WO2023198575A1 - Exhaust gas system for predominantly stoichiometrically operated internal combustion engines, comprising a catalyst for reducing ammonia emissions - Google Patents

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

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Publication number
WO2023198575A1
WO2023198575A1 PCT/EP2023/059084 EP2023059084W WO2023198575A1 WO 2023198575 A1 WO2023198575 A1 WO 2023198575A1 EP 2023059084 W EP2023059084 W EP 2023059084W WO 2023198575 A1 WO2023198575 A1 WO 2023198575A1
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WIPO (PCT)
Prior art keywords
ammonia
exhaust system
catalyst
exhaust gas
oxide
Prior art date
Application number
PCT/EP2023/059084
Other languages
German (de)
French (fr)
Inventor
Julius KOEGEL
Massimo Colombo
Sonja Buchberger
Marcus Schmidt
Original Assignee
Umicore Ag & Co. Kg
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Publication date
Priority claimed from DE102023101768.3A external-priority patent/DE102023101768A1/en
Application filed by Umicore Ag & Co. Kg filed Critical Umicore Ag & Co. Kg
Publication of WO2023198575A1 publication Critical patent/WO2023198575A1/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9404—Removing only nitrogen compounds
    • B01D53/9436—Ammonia
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9445—Simultaneously removing carbon monoxide, hydrocarbons or nitrogen oxides making use of three-way catalysts [TWC] or four-way-catalysts [FWC]
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
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    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9459—Removing one or more of nitrogen oxides, carbon monoxide, or hydrocarbons by multiple successive catalytic functions; systems with more than one different function, e.g. zone coated catalysts
    • B01D53/9463—Removing one or more of nitrogen oxides, carbon monoxide, or hydrocarbons by multiple successive catalytic functions; systems with more than one different function, e.g. zone coated catalysts with catalysts positioned on one brick
    • B01D53/9468—Removing one or more of nitrogen oxides, carbon monoxide, or hydrocarbons by multiple successive catalytic functions; systems with more than one different function, e.g. zone coated catalysts with catalysts positioned on one brick in different layers
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    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9459—Removing one or more of nitrogen oxides, carbon monoxide, or hydrocarbons by multiple successive catalytic functions; systems with more than one different function, e.g. zone coated catalysts
    • B01D53/9477—Removing one or more of nitrogen oxides, carbon monoxide, or hydrocarbons by multiple successive catalytic functions; systems with more than one different function, e.g. zone coated catalysts with catalysts positioned on separate bricks, e.g. exhaust systems
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
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    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
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    • F01N2570/00—Exhaust treating apparatus eliminating, absorbing or adsorbing specific elements or compounds
    • F01N2570/18—Ammonia

Definitions

  • 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.
  • 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 ⁇ (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 ⁇ ⁇ 1 (e.g. 0.9) this means “lack of air”, one speaks of a rich exhaust gas mixture, ⁇ > 1 (e.g. 1.1) means “excess air” and the exhaust gas mixture is considered lean designated.
  • ⁇ 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.
  • 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.
  • 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).
  • CNG “Compressed Natural Gas”
  • SCR SCR catalytically active component
  • 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.
  • the corresponding limit values for NH 3 and N 2 O should be safely adhered to in addition to the traditional ones for CO, HC and NOx.
  • 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 as possible to produce.
  • Claims 2 - 10 relate to preferred embodiments of the exhaust system and can accordingly also be applied to the method according to the invention.
  • 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, on the downstream side, a catalytic converter for reducing ammonia emissions, which has the following components: - a first component comprising 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 component has a mixture of small-pore and large-pore zeolites or zeotypes, 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 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 sufficiently robust to meet these requirements over a sufficient period of time become. In addition, it is more advantageous to produce in terms of production technology.
  • the large-pore zeolites or zeotypes can serve as hydrocarbon traps and thus help to reduce HC emissions during, for example, cold start conditions.
  • 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 ⁇ m and an average fiber length of 2 to 30 mm. Fibrous heat-resistant materials made of silicon dioxide, especially glass fibers, are preferred.
  • 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.
  • 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.
  • the components are present as separate coatings one above the other on the substrate. It is preferred if the second component is completely over 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.
  • 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.
  • 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.
  • zeolites and zeotypes are present in the first component of the catalyst to reduce ammonia emissions.
  • 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.
  • zeolites or zeotypes Two- and/or three-dimensional zeolites or zeotypes are preferred in the present invention for use [Ch. Baerlocher, WM Meier and DH Olson, Atlas of Zeolite Framework Types, Elsevier, 2001].
  • 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.
  • zeolite therefore comprises a network made up of tetrahedra that encloses channels and cavities.
  • 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 .
  • 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.
  • 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.
  • 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 units.
  • 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 zeol types for storing ammonia are preferably selected from the group consisting of BEA, FAU or MOR. BEA is particularly preferred in this context.
  • 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.
  • the exhaust gas composition is also different compared to lean-burn engine exhaust.
  • 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 small-pore zeolites used preferably have a SAR value (silica-to-alumina ratio) or the zeotypes have a ratio corresponding to this value of >15 to ⁇ 45, very preferably of >20 to ⁇ 35. Extremely preferred is a Range from 20 – 30 in this context.
  • the large-pore zeolites used preferably have a SAR value (silica-to-alumina ratio) or the zeotypes have a ratio corresponding to this value of >10 to ⁇ 50, very preferably from >10 to ⁇ 40. Extremely preferred is a range of 10 – 30 in this context.
  • 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.
  • 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.
  • the zeolite or zeotype described acts as a catalyst for selective catalytic reduction (SCR) (see WO2008106518A2, WO2017187344A1, US2015290632AA, US2015231617AA, WO2014062949A1, US2015231617AA).
  • SCR capability is understood to mean the ability to selectively convert NOx and NH3 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 0.05 - 0.8, preferably between 0.2 - 0.5 and most preferably between 0.3 - 0.5 for zeolites.
  • 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.
  • ion-exchanged zeolites or zeotypes are already introduced into the first component.
  • 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).
  • ion exchange represents a challenge for small-pore zeolites. While ion exchange for large-pore zeolites can be carried out as part of the washcoat process, since the ions readily penetrate into the large zeolite pores and occupy the ion exchange positions of the material, the occupation of small-pore zeolites requires porous zeolites require a separate process step. This can be an incipient wetness process or an ion exchange process with, if necessary, subsequent spray drying.
