Catalyst composition comprising cobalt for the synthesis of carbon nanotubes

Abstract

The present invention relates to a catalyst composition with improved activity for the synthesis of carbon nanotubes, to the methods for obtaining these catalysts, and to the carbon nanotubes obtained by using these catalysts.

Description

[0001] DESCRIPTION

[0002] TITLE: Cobalt-containing catalyst composition for the synthesis of carbon nanotubes

[0003] 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.

[0004] 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.

[0005] The quality of the nanotubes produced 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.

[0006] Catalyst compositions consist of 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. Iron and cobalt have been widely used alone or in combination with other metals, while inorganic supports such as aluminas, zeolites, magnesias, and silicas, among others, have also been described.

[0007] The applicant has excluded iron from the present invention because iron generates subsequent difficulties when using the carbon nanotubes obtained in lithium-ion batteries.

[0008] Carbon nanotubes are synthesized by decomposing a hydrocarbon such as 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) present either zero oxidation states because they are reduced by hydrogen and / or positive oxidation states as unreduced oxides during the synthesis.

[0009] 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.

[0010] Thus, among the many 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.

[0011] In catalyst preparation, various types and methods of preparing metal / support compositions can be used. These include impregnation of metal salt solutions onto the support, coprecipitation methods, sol-gel methods, coating, and CVD (chemical vapor deposition).

[0012] The applicant restricted the mineral support to aluminas and a combination of metals including cobalt.

[0013] Aluminas are among the most interesting supports because they combine good dimensional stability with varied characteristics of pore volume, pore size and type, and apparent specific surface area depending on the type (alpha, beta, gamma...), with good industrial availability.

[0014] The method of combining metals on alumina and the choice of alumina characteristics indicate that certain combinations are favored. Metals are combined with alumina in the form of metal salts in solvent solution using methods known to those skilled in the art. The solvent is then evaporated. Evaporation can be carried out concurrently with the continuous introduction of 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 performed to obtain metal oxides on the surface of the alumina.

[0015] Within the framework 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), as it has been observed that a catalyst with good reduction capacity exhibits the best characteristics. It is therefore necessary to reduce the metals as much as possible for the carbon nanotubes to grow. The metal oxides present on the surface are thus reduced to their metallic 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 catalytic preparations in which a significant proportion of unreduced cobalt remains after the carbon nanotube growth process.

[0017] This significant amount of unreduced, and therefore unused, cobalt ends up in the synthesized carbon nanotubes because the catalyst remains bound to the carbon nanotubes as they exit the reactor. Given the economic and environmental costs associated with cobalt use in industry, it is important to use it optimally.

[0018] The applicant therefore sought to maximize the reducibility of cobalt oxides so that a minimum amount of cobalt is used.

[0019] 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).

[0020] 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.

[0021] With cobalt potentially combined with other metals, the applicant determined that for the synthesis of carbon nanotubes with a ratio of carbon nanotube masses to catalyst mass 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 temperature-programmed reduction (TPR) using hydrogen at a heating rate of 5°C / min and a hydrogen concentration of 5% by volume. Consequently, compositions with a reduction rate greater than 40% are those that allow for better conversion to carbon nanotubes. Ideally, compositions with reduction rates ranging from 70% to 90% are preferred because the carbon nanotube / catalyst ratio is greater than 30.

[0022] Numerous other metal combinations are possible with cobalt. These include, but are not limited to, platinum, silver, cerium, and lanthanum. The applicant has tested several combinations of cobalt with so-called "cooperative" metals from groups IB, IVb, Vb, VI, VIIIIb, Ib, and IIIb of Mendeleev's periodic table.

[0023] It has emerged that these metals all contribute a more or less marked positive cooperation to the catalytic activity of catalysts containing cobalt.

[0024] While in the case of aluminas a systematic improvement in 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.

[0025] 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.

[0026] 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.

[0027] The present invention also relates to the processes for obtaining these catalysts.

[0028] The present invention also relates to carbon nanotubes obtained with the catalysts of the invention and according to the processes of the invention.

