System and method for continuous production of arrayed carbon nanotubes

Abstract

This disclosure belongs to the field of carbon nanotube preparation technology, and provides a system and method for the continuous preparation of array-type carbon nanotubes. The system includes a reaction vessel, an atomization system, a pyrolysis furnace, a classifier, a feeder, and a CVD fluidized bed connected in sequence. Preparation method: The catalyst is designed with the chemical formula Fe. a Co b Ni c Mg d Al e Mo f Mn g V h The catalyst solution was prepared according to the stoichiometric ratio of the above chemical formula, and the reaction was carried out sequentially in the above system to finally obtain arrayed carbon nanotubes. This disclosure utilizes the rapid and one-step characteristics of spray pyrolysis, combined with precise catalyst structure design, to obtain a catalyst with high-density active sites and small active site particle size, achieving efficient, continuous, and controllable preparation from molecular precursors to high-performance arrayed carbon nanotubes. Furthermore, the spray pyrolysis method can be used to prepare catalysts with high activity and low bulk density, which is beneficial for CVD fluidized bed production of arrayed carbon nanotubes.

Description

Technical Field

[0001] This disclosure relates to the field of carbon nanotube preparation technology, and in particular to a system and method for the continuous preparation of array-type carbon nanotubes. Background Technology

[0002] Carbon nanotubes, as carbon materials with unique one-dimensional nanostructures, have shown broad application prospects in many fields, such as conductive materials for secondary batteries and reinforcements for composite materials, due to their excellent electrical, thermal, and mechanical properties. Carbon nanotubes are divided into single-walled and multi-walled types, with multi-walled nanotubes further divided into clustered, wound types and oriented array types. Arrayed carbon nanotubes, due to their consistent orientation and large aspect ratio, can better maintain their structural integrity during dispersion, forming a highly efficient conductive network. Their conductivity is significantly higher than that of wound carbon nanotubes, making them the preferred material for high-performance applications.

[0003] Catalysts are the core component in the preparation of arrayed carbon nanotubes, and their performance directly determines the structure, size, and properties of the carbon nanotubes. An ideal carbon nanotube catalyst should possess characteristics such as high-density active sites and small-size active sites, and the preparation process should meet the requirements of environmental friendliness, high efficiency, and ease of industrialization. Currently, traditional methods for preparing carbon nanotube catalysts mainly include impregnation, homogeneous co-precipitation, and sol-gel methods, but these methods all have significant drawbacks, limiting the large-scale, high-quality preparation of arrayed carbon nanotubes. While impregnation can produce arrayed carbon nanotubes with good orientation, it requires the introduction of a substrate with a smooth surface and sufficient strength as the catalyst support. This process inevitably introduces impurities that are difficult to remove, which severely affect the purity and core properties of carbon nanotubes, such as conductivity and mechanical properties, reducing their application value in high-end fields. The homogeneous coprecipitation method can self-assemble layered catalysts without introducing impurity elements. However, this method generates a large amount of industrial wastewater during preparation, causing serious pollution to the ecological environment and contradicting the trend of green production. Furthermore, its preparation process is complex, time-consuming, and inefficient, making it unsuitable for large-scale industrial production. The sol-gel method can also synthesize impurity-free layered catalysts, but it requires the addition of auxiliary reagents such as organic complexing agents and foaming agents. These reagents generate a large amount of gas during subsequent calcination, causing the catalyst to expand and form a layered structure. However, this method requires multiple complex steps, including evaporation and concentration, and high-temperature calcination, making process control difficult and energy consumption high. This not only increases production costs but also further limits its industrial application.

[0004] Furthermore, the diameter of carbon nanotubes is one of the key factors affecting their conductivity; the smaller the diameter, the better the conductivity. Applying small-diameter carbon nanotubes to fields such as secondary batteries can significantly improve battery capacity and cycle life. However, catalysts prepared by traditional methods suffer from uneven distribution and low density of active sites, making it difficult to precisely control the growth process of carbon nanotubes. This results in synthesized carbon nanotubes with larger diameters and wider distributions, failing to meet the stringent size requirements of high-performance applications.

[0005] Therefore, how to prepare specialized catalysts with high-density active sites and small-particle-size active sites through precise catalyst composition design and process parameter optimization, so as to achieve efficient, continuous and controllable preparation of array-type carbon nanotubes, while solving the problems of high pollution, complicated processes and poor product performance of traditional methods, has become a technical bottleneck that urgently needs to be overcome in the field of carbon nanotube materials. Summary of the Invention

[0006] This disclosure provides a system and method for the continuous preparation of array-type carbon nanotubes, in order to at least solve the above-mentioned technical problems existing in the prior art.

[0007] According to a first aspect of this disclosure, a system for the continuous preparation of arrayed carbon nanotubes is provided, comprising a reaction vessel, an atomization system, a pyrolysis furnace, a classifier, a feeder, and a CVD (chemical vapor deposition) fluidized bed; one end of the atomization system is connected to the reaction vessel, and the other end is connected to the pyrolysis furnace, for atomizing the catalyst solution prepared in the reaction vessel and feeding it into the pyrolysis furnace; the pyrolysis furnace is connected to the classifier, for conveying the decomposed catalyst powder to the classifier for particle size classification; one end of the feeder is connected to the classifier, and the other end is connected to the CVD fluidized bed, for feeding the classified catalyst powder into the CVD fluidized bed to prepare the arrayed carbon nanotubes.

[0008] In one embodiment, the atomization system is selected from either a two-fluid nozzle atomization device or an ultrasonic atomization device.

[0009] In one possible implementation, the classifier is selected from a cyclone classifier.

[0010] In one possible implementation, the feeder is selected from powder feeders.

[0011] In one embodiment, the system further includes a gas source, which includes a first gas source and a second gas source. The first gas source is connected to the atomization system and is used to provide a first carrier gas to the atomization system. The second gas source is connected to the CVD fluidized bed and is used to provide a second carrier gas to the CVD fluidized bed.

[0012] In one embodiment, the first carrier gas is selected from at least one of air and oxygen.