  • One approach to saving this additional step and the associated costs and production time is to coat with a mixture of a small-pore zeolite, a large-pore zeolite and a soluble salt of the ions to be exchanged (e.g. Fe(NO3)3 x 9 H2O, Cu acetate) with, if necessary, a common binder system onto a substrate.
  • the dissolved ions are first absorbed by the large-pore zeolite.
  • a distribution of the ions between large and small pore zeolite then occurs during the calcination process.
  • the resulting catalyst benefits from the high stability of the ammonia storage function of the small-pore zeolite as well as from the usually high SCR activity of the large-pore exchanged zeolite and thus shows good performance and selectivity in the application described.
  • 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 via the then active three-way catalysts or oxidation catalysts. The first component has large and small pore zeolites.
  • the order of the layers of large and small pore zeolites and the corresponding loading amounts can be varied.
  • the quantitative ratio of the different types of zeolites to one another can be varied.
  • the small-pore zeolite can be used in a transition metal-free form or coated with transition metals, preferably iron or copper.
  • the large and small pore zeolites are preferably used in a weight ratio of 5:1 to 1:5, more preferably 2:1 to 1:2.
  • the first component can preferably have other non-catalytically active components, such as binders.
  • binders 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 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 first component should increase the ammonia storage capacity of the exhaust gas purification system to at least 0.25 g of ammonia per L of carrier volume (measured in the fresh state).
  • the storage capacity of the ammonia storage components used in the form of zeolites or zeotypes should be sufficient to ensure that there is between 0.25 and 10.0 g of NH3 per liter of carrier volume in the system, preferably between 0.5 and 8.0 g of NH3 per liter of carrier volume and particularly preferably between 0.5 and 5.0 g of NH 3 /liter of carrier volume of ammonia can be stored (always based on the fresh state).
  • the zeolites or zeotypes are present in a sufficient amount 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-free precious metal catalyst and/or an OSC-containing noble metal catalyst.
  • Precious metal refers in particular to the platinum group metals platinum, palladium and rhodium.
  • 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.
  • 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.
  • 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 N2O will be formed from ammonia oxidation.
  • the oxidation power can be adjusted, among other things, by the amount of platinum and the Pt:Pd and/or Pt: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.
  • 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 coating layer. 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.
  • 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 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 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 La2O3 and based on the weight of the stabilized Aluminum oxide is used.
  • the proportion of barium oxide 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.
  • 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.
  • 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.
  • oxygen storage materials are also present in the precious metal catalyst (containing OSC).
  • Cerium or cerium-zirconium mixed oxides 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 (NH 3 , 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 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 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.
  • 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. 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.
  • the lambda value of the exhaust gas is adjusted with a very short cycle time (approx.
  • 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 used as Cerium/zirconium/rare earth metal mixed oxides are referred to.
  • 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 that are not completely homogeneous may not be created, 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.
  • rare earth metal oxides in the cerium/zirconium/rare earth metal mixed oxides are 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 lanthanum oxide and/or yttrium oxide and very particularly preferred is the joint presence of lanthanum oxide and yttrium oxide, yttrium oxide and praseodymium oxide, as well as lanthanum oxide and praseodymium oxide in the cerium/zirconium/rare earth metal mixed oxide.
  • 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.
  • 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.
  • 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 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.
  • 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 the oxidation of 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.
  • 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.
  • both coatings are of the same length. It is particularly preferred if, for further improved three-way activity, 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 there is a further thin, separate layer of inert, temperature-stable, high-surface metal oxides between the two layers/components just mentioned. The expert is guided by the coating methods mentioned above for their production.
  • This thin layer which is between 5 ⁇ m and 200 ⁇ m, preferably between 10 ⁇ m and 150 ⁇ m high, helps to further increase the aging stability of the catalyst to reduce ammonia emissions.
  • a disadvantage of the known systems for reducing ammonia emissions can be that the transition metals in the first component, such as iron and/or copper, become part of the exhaust system of a predominantly stoichiometrically operated internal combustion engine after a long period of use tend to diffuse into the ammonia oxidation component and poison it. The result is a lower activity of the ammonia-storing and oxidative components.
  • 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 in 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, 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.
  • 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 is delivered.
  • 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.
  • TWC three-way catalytic converter
  • 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.
  • 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 three-way catalyst and a possibly catalytically coated wall flow filter (GPF) in front of the catalyst to reduce ammonia emissions.
  • GPF catalytically coated wall flow filter
  • 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.
  • 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 WO2019219816A1).
  • the present invention relates to a method for reducing harmful exhaust gas components from predominantly stoichiometric operated internal combustion engines, in particular spark-ignited gasoline engines, in which the exhaust gas is passed through an exhaust system according to the invention.
  • the present invention is directed to an exhaust gas purification system, in particular for stoichiometrically operated internal combustion engines.
  • 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.
  • the ammonia storage catalyst is also easier to produce, since only a simplified ion exchange step is required in the production of the first component. Nevertheless, even after intensive aging, the system is robust enough to fully meet the Euro 7 requirements.
  • Fig.1 Chart to explain the measurement of ammonia storage capacity.
  • Fig.2 Catalyst for reducing ammonia emissions (1), coating with metal-free zeolites or zeotypes for storing ammonia (2) and a coating with an OSC-free noble 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 Reduction in ammonia emissions through catalytic converters A and B compared to a system without an underbody catalytic converter.
  • Fig.5 Reduction in hydrocarbon emissions through catalytic converters A and B compared to a system without an underbody catalytic converter.
  • 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).
  • 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 .
  • 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.
  • the production of the iron-containing zeolite coating was carried out using a washcoat consisting of a large-pore zeolite of the BEA structural type, iron (III) nitrate solution and a suitable amount of a binder system consisting of a Al2O3 and a SiO2 component.
  • the desired amount of washcoat was coated in one step over 100% of the substrate length.
  • 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, further layers can be applied as a top layer to the now coated carrier. B2.
  • a silicon-aluminum mixed oxide which consists of 95% by weight aluminum oxide and 5% 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 and tempered.