[0029] Superior activity refers to an increased quantity of carbon nanotubes produced relative to the amount of catalyst used, as well as improved resistivity, and therefore better electrical conductivity. Summary of the invention:

[0030] The invention relates to a catalyst composition for the synthesis of carbon nanotubes comprising:

[0031] - At least one alumina,

[0032] - One or more cobalt compounds,

[0033] - One or more compounds of metals from groups IIIB, IVb, Vb, VIb, VI, I, VIIb, Ib and IIIb of Mendeleev's periodic table other than cobalt or iron,

[0034] - 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.

[0035] Detailed description:

[0036] 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.

[0037] The growth of single-walled carbon nanotubes can be promoted by selecting specific carbon sources such as methane, carbon monoxide, or carbon dioxide.

[0038] 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.

[0039] The reducibility rate of metals refers to the capacity of one or more metals to be reduced to a zero oxidation state, as measured by programmed temperature reduction. This expresses the proportion of metals in a zero oxidation state, with the unreduced portion remaining in a non-zero oxidation state. Thus, within the scope of this invention, the catalysts exhibit an overall reducibility rate of metals, and therefore a percentage of metal in a zero oxidation state, greater than 40%, preferably greater than 50%, and ideally greater than 60%, and ideally greater than 70%, following a programmed temperature reduction measurement.

[0040] The supports used in the invention are of the gamma, delta, eta, theta alumina type, or silica-doped aluminas with at least one lanthanide, including lanthanum, cerium, titanium, zirconium, and molybdenum, these doped or undoped aluminas having apparent specific surface areas (ASA) between 20 and 500 M 2 / g data from suppliers (BET method corresponding to ASTM D 4641-17).

[0041] Preferably, this is gamma alumina, doped or undoped, and preferably undoped, with an SSA between 50 and 250M 2 / g and preferably between 100 and 200 M 2 / g.

[0042] Catalyst preparation.

[0043] According to one method, alumina is impregnated with all the metallic salts using the classic dry or wet impregnation processes in one step. (Impregnancy with nascent moisture or impregnation in suspension phase).

[0044] 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.

[0045] 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.

[0046] The process then involves calcination at a temperature between 200 and 800 °C, and preferably between 400 and 500 °C, at atmospheric pressure for a minimum of 2 hours. This first method allows for a significant improvement in catalyst activity, i.e., improved carbon nanotube yields compared to the use of cobalt alone, accompanied by a measured metal reduction rate exceeding 40% in certain cases.

[0047] According to a second method, 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 hours.

[0048] 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.

[0049] 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 hours. A second calcination is then carried out at a temperature between 200 and 800°C and preferably between 400 and 500°C.

[0050] 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%.

[0051] Alternatively, the applicant unsuccessfully tested pre-impregnation of cobalt with other metals.

[0052] 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.

[0053] Calcination is carried out at temperatures between 200 and 800°C, and preferably between 400 and 500°C. The cobalt salts (M1) may be organic, inorganic, or mixtures of organic and inorganic salts. M1 is present in the 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.

[0054] The second metal (M2) or third metal (M3) is chosen from among 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, 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.

[0055] With the metals well chosen and / or introduced in the right order, the catalysts of the invention provide improved and maximized yields of carbon nanotubes.

[0056] These catalysts, combining cobalt and cooperative metals introduced in a particular order, exhibit the highest metal reduction values.

[0057] The use of alumina doped with certain metals such as cerium, titanium, zirconium, molybdenum and lanthanides, and in particular lanthanum, can also be used within the framework of the present invention.

[0058] With the compositions of the invention, carbon nanotube yields are improved, and concomitantly, a better reduction rate of all metallic species present within the catalyst is observed. The invention therefore also relates to a process for preparing the catalysts of the invention, enabling the best metal reduction values ​​and thus the highest carbon nanotube yields, said process comprising the following steps:

[0059] 1) Impregnation of at least one metal salt from groups IIIB, IVb, Vb, Vlb, VI, I, Vlllb, Ib and IIIb of Mendeleev's periodic table other than cobalt or iron on alumina,

[0060] -Drying,

[0061] -Calcination,

[0062] 2) Impregnation of at least one cobalt salt onto the product obtained in 1) - Drying

[0063] -Calcination.

[0064] The invention also relates to carbon nanotubes obtained using the catalysts of the invention because they exhibit unprecedented electrical resistance values.

