EP1968742A1 - Catalyst based on a hexaaluminate for the combustion of hydrocarbons and fuel cell arrangement with exhaust burner - Google Patents

Abstract

The invention relates to a method for production of a catalyst for the catalytic combustion of hydrocarbons, in particular, methane, wherein a hexaaluminate of formula MO 6 Al<SUB>2</SUB>O<SUB>3</SUB> is prepared,where M is at least one earth alkali metal and the at least one earth alkali metal and eth aluminium can be proportionately replaced by at lest one further metal and the hexaaluminate is milled to give a mean particle size D<SUB>50</SUB> of less than 3 µm. The invention further relates to a catalyst as obtained by said method, a catalytic burner, comprising said catalyst, and a fuel cell arrangement including the catalytic burner.

Description

 CATALYST BASED ON A HEXAALUMINATE FOR THE COMBUSTION OF HYDROCARBONS AND FUEL CELL ARRANGEMENT WITH EXHAUST BURNER

description

The invention relates to a process for the preparation of a catalyst for the catalytic combustion of hydrocarbons, in particular methane, a catalyst, as can be obtained with the method, a catalytic burner, and a fuel cell assembly.

Fuel cells offer the possibility with high efficiency of obtaining electric power from the controlled combustion of hydrogen. However, since hydrogen is difficult to store or transport due to its high explosive hazard, methanol or hydrocarbons are currently used as a source of hydrogen, from which hydrogen is then produced in an upstream reformer. Methanol is liquid under normal conditions and can therefore be transported and stored without major problems. Hydrocarbons are also present either under normal conditions liquid form before or they can easily liquefy under pressure. For natural gas, which consists essentially of methane, an appropriate infrastructure already exists, so that stationary energy aggregates could be operated on the basis of fuel cells with methane as raw material without further notice.

Methane can be released by steam reforming hydrogen. The resulting gas contains traces of unreacted methane and water, essentially hydrogen, carbon dioxide and carbon monoxide. This gas can be used as fuel gas for a fuel cell. To shift the equilibrium in the steam reforming on the side of the hydrogen, it must be carried out at temperatures of about 650 ⁰ C, with a constant composition of the fuel gas, this temperature should be maintained as accurately as possible.

A fuel cell arrangement, in which the fuel gas generated from methane and water can be used to generate energy, is described for example in DE 197 43 075 Al. Such an arrangement includes a number of fuel cells disposed in a fuel cell stack within a closed protective housing. Via an anode gas inlet, fuel gas is supplied to the fuel cells, which essentially consists of hydrogen, carbon dioxide, carbon monoxide and residues of methane and water. The fuel gas is generated in an upstream reformer of methane and water. On the anode side, the fuel gas is burned, wherein electrons are generated according to the following reaction equations:

CO ₃ ^{2 "} + H ₂ > H ₂ O + CO ₂ + 2 e ^"

CO ₃ ^{2 '} + CO> 2 CO ₂ + 2 e ^" In order to achieve a high efficiency of the fuel cell, the reaction is conducted so that it does not run completely. The anode exhaust gas therefore contains not only the reaction products carbon dioxide and water but also fractions of hydrogen and methane gas. To remove residual hydrogen, therefore, the anode exhaust gas is first mixed with air and then fed to a catalytic exhaust gas burner, in which the remaining methane and traces of hydrogen are burned to water and carbon dioxide. The released thermal energy is used to heat the carbonate present in molten form in the fuel cell. As catalysts, noble metals are used, which are provided on a suitable support in finely divided form. The catalytic combustion has the advantage that it is very uniform and without temperature peaks. The combustion of palladium catalysts proceeds at temperatures ranging from about 450 to 550 ⁰ C. At higher temperatures beyond about 800-900 ⁰ C, the equilibrium shifts Pd / PdO favor of palladium metal, thereby decreasing the activity of the catalyst. (See Catalysis Today 47 (1999) 29-44) The ideal temperature range for operating a fuel cell is in the range of about 600 to 650 ^° C. The heat produced by the combustion can then be used to recover the fuel gas by steam reforming. The completely oxidized anode exhaust gas, which in particular no longer contains any hydrogen gas, is fed to the cathode as cathode gas after it leaves the burner. The carbon dioxide contained in the cathode gas reacts with the oxygen taking up two electrodes according to the following equation:

1/2 O ₂ + CO ₂ + 2 e ^" > CO ₃ ^2"

The current concepts for fuel cells are still relatively expensive, so that the currently operated plants have only pilot character. To find a wide application Therefore, the cost of providing a power generation unit based on fuel cells must be drastically reduced.

A significant cost in the fuel cell assembly described above is the precious metal catalyst needed for the combustion of the anode exhaust gas. It is therefore endeavored to find catalysts which have a sufficiently high activity at the required temperatures of about 650 ⁰ C and do not include expensive precious metals.

The catalytic combustion of methane has also been proposed for the power generation of, for example, turbines or other heat sources, although very high temperatures of possibly more than 1000 ⁰ C are generated.

EP 0 270 203 A1 describes a heat-stable catalyst for the catalytic combustion of, for example, methane. The catalyst is based on alkaline earth hexaaluminates which contain fractions of Mn, Co, Fe, Ni, Cu or Cr. These catalysts are characterized by a high activity and resistance even at temperatures of more than 1200 ⁰ C. However, the activity of the catalyst is relatively low at lower temperatures. In order to be able to provide sufficient catalytic activity even at lower temperatures, small amounts of platinum metals are added, for example Pt, Ru, Rh or Pd.