  • Aluminum oxide stabilized with lanthana oxide was prepared together with an oxygen storage component containing 24 wt.% cerium oxide, 60 wt.% zirconium oxide, 3.5 wt.% lanthanum oxide and 12.5 wt. % 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.
  • the suspension thus obtained was then mixed with constant stirring Rhodium nitrate solution added.
  • 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 can be, for example, 122 g/L, the precious metal loading 0.177 g/L (5 g/ft 3 ).
  • the coated catalyst thus obtained was dried and then calcined and tempered. If necessary, a layer free of precious metals can be applied as a top layer to the now coated carrier.
  • Catalysts were prepared as shown schematically in Figure 3.
  • 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.

Abstract

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

Description

Abgassystem für überwiegend stöchiometrisch betriebene Verbrennungsmoto- ren aufweisend einen Katalysator zur Verminderung der Ammoniakemissionen Vorliegende Erfindung richtet sich auf ein Abgassystem zur Verminderung der Abgase und insbesondere der Ammoniakemissionen im Abgasstrang eines überwiegend stöch- iometrisch betriebenen Fremdzündungsmotors. Abgase von mit überwiegend (>50% der Betriebszeit) stöchiometrischem Luft/Kraftstoff- Gemisch betriebenen Verbrennungsmotoren, also z. B. mit Benzin oder Erdgas betrie- bene Fremdzündungsmotoren oder Ottomotoren, werden in herkömmlichen Verfahren mit Hilfe von Dreiwegekatalysatoren (three-way-catalyst; TWC) gereinigt. Diese sind in der Lage, die drei wesentlichen gasförmigen Schadstoffe des Motors, nämlich Kohlen- wasserstoffe, Kohlenmonoxid und Stickoxide, gleichzeitig zu unschädlichen Komponen- ten umzusetzen. Stöchiometrisch heißt, dass im Mittel genau so viel Luft zur Verbren- nung des im Zylinder vorhandenen Kraftstoffs zur Verfügung steht, wie für eine vollstän- dige Verbrennung benötigt wird. Das Verbrennungsluftverhältnis λ (A/F-Verhältnis; Luft/Kraftstoffverhältnis) setzt die tatsächlich für eine Verbrennung zur Verfügung ste- hende Luftmasse mL,tats ins Verhältnis zur stöchiometrischen Luftmasse mL,st:
Figure imgf000003_0001
Ist λ < 1 (z. B.0,9) bedeutet dies „Luftmangel“, man spricht von einem fetten Abgasge- misch, λ > 1 (z. B.1,1) bedeutet „Luftüberschuss“ und das Abgasgemisch wird als mager bezeichnet. Die Aussage λ = 1,1 bedeutet, dass 10% mehr Luft vorhanden ist, als zur stöchiometrischen Reaktion notwendig wäre. Gleiches gilt für das Abgas von Verbren- nungsmotoren. Als katalytisch aktive Materialien werden in den bekannten Dreiwegekatalysatoren in der Regel Platingruppenmetalle, insbesondere Platin, Palladium und Rhodium eingesetzt, die beispielsweise auf ^-Aluminiumoxid als Trägermaterial vorliegen. Daneben enthalten Dreiwegekatalysatoren Sauerstoffspeichermaterialien, beispielsweise Cer/Zirkonium- Mischoxide. In letzteren stellt Ceroxid die für die Sauerstoffspeicherung grundlegende Komponente dar. Neben Zirkoniumoxid und Ceroxid können diese Materialien zusätzli- che Bestandteile wie weitere Seltenerdmetalloxide oder Erdalkalimetalloxide enthalten. Sauerstoffspeichermaterialien werden durch Aufbringen von katalytisch aktiven Materialien wie Platingruppenmetallen aktiviert und dienen somit auch als Trägermate- rial für die Platingruppenmetalle. Im Rahmen der zur Mitte der 2020er Jahre in Kraft tretenden Euro 7-Gesetzgebung wer- den erstmals die Emissionen von Ammoniak (NH3) und Lachgas (N2O) für stöchiomet- risch arbeitende Verbrennungsmotoren reguliert. Der giftige Ammoniak und das starke Treibhausgas N2O werden als Sekundäremissionen bezeichnet und ihr Ausstoß kann durch aktuelle Abgasnachbehandlungssysteme nicht ausreichend reduziert werden. Die Einhaltung der strengen Grenzwerte für Sekundäremissionen über einen breiten Bereich von Fahrsituationen erfordert die Entwicklung einer robusten technischen Lösung in Form eines neuen Katalysators für den Benzinabgasstrang. Eine große Herausforde- rung stellen insbesondere die extrem dynamischen Umgebungsbedingungen gerade im Unterboden eines Benzin-PKWs dar. Die Einhaltung der strengen Emissionswerte für Ammoniak erfordert insbesondere für niedrige und mittlere Temperaturbereiche die Verwendung eines Speichermaterials zur Einspeicherung von NH3 während der fetten Betriebsbedingungen des Verbrennungs- motors, da der Ammoniak hauptsächlich unter diesen Abgasbedingungen gebildet wird. Die Umsetzung des gespeicherten Ammoniaks erfolgt dann während magerer Betriebs- punkte durch Oxidation auf einer edelmetalthaltigen Schicht und/oder im Rahmen einer SCR-Reaktion. Hierbei wird eine möglichst geringe Selektivität zu N2O angestrebt. Eine besondere Anforderung an den hier betrachteten Katalysatoren stellt die hohe Alterungs- stabilität der verwendeten Materialien dar: Über die Stabilität gegenüber mageren Gas- bedingungen hinaus erfordert ihre Anwendung im Abgasstrang von stöchiometrisch be- triebenen Verbrennungsmotoren, dass diese auch im Abgas mit fetter oder stöchiomet- rischer Zusammensetzung unter hydrothermalen Abgasbedingungen stabil