructures is required.

[0004] On the other hand, commercial palladium catalysts usually choose to support palladium on carbon carriers, which can improve the utilization rate of palladium and, to a certain extent, improve its electrocatalytic performance. However, due to the high graphitization of carbon carriers, they are chemically inert and cannot directly participate in the electrooxidation process. In addition, the lack of active growth points on the surface of some carbon matrices greatly hinders the synergistic effect between the matrix and palladium metal. In recent years, two-dimensional titanium carbide nanosheet materials have attracted much attention due to their high specific surface area, excellent metallic conductivity, good hydrophilicity, and adjustable surface chemical properties, and are considered ideal new-generation catalyst carriers. Research has found that introducing a titanium carbide carrier can optimize the electronic structure of the palladium component, thereby improving its intrinsic electrocatalytic performance.

[0005] Therefore, adjusting the morphology and structure of the palladium catalyst component on the surface of titanium carbide, improving the aggregation and stacking problems of two-dimensional palladium nanostructures, further optimizing their electronic structures, and enhancing the interfacial connection and synergistic coupling between palladium metal and the surface of titanium carbide are one of the hotspots and difficulties in the current field of electrocatalysis. Summary of the Invention

[0006] The object of the present invention is to overcome the deficiencies in the prior art and provide a preparation method of a palladium nanosheet electrode catalyst supported on titanium carbide. The palladium nanosheet electrode catalyst supported on titanium carbide prepared thereby has the advantages of a large specific surface area, a high electron mobility, sufficient interfacial bonding, and an optimized electronic structure, etc., and can effectively improve the electrocatalytic activity.

[0007] To achieve the above object, the present invention is implemented by the following technical solution: A preparation method of a palladium nanosheet electrode catalyst supported on titanium carbide, comprising the following steps,

[0008] S1. Prepare a reaction precursor of titanium carbide nanosheets;

[0009] S2. Add the reaction precursor of titanium carbide nanosheets into a 1-methyl-2-pyrrolidone solution, and perform ultrasonic dispersion to obtain a mixed solution I. The mass ratio of the 1-methyl-2-pyrrolidone solution to the reaction precursor is 824 - 1030:1;

[0010] S3. Add polyvinylpyrrolidone and molybdenum hexacarbonyl into the mixed solution I, and perform ultrasonic mixing to obtain a uniform mixed solution II. The mass ratio of polyvinylpyrrolidone, molybdenum hexacarbonyl to the reaction precursor contained in the mixed solution I is 8 - 12:0.2 - 0.8:1;

[0011] S4. Add palladium acetylacetonate into the mixed solution II, mix evenly, then add acetic acid, and perform an oil bath reaction to obtain a black precipitate. After centrifugation, washing, and freeze-drying, a palladium nanosheet / titanium carbide catalyst is obtained. Among them, the mass ratio of palladium acetylacetonate, acetic acid to the reaction precursor contained in the mixed solution II is 0.11 - 0.43:0.1 - 0.3:1.

[0012] Further, the preparation method of the reaction precursor of titanium carbide nanosheets specifically includes:

[0013] a. Add aluminum titanium carbide into a mixed solution of lithium fluoride and hydrochloric acid for etching to obtain a multi-layer titanium carbide precipitate;

[0014] b. Add distilled water into the multi-layer titanium carbide precipitate, perform centrifugal washing until neutral, the centrifugal speed is 3500 - 8000 rpm, then perform ultrasonic peeling, and then perform centrifugal screening, the centrifugal speed is 5000 - 8000 rpm, and take the supernatant for freeze-drying to obtain the reaction precursor of titanium carbide nanosheets.

[0015] Further, in step a, the etching time is 24 - 48 h, and the reaction temperature is 40 - 50 °C.

[0016] Further, in step b, the ultrasonic stripping conditions are as follows: the ultrasonic time is 0.5 - 1 h, and the protective gas is an argon atmosphere.

[0017] Further, in step S2, the mass ratio of the 1-methyl-2-pyrrolidone solution to the reaction precursor is 1030:1.

[0018] Further, in step S3, the mass ratio of polyvinylpyrrolidone, molybdenum hexacarbonyl to the reaction precursor contained in the mixed solution I is 10:0.5:1.

[0019] Further, in step S4, the mass ratio of palladium acetylacetonate, acetic acid to the reaction precursor contained in the mixed solution II is 0.25:0.2:1.

