RU2427423C1 - Metal-oxide catalyst for growing bundles of carbon nanotubes from gaseous phase - Google Patents

Abstract

FIELD: chemistry. ^ SUBSTANCE: invention relates to chemical catalysts for producing carbon nanotubes via catalytic pyrolysis of hydrocarbons. Described is a metal-oxide catalyst for growing bundles of carbon nanotubes from gaseous phase, containing iron, cobalt and aluminium oxides, molybdenum oxide in atomic ratio of molybdenum to iron, cobalt and aluminium from 1:10 to 1:50, wherein the atomic ratio of iron to cobalt ranges from 3:1 to 1:3. ^ EFFECT: catalyst enables to obtain bundles of carbon nanotubes with high output. ^ 3 dwg, 2 ex

Description

The invention relates to chemical technology for producing catalysts for the production of carbon nanotubes (CNTs) by the method of catalytic pyrolysis of hydrocarbons.

Further in the description, the following terms are used, which, although they are generally accepted by specialists in this field of technology, however, require clarification in the context of the claimed invention.

The term “catalyst containing oxides ... (of said metals)” means that the catalyst may contain oxides of these metals both in the form of a single crystalline or amorphous phase, and in the form of several phases, as well as in the form of clusters that do not form a phase. For example, the catalyst may comprise phases of alumina, iron oxide, cobalt ferrite; or the phases of aluminum oxide, iron molybdate, aluminum-cobalt spinel; or other combinations of phases. Some elements, such as molybdenum, with a low content in the catalyst, can be in the form of metal oxide clusters concentrated at the phase boundaries or on the surface of the catalyst particles. Moreover, the claimed limits of the content of elements correspond to the total composition of the catalyst, regardless of its structure and phase composition. However, for one purpose or another, a mechanical mixture of the particles of the inventive catalyst with some other components, for example, powders or fibers of various nature, can be prepared. This can be done to regulate certain technological parameters of the catalyst or carbon nanotubes grown on it. Also, the inventive catalyst can be deposited on the surface of various inert carriers (plates, powders, microspheres, macroporous carriers, fibers, and others) by impregnating them with a solution of precatalyst substances with subsequent heat treatment. In all these and similar cases, the preparation of these compositions, if carried out by methods known in the art, means using the claimed invention if the composition of the catalyst particles used to prepare their mechanical mixture with other components, or the composition of the surface metal oxide layer deposited on the surface of the carrier, or the composition of the solution of precatalysts falls within the range of compositions claimed in the present invention.

It should also be borne in mind that although the initial catalysts, usually used for growing carbon nanotubes by the method of catalytic pyrolysis of hydrocarbons, are usually a mixture of metal oxides, the real catalytically active particles on which carbon nanotubes grow are reduced forms of these catalysts containing transition metal particles. Therefore, taking into account the interconversions of transition metals and their oxides depending on the redox properties of the gas medium and temperature, sometimes the composition of the catalysts is arbitrarily designated as, for example, Al / Mo / Al ₂ O ₃ , Ni-Y / Mo, Mo / MgO, Ni / Mo / MgO, and the like, while implying that in the initial catalyst the transition metals are in the form of oxides, and in the reducing atmosphere of the carbon nanotube synthesis reactor, the transition metal oxides are reduced to metals.

It is known that single-walled CNTs, as a rule, form bundles consisting of many parallel oriented individual CNTs. The multiwalled CNTs obtained by the catalytic pyrolysis of hydrocarbons are usually randomly entangled aggregates of nanotubes, like cotton wool. For a number of applications, for example, in electrode materials of chemical current sources, it would be of interest to obtain beams of approximately parallel oriented multiwalled CNTs.

It is known that parallel oriented carbon nanotubes in the form of a "forest" grow on the surface of flat substrates, on which a thin catalyst layer is deposited in one way or another. An example is the work [1], in which a forest of multi-walled carbon nanotubes 10–100 μm long and 10–20 nm in diameter was grown on a silicon wafer with a surface layer of silicon dioxide on which a thin layer of iron catalyst was deposited (islands). A lot of similar publications are known. Such methods for producing arrays of oriented CNTs are used in electronics and in some other fields. The mass productivity of these methods is small. Therefore, the mass production of CNTs requires the use of dispersed catalysts with a developed surface. However, only a few of the known dispersed catalysts make it possible to obtain multiwalled CNTs in the form of beams.

