Coprecipitation preparation method and application of catalyst for synthesizing carbon nanotubes

Abstract

The invention relates to the technical field of carbon nanotubes, in particular to a coprecipitation preparation method and application of a catalyst for synthesizing carbon nanotubes. Three metal ions of magnesium (Mg), cobalt (Co) and molybdenum (Mo) are synchronously and uniformly precipitated in a solution state through a coprecipitation method, so that highly uniform mixing of active components (Co and Mo) and a carrier precursor (Mg) is realized on a molecular or atomic scale. Compared with a traditional step-by-step loading process such as an impregnation method, the problems that active metal particles are not uniformly distributed on the surface of the carrier and are easy to migrate and agglomerate are solved from the source. After roasting, the obtained catalyst has the characteristics of uniform distribution of active sites (Co particles) and tight combination with the carrier (MgO). And in combination with a specific molar ratio of magnesium (Mg), cobalt (Co) and molybdenum (Mo), the prepared catalyst can be used for preparing high-orientation array type carbon nanotubes.

Description

Technical Field

[0001] This invention relates to the field of carbon nanotube technology, and in particular to a co-precipitation preparation method for a catalyst used in the synthesis of carbon nanotubes and its application. Background Technology

[0002] Carbon nanotubes, due to their unique mechanical, electrical, and thermal properties, have shown great application potential in various fields such as composite materials, electronic devices, and energy storage. Among them, array-type carbon nanotubes, with their highly ordered orientation structure, have significant advantages in thermal and electrical conductivity anisotropy, field emission properties, and macroscopic material construction, making them a hot topic in research and application. The growth of array-type carbon nanotubes mainly relies on chemical vapor deposition (CVD), and the core of this method lies in efficient, stable, and controllable catalysts. The performance of the catalyst directly determines the morphology, structure, purity, and yield of carbon nanotubes.

[0003] Traditional catalyst preparation methods, such as impregnation, while simple, have significant limitations: the active components (such as Co and Fe) are unevenly distributed on the support, easily agglomerating, resulting in a wide range of catalyst particle sizes and rapid deactivation under the high-temperature environment of CVD. This directly leads to uneven diameter, short length, and easy bending of the grown carbon nanotubes, accompanied by a large amount of amorphous carbon, making it difficult to form high-quality, highly oriented array structures.

[0004] To address the aforementioned issues, the co-precipitation method for catalyst preparation exhibits unique advantages. This method achieves uniform mixing of active and support components at the molecular / atomic level by simultaneously and uniformly precipitating multiple metal ions in solution. After calcination, a composite metal oxide catalyst with highly dispersed active sites, uniform particle size, and excellent thermal stability can be obtained. This is crucial for achieving controllable, consistent, and high-yield growth of carbon nanotubes, especially for forming demanding array structures. The scientific ratio and functional synergy of the catalyst components are the chemical basis for achieving the growth of carbon nanotubes with specific morphologies. However, in existing technologies, carbon nanotube growth catalyst systems are mainly iron-based (such as pure iron) and iron-cobalt-based catalysts, all of which inevitably introduce iron as an active component.

[0005] Therefore, it is necessary to optimize the metal elements and develop a technical solution for preparing catalysts that can synthesize array-type carbon nanotubes using a co-precipitation method. Summary of the Invention

[0006] Therefore, based on the above background, the present invention provides a co-precipitation preparation method for catalysts used in the synthesis of carbon nanotubes and its application. The present invention prepares catalysts that can synthesize highly oriented carbon nanotubes by means of a reasonable combination of metal elements and a co-precipitation method.