[0020] Further, in steps S2 and S3, the ultrasonic conditions are ultrasonic treatment for 10 - 30 min in the temperature range of 0 - 25 °C.

[0021] Further, in step S4, the oil bath conditions are reaction for 2 - 5 h at 140 °C; the centrifugal washing conditions are as follows: the solvent is ethanol, the number of washing times is 3 - 5 times, and the centrifugal rotation speed is 6000 - 9000 rpm; the drying pressure for freeze-drying is 0 - 200 Pa.

[0022] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0023] For the titanium carbide supported palladium nanosheet electrode catalyst prepared by the present invention, a uniform two-dimensional hexagonal palladium nanosheet structure is synthesized by a solvent-induced method. Through the coupling effect between the palladium nanosheets and titanium carbide, the electrochemically active performance of the catalyst is greatly improved;

[0024] By using titanium carbide as the substrate to support the palladium nanosheet structure, the stacking and aggregation of palladium can be effectively prevented, and the large layer spacing can ensure sufficient contact between the active sites and the reaction medium, which is beneficial to improving the catalytic activity;

[0025] The titanium carbide supported palladium nanosheet electrode catalyst prepared by the present invention has a uniform morphology, a uniform size distribution, and a high crystallinity of the formed grains. After testing, it has excellent electrocatalytic performance and durability, and has good application prospects and economic benefits in the fields such as direct methanol fuel cells;

[0026] The preparation method provided by the present invention is simple, controllable, has good repeatability, and low cost, which is conducive to large-scale industrial production. Description of the Drawings

[0027] Figure 1Schematic diagram of the preparation process of the embodiments of the present invention;

[0028] Figure 2 X-ray diffraction (XRD) pattern and X-ray photoelectron spectroscopy (XPS) schematic diagram of the titanium carbide-supported palladium nanosheet electrode catalyst prepared in the embodiments of the present invention;

[0029] Figure 3 Field emission scanning electron microscope (TEM) schematic diagram of the titanium carbide-supported palladium nanosheet electrode catalyst prepared in the embodiments of the present invention;

[0030] Figure 4 For the titanium carbide-supported palladium nanosheet electrode catalyst (PdNSs / Ti3C2T x ) and palladium / titanium carbide (Pd / Ti3C2T x ), palladium / graphene (Pd / RGO), palladium / carbon nanotube (Pd / CNT) and palladium / carbon black (Pd / C) materials, schematic diagram of cyclic voltammograms in 0.5 M NaOH solution (Figure a) and schematic diagram of cyclic voltammograms in a mixed solution of 0.5 M NaOH and 1 M CH3OH (Figure b);

[0031] Figure 5 For the titanium carbide-supported palladium nanosheet electrode catalyst (PdNSs / Ti3C2T x ) and palladium / titanium carbide (Pd / Ti3C2T x ), palladium / graphene (Pd / RGO), palladium / carbon nanotube (Pd / CNT) and palladium / carbon black (Pd / C) materials, schematic diagram of chronopotentiometry test curves in 0.5 M NaOH and 1 M CH3OH (Figure a); schematic diagram of electrochemical impedance spectroscopy test (Figure b). Detailed implementation manners

[0032] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention and cannot be used to limit the protection scope of the present invention.

[0033] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0034] For the purposes of this specification and the appended claims, unless otherwise stated, all numbers expressing quantities, percentages, or ratios, and other numerical values used in this specification and the appended claims are understood to be modified in all instances by the term "about". In addition, all ranges disclosed herein include the endpoints and are combinable independently.

[0035] A preparation method of a palladium nanosheet electrode catalyst supported on titanium carbide includes the following steps:

[0036] S1. Prepare a reaction precursor of titanium carbide nanosheets. The specific steps are as follows:

[0037] a. Add aluminum titanium carbide to a mixed solution of lithium fluoride and hydrochloric acid for etching to obtain a multi-layer titanium carbide precipitate;

[0038] b. Add distilled water to the multi-layer titanium carbide precipitate and perform centrifugal washing until neutral. The centrifugal speed is 3500 - 8000 rpm. Then perform ultrasonic exfoliation, and then perform centrifugal screening. The centrifugal speed is 5000 - 8000 rpm. Take the supernatant and freeze-dry it to obtain the reaction precursor of titanium carbide nanosheets;

[0039] S2. Add the reaction precursor of titanium carbide nanosheets to a 1-methyl-2-pyrrolidone solution, and perform ultrasonic dispersion to obtain a mixed solution I. The mass ratio of the 1-methyl-2-pyrrolidone solution to the reaction precursor is 824 - 1030:1;