M. Machida, H. Kawasaki, K. Eguchi, H. Arai, Chem. Lett. , 1988, 1461-1464, describe substituted with manganese Hexaalumina- te Ai _{X X} A ¹ MnAl ₁₁ Oi Q _9, which have a high specific surface area even after calcination at temperatures of about 1300 ⁰ C. The hexaaluminates are prepared by the sol-gel method by reacting under a nitrogen atmosphere with appropriate amounts of Ba (OC ₃ H ₇ J ₂ , Sr (OC ₃ H ₇ ) ₂ , Ca (OC ₃ H ₇ ) ₂ , La (OC ₃ H ₇ ) ₃ , and / or Al (OC ₃ H ₇ ) _{3 are dissolved} in isopropanol. An aqueous solution of manganese nitrate and potassium nitrate is added. The resulting gel is decomposed at 500 ⁰ C and then calcined at 1300 ⁰ C under air. With this synthetic route, for Sr ₀ , 8La ₀ , 2MnAliiOi9-α a specific surface area of 23.8 m ² / g and a conversion of 10% at a temperature Tio% of 500 ⁰ C is reached. This material has also been tested in a practical application. H. Sadamori, T. Taniaka, T. Matsuhisa, Catalysis Today, 26 (1995) 337-344, describe the use of the hexaaluminate described above in a catalytic burner upstream of a gas turbine. The catalyst was prepared by the alkoxide process and calcined after precipitation at 1100 ⁰ C. The powdery catalyst is kneaded with water and an organic binder and then extruded to a honeycomb structure. After shaping, the catalyst is calcined again at 1200 or 1300 ° C. However, in the combustion of methane, the ceramic catalyst exhibits a relatively high ignition temperature of above 600 ^° C. The ceramic catalyst is therefore preceded by sections in which a noble metal-containing catalyst is arranged. The catalyst system was successfully tested in a 160 kW burner coupled to a gas turbine. During operation of the experimental setup, the temperatures in the catalyst reach values of up to about 1100 ^° C.

The production of hexaaluminates via the sol-gel process is very complicated and therefore too expensive for large-scale application. G. Groppi, M. Bellotto, C. Cristiani, P. Forzatti and PL Villa, in Applied Catalysis A: General, 104 (1993) 101-108, describe a process for producing high specific surface area hexaaluminates. Barium and possibly manganese nitrate are dissolved in water and the solution is acidified to pH 1 with nitric acid. To this mixture is added aluminum nitrate. The clear solution is added to an excess of ammonium carbonate in water, selecting a pH of 7.5 to 8.0 is maintained throughout the precipitation reaction. The precipitate is washed, dried and then calcined. Samples which were calcined at temperatures of 1300 ^° C. showed comparable activity to the catalysts obtained via the alkoxide route.

Another way of producing catalytically active hexaalminates is described by AJ Zarur and JY Ying in Nature, 403, (2000), 65-67. It used an inverse emulsion technique for the preparation of the compounds. For this purpose, barium and aluminum alkoxides are dissolved in isooctane. Using surfactants, an inverse emulsion was prepared with water. The size of the resulting Bariumhexaaluminatpartikel is thereby influenced by the added amount of water. The hydrolysis of the metal alkoxides is determined by the diffusion of these compounds from the organic phase to the aqueous phase and proceeds very slowly and uniformly. The nanocrystals obtained in the hydrolysis are separated from the solvent by lyophilization and then dried under supercritical conditions in order to minimize particle growth of the nanocrystals. The nanocrystalline Bariumhexaaluminat shows for methane an ignition temperature of 590 ⁰ C and a complete conversion of methane within a temperature range of 710 to 1300 ⁰ C. By adding cerium, the ignition temperature can be lowered to about 400 ⁰ C. Although the process makes it possible to produce hexaaluminates which show a high catalytic activity during the combustion of methane. For a production of these catalysts on an industrial scale, however, the process is too expensive.

In order to help fuel cell assemblies to a wide application and thus to an economic breakthrough, it is necessary to reduce the manufacturing cost considerably. As already explained above, are currently noble metal-containing catalysts used for the combustion of the anode exhaust gas. Although they have a low ignition temperature and a suitable operating temperature in order to be able to be coupled with fuel cells. However, their production is very expensive due to the high price of precious metals.

It is therefore a first object of the invention to provide a process which enables the simple provision of a catalyst for the combustion of hydrocarbons, in particular methane, the catalyst having a low ignition temperature and an operating temperature of preferably in the range from 500 to 700 ^° C. should.

This object is achieved by a method having the features of patent claim 1. Advantageous embodiments of the method are the subject of the dependent claims.

Surprisingly, it has been found that by intensive grinding of hexaaluminates their activity can be increased so far that in the combustion of methane ignition temperatures in the range of 300 to 500 ⁰ C and operating temperatures in the range of about 500 to 1100 ⁰ C can be achieved. This high activity allows, for example, the use of these catalysts in catalytic exhaust gas burners, as required for fuel cell assemblies. The preparation of the catalysts is very favorable because they do not rely on the addition of precious metals in order to achieve high activity can. Preferably, the catalysts are free of noble metals, in particular subgroup VIII of the Periodic Table of the Elements. The combustion of the anode exhaust gas is complete even at low temperatures, so that the waste heat can be used directly for the steam reforming or for the production of the fuel gas. The combustion can ideally be controlled so that the waste heat No energy has to be dissipated from the anode exhaust stream to reach the temperatures required for steam reforming.

