WO2023198574A1

WO2023198574A1 - Exhaust gas system for predominantly stoichiometrically operated internal combustion engines, comprising a catalyst for reducing ammonia emissions

Info

Publication number: WO2023198574A1
Application number: PCT/EP2023/059083
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: WO2023198577A1; WO2023198570A1; WO2023198569A1; WO2023198573A1; WO2023198572A1; WO2023198571A1; 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 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 created by applying catalytically active Materials such as platinum group metals are activated and thus also serve 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 CNG engines (CNG = “Compressed Natural Gas”) (EP24258A1). These catalysts often consist of an SCR catalytically active component and a component that catalyzes the oxidation of ammonia. They are usually located in the underbody at the last point of the exhaust system. If for oxidation of the If there are not enough nitrogen oxides in the system from the stored ammonia, the ammonia can also be converted into nitrogen via the ASC with the oxygen present. It is an object of the present invention to present new exhaust gas purification systems which allow the operation of an internal combustion engine, in particular a predominantly stoichiometrically operated, spark-ignited internal combustion engine, even under the new Euro 7 legislation. In particular, the corresponding limit values for NH ₃ and N ₂ O in addition to the limit values for CO, HC and NOx should be safely adhered to. 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. It should also be as cost-effective to produce as possible. 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 providing an exhaust system for reducing harmful exhaust gas components from internal combustion engines, in particular predominantly stoichiometrically operated gasoline engines, having a first three-way catalytic converter and, downstream of this, a catalyst for reducing ammonia emissions, which has the following components: - a first component with a transition metal exchanged zeolite and/or zeotypes for storing ammonia; - a second component with an OSC-free noble metal catalyst and/or an OSC-containing noble metal catalyst, where the first and second components are present in a homogeneous mixture, the solution to the task is achieved relatively easily, but no less surprisingly. The system according to the invention is characterized by extremely good performance in terms of reducing CO, HC and NOx emissions as well as NH3 and N2O emissions. It reacts well to the dynamic requirements in the exhaust system of a gasoline engine and is robust enough to meet these requirements for a sufficient period of time. Furthermore, it is In terms of production technology, it is more advantageous to produce it since the system described can be applied to the carrier substrate in just one coating step compared to a catalyst with the first and second components in separate layers. In a preferred embodiment, the first component consists of a homogeneous mixture of transition metal-exchanged zeolites and/or zeotypes for storing ammonia and the second component consists only of an OSC-free noble metal catalyst. According to the invention, these two components are then present in a homogeneous mixture as a coating on the catalyst to reduce ammonia emissions. An OSC-containing noble metal catalyst is located as a separate layer on this homogeneous mixture (Fig. 3 c). Such a system is particularly suitable for use in an exhaust system of a predominantly stoichiometric combustion engine, as it also exhibits excellent three-way activity. 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 coating 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 coating (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 in a homogeneous coating on the substrate. The components are mixed before coating the substrate and then applied to the substrate and dried. The length of the layer can be chosen by the specialist. It is preferably located on a flow-through substrate and here takes 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. Another well-known property of the ion-exchanged large-pore zeolites is their ability to store hydrocarbons, which gives the formulated ammonia storage the properties of a so-called hydrocarbon trap. During the cold start, incoming hydrocarbons can be captured, which then desorb at a higher temperature and can be converted over the existing active three-way catalysts or oxidation catalysts. 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. In the present invention, two- and/or three-dimensional zeolites or zeotypes are preferably used [Ch. Baerlocher, WM Meier and DH Olson, Atlas of Zeolite Framework Types, Elsevier, 2001]. According to the invention, the term “zeolite” means porous materials with a lattice structure of corner-connected AlO ₄ and SiO ₄ tetrahedra according to the general formula (WM Meier, Pure & Appl. Chem., Vol.58, No.10, pp.1323 -1328, 1986): M _m/z [m AlO ₂ * n SiO ₂ ] * q H ₂ O The structure of a zeolite thus comprises 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 zeolites come from the group of two-dimensional or three-dimensional zeolites/zeotypes. They preferably belong to the structure types ACO, AEI, AEN, AFN, AFT, AFX, ANA, APC, APD, ATT, BEA, BIK, CDO, CHA, DDR, DFT, EAB, EDI, EPI, ERI, ESV, ETL, FER , GIS, GOO, IHW, ITE, ITW, LEV, KFI, MER, MON, NSI, OWE, PAU, PHI, RHO, RTH, SAT, SAV, SIV, THO, TSC, UEI, UFI, VNI, YUG, ZON at. It is particularly preferred if the zeolites or zeotypes in the car exhaust gas catalyst according to the invention are selected from