EP2512659A2

EP2512659A2 - Nitrogen doped carbon nanotubes with metal nanoparticles

Info

Publication number: EP2512659A2
Application number: EP10787809A
Authority: EP
Inventors: Jens Assmann; Aurel Wolf; Leslaw Mleczko; Oliver Felix-Karl SCHLÜTER
Original assignee: Bayer Intellectual Property GmbH
Current assignee: Bayer Intellectual Property GmbH
Priority date: 2009-12-18
Filing date: 2010-12-14
Publication date: 2012-10-24
Also published as: SG181428A1; US20120252662A1; DE102009058833A1; WO2011080066A3; CN102821846A; KR20120095423A; WO2011080066A2; JP2013514164A

Abstract

The invention relates to nitrogen-doped carbon nanotubes (NCNT), the surface of which is charged with metal nanoparticles, and to a method for the production thereof and use thereof as a catalyst.

Description

Nitrogen-doped carbon nanotubes with metal nanoparticles

The invention relates to nitrogen-doped carbon nanotubes (NCNT), which are loaded on their surface with metal nanoparticles, and a process for their preparation and their use as a catalyst. Carbon nanotubes are well known to those skilled in the art, at least since their description in 1991 by Iijima (S.Iijima, Nature 354, 56-58, 1991). Since then, carbon nanotubes have been comprised of cylindrical bodies comprising carbon with a diameter between 3 and 80 nm and a length which is a multiple, at least 10 times, of the diameter. Also characteristic of these carbon nanotubes are layers of ordered carbon atoms, with the carbon nanotubes usually having a different core in morphology. Synonyms for carbon nanotubes are, for example, "carbon fibrils" or "hollow carbon fibers" or "carbon bamboos" or (in the case of wound structures) "nanoscrolls" or "nanorolls".

These carbon nanotubes have a technical importance for the production of composite materials due to their dimensions and their special properties. Substantial further possibilities are in electronics and energy applications, as they are generally characterized by a higher specific conductivity than graphitic carbon, e.g. in the form of Leitruß. The use of carbon nanotubes is particularly advantageous if they are as uniform as possible in terms of the abovementioned properties (diameter, length, etc.).

Also known is the possibility of these carbon nanotubes by heteroatoms, e.g. of the fifth main group (such as nitrogen) during the carbon nanotube manufacturing process.

The well-known methods for the production of nitrogen-doped carbon nanotubes are based on the conventional production methods for the classical carbon nanotubes such as arc, laser ablation and catalytic processes. Among other things, arc and laser ablation processes are characterized in that carbon black, amorphous carbon and high-diameter fibers are formed as by-products in the course of these production processes, with which the resulting carbon nanotubes usually have to be subjected to elaborate after-treatment steps, which is the products obtained from these processes and thus these Makes the process economically unattractive. On the other hand, catalytic processes offer advantages for economical production of carbon nanotubes, since by these processes it is optionally possible to produce a product of high quality in good yield.

Such a catalytic process, in particular a fluidized bed process, is disclosed in DE 10 2006 017 695 Al. In particular, the process disclosed therein comprises an advantageous mode of operation of the fluidized bed, by means of which carbon nanotubes can be produced continuously while supplying new catalyst and removing product. It is also disclosed that the starting materials used may comprise heteroatoms. A use of starting materials, which would result in a nitrogen doping of the carbon nanotubes, is not disclosed. A similar process for the targeted, advantageous production of nitrogen-doped carbon nanotubes (NCNT) is disclosed in WO 2009/080204. In WO 2009/08204, it is also known that nickel-doped carbon nanotubes (NCNTs) may still contain residues of the catalyst material for their production. These residues of the catalyst material may be metal nanoparticles. A method of retrofilling the nitrogen-doped carbon nanotubes (NCNT) is not disclosed. According to the method according to WO 2009/080204, it is further preferred to remove the residues of the catalyst material.

