A rare earth catalyst for preparing an array type carbon nanotube and a method of preparing the same

Abstract

The application provides a rare earth catalyst for preparing array type carbon nanotubes and a preparation method thereof, and relates to the technical field of rare earth catalysts.The preparation method comprises the following steps: mixing hydrolysis of ethyl orthosilicate and bayerite to form a composite gel;after calcination of the composite gel, a porous carrier is prepared; after the porous carrier is immersed in a ligand solution, a cerium-containing liquid is added and stirred and mixed, and a catalytic precursor is separated; after the catalytic precursor is immersed in a complex solution, an iron-containing liquid is added and stirred and mixed, and a catalytic intermediate is separated; after a carbon coating layer is deposited on the surface of the catalytic intermediate, the rare earth catalyst is prepared by reduction under a hydrogen atmosphere.The catalyst provided by the application can make the carbon tube diameter distribution concentrated in the growth process of the array type carbon nanotubes, and the Fe-Ce-O interface can induce directional assembly of carbon atoms and reduce the pipe wall defect density.

Description

Technical Field

[0001] This invention relates to the field of rare earth catalyst technology, and in particular to a rare earth catalyst for preparing arrayed carbon nanotubes and its preparation method. Background Technology

[0002] Arrayed carbon nanotubes, with their high orientation, large aspect ratio, and regular topology, have become a research hotspot in the field of nanomaterials. Chemical vapor deposition (CVD) is the mainstream process for preparing arrayed carbon nanotubes, and the catalyst plays a dual role in this process: on the one hand, it acts as an active center for the cracking of carbon sources (such as methane and ethylene), regulating the dynamic adsorption and dissociation efficiency of carbon atoms; on the other hand, by controlling the size, morphology, and crystal orientation of the catalyst particles, the nucleation sites, growth direction, and wall structure of the carbon nanotubes can be precisely controlled. Therefore, the dispersion, thermal stability, and interfacial characteristics of the catalyst directly determine the orientation consistency, defect density, and macroscopic properties of the array.

[0003] Current catalyst systems for preparing arrayed carbon nanotubes mainly include supported catalysts and composite catalysts. Active components are anchored to the surface of high-melting-point oxide supports via physical vapor deposition, sol-gel, or impregnation methods to suppress particle migration at high temperatures. In recent years, rare earth elements (such as lanthanum, cerium, and yttrium) have been introduced into catalytic systems due to their unique electronic structure and oxygen defect modulation capabilities, forming transition metal-rare earth oxide composite structures. These designs capture carbon clusters through oxygen vacancies in rare earth oxides or enhance the dispersion of active components through strong metal-support interactions.

[0004] Although existing catalyst systems have made some progress, they have revealed several technical shortcomings in large-scale preparation. First, under high-temperature reaction conditions, transition metal nanoparticles are prone to Ostwald ripening and sintering due to excessively high surface energy, leading to particle size dispersion and subsequently causing uneven carbon nanotube diameter, disordered array orientation, and a surge in tube wall defects. Second, while the introduction of rare earth additives can suppress particle agglomeration through electron doping, traditional co-precipitation or impregnation methods are insufficient to achieve single-atom-level dispersion of rare earth oxides on the support surface, easily forming micron-sized agglomerates. This not only weakens catalytic activity but also causes support structural collapse due to localized stress concentration. Therefore, a solution is needed to address these issues. Summary of the Invention

[0005] The purpose of this invention is to provide a rare earth catalyst for preparing arrayed carbon nanotubes and a method thereof, which can effectively improve the structural stability of the prepared rare earth catalyst during use, and induce carbon atoms to orientedly assemble into arrayed carbon nanotubes with high orientation during the preparation of carbon nanotubes, and help reduce defects in the tube wall of carbon nanotubes.

[0006] In a first aspect, the present invention provides a method for preparing a rare earth catalyst for preparing arrayed carbon nanotubes, comprising: mixing and hydrolyzing tetraethyl orthosilicate and gibbsite to form a composite gel; calcining the composite gel to obtain a porous support; impregnating the porous support in a ligand solution and then adding a cerium-containing liquid and stirring to obtain a catalytic precursor; impregnating the catalytic precursor in a complexing solution and then adding an iron-containing liquid and stirring to obtain a catalytic intermediate; depositing a carbon coating layer on the surface of the catalytic intermediate and then reducing it under a hydrogen atmosphere to obtain a rare earth catalyst.

