HK1063772A - Method of producing carbon nanotubes and catalysts therefor - Google Patents

Description

Method and catalyst for producing carbon nanotubes

The application is a divisional application of Chinese patent application 00808276.6 (application date 2000, 6, 1).

Background

The present invention relates to a method and catalyst for producing carbon nanotubes, and more particularly, but not by way of limitation, to a method and catalyst for producing single-walled carbon nanotubes.

Carbon nanotubes (also known as carbon fibrils) are graphite sheet seamless tubes with a complete fullerene shield, which are first multi-layered concentric tubes or multi-walled carbon nanotubes and then single-walled carbon nanotubes in the presence of a transition metal catalyst. Carbon nanotubes show promising applications including nanoscale electronics, high-strength materials, electron field emission, the tip of scanning probe microscopy, and gas storage.

In general, single-walled carbon nanotubes are preferred over multi-walled carbon nanotubes in these applications because the former have fewer defects and are therefore stronger and more conductive than multi-walled carbon nanotubes of similar diameter. Single-walled carbon nanotubes are less prone to defects than multi-walled carbon nanotubes because multi-walled carbon nanotubes survive accidental defects by forming bridges between unsaturated carbon valences, while single-walled carbon nanotubes have no septa that can compensate for defects.

However, the feasibility of these new single-walled carbon nanotubes in the amounts required for practical techniques remains problematic. There remains a need for large scale processes for producing high quality single-walled carbon nanotubes.

Currently, there are three main methods for synthesizing carbon nanotubes. These include laser ablation of carbon (Thess, A et al, Science 273, 483(1996)), arc discharge of graphite rods (Journet, C et al, Nature 388, 756(1997)) and chemical vapor deposition of hydrocarbons (Ivanov, V et al, chem. Phys. Lett 223, 329 (1994); LiA et al, Science274, 1701 (1996)). The production of multi-walled carbon nanotubes by catalytic hydrocarbon cracking has now reached industrial scale (us patent 5578543), while the production of single-walled carbon nanotubes has been produced on gram scale by laser (Rinzler, a.g. et al, appl.phys.a, 67, 29(1998)) and arc (Haffner, j.h. et al, chem.phys.lett.296, 195(1998)) techniques.

Unlike laser and arc techniques, carbon vapor deposition on transition metal catalysts tends to produce multi-walled carbon nanotubes as the primary product, rather than single-walled carbon nanotubes. However, there has been some success in producing single-walled carbon nanotubes from catalytic hydrocarbon cracking processes. Dai et al (Dai, H et al, chem. phys. lett.260, 471(1996)) demonstrated the production of reticulated single-walled carbon nanotubes by disproportionation of carbon monoxide (CO) and molybdenum (Mo) supported on alumina catalyst heated to 1200 ℃. From the reported electron microscopy images, Mo metal apparently attached to the tip of the nanotube. The reported diameter of single-walled carbon nanotubes is typically 1-5 nm and is controlled by the particle size of Mo. Catalysts containing iron, cobalt or nickel were used at temperatures of 850 ℃ -. More recently, rope-like bundles of single-walled carbon nanotubes have been produced by thermal cracking of benzene with iron catalyst and sulfur additive at temperatures of 1100-. The synthesized single-walled carbon nanotubes are arranged and woven together in roughly bundles, similar to those obtained from laser gasification or arc processes. It has been proposed to use a laser target comprising one or more group VI or group VIII transition metals to form single-walled carbon nanotubes (WO 98/39250). The use of metal catalysts comprising iron and at least one element selected from the group consisting of group V (V, Nb and Ta), group VI (Cr, Mo and W), group VII (Mn, Tc and Re) or the lanthanide series has been proposed (us patent 5707916). However, methods using these catalysts do not teach the production of large numbers of nanotubes having a high ratio of single-walled carbon nanotubes to multi-walled carbon nanotubes.

In addition, the separation step before or after the reaction step accounts for most of the capital and operating costs required to produce the carbon nanotubes. Therefore, the purification of single-walled carbon nanotubes from multi-walled carbon nanotubes and contaminants (i.e., amorphous and graphitic carbon) requires more time and expense than the actual production of carbon nanotubes.

In addition, one of the greatest limitations in the current art is the inability to simply and directly quantify the different forms of carbon contained in a particular synthesis. Currently, Transmission Electron Microscopy (TEM) is the most widely used characterization technique for determining the fraction of single-walled carbon nanotubes in a particular sample. However, transmission electron microscopy can only provide a qualitative description of the type of carbon species produced. It is difficult to determine how much total yield a given transmission electron micrograph represents. Semi-quantitatively determining the distribution of different carbon species in a sample using any statistical data is time consuming and cannot be used as a routine quality control means for large scale operations using transmission electron microscopy.

Therefore, there is a need for new and improved methods of producing nanotubes that can synthesize substantially pure single-walled carbon nanotubes in industrial scale quantities at lower temperatures than existing methods, as well as for direct quantification of different forms of carbon in a particular synthesis. The present invention relates to such a method for producing carbon nanotubes and quantifying the synthesized product.

Summary of The Invention

According to the present invention, there are provided a catalyst and a method for producing carbon nanotubes, which can avoid the drawbacks and disadvantages of the prior art. Specifically, the method includes contacting in a reactor unit metallic catalytic particles with an effective amount of a carbon-containing gas at a temperature sufficient to catalytically produce carbon nanotubes, wherein a substantial portion of the carbon nanotubes are single-walled carbon nanotubes, the metallic catalytic particles comprising a group VIII metal other than iron and a group VIb metal.

In addition, in accordance with the present invention, methods are provided for determining catalyst composition and reaction conditions to optimize the production of single-walled carbon nanotubes. Specifically, the method includes contacting a sample of the carbon nanotube-containing product with an effective amount of an oxygen-containing gas in a reactor unit to oxidize carbon present in the sample while increasing the temperature within the reactor unit. The amount of carbon dioxide released from the sample is detected and the presence of a particular carbon species in the sample is determined by the carbon dioxide released from the sample at a particular temperature. The catalyst composition and/or reaction conditions are varied until the single-walled carbon nanotubes are present in amounts significantly higher than all other carbon species in the carbon nanotube-containing product sample.

