EP3180107A1

EP3180107A1 - Catalyst system for reducing nitrogen oxides

Info

Publication number: EP3180107A1
Application number: EP15757157.1A
Authority: EP
Inventors: Ruediger Hoyer; Thomas UTSCHIG; Michael Schiffer; Michael Seyler; Frank-Walter Schuetze
Original assignee: Umicore AG and Co KG
Current assignee: Umicore AG and Co KG
Priority date: 2014-08-13
Filing date: 2015-08-12
Publication date: 2017-06-21
Also published as: CN107148310A; WO2016023928A1; US20170218809A1; JP2017524520A; US10443463B2; EP2985068A1; JP6700250B2; KR20170040352A

Abstract

The invention relates to a catalyst system for reducing nitrogen oxides, which comprises a nitrogen oxide storage catalyst and an SCR catalyst, wherein the nitrogen oxide storage catalyst comprises at least two catalytically active washcoat layers on a supporting body, wherein a lower washcoat layer A contains cerium oxide, an alkaline earth compound and/or alkali compound, and platinum and palladium and an upper washcoat layer B, which is arranged over the washcoat layer A, contains cerium oxide, platinum and palladium and no alkli compound and no alkaline earth compound. The invention also relates to a method for converting NOx in exhaust gases of motor vehicles that are operated by means of engines that are operated in a lean manner.

Description

Catalyst system for the reduction of nitrogen oxides

The present invention relates to a catalyst system for the reduction of nitrogen oxides, which is contained in the exhaust lean burned internal combustion engines.

Exhaust gases from motor vehicles with a predominantly lean-burn internal combustion engine contain, in addition to particulate emissions, in particular the primary emissions carbon monoxide CO, hydrocarbons HC and

Nitrogen oxides NOx. Due to the relatively high oxygen content of up to 15% by volume, carbon monoxide and hydrocarbons can easily be rendered harmless by oxidation, and the reduction of nitrogen oxides too

However, nitrogen is much more difficult. It is known to neutralize nitrogen oxides from exhaust gases in the presence of oxygen by means of nitrogen oxide storage catalysts, for which the terms "lean NOx trap" or LNT are customary, whose cleaning effect is based on the fact that in a lean operating phase of the engine

Nitrogen oxides are stored by the storage material of the storage catalyst predominantly in the form of nitrates and this decomposed in a subsequent rich operating phase of the engine again and the thus released nitrogen oxides are reacted with the reducing exhaust gas components on the storage catalyst to nitrogen, carbon dioxide and water. This procedure is described for example in SAE SAE 950809.

As storage materials are in particular oxides, carbonates or

Hydroxides of magnesium, calcium, strontium, barium, the alkali metals, the rare earth metals or mixtures thereof in question. Due to their basic properties, these compounds are capable of forming nitrates with the acidic nitrogen oxides of the exhaust gas and of storing them in this way. They are suitable for generating a large interaction area with the exhaust gas in the highest possible dispersion

Support materials deposited. In addition, nitrogen oxide storage catalysts usually contain precious metals such as platinum, palladium and / or Rhodium as catalytically active components. Their task is, on the one hand, to oxidise NO to NO 2, CO and HC to CO 2 under lean conditions and, on the other hand, to reduce released NO 2 to nitrogen during the rich operating phases in which the nitrogen oxide storage catalyst is regenerated.

Another known method of removing nitrogen oxides from exhaust gases in the presence of oxygen is the Selective Catalytic Reduction (SCR) process using ammonia on a suitable catalyst, the SCR catalyst. In this method, the nitrogen oxides to be removed from the exhaust gas are reacted with ammonia to nitrogen and water.

The ammonia used as the reducing agent can be made available by metering in an ammonia-decomposable compound such as urea, ammonium carbamate or ammonium formate into the exhaust gas line and subsequent hydrolysis.

In addition, it is known to produce ammonia during a rich operating phase of the engine on a catalyst upstream of the SCR catalyst as a secondary emission and in the SCR catalyst to the

To temporarily store the consumption time during the lean operating phases. For the production of ammonia, for example

Nitrogen oxide storage catalysts are used, the nitrogen oxides

at least in part not only to the stage of nitrogen, but also to reduce ammonia. Catalyst systems which comprise an upstream nitrogen oxide storage catalyst and a downstream SCR catalyst are described, for example, in DE 100 11 612 A1, EP 0 957 242 A2, EP 1 027 919 A2, EP 2 698 193 A1, US Pat. No. 7,332,135,

WO2004 / 076829 Al, WO2005 / 047663 A2 and WO2010 / 114873 A2.

