WO2005064130A1

WO2005064130A1 - Device and process for removing nitrogen oxides from the exhaust gas of internal combustion engines with the aid of catalytically generated ammonia

Info

Publication number: WO2005064130A1
Application number: PCT/EP2004/010261
Authority: WO
Inventors: Marcus Pfeifer; Nicola SÖGER; Paul Spurk; Roger Staab; Christian Kühn; Jürgen GIESHOFF; Egbert Lox; Thomas Kreuzer
Original assignee: Umicore AG and Co KG
Current assignee: Umicore AG and Co KG
Priority date: 2003-12-23
Filing date: 2004-09-14
Publication date: 2005-07-14
Anticipated expiration: 2006-06-23
Also published as: DE10360955A1

Abstract

The invention relates to an exhaust-gas purification system for a lean-burn internal combustion engine, comprising at least a first catalyst (1) and a downstream second catalyst (2), the first catalyst (1) generating ammonia from corresponding exhaust-gas constituents when the exhaust-gas composition is rich, and the second catalyst (2) temporarily storing the ammonia generated by the first catalyst (1) when the exhaust-gas composition is rich and subjecting the nitrogen oxides (NOx) contained in the exhaust gas to a reduction reaction using the temporarily stored ammonia as reducing agent when the exhaust-gas composition is lean. The exhaust-gas purification system is characterized in that the exhaust-gas purification system has, downstream of the second catalyst (2), a third, precious-metal-containing catalyst (3), which contains at least one of the platinum group metals platinum, palladium and rhodium on support materials, with the support materials being able to store ammonia when the exhaust-gas composition is rich and to release ammonia when the exhaust-gas composition is lean.

Description

DEVICE AND PROCESS FOR REMOVING NITROGEN OXIDES FROM THE EXHAUST GAS OF INTERNAL COMBUSTION ENGINES WITH THE AID OF CATALYTICALLY GENERATED AMMONIA

Description The invention relates to an exhaust-gas purification system for removing nitrogen oxides from the exhaust gas of internal combustion engines with the aid of catalytically generated ammonia, and to a process for purifying the exhaust gases from lean-burn internal combustion engines, in particular from diesel engines.

Internal combustion engines which are operated in lean-burn mode are also referred to below as lean-burn engines. They are operated with a lean air/fuel mixture. Therefore, in addition to the usual pollutants, namely carbon monoxide (CO), nitrogen oxides (NOx) and unburnt hydrocarbons (HC) and particulates (PM), their exhaust gas also contains a high proportion, amounting to up to 15% by volume, of oxygen, and consequently the exhaust gas has a net oxidizing action. Therefore, the exhaust-gas purification processes by means of three-way catalysts which are customary for stoichiometrically operated internal combustion engines cannot be employed. In particular, conversion of the nitrogen oxides into nitrogen presents considerable difficulties in the oxidizing exhaust-gas atmosphere.

The main components of the nitrogen oxides in the exhaust gas from lean-burn engines are nitrogen monoxide (NO) and nitrogen dioxide (NO₂), with nitrogen monoxide forming the majority. Depending on the operating conditions of the internal combustion engine, the nitrogen monoxide makes up from 60 to 95% by volume of the nitrogen oxides as a whole.

The selective catalytic reduction (SCR) process has long been known for reducing nitrogen oxides in oxidizing exhaust gases. In this process, ammonia is added to the exhaust gas as reducing agent, and this gas mixture is then passed over a catalyst for selective catalytic reduction (SCR catalyst). At the SCR catalyst, the nitrogen oxides are selectively reacted with the ammonia to form nitrogen and water. This process is nowadays employed on a large industrial scale for the purification of power plant exhaust gases. Typical SCR catalysts contain, as catalytically active components, by way of example solid-state acids selected from the TiO₂/WO₃/MoO₃/N₂O5/SiO₂/SO₃ system. Other SCR catalysts are based on acid-resistant zeolites which have been exchanged with transition metals, such as for example dealuminized Y-zeolite, mordenite, silicalite or ZSM-5. Furthermore, the catalysts may contain further components, such as for example copper, iron, cerium and manganese.

SCR catalysts based on solid-state acid systems or based on zeolites are referred to below as standard SCR catalysts. They also always have a certain ability to store ammonia. Their operating temperature is approximately between 300 and 500°C.