In one aspect of the invention, the metallic catalytic particles are bimetallic catalysts deposited on a support, such as silica. The ratio of group VIII metal to group VIb metal in the bimetallic catalyst is in the range of about 1: 5 to about 2: 1.

It is an object of the present invention to provide a method for producing single-walled carbon nanotubes in relatively large quantities and at relatively low temperatures.

It is another object of the present invention to provide a method for quantitatively determining the different forms of carbon present in a sample, including single-walled carbon nanotubes, multi-walled carbon nanotubes, and amorphous carbon, and thereby determining the selectivity of a particular catalyst and optimizing the reaction conditions for producing carbon nanotubes.

Other objects, features and advantages of the present invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings and the appended claims.

Brief Description of Drawings

FIG. 1 is a graph obtained by passing through SiO₂Transmission electron micrograph (magnification about 100000) of single-walled carbon nanotubes obtained by Co disproportionation catalyzed by Co/Mo catalyst on top at about 700 ℃.

FIG. 2 is a transmission electron micrograph (at approximately 400000 magnification) of the sample used in FIG. 1 at higher resolution showing bundles of single-walled carbon nanotubes (SWNTs).

Fig. 3 is a transmission electron micrograph of the sample used in fig. 1 showing aligned single-walled carbon nanotubes growing in the beam.

FIG. 4 is a transmission electron micrograph of the sample used in FIG. 1 showing an end view of a bundle of single walled carbon nanotubes.

FIG. 5 is a scanning electron micrograph of the sample used in FIG. 1 showing single walled carbon nanotube bundles grown from the catalytic surface.

FIG. 6 is a Co: Mo/SiO solid solution₂Temperature programmed oxidation profile of the product obtained from the CO disproportionation reaction catalyzed by the catalyst at about 700 ℃.

FIG. 7 is a graph showing a chemical composition formed of SiO₂Co-on-SiO catalyst₂Mo on catalyst and on SiO₂Temperature programmed oxidation profile of the product obtained from CO disproportionation catalyzed by the above Co: Mo catalyst at about 700 ℃.

FIG. 8 is a graph showing a chemical composition formed of SiO₂The Co: Mo over Co catalyst resulted in a temperature programmed oxidation profile of the product from Co disproportionation catalyzed at about 700 c, with varying Co: Mo molar ratios.

FIG. 9 is a Co: Mo/SiO solid phase solution₂Temperature programmed oxidation profile of the product from a catalyst catalyzed CO disproportionation reaction in which the reaction temperature is varied.

FIG. 10 is a Co: Mo/SiO solid phase solution₂Temperature programmed oxidation profile of product from CO disproportionation catalyzed by catalyst at about 700 deg.CWherein the percentage of CO in the carbon-containing gas used in the CO disproportionation reaction is varied.

FIG. 11 is a graph of Co: Mo/SiO₂Temperature programmed oxidation profile of the product obtained from the CO disproportionation reaction catalyzed by the catalyst at about 700 ℃, wherein the time of the CO disproportionation reaction is varied.

Detailed Description

The present invention relates to a catalyst and method for producing large quantities of single-walled carbon nanotubes, wherein an effective amount of a carbon-containing gas is passed through bimetallic catalytic particles comprising at least one group VIII metal and at least one group VIb metal at a lower temperature; and to a method for reliably and quantitatively detecting the yield of single-walled carbon nanotubes in a product containing carbon nanotubes.

Specifically, the method for producing single-walled carbon nanotubes comprises contacting bimetallic catalytic particles comprising a group VIII metal and a group VIb metal with an effective amount of a carbon-containing gas in a reactor heated to a temperature of about 500 ℃ -. The carbon-containing gas may be continuously supplied into the reactor, or the carbon-containing gas may be maintained in the reactor in a static atmosphere.

As used herein, the phrase "an effective amount of a carbon-containing gas" refers to a sufficient amount of gaseous carbon species present to cause carbon to be deposited on the metal catalytic particles at higher temperatures as described below to form carbon nanotubes.

The metal catalyst particles used herein include a catalyst component. The catalysts provided and used in the present invention are bimetallic. The bimetallic catalyst contains at least one metal other than iron selected from group VIII, including Co, Ni, Ru, Rh, Pd, Ir, Pt, and mixtures thereof, and at least one metal selected from group VIb, including Cr, W, Mo, and mixtures thereof. Specific examples of bimetallic catalysts useful in the present invention include Co-Cr, Co-W, Co-Mo, Ni-Cr, Ni-W, Ni-Mo, Ru-Cr, Ru-W, Ru-Mo, Rh-Cr, Rh-W, Rh-Mo, Pd-Cr, Pd-W, Pd-Mo, Ir-Cr, Ir-W, Ir-Mo, Pt-Cr, Pt-W, and Pt-Mo. Particularly preferred catalysts of the present invention include Co-Mo, Co-W, Ni-Mo and Ni-W.

There is a synergy between the two metal components of the bimetallic catalyst because the metallic catalytic particles containing the bimetallic catalyst are more effective for producing single-walled carbon nanotubes than are metallic catalytic particles containing one of the group VIII or group VIb metals as the catalyst. The synergistic effect of the bimetallic catalyst will be described in more detail below.

The ratio of group VIII metal to group VIb metal in the metallic catalytic particles also affects the selectivity of the process of the invention for producing single-walled carbon nanotubes. The ratio of group VIII metal to group VIb metal is preferably from about 1: 10 to about 15: 1, more preferably from about 1: 5 to about 2: 1. Generally, the concentration of group VIb metals (e.g., Mo) will exceed the concentration of group VIII metals (e.g., Co) in the metallic catalytic particles used to selectively produce single-walled carbon nanotubes.

The metallic catalytic particles may contain more than one metal selected from both group VIII and group VIb, as long as at least one metal selected from each of the groups is present. For example, the metallic catalytic particles may contain (1) more than one group VIII metal and one group VIb metal, (2) one group VIII metal and more than one group VIb metal, or (3) more than one group VIII metal and more than one group VIb metal.

Bimetallic catalysts can be made by simply mixing two metals. Bimetallic catalysts may also be formed in situ by decomposition of precursor compounds, such as bis (cyclopentadienyl) cobalt or bis (cyclopentadienyl) molybdenum chlorides.

