WO2018162378A1

WO2018162378A1 - Sulphur-doped carbon nanotubes and method for preparing same

Info

Publication number: WO2018162378A1
Application number: PCT/EP2018/055280
Authority: WO
Inventors: Stéphane LOUISIA; Marie Heitzmann; Pierre-André Jacques; Philippe Serp; Marie Rosa AXET
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2017-03-08
Filing date: 2018-03-05
Publication date: 2018-09-13
Also published as: FR3063721B1; FR3063721A1

Abstract

The present invention relates, mainly, to a method for preparing sulphur-doped multi-wall carbon nanotubes (MWCNT)<i />) by a CVD technique, involving the thermal decomposition of a gaseous stream, source of carbon and sulphur, in contact with the reduced form of a metal catalyst, characterised in that, prior to being placed in contact with said gaseous stream, source of carbon and sulphur, said catalyst is placed in contact with a gaseous stream which contains at least one hydrocarbon compound and is free of heteroatom source compound, under favourable conditions for initiating the growth of carbon nanotubes. The invention also relates to sulphur-doped multi-wall carbon nanotubes (MWCNT) according to the invention.

Description

Sulfur doped carbon nanotubes and process for their preparation

The present invention relates to a new type of multi-walled carbon nanotubes (NTC) doped (MWCNT, acronym for "Multi Wall Carbon Nano Tubes") sulfur and proposes for this purpose an original process for accessing it.

Over the last few years, CNTs have been the object of intensive research because they have good mechanical properties and are capable of providing improved electrical and / or thermal conduction properties to any composite material containing them.

Specifically, CNTs consist of one or more graphitic sheets arranged concentrically about a longitudinal axis. For nanotubes composed of a single sheet, we speak of SWNT (acronym for "Single Wall Nanotubes") and for nanotubes composed of several concentric layers, this is called MWNT (acronym for "Multi Wall Nanotubes").

The invention is more particularly MWNT.

Generally speaking, these NTCs are manufactured using the CVD process

(acronym for "Chemical Vapor Deposition"). This method consists of injecting a source of carbon-rich gas, typically methane, ethane, ethylene, acetylene or benzene, into a reactor containing a metal catalyst carried at high temperature, generally iron, cobalt or nickel, most often supported by a solid substrate, such as alumina, silica, magnesite or carbon. This technology is notably described in the document WO 86/03455 of Hyperion Catalysis International Inc.

It is known that the electrochemical performance of CNTs can be improved by acting on the electronic properties of their sp ² carbons and a large number of developments have been made in this regard in recent years. This improvement is in particular obtained by insertion of heteroatoms, like nitrogen and sulfur, within the CNT structure. These CNTs with heteroatoms incorporated into their structures are proposed for various applications such as in the fields of biomass conversion, for catalyst support, in optoelectronics, solar panels, or energy storage. .

Thus, the publication Yingchun Zhu et al. "Synthesis of Sulfur-Doped Carbon Nanotubes by Liquid Precursor" Materials Focus 2, 44-47 (2013) describes the preparation of MWCNT sulfur doped which is carried out according to a procedure involving the decomposition at high temperature (1000 ° C) of methylsulfide on a cobalt catalyst This process requires the use of a high flow of inert gas (argon 700 ml / min) to bubble and vaporize in the reaction chamber satisfactorily. However, the exposed structures of the compounds formed according to this procedure appear to be assemblies of nanotubes coated with sulfur-containing sheaths and not nanotubes containing within their structure sulfur atoms. The publication of Kun-Hong Lee et al. "Synthesis of high-quality carbon nanotube fibers by controlling the effects of sulfur on the catalyst agglomeration during the direct spinning process" RSC Adv. 5, 41894 (2015) also mentions a method, implementing, besides an iron catalyst, thiophene for the formation of CNTs at very high temperature (1170 ° C.). However, thiophene is in this case used for catalyst activation purposes and not for doping the carbon nanotubes that are formed during this process. Thus, the EELS (Electron Energy Loss Spectroscopy) analysis carried out in this work makes it possible to observe the presence of encapsulated sulfur on the surface of the metal, and not in the walls of the CNTs.

