Abstract:
A method of producing carbon nanoparticles comprises the steps of: passing a gaseous carbon source through a heated reactor; and adding catalyst supported on substrate particles or thermally decomposable catalyst precursor supported on substrate particles to the heated reactor to form a fluidised bed; such that carbon nanoparticles are formed in the heated reactor.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method of producing carbon nanoparticles, and to carbon nanoparticles so produced.  
       BACKGROUND OF THE INVENTION  
       [0002]     Carbon nanoparticles may be produced by various routes, including catalytic vapour deposition (CVD), arc discharge and laser ablation.  
         [0003]     The CVD route has advantages of low cost and scalability. There has therefore been significant interest in this route.  
         [0004]     Typically, in the CVD route, a gaseous carbon source such as a hydrocarbon or carbon monoxide is decomposed by a metallic catalyst in a heated reactor under suitable reaction conditions. Carbon nanoparticles (for example carbon nanotubes) are deposited.  
         [0005]     The catalyst may be either supported by a substrate or suspended in the gas stream. The catalyst may be introduced into the reactor in the following ways: 
        1. Placing a supported catalyst or catalyst precursor (e.g. ferrocene, iron pentacarbonyl) into the reactor and then introducing a gaseous carbon source into the reactor. This is the fixed bed method. The supported catalyst may be made by sputtering catalyst metal onto a substrate, by oxidation of a metal salt followed by reduction (WO 00/73205), by impregnation of a metal salt into a high surface area substrate [Geng 02], or by a sol-gel reaction using precursors containing the catalyst elements and the support materials [Su 00, Flauhaut 99], or by in situ thermal decomposition of a supported catalyst precursor.     2. Introducing the catalyst in the form of a precursor directly into the gaseous carbon source in the heated reactor to produce catalytic metal particles in situ by thermal decomposition. The metal catalyst is suspended in the reaction gas mixture [WO 00/26138].     3. Introducing the catalyst in the form of a precursor directly into the gaseous carbon source in the heated reactor to deposit catalytic metal particles onto solid supports held within the reactor [Singh 03].     4. Introducing the catalyst and substrate into the furnace, where upon they react, as described in WO02/092506.        
 
         [0010]     The most commonly used process for synthesizing nanoparticles is the fixed-bed method. In the fixed-bed method, the supported catalyst is heated slowly within the heated reactor. Some fixed-bed supported catalyst systems produce nanotubes, while others yield only amorphous carbon or carbon capsulated metal particles. There is often failure to produce carbon nanotubes, and in particular failure to produce single-walled nanotubes [Li and summary of WO 0017102].  
         [0011]     A promising process for large-scale synthesis of carbon nanotubes is the fluidised bed method. Fluidised-bed processes are well-established in chemical engineering. Such processes have the advantage of enhancing gas-solid mixing so as to increase reaction efficiency and provide uniform products.  
         [0012]     Fluidised bed methods have been used for production of multi-walled carbon nanotubes. These methods have been carried out by introducing supported catalyst into a heated fluidised bed reactor followed by slow heating to a synthesis temperature [Wang, Carbon].  
         [0013]     Some workers use an additional reduction step in hydrogen prior to the nanotube synthesis reaction [Wang,Bachilo]). Recently, Bachilo et al. and Mauron et al. have reported the production of single-walled nanotubes from salt impregnated silica which was oxidised and then reacted [Bachilo, Mauron].  
         [0014]     Fluidised bed methods also suffer from the disadvantage which applies to the fixed-bed method of failure to produce carbon nanotubes and in particular failure to produce single-walled carbon nanotubes.  
       SUMMARY OF THE INVENTION  
       [0015]     In a first aspect, the present invention provides a method of producing carbon nanoparticles, comprising the steps of: 
        passing a gaseous carbon source through a heated reactor; and     adding catalyst supported on substrate particles or thermally decomposable catalyst precursor supported on substrate particles to the heated reactor;     maintaining a fluidised bed of the substrate particles in the heated reactor; and     forming carbon nanoparticles in the heated reactor.        
 
         [0020]     The particles bearing the catalyst or precursor are added to the heated reactor in the presence of the gaseous carbon source. Preferably, the particles are thereby rapidly heated from a temperature at which they can be stored without deterioration in their nanoparticles forming properties to the temperature of the heated reactor. In an example described below is of the order of 10 2 -10 3 ° C./min. More generally, a heating rate between 10 and 10 4 ° C./min should be acceptable, but more preferably it should be above 10 2 ° C./min. Preferably, the particles are subjected to said rapid heating from a starting temperature not above 300° C., more preferably not above 100° C., e.g. from around room temperature. Suitably, the heating time from the safe starting temperature to the temperature of the heated reactor is from 0.01-60 seconds, more preferably not exceeding 20 seconds. Generally, the difference in temperature between the storage of the particles before injection and the heated reactor should be from 100 to 1200° C., more preferably 500 to 1000° C.  
