Carbon nanotube catalyst, method for preparing the same, and use thereof

Abstract

The application provides a preparation method of a carbon nanotube catalyst, which comprises the following steps: preparing a mixed solution containing Co 2+ : Fe 3+ : La 3+ : Al 3+ , wherein the molar ratio of the metal ions is Co: Fe: La: Al = 1: (1-3): (0.1-0.5): (1-8); adding a complexing agent and mixing, keeping the solution temperature at 70-80 DEG C, and the molar ratio of the complexing agent to the metal ions in the solution is 1-10:10; adding ammonia water to adjust the pH to 7-8, keeping the solution temperature at 70-80 DEG C, stirring until the solution becomes sol; allowing the sol to stand to form a gel, and heating the gel at 100-300 DEG C for 3-6 hours to initiate self-spread combustion, forming a loose precursor powder; calcining the precursor powder at a temperature of 700-900 DEG C for 1-3 hours; cooling in the furnace, and when the temperature decreases to 400-500 DEG C, passing hydrogen gas to reduce and calcine under the protection of argon to obtain the carbon nanotube catalyst. The application also provides a carbon nanotube catalyst prepared by the method, and a carbon nanotube preparation method using the catalyst.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of catalysts, in particular to a carbon nanotube catalyst, a preparation method and application thereof, and particularly to a preparation method and application of a cobalt ferrite carbon nanotube catalyst. BACKGROUND

[0002] Carbon nanotubes are one-dimensional quantum materials with special structures, and their unique structures and excellent mechanical, electrical and chemical properties have attracted widespread attention from scientists in the fields of materials, physics, electronics and chemistry, and are the research frontier and hotspot in the field of international new materials.

[0003] At present, great progress has been made in the research on the properties and preparation methods of carbon nanotubes, and the focus is shifting to the research on its large-scale production and application field. The main methods for industrial production of carbon nanotubes include arc discharge method, laser evaporation method and chemical vapor deposition method, among which the chemical vapor deposition method (CVD) has become the industry mainstream due to its low cost, large output, and easy realization of physical and chemical conditions.

[0004] The key to the preparation of carbon nanotubes is a catalyst with high synthesis yield, low cost, high catalytic activity and good carbon tube morphology. Iron, cobalt and nickel are common metal catalyst elements, and alumina and magnesium oxide are commonly used as carriers, and metals such as molybdenum, lanthanum, yttrium and manganese are added as additives.

[0005] Patent CN114524466A provides a preparation method of carbon nanotube catalyst, which comprises adding two different complexing agents in sequence, carbonizing, calcining, crushing and sieving to obtain a metal oxide catalyst. This method is complicated, and the temperature needs to be strictly controlled below 500℃, otherwise the catalyst is easy to sinter and deactivate. The growth temperature of carbon nanotubes is generally 680-1200℃, which is much higher than the calcination temperature, resulting in low efficiency.

[0006] The quality of the catalyst depends largely on the size of the particles. The catalyst with small particle size can facilitate the adsorption of carbon source gas on the surface of the particles during high-temperature cracking, and the nanoscale catalyst particles are more conducive to capturing carbon atoms.

[0007] Patent CN110586115A provides a carbon nanotube catalyst and a preparation method thereof. The method limits the migration and coalescence of the first phase crystal grains after reduction by adding second phase metal catalyst crystal grain salt and pore-forming agent, thereby reducing the size of the catalyst powder. The second phase metal catalyst crystal grain salt introduced in this method has a large content, and the carbon nanotubes prepared have more impurity metal residues, which is not conducive to further processing.