sind. Insbesondere im Dieselbereich oder für den Einsatz in mager verbrennenden DI Ben- zinmotoren wurde der Einsatz von Katalysatoren, welche bevorzugt Ammoniak zu Stick- stoff umsetzen, schon diskutiert (US5120695; EP1892395A1; EP1882832A2; EP1876331A2; WO12135871A1; US2011271664AA; WO11110919A1, EP3915679A1). Auch im Bereich der CNG-Motoren (CNG = „Compressed Natural Gas“) wurde die Ver- wendung von Ammonia Slip Catalysts, kurz ASCs, bereits beschrieben (EP24258A1). Diese Katalysatoren bestehen häufig aus einer SCR-katalytisch aktiven und einer die Oxidation von Ammoniak katalysierenden Komponente. Sie befinden sich regelmäßig im Unterboden an letzter Stelle des Abgassystems. Sofern zur Oxidation des eingespeicherten Ammoniaks nicht genügend Stickoxide im System vorhanden sein soll- ten, kann der Ammoniak über dem ASC auch mit vorhandenem Sauerstoff zu Stickstoff umgesetzt werden. Es ist eine Aufgabe der vorliegenden Erfindung neue Abgassysteme vorzustellen, wel- che den Betrieb eines Verbrennungsmotors, insbesondere eines überwiegend stöchio- metrisch betriebenen, fremdgezündeten Verbrennungsmotors, auch unter der neuen Euro 7-Gesetzgebung erlauben. Insbesondere sollten die entsprechenden Grenzwerte für NH3 und N2O neben den angestammten für CO, HC und NOx sicher einzuhalten sein. Darüber hinaus sollte das System aber auch entsprechend robust und agil sein, um den Arbeitsbedingungen im Abgasstrang eines entsprechenden Automobils für eine ausrei- chende Zeit standhalten zu können. Es sollte auch möglichst kostengünstig zu produ- zieren sein. Diese und weitere, sich für den Fachmann aus dem Stand der Technik ergebenden Auf- gaben werden durch ein Abgassystem und ein Verfahren zur Abgasreinigung gemäß den Ansprüchen 1 bzw.11 gelöst. Die Ansprüche 2 – 10 beziehen sich auf bevorzugte Ausgestaltungen des Abgassystems und sind entsprechend auch auf das erfindungsge- mäße Verfahren anwendbar. Dadurch, dass man ein Abgassystem zur Verminderung von schädlichen Abgasbestand- teile von Verbrennungsmotoren, insbesondere überwiegend stöchiometrisch betriebe- nen Benzinmotoren, aufweisend einen ersten Dreiwegekatalysator und abstromseitig hierzu einen Katalysator zur Verminderung der Ammoniak-Emissionen zur Verfügung stellt, wobei dieser folgende Bestandteile aufweist: - eine erste Komponente mit einem übergangsmetallausgetauschten Zeolithen und/oder Zeotypen zur Speicherung von Ammoniak; - eine zweite Komponente mit einem OSC-freien Edelmetallkatalysator und/oder einem OSC-haltigen Edelmetallkatalysator, wobei die erste Komponente eine Mischung aus kleinporigen und großporigen Zeolithen bzw. Zeotypen aufweist, gelangt man relative einfach, dafür aber nicht minder überraschend zur Lösung der gestellten Aufgabe. Das erfindungsgemäße System zeichnet sich durch eine extrem gute Performance hinsichtlich der Verminderung der CO, HC und NOx- Emissionen wie auch der NH3- und N2O-Emissionen aus. Es reagiert gut unter den dy- namischen Anforderungen im Abgasstrang eines Benzinmotors und es ist entsprechend robust, um auch über eine ausreichende Dauer diesen Anforderungen gerecht zu werden. Darüber hinaus ist es produktionstechnisch vorteilhafter herzustellen. Zudem können die großporigen Zeolithe bzw. Zeotype als Kohlenwasserstofffallen dienen und so helfen, die HC-Emissionen während z.B. der Kaltstartbedingungen zu vermindern. Die Komponenten des Katalysator zur Verminderung der Ammoniak-Emissionen werden durch einen dem Fachmann geläufigen Beschichtungsschritt auf einen Träger, vorzugs- weise auf ein Durchflusssubstrat aufgebracht (DE102019100099A1 sowie dort zitierte Literatur). Ein Filtersubstrat wie ein Wandflussfilter ist in diesem Zusammenhang auch möglich. Durchflusssubstrate sind im Stand der Technik übliche Katalysatorträger, die aus Metall z.B. WO17153239A1, WO16057285A1, WO15121910A1 und darin zitierte Literatur) oder keramischen Materialien bestehen können. „Corrugated substrates“ kön- nen auch als Durchflusssubstrate angesehen werden. Diese sind dem Fachmann als Träger aus gewellten Blättern, welche aus inerten Materialien bestehen, bekannt. Ge- eignete inerte Materialien sind zum Beispiel faserförmige Materialien mit einem durch- schnittlichen Faserdurchmesser von 50 bis 250 µm und einer durchschnittlichen Faser- länge von 2 bis 30 mm. Bevorzugt sind faserförmige hitzebeständige Materialien aus Siliziumdioxid, insbesondere aus Glasfasern. Bevorzugt werden jedoch feuerfeste Kera- miken wie zum Beispiel Cordierit, Siliziumcarbit oder Aluminiumtitanat etc. als Ho- neycomb-Träger eingesetzt. Die Anzahl der Kanäle der Träger pro Fläche wird durch die Zelldichte charakterisiert, welche üblicher Weise zwischen 300 und 900 Zellen pro Quad- rat inch (cells per square inch, cpsi) liegt. Die Wanddicke der Kanalwände beträgt bei Keramiken zwischen 0,5 – 0,05 mm. Die Gesamtmenge der Beschichtung im Katalysator zur Verminderung der Ammoniak- Emissionen wird so ausgewählt, dass der erfindungsgemäße Katalysator insgesamt möglichst effizient genutzt wird. Im Falle eines oder mehrerer Durchflusssubstrat(e) kann beispielsweise die Gesamtmenge der