The process according to the invention for the preparation of a catalyst for the catalytic combustion of hydrocarbons, in particular methane, is carried out by:

a hexaaluminate of the formula MO • 6 Al ₂ O _{3 is} provided in which M is at least one alkaline earth metal, wherein the at least one alkaline earth metal and the aluminum may also be proportionally replaced by one or more other metals, - and

the hexaaluminate is ground to an average particle size D ₅₀ of less than 3 μm.

The hexaaluminate can first be prepared in any desired manner. It is not necessary that the hexaaluminate is obtained in nano- or microcrystalline form. The hexaaluminate can be washed and dried in the usual way. If the hexaaluminate is obtained in the form of larger agglomerates, it can first be coarsely crushed. The agglomerates can have a very high hardness so that correspondingly robust devices are used for their comminution, for example jaw crushers. After any pre-crushing, the agglomerates are then ground intensively, preferably wet, in a suitable mill. The grinding can be carried out in a single pass or preferably in several passes. As mills such devices are used, which also allow a crushing of very hard aggregates. The hexaaluminate is ground to the extent that it has a mean particle size D ₅₀ of less than 3 microns. An average particle size D ₅₀ is understood to be the value at which 50% of the particles kel smaller particle size and 50% have a larger particle size. The activity of the catalyst increases with increasing degree of comminution of the hexaaluminate. Preferably, the average particle size D _{50 is} less than 2.5 microns, more preferably less than 2.0 microns and most preferably selected in the range of 0.5 to 1.9 microns. It has been found that the particle size distribution exerts no extraordinary influence on the activity of the catalyst. It is therefore not necessary to strive for the narrowest possible particle size distribution. On the other hand, however, the particle size distribution should not be chosen too broad, in particular to make the proportion of coarser particles not too large. D _{70 is} preferably chosen to be less than 6 μm, more preferably less than 4 μm, particularly preferably less than 3.8 μm, and particularly preferably in the range from 3.7 to 4.2 μm.

The composition of the hexaaluminate per se is that of hexaaluminates already proposed for use in the catalytic combustion of hydrocarbons. Preferably alkaline earth hexaaluminates are used, wherein both the alkaline earth metal and the aluminum may be partially replaced by other metals. Such other metals may be, for example, alkali metals or subgroup metals.

Preferably, the hexaaluminate is calcined before grinding at a temperature of more than 1050 ⁰ C, more preferably more than 1100 ⁰ C. Above 1000 ⁰ C may advantageously form a mineral phase. By this measure, the hexaaluminate is largely insensitive to stresses below the calcination temperature. When used in a catalytic burner for the anode exhaust gas of a fuel cell assembly, the hexaaluminate, for example, in the above-described fuel cell assembly of a fuel cell assembly. exposure temperature in the range of 500 to 700 ⁰ C, in particular about 650 ⁰ C exposed. Since the hexaaluminate has already been calcined at a higher temperature, therefore, a long life of the catalytic burner is ensured.

The calcination temperature can also be set higher if, for example, the catalytic burner is to be operated at a higher temperature than is required in combination with a fuel cell. For example, calcining temperatures of up to 1300 ^° C. are also possible.

Particularly preferably, the hexaaluminate is already calcined before grinding.

In particular, it is carried out in such a way that the hexaaluminate is first subjected to a precalcination at low temperature and then a Hauptcalcinierung at a higher temperature. The calcination temperature of the precalcination is chosen to be relatively low, preferably in the range of 500 to 900 ⁰ C, particularly preferably in the range of 600 to 800 ⁰ C. In the main calcination, the hexaaluminate is then preferably calcined again, with higher temperatures are used than at precalcination. The temperature at Hauptcalcinieren in the range 900-1400 ⁰ C is preferred, particularly preferably in the range from 1050 to 1200 ⁰ C selected. At these high temperatures, a mineral phase of the hexaaluminates can form.

As already explained, the hexaaluminates are characterized by a high hardness. Preferably, therefore, the procedure during grinding is such that the hexaaluminate is slurried in water for grinding and then ground in a suitable mill, preferably a ball mill.

The grinding process is preferably carried out in stages, the particle size of the hexaaluminate being gradually reduced. is being rectified. Thus, coarse grinding is preferably carried out between pre- and maincalcining. This coarse grinding is preferably carried out dry. In the coarse milling, the particle size D _{50 of} the coarse powder is preferably set in a range of 5 to 50 μm. Thereafter, this coarse powder is preferably slurried in water and ground wet in a fine grinding step to a very fine particle size.

Preferably, the preparation of the hexaaluminate is carried out in such a way that

an aqueous solution of at least one alkaline earth nitrate is prepared;

the aqueous solution of the at least one alkaline earth nitrate is acidified to a pH of less than 2;

adding to the acidified aqueous solution of the at least one alkaline earth metal nitrate an aluminum salt to give a clear aluminum-containing solution;

an aqueous solution of (NH ₄ ) ₂ CO _{3 is} provided which contains an excess of (NH ₄ ) ₂ CO ₃ ;

the clear aluminum-containing solution is added to the (NH ₄ J ₂ CO ₃ solution, during which a pH of more than 7.5 is maintained, and

the precipitated hexaaluminate is separated from the resulting mixture.

