Catalyst for reducing nitrogen oxides

0001The present invention relates to a catalytic converter for reducing nitrogen oxides, which is contained in the exhaust gas of lean-burn internal combustion engines.

0002The exhaust gas from motor vehicles that are operated with lean-burn internal combustion engines, for example diesel engines, contains carbon monoxide (CO) and nitrogen oxides (NO_x) also components that result from the incomplete combustion of the fuel in the combustion chamber of the cylinder. In addition to residual hydrocarbons (HC), which are mostly also predominantly gaseous, these also include particle emissions, also known as "diesel soot" or "soot particles". These are complex agglomerates of predominantly carbon-containing solid particles and an adhering liquid phase, the majority of which consists of long-chain hydrocarbon condensates. The liquid phase adhering to the solid components is also referred to as "Soluble Organic Fraction SOF" or "Volatile Organic Fraction VOF".

0003To clean these exhaust gases, the components mentioned must be converted as completely as possible into harmless compounds, which is only possible with the use of suitable catalysts.

0004So-called nitrogen oxide storage catalytic converters, for which the term “Lean NOx Trap” or “LNT” is also common, are known for removing the nitrogen oxides. Their cleaning effect is based on the fact that in a lean operating phase of the engine the nitrogen oxides are mainly stored in the form of nitrates by the storage material of the storage catalytic converter and these are decomposed again in a subsequent rich operating phase of the engine and the nitrogen oxides released in this way with the reducing exhaust gas components on the storage catalytic converter to nitrogen , Carbon dioxide and water are converted. This mode of operation is described in SAE publication SAE 950809, for example.

0005Particularly suitable storage materials are oxides, carbonates or hydroxides of magnesium, calcium, strontium, barium, alkali metals, rare earth metals or mixtures thereof. Due to their basic properties, these compounds are able to form nitrates with the acidic nitrogen oxides in the exhaust gas and to store them in this way. In order to generate a large area of ​​interaction with the exhaust gas, they are deposited in the highest possible dispersion on suitable carrier materials. In addition, nitrogen oxide storage catalytic converters generally contain precious metals such as platinum, palladium and / or rhodium as catalytically active components. Their task is on the one hand to convert NO to NO under lean conditions₂, as well as CO and HC to CO₂ to oxidize and on the other hand, released NO during the rich operating phases in which the nitrogen oxide storage catalytic converter is regenerated₂ to reduce nitrogen.

0006Another process for removing nitrogen oxides from exhaust gases in the presence of oxygen is selective catalytic reduction (SCR process) using ammonia on a suitable catalytic converter. In this process, the nitrogen oxides to be removed from the exhaust gas are converted to nitrogen and water with ammonia. The ammonia used as reducing agent can be made available by metering an ammonia precursor compound, such as urea, ammonium carbamate or ammonium formate, into the exhaust gas line and subsequent hydrolysis.

0007Certain metal-exchanged zeolites, for example, can be used as SCR catalysts, with iron-exchanged β-zeolites and copper-exchanged small-pore zeolites, for example chabazites, being used, in particular, depending on the area of ​​application. So-called mixed oxide catalysts are also used as SCR catalysts. These are in particular compounds containing vanadium oxide, tungsten oxide and, if appropriate, further oxides, which as a rule contain titanium dioxide as the carrier oxide.

0008In order to comply with Euro 6d and future exhaust gas legislation, exhaust systems must be used that show high CO and NOx conversion rates over a wide temperature range. Such systems can be a combination of nitrogen oxide storage catalysts and SCR catalysts, the nitrogen oxide storage catalyst in particular having to meet high requirements. Efficient NOx storage and conversion is a major challenge, especially when driving around town, where the exhaust gas temperatures do not exceed 200 ° C. In addition, the nitrogen oxide storage catalytic converter must have sufficient NO for effective NOx conversion by the SCR catalytic converter₂ provide.