[0040] S3. Add polyvinylpyrrolidone and molybdenum hexacarbonyl to the mixed solution I, and perform ultrasonic mixing to obtain a uniform mixed solution II. The mass ratio of polyvinylpyrrolidone, molybdenum hexacarbonyl to the reaction precursor contained in the mixed solution I is 8 - 12:0.2 - 0.8:1;

[0041] S4. Add palladium acetylacetonate to the mixed solution II, mix evenly, then add acetic acid, and perform an oil bath reaction to obtain a black precipitate. After centrifugal washing and freeze-drying, a palladium nanosheet / titanium carbide catalyst is obtained. Among them, the mass ratio of palladium acetylacetonate, acetic acid to the reaction precursor contained in the mixed solution II is 0.11 - 0.43:0.1 - 0.3:1.

[0042] There have been many reports in the prior art on the influence of the etching and ultrasonic dispersion conditions of titanium carbide on its dispersion degree, which will not be elaborated in this application. Therefore, the etching reaction temperature in step a is 40°C and the reaction time is 36 h; in step b, centrifugal water washing is performed at a speed of 6000 rpm, and ultrasonic exfoliation is performed at room temperature for 0.5 h, which are used as the preparation conditions of this application. The following examples all use these conditions.

[0043] Meanwhile, through preliminary experiments, within the scope of this application, changes in the stirring temperature and time, number of washing times, centrifugation speed, etc. in steps S2 - S4 of the present invention have little effect on the grain morphology and size of palladium nanosheets and their electrochemical performance, and thus will not be discussed separately in the present invention. Except for the raw material addition amounts, the preparation methods of the following examples are all carried out according to the preferred scheme of Example 1.

[0044] Example 1:

[0045] A preparation method of a titanium carbide - supported palladium nanosheet electrode catalyst, comprising the following steps:

[0046] S1. Prepare a reaction precursor of titanium carbide nanosheets;

[0047] S2. Add the reaction precursor of titanium carbide nanosheets into a 1 - methyl - 2 - pyrrolidone solution, and perform ultrasonic dispersion to obtain a mixed solution I. The mass ratio of the 1 - methyl - 2 - pyrrolidone solution to the reaction precursor is 1030:1;

[0048] S3. Add polyvinylpyrrolidone and molybdenum hexacarbonyl to the mixed solution I, and perform ultrasonic mixing to obtain a uniform mixed solution II. The mass ratio of polyvinylpyrrolidone, molybdenum hexacarbonyl to the reaction precursor contained in the mixed solution I is 10:0.5:1;

[0049] S4. Add palladium acetylacetonate to the mixed solution II, mix evenly, then add acetic acid, and perform an oil - bath reaction to obtain a black precipitate. After centrifugation and washing, and freeze - drying, a palladium nanosheet / titanium carbide catalyst is obtained. Among them, the mass ratio of palladium acetylacetonate, acetic acid to the reaction precursor contained in the mixed solution II is 0.25:0.2:1.

[0050] Example 2:

[0051] The difference between this example and Example 1 is that in step S4, the mass ratio of palladium acetylacetonate to the reaction precursor contained in the mixed solution I is 0.11:1.

[0052] Example 3:

[0053] The difference between this example and Example 1 is that in step S4, the mass ratio of palladium acetylacetonate to the precursor contained in the mixed solution 1 is 0.43:1.

[0054] Comparative Example 1:

[0055] The difference between this comparative example and Example 1 is only that an equal amount of ethylene glycol solvent is used to replace the 1 - methyl - 2 - pyrrolidone solution to obtain a palladium / titanium carbide (Pd / Ti3C2T x ) material sample.

[0056] Comparative Example 2:

[0057] The difference between this comparative example and Example 1 is only that an equal amount of graphene oxide is used to replace the titanium carbide nanosheets to obtain a palladium / graphene (Pd / RGO) material sample.

[0058] Comparative Example 3:

[0059] The difference between this comparative example and Example 1 is only that an equal amount of carbon nanotubes is used to replace the titanium carbide nanosheets to obtain a palladium / carbon nanotube (Pd / CNT) material sample.

[0060] Comparative Example 4:

[0061] The difference between this comparative example and Example 1 is only that an equal amount of carbon black is used to replace the titanium carbide nanosheets to obtain a palladium / carbon black (Pd / C) material sample.

