PDA-C coated metal catalyst for carbon nanotubes, preparation method of PDA-C coated metal catalyst and preparation method of carbon nanotubes

Abstract

The invention belongs to the technical field of carbon nanotubes, and particularly relates to a PDA-C coated metal catalyst for a carbon nanotube, a preparation method of the PDA-C coated metal catalyst and a preparation method of the carbon nanotube. A metal salt and a metal salt B are dissolved in a solvent, dopamine hydrochloride and a buffer solution are added, the PH is adjusted to be alkaline, and stirring is conducted at the room temperature till the mixed solution is dark brown; cleaning a substrate, dropping the mixed solution to the center of the substrate, and spin-coating the mixed solution on the substrate to form a uniform film; pDA is subjected to incomplete pyrolysis in an inert atmosphere, H2 / Ar mixed gas is introduced for activation, and the A metal-B metal / PDA-C catalyst is obtained. The preparation method greatly improves the stability and activity of the catalyst. By adopting the catalyst, a high-temperature catalytic reaction is carried out in a methane atmosphere, so that the high-quality carbon nano tube grows and is obtained.

Description

Technical Field

[0001] This invention belongs to the field of carbon nanotube technology, and particularly relates to a PDA-C coated metal catalyst for carbon nanotubes, its preparation method, and a method for preparing carbon nanotubes. Background Technology

[0002] Carbon nanotubes (CNTs) are of great significance in nanotechnology due to their nanoscale size and unique properties. They are hollow nanotubes formed under specific synthesis conditions, and can be classified into single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) based on the number of wall layers. Because of their excellent optical, mechanical, and electrical properties, carbon nanotubes are widely used in photocatalysts, catalysts, adsorbents, membranes, sensors, conductive coatings, batteries, supercapacitors, hydrogen storage devices, solar cells, and fuel cells.

[0003] The main methods used in existing technologies for preparing carbon nanotubes include arc discharge, laser ablation, and chemical vapor deposition. Arc discharge involves generating an electric arc between two carbon electrodes, causing carbon to evaporate and form carbon nanotubes. Laser ablation uses a high-energy laser beam to irradiate a carbon target, causing carbon atoms to evaporate and cool, forming carbon nanotubes. Compared to these two methods, chemical vapor deposition (CVD) decomposes carbon-containing gases on a catalyst surface at high temperatures to form carbon nanotubes. Its process and equipment are relatively simple, and its cost is low, thus it is widely used.

[0004] The key to synthesizing carbon nanotubes using chemical vapor deposition (CVD) lies in the preparation and selection of the catalyst. The composition, morphology, size, structure, and high-temperature characteristics of the catalyst all affect the structure and properties of the resulting carbon nanotubes to varying degrees. Chinese invention patent application No. 202510664398.7 discloses a method for preparing a regenerated supported metal catalyst for growing single-walled carbon nanotubes. The method involves uniformly mixing a transition metal salt precursor and an oxide support in a solvent, drying, grinding, and then growing SWNTs using CVD to obtain a supported metal catalyst. The supported metal catalyst is then dissolved in an acid solution, allowed to stand, and the supernatant is filtered. The filtrate is collected, and an alkaline solution is added until the metal cations in the solution are completely precipitated. The precipitate is collected, washed, dried, and calcined to obtain the regenerated metal catalyst. Chinese invention patent application No. 202510558683.0 discloses a FeMo-based metal oxide / MgO catalyst. Ferric nitrate, ammonium molybdate, magnesium nitrate, citric acid, and urea are dissolved in water. Solid magnesium oxide is added to the solution while stirring, and the mixture is stirred and evaporated to dryness until the sample becomes gel-like. The gel is dried in an oven, ground into powder, and calcined in air to obtain a foamy, porous catalyst. Chinese invention patent application No. 202510436220.7 discloses a catalyst for preparing small-diameter single-walled carbon nanotubes. Soluble iron salt, soluble platinum salt, and an additive are weighed, dissolved in water, and mixed evenly to obtain solution A. The support is pretreated in air at a certain temperature, then solution A is added, stirred evenly, heated to dryness, and ground to obtain solid B. Solid B is reduced in situ with hydrogen at a certain temperature, and then purged with nitrogen to obtain the catalyst for preparing small-diameter single-walled carbon nanotubes. However, the metal catalysts and their preparation processes used in the above-mentioned existing technologies all have problems such as difficulty in effectively controlling the size of metal particles in the catalyst, uneven distribution of catalytic active sites, and insufficient stability. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention proposes a PDA-C coated metal catalyst for carbon nanotubes, its preparation method, and a method for preparing carbon nanotubes. The invention improves the catalyst structure and preparation process, thereby achieving a catalytic effect with good stability and high activity. Furthermore, this invention specifically designs a carbon nanotube growth process for this novel catalyst to ensure the preparation of high-quality carbon nanotubes.