However, according to WO 2009/080204, it is always the case that only small amounts of the catalyst material are contained in the resulting nitrogen-doped carbon nanotubes (NCNT). The list of possible catalyst materials which may be present in minor proportions in the produced nitrogen-doped carbon nanotubes (NCNT) consists of Fe, Ni, Cu, W, V, Cr, Sn, Co, Mn and Mo, and optionally Mg, Al , Si, Zr, Ti, as well as other Mischmetalloxid forming elements known in the art and their salts and oxides.

Furthermore, WO 2009/080204 does not disclose in which forms the nitrogen may be contained in the nitrogen-doped carbon nanotubes (NCNT).

That carbon nanotubes without heteroatoms can be subsequently loaded with silver on their surface is disclosed by Yan et al. in "Materials of a high dispersion of silver nanoparticles on surface-functionalized multi-walled carbon nanotubes using an electrostatic technology", in Materials Letters 63 (2009) 1 7 1-173, it is possible to subsequently load carbon nanotubes with silver, by first functionalizing them with oxidizing acids such as nitric acid and sulfuric acid on their surface According to the disclosure of Yan et al Carbon nanotubes with the oxidizing acids on their surfaces functional groups that serve as "anchorages" for the deposited on these silver nanoparticles.

The method of loading the oxidized carbon nanotubes according to the disclosure of Yan et al. consists of the steps of dispersing the oxidized carbon nanotubes in dimethyl sulfoxide, adding silver nitrate and reducing the silver with sodium citrate on the surface of the oxidized carbon nanotubes.

Since, according to Yan et al. the oxidizing property of the acids is relevant, according to the disclosure of Yan et al. It can be assumed that the heteroatoms are oxygen and thus no nitrogen-doped carbon nanotubes (NCNT) are disclosed as starting point for the silver-loaded carbon nanotubes.

WO 2008/138269 discloses nitrogen-containing carbon nanotubes with platinum or ruthenium metal nanoparticles having a nitrogen content of 0.01 to 1.34, expressed in the ratio of nitrogen to carbon (CN _X with x = 0.01-1, 34). , exhibit. According to WO 2008/138269, the platinum or ruthenium metal nanoparticles have a diameter of 0.1 to 15 nm and are present in a proportion of 1 to 100% of the total mass of the nitrogen-containing carbon nanotubes.

The disclosed method of producing the nitrogen-containing carbon nanotubes with platinum or ruthenium metal nanoparticles comprises dissolving salts of platinum and ruthenium in a solution, introducing the nitrogen-containing carbon nanotubes into this solution, and reducing the salts adsorbed on the surface of the nitrogen-containing carbon nanotubes of platinum and ruthenium by a chemical reducing agent.

It is not disclosed in WO 2008/138269 that metal nanoparticles other than platinum or ruthenium may be present. Furthermore, WO 2008/138269 does not disclose the nature of the nitrogen in the nitrogen-containing carbon nanotubes.

According to the prior art, it is therefore only a few areas solved problem, subsequently applied to carbon nanotubes and in particular on nitrogen-doped carbon nanotubes (NCNT) metal nanoparticles.

In particular, there is the problem of providing nitrogen-doped carbon nanotubes (NCNT) as well as a process for their preparation on which any metal nanoparticles are applied in finely divided, dispersed form. Such finely dispersed metal nanoparticles on Nitrogen-doped carbon nanotubes would be particularly useful as catalyst materials.

It has now surprisingly been found, as a first subject of the present invention, that this object is achieved by a catalyst comprising nitrogen-doped carbon nanotubes (NCNT) with a proportion of at least 0.5% by weight of nitrogen which is at least 40 mol% pyridinic nitrogen , is present in the nitrogen-doped carbon nanotubes (NCNT) and wherein on the surface of the nitrogen-doped carbon nanotubes (NCNT) from 2 to 60 wt .-% metal nanoparticles of an average particle size in the range of 1 to 10 nm are dissolved can. The nitrogen-doped carbon nanotubes (NCNT) preferably have a nitrogen content in the range of 0.5% to 18% by weight, and more preferably in the range of 1% to 16% by weight.

