EP2185280A1

EP2185280A1 - Catalyst and method for the production and use thereof

Info

Publication number: EP2185280A1
Application number: EP08786740A
Authority: EP
Inventors: Stefan Kotrel; Gerhard Cox; Ekkehard Schwab; Alexander Panchenko
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2007-08-24
Filing date: 2008-08-01
Publication date: 2010-05-19
Also published as: CA2697118A1; KR20100065160A; JP2010536548A; US20110118110A1; CN101808734A; WO2009027171A1

Abstract

The invention relates to a catalyst, comprising an alloy made of at least two different metals, wherein at least one metal is a metal of the subgroup VIII. The alloy is present in at least two phases in different alloy qualities. The invention further relates to a method for the production of the catalyst, and a use of the catalyst.

Description

Catalyst and process for its preparation and use

description

The invention relates to a catalyst comprising an alloy of at least two different metals, wherein at least one metal is a metal of the VIII. Subgroup of the Periodic Table of the Elements according to the old name. The invention further relates to a process for the preparation of the catalyst and its use.

Fuel cells are energy converters that convert chemical energy into electrical energy. In a fuel cell, the principle of electrolysis is reversed. Today, various types of fuel cells are known, which generally differ in operating temperature from each other. The structure of the cells is basically the same for all types. They are generally composed of two electrode layers, an anode and a cathode, where the reactions take place, and an electrolyte between the two electrodes in the form of a membrane. This has three functions. It establishes the ionic contact, prevents the electronic contact and also ensures the separation of the media supplied to the electrode layers. The electrode layers are usually supplied with gases or liquids, which are reacted in the context of a redox reaction. For example, the anode is supplied with hydrogen or methanol and the cathode with oxygen. To ensure this, the electrode layers are usually contacted with electronically conductive gas distribution layers. These are e.g. Plates with a grid-like surface structure consisting of a system of fine channels. The overall reaction can be broken down into anodic and a cathodic sub-step in all fuel cells. With regard to the operating temperature, the electrolyte used and the possible fuels, there are differences between the different cell types.

According to the current state of the art, all fuel cells have gas-permeable, porous, so-called three-dimensional electrodes. These are listed under the collective term gas diffusion electrodes (GDE), and comprise the gas distributor devices and the electrode layer. Through the gas distribution layers, the respective reaction gases are led to close to the membrane, the electrolyte. Adjacent to the membrane are electrode layers in which there are generally catalytically active species that catalyze the reduction or oxidation reaction. The electrolyte present in all fuel cells ensures ionic current transport in the fuel cell. He also has the task of forming a gas-tight barrier between the two electrodes. In addition, the electrolyte guarantees and supports a stable 3-phase layer in which the electrolytic reaction can take place. The polymer electrolyte fuel cell uses organic lo nenaustauschermembranen, in the technically realized cases in particular perfluorinated cation exchange membranes, as electrolytes. A membrane electrode assembly, which is generally composed of a membrane and two electrode layers respectively adjacent to one side of the membrane, is referred to as a membrane electrode assembly or MEA.

Catalysts containing an alloy of at least two different metals, where at least one metal is a VIII subgroup metal, are used e.g. used as electrocatalysts in fuel cells. In particular, such catalysts are suitable for use as cathode catalyst in direct methanol fuel cells (direct methanol fuel cells, DMFC). In addition to a high current density for the reduction of oxygen, further requirements are placed on cathode catalysts in DMFCs. In the operation of a DMFC, the diffusion of methanol across the membrane to the cathode (crossover), which occurs when a fuel cell is operated with organic, water-soluble fuels, is problematic. As a result, the organic molecule at the catalytically active center of the cathode catalyst is burnt with oxygen directly to carbon dioxide and water. The active sites occupied by the combustion of organic molecules are no longer available for the actual electrochemical reaction - the electrochemical reduction of oxygen - so that the total activity of the cathode layer decreases. In addition, the direct oxidation of the organic molecule with oxygen lowers the electrochemical potential of the cathode layer and reduces the total voltage that can be tapped at the fuel cell. Since oxygen reduction and oxidation of the organic molecule occur at the same electrochemically active center, a so-called mixed potential is formed, which is lower than that of the oxygen reduction. The driving force (EMF) is lowered, the total cell voltage and thus the power is lowered. The cathode catalyst used must therefore be as inactive as possible against the methanol oxidation. That is, it must have a high selectivity for the oxygen reduction over the methanol oxidation.