The catalyst is preferably deposited on a support, for example Silica (SiO)₂) MCM-41(Mobil crystalline material-41), alumina (Al)₂O₃) MgO, Mg (Al) O (aluminum-stabilized magnesium oxide), ZrO₂Molecular sieve zeolites, or other oxidic supports known in the art.

Metal catalytic particles, i.e. catalysts deposited on a support, can be made by evaporating a mixture of metals on a planar substrate such as quartz, glass, silicon and silica surfaces in a manner well known to those skilled in the art.

The total amount of bimetallic catalyst deposited on the support may vary over a wide range, but generally comprises from about 1 to about 20 percent by weight of the total weight of the metal catalytic particles, and more preferably from about 3 to about 10 percent by weight of the total weight of the metal catalytic particles.

In another embodiment of the invention, the bimetallic catalyst may not be deposited on a support, in which case the metal component contains substantially about 100% of the metal catalytic particles.

Examples of suitable carbon-containing gases include: saturated and unsaturated aliphatic hydrocarbons such as methane, ethane, propane, butane, hexane, ethylene and propylene; carbon monoxide; oxygenated hydrocarbons such as acetone, acetylene and methanol; aromatic hydrocarbons such as toluene, benzene, and naphthalene; mixtures of the above, such as carbon monoxide and methane. The use of acetylene promotes the formation of multi-walled carbon nanotubes, while CO and methane are preferred feed gases for the formation of single-walled carbon nanotubes. The carbon-containing gas may optionally be mixed with a diluent gas, such as helium, argon or hydrogen.

In a preferred embodiment of the invention, the bimetallic catalytic particles are placed in a reactor unit, such as a quartz tube placed in a furnace or oven, and a carbon-containing gas is passed into the reactor unit. Alternatively, the sample may be heated by microwave radiation. The process may be continuous, wherein the metallic catalytic particles and the carbon-containing gas are continuously fed and mixed within the reactor, or the process may be a batch process, wherein the carbon-containing gas and the metallic catalytic particles are placed within the reactor unit and remain within the reactor unit during the reaction.

Alternatively, the metallic catalytic particles may be mixed with electrodes in an arc discharge system to produce single-walled and/or multi-walled carbon nanotubes. Alternatively, the metal catalytic particles may be used in a system exposed to a microwave-induced plasma discharge. After the catalytic process is completed, the metallic catalytic particles and nanotubes are removed from the reactor. The nanotubes are separated from the metal catalytic particles by methods well known to those of ordinary skill in the art. It is not necessary here to discuss this method of separating the carbon nanotubes from the metallic catalytic particles further.

The single-walled carbon nanotubes produced herein typically have an outer diameter of about 0.7 to 5 nanometers. The multi-walled carbon nanotubes produced herein typically have an outer diameter of about 2 to 50 nanometers.

Methods for reliable quantitative detection of single-walled carbon nanotubes are straightforward and easy to perform, allowing changes in selective or steady-state production to be detected, making reproducibility and quality control easy to implement. The method is based on the Temperature Programmed Oxidation (TPO) technique (Krishhnankutty, N et al, Catalysis Today, 37, 295 (1997)). This technique is commonly used to assess the crystallinity of carbon and is based on the concept that highly graphitic materials will be more resistant to oxidation than materials with short-range crystalline order. In the present invention, this technique is employed to provide a method of determining the selectivity of producing single-walled carbon nanotubes over multi-walled carbon nanotubes, and the percentage of total solid products by each carbon species, including not only single-walled carbon nanotubes and multi-walled carbon nanotubes, but also amorphous carbon and graphitic carbon species. Therefore, this method, in combination with the above-described method for producing carbon nanotubes, will allow for the controlled production of single-walled carbon nanotubes. However, it should be understood that the method can also be used to analyze any sample containing carbon nanotubes.

Specifically, the method includes passing a continuous flow of an oxygen-containing gas, such as 5% oxygen in helium, dispersed in a carrier gas over a carbon nanotube-containing sample, such as a catalyst containing carbonaceous deposits, while linearly increasing the temperature from room temperature to about 800 ℃. The amount of oxygen-containing gas is effective to oxidize carbon species present in the sample. The oxidation of carbon species results in the formation of carbon dioxide, each carbon species, such as single or multi-walled carbon nanotubes, amorphous carbon or graphite, being oxidized at different temperatures. The carbon dioxide generated by the oxidation of each carbon species present in the sample is monitored by mass spectrometry. The carbon dioxide generated was corrected by oxidation with a pulse of a known amount of pure carbon dioxide and a known amount of graphite to directly detect the amount of carbon oxidized at each temperature. That is, 1 mole of carbon dioxide as measured by mass spectrometry corresponds to 1 mole of a particular species of carbon that is oxidized at a given temperature.

The quantitative method using temperature programmed oxidation is hereinafter referred to as temperature programmed oxidation method, which is particularly suitable for quantitatively characterizing single-walled carbon nanotubes because single-walled carbon nanotubes are oxidized in a narrow temperature range, which is above the oxidation temperature of amorphous carbon and below the oxidation temperature of multi-walled carbon nanotubes and graphitic carbon. For example, by this method, the oxidation temperature ratio C of the single-walled carbon nanotube is measured₆₀The fullerene has an oxidation temperature about 100 ℃ higher and about 100 ℃ lower than that of the multi-walled carbon nanotube. Similar results were obtained by thermogravimetric analysis (TGA) (Rinzler, a.g., appl.phys.a, 67, 29(1998)), confirming the applicability of this method for the quantification of single-walled carbon nanotubes.

The temperature programmed oxidation analysis methods described herein can be used to rapidly test the operating conditions of different catalyst formulations and carbon nanotube production processes in order to optimize the production of single-walled carbon nanotubes. For example, the optimum bimetallic catalyst present in the metal catalytic particle, and the optimum molar ratio of the two metals, can be determined by temperature programmed oxidation. Temperature programmed oxidation may also be used to optimize reaction conditions such as temperature, time, and carbon concentration in the carbon-containing gas. For example, temperature programmed oxidation of the product at different reaction temperatures shows that the amount of carbon deposited increases with decreasing temperature, but the selectivity for producing single-walled carbon nanotubes is lower at lower temperatures. Therefore, temperature programmed oxidation can be used to find the optimum reaction temperature for any particular catalyst.

