Properties of carbon nanotubes. Nanotechnology: Carbon Nanotubes Carbon Nanotube Cleaning
The invention relates to the field of sorption treatment of surface and underground waters with a high content of titanium and its compounds and can be used to purify water to produce safe drinking water. A method for cleaning surface and groundwater from titanium and its compounds involves contacting contaminated water with an adsorbent, where carbon nanotubes are used as adsorbent, which are placed in an ultrasonic bath and act on carbon nanotubes and purified water in a mode of 1-15 minutes, with a frequency ultrasound 42 kHz and a power of 50 watts. The technical result consists in 100% water purification from titanium and its compounds due to the very high adsorption performance of carbon nanotubes. 4 ill., 2 tablets, 4 ave.
Figures to the patent of the Russian Federation 2575029
The invention relates to the field of sorption treatment of surface and underground waters with a high content of titanium and its compounds and can be used to purify water from titanium and its compounds to obtain safe drinking water.
A known method of purifying water from heavy metal ions, according to which calcined activated natural adsorbent is used as adsorbent, which is siliceous rock of mixed mineral composition of the deposits of Tatarstan, containing wt.%: Opalcristobolite 51-70, zeolite 9-25, clay component - mont morillonite, hydromica 7-15, calcite 10-25, etc. [RF patent 2150997, IPC B01G 20/16, B01G 20/26, publ. 06/20/2000]. A disadvantage of the known method is the use of hydrochloric acid for activation of the material, which requires equipment that is resistant to aggressive environments. In addition, the method uses a rather rare rock of complex mineral composition and there is no data on the content of titanium and its compounds.
A known method of producing granular adsorbent based on shungite [Auth. USSR No. 822881, IPC B01G 20/16, publ. 04/23/1981].
The disadvantage of this method is the use of the rare mineral shungite, which is pre-modified with ammonium nitrate, calcination at high temperature, which requires appropriate equipment and energy consumption, as well as processing in aggressive environments. There is no data on the effectiveness of water purification from titanium.
The known method, taken as an analogue, of obtaining organomineral sorbents based on natural aluminosilicates, namely zeolite, by modifying pre-treated aluminosilicate with polysaccharides, in particular chitosan [RF Patent No. 2184607, IPC C02F 1/56, B01J 20/32, B01J 20/26 , B01J 20/12, publ. 07/10/2002]. The method allows to obtain sorbents suitable for the effective purification of aqueous solutions of metal ions and organic dyes of various nature.
The disadvantages of the sorbents obtained by the described method are their high degree of dispersion, which does not allow for the purification of water by current through the sorbent layer (the filter quickly clogs), as well as the possibility of washing away the chitosan layer from the sorbent over time due to the lack of fixing it on a mineral basis data on effective cleaning of compounds of heavy metals, such as titanium and its compounds.
Describes a method of clarification and disposal of industrial water filter structures of water treatment plants [Patent for invention RU No. 2372297, IPC C02F 1/5, C02F 103/04, publ. November 10, 2009].
The invention consists in the use of a complex coagulant, which is a mixture of aqueous solutions of aluminum sulfate and oxychloride in a dose ratio of 2: 1 on aluminum oxide.
This patent provides examples of groundwater treatment for drinking water supply.
The disadvantage of the described method is the poor efficiency of purification from impurities, 46% of the sediment has surfaced, and the rest was in suspension.
A known method of water treatment by treatment in the feed pipe with a cationic flocculant [RF Patent No. 2125540, IPC C02F 1/00, publ. 01/27/1999].
The invention relates to methods for treating water from surface drains and can be used in the field of domestic, drinking or industrial water supply.
The essence of the invention: in addition to the flocculant, mineral coagulant is introduced into the pipeline in a mass ratio to the flocculant from 40: 1 to 1: 1.
The method provides an increase in the efficiency of aggregation of suspended solids, which allows to reduce the turbidity of standing water by 2-3 times. After using this method, further complete sedimentation in sedimentation tanks is necessary. Thus, according to the described method, 100% purification from metals was not achieved, water hardness decreased from 5.7 mEq / L to 3 mEq / L, turbidity decreased to 8.0 mg / L.
A disadvantage of the analogue is the low efficiency of cleaning metals and organic impurities; there is no data on the titanium content.
The sorption efficiency of carbon nanotubes (CNTs) is described as the basis of an innovative technology for the purification of water-ethanol mixtures [Zaporotskova NP et al. Vestnik VolSU, series 10, no. 5, 2011, 106 pp.].
Quantum-mechanical studies of adsorption of heavy alcohol molecules on the outer surface of single-walled carbon nanotubes are performed.
The drawback of the described sorption activity of CNTs is only theoretical quantum-mechanical calculations, and experimental studies have been carried out for alcohols. There are no examples for cleaning metals.
The positive effect of carbon nanotubes on the process of purification of water-ethanol mixtures is proved.
Currently, special hopes in the development of many fields of science and technology are associated with carbon nanotubes of CNTs [Harris P. Carbon nanotubes and related structures. New materials of the XXI century. - M .: Technosphere, 2003. - 336 p.].
A remarkable feature of CNTs is associated with their unique sorption characteristics [A. Yeletsky Sorption properties of carbon nanostructures. - Advances in physical sciences. - 2004. -T. 174, No. 11. - S. 1191-1231].
A filter based on carbon nanotubes for cleaning alcohol-containing liquids is described [Polikarpova NP et al. Vestnik VolSU, series 10, no. 6, 2012, 75 pp.]. Experiments on the purification of alcohol-containing liquids by filtration and transmission methods were carried out, the mass fraction of CNTs was established, leading to the best result.
The performed experimental studies have proved that the treatment of a water-ethanol mixture of CNTs reduces the content of fusel oils and other substances. The disadvantage of this analogue is the lack of data on water purification from metals.
We studied the sorption / desorption of Zn (II) in successive cycles with activated carbon and CNTs. Adsorption of Zn (II) by activated carbon sharply decreased after several cycles, which is explained by the low removal of metal ions from the inner pore surface of activated carbon.
The hydrophobic nature of CNTs determines their weak interaction with water molecules, creating conditions for its free flow.
Noy A., Park N.G., Fornasiero F., Holt J.K., Grigoropoulos C.P. and Bakajin O. Nanofluidics in carbon nanotubes // Nano Today. 2007, vol. 2, no. 6, pp. 22-29.
The adsorption capacity of CNTs depends on the presence of functional groups on the surface of the adsorbent and the properties of the adsorbate.
So, for example, the presence of carboxyl, lactone and phenolic groups increases the adsorption capacity for polar substances.
CNTs, on the surface of which there are no functional groups, are characterized by a high adsorption capacity for non-polar pollutants.
One way to create a membrane is to grow CNTs on a silicon surface using carbon-containing vapors using nickel as a catalyst.
CNTs are molecular structures resembling straws made of carbon sheets a fraction of a nanometer 10 -9 m thick; in fact, this is an atomic layer of ordinary graphite, twisted into a tube, one of the most promising materials in the field of nanotechnology. CNTs can also have a detailed structure [WCG website http://www.worldcommunitygrid.org/].
Membrane technology, which is widely used to produce drinking water for the inhabitants of our planet.
There are two significant drawbacks - energy consumption and membrane fouling, which can only be removed by chemical means.
Productive and antifouling membranes can be created on the basis of carbon nanotubes or graphene [M. Majumder et al. Nature 438, 44 (2005)].
Closest to the claimed invention in technical essence and the achieved result is a method for producing sorbents for water purification [RF Patent 2277013 C1, IPC B01J 20/16, B01J 20/26, B01J 20/32, publ. 12/01/2004]. This patent is taken as a prototype. This method relates to the field of sorption water purification, specifically to the production of sorbents and purification methods, and can be used to purify drinking or industrial water with a high content of heavy metal ions and polar organic substances. The method includes treating natural aluminosilicate with a solution of chitosan in dilute acetic acid in a ratio of aluminosilicate to a solution of chitosan equal to 1: 1, at a pH of 8-9.
In the table. 1 shows a comparative characteristic of the sorbents obtained according to the invention, taken as a prototype [Patent 2277013]. Examples are given for sorption with respect to dyes and for sorption of copper, iron and other metal ions from solutions.