Temperature-treated porphyrin-transition metal complexes, e.g. from J. Applied Electrochemistry (1998), pp. 673-682, or transition metal sulfides, for example ReRuS or MoRuS systems, as described e.g. from J. Electrochem. Soc, 145 (10), 1998, pages 3463-3471, see e.g. a high current density for the oxygen reduction and show a good tolerance to methanol. However, these catalysts do not achieve the activity of Pt-based catalysts and are also not stable enough to ensure a sufficient current density in the acidic environment of a fuel cell for a long time.

From US-A 2004/0161641 it is known that Pt catalysts, which are alloyed with transition metals, have a good methanol tolerance and a sufficiently high Ensure current density for the oxygen reduction. For example, it is known from US-A 2004/0161641 that an active methanol-tolerant cathode catalyst should have the highest possible oxygen-binding energy with simultaneously low hydrogen binding energy. High oxygen binding energy ensures high current density for oxygen reduction, while low hydrogen binding energy attenuates the electro-oxidative dehydrogenation of methanol to carbon monoxide, thereby increasing methanol tolerance. These properties have according to US-A 2004/0161641 alloys of the elements Fe, Co, Ni, Rh, Pd, Pt, Cu, Ag, Au, Zn and Cd. However, a concrete example of an alloy composition suitable as a methanol-tolerant cathode catalyst is not given.

Alternatively to the use of a methanol-tolerant catalyst, e.g. in Platinum Metals Rev. 2002, 46, (4), the possibility of reducing the methanol throughput by choosing a more suitable membrane. For this, e.g. thicker Nafion membranes are used. However, the lower methanol passage at the same time leads to an increase in the membrane resistance, which ultimately leads to a loss of power of the fuel cell.

The object of the present invention is to provide a catalyst which is suitable for the cathodic reduction of oxygen in methanol fuel cells and is sufficiently stable and as insensitive as possible to methanol impurities in the acidic environment of the fuel cell. Another object of the invention is to provide a process for the preparation of the catalyst.

The object is achieved by a catalyst comprising an alloy of at least two different metals, wherein at least one metal is a metal of the VIII. Subgroup. The alloy is present in at least two phases with different degrees of alloying.

An alloy is a homogeneous, solid solution of at least two different metals, with one element as the basic element and the others as alloying elements. The basic element is the element that has the largest mass fraction within the alloy. For alloys containing the same basic element and the same alloying elements, different phases result due to a different composition. Thus, in the individual phases, the proportion of alloying elements in the basic element differs. Optionally, it is even possible that in one phase the proportion of the base element is smaller than the proportion of at least one alloying element. In a preferred embodiment, the catalyst contains an alloy of two different metals, wherein at least one of the two metals is a metal of VIII. Subgroup of the Periodic Table of the Elements according to old name.

The metal of VIII. Subgroup preferably forms the basic element of the alloy. Metals of Group VIII, which are useful, are iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum. The base metal of the alloy is particularly preferably platinum or palladium.

In the case of alloys which consist of two different metals, both metals are preferably elements of subgroup VIII of the Periodic Table of the Elements. Particularly preferred is the alloy containing the catalyst selected from the group consisting of PtCo, PtNi, PtFe, PtRu, PtPd, PdFe.

According to the invention, the alloy is present in at least two phases with different degrees of alloying. The individual phases in each case form metal crystallites which lie next to one another in disorder. The result is a heterogeneous structure of metal crystallites of the individual phases of the alloy.