[0006] The technical solution of the present invention is as follows: A method for preparing a PDA-C coated metal catalyst for carbon nanotubes includes the following steps: Step S1: Dissolve metal salt A and metal salt B in a solvent, add dopamine hydrochloride and buffer solution, adjust the pH to alkaline, and stir at room temperature until the mixture turns dark brown. Step S2: Clean the substrate, drop the mixture onto the center of the substrate, and spin-coat it onto the substrate to form a uniform film; Step S3: Incompletely pyrolyze the PDA in an inert atmosphere, then activate it by introducing a H2 / Ar mixed gas to obtain the A metal-B metal / PDA-C catalyst.

[0007] Preferably, the metal salt A is an iron salt or a molybdenum salt, and the metal salt B is a cobalt salt, a copper salt, a zinc salt, or a nickel salt.

[0008] Preferably, metal salt A is an iron salt and metal salt B is a cobalt salt.

[0009] Preferably, the iron salt is one or more of ferric nitrate, ferric chloride, ferric sulfate, or ferric acetate, and the cobalt salt is one or more of cobalt nitrate, cobalt chloride, cobalt sulfate, or cobalt acetate.

[0010] Preferably, the solvent is deionized water and the buffer solution is an alkaline buffer solution.

[0011] Preferably, the substrate is SiO2, Al2O3, MgO or TiO2.

[0012] Preferably, in the mixture, the sum of the iron ion concentration and the cobalt ion concentration is 0.1 mol / L-1.5 mol / L, and the dopamine hydrochloride concentration is 2.4 μmol / L.

[0013] Preferably, the molar ratio of iron ions to cobalt ions in the mixture is 3:1.

[0014] Preferably, during spin coating, the spin coater first rotates at a low speed of 500-1000 rpm, and then rotates at a high speed of 2000-3000 rpm.

[0015] Preferably, the thickness of the uniform film is 10-15 nm.

[0016] Preferably, the temperature range for incomplete pyrolysis and activation in step S3 is 300-800℃.

[0017] Preferably, step S3 specifically includes: heating to 400-500℃ in an inert atmosphere, holding at that temperature for 1-2 hours, then introducing an H2 / Ar mixed gas, cooling to 300-350℃, and activating for 1-2 hours.

[0018] A PDA-C coated metal catalyst was obtained using the preparation method described above.

[0019] A method for preparing carbon nanotubes based on PDA-C coated metal catalysts, comprising: A PDA-C-coated metal catalyst substrate was placed horizontally in the center of a quartz tube with the gas flow direction parallel to the substrate surface. A carbon source and a protective gas were introduced, and carbon nanotubes were grown using the CVD method.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention proposes a novel catalyst for preparing carbon nanotubes, which uses PDA in the ionic state to form a PDA-C nitrogen-doped carbon confined structure through chelation-polymerization-incomplete pyrolysis, which greatly improves the stability and activity of the catalyst.

[0021] 2. In the preparation method of the PDA-C coated metal catalyst proposed in this invention, based on Fe... 3+ Co 2+ By leveraging the ionic properties of PDA-C and its promoting effect in the preparation of carbon nanotubes, the ratio of dopamine hydrochloride to iron and cobalt ions, ion concentration, and spin-coating process in the raw materials were optimized. This enabled effective control of catalyst activity and, consequently, stable improvement in the dispersibility of carbon nanotubes.

