WO2014092638A1

WO2014092638A1 - Catalytic converter for treatment of exhausts and an aftertreatment system including such a catalytic converter

Info

Publication number: WO2014092638A1
Application number: PCT/SE2013/051495
Authority: WO
Inventors: Björn WESTERBERG
Original assignee: Scania Cv Ab
Priority date: 2012-12-14
Filing date: 2013-12-12
Publication date: 2014-06-19
Also published as: EP2932057A1; CN104919152A; SE537367C2; EP2932057A4; SE1251420A1

Abstract

The present invention concerns a catalytic converter for treating exhaust from a combustion engine (1). The catalytic converter (5) comprises wall elements (5b) that form a plurality of longitudinal channels (5a) in the catalytic converter for receiving exhaust. The wall elements (5b) comprise a first outer layer (5b₁) that presents an external surface of the longitudinal channels (5a) and a second inner layer (5b₂) that is arranged internally around the first layer (5b₁). The first layer comprises a first active catalytic material that promotes a catalytic reaction between nitrogen oxide and ammonia. The second layer (5b₂) comprises a material that has the ability to store nitrogen oxide and a second active catalytic material that has a greater ability to promote the oxidation of nitrogen monoxide into nitrogen dioxide than the first catalytic material.

Description

Catalytic converter for treatment of exhausts and m\ aftertreatment system including such a catalytic converter BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention concerns a catalytic converter for treating exhaust according to the preamble to claim 1 , and an exhaust system that contains such a catalytic converter. A technology known as SCR (Selective Catalytic Reduction) is used, among others, to reduce emissions of nitrogen oxide ΝΟχ from combustion engines. This technology involves the addition of a specific dose of a solu tion of urea to the exhaust in an exhaust line. The urea solution can be sprayed into the exhaust line, after which the finely distributed urea solution vaporizes upon contact with the hot exhaust, so that ammonia is formed. The mixture of ammonia and exhaust is then led through an SCR catalytic converter. There the nitrogen in the nitrogen oxide in the exhaust reacts with the nitrogen in the ammonia so that nitrogen gas is formed. The oxygen in the nitrogen oxide reacts with the hydrogen in the ammonia so that water is formed. The nitrogen oxide in the exhaust is thus reduced to nitrogen gas and water vapor in the catalytic converter. With a correct proportioning of urea, the nitrogen oxide emissions from the combustion engine can be reduced to a large extent,

The ability of a conventional SCR catalytic converter to remove nitrogen oxide is related to the exhaust temperature. The optimum temperature is dependent upon the type of active catalytic material used in the SCR catalytic converter. The ability of conventional SCR catalytic converters to remove nitrogen oxide from exhaust is problematic mainly at low temperatures. Nitrogen oxide ΝΟχ in exhaust consists of nitrogen monoxide NO and nitrogen dioxide NO?. The ability of conventional SCR catalytic converters to remove nitrogen oxide from exhaust is also dependent upon the ratio between nitrogen monoxide NO and nitrogen dioxide N0₂. The ability of an SCR catalytic converter to reduce the amount of nitrogen oxide in exhaust is optimum when the exha u st contains equal amounts of nitrogen monoxide and nitrogen dioxide.

Exhaust from, in particular, diesel engines normally contains a significantly lower proportion of nitrogen dioxide than of nitrogen monoxide. The exhaust flow through the SCR catalytic converter is another factor that affects the capacity of the SCR catalytic converter. The aiTangeraent of an oxidation catalytic converter DOC (Diesel Oxidation Catalyst) in the exhaust line upstream of the SCR catalytic con verter is known in order to increase the proportion of nitrogen dioxide in the exhaust. An oxidation catalytic converter oxidizes nitrogen monoxide NO into nitrogen dioxide NO?. The proportion of nitrogen dioxide NO? in the exhaust can thus be increased. However, the ability of an oxidation catalytic converter to oxidize nitrogen monoxide NO into nitrogen dioxide N0₂ varies with the exhaust temperature and flow. As a result, an oxidation catalytic converter cannot always deliver the desired distribution between nitrogen monoxide and nitrogen dioxide. An oxidation catalytic converter also creates counterpressure in the exhaust line.