On account of the need to add a reducing agent to the exhaust gas, the SCR process is highly complex for use in mobile applications. Therefore, the NOx storage technology has been developed as an alternative to the SCR process. In this technology, the nitrogen oxides contained in the lean exhaust gas are temporarily stored in the form of nitrates on a nitrogen oxide storage catalyst. Once the storage capacity of the storage catalyst is exhausted, the catalyst has to be regenerated. For this purpose, the internal combustion engine is briefly operated with a rich air/fuel mixture, i.e. more fuel is fed to the air/fuel mixture than can be completely burnt by the combustion air — the exhaust gas is rich. It therefore still contains unburnt hydrocarbons. In the rich exhaust gas, the stored nitrates are decomposed to form nitrogen oxides and are reacted with the unburnt hydrocarbons contained in the rich exhaust gas as reducing agents to form nitrogen and water.

To store the nitrogen oxides in the form of nitrates, nitrogen oxide storage catalysts contain basic components, such as the metal oxides of the alkali metals and of the alkaline-earth metals, or also rare earths, such as cerium oxide and lanthanum oxide. It is preferable to use barium oxide and strontium oxide. Moreover, the nitrogen oxide storage catalysts also contain catalytically active precious metals, generally platinum. The role of these precious metals is to oxidize the nitrogen monoxide, which is the dominant nitrogen oxide in the exhaust gas, to form nitrogen dioxide. Only nitrogen dioxide is able to react with the storage components to form nitrates with the aid of the steam which is present in the exhaust gas. During the regeneration of the storage catalyst, the desorbed nitrogen oxides are reduced at the catalytically active precious metals to form nitro ^Jgae"n and water.

Cyclical operation of the engine with a lean air/fuel mixture and a rich air/fuel mixture is crucial if the nitrogen oxide storage technology is to be used to purify the exhaust gases from lean-burn engines. The lean-burn mode is in this case the normal running mode of the lean-burn engine. During this operating phase, the nitrogen oxides in the exhaust gas are stored by the storage catalyst (storage phase). During the rich-burn mode, the nitrogen oxides are desorbed again and converted (desorption phase). The storage phase usually lasts from 1 to 2 minutes, whereas the desorption phase requires only a short time, of from 1 to 20 seconds.

Drawbacks of the nitrogen oxide storage technology include the fact that the storage components can easily become poisoned by sulfur, and the relatively low conversion rates of at most 60 to 70%. The SCR technology is superior to the nitrogen oxide storage technology in terms of its conversion rates, its temperature activity range and its durability, but requires the use of a second operating medium - ammonia or a precursor compound which can be decomposed to form ammonia, for example urea or ammonium carbamate.

It has been proposed by EP 0 773 354 A 1 to synthesize the ammonia required for the SCR reaction from the exhaust-gas constituents on board the vehicle. For this purpose, the exhaust gas is passed over a three-way catalyst and then over an SCR catalyst, and the engine is alternately operated with lean and rich air/fuel mixtures. During the operating phases with rich air/fuel mixtures, the three-way catalyst forms ammonia from the nitrogen oxides contained in the exhaust gas, and this ammonia is temporarily stored on the SCR catalyst. During the operating phases with lean air/fuel mixtures, the nitrogen oxides contained in the exhaust gas pass through the three-way catalyst virtually unchanged and are reduced to nitrogen and water by the ammonia which has been adsorbed by the SCR catalyst.

DE 198 20 828 Al describes an improvement to this process, which consists in a nitrogen oxide storage catalyst being introduced into the exhaust system as a third catalyst upstream of the two catalysts, this nitrogen oxide storage catalyst temporarily storing the nitrogen oxides contained in the exhaust gas when the exhaust-gas composition is lean and releasing them again, and partially reducing them directly with the reducing agents present in the rich exhaust gas, such as carbon monoxide or hydrogen, when the exhaust-gas composition is rich. This is supposed to be able to achieve a higher overall conversion rate for the nitrogen oxides, since some of the nitrogen oxides which are present are already reduced by the nitrogen oxide storage catalyst. If the exhaust-gas composition is lean, the nitrogen oxides which have not been stored by the nitrogen oxide storage can then be converted at the SCR catalyst, consuming the ammonia which has been stored previously.

EP 1 226 861 Al proposes, as a further possible improvement, the integration of an oxidation catalyst in the exhaust system upstream of the SCR catalyst. With the aid of this oxidation catalyst, a proportion of the nitrogen oxides, which are predominantly present as NO in the exhaust gas, is oxidized to form NO₂ under lean operating conditions. It is known from prior art that at temperatures below approximately 300°C an NO/NO₂ mixture is more reactive in the SCR reaction than pure NO, and conse- quently the SCR catalyst achieves significantly higher conversion rates even at lower temperatures.