The nitrogen present in the nitrogen-doped carbon nanotubes (NCNT) of the present invention is incorporated in the graphitic layers and is present therein at least in part as pyridinic nitrogen. However, the nitrogen present in the nitrogen-doped carbon nanotubes (NCNT) according to the invention can also be present as nitro nitrogen and / or as nitroso nitrogen and / or pyrrolic nitrogen and / or amine nitrogen and / or as quaternary nitrogen.

The proportions of quaternary nitrogen and / or nitro and / or nitroso and / or amine and / or pyrrolic nitrogen are of secondary importance to the present invention in that their presence does not significantly hinder the invention as long as the proportions of pyridinic nitrogen described Nitrogen are present.

The proportion of pyridinic nitrogen in the catalyst according to the invention is preferably at least 50 mol%. In the context of the present invention, "pyridinic nitrogen describes nitrogen atoms present in a heterocyclic compound consisting of five carbon atoms and the nitrogen atom in the nitrogen-doped carbon nanotubes (NCNT). An example of such a pyridinic nitrogen is shown in Figure (I) below.

(I) However, pyridinic nitrogen refers not only to the aromatic form of the aforementioned heterocyclic compound shown in Figure (I), but also the mono- or polyunsaturated compounds of the same molecular formula.

In addition, other compounds of the term "pyridinic nitrogen are also included when such other compounds comprise a heterocyclic compound consisting of five carbon atoms and the nitrogen atom. An example of such pyridinic nitrogen is shown in Figure (II).

For example, Figure (II) shows three pyridinic nitrogen atoms that are part of a multicyclic compound. One of the pyridinic nitrogens is part of a non-aromatic heterocyclic compound.

By contrast, in the context of the present invention, "quaternary nitrogen denotes nitrogen atoms which are covalently bonded to at least three carbon atoms. For example, such quaternary nitrogen may be part of multicyclic compounds as shown in Figure (III).

Pyrrolic nitrogen in the context of the present invention describes nitrogen atoms present in a heterocyclic compound consisting of four carbon atoms and the nitrogen atom in the nitrogen-doped carbon nanotubes (NCNT). An example of a pyrrole compound in connection with the present invention is shown in Figure (IV):

Also in the context of "pyrrolic nitrogen, it is understood that this is not to be understood as being limited to the heterocyclic unsaturated compound shown in Figure (IV), but also to saturated compounds having four carbon atoms and a nitrogen atom in cyclic arrangement in the context of the present invention are included.

Nitro- or nitroso-nitrogen in the context of the present invention refers to nitrogen atoms in the nitrogen-doped carbon nanotubes (NCNT), which are bound to at least one oxygen atom regardless of their further covalent bonds. A special manifestation of such a nitro or nitroso nitrogen is shown in Figure (V), which is intended to explain in particular the distinction from the abovementioned pyridinic nitrogen.

From the figure (V) it is apparent that in contrast to the compounds which comprise a "pyridinic nitrogen in the context of the present invention, here the nitrogen is also covalently bonded to at least one oxygen atom. Thus, the heterocyclic compound no longer consists of only five carbon atoms and the nitrogen atom, but consists of five carbon atoms, the nitrogen atom and an oxygen atom.

In addition to the compound shown in Figure (V), the term nitro or nitroso nitrogen in connection with the present invention also covers the compounds which consist only of nitrogen and oxygen. The appearance of nitro or nitroso nitrogen shown in Figure (V) is also referred to as oxidized pyridinic nitrogen.

Amine nitrogen in connection with the present invention denotes nitrogen atoms which are present in the nitrogen-doped carbon nanotubes (NCNT) bound to at least two hydrogen atoms and to at most one carbon atom, but which are not bound to oxygen.

The presence of pyridinic nitrogen in the stated proportions is particularly advantageous because it was surprisingly found that in particular the pyridinic nitrogen simplifies a later loading of the surface of the nitrogen-doped carbon nanotubes (NCNT) with metal nanoparticles and that these nitrogen species, especially in the presence of the abovementioned proportions, result in a finely divided dispersion of the metal nanoparticles on the Surface of the nitrogen-doped carbon nanotube leads, which is because of the resulting high specific surface area of the metal nanoparticles of particular advantage.