As a rule, an SCR catalytic converter accumulates ammonia, which then reacts with the nitrogen oxide in the exhaust. More ammonia accumulates in the SCR catalytic converter at low temperatures that at high temperatures. This means that accumulated ammonia can, in connection with rapid temperature increases in the exhaust, be released and led out of the SCR catalytic converter. To eliminate such emissions of ammonia, an ammonia slip catalytic converter can be arranged downstream of the SCR catalytic converter in the exhaust line. An ammonia slip catalytic converter generally comprises a coating of a precious metal such as platinum, which oxidizes ammonia into nitrogen gas, nitrogen oxide and nitrous oxide. Nitrous oxide is a powerful greenhouse gas. An ammonia slip catalytic converter also creates a counterpressure in the exhaust line.

In a conventional exhaust system with an oxidation catalytic converter DOC, a particulate filter DPF, an SCR catalytic converter and an ammonia slip catalytic converter ASC, it takes a relatively long time after a cold start until a temperature is reached at which the urea solution can start to be added. No elimination of nitrogen oxide from the exhaust occurs during this warming-up period. US 7,431,895 describes an exhaust system that comprises an SCR catalytic converter with active catalytic material arranged in two layers. An SCR component can be arranged in an outer layer and a nitrogen oxide-storing component can be arranged in an inner layer. Nitrogen oxide can be stored temporarily in the SCR catalytic converter by means of the nitrogen oxide-storage component when the exhaust has a lower temperature, and released when the exhaust reaches a higher temperature. The presence of an oxidation catalytic converter that provides the SCR catalytic converter with nitrogen oxide containing essentially equal parts nitrogen monoxide and nitrogen dioxide is, however, a condition for this SCR catalytic converter to work well. If, on average, a surplus of nitrogen dioxide is present over time, the storage capacity will not be utilized efficiently, as it will generally be fully utilized and lack the capacity for additional storage.

SUMMARY OF THE INVENTION

The main object of the present invention is to provide a catalytic converter that has the capacity to store nitrogen oxides and reduce nitrogen oxide emissions in connection with cold starts. Other objects are to provide good elimination of nitrogen oxide at essentially ail operating temperatures, and to essentially prevent ammonia emissions, known as "ammonia slip." These objects are achieved by means of the catalytic converter of the aforementioned type, which is characterized by the fea tures specified in the characterizing part of claim 1. The injection of a reducing agent begins only after a certain temperature is reached in the exhaust system. During cold starts, the exhaust thus initially streams through the catalytic converter with no addition of ammonia. The inner layer thus comprises a nitrogen oxide-storing material and a catalytic material that has a good capacity to oxidize nitrogen monoxide into nitrogen dioxide on a carrier material. Such a catalytic converter can be used as an ammonia-slip catalytic converter (ASC) in a system with an oxidation catalytic converter disposed upstream, a subsequent particulate filter and an SCR catalytic converter. During a cold start, the oxidation catalytic converter is heated up first, whereupon it begins to produce nitrogen dioxide. During this stage the particulate filter, the SCR catalytic converter and the ammonia slip catalytic converter are still cold. Nitrogen dioxide is then stored in the nitrogen oxide-storing material. Once the particulate filter has become warm and the SCR catalytic converter has begun to become warm, urea can start to be added, and the nitrogen oxides can begin to be reduced. Stored nitrogen oxide can be reduced once the heat reaches the ammonia slip catalytic converter. This catalytic converter can also be used as an SCR catalytic converter in corresponding systems. In this case a higher capacity to store nitrogen oxides during cold starts is achieved. Nitrogen oxide emissions can be reduced during a cold start by means of such a nitrogen oxide-storing function until a temperature at which it is possible to add a reducing agent is reached. The injection of the reducing agent begins as soon as a lowest necessary operating temperature is reached, so that ammonia is formed in the exhaust. The reducing agent is advantageously a urea solution. Exhaust and the ammonia that reaches the catalytic converter will initially come into contact with the outer first layer. The ammonia diffuses into the first layer and is absorbed in the so-called active seats. The nitrogen oxide in the exhaust also diffuses into the first layer, where it reacts with ammonia in said active seats so that water and nitrogen gas are formed. The concentration of ammonia and nitrogen oxide decreases with the distance from the surface of the first l ayer. However, the concen tration of ammonia decreases more rapidly with the distance from the surface t han does the concentration of nitrogen dioxide, as the ammonia is absorbed in said seats, while the nitrogen oxide can diffuse essentially freely in the first layer as long as it does not react with ammonia. A portion of the nitrogen oxide also penetrates into the second layer. The tendency of the nitrogen oxide and ammonia to react is low in operating situations in which ammonia is being added to the exhaust at a relatively low temperature. The nitrogen oxide in the first layer will thus react to a significantly lesser extent with ammonia that is adsorbed into said seats. A relati vely large amount of nitrogen oxide will thus diffuse into the second layer, where a portion of the nitrogen oxide will be oxidized into nitrogen dioxide by the active catalytic material that promotes the oxidation of nitrogen monoxide into nitrogen dioxide in the second layer. The nitrogen dioxide in the second layer diffuses back to the first layer, which results in a highly increased number of reactions between ammonia and nitrogen oxide at a relatively deep depth in the first layer. In situations where the exhaust has a low temperature, the second layer will contribute to increasing the capacity of the catalytic converter markedly, and to eliminating nitrogen oxide from the exhaust. When the exhaust has a low temperature, this is extremely desirable, and results in the catalytic converter being able to have a high capacity to eliminate nitrogen oxide, even at low operating temperatures.