The preparation by precipitation of the hexaaluminate from aqueous solution essentially follows the synthetic route described above, as described by Groppi et al. loc. cit. was found. This enables a cost-effective provision of the hexa-luminate, which can also be done on an industrial scale.

In carrying out an aqueous solution of an alkaline earth nitrate is first prepared. For this purpose, the corresponding alkaline earth nitrate is dissolved in deionized water. Possibly. can also be added to this solution, other metal salts, preferably also in the form of their nitrates. The alkaline earth metal nitrate solution is then acidified to a pH of less than 2, more preferably less than 1. Preferably, nitric acid is used for this purpose. An aluminum salt is then added to the acidified aqueous alkaline earth metal nitrate solution, forming a clear solution. Preferably, the aluminum salt used is aluminum nitrate, particularly preferably the nonahydrate. In an original, an aqueous solution of (NH ₄ ) ₂ CO _{3 is} provided, the concentration being selected to be high enough to provide an excess of (NH ₄ J ₂ CO ₃ relative to the neutralization of the acidic aluminum-containing solution The (NH ₄ ) ₂ CO ₃ solution is then introduced, preferably with vigorous stirring, into the clear aluminum-containing solution, the mixture being able to be heated, preferably to temperatures in the range from 50 to 80 ^° C., preferably 55 to 70 ^° C. during the addition of (NH _{4) 2} CO ₃ .Lösung is observed a vigorous evolution of gas. during the addition of (NH _{4) 2} CO ₃ solution, the pH of the mixture in the range of more than 7.5, preferably in The rate of addition of the acidic aluminum-containing solution may be adjusted accordingly.

According to a preferred embodiment, the resulting precipitate is aged. The aging is preferably carried out for a period of 30 minutes to 8 hours, more preferably for a period of 3 to 6 hours. The temperature of the slurry is preferably maintained in the range of 40 to 80 ⁰ C, particularly preferably 50 to 70 ⁰ C. The precipitate is then separated, preferably by filtration, washed with deionized water, preferably until nitrate can no longer be detected in the filtrate, and then dried at temperatures of preferably 90 to 140 ⁰ C, particularly preferably 100 to 120 ⁰ C.

After drying, the solid obtained may optionally be pre-crushed to then, as described above, to be precalcined first at 500- 900 ⁰ C, then dry pre-ground and after the main calcination at 1050-1200 ⁰ C wet-ground wet.

According to a particularly preferred embodiment, the hexaaluminate is a compound of the formula A 1 _-z B _z C _x Al ₂ - _y 0i ₉ - _α , in which

A: at least one element from the group of Ca, Sr, Ba and

La;

B: at least one element from the group of K and Rb C: at least one element from the group of Mn, Co, Fe and

Cr z: a value between 0 and 0.4; x: a value between 0.1 and 4, - y: a value between x and 2x; and oc: a value determined by the valences of the other elements.

In the formula shown above, α corresponds to a value which is determined by the valences of the other constituents and their content in the hexaaluminate. The value α results from:

α = 1 - M {x - z (x - y) + XZ - 3y}.

In the production of the hexaaluminate, the individual components, ie the metal salts, in particular nitrates of the metals A, B and C, are preferably in a ratio Al: (A * B): C of 100: (7 -10): (0.1 -4) used.

Particularly preferred materials are BaMnAlnOig _x and LaMnAliiOi _9-x , where x corresponds to the oxygen deficit.

The hexaaluminate obtained by the process according to the invention has a low ignition point for the combustion of methane and shows a high activity even at relatively low temperatures, in particular in the range of 600 to 700 ⁰ C, so that the combustion of the methane is complete and at high conversion rates. The invention therefore also relates to a catalyst as obtained by the process described above.

According to a particularly preferred embodiment, the catalyst has a specific surface area of more than 6 m ² / g, particularly preferably more than 15 m ² / g.

The catalyst according to the invention has a particle size with a D ₅₀ of less than 3 μm at the end due to the intensive grinding carried out during the production. This particle size can also be determined on the finished catalyst. For this purpose, the finished catalyst can for example be scraped off a surface and the recovered powder can be screened to a particle size of 80 to 250 microns. The sieved powder is slurried in distilled water in an Erlenmeyer flask and the Erlenmeyer flask is kept in an ultrasonic bath for 30 minutes. Subsequently, the particle size distribution is measured under ultrasound and stirring for 1 minute.

As already explained, the catalyst according to the invention has a low ignition temperature for the combustion of methane and preferably an operating temperature in the range from about 600 to 700 ^° C. The subject of the invention is therefore also a catalytic burner which satisfies the above-described contains a catalyst.

The catalyst may be provided in particulate form, as a shaped body or preferably as a coating on a suitable carrier. For example, the catalyst can be mixed with water and a suitable organic or inorganic binder and then extruded into Formkδrpern. Preferably, however, the catalyst, if appropriate mixed with a suitable binder, is slurried in water or another suitable solvent, for example an alcohol or alcohol / water mixture, and then applied to a suitable support by brushing, spraying or dipping.

The carrier is preferably formed as a monolith. The catalyst can then be applied by simple dipping onto the surface of the monolith. After drying, the coating may optionally be fixed to the surface of the support by a calcination step. Such a coated monolith can be very easily installed in the exhaust stream, such as a fuel cell.

