Preparation method of super-large-diameter carbon nanotube

Abstract

The present application relates to the technical field of carbon nanotube material preparation, and provides a preparation method of super-large-diameter carbon nanotubes, which comprises the following steps: forming a catalyst precursor by attaching active substances and accelerators to the surface of fumed white carbon black, adding the catalyst precursor into a fluidized bed or a CVD tubular furnace reactor for reaction after high-temperature annealing treatment, performing pre-oxidation treatment after the reaction is completed, and finally obtaining high-purity super-large-diameter carbon nanotubes through acid washing purification. The method realizes the size control of catalyst particles by controlling the concentration of ammonium carbonate, so as to realize the leap from traditional carbon nanotubes to super-large-diameter carbon nanotubes. The present application solves the problems of catalyst particle agglomeration and environmental pollution existing in the traditional method, optimizes the concentration scheme of ammonium carbonate, realizes the best catalyst complexing condition, and provides an efficient and controllable process scheme for the preparation of super-large-diameter carbon nanotubes.

Description

Technical Field

[0001] This invention relates to the field of carbon nanotube material preparation technology, specifically to a method for preparing ultra-large diameter carbon nanotubes. Background Technology

[0002] Carbon is one of the most abundant elements in nature. Carbon atoms can form complex cross-linked network structures, which are crucial for organic chemistry and the existence of life. Carbon materials are special materials that can exist in nature in zero-dimensional to three-dimensional forms. Furthermore, carbon allotropes possess diverse physicochemical properties. From the superhard material diamond to the soft material graphite, from the insulating material diamond to the semiconductor material graphite and the conductive material carbon nanotubes, carbon allotropes play important roles. Although carbon allotropes have different structures and properties, one remarkable carbon allotrope holds a unique and special position in the field of materials science: carbon nanotubes. Carbon nanotubes offer significant advantages in areas such as biosensors, lithium-ion batteries, drug delivery materials, and composite materials.

[0003] Large-diameter carbon nanotubes are carbon-based nanomaterials with diameters ranging from 100 nanometers to micrometers. Compared to conventional materials, large-diameter carbon nanotubes exhibit superior electrical conductivity, tensile strength >4 GPa, and dispersibility (low specific surface area reduces agglomeration). Large-diameter carbon nanotubes show broad prospects in new energy, composite materials, and electronic devices. Studies have shown that as a conductive additive for lithium batteries, they can increase energy density by more than 20%. When used in aerospace composite materials, they can reduce weight by 30% and enhance structural strength. They are also irreplaceable in chip heat dissipation, flexible sensors, and biological drug delivery systems. They fill the performance gap between nanomaterials and macromaterials, solve the pain points of high cost and difficulty in dispersion of traditional carbon fibers, and are expected to become a core component of next-generation high-performance materials, driving technological innovation in multiple fields.

[0004] In the preparation process of large-diameter carbon nanotubes, the core breakthrough lies in achieving a leap from the diameter of traditional carbon nanotubes (<100nm) to the carbon fiber scale (>1μm) through catalyst design and dynamic process control (staged carbon source introduction and gradient annealing).

[0005] For example, patent application CN103721750B discloses a catalyst for preparing large-diameter carbon nanotubes, which is formulated from the following raw materials in weight percentages: lanthanum nitrate 5.6%–47.8%, cobalt nitrate 2.3%–39.5%, ferric nitrate 1.7%–46.7%, calcium nitrate 2.7%–41.6%, citric acid 2.7%–41.6%, and ethylenediaminetetraacetic acid 2.7%–41.6%. This patent also discloses a method for preparing the catalyst for large-diameter carbon nanotubes. Adding large-diameter carbon nanotubes to the positive electrode of a battery significantly outperforms traditional graphite and small-diameter carbon nanotubes. The lower resistivity significantly helps reduce the internal resistance of the battery electrode, thus improving the battery's cycle life and energy density. The smaller specific surface area facilitates the dispersion of carbon nanotubes.

[0006] Although this method can achieve the preparation of large-diameter carbon nanotubes, the diameter is basically distributed below 150nm, which is difficult to meet the application scenarios that require larger diameter carbon nanotubes. In addition, the preparation method uses sol-gel, which causes significant environmental pollution. The auxiliary agent EDTA used is also harmful to the environment and human body. The catalyst prepared by this method has a low bulk density and is loose, which poses significant problems in practical applications.

[0007] This invention utilizes a special catalyst preparation process to produce high-quality, ultra-large diameter carbon nanotubes with diameters ranging from 180 to 380 nm. Using a corresponding reactor process, the diameter can reach 500 nm. Further processing yields the final product, preserving its excellent structure to ensure optimal performance. It can be used as a conductive additive in various materials, exhibiting excellent thermal conductivity and reinforcing effects. Summary of the Invention

[0008] To address the problems existing in the background art, the present invention provides a method for preparing ultra-large diameter carbon nanotubes, comprising the following steps:

[0009] S1. Active substances and promoters are attached to the surface of fumed silica to prepare a catalyst precursor.

[0010] S2. Filter and dry the precursor solution;

[0011] S3. The dried precursor is subjected to high-temperature annealing.

[0012] S4. The precursor that has undergone high-temperature annealing is added as a catalyst to a fluidized bed or CVD tubular furnace reactor for reaction.

[0013] S5. After the reaction is completed, a pre-oxidation treatment is performed to obtain ultra-large diameter carbon nanotube raw powder;

[0014] S6. Carbon nanotube raw powder is purified by acid washing to obtain high-purity ultra-large diameter carbon nanotubes.

[0015] In the preferred embodiment, step S1 specifically includes: soaking fumed silica in a nickel nitrate solution of 0.2–0.5 g / ml, titrating it with an ammonium carbonate solution of 0.02–0.06 g / ml, and allowing the reaction to proceed fully.

[0016] In the preferred embodiment, step S1 specifically includes: soaking fumed silica in a nickel nitrate solution of 0.2-0.5 g / ml for 20 min, then titrating with a 0.04 g / ml ammonium carbonate solution, and allowing the reaction to proceed fully for 30 min.

[0017] In the preferred embodiment, in step S2, the filtered product is dried at 120°C for 180 min.