The inventively formed catalyst containing the alloy of at least two different metals, wherein the alloy is present in at least two phases with different degrees of alloying, is stable to acids and has a high current density for oxygen reduction, as desired in direct methanol fuel cells , In addition, the catalyst formed according to the invention is also very tolerant of methanol impurities.

In order to achieve a sufficiently good catalytic activity, it is necessary that the catalyst has a large specific surface area. This is preferably achieved by the catalyst further containing a carrier, wherein the alloy is applied to the carrier or is heterogeneously mixed with the carrier. To achieve a large surface area, it is preferred if the support is porous

When the catalyst is heterogeneously mixed with the carrier, individual catalyst particles are distributed in the carrier material. When the catalyst is supported on the carrier, individual particles of the catalyst material are generally contained on the carrier surface. Usually, the catalyst is not present as a continuous layer on the support surface.

Suitable supports are, for example, ceramics or carbon. Particularly preferred as carrier material is carbon. The advantage of carbon as a carrier material is that it is electrically conductive. When the catalyst is used as an electrocatalyst in a fuel Cell is used, for example, as the cathode of the fuel cell, it is necessary that it is electrically conductive to ensure the function of the fuel cell.

Other suitable carrier materials are e.g. Tin oxide, preferably semiconductive tin oxide, γ-alumina, which is optionally C-coated, titanium dioxide, zirconium oxide, silicon dioxide, the latter preferably being highly dispersed, the primary particles having a diameter of 50 to 200 nm.

Also suitable are tungsten oxide and molybdenum oxide, which may also be present as bronzes, that is to say as substoichiometric oxides. Also suitable are carbides and nitrides of metals of IV. To VII. Subgroup of the Periodic Table of the Elements, preferably of tungsten and molybdenum.

When carbon is used as the material for the carrier, it is preferably present as carbon black or graphite. The carbon may alternatively also be present as activated carbon or as so-called nanostructured carbon. One representative of the nanostructured carbons are, for example, carbon nanotubes.

To produce an electrode of a fuel cell, in particular a cathode of a fuel cell, a catalyst layer is applied either to the membrane or to the gas diffusion layer. The application of the catalyst layer is carried out by techniques known in the art. Suitable techniques include, for example, printing, spraying, knife coating, rolling, brushing and brushing. Furthermore, the catalyst layer can be applied by physical vapor deposition (PVD), chemical vapor deposition (CVD) or sputtering. A "DecaT" process, in which the catalyst layer is first prepared onto a "release" film and then relaminated onto the membrane, can also be used, for which a homogenized process is applied analogously to the direct application of the catalyst layer to the polymer electrolyte membrane In general, it is preferred to use at least one catalytic active species, optionally applied to a suitable carrier, at least one ionomer and at least one solvent suitable solvents are water, monohydric and polyhydric alcohols, nitrogen-containing polar solvents, glycols and glycol ether alcohols and glycol ethers. Particularly suitable are, for example, propylene glycol, dipropylene glycol, glycerol, ethylene glycol, hexylene glycol, dimethylacetamide, N-methylpyrrolidone and mixtures thereof.

In a preferred embodiment, the phases in which the alloy is present are cubic phases with different lattice constants. The lattice constant describes the mean distance of the atoms at the corners of the cubic lattice, which forms the cubic phase. Since different metal atoms each have a different diameter, the lattice constants differ in different composition of the alloy. In this way, the different phases can be characterized.

The crystallite size of the individual phases is preferably in the range from 1 to 10 nm, particularly preferably 2 to 5 nm.

In a particularly preferred embodiment, the alloy containing the catalyst is a PtCo alloy. The phases of the PtCo alloy preferably have lattice constants of 0.388 nm and 0.369 nm. At a grating cost of 0.38 nm, the Co content in the alloy is about 11 atomic percent. The proportion of Co in the alloy with a lattice constant of 0.369 nm is approximately 41 ± 5 atomic percent.

