EP2297386A2

EP2297386A2 - A highly efficient gas phase method for modification and functionalization of carbon nanofibres with nitric acid vapour

Info

Publication number: EP2297386A2
Application number: EP09772124A
Authority: EP
Inventors: Martin Muhler; Wei Xia
Original assignee: Bayer MaterialScience AG
Current assignee: Bayer Intellectual Property GmbH
Priority date: 2008-07-03
Filing date: 2009-06-27
Publication date: 2011-03-23
Also published as: BRPI0915597A2; DE102008031579A1; CN102099515A; AU2009266063A1; CA2729693A1; US20110104492A1; KR20110027723A; JP2011526331A; WO2010000424A2; RU2011103676A; WO2010000424A3

Abstract

The present invention relates to a method for the functionalization of carbon fibres using the vapour of nitric acid, carbon fibres thus modified and use thereof.

Description

A highly efficient gas phase process for the modification and functionalization of carbon nanofibers with nitric acid vapor

The present invention relates to a process for the functionalization of carbon fibers with the vapor of nitric acid, so modified carbon fibers and their use.

Under carbon nanofibers are understood in the prior art mainly cylindrical carbon tubes with a diameter between 3 and 100 nm and a length which is a multiple of the diameter. These tubes consist of one or more layers of ordered carbon atoms and have a different nucleus in morphology. These carbon nanofibers are also referred to as "carbon fibrils" or "hollow carbon fibers", for example.

Carbon nanofibers have long been known in the literature. Although Iijima (Publication: S. Iijima, Nature 354, 56-58, 1991) is generally referred to as the discoverer of nanotubes, these materials, especially fibrous graphite materials with multiple layers of graphite, have been around since the 1970s and early 1980s, respectively known. Tates and Baker (GB 1469930A1, 1977 and EP 56004 A2) described for the first time the deposition of very fine fibrous carbon from the catalytic decomposition of hydrocarbons. However, the carbon filaments made from short-chain hydrocarbons are no longer characterized in terms of their diameter.

Typical structures of these carbon nanofibers are those of the cylinder type. In the cylindrical structures, a distinction is made between the single walled carbon nanotubes and the multiwalled carbon nanotubes (Multi Wall Carbon Nano Tubes) Tubes). Common processes for their production include arc discharge, laser ablation, chemical vapor deposition (CVD) and chemical vapor deposition (CCVD process).

From Iijima, Nature 354, 1991, 56-8, the formation of carbon fibers in the arc process is known, consisting of two or more graphene layers and rolled up into a seamlessly closed cylinder and nested. Depending on the roll-up vector, chiral and achiral arrangements of the carbon atoms relative to the longitudinal axis of the carbon fiber are possible.

Structures of carbon fibers in which a single contiguous graphene layer (so-called scroll type) or interrupted graphene layer (so-called onion type) is the basis for the construction of the nanotube were first reported by Bacon et al., J. Appl. Phys. 34, 1960, 283-90. The structure is called Scroll Type. Later, corresponding structures were also found by Zhou et al., Science, 263, 1994, 1744-47 and by Lavin et al., Carbon 40, 2002, 1123-30.

Because of the inert and hydrophobic properties of the carbon nanofibers, the modification and functionalization of the surfaces are essential for their applications, especially in catalysis (Toebes, ML et al., J. Catal. 214: 78-87 (2003); Jong KP, Geus JW, Catal., Rev. Sci. Eng 42: 481-510 (2000); Serp P. et al., Appl. Catal. A 253: 337-58 (2003); Nhut, JM et al Appl. Catal. A 254: 345-63 (2003)). One of the most commonly used methods for surface modification is the generation of oxygenated functional groups by partial oxidation. On the one hand, the carbon nanofibers become hydrophilic due to the oxidation, which makes possible an aqueous catalyst preparation because of the improved wetting properties. On the other hand, those on the surface produced oxygen-containing functional groups serve as anchor sites for catalyst precursor complexes. A key function is attributed to the carboxyl groups (Boehm, HP Carbon 32: 759: 69 (1994)).

The literature describes a large number of methods for the treatment of carbon nanofibres. These include the oxygen (Morishita, K. Takarada T., Carbon 35: 977-81 (1997); Ajayan, PM et al., Nature 362: 522-5 (1993); Ebbesen, TW et al., Nature 367 : 519-9 (1997)), ozone (ByI, O. et al., Langmuir 21: 4200-4 (2005)), carbon dioxide (Tsang, SC et al., Nature 262: 520-2 (1993) Seo, K. et al., J. Am. Chem. Soc. 125: 13946-7 (2003)), water (Xia, W. et al., Mater 19: 3648-52 (2007)), hydrogen peroxide (Xu, C. et al., Adv. Engineering Mater 8: 73-77 (2006)) and Plasma Treatment (Bubert, H. et al., Anal. Bioanal Chem 374: 1237-41 (2002). and the most commonly used nitric acid treatment (Lakshminarayan, PV et al., Carbon 42: 2433-42 (2004); Darmstadt, H. et al., Carbon 36: 1183-90 (1998); Darmstadt, H et al., Carbon 35: 1581-5 (1997)). For the processing of classic carbon materials such. Nitrogen dioxide is used (Jacquot, F. et al., 40: 335-43 (2002); Jeguirim, M. et al., Fuel 84: 1949-56 (2005)). An objective of these treatments may also be the purification, cutting and opening of the carbon nanofibers (Liu, J. et al., 280: 1253-6 (1998)).

