US20080193367A1 - Method for Selectively Producing Ordered Carbon Nanotubes - Google Patents

Method for Selectively Producing Ordered Carbon Nanotubes Download PDF

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US20080193367A1
US20080193367A1 US11/629,028 US62902805A US2008193367A1 US 20080193367 A1 US20080193367 A1 US 20080193367A1 US 62902805 A US62902805 A US 62902805A US 2008193367 A1 US2008193367 A1 US 2008193367A1
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ferrous metal
metal coating
catalyst
particles
iron
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Philippe Kalck
Philippe Serp
Massimiliano Corrias
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Institut National Polytechnique de Toulouse INPT
Arkema France SA
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/70—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper
    • B01J23/74—Iron group metals
    • B01J23/745—Iron
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82B—NANOSTRUCTURES FORMED BY MANIPULATION OF INDIVIDUAL ATOMS, MOLECULES, OR LIMITED COLLECTIONS OF ATOMS OR MOLECULES AS DISCRETE UNITS; MANUFACTURE OR TREATMENT THEREOF
    • B82B3/00—Manufacture or treatment of nanostructures by manipulation of individual atoms or molecules, or limited collections of atoms or molecules as discrete units
    • B82B3/0009—Forming specific nanostructures
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y40/00—Manufacture or treatment of nanostructures
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/15—Nano-sized carbon materials
    • C01B32/158—Carbon nanotubes
    • C01B32/16—Preparation
    • C01B32/162—Preparation characterised by catalysts
    • B01J35/393—
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2202/00—Structure or properties of carbon nanotubes
    • C01B2202/06—Multi-walled nanotubes
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2202/00—Structure or properties of carbon nanotubes
    • C01B2202/20—Nanotubes characterized by their properties
    • C01B2202/36—Diameter

Definitions

  • the invention relates to the manufacture of ordered carbon nanotubes.
  • the ordered carbon nanotubes have a tubular structure with a diameter between 0.4 nm and 30 nm and a length of greater than 100 times their diameter, especially between 1000 and 100 000 times their diameter. They may either be associated with metal catalyst particles or be free of such particles (after purification).
  • Carbon nanotubes were described a long time ago (S. Iijima “ Helical nanotubes of graphitic carbon ”, Nature, 354, 56 (1991)), but they have still not been exploited on an industrial scale. However, they could be used for many applications, and especially be very useful and advantageous in the manufacture of composites, flat screens, tips for atomic force microscopes, the storage of hydrogen or other gases, as catalyst supports, etc.
  • WO-03/002456 describes a process for the selective manufacture of ordered carbon nanotubes in a fluidized bed in the presence of a supported catalyst formed from iron on alumina, comprising from 1 to 5% by weight of highly dispersed atomic iron by fluidized-bed CVD on alumina grains about 120 ⁇ m or 150 ⁇ m in size.
  • the iron particles deposited are dispersed and have a dimension of around 3 to 6 nm. This process makes it possible to obtain a good selectivity and a good yield (greater than 90%) relative to the carbon source.
  • reaction it is desirable not only for the reaction to be selective in terms of nanotubes (as opposed to other forms of carbon that may be produced, namely soot, fibers, etc.) and for the activity to be high so that the reaction is rapid, but also for its productivity to be high in order to avoid the need for purification steps, in order to separate the catalyst from the nanotubes, and the costs incurred.
  • the object of the invention is therefore to alleviate these drawbacks by proposing a process using a catalyst of astonishingly high performance. More particularly, the aim of the invention is to propose a process for obtaining, simultaneously, a high productivity, especially of about 25 or higher, a high activity, especially of about 10 or higher, and a very high selectivity, especially greater than 90%, or even close to 100%, in terms of carbon nanotubes produced, especially multiwalled nanotubes.
  • the object of the invention is more particularly to propose a process for manufacturing ordered carbon nanotubes, especially multiwalled nanotubes, having a production rate and a yield that are compatible with the constraints of exploitation on an industrial scale.
  • the invention relates to a process for the selective manufacture of ordered carbon nanotubes by decomposition of a carbon source in the gaseous state brought into contact with at least one supported solid catalyst in the form of particles, called catalyst particles, consisting of a porous alumina support bearing an unoxidized metal coating of at least one transition metal, including iron, referred to as ferrous metal coating, characterized in that a supported catalyst is used that is mainly formed from catalyst particles:
  • the ferrous metal coating is in the form of at least one cluster formed from a plurality of agglutinated metal bulbs.
  • the ferrous metal coating forms a homogeneous continuous ferrous metal surface layer formed from metal bulbs.
  • Each cluster, especially the ferrous metal layer, is formed from bulbs, that is to say mutually agglutinated rounded globules.
  • the inventors have in fact found that the specific catalyst formed by an unoxidized ferrous metal coating, especially produced in the form of clusters, or of a continuous layer, of bulbs covering more than 75% of the alumina support has a very greatly superior performance than the known catalysts, in particular making it possible simultaneously to obtain a high activity and a high productivity with a carbon nanotube selectivity close to 100%.
  • the ferrous metal coating is designed to cover the alumina support in such a way that its pores are made inaccessible. It should be noted that the fact that these pores (mesopores in the case of a mesoporous alumina) are made inaccessible by the metal coating may be easily verified by simply measuring the change in specific surface area due to the presence of the ferrous metal coating and/or by calculating the volume of residual mesopores and/or micropores and/or by XPS analysis, making it possible to demonstrate that the constituent chemical elements of the alumina support are no longer accessible on the surface.
  • the composition according to the invention has a specific surface area corresponding to that of grains whose pores are inaccessible.
  • each catalyst particle has an unoxidized ferrous metal coating forming a homogeneous continuous surface layer extending over at least one portion of a closed surface around a porous alumina core.
  • continuous layer denotes the fact that it is possible to pass continuously over the entire surface of this layer without having to pass through a portion of another nature (especially a portion containing no unoxidized ferrous metal coating).
  • the ferrous metal coating is not dispersed on the surface of each alumina grain but on the contrary forms a continuous layer with an apparent area corresponding substantially to that of the grains.
  • This layer is also “homogeneous” in the sense that it is formed from iron or from a plurality of metals including iron, and has an identical solid composition throughout its volume.
  • closed surface is used in the topological sense of the term, that is to say it denotes a surface that delimits and surrounds a finite internal space, which is the core of the grain, and can adopt various shapes (sphere, polyhedron, prism, torus, cylinder, cone, etc.)