the group AEI, AFT, AFX, CHA, DDR, ERI, ESV, ETL, FER, KFI, LEV, UFI and the corresponding zeotypes of these structural types, such as e.g.: SAPO. Mixtures of the same can also be present. Zeolites or zeotypes can also be classified according to their pore structure. A distinction is made between small-pore, medium-pore and large-pore zeolites. The extra-large pore zeolites are of academic interest. Small-pore zeolites are those with a largest ring size of 8 tetrahedral atoms. Large-pore zeolites have a top ring size of 12 tetrahedral atoms (https://en.wikipedia.org/w/index.php?title=Zeolite&oldid=1103217432 and the literature cited there). CHA, AEI, AFX, BIK, DDR, ERI, LEV or LTA are particularly preferred as small-pore zeolites or zeotypes for storing ammonia. CHA is extremely preferred in this context. Large-pore zeolites or zeotypes for storing ammonia are preferably selected from the group consisting of BEA, FAU or MOR. BEA is preferred in this context. As already mentioned above, large-pore zeolites or zeotypes are particularly preferred in connection with HC storage. 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 is therefore particularly in demand. The zeolites preferably have a SAR value (silica-to-alumina ratio) or Zeotype has a ratio corresponding to this value of >15 to <45, most preferably from >25 to <40. A range of 28 - 38 is extremely preferred in this context. To calculate the SAR value for zeolites, the amount of silicon atoms remaining in the framework is related to the substitution atoms. This results in the number of negative charges in the base body and thus a measure of the number of counterions to be absorbed until electroneutrality is achieved. A corresponding ratio can be determined for Zeotype. 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 0.8-7 and most preferably 1.5-5.0% 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 have other non-catalytically active components, such as binders. As binders are For example, not or only slightly catalytically active temperature-stable metal oxides, such as SiO2, Al2O3 and ZrO2, are suitable. The expert knows which materials come into question here. The proportion of such binders in the first coating can, for example, be up to 15% by weight, preferably up to 10% by weight, of the coating. The binder can also contain the above-mentioned transition metals, in particular iron and/or copper. Binders are suitable for ensuring stronger adhesion of the coating to a carrier or another coating. For this purpose, a certain particle size of the metal oxides in the binder is advantageous. This can be adjusted accordingly by a specialist. 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 first component 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 so that 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 below. The second component consists of an OSC-free precious metal catalyst and/or an OSC-containing noble metal catalyst. The second component can optionally also have the binders shown above for the first component. Precious metal refers in particular to the platinum group metals platinum, palladium and rhodium. Accordingly, the noble metals in the OSC-free or OSC-containing noble metal catalyst are preferably selected from the group consisting of palladium, platinum, rhodium. OSC means Oxygen Storage Component. An OSC-containing noble metal catalyst therefore has oxygen storage materials. The OSC-free precious metal catalyst, on the other hand, essentially has no function of storing oxygen in the exhaust gas of the internal combustion engine. In particular, this component has oxygen storage materials, in particular cerium-zirconium mixed oxides, of less than 10 g/L, preferably less than 5 g/L and most preferably less than 2 g/L carrier volume. The entire amount of cerium or cerium-zirconium mixed oxides, for example, is considered the storage material, including the doping elements present. Corresponding OSC-free precious metal catalysts have the ability to have an oxidative effect on the substances present (NH3, HC, CO) in the already slightly lean exhaust gas of a predominantly stoichiometrically operated combustion engine. 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 N ₂ O will be formed from ammonia oxidation. The oxidation effect can be regulated, for example, via the Pt content and/or the Pt:Pd/Rh ratio. The second component in the form of an OSC-free precious metal catalyst therefore contains materials that have an oxidative effect on, among other things, ammonia. Normally, this component contains a temperature-stable, high-surface metal oxide and at least one noble metal selected from the group rhodium, platinum and palladium. The total precious 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. The precious metals platinum or palladium, or platinum and palladium together, are particularly suitable for use in this component that has an oxidative effect on ammonia. The person skilled in the art can preferably choose whether to use the strongly oxidative platinum alone or, if necessary, in conjunction with palladium in the second component. 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. Palladium can be present in the coating between 0.015 - 1.42 g/L, preferably 0.035 - 0.35 g/L carrier volume. The weight ratio of platinum to palladium should be between 1:0 and 1:5, more preferably 1:0 and 1:4 and most preferably 1:0 and 1:2. As mentioned, the precious metals in the OSC-free second component are fixed on one or more temperature-stable, high-surface metal oxides as carrier materials. All materials familiar to those 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 from 30 to 250 m ² /g, preferably from 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 these. 