Catalyst, catalyst precursor, production method, and high-purity, morphology-controlled carbon nanotubes

Abstract

Carbon nanotubes grown using a catalyst, a catalyst precursor, and a die catalyst. The catalyst includes an alumina-containing support and cobalt species on the surface of the support, where the cobalt is the only active catalytic species for carbon nanotube growth. The support surface does not include iron.

Description

[Technical Field]

[0001] This application claims priority to Provisional Application No. 63 / 446,647, filed February 17, 2023. The entire disclosure of the priority application is incorporated herein by reference for all purposes. [Background technology]

[0002] The present disclosure relates to carbon nanotubes (CNTs) and catalysts for producing CNTs.

[0003] Numerous commercial applications take full advantage of CNT material properties due to their unique structural and morphological characteristics, electrical and thermal conductivity capacities, and mechanical properties. An example of one such application is energy storage, where CNTs have demonstrated great potential as anode and cathode materials and as conductive additives for lithium-ion batteries (LiBs). The measured reversible lithium-ion capacity of CNT-based electrodes has been significantly improved compared to conventional graphite-based anodes. The capacity of LiBs using graphite as the anode material is limited because lithium ions can only bind to one out of every two six-membered carbon rings in the graphite sheet. Lithium intercalation into graphite involves one lithium atom per six carbon atoms, i.e., LiC6, resulting in a limited specific capacity of 372 mAh / g and an observed capacity of 280–330 mAh / g, depending on the type of graphite used. Carbon nanotubes have been reported to significantly improve Li capacity compared to graphite due to their tubular carbon structure and properties.

[0004] There are two types of carbon nanotubes: single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). These CNT materials are 10 6 S / m and 10 5CNTs have a high electrical conductivity of 1.5 S / m, a tensile strength of 63 GPa, and a Young's modulus of approximately 1.2 TPa. These properties make them one of the strongest and stiffest conductive materials. The inner and outer diameters of the tubes are important parameters in the structure of CNTs and can affect the lithium adsorption capacity and diffusion both inside and outside the CNT. Lithium ions intercalated in the CNTs interact with the CNTs, and charge transfer between the CNTs and lithium ions further enhances these interactions. The interaction is related to the tube curvature, which will result in different lithium capacities within the CNTs. The desorption of lithium from MWCNTs during electrochemical insertion also depends on the inner diameter of the tube, which will consequently affect the lithium capacity, but the intercalated Li / Li + It was found that the ions are localized near the CNT sidewalls rather than at the center of the tube. Therefore, when used in batteries, very large diameter MWCNTs with internal wasted space are at a disadvantage compared to smaller diameter ones. As the tube diameter increases, the intercalated lithium atoms tend to form a multishell structure in equilibrium, consisting of coaxial tubes with linear chains at the axis, improving lithium capacity. Other studies have shown that CNTs with diameters greater than 5 Å are ideal candidates for lithium storage as anode materials in LIBs due to their large interaction energy. In summary, very large diameter MWCNTs are at a disadvantage compared to smaller diameters due to the availability of unoccupied space for lithium retention. There may also be an optimal number of walls corresponding to greater interlayer lithium retention. MWCNTs are believed to have an optimal outer diameter that provides higher electrical conductivity. MWCNTs with diameters greater than 15 nm have been observed to exhibit poor conductivity. The number of concentric tubes for an outer diameter of 15 nm and an inner diameter of 2.5 nm is approximately 13.

[0005] CNT length is another important morphological factor that can affect lithium ion diffusion in CNTs. It has been observed that short MWCNTs provide a short reversible path for Li ions and abundant open ends for additional intercalation sites for Li ion storage. In practice, chemical and ball milling treatments have been widely used to increase lithium insertion and reversible capacity. However, chemical treatments reduce Coulombic efficiency due to the introduction of large structural defects, while ball milling treatments result in large voltage hysteresis due to the amount of surface functional groups formed on the CNTs.

[0006] The use of CNTs as a conductive additive in the cathode of lithium-ion batteries has proven to be a unique choice for improving electrochemical properties. The high aspect ratio (L / D) of CNTs compared to other carbon additives allows them to achieve comparable percolation thresholds at lower load levels. Therefore, the electrode's conductivity and electrochemical properties can be improved with a lower amount of CNTs, leading to higher energy capacity, power density, and longer charge-discharge cycles.

[0007] CNTs also have significant thermal conductivity, which can promote effective heat dissipation within the composite, potentially enhancing safety over composite electrodes containing inferior carbon additives. Furthermore, the mechanical properties of CNTs exhibit both strength and flexibility, both of which are attractive features for preventing cracking during operation and under vibration. Furthermore, the ability to entrap anode and cathode particles using CNTs can avoid the problem of active particle separation over long cycles.

[0008] High carbon nanotube purity is another parameter that improves LIB performance. It has been found that the presence of certain impurities, such as metal ions (e.g., iron, sodium, aluminum, and nickel) or substances containing active hydrogen in the electrolyte molecules, can degrade the electrochemical performance of the battery. Some metal impurity ions have a lower reduction potential than lithium ions. Therefore, during charging, metal impurity ions initially embed themselves in the carbon anode, reducing the number of lithium ion insertion sites and reducing the reversible capacity of the lithium-ion battery. Iron deposition accelerates electrolyte decomposition and exacerbates the disorder of the graphite surface structure. This results in a thicker solid electrolyte interface (SEI) film and an increased interfacial impedance of the graphite electrode. These effects contribute to capacity loss during cycling. High concentrations of metal impurity ions not only reduce the reversible specific capacity of lithium-ion batteries, but also prevent the formation of an effective passivation layer on the graphite electrode surface, potentially destroying the entire battery. Therefore, it is generally recognized that the content of each metal impurity ion in the organic electrolyte should be less than approximately 0.007 wt%.

[0009] Despite the high energy capacity of LIBs, the development of a new generation of LIBs faces many challenges, including improving the charge capacity rate, reducing the weight and size of the battery, and extending the cycle life of the electrode materials. To improve the performance of conventional materials, it is necessary to develop CNT materials with controlled structure and morphology and high purity properties. Currently, there are clear advantages to using CNTs in LIBs as additives in composite electrodes to increase reversible capacity, enhance rate capability, and improve cyclability. Summary of the Invention

[0010] Aspects and examples relate to catalyst precursors, catalysts, methods for preparing them, and CNT compositions and purified CNT compositions.

