A high-quality carbon nanotube preparation device and method

Abstract

This invention discloses a carbon nanotube preparation apparatus and method. The apparatus consists of a raw material supply unit, an in-situ conversion reaction unit, and a chemical vapor deposition (CVD) growth unit connected in series. This invention converts saturated hydrocarbons or carbon oxides in situ at the front end into a mixed carbon source containing unsaturated hydrocarbons and Cl gas, which is then directly introduced into the back-end CVD unit. By dynamically adjusting the front-end operating parameters, dynamic hierarchical control of the high concentration of active carbon source during the nucleation phase and the low concentration during the growth phase of carbon nanotubes is achieved. Simultaneously, hydrogen, a dehydrogenation byproduct, is used as the CVD reducing atmosphere, and oxygen-containing gas is used to etch amorphous carbon in situ. This invention seamlessly couples the front-end catalysis with the back-end CVD, resolving the contradiction in carbon source activity and achieving "hydrogen-carbon" recycling. Under mild and safe conditions, it significantly improves the yield, purity, and aspect ratio of carbon nanotubes.

Description

Technical Field

[0001] This invention relates to the field of carbon nanomaterial preparation technology, and more specifically, to a carbon nanotube preparation device and method based on in-situ carbon source activation regulation. Background Technology

[0002] Carbon nanotubes (CNTs) have broad application prospects in composite materials, microelectronic devices, energy storage, and catalyst supports due to their excellent mechanical, electrical, and thermal properties. Currently, chemical vapor deposition (CVD) has become the mainstream technology for preparing carbon nanotubes because of its relatively mild reaction conditions, ease of large-scale production, and ability to control the structure of carbon nanotubes.

[0003] In traditional CVD growth processes, the type and supply method of the carbon source gas play a decisive role in the growth rate, morphology, and quality of carbon nanotubes. However, existing CVD preparation technologies still face the following technical bottlenecks: 1. The contradiction between carbon source activity, energy consumption, and catalyst lifetime: When using saturated hydrocarbons such as methane and propane as carbon sources, their stable molecular structure and high activation energy for decomposition typically require high reaction temperatures (>700°C). High temperatures not only increase energy consumption but also easily lead to thermal sintering and deactivation of CVD catalyst particles. Conversely, if unsaturated hydrocarbons such as ethylene and propylene are used as active carbon sources, although the growth temperature can be lowered, they are prone to excessive decomposition on the catalyst surface. The excessively rapid carbon deposition rate often exceeds the dissolution and diffusion rate of carbon in the catalyst particles, causing amorphous carbon to rapidly encapsulate the catalyst, prematurely terminating the growth of carbon nanotubes.

[0004] 2. Mismatch between nucleation and growth kinetics: Carbon nanotube growth typically involves two stages: induced nucleation and steady-state growth. The nucleation stage requires a high concentration of active carbon to achieve rapid, high-density initiation; however, the steady-state growth stage requires a reduced concentration of active carbon to maintain the axial linear growth of the carbon nanotubes and suppress the formation of defects in the tube wall. Traditional CVD devices typically use a single mixed gas with constant composition and flow rate for direct feeding, making it impossible to dynamically and stepwise control the type and concentration of carbon during the growth process.

[0005] 3. Cost and Control Challenges of Introducing Reducing and Etching Gases: To maintain the metallic active state of the CVD catalyst and etch the generated amorphous carbon, traditional processes typically require a continuous introduction of large amounts of external hydrogen into the reaction system or the addition of trace amounts of weak oxidants. The introduction of large amounts of external hydrogen not only increases production costs but also poses significant safety hazards. Furthermore, it is difficult to accurately balance the thermodynamic relationship between carbon source decomposition and amorphous carbon etching within a single reactor.

[0006] 4. Limitations of Single Carbon Source Routes: Most existing technologies are limited to single hydrocarbon cracking routes. With the advancement of the "dual carbon" goal, utilizing the greenhouse gas carbon dioxide (CO2) as a carbon source to prepare high-value-added carbon nanotubes has become a research hotspot. However, CO2 molecules are extremely stable, and direct cracking has an excessively high energy barrier, typically requiring extremely high temperatures or special plasma assistance. Furthermore, using CO2 alone makes it difficult to achieve efficient CNT growth. Currently, there is a lack of methods for preparing carbon nanotubes using CO2 as the carbon source material.