  • the ferrous metal coating forms the outer layer of the catalyst particles, immediately after its manufacture and if the catalyst composition is not brought into the presence of an oxidizing medium. If the catalyst composition is in contact with the atmospheric air, an oxide layer may form on the periphery. This oxide layer may if necessary be removed by a reduction step, before the catalyst particles are used.
  • the ferrous metal coating results from elemental metal deposition (i.e. in which one (or more) metal(s) is (are) deposited in the elemental state, that is to say in atomic or ionic form) carried out in a single step on the alumina support.
  • the ferrous metal layer forms part of an elemental ferrous metal coating deposited in a single step on the solid alumina support.
  • an elemental metal coating deposited in a single step may result in particular from a vacuum evaporation deposition (PVD) operation or a chemical vapor deposition (CVD) operation or an electroplating operation.
  • PVD vacuum evaporation deposition
  • CVD chemical vapor deposition
  • the catalyst composition used in a process according to the invention is distinguished in particular from a composition obtained by milling pieces of pure metal manufactured metallurgically.
  • An elemental metal coating deposited in a single step is formed from crystalline microdomaines of the metal(s). Such an elemental metal coating is formed from mutually agglutinated metal bulbs (rounded globules).
  • the bulbs have a mean dimension of between 10 nm and 1 ⁇ m, especially between 30 nm and 100 nm.
  • the ferrous metal coating covers 90% to 100% of the surface of the macroscopic form (the envelope surface considered without taking the porosity into account) of the particles which is itself a closed surface.
  • This coverage of the surface of the alumina support by the ferrous metal coating may be determined by XPS analysis.
  • the ferrous metal coating thus extends over 90% to 100% of a closed surface.
  • the ferrous metal coating extends over a thickness of greater than 0.5 ⁇ m, especially around 2 to 20 ⁇ m. Furthermore, advantageously and according to the invention, the ferrous metal coating of each catalyst particle extends superficially with a mean apparent area (on the external surface of the particle) of greater than 2 ⁇ 10 3 ⁇ m 2 . More particularly, advantageously and according to the invention, the ferrous metal coating of each catalyst particle extends superficially with a mean apparent area of between 10 4 ⁇ m 2 and 1.5 ⁇ 10 5 ⁇ m 2 .
  • the unoxidized ferrous metal coating of each catalyst particle extends superficially with a developed overall mean dimension of greater than 35 ⁇ m.
  • the developed overall mean dimension is the equivalent radius of the disk circumscribing the ferrous metal coating after it has been virtually developed in a plane.
  • the unoxidized ferrous metal coating of each catalyst particle extends superficially with a developed overall mean dimension of between 200 ⁇ m and 400 ⁇ m.
  • a process according to the invention is characterized in that a supported catalyst is used in the form of particles whose shapes and dimensions are adapted so as to allow the formation of a fluidized bed of these catalyst particles, in that a fluidized bed of the catalyst particles is formed in a reactor and in that the carbon source is continuously delivered into the reactor, contacting the catalyst particles under conditions suitable for fluidizing the bed of catalyst particles and for ensuring that the decomposition reaction and the formation of nanotubes take place.
  • a supported catalyst having a mean particle size (D 50 ) of between 100 ⁇ m and 200 ⁇ m is used.
  • the shape of the catalyst particles may or may not be substantially spherical overall.
  • the invention also applies to a process in which catalyst particles of relatively flat shape (flakes, disks, etc.) and/or of elongate shape (cylinders, rods, ribbons, etc.) are used.
  • each particle comprises an alumina core covered with a shell formed from said ferrous metal coating.
  • the ferrous metal coating forms a metal shell covering the entire surface of the porous alumina support and making its pores inaccessible.
  • each particle depends on that of the alumina core and on the conditions under which the ferrous metal coating is formed on this core.
  • the alumina has a specific surface area of greater than 100 m 2 /g, but the supported catalyst has a specific surface area of less than 25 m 2 /g.
  • the thickness of the ferrous metal coating may extend, at least partly, into the thickness of the porous alumina core and/or, at least partly, as an overthickness relative to the porous core.
  • a supported catalyst comprising more than 20% by weight, especially around 40% by weight, of ferrous metal coating is used.
  • the ferrous metal coating consists exclusively of iron.
  • the ferrous metal coating is formed from iron and from at least one metal chosen from nickel and cobalt. This is because it is known in particular that an Fe/Ni or Fe/Co bimetallic catalyst can be used with similar results to a pure iron catalyst, all other things being equal.
  • the ferrous metal coating consists mainly of iron.
  • the supported catalyst composition used in a process according to the invention is advantageously formed mainly from such particles, that is to say it contains more than 50% of such particles, preferably more than 90% of such particles.
  • the invention also relates to a process for the selective manufacture of ordered carbon nanotubes, in which a supported catalyst composition formed exclusively, apart from impurities, from such particles is used, that is to say the particles of which catalyst composition are all in accordance with all or some of the features defined above or below.
  • a quantity of carbon source such that the ratio of the mass of the initial carbon source, especially the mass of carbon introduced into the reactor per hour, to the mass of metal of the supported catalyst, especially when present in the reactor, is greater than 100 is used.
  • the carbon source is ethylene.
  • Other carbon-containing gases may be used.
  • FIG. 1 is a diagram of an embodiment of an installation for manufacturing a catalyst composition that can be used in a process according to the invention
  • FIG. 2 is a diagram of an embodiment of an installation for producing carbon nanotubes with a process according to the invention
  • FIG. 3 is a micrograph of the surface of a particle of a catalytic composition that can be used in a process according to the invention, obtained in example 1;
  • FIGS. 4 and 5 are micrographs of the surface of the particles of a catalytic composition obtained in example 2 that can be used in a process according to the invention
  • FIG. 6 is a graph showing the diameter distribution of the nanotubes obtained in example 4.
  • FIGS. 7 a and 7 b are micrographs on two different scales showing nanotubes obtained in example 4.
  • FIG. 1 is a diagram of an installation for the implementation of a process for manufacturing a divided solid catalytic composition used in a process according to the invention.
  • This installation comprises a reactor, called a deposition reactor 20 , for synthesizing the catalytic composition by chemical vapor deposition (CVD), which includes a glass sublimator 1 into which the organometallic precursor is introduced.
  • This sublimator comprises a sintered plate and can be heated to the desired temperature by a heated bath 2 .