For the preparation of oxygen-containing functional groups are only strong oxidizing agents such. Nitric acid or a mixture of nitric acid with sulfuric acid under aggressive reaction conditions, especially when a large amount of carboxyl groups is required (Toebes, ML et al., Carbon 42: 307-15; Ros, TG et al., 8: 1151-62 (2002)). However, structural damage to the carbon nanofibers often occurs in this oxidation by means of corrosive acids in the liquid phase (Ros, TG et al., 8: 1151-62 (2002); Zhang, J. et al., J. Phys. Chem B 107: 3712-8 (2003)), which is at least partly due to the mechanical stress due to the refluxing and stirring. In addition, separation of the treated carbon nanofibers from the acid is particularly difficult for small diameter carbon nanofibers. Normally, the separation is by filtration, but a considerable amount of carbon nanofibers is lost. The subsequent drying often leads to the agglomeration of carbon nanofibers and thus influences their applicability.

The gas phase treatment seems to be an attractive alternative to avoid these problems. However, conventional gas phase treatments with air, ozone, oxygen or plasma are usually less effective than treatment with nitric acid (Ros, T. G. et al., 8: 1151-62 (2002)). In WO 06/135439, a maximum surface concentration of oxygen of 0.069 measured with XPS is achieved with the various oxidation methods used. In addition, it is known that more carbonyl groups than carboxyl groups are generated by these methods due to the lack of water, and thus the carbon nanofibers are less efficiently functionalized.

At present, oxidative treatment with corrosive acids in aqueous solution is the most effective method. The biggest disadvantages are:

1. Mechanical stress caused by stirring and refluxing is at least partially responsible for structural damage to the carbon nanofibers.

2. The separation by filtration of the acid-treated carbon nanofibers, especially of small diameter nanofibers, is associated with high losses.

3. The subsequent drying process also leads to the agglomeration of the carbon nanofibers and reduces the usability. Gas phase methods are an attractive alternative to the traditional method of treatment as they avoid the problems listed above. However, conventional gas phase treatments (ozone, air, and plasma, etc.) are less effective than treatment with nitric acid. It is also known that, due to the lack of water, it has hitherto been preferred to form carbonyl groups and, less preferably, carboxyl groups.

US 04/0253374 describes a process for purifying and reinforcing carbon nanofibers with a pretreated dilute aqueous nitric acid solution and by using helium as a carrier gas in a fluidized bed reactor at temperatures of 400 ⁰ C, resulting in the nitro groups on the surface. A disadvantage of this process is the use of large amounts of helium, which is necessary to keep the carbon nanofiber agglomerates in suspension and the abrasion caused by the rubbing together of the carbon particles, which is carried out with the carrier gas.

WO 02/45812 A2 describes a purification process for carbon nanofibers, wherein the steam is condensed before the treatment of the fibers and thus a filtration of the fibers is necessary.

It is therefore an object of the present invention to provide a very simple, but highly efficient gas phase process, which allows the modification and the functionalization of carbon fibers without structural and morphological changes.

In a first embodiment, the object underlying the invention is achieved by a process for the functionalization of carbon fibers, wherein a) carbon fibers 1 are introduced into a reactor 2, which has an inlet 3 and an outlet 4, b) heating the reactor 2 to a temperature in a range of 125 to 500 ^° C, c) passing steam of nitric acid 5 through the reactor 2, and d) then drying the treated carbon fibers.

"Nitric acid" in the sense of the invention does not exclude that it is diluted with water or used in combination with sulfuric acid, for example.

Thus, a simple but highly effective method of functionalizing carbon fibers by treatment with nitric acid vapor is provided, thereby avoiding the problematic separation by filtration. Compared to conventional wet HNO ₃ treatment, a significantly larger amount of oxygen species on the surface can be detected by X-ray photoelectron spectroscopy (XPS). The morphology and the degree of agglomeration are not worsened by the treatment.