[0007] The technical solution provided by this invention is as follows: A coprecipitation preparation method for a catalyst used in the synthesis of carbon nanotubes, comprising the following steps: ① Prepare the first solution by dissolving water-soluble magnesium salts, cobalt salts and molybdates in deionized water with a Mg:(Co+Mo) molar ratio of (2-10):(20-45) and a Mo:Co ratio of 1:(1-5); ②At a temperature of 40-60℃, under stirring conditions, the alkaline precipitate is slowly added to the first solution to obtain the precursor solution; ③ Stir the precursor fluid at 40-60℃ for a period of time; ④ Filter the precursor fluid and collect the filtrate; ⑤ After washing the filtered solid to neutrality, calcine it at 450-600℃ for 3-6 hours, and then let it cool naturally to room temperature to obtain the catalyst for carbon nanotube synthesis.

[0008] Furthermore, in step ②, the alkaline precipitate is selected from at least one of ammonium carbonate solution, sodium carbonate solution, sodium hydroxide solution, and ammonia solution; The amount of alkaline precipitate added is such that the pH of the precursor fluid is 8.5-9.5.

[0009] Furthermore, in step ①, the magnesium salt is selected as magnesium nitrate hexahydrate, the cobalt salt is selected as cobalt nitrate hexahydrate, and the molybdate is selected as ammonium tetramolybdate.

[0010] Further, in step ①, cobalt salt and molybdate are prepared with a Mo:Co ratio of 1:(3-5).

[0011] Based on the same inventive concept, the present invention also provides a catalyst for the synthesis of carbon nanotubes prepared by a co-precipitation preparation method for a catalyst used in the synthesis of carbon nanotubes.

[0012] Based on the same inventive concept, the present invention also provides the application of the catalyst for carbon nanotube synthesis in the preparation of carbon nanotubes.

[0013] Furthermore, the carbon nanotubes are array-type carbon nanotubes.

[0014] Based on the same inventive concept, the present invention also provides a method for preparing arrayed carbon nanotubes, comprising: The catalyst for carbon nanotube synthesis is introduced into the reactor, and carbon source gas or a mixture of carbon source gas and carrier gas is injected into the reactor at a temperature of 600-900°C. Then, carbon nanotubes are grown by decomposing the injected carbon source on the surface of the non-ferrous catalyst.

[0015] Based on the same inventive concept, the present invention also provides a carbon nanotube, which is prepared by the aforementioned method for preparing an array-type carbon nanotube.

[0016] The beneficial effects achieved by this invention are as follows: This invention utilizes a co-precipitation method to simultaneously and uniformly precipitate magnesium (Mg), cobalt (Co), and molybdenum (Mo) ions in solution, achieving a highly uniform mixture of the active components (Co, Mo) and the support precursor (Mg) at the molecular or atomic scale. Compared to traditional stepwise loading processes such as impregnation, this method avoids the problems of uneven distribution and easy migration and aggregation of active metal particles on the support surface. After calcination, the resulting catalyst exhibits a uniform distribution of active sites (Co particles) and a tight bond with the support (MgO). Furthermore, by combining specific molar ratios of magnesium (Mg), cobalt (Co), and molybdenum (Mo), the prepared catalyst can produce highly oriented array-type carbon nanotubes. Attached Figure Description

[0017] Figure 1 This is a microscopic morphology diagram of the carbon nanotubes prepared in the embodiments of the present invention. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] The following are specific embodiments of the present invention.

[0020] Example 1: Preparation of catalysts for the synthesis of carbon nanotubes ① Prepare the first solution by dissolving water-soluble magnesium salt, cobalt salt and molybdate in deionized water with a Mg:(Co+Mo) molar ratio of 8:18 and a Mo:Co ratio of 1:5; ② At a temperature of 40-60℃, under stirring conditions, the alkaline precipitate is slowly added to the first solution to obtain the precursor liquid; the alkaline precipitate is ammonia water with a mass concentration of 40%.

[0021] ③ Stir the precursor fluid at 40-60℃ for 2 hours; ④ Filter the precursor fluid and collect the filtrate; ⑤ After washing the filtered solid until it is neutral, calcine it at 550°C for 5 hours, and then let it cool naturally to room temperature to obtain the catalyst for carbon nanotube synthesis.