The publication of D. Demendoza et al. Synthesis of carbon nanofibers and nanotubes using carbon disulfide as the precursor REVISTA MEXICANA DE FISICA S 53, 5, 9-12 (2007) discloses a process using an iron catalyst and decomposing carbon disulfide at 900 ° C . The nano fibers obtained have a diameter of between 50 nm and 500 nm and contain little sulfur (0.25 atomic%, ie 0.7% by weight).

Therefore, and to the knowledge of the inventors, there is currently no effective method for dope satisfactorily and homogeneously MWCNT with sulfur.

The object of the invention is precisely to meet this need.

Thus, according to one of its aspects, the invention relates to a process for the preparation of multi-walled carbon nanotubes (MWNT) and sulfur doped by a CVD technique, implementing the thermal decomposition of a stream. gaseous source of carbon and sulfur, in contact with the reduced form of a metal catalyst, characterized in that, prior to its contact with said gas stream, source of carbon and sulfur, said catalyst is brought into contact with a gas stream that contains at least one compound hydrocarbon and is free of heteroatom source compound, under conditions conducive to the initiation of the growth of carbon nanotubes.

Within the meaning of the invention, the operation of priming or activation of the catalyst is an operation in which a gas carrying a hydrocarbon compound, carbon source only because denuded heteroatom and therefore devoid of sulfur element, will be contacted with the catalyst for a short time to promote the thermal decomposition of the hydrocarbon compound to initiate the growth of carbon nanotubes. In contact with the catalyst, the carbon of the hydrocarbon derivative will begin to rearrange in the form of nanotubes.

Unexpectedly, the inventors have indeed found that the implementation of a step of priming / activation of the catalyst by at least one gas flow, also called gaseous priming flow, containing a hydrocarbon gas and not sulfur, previously the interaction of this catalyst with a specific sulfur-containing hydrocarbon gas, and under controlled operating conditions is particularly critical to access MWCNT doped homogeneously and not only at the surface, sulfur.

Advantageously, the priming gas flow comprises and preferably consists of a single hydrocarbon compound which is devoid of heteroatom.

According to a preferred embodiment, the hydrocarbon compound of said gaseous initiation stream is selected from ethylene, methane and ethane and preferably is ethylene gas.

Advantageously, the gaseous priming flow is preferably implemented in conjunction with a hydrogen flow.

As regards the source gas stream of carbon and sulfur, it is advantageously a gaseous mixture of hydrogen and inert gas carrying at least one sulfur-containing hydrocarbon. Advantageously, the sulfur-containing hydrocarbon may be chosen from sulfur-containing hydrocarbons which are liquid at ambient temperature and have a low boiling point. Dimethyl disulfide, carbon disulfide and more preferably thiophene are suitable for this purpose.

Of course, all the gaseous flows considered according to the invention are controlled and optimized in order to reduce the degradation or saturation of the catalyst and the formation of reaction by-products, while keeping the catalyst in its reduced active form. More specifically, the method according to the invention comprises at least the steps of

a) disposing, in a furnace reactor heated to a temperature ranging from 600 ° C to 800 ° C, preferably about 750 ° C and under a stream of an inert gas, the reduced form of at least a metal catalyst suitable for the preparation of multi-walled carbon nanotubes (MWNT) by the CVD technique,

b) exposing said catalyst of step a) in contact with a gaseous flow that contains at least one hydrocarbon compound and is free of heteroatom source compound, under conditions conducive to the initiation of the growth of carbon nanotubes , and

c) exposing said activated catalyst at the end of step b) to a source gas stream of carbon and sulfur under conditions conducive to the formation of sulfur-doped multi-wall carbon nanotubes (MWNT). Advantageously, the reduced form of the catalyst of step a) is obtained beforehand, by exposure in the reactor of said catalyst, to hydrogen gas.

The present invention relates, according to another of its aspects, to a sulfur-doped multi-walled carbon nanotube (MWCNT) characterized in that it contains more than 2% by weight, and preferably more than 3% or even more than 5% by weight of sulfur element distributed within its structure and has a decomposition temperature below 600 ° C.