Thus, a new gas phase method for the oxidation and functionalization of carbon nanofibers is provided. The treatment with nitric acid vapor proves to be a more effective method for generating oxygen-containing functional groups on, for example, carbon nanofiber surfaces, compared to the conventional liquid nitric acid method, without aggravating the morphology and degree of agglomeration and choosing the temperature of the treatment can be. In addition, the use of HNO ₃ gas phase treatment is more advantageous because filtration, washing and drying steps are avoided.

Advantageously, the carbon fibers used are carbon nanofibers, in particular those having an outer diameter in the range from 3 up to 500 nm. The diameter can be determined, for example, by means of transmission electron microscopy (TEM). If one uses carbon fibers with a diameter below the preferred range, it may happen that in the treatment, the carbon fibers are destroyed or at least severely impaired in their mechanical properties. Using carbon fibers with an outer diameter above the preferred range, the BET specific surface area may become too low for certain applications such as catalysis.

Carbon nanofibers for the purposes of the invention are all single-walled or multi-walled carbon nanotubes of the cylinder type, scroll type or onion-like structure. Preference is given to using multi-walled carbon nanotubes of the cylinder type, scroll type or mixtures thereof. Particular preference is given to using carbon nanofibers having a length to outside diameter ratio of greater than 5, preferably greater than 100.

The carbon nanofibers are particularly preferably used in the form of agglomerates, the agglomerates in particular having an average diameter in the range from 0.05 to 5 mm, preferably 0.1 to 2 mm, particularly preferably 0.2 to 1 mm.

The carbon nanofibers to be used preferably have essentially an average diameter of 3 to 100 nm, more preferably 5 to 80 nm, particularly preferably 6 to 60 nm.

In contrast to the initially mentioned known CNTs of the scroll type with only one continuous or interrupted graphene layer, the applicant has also found C NT structures which consist of several graphene layers which are combined into a stack and rolled up (multiscroll type). These carbon nanotubes and carbon nanotube agglomerates thereof, for example, are still the subject matter unpublished German patent application with the official file reference 102007044031.8. Their content is hereby incorporated with respect to the CNT and its preparation to the disclosure of this application. This CNT structure is similar to the simple scroti type carbon nanotubes in comparison with the structure of multi-walled cylindrical monocarbon nanotubes (cylindrical MWNT) to the structure of single-walled cylindrical carbon nanotubes (cylindrical SWNT).

Unlike the onion type structure, the individual graphene or graphite layers in these carbon nanofibers, viewed in cross-section, evidently run continuously from the center of the CNT to the outer edge without interruption. This can be z. For example, it will be possible to provide improved and faster intercalation of other materials in the tube framework, as more open edges than the entry zone of the intercalates are available compared to single scroll CNTs (Carbon 34, 1996, 1301-3) or onion-like CNTs (Science 263, 1994, 1744-7).

The methods known today for the production of carbon nanotubes include arc, laser ablation and catalytic Vertanren. In many of these processes, carbon black, amorphous carbon and high diameter fibers are formed as by-products. In the catalytic process, a distinction can be made between the deposition of supported catalyst particles and the deposition of in-situ formed metal centers with diameters in the nanometer range (so-called flow processes). In the production via the catalytic deposition of carbon at reaction conditions gaseous hydrocarbons (hereinafter CCVD, Catalytic Carbon Vapor Deposition) are given as possible carbon donors acetylene, methane, ethane, ethylene, butane, butene, butadiene, benzene and other carbon-containing reactants , Preference is therefore given to using CNTs obtainable from catalytic processes. The catalysts usually include metals, metal oxides or decomposable or reducible metal components. For example, in the prior art, the metals mentioned for the catalyst are Fe, Mo, Ni, V, Mn, Sn, Co, Cu and other subgroup elements. Although the individual metals usually have a tendency to support the formation of carbon nanotubes, according to the prior art high yields and low proportions of amorphous carbons are advantageously achieved with metal catalysts based on a combination of the abovementioned metals. CNTs obtainable using mixed catalysts are therefore preferred to use. Particularly advantageous catalyst systems for the production of CNTs are based on combinations of metals or metal compounds containing two or more elements from the series Fe, Co, Mn, Mo and Ni.

The formation of carbon nanotubes and the properties of the tubes formed are known to be complex in a manner dependent on the metal component used as a catalyst or a combination of several metal components, the catalyst support optionally used and the catalyst-carrier interaction, the reactant gas and partial pressure, an admixture of hydrogen or other gases, the reaction temperature and the residence time or the reactor used. A particularly preferred method for the production of carbon nanotubes is known from WO 2006/050903 A2.

In the various processes mentioned so far, using different catalyst systems, carbon nanotubes of various structures are produced, which can be removed from the process predominantly as carbon nanotube powder.