0009It has already been shown in the past that nitrogen oxide storage catalytic converters, for example, show the best possible activity when the different components or functions of the catalytic converter are spatially separated and undesired interactions are thus avoided. So reveals the <patcit id="pcit0001" dnum="WO2011154913A"><text>WO2011 / 154913</text></patcit> a catalyst made up of three layers on top of each other on a catalyst substrate, the bottom layer containing a nitrogen oxide storage material, the middle layer containing a material for converting nitrogen oxides and the upper layer containing a hydrocarbon storage medium. The <patcit id="pcit0002" dnum="WO2017191099A"><text>WO2017 / 191099</text></patcit> describes a catalyst in which three material zones are arranged in a certain way on a carrier body. Further nitrogen oxide storage catalysts made of at least two layers are in<patcit id="pcit0003" dnum="EP0885650A2"><text>EP0885650 A2</text></patcit>, <patcit id="pcit0004" dnum="WO2009158453A1"><text>WO2009 / 158453A1</text></patcit>, <patcit id="pcit0005" dnum="WO2012029050A1"><text>WO2012 / 029050 A1</text></patcit>, <patcit id="pcit0006" dnum="WO2014108362A1"><text>WO2014 / 108362 A1</text></patcit>, <patcit id="pcit0007" dnum="WO2017134065A1"><text>WO2017 / 134065 A1</text></patcit> and <patcit id="pcit0008" dnum="WO2017144426A1"><text>WO2017 / 144426 A1</text></patcit> disclosed.

0010In the future, however, further developed nitrogen oxide storage catalytic converters will be required, in which especially NOx storage, NO₂Formation and reducing power are further improved and coordinated.

0011The present invention relates to a nitrogen oxide storage catalytic converter which comprises a support body and washcoat layers A, B, C and D, wherein<ul id="ul0001" list-style="dash" compact="compact"><li>Washcoat layer A is arranged on the support body and contains cerium oxide, an alkaline earth compound and / or an alkali compound as well as platinum or platinum and palladium in a weight ratio of> 5: 1;</li><li>Washcoat layer B is arranged on washcoat layer A and contains cerium oxide, as well as platinum or platinum and palladium in a weight ratio> 5: 1 and is free from alkali compounds and compounds of calcium, strontium and barium;</li><li>Washcoat layer C is arranged on washcoat layer B and comprises palladium or palladium and platinum supported on cerium oxide in a weight ratio of> 2: 1, as well as rhodium; and</li><li>Washcoat layer D is arranged on washcoat layer C and comprises platinum or platinum and palladium.</li></ul>

0012In one embodiment of the present invention, washcoat layer B is free from alkali and alkaline earth compounds.

0013In another embodiment of the present invention, washcoat layer B comprises magnesium oxide (MgO). In this case, washcoat layer B comprises magnesium oxide, preferably in quantities of 3 to 20 g / l, based on the volume of the support body.

0014In a further embodiment of the nitrogen oxide storage catalytic converter according to the invention, the washcoat layers A and B are combined to form a single layer comprising cerium oxide, an alkaline earth compound and / or an alkali compound as well as platinum or platinum and palladium in a weight ratio of> 5: 1. This washcoat layer is referred to below as AB.

0015The cerium oxide used in the washcoat layers A, B and C can be of commercial quality, ie have a cerium oxide content of 90 to 100% by weight. Cerium oxide is preferred, which even after calcination for 6 hours at 1150 ° C. still has a surface of at least 10 m²/ g. Cerium oxide with these surface properties is known and for example in<patcit id="pcit0009" dnum="EP1527018B1"><text>EP 1 527 018 B1</text></patcit> described.

0016In embodiments of the present invention, cerium oxide is used in the washcoat layer A in an amount of 110 to 160 g / l, for example 125 to 145 g / l, based on the volume of the support body. In the washcoat layer B, cerium oxide is used in quantities of 22 to 120 g / l, for example 40 to 100 g / l or 45 to 65 g / l, based on the volume of the support body.

0017In the embodiment of the nitrogen oxide storage catalytic converter according to the invention, in which the washcoat layers A and B are combined to form a single washcoat layer AB, cerium oxide is used in washcoat layer AB in an amount of 132 to 280 g / l, for example 165 to 210 g / l, based on the volume of the support body, used.

0018In washcoat layer C, the amount of cerium oxide is in particular 20 to 120 g / l, preferably 40 to 70 g / l, based on the volume of the support body.

