Catalyst composition including cobalt for the synthesis of carbon nanotubes.

Abstract

TITLE: Cobalt-containing catalyst composition for the synthesis of carbon nanotubes The present invention relates to an enhanced activity catalyst composition for the synthesis of carbon nanotubes and processes for obtaining these catalysts, as well as the carbon nanotubes obtained by the use of these catalysts.

Description

Title of the invention: Cobalt-containing catalyst composition for the synthesis of carbon nanotubes

[0001] The present invention relates to a catalyst composition with improved activity for the synthesis of carbon nanotubes and processes for obtaining these catalysts, as well as the carbon nanotubes obtained by the use of these catalysts.

[0002] The compositions of catalysts are among the factors that determine the quality of carbon nanotubes as well as the conversion to carbon in the form of nanotubes that can be obtained in relation to the amount of catalyst used.

[0003] The quality of the nanotubes obtained can be crucial for certain applications, such as those found in batteries (lead-acid, lithium-ion, lithium-sulfur, etc.). For example, some metallic elements present in carbon nanotubes are unsuitable for the proper functioning of certain battery devices due to electrochemical incompatibility. The selection of usable metals may therefore be limited.

[0004] Catalyst compositions comprise a mineral support on the surface of which one or more transition metals are deposited. The possibilities for combining different types of supports and different types of metals are numerous and have been the subject of extensive research over the last 35 years.

[0005] Thus iron and cobalt have been widely used alone or in combination with other metals while inorganic supports such as aluminas, zeolites, magnesias, silicas among others have been described.

[0006] The applicant has excluded iron from the present invention because iron generates subsequent difficulties when using the carbon nanotubes obtained in lithium-ion batteries.

[0007] The synthesis of carbon nanotubes is carried out by decomposing a hydrocarbon such as, for example, methane, ethylene, acetylene, or ethanol in a reactor at a temperature between 500 and 1000°C in the presence of these catalysts and hydrogen. The transition metal(s) have zero oxidation states because they are reduced by hydrogen and / or positive oxidation states as unreduced oxides during the synthesis.

[0008] For the growth of carbon nanotubes to be initiated and propagated, it is not clearly identified whether this takes place on the supported metal(s) in the zero or positive oxidation state, with combinations in the form of metal carbides, etc.

[0009] Thus among the multiple possible associations, it appeared to the applicant that certain metal-alumina associations were more or less favorable to the growth of carbon nanotubes under good conditions, but also in the way they were associated.

[0010] In the preparation of catalysts, different types and methods of preparing the metal / support compositions can be used. These include impregnation of metal salt solutions onto the support, coprecipitation methods, sol-gel methods, coating, or CVD (chemical vapor deposition).

[0011] The applicant restricted the mineral support to aluminas and a combination of metals including cobalt.

[0012] Aluminas are among the most interesting supports because they combine good dimensional stability with porous volume characteristics, pore size and type, and apparent specific surface area which vary according to type (alpha, beta, gamma...), with good industrial availability.

[0013] The way in which metals are combined on aluminas and the choice in the characteristics of aluminas shows that certain combinations are favoured.

[0014] The metals are combined with alumina in the form of metal salts in solution in a solvent by processes known to those skilled in the art. The solvent is then evaporated. Evaporation can be carried out concurrently with the continuous introduction of the metal salt solutions onto the alumina, for example in a fluidized bed, dry impregnation followed by a drying step, or wet impregnation, by immersing the alumina in the metal salt solution followed by solvent removal in a suitable device, and then a drying step. In all cases, calcination is subsequently carried out to obtain metal oxides on the surface of the alumina.

[0015] In the context of this invention, it has been shown that the active species required for the synthesis of carbon nanotubes consists of these metals in reduced form (zero oxidation state) because it has been observed that a catalyst with good reduction properties exhibits the best characteristics. It is therefore necessary to reduce the metals as much as possible for the carbon nanotubes to develop. The metal oxides present on the surface are thus reduced to their metal form, but a proportion of these metal oxides, up to 85%, always remains unreduced.

[0016] Thus, even though cobalt is known to be a good catalyst for the growth of carbon nanotubes, in the case of alumina it is necessary to use large quantities relative to the alumina. Cobalt / aluminum atomic ratios of up to 2 can be used. These cases correspond to preparations catalytics of which a significant portion of unreduced cobalt remains during the growth process of carbon nanotubes.

[0017] This significant portion of unreduced and therefore unused cobalt is found in the synthesized carbon nanotubes because the catalyst remains bound to the carbon nanotubes at the reactor outlet.