[0018] In the preferred embodiment, in step S3, after drying, the product is placed in a dual-cavity heating furnace for high-temperature annealing. The initial heating rate is 10℃ / min for the first 500℃, and the heating rate increases by 5℃ / min from 500℃ to 900℃ until the heating rate reaches 55℃ / min. Then, the product is heated to 900℃ at a uniform rate, held at that temperature for 30 minutes, and then allowed to cool naturally to room temperature.

[0019] In the preferred embodiment, step S41, in which the precursor treated with high-temperature annealing is added as a catalyst to the fluidized bed reactor for reaction, is as follows:

[0020] S411. Reactor preheating: Under a nitrogen protective atmosphere, the fluidized bed reactor is heated to 660°C; the low-temperature preheater is heated to 500°C to preheat hydrogen and propylene gas; the high-temperature preheater is heated to 850°C to fully decompose the carbon source gas.

[0021] S412, Catalyst addition and fluidization: The precursor after high-temperature annealing is added as a catalyst to the fluidized bed reactor; nitrogen gas is introduced to keep the catalyst particles in a suspended fluidized state;

[0022] S413, Reduction Reaction: Hydrogen gas is introduced to reduce the metal oxide in the catalyst to the active element at 660℃, continuing until the reduction is complete; during this process, the carbon source is introduced in stages; for the first 5 minutes: propylene is introduced as the carbon source at a flow rate of 10L / min to initiate carbon nanotube nucleation; for the middle 10 minutes: the propylene flow rate is increased to 35L / min to promote rapid carbon nanotube growth; for the last 5 minutes: the propylene flow rate is reduced to 20L / min to optimize the integrity of the tube wall structure; total reaction time: 20 minutes, with hydrogen gas introduced as the reducing gas throughout the process; after the reaction is completed, the propylene supply is stopped, and nitrogen gas is introduced for another 5 minutes to remove residual gas in the reactor.

[0023] In the preferred embodiment, step S42, in which the precursor treated with high-temperature annealing is added as a catalyst to the CVD tube furnace reactor for reaction, is as follows:

[0024] S421. Reactor preheating: Under a nitrogen protective atmosphere, the CVD tubular furnace reactor is heated to 660°C; the low-temperature preheater is heated to 500°C to preheat hydrogen and propylene gas; the high-temperature preheater is heated to 850°C to fully decompose propylene into active carbon atoms.

[0025] S422, Catalyst loading: The high-temperature annealed precursor is evenly spread in a quartz boat as a catalyst and placed in the constant temperature zone of the reaction tube.

[0026] S423, Reduction Reaction: Hydrogen and nitrogen are introduced, and the catalyst reduction is completed at 660℃. During this process, the carbon source is introduced in stages: First 5 minutes: propylene is introduced at a flow rate of 10L / min, carbon nanotube nucleation stage; Middle 10 minutes: propylene flow rate is increased to 35L / min, main growth stage; Last 5 minutes: propylene flow rate is reduced to 20L / min, structure optimization stage; Total reaction time is 20 minutes, with hydrogen introduced throughout; After the reaction is completed, the propylene supply is stopped, and nitrogen is introduced for another 5 minutes to purge the gas in the reaction tube.

[0027] In the preferred embodiment, the specific process of step S5 includes: under nitrogen positive pressure protection, the product of step S4 is transported to the pre-oxidation equipment; nitrogen and air are mixed at a volume ratio of 1:1 and oxidized at 500°C for 20 minutes, and then transferred to the storage tank after completion.

[0028] In the preferred embodiment, step S6 specifically includes:

[0029] S61. Place the pre-oxidized product into a prepared acidic solution of a specific concentration to remove residual reactive metals, peel off the product, and wash and filter twice with water.

[0030] S62. The filtered product is placed in an acidic solution of a specific concentration for impurity removal.

[0031] S63. After impurity removal and filtration, the product is washed with water. After the pH is close to 7.0, it is filtered and dried.

[0032] In the preferred embodiment, in step S61, the mass ratio of the product to the acidic component in the acidic solution of a specific concentration is set as follows:

[0033] Products: hydrochloric acid: nitric acid: hydrofluoric acid = 1:2:1:0.5;

[0034] In S62, the mass ratio of the product to the acidic component in a specific concentration of acidic solution is set as follows:

[0035] Product: Hydrochloric acid: Nitric acid: Hydrofluoric acid = 1:1:0.5:0.25.

[0036] The beneficial effects achieved by this invention are as follows:

[0037] First, this invention uses fumed silica as a catalyst support, forming uniformly dispersed nickel active particles through the complexation reaction of nickel nitrate and ammonium carbonate. Compared with traditional sol-gel methods (such as the use of EDTA in the background art), this avoids harmful chemicals, and by controlling the concentration of ammonium carbonate (0.02-0.06 g / ml), the catalyst particle size can be controlled, achieving a tube diameter of 180-380 nm (fluidized bed process) to 520 nm (CVD process), far exceeding the existing technology (<150 nm in the background art).

[0038] Second, by adding ammonium carbonate solution in stages (0.02–0.06 g / ml), the complexation balance between nickel ions and carbonate ions is precisely controlled to form a stable, soluble complex, rather than directly generating a precipitate. This process, verified in Example 12, solves the problem of catalyst particle agglomeration in traditional processes.

[0039] Third, the dual-cavity heating furnace employs a staged heating strategy (10℃ / min for the first 500℃, gradually increasing the heating rate to 55℃ / min from 500-900℃) to achieve lattice recombination of catalyst particles and the formation of highly active sites. Compared with the traditional constant heating rate process, this significantly improves the thermal stability and activity of the catalyst.

[0040] Fourth, in the fluidized bed / CVD reactor, the propylene flow rate is controlled in three stages (10 L / min during nucleation, 35 L / min during growth, and 20 L / min during optimization), corresponding to the nucleation, rapid growth, and structural optimization of carbon nanotubes. Compared to the traditional single-stage introduction, this reduces the generation of amorphous carbon and improves the crystallinity of the tube wall. The parallel selection of the fluidized bed and the CVD tube furnace allows for high yield through the fluidized bed and ultra-large tube diameter (520 nm) through CVD, flexibly meeting different application requirements.

