SE537367C2 - Catalyst for the treatment of exhaust gases and an exhaust system comprising such a catalyst - Google Patents

Abstract

Summary The present invention relates to a catalyst for treating exhaust gases from an internal combustion engine (1). The catalyst (5) comprises cradle elements (5b) forming a plurality of elongate channels (5a) of the exhaust gas receiving catalyst, the cradle elements (5b) comprising a first outer layer (5bi) providing an outer surface of the elongate channels (5a) and a second inner layer (5b2) disposed internally about the first layer (5b1), the first layer (5bi) comprising a first active catalyst material which promotes a catalytic reaction between nitrogen oxide and ammonia, and wherein the second layer (5b2) comprises a material capable of storing nitrogen oxide and a second active catalyst material having a higher capacity to promote oxidation of nitrogen monoxide to nitrogen dioxide than the first catalyst material. The second layer (5b2) comprises a nitrogen oxide storage material consisting of an oxide of an alkaline earth metal or a combination of Adana oxides.

Description

BACKGROUND OF THE INVENTION AND PRIOR ART The present invention relates to a catalyst for the treatment of exhaust gases according to the preamble of claim 1 and an exhaust system comprising such a catalyst.

In order to reduce the emissions of nitrogen oxide NOx from internal combustion engines, e.g. a technique called bendmns SCR (Selective Catalytic Reduction). This technique meant that a solution of urea was supplied at a certain dose to the exhaust gases in an exhaust line. The urea solution can be sprayed into the exhaust line, after which the finely divided urea solution is evaporated in contact with the hot exhaust gases so that ammonia is formed. The mixture of ammonia and exhaust gases is then passed through an SCR catalyst. The nitrogen of the nitrogen oxide in the exhaust gases reacts with the nitrogen in the ammonia so that nitrogen gas is formed. The oxygen of the nitrogen oxide reacts with the water in the ammonia to form water. The nitrogen oxide in the exhaust gases is thus reduced in the catalyst to nitrogen gas and water vapor.

With the correct dosage of urea, the emissions of carbon dioxide from the internal combustion engine can be greatly reduced.

The design of a conventional SCR catalyst is that nitrogen oxide precipitation is related to the temperature of the exhaust gases. The optimum temperature depends on the type of active catalyst material used in the SCR catalyst. The ability of conventional SCR catalysts to emit carbon dioxide from exhaust gases is above all a problem at low temperatures. Nitric oxide NOx in exhaust gases consists of nitrogen monoxide NO and nitrogen dioxide NO /. The ability of conventional SCR catalysts to emit nitrogen oxide from exhaust gases is Wen dependent on the ratio of nitrogen monoxide NO to nitrogen dioxide NO /. The ability of an SCR catalyst to reduce the amount of nitrogen oxide in exhaust gases is optimal as the exhaust gases contain the same amount of nitrogen monoxide and 1 nitrogen dioxide. Exhaust gases from diesel engines in particular usually contain a much smaller proportion of nitrogen dioxide than nitrogen monoxide. The exhaust flow through the SCR catalyst is also a factor that affects the capacity of the SCR catalyst.

In order to increase the proportion of nitrogen dioxide in the exhaust gases which is led to an SCR catalyst, it is advisable to arrange an oxidation catalyst DOC (Diesel Oxidation Catalyst) in the exhaust line upstream of the SCR catalyst. An oxidation catalyst oxidizes nitrogen monoxide NO to nitrogen dioxide NO2. Thus, the proportion of nitrogen dioxide NO2 in the exhaust gases can be increased. However, the shape of an oxidation catalyst to oxidize nitrogen monoxide NO to nitrogen dioxide NO2 varies with the temperature and flow of the exhaust gases. Thus, an oxidation catalyst can not always deliver the desired distribution between nitrogen monoxide and nitrogen dioxide. An oxidation catalyst also creates back pressure in the exhaust line.