[0062] Next, performance tests were carried out on the electrode catalyst samples obtained in Examples 1 to 3 and Comparative Examples 1 to 4.

[0063] First, the methanol oxidation reaction tests were carried out on the electrode catalysts prepared in Examples 1 to 3, and the results are shown in Table 1.

[0064] Table 1: Performance indicators of the electrode catalysts prepared in Examples 1 to 3 for the methanol oxidation reaction

[0065]

[0066]

[0067] In Examples 1 to 3, three different electrode catalysts were prepared by adjusting the ratio of palladium element and titanium carbide precursor. As can be seen from Table 1, all three examples have high catalytic activity. Comparing the data of Examples 1 to 3, it can be known that as the noble metal Pd loading increases, the active surface area and mass activity of the catalyst show a trend of first increasing and then decreasing.

[0068] This is because the catalytic reaction is only a surface reaction, and only the surface atoms can play a catalytic role, while the internal atoms do not participate in the reaction. In Example 2, the Pd loading is too low to achieve a good catalytic effect. In Example 3, when the Pd content is further increased, it will cause Pd atoms to stack and aggregate, which is not conducive to the improvement of catalytic efficiency; therefore, the addition amount of palladium can only have good catalytic activity under appropriate conditions.

[0069] Since the performance of the electrode catalyst prepared in Example 1 is the best and most stable among Examples 1 to 3, the palladium nanosheet electrode catalyst supported on titanium carbide prepared by the method of Example 1 is selected as an example for performance comparison with Comparative Examples 1 to 4 below.

[0070] 1) Structural characterization analysis

[0071] Combined with Figure 2 , where Figure a is the X-ray powder diffraction pattern (XRD pattern) of the palladium nanosheet electrode catalyst supported on titanium carbide prepared by the method of Example 1. From the figure, the characteristic peaks of metallic palladium and titanium carbide can be clearly seen. Figure b is the X-ray photoelectron spectroscopy (XPS pattern) of the palladium nanosheet electrode catalyst supported on titanium carbide prepared by the method of Example 1. Element signals such as C, N, O, Pd, Ti, and F can be seen from the measured spectrum.

[0072] 2) Transmission electron microscopy analysis

[0073] Figure 3 is the transmission electron micrograph of the palladium nanosheet electrode catalyst supported on titanium carbide. As can be seen from Figure 3 (a), palladium nanosheets are evenly distributed on the titanium carbide support; as can be seen from Figure 3 (b), the average diameter of the palladium nanosheets is about 25 nm.

[0074] The above results show that the palladium nanosheet electrode catalyst supported on titanium carbide of the present invention has an anti-stacking two-dimensional structure and a large specific surface area. The two-dimensional sheet structure can expose more active sites, so the catalyst has higher catalytic performance and electrochemical activity.

[0075] 3) Catalytic activity test

[0076] All material samples were subjected to electrochemical tests on a CHI760E electrochemical workstation using a conventional three-electrode test system (a platinum wire as the counter electrode, a saturated calomel electrode as the reference electrode, and a glassy carbon electrode with a diameter of 3 mm coated with the active material as the working electrode) for testing.

[0077] First, the electrochemically active surface area (ECSA) and the catalytic activity of methanol oxidation of the catalyst were both measured by cyclic voltammetry. The electrolytes were 0.5 mol / L NaOH solution and a mixed solution of 0.5 mol / L NaOH and 1 mol / L CH3OH, and the scanning rate was 50 mV / s -1 .

[0078] By calculation Figure 4 (a) curve, it can be obtained that the palladium nanosheet electrode catalyst supported on titanium carbide has the highest electrochemically active surface area (151.5 m 2 / g). At the same time, the catalytic performance of the palladium nanosheet electrode catalyst supported on titanium carbide for methanol oxidation was tested, as shown in Figure 4 (b). The mass activity of this catalyst is 1955.5 mA / mg. To further illustrate that this catalyst has high catalytic activity, a cyclic voltammetry test of the methanol oxidation reaction of different materials was also compared. FromFigure 4 It can be seen that both the active surface area and the mass activity of the palladium-loaded titanium carbide nanosheet electrode catalyst are significantly higher than those of the other four comparative samples, indicating its highest catalytic activity.

[0079] Next, the potentiostatic oxidation method and the chronopotentiometry method were used to evaluate the stability and methanol tolerance of the catalyst. The conductivity of the catalyst was studied by electrochemical impedance spectroscopy with a frequency range from 105 to 0.02 Hz and an amplitude of 10 mV.

