A method for encapsulating catalyst pretreatment and a method for preparing small-diameter single-walled carbon nanotubes

Abstract

A pre-treatment method of encapsulated catalyst, the encapsulated catalyst is placed in a tube furnace, first heat treatment at 400-900 DEG C in air atmosphere for 1-3h, after cooling to room temperature, in the atmosphere of mixed gas of inert gas and carbon dioxide gas, heat to 400-900 DEG C, heat treatment for 1-3h after cooling to room temperature, obtain the required catalyst;The catalyst is placed in a tube furnace, heated to 660-900 DEG C in inert gas, the mixed gas containing carbon source and reducing gas is introduced into the reactor, after sufficient reaction, stop the reaction and continue to cool in the protective atmosphere, obtain small tube diameter single wall carbon nanotube.The present application realizes the structure modification of encapsulated catalyst by two-step heat treatment method, induces its structure to break in the process of catalytic carbon nanotube and exposes the internal catalytic metal, reduces the aggregation of active metal particles;By adjusting the pre-treatment process and CVD process, mutual coordination is realized to control the content and tube diameter distribution of single wall carbon nanotube, to realize the controllable growth of carbon nanotube.

Description

Technical Field

[0001] This invention belongs to the field of new material preparation, specifically relating to an encapsulated catalyst pretreatment method based on air and CO2 and a method for preparing small-diameter single-walled carbon nanotubes. Background Technology

[0002] Carbon nanotubes, as one-dimensional nanomaterials within the carbon nanomaterial family, possess numerous excellent mechanical, thermal, electrical, and chemical properties, demonstrating broad application prospects in various fields such as semiconductors, rubber, and conductive pastes. Based on the number of walls, carbon nanotubes can be classified into multi-walled carbon nanotubes and single-walled carbon nanotubes. Compared to multi-walled carbon nanotubes, single-walled carbon nanotubes exhibit superior electrical and thermal conductivity, which is beneficial for further performance improvements. Based on efforts over the past two decades, it has been recognized that small-diameter single-walled carbon nanotubes place extremely stringent requirements on catalyst size. However, the liquefaction of metal nanoparticles at high temperatures leads to the aggregation and coarsening of adjacent nanoparticles, posing a significant challenge to the preparation of small-diameter single-walled carbon nanotubes.

[0003] Chemical vapor deposition (CVD) synthesis based on solid-encapsulated catalysts has become an attractive method due to its low cost, good controllability, and potential for large-scale synthesis with controlled structures. Although many core-shell, multilayer, and pinning techniques have been developed to inhibit catalyst agglomeration, metal nanoparticles reduced on the support surface still tend to agglomerate under high-temperature conditions. Therefore, the synthesis of small-diameter single-walled carbon nanotubes remains challenging.

[0004] Therefore, in order to limit the aggregation of metal nanoparticles during CVD, it is urgent to design a strategy that can destroy the encapsulated catalyst structure to directly expose the internal catalytic metal for the growth of single-walled carbon nanotubes, thus avoiding their coarsening caused by precipitation on the support surface. Summary of the Invention

[0005] The technical solution of this invention is: One objective of this invention is to provide a pretreatment method for an encapsulated catalyst based on air and CO2, the encapsulated catalyst pretreatment method comprising the following steps: (1) Fe-Mg-Al hydrotalcite is placed in a tube furnace and a mixture of inert gas and air is introduced throughout the process. The temperature is raised to 400-900℃, held for 1-3 hours, and then naturally cooled to room temperature to obtain the precursor. (2) Place the precursor in a tube furnace and pass a mixture of inert gas and CO2 gas through it throughout the process. Heat the furnace to 400-900℃, keep it at that temperature for 1-3 hours, and then let it cool naturally to room temperature to obtain the desired catalyst.

[0006] In step (1), the molar ratio of the catalytic metal in Fe-Mg-Al hydrotalcite to all metals in Fe-Mg-Al hydrotalcite catalyst is 5%-40%; in the mixed gas in step (1), the air flow rate is 1-100%; in the mixed gas in step (2), the CO2 flow rate is 1-100%.

[0007] The preparation method described in this invention uses Fe-Mg-Al hydrotalcite as a catalyst. This encapsulated catalyst has the following advantages: 1. The perfect thin-film structure provides ample open surface for catalyst modification, reduces the energy required for catalyst structure destruction, and provides the possibility for the release of internal catalytic metal; 2. The catalytic metal is encapsulated by inactive metals (Mg, Al, O), and the catalytic metal exists in atomic form inside the catalyst. Multiple heat treatments only modify the surface catalytic metal, while the internal catalytic metal still exists in a dispersed form; 3. The elemental composition is simple and easy to prepare. These advantages enable the grown single-walled carbon nanotubes to have a small diameter.

