U.S. patent application number 10/481602 was filed with the patent office on 2004-11-25 for method for the selective production of ordered carbon nanotubes in a fluidised bed.
Invention is credited to Feurer, Roselyne, Kalck, Philippe Joseph, Serp, Philippe Gilles, Vahlas, Constantin.
Application Number | 20040234445 10/481602 |
Document ID | / |
Family ID | 8864856 |
Filed Date | 2004-11-25 |
United States Patent
Application |
20040234445 |
Kind Code |
A1 |
Serp, Philippe Gilles ; et
al. |
November 25, 2004 |
Method for the selective production of ordered carbon nanotubes in
a fluidised bed
Abstract
A method for the selective production of ordered carbon
nanotubes includes decomposition of a carbon source in the gaseous
state in contact with at least one solid catalyst, taking the form
of metallic particles borne by carrier grains. The catalyst grains
are adapted so as to be able to form a fluidised bed containing
between 1% and 5% by weight of metallic particles having average
dimensions of between 1 nm and 10 nm. The decomposition takes place
in a fluidised bed of catalyst grains. The method can be used to
obtain pure nanotubes with predetermined dimensions in a high
yield.
Inventors: |
Serp, Philippe Gilles;
(Toulouse, FR) ; Feurer, Roselyne; (Montlaur,
FR) ; Vahlas, Constantin; (Toulouse, FR) ;
Kalck, Philippe Joseph; (Auzeville-Tolosane, FR) |
Correspondence
Address: |
YOUNG & THOMPSON
745 SOUTH 23RD STREET
2ND FLOOR
ARLINGTON
VA
22202
US
|
Family ID: |
8864856 |
Appl. No.: |
10/481602 |
Filed: |
June 4, 2004 |
PCT Filed: |
June 25, 2002 |
PCT NO: |
PCT/FR02/02195 |
Current U.S.
Class: |
423/447.3 |
Current CPC
Class: |
C01B 32/162 20170801;
C01B 2202/08 20130101; B82Y 30/00 20130101; B01J 37/0238 20130101;
B01J 23/745 20130101; B82Y 40/00 20130101 |
Class at
Publication: |
423/447.3 |
International
Class: |
D01C 005/00; D01F
009/12 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 28, 2001 |
FR |
01 08511 |
Claims
1. A process for the selective production of ordered carbon
nanotubes by decomposition of a source of carbon in the gaseous
state in contact with at least one solid catalyst in the form of
metallic particles comprising at least one transition metal carried
on granules of solid support, so-called catalyst granules, capable
of being able to form a fluidised bed, the metallic particles
having a mean dimension between 1 nm and 10 nm as measured after
activation by heating to 750.degree. C., in which a fluidised bed
of the catalyst granules is formed in a reactor, the so-called
growth reactor (30), and the carbon source is added continuously to
the growth reactor (30) in contact with the catalyst granules under
conditions capable of ensuring the fluidisation of the bed of
catalyst granules, the decomposition reaction and the formation of
nanotubes, wherein: the catalyst granules of each catalyst are
produced beforehand by deposition of metallic particles on support
granules in a fluidised bed of the support granules formed in a
reactor, the so-called deposition reactor (20), fed with at least
one precursor capable of forming the metallic particles, and so as
to obtain catalyst granules comprising a proportion by weight of
the metallic particles of between 1% and 5%, the catalyst granules
are then placed in the growth reactor (30) without contact with the
external atmosphere, followed by the formation of the fluidised bed
of the catalyst granules and the formation of nanotubes in the
growth reactor (30).
2. A process as claimed in claim 1, wherein the catalyst granules
are produced having a mean dimension of the metallic particles of
between 2 nm and 8 nm, and in which, for at least 97% by number of
the metallic particles, the difference between their dimension and
the mean dimension of the metallic particles is less than or equal
to 5 nm.
3. A process as claimed in claim 1, wherein the catalyst granules
are produced with a mean dimension of the particles of the order of
4 nm to 5 nm, and in which, for at least 97% by number of the
metallic particles, the difference between their dimension and the
mean dimension of the metallic particles is of the order of 3
nm.
4. A process as claimed in claim 1, wherein the catalyst granules
are produced with a dimension of the metallic particles of less
than 50 nm.
5. A process as claimed in claim 1, wherein the fluidised bed is
situated in the growth reactor (30) at a temperature between
600.degree. C. and 800.degree. C.
6. A process as claimed in claim 1, wherein the metallic particles
consist in an amount of at least 98% by weight of at least one
transition metal and are substantially free of non-metallic
elements apart from traces of carbon and/or oxygen and/or hydrogen
and/or nitrogen.
7. A process as claimed in claim 1, wherein the metallic particles
consist of a pure metallic deposit of at least one transition
metal.
8. A process as claimed in claim 1, wherein the catalyst granules
are produced with a mean dimension between 10.mu. and 1000.mu..
9. A process as claimed in claim 1, wherein the difference between
the dimension of the catalyst granules and the mean dimension of
the produced catalyst granules is less than 50% of the value of the
said mean dimension.
10. A process as claimed in claim 1, wherein the support has a
specific surface greater than 10 m.sup.2/g.
11. A process as claimed in claim 1, wherein the support is a
porous material having a mean pore size greater than the mean
dimension of the metallic particles.
12. A process as claimed in claim 1, wherein the support is chosen
from alumina, an activated carbon, silica, a silicate, magnesia,
titanium dioxide, zirconia, a zeolite or a mixture of granules of
several of these materials.
13. A process as claimed in claim 1, wherein the metallic particles
consist of pure iron deposited in the dispersed state on alumina
granules.
14. A process as claimed in claim 1, wherein the deposition reactor
(20) and the growth reactor (30) are different.
15. A process as claimed in claim 14, wherein the deposition
reactor (20) and the growth reactor (30) are joined by at least one
gas-tight line (25a, 26, 25b) and wherein the growth reactor (30)
is fed with catalyst granules through this line (25).
16. A process as claimed in claim 1, wherein the catalyst granules
are produced by chemical deposition in the vapour phase of the
metallic particles on the support granules in a fluidised bed of
the support granules in the deposition reactor (20).
17. A process as claimed in claim 1, wherein the deposition of the
particles on the support granules is carried out at a temperature
between 200.degree. C. and 300.degree. C.
18. A process as claimed in claim 1, wherein the fluidised bed of
the support granules in the deposition reactor (20) is fed with at
least one organometallic precursor.
19. A process as claimed in claim 18, wherein Fe(CO).sub.5 is used
as organometallic precursor.
20. A process as claimed in claim 1, wherein the precursor(s) is
continuously diluted in the vapour phase in a gaseous mixture that
is continuously fed to the deposition reactor (20) under conditions
suitable for ensuring the fluidisation of the support granules.
