CA2596938A1

CA2596938A1 - Catalysts for the large scale production of high purity carbon nanotubes with chemical vapor deposition

Info

Publication number: CA2596938A1
Application number: CA002596938A
Authority: CA
Inventors: Stratigoula Mitri; Efstratios Sotirchos (Deceased)
Original assignee: Individual
Current assignee: FOUNDATION OF RESEARCH AND TECHNOLOGY HELLAS / INSTITUTE OF CHEMICAL ENGINEERING AND HIGH TEMPERATURE CHEMICAL PROCESSES
Priority date: 2005-02-15
Filing date: 2005-06-10
Publication date: 2006-08-24
Also published as: WO2006087590A8; WO2006087590A1; EP1848531A1

Abstract

In the present invention, advanced catalysts for the production of high purity and high quality carbon nanotubes with the technique of chemical vapor deposition are described. The use of these catalysts and the described methodology allows the production of carbon nanotubes at high rates and with yields per mass unit of catalytic material, which are much higher than those achieved with other methods of carbon nannotubes production and other catalysts. The high yields, the high production rates and the very low cost of the catalysts that are employed in the developed method lead to the production of materials that cost much less than commercially available materials of similar or lower quality. The catalyst or the catalytic substrate on which carbon is deposited and grows in the form of nanotubes consists of the carrier or the substrate, which is aluminum oxide (alumina) or one of the other metal oxides that are usually employed as catalytic media, the active phase which is iron oxide (preferably hematite but also any other form) and a promoter, such as molybdenum oxide. The ratio of these three components plays a very important role in the composition of the catalyst.

Description

Catalysts f r the Large Scale Fa9'tDd$L9CtiO611 of High Purity Carbon Nanotubes with ChemicaC Vapor Deposition In the present invention, advanced catalysts fQr the production of high purity and high quality carbon nanotubes vvith the technique of chemical vapor deposition are described. -iuhe use of these catalysts and the described methodology allows the production of carbon nanotubes at high rates and wit'ri yields per mass unit of catalytic material, which are rnuch higiner than those achieved with other methods of carbori ri ,anc}t z.ab es production and other catalysts. The high yieids, tl-1 E~ 11-1 igh production rates and the very low cost of the catalysts that are employed in the developed method lead to the produ-r:=tion of materials that cost much less than commercially s:vai'~able materials of similar or lower quality. The catalyst or the catalytic substrate on which carbori is deposited and clrovsrs in the form of nanotubes consists of the carrier or the :;U4:.>strate, which is aluminum oxide (alumina) or one of the o=tl~ier rjietal oxides that are usually employed as catalytic media, the active phase which is iron oxide (preferably hematite but also i=any other form) and a promoter, such as molybdenurri o;cide, "I"I-ie ratio of these three componerits plays a very irnportai-ii: rOiE,~ in the composition of the catalyst.

Background of the invention The present inventiori refers to a method for the developmt,rit of catalysts and catalytic substrates and their use r"or the large-scale production of carbon nanotubes with c;hernical vapor deposition. The developed nanotubes are eii:hc,r ciryc~lE-wall or multi-wall, depending on the employed catalyst and the employed hydrocarbon. In particular, the present inven=tion refers to a procedure that leads to the developrnent of catalytic substrates o=F specific composition. The catalytic substr~ate:a are used for the deposition of carbon in nanotubes form on their surface, in a way that provides the capability of hiqh yield relative to the initial weight of the substrate. Carbon deposition and nanotubes synthesis take place with the methcd of chemical vapor deposition. This method ensures the production of carbon nanotubes of constant quali -l:y ,3- t a relatively low temperature and atmospheric pressure arid gives solution to the problem of carbon nanotubes higi,i pri:adUction cost.

Many scientists characterize carbon nanotubes as the "materials of the future". The nanotubes - cylindrical carbon structures with diameters that range from 0.6 nanometers. {0.6 x 10-6 m) to 300 nanometers - are materials that ~:,,ornbine exceptional mechanical and electrical properties. For this reason, there is a lot of interest in the production r.if such materials that can find many applications, as it can be seen in various publications [1-3].

Carbon nanotubes are materials that exhibit unique propY.=Prties such as high electrical and thermal conductivity. They also have exceptionally high mechanical strength (100 tirnes, higher than that of steel) and combine high surface area with low weight. Thus, they can be employed in a variety of applications including microelectronics (since they behave as conductive or semiconductive materials depending on their structure), batteries (Li storage), flat panel displays, hydrogen fLiel cells, adsorption materials and membranes for separations. Carbon nanotubes can also be used as components of composite materia(s for reinforcement or modification of properties (e.g.
electrical conductivity of piastics), as microscope probes, in materials of electromagnetic shields, and in high==;:strength structures and applications.