[0013] Specifically, the core function of the atomization system is to disperse the catalyst solution into small droplets. Air / oxygen can be used as the atomizing carrier gas, which can carry the droplets into the pyrolysis furnace on the one hand, and assist in the oxidative decomposition of the catalyst precursor in the subsequent pyrolysis stage, which is in line with the conventional process design of spray pyrolysis.

[0014] In one embodiment, the second carrier gas includes a reducing carrier gas and a growing carrier gas; the growing carrier gas is selected from at least one of an inert gas and nitrogen, and the inert gas is selected from at least one of argon and helium; the reducing carrier gas is a mixture of inert gas / nitrogen and hydrogen, and the volume ratio of hydrogen to inert gas / nitrogen is 1:5 to 1:10.

[0015] Specifically, the preparation of carbon nanotubes in a CVD fluidized bed requires an oxygen-free / low-oxygen environment to avoid carbon source oxidation and catalyst deactivation. Therefore, inert gases or nitrogen must be used as carrier / protective gases. These gases also fluidize the catalyst powder and promote uniform mass and heat transfer. In addition, hydrogen, as a reducing gas, reduces the catalyst powder to a metallic catalyst (elemental form).

[0016] In one embodiment, the total flow rate of both the reducing carrier gas and the growing carrier gas is controlled at 0.5~5 L / min.

[0017] According to a second aspect of this disclosure, a method for continuously preparing array-type carbon nanotubes using the above-described system is provided, comprising the following steps:

[0018] S1: The chemical formula of the catalyst is Fe. a Co b Ni c Mg d Al e Mo f Mn g V h Where 0≤a<1, 0≤b<1, 0≤c<1, 0<d<10, 0<e<5, 0≤f<0.5, 0≤g<0.5, 0≤h<0.5, the catalyst must include at least one of Fe, Co, and Ni, and at least two of Mo, Mn, and V; S2: According to the stoichiometric ratio of step S1, at least three of the soluble salts corresponding to Fe, Co, Ni, Mg, Al, Mo, Mn, and V are mixed in a reaction vessel to prepare a catalyst solution; among the at least three soluble salts, at least one of the soluble salts corresponding to Fe, Co, and Ni, and at least two of the soluble salts corresponding to Mo, Mn, and V must be included. S3: The catalyst solution is fed into a thermal decomposition furnace through an atomization system for decomposition to obtain catalyst powder; S4: The catalyst powder enters a classifier for particle size classification. S5: The catalyst powder after particle size classification in step S4 is fed into the CVD fluidized bed through a feeder to provide carbon source gas, and the array-type carbon nanotubes are obtained through fluidized reaction.

[0019] Specifically, in the preparation of array-type carbon nanotubes, the selection of catalyst components is not a random combination, but based on the metal catalysis mechanism and the growth requirements of carbon nanotubes. Through the synergistic design of active components (Fe, Co, Ni) + structural aids (Mg, Al) + electronic aids (Mo, Mn, V), the defects of single metal catalysis are solved, and the goal of high density small particle size active sites, high catalytic efficiency and low impurities is finally achieved. The functions and synergistic effects of each component are as follows: Fe, Co, and Ni are irreplaceable cores for the growth of array-type carbon nanotubes. Their role runs through the entire process of carbon source decomposition, carbon species diffusion to carbon tube nucleation and growth, which is due to the unique electronic structure and crystal characteristics of the three. Specifically: (1) The first step in the growth of array-type carbon nanotubes is to decompose the carbon source into usable carbon atoms or small molecule carbon species. Fe, Co, and Ni have unpaired electrons in their 3d orbitals. This special electronic structure enables them to form a strong adsorption effect with carbon source molecules. This not only firmly holds the carbon source molecules, preventing them from leaving the catalytic system before reacting, but also reduces the breaking energy of the C-C and CH bonds in the carbon source molecules (i.e., the activation energy of carbon source decomposition) through electron transfer. Compared with other metals (such as Cu, Pt, and Au), Fe, Co, and Ni have a more significant effect in reducing activation energy, which means that at the same temperature, Fe, Co, and Ni can rapidly decompose the carbon source, providing sufficient carbon species for the growth of carbon nanotubes. If other metals are used instead, they either cannot adsorb the carbon source (such as Au) or the efficiency of decomposing the carbon source is extremely low (such as Al and Mg), neither of which can meet the raw material requirements for the continuous growth of carbon nanotubes. (2) The decomposed carbon atoms need to diffuse and accumulate inside / on the surface of the metal particles until they reach saturation, and then nucleate on the surface of the metal particles by precipitation, gradually growing into carbon nanotubes. Fe, Co, and Ni are all face-centered cubic or body-centered cubic crystals. Their lattice constants are highly compatible with the hexagonal lattice of carbon, providing a fast diffusion channel for carbon atoms and preventing carbon atoms from accumulating on the particle surface to form amorphous carbon. The saturated carbon solubility of the three is moderate, which will not lead to insufficient carbon species due to low solubility, nor will it lead to carbon agglomeration into amorphous carbon due to high solubility. More importantly, Fe, Co, and Ni can stably form small-diameter particles and are not easy to agglomerate at high temperatures. The particle size of the metal particles directly determines the diameter of the carbon nanotubes. Small-diameter active sites are the core prerequisite for preparing small-diameter, highly conductive array-type carbon nanotubes. (3) Array-type carbon nanotubes need to grow in a uniform direction. Fe, Co, and Ni can maintain a stable metallic state (non-oxidized, non-melting) at high temperatures, which can continuously guide the growth of carbon nanotubes. The binding force between the three and the planar structure of the catalyst itself is moderate. If the binding is too weak, the metal particles will fall off with the growth of the carbon nanotubes and cannot continuously guide the orientation. If the binding is too strong, it will restrict the upward growth of the carbon nanotubes, resulting in the carbon nanotubes being messily entangled. The bonding forces of Fe, Co, and Ni are precisely in the equilibrium range, ensuring that carbon nanotubes grow in a uniform direction and form an array structure.