[0008] As a preferred technical solution of the present invention, the Fe molar ratio of Fe in the Fe-Mg-Al hydrotalcite in step (1) is 5%-40%, such as 5%, 10%, 15%, 20%, 25%, 30%, 35% or 40%, etc., but not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0009] Preferably, the Fe-Mg-Al hydrotalcite in step (1) is a plate-like hexagonal shape.

[0010] As a preferred technical solution of the present invention, in step (1), a mixture of inert gas and air is introduced throughout the process, with an air flow rate of 1-100%, such as 5%, 10%, 15%, 20%, 25%, 30%, 35% or 40%, etc. The temperature is raised to 400-900℃, such as 400℃, 450℃, 500℃, 550℃, 600℃, 650℃ or 700℃, etc. The temperature is maintained for 1-3 hours, such as 1 hour, 1.5 hours, 2 hours, 2.5 hours or 3 hours, etc., but not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0011] Preferably, the heating rate in step (1) is 10°C / min.

[0012] As a preferred technical solution of the present invention, in step (2), a mixture of inert gas and CO2 gas is introduced throughout the process, with an air flow rate of 1-100%, such as 6%, 16%, 26%, 36%, 46%, 56%, 66% or 76%, etc. The temperature is raised to 400-900℃, such as 500℃, 550℃, 600℃, 650℃, 700℃, 750℃ or 800℃, etc. The temperature is maintained for 1-3 hours, such as 1 hour, 1.5 hours, 2 hours, 2.5 hours or 3 hours, etc., but not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0013] The second objective of this invention is to provide a method for preparing small-diameter single-walled carbon nanotubes, which are prepared by the method described in the first objective and grown on the nanotubes by chemical vapor deposition.

[0014] This invention achieves control over the diameter and content of small-diameter single-walled carbon nanotubes by regulating the pretreatment process and the CVD process. By adjusting the temperature and CO2 ratio during the pretreatment, layered catalysts with different surface structures can be obtained. In the CVD process, the reaction temperature is adjusted, and the type and flow rate of the carbon source are controlled. The two processes work together to achieve control over the content and size of small-diameter single-walled carbon nanotubes.

[0015] As a preferred technical solution of the present invention, the conditions for chemical vapor deposition include: a deposition temperature of 660℃-900℃; and simultaneous introduction of hydrogen and carbon source to ensure the continuous flow of the carrier throughout the process.

[0016] Preferably, the carbon source gas includes any one or a combination of at least two of methane and ethylene.

[0017] Preferably, the carrier includes nitrogen and / or argon.

[0018] Preferably, the flow rate ratio of hydrogen, carbon source, and carrier is (0-200):(10-50):200, for example, 200:10:200, 200:20:200, or 200:30:200, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0019] Compared with existing technical solutions, the present invention has at least the following beneficial effects: (1) The catalyst obtained by the present invention through the encapsulated catalyst pretreatment method based on air and CO2 has a modified surface structure that facilitates the breakage of the catalyst structure during the growth of catalytic carbon nanotubes, resulting in the release of the encapsulated catalytic metal, thereby obtaining small-diameter single-walled carbon nanotubes.

[0020] (2) In the preparation method of small-diameter single-walled carbon nanotubes described in this invention, the diameter of the single-walled carbon nanotubes is mainly achieved through the combined use of the loading process and the CVD process. By adjusting the temperature, time and CO2 flow ratio of the two-step heat treatment, encapsulated catalysts with different surface modifications can be obtained. In the CVD process, the reaction temperature is adjusted and the type and flow rate of the carbon source are controlled. The two processes work together to achieve control of the content and size of small-diameter single-walled carbon nanotubes. Attached Figure Description

[0021] Figure 1 This is a scanning electron microscope image of the Fe-Mg-Al hydrotalcite encapsulated catalyst prepared in Example 1 of this invention; Figure 2 This is a scanning electron microscope image of the catalyst after pretreatment with air and CO2 based on the encapsulated catalyst of Example 1 of the present invention; Figure 3 This is a scanning electron microscope image of the small-diameter single-walled carbon nanotubes prepared in Example 1 of this invention; Figure 4 This is a Raman image of the small-diameter single-walled carbon nanotubes prepared in Example 1 of this invention; Figure 5 This is a Raman image of the small-diameter single-walled carbon nanotubes prepared in Example 2 of this invention; Figure 6 This is a Raman image of the small-diameter single-walled carbon nanotubes prepared in Example 3 of this invention; Figure 7 This is a scanning electron microscope image of the small-diameter single-walled carbon nanotubes prepared in Comparative Example 1 of this invention; Figure 8 This is a Raman image of the small-diameter single-walled carbon nanotubes prepared in Comparative Example 1 of this invention; Figure 9 This is a Raman image of the small-diameter single-walled carbon nanotubes prepared in Comparative Example 2 of this invention; Figure 10 This is a Raman image of the small-diameter single-walled carbon nanotubes prepared in Comparative Example 3 of this invention; Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and 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.