A characteristic of the importance that is being given in the potential applications of carbon nanotubes in the last years is the data provided by several companies that are active in the fields of investments and economic analysis [4-9]. Accordirig to these data, 5-10 million dollars were invested on research for the development and production of carbon nanotubes in 2002, while several tons of carbon nanotubes were globally produced, the total cost of which amounts to millions of dollars.

Carbon nanotubes are mainly produced by 1) sublimation of graphite rodslelectrodes with arc discharge, 2) laser ablation, 3) catalytic decomposition of carbon-bearing gases (usuaiiy hydrocarbons or carbon monoxide) with the use of inetal catalysts supported on metal oxide substrates or suspended in the gas phase (catalytic chemical vapor deposition), and 4) decomposition of gaseous or liquid compounds with arc discharge. Metal catalysts are not only employed in chernical vapor deposition but in other techniques as well. The carbon products that are obtained with arc discharge are mixtures of single-wall and multi-wall nanotubes, fulierenes and relatively high amounts of amorphous carbon. Similai- drawbacks are encountered in the method of iaser ablation where iarge amounts of amorphous carbon and multi-wall nanotube:::; with a lot of structural defects are produced. Laser ablation i,,:. also a costly technique with high power requirements. Despit(:,l the fact that these methods can produce significant quantities of nanostructured carbon, they consume a lot of energy and their products have a low concentration of single-wall nanotubes and a high concentration of multi-wall nanotubes. Nanotube enrichment techniques have been developed, but ii-reir complexity and their high cost affect significantly the final cost of pure products, the prices of which prohibit their use in a wide range of applications.

Methods based on chemical vapor deposition (CVD) can be employed for the production of high quality carbon nanotubes for various applications because of their capability for large scale production and control of the synthesis procedure vvith the use of the appropriate catalyst. Chemical vapor deposition can lead to long (almost 2mm) carbon nanotubes of relatively high purity, good alignment and uniformity throughout their length, and high carbon yield (in percentage of the overall feed) in relatively mild conditions, compared to the methods of arc discharge and laser ablation. For these reasons, c~VE) is the most attractive method for the production of carbon nanotubes in industrial scale.

The basic problem in the wide use of carbon nanotubes is their high production cost, which is multiple of the cost of ~:aoid per gram. The high production cost is mainly attributed tt) the use of unsuitable and energy-consuming methods, as well as unmanageable systems of reactors and catalysts, the nanostructures production yield of which is lirnited. The high cost of carbon nanotubes renders research in the field t:;f their potential applications almost prohibitive.
The unique properties of these materials though, make thiam very attractive for applications that involve composite materials, and specifically, changes in their mechansr;al arzd electrical properties with the incorporatiori of carbon nanotubes in their structure. The use of carbon nanol:Uibe., in hydrogen storage for fuel cell applications as well as in the fabrication of nanoelectronic materials and parts has br-;en also suggested.

The present invention erisures a very high yield in the process of carbon nanotubes production, which is capable of pr=oviding large quantities of these materials in a very short timE:, with a relatively easy and inexpensive way. With the use of the new catalysts that are described in the present inventiori and the application of the presented method, the production cost of carbon nanotubes is significantly reduced, at least by 20 times.
An additional advantage is that the production of nanotubes takes place without the generation and emission of sigriificant pollutants. The exceptionally high yield of the process results from the large activity of the catalysts and the r::a talytic substrates, and their capability to absorb all carbon present in the gaseous precursors.

In addition, the high yield of the method ensures a vey-y clean 5 product that excludes the need for any further purification process, which in other cases is necessary. The purification process increases the material cost, is time-consuming and can cause degradation of the quality of nanotubes.

Description of the invention Figures Caption Figure 1. Schematic representation of the experimental apparatus (vertical reactor) for chemical vapoi- deposition of carbon nanotubes.
Figure 2. Scanning Electron Microscope image of carbon nanotubes deposited frorn 31% C2H4 at 700 C on catalytic substrate of A1203 I Fe /Mo.