Carbon nanofibers more preferred for the invention are obtained by methods basically described in the following references: The production of carbon nanotubes with diameters smaller than 100 nm is described for the first time in EP 205 556 Bl. For the preparation here light (ie, short- and medium-chain aliphatic or mononuclear or dinuclear aromatic) hydrocarbons and an iron-based catalyst are used, are decomposed at the carbon carrier compounds at a temperature above 800-900 ⁰ C.

WO86 / 03455A1 describes the production of carbon filaments having a cylindrical structure with a constant diameter of 3.5 to 70 nm, an aspect ratio (length to diameter ratio) greater than 100 and a core region. These fibrils consist of many continuous layers of ordered carbon atoms arranged concentrically around the cylindrical axis of the fibrils. These cylindrical nanotubes were produced by a CVD process from carbonaceous compounds by means of a metal-containing particle at a temperature between 850 ⁰ C and 1200 ⁰ C.

From WO2007 / 093337A2 a method for the preparation of a catalyst is known, which is suitable for the production of conventional carbon nanotubes with a cylindrical structure. When using this catalyst in a fixed bed, higher yields of cylindrical carbon nanotubes with a diameter in the range of 5 to 30 nm are obtained.

An entirely different way of producing cylindrical carbon nanofibers has been described by Oberlin, Endo and Koyam (Carbon 14, 1976, 133). In this case, aromatic hydrocarbons, such as benzene, are reacted on a metal catalyst. The resulting carbon tube shows a well-defined, graphitic, hollow core that is approximately the diameter of the catalyst particle, on which more less graphitic ordered carbon atoms are present. is located. The entire tube can be graphitized by treatment at high temperature (2500 ^° C - 3000 ^° C).

Most of the above processes (with arc, spray pyrolysis or CVD) are used today for the production of carbon nanotubes. The production of single-walled cylindrical carbon nanotubes, however, is very complex in terms of apparatus and proceeds according to the known processes with very low formation rate and often also with many secondary reactions which lead to a high proportion of undesired impurities, i. the yield of such processes is comparatively low. Therefore, the production of such carbon nanotubes is still extremely technically complicated today and they are therefore used mainly for highly specialized applications in small quantities. Their application, however, is conceivable for the invention, but less preferred than the use of multi-wall CNTs of the cylinder or scroll type.

The production of multi-walled carbon nanotubes, in the form of nested seamless cylindrical nanotubes or also in the form of the described scroll or onion structures, today takes place commercially in large quantities, predominantly using catalytic processes. These processes usually show a higher yield than the above-mentioned arc and other processes and today are typically carried out on the kg scale (several hundred kilo / day worldwide). The MW carbon nanotubes produced in this way are generally much cheaper than the single-walled nanotubes and are therefore used, for example. used as a performance-enhancing additive in other materials.

Therefore, it is preferable to use carbon fibers having a BET surface area in a range of 10 to 500 m ² / g _{/ more} preferably in a range of 20 to 200 m ² / g. The BET specific surface area can be determined, for example, using the device Sorptomatic 1990 from Porotec according to DIN 66131 be determined. If one uses carbon fibers with a BEΞT surface below the preferred range, this can - as already indicated - lead to the fact that the carbon fibers for certain applications such as catalysis are no longer suitable. Using carbon fibers with a BET surface area in excess of the preferred range may cause the carbon fibers to be excessively attacked or even destroyed by treatment with the nitric acid vapor.

Preferably, in the process according to the invention, after the outlet of the reactor 4, a condenser 6 is provided, the outlet of the condenser 7 for the condensate being connected via a return line 8 to a reservoir 9 for the nitric acid 5. As a result, it is possible to prevent condensed nitric acid in the liquid state from wetting the carbon fibers present in the reactor. By treating it in the vapor phase of nitric acid, the surface of carbon fibers can be modified much better with oxygen than in the liquid phase.

Preferably, a glass bulb is used as the reservoir 9 for the nitric acid, which is heated in particular with an oil bath 10. This reservoir 9 is advantageously arranged below the reactor 2. Thereby, the steam of the nitric acid, when heated in the glass bulb by the oil bath, may come into contact with the carbon fibers through the inlet of the reactor. The reactor is therefore preferably arranged vertically, wherein the inlet for the vapor of nitric acid is arranged below the carbon fibers and the outlet is arranged above the carbon fibers. The steam can thus flow through the reactor and pass through the outlet of the reactor into the condenser, where the nitric acid is subsequently condensed and returned to the reservoir. The reactor 2 is heated, for example, with a heater 11. After step (b), the reactor is left for a period in the range of 3 to 20 hours, in particular in a range of 5 to 15 hours at this temperature. If one falls short of this period, then there is too little modification of the surface. Exceeding this preferred range shows no further improvement in surface modification. In particular, the temperature for the period of the treatment to a temperature below 250 ⁰ C and independently of a temperature above 150 ⁰ C. These temperatures have been found to be particularly suitable for the surface modification of carbon fibers with oxygen.