In particular, this particularly good distribution of the metal nanoparticles on the surface of the nitrogen-doped carbon nanotubes (NCNT) means that many catalytically active centers are simultaneously available for a reaction at the catalyst surface. This is particularly advantageous for later use of the nitrogen-doped carbon nanotubes (NCNT) according to the invention with metal nanoparticles as catalysts in heterogeneous catalytic chemistry.

Without wishing to be bound by theory, it would seem that the pyridinic nitrogen groups present anisotropically on the surface of the nitrogen-doped carbon nanotubes (NCNT) would have condensation sites for the later metal nanoparticles, especially at the pyridinic nitrogen groups The metal nanoparticles adhere particularly well because of a molecular interaction between the metal nanoparticles and the pyridinic backbone groups, that is, at the surface of the backbone-doped carbon nanotubes (NCNT) are created arises. In particular, that molecular interaction probably entails the advantageous use of the nitrogen-doped carbon nanotubes (NCNT) with metal nanoparticles over the pure metal nanoparticles as improved catalysts, as is also the subject of the present invention.

The metal nanoparticles can be selected from a metal selected from among Fe, Ni, Cu, W, V, Cr, Sn, Co, Mn, Mo, Mg, Al, Si, Zr, Ti, Ru, Pt, Ag, Au, Pd, Rh, Ir, Ta, Nb, Zn and Cd exist.

The metal nanoparticles preferably consist of a metal selected from the list consisting of Ru, Pt, Ag, Au, Pd, Rh, Ir, Ta, Nb, Zn and Cd.

Particularly preferably, the metal nanoparticles consist of a metal selected from the list consisting of Ag, Au, Pd, Pt, Rh, Ir, Ta, Nb, Zn and Cd.

Most preferably, the metal nanoparticles of platinum (Pt). The mean particle size of the metal nanoparticles is preferably in the range of 2 to 5 nm.

The proportion of the metal nanoparticles on the catalyst comprising nitrogen-doped carbon nanotubes (NCNT) with metal nanoparticles is preferably from 20 to 50% by weight.

Another object of the present invention is a process for the preparation of nitrogen-doped carbon nanotubes (NCNT) with metal nanoparticles on their surface, characterized in that it comprises at least the steps: a) introducing nitrogen-doped carbon nanotubes (NCNT) with a Proportion of at least 0.5% by weight of nitrogen which is at least 40 mol% pyridinic nitrogen, in a solution (A) comprising a metal salt, b) reduction of the metal salt in the solution (A) in the presence of the nitrogen-doped Carbon nanotube (NCNT) optionally with addition of a chemical reducing agent (R), and c) separation of the nitrogen-doped carbon nanotubes (NCNT), now loaded with metal nanoparticles from solution (A).

The nitrogen-doped carbon nanotubes (NCNT) used in step a) of the process of the invention are usually those which can be obtained from the processes according to WO 2009/080204. In a first preferred embodiment of the process, there are nitrogen-doped carbon nanotubes (NCNT) which have a nitrogen content in the range from 0.5% to 1 8% by weight. Preferably, it is nitrogen-doped carbon nanotubes (NCNT), which have a nitrogen content in the range of 1 wt .-% to 16 wt .-%.

In a second preferred embodiment of the process, nitrogen-doped carbon nanotubes (NCNTs) have at least 50 mole% of pyridinic nitrogen at the nitrogen contained in the nitrogen-doped carbon nanotube (NCNT).

It has surprisingly been found that prior treatment of the nitrogen-doped carbon nanotubes (NCNT) prior to reduction of the metal salt in their presence in the Contrary to the use of other carbon nanotubes is not necessary. This is especially true when the preferred nitrogen-doped carbon nanotubes (NCNT) described above are used. Compared to the prior art, this represents a significant simplification of the production process. This, without being bound thereby to a theory, apparently made possible in that in particular the pyridinic surface structures of the nitrogen-doped carbon nanotubes (NCNT) in contrast to the surface structures of " normal "carbon nanotubes are preferred condensation / adsorption sites for metal salts / metal.

