A catalyst for synthesizing discrete distribution carbon nanotubes, a preparation method thereof and an application thereof

Abstract

The present invention discloses a catalyst for synthesizing discrete distribution carbon nanotubes, its preparation method and application, belonging to the technical field of catalysts. The catalyst of the present invention consists of three parts: an active component, a carbon nanotube promoter, and a carrier. The carrier is MgO, the metal active component is metal elements such as Fe, Co, Ni, Cr, etc., the growth promoter of carbon nanotubes is Mo, the molar ratio of the metal active component to Mo is (1-5):1, and the content of the active metal and the promoter is 5-30 wt% of the mass of the carrier. By making a gel of the active metal and the carrier through the sol-gel method and foaming it into a porous material in an oven, the material is calcined in an inert atmosphere at 700-900 °C and then calcined in air at 400-600 °C to obtain a catalyst for synthesizing carbon nanotubes. This preparation method has the advantages of simple preparation process and convenient operation. The obtained catalyst has a high yield of synthesized carbon nanotubes and a uniform particle size distribution of the carbon nanotubes.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalysts, and particularly relates to a catalyst for synthesizing discretely distributed carbon nanotubes, a preparation method thereof, and an application thereof. Background Art

[0002] Carbon nanotubes are a supermaterial with great potential and are ideal core basic materials for constructing future ultra-strong structures and carbon-based semiconductor devices. Assembling carbon nanotubes into macroscopic bodies (such as fibers, films, and foams, etc.) is one of the important ways to realize the large-scale application of carbon nanotubes. Carbon nanotubes are a kind of nanoscale tubular carbon material with graphite crystallization discovered by Iijima, an electron microscopy expert from NEC Corporation of Japan, in a high-resolution transmission electron microscope in 1991. Since the discovery of carbon nanotubes for 30 years, there are dozens of methods for preparing carbon nanotubes represented by arc discharge method, laser evaporation method, and chemical vapor deposition method (CVD method). Among them, the CVD method is widely used because of its lower reaction temperature, easy control of parameters, and ability to prepare a large number of discrete and high-quality carbon nanotubes. The key to preparing carbon nanotubes by the CVD method lies in the preparation of the catalyst. Preparing a metal catalyst with controllable particle size is the key to preparing discretely distributed and high-quality carbon nanotubes. Therefore, we need to optimize the selection of the catalyst for growing carbon nanotubes. Currently, the combination of the catalytic method and the CVD method is considered to be the most promising preparation method for the industrialization of carbon nanotubes.

[0003] The catalytic method for preparing carbon nanotubes has the advantages of high yield, low cost, and easy control of process parameters. However, the composition, preparation conditions, and activation conditions of the catalyst in the catalytic method are important factors affecting the morphology and yield of carbon nanotubes. The elements commonly used for growing carbon nanotubes are one or several of metal elements such as Fe, Co, Ni, Cr, etc. In order to obtain the optimal composition elements and preparation conditions for growing carbon nanotubes, it is necessary to study the preparation components and conditions of the catalyst to promote the industrial popularization of carbon nanotubes. With the development of carbon nanotube preparation technology, various methods have been applied to prepare catalysts for growing carbon nanotubes. Since the growth of carbon nanotubes is relatively demanding on the physical and chemical properties of the catalyst, the diameter of the prepared carbon nanotubes and the width of its distribution range are directly related to the composition and particle size of the metal catalyst. For example, the particle size, bulk density, particle size distribution, and particle uniformity of the metal catalyst all affect the growth of carbon nanotubes; and the catalyst prepared during production or experiments often has a low first-pass yield and needs to be pulverized and sieved twice or multiple times. The prepared metal catalyst is not only complex and difficult to control in process, but also causes waste of the catalyst. For example, a large amount of catalysts with too large mesh numbers do not meet the production conditions after pulverization, resulting in waste of the catalyst and high costs. How to prepare a metal catalyst with uniform particle size, narrow distribution range, and high mechanical strength is a difficult problem in the preparation of metal catalysts and is also the technical key to preparing carbon nanotubes with high crystallinity and uniform diameter using a fluidized bed. Summary of the Invention

[0004] To solve the technical problems in the background art, the present invention provides a catalyst for synthesizing discretely distributed carbon nanotubes, which has a simple manufacturing process, good thermal stability, and uniform particle size, as well as a preparation method and an application thereof. Compared with the traditional method, this method has a lower cost and is easy to operate, and the obtained catalyst can be used to synthesize carbon nanotubes with high crystallinity and uniform tube diameters.