[0007] In fact, theoretically, coupling the dehydrogenation reaction of saturated hydrocarbons with the hydrogenation reaction of carbon dioxide can generate a mixture rich in unsaturated hydrocarbons, carbon monoxide, and hydrogen. This mixture is an ideal carbon source and atmosphere for CVD. However, the industry currently lacks a device that can effectively integrate the front-end "in-situ carbon source activation and conversion" with the back-end "CVD directional growth" in terms of physical space and process control. Existing technologies cannot utilize the hydrogen generated in-situ from the front-end dehydrogenation reaction to maintain the metallic state of the back-end catalyst, nor can they dynamically adjust the depth of the front-end conversion reaction to precisely control the proportion of active carbon source entering the CVD growth zone in real time.

[0008] Therefore, there is an urgent need for a novel carbon nanotube preparation device and method that can overcome the shortcomings of single carbon source and static feeding, realize the in-situ generation and dynamic control of mixed carbon sources, and thus prepare carbon nanotubes with high efficiency and high quality under mild conditions. Summary of the Invention

[0009] To address the shortcomings of existing CVD methods for preparing carbon nanotubes, such as difficulty in balancing the activity of a single carbon source, easy carbon deposition and deactivation of catalysts, high cost and safety hazards of introducing external hydrogen, limitations of the single carbon source route, and inability to dynamically control the carbon source concentration during the growth process, the present invention aims to provide a carbon nanotube preparation device and method based on in-situ carbon source activation regulation.

[0010] This invention integrates in-situ catalytic conversion at the front end with CVD directional growth at the back end. It utilizes the front-end reaction to generate and adjust an ideal mixed carbon source and reducing gas in real time, perfectly matching the kinetic requirements of different stages of carbon nanotube “induced nucleation” and “steady-state growth”, thereby achieving efficient and controllable preparation of high-quality, low-defect carbon nanotubes under mild conditions.

[0011] To achieve the above-mentioned objective, a first aspect of the present invention provides a carbon nanotube preparation apparatus, characterized in that it comprises: 1. Raw material supply unit: used to provide saturated hydrocarbons (such as saturated hydrocarbons that are gaseous at atmospheric pressure and 600°C) or carbon oxides (such as carbon dioxide) as gaseous raw materials.

[0012] 2. In-situ conversion reaction unit: Its inlet is connected to the raw material supply unit, and it is equipped with a catalytic conversion medium or heating component. This unit is configured to convert part or all of the saturated hydrocarbon or carbon oxide gaseous raw material into unsaturated hydrocarbon gas or C1 gas (such as carbon monoxide, methane), and output a mixed carbon source gas.

[0013] The in-situ conversion reaction unit includes a reaction tube filled with a bifunctional catalyst or a layered catalyst bed. The catalyst used is selected from at least one of supported noble metal catalysts, transition metal oxides, or zeolite molecular sieves. In practical applications, supported catalysts with high thermal stability and anti-sintering properties are preferred to cope with the high-temperature conversion environment and extend the continuous operation cycle of the unit.

[0014] The amount of catalytic conversion medium or the heating temperature of the reaction tube is configured to precisely control the conversion rate of the dehydrogenation or carbon dioxide hydrogenation reaction within the range of 10% to 90%.

[0015] This unit can be configured with an independent heating unit, or integrated with the CVD growth unit in the same heating furnace. It can also directly utilize the waste heat of the CVD growth unit for heating to optimize system energy efficiency.

[0016] 3. CVD growth unit: Its inlet is connected to the outlet of the in-situ conversion reaction unit, and a catalyst substrate is provided inside. It is configured to grow carbon nanotubes on the catalyst substrate using the mixed carbon source gas.

[0017] 4. Real-time control configuration: The device is configured to control the ratio of unsaturated hydrocarbons to C1 gas in the mixed carbon source gas entering the CVD growth unit in real time by adjusting the operating parameters (such as temperature, space velocity, gas composition, etc.) of the in-situ conversion reaction unit.

[0018] A second aspect of the present invention provides a method for preparing carbon nanotubes using the above-described apparatus, comprising the following steps: S1: Saturated hydrocarbons or carbon oxides and hydrogen are introduced into the in-situ conversion reaction unit as gaseous feedstock.

[0019] S2: Dehydrogenation or carbon dioxide hydrogenation reaction is carried out in the in-situ conversion reaction unit to generate a mixed carbon source gas containing saturated hydrocarbons, unsaturated hydrocarbons, and C1 gas.

