DE10035330A1 - Catalyst used for decomposing nitrogen oxide waste gas from vehicles and power stations comprises absorber body with channels and electrolyte - Google Patents

Abstract

A catalyst comprises an absorber body (1) with channels (2) and an electrolyte (4). At least two chemically different solid body materials are present within the absorber body and are in electrical contact with each other so that an electrical contact potential is formed. The solid body materials are wetted with the electrolyte. Preferred Features: A powder, preferably made of a mixture of metals and/or electrically conducting oxides, is arranged within the channels. The metals and oxides are selected from nickel, iron, copper, gold, manganese metals and/or oxides. The absorber body is a porous non-electrically conducting ceramic sintered body and comprises lithium aluminate, zirconium dioxide or cordierite.

Description

The present invention relates to a catalyst for the decomposition of nitrogen oxides according to the preamble of claim 1.

Nitrogen oxides (NO _x ) are formed endothermically in all combustion processes in the presence of air. They are thermodynamically unstable at ambient temperatures and would have to decompose to nitrogen and oxygen. However, this decay is kinetically inhibited and no catalyst has been found to accelerate the actual decay reaction sufficiently. On a conventional catalyst surface (e.g. in a three-way catalyst), nitrogen oxides are atomically split into nitrogen and oxygen, but the further reaction to form molecular nitrogen has a relatively high activation energy, so that nitrogen oxide is always produced again during the desorption ,

Since the emission of nitrogen oxides represents a significant environmental impact, attempts are made to to limit these by steadily tightening legal requirements. Procedure for Removal or reduction of the emission of nitrogen oxides and corresponding Devices are therefore being examined intensively. For denitrification of exhaust gases for the formation of molecular nitrogen initially the atomically bound oxygen of the catalyst surface and thus removed from the atomically bound nitrogen become. This happens e.g. B. by the reaction with hydrogen, formed in the water or by electrochemical denitrification. The latter are oxides, carbonates or hydroxides on the. Anode is oxidized and nitrogen oxides or Nitrates / nitrites reduced. However, both methods are not one catalytic decomposition of nitrogen oxides, but a chemical implementation that overall requires an energy expenditure.

A known device for removing NO _x from the exhaust gases of internal combustion engines is described, for example, in DE 196 51 492 A1. The device disclosed therein comprises a porous absorber with gas-carrying channels, in which electrodes are arranged in pairs for electrochemical decomposition. The porous absorber is soaked in a liquid, alkaline salt. Due to the molten salt, nitrogen oxides are initially absorbed from the exhaust gas. The nitrate or nitrite formed during absorption is then electrochemically reduced to elemental nitrogen. The voltage required for the electrochemical decomposition is applied via the pair of electrodes. For this purpose, the electrode rods are connected at one end to a power supply which is connected to the battery or the electrical system of the motor vehicle. The applied voltage reaches the decomposition potential of the nitrate and nitrite, which, however, is essentially not exceeded.

The previously used devices based on an electrochemical Denitrification processes are based, have the disadvantage that an external energy supply is required. Special precautions are required, for example in pairs Absorber-arranged electrodes, via which the decomposition potential is provided. The energy for the decomposition potential is the battery or the electrical system of the Removed from the vehicle.

Furthermore, in the known devices, precise engine management (e.g. in the Lambda control) required. In addition, there are additional equipment precautions for controlled metering of fuel or other reducing agents required. This typically has an increased consumption of fuel or Reductants follow.

Starting from the prior art, it is therefore the object of the present invention to create a catalyst for the decomposition of nitrogen oxides, which is an effective Decomposition made possible in a simple manner, without the need for external energy supply is required.

This problem is solved by a device with the features of Claim 1.  

Further advantageous configurations of the device according to the invention result from subclaims 2 to 15.

