Title:
Process for processing carbon material
Kind Code:
A1


Abstract:
This invention relates to a process for the preparation of fibrous carbon which comprises contacting a metallic catalyst with a carbon-containing gas at elevated temperature, characterized in that said catalyst is a porous metal comprising a transition metal or an alloy thereof.



Inventors:
Edwin, Emil (Stavanger, NO)
Rusten, Bernt Henning (Stavanger, NO)
Arnesen, Tore (Stavanger, NO)
Application Number:
10/514238
Publication Date:
01/19/2006
Filing Date:
05/22/2003
Assignee:
STATOIL ASA (Stavanger, NO)
Primary Class:
International Classes:
D01F9/12; C01B3/00; D01F9/127; B01J25/02
View Patent Images:



Primary Examiner:
MCCRACKEN, DANIEL
Attorney, Agent or Firm:
BACON & THOMAS, PLLC (ALEXANDRIA, VA, US)
Claims:
1. A process for the preparation of fibrous carbon which comprises contacting a metallic catalyst with a carbon-containing gas at elevated temperature, characterized in that said catalyst is a porous metal comprising a transition metal or an alloy thereof.

2. A process as claimed in claim 1 wherein said catalyst is particulate.

3. A process as claimed in claim 1 wherein said catalyst is a Raney metal.

4. A process as claimed in claim 1 wherein said catalyst comprises an element selected from Ni, Co and Fe.

5. A process as claimed in claim 1 wherein said catalyst is particulate with a mode particle size of 1 to 300 μm.

6. A process as claimed in claim 1 wherein said catalyst forms part of the surface of a metal article.

7. A process for the production of fibrous carbon which comprises contacting a particulate catalyst comprising a transition metal or an alloy thereof with a carbon-containing gas at elevated temperature, characterized in that said gas comprises natural gas.

8. A process for the production of particulate carbon by contacting a carbon-containing gas with a transition metal catalyst at elevated temperature, which process comprises selecting the temperature and gas pressure at which gas:catalyst contact occurs so as to produce particulate carbon having a desired ration of graphitic to amorphous carbon.

9. A process for the preparation of carbon nanofibres which comprises contacting a catalyst for carbon nanofibre production with a carbonaceous gas, characterized in that said gas comprises methane and carbon monoxide in a mole ratio of from 1:99 to 99:1.

10. A process for the preparation of carbon nanofibres which comprises in a first stage contacting a catalyst for carbon nanofibre production with a first carbonaceous gas at a first temperature for a first time period and subsequently containing said catalyst with a second carbonaceous gas at a second temperature for a second time period, characterized in that said first gas has a lower hydrogen mole percentage than said second gas, said first temperature is lower than said second temperature, and said first period is shorter than said second period.

11. A process as claimed in claim 7 wherein said catalyst is a porous metal comprising a transition metal or an alloy thereof.

12. A process as claimed in claim 1 wherein said catalyst-comprises a leached intermetal.

13. A process as claimed in claim 1 wherein said catalyst comprises at least one of nickel, iron and cobalt.

14. A process as claimed in claim 1 wherein said gas comprises methane.

15. A process as claimed in claim 1 wherein said temperature is in the range 350 to 1200° C.

16. A process as claimed in claim 1 wherein the catalytically produced carbon product is further treated or formulated.

17. A process as claimed in claim 16 wherein said carbon product is fragmented.

18. A process as claimed in claim 16 wherein said carbon product is compacted.

19. A process as claimed in claim 16 wherein said carbon product is treated to reduce the metal content thereof.

20. A carbon article formed from carbon generated in a process as claimed in claim 1.

21. An article comprising a metal substrate bearing on a surface thereof carbon generated in a process as claimed in claim 1.

22. An article as claimed in claim 21 in the form of a hydrogen reservoir.

23. A metal catalyst comprising particles of a carbon-containing metal having deposits of carbon on the surfaces thereof, prepared by exposure of said metal to a gas comprising methane and/or carbon monoxide for a period of 1 to 60 minutes at a temperature of 400 to 600° C. and a pressure of 5 to 10 bar, said gas containing less than 2% mole of hydrogen.

Description:

This invention relates to a process for the production of carbon, in particular carbon comprising carbon nanofibres (CNF), otherwise known as filamentous carbon or carbon fibrils.