[0023] Example 1 A pretreatment method for an encapsulated catalyst based on air and CO2, the preparation method comprising the following steps: (1) 4.69 g aluminum nitrate, 6.41 g magnesium nitrate, 1.01 g ferric nitrate, 45.05 g urea, and 250 ml water were placed in a round-bottom flask and heated in a water bath at 100 °C for 9 h with stirring. Then, the mixture was heated at 95 °C for 14 h after standing. After cooling, the mixture was filtered and freeze-dried. The temperature was increased to 450 °C at 1 °C / min under an argon atmosphere and held for 1 h before being naturally cooled to room temperature to obtain the Fe-Mg-Al hydrotalcite encapsulated catalyst. The scanning electron microscope image of this support is shown below. Figure 1 As shown, its main structure is a uniform sheet-like structure; (2) Weigh 1g of the Fe-Mg-Al hydrotalcite encapsulated catalyst described in step (1), place it in a tube furnace, heat it to 500℃ at 10℃ / min, keep it at that temperature for 1h, and then cool it naturally to room temperature. Air is circulated throughout the process to obtain the precursor. (3) The precursor described in step (2) is placed in a tube furnace and heated to 600°C at a rate of 10°C / min. After holding at this temperature for 1 hour, it is naturally cooled to room temperature. A mixture of 50% nitrogen and 50% CO2 is passed through the furnace throughout the process to obtain the desired catalyst. The scanning electron microscope image of the catalyst is shown below. Figure 2 As shown, its main structure is still a sheet-like structure.

[0024] A method for preparing small-diameter single-walled carbon nanotubes involves placing the air- and CO2-encapsulated catalyst prepared in this embodiment into a tube furnace and heating it to 800°C at a heating rate of 10°C / min. Propylene and hydrogen are simultaneously introduced, and the reaction is carried out for 40 min. Afterward, the hydrogen and propylene are turned off, and the mixture is allowed to cool naturally to room temperature. Nitrogen gas is purged throughout the process, with a nitrogen, propylene, and hydrogen flow rate ratio of 200:30:200, resulting in small-diameter single-walled carbon nanotubes. Scanning electron microscopy images of the obtained small-diameter single-walled carbon nanotubes are shown below. Figure 3 As shown, Raman diagram Figure 4 As shown.

[0025] Example 2 1g of the catalyst pretreated with air and CO2 in Example 1 was weighed and placed in a tube furnace. The temperature was increased to 800°C at a rate of 10°C / min, while methane and hydrogen were simultaneously introduced. After reacting for 40 minutes, the hydrogen and methane were turned off, and the mixture was allowed to cool naturally to room temperature. Nitrogen gas was introduced throughout the process, with a flow rate ratio of nitrogen, methane, and hydrogen of 200:30:200, resulting in small-diameter single-walled carbon nanotubes. The Raman spectrum of the obtained small-diameter single-walled carbon nanotubes is shown below. Figure 5 As shown.

[0026] Example 3 1g of the catalyst pretreated with air and CO2 in Example 1 was weighed and placed in a tube furnace. The temperature was increased to 800°C at a rate of 10°C / min, while methane, propylene, and hydrogen were simultaneously introduced. After reacting for 40 minutes, the hydrogen, propylene, and methane were turned off, and the furnace was allowed to cool naturally to room temperature. Nitrogen gas was purged throughout the process, with a flow rate ratio of nitrogen, methane, propylene, and hydrogen of 200:15:15:200, resulting in small-diameter single-walled carbon nanotubes. The Raman spectrum of the obtained small-diameter single-walled carbon nanotubes is shown below. Figure 6 As shown.

[0027] Comparative Example 1 The Fe-Mg-Al hydrotalcite encapsulated catalyst from Example 1 was directly placed inside a tube furnace for the growth of single-walled carbon nanotubes, maintaining the growth process unchanged. Scanning electron micrographs of the obtained single-walled carbon nanotubes are shown below. Figure 7 As shown, Raman diagram Figure 8 As shown.

[0028] Comparative Example 2 The Fe-Mg-Al hydrotalcite encapsulated catalyst from Example 2 was directly placed inside a tube furnace for the growth of single-walled carbon nanotubes, maintaining the growth process unchanged. The Raman spectra of the obtained single-walled carbon nanotubes are shown below. Figure 9 As shown.