The tendency of the nitrogen oxide and ammonia to react is high in operating situations in which the reducing agent is being injected into the exhaust and a high temperature prevails. The nitrogen oxide that diffuses into the first layer thus reacts almost immediately with the ammonia that has been adsorbed in the first layer. The reactions mainly occur in proximity to the surface of the first layer. Only a small portion of the nitrogen oxide reaches the second layer. The second layer in this case receives only a relatively small amount of nitrogen oxide, In this case the presence of the second layer contributes to only a marginal extent to increasing the capacity of the catalytic converter to eliminate nitrogen oxide from the exhaust. Because a catalytic converter normally already has a high capacity to eliminate nitrogen oxide from the exhaust at high temperature, this is not necessary. The catalytic converter thus has a good capacity to eliminate nitrogen oxide from the exhaust at both low and high exhaust temperatures.

Relatively large amounts of ammonia can be stored in the first layer in operating situations in which a reducing agent is being injected and a low temperature prevails in the catalytic converter. Because the catalytic converter comprises a second layer with a nitrogen oxide-storing material and a catalytic material that has the ability to promote the oxidation of nitrogen monoxide into nitrogen dioxide, the second layer can simultaneously contain a large amount of stored nitrogen dioxide. The capacity of the catalytic converter to adsorb ammonia is reduced as a result of engine depression of the gas pedal. Accumulated ammonia will thus be released from the seats in the first layer, A portion of the released ammonia diffuses in the direction of the second layer, where it reacts with the stored nitrogen oxide that contains a high proportion of nitrogen dioxide, This means that a significant portion of the temporarily released ammonia can be eliminated, as thus can the ammonia emissions downstream of the catalytic converter. Preventing ammonia slip by means of stored nitrogen oxide has the advantage that the ammonia is eliminated without producing nitrous oxide.

