Amorphous carbon nanotube catalyst, preparation method and application thereof

Abstract

The application relates to an amorphous carbon nanotube catalyst, a preparation method and application thereof. The preparation method is as follows: iron salt and aluminum salt are prepared by a coprecipitation method; and the amorphous carbon nanotube catalyst is prepared by controlling the total metal ion concentration, the Fe / Al molar ratio, the precipitation pH value, the catalyst precursor calcination temperature and the calcination time. The catalyst has more active centers and the active centers can be more uniformly distributed, and the carbon nanotube yield is improved by the catalyst.

Description

Technical Field

[0001] This invention relates to the field of carbon nanotube production and preparation, specifically to an amorphous carbon nanotube catalyst, its preparation method, and its application. Background Technology

[0002] Carbon nanotubes, due to their unique one-dimensional structure, possess excellent electrical and thermal conductivity, making them widely used as conductive agents in lithium-ion batteries. The primary production method for carbon nanotubes is chemical vapor deposition (CVD). Catalysts are crucial for CVD production, with transition metals such as Fe, Co, and Ni exhibiting excellent activity for carbon nanotube growth. To improve catalyst reactivity, previous researchers have conducted extensive studies on the active components, supports, and structural properties of catalysts. Gulino et al. in France prepared catalysts by impregnating Fe onto an Al₂O₃ support, synthesizing multi-walled carbon nanotubes from ethane. The resulting carbon nanotubes exhibited high purity and high yield, with diameters ranging from approximately 20 to 40 nm. Couteau et al. at the Swiss Institute of Physics prepared Co / CaCO₃, Fe / CaCO₃, and Fe / Co / CaCO₃ catalysts using calcium carbonate as a support via impregnation, synthesizing multi-walled carbon nanotubes from acetylene at 720 °C. Because calcium carbonate has almost no pores, it can effectively avoid the formation of amorphous carbon, improving the selectivity and yield of CNTs. Research by Du et al. at the University of California found that introducing more active sites at high temperatures can significantly improve the yield of carbon nanotubes. Chinese patent CN114405510A proposes a method for preparing an iron-based carbon nanotube catalyst. This catalyst is prepared by co-precipitation, and the introduction of nickel atoms improves the catalytic activity. However, this method requires strict control of the metal ratios in the catalyst; otherwise, it will affect the catalyst conversion rate and carbon nanotube yield, resulting in poor flexibility. Chinese patent CN115041180A provides a method for preparing a carbon nanotube catalyst for fluidized beds. This method uses co-precipitation to obtain a catalyst with a core-shell structure, improving the uniformity of catalyst particle size and mechanical strength to adapt to high-throughput gas operations in fluidized bed reactors. Chinese patent CN115337934A provides a method for preparing a low-walled carbon nanotube catalyst. A hydrothermal method is used to adjust the dispersion of the active material on the support surface, thereby obtaining high-quality low-walled carbon nanotubes.

[0003] Existing carbon nanotube catalysts are generally crystalline catalysts, in which the distribution of active metal components is relatively fixed. To avoid the aggregation of active components in the catalyst, the content of active components is usually limited. However, in the synthesis of carbon nanotubes, active metals such as Fe and Ni are the active reaction centers, and the number of active centers directly determines the yield of carbon nanotubes. Limiting the content of active metals in the catalyst also limits the yield of carbon nanotubes, but increasing the content of active metals in the catalyst will lead to uneven distribution of active components. Summary of the Invention

[0004] The present invention aims to provide an amorphous carbon nanotube catalyst, a preparation method and an application, which enables the catalyst to have more active centers and make the active centers more uniformly distributed, and improves the yield of carbon nanotubes.

[0005] To solve the above technical problems, the specific solution adopted by the present invention is as follows: a method for preparing an amorphous carbon nanotube catalyst, which is prepared by co-precipitation of iron salt and aluminum salt, and the amorphous carbon nanotube catalyst is prepared by controlling the total metal ion concentration, Fe / Al molar ratio, precipitation pH value, calcination temperature and calcination time of catalyst precursor.

[0006] The Fe / Al molar ratio is (2–0.1):1, the total metal ion concentration is 0.1–2 mol / L, the hydroxide concentration in the precipitant solution is 0.5–2.5 mol / L, and the hydroxide concentration in the precipitant solution is higher than the total metal ion concentration. The endpoint pH value at which the precipitant solution stops adding is 6.5–10.0. The calcination temperature of the colloidal precipitate of the catalyst precursor is 400–600℃, and the calcination time is 1–5 h.

[0007] Preferably, the iron salt is ferric nitrate, ferric chloride, or ferric sulfate, the aluminum salt is aluminum nitrate, aluminum chloride, or aluminum sulfate, and the precipitant is ammonia, ammonium carbonate, sodium carbonate, sodium hydroxide, or potassium hydroxide.

[0008] Preferably, it includes the following steps:

[0009] 1) Iron salt and aluminum salt are dissolved together in deionized water to obtain a mixed metal salt solution of active component and carrier component;

[0010] 2) Add the mixed metal solution obtained in step 1) into a stirred reactor. While stirring continuously, add the precipitant dropwise into the reactor. Stop when the final pH value is reached in the reactor and allow the sol system in the reactor to be evenly dispersed. After standing, a gel-like precipitate is formed.

[0011] 3) After filtering the solution in the reactor, a colloidal precipitate is obtained. After washing, drying, calcining and grinding, the amorphous carbon nanotube catalyst is obtained.

[0012] Preferably, the stirring rate of the reactor is 100-500 r / min, the dropping rate of the precipitant solution is 1-3 ml / min, and stirring continues for 0.5-2 h after the final pH value is reached in the reactor, followed by standing for 12-48 h, and then drying is carried out; the drying temperature is 80-120℃, and the drying time is 6-12 h.

[0013] Preferably, the calcination temperature of the colloidal precipitate of the catalyst precursor is 490–510 °C.

[0014] Preferably, the endpoint pH value at which the precipitant solution stops being added is 8 to 9.5.

[0015] Preferably, the precipitant is added dropwise to the reaction vessel using a dropping funnel or a peristaltic pump.