Within the meaning of the invention, the expression "distributed within the structure" means that the sulfur element is present on the outer surface of the walls of the nanotubes and also at the depth of these walls, that is to say in their thickness.

FIG. 2 notably makes it possible to visualize this homogeneous distribution of the sulfur element at the surface and in the multiple walls constituting the doped carbon nanotubes according to the invention.

More specifically, after purification of the multi-walled carbon nanotubes doped according to the invention, the sulfur element can be characterized in at least two forms, preferably three forms and more preferably four distinct chemical forms. chosen from thiol, disulfide, sulphide, thioester, sulphoxide, sulphite or sulphate units and sulphonic acid entities and preferably thioester, sulphoxide and sulphite.

This diversity of patterns is particularly characterized in Table 1 below.

According to another of its aspects, the present invention relates to the use of the sulfur-doped multi-walled carbon nanotubes according to the invention for the support of catalysts, in the fields of optoelectronics, solar panels and fuel cells. .

PROCESS ACCORDING TO THE INVENTION

As stated above, the process according to the invention is based on the use of the CVD technique for the formation of sulfur-doped carbon nanotubes.

This technique is based more particularly on the high temperature decomposition of a carbon source in the gaseous state in contact with a metal catalyst generally supported and contained in a reactor. This CVD technique is in particular illustrated in the document WO 2011/020970 A2.

An experimental device that is suitable for the invention is in particular represented in FIG.

It is a fluidized bed reactor consisting of a quartz column equipped with a (sintered) distributor plate placed inside a vertical furnace. The catalyst is placed in the enclosure and the latter is supplied in a controlled manner with the gas flows required according to the invention. The whole is heated to temperatures between 600 ° C and 800 ° C.

More precisely, the process for preparing the sulfur-doped multi-walled carbon nanotubes according to this technique can be detailed as follows:

The catalyst is heated to a temperature below 800 ° C and preferably above 700 ° C under a flow of inert gas in the furnace reactor.

If necessary, the catalyst is first reduced by contacting with hydrogen so that it is in an active form.

A first synthesis step consisting of priming the catalyst with a hydrocarbon gas (s) devoid of heteroatom is then carried out to initiate the growth of carbon nanotubes. A second synthesis step consisting in the growth of sulfur-doped MWCNTs is then carried out by introducing into the chamber a second source gas stream of carbon and sulfur. This gaseous flow is advantageously generated by bubbling, at a temperature preferably below ambient temperature, of an inert gas / H 2 mixture at a controlled rate in a hydrocarbon solvent containing sulfur and which must be liquid at ambient temperature and have a low temperature. boiling.

Preferably, the sulfur-doped MWCNT thus obtained can then be purified in a heated water / sulfuric acid mixture.

The main originality of the process therefore lies in the preliminary contacting of the catalyst with a gas stream which contains at least one hydrocarbon compound and is free of heteroatom source compound. In other words, this gas stream is only a source of the carbon element. This step is performed according to the invention, prior to bringing the activated catalyst into contact with a gaseous flow source of carbon and sulfur elements. The inventors have indeed found that contacting the catalyst only with a source gas stream of carbon and sulfur, that is to say without this initiation / initiation step with this first gas stream does not allow access to carbon nanotubes doped with sulfur according to the invention. a) priming gas flow

The gaseous initiation stream comprises, as carbon source, at least one hydrocarbon derivative chosen from alkanes, in particular methane or ethane, and alkenes, preferably ethylene, isopropylene, propylene, butene, butadiene, and mixtures thereof.

As pointed out above, this gas stream does not carry a heteroatom capable of interacting during the initiation of the growth of carbon nanotubes. It is thus devoid of any sulfur-containing hydrocarbon derivative but also of nitrogen-containing hydrocarbon derivative.

Advantageously, this gaseous source stream carbon and denuded heteroatom and therefore sulfur, comprises at least and preferably consists of ethylene.

The gaseous priming flow is implemented in the process of the invention with a speed ranging from 2.3 mm / s to 7.0 mm / s and preferably from 4.6 mm / s to 5.75 mm and usually for less than 10 minutes. The speed unit, expressed in mm / s, represents the distance traveled by the ignition gas flow, within the reactor, in one second.