Although all three proposals do lead to improved conversion rates of the nitrogen oxides emitted by lean-burn and diesel engines, further improvement to the nitrogen oxide conversion is required in order to comply with future exhaust-gas standards. Particularly in the case of diesel engines, the low-temperature activity of the catalyst system in the temperature range between 150°C and 250°C needs to be increased. It is an object of the present invention to provide an exhaust-gas purification system which allows conversion of nitrogen oxides in the exhaust gas from lean-burn and diesel engines to be improved further.

This object is achieved by an exhaust-gas purification system for an internal combustion engine, which includes, in the direction of flow of the exhaust gas, at least a first catalyst and a downstream second catalyst, the first catalyst generating ammonia from corresponding exhaust-gas constituents when the exhaust-gas composition is rich, and the second catalyst temporarily storing the ammonia generated by the first catalyst when the exhaust-gas composition is rich and subjecting the nitrogen oxides (NOx) contained in the exhaust gas to a reduction reaction using the temporarily stored ammonia as reducing agent when the exhaust-gas composition is lean. This exhaust-gas purification system is characterized in that the exhaust-gas purification system has, downstream of the second catalyst, a third, precious-metal-containing catalyst, which contains at least one of the platinum group metals platinum, palladium and rhodium on support materials which are able to store ammonia when the exhaust-gas composition is rich and to release ammonia when the exhaust-gas composition is lean.

Catalysts containing platinum group metals, in particular catalysts containing platinum or palladium, are highly reactive with regard to the SCR reaction in the temperature range between 150 and 250°C. However, these catalysts cannot be used effectively at higher temperatures, since at these temperatures they preferentially convert the ammonia into nitrogen or nitrogen oxides. According to the invention, therefore, two different SCR catalysts are combined with one another: a standard SCR catalyst with a precious-metal-containing catalyst. This makes it possible to boost the temperature activity ranges of both SCR catalysts; the precious-metal-containing SCR catalyst covers the temperature range between 150 and 250°C, and the standard SCR catalyst covers the temperature range above 250°C. With regard to the order, it is crucial that the standard SCR catalyst be arranged upstream of the precious-metal-containing catalyst, since otherwise the ammonia formed in the rich phases would at higher temperatures already be undesirably oxidized to form nitrogen or nitrogen oxides by the precious- metal-containing SCR catalyst and would therefore no longer be available to the standard SCR catalyst as reducing agent.

In the rich phases, the ammonia formed is stored in the standard SCR catalyst. In lean- burn mode, at higher temperatures between approximately 300°C and 500°C, the stored ammonia is reacted with the nitrogen oxides in the exhaust gas directly on the standard SCR catalyst. At temperatures from approximately 200 to 300°C, i.e. below the activity range of the standard SCR catalyst, the ammonia which has been stored during the rich phases is reacted with the nitrogen oxides in the exhaust gas by the precious-metal- containing third catalyst.

The proposed exhaust-gas purification system enables the nitrogen oxide conversion levels to be increased considerably in particular at low temperatures. The exhaust-gas purification system according to the invention is preferably used to purify the exhaust gases from diesel engines.

It has been found in various tests that for the application described it is not enough to use standard, precious-metal-containing catalysts, such as for example diesel oxidation catalysts, as SCR catalysts. Catalysts of this type contain, for example, platinum on an active aluminum oxide as support material.

Unlike in the standard SCR process, in which ammonia or a compound which can be decomposed to form ammonia is fed continuously to the exhaust gas, in the present application the SCR catalyst has to have a correspondingly high ammonia storage capacity, enabling it to store ammonia formed in the rich phases and to react the stored ammonia with the nitrogen oxides in the exhaust gas in the lean phases. Whereas standard SCR catalysts have an ammonia storage capacity of this nature, such a capacity is not present in typical, precious-metal-containing catalysts, such as for example diesel oxidation catalysts, since the support materials used for the precious metals in these catalysts have only a low ability to store ammonia.

According to the invention, therefore, the platinum group metals of the third catalyst are applied to support materials with a correspondingly high ammonia storage capacity. Suitable support materials with a capacity to store ammonia include, for example, the oxidic materials selected from the group consisting of titanium oxide, titanium oxide/aluminum oxide, titanium oxide/silicon dioxide, vanadium, vanadium/tungsten oxide, vanadium/molybdenum oxide or zeolites or mixtures thereof used for standard SCR catalysts.