Preferably, the step (c) of drying is carried out over a period of time in a range of 0.5 to 4 hours and independently at a temperature in the range of 80 to 150 ⁰ C. The easiest way to dry them is to prevent the nitric acid in the storage tank from being heated, so that no further steam is generated.

For example, in the reactor, the carbon fibers may be positioned in the vapor stream with a fixture 12. This holding device may be, for example, a sieve, grid or even rust.

Compared to the conventional treatment with liquid nitric acid, for example, the five-hour treatment with nitric acid vapor at 125 ⁰ C seems to be an efficient method, for example, to use the carbon nanofibers as a support for catalysts that can be applied by impregnation.

In a further embodiment, the object underlying the invention is achieved by carbon fibers, characterized in that the ratio of oxygen atoms to carbon atoms derived from the atomic surface concentrations measured with XPS is greater than 0.18. With the methods known hitherto, it has not been possible to produce carbon fibers with such a high surface concentration of oxygen. These carbon fibers could therefore be made available for the first time surprisingly. In comparison with previously known surface-modified carbon fibers, it is possible for the first time to provide a material with the carbon fibers according to the invention which opens up completely new fields of application by further modifying the surface by means of organic molecules.

Therefore, those carbon fibers are particularly preferred in which the ratio of oxygen atoms to carbon atoms, derived from the atomic surface concentrations measured with XPS, is greater than 0.2. XPS is for the purposes of the invention for "X-ray photoelectron spectroscopy".

For the subsequent use of the functionalized carbon nanofibers, it is desirable that the functional groups produced in the nitric acid gas phase treatment on the surface of the carbon nanofibers be as reactive as possible for further subsequent reaction steps. Particularly reactive are free unesterified carboxyl or carboxylic acid groups, which are to be contained in the highest possible number, as well as carboxylic anhydride groups, which also have a sufficient reactivity.

Surprisingly, for the first time, carbon fibers with a particularly high proportion of carboxylic acid groups have become accessible through the use of the novel oxidation process.

Therefore, carbon fibers containing more than 400 μmol as the sum of carboxylic acid groups and carboxylic anhydride groups per g of carbon are further preferable. Particularly preferred are those Carbon fibers containing more than 350 μmol of carboxylic acid groups per g of carbon chemically bound.

The lowest possible leaving temperature in the TPD analysis is a sure sign of the best possible reactivity of the cleaving functional group for subsequent reactions. Since CO _{2 is} split off for the most part at lower temperatures than CO, carbon nanofibers, which split off more than 45% of their chemically bound oxygen as CO ₂ in the TPD analysis, are furthermore preferred. Very particular preference is given to carbon fibers which contain more oxygen in CO _2- releasing or desorbing groups than are bonded in CO-releasing groups.

In a further embodiment, the object underlying the invention is achieved by carbon fibers obtainable by the process according to the invention.

In yet another embodiment, the object underlying the invention is achieved by the use of the carbon fibers according to the invention in composite materials, energy storage devices, as sensors, as adsorbents, as carriers for heterogeneous catalysts or as catalytically active material.

Figure 1 shows the schematic representation of the structure of the treatment of carbon nanofibers with the vapor of nitric acid. The heating of the tubular reactor is carried out by a resistance heating band, that of the round bottom flask with an oil bath.

Figure 2 shows the following XPS spectra: (a) XPS overview spectrum, (b) C Is and (c) O Is-XP spectrum of carbon nanofibers treated with HNO ₃ vapor at different temperatures for 15 hours , The O is spectrum of the carbon nanofibers, which is 1.5 hours after the conventional method with liquid HNO ₃ were treated at 120 ⁰ C, is shown in (d) for comparison.

Figure 3 shows the oxygen to carbon ratio derived from the atomic surface concentrations (XPS) of the carbon nanofibers treated with HNO ₃ vapor for different times at different temperatures. The oxygen / carbon ratio after the conventional treatment is indicated for comparison.

FIG. 4 shows SEM images (a) of the untreated carbon nanofibers and (b) the carbon nanofibers treated with HNO ₃ vapor at 200 ^° C. for 15 hours.

FIG. 5 shows the comparison of the TPD cleavage profiles of carbon nanofibers during treatment by means of HNO 3 gas phase, NO 2, NO 2: O 2 (1: 1) and HNO 3 liquid. All treatments were carried out for 3 hours each. The graphics are normalized to 1 g carbon fiber.

FIG. 6 shows an overview of the various chemically bonded oxygen-containing groups on carbon nanofibers.

Figure 7 illustrates the method of peak fitting for the TPD profiles ((a) CO profile, (b) CO2 profile) using the example of gas phase treatment with HNO3 at 200 ° C for 15 h.