The nitrogen oxide storage catalysts used in the catalyst system of WO2010 / 114873 A2 have two layers, wherein the lower layer is directly on a flow monolith and, inter alia, ceria, and Containing alumina having ceria and barium oxide. The upper layer is on the lower layer and also contains ceria, but in a smaller amount than the lower layer. Both layers also contain the precious metals platinum and palladium, in the upper layer the ceria also carries rhodium.

Two-layer nitric oxide storage catalysts are also described elsewhere. For example, in EP 0 885 650 A2, from an exhaust gas purification catalyst for internal combustion engines with two catalytically active layers on a support body is known. The layer located on the support body comprises one or more highly dispersed alkaline earth oxides, at least one platinum group metal, as well as at least one finely divided, oxygen-storing material. The platinum group metals are in close contact with all components of the first layer. The second layer is in direct contact with the exhaust gas and contains at least one platinum group metal, as well as at least one finely divided, oxygen-storing material. Only part of the finely divided solids of the second layer serve as a carrier for the platinum group metals. The catalyst is a three-way catalytic converter that controls the harmful components of the exhaust gas

Essentially at stoichiometric conditions, i. at the air ratio λ of 1.

From US2009 / 320457 a nitrogen oxide storage catalyst is known which comprises two superimposed catalyst layers on a carrier substrate. The lower layer lying directly on the carrier substrate comprises one or more noble metals, as well as one or more nitrogen oxide storage components. The upper layer comprises one or more

Precious metals, as well as cerium oxide and is free of alkali or alkaline earth components.

Catalyst substrates containing nitrogen oxide storage materials and having two or more layers are also described in WO 2012/029050. The first layer is located directly on the carrier substrate and includes platinum and / or palladium while the second layer is on the first and comprises platinum. Both layers also contain one or more oxygen storage materials and one or more nitric oxide storage materials comprising one or more alkali and / or alkaline earth metals. The total amount of alkali and alkaline earth metal in the nitrogen oxide storage materials is 0.18 to 2.5 g / in ³ calculated as alkali metal oxide M ₂ O and alkaline earth metal oxide MO.

The present invention relates to a catalyst system for the reduction of nitrogen oxides, which comprises a nitrogen oxide storage catalyst and an SCR

Catalyst, wherein the SCR catalyst is a small-pore

Contains zeolites having a maximum ring size of eight tetrahedral atoms and a transition metal and wherein the

Nitrogen oxide storage catalyst consists of at least two catalytically active Washcoatschichten A and B on a support body and

the washcoat layer A is arranged directly on the support body and contains cerium oxide, an alkaline earth compound and / or an alkali compound, as well as platinum and palladium; and

- The washcoat layer B is disposed over the Washcoatschicht A and contains cerium oxide, as well as platinum and palladium and is free of alkali and

alkaline earth compounds;

characterized in that

the ratio of ceria in washcoat B to ceria in

Washcoat A, calculated in g / l and based on the

Volume of the support body, 1: 2 to 3: 1, wherein the sum of cerium oxide in washcoat A and washcoat B, calculated in g / l and based on the volume of the support body, 100 to 240 g / l.

In the context of the present invention, the term ceria is understood to mean ceria of commercial quality, ie. Ceria with a cerium oxide content of 90 to 100 wt .-%. The term thus also includes doped cerium oxides. On the other hand, the term "cerium oxide" in the context of the present invention also means cerium mixed oxides which have a cerium content of less than 90% by weight, for example 20 to 90% by weight or 50 to 90% by weight. Advantageously, however, the cerium content is not less than 20% by weight.

Examples of cerium mixed oxides which can be used according to the invention are in particular cerium-zirconium mixed oxides and cerium-aluminum mixed oxides.

In embodiments of the present invention, the ratio of cerium oxide in washcoat B to cerium oxide in washcoat A, calculated in each case in g / l and based on the volume of the support body, is 1: 2 to 2.5: 1, in particular 1: 1 to 2: 1.

