See Dresselhaus, M. Dresselhaus, and P. See Endo, M. See Tibbetts, G. In some of Endo's early experiments it is clear that small amounts of single-wall carbon nanotubes were produced as well. But until recently no means has been found to adapt these gas phase methods to produce primarily single-wall carbon nanotubes.
See Sen, R. However, all these methods involving hydrocarbon feed stocks suffer unavoidably from the simultaneous production of multi-walled carbon nanotubes, amorphous carbon, and other products of hydrocarbon pyrolysis under the high temperature growth conditions necessary to produce high quality single-wall carbon nanotubes.
Therefore, there remains a need for improved methods of producing singlewall nanotubes of greater purity and homogeneity. The present invention provides a method and apparatus for the efficient, industrial scale production of single-wall carbon nanotubes SWNTs from all gaseous reactants and which product is substantially free of solid contaminants or by-products e.
This process is based on the use of high pressure CO as the carbon source and an appropriate gaseous transition metal catalyst precursor. The present invention further provides a composition of matter comprising single-wall carbon nanotubes having a tube diameter in the range of 0. The present invention further provides a composition of matter comprising 5,5 single-wall carbon nanotubes.
- 1. Introduction.
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- US7204970B2 - Single-wall carbon nanotubes from high pressure CO - Google Patents!
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The process involves supplying high pressure e. In this mixing zone, the catalyst precursor is rapidly heated to a temperature that results in 1 precursor decomposition, 2 formation of active catalyst metal atom clusters of the appropriate size, and 3 favorable growth of SWNTs on the catalyst clusters. Preferably a catalyst cluster nucleation agency is employed to enable rapid reaction of the catalyst precursor gas to form many small, active catalyst particles instead of a few large, inactive ones.
Such nucleation agencies can include auxiliary metal precursors that cluster more rapidly than the primary catalyst, or through provision of additional energy inputs e. The SWNTs thus formed may be recovered directly or passed through a growth and annealing zone maintained at an elevated temperature e.
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The SWNT products can be separated from the gaseous stream and recovered. The gaseous stream, which is primarily unreacted CO can be recovered and recycled. The process of this invention also provides the ability to reproducibly control the diameter of SWNT products produced. This process also provides the first SWNT process that can produce a product that is substantially made up of small diameter nanotubes e.
Carbon has, from its very essence, not only the propensity to self-assemble from a high temperature vapor to form perfect spheroidal closed cages of which C 60 is prototypical , but also with the aid of a transition metal catalyst to assemble into perfect single-wall cylindrical tubes which may be sealed perfectly at one or both ends with a semifullerene dome.
These tubes, which may be thought of as one-dimensional single crystals of carbon, are true fullerene molecules. Single-wall nanotubes are much more likely to be free of defects than multi-wall carbon nanotubes. Defects in single-wall carbon nanotubes are less likely than defects in multi-walled carbon nanotubes because the latter can survive occasional defects, while the former have no neighboring walls to compensate for defects by forming bridges between unsaturated carbon valances. Since single-wall carbon nanotubes will have fewer defects, they are stronger, more conductive, and therefore more useful than multi-wall carbon nanotubes of similar diameter.
The primary carbon source employed in the process of the present invention is carbon monoxide. CO is a readily available industrial gas that can be used with minimal pretreatment in the process of the present invention. Typically, filtration to remove unwanted particulate contaminants is all that is required. Alternatively, if desired, other purification processes such as sorption can be employed to remove unwanted gaseous contaminants in the CO feedstock.
As described in more detail below, a major portion of the CO feed gas stream may result from recycling the gaseous effluent from the process. According to the present invention, a gaseous catalyst precursor from which the catalyst cluster forms may be a metal-containing compound that is in the gaseous state under the reaction conditions.
As described below the size of this catalyst metal atom cluster has an important influence on the nature of the product produced and in the selectivity of the process to produce SWNTs. The preferred catalyst precursor compounds are metal carbonyls e. As described in greater detail below, the process of the present invention is based in part on the provision of rapid near simultaneous 1 formation of the active catalyst metal atom cluster of the appropriate size and 2 initiation of SWNT growth.
In order to form clusters of Fe atoms from dissociated precursor molecules e. Aggregation is effected at this early stage by how tightly the COs are bound to the Fe atom in the precursor and how tightly the initially formed Fe dimer is bound. The Fe dimer binding energy is relatively low on the order of 1 eV.
More rapid nucleation can be effected by including a nucleating agent in the gas feed stream. Such a nucleating agent can be a precursor moiety that under the reaction conditions stimulates clustering by decomposing more rapidly or binding to itself more tightly after dissociation. One such nucleating agent that has been shown to substantially improve the performance of Fe catalysts is Ni CO 4.