21. A process as claimed in claim 20, wherein the gaseous mixture
comprises a neutral gas and at least one reactive gas.
22. A process as claimed in claim 21, wherein steam is used as
reactive gas.
23. A process as claimed in claim 1, wherein the fluidised bed of
the catalyst granules is formed in a cylindrical growth reactor
(30) of diameter greater than 2 cm and having a wall height capable
of containing 10 to 20 times the volume of the initial,
non-fluidised bed of the catalyst granules as measured in the
absence of any gaseous feed.
24. A process as claimed in claim 1, wherein a fluidised bed of the
catalyst granules is formed in the growth reactor (30) under a
bubbling regime that is at least substantially free of leakage.
25. A process as claimed in claim 1, wherein in order to form the
fluidised bed of catalyst granules in the growth reactor (30): a
bed of catalyst granules is formed in the bottom of the growth
reactor (30), the growth reactor (30) is fed from underneath the
bed of catalyst granules with at least one gas whose velocity is
greater than the minimum velocity of fluidisation of the bed of
catalyst granules and less than the minimum velocity for the
occurrence of a plunger-type rgime.
26. A process as claimed in claim 1, wherein in order to form the
fluidised bed of the catalyst granules in the growth reactor (30),
the growth reactor (30) is fed from underneath the catalyst
granules with the carbon source in the gaseous state and with at
least one neutral carrier gas.
27. A process as claimed in claim 1, wherein the growth reactor is
fed with at least one carbon-containing precursor forming the
carbon source, with at least one reactive gas and with at least one
neutral gas, which are mixed before being introduced into the
growth reactor (30).
28. A process as claimed in claim 1, wherein the carbon source
comprises at least one carbon-containing precursor chosen from
hydrocarbons.
29. A process as claimed in claim 1, wherein the growth reactor
(30) is fed with hydrogen as reactive gas.
30. A process as claimed in claim 27, wherein the molar ratio of
the reactive gas(es) to the carbon-containing precursor(s) is
greater than 0.5 and less than 10, and in particular is of the
order of 3.
31. A process as claimed in claim 27, wherein the growth reactor
(30) is fed at a flow rate of carbon-containing precursor(s) of
between 5% and 80%, in particular of the order of 25%, of the
overall gaseous flow rate.
32. A process for the preparation of a catalytic granular
composition comprising metallic particles containing at least one
transition metal carried on solid support granules, so-called
catalyst granules, in which there is effected a chemical deposition
in the vapour phase of the metallic particles on the support
granules, wherein the deposition of the metallic particles on the
support granules is carried out in a fluidised bed of the support
granules fed with at least one precursor capable of forming the
metallic particles, and wherein the support granules are chosen and
the parameters of the deposition are adjusted so that: the catalyst
granules are capable of being able to form a fluidised bed, the
proportion by weight of the metallic particles is between 1% and
5%, the metallic particles have a mean particle dimension between 1
nm and 10 nm as measured after activation by heating to 750.degree.
C.
33. A process as claimed in claim 32, wherein the deposition is
carried out in the form of a chemical deposition in the vapour
phase.
34. A process as claimed in claim 32, wherein the deposition is
carried out at a temperature between 200.degree. C. and 300.degree.
C.
35. A process as claimed in claim 32, wherein the fluidised bed of
the support granules is fed with at least one organometallic
precursor.
36. A process as claimed in claim 32, wherein Fe(CO).sub.5 is used
as organometallic precursor.
37. A process as claimed in claim 32, wherein the precursor(s) is
continuously diluted in the vapour state in a gaseous mixture that
is continuously fed to a deposition reactor (20) under conditions
capable of ensuring the fluidisation of the support granules.
38. A process as claimed in claim 37, wherein the gaseous mixture
comprises a neutral gas and at least one reactive gas.
39. A process as claimed in claim 38, wherein steam is used as
reactive gas.
Description
[0001] The present invention relates to the production of ordered
carbon nanotubes.
[0002] Ordered carbon nanotubes within the meaning of the present
invention have a tubular structure of diameter between 0.4 nm and
50 nm and a length greater than 100 times their diameter, in
particular between 1000 and 100,000 times their diameter. They may
exist either combined with particles of metallic catalyst, or
separate from these particles. Carbon nanotubes have been described
for a long time (S. Iijima "Helical nanotubules of graphitic
carbon" Nature 354, 56 (1991)), but are still not exploited on an
industrial scale. They could nevertheless be employed in numerous
applications and in particular could be extremely useful and
advantageous in the production of composite materials, flat
screens, tips for nuclear power microscopes, storage of hydrogen or
other gases, as catalyst supports, etc.
[0003] U.S. Pat. No. 4,663,230 and U.S. Pat. No. 5,500,200 describe
a process for the catalytic preparation of carbon fibrils by high
temperature decomposition of a source of gaseous carbon in contact
with a solid catalyst in the form of metallic particles of size 3.5
nm to 70 nm, comprising at least one transition metal, carried by
granules of solid support of size less than 400 .mu.m. According to
these documents the fibrils obtained should comprise an internal
core of less ordered carbon surrounded by an external region of
ordered carbon, and should have a diameter varying between 3.5 nm
and 70 nm. U.S. Pat. No. 5,500,200 discloses that the process for
obtaining these fibrils may be carried out in a fluidised bed, but
does not provide any example of such a process. All the examples
mentioned are carried out with a fixed bed, produce a moderate
yield with respect to the carbon source (<20% by weight), and
the actual characteristics of the products obtained are not given.
These documents therefore do not provide any real information
relating to the production of real nanotubes of ordered carbon
and/or the use of a fluidised bed for the production of such
nanotubes.
[0004] Other documents disclose the production of nanotubes of
single-wall carbon by means of a catalytic composition formed from
metallic particles that are either carried by support granules
deposited in a crucible (WO-0017102) or introduced in the form of
an aerosol (WO-9906618) to a reactor fed with a gaseous source of
carbon such as carbon monoxide or ethylene. The yields obtained
(nanotubes produced with respect to the source of carbon) with such
processes are very low, and a certain amount of particles of
pyrolitic or amorphous carbon is produced. However, it is important
for the practical industrial exploitation of carbon nanotubes to be
able to control precisely and simultaneously the dimensional
characteristics, the production yields and the purity of the
product obtained.
[0005] WO 01/94260, published on 13 Dec. 2001, describes a process
and an apparatus for the production of carbon nanotubes in several
stages, in which a preliminary treatment stage of the catalyst in
order to extract air from the latter is followed by a stage
involving the reduction of the catalyst. In such a process it is
also necessary to eliminate the amorphous carbon formed in the
reaction, which is thus not selective with regard to the nanotubes
that are formed.