In particular, cerium oxide is used in the washcoat layer B in an amount of 46 to 180 g / l, preferably of 46 to 90 g / l and particularly preferably of 46 to 70 g / l, in each case based on the volume of the support body. In the washcoat layer A, cerium oxide is used in amounts of from 14 to 95 g / l, preferably from 25 to 95 g / l and particularly preferably from 46 to 95 g / l, in each case based on the volume of the support body. In further embodiments, cerium oxide is used in washcoat A in an amount of 25 to 120 g / l and in washcoat B in an amount of 50 to 180 g / l, in each case based on the volume of the support body.

The total washcoat load of the support body is in

Embodiments of the present invention 300 to 600 g / l, based on the volume of the support body. In particular, the loading with washcoat layer A is 150 to 500 g / l and the loading with washcoat layer B is 50 to 300 g / l, in each case based on the volume of the support body.

In further embodiments of the present invention, the loading with washcoat layer A is 250 to 300 g / l, and with washcoat layer B 100 to 200 g / l, in each case based on the volume of the support body. The ratio of platinum to palladium in the washcoat layers A and B in embodiments of the present invention is, for example, 2: 1 to 18: 1 or 6: 1 to 16: 1, for example 5: 1, 8: 1, 10: 1, 12: 1 or 14: 1.

In embodiments of the present invention, the sum of platinum and palladium, calculated in each case in g / l and based on the volume of the support body, in washcoat A and washcoat B is the same.

In a further embodiment of the present invention, the ratio of the concentrations of platinum and palladium in washcoat B to platinum and palladium in washcoat A, in each case based on the total weight of the respective washcoat, calculated in each case in g / l and based on the volume of the support body, 1: 1 to 1: 5.

Included in embodiments of the present invention

Washcoat A and / or Washcoat B as another noble metal rhodium. Rhodium is in this case, in particular in amounts of 0.003 to 0.35 g / l, based on the volume of the support body before.

Both in the washcoat layer A and in the washcoat layer B, the noble metals are platinum and palladium and optionally rhodium

usually on suitable carrier materials. As such, high surface area, high melting oxides are used, for example

Aluminum oxide, silicon dioxide, titanium dioxide, but also mixed oxides such as aluminum-silicon mixed oxides and cerium-zirconium mixed oxides. In embodiments of the present invention, alumina is used as support material for the noble metals, in particular those stabilized by 1 to 6 wt .-%, in particular 4 wt .-%, lanthanum oxide.

It is preferred if the noble metals platinum, palladium or rhodium are supported only on one or more of the abovementioned carrier materials and thus not with all components of the respective

Washcoat layer in close contact. In particular, it is preferable when the noble metals platinum, palladium or rhodium are not supported on ceria and are not in close contact with ceria.

Suitable alkaline earth compounds in the washcoat A are, in particular, oxides, carbonates or hydroxides of magnesium, strontium and barium, in particular magnesium oxide, barium oxide and strontium oxide.

Suitable alkali compounds in the washcoat A are, in particular, oxides, carbonates or hydroxides of lithium, potassium and sodium. In embodiments of the present invention, the alkaline earth or alkali compound is present in amounts of 10 to 50 g / l, especially 15 to 20 g / l, calculated as alkaline earth or alkali oxide.

In further embodiments of the present invention is the

Alkaline earth or alkali compound not supported on ceria.

In a preferred embodiment, the nitrogen oxide storage catalyst contains at least two catalytically active washcoat layers on a support body,

in which

a lower washcoat layer A

o cerium oxide in an amount of 14 to 95 g / l,

o platinum and palladium in the mass ratio of 8: 1 to 10: 1, and o magnesium oxide and / or barium oxide; contains and

an upper washcoat layer B over the lower washcoat layer A.

is arranged and

o no alkaline earth compound and no alkali compound,

o platinum and palladium in the mass ratio 8: 1 to 10: 1, as well as

o Cerium oxide in an amount of 46 to 180 g / l

contains.

It is particularly preferred if in the mentioned embodiment, the washcoat layer B in amounts of 250 to 350 g / l and the washcoat layer A is present in amounts of 80 to 130 g / l, with the quantity g / l in each case refers to the volume of the support body.

In a further preferred embodiment of the present invention, it relates to a catalyst system for the reduction of nitrogen oxides, which comprises a nitrogen oxide storage catalyst and an SCR catalyst, wherein the nitrogen oxide storage catalyst at least two catalytically active

Contains washcoat layers on a support body,

in which

a lower washcoat layer A

o cerium oxide in an amount of 25 to 120 g / l,

an upper washcoat layer B is disposed over the lower washcoat layer A and

o no alkaline earth compound and no alkali compound,

o platinum and palladium in the mass ratio 8: 1 to 10: 1, as well as

o Ceria in an amount of 50 to 180 g / l

contains

In the present invention, SCR catalysts containing a small pore zeolite having a maximum ring size of eight

tetrahedral atoms and a transition metal used. Such SCR catalysts are described, for example, in EP 2 117 707 A1 and WO2008 / 132452.

