Preparation method of high-purity single-walled carbon nanotube

Abstract

The invention discloses a preparation method of a high-purity single-walled carbon nanotube. The preparation method comprises the following steps: cyclically executing a growth pulse and an etching-relaxation pulse, wherein the growth pulse supplies a carbon source gas; the etch-relaxation pulse supplies an etching gas such as carbon dioxide to remove amorphous carbon by-products from the surface of the catalyst. The method is characterized in that a signal indicating a product is monitored on line during etching, and the duration of a next growth pulse is adaptively adjusted through a feedback control system according to the signal. The invention also discloses a catalyst suitable for the method, the catalyst comprises an iron-based active component loaded on a magnesium oxide carrier, and the mass loading amount of an iron element is 0.5-2.0%. According to the invention, the growth conditions are regulated in real time, the activity of the catalyst is maintained in an optimal state, and the service life of the catalyst is effectively prolonged, so that the purity and yield of the single-walled carbon nanotube are remarkably improved, and the controllability and repeatability of the preparation process are enhanced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of carbon nanomaterials, and particularly to a method for preparing high-purity single-walled carbon nanotubes. BACKGROUND

[0002] Single-walled carbon nanotubes (SWCNT) have shown great potential in applications such as composite materials, microelectronic devices, and energy storage due to their unique structure and excellent mechanical, electrical, and thermal properties. Among various preparation methods, chemical vapor deposition (CVD) is considered the most promising approach for large-scale production of single-walled carbon nanotubes. This method typically involves using a transition metal catalyst to catalyze the cracking of carbon-containing gas at high temperatures, resulting in the growth of carbon nanotubes on the surface of the catalyst particles.

[0003] However, existing chemical vapor deposition processes still face severe challenges in producing high-purity single-walled carbon nanotubes. During the growth process, in addition to the target product single-walled carbon nanotubes, a large amount of by-products such as amorphous carbon and multi-walled carbon nanotubes are inevitably generated. These by-products not only severely affect the performance of the final product, but also make the subsequent separation and purification process extremely complex, often accompanied by damage to the structure of single-walled carbon nanotubes. The core reason for this problem lies in the difficulty of maintaining the active state of the catalyst. During the reaction, the active sites of the catalyst are gradually covered by the generated amorphous carbon, leading to a rapid decline in its catalytic activity or even complete loss of activity, a phenomenon known as catalyst deactivation.

[0004] To delay catalyst deactivation, researchers have proposed pulse chemical vapor deposition technology, which involves alternating the introduction of carbon source gas and weak oxidizing gas to remove amorphous carbon from the catalyst surface during the growth pause. Although this method extends the life of the catalyst to some extent, it generally uses fixed pulse time parameters. This pre-set, static control strategy cannot adapt to the dynamic changes in the state of the catalyst during the reaction. The activity of the catalyst is not constant and evolves with factors such as reaction time and local temperature fluctuations. Fixed etching time may cause excessive oxidation damage to the catalyst at the beginning of the reaction, while in the later stage of the reaction, it may not be sufficient to remove the accumulated amorphous carbon, ultimately still unable to avoid catalyst deactivation and product purity decline. Therefore, the existing technology lacks a mechanism that can respond to the state of the catalyst in real time and perform dynamic control, which limits the controllability, repeatability, and further improvement of product quality in the preparation process of single-walled carbon nanotubes. SUMMARY

[0005] In view of the deficiencies of the prior art, the application provides a preparation method of high-purity single-wall carbon nanotubes, which solves the problems of catalyst deactivation, low product purity and poor repeatability in the prior art chemical vapor deposition method due to the inability to respond and control the dynamic activity state of the catalyst in real time because of the fixed static process parameters.

[0006] To achieve the above object, the application is implemented by the following technical solutions: a high-purity single-wall carbon nanotube comprising an iron-based active component and a magnesium oxide carrier, wherein the mass loading of the iron-based active component in terms of iron element is 0.5-2.0% relative to the magnesium oxide carrier.

[0007] Preferably, the mass loading of the iron element is 0.8-1.2%.

[0008] Preferably, a solid solution or a strong interaction interface is formed between the iron-based active component and the magnesium oxide carrier.