It has long been known that the interaction of hydrocarbon gas and metal surfaces can give rise to dehydrogenation and the growth of carbon “whiskers” on the metal surface. More recently it has been found that such carbon whiskers, which are carbon fibres having a diameter of about 3 to 100 nm and a length of about 0.1 to 1000 μm, have interesting and potentially useful properties, eg the ability to act as reservoirs for hydrogen storage (see for example Chambers et al. in J. Phys. Chem. B 102: 4253-4256 (1998) and Fan et al. in Carbon 37: 1649-1652 (1999)).

Several researchers have thus sought to produce carbon nanofibres and to investigate their structure, properties and potential uses and such work is described in a review article by De Jong et al in Catal. Rev.—Sci. Eng. 42: 481-510 (2000) which points out that the cost of the CNF is still relatively high (ca. US $50/kg or more). There is thus a need for a process by which CNF may be produced more efficiently.

As described by De Jong et al. (supra) and in a further review article by Rodriguez in J. Mater. Res. 8: 3233-3250 (1993), transition metals such as iron, cobalt, nickel, chromium, vanadium and molybdenum, and their alloys, catalyse the production of CNF from gases such as methane, carbon monoxide, synthesis gas (ie H2/CO), ethyne and ethene. In this reaction, such metals may take the form of flat surfaces, of microparticles (having typical sizes of about 100 nm) or of nano-particles (typically 10-50 nm in size) supported on an inert carrier material, eg silica or alumina. The metal of the catalyst must be one which can dissolve carbon or form a carbide.

Both De Jong et al (supra) and Rodriguez (supra) explain that carbon absorption and CNF growth is favoured at particular crystallographic surfaces of the catalyst metal.

We have now surprisingly found that for given CNF production processes CNF yield is significantly enhanced by the use of a porous transition metal catalyst.

Thus viewed from one aspect the present invention provides a process for the preparation of fibrous carbon which comprises contacting a metallic catalyst with a carbon-containing gas at elevated temperature, characterized in that said catalyst is a porous metal comprising a transition metal or an alloy thereof.

By porous is meant metals with a high surface area, typically Raney metals which are produced by leaching one metal out of a metal alloy. The person skilled in the art will readily understand that the term porous is not applicable in this context to grids or meshes formed from solid, i.e. non porous, metals. For particulate porous metal catalysts, surface area (e.g. determined by gas adsorption) will typically be at least 20 m2/g, more preferably at least 40 m2/g, especially at least 50 m2/g e.g. up to 200 m2/g, for example 50-100 m2/g. The mode particle size, before CNF formation begins, will typically be in the range 1 to 300 μm, preferably 5 to μm, especially 10 to 80 μm, more especially 20 to 40 μm. Porous moreover refers to the metal catalyst rather than any catalyst support,.i.e. a solid metal catalyst deposited on a porous support, e.g. silica or alumina, is not a porous metal catalyst.

The metal catalyst used according to the invention preferably is selected from group 5 to 10 metals, eg nickel, iron, cobalt, vanadium, molybdenum, chromium and ruthenium and alloys thereof, eg Fe/Ni, Cu/Ni etc alloys. Lanthanides may also be used. In general the requirement seems to be that the metal is able to form carbides which are unstable at the temperatures used in the CNF production process. Precious metals, such as Pt, Au and Ag may also be deposited on such metals or alloys. Especially preferably the transition metal of the catalyst is nickel, iron or cobalt or a mixture of two or three thereof, eg Ni/Fe. Particularly preferably the transition metal content of the catalyst metal is at least 50% wt nickel, eg 70% Ni/30% Fe or 100% Ni. Especially preferably the metal catalyst is produced by total or partial removal of one metallic element from an alloy, eg removal of aluminium from an aluminium-transition metal alloy. Such aluminium-transition metal alloys or intermetals from which aluminium has been removed are available commercially (eg under the trade name Amperkat® from H.C. Starck GmbH & Co AG, Goslar, Germany) or may be prepared from the aluminium alloys by leaching with acid, eg nitric acid. Examples of Amperkat® catalysts available from H.C. Starck include Amperkat SK—NiFe 6816, SK—Ni 3704, SK—Ni 5544, and SK—Ni 5546 which contain respectively 4-7% wt Al: 62-67% wt Ni: 26-30% wt Fe, 4-7% wt Al: 93-96% wt Ni: <1% wt Fe, 5-9% wt Al: 90-95% wt Ni: <0.6% wt Fe, and 5-9% wt Al: 90-95% wt Ni: <0.6% wt Fe. These Amperkat catalysts have a grain size of about 80 μm (i.e. 80-90% below 80 μm), a solid concentration of about 20-50% and an apparent density (by watery catalyst slurry) of about 1300 to 1800 kg/m3. The use of SK—Ni 5546 is preferred.