[0007] The preparation method provided by this invention inhibits metal migration through the spatial confinement of a porous support, achieves rare earth single-atom dispersion through ligand competitive adsorption, enhances the metal-support interaction through the Fe-Ce-O interface, provides thermal stability through the carbon coating layer, and ensures that Ce is uniformly distributed on the support surface in a single-atom state in the catalyst. Therefore, during the growth of array-type carbon nanotubes, the confined structure concentrates the diameter distribution of the carbon nanotubes, and the Fe-Ce-O interface induces the directional assembly of carbon atoms, reducing the defect density of the tube wall.

[0008] Optionally, tetraethyl orthosilicate and gibbsite are mixed at a molar ratio of (2-5):1, and then a mesoporous template agent is added for mixing and hydrolysis to form a composite gel; the mesoporous template agent includes block copolymer F127.

[0009] Optionally, the composite gel is pretreated at 400℃-500℃ for 1h-3h and then calcined at 650℃-750℃ for 1h-2h to obtain a porous carrier.

[0010] Optionally, the ligand solution comprises an ethanol solution of ethylenediaminetetraacetic acid.

[0011] Optionally, the porous support is impregnated in a ligand solution and then ultrasonically dispersed.

[0012] Optionally, the cerium concentration in the cerium-containing solution is 0.01 mol / L to 0.05 mol / L.

[0013] Optionally, the cerium-containing solution includes an ethanol solution of cerium nitrate.

[0014] Optionally, after adding cerium-containing liquid and stirring to mix, the mixture is filtered and separated, and then dried at 70℃-90℃ to obtain the catalytic precursor.

[0015] Optionally, the complexing solution includes a sodium citrate solution.

[0016] Optionally, the catalytic precursor is impregnated in a complexing solution and then ultrasonically dispersed.

[0017] Optionally, the iron concentration in the iron-containing liquid is 0.01 mol / L to 0.1 mol / L.

[0018] Optionally, the iron-containing liquid includes a ferrous sulfate solution.

[0019] Optionally, the iron-containing liquid also includes cobalt ions at a concentration of 0.01 mol / L to 0.05 mol / L.

[0020] Optionally, after adding iron-containing liquid and stirring, the pH is adjusted to 7-8, and the mixture is separated after 1-2 hours in a microwave environment.

[0021] Optionally, after depositing the catalytic intermediate on the surface for 5 min to 10 min in a mixed atmosphere of ethylene and hydrogen at 550℃-650℃, the rare earth catalyst is obtained by reduction in a hydrogen atmosphere.

[0022] Optionally, the volume ratio of ethylene to hydrogen in the mixed atmosphere is 1:(3-5).

[0023] Optionally, the pressure of the mixed atmosphere is 0.1 MPa to 0.5 MPa.

[0024] Optionally, after depositing on the surface of the catalytic intermediate for 5-10 minutes, the mixture is kept at an inert atmosphere of 800-850℃ for 1-2 hours.

[0025] Secondly, the present invention also provides a rare earth catalyst prepared by any of the above-mentioned optional preparation methods, wherein the rare earth catalyst is used to prepare array-type carbon nanotubes.

[0026] The present invention provides a method for preparing rare earth catalysts for arrayed carbon nanotubes, which has at least one of the following advantages compared with the prior art:

[0027] 1. The preparation method provided by the present invention does not require the use of complex mechanical equipment, which can effectively improve the production convenience of rare earth catalysts. At the same time, the raw materials used are simple and readily available, which helps to reduce production costs and facilitates large-scale industrial production.

[0028] 2. The preparation method provided by the present invention inhibits the migration of metal active components through spatial confinement, and at the same time enables the metal active elements to be uniformly distributed on the support. This not only helps to improve catalytic activity and catalytic efficiency, but also improves the structural stability of the metal active components on the support during long-term use.