[0011] This disclosure describes a method for preparing heterogeneous catalysts with large specific surface areas and low active phase compositions compared to conventional catalysts, which can produce carbon nanotubes with high purity and selectivity to tubular carbon, high surface areas, and high aspect ratios (L / D) in high yields in short reaction times. Methods for producing these carbon nanotubes using different catalytic reactor types are also described. Nanotubes obtained using this catalyst and production method are desirable for various industrial applications, particularly those requiring carbon nanotubes with controlled morphology (internal and external diameter distribution, length, number of walls, bundle size), including lithium-ion battery electrodes, supercapacitors, CNT membranes, printed electronics, and composite materials.

[0012] Catalyst preparation for the desired CNTs involves the following sequential steps: a) contacting an aqueous solution containing metal salts of the active components with aluminum hydroxide (e.g., boehmite, gibbsite, bayerite, aluminum alkoxide) using conventional impregnation techniques (pore volume, ion exchange, mixing in a high-speed mixer); b) forming a paste or granules of the impregnated material and aging it for approximately two hours under controlled humidity and temperature; c) drying the impregnated material in the presence of air flow at temperatures between 25°C, 60°C, and 120°C; d) sieving the material to a particle size between 30 and 500 microns, depending on the type of reactor used; and e) calcining the catalyst powder in an oven in the presence of nitrogen or air and nitrogen flow at temperatures between 350°C and 550°C. The active metal composition can be adjusted to control the morphology and yield of CNTs. For example, the composition of the active metal on the support surface can be optimized to control the size and density of the active phase nanoparticles on the surface. The size of the nanoparticles controls the tube diameter, while their density and surface composition control the CNT morphology.

[0013] When using ion exchange catalyst preparation, an excess of metal solution (approximately 3-6 times the porous volume of the catalyst support) is brought into contact with the support in a closed vessel equipped with a reflux system. The ion exchange is carried out at temperatures ranging from 45 to 75 °C for several hours (t > 2 h). The liquid is separated from the solid by filtration. The resulting paste is dried, then sieved and calcined according to the protocol described above.

[0014] Various types of carbon nanotubes (single-walled, double-walled, or multi-walled) can be synthesized using different catalyst compositions, carbon sources (CO, CH, C, H ... 10 The CNTs can be obtained using synthesis reactors in a fluidized bed or rotating tube reactor, etc. Residual catalyst particles are removed from the CNT product by leaching with concentrated inorganic acid solutions (e.g., HF, HCl, H2SO4, HNO3, and mixtures thereof), followed by filtration, washing with deionized water, and drying. The process for catalyst production, synthesis, and purification of CNTs of the present invention is commercially scalable.

[0015] Most commercial carbon nanotubes produced today are synthesized using iron in combination with other active metals (Ni, Co, Mo, etc.) in catalyst formulations. Exemplary catalyst precursor and catalyst compositions and catalyst preparation methods are disclosed, for example, in U.S. Pat. No. 9,084,990, the disclosure of which is incorporated herein by reference in its entirety and for all purposes. As previously mentioned, iron and nickel ions are elemental impurities that moderate the electrochemical performance of lithium-ion batteries by reducing lithium ion insertion into the anode, affecting their reversible capacity. Combinations of Fe and Co or Fe and Mo provide high-carbon yield catalysts for the synthesis of double-walled (DWCNT) and multi-walled carbon nanotubes. These are primarily due to the formation of oxide precursors during the calcination process; nanocrystals such as CoFe2O4 and FeMoO4 form highly dispersed active metal nanoparticles on the surface of the catalyst support upon contact with a carbon source. Iron in the catalysts can be present in metallic form or in the form of iron carbide (Fe3C) or cobalt (Co xFe 3-x C) or molybdenum (Mo 2-x Fe x C) to form a non-stoichiometric mixed phase.

[0016] Iron is found in most catalysts used in CNT synthesis, with higher compositions than cobalt, nickel, or molybdenum. During the purification stage of carbon nanotubes, strong inorganic acids are used to remove active metals and catalyst support particles. However, metal carbides, such as iron carbide, are very stable and have very low solubility in these acids (especially iron carbide, a component of stainless steel, must be removed from the product by ultra-high-temperature treatment in a vacuum). These treatments can cause significant structural and morphological changes (such as graphitization of the tubes, resulting in defects) that affect their electrical conductivity and mechanical properties. These results make them unsuitable for use in lithium-ion battery electrodes.

[0017] In preparing heterogeneous catalysts for carbon nanotube production, it can be important to control the type and distribution of the supported metal oxide and the size of the active metal nanoparticles, which determine the diameter and surface dispersion of the nanotubes. To address these limitations, new catalyst formulations using active metals that do not alter the electrochemical capacity of lithium batteries are needed. There have been no reports demonstrating that cobalt ions affect the electrochemical capacity of lithium batteries. In fact, cobalt is one of the key components used in cathodes. Because cobalt is used in commercial catalyst formulations for the production of SWCNTs and MWCNTs, this metal offers advantages for the production of MWCNTs with controlled morphology and high purity.

[0018] The key to developing the new catalyst is the ability to control the surface dispersion of metal nanoparticles and prevent cobalt from reacting with the aluminum-containing catalyst support to form cobalt aluminate (CoAl2O4, which exhibits a spinel-like structure), an inactive phase during catalytic reactions.

[0019] The present invention differs from prior art in at least the following aspects: The Co / MgO-Al2O3 catalyst preparation method allows for the use of much lower amounts of cobalt (at least 50 wt% lower) than prior art catalysts. The catalyst is prepared from aluminum hydroxide instead of alumina, allowing for greater surface metal dispersion. The preparation method is commercially scalable, using fewer steps, reducing production time and costs. The catalyst results in higher yields of carbon nanotubes in shorter reaction times. Synthesis can be performed in both fluidized bed and rotating tube reactors. The produced carbon nanotubes have higher aspect ratios than commercially available ones. The lower cobalt loading in the catalyst compared to prior art catalysts provides the following advantages: a) more efficient metal removal from the product during leaching; b) better control of tube morphology characteristics (uniformity, diameter distribution, bundle size); c) selectivity to tubular carbon; and d) improved dispersibility characteristics. Hydrogen formed during the catalytic decomposition of a carbon source can be separated using selective membranes and subsequently used for other industrial processes or to generate power and heat. In one aspect, to suppress the formation of cobalt aluminate during catalyst preparation, the present invention can add cations from Group II of the periodic table (such as Mg or Ca) to block the tetrahedral sites of alumina by forming a more stable spinel-like structure. These additives reduce the surface acidity of alumina and prevent the formation of non-tubular carbon species by cracking the carbon source molecules.