The solution (A) of a metal salt into which the nitrogen-doped carbon nanotubes obtained in step a) is introduced is usually a solution of a salt of one of the metals selected from the list consisting of Fe, Ni, Cu, W, V, Cr , Sn, Co, Mn, Mo, Mg, Al, Si, Zr, Ti, Ru, Pt, Ag, Au, Pd, Rh, Ir, Ta, Nb, Zn and Cd.

Preferably, the metals are selected from the list consisting of Ru, Pt, Ag, Au, Pd, Rh, Ir, Ta, Nb, Zn and Cd. The metals are particularly preferably selected from the list consisting of Ag, Au, Pd, Pt, Rh, Ir, Ta, Nb, Zn and Cd. Most preferably, the metal is platinum (Pt).

The metal salts are usually salts of the aforementioned metals with a compound selected from the list consisting of nitrate, acetate, chloride, bromide, iodide, sulfate. Preference is given to chloride or nitrate.

The metal salts are present in the solution (A) usually in a concentration in the range of 1 to 100 mmol / l, preferably in the range of 5 to 50 mmol / l, particularly preferably in the range of 5 to 15 mmol / l.

The solvent of the solution (A) is usually one selected from the list consisting of water, ethylene glycol, monoalcohols, dimethyl sulfoxide (DMSO), toluene and cyclohexane.

The solvents are preferably selected from the list consisting of water, DMSO, ethylene glycol and monoalcohols.

The monohydric alcohols are usually methanol or ethanol, as well as their mixtures.

By using the particularly advantageous nitrogen-doped carbon nanotubes (NCNT) with a high proportion of pyridinic nitrogen on their surface, can be dispensed with the further addition of additives, such as for colloid stabilization. Nevertheless, the Further improvement of the catalysts obtained from the process, the addition of such additives, such as for colloid stabilization, be advantageous.

The reduction according to step b) of the process according to the invention is usually carried out using a chemical reducing agent (R) selected from the list consisting of ethylene glycol, monoalcohols, citrates, borohydrides, formaldehydes, DMSO and hydrazine.

It will thus be appreciated that the amount of possible solvents of solution (A) disclosed above is in part congruent with the amount of the possible reducing agents (R) just disclosed. Accordingly, in many cases, the further addition of a reducing agent (R) can be dispensed with. It is therefore preferred that the chemical reducing agent (R) and the solvent of the solution (A) are at least partially identical.

It has surprisingly been found that in many embodiments of the process according to the invention the process can be simplified by virtue of the fact that the solvent and the chemical reducing agent (R) are at least partially identical, which is possible by virtue of the fact that He already used nitrogen-doped carbon nanotubes (NCNT) already have high levels of pyridinic nitrogen on their surface, which serves, as just described as an active center / adsorption point for the deposition of the metal on its surface. This high affinity also eliminates the need for further addition of a reducing agent (R) in many embodiments. From the above, it will be understood that reduction in the presence of the nitrogen-doped carbon nanotubes (NCNT) according to step b) of the process means both the reduction of a metal salt on the surface of the nitrogen-doped carbon nanotubes (NCNT), as well as the reduction the metal salt in the solution (A) under in the same solution (A) thereafter taking place adsorption of the metal nanoparticle nuclei formed. A precise distinction is often not possible because, for example, in the case of reduction in the solution with subsequent adsorption on the surface, these processes take place partly simultaneously.

In particular, however, a distinction is also not necessary, since in both cases, in particular the advantageous properties of the nitrogen-doped carbon nanotubes used (NCNT) cause precipitate on the surface thereof finely dispersed metal nanoparticles, so that with the inventive method obtained catalyst In particular, characterized by an extremely high specific surface of the metal nanoparticles and wherein a subsequent sintering of the metal nanoparticles by the immobilization of the same on the surface at the aforementioned condensation centers of the nitrogen-doped carbon nanotubes (NCNT) also prevented or at least greatly reduced. The separation according to step c) of the process according to the invention is usually carried out by methods which are generally known to the person skilled in the art. Non-exhaustive example of such separation is about the filtration.