Particularly preferred zeolites belong to the framework types AEI, CHA, KFI, ERI, LEV, MER or DDR and are particularly preferred with cobalt, iron, copper or mixtures of two or three of these metals

replaced.

In the context of the present invention, the term zeolites also includes molecular sieves, sometimes also referred to as "zeolite-like" compounds become. Molecular sieves are preferred if they belong to one of the above types of scaffolding. Examples are silica-aluminum-phosphate zeolites known by the term SAPO and aluminum phosphate zeolites known by the term AIPO.

These are also particularly preferred when they are exchanged with cobalt, iron, copper or mixtures of two or three of these metals.

Preferred zeolites or molecular sieves are furthermore those which have a SAR (silica-to-alumina) value of 2 to 100, in particular of 5 to 50.

The zeolites or molecular sieves contain transition metal, in particular in amounts of 1 to 10 wt .-%, in particular 2 to 5 wt .-%, calculated as metal oxide, that is, for example, as Fe ₂ C> 3 or CuO.

Preferred embodiments of the present invention include as SCR catalysts with copper, iron or copper and iron exchanged zeolites or molecular sieves of the chabazite type. Corresponding zeolites or molecular sieves are known, for example, under the designations SSZ-13, SSZ-62, SAPO-34 or AIPO-34, see for example US Pat. No. 6,709,644 and US Pat

8,617,474.

In a particularly preferred embodiment of the present invention

The invention relates to a catalyst system for the reduction of nitrogen oxides, which comprises a nitrogen oxide storage catalyst and an SCR catalyst, wherein the nitrogen oxide storage catalyst contains at least two catalytically active Washcoatschichten on a support body,

in which

a lower washcoat layer A

o cerium oxide in an amount of 14 to 95 g / l,

o platinum and palladium in the mass ratio of 8: 1 to 10: 1, and o magnesium oxide and / or barium oxide; contains and an upper washcoat layer B is disposed over the lower washcoat layer A and

o no alkaline earth compound and no alkali compound,

o platinum and palladium in the mass ratio 8: 1 to 10: 1, as well as

o Cerium oxide in an amount of 46 to 180 g / l

contains and where

the SCR catalyst comprises a zeolite or chalcazite-structured molecular sieve containing copper in an amount of 1 to 10 wt%, calculated as CuO and based on the SCR catalyst.

In a further particularly preferred embodiment of the present invention, it relates to a catalyst system for the reduction of nitrogen oxides, which comprises a nitrogen oxide storage catalyst and an SCR catalyst, wherein the nitrogen oxide storage catalyst contains at least two catalytically active washcoat layers on a support body,

in which

a lower washcoat layer A

o cerium oxide in an amount of 25 to 120 g / l,

an upper washcoat layer B is disposed over the lower washcoat layer A and

o no alkaline earth compound and no alkali compound,

o platinum and palladium in the mass ratio 8: 1 to 10: 1, as well as

o Ceria in an amount of 50 to 180 g / l

contains and where

In the catalyst system of the present invention, the

Nitrogen oxide storage catalyst comprising at least two catalytically active Washcoatschichten A and B on a support body. The application of the catalytically active Washcoatschichten A and B on the support body is carried out according to the usual dip coating method or pump and suction coating method with subsequent thermal

Post-treatment (calcination and optionally reduction with forming gas or hydrogen). These methods are well known in the art.

Suitable supporting bodies are in principle all known catalytically inert support bodies for heterogeneous catalysts. Preference is given to monolithic and monolithic flow-through honeycomb bodies made of ceramic and metal, as well as particle filter substrates, as is customary for the purification of

Diesel engine exhaust can be used. Very particular preference is given to ceramic flow honeycomb bodies and ceramic wall flow filter substrates made of cordierite, aluminum titanate or silicon carbide.