[0029] 3. The preparation method provided by the present invention can protect the metal active components on the surface of the catalytic intermediate by depositing a carbon coating layer on the surface of the catalytic intermediate, avoid the stripping of the metal active components during the formation of array carbon nanotubes, and at the same time help to adjust the orientation of rare earth catalysts to array carbon nanotubes during the preparation. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Unless otherwise defined, the technical or scientific terms used herein should have the ordinary meaning understood by those skilled in the art to which this invention pertains.

[0031] This invention provides a method for preparing rare earth catalysts for arrayed carbon nanotubes, comprising the following steps:

[0032] S1. Tetraethyl orthosilicate and gibbsite are mixed and hydrolyzed to form a composite gel;

[0033] S2. A porous carrier is prepared by calcining the composite gel.

[0034] S3. After impregnating the porous support in the ligand solution, add the cerium-containing material solution and stir to mix, then separate to obtain the catalytic precursor;

[0035] S4. After impregnating the catalytic precursor in a complexing solution, add an iron-containing liquid and stir to mix, then separate to obtain the catalytic intermediate;

[0036] S5. A rare earth catalyst is prepared by depositing a carbon coating layer on the surface of a catalytic intermediate and then reducing it under a hydrogen atmosphere.

[0037] In fact, the preparation method provided by the present invention utilizes the hydrolysis and condensation reaction of tetraethyl orthosilicate and gibbsite to form a composite gel with a Si-O-Al network structure, and then forms a SiO2-Al2O3 porous support with a high specific surface area by calcination. The porous support is then impregnated in a ligand solution. Through the competitive adsorption of hydroxyl groups on the surface of the porous support by the ligand, high-energy active sites can be preferentially occupied. After the introduction of cerium-containing liquid, free cerium ions are adsorbed at the defect sites of the support, thereby forming an atomically dispersed Ce-O-Si / Al bonded structure.

[0038] Furthermore, treatment in a complexing solution and an iron-containing liquid can enable Fe... 2+ / Fe 3+ It forms a stable complex with the complexing agent and gradually replaces the weakly bound Ce on the support surface through ion exchange. 3+By utilizing the strong stability of rare earth-carrier bonds, Fe species are preferentially deposited in the vicinity of Ce atoms, forming a Fe-Ce-O ternary interface. This enhances the electronic coupling effect between the active component and the carrier. Finally, the surface energy of Fe nanoparticles is reduced by forming a carbon coating structure on the surface to physically block the flow. At the same time, Fe-C chemical bonds are formed between the carbon layer and the metal to further fix the particle position. Finally, hydrogen reduction eliminates the surface oxide layer, exposing the highly active metal crystal facet, while the carbon layer remains intact.

[0039] In some embodiments, during step S1, tetraethyl orthosilicate and boehmite are mixed at a molar ratio of (2-5):1, and then a mesoporous template agent is added for hydrolysis to form a composite gel. Specifically, the mesoporous template agent includes block copolymer F127. In practice, the chemical properties of the support are optimized by adjusting the Si / Al ratio. For example, increasing the Si content can improve the thermal stability and surface hydroxyl density of the support, providing more active sites; while increasing the Al content can enhance the mechanical strength of the support and promote the electrostatic adsorption of metal ions on the support surface.

[0040] In some embodiments, during step S2, the composite gel is pretreated at 400℃-500℃ for 1-3 hours, and then calcined at 650℃-750℃ for 1-2 hours to obtain a porous support. In fact, the low-temperature pretreatment process removes the template agent and incompletely hydrolyzed tetraethyl orthosilicate, while the gradient heating helps to slow down the gas generation rate, avoiding pore defects and collapse caused by rapid gas removal.

[0041] In some embodiments, the ligand solution used in step S3 includes an ethanol solution of ethylenediaminetetraacetic acid, and the porous support is impregnated in the ligand solution and then ultrasonically dispersed during step S3. In fact, impregnating the porous support improves the uniformity of the ligand on the surface of the porous support. Simultaneously, the ultrasonic cavitation effect generates microjets and localized high pressure, driving ligand molecules to penetrate deep into the micropores of the support, thereby ensuring that the inner walls of the pores are fully covered by ligand modification.