An SCR catalyst usually accumulates ammonia which then reacts with the nitrogen oxide in the exhaust gases. At low temperatures more ammonia accumulates in the SCR catalyst than at high temperatures. This meant that accumulated ammonia, at rapid temperature rises in the exhaust gases, could be released and discharged from the SCR catalyst. To eliminate such ammonia emissions, an ammonia grinding catalyst can be provided downstream of the SCR catalyst in the exhaust line. An ammonia abrasive catalyst usually comprises a coating of a noble metal such as platinum which oxidizes ammonia to nitrogen gas, nitrogen oxide and nitrous oxide. Nitrous oxide is a strong greenhouse gas. An ammonia grinding catalyst also creates a back pressure in the exhaust line.

In a conventional exhaust system with an oxidation catalyst DOC, a particulate filter DPF, an SCR catalyst and an ammonia abrasive catalyst ASC, it takes a relatively long time after a cold start before a temperature is reached at which the urea solution can begin to be added. During this heating period, no nitrogen oxide is eliminated from the exhaust gases.

US 7,431,895 discloses an exhaust system comprising an SCR catalyst with active catalyst materials arranged in two. layer. An SCR component may be arranged in an outer layer and a nitric oxide storage component may be arranged in an inner layer. With the aid of the nitrogen oxide storage component, nitrogen oxide can be stored temporarily in the SCR catalyst when the exhaust gases have a low temperature and are released when the 2 exhaust gases reach a higher temperature. A prerequisite for this SCR catalyst to function, however, is the presence of an oxidation catalyst which supplies the SCR catalyst with nitrogen oxide with substantially equal proportions of nitrogen monoxide and nitrogen dioxide. If it over time fathers an excess of nitrogen dioxide on average, the storage form will not be used efficiently, as it will usually be filled and not have the form to store further.

SUMMARY OF THE INVENTION The primary object of the present invention is to provide a catalyst having the capacity to store nitrogen oxides to reduce nitrogen oxide emissions during cold starts. Other objects are to provide a good elimination of nitrogen oxide yid substantially all operating temperatures and to substantially prevent the emission of ammonia, so-called ammonia lip.

These objects are achieved with the catalyst of the kind mentioned in the introduction, which can be characterized by the features stated in the characterizing part of claim 1. The injection of a reducing agent only starts after a certain temperature has been reached in the exhaust system. Thus, in cold starts, the exhaust gases initially flow through the catalyst without flaking addition of ammonia. The second inner layer thus comprises a nitrogen oxide-storing material and a catalyst material which has a good ability to oxidize nitrogen monoxide to nitrogen dioxide. Such a catalyst can be used as an ammonia abrasive catalyst (ASC) in a system with an upstream oxidation catalyst, a subsequent particulate filter and SCR catalyst. At a cold start, the oxidation catalyst first warms up and begins to produce nitrogen dioxide. At this stage, the particulate filter, SCR catalyst and ammonia abrasive catalyst are still cold. Nitrogen dioxide is then stored in the nitrogen oxide storage material. When the particle filter has become hot and the SCR catalyst has started to get hot, urea can start to be dosed and the nitrogen oxides can start to be reduced. When the heat reaches the ammonia abrasive catalyst, the stored nitrogen oxide can be reduced. This catalyst can be Liven anyandas as SCR catalyst in the corresponding system. In this case, a stone capacity is achieved to store nitrogen oxides at cold start. With such a nitrogen oxide storage function, the emissions of nitrogen oxide can be reduced during a cold start before a temperature is reached at which it is possible to supply a reducing agent. 3 As soon as a minimum required operating temperature has been reached, the injection of the reducing agent starts so that ammonia is formed in the exhaust gases. The reducing agent is advantageously a urea solution. Exhaust gases and the ammonia reaching the catalyst initially come into contact with the outer first layer. The ammonia diffuses into the first layer and is adsorbed on so-called active saten. The nitrogen oxide in the exhaust gases diffuses Wen into the first layer where it reacts with ammonia on the said active compound so that water and nitrogen gas are formed. The concentration of ammonia and nitrogen oxide decreases with distance from the surface of the first layer. However, the concentration of ammonia decreases more rapidly with the distance from the surface than the concentration of nitric oxide because the ammonia is adsorbed in the said way while the nitrogen oxide can diffuse substantially freely in the first layer as long as it does not react with ammonia. Some of the nitrogen oxide penetrates Wen into the second layer.

