EP1448477A2

EP1448477A2 - Composites based on carbon nanotubes or nanofibers deposited on an activated support for use in catalysis

Info

Publication number: EP1448477A2
Application number: EP20020801071
Authority: EP
Inventors: Huu-Cuong PHAM; Ricardo Vieira; Marc Ledoux; Loic Charbonnier; Raymond Ziessel; Charlotte Phan-Huu
Original assignee: Centre National de la Recherche Scientifique CNRS; Universite Louis Pasteur Strasbourg I; SICAT Societe Industrielle de Creations et d'Applications Textiles SARL
Current assignee: Centre National de la Recherche Scientifique CNRS; Universite Louis Pasteur Strasbourg I; SICAT Societe Industrielle de Creations et d'Applications Textiles SARL
Priority date: 2001-11-23
Filing date: 2002-11-20
Publication date: 2004-08-25
Also published as: WO2003048039A2; FR2832649A1; WO2003048039A3; FR2832649B1; US20050103990A1; AU2002364790A1; AU2002364790A8; US7799726B2

Abstract

The invention concerns a composite comprising an activated support and carbon nanotubes and nanofibers formed by vapour deposition, and the use of said composites as catalysts of chemical reactions in gas or liquid media, in particular in the chemical or petrochemical industry and for depollution of motor vehicle exhaust gases.

Description

Composites based on carbon nanotubes or nanofibers deposited on an activated support for application in catalysis

Technical field of the invention

The invention relates to the technical field of composites based on carbon nanotubes or nanofibers with large specific surfaces intended to be used as catalyst support or as catalyst for the chemical or petrochemical industry, in the depollution of vehicle exhaust gases. powered or in satellite propulsion systems. Their advantage is to combine the intrinsic properties of materials based on nanofibers and carbon nanotubes with that of macroscopic structures that are easily manipulated.

State of the art

The catalysts currently used in the fields of the chemical or petrochemical industry or in the depollution of exhaust gases from motor vehicles are essentially in the form of grains, extrudates, barrels or monoliths. These materials in the form of grains, extruded, barrels, monoliths can fulfill a catalyst support function, in which case an active phase is applied to said support to form a catalyst. This active phase is often made up of metals or metal oxides. Said materials can also show as such a catalytic activity, and in this case they constitute the catalyst. Research into new catalysts that are more selective, more efficient, more durable and more practical to use concerns both the supports and the active phases.

Nanostructured compounds (average diameter typically varying between 2 and 200 nm) based on carbon, such as nanotubes and nanofibers, exhibit on the one hand a strong intrinsic mechanical resistance, and on the other hand a large external exchange surface and good interaction with the active phase deposited, which allows a strong dispersion of the latter. These new materials thus have properties interesting physico-chemical for their uses in various fields such as catalysis or in reinforcement materials. According to the state of the art (see the article by NM Rodriguez, A. Chambers and RTK Baker "Catalytic engineering of carbon nanostructures" published in the journal Langmuir, vol. 11, p. 3862-3866 in 1995), these compounds carbon-based nanostructures are deposited from a gas phase containing ethylene, or a CO + H mixture, on a substrate consisting of either a metal powder or a solid support of silica impregnated with a aqueous solution of iron nitrate which is then calcined and reduced to iron to form an active phase. In both cases, the metal (copper or iron) acts as a catalyst for the formation of carbon nanotubes or nanofibers from a vapor phase. Patent application WO 01/51201 (Hyperion Catalysis International) gives other methods for preparing carbon nanotubes and nanofibers and indicates their possible use as a catalyst.

There are also known processes for manufacturing single-wall nanotubes in which the nanotubes are deposited by vapor deposition on an alumina airgel having a very high specific surface (of the order of 600 m / g) comprising a growth catalyst of the type Fe / Mo (see the article "A scalable CVD method for the synthesis of single-walled carbon nanotubes with high catalyst productivity" by Ming Su, Bo Zheng and Jie Liu, published in Chemical Physics Letters 322, p. 321-326 ( 2000)).