It will now be appreciated that although the optimization of single-walled carbon nanotube production has been discussed in detail, the same method can be used to optimize multi-walled carbon nanotube production.

The amount of graphite, amorphous carbon and other carbon residues formed during the catalysis process is minimized because lower temperatures are used. The weight of the graphitic or amorphous carbon is less than about 40%, more preferably less than about 10%, of the weight of the total solid material formed during the process. Most preferably, the amount of graphite, amorphous carbon and other solid carbon residues is less than about 5% of the total solid product of the catalytic process.

The temperature programmed oxidation method described herein appears to be the first method to be able to determine not only the carbon species present in the sample, but also the percentage of each carbon species present in the sample. This is particularly helpful in determining what purification steps, if any, should be taken before the single-walled carbon nanotubes are used in various applications. The value of the temperature programmed oxidation process is evident because the purification step is very time consuming and expensive compared to the actual carbon nanotube production itself.

The carbon nanotubes produced herein can be used for various purposes. For example, they may be used as reinforcement in fiber-reinforced composite structures or hybrid composite structures (i.e., composites containing reinforcement, such as continuous fibers, in addition to carbon nanotubes). The composite material may further contain fillers such as carbon black, silica and mixtures thereof. Examples of matrix materials that can be reinforced include inorganic and organic polymers, ceramics (e.g., portland cement), carbon, and metals (e.g., lead or copper). When the matrix is an organic polymer, this may be: thermosetting resins such as epoxy, bismaleimide, polyimide, or polyester resins; a thermoplastic resin; or a reaction injection molding resin. Carbon nanotubes can also be used to reinforce continuous fibers. Examples of continuous fibers that may be reinforced or included in the hybrid composite include aramid, carbon, glass fibers, and mixtures thereof. The continuous fibers may be woven, braided, crimped, or direct.

The present invention will be illustrated in more detail by the following examples. However, the examples are merely illustrative of desirable aspects of the present invention and are not intended to limit the scope of the present invention.

Example 1:

bimetallic catalytic particles containing about 10 wt.% mixed cobalt and molybdenum (about 1: 1 ratio) on a silica substrate were prepared by an incipient wetness impregnation process in which the appropriate amounts of cobalt nitrate and ammonium heptamolybdate tetrahydrate were dissolved together in deionized water and then gradually added dropwise to the silica. A ceramic sand and pestle is used to disperse the metal on the silica. The resulting bimetallic catalytic particles are then dried under ambient conditions for several hours. The partially dried bimetallic catalytic particles are then dried in an oven at about 80 ℃ for about 12 hours. The dry bimetallic catalytic particles are then calcined in flowing air at about 450 ℃.

To produce carbon nanotubes, about 0.1 grams of the calcined bimetallic catalytic particles were placed in a vertical quartz tube reactor having an arc inside diameter of about 8 millimeters. The vertical quartz tube reactor containing the calcined bimetallic catalytic particles was placed in a furnace equipped with a thermocouple and temperature control. Make hydrogen (about 85 cm)³Per minute) was passed into the reactor from the top thereof. The furnace temperature was linearly increased from room temperature to about 450 c at a rate of about 20 c/min. After reaching about 450 ℃, the hydrogen stream was re-introduced into the reactor for about 30 minutes. The reactor temperature was then raised to about 600-700 ℃ in helium. Then, carbon monoxide gas (about 50% carbon monoxide/50% helium) is supplied at about 100 cm³The flow rate per minute was passed into the reactor. The contact time of the CO with the calcined bimetallic catalytic particle is from about 15 minutes to about 2 hours. After the above contact time had elapsed, the furnace was shut down and the product was cooled to room temperature in helium.

After the reaction, the color of the sample returned to dark black. For transmission electron microscopy analysis of the product, a portion of the product was suspended in distilled water and irradiated with ultrasound. A few drops of this suspension were deposited on the lacey carbon supported on a copper grid. A portion of the product was then dried and observed at about 200KV using a JEOL JEM-2000FX type transmission electron microscope. As shown in the transmission electron microscope images (fig. 1-4), a large number of single-walled carbon nanotubes can be clearly seen. These single-walled carbon nanotubes were observed to be stacked together, roughly arranged in bundles. Transmission electron microscopy images also show that bundles of single-walled carbon nanotubes are coated with amorphous carbon, similar to other methods. Most of the tubes have a diameter of about 1 nanometer, and a small portion of the tubes have a larger diameter, up to about 3.2 nanometers.

After transmission electron microscopy analysis, the product was scanned with a JEOL JSM-880 type scanning electron microscope. The scanning electron microscope image in fig. 5 shows bundles of single-walled carbon nanotubes on the surface of silica.

Example 2:

metallic catalytic particles containing Ni, Co or Mo single metal catalysts supported on silica were also prepared by the same method as described in example 1, and their catalytic performance was compared with that of metallic catalytic particles containing bimetallic catalysts. After the same CO treatment at 700 c as described in example 1, the same transmission electron microscopy analysis was performed and no single-walled carbon nanotubes were observed in these samples. This result indicates that there is indeed a synergistic effect between CO and Mo making the combination of the two metals a very effective formulation, while at this temperature the metals alone are not capable of producing single-walled carbon nanotubes.

Example 3:

on different carriers (SiO)₂、MCM-41、Al₂O₃Mg (Al) O and ZrO₂) A series of metal catalyst particles containing about 6 wt% of Co — Mo bimetallic catalyst were prepared and compared for their carbon nanotube productivity, and then the same Co disproportionation process as in example 1 was employed. Table I summarizes the results of these experiments.

Example 4:

following the same procedure as in example 1, it was observed to contain deposits on SiO₂Metal catalytic particles of a bimetallic catalyst of Co-W on, wherein the molar ratio of Co/W isAbout 1.0, obtained with Co-Mo/SiO₂Similar single-walled carbon nanotube yields for metal catalytic particles. In the case of the Co-Mo series, it was observed that only W/SiO was contained₂But the Co-free metal catalytic particles cannot form single-walled carbon nanotubes.