0019Particularly suitable alkaline earth compounds in the washcoat layer A are oxides, carbonates and / or hydroxides of magnesium, strontium and / or barium, especially magnesium oxide, barium oxide and strontium oxide. Particularly suitable alkali compounds in the washcoat layer A are oxides, carbonates and / or hydroxides of lithium, potassium and / or sodium.

0020In embodiments of the present invention, the alkaline earth or alkali compound is present in washcoat layer A in amounts of 10 to 50 g / l, particularly 15 to 20 g / l, calculated as alkaline earth or alkali oxide and based on the volume of the support body.

0021In the embodiment of the nitrogen oxide storage catalyst according to the invention, in which the washcoat layers A and B are combined to form a single washcoat layer AB, this contains the alkaline earth or alkali compound preferably in amounts of 10 to 50 g / l, especially 15 to 20 g / l , calculated as alkaline earth or alkali oxide and based on the volume of the support body.

0022In one embodiment of the present invention, washcoat layer C comprises an iron compound. The iron compound is preferably an iron oxide, especially Fe₂O₃. Iron is present in washcoat layer C in particular in amounts of 1 to 10% by weight, preferably 2 to 7% by weight, based on the weight of washcoat layer C and calculated as Fe₂O₃, in front.

0023The iron compound, be it iron oxide or another iron compound, preferably does not serve as a carrier material, neither for the noble metals platinum, palladium and rhodium, nor for any other component of washcoat layer C.

0024Washcoat layer A comprises platinum or platinum and palladium and is preferably free of rhodium. If it contains platinum and palladium, the weight ratio Pt: Pd> 5: 1 is preferably ≥ 8: 1, for example 8: 1 to 20: 1, for example 8: 1, 10: 1, 12: 1, 14: 1, 16: 1, 18: 1 or 20: 1.

0025Washcoat layer B also comprises platinum or platinum and palladium and is preferably free of rhodium. If it contains platinum and palladium, the weight ratio Pt: Pd> 5: 1 is preferably ≥ 8: 1, for example 8: 1 to 20: 1, for example 8: 1, 10: 1, 12: 1, 14: 1, 16: 1, 18: 1 or 20: 1.

0026The contents of platinum or platinum and palladium and their weight ratios in washcoat layer A and washcoat layer B are independent of one another

0027Usual platinum loads for the washcoat layers A and B are in the range from 0.7 to 1.5 g / l, the palladium loadings in the range from 0.035 to 0.3 g / l, each based on the volume of the support body.

0028Washcoat layer C preferably comprises palladium or palladium and platinum, preferably in a weight ratio of 2: 1 to 8: 1, preferably 4: 1 to 6: 1. The palladium or the palladium and platinum mass concentration based on the amount of cerium oxide in the washcoat layer C is usually 0.2 to 1%, preferably 0.4 to 0.6%. In addition, washcoat layer C also comprises rhodium, preferably in an amount of 0.003 to 0.35 g / l, in particular 0.18 to 0.26 g / l, in each case based on the volume of the support body. Washcoat layer D comprises platinum or platinum and palladium and is preferably free of rhodium. If it contains platinum and palladium, the weight ratio Pt: Pd> 5: 1 is preferably 8: 1, for example 8: 1 to 18: 1, for example 8: 1, 10: 1, 12: 1 or 14: 1.

0029The ratio of the amount of platinum and palladium in the washcoat layer D to the sum of the amount of platinum and palladium in the washcoat layers A and B is usually 0.3-1.2, preferably 0.35-0.5.

0030The total amount of noble metal, ie of platinum, palladium and optionally rhodium, in the nitrogen oxide storage catalyst according to the invention is in embodiments of the present invention from 2.12 to 7.1 g / l (60 to 200 g / ft³), based on the volume of the supporting body.

0031Both in the washcoat layer A, as well as in the washcoat layer B, and in the washcoat layer D, the noble metals platinum or platinum and palladium are usually present on suitable carrier materials. Oxides with a BET surface area of ​​30 to 250 m²/ g, preferably from 100 to 200 m²/ g (determined in accordance with DIN 66132), 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, aluminum oxide is used as the carrier material for the noble metals platinum or platinum and palladium, in particular that which is stabilized by 1 to 6% by weight, in particular 4% by weight, of lanthanum oxide.