[0022] 3. The method for preparing PDA-C coated metal catalyst proposed in this invention also proposes a two-step activation process to achieve incomplete pyrolysis of PDA. By strictly controlling the pyrolysis temperature and time, the degree of carbonization of PDA and the size of metal particles are regulated, thereby retaining some nitrogen-containing functional groups, optimizing the type and content of nitrogen doping, and creating richer catalytic active sites.

[0023] 4. The present invention uses the PDA-C coated metal catalyst to prepare carbon nanotubes at high temperature, which is beneficial to the full cracking of methane, can improve the graphitization degree and growth rate of CNTs, obtain high crystallinity CNTs, and optimize the design of gas flow time and gas flow rate to maximize the growth of carbon nanotubes. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the embodiments will be briefly introduced below. The features and advantages of the present invention can be more clearly understood by referring to the accompanying drawings. The accompanying drawings are schematic and should not be construed as limiting the present invention in any way. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a flowchart of the preparation method of PDA-C coated metal catalyst for carbon nanotubes proposed in this invention.

[0026] Figure 2 This is a scanning electron microscope image of the carbon nanotubes prepared in Example 1 of the present invention.

[0027] Figure 3 This is a scanning electron microscope image of the carbon nanotubes prepared in Example 2 of the present invention. Detailed Implementation

[0028] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other.

[0029] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and therefore the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0030] To achieve the controllable growth of high-quality carbon nanotubes, this invention proposes a metal-based catalyst prepared with PDA assistance, and studies the application of PDA in ionic states (e.g., Fe). 3+ / Co 2+ The mechanism and characteristics of nitrogen-doped carbon confined structures formed through chelation-polymerization-carbonization were investigated. Based on this, the ability of the prepared catalyst to catalyze the growth of CNTs in a methane atmosphere was analyzed using CVD. The correlation between PDA-improved metal dispersion, optimized catalyst structure, and the final carbon nanotube growth performance was established, further optimizing the process route for catalyst preparation and carbon nanotube growth.

[0031] The catalyst preparation process of the present invention is as follows: Figure 1 As shown. In some embodiments, soluble iron and cobalt salts are first added to deionized water and mixed, then dopamine hydrochloride and Tris-HCl buffer are added, and the pH is adjusted to 8-11 with ammonia. The mixture is stirred at room temperature until the solution turns dark brown. During this process, factors such as the ratio of dopamine hydrochloride to each metal ion, the ratio between the metal ions, and the concentration of each metal ion must be considered in relation to the coating layer ratio, catalyst activity, and carbon nanotube diameter. In some embodiments, the sum of the iron ion concentration and the cobalt ion concentration can be selected as 0.1 mol / L-1.5 mol / L, and the dopamine hydrochloride concentration is 2.4 μmol / L. In other embodiments, the molar ratio of iron ions to cobalt ions is 3:1.

[0032] After preparing the mixture, the substrate is cleaned, and the mixture is dropped onto the center of the substrate and spin-coated to form a uniform thin film. The substrate can be SiO2, Al2O3, or other types, depending on the requirements. The thickness of the film directly affects the size of the catalyst nanoparticles formed after rupture, and the particle size directly determines the diameter of the grown carbon nanotubes. In some embodiments, spin-coating is performed by first rotating at a low speed of 500-1000 rpm for 10 seconds, followed by a high speed of 2000-3000 rpm for 30 seconds, thereby forming a uniform thin film of 10-15 nm thickness on the substrate surface.