Advantageously and according to the invention, this gaseous ignition source, carbon source, and preferably consisting of ethylene, is implemented jointly in the presence of a gaseous flow of hydrogen.

The speed of the gaseous flow of hydrogen is preferably chosen between 3 mm / s and

7.0 mm / s.

As stated above, the gaseous flows considered according to the invention are controlled and optimized in order to reduce the degradation or saturation of the catalyst and the formation of reaction by-products, while keeping the catalyst in its reduced active form. Thus, advantageously, the priming step is carried out with a gaseous carbon source stream / gaseous hydrogen flow in a ratio of 3/2.

Advantageously, the catalyst is brought into contact with the ignition gas stream at a temperature of less than 900 ° C., preferably ranging from 600 ° C. to 800 ° C.

This initiation step is carried out the time required to initiate the growth of carbon nanotubes.

Commonly, this initiation requires a treatment time on a minute scale, of at least 30 seconds to 10 minutes and preferably 1 to 4 minutes.

Initiation of carbon nanotube growth can be monitored by transmission electron microscopy (TEM). b) carbon and sulfur source gas stream

As a result of the ignition gas flow, the catalyst is brought into contact with a second source gas stream of carbon and sulfur.

This second source gas stream of carbon and sulfur preferably consists of a gaseous mixture of hydrogen and inert gas carrying at least one sulfur-containing hydrocarbon.

The inert gas is preferably selected from argon (Ar) or nitrogen.

The speed of this inert gas is preferably chosen between 2.3 mm / s and 7.0 mm / s.

The hydrogen velocity is preferably chosen between 2.3 mm / s and 7.0 mm s. The sulfur-containing hydrocarbon is advantageously chosen from sulfur-containing hydrocarbons which are liquid at ambient temperature and have a low boiling point, such as dimethyl disulfide or carbon disulfide and preferably thiophene.

This second gas stream is advantageously generated by bubbling a gaseous mixture of hydrogen and inert gas into a container containing the sulfur-containing hydrocarbon, which is liquid at ambient temperature and has a low boiling point, chosen according to the invention. .

The temperature of the sulfur-containing hydrocarbon solvent is advantageously adjusted as a function of the gas flow rates in order to entrain enough of it to allow growth but not in excess to avoid the poisoning of the catalyst and the excess of this solvent at the outlet . Thus, in the specific case of thiophene it is carried out at a temperature between 5 ° C and 17 ° C, preferably at 17 ° C.

Preferably, the flow rates of inert gas and hydrogen are also adjusted to obtain the growth of sulfur-doped MWCNT by limiting the amount of amorphous carbon and encapsulated catalyst around them at the end of the reaction.

Thus, this second carbon and sulfur source gas stream is advantageously implemented with a speed ranging from 4.3 mm / s to 13.8 mm / s and preferably from 7.0 mm / s to 9.3 mm. / s.

This second gas stream enriched in carbon and sulfur is maintained in contact with the catalyst until the desired end of the growth of sulfur-doped MWCNTs.

In general, this sulfur doping step of the MWCNT is completed after a period of 30 minutes. Beyond this period, inactivation of the catalyst occurs because of its prolonged exposure to the sulfur element.

All the stages of initiation and growth of carbon nanotubes can be carried out at a temperature of between 600 ° C. and 800 ° C., preferably around 700 ° C. In other words, the contacting of the catalyst with said gaseous stream, a source of carbon and sulfur, is advantageously carried out at a temperature of between 600 ° C. and 800 ° C., preferably 700 ° C.

The catalytic yield of a synthesis of carbon nanotubes doped with sulfur according to the invention is at least 25% by weight of nanotubes expressed relative to the weight of catalyst. c) Catalyst

According to an advantageous embodiment, the catalyst may be chosen from iron (Fe), nickel (Ni), cobalt (Co), molybdenum (Mo) or mixtures thereof in the image of AlFeCo or FeMo in proportions ranging from 5% to 20%. Advantageously, the catalyst is chosen from iron (Fe), nickel (Ni) or cobalt (Co) and preferably is an iron-based catalyst preferably supported on alumina.