[0008] Patent CN102010577A discloses a preparation method of a rare earth doped ferrite / polythiophene / carbon nanotube microwave absorber. The method is to obtain a composite material with good frequency characteristics and wave absorption performance. The doped ferrite composite is obtained by self-propagating-sol-gel method, and then the composite is dissolved with existing polythiophene and carbon nanotube through an organic solvent and ultrasonic dispersion to realize the light weight of the composite material. In the technical solution, the self-propagating-sol-gel method is used to prepare the ferrite composite, and the purpose is to obtain uniformly distributed active sites, avoid local excessive activity or inactivation, and facilitate the preparation of uniform composite material when the carbon nanotube is compounded later. Among them, metal iron, barium and other metals are used as wave absorbing materials. In order to play the role of each metal component iron and barium in wave absorption performance, a certain proportion of lanthanum and neodymium is added, and the component ratio and reaction temperature are adjusted, so that the rare earth elements can interact with the iron-barium alloy phase, increase the energy of the grain boundary, promote the growth of the grain, increase the average grain size of the crystal, and reduce the grain boundary resistivity. The preparation of the doped ferrite composite in the technical solution is opposite to the requirement of the carbon nanotube catalyst.

[0009] The purpose of the present application is to provide a cobalt ferrite carbon nanotube catalyst with small grain size and high catalytic activity. SUMMARY

[0010] The technical problem to be solved by the present application is to provide a carbon nanotube catalyst and a preparation method thereof. The prepared catalyst has uniform particles, small grain size, high catalytic activity, and greatly reduces the growth temperature of carbon nanotubes.

[0011] The first aspect of the present application is to provide a preparation method of a carbon nanotube catalyst, comprising the following steps:

[0012] Step S1, configuring a mixed solution containing Co 2+ : Fe 3+ : La 3+ : Al 3+ , wherein the molar ratio of each metal ion is Co: Fe: La: Al = 1: (1-3): (0.1-0.5): (1-8);

[0013] Specifically, the molar ratio of Co:Fe:La:Al can be 1:1:0.1:1, 1:1:0.1:5, 1:1:0.1:8, 1:1:0.3:1, 1:1:0.3:3, 1:1:0.3:5, 1:1:0.3:8, 1:1:0.5:1, 1:1:0.5:3, 1:1:0.5:5, 1:1:0.5:8, 1:2:0.1:1, 1:2:0.1:5, 1:2:0.1:8, 1:2:0.3:1, 1:2:0.3:3, 1:2:0.3:5, 1:2:0.3:8, 1:2:0.5:1, 1:2:0.5:3, 1:2:0.5:5, 1:2:0.5:8, 1:3:0.1:1, 1:3:0.1:5, 1:3:0.1:8, 1:3:0.3:1, 1:3:0.3:3, 1:3:0.3:5, 1:3:0.3:8, 1:3:0.5:1, 1:3:0.5:3, 1:3:0.5:5, 1:3:0.5:8, or other ratio values within the range.

[0014] In step S2, the complexing agent is added and mixed, and the solution temperature is maintained at 70-80℃; wherein the complexing agent is one of citric acid, tartaric acid, and gluconic acid, and the molar ratio of the complexing agent to the metal ions in the solution is 1-10:10;

[0015] Specifically, the molar ratio of the complexing agent to the metal ions in the solution can be 1:10, 2:10, 3:10, 4:10, 5:10, 6:10, 7:10, 8:10, 9:10, or 1:1, and can also be other values within the range;

[0016] The solution temperature can be 70℃, 75℃, or 80℃, and can also be other temperature values within the range.

[0017] In step S3, ammonia water is added to adjust the pH to 7-8, the solution temperature is maintained at 70-80℃, and stirring is performed until the solution becomes sol-like;

[0018] Through the complexation of the organic acid, the solution continuously undergoes hydrolysis and condensation reactions during stirring, and then a uniformly dispersed sol system is formed; the ammonia water is used to neutralize the excess complexing agent. The sol formation temperature can be 70℃, 75℃, or 80℃, and can also be other temperature values within the range.