The central idea here is that the decomposition potential required for electrochemical denitrification within the absorber body is generated by the contact voltage between at least two chemically different solid materials which are in electrically conductive contact with one another. The electrical contact voltage that arises between the at least two chemically different, touching solid materials serves as decomposition potential, one body charging positively and the other negatively. In addition, the exergony of the decomposition of nitrate / nitrite ions in the presence of CO _{2 is} exploited.

In this way, the potential required for electrochemical decomposition becomes generated automatically within the absorber body without external energy supply. The by two chemically different, in contact with each other Solid contact potential is sufficient to achieve an electrochemical To cause decomposition.

A major advantage of the invention is that it is external Power supply and consequently also on electrodes arranged in the absorber can be dispensed with. At the same time, this means that additional control devices are not required. The device according to the invention thus assigns in comparison known devices on a simplified structure.

Another advantage is that the device according to the invention can be used with both liquid as well as with a solid electrolyte can be used. Next to it is the simultaneous use of liquid and solid electrolytes possible. This allows a more flexible use of the device according to the invention.

According to a first embodiment, the absorber body consists of an electrical non-conductive ceramics, for example zirconium oxide, lithium aluminate or cerium oxide. On Such an absorber body is described in DE 196 51 492 A1. In the absorber body  gas channels are arranged in the longitudinal direction through which the exhaust gas to be cleaned flows. A powder is applied to the inside of the channels, which consists of a mixture consists of at least two chemically different solids. The Solid materials are electrically conductive and, for example, metals are preferred Precious metals, or electrically conductive oxides. It can e.g. B. nickel, iron, copper, gold, Manganese and the like or their oxides can be used. Touch the powder grains themselves and are in electrically conductive contact. This creates the contact potential. Such powder mixtures of metals, metal oxides or mixtures are preferred from metals and metal oxides to use due to the electrochemical Voltage series generate the highest contact potentials. That is on the inner wall of the Powder mixture located in channels is additionally wetted by an electrolyte. This Electrolyte can be an alkaline liquid electrolyte, for example consisting of molten alkali and / or alkaline earth carbonate, or an oxide ion conductive Solid electrolyte. Liquid and solid electrolyte can also be used together become.

According to a further embodiment, the absorber body itself consists of a Mixture of at least two chemically different solids. The Absorber is e.g. B. a porous, sintered body with gas-carrying channels, the a homogeneous mixture of at least two electrically conductive, chemically contains different substances. Alternatively, the porous absorber body consists of a Solid state and is with at least a second, different from it Solid material coated. Thus, no powder is required in this embodiment. The absorber body is made with a liquid, a solid or with a liquid and solid electrolyte used. The liquid electrolyte becomes capillary from the porous Absorber body sucked up.

According to a further embodiment, the absorber body contains at least two chemically different solid materials, as well as a solid electrolyte further solid component. This way, you can go to a separate one Solid electrolytes are dispensed with, since the solid electrolyte component already is contained in the sintered body. This is realized, for example, in that  Solid state electrolyte component is added to a ceramic, which at higher Temperatures conducts oxygen ions.

If the absorber body is used together with a liquid electrolyte, be used the nitrogen oxides advantageously through the liquid electrolyte in the form of nitrate or Nitrite bound. The liquid electrolyte stores or buffers the bound nitrogen oxides. It is possible to first bind the nitrates / nitrites in the liquid electrolyte and then, at different times from the absorption reaction, electrochemical denitrification perform. On the other hand, the porous structures of the absorber body wetted due to the capillary suction of the liquid electrolyte, whereby the Surface of the absorber body is reduced and the effectiveness of the decomposition process is reduced. The latter, however, occurs when using a solid electrolyte avoided since the porous structure of the absorber body is not affected. The Absorber body maintains its high porosity of approximately 70% to 80% and has one very large surface area, which is required for effective denitrification of exhaust gases.

Another advantage of solid electrolytes is that denitrification is free upstream absorption reaction takes place. The nitrogen oxides are reduced directly.

The device according to the invention is not limited to the geometry described, but can also be used in a different form. In addition, the Device according to the invention can be used variably. You can not only Exhaust gas denitrification used by internal combustion engines, but also for the exhaust gas denitrification of power plants or the like.