Another object of the present invention is the use of nitrogen-doped Kohlenstoffhanoröhrchen (NCNT) with a proportion of at least 0.5 wt .-% of nitrogen, which is at least 40 mol% pyridinic nitrogen and on which on the surface of the nitrogen doped carbon nanotubes (NCNT) from 2 to 60% by weight of metal nanoparticles with a particle size of 1 to 10 nm are used as catalysts.

Preferred is a use as catalysts in the electrolysis.

In the following, the process according to the invention and the catalysts according to the invention comprising nitrogen-doped carbon nanotubes (NCNT) with metal nanoparticles are illustrated by means of some examples, the examples, however, not being understood as limiting the inventive idea.

In addition, the invention will be further illustrated with reference to figures, without, however, limiting them thereto. 1 shows an excerpt of an X-ray photoelectron spectroscopy (ESCA) investigation of the nitrogen-doped carbon nanotubes used in the context of Example 1. In particular, the spectrum of the nitrogen-doped carbon nanotubes used in the context of Example 1 in the range of a binding energy [B] of 390 to 410 eV is shown. Below the measured spectrum (O: black, thick, hatched, solid line) are the approximated ideal measurement signals of a pyridinic nitrogen species (A: black, thin, long dashed line), a pyrrole nitrogen species (B: black, thin, short dashed line), a first quaternary nitrogen species (C: black, thin solid line), a second quaternary nitrogen spike (D: gray, thick solid line), a nitroso nitrogen species, and oxidized pyridinic nitrogen species, respectively (E : gray, thick, hatched line) and a nitro nitrogen species (F: dark gray, thick solid line). Also indicated on the x-axis are the respective values of the binding energy (eV) at which the measured value maximum is present for a particular nitrogen species. The sum of Approximated ideal measurement signals result in addition a smooth representation of the measured spectrum (O).

FIG. 2 shows a first transmission electron microscopic (TEM) image of the catalyst prepared according to Example 1.

FIG. 3 shows a second transmission electron microscopic (TEM) image of the catalyst prepared according to Example 1.

4 shows a first transmission electron microscopic (TEM) image of the catalyst prepared according to Example 2.

FIG. 5 shows a second transmission electron microscopic (TEM) image of the catalyst prepared according to Example 2.

FIG. 6 shows a transmission electron microscopic (TEM) image of the catalyst prepared according to Example 3.

Examples:

Example 1: Preparation of a catalyst according to the invention

The nitrogen-doped carbon nanotubes were prepared according to Example 5 of WO 2009/080204 with the only difference that apart from pyridine was used as starting material, the reaction was carried out at a reaction temperature of 700 ° C and the reaction time was limited to 30 minutes ,

Residuals of the catalyst used (a catalyst according to Example 1 of WO2009080204 was prepared and used) were removed by washing the resulting nitrogen-doped carbon nanotubes in 2 molar hydrochloric acid for 3 hours under reflux.

The resulting nitrogen-doped carbon nanotubes were partially subjected to the test according to Example 4.

The nitrogen-doped carbon nanotubes thus obtained were subsequently dispersed in 467 ml of ethylene glycol by placing them in this liquid and stirring them for 10 minutes at 3000 rpm with a SILVERSON stirrer-type stirrer.

To the obtained dispersion of the nitrogen-doped carbon nanotubes were subsequently added 187 ml of a silver salt solution of 2.5 g of hexachloroplatinic acid hydrate (Umicore) in distilled water at a rate of about 1 ml / min. The dispersion was further stirred. After the platinum salt solution had been added, a pH of 11-12 in the dispersion was adjusted with 1, 5-molar NaOH solution in ethylene glycol.

Subsequently, the dispersion thus obtained was transferred to a 3-neck flask and reacted therein for 3 hours at about 140 ° C under reflux and inert gas atmosphere (argon).