[0022] Example 2: Preparation of catalysts for the synthesis of carbon nanotubes ① Prepare the first solution by dissolving water-soluble magnesium salt, cobalt salt and molybdate in deionized water with a Mg:(Co+Mo) molar ratio of 2:42 and a Mo:Co ratio of 1:5; ② At a temperature of 40-60℃, under stirring, the alkaline precipitate is slowly added to the first solution to obtain the precursor fluid; the alkaline precipitate is ammonia water with a mass concentration of 40%.

[0023] ③ Stir the precursor fluid at 40-60℃ for 2 hours; ④ Filter the precursor fluid and collect the filtrate; ⑤ After washing the filtered solid until it is neutral, calcine it at 550°C for 5 hours, and then let it cool naturally to room temperature to obtain the catalyst for carbon nanotube synthesis.

[0024] In the catalyst preparation process, magnesium exists as magnesium oxide (MgO) after calcination, and its main role is as a support and structural aid. Cobalt (Co) in the catalyst serves as the main active center. Its 3d electron orbital structure gives it suitable adsorption and catalytic decomposition capabilities for carbon source gases (such as acetylene and ethylene). It is the core active source for carbon atom deposition and nucleation and growth of graphitized tubular structures. Molybdenum acts as a co-catalyst, adjusting the electron cloud density of cobalt, optimizing its adsorption and diffusion performance for carbon species, and effectively inhibiting the excessive deposition and encapsulation of amorphous carbon on cobalt active sites. This ensures the continuous and straight growth of carbon nanotubes, which is beneficial for the preparation of highly oriented array-type carbon nanotubes. Therefore, the Mo / Co ratio is crucial. If the ratio is too low, Mo's co-catalytic effect is weak, Co is easily deactivated, and carbon nanotubes tend to be short and flexible. If the ratio is too high, Mo may cover too many active sites, reducing overall activity. Furthermore, if the Mg content is too high, a large amount of MgO support will easily form, leading to excessive dispersion and isolation of Co particles, insufficient active site density, low catalytic efficiency, and the inability of the carbon source to efficiently graphitize on limited active sites, easily forming disordered pyrolytic carbon and producing coarse, entangled fibrous structures. Conversely, if the Mg content is too low, the MgO support content will be insufficient, failing to provide enough attachment sites for Co and Mo nanoparticles. This can easily lead to high surface energy Co particles migrating and merging into large aggregates during carbon nanotube preparation, resulting in disordered carbon nanotube growth orientation. Therefore, the catalyst prepared in Example 2 above was prepared by using a reasonable ratio of metal elements.

[0025] Carbon nanotubes were prepared using the catalysts prepared in Examples 1 and 2, respectively. The specific experiments are shown below: Take 0.1 g of the catalyst powder obtained in the example and spread it evenly in a quartz boat in a tube furnace. Place the quartz boat in the isothermal zone of the tube furnace. After sealing the reaction system, introduce high-purity nitrogen (N2, 99.999%) as a protective gas at a flow rate of 200 mL / min. Raise the furnace temperature to 500 °C at a heating rate of 10 °C / min and maintain this temperature for 30 minutes to remove moisture and impurities adsorbed on the catalyst surface.

[0026] Maintain the temperature at 500 °C, switch the gas to a hydrogen-argon mixture (H2 / Ar volume ratio of 1:4, total flow rate of 200 mL / min), and reduce for 60 minutes. This step aims to partially reduce the catalyst precursor (mainly oxides of Co and Mo) to a catalytically active metal or a lower valence state.

[0027] After reduction, the reaction temperature was adjusted to the preset 750℃, the hydrogen gas supply was stopped, and a mixture of carbon source gas and argon gas was switched (gas flow rate and ratio C2H2:Ar = 50 mL / min : 200 mL / min). Chemical vapor deposition was carried out under this atmosphere for 30 minutes.