[0020] All examples and features described below can be combined in any way technically possible.

[0021] In one embodiment, a catalyst precursor composition includes a support comprising alumina and cobalt species on the surface of the support, where the cobalt is the only active catalytic species for carbon nanotube (CNT) growth.

[0022] In another embodiment, a catalyst composition for growing carbon nanotubes (CNTs) comprises an alumina support and cobalt on the surface of the support, wherein the cobalt is the only active catalytic species for CNT growth.

[0023] Some examples include one of the above and / or below features, or any combination thereof. In one example, the surface of the support is free of iron. In one example, the support further includes an element from Group IIA of the periodic table. In one example, the element from Group IIA of the periodic table includes magnesium. In one example, the cobalt species includes cobalt oxide. In one example, the cobalt species is less than 15% by weight of the catalyst precursor. In one example, the cobalt species is about 10% by weight of the catalyst precursor.

[0024] Some examples include one or any combination of the above and / or the following features. In one example, the catalyst precursor composition comprises 300 ml 2 In one example, the catalyst precursor composition has a pore volume of at least about 0.25 cc / g.

[0025] Some examples include one or any combination of the above and / or the following features. In one example, the catalyst precursor composition is configured to obtain CNTs in at least about 85% yield. In one example, the catalyst precursor composition is configured to produce multi-walled CNTs. In one example, the catalyst precursor composition is configured to produce 300 m 2 / g。 In one example, the catalyst precursor composition is configured to produce CNTs having a BET surface area of ​​at least about 2 cc / g. In one example, the catalyst precursor composition is configured to produce CNTs having a pore volume of at least about 2 cc / g. In one example, the catalyst precursor composition is configured to produce CNTs having a bulk density of at least about 0.08 g / cc. In one example, the catalyst precursor composition is configured to produce CNTs having a diameter range of about 8 nm to about 12 nm. In one example, the catalyst precursor composition is configured to produce CNTs having a G / D relative Raman intensity ratio of at least about 1.0 measured using a 638 nm laser source and about 0.9 measured using a 532 nm laser source. In one example, the catalyst precursor composition is configured to produce CNTs having a length of at least about 10 microns.

[0026] In another aspect, a method for preparing a catalyst precursor composition comprising a support comprising alumina and cobalt species on the surface of the support, wherein cobalt is the only active catalytic species for carbon nanotube (CNT) growth, comprises providing an aluminum hydroxide support precursor, contacting the aluminum hydroxide support precursor with a solution comprising at least a cobalt salt to form a paste, and drying and calcining the paste to form the catalyst precursor.

[0027] Some examples include any of the above and / or the following features, or any combination thereof. In one example, the solution includes a cobalt salt and a magnesium salt. In one example, the aluminum hydroxide includes one or more of gibbsite, bayerite, boehmite, and those obtained by hydrolysis of aluminum alkoxides. In one example, the contacting step includes mixing the aluminum hydroxide support precursor and the solution in a mixer. In one example, drying the paste includes aging the paste at room temperature. In one example, drying the paste further includes exposing the aged paste to an elevated temperature for several hours to produce a dried paste.

[0028] Some examples include one or any combination of the above and / or below features. In one example, the firing step includes exposing the dried paste to a temperature of at least about 400°C in flowing air for at least about 2 hours. In one example, the firing step is performed at about 450°C. In one example, the molar ratio composition of Co:MgO:Al2O3 is about 14:up to about 1:up to about 70, respectively. In one example, the method further includes sizing the dried paste prior to the firing step. In one example, the sizing step includes producing dried solid particles of at least about 100 microns. In one example, the dried solid particles are produced in a size range of about 100 microns to about 300 microns.

[0029] In another embodiment, the purified carbon nanotube (CNT) composition comprises at least about 99% CNTs and about 0.5% or less of a metal catalyst, with a residual mass of about 0.80% or less by weight as measured by an ashing method.

[0030] Some examples include one or any combination of the above and / or the following features: In one example, the purified CNT composition comprises at least about 99.7% CNTs; In one example, the purified CNT composition comprises about 0.3% or less catalyst; In one example, the purified CNT composition comprises at least about 85% CNTs prior to purification; In one example, the purified CNT composition comprises multi-walled CNTs.

[0031] Some examples include one or any combination of the above and / or the following features. In one example, the CNTs are 300 mm 2 / g. In one example, the CNTs have a pore volume of at least about 2 cc / g. In one example, the CNTs have a bulk density of at least about 0.08 g / cc. In one example, the CNTs have a diameter range of about 8 nm to about 12 nm. In one example, the CNTs have a G / D relative Raman intensity ratio of at least about 1 measured using a 638 nm laser source and at least about 0.9 measured using a 532 nm laser source. In one example, the CNTs have a length of at least about 10 microns. In one example, at least a majority of the CNTs have a length after synthesis and chemical purification of at least about 10 microns. In one example, at least a majority of the CNTs have a length after dispersion ranging from about 1 micron to about 5 microns. In one example, at least a majority of the CNTs have a bulk density ranging from about 0.06 to about 0.09 g / cc. [Brief explanation of the drawings]

[0032] Various aspects of at least one embodiment are described below with reference to the accompanying drawings, which are not intended to be drawn to scale. The drawings are included to provide explanation and further understanding of various aspects and embodiments, and are incorporated into and constitute a part of this specification, but are not intended as a definition of the limits of the invention. In the drawings, identical or nearly identical components shown in various drawings may be represented by like reference characters or numerals. For clarity, not every component is labeled in every figure. In the drawings:

[0033] [Figure 1] FIG. 1 is a schematic diagram of a prior art catalyst preparation procedure. [Figure 2] FIG. 2 is a schematic diagram of the catalyst preparation procedure of the present disclosure. [Figure 3A] FIG. 3A is a Raman spectrum using a 638 nm laser source for prior art CNTs containing 96.90% MWCNTs. [Figure 3B]FIG. 3B shows similar Raman spectra for two examples of purified CNTs of the invention produced using a Co / MgO-Al2O3-based catalyst at two different carbon purities, 99.70% and 99.17% MWCNT, respectively. [Figure 3C] FIG. 3C shows similar Raman spectra for two examples of purified CNTs of the invention produced using a Co / MgO-Al2O3-based catalyst at two different carbon purities, 99.70% and 99.17% MWCNT, respectively. [Figure 4A] FIG. 4A is a Raman spectrum using a 532 nm laser source for prior art CNTs containing 96.90% MWCNTs as reported in FIG. 3A. [Figure 4B] Figures 4B and 4C show similar Raman spectra, but for two examples of purified CNTs of the present invention, as reported in Figures 3B and 3C, at two different carbon purities, 99.70% and 99.17% MWCNT, respectively. [Figure 4C] FIG. 4C shows similar Raman spectra, but for two examples of purified CNTs of the present invention, as reported in FIGS. 3B and 3C, at two different carbon purities, 99.70% and 99.17% MWCNT, respectively. [Figure 5A] FIG. 5A shows Raman spectra of an example of purified CNTs of the present invention produced using a Co—Al 2 O 3 based catalyst using a 532 nm laser source and a 638 nm laser source, respectively. [Figure 5B] FIG. 5B shows Raman spectra of an example of purified CNTs of the present invention produced using a Co—Al 2 O 3 based catalyst using a 532 nm laser source and a 638 nm laser source, respectively. [Figure 6]Figure 6 contains nine SEM images. The top row was taken at 1KX, the middle row at 5KX, and the bottom row at 25KX. The leftmost column contains three images of CNTs of the present invention produced based on a 10-minute reaction time, the middle column contains three images of CNTs of the present invention produced based on a 20-minute reaction time, and the rightmost column contains three images of CNTs of the present invention produced based on a 30-minute reaction time. [Figure 7] FIG. 7 is an SEM image taken at 10KX magnification corresponding to the CNT capturing supported catalyst synthesized in the present invention. [Figure 8] FIG. 8 is an SEM image taken at 10KX magnification corresponding to a CNT purified sample at 99.17 wt % carbon, details of which are provided in other figures as indicated herein. [Figure 9] FIG. 9 is an SEM image taken at 10KX magnification of purified CNTs of the present invention produced using a Co / Al 2 O 3 catalyst. [Figure 10] Figure 10 was constructed in the same manner as Figure 6, but includes nine SEM images of prior art CNTs, where the top row was taken at 1KX, the middle row at 5KX, and the bottom row at 25KX. The leftmost column contains three images of prior art CNTs made based on a 10 minute reaction time, the middle column contains three images of prior art CNTs made based on a 20 minute reaction time, and the rightmost column contains three images of prior art CNTs made based on a 30 minute reaction time. [Figure 11] FIG. 11 shows the CNT length distribution in an aqueous dispersion of CNTs of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0034] The examples of materials, systems, methods, and devices described herein are not limited in application to the details of construction and arrangement of components set forth in the following description or illustrated in the accompanying drawings. The materials, systems, methods, and devices may be implemented in other examples and may be implemented in various ways. The specific examples are provided herein for illustrative purposes only and are not intended to be limiting. In particular, functions, components, elements, and features described in connection with any one or more examples are not intended to be excluded from a similar role in any other examples.

[0035] Examples disclosed herein may be combined with other examples in any manner consistent with at least one of the principles disclosed herein, and references to "one example," "some examples," "alternative examples," "various examples," "one example," etc. are not necessarily mutually exclusive and are intended to indicate that a particular feature, structure, or characteristic described may be included in at least one example. Appearances of such terms herein do not necessarily all refer to the same example.

[0036] Additionally, the phraseology and terminology used herein are for purposes of description and should not be considered limiting. References herein to examples, components, elements, acts, or functions of computer program products, systems, and methods in the singular can also encompass embodiments that include the plural, and references herein to any example, component, element, act, or function in the plural can also encompass examples that include only the singular. Thus, singular or plural references are not intended to limit the presently disclosed systems or methods, their components, acts, or elements. Uses of "including," "comprising," "having," "containing," "involving," and variations thereof herein are meant to encompass the items listed thereafter, and equivalents thereof, as well as additional items. References to "or" can be interpreted as inclusive, such that any term described using "or" can refer to either one, more than one, or all of the described terms.

[0037] In this invention, these limitations have been overcome by preparing low Co-loading catalysts supported on MgAl2O4, MgO-Al2O3, and methods for using the catalysts to produce CNTs with high purity and controlled morphology. These CNTs are useful in energy storage and other commercial applications requiring or benefiting from these characteristics.

[0038] Unlike the prior art, in the examples herein, a solution containing either Co salt alone or a combination of Co and Mg salts is contacted with an alumina precursor (Al(OH)3) to form a homogeneous paste in a mixing device. +2 and Mg +2 The ion is Al(OH)3 OH - After a certain aging period during which the salts are exchanged with the bases, the paste is dried at different temperatures to control the thermal decomposition of the salts. The drying step helps prevent exothermic decomposition of the active metal surface dispersion, which can occur if the removal of water and the reaction of the metal salts with nitrates or acetates are carried out more quickly, such as in a reactor. The excess heat generated by the exothermic decomposition reaction of the nitrates or acetates can be controlled by using N2 or air diluted with N2 flow during the drying step.

[0039] The dried solid is crushed and sieved to the desired particle size, depending on the reactor used for CNT synthesis, and then calcined at a moderate temperature (e.g., 450 °C) to control the textural characteristics (specific surface area, pore size distribution, pore volume) and surface properties (surface acidity, surface charge) of the catalyst support and to prevent the active metal from reacting with the alumina of the catalyst support to form the inactive CoAl2O4 phase. Depending on the type of aluminum hydroxide used (boehmite, gibbsite, or bayerite), different types of transition alumina (γ-Al2O3, χ-Al2O3, or η-Al2O3) can be produced after calcination at temperatures above 250 °C. This is the importance of a well-controlled calcination process.