[0041] Fifth, amorphous carbon is removed by pre-oxidation with a nitrogen-air mixture (500℃), combined with two-stage pickling (hydrochloric acid: nitric acid: hydrofluoric acid = 1:2:1:0.5 → 1:1:0.5:0.25), which removes metallic impurities while preserving the structural integrity of the carbon nanotubes. Compared with traditional single pickling, the metal residue rate is reduced to <0.5wt%.

[0042] Sixth, this invention allows for predictable changes in pipe diameter (e.g., 0.02 g / ml → 240 nm, 0.04 g / ml → 380 nm) by adjusting the ammonium carbonate concentration (0.02–0.06 g / ml), meeting the needs of different application scenarios. Through extensive experimentation, this invention has achieved a technical solution using 0.04 g / ml ammonium carbonate, resulting in a maximum pipe diameter of 380 nm and a BET specific surface area of ​​43.23 m². 2 / g provides optimal complexation conditions. Attached Figure Description

[0043] Figure 1 This diagram shows the process flow for preparing ultra-large diameter carbon nanotubes.

[0044] Appendix Figure 2 The macroscopic morphology and microstructure of the ultra-large diameter carbon nanotubes prepared in Example 1 are shown (scanning electron microscope images), revealing the basic morphology of the carbon nanotubes.

[0045] Appendix Figure 3 The microstructure of the product shown in Example 2 is that of the attached... Figure 2 A magnified image of the sampling area shows a more detailed distribution of pipe diameters.

[0046] Appendix Figure 4 The microstructure of the product of Example 3 is shown (scanning electron microscope image), demonstrating the morphology of carbon nanotubes after adjustment with different concentrations of ammonium carbonate.

[0047] Appendix Figure 5 The microstructure of the product of Example 4 is shown in the scanning electron microscope image, revealing the diameter and length distribution of the carbon nanotubes.

[0048] Appendix Figure 6 The microstructure of the product of Example 5 is shown (scanning electron microscopy image), demonstrating the morphology of carbon nanotubes under high concentration ammonium carbonate conditions.

[0049] Appendix Figure 7 The microstructure of the product of Example 6 is shown in scanning electron microscopy (SEM) images, revealing the morphology and distribution of short and thick carbon nanotubes.

[0050] Appendix Figure 8 The microstructure of the product of Example 7 is shown (scanning electron microscopy image), demonstrating the morphology of carbon nanotubes under high concentration ammonium carbonate conditions.

[0051] Appendix Figure 9 The microstructure (scanning electron microscope image) of the product of Example 8 is shown, revealing the morphology of the product under low concentration ammonium carbonate conditions.

[0052] Appendix Figure 10 The microstructure (scanning electron microscope image) of the product of Example 9 is shown, demonstrating the morphology of carbon-coated particles under extremely low concentration ammonium carbonate conditions.

[0053] Appendix Figure 11 The microstructure (scanning electron microscope image) of the product of Example 10 is shown, showing the morphology of carbon nanotubes after replacing fumed silica with magnesium nitrate.

[0054] Appendix Figure 12 The microstructure (scanning electron microscope image) of the product of Example 11 is shown, showing the morphology of carbon nanotubes prepared using a CVD tube furnace reactor.

[0055] Appendix Figure 13 for Figure 2 Magnified microstructure of ultra-large diameter carbon nanotubes;

[0056] Appendix Figure 14 This is a state diagram of the reactants during the reaction process in Example 12. Detailed Implementation

[0057] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. In addition, the forms of the various structures described in the following embodiments are merely illustrative. The present invention is not limited to the structures described in the following embodiments. All other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0058] refer to Figure 1 This invention presents a method for preparing ultra-large diameter carbon nanotubes, comprising the following steps:

[0059] S1. An active substance and a promoter are attached to the surface of fumed silica to prepare a catalyst precursor. The specific process of step S1 includes: soaking the fumed silica in a 0.2-0.5 g / ml nickel nitrate solution, followed by titration with a 0.02-0.06 g / ml ammonium carbonate solution, and allowing the reaction to proceed fully. A preferred specific process for step S1 is: soaking the fumed silica in a 0.2-0.5 g / ml nickel nitrate solution for 20 min, followed by titration with a 0.02-0.06 g / ml ammonium carbonate solution, and allowing the reaction to proceed fully for 30 min.

[0060] S2. Filter and dry the precursor solution; in step S2, after filtration, dry at 120°C for 180 min.

[0061] S3. The dried precursor is subjected to high-temperature annealing treatment. In step S3, after drying, it is placed in a double-cavity heating furnace for high-temperature annealing treatment. The heating rate is 10℃ / min for the first 500℃, and the heating rate is increased by 5℃ / min from 500 to 900℃ until the heating rate is 55℃ / min. Then, the temperature is uniformly increased to 900℃ and held at that temperature for 30 minutes, and then naturally cooled to room temperature.

[0062] S4. The precursor treated with high-temperature annealing is added as a catalyst to a fluidized bed or CVD tubular furnace reactor for reaction; Step S41, in which the precursor treated with high-temperature annealing is added as a catalyst to a fluidized bed reactor for reaction, is as follows:

[0063] S411. Reactor preheating: Under a nitrogen protective atmosphere, the fluidized bed reactor is heated to 660°C; the low-temperature preheater is heated to 500°C to preheat hydrogen and propylene gas; the high-temperature preheater is heated to 850°C to fully decompose the carbon source gas.

[0064] S412, Catalyst addition and fluidization: The precursor after high-temperature annealing is added as a catalyst to the fluidized bed reactor; nitrogen gas is introduced to keep the catalyst particles in a suspended fluidized state;

[0065] S413, Reduction Reaction: Hydrogen gas is introduced to reduce the metal oxide in the catalyst to the active element at 660℃, continuing until the reduction is complete; during this process, the carbon source is introduced in stages; for the first 5 minutes: propylene is introduced as the carbon source at a flow rate of 10L / min to initiate carbon nanotube nucleation; for the middle 10 minutes: the propylene flow rate is increased to 35L / min to promote rapid carbon nanotube growth; for the last 5 minutes: the propylene flow rate is reduced to 20L / min to optimize the integrity of the tube wall structure; total reaction time: 20 minutes, with hydrogen gas introduced as the reducing gas throughout the process; after the reaction is completed, the propylene supply is stopped, and nitrogen gas is introduced for another 5 minutes to remove residual gas in the reactor.