Problem

According to the state of the art, carbon nanostructure compounds are only synthesized with low yields. In addition, their nanometric size makes their shaping and their use difficult and creates problems of generation of dust during transport and loading; it also makes it impossible to use them in fixed bed reactors due to pressure drop problems. Consequently, not only the cost price of these compounds is high, but also their use as catalyst or catalyst support in industrial processes is difficult and ineffective. The object of the present invention is to propose new composites based on carbon nanotubes or nanofibers which retain the advantages of these nanotubes or nanofibers, namely their ability to serve as a support for an active phase for catalysis, and their intrinsic catalytic activity. , without having the known drawbacks of said nanotubes or nanofibers, namely the difficulty of their shaping, the generation of dust, the difficulty of their use in a fixed bed reactor, and their cost.

Objects of the invention

The Applicant has found a new class of composite materials with a high specific surface which can be used as catalyst or active phase support in various fields such as catalysis, propulsion and electrochemistry.

This class of materials consists of a composite comprising an activated support and carbon nanotubes or nanofibers formed by vapor deposition. The support can be a macroscopic support in the form of balls, felts, fibers, foams, extruded, monoliths, pellets etc. The surface of the support intended to receive the deposit of carbon nanotubes or nanofibers must be activated beforehand by depositing an active phase. Said composites combine the advantages acquired on macroscopic supports and those of the isolated nanoscopic compounds that are carbon nanotubes and nanofibers; they show in particular a high specific surface.

The first object of this patent application is a composite comprising a support activated by impregnation and carbon nanotubes or nanofibers formed by vapor deposition, characterized in that the mass of said carbon nanotubes or nanofibers formed on said activated support is at least equal at 10%, preferably greater than 20% and even more preferably greater than 30% of the total mass of the composite.

Another object of the present invention is the use of a composite comprising carbon nanotubes or nanofibers deposited on a support activated by impregnation as a catalyst support for chemical reactions in liquid or gaseous medium.

Another object of the present invention is the use of a composite comprising carbon nanotubes or nanofibers vaporized on a support activated by impregnation and an active phase deposited on the surface of said nanofibers or nanotubes, as catalyst for chemical reactions in liquid medium or gaseous.

Yet another object of the present invention is the use of a composite comprising carbon nanotubes or nanofibers deposited on a support activated by impregnation as an electrode in electrochemical processes or devices.

Description of the figures

FIG. 1 shows two images of scanning electron microscopy with the same magnification (see example 1). Figure la shows an activated support made of carbon felt impregnated with nickel. Figure 1b shows the same support after growth of carbon nanofibers.

FIG. 2 shows the porous distribution of two composites according to the invention (see example 2).

Figure 3 shows the scanning electron microscopy image of carbon nanofibers formed on the surface of a graphite electrode (see example 3).

FIG. 4 shows a comparative test of catalytic decomposition of hydrazine with a catalyst according to the invention and a catalyst according to the prior art. Detailed description of the invention

The terms "carbon nanotubes or nanofibers" and "carbon-based nanostructure compounds" refer here to tubes or fibers of highly ordered atomic structure, composed of hexagons of graphitic type, which can be synthesized under certain conditions (see article "Carbon nanotubes" by S. Iijima, para in the journal MRS Bulletin, p. 43 - 49 (1994)). It is known that according to the conditions of synthesis by vapor deposition, and in particular according to the catalysts used, it is possible to obtain either hollow tubes, possibly formed of several concentric tubes of different diameter, or solid fibers, also filiform, but containing graphitic carbon in a typically less ordered form. Said tubes or fibers can have a diameter typically between 2 and 200 nm, this diameter being substantially uniform over the entire length of each tube or fiber.

1. Preparation of the composites according to the invention

The macroscopic support must have sufficient thermal stability in a reducing medium, preferably up to at least 1000 ° C. It can be in the form of beads, fibers, felts, extruded, foam, monoliths or pellets. It can advantageously be chosen from alumina, silica, silicon carbide, titanium oxide, zirconium oxide, cordierite and carbon (in particular graphite and activated carbon) in the various forms indicated above. . The specific surface of said macroscopic supports can vary quite widely according to their origins. Advantageously, said specific surface, determined by the BET method of nitrogen adsorption at the temperature of liquid nitrogen (standard NF X 11-621), can represent 1 m ² / g to 1000 m ² / g and more especially 5 m ² / g to 600 m ² / g. Substrates having a specific surface of between 7 m ² / g and 400 m ² / g are preferred.