[0018] Given the economic and environmental costs associated with the use of cobalt in industry, it is important to use it optimally.

[0019] The applicant therefore sought to maximize the reducibility of cobalt oxides so that a minimum of cobalt is used.

[0020] Given the difficulty in obtaining good conversions with cobalt deposited alone on alumina, it has been shown that combining cobalt with iron, vanadium, and molybdenum improves the catalyst activity. This is described and explained by Willems et al. (Chemical Physics Letters 317 (2000) 71-76).

[0021] The association of cobalt with molybdenum is also described as beneficial to the synthesis of carbon nanotubes as described by Nguyen et al. (Advances in Natural science: Nanoscience and Nanotechnology 4 (2013) 035018.

[0022] With cobalt possibly combined with other metals, the applicant has determined that for the synthesis of carbon nanotubes with a ratio of the mass of carbon nanotubes obtained to the amount of catalyst used greater than 10.20 and even 30 using the minimum amount of cobalt, the cobalt reduction rate must be greater than 40%. This reduction rate was measured during a programmed temperature reduction (PTR) using hydrogen at a heating rate of 5°C / min and a hydrogen content of 5% by volume. Consequently, compositions with a reduction rate greater than 40% are those that allow for better conversion into carbon nanotubes. Ideally, compositions with reduction rates ranging from 70% to 90% are preferred because the carbon nanotube / catalyst ratio is greater than 30.

[0023] Many other metal combinations are possible with cobalt. Platinum, silver, cerium, and lanthanum are examples, but are not limited to these.

[0024] The applicant has thus tested several combinations of cobalt with so-called “cooperative” metals from groups IIIB, IVb, Vb, VIb, VI, I, VIIb, Ib and Ilb of the Mendeleev periodic table.

[0025] It has appeared that these metals all contribute a more or less marked positive cooperation to the catalytic activity of catalysts including cobalt.

[0026] If in the case of aluminas a systematic improvement of catalytic activity is observed with these combinations compared to the use of cobalt alone, the best combinations are obtained by depositing the cooperative metal(s) prior to the cobalt.

[0027] The applicant was able to determine that a high reduction rate measured during a programmed temperature reduction corresponds to the best yields in carbon nanotubes.

[0028] The present invention thus relates to a type of catalyst exhibiting an activity superior to those known in the prior art and enabling the production of carbon nanotubes particularly suited to applications where good electrical conductivity is required.

[0029] The present invention also relates to the processes for obtaining these catalysts.

[0030] The present invention also relates to carbon nanotubes obtained with the catalysts of the invention and according to the processes of the invention.

[0031] By superior activity we mean an improved quantity of carbon nanotubes produced in relation to the quantity of catalyst used as well as an improved resistivity, therefore better electrical conductivity. Summary of the invention

[0032] The invention relates to a catalyst composition for the synthesis of carbon nanotubes comprising:

[0033] - At least one alumina,

[0034] - One or more cobalt compounds,

[0035] - One or more compounds of metals from groups IIIB, IVb, Vb, VIb, VI, I, VIIIb, Ib and IIb of Mendeleev's periodic table, other than cobalt or iron,

[0036] - and exhibiting an overall metal reducibility rate greater than 40% during a programmed temperature reduction (PTR) of hydrogen, at a heating rate of 5°C / min up to 700°C and at a hydrogen content of 5% by volume in an inert gas stream. Description of the implementation methods

[0037] The carbon nanotubes that can be prepared with the catalysts of the invention can be of the single-walled or multi-walled type. They exhibit very good resistivities.

[0038] The growth of single-walled carbon nanotubes can be promoted by selecting particular carbon sources such as methane, carbon monoxide or carbon dioxide.

[0039] The growth of multi-walled carbon nanotubes can be promoted by using ethylene, ethane, propane as a carbon source, but other sources of organic compounds are possible.

[0040] By the reducibility rate of metals, we mean the capacity of one or more metals to be reduced to an oxidation state of zero, as measured by programmed temperature reduction. The proportion is thus expressed as a percentage. metals in a state of zero oxidation, the unreduced part remaining in a non-zero oxidation state.

[0041] Thus, within the framework of the invention, the catalysts have an overall metal reducibility rate and therefore a percentage of metal in the form of zero oxidation state greater than 40%, preferably greater than 50% and preferably greater than 60%, ideally greater than 70% following a programmed temperature reduction measurement.