On the one hand, to provide as little as possible resistance to the gas flow and, on the other hand, to provide the largest possible catalyst area, the monolith preferably has a honeycomb structure. The catalytic burner according to the invention then has a plurality of parallel exhaust gas channels, which can also be connected through connection openings in the side surfaces of the channels in order to be able to compensate for pressure fluctuations within the gas flow.

The monolith is preferably made of a ceramic material or of a suitable metal, for example a stainless steel, which can withstand the temperatures occurring in the gas stream during combustion, so that a long operating time of the catalytic burner is ensured. The catalytic burner according to the invention has a low ignition temperature for hydrocarbons, in particular methane. Furthermore, it is characterized by high conversion rates and an almost complete combustion of the supplied gases. It is therefore possible to reliably burn even small amounts of hydrocarbons, in particular methane, and hydrogen with the catalytic burner according to the invention. The catalytic burner according to the invention is therefore particularly suitable for use in a fuel cell arrangement for nearly complete combustion of the anode exhaust gas.

The invention therefore also relates to a fuel cell arrangement with a number of fuel cells, which are arranged in a fuel cell stack, with an anode inlet for supplying fuel gas to the anodes of the fuel cell, an anode outlet for discharging the combusted fuel gas from the anodes, a cathode inlet to Supplying cathode gas to the cathodes of the fuel cells and a cathode outlet for discharging the spent cathode gas from the cathodes. According to the invention, a catalytic burner is provided at the anode outlet of the fuel cells, as described above, in which residues of hydrocarbons, in particular methane, and of hydrogen are burnt in after addition of air.

Preferably, a reformer for steam reforming of hydrocarbons, especially methane, is provided, and a heat exchanger, with which heat generated in the catalytic burner is supplied to the reformer. In this way, the heat generated in the catalytic burner can be used for the treatment of the fuel gas.

According to a preferred embodiment, the fuel cells are designed as Molten Carbonate fuel cell (MCFC = Molten Carbonate Fuel Cell). In this type of fuel cell For example, the membrane between the anode and cathode compartment of the fuel cell is formed by a layer of a molten carbonate such as lithium potassium carbonate. The waste heat of the catalytic burner can be used to keep the carbonate in molten form.

Preferably, a cathode gas guide is provided, with which the exhaust gas emerging from the catalytic burner is supplied as cathode gas to the cathode inlet. The MCFC requires the provision of carbon dioxide to regenerate the carbonate on the cathode side. By recycling the exhaust gas generated in the catalytic burner as a cathode gas, the carbon dioxide contained can be used for the regeneration of the molten carbonate membrane.

According to a particularly preferred embodiment, a protective housing is provided, which surrounds the fuel cell stack and the catalytic burner, wherein the fuel gas flow circulates within the protective housing. In this way, the fuel cell assembly can on the one hand be made very compact and on the other hand, the circulating gas streams can be optimally used for energy production.

The invention will be further elucidated with reference to examples and with reference to the accompanying figures. Showing:

Fig. 1: a fuel cell assembly as described in DE 197 43 075 Al.

Fig. 2: a particle size distribution of a hexaaluminate after four passes through a ball mill;

Fig. 3: a particle size distribution of a hexaaluminate after ten passes through a ball mill; 4 shows a representation of the methane conversion rate as a function of the temperature for a measurement gas which contains 1000 ppm of methane in air;

Fig. 5: a representation of the methane conversion rate as a function of the temperature for a measurement gas which contains 1000 ppm of methane and 3000 ppm of hydrogen in air;

6 shows a particle size distribution of a catalyst powder obtained from a washcoat, the following samples being used for the production of the washcoat:

a) BaMnAl ₁₁ Oi _9-X unmilled (Ex 3001.1)

b) grinding BaMnAl ₁₁ O _19-X 10 x (Ex 3001.1)

c) LaMnAl ₁₁ O ₁₉ - _X unmixed (Ex 3016)

d) LaMnAl ₁₁ O ₁₉ - _X 10 x ground (Ex 3016)