Beschichtung (Feststoffanteil) pro Trägervolumen (Gesamtvolumen des Trägers) zwischen 100 und 600 g/L sein, insbesondere zwischen 150 und 400 g/L. Die erste Komponente wird bevorzugt in einer Menge von 50 bis 350 g/L, insbesondere zwischen 120 und 250 g/L, besonders bevorzugt von etwa 145 – 230 g/L Trägervolumen, eingesetzt. Die zweite Komponente wird vorzugsweise von 50 bis 350 g/L, insbesondere zwischen 120 und 250 g/L, besonders bevorzugt von etwa 145 – 230 g/L Trägervolumen, eingesetzt. Erfindungsgemäß liegen die Komponenten als separate Beschichtungen übereinander liegend auf dem Substrat vor. Bevorzugt ist, wenn die zweite Komponente komplett über der ersten liegt und diese vollständig überdeckt. Hierunter wird die Tatsache verstanden, dass die erste Komponente an keinem Ende über die zweiten Komponente hinausragt. Besonders bevorzugt ist, wenn die beiden Beschichtungen mit den jeweiligen Kompo- nenten gleich lang sind (Fig.2). Die Länge der Schichten kann vom Fachmann gewählt werden. Sie befinden sich auf bevorzugt auf einem Durchflusssubstrat und nehmen hier eine Länge von mindestens 10% und maximal 100%, mehr bevorzugt 20% - 90% äu- ßerst bevorzugt 30% - 80% der Substratlänge ein. Eine Beschichtung, die über einer anderen liegt, kommt vorliegend vor letzterer als erstes mit dem Abgas in Berührung. Wie oben schon angedeutet besteht eine erste Komponente des Katalysator zur Ver- minderung der Ammoniak-Emissionen aus Zeolithen und/oder Zeotypen zur Speiche- rung von Ammoniak. Prinzipiell sind dem Fachmann die hierfür zur Verfügung stehenden Zeolithe und Zeotype aus dem Dieselsektor bekannt. Die Arbeitsweise der Zeolithe bzw. Zeotype beruht dabei darauf, dass sie Ammoniak in Betriebszuständen des Abgasreini- gungssystems zwischenspeichern können, in denen Ammoniak z.B. durch Überreduk- tion von Stickoxiden über einem anstromseitig verbauten Dreiwegekatalysator entsteht, dieses aber nicht von weiteren herkömmlichen Dreiwegekatalysatoren umgesetzt wer- den kann, beispielsweise wegen des Mangels an Sauerstoff oder ungenügenden Be- triebstemperaturen. Der so gespeicherte Ammoniak kann dann bei verändertem Be- triebszustand des Abgasreinigungssystems ausgespeichert und anschließend oder di- rekt umgesetzt werden, beispielsweise dann, wenn genügend Sauerstoff oder Stickoxide vorhanden sind. Zeolithe und Zeotype sind erfindungsgemäß in der ersten Komponente des Katalysators zur Verminderung der Ammoniak-Emissionen zugegen. Gemäß der Klassifizierung der IZA (https://europe.iza-structure.org/IZA-SC/ftc_table.php), der internationalen Zeo- lithvereinigung, können Zeolithe bzw. Zeotype in unterschiedliche Klassen eingeteilt wer- den. Danach werden Zeolithe z.B. gemäß ihres Kanalsystems und ihrer Gerüststruktur unterteilt. Beispielsweise werden Laumontit und Mordenit den Zeolithen zugeordnet, die über ein eindimensionales System von Kanälen verfügen. Ihre Kanäle haben keine Ver- bindung untereinander. Zeolithe mit zweidimensionalem Kanalsystem zeichnen sich dadurch aus, dass ihre Kanäle untereinander in einer Art schichtförmigem System ver- bunden sind. Eine dritte Gruppe weist eine dreidimensionale Gerüststruktur auf mit schichtübergreifenden Verbindungen der Kanäle untereinander. In der vorliegenden Er- findung kommen vorzugsweise zwei- und/oder dreidimensionale Zeolithe bzw. Zeotype zum Einsatz [Ch. Baerlocher, W.M. Meier and D.H. Olson, Atlas of Zeolite Framework Types, Elsevier, 2001]. Unter dem Begriff „Zeolith“ versteht man erfindungsgemäß poröse Materialien mit einer Gitterstruktur aus eckenverknüpften AlO4- und SiO4-Tetraedern gemäß der allgemeinen Formel (W.M. Meier, Pure & Appl. Chem., Vol.58, No.10, pp.1323-1328, 1986): Mm/z [m AlO2 * n SiO2] * q H2O Die Struktur eines Zeolithen umfasst somit ein aus Tetraedern aufgebautes Netzwerk, das Kanäle und Hohlräume umschließt. Man unterscheidet natürlich vorkommende und synthetisch hergestellte Zeolithe. Unter dem Begriff „Zeotyp“ wird eine zeolithähnliche Verbindung verstanden, die denselben Strukturtyp aufweist, wie eine natürlich vorkom- mende oder eine synthetisch hergestellte Zeolithverbindung, die sich von solchen jedoch dadurch unterscheidet, dass die entsprechende Käfigstruktur nicht ausschließlich aus Aluminium- und Siliziumgerüstatomen aufgebaut ist. In solchen Verbindungen werden die Aluminium- und/oder Siliziumgerüstatome anteilig durch andere drei-, vier- oder fünf- wertige Gerüstatome wie beispielsweise B(III), Ga(III), Ge(IV), Ti(IV) oder P(V) ersetzt. In der Praxis am häufigsten zur Anwendung kommt der Ersatz von Aluminium- und/oder Siliziumgerüstatomen durch Phosphoratome, beispielsweise in den Siliziumaluminiump- hosphaten oder in den Aluminiumphosphaten, die in Zeolithstrukturtypen kristallisieren. Beispiele geeigneter Zeolithe kommen aus der Gruppe der zweidimensionalen oder drei- dimensionalen Zeolithe/Zeotype. Bevorzugt gehören sie den Strukturtypen 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 an. Besonders bevorzugt ist, wenn die Zeolithe bzw. Zeotype in dem erfindungsgemäßen Autoabgaskatalysator ausgewählt sind aus der Gruppe