In the present invention, advanced catalysts for lhe production of high purity and high quality carbon nanotubes with the technique of chemical vapor deposition are desciibed i ^" h ^p use of these catalysts and the described methodology all sws lhe production of carbon nanotubes at high rates and wιtι> yields per mass unit of catalytic material, which are much higher Hi an those achieved with other methods of carbon nanutubes production and other catalysts The high yields, high production rates and the very low cost of the catalysts thai are employed in the developed method lead to the piodurhon of materials that cost much less than commercially avai'able materials of similar or lower quality The catalyst or the catalytic substrate on which carbon is deposited and ηrows in the form of nanotubes consists of the carrier or the subsl i ate, which is aluminum oxide (alumina) or one of the othei metal oxides that are usually employed as catalytic media, lha active phase which is iron oxide (preferably hematite but cilso any other form) and a promoter, such as molybdenum oxide The ratio of these three components plays a very importan! role in the composition of the catalyst

Background of the invention

The present invention refers to a method for the development of catalysts and catalytic substrates and their use for the large-scale production of carbon nanotubes with chemical vapor deposition The developed nanotubes are eiiher single- wall or multi-wall, depending on the employed catalyst and the employed hydrocarbon in particular, the present invention refers to a procedure that leads to the development of r.αtalytic substrates of specific composition The catalytic substr files are used for the deposition of carbon in nanotubes form on their surface, in a way that provides the capability of high yield relative to the initial weight of the substrate Carbon

deposition and nanotubes synthesis take place with the method of chemical vapor deposition This method ensuies fhe production of carbon nanotubes of constant qualily at a relatively low temperature and atmospheric pressure and gives solution to the problem of carbon nanotubes high production cost

Many scientists characterize carbon nanotubes as the "materials of the future" The nanotubes - cylindrical carbon structures with diameters that range from 0 6 nanometer'- f0 6 x 10 ⁶ m) to 300 nanometers - are materials that combine exceptional mechanical and electrical properties For ihis reason, there is a lot oF interest in the production of such materials that can find many applications, as it can be seen in various publications [1-3]

Carbon nanotubes are materials that exhibit unique pioporlies such as high electrical and thermal conductivity They also have exceptionally high mechanical strength (100 times 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 fuel cells, adsorption materials and membranes for separations Carbon nanotubes can also be used as components of composite materials for reinforcement or modification of properties (e g electrical conductivity of plastics), 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] According 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 rods/electrodes with arc discharge, 2) laser ablation, 3) catalytic decomposition of carbon-bearing gases (usually hydrocarbons or carbon monoxide) with the use of metal catalysts supported on metal oxide substrates or suspended in the gas phase (catalytic chemical vapor deposition), and 4) decomposition of gaseous or liquid compounds wifh arc discharge Metal catalysts are not only employed in chemical 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, fullerenes and relatively high amounts of amorphous carbon Similar drawbacks are encountered in the method of laser ablation whore iarge amounts of amorphous carbon and multi-wall nanotube ■> wth a lot of structural defects are produced Laser ablation i <-. also a costly technique with high power requirements Despite 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 nanoiubes and a high concentration of multi-wall nanotubes Nanotube enrichment techniques have been developed, but their 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 wiih the use of the appropriate catalyst Chemical vapor deposmori 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, CVD 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 > j o 1 cl per gram. The high production cost is mainly attributed to the use of unsuitable and energy-consuming methods, as well as unmanageable systems of reactors and catalysis, Ihe nanostructures production yield of which is limited The high cost of carbon nanotubes renders research in the field of their potential applications almost prohibitive.

The unique properties of these materials though, make them very attractive for applications that involve composite materials, and specifically, changes in their mechanical and electrical properties with the incorporation of carbon nanotubes in their structure. The use of carbon nanolubos in hydrogen storage for fuel cell applications as well as in the fabrication of nanoelectronic materials and parts has been also suggested.