[0020] S3: The mixed carbon source gas is directly introduced into the CVD growth unit to nucleate and grow carbon nanotubes on the catalyst substrate.

[0021] S4: Implement a dynamic hierarchical control strategy during carbon nanotube growth: Nucleation induction period: Control the front-end conversion parameters to maintain the total volume fraction of unsaturated hydrocarbons, methane, and carbon monoxide in the mixed gas entering the CVD growth unit at a high level, so as to promote the rapid formation of high-density carbon nanotube nucleation sites on the surface of catalyst particles.

[0022] Steady-state growth period: After detecting pressure fluctuations or changes in exhaust hydrocarbon concentration within the CVD unit that meet the standards, the front-end parameters are changed to reduce the total integral number to a lower level in order to maintain axial linear growth and suppress amorphous carbon encapsulation.

[0023] Atmosphere self-regulation and in-situ utilization: During growth, the concentration of oxygen-containing carbon dioxide is increased to etch amorphous carbon, reducing defects in carbon nanotubes. Simultaneously, hydrogen, a byproduct of the dehydrogenation reaction within the in-situ conversion unit, is directly used as the hydrogen source for the concurrently occurring carbon dioxide hydrogenation reaction, and as the reducing atmosphere for subsequent CVD growth units. By controlling the conversion rate, the molar ratio of hydrogen to mixed carbon source gas is maintained between 0.5:1 and 5:1.

[0024] Compared with the prior art, the advantages of the present invention are: 1. Resolving the contradiction between carbon source activity and CVD catalyst lifetime. This invention overcomes the limitations of traditional CVD feedstocks consisting of only saturated or unsaturated hydrocarbons, cleverly coupling in-situ catalytic conversion at the front end with CVD growth at the back end. It converts inert, difficult-to-crack saturated hydrocarbons in situ into highly reactive unsaturated hydrocarbons, significantly reducing the growth temperature of the CVD process and avoiding the problem of excessively rapid carbon deposition caused by directly introducing high-concentration, highly reactive carbon sources. This ensures an extremely stable carbon source supply to the CVD unit, greatly extending the continuous operation cycle of the entire preparation system.

[0025] 2. Dynamic hierarchical carbon control for precise matching of carbon nanotube growth kinetics This invention proposes a dynamic hierarchical control strategy for mixed carbon sources. By adjusting the front-end reaction temperature or space velocity in real time, a high concentration of active carbon source is provided during the "nucleation induction period" of carbon nanotube growth, achieving rapid and high-density nucleation on the catalyst surface. Conversely, during the "steady-state growth period," the concentration of active carbon source is rapidly and seamlessly reduced, effectively preventing amorphous carbon from encapsulating active sites due to excessive carbon precipitation. This design, dynamically matching the growth kinetics requirements, significantly improves the yield, aspect ratio, and structural uniformity of carbon nanotubes.

[0026] 3. In-situ self-regulation of hydrogen source enables inherent process safety and significant cost reduction. Traditional CVD processes rely on external hydrogen sources to maintain a reducing atmosphere, which is costly and poses significant safety risks. This invention fully utilizes hydrogen, a byproduct of the front-end dehydrogenation reaction, directly introducing it with the gas flow into the back-end as a reducing atmosphere for the CVD growth unit. By controlling the front-end conversion rate, the molar ratio of hydrogen to mixed carbon source gas within the system is stabilized between 0.5:1 and 5:1. This not only maintains the metallic active state of the CVD catalyst and achieves atom economy but also fundamentally eliminates the need for large-scale external hydrogen introduction, greatly improving the safety and economy of industrial production.

[0027] 4. Introducing a gentle in-situ etching mechanism significantly improves the crystallinity and purity of carbon nanotubes. This invention introduces carbon oxides, such as carbon dioxide, into a mixed gas. On one hand, carbon dioxide can assist in the dehydrogenation of saturated hydrocarbons at the front end, breaking the thermodynamic equilibrium and improving the conversion rate. On the other hand, the unreacted carbon dioxide, after entering the back-end CVD unit, acts as a mild oxygen-containing etchant, selectively oxidizing and removing amorphous carbon and defective carbon structures attached to the carbon nanotube walls. This in-situ etching mechanism effectively heals carbon nanotube defects, significantly improving the graphitization degree and purity of the final product, making it more advantageous in high-end applications such as high-performance electrical conductivity, thermal conductivity, and catalyst support.