It is believed that the high CNF yields in the process of the invention using porosified intermetal catalysts may result at least in part from the “unnatural” structure of the remaining metal. While not wishing to be bound by theory it is possible that the leached intermetal presents metal surfaces effective in carbon absorption and/or CNF growth that are not presented by the metal or alloy in its normal form.

The catalyst is preferably particulate, conveniently having a particle size as described above, or of from 10 nm to 100 μm, preferably 50 nm to 1000 nm, especially 80 to 200 nm. Alternatively however the catalyst may be a macrostructure having a desired shape (eg tube, ring, rod, etc), optionally a structure which is porous only on desired surfaces, eg the catalyst may be tubular with the interior surface porosified or it may be of any desired shape fully porosified. In this way CNF growth will yield a tube fully or partially filled with CNF or a predetermined shape consisting substantially entirely of CNF. Such CNF-filled tubes could be used as hydrogen reservoirs, eg after capping and provision of a valve. Such CNF or CNF-containing structures form a further aspect of the present invention.

The gas used in the process of the invention may be any carbon-containing gas suitable for CNF production, eg C1-3 hydrocarbons (such as for example methane, ethene, ethyne, etc), carbon monoxide, synthesis gas etc. Preferably the gas is or comprises methane. In one especially preferred embodiment the gas comprises methane and carbon monoxide as this lowers the energy supply needed since the CNF production reaction is less endothermic with carbon monoxide than with methane alone. In particular it is especially preferred that the feed gas comprise methane and carbon monoxide in a mole ratio of 1:99 to 99:1, more particularly 10:90 to 90:10. This use of a mixed methane/carbon monoxide mixture is novel, irrespective of the CNF production catalyst used, and forms a further aspect of the present invention. Viewed from this aspect the invention provides a process for the preparation of carbon nanofibres which comprises contacting a catalyst for carbon nanofibre production with a carbonaceous gas, characterized in that said gas comprises methane and carbon monoxide in a mole ratio of from 1:99 to 99:1, and preferably in that said gas is introduced into the reaction vessel in at least two streams, at least one stream being substantially carbon monoxide free and being at a higher temperature than another carbon monoxide-containing stream, e.g. using a first, carbon monoxide containing, stream at a temperature of 300° C. or less and a second, methane-containing, stream at a temperature of 600° C. or more, or using a single gas stream at a temperature of 300° C. or less, e.g. 200-290° C.

It is also especially preferred that the gas fed into the CNF production reactor should, for at least part of the reaction period, contain a small proportion of hydrogen, e.g. 1 to 20% mole, more preferably 2 to 10% mole. This has the effect of reducing the carbon activity of the catalyst metal (i.e. the rate of carbon uptake by the metal) and serves to prolong CNF production, increase total yield and reduce the weight percentage of the CNF product which is in the form of amorphous carbon. Hydrogen can be added to the gas feed to the reactor or off-gas from the reactor (which is hydrogen containing if the feed gas contains a hydrocarbon) may be, at least in part, recycled into the reactor to provide the desired hydrogen content. Depending on reactor design however, especially for continuous rather than batch operating reactors, the hydrogen generated by CNF production may be sufficient to provide an appropriate hydrogen content in the CNF/catalyst bed.

Using the porous catalyst according to the process of the invention it is possible to use natural gas without requiring unusual purification—this is a novel reagent for CNF production and this use of natural gas represents a further aspect of the invention. Viewed from this aspect the invention provides a process for the production of fibrous carbon which comprises contacting a particulate catalyst comprising a transition metal or an alloy thereof with a carbon-containing gas at elevated temperature, characterized in that said gas comprises natural gas.