[0019] In step S4, the sol is allowed to stand to form a gel, and the gel is heated at 100-300℃ for 3-6h, the nitrate and the complexing agent undergo a redox reaction, which in turn triggers a self-propagating combustion, forming a loose precursor powder;

[0020] Specifically, with the hydrolysis and condensation process, the solvent is continuously evaporated and consumed, the concentration is increased, the solution is concentrated, and the stability of the suspension system is destroyed, so that the gelation occurs. Through low-temperature drying foaming, the nitrate and the complexing agent undergoes redox reaction, in which NO3 - An oxidizing atmosphere is provided, the carboxyl group in the complexing agent acts as fuel, and the two undergo "in-situ" oxidation-reduction reaction, which in turn triggers self-sustaining combustion, forming a loose precursor powder. The heating temperature can be 100℃, 150℃, 200℃, 250℃ or 300℃, or other values within the range; the heating time can be 3h, 4h, 5h or 6h, or other time values within the range.

[0021] Step S5, the precursor powder is calcined at a temperature of 700-900℃ for 1-3h to form a loose flaky surface microstructure; the furnace is cooled down, when the temperature is reduced to 400-500℃, hydrogen is introduced for reduction calcination under argon protection to obtain a carbon nanotube catalyst, the surface of the carbon nanotube catalyst is flaky stacked particles.

[0022] Specifically, the calcination of the catalyst is divided into two stages, wherein the calcination temperature of the first stage can be 700℃, 750℃, 800℃, 850℃ or 900℃, or other values within the range; the calcination temperature of the second stage can be 400℃, 420℃, 450℃, 480℃ or 500℃, or other values within the range.

[0023] Further, in step S5, the temperature is raised to the calcination temperature at a heating rate of 12-15℃ / min; during the reduction calcination process, the flow rate ratio of argon to hydrogen is 10-20:1, and the hydrogen reduction time is 1-2h. Specifically, the heating rate can be 12℃ / min, 13℃ / min, 14℃ / min or 15℃ / min, or other values within the range; the flow rate ratio of argon to hydrogen can be 10:1, 15:1 or 20:1, or other values within the range.

[0024] Further, Co 2+ , Fe 3+ , La 3+ , Al 3+ are all selected from nitrate compounds, such as: Co 2+ provided by cobalt nitrate hexahydrate, Fe 3+ provided by iron nitrate nonahydrate, La 3+ provided by lanthanum nitrate, Al 3+ provided by aluminum nitrate.

[0025] The second aspect of the present application is to provide a carbon nanotube catalyst prepared by the method of the first aspect.

[0026] The third aspect of the present application provides a use of the catalyst of the second aspect in the preparation of carbon nanotubes.

[0027] Further, a preparation method of carbon nanotubes comprises the following steps:

[0028] Step S1, placing the carbon nanotube catalyst of the second aspect into a tube furnace;

[0029] Step S2, heating, keeping the heating rate in the furnace at 12-15℃ / min, passing in argon when the temperature is 100-200℃, passing in hydrogen when the temperature is 200-600℃, and then passing in carbon source gas when the temperature is 500-800℃, controlling the flow rate ratio of argon flow rate:hydrogen flow rate:carbon source flow rate=(15-20):(1-5):(5-10), and cooling to room temperature after passing in the carbon source gas for 25-35min to prepare the carbon nanotubes.

[0030] Specifically, the heating rate can be 12℃ / min, 13℃ / min, 14℃ / min or 15℃ / min, or other values in the range; the temperature in the furnace when argon is passed in can be 100℃, 150℃ or 200℃, or other values in the range; the temperature in the furnace when hydrogen is passed in can be 200℃, 250℃, 300℃, 350℃, 400℃, 450℃, 500℃, 550℃ or 600℃, or other values in the range; the temperature in the furnace when carbon source gas is passed in can be 500℃, 550℃, 600℃, 650℃, 700℃, 750℃ or 800℃, or other values in the range; the flow rate ratio of argon flow rate:hydrogen flow rate:carbon source flow rate can be 15:1:5, 15:2:8, 15:5:10, 20:1:5 or 20:5:10, or other values in the range; and the reaction time can be 25min, 30min or 35min, or other values in the range.

[0031] Further, the carbon source gas is one of ethylene, propylene, methane, propane and acetylene.