According to one embodiment of the invention, the second layer comprises a precious metal. As a rule, precious metals have a high capacity to serve as catalysts in connection with the oxidation of nitrogen monoxide into nitrogen dioxide. The second layer advantageously comprises platinum. Platinum is an extremely good catalyst for this purpose. Palladium and rhodium are other alternative precious metals. However, their capacity to oxidize nitrogen oxide is clearly lower than that of platinum. Precious metals are used in conventional ammonia slip catalytic converters to oxidize ammonia into nitrogen gas, nitrous oxide and nitrogen monoxide. In those cases where the stored nitrogen oxide does not suffice to eliminate a temporary surplus of ammonia, the precious metals can help to do so. According to one embodiment of the invention, the second layer comprises a nitrogen oxide-storing material that consists of one or a combination of oxides of alkaline earth metals, alkaline metals or rare earth metals. As a rule, oxides of alkaline earth metals, alkaline metals and rare earth metals have a very high capacity for storing nitrogen oxide. Suitable such oxides comprise magnesium oxide MgO, calcium oxide CaO, strontium oxide SrQ, barium oxide BaO, lithium oxide Li₂0, sodium oxide Na₂0, potassium oxide K₂0, cesium oxide Cs₂0, lanthanum oxide La₂0₃ and yttrium oxide Y2O3.

According to one embodiment of the invention, the first layer comprises an active catalytic material that promotes the catalytic reaction between ammonia and nitrogen oxide. The first active catalytic material should also have the ability to accumulate ammonia. The first catalytic material advantageously contains vanadium pentoxide V₂O₅. As opposed to other catalytic materials such as zeolites, vanadium pentoxide has a relatively good ability to promote the catalytic reaction between ammonia and nitrogen oxide at relatively low temperatures, and when a deficit of nitrogen dioxide is present. SCR catalytic converters based on iron-zeolite deliver poorer performance than a vanadium-based SCR catalytic converter at low temperatures. SCR catalytic converters based on copper-zeolite offer poorer selectivity than vanadium-based SCR catalytic converters at high temperatures, as they oxidize ammonia to a much greater extent. Both types of zeolite catalytic converters produce more nitrous oxide than a vanadium-based SCR ca talytic converter, especially in the case of the copper-zeolite- based converters. The first active catalytic material also advantageously contains wolfram oxide. The first catalytic material thus acquires additionally improved properties in terms of catalyzing the reaction between ammonia and nitrogen oxide in the first layer.

According to one embodiment of the in vention, said wall elements are formed by a carrier body that carries the two layers. As a rule, the two layers are not themselves capable of forming the structure of the catalytic converter. Such a carrier body advantageously consists of a monolithic structure that forms a large number of longitudinal parallel channels in a cohesi ve structure. The carrier body can consist of a ceramic material, which can consist of cordierite (a blended oxide of magnesium, silicon and aluminum) or of a metal. The carrier body can alternatively comprise at least one of the two layers. This can occur in that the carrier body is extruded from the active material in one of the layers, or in that the active material is applied to a corrugated rolled-up fiberglass fabric and a binding agent. According to one embodiment of the invention, the second layer comprises a carrier material in the form of aluminum oxide. Aluminum oxide also has a certain ability to store nitrogen oxide. In addition to one or a plurality of active materials, the first and second layers also comprise a porous carrier material. The purpose of the carrier material is to provide a large total contact surface for the exhaust. The contact surface for a normal carrier material is on the order of 100 m²/g. The active material is spread out as a thin layer or as many small particles in the pores of the carrier material. The carrier material in the first layer can consist of titanium dioxide (anathase). According to another embodiment of the invention, the first lay er is designed so that it has the capacity to receive all the ammonia during normal operation, so that the ammonia penetrates into the second layer only in operating situations in which a temporary surplus of ammonia arises. In this case, all reactions between ammonia and nitrogen oxide will occur in the first layer during normal operation. When a surplus of ammonia arises, e.g. in connection with a rapid increase in the exhaust temperature, released ammonia can react with stored nitrogen oxide. Released ammonia can also be oxidized in the second layer by the second active catalytic material, as it consists of a precious metal. The catalytic converter can consist of an SCR catalytic converter in an exhaust system. The material in the first layer ensures that the catalytic converter has a function corresponding to that of a conventional SCR catalytic converter. The material in the second layer that promotes the oxidation of nitrogen monoxide into nitrogen dioxide results in the elimination, or at least the reduction, of the need of the catalytic converter for an oxidation catalytic converter arranged upstream, The material in the second layer that promotes the storage of nitrogen oxide also results in the catalytic converter acquiring a very good ability to reduce temporary surpluses of ammonia. The need for an ammonia-slip catalytic converter arranged downstream is thereby eliminated or at least reduced.