[0042] The supports used in the invention are of the gamma, delta, eta, theta alumina type or also silica-doped aluminas with at least one lanthanide including lanthanum, cerium, titanium, zirconium, molybdenum, these doped or undoped aluminas having apparent specific surface areas (ASA) between 20 and 500 M2 / g.

[0043] Preferably, it is gamma alumina doped or undoped, and preferably undoped, whose SSA is between 50 and 250M2 / g and preferably between 100 and 200 M2 / g.

[0044] Catalyst preparation.

[0045] According to a first method, the alumina is impregnated with all the metallic salts according to the conventional dry or wet impregnation processes in one step. (Impregnancy with nascent moisture or impregnation in suspension phase).

[0046] Depending on the solubility of the salts in the solvent, several impregnations of this set of salt solutions can be carried out in more dilute solutions.

[0047] Drying is then carried out at a temperature above 100°C and preferably above 200°C at atmospheric pressure for a minimum of 2 hours.

[0048] Calcination is then carried out at a temperature between 200 and 800 °C and preferably between 400 and 500 °C at atmospheric pressure for a minimum of 2 hours.

[0049] This first method allows for a good improvement in the activity of the catalyst, i.e. improved yields in carbon nanotubes compared to the use of cobalt alone, accompanied by a measured metal reduction rate greater than 40% in some cases only.

[0050] According to a second method of doing, the chosen alumina is impregnated with the metal salt(s) not containing cobalt and then dried at 200°C at atmospheric pressure for a minimum of 2 h.

[0051] This first impregnation is followed by calcination at a temperature between 200 and 800 °C and preferably between 400 and 500 °C for a minimum of 2 hours.

[0052] The cobalt salt is then impregnated onto this preparation, and the resulting product is dried at 200°C at atmospheric pressure for a minimum of 2 h. A second calcination is then carried out at a temperature between 200 and 800 °C and preferably between 400 and 500 °C.

[0053] This second method is the preferred method because it allows obtaining a catalyst with optimal desired properties, i.e. maximum yields accompanied by a measured reduction rate that can be greater than 70%.

[0054] Alternatively, the applicant unsuccessfully tested pre-impregnation of cobalt with other metals.

[0055] The pH of the metal salt solution can be lowered using a mineral acid (nitric, hydrochloric, sulfuric) or an organic acid, such as citric, succinic, oxalic, acetic, glycolic, and preferably citric acid. This can improve the solubility of the salts in the impregnation solvent.

[0056] Calcination is carried out at temperatures between 200 and 800°C and preferably between 400 and 500°C.

[0057] As regards cobalt salts (Ml), they may be organic, inorganic, or mixtures of organic and inorganic salts. Ml is present in a predominant quantity relative to the other metals. The Co / Al atomic ratio is between 0.5 / 10 and 5 / 10, preferably between 1 / 10 and 4 / 10, and even more preferably between 1 / 10 and 2 / 10.

[0058] The second metal (M2) or third metal (M3) is selected from molybdenum, vanadium, platinum, iridium, tungsten, nickel, palladium, copper, hafnium, zirconium, magnesium, barium, lanthanum, rhenium, cerium, manganese, and zinc, and preferably platinum, vanadium, silver, molybdenum, and lanthanum, whether combined or not. These metals M2 and M3 are present in minor quantities relative to cobalt, and in a cobalt / other metals atomic ratio ranging from 1 / 20 to 1 / 5, and preferably from 1 / 7 to 1 / 12, and even more preferably from 1 / 8 to 1 / 10. The M2 / M3 atomic ratio may vary from 1 / 4 to 4 / 1.

[0059] With the metals well chosen and / or introduced in the right order, the catalysts of the invention provide improved and maximized yields in carbon nanotubes.

[0060] These catalysts combining cobalt and cooperative metals introduced in a particular order exhibit the highest metal reduction values.

[0061] The use of alumina doped with certain metals such as cerium, titanium, zirconium, molybdenum and lanthanides and in particular lanthanum can also be used in the context of the present invention.

[0062] With the compositions of the invention, the yields of carbon nanotubes are improved, and concomitantly a better rate of reduction of all the metallic species present within the catalyst is observed.

[0063] The invention therefore also relates to a process for preparing the catalysts of the invention allowing the best metal reduction values ​​to be obtained and thus the highest yields of carbon nanotubes, said process comprising the following steps:

[0064] 1) Impregnation with at least one metal salt from groups IIIB, IVb, Vb, VIb, VI, I VIIb, Ib and IIb of Mendeleev's periodic table other than cobalt or iron on alumina,

[0065] -Drying,

[0066] -Calcination,

[0067] 2) Impregnation of at least one cobalt salt on the product obtained in 1)

[0068] -Drying

[0069] -Calcination.