The dispersion was then cooled to room temperature by simply allowing it to stand under ambient conditions (1013 hPa, 23 ° C) and then passed through a filter paper (Blauband Rundfilter, Schleicher & Schull) and washed once with distilled water, whereby the catalyst of the invention from Dispersion was separated. The resulting, still moist solid was then dried for a further 12 hours in a vacuum oven (pressure - 10 mbar) and 80 ° C.

The catalyst according to the invention was subsequently fed to the investigation according to example 5. Example 2: Preparation of a first catalyst not according to the invention

Nitrogen-doped carbon nanotubes were prepared analogously to Example 1, with the only difference that the reaction was carried out for 120 min. was executed.

The nitrogen-doped carbon nanotubes were also partially subjected to the test according to Example 4 prior to dispersion in 467 ml of ethylene glycol.

Thereafter, a treatment similar to that according to Example 1 followed again. The catalyst obtained was subsequently also subjected to the test according to Example 5.

Example 3: Preparation of another catalyst not according to the invention

An experiment similar to that in Example 1 was carried out with the only difference that instead of the nitrogen-doped carbon nanotubes used there, commercially available carbon nanotubes (BayTubes®, BayTubes) were used.

It follows that from the washing of the carbon nanotubes in 2 molar hydrochloric acid, the preparation of the catalyst was carried out in the same way.

An investigation according to Example 4 was not carried out in the absence of nitrogen constituents in the commercial carbon nanotubes.

The catalyst obtained was subsequently also subjected to the test according to Example 5.

Example 4: X-ray Photoelectron Spectroscopy (ESCA) Examination of the Catalysts According to Example 1 and Example 2 By means of X-ray photoelectron spectroscopy (ESCA; apparatus: ThermoFisher, ESCALab 220iXL; method: according to the manufacturer) investigation was made for the nitrogen-doped carbon nanotubes as described In the course of Example 1 and Example 2 were obtained, the mass fraction of nitrogen at the nitrogen-doped carbon nanotubes, and determined within the determined mass fraction of nitrogen of the nitrogen-doped carbon nanotubes, the molar fraction of various nitrogen species. The values determined are summarized in Table 1.

The determination of the molar fractions of the various nitrogen species or the binding state of the nitrogen species was carried out by surface approximation under the respective binding energy value of the N 1 spectrum characteristic of the nitrogen species. Measured values according to Example 4

Sample pyridine

Pyridine Amine Pyrrole Quaternary Quaternary

N component (oxidized) NOx

N N N N N

[% By weight] N ⁺ -0 [mol%]

[mol%] [mol%] [mol%] [mol%] [mol%]

[Mol .-%]

Example 7,72,72 0 24,21 10,07 9,71 4,92 2,36

Example 2 10 39.66 0 6.5 40.54 4.05 6.67 2.58

For this purpose, it was assumed that the individual values characteristic of the nitrogen species were superimposed, with the resulting proportions being mathematically adapted to the measured spectrum. This is shown by way of example for the measured spectrum of the nitrogen-doped carbon nanotubes used according to the invention according to Example 1 in FIG.

The comparison of the nitrogen-doped carbon nanotubes according to the invention used in Example 1 with the non-inventive nitrogen-doped carbon nanotubes used according to Example 2 shows that the proportion of nitrogen in the nitrogen-doped carbon nanotubes according to Example 2 is higher than in the case of the nitrogen-doped carbon nanotubes according to Example 1, but that the molar proportion of the pyridinic nitrogen species in the nitrogen-doped carbon nanotube according to Example 1 is again greater than that in the nitrogen-doped carbon nanotubes according to Example 2. Conversely, in turn It is with the proportions of quaternary nitrogen. Example 5: Transmission electron microscopic (TEM) examination of the catalysts according to Example 1, Example 2 and Example 3

