Preparation method of alloy-loaded Ru nano-particle carbon-based electrocatalyst

Abstract

The invention relates to the field of electrocatalysts, in particular to a preparation method of an alloy-loaded Ru nanoparticle carbon-based electrocatalyst, Pt, Ru, other transition metal salts and C60 are adopted to synthesize a metal + carbon substrate structure, and a high-temperature sintering method is utilized to crystallize noble metal and rearrange the carbon substrate structure, so that the high activity brought by a pure noble metal catalyst is overcome; and the cheap transition metal is utilized to improve the electron coordination environment of the Pt-based metal, so that the cost of the noble metal is greatly saved, and the electrocatalyst with a good catalytic effect is obtained.

Description

Technical Field

[0001] The present invention relates to the field of electrocatalysts, and particularly to a preparation method of an alloy-supported Ru nanoparticle carbon-based electrocatalyst. Background Art

[0002] The problems of ecological pollution and energy depletion caused by the consumption of fossil energy are becoming increasingly serious, and renewable energy hydrogen with high energy density has attracted wide attention. Traditional hydrogen production processes often have many disadvantages such as high energy consumption and large pollution. In contrast, electrocatalytic water splitting has become a highly potential new hydrogen production route due to its significant characteristics of environmental friendliness and high hydrogen production efficiency. However, in the process of electrocatalytic water splitting, although the commonly used platinum (Pt)-based catalysts have excellent catalytic performance, due to the scarcity of Pt resources and high cost, the large-scale application of this technology is greatly restricted. Summary of the Invention

[0003] The purpose of the present invention is to provide a preparation method of an alloy-supported Ru nanoparticle carbon-based electrocatalyst, which is simple and feasible, can not only reduce the noble metal cost but also has universality, and at the same time, the obtained electrocatalyst has good catalytic performance.

[0004] The present invention is realized through the following technical solutions:

[0005] A preparation method of an alloy-supported Ru nanoparticle carbon-based electrocatalyst includes the following steps:

[0006] S1: 130 - 140 mg of imidazole and 340 - 360 mg of C 60 are uniformly dispersed in 50 mL of chlorobenzene by ultrasonic treatment to obtain a mixed solution A;

[0007] S2: 60 - 70 mg of RuCl3 is dissolved in 10 mL of ethanol to obtain a mixed solution B;

[0008] S3: The mixed solution B is added dropwise to the mixed solution A under stirring to obtain a mixed solution C;

[0009] S3: The mixed solution C is transferred to a reaction kettle at 150 - 220 °C for hydrothermal reaction. After cooling to room temperature, it is centrifuged, washed, and vacuum dried to obtain a ruthenium-carbon composite substrate;

[0010] S4: 150 - 170 mg of the ruthenium-carbon composite substrate is dissolved in 20 mL of absolute ethanol and stirred evenly to obtain a composite alcohol solution. At the same time, a mixture of 0.1 mmol of H2PtCl6·6H2O and 0.1 mmol of transition metal chloride is dissolved in 20 mL of ethanol and mixed and ultrasonically treated to obtain a mixed solution D. The mixed solution D is added dropwise to the composite alcohol solution under stirring and mixed evenly to obtain a mixed solution E;

[0011] S5: Transfer the mixed solution E to a reaction kettle at 150 - 220 °C for hydrothermal reaction. After cooling to room temperature, centrifuge, wash, and then vacuum dry to obtain the composite powder.

[0012] S6: Heat the obtained composite powder to 800 °C in an Ar atmosphere in a vacuum tube furnace and carbonize for 1.5 - 3 hours. After cooling, the final catalyst is obtained.

[0013] Further, the time of the ultrasonic treatment in step S1 is 10 - 30 minutes.

[0014] Further, the hydrothermal reaction time in step S3 is 10 - 36 h.

[0015] Further, the specific operation of the washing in step S3 is to wash three times with toluene and absolute ethanol.

[0016] Further, the metal in the transition metal chloride in step S4 is one or more of iron, cobalt, or nickel.

[0017] Further, the hydrothermal reaction time in step S5 is 2 - 5 h.

[0018] Further, the specific operation of the washing in step S5 is to wash three times with absolute ethanol.

[0019] Further, the heating rate of the vacuum tube furnace in step S6 is 5 - 10 °C / min.