[0020] Mg and Al, as structural aids, have the following specific functions: (1) Inhibiting the aggregation of active sites: Fe, Co, and Ni are prone to aggregation at high temperatures due to excessively high surface energy, leading to an increase in the particle size of active sites. Mg, on the other hand, inhibits the aggregation of active sites. 2+ Al 3+ The ionic radius is much smaller than that of Fe. 2+ Co 2+ Ni 2+ , can be embedded in the gaps between Fe / Co / Ni particles to form stable composite oxide bonds such as Mg-O-Fe and Al-O-Co. This bonding effect can lock the active metal particles, prevent them from fusing and agglomerating at high temperatures, and ensure that the active sites always maintain small particle size and high density, providing sufficient and uniform catalytic centers for the growth of array-type carbon nanotubes. (2) Constructing porous / planar structures: Al oxides have high specific surface area and excellent thermal stability, and can construct porous or planar framework structures inside the catalyst. Porous structures can increase the contact area between the carbon source and the active sites, and improve the utilization rate of the carbon source; planar structures can serve as the growth substrate for array-type carbon nanotubes, allowing Fe / Co / Ni particles to be uniformly loaded on the plane, ensuring that the carbon nanotubes grow upward from the same plane, and further strengthening the array orientation. Mg oxides can adjust the bulk density of the catalyst, Mg 2+ The introduction of [a specific catalyst] can reduce the overall density of the catalyst, avoid poor mass transfer in the CVD fluidized bed due to excessive density, and adapt to the needs of continuous production.

[0021] Mo, Mn, and V serve as electronic additives, with the following specific functions: (1) Regulating electronic structure: The outer electrons of Mo, Mn, and V are easily transferred to the 3d orbitals of Fe / Co / Ni, changing the electron cloud density on the surface of the active metal, enhancing the adsorption capacity for carbon sources, and further reducing the activation energy of carbon source decomposition, allowing the carbon source to decompose rapidly at a lower temperature, thus improving catalytic efficiency; (2) Inhibiting the formation of amorphous carbon: During the growth of carbon nanotubes, excess carbon species agglomerate to form amorphous carbon, covering active sites and reducing the purity of carbon nanotubes. Mo, Mn, and V can form stable carbides with excess carbon. These carbides are not only non-toxic but can also serve as carbon storage reservoirs, temporarily storing excess carbon species and preventing them from depositing as amorphous carbon on the surface of active sites. At the same time, the presence of these carbides can also refine the particle size of Fe / Co / Ni particles, further improving the purity and conductivity of carbon nanotubes.

[0022] In summary, this combination ensures the core catalytic function through active components, guarantees stability and structure through structural aids, and optimizes efficiency and purity through electronic aids, achieving the goals of high-density small-particle-size active sites, high catalytic efficiency, and low impurities, perfectly meeting the needs of continuous preparation of array-type carbon nanotubes.

[0023] Specifically, the catalysts mentioned above include at least two of Mo, Mn, and V. The synergistic effect of these three metal elements is used to optimize the activity, stability, and directionality of carbon nanotube growth, ultimately improving the performance and preparation efficiency of carbon nanotubes. Specifically, as follows: (1) Mo, Mn, and V are all transition metal elements. Their oxides or composite salts can form multi-component composite active centers with the Fe / Co / Ni main catalyst at high temperatures. When one of them is added alone, the modification effect on the active sites of the main catalyst is limited, and it can only slightly increase the carbon source cracking rate. When two or more are combined, the electronic structures of different metal ions can be mutually regulated, increasing the number of active sites on the catalyst surface and making the energy level more matched to the cracking requirements of the carbon source gas, greatly improving the carbon atom precipitation efficiency, and thus accelerating the growth rate of carbon nanotubes. (2) In spray pyrolysis and CVD reaction processes, single Mo / Mn / V doping easily leads to sintering and agglomeration of catalyst particles at high temperatures, reducing the exposure rate of active sites; while when two or more elements are synergistically doped, a composite crystal form with smaller lattice distortion and higher thermal stability can be formed, inhibiting the agglomeration of catalyst particles in fluidized reaction, extending the service life of the catalyst, and ensuring the stability of continuous production. (3) Mo, Mn, and V have different guiding effects on the growth of carbon nanotubes: Mo tends to induce the formation of carbon nanotubes with uniform diameter and thinner walls; Mn can inhibit amorphous carbon deposition on the walls of carbon nanotubes and improve product purity; V can regulate the growth direction of carbon nanotubes and reduce bending and defect structures. When two or more elements are combined, multiple effects such as diameter control, purity improvement, and defect reduction can be achieved simultaneously, and high-performance carbon nanotubes can be prepared. If only one element is added, it is difficult to achieve multi-dimensional morphology optimization at the same time. (4) Spray pyrolysis requires that the metal ions in the catalyst solution have good solubility and co-precipitation consistency. The salts of Mo, Mn and V have complementary solubility characteristics in aqueous solution. When two or more are combined, the solution layering or precipitation problem caused by excessive concentration of a single salt can be avoided, ensuring that the catalyst powder obtained after spray pyrolysis has uniform particle size and consistent composition distribution.

[0024] In one embodiment, the soluble salt in step S2 is selected from any one of the carbonates, nitrates, ammonium salts, oxalates, acetates, citrates, and sulfates corresponding to Fe, Co, Ni, Mg, Al, Mo, Mn, and V.

[0025] In one embodiment, the metal ion concentration of the catalyst solution in step S2 is 0.001~0.1 mol / L.

[0026] In one embodiment, in step S2, a complexing agent is added when preparing the catalyst solution. The concentration of the complexing agent is 0.004~0.008 mol / L, and the ratio of the molar amount of the complexing agent to the total molar amount of metal ions in the catalyst solution is 1:5~1:1.

[0027] Specifically, the complexing agent is selected from at least one of anhydrous citric acid, tartaric acid, malic acid, EDTA, and triethanolamine.