[0066] In step S4, the process of adding the high-temperature annealed precursor as a catalyst to the CVD tube furnace reactor for reaction is as follows:

[0067] S421. Reactor preheating: Under a nitrogen protective atmosphere, the CVD tubular furnace reactor is heated to 660°C; the low-temperature preheater is heated to 500°C to preheat hydrogen and propylene gas; the high-temperature preheater is heated to 850°C to fully decompose propylene into active carbon atoms.

[0068] S422, Catalyst loading: The high-temperature annealed precursor is evenly spread in a quartz boat as a catalyst and placed in the constant temperature zone of the reaction tube.

[0069] S423, Reduction Reaction: Hydrogen and nitrogen are introduced, and the catalyst reduction is completed at 660℃. During this process, the carbon source is introduced in stages: First 5 minutes: propylene is introduced at a flow rate of 10L / min, carbon nanotube nucleation stage; Middle 10 minutes: propylene flow rate is increased to 35L / min, main growth stage; Last 5 minutes: propylene flow rate is reduced to 20L / min, structure optimization stage; Total reaction time is 20 minutes, with hydrogen introduced throughout; After the reaction is completed, the propylene supply is stopped, and nitrogen is introduced for another 5 minutes to purge the gas in the reaction tube.

[0070] S5. After the reaction is completed, a pre-oxidation treatment is performed to obtain ultra-large diameter carbon nanotube powder. The specific process of step S5 includes: under the protection of nitrogen positive pressure, the product of step S4 is transported to the pre-oxidation equipment; nitrogen and air are mixed at a volume ratio of 1:1 and oxidized at 500°C for 20 minutes, and then transferred to the storage tank after completion.

[0071] S6. The carbon nanotube powder is purified by acid washing to obtain high-purity, ultra-large diameter carbon nanotubes. The specific process of step S6 includes:

[0072] S61. Place the pre-oxidized product into a prepared acidic solution of a specific concentration to remove residual reactive metals, peel off the product, and wash and filter twice with water.

[0073] S62. The filtered product is placed in an acidic solution of a specific concentration for impurity removal.

[0074] S63. After impurity removal and filtration, the product is washed with water. After the pH is close to 7.0, it is filtered and dried.

[0075] In S61, the mass ratio of the product to the acidic component in a specific concentration of acidic solution is set as follows:

[0076] Products: hydrochloric acid: nitric acid: hydrofluoric acid = 1:2:1:0.5;

[0077] In S62, the mass ratio of the product to the acidic component in a specific concentration of acidic solution is set as follows:

[0078] Products: hydrochloric acid: nitric acid: hydrofluoric acid = 1:1:0.5:0.25;

[0079] Example 1: A method for preparing ultra-large diameter carbon nanotubes in this example includes the following steps:

[0080] Step 1: Precursor preparation: Select a 0.2 g / ml nickel nitrate solution, totaling 2 L of solution, soak 10 g of fumed silica in it, and after 20 min, titrate with a 0.02 g / ml ammonium carbonate solution, adding 0.2 L of solution. After titration, react for 30 min.

[0081] Step 2: After the reaction is complete, filter the solution and dry the attached catalyst precursor at 120℃ for 180 min.

[0082] Step 3: After drying, the catalyst is placed in a dual-chamber heating furnace for high-temperature annealing. The heating rate is 10℃ / min for the first 500℃, and then 5℃ / min for 500-900℃ until the heating rate is 55℃ / min. After that, the temperature is uniformly raised to 900℃ and held for 30 minutes. The catalyst is then allowed to cool naturally to room temperature.

[0083] Step 4: After the initial treatment, the fluidized bed reactor is heated to 660°C with protective gas introduced, the low-temperature preheater is heated to 500°C, and the high-temperature preheater is heated to 850°C. After heating, 10g of catalyst is added, and the carrier gas, reducing gas, and carbon source are introduced quantitatively to carry out the reaction (the reduction reaction is carried out first, and the carbon source is introduced after the reduction reaction is completed). The carrier gas flow rate is 80L / min, the reducing gas flow rate is 40L / min, and the carbon source flow rate is 10L / min for the first 5 minutes, 35L / min for the middle 10 minutes, and 20L / min for the last 5 minutes. The reaction time is 20 minutes. The carbon source gas is propylene, the reducing gas is hydrogen, and the protective gas and carrier gas are nitrogen. After the reaction is completed, the supply of carbon source gas is stopped, the carrier gas is introduced for another 5 minutes, and then the material is conveyed to the pre-oxidation equipment under positive pressure to complete this preparation and begin the next preparation.

[0084] Step 5: After the material is transported to the pre-oxidation equipment, the pre-oxidation treatment begins. The ratio of nitrogen to air is 1:2, the temperature is 500℃, and the time is 20 minutes. After the pre-oxidation is completed, the material is transported to the storage tank to obtain 117g of product.

[0085] Step 6: Place the pre-oxidized product into a prepared acidic solution of a specific concentration (product: hydrochloric acid: nitric acid: hydrofluoric acid = 1:2:1:0.5) to remove residual reactive metals and peel off the product. Wash and filter twice with water. Place the filtered product into an acidic solution of a specific concentration (product: hydrochloric acid: nitric acid: hydrofluoric acid = 1:1:0.5:0.25) for impurity removal. After impurity removal and filtration, wash the product with water. When the pH is close to 7.0, filter again and dry to obtain 105g of high-purity ultra-large diameter carbon nanotubes. (See appendix for macroscopic and microstructures) Figure 2 )

[0086] Example 2: This example differs from Example 1 in that the concentration of ammonium carbonate in step 1 is increased from 0.02 g / ml to 0.03 g / ml. Other steps and parameters are the same as in Example 1. After the reaction, 140 g of the unpurified product and 125 g of the purified product were obtained. The microstructure is shown in the attached figure. Figure 3 .

