Rare earth catalyst, preparation method and prepared carbon nanotube

Abstract

The invention provides a rare earth catalyst, a preparation method and a prepared carbon nanotube, and relates to the technical field of rare earth catalysts. The preparation method provided by the invention comprises the following steps: mixing a carbon source and rare earth nitrate in an ethanol-water system, then adding a mesoporous template agent, and carrying out hydrothermal reaction to prepare rare earth gel; carbonizing the rare earth gel in an inert gas atmosphere to obtain a carbonized carrier; taking the carbonized carrier as a working electrode, and performing electrochemical deposition in electrolyte of iron ions and cobalt ions to obtain a composite carrier; and depositing a carbon coating layer on the surface of the composite carrier, and reducing in a hydrogen atmosphere to prepare the rare earth catalyst. Through carrier-metal-carbon coated multistage interface coupling, a good protection effect on metal active components can be achieved, oxidation stripping of active metal is inhibited, the service life of the catalyst in the recycling process is effectively prolonged, and meanwhile the growth rate and orientation consistency of the carbon nanotubes can be improved.

Description

Technical Field

[0001] The present invention relates to the technical field of rare earth catalysts, and in particular to a rare earth catalyst, a preparation method thereof, and carbon nanotubes prepared thereby. Background Art

[0002] Due to its highly ordered one-dimensional structure, array-type carbon nanotubes exhibit unique performance advantages in the fields of electronic devices, energy storage materials, composite materials, etc. The characteristics of the catalyst used to prepare array-type carbon nanotubes directly determine the growth quality and structural controllability of array-type carbon nanotubes, including key parameters such as carbon nanotube diameter, number of walls, arrangement density, and defect degree. Therefore, by regulating the composition, morphology, and active site distribution of the catalyst, the dynamic balance of nucleation and extension during the growth process of carbon nanotubes can be achieved, thereby optimizing the uniformity and functionality of the array structure.

[0003] Currently, the mainstream catalyst system for preparing array-type carbon nanotubes usually uses transition metals (such as iron, cobalt, nickel) or their alloys as active components, and porous materials such as alumina and silica are used as carriers. Typical catalyst structures include core-shell type, multi-layer stacked type, or supported nanoparticles. By constructing a periodically distributed catalyst array, the directional growth of carbon nanotubes can be achieved. However, the metal active components are prone to migration or sintering during the high-temperature reaction process, resulting in uneven particle size distribution, and further causing problems such as large diameter dispersion of carbon nanotubes and chaotic array arrangement. Secondly, the interfacial binding force between the carrier and the active component is insufficient, and component peeling is prone to occur during long-term reaction, resulting in catalyst deactivation. Therefore, there is an urgent need to provide a solution to improve the above problems. Summary of the Invention

[0004] The purpose of the present invention is to provide a rare earth catalyst, a preparation method thereof, and carbon nanotubes prepared thereby. Through the multi-level interface coupling of carrier-metal-carbon coating, a good protective effect can be achieved on the metal active component, inhibiting the oxidation and peeling of the active metal, effectively improving the service life of the catalyst during the recycling process, and at the same time, the growth rate and orientation consistency of carbon nanotubes can be improved.

[0005] In a first aspect, a preparation method of a rare earth catalyst provided by the present invention includes: mixing a carbon source and rare earth nitrate in an ethanol-water system, adding a mesoporous template agent, and obtaining a rare earth gel through hydrothermal reaction; carbonizing the rare earth gel in an inert gas atmosphere to obtain a carbonized carrier; using the carbonized carrier as a working electrode, and performing electrochemical deposition in an electrolyte solution of iron ions and cobalt ions to obtain a composite carrier; depositing a carbon coating layer on the surface of the composite carrier and reducing it in a hydrogen atmosphere to obtain a rare earth catalyst.

[0006] The preparation method provided by the present invention forms a three-dimensional network-like rare-earth doped gel through hydrothermal crosslinking of a carbon source and a rare-earth catalyst in a mesoporous templating agent in an ethanol-water solvent system, and after carbonization treatment, the rare earth is in-situ embedded in the pore walls of the carbon skeleton, obtaining a carbonized carrier with a high specific surface area, conductivity, and rare-earth anchoring sites. Through electrochemical deposition, metal ions can be directionally reduced in the mesoporous channels of the carbonized carrier, and iron ions and cobalt ions are preferentially adsorbed with the rare earth as the anchor point to induce heterogeneous nucleation, forming highly dispersed alloy nanoparticles. Therefore, the migration and aggregation of metal particles can be inhibited through the strong bonding between rare-earth oxides and the carbon skeleton. In addition, by depositing a carbon layer on the surface of the composite carrier, triple interfacial coupling structures can be constructed through re-bonding with rare earth, which can play a good protective role for the active metal components in the catalyst.