[0028] After the growth stage, the carbon source gas (C2H2) is turned off, and the tube furnace is naturally cooled to room temperature (below 100℃) under the protection of pure argon (200 mL / min). The reactor is then opened, and the quartz boat is removed to obtain the black flocculent product supported on the catalyst, which is the crude carbon nanotube product.

[0029] The crude product was placed in a 6 M hydrochloric acid solution and stirred at 60 °C for 6 hours to dissolve and remove the catalyst support (MgO) and metal particles (Co, Mo). It was then repeatedly washed with deionized water until the filtrate was neutral, and finally dried in a vacuum oven at 80 °C for 12 hours to obtain a pure carbon nanotube sample.

[0030] The carbon nanotubes prepared in Examples 1 and 2 were scanned by electron microscopy, and the results are shown in the figure. Figure 1 As shown in the figure, the catalyst prepared by co-precipitation method using a reasonable proportion of metal elements in Example 2 can produce highly oriented array-type carbon nanotubes.

[0031] The present invention and its embodiments have been described above. This description is not restrictive, and the embodiments shown are only one of the embodiments of the present invention. The actual structure is not limited to this. In conclusion, if those skilled in the art are inspired by this description and design similar structures and embodiments without departing from the spirit of the present invention, they should all fall within the protection scope of the present invention.

Claims

1. A method for co-precipitation preparation of a catalyst for synthesizing carbon nanotubes, characterized in that, It includes the following steps: ① Prepare the first solution by dissolving water-soluble magnesium salts, cobalt salts and molybdates in deionized water with a Mg:(Co+Mo) molar ratio of (2-10):(20-45) and a Mo:Co ratio of 1:(1-5); ②At a temperature of 40-60℃, under stirring conditions, the alkaline precipitate is slowly added to the first solution to obtain the precursor solution; ③ Stir the precursor fluid at 40-60℃ for a period of time; ④ Filter the precursor fluid and collect the filtrate; ⑤ After washing the filtered solid to neutrality, calcine it at 450-600℃ for 3-6 hours, and then let it cool naturally to room temperature to obtain the catalyst for carbon nanotube synthesis.

2. The method for co-precipitation preparation of a catalyst for synthesizing carbon nanotubes according to claim 1, characterized in that, In step ②, the alkaline precipitate solution is selected from at least one of ammonium carbonate solution, sodium carbonate solution, sodium hydroxide solution, and ammonia solution; The amount of alkaline precipitate added is such that the pH of the precursor fluid is 8.5-9.

5.

3. The method for co-precipitation preparation of a catalyst for synthesizing carbon nanotubes according to claim 1, characterized in that, In step ①, the magnesium salt is magnesium nitrate hexahydrate, the cobalt salt is cobalt nitrate hexahydrate, and the molybdate is ammonium tetramolybdate.

4. The method for co-precipitating a catalyst for the synthesis of carbon nanotubes according to claim 1, characterized in that, In step ①, cobalt salt and molybdate are prepared with a Mo:Co ratio of 1:(3-5).

5. The catalyst for carbon nanotube synthesis prepared by the coprecipitation preparation method of the catalyst for synthesizing carbon nanotubes according to any one of claims 1 to 4.

6. The application of the catalyst for carbon nanotube synthesis according to claim 5 in the preparation of carbon nanotubes.

7. The application according to claim 6, characterized in that, The carbon nanotubes are array-type carbon nanotubes.

8. A method for preparing arrayed carbon nanotubes, characterized in that, include: The catalyst for carbon nanotube synthesis as described in claim 6 is introduced into the reactor, and carbon source gas or a mixture of carbon source gas and carrier gas is injected into the reactor at a temperature of 600-900°C; then, carbon nanotubes are grown by decomposing the injected carbon source on the surface of the non-ferrous catalyst.

9. A carbon nanotube, characterized in that, It is prepared by the method for preparing array-type carbon nanotubes as described in claim 8.

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