[0009] A preparation method of high-purity single-wall carbon nanotubes, comprising the following steps: a) providing a catalyst in a reactor; b) performing reduction treatment on the catalyst; c) cyclically performing a growth pulse and an etching-relaxation pulse at a preset reaction temperature; wherein the duration of the growth pulse is , and during the duration, a carbon source gas is supplied to the catalyst; the etching-relaxation pulse comprises stopping the supply of the carbon source gas and supplying an etching gas to the catalyst; and the method further comprises: monitoring a signal of an indicative product generated by the reaction of the etching gas with the catalyst surface deposits during the etching-relaxation pulse; adjusting the duration of the next growth pulse according to the signal of the indicative product.

[0010] Preferably, the steps b) and c) are performed at a temperature of 850-900℃.

[0011] Preferably, the carbon source gas is ethanol vapor, and the etching gas is carbon dioxide.

[0012] Preferably, the indicative product is carbon monoxide; and the adjustment comprises shortening the duration when the signal of the carbon monoxide is higher than a preset target interval , and lengthening the duration when the signal of the carbon monoxide is lower than a preset target interval .

[0013] Preferably, in the step c), the duration of the growth pulse initially set is 1.0-2.0 seconds, and the duration of the etching-relaxation pulse is 5.0-8.0 seconds.

[0014] Preferably, the reduction treatment in the step b) is carried out under a hydrogen atmosphere.

[0015] The present application provides a method for preparing high-purity single-walled carbon nanotubes. The present application has the following advantages: 1. The present application introduces an adaptive feedback control system based on etching product monitoring, which significantly improves the purity of single-walled carbon nanotube products. After each growth pulse, the method uses etching gas to selectively gasify the amorphous carbon and other by-products covered on the surface of the catalyst, and monitors the indicative products of the gasification reaction in real time. According to the intensity of the signal, the control system dynamically adjusts the duration of the subsequent growth pulse, so as to accurately maintain the activity of the catalyst within the dynamic balance window that is conducive to the growth of single-walled carbon nanotubes rather than the deposition of amorphous carbon. This real-time inhibition of by-product generation fundamentally reduces the impurity content in the final product.

[0016] 2. The present application effectively prolongs the activity life of the catalyst by dynamically adjusting the growth and etching process, thereby improving the overall yield of single-walled carbon nanotubes. In traditional continuous growth or fixed pulse growth, the catalyst will quickly lose activity due to rapid coverage of amorphous carbon. The etching-relaxation pulse in the present application can periodically remove these coverings, exposing the active sites of the catalyst again. More importantly, the adaptive adjustment mechanism avoids the deactivation caused by insufficient etching or the damage to the catalyst particles caused by excessive etching, ensuring that the catalyst remains in a high-activity state throughout the reaction period, thereby maximizing the use of catalyst efficiency.

[0017] 3. The closed-loop feedback control method established in the present application greatly enhances the controllability and batch-to-batch repeatability of the single-walled carbon nanotube preparation process. The system can automatically compensate for changes in catalyst performance caused by factors such as batch-to-batch differences in catalyst, fluctuations in the microenvironment within the reactor, etc. When the catalyst activity drifts, the system automatically corrects by adjusting the growth pulse time, so that the entire growth process tends to a stable, pre-set optimal state. This adaptive ability reduces the extreme dependence of the process on the accuracy of the initial conditions, making the scaling-up process from laboratory scale to industrial production more reliable and ensuring the consistency of product quality. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 is a flowchart of the method steps of the present application; Figure 2Transmission electron microscope (TEM) image of high purity single-walled carbon nanotubes prepared in Example 1 of the present application; Figure 3 Transmission electron microscope (TEM) image of high purity single-walled carbon nanotubes prepared in Example 2 of the present application; Figure 4 Transmission electron microscope (TEM) image of high purity single-walled carbon nanotubes prepared in Example 3 of the present application; Figure 5 Transmission electron microscope (TEM) image of high purity single-walled carbon nanotubes prepared in Example 4 of the present application. DETAILED DESCRIPTION

[0019] The technical solutions in the embodiments of the present application will be apparently and completely described below with reference to the drawings in the specification of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all the other embodiments obtained by a person of ordinary skill in the art without any creative work fall within the protection scope of the present application.

[0020] Please refer to the drawings in the specification of the present application Figure 1 The high purity single-walled carbon nanotubes of the present application comprise an iron-based active component and a magnesium oxide carrier, wherein the mass loading of the iron-based active component in terms of iron element is 0.5-2.0% relative to the magnesium oxide carrier. This component is the active center for catalyzing the cracking and nucleation growth of single-walled carbon nanotubes from carbon source. Before being used in the final chemical vapor deposition reaction, i.e. in the state after calcination, the iron species mainly exists in the form of highly dispersed iron oxide (such as Fe2O3), or partly forms a magnesium-iron solid solution (such as Mg(Fe)O) or a spinel phase (such as MgFe2O4) with the magnesium oxide carrier. In the initial reduction step for growing single-walled carbon nanotubes, these iron oxide species are in-situ reduced to zero-valent metallic iron (Fe) nanoparticles, which are the direct catalytic active sites.