  • the inert carrier gas 3 for example helium, which entrains the vapor of the organometallic precursor used, is stored in a bottle and admitted into the sublimator 1 via a flow regulator (not shown).
  • the sublimator 1 is connected to a lower gas compartment 4 , which comprises a sintered plate, into which compartment water vapor is introduced, which serves to activate the decomposition of the organometallic precursor.
  • the presence of water makes it possible to obtain an unoxidized metal coating (thanks to the gas-to-water displacement reaction) containing no impurities, and thus a highly active catalyst.
  • the compartment 4 has a jacket thermostatted to a temperature that can be adjusted by means of a temperature regulator (not shown).
  • the water vapor is entrained by and with an inert carrier gas 5 , for example nitrogen, stored in a bottle and admitted into the compartment 4 via a flow regulator (not shown).
  • a supply of inert carrier gas 6 for example nitrogen, is intended to adjust the flow rates so as to obtain the fluidization conditions.
  • This carrier gas 6 is stored in a bottle and admitted into the compartment 4 via a flow regulator (not shown).
  • the top of the compartment 4 is connected in a sealed manner to a glass fluidization column 7 , for example 5 cm in diameter, which is provided at its base with a gas distributor.
  • This jacketed column 7 is thermostatted at a temperature which may be adjusted by means of a temperature regulator 8 .
  • the top of the column 7 is connected to a vacuum pump 9 via a trap, in order to retain the decomposition gases released.
  • the operating protocol for the embodiments relating to the production of the catalysts according to the invention by CVD is the following.
  • a mass M p of precursor is introduced into the sublimator 1 .
  • a mass M g of alumina support grains is poured into the column 7 and a quantity (for example around 20 g) of water is introduced into the compartment 4 using a syringe.
  • a vacuum is created in the assembly formed by the compartment 4 and the column 7 .
  • the temperature of the bed is brought to T 1 .
  • the sublimator 1 is brought to the temperature T s and the pressure is set to the value P a throughout the apparatus by introducing the carrier gases 3 , 5 and 6 (total flow rate Q). The deposition then starts and lasts for a time t d .
  • the temperature is brought back down to room temperature by slow cooling, and the vacuum pump 9 is stopped.
  • the catalytic granular composition is removed from the column 7 under an inert gas atmosphere (for example a nitrogen atmosphere).
  • the composition is ready to be used for manufacturing nanotubes in a growth reactor 30 .
  • the growth reactor 30 consists of a quartz fluidization column 10 (for example 2.6 cm in diameter) provided in the middle of it with a distributing plate 11 (made of quartz frit) on which the powder of catalytic granular composition is placed.
  • the column 10 may be brought to the desired temperature using, an external oven 12 , which may slide vertically along the fluidization column 10 . In the protocol used, this oven 12 has either a high position, where it does not heat the fluidized bed, or a low position where it heats the bed.
  • the gases 13 ininert gas such as helium, carbon source and hydrogen
  • flow regulators 14 are stored in bottles and admitted into the fluidization column via flow regulators 14 .
  • the fluidization column 10 is connected in a sealed manner to a trap 15 designed to collect any fines of the catalytic granular composition or a catalytic granular composition/nanotube mixture.
  • the height of the column 10 is adapted so as to contain, in operation, the fluidized bed of catalyst particles. In particular, it is at least equal to 10 to 20 times the gas height, and must correspond to the heated zone. In the embodiments, a column 10 having a total height of 70 cm, heated over a height of 60 cm by the oven 12 , is chosen.
  • a mass M c of granular supported catalyst is introduced into the fluidization column 10 with an atmosphere of inert gas.
  • the oven 12 When the oven 12 is in the low position relative to the catalyst bed, its temperature is brought to the desired temperature T n for synthesizing the nanotubes, either in an inert gas atmosphere or in an inert gas/hydrogen (reactive gas) mixture.
  • the carbon source, the hydrogen and an addition of inert gas are introduced into the column 10 .
  • the total flow rate Q T ensures that the bed is in a bubbling regime at the temperature T n , without expulsion.
  • the growth of the nanotubes then starts, and lasts for a time t n .
  • the oven 12 is placed in the high position relative to the catalyst bed, the flows of gases corresponding to the carbon source and hydrogen are stopped, and the temperature is brought back down to room temperature by slow cooling.
  • the carbon nanotubes associated with the metal particles and attached to the support grains are extracted from the growth reactor 30 and stored without taking any particular precaution.
  • the quantity of carbon deposited is measured by weighing and by thermogravimetric analysis.
  • the nanotubes thus manufactured are analyzed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) for the size and dispersion measurements and by X-ray crystallography and Raman spectroscopy for evaluating the crystallinity of the nanotubes.
  • TEM transmission electron microscopy
  • SEM scanning electron microscopy
  • a catalyst composition containing 24 wt % Fe/Al 2 O 3 was prepared by the fluidized-bed CVD technique described above.
  • the carrier gas was nitrogen.
  • the organometallic precursor was iron pentacarbonyl and the support was mesoporous ⁇ -alumina (pore volume: 0.54 cm 3 /g) that had been screened between 120 ⁇ m and 150 ⁇ m and had a specific surface area of 160 m 2 /g.
  • the operating conditions were the following:
  • the composition obtained was formed from alumina grains covered with clusters of iron bulbs (the mean size of the bulbs is around 20 nm), covering the surface of the alumina with a surface composition having 22% aluminum as measured by XPS analysis ( FIG. 3 ).
  • the purpose of this example is to prepare a supported catalyst composition consisting of 40 wt % iron on alumina (Al 2 O 3 ) as indicated in example 1, but with the following operating conditions:
  • the composition obtained was formed from alumina grains completely covered with an iron shell consisting of clusters of iron bulbs 30 nm to 300 nm in size ( FIGS. 4 and 5 ).
  • the specific surface area of the final material was 8 m 2 /g and the XPS analyses showed that aluminum was no longer present on the surface.
  • Multiwalled carbon nanotubes were manufactured from the 24% Fe/Al 2 O 3 catalyst of example 1 in an installation according to FIG. 2 , using gaseous ethylene as carbon source.
  • the operating conditions were the following:
  • the selectivity was close to 100% in terms of multiwalled nanotubes.
  • Multiwalled carbon nanotubes were manufactured from the 40% Fe/Al 2 O 3 catalyst of example 2 in an installation according to FIG. 2 , using gaseous ethylene as carbon source.