To minimize the pressure losses in the exhaust-gas purification system, the third catalyst is preferably applied to a downstream-side zone of the second catalyst, this zone amounting to 5 to 50% of the overall length L of the second catalyst. In this context, a distinction is to be drawn between three cases:

• The third catalyst is formed by platinum group metals which have previously been deposited on support materials with a capacity to store ammonia and is in the form of a coating on the downstream-side zone of the second catalyst.

• The third catalyst is formed by platinum group metals which have previously been deposited on support materials without or with only a slight capacity to store ammo- nia. In this case too, the third catalyst can be applied in the form of a coating to the downstream-side zone of the second catalyst. The required ammonia storage capacity is in this case provided by the second catalyst beneath it.

• However, it is particularly preferable for the third catalyst to be produced by impregnating the downstream-side zone of the second catalyst with compounds containing platinum, palladium or rhodium. In this case, therefore, the materials of the second catalysts form the support materials for the platinum group metals of the , third catalyst and at the same time provide the required ammonia storage capacity.

It has proven advantageous to apply the third catalyst to a downstream-side zone of the second catalyst. The ammonia consumed in the downstream-side zone during the conversion of the nitrogen oxides is evidently replaced by being topped up from the remaining part of the second catalyst.

Producing the third catalyst in the form of a coating on the downstream-side zone of the second catalyst leads to a slightly increased exhaust-gas back pressure. However, this drawback does not occur if the third catalyst is produced by impregnation of the downstream-side zone.

As has already been explained, the second catalyst is a standard SCR catalyst which includes at least one zeolite which has been exchanged with a transition metal or contains a solid-state acid system selected from the group consisting of titanium oxide or titanium oxide/aluminum oxide or titanium oxide/silicon dioxide in combination with vanadium, vanadium/tungsten oxide or vanadium/molybdenum oxide or zeolites or mixtures thereof.

The catalytically active components of the SCR catalyst (zeolites or solid-state acid system) may on the one hand be applied in the form of a coating to the flow passages of an inert honeycomb carrier made from cordierite or metal, in which case the SCR catalyst takes the form of what is known as a coated catalyst. Alternatively, the catalytically active components may also be processed to form an extrudable compound and extruded to form a honeycomb carrier with flow passages for the exhaust gas. This may be called an extruded catalyst.

The first catalyst of the exhaust-gas purification system according to the invention serves the purpose of forming ammonia from the components of the exhaust gas when the exhaust-gas composition is rich; this ammonia is then stored on the downstream catalysts and consumed for reduction of the nitrogen oxides during the lean phases. Conventional three-way catalysts are eminently suitable for this purpose, but it is also possible to use other catalysts which perform this function.

The invention will now be explained in more detail with reference to Figures 1 to 7 and the Examples. In the drawing:

Figures 1 to 5: show block diagrams illustrating various embodiments of the exhaust- gas purification system according to the invention

Figure 6: shows the NOx conversion curve in the synthesized exhaust gas for an exhaust-gas purification system a) having ah NOx storage catalyst (aged) and b) having an NOx storage catalyst (aged) and a downstream standard SCR catalyst based on zeolites exchanged with metal ions

Figure 7: shows the NOx conversion curve in the synthesized exhaust gas for an exhaust-gas purification system a) having an NOx storage catalyst (aged), b) having an NOx storage catalyst (aged) + standard SCR catalyst + downstream diesel oxidation catalyst c) having an NOx storage catalyst, (aged) + standard SCR catalyst with Pt-containing zone coating. Figure 1 shows a block diagram of an embodiment of the exhaust-gas purification system according to the invention. It includes, arranged one behind the other in the direction of flow of the exhaust gas, a three-way catalyst (1), a standard SCR catalyst (2) and a catalyst (3) containing platinum group metals, the catalysts (2) and (3) each being able to store the ammonia formed by catalyst (1) during the rich phases.

Figure 2 shows a preferred variant of the exhaust-gas purification system shown in Figure 1. In this case, catalyst (3) is applied to a downstream-side zone of the catalyst (2). The width of this zone amounts to 5 to 50% of the length L of the catalyst (2).

Figure 3 shows a further embodiment of the exhaust-gas purification system according to the invention. A fourth catalyst (4), which is a nitrogen oxide storage catalyst, i.e. this catalyst stores the nitrogen oxides contained in the exhaust gas when the exhaust- gas composition is lean and releases them again when the exhaust-gas composition is rich, so that they can then be at least partially reduced with the aid of the reducing agents which are present in the rich exhaust gas, such as hydrocarbons, hydrogen or carbon monoxide, is arranged between the first and second catalysts.