Also, the SCR catalyst is preferably applied to a catalytically inert support body and that by the usual dip coating method or pump and suction coating method with it

subsequent thermal aftertreatment (calcination and

optionally reduction with forming gas or hydrogen). In principle, all known catalytically inert support bodies for heterogeneous catalysts are also suitable as support bodies, monolithic and

ceramic and metal monolithic flow honeycomb bodies, and particulate filter substrates, such as those commonly used to clean

Diesel engine exhaust gases are used, are preferred. Very particular preference is given to ceramic flow honeycomb bodies and ceramic

Wall flow filter substrates of cordierite, aluminum titanate or silicon carbide.

Between the nitrogen oxide storage catalyst and the SCR catalyst may also be placed a filter for reducing particulate emissions.

By filter is meant in particular a ceramic wall flow filter substrate made of, for example, cordierite. The filter may also be coated with a HC / CO conversion catalyst. In a preferred Embodiment, the filter contains no OSC (Oxygen Storage Compound) material.

In the catalyst system according to the invention, nitrogen oxide storage catalyst and SCR catalyst can not only be applied to two separate catalytically inert support bodies. They can also be applied together on a single catalytically inert support body. In this case, for example, the nitrogen oxide storage catalyst is coated from one end of the support body over less than its entire length, while the SCR catalyst also extends from the other end of the support body over less than its entire length

is coated.

In embodiments of the present invention, the length E of the catalytically active zone comprising the nitrogen oxide storage catalyst is 20 to 70%, 40 to 60% or 45 to 50% of the total length L of the support body. The length Z of the catalytically active zone comprising the SCR catalyst is in embodiments of the present invention 20 to 70%, 40 to 60% or 45 to 50% of the total length L of the support body. In

In preferred embodiments, the lengths E and Z are both 50% of the total length L.

The sum of the length E of the first catalytically active zone and the length Z of the second catalytically active zone can correspond exactly to the total length L. In particular, for production reasons, however, it may be smaller than the total length L in embodiments of the present invention. In these cases, a certain length is the

Total length L between the coated lengths E and Z uncoated. For example, the sum of the length E of the first catalytically active zone and the length Z of the second catalytically active zone L x 0.8 to L x 0.999. When the intended use of the invention

Catalyst system, the exhaust gas must first flow through the nitrogen oxide storage catalyst and then the SCR catalyst. Under this condition, it is excellently suited for the conversion of NOx into exhaust gases of motor vehicles operated with lean-burn engines, such as diesel engines. It achieves good NOx conversion at temperatures of around 200 to 450 ° C without being adversely affected at high temperatures. The catalyst system according to the invention is thus suitable for Euro 6 applications.

The present invention also relates to a method for converting NOx into exhaust gases of motor vehicles operated with lean-burn engines, such as diesel engines, characterized in that the exhaust gas is passed through a nitrogen oxide reduction catalyst system comprising a nitrogen oxide storage catalyst and an SCR catalyst, wherein the SCR catalyst has a small pore

Containing zeolites with a maximum ring size of eight tetrahedral atoms and a transition metal and the nitrogen oxide storage catalyst consists of at least two catalytically active Washcoatschichten A and B on a support body and

alkaline earth compounds;

characterized in that

the ratio of ceria in washcoat B to ceria in

Washcoat A, calculated in g / l and based on the

Volume of the support body, 1: 2 to 3: 1, wherein the sum of ceria in washcoat A and washcoat B, calculated in g / l and based on the volume of the support body, 100 to 240 g / l; and wherein the exhaust gas is passed through the catalyst system such that it first flows through the nitrogen oxide storage catalyst and then the SCR catalyst.

For regeneration of the nitrogen oxide storage catalyst and to form ammonia, which in the subsequent phase under lean

Exhaust conditions converts nitrogen oxides to nitrogen, the exhaust gas is periodically set to lambda <1 (rich exhaust gas). The change to lambda <1 can be done within the engine by e.g. Nacheinspritzung of fuel or even by injection of reducing agent directly in front of the nitrogen oxide storage catalyst.

Embodiments of the process according to the invention with regard to the catalyst system, including the nitrogen oxide storage catalyst and the SCR catalyst, correspond to the above descriptions.

The invention is explained in more detail in the following examples and figures.