[0028] 5. Expand the adaptability of raw materials to facilitate the resource utilization of greenhouse gases. The in-situ conversion reaction unit design of this invention allows the device to move beyond the limitations of a single high-valent carbon source of specific purity. By employing bifunctional catalysts or layered catalyst beds, this invention can directly utilize inexpensive low-carbon alkanes and even industrial waste gases, while consuming large quantities of the greenhouse gas carbon dioxide as a co-reactant. This not only broadens the raw material sources for CVD preparation of carbon nanotubes but also provides a promising green process route for the high-value-added conversion and resource utilization of carbon dioxide.

[0029] 6. Thermodynamic coupling and high optimization of system energy efficiency This invention fully considers the energy integration of chemical processes in its device structural design. The front-end in-situ conversion unit can be independently temperature-controlled, or integrated with the CVD growth unit into the same furnace. It can even directly utilize the waste heat from the high-temperature exhaust gas generated by the CVD unit to preheat the front-end feed and drive endothermic conversion reactions. This ingenious thermodynamic coupling and system-level energy efficiency optimization significantly reduces the overall energy consumption of the device, laying a solid engineering foundation for the large-scale, continuous industrial scaling up of this technology. Attached Figure Description

[0030] Figure 1 This is a diagram of the apparatus for preparing high-quality carbon nanotubes by in-situ carbon source activation regulation as described in this invention.

[0031] Figure 2 This is a flowchart of the method for preparing high-quality carbon nanotubes according to the present invention.

[0032] Figure 3 The images shown are SEM and Raman images of the carbon nanotubes prepared in Example 1 of this invention.

[0033] Figure 4 The images shown are SEM and Raman images of the carbon nanotubes prepared in Comparative Example 1 of this invention. Detailed Implementation

[0034] The present invention will be further described in detail below with reference to specific embodiments. These embodiments are only for illustrating the present invention and are not intended to limit the scope of the present invention.

[0035] Unless otherwise stated, the gas composition described in this invention should be as simple and free of impurities as possible.

[0036] In this invention, the dynamic adjustment strategy can be flexibly and freely adjusted according to the different catalysts used in the in-situ conversion unit and the different requirements of the CVD growth unit for mixed carbon source gas.

[0037] Equipment for preparing high-quality carbon nanotubes based on in-situ carbon source activation regulation, such as Figure 1 As shown: The gas cylinder serves as the raw material supply unit, providing propane as a saturated hydrocarbon and carbon dioxide as a carbon oxide to the in-situ conversion reaction unit. A mass flow meter controls the gas flow rate. The reaction tube with a heater is the in-situ conversion unit; its inlet is connected to the outlet of the raw material supply unit, and the heater temperature can reach up to 1000 degrees Celsius. Its outlet is connected to the inlet of the CVD growth unit. The CVD growth unit is a tubular reactor with a maximum heating temperature of 1000 degrees Celsius, and its exhaust gas is released into the atmosphere.

[0038] The steps for preparing high-quality carbon nanotubes based on in-situ carbon source activation regulation are as follows: A bifunctional catalyst for carbon dioxide hydrogenation and propane dehydrogenation is placed in the in-situ conversion reaction unit. A CVD catalyst for producing carbon nanotubes is placed in the CVD growth unit, ensuring the circuit is connected. Hydrogen and an inert gas are then introduced as protective gases to slowly raise the temperature of both the in-situ conversion reaction unit and the CVD growth unit to the desired temperature. Propane, carbon dioxide, and hydrogen are then introduced to initiate the reaction. After the reaction is complete, the gas is switched to an inert gas, and the heater is turned off, allowing the temperature of both the in-situ conversion reaction unit and the CVD growth unit to slowly cool to room temperature. The carbon nanotubes produced in the CVD growth unit are then removed.

[0039] Example 1 The first step is catalyst loading and pretreatment: 0.5g of PtSn / SiO2 bifunctional catalyst is loaded into the metal tube of the in-situ conversion reaction unit (second unit); a substrate coated with 5g of Fe / Al2O3 catalyst is placed in the CVD growth unit (third unit), all units are connected in sequence, and the device is sealed.

[0040] The second step is to heat up the reaction equipment and activate the catalyst: Inert argon gas is introduced through the raw material supply unit to purge the air from all the equipment. The temperature of the second unit is set to 580°C and the temperature of the third unit is set to 650°C. The temperature is slowly increased at a rate of 5°C / min. After the predetermined temperature is reached, hydrogen gas is switched to a flow rate of 5 ml / min and maintained for 30 minutes to activate the catalyst.