  • the operating conditions were the following:
  • the multiwalled-nanotube selectivity was close to 100%.
  • FIG. 6 also shows that the diameter of the nanotubes obtained in example 4 is predominantly around 10 nm to 25 nm, whereas the particles of the composition had a diameter of around 150 ⁇ m and the iron bulbs had sizes from 30 to 300 nm.
  • FIGS. 7 a and 7 b show the high selectivity in the nanotubes produced in example 4, which can thus be used directly, in particular taking into account the low proportion of residual porous support in the nanotubes that it was necessary to remove in the previously known processes.
  • Multiwalled carbon nanotubes were manufactured from a 5% Fe/Al 2 O 3 catalyst obtained as indicated in example 1 with the following operating conditions:
  • the carbon nanotubes were prepared in an installation as shown in FIG. 2 using gaseous ethylene as carbon source.
  • the operating conditions for manufacturing the nanotubes were the following:
  • a 20 wt % Fe/Al 2 O 3 catalyst composition was prepared by the fluidized-bed CVD technique described above.
  • the carrier gas was nitrogen.
  • the organometallic precursor was iron pentacarbonyl and the support was nonporous ⁇ -alumina (specific surface area (BET method): 2 m 2 /g).
  • the operating conditions were the following:
  • the composition obtained was formed from alumina particles covered with a shell formed by a cluster of iron bulbs entirely covering the surface of the alumina with a surface composition in which aluminum was absent, as measured by XPS analysis.
  • Multiwalled carbon nanotubes were manufactured from this iron/nonporous alumina catalyst in an installation as shown in FIG. 2 , using gaseous ethylene as carbon source.
  • the operating conditions were the following:
  • the invention may be the subject of many alternative embodiments and applications other than those of the examples mentioned above.

Abstract

The invention relates to a method for selectively producing nanotubes made of carbon ordered by decomposing a gaseous carbon source in contact with at least one solid catalyst in the form of catalyst grains which are made of an alumina porous support provided with a metallic ferrous non-oxidised deposit and whose mean grain-size ranges from 25 μm to 2.5 mm and on which said metallic ferrous deposit covers more than 75% of the surface of the microscopic alumina support and is embodied in the form of at least one cluster formed by a plurality of metallic agglutinated bulbs.

Description

  • The invention relates to the manufacture of ordered carbon nanotubes.
  • For the purpose of the present invention, the ordered carbon nanotubes have a tubular structure with a diameter between 0.4 nm and 30 nm and a length of greater than 100 times their diameter, especially between 1000 and 100 000 times their diameter. They may either be associated with metal catalyst particles or be free of such particles (after purification). Carbon nanotubes were described a long time ago (S. Iijima “Helical nanotubes of graphitic carbon”, Nature, 354, 56 (1991)), but they have still not been exploited on an industrial scale. However, they could be used for many applications, and especially be very useful and advantageous in the manufacture of composites, flat screens, tips for atomic force microscopes, the storage of hydrogen or other gases, as catalyst supports, etc.
  • WO-03/002456 describes a process for the selective manufacture of ordered carbon nanotubes in a fluidized bed in the presence of a supported catalyst formed from iron on alumina, comprising from 1 to 5% by weight of highly dispersed atomic iron by fluidized-bed CVD on alumina grains about 120 μm or 150 μm in size. The iron particles deposited are dispersed and have a dimension of around 3 to 6 nm. This process makes it possible to obtain a good selectivity and a good yield (greater than 90%) relative to the carbon source.
  • In particular in the case of unoxidized metals used to catalyze the formation of carbon nanotubes by thermal decomposition in a gas phase of a carbon source, it is considered necessary to provide a multiplicity of discontinuous metal catalyst sites dispersed to the maximum on grains of a support, the size of the dispersed metal sites corresponding to the diameter of the nanotubes to be formed. A very considerable amount of research has been carried out in this regard. Another solution would be to use isolated catalyst particles of a size equivalent to the diameter of the nanotubes to be formed. This is because a metal particle is entrained to the end of each nanotube.
  • Highly dispersed catalysts with a low metal content make it possible to achieve a good metal catalyst activity A* (grams of nanotubes formed per gram of metal per hour) and a rather moderate catalytic activity A (grams of nanotubes formed per gram of catalytic composition per hour). However, this good activity is obtained to the detriment of a low productivity (grams of nanotubes formed per gram of catalytic composition). For example, the process described in WO-03/002456 makes it possible to achieve at best an activity A* 13.1 and an activity A of 0.46 for a productivity of 0.46.
  • Now, from the economic and industrial standpoints, it is desirable not only for the reaction to be selective in terms of nanotubes (as opposed to other forms of carbon that may be produced, namely soot, fibers, etc.) and for the activity to be high so that the reaction is rapid, but also for its productivity to be high in order to avoid the need for purification steps, in order to separate the catalyst from the nanotubes, and the costs incurred.
  • Certain authors (Lyudmila B. Avdeeva et al in “Iron-containing catalysts of methane decomposition: accumulation of filamentous carbon”, Applied Catalysis A: General 228, 53-63 (2002)) have recently proposed a use of alumina catalysts with a high iron or iron/cobalt content produced by precipitation or coprecipitation or impregnation. The best results announced with an Fe/Co/Al2O3 catalyst containing 50% iron by weight and 6% cobalt by weight make it possible to obtain, after 40 hours, a productivity of 52.4 for an activity A of 1.31 and an activity A* of 2.34, and with a material produced that contains both carbon nanotubes and other fibrous structures (poor selectivity).
  • Thus, it may be thought that a high proportion of metal on a catalyst produced by impregnation or precipitation makes it possible to increase the productivity, but to the detriment of the activity and/or the nanotube production selectivity.
  • It remains the case that the mechanisms involved in the catalysis of carbon nanotube formation are still largely unexplained and poorly controlled, and the processes and catalysts envisaged are defined in an essentially empirical manner.
  • The object of the invention is therefore to alleviate these drawbacks by proposing a process using a catalyst of astonishingly high performance. More particularly, the aim of the invention is to propose a process for obtaining, simultaneously, a high productivity, especially of about 25 or higher, a high activity, especially of about 10 or higher, and a very high selectivity, especially greater than 90%, or even close to 100%, in terms of carbon nanotubes produced, especially multiwalled nanotubes.
  • The object of the invention is more particularly to propose a process for manufacturing ordered carbon nanotubes, especially multiwalled nanotubes, having a production rate and a yield that are compatible with the constraints of exploitation on an industrial scale.