The present invention ensures a very high yield in the process of carbon nanotubes production, which is capable of providing 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 invention 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 significant pollutants. The exceptionally high yield of the process results from the large activity of the catalysts and the catalytic

substrates, and their capability to absorb all carbon present in the gaseous precursors

In addition, the high yield of the method ensures a v e > y clean product that excludes the need for any further purification process, which in other cases is necessaiy The puπiscation process increases the malerial 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 ex pen mental apparatus (vertical reactor) for chemical vapor deposition of carbon nanotubes

Figure 2 Scanning Electron Microscope image of carbon nanotubes deposited from 31% C ₂ H ₄ at 700 ⁰ C on <. ata lytic substrate of AI ₂ O ₃ / Fe /Mo

The present invention involves catalysts and catalytic substrates for the production of carbon nanotubes wifh the technique of chemical vapor deposition, by employing hydrocarbons, alcohols as well as other molecules thai contain carbon The catalyst or the catalytic substrate on which cirbon 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 of which is iron or iron oxide (preferably hematite but also any other form) and a promoter, such as molybdenum or molybdenum 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 AI ₂ O ₃ ) 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 in which ihe

AI ₂ O ₃ ZFe ₂ O ₃ ratio is 26 4/73 6 The ratio of molybdenum over iron (or of their respective oxides) is between 0 and 1/1, preferably between 1/10 and 1/3

The preparation of the catalytic substrate takes plac^ through dissolution of the right amounts of hydrous nitric sails of non (Fe(NOs) ₃ • 9H ₂ O) and aluminum (Al(NO ₃ ) ₃ • 9H ₂ O) m a small volume of methanol or water The resulting mixtute iu mixed with an aqueous solution that contains ammonium rnnlybdate tetrahydrate ((NH ₄ ) ₆ Mθ7θ _?4 ° 4H ₂ O) The solution dries in room temperature for a week until complete evaporation of molhaiiol is achieved, and the remaining mud is baked at 300-700 ⁰ C lor 30 minutes under helium flow The baked material i ε cooled under inert gas flow, and subsequently it is ground in a compact mortar until it turns into a fine red powdπr This powder is the catalyst, which is then placed in the ruactoi for the production of carbon nanotubes

With the above described method, the iron, aluminum and molybdenum of the catalytic substrate are converted to ihe respective oxides during the heating of the mafonal Subsequently, the iron oxide (possibly hematite (Fe ₂ O _n )) thai has been formed should be reduced to iron or iron eaibide in order to initiate the deposition process The reduction or κon oxide can take place either with the use of the hydi occirbon that is employed for the production of carbon nanotubes or with the use of hydrogen prior to the beginning of the production process There are two alternative ways that can be employed for this purpose 1) a two-step process which includes (a) heating of the catalyst (at 500-900 ⁰ C) in ineri atmosphere and (b) exposure of the catalyst in a mixture of a hydrocarbon and hydrogen that results in the reduction ot hematile, and 2) a three-step procedure which includes (a) heating ot the calalysl in inert atmosphere, as above, (b) reduction of the calalysl in inert gas/hydrogen flow at a temperature that ranges ftom 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 species, hydrogen and an inert gas In the above mentioned procedures, the carbon-bearing gas (hydrocarbon) is preferably ethylene 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 oi the catalytic substrate that was prepared according Io flv- above described method is placed in a suitable reactor, as for example the one that is shown in Figure 1 The specific reactor consists of a vertical quartz tube (1) with I5 mm inlernal diameter, which is heated by a two-temperature zone furnace with 22 cm length (2) Two K-thermocouples (3) are employed for temperalure measurements and are placed at the ceni^t of each heating zone The temperature is controlled by a temperature controller (4) The rate of the deposition oi caibon nanotubes on the catalytic substrate (5) is measured gravimetrically by recording the change in the weight of ihe catalytic substrate In the apparatus that is presented in Figure 1, the reactor tube is being coupled to an e!ectι<»nιc microbalance of 1 microgram (μg) sensitivity (CAHN D-IOI) (6) for continuous monitoring of the weight of the deposit, until 100 grams (7) The catalytic substrate is placed in a shallow container made of quartz or platinum, or other men and resistant material, which is hung from the sample arm ol the microbalance with a thin wire (8) aligned to the reacLor axis

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 ιε not necessary, a vertical or horizontal quartz tube of larger diameter is employed as the reactor, withoul the use of a sensitive microbalance The catalytic substrate is inseried in a

suitable quartz container, which is placed in the middle of the quartz tube

The gas mixture that contains the carbon source is supplied to the reactor through an appropriate system of pressure controllers (9, 10), valves (11), a pump (12) and m?ust> 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 alcohol or carbon monoxide) (14) is mixed with an inert gas (15) and if chosen with hydrogen (16), and the total stream is d r i */ P n into the reactor where it flows above the quartz container that encloses the catalyst The gas comes in contact with the catalyst and carbon nanotubes are produced The gaseous 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 given as an example Any suitable arrangement and any hydrocarbon wυuld produce carbon nanotubes at the same rate and with the same quality, provided that the employed catalyst was Hie one described in the present invention