[0040] Carbon nanotube synthesis is carried out in a fluidized bed or rotating tube reactor at temperatures between 600°C and 800°C in the presence of a gaseous mixture of a carbon source (e.g., ethylene) and hydrogen. Ethane, propane, propylene, and butane, and possibly other gaseous hydrocarbons, alone or in any desired combination, can also be used as carbon sources. In some instances, the catalyst residence time in a rotating tube reactor is 10 minutes, compared to 30 minutes for reactions in prior art fluidized bed reactors. Under these reaction conditions, carbon yields of over 90% are obtained, a conversion level comparable to that of the prior art.

[0041] After removing metals and support particles from the catalyst using an inorganic acid solution, a product with a carbon purity of greater than 99.8 wt% is obtained.

[0042] Examples include:

[0043] Example 1:

[0044] In this example, a prior art CoMo / Al2O3 supported catalyst was prepared according to methods described in the prior art (e.g., patents incorporated herein by reference), as shown in Figure 1. Aluminum hydroxide (Al(OH)3) was calcined at 400°C for 4 hours to obtain an alumina (Al2O3) catalyst support. The calcined material was then impregnated with a solution containing cobalt nitrate and ammonium molybdate (active metal solution). The catalyst had a Co:Mo:Al2O3 molar ratio of 5:1:15. Citric acid was added to the solution to prevent precipitation of cobalt molybdate. The impregnated material was then aged with stirring in a thermostatic oven at 60°C for 30 minutes, after which excess water was removed from the solid under vacuum to form a wet cake. Drying was completed at 120°C, and the solid was calcined at 300°C for 4 hours. The catalyst was sieved to a particle size between 100 and 300 microns to produce the final catalyst.

[0045] Two catalysts according to the present invention were also prepared. One was based on Co / Al2O3, and the other was based on Co / MgO-Al2O3. These catalysts were prepared according to the following procedure, outlined in Figure 2. For the first catalyst, a solution containing cobalt nitrate was contacted with Al(OH)3 in a mixer to form a paste. For the second catalyst, magnesium nitrate salt was added to the cobalt nitrate solution. The molar compositions of Co, MgO, and Al2O3 in the catalyst (Co:Al2O3 and Co:MgO:Al2O3) were 14:70 and 14:1:70, respectively. The impregnated material was then aged at room temperature for 2 hours, dried at 60°C for 3 hours, and finally dried in a stream of air at 120°C for 2 hours. The dried solid particles were sieved to a particle size between 100 and 300 microns and calcined in a stream of air at 450°C for 3 hours to produce the final catalyst.

[0046] These catalysts were used for the synthesis of carbon nanotubes carried out in a fluidized bed reactor (FBR) and a rotating tube reactor (RR) under the following conditions:

[0047] For the fluidized bed reactor, the process gas contained 80v% ethylene and 20v% H2, the space velocity (WHSV) was 180 L C2H4 / g catalyst hour, the reaction temperature was 675°C, and the reaction times were 10, 20, and 30 min.

[0048] For the rotating tube reactor, the process gas contained 60v% ethylene, 20v% H2 and 20v% N2, the catalyst residence time in the reaction zone was about 10 minutes, and the ethylene flow rate / catalyst feed ratio was 3 liters / gram.

[0049] Table 1 contains the CNT synthesis results for catalysts prepared using the prior art catalyst and both Co / Al2O3 and Co / MgO-Al2O3, all performed in a fluidized bed reactor (FBR) at different reaction times, as described above. Ethylene conversion was observed to gradually increase with reaction time. The catalyst prepared by the method of the present invention exhibited the highest MWCNT yield, despite the fact that the active phase composition of the catalyst prepared by the present invention was 5 wt% lower, as explained below. The maximum MWCNT content reached for the catalysts of the present invention was 94.2 wt% for the Co / MgO-Al2O3 catalyst and 96.4 wt% for the Co / Al2O3 catalyst, corresponding to CNT / catalyst productivity ratios of approximately 16.2 and 17.9, respectively.

[0050] The same table shows the results of MWCNT yield obtained after 10 minutes of reaction in a rotating tube reactor (RR) for the Co / MgO-Al2O3 catalyst of the present invention. It is observed that the MWCNT yield obtained in the rotating reactor is lower than that in the fluidized bed reactor (88.4 wt% vs. 92.0 wt%). This is mainly due to better heat and mass transfer and optimal contact between the ethylene molecules and the active surface of the catalyst in the fluidized bed reactor. However, this catalyst is much more active than that of the prior art, which showed an MWCNT yield of only 75.5 wt%. [Table 1]

[0051] Example 2: Catalyst properties

[0052] In this example, we compare the textural characteristics of alumina supports and prepared cobalt-supported catalysts (Table 2). The surface area of ​​the alumina and aluminum hydroxide used in the synthesis of the catalyst is approximately 5 m. 2 When Al(OH)3 is calcined at 400°C, the specific surface area is approximately 369 m 2 / g, and the pore volume increased to about 0.29 cc / g. The average pore diameter was 27 A°. In the prior art catalyst preparation, when active metals were deposited on the Al2O3 surface, the specific surface area and pore volume were about 267 m2 / g and about 0.22 cc / g. However, the catalyst of the present invention has a surface area and pore volume (348 m) comparable to that of the Al2O3 support. 2 / g and 0.27 cc / g), indicating a lower cobalt content (about 33% less cobalt) than the prior art catalyst. Despite the lower active phase composition of the catalyst of the present invention, its MWCNT yield is significantly higher. [Table 2]

[0053] Example 3: Properties of MWCNT purified material.

[0054] To compare the intrinsic properties of carbon nanotube synthesis, the CNT samples obtained after 30 minutes of reaction time were subjected to a chemical purification process. An acid solution containing 22.5% v / v HF was used to digest the catalyst particles. This process was carried out overnight at room temperature under stirring. The solid was then separated from the HF acid solution by vacuum filtration. The purified CNTs were washed several times with copious amounts of deionized water until they reached a neutral pH, and then dried using a freeze dryer.