[0032] Compared with the prior art, the carbon nanotube catalyst, the preparation method and the application thereof provided by the present application have the beneficial effects that:

[0033] Firstly, the carbon nanotube catalyst provided by the present application takes Co 2+ , Fe 3+ , La 3+ and Al 3+ as metal ions, wherein Co 2+ , Fe 3+ serves as the active ingredient of the catalyst, and Al 3+La as catalyst adjuvant for providing more active sites 3+ As a doping element; with citric acid as a complexing agent, by adjusting the ratio of metal ions, and using self-propagating-sol-gel method, the catalyst particles are micron-sized after self-propagating combustion, the surface is a small scale stacked particles, which provides more attachment sites for the growth of carbon nanotubes, thereby having good catalytic activity.

[0034] Secondly, the preparation method of the carbon nanotube catalyst provided by the application uniformly disperses lanthanum elements in the whole solution system by using a sol-gel method, interacts with cobalt, iron and aluminum alloys, reduces the interface energy of the crystal grains, and inhibits the growth of the crystal grains, so that the nanometer particles with small grain sizes are obtained; and the low-temperature self-propagating combustion further disperses the particles, so that the subsequent heat treatment is facilitated to obtain the non-agglomerated and high-specific-surface-area sheet-shaped catalyst. The prepared catalyst has more active sites, which is conducive to the combination with the cracked carbon source gas, and the carbon nanotubes grow according to the gas-solid (VS for short) mechanism.

[0035] Thirdly, the preparation method of the carbon nanotube catalyst provided by the application, the precursor powder obtained by self-propagating combustion is calcined twice, the temperature of the first calcination is 700-900 DEG C, the purpose of this stage is to remove the water in the hydrotalcite interlayer, and the loose scale-shaped surface microstructure is formed in the dehydration process, and the layered double hydroxide is changed into the double metal oxide cobalt ferrite. The second stage of calcination is at a temperature of 400-500 DEG C, and hydrogen is introduced for reduction calcination under the protection of argon, so that the catalyst with scale-shaped stacked particles on the surface is obtained. Under this condition, the crystal structure is adjusted, the active sites on the surface of the catalyst are changed, and thus the growth direction, diameter and structure of the carbon nanotubes can be affected.

[0036] Fourthly, the preparation method of the carbon nanotube catalyst provided by the application, by adjusting the composition and ratio of the catalyst and the calcination process and other conditions, the prepared catalyst can reduce the growth temperature of the carbon nanotubes, not only ensures the catalytic efficiency of the catalyst, but also reduces the energy consumption of the production of the carbon nanotubes. BRIEF DESCRIPTION OF DRAWINGS

[0037] In order to more clearly illustrate the technical solutions in the embodiments of the application, the following will briefly introduce the drawings needed to be used in the embodiment description, and the drawings in the following description are only some embodiments of the application, and for those skilled in the art, other drawings can also be obtained according to these drawings without paying creative labor.

[0038] Figure 1 The XRD pattern of the catalyst prepared for the first embodiment of the application;

[0039] Figure 2The image shows a scanning electron microscope (SEM) image of the catalyst prepared in Example 1 of this invention, with a scale bar of 10 μm.

[0040] Figure 3 The image shows a scanning electron microscope (SEM) image of the catalyst prepared in Example 1 of this invention, with a scale bar of 500 nm.

[0041] Figure 4 This is a scanning electron microscope image of carbon nanotubes grown by the catalyst prepared in Example 1 of the present invention, with a scale bar of 500 nm. Detailed Implementation

[0042] To enable those skilled in the art to better understand the technical solutions in the embodiments of the present invention, and to make the above-mentioned objectives, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be further described below.

[0043] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0044] Example 1

[0045] A method for preparing a carbon nanotube catalyst includes the following steps:

[0046] Step S1: Take 0.005 mol (1.46 g) cobalt nitrate hexahydrate, 0.01 mol (4.04 g) ferric nitrate nonahydrate, 0.0015 mol (0.49 g) lanthanum nitrate, 0.04 mol (15.00 g) aluminum nitrate and 1.19 g citric acid. The molar ratio of each metal ion Co:Fe:La:Al = 1:2:0.3:8, and the molar ratio of citric acid to total gold ions is 1:10.