[0065] Examples:

[0066] Temperature-programmed reduction (TPR) allows for the study of hydrogen reduction of supported oxide phases by monitoring hydrogen consumption as a function of temperature. Analyses were performed on a Micromeritics Autochem II 2920 series instrument. Knowing the amount of hydrogen consumed during TPR allows for the evaluation of the reduction rate of the metals present on the support (% reduction).

[0067] All analyses were therefore carried out under the same conditions. The calcined samples (50 mg) were first degassed under argon at 300°C to eliminate all traces 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 of 5% hydrogen diluted in argon. The heating rate was 5°C / min and the flow rate 3 liters per hour. Hydrogen consumption was measured using a heat-discharge meter. The amount of hydrogen consumed when a phase was reduced was then obtained by integrating the area corresponding to the hydrogen consumption. Two measurements were taken per test. The average of the two tests was calculated.It is assumed that cobalt is in the form CO3O4 in calcined catalysts, platinum in the form PtO2, silver in the form Ag2O, vanadium in the form V2O5, molybdenum in the form MoOs, lanthanum in the form La2O3, cerium in the form Ce2Os, zinc in the form ZnO, magnesium in the form MgO, barium in the form BaO.

[0068] Catalyst impregnation procedure:

[0069] The alumina used in the examples is of the gamma type, with an apparent specific surface area of ​​150 m². 2 / g.

[0070] The metallic salts used are as follows:

[0071] For cobalt: cobalt nitrate.

[0072] For platinum: platinum nitrate II tetramine.

[0073] For silver: silver nitrate.

[0074] For vanadium: ammonium vanadate.

[0075] For molybdenum: ammonium molybdate.

[0076] For lanthanum: lanthanum nitrate.

[0077] For cerium: cerium sulfate 4.

[0078] For zinc: zinc chloride.

[0079] For magnesium: magnesium nitrate.

[0080] For barium: barium nitrate.

[0081] 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 ammonium vanadate.

[0082] The process involves dry impregnation by continuously introducing the metallic salt solution into a fluidized bed of alumina and continuously removing the solvent, then the entire resulting mass is dried at 200°C for 2 hours.

[0083] Calcination procedure:

[0084] The impregnated products are calcined in a muffle furnace for 2 hours at a temperature indicated in the table of examples.

[0085] Carbon nanotube synthesis procedure:

[0086] 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. The reactor is heated to 700°C under hydrogen and 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 maintained, corresponding to an ethylene partial pressure of 0.75.

[0087] The gas flow rate is sufficient to ensure that the solid is well above the limiting fluidization velocity, while remaining below the flight velocity.

[0088] After 60 minutes, the heating is stopped and the result of the quantity of carbon nanotubes formed is evaluated.

[0089] The activity of the catalysts is evaluated by measuring the mass ratio of carbon nanotubes obtained divided by the amount of catalyst used (CNT / catalyst).

[0090] Comparative Example 1:

[0091] In this example, catalysts are synthesized using cobalt alone on alumina (control). The catalyst's reduction capacity is then evaluated by measuring the reduction in programmed temperature, allowing the assessment of the catalyst's ability to be reduced as a percentage of metal in the form of zero oxidation state (% reduction). Next, carbon nanotubes are synthesized, and the yields obtained are evaluated by the ratio of the mass quantity of carbon nanotubes to the amount of catalyst used (grams of carbon nanotubes per gram of calcined catalyst). Gamma alumina has an apparent specific surface area of ​​150 M⁻¹. 2 / g is impregnated with a cobalt nitrate solution in the atomic proportions listed in Table 1, corresponding to tests 1 to 6. These impregnated aluminas are then dried for 2 hours at 200°C. They subsequently undergo calcination at 450 or 650°C and are then tested for the synthesis of carbon nanotubes. The results are given in Table 1; the metals are expressed in atomic quantities.

[0092] [Table 1] It is observed that with cobalt alone, the reduction rate remains low accompanied by low yields in carbon nanotubes.

[0093] Example 2:

[0094] In this example, catalyst synthesis is carried out by simultaneously impregnating the cobalt salt and another metal salt, followed by drying for 2 hours at 200°C and calcination for 2 hours at 450°C.