Below is the device according to the invention with reference to the accompanying drawings explained in more detail. It shows:

Fig. 1 shows a cross section through a cylindrical absorber body;

FIG. 2 shows an enlarged detail of the absorber body shown in FIG. 1 according to a first embodiment;

Fig. 3 is a schematic representation of the powder mixture used according to a first embodiment.

Fig. 1 shows the cross section of a cylindrical absorber body 1, as it is known in principle from DE 196 51 492 A1. The porous absorber body 1 is traversed in its longitudinal direction with gas channels 2 through which the exhaust gas to be cleaned flows. The gas channels 2 can be rectangular, circular or in another shape. The absorber body 1 consists of an electrically non-conductive ceramic, for. As lithium aluminate, zirconium oxide or cordierite, and is produced by extrusion and subsequent sintering. The absorber body has a high porosity, so that the largest possible surface for the decomposition of nitrogen oxides is available in a small space.

FIG. 2 shows an enlarged illustration of a section of the absorber body 1 shown in FIG. 1 . On the inner wall of the gas-carrying channels 2 there is a powder 3 which, for. B. is applied with a wash coat process. The powder 3 consists of a mixture of at least two chemically different solid materials which are sufficiently electrically conductive at an operating temperature of typically 200-900 ° C. The powder mixture 3 consists, for. B. from at least two different metals, such as nickel, copper, gold, iron, manganese and the like. The powder mixture 3 can also consist of two different metal oxides of the above-mentioned metals. A mixture of metals and metal oxides with chemically different properties is also possible. The powder grains of the powder mixture 3 touch and are in electrically conductive contact. Since the at least two solid materials of the powder 3 have a different chemical structure, a contact potential arises due to the electrochemical series of voltages. The contact potential is greater the further the solid materials used in the electrochemical voltage series are from each other. The powder 3 is additionally wetted with an electrolyte 4 , which is indicated in FIG. 2 by hatched lines.

Fig. 3 shows diagrammatically a powder particle that consists of at least two chemically different solid materials 3 a and 3 b and is wetted with the electrolyte 4. The electrolyte can be liquid and / or solid.

In the case of a liquid electrolyte, it becomes porous, ceramic Absorber body sucked up, causing the porous hole structure of the absorber body with the liquid is wetted such that the surface of the porous absorber body is reduced. Any substance that can be used as a liquid electrolyte can be used Operating temperature of approx. 200-900 ° C liquid or made liquid by additives can be ionic itself or can be made ionic by additives can and dissolves nitrogen oxides physically or by chemical reaction. The liquid Electrolyte is, for example, a molten salt from a ternary or quaternary Mixture of the carbonates of lithium, sodium, potassium and / or rubidium, or a eutectic mixture of alkali and / or alkaline earth metal carbonates with alkali and / or Alkaline earth nitrates and / or nitrites.

The decomposition process for the decomposition of nitrogen oxides initially takes place in such a way that the nitrogen oxides contained in the exhaust gas are absorbed by the basic liquid electrolyte, so that nitrates and nitrites are formed. Starting from a carbonate-containing salt, this is the well-known absorption reaction

2NO ₂ + CO ₃ ^2- → NO ₂ ^- + NO ₃ ^- + CO ₂ . (I)

In the next step, the nitrate or nitrite on the powder disintegrates and is reduced to nitrogen, with the reaction on the negatively charged powder grains (cathode)

10e ^- + 2NO ₃ ^- + 6CO ₂ → N ₂ + 6CO ₃ ^2- (starting from nitrate) (II)

respectively.