[0028] Specifically, the complexing agent has the following functions: (1) The complexing agent contains polydentate coordination groups, which can form spatially stable chelates with metal ions in the catalyst. This chelation effect can encapsulate metal ions, preventing the aggregation of ions into large particles due to electrostatic attraction or hydrolysis, thus laying the foundation for the formation of nanoscale active sites in subsequent pyrolysis. (2) Most metal-complexing agent chelates are negatively charged in aqueous solution. Chelates with the same charge will generate electrostatic repulsion, further preventing particle aggregation and ensuring long-term uniformity of the catalyst solution, which is suitable for the process requirements of uniform atomization in spray pyrolysis. (3) Under the high temperature environment of spray pyrolysis, the complexing agent will gradually decompose into gases such as CO2, H2O, and small molecule organic acids. When these gases escape rapidly inside the catalyst particles, they will naturally form a large number of micron / nanoscale pores. The porous structure can significantly increase the specific surface area of ​​the catalyst, increase the exposure of active sites, and provide more sufficient diffusion channels for carbon source gases, thereby improving catalytic efficiency.

[0029] In one embodiment, step S3 introduces a first carrier gas into the atomization system via a first gas source; during atomization, the flow ratio of the first carrier gas to the catalyst solution is 10~20:1.

[0030] In one embodiment, the decomposition temperature in step S3 is 600~900℃ and the time is 10~30s.

[0031] Specifically, spray pyrolysis is the only core path to achieve high-efficiency preparation: (1) The complete process of spray pyrolysis (solution atomization - high-temperature pyrolysis - gas-solid separation) only takes 10~30s, without the cumbersome steps of traditional methods such as water bath concentration, long-term calcination, crushing and sieving. (2) In spray pyrolysis, the catalyst solution is directly atomized and then dehydrated and dried at high temperature and the component decomposition is completed simultaneously, producing only trace amounts of water vapor. No washing step is required, thus avoiding wastewater from the source. Moreover, if no organic additives are added, zero organic waste gas can even be achieved. This environmental advantage is something that traditional methods cannot achieve and is also the core guarantee for the potential of industrial application of this technology.

[0032] Spray pyrolysis defines the core performance of the catalyst with high density and small particle size active sites and planar growth regions: (1) In spray pyrolysis, the catalyst solution is atomized into micron-sized droplets. Each droplet is equivalent to an independent micro-reaction unit. In the high temperature range of 600~900℃, the droplets are instantly dehydrated and dried, and the internal metal ions are rapidly decomposed into oxides. Because the pyrolysis rate is faster than the ion diffusion rate, the metal oxide particles do not have time to agglomerate, thus forming active sites with uniform particle size and high density dispersion. (2) The catalyst powder produced by spray pyrolysis is a loose, sheet-like powder. This structure originates from the spherical symmetrical pyrolysis of the atomized droplets. The droplets shrink uniformly at high temperature, and the decomposed metal oxides form microspheres / sheets with flat surfaces and no obvious protrusions, naturally possessing planar growth regions.

[0033] In one embodiment, the particle size classification process in step S4 is carried out in a dry, inert atmosphere with an inlet air velocity of 10-30 m / s; the temperature of the particle size classification process is 25-80°C; and the rotational speed of the classifier is 1000-3000 r / min.

[0034] In one embodiment, after particle size classification in step S4, the particle size of the collected catalyst powder is 0.5~50μm.

[0035] In a preferred embodiment, after particle size classification in step S4, the collected catalyst powder has a particle size of 0.5~10μm or 10~50μm.

[0036] In one embodiment, the step of preparing the array-type carbon nanotubes after the catalyst powder in step S5 enters the CVD fluidized bed through the feeder includes: introducing a reduced carrier gas into the CVD fluidized bed through a second gas source, heating the CVD fluidized bed to 500~900℃, holding it at that temperature for 10~30min, and reducing it to obtain a metal catalyst; stopping the introduction of the reduced carrier gas, switching to a growth carrier gas for continuous introduction, introducing a carbon source gas into the CVD fluidized bed, and reacting for 30~180min to obtain the array-type carbon nanotubes.

[0037] Specifically, the formation process of the aforementioned metal catalyst is illustrated in the following diagram. Figure 5As shown, the process follows the path of "droplet formation - solid-phase transformation - sintering and granulation - reduction and activation", specifically as follows: (1) Precursor droplet formation (initial state): The catalyst precursor (i.e., catalyst solution) is dispersed into micron-sized precursor droplets by atomization and other means, laying the foundation for subsequent uniform nucleation. (2) Solvent evaporation (physical dehydration): The precursor droplets enter the high-temperature reaction zone. Under the action of the thermal field, the solvent in the droplets evaporates rapidly and is removed. The droplets shrink and solidify to form dry precursor solid particles. (3) Thermal decomposition (chemical transformation): The solid particles undergo a high-temperature thermal decomposition reaction in an oxidizing atmosphere (first carrier gas). The organic ligands are burned off, and the metal salts are decomposed and oxidized to generate a nascent powder composed of metal oxides (active components) and auxiliary oxides (carriers). (4) Sintering and Shaping (Structure Construction): The nascent oxide powder undergoes sintering and crystal reconstruction at high temperature. The auxiliary oxide forms a porous spherical or near-spherical framework structure, while the active component oxide is highly dispersed in the form of nanocrystals and anchored on the inner and outer surfaces of the framework, forming oxide catalyst particles with uniform particle size and stable structure. (5) Reduction and Activation (Active Site Generation): After the sintered crystal particles enter the CVD fluidized bed, a reduction reaction occurs in a reducing atmosphere (hydrogen in the second carrier gas). The active component oxide on the framework is reduced to catalytically active metal elemental nanoparticles; while the auxiliary oxide framework remains chemically inert and continues to serve as a support for metal sites. Finally, a high density of metal active sites is formed on the surface of the support, resulting in a metal catalyst that can be directly used for the catalytic growth of carbon nanotubes.

[0038] Specifically, the carbon source gas is selected from at least one of methane, ethylene, propylene, and acetylene.

[0039] Specifically, the volume ratio of the carbon source gas to the second carrier gas is 1:5 to 1:20.