In this context, by “natural gas” is meant methane-containing gas from a hydrocarbon well, optionally treated to remove hydrocarbons having four or more carbon atoms per molecule, water, nitrogen and carbon dioxide, and preferably treated to remove catalyst poisons, e.g. sulphur compounds and possibly halogens. Conventional means for poison removal may be used. The methane content of such natural gas will generally lie in the range 80 to 95% mole. Liquefied natural gas (LNG or LPG) may be used as the source of the natural gas for the process.

In the process of the invention, CNF production is preferably effected so as to yield carbon in an amount of at least 1 g carbon per gram metal catalyst, more preferably at least 10 g/g, still more preferably at least 50 g/g, especially at least 100 g/g, more especially at least 150 g/g, eg 100 to 400 g/g, typically 150 to 250 g/g.

The process of the invention will typically be effected by flowing the carbon-containing gas past the catalyst. The CNF growth results in hydrogen release where the gas contains a hydrocarbon such as methane and the hydrogen generated is an important by-product of the process.

The hydrogen produced may be separated out of the gas flow from the catalyst. However it may also be burned to provide a heat source for the reaction. Moreover it is preferred that the gas flow to the catalyst should contain hydrogen, eg 1 to 20% mole, for example 5 to 15% mole, preferably 8 to 11%, and to this end it is preferred that a part of the gas flow from the catalyst be drawn off and mixed with the carbon-containing gas flow to the catalyst. The hydrogen produced may also be separated out from the gas flow inside the reactor. One option is to separate the hydrogen from the catalyst bed by use of membranes (for example ceramic membranes) followed by a subsequent separation and discharge of the carbon product. This is of particular interest if high purity hydrogen is a desired by-product of the process.

The process of the invention is performed at elevated temperature, typically 350 to 1200° C., preferably 400 to 700° C., more preferably 500 to 680° C., especially 525 to 630° C., eg about 600° C. Particularly preferably the temperature is below 900° C., more especially below 850° C., particularly below 800° C., especially below 750° C., e.g. below 700° C., and above 550° C., especially above 600° C., particularly above 630° C. Operating temperatures of between 630 and 680° C. have been found to give rise to especially good CNF production rates and yields.

The gas flow to the catalyst is preferably at elevated pressure, eg 2 to 10 bar, especially 3 to 6 bar. The use of pressures above 10 bar is not preferred when methane is the source gas for the carbon, due to undue methane adsorption.

Quite surprisingly, catalyst activity and yield may be maintained if reaction temperature is increased by also increasing gas pressure and vice versa. However use of prolonged reaction time (or residence time in the reactor for a continuous production process) tends to increase the percentage of amorphous carbon in the CNF product. The reaction time (or residence time as appropriate) is preferably up to 30 hours, more preferably up to 10 hours, especially up to 3 hours.

The catalyst may be presented as a fluidized bed with gas flow from bottom to top. However, alternatively, the gas is passed through a catalyst bed in a generally horizontal direction, To this end the reactor in which the CNF generation may be a substantially horizontal tube, optionally having a cross section which increases in the gas flow direction. Since the catalyst bed will expand as CNF generation proceeds, since CNF coating on catalyst particles causes the particulate to adhere to the reactor walls, and since compression of the catalyst bed reduces CNF growth rate, the lower wall of the reactor may be provided with a downward slope in at least one portion following the initial location of the catalyst bed. Such a horizontal reactor design has the benefit that the carbon product compacts naturally during production without any significant adverse effect on carbon yield. Typically the carbon may compact in this way to a density of about 0.4 to 0.9 g/cm3, more typically 0.5 to 0.7 g/cm3. Alternatively the catalyst/carbon bed may be mechanically agitated, for example to improve gas and heat distribution and/or to facilitate flow of the CNF product towards an outlet.

The process of the invention may be performed continuously or batchwise. In the former case the reactor in which the process is carried out is preferably provided with means for introducing fresh catalyst at the downstream end of the catalyst bed and for removing CNF from the upstream end of the catalyst bed, eg isolatable settling legs of the type commonly provided in the loop reactors used for olefin polymerization. In the production of carbon products, particularly for bulk applications, a reactor design similar to the reactor designs used in the polyolefin-industry could be used. These reactors are designed to achieve a favourable mass transport and enhance the reactivity of the reacting gas molecules at the catalytically active metal surfaces.