The support must be activated by depositing an active growth phase; this promotes the growth of nanotubes and carbon nanofibers in the presence of a mixture containing a source of hydrocarbon and hydrogen. In the context of this invention, this deposition is carried out by impregnation of the surface intended to receive the deposition of carbon nanofibers or nanotubes with a solution of one or more transition metal salts. In a preferred embodiment of the invention, this metal is chosen from the group comprising Fe, Ni, Co, Cu; bi or tri-metallic mixtures of these metals are also suitable. The concentration of the active phase, expressed by weight of metal, advantageously represents 0.1 to 20%, more preferably 0.2 to 15% and even more preferably 0.5 to 3% of the weight of said support. The low values are advantageously chosen for supports whose specific surface is rather low, while the high values are advantageously chosen for supports whose specific surface is rather high. According to the Applicant's findings, the use of too much metal can interfere with the catalytic activity of the composite if the metals used for the two catalytic functions (catalysis of the growth of nanotubes or nanofibers, and catalysis of the chemical reaction targeted in the industrial application of the composite) are different.

Thanks to its activation by the deposition of an appropriate active phase, the activated support catalyzes the growth of carbon nanotubes or nanofibers. Such an activated suppport can be prepared for example in the following way:

First of all, the support is impregnated, in the form of powder, pellets, granules, extrudates, foam, monoliths or other agglomerated forms, by means of a solution or a sol formed of a solvent such as water or any other organic solvent such as dichloromethane or toluene, and of the desired metal or metals in the form of salts.

The support thus impregnated is dried and the dried support is calcined at temperatures which can range from 250 ° C. to 500 ° C., with or without operating under an inert atmosphere. The support is then brought into contact with a reducing agent consisting of pure hydrogen or mixed with an inert gas, or any other gaseous source containing a reducing agent. The reduction is carried out at temperatures below 600 ° C and preferably between 300 and 400 ° C and for a period between 0.2 hours and 3 hours and preferably between 0.5 hours and 1 hour. The stage of reduction can be carried out either outside the synthesis reactor followed by storage in air of the resulting solid, or directly in the reactor just before the synthesis of nanotubes or nanofibers.

The growth of carbon nanotubes or nanofibers is carried out by subjecting the solid to a gas flow containing hydrogen and a carbon source at a temperature above 500 ° C, preferably between 500 and 1000 ° C and more preferably between 550 and 700 ° C, and under a pressure between 1 and 10 atmospheres, and preferably between 1 and 3 atmospheres. The gas containing free hydrogen or an inert gas and the gas containing the carbon source can be brought separately into contact with the macroscopic catalyst. However, in order to obtain a very homogeneous gaseous reaction medium during contact with the activated support, it is preferable to firstly mix the gas containing the carbon source with the gas containing hydrogen or the inert and bringing the mixture thus formed into contact with the activated support.

The gas containing free hydrogen or an inert gas is used in an amount suitable for providing an H ₂ : C molar ratio ranging from 0.05 to 10, and preferably from 0.1 to 5, and more preferably from 0.1 to 1 in the reaction medium arriving in contact with the activated support. The carbon source can be any molecule containing at least one carbon atom, but preferably either a hydrocarbon or carbon monoxide diluted in a flow of inert gas in the presence of hydrogen. The hydrocarbon can be any saturated or olefinic hydrocarbon ranging from Ci to C ₆ , preferably a saturated hydrocarbon ranging from Ci to C ₄ and more preferably a saturated hydrocarbon therefore the length of the chain will be between Ci and C ₃ . Methane and ethane are preferred among these different families of hydrocarbons.