The further object is achieved by a process for producing a catalyst comprising an alloy of at least two different metals, where at least one metal is a metal of transition group VIII, which comprises the following steps:

(a) depositing at least one further metal on the metal of the VIII subgroup,

(b) annealing to form an alloy at a temperature above the Tammann temperature and below the melting temperature of the alloy.

By annealing the alloy at a temperature above the Tammann temperature and below the melting temperature, the individual atoms within the metal lattice of the alloy have sufficient mobility to be able to reorient themselves. In this way, it is possible for individual atoms to leave their lattice site and exchange with other atoms.

Tammann temperature is the temperature at which atoms within the lattice have sufficient mobility to reorient. Usually, the Tammann temperature is at a level of about 30 to 50% of the melting temperature of the alloy.

By annealing at a temperature above the Tammann temperature and below the melting temperature, individual atoms in the solid metal can reorient themselves in such a way that a new metal phase is formed.

It is further preferred if the selected temperature at which the alloy is annealed, below the stability limit of the at least two phases with different is different degrees of alloy, which are to be adjusted by the method according to the invention. In order to form the two different phases, it is necessary in forming the alloy that the proportion of the alloying element is greater than the proportion of the alloying element in the low alloying element phase and less than the alloying element content in the alloying element Phase with the larger proportion of the alloying element. The ratio of the phases to one another can likewise be adjusted by the proportion of the alloying element in the alloy formed.

The alloy is formed by any method known to those skilled in the art. For this purpose, the at least one further metal is first deposited on the metal of subgroup VIII. The deposition of the at least one further metal can take place, for example, in solution. For this purpose, for example, metal compounds may be dissolved in a solvent. The metal may be bound covalently, ionically or complexed. The metal can be deposited, for example, reductively or alkaline by precipitating the corresponding hydroxide. Further possibilities for depositing the at least one further metal are also impregnations with a solution containing the metal (incipient wetness), chemical vapor deposition (CVD) or physical vapor deposition (PVD) processes as well as all other processes known to the person skilled in the art a metal can be deposited.

When a carrier is provided, it is preferred that the base member, i. H. the metal of VIII. Subgroup is first deposited on the carrier. This is preferably the same as described above for the at least one further metal. Preferably, first a salt of the base element and then a salt of the alloying element is precipitated. After precipitation, drying and temperature treatment are performed to form the alloy. It is possible that the temperature treatment at the same time includes the annealing in step (b).

In a preferred embodiment, the deposition of the further metal on the metal of the VIII. Subgroup in step (a) by precipitating a corresponding metal salt from a solution in the presence of a carrier. By annealing at a temperature below the melting temperature in step (b), the alloy is formed.

To remove the solvent of the solution, the solution is preferably filtered off after precipitation and the catalyst is washed. By drying in the presence of an inert gas or in vacuo, the residual water content in the solvent is further reduced, typically to less than 5% by weight. The result is a pulverulent catalyst precursor. As the solvent in which the precipitation is carried out, any solvent is suitable. It is only necessary to ensure that the salts of the metals that form the alloy dissolve in the solvent. Preferred as a solvent is water. Alcohols, especially ethanol, serve to reduce, for example, platinum. When cobalt is used, the precipitation also takes place in aqueous solution, in contrast to platinum, preferably alkaline and not reductive.

The protective gas in whose presence the drying is carried out is preferably nitrogen or argon. Also, a drying under vacuum is possible. When drying under reducing conditions is desired, drying is generally carried out under a hydrogen atmosphere. The hydrogen can be either pure or as a mixture with nitrogen or argon.

In order to reduce the metal salts which have been precipitated to the desired alloy, the annealing step is preferably carried out in the presence of hydrogen. Furthermore, however, it is also possible to carry out the annealing step in the presence of nitrogen.

In order to produce an alloy of platinum and cobalt, it is preferable to use Pt (NO ₂ ) ₂ as the salt of the first metal and Co (NO ₂ ) ₂ as the salt of the second metal. For the preparation of the catalyst with the alloy of platinum and cobalt, carbon black in water is preferably initially introduced in a first step. A solution of Pt (NOs) ₂ in water and ethanol is combined with the carbon black suspension. The resulting reaction mixture is then stirred and then heated. This precipitates on the carbon platinum. The precipitated on the carbon platinum is filtered off and then washed with water nitrate-free. Finally, drying takes place under a nitrogen atmosphere.