AEI, AFT, AFX, CHA, DDR, ERI, ESV, ETL, FER, KFI, LEV, UFI und den entsprechenden Zeotypen dieser Strukturtypen, wie z.B: SAPO. Es können auch Mischungen derselben vorliegen. Zeolithe bzw. Zeotype können auch hinsichtlich ihrer Porenstruktur klassifiziert werden. Es wird unterschieden hinsichtlich kleinporiger, mittelporiger und großporiger Zeolithe. Von akademischen Interesse sind die extra-großporigen Zeolithe. Kleinporige Zeolithe sind solche mit einer größten Ringgröße von 8 Tetraeder-Einheiten. Großporige Zeolithe besitzen eine oberste Ringgröße von 12 Tetraeder-Einheiten (https://en.wikipedia.org/w/index.php?title=Zeolite&oldid=1103217432 sowie dort zitier- ter Literatur). Als kleinporige Zeolithe oder Zeotype zum Speichern von Ammoniak kom- men ganz besonders bevorzugt CHA, AEI, AFX, BIK, DDR, ERI, LEV oder LTA zum Einsatz. Äußerst bevorzugt ist CHA in diesem Zusammenhang. Großporige Zeolithe oder Zeoltype zum Speichern von Ammoniak sind vorzugsweise ausgewählt aus der Gruppe bestehend aus BEA, FAU oder MOR. Ganz besonders bevorzugt ist BEA in diesem Zusammenhang. Die Alterungsstabilität der verwendeten Zeolithe bzw. Zeotype im Abgasstrang von über- wiegend stöchiometrisch verbrennenden Motoren ist vorliegend besonders im Fokus, da hier gemeinhin höhere Temperaturen als in einem mager verbrennenden Motor vorherr- schen. Insofern sind solche Materialien gewünscht, welche den teils sehr hohen und stark wechselnden hydrothermalen Bedingungen möglichst lange standhalten können. Auf der anderen Seite ist aber auch die Abgaszusammensetzung eine andere verglichen mit Magermotorenabgas. Die Konzentration insbesondere von Kohlenwasserstoffen und Kohlenmonoxid, welche am erfindungsgemäßen Katalysator ankommen, sind zum einen höher als bei Magermotoren und die Zusammensetzung wechselt auch je nach Fahr- weise um den stöchiometrischen Bereich herum (fett/mager-Wechsel). Die hydrother- male Temperaturstabilität von Zeolithen und Zeotypen ist demnach besonders gefragt. Bevorzugt besitzen die verwendeten kleinporigen Zeolithe einen SAR-Wert (silica-to-alu- mina-ratio) bzw. die Zeotype einen diesem Wert entsprechendes Verhältnis von > 15 bis < 45, ganz bevorzugt von > 20 bis < 35. Äußerst bevorzugt ist ein Bereich von 20 – 30 in diesem Zusammenhang. Bevorzugt besitzen die verwendeten großporigen Zeolithe einen SAR-Wert (silica-to-alumina-ratio) bzw. die Zeotype einen diesem Wert entspre- chendes Verhältnis von > 10 bis < 50, ganz bevorzugt von > 10 bis < 40. Äußerst bevor- zugt ist ein Bereich von 10 – 30 in diesem Zusammenhang. Zur Berechnung des SAR- Wertes wird bei Zeolithen die Menge an im Gerüst verbleibenden Siliziumatomen zu den Substitutionsatomen ins Verhältnis gesetzt. Es ergibt sich hieraus die Anzahl der nega- tiven Ladungen im Grundkörper und damit ein Maß für die aufzunehmende Anzahl an Gegenionen, bis Elektroneutralität hergestellt ist. Für Zeotype ist ein entsprechendes Verhältnis bestimmbar. Erfindungsgemäß ist der eingesetzte Zeolith oder Zeotyp mit Übergangsmetallionen io- nenausgetauscht. Letztere sind vorzugsweise ausgewählt aus der Gruppe bestehend aus Eisen und/oder Kupfer. Eisen ist besonders bevorzugt, da er im Vergleich zu Kupfer weniger oxidierend auf Ammoniak wirkt. Diese Verbindungen besitzen die Möglichkeit, im Abgas vorhandene Stickoxide und den eingespeicherten Ammoniak im Mageren zu Stickstoff zu komproportionieren. In diesem Fall wirkt der beschriebene Zeolith bzw. Ze- otyp wie ein Katalysator zur selektiven katalytischen Reduktion (SCR) (siehe WO2008106518A2, WO2017187344A1, US2015290632AA, US2015231617AA, WO2014062949A1, US2015231617AA). Unter SCR-Fähigkeit wird vorliegend die Fä- higkeit verstanden, selektiv NOx und NH3 im mageren Abgas in Stickstoff umzuwandeln. Die vorteilhafter Weise im Katalysator zur Verminderung der Ammoniak-Emissionen vor- kommenden Metalle, wie Eisen und/oder Kupfer sind in der ersten Komponente in einem bestimmten Anteil vorhanden. Dieser liegt bei 0,4 – 10, mehr bevorzugt 0,8 – 7 und ganz bevorzugt 1,5 – 5,0 Gew.-% an der ersten Komponente. Das Eisen- und/oder Kupfer- zu-Aluminiumverhältnis liegt zwischen 0,05 – 0,8, vorzugsweise zwischen 0,2 – 0,5 und ganz bevorzugt zwischen 0,3 – 0,5 für Zeolithe. Für den Zeotypen gilt ein entsprechen- des Verhältnis für die dort zur Verfügung stehenden Austauschplätze. Die Metalle liegen dabei zumindest teilweise in ionenausgetauschter Form in den Zeolithen bzw. Zeotypen vor. Vorzugsweise werden bereits ionenausgetauschte Zeolithe bzw. Zeotype in die erste Komponente eingebracht. Es kann jedoch auch sein, dass die Zeolithe bzw. Zeo- type mit z.B. einem Bindemittel und einer Lösung der Metallionen in einer Flüssigkeit, bevorzugt Wasser, zusammengebracht werden und dann (vorzugsweise sprüh-) ge- trocknet werden. Hier finden sich dann gewisse Anteile der Metalle in Form der Oxide auch auf dem Bindemittel wieder. Beide Vorgehensweisen sind möglich. Eine Herausforderung stellt für kleinporige Zeolithe allerdings der Ioneneintausch dar. Während der Ioneneintausch für großporige Zeolithe im Rahmen des Washcoat-Prozes- ses erfolgen kann, da die