The disadvantage of the prototype is the low adsorption capacity with respect to heavy metals (SOE) mg / l for copper Cu +2 (from 3.4 to 5.85), there is no data on the adsorption of titanium and its compounds. SOE, mg / L for Fe +3 varies from 3.4 to 6.9.
The objective of the invention is to develop a method for purifying surface and groundwater from titanium and its compounds using carbon nanotubes and the action of ultrasound, which will provide high-quality clean drinking water, increase the efficiency of surface and ground water treatment due to the high adsorption rates of CNTs.
The problem is solved by the proposed method for purifying surface and groundwater from titanium and its compounds using CNTs by exposure to 50 W ultrasound with an ultrasound frequency of 42 kHz for 1-15 minutes.
The method is as follows. The adsorbent is a single-walled carbon nanotube with the ability to enter into active interaction with titanium atoms and its cations (Ti, Ti + 2, Ti + 4).
One gram of CNTs of 98% purity is added to 99 g of water for cleaning from Ti, Ti + 2, Ti + 4, and then all the contents are placed in an ultrasonic bath UH-3560 and sonicated for 1-15 minutes with a power of 50 watts and with a frequency ultrasound 42 kHz.
After filtering, water samples taken for analysis are examined. Atomic emission analysis is used to determine the content of titanium and its compounds in water samples before processing CNTs and after processing water samples of CNTs in an ultrasonic bath.
The proposed "Method for the purification of surface and groundwater from titanium and its compounds using carbon nanotubes and ultrasound" is confirmed by the examples that will be described later.
The implementation of the method in accordance with the specified conditions allows to obtain absolutely pure water with a zero content of titanium and its compounds (Ti, Ti +2, Ti +4).
The technical result is achieved by the fact that CNTs act as a capillary, sucking in Ti atoms and titanium cations Ti +2 and Ti +4, the sizes of which are comparable to the inner diameter of CNTs. The diameter of CNTs varies from 4.8 Å to 19.6 Å depending on the conditions for obtaining CNTs.
It has been experimentally proved that CNT cavities are actively filled with various chemical elements.
An important feature that distinguishes CNTs from other known materials is the presence of an internal cavity in the nanotube. The Ti atom and its cations Ti + 2, Ti + 4 penetrate into the CNT under the influence of external pressure or as a result of the capillary effect and are retained there due to sorption forces [Dyachkov PN Carbon nanotubes: structure, properties, application. - M .: Binom. Laboratory of Knowledge, 2006. - 293 p.].
This provides the possibility of selective adsorption by nanotubes. In addition, the highly curved surface of CNTs allows the adsorption of rather complex atoms and molecules, in particular Ti, Ti +2, Ti +4, on its surface.
Moreover, the efficiency of nanotubes is ten times greater than the activity of graphite adsorbents, which are by far the most common means of purification. CNTs can adsorb impurities both on the outer surface and on the inside, which allows selective adsorption.
Therefore, CNTs can be used for the final purification of various liquids from impurities of ultra-low concentrations.
For CNTs, the high specific surface area of \u200b\u200bCNT material is attractive, reaching values \u200b\u200bof 600 m 2 / g or more.
Such a high specific surface area, several times higher than the specific surface area of \u200b\u200bthe best modern sorbents, opens the possibility of their use for cleaning surface and groundwater from heavy metals, in particular Ti, Ti +2, Ti +4.
CNT synthesis. Using a CVDomna carbon nanotube synthesis unit, carbon nanotubes CNTs were obtained, which was used to purify titanium and its compounds from surface and underground waters.
Experimental studies on the purification of water from titanium and its compounds have been carried out.
To determine the optimal amount of CNTs, it is necessary to bring the content of titanium and its compounds to extremely small amounts. Such a concentration of CNTs was found, and in subsequent experiments, the optimal concentration was used in an amount of 0.01 g per 1 liter of analyzed water.
Atomic emission analysis showed the presence of atomic Ti and its cations (Ti + 2, Ti + 4) in the studied water samples, from which we can conclude that it is titanium and Ti + 2, Ti + 4 cations that interact with carbon nanotubes. The radius of the Ti atom is 147 pm, i.e. titanium cations can both intercalate into the cavity of a carbon nanotube and adsorb inside (Fig. 1), and adsorb on its outer surface, also forming a bridge structure with carbon atoms of hexagons (Fig. 2), forming bound molecular structures.
The introduction of Ti and its cations into the CNT cavity is possible by stepwise approximation of Ti to the nanotube along its main longitudinal axis and the penetration of titanium atoms and its cations into the nanotube cavity with their further adsorption on the inner surface of the CNT. Another variant of Ti adsorption is also known, according to which one titanium atom can create stable Ti-C bonds with carbon atoms on the outside of a carbon nanotube in two simple cases when Ti is in 1/4 and 1/2 of all hexagons (Fig. 3) .
That is, the adsorption of titanium and its cations on the surface of CNTs is not only theoretically proven fact, but also experimentally proven in studies.
The inventive sorbent is a conglomerate of single-walled carbon nanotubes with the ability to actively interact with titanium and its cations, forming stable bonds, and the possibility of adsorption of titanium atoms and its compounds on the inner and outer surfaces of CNTs with the formation of bridge structures with two Ti-C bonds, if Ti +2 or four for Ti +4. When treating water contaminated with titanium and its compounds, CNTs are used, titanium is adsorbed on the surfaces of CNTs due to the van der Waals forces, i.e., titanium and its compounds from a free atom and cations Ti + 2 and Ti + 4 become molecular connection (Fig. 4).
The possibility of carrying out the invention is illustrated by the following examples.
Example 1. Underground water from a well 1) 40 m deep was taken for research on the content of high-quality elemental composition, as well as a quantitative analysis on the content of titanium and its compounds before purification using CNTs and after adsorption of CNTs and sonication. Ultrasound exposure time 15 min. The content of Ti and its compounds after purification is 0% (table. 2).
Example 2. Groundwater from a well 2) 41 m deep, unlike well 1) this water was 200 m from well 1) of the Bereslavsky reservoir (Volgograd). Ultrasound exposure time 15 min. The content of Ti and its compounds after purification 0% according to the invention (table. 2).
Example 3. Water was taken from a faucet (Sovetsky district, Volgograd) was cleaned using CNTs and exposed to ultrasound for 15 minutes, a power of 50 W and an operating frequency of ultrasound of 42 kHz (Table 2).
Example 4. Everything is the same as in example 1, but the exposure time of ultrasound is 1 min.
Example 5. Groundwater from a well 1) 40 m deep was taken for analysis for the content of titanium and its compounds, and then subjected to purification according to the prototype [Patent RU 2277013].
The time of exposure to ultrasound is 15 minutes (experiment 1, 2, 3, 5). Ultrasound exposure time 1 min (experiment 4).
The advantages of the claimed method based on CNTs include a very high degree of adsorption of titanium and its compounds. According to the results of the experiment, 100% purification of the studied waters from titanium and its compounds under optimal conditions is provided.
CLAIM
A method of purifying surface and groundwater from titanium and its compounds using carbon nanotubes (CNTs) and ultrasound, including contacting contaminated water with adsorbents for trapping heavy metals, characterized in that carbon nanotubes are used as adsorbent and placed in an ultrasonic bath acting on CNTs and purified water in the regime of 1-15 minutes, with an ultrasound frequency of 42 kHz and a power of 50 watts.
Owners of patent RU 2430879:
The invention relates to nanotechnology and can be used as a component of composite materials. Multilayer carbon nanotubes are obtained by the pyrolysis of hydrocarbons using catalysts containing Fe, Co, Ni, Mo, Mn and their combinations as active components, as well as Al 2 O 3, MgO, CaCO 3 as carriers. The obtained nanotubes are purified by boiling in a solution of hydrochloric acid with further washing with water. After acid treatment, heating is carried out in a stream of high-purity argon in a furnace with a temperature gradient. In the working area of \u200b\u200bthe furnace, the temperature is 2200-2800 ° C. At the edges of the furnace, the temperature is 900-1000 ° C. The invention allows to obtain multilayer nanotubes with a metal impurity content of less than 1 ppm. 3 s.p. f-ly, 9 ill., 3 tab.