The present invention involves catalysts and catalytic substrates for the production of carbon nanotubes vvith fi.he technique of chemical vapor deposition, by errti) loying hydrocarbons, alcohols as well as other molecules that contain carbon. The catalyst or the catalytic substrate on which carbon is deposited and grows in the form of nanotubes consists of the carrier or the substrate, which is aluminum oxide (alumina) or one of the other metal oxides that are usuaiiy employed as catalytic media, the active phase of which is irori or iron oxide (preferably hematite but also any other form) and a pr-orrioter, such as molybdenum or niolybdenum oxide. The ratio of these three components plays a very important role in the composition of the catalyst. The concentration of iron or its oxide in the carrier (e.g. AI203) is between 5 and 90%, preferably between 25 and 75%. For example, the employed catalyst can be a natural material that contains alumina and iron at the desired ratio, as the red mud iri which i:he AIzO3/FeZO3 ratio is 26.4/73.6. The ratio of molybderiurn over iron (or of their i-espective oxides) is between 0 and 1/'( , preferably between 1/10 and 1/3.

The preparation of the catalytic substrate takes place through dissolution of the right amounts of hydrous nitric salts of iron (Fe(NO3)3 9H20) and aluminum (A!(N03)3 OFI2O) in asn'iall volume of methanol or water. The resulting mixture is mixed with an aqueous solution that contains ammonium rnr.aly'bdate tetrahydrate ((NH4)6Mo7O24 4H2O). The solution dries in room temperature for a week until complete evaporation oF ao:-)1:hanol is achieved, and the remaining mud is baked at: 300-700 :1 for 30 minutes under heliurn flow. The baked material is cooled under inert gas flow, and subsequently it is groi.ind in a compact mortar until it turns into a fine red powdr:~~r. This powder is the catalyst, which is theri placed in the reactor tt:ar the production of carbon nanotubes.

With the above described method, the iron, alurninurn and molybdenum of the catalytic substrate are converted to the respective oxides during the heating of the rria teria i.
Subsequently, the iron oxide (possibly hematite (Fe203)) that has been formed should be reduced to iron or iron c,arbide in order to initiate the deposition process. The reductioVI of iron oxide can take place either with the use of the hydroc:virbon that is employed for the production of carbon nanotubes or with the use of hydrogen prior to the beginnirig of the production process. There are two alternative ways that can be ei-nployed for this purpose: '1 ) a two-step process which includes (a) heating of the catalyst (at 500-900 C) in ineri: atmosphere and (b) exposure of the catalyst in a mixture of a hydrocarboei and hydrogen that results in the reduction of hematite, anci 2) a three-step procedure which includes (a) heating of the catalyst in inert atmosphere, as above, (b) reduction of the cai:alyst in inert gas/hydrogen flow at a temperature that ranges from 200 to 700 0 C and (c) exposure of the catalyst in a mixture that consists of a hydrocarbon species and an inert gas (e.g., helium or nitrogen) or a mixture of a hydrocarbon sl::recivs, hydrogen and an inert gas. In the above mentioned procedures, the carbon-bearing gas (hydrocarbon) is preferably ethyiene or methane, and the mixture at which the catalyst is exposed contains hydrogen at a percentage that ranges between 10 and 200 % of the hydrocarbon concentration.

For the production of carbon nanotubes, the catalyst or the catalytic substrate that was prepared according to thr: above described method is placed in a suitable reactor, a;~~ for example the one that is shown in Figure 1. The specific reactor consists of a vertical quartz tube (1) with 15 mm tnl:ernal diameter, which is heated by a two-temperature zone furnace with 22 cm length (2). Two K-thermocouples (3) are ernployed for temperature measurements and are placed at the center of each heating zone. The temperature is coritrollec.l by a temperature controller (4). The rate of the deposition of carbon nanotubes on the catalytic substrate (5) is measured gravimetrically by i-ecording the change in the weight of the catalytic substrate. In the apparatus that is presented in Figure 1, the reactor tube is being coupled to an electrccnic microbalance of 1 microgram (pg) sensitivity (CAHN D-101)(5) for continuous monitoring of the weight of the deposit, ur-itil 100 grams (7). The catalytic substrate is placed iri a::ah;Wow container made of quartz or platinum, or other inert and resistant material, which is hung from the sample arni of the microbalance with a thin wire (8) aligned to the reactor axi.,ti.

For the production of carbon nanotubes in a larger scale, and provided that the precise monitoring of the rate of carbon nanotubes deposition on the catalytic substrate is riot necessary, a vertical or horizontal quartz tube of larger diameter is employed as the reactor, without the use of a sensitive microbalance. The catalytic substrate is inseri:ed in a suitable quartz container, which is placed in the midclie o-f the quartz tube.