  • To do this, the invention relates to a process for the selective manufacture of ordered carbon nanotubes by decomposition of a carbon source in the gaseous state brought into contact with at least one supported solid catalyst in the form of particles, called catalyst particles, consisting of a porous alumina support bearing an unoxidized metal coating of at least one transition metal, including iron, referred to as ferrous metal coating, characterized in that a supported catalyst is used that is mainly formed from catalyst particles:
      • having a mean particle size of between 25 μm and 2.5 mm;
      • on which the ferrous metal coating covers more than 75% of the surface of the macroscopic form of the alumina support (without taking the porosity into account).
  • Advantageously, and according to the invention, the ferrous metal coating is in the form of at least one cluster formed from a plurality of agglutinated metal bulbs.
  • Advantageously, and according to the invention, the ferrous metal coating forms a homogeneous continuous ferrous metal surface layer formed from metal bulbs. Each cluster, especially the ferrous metal layer, is formed from bulbs, that is to say mutually agglutinated rounded globules.
  • Inexplicably and in complete contradiction with the teaching of the prior art, the inventors have in fact found that the specific catalyst formed by an unoxidized ferrous metal coating, especially produced in the form of clusters, or of a continuous layer, of bulbs covering more than 75% of the alumina support has a very greatly superior performance than the known catalysts, in particular making it possible simultaneously to obtain a high activity and a high productivity with a carbon nanotube selectivity close to 100%.
  • Advantageously, and according to the invention, the ferrous metal coating is designed to cover the alumina support in such a way that its pores are made inaccessible. It should be noted that the fact that these pores (mesopores in the case of a mesoporous alumina) are made inaccessible by the metal coating may be easily verified by simply measuring the change in specific surface area due to the presence of the ferrous metal coating and/or by calculating the volume of residual mesopores and/or micropores and/or by XPS analysis, making it possible to demonstrate that the constituent chemical elements of the alumina support are no longer accessible on the surface. Thus, in particular, the composition according to the invention has a specific surface area corresponding to that of grains whose pores are inaccessible.
  • Advantageously, and according to the invention, each catalyst particle has an unoxidized ferrous metal coating forming a homogeneous continuous surface layer extending over at least one portion of a closed surface around a porous alumina core.
  • The term “continuous” layer denotes the fact that it is possible to pass continuously over the entire surface of this layer without having to pass through a portion of another nature (especially a portion containing no unoxidized ferrous metal coating). Thus, the ferrous metal coating is not dispersed on the surface of each alumina grain but on the contrary forms a continuous layer with an apparent area corresponding substantially to that of the grains. This layer is also “homogeneous” in the sense that it is formed from iron or from a plurality of metals including iron, and has an identical solid composition throughout its volume.
  • The expression “closed surface” is used in the topological sense of the term, that is to say it denotes a surface that delimits and surrounds a finite internal space, which is the core of the grain, and can adopt various shapes (sphere, polyhedron, prism, torus, cylinder, cone, etc.)
  • The ferrous metal coating forms the outer layer of the catalyst particles, immediately after its manufacture and if the catalyst composition is not brought into the presence of an oxidizing medium. If the catalyst composition is in contact with the atmospheric air, an oxide layer may form on the periphery. This oxide layer may if necessary be removed by a reduction step, before the catalyst particles are used.
  • Advantageously, and according to the invention, the ferrous metal coating results from elemental metal deposition (i.e. in which one (or more) metal(s) is (are) deposited in the elemental state, that is to say in atomic or ionic form) carried out in a single step on the alumina support.
  • Thus, the ferrous metal layer forms part of an elemental ferrous metal coating deposited in a single step on the solid alumina support. Such an elemental metal coating deposited in a single step may result in particular from a vacuum evaporation deposition (PVD) operation or a chemical vapor deposition (CVD) operation or an electroplating operation.
  • However, this coating cannot result from a process carried out in several steps in liquid phase, especially by precipitation or impregnation, or by deposition in the molten state and solidification, or by deposition of one or more metal oxides followed by a reduction step. The catalyst composition used in a process according to the invention is distinguished in particular from a composition obtained by milling pieces of pure metal manufactured metallurgically.
  • An elemental metal coating deposited in a single step is formed from crystalline microdomaines of the metal(s). Such an elemental metal coating is formed from mutually agglutinated metal bulbs (rounded globules).
  • Furthermore, advantageously and according to the invention, the bulbs have a mean dimension of between 10 nm and 1 μm, especially between 30 nm and 100 nm.
  • Advantageously, and according to the invention, the ferrous metal coating covers 90% to 100% of the surface of the macroscopic form (the envelope surface considered without taking the porosity into account) of the particles which is itself a closed surface. This coverage of the surface of the alumina support by the ferrous metal coating may be determined by XPS analysis. The ferrous metal coating thus extends over 90% to 100% of a closed surface.
  • Advantageously, and according to the invention, the ferrous metal coating extends over a thickness of greater than 0.5 μm, especially around 2 to 20 μm. Furthermore, advantageously and according to the invention, the ferrous metal coating of each catalyst particle extends superficially with a mean apparent area (on the external surface of the particle) of greater than 2×103 μm2. More particularly, advantageously and according to the invention, the ferrous metal coating of each catalyst particle extends superficially with a mean apparent area of between 104 μm2 and 1.5×105 μm2.
  • Furthermore, advantageously and according to the invention, the unoxidized ferrous metal coating of each catalyst particle extends superficially with a developed overall mean dimension of greater than 35 μm. The developed overall mean dimension is the equivalent radius of the disk circumscribing the ferrous metal coating after it has been virtually developed in a plane. Advantageously, and according to the invention, the unoxidized ferrous metal coating of each catalyst particle extends superficially with a developed overall mean dimension of between 200 μm and 400 μm.
  • Advantageously, a process according to the invention is characterized in that a supported catalyst is used in the form of particles whose shapes and dimensions are adapted so as to allow the formation of a fluidized bed of these catalyst particles, in that a fluidized bed of the catalyst particles is formed in a reactor and in that the carbon source is continuously delivered into the reactor, contacting the catalyst particles under conditions suitable for fluidizing the bed of catalyst particles and for ensuring that the decomposition reaction and the formation of nanotubes take place.