Thus, the Ni / Mo / MgO catalyst was described in [2], which produces multi-walled CNTs with a diameter of 9–20 nm with a yield of about 45 parts by weight. CNTs from 1 parts by weight catalyst with a growth time of 60 minutes The process of growing nanotubes was carried out in a tubular reactor at 1000 ° C using a mixture of methane and hydrogen. As can be judged from the images presented in this work, the CNT bundles had a diameter of approximately 0.2-1 μm and a length of several tens of microns.

Morphology-like bundles of multi-walled CNTs were obtained from a mixture of methane with hydrogen at 1000 ° С on a Ni-Y / Mo catalyst [3] with a yield of about 30 wt.h. CNTs from 1 parts by weight catalyst at a growth time of 30 minutes The external diameter of individual nanotubes was in the range of 5–20 nm.

In [4], multiwalled CNTs were obtained from methane at 900–1200 ° С on a Mo / MgO catalyst. Depending on the conditions, the nanotubes grew in the form of beams or in the form of tangled individual tubes.

In [5], multiwalled CNT bundles were obtained by catalytic pyrolysis of methane at 900 ° С on a Mo / MgO catalyst.

Common essential features of the above technical solutions and the claimed invention is the presence in the composition of the catalyst of transition metal oxides and metal oxides not restored by hydrogen in the synthesis of carbon nanotubes.

The disadvantage of the above catalysts is that they operate at too high a temperature (900-1000 ° C), which makes it difficult to scale this process to industrial production.

In [6], a catalyst was described for growing bundles of multi-walled carbon nanotubes of the composition Fe / Al ₂ O ₃ . The catalyst is a mixed oxide of iron and aluminum. The atomic ratio of iron to aluminum was 1: 4. Using this catalyst, beams of multi-walled carbon nanotubes were obtained by catalytic pyrolysis of acetylene at a temperature of 800 ° C. Individual nanotubes had an average diameter of 35 nm, the beams, judging by the images given in [6], had a thickness of about 1 to 20 μm and a length of up to several tens of microns. The yield of CNTs was 24 parts by weight. from 1 parts by weight catalyst.

Common essential features of this catalyst and the claimed catalyst are the presence of iron and aluminum oxides in their composition.

The disadvantage of the considered catalyst is that it gives too thick nanotubes, and also, as our experiments have shown, a catalyst of this composition does not produce bundles of nanotubes when propylene and propane-butane are used as carbon source gases. Acetylene is explosive and its sales brands contain difficult to remove impurities, which makes it difficult to work with it. In addition, as our experience with acetylene shows, when acetylene is used to produce carbon nanotubes and nanofibers, the number of by-products, polycyclic aromatic hydrocarbons, which are environmentally harmful, increases. Propylene and propane-butane to a lesser extent produce harmful by-products.

Closest to the claimed invention is a catalyst composition FeCo / Al ₂ O ₃ described in the patent [7] (prototype). This work also describes a number of other catalyst compositions for growing carbon nanotubes, namely, FeCo / MgO, FeCo / Al ₂ O ₃ , FeMn / MgO, FeMo / MgO, NiCo / MgO, Co / MgO, FeCo / MgO, FeMn / MgO, FeMo / MgO, NiCo / MgO, Co / MgO, FeCo / MgO, as well as a number of catalysts in which compounds of catalytically active metals are supported on CaCO ₃ . However, there is no data on the possibility of growing beams of multi-walled carbon nanotubes on a FeCo / Al ₂ O ₃ catalyst. In our experiments on a catalyst of this composition, the formation of carbon nanotube bundles was not observed.

The basis of the claimed invention is the task, by introducing an additional component into the prototype catalyst and selecting the ratio of components, to ensure the production of carbon nanotubes in the form of bundles.

The problem is solved in that the metal oxide catalyst for growing bundles of carbon nanotubes from the gas phase, containing oxides of iron, cobalt and aluminum, additionally contains molybdenum oxide with an atomic ratio of molybdenum to the sum of iron, cobalt and aluminum from 1:10 to 1:50, and the atomic ratio of iron to cobalt is in the range from 3: 1 to 1: 3.

The following is information confirming the possibility of implementing the claimed invention.

For the implementation of the invention used the following materials and equipment.

Iron (III) nitrate Fe (NO ₃ ) ₃ · 9H ₂ O, grade "H".

Cobalt (II) nitrate Co (NO ₃ ) ₂ · 6H ₂ O, grade "H".

Ammonium molybdate (NH ₄ ) ₆ Mo ₇ O ₂₄ · 4H ₂ O, grade “ChDA”.

Aluminum nitrate Al (NO ₃ ) ₃ · 9H ₂ O, grade “ChDA”.