[0040] Specifically, after introducing the carbon source gas, the total gas flow rate is controlled at 0.5~5L / min, so that the carbon source gas undergoes catalytic cracking on the surface of the metal catalyst particles, and carbon elements are deposited and grown to form the array-type carbon nanotubes.

[0041] According to one possible implementation of this disclosure, at least the following beneficial effects are achieved: 1. The staged treatment disclosed herein improves the dispersibility of catalyst particles, avoids the coating of active sites caused by agglomeration, allows more active sites to contact the carbon source gas, provides a uniform growth starting point for carbon nanotubes, and fundamentally ensures the consistency of carbon nanotube growth direction, reducing morphological defects. The synergistic effect of Fe / Co / Ni-based multi-component components can regulate the electronic structure of the catalyst, reduce the activation energy of carbon source gas cracking, and promote the rapid and directional deposition of carbon elements. Compared with single-component catalysts, this is more conducive to the axial growth of carbon nanotubes and improves the aspect ratio.

[0042] 2. The system disclosed herein employs a powder feeding device for precise delivery of graded catalysts, coupled with a closed CVD fluidized bed reaction chamber, forming an integrated feed-reaction-discharge process. This integrated design avoids material transfer losses associated with segmented operations, while ensuring the continuity of the reaction atmosphere and preventing external impurities from affecting product purity. Furthermore, the fluidized environment of the fluidized bed allows catalyst particles to suspend and move under the influence of airflow, forming dynamic and sufficient contact with the carbon source gas. This avoids the problem of excessively high or low local carbon source concentrations in traditional fixed-bed processes, ensuring uniform carbon nanotube growth rates and reducing diameter fluctuations. The airflow stirring effect within the fluidized bed eliminates local temperature gradients, ensuring a uniform temperature in the reaction zone. This prevents carbon nanotube structural collapse due to localized high temperatures or incomplete growth due to localized low temperatures, ensuring product structural stability. Additionally, the closed fluidized bed facilitates inert atmosphere replacement, completely isolating oxygen and preventing high-temperature oxidation of the carbon source gas and oxidation failure during carbon nanotube growth. It also prevents catalyst deactivation through oxidation, extending the catalytic cycle.

[0043] 3. Each step of the preparation method disclosed herein is matched to the growth characteristics of carbon nanotubes. The preheating stage removes impurities adsorbed on the catalyst surface and activates active sites; the growth stage precisely controls the carbon source supply rate to ensure the continuity of axial growth of carbon nanotubes. The method disclosed herein adopts a continuous feeding, continuous reaction, and continuous discharge mode. Compared with batch processes, it avoids damage to equipment and catalysts caused by repeated heating and cooling, keeping the entire preparation process in a steady state and ensuring the performance consistency of different batches of carbon nanotubes, making it more suitable for industrial mass production.

[0044] It should be understood that the description in this section is not intended to identify key or essential features of the embodiments of this disclosure, nor is it intended to limit the scope of this disclosure. Other features of this disclosure will become readily apparent from the following description. Attached Figure Description

[0045] The above and other objects, features, and advantages of this disclosure will become readily apparent from the following detailed description of exemplary embodiments, taken in conjunction with the accompanying drawings. Several embodiments of this disclosure are illustrated in the drawings by way of example and not limitation, in which: In the accompanying drawings, the same or corresponding reference numerals indicate the same or corresponding parts.

[0046] Figure 1 A schematic diagram of the system for continuously preparing array-type carbon nanotubes in Embodiment 1 of this disclosure is shown; Figure 2 A scanning electron micrograph of the catalyst powder in Example 2 of this disclosure is shown; the scale bar is 40 μm. Figure 3A scanning electron micrograph of the array-type carbon nanotubes in Embodiment 2 of this disclosure is shown; the scale bar is 40 μm. Figure 4 A scanning electron micrograph of the catalyst powder in Example 10 of this disclosure is shown; the scale bar is 40 μm. Figure 5 A schematic diagram of the formation process of the metal catalyst disclosed herein is shown.

[0047] Figure reference numerals: 1-Reaction vessel; 2-Atomization system; 3-Thermal decomposition furnace; 4-Cyclone classifier; 5-Powder feeder; 6-CVD fluidized bed; 7-First gas source; 8-Second gas source. Detailed Implementation

[0048] To make the objectives, features, and advantages of this disclosure more apparent and understandable, the technical solutions in the embodiments of this disclosure will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this disclosure, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this disclosure without creative effort are within the scope of protection of this disclosure.

[0049] Example 1 This embodiment provides a system for the continuous preparation of array-type carbon nanotubes, the structural schematic diagram of which is shown below. Figure 1 As shown.

[0050] The system includes a reactor 1, an atomization system 2, a pyrolysis furnace 3, a cyclone classifier 4, a powder feeder 5, and a CVD fluidized bed 6. One end of the atomization system 2 is connected to the reactor 1, and the other end is connected to the pyrolysis furnace 3. It is used to atomize the catalyst solution prepared in the reactor 1 and feed it into the pyrolysis furnace 3. The atomization system 2 is selected from either a two-fluid nozzle atomization device or an ultrasonic atomization device. The pyrolysis furnace 3 is connected to the cyclone classifier 4 and is used to transport the decomposed catalyst powder to the cyclone classifier 4 for particle size classification. One end of the powder feeder 5 is connected to the cyclone classifier 4, and the other end is connected to the CVD fluidized bed 6. It is used to feed the classified catalyst powder into the CVD fluidized bed 6 to prepare array-type carbon nanotubes.

[0051] The system also includes gas sources, namely a first gas source 7 and a second gas source 8. The first gas source 7 is connected to the atomization system 2 and is used to provide a first carrier gas to the atomization system 2. The first carrier gas is selected from at least one of air and oxygen. The second gas source 8 is connected to the CVD fluidized bed 6 and is used to provide a second carrier gas to the CVD fluidized bed 6. The second carrier gas includes a reducing carrier gas and a growth carrier gas. The growth carrier gas is selected from at least one of inert gas and nitrogen. The inert gas is selected from at least one of argon and helium. The reducing carrier gas is a mixture of inert gas / nitrogen and hydrogen, and the volume ratio of hydrogen to inert gas / nitrogen is 1:5 to 1:10.