TABLE I influence of catalyst support on morphology of carbon deposition
		Catalyst and process for preparing same	Morphology of observed carbon deposits
Co∶Mo/SiO2	Major amount of single-walled carbon nanotubes, minor amount of multi-walled carbon nanotubes and graphite
		Co∶Mo/MCM-41	Major amount of single-walled carbon nanotubes, minor amount of multi-walled carbon nanotubes and graphite
Co∶Mo/Al2O3	Small amount of single-walled carbon nanotubes, multi-walled carbon nanotubes and graphite
		Co∶Mo/Mg(Al)O	Small amount of graphite and single-wall carbon nanotube
Co∶Mo/ZrO2	Small amount of graphite and single-wall carbon nanotube

Example 5:

carbon material produced by the same CO disproportionation process as described in example 1 using metal catalytic particles containing about 6 wt% CO-Mo bimetallic catalyst supported on silica (about 1: 2 ratio) was analyzed by the temperature programmed oxidation method as shown in fig. 6.

For temperature programmed oxidation analysis, a sample of about 50 mg obtained from CO treatment of the product at about 700 ℃ was placed in a quartz tube reactor similar to that used in example 1. A continuous flow of about 5% oxygen/95% helium was passed into the reactor and the furnace temperature was raised from room temperature to about 800 c at a rate of about 11 c/min and then held at about 800 c for about 1 hour. CO formed₂Mass spectrometry was used to determine the amount of carbon species oxidized at each temperature.

Mass spectrometric detection of CO₂The partial pressure in the quartz tube gives the absolute value. The values were then normalized by subtracting the baseline values, using approximately 100 microliters of CO₂Pulses and known amounts of graphite oxidation are corrected and then calculated. Adjusted value and CO oxidized at specific temperature₂The molar amount is directly proportional, which in turn is directly proportional to the molar amount of the particular carbon species present in the sample. From these values, the percentage of single-walled carbon nanotubes in the total solid product of the catalytic process can be calculated.

In the ratio of Co to Mo/SiO₂The temperature programmed oxidation curve of the carbon species produced on the metal catalytic particles (labeled "Co: Mo 1: 2") is represented by a small oxidation peak centered at about 330 ℃ that is attributable to the oxidation of amorphous carbon, and a major peak centered at about 510 ℃ that is indicated by the arrow in the figure and attributable to the oxidation of single-walled carbon nanotubes.

The two reference samples were also observed by the temperature programmed oxidation method, and the curves are shown in fig. 6. The first reference sample (labeled "graphite") was Co: Mo/SiO₂Graphite powder with metal catalytic particles physically mixed. This form of carbon oxidation is carried out at very high temperatures, beginning at about 700 c and completing after about 800 c for about 30 minutes.

The second reference sample was a commercial sample of purified single-walled carbon nanotubes, purchased from Tubes  Rice (Rice University, Houston, Tex.). The sample was provided as a liquid suspension containing the nonionic surfactant Triton X-100 at about 5.9 g/L. For temperature programmed oxidation analysis, Co: Mo/SiO₂The metal catalytic particles were impregnated with a suspension of single-walled carbon nanotubes in a liquid/catalyst weight ratio of about 1: 1 to yield about 0.6 wt% single-walled carbon nanotubes on the sample. The temperature program oxidation curve of the impregnated sample (labeled "Tubes  Rice") shows two peaks, the low temperature peak corresponding to oxidation of the surfactant and the second peak at about 510 ℃, corresponding exactly to oxidation of single-walled carbon nanotubes. To determine that the first peak is indeed attributable to surfactant oxidation, samples of surfactant-only blank solutions with the same concentration were prepared. The temperature program oxidation curve (labeled "blank solution") followed the first peak of the "Tubes  Rice" curve, confirming that this peak indeed corresponds to surfactant Triton.

From CO by temperature-programmed oxidation₂Quantifying the amount of single-walled carbon nanotubes in the "Tubes  Rice" reference sample yielded a value of about 0.64 wt%, which corresponds well to the amount of single-walled carbon nanotubes supported in the sample (about 0.6 wt%). This result demonstrates that the temperature programmed oxidation process of the present invention can be used to directly quantify the percentage of specific carbon species, such as single-walled carbon nanotubes, multi-walled carbon nanotubes, and amorphous carbon present in the product resulting from the nanotube production process. Currently, there are no other methods to directly quantify the fraction of a particular carbon species in the total solid product obtained from nanotube production.

Example 6:

the temperature-programmed oxidation curve of the product obtained by CO disproportionation catalyzed by a metal catalyst particle containing a Co or Mo monometallic catalyst supported on silica was obtained by the method of example 5 and compared with the temperature-programmed oxidation curve of the product obtained by CO disproportionation catalyzed by a bimetallic catalyst. The temperature-programmed oxidation process clearly demonstrates the synergistic effect shown by Co and Mo, which is also observed by transmission electron microscopy in example 2.

As shown in FIG. 7, contains Mo/SiO₂The temperature program oxidation curve of a sample of metal catalytic particles (labeled "Mo") indicates that Mo alone cannot produce carbon nanotubes; the "Mo" temperature program oxidation curve includes only a small low temperature peak, corresponding to amorphous carbon. Similarly, containing Co/SiO₂The temperature programmed oxidation curve of a sample of metal catalytic particles (labeled "Co") indicates that Co alone is not selective for producing single-walled carbon nanotubes and produces predominantly graphitic carbon and multi-walled carbon nanotubes, which oxidize at higher temperatures than single-walled carbon nanotubes, as described above. In contrast, the combination of the two metals results in high selectivity to single-walled carbon nanotubes, containing Co: Mo/SiO₂A sample of metal catalytic particles (labeled "Co: Mo ═ 1: 2", where the Co: Mo ratio is about 1: 2) showed a large peak centered at about 510 ℃, attributed to single-walled carbon nanotubes. Because there are no other significant peaks, it can be assumed that single-walled carbon nanotubes represent a large percentage of the total solid product resulting from carbon nanotube production.

The percentages of single-walled carbon nanotubes, amorphous carbon, multi-walled carbon nanotubes and graphite present in the catalytic product are listed in table II, where all numbers and measurements are approximate.