[0016] An amorphous carbon nanotube catalyst was prepared by the above preparation method.

[0017] An application of an amorphous carbon nanotube catalyst involves adding the amorphous carbon nanotube catalyst to a fixed-bed reactor, reducing it with hydrogen, and then synthesizing carbon nanotubes using low-carbon hydrocarbons. After the reaction, the crude product is subjected to alkali and acid washing to remove the catalyst support and active metal. It is then washed with deionized water and anhydrous ethanol, filtered, and dried in an oven to obtain a pure carbon nanotube product.

[0018] Preferably, gas chromatography is used to analyze the composition of gaseous products during the reaction to calculate the reaction conversion rate.

[0019] Preferably, the reduction temperature of the amorphous carbon nanotube catalyst is 650–750 °C, the reduction time is 10–60 min, and the hydrogen mass hourly space velocity (HHSV) during the reduction process is 0.5–5 h⁻¹. -1 ;

[0020] Preferably, the raw materials for carbon nanotube synthesis are one or more low-carbon hydrocarbons such as methane, ethane, ethylene, propane, and propylene; the reaction temperature is 650–750°C, consistent with the reduction temperature of the amorphous carbon nanotube catalyst; and the mass hourly space velocity (HSV) of the raw material gas during the reaction is 0.5–5 h⁻¹. -1 .

[0021] Preferably, the alkaline washing process uses a sodium hydroxide or potassium hydroxide solution with a concentration of 0.5–3 mol / L, a solution mass to carbon nanotube mass ratio of (10–30):1, an alkaline washing temperature of 60–80℃, and an alkaline washing time of 12–24 h.

[0022] Preferably, the pickling process uses one or more of hydrochloric acid, nitric acid and sulfuric acid, with a solution concentration of 0.5 to 5 mol / L, a solution mass to carbon nanotube mass ratio of (10 to 30): 1, a pickling temperature of 60 to 80°C, and a pickling time of 12 to 24 hours.

[0023] The carbon nanotube catalyst prepared by this invention is amorphous. Compared with crystalline catalysts with a fixed atomic ratio, amorphous catalysts allow for a wider range of adjustment of the metal atom ratio, which helps to increase the number of active centers in the catalyst. Amorphous catalysts exhibit short-range order and long-range disorder. This disordered structure alters the contact properties of the catalyst surface, promotes the uniform distribution of active metal components, and helps to improve catalytic activity. This addresses the problems of limited active component content and uneven distribution in existing crystalline catalysts. Specifically:

[0024] The amorphous Fe / Al₂O₃ catalyst prepared in this invention, i.e., an amorphous mixture of Fe₂O₃ and Al₂O₃, exhibits an XRD pattern as a curve with a gradually changing intensity. From the perspective of the Scherrer equation, this curve can be considered as the result of extremely fine grains leading to a significant broadening, overlap, and blurring of the diffraction peaks, indicating that the catalyst mainly consists of Fe₂O₃ and Al₂O₃ microcrystals, exhibiting a unique structure of short-range order and long-range disorder. This unique structure suggests that large grains have not formed in the catalyst, promoting the uniform dispersion of Fe and Al within it.

[0025] After reduction, the amorphous Fe2O3 in the amorphous Fe / Al2O3 catalyst is reduced to crystalline elemental Fe, and its XRD pattern shows obvious Fe diffraction peaks. Due to the higher specific surface area in the amorphous phase, amorphous Fe2O3 microcrystals are more catalytically active than nanocrystalline particles of the same diameter, and are more easily reduced to elemental Fe during the catalyst reduction process, serving as the active centers of the catalyst.

[0026] In summary, the advantages of this invention compared to existing carbon nanotube catalyst preparation processes are as follows:

[0027] (1) The amorphous carbon nanotube catalyst prepared by the present invention exhibits a special structure of short-range order and long-range disorder under microscopic conditions. This special structure is beneficial to the uniform distribution of Fe and Al in the catalyst.

[0028] (2) Compared with crystalline catalysts with a fixed atomic ratio, amorphous carbon nanotube catalysts can adjust the metal atomic ratio within a wider range, which is beneficial to increase the number of active centers in the catalyst and improve catalytic activity;

[0029] (3) Due to the higher specific surface area in the amorphous phase, amorphous Fe2O3 microcrystals are more catalytically active than nanocrystals of the same diameter, and amorphous catalysts can more easily obtain more active centers through the reduction process.

[0030] (4) The process of producing catalysts is easy to scale up. Attached Figure Description

[0031] Figure 1 The XRD patterns of the catalysts in Example 2 and Comparative Example 1 are shown below.

[0032] Figure 2 The XRD patterns of the catalysts in Example 2 and Comparative Example 1 after reduction are shown below.

[0033] Figure 3 SEM and SEM-mapping images of the catalysts in Example 2(a) and Comparative Example 1(b);

[0034] Figure 4 This is a graph showing the change in ethane conversion rate as a function of reaction time for the catalysts of Example 2 and Comparative Example 1.

[0035] Figure 5 This is a graph showing the change in gaseous product composition over reaction time in Example 2.

[0036] Figure 6 Here is a SEM image of the carbon nanotubes synthesized in Example 2;

[0037] Figure 7 TEM image of carbon nanotubes synthesized in Example 2;

[0038] Figure 8 The Raman spectrum of the carbon nanotubes synthesized in Example 2;

[0039] Figure 9 The XRD patterns of the catalysts in Example 2, Comparative Examples 3 and 4 are shown. Detailed Implementation

[0040] The following 11 examples and 4 comparative examples illustrate an amorphous carbon nanotube catalyst, its preparation method, and its applications according to the present invention. In the examples, the iron source is ferric nitrate, the aluminum source is aluminum nitrate, and the precipitant is an aqueous ammonia solution. Carbon nanotubes are synthesized using the amorphous catalyst prepared above in a fixed-bed reactor. The reactor used is a quartz glass reactor with dimensions of 40 mm × 1400 mm.