[0029] Comparative Example 3 The Fe-Mg-Al hydrotalcite encapsulated catalyst from Example 3 was directly placed inside a tube furnace for the growth of single-walled carbon nanotubes, maintaining the growth process unchanged. The Raman spectra of the obtained single-walled carbon nanotubes are shown below. Figure 10 As shown.

[0030] According to Example 1 and Comparative Example 1, when methane is used as the carbon source for the untreated catalyst and the pretreated catalyst, the content of single-walled carbon nanotubes grown by the untreated catalyst is lower, while the content of single-walled carbon nanotubes grown by the pretreated catalyst is higher.

[0031] According to Example 2 and Comparative Example 2, when propylene is used as the carbon source for the untreated and pretreated catalysts, the single-walled carbon nanotubes grown by the untreated catalyst are concentrated in large-diameter single-walled carbon nanotubes, while the single-walled carbon nanotubes grown by the pretreated catalyst are concentrated in small-diameter single-walled carbon nanotubes.

[0032] According to Example 1 and Comparative Example 1, when methane and propylene are used as carbon sources for the untreated catalyst and the pretreated catalyst, respectively, the content of small-diameter single-walled carbon nanotubes grown by the untreated catalyst is lower, while the content of small-diameter single-walled carbon nanotubes grown by the pretreated catalyst is higher.

[0033] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, alterations, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for pretreating an encapsulated catalyst, characterized in that, The encapsulated catalyst is subjected to a two-step heat treatment to change the functional groups of the active sites on the catalyst surface, thereby inducing the exposure of the active sites inside the encapsulated catalyst. The encapsulated catalyst includes at least one spinel catalyst and a hydrotalcite catalyst. In the encapsulated catalyst, the catalytic metal accounts for 5%-40% of the total metal content of the encapsulated catalyst. In the two-step heat treatment, the atmosphere for the first heat treatment is a mixture of air and inert gas, and the atmosphere for the second heat treatment is a mixture of CO2 and inert gas.

2. The encapsulated catalyst pretreatment method according to claim 1, characterized in that, The encapsulated catalyst is preferably Fe-Mg-Al hydrotalcite; The steps include: (1) Fe-Mg-Al hydrotalcite is placed in a tube furnace and a mixture of inert gas and air is introduced throughout the process. The temperature is raised to 400-900℃, held for 1-3 hours, and then naturally cooled to room temperature to obtain the precursor. (2) The precursor is placed in a tube furnace and a mixture of inert gas and CO2 gas is introduced throughout the process. The temperature is raised to 400-900℃, held for 1-3 hours, and then naturally cooled to room temperature to obtain the desired catalyst. In step (1), the molar ratio of the catalytic metal in Fe-Mg-Al hydrotalcite to all metals in Fe-Mg-Al hydrotalcite catalyst is 5%-40%; in the mixed gas in step (1), the air flow rate is 1-100%; in the mixed gas in step (2), the CO2 flow rate is 1-100%.

3. The encapsulated catalyst pretreatment method according to claim 2, characterized in that, The synthesis steps of Fe-Mg-Al hydrotalcite in step (1) are as follows: Aluminum nitrate, magnesium nitrate, ferric nitrate, urea, and water were placed in a round-bottom flask, stirred and heated in a water bath for 9 hours, then allowed to stand and heated for 14 hours. After cooling, the mixture was filtered, freeze-dried, and calcined to obtain Fe-Mg-Al hydrotalcite.

4. A method for preparing small-diameter single-walled carbon nanotubes, characterized in that, Prepare a catalyst prepared by the preparation method according to any one of claims 1-3, and grow small-diameter single-walled carbon nanotubes on the catalyst by chemical vapor deposition.

5. The method for preparing small-diameter single-walled carbon nanotubes according to claim 4, characterized in that, The conditions for chemical vapor deposition include: a deposition temperature of 660℃-900℃; and simultaneous introduction of hydrogen and carbon source to ensure continuous carrier flow throughout the process. Preferably, the carbon source gas includes any one or a combination of at least two of methane and ethylene; Preferably, the carrier includes nitrogen and / or argon.

6. The method for preparing small-diameter single-walled carbon nanotubes according to claim 4, characterized in that, The flow rate ratio of hydrogen, carbon source, and carrier is (0-200):(10-50):200; Preferably, the flow rate ratio of the hydrogen, carbon source, and carrier is 200:30:

200.

7. The method for preparing small-diameter single-walled carbon nanotubes according to claim 4, characterized in that, The catalyst prepared by the method according to any one of claims 1-3 exhibits a fragmentation characteristic after catalyzing the growth of single-walled carbon nanotubes.

8. The method for preparing small-diameter single-walled carbon nanotubes according to claim 4, characterized in that, The diameter of the prepared small-diameter single-walled carbon nanotubes is 0.8-1.2 nm.

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