[0055] The characterization results of these purified carbon nanotube samples are shown in Table 3. While the CNT samples corresponding to the prior art have a carbon purity of about 96.84 wt% (i.e., a residual metal content of about 3.16 wt%), the residual metals in the CNTs synthesized using the catalyst of the present invention are about 1 / 10 of this, i.e., between 0.30-0.35 wt%, which represents a significantly higher carbon purity of about 99.65 to 99.70 wt%. The specific surface area (BET surface area) results obtained for the CNT purified samples are significantly higher than those obtained for the prior art and catalysts used in the present invention (264 m, respectively). 2 / g vs 267m 2 / g and 337m 2 / g vs 348m 2The pore volume of the pure samples showed only slight differences in pore volume (cc / g). However, the pore volume of the pure samples increased significantly relative to the values ​​observed for each catalyst. The largest increase in pore volume was observed for the CNTs synthesized with the catalyst of the present invention (2.5 cc / g for the present invention vs. 0.92 cc / g for the prior art). The tapped bulk density value was lower for the CNTs of the prior art (0.056 g / cc). Scanning electron microscopy (SEM) revealed that both purified carbon nanotubes had outer diameters between 10 and 14 nm for the prior art and between 8 and 12 nm for the present invention. The conductive properties of the purified samples were determined by measuring surface resistivity using a four-probe electrode. The results in Table 3 clearly show that the CNTs synthesized using the catalyst of the present invention have higher conductivity. Note that the same CNT content was used in the sample preparation for surface resistivity measurements. This is because the carbon purity differs between the two samples, and the amount of material used in sample preparation needs to be standardized. [Table 3]

[0056] Raman spectrum:

[0057] Raman spectroscopy is a useful technique for characterizing carbon nanotubes, where the purity of the tubular carbon species and the number of structural defects can be determined by measuring the G / D ratio intensity. Analysis was performed using 532 and 638 nm laser sources. -1 The D band located at approximately 1585 cm is due to the presence of amorphous carbon and / or structural defects in carbon nanotubes. -1The G band located at corresponds to tubular carbon. The G / D ratio values ​​obtained using a 532 nm laser source were lower compared to those obtained using a 638 nm laser source. Figures 3A-3C (using a 638 nm laser source) and Figures 4A-4C (using a 532 nm laser source) show Raman spectra corresponding to a prior art CNT-purified sample and a CNT-purified sample of the present invention obtained using a Co / MgO-Al2O3 catalyst. When analyzed using both laser sources, a higher G / D ratio was clearly observed for the sample of the present invention, indicating that the CNTs have higher purity and / or fewer structural defects in the tubular carbon. Figures 5A and 5B show Raman spectra corresponding to a purified CNT sample obtained using a Co / Al2O3 catalyst. The G / D ratio values ​​obtained using a 532 nm laser are similar to those obtained using a Co / MgO-Al2O3 catalyst. When using a 638 nm laser, the obtained G / D ratio values ​​are significantly higher than those of the prior art CNT-purified product.

[0058] ICP (inductively coupled plasma) spectroscopy is a useful technique for analyzing the chemical composition of materials. Analysis was performed on a sample containing 99.70 wt% MWCNTs, and the results showed that the total metal content was less than 2000 ppm, with approximately 1814 ppm (about 90%) of this metal being Co. Table 4 provides a more complete description of the metals observed in the sample. [Table 4]

[0059] Example 4: Morphological characterization of CNTs.

[0060] In this example, CNTs synthesized with the Co / MgO-Al2O3 catalyst at different reaction temperatures in Example 1 were analyzed by scanning electron microscopy (SEM) to examine their morphological characteristics. Figure 6 contains SEM images obtained at 1KX, 5KX, and 25KX magnifications corresponding to CNTs obtained in the present invention. Figure 10 provides the same images for CNTs obtained using the prior art catalyst described above. The differences in the morphological characteristics of the CNTs are clearly observed. The carbon nanotubes of the present invention form long, rod-like structures with lengths exceeding 10 microns and CNT diameters varying between 8 and 12 nm. As the reaction time increases from 10 to 30 minutes, these rods become longer, while the CNT diameter remains unchanged (Figure 6).

[0061] Figure 7 shows an SEM image taken at 10KX magnification of the CNTs synthesized in this invention, which shows the growth of MWCNT rods on catalyst particles.

[0062] Figure 8 shows the SEM image corresponding to the MWCNT purified sample, where no residual catalyst particles are visible and the tubes maintain the same morphology before and after purification.

[0063] Figure 9 shows an SEM image taken at 10KX magnification of the CNTs of the present invention synthesized using a Co / Al2O3 catalyst. The morphology appeared similar compared to the CNTs synthesized using a Co / MgO-Al2O3 catalyst.

[0064] Carbon nanotubes synthesized according to the prior art exhibit an aggregated, cotton-ball-like structure (Figure 10). The tubes are short (<10 microns) and their diameters vary between 10 and 14 nm. These differences in CNT morphology can affect their dispersibility and electrical conductivity properties, which are important for the fabrication of lithium-ion battery electrodes.

[0065] Example 5: Method for dispersing carbon nanotubes.

[0066] An aqueous dispersion of purified CNTs was prepared using polyvinylpyrrolidone (PVP) and N-methyl-2-pyrrolidone (NMP) as dispersants, the composition of which is shown in Table 5. [Table 5]

[0067] The procedure for preparing the CNT dispersion was as follows.

[0068] 1) Weigh the desired amount of purified CNTs and PVP stock (9.1 wt% PVP in NMP) solution into a cup.

[0069] 2) Apply a high speed mixer and mill using a paste composition of 6.26 wt% purified CNT, 6.24 wt% PVP and 87.49% NMP.

[0070] 3) further dilution with NMP to obtain a final composition of 1.53 wt% purified CNTs, 1.52 wt% PVP, and 96.95 wt% NMP; and

[0071] 4) Sonicate the above solution using an energy of 8600 kJ / L.

[0072] The viscosity of the prepared CNT dispersion is about 3800 cps. The prepared CNT dispersion was tested using a Hegman gauge, which showed a uniform particle size distribution.