[0087] Example 3: This example differs from Example 1 in that the concentration of ammonium carbonate in step 1 is increased from 0.02 g / ml to 0.04 g / ml. Other steps and parameters are the same as in Example 1. After the reaction, 195 g of the unpurified product and 177 g of the purified product were obtained. The microstructure is shown in the attached figure. Figure 4 .

[0088] Example 4: This example differs from Example 1 in that the concentration of ammonium carbonate in step 1 is increased from 0.02 g / ml to 0.05 g / ml. Other steps and parameters are the same as in Example 1. After the reaction, 121 g of the unpurified product and 101 g of the purified product were obtained. The microstructure is shown in the attached figure. Figure 5 .

[0089] Example 5: This example differs from Example 1 in that the concentration of ammonium carbonate in step 1 is increased from 0.02 g / ml to 0.06 g / ml. Other steps and parameters are the same as in Example 1. After the reaction, 82 g of the unpurified product and 69 g of the purified product were obtained. The microstructure is shown in the attached figure. Figure 6 .

[0090] Example 6: This example differs from Example 1 in that the concentration of ammonium carbonate in step 1 is increased from 0.02 g / ml to 0.07 g / ml. Other steps and parameters are the same as in Example 1. After the reaction is complete, 45 g of the unpurified product and 39 g of the purified product are obtained. The microstructure is shown in the appendix. Figure 7 .

[0091] Example 7: This example differs from Example 1 in that the concentration of ammonium carbonate in step 1 is increased from 0.02 g / ml to 0.08 g / ml. Other steps and parameters are the same as in Example 1. After the reaction, 31 g of the unpurified product and 19 g of the purified product were obtained. The microstructure is shown in the appendix. Figure 8 .

[0092] Example 8: This example differs from Example 1 in that the concentration of ammonium carbonate in step 1 is increased from 0.02 g / ml to 0.01 g / ml. Other steps and parameters are the same as in Example 1. After the reaction is complete, 22 g of the unpurified product and 15 g of the purified product are obtained. The microstructure is shown in the appendix. Figure 9 .

[0093] Example 9: This example differs from Example 1 in that the concentration of ammonium carbonate in step 1 is increased from 0.02 g / ml to 0.005 g / ml. Other steps and parameters are the same as in Example 1. After the reaction, 13 g of the unpurified product and 8 g of the purified product were obtained. The microstructure is shown in the appendix. Figure 10 .

[0094] Example 10: This example differs from Example 1 in that the fumed silica in step 1 is replaced with magnesium nitrate. Other steps and parameters are the same as in Example 1. After the reaction is complete, 55g of the unpurified product and 41g of the purified product are obtained. The microstructure is shown in the appendix. Figure 11 .

[0095] Example 11: This example differs from Example 3 in that the fluidized bed reactor in step 4 is replaced with a CVD tubular furnace reactor. Other steps and parameters are the same as in Example 3. After the reaction, 225g of the unpurified product and 200g of the purified product were obtained. The microstructure is shown in the attached diagram. Figure 12 The specific process is as follows:

[0096] Step S1: Preparation of catalyst precursor;

[0097] First, prepare 2 L of a 0.2 g / ml nickel nitrate solution and immerse 10 g of fumed silica in it for 20 minutes to allow the support to fully adsorb the active component. Then, titrate 0.2 L of a 0.04 g / ml ammonium carbonate solution (consistent with the concentration in Example 3) with slow stirring during the addition to ensure a uniform reaction. After titration, continue the reaction for 30 minutes to allow the nickel ions to fully combine with the ammonium carbonate, forming a stable catalyst precursor complex.

[0098] Step S2: Precursor filtration and drying;

[0099] After the reaction, the solution and the solid support carrying the catalyst were separated by a filtration device to remove unreacted residual liquid. The filtered precursor was then placed in a 120°C oven and dried for 180 minutes to ensure that the catalyst precursor was completely dry and to avoid structural defects caused by residual moisture during the subsequent high-temperature annealing process.

[0100] Step S3: High-temperature annealing treatment;

[0101] The dried precursor was transferred to a dual-chamber furnace for staged heat treatment. Initially, the temperature was increased to 500°C at a rate of 10°C / min; subsequently, the rate was increased by 5°C / min for every 100°C increase, until a constant rate of 55°C / min was reached, at which point the rate was maintained until a final temperature of 900°C. The catalyst was held at 900°C for 30 minutes to promote catalyst particle crystallization and the formation of highly active sites, followed by natural cooling to room temperature to obtain a structurally stable catalyst.

[0102] Step S4: CVD tube furnace reaction process;

[0103] Reactor preheating: Under nitrogen protection, the CVD tube furnace is heated to 660℃; a low-temperature preheater (500℃) is used to preheat hydrogen and propylene gases, and a high-temperature preheater (850℃) ensures that propylene is fully decomposed into active carbon atoms. Catalyst loading: The annealed catalyst is evenly spread in a quartz boat and placed in the isothermal zone of the reaction tube. Staged carbon source introduction: Propylene is introduced at 10L / min for the first 5 minutes to initiate carbon nanotube nucleation; the rate is increased to 35L / min for the middle 10 minutes to accelerate growth; and the rate is reduced to 20L / min for the last 5 minutes to optimize the tube wall structure. The total reaction time is 20 minutes, with hydrogen introduced throughout to assist reduction. Post-reaction treatment: After stopping the propylene supply, nitrogen is continuously introduced for 5 minutes to remove residual gases and prevent side reactions.

[0104] Step S5: Pre-oxidation treatment;

[0105] The reaction product was transferred to a pre-oxidation device under nitrogen positive pressure protection, and a mixture of nitrogen and air (volume ratio 1:1) was introduced for oxidation at 500°C for 20 minutes. This process removes amorphous carbon impurities and enhances the surface activity of carbon nanotubes. After pre-oxidation, 225g of raw carbon nanotube powder (pre-purification product) was collected and temporarily stored in a sealed storage tank.