[0052] Example 2 This embodiment uses the system from Example 1 to prepare an array of carbon nanotubes, as detailed below: (1) Based on the chemical formula Co of the catalyst 0.5 Al 2.5 Mo 0.05 V 0.05 Weigh out 0.5 mol of cobalt nitrate hexahydrate, 2.5 mol of aluminum nitrate nonahydrate, 0.05 mol of ammonium heptamolybdate tetrahydrate, and 0.05 mol of ammonium metavanadate, and prepare a catalyst solution with a metal ion concentration of 0.001 mol / L in a reaction vessel.

[0053] (2) The catalyst solution in step (1) enters the thermal decomposition furnace through a two-fluid nozzle, wherein the first gas source provides the first carrier gas (oxygen) to the two-fluid atomizing device, and the flow ratio of the first carrier gas to the catalyst solution is 20:1; the decomposition temperature in the thermal decomposition furnace is 850℃ and the time is 20s, and finally the catalyst powder is obtained.

[0054] (3) The catalyst powder is fed into a cyclone classifier for particle size classification, which is carried out in dry argon gas at an inlet velocity of 20 m / s. The processing temperature is 50 °C and the rotation speed of the classifier is 2000 r / min. After the above classification process, catalyst powder with a particle size of 0.5~10 μm is collected.

[0055] (4) The catalyst powder obtained from the graded treatment in step (3) is fed into the CVD fluidized bed through a powder feeder. The second gas source introduces the reduced carrier gas (total flow rate of 2L / min, a mixture of nitrogen and hydrogen, with a volume ratio of hydrogen to nitrogen of 1:5) into the CVD fluidized bed. The CVD fluidized bed is heated to 800℃ and kept at that temperature for 20min to reduce and obtain the metal catalyst. The reduced carrier gas is stopped, and the growth carrier gas (total flow rate of 2L / min, nitrogen) is continuously introduced. Ethylene is introduced into the CVD fluidized bed as a carbon source gas. The volume ratio of ethylene to the second carrier gas is 1:10. The total gas flow rate is controlled at 3L / min, so that ethylene undergoes catalytic cracking on the surface of the catalyst particles, and carbon elements are deposited and grown. After 40min, an array of carbon nanotubes is formed.

[0056] The microstructure of the catalyst powder prepared above was observed under a scanning electron microscope, and the results are as follows: Figure 2 As shown. Figure 2 The catalyst is mainly composed of spherical or near-spherical particles with a relatively uniform particle size distribution. This is a typical characteristic of spray pyrolysis process, where the catalyst precursor liquid is atomized into droplets, which then rapidly shrinks and solidifies at high temperature to form spherical precursors, which eventually decompose into metal oxide particles.

[0057] This morphology and structure play a key role in improving catalyst performance: (1) Spherical particles have a large specific surface area and uniform surface energy, making them less prone to agglomeration during fluidization in a fluidized bed, allowing for full contact with the carbon source gas and improving mass transfer efficiency; at the same time, the spherical structure ensures that active sites are uniformly distributed on the particle surface / inside, avoiding the problem of active site aggregation in traditional irregular particles. (2) The presence of pores significantly increases the specific surface area of ​​the catalyst, exposing more Fe / Co / Ni active sites, directly improving the efficiency of carbon source decomposition and carbon nanotube growth. (3) Uniform particle size ensures consistent particle size of active sites, thereby growing array carbon nanotubes with uniform diameter, avoiding amorphous carbon caused by uneven particle size.

[0058] The array-shaped carbon nanotubes prepared above were placed under a scanning electron microscope to observe their microstructure. The results are as follows: Figure 3 As shown. Figure 3 The results show that the carbon nanotubes grow in a nearly vertical direction, exhibiting a distinct array morphology with uniform diameter, relatively long length, and parallel structure. This demonstrates that the catalyst prepared in this disclosure (high-density small-particle-size active sites + planar structure) can be used to grow directionally arranged, small-diameter, and long carbon nanotubes.

[0059] Example 3 This embodiment uses the system of Example 1 to prepare an array of carbon nanotubes. The difference from Example 2 is that the metal ion concentration of the catalyst solution in this embodiment is 0.01 mol / L. The rest is the same as in Example 2 and will not be repeated here.

[0060] Example 4 This embodiment uses the system of Example 1 to prepare an array of carbon nanotubes. The difference from Example 2 is that the metal ion concentration of the catalyst solution in this embodiment is 0.1 mol / L. The rest is the same as in Example 2 and will not be repeated here.

[0061] Example 5 This embodiment uses the system of Example 1 to prepare an array of carbon nanotubes. Unlike Example 2, this embodiment also adds anhydrous citric acid at a concentration of 0.008 mol / L when preparing the catalyst solution. Based on the catalyst chemical formula Co... 0.5Al 2.5 Mo 0.05 V 0.05 Weigh out 0.5 mol of cobalt nitrate hexahydrate, 2.5 mol of aluminum nitrate nonahydrate, 0.05 mol of ammonium heptamolybdate tetrahydrate, and 0.05 mol of ammonium metavanadate. Then add anhydrous citric acid in an amount 0.4 times the total molar amount of the above metal ions (i.e., 1.24 mol) to prepare a catalyst solution with a metal ion concentration of 0.001 mol / L in a reaction vessel. The rest is the same as in Example 2, and will not be repeated here.

[0062] Example 6 This embodiment uses the system of Example 1 to prepare an array of carbon nanotubes. Unlike Example 2, this embodiment also adds anhydrous citric acid at a concentration of 0.004 mol / L when preparing the catalyst solution. Based on the catalyst chemical formula Co... 0.5 Al 2.5 Mo 0.05 V 0.05 Weigh out 0.5 mol of cobalt nitrate hexahydrate, 2.5 mol of aluminum nitrate nonahydrate, 0.05 mol of ammonium heptamolybdate tetrahydrate, and 0.05 mol of ammonium metavanadate. Then add anhydrous citric acid in an amount 0.4 times the total molar amount of the above metal ions (i.e., 1.24 mol) to prepare a catalyst solution with a metal ion concentration of 0.001 mol / L in a reaction vessel. The rest is the same as in Example 2, and will not be repeated here.