6e ^- + 2NO ₂ ^- + 4CO ₂ → N ₂ + 4CO ₃ ^2- (starting from nitrite) (III)

and the reaction on the positively charged powder grains (anode)

expires. The electrons flow here via the contact point denoted by K in FIG. 3 between the two different phases, ie over the interface of the two different solid materials, through which the contact voltage is generated. Due to the presence of carbon dioxide (CO ₂ ), which contains approx. 10 vol.% In the exhaust gas, the decomposition of nitrate or nitrite ions takes place exergonically within a wide temperature window, ie the reaction proceeds voluntarily.

Due to the exploitation of the contact potential between two different, electrically conductive solid materials, a charge exchange takes place and thus the electrochemical decomposition of the nitrate or nitrite without the need for electrodes in the absorber body, to which a voltage from the outside would have to be applied. Consequently, the absorber body for the decomposition of nitrogen oxides has a simpler structure than known devices for NO _x denitrification, since additional components can be dispensed with, and it does not require any additional energy supply.

If the absorber body 1 described above is used with a liquid electrolyte, it is advantageously possible to dispense with additional electrodes with an external voltage supply. Another advantage is that an absorption / decomposition cycle is created that is automatically renewed due to the continuous post-production of carbonate ions. It is also advantageous in such an arrangement that the nitrogen oxides are first stored in the liquid electrolyte before the nitrates or nitrites are subsequently decomposed. The decomposition can be carried out after the absorption, so that there is a buffer possibility.

However, when using a liquid electrolyte, it should be noted that the Porosity of the absorber structure due to the capillary suction of the liquid  Electrolyte is reduced, so that the surface of the absorber and consequently the Effectiveness of denitrification can decrease.

Next, the use of the absorber body 1 described in Figs. 1-3 with a solid electrolyte will be described. Any solid, preferably porous substance that is itself an oxide ion and / or hydroxide ion conductor and / or a carbon ion conductor or can be made correspondingly conductive by additives can be used as the solid electrolyte. In the case of a solid electrolyte, the nitrogen is reduced directly in the nitrogen oxide on the negatively charged solid substance of the powder mixture 3 . The negatively charged solid body corresponds to the cathode. The resulting oxide ions are passed on in the solid electrolyte to the positively charged solid material (anode), where they are oxidized to elemental oxygen. This ensures the required removal of oxygen from nitrogen without the need for an upstream absorption reaction. In principle, it is possible to remove oxygen with all the oxygen-containing ions that form, e.g. B. with hydroxide ions by further reaction of the oxide with water (O ^2- + H ₂ O → 2OH ^- ) and / or formed carbonate ions by further reaction of the oxide with carbon dioxide (O ^2- + CO ₂ → CO ₃ ^2- ), if provided the solid electrolyte can transport these ions. The electrons released at the anode, ie the positively charged solid material of the powder mixture, flow back to the cathode and reduce the nitrogen bound there to elemental nitrogen.

The advantage of a solid electrolyte is that the porosity of the absorber body 1 is not adversely affected by the presence of the solid electrolyte. Accordingly, the absorber body used in Figs. 1 and 2 has a high efficiency when used with a solid electrolyte. An absorption reaction is not required for solid electrolytes. At the same time, this means that nitrogen oxides cannot be stored or buffered before the actual decomposition reaction.

The absorber body 1 shown in FIGS. 1 and 2 can also be used with an electrolyte which consists of a mixture of solid and liquid electrolytes.

A simultaneous combination of liquid and solid electrolyte leads to the fact that the reactions described above can run in parallel. In this case too The driving force of the reactions taking place is exergony and Contact potential difference of the two contacting solid phases.

In a further embodiment, the powder 3 shown in FIGS. 2 and 3 can be dispensed with. In this case shown in Fig. 1 absorber body 1 itself consists of a homogeneous mixture of at least two chemically different substances or solid, with a second chemically different solid material is coated from a material at least partially. The at least two different solid materials must touch and be made of electrically conductive material so that a contact potential is formed due to their electrical contact. Metals and / or metal oxides of iron, copper, gold, nickel, manganese and the like can be used as solid materials. The production of the absorber body consisting of at least two chemically different solid materials takes place, for. B. by mixing the solids in powder form. A binder or filler is added to this mixture so that it is extruded into the later form. After the extrusion and drying, the molded body is fired. The binder or filler evaporates, evaporates or burns. What remains is a porous sintered body.