The invention relates to the field of producing high-purity multilayer carbon nanotubes (MWCNTs) with a metal impurity content of less than 1 ppm, which can be used as components of composite materials for various purposes.
For mass production of MWCNTs, methods based on the pyrolysis of hydrocarbons or carbon monoxide in the presence of metal catalysts based on metals of the iron subgroup are used [T.W. Ebbesen // Carbon nanotubes: Preparation and properties, CRC Press, 1997, p.139-161; V.Shanov, Yeo-Heung Yun, M.J. Schuiz // Synthesis and characterization of carbon nanotube materials (review) // Journal of the University of Chemical Technology and Metallurgy, 2006, No. 4, v.41, p.377-390; J.W. Seo; A. Magrez; M. Milas; K. Lee, V Lukovac, L. Forro // Catalytically grown carbon nanotubes: from synthesis to toxicity // Journal of Physics D (Applied Physics), 2007, v.40, n.6]. Because of this, MWCNTs obtained with their help contain metal impurities of the used catalysts. At the same time, for a number of applications, for example, to create electrochemical devices and to obtain composite materials for various purposes, high-purity MWCNTs containing no metal impurities are required. High-purity MWCNTs are primarily required to obtain composite materials subjected to high-temperature processing. This is due to the fact that inorganic inclusions can be catalysts for local graphitization and, as a result, initiate the formation of new defects in the carbon structure [A.S. Fialkov // Carbon, interlayer compounds and composites based on it, Aspect Press, Moscow, 1997, p. 588 -602]. The mechanism of the catalytic action of metal particles is based on the interaction of metal atoms with a carbon matrix with the formation of metal-carbon particles with the subsequent release of new graphite-like formations that can destroy the structure of the composite. Therefore, even small impurities of metals can lead to a violation of the homogeneity and morphology of the composite material.
The most common methods for cleaning impurities from catalytic carbon nanotubes are based on their treatment with a mixture of acids with different concentrations when heated, and also in combination with exposure to microwave radiation. However, the main drawback of these methods is the destruction of the walls of carbon nanotubes as a result of exposure to strong acids, as well as the appearance of a large number of undesirable oxygen-containing functional groups on their surface, which complicates the selection of conditions for acid treatment. The purity of the obtained MWCNTs is 96-98 wt.%, Since metal catalyst particles are encapsulated in the inner cavity of a carbon nanotube and are inaccessible to reagents.
The purity of MWCNTs can be improved by heating them at temperatures above 1500 ° C while maintaining the structure and morphology of carbon nanotubes. These methods can not only clean the MWCNTs of metal impurities, but also help to order the structure of carbon nanotubes by annealing small defects, increase Young's modulus, decrease the distance between graphite layers, and remove surface oxygen, which further ensures more uniform dispersion of carbon nanotubes in polymer matrix, necessary to obtain better composite materials. Calcination at a temperature of about 3000 ° C leads to the formation of additional defects in the structure of carbon nanotubes and the development of existing defects. It should be noted that the purity of carbon nanotubes obtained using the described methods is not more than 99.9%.
The invention solves the problem of developing a method for cleaning multilayer carbon nanotubes obtained by catalytic pyrolysis of hydrocarbons, with almost complete removal of catalyst impurities (up to 1 ppm), as well as impurities of other compounds that may appear during the acid treatment of MWNTs, while maintaining the morphology of carbon nanotubes.
The problem is solved by the method of purification of multilayer carbon nanotubes obtained by the pyrolysis of hydrocarbons using catalysts containing Fe, Co, Ni, Mo, Mn and their combinations as active components, as well as Al 2 O 3, MgO, CaCO 3 as carriers, which is carried out boiling in a solution of hydrochloric acid with further washing with water, after acid treatment, heating is carried out in a stream of high-purity argon in a furnace with a temperature gradient, in the working zone the temperature is 2200-2800 ° C, at the edges of the furnace the temperature is 900-1000 ° C, in p which result obtained multilayer nanotubes with a content of metallic impurities of less than 1 ppm.
Warming up is carried out in ampoules made of high-purity graphite.
The warm-up time in an argon flow is, for example, 15-60 minutes.
Use argon with a purity of 99.999%.
A significant difference of the method is the use of a temperature gradient furnace for cleaning MWCNTs, where metal impurities evaporate in the hot zone and metal particles condense in the form of small balls in the cold zone. To carry out the transfer of metal vapors, a stream of high-purity argon (with a purity of 99.999%) with a gas flow rate of about 20 l / h is used. The furnace is equipped with special seals to prevent exposure to atmospheric gases.
Preliminary desorption of water and oxygen from the surface of MWCNTs and a graphite ampoule, in which the sample is placed in a graphite furnace, as well as purging them with high-purity argon, avoid the effect of gas-transport reactions involving hydrogen and oxygen-containing gases that lead to redistribution of carbon between its finely dispersed carbon nanotubes forms and well crystallized graphite-like forms with low surface energy (VLKuznetsov, Yu.V. Butenko, VIZaikovskii and ALChuvilin // Carbon redistribution processes in nanocarbons // Carbon 42 (20 04) pp.1057-1061; A.S. Fialkov // Processes and apparatus for the production of carbon powder graphite materials, Aspect Press, Moscow, 2008, p. 510-514).
Catalytic carbon multilayer nanotubes are obtained by the pyrolysis of hydrocarbons using catalysts containing Fe, Co, Ni, Mo and their combinations as active components, as well as Al 2 O 3, MgO, CaCO 3 as carriers (T.W. Ebbesen // Carbon nanotubes: Preparation and properties, CRC Press, 1997, p. 139-161; V. Shanov, Yeo-Heung Yun, MJ Schuiz // Synthesis and characterization of carbon nanotube materials (review) // Journal of the University of Chemical Technology and Metallurgy, 2006, No. 4, v.41, p. 377-390; JWSeo; A. Magrez; M. Milas; K. Lee, V Lukovac, L. Forro // Catalytically grown carbon nanotubes: from synthesis to toxicity / / Journal of Physics D (Applied Physics), 2007, v. 40, n. 6).
In the proposed method, to demonstrate the possibility of removing impurities of the most typical metals, purification is carried out for two types of MWCNTs synthesized on Fe — Co / Al 2 O 3 and Fe — Co / CaCO 3 catalysts containing Fe and Co in a 2: 1 ratio. One of the most important features of using these catalysts is the absence of carbon phases other than MWCNTs in the synthesized samples. In the presence of a Fe — Co / Al 2 O 3 catalyst, MWCNTs with average external diameters of 7–10 nm are obtained, and in the presence of a Fe — Co / CaCO 3 catalyst, MWCNTs with large average external diameters of 22–25 nm are obtained.
The obtained samples are examined by transmission electron microscopy, X-ray spectral fluorescence method on an ARL-Advant "x analyzer with an Rh-anode of an X-ray tube (measurement accuracy ± 10%), and the specific surface area of \u200b\u200bthe samples is measured by the BET method.
According to TEM, the initial samples consist of highly defective MWCNTs (Figs. 1, 6). Fragments of the tubes in the bend area have smooth, rounded contours; a large number of fullerene-like formations are observed on the surface of the tubes. Graphene-like layers of nanotubes are characterized by the presence of a large number of defects (discontinuities, Y-like compounds, etc.). In some sections of the tubes, there is a mismatch in the number of layers on different sides of the MWCNTs. The latter indicates the presence of open extended graphene layers, mainly localized inside the tubes. Electron microscopic images of heated MWNTs in a stream of high-purity argon at temperatures of 2200 ° C - Fig.2, 7; 2600 ° C - Fig. 3, 8; 2800 ° С - Figs. 4, 5, 9. In the samples after calcination, more even MWNTs with a smaller number of both internal and surface defects are observed. The tubes consist of rectilinear fragments of the order of hundreds of nanometers with clearly defined kinks. With an increase in the calcination temperature, the sizes of straight sections increase. The number of graphene layers in the walls of the tubes from different sides becomes the same, which makes the structure of MWCNTs more ordered. The inner surface of the tubes also undergoes significant changes - metal particles are removed, the internal partitions become more ordered. Moreover, the ends of the tubes are closed — graphene layers forming the tubes are closed.