[0041] Example 1:

[0042] 15g of Al(NO3)3·9H2O and 3.5g of Fe(NO3)3·9H2O were dissolved in 200mL of deionized water, with a Fe / Al molar ratio of 1:4.6 and a total metal ion concentration of 0.24mol / L. The mixed metal salt solution was added to a stirred reactor at a stirring rate of 150r / min. A 1.5mol / L ammonia solution was used as the precipitant, and the solution was added dropwise to the reactor at a rate of 1mL / min using a dropping funnel. The addition was stopped when the pH of the solution reached 8. After stopping the addition of ammonia, the solution was stirred for 1 hour and then allowed to stand for 24 hours. The settled solution was filtered, and the filter cake was washed with deionized water and anhydrous ethanol until neutral. The precipitate was dried at 90℃ for 12 hours and calcined in air at 500℃ for 3 hours to obtain an amorphous catalyst.

[0043] The catalyst described above was added to a fixed-bed reactor at a dosage of 1.5 g. Reduction with hydrogen gas at 700 °C and a flow rate of 90 mL / min was carried out for 1 h. Then, carbon nanotubes were synthesized using ethane as a raw material. The reaction temperature for carbon nanotube synthesis was 700 °C, and the ethane flow rate was 90 mL / min. The composition of the gaseous products was analyzed using gas chromatography. The reaction was stopped after catalyst deactivation. The reacted carbon nanotubes were collected and alkali-washed with 2 mol / L potassium hydroxide solution at 80 °C for 24 h. After alkali washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. Then, they were acid-washed with 2 mol / L hydrochloric acid solution at 80 °C for 24 h. After acid washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. The washed carbon nanotubes were dried at 90 °C for 12 h to obtain a pure carbon nanotube product.

[0044] Example 2:

[0045] 15g of Al(NO3)3·9H2O and 8.1g of Fe(NO3)3·9H2O were dissolved in 200mL of deionized water, with a Fe / Al molar ratio of 1:2 and a total metal ion concentration of 0.3mol / L. The mixed metal salt solution was added to a stirred reactor at a stirring rate of 150r / min. A 1.5mol / L ammonia solution was used as the precipitant, and the precipitant solution was added dropwise to the reactor at a rate of 1mL / min using a dropping funnel. The addition was stopped when the pH of the solution reached 8. After stopping the addition of ammonia, the solution was stirred for 1 hour and then allowed to stand for 24 hours. The settled solution was filtered, and the filter cake was washed with deionized water and anhydrous ethanol until neutral. The precipitate was dried at 90℃ for 12 hours and calcined in air at 500℃ for 3 hours to obtain an amorphous catalyst.

[0046] The catalyst described above was added to a fixed-bed reactor at a dosage of 1.5 g. Reduction with hydrogen gas at 700 °C and a flow rate of 90 mL / min was carried out for 1 h. Then, carbon nanotubes were synthesized using ethane as a raw material. The reaction temperature for carbon nanotube synthesis was 700 °C, and the ethane flow rate was 90 mL / min. The composition of the gaseous products was analyzed using gas chromatography. The reaction was stopped after catalyst deactivation. The reacted carbon nanotubes were collected and alkali-washed with 2 mol / L potassium hydroxide solution at 80 °C for 24 h. After alkali washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. Then, they were acid-washed with 2 mol / L hydrochloric acid solution at 80 °C for 24 h. After acid washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. The washed carbon nanotubes were dried at 90 °C for 12 h to obtain a pure carbon nanotube product.

[0047] Example 3:

[0048] 15 g of Al(NO3)3·9H2O and 10.8 g of Fe(NO3)3·9H2O were dissolved in 200 mL of deionized water, with a Fe / Al molar ratio of 1:1.5 and a total metal ion concentration of 0.33 mol / L. The mixed metal salt solution was added to a stirred reactor at a stirring rate of 150 r / min. A 1.5 mol / L ammonia solution was used as the precipitant, and the precipitant solution was added dropwise to the reactor at a rate of 1 mL / min using a dropping funnel. The addition was stopped when the pH of the solution reached 8. After stopping the addition of ammonia, the solution was stirred for 1 h and then allowed to stand for 24 h. The settled solution was filtered, and the filter cake was washed with deionized water and anhydrous ethanol until neutral. The precipitate was dried at 90 °C for 12 h and calcined in air at 500 °C for 3 h to obtain an amorphous catalyst.

[0049] The catalyst described above was added to a fixed-bed reactor at a dosage of 1.5 g. Reduction with hydrogen gas at 700 °C and a flow rate of 90 mL / min was carried out for 1 h. Then, carbon nanotubes were synthesized using ethane as a raw material. The reaction temperature for carbon nanotube synthesis was 700 °C, and the ethane flow rate was 90 mL / min. The composition of the gaseous products was analyzed using gas chromatography. The reaction was stopped after catalyst deactivation. The reacted carbon nanotubes were collected and alkali-washed with 2 mol / L potassium hydroxide solution at 80 °C for 24 h. After alkali washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. Then, they were acid-washed with 2 mol / L hydrochloric acid solution at 80 °C for 24 h. After acid washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. The washed carbon nanotubes were dried at 90 °C for 12 h to obtain a pure carbon nanotube product.

[0050] Example 4:

[0051] 15g of Al(NO3)3·9H2O and 16.2g of Fe(NO3)3·9H2O were dissolved in 200mL of deionized water, with a Fe / Al molar ratio of 1:1 and a total metal ion concentration of 0.4mol / L. The mixed metal salt solution was added to a stirred reactor at a stirring rate of 150r / min. A 1.5mol / L ammonia solution was used as the precipitant, and the precipitant solution was added dropwise to the reactor at a rate of 1mL / min using a dropping funnel. The addition was stopped when the pH of the solution reached 8. After stopping the addition of ammonia, the solution was stirred for 1 hour and then allowed to stand for 24 hours. The settled solution was filtered, and the filter cake was washed with deionized water and anhydrous ethanol until neutral. The precipitate was dried at 90℃ for 12 hours and calcined in air at 500℃ for 3 hours to obtain an amorphous catalyst.