Table 1 shows the values for the quantification of the different functional groups from the TPD measurements for CO2 cleavage. The amounts are shown in μmol / g (10-6 mol / g). Table 2 shows the values for the quantification of the various functional groups from the TPD measurements for CO cleavage. The amounts are shown in μmol / g (10-6 mol / g).

Examples

The construction used for the HNO ₃ gas phase treatment is shown in FIG. Typically, 200 mg carbon nanofibers 1 (50-200 nm diameter, Applied Sciences, Ohio, USA) were incorporated into the reactor 2 and in various experiments to a temperature of 125 ⁰ C, 150 ⁰ C, 175 ° C, 200 ⁰ C 250 ⁰ C heated. The round bottom flask 9 was concentrated with 150 ml. HNO ₃ 5 filled and heated to 125 ⁰ C with stirring. The attached Gegenstromküh- ler 6 was connected to the exhaust. After a defined time period of 5, 10 and 15 hours, the heating of the oil bath 10 was turned off and the heating of the reactor 1 was held at 110 ° C for an additional 2 hours to dry the treated carbon nanofibers. Subsequently, the carbon nanofibers 1 were comprehensively characterized. The construction used effectively prevents the return of condensed liquid nitric acid within the cooler back over the sample. Accordingly, the treatment was completely carried out under gas-phase conditions, since wetting of the carbon nanofibers with liquid nitric acid was completely avoided. The morphology of carbon nanofibers was analyzed by scanning electron microscopy (LEO Gemini 1530). X-ray photoelectron spectroscopy (XPS) was performed in an ultra-high vacuum system with a Gamma-Data-Scienta SES 2002 analyzer. The pressure in the measuring chamber was 2xlQ ^{~ 10} rrsbar. As X-radiation, Al K ₀ radiation (1486.6 eV, 14 kV, 55 mA) was used at a transmission energy of 200 eV, whereby an energy resolution of better than 0.5 eV could be achieved. Possible charging effects were compensated by the use of a slow electron source. The binding energies were calibrated to the position of the main signal of the carbon (C Is) at 284.5 eV.

XP spectroscopy is a proven method for the characterization of oxygen-containing functional groups. Different oxygenated groups can be distinguished by the C Is and O ls spectra (Okpalugo, TIT et al., Carbon 43: 153-61 (2005); Martinez, MT et al., Carbon 41: 2247-56 (2003)). As an example of this, the XP spectra of carbon nanofibers are shown, which were treated for 15 hours at different temperatures. Figure 2 (a) shows the XPS overview spectra of the carbon nanofibers after 15 hours of HNO ₃ gas phase treatment at different temperatures. The signals in the C Is, O Is and O KLL regions are clearly visible. The presence of nitrogen is indicated by a weak N Is signal at about 400 eV. The intensity of the O ls signal increases with increasing temperature, while that of the C ls signal decreases accordingly.

The assignment of the signals in the C ls region is made in the literature as follows (Lakshminarayanan, PV et al., Carbon 42: 2433-42 (2004); Okpalugo, TIT et al., Carbon 43: 153-61 (2005 ): Carbon in graphite at 284.5 eV, carbon easily bound to oxygen in phenols and ethers (CO) at 286.1 eV, carbon doubly bound to oxygen in ketones and quinones (C = O) at 287.5 eV, carbon bonded to two oxygens. atoms in carboxyl groups, carboxylic and esters (COO) at 288τ7 ^"eV Uend ^{^'~} the ^~ ^ chBrakteristische- shake üp" -tinie of carbon-in-aromatic compounds at 190.5 eV (π -> π ^* transitions ). The C ls spectrum after 15 hours of HNO ₃ gas phase treatment is shown in Fig. 2 (b). The shoulder increasing in temperature with increasing temperature at higher bond energies of the C Is main signal at 284.5 eV becomes apparent by comparing the symmetry of the signals. Even clearer is the strong growth of the signal at 288.7 eV, which signals a sharp increase in the amount of -COO groups. These are mainly carboxyl groups and anhydrides, which are among the most important oxygen-containing functional groups on carbon surfaces for various applications.