The carbon thus produced with platinum precipitated thereon is subsequently placed in water. This suspension is a mixture of Co (NO ₃₎ ₂ ^* 6H ₂ O dissolved in water was added. By adding a soda solution, the pH is kept constant. Cobalt precipitates on the platinized carbon. The solid is filtered off and then dried under nitrogen atmosphere. To produce an alloy on the carbon support, the solid is then tempered at elevated temperature. The temperature is preferably above the Tammann temperature of the alloy. The temperature treatment is preferably carried out in the presence of nitrogen and hydrogen. After the temperature treatment, preference is given to passivation at room temperature in the presence of a nitrogen and air atmosphere. To remove excess, non-acid stable cobalt, the thermally treated catalyst is preferably slurried in sulfuric acid and stirred under a nitrogen atmosphere. For slurrying preferably 0 to 1 M, preferably 0.4 to 0.6 M sulfuric acid is used. The temperature is in the range between 60 and 100 ⁰ C, preferably between 85 and 95 ⁰ C. The catalyst is finally filtered off from the solution and dried under vacuum.

The catalyst prepared according to the invention is suitable e.g. for use as electrode material in a fuel cell. Suitable fields of application are the electrooxidation of methanol or hydrogen and / or the electroreduction of oxygen. Also for other electrochemical processes, such as the chloralkali electrolysis and water electrolysis, the catalyst of the invention is applicable. The catalyst according to the invention can also be used, for example, in auto exhaust gas catalysis, for example as a 3-way catalyst or diesel oxidation catalyst, or for catalytic hydrogenation or dehydrogenation in the chemical industry. These include, for example, hydrogenations of unsaturated aliphatic, aromatic and heterocyclic compounds. The hydrogenation of carbonyl, nitrile, nitro groups and of carboxylic acids and their esters, aminating hydrogenations, hydrogenations of mineral oils and carbon monoxide. Examples of dehydrations include the dehydrogenation of paraffins, naphthenes, alkylaromatics and alcohols. The hydrogenation or dehydrogenation can be carried out both in the gas phase and in the liquid phase.

In a particularly preferred embodiment, the catalyst according to the invention is used for an electrode in a direct methanol fuel cell. The electrode for which the catalyst is used is in particular a cathode of the direct methanol fuel cell. When used as the cathode of a direct methanol fuel cell, the catalyst according to the invention exhibits a high current density for the oxygen reduction. In addition, the catalyst according to the invention is tolerant of methanol impurities. This means that the catalyst according to the invention is essentially inactive towards the methanol oxidation. Thus, it has been shown that the current density for the oxygen reduction in the presence of 0.1 M methanol drops by less than 5%, while the current density for the oxygen reduction in a catalyst in which the alloy is in only one phase, in the presence of Me - ethanol partially decreases by more than 50%. Examples

Preparation example

a) Precipitation of Platinum on Carbon (Pt / C)

75 g of EC300J carbon black were introduced into 3.5 l of water, homogenized for 2 min with an Ultra-Turrax T25 at 1 10,000 rpm and then stirred for 13 min with an IKA stirrer with double stirrer. Subsequently, 130 g of Pt (NO ₂ ) ₂ were dissolved in 1.5 l of water and mixed with 5 l of ethanol. This solution was combined with the carbon black suspension to produce a reaction mixture. The reaction mixture was then stirred for 30 min at room temperature and then boiled under reflux for 5 h. The resulting Pt / C was filtered off and washed with 242 l of water nitrate-free within 16 h. Finally, drying takes place in a rotary kiln at 100 ^° C. for 54 h under a nitrogen atmosphere with a throughput of 50 l / h of nitrogen.