[0020] The present invention has the following beneficial effects:

[0021] (1) The present invention uses the hydrothermal method to prepare an alloy-supported Ru nanoparticle carbon-based electrocatalyst. Specifically, Pt, Ru, and other transition metal salts are used to synthesize a metal + carbon substrate structure with C. By using the method of high-temperature sintering, the noble metal is crystallized and the carbon substrate structure is rearranged, significantly improving the problem of excessively high activity of pure noble metal catalysts. Moreover, the use of inexpensive transition metals to improve the electronic coordination environment of Pt-based metals not only greatly saves the cost of noble metals but also obtains an electrocatalyst with good catalytic effects. 60 (2) The present invention improves the electronic coordination environment of Pt-based metals through transition metals to achieve the purpose of saving the cost of noble metals. The transition metals can be selected from one or more of common iron, cobalt, and nickel, and the catalytic effects of the electrocatalysts of the final products are not affected. Therefore, the method of the present invention has a certain universality.

[0022] (3) The Ru / C prepared in Example 1 of the present invention Description of the Drawings

[0023] Figure 1 is the Ru / C prepared in Example 1 of the present invention 60XRD patterns of the composite substrate and the PtNi / RuC composite;

[0024] Figure 2 Among them, (a) and (b) are Ru / C prepared in Example 1 of the present invention respectively 60 TEM characterization diagrams of the composite substrate and the PtNi / RuC composite;

[0025] Figure 3 It is a comparison diagram of the LSV performance curves of the catalysts prepared in Example 1 and Example 2 and Pt / C. Detailed implementation manners

[0026] The following further describes the present invention in detail with specific examples, which are explanations rather than limitations of the present invention.

[0027] Based on certain experimental research, it is found that the energy level structure of Pt can be finely adjusted by alloying with transition metals (such as Fe, Co, Ni) anchored on Ru, and its hydrogen binding strength is similar to that of Pt. This strategy significantly improves the problem of excessively high activity of pure noble metal catalysts. Using cheap transition metals to improve the electronic coordination environment of Pt-based metals not only greatly saves the cost of noble metals, but also brings more excellent catalytic activity. The PtM-Ru interaction leads to precise adjustment of the d-band center, which is beneficial to the inherent electrochemistry hydrogen adsorption-desorption ability.

[0028] C of the fullerene family 60 The highly symmetric cage structure endows it with high stability. The five-membered and six-membered rings of fullerenes can serve as ligands of metal complexes. Based on this, the present invention uses a fullerene-supported metal modification strategy to construct electrocatalysts with high intrinsic activity and high stability. These strategies aim to reduce the loading of Pt / Ru noble metals to save costs, improve the EMSI effect, and give full play to the synergistic effect between different components, etc., providing a feasible solution for the preparation of industrial catalysts.

[0029] The present invention adopts a two-step hydrothermal method to synthesize the precursor of PtM (M = Fe, Co, Ni) supported on a Ru carbon substrate, and then uses a high-temperature sintering method to obtain a highly crystalline carbon-based electrocatalyst.

[0030] Example 1

[0031] A preparation method of an alloy-supported Ru nanoparticle carbon-based electrocatalyst, specifically:

[0032] S1: Weigh 130 mg of imidazole and 340 mg of C 60 And uniformly disperse them in 50 mL of chlorobenzene by ultrasonic treatment for 10 minutes to obtain a mixed solution A;

[0033] S2: Dissolve 60 mg of RuCl3 in 10 mL of ethanol to obtain a mixed solution B;

[0034] S3: Dropwise add the mixed solution B into the mixed solution A under stirring to obtain a mixed solution C;

[0035] S3: Transfer the mixed solution C to a reaction kettle at 150 °C and react for 36 h. After cooling to room temperature, wash it three times with toluene and absolute ethanol, and dry it at 60 °C for 24 hours to obtain a ruthenium-carbon composite substrate;

[0036] S4: Dissolve 150 mg of the ruthenium-carbon composite substrate in 20 mL of absolute ethanol and stir evenly to obtain a composite alcohol solution. At the same time, dissolve a mixture of 0.1 mmol of H2PtCl6·6H2O and 0.1 mmol of NiCl2 in 20 mL of ethanol, mix and ultrasonically treat it for 20 min to obtain a mixed solution D. Dropwise add the mixed solution D into the composite alcohol solution under stirring and mix evenly to obtain a mixed solution E;

[0037] S5: Transfer the mixed solution E to a reaction kettle at 150 °C and react for 5 h. After cooling to room temperature, centrifuge it, wash it three times with absolute ethanol, and then vacuum dry it to obtain a composite powder;

[0038] S6: Heat the obtained composite powder in a vacuum tube furnace under an Ar atmosphere at a rate of 5 °C / min to 800 °C and carbonize it for 2 hours. After cooling, obtain the final catalyst.