The reactor used in the process of the invention will conveniently have a volume of 10 to 100 m3, preferably 50 to 70 m3 allowing a total product content in the thousands of kilograms. For continuous operation, methane feed rates of 500 to 2000 kg/hour, eg 1000 to 1500 kg/hour, and carbon removal rates of 200 to 2000 kg/hour, eg 750 to 1250 kg/hour may thus typically be achieved. The energy supply necessary to operate such a reactor will typically be in the hundreds of kW, eg 100 to 1000 kW, more typically 500 to 750 kW. Alternatively expressed, the energy demand will typically be in the range 1 to 5 kW/kgC, e.g. 2-3.5 kW/kgC. On the small scale, energy supply into the reactor may be achieved by external heating of the reactors or by inclusion within the reactor of heating means or heat exchange elements connected to a heat source. As reactor size increases however it will become more necessary to heat the feed gas that is supplied into the reactor, e.g. to temperatures of 300 to 1200° C., more preferably 300 to 1000° C., especially 500 to 900° C., more especially 800 to 850° C. To minimize catalyst deactivation, heated feed gas is preferably fed into an agitated catalyst/carbon bed at a plurality of points or over the entire undersurface of a gas-fluidized bed. Where the feed gas includes carbon monoxide and methane, the carbon monoxide is preferably introduced at a lower temperature (e.g. <300° C.), for example through a separate feed line, e.g. to avoid dusting of ferrous metal feed lines.

Since, as mentioned above, compression of the catalyst bed slows CNF formation, the reactor in which the process of the invention is carried out is preferably provided with means for agitating the catalyst bed. Where the catalyst bed is a horizontal fluidized bed such agitation may be effected by the gas flow through the bed. However, where gas flow is substantially horizontal, the reactor is preferably provided with moving or static mixers downstream of the start of the catalyst bed. Where the process is to be performed batchwise, the CNF generation process may be slowed down or halted towards the end of each batch by compression of the catalyst/CNF bed, either actively or passively by allowing the catalyst/CNF bed to compress itself against the end of the reaction zone in the reactor.

In general, CNF produced by the process of the invention will be subjected to compaction following production and/or to mechanical agitation (e.g. milling) following production. The CNF product is in the form of fibrous particles (e.g. “furballs”)—milling can release the fibres if a fibrous product is desired while compaction can be used to increase the density and mechanical strength of the product.

As mentioned above, CNF generation from a hydrocarbon gas such as methane results in hydrogen generation. If the hydrogen concentration in the catalyst bed increases unduly, the CNF production rate decreases. Accordingly, if desired, the reactor may be provided with means along the gas flow path for gas removal (in order to maintain a relatively low hydrogen concentration in the gas) and/or gas introduction (to introduce further carbon-containing gas).

Gas removed from the reactor is preferably passed through a separator in which hydrogen is removed by metallic hydride formation. Pellets of a metallic hydride in a column absorb the produced hydrogen at a low temperature, and the absorbed hydrogen can then be recovered by raising the temperature in the column. Alternatively, the hydrogen can be removed by passage of the gas through a hydrogen-permeable membrane, eg a palladium membrane, which is not permeable to the carbon-containing components of the gas. Pressure Swing Adsorption (PSA) is also an alternative separation principle that may be employed. Another separation method which may be used involves the use of polymer membranes. Such polymer membranes are commercially available for separation of hydrogen and other gas components. The resulting gas with a reduced hydrogen contact may then be recycled into the reactor or burned at least in part to heat the reactor and the gas flowing into the reactor.

The hydrogen may be absorbed using other metals if desired, e.g. Mg, Mg/Ni, Ca/Ni, La/Ni, Fe/Ti, Ti/Cr, etc.

It is of course possible to operate the process without recirculation, i.e. as a “once through” process, none of the produced hydrogen or unused feed gas being recycled and all being used for other purposes, e.g. as fuel or in other industrial applications.

The carbon produced in the reactor bed generally comprises graphitic and amorphous carbon. The weight ratio between the two, which may be determined for example from the X-ray diffraction pattern of the carbon product, may be adjusted by varying the temperature and gas pressure in the catalyst bed. This adjustment of reactor operating conditions to produce a carbon product having a desired graphitic:amorphous carbon weight ratio forms a further aspect of the present invention.