  • More particularly, advantageously and according to the invention, a supported catalyst having a mean particle size (D50) of between 100 μm and 200 μm is used. The shape of the catalyst particles may or may not be substantially spherical overall. The invention also applies to a process in which catalyst particles of relatively flat shape (flakes, disks, etc.) and/or of elongate shape (cylinders, rods, ribbons, etc.) are used.
  • Advantageously, and according to the invention, each particle comprises an alumina core covered with a shell formed from said ferrous metal coating. Thus, advantageously and according to the invention, the ferrous metal coating forms a metal shell covering the entire surface of the porous alumina support and making its pores inaccessible.
  • The shape of each particle depends on that of the alumina core and on the conditions under which the ferrous metal coating is formed on this core.
  • Advantageously, and according to the invention, the alumina has a specific surface area of greater than 100 m2/g, but the supported catalyst has a specific surface area of less than 25 m2/g.
  • It should be noted that the thickness of the ferrous metal coating may extend, at least partly, into the thickness of the porous alumina core and/or, at least partly, as an overthickness relative to the porous core. However, it is not always easy to precisely and clearly determine the interface between the porous alumina core impregnated with the ferrous metal coating and the pure ferrous metal layer extending away from the alumina core and their relative disposition.
  • Furthermore, advantageously and according to the invention, a supported catalyst comprising more than 20% by weight, especially around 40% by weight, of ferrous metal coating is used.
  • Advantageously, and according to the invention, the ferrous metal coating consists exclusively of iron.
  • As a variant, advantageously and according to the invention, the ferrous metal coating is formed from iron and from at least one metal chosen from nickel and cobalt. This is because it is known in particular that an Fe/Ni or Fe/Co bimetallic catalyst can be used with similar results to a pure iron catalyst, all other things being equal. Preferably, the ferrous metal coating consists mainly of iron.
  • The supported catalyst composition used in a process according to the invention is advantageously formed mainly from such particles, that is to say it contains more than 50% of such particles, preferably more than 90% of such particles.
  • The invention also relates to a process for the selective manufacture of ordered carbon nanotubes, in which a supported catalyst composition formed exclusively, apart from impurities, from such particles is used, that is to say the particles of which catalyst composition are all in accordance with all or some of the features defined above or below.
  • The use of such a high-performance supported catalyst makes it possible in particular for the quantity of initial carbon source to be considerably increased.
  • Thus, in a process according to the invention, a quantity of carbon source such that the ratio of the mass of the initial carbon source, especially the mass of carbon introduced into the reactor per hour, to the mass of metal of the supported catalyst, especially when present in the reactor, is greater than 100 is used. Advantageously, and according to the invention, the carbon source is ethylene. Other carbon-containing gases may be used.
  • Other objects, features and advantages of the invention will become apparent on reading the following description of its embodiments, with reference to the appended figures in which:
  • FIG. 1 is a diagram of an embodiment of an installation for manufacturing a catalyst composition that can be used in a process according to the invention;
  • FIG. 2 is a diagram of an embodiment of an installation for producing carbon nanotubes with a process according to the invention;
  • FIG. 3 is a micrograph of the surface of a particle of a catalytic composition that can be used in a process according to the invention, obtained in example 1;
  • FIGS. 4 and 5 are micrographs of the surface of the particles of a catalytic composition obtained in example 2 that can be used in a process according to the invention;
  • FIG. 6 is a graph showing the diameter distribution of the nanotubes obtained in example 4; and
  • FIGS. 7 a and 7 b are micrographs on two different scales showing nanotubes obtained in example 4.
    •
  • FIG. 1 is a diagram of an installation for the implementation of a process for manufacturing a divided solid catalytic composition used in a process according to the invention. This installation comprises a reactor, called a deposition reactor 20, for synthesizing the catalytic composition by chemical vapor deposition (CVD), which includes a glass sublimator 1 into which the organometallic precursor is introduced. This sublimator comprises a sintered plate and can be heated to the desired temperature by a heated bath 2.
  • The inert carrier gas 3, for example helium, which entrains the vapor of the organometallic precursor used, is stored in a bottle and admitted into the sublimator 1 via a flow regulator (not shown).
  • The sublimator 1 is connected to a lower gas compartment 4, which comprises a sintered plate, into which compartment water vapor is introduced, which serves to activate the decomposition of the organometallic precursor. The presence of water makes it possible to obtain an unoxidized metal coating (thanks to the gas-to-water displacement reaction) containing no impurities, and thus a highly active catalyst. The compartment 4 has a jacket thermostatted to a temperature that can be adjusted by means of a temperature regulator (not shown). The water vapor is entrained by and with an inert carrier gas 5, for example nitrogen, stored in a bottle and admitted into the compartment 4 via a flow regulator (not shown). A supply of inert carrier gas 6, for example nitrogen, is intended to adjust the flow rates so as to obtain the fluidization conditions. This carrier gas 6 is stored in a bottle and admitted into the compartment 4 via a flow regulator (not shown).
  • The top of the compartment 4 is connected in a sealed manner to a glass fluidization column 7, for example 5 cm in diameter, which is provided at its base with a gas distributor. This jacketed column 7 is thermostatted at a temperature which may be adjusted by means of a temperature regulator 8.
  • The top of the column 7 is connected to a vacuum pump 9 via a trap, in order to retain the decomposition gases released.
  • The operating protocol for the embodiments relating to the production of the catalysts according to the invention by CVD is the following.
  • A mass Mp of precursor is introduced into the sublimator 1.
  • A mass Mg of alumina support grains is poured into the column 7 and a quantity (for example around 20 g) of water is introduced into the compartment 4 using a syringe. A vacuum is created in the assembly formed by the compartment 4 and the column 7. The temperature of the bed is brought to T1.
  • The sublimator 1 is brought to the temperature Ts and the pressure is set to the value Pa throughout the apparatus by introducing the carrier gases 3, 5 and 6 (total flow rate Q). The deposition then starts and lasts for a time td.
  • At the end of deposition, the temperature is brought back down to room temperature by slow cooling, and the vacuum pump 9 is stopped. Once the system has returned to room temperature and atmospheric pressure, the catalytic granular composition is removed from the column 7 under an inert gas atmosphere (for example a nitrogen atmosphere). The composition is ready to be used for manufacturing nanotubes in a growth reactor 30.
  • The growth reactor 30 consists of a quartz fluidization column 10 (for example 2.6 cm in diameter) provided in the middle of it with a distributing plate 11 (made of quartz frit) on which the powder of catalytic granular composition is placed. The column 10 may be brought to the desired temperature using, an external oven 12, which may slide vertically along the fluidization column 10. In the protocol used, this oven 12 has either a high position, where it does not heat the fluidized bed, or a low position where it heats the bed. The gases 13 (inert gas such as helium, carbon source and hydrogen) are stored in bottles and admitted into the fluidization column via flow regulators 14.