0032In washcoat layer C, the noble metals palladium and platinum are supported on cerium oxide, while rhodium is preferably present on one of the above-mentioned customary support materials, in particular aluminum oxide or aluminum oxide stabilized with lanthanum oxide.

0033It is preferred if the noble metals platinum or platinum and palladium and rhodium are only supported on one or more of the above-mentioned carrier materials and are therefore not in close contact with all components of the respective washcoat layer. In particular, iron preferably does not serve as a carrier for palladium, platinum and rhodium.

0034The total washcoat loading of the support body in embodiments of the present invention is 300 to 600 g / l, based on the volume of the support body.

0035In a preferred embodiment, the present invention relates to a nitrogen oxide storage catalytic converter which comprises a support body and washcoat layers A, B, C and D, wherein<ul id="ul0002" list-style="dash" compact="compact"><li>Washcoat layer A is arranged on the support body and<ul id="ul0003" list-style="none" compact="compact"><li>∘ Cerium oxide in an amount of 100 to 160 g / l, based on the volume of the support body,</li><li>∘ platinum or platinum and palladium in a weight ratio of 10: 1, and</li><li>∘ contains magnesium oxide and / or barium oxide;</li></ul></li><li>Washcoat layer B is arranged on washcoat layer A and<ul id="ul0004" list-style="none" compact="compact"><li>∘ no alkali compounds and no compounds of calcium, strontium and barium,</li><li>∘ platinum or platinum and palladium in a weight ratio of 10: 1, and</li><li>∘ contains cerium oxide in an amount of 45 to 65 g / l, based on the volume of the support body;</li></ul></li></ul><ul id="ul0005" list-style="bullet" compact="compact"><li>Washcoat layer C is arranged on washcoat layer B and<ul id="ul0006" list-style="none" compact="compact"><li>∘ 40 to 70 g / l cerium oxide to the</li><li>∘ Palladium and platinum are supported in an amount of 0.16 to 0.72 g / l, based on the volume of the support body,</li><li>∘ Rhodium in an amount of 0.18 to 0.26 g / l, based on the volume of the support body, and</li><li>∘ Fe₂O₃ in an amount of 2 to 7 g / l, based on the volume of the support body; and</li></ul></li><li>Washcoat layer D is arranged on washcoat layer C and o Contains platinum or platinum and palladium in a weight ratio of 10: 1.</li></ul>

0036The application of the catalytically active washcoat layers A, B, C and D to the support body takes place according to the usual dip coating processes or pump and suction coating processes with subsequent thermal aftertreatment (calcination and, if necessary, reduction with forming gas or hydrogen). These methods are sufficiently known from the prior art. The coating suspensions required for this can be obtained by methods known to the person skilled in the art. The constituents of the individual washcoat layers, such as cerium oxide, alkaline earth and / or alkali compounds, precious metals supported on suitable carrier materials, as well as iron compounds are suspended in water in the appropriate quantities and in a suitable mill, in particular a ball mill, to a particle size of dso = 3 ground to 5 µm.

0037The nitrogen oxide storage catalytic converters according to the invention are outstandingly suitable for converting NO_x in exhaust gases from motor vehicles that are operated with lean-burn engines, such as diesel engines. You achieve good NOx conversion at temperatures of around 200 to 450 ° C, without the NOx conversion being negatively affected at high temperatures. The nitrogen oxide storage catalytic converters according to the invention are therefore suitable for Euro 6 applications. The present invention thus also relates to a method for converting NO_x in exhaust gases from motor vehicles that are operated with lean-burn engines, such as diesel engines, which is characterized in that the exhaust gas is passed over a nitrogen oxide storage catalyst according to the invention

0038Refinements of the method according to the invention with regard to the nitrogen oxide storage catalytic converter correspond to the descriptions above.

0039The present invention further relates to an exhaust system that<ol id="ol0001" compact="compact"><li>a) a nitrogen oxide storage catalytic converter which comprises a support body and washcoat layers A, B, C and D, wherein<ul id="ul0007" list-style="dash" compact="compact"><li>Washcoat layer A is arranged on the support body and contains cerium oxide, an alkaline earth compound and / or an alkali compound as well as platinum or platinum and palladium in a weight ratio of> 5: 1;</li><li>Washcoat layer B is arranged on washcoat layer A and contains cerium oxide, as well as platinum or platinum and palladium in a weight ratio> 5: 1 and is free from alkali compounds and compounds of calcium, strontium and barium;</li><li>Washcoat layer C is arranged on washcoat layer B and comprises palladium or palladium and platinum supported on cerium oxide in a weight ratio of> 2: 1, as well as rhodium; and</li><li>Washcoat layer D is arranged on washcoat layer C and comprises platinum or platinum and palladium.</li></ul>and</li><li>b) an SCR catalytic converter</li></ol>includes.