[0052] The catalyst described above was added to a fixed-bed reactor at a dosage of 1.5 g. Reduction with hydrogen gas at 700 °C and a flow rate of 90 mL / min was carried out for 1 h. Then, carbon nanotubes were synthesized using ethane as a raw material. The reaction temperature for carbon nanotube synthesis was 700 °C, and the ethane flow rate was 90 mL / min. The composition of the gaseous products was analyzed using gas chromatography. The reaction was stopped after catalyst deactivation. The reacted carbon nanotubes were collected and alkali-washed with 2 mol / L potassium hydroxide solution at 80 °C for 24 h. After alkali washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. Then, they were acid-washed with 2 mol / L hydrochloric acid solution at 80 °C for 24 h. After acid washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. The washed carbon nanotubes were dried at 90 °C for 12 h to obtain a pure carbon nanotube product.

[0053] Example 5:

[0054] 15g of Al(NO3)3·9H2O and 16.2g of Fe(NO3)3·9H2O were dissolved in 200mL of deionized water, with a Fe / Al molar ratio of 2:1 and a total metal ion concentration of 0.6mol / L. The mixed metal salt solution was added to a stirred reactor at a stirring rate of 150r / min. A 1.5mol / L ammonia solution was used as the precipitant, and the precipitant solution was added dropwise to the reactor at a rate of 1mL / min using a dropping funnel. The addition was stopped when the pH of the solution reached 8. After stopping the addition of ammonia, the solution was stirred for 1 hour and then allowed to stand for 24 hours. The settled solution was filtered, and the filter cake was washed with deionized water and anhydrous ethanol until neutral. The precipitate was dried at 90℃ for 12 hours and calcined in air at 500℃ for 3 hours to obtain an amorphous catalyst.

[0055] The catalyst described above was added to a fixed-bed reactor at a dosage of 1.5 g. Reduction with hydrogen gas at 700 °C and a flow rate of 90 mL / min was carried out for 1 h. Then, carbon nanotubes were synthesized using ethane as a raw material. The reaction temperature for carbon nanotube synthesis was 700 °C, and the ethane flow rate was 90 mL / min. The composition of the gaseous products was analyzed using gas chromatography. The reaction was stopped after catalyst deactivation. The reacted carbon nanotubes were collected and alkali-washed with 2 mol / L potassium hydroxide solution at 80 °C for 24 h. After alkali washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. Then, they were acid-washed with 2 mol / L hydrochloric acid solution at 80 °C for 24 h. After acid washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. The washed carbon nanotubes were dried at 90 °C for 12 h to obtain a pure carbon nanotube product.

[0056] Example 6:

[0057] 15 g of Al(NO3)3·9H2O and 8.1 g of Fe(NO3)3·9H2O were dissolved in 200 mL of deionized water, with a Fe / Al molar ratio of 1:2 and a total metal ion concentration of 0.3 mol / L. The mixed metal salt solution was added to a stirred reactor at a stirring rate of 150 r / min. A 1.5 mol / L ammonia solution was used as the precipitant, and the precipitant solution was added dropwise to the reactor at a rate of 1 mL / min using a dropping funnel. The addition was stopped when the pH of the solution reached 8. After stopping the addition of ammonia, the solution was stirred for 1 h and then allowed to stand for 24 h. The settled solution was filtered, and the filter cake was washed with deionized water and anhydrous ethanol until neutral. The precipitate was dried at 90 °C for 12 h and calcined in air at 400 °C for 3 h to obtain an amorphous catalyst.

[0058] The catalyst described above was added to a fixed-bed reactor at a dosage of 1.5 g. Reduction with hydrogen gas at 700 °C and a flow rate of 90 mL / min was carried out for 1 h. Then, carbon nanotubes were synthesized using ethane as a raw material. The reaction temperature for carbon nanotube synthesis was 700 °C, and the ethane flow rate was 90 mL / min. The composition of the gaseous products was analyzed using gas chromatography. The reaction was stopped after catalyst deactivation. The reacted carbon nanotubes were collected and alkali-washed with 2 mol / L potassium hydroxide solution at 80 °C for 24 h. After alkali washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. Then, they were acid-washed with 2 mol / L hydrochloric acid solution at 80 °C for 24 h. After acid washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. The washed carbon nanotubes were dried at 90 °C for 12 h to obtain a pure carbon nanotube product.

[0059] Example 7:

[0060] 15g of Al(NO3)3·9H2O and 8.1g of Fe(NO3)3·9H2O were dissolved in 200mL of deionized water, with a Fe / Al molar ratio of 1:2 and a total metal ion concentration of 0.3mol / L. The mixed metal salt solution was added to a stirred reactor at a stirring rate of 150r / min. A 1.5mol / L ammonia solution was used as the precipitant, and the precipitant solution was added dropwise to the reactor at a rate of 1mL / min using a dropping funnel. The addition was stopped when the pH of the solution reached 8. After stopping the addition of ammonia, the solution was stirred for 1 hour and then allowed to stand for 24 hours. The settled solution was filtered, and the filter cake was washed with deionized water and anhydrous ethanol until neutral. The precipitate was dried at 90℃ for 12 hours and calcined in air at 600℃ for 3 hours to obtain an amorphous catalyst.

[0061] The catalyst described above was added to a fixed-bed reactor at a dosage of 1.5 g. Reduction with hydrogen gas at 700 °C and a flow rate of 90 mL / min was carried out for 1 h. Then, carbon nanotubes were synthesized using ethane as a raw material. The reaction temperature for carbon nanotube synthesis was 700 °C, and the ethane flow rate was 90 mL / min. The composition of the gaseous products was analyzed using gas chromatography. The reaction was stopped after catalyst deactivation. The reacted carbon nanotubes were collected and alkali-washed with 2 mol / L potassium hydroxide solution at 80 °C for 24 h. After alkali washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. Then, they were acid-washed with 2 mol / L hydrochloric acid solution at 80 °C for 24 h. After acid washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. The washed carbon nanotubes were dried at 90 °C for 12 h to obtain a pure carbon nanotube product.