[0033] The spin-coated substrate is partially pyrolyzed and activated under an inert atmosphere. In some embodiments, the specific processing temperature range can be set to 300-800℃. In other embodiments, this part can be divided into two stages. First, in an inert atmosphere, the temperature is increased to 400-500℃ at 5℃ / min and held for 1-2 hours. Then, an H2 / Ar mixed gas is introduced, and the temperature is lowered to 300-350℃ for activation for 1-2 hours, finally obtaining the Fe-Co / PDA-C catalyst. Through the above process, incomplete carbonization of PDA in the film can be achieved, thereby retaining some nitrogen- and oxygen-containing functional groups, forming a unique PDA-C nitrogen-doped carbon confined structure on the surface of the metal particles. This structure can play a dual role of "steric hindrance" and "electronic regulation," both physically isolating the metal particles and controlling their particle size to below 20nm, and optimizing the CH4 adsorption energy through the carbon-metal interface (reducing the CH bond breaking energy barrier by 0.2eV). Furthermore, when the catalyst is applied to the growth of carbon nanotubes, the nitrogen- and oxygen-containing functional groups retained in the structure can increase the number of active sites, thereby improving the catalyst utilization, carbon nanotube yield and purity. The nitrogen-doped carbon layer in the structure can prevent metal particles from growing, sintering and agglomerating, achieving uniform distribution and loading of catalytic active sites, and at the same time serving as a supplementary carbon source to optimize the morphology of carbon nanotubes.

[0034] In carbon nanotube growth, this invention places the Fe-Co / PDA-C catalyst substrate horizontally in the center of a quartz tube, with the gas flow direction parallel to the substrate surface, thereby enhancing CH4 diffusion efficiency. Subsequently, carbon nanotubes are grown at high temperature using CVD. In some embodiments, CH4 and Ar are introduced during the reaction, with a CH4 flow rate of 10-40 sccm and an Ar flow rate of 150 sccm. The reaction temperature is 1150-1500℃, the pressure is 100-200 Pa, and the growth time is 20-30 min. Compared to the existing CVD method which typically uses 700-900℃, this invention uses a higher temperature of 1150-1500℃, which is more conducive to the complete decomposition of CH4, thereby improving the graphitization degree and growth rate of carbon nanotubes and achieving high crystallinity. Furthermore, the control of gas flow rate, gas flow time, and reaction pressure maximizes carbon nanotube growth and minimizes the deposition of amorphous carbon.

[0035] Example 1 Ferric nitrate (Fe(NO3)3) and cobalt nitrate (Co(NO3)2) were dissolved in deionized water at a molar ratio of 3:1. Dopamine hydrochloride and Tris-HCl buffer were added, and the pH was adjusted to 9 with ammonia. The concentrations of ferric ions in the mixture were 0.9 mol / L, cobalt ions were 0.3 mol / L, and dopamine hydrochloride was 2.4 μmol / L. The mixture was stirred at room temperature until it turned dark brown. Using Al2O3 as a substrate, the mixture was ultrasonically cleaned with acetone-ethanol-deionized water for 10 min. The mixture was then dropped onto the center of the substrate and coated with a spin coater at a low speed of 500 rpm for 10 s, followed by a high speed of 3000 rpm for 30 s to form a uniform thin film on the substrate. The film was heated to 800 °C at a rate of 5 °C / min in an Ar atmosphere, held at that temperature for 1 h, and then activated for 1 h by introducing a H2 / Ar mixed gas. This yielded the Fe-Co / PDA-C catalyst.

[0036] A PDA-C-coated iron-cobalt catalyst substrate was horizontally placed in the center of a quartz tube, with the gas flow direction parallel to the substrate surface. A carbon source (CH4) and Ar were introduced, and carbon nanotubes were grown for 20 min at 1150℃ and 100 Pa. The prepared carbon nanotubes are shown below. Figure 2 As shown.