[0106] Step S6: Acid washing and purification;

[0107] First acid wash: The raw powder was immersed in a mixed acid solution (product: hydrochloric acid: nitric acid: hydrofluoric acid = 1:2:1:0.5) to remove residual metal catalyst. It was then washed twice with water and filtered to separate the acid solution from the product. Second acid wash: A diluted acid solution (product: hydrochloric acid: nitric acid: hydrofluoric acid = 1:1:0.5:0.25) was used for further impurity removal. The product was washed with water until the pH of the filtrate was close to 7.0. The filtered product was dried at 120℃ to obtain 200g of high-purity, ultra-large diameter carbon nanotubes with uniform diameter distribution and intact structure (microscopic morphology as shown in the attached image). Figure 12 (As shown).

[0108] The specific surface area of ​​carbon nanotubes refers to the total surface area per unit mass or unit volume of carbon nanotubes, usually expressed in square meters per gram (m²). 2 The unit is g. Its value is determined by the combined surface area of ​​the inner and outer walls of the carbon nanotubes, with single-walled carbon nanotubes (SWCNTs) having a theoretical specific surface area as high as 1315 m². 2 / g, while multi-walled carbon nanotubes (MWCNTs) have a relatively low specific surface area due to their multilayered structure. In actual preparation, because carbon nanotubes tend to bundle and aggregate and have a low opening ratio, the measured specific surface area is usually significantly lower than the theoretical value.

[0109] The specific surface area of ​​carbon nanotubes is negatively correlated with their diameter. Theoretically, decreasing the diameter significantly increases the proportion of surface atoms per unit mass. For example, for every 1 nm decrease in the diameter of a single-walled carbon nanotube, the specific surface area can increase by approximately 200 m² / s.2 / g. This pattern has also been confirmed experimentally: fine-diameter carbon nanotubes (such as samples with a diameter of 2 nm) prepared through catalyst design and process optimization can achieve a specific surface area of ​​up to 600 m². 2 / g or more.

[0110] While small diameter nanotubes offer the advantage of high specific surface area, excessively small diameters (e.g., <5 nm) can lead to an increased aspect ratio, higher surface energy, and exacerbated aggregation. For example, carbon nanotubes with a diameter of 2 nm have an excessively large specific surface area (>80 nm). 2 / g), which is prone to agglomeration in conductive slurries, leading to difficulties in dispersion and a decrease in actual conductivity. Therefore, a balance needs to be struck between pipe diameter and dispersibility in industrial applications.

[0111] For the samples obtained in Examples 1 to 11, the specific surface area of ​​each sample was determined by nitrogen adsorption method using a NOVA series canta specific surface area meter based on the Brunauer-Emmett-Teller (BET) theory.

[0112] Sample pretreatment: Place approximately 0.1–0.3 g of carbon nanotube sample in a sample tube and heat it to 200–300 °C in a vacuum or flowing nitrogen environment for 2–4 hours to degas the sample, thoroughly removing adsorbed moisture, gas and impurities from the surface, ensuring that the sample is clean and dry before testing.

[0113] Nitrogen adsorption experiment: The sample tube is immersed in liquid nitrogen (-196℃) to maintain a low temperature. The nitrogen pressure is gradually adjusted by a precision flow controller, and the amount of nitrogen adsorbed by the sample at different pressures is recorded in real time to generate adsorption-desorption isotherms.

[0114] BET equation calculation: Based on adsorption isotherm data, the BET equation is used to fit the monolayer adsorption capacity, combined with the cross-sectional area of ​​nitrogen molecules (0.162 nm). 2 The specific surface area is calculated using a formula based on the sample mass and the total surface area of ​​the material, directly reflecting the correlation between the pipe diameter and pore size.

[0115] Data validation and optimization: The samples were measured three times, outliers were removed and the mean was taken. The contribution of micropores / mesopores was verified by comparing with theoretical models (such as the t-plot method) to ensure the accuracy and repeatability of the specific surface area data.

[0116] The samples obtained in Examples 1-11 were selected, and the average pipe diameter was measured according to the following steps:

[0117] The carbon nanotube sample was dispersed in an ethanol solution and sonicated for 10–20 minutes to break up agglomerates and ensure that individual carbon nanotubes were fully dispersed. Subsequently, the dispersion was dropped onto the surface of a silicon wafer covered with conductive adhesive. After drying, the sample conductivity was enhanced by gold (or carbon) sputtering to avoid image distortion caused by charge accumulation during SEM imaging.

[0118] Using scanning electron microscopy (SEM), multiple representative regions were selected at accelerating voltages of 5-20 kV to capture the morphology of carbon nanotubes in high-resolution mode. The surface structure and diameter details of the carbon nanotubes were clearly revealed through secondary electron signal (SE) or backscattered electron (BSE) modes.

[0119] At least 100 carbon nanotubes were randomly selected from the SEM images, and the diameter of each tube was measured using ImageJ image analysis software along a direction perpendicular to the carbon nanotube axis. Data was recorded using manual scales or automatic recognition functions, excluding broken, overlapping, or contaminated areas to ensure that the measured objects were complete and independent carbon nanotubes.

[0120] The arithmetic mean and standard deviation of all valid measurements were calculated to generate a histogram of pipe diameter distribution. For key samples (such as ultra-thick pipes), high-precision local re-measurement using transmission electron microscopy (TEM) was employed to verify the accuracy of the SEM data. Finally, the average pipe diameter result was obtained, and the error range was marked. The specific surface area and average pipe diameter measurement results of each sample in Examples 1-11 are shown in Table 1.

[0121] Table 1. Summary of experimental data from Examples 1 to 11

[0122]

[0123]

[0124] Example 1 (using fumed silica), the product contained large-diameter carbon nanotubes with an average diameter of over 200 nm (attached). Figure 2 Example 10 (replaced with magnesium nitrate): only fine carbon nanotubes with a diameter <50 nm were produced (see attached). Figure 11 The absence of large-diameter products indicates that fumed silica is a key condition for the formation of large-diameter carbon nanotubes. Fumed silica possesses a porous, high-specific-surface-area silica (SiO2) structure, enabling it to uniformly load nickel (Ni) active materials, forming dispersed catalyst particles and providing uniform nucleation sites for carbon nanotubes. In contrast, active materials in existing technologies, such as magnesium nitrate, lack porous structures and sufficient surface active sites, leading to catalyst particle agglomeration and the formation of small-sized particles, resulting only in the formation of fine-diameter carbon nanotubes.