[0047] Step S2: Place the weighed nitrate into a beaker, dissolve it in an appropriate amount of deionized water, stir it on a magnetic stirrer at 75°C for 15 minutes, and then add citric acid as a complexing agent.

[0048] Step S3: Add ammonia to adjust the pH to 7, keep the solution temperature at 75℃, and stir for 4 hours until it becomes a sol state;

[0049] Step S4: While still hot, transfer the sol to a crucible and let it stand to form a gel. Then place the crucible in a muffle furnace and heat it to 200°C for 3 hours. At this time, the gel will be calcined by self-propagating combustion to obtain a fluffy precursor powder. Take it out and grind it.

[0050] Step S5: The powder was placed in a tube furnace and calcined at 800℃ for 1h at a temperature rising rate of 15℃ / min; the furnace was cooled down, and when the temperature dropped to 500℃, argon gas with a flow rate of 400sccm was used as a protective gas, and hydrogen gas with a flow rate of 20sccm was used as a reducing gas to be introduced into the tube furnace, and the calcination was performed for 2h to obtain the catalyst.

[0051] Reference should be made to Figures 1-3 wherein Figure 1 is an XRD pattern of the catalyst prepared in Example 1 of the present application; Figure 2 is a scanning electron microscope (SEM) image of the catalyst prepared in Example 1 of the present application, with a scale of 10μm; Figure 3 is a scanning electron microscope (SEM) image of the catalyst prepared in Example 1 of the present application, with a scale of 500nm. Figures 1-3 It can be seen that the catalyst prepared in the present example has a spinel structure, and the surface thereof is a flaky stacked particle, and the catalyst has fine and uniform crystal grains.

[0052] Comparative Example 1

[0053] A method for preparing a carbon nanotube catalyst is basically the same as that in Example 1, except that no lanthanum nitrate is added to the salt solution, the molar ratio of the remaining metal ions is Co:Fe:Al=1:2:8, the molar ratio of citric acid to total metal ions is 1:10, and the amount of citric acid used is 1.16g, and the remaining steps are the same as those in Example 1.

[0054] Comparative Example 2

[0055] A method for preparing a carbon nanotube catalyst is basically the same as that in Example 1, except that no aluminum nitrate is added to the salt solution, the molar ratio of the remaining metal ions is Co:Fe:La=1:2:0.3, the molar ratio of citric acid to total metal ions is 1:10, and the amount of citric acid used is 0.35g, and the remaining steps are the same as those in Example 1.

[0056] Comparative Example 3

[0057] A method for preparing a carbon nanotube catalyst is basically the same as that in Example 1, except that no iron nitrate nonahydrate is added to the salt solution, the molar ratio of the remaining metal ions is Co:La:Al=1:0.3:8, the molar ratio of citric acid to total metal ions is 1:10, and the amount of citric acid used is 0.98g, and the remaining steps are the same as those in Example 1.

[0058] Comparative Example 4

[0059] A preparation method of a carbon nanotube catalyst, which is basically the same as the method of Example 1, except that no cobalt nitrate is added to the salt solution, and the molar ratio of the metal ions is Fe:La:Al = 2:0.3:8, and the amount of iron nitrate used is 0.01 mol, and the molar ratio of citric acid to total metal ions is 1:10, and the amount used is 1.08 g, and the remaining steps are the same as in Example 1.

[0060] Comparative Example 5

[0061] A preparation method of a carbon nanotube catalyst, which is basically the same as the method of Example 1, except that the molar ratio of the metal ions is Co:Fe:La:Al = 1:2:0.8:8, and the molar ratio of citric acid to total metal ions is 1:10, and the amount used is 1.24 g, and the remaining steps are the same as in Example 1. Compared to Example 1, the amount of lanthanum ions is increased in Comparative Example 5.