[0006] U.S. Pat. No. 4,650,657 and U.S. Pat. No. 4,767,737 describe
a process for the production of a fibrous carbon-containing
material containing a ferrous metallic component in a fluidised bed
by decomposition of carbon monoxide in the presence of hydrogen and
a neutral gas such as nitrogen, a powder of ferrous metallic
catalyst and in the presence of an abrasive such as alumina that
acts as a support. These documents mention that the effect of such
a fluidised bed is to remove the carbon formed from the surface of
the granules, to promote the fragmentation and to minimise the size
of the reactive mass of the fluidised bed. These documents do not
describe a process that can be applied to the production of carbon
nanotubes. On the contrary, the products obtained are particles of
carbon of average dimension 1.mu. to 50.mu. (Table 1 of U.S. Pat.
No. 4,650,657).
[0007] The publication "Fe-catalyzed carbon nanotubes formation" by
K. Hernadi et al., Carbon, 34, No. 10, (1996), 1249-1257 describes
a process for the production of carbon nanotubes on various
catalysts in a fixed bed or in a so-called "fluidised bed" reactor
of 6.4 mm diameter. Such a diameter cannot produce a true fluidised
bed. The catalysts are prepared by impregnation. This process
limits the amorphous carbon produced to a laboratory scale
exploitation and teaches that the use of such a "fluidised bed"
would be less suitable than the use of a fixed bed.
[0008] In addition FR-2 707 526 describes a process for the
preparation of a catalyst by chemical deposition in the vapour
phase of metallic particles of size less than 2 nm in a fluidised
bed of porous support granules at a temperature of less than
200.degree. C. This document describes more particularly the
preparation of a rhodium-containing catalyst and does not describe
a catalyst suitable for the production of carbon nanotubes.
[0009] The object of the invention is thus to provide a process for
the selective production of true nanotubes of ordered carbon of
homogeneous average dimensions (varying only slightly around a mean
value) under conditions compatible with an industrial-scale
exploitation, particularly in terms of yield with respect to the
carbon source, catalytic activity and production costs, and of
purity in nanotubes of the product obtained.
[0010] The invention also provides such a process in which the
characteristics of the nanotubes produced may be predetermined and
adjusted by simple modification of the parameters involved in the
implementation of the process.
[0011] The invention provides more particularly such a process in
which the yield of produced nanotubes with respect to the carbon
source is equal to or greater than 80% by weight.
[0012] The invention also provides a catalytic granular composition
that may be used in a process for the production of ordered carbon
nanotubes according to the invention, as well as a process for the
preparation of such a catalytic granular composition.
[0013] (Throughout the text, all the terms and criteria relating to
the characteristics of the fluidised bed are adopted within the
meaning given in the reference work "Fluidization Engineering",
Kunii, D.; Levenspiel, O.; Butterworth-Heinemann Edition 1991.)
[0014] To this end, the invention relates to a process for the
selective production of ordered carbon nanotubes by decomposition
of a source of carbon in the gaseous state in contact with at least
one solid catalyst in the form of metallic particles comprising at
least one transition metal carried on granules of solid support,
these support granules carrying the metallic particles, so-called
catalyst granules, capable of being able to form a fluidised bed,
the metallic particles having a mean dimension between 1 nm and 10
nm as measured after activation by heating to 750.degree. C., in
which a fluidised bed of the catalyst granules is produced in a
reactor, the so-called growth reactor (30), and the carbon source
is supplied continuously to the said growth reactor (30) in contact
with the catalyst granules under conditions suitable for ensuring
the fluidisation of the bed of catalyst granules, the decomposition
reaction and the formation of nanotubes, characterised in that:
[0015] the catalyst granules of each catalyst are produced
beforehand by deposition of metallic particles on support granules
in a fluidised bed of support granules formed in a reactor, the
so-called deposition reactor (20) supplied with at least one
precursor capable of forming the metallic particles, and in such a
way as to obtain catalyst granules comprising a proportion by
weight of the metallic particles of between 1% and 5%,
[0016] the catalyst granules are then placed in the growth reactor
(30) without coming into contact with the external atmosphere,
which is followed by the formation of the fluidised bed of the
catalyst granules and the formation of nanotubes in the growth
reactor (30).
[0017] The inventors have surprisingly found that, contrary to the
teaching of U.S. Pat. No. 4,650,657 and U.S. Pat. No. 4,767,737,
the use of one fluidised bed to produce the catalyst(s) and of
another fluidised bed to produce the nanotubes, without the
catalyst(s) coming into contact with the atmosphere, under the
conditions of the invention, not only does not result in the
fragmentation of the carbon-containing products growing on the
granules, but on the contrary enables ordered carbon nanotubes of
very homogeneous dimensions (varying only slightly around the mean
value) to be selectively produced, and in a yield of more than 80%
by weight with respect to the carbon source.
[0018] The catalyst is not subjected to any atmospheric pollution,
and in particular is not oxidised between its preparation and its
use in the growth reactor.
[0019] Advantageously and according to the invention, the
deposition reactor and the growth reactor are separate.
Advantageously and according to the invention, the deposition
reactor and the growth reactor are connected by at least one
airtight line and the growth reactor is supplied with catalyst
granules through this line. As a variant, the granules of the
catalyst may be recovered and transferred from the deposition
reactor under an inert atmosphere. Advantageously and according to
the invention, the catalyst granules are produced by chemical
deposition in the vapour phase.
[0020] According to another possible variant of the invention, one
and the same reactor may be used both as deposition reactor and as
growth reactor. In other words, the two stages of preparation of
the catalyst granules (deposition) followed by production of the
carbon nanotubes (growth) may be carried out successively in one
and the same reactor by modifying the gases and reactants at the
inlet of the reactor as well as the operating parameters between
the two stages.
[0021] Advantageously and according to the invention, the fluidised
bed of the catalyst granules is formed in a cylindrical growth
reactor of diameter greater than 2 cm and having a wall height
capable of containing 10 to 20 times the volume of the initial
non-fluidised bed of the catalyst granules as determined in the
absence of any gaseous feed. Such a reactor enables a true
fluidised bed to be formed.
[0022] Advantageously and according to the invention, a fluidised
bed of the catalyst granules is formed under a bubbling regime at
least substantially free of leakage.
[0023] Furthermore, advantageously and according to the invention,
in order to form the fluidised bed of catalyst granules:
[0024] a bed of catalyst granules is formed at the bottom of the
growth reactor,
[0025] the growth reactor is fed underneath the bed of catalyst
granules with at least one gas whose velocity is greater than the
minimal velocity of fluidisation of the bed of catalyst granules
and less than the minimal velocity of occurrence of a plunger-type
regime.
[0026] Advantageously and according to the invention, in order to
form the fluidised bed of the catalyst granules, the growth reactor
is fed underneath the catalyst granules with the carbon source in
the gaseous state, and with at least one neutral carrier gas.