[0073] SEM analysis was used to characterize the morphology of the CNTs in the dispersion. To analyze individual CNTs, the CNT dispersion was diluted with DI water, and the diluted sample was then vacuum filtered through a polycarbonate or alumina filter and rinsed several times with water and alcohol to remove residual dispersant. To measure the length of the dispersed CNTs, approximately 50 measurements were taken at several spots on the filter. The results of the length distribution analysis are shown in Figure 11 and discussed below.

[0074] As shown in the SEM images in Figures 6 and 7 (as prepared) and Figure 8 (after purification), the length of the tubes after synthesis and purification is greater than 10 microns. However, SEM images of the CNTs after dispersion preparation show a length distribution almost entirely in the range of 1 to 5 microns.

[0075] During dispersion, CNTs become entangled, potentially resulting in breakage. If the tubes are sufficiently long to begin with (as in this case) and not sufficiently entangled to begin with (as in this case), it is possible to create a dispersion containing individualized CNTs with an average length of at least about 1 micron (as in this case, the length averages about 2 microns). Note that, assuming the CNTs are well dispersed, a longer average length is desirable. This is to lower the percolation threshold for the CNT electrically conductive network (i.e., to achieve a low electrical resistivity network, a lower CNT loading is required). However, it may be best to ensure that relatively few CNTs in the dispersion are longer than about 5 microns. This is to minimize CNT toxicity. Should CNTs be inhaled into the lungs, CNTs longer than about 5 microns are not readily ingested by macrophages, which remove debris from the lungs. This example clearly demonstrates a dispersion with an average length greater than about 1 micron, with negligible fraction longer than about 5 microns.

[0076] CNTs produced using the catalysts of this disclosure are well suited for use in LiBs. For one thing, CNTs do not contain iron, which is the cause of many problems in LiBs. Battery manufacturers prefer iron-free conductive carbon materials. Additionally, the metal loading levels of the purified CNTs (primarily Co, or Co and Mg) are very low, in the 2,000 ppm range (see Table 4), with the majority being Co (1814 ppm).

[0077] Typically, "as-produced" CNT products contain approximately 90 wt% carbon and approximately 10 wt% supported catalyst. Current catalyst supports typically consist of either alumina or magnesium oxide / alumina. Because the catalyst support typically represents more than 90 wt% of the supported catalyst, cobalt metal represents less than 10 wt% of the supported catalyst and less than 1 wt% of the as-produced product. Importantly, the vast majority of the cobalt metal ultimately becomes active catalytic sites. Any active sites ultimately nucleate CNT growth and are fully encapsulated by carbon during CNT synthesis. Based on high CNT yields and the amount of cobalt in the catalyst composition, it is expected that at least 80% of the cobalt metal ultimately becomes active catalytic sites. Therefore, less than approximately 20 wt% of the cobalt ultimately becomes encapsulated by carbon. During chemical refining, essentially 100% of the catalyst support is removed, leaving a purified CNT product composition with greater than approximately 99.5 wt% carbon. Consistent with this, ICP analysis indicates that the cobalt content in the purified CNT product is approximately 1814 ppm, or 0.18 wt%. If only 20 wt% of this cobalt is "inert" and therefore not encapsulated by carbon, this corresponds to 0.036 wt%. Typical conductive CNT additive loadings in LiB cathodes or anodes are expected to be approximately 0.5 wt% or less. Therefore, the cobalt impurity not encapsulated by carbon is less than approximately (0.036% x 0.5%) = 0.00018 wt%, or approximately 1.8 ppm of the electrode weight. This level of unencapsulated cobalt impurity is not expected to be a concern for battery cell manufacturers and battery industry professionals. Furthermore, Co is already present in LiBs and is generally a harmless element present in the conductive carbon (CNT) used in cathodes and anodes.

[0078] While several aspects of at least one example have been described above, it should be understood that various changes, modifications, and improvements will readily occur to those skilled in the art. Such changes, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention. Accordingly, the foregoing description and drawings are by way of example only, and the scope of the invention should be determined from proper construction of the appended claims and their equivalents.

Claims

1. a support comprising alumina; and cobalt seeds on the surface of the support, wherein the cobalt is the only active catalytic species for carbon nanotube (CNT) growth; A catalyst precursor composition comprising:

2. The catalyst precursor composition of claim 1 , wherein the surface of the support is iron-free.

3. 10. The catalyst precursor composition of claim 1, wherein said support further comprises an element from Group IIA of the periodic table.

4. 4. The catalyst precursor composition of claim 3, wherein said element from Group IIA of the periodic table comprises magnesium.

5. The catalyst precursor composition of claim 1 , wherein the cobalt species comprises cobalt oxide.

6. 10. The catalyst precursor composition of claim 1, wherein said cobalt species is less than 15% by weight of said catalyst precursor.

7. 2. The catalyst precursor composition of claim 1, wherein said cobalt species is about 10% by weight of said catalyst precursor.

8. 300m ２ 10. The catalyst precursor composition of claim 1 having a BET surface area of ​​greater than 1000 W / g.

9. 10. The catalyst precursor composition of claim 1 having a pore volume of at least about 0.25 cc / g.

10. 10. The catalyst precursor composition of claim 1 configured to provide CNTs in at least about 85% yield.

11. 10. The catalyst precursor composition of claim 1 configured to produce multi-walled CNTs.

12. 300m ２ 10. The catalyst precursor composition of claim 1 configured to produce CNTs having a BET surface area of ​​greater than 1000 W / g.

13. 10. The catalyst precursor composition of claim 1 configured to produce CNTs having a pore volume of at least about 2 cc / g.

14. 10. The catalyst precursor composition of claim 1 configured to produce CNTs having a bulk density of at least about 0.08 g / cc.

15. 10. The catalyst precursor composition of claim 1 configured to produce CNTs having a diameter range of about 8 nm to about 12 nm.

16. 10. The catalyst precursor composition of claim 1 configured to produce CNTs having a G / D relative Raman intensity ratio of at least about 1.0 measured with a 638 nm laser source and a G / D relative Raman intensity ratio of at least 0.9 measured with a 532 nm laser source.

17. 10. The catalyst precursor composition of claim 1 configured to produce CNTs having a length of at least about 10 microns.