[0005] To solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0006] A preparation method of a catalyst for synthesizing discretely distributed carbon nanotubes mainly includes the following steps:

[0007] (1) Dissolve the salt of active metal A and the salt of carbon nanotube promoter B into a solvent, and the molar ratio of the metal ions of active metal A to carbon nanotube promoter B is (1-5):1;

[0008] (2) Add the salt of carrier metal C, and the total mass of active metal A and carbon nanotube promoter B is 5-30 wt% of the total mass of the carrier;

[0009] (3) Add metal ion complexing agent D, stir in a water bath to form a homogeneous solution until a sol is formed, and finally a gel is formed;

[0010] (4) Place the gel obtained in step (3) at 50-150 °C to foam and form a porous material;

[0011] (5) Grind the porous material obtained in step (4) into powder, and calcine it in an inert atmosphere at 700-1000 °C for 1-10 h;

[0012] (6) After the calcination is completed, cool it to room temperature, and then calcine it in air by heating to 300-600 °C for 1-10 h to obtain a catalyst for synthesizing discretely distributed carbon nanotubes.

[0013] Further, in step (1), the active metal A is one or a combination of two or more of Fe, Co, Ni, and Cr; the carbon nanotube promoter B is Mo; and the solvent is one or a combination of two or more of water, methanol, ethanol, and acetonitrile.

[0014] Further, in step (1), the salt of active metal A is one or a combination of two or more of the sulfate, nitrate, and chloride salts of active metal A; the salt of carbon nanotube promoter B is one or a combination of two of ammonium molybdate and ammonium heptamolybdate.

[0015] Further, in step (2), the carrier metal C is one or a combination of two or more of Mg, Al, and Si.

[0016] Further, the metal ion complexing agent D in step (3) is one or a combination of two or more of citric acid, oxalic acid, and tartaric acid.

[0017] Further, the molar amount of the metal ion complexing agent D in step (3) is ≥ the total molar amount of the active metal A, the carbon nanotube promoter B, and the carrier metal C, and the amount of the complexing agent added should meet the function of completely dispersing metal ions.

[0018] Further, in step (5), the calcination temperature is 700 - 900 °C, the calcination time is 1 - 4 h, and the inert atmosphere is one or a combination of two or more of argon, helium, nitrogen, etc.

[0019] Further, in step (6), the calcination temperature is 400 - 600 °C, the calcination time is 2 - 6 h, and calcination at different temperatures can affect the particle size of the metal active component in the catalyst and the specific surface area of the carrier.

[0020] On the other hand, the present invention provides a catalyst for synthesizing discrete carbon nanotubes prepared by the above preparation method.

[0021] Further, the particle size of the catalyst is 5 - 100 nm.

[0022] Further, the carrier of the catalyst is one or a combination of two or more of MgO, Al2O3, and SiO2.

[0023] The present invention also provides the application of the above catalyst in the preparation of carbon nanotubes.

[0024] Further, the carbon nanotubes are prepared by chemical vapor deposition.

[0025] The beneficial effects of the present invention compared with the prior art are as follows:

[0026] 1. The present invention uses the method of synthesizing metal active components + carbon nanotube promoters + carriers by sol - gel method as a catalyst, which can be used for synthesizing carbon nanotubes by chemical vapor deposition (CVD). Compared with traditional methods, this method has lower costs and is easier to operate. Only by setting a set of procedures can the preparation of the catalyst and the growth of a large number of high - quality, discrete carbon nanotubes on the catalyst be achieved on a single CVD furnace.