[0039] See Figure 1 The XRD characterization diagrams of the Ru / C 60 composite substrate and the PtNi / RuC composite. The Ru / C and PtNi / RuC samples show diffraction peaks at approximately 10.8°, 17.7°, and 20.8° respectively, corresponding to the (111), (220), and (311) planes of C 60 (044 - 0558), indicating the preservation of the C 60 structure. The corresponding XRD pattern of PtNi / RuC shows two characteristic peaks at approximately 41.33° and 48.09°, corresponding to the (111) and (200) crystal planes of PtNi (JCPDS No. 04 - 003 - 4660), indicating the formation of the PtNi alloy. The small peaks at 38.5°, 42.1°, and 44.2° can be attributed to the (100), (002), and (101) planes of Ru (JCPDS No. 06 - 0663) respectively, indicating the successful combination of the substrate and the alloy.

[0040] And referring to Figure 2 (a) and (b), it can be seen that the PtNi alloy nanoparticles are distributed on the surface of the Ru / C substrate.

[0041] Example 2

[0042] A preparation method of an alloy-supported Ru nanoparticle carbon-based electrocatalyst is as follows:

[0043] S1: Weigh 135 mg of imidazole and 353 mg of C 60 and disperse them evenly in 50 mL of chlorobenzene by ultrasonic treatment for 20 minutes to obtain a mixed solution A;

[0044] S2: Dissolve 62.3 mg of RuCl3 in 10 mL of ethanol to obtain a mixed solution B;

[0045] S3: Dropwise add the mixed solution B into the mixed solution A under stirring to obtain a mixed solution C;

[0046] S3: Transfer the mixed solution C to a reaction kettle at 180 °C and react for 24 h. After cooling to room temperature, wash it three times with toluene and absolute ethanol, and dry it at 60 °C for 24 hours to obtain a ruthenium-carbon composite substrate;

[0047] S4: Dissolve 155.8 mg of the ruthenium-carbon composite substrate in 20 mL of absolute ethanol and stir evenly to obtain a composite alcohol solution. At the same time, dissolve a mixture of 0.1 mmol of H2PtCl6·6H2O and 0.1 mmol of CoCl2 in 20 mL of ethanol, mix and perform ultrasonic treatment for 20 min to obtain a mixed solution D. Dropwise add the mixed solution D into the composite alcohol solution under stirring and mix evenly to obtain a mixed solution E;

[0048] S5: Transfer the mixed solution E to a reaction kettle at 200 °C and react for 4 h. After cooling to room temperature, centrifuge it, wash it three times with absolute ethanol, and then obtain a composite powder through vacuum drying;

[0049] S6: Heat the obtained composite powder in a vacuum tube furnace under an Ar atmosphere to 800 °C at a rate of 10 °C / min and carbonize it for 3 hours, and obtain the final catalyst after cooling.

[0050] Compare the PtNi / RuC catalyst prepared in Example 1 and the PtCo / RuC catalyst prepared in Example 2 with the traditional Pt / C catalyst, and use an electrochemical workstation to perform linear sweep voltammetry (LSV) tests to evaluate the electrocatalytic performance of the three catalysts.

[0051] In the present invention, a current density of 10 mA cm -2 is selected as the comparison basis because when solar energy is converted into hydrogen, when the current density reaches 10 mA cm -2In this case, the efficiency can reach the cost - advantageous expected value of 12.3%. The linear sweep voltammetry curve can reflect the voltage - current density relationship on the working electrode of the catalyst, where the current density changes linearly and regularly with the applied voltage.