0040The SCR catalytic converter in the exhaust system according to the invention can in principle be selected from all catalytic converters active in the SCR reaction of nitrogen oxides with ammonia, in particular from those which are known to the person skilled in the art of automobile exhaust gas catalysis as customary. This includes catalysts of the mixed oxide type as well as catalysts based on zeolites, in particular zeolites exchanged with transition metal, for example zeolites exchanged with copper, iron or copper and iron.

0041In embodiments of the present invention, SCR catalysts are used, which are a small-pore zeolite with a maximum ring size of eight tetrahedral atoms and a transition metal, for example copper, iron or copper and iron. Such SCR catalysts are for example in<patcit id="pcit0010" dnum="WO2008106519A1"><text>WO2008 / 106519 A1</text></patcit>, <patcit id="pcit0011" dnum="WO2008118434A1"><text>WO2008 / 118434 A1</text></patcit> and <patcit id="pcit0012" dnum="WO2008132452A2"><text>WO2008 / 132452 A2</text></patcit> described. In addition, however, large and medium-pore zeolites can also be used, in particular those of the BEA structure type being considered. Iron BEA and copper BEA are of interest.

0042Particularly preferred zeolites belong to the skeleton types BEA, AEI, CHA, KFI, ERI, LEV, MER or DDR and are particularly preferably exchanged for copper, iron or copper and iron.

0043In the context of the present invention, the term zeolites also includes molecular sieves, which are sometimes also referred to as "zeolite-like" compounds. Molecular sieves are preferred if they belong to one of the structural types mentioned above. Examples are silica aluminum phosphate zeolites, which are known by the term SAPO, and aluminum phosphate zeolites, which are known by the term AIPO. These are also particularly preferred when they are exchanged for copper, iron or copper and iron.

0044Preferred zeolites are also those which have an SAR (silica-to-alumina ratio) value of 2 to 100, in particular 5 to 50.

0045The zeolites or molecular sieves contain transition metal in particular in amounts of 1 to 10% by weight, in particular 2 to 5% by weight, calculated as metal oxide, for example as Fe₂O₃ or CuO.

0046Preferred embodiments of the present invention contain as SCR catalysts with copper, iron or copper and iron-exchanged zeolites or molecular sieves of the beta type (BEA), Chabazite type (CHA) or of the Levyne type (LEV). Corresponding zeolites or molecular sieves are, for example, under the names ZSM-5, Beta, SSZ-13, SSZ-62, Nu-3, ZK-20, LZ-132, SAPO-34, SAPO-35, AIPO-34 and AIPO-35 known, see about<patcit id="pcit0013" dnum="US6709644B"><text>U.S. 6,709,644</text></patcit> and <patcit id="pcit0014" dnum="US8617474B"><text>U.S. 8,617,474</text></patcit>.

0047In one embodiment of the exhaust system according to the invention, an injection device for reducing agent is located between the catalyst according to the invention and the SCR catalyst. The injection device can be selected as desired by the person skilled in the art, and suitable devices can be found in the literature (see, for example, T. Mayer, Solid SCR System based on Ammonium Carbamate, dissertation, TU Kaiserslautern, 2005). The ammonia can be introduced into the exhaust gas flow via the injection device as such or in the form of a compound from which ammonia is formed under the ambient conditions. Examples of these are aqueous solutions of urea or ammonium formate, as well as solid ammonium carbamate. As a rule, the reducing agent or a precursor thereof is kept in stock in a container that is carried along and is connected to the injection device.

0048The SCR catalytic converter is preferably in the form of a coating on a support body, which can be a flow-through substrate or a wall-flow filter and, for example, can consist of silicon carbide, aluminum titanate or cordierite. Alternatively, however, the support body itself can also consist of the SCR catalytic converter and a matrix component as described above, that is to say it can be in extruded form.