[0021] The iron source precursor used for preparing the catalyst is preferably an iron salt capable of being well dissolved in a solvent, such as ferric nitrate (e.g. ferric nitrate nonahydrate), ferric chloride, ferric sulfate or ferric acetate, etc. The use of these soluble salts can ensure that the iron species is uniformly distributed on the surface of the carrier in the form of ions or small molecular clusters during the impregnation process.

[0022] The role of the magnesium oxide carrier is far more than providing physical support. It effectively anchors the iron nanoparticles at high temperature by forming a strong interaction with the iron-based active component, thereby inhibiting the migration and agglomeration of the iron nanoparticles, and thus maintaining the high dispersity and size uniformity of the active sites. To achieve this purpose, the selected magnesium oxide carrier preferably has a relatively high specific surface area, such as 50-300 m 2The mass loading of iron element is strictly controlled in the range of 0.5-2.0% relative to the magnesium oxide support, which is the key to achieve high selective catalytic synthesis of single-walled carbon nanotubes.

[0023] The mass loading of iron element is strictly controlled in the range of 0.5-2.0% relative to the magnesium oxide support, which is the key to achieve high selective catalytic synthesis of single-walled carbon nanotubes.

[0024] When the loading is lower than 0.5%, the number of effective active sites per unit mass of catalyst is too small, resulting in extremely low yield of carbon nanotubes, which lacks industrial application value.

[0025] When the loading is higher than 2.0%, even with the dispersion effect of the support, the initial spacing of iron species is too close, and it is prone to agglomeration during high-temperature reduction and growth, forming large-sized metal particles. These large-sized particles tend to catalyze the formation of multi-walled carbon nanotubes or amorphous carbon, thereby significantly reducing the purity of the target product single-walled carbon nanotubes.

[0026] Therefore, the range of 0.5-2.0% is the best balance between ensuring catalytic activity and inhibiting particle growth. A more preferred range is 0.7-1.5%, in which range the catalyst exhibits more excellent overall performance.

[0027] The typical preparation method of the catalyst includes impregnation, drying, and calcination steps.

[0028] Impregnation: The pretreated magnesium oxide powder is added to a solution containing an iron source precursor (such as an ethanol solution of iron nitrate), and impregnation is carried out under sufficient stirring. This step aims to uniformly penetrate the iron salt solution into the pores and surface of the magnesium oxide support.

[0029] Drying: The impregnated mixture is dried at a temperature of 60-120°C to slowly remove the solvent, allowing the iron species to be deposited on the support in a highly dispersed state.

[0030] Calcination: The dried powder is calcined in an air atmosphere at a temperature of 400-550°C. The purpose of this step is: first, to decompose the iron salt precursor (such as iron nitrate) into iron oxide; second, to promote the solid-phase reaction between the iron oxide and the magnesium oxide support, forming the aforementioned solid solution or spinel and other strong interaction interfaces. It is this step that endows the catalyst with excellent high-temperature structural stability, which is the fundamental reason for its ability to remain active under harsh growth conditions.

[0031] Example 1 This embodiment provides a method for preparing high-purity single-walled carbon nanotubes using a self-adaptive pulsed chemical vapor deposition method, specifically comprising the following steps: In the center of the constant temperature zone of the quartz tube reactor of the horizontal tube furnace, 20 mg of Fe / MgO-M catalyst prepared in Preparation Example 2 was laid flat.

[0032] The reactor was sealed and purged with high-purity argon at a flow rate of 200 sccm for 30 minutes.

[0033] The reaction zone was heated to 850°C at a rate of 10°C / min under an argon atmosphere.

[0034] After reaching 850°C, the atmosphere was switched to an Ar / H2 mixture (H2 volume fraction of 10%, total flow rate of 200 sccm), and the catalyst was reduced in situ for 30 minutes under this condition.

[0035] After the reduction was complete, the H2 supply was stopped. The initial pulse parameters were set in the central control system: growth pulse time = 2.0 seconds, etch-relaxation pulse time = 5.0 seconds. The target interval for the CO signal of the feedback control algorithm and the adjustment step size were also set to 0.1 seconds.