[0042] In some embodiments, the cerium concentration in the cerium-containing solution in step S3 is 0.01 mol / L to 0.05 mol / L. In fact, by adjusting the cerium concentration in the cerium-containing solution, the cerium content modified onto the porous support surface can be adjusted, thereby ensuring the uniformity of cerium modification on the porous surface and avoiding oversaturation to prevent the formation of polymerization sites. This facilitates the formation of strong single-atom cerium-support bonding structures on the porous support surface, providing a foundation for the subsequent directional anchoring of Fe on the support and the generation of array-type carbon nanotubes.

[0043] In some embodiments, the cerium-containing solution in step S3 includes an ethanol solution of cerium nitrate. In practice, the cerium-containing solution can also be other cerium salts, such as sulfates, chlorides, etc.

[0044] In some embodiments, after adding cerium-containing liquid and stirring in step S3, the mixture is filtered and dried at 70°C-90°C to obtain the catalytic precursor. In fact, by using a cerium nitrate ethanol solution instead of a traditional aqueous system, the cerium loading process is transformed from "rapid hydrolysis-random deposition" to "slow-release adsorption-site anchoring" through multiple interactions between the solvent, solute, and support. This helps maintain the pore structure and surface hydroxyl activity, providing an ideal interfacial environment for the subsequent confined loading of Fe.

[0045] In some embodiments, the complexing solution in step S4 includes a sodium citrate solution. In fact, the sodium citrate complexing system, through the synergy of chemical coordination and physical confinement, upgrades the transition metal loading from "random deposition" to "directional assembly," which can improve the problem of uncontrolled particle size and enhance the directional guidance performance of carbon nanotube growth on the catalyst surface through the regulation of interfacial electronic structure.

[0046] In some embodiments, the catalytic precursor is impregnated in a complexing solution and then ultrasonically dispersed in step S4. In fact, during ultrasonic impregnation, sodium citrate in the complexing solution competitively coordinates with the rare earth single atoms (such as Ce-O-Al / Si) pre-loaded on the surface of the catalytic precursor, preferentially binding to weakly anchored Ce. 3+ This replaces the low-stability site, releasing the high-energy anchoring site as Fe. 2+ Provides high-quality adsorption sites.

[0047] In some embodiments, the iron-containing solution in step S4 includes ferrous sulfate at a concentration of 0.01 mol / L to 0.1 mol / L, and also includes cobalt ions at a concentration of 0.01 mol / L to 0.05 mol / L. In fact, a Fe-Co dual-active-phase system can be constructed through bimetallic adsorption. The resulting metal nanoparticles possess both high catalytic activity and anti-sintering properties, improving the high-temperature deactivation problem of the catalyst and enhancing the orientation consistency of carbon nanotube formation through a bimetallic synergistic catalytic mechanism.

[0048] In some embodiments, after adding iron-containing liquid and stirring in step S4, the pH is adjusted to 7-8, and the mixture is stirred in a microwave environment for 1-2 hours before separating the catalytic intermediate. In fact, by reducing the oxidation potential of ferrous ions in a weakly alkaline environment and forming a complex with citrate ions, the formation of ferrous precipitate is avoided. Furthermore, by performing interfacial assembly in a microwave field, the dispersion uniformity of iron nanoclusters can be improved.

[0049] In some embodiments, during step S5, after depositing the catalytic intermediate on its surface for 5-10 minutes in a mixed atmosphere of ethylene and hydrogen at 550°C-650°C, the rare earth catalyst is reduced in a hydrogen atmosphere to obtain the catalyst. In fact, by confined carbon deposition in an ethylene / hydrogen mixed atmosphere followed by hydrogen reduction activation, the problems of high-temperature sintering and disordered carbon nanotube growth can be improved. Furthermore, through the synergistic effect of carbon layer porosity and rare earth electronic effects, ultra-long-range ordered growth of array-type carbon nanotubes is achieved.

[0050] In some embodiments, the volume ratio of ethylene to hydrogen in the mixed atmosphere is 1:(3-5), and the pressure is 0.1 MPa-0.5 MPa. Furthermore, after deposition on the surface of the catalytic intermediate for 5-10 minutes, it is kept at an inert atmosphere at 800-850°C for 1-2 hours.