[0041] The third step involves the injection of a high-concentration active carbon source: a mixture of propane, carbon dioxide, and hydrogen is introduced into the second unit at a flow rate of 5 ml / min. Under these conditions, propane and carbon dioxide undergo an assisted dehydrogenation reaction in the presence of a PtSn / SiO2 catalyst, generating a mixed gas containing propane, propylene, carbon dioxide, carbon monoxide, and hydrogen in situ. At this point, the propane conversion rate and carbon dioxide conversion rate in the second unit are approximately 60%, resulting in a total volume fraction of propylene and carbon monoxide of 40% in the mixed carbon source gas entering the third unit. This state is maintained for 10 minutes to promote the rapid formation of high-density carbon nanotube nucleation sites on the surface of the CVD catalyst.

[0042] The fourth step involves dynamically reducing the concentration of the active carbon source: After the nucleation period, to prevent excessive deposition of amorphous carbon leading to catalyst deactivation, the system is dynamically adjusted. Maintaining a constant total inlet flow rate, the temperature of the second unit is reduced to 500°C. This operation lowers the dehydrogenation conversion rate of the first unit to 30% and the carbon dioxide hydrogenation conversion rate to 40%, reducing and maintaining the total volume fraction of propylene and carbon monoxide in the mixed carbon source gas entering the third unit at 24%. Under this low-concentration active carbon source and in-situ generated hydrogen reducing atmosphere, the reaction continues for 50 minutes, allowing carbon nanotubes to grow linearly in one dimension along the axial direction. Simultaneously, unconverted carbon dioxide is used to gently etch the generated amorphous carbon in situ at 650°C.

[0043] The entire reaction process takes 1 hour.

[0044] Step 5, Product Collection and Characterization: After the reaction is complete, the substrate is allowed to cool naturally to room temperature under Ar atmosphere. The black flocculent powder on the substrate is collected, which is the carbon nanotube product.

[0045] The catalyst obtained in this embodiment was characterized by scanning electron microscopy (SEM), and the average diameter of the obtained carbon nanotubes was 24 nm.

[0046] Raman spectroscopy was used to characterize its defect rate and degree of graphitization. D / I G A ratio of 0.20 indicates that the product has high crystallinity and low amorphous carbon content.

[0047] Comparative Example 1 To verify the regulatory effect of the present invention, Comparative Example 1 was set up, with 5g of Fe / Al2O3 catalyst and no in-situ conversion reaction unit set up as a blank control group to evaluate the performance of carbon nanotubes produced by mixed gaseous carbon sources that have not been activated by in-situ carbon sources.

[0048] The reactor heating and catalyst activation steps were the same as in Example 1. Subsequently, the comparative example was subjected to a CVD reaction at a reaction temperature of 650 degrees Celsius. The feed conditions were C3H8:CO2:H2 = 1:1:1, the total feed rate was 15 ml / min, and the reaction time was 1 hour. Ar gas was then introduced as a protective gas, and the reaction products were collected after cooling to room temperature.

[0049] The catalyst obtained in this comparative example was characterized by SEM, and the average diameter of the obtained carbon nanotubes was 43 nm.

[0050] Raman spectroscopy was used to characterize its defect rate and degree of graphitization. D / I G The ratio of 1.10 indicates that carbon nanotubes produced by methods without in-situ carbon source activation have more defects.

[0051] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A carbon nanotube preparation apparatus, characterized in that, include: (1) Raw material supply unit, used to provide saturated hydrocarbons or carbon oxides as gaseous raw materials; (2) An in-situ conversion reaction unit, whose inlet is connected to the raw material supply unit, and is equipped with a catalytic conversion medium or heating component, configured to convert part or all of the saturated hydrocarbon or carbon oxide gas raw material into unsaturated hydrocarbon gas or Cl gas, and output mixed carbon source gas. (3) Chemical vapor deposition (CVD) growth unit, whose inlet is connected to the outlet of the in-situ conversion reaction unit, and whose interior is provided with a catalyst substrate, configured to grow carbon nanotubes on the catalyst substrate using the mixed carbon source gas; (4) wherein the device is configured to control the ratio of unsaturated hydrocarbons to C1 gas in the mixed carbon source gas entering the CVD growth unit in real time by adjusting the operating parameters of the in-situ conversion reaction unit.