Ionen bereitwillig in die großen Zeolithporen eindringen und die loneneintauschpositionen des Materials besetzen, erfordert die Belegung von klein- porigen Zeolithen einen separaten Prozessschritt. Hierbei kann es sich etwa um einen Incipient-Wetness-Prozess oder einen Ioneneintausch-Prozess mit ggf. nachgeschalte- ter Sprühtrocknung handeln. Ein Ansatz, diesen zusätzlichen Schritt und die damit ver- bundenen Kosten und Produktionszeit einzusparen, stellt die Beschichtung mit einer Mi- schung eines kleinporigen Zeolithen, eines großporigen Zeolithen und eines löslichen Salzes der einzutauschenden Ionen (z.B. Fe(NO3)3 x 9 H2O, Cu-Acetat) mit ggf. einem gängigen Binder-System auf ein Substrat dar. Die Aufnahme der gelösten Ionen erfolgt zunächst durch den großporigen Zeolithen. Eine Verteilung der Ionen zwischen groß- und kleinporigem Zeolith stellt sich dann während des Kalzinierungsprozesses ein. Ne- ben der Einsparung eines Prozessschrittes wird erwartet, dass ein auf diese Weise erhaltener Katalysator von der hohen Stabilität der Ammoniak-Speicherfunktion des kleinporigen Zeoliths sowie von der üblicherweise hohen SCR-Aktivität des großporigen ausgetauschten Zeolithen profitiert und somit eine gute Performance und Selektivität in der beschriebenen Anwendung zeigt. Eine weitere bekannte Eigenschaft der ionenaus- getauschten großporigen Zeolithe ist ihre Fähigkeit, Kohlenwasserstoffe einzuspeichern, was dem formulierten Ammoniakspeicher die Eigenschaften einer sogenannten Kohlen- wasserstofffalle verleiht. Während des Kaltstarts können so ankommende Kohlenwas- serstoffe eingefangen werden, welche bei höherer Temperatur dann desorbieren und über den vorhandenen dann aktiven Dreiwegekatalysatoren oder Oxidationskatalysato- ren umgesetzt werden können. Die erste Komponente weist groß- und kleinporige Zeolithe auf. Diese können in ge- trennten Schichten auf dem Substrat abgeschieden vorliegen. Die Reihenfolge der Schichten aus groß- und kleinporigen Zeolithen und die entsprechenden Beladungsmen- gen können variiert werden. Darüber hinaus ist es möglich, die unterschiedlichen Zeo- lithtypen zu mischen und im Rahmen einer homogenen Beschichtung auf den Träger aufzubringen. Das Mengenverhältnis der unterschiedlichen Zeolithtypen zueinander kann variiert werden. Der kleinporige Zeolith kann in übergangsmetallfreier Form sowie belegt mit Übergangsmetallen, bevorzugt Eisen oder Kupfer, eingesetzt werden. Die groß- und kleinporigen Zeolithe werden vorzugsweise im Gewichtsverhältnis 5:1 bis 1:5 eingesetzt, mehr vorzugsweise 2:1 bis 1:2. Ein Abgleich der Fähigkeit der HC-Einspei- cherung und der Ammoniakspeicherfähigkeit kann durch das Verhältnis der großporigen und kleinporigen Zeolithe oder Zeotype in der Mischung oder den Schichten gezielt ein- gestellt und an die vorliegende Applikation angepasst werden. Die erste Komponente kann bevorzugt neben den Zeolithen bzw. Zeotypen weitere nicht katalytisch aktive Komponenten aufweisen, wie z.B. Bindemittel. Als Bindemittel sind beispielsweise nicht oder nur wenig katalytisch aktive temperaturstabile Metalloxide, wie SiO2, Al2O3 und ZrO2, geeignet. Der Fachmann weiß, welche Materialien hier in Frage kommen. Der Anteil solcher Binder in der ersten Beschichtung kann beispielsweise bis zu 15 Gew.-%, vorzugsweise bis zu 10 Gew.-% an der Beschichtung ausmachen. Auch das Bindemittel kann die oben angegebenen Übergangsmetalle, insbesondere Eisen und/oder Kupfer, aufweisen. Bindemittel sind dazu geeignet, ein stärkeres Anhaften der Beschichtung auf einem Träger oder einer weiteren Beschichtung zu gewährleisten. Hierzu ist eine bestimmte Partikelgröße der Metalloxide im Bindemittel vorteilhaft. Diese kann vom Fachmann entsprechend eingestellt werden. Die im Rahmen dieser Erfindung angesprochene Ammoniakspeicherfähigkeit bzw. -ka- pazität wird als Quotient aus gespeicherter Masse Ammoniak pro Liter Katalysatorträ- gervolumen angegeben. Durch die erste Komponente sollte die Ammoniakspeicherfä- higkeit des Abgasreinigungssystems auf mindestens 0,25 g Ammoniak pro L Trägervo- lumen erhöht werden (gemessen im Frischzustand). Insgesamt sollte die Speicherkapa- zität der eingesetzten Ammoniakspeicherkomponenten in Form der Zeolithe oder Zeo- type ausreichen, damit im System zwischen 0,25 und 10,0 g NH3 pro Liter Trägervolu- men, bevorzugt zwischen 0,5 und 8,0 g NH3 pro Liter Trägervolumen und besonders bevorzugt zwischen 0,5 und 5,0 g NH3/Liter Trägervolumen Ammoniak gespeichert wer- den kann (immer bezogen auf den Frischzustand). Die Zeolithe bzw. Zeotype sind in einer ausreichenden Menge im Katalysator zur Verminderung der Ammoniak-Emissio- nen vorhanden. Die Bestimmung der Ammoniakspeicherfähigkeit ist weiter hinten dar- gestellt. Die zweite Komponente besteht aus einem OSC-freien Edelmetallkatalysator und/oder einem OSC-haltigen Edelmetallkatalysator. Unter Edelmetall werden insbesondere die Platingruppenmetalle Platin, Palladium und Rhodium verstanden. Demgemäß sind die Edelmetalle im OSC-freien bzw. OSC-haltigen Edelmetallkatalysator bevorzugt ausge- wählt aus der Gruppe bestehend aus