[0027] More particularly, advantageously and according to the
invention, the growth reactor is fed with at least one
carbon-containing precursor forming the carbon source, at least one
reactive gas, and at least one neutral gas, which are mixed before
being introduced into the growth reactor. The term "reactive gas"
is understood to denote a gas such as hydrogen that is capable of
participating in and promoting the production of nanotubes.
[0028] Advantageously and according to the invention, the source of
carbon comprises at least one carbon-containing precursor selected
from hydrocarbons. Among the hydrocarbons that may advantageously
be used, there may be mentioned ethylene and methane. As a variant
or in combination, there may however also be used an oxide of
carbon, in particular carbon monoxide.
[0029] Advantageously and according to the invention, the molar
ratio of the reactive gas(es) to the carbon-containing precursor(s)
is greater than 0.5 and less than 10, and in particular is of the
order of 3.
[0030] Advantageously and according to the invention, the growth
reactor (30) is fed at a flow rate of carbon-containing
precursor(s) of between 5% and 80%, in particular of the order of
25%, of the total gas flow rate.
[0031] Advantageously and according to the invention the fluidised
bed is heated to a temperature between 600.degree. C. and
800.degree. C.
[0032] The invention also covers a catalytic granular composition
suitable for the implementation of a production process according
to the invention.
[0033] The invention thus relates to a catalytic granular
composition comprising metallic particles containing at least one
transition metal carried by granules of solid support, so-called
catalyst granules, characterised in that:
[0034] the catalyst granules are capable of being able to form a
fluidised bed,
[0035] the proportion by weight of metallic particles is between 1%
and 5%,
[0036] the metallic particles have a mean particle dimension of
between 1 nm and 10 nm as measured after heating at 750.degree.
C.
[0037] Throughout the text the expression "mean dimension" of the
particles or granules denotes the mean value (maximum of the
distribution curve of the dimensions of the particles or granules)
of the dimensions of all the particle or granules as determined by
conventional granulometry, in particular by the sedimentation rate,
before use. The term "dimension" used in isolation denotes, for a
given particle or a given granule, its largest real dimension as
determined for example by static measurements obtained by
observations with a scanning or transmission electron microscope,
also before use.
[0038] As regards the metallic particles, the values of the
dimension or of the mean dimension that are given throughout the
text are those measured before use for the production of the
nanotubes, but after heating the catalytic composition to
750.degree. C. The inventors have in fact found that the dimensions
of the particles before heating are not, in general, capable of
analysis, the particles being invisible under a microscope. This
operation is effected by contact with a neutral atmosphere, for
example helium and/or nitrogen, at 750.degree. C., for a sufficient
time in order to obtain stable values of dimensions. This time is
in practice very low (of the order of a minute or a few minutes).
The activation may be effected in a fluidised bed (in the fluidised
bed of the catalyst granules before feeding the carbon source) or
in any other way, for example in a fixed bed. Furthermore the
temperature of 750.degree. C. should be regarded solely as a value
for the measurement of the size of the particles and does not
correspond to a temperature value that should necessarily be used
in a process according to the invention or in order to obtain a
catalytic composition according to the invention (even if this
value may advantageously be that used in certain embodiments of the
invention). In other words, it enables the invention to be
characterised uniquely by dimensional criteria, although a
catalytic composition not subjected to this specific temperature
may also be in accordance with the invention.
[0039] Advantageously the catalytic granular composition according
to the invention is characterised in that the mean dimension of the
metallic particles is between 2 nm and 8 nm, in particular of the
order of 4 to 5 nm, and in that for at least 97% by number of the
metallic particles, the difference between their dimension and the
mean dimension of the metallic particles is less than or equal to 5
nm, and in particular is of the order of 3 nm.
[0040] The catalytic granular composition may comprise a small
amount of metallic particles of dimension very much greater than
the mean dimension (typically more than 200% of the mean
dimension). Nevertheless, advantageously and according to the
invention the dimension of the metallic particles is less than 50
nm as measured before use and installation in the fluidised bed,
and after activation at 750.degree. C.
[0041] Advantageously and according to the invention, the metallic
particles consist in an amount of at least 98 wt. % of at least one
transition metal and are substantially free of non-metallic
elements other than traces of carbon and/or oxygen and/or hydrogen
and/or nitrogen. Several different transition metal may be used in
order to be deposited on the support granules. Likewise, several
different catalytic compositions according to the invention (whose
support granules and/or metallic particles have distinct
characteristics) may be used as a mixture. The traces of impurity
may derive from the preparation process of the metallic particles.
Apart from these traces, the 2% maximum remaining amount may
comprise one or more metallic elements other than a transition
metal. Preferably, advantageously and according to the invention,
the metallic particles consist of a pure metallic deposit of at
least one transition metal, with the exception of traces of
impurity. Advantageously and according to the invention, the
proportion by weight of metallic particles, in particular of iron,
is between 1.5% and 4%.
[0042] Advantageously and according to the invention, the catalyst
granules have a mean dimension between 10.mu. and 1000.mu..
Advantageously and according to the invention, the difference
between the dimension of the catalyst granules and the mean
dimension of the catalyst granules is less than 50% of the value of
the said mean dimension.
[0043] It has been found in fact that these dimensional
distributions of the metallic particles and of the granules enable
excellent results to be obtained within the context of a fluidised
bed.
[0044] Furthermore, advantageously and according to the invention,
the support has a specific surface greater than 10 m.sup.2/g.
[0045] Advantageously and according to the invention, the support
is a porous material having a mean pore size greater than the mean
dimension of the metallic particles. Advantageously and according
to the invention, the support is a mesoporous material, the pores
having a mean size of less than 50 nm. Advantageously and according
to the invention, the support is chosen from alumina
(Al.sub.2O.sub.3), an activated carbon, silica, a silicate,
magnesia (MgO), titanium dioxide (TiO.sub.2), zirconia (ZrO.sub.2),
a zeolite or a mixture of granules of several of these
materials.
[0046] In particular, in the case where the carbon source is
ethylene, advantageously and according to the invention the
metallic particles consist of pure iron deposited in the dispersed
state on granules of alumina.
[0047] Advantageously, in a process for the production of nanotubes
according to the invention, the catalyst granules are produced
beforehand by chemical deposition in the vapour phase of the
metallic particles on the support granules in a fluidised bed of
the support granules fed with at least one precursor capable of
forming the metallic particles.
[0048] The invention also covers a process for the preparation of a
catalytic granular composition according to the invention.