[0052] When conducting the test, to ensure the accuracy and stability of the test results, a relatively low scan rate is usually adopted, generally set at 5 mV / s. In addition, to visually compare the performance of different catalysts in different electrolytes, the voltage values on the abscissa are uniformly converted to values relative to the standard hydrogen electrode (RHE) for processing. As Figure 3 shown, it can be seen that for PtNi / RuC, PtCo / RuC, and Pt / C to reach a current density of 10 mA cm -2 in a 1M KOH alkaline environment, 25 mV, 61 mV, and 36 mV are respectively required. Therefore, the catalyst prepared by the present invention has excellent electrochemical performance comparable to commercial Pt / C, demonstrating its competitiveness in electrocatalytic activity.

[0053] Example 3

[0054] A preparation method of an alloy - supported Ru nanoparticle carbon - based electrocatalyst, specifically:

[0055] S1: Weigh 137.7 mg of imidazole and 360 mg of C 60 and disperse them evenly in 50 mL of chlorobenzene through 30 - minute ultrasonic treatment to obtain a mixed solution A;

[0056] S2: Dissolve 63.5 mg of RuCl3 in 10 mL of ethanol to obtain a mixed solution B;

[0057] S3: Dropwise add the mixed solution B into the mixed solution A under stirring to obtain a mixed solution C;

[0058] S3: Transfer the mixed solution C to a reaction kettle at 220 °C and react for 10 h. After cooling to room temperature, wash it three times with toluene and absolute ethanol, and dry it at 60 °C for 24 hours to obtain a ruthenium - carbon composite substrate;

[0059] S4: Dissolve 158.8 mg of the ruthenium - carbon composite substrate in 20 mL of absolute ethanol and stir evenly to obtain a composite alcohol solution. At the same time, dissolve a mixture of 0.1 mmol of H2PtCl6·6H2O and 0.1 mmol of NiCl2 in 20 mL of ethanol, mix and ultrasonically treat for 20 min to obtain a mixed solution D, and dropwise add the mixed solution D into the composite alcohol solution under stirring and mix evenly to obtain a mixed solution E;

[0060] S5: Transfer the mixed solution E to a reactor at 180 °C and react for 3 h. After cooling to room temperature, centrifuge, wash three times with absolute ethanol, and then obtain the composite powder through vacuum drying;

[0061] S6: Heat the obtained composite powder to 800 °C at a rate of 5 °C / min under an Ar atmosphere in a vacuum tube furnace and carbonize for 2 h, and then obtain the final catalyst after cooling.

[0062] Example 4

[0063] A preparation method of an alloy-supported Ru nanoparticle carbon-based electrocatalyst, specifically:

[0064] S1: Weigh 140 mg of imidazole and 360 mg of C 60 Disperse them evenly in 50 mL of chlorobenzene through 15 minutes of ultrasonic treatment to obtain a mixed solution A;

[0065] S2: Dissolve 70 mg of RuCl3 in 10 mL of ethanol to obtain a mixed solution B;

[0066] S3: Dropwise add the mixed solution B to the mixed solution A under stirring to obtain a mixed solution C;

[0067] S3: Transfer the mixed solution C to a reactor at 220 °C and react for 10 h. After cooling to room temperature, wash three times with toluene and absolute ethanol, and then dry at 60 °C for 24 hours to obtain a ruthenium-carbon composite substrate;

[0068] S4: Dissolve 170 mg of the ruthenium-carbon composite substrate in 20 mL of absolute ethanol and stir evenly to obtain a composite alcohol solution. At the same time, dissolve a mixture of 0.1 mmol of H2PtCl6·6H2O and 0.1 mmol of FeCl3 in 20 mL of ethanol, mix and perform ultrasonic treatment for 20 min to obtain a mixed solution D. Dropwise add the mixed solution D to the composite alcohol solution under stirring and mix evenly to obtain a mixed solution E;

[0069] S5: Transfer the mixed solution E to a reactor at 220 °C and react for 2 h. After cooling to room temperature, centrifuge, wash three times with absolute ethanol, and then obtain the composite powder through vacuum drying;

[0070] S6: Heat the obtained composite powder to 800 °C at a rate of 6 °C / min under an Ar atmosphere in a vacuum tube furnace and carbonize for 1.5 h, and then obtain the final catalyst after cooling.