Viewed from this aspect the present invention provides a process for the production of particulate carbon by contacting a carbon-containing gas with a transition metal catalyst at elevated temperature, which process comprises selecting the temperature and gas pressure at which gas:catalyst contact occurs so as to produce particulate carbon having a desired ratio of graphitic to amorphous carbon.

(The term graphitic is used above since the walls of the CNF comprise carbon having a graphite or graphite-like structure as shown by the X-ray diffraction pattern).

The process may thus be subject to feedback control, whereby carbon is removed from the catalyst bed in the reactor, its graphite to amorphous carbon weight ratio determined, eg by X-ray diffraction, and the temperature, pressure and/or carbon-containing gas:hydrogen ratio adjusted if the ratio is above or below the desired value. In this regard, if the ratio is too high the temperature should be increased and/or the pressure increased (or the temperature should be decreased and/or the pressure should be decreased if the ratio is too low). As a result, a reactor may be used to produce successive batches of carbon with different desired graphite:amorphous carbon ratios, or in the case of continuous reactor operation the changeover period between production of carbon with two different desired values of the ratio may be controlled to minimize wastage, ie material not meeting the desired ratio requirements.

Generally, in this way, a desired graphitic:amorphous carbon ratio of at least 5:95 may readily be achieved. A ratio of 5:95 to 90:10, especially 50:50 to 80:20 particularly 50:50 to 70:30 is especially preferred.

In a particularly preferred aspect, the catalyst is subjected to an initiation or pretreatment. This serves to increase CNF production rate and CNF yield and may be achieved with any CNF production catalyst, i.e. not just porous metal catalysts, by a limited period of exposure to a feed gas with reduced or no hydrogen content at a lower temperature than the reaction temperature in the main CNF production stage. Such pretreatment is preferably under process conditions under which the carbon activity of the catalyst is greater than in the main CNF production stage. Thus viewed from a further aspect the invention provides a process for the preparation of carbon nanofibres which comprises in a first stage contacting a catalyst for carbon nanofibre production with a first carbonaceous gas at a first temperature for a first time period and subsequently contacting said catalyst with a second carbonaceous gas at a second temperature for a second time period, characterized in that said first gas has a lower hydrogen (H2) mole percentage than said second gas, said first temperature is lower than said second temperature, and said first period is shorter than said second period. If a higher graphitic contact of the CNF product is desired, the first temperature may be reduced and/or the second temperature may be increased.

In this aspect of the invention, the catalyst is preferably a transition or lanthanide metal or an alloy thereof, especially a transition metal and more especially a porous metal, in particular a nickel containing metal, especially a Raney metal. The temperature, pressure and gas composition, in the second period are preferably as described above for CNF production. The temperature in the first period is preferably in the range 400 to 600° C., especially 450 to 550° C., more especially 460 to 500° C. The hydrogen mole percentage in the first period is preferably 0 to 2% mole, especially 0 , to 1% mole, more especially 0 to 0.25% mole, particularly 0 to 0.05% mole. The pressure in the first period is preferably 5 to 10 bar, especially 6 to 9 bar. The duration of the first period is preferably 1 to 60 minutes, more especially 2 to 40 minutes, particularly 5 to 15 minutes.

This pretreatment or initiation of the catalyst causes the catalyst to become a catalyst/carbon agglomerate comprising particles of a carbon-containing metal having carbon on the surfaces thereof.

Before this pretreatment, the catalyst may if desired be treated with hydrogen at elevated temperature, e.g. to reduce any surface oxide.

Such pretreated catalysts and their use in CNF production are novel and form further aspects of the present invention.

Thus viewed from one aspect the invention provides a metal catalyst comprising particles of a carbon-containing metal having deposits of carbon on the surfaces thereof, prepared by exposure of said metal to a gas comprising methane and/or carbon monoxide for a period of 1 to 60 minutes at a temperature of 400 to 600° C. and a pressure of 5 to 10 bar, said gas containing less than 2% mole of hydrogen and preferably less than 10% mole of other carbonaceous compounds, especially less than 2% mole of other carbonaceous compounds.