1 shows in a front view a section through a fuel cell arrangement, as shown in FIG. 2a of DE 197 430 75 A1, which, however, comprises the catalytic burner according to the invention. In a protective housing 1, which is gas-tight and thermally insulating, there is a fuel cell assembly whose main component consists of a number of fuel cells arranged in a fuel cell stack 2. The fuel cells contain anodes (not shown), which are traversed vertically from bottom to top by a fuel gas B, and cathodes (not shown), which are flowed through in cross-flow to the anodes in the horizontal direction of a cathode gas. For this purpose, a fuel gas scrubber 3 is provided at the lower end of the fuel cell stack, via which fuel gas is fed to an anode inlet 4. The fuel gas scrubber separates the anode inlet 4 gas-tight from the interior of the protective housing 1. The fuel gas was in a reformer (not shown) produced from methane and water according to the water gas equilibrium and contains essentially hydrogen, carbon monoxide and carbon dioxide in addition to residues of unreacted methane and water. The fuel gas flows through the anode chamber of the fuel cell in the vertical direction and exits at the anode outlet 5 again from the anode chamber. After passing through a diffuser 6, in which a uniform gas flow is generated, the anode exhaust gas stream is sucked by an ejector 7. The ejector 7 is driven on the principle of a water jet pump by an air flow which is compressed by means of an external pump and then ejected through a nozzle. The ejector 7 is arranged in a separate space, which is bounded by the collecting hood 8 and the outer wall of the protective housing 1. Subsequent to the outlet side of the ejector 7, the gas stream mixed with air is fed to a catalytic burner 9, in which combustible fractions of the anode exhaust gas still present in the gas stream are catalytically combusted. The combustion chamber of the catalytic burner 9 is coated with a hexaaluminate, which has been produced by the process according to the invention. The gas stream leaving the catalytic burner 9 is fed to the cathode inlet 10. In front of the cathode inlet 10, a preheater and diffuser 11 is arranged, in which a uniform gas flow is generated. When starting the fuel cell assembly, the gas flow with the aid of the preheater and diffuser 11 can be heated to the required operating temperature. During regular operation, thermal energy is sufficiently generated by the catalytic burner 9, so that the gas flow need not be additionally heated. The cathode gas stream enters the cathode compartments of the fuel cells at the cathode inlet 10 and passes through them in the horizontal direction to emerge from the cathode compartment again at the cathode outlet 12. After the cathode output 12, the spent cathode exhaust gas passes through a heat exchanger 13, in which the cathode exhaust heat is removed. This heat is used to preheat the fuel gas. Subsequently, the cathode exhaust gas passes through a further heat exchanger 14, through which further heat is decoupled from the circulating cathode exhaust gas. This heat can be used for example for the production of the fuel gas in a reformer. Excess cathode exhaust gas is led out of the protective housing 1 via the exhaust gas guide A. The exhaust gas corresponds to the chemical equivalent of the fuel cell assembly supplied streams of fresh air L and fuel gas B. By the decoupled heat exchanger 14 and the heat dissipated exhaust gas A, the operating point of the fuel cell assembly is set as the equilibrium state. The remaining cathode exhaust gas is mixed with the anode exhaust gas and fed via the feed hood 8 again to the ejector 7.

analysis methods

Surface area / pore volume:

The specific surface area is carried out on a fully automatic nitrogen porosimeter from Micromeritics, type ASAP 2010, in accordance with DIN 66131.

Determination of mean particle size

The particle size is determined on the "Fritsch Particle Sizer Analysette 22" under ultrasound and stirring by laser diffraction, indicating the D ₅₀ value, ie 50% of all particles <x μm.

Production of hexaaluminates:

The preparation of the hexaaluminates was carried out according to the method described by Groppi et al. loc. cit. published manufacturing specification. Example 1. BaMnAIi ₁ Oi _9-X 4x ground with Dyno-Mill

786 g of barium nitrate (52.4% Ba) and 1074 g of manganese nitrate (50% manganese) are placed in a first container and the container is filled with demineralized water to 30 1. The pH of the solution is washed with conc. Nitric acid adjusted to 1. Then 12.38 kg of aluminum nitrate 9-hydrate are dissolved therein and the solution is heated to 70 ⁰ C.

In a second container, a solution of 12 kg of ammonium carbonate in 60 1 of demineralized water is prepared and heated to 55 ⁰ C. In a precipitation vessel with stirrer, a little water is introduced and installed a pH probe. Then the two solutions are pumped simultaneously into the precipitation vessel at such speeds that the pH is maintained between 7.5 and 8. It is ensured that the pH does not fall below 7.5.

The resulting suspension is aged for one hour and then the precipitate is separated by means of a filter press. The precipitate is washed with water at 70 ^° C. and then separated by filtration. Subsequently, the precipitate is dried in a drying oven at 120 ⁰ C for 48 h. The dried precipitate is coarsely ground _(D50 <20 microns) precalcined at 750 ⁰ C and 3 h. The precalcined precipitate is then calcined again for 16 h at 1150 ⁰ C.

The powdered hexaaluminate is slurried in 4 liters of water and the slurry ground in a ball mill (Dyno-Mill, WA Bachofen, Switzerland), which has a grinding chamber with a volume of 250 ml, with Zirkonoxidku- gel with a diameter of 1 mm is filled. The suspension is pumped 4 times through the grinding chamber of 250 ml, whereby the speed of the mill is set to 4000 min ^-1 The particle size distribution of the suspension after grinding is shown in FIG. The finely ground suspension is dried at 120 ⁰ C.

From the dried powder, a suspension with a solids content of 50% is prepared in water. This suspension is either coated on catalyst support, such as honeycomb or placed in a mold in which the catalyst is to be used. After coating or shaping is dried again and calcined again at 450 ⁰ C to achieve sufficient adhesion of the powder particles.

For the activity tests, a granulate is produced whose particles have a diameter of 250 to 500 μm. The granulated catalyst has a BET surface area of 19 m ² / g.

Example 2. BaMnAInO ₁₉ - _X IQx ground with Fryma ^β mill

Analogously to Example 1 is first BaMnAlnOi ₉ - _x produced. The calcined at 1150 ⁰ C hexaaluminate is slurried in 5 1 of water. This suspension is pumped 10 times through a ball mill from the company Fryma (Rheinfelden) type MS-32 with 3.4 1 grinding chamber (2.4 1 balls lmm). This ball mill is an even more efficient annular gap ball mill. After the grinding process, the suspension has a mean particle size D ₅₀ of 0.93 microns. The particle size distribution is shown in FIG. 3a. The finely ground suspension is then dried at 120 ⁰ C.