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 La ₂ O ₃ and based on that Weight of stabilized aluminum oxide is used. 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 is widely described in the literature and is available on the market. The catalyst for reducing ammonia emissions has an OSC-containing noble metal catalyst as an alternative or cumulative to the OSC-free precious metal catalyst. In addition to the precious metals and the above-mentioned temperature-stable, high-surface metal oxides, oxygen storage materials are also present in the precious metal catalyst (containing OSC). 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 10 g/L, preferably more than 20 g/L and most preferably more than 25 g/L carrier volume. The entire cerium-zirconium mixed oxide with all its components is included. Corresponding OSC-containing precious metal catalysts have the ability to have an oxidative effect on the substances present (NH3, HC, CO) in the already slightly rich exhaust gas of a predominantly stoichiometrically operated combustion engine. This component is preferably designed so that it becomes active at correspondingly low temperatures. The ammonia stored in the zeolite or zeotype is transferred here This component is preferably converted into non-harmful nitrogen. 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 noble metals in the OSC-containing noble metal catalyst are preferably selected from the group consisting of palladium or rhodium or platinum, platinum and rhodium, palladium and rhodium or palladium and rhodium and platinum together. This catalyst is preferably a coating equipped with three-way catalytic capability. This particularly preferably has precious metals selected from the group of platinum and rhodium, palladium and rhodium, preferably rhodium alone. 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 support materials mentioned and on the oxygen storage materials. If rhodium is present in this component (whether alone or in combination with the other aforementioned precious metals), this should preferably be in the range of 0.035 - 1.0 g/L, more preferably 0.1 - 0.35 g/L. L carrier volume is located in the respective component. If palladium and/or platinum are also present in this component, the ranges mentioned above for the OSC-free precious metal catalysts apply to these metals. Suitable three-way catalytically active coatings are described, for example, in DE102013210270A1, DE102020101876A1, EP3247493A1, EP3727655A1. 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, the oxygen storage materials contained in the catalytic converter are the same Deviations occur to a certain extent by absorbing oxygen from the exhaust gas or releasing it into the exhaust gas as required (Catalytic Air Pollution Control, Commercial Technology, R. Heck et al., 1995, p.90). The OSC-containing noble metal catalysts (such as modern three-way catalysts) 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. It is, for example, 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 10 to 120 g/l, based on the volume of the carrier or substrate. The OSC-containing noble metal catalysts also have the temperature-stable, high-surface support materials mentioned for the OSC-free noble metal catalysts and, in addition to these, oxygen-storing materials. The mass ratio of temperature-stable, high-surface support 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. The first and second components preferably form an ammonia storage 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 using the second component with the oxygen present. 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, they are present in a homogeneous mixture on the substrate. The components are mixed before coating the substrate and then applied together to it using methods known to those skilled in the art. The length of the coating can be freely chosen by the person skilled in the art within the scope of the invention. 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, 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. 6). GPF are gasoline particle filters and are well known to those skilled in the art (EP3737491A1, EP3601755A1). An exhaust gas design in which the first three-way catalytic converter and the GPF are installed in a position close to the engine is particularly preferred. 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). The TWCs have zoned or Layered embodiments are now the norm. 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 one additional three-way catalytic converter and a possibly catalytically coated wall flow filter (GPF) in the exhaust system in front of the catalytic converter 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 there is an addition unit for secondary air in the exhaust system upstream of the catalytic converter to reduce ammonia emissions or 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 catalytic converter to reduce ammonia emissions are then oxidized to nitrogen under slightly oxidizing conditions. Here too, care must be taken to ensure that over-oxidation to N2O does not occur. Even after intensive aging, the exhaust system is robust enough to fully meet Euro 7 requirements. In particular, it may come as a surprise that the catalysts advocated here do not have to be applied in layers to a substrate, but rather as a homogeneous mixture in one layer. This has the advantage of being easier to manufacture. Nevertheless, a good conversion of the harmful exhaust gas components is achieved even after harsh aging conditions. This promises a long active lifespan for the targeted exhaust system.