[0062] Example 8:

[0063] 15g of Al(NO3)3·9H2O and 8.1g of Fe(NO3)3·9H2O were dissolved in 200mL of deionized water, with a Fe / Al molar ratio of 1:2 and a total metal ion concentration of 0.3mol / L. The mixed metal salt solution was added to a stirred reactor at a stirring rate of 150r / min. A 1.5mol / L ammonia solution was used as the precipitant, and the precipitant solution was added dropwise to the reactor at a rate of 1mL / min using a dropping funnel. The addition was stopped when the pH of the solution reached 7. After stopping the addition of ammonia, the solution was stirred for 1 hour and then allowed to stand for 24 hours. The settled solution was filtered, and the filter cake was washed with deionized water and anhydrous ethanol until neutral. The precipitate was dried at 90℃ for 12 hours and calcined in air at 500℃ for 3 hours to obtain an amorphous catalyst.

[0064] The catalyst described above was added to a fixed-bed reactor at a dosage of 1.5 g. Reduction with hydrogen gas at 700 °C and a flow rate of 90 mL / min was carried out for 1 h. Then, carbon nanotubes were synthesized using ethane as a raw material. The reaction temperature for carbon nanotube synthesis was 700 °C, and the ethane flow rate was 90 mL / min. The composition of the gaseous products was analyzed using gas chromatography. The reaction was stopped after catalyst deactivation. The reacted carbon nanotubes were collected and alkali-washed with 2 mol / L potassium hydroxide solution at 80 °C for 24 h. After alkali washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. Then, they were acid-washed with 2 mol / L hydrochloric acid solution at 80 °C for 24 h. After acid washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. The washed carbon nanotubes were dried at 90 °C for 12 h to obtain a pure carbon nanotube product.

[0065] Example 9:

[0066] 15g of Al(NO3)3·9H2O and 8.1g of Fe(NO3)3·9H2O were dissolved in 200mL of deionized water, with a Fe / Al molar ratio of 1:2 and a total metal ion concentration of 0.3mol / L. The mixed metal salt solution was added to a stirred reactor at a stirring rate of 150r / min. A 1.5mol / L ammonia solution was used as the precipitant, and the precipitant solution was added dropwise to the reactor at a rate of 1mL / min using a dropping funnel. The addition was stopped when the pH of the solution reached 9. After stopping the addition of ammonia, the solution was stirred for 1 hour and then allowed to stand for 24 hours. The settled solution was filtered, and the filter cake was washed with deionized water and anhydrous ethanol until neutral. The precipitate was dried at 90℃ for 12 hours and calcined in air at 500℃ for 3 hours to obtain an amorphous catalyst.

[0067] The catalyst described above was added to a fixed-bed reactor at a dosage of 1.5 g. Reduction with hydrogen gas at 700 °C and a flow rate of 90 mL / min was carried out for 1 h. Then, carbon nanotubes were synthesized using ethane as a raw material. The reaction temperature for carbon nanotube synthesis was 700 °C, and the ethane flow rate was 90 mL / min. The composition of the gaseous products was analyzed using gas chromatography. The reaction was stopped after catalyst deactivation. The reacted carbon nanotubes were collected and alkali-washed with 2 mol / L potassium hydroxide solution at 80 °C for 24 h. After alkali washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. Then, they were acid-washed with 2 mol / L hydrochloric acid solution at 80 °C for 24 h. After acid washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. The washed carbon nanotubes were dried at 90 °C for 12 h to obtain a pure carbon nanotube product.

[0068] Example 10:

[0069] 15g of Al(NO3)3·9H2O and 8.1g of Fe(NO3)3·9H2O were dissolved in 200mL of deionized water, with a Fe / Al molar ratio of 1:2 and a total metal ion concentration of 0.6mol / L. The mixed metal salt solution was added to a stirred reactor at a stirring rate of 150r / min. A 1.5mol / L ammonia solution was used as the precipitant, and the precipitant solution was added dropwise to the reactor at a rate of 1mL / min using a dropping funnel. The addition was stopped when the pH of the solution reached 8. After stopping the addition of ammonia, the solution was stirred for 1 hour and then allowed to stand for 24 hours. The settled solution was filtered, and the filter cake was washed with deionized water and anhydrous ethanol until neutral. The precipitate was dried at 90℃ for 12 hours and calcined in air at 500℃ for 3 hours to obtain an amorphous catalyst.

[0070] The catalyst described above was added to a fixed-bed reactor at a dosage of 1.5 g. Reduction with hydrogen gas at 700 °C and a flow rate of 90 mL / min was carried out for 1 h. Then, carbon nanotubes were synthesized using ethane as a raw material. The reaction temperature for carbon nanotube synthesis was 700 °C, and the ethane flow rate was 90 mL / min. The composition of the gaseous products was analyzed using gas chromatography. The reaction was stopped after catalyst deactivation. The reacted carbon nanotubes were collected and alkali-washed with 2 mol / L potassium hydroxide solution at 80 °C for 24 h. After alkali washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. Then, they were acid-washed with 2 mol / L hydrochloric acid solution at 80 °C for 24 h. After acid washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. The washed carbon nanotubes were dried at 90 °C for 12 h to obtain a pure carbon nanotube product.

[0071] Example 11:

[0072] 15 g of Al(NO3)3·9H2O and 8.1 g of Fe(NO3)3·9H2O were dissolved in 200 mL of deionized water, with a Fe / Al molar ratio of 1:2 and a total metal ion concentration of 1.2 mol / L. The mixed metal salt solution was added to a stirred reactor at a stirring rate of 150 r / min. A 1.5 mol / L ammonia solution was used as the precipitant, and the precipitant solution was added dropwise to the reactor at a rate of 1 mL / min using a dropping funnel. The addition was stopped when the pH of the solution reached 8. After stopping the addition of ammonia, the solution was stirred for 1 h and then allowed to stand for 24 h. The settled solution was filtered, and the filter cake was washed with deionized water and anhydrous ethanol until neutral. The precipitate was dried at 90 °C for 12 h and calcined in air at 500 °C for 3 h to obtain an amorphous catalyst.