The adjustment of the amount of catalyst is clearly within the skill of those skilled in the art. It is generally adapted to obtain good efficiency in sulfur-doped MWCNT while being careful not to form too much amorphous carbon at the end of the reaction. An amount of less than 5 g of catalyst may in particular be considered for a catalyst of Fe (5%) / Ab03 type.

As can be seen from the foregoing, the catalyst is operated in its reduced form. This is generally obtained by exposing the catalyst according to the invention to hydrogen. In this way, the catalyst material is reduced in situ in said reactor, and its catalytic layer is not oxidized at the moment when it is used for the synthesis of carbon nanotubes.

This catalyst is advantageously supported on a porous substrate which is of course chosen for its chemical inertness under the operating conditions of the CNT synthesis method by the CVD technique.

This substrate may represent from 30% to 70% by total weight of the catalyst.

Advantageously, this substrate may be inorganic. It is especially chosen from alumina, an active carbon, silica, a silicate, magnesia, titanium oxide, zirconia, a zeolite or even carbon fibers. According to an advantageous embodiment, the substrate is alumina.

Advantageously, the catalyst supported on porous substrate is, after its introduction into the reactor first heated under a flow of inert gas, and then reduced with the introduction of hydrogen at a controlled rate, while maintaining the flow rate of inert gas. d) Purification. The sulfur-doped MWCNTs are preferably isolated at the end of the reaction and then purified.

This purification step is in particular carried out in order to eliminate the amorphous carbon that may have formed during the reaction, as well as the catalyst remaining around the sulfur-doped MWCNTs formed.

This purification can be carried out by heating in a water / acid mixture the supported catalyst as well as the dark matter which has formed around it.

Preferably, according to the method of the invention, the nanotubes obtained at the end of step c) are purified and recovered in an H2SO4 / H2O mixture for 3 hours at 140 ° C., as illustrated in Example 1 below.

MWNCT SULFUR DOPES ACCORDING TO THE INVENTION

The invention also relates to sulfur-doped multi-wall carbon nanotubes (MWCNT) obtainable by the method described and detailed above.

It is advantageously multi-walled carbon nanotubes, comprising for example from 5 to 15, and preferably from 7 to 10, graphene sheets wound concentrically.

The nanotubes obtained according to the invention usually have a mean diameter ranging from 0.1 to 50 nm, more preferably from 0.4 to 50 nm and better still from 1 to 30 nm and advantageously a length of more than 0.1 μιη. and advantageously from 0.1 to 20 μιη, for example about 6 μιη. Their length / diameter ratio is advantageously greater than 10 and most often greater than 100. Their specific surface area is for example between 100 and 600 m ² / g and their apparent density may especially be between 0.01 and 0.5 g. / cm ³ and more preferably between 0.07 and 0.2 g / cm ³ .

As explained above, the sulfur-doped carbon nanotubes according to the invention have the specificity of having a distribution of the sulfur element which is not only located on the surface of the nanotube walls. This element is integrated in the structure of these compounds, as illustrated in FIG.

The sulfur-doped multi-wall carbon nanotubes according to the invention are also characterized in that the sulfur element is present in at least two forms, preferably three forms and more preferably four forms. chemical moieties selected from thiol, disulfide, sulfide, thioester, sulfoxide, sulfite or sulfate moieties and sulfonic acid moieties.

This is particularly characterized by XPS analysis (acronym for "X-ray photoelectron spectroscopy"). An example of this type of analysis is particularly detailed in Example 1 below.

The sulfur-doped multi-wall carbon nanotubes according to the invention are also characterized by elemental analysis in that they contain more than 2%, preferably more than 3%, or even more than 5% by weight of sulfur element distributed within their structures. In particular, the sulfur-doped multi-wall carbon nanotubes produced contain at least 6%, preferably at least 7% by weight of sulfur element relative to their total weight.