0049The present invention also relates to a method for cleaning exhaust gases from motor vehicles which are operated with lean-burn engines, for example diesel engines, which is characterized in that the exhaust gas is passed through an exhaust system according to the invention.

0050The invention is explained in more detail in the following examples and figures.

0051<figref idref="f0001">Figure 1</figref> shows the NOx storage capacity at 70% NOx conversion as a function of the temperature upstream of the catalytic converter for catalytic converters RK1, RK2, K1 and K2

Comparative example 1

0052<ol id="ol0002" compact="compact"><li>a) For the washcoat layer A, an aqueous suspension was produced. 0.7 g / l platinum, based on the volume of the support, and 0.07 g / l palladium as tetraethylammonium hydroxide or nitrate salt were added to an aqueous suspension of 34 g / l aluminum oxide. Then 19.5 g / l cerium oxide coated with 17% BaO based on the cerium oxide amount and 45.5 g / l cerium oxide were added to the suspension. 3 g / l aluminum oxide was added as an alumina sol. Then 102.5 g / l of solids were applied by dipping a flow-through monolith, dried and calcined at 550 ° C. for 2 hours.</li><li>b) To produce the coating suspension B, 0.7 g / l of platinum as tetraethylammonium hydroxide and based on the amount of solids in the suspension and 40 g / l of aluminum oxide were mixed. 148 g / l cerium oxide, 0.07 g / l palladium as Pd nitrate and 10 g / l magnesium oxide as Mg acetate were then added. An iron oxide component was used as a pre-set powder. For this purpose, an aluminum oxide with 25 percent by weight iron oxide was produced by impregnating aluminum oxide with iron nitrate solution and subsequent calcination at 550 ° C. 20 g / l of this material was used in the coating suspension. In addition, 8 g / l aluminum oxide and 0.18 g / l rhodium as Rh nitrate were added to the suspension. 227 g / l were applied to washcoat layer A as washcoat layer B. The layer was then dried and calcined at 550 ° C. for 2 hours.</li><li>c) The washcoat layer C was produced by mixing 1.8 g / l platinum based on the weight of the solids in the washcoat suspension C as Pt nitrate with 0.18 g / l palladium as Pd nitrate with 70 g / l Alumina. 72 g / l of this suspension were applied to the washcoat layer B, dried and then calcined at 550 ° C. for 2 hours.</li></ol>The catalyst obtained in this way is hereinafter referred to as RK1.

Comparative example 2

0053Comparative example 1 was repeated with the difference that in the washcoat layer B 50 g / l cerium oxide were coated with 0.5 percent by weight of palladium based on the cerium oxide before the addition to the washcoat dispersion B, so that the layer B 98 g / l cerium oxide and 50 g / l Pd-coated cerium oxide contained. The catalyst obtained in this way is called RK2 below.

example 1

0054<ol id="ol0003" compact="compact"><li>a) For the preparation of a catalyst according to the invention, the washcoat layer A was coated as in Comparative Examples 1 and 2.</li><li>b) To produce the coating suspension B, 0.7 g / l of platinum as tetraethylammonium hydroxide and based on the amount of solids in the suspension and 40 g / l of aluminum oxide were mixed. Then 100 g / l cerium oxide, 0.07 g / l palladium as Pd nitrate and 10 g / l magnesium oxide as Mg acetate were added. 3 g / l aluminum oxide was added as an alumina sol. 154 g / l were applied to washcoat layer A as washcoat layer B. The layer was then dried and calcined at 550 ° C. for 2 hours.</li><li>c) To produce the washcoat layer C, 0.25 g / l palladium were adsorbed as Pd nitrate in aqueous suspension on 50 g / l cerium oxide. An iron oxide component was added to this suspension as a pre-fixed powder. For this purpose, an aluminum oxide with 25 percent by weight iron oxide was produced by impregnating aluminum oxide with iron nitrate solution and subsequent calcination at 550 ° C. 20 g / l of this material was used in the coating suspension. In addition, 5 g / l aluminum oxide and 0.18 g / l rhodium were added as Rh nitrate to the suspension. 75 g / l were applied to washcoat layer B as washcoat layer C. The layer was then dried and calcined at 550 ° C. for 2 hours.</li><li>d) Washcoat layer D corresponded to washcoat layer C in Comparative Examples 1 and 2.</li></ol>The catalyst obtained in this way is called K1 below.