The invention also extends to the products of the process of the invention. Viewed from this aspect the invention provides a carbon article formed from carbon generated in a process according to the invention. Viewed from another aspect the invention provides an article comprising a metal substrate bearing on a surface thereof carbon generated in a process according to the invention.

The carbon produced in the process of the invention may be processed after removal from the reactor, eg to remove catalyst material, to separate CNF from amorphous material, to mix in additives, or by compaction. Such steps are considered to be further steps of the process of the invention. Catalyst removal typically may involve acid or base treatment; CNF separation may for example involve dispersion in a liquid and sedimentation (eg centrifugation), possibly in combination with other steps such as magnetic separation; additive treatment may for example involve deposition of a further catalytically active material on the carbon, especially the CNF, whereby the carbon will then act as a catalyst carrier, or absorption of hydrogen into the carbon; and compaction may be used to produce shaped carbon items, eg pellets, rods, etc.

Processing of the carbon product to reduce the catalyst content therein may also be achieved by heating, e.g. to a temperature above 1000° C., preferably above 2000° C., for example 2200 to 3000° C. Thus for example a sample of carbon produced using the process of the invention and having a nickel content of 1.0% wt was heat treated at 2500° C. which reduced the nickel content to 0.0017% wt. The total ash content was also significantly reduced by this treatment.

Catalyst removal from the CNF product may also be effected by exposure to a flow of carbon monoxide, preferably at elevated temperature and pressure, e.g. at least 50° C. and at least 20 bar, preferably 50 to 200° C. and 30 to 60 bar. The CO stream may be recycled after deposition of any entrained metal carbonyls at an increased temperature, e.g. 230° to 400° C.

As a result of such temperature and/or carbon monoxide treatment an especially low metal content CNF may be produced. This forms a further aspect of the present invention. Viewed from this aspect the invention provides nanofibrous carbon having a metal content of less than 0.2% wt, especially less than 0.1% wt, particularly less than 0.05% wt, more particularly less than 0.01% wt, e.g. as low as 0.001% wt.

Publications referred to herein are hereby incorporated by reference.

The process of the invention will now be described further with reference to the following non-limiting Examples.

EXAMPLE 1

CNF Production

Carbon containing gas (90% mol methane and 10% mol hydrogen) at a pressure of 5 bar was introduced at a flow rate of 400 mL/minute and a temperature of 550° C. into a horizontal tubular reactor having a conical section increasing in cross-section in the flow direction. Before the reaction began, 0.3 g of a aluminium-leached nickel:aluminium intermetal catalyst (Amperkat® SK Ni 3704 from H. C. Starck GmbH & Co KG, Goslar, Germany) was placed at the narrowest point of the reactor. The gas flow was maintained for 30 hours by which time CNF generation had ceased.

EXAMPLE 2

CNF Production

Carbon containing gas (90% mol methane and 10% mol hydrogen) at a pressure of 5 bar was introduced at a flow rate of 400 mL/minute and a temperature of 550° C. into a horizontal tubular reactor having a conical section increasing in cross-section in the flow direction. Before the reaction began, 0.3 g of a aluminium-leached 68% Nickel/32% Iron:aluminium intermetal catalyst (Amperkat® SK Ni Fe 6816 from H.C. Starck GmbH & Co KG, Goslar, Germany) was placed at the narrowest point of the reactor. The gas flow was maintained for 30 hours by which time CNF generation had ceased.

EXAMPLE 3

CNF Production

0.04 g of intermetal catalyst (SK—Ni 5546 from H.C. Starck GmbH & Co KG as described earlier) was placed in a horizontal tubular reactor. The reactor was heated to 480° C. with a nitrogen:hydrogen (1:1 by mole) mixture at a rate of 400 C.°/hour. Then methane at 480° C. and 6 bar was flowed through the reactor for 30 minutes at 1.6 L/min. The reactor temperature was raised to 630° C. at 600 C.°/hour and a gas mixture comprising 1.6 L/min CH4, 250 mL/min hydrogen and 40 mL/min nitrogen was flowed through the reactor at 630° C. and 6 bar for 24 hours. The carbon product yield was in the range 13.6 to 15 g C, i.e. 340 to 375 g C/g catalyst. Using a 3 hour production run, 6-8 gC may be produced analogously.