The contact time between the reactants and the solid is between 0.5 seconds and several minutes, preferably between 0.5 and 60 seconds and more preferably between 1 and 30 seconds. The total pressure of the synthesis can be variable and between 1 and 10 atmospheres, preferably between 1 and 5 atmospheres and more preferentially between 1 and 3 atmospheres. The duration of the synthesis is between 1 hour and 24 hours, preferably between 2 hours and 12 hours and more preferably between 2 hours and 6 hours. In a preferred variant, this duration is chosen so that the quantity deposited in the form of nanotubes or nanofibers, by weight of carbon, is at least five times, and preferably between twenty times and one hundred times and even more preferably between one hundred and fifty times and a thousand times greater than the weight of the active phase, expressed in weight of metal,

After synthesis, the solid is cooled under reaction mixture to 200 ° C., then the mixture is then replaced with pure hydrogen up to room temperature. The solid is then discharged and stored in air at room temperature.

2. Characteristics and advantages of the composites according to the invention

The morphology of the carbon nanotubes or nanofibers according to the invention is characterized by nanostructured carbon in the form of nanotubes or nanofibers with an average diameter of between 5 nm and 200 nm. The average diameter of the nanotubes or carbon nanofibers can vary quite widely depending on the starting catalysts used in the active phase, and according to the synthesis conditions. Advantageously, said diameter, determined by scanning and transmission electron microscopy, varies between 0.01 micrometer and 20 micrometers, and more preferably 0.05 micrometer to 10 micrometers. The average length of these fibers and tubes is between a few tens and a few hundred micrometers. The macroscopic morphology of the starting supports is preserved.

The specific surface, measured by the BET method of nitrogen adsorption at the temperature of liquid nitrogen (standard NF X 11-621), of the composite materials according to the invention is typically between 1 and 1000 m ² / g; it is preferably greater than 10 m ² / g. For most of the industrial applications envisaged, composites can be used with a value of between 10 m ² / g and 100 m ² / g. Their porous distribution is essentially mesoporous, with an average size of between 5 and 60 nm. It is preferable that the microporous surface is as small as possible and represents less than 10% of the total contribution of the surfaces. In a preferred variant of the invention, the mass of the carbon nanotubes or nanofibers formed on the support is at least equal to 10%, preferably greater than 20% and even more preferably greater than 30% of the total mass of the composite.

The hardness of the composites according to the invention is much higher than those of starting materials, due to the formation of carbon nanotubes or nanofibers on the surface and in the matrices of said starting materials.

3. Advantages of the composites according to the invention

The composites according to the invention have numerous advantages compared, on the one hand, to known supports, and, on the other hand, compared to known carbon nanotubes or nanofibers. Their handling is easy because the macroscopic shape of the support is preserved, the carbon nanotubes or nanofibers deposited in no way modifying the morphology of the support. Their external exchange surface is large, as is their specific surface, relative to that of the starting solid, due to the presence of nanotubes or carbon nanofibers on the external surface. Their thermal and electrical conductivities are good due to the presence of nanotubes or carbon nanofibers on the surface (case of monoliths). The strong interaction between nanotubes or carbon nanofibers and the precursor salts of the active phase ensures good dispersion of the latter. Thanks to the strong interaction between nanofibers or nanotubes and the macroscopic support, we are not confronted with the problem of dust generation during handling of these materials, which is one of the drawbacks of known carbon-based nanostructure compounds; this absence of powder also facilitates the separation of the catalysts and of the reaction products, which is a primary property for the reactions taking place in the liquid phase. Similarly, the small size of the nanotubes or carbon nanofibers makes it possible to considerably reduce the mass transfer phenomena. The composites according to the invention also have a very high resistance to the sintering problems caused by water or thermal vapor, compared to that of the supports. traditional solid oxides such as alumina (Al ₂ O ₃ ), silica (SiO ₂ ), TiO or ZrO ₂ .

All of these new properties give the composites according to the invention a high potential for application in various fields such as catalysis, propulsion and electrochemistry or in the fields of mechanical reinforcement of materials working under high stress or under friction.