TABLE II synergistic effects of Co and Mo
				Catalyst and process for preparing same	Amorphous carbon%	Single-walled carbon nanotubes%	Multi-walled carbon nanotubes and graphite%
Co	38	11	51
				Mo	95	5	0
Co∶Mo(1∶2)	8	88	4

Example 7:

temperature programmed oxidation profiles of CO disproportionation products catalyzed by metal catalyst particles containing Co: Mo bimetallic catalysts were compared, wherein the Co: Mo ratios were about 1: 4, about 1: 2, about 1: 1, and about 2: 1, to determine changes in Co: Mo/SiO₂The mol ratio of Co to Mo in the metal catalytic particles. A temperature programmed oxidation curve was obtained by the same method as described in example 5. As shown in FIG. 8, a Co: Mo/SiO mixture containing Co: Mo in a molar ratio of about 1: 2 and about 1: 4₂The metal catalytic particles show the highest selectivity for single-walled carbon nanotubes. The arrow indicates the center of the peak corresponding to oxidation of the single-walled carbon nanotube. The temperature programmed oxidation curves for these samples show that these catalysts produce a large fraction of single-walled carbon nanotubes and a small amount of amorphous carbon. An increase in the Co: Mo ratio does not increase the yield of single-walled carbon nanotubes, but does accelerate the formation of multi-walled carbon nanotubes and graphitic carbon as shown by the increase in peak size in the region of about 600-700 of the programmed temperature oxidation curve labeled "Co: Mo 2: 1".

The selectivity values for each catalyst were estimated from the temperature oxidation curves of the program of fig. 8 and are shown in table III, where all numbers and measurements are approximate.

TABLE III Effect of Co to Mo molar ratio on Single-walled carbon nanotube production
				Molar ratio of Co to Mo catalyst	Amorphous carbon%	Single-walled carbon nanotubes%	Multi-walled carbon nanotubes and graphite%
2∶1	12	57	31
				1∶1	16	80	4
1∶2	8	88	4
				1∶4	5	94	1

Example 8:

FIGS. 9-11 show the use of temperature programmed oxidation techniques to optimize reaction conditions. Co, Mo/SiO for CO disproportionation₂The metal catalyst particles (about 1: 1 molar ratio) were catalyzed and the method used was similar to that described in example 1, except that the reaction temperature was varied in fig. 9, the CO concentration was varied in fig. 10, and the reaction time was varied in fig. 11. The CO disproportionation product was analyzed using the temperature programmed oxidation method described in example 5.

In fig. 9, the temperature program oxidation curves of the carbon species produced when the reactor temperature was about 600 ℃, about 700 ℃ and about 800 ℃ are shown. These curves demonstrate that the amount of carbon deposition increases with decreasing temperature; but at lower temperatures, the selectivity to single-walled carbon nanotubes is lower. Temperature programmed oxidation may be used to determine the optimum reaction temperature for any particular catalyst, in which case the optimum temperature is about 700 ℃. The percentages of single-walled carbon nanotubes, amorphous carbon, multi-walled carbon nanotubes and graphite in the catalytic product are listed in table IV, where all numbers and measurements are approximate.

In fig. 10, a temperature program oxidation curve of the carbon species produced when the CO concentration in the carbon-containing gas was about 1%, about 20%, about 35%, and about 50% is shown. These curves demonstrate that the yield of single-walled carbon nanotubes is strongly related to the CO concentration in the carbon-containing gas.

TABLE IV Effect of reactor temperature on Single-walled carbon nanotube production
				Temperature of	Amorphous carbon%	Single-walled carbon nanotubes%	Multi-walled carbon nanotubes and graphite%
600℃	16	55	29
				700℃	16	80	4
800℃	25	61	14

In fig. 11, the temperature programmed oxidation curves of the carbon species produced when the reaction time was about 3 minutes, about 10 minutes and about 1 hour are shown. Reaction time refers to the time the reactor is maintained at about 700 ℃ and the CO is in contact with the metallic catalytic particles. These temperature program oxidation curves demonstrate that the yield of single-walled carbon nanotubes increases significantly over time during the first approximately 10 minutes, but beyond that time the magnitude of the increase is less significant.

It should now be understood that the temperature programmed oxidation process is a catalytic process in which the metals present in the sample catalyze the oxidation of carbon species. Therefore, if the properties of the catalyst are significantly changed, the position of the oxidation peak may be shifted from the peak position described in the above examples, although the carbon structures represented by the peaks are the same. For example, it has been observed that changes in the catalyst support can cause such displacements. Therefore, for each catalyst used in the process of the present invention, the complete temperature program oxidation curve of the catalyst and the operating conditions should be carried out with suitable references to determine the peak shifts and optimum operating conditions.

Example 9:

in a particularly preferred embodiment of the process of the present invention, the catalyst composition is a Co-Mo/silica catalyst wherein the molar ratio of Co to Mo is about 1: 2. Single metal Co catalysts or catalysts with higher molar ratios of Co to Mo tend to give low selectivity, significantly producing unfavorable multi-walled carbon nanotubes and graphite. In the temperature range studied, Mo is essentially inert to nanotube production in the absence of Co. The catalyst is pretreated in hydrogen, for example at about 500 ℃, to partially reduce Mo, but not Co. Without this pretreatment step, or with pre-reduction at higher temperatures (i.e., insufficient reduction or excessive reduction), the catalyst is ineffective and produces less SWNTs. Other supports such as alumina can result in poor Co-Mo interactions, with losses in selectivity and yield.

High space velocity (at about 30000 hours)^-1Above) is preferred, so that CO is present₂Is minimized in concentration, CO₂Is a by-product of the reaction which inhibits the conversion to nanotubes. High CO concentrations are preferred to minimize the formation of amorphous carbon deposits, since such deposit formation occurs at low CO concentrations. Preferred temperature ranges are characterized by low selectivity to SWNTs below about 650 ℃; above about 850 c, the conversion is low because of the reversibility of the reaction (exotherm) and deactivation of the catalyst. Therefore, the optimum temperature is about 700-; more preferably about 725-.

The production process is designed in such a way that the preferred catalyst formulation is rapidly contacted with a highly concentrated CO stream at about 750 ℃. Otherwise, the yield and selectivity will be greatly affected. The quality of SWNTs produced by this process can be determined by a combination of characterization techniques including raman spectroscopy, Temperature Programmed Oxidation (TPO), and electron microscopy (TEM).