According to another embodiment, the catalytic converter consists of an ammonia slip catalytic converter. Because the catalytic converter has such good properties in terms of eliminating temporary surpluses of ammonia, it can be used as an ammonia slip catalytic con verter and disposed downstream of a conventional SCR catalytic converter. Compared to a conventional ammonia slip catalytic converter, the catalytic converter has the advantage that it can utilize stored nitrogen oxide in order to eliminate a temporary_' surplus of ammonia without producing nitrous oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments are described below as examples with reference to the accompanying drawings, in which:

Fig. 1 shows a part of an exhaust line that contains a catalytic converter

according to the present invention in the form of an SCR catalytic converter,

Fig. 2 shows a longitudinal cross-section view of a part of the catalytic

converter in Fig. 1 and,

Fig. 3 shows a part of an exhaust line that contains a catalytic converter

according to the present invention in the form of an ammonia slip catalytic converter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Fig. 1 shows a combustion engine in the form of a diesel engine 1. The diesel engine 1 can be intended as the power plant for a heavy vehicle. The diesel engine 1 is equipped with an exhaust line 2 that comprises a container 3 for exhaust-treating components. The container 3 can be a muffler. The container 3 in this case comprises a first exhaust-treating component in the form of a particulate filter 4, which can be designated as DPF (Diesel Particulate Filter). A particulate filter 4 comprises longitudinal parallel channels with stop surfaces disposed in suitable locations. The stop surfaces force the exhaust to be led into adjacent longitudinal channels in the particulate filter 4. The wal ls of the channels are made of a porous material with fine channels that permit the passage of exhaust gases, but not of soot particles. The soot particles thus lodge inside the particulate filter 4. The particle tilterparticulate filter 4 is continuously regenerated without active measures in that the soot particles are oxidized with N0₂ and/or actively by means of heating measures that accelerate the oxidation with either N0₂ or oxygen. The container 3 comprises a second exhaust-purifying component in the form of a catalytic converter 5 according to the present invention. The catalytic converter here consists of an SCR catalytic converter for catalytic exhaust purification according to the method known as SCR. (Selective Catalytic Reduction). This method entails that a reducing agent in the form of a urea solution is injected into the exhaust. In this case the urea solution is stored in a tank 6 and led, by means of a line 7, to an injection element 8 that injects the urea solution into a space 3a in the container. A control unit 9 controls the supply of the urea solution based on information concerning specific engine parameters 10. A pump 1 1 transports the urea solution to the injection element 8.

Fig. 2 shows a longitudinal cross-section of two channels 5a in the SCR. catalytic converter 5. The channels 5a are defined by wall elements 5b. The wall elements 5b comprise a first outer layer 5b-. that defines a surface of the channel 5a, a second inner layer 5b₂ and a carrier body 5b₃ that carries the first layer 5bj and the second layer 5b?.. The first layer 5b] comprises a first active catalytic material that has the ability to accumulate ammonia and to promote a catalytic reaction between nitrogen oxide and ammonia. The first layer advantageously consists of vanadium pentoxide and wolfram trioxide on a carrier material consisting of titanium dioxide. The second layer 5b?. consists of a material, that has nitrogen oxide-storing properties and an acti ve catalytic material that promotes the oxidation of nitrogen monoxide into nitrogen dioxide. The second layer can comprise a blend of one or a plurality of oxides of alkaline earth metals, alkaline metals and rare earth metals that have nitrogen oxide-storing properties and a precious metal such as platinum that promotes the oxidation of nitrogen monoxide into nitrogen dioxide on a carrier materia! consisting of aluminum oxide. Oxides of alkaline earth metals, alkaline metals and rare earth metals are especially suitable as nitrogen-storing materials because they have a high nitrogen oxide-storing capacity even at the relatively high temperatures that are relevant in connection with SCR systems. Aluminum oxide also has a certain ability to store nitrogen oxide. In connection with nitrogen oxide storage in the second layer, mainly nitrogen dioxide is taken up by the metal oxide, whereupon nitrites (N0₂ ^") and nitrates (NO₃ ^") are formed. The nitrites can be oxidized further to nitrates by additional.