The oil body level spectrum of the same batch of treated carbon nanofibers is shown in Fig. 2 (c). The two main contributions are indicated by the dashed lines and are on the one hand twice Assigned carbon-bound oxygen atoms (C = O) in quinones, ketones or aldehydes at 531.5 eV, and on the other hand, the simply bound to carbon oxygen atoms (CO) in ethers, hydroxyl groups or phenols at 533.2 eV (Bubert, H. et Anal., Bioanal Chem. 374: 1237-41 (2002); Zhang, J. et al., J. Phys. Chem. B 107: 3712-8 (2003)). Since both simple and double carbon-bonded oxygen atoms occur in esters, carboxyl groups, anhydrides or pyrans, both oxygen atoms of these groups contribute to the two oil signals. In the O Is spectra, it becomes clear that the main signal is dominated by the CO single bond at lower treatment temperatures, presumably due to the preferential formation of hydroxyl groups at low temperatures. With increasing temperature, the formation of C = O double bonds increases strongly. For comparison, the oil spectrum of the carbon nanofibers in conventional HNO ₃ treatment is shown in FIG. 2 (d). Here the contribution to the signal at 533.2 eV is greater than at 531.6 eV and is similar to the spectrum of HNO ₃ gas phase treatment at low temperatures. In the literature, results have been obtained with a similar trend in the conventional wet HNO ₃ method, ie the signal at 533.2 eV was greater than that at 531.6 eV (Martinez, MT et al., Carbon 41: 2247-56 (2003)). , Therefore, compared to the conventional HNO ₃ method, HNO ₃ gas phase treatment not only improves the yield but also alters the number of different oxygen-containing functional groups on the carbon nanofibers. It is known that the formation of various oxygen species such. B. C = O is strongly temperature dependent. Due to the azeotropic boiling point limit of concentrated HNO ₃ of 122 ⁰ C, it is not possible to perform the conventional HNO ₃ treatment at temperatures higher than 122 ⁰ C and atmospheric pressure, whereby the production of certain species within a given reaction time is limited. The atomic surface concentrations of carbon and oxygen were determined by XPS measurements (Xia, W. et al., Catal. Today 102-103: 34-9 (2005)). The ratio of oxygen to carbon (O / C) of the carbon nanofibers after different treatments is shown in FIG. It can be seen that the O / C ratio after HNO ₃ treatment at 125 ⁰ C is about 0.155, which is slightly higher than in the conventional HNO ₃ treatment at 120 ⁰ C and 1.5 hours duration and slightly lower than in the conventional mixed acid treatment (HNO ₃ and H ₂ SO ₄₎ at 120 ⁰ C and 1.5 hours duration. The ratio increases with increasing temperature and duration of treatment. After 15 hours of treatment at 175 ⁰ C or 200 ⁰ C, the ratio is more than 0.21. Under these conditions, the amount of oxygen on the carbon nanofibers seems to reach the saturation limit, which becomes apparent by the flattening of the correlation curve.

After the HNO ₃ gas phase treatment, the carbon nanofibers could be used without additional steps such. As filtration, washing and drying continue to be used. There was no change in the bulk density of the carbon nanofibers after the treatment ^" and the SEM images show that no morphological changes of the carbon nanofibers occurred through the treatment (Figure 4) .The frequent agglomeration by conventional treatment with liquid HNO ₃ was not observed in the HNO ₃ gas-phase treatment, nor is the morphology of the carbon nanofibers altered by the gas-phase treatment (Figure 3) and the treatment of carbon nanofibers grown on various carbon substrates, such as Graphite foil or carbon fibers (Briggs, D. et al., John Wiley & Sons 635-6 (2994); Li, N. et al., Adv. Mater. 19: 2957-60 (2007)) 1.5 hours of refluxing in a stirred HNO ₃ solution, the carbon nanofibers were largely detached from the substrate, so that a dark suspension was present.The carbon nanofibers bli just after the ENT ₃ - Gas phase treatment, however, intact on the substrate. This result is particularly important for the applications of carbon nanofibers where the secondary structure is to be maintained, e.g. As in vertically oriented carbon nanofibers or branched carbon nanofiber networks.

In order to obtain information about the type of functional groups reacted on the carbon nanofibers, TPD measurements (temperature programmed desorption) were carried out.

For this purpose, about 150-200 mg of the functionalized carbon nanofibers (Baytubes C150P, treated with HNO ₃ GaS for 3 hours at 300 ⁰ C) in a horizontal quartz tube with 10 mm inner diameter and helium (99.9999% purity, flow rate 30 sccm) as a carrier gas over it. The sample was then heated from room temperature to 1000 ^° C. at a heating rate of 2 K / min and the amounts of CO and CO _{2 released were determined} by means of an online infrared detector (Binos) in the gas stream. The temperature was held at 1000 ⁰ C for a total of one hour before being cooled back to room temperature. The detector itself had previously been calibrated with the gases mentioned for a measuring range of 0 to 4000 ppm.

For comparison with other methods for oxidative functionalization, carbon nanofibers (Baytubes C150P) were conventionally treated in liquid phase with HNO ₃ as well as in the gas phase with NO ₂ and a mixture of NO ₂ and O ₂ . These gas phase treatments were carried out in a vertical quartz tube having an inner diameter of 20 mm. In one experiment, NO ₂ (10 vol% in helium) was passed through the bed of carbon nanofibers at a flow rate of 10 sccm. For the treatment with NO ₂ + O ₂ , additional oxygen (20.5% by volume in N ₂ , 5 sccm) was passed into the NO ₂ / He gas stream in order to set a ratio of NO ₂ : O ₂ of 1: 1 in the carrier gas. For the treatment in liquid phase, the coal The substance nanofibers are refluxed for 3 hours in concentrated nitric acid (65%, JTBaker).