Furthermore, the Raman analysis of the sulfur-doped carbon nanotubes according to the invention makes it possible to highlight two bands ID and IG, (ID for the disordered carbon and Io for the graphitized carbon) respectively present at 1330 cm ^-1 and 1575 cm ^-1 . The ratio of the intensities of these bands (ID / IG) is characteristic of the synthesized material but also of the synthesis method. Thus, the ID / IG ratio is equal to 1.7 for undoped MWCNT and less than 1.7 or even less than 1.5 and in particular equal to about 1 for the sulfur-doped MWCNTs according to the invention.

The sulfur-doped carbon nanotubes according to the invention also have a decomposition temperature of less than 600 ° C., preferably 570 ° C., which is lower than that of nanotubes composed solely of carbon. This is particularly illustrated in FIG. 3, which reports the thermogravimetric analysis of sulfur-doped carbon nanotubes obtained according to example 1.

Thus, advantageously, a sulfur-doped multi-wall carbon nanotube prepared according to the process of the invention contains more than 5% by weight of sulfur distributed within its structure and has a decomposition temperature of less than 600 ° C.

USE OF MWCNT DOPES WITH SULFUR ACCORDING TO

THE INVENTION The invention also relates to the use of the sulfur-doped MWCNTs according to the invention in the fields of optoelectronics, solar panels and fuel cells, metallic nanoparticles and also support catalysts for catalysts in various fields of the invention. chemistry.

Thus, the invention also relates to the use of the sulfur-doped MWCNTs according to the invention in composite materials in order to impart to them improved electrical conduction properties and / or mechanical properties, especially resistance to elongation.

In particular, the sulfur-doped MWCNTs according to the invention can be used in macro-molecular compositions intended for the packaging of electronic components or the manufacture of fuel lines or antistatic coatings or paints. or in thermistors or electrodes for supercapacities or for the manufacture of structural parts for aeronautical, nautical or automotive fields, or as a catalyst support.

Other advantages and characteristics of the invention will emerge more clearly on reading the detailed description of exemplary embodiments of the invention which are presented as non-limiting illustrations of the invention and with reference to the following figures:

Figure 1 is a diagram of the furnace / reactor suitable for the invention.

FIG. 2 is a TEM micrograph image of the sulfur-doped carbon nanotubes prepared according to Example 1.

FIG. 3 reports the result of the ATG analyzes obtained for the sulfur-doped MWCNTs according to the invention (dotted patterns) and for conventional CNTs (black outline).

FIG. 4 shows the plots of the spectra obtained by Raman spectroscopy for the sulfur-doped MWCNTs prepared in Example 1 (dotted patterns) as well as those obtained for conventional CNTs (black plot). Figure 5 shows the XPS analysis of the sulfur-doped MWCNTs prepared in Example 1.

FIG. 6 shows the images obtained by TEM micrograph of Co / Pt nanoparticles deposited on carbon nanotubes doped with sulfur according to the invention. FIG. 7 presents a comparison of the catalytic activity of Vulcan-grafted PtCo catalysts (solid histogram) and sulfur-doped MWCNT grafts prepared in Example 1 (histogram TIRETS) as measured by RRDE (acronym for "Rotating Ring-Disk Electrode"). 10 μg / cm ² at a speed of 5 mV / sec.

EXAMPLE 1

Preparation of sulfur-doped MWCNTs according to the invention a) Synthesis of the Fe / AhC catalyst

In an Erlenmeyer flask of 11, 72.1 mg of Fe (NO ₃ ) 3.9F ₂ C) (98%, Strem chemicals) are dissolved in 500 ml of distilled water. 5 g of Al ₂ O ₃ alumina (Pural NW from Sasol and supplied by Arkema) are added to the reaction mixture. The metal salt and the support are brought into contact with stirring for 2 hours at room temperature. The solvent is then evaporated by rotary evaporator and the powder is dried under vacuum for 2 hours. This powder is then studied at 110 ° C. for 15 hours. The final catalyst is obtained by calcination in air at 450 ° C. for 8 hours. b) Preparation