4. Industrial applications of the composites according to the invention

The composites according to the invention can have numerous industrial applications. They can be used as catalyst supports or directly as chemical reaction catalysts in the chemical industry, the petrochemical industry or in the depollution of exhaust gases from motor vehicles. They have increased mechanical and chemical resistance under working conditions in the presence of high water vapor pressure or in a humid atmosphere. By way of example, the composites according to the invention can directly catalyze Friedel-Crafts acylation in a liquid medium. They can, after application of an appropriate active phase, catalyze the decomposition of hydrazine and its derivatives and hydrogen peroxide (active phase: Ir (preferably) or Ru), the synthesis of ammonia in the presence of N ₂ and H ₂ (active phase: Ru (preferably) or Fe), selective or total oxidation such as the oxidation of CO to CO ₂ (active phase: Ni or Fe), hydrogenation-dehydrogenation such that l hydrogenation of nitro-aromatics or aromatics (active phase: Pt or Pd).

The Applicant has demonstrated that the composites according to the invention can be used as a catalyst for the decomposition of hydrazine under conditions close to those used in satellite propulsion systems.

Thanks to their large specific surface, the composites according to the invention can also be used as an electrode in electrochemical processes or devices. Thanks to their increased mechanical resistance, they can be used in fields other than that of catalysis, for example as reinforcing compounds in materials working under high friction stress. Furthermore, the deposition of nanotubes or carbon nanofibers considerably increases the mechanical resistance to crushing of the final composite compared to that of the starting material; this surface deposition according to the invention can therefore be used as a surface treatment to protect the substrate. Thus, the comopsites according to the invention can be used as reinforcement or protection of materials or elements working under friction.

Examples

To complete the above description, a series of examples illustrating the invention is given below, without implied limitation.

Example 1:

Preparation of a composite based on carbon nanofibers deposited on a carbon felt support

The carbon felt support is composed of a network of carbon microfibers having an external diameter centered on 0.01 mm and a specific surface area measured by the BET method of 10 m / g. The felts are first treated in a mixture of aqua regia (HC1, HNO ₃ ) at room temperature for 6 hours in order to prepare their surface (which is originally hydrophobic) for impregnation. Then, nickel in the form of nitrate (using distilled water as solvent) or acetylacetonate (using toluene as solvent) is deposited on the surface by successive impregnations. The impregnated supports are then dried in air at 100 ° C for 6 hours followed by calcination in air at 400 ° C for 2 hours, which transforms the nickel salts into oxide. The samples are then placed in a tabular oven and swept under a stream of argon at room temperature for 1 hour. Argon is replaced by hydrogen and the temperature gradually rises from ambient to 400 ° C (heating gradient of 5 ° C / min) and kept at this temperature for 2 hours; nickel oxide is reduced to metal. The temperature then rose from 400 ° C to 700 ° C and the hydrogen flow is replaced by the reaction mixture containing hydrogen and emane. The total flow rate is fixed at 150 ml / min (H ₂ : 100 ml / min and nC H ₆ : 50 ml / min). The duration of the synthesis is fixed at 6 hours. After synthesis, the samples are cooled under reaction mixture to 200 ° C. and then the mixture is replaced by pure hydrogen.

Figure la shows a scanning electron microscopy image of carbon felts previously impregnated with nickel. The diameters of the fibers that form the felt are centered around 10 micrometers. Figure 1b represents the morphology of the carbon nanofibers composite on carbon felt obtained after growth under a stream of hydrogen and ethane at 700 ° C. The diameter of the filaments in the composite is greatly increased; it is now on the order of four times that of the starting filaments (Figure la). The presence of carbon nanofibers is clearly visible in the form of small filaments. A higher magnification observation gives a diameter of the nanofibers between 80 and 100 nm.

The different characteristics of the samples obtained at 700 ° C. and with a synthesis time of two and six hours are presented in Table 1. The mass gain resulting from the formation of carbon nanofibers on the surface of graphite felts varies little depending on the concentration of nickel while the duration of the synthesis has a significant influence on the specific surface of carbon nanofibers. The formation of carbon nanofibers on the surface of the support significantly increases the specific surface area measured by the BET method of the samples as well as its porosity, essentially in the region of the mesopores between 3 and 20 nm, as shown by the distribution porous composites based on carbon nanofibers deposited on graphite felts (Figure 2). Table 1:

Characteristics of the composites obtained in Example 1

(Conditions: 700 ° C, H ₂ : 100 ml / min, C ₂ H ₆ : 50 ml / min)

This increase in surface is attributed to the formation of carbon nanofibers on the surface of the macroscopic starting support. The basal planes of graphite present in nanofibers contribute to the increase in the specific surface observed in composites. Nevertheless, the total specific surface area of the composites decreases appreciably when the duration of the synthesis increases from 2 hours to 6 hours. The mechanical hardness of the composites obtained is greatly improved compared to that of the starting graphite felt.