[0062] Comparative Example 6

[0063] A preparation method of a carbon nanotube catalyst, which is basically the same as the method of Example 1, except that the molar ratio of the metal ions is Co:Fe:La:Al = 1:2:0.3:14, and the molar ratio of citric acid to total metal ions is 1:10, and the amount used is 1.82 g, and the remaining steps are the same as in Example 1. Compared to Example 1, the amount of aluminum ions is increased in Comparative Example 6.

[0064] Comparative Example 7

[0065] A preparation method of a carbon nanotube catalyst, which is basically the same as the method of Example 1, except that the molar ratio of the metal ions is Co:Fe:La:Al = 1:6:0.3:8, and the molar ratio of citric acid to total metal ions is 1:10, and the amount used is 1.60 g, and the remaining steps are the same as in Example 1. Compared to Example 1, the amount of iron ions is increased in Comparative Example 7.

[0066] Comparative Example 8

[0067] A preparation method of a carbon nanotube catalyst, which is basically the same as the method of Example 1, except that the molar ratio of the metal ions is Co:Fe:La:Al = 3:2:0.3:8, and the amount of cobalt nitrate used is 0.015 mol, and the molar ratio of citric acid to total metal ions is 1:10, and the amount used is 1.40 g, and the remaining steps are the same as in Example 1. Compared to Example 1, the amount of cobalt ions is increased in Comparative Example 8.

[0068] Comparative Example 9

[0069] A method for preparing a carbon nanotube catalyst is basically the same as that in Example 1, except that the molar ratio of citric acid to total metal ions is 2:1, and the amount used is 23.75 g. The remaining steps are the same as in Example 1. Compared with Example 1, the amount of complexing agent citric acid in Comparative Example 9 is increased.

[0070] Example 2

[0071] A method for preparing a carbon nanotube catalyst includes the following steps:

[0072] Step 1: Take 0.005 mol (1.46 g) cobalt nitrate hexahydrate, 0.005 mol (2.02 g) ferric nitrate nonahydrate, 0.0015 mol (0.49 g) lanthanum nitrate, 0.025 mol (9.38 g) aluminum nitrate and 3.84 g citric acid. The molar ratio of each metal ion is 1:1:0.3:5, and the molar ratio of citric acid to total metal ions is 1:2.

[0073] Step 2: Place the weighed nitrate into a beaker, dissolve it in an appropriate amount of deionized water, stir it on a magnetic stirrer at 75°C for 15 minutes, and then add citric acid as a complexing agent.

[0074] Step 3: Add ammonia to adjust the pH to 7, keep the solution temperature at 75℃, and stir for 4 hours until it becomes a sol state;

[0075] Step 4: While still hot, transfer the sol to a crucible and let it stand to form a gel. Then place the crucible in a muffle furnace and heat it to 150°C for 5 hours. At this time, the gel will undergo self-propagating combustion to obtain a fluffy precursor powder. Take it out and grind it.

[0076] Step 5: Place the powder in a tube furnace and calcine it to 800℃ for 1 hour at a heating rate of 15℃ / min. Cool the furnace and when the temperature drops to 400℃, introduce 300 sccm of argon as a protective gas and 30 sccm of hydrogen as a reducing gas into the tube furnace and calcine for 1 hour to obtain the catalyst.

[0077] Comparative Example 10

[0078] A method for preparing a carbon nanotube catalyst is basically the same as that in Example 2, except that the calcination temperature in the first stage of step S5 is different; the powder is placed in a tube furnace and calcined at 600°C for 1 hour. The remaining steps are the same as in Example 2.

[0079] Comparative Example 11

[0080] A method for preparing a carbon nanotube catalyst is basically the same as that in Example 2, except that the calcination temperature in the second stage of step S5 is different. When the temperature drops to 700°C, argon gas at 300 sccm is used as a protective gas and hydrogen gas at 30 sccm is used as a reducing gas in a tube furnace, and the catalyst is obtained by calcination for 1 hour. The remaining steps are the same as in Example 2.