[0070] The invention also relates to carbon nanotubes obtained using the catalysts of the invention because they exhibit novel electrical resistance values. Examples

[0071] Temperature-programmed reduction (TPR) allows the study of hydrogen reduction of supported oxide phases by monitoring the hydrogen consumed as a function of temperature. The analyses were performed on a Micromeritics Autochem II 2920 series instrument. Knowing the amount of hydrogen consumed during TPR allows for an evaluation of the reduction rate of the metals present on the support (% reduction).

[0072] All analyses were therefore carried out under the same conditions. The

[0073] Calcined samples (50 mg) were first degassed under argon at 300°C to eliminate any trace of water, then cooled to room temperature. The volume of hydrogen consumed was then evaluated during a temperature increase from room temperature to 700°C under a mixture consisting of 5% hydrogen diluted in argon. The heating rate was 5°C / min and the flow rate was 3 liters per hour. Hydrogen consumption was measured using a heat transfer analyzer.

[0074] The amount of hydrogen consumed when a phase is reduced is then obtained by integrating the surface area corresponding to the hydrogen consumption. Two measurements are taken per test. The average of the two tests is calculated. It is assumed that cobalt is in the form Co3O4 in the calcined catalysts, platinum in the form PtO2, silver in the form Ag2O, vanadium in the form V2O5, molybdenum in the form MoO3, lanthanum in the form La2O3, cerium in the form Ce2O3, zinc in the form ZnO, magnesium in the form MgO, and barium in the form BaO.

[0075] Catalyst impregnation procedure:

[0076] The alumina used in the examples is of the gamma type, with an apparent specific surface area of ​​150 m2 / g.

[0077] The metallic salts used are as follows:

[0078] For cobalt: cobalt nitrate.

[0079] For platinum: platinum nitrate II tetramine.

[0080] For silver: silver nitrate.

[0081] For vanadium: ammonium vanadate.

[0082] For molybdenum: ammonium molybdate.

[0083] For lanthanum: lanthanum nitrate.

[0084] For cerium: cerium sulfate 4.

[0085] For zinc: zinc chloride.

[0086] For magnesium: magnesium nitrate.

[0087] For barium: barium nitrate.

[0088] The metals are impregnated in the proportions given in the example tables with sufficient water or a water-ethanol mixture to achieve good solubilization of the metal salts. Citric acid may be added to facilitate the dissolution of the salts, particularly with ammonium vanadate.

[0089] The process is carried out by dry impregnation by continuously introducing the solution of metallic salts into a fluidized bed of alumina and continuously removing the solvent, then the entire resulting mass is dried at 200°C for 2h.

[0090] Calcination procedure:

[0091] The impregnated products are calcined in a muffle furnace for 2 hours at a temperature indicated in the table of examples.

[0092] Procedure for synthesizing carbon nanotubes:

[0093] A catalytic test is performed by placing approximately 150 g of catalyst in a layer in a reactor 25 cm in diameter and 1 m in effective height, equipped with a disengagement device to prevent the downstream carryover of fine particles. It is heated to 700°C under hydrogen and

[0094] nitrogen (20% / 80% vol. / vol.) for 2 hours. At this temperature, an ethylene flow rate of 3000 NL / h and a hydrogen flow rate of 1000 NL / h are introduced, which corresponds to a partial pressure of ethylene of 0.75.

[0095] The gas flow rate is sufficient for the solid to be well beyond the limiting fluidization velocity, while remaining below the flight velocity.

[0096] After 60 minutes, the heating is stopped and the result of the quantity of carbon nanotubes formed is evaluated.

[0097] The activity of the catalysts is evaluated by measuring the mass ratio of carbon nanotubes obtained divided by the amount of catalyst used (NTC / catalyst).

[0098] Comparative Example 1:

[0099] In this example, catalysts are synthesized with cobalt alone on alumina (control), then the reduction capacity of the catalyst is evaluated by measuring it as a programmed temperature reduction, which allows the catalyst's ability to be reduced as a percentage of metal in the form of oxidation state. zero (% reduction). We then carry out a synthesis of carbon nanotubes and we evaluate the yields obtained by the ratio between the mass quantity of carbon nanotubes and the quantity of catalyst used (grams of carbon nanotubes per gram of calcined catalyst).