Citric Acid Food Monohydrate.

Hydrazine hydrate.

Electric stove with thermostat.

The muffle furnace.

Standard laboratory glassware and porcelain.

Laboratory horizontal quartz tube reactor.

Industrial reactor for producing carbon nanomaterials, NanoTechCenter LLC, Tambov.

An example of a catalyst synthesis.

3.31 g of ammonium molybdate in 40 ml of water was dissolved in a wide glass of heat-resistant glass with a capacity of 1 l. Then, with stirring, 72.0 g of iron nitrate of nine-water was added and mixed until completely dissolved. Then 223.2 g of citric acid, 36.4 g of cobalt nitrate, hexahydrate and 140.6 g of aluminum nitrate, nine-hydrous, were successively added. The mixture was heated on a tile with stirring. Within an hour, the temperature reached 60 ° C, a clear solution was obtained.

After complete dissolution of the components, the solution was allowed to cool to 50 ° C and 80 ml of hydrazine hydrate were added dropwise gradually over 20 minutes with continuous stirring. At the same time, cooling the glass in a water bath, it was ensured that the temperature of the solution, reaching 60 ° C, then stayed near this value (an exothermic reaction with hydrazine occurs). After all hydrazine hydrate was added, the temperature was maintained at 60 ° С for another 30 min. Then, the temperature of the solution was raised to 80 ° C for 15 minutes and the reaction mixture was kept for 1 hour with continuous stirring. The temperature was maintained by heating the beaker with the reaction mixture on a tile or by cooling it in a bath of cold water, depending on the reaction time (the reaction is exothermic at first, then the heat generation decreases). Gas evolution was observed (probably nitrogen due to the oxidation of hydrazine by nitro groups).

The resulting pink-brown clear solution was heat treated in small portions in porcelain cups by placing a cup with a portion of the solution in a muffle furnace at a temperature of 500 ° C. Then the catalyst was kept in a muffle furnace for 2 hours at 600 ° C. The finished catalyst was ground in a mortar until passing through a 0.1 mm sieve. The catalyst yield was as calculated (42.7 g). The catalyst was a light brown-gray powder with a bulk density of about 0.2 g / cm ³ . Under a microscope, the catalyst consists of thin flakes. Probably, these flakes are formed during foaming of a viscous solution during its heat treatment.

In the catalyst, the synthesis procedure of which is described above, the atomic ratio of metals is:

Mo: (Fe + Co + Al) = 1: 36.2.

Fe: Co = 1: 0.70.

Catalysts with other ratios of metal atoms were synthesized in the same way, while the calculated quantities of the starting components were taken.

Since there is no separation of components in this synthesis method, the atomic ratio of metals in the final product corresponds to the molar ratio of the starting metal salts taken for synthesis. Organic components, nitro groups and water decompose during heat treatment and are completely removed.

Getting carbon nanotubes

Carbon nanotubes were obtained by the method of catalytic pyrolysis of hydrocarbons. Propylene experiments were carried out in a horizontal flow quartz tube reactor with a diameter of 40 mm. The reactor was in a PT-1.2-70 tube furnace. A portion of the catalyst (20 mg) was placed on a graphlex substrate “Graphlex”. After connecting the nozzles for the input and output of gases, the reactor was purged with argon and heated to operating temperature (650 ° C). Then, for 30 minutes, the working gas mixture was passed (propylene 600 ml / min, hydrogen 800 ml / min, N.U.). After the end of the process, propylene and hydrogen were turned off, the reactor was purged with argon, and the substrate with carbon nanotubes grown on it was removed. The mass of the product (CNT + catalyst residues) was 650 mg. Thus, the mass of carbon nanotubes (630 mg) is 31.5 times the mass of the initial catalyst.

Experiments using a propane-butane mixture were carried out in an industrial reactor of NanoTechCenter LLC (Tambov). On the working surface of the reactor, a uniform layer of 6 g of catalyst was applied. The reactor was purged with argon and heated to 650 ° C, after which the propane-butane mixture was started at a rate of 11.25 l / min. The process was carried out for 40 min, after which the reactor was purged with argon. After cooling, the reactor was opened and the product was unloaded. The mass of the product was 180 g. Thus, the yield of CNTs is 180-6 = 172 g, which is 28.7 times the mass of the initial catalyst. The apparent (bulk) volume of the product was 12 dm ³ . Thus, the bulk density of the product was 15 g / DM ³ . It should be noted that the values of apparent volume and bulk density are approximate, since they depend on the degree of compaction of the product.