[0051] Example 1

[0052] This embodiment 1 provides a method for preparing a rare earth catalyst for preparing arrayed carbon nanotubes, including the following steps:

[0053] S1. Tetraethyl orthosilicate and gibbsite are mixed at a molar ratio of 3:1 to obtain a preliminary mixture. 10% by mass of F127 mesoporous template agent is added to the preliminary mixture and mixed. The mixture is then added to deionized water at a solid-liquid ratio of 0.08 g / mL for hydrolysis and condensation to form a composite gel.

[0054] S2. After calcining the composite gel at 450℃ for 2 hours, transfer it to 700℃ for 1 hour and allow it to cool naturally to room temperature to obtain a porous carrier.

[0055] S3. The porous support was impregnated in ethylenediaminetetraacetic acid (EDTA) ethanol solution at a solid-liquid ratio of 0.05 g / mL. After ultrasonic treatment at 200 W for 15 min, the mixture was filtered and separated. The solution was then added to 0.03 mol / L cerium nitrate ethanol solution and stirred at 300 rpm for 2 h. After filtration and separation, the mixture was dried in an oven at 80 °C to constant weight to obtain the catalytic precursor. The molar ratio of the porous support to EDTA and cerium nitrate was 5:1.2:1.

[0056] S4. The catalytic precursor was impregnated in an aqueous solution of sodium citrate at a solid-liquid ratio of 0.05 g / mL. After ultrasonic treatment at 200 W for 15 min, it was separated by filtration. The precursor was then mixed with an iron-containing solution (solvent: water) containing 0.05 mol / L ferrous sulfate and 0.03 mol / L cobalt chloride. The pH was adjusted to 8, and the mixture was stirred in a microwave environment (2.45 GHz, 500 W) for 2 h before separation to obtain the catalytic intermediate. The mass ratio of the catalytic precursor to sodium citrate and iron ions was 1:0.2:0.2.

[0057] S5. The catalytic intermediate is transferred to a tube furnace and a mixed atmosphere of 0.3 MPa (the volume ratio of ethylene to hydrogen is 1:4) is introduced. After surface deposition at 600°C for 8 min, it is kept at 800°C under an argon atmosphere for 2 h, and then reduced under a hydrogen atmosphere to obtain the rare earth catalyst.

[0058] Example 2

[0059] Example 2 provides a method for preparing a rare earth catalyst for preparing arrayed carbon nanotubes. The difference from Example 1 is that the iron-containing solution in step S4 does not contain 0.03 mol / L cobalt chloride.

[0060] Example 3

[0061] This embodiment 3 provides a method for preparing a rare earth catalyst for preparing arrayed carbon nanotubes. The difference from embodiment 1 is that in step S4, the mixture is not mixed in a microwave environment, but is ultrasonically treated at 200W for 10 min, stirred at 300rpm for 20 min, and the ultrasonic treatment-stirring treatment is repeated for a total of 120 min (a total of four cycles) to separate the catalytic intermediate.

[0062] Comparative Example 1

[0063] Comparative Example 1 provides a method for preparing a rare earth catalyst for preparing arrayed carbon nanotubes. The difference from Example 1 is that step S5 is not performed, and the rare earth catalyst is prepared in step S4.

[0064] Comparative Example 2

[0065] Comparative Example 2 provides a method for preparing a rare earth catalyst for arrayed carbon nanotubes. The difference from Example 1 is that steps S3 and S4 are not performed. In step S4, a porous support is added to a mixed solution containing 0.03 mol / L cerium nitrate and 0.05 mol / L ferrous sulfate at a solid-liquid ratio of 0.05 g / mL for impregnation and then calcined to obtain a catalytic intermediate. The mass ratio of the porous support to cerium nitrate is 1:0.2.

[0066] Performance testing

[0067] The rare earth catalysts prepared in Examples 1 to 3 and Comparative Examples 1 to 2 were used to synthesize arrayed carbon nanotubes. The process included: spreading the catalyst flat in a quartz boat and placing it in a tube furnace; heating to 650°C under nitrogen protection and introducing propylene at a flow rate of 500 sccm for 30 min; and then detecting the reaction yield, as shown in Table 1. The rare earth catalysts prepared in Examples 1 and Comparative Examples 1 to 2 were then used in a long-term cycle for 100 h, and the retention rate of the reaction yield compared to the first cycle was detected, as shown in Table 1.