[0063] Example 7 This embodiment uses the system of Example 1 to prepare an array of carbon nanotubes. The difference from Example 2 is that in step (2) of this embodiment, the flow ratio of the first carrier gas to the catalyst solution is adjusted to 15:1. The rest is the same as in Example 2, and will not be repeated here.

[0064] Example 8 This embodiment uses the system of Example 1 to prepare an array of carbon nanotubes. The difference from Example 2 is that in step (2) of this embodiment, the flow ratio of the first carrier gas to the catalyst solution is adjusted to 10:1. The rest is the same as in Example 2, and will not be repeated here.

[0065] Example 9 This embodiment uses the system of Example 1 to prepare an array of carbon nanotubes. The difference from Example 2 is that in step (2) of this embodiment, the catalyst solution is introduced into the thermal decomposition furnace through an ultrasonic atomization device. The rest is the same as in Example 2, and will not be repeated here.

[0066] Example 10 This embodiment uses the system of Example 1 to prepare an array of carbon nanotubes. The difference from Example 2 is that in step (3) of this embodiment, catalyst powder with a particle size of 10~50μm is collected. The rest is the same as in Example 2, and will not be repeated here.

[0067] The catalyst powder prepared in this embodiment was observed under a scanning electron microscope to examine its microstructure. The results are as follows: Figure 4 As shown. Figure 4 Similarly, it was shown that the catalyst was mainly composed of spherical or near-spherical particles with a relatively uniform particle size distribution.

[0068] Comparative Example 1 This comparative example prepared an array of carbon nanotubes. Unlike Example 2, this comparative example did not use the system of Example 1 to directly prepare array of carbon nanotubes. Instead, it first prepared catalyst powder and then used the catalyst powder to prepare array of carbon nanotubes.

[0069] Based on the catalyst chemical formula Co 0.5 Al 2.5 Mo 0.05 V 0.05 Weigh out the corresponding amount of salt, and use the uniform precipitation method to obtain the catalyst hydroxide. Dry it in a 120℃ forced-air drying oven for 8 hours, grind and pulverize it, and then calcine it at 450℃ for 2 hours to obtain the final catalyst powder.

[0070] Test case The yield and density of the catalysts prepared in Examples 2-10, and the yield and I of the arrayed carbon nanotubes were tested. G / I D (Raman characteristic peak intensity ratio), specific surface area, and pipe diameter: 1. Catalyst yield = (weight of catalyst powder of corresponding particle size (g) / theoretical yield of catalyst (g)) × 100%.

[0071] 2. Catalyst density test: The true density of the catalyst was tested using a true density meter and the helium displacement method. The test conditions were room temperature and atmospheric pressure, and the test was repeated 3 times and the average value was taken.

[0072] 3. Carbon nanotube yield test: The mass difference subtraction method was adopted, that is, the mass of the catalyst was weighed before the reaction, and the carbon nanotubes were collected and dried to constant weight after the reaction. Carbon nanotube yield = (weight of carbon nanotube powder (g) - weight of catalyst powder (g)) / weight of catalyst powder (g).

[0073] 4. Carbon nanotubes I G / I D Test: A Raman spectrometer was used with a laser wavelength of 532 nm and a scanning range of 1000~2000 cm⁻¹. -1 The intensities of the G peak and the D peak were read separately, and the ratio I between them was calculated.G / I D .

[0074] 5. Carbon nanotube specific surface area test: The BET specific surface area analyzer was used to test the specific surface area using the nitrogen adsorption-desorption method. Before the test, the sample was degassed at 200℃ for 2 hours, and the specific surface area was calculated using the BET model.

[0075] 6. Carbon nanotube diameter test: Using a transmission electron microscope, randomly select more than 50 carbon nanotubes, observe and measure their diameter, and take the average value as the final diameter.

[0076] The test results are shown in Table 1.

[0077] Table 1

[0078] Table 1 shows that in Examples 2, 3, and 4, the concentration of metal ions in the catalyst solution gradually increased, the density of the corresponding catalyst gradually increased, the diameter of the carbon nanotubes gradually increased, and the yield of carbon nanotubes gradually decreased. This indicates that the lower the concentration of the catalyst solution, the lower the corresponding catalyst bulk density, and the relatively smaller the diameter of the carbon nanotubes.

[0079] Examples 5 and 6, which added anhydrous citric acid, showed that the specific surface area of ​​the carbon nanotubes was much higher than that of Example 2, indicating that the use of a complexing agent can increase the specific surface area of ​​carbon nanotubes.

[0080] Examples 7 and 8 reduced the flow ratio of the first carrier gas to the catalyst solution, resulting in a significantly lower carbon nanotube yield compared to Example 2. This indicates that insufficient first carrier gas weakens the atomization effect, increases droplet size, and makes the catalyst particles formed after thermal decomposition prone to agglomeration, ultimately leading to larger catalyst particles with poor dispersibility. After the catalyst enters the CVD fluidized bed, the adhesion between catalyst particles can coat some active sites. Simultaneously, the smaller specific surface area of ​​larger catalyst particles prevents the carbon source gas from fully contacting the active sites, thus reducing the growth efficiency of carbon nanotubes.

[0081] Example 9 employed ultrasonic atomization, achieving a catalyst yield of 85.63% for the 0.5–10 μm range, significantly higher than that of Example 2; furthermore, the specific surface area and I0.05 of the carbon nanotubes were also improved. G / I D Compared to Example 2, all results show improvements, with the smallest tube diameter range. This indicates that ultrasonic atomization increases the yield of small-particle catalysts, increases the specific surface area of ​​carbon nanotubes, improves the crystallinity of carbon nanotubes, and reduces the tube diameter of carbon nanotubes.