Annealing of the samples at 2800 ° C leads to the formation of a small number of enlarged carbon formations of a cylindrical shape, consisting of graphene layers embedded in each other, which can be associated with the transfer of carbon over short distances due to an increase in the vapor pressure of graphite.
Studies of the samples of the initial and heated MWCNTs by the X-ray fluorescence method showed that after heating the samples of multilayer carbon nanotubes at temperatures in the range 2200-2800 ° C, the amount of impurities decreases, which is also confirmed by transmission electron microscopy. The heating of MWCNT samples at 2800 ° С provides almost complete removal of impurities from the samples. At the same time, not only the impurities of the catalyst metals are removed, but also the impurities of other elements entering the MWCNTs at the stages of acid treatment and washing. In the initial samples, the ratio of iron to cobalt is approximately 2: 1, which corresponds to the initial composition of the catalysts. The aluminum content in the initial tubes obtained on Fe — Co / Al 2 O 3 catalyst samples is small, which is associated with its removal during the treatment of the nanotubes with acid during the washing of the catalyst. The results of the study of the content of impurities by x-ray fluorescence method are shown in tables 1 and 2.
Measurement of the specific surface by the BET method showed that with increasing temperature the specific surface of the MWCNT samples changes insignificantly with the structure and morphology of carbon nanotubes being preserved. According to TEM, a decrease in the specific surface can be attributed both to the closure of the ends of the MWCNTs and to a decrease in the number of surface defects. With increasing temperature, a small fraction of enlarged formations of a cylindrical shape with an increased number of layers and a length to width ratio of approximately 2-3 is possible, which also contributes to a decrease in the specific surface. The results of the study of the specific surface are shown in table 3.
The invention is illustrated by the following examples, tables (tables 1-3) and illustrations (Fig.1-9).
A portion of MWCNTs (10 g), obtained by pyrolysis of ethylene in the presence of an Fe-Co / Al 2 O 3 catalyst in a flowing quartz reactor at a temperature of 650-750 ° C, is placed in a graphite ampoule 200 mm high and with an external diameter of 45 mm and closed with a lid ( 10 mm in diameter) with a hole (1-2 mm in diameter). A graphite ampoule is placed in a quartz ampoule and the air is pumped out with a vacuum pump to a pressure of at least 10 -3 Torr, followed by purging with high-purity argon (purity 99.999%), first at room temperature, and then at a temperature of 200-230 ° C to remove oxygen-containing surface groups and traces of water. The sample is heated at a temperature of 2200 ° C for 1 h in a stream of high-purity argon (~ 20 l / h) in a furnace with a temperature gradient, where in the working zone the temperature is kept at 2200 ° C and the temperature at the edges of the furnace is 900-1000 ° FROM. Metal atoms evaporated during heating from MWNTs are removed from the hot part of the furnace to the cold stream of argon, where metal is deposited in the form of small balls.
After calcination, a study of the obtained material is carried out by transmission electron microscopy and X-ray fluorescence method. Figure 1 shows the electron microscopic images of the original MWCNTs, and Fig.2 - heated MWCNTs at 2200 ° C. Using the BET method, the specific surface area of \u200b\u200bMWCNT samples is determined before and after calcination. The data obtained indicate a slight decrease in the specific surface of the samples after calcination when compared with the specific surface of the initial sample of MWCNTs.
Analogously to example 1, characterized in that a portion of the initial MWCNTs is heated at 2600 ° C for 1 h in a stream of high-purity argon (~ 20 l / h) in a furnace with a temperature gradient, where the temperature remains in the working zone and is 2600 ° C, the edges of the furnace temperature is 900-1000 ° C. Images of heated MWNTs obtained by transmission electron microscopy are shown in FIG. 3. In high resolution TEM images, the closed ends of nanotubes are visible.
Analogously to example 1, characterized in that a portion of the initial MWCNTs is heated at 2800 ° C for 15 min in a stream of high-purity argon (~ 20 l / h) in a temperature gradient furnace, where the temperature remains in the working zone and is 2800 ° C, the edges of the furnace temperature is 900-1000 ° C. Images of heated MWNTs obtained by transmission electron microscopy are shown in FIG. 4.
Annealing at 2800 ° C leads to the formation of a small number of enlarged formations of a cylindrical shape with an increased number of layers and a length to width ratio of approximately 2-3. These enlargements are visible in TEM images (Figure 5).
Analogously to example 1, characterized in that the initial MWCNTs are obtained in the presence of a Fe-Co / CaCO 3 catalyst. Images of the initial MWCNTs and MWCNTs heated at 2200 ° С obtained by transmission electron microscopy are shown in Figs. 6, 7, respectively. The TEM images of the initial MWCNTs show metal particles encapsulated in the channels of the tubes (marked by arrows).
Analogously to example 4, characterized in that the sample of the initial MWCNT is heated at 2600 ° C. Images of heated MWNTs obtained by transmission electron microscopy are shown in Fig. 8. In high resolution TEM images, the closed ends of nanotubes are visible.
Analogously to example 4, characterized in that a portion of the initial MWCNT was heated at 2800 ° C for 15 minutes. Images of heated MWNTs obtained by transmission electron microscopy are shown in Fig.9. The images show the formation of a small proportion of enlargement.
Table 1 | ||||
Data of the X-ray spectral fluorescence method on the content of impurities in MWCNTs after heating, obtained using a Fe-Co / Al 2 O 3 catalyst | ||||
Element | ||||
Source MWCNTs | MWNT_2200 ° C example 1 | MWNT_2600 ° C example 2 | MWNT_2800 ° C example 3 | |
Fe | 0.136 | 0.008 | traces | traces |
With | 0.0627 | traces | traces | traces |
Al | 0.0050 | traces | traces | traces |
Sa | traces | 0.0028 | 0.0014 | traces |
Ni | 0.0004 | traces | traces | traces |
Si | 0.0083 | 0.0076 | traces | not |
Ti | not | 0.0033 | traces | traces |
S | traces | not | not | not |
Cl | 0.111 | not | not | not |
Sn | 0.001 | 0.001 | traces | traces |
Ba | not | not | not | not |
Cu | 0.001 | 0.001 | traces | traces |
traces - element content below 1 ppm |
table 2 | ||||
X-ray fluorescence data on the content of impurities in MWCNTs after heating, obtained using a Fe-Co / CaCO 3 catalyst | ||||
Element | Estimation of the content of impurities, wt.% | |||
Source MWCNTs | MWNT_2200 ° C example 4 | MWNT_2600 ° C example 5 | MWNT_2800 ° C example 6 | |
Fe | 0.212 | 0.0011 | 0.0014 | 0.001 |
With | 0.0936 | traces | traces | traces |
Al | 0.0048 | traces | traces | traces |
Sa | 0.0035 | 0.005 | 0.0036 | traces |
Ni | 0.0003 | traces | traces | traces |
Si | 0.0080 | 0.0169 | 0.0098 | traces |
Ti | not | traces | 0.0021 | 0.0005 |
S | 0.002 | not | not | not |
Cl | 0.078 | not | not | not |
Sn | 0.0005 | traces | traces | traces |
Ba | 0.008 | not | not | not |
Cu | traces | traces | traces | traces |
Table 3 | |
BET specific surface area of \u200b\u200bthe initial and heated images of MWCNTs | |
MWCNT sample (catalyst) | S beats, m 2 / g (± 2.5%) |
MWNT_ex (Fe-Co / Al 2 O 3) | 390 |
MWCNT_2200 (Fe-Co / Al 2 O 3) example 1 | 328 |
MWNT_2600 (Fe-Co / Al 2 O 3) example 2 | 302 |
MWCNT_2800 (Fe-Co / Al 2 O 3) example 3 | 304 |
MWCNT_ex (Fe-Co / CaCO 3) | 140 |
MWCNT_2200 (Fe-Co / CaCO 3) example 4 | 134 |
MWNT_2600 (Fe-Co / CaCO 3) example 5 | 140 |
MWNT_2800 (Fe-Co / CaCO 3) example 6 | 134 |
Captions to figures:
Figure 1. Electron microscopic images of the initial sample of MWCNTs synthesized on a Fe — Co / Al 2 O 3 catalyst. On the left is a low resolution TEM image. Right, bottom - high resolution TEM image on which defective walls of MWCNTs are visible.