In the case of operation when ammonia is added to the exhaust gases at a relatively low temperature, the tendency of nitrogen oxide and ammonia to react is low. The nitrogen oxide in the first layer reacts with the dam to a much lesser extent with ammonia adsorbed on the said seed. Relatively much nitrogen oxide thus diffuses into the second layer where a part of the nitrogen monoxide is oxidized to nitrogen dioxide by the active catalyst material which promotes oxidation of nitrogen monoxide to nitrogen dioxide in the second layer. The nitrogen dioxide in the second layer diffuses back to the first layer, which results in a greatly increased number of reactions between ammonia and nitrogen oxide at a relatively large depth in the first layer. In cases where the exhaust gases have a low temperature, the second layer contributes significantly to shaping the catalyst and eliminating nitrogen oxide from the exhaust gases. As the exhaust gases have a low temperature, this is very unfavorable and it results in the catalyst having a high capacity to eliminate nitric oxide Wen at low operating temperatures.

In operational cases when the reducing agent is injected into the exhaust gases and it raids a high temperature, the tendency of nitrogen oxide and ammonia to react is high.

The nitrogen oxide which diffuses into the first layer reacts immediately with the ammonia adsorbed in the first layer. The reactions take place mainly near the surface of the first layer. Only a small part of the nitrogen oxide reaches the second layer. The second layer in this case receives only a relatively small amount of nitric oxide. In this case, the presence of the second layer contributes only to an extent to the Oka catalyst's ability to eliminate nitrogen oxide from the exhaust gases.

Since a catalyst normally already has a high capacity to eliminate nitric oxide from the 4 exhaust gases at a high temperature, this is not necessary. The catalyst thus has a good capacity to eliminate nitrogen oxide from the exhaust gases at both low and high exhaust gas temperatures.

In the case of operation when a reducing agent is injected and it radiates a low temperature in the catalyst, relatively large amounts of ammonia can be stored in the first layer. Since the catalyst comprises a second layer with a nitrogen oxide storage material and a catalyst material having the ability to promote oxidation of nitrogen monoxide to nitrogen dioxide, the second layer may simultaneously contain a large amount of stored nitrogen dioxide. In operating cases where the temperature of the exhaust gases rises rapidly, due to an engine path, the ability of the catalyst to adsorb ammonia decreases. Accumulated ammonia will thus be released from the satin in the first layer. Some of the released ammonia diffuses towards the other layer where it reacts with the stored nitrogen oxide which has a high proportion of nitrogen dioxide. This meant that a substantial portion of the temporarily released ammonia could be eliminated and the ammonia emission downstream of the catalyst clambered. Preventing ammonia grinding with the help of stored nitric oxide has the advantage that ammonia is eliminated without creating nitrous oxide.

According to a preferred embodiment of the invention, the second layer comprises a noble metal. Noble metals generally have a good ability to act as a catalyst in the oxidation of nitrogen monoxide to nitrogen dioxide. The second layer advantageously comprises platinum. Platinum is a very good catalyst for this andamal Other alternative precious metals are palladium and rhodium. However, they have a much better ability to oxidize nitric oxide than platinum. Noble metals are used in conventional ammonia abrasive catalysts to oxidize ammonia to nitrogen gas, nitrous oxide and nitrogen monoxide. In cases where the stored nitrogen oxide is not sufficient to eliminate a temporary excess of ammonia, the noble metal can help with this.

According to a preferred embodiment of the invention, the second layer comprises a nitrogen oxide storage material consisting of one or a combination of oxides of alkaline earth metals. Oxides of alkaline earth metals generally have a very good ability to store nitrogen oxide. Suitable side oxides are Magnesium Oxide MgO, Calcium Oxide CaO, Strontium Oxide SrO and Barium Oxide BaO.