The preferred process therefore involves contacting the CO gas stream (high concentration) with catalytic particles at a high space velocity (above about 30000 hours) at about 750 deg.C^-1) Under high pressure (above about 4826322.99Pa (i.e., above about 4826322.99N-m)^-2(70psi))) for about 1 hour.

If the above conditions are followed, high yields of SWNTs (about 20-25 grams of SWNTs per about 100 grams of initial catalyst loaded in the reactor) and high selectivities (greater than about 90%) will be obtained.

Changes may be made in the various components, elements, and combinations or in the steps or in the sequence of steps of the method described without departing from the spirit and scope of the invention as defined in the appended claims.

The invention described herein may suitably be practiced in the absence of any element which is not specifically disclosed herein.

The following claims are to be accorded the widest possible scope consistent with the present application. The claims should not necessarily be limited to the preferred embodiments or embodiments shown in the examples.

Claims (62)

1. A method of producing carbon nanotubes, comprising:

contacting in a reactor unit metallic catalytic particles comprising at least one group VIII metal other than iron and at least one group VIb metal with a carbon-containing gas in an amount effective and at a temperature sufficient to catalytically produce carbon nanotubes such that a substantial portion of the carbon nanotubes are single-walled carbon nanotubes.

2. The process according to claim 1 wherein the group VIII metal is selected from the group consisting of Co, Ni, Ru, Rh, Pd, Ir, Pt, and mixtures thereof.

3. The method of any one of claims 1 or 2 wherein the group VIb metal is selected from Cr, Mo, W, and mixtures thereof.

4. The process according to claim 1 wherein the group VIII metal is selected from the group consisting of Co, Ni, Ru, Rh, Pd, Ir, Pt and mixtures thereof, and wherein the group VIb metal is selected from the group consisting of Cr, Mo, W and mixtures thereof.

5. A method according to any one of claims 1 to 4 wherein the metallic catalytic particles further comprise a support for the deposited metal.

6. A process according to claim 5 wherein the support is selected from the group consisting of silica, MCM-41, alumina, MgO, Mg (Al) O, ZrO₂And molecular sieve zeolites.

7. The method of any of claims 1-6 wherein the ratio of group VIII metal to group VIb metal is from about 1: 10 to about 15: 1.

8. The method of any of claims 1-7 wherein the ratio of group VIII metal to group VIb metal is from about 1: 5 to about 2: 1.

9. The method of any of claims 5 or 6 wherein the catalytic particles comprise from about 1 to about 20 wt% metal.

10. The process according to any one of claims 1 to 9, wherein the carbon-containing gas is selected from the group consisting of saturated hydrocarbons, aliphatic hydrocarbons, oxygenated hydrocarbons, aromatic hydrocarbons, carbon monoxide and mixtures thereof.

11. The method of any one of claims 1 to 9 wherein the carbon-containing gas further comprises a diluent gas.

12. The method of any one of claims 1-11, wherein the temperature is sufficiently below the thermal decomposition temperature of the carbon-containing gas so as to avoid substantial formation of pyrolized carbon.

13. The method as set forth in any one of claims 1 to 12 wherein the temperature is about 500-1200 ℃.

14. The method as set forth in any one of claims 1 to 13, wherein the temperature is about 600-850 ℃.

15. The method as set forth in any one of claims 1 to 14 wherein the temperature is about 650-750 ℃.

16. The method according to any one of claims 1-15, wherein the catalytically produced carbon nanotubes further comprise multi-walled carbon nanotubes.

17. The method of any one of claims 1-16 wherein single-walled carbon nanotubes comprise at least about 60% to at least about 95% of the catalytically produced nanotubes.

18. The method of any one of claims 1-17 wherein the group VIII metal is Co.

19. The method of any one of claims 1-17 wherein the group VIII metal is Ni.

20. The method of any one of claims 1-17 wherein the group VIII metal is Ru.

21. The method of any one of claims 1-17 wherein the group VIII metal is Rh.

22. The process of any one of claims 1-17 wherein the group VIII metal is Pd.

23. The method of any one of claims 1-17 wherein the group VIII metal is Ir.

24. The method of any one of claims 1-17 wherein the group VIII metal is Pt.

25. The method of any one of claims 1-24 wherein the group VIb metal is Cr.

26. The method of any one of claims 1-24 wherein the group VIb metal is Mo.

27. The method of any one of claims 1-24 wherein the group VIb metal is W.

28. The method of any of claims 1-27 wherein the metallic catalytic particle comprises at least one additional group VIII metal.

29. The method of any of claims 1-28 wherein the metallic catalytic particles comprise at least one additional group VIb metal.

30. The method of any of claims 1-29 wherein the metallic catalytic particles comprise at least one additional group VIII metal and at least one additional group VIb metal.

31. The method of any one of claims 1 to 30 wherein the metallic catalytic particles are supplied substantially continuously into the stream of carbon-containing gas.

32. The process defined in any one of claims 1 to 31 wherein the carbonaceous gas is supplied to a reactor unit having catalytic particles therein.

33. A method of determining catalyst composition to optimize single-walled carbon nanotube production, comprising:

providing a product of single-walled carbon nanotube production wherein metallic catalytic particles having a composition comprising a group VIII metal other than iron and a group VIb metal with a predetermined ratio between the group VIII metal and the group VIb metal are used in the production;

taking out a product sample containing the single-walled carbon nanotubes;

contacting a product sample containing single-walled carbon nanotubes with an effective amount of an oxygen-containing gas in a reactor unit to oxidize carbon species present in the sample;

raising the temperature within the reactor unit from about room temperature to about 800 ℃;

detecting the amount of carbon dioxide released from the sample at a given temperature of about room temperature to about 800 ℃;

determining the presence of a particular carbon species in the sample by the amount of carbon dioxide released from the sample at the detection temperature; and

the composition of the metallic catalytic particles is altered by at least one of altering the group VIII metal, altering the group VIb metal, and altering the predetermined ratio of the two metals such that the single-walled carbon nanotubes are present in an amount significantly greater than all other carbon species in the carbon nanotube-containing product sample.

34. A metal catalytic particle having a composition determined according to the method of claim 33, wherein the metal catalytic particle produces a product wherein at least about 60% to at least about 95% of the carbon species present are single-walled carbon nanotubes.