Figure 2. Electron microscopic images of a sample of MWCNTs heated at a temperature of 2200 ° C synthesized on a Fe — Co / Al 2 O 3 catalyst. On the left is a low resolution TEM image. On the bottom right is a high resolution TEM image. The structure of MWCNTs becomes less defective, the ends of the nanotubes are closed.
Figure 3. Electron microscopic images of a sample of MWCNTs heated at a temperature of 2600 ° C synthesized on a Fe — Co / Al 2 O 3 catalyst. On the left is a low resolution TEM image. Right, bottom - high resolution TEM image on which the closed ends of MWCNTs are visible. The walls of MWCNTs become more even and less defective.
Figure 4. Electron microscopic images of a sample of MWCNTs heated at a temperature of 2800 ° C synthesized on a Fe — Co / Al 2 O 3 catalyst. On the left is a low resolution TEM image. On the right, below is a high resolution TEM image on which less defective walls of MWCNTs are visible.
Figure 5. Electron microscopic images of a MWCNT sample heated at a temperature of 2800 ° С synthesized on a Fe-Co / Al 2 O 3 catalyst, showing the appearance of defects in the MWCNT structure, which are cylindrical formations consisting of graphene layers embedded in one another, which are displayed on the right High resolution TEM image.
6. Electron microscopic images of the initial sample of MWCNTs synthesized on a Fe-Co / CaCO 3 catalyst. On the left is a low resolution TEM image. Right, bottom - high resolution TEM image on which the uneven surface of the MWCNT is visible. On the right, catalyst particles encapsulated inside the channels of carbon nanotubes (marked by arrows) are visible at the top.
7. Electron microscopic images of a sample of MWCNTs heated at a temperature of 2200 ° C synthesized on a Fe-Co / CaCO 3 catalyst. On the left is a low resolution TEM image. On the right, below is a high resolution TEM image on which more even walls of the MWCNTs are seen.
Fig. 8. Electron microscopic images of a sample of MWCNTs heated at a temperature of 2600 ° C synthesized on a Fe-Co / CaCO 3 catalyst. On the left is a low resolution TEM image. Right, bottom - high resolution TEM image on which the closed ends of the MWCNTs are visible. The walls of MWCNTs become more even and less defective.
Fig.9. Electron microscopic images of a sample of MWCNTs heated at a temperature of 2800 ° С synthesized on a Fe-Co / CaCO 3 catalyst. On the left is a low resolution TEM image. On the bottom right is a high resolution TEM image.
1. The method of purification of multilayer carbon nanotubes obtained by the pyrolysis of hydrocarbons using catalysts containing Fe, Co, Ni, Mo, Mn and their combinations as active components, as well as Al 2 O 3, MgO, CaCO 3 as carriers, by boiling in a solution of hydrochloric acid with further washing with water, characterized in that after acid treatment they are heated in a stream of high-purity argon in a temperature gradient furnace, where in the working zone the temperature is 2200-2800 ° С, at the edges of the furnace the temperature is 900-1000 ° С as a result of They obtain multilayer nanotubes with a metal impurity content of less than 1 ppm.
2. The method according to claim 1, characterized in that the heating is carried out in ampoules made of high-purity graphite.
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1 TECHNICAL INNOVATION UDC BBC 30.6 FILTER BASED ON CARBON NANOTUBES FOR CLEANING ALCOHOL-CONTAINING LIQUIDS N.P. Polikarpova, I.V. Zaporotskova, T.A. Ermakova, P.A. Zaporotskov. Experiments on the purification of alcohol-containing liquids by filtration and transmission methods were carried out, the mass fraction of carbon nanotubes was determined, leading to the best result. A filter mockup was created on the basis of nanomaterial enclosed in the space between the layers of porous glass and its structural features were determined. Polikarpova N.P., Zaporotskova I.V., Ermakova T.A., Zaporotskov P.A., 2012 Keywords: carbon nanotubes, alcohol-containing liquid, adsorption, filter, porous glass, porous ceramics. Introduction Purification of alcohol-containing liquids, which include products of the food industry of vodka, plays an important role in the process of their production. Each manufacturer is trying to use the most effective methods for cleaning alcohol-containing liquids from impurities and fusel oils. Fusel oils, aldehydes, mineral salts and other impurities are removed from the product by filtration using charcoal, silica sand, silver dust, platinum filters, milk powder, egg white. Many of the manufacturers of expensive varieties of vodka repeat the cleaning process many times, combining different options. Each subsequent cleaning even more relieves the product of fusel oils and other impurities. Double or triple degree of purification significantly improves the taste, but also significantly increases the manufacturing process. Currently, alcoholic beverage enterprises use various methods of purification of alcohol-containing products. The most common of these are cleaning with charcoal filters, cleaning with milk and egg whites, “silver filtration” and cleaning with gold and precious stones. In the works of I.V. Zaporotskova and N.P. Zaporotskova presented the results of theoretical calculations of the adsorption interaction of carbon nanotubes (CNTs) with molecules of heavy organic alcohols that are part of alcohol-containing liquids in the form of undesirable impurities, and the possibility of their sorption on the surface of nanotubes is proved. This allowed us to offer an innovative method of purification of water-ethanol mixtures, which include vodka, using carbon nanomaterial. As you know, graphite sorbents and charcoal purify the product from harmful impurities by 60%, milk by 70%, precious metals (silver, gold) by 75%. The use of carbon nanotubes as the sorbing material will make it possible to purify alcohol-containing liquid from impurities by 98%. Also, the advantages of the claimed CNT-based filters include: 1) high productivity of the process at low cost; 2) ten times smaller volume of adsorbing substance; 3) the absence of side effects from the use of adsorbents of graphite nature with the preservation and multiple increase in the activity of the process; Bulletin of VolSU. Series 10. Vol.