[0100] Gamma alumina with an apparent specific surface area of ​​150 m² / g is impregnated with a cobalt nitrate solution in the atomic proportions of Table 1 corresponding to tests 1 to 6. These impregnated aluminas are then dried for 2 hours at 200°C. They are subsequently calcined at 450 or 650°C and then tested for the synthesis of carbon nanotubes. The results are given in Table 1, with the metals expressed in atomic quantities.

[0101] [Tables 1] 1-1 100 20 450 23 6 1-5 5 ' 450 <15 3 1-ë iôè 5 65Ô <Ï5 è

[0102] It is observed that with cobalt alone, the reduction rate remains low accompanied by low yields in carbon nanotubes.

[0103] Example 2:

[0104] In this example, the synthesis of catalysts is carried out by simultaneously impregnating the cobalt salt and that of another metal, followed by drying for 2 hours at 200°C and calcination for 2 hours at 450°C.

[0105] In this example, the impregnation of the metals on the alumina is carried out by a solution containing all the metals.

[0106] The reduction capacity of the catalyst is evaluated by measuring it at a programmed temperature reduction, which allows for the assessment of the catalyst's ability to be reduced as a percentage of metal in the form of zero oxidation state. Carbon nanotubes are then synthesized, and the yields obtained are evaluated by the ratio between the mass quantity of carbon nanotubes and the quantity of catalyst used (grams of carbon nanotubes per gram of calcined catalyst).

[0107] The compositions of these results from these examples are given in Table 2; the metals are expressed in atomic quantities:

[0108] [Tables2] table 2 Ai Ob' h Ag V Kto G® Ba % rëdücaœ RTCÂætàlyssùr 2-1 fOQ wi 42.5 10.1 2-2 100 5 42.6 8.3 1ÔÔ 1Ô lÔ.ë 2-4 100 6.5 40 10.3 2-5 ■Ci) '10 i 36 10 É-6 £ Ô.5 34 5 iô 1ÙÔ 1Ù 3b ww 5 0.5 35.1 8 1ÔÔ 1Ô 1 35 8 ëdô IM 1Ô 1 'âd & 2-11 100 1Ô 1 30 2-12 TOO w 1 34 8 è-13 idë ib J ...........3'2.......... i

[0109] Example 3:

[0110] In this example, two impregnations are carried out:

[0111] 1)-Impregation of a metal salt from groups IIIB, IVb, Vb, VIb, VI, I, VIIIb, Ib and IIb of Mendeleev's periodic table other than cobalt or iron on an alumina followed by drying for 2 hours at 200°C, and calcination for 2 hours at 450°C,

[0112] 2) Impregnation of a cobalt salt onto the product obtained in 1) followed by drying from 2 hours at 200°C, and from a calcination 2 hours at 450°C.

[0113] The reduction capacity of the catalysts is evaluated by measurement in programmed temperature reduction which makes it possible to evaluate the capacity of the catalyst to be reduced in percentage of metal in the form of zero degree of oxidation.

[0114] Carbon nanotubes are then synthesized and the yields obtained are evaluated by the ratio between the mass quantity of carbon nanotubes obtained and the quantity of catalyst used (grams of carbon nanotubes per gram of calcined catalyst).

[0115] The results of these tests are given in Table 3; the metals are expressed in atomic quantities:

[0116] [Tables3] tablêàü 3 Al |Cb Pi Ag ¥ Mc La Ce Zn 5a I^CfëâWÿsëür 3-1 100 p0 1 79.5 35.2 3-2 Wt | a ü.S âO 33.8 à'à 1CÔ 110 1 76 33 3-4 100 | 5 0.5 74.3 31 .........w.......|w ■j <1 28 ' 3-6 100 [ 5 'O 68.6 25.6 3-7 O |iü 50.2 éê 3-8 100 I 5 52 22.3 3-9 100 110 1 48 32 i Mô làà । m 1: w 54 3-11 100 110 1 43.3 •23.7 i 1ÔÔ I# 44.8 ■ ; 3-13 100 |lQi 1 41.4 24.1

[0117] In this example, it can be seen that the reducibility rate is significantly improved when cobalt is impregnated in a second step; the prior introduction of other metals is favorable to the preparation of catalysts exhibiting the best activities.