[0125] Example 10 Specific surface area (83.13 m²) 2 / g) was significantly higher than that of Example 1 (54.37m). 2The presence of small-diameter carbon nanotubes ( / g) indicates a high number of fine-diameter carbon nanotubes with high porosity, indirectly verifying the difference in tube diameter. The growth of large-diameter carbon nanotubes requires larger catalyst particles. Fumed silica stabilizes and disperses Ni particles, achieving a size greater than 200 nm, thus supporting large-diameter growth. Magnesium nitrate cannot inhibit particle aggregation; its catalyst particle size is small (<50 nm), and it can only generate fine-diameter carbon nanotubes.

[0126] The concentration of ammonium carbonate is a key factor in controlling the tube diameter. Low concentration (0.005-0.01 g / mL): the product is carbon-coated catalyst particles, and no carbon nanotubes are generated (Examples 8-9).

[0127] Medium concentration (0.02–0.04 g / mL): Large-diameter carbon nanotubes (100–380 nm) are generated, with the diameter initially increasing and then decreasing as the concentration increases (Examples 1–3). High concentration (0.06–0.08 g / mL): Carbon nanotubes gradually disappear, and the product is short, thick carbon nanotubes or amorphous carbon (Examples 6–7).

[0128] The complexation equilibrium between nickel nitrate and ammonium carbonate is crucial. Insufficient ammonium carbonate leads to the formation of NiCO3 precipitate (excessively large particles), preventing the formation of an active catalyst and resulting in carbon-coated carbon nanotubes (Examples 8-9). This leads to the formation of soluble [Ni(CO3)2]. 2- The complex provides uniformly dispersed Ni nanoparticles (50-300 nm) to support the growth of large-diameter carbon nanotubes. Excess ammonium carbonate leads to NH4+... + Hydrolysis generates Ni(OH)2 precipitate, which destroys the complex structure and forms small-sized catalyst particles (<50nm), producing only short and thick carbon nanotubes.

[0129] Carbon-coated particles (Examples 8-9), the particle surface is covered with amorphous carbon, and no carbon nanotubes are formed (see attached). Figure 9-10 Small specific surface area (<10m²) 2 / g), due to the lack of pores between particles, the adsorption capacity of gas molecules is low. Short and thick carbon nanotubes (Examples 6-7); short carbon nanotubes (length <1μm) are closely packed, with low porosity (see attached). Figure 7-8 Low specific surface area (15-21m²) 2 / g), because short carbon nanotubes cannot form an open porous network. In summary, as can be seen from Table 1:

[0130] The key support for catalyst nucleation is fumed silica. This material serves as a catalyst nucleation support to control the particle size of the basic catalyst active material, ensuring that a suitable catalyst exists during the carbon nanotube growth process and maintaining the minimum large-diameter carbon nanotube catalyst particle size.

[0131] An appropriate ammonium carbonate concentration, whether too low or too high, will not produce large-diameter carbon nanotubes.

[0132] Example 12: To further determine the influence of ammonium carbonate concentration changes on the formation of large-diameter carbon nanotubes, this example selected nickel nitrate solution and ammonium carbonate solution, and designed an experiment. Using the 0.2 g / ml nickel nitrate solution from Example 1 as the base solution, ammonium carbonate was gradually added, and the reaction changes were observed. The experimental reaction points are as follows: Figure 14 As shown, after ammonium carbonate is added, a green precipitate first appears (No. 1). As ammonium carbonate is continuously added, the precipitate disappears and the whole becomes a green gel (No. 2). If more ammonium carbonate is added, the gel disappears and it turns back into a green precipitate (No. 3).

[0133] The phenomenon observed when ammonium carbonate is added dropwise to a nickel nitrate solution initially results in a precipitate, which then disappears and reappears. The underlying mechanism is as follows:

[0134] 1. Precipitation occurs first;

[0135] Nickel nitrate (Ni(NO3)2) reacts with ammonium carbonate (NH4)2CO3 to form nickel carbonate (NiCO3) precipitate. The ionic equation is Ni... 2+ +CO3 2- =NiCO3 precipitate. At this point, the pH of the solution is around 7. Because the ammonium carbonate solution is slightly alkaline and the nickel nitrate solution is slightly acidic, the pH is close to neutral after the two are mixed.

[0136] 2. The precipitate disappears;

[0137] Continue adding ammonium carbonate, because the carbonate ions CO32- 2- As the concentration increases, it will react with the generated nickel carbonate precipitate to form soluble [Ni(CO3)2]. 2- The complex ion dissolves the precipitate; the ionic equation is NiCO3 + CO32-. 2- =[Ni(CO3)2] 2- During this process, the pH value of the solution will increase because the concentration of carbonate ions increases, the alkalinity of the solution increases, and the pH value rises to about 8-9.5.

[0138] 3. More sediment appeared;

[0139] When ammonium carbonate is in excess, the ammonium ions (NH4+) in the solution... + The concentration also increases significantly, and it will undergo a hydrolysis reaction with carbonate ions to produce a large amount of bicarbonate ions (HCO3-). - and hydroxide ions OH - When the concentration of hydroxide ions increases to a certain level, they react with nickel ions to form nickel hydroxide Ni(OH)₂ precipitate. The ionic equation is Ni 2+ +2OH -=Ni(OH)2 precipitate. At this point, the pH of the solution continues to rise, possibly reaching above 9.5, and nickel hydroxide precipitate forms.

[0140] Based on the above mechanism and the fluctuations in ammonium carbonate, corresponding to the microstructure and specific surface properties of the product, we can conclude that when nickel nitrate solution is added dropwise to ammonium carbonate solution, a precipitate is first formed, coating the nucleation support, but carbon nanotubes cannot grow (Examples 8 / 9). Excessive addition of carbonic acid also results in a precipitate formation, coating the nucleation support, but large-diameter carbon nanotubes cannot grow (Examples 6 / 7). Only during the formation of a complex (Examples 1-5) do large-diameter carbon nanotubes grow. Furthermore, as ammonium carbonate is continuously added, the complex begins to decrease, and the number of large-diameter carbon nanotubes also decreases. This complex is the key to preparing large-size catalysts, which is the appropriate concentration of ammonium carbonate. According to the results in Table 1, the optimal ammonium carbonate concentration of 0.04 g / ml was chosen in this invention, yielding unexpected results.