2 4) the possibility of selective adsorption. It should be noted that the introduction of a filter based on nanomaterials in the finished production cycle at the final stage without fundamentally changing the technological process provides almost 100 percent purification of the product of water-ethanol mixtures without significantly increasing the cost of production. 1. Determination of the optimal amount of carbon nanomaterial for the purification of liquids Before proceeding directly to laboratory experiments on the purification of alcohol-containing liquids (domestic vodka), it was necessary to determine the optimal amount of nanomaterial that leads to the desired effect of a high degree of purification. As an object of research, vodka “Let's drink for” was chosen, which belongs to the class of ordinary vodkas of low cost. Liquid studies were performed by the titrimetric method until the minimum mass of nanotubes necessary for the effective purification of 50 ml of vodka was detected. The selection was carried out using the “from larger to smaller” method, the initial number of carbon nanotubes was 1 g. The accuracy of weighing CNTs was determined by the accuracy of the used analytical weights and amounted to 0.0001 g. The decrease in the number of nanotubes was carried out until the moment was fixed when the alkalinity of vodka ceased to decrease. According to the standards of GOST R “Vodka and special vodka. General specifications ", the alkalinity of vodka should not exceed 2.5 3.0 ml. Prior to purification, the alkalinity of the selected vodka was 2.5 ml. The results of titrimetric studies are presented in the table. An analysis of the results showed that passing an alcohol-containing liquid through a filter with carbon nanotubes reduces the alkalinity by an average of 98% (2.45 ml). The minimum amount of required nanomaterial is 0.001 g, since with a decrease in this amount, alkalinity increases sharply, and with a larger amount, its decrease is insignificant. 2. Selection of material for creating a filter shell based on carbon nanotubes In the production of vodka, filters with porous glass, such as Schott filters, and ceramic filters can be used as filters. These porous materials can also be used as materials for creating a filter shell based on carbon nanotubes. Consider the features of these materials. Porous glass is a glassy porous material with a spongy structure and a silicon oxide SiO 2 content of about 96% (mass.). Porous glass is the result of heat and chemical treatment of glasses of a special composition. Porous glasses can only be obtained from glasses with a sufficiently high Na 2 O content, in which coexisting phases after long-term heat treatment form interpenetrating frames. A prerequisite for the production of porous glasses is also a content in the initial glasses of at least 40% (mass.) Of silicon dioxide, which ensures the formation of a continuous spatial network of SiO 2 in the glass. Glass filters are plates of crushed and fused glass. For their manufacture, the glass is ground in ball mills and sieved using a set of sieves. Glass powder is sintered by heating in a furnace in metal or ceramic forms. The resulting plates are soldered into tubes, glasses, funnels, crucibles and other glass dishes of the same composition. Hot plates, concentrated acids and dilute alkalis can be filtered through such plates, since such filters are resistant to aggressive media. Filter plates are distinguished by porosity. Several classes of filters are made depending on the pore size. Glass filters, or so-called Schott filters, are available in the following types: 1 pore size is microns, it is used to work with coarse-grained precipitates; 7 6 N.P. Polikarpova et al. Filter based on carbon nanotubes
3 2 the pore size is microns, it is used to work with srednekristallnymi precipitation; 3, the pore size is microns; it is used to work with fine crystalline precipitates; 4, the pore size is 4 × 10 μm; it is used to work with very fine crystalline precipitates. Ceramic membranes are porous fine filters made by sintering cermet materials, such as alumina, titanium dioxide or zirconium (Fig. 1), at extremely high temperatures. Ceramic membranes usually have an asymmetric structure supporting the active membrane layer (Fig. 2). Porous ceramics consist of bound particles of approximately the same size, which creates a uniform, permeable material that provides tortuous channels for fluid flow. The most commonly used filters are silica and alumina, although the choice of material, size and shape is almost unlimited. Ceramic filters are usually classified by average pore diameter and / or permeability. The average pore diameter is the average minimum pore diameter, measured in microns. The dimensions of the membranes of ceramic filters: - microfiltration: 1.2 μm 0.5 μm 0.2 μm 0.1 μm; - ultrafiltration: 50 nm 20 nm. Macroporous materials provide mechanical stability, while the active membrane layer provides separation: microfiltration, ultrafiltration, nanofiltration. Ceramic membrane filters always operate in tangential filtration mode with optimal hydrodynamic modes. Turbid liquid passes through the membrane layer inside a single or multichannel membrane at high speed. Under the action of transmembrane pressure (TMD), the micromolecules and water pass vertically through the membrane layer, forming a permeate flow. Suspended substances and macromolecular compounds are retained inside the membrane, forming a flow of concentrate. Thus, the cleaning of contaminated liquids. Ceramic membranes allow the physical method to separate mixtures of components without the use of additives. The introduction of carbon nanotube material into these systems can further increase the efficiency of such a filter. 3. A prototype of a filter based on carbon nanotubes in a shell made of porous glass. To create a prototype of a filter through which the alcohol-containing liquid was vertically passed (Fig. 3), we used Schott glass filters made of porous glass with carbon nanotubes placed inside carbon nanomaterial obtained on CVDomna installation according to the method described in I. V. Zaporotskova. The filter part of the filters used is a glass porous substance. 1. Porous ceramics Fig. 2. Ceramic filter Bulletin of VolSU. Series 10. Vol.
4 with a membrane size of 4 10 μm. For the preliminary layout, two Schott filters of different diameters were used, which joined together to form a closed filtering system. Between the glass plates, the pore sizes of which were 4 10 μm, a layer of carbon nanotubes was placed. An enlarged image of the porous glass is shown in Figure 4. To ensure closure, carbon nanotubes were additionally placed between the layers of filter paper. The product under test drink vodka “We Drink For” freely vertically flowed through the filter thus created under the influence of gravity. The amount of filtering carbon nanomaterial and the amount of alcohol-containing liquid flowing through the manufactured filter were selected in accordance with the previously obtained results: 0.001 g of CNTs for the purification of 50 ml of vodka. These types of filters turned out to be sufficiently effective to ensure the free flow of a water-ethanol mixture through them without penetration of carbon nanomaterial through the glass, which can be explained by a random arrangement of pores in the shell. The further studies of the quality of the purified product using molecular spectroscopy and liquid chromatography methods (Fig. 5, 6) confirmed the high degree of purification of vodka from impurities of high molecular weight alcohol fusel oils: there are no peaks on the spectra related to these alcohols. The results of titration of vodka “Let's drink for” a different amount of carbon nanotubes 3. Filter layout with porous glass plates. Fig. 4. The type of glass plate with a pore size of 4 10 microns with increasing x N.P. Polikarpova et al. Filter based on carbon nanotubes
5 Transmission,% Wave number, cm -1 Fig. 5. IR spectra of vodka “Let's drink for”: red spectrum before purification; violet spectrum after purification by passing through a filter with carbon nanotubes a Conclusion The performed experimental studies have proved that the treatment of the water-ethanol mixture with carbon nanotubes helps to reduce the content of fusel oils and other impurities, while retaining 6. Chromatograms of vodka “Drink Over”: a) before purification; b) after purification by passing through a filter with carbon nanotubes, the content of the main useful component of the product is ethyl alcohol. The created and tested model of the filter based on carbon nanotubes enclosed in a shell of porous glass can be used as the basis for creating an industrial filter. Further research Vestnik VolSU. Series 10. Vol.
6 will be aimed at creating a mock-up of a filter with a ceramic shell, the smaller pore sizes of which (compared with the pores of the glass shell) can provide better protection of the product being cleaned from carbon nanoparticles entering it. REFERENCES 1. Berkman, A. S. Porous permeable ceramics / A. S. Berkman. M.: Gosstroyizdat, p. 2. Vasiliev, V. P. Analytical chemistry. Titrimetric and gravimetric methods of analysis: a textbook / V.P. Vasiliev. M.: Bustard, p. 3. Garmash, E.P. Ceramic membranes for ultrafiltration and microfiltration / E.P. Garmash, Yu. N. Kryuchkov, V.P. Pavlikov // Glass and ceramics С GOST R Special vodkas and vodkas. General specifications. State standard of the Russian Federation. M.: Gosstandart of Russia, p. 5. Zaporotskova, I.V. Perspective carbon-based nanomaterials / I.V. Zaporotskova, L.V. Kozhitov, V.V. Kozlov // Vestn. Volgogr. state un-that. Ser. 10, Innovation activity S. Zaporockova, I. V. Sorption activity of carbon nanotubes as the basis of innovative technology for the purification of water-ethanol mixtures / I. V. Zaporotskova, N. P. Zaporotskova, T. A. Ermakova // Vestn. Volgogr. state un-that. Ser. 10, Innovation activity S Zaporotskova, I.V. Carbon and non-carbon nanomaterials and composite structures based on them: structure and electronic properties / I.V. Zaporotskova. Volgograd: From VolSU, p. 8. The study of the influence of carbon nanotubes on the process of purification of alcohol-containing liquids / I. V. Zaporotskova [et al.] // Vestn. Volgogr. state un-that. Ser. 10, Innovation activity S. Kazitsyna, L. A. Application of UV, IR, NMR spectroscopy in organic chemistry: textbook. manual for universities / L. A. Kazitsyna, N. B. Kupletskaya. M.: Higher. school, sec. 10. Sychev, S. N. High-performance liquid chromatography as a method for determining falsification and product safety / S. N. Sychev, V. A. Gavrilina, R. S. Murzalevskaya. M.: DeLi print, p. 11. Chemical Encyclopedia / Ed. I. L. Knunyantsa. M.: Soviet Encyclopedia, Dresselhaus, M. S. / M. S. Dresselhaus, G. Dresselhaus, P. Avouris // Carbon nanotubes: synthesis, structure, properties, and application. Springer-Verlag, p. 13. Zaporotskova, I. V. Active properties of nanotubular carbon structures with respect to heavy organic molecules / I. V. Zaporotskova // Nanoscience & nanotechnology-2011: Book of abstract. Frascati National Laboratories INFN. Frascati, Sept, Frascati: INFN, P Zaporotskova, N. P. Investigation of carbon nanotube activity to heavy organic molecules / N. P. Zaporotskova, I. V. Zaporotskova, T. A. Ermakova // Fullerenes and Atomic clusters. Abstracts of invited lectures & contributed papers. St. Peterburg, July 4-8, St. Peterb., P THE FILTER ON THE BASIS OF CARBON NANOTUBES FOR PURIFICATION OF ALCOHOL-CONTAINING LIQUIDS N.P. Polikarpova, I.V. Zaporotskova, T.A. Ermakova, P.A. Zaporotskov Experiments on purification of alcohol-containing liquids by filtration and transmission methods are made, the mass fraction of carbon nanotubes leading to the best result is established. The filter model on the basis of a nanomaterial concluded in space between layers of porous glass is created, and its constructional features are defined. Key words: carbon nanotubes, alcohol-containing liquids, adsorption, filter, porous glass, porous ceramics. 8 0 N.P. Polikarpova et al. Filter based on carbon nanotubes
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None of the common methods for producing CNTs makes it possible to isolate them in pure form. Impurities to NT can be fullerenes, amorphous carbon, graphitized particles, and catalyst particles.