[0071] Example 5

[0072] A preparation method of an alloy-supported Ru nanoparticle carbon-based electrocatalyst, specifically:

[0073] S1: Weigh 140 mg of imidazole and 360 mg of C 60A mixed solution A was obtained by ultrasonic treatment for 20 minutes to uniformly disperse it in 50 mL of chlorobenzene;

[0074] S2: 70 mg of RuCl3 was dissolved in 10 mL of ethanol to obtain a mixed solution B;

[0075] S3: The mixed solution B was added dropwise to the mixed solution A under stirring to obtain a mixed solution C;

[0076] S3: The mixed solution C was transferred to a reaction kettle at 150 °C and reacted for 24 h. After cooling to room temperature, it was washed three times with toluene and absolute ethanol, and dried at 60 °C for 24 hours to obtain a ruthenium-carbon composite substrate;

[0077] S4: 170 mg of the ruthenium-carbon composite substrate was dissolved in 20 mL of absolute ethanol and stirred evenly to obtain a composite alcohol solution. At the same time, a mixture of 0.1 mmol of H2PtCl6·6H2O, 0.05 mmol of CoCl2 and 0.05 mmol of FeCl3 was dissolved in 20 mL of ethanol and mixed and ultrasonically treated for 20 min to obtain a mixed solution D. The mixed solution D was added dropwise to the composite alcohol solution under stirring and mixed evenly to obtain a mixed solution E;

[0078] S5: The mixed solution E was transferred to a reaction kettle at 220 °C and reacted for 2 h. After cooling to room temperature, it was centrifuged, washed three times with absolute ethanol, and then vacuum dried to obtain a composite powder;

[0079] S6: The obtained composite powder was heated to 800 °C at a rate of 8 °C / min in an Ar atmosphere in a vacuum tube furnace and carbonized for 1.5 hours, and the final catalyst was obtained after cooling.

[0080] Example 6

[0081] A preparation method of an alloy-supported Ru nanoparticle carbon-based electrocatalyst, specifically:

[0082] S1: Weigh 130 mg of imidazole and 340 mg of C 60 A mixed solution A was obtained by ultrasonic treatment for 20 minutes to uniformly disperse it in 50 mL of chlorobenzene;

[0083] S2: 65 mg of RuCl3 was dissolved in 10 mL of ethanol to obtain a mixed solution B;

[0084] S3: The mixed solution B was added dropwise to the mixed solution A under stirring to obtain a mixed solution C;

[0085] S3: The mixed solution C was transferred to a reaction kettle at 150 °C and reacted for 24 h. After cooling to room temperature, it was washed three times with toluene and absolute ethanol, and dried at 60 °C for 24 hours to obtain a ruthenium-carbon composite substrate;

[0086] S4: Dissolve 150 mg of the ruthenium-carbon composite substrate in 20 mL of absolute ethanol and stir evenly to obtain a composite alcohol solution. At the same time, dissolve a mixture of 0.1 mmol of H2PtCl6·6H2O, 0.03 mmol of NiCl2, and 0.07 mmol of FeCl3 in 20 mL of ethanol, and after mixing and ultrasonic treatment for 20 min, obtain solution D. Dropwise add solution D into the stirred composite alcohol solution and mix evenly to obtain a mixed solution E;

[0087] S5: Transfer the mixed solution E to a reaction kettle at 220 °C and react for 2 h. After cooling to room temperature, centrifuge, wash three times with absolute ethanol, and then vacuum dry to obtain a composite powder;

[0088] S6: Heat the obtained composite powder to 800 °C at a rate of 5 °C / min under an Ar atmosphere in a vacuum tube furnace and carbonize for 2 hours, and then obtain the final catalyst after cooling.

[0089] Example 7

[0090] A preparation method of an alloy-supported Ru nanoparticle carbon-based electrocatalyst, specifically:

[0091] S1: Weigh 140 mg of imidazole and 360 mg of C 60 Disperse evenly in 50 mL of chlorobenzene by ultrasonic treatment for 20 minutes to obtain a mixed solution A;

[0092] S2: Dissolve 68 mg of RuCl3 in 10 mL of ethanol to obtain a mixed solution B;

[0093] S3: Dropwise add the mixed solution B into the stirred mixed solution A to obtain a mixed solution C;

[0094] S3: Transfer the mixed solution C to a reaction kettle at 150 °C and react for 24 h. After cooling to room temperature, wash three times with toluene and absolute ethanol, and dry at 60 °C for 24 hours to obtain a ruthenium-carbon composite substrate;