According to one embodiment of the invention, the first layer comprises an active catalyst material which promotes the catalytic reaction between ammonia and nitrogen oxide. The first active catalyst material should also have the capacity to accumulate ammonia. The first catalyst material advantageously comprises Vanadium pentoxide V205. Unlike other catalyst materials, such as zeolites, vanadium pentoxide has a relatively good ability to promote the catalytic reaction between ammonia and nitrous oxide at relatively low temperatures and then there is a deficit of nitrogen dioxide. Iron-zeolite-based SCR catalysts have a better performance than a vanadium-based SCR catalyst at low temperatures. SCR catalysts based on copper zeolite have a lower selectivity than vanadium-based SCR catalysts at high temperatures, as it oxidises ammonia to a much greater extent. Both types of zelite catalyst produce more nitrous oxide, and in particular the copper-zeolite-based, vanadium-based SCR catalyst. The first active catalyst material advantageously comprises alien tungsten oxide. Thereby, the first catalyst material acquires further improved properties to catalyze the reaction between ammonia and nitrous oxide in the first layer.

According to an embodiment of the invention, said cradle elements are formed by a supporting body which carried the two layers. As a rule, the two layers themselves are not capable of shaping the structure of the catalyst. Such a bearing body advantageously consists of a monolith structure which forms a large number of longitudinal parallel channels in a cohesive structure. The bearing body may consist of a ceramic material which may be cordierite (a mixed oxide of magnesium, silicon and aluminum) or of a metal. Alternatively, the bearing body may comprise at least one of the two layers. This can be done by the bearing body being extruded from the active material in some of the layers or by the active material being applied to a corrugated rolled fiberglass and a binder.

According to an embodiment of the invention, the second layer comprises a bare material in the form of alumina. Aluminum oxide, Liven has a certain ability to store nitrogen oxide.

The first and second layers comprise, in addition to one or more active materials, also a porous bar material. The purpose of the bare material is to provide a large total contact area between the first layer and the exhaust gases. For a normal bare material, the contact surface is in the order of 100 m2 / g. The active material is spread as a thin layer or as many small particles in the pores of the bare material. The bare material in the first layer may be titanium dioxide or anatase. According to another embodiment of the invention, the first layer is dimensioned so that it has the capacity to receive all ammonia during normal operation so that ammonia only penetrates into the second layer during operating cases when a temporary excess of ammonia arises. In this case, all reactions between ammonia and nitric oxide will take place in the first layer during normal operation. When an excess of ammonia arises, for example at a rapid rise in the temperature of the exhaust gases, liberated ammonia can react with stored nitrogen oxide. Released ammonia, Wen can be oxidized in the second layer of the second active catalyst material as it consists of a noble metal.

The catalyst may be an SCR catalyst in an exhaust system. The material in the first layer ensures that the catalyst has a corresponding function as a conventional SCR catalyst. The material in the second layer which promotes the oxidation of nitrogen monoxide to nitrogen dioxide eliminates or at least reduces the catalyst's need for an upstream oxidation catalyst. The material in the second layer which promotes the storage of nitric oxide results in Wen obtaining a very good catalyst to reduce temporary excess ammonia. This eliminates or at least reduces the need for a downstream ammonia abrasive catalyst.

According to another embodiment, the catalyst is an ammonia abrasive catalyst. Because the catalyst has such good properties to eliminate temporary excess ammonia, it can be used as an ammonia abrasive catalyst and placed downstream of a conventional SCR catalyst. The catalyst has the advantage in relation to a conventional ammonia abrasive catalyst that it can use stored nitrogen oxide to eliminate a temporary excess of ammonia without forming nitrous oxide.

BRIEF DESCRIPTION OF THE DRAWINGS In the following, by way of example, preferred embodiments of the invention are described with reference to the accompanying drawings, in which: Fig. 1 shows a portion of an exhaust line comprising a catalyst of the present invention in the form of an SCR catalyst; Fig. 2 shows a longitudinal cross-sectional view of a part of the catalyst in Fig. 1 and.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Fig. 1 shows an internal combustion engine in the form of a diesel electric motor 1. The diesel engine 1 may be intended as a drive motor for a heavier vehicle. The diesel engine 1 is provided with an exhaust line 2 which contains a container 3 for exhaust gas treatment components.