[0049] The invention thus relates to a process for the preparation
of a catalytic granular composition comprising metallic particles
containing at least one transition metal carried on solid support
granules, so-called catalyst granules, in which a chemical
deposition in the vapour phase of the metallic particles on the
support granules is performed in the vapour phase, characterised in
that the deposition, particularly in the form of a chemical
deposition, of the metallic particles on the support granules is
carried out in a fluidised bed of the support granules fed with at
least one precursor capable of forming the said metallic particles,
and in that the support granules are chosen and the parameters of
the deposition are adjusted so that:
[0050] the catalyst granules are capable of being able to form a
fluidised bed,
[0051] the proportion by weight of the metallic particles is
between 1% and 5%,
[0052] the metallic particles have a mean particle dimension
between 1 nm and 10 nm as measured after heating to 750.degree.
C.
[0053] Advantageously and according to the invention the deposition
is carried out at a temperature between 200.degree. C. and
300.degree. C.
[0054] Advantageously and according to the invention the fluidised
bed of the support granules is fed with at least one organometallic
precursor, in particular Fe(CO).sub.5.
[0055] Advantageously and according to the invention, the
precursor(s) in the vapour state is/are continuously diluted in a
gaseous mixture that is supplied continuously to a deposition
reactor under conditions capable of ensuring the fluidisation of
the support granules. Thus, advantageously and according to the
invention, the fluidised bed is fed continuously with precursor(s).
Advantageously and according to the invention, the gaseous mixture
comprises a neutral gas and at least one reactive gas.
Advantageously and according to the invention, steam (water vapour)
is used as reactive gas. Between 200.degree. C. and 300.degree. C.
the steam in fact enables the precursor Fe(CO).sub.5 to be
decomposed, releasing atoms of Fe. In addition all manifestations
of fritting and agglomeration of the metallic catalyst into
excessively large metallic particles is avoided.
[0056] The invention also relates to a process for the production
of nanotubes, a catalytic granular composition and a process for
the preparation of a catalytic granular composition, characterised
by a combination of all or some of the characteristics mentioned
hereinbelow or hereinafter.
[0057] Other objects, advantages and characteristics of the
invention are disclosed in the following description and examples,
which refer to the accompanying drawings in which:
[0058] FIG. 1 is a diagram of a first variant of an installation
for implementing a process for producing nanotubes according to the
invention,
[0059] FIG. 2 is a diagram of a second variant of an installation
of a process for producing nanotubes according to the
invention,
[0060] FIG. 3 is a histogram of the dimensions of the metallic
particles of a catalytic composition according to the invention
obtained in Example 5,
[0061] FIGS. 4 and 5 are micrographs of the nanotubes obtained
according to the invention as described in Example 9.
[0062] FIG. 1 is a diagram of an installation enabling a process
for producing nanotubes according to the invention to be
implemented. This installation comprises two reactors: a reactor,
so-called deposition reactor 20, for the synthesis of the catalyst,
and a reactor, so-called growth reactor 30, for the preparation of
the nanotubes.
[0063] The deposition reactor 20 for the synthesis of the catalyst
by chemical deposition in the vapour phase (CVD) comprises a glass
sublimator 1 to which is added the organometallic precursor. This
sublimator comprises a fritted plate and may be heated to the
desired temperature by a heating bath 2.
[0064] The neutral carrier gas 3, for example helium, which
entrains the vapours of the organometallic precursor that is used
is stored in a cylinder and introduced into the sublimator 1 with
the aid of a flow regulator (not shown).
[0065] The sublimator 1 is connected to a lower glass compartment 4
that comprises a fritted plate into which is introduced steam that
serves to activate the decomposition of the organometallic
precursor. The presence of steam enables a very active catalyst to
be obtained. This compartment 4 comprises a double jacket that is
thermostatically controlled at a temperature that may be adjusted
by means of a temperature regulator (not shown). The steam is
entrained by a neutral carrier gas 5, for example nitrogen, stored
in a cylinder and added to the compartment 4 with the aid of a flow
regulator (not shown). A feed of neutral carrier gas 6, for example
nitrogen, is intended to adjust the flow rates to those prevailing
under the fluidisation conditions. This carrier gas 6 is stored in
a cylinder and added to the compartment 4 by means of a flow
regulator (not shown).
[0066] The upper part of the compartment 4 is connected in a
gas-tight manner to a glass fluidisation column 7 of 5 cm diameter,
which is equipped at its base with a gas distributor. This double
jacket column 7 is thermostatically controlled to a temperature
that may be adjusted by means of a temperature regulator 8.
[0067] The upper part of the column 7 is connected to a vacuum pump
9 via a trap in order to retain the released decomposition
gases.
[0068] The procedure for implementing the examples relating to the
preparation of the catalysts by CVD is as follows: A mass Ma of
precursor is added to the sublimator 1.
[0069] A mass Ms of support granules Ms is added to the column 7
and a mass Me of water is added to the compartment 4 by means of a
syringe. A vacuum is applied to the arrangement consisting of the
compartment 4 and the column 7. The temperature of the bed is
raised to T1.
[0070] The sublimator 1 is heated to the temperature Ts and the
pressure is fixed at the value Pa in the whole apparatus by
introducing carrier gases 3, 5 and 6 (total flow rate Q). The
deposition then starts and lasts for a time t.sub.c.
[0071] At the end of the deposition, the temperature is restored to
ambient temperature by slow cooling and the vacuum pump 9 is
switched off. Once the system has returned to ambient temperature
and atmospheric pressure, the catalytic granular composition is
removed from the column 7 under an inert gas atmosphere (for
example nitrogen); it is then ready to be used for the production
of the nanotubes.
[0072] Two variants of the growth reactor 30, of different
diameters, were used in the examples for growing the nanotubes.
[0073] In the first variant, shown in FIG. 1, the growth reactor 30
consists of a quartz fluidisation column (2.5 cm diameter) 10
equipped in its middle part with a distributing plate (quartz
fritted plate) 11 on which is placed the powder of the catalytic
granular composition. The column 10 may be heated to the desired
temperature by means of an external heater 12 that can slide
vertically along the fluidisation column 10. In the procedure that
is employed this heater 12 is arranged either at an upper position,
where it does not heat the fluidised bed, or at a lower position,
where it heats the bed. The gases 13 (neutral gas such as helium,
carbon source, and hydrogen) are stored in cylinders and are added
to the fluidisation column by means of flow regulators 14.
[0074] In the upper part, the fluidisation column 10 is connected
in a gas-tight manner to a trap 15 intended to collect any fine
particles of catalytic granular composition or a mixture of
catalytic granular composition and nanotubes.
[0075] The height of the column 10 is adapted so as to contain,
during operation, the fluidised bed of the catalyst granules. In
particular, the height is at least equal to 10 to 20 times the
initial height of the bed of catalyst granules measured in the
absence of the gaseous feed, and should correspond to the heated
zone. In the examples a column 10 to 70 cm in total height is
chosen, which is heated over 60 cm of its height by the heater
12.