[0027] 2. In the present invention, a complexing agent is added to the metal salt solution as a metal ion dispersant, enabling the metal ions to be well-dispersed on the carrier, preventing the active metal from agglomerating and resulting in uneven active metal particles. This avoids the problems of messy, intertwined, and difficult-to-disperse carbon nanotube samples and a large amount of amorphous carbon and other impurities in the subsequent growth of carbon nanotubes using this catalyst. At the same time, the complexing agent can also maintain the uniformity of metal particles. When loaded on the carrier, it can reduce the surface tension, promote a more uniform distribution of metal elements, and prevent the metal particles from aggregating and depositing on the surface of the carrier. This type of complexing agent can be easily removed during the growth of carbon nanotubes without damaging the surface properties of the catalyst carrier, and still maintain the dispersed state.

[0028] 3. The preparation method of the catalyst of the present invention is simple, and the calcination process is simple, simplifying the process flow for preparing carbon nanotubes. Carbon nanotubes with uniform diameters are obtained. The diameters of the carbon nanotubes are mainly distributed in the range of 7 - 9 nm, with a long tube diameter and a significantly increased yield. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] To more clearly illustrate the embodiments of the present invention, the accompanying drawings related to the embodiments will be briefly introduced below.

[0030] Figure 1 It is the XRD pattern of the catalyst of Example 4 of the present invention.

[0031] Figure 2 It is the TEM image (a) and particle size distribution diagram (b) of the catalyst of Example 4 of the present invention.

[0032] Figure 3 It is the SEM image (a) and tube diameter distribution diagram (c) of the carbon nanotubes prepared from the catalyst of Example 1 of the present invention, and the SEM image (b) and tube diameter distribution diagram (d) of the carbon nanotubes prepared from the catalyst of Comparative Example 1.

[0033] Figure 4 It is the SEM image (a) and TEM images (b, c) of the carbon nanotubes prepared from the catalyst of Example 4 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0034] The present invention will be described in detail below in conjunction with the embodiments. However, the embodiments of the present invention are not limited thereto. Obviously, the embodiments described below are only part of the embodiments of the present invention. For those skilled in the art, without creative efforts, obtaining other similar embodiments will fall within the protection scope of the present invention.

[0035] Example 1:

[0036] A preparation method of a catalyst for synthesizing discrete distribution carbon nanotubes. The catalyst consists of three parts: an active component, a carbon nanotube promoter, and a carrier. The preparation steps are as follows:

[0037] Step 1: According to the total mass of Fe and Mo being 20 wt% of the total mass of the carrier and the molar ratio of Fe / Mo being 5:1, dissolve 1.0746 g of Fe(NO3)3·9H2O and 0.0939 g of ammonium molybdate in water;

[0038] Step 2: Add the fixed carrier metal salt Mg(NO3)2·6H2O with a mass of 6.4100 g;

[0039] Step 3: Add the metal ion complexing agent citric acid. Weigh 22.7357 g of citric acid, stir in a water bath to form a homogeneous solution, then form a sol, and finally form a gel;

[0040] Step 4: Put the gel obtained in Step 3 into an oven at 100 °C for foaming and forming to make a porous material;

[0041] Step 5: Grind the porous material in Step 4 into powder and calcine it in an inert atmosphere at 800 °C for 2 h;

[0042] Step 6: After the calcination is completed and cooled to room temperature, then in the air, heat it up to 500 °C and calcine for 4 h to obtain a catalyst for growing discrete distribution carbon nanotubes.

[0043] Step 7: The catalyst obtained in Step 6 is used for the carbon nanotube yield test during growth:

[0044] The growth of carbon nanotubes using the catalyst is carried out in a reaction tube with an inner diameter of 48 mm. Weigh 0.5 g of the catalyst and evenly spread it on a quartz boat. Purge it with Ar at a flow rate of 80 mL min -1 for 0.5 h to exhaust the air in the reaction tube, then heat it up to 500 °C at a rate of 5 °C min -1 and reduce the catalyst with pure hydrogen at a flow rate of 80 mL min -1 for 2 h; then heat it up to 800 °C. The reaction gas is a mixture of CH4 and H2 (V CH4 :V H2 = 40:80), and the total flow rate is 120 mL min -1 , and react for 2 h to grow carbon nanotubes. Then naturally cool it to room temperature in an Ar atmosphere and calculate the yield of carbon nanotubes in the reaction. The yield is calculated in g·g cat -1 ·h -1 . The calculation formula is as follows: m before is the mass of the catalyst before growing carbon nanotubes, m after$m$ is the total mass of the catalyst and carbon nanotubes after growing carbon nanotubes, $t$ is the reaction time for carbon nanotube growth. The carbon nanotube yield synthesized by the catalyst prepared in Example 1 is shown in Table 1.

[0045] Example 2:

[0046] A preparation method of a catalyst for synthesizing discretely distributed carbon nanotubes. The catalyst consists of three parts: an active component, a carbon nanotube promoter, and a support. The preparation steps are as follows:

[0047] Step 1: According to the total mass of Fe and Mo being 20 wt% of the total mass of the support, and the molar ratio of Fe / Mo being 3:1, dissolve 0.9187 g of Fe(NO3)3·9H2O and 0.1338 g of ammonium molybdate in water;

[0048] Step 2: The same as in Example 1;

[0049] Step 3: Add the metal ion complexing agent citric acid. Weigh 22.3876 g of citric acid, stir in a water bath to form a homogeneous solution, then form a sol, and finally form a gel;

[0050] Step 4: The gel obtained in Step 3 is placed in an oven at 100 °C for foaming and forming to make a porous material;

[0051] Step 5: Grind the porous material in Step 4 into powder and calcine it in an inert atmosphere at 800 °C for 2 h;

[0052] Step 6: After the calcination is completed and cooled to room temperature, then in air, heat it up to 500 °C and calcine it for 4 h to obtain a catalyst for growing discretely distributed carbon nanotubes.

[0053] Step 7: The catalyst is used for testing the carbon nanotube growth yield in the same way as in Example 1. The carbon nanotube growth yield of the catalyst is shown in Table 1.

[0054] Example 3:

[0055] A preparation method of a catalyst for synthesizing discretely distributed carbon nanotubes. The catalyst consists of three parts: an active component, a carbon nanotube promoter, and a support. The preparation steps are as follows:

[0056] Step 1: According to the total mass of Fe and Mo being 20 wt% of the total mass of the support, and the molar ratio of Fe / Mo being 1:1, dissolve 0.5333 g of Fe(NO3)3·9H2O and 0.2330 g of ammonium molybdate in water;

[0057] Step 2: The same as in Example 1;

[0058] Step 3: Add the metal ion complexing agent citric acid. Weigh 21.5290 g of citric acid, stir it in a water bath to make a homogeneous solution, then form a sol, and finally form a gel;

[0059] Step 4: Put the gel obtained in Step 3 into an oven at 100 °C for foaming and forming to make a porous material;

[0060] Step 5: Grind the porous material in Step 4 into powder and calcine it in an inert atmosphere at 800 °C for 2 h;

[0061] Step 6: After the calcination is completed and cooled to room temperature, then in air, heat it up to 500 °C and calcine it for 4 h to obtain a catalyst for growing discretely distributed carbon nanotubes.

[0062] Step 7: The yield test of the catalyst for growing carbon nanotubes is the same as that in Example 1. The yield of the catalyst for growing carbon nanotubes is shown in Table 1.

[0063] Example 4:

[0064] A preparation method of a catalyst for synthesizing discretely distributed carbon nanotubes. This catalyst consists of three parts: an active component, a carbon nanotube promoter, and a carrier. The preparation steps are as follows:

[0065] Step 1: According to the total mass of Fe and Mo being 5 wt% of the total mass of the carrier and the molar ratio of Fe / Mo being 5:1, dissolve 0.2687 g of Fe(NO3)3·9H2O and 0.0235 g of ammonium molybdate in water;

[0066] Step 2: The same as in Example 1;

[0067] Step 3: Add the metal ion complexing agent citric acid. Weigh 20.1065 g of citric acid, stir it in a water bath to make a homogeneous solution, then form a sol, and finally form a gel;

[0068] Step 4: Put the gel obtained in Step 3 into an oven at 100 °C for foaming and forming to make a porous material;

[0069] Step 5: Grind the porous material in Step 4 into powder and calcine it in an inert atmosphere at 800 °C for 2 h;

[0070] Step 6: After the calcination is completed and cooled to room temperature, then in air, heat it up to 500 °C and calcine it for 4 h to obtain a catalyst for growing discretely distributed carbon nanotubes.