The container 3 can be a muffler. The container 3 in this case contains a first exhaust gas treatment components in the form of a particle filter 4 which can be called DPF (Diesel Particulate Filter). A particle filter 4 comprises elongate parallel channels with stop surfaces arranged on suitable stalls. The stop surfaces force the exhaust gases to be led into adjacent elongate channels in the particulate filter 4. The cradles of the channels are made of a porous material with fine channels that tinker the passage of exhaust gases but not of soot particles. The soot particles then get stuck inside the particle filter 4. The particle filter 4 is regenerated continuously without active action by oxidizing the soot particles with NO2 and / or actively by heat-raising & Orders which accelerate the oxidation with either NO2 or oxygen. The container 3 contains a second exhaust gas purifying component in the form of a catalyst 5 according to the present invention. The catalyst consists of an SCR catalyst for catalytic exhaust gas purification according to the method called SCR (Selective Catalytic Reduction). This method meant that a reducing agent in the form of a urea solution was injected into the exhaust gases. In this case, urea discharge is stored in a tank 6 and is led, via a line 7, to an injection means 8 which injects the urea discharge into a space 3a in the container. A control unit 9 controls the supply of the urea discharge with information regarding specific motor parameters 10. A pump 11 transports the urea indentation to the injection means 8.

Fig. 2 shows a longitudinal section of two. channels 5a of the SCR catalyst 5. The channels 5a are defined by cradle elements 5b. The cradle elements 5b comprise of a first outer layer 5bi defining a surface of the channel 5a, a second inner layer 5b2 and a bearing body 51); which carried the first layer 5b1 and the second layer 5b2. The first layer 5bi comprises a first active catalyst material which has the ability to accumulate ammonia and to promote a catalytic reaction between nitrogen oxide and ammonia. The first layer consists advantageously of vanadium pentoxide and of tungsten trioxide on a bare material containing titanium dioxide. The second layer 5b2 consists of a material 8 having nitrogen oxide storage properties and an active catalyst material which promotes oxidation of nitrogen monoxide to nitrogen dioxide. The second sieve may contain a mixture of one or more oxides of alkaline earth metals having nitrogen oxide storing properties and a noble metal such as platinum which promotes oxidation of nitrogen monoxide to nitrogen dioxide on a bar material consisting of alumina.

Alumina also has a certain property of storing nitric oxide. The bearing body 51); consists of cordierite or other suitable ceramic material.

During operation of the internal combustion engine 1, the control unit 9 calculates with the aid of information regarding engine parameters 10 such as load and speed the amount of urea solution that needs to be added in order for the nitrogen oxide in the exhaust gases to be reduced in an optimal way. The control unit 9 activates the pump 11 which transports the urea solution in the grounded quantity to the injector 8 which injects the urea solution into the exhaust gases. The supplied urea solution is heated by the exhaust gases in the container 3 so that it evaporates and is converted to ammonia. The mixture of ammonia and the exhaust gases is introduced into catalyst 5. In catalyst 5, the nitrogen of the nitrogen oxide in the exhaust gases reacts with the nitrogen in the ammonia so that nitrogen gas is formed. The oxygen of the nitrogen oxide reacts with the water in the ammonia to form water. The nitrogen oxide in the exhaust gases is thus reduced in the catalyst to nitrogen gas and water vapor.

However, the Ureal solution only begins to be injected into the exhaust gases after a minimum acceptable operating temperature has been reached. Thus, for example, in a cold start when a lower temperature rows in the exhaust system, exhaust gases are thus led to the catalyst 5 without any addition of ammonia. The nitrogen oxide in the exhaust gases diffuses has the first layer 5bi and the second layer 5b2 of the SCR catalyst 5. Since the second layer 5b2 comprises a material with nitrogen oxide storage properties, it becomes at least a part of the exhaust gases which the SCR catalyst 5 remains in the second layer 5b2.

When an operating temperature is reached, the injection of the urea solution starts so that ammonia is formed in the exhaust gases. Exhaust gases and ammonia initially come into contact with the catalyst 5 in the first layer Sbi which contains the first active catalyst material. The ammonia diffuses into the first layer 5b1 and adsorbs on the active seeds. Nitric oxide also diffuses into the first layer 5bi. The nitrogen oxide reacts there with ammonia on the said active way so that water and nitrogen gas are formed. The concentration of ammonia and nitrogen oxide decreases with distance from the surface of the first layer 5bi. The concentration of ammonia decreases more rapidly with the distance 9 from the surface than the concentration of nitric oxide because the ammonia is adsorbed in the said way while the nitrogen oxide can diffuse freely in the first layer as long as it has not reacted with ammonia. Nitric oxide penetrates Wen into the second layer 5130.

In operating cases when the urea solution is injected and it fades a relatively low temperature in the catalyst, the tendency of the nitrogen oxide and ammonia to react is low. The nitrogen oxide penetrating into the first layer 5b1 thus reacts to a much lesser extent with the ammonia adsorbed in the first layer 5b1. Since the second layer 5b2 comprises a catalyst material which promotes oxidation of nitrogen monoxide to nitrogen dioxide, a relatively large amount of nitrogen dioxide is created in the second layer 5b2. After a cold start, there may be a relatively large amount of nitrogen dioxide stored in the second layer 5b2. The nitrogen dioxide in the second layer 5b2 then gradually diffuses back to the first layer 5b1, 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 cases where the exhaust gases have a low temperature, the second layer 5b2 helps to significantly shape the catalyst 5 to eliminate the nitric oxide from the exhaust gases. As a result, the catalyst 5 may have a high capacity to eliminate nitric oxide at 18ga operating temperatures.

In the case of operation when the reducing agent is injected into the exhaust gases and it fades a relatively high temperature in the catalyst, the tendency of nitrogen oxide and ammonia to react is high. The kvWe oxide which diffuses into the first layer 5bi reacts almost immediately with the ammonia adsorbed in the first layer 5b1. The reactions take place substantially near the surface of the first layer 5bi. Only a small part of the nitrogen oxide reaches the second layer 5b2. The second layer 5b2 in this case receives only a small amount of nitrogen monoxide which can be oxidized to nitrogen dioxide. In this case, the presence of the second layer 5b2 contributes only to a small extent to the shape of the Oka catalyst 5 to eliminate nitrogen oxide from the exhaust gases. Since catalyst 5 already has a high capacity to eliminate nitric oxide from the exhaust gases at a high temperature range, this is not necessary either.

In operating cases when the urea solution is injected and it has a low temperature in the catalyst 5, relatively large amounts of ammonia can be stored in the first layer 5bi.

Since the catalyst 5 comprises a second layer 5b2 with a nitrous oxide storage material and after catalyst material which is capable of promoting oxidation of nitrous oxide to nitrous oxide, the second layer 5b2 may contain a large amount of stored nitrogen dioxide. In the case of operation where the temperature of the exhaust gases rises rapidly, due to, for example, an engine path, the ability of the catalyst 5 to adsorb ammonia decreases. Accumulated ammonia in the first layer 5 thus becomes released and diffuses in different directions. Ammonia which diffuses in the direction of the second layer 5b2 reacts with the nitrogen oxide in the second layer 5b2 which has a suitably high proportion of nitrogen dioxide. This meant that a substantial part of the temporarily released amount of ammonia can be reduced with the aid of stored nitrogen oxide, which eliminates the emission of ammonia downstream of the catalyst 5. In this case then. Catalyst 5 is an SCR catalyst, no ammonia abrasive catalyst is provided downstream of catalyst 5.

Fig. 3 shows an alternative embodiment of the catalyst 5. In this case the exhaust line 3 is provided with a conventional oxidation catalyst 12, a particulate filter 4, a conventional SCR catalyst 13 and an ammonia abrasive catalyst in the form of the catalyst 5 according to the invention. The catalyst 5 is located downstream SCR catalyst which is conventionally formed with a layer of an active catalyst material. The catalyst 5 is also formed with channels 5a defined by cradle elements 5b which comprise a first outer layer 5bi, a second inner layer 5b3 and a supporting body 5b3 in the same manner as in Fig. 2.