From the dried powder, a suspension is prepared in water, which has a solids content of 50%. With this suspension either catalyst support, such as honeycomb bodies are coated or the suspension is brought into a form in which the catalyst is to be used. After the coating or shaping is dried again and calcined again at 450 ⁰ C to produce a firm cohesion between the powder particles. For the activity tests, a granulate is produced which has a diameter of 250-500 μm.

Example 3. LaMnAIi ₁ Oi _9-X IQx ground with a Fryma mill

In a first container, 650 g (2 mol) of lanthanum nitrate and 716 g of manganese nitrate (50% manganese) (2 mol) are initially charged and the container is made up to 25 l with demineralized water. The pH of the solution is washed with conc. Nitric acid adjusted to 1. Then 8.257 kg Aluminiumnitrat- 9-hydrate in the solution is added and the solution is heated to 60 ⁰ C.

In a second container, a solution of 8 kg of ammonium carbonate in 45 1 of demineralized water is prepared and heated to 50 ⁰ C. In a precipitation vessel with stirrer, a little water is introduced and installed a pH probe. Then the two solutions are pumped simultaneously into the precipitation vessel at such speeds that the pH remains between 7.5 and 8. The addition is carried out so that the pH does not fall below 7.5.

The suspension is aged for one hour and then the precipitate is separated by filtration through a filter press. The solid obtained is washed once with 70 ⁰ C with hot water and then again separated by filtration. Finally, the precipitate is dried in a drying oven at 120 ⁰ C for 48 h. The dried precipitate is coarsely ground to an average particle size D ₅₀ <20 microns and the milled solid vorcalci- defined 3h at 750 ⁰ C. Subsequently, the precalcined solid is again calcined 16h at 1100 ⁰ C.

The calcined at 1100 ⁰ C hexaaluminate is slurried in 5 1 of water. This suspension is pumped 10 times through a ball mill from the company Fryma (Rheinfelden) type MS-32 with 3.4 1 grinding chamber (2.4 1 balls lmm). The mean particle size D ₅₀ of the solid contained in the suspension was found to be 0.88 μm. The particle size distribution is shown in FIG. 3b.

The finely ground suspension is dried at 120 ⁰ C and made from the dry powder again a suspension in water with 50% solids. With this suspension, either a catalyst support, such as a honeycomb body can be coated or the hexaaluminate is brought into a form in which the catalyst is to be used. The catalyst is then dried again and calcined again at 450 ^° C. in order to achieve sufficient adhesion between the grains of the catalyst.

For the activity tests, a granulate with a diameter of 250 to 500 μm is produced from the powdered hexaaluminate.

Example 4: (Comparative Example)

Analogously to Example 1, a hexaaluminate is prepared. After calcining at 1150 ⁰ C only dry is also ground to the grain size 250-500 microns.

For the activity tests, a granulate with a diameter of 250 to 500 μm is produced from the powdered hexaaluminate.

Example 5: Activity Measurements

The catalyst granules prepared in Examples 1 to 4 are tested in a microreactor for the oxidation of methane and hydrogen. The granules are introduced into a reactor with 8 mm inner diameter. A gas mixture of 1000 ppm methane in synthetic air (20% O ₂ /80% N ₂ ) or a gas mixture with 1000 ppm methane and 3000 ppm hydrogen in synthetic air is used at a space velocity of 40,000 hours ^"1 through the reactor, with the aid of this gas mixture conversion curves being produced as a function of the temperature are shown in FIGS. 4 and 5.

4 shows the methane conversion for three differently ground barium hexaaluminate samples and a lanthanum hexaaluminate sample as a function of the temperature. The test gas used was a gas containing 1000 ppm of methane in air. The GHSV was 40000 h ^'1 . It can be seen that the activity of the catalyst increases with decreasing particle size.

In another series of experiments, the combustion of a gas mixture was examined, which contained 3000 ppm of hydrogen and 1000 ppm of methane in air. This ratio of 3/1 hydrogen / methane is found approximately at the outlet of most fuel cell systems.

FIG. 5 shows the conversion of methane as a function of the temperature for catalysts ground to a different degree. The conversion rate for methane is only slightly affected by the presence of hydrogen. The catalyst LaMnAli ₁ Oi ₉ - _X shows at 600 ⁰ C a conversion rate of more than 90%. At 450 ⁰ C, a conversion rate of 20% is already achieved. The ignition temperature for hydrogen is 200 ⁰ C. The catalyst is therefore suitable at temperatures of more than 200 ⁰ C as a catalytic burner for the anode exhaust gas of a fuel cell. It is only necessary to ignite the catalyst to heat it to a temperature of about 200 ⁰ C. Subsequently, the required temperature is adjusted and maintained by controlling the mass and heat flows in the fuel cell. For preheating the catalytic burner, for example, a small platinum precatalyst may be provided, in which the hydrogen can be ignited at lower temperatures. Example 6 Determination of the Particle Size Distribution of a Catalyst from the Washcoat after Coating

From the various hexaaluminates BaMnAlnOi ₉ -χ and LaMnAlnOi _9-x obtained in Examples 1 to 4, a suspension is prepared in each case. For this purpose, the unmilled and the 10-times ground powdery material is used in each case.