In Figure 4, the nitrogen oxide storage catalyst (4) also performs the function of the three-way catalyst (1), and consequently there is no need for the latter. This arrangement is particularly advantageous for purifying the exhaust gas from diesel engines.

Figure 5 shows a further variant of the embodiment from Figure 3. In this case, the nitrogen oxide storage catalyst (4) is replaced by an oxidation catalyst (5), which partially oxidizes the nitrogen oxides contained in the exhaust gas to form nitrogen dioxide when the exhaust-gas composition is lean and thereby improves the conversion of the nitrogen oxides at the downstream SCR catalyst.

Examples:

Four catalysts having the following properties were produced by coating conventional honeycomb carriers made from cordierite (cell density 62 cm^" ): NOx storage catalyst Precious metals: Pt/Rh Concentration: 1.94 g per litre of honeycomb carrier volume (g/1); corresponds to 55 g/ft³; Weight ratio: Pt/Rh = 10:1 Composition: Pt/Rh on zirconium-stabilized cerium oxide Aluminium oxide Barium carbonate Dimensions: 0 = 25.4 mm; length = 76.2 mm (1" x 3")

SCR catalyst Precious metals: None Composition: ZSM-5 zeolite exchanged with iron Dimensions: 0 = 25.4 mm; length = 76.2 mm (1" x 3")

3. Diesel oxidation catalyst Precious metals: Pt Concentration: 0.35 g per litre of honeycomb carrier volume (g/1); corresponds to 10 g/ft³; Composition: Pt on silicon-stabilized aluminium oxide; zeolite; WC loading 70 g/1 Dimensions: 0 = 25.4 mm; length = 25.4 mm (l" x 1")

4. SCR catalyst with Pt-containing zone impregnation Compositon: ZSM-5 zeolite exchanged with iron Dimensions: 0 = 25.4 mm; length = 76.2 mm (1" x 4") Pt coating: downstream side over a length of 25.4 mm with diesel oxidation catalyst as described under 3 (WC loading 70 g/1)

The catalysts were aged hydrothermally for 10 hours at a temperature of 800°C in a furnace, and their ability to remove the nitrogen oxides from an oxygen-rich exhaust gas was then tested in various combinations.

The catalytic activity was checked with the aid of a model exhaust gas having the following compositions: Table: Measurement conditions for determining the catalytic activity at a model gas system:

λ: air/fuel ratio standardized to stoichiometric conditions

Figure 6 shows that the NOx conversion for the nitrogen oxide storage catalyst (curve a)) in lean/rich operation can be considerably increased by a downstream standard SCR catalyst (curve b)) based on zeolites exchanged with iron, in particular in the temperature range over 230°C. Below this temperature limit, the NOx conversions achieved by the NOx storage catalyst alone and in combination with the SCR catalyst are practically identical. At these low temperatures, in the present NO₂-free model exhaust gas, therefore, the standard SCR catalyst has scarcely any activity.

Figure 7 compares the NOx conversion of the same NOx storage catalyst in lean/rich mode (curve a)) with a combination of the NOx storage catalyst together with the downstream standard SCR catalyst and a diesel oxidation catalyst arranged further downstream (curve c)). The figure also shows the NOx conversion curve for the same NOx storage catalyst in lean/rich mode with the downstream standard SCR catalyst, the standard SCR catalyst additionally having been provided over 25% of its length, on the downstream side in accordance with the invention, with a Pt-containing coating (curve d)). The quantitative loading, precious metal content of (0.35 g/1) of platinum and the chemical composition of this Pt-containing coating precisely correspond to the diesel oxidation catalyst described above, and consequently differences in the NOx conversion curves achieved by the two systems cannot be attributed to different activities on the part of the precious-metal-containing coating.

It can be seen that the additional diesel oxidation catalyst arranged downstream of the SCR catalyst does not bring any benefit to the system. At medium temperatures, there is even evidence of a deterioration in the NOx conversion compared to the NOx storage catalyst/SCR catalyst system (curve b from Figure 6), which is attributable to oxidation of ammonia breaking through the SCR catalyst to form NO. By contrast, the conversion at temperatures below approx. 350°C is considerably increased in the system comprising NOx storage catalyst/SCR catalyst with Pt-containing zone coating. This system enables conversion levels of approx. 50% to be achieved even at temperatures of 150°C. In the system given, the precious metal catalyst can make use of the ammonia store provided by the standard SCR catalyst in contact with it; therefore, unlike in the system comprising the pure diesel oxidation catalyst, sufficient ammonia is available to it to reduce the nitrogen oxides. On account of its high activity even at low temperatures, therefore, it is particularly recommended to use this system for purifying the exhaust gases from diesel engines.

