Patent Application: US-65255403-A

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:
the invention will be further described with reference to a preferred embodiment of the invention ( example 2 ) and to the figures , in which : fig1 shows in schematic sectional side elevation an apparatus for use in the invention . fig2 shows raman spectra of the products synthesized in ( a ) example 2 and b ) comparative example 1 . fig3 shows sem micrographs of the products synthesized in ( a ) example 2 and b ) comparative example 1 . the apparatus shown in fig1 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 . 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 . 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 . the invention will be further described with reference to the following non - limiting examples . 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 . 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 . 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 ( fig1 b ), fig2 b )). this showed that only amorphous carbon was formed on the surface of the silica gel particles . 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 . a hot - injection synthesis was conducted using the same supported catalyst of example 1 . 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 . 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 . 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 . sem observation ( fig1 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 ( fig1 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 . 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 . 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 . 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 . 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 ]). 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 . 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 . 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 . luo — g . luo , z . li , f . wei , l . xiang , xiangyi deng , yong jin . catalysts effect on morphology of carbon nanotubes prepared by catalytic chemical vapor depostion in a nano - agglomerate bed , phyisca b , 323 , 2002 , 314 - 317 wang — y . wang , fei wei , guansheng gu , hao yu , agglomerated carbon nanotubes and its mass production in a fluidised - 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