[0080] From Figure 5 (a), it can be known that during the 6000 s test time, the palladium-loaded titanium carbide nanosheet electrode catalyst maintained the lowest current decay rate and the highest oxidation current density, indicating good catalytic durability. Figure 5 (b) shows the AC impedance diagrams of different catalysts. The semicircle diameter presented by the palladium-loaded titanium carbide nanosheet electrode catalyst is smaller than that of other catalysts, indicating that the palladium-loaded titanium carbide nanosheet electrode catalyst has the smallest charge transfer resistance. Therefore, this catalyst can exhibit better catalytic activity.

[0081] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the technical principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A preparation method of a palladium nanosheet electrode catalyst supported on titanium carbide, characterized in that: It includes the following steps: S1. Prepare the reaction precursor of titanium carbide nanosheets, specifically including: a. Add aluminum titanium carbide to the mixed solution of lithium fluoride and hydrochloric acid for etching to obtain multi-layer titanium carbide precipitates; b. Add distilled water to the multi-layer titanium carbide precipitates, centrifuge and wash until neutral, with a centrifuge speed of 3500 - 8000 rpm, then perform ultrasonic exfoliation, and then screen by centrifugation with a centrifuge speed of 5000 - 8000 rpm. Take the supernatant and freeze-dry it to obtain the reaction precursor of titanium carbide nanosheets; S2. Add the reaction precursor of titanium carbide nanosheets to the 1-methyl-2-pyrrolidone solution, and ultrasonically disperse to obtain mixed solution I. The mass ratio of the 1-methyl-2-pyrrolidone solution to the reaction precursor is 824 - 1030:1; S3. Add polyvinylpyrrolidone and molybdenum hexacarbonyl to mixed solution I, and ultrasonically mix evenly to obtain mixed solution II. The mass ratio of polyvinylpyrrolidone, molybdenum hexacarbonyl to the reaction precursor contained in mixed solution I is 8 - 12:0.2 - 0.8:1; S4. Add palladium acetylacetonate to mixed solution II, mix evenly, then add acetic acid, and perform an oil bath reaction to obtain black precipitates. After centrifugal washing and freeze-drying, palladium nanosheet / titanium carbide catalyst is obtained. Among them, the mass ratio of palladium acetylacetonate, acetic acid to the reaction precursor contained in mixed solution II is 0.11 - 0.43:0.1 - 0.3:

1.

2. The preparation method of a palladium nanosheet electrode catalyst supported on titanium carbide according to claim 1, characterized in that: In step a, the etching time is 24 - 48 h, and the reaction temperature is 40 - 50 °C.

3. The preparation method of a palladium nanosheet electrode catalyst supported on titanium carbide according to claim 1, characterized in that: In step b, the ultrasonic exfoliation conditions are: ultrasonic time 0.5 - 1 h, and the protective gas is an argon atmosphere.

4. The preparation method of a palladium nanosheet electrode catalyst supported on titanium carbide according to claim 1, wherein: In step S2, the mass ratio of the 1-methyl-2-pyrrolidone solution to the reaction precursor is 1030:

1.

5. The preparation method of a palladium nanosheet electrode catalyst supported on titanium carbide according to claim 1, characterized in that: In step S3, the mass ratio of polyvinylpyrrolidone, molybdenum hexacarbonyl to the reaction precursor contained in mixed solution I is 10:0.5:

1.

6. The preparation method of a palladium nanosheet electrode catalyst supported on titanium carbide according to claim 1, characterized in that: In step S4, the mass ratio of palladium acetylacetonate, acetic acid to the reaction precursor contained in mixed solution II is 0.25:0.2:

1.

7. The preparation method of a palladium nanosheet electrode catalyst supported on titanium carbide according to claim 1, characterized in that: The ultrasonic conditions in steps S2 and S3 are to ultrasonically for 10 - 30 min within the temperature range of 0 - 25 °C.

8. The preparation method of a palladium nanosheet electrode catalyst supported on titanium carbide according to claim 1, wherein: In step S4, the oil bath conditions are to react at 140 °C for 2 - 5 h; the centrifugal washing conditions are: the solvent is ethanol, the washing times are 3 - 5 times, and the centrifuge speed is 6000 - 9000 rpm; the drying pressure for freeze-drying is 0 - 200 Pa.

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