b) Preparation of a mixture of platinum and cobalt on carbon (PtCo / C)

16 g of the above-prepared Pt / C material were placed in 1, 5 l of water and stirred for 30 min. Subsequently, 20 g Co (NO3) ₂ ^* 6H ₂ O, which was dissolved in 50 ml of water was added. The pH of this mixture is kept constant at 5.6 by addition of a 5% sodium carbonate solution. After the addition of Co (NO ₃ ) ₂ was stirred with air at 60 ⁰ C for one hour, the pH dropped to 4.3. After one hour, the pH was adjusted to 7.5 with a 5% sodium carbonate solution. Subsequently, the PtCo / C was filtered off and washed with 12 l of water nitrate-free. This was followed by drying in a rotary kiln at 100 ° C for 16 h under a nitrogen atmosphere with a nitrogen flow rate of 50 l / h.

In the further course this material is called ES 271.

temperature treatment

4 g of the PtCo / C material ES 271 were brought to 600 ⁰ C in a rotary kiln in 3 h and maintained at this temperature for 3 h. During this temperature treatment, the sample was rinsed with 5 l / h of nitrogen and 10 l / h of hydrogen, the addition of nitrogen and hydrogen taking place simultaneously. After the temperature treatment, the sample was passivated at room temperature with 15 l / h of N ₂ and 3 l / h of air. For this purpose, it was first flooded with pure nitrogen to completely remove the hydrogen from the rotary kiln and then air was added to the nitrogen. To remove excess, non-acid-stable Co, the thermally treated catalyst was then slurried with 0.5 MH ₂ SO ₄ and stirred under nitrogen for one hour at 90 ⁰ C. The catalyst was then filtered off with suction and dried under vacuum.

The catalyst thus prepared was examined by X-ray diffractometry. In the diffractogram, double lines occur at 40.3 ° and 41.7 °, at 46.3 ° and 48.5 °, at 68.2 ° and 71.4 °, and at 82.3 ° and 86.6 °, respectively. From the diffractogram, the crystal size and the lattice constant of the two phases can be determined as follows:

Phase 1: crystallite size 3.0nm; Lattice constant: 0.388nm phase 2: crystallite size 8.4nm; Lattice constant: 0.369nm

In the further course this material is called ES 294.

1st comparative example for the preparation

In contrast to the above-described preparation of the catalyst according to the invention, the PtCo / C material ES has not been heat-treated at 600 ⁰ C but under otherwise identical conditions at 400 ⁰ C 271st In this case, it has been shown that no double phase occurs. The crystallite size of the single resulting phase is 2.9nm and the lattice constant is 0.38nm.

This catalyst was also slurried after the thermal treatment with 0.5 MH ₂ SO ₄ and stirred at 90 ⁰ C under nitrogen for one hour. Finally, the catalyst was filtered off with suction and dried. In the following, the material produced in this way is called ES 275.

In contrast to the catalyst ES 294 according to the invention, the diffractogram for ES 275 shows only single lines and no appearance of double lines. It can be seen from this that the temperature-treated at 400 ⁰ C material is only single-phase.

2nd comparative example for the preparation

A portion of the inventively prepared catalyst ES 294, the preparation of which was described in the preparation example, was repeatedly slurried with sulfuric acid and stirred at 90 ° C for one hour. After each acid treatment, the catalyst was sucked off, dried, and the phase composition of the PtCo metal was examined by means of X-ray diffraction. After three times acid treatment, the material no longer had the characteristic double lines in the X-ray diffractogram. The material therefore only has the status as phase 1 in the example. Phase 2 was completely dissolved out of the material by repeated acid treatment.

This material, with the phase 2 dissolved out, is referred to below as ES 297.

The X-ray diffractogram shows that the material ES 297 no longer shows double lines but only single lines.