[0073] The catalyst described above was added to a fixed-bed reactor at a dosage of 1.5 g. Reduction with hydrogen gas at 700 °C and a flow rate of 90 mL / min was carried out for 1 h. Then, carbon nanotubes were synthesized using ethane as a raw material. The reaction temperature for carbon nanotube synthesis was 700 °C, and the ethane flow rate was 90 mL / min. The composition of the gaseous products was analyzed using gas chromatography. The reaction was stopped after catalyst deactivation. The reacted carbon nanotubes were collected and alkali-washed with 2 mol / L potassium hydroxide solution at 80 °C for 24 h. After alkali washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. Then, they were acid-washed with 2 mol / L hydrochloric acid solution at 80 °C for 24 h. After acid washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. The washed carbon nanotubes were dried at 90 °C for 12 h to obtain a pure carbon nanotube product.

[0074] Comparative Example 1:

[0075] Boehmite (SB powder) was calcined in air at 550°C for 3 hours to obtain a γ-Al₂O₃ support. Fe(NO₃)₃·9H₂O was dissolved in deionized water and added dropwise to the γ-Al₂O₃ support, resulting in a Fe / Al molar ratio of 1:2 in the solid after impregnation. The wet material was allowed to stand for 12 hours, then dried at 90°C for 5 hours, and finally calcined in air at 550°C for 3 hours to obtain a crystalline catalyst.

[0076] The catalyst described above was added to a fixed-bed reactor at a dosage of 1.5 g. Reduction with hydrogen gas at 700 °C and a flow rate of 90 mL / min was carried out for 1 h. Then, carbon nanotubes were synthesized using ethane as a raw material. The reaction temperature for carbon nanotube synthesis was 700 °C, and the ethane flow rate was 90 mL / min. The composition of the gaseous products was analyzed using gas chromatography. The reaction was stopped after catalyst deactivation. The reacted carbon nanotubes were collected and alkali-washed with 2 mol / L potassium hydroxide solution at 80 °C for 24 h. After alkali washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. Then, they were acid-washed with 2 mol / L hydrochloric acid solution at 80 °C for 24 h. After acid washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. The washed carbon nanotubes were dried at 90 °C for 12 h to obtain a pure carbon nanotube product.

[0077] Comparative Example 2:

[0078] 15g of Al(NO3)3·9H2O and 8.1g of Fe(NO3)3·9H2O were dissolved in 200mL of deionized water, with a Fe / Al molar ratio of 1:2 and a total metal ion concentration of 0.3mol / L. The mixed metal salt solution was added to a stirred reactor at a stirring rate of 150r / min. A 1.5mol / L ammonia solution was used as the precipitant, and the precipitant solution was added dropwise to the reactor at a rate of 1mL / min using a dropping funnel. The addition was stopped when the pH of the solution reached 8. After stopping the addition of ammonia, the solution was stirred for 1 hour and then allowed to stand for 24 hours. The settled solution was filtered, and the filter cake was washed with deionized water and anhydrous ethanol until neutral. The precipitate was dried at 90℃ for 12 hours and calcined in air at 800℃ for 5 hours to obtain a crystalline catalyst.

[0079] The catalyst described above was added to a fixed-bed reactor at a dosage of 1.5 g. Reduction with hydrogen gas at 700 °C and a flow rate of 90 mL / min was carried out for 1 h. Then, carbon nanotubes were synthesized using ethane as a raw material. The reaction temperature for carbon nanotube synthesis was 700 °C, and the ethane flow rate was 90 mL / min. The composition of the gaseous products was analyzed using gas chromatography. The reaction was stopped after catalyst deactivation. The reacted carbon nanotubes were collected and alkali-washed with 2 mol / L potassium hydroxide solution at 80 °C for 24 h. After alkali washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. Then, they were acid-washed with 2 mol / L hydrochloric acid solution at 80 °C for 24 h. After acid washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. The washed carbon nanotubes were dried at 90 °C for 12 h to obtain a pure carbon nanotube product.

[0080] Comparative Example 3:

[0081] 15g of Al(NO3)3·9H2O and 8.1g of Fe(NO3)3·9H2O were dissolved in 200mL of deionized water, with a Fe / Al molar ratio of 1:2 and a total metal ion concentration of 0.3mol / L. The mixed metal salt solution was added to a stirred reactor at a stirring rate of 150r / min. A 1.5mol / L ammonia solution was used as the precipitant, and the precipitant solution was added dropwise to the reactor at a rate of 1mL / min using a dropping funnel. The addition was stopped when the pH of the solution reached 10.5. After stopping the addition of ammonia, the solution was stirred for 1 hour and then allowed to stand for 24 hours. The settled solution was filtered, and the filter cake was washed with deionized water and anhydrous ethanol until neutral. The precipitate was dried at 90℃ for 12 hours and calcined in air at 500℃ for 3 hours to obtain a crystalline catalyst.

[0082] The catalyst described above was added to a fixed-bed reactor at a dosage of 1.5 g. Reduction with hydrogen gas at 700 °C and a flow rate of 90 mL / min was carried out for 1 h. Then, carbon nanotubes were synthesized using ethane as a raw material. The reaction temperature for carbon nanotube synthesis was 700 °C, and the ethane flow rate was 90 mL / min. The composition of the gaseous products was analyzed using gas chromatography. The reaction was stopped after catalyst deactivation. The reacted carbon nanotubes were collected and alkali-washed with 2 mol / L potassium hydroxide solution at 80 °C for 24 h. After alkali washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. Then, they were acid-washed with 2 mol / L hydrochloric acid solution at 80 °C for 24 h. After acid washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. The washed carbon nanotubes were dried at 90 °C for 12 h to obtain a pure carbon nanotube product.

[0083] Comparative Example 4:

[0084] 15g of Al(NO3)3·9H2O and 32.3g of Fe(NO3)3·9H2O were dissolved in 200mL of deionized water, with a Fe / Al molar ratio of 5:1 and a total metal ion concentration of 1.2mol / L. The mixed metal salt solution was added to a stirred reactor at a stirring rate of 150r / min. A 1.5mol / L ammonia solution was used as the precipitant, and the solution was added dropwise at a rate of 1mL / min using a dropping funnel. The addition was stopped when the pH of the solution reached 8. After stopping the addition of ammonia, the solution was stirred for 1 hour and then allowed to stand for 24 hours. The settled solution was filtered, and the filter cake was washed with deionized water and anhydrous ethanol until neutral. The precipitate was dried at 90℃ for 12 hours and calcined in air at 500℃ for 3 hours to obtain a crystalline catalyst.