[0071] Step 7: The yield test of the catalyst for growing carbon nanotubes is the same as that in Example 1. The yield of the catalyst for growing carbon nanotubes is shown in Table 1.

[0072] The catalyst prepared in Example 4 is an Fe-Mo / MgO catalyst (such asFigure 1 As shown in [Figure X], the TEM image and particle size distribution of the catalyst are as follows Figure 2 As shown in [Figure X], it can be seen that the obtained catalyst has a relatively uniform particle size distribution. When used for the growth of carbon nanotubes at 800 °C, the SEM image and TEM image of the obtained carbon nanotubes are as follows Figure 4 As shown in [Figure X], it can be seen that high-quality discrete carbon nanotubes are obtained.

[0073] Example 5:

[0074] A preparation method of a catalyst for synthesizing discrete carbon nanotubes. The catalyst consists of an active component, a carbon nanotube promoter, and a support. The preparation steps are as follows:

[0075] Step 1: According to the total mass of Fe and Mo being 10 wt% of the total mass of the support, and the molar ratio of Fe / Mo being 5:1, dissolve 0.5373 g of Fe(NO3)3·9H2O and 0.047 g of ammonium molybdate in water;

[0076] Step 2: The same as in Example 1;

[0077] Step 3: Add the metal ion complexing agent citric acid. Weigh 20.9829 g of citric acid, stir in a water bath to form a homogeneous solution, then form a sol, and finally form a gel;

[0078] Step 4: Put the gel obtained in Step 3 into an oven at 100 °C, foam and mold it to form a porous material;

[0079] Step 5: Grind the porous material in Step 4 into powder and calcine it in an inert atmosphere at 800 °C for 2 h;

[0080] Step 6: After the calcination is completed and cooled to room temperature, then in air, heat it to 500 °C and calcine it for 4 h to obtain a catalyst for growing discrete carbon nanotubes.

[0081] Step 7: The test of the carbon nanotube production using the catalyst is the same as in Example 1. The carbon nanotube production of the catalyst is shown in Table 1.

[0082] Example 6:

[0083] A preparation method of a catalyst for synthesizing discrete carbon nanotubes. The catalyst consists of an active component, a carbon nanotube promoter, and a support. The preparation steps are as follows:

[0084] Step 1: According to the total mass of Fe and Mo being 30 wt% of the total mass of the support, and the molar ratio of Fe / Mo being 5:1, dissolve 1.6119 g of Fe(NO3)3·9H2O and 0.1409 g of ammonium molybdate in water;

[0085] Step 2: The same as in Example 1;

[0086] Step 3: Add the metal ion complexing agent citric acid. Weigh 24.4886 g of citric acid, stir it in a water bath to form a homogeneous solution, then form a sol, and finally form a gel.

[0087] Step 4: Put the gel obtained in Step 3 into an oven at 100 °C for foaming and forming to make a porous material.

[0088] Step 5: Grind the porous material in Step 4 into powder and calcine it in an inert atmosphere at 800 °C for 2 h.

[0089] Step 6: After the calcination is completed and cooled to room temperature, then heat it up to 500 °C in the air and calcine it for 4 h to obtain a catalyst for growing discretely distributed carbon nanotubes.

[0090] Step 7: The yield test of the catalyst for growing carbon nanotubes is the same as that in Example 1. The yield of carbon nanotubes grown by the catalyst is shown in Table 1.