         [0021]     Preferably, the catalyst or catalyst precursor supported on substrate particles is introduced into the heated reactor via a gravity-feed hopper. Alternatively or additionally, the catalyst or catalyst precursor supported on substrate particles may be introduced into the heated reactor via an injection gas flow. Thus, the injection gas flow may be used to entrain and carry particles released from a hopper to fall into the gas flow, or the gas flow may be used to lift particles from a bed of particles to carry them into the heated reactor.  
         [0022]     It may be that the injection gas flow reverses the direction of gas flow through the heated reactor or in a portion thereof during injection.  
         [0023]     The injection gas is suitably an inert gas but may also be or may comprise a gaseous carbon source.  
         [0024]     The reactor heated reactor is suitably at a temperature between 500 and 1200° C., more preferably at a temperature between 700 and 900° C.  
         [0025]     When a catalyst precursor is present it is suitably a metal salt, an organometallic species or a metal carbonyl. Such a catalyst precursor may comprise one or more of nickel, iron, molybdenum, platinum and cobalt. Suitably, the catalyst precursor is a metal salt and comprises a counter ion consisting of nitrate, stearate, formate, oxalate, acetate or chloride. The organic counter ions are preferred, for instance C 2  to C 30  carboxylate.  
         [0026]     The carbon nanoparticles may contain a non-carbon dopant such as nitrogen.  
         [0027]     The gaseous carbon source is suitably one or more of acetylene, alcohol, alkane, alkene, CO, benzene, toluene, xylene, cumene, ethylbenzene, naphthalene, phenanthrene, anthracene, formaldehyde, acetaldehyde, or acetone.  
         [0028]     Preferably, the gaseous carbon source is mixed with a diluent gas and preferably the mixture of these gases fluidises the bed of substrate particles. The diluent gas is preferably one or more of hydrogen, ammonia, nitrogen, helium and argon.  
         [0029]     The ratio of gaseous carbon source to diluent gas is preferably reduced while the catalyst or catalyst precursor supported on substrate particles is introduced into the heated reactor, e.g. so that the proportion of the gaseous carbon source in the mixture drops by a from 20 to 100%, so that for instance if during nanoparticles production the ratio is say 1:2 carbon source to diluent, during particle addition the amount of carbon source fed might be reduced so that the ratio is from 2/3:2 (33% reduction) down to 0:2 (100% reduction). More preferably said reduction might be by from 40 to 60%.  
         [0030]     The substrate particles may comprise or consist of one or more of silica, alumina, MCM (a family of mesoporous aluminosilicate molecular sieve materials, including MCM-41), and magnesium oxide. Suitably therefore, the substrate particles comprise a halide, nitrate, sulphate, carbonate, aluminate, aluminium chloride, arsenate, arsenite, borate, chromate, fluoroaluminate, silicate, sulphide, telluride, tungstate, vanadate or phosphate of a Group 1 or Group 2 metal. The Group 1 or Group 2 metal may be lithium, sodium, potassium, calcium or magnesium.  
         [0031]     Suitably, the average dimension of the substrate particles is between 20 microns and 1 mm, more preferably between 40 microns and 200 microns.  
         [0032]     The method according to this first aspect of the invention preferably further comprises the step of removing nanoparticles from the heated reactor. This may be done by the use of vacuum to suck out the particles bearing the nanotubes or by the use of pressure, e.g. the use of a gas jet to blow the particles off the top of the bed for collection.  
         [0033]     The process is preferably operated continuously with continuous or repeated introduction of catalyst or catalyst precursor supported on substrate particles and optionally simultaneous similarly continuous or continual removal of nanoparticles.  
         [0034]     Alternatively, the method is operated non-continuously with alternating batch wise introduction of catalyst or catalyst precursor supported on substrate particles and removal of nanoparticles.  
         [0035]     The carbon nanoparticles produced may be nanotubes and/or nanofibres. Subtle variations in conditions can be used to produced nanoparticles selectively of a desired kind. The nanotubes may be single-walled nanotubes or multi-walled nanotubes.  
         [0036]     In an alternative aspect, the invention includes a method of producing carbon nanoparticles, comprising the steps of: 
        passing a non-carbon-containing gas through a heated reactor; and     adding catalyst or catalyst precursor supported on substrate particles to the heated reactor;     maintaining a fluidised bed of said substrate particles in the heated reactor;     passing a gaseous carbon source through the heated reactor; and     forming carbon nanoparticles in the heated reactor.        
 
         [0042]     In preferred methods according to either aspect of the invention, efficiency is enhanced by the fact that the supported product particles have a lower density that the supported catalyst particle, and hence are preferentially carried out of the reactor by the fluidising gas flow. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0043]     The invention will be further described with reference to a preferred embodiment of the invention (Example 2) and to the figures, in which:  
         [0044]      FIG. 1  shows in schematic sectional side elevation an apparatus for use in the invention.  
         [0045]      FIG. 2  shows Raman spectra of the products synthesized in (a) Example 2 and b) Comparative Example 1.  
         [0046]      FIG. 3  shows SEM micrographs of the products synthesized in (a) Example 2 and b) Comparative Example 1. 
     
    
       [0047]     The apparatus shown in  FIG. 1  comprises a resistance tube furnace  10  extending vertically and annularly surrounding a vertically running quartz tube  12  having upper and lower end caps  14 ,  16 . Within the quartz tube  12  is an inner quartz tube  18  running coaxially therewith from the lower end cap  16  on which it is supported and stopping short of the upper end cap  14 . Approximately half way along its length, the inner quartz tube  18  has a disc  20  of porous silica frit bridging across its bore. An inlet tube  22  for the introduction of a mixture of gaseous carbon source and diluent gas extends through the lower end cap  16  axially into the lower end of the inner quartz tube  18 . An outlet tube  24  for venting gas from the reactor extends from the annular space between quartz tubes  12  and  18  through the lower end cap  16 .  
         [0048]     An inlet tube  26  extends axially through the upper end cap  14  to reach down into the upper part of the inner quartz tube  18 . A hopper  28  for the gravity feed of substrate particles is connected via a ball valve  30  to a port at the top of a horizontal run of the tube  26  and a side arm of said tube leading to said port is connected to a supply  32  of carrier gas.  
         [0049]     In use, the furnace is heated to heat the quartz tubes  12  and  16  to the desired nanotubes forming reaction temperature and a flow of carbon containing gas and diluent gas mixture is established through inlet  22 . Thereafter, substrate particles are dropped from the hopper  28  and displaced by a flow of carrier gas from the side arm of tube  26  to fall into the reaction zone where they are supported on the frit  20  and form a fluidised bed  34 . Carbon nanoparticles then form on the substrate particles.  
         [0050]     The invention will be further described with reference to the following non-limiting examples.  
       EXAMPLES  
     Comparative Example 1  
       [0051]     Nickel formate/silica gel particles were prepared by impregnating porous silica gel particles (50 micron in diameter) with a nickel formate aqueous solution. A nickel loading of 3.0 wt % was obtained.  
         [0052]     100 mg of the supported catalyst particles were placed onto the bed of a fluidised bed reactor containing a porous frit at room temperature. The reactor was purged with argon and was then heated at 10° C./min to the synthesis temperature of 860 C.  
         [0053]     The supported catalyst particles were then fluidised by passing a stream of methane and argon (ratio 1:2) through the bed at a flow rate of 2.0 l/min. After 20 min and subsequent cooling of the system, the products were collected from inside the fluidised reactor and were characterized by Raman spectrometry and scanning electronic microscopy ( FIG. 1   b ),  FIG. 2   b )). This showed that only amorphous carbon was formed on the surface of the silica gel particles.  
         [0054]     A similar synthesis was conducted in a horizontal reactor by a fixed-bed method. An identical supported catalyst was placed in an alumina crucible then heated to the reaction temperature in the reaction gas mixture described above. Again, in this case, only amorphous carbon was formed.  
       Example 2  
       [0055]     A hot-injection synthesis was conducted using the same supported catalyst of Example 1.  
         [0056]     The supported catalyst was held outside the reactor under an inert argon atmosphere whilst the fluidised bed reactor was heated to 860° C. Once the reactor had reached this temperature, the supported catalyst particles were blown into the top of the vertical reactor using argon (600 ml/min) as the carrier gas.  
         [0057]     During addition of the supported catalyst, a methane-argon mixture (ratio 1:2, 2.0 l/min) was kept flowing through the bed. The catalyst particles were fluidized on the bed in a 1:1 methane-argon mixture, at a flow rate of 2.0 l/min, at 860° C. for 20 min.  
         [0058]     As the catalyst was exposed to the carbon source at the high temperature, an immediate colour change of the catalyst particles from their original green colour to brown or black was observed on those particles which were swept out of the fluidised bed reactor.  
         [0059]     SEM observation ( FIG. 1   a )) of the black products collected inside the fluidized bed reactor revealed a distribution of fibrous carbon products on the silica gel particles, and Raman analysis ( FIG. 1   b )) showed that these particles were single walled nanotubes, as demonstrated by the presence of a strong G band at 1585 cm-1 and radial breathing modes at the low frequencies.  
       Example 3  
       [0060]     The supported catalyst injection method of Example 2 was carried out using pure methane rather than a mixture of methane and argon as the injection gas. The synthesis was carried out under the same conditions as Example 2, using 1:1 methane-argon. Multi-walled carbon nanotubes were grown on the surface of the silica-gel particles rather than single-walled nanotubes.  
         [0061]     The advantages of the method of Example 2 include: 
        1. The method improves the efficiency of the catalyst, that is, the percentage of catalyst which produces single-walled nanotubes.     2. The addition and subsequent removal of the catalyst while the reactor is hot means that the plant is run more efficiently than a conventional fluidised bed reactor plant.     3. The plant can be run in a continuous or semi-continuous mode. A conventional fluidised bed reactor plant is run in a batchwise mode.        
 
         [0065]     Without wishing to be bound by theory, the applicants believe that good results are achieved in the method of Example 2 for the following reasons.  
         [0066]     In the fixed-bed method, catalyst particles are formed by thermal decomposition of catalyst precursor during heating. The nature of the catalyst particles is affected by the rate of heating. In particular, slow heating may result in larger catalyst particles because of slow decomposition of the catalyst precursor and ripening of the catalyst particles on the substrate surface after decomposition. This can lead to failure to produce carbon nanotubes, and in particular to failure to produce single-walled nanotubes whose growth requires catalyst particles of similar diameters to the nanotubes (a few nanometres) [Li and summary of WO 0017102]).  
         [0067]     In order to produce carbon nanotubes, it is necessary to form catalyst particles of small size. This can be achieved by rapid heating of the supported catalyst in a highly dispersed state. This leads to the formation of small catalyst particles due to the impeded decomposition of the catalyst precursors. The impeded           n heat exposure of the           perature so that the           hesis high temperature           which would be           furnace temperature as           bearing substrate           ctor from cold.           er in the vapour phase           on the substrates.           heating may generate           rve the small metallic           nucleation of           ce means that ripening           urface of the substrate           ts as soon as it           . This condition is met           ng rate achieved when           the reactor at 500 to           rted to produce a           sintering since both           e inhibited. Therefore           .           orted catalyst in a           post-interparticulate           d-bed method where the           ause the particles are           not contact each           of the catalyst in           a fixed bed condition,           able to cause           icles through either           ripening on the           s.  
         [0068]     Rapid heating of a catalyst precursor has been used in a floating catalyst method to synthesize nanotubes. In this method, a preheated gas was injected into a heated reactor with a catalyst precursor from a cooled nozzle [WO 00/26318]. No catalyst support was used. In the preferred embodiment of the present invention, the catalyst support plays an essential role.  
         [0069]     Whilst the invention has been described with reference to a preferred embodiment, it will be appreciated that various modifications are possible within the scope of the invention.  
       REFERENCES  
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         [0071]     Wang—Y. Wang, Fei Wei, Guansheng Gu, hao Yu, Agglomerated carbon nanotubes and its mass production in a fluidised-bed reactor, Physica B, 323, 2002, 327-329 
        The large-scale production of carbon nanotubes in a nano-agglomerate fluidized-bed reactor,  CPL,  364 (5-6), 2002, pp 568-572 Yao Wang, Fei Wei, Guohua Luo, Hao Yu and Guangsheng Gu        
 
         [0073]     Carbon—Vengoni D, Serp P, Feurer, R, Yolande K, Vahlas C, Kalck P, Parametric study for the growth of carbon nanotubes by catalytic chemical vapor deposition in a fludised bed reactor, Carbon 40, 2002, pp 1799-1807  
         [0074]     “Narrow (n,m)-Distribution of Single-Walled Carbon Nanotubes Grown using a Solid Supported Catalyst” S. M. Bachilo, L. Balzano, J. E. Herrera, F. Pompeo, D. E. Resasco and R. B. Weisman, J. Am. Chem. Soc. Submitted (available at http://www.ou.edu/engineering/nanotube/publications.html)  
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