[0068] Table 1 Catalytic performance and long-term performance of rare earth catalysts

[0069] Reaction yield / % Retention rate after 100 hours / % Example 1 43.5 95.47 Example 2 38.6 86.72 Example 3 35.3 79.54 Comparative Example 1 21.7 64.28 Comparative Example 2 36.8 76.52

[0070] As shown in Table 1, and in conjunction with Example 1 and Comparative Example 2, the rare earth catalyst prepared in Example 1 of this invention exhibits good reaction yield in the preparation of arrayed carbon nanotubes and maintains good catalytic activity during long-term use, thus facilitating the industrial production of arrayed carbon nanotubes. Furthermore, as shown in Examples 1 and 2, introducing Co into the catalyst effectively improves catalytic activity and enhances the structural stability of the metal active component on the support. Examples 1 and 3 demonstrate that microwave treatment helps improve the uniformity and stability of the metal active component on the support surface. Simultaneously, Examples 1 and Comparative Example 1 show that the absence of a carbon coating layer and reduction treatment lead to a sharp decrease in the catalyst's reaction yield and a significant decrease in the stability of the active component, making it prone to detachment during catalysis.

[0071] While embodiments of the present invention have been described in detail above, it will be apparent to those skilled in the art that various modifications and variations can be made to these embodiments. However, it should be understood that such modifications and variations fall within the scope and spirit of the invention as set forth in the claims. Furthermore, the invention described herein may have other embodiments and can be implemented or carried out in various ways.

Claims

1. A method for preparing a rare earth catalyst for arrayed carbon nanotubes, characterized in that, include: Tetraethyl orthosilicate and boehmite were mixed at a molar ratio of (2-5):1, and then a mesoporous template agent was added for hydrolysis to form a composite gel. The composite gel was pretreated at 400℃-500℃ for 1-3 hours, and then calcined at 650℃-750℃ for 1-2 hours to obtain a porous support. The porous support was impregnated in a ligand solution, and then a cerium-containing solution was added, stirred, mixed, and separated to obtain a catalytic precursor. The ligand solution included an ethanolic solution of ethylenediaminetetraacetic acid, and the cerium-containing solution included an ethanolic solution of cerium nitrate. The catalytic precursor was then subjected to complexation in a complexing solution. After impregnation in the liquid, an iron-containing solution is added and stirred to adjust the pH to 7-8. After mixing in a microwave environment for 1-2 hours, the catalytic intermediate is separated. The complexing solution includes sodium citrate solution, and the iron-containing solution includes ferrous sulfate solution and cobalt ions with a concentration of 0.01 mol / L-0.05 mol / L. After deposition on the surface of the catalytic intermediate for 5-10 minutes in a mixed atmosphere of ethylene and hydrogen at 550℃-650℃, the rare earth catalyst is obtained by reduction in a hydrogen atmosphere.

2. The preparation method according to claim 1, characterized in that, The mesoporous template agent includes block copolymer F127.

3. The preparation method according to claim 1, characterized in that, The porous support is impregnated in a ligand solution and then ultrasonically dispersed; and / or, the cerium concentration in the cerium-containing solution is 0.01 mol / L - 0.05 mol / L.

4. The preparation method according to claim 1, characterized in that, After adding cerium-containing liquid and stirring to mix, the mixture is filtered and separated, and then dried at 70℃-90℃ to obtain the catalytic precursor.

5. The preparation method according to claim 1, characterized in that, The catalytic precursor was impregnated in a complexing solution and then ultrasonically dispersed. And / or, the iron concentration in the iron-containing liquid is 0.01 mol / L - 0.1 mol / L.

6. The preparation method according to claim 1, characterized in that, The volume ratio of ethylene to hydrogen in the mixed atmosphere is 1:(3-5); and / or the pressure of the mixed atmosphere is 0.1Mpa-0.5Mpa; and / or, after deposition on the surface of the catalytic intermediate for 5min-10min, it is kept at an inert atmosphere of 800℃-850℃ for 1h-2h.

7. A rare earth catalyst prepared by any one of claims 1 to 6, wherein the rare earth catalyst is used to prepare arrayed carbon nanotubes.