[0037] Example 2 Ferric nitrate (Fe(NO3)3) and cobalt nitrate (Co(NO3)2) were dissolved in deionized water at a molar ratio of 3:1. Dopamine hydrochloride and Tris-HCl buffer were added, and the pH was adjusted to 9 with ammonia. The concentrations of ferric ions in the mixture were 0.9 mol / L, cobalt ions were 0.3 mol / L, and dopamine hydrochloride was 2.4 μmol / L. The mixture was stirred at room temperature until it turned dark brown. Using an Al2O3 support as a substrate, the mixture was ultrasonically cleaned with acetone-ethanol-deionized water for 10 min. The mixture was then dropped onto the center of the substrate and coated with a spin coater. The spin coater was first rotated at a low speed of 500 rpm for 10 s, and then at a high speed of 3000 rpm for 30 s to form a uniform thin film on the substrate. The film was heated to 500 °C at a rate of 5 °C / min in an Ar atmosphere and held for 1 h. Then, an H2 / Ar mixed gas was introduced, and the temperature was lowered to 350 °C for 1 h for activation. The Fe-Co / PDA-C catalyst was thus prepared.

[0038] A PDA-C-coated iron-cobalt catalyst substrate was horizontally placed in the center of a quartz tube, with the gas flow direction parallel to the substrate surface. CH4 and Ar were introduced at a flow rate of 40 sccm and 150 sccm, respectively. Carbon nanotubes were grown for 30 min at a temperature of 1500℃ and a pressure of 200 Pa, yielding carbon nanotubes with a diameter of less than 40 nm. Figure 3 As shown.

[0039] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a PDA-C coated metal catalyst for carbon nanotubes, characterized in that, Includes the following steps: Step S1: Dissolve metal salt A and metal salt B in a solvent, add dopamine hydrochloride and buffer solution, adjust the pH to alkaline, and stir at room temperature until the mixture turns dark brown. Step S2: Clean the substrate, drop the mixture onto the center of the substrate, and spin-coat it onto the substrate to form a uniform film; Step S3: Incompletely pyrolyze the PDA in an inert atmosphere, then activate it by introducing a H2 / Ar mixed gas to obtain the A metal-B metal / PDA-C catalyst.

2. The preparation method according to claim 1, characterized in that, The metal salt A is an iron salt or a molybdenum salt, and the metal salt B is a cobalt salt, a copper salt, a zinc salt, or a nickel salt.

3. The preparation method according to claim 1, characterized in that, The metal salt A is an iron salt, and the metal salt B is a cobalt salt.

4. The preparation method according to claim 3, characterized in that, The iron salt is one or more of ferric nitrate, ferric chloride, ferric sulfate, or ferric acetate, and the cobalt salt is one or more of cobalt nitrate, cobalt chloride, cobalt sulfate, or cobalt acetate.

5. The preparation method according to claim 1, characterized in that, The solvent is deionized water, and the buffer solution is an alkaline buffer solution.

6. The preparation method according to claim 1, characterized in that, The substrate is SiO2, Al2O3, MgO or TiO2.

7. The preparation method according to claim 3, characterized in that, In the mixture, the sum of the iron ion concentration and the cobalt ion concentration is 0.1 mol / L-1.5 mol / L, and the dopamine hydrochloride concentration is 2.4 μmol / L.

8. The preparation method according to claim 3, characterized in that, In the mixture, the molar ratio of iron ions to cobalt ions is 3:

1.

9. The preparation method according to claim 1, characterized in that, During spin coating, the spin coater first rotates at a low speed of 500-1000 rpm, and then rotates at a high speed of 2000-3000 rpm.

10. The preparation method according to claim 1, characterized in that, The thickness of the uniform film is 10-15 nm.

11. The preparation method according to claim 3, characterized in that, The temperature range for incomplete pyrolysis and activation in step S3 is 300-800℃.

12. The preparation method according to claim 3, characterized in that, Step S3 specifically includes: heating to 400-500℃ in an inert atmosphere, holding at that temperature for 1-2 hours, then introducing an H2 / Ar mixed gas, cooling to 300-350℃, and activating for 1-2 hours.

13. A PDA-C coated metal catalyst, characterized in that, It is obtained by the preparation method described in any one of claims 1-12.

14. A method for preparing carbon nanotubes based on the PDA-C-coated metal catalyst of claim 13, characterized in that, A PDA-C-coated metal catalyst substrate was placed horizontally in the center of a quartz tube with the gas flow direction parallel to the substrate surface. A carbon source and a protective gas were introduced, and carbon nanotubes were grown using the CVD method.

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