[0081] 0.15 g of each of the catalysts prepared in Examples 1-2 and Comparative Examples 1-11 were spread evenly on a silicon-based glass slide and placed in a quartz tube furnace with a diameter of 60 mm. The furnace was heated at a rate of 15 °C / min. When the temperature reached 100 °C, argon gas was introduced at a flow rate of 400 sccm. When the temperature rose to 200 °C, hydrogen gas was introduced at a flow rate of 20 sccm. When the temperature reached 650 °C, acetylene gas was introduced at a flow rate of 80 sccm. Carbon nanotubes were grown at this time. After reacting for 30 min, the carbon nanotubes were collected after cooling to room temperature.

[0082] The carbon nanotubes from each embodiment were collected and weighed. The carbon nanotubes prepared in Examples 1-2 and Comparative Examples 1-11 were subjected to electron microscopy (EMS). The EMS image of the carbon nanotubes corresponding to the catalyst in Example 1 is shown below. Figure 4 As shown in the figure. One hundred carbon nanotubes were selected from the scanning electron microscope images of different embodiments, and their diameters were measured. The diameter measurement data and the corresponding yields of the catalysts in each embodiment are shown in Table 1.

[0083] Table 1: Catalyst yield and carbon nanotube size of each example

[0084]

[0085]

[0086] Wherein, catalyst yield = (mass of carbon nanotubes obtained / mass of catalyst precursor used) × 100%

[0087] Based on the above data analysis, we can conclude that:

[0088] 1. As can be seen from Examples 1, 1, and 5, the addition of lanthanum nitrate significantly affects the catalyst yield. When the amount of lanthanum nitrate exceeds a certain value, the catalyst yield does not increase but rather decreases. Therefore, excessive lanthanum does not significantly improve the catalyst yield. Lanthanum is a rare earth element with a high price, and increasing its amount increases the cost. On the other hand, when the amount of lanthanum nitrate exceeds a certain value, the effect of lanthanum on the crystal structure changes from inhibition to promotion of grain growth. At this time, the diameter of carbon nanotubes increases, affecting the thermal conductivity and mechanical strength of carbon nanotubes. As the amount of lanthanum nitrate further increases, the diameter of carbon nanotubes will further increase.

[0089] 2. As can be seen from Examples 1, 2, and 6, the amount of aluminum ions affects the diameter of carbon nanotubes to a certain extent and significantly influences the catalyst yield. In Comparative Example 2, when no aluminum ions were added, the catalyst yield decreased to 660%, indicating that aluminum nitrate, as a catalyst promoter, can provide more active sites, thereby increasing the yield. In Comparative Example 6, when the aluminum ion content was excessive, the catalyst yield was also low, approximately 1360%. This is because excessive aluminum nitrate would coat the active metal particles, reducing the contact area with carbon atoms and affecting the yield and nanotube diameter.

[0090] 3. As shown in Examples 1, 3, 4, 7, and 8, both cobalt and iron can be used individually as active metals for catalyzing carbon nanotube growth, with cobalt exhibiting better catalytic performance than iron. However, regardless of the specific catalyst system used, the yield and nanotube diameter parameters are not as good as those achieved with the synergistic catalysis of cobalt ferrite. Furthermore, increasing the amount of cobalt or iron beyond a certain point will affect the size of the generated carbon nanotubes and reduce the catalyst yield, thus impacting the catalyst's effectiveness. Therefore, to synthesize uniform cobalt ferrite during calcination, the ratio of cobalt to iron, the active metals, must be strictly controlled.

[0091] 4. As can be seen from Example 1 and Comparative Example 9, the amount of citric acid used as a complexing agent depends on the metal ions that need to be complexed in the salt solution. When the amount of citric acid is insufficient, it cannot be fully complexed. When the amount of citric acid is excessive, too much ammonia needs to be added to adjust the pH value, which causes a large amount of pH-sensitive aluminum ions to precipitate out, reducing the content of catalyst promoters, thereby reducing the active sites provided by aluminum and reducing the yield of the catalyst.