[0141] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a super-large diameter carbon nanotube, characterized by, The method comprises the following steps: S1, attaching active substance and promoter on the surface of fumed white carbon black to prepare catalyst precursor; the specific process of step S1 comprises the following steps: soaking fumed white carbon black in 0.2-0.5 g / ml nickel nitrate solution, titrating with 0.02-0.06 g / ml ammonium carbonate solution, and fully reacting; S2, filtering and drying the precursor solution; S3, high-temperature annealing treatment of the dried precursor; S4, adding the precursor after high-temperature annealing treatment as catalyst into a fluidized bed or CVD tube furnace reactor for reaction; S5, pre-oxidation treatment after the reaction is completed to obtain ultra-large tube diameter carbon nanotube raw powder; S6, high-purity ultra-large tube diameter carbon nanotube is obtained by acid washing and purifying the carbon nanotube raw powder.

2. The method of claim 1, wherein the carbon nanotubes have a diameter of 50 nm or more. The specific process of step S1 comprises the following steps: soaking fumed white carbon black in 0.2-0.5 g / ml nickel nitrate solution for 20 min, titrating with 0.04 g / ml ammonium carbonate solution, and fully reacting for 30 min.

3. The method of claim 1, wherein the carbon nanotubes have a diameter of 50 nm or more. In step S2, the filter is dried at 120℃ for 180 min.

4. The method of claim 1, wherein the carbon nanotubes have a diameter of 50 nm or more. In step S3, after drying, the high-temperature annealing treatment is carried out in a double-cavity heating furnace, the temperature is raised at a rate of 10℃ / min from 500℃ to 900℃, then the temperature is raised at a rate of 5℃ / min until the temperature raising rate is 55℃ / min, then the temperature is kept at 900℃ for 30 min, and then the temperature is naturally cooled to room temperature.

5. The method of claim 1, wherein the carbon nanotubes have a diameter of 50 nm or more. In step S4, the precursor after high-temperature annealing treatment is added as catalyst into a fluidized bed reactor for reaction, and step S41 is as follows: S411, reactor preheating: the fluidized bed reactor is heated to 660℃ under nitrogen protection atmosphere, the low-temperature preheater is heated to 500℃ to preheat hydrogen and propylene gas, and the high-temperature preheater is heated to 850℃ to make the carbon source gas fully split; S412, catalyst addition and fluidization: the precursor after high-temperature annealing is added as catalyst into the fluidized bed reactor, and nitrogen is introduced to make the catalyst particles in a suspended fluidized state; S413, reduction reaction: hydrogen is introduced to reduce the metal oxide in the catalyst to active elements at 660℃ until the reduction is completed; In this process, the carbon source is introduced in stages; in the first 5 minutes: propylene is introduced as carbon source at a flow rate of 10 L / min to start carbon nanotube nucleation; in the middle 10 minutes: the flow rate of propylene is increased to 35 L / min to promote rapid growth of carbon nanotube; in the last 5 minutes: the flow rate of propylene is reduced to 20 L / min to optimize the structure integrity of the tube wall; the total reaction time is 20 minutes, and hydrogen is introduced as reducing gas throughout the process; after the reaction is completed, the supply of propylene is stopped, and nitrogen is continuously introduced for 5 minutes to remove residual gas in the reactor.

6. The method of claim 1, wherein the carbon nanotubes have a diameter of 50 nm or more. In step S4, the precursor after high-temperature annealing treatment is added as catalyst into a CVD tube furnace reactor for reaction, and step S42 is as follows: S421, reactor preheating: the CVD tube furnace reactor is heated to 660℃ under nitrogen protection atmosphere, the low-temperature preheater is heated to 500℃ to preheat hydrogen and propylene gas, and the high-temperature preheater is heated to 850℃ to make propylene fully split into active carbon atoms; S422, catalyst loading: the precursor after high-temperature annealing is uniformly laid in the quartz boat as a catalyst and placed in the constant temperature zone of the reaction tube; S423, reduction reaction: hydrogen and nitrogen are introduced, and the catalyst reduction is completed at 660°C. In this process, the carbon source is introduced in stages. The first 5 minutes: propylene is introduced at a flow rate of 10 L / min, and the carbon tube nucleation stage is completed. The next 10 minutes: the propylene flow rate is increased to 35 L / min, and the main growth stage is completed. The last 5 minutes: the propylene flow rate is reduced to 20 L / min, and the structure optimization stage is completed. The total reaction time is 20 minutes, and hydrogen is introduced throughout the process. After the reaction is completed, stop supplying propylene and continue to introduce nitrogen for 5 minutes to remove the gas in the reaction tube. The specific process of step S5 includes: under the protection of nitrogen overpressure, the product of step S4 is transported to the pre-oxidation equipment; nitrogen and air are mixed at a volume ratio of 1:1, and oxidized at 500°C for 20 minutes. After completion, it is transferred to the storage tank.

7. The method of claim 1, wherein the carbon nanotubes have a diameter of 50 nm or more. The specific process of step S6 includes:

8. The method of claim 1, wherein the carbon nanotubes have a diameter of at least 50 nm. S61, the pre-oxidized product is placed in a specially configured acidic solution of a certain concentration to remove residual reaction metals while peeling the product, and then washed with water and filtered twice; S62, the filtered product is placed in a specially configured acidic solution of a certain concentration for a second time to remove impurities; S63, the filtered product is washed with water until the pH approaches 7.0, then filtered and dried.

9. The method for preparing ultra-large diameter carbon nanotubes according to claim 8, wherein: In S61, the mass ratio of the product to the acidic components in the acidic solution of a certain concentration is set as follows: Product: hydrochloric acid: nitric acid: hydrofluoric acid = 1:2:1:0.5; In S62, the mass ratio of the product to the acidic components in the acidic solution of a certain concentration is set as follows: Product: hydrochloric acid: nitric acid: hydrofluoric acid = 1:1:0.5:0.

25. ​

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