  • At the top, the fluidization column 10 is connected in a sealed manner to a trap 15 designed to collect any fines of the catalytic granular composition or a catalytic granular composition/nanotube mixture.
  • The height of the column 10 is adapted so as to contain, in operation, the fluidized bed of catalyst particles. In particular, it is at least equal to 10 to 20 times the gas height, and must correspond to the heated zone. In the embodiments, a column 10 having a total height of 70 cm, heated over a height of 60 cm by the oven 12, is chosen.
  • The operating protocol for the embodiments relating to the manufacture of nanotubes according to the invention is the following:
  • A mass Mc of granular supported catalyst is introduced into the fluidization column 10 with an atmosphere of inert gas.
  • When the oven 12 is in the low position relative to the catalyst bed, its temperature is brought to the desired temperature Tn for synthesizing the nanotubes, either in an inert gas atmosphere or in an inert gas/hydrogen (reactive gas) mixture.
  • When this temperature is reached, the carbon source, the hydrogen and an addition of inert gas are introduced into the column 10. The total flow rate QT ensures that the bed is in a bubbling regime at the temperature Tn, without expulsion.
  • The growth of the nanotubes then starts, and lasts for a time tn.
  • After the growth, the oven 12 is placed in the high position relative to the catalyst bed, the flows of gases corresponding to the carbon source and hydrogen are stopped, and the temperature is brought back down to room temperature by slow cooling.
  • The carbon nanotubes associated with the metal particles and attached to the support grains are extracted from the growth reactor 30 and stored without taking any particular precaution.
  • The quantity of carbon deposited is measured by weighing and by thermogravimetric analysis.
  • The nanotubes thus manufactured are analyzed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) for the size and dispersion measurements and by X-ray crystallography and Raman spectroscopy for evaluating the crystallinity of the nanotubes.
    • EXAMPLES Example 1
  • A catalyst composition containing 24 wt % Fe/Al2O3 was prepared by the fluidized-bed CVD technique described above. The carrier gas was nitrogen. The organometallic precursor was iron pentacarbonyl and the support was mesoporous γ-alumina (pore volume: 0.54 cm3/g) that had been screened between 120 μm and 150 μm and had a specific surface area of 160 m2/g.
  • The operating conditions were the following:
      • Mg=50 g;
      • Mp=15.8 g;
      • T1=220° C.;
      • Pa=40 torr;
      • Ts=35° C.;
      • Q=250 cm3/min;
      • td=95 min.
  • The composition obtained was formed from alumina grains covered with clusters of iron bulbs (the mean size of the bulbs is around 20 nm), covering the surface of the alumina with a surface composition having 22% aluminum as measured by XPS analysis (FIG. 3).
    • Example 2
  • The purpose of this example is to prepare a supported catalyst composition consisting of 40 wt % iron on alumina (Al2O3) as indicated in example 1, but with the following operating conditions:
      • Mg=25 g;
      • Mp=58.5 g;
      • T1=220° C.;
      • Pa=40 torr;
      • Ts=35° C.;
      • Q=250 cm3/min;
      • td=200 min.
  • The composition obtained was formed from alumina grains completely covered with an iron shell consisting of clusters of iron bulbs 30 nm to 300 nm in size (FIGS. 4 and 5). The specific surface area of the final material was 8 m2/g and the XPS analyses showed that aluminum was no longer present on the surface.
    • Example 3
  • Multiwalled carbon nanotubes were manufactured from the 24% Fe/Al2O3 catalyst of example 1 in an installation according to FIG. 2, using gaseous ethylene as carbon source.
  • The operating conditions were the following:
      • Mc=0.100 g;
      • Tn=650° C.;
      • Q(H2)=100 cm3/min;
      • Q(C2H4)=200 cm3/min;
      • Z=500 (ratio of the mass of carbon introduced per hour to the mass of iron present in the reactor);
      • for tn=120 min:
      • A=13.4 (activity expressed in grams of nanotubes produced per gram of catalytic composition per hour);
      • P=26.8 (productivity expressed in grams of nanotubes produced per gram of catalytic composition).
  • The selectivity was close to 100% in terms of multiwalled nanotubes.
    • Example 4
  • Multiwalled carbon nanotubes were manufactured from the 40% Fe/Al2O3 catalyst of example 2 in an installation according to FIG. 2, using gaseous ethylene as carbon source.
  • The operating conditions were the following:
      • Mc=0.100 g;
      • Tn=650° C.;
      • Q(H2)=100 cm3/min;
      • Q(C2H4)=200 cm3/min;
      • Z=300;
      • for tn=120 min: A=15.6 and P=30.3;
      • for tn=240 min: A=9.9 and P=39.6.
  • In all cases, the multiwalled-nanotube selectivity was close to 100%.
  • What was thus obtained was both a high catalytic activity A (expressed in grams of nanotubes produced per gram of catalytic composition and per hour), of the order of or greater than 10, and, simultaneously, also a high productivity P (expressed in grams of nanotubes produced per gram of catalytic composition), of the order of or greater than 25, and to do so with a nanotube selectivity close to 100%.
  • The result is extremely surprising in so far as, with all known catalysts, either a good activity A* is obtained to the detriment of a low productivity (the case for catalysts having a low proportion of metal on the support) or, on the contrary, a high productivity to the detriment of a low activity (the case for catalysts having a high proportion of metal). Now, these parameters are both important in the context of an industrial production line. The productivity associated with the selectivity makes it possible to dispense with the subsequent purification steps. A high activity allows the reaction time to be minimized.
  • FIG. 6 also shows that the diameter of the nanotubes obtained in example 4 is predominantly around 10 nm to 25 nm, whereas the particles of the composition had a diameter of around 150 μm and the iron bulbs had sizes from 30 to 300 nm. Here again, this result is surprising and inexplicable, going counter to all the prior teaching.
  • FIGS. 7 a and 7 b show the high selectivity in the nanotubes produced in example 4, which can thus be used directly, in particular taking into account the low proportion of residual porous support in the nanotubes that it was necessary to remove in the previously known processes.
    • Comparative Example 5
  • Multiwalled carbon nanotubes were manufactured from a 5% Fe/Al2O3 catalyst obtained as indicated in example 1 with the following operating conditions:
      • Mg=100 g;
      • Ma=18.45 g;
      • td=21 min.
  • The carbon nanotubes were prepared in an installation as shown in FIG. 2 using gaseous ethylene as carbon source.
  • The operating conditions for manufacturing the nanotubes were the following:
      • Mc=0.100 g;
      • Tn=650° C.;
      • Q(H2)=100 cm3/min;
      • Q(C2H4)=200 cm3/min;
      • Z=2400;
      • for tn=30 min: A=1.6 and P=0.8.
  • As may be noticed, using a catalyst less charged, covering 75% of the surface of the particles, while keeping a nanotube selectivity close to 100%, it is impossible to obtain high values of A and P.
    • Comparative Example 6
  • A 20 wt % Fe/Al2O3 catalyst composition was prepared by the fluidized-bed CVD technique described above. The carrier gas was nitrogen. The organometallic precursor was iron pentacarbonyl and the support was nonporous α-alumina (specific surface area (BET method): 2 m2/g).
  • The operating conditions were the following:
      • Mg=50 g;
      • Ma=14 g;
      • T1=220° C.;
      • Pa=40 torr;
      • Ts=35° C.
      • Q=250 cm3/min;
      • td=15 min.
  • The composition obtained was formed from alumina particles covered with a shell formed by a cluster of iron bulbs entirely covering the surface of the alumina with a surface composition in which aluminum was absent, as measured by XPS analysis.
  • Multiwalled carbon nanotubes were manufactured from this iron/nonporous alumina catalyst in an installation as shown in FIG. 2, using gaseous ethylene as carbon source.
  • The operating conditions were the following:
      • Mc=0.100 g;
      • Tn=650° C.;
      • Q(H2)=100 cm3/min;
      • Q(C2H4)=200 cm3/min;
      • Z=500;
      • for tn=60 min: A=0.9 and P=0.2.
  • These results are 30 times inferior to those obtained in accordance with the invention for a catalyst according to the invention (example 1) and under the same operating conditions. In addition, the selectivity obtained, as evaluated by transmission electron microscopy and thermogravimetric analysis, was poor.
  • These results cannot be explained in so far as the sole difference between the two catalytic compositions lies in the porous or nonporous nature of the core, which is not accessible on the surface on account of the metal shell.
  • The invention may be the subject of many alternative embodiments and applications other than those of the examples mentioned above.

Claims (24)

1. A process for the selective manufacture of ordered carbon nanotubes by decomposition of a carbon source in the gaseous state brought into contact with catalyst particles comprising at least one supported solid catalyst in the form of particles consisting of a porous alumina support bearing an unoxidized ferrous metal coating of at least one transition metal, including iron, characterized in that said supported catalyst particles:
have a mean particle size of between 25 μm and 2.5 mm; and
said ferrous metal coating covers more than 75% of the surface of the macroscopic form of the porous alumina support.
2. The process as claimed in claim 1, characterized in that the ferrous metal coating is in the form of at least one cluster formed from a plurality of agglutinated metal bulbs.
3. The process as claimed in claim 1, characterized in that the ferrous metal coating forms a homogeneous continuous ferrous metal surface layer formed from metal bulbs.
4. The process as claimed in claim 1, characterized in that the ferrous metal coating is designed to cover the alumina support in such a way that its pores are made inaccessible.
5. The process as claimed in claim 1, characterized in that the ferrous metal coating results from elemental metal deposition carried out in a single step on the alumina support.
6. The process as claimed in claim 1, characterized in that the bulbs have a mean dimension of between 10 nm and 1 μm.
7. The process as claimed in claim 1, characterized in that the unoxidized ferrous metal coating on each catalyst particle extends superficially with a developed overall mean dimension of greater than 35 μm.
8. The process as claimed in claim 7, characterized in that the unoxidized ferrous metal coating of each catalyst particle extends superficially with a developed overall mean dimension of between 200 μm and 400 μm.
9. The process as claimed in claim 1, characterized in that the ferrous metal coating of each catalyst particle extends superficially with a mean apparent area of each catalyst particle greater than 2×103 μm2.
10. The process as claimed in claim 9, characterized in that the ferrous metal coating of each catalyst particle extends superficially with a mean apparent area of between 104 μm2 and 1.5×105 μm2.
11. The process as claimed in claim 1, characterized in that a supported catalyst is used in the form of particles whose shapes and dimensions are adapted so as to allow the formation of a fluidized bed of these catalyst particles, in that a fluidized bed of the catalyst particles is formed in a reactor and in that the carbon source is continuously delivered into the reactor, contacting the catalyst particles under conditions suitable for fluidizing the bed of catalyst particles and for ensuring that the decomposition reaction and the formation of nanotubes take place.
12. The process as claimed in claim 1, characterized in that a supported catalyst having a mean particle size of between 100 μm and 200 μm is used.
13. The process as claimed in claim 1, characterized in that the ferrous metal coating covers 90% to 100% of the surface of the particles.
14. The process as claimed in claim 1, characterized in that the ferrous metal coating forms a metal shell covering the entire surface of the porous alumina support and making its pores inaccessible.
15. The process as claimed in claim 1, characterized in that the ferrous metal coating extends over a thickness of greater than 0.5 μm.
16. The process as claimed in claim 1, characterized in that the alumina core has a specific surface area of greater than 100 m2/g and in that the supported catalyst has a specific surface area of less than 25 m2/g.
17. The process as claimed in claim 1, characterized in that a supported catalyst comprising more than 20% by weight of unoxidized ferrous metal coating is used.
18. The process as claimed in claim 1, characterized in that the ferrous metal coating consists mainly of iron.
19. The process as claimed in claim 1, characterized in that the ferrous metal coating consists exclusively of iron.
20. The process as claimed in claim 1, characterized in that the ferrous metal coating is formed from iron and from at least one metal chosen from nickel and cobalt.
21. The process as claimed in claim 1, characterized in that a quantity of carbon source such that the ratio of the mass of carbon of the initial carbon source introduced per hour to the mass of metal of the supported catalyst is greater than 100 is used.
22. The process as claimed in claim 1, characterized in that the carbon source is ethylene.
23. The process as claimed in claim 1, characterized in that the bulbs have a mean dimension of between 30 nm and 100 nm.
24. The process as claimed in claim 1, characterized in that the ferrous metal coating extends over a thickness of around 2 to 20 μm.
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