[0036] The adaptive pulse growth program was started, and a total of 60 minutes of operation was performed. The program was executed in a loop: Growth pulse: Ar carrier gas carrying ethanol vapor was supplied (flow rate of 30 sccm), and the total flow rate was maintained at 200 sccm.

[0037] Etch-relaxation pulse: The ethanol carrier gas was stopped, and CO2 was supplied (flow rate of 2 sccm), and the total flow rate was maintained at 200 sccm. During this period, the CO signal was monitored by an online gas analyzer, and the of the next growth pulse was adaptively adjusted by the control system.

[0038] After the growth was complete, all reaction gas supplies were stopped, and only 200 sccm of argon was supplied, and the furnace was naturally cooled to room temperature. The product was removed, and a black fluffy powder was obtained.

[0039] Example 2 The present embodiment provides a method for preparing high-purity single-walled carbon nanotubes using an adaptive pulse chemical vapor deposition method, specifically comprising the following steps: The catalyst used in Example 1 was replaced with 20 mg of Fe / MgO-L catalyst prepared in Preparation Example 1. All other steps, including reaction temperature, gas flow rate, initial pulse parameter setting, feedback control strategy, and reaction time, were exactly the same as in Example 1.

[0040] Example 3 The embodiment provides a method for preparing high-purity single-walled carbon nanotubes by using an adaptive pulse chemical vapor deposition method, and specifically comprises the following steps. The target temperature, the reduction temperature and the growth temperature in the embodiment 1 are all set to 900 DEG C. All the remaining steps, including the used catalyst (Fe / MgO-M), the gas flow, the initial pulse parameter setting, the feedback control strategy and the reaction time, are all completely same as those in the embodiment 1.

[0041] Embodiment 4 The embodiment provides a method for preparing high-purity single-walled carbon nanotubes by using an adaptive pulse chemical vapor deposition method, and specifically comprises the following steps. In step 5 of the embodiment 1, the initial pulse parameters set by the central control system are changed to: the growth pulse time =1.0 second, and the etching-relaxation pulse time =8.0 seconds. All the remaining steps, including the used catalyst (Fe / MgO-M), the reaction temperature, the gas flow, the feedback control strategy and the reaction time, are all completely same as those in the embodiment 1.

[0042] Comparative example 1 Compared with the embodiment 1, the difference lies in that: a traditional continuous flow chemical vapor deposition method is used, after the catalyst is reduced, the Ar carrier gas carrying ethanol vapor is continuously introduced, without any pulse operation, and without introducing CO2 gas.

[0043] Comparative example 2 Compared with the embodiment 1, the difference lies in that: the adaptive feedback control system is closed. The growth pulse time and the etching-relaxation pulse time are constantly set to the initial setting of 2.0 seconds and 5.0 seconds, without dynamic adjustment.

[0044] Comparative example 3 Compared with the embodiment 1, the difference lies in that: in the etching-relaxation pulse stage, the CO2 gas is not introduced, and only the Ar gas is introduced for purging, and the remaining pulse operation and parameters are kept unchanged.

[0045] Comparative example 4 Compared with the embodiment 1, the difference lies in that: a reactant co-feeding method is used. After the catalyst is reduced, the Ar carrier gas carrying ethanol vapor and the CO2 gas (flow rate 1.5 sccm) are simultaneously and continuously introduced into the reactor for reaction.

[0046] Test example: adaptive control system function verification Experimental description This test example aims to verify the actual functioning of the adaptive feedback control system during the growth of single-walled carbon nanotubes (SWCNTs). The experimental procedure is based on the process conditions of Example 1, with a focus on data acquisition and analysis.

[0047] The experimental procedure is as follows: During the growth step of Example 1, the central control computer was set to continuously record two key parameters: one is the integrated value of the CO signal (m / z = 28) monitored by the online quadrupole mass spectrometer during each etch-relaxation pulse cycle; the other is the adjusted growth pulse time calculated by the control system based on the integrated value and applied to the next growth pulse ).

[0048] The integrated value of the CO signal was used as an indirect measure of the amount of amorphous carbon deposited on the catalyst surface. At the end of each etch-relaxation pulse, the control algorithm compared the integrated value to a pre-set target interval representing the optimal growth state.

[0049] If the integrated value deviated from the target interval, the algorithm would correct by a pre-set adjustment step (0.1 s) and record the corrected value along with the corresponding CO signal integrated value and time stamp.

[0050] Data was collected at fixed time intervals (every 5 minutes) to demonstrate the dynamic response behavior of the system throughout the growth process.