[0092] 5. As can be seen from Examples 2, 10, and 11, the stepwise calcination temperature of the catalyst precursor has a significant impact on the catalyst performance. The first calcination removes moisture from the interlayers of the hydrotalcite, forming a loose, scaly surface microstructure during dehydration, transforming the original layered double hydroxide into a bimetallic oxide cobalt ferrite. In Comparative Example 10, when the calcination temperature is below a certain value, incomplete removal of moisture from the hydrotalcite interlayers leads to agglomeration on the catalyst surface, reducing the specific surface area of ​​the catalyst compared to Example 2, affecting the active sites and consequently the catalyst's activity. In Comparative Example 11, the second calcination involves reducing the catalyst by introducing hydrogen gas. When the temperature exceeds a certain value, the reduction reaction rate is too fast, leading to excessively rapid phase transitions within the catalyst. This results in an uneven crystal structure, manifested as an increased carbon nanotube diameter distribution and a decreased number of active sites, thus reducing the catalyst's activity and significantly decreasing its yield.

[0093] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations made to these embodiments without departing from the principles and spirit of the present invention still fall within the protection scope of the present invention.

Claims

1. A method for preparing a carbon nanotube catalyst, characterized in that, Includes the following steps: Step S1, configure the configuration containing Co 2+ Fe 3+ :La 3+ Al 3+ A mixed solution in which the molar ratio of each metal ion Co:Fe:La:Al = 1:(1-3):(0.1-0.5):(1-8); Step S2: Add the complexing agent and mix well, keeping the solution temperature at 70-80℃; wherein the complexing agent is one of citric acid, tartaric acid, and gluconic acid, and the molar ratio of the complexing agent to the metal ions in the solution is 1-10:

10. Step S3: Add ammonia to adjust the pH to 7-8, keep the solution temperature at 70-80℃, and stir until the solution becomes a sol. Step S4: The sol is allowed to stand to form a gel, and the gel is heated at 100-300℃ for 3-6 hours. The nitrate and the complexing agent undergo an oxidation-reduction reaction, which then initiates self-propagating combustion to form a loose precursor powder. Step S5: The precursor powder is calcined at 700-900℃ for 1-3 hours to form a loose, scaly surface microstructure; the furnace is cooled, and when the temperature drops to 400-500℃, hydrogen is introduced under argon protection for reduction calcination to obtain a carbon nanotube catalyst, wherein the surface of the carbon nanotube catalyst is composed of scaly stacked particles.

2. The method for preparing the carbon nanotube catalyst according to claim 1, characterized in that, In step S5, the temperature is increased to the calcination temperature at a heating rate of 12-15℃ / min; during the reduction calcination process, the flow rate ratio of argon to hydrogen is 10-20:1, and the hydrogen reduction time is 1-2h.

3. The method for preparing the carbon nanotube catalyst according to claim 1, characterized in that, Co 2+ Fe 3+ La 3+ Al 3+ All were selected from nitrate compounds.

4. A carbon nanotube catalyst, characterized in that, It is prepared by the method described in claim 1.

5. The application of the carbon nanotube catalyst as described in claim 4 in the preparation of carbon nanotubes.

6. A method for preparing carbon nanotubes, characterized in that, Includes the following steps: Step S1: Place the carbon nanotube catalyst according to claim 4 into a tube furnace; Step S2: Heat up the furnace at a rate of 12-15℃ / min. When the temperature reaches 100-200℃, introduce argon gas. When the temperature reaches 200-600℃, introduce hydrogen gas. When the temperature reaches 500-800℃, introduce carbon source gas. Control the flow rate ratio as argon flow rate: hydrogen flow rate: carbon source gas flow rate = (15-20): (1-5): (5-10). After reacting with the carbon source gas for 25-35 minutes, cool to room temperature to prepare carbon nanotubes.

7. The method for preparing carbon nanotubes according to claim 6, characterized in that, The carbon source gas is one of ethylene, propylene, methane, propane, and acetylene.

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