2. The apparatus according to claim 1, characterized in that: The in-situ conversion reaction unit includes a reaction tube filled with a bifunctional catalyst or a layered catalyst bed.

3. The apparatus according to claim 2, characterized in that: The bifunctional catalyst or the layered catalyst is selected from at least one of supported noble metal catalysts, transition metal oxides or zeolite molecular sieves; preferably, the loading amount of the catalytic conversion medium or the heating temperature of the reaction tube is configured to control the conversion rate of the dehydrogenation or carbon dioxide hydrogenation reaction within the range of 10% to 90%.

4. The apparatus according to claim 1, characterized in that: The in-situ conversion reaction unit has an independent heating unit, or it is integrated with the CVD growth unit in the same heating furnace, or the in-situ conversion reaction unit uses the waste heat of the CVD growth unit for heating.

5. The apparatus according to claim 1, characterized in that: The saturated hydrocarbon gaseous feedstock is a saturated hydrocarbon that can be gaseous at 600°C and normal pressure, and the unsaturated hydrocarbon gaseous feedstock is an unsaturated hydrocarbon or a mixture thereof that can be gaseous at 600°C and normal pressure; the carbon oxide gaseous feedstock is carbon dioxide, and the Cl gas is carbon monoxide or methane or a mixture thereof.

6. A method for preparing carbon nanotubes using the apparatus according to any one of claims 1 to 5, characterized in that, Includes the following steps: S1: Saturated hydrocarbons or carbon oxides and hydrogen are introduced into the in-situ conversion reaction unit as gaseous feedstock; S2: Dehydrogenation or carbon dioxide hydrogenation reaction is carried out in the in-situ conversion reaction unit to generate a mixed carbon source gas containing saturated hydrocarbons, unsaturated hydrocarbons, and C1 gas. S3: The mixed carbon source gas is directly introduced into the CVD growth unit to nucleate and grow carbon nanotubes on the catalyst substrate. S4: During the growth of carbon nanotubes, the temperature, gas space velocity, and gas composition of the in-situ conversion reaction unit are dynamically adjusted to change the concentration of unsaturated hydrocarbons and C1 gas in the mixed carbon source gas, thereby regulating the growth rate or structural morphology of carbon nanotubes.

7. The method according to claim 6, characterized in that, The dynamic adjustment mentioned in step S4 specifically includes the following hierarchical control strategies: Nucleation induction period: The temperature and gas composition of the in-situ conversion reaction unit are controlled to maintain the total volume fraction of unsaturated hydrocarbons, methane, and carbon monoxide in the mixed carbon source gas entering the CVD growth unit at V1, so as to promote the rapid formation of high-density carbon nanotube nucleation sites on the surface of catalyst particles. Preferably, V1 is 40-80%. Steady-state growth period: After detecting pressure fluctuations within the CVD growth unit or changes in hydrocarbon concentration in the exhaust gas that reach a preset threshold, the temperature or gas composition of the in-situ conversion reaction unit is changed to maintain the total volume fraction of unsaturated hydrocarbons, methane, and carbon monoxide in the mixed carbon source gas entering the CVD growth unit at V2, so as to maintain the axial linear growth of carbon nanotubes and inhibit the encapsulation of amorphous carbon. Preferably, V2 is 10-39%.

8. The method according to claim 6, characterized in that: During the growth of carbon nanotubes, the temperature and gas composition of the in-situ conversion reaction unit are controlled to increase the concentration of oxygen-containing carbon dioxide, thereby etching amorphous carbon and reducing defects in the carbon nanotubes.

9. The method according to any one of claims 6-8, characterized in that: Hydrogen, a byproduct of the dehydrogenation reaction within the in-situ conversion reaction unit, is used as the hydrogen source for the concurrent carbon dioxide hydrogenation reaction and as the reducing atmosphere for the subsequent CVD growth unit. This reduces the input of external hydrogen while ensuring the maintenance of the catalyst's metallic state and the etching of amorphous carbon in the carbon nanotube products. Specifically, the gas composition and conversion rate of the in-situ conversion reaction unit are controlled so that the molar ratio of hydrogen to mixed carbon source gas is maintained between 0.5:1 and 5:

1.

10. The method according to claim 6, characterized in that, The temperature adjustment range of the in-situ conversion reaction unit in step S4 is 300-900°C, preferably 550-620°C.

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