[0085] The catalyst described above was added to a fixed-bed reactor at a dosage of 1.5 g. Reduction with hydrogen gas at 700 °C and a flow rate of 90 mL / min was carried out for 1 h. Then, carbon nanotubes were synthesized using ethane as a raw material. The reaction temperature for carbon nanotube synthesis was 700 °C, and the ethane flow rate was 90 mL / min. The composition of the gaseous products was analyzed using gas chromatography. The reaction was stopped after catalyst deactivation. The reacted carbon nanotubes were collected and alkali-washed with 2 mol / L potassium hydroxide solution at 80 °C for 24 h. After alkali washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. Then, they were acid-washed with 2 mol / L hydrochloric acid solution at 80 °C for 24 h. After acid washing, the carbon nanotubes were washed with deionized water and anhydrous ethanol until neutral. The washed carbon nanotubes were dried at 90 °C for 12 h to obtain a pure carbon nanotube product.

[0086] Examples 1-11 are amorphous catalysts, and Comparative Examples 1-4 are crystalline catalysts. Table 1 shows the catalytic performance and carbon nanotube purity of the catalysts in each example and comparative example. The carbon nanotube yield in Examples 1-5 first increased and then decreased. This is because as the Fe content in the catalyst increases, the amorphous catalyst has more active centers. However, with further increases in Fe content, Fe2O3 microcrystals aggregate to form larger crystallites. This leads to the aggregation of active components after reduction, resulting in a decrease in catalyst activity and thus a reduction in carbon nanotube yield. The low conversion rate in Example 1 is due to its excessively low Fe content. In Examples 2, 6, and 7, the carbon nanotube yield first increased and then decreased with increasing catalyst calcination temperature, indicating that 500℃ is a suitable calcination temperature under current conditions. The carbon nanotube yield in Examples 2, 8, and 9 increased with increasing pH at the precipitation endpoint, indicating that increasing the pH at the precipitation endpoint is beneficial for improving the catalyst structure and increasing carbon nanotube yield. The carbon nanotube yields in Examples 2, 10, and 11 decreased with increasing total metal ion concentration in the solution, indicating that an increase in total metal ion concentration leads to larger precipitate particle size, which is detrimental to obtaining high carbon nanotube yields. The carbon nanotube yield in Example 2 was significantly higher than that in Comparative Examples 1, 2, 3, and 4, indicating that the amorphous catalyst has more active centers, resulting in better catalytic activity and higher carbon nanotube yield.

[0087] Figure 1 The XRD patterns of the catalysts in Example 2 and Comparative Example 1 are shown below. Figure 1 As can be seen, Example 2 shows a curve with a gradual change in intensity, indicating that the catalyst is mainly composed of Fe2O3 microcrystals and Al2O3 microcrystals, exhibiting a short-range ordered and long-range disordered structure. Comparative Example 1 shows diffraction peaks of the γ-Al2O3 phase at 45.9° and 67.0°, and diffraction peaks of the α-Fe2O3 phase at 24.1°, 33.2°, and 35.6°, indicating that it is a crystalline catalyst. Figure 2 The images show the XRD patterns of the catalysts from Example 2 and Comparative Example 1 after reduction. Figure 2 It can be seen that the diffraction peaks of both at 44.7°, 64.9°, and 82.3° are those of elemental Fe, while the diffraction peaks at 30.8°, 36.4°, 55.1°, and 58.8° are those of the iron-aluminum spinel phase. This indicates that after reduction, the iron oxides were reduced to elemental Fe, and some Fe3+ was also reduced. + Reduced to Fe 2+Iron-aluminum spinel is formed. In samples containing multiple phases, the XRD diffraction intensity of a certain phase increases with its content in the sample. Since elemental Fe is the active center for the catalytic cracking of low-carbon hydrocarbons to synthesize carbon nanotubes, the intensity of the diffraction peak at 44.7° can qualitatively compare the number of active centers in the reduced catalyst. The catalyst in Example 2 has a significantly higher number of active centers than that in Comparative Example 1. The amorphous catalyst has more active centers, while in the crystalline catalyst, a large amount of iron oxides failed to be reduced to effective active centers. Figure 3 SEM and SEM-mapping images of the catalysts in Example 2 and Comparative Example 1. From Figure 3 It can be seen that in Example 2, the distribution of Fe and Al elements is consistent, and the Fe dispersion is good, while in Comparative Example 1, the distribution of Fe and Al elements is significantly different. In Example 2, the amorphous catalyst exists in the form of Fe2O3 microcrystals and Al2O3 microcrystals. The smaller crystallites can promote the uniform distribution of active components. In Comparative Example 1, the crystalline catalysts Fe2O3 and Al2O3 gradually aggregate and grow during calcination, resulting in the aggregation of active components and uneven Fe distribution. Figure 4 The chart shows the change in ethane conversion rate with reaction time for the catalysts of Example 2 and Comparative Example 1. From... Figure 4 It can be seen that the conversion rate of both gradually decreases as the reaction proceeds. The conversion rate and activity time of Example 2 are better than those of Comparative Example 1, which is because Example 2 has more active centers. Figure 5 This shows the trend of gaseous product composition changing with reaction time in Example 2. Figure 5 It can be seen that the main products in the gaseous products are hydrogen and methane. As the reaction proceeds, the catalyst conversion rate gradually decreases, the hydrogen content in the products decreases, and the content of methane, ethane, and ethylene gradually increases, indicating a shift from the reaction to synthesize carbon nanotubes to the reaction to generate methane and ethylene. Figure 6 and Figure 7 SEM and TEM images of the carbon nanotubes synthesized in Example 2. Figure 6 and Figure 7 It can be seen that there are no traces of amorphous carbon in the synthesized carbon nanotubes, the carbon nanotubes have high purity and uniform diameter distribution. Figure 8 The image shows the Raman spectrum of the carbon nanotubes synthesized in Example 2. Figure 8 It can be seen that the intensity ratio of the D peak and G peak of the carbon nanotubes synthesized in Example 2, ID / IG, is 0.6. This value is often used to evaluate the graphitization degree of carbon nanotubes, indicating that the carbon nanotubes of Example 2 have fewer defects and a higher degree of graphitization. Figure 9It can be seen that Comparative Example 3 has two sharp peaks at 46.0° and 66.7°, indicating that Comparative Example 3 contains the η-Al2O3 phase and is a crystalline catalyst. This is because, under pH>10 conditions, the amorphous colloidal precipitate of aluminum hydroxide is converted into β-Al2O3·3H2O, which forms the η-Al2O3 phase after calcination. Comparative Example 4 shows diffraction peaks of the α-Fe2O3 phase at 24.1°, 33.2°, and 35.6°, indicating that it is a crystalline catalyst. This is because the Fe content in the catalyst is too high, and during the calcination process, Fe2O3 microcrystals aggregate to form the α-Fe2O3 phase, resulting in uneven distribution of Fe and Al in the catalyst. This leads to the aggregation of active components after reduction, causing a decrease in catalyst activity.