The gas mixture that contains the carbon source is supplied to the reactor through an appropriate system of preS sure controllers (9, '10), valves (11), a pump (12) and mass flow controllers (13). This system determines the gas composition and flow. The stream that contains the carbon source (e.g,, ethylene, methane or another hydrocarbon, or alr..nhol or carbon monoxide) (14) is mixed with an inert gas (15), and if chosen with hydrogen (16), and the total stream is driven into the reactor where it flows above the quartz contairi(:~r that encloses the catalyst. T'he gas comes in contact with the catalyst and carbon nanotubes are produced. The clasead.i,,, by==
products of the production reaction are safely driven to the exhaust line (17). It should be pointed out that the apparatus described above, as well as the reactor are only giveti as an example. Any suitable arrangement and any hydrocarbon would produce carbon nanotubes at the same rate and with t1-1i-; same quality, provided that the employed catalyst was -rhe one described in the present invention.

Any hydrocarbon or alcohol or other organic or irtorganic material that contains carbon can be used as carbon source, Better results are obtained when employing ethylene. For example, when the above ethylene mixture, with a concentration of 31% in ethylene, is supplied to the reactor that contains the above described catalyst, the yield of the production of carbon nanotubes surpasses 2000% -relative to the initial weight of the mixture of the oxides that comprise; the catalytic substrate - in less than 20 minutes. The nanotubes that are produced this way are multi-wall carbon nanotubes, and their purity exceeds 95%. Their diameter ranges frorn 'i 5 to nanometers, and their length is of several micrometers as it 35 is shown in the pictures that were taken with a scanning electron microscope (Figure 2).

The use of methane as hydrocarbon and a sir-nilar procedure lead to the production of mixtures of single-wall and muli:i-wall carbon nanotubes with a 700% yield relative to the initial weight of the mixture of the oxides that comprise the c:atalytic substrate. The methane concentration in the mixture of tf7e deposition reaction is 36%. The rate of the deposition reaction for this case is lower than that of the case where ethylene is employed. However, the purity (88%) and the quality of the nanotubes are very good. Scanning electron microscopy revealed the presence of carbon tubes with diameters in the range of 10-40 nariometers. The use of Raman spectroscopy proved the existence of single-wall nanotubes.

Example 1: Production of Mul$i-waiB Carbon Nanot:ubas =t'rorru Ethylene Preparation of the Catalytic Substrate The catalytic substrate for carbon deposition was prepared as following: in high purity methanol solution of approximately 8-10 ml volurne, 3.71 g of iron nitrate (Fe(N03)3 9H20) and 1.948 g of aluminum nitrate (AI(N03) 9H20) were dissoived.
In addition, 0.18 g of ammonium molybdate tetrahydrate ((NH4)6M07024 4H2O) were initially dissolved in 3-5 mf of water and subsequently added to the methanol solution. The resulting solution was left in environmental conditiorIs urrtil methanol evaporated, and the generated mud was placed in a shallow quartz container and heated at 700 C for 30 rriinutes under helium flow. Subsequently, the solid material was cooled slowly to room temperature and it was then ground in a compact mortar. This procedure led to the creation of a red powder, which contained Fe203 and A1203 in a ratio of 74/26 whereas the Fe/Mo ratio was equal to 5/1.
Production of Carbon Nanotubes The material that resulted from the above procedure was employed as the catalytic substrate in the process of carbon nanotubes production. A catalyst quantity equal to 2.8 rn'j was placed in a shallow platinum container, which was hung from 5 the sample arm of the microbalance with a thin wire aligned to the axis of the reactor, which was placed inside a furnace. The reactor was a quartz tube of 15 mm internal diameter and 22 cm length. The catalytic substrate was heated under 200 sccm helium (He) flow until the temperature reached 700 l:".. After 10 approximately 30 min and with the temperature having reached 700 C, an ethylene-helium mixture, in which the C2H4/He ratio was 63/137, was allowed to enter the reactor at 200 scorn total flow by opening the respective valve. Initially, and fc7r .:ibout one minute, a slight weight loss (0.15 mg) was observed that is correlated to the procedure of catalyst activation via the process of the reduction of the initial oxide. Subsequently, a continuous increase of the weight of the material iri the container was observed and after 7 min it reached 48.6 n-ig.
The increase of the weight of the material in the contairier as a result of the carbon deposition was more than 18 tirne s the weight of the catalytic substrate that was initially place(:l in it.
The material was characterized without being previously subjected to any treatment for the removal of the catalytic substrate, soot, or other carbon forms that were possibly generated during the deposition process. The presc,ance of multi-wall carbon nanotubes was confirmed with scanning electron microscopy (SEM). The average diameter of the nanotubes was estimated to be 10-20 nm and their lengf:h a few pm. The characterization of the material - as obtained afte:r the deposition process - with Raman spectroscopy revealed characteristics of graphitic forms of carbon. The specific surface area of the material was measured to be 230 m2/g.