4.64 g of the iron-alumina catalyst Fe (5%) / Al ₂ O ₃ prepared above are heated at 750 ° C. under a flow of argon (at a speed of 7.0 mm / s) in a furnace reactor. vertical diameter of 3 cm and reduced by hydrogen mixed with argon (Ar at a speed of 5.3 mm / s and H ₂ at a speed of 3.5 mm / s). This catalyst is then initiated in the presence of H ₂ (at a rate of 3.5 mm / s) by contact with ethylene gas (C ₂ H ₄ at a speed of 5.3 mm / s) for 3 hours. minutes. The growth of the MWCNT is continued by bubbling an Ar / H ₂ mixture (Ar at a rate of 3.5 mm / s and H ₂ at a rate of 5.3 mm / s) in thiophene at 17 ° C. for 27 hours. min. A purification step of the 1.08 g of sulfur-doped MWCNT thus obtained is carried out by immersing them in a sulfuric acid / water mixture (H ₂ SO ₄ / H ₂ O) for 3 h at 140 ° C. c) Physicochemical characterization of sulfur-doped MWCNTs The sulfur-doped multi-wall carbon nanotubes obtained according to this method contain 7.7% sulfur according to elemental analysis (CFiN Perkin Elmer elemental analyzer).

They have a decomposition temperature of 570 ° C characterized by thermogravimetric analysis (Figure 3), (Perkin Elmer Diamond Thermobalance TG / TDA 25 ° C up to 1000 ° C at 10 ° C / min in air).

These nanotubes were also characterized by TEM analysis (FIG. 2), by Raman spectrometry (FIG. 4), (JEOL JEM-1011 at 100 kV for TEM). XPS analysis (Figure 5), (K-alpha Thermo S cientific apparatus with a non-chromatic source Mg K source (1253.6 eV, 300 W) with a 20 eV passing energy) doped nanotubes formed reveals the presence different sulfur groups.

Table 1 below shows the various sulfurized units characterized.

In parallel these nanotubes have been characterized in terms of composition as follows.

d) Characterization of sulfur-doped MWCNTs in terms of activity

i) Preparation of Pt / Co nanoparticles on MWCNT

200 mg of sulfur-doped nanotubes according to the invention are introduced into a reactor of Schlenk type and placed under vacuum for 30 minutes, then under argon. 0.48 mmol of [bmim] [Tf2N] (99% Solvionic) are added to the reactor. The reaction is under an inert atmosphere. 2.04 mmol of CoCl 2 · 6H 2 O (98% Strem chemicals) are dissolved in 60 ml of ethanol and added to the reaction medium. The reaction medium is placed under ultrasound for 20 minutes. 60 ml of a 0.15 mol.l ^{-1 solution} of NaBH ₄ (99.99% Sigma Aldrich) in ethanol are added to the reaction medium After 30 minutes, 100 ml of distilled water is added to the mixture. 3 hours later, 1.27 mmol of K ₂ PtCl ₄ (99.9% Strem Chemicals) are dissolved in 60 ml of distilled water and added to the reaction mixture After stirring overnight, the solution is filtered and washed with a mixture of distilled water and ethanol and then dried for 24 hours at 80 ° C. The particles obtained at the end of the reaction were studied by TEM micrograph and the images obtained are shown in FIG.

ii) Comparison of activity in RRDE

The catalyst thus prepared is compared with a commercial reference by RRDE. 10 mg of catalyst prepared in step i) are dispersed in 4 ml of a mixture Isopropanol / distilled water / Nafion® (D-2020 Dupont Fluoroproduct, 90 / 19.5 / 0.5) and placed under ultrasound for 30 min. . 30 μl of the ink thus prepared is deposited in three times on a vitreous carbon disk type electrode, previously polished. A fine catalyst deposit loaded at 100 μg Pt / cm ² is obtained after evaporation of the solvent in air at room temperature.

The specific activity of the catalyst is measured 0.9 V / rhe by cyclic voltammetry in a solution made of H ₂ S0 ₄ to 0.5 M saturated with N ₂ 5 mV / s between 0.04 and 1.2 V / RHE at 900 rpm. The same procedure is performed for the commercial reference Pt ₃ Co / Vulcan XC-72.

It can be noted that the catalyst prepared with the sulfur-doped nanotubes has a greater activity for the reduction of oxygen (Figure 7).