[0091] Comparative Example 1:

[0092] A preparation method of a catalyst for synthesizing carbon nanotubes. The catalyst is composed of an active component, a carbon nanotube promoter, and a carrier. The preparation steps are as follows:

[0093] Step 1: According to the total mass of Fe and Mo being 20 wt% of the total mass of the carrier and the molar ratio of Fe / Mo being 5:1, dissolve 1.0746 g of Fe(NO3)3·9H2O and 0.0939 g of ammonium molybdate in water.

[0094] Step 2: The same as in Example 1.

[0095] Step 3: Add the metal ion complexing agent citric acid. Weigh 22.7357 g of citric acid, stir it in a water bath to form a homogeneous solution, then form a sol, and finally form a gel.

[0096] Step 4: Put the gel obtained in Step 3 into an oven at 100 °C for foaming and forming to make a porous material.

[0097] Step 5: Grind the porous material in Step 4 into powder and calcine it in an air atmosphere at 500 °C for 4 h to obtain a catalyst for growing carbon nanotubes.

[0098] Step 6: The yield test of the catalyst for growing carbon nanotubes is the same as that in Example 1. The yield of carbon nanotubes grown by the catalyst is shown in Table 1.

[0099] Table 1: Comparison of the yields of carbon nanotubes synthesized by the catalysts of Examples 1 - 6 and Comparative Example 1

[0100]

[0101]

[0102] As can be seen from Table 1, the catalyst calcined with inert gas has a high activity for carbon nanotube growth. The catalysts in Examples 1-6 all show good carbon nanotube growth activity. Among them, the catalyst in Example 6 has the highest carbon nanotube production. 0.5 g of the catalyst reacts with methane gas with a content of 33.33% at 800 °C for 2 h, and the carbon nanotube production is 4.68 g·g cat -1 ·h -1 。

[0103] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a catalyst for synthesizing discrete distribution carbon nanotubes, characterized in that, It includes the following steps: (1) Dissolve the salt of active metal A and the salt of carbon nanotube promoter B into a solvent, and the molar ratio of the metal ions of active metal A to carbon nanotube promoter B is (1-5):1; (2) Add the salt of support metal C, and the total mass of active metal A and carbon nanotube promoter B is 5-30 wt% of the total mass of the support; (3) Add metal ion complexing agent D, stir in a water bath to form a homogeneous solution until a sol is formed and finally a gel is formed; (4) Place the gel obtained in step (3) under the condition of 50-150 °C for foaming and molding to obtain a porous material; (5) Grind the porous material obtained in step (4) into powder and calcine it in an inert atmosphere at 700-1000 °C for 1-10 h; (6) After the calcination is completed, cool it to room temperature, and then heat it to 300-600 °C in air for calcination for 1-10 h to obtain a catalyst for synthesizing discretely distributed carbon nanotubes; In step (1), the active metal A is Fe; the carbon nanotube promoter B is Mo; the support metal C is Mg; the salt of the active metal A in step (1) is one or a combination of two or more of the sulfates, nitrates, and chlorides of the active metal A; the salt of the carbon nanotube promoter B is one or a combination of two of ammonium molybdate and ammonium heptamolybdate; the salt of the support metal C is Mg(NO3)2·6H2O; In step (3), the metal ion complexing agent D is one or a combination of two or more of citric acid, oxalic acid, and tartaric acid; the molar amount of the metal ion complexing agent D ≥ the total molar amount of active metal A, carbon nanotube promoter B, and support metal C.

2. The preparation method according to claim 1, characterized in that, In step (1), the solvent is one or a combination of two or more of water, methanol, ethanol, and acetonitrile.

3. The preparation method according to claim 1, wherein In step (5), the calcination temperature is 700-900 °C, the calcination time is 1-4 h, and the inert atmosphere is one or a combination of two or more of argon, helium, and nitrogen.

4. The preparation method according to claim 1, characterized in that, In step (6), the calcination temperature is 400-600 °C, and the calcination time is 2-6 h.

5. The catalyst prepared by the preparation method according to any one of claims 1-4.

6. The catalyst according to claim 5, characterized in that, The particle size of the catalyst is 5-100 nm.

7. The application of the catalyst according to claim 5 or 6 in the preparation of carbon nanotubes.

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