Example 2: Production of Single-wao8 Carfooru Nanratubes from Methane In a second procedure, for the production of single-wall carbon nanotubes, a quantity of the catalytic substrate (tE-iat was prepared with the procedure described in Example 1) equa{ to 2.4 mg was placed in a shallow platinum container and positioned in the same apparatus that was describeca in Example 1. The catalytic substrate was heated under 200 sc.crn helium (He) flow until 700 C. After approximately 30 min and with the temperature having reached 700 C, a rneth ei ri r.,-helium-hydrogen mixture, with CH4/H2/He ratio ecl u ,.Ql to 73/67/60, was allowed to enter the reactor at 200 scc.;rn total flow by opening the respective valve. Initially, and for about five minutes, a slight weight loss (around 0.3 mg) was observed that is correlated to the procedure of catalyst activation. Subsequently, a continuous increase of the ve,ffwight of the material in the container was observed and after 22 miri it reached 4.8 mg.

The material was characterized without being previously subjected to any treatment for the removal of the catalytic substrate, soot, or other carbon forms that were possibly generated during the deposition process. The diameter of the observed tubes was 15 rim, which is a characteristic size of single-wall carbon nanotube bundles. The size r.) f the nanotubes is considered to be less thari 2 nm whereas their length was estimated to be a few pm. The characterization of the material - as obtained after the deposition process - with Raman spectroscopy revealed the absence of an-i orphous carbon and structural defects, as well as the presence ol' Pyllõiiti-wall carbon rianotubes.

References 1. R.H. Baughman, A.A. Zakhidov and W.A. cle Heer, "Carbon Nanotubes - The Route toward Applications", Science, 297 (2002).

2. J. Kong, A.M. Cassell and H. Dai, Chem. Phys. Lett., 292, 567-574 (1998).
3. M. Su, B. Zheng and J. Liu, Chem. Phys. Lett., 322, -321-326 (2000).
4. http://www.nanospace.orq/new page 64.htrn 5. http://www.researchandmarkets.com/reports/7730/
6. http://bcc.ecnext.com/coms2/summary_0002_001 972000000 _000000_000_1 7. http://www.ieccomposites.com/news/news fiche.asp?ir!~~(012 &
8. http://www.tappi.org/index.asp?pid=25961&bhcd2='I0t31352 9. http://nanotech-now.com/nanotube-survey-apri12003.1-itni

Claims

1. A catalyst with feeble consolidated structure for the large-scale production of high-purity carbon nanotubes with the method of chemical vapor deposition, or any other method, using hydrocarbons as carbon bearing precursors. The catalyst consists of Al2O3, Fe2O3 and Mo at a Fe2O3 concentration of 75-9~% and a Fe/Mo ratio of 8-12.

2. The procedure for preparing the catalyst of Claim 1, which comprises:
- Mixing of the ~ight amounts of hydrous nitric salts of iron (Fe(NO3)3 .cndot. 9H2O an aluminum (Al(NO3)3 .cndot. 9Hz0), and the right amount of ammonium ~olybdate tetrahydrate ((NH4)6Mo7O24 = 4H2O) - Heating of the mixture at a temperature of about 300 C to 600 C for 10-25 minutes.
-Grinding of the resulting material until it turns into a fine red powder, which is used as the catalyst for the production of carbon nanotubes

3. The concentration th gaseous reactive mixture employed for the Oroquction of carbon rran ubes with chemical vapor deposition using the batalyst of Claims 1 & 2, which is about:
-C2H4/He=63/137 f~n the production of multi-wall carbon nanotubes -CH4/H2/He=73/67~0 for the production of single-wall carbon nanotubes.

To whom it may concern:

Please, find below the explanations of the amendments made in the claims of the international application with numb OCT R2005/000022. (Applicant: The Foundation of Research and Technology, Hellas) The claims 1 to 7 of the initially submitted application have been replaced by t amended claims 1 to 3.

The amendments were made having tabkin under consideration the written opinion of the international searching authority;

1. The concentration of the claimed catalyst has been clearly defined and it does not coincide with the concentration or characteristics of previously claimed or prepared catalysts.
2. The claimed catalyst is not c~nstrued as a "natural" material.
3. The methodology of the catalyst's reparation has been clearly defined and differentiated from the metholologies mentioned in the international search report.

4. The thermal method of carbon nanotubes synthesis using the developed catalyst is not claimed in this international application, but it is rather stated in its general description.

5. The concentration of the gas~dus reactive mixture employed for the production of carbon nanotubes using the claimed catalyst has been added as a claim to the new (amended) list of claims The amendments do not have any i~ pact on the drawings and the description of the international application.