Claims

1. Process for the preparation of multi-walled carbon nanotubes (MWNT) and sulfur-doped by a CVD technique, implementing the thermal decomposition of a gaseous stream, source of carbon and sulfur, in contact with the form reduced by a metal catalyst, characterized in that, prior to being brought into contact with said carbon and sulfur source gas stream, said catalyst is brought into contact with a gas stream which contains at least one hydrocarbon compound and is free from source compound heteroatom, under conditions conducive to the initiation of the growth of carbon nanotubes.

2. Method according to the preceding claim comprising at least the steps of

c) exposing said activated catalyst at the end of step b) to a source gas stream of carbon and sulfur under conditions conducive to the formation of sulfur-doped multi-wall carbon nanotubes (MWNT).

3. Method according to the preceding claim wherein the reduced form of the catalyst of step a) is previously obtained by exposure within the reactor of said catalyst to hydrogen gas.

4. Method according to any one of claims 2 or 3 wherein the nanotubes obtained in step c) are purified and recovered in an H2SO4 / H2O mixture for 3 hours at 140 ° C.

5. Process according to any one of the preceding claims, in which said priming gas flow comprises and preferably consists of a single hydrocarbon compound which is devoid of heteroatom.

6. Method according to any one of the preceding claims wherein the hydrocarbon compound of said gaseous initiation stream is selected from ethylene, methane and ethane and preferably is ethylene gas.

7. Process according to any one of the preceding claims, in which the bringing of said catalyst into contact with said gaseous initiation flux is carried out at a temperature of less than 900 ° C, preferably ranging from 600 ° C to 800 ° C.

8. A method according to any one of the preceding claims wherein said gaseous priming flux is implemented with a speed ranging from 2.3 mm / s to 7.0 mm / s and preferably 4.6 mm / sec. s at 5.75 mm / s for a time less than 10 minutes.

9. Method according to any one of the preceding claims wherein said gaseous priming flow is carried out in the presence of a gaseous flow of hydrogen.

10. Method according to any one of the preceding claims wherein said source gas stream of carbon and sulfur consists of a gaseous mixture of hydrogen and inert gas carrying at least one sulfur-containing hydrocarbon.

11. Process according to the preceding claim, in which the sulfur-containing hydrocarbon is chosen from sulfur-containing hydrocarbons which are liquid at ambient temperature and have a low boiling point, such as dimethyl disulfide or carbon disulfide and preferably thiophene. .

12. Process according to any one of the preceding claims, in which the source gas stream of carbon and sulfur is used with a speed ranging from 4.3 mm / s to 13.8 mm / s and preferably from 7. 0 mm / s to 9.3 mm / s.

13. Process according to any one of the preceding claims, in which the bringing into contact of said catalyst with said source gas stream of carbon and sulfur is carried out at a temperature of between 600 ° C. and 800 ° C., preferably 700 ° C. vs.

14. Process according to any one of the preceding claims, in which the catalyst is chosen from iron (Fe), nickel (Ni) or cobalt (Co) and preferably is an iron-based catalyst preferably supported on alumina. .

15. Sulfur doped multi-walled carbon nanotube characterized in that it contains more than 5% by weight of sulfur distributed within its structure and has a decomposition temperature below 600 ° C.

16. sulfur-doped multi-wall carbon nanotube according to claim 16, characterized in that the sulfur element is present in at least two forms, preferably three forms and more preferably four different chemical forms chosen from thiol, disulfide units. , sulfide, thioester, sulfoxide, sulfite or sulfate and sulfonic acid moieties.

17. A sulfur-doped multi-wall carbon nanotube according to claim 15 or 16 containing at least 6%, preferably at least 7%, by weight of sulfur element relative to its total weight.

18, multi-walled carbon nanotube doped with sulfur according to any one of claims 15, 16 or 17, the Raman analysis has two bands ID and IG, respectively present at 1330 cm ^"1 and 1575 cm ^_1 -and which the ratio ID / IG is less than 1.7 or even less than 1.5 and in particular equal to about 1.

19. Use of sulfur-doped multi-wall carbon nanotubes obtained according to any one of claims 1 to 18, in the fields of optoelectronics, solar panels and fuel cells and in particular for supporting catalysts.