Such, consisting of at least two different solid materials Absorber body can be with both a liquid and a solid electrolyte be used. Likewise, simultaneous use with liquid and solid Electrolytes possible.

According to a further embodiment, the absorber body 1 shown in FIGS. 1 and 2 consists of two chemically different solid materials and an additional component, a solid electrolyte component. As a result, the solid electrolyte is integrated directly into the porous absorber body. This is achieved, for example, by adding an oxygen-conducting oxide to the mixture before extruding the mixture of at least two different solid materials. This addition represents the solid electrolyte component and is e.g. B. zirconia doped with yttrium. However, other components can also be used which conduct oxygen ions at higher temperatures. This ensures that the oxide ion-conducting part of the solid electrolyte is already contained in the finished sintered body, so that an additional solid electrolyte is not necessary. This represents a further reduction in the components of the catalyst according to the invention required for the decomposition of oxides.

The device according to the invention can be used, for example, for the decomposition of Nitrogen oxide emissions from motor vehicles and commercial vehicles are used. For this, the Device usually installed in the exhaust system of the vehicle. Furthermore, the Device according to the invention for denitrification of emissions from power plants and the like can be used.

The present invention enables effective decomposition of nitrogen oxides for example through the exhaust gas emissions from motor vehicles or power plants be expelled. The device according to the invention is particularly distinguished in that it has a reduced compared to conventional devices Has number of components and not on the external supply electrical or chemical energy is required to decompose the nitrogen oxides. This results in no additional energy consumption of the cleaning system and additional components omitted, for example in conventional systems for controlling the Engine management are required.

Claims (11)

1. Catalyst for the decomposition of nitrogen oxides, comprising an absorber body ( 1 ) with channels ( 2 ) and an electrolyte ( 4 ), characterized in that
within the absorber body (I) at least two chemically different solid materials (3 a, 3 b) are located, which are in electrically conductive contact with each other, then an electrical contact potential that forms; and
the at least two chemically different solid materials ( 3 a, 3 b) are wetted by the electrolyte ( 4 ). 2. Catalyst according to claim 1, characterized in that a powder ( 3 ) is arranged within the channels ( 2 ), which contains the at least two chemically different solids ( 3 a, 3 b). 3. Catalyst according to claim 2, characterized in that the absorber body ( 1 ) is a porous, non-electrically conductive, ceramic sintered body and consists of lithium aluminate, zirconium dioxide or cordierite. 4. Catalyst according to claim 2, characterized in that the powder ( 3 ) is chemically and electrochemically stable and consists of a mixture of different metals and / or electrically conductive oxides. 5. A catalyst according to claim 4, characterized in that the metals or oxides nickel, iron, copper, gold, manganese metals or oxides are. 6. Catalyst according to claim 1, characterized in that the absorber body ( 1 ) consists of the at least two chemically different, electron-conducting solid materials ( 3 a, 3 b). 7. Catalyst according to claim 1, characterized in that the absorber body ( 1 ) consists of a solid material ( 3 a) and is coated with at least a second chemically different solid material ( 3 b). 8. Catalyst according to claim 1, characterized in that the absorber body ( 1 ) consists of at least two chemically different solid materials and a solid electrolyte component, the solid electrolyte component being an oxygen-ion-conducting ceramic. 9. A catalyst according to claim 1, characterized in that the electrolyte is chemically stable and is in liquid and / or solid form, where the liquid electrolyte is ionic and the solid electrolyte is ionic and / or is conductive to hydroxide ions and / or is conductive to carbon ions. 10. Use of the device according to one of the preceding claims Decomposition of nitrogen oxide emissions from commercial and motor vehicles. 11. Use of the device according to one of the preceding claims Decomposition of nitrogen oxide emissions from power plants.