[0051] Experimental data Table 1: Partial data record of the adaptive control process in Example 1: Time (min) CO signal integral value (arbitrary units, a.u.) Adjusted growth pulse time tg (s) 0 - 2.0 (initial value) 5 48.5 2.1 10 65.2 2.1 15 51.3 2.1 20 89.7 2 25 61.8 2 30 43.6 2.1 35 75.4 2 40 68.9 2 45 59 2 50 81.2 1.9 55 63.5 1.9 60 66.1 1.9 The data in Table 1 shows a clear negative correlation between the integrated value of the CO signal and the subsequent adjusted growth pulse time . The integrated value of the CO signal reflects the amount of amorphous carbon gasified by CO2 during the etch-relaxation phase. When the integrated value of the CO signal exceeds the pre-set upper limit (e.g., at t = 20 min, the signal value is 89.7 a.u.), it indicates that too much amorphous carbon was deposited during the previous growth pulse, and the control system immediately reduces the duration of the next growth pulse (from 2.1 s to 2.0 s) to suppress the excessive generation of amorphous carbon. Conversely, when the integrated value of the CO signal is below the pre-set lower limit (e.g., at t = 5 min, the signal value is 48.5 a.u.), it indicates that the catalyst activity is high or that there is insufficient carbon deposition, and the system increases the growth pulse time (from 2.0 s to 2.1 s) to ensure sufficient carbon supply.

[0052] The core of this dynamic adjustment mechanism is to indirectly evaluate the instantaneous state of the catalyst by real-time monitoring of the etching product, and to adjust the supply amount of carbon source accordingly. This process can maintain the activity of the catalyst within a dynamic balance window, avoiding both rapid deactivation of the catalyst due to excessive amorphous carbon coating and low growth efficiency due to excessive etching or insufficient carbon source supply. Therefore, this test example proves that this technical solution can realize real-time and autonomous optimization of growth conditions, which is the key to realizing controllable preparation of high-purity single-walled carbon nanotubes.

[0053] While embodiments of the present application have been shown and described, it is to be understood that the embodiments described are merely divergences, modifications, replacements and variations of the embodiments, and the scope of the present application is defined by the appended claims and their equivalents.

Claims

1. A high purity single-walled carbon nanotube, characterized by, A catalyst comprising an iron-based active component and a magnesium oxide support, wherein the mass loading of the iron-based active component, calculated as elemental iron, is 0.5-2.0% relative to the magnesium oxide support.

2. The high purity single-walled carbon nanotubes of claim 1, wherein, The mass loading of the elemental iron is 0.8-1.2%.

3. The high purity single-walled carbon nanotubes of claim 1, wherein, A solid solution or a strong interfacial interaction is formed between the iron-based active component and the magnesium oxide support.

4. A method for producing high purity single-walled carbon nanotubes, for producing high purity single-walled carbon nanotubes as claimed in any one of claims 1 to 3, characterized by, The method comprises the following steps: a) providing a catalyst in a reactor; b) reducing the catalyst; c) cyclically performing a growth pulse and an etch-relaxation pulse at a predetermined reaction temperature; wherein the duration of the growth pulse is a carbon source gas is supplied to the catalyst during the duration. The etch-relaxation pulse comprises: stopping the supply of the carbon source gas, and supplying an etching gas to the catalyst; And the method further comprises: During the etch-relaxation pulse, monitoring the signal of an indicative product generated by the reaction of the etching gas with the catalyst surface deposits; According to the signal of the indicative product, the duration of the next growth pulse is adjusted.

5. The method for preparing high-purity single-walled carbon nanotubes according to claim 4, characterized in that, The steps b) and c) are performed at a temperature of 850-900℃.

6. The method for preparing high-purity single-walled carbon nanotubes according to claim 4, characterized in that, The carbon source gas is ethanol vapor, and the etching gas is carbon dioxide.

7. The method for preparing high-purity single-walled carbon nanotubes according to claim 4, characterized in that, The indicative product is carbon monoxide; the adjustment comprises: when the signal of the carbon monoxide is higher than a preset target interval, shortening the duration ; when the signal of the carbon monoxide is lower than a preset target interval, lengthening the duration .

8. The method for preparing high-purity single-walled carbon nanotubes according to claim 4, characterized in that, In said step c) the duration of the growth pulse initially set is 1.0-2.0 seconds and the duration of the etch-relaxation pulse is 5.0-8.0 seconds.

9. The method for preparing high-purity single-walled carbon nanotubes according to claim 4, characterized in that, The reducing treatment of step b) is performed in a hydrogen atmosphere.

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