[0076] In the second variant (not shown) the growth reactor
consists of a stainless steel fluidisation column (5 cm diameter
and 1 m total height, heated over the whole height) provided at its
base with a distributor plate (stainless steel) on which is placed
the catalyst powder. The column may be heated to the desired
temperature by means of two fixed heaters and the said desired
temperature is controlled by a thermocouple dipping into the
fluidised bed. The gases (neutral gas, carbon source and hydrogen)
are stored in cylinders and are fed to the fluidisation column by
means of flow regulators.
[0077] FIG. 2 shows a variant of a process according to the
invention in which the catalytic granular composition is prepared,
according to the invention, continuously in the deposition reactor
20, removed continuously from this deposition reactor 20 through a
line 25a via which it is introduced to an intermediate buffer
reservoir 26, from which it is fed continuously, through a line
25b, to the growth reactor 30 where the nanotubes are produced. The
deposition reactor 20 is fed continuously with support granules
through a line 19 from a reservoir 18. The powder of catalyst
granules on which the nanotubes are attached is removed
continuously from the growth reactor 30 through an extraction line
27 that terminates in a buffer reservoir 28. The nanotubes may then
be separated from the support granules and metallic particles in a
known manner, following which they are stored in a storage
reservoir 29.
[0078] In the variants shown in the Figures, a growth reactor 30
different from the deposition reactor 20 is employed. By way of
variation (not shown), the deposition reactor 20 may then be used
for growing the nanotubes in a subsequent stage. However, this
latter variant means that the two stages have to be successively
carried out with different operating parameters, and there is a
risk of interference in the growth reaction, particularly in its
initial phase, due to residual byproducts from the deposition
phase.
[0079] The procedure for implementing the examples relating to the
production of nanotubes according to the invention is as
follows:
[0080] a mass Mc of catalyst (catalytic granular composition
according to the invention) is added to the fluidisation column 10
under an inert gas atmosphere.
[0081] With the heater 12 in its low position with respect to the
catalytic bed, its temperature is raised to the desired value Tn
for the synthesis of the nanotubes, either under an inert gas
atmosphere or under an atmosphere of a mixture of inert gas and
hydrogen (reactive gas).
[0082] When this temperature is reached, the carbon source, the
hydrogen and a neutral gas supplement are added to the column 10.
The overall flow rate QT ensures a bubbling regime of the bed at
the temperature T.sub.n, without any leakage.
[0083] The growth of the nanotubes then starts and lasts for a time
t.sub.n.
[0084] At the end of the growth stage the heater 12 is placed in
the high position with respect to the catalytic bed, the gas flow
rates corresponding to the carbon source and hydrogen are stopped,
and the temperature is slowly restored to ambient temperature.
[0085] The procedure is similar in the case of reactors with fixed
heaters.
[0086] The carbon nanotubes associated with the metallic particles
and fixed to the support granules are removed from the growth
reactor 30 and stored without any particular precautions. The
carbon nanotubes may then be separated from the metallic particles
and support granules so that they can be obtained in the pure
state, for example by dissolution with acid as described in WO
01/94260.
[0087] The amount of carbon deposited is measured by weighing and
by gravimetric thermal analysis.
[0088] The nanotubes produced in this way are analysed by
transmission electron microscopy (TEM) and scanning electron
microscopy (SEN) for the size and dispersion measurements, and by
X-ray crystallography and Raman spectroscopy in order to evaluate
the crystallinity of the nanotubes.
EXAMPLES
[0089] Preparation of the Catalysts
Comparison Example 1
[0090] A catalyst containing 2.6% Fe/Al.sub.2O.sub.3 is prepared by
a known method of liquid impregnation of metallic salts. The iron
precursor is hydrated iron nitrate Fe(NO.sub.3).sub.3,9H.sub.2O.
The support granules of alumina have a mean grain size of 120.mu.,
a density of 1.19 g/cm.sup.3 and a specific surface of 155
m.sup.2/g. The carrier gas is nitrogen. The implementation of the
preparation of the catalyst is as follows:
[0091] The support is a mesoporous alumina. 100 g of this support
are dehydrated in vacuo for 120 minutes. The appropriate amount of
salt in order to obtain 2.6% Fe/Al.sub.2O.sub.3 is contacted with
the alumina in 250 cm.sup.3 of deaerated ethanol. After 3 hours'
contact time, the solvent is evaporated and the catalyst is dried
overnight under reduced pressure (0.1 Torr). The catalyst is then
calcined at 500.degree. C. for 2 hours, following which it is
reduced by a mixture of nitrogen and hydrogen (80/20 by volume) for
2 hours at 650.degree. C.
[0092] The product obtained has a mean dimension of the metallic
particles equal to 13 nm and the variation of the dimensions of the
metallic particles with respect to this value is, for at least 98%
of the particles, at most of the order of 11 nm.
Example 2
[0093] A catalyst containing 2.6% Fe/Al.sub.2O.sub.3 is prepared in
accordance with the process according to the invention, in the
deposition reactor 20, as described hereinbefore but without using
water to activate the decomposition of the precursor. The
organometallic precursor used is the complex Fe(CO).sub.5, while
the support granules and the carrier gas that are used are the same
as in Example 1. The various parameters are adjusted as
follows:
[0094] Ma=9.11 g,
[0095] Ms=100 g,
[0096] Tl=220.degree. C.,
[0097] Pa=22 Torr,
[0098] Ts=35.degree. C.,
[0099] Q=82 cm.sup.3/min,
[0100] t.sub.c=15 min.
[0101] The product obtained (catalytic granular composition
according to the invention) comprises metallic particles deposited
on the granules. The dimension of the metallic particles after
heating under nitrogen at 750.degree. C. for 5 minutes is equal to
4 nm, and the variation of the dimensions of the metallic particles
with respect to this value is, for at least 97% of the particles,
at most of the order of 3.5 nm.
Example 3
[0102] A catalyst containing 1.3% of Fe/Al.sub.2O.sub.3 is prepared
according to the invention. The carrier gas is nitrogen. The
organometallic precursor, the support granules and the carrier gas
used are the same as in Example 2. The various parameters are
adjusted as follows:
[0103] Ma=7.12 g,
[0104] Ms=150 g,
[0105] Me=10 g,
[0106] Tl=220.degree. C.,
[0107] Pa=26 Torr,
[0108] Ts=35.degree. C.,
[0109] Q=82 cm.sup.3/min,
[0110] t.sub.c=7 min.
[0111] The product obtained has a mean dimension of the particles
equal to 3 nm and the variation of the dimensions of the metallic
particles with respect to this value is, for at least 98% of the
particles, at most of the order of 2.5 nm.
Example 4
[0112] This example relates to the preparation of a catalyst
containing 2.5% Fe/Al.sub.2O.sub.3. The organometallic precursor,
the support granules and the carrier gas used are the same as in
Example 2. The various parameters are adjusted as follows:
[0113] Ma=17.95 g,
[0114] Ms=200 g,
[0115] Me=25 g,
[0116] Tl=220.degree. C.,
[0117] Pa=20 Torr,
[0118] Ts=35.degree. C.,
[0119] Q=82 cm.sup.3/min,
[0120] t.sub.c=18 min.
[0121] The product obtained has a mean dimension of the metallic
particles equal to 4 nm and the variation of the dimensions of the
metallic particles with respect to this value is, for at least 98%
of the particles, at most of the order of 3.5 nm.
Example 5
[0122] This example relates to the preparation of a catalyst
containing 3.5% Fe/Al.sub.2O.sub.3. The organometallic precursor,
the support granules and the carrier gas used are the same as in
Example 2. The various parameters are adjusted as follows:
[0123] Ma=12.27 g,
[0124] Ms=100 g,
[0125] Me=25 g,
[0126] Tl=220.degree. C.,
[0127] Pa=24 Torr,
[0128] Ts=35.degree. C.,
[0129] Q=82 cm.sup.3/min,
[0130] t.sub.c=20 min.
[0131] The product obtained has a mean dimension of the particles
equal to 5 nm and the variation of the dimensions of the metallic
particles with respect to this value is, for at least 98% of the
particles, at most of the order of 4.5 nm. A histogram of the sizes
of particles is given in FIG. 3.
[0132] In this figure the mean dimension of the particles is
plotted along the x axis and their number is plotted along the y
axis.
Example 6
[0133] This example relates to the preparation of a catalyst
containing 5.65% Fe/Al.sub.2O.sub.3. The organometallic precursor,
the support granules and the carrier gas used are the same as in
Example 2. The various parameters are adjusted as follows:
[0134] Ma=9.89 g,
[0135] Ms=100 g,
[0136] Me=15 g,
[0137] Tl=220.degree. C.,
[0138] Pa=23 Torr,
[0139] Ts=35.degree. C.,
[0140] Q=82 cm.sup.3/min,
[0141] t.sub.c=23 min.
[0142] The product obtained has a mean dimension of the particles
equal to 6 nm and the variation of the dimensions of the metallic
particles with respect to this value is, for at least 98% of the
particles, at most of the order of 5.5 nm.
[0143] The results of Examples 1 to 6 are summarised in the
following Table I.
1TABLE I Size of the Metallic Example Precursor Method % Fe
Particles (nm) 1 Fe(NO.sub.3).sub.3, 9H.sub.2O impregnation 2.6 13
.+-. 11 2 Fe(CO).sub.5 CVD* 2.6 4.5 .+-. 4 3 Fe(CO).sub.5 CVD 1.3 3
.+-. 2.5 4 Fe(CO).sub.5 CVD 2.5 4 .+-. 3.5 5 Fe(CO).sub.5 CVD 3.50
5 .+-. 4.5 6 Fe(CO).sub.5 CVD 5.65 6 .+-. 5.5 *Preparation without
addition of water
[0144] Production of the Nanotubes
Comparison Example 7
[0145] Multi-walled nanotubes are produced using the catalyst of
comparison example 1 containing 2.6% Fe/Al.sub.2O.sub.3. In this
test the amount of catalyst was intentionally reduced so as not to
obtain large yields and, more particularly, so as to be better able
to determine the influence of the method of preparation of the
catalyst. The various parameters are adjusted as follows:
[0146] Mc=5 g,
[0147] Tn=750.degree. C.
[0148] Q.sub.T=320 cm.sup.3/min,
[0149] Amount of carbon added=3 g,
[0150] t.sub.n=60 min.
[0151] Under these conditions, the amount of carbon deposited is
0.16 g, which should be compared with the result obtained in test 5
of Example 12 (same percentage of iron and identical conditions),
namely 1.57 g. The height of the bed remains substantially the
same, whereas it changes from about 1 cm to 8.7 cm in test 5 of
Example 12. The SEM and TEM analyses show that the multi-walled
nanotubes comprise only a part of the deposit and that the
encapsulated particles are in this case extremely numerous. Thus,
only a catalyst composition according to the invention permits the
selective production of multi-walled nanotubes of homogeneous mean
dimensions.
Example 8
[0152] Multi-walled nanotubes are produced using the catalyst of
Example 2 containing 2.6% Fe/Al.sub.2O.sub.3 prepared without using
water to activate the decomposition of the precursor. In this test,
the amount of catalyst was intentionally reduced so as not to
obtain high yields, and more specifically so as to be better able
to determine the influence of the activation of the catalyst by
water. The various parameters are adjusted as follows:
[0153] Mc=5 g,
[0154] Tn=750.degree. C.
[0155] Q.sub.T=320 cm.sup.3/min,
[0156] Amount of carbon added=3 g,
[0157] t.sub.n=60 min.
[0158] Under these conditions, the amount of carbon deposited is
0.88 g, which should be compared with the result obtained in test 5
of Example 12 (same percentage of iron and identical conditions
except for the addition of water), namely 1.57 g.
[0159] The activation of the catalyst by water thus promotes a high
yield of nanotubes.
[0160] The TEM and SEM analyses show that the multi-walled
nanotubes constitute the only product of the deposition
reaction.
Example 9
[0161] Nanotubes are produced using the catalyst of Example 4
containing 2.5% Fe/Al.sub.2O.sub.3 and ethylene, and using a
stainless steel reactor of 5 cm internal diameter. Five tests were
carried out under the same conditions so as to verify the
reproducibility of the results.
[0162] The various parameters are adjusted as follows:
[0163] Mc=100 g,
[0164] Tn=650.degree. C.
[0165] Q.sub.T=1200 cm.sup.3/min,
[0166] Amount of carbon added=30 g,
[0167] t.sub.n=120 min.
[0168] Under these conditions, the amount of carbon deposited is
27.+-.0.2 g in all the tests carried out, i.e. a yield of 90% with
respect to the added carbon. The SEM and TEM analyses show that the
multi-walled nanotubes constitute the only product of the reaction.
The pyrolytic carbon or the encapsulated metal particles are
largely absent from the deposit. TEM micrographs of the nanotubes
obtained are shown in FIGS. 4 and 5. In FIG. 4 the scale
represented by the continuous line is 400 nm. In FIG. 5 the scale
represented by the continuous line is 20 nm. The external diameter
of the nanotubes is 20.+-.5 nm and their internal diameter is
4.+-.2 nm, which corresponds substantially to the mean dimension of
the metallic particles. The X-ray and Raman analyses of the
nanotubes obtained show the good degree of graphitisation of the
latter; this can also be seen in FIG. 5, where the planes of the
graphite can be observed.