FIG. 1: NOx conversion of the catalyst systems KS1, KS2, KS3 and VKS1 as a function of the temperature.

example 1

a) To produce a catalyst system according to the invention is a honeycomb ceramic carrier with a first Washcoatschicht A

coated, the Pt and Pd supported on a lanthanum-stabilized

Alumina, ceria in an amount of 47 g / l, and 17 g / l barium oxide and 15 g / l magnesium oxide. Neither barium oxide nor magesium oxide are supported on the cerium oxide. The loading of Pt and Pd is 1.77 g / l or 0.177 g / l and the total loading of the washcoat 181 g / l based on the volume of the ceramic support.

b) On the first washcoat layer a further washcoat layer B is applied which contains Pt, Pd and Rh supported on a lanthanum-stabilized alumina. The loading of Pt, Pd and Rh in this Washcoat layer is 1.77 g / l or 0.177 g / l or 0.177 g / l. The washcoat layer B also contains 94 g / l of cerium oxide at one

Washcoat loading of layer B of 181 g / l.

The catalyst thus obtained is hereinafter called SPKl. c) To produce the SCR catalyst is a honeycomb

Ceramic support coated with a chabazite-type zeolite with a SAR of 28 and exchanged with copper. The washcoat is composed of 85% by weight of zeolite, 3% by weight of CuO and 12% by weight.

Alumina.

The catalyst thus obtained is hereinafter called SCRKl. d) The catalysts SPK1 and SCRK1 are combined to form a catalyst system, referred to below as KS1.

Example 2

The steps a) to d) of Example 1 are repeated, with the

Difference that in step a) cerium oxide in an amount of 70 g / l and in step b) cerium oxide in an amount of 70 g / l is used.

The catalyst system thus obtained is called KS2 below.

Example 3

The steps a) to d) of Example 1 are repeated, with the

Difference that in step a) cerium oxide in an amount of 93 g / l and in step b) cerium oxide in an amount of 47 g / l is used.

The catalyst system thus obtained is hereinafter called KS3.

Comparative Example 1

The steps a) to d) of Example 1 are repeated, with the

Difference that in step a) cerium oxide in an amount of 116 g / l instead of 47 g / l and in step b) ceria in an amount of 24 g / l instead of 94 g / l is used.

The catalyst system thus obtained is hereinafter called VKS1. Examples 4 to 6

For the preparation of further catalyst systems according to the invention, the nitrogen oxide storage catalysts mentioned in Table 1 or the SCR catalysts listed in Table 2 were prepared analogously to Example 1 a) or b) and to those mentioned in Table 3

Catalyst systems combined.

Table 1

Table 2

Table 3

Nitrogen oxide storage SCR

catalyst

catalyst catalyst

KS4 SPK4 SCRK4

KS5 SPK5 SCRK2

KS6 SPK6 SCRK1 Determination of NOx Conversion of KSl, KS2, KS3 and VKS1 a) First, KSl, KS2, KS3 and VKS1 were heated at 800 ° C for 16h

hydrothermal atmosphere aged.

b) The NOx conversion of the catalyst system according to the invention KSl, KS2, KS3 and the comparative catalyst system VKS1 as a function of the temperature before the catalyst was in a model gas reactor in the so-called. NOx turnover test determined.

This is synthetic exhaust gas with a nitrogen monoxide concentration of 500 ppm, 10 vol .-% carbon dioxide and

Water, a concentration of 50ppm of a short-chain

Hydrocarbon mixture (consisting of 33 ppm propene and 17 ppm propane) and a residual oxygen content of 7 vol .-% at one

Space velocity of 50k / h passed in a model gas reactor over the respective catalyst sample, wherein the gas mixture alternately an oxygen excess 80s long ("lean" gas mixture with air ratio λ of 1.47) while nitrogen oxides are stored, and to regenerate the catalyst sample for 10s long an oxygen deficit ("rich" gas mixture with air ratio λ of 0.92, by mixing 5.5% by volume of carbon monoxide while reducing the residual oxygen content to 1% by volume).

The temperature is lowered at 7.5 ° C / min from 600 ° C to 150 ° C and determines the conversion over each 90s long lean-fat cycle. The NOx regeneration capability at 200 ° C is important to map driving behavior in the urban area, at 450 ° C for highway driving. In order to meet the Euro 6 emission standard, it is particularly important to show a high NOx regeneration capability over this entire temperature range.

FIG. 1 shows the NO x conversion of the catalyst systems KS 1, KS 2, KS 3 and the comparison system VKS 1 thus determined.

It follows that the NOx conversion of the comparative catalyst system VKS1 at temperatures up to about 350 ° C is significantly worse than that of the catalyst systems according to the invention KSl to KS3. For example, the NOx conversion of VKSl at 250 ° C is about 67%, while at KSl to KS3 it is about 75%.