The resistance of the carbon nanofibers formed on the graphite felts is characterized by subjecting the composite obtained to sonication in an ultrasonic water bath for a period of at least half an hour with a nominal power of 1100 W at a frequency of 35 kHz. The absence of residues in the solution indicates that the fibers did not come off the surface of the composite during the operation, thus indicating the high resistance to attrition of said composites. Example 2:

Preparation of a composite of carbon nanofibers deposited on a monolithic TiO support?

The TiO ₂ monolith is characterized by square channels of 3 mm side and a wall with a thickness of approximately 0.5 mm. The starting material has a specific surface area measured by nitrogen adsorption of the order of 100 m ² / g. Before synthesis, the material is subjected to a heat treatment at 700 ° C; filtering and phase transition phenomena lead to a significant loss of the initial specific surface, which increases to 45 m ² / g (Table 2). It is impregnated and treated in the same manner as that used in Example 1. The synthesis is carried out under a mixture containing 100 ml / min of hydrogen and 100 ml / min of ethane. The duration of the synthesis is fixed at two hours. After synthesis, the samples are cooled under reaction mixture to 200 ° C and then under pure hydrogen to room temperature.

The physical characteristics of the samples obtained at 700 ° C. and two hours of synthesis are presented in Table 2. The formation of carbon nanofibers on the surface of the monolithic support considerably increases its specific surface which goes from 45 m ² / g to almost 100 m ² / g. The porous distribution was also modified due to the formation of carbon nanofibers.

Table 2: Characteristics of the composites obtained in Example 2

The porous distribution of the composites obtained is presented in Figure 3 as a function of the temperature and the duration of the synthesis. In this case, it should also be noted that the formation of carbon nanofibers contributed significantly to the mechanical strength of said composites.

Example 3:

Preparation of a composite of carbon nanofibers deposited on a glassy carbon disc for electrochemical applications

A glassy carbon disc with a diameter of 2 cm and a thickness of 0.4 cm is washed beforehand by soaking in a mixture of aqua regia (HCl / HNO ₃ ), followed by rinsing thoroughly with water. distilled water and drying at 100 ° C. One of the surface of the disc is then impregnated by deposition of a solution of nickel nitrate in water (1.4 mg / 1 ml), followed by the evaporation of the water at 100 ° C. in air for a night. The sample is then calcined at 300 ° C for two hours in air, then for one hour at 400 ° C under a stream of hydrogen. The formation of nanofibers is obtained by treating the sample under a flow containing a mixture of hydrogen (100 ml / min) and ethane (50 ml / min) at 650 ° C for 2 hours. During this step, the sample is placed horizontally in the tabular oven with the nickel-treated side up. After synthesis, the sample is cooled under reaction mixture to 200 ° C. and then under a stream of pure hydrogen up to room temperature. The sample is then discharged and stored in air.

The composite thus obtained is composed of a smooth face, while the other face has growths visible to the naked eye. The mass of the disc has increased by 10%. Figure 3 shows a scanning electron microscopy image of carbon nanofibers formed on the surface of a graphite electrode under a stream of hydrogen and ethane at 650 ° C. We distinguish the formation on the surface of the disc of tangles of carbon nanofibers so the diameter varies from a few tens of nanometers to several hundred nanometers. These modified electrodes are electroactive. It was in particular measured by cyclic voltammetry under Ar and under CO in acetonitrile containing 20% water and in a purely aqueous medium of catalytic currents corresponding to the reduction of CO ₂ to CO. The current densities j are between 1.6 mA / cm ² and 10.6 mA / cm ² .