Example 10
[0169] Nanotubes are produced using the catalyst of Example 4
containing 2.5% Fe/Al.sub.2O.sub.3 and ethylene, and using a
stainless steel reactor of 5 cm internal diameter.
[0170] The various parameters are adjusted as follows:
[0171] Mc=100 g,
[0172] Tn=650.degree. C.
[0173] Q.sub.T=1200 cm.sup.3/min,
[0174] Amount of carbon added=45 g,
[0175] t.sub.n=180 min.
[0176] Under these conditions, the amount of carbon deposited is 44
g, i.e. a yield of 97% with respect to the added carbon. The SEM
and TEM analyses show that the multi-walled nanotubes constitute
the only product of the reaction.
Example 11
[0177] A series of tests was carried out in a reactor of 2.5 cm
diameter so as to investigate the influence of the amount of metal
on the preparation of multi-walled nanotubes using the catalysts of
Examples 3 to 6 and a catalyst containing 0.5% of iron prepared in
a similar manner, and with ethylene as carbon source. In these
tests the amount of catalyst was intentionally reduced so as not to
obtain large yields, and specifically so as to be able better to
determine the influence of the amount of metal.
[0178] The various parameters are adjusted as follows:
[0179] Mc=5 g,
[0180] Tn=750.degree. C.
[0181] Q.sub.T=320 cm.sup.3/min,
[0182] Amount of carbon added=3 g,
[0183] t.sub.n=60 min.
[0184] The tests 1 to 5 of this example are summarised in the
following Table II.
2TABLE II Height of Bed Deposited after Deposition Test % Fe Carbon
(g) (cm) TEM Observation 1 0.5 0.52 3.2 multi-walled nanotubes 1
1.3 1.13 4 multi-walled nanotubes 2 2.5 1.90 6.2 multi-walled
nanotubes 3 3.5 2.29 8.6 multi-walled nanotubes 4 5.65 1.37 3
nanotubes + particles of encapsulated iron
[0185] The TEM and SEM analyses show that the multi-walled
nanotubes constitute the only product or virtually the only product
of the deposition reaction. The pyrolytic carbon or the particles
of encapsulated metal are particularly absent in tests 1 to 5. In
test 1, since the concentration of iron is low (0.5%) the yield is
greatly affected. In test 5, since the concentration of iron is
high the size of the iron particles is large and the formation of
particles of encapsulated iron can be seen.
Example 12
[0186] A series of tests was carried out in a reactor of 2.5 cm
diameter so as to investigate the influence of the temperature on
the preparation of multi-walled nanotubes using the catalyst of
Example 4 containing 2.5% Fe/Al.sub.2O.sub.3 and ethylene as carbon
source. In these tests the amount of catalyst was intentionally
reduced so as not to obtain high yields, and so as to be better
able to determine the influence of the temperature.
[0187] The various parameters are adjusted as follows:
[0188] Mc=5 g,
[0189] Tn=variable from 500 to 850.degree. C.
[0190] Q.sub.T=320 cm.sup.3/min,
[0191] Amount of carbon added=3 g,
[0192] t.sub.n=60 min.
[0193] The tests 1 to 6 of this example are summarised in Table
III.
3TABLE III Deposited Height of Bed Temp. Carbon after Deposition
Test (.degree.C.) (g) (cm) TEM Observation 1 500 0.05 1.9
multi-walled nanotubes 2 600 1.05 4.4 multi-walled nanotubes 3 650
1.13 5.5 multi-walled nanotubes 4 700 1.29 4.7 multi-walled
nanotubes 5 750 1.57 8.7 multi-walled nanotubes 6 850 1.86 4.7
nanotubes + pyrolytic carbon + particles of encapsulated iron
[0194] The TEM and SEM analyses show that the multi-walled
nanotubes constitute the only product or virtually the only product
of the deposition reaction. The pyrolytic carbon or the particles
of encapsulated metal are particularly absent in tests 1 to 5. In
test 1, the temperature is too low for the reaction to proceed
properly. In test 6, the temperature is too high and a thermal
decomposition of the ethylene leads to the formation of pyrolytic
carbon.
Example 13
[0195] This example relates to the preparation of nanotubes using
the catalyst of Example 4 containing 2.5% Fe/Al.sub.2O.sub.3 and
ethylene, and using a stainless steel growth reactor of 5 cm
internal diameter.
[0196] The various parameters are adjusted as follows:
[0197] Mc=100 g,
[0198] Tn=650.degree. C.,
[0199] Q.sub.T=1405 cm.sup.3/min,
[0200] Amount of carbon added=48.5 g,
[0201] t.sub.n=120 min.
[0202] Under these conditions, the amount of carbon deposited is
46.2 g, i.e. a yield of 95% with respect to the added carbon. The
TEM and SEM analyses show that the multi-walled nanotubes
constitute the only product of the reaction.
Example 14
[0203] This example relates to the preparation of nanotubes using a
catalyst containing 0.5% Fe/Al.sub.2O.sub.3 prepared according to
the procedure described in Example 4 and ethylene, and using a
stainless steel growth reactor of 5 cm internal diameter.
[0204] The various parameters are adjusted as follows:
[0205] Mc=100 g,
[0206] Tn=650.degree. C.,
[0207] Q.sub.T=1405 cm.sup.3/min,
[0208] Amount of carbon added=48.5 g,
[0209] t.sub.n=120 min.
[0210] Under these conditions, the amount of carbon deposited is
20.4 g, i.e. a yield of 42% with respect to the added carbon. The
TEM and SEM analyses show that the multi-walled nanotubes
constitute the only product of the reaction. This example confirms
the poor performances of the catalyst containing 0.5% of iron.
Example 15
[0211] This example relates to the purification of nanotubes
produced using a catalyst containing 2.5% Fe/Al.sub.2O.sub.3 and
ethylene, and using a stainless steel growth reactor of 5 cm
internal diameter according to the procedure described in Example
9. The solid powder leaving the reactor is added to a 2 l capacity
flask in the presence of 500 ml of water and 500 ml of 98% sulfuric
acid.
[0212] The various parameters are adjusted as follows:
[0213] M (nanotube powder+catalyst)=75 g,
[0214] V(H.sub.2O)=500 ml,
[0215] V(H.sub.2SO.sub.4, 98%)=500 ml,
[0216] T=140.degree. C.,
[0217] t.sub.n=120 min.
[0218] After dissolving the alumina for 2 hours with acid, the
solution is filtered, the nanotubes are washed several times with
water and then dried in a stove. The dry product (thermogravimetric
analysis) consists of 97% by weight of carbon nanotubes and 3% of
iron.
* * * * *