Three groups of CNT purification methods are used:
1) destructive,
2) non-destructive,
3) combined.
Destructive methods use chemical reactions, which can be oxidative or reducing, based on differences in reactivity of different carbon forms. Either solutions of oxidizing agents or gaseous reagents are used for oxidation; hydrogen is used for reduction. The methods allow the isolation of high purity CNTs, but are associated with tube losses.
Non destructive methods include extraction, flocculation and selective precipitation, cross-flow microfiltration, size exclusion chromatography, electrophoresis, selective interaction with organic polymers. As a rule, these methods are inefficient and inefficient.
At the same time, it was shown that the purification of SWCNTs obtained by the laser-thermal method by filtration with sonication allows obtaining material with a purity of more than 90% with a yield of 30–70% (depending on the purity of the initial carbon black).
Extraction is used exclusively to remove fullerenes, with a large amount of which they are extracted with carbon disulfide or other organic solvents.
The bulk of the catalyst and catalyst carrier is removed by washing in sulfuric and nitric acids, as well as mixtures thereof. If the catalyst carrier is silica gel, quartz or zeolites, hydrofluoric acid or alkali solutions are used. Concentrated alkali solutions are used to remove alumina. Catalyst metals occluded in the CNT cavity or surrounded by a graphite shell are not removed.
Amorphous carbon is removed either by oxidation or by reduction. Hydrogen is used for reduction at a temperature not lower than 700 ° C; for oxidation, air, oxygen, ozone, carbon dioxide or aqueous solutions of oxidizing agents are used. Oxidation in air begins to occur at 450 ° C. In this case, part of the CNTs (mainly of the smallest diameter) is completely oxidized, which helps to open the remaining tubes and remove catalyst particles not removed during the primary acid treatment. The latter is removed by secondary washing in acid. To obtain the purest product, acid and gas purification operations can be repeated several times, combined with each other and with physical methods.
In some cases, the initial acid purification is carried out in two stages, using first diluted acid (to remove the bulk of the catalyst and support), and then concentrated (to remove amorphous carbon and clean the surface of the CNTs) with intermediate filtration and washing operations.
Since metal oxide particles catalyze the oxidation of CNTs and cause a decrease in the yield of the purified product, an additional operation of their passivation is used by conversion to fluorides by the action of SF 6 or other reagents. The yield of purified CNTs increases.
To clean the materials obtained by the arc and laser-thermal method at the University of Rice (USA), several methods have been developed. The “old” method included the oxidation of 5 M HNO 3 (24 h, 96 ° C), neutralization of NaOH, dispersion in a 1% aqueous solution of Triton X-100, and cross-flow filtration. Its disadvantages include the coprecipitation of Ni and Co hydroxides together with CNTs, difficulties in removing graphitized particles and organic Na salts, foaming during drying in vacuum, low filtration efficiency, long process time, and low yield of purified tubes.
The “new” method involved the oxidation of 5 M HNO 3 for 6 h, centrifugation, washing and neutralization of the NaOH precipitate, re-oxidation of HNO 3 with repeated centrifugation and neutralization, washing with methanol, dispersion in toluene and filtering. This method also does not allow complete purification, although the yield of CNTs (50–90%) exceeds the “old” method.
The use of organic solvents immediately after boiling in acid allows the removal of 18–20% of impurities, half of which are fullerenes and the other is soluble hydrocarbons.
The SWCNTs obtained by the arc method (5% of a catalyst consisting of Ni, Co, and FeS with a ratio of 1: 1: 1) were first oxidized in air at 470 ° С for 50 min in a rotary laboratory furnace, then metal impurities were removed by repeated washing with 6 M HCl achieving complete discoloration of the solution. The yield of SWCNTs containing less than 1 wt.% Non-volatile residue was 25–30%.
A process has been developed for the purification of arc SWCNTs, which, in addition to oxidation in air and boiling in HNO 3, is treated with a HCl solution and neutralizes ultrasonic dispersion in dimethylformamide or N-methyl-2-pyrrolidone, followed by centrifugation, evaporation of the solvent and vacuum annealing at 1100 about C.
The purification of pyrolytic SWCNTs and MWCNTs in two stages is described: by prolonged (12 h) sonication at 60 ° С in a Н 2 О 2 solution to remove carbon impurities in the first stage and by sonication for 6 hours in HCl to remove Ni impurity in the second. After each stage, centrifugation and filtration were performed.
For purification of SWCNTs obtained by the HiPco method and containing up to 30 wt.% Fe, a two-stage method is also described, including oxidation in air (in particular, in a microwave oven) and subsequent washing with concentrated HCl.
An even greater number of stages (dispersion in hot water with sonication, interaction with bromine water at 90 ° C for 3 h, oxidation in air at 520 ° C for 45 min, treatment with 5 M HCl at room temperature) was used to purify MWCNTs, obtained by pyrolysis of a solution of ferrocene in benzene and containing up to 32 wt.% Fe. After washing and drying at 150 ° C for 12 hours, the Fe content decreased to a few percent, and the yield was up to 50%.
Gas oxidation can lead to the development of porosity of NT and HB, prolonged boiling in nitric acid - to the complete degradation of these substances.
With a relatively large amount of silicon (laser-thermal method), the primary product is heated in concentrated hydrofluoric acid, then HNO 3 is added and processing is carried out at 35–40 ° С for another 45 minutes. Operations are associated with the use of highly corrosive media and the release of toxic gases.
To remove the zeolite used in the preparation of SWCNTs by catalytic pyrolysis of ethanol vapor, the product oxidized in air is treated with an aqueous solution of NaOH (6 N) with short-term (5 min) sonication, and the residue collected on the filter is washed with HCl (6 N).
The separation of SWCNTs from impurities of other forms of carbon and metal particles can be carried out by ultrasonic dispersion of the tubes in a solution of polymethyl methacrylate in monochlorobenzene, followed by filtration.
For the purification of SWCNTs, it is often recommended to use their functionalization. In particular, a method is described that includes three sequential operations: functionalization using azomethinilide in dimethylformamide (see Section 4.5), slow precipitation of functionalized SWCNTs when diethyl ether is added to a solution of tubes in chloroform, removal of functional groups, and regeneration of SWCNTs by heating at 350 о С and annealing at 900 о С. At the first stage, metal particles are removed, at the second - amorphous carbon. The Fe content in HiPco tubes purified by this method is reduced to 0.4 wt.%.
Interaction with DNA can be used to separate metal SWCNTs from semiconductor ones. In laboratories there is a wide range of various single-stranded DNAs, choosing which one succeeds in achieving selective enveloping and subsequent separation of the initial mixture into fractions by chromatographic method.
Physical methods include the transfer of the initial mixture into an aqueous solution using a long ultrasound treatment in the presence of surfactants or enveloping soluble polymers, microfiltration, centrifugation, high performance liquid chromatography, gel permeation chromatography. The preparation of dispersions suitable for chromatography was performed by grafting zwitterionic ions (see Section 4.5).