nitrogen dioxide, and the nitrates can be reduced to nitrites by nitrogen monoxide. The nitrates bind nitrogen oxides at higher temperatures than do the nitrites. Also having a precious metal such as platinum in the nitrogen oxide-storing layer results in an improved ability to store nitrogen oxides, as platinum oxidizes nitrogen monoxide into nitrogen dioxide. By choosing one or a combination of oxides of alkaline earth metals, alkaline metals or rare earth metals, the temperature stability of the stored nitrogen oxide can be selected so that it fits the application in question, The carrier body 5b₃ consists of eordierite or another suitable ceramic material.

During operation of the combustion engine 1 the control unit 9 calculates, based on information concerning engine parameters 10 such as load and rpm, the amount of urea solution that needs to be added in order for the nitrogen oxide in the exhaust to be reduced in an optimal manner. The control unit 9 activates the pump 1 1 , which transports the calculated amount of urea solution to the injection element 8, which injects the urea solution into the exhaust, The added urea solution is heated by the exhaust in the container 3 so that it vaporizes and is converted into ammonia. The blend of ammonia and exhaust is led into a catalytic converter 5. In the catalytic converter 5 the nitrogen in the nitrogen oxide in the exhaust reacts with the nitrogen in the ammonia so that nitrogen gas is formed, The oxygen in the nitrogen oxide reacts with the hydrogen in the ammonia so that water is formed. The nitrogen oxide in the exhaust is thus reduced to nitrogen gas and water vapor in the catalytic converter 5.

However, the urea solution should not be injected into the exhaust until a minimum acceptable operating temperature has been reached. During, for example, a cold start when a low temperature prevails in the exhaust system, exhaust is led to the catalytic converter 5 with no added ammonia. There the nitrogen oxide in the exhaust diffuses into the first layer 5bi and the second layer 5h₂ in the SCR catalytic converter 5.

Because the second layer 5b₂ comprises a material with nitrogen oxide-storing properties, at least part of the exhaust that reaches the SCR catalytic converter 5 remains in the second layer 5b₂.

Once an operating temperature is reached, the injection of urea solution begins, so that ammonia is formed in the exhaust. Exhaust and ammonia will initially, in the catalytic converter 5, come into contact with the first layer 5bi, which contains the first active catalytic material. The ammonia diffuses into the first layer 5bi and adsorbs into the active seats. Nitrogen oxide also diffuses into the first layer

The nitrogen oxide reacts there with ammonia in said active seats, so that water and nitrogen gas are formed. The concentration of ammonia and nitrogen oxide decreases with the distance from the surface of the first layer 5bi. The concentration of ammonia decreases more rapidly with the distance from the surface than does the concentration of nitrogen oxide, as the ammonia is adsorbed in said seats while the nitrogen oxide can diffuse freely in the first layer as long as it has not reacted with ammonia, Nitrogen oxide also penetrates into the second layer 5b₂.

The tendency of the nitrogen oxide and ammonia to react is low in operating situations in which the urea solution is being injected and a relatively low temperature prevails in the catalytic converter. The nitrogen oxide that penetrates into the first layer 5bi thus reacts to a significantly lesser extent with the ammonia that has been adsorbed in the first layer 5bi, Because the second layer 5b₂ comprises a catalytic material that promotes the oxidation of nitrogen monoxide into nitrogen dioxide, a relatively large amount of nitrogen dioxide is created in the second layer 5b₂. After a cold start there can also be a relatively large amount of stored nitrogen dioxide in the second layer 5b₂. The nitrogen dioxide in the second layer 5b? then gradually diffuses back to the first layer 5b i which results in a greatly increased number of reactions between ammonia and nitrogen oxide at a relatively large distance from the surface of the first layer 5bi. In the event that the exhaust has a low temperature, the second layer 5b₂ contributes to a marked increase in the ability of the catalytic converter 5 to eliminate nitrogen oxide from the exhaust. This results in the catalytic converter 5 being able to have a high capacity to eliminate nitrogen oxide at low operating temperatures. The tendency of the ni trogen oxide and the ammonia to react is high in operating situations in which the reducing agent is being injected into the exhaust and a relatively high temperature prevails in the catalytic converter 5. The nitrogen oxide that diffuses into the first layer 5b] thus reacts almost immediately with the ammonia that has been adsorbed in the first layer 5b i. The reactions occur mainly in proximity to the surface of the first layer 5b Only a small portion of the nitrogen oxide reaches the second layer 5b₂. The second layer 5b₂ in this case receives only a small amount of nitrogen monoxide that can be oxidized into nitrogen dioxide. In this case the presence of the second layer 5b₂ contributes only to a small extent to increasing the ability of the catalytic converter 5 to eliminate nitrogen oxide from the exhaust. Nor is this necessary_', as the catalytic converter 5 already has a high capacity for eliminating nitrogen oxide from the exhaust when a high temperature prevails.

Relatively large amounts of ammonia can be stored in the first layer 5bi in operating situations in which the urea solution is being injected and a low temperature prevails in the catalytic converter 5. Because the catalytic converter 5 comprises a second layer

5b₂ with a nitrogen oxide-storing material and a catalytic material that has the ability to promote the oxidation of nitrogen monoxide into nitrogen dioxide, the second layer 5b₂ can contain a large amount of stored nitrogen dioxide. The ability of the catalytic converter 5 to adsorb ammonia decreases in operating situations in which the exhaust temperature is rapidly increased, due to, for example, depression of the gas pedal, Accumulated ammonia in the first layer 5b] will thus be released and diffuse in different directions. Ammonia that diffuses in a direction toward the second layer 5b₂ reacts with the nitrogen oxide in the second layer 5b_{2 1} which has a suitable high proportion of nitrogen dioxide. This means that a significant portion of the temporarily released amount of ammonia can be reduced by means of stored nitrogen oxide, which eliminates emissions of ammonia downstream of the catalytic converter 5. In the event that the catalytic converter 5 consists of an SCR catalytic converter, no ammonia slip catalytic converter need be arranged downstream of the catalytic converter 5.

Fig. 3 shows an alternati ve embodiment of the catalytic con verter 5. In this case the exhaust line 3 is equipped with a conventional oxidation catalytic converter 12, a particulate filter 4, a conventional SCR catalytic converter 13 and an ammonia slip catalytic converter in the form of the catalytic converter 5 according to the invention. Here the catalytic con verter 5 is disposed do wnstream of the SCR catalytic con verter, which is designed in a conventional manner with a layer of active catalytic material. Here again the catalytic converter 5 is designed with channels 5a that are defined by wall elements 5b that comprise a first outer layer 5bi, a second inner layer 5bs and a carrier body 5b₃ in the same way as in Fig. 2.

Because the catalytic converter 5 can store nitrogen oxide, it can eliminate or at least reduce the emissions of nitrogen oxide in connection with cold starts before urea solution hasstarted to be injected into the exhaust. In operating situations in which urea solution is being injected and the exhaust has a low temperature, it has a significantly greater capacity to eliminate nitrogen oxide than a conventional SCR catalytic converter 13, and thus constitutes a very good complement to the conventional SCR. catalytic converter 13. Because the SCR catalytic converter 13 has a conventional design, it allows ammonia to pass through in connection with rapid exhaust temperature increases. The catalytic converter 5 here eliminates the temporary surplus of ammonia by means of stored nitrogen oxide in the second layer. Because the second active catalytic material contains a precious metal, it can also oxidize the surplus of ammonia that penetrates into the second layer. Some tradeoffs need to be made in designing the catalytic converter 5 when it consists of an ammonia slip catalytic converter with a nitrogen oxide-storing function. The ability to store NO_x must decrease and be low above temperatures at which nitrogen oxide reduction is achieved in the SCR catalytic converter and the ammonia slip catalytic con verter. This is accomplished through the choice of materials or combination of materials for nitrogen oxide storage (the thermal stability of nitrates of alkaline earth metals decreases moving upward in the periodic table). After a period of hot operation, the nitrogen oxide-storing component is depleted and can once again store nitrogen oxide at the next cold start. Additional tradeoffs need to be made when designing the catalytic converter 5 when it consists of an SCR catalytic converter with a nitrogen oxide-storing function. The relationship between the thickness of the layers and the precious metal content of the inner layer should be chosen so that the reaction between ammonia and NO_x in the outer layer consumes all the ammonia in most operating cases. The thickness of the inner layer can be in the range of 5-50% of the total thickness of both layers. The precious metal content can be in the range of 0.25- 10 g/ft3.

The invention is not limited to the embodiments described above, but can be varied freely within the scope of the claims.

Claims

1 , A catalytic converter for treating exhaust from a combustion engine (1), wherein the catalytic converter (5) comprises wail elements (5b) that form a plurality of longitudmal channels (5a) in the catalytic converter for receiving exhaust, wherein the wail elements (5b) comprise a first outer layer (5bi) that presents an external surface in the longitudinal channels (5a) and a second inner layer (5b₂) that is arranged internally around the first surface (5bi), wherein the first layer (5bi) comprises a first active catalytic material that promotes a catalytic reaction between nitrogen oxide and ammonia, characterized in that the second layer (5b₂) comprises a carrier material, a material that has the ability to store nitrogen oxide and a second active catalytic material that has a greater ability to promote the oxida tion of nitrogen monoxide into nitrogen dioxide than the first catalytic material,

2. A catalytic converter for purifying exhaust gases according to claim 1, characterized in that the second layer (5b₂) comprises an active catalytic material in the form of a precious metal,

3. A catalytic converter according to claim 2, characterized in that the second layer (5b₂) comprises an active ca talytic material in the form of pla tinum.

4. A catalytic con verter according to any of the preceding claims, characterized in that the second layer (5b₂) comprises a nitrogen oxide-storing material that consists of one or a combination of oxides of alkaline earth metals, alkaline metals or rare earth metals.

5. A catalytic converter according to claim 4, characterized in that the first catalytic material in the first layer (5b]) comprises vanadium pentoxide.

6. A catalytic converter according to claim 5, characterized in that the first catalytic material in the first layer (5bi) also comprises wolfram oxide.

7. A. catalytic converter according to any of the preceding claims, characterized in that said wall elements (5b) are formed by a carrier body (5b₃) that carries the two layers (5b) , 5b₂).

8. A. catalytic converter according to any of the preceding claims, characterized in that the material in at least one of the layers is contained in a carrier body (5b₃).

9. A catalytic converter according to any of the preceding claims, characterized in that the second layer (5b₂) comprises a carrier material in the form of aluminum oxide.

10. A catalytic converter according to any of the preceding claims, characterized in that the first layer (5b s) is designed so that it has the capacity to take up all the ammonia during normal operation so that ammonia penetrates into the second layer (5b₂) only in operating situations in which a temporary surplus of ammonia arises.

11. An exhaust system for a combustion engine, characterized in that it contains a catalytic con verter (5) according to any of claims 1 -10,

12. An exhaust system for a combustion engine according to claim 1 1 , characterized in that the catalytic converter (5) consists of an SCR catalytic converter in the exhaus t system.

13. An exhaust system for a combustion engine according to claim 11, characterized in that the catalytic converter (5) consists of an ammonia slip catalytic converter in the exhaust system.