Example 2

0055Example 2 was repeated with the difference that the washcoat layer B contained 0.65 g / l platinum and the washcoat layer C additionally contained 0.05 g / l platinum. The catalyst obtained in this way is called K2 below.

Testing:

0056To test the catalyst activity of the catalysts RK1, RK2, K1 and K2, the samples were first subjected to a thermal pretreatment for 16 hours at 800 ° C. in an atmosphere of 10% oxygen, 10% water and the remainder nitrogen. They were then heated to 650 ° C. and the gas composition according to Tab. 1 exposed, 150 seconds the lean (oxidizing) and 30 seconds the rich (reducing) gas atmosphere was passed over the catalyst sample. The NOx concentration in the gas flow behind the catalytic converter was measured using FTIR.<tables id="tabl0001" num="0001"><table frame="all"><title>Tab. 1: Gas composition of the catalyst activity test</title><tgroup cols="3"><colspec colnum="1" colname="col1" colwidth="40mm" /><colspec colnum="2" colname="col2" colwidth="28mm" /><colspec colnum="3" colname="col3" colwidth="28mm" /><thead><row><entry valign="top" /><entry valign="top">skinny</entry><entry valign="top">fat</entry></row></thead><tbody><row><entry>GHSV [1 / h]</entry><entry>50000</entry><entry>50000</entry></row><row><entry>NO [ppm]</entry><entry>500</entry><entry>500</entry></row><row><entry>NO2 [ppm]</entry><entry>0</entry><entry>0</entry></row><row><entry>O₂ [vol%]</entry><entry>8</entry><entry>0,7</entry></row><row><entry>CO [vol%]</entry><entry>0</entry><entry>2,23</entry></row><row><entry>H₂ [vol%]</entry><entry>0</entry><entry>0,67</entry></row><row><entry>HC [ppm C₃] (C₃H₆/ C₃H₈)</entry><entry>746 (373/373)</entry><entry>746 (373/373)</entry></row><row><entry>CO2 [vol%]</entry><entry>13,3</entry><entry>13,3</entry></row><row><entry>HO [vol%]</entry><entry>10</entry><entry>10</entry></row></tbody></tgroup></table></tables>

0057To determine the NOx conversion activity, the stored NOx mass in mg per catalyst volume was determined for each lean phase when the NOx conversion had reached a value of 70%. The NOx conversion is calculated using the following formula:<maths id="math0001"><math display="block"><mrow><mi mathvariant="normal">NOx</mi><mo>−</mo><mi mathvariant="normal">Conversion</mi><mspace width="1em" /><mfenced open="[" close="]"><mo>%</mo></mfenced><mo>=</mo><mn mathvariant="normal">100</mn><mo>×</mo><mfenced><mi mathvariant="normal">c</mi><mfenced><msub><mi mathvariant="normal">NOx</mi><mi mathvariant="normal">in</mi></msub></mfenced><mo>−</mo><mi mathvariant="normal">c</mi><mfenced><msub><mi mathvariant="normal">NOx</mi><mi mathvariant="normal">out</mi></msub></mfenced></mfenced><mo>/</mo><mi mathvariant="normal">c</mi><mfenced><msub><mi mathvariant="normal">NOx</mi><mi mathvariant="normal">out</mi></msub></mfenced></mrow></math><img file="EP3695902A1_D0001.tif" /></maths> The stored NOx mass is calculated from the difference between the NOx mass in the gas upstream of the catalytic converter and the NOx mass in the gas downstream of the catalytic converter from the start of the lean phase to the point in time at which 70% NOx conversion has been reached.<figref idref="f0001">Figure 1</figref> shows the NOx storage values ​​at 70% NOx conversion as a function of the temperature for the reference catalytic converters RK1 and RK2 and the inventive catalytic converters K1 and K2. In the range from 175 to 250 ° C., the 4-layer design of the catalysts according to the invention achieves a higher amount of NOx storage. The use of palladium and platinum in catalytic converter K2 leads to an additional increase in the amount of NOx storage