It is assumed that the development of chromatographic methods will allow separation of CNTs not only in length and diameter, but also in chirality, and to separate tubes with metallic properties from tubes with a semiconductor type of conductivity. To separate SWCNTs with different electronic properties, the selective deposition of metal tubes in a solution of octadecylamine in tetrahydrofuran was tested (the amine is more strongly adsorbed on semiconductor tubes and leaves them in solution).
An example of the use of non-destructive methods of cleaning and size separation of CNTs is also the method developed by scientists from Switzerland and the USA. The starting material obtained by the arc method was transferred using sodium dodecyl sulfate to an aqueous colloidal solution (the surfactant concentration was slightly higher than the critical micelle concentration). With an increase in the surfactant concentration, CNT aggregates were obtained, which were filtered off with intensive sounding through track membranes with 0.4 μm pores. After repeated dispersion in water, the operation was repeated several times, achieving the desired degree of purification of CNTs.
The method of capillary electrophoresis is inefficient, although it allows not only to clean the CNTs, but also to separate them by length or diameter. For the separation, dispersions stabilized by surfactants or soluble polymers are used. For purification and separation of CNTs by dielectrophoresis, see Sec. 4.13.
A non-destructive method has been developed for separating cleaned and shortened CNTs into fractions with tubes of different sizes in cross (asymmetric) fluid flows.
To enlarge the particles of metal catalysts, annealing is carried out in hydrogen at 1200 ° C, after which the metals are dissolved in acid. The complete removal of metal catalysts and catalyst supports regardless of their form in the mixture can be carried out by high-temperature (1500–1800 о С) vacuum annealing. At the same time, fullerenes are also removed, and CNTs increase in diameter and become less defective. For complete annealing of defects, temperatures above 2500 ° C are required. Vacuum annealing at 2000 ° C is used to increase the resistance of MWCNTs to acid treatment.
For purification from impurities of carbon fibers formed during the pyrolysis of hydrocarbons, freezing with liquid nitrogen is recommended.
The choice of one or another cleaning option depends on the composition of the mixture being cleaned, the structure and morphology of NT, the amount of impurities and the requirements for the final product. Pyrolytic CNTs and especially CNFs contain less or no amorphous carbon.
In assessing the purity of CNTs, the determination of the content of amorphous carbon impurities presents the greatest difficulty. Raman spectroscopy (see Ch. 8) gives only a qualitative picture. A more reliable, but at the same time time-consuming method is near-infrared spectroscopy (Itkis, 2003).
In the United States, the purity standard for SWCNTs has been created.
Carbon Nanotube Cleaning
None of the common methods for producing CNTs makes it possible to isolate them in pure form. Impurities to NT can be fullerenes, amorphous carbon, graphitized particles, and catalyst particles.
Three groups of CNT purification methods are used:
destructive
non-destructive
combined.
Destructive methods use chemical reactions, which can be oxidative or reducing, based on differences in reactivity of different carbon forms. Either solutions of oxidizing agents or gaseous reagents are used for oxidation; hydrogen is used for reduction. The methods allow the isolation of high purity CNTs, but are associated with tube losses.
Non-destructive methods include extraction, flocculation and selective precipitation, cross-flow microfiltration, size exclusion chromatography, electrophoresis, and selective interaction with organic polymers. As a rule, these methods are inefficient and inefficient.
Properties of carbon nanotubes
Mechanical. Nanotubes, as has been said, are extremely durable material, both in tension and in bending. Moreover, under the action of mechanical stresses exceeding critical, nanotubes do not "break", but are rebuilt. Based on such a property of nanotubes as high strength, it can be argued that they are the best material for a space elevator cable at the moment. As the results of experiments and numerical simulations show, the Young's modulus of a single-walled nanotube reaches values \u200b\u200bof the order of 1-5 TPa, which is an order of magnitude greater than that of steel. The graph below shows a comparison of a single-walled nanotube and high-strength steel.
1 - The space elevator cable is estimated to withstand mechanical stress of 62.5 GPa
2 - Tensile diagram (dependence of mechanical stress y on elongation e)
To demonstrate the significant difference between the currently most durable materials and carbon nanotubes, let us conduct the following thought experiment. Imagine that, as previously assumed, a certain wedge-shaped homogeneous structure consisting of the most durable materials today will serve as a cable for the space elevator, then the diameter of the cable at GEO (geostationary Earth orbit) will be about 2 km and will narrow to 1 mm at the surface Of the earth. In this case, the total mass will be 60 * 1010 tons. If carbon nanotubes were used as the material, the cable diameter at GEO was 0.26 mm and 0.15 mm at the Earth's surface, and therefore the total mass was 9.2 tons. As can be seen from the above facts, carbon nanofiber is just the material that is needed when building a cable, the actual diameter of which will be about 0.75 m in order to withstand the electromagnetic system used to move the space elevator car.
Electric. Due to the small size of carbon nanotubes, it was only in 1996 that it was possible to directly measure their electrical resistivity in a four-contact way.
Gold strips were applied to a polished surface of silicon oxide in vacuum. In the interval between them, nanotubes 2-3 μm long were sprayed. Then, 4 tungsten conductors 80 nm thick were deposited on one of the nanotubes selected for measurement. Each of the tungsten conductors had contact with one of the gold stripes. The distance between the contacts on the nanotube ranged from 0.3 to 1 μm. The results of direct measurements showed that the specific resistance of nanotubes can vary significantly - from 5.1 * 10 -6 to 0.8 Ohm / cm. The minimum resistivity is an order of magnitude lower than that of graphite. Most nanotubes have metallic conductivity, while the smaller one exhibits semiconductor properties with a band gap of 0.1 to 0.3 eV.
French and Russian researchers (from IPTM RAS, Chernogolovka) discovered yet another property of nanotubes, as superconductivity. They measured the current – \u200b\u200bvoltage characteristics of a single single-walled nanotube with a diameter of ~ 1 nm, rolled into a bundle of a large number of single-walled nanotubes, as well as individual multilayer nanotubes. A superconducting current at a temperature close to 4K was observed between two superconducting metal contacts. The features of charge transfer in a nanotube are significantly different from those inherent in ordinary, three-dimensional conductors and, apparently, are explained by the one-dimensional nature of the transfer.
Also de Geer from the University of Lausanne (Switzerland) discovered an interesting property: a sharp (about two orders of magnitude) change in conductivity with a small, by 5-10 °, bending of a single-walled nanotube. This property can expand the scope of nanotubes. On the one hand, a nanotube turns out to be a ready-made highly sensitive transducer of mechanical vibrations into an electrical signal and vice versa (in fact, it is a telephone tube several microns in length and about a nanometer in diameter), and, on the other hand, it is a practically ready sensor of the smallest deformations. Such a sensor could find application in devices that monitor the condition of mechanical components and parts, on which the safety of people depends, for example, passengers of trains and aircraft, personnel of nuclear and thermal power plants, etc.
Capillary. As experiments have shown, an open nanotube has capillary properties. To open the nanotube, you need to remove the upper part - the lid. One of the removal methods is to anneal the nanotubes at a temperature of 850 ° C for several hours in a stream of carbon dioxide. As a result of oxidation, about 10% of all nanotubes are open. Another way to destroy the closed ends of nanotubes is by exposure to concentrated nitric acid for 4.5 hours at a temperature of 2400 C. As a result of this treatment, 80% of the nanotubes become open.
The first studies of capillary phenomena showed that a liquid penetrates the nanotube channel if its surface tension is not higher than 200 mN / m. Therefore, solvents having a low surface tension are used to introduce any substances into the nanotubes. So, for example, concentrated nitric acid, whose surface tension is low (43 mN / m), is used to introduce some metals into the channel of a nanotube. Then annealing is carried out at 4000 C for 4 hours in a hydrogen atmosphere, which leads to the reduction of the metal. Thus, nanotubes containing nickel, cobalt and iron were obtained.
Along with metals, carbon nanotubes can be filled with gaseous substances, for example, hydrogen in molecular form. This ability is of practical importance, because it opens up the possibility of safe storage of hydrogen, which can be used as environmentally friendly fuel in internal combustion engines. Scientists were also able to place a whole chain of fullerenes inside the nanotube with the gadolinium atoms already embedded in them (see Fig. 5).
Fig. 5. Inside C60 Inside a Single Layer Nanotube