[0095] In this example, the impregnation of metals onto alumina is carried out by a solution containing all the metals.

[0096] The catalyst's reduction capacity is evaluated by measuring the reduction in programmed temperature, which allows for the assessment of the catalyst's ability to be reduced to 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 amount of catalyst used (grams of carbon nanotubes per gram of calcined catalyst).

[0097] The compositions of these results from these examples are given in Table 2; the metals are expressed in atomic quantities:

[0098]

[0099] Example 3:

[0100] In this example, two impregnations are carried out:

[0101] 1) - Impregnation of a metal salt from groups IB, IVb, Vb, Vlb, VI, I, Vlllb, Ib and IIIb 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,

[0102] 2)- Impregnation of a cobalt salt on the product obtained in 1) followed by drying for 2 hours at 200°C, and calcination for 2 hours at 450°C.

[0103] The reduction capacity of catalysts is evaluated by measuring programmed temperature reduction, which allows us to assess the catalyst's ability to be reduced as a percentage of metal in the form of zero oxidation state.

[0104] We then synthesize carbon nanotubes and evaluate the yields obtained 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).

[0105] The results of these tests are given in Table 3; the metals are expressed in atomic quantities:

[0106] [Table 3]

[0107] This example shows that the reducibility rate is significantly improved when cobalt is impregnated in a second step; the prior introduction of other metals is beneficial for preparing catalysts with the best activities. Example 4:

[0108] In this example, two impregnations are carried out:

[0109] 1) Impregnation of two metal salts from the groups (Pt and La, Pt and Mo, Ag and La, Ag and Mo) on alumina, followed by drying for 2 hours at 200°C, and calcination for 2 hours at 450°C,

[0110] 2)- Impregnation of the cobalt salt on the product obtained in 1) followed by drying for 2 hours at 200°C, and calcination for 2 hours at 450°C.

[0111] The reduction capacity of catalysts is evaluated by measuring programmed temperature reduction, which allows us to assess the catalyst's ability to be reduced as a percentage of metal in the form of zero oxidation state.

[0112] We then synthesize carbon nanotubes and evaluate the yields obtained 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).

[0113] The results of these tests are given in Table 3; the metals are expressed in atomic quantities:

[0114] [Table 4]

[0115] Example 5: In this example, two impregnations are carried out:

[0116] 1) Impregnation with cobalt salt, followed by drying for 2 hours at 200°C, and calcination for 2 hours at 450°C,

[0117] 2) Impregnation of the product obtained in 1) with a metal salt from the groups

[0118] 111 B, IVb, Vb, Vlb, VI, I, Vlllb, Ib, and llb of Mendeleev's periodic table, other than cobalt or iron, are heated on alumina, followed by drying for 2 hours at 200°C and calcination for 2 hours at 450°C. The reduction capacity of the catalysts is evaluated by measuring the programmed temperature reduction, which allows for the assessment of the catalyst's ability to be reduced as a percentage of metal in its zero oxidation state. Carbon nanotubes are then synthesized, and the yields obtained are evaluated by the ratio between the mass of carbon nanotubes produced and the amount of catalyst used (grams of carbon nanotubes per gram of catalyst calcined).

[0119] The results of these tests are given in Table 5; the metals are expressed in atomic quantities:

[0120] [Table 5]

[0121] We can see 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 metallic compounds from groups IB, IVb, Vb, VI, VI, VIIb, Ib and IIIb of Mendeleev's 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 in which one or more metal compounds from groups IB, IVb, Vb, Vlb, VI, I Vlllb, Ib and llb of the periodic table of Mendeleev 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 3 in which gamma alumina has an apparent specific surface area of ​​between 50 and 250 M 2 / g measured by the BET method (ASTM D 4641-17).

5. A catalyst preparation process according to claim 1 to 5 comprising the following steps: 1) Impregnation of at least one metal salt from groups IIIB, IVb, Vb, VIb, VI, I, VIIb, Ib and IIIb of Mendeleev's periodic table other than cobalt or iron on alumina, -Drying, -Calcination, 2) Impregnation of at least one cobalt salt onto the product obtained in 1) -Drying -Calcination.

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