Claims

claims

A catalyst system for reducing nitrogen oxides comprising a nitrogen oxide storage catalyst and an SCR catalyst, the SCR catalyst comprising a small pore zeolite having a maximum ring size of eight tetrahedral atoms and a transition metal, and wherein the nitrogen oxide storage catalyst comprises at least two catalytically active washcoat layers A and B is on a support body and

the washcoat layer B is disposed over the washcoat layer A and contains cerium oxide, as well as platinum and palladium, and is free from alkali and alkaline earth compounds;

characterized in that

the ratio of ceria in washcoat B to ceria in

Washcoat A, calculated in g / l and based on the

2. A catalyst system according to claim 1, characterized in that the Washcoatschicht B contains cerium oxide in an amount of 46 to 180 g / l.

3. Catalyst system according to claim 1, characterized in that the Washcoatschicht A contains ceria in an amount of 14 to 95 g / l.

4. Catalyst system according to one or more of claims 1 to 3, characterized in that the total Washcoatbeladung the

Carrying body 300 to 600g / l, based on the volume of the support body is.

5. Catalyst system according to claim 4, characterized in that the loading with Washcoatschicht A 150 to 500 g / l and the loading with Washcoat layer B 50 to 300 g / l, in each case based on the volume of the support body is.

6. A catalyst system according to claim 4, characterized in that the loading with Washcoatschicht A 250 to 300g / l and with Washcoatschicht B 100 to 200 g / l, each based on the volume of the support body is.

7. Catalyst system according to one or more of claims 1 to 6, characterized in that the ratio of platinum to palladium in the Washcoatschichten A and B is 2: 1 to 18: 1.

8. Catalyst system according to one or more of claims 1 to 7, characterized in that the alkaline earth compound in washcoat A is magnesium oxide, barium oxide and / or strontium oxide.

9. A catalyst system according to one or more of claims 1 to 8, characterized in that the SCR catalyst contains a zeolite belonging to the framework type AEI, CHA, KFI, ERI, LEV, MER or DDR and with cobalt, iron, copper or Mixtures of two or three of these metals is exchanged.

10. Catalyst system according to one or more of claims 1 to 9, characterized in that the SCR catalyst contains a zeolite of chabazite type, with copper, iron or copper and iron

is exchanged.

11. The catalyst system according to claim 1, characterized in that the nitrogen oxide storage catalyst

a lower washcoat layer A

o cerium oxide in an amount of 14 to 95 g / l,

o no alkaline earth compound and no alkali compound,

o platinum and palladium in the mass ratio 8: 1 to 10: 1, as well as

o Cerium oxide in an amount of 46 to 180 g / l

contains and where

12. Catalyst system according to one or more of claims 1 to 10, characterized in that the nitrogen oxide storage catalyst contains at least two catalytically active Washcoatschichten on a support body, wherein

a lower washcoat layer A

o cerium oxide in an amount of 25 to 120 g / l,

an upper washcoat layer B is disposed over the lower washcoat layer A and

o no alkaline earth compound and no alkali compound,

o platinum and palladium in the mass ratio 8: 1 to 10: 1, as well as

o Ceria in an amount of 50 to 180 g / l

contains and where

13. Catalyst system according to one or more of claims 1 to 12, characterized in that the nitrogen oxide storage catalyst and SCR catalyst are arranged on different support bodies or that nitrogen oxide storage catalyst and SCR catalyst on the same

Support body are arranged.

14. A method for converting NO _x in exhaust gases of motor vehicles, which are operated with lean-burn engines, characterized

characterized in that the exhaust gas is passed through a nitrogen oxide reduction catalyst system comprising a nitrogen oxide storage catalyst and an SCR catalyst, the SCR catalyst having a nitrogen oxide storage catalyst and a SCR catalyst

small pore zeolites with a maximum ring size of eight

tetrahedral atoms and a transition metal and wherein the nitrogen oxide storage catalyst of at least two catalytically active

Washcoatschichten A and B is on a support body and

alkaline earth compounds; and

characterized in that the ratio of ceria in

Washcoat B to cerium oxide in washcoat A, calculated in each case in g / l and based on the volume of the support body, 1: 2 to 3: 1, wherein the sum of ceria in washcoat A and washcoat B, calculated in g / l and relative to the volume of the supporting body, 100 to 240 g / l; and

wherein the exhaust gas is passed through the catalyst system so that it first flows through the nitrogen oxide storage catalyst and then the SCR catalyst.