Palladium, Platin, Rhodium. OSC bedeutet Oxygen Storage Component - Sauerstoffspeicherkomponente. Ein OSC-haltiger Edelmetallkata- lysator weist demnach Sauerstoffspeichermaterialien auf. Der OSC-freie Edelmetallkatalysator weist hingegen im Wesentlichen keine den Sauer- stoff im Abgas des Verbrennungsmotors speichernde Funktion auf. Insbesondere weist diese Komponente Sauerstoffspeichermaterialien, insbesondere Cer-Zirkonium- Mischoxide, von weniger als 10 g/L, bevorzugt weniger als 5 g /L und ganz bevorzugt weniger als 2 g/L Trägervolumen. Als Speichermaterial wird die gesamte Menge an z.B. Cer- oder Cer-Zirkonium-Mischoxiden angesehen, samt der vorhandenen Dotierungs- elemente. Entsprechende OSC-freie Edelmetallkatalysatoren haben die Befähigung im schon leicht mageren Abgas eines überwiegend stöchiometrisch betriebenen Verbrennungs- motors oxidativ auf die vorhandenen Stoffe (NH3, HC, CO) zu wirken. Diese Komponente ist dabei bevorzugt so ausgelegt, dass sie bei entsprechend niedrigen Temperaturen schon aktiv wird. Der im Zeolith bzw. Zeotyp eingespeicherte Ammoniak wird hier über diese Komponente bevorzugt in nichtschädlichen Stickstoff umgewandelt. Die Oxidationswirkung sollte nicht zu groß sein, da ansonsten aus der Ammoniakoxidation ein gewisser Anteil an dem starken Treibhausgas N2O gebildet wird. Die Oxidationskraft kann u.a. durch die Menge an Platin und das Pt:Pd- und/oder Pt:Rh-Verhältnis eingestellt werden. Die zweite Komponente in Form eines OSC-freien Edelmetallkatalysators weist dem- nach Materialien auf, die oxidativ auf u.a. Ammoniak wirken. Normalerweise enthält diese Komponente ein temperaturstabiles, hochoberflächiges Metalloxid und mindes- tens ein Edelmetall ausgewählt aus der Gruppe Rhodium, Platin und Palladium. Der Ge- samtedelmetallgehalt dieser Komponente beträgt vorzugsweise von 0,015 – 5 g/L, mehr bevorzugt von 0,035 – 1,8 g/L und besonders bevorzugt von 0,07 – 1,2 g/L Trägervolu- men. Für den Einsatz in dieser oxidativ auf Ammoniak wirkenden Komponente bieten sich insbesondere die Edelmetalle Platin oder Palladium, oder Platin und Palladium zu- sammen an. Dabei kann der Fachmann vorzugsweise wählen, ob er das stark oxidativ wirkende Platin alleine oder ggf. in Verbindung mit Palladium in der zweiten Beschich- tungsschicht einsetzt. Kommt Platin und/oder Palladium zum Einsatz, so sollte sich ers- teres im Bereich von 0,015 – 1,42 g/L, mehr bevorzugt 0,035 – 0,35 g/L Trägervolumen in der Beschichtung befinden. Palladium kann bei Vorhandensein in der Beschichtung zwischen 0,015 – 1,42 g/L, bevorzugt 0,035 – 0,35 g/L Trägervolumen zugegen sein. Das Gewichtsverhältnis von Platin zu Palladium sollte zwischen 1:0 und 1:5, mehr be- vorzugt 1:0 und 1:4 und ganz bevorzugt 1:0 und 1:2 betragen. Die Edelmetalle in der OSC-freien zweiten Komponente sind wie gesagt auf einem oder mehreren temperaturstabilen, hochoberflächigen Metalloxiden als Trägermaterialien fi- xiert. Als Trägermaterialien kommen alle dem Fachmann für diesen Zweck geläufigen Materialien in Betracht. Solche Materialien sind insbesondere Metalloxide mit einer BET- Oberfläche von 30 bis 250 m2/g, bevorzugt von 100 bis 200 m2/g (bestimmt nach DIN 66132 – neueste Fassung am Anmeldetag). Besonders geeignete Trägermaterialien für die Edelmetalle sind ausgewählt aus der Reihe bestehend aus Aluminiumoxid, dotiertes Aluminiumoxid, Siliziumoxid, Titandioxid und Mischoxiden aus einem oder mehreren da- von. Dotierte Aluminiumoxide sind beispielsweise Lanthanoxid-, Zirkoniumoxid-, Bari- umoxid- und/oder Titanoxid-dotierte Aluminiumoxide. Mit Vorteil wird Aluminiumoxid oder Lanthan-stabilisiertes Aluminiumoxid eingesetzt, wobei im letztgenannten Fall Lan- than in Mengen von insbesondere 1 bis 10 Gew.-%, bevorzugt 3 bis 6 Gew.-%, jeweils berechnet als La2O3 und bezogen auf das Gewicht des stabilisierten Aluminiumoxides, verwendet wird. Auch im Fall von mit Bariumoxid dotiertem Aluminiumoxid ist der Anteil an Bariumoxid insbesondere 1 bis 10 Gew.-%, bevorzugt 3 bis 6 Gew.-%, jeweils be- rechnet als BaO und bezogen auf das Gewicht des stabilisierten Aluminiumoxides. Ein weiteres geeignetes Trägermaterial ist Lanthan-stabilisiertes Aluminiumoxid, dessen Oberfläche mit Lanthanoxid, mit Bariumoxid und/oder mit Strontiumoxid beschichtet ist. Diese Komponente umfasst bevorzugt mindestens ein Aluminiumoxid oder dotiertes Alu- miniumoxid. Vorteilhaft ist in diesem Zusammenhang insbesondere La-stabilisiertes ^- Aluminiumoxid mit einer Oberfläche von 100 bis 200 m2/g. Solches aktives Alumini- umoxid ist in der Literatur vielfach beschrieben und am Markt erhältlich. Der Katalysator zur Verminderung der Ammoniak-Emissionen weist alternativ oder ku- mulativ zum OSC-freien Edelmetallkatalysator einen OSC-haltigen Edelmetallkatalysa- tor auf. Hier sind neben den Edelmetallen und den eben genannten temperaturstabilen, hochoberflächigen Metalloxiden auch Sauerstoffspeichermaterialien im Edelmetallkata- lysator vorhanden (OSC-haltig). Als Sauerstoffspeichermaterialien werden durchweg Cer- oder Cer-Zirkonium-Mischoxide (siehe weiter unten) verwendet. Demzufolge zeich- net sich ