WO2023198572A1

WO2023198572A1 - Exhaust gas system for predominantly stoichiometrically operated internal combustion engines, comprising a catalyst for reducing ammonia emissions

Info

Publication number: WO2023198572A1
Application number: PCT/EP2023/059081
Authority: WO
Inventors: Julius KOEGEL; Massimo Colombo; Sonja Buchberger; Marcus Schmidt
Original assignee: Umicore Ag & Co. Kg
Priority date: 2022-04-11
Filing date: 2023-04-06
Publication date: 2023-10-19
Also published as: WO2023198569A1; WO2023198574A1; WO2023198570A1; WO2023198571A1; WO2023198577A1; WO2023198573A1; WO2023198575A1

Abstract

The present invention is directed to an exhaust gas system for reducing exhaust gas emissions and in particular ammonia emissions in the exhaust train of a predominantly stoichiometrically operated spark ignition engine.

Description

Exhaust system for predominantly stoichiometrically operated internal combustion engines having a catalyst for reducing ammonia emissions. The present invention is aimed at an exhaust system for reducing exhaust gases and in particular ammonia emissions in the exhaust system of a predominantly stoichiometrically operated spark ignition engine. Exhaust gases from internal combustion engines operated with predominantly (>50% of the operating time) stoichiometric air/fuel mixture, i.e. e.g. B. Spark ignition engines or gasoline engines powered by gasoline or natural gas are cleaned in conventional processes using three-way catalysts (TWC). These are able to simultaneously convert the engine's three main gaseous pollutants, namely hydrocarbons, carbon monoxide and nitrogen oxides, into harmless components. Stoichiometric means that, on average, there is as much air available to burn the fuel present in the cylinder as is required for complete combustion. The combustion air ratio λ (A/F ratio; air/fuel ratio) relates the air mass m _L,tats actually available for combustion to the stoichiometric air mass m _L,st :

If λ < 1 (e.g. 0.9) this means “lack of air”, one speaks of a rich exhaust gas mixture, λ > 1 (e.g. 1.1) means “excess air” and the exhaust gas mixture is considered lean designated. The statement λ = 1.1 means that 10% more air is present than would be necessary for the stoichiometric reaction. The same applies to the exhaust gases from internal combustion engines. The catalytically active materials used in the known three-way catalysts are generally platinum group metals, in particular platinum, palladium and rhodium, which are present, for example, on ^-aluminum oxide as a support material. In addition, three-way catalysts contain oxygen storage materials, for example cerium/zirconium mixed oxides. In the latter, cerium oxide, a rare earth metal oxide, is the fundamental component for oxygen storage. In addition to zirconium oxide and cerium oxide, these materials can contain additional components such as other rare earth metal oxides or alkaline earth metal oxides. Oxygen storage materials are made by Application of catalytically active materials such as platinum group metals activates and thus also serves as a carrier material for the platinum group metals. As part of the Euro 7 legislation, which will come into force in the mid-2020s, the emissions of ammonia (NH3) and nitrous oxide (N2O) for stoichiometric combustion engines will be regulated for the first time. The toxic ammonia and the powerful greenhouse gas N2O are referred to as secondary emissions and their emissions cannot be sufficiently reduced by current exhaust aftertreatment systems. Compliance with strict secondary emissions limits across a wide range of driving situations requires the development of a robust technical solution in the form of a new catalyst for the gasoline exhaust system. The extremely dynamic environmental conditions, especially in the underbody of a gasoline car, represent a major challenge. Compliance with the strict emission values for ammonia requires the use of a storage material to store NH ₃ during the rich operating conditions of the combustion engine, particularly in low and medium temperature ranges. engines, as the ammonia is mainly formed under these exhaust gas conditions. The stored ammonia is then converted during lean operating points by oxidation on a layer containing precious metal and/or as part of an SCR reaction. The aim here is to achieve the lowest possible selectivity to N ₂ O. A special requirement for the catalysts considered here is the high aging stability of the materials used: In addition to the stability against lean gas conditions, their use in the exhaust system of stoichiometrically operated internal combustion engines also requires that they also be used in the exhaust gas with rich or stoichiometric ric composition are stable under hydrothermal exhaust gas conditions. The use of catalysts, which preferentially convert ammonia into nitrogen, has already been discussed, particularly in the diesel sector or for use in lean-burning DI petrol engines (US5120695; EP1892395A1; EP1882832A2; EP1876331A2; WO12135871A1; US2011271664AA; WO1111091 9A1, EP3915679A1). The use of ammonia slip catalysts, or ASCs for short, has also already been described in the area of LNG gasoline engines (EP24258A1). These catalysts often consist of an SCR catalytically active component and a component that catalyzes the oxidation of ammonia. These catalytic converters are usually located in the underbody at the last point of the exhaust system. If for oxidation of the stored If there are not enough nitrogen oxides in the system, the ammonia can also be converted into nitrogen via the ASC with the oxygen present. As it turns out, the aging stability of ASC catalytic converters also depends largely on their design. Accordingly, it is an object of the present invention to present new exhaust systems which allow the operation of an internal combustion engine, in particular a predominantly stoichiometrically operated internal combustion engine, even under the new Euro 7 legislation. In particular, the corresponding limit values should be safely adhered to, especially for NH ₃ and N ₂ O. In addition, the system should also be robust and agile in order to be able to withstand the working conditions in the exhaust system of a corresponding automobile for a sufficient period of time. These and other tasks arising from the prior art for a person skilled in the art are solved by an exhaust system and a method for exhaust gas purification according to claims 1 and 11, respectively. Claims 2 - 10 relate to preferred embodiments of the exhaust system and can accordingly also be applied to the method according to the invention. By proposing an exhaust system for reducing harmful exhaust gas components from internal combustion engines, in particular predominantly stoichiometrically operated such as spark-ignited gasoline engines, having a first three-way catalytic converter and, on the downstream side, a catalyst for reducing ammonia emissions, which has the following components comprises: - a first component with a transition metal-exchanged zeolite and/or zeotypes with a three-dimensional framework structure; - a second component with an OSC-containing noble metal catalyst which has rhodium; and where the two components are applied as layers on top of one another on a substrate, the solution to the problem is achieved relatively easily, but no less surprisingly. The system according to the invention is characterized by extremely good performance both in terms of the original exhaust gas components and in terms of the NH3 and N2O emissions. It reacts well to the dynamic requirements in the exhaust system of a spark-ignition engine and is sufficiently robust to meet these requirements for a sufficient period of time. The components of the catalyst for reducing ammonia emissions are applied to a carrier, preferably to a flow-through substrate, using a coating step familiar to those skilled in the art (DE102019100099A1 and the literature cited there). A filter substrate such as a wall flow filter is also possible in this context. Flow-through substrates are catalyst supports that are common in the prior art and can consist of metal, for example WO17153239A1, WO16057285A1, WO15121910A1 and the literature cited therein) or ceramic materials. “Corrugated substrates” can also be viewed as flow-through substrates. These are known to those skilled in the art as carriers made of corrugated sheets made of inert materials. Suitable inert materials are, for example, fibrous materials with an average fiber diameter of 50 to 250 µm and an average fiber length of 2 to 30 mm. Fibrous heat-resistant materials made of silicon dioxide, especially glass fibers, are preferred. However, refractory ceramics such as cordierite, silicon carbite or aluminum titanate etc. are preferably used as honeycomb carriers. The number of carrier channels per area is characterized by the cell density, which is usually between 300 and 900 cells per square inch (cells per square inch, cpsi). The wall thickness of the channel walls for ceramics is between 0.5 – 0.05 mm. The total amount of coatings in the catalyst to reduce ammonia emissions is selected so that the catalyst according to the invention is used as efficiently as possible overall. In the case of one or more flow-through substrates, for example, the total amount of coatings (solids content) per carrier volume (total volume of the carrier) can be between 100 and 600 g/L, in particular between 150 and 400 g/L. The first component is preferably used in an amount of 50 to 350 g/L, in particular between 120 and 250 g/L, particularly preferably about 145 - 230 g/L of carrier volume. The second component is preferably used from 50 to 350 g/L, in particular between 120 and 250 g/L, particularly preferably from about 145 - 230 g/L carrier volume. According to the invention, the components are present as separate coatings one above the other on the substrate. It is preferred if the second component lies completely above the first and completely covers it. This is understood to mean the fact that the first component does not protrude beyond the second component at any end. It is particularly preferred if the two coatings with the respective components are of the same length (Fig. 2). The length of the layers can be chosen by the specialist become. They are preferably located on a flow-through substrate and here take up a length of at least 10% and a maximum of 100%, more preferably 20% - 90%, extremely preferably 30% - 80% of the substrate length. As already indicated above, a first component of the catalyst for reducing ammonia emissions consists of zeolites and/or zeotypes for storing ammonia. In principle, those skilled in the art are familiar with the zeolites and zeotypes available for this purpose from the diesel sector. The way the zeolites or zeotypes work is based on the fact that they can temporarily store ammonia in operating states of the exhaust gas purification system in which ammonia is produced, for example, by overreduction of nitrogen oxides via a three-way catalytic converter installed on the upstream side, but this is not converted by other conventional three-way catalytic converters. this can happen, for example due to a lack of oxygen or insufficient operating temperatures. The ammonia stored in this way can then be stored out when the operating state of the exhaust gas purification system changes and subsequently or directly converted, for example when sufficient oxygen or nitrogen oxides are present. According to the invention, zeolites and zeotypes are present in the first component of the catalyst to reduce ammonia emissions. According to the classification of the IZA (https://europe.iza-structure.org/IZA-SC/ftc_table.php), the international zeolite association, zeolites or zeotypes can be divided into different classes. Zeolites are then divided, for example, according to their channel system and their framework structure. For example, laumontite and mordenite are classified as zeolites, which have a one-dimensional system of channels. Your channels have no connection with each other. Zeolites with a two-dimensional channel system are characterized by the fact that their channels are connected to one another in a kind of layered system. A third group has a three-dimensional framework structure with cross-layer connections between the channels. According to the invention, three-dimensional zeolites or zeotypes are used in the present invention [Ch. Baerlocher, WM Meier and DH Olson, Atlas of Zeolite Framework Types, Elsevier, 2001]. According to the invention, the term “zeolite” refers to porous materials with a lattice structure of corner-linked AlO4 and SiO4 tetrahedra according to the general formula (WM Meier, Pure & Appl. Chem., Vol.58, No.10, pp.1323-1328 , 1986): Mm/z [m AlO2 * n SiO2] * q H2O The structure of a zeolite therefore includes a network made up of tetrahedra that encloses channels and cavities. A distinction is made between naturally occurring and synthetically produced zeolites. The term “zeotype” is understood to mean a zeolite-like compound that has the same structural type as a naturally occurring or synthetically produced zeolite compound, but which differs from such compounds in that the corresponding cage structure is not made up exclusively of aluminum and silicon framework atoms . In such compounds, the aluminum and/or silicon framework atoms are proportionally replaced by other trivalent, quadrivalent or pentavalent framework atoms such as B(III), Ga(III), Ge(IV), Ti(IV) or P(V) replaced. The most common method used in practice is the replacement of aluminum and/or silicon framework atoms by phosphorus atoms, for example in the silicon aluminum phosphates or in the aluminum phosphates, which crystallize in zeolite structure types. Examples of suitable three-dimensional zeolites belong to the structural types CHA, AEI, BEA, AFX. It is particularly preferred if the zeolites or zeotypes in the automobile exhaust catalyst according to the invention are selected from the group of three-dimensional zeolites, such as CHA, AEI and the corresponding zeotypes of these structural types, such as: SAPO. Mixtures of the same can also be present. The use of CHA is particularly preferred. The aging stability of the zeolites or zeotypes used in the exhaust system of predominantly stoichiometrically burning engines is particularly in focus here, since higher temperatures generally prevail here than in a lean-burning engine. In this respect, materials are desired that can withstand the sometimes very high and rapidly changing hydrothermal conditions for as long as possible. On the other hand, the exhaust gas composition is also different compared to lean-burn engine exhaust. The concentration, in particular of hydrocarbons and carbon monoxide, which arrive at the catalyst according to the invention is, on the one hand, higher than in lean-burn engines and the composition also changes depending on the driving style around the stoichiometric range (rich/lean change). The hydrothermal temperature stability of zeolites and zeotypes depends heavily on the SAR value (silica-to-alumina ratio) of the zeolite or the ratio corresponding to this value for zeotypes. The amount of silicon atoms remaining in the framework is then related to the substitution atoms. It has proven to be beneficial proven when the zeolites have a SAR value of 10 - 50, preferably 12 - 35 and most preferably 13 - 30. The same applies to the zeotype with the corresponding ratio. According to the invention, the zeolite or zeotype used is ion-exchanged with transition metal ions. The latter are preferably selected from the group consisting of iron and/or copper. Iron is particularly preferred because it has a less oxidizing effect on ammonia compared to copper. These compounds have the possibility of comproportioning nitrogen oxides present in the exhaust gas and the stored ammonia into nitrogen when lean. In this case, the zeolite or zeotype described acts as a catalyst for selective catalytic reduction (SCR) (see WO2008106518A2, WO2017187344A1, US2015290632AA, US2015231617AA, WO2014062949A1, US2015231617AA). In the present case, SCR capability is understood to mean the ability to selectively convert NO _x and NH ₃ in the lean exhaust gas into nitrogen. The metals, such as iron and/or copper, which advantageously occur in the catalyst to reduce ammonia emissions, are present in a certain proportion in the first component. This is 0.4-10, more preferably 0.8-6 and very preferably 1.5-4.8% by weight of the first component. The iron and/or copper to aluminum ratio is between 0.15 - 0.8, preferably between 0.2 - 0.5 and most preferably between 0.3 - 0.5 for zeolites. For the zeotype, a corresponding ratio applies to the exchange places available there. The metals are at least partially present in ion-exchanged form in the zeolites or zeotypes. Preferably, ion-exchanged zeolites or zeotypes are already introduced into the first component. However, it can also be the case that the zeolites or zeotypes are brought together with, for example, a binder and a solution of the metal ions in a liquid, preferably water, and then dried (preferably by spraying). Here, certain proportions of the metals can also be found on the binder in the form of oxides. Both approaches are possible. In addition to the zeolites or zeotypes, the first component can preferably contain further components, in particular non-catalytically active components, such as binders. For example, temperature-stable metal oxides that are not or only slightly catalytically active, such as SiO2, Al2O3 and ZrO2, are suitable as binders. The expert knows which materials come into question here. The proportion of such binders in the first component can, for example, be up to 15% by weight, preferably up to 10% by weight Make up component. As mentioned, the binder can also contain the metals specified above. Binders are suitable for ensuring stronger adhesion of the coating to a carrier. For this purpose, a certain particle size of the metal oxides in the binder is advantageous. This can be adjusted accordingly by an expert. The ammonia storage ability or capacity addressed in the context of this invention is given as a quotient of the stored mass of ammonia per liter of catalyst support volume. The zeolites or zeotypes should increase the ammonia storage capacity of the exhaust gas purification system to at least 0.25 g of ammonia per liter of carrier volume (measured in the fresh state). Overall, the storage capacity of the ammonia storage components used should be sufficient for the system to contain between 0.25 and 10.0 g of NH ₃ per liter of carrier volume, preferably between 0.5 and 8.0 g of NH ₃ per liter of carrier volume and particularly preferably between 0. 5 and 5.0 g NH ₃ /liter carrier volume of ammonia can be stored (always based on the fresh state). The zeolites or zeotypes are present in sufficient quantities in the catalyst to reduce ammonia emissions. The determination of the ammonia storage capacity is shown further below. The second component consists of an OSC-containing noble metal catalyst containing rhodium. Precious metal refers in particular to the platinum group metals platinum, palladium and rhodium. OSC stands for Oxygen Storage Catalyst. An OSC-containing noble metal catalyst therefore has oxygen storage materials. The OSC-containing precious metal catalyst has the function of storing oxygen in the exhaust gas of the internal combustion engine. Cerium or cerium-zirconium mixed oxides (see below) are consistently used as oxygen storage materials. Accordingly, an OSC-containing noble metal catalyst is characterized by the presence of a certain amount of these oxygen storage materials. In particular, this component has oxygen storage materials in an amount of more than 5 g/L, preferably more than 10 g/L and most preferably more than 20 g/L carrier volume. This includes the entire cerium-zirconium mixed oxide with all its components. Corresponding OSC-containing precious metal catalysts are capable of operating in the already slightly rich exhaust gas of a predominantly stoichiometric engine Internal combustion engine has an oxidative effect on the substances present (NH3, HC, CO). This component is preferably designed so that it becomes active at correspondingly low temperatures. The ammonia stored in the zeolite or zeotype is preferably converted into non-harmful nitrogen via this component. The oxidation effect should not be too great, otherwise a certain proportion of the powerful greenhouse gas N2O will be formed from ammonia oxidation. The second component in the form of an OSC-containing noble metal catalyst therefore has materials that have an oxidative effect on, among other things, ammonia. This component preferably contains a temperature-stable, high-surface metal oxide, oxygen storage material and at least the noble metal rhodium. Platinum and/or palladium may also be present. However, only rhodium is most preferably present in the second component. The total noble metal content of this component is preferably from 0.015 - 5 g/L, more preferably from 0.035 - 1.8 g/L and particularly preferably from 0.07 - 1.2 g/L carrier volume. If platinum and/or palladium is used, the former should be in the range of 0.015 - 1.42 g/L, more preferably 0.035 - 0.35 g/L carrier volume in the coating. When present in the coating, palladium can be present in between 0.015 - 1.42 g/L, preferably 0.035 - 0.35 g/L carrier volume. Rhodium is present in the second component according to the invention (alone or in combination with the other aforementioned noble metals). The carrier volume in this component should preferably be in the range of 0.035 - 1.0 g/L, more preferably 0.1 - 0.35 g/L. If palladium and/or platinum are also present in this component, the above ranges apply to these metals. Equipped in this way, this component has three-way activity. Suitable three-way catalytically active coatings (TWC) are described, for example, in DE102013210270A1, DE102020101876A1, EP3247493A1, EP3727655A1. The noble metals in the OSC-containing second component are usually fixed on one or more temperature-stable, high-surface metal oxides as carrier materials. All materials familiar to a person skilled in the art for this purpose can be considered as carrier materials. Such materials are in particular metal oxides with a BET surface area of 30 to 250 m ² /g, preferably 100 to 200 m ² /g (determined according to DIN 66132 - latest version on the filing date). Particularly suitable carrier materials for the precious metals are selected from the series consisting of aluminum oxide, doped aluminum oxide, silicon oxide, titanium dioxide and mixed oxides from one or more of them. Doped aluminum oxides are, for example, lanthanum oxide, zirconium oxide, barium oxide and/or titanium oxide-doped aluminum oxides. Aluminum oxide or lanthanum-stabilized aluminum oxide is advantageously used, in the latter case lanthanum in amounts of in particular 1 to 10% by weight, preferably 3 to 6% by weight, in each case calculated as La2O3 and based on the weight of stabilized aluminum oxide. Even in the case of aluminum oxide doped with barium oxide, the proportion of barium oxide is in particular 1 to 10% by weight, preferably 3 to 6% by weight, in each case calculated as BaO and based on the weight of the stabilized aluminum oxide. Another suitable carrier material is lanthanum-stabilized aluminum oxide, the surface of which is coated with lanthanum oxide, barium oxide and/or strontium oxide. This component preferably comprises at least one aluminum oxide or doped aluminum oxide. La-stabilized aluminum oxide with a surface area of 100 to 200 m ² /g is particularly advantageous in this context. Such active aluminum oxide has been widely described in the literature and is available on the market. Modern gasoline engines are operated under conditions with a discontinuous course of the air ratio ^. They are subject in a defined manner to a periodic change in the air ratio ^ and thus to a periodic change in oxidizing and reducing exhaust gas conditions. In both cases, this change in the air ratio ^ is essential for the exhaust gas purification result. For this purpose, the lambda value of the exhaust gas is adjusted with a very short cycle time (approx. 0.5 to 5 Hertz) and an amplitude ^ ^ of 0.005 ^ ^ ^ ^ 0.05 by the value ^ = 1 (reducing and oxidizing exhaust gas components are stoichiometric relationship to each other). Due to the dynamic operation of the engine in the vehicle, deviations from this state also occur. So that the deviations from the stoichiometric point mentioned do not have a negative impact on the exhaust gas purification result when the exhaust gas is passed over the three-way catalytic converter, oxygen storage materials contained in the catalytic converter compensate for these deviations to a certain extent by absorbing oxygen from the exhaust gas as required or released into the exhaust gas (Catalytic Air Pollution Control, Commercial Technology, R. Heck et al., 1995, p.90). The OSC-containing noble metal catalysts (modern three-way catalysts) of the second component therefore contain oxygen storage materials, in particular cerium or Ce/Zr mixed oxides. The mass ratio of cerium oxide to zirconium oxide can vary within wide limits in these mixed oxides. For example, it is 0.1 to 1.5, preferably 0.15 to 1 or 0.2 to 0.9. Preferred cerium/zirconium mixed oxides include one or more rare earth metal oxides and can therefore be referred to as cerium/zirconium/rare earth metal mixed oxides. The term “cerium/zirconium/rare earth metal mixed oxide” in the sense of the present invention excludes physical mixtures of cerium oxide, zirconium oxide and rare earth oxide. Rather, “cerium/zirconium/rare earth metal mixed oxides” are characterized by a largely homogeneous, three-dimensional crystal structure, which is ideally free of phases made of pure cerium oxide, zirconium oxide or rare earth oxide (so-called solid solution). Depending on the manufacturing process, however, products may not be completely homogeneous, which can generally be used without disadvantage. The same applies to cerium/zirconium mixed oxides that do not contain any rare earth metal oxide. Furthermore, the term rare earth metal or rare earth metal oxide in the sense of the present invention does not include cerium or cerium oxide. Possible rare earth metal oxides in the cerium/zirconium/rare earth metal mixed oxides are, for example, lanthanum oxide, yttrium oxide, praseodymium oxide, neodymium oxide and/or samarium oxide. Lanthanum oxide, yttrium oxide and/or praseodymium oxide are preferred. Particularly preferred rare earth metal oxides are lanthana and/or yttrium oxide and very particularly preferred is the joint presence of lanthana and yttrium oxide, yttrium oxide and praseodymium oxide, as well as lanthana and praseodymium oxide in the cerium/zirconium/rare earth metal mixed oxide. In a preferred embodiment, this noble metal catalyst has two different cerium/zirconium/rare earth metal mixed oxides, preferably one doped with La and Y and one doped with La and Pr. In embodiments of the present invention, the oxygen storage components are preferably free of neodymium oxide. The proportion of rare earth metal oxide(s) in the cerium/zirconium/rare earth metal mixed oxides is advantageously 3 to 20% by weight based on the cerium/zirconium/rare earth metal mixed oxide. If the cerium/zirconium/rare earth metal mixed oxides contain yttrium oxide as the rare earth metal, its proportion is preferably 4 to 15% by weight based on the cerium/zirconium/rare earth metal mixed oxide. If the cerium/zirconium/rare earth mixed oxides contain praseodymium oxide as the rare earth metal, its proportion is preferably 2 to 10% by weight based on the cerium/zirconium/rare earth mixed oxide. If the cerium/zirconium/rare earth metal mixed oxides contain lanthanum oxide and another rare earth oxide as the rare earth metal, such as yttrium oxide or praseodymium oxide, their mass ratio is in particular 0.1 to 1.25, preferably 0.1 to 1. This noble metal catalyst usually contains oxygen storage materials in amounts of 15 to 120 g/l, based on the volume of the carrier or substrate. The OSC-containing noble metal catalysts therefore have the temperature-stable, high-surface support materials mentioned and, in addition to these, the oxygen-storing materials just explained. The mass ratio of temperature-stable, high-surface carrier materials and oxygen storage components in this component is usually 0.25 to 1.5, for example 0.3 to 1.3. In an exemplary embodiment, the weight ratio of the sum of the masses of all support materials, such as aluminum oxides (including doped aluminum oxides) to the sum of the masses of all cerium/zirconium mixed oxides in the OSC-containing noble metal catalyst is 10:90 to 75:25, preferably 20: 80 to 65:35. In the OSC-containing noble metal catalyst, the noble metals can only be deposited on the temperature-stable, high-surface support materials. However, it is preferred if the noble metals are deposited both on the carrier materials mentioned and on the oxygen storage materials. The first and second components preferably form an ammonia storage with SCR functionality and a function for oxidizing ammonia to nitrogen (eg as in WO2008106523A2). If there are not enough nitrogen oxides in the system to oxidize the stored ammonia, the ammonia can also be converted into nitrogen with the oxygen present via the second component. In both cases, if possible, no ammonia or N ₂ O is released into the environment. In the broadest sense, the first component and the second component of the catalyst for reducing ammonia emissions can therefore preferably consist of an ammonia-storing coating paired with a second coating that has an oxidative effect on ammonia. As such, according to the invention, they are present in separate coatings one above the other on the substrate. It is particularly preferred if both coatings are of the same length. It is particularly preferred if the OSC-containing noble metal catalyst of component two is located as a top layer over the first component made of zeolites and / or zeotypes for storing ammonia as a bottom layer. Most preferably, no further layers are present below or above these two coatings on the substrate. In a further preferred embodiment, it has proven to be advantageous if a thin, additional, separate layer of inert, temperature-stable, high-surface metal oxides, as mentioned above, is present between the two layers just mentioned. The expert is guided by the coating methods mentioned above for their production. This thin between 5 µm and 200 µm, preferably between 10 µm and 150 µm high layer helps to further increase the aging stability of the catalyst to reduce ammonia emissions. As has been shown, a disadvantage of the known systems for reducing ammonia emissions can be that the transition metals in the SCR component, such as iron and/or copper, become predominantly stoichiometric after a long period of use in the exhaust system operated internal combustion engine tend to diffuse into the component for the oxidation of ammonia and poison it. The result is a lower activity of the SCR and the oxidative component. Suitable materials for this layer are those selected from the group consisting of aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, zeolites or mixtures thereof. Very particularly preferred in this context is a layer made of aluminum oxide or silicon oxide, which is preferably located on the substrate at the same length above the lower layer and under the upper layer. The present exhaust system has a first three-way catalyst and a catalyst positioned downstream to reduce ammonia emissions. The first three-way catalyst can have the same components as the OSC-containing noble metal catalyst of the second component. It is preferably constructed as described in DE102013210270A1, DE102020101876A1, EP3247493A1, EP3727655A1. Zoned or layered versions are now the norm for TWCs. In the car exhaust system according to the invention, the first catalyst with three-way activity has, in a further preferred embodiment, a 2-layer structure with two different three-way coatings, preferably as described in EP3247493A1. Downstream refers to the fact that the exhaust gas flow first hits the upstream catalytic converter and then the downstream catalytic converter. The reverse applies to the upstream side. With regard to the Euro 7 legislation, it has proven to be advantageous if an exhaust system for a predominantly stoichiometric engine has a unit for filtering small soot and ash particles. Preference is therefore given to an exhaust system that additionally has a possibly catalytically coated GPF between the first three-way catalytic converter and the catalytic converter to reduce ammonia emissions (Fig. 8). GPF are gasoline particle filters and are well known to those skilled in the art (EP3737491A1, EP3601755A1). Particularly preferred is an exhaust gas design in which the first three-way catalytic converter and the GPF on the downstream side are installed in a housing close to the engine, if necessary. Close to the engine in the sense of the invention refers to an area in the exhaust system that is in a position close to the engine, i.e. approx. 10 - 80 cm, preferably 20 - 60 cm away from the engine outlet. It has proven to be advantageous if the catalytic converter is installed last in the exhaust direction in the underbody of a vehicle to reduce ammonia emissions, so that the exhaust gas is then released into the ambient air. The exhaust system can also have additional exhaust units such as additional three-way catalytic converters or hydrocarbon storage (HC traps) or nitrogen oxide storage (LNT). The underbody is the area below the driver's cab. In a further preferred embodiment, there is at least a second three-way catalytic converter (TWC) between the first three-way catalytic converter and in front of the catalytic converter to reduce ammonia emissions in the car exhaust system according to the invention. The three-way activity has already been described earlier. There is explicit reference to what is stated there, especially with regard to the type and quantity of the individual components. This three-way catalyst is preferably one as described in the prior art (DE102013210270A1, DE102020101876A1, EP3247493A1, EP3727655A1). Zoned or layered versions are now the norm for TWCs. In a further preferred embodiment, in the car exhaust system according to the invention, at least one of the additional catalysts with three-way activity has a 2-layer structure with two different three-way coatings, preferably as described in EP3247493A1. The at least second three-way catalytic converter just described in the exhaust system according to the invention can be installed in the underbody of the vehicle, but it can also be in a position close to the engine. The range of possible Euro 7 systems is large. There can be up to 4 three-way catalytic converters in front of the catalytic converter per exhaust system to reduce ammonia emissions. In an alternative embodiment, there is at least a second three-way catalyst and a possibly catalytically coated wall flow filter (GPF) in front of the catalyst to reduce ammonia emissions. The catalyst for reducing ammonia emissions is preferably located last in the underbody and in fluid communication with the further catalyst or catalysts or the filter of the car exhaust system. The car exhaust system preferably has no additional injection device for ammonia or a precursor compound for ammonia. However, it is possible that in the exhaust system upstream of the catalytic converter Reduction of ammonia emissions or there is an addition unit for secondary air upstream of the wall flow filter (analogous to WO2019219816). In a further aspect, the present invention relates to a method for reducing harmful exhaust gas components from predominantly stoichiometrically operated internal combustion engines, in particular spark-ignited gasoline engines, in which the exhaust gas is directed via an exhaust system according to the invention. It should be noted that the preferred embodiments of the car exhaust system also apply mutatis mutandis to the present method. The present invention is directed to an exhaust gas purification system, in particular for stoichiometrically operated internal combustion engines. There are operating points of a stoichiometrically burning engine in which a rich exhaust gas is produced within a certain temperature interval. This can lead to nitrogen oxides arriving via a three-way catalytic converter being over-reduced to ammonia. This ammonia should not be released into the environment. The ammonia is therefore stored above the catalyst to reduce ammonia emissions and is then oxidized to nitrogen under slightly oxidizing conditions. Here too, care must be taken to ensure that no over-oxidation to N ₂ O occurs. Even after intensive aging, the exhaust system is robust enough to fully meet Euro 7 requirements. The coating design shown here leads to a significantly improved suppression of ammonia emissions and the formation of nitrous oxide. This promises a long active lifespan of the targeted exhaust system.

Figures: Fig.1: Chart to explain the measurement of ammonia storage capacity. Fig.2: Catalyst for reducing ammonia emissions (1), coating with an OSC-containing noble metal catalyst containing rhodium (2), coating with transition metal-exchanged zeolites or zeotypes for storing ammonia (3). Fig.3: Schematic representation of the catalysts tested in the underbody position, with TWC and SCR coatings being combined in different ways. Fig.4: Schematic representation of the catalysts tested in the underbody position with a rhodium-containing TWC coating or a platinum-containing oxidation layer with different layouts Fig.5: Emission values for the catalysts shown in Fig.3 in comparison Fig.6: Emission values for the catalysts shown in Fig.4 in comparison Fig.7: N ₂ O selectivity for the catalysts shown in Fig.4 Catalysts shown in comparison Fig.8: Preferred exhaust system with TWC close to the engine followed by a possibly catalytically coated GPF and a catalytic converter to reduce ammonia emissions in the underbody area

Examples: A. Determination of the ammonia storage capacity This is determined experimentally in a flow tube reactor. To avoid undesirable ammonia oxidation on the reactor material, a reactor made of quartz glass is used. A drill core is taken as a test specimen from the area of the catalytic converter whose ammonia storage capacity is to be determined. A drill core with a diameter of 1 inch and a length of 3 inches is preferably taken as a test specimen. The drill core is inserted into the flow tube reactor and at a temperature of 600 ° C in a gas atmosphere consisting of 500 ppm nitrogen monoxide, 5 vol.% oxygen, 5 vol.% water and the rest nitrogen with a space velocity of 30000 h ^-1 for 10 minutes conditioned. The measuring temperature of 200 °C is then reached in a gas mixture of 0 vol.% oxygen, 5 vol.% water and the rest nitrogen at a space velocity of 30,000 h ^-1 . After the temperature has stabilized, the NH ₃ storage phase is initiated by switching on a gas mixture of 450 ppm ammonia, 0 vol.% oxygen, 5 vol.% water and the rest nitrogen at a space velocity of 30,000 h ^-1 . This gas mixture remains switched on until a stationary ammonia breakthrough concentration is recorded on the downstream side of the test specimen. The mass of ammonia stored on the test specimen is calculated from the recorded ammonia breakthrough curve by integration from the start of the NH ₃ storage phase until stationarity is reached, taking into account the measured stationary NH ₃ breakthrough concentration and the known volume flow (hatched area in Figure 1). The ammonia storage capacity is calculated as the quotient of the stored mass of ammonia divided by the volume of the tested core.

B. Production of the ammonia-storing SCR layers B1. Preparation of the Cu-loaded zeolite The zeolite was coated with copper using a copper(II) nitrate solution in a solids mixer using an incipient wetness process. This was followed by treatment in the oven for 8 hours at 120°C and for 5 hours at 600°C in air. For the zeolite of the Levyn structural type (LEV), a composition with 3.5 wt% CuO based on the total mass of zeolite and CuO was prepared. B2. Preparation of the Fe-loaded zeolite The zeolite was coated with iron using an incipient wetness process with an iron(III) nitrate solution in a solids mixer. This was followed by treatment in the oven for 8 hours at 120°C and for 2 hours at 600°C in air. For the zeolite with the structure type chabazite (CHA), a composition with 4.0 wt% Fe ₂ O ₃ based on the total mass of zeolite and Fe ₂ O ₃ was prepared. B3. Production of a copper-containing zeolite coating The coating with a Cu-loaded zeolite was carried out after co-grinding with Nyacol ^® -AL20 binder on a cordierite substrate with a 150 g/L washcoat load (88% zeolite, 12% binder). The coated catalyst thus obtained was dried at 90 °C and then calcined at 350 °C for 15 min and annealed in air at 550 °C for 2 h. If necessary, a layer containing precious metal can be applied as a top layer to the now coated carrier. B4. Production of an iron-containing zeolite coating The coating with an Fe-loaded zeolite was carried out after co-grinding with Nyacol ^® -AL20 binder on a cordierite substrate with 164.8 g/L washcoat loading (88% zeolite, 12% binder). The coated catalyst thus obtained was dried at 90 °C and then calcined at 350 °C for 15 min and annealed in air at 550 °C for 2 h. If necessary, a layer containing precious metal can be applied as a top layer to the now coated carrier. C. Production of the precious metal-containing coatings with TWC activity Lanthanum oxide stabilized alumina was added along with an oxygen storage component comprising 24 wt% ceria, 60 wt% zirconium oxide, 3.5 wt% lanthanum oxide and 12.5 wt% yttria, and lanthanum acetate as an additional source of lanthanum oxide Water suspended. The weight ratio of aluminum oxide to oxygen storage component to additional lanthanum oxide was 43.6:55.7:0.7. A rhodium nitrate solution was then added to the suspension thus obtained with constant stirring. The resulting coating suspension was used directly to coat a commercially available substrate, with the coating taking place over 100% of the substrate length. The total loading of this washcoat on the catalyst was 122 g/L, the precious metal loading was 0.177 g/L (5 g/ft ³ ). The coated catalyst thus obtained was dried and then calcined. If necessary, a layer free of precious metals can be applied as a top layer to the now coated carrier. D. Production of the platinum-containing SiO ₂ /Al ₂ O ₃ layer without TWC activity: A silicon-aluminum mixed oxide, which consists of 95% by weight of aluminum oxide and 5% of silicon oxide, was suspended in water. After adjusting the pH to 7.6 ± 0.4, the resulting suspension was mixed with an EA platinum solution with constant stirring. The resulting suspension was ground and, after stabilization with ammonium acetate, used to coat a commercially available carrier, with the coating taking place over 100% of the carrier length. The total loading of this washcoat on the catalyst was 25 g/L, the precious metal loading was 0.106 g/L (3 g/ft ³ ). The coated catalyst thus obtained was dried and then calcined. If necessary, further layers can be applied as a top layer to the now coated carrier. Catalysts as shown schematically in FIGS. 3 and 4 were produced or combined with one another. E. Aging and testing of the ASCs Aging conditions: To determine the catalytic properties of the catalysts according to the invention, they were first aged in an engine test bench aging behind a TWC close to the engine in the underbody position (“fuel-cut aging”). The aging consists of fuel cut-off aging with an exhaust gas temperature of 950 °C in front of the inlet of the TWC near the engine (maximum bed temperature 1030 °C). The aging period and the inlet temperature for the catalytic converter in the underbody position are specified individually for each test. Test conditions: The different catalytic converters were tested in the underbody position on a highly dynamic engine test bench in a WLTC driving cycle. Here, a series-produced TWC containing Pd/Rh was placed in an aged state in a position close to the engine. The value “reduction in NH ₃ emissions” refers to the NH ₃ emissions of a system with one of the catalytic converters shown in the underbody position over the entire driving cycle in relation to the emissions of the corresponding system in the absence of a catalytic converter in underbody position. The value “Selectivity to N ₂ O” relates the additional N ₂ O molecules formed by the ASC in the bottom position to the NH ₃ molecules converted by the ASC according to the formula: Selectivity to N ₂ O = 100 ∙ 2 ∙ [(amount of substance N ₂ O after ASC) – (amount of substance N ₂ O before ASC)] / [(amount of substance NH ₃ before ASC) – (amount of substance NH ₃ after ASC)].

F. Results Comparison of different combinations of SCR and TWC layers: See Figures 3 and 5. All catalysts with TWC layer contain 5 g/ft ³ Rh. Aging: Fuel-cut aging, 38 h, 800 °C inlet temperature for the catalysts in the underbody position Volume of the underbody catalyst: 0.83 L A catalyst in which an SCR layer is combined in a layered design with a TWC layer with 5 g/ft ³ Rh shows improved catalytic performance compared to catalysts in which the SCR and TWC layers are arranged one after the other. Comparison of catalysts with SCR and rhodium-containing TWC coating with a catalyst with SCR and platinum-containing oxidation coating: See Figures 4, 6 and 7 The catalysts with TWC coating contain 5 g/ft ³ Rh, the catalyst with oxidation coating contains 3 g/ft ³ pt. Aging: Fuel-cut aging, 19 h, 830 °C Inlet temperature for the catalysts in the underbody position Volume of the underbody catalyst: 0.83 L The catalysts in which an SCR layer in a layered design with a TWC layer with 5 g/ft ³ Rh show improved catalytic performance and reduced selectivity to N2O compared to a catalyst in which the SCR layer is combined with a typical oxidation coating with 3 g/ft ³ Pt . In addition, a coating in which the TWC coating is over the SCR coating shows better performance with regard to NH3 and N2O. A design with SCR catalysts with a three-dimensional framework structure is also preferred. Light-off behavior of TWC catalysts with different precious metal loadings (see Table 1): Catalysts with a TWC layer with different precious metal contents are compared with each other. Table 1 shows the temperatures at which the catalysts show 50% conversion for hydrocarbons, carbon monoxide and nitrogen oxides in a light-off test after fuel-cut aging in the underbody position. A lower T50 value corresponds to a higher catalytic activity. A catalytic converter containing only rhodium shows the best starting behavior in this test. Tab.1:

Claims

Claims 1. Exhaust system for reducing harmful exhaust gas components from predominantly stoichiometrically operated internal combustion engines, in particular spark-ignited gasoline engines, having a first three-way catalytic converter and, downstream of this, a catalytic converter for reducing ammonia emissions, characterized in that it has the following components: - a first component with a transition metal-exchanged zeolite and/or zeotypes with a three-dimensional framework structure; - a second component with an OSC-containing noble metal catalyst which has rhodium; and wherein the two components are applied as superimposed layers on a substrate. 2. Exhaust system according to claim 1, characterized in that the second component lies completely above the first and completely covers it. 3. Exhaust system according to one of the preceding claims, characterized in that the two layers are the same length. 4. Exhaust system according to one of the preceding claims, characterized in that iron and / or copper are present as transition metals. 5. Exhaust system according to claim 1 or 2, characterized in that the zeolite or zeotype with a three-dimensional framework structure selected from the group consisting of CHA, AEI, BEA and AFX can be considered. 6. Exhaust system according to one of the preceding claims, characterized in that the zeolite or zeotype has an ammonia storage capacity of between 0.25 and 10.0 g NH3 per liter of carrier volume. 7. Exhaust system according to one of the preceding claims, characterized in that the precious metals in the precious metal catalyst are deposited both on temperature-stable, high-surface support materials and on oxygen storage materials. 8. Exhaust system according to one of the preceding claims, characterized in that it additionally has a GPF between the first three-way catalytic converter and the catalytic converter for reducing ammonia emissions. 9. Exhaust system according to claim 8, characterized in that the first three-way catalytic converter and the GPF are installed in a position close to the engine. 10. Exhaust system according to one of the preceding claims, characterized in that the catalytic converter is installed last in the exhaust direction in the underbody of a vehicle to reduce ammonia emissions. 11. A method for reducing harmful exhaust gas components from predominantly stoichiometrically operated internal combustion engines, in particular spark-ignited gasoline engines, characterized in that the exhaust gas is passed through an exhaust system according to one of the preceding claims.