The binding energy of the Ni dimer is on the order of 2 eV and thus Ni dimers are more likely than Fe dimers to facilitate rapid aggregation to the critical 4 or 5 atom cluster level. Fe atom clusters thus may be formed homogeneously or on seed clusters of Ni atoms. Any metal-containing precursor that facilitates this rapid nucleation can be employed.
Preferred are ratios in the range of about to with the most preferred ratio being about The use of nucleating agents can increase the productivity of the process significantly e. This increase is especially unexpected since Ni CO 4 alone has no appreciable catalytic effect in the high pressure CO process of the present invention under conditions typically employed. As shown in FIG. After optional cleanup in filtration unit 2 , the high pressure CO is divided into undiluted stream 3 and catalyst carrier stream 4.
Catalyst precursor is supplied via stream 5 from a suitable source, shown here as catalyst supply vessel 6. A catalyst-containing CO stream 7 is then formed by combining streams 4 and 5. The gas phase process of the present invention operates at high i. Since the gaseous reactants are predominantly CO, the reaction pressure parameters can be best discussed in terms of the partial pressure of CO, i. In general, it is preferred to employ P CO in the range of from about 3 to about atm. More preferred are P CO values in the range of about 5 to atm, with most preferred values being in the range of 10 to atm.
In general, higher P CO values in these ranges are preferred.
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As the P CO of the reaction is increased, at least three benefits are achieved. Firstly, the partial pressure of catalyst precursor P CAT can be increased as the P CO is increased resulting in more catalyst clusters and better productivity. Secondly, the Boudouard reaction is faster at higher pressures and this facilitates the rapid growth of SWNTs. Finally, at higher P CO values the catalyst precursor e. The concentration of catalyst precursor in the total CO gas feed should be in the range of from about 1 to ppm, and preferably about 5 to 50 ppm.
Typical concentrations in the range of 10—30 ppm may be employed in a most preferred embodiment of the process. It is convenient to refer to the catalyst precursor feed concentration in terms of its partial pressure, P CAT. This value can in general range from about mTorr up to Torr.
Preferred P CAT ranges are from 0.
While flow rate necessary to achieve the partial pressures described above will vary with the particular design and scale of the apparatus employed, typical flow rates for the apparatus schematically shown in FIG. Catalyst-containing stream 7 and undiluted CO stream 3 are forwarded to a mixing zone 8. Although not shown in this figure and as described in more detail below, stream 3 should be preheated prior to or in combination with its introduction into the mixing zone.
Any suitable means normally employed to preheat gas streams may be employed. Stream 7 should be kept below the decomposition temperature of the catalyst precursor. This can be accomplished, if necessary, by using known cooling methods such as air or water cooling. If the temperature exceeds the catalyst decomposition temperature, clusters may form too early in the process and become inactivated before they can participate in the SWNT growth process. Of course, the temperature range may vary depending on the precise catalyst or catalyst mixture employed. Streams 3 and 7 are then combined in mixing zone 8 where nucleation and growth of SWNTs take place.
The mixing zone 8 should be configured to provide rapid mixing of preheated CO stream with catalyst precursor containing stream 7.
US7204970B2 - Single-wall carbon nanotubes from high pressure CO - Google Patents
Extremely short mixing times are desired and can be referred to as nearly simultaneous. These mixing times should preferably be below about 1 msec and preferably on the order of 1 to usec. The object of this fast mixing is the fast and uniform heating of the catalyst precursor. Accordingly, turbulent mixing conditions are preferred since heat transfer is promoted thereby.
As a result of these rapid mixing conditions, the volume of the mixing zone will not be large. Flow rates to the mixing zone can be controlled for a given mixing zone configuration to provide the requisite turbulence and are typically subsonic although supersonic mixing may be employed. The mixture of SWNTs freely suspended in gas leaving the mixing zone enters growth and annealing zone 9. This zone is preferably kept at an elevated temperature by enclosing it in an oven 10 , containing heating elements 11 of any suitable kind.
The oven 10 is preferably supplied with a pressure equalizing gas, e. This gas should equal or slightly exceed the operating pressure in the system. In the growth and annealing zone, additional growth of previously formed SWNTs may take place, as may the formation of new tubes. In this zone, the formed tubes may also aggregate and remain bound to one another by van der Waals forces to form ropes i. After leaving growth and annealing zones, the mixture of gas primarily unreacted CO and CO 2 containing suspended SWNT products mostly ropes is forwarded to a product recovery zone In the product recovery zone, the solid product 13 is removed from the gas stream by any suitable means and the separated gas stream 14 can be recycled.
To facilitate continuous operation, an endless belt or drum-type filter carrier can be employed in a known manner.