[0007] Optionally, the carbon source includes at least one of glucose and phenolic resin.

[0008] Optionally, the rare-earth nitrate includes one of yttrium nitrate and gadolinium nitrate.

[0009] Optionally, the mass ratio of the carbon source to the rare-earth nitrate is 1:(0.1 - 1).

[0010] Optionally, the hydrothermal reaction is carried out at 160°C - 200°C.

[0011] Optionally, the rare-earth gel is obtained by hydrothermal reaction for 12h - 24h.

[0012] Optionally, the gas used in the inert gas atmosphere includes at least one of argon and nitrogen.

[0013] Optionally, the rare-earth gel is carbonized at 700°C - 900°C.

[0014] Optionally, the rare-earth gel is carbonized for 1h - 2h.

[0015] Optionally, the carbonized carrier is used as the working electrode, and Ag / AgCl is used as the reference electrode.

[0016] Optionally, a pulsed voltage of -0.5V to 1.2V is applied during electrochemical deposition.

[0017] Optionally, 2mol / L - 5mol / L of potassium chloride is dissolved in the electrolyte.

[0018] Optionally, the molar ratio of iron ions to cobalt ions in the electrolyte is (1 - 3):1.

[0019] Optionally, at least one of iron nitrate, iron sulfate, and iron chloride is dissolved in the electrolyte.

[0020] Optionally, at least one of cobalt sulfate, cobalt nitrate, and cobalt chloride is dissolved in the electrolyte.

[0021] Optionally, the volume fraction of ethanol in the ethanol-water system is 10%-70%.

[0022] Optionally, the mesoporous template agent includes one of F127 and CTAB.

[0023] Optionally, in a mixed atmosphere of ethylene and hydrogen and an environment of 550°C - 650°C, after depositing on the surface of the composite support for 5 min - 10 min, reduction is carried out in a hydrogen atmosphere to obtain the rare earth catalyst.

[0024] Optionally, the volume ratio of ethylene to hydrogen in the mixed atmosphere is 1:(3 - 5).

[0025] Optionally, the air pressure of the mixed atmosphere is 0.1 Mpa - 0.5 Mpa.

[0026] Optionally, after depositing on the surface of the composite support for 5 min - 10 min, it is kept warm for 1 h - 2 h in an inert gas atmosphere of 800°C - 850°C.

[0027] In a second aspect, the present invention also provides a rare earth catalyst prepared by any one of the above optional preparation methods.

[0028] In a third aspect, the present invention also provides carbon nanotubes prepared by using the rare earth catalyst prepared by any one of the above optional preparation methods.

[0029] The preparation method of a rare earth catalyst provided by the present invention has at least one of the following beneficial technical effects compared with the prior art:

[0030] 1. The preparation method provided by the present invention does not require the use of complex mechanical equipment, and the raw materials used are cheap and easily available. It has a high batch production rate when preparing rare earth catalysts and can be used for large-scale industrial production, which is beneficial for the production and application of array-type carbon nanotubes. At the same time, the solvent system adopted by the present invention is safe and non-toxic, which is beneficial for reducing the release of toxic and harmful substances during the production process and has high environmental protection.

[0031] 2. The rare earth catalyst prepared by the present invention has a stable carbon skeleton structure, can form a stable rare earth-carbon skeleton on the surface of the support, and the surface-coated carbon layer is beneficial for improving the stability of rare earth elements during the catalytic use process and avoiding peeling. Detailed implementation mode

[0032] 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 described clearly and completely below. Apparently, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention. Unless otherwise defined, the technical terms or scientific terms used herein shall have the ordinary meanings understood by those of ordinary skill in the art belonging to the field of the present invention.

[0033] The present invention provides a method for preparing a rare earth catalyst, comprising the following steps:

[0034] S1. Mix a carbon source and rare earth nitrate in an ethanol-water system, then add a mesoporous template agent, and obtain a rare earth gel after hydrothermal reaction;

[0035] S2. Carbonize the rare earth gel in an inert gas atmosphere to obtain a carbonized carrier;

[0036] S3. Use the carbonized carrier as a working electrode, and perform electrochemical deposition in an electrolyte solution of iron ions and cobalt ions to obtain a composite carrier;

[0037] S4. Deposit a carbon coating layer on the surface of the composite carrier and then reduce it in a hydrogen atmosphere to obtain a rare earth catalyst.