Figures: Fig.1: Chart to explain the measurement of ammonia storage capacity. Fig.2: Catalyst to reduce ammonia emissions (1), coating with zeolites or zeotypes to store ammonia (2) and a coating with an OSC-free precious metal catalyst and/or an OSC-containing noble metal catalyst (3). Fig.3: Schematic representation of the catalysts tested in the underbody position. Fig.4: Emission values for catalytic converters A, B and C in comparison. Fig.5: Selectivity of the NH ₃ conversion to N ₂ O for the catalysts A, B and C in comparison. Fig.6: Exhaust system according to the invention with a three-way catalytic converter close to the engine (A), GPF close to the engine (B) and the following catalytic converter to reduce ammonia emissions (C).

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 Fe-loaded zeolite: The zeolite CHA (SAR=25) 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 h at 120 ℃ and for 2 h at 550 ℃ in air. For the Zeolite of the chabazite structural type was produced in a composition with 4.0 wt% Fe2O3 based on the total mass of zeolite and Fe2O3. B2. Production of the iron-containing SCR coating (1) The coating with an Fe-loaded zeolite was carried out after joint grinding with Nyacol ^® -AL20 binder on a cordierite substrate with the desired washcoat loading (88% zeolite, 12% binder). B3. Production of the platinum-containing SiO2/Al2O3 washcoat (2) A silicon-aluminum mixed oxide, which consists of 95% by weight aluminum oxide and 5% silicon oxide, was suspended in water. After the pH had been adjusted to 7.6 ± 0.4, a platinum solution was added to the resulting suspension with constant stirring. The resulting solution was ground and stabilized with ammonium acetate. B4. Production of the homogeneous coatings from the washcoats (1) and (2) The washcoats (1) and (2) were mixed together in the desired ratio and ground linearly. 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 support coated in this way was dried at 90 °C, calcined at 350 °C for 15 min and then annealed in air at 600 °C for 2 h. The total load of the washcoat on the catalyst was 189.8 g/L, the precious metal load was 0.106 g/L. If necessary, a further layer can be applied as a top layer to the carrier coated in this way. C. Preparation of the precious metal-containing coatings with TWC activity Aluminum oxide stabilized with lanthana oxide was prepared together with an oxygen storage component containing 24% by weight of cerium oxide, 60% by weight of zirconium oxide, 3.5% by weight of lanthanum oxide and 12.5% by weight of cerium oxide. % yttrium oxide, and lanthanum acetate suspended in water as an additional source of lanthanum oxide. 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, whereby the Coating took 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. Catalysts as shown schematically in FIG. 3 were produced using methods common to those skilled in the art. 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. F. Results Comparison of an ASC coated in two steps with separate layers (A) with an ASC homogeneously coated in one step (B) and a homogeneously coated ASC with additional TWC coating (C): (See Figs. 4 and 5 ) All catalysts with an oxidation layer contain 3 g/ft ³ pt. The catalyst C contains a TWC coating with 5 g/ft ³ Rh. Aging: Fuel-cut aging, 38 h, 860 °C bed temperature for the catalysts in the underbody position. Volume of the underbody catalyst: 0.83 L A catalyst in which an iron-containing zeolite layer is combined in a layered design with a 3 g/ft ³ Pt oxidation layer (A) shows comparable catalytic performance compared to a catalyst homogeneously coated in one step (B) . Catalyst C has slightly lower performance in terms of NH ₃ conversion, but has additional three-way catalytic activity. The catalysts B and C show a reduced selectivity of the NH ₃ conversion to N ₂ O. Light-off behavior of catalyst C with TWC coating (see Table 1): Table 1 shows the temperatures at which the catalyst C after a fuel cut Aging in the underbody position shows 50% conversion for hydrocarbons, carbon monoxide and nitrogen oxides in a light-off test. A lower T ₅₀ value corresponds to a higher catalytic activity. Catalysts A and B do not have a coating with three-way catalytic activity. Tab.1: Start-up behavior of catalyst C after the aging described.

Claims

Claims 1. Exhaust system for reducing harmful exhaust gas components from internal combustion engines, in particular predominantly stoichiometrically operated 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 for storing ammonia; - a second component with an OSC-free noble metal catalyst and/or an OSC-containing noble metal catalyst, the first and second components being present in a homogeneous mixture.

2. Exhaust system according to claim 1, characterized in that the first component consists of a homogeneous mixture of transition metal-exchanged zeolites and / or zeotypes for storing ammonia and the second component consists of an OSC-free noble metal catalyst, one containing OSC Precious metal catalyst is located as a separate layer on this homogeneous mixture.

3. Exhaust system according to one of the preceding claims, characterized in that the precious metals in the OSC-free or OSC-containing noble metal catalyst are selected from the group consisting of palladium, platinum, rhodium.

4. Exhaust system according to one of the preceding claims, characterized in that the metal-free zeolites or zeotypes for storing ammonia are selected from the group consisting of CHA, AEI, AFX, BEA, BIK, DDR, ERI, FER, LEV or LTA .

5. Exhaust system according to one of the preceding claims, characterized in that iron and / or copper are present as transition metals.

6. Exhaust system according to one of the preceding claims, characterized in that the first component 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 in the case of the presence of OSC-containing precious metal catalysts, the precious metals are deposited both on temperature-stable, high-surface carrier materials and on the 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 one of the preceding claims, 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 conducted via an exhaust system according to one of the preceding claims.