Example 4:

Preparation of a composite of carbon nanotubes deposited on a carbon felt support

The support is similar to that of Example 1, and prepared according to the same procedure. The nanotube growth catalyst is iron, which is deposited on the surface of the carbon felts according to the same impregnation method as that used in Example 1. The heat treatments are also identical.

The samples are then placed in a tabular oven and swept under a stream of argon at room temperature for 1 hour. Argon is replaced by hydrogen and the temperature gradually rises from ambient to 400 ° C (heating slope of 5 ° C / min) and kept at this temperature for 2 hours; iron oxide is reduced to metal. The temperature then rose from 400 ° C to 750 ° C and the hydrogen flow is replaced by the reaction mixture containing hydrogen and ethane. The total flow rate is fixed at 100 ml / min (H: 50 ml / min and nC ₂ H ₆ : 50 ml / min). The duration of the synthesis is fixed at 6 hours. After synthesis, the samples are cooled under reaction mixture to 300 ° C., then the mixture is replaced by pure hydrogen.

The composite obtained has the same characteristics as that based on carbon nanofibers described in Example 1. The total specific surface of the composite is slightly lower and varies between 20 and 40 m / g. Example 5:

Acylation of anisole with benzoyl chlorare on composite based on carbon nanotubes

The carbon nanotabes composite on graphite felt is used without any prior treatment in the acylation reaction of anisole with benzoyl chloride according to the following equation:

+

The reagents are dissolved in a chlorobenzene solution. The concentration of anisole is 2 millimoles and that of benzoyl chloride is 3 millimoles. The solution is then degassed under a stream of argon at room temperature for 30 minutes. 0.2 grams of the composite are introduced into the flask, then the reactor atmosphere is purged under a stream of argon at room temperature for 30 minutes. The flask is closed and the temperature is brought to 120 ° C. The acylation is followed by gas chromatography. The results obtained as a function of time are reported in Table 3. Table 3: Friedel-Crafts reaction on carbon nanotube composite

Example 6:

Decomposition of hydrazine

The breakdown of hydrazine is a process used industrially in satellite propulsion systems. The main industrial catalyst is lr-37% / Alumina.

A composite of carbon nanofibers on carbon felt was prepared according to a procedure similar to those of the previous examples. The composite was sonicated (1100 W, 35 kHz) for 30 minutes to remove any fibers that were not well attached to the surface of the felt. 267 mg of this composite were impregnated with a solution of H IrCl ₆ .6H ₂ 0 containing 215 mg • of iridium. The product was dried at 100 ° C and then calcined in air at 300 ° C for 2 hours in order to convert the iridium salt to oxide. Then, the oxide was reduced under a stream of hydrogen at 400 ° C. The final catalyst thus obtained (catalyst A) contained 30% by mass of metallic iridium.

The hydrazine decomposition tests were carried out by injecting 0.4 ml of hydrazine of 99.9% purity into a reactor which contained the same amount (120 mg) of catalyst, namely either catalyst A (according to l invention, containing 30% by mass of metallic iridium), ie an iridium-based catalyst (37% by mass of metallic iridium) on alumina according to the prior art (catalyst B). The results are shown in FIG. 4. It can be seen that for the same quantity of catalyst (corresponding to a amount of very similar metallic iridium) and the same amount of hydrazine injected, the gas pressure generated by the decomposition of hydrazine is approximately 3 times greater with catalyst A (according to the invention) than with the catalyst B (according to the prior art). This decomposition is provided in a sufficiently short time interval to allow the use of the catalyst according to the invention in a propulsion system, for example for the precise positioning of satellites.

Claims

1. Composite comprising a support activated by impregnation and carbon nanotubes or nanofibers formed by vapor deposition, characterized in that the mass of said carbon nanotabes or nanofibers formed on said activated support is at least equal to 10%, preferably greater than 20% and even more preferably greater than 30% of the total mass of the composite.

2. Composite according to claim 1, characterized in that said support is in the form of balls, fibers, felts, extruded, foams, monoliths or pellets, and in that it has been activated by impregnation of an aqueous solution d one or more transition metal salts, followed by drying, calcination and a reducing treatment by contacting with a reducing gas.

3. Composite according to any one of claims 1 or 2, characterized in that said support is chosen from the group consisting of alumina, silica, titanium oxide, zirconium oxide, cordierite and carbon.

4. Composite according to any one of claims 1 to 3, characterized in that said support has, before impregnation, a BET surface of between 1 m ² / g and 1000 m ² / g and preferably between 5 m ² / g and 600 m ² / g.

5. Composite according to claim 4, characterized in that said support has, before impregnation, a BET surface of between 7 m ² / g and 400 m ² / g.

6. Use of a composite comprising carbon nanotubes or nanofibers vaporized on a support activated by impregnation as a support for catalyst for chemical reactions in liquid or gaseous medium.

7. Use of a composite comprising carbon nanotubes or nanofibers vaporized on a support activated by impregnation and an active phase deposited on the surface of said nanofibers or nanotubes, as a catalyst for chemical reactions in a liquid or gaseous medium.

8. Use of a composite comprising carbon nanotubes or nanofibers formed by vapor deposition on a support activated by impregnation as an electrode in electrochemical processes or devices.

9. Use of a composite comprising carbon nanotubes or nanofibers formed by vapor deposition on a support activated by impregnation as reinforcement or protection of materials or elements working under friction.

10. Use according to any one of claims 6 to 9, characterized in that said support intended to be activated by impregnation is chosen from alumina, silica, silicon carbide, titanium oxide, zirconium oxide , cordierite and carbon.

11. Use according to any one of claims 6 to 10, characterized in that said support is in the form of balls, fibers, felts, extruded, foam, monoliths or pellets.

12. Use according to any one of claims 6 to 11, characterized in that said support has, before impregnation, a BET surface area between 1 m ² / g and 1000 m ² / g, preferably between 5 m ² / g and 600 m ² / g and even more preferably between 7 m ² / g and 400 m ² / g.

13. Use according to any one of claims 6 to 12, characterized in that the surface of said support intended to receive the deposit of carbon nanofibers or nanotubes has been impregnated with a solution of one or more transition metal salts.

14. Use according to claim 13, characterized in that said transition metal is chosen from the group comprising Ni, Fe, Co and Cu.

15. Use according to any one of claims 13 to 14, characterized in that the impregnation was followed by drying, calcination and a reducing treatment by contacting with a reducing gas.

16. Use according to any one of claims 6 to 15, characterized in that said carbon nanofibers or nanotabes were formed from a gas mixture containing molecular hydrogen or an inert gas, and a gas containing carbon.

17. Use according to claim 16, characterized in that said carbon-containing gas contains carbon monoxide or a hydrocarbon, and in the case where it contains a hydrocarbon, preferably a linear or branched, saturated or olefinic hydrocarbon, with 1, 2, 3, 4, 5, or 6 carbon atoms and even more preferably with 1, 2 or 3 carbon atoms.

18. Use according to any one of claims 6 to 17, characterized in that the mass of said carbon nanotubes or nanofibers formed on said support is at least equal to 10%, preferably greater than 20% and even more preferably greater than 30 % of the total mass of said composite.

19. Use according to any one of claims 6 to 18, characterized in that the specific surface of said composite, determined by the BET method of nitrogen adsorption at the temperature of liquid nitrogen according to standard NF X

11-621, is greater than 10 m / g, preferably greater than 20 m / g and even more preferably greater than 50 m ² / g.

20. Use according to any one of claims 6, 7 or 10 to 19 as catalyst or catalyst support in the chemical industry, the petrochemical industry or in the depollution of exhaust gases from motor vehicles.

21. Use according to any one of claims 6, 7 or 10 to 20 as catalyst or catalyst support, characterized in that said chemical reactions in liquid or gaseous medium are chosen from Friedel-Crafts acylation, decomposition of the hydrazine and its derivatives, decomposition of hydrogen peroxide, synthesis of ammonia in the presence of N ₂ and H, oxidation of CO to CO ₂ .

22. Use according to claim 21 for the decomposition of hydrazine, characterized in that an active iridium phase has previously been deposited on the surface of said carbon nanofibers or nanotubes.

23. Use according to claim 22 in a propulsion system for satellites.