[0038] In fact, in step S1 of the present invention, a carbon source and rare earth nitrate are used as raw materials to undergo hydrothermal crosslinking under the guidance of a mesoporous template agent, thereby forming a three-dimensional network-like rare earth-doped gel - rare earth gel. During this hydrothermal reaction process, rare earth ions are combined with the carbon source through coordination, so that they can be evenly dispersed in the rare earth gel framework, and the carbon source is decomposed into a conductive carbon framework during the subsequent carbonization process, and the rare earth ions are sintered with the carbon framework to form a rare earth-carbon bonding structure.

[0039] In some embodiments, the carbon source used when performing step S1 includes at least one of glucose and phenolic resin. In fact, using different carbon sources can form carbon framework structures with different structures in the rare earth gel. For example, after carbonization with glucose, amorphous carbon can be generated with a higher density of surface active sites, while the conductivity is better after carbonization with phenolic resin. In the ethanol-water system, the volume fraction of ethanol is 10% - 70%, and the mesoporous template agent includes one of F127 and CTAB.

[0040] In some embodiments, the rare earth nitrate used in performing step S1 includes one of yttrium nitrate and gadolinium nitrate. In fact, after carbonization of yttrium nitrate and gadolinium nitrate, Y2O3 and Gd2O3 are respectively formed. Both of them are rare earth oxides with a cubic crystal system, having high thermal stability and chemical inertness. They can not only serve as anchor sites to fix transition metal particles, but also avoid blocking active sites due to excessive adsorption of carbon sources, and can act together with the carbon skeleton to construct a highly stable and highly active carbonized carrier.

[0041] In some embodiments, the mass ratio of the carbon source to the rare earth nitrate in performing step S1 is 1:(0.1 - 1). In fact, when the amount of the rare earth nitrate used is relatively large, cross-linked rare earth nanowires can be formed within the carbon skeleton, and the structural properties of the carbonized carrier can be effectively improved by intertwining with the carbon skeleton. When the amount of the rare earth nitrate used is relatively small, rare earth oxides are embedded in the carbon skeleton in the form of isolated nanoclusters, which can provide sufficient metal anchor sites and avoid blocking mesoporous channels.

[0042] In some embodiments, after mixing the carbon source, the rare earth nitrate and the mesoporous templating agent in performing step S1, a rare earth gel is prepared by hydrothermal reaction at 160 °C - 200 °C for 12 h - 24 h. In fact, in an ethanol-water mixed solvent, the mesoporous templating agent undergoes hydrophilic-hydrophobic segment rearrangement at high temperature to form a micelle or liquid crystal phase template. Carbon source molecules are adsorbed on the surface of the templating agent micelles, and at the same time, rare earth ions coordinate with the carbon source to form a ternary composite system.

[0043] In some embodiments, when performing the carbonization treatment in step S2, the inert gas atmosphere used includes at least one of argon and nitrogen. In addition, the rare earth gel is carbonized at 700 °C - 900 °C for 1 h - 2 h. In fact, by carbonizing the above ternary composite system, the rare earth gel is transformed into a carbonized carrier with high conductivity, high specific surface area and strong interfacial bonding, which can accurately control the graphitization degree of the carbon skeleton, the rare earth dispersion state and the mesoporous structure.

[0044] In some embodiments, when performing step S3, the carbonized carrier is used as the working electrode, Ag / AgCl is used as the reference electrode, and a pulsed voltage of -0.5 V to 1.2 V is applied. In some embodiments, when performing electrochemical deposition in step S3, the electrolyte solution contains 2 mol / L - 5 mol / L of potassium chloride, and also contains at least one of iron nitrate, iron sulfate, and iron chloride, and at least one of cobalt sulfate, cobalt nitrate, and cobalt chloride. In addition, the molar ratio of iron ions to cobalt ions in the electrolyte solution is (1 - 3):1.

[0045] In some embodiments, when performing step S4, in a mixed atmosphere of ethylene and hydrogen and at an environment temperature of 550°C - 650°C, after depositing on the surface of the composite support for 5 min - 10 min, reduction is carried out in a hydrogen atmosphere to obtain the rare earth catalyst.

[0046] In some embodiments, when performing step S4, the volume ratio of ethylene to hydrogen in the mixed atmosphere is 1:(3 - 5), and the pressure of the mixed atmosphere is 0.1 Mpa - 0.5 Mpa. In addition, after depositing on the surface of the composite support for 5 min - 10 min, heat preservation is carried out for 1 h - 2 h in an inert gas atmosphere at 800°C - 850°C.

[0047] Example 1

[0048] This Example 1 provides a preparation method of a rare earth catalyst, including the following steps:

[0049] S1. Glucose and yttrium nitrate are mixed at a mass ratio of 1:0.5, then added to an ethanol - water system (the volume fraction of ethanol in the system is 50%) at a solid - liquid ratio of 0.08 g / mL for mixing and dissolving. After adding mesoporous template agent F127 with a mass fraction of 20% of glucose and stirring for mixing, hydrothermal reaction is carried out at 180°C for 18 h, and then rare earth gel is separated;

[0050] S2. The rare earth gel is placed in a tube furnace. After gas replacement with argon, the tube furnace is heated to 800°C at a rate of 10°C / min and then calcined for h, and then cooled to room temperature with the furnace to obtain a carbonized support;

[0051] S3. Using the carbonized support as the working electrode, Ag / AgCl as the reference electrode, and 3 mol / L potassium chloride as the electrolyte (which also dissolves 2 mol / L ferric chloride and 1 mol / L cobalt chloride), electrochemical deposition is carried out at a pulsed voltage of - 0.5 V - 1.2 V for 1 h to obtain a composite support;

[0052] S4. The composite support is transferred to the furnace chamber of the tube furnace, and a mixed atmosphere of 0.3 Mpa (the volume ratio of ethylene to hydrogen is 1:4) is introduced. After depositing on the surface at 600°C for 8 min, heat preservation is carried out for 2 h in an argon atmosphere at 800°C, and then reduction is carried out in a hydrogen atmosphere to obtain the rare earth catalyst.

[0053] Example 2

[0054] This Example 2 provides a preparation method of a rare earth catalyst. The difference from Example 1 is that in step S1, phenolic resin is used as the carbon source to replace glucose, and the mass ratio of phenolic resin to yttrium nitrate is 1:0.5.

[0055] Example 3

[0056] Example 3 provided a preparation method of a rare earth catalyst. The difference from Example 1 was that in step S3, a constant DC voltage of 1.0 V was used for electrochemical deposition for 1 h.

[0057] Comparative Example 1

[0058] Comparative Example 1 provided a preparation method of a rare earth catalyst. The difference from Example 1 was that in step S3, the carbonized carrier was added into a mixed aqueous solution dissolving ferric chloride and cobalt chloride (the molar ratio of ferric ions to cobalt ions was 2:1, and the mass ratio of the carbonized carrier to ferric ions was 1:0.2) at a solid-liquid ratio of 0.05 g / mL for impregnation and then sintered to obtain a composite carrier.

[0059] Comparative Example 2

[0060] Comparative Example 2 provided a preparation method of a rare earth catalyst. The difference from Example 1 was that in step S3, 3 mol / L potassium chloride was used as the electrolyte, and 2 mol / L ferric chloride was dissolved in the electrolyte.

[0061] Comparative Example 3

[0062] Comparative Example 3 provided a preparation method of a rare earth catalyst. The difference from Example 1 was that in step S3, 3 mol / L potassium chloride was used as the electrolyte, and 2 mol / L ferric chloride and 1 mol / L nickel chloride were dissolved in the electrolyte.

[0063] Comparative Example 4

[0064] Comparative Example 4 provided a preparation method of a rare earth catalyst. The difference from Example 1 was that in step S3, 3 mol / L potassium chloride was used as the electrolyte, and 2 mol / L ferric chloride and 1 mol / L zinc chloride were dissolved in the electrolyte.

[0065] Performance Detection

[0066] The rare earth catalysts prepared in Examples 1 to 3 and Comparative Examples 1 to 4 were used for the synthesis of array-type carbon nanotubes, including: 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 with a flow rate of 500 sccm for reaction for 30 min, and then detecting the reaction yield as shown in Table 1 below; the rare earth catalysts prepared in Example 1 and Comparative Examples 1 to 2 were recycled for 100 h, and then the retention rate of the reaction yield compared with the first time was detected as shown in Table 1 below.

[0067] Table 1 Catalytic Performance and Long-Term Use Performance of Rare Earth Catalysts

[0068] Reaction yield / % Retention rate after 100 h / % Example 1 54.82 97.62 Example 2 51.39 98.13 Example 3 45.22 94.67 Comparative Example 1 34.58 72.18 Comparative Example 2 21.87 73.52 Comparative Example 3 36.75 83.41 Comparative Example 4 33.68 79.52

[0069] As can be seen from Table 1, the rare earth catalyst prepared in Example 1 of the present invention has good catalytic activity in the preparation of array-type carbon nanotubes, can effectively adjust the orientation of carbon nanotubes, and can maintain good catalytic activity during long-term use. In Example 2, using phenolic resin as the carbon source helps to improve the structural stability of the catalyst, thereby increasing the retention rate of catalytic performance. In Example 3, using a DC power supply for deposition has a negative impact on the loading amount and structural stability of the active metal on the catalyst surface. In Comparative Example 1, the loading amount and stability of the catalyst prepared by the impregnation method show a large attenuation. In Comparative Example 2, the electrolyte containing only iron ions has a huge impact on the catalytic activity of the catalyst. As can be seen from Example 1 and Comparative Examples 3 to 4, loading iron-cobalt active components on the rare earth catalyst has a positive effect on the catalytic activity and structural stability of the catalyst.

[0070] Although the embodiments of the present invention have been described in detail above, it is obvious to those skilled in the art that various modifications and changes can be made to these embodiments. However, it should be understood that such modifications and changes are all within the scope and spirit of the present invention described in the claims. Moreover, the present invention described herein can have other embodiments and can be implemented or realized in various ways.

Claims

1. A preparation method of a rare earth catalyst, characterized in that, Including: Mix a carbon source and rare earth nitrate in an ethanol-water system, then add a mesoporous template agent, and obtain a rare earth gel after hydrothermal reaction; carbonize the rare earth gel in an inert gas atmosphere to obtain a carbonized support; use the carbonized support as a working electrode and perform electrochemical deposition in an electrolyte solution of iron ions and cobalt ions to obtain a composite support; deposit a carbon coating layer on the surface of the composite support and then reduce it in a hydrogen atmosphere to obtain a rare earth catalyst.

2. The preparation method according to claim 1, characterized in that, The carbon source includes at least one of glucose and phenolic resin; and / or, the rare earth nitrate includes one of yttrium nitrate and gadolinium nitrate; and / or, the mass ratio of the carbon source to the rare earth nitrate is 1:(0.1 - 1).

3. The preparation method according to claim 1, characterized in that, Perform hydrothermal reaction at 160°C - 200°C; and / or, obtain the rare earth gel by hydrothermal reaction for 12h - 24h; and / or, the gas in the inert gas atmosphere includes at least one of argon and nitrogen; and / or, carbonize the rare earth gel at 700°C - 900°C; and / or, perform carbonization treatment on the rare earth gel for 1h - 2h.

4. The preparation method according to claim 1, wherein Use the carbonized support as a working electrode and Ag / AgCl as a reference electrode; and / or, apply a pulsed voltage of -0.5V to 1.2V during electrochemical deposition; and / or, 2mol / L - 5mol / L of potassium chloride is dissolved in the electrolyte solution.

5. The preparation method according to claim 1, characterized in that, The molar ratio of iron ions to cobalt ions in the electrolyte solution is (1 - 3):1; and / or, at least one of iron nitrate, iron sulfate, and iron chloride is dissolved in the electrolyte solution; and / or, at least one of cobalt sulfate, cobalt nitrate, and cobalt chloride is dissolved in the electrolyte solution.

6. The preparation method according to claim 1, wherein, The volume fraction of ethanol in the ethanol-water system is 10% - 70%; and / or, the mesoporous template agent includes one of F127 and CTAB.

7. The preparation method according to claim 1, characterized in that, In a mixed atmosphere of ethylene and hydrogen and an environment of 550°C - 650°C, after depositing on the surface of the composite support for 5min - 10min, perform reduction in a hydrogen atmosphere to obtain a rare earth catalyst.

8. The preparation method according to claim 1, wherein The volume ratio of ethylene to hydrogen in the mixed atmosphere is 1:(3 - 5); and / or, the air pressure of the mixed atmosphere is 0.1Mpa - 0.5Mpa; and / or, after depositing on the surface of the composite support for 5min - 10min, keep it warm for 1h - 2h in an inert gas atmosphere of 800°C - 850°C.

9. A rare earth catalyst prepared by the preparation method according to any one of claims 1 to 8.

10. A carbon nanotube prepared by using the rare earth catalyst according to claim 9.