[0118] Example 4:

[0119] In this example, two impregnations are carried out:

[0120] 1)-Impregation of two metal salts from the groups (Pt and La, Pt and Mo, Ag and La, Ag and Mo) on an alumina, followed by drying for 2 hours at 200°C, and calcination for 2 hours at 450°C,

[0121] 2) Impregnation of the cobalt salt onto the product obtained in 1) followed by drying 2 hours at 200°C, and calcination 2 hours at 450°C.

[0122] The reduction capacity of the catalysts is evaluated by measurement in programmed temperature reduction, which allows the ability of the catalyst to be reduced in percentage of metal in the form of zero degree of oxidation to be evaluated.

[0123] Carbon nanotubes are then synthesized and the yields obtained are evaluated by the ratio between the mass quantity of carbon nanotubes obtained and the quantity of catalyst used (grams of carbon nanotubes per gram of calcined catalyst).

[0124] The results of these tests are given in Table 3; the metals are expressed in atomic quantities:

[0125] [Tables4] 4| Al Co Pt Ag La Mo % redacïiôrt NTC / caSysèOt | 1 1100 è 0.5 ô.3 85 36.3 | 4-S |1ÔÔ S i 0.5 0.3 88 35.5 1........o........|'W| S; Ô.S 1 A4 |wo 5 ; 0s5 0.3 79 35

[0126] Example 5:

[0127] In this example, two impregnations are carried out:

[0128] 1)-Impregation with cobalt salt, followed by drying for 2 hours at 200°C, and calcination for 2 hours at 450°C,

[0129] 2)- Impregnation of the product obtained in 1) with a metal salt from groups IIIB, IVb, Vb, VIb, VI, I, VIIb, Ib and Ilb of Mendeleev's periodic table other than cobalt or iron on alumina followed by drying for 2 hours at 200°C, and calcination for 2 hours at 450°C.

[0130] The reduction capacity of the catalysts is evaluated by measurement in programmed temperature reduction, which allows the ability of the catalyst to be reduced in percentage of metal in the form of zero degree of oxidation to be evaluated.

[0131] Carbon nanotubes are then synthesized and the yields obtained are evaluated by the ratio between the mass quantity of carbon nanotubes obtained and the quantity of catalyst used (grams of carbon nanotubes per gram of calcined catalyst).

[0132] The results of these tests are given in Table 5; the metals are expressed in atomic quantities:

[0133] [Tables5] tablsau 5 Al Ch Pt As V Mo La Ce Ba 5-1 U® la 1 30 ..............w............ 8 3 wo 10 1 31.2 8 SS .........ÏSS........ ià 1 .........'10........ “.s 5-7 ■Wh B 1 29 11 ; 5-9 WO 10: 1 26 9 5 13 190 10; 1 3Ô 9.5 5-11 100 10 1 27.3 8 2 ] ÿ U'Ô 1Ô;

[0134] It can be seen in this series of examples that although the reducibility remains improved compared to the trials in example 1, the improvements are significantly less good than in the case of examples 2, 3 and 4.

Claims

Demands

1. Catalyst composition for the synthesis of carbon nanotubes comprising: - At least one alumina, - One or more cobalt compounds, - One or more metal compounds from groups IIIB, IVb, Vb, VIb, VI, I, VIIb, Ib and IIb of the Mendeleev periodic table other than cobalt or iron, - and exhibiting an overall metal reducibility rate greater than 40% during a programmed temperature reduction (PTR) of hydrogen, at a heating rate of 5°C / min up to 700°C and at a hydrogen content of 5% by volume in an inert gas stream.

2. Composition according to claim 1 wherein one or more metal compounds from groups IIIB, IVb, Vb, VIb, VI, I, VIIb, Ib and IIb of the Mendeleev periodic table are selected from platinum, silver, vanadium, lanthanum and molybdenum.

3. Composition according to claim 1 and or 2 wherein the alumina is gamma alumina.

4. Composition according to claim 1 or 2 wherein the alumina is doped alumina

5. Composition according to claim 3 wherein gamma alumina has an apparent specific surface area of ​​between 50 and 250 M2 / g.

6. Process for preparing catalyst according to claim 1 to 5 comprising the following steps: 1)-Impregation of at least one metal salt from groups IIIB, IVb, Vb, VIb, VI, I, VIIIb, Ib and IIb of the periodic table of Mendeleev other than cobalt or iron on an alumina, -Drying, -Calcination, 2)- Impregnation of at least one cobalt salt on the product obtained in 1) -Drying -Calcination.

7. Carbon nanotube obtained using a catalyst obtained using the process of claim 6.