18. 1. A catalyst composition for growing carbon nanotubes (CNTs), comprising: an alumina support; and cobalt on the surface of the support, wherein the cobalt is the only active catalytic species for CNT growth; A catalyst composition comprising:

19. 20. The catalyst composition of claim 18, wherein the surface of the support is iron-free.

20. 20. The catalyst composition of claim 18, wherein said support further comprises an element from Group IIA of the periodic table.

21. 21. The catalyst composition of claim 20, wherein the element from Group IIA of the periodic table comprises magnesium.

22. 20. The catalyst composition of claim 18, wherein the cobalt species comprises cobalt oxide.

23. 20. The catalyst composition of claim 18, wherein said cobalt species is less than 15% by weight of said catalyst precursor.

24. 20. The catalyst composition of claim 18, wherein said cobalt species is about 10% by weight of said catalyst precursor.

25. 300m ２ 20. The catalyst composition of claim 18 having a BET surface area of ​​greater than 1 / g.

26. 20. The catalyst composition of claim 18 having a pore volume of at least about 0.25 cc / g.

27. 20. The catalyst composition of claim 18 configured to obtain CNTs in at least about 85% yield.

28. 20. The catalyst composition of claim 18 configured to produce multi-walled CNTs.

29. 300m ２ 20. The catalyst composition of claim 18 configured to produce CNTs having a BET surface area of ​​greater than 1000 W / g.

30. 20. The catalyst composition of claim 18 configured to produce CNTs having a pore volume of at least about 2 cc / g.

31. 20. The catalyst composition of claim 18 configured to produce CNTs having a bulk density of at least about 0.08 g / cc.

32. 20. The catalyst composition of claim 18 configured to produce CNTs having a diameter range of about 8 nm to about 12 nm.

33. 20. The catalyst composition of claim 18 configured to produce CNTs having a G / D relative Raman intensity ratio of at least about 1 as measured with a 638 nm laser source and a G / D relative Raman intensity ratio of at least 0.9 as measured with a 532 nm laser source.

34. 20. The catalyst composition of claim 18 configured to produce CNTs having a length of at least about 10 microns.

35. 1. A method for preparing a catalyst precursor composition comprising a support comprising alumina and a cobalt species on a surface of the support, the method comprising: Cobalt is the only active catalytic species for carbon nanotube (CNT) growth, and the method comprises: providing an aluminum hydroxide support precursor; contacting the aluminum hydroxide support precursor with a solution comprising at least a cobalt salt to form a paste; drying and calcining the paste to form a catalyst precursor; A method comprising:

36. 36. The method of claim 35, wherein the solution comprises a cobalt salt and a magnesium salt.

37. 36. The method of claim 35, wherein said contacting step comprises mixing said aluminum hydroxide support precursor and said solution in a mixer.

38. 36. The method of claim 35, wherein drying the paste comprises aging the paste at room temperature.

39. 39. The method of claim 38, wherein drying the paste further comprises exposing the aged paste to an elevated temperature for several hours to produce a dried paste.

40. 40. The method of claim 39, wherein said firing step comprises exposing said dried paste to a temperature of at least about 400°C for at least about 2 hours in flowing air.

41. 41. The method of claim 40, wherein the firing step is performed at about 450°C.

42. Co:MgO:Al:about 14:up to about 1:up to about 70, respectively ２ O ３ 36. The method of claim 35 having a molar ratio composition.

43. 36. The method of claim 35, further comprising the step of sizing the dried paste prior to the firing step.

44. 44. The method of claim 43, wherein the size-classifying step comprises producing dry solid particles of at least about 100 microns.

45. 45. The method of claim 44, wherein the dry solid particles are produced in a size range of about 100 microns to about 300 microns.

46. 36. The method of claim 35, wherein the aluminum hydroxide comprises one or more of gibbsite, bayerite, boehmite, and those obtained by hydrolysis of aluminum alkoxides.

47. A purified carbon nanotube (CNT) composition comprising: at least about 99% CNTs; and about 0.5% or less of a metal catalyst Including, Here, the residual mass measured by the ashing method is about 0.80 wt % or less. A purified carbon nanotube composition.

48. 48. The purified CNT composition of claim 47, comprising at least about 99.7% CNTs.

49. 48. The purified CNT composition of claim 47, comprising about 0.3% or less of a catalyst.

50. 48. The purified CNT composition of claim 47, comprising at least about 85% CNTs prior to purification.

51. 48. The purified CNT composition of claim 47, wherein the CNTs comprise multi-walled CNTs.

52. The CNT is 300 m ２ 48. The purified CNT composition of claim 47, having a BET surface area of ​​greater than 1 / g.

53. 48. The purified CNT composition of claim 47, wherein the CNTs have a pore volume of at least about 2 cc / g.

54. 48. The purified CNT composition of claim 47, wherein the CNTs have a bulk density of at least about 0.08 g / cc.

55. 48. The purified CNT composition of claim 47, wherein the CNTs have a diameter range of about 8 nm to about 12 nm.

56. 48. The purified CNT composition of claim 47, wherein the CNTs have a G / D relative Raman intensity ratio of at least about 1 measured using a 638 nm laser source and a G / D relative Raman intensity ratio of at least 0.9 measured using a 532 nm laser source.

57. 48. The purified CNT composition of claim 47, wherein the CNTs have a length of at least about 10 microns.

58. 48. The purified CNT composition of claim 47, wherein at least a majority of the CNTs have a length after synthesis and chemical purification of at least about 10 microns.

59. 48. The purified CNT composition of claim 47, wherein at least a majority of the CNTs have a length after dispersion in the range of about 1 micron to about 5 microns.

60. 48. The purified CNT composition of claim 47, wherein at least a majority of the CNTs have a bulk density in the range of about 0.06 to about 0.09 g / cc.

61. 48. The purified CNT composition of claim 47, made in a fluidized bed reactor or a rotating tube reactor using a catalyst comprising an alumina support and cobalt on the surface of the support, wherein cobalt is the only active catalytic species for CNT growth, and the surface of the support is iron-free.

62. A carbon source for CNTs is provided to the reactor, the carbon source being CO, CH ４ , C ２ H ２ , C ２ H ４ , C ３ H ６ , C ２ H ６ , C ３ H ８ , and C ４ H １０ 62. The purified CNT composition of claim 61, comprising one or more of:

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