The results (FIG. 5) show a significantly different release of CO and CO ₂ as a function of the temperature for the differently functionalized carbon nanofibers. It is clear that the carbon nanofibers treated with HNO ₃ in the gas phase release larger amounts of both CO and CO ₂ , indicating an overall higher surface functionalization with oxygen-containing groups. In addition, the sample treated with HNO ₃ in the gas phase shows a high rate of release of CO and CO ₂ at about 600 ^° C., which points in particular to a high proportion of carboxylic anhydride functionalities.

However, the release curves in FIG. 5 also show that CO is released at much higher temperatures than CO ₂ . This is due to the higher bond strength of the functional groups from which CO is split off. An overview of the functional groups usually present in oxidized carbon nanofibers is shown in FIG. 6. From the literature, the following assignment for the cleavage temperatures can be made:

CO ₂ : chemisorbed CO ₂ below 250 ^° C.

Carboxylic acid 310 ^° C.

Carboxylic anhydride 42O ⁰ C

La cto n 580 ⁰ C

CO: aldehyde, ketone below 300 ^° C.

Carboxylic 420 ⁰ C.

Phenol, ether 700 ^° C

Pyron 830 ⁰ C

Based on these assignments, a sum of curves with Gaussian normal distribution were adapted to the TPD curves (FIG. 7) and from this the quantitative assignment (Tables 1 and 2) to the functional groups originally contained in the carbon nanofibers.

Table 1

Sample CO ₂ carboxylic acid carboxylic acid La cto n chemisorbed anhydride

15 h at 200 ^° C. 87 546 142 47 HNO ₃ liquid 117 305 58 46 NO ₂ gas, 3 h at 200 ^° C. 8,131,240

Table 2

Pattern ketone, phenol, pyrone

Carboxylic acid aldehyde anhydride ether

15 h at 200 ^° C. 28 142 1023 317 HNO ₃ liquid 58 105 741 197 NO ₂ gas, 3 h at 200 ^° C. 12 35 250 87

Claims

claims

A process for the functionalization of carbon fibers (1), wherein a) Kohienstofffasern (1) in a reactor (2) having an inlet (3) and an outlet (4), b) the reactor (2) to a Heated temperature in a range of 125 to 500 ⁰ C, c) steam of nitric acid (5) passes through the reactor (2), and d) then the treated carbon fibers (1) is dried.

2. The method according to claim 1, characterized in that one uses as carbon fibers (1) carbon nanofibers, in particular those having an outer diameter in a range of 3 to 500 nm.

^'3. A method according to claim 1, characterized in that as the carbon fibers (1) those having a BET surface area in a range of- lθ-bis- θ ^~ 56 m ² / GYH nsb it θfi d ere-i-n emem -Receive of 2-0-bi s-200 m ² / g.

4. The method according to claim 1, characterized in that after the outlet of the reactor (4) provides a condenser (6), wherein the outlet of the condenser (7) for the condensate via a return line (8) with a reservoir (9) connected to the nitric acid.

5. The method according to claim 4, characterized in that one uses as a Vorratsbehäiter (9) for the nitric acid, a glass bulb which is heated in particular with an oil bath (10).

6. The method according to claim 1, characterized in that after step b) leaves the reactor (2) for a period in a range of 3 to 20 h, in particular in a range of 5 to 15 h at this temperature.

7. The method according to claim 1, characterized in that one carries out the step c) over a period of time in a range of 0.5 to 4 hours and independently at a temperature in a range of 80 to 150 0C.

8. carbon fibers, characterized in that the ratio of oxygen atoms to carbon atoms derived from the atomic surface concentrations measured with XPS is greater than 0.18.

9. carbon fibers, characterized in that they contain more than 350 .mu.mol carboxylic acid groups per g of carbon chemically bonded.

10. carbon fibers according to claim 9, characterized in that they contain more than 400 .mu.mol chemically bound as the sum of carboxylic acid groups and carboxylic anhydride groups per g of carbon.

11. carbon fibers according to claim 9 or 10, characterized in that they split off more than 45% of the chemically bound oxygen in the TPD analysis as CO2.

12. carbon fibers according to claim 8, characterized in that the fibers have a mean diameter of 3 to 500 nm, and a length to diameter ratio of at least 5: 1.

13. carbon fibers according to any one of claims 8 to 12, obtainable by the method according to claim 1.

14. Use of the carbon fibers according to one of claims 8 to 13 in composite materials, energy storage devices, as sensors, as adsorbents, as carriers for heterogeneous catalysts or as catalytically active material.