[0082] Example 10 collected catalyst powder of 10-50 μm, with corresponding carbon nanotube diameters of 10-20 nm, significantly larger than those in Example 2, indicating that large-particle-size catalysts readily produce carbon nanotube products with larger diameters. The carbon nanotube yield in Example 10 was close to that in Example 2, demonstrating that the 10-50 μm catalyst in this disclosure can also maintain a high carbon nanotube yield.

[0083] The steps for obtaining the catalyst in Comparative Example 1 are cumbersome and time-consuming, and the carbon nanotube yield is low with a relatively large diameter. This shows that the system of this disclosure has a simpler steps for obtaining the catalyst and a higher carbon nanotube yield.

[0084] It should be understood that the various forms of processes shown above can be used to rearrange, add, or delete steps. For example, the steps described in this disclosure can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution of this disclosure can be achieved, and this is not limited herein.

[0085] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this disclosure, "a plurality of" means two or more, unless otherwise explicitly specified.

[0086] The above description is merely a specific embodiment of this disclosure, but the scope of protection of this disclosure is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this disclosure should be included within the scope of protection of this disclosure. Therefore, the scope of protection of this disclosure should be determined by the scope of the claims.

Claims

1. A system for the continuous preparation of array-type carbon nanotubes, characterized in that, The system includes a reactor, an atomization system, a pyrolysis furnace, a classifier, a feeder, and a CVD fluidized bed. One end of the atomization system is connected to the reactor, and the other end is connected to the pyrolysis furnace, for atomizing the catalyst solution prepared in the reactor and feeding it into the pyrolysis furnace. The pyrolysis furnace is connected to the classifier, for conveying the decomposed catalyst powder to the classifier for particle size classification. One end of the feeder is connected to the classifier, and the other end is connected to the CVD fluidized bed, for feeding the classified catalyst powder into the CVD fluidized bed to prepare the arrayed carbon nanotubes.

2. The system according to claim 1, characterized in that, The atomization system is selected from either a two-fluid nozzle atomization device or an ultrasonic atomization device.

3. The system according to claim 1, characterized in that, The system also includes a gas source, which includes a first gas source and a second gas source. The first gas source is connected to the atomization system and is used to provide a first carrier gas to the atomization system. The second gas source is connected to the CVD fluidized bed and is used to provide a second carrier gas to the CVD fluidized bed. The first carrier gas is selected from at least one of air and oxygen; the second carrier gas includes a reducing carrier gas and a growth carrier gas; the growth carrier gas is selected from at least one of inert gas and nitrogen, and the inert gas is selected from at least one of argon and helium; the reducing carrier gas is a mixture of inert gas / nitrogen and hydrogen, and the volume ratio of hydrogen to inert gas / nitrogen is 1:5 to 1:

10.

4. A method for continuously preparing arrayed carbon nanotubes using the system according to any one of claims 1 to 3, characterized in that, Includes the following steps: S1: The chemical formula of the catalyst is Fe. a Co b Ni c Mg d Al e Mo f Mn g V h Where 0≤a<1, 0≤b<1, 0≤c<1, 0<d<10, 0<e<5, 0≤f<0.5, 0≤g<0.5, 0≤h<0.5, the catalyst must include at least one of Fe, Co, and Ni, and at least two of Mo, Mn, and V; S2: According to the stoichiometric ratio of step S1, at least three of the soluble salts corresponding to Fe, Co, Ni, Mg, Al, Mo, Mn, and V are mixed in a reaction vessel to prepare a catalyst solution; among the at least three soluble salts, at least one of the soluble salts corresponding to Fe, Co, and Ni, and at least two of the soluble salts corresponding to Mo, Mn, and V must be included. S3: The catalyst solution is fed into a thermal decomposition furnace through an atomization system for decomposition to obtain catalyst powder; S4: The catalyst powder enters a classifier for particle size classification. S5: The catalyst powder after particle size classification in step S4 is fed into the CVD fluidized bed through a feeder to provide carbon source gas, and the array-type carbon nanotubes are obtained through fluidized reaction.

5. The method according to claim 4, characterized in that, In step S2, the soluble salt is selected from any one of the carbonates, nitrates, ammonium salts, oxalates, acetates, citrates, and sulfates corresponding to Fe, Co, Ni, Mg, Al, Mo, Mn, and V; The concentration of metal ions in the catalyst solution is 0.001~0.1 mol / L.

6. The method according to claim 5, characterized in that, In step S2, a complexing agent is added during the preparation of the catalyst solution. The concentration of the complexing agent is 0.004~0.008 mol / L. The ratio of the molar amount of the complexing agent to the total molar amount of metal ions in the catalyst solution is 1:5~1:

1. The complexing agent is selected from at least one of anhydrous citric acid, tartaric acid, malic acid, EDTA, and triethanolamine.

7. The method according to claim 4, characterized in that, In step S3, the first carrier gas is introduced into the atomization system via the first gas source; during the atomization process, the flow ratio of the first carrier gas to the catalyst solution is 10~20:1; The decomposition temperature in step S3 is 600~900℃, and the time is 10~30s.

8. The method according to claim 4, characterized in that, In step S4, the particle size classification process is carried out in a dry, inert atmosphere with an inlet air velocity of 10-30 m / s; the temperature of the particle size classification process is 25-80℃; and the rotational speed of the classifier is 1000-3000 r / min.

9. The method according to claim 4, characterized in that, After particle size classification in step S4, the collected catalyst powder has a particle size of 0.5~50μm.

10. The method according to claim 4, characterized in that, The step S5, where the catalyst powder is fed into the CVD fluidized bed via the feeder to prepare the array-type carbon nanotubes, includes: introducing a reducing carrier gas into the CVD fluidized bed via a second gas source, heating the CVD fluidized bed to 500-900°C, holding at that temperature for 10-30 min, and reducing to obtain a metal catalyst; stopping the introduction of the reducing carrier gas, switching to a growth carrier gas for continuous introduction, introducing a carbon source gas into the CVD fluidized bed, and reacting for 30-180 min to obtain the array-type carbon nanotubes; The carbon source gas is selected from at least one of methane, ethylene, propylene, and acetylene; The volume ratio of the carbon source gas to the second carrier gas is 1:5 to 1:20.

Images