[0088] Table 1. Catalytic performance of catalysts and purity of carbon nanotubes in each example and comparative example.

[0089]

Claims

1. A method for preparing an amorphous carbon nanotube catalyst, characterized in that: Amorphous carbon nanotube catalysts were prepared by co-precipitation of iron and aluminum salts, and by controlling the total metal ion concentration, Fe / Al molar ratio, precipitation pH endpoint, calcination temperature and calcination time of the catalyst precursor. The Fe / Al molar ratio is (2~0.1):1, the total metal ion concentration is 0.1~2 mol / L, the hydroxide concentration in the precipitant solution is 0.5~2.5 mol / L, and the hydroxide concentration in the precipitant solution is higher than the total metal ion concentration. The endpoint pH value at which the precipitant solution stops adding is 6.5~10.

0. The calcination temperature of the colloidal precipitate of the catalyst precursor is 400~600 ℃, and the calcination time is 1~5 h. Includes the following steps: 1) Iron salt and aluminum salt are dissolved together in deionized water to obtain a mixed metal salt solution of active component and carrier component; 2) Add the mixed metal salt solution obtained in step 1) into the stirred reactor. While stirring continuously, add the precipitant dropwise into the reactor. Stop stirring when the final pH value is reached in the reactor and allow the sol system in the reactor to be evenly dispersed. After standing, a gel-like precipitate is formed. 3) After filtering the solution in the reactor, a colloidal precipitate is obtained. After washing, drying, calcining and grinding, the amorphous carbon nanotube catalyst is obtained.

2. The method for preparing an amorphous carbon nanotube catalyst as described in claim 1, characterized in that: The iron salts are ferric nitrate, ferric chloride, or ferric sulfate; the aluminum salts are aluminum nitrate, aluminum chloride, or aluminum sulfate; and the precipitating agents are ammonia, ammonium carbonate, sodium carbonate, sodium hydroxide, or potassium hydroxide.

3. The method for preparing an amorphous carbon nanotube catalyst as described in claim 1, characterized in that: The stirring rate of the reactor is 100~500 r / min, the dropping rate of the precipitant solution is 1~3 ml / min, and stirring continues for 0.5~2 h after the final pH value is reached in the reactor. Then, it is allowed to stand for 12~48 h and then dried. The drying temperature is 80~120 ℃ and the drying time is 6~12 h.

4. The method for preparing an amorphous carbon nanotube catalyst as described in claim 1, characterized in that: The calcination temperature of the colloidal precipitate of the catalyst precursor is 490~510 ℃.

5. The method for preparing an amorphous carbon nanotube catalyst as described in claim 1, characterized in that: The endpoint pH value at which the addition of the precipitant solution stops is 8~9.

5.

6. The method for preparing an amorphous carbon nanotube catalyst as described in claim 1, characterized in that: The precipitant is added dropwise to the reaction vessel using a dropping funnel or peristaltic pump.

7. An amorphous carbon nanotube catalyst, characterized in that: Prepared by any of the preparation methods of the amorphous carbon nanotube catalysts described in claims 1-6.

8. The application of the amorphous carbon nanotube catalyst as described in claim 7, characterized in that: Amorphous carbon nanotube catalyst was added to a fixed-bed reactor and reduced with hydrogen. Then, low-carbon hydrocarbons were used to synthesize carbon nanotubes. After the reaction, the crude product was washed with alkali and acid to remove the catalyst support and active metal. It was then washed with deionized water and anhydrous ethanol and filtered. After drying in an oven, a pure carbon nanotube product was obtained.

9. The application of the amorphous carbon nanotube catalyst as described in claim 8, characterized in that: During the reaction, gas chromatography was used to analyze the composition of the gaseous products in order to calculate the reaction conversion rate.

10. The application of the amorphous carbon nanotube catalyst as described in claim 8, characterized in that: The reduction temperature of the amorphous carbon nanotube catalyst is 650–750 °C, the reduction time is 10–60 min, and the hydrogen mass hourly space velocity (HHSV) during the reduction process is 0.5–5 h⁻¹. -1 .

11. The application of the amorphous carbon nanotube catalyst as described in claim 8, characterized in that: The raw materials for the synthesis of carbon nanotubes are one or more low-carbon hydrocarbons such as methane, ethane, ethylene, propane, and propylene. The reaction temperature is 650–750 °C, which is consistent with the reduction temperature of the amorphous carbon nanotube catalyst. The mass hourly space velocity (HHSV) of the raw material gas during the reaction process is 0.5–5 h⁻¹. -1 .

12. The application of the amorphous carbon nanotube catalyst as described in claim 8, characterized in that: The alkaline washing process uses sodium hydroxide or potassium hydroxide solution with a concentration of 0.5~3 mol / L, a solution mass to carbon nanotube mass ratio of (10~30):1, an alkaline washing temperature of 60~80 ℃, and an alkaline washing time of 12~24 h.

13. The application of the amorphous carbon nanotube catalyst as described in claim 8, characterized in that: The pickling process uses one or more of hydrochloric acid, nitric acid, and sulfuric acid, with a solution concentration of 0.5~5 mol / L, a solution mass to carbon nanotube mass ratio of (10~30):1, a pickling temperature of 60~80 ℃, and a pickling time of 12~24 h.

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