Methanol tolerance

The catalysts ES 275, ES 294, ES 297 prepared according to the preparation example and the two comparative examples were each processed into an ink. For this purpose, in each case 6 mg of the catalyst, 1 g of 5% strength Nafion solution and 7.07 g of isopropanol were mixed. 200 .mu.l of this ink were applied in 20 .mu.l portions of a measuring head with a cross-sectional area of 100 mm _{2 of} a 3-electrode assembly with a calomel reference electrode to a ring disk electrode and dried with a hot air dryer. The methanol tolerance experiments were carried out in 1 MH ₂ SO ₄ at 70 ⁰ C. The electrolyte was saturated with oxygen for one hour before starting the measurement. To bring the catalyst layers into a defined initial state, two cyclovoltametric scans of -150 mV to 850 mV and back to -150 mV were carried out against calomel at a rate of increase of 50 mV / sec and a speed of 600 rpm before the actual measurement. For the actual measurement, the electrode potential of the working electrode was kept constant at 500 mV against the calomel electrode and recorded the time course of the cathode current. The average current density normalized to the noble metal content between 1700 and 1710 seconds after the start of the test served as a measure of the current density for the oxygen reduction of the catalysts investigated. In order to investigate the influence of methanol, the experiment was first determined in a pure sulfuric acid electrolyte and subsequently in a methanol-containing electrolyte, the methanol-containing electrolyte containing 3 mM methanol.

The measured current densities for the oxygen reduction for the three catalysts prepared are shown in Table 1.

Table 1: Oxygen reduction current densities for the three catalysts

From the experiment it can be seen that the current density for the oxygen reduction in the inventively prepared catalyst is more than twice as high as in a non-temperature-treated catalyst and still greater than 30% greater than in a catalyst in which the 2. Phase has been dissolved out. Furthermore, it is found that the measured O ₂ reduction currents hardly differ in a solution without methanol and in a solution with 0.1 M methanol for the catalyst according to the invention, whereas in the non-temperature-treated catalyst a drop in the current density of about 40% can be observed and the catalyst with the phase leached out shows a drop of about 62%.

Claims

claims

1. A catalyst comprising an alloy of at least two different metals, wherein at least one metal is a metal of VIII. Subgroup, characterized in that the alloy is present in at least two phases with different degrees of alloying.

2. Catalyst according to claim 1, characterized in that the alloy is selected from the group consisting of PtCo, PtNi, PtFe, PtRu, PtPd, PtCu and PdFe.

3. A catalyst according to claim 1 or 2, characterized in that the catalyst further comprises a carrier, wherein the alloy is applied to the carrier or is heterogeneously mixed with the carrier.

4. Catalyst according to claim 3, characterized in that the carrier is a carbon carrier.

5. Catalyst according to one of claims 1 to 4, characterized in that the phases are cubic phases with different lattice constants.

6. Catalyst according to one of claims 1 to 5, characterized in that the alloy is a PtCo alloy, wherein the phases have lattice constants of 0.388 nm and 0.369 nm.

7. A process for preparing a catalyst comprising an alloy of at least two different metals, wherein at least one metal is a metal of VIII. Subgroup, comprising the following steps:

a. Forming an alloy of the metal of the VIII. Subgroup and the at least one other metal,

b. Annealing the alloy at a temperature above the Tammann temperature

Temperature and below the melting temperature of the alloy.

A method according to claim 7, characterized in that the alloy is formed in step (a) by sequentially precipitating salts of the metals forming the alloy from a solution in the presence of a carrier and then reducing it to a temperature below the melting temperature.

9. The method according to claim 8, characterized in that after precipitation drying in the presence of a protective gas is performed.

10. The method according to claim 9, characterized in that the protective gas is N ₂ .

1 1. A method according to any one of claims 7 to 10, characterized in that the salt of the first metal is Pt (NO ₃ ) ₂ and the salt of the second metal Co (NO ₃ ) ₂ .

12. Use of the catalyst according to one of claims 1 to 6 as electrode material in a fuel cell.

13. Use according to claim 12, characterized in that the fuel cell is a methanol fuel cell.

14. Use according to claim 12 or 13, characterized in that the E- lektrode for which the catalyst is used, is a cathode.