[0095] S4: Dissolve 170 mg of the ruthenium-carbon composite substrate in 20 mL of absolute ethanol and stir evenly to obtain a composite alcohol solution. At the same time, dissolve a mixture of 0.1 mmol of H2PtCl6·6H2O, 0.065 mmol of NiCl2, and 0.035 mmol of CoCl2 in 20 mL of ethanol, and after mixing and ultrasonic treatment for 20 min, obtain solution D. Dropwise add solution D into the stirred composite alcohol solution and mix evenly to obtain a mixed solution E;

[0096] S5: Transfer the mixed solution E to a reaction kettle at 220 °C and react for 2 h. After cooling to room temperature, centrifuge, wash three times with absolute ethanol, and then vacuum dry to obtain a composite powder;

[0097] S6: Heat the obtained composite powder to 800 °C at a rate of 5 °C / min in an Ar atmosphere in a vacuum tube furnace and carbonize for 2 hours, and obtain the final catalyst after cooling.

[0098] The above content is a further detailed description of the present invention in combination with specific embodiments. It cannot be determined that the specific implementation of the present invention is only limited to these embodiments. For those of ordinary skill in the technical field of the present invention, without departing from the concept of the present invention, several simple deductions or substitutions can be made, which should all be regarded as belonging to the protection scope of the present invention.

Claims

1. A preparation method of an alloy-supported Ru nanoparticle carbon-based electrocatalyst, characterized in that, It includes the following steps: S1: Disperse 130 - 140 mg of imidazole and 340 - 360 mg of C 60 in 50 mL of chlorobenzene by ultrasonic treatment to obtain a mixed solution A; S2: Dissolve 60 - 70 mg of RuCl3 in 10 mL of ethanol to obtain a mixed solution B; S3: Dropwise add the mixed solution B into the mixed solution A under stirring to obtain a mixed solution C; S3: Transfer the mixed solution C to a hydrothermal reactor at 150 - 220 °C for hydrothermal reaction. After cooling to room temperature, centrifuge, wash, and vacuum dry to obtain a ruthenium-carbon composite substrate; S4: Dissolve 150 - 170 mg of the ruthenium-carbon composite substrate in 20 mL of absolute ethanol and stir evenly to obtain a composite alcohol solution. At the same time, dissolve a mixture of 0.1 mmol of H2PtCl6·6H2O and 0.1 mmol of transition metal chloride in 20 mL of ethanol, mix and ultrasonically treat to obtain a mixed solution D. Dropwise add the mixed solution D into the composite alcohol solution under stirring and mix evenly to obtain a mixed solution E; S5: Transfer the mixed solution E to a hydrothermal reactor at 150 - 220 °C for hydrothermal reaction. After cooling to room temperature, centrifuge, wash, and vacuum dry to obtain a composite powder; S6: Heat the obtained composite powder to 800 °C in an Ar atmosphere in a vacuum tube furnace and carbonize for 1.5 - 3 hours, and obtain the final catalyst after cooling; 2. The preparation method of an alloy-supported Ru nanoparticle carbon-based electrocatalyst according to claim 1, characterized in that, The time of the ultrasonic treatment described in step S1 is 10 - 30 minutes.

3. The preparation method of an alloy-supported Ru nanoparticle carbon-based electrocatalyst according to claim 1, wherein The hydrothermal reaction time described in step S3 is 10 - 36 h.

4. The preparation method of an alloy-supported Ru nanoparticle carbon-based electrocatalyst according to claim 1, characterized in that, The specific operation of the washing described in step S3 is to wash three times with toluene and absolute ethanol.

5. The preparation method of an alloy-supported Ru nanoparticle carbon-based electrocatalyst according to claim 1, characterized in that, The metal in the transition metal chloride described in step S4 is one or more of iron, cobalt, or nickel.

6. The preparation method of an alloy-supported Ru nanoparticle carbon-based electrocatalyst according to claim 1, characterized in that, The hydrothermal reaction time described in step S5 is 2 - 5 h.

7. The preparation method of an alloy-supported Ru nanoparticle carbon-based electrocatalyst according to claim 1, characterized in that, The specific operation of the washing described in step S5 is to wash three times with absolute ethanol.

8. The preparation method of an alloy-supported Ru nanoparticle carbon-based electrocatalyst according to claim 1, characterized in that, The heating rate of the vacuum tube furnace described in step S6 is 5 - 10 °C / min.

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