35. The metallic catalytic particle of claim 34, wherein the catalyst composition comprises Co and Mo, wherein the predetermined ratio of Co to Mo is from about 1: 10 to about 15: 1.

36. The method of determining catalyst composition to optimize single-walled carbon nanotube production according to claim 33 wherein in the step of providing a product of single-walled carbon nanotube production wherein metallic catalytic particles are used in the production, the method of producing single-walled carbon nanotubes comprises contacting the metallic catalytic particles with an effective amount of a carbon-containing gas in a reactor unit at a temperature sufficient to catalytically produce the product containing single-walled carbon nanotubes.

37. A method of optimizing reaction conditions in a method of producing single-walled carbon nanotubes, comprising:

providing a product of single-walled carbon nanotube production, wherein a set of reaction conditions is used including at least one of temperature, time, and carbon concentration in the carbon-containing gas;

taking out a product sample containing the single-walled carbon nanotubes;

contacting a product sample containing single-walled carbon nanotubes with an effective amount of an oxygen-containing gas in a reactor unit to oxidize carbon species present in the sample;

raising the temperature within the reactor unit from about room temperature to about 800 ℃;

detecting the amount of carbon dioxide released from the sample at a given temperature of about room temperature to about 800 ℃;

determining the presence of a particular carbon species in the sample by the amount of carbon dioxide released from the sample at the detection temperature; and

the reaction conditions are modified by varying at least one of the temperature, time, and carbon concentration in the carbon-containing gas such that the single-walled carbon nanotubes are present in an amount significantly greater than all other carbon species in the carbon nanotube-containing product sample.

38. The method of optimizing reaction conditions in a method of producing single-walled carbon nanotubes of claim 37 wherein in the step of providing a single-walled carbon nanotube product, the method of producing single-walled carbon nanotubes comprises contacting in a reactor unit metallic catalytic particles with an effective amount of a carbon-containing gas at a temperature sufficient to catalytically produce a product containing single-walled carbon nanotubes, wherein the metallic catalytic particles comprise a group VIII metal and a group VIb metal other than iron.

39. A catalytic particle for producing carbon nanotubes comprising at least one group VIII metal other than iron and at least one group VIb metal.

40. Catalytic particles according to claim 39, wherein the group VIII metal is selected from the group consisting of Co, Ni, Ru, Rh, Pd, Ir, Pt and mixtures thereof.

41. Catalytic particles according to claim 39 or 40.

42. A catalytic particle according to any of claims 39 to 41, wherein the particle further comprises a support for the deposited metal.

43. The catalytic particle of claim 42, wherein the support is selected from the group consisting of silica, MCM-41, alumina, MgO, Mg (Al) O, ZrO₂And molecular sieve zeolites.

44. The catalytic particle of any of claims 39-43, wherein the ratio of group VIII metal to group VIb metal is from about 1: 10 to about 15: 1.

45. The catalytic particle of any of claims 39-44, wherein the ratio of group VIII metal to group VIb metal is from about 1: 5 to about 2: 1.

46. The catalytic particle of any of claims 42 or 43, wherein the catalytic particle comprises from about 1 to about 20 wt% metal.

47. The catalytic particle of any of claims 39-46, wherein the catalytic particle comprises at least one additional group VIII metal.

48. The catalytic particle of any of claims 39-47, wherein the catalytic particle comprises at least one additional group VIb metal.

49. A method of producing carbon nanotubes, comprising:

contacting in a reactor unit metal catalytic particles comprising at least one metal with an effective amount of a gas at a temperature sufficient to catalytically produce carbon nanotubes.

50. A method of determining catalyst composition, comprising:

providing a product of nanotube production, wherein metal catalytic particles are used in the production;

taking out a product sample;

contacting the product sample with an effective amount of a gas in a reactor unit to oxidize carbon species present in the sample;

raising the temperature within the reactor unit above about room temperature;

determining the presence of specific carbon species in the sample; and

the composition of the metal catalytic particles is changed.

51. A method of optimizing reaction conditions in a method of producing nanotubes, comprising:

providing a product of carbon nanotube production, wherein a set of reaction conditions is used including at least one of temperature, time, and carbon concentration in a carbon-containing gas;

taking out a product sample;

contacting the product sample with an effective amount of a gas in a reactor unit to oxidize carbon species present in the sample;

raising the temperature within the reactor unit above about room temperature;

determining the presence of specific carbon species in the sample; and

the reaction conditions are modified by varying at least one of the temperature, time, and carbon concentration in the carbon-containing gas.

52. A catalytic particle for producing carbon nanotubes comprising at least one metal.

53. The catalytic particle of claim 39, comprising:

co and Mo in a ratio of 1 part Co to at least 2 parts Mo or more; and

a support material, wherein Co, Mo and the support material are combined to have a granular shape.

54. The catalytic particle of claim 53 further comprising an additional group VIII metal.

55. Catalytic particles according to claim 53 or 54, further comprising an additional group VIb metal.

56. A catalytic particle according to any of claims 53 to 55, wherein the support material is selected from silica, MCM-41, alumina, MgO, ZrO, and alumina₂Aluminum stabilized magnesia and molecular sieve zeolites.

57. The method of any of claims 53 to 56, comprising about 1 to 20 wt.% Co and Mo.

58. The catalytic particle of claim 39, wherein the group VIb metal is at least one of Cr, Mo, and W; wherein the catalytic particles further comprise a support material; and wherein at least one of the group VIII metals, Cr, Mo, and W, is combined with a support material to have a particulate shape, excluding catalytic particles consisting of the support material and Co and W or Co and Mo.

59. Catalytic particle according to claim 58, wherein the at least one group VIII metal is selected from Co, Ni, Ru, Rh, Pd, Ir and Pt.

60. Catalytic particles according to claim 58 or 59, wherein the support material is selected from silica, MCM-41, alumina, MgO, ZrO, alumina₂Aluminum stabilized magnesia and molecular sieve zeolites.

61. The catalytic particle of any of claims 58-60, comprising about 1-20 wt% of at least one group VIII metal and at least one of Cr, Mo, and W.

62. Catalytic particles according to any of claims 39 to 48 and 53 to 61 comprising a silica support material.