Electronic images of carbon nanotubes were obtained using a JEOL JSM - 6380 LV scanning electron microscope. Figure 1, 2 shows with different magnification the image of the bundles of nanotubes obtained in a laboratory horizontal tubular reactor using propylene as a carbon source. Figure 3 shows with different magnification the image of the bundles of nanotubes obtained in the industrial reactor of NanoTechcenter LLC using a propane-butane mixture as a carbon source. A significant part of the carbon nanotubes obtained using the inventive catalyst are obtained in the form of beams. As can be seen from figure 2, as the growth of the beams branch out, and at the final stage of growth are divided into separate nanotubes, forming a randomly interwoven structure. This form of nanotube aggregates, as can be assumed, is associated with the mechanism of their growth. It is likely that initially the growth of the nanotube bundle occurs on the external geometric surface of the catalyst flakes. However, as is known from the literature [8], during the growth of carbon nanotubes on Fe-Mo / Al ₂ O ₃ catalysts, the catalyst particles spontaneously split into smaller particles, up to nanosized particles. This process occurs due to the repulsion of catalyst particles by growing carbon nanotubes. Thus, the beam splitting observed in the above images corresponds to the splitting of the catalyst particles (assuming the root growth mechanism of nanotubes). When the splitting reaches very thin beams, they become quite flexible and form a randomly entangled structure.

The diameter of individual nanotubes grown using propylene is 10 nm, while from propane-butane on the same catalyst thicker tubes with a large diameter spread (about 30-60) nm are obtained.

The experiments showed that the growth effect of carbon nanotube bundles is observed when the atomic ratio of molybdenum to the sum of iron, cobalt and aluminum is from 1:10 to 1:50 and the atomic ratio of iron and cobalt is in the range from 3: 1 to 1: 3.

The inventive catalysts allow to obtain in high yield bundles of high-quality carbon nanotubes. The invention may find application for the industrial production of carbon nanotubes.

Information sources

1. Bronikowski M.J. CVD growth of carbon nanotube bundle arrays // Carbon, 2006, vol. 44, p. 2822-2832.

2. Li Y, Zhang X.B., Tao X. Y., Xu J.M., Huang W.Z, Luo J.H., Luo Z.Q., Li T., Liu F., Bao Y., Geise H.J. Mass production of high-quality multi-walled carbon nanotube bundles on a Ni / Mo / MgO catalyst // Carbon, 2005, vol. 43, p. 295-301.

3. Perez-Mendoza M., Valles C., Maser W.K., Martmez M.T., Langlois S., Sauvajol J.L., Benito A.M. Ni-Y / Mo catalyst for the large-scale CVD production of multi-wall carbon nanotubes // Carbon, 2005, vol. 43, p.3034-3037.

4. Li Y., Zhang X., Tao X., Xu J., Chen F., Huang W., Liu F. Growth mechanism of multi-walled carbon nanotubes with or without bundles by catalytic deposition of methane on Mo / MgO // Chemical Physics Letters, 2004, vol. 386, p. 105-110.

5. Jia Y., He L., Kong L., Liu J., Guo Z., Meng F., Luo T., Li M., Liu J. Synthesis of close-packed multi-walled carbon nanotube bundles using Mo as catalyst // Carbon, 2009, v. 47, p. 1652-1658.

6. Wang XQ, Li L., Chu NJ, Liu YP, Jin HX, Ge HL Lamellar Fe / Al ₂ O ₃ catalyst for high-yield production of multi-walled carbon nanotubes bundles // Materials Research Bulletin, 2009, vol. 44, p. 422-425.

7. Pat. RF 2373995. Kuznetsov V.L., Usoltseva A.N. A method of obtaining highly supported supported catalysts and the synthesis of carbon nanotubes. 11/27/2009. IPC B01J 37/00, B01J 23/74, C01B 31/00, B82B 3/00, B01J 21/00.

8. Hao Y., Qunfeng Z., Fei W., Weizhong Q., Guohua L. Agglomerated CNTs synthesized in a fluidized bed reactor: Agglomerate structure and formation mechanism // Carbon, 2003, vol. 41, p. 2855-2863 .

Claims (1)

A metal oxide catalyst for growing bundles of carbon nanotubes from the gas phase, containing iron, cobalt and aluminum oxides, characterized in that it further comprises molybdenum oxide with an atomic ratio of molybdenum to the sum of iron, cobalt and aluminum from 1:10 to 1:50, the atomic ratio iron and cobalt ranges from 3: 1 to 1: 3.

Images