Any hydrocarbon or alcohol or other organic or morticimc material that contains carbon can be used as carbon iource 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 from 15 to 40 nanometers, and their length is of several micrometPis as ii is shown in the pictures that were taken with a scanning electron microscope (Figure 2)

The use of methane as hydrocarbon and a similar piocedυre lead to the production of mixtures of single-wall and mulli wall carbon nanotubes with a 700% yield relative to the initial weight of the mixture of the oxides that comprise the catalytic substrate The methane concentration in the mixture of the deposition reaction is 36% The rate of the deposition ieaciion for this case is lower than that of the case where eth/lerte 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 nanometers The use of Raman spectroscopy proved the existence of single-wall nanotubes

Example 1: Production of MuJti-wall Carbon Nanotubes From Ethylene

Preparation of the Catalytic Substrate

The catalytic substrate for carbon deposition was prepared as following in high purity methanol solution of approximately B- 10 m\ volume, 3 71 g of iron nitrate (Fe(NO ₃ ) ₃ ° 9H O) and 1 948 g of aluminum nitrate (AI(NO ₃ J ₃ ^β 9H ₂ O) were dissolved In addition, 0 18 g of ammonium molybdate tetra hydrate ((NH ₄ J ₆ Mo ₇ O ₂₄ ^β 4H ₂ O) were initially dissolved in 3-5 mi of water and subsequently added to the methanol solution The resulting solution was left in environmental conditions until methanol evaporated, and the generated mud was plated in a shallow quartz container and heated at 700 ⁰ C for 30 minutes 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 ot a red powder, which contained Fe ₂ O ₃ and AI ₂ O ₃ in a ratio oi 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 oi carbon nanotubes production A catalyst quantity equal to 2 8 nig was placed in a shallow platinum container, which was hung from the sample arm of the microbalance with a thin wire aligned io 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 scorn helium (He) flow until the temperature reached 700 ⁰ C After approximately 30 mm and with the temperature having reached 700 ⁰ C, an ethylene-helium mixture, in which the C ₂ H ₄ / He ratio was 63/137, was allowed to enter the reactor at 200 scorn total flow by opening the respective valve Initially, and fen about one minute, a slight weight loss (0 15 mg) was observe 'I that is correlated to the procedure of catalyst activation via the process of the reduction of the initial oxide SubsequenUy, a continuous increase of the weight of the materia! in the container was observed and after 7 mm it reached 48 6 mg The increase of the weight of the material in the container as a result of the carbon deposition was more than 18 tunes the weight of the catalytic substrate that was initially placed 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 presence of multi-wall carbon nanotubes was confirmed with scanning electron microscopy (SEM) The average diameter oi the nanotubes was estimated to be 10-20 nm and their length a few μm. The characterization of the material - as obtained after 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 m ² /g

Example 2: Production of Single-wall Carbon Nanotubes from Methane

In a second procedure, for the production of single-waM carbon nanotubes, a quantity of the catalytic substrate (thai was prepared with the procedure described in Example 1) equal to 2.4 mg was places d in a shallow platinum container and positioned in the same apparatus that was desciibed in Example 1. The catalytic substrate was heated under 200 seem helium (He) flow until 700 ⁰ C. After approximately 30 mm and with the temperature having reached 700 ⁰ C, a methane- helium-hydrogen mixture, with CH ₄ /H ₂ /He ratio equal Io 73/67/60, was allowed to enter the reactor at 200 seem 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 weight of the material in the container was observed and after 22 min 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 thai were possibly generated during the deposition process. The diameter of lhe observed tubes was 15 nm, which is a characteristic size of single-wall carbon nanotube bundles. The size of the nanotubes is considered to be less than 2 nm whereas iheir length was estimated to be a few μm. The characterization of the material - as obtained after the deposition process - with Raman spectroscopy revealed the absence o1 amorphous carbon and structural defects, as well as the presence of multi- wall carbon nanotubes.

References

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

J Kong, A M Cassell and H Dai, Chem Phys Lett , 292, 567-574 (1998) M Su, B Zheng and J Liu, Chem Phys Lett 322, 32 t -326 (2000) http //www nanospace org/new page 64 htm http //www researchandmarkets com/reports//730/ http //bcc ecnext com/coms2/summary_0002_00197?_ 000000 _000000_000_1 http //www ieccom posites com/news/news fiche asp?κl= I 012 S ₁ http //www tappi orq/index asp?pιd = 25961 &bhcd2=10H91352

31 http //nanotech-now com/nanotube-survey-apnl2003 h t ni