In each case, 500 g of the powder are stirred in portions using 500 ml of water using an Ultra-Turrax-stirrer. As soon as the viscosity of the suspension increases, a total of 6 g of concentrated acetic acid are added dropwise in each case. The suspension is then ground again briefly in a ball mill (Dyno-mill ^<s , WAB, Switzerland) filled with 1 mm diameter zirconia balls. Subsequently, the suspension is spread to a thin film and dried. The dried coating is scraped off and the recovered powder sieved to a particle size of 80 to 250 microns.

100 mg of the sieved powder are slurried in 100 ml of distilled water in an Erlenmeyer flask and the Erlenmeyer flask is kept in an ultrasonic bath for 30 minutes. The particle size distribution is then measured in a Fritsch Particle Sizer Analysette 22 under ultrasound and with stirring for 1 minute.

The measured curves are reproduced in FIGS. 6 a to d. Only in the case of the previously finely ground samples is a reduced mean particle diameter D ₅₀ of less than 1.5 μm found in the removed washcoat. For BaMnAlnO ₁₉ - _x , a D ₅₀ value of 3.06 μm is found for the unmilled catalyst and a D ₅₀ value of 1.16 μm for the milled catalyst. For LaMnAl ₁₁ Oi ₉ - _X , a D ₅₀ value of 5.21 μm is found for the unmilled catalyst, and a D ₅₀ value of 1.44 μm for the milled catalyst. The particle size of the catalyst leaves Therefore, they also determine from the catalyst coating applied on a support.

Claims

claims 1. A process for the preparation of a catalyst for the catalytic combustion of hydrocarbons, in particular methane, wherein: a hexaaluminate of the formula MO • 6 Al ₂ O _{3 is} provided, in which M is at least one alkaline earth metal, and wherein the at least one alkaline earth metal and the aluminum may also be proportionally replaced by at least one other metal; and the hexaaluminate is ground to an average particle size D ₅₀ of less than 3 μm. 2. The method according to claim 1, characterized in that the hexaaluminate is calcined before grinding at a temperature of more than 1050 ⁰ C. 3. The method according to any one of claims 1 or 2, characterized in that the hexaaluminate is slurried for grinding in water and then ground in a ball mill. 4. The method according to any one of the preceding claims, characterized in that for the production of the hexaaluminate an aqueous solution of at least one alkaline earth nitrate is prepared; the aqueous solution of the at least one alkaline earth nitrate is acidified to a pH of less than 2; adding to the acidified aqueous solution of the at least one alkaline earth metal nitrate an aluminum salt to give a clear aluminum-containing solution; an aqueous solution of (NH ₄ J ₂ CO _{3 is} provided which contains an excess of (NH ₄ ) ₂ CO ₃ ; the clear aluminum-containing solution is added to the (NH ₄ J ₂ CO ₃ solution, during which a pH of more than 7.5 is maintained, and the precipitated hexaaluminate is separated from the resulting mixture. 5. The method according to claim 4, characterized in that after the combination of the clear aluminum-containing solution and the (NH ₄ ) ₂ CO ₃ solution, the resulting precipitate is aged. 6. The method according to any one of the preceding claims, characterized in that the hexaaluminate is a compound of the formula Ai _-z B _z C _x Ali ₂ -y0i9- _α in which means: A: at least one element from the group of Ca, Sr, Ba and La; B: at least one element from the group of K and Rb C: at least one element from the group of Mn, Co, Fe and Cr z: a value between 0 and 0.4; x: a value between 0.1 and 4; y: a value between x and 2x; and α: a value determined by the valences of the other elements and corresponding to the oxygen deficit. 7. Catalyst for the combustion of hydrocarbons, in particular methane, obtained by a process according to one of claims 1 to 6. 8. Catalyst according to claim 7, characterized in that the hexaaluminate has a specific surface area of more than 10 m ² / g. 9. A catalytic burner, in particular for anode exhaust gas of a fuel cell assembly, characterized in that the burner comprises a catalyst according to one of claims 7 or 8. 10. A catalytic burner according to claim 9, characterized in that the catalyst is applied as a coating on a support. 11. Catalytic burner according to one of claims 9 or 10, characterized in that the carrier is designed as a monolith. 12. The catalytic burner according to claim 11, characterized in that the monolith has a honeycomb structure. 13. Catalytic burner according to one of claims 9 to 11, characterized in that the catalyst does not comprise a noble metal. 14. A fuel cell assembly having a number of fuel cells arranged in a fuel cell stack with an anode inlet for supplying fuel gas to the anodes of the fuel cells, an anode outlet for discharging the combusted fuel gas from the anodes, a cathode inlet for supplying cathode gas to the cathodes of Fuel cells and a cathode outlet for discharging the spent cathode gas from the cathode, characterized in that at the anode outlet, a catalytic burner is provided according to one of claims 9 to 13. 15. A fuel cell assembly according to claim 14, characterized in that a reformer for steam reforming of hydrocarbons, in particular methane is provided, and a heat exchanger, with which heat generated in the catalytic burner is supplied to the reformer. 16. Fuel cell assembly according to claim 14 or 15, characterized in that the fuel cells are designed as MCFC fuel cell. 17. Fuel cell arrangement according to one of claims 14 to 16, characterized in that a cathode gas guide is provided, with which the exhaust gas emerging from the catalytic burner is supplied to the cathode inlet. 18. Fuel cell assembly according to one of claims 14 to 17, characterized in that a protective housing is provided, which surrounds the fuel cell stack and the catalytic burner, wherein the fuel gas flow circulates within the protective housing.