Claims

Patent Claims

1. Exhaust-gas purification system for an internal combustion engine, which includes, in the direction of flow of the exhaust gas, at least a first catalyst and a downstream second catalyst, the first catalyst generating ammonia from corre- . sponding exhaust-gas constituents when the exhaust-gas composition is rich, and the second catalyst temporarily storing the ammonia generated by the first catalyst when the exhaust-gas composition is rich and subjecting the nitrogen oxides (NOx) contained in the exhaust gas to a reduction reaction using the temporarily stored ammonia as reducing agent when the exhaust-gas composition is lean, wherein the exhaust-gas purification system has, downstream of the second catalyst, a third, precious-metal-containing catalyst, which contains at least one of the platinum group metals platinum, palladium and rhodium on support materials, with the support materials being able to store ammonia when the exhaust-gas composition is rich and to release ammonia when the exhaust-gas composition is lean.

2. Exhaust-gas purification system according to Claim 1, wherein the support materials of the third catalyst are selected from the group consisting of titanium oxide, titanium oxide/aluminum oxide, titanium oxide/silicon dioxide, vanadium, vanadium/tungsten oxide, vanadium/molybdenum oxide and zeolites or mixtures thereof.

3. Exhaust-gas purification system according to Claim 1, wherein the second catalyst is formed by a standard SCR catalyst and has an overall length L in the direction of flow of the exhaust gas.

4. Exhaust-gas purification system according to Claim 3, wherein the third catalyst forms a downstream-side zone on the second catalyst, and this zone amounts to 5 to 50% of the overall length L of the second catalyst.

5. Exhaust-gas purification system according to Claim 4, wherein the third catalyst is applied in the form of a coating to the downstream-side zone of the second catalyst.

6. Exhaust-gas purification system according to Claim 4, wherein the third catalyst is obtained on the downstream-side zone of the second catalyst by impregnating this zone with compounds containing rhodium, palladium or platinum.

7. Exhaust-gas purification system according to one of Claims 1 to 6, wherein the second catalyst includes at least one zeolite which has been exchanged with a transition metal.

8. Exhaust-gas purification system according to one of Claims 1 to 6, wherein the second catalyst includes a solid-state acid system selected from the group consisting of titanium oxide or titanium oxide/aluminum oxide or titanium oxide/silicon dioxide in combination with vanadium, vanadium/tungsten oxide or vanadium molybdenum oxide.

9. Exhaust-gas purification system according to one of Claims 1 to 6, wherein the second catalyst is a coated catalyst or an extruded catalyst.

10. Exhaust-gas purification system according to one of Claims 1 to 6, wherein the first catalyst is a three-way catalyst.

11. Exhaust-gas purification system according to one of Claims 1 to 6, wherein the first catalyst is a nitrogen oxide storage catalyst.

12. Exhaust-gas purification system according to Claim 1, wherein a fourth catalyst is arranged between the first and second catalysts, and this fourth catalyst, when the exhaust-gas composition is lean, temporarily stores the nitrogen oxides contained in the exhaust gas and, when the exhaust-gas composition is rich, releases nitrogen oxides which have previously been temporarily stored again and partially reduces them with the aid of the reducing agents which are present in the rich exhaust gas, such as hydrocarbons, hydrogen or carbon monoxide.

13. Exhaust-gas purification system according to Claim 1, wherein a fourth catalyst is arranged between the first and second catalysts, this fourth catalyst partially oxidizing the nitrogen oxides contained in the exhaust gas to form nitrogen dioxide when the exhaust-gas composition is lean.

14. Process for converting the nitrogen oxides contained in the oxygen-containing exhaust gas from lean-burn internal combustion engines by cyclically changing the air/fuel ratio supplied to the internal combustion engine from lean to rich, wherein the exhaust gas is treated in an exhaust-gas purification system according to one of Claims 1 to 13 before it is released to atmosphere.

15. Process according to Claim 14, wherein the internal combustion engine is a diesel engine.

16. Use of the exhaust-gas purification system according to Claim 1 to purify the exhaust gases from a diesel engine in the temperature range between 150 and 600°C.