The catalysts obtained according to Examples 1 to 3 were subsequently examined optically for their loading with platinum under the aid of a transmission electron microscope (TEM; P hil ip s TE CNA I 20, with a 200 kV acceleration voltage). 2 and 3, the catalysts of the invention are gem. Example 1 shown. It can be seen that the nitrogen-doped carbon nanotubes are loaded with finely divided particles of platinum dispersed on their surface in a size of about 2 to 5 nm. The loading of the nitrogen-doped Kohlenstoffhanoröhrchen with platinum is about 50 wt .-% platinum, based on the total mass of the catalyst according to the invention. In contrast to FIGS. 2 and 3 of the catalyst according to the invention, FIGS. 4 and 5 of the first catalyst not according to the invention show that there has not been a finely divided distribution of particles of platinum on the surface of the nitrogen-doped carbon nanotubes. The particles of platinum are predominantly larger than 10 nm and some of them are also present as agglomerates, whose size even exceeds the diameter of the nitrogen-doped carbon nanotubes. Accordingly, the difference in pyridinic nitrogen in the nitrogen-doped carbon nanotubes alone seems to be crucial for the desired finely divided disp of the metal on the surface of the nitrogen-doped carbon nanotubes.

This assumption is further substantiated by the results of the measurement of the catalysts according to Example 3. These are shown in FIG.

These results illustrate that the inventive method successfully demonstrated in Example 1 for preparing the catalysts of the invention without subsequent functionalization of the carbon nanotubes and without the use of additives for the stabilization of metal nanoparticles, but the use of carbon nanotubes without nitrogen and in particular without a nitrogen content in the form of pyridinic species does not result in the desired high dispersion of metal nanoparticles on the carbon nanotubes. The commercial carbon nanotubes not used according to the invention remain largely uncovered and the platinum is found predominantly in the form of agglomerates again.

Claims

claims

A catalyst comprising nitrogen-doped carbon nanotubes (NCNT) in an amount of at least 0.5% by weight of nitrogen which is at least 40 mole% pyridinic nitrogen present in the nitrogen-doped carbon nanotubes (NCNT) and wherein the surface of the nitrogen-doped carbon nanotubes (NCNT) from 2 to 60 wt .-% metal nanoparticles of an average particle size in the range of 1 to 10 nm.

2. A catalyst according to claim 1, characterized in that the proportion of pyridinic nitrogen is at least 50 mol%.

3. A catalyst according to claim 1 or 2, characterized in that the metal nanoparticles of a metal selected from the list consisting of Fe, Ni, Cu, W, V, Cr, Sn, Co, Mn, Mo, Mg, Al, Si, Zr, Ti, Ru, Pt, Ag, Au, Pd, Rh, Ir, Ta, Nb, Zn and Cd.

4. A catalyst according to claim 3, characterized in that the metal is platinum (Pt).

5. Catalyst according to one of the preceding claims, characterized in that the metal nanoparticles have an average particle size in the range of 2 to 5 nm.

6. A process for the preparation of nitrogen-doped carbon nanotubes (NCNT) with metal nanoparticles on their surface, characterized in that it comprises at least the steps of: a) introducing nitrogen-doped carbon nanotubes (NCNT) in a proportion of at least 0.5 Wt .-% nitrogen, which is at least 40 mol% pyridine nitrogen, in a solution (A) comprising a metal salt, b) reduction of the metal salt in the solution (A) in the presence of the nitrogen-doped carbon nanotubes (NCNT) optionally under Adding a chemical reducing agent (R), and c) separating the nitrogen-doped carbon nanotubes (NCNT), now loaded with metal nanoparticles from solution (A).

7. The method according to claim 6, characterized in that nitrogen-doped carbon nanotubes (NCNT) have a nitrogen content in the range of 0.5 wt .-% to 18 wt .-%.

8. The method according to claim 6 or 7, characterized in that the nitrogen-doped carbon nanotubes (NCNT) a proportion of pyridinic nitrogen of at least

50 mol%.

9. The method according to any one of claims 6 to 8, characterized in that the chemical reducing agent (R) and the solvent of the solution (A) are at least partially identical.

10. The use of nitrogen-doped carbon nanotubes (NCNT) with at least 0.5% by weight of nitrogen which is at least 40 mol% of pyridinic nitrogen and on which the surface of the nitrogen-doped carbon nanotubes (NCNT) from 2 to 60 wt .-% metal nanoparticles of a particle size of 1 to 10 nm, as catalysts.