Since the catalyst 5 may have stored nitric oxide, it can reduce the emission of nitric oxide at cold starts before urea release (5b1) has started to be injected into the exhaust gases. In operational cases where urea discharge is injected and when the exhaust gases have a low temperature, it has a much higher capacity to eliminate nitrogen oxide than the conventional SCR catalyst 13 and it is therefore a very good complement to the conventional SCR catalyst 13. Since the SCR catalyst 13 has a conventional design, it ships through ammonia at rapid temperature rises in the exhaust gases. The catalyst 5 eliminates the temporary excess of ammonia by means of stored nitrogen oxide in the second layer. Since the second active catalyst material contains a noble metal, it can reduce the excess ammonia that penetrates into the second layer.

When designing the catalyst 5 as it consists of an ammonia abrasive catalyst with a nitrogen oxide storage function, some balances need to be made. The capacity to store NOx must decrease and be small. Over temperatures when the nitrogen oxide reduction is achieved in the SCR catalyst and ammonia abrasive catalyst. This is achieved by the choice of material or combination of materials for nitrogen oxide storage (the thermal stability of nitrates of alkaline earth metals decreases upwards in the periodic table). After a while of hot operation, the nitrogen oxide storage component is empty and it can store nitrogen oxide again at the next cold start. When designing the catalyst 5 as it constitutes an SCR catalyst with a nitrogen oxide storage function, further balances need to be made. The ratio between the thickness of the layers and the noble metal content in the inner layer should be chosen so that the reaction between ammonia and NOx in the outer layer in most operating cases consumes all ammonia. The thickness of the inner layer can be in the range 5-50% of the sum of the thickness of both layers. The noble metal content can be in the range 0.25-10 g / ft3.

The invention is not limited to the embodiment described above, but it can be varied freely within the scope of the patent claims. 12

Claims (12)

Patent claims A catalyst for treating exhaust gases from an internal combustion engine (1), the catalyst (5) comprising rocker elements (5b) forming a plurality of elongate channels (5a) of the catalyst for receiving exhaust gases, the rocker elements (5b) comprising a first outer layer (5b1) providing an outer surface of the elongate channels (5a) and a second inner layer (5b2) disposed internally about the first layer (5b1), the first layer (5b1) comprising a first active catalyst material which promotes a catalytic reaction between nitrogen oxide and ammonia, and wherein the second layer (5b2) comprises a material capable of storing nitrogen oxide and a second active catalyst material having a higher capacity to promote oxidation of nitrogen monoxide to nitrogen dioxide than the first catalyst material, characterized in that the second the layer (5b2) comprises a nitrogen oxide-storing material consisting of an oxide of an alkaline earth metal or a combination of sa dana oxides. Catalyst for exhaust gas purification according to claim 1, characterized in that the second layer (5b2) comprises an active catalyst material in the form of a noble metal. Catalyst according to claim 2, characterized in that the second layer (5b2) comprises an active catalyst material in the form of platinum. Catalyst according to the preceding claim, characterized in that the first catalyst material in the first layer (5b1) comprises vanadium pentoxide. S. Catalyst according to claim 4, characterized in that the first catalyst material in the first layer (5b1) of the Life comprises tungsten oxide. Catalyst according to any one of the preceding claims, characterized in that said rock element (5b) is formed by a supporting body (5b3) which supported the two layers (5bi, 5b2). Catalyst according to any one of the preceding claims, characterized in that the material in at least one of the layers is included in a bearing body (5b3). 13 Catalyst according to one of the preceding claims, characterized in that the second layer (5b2) comprises a support material in the form of alumina. Catalyst according to any one of the preceding claims, characterized in that the first layer (5b1) is dimensioned so that it has the capacity to receive all ammonia during normal operation so that ammonia only penetrates into the second layer (5b2) at operating times when it arises a temporary excess of ammonia. Exhaust system for an internal combustion engine, characterized in that it comprises a catalyst (5) according to any one of claims 1-10. Exhaust system for an internal combustion engine according to claim 10, characterized in that the catalyst (5) constitutes an SCR catalyst in the exhaust system. Exhaust system for an internal combustion engine according to claim 10, characterized in that the catalyst (5) constitutes an ammonia abrasive catalyst in the exhaust system. 14 1 4 8 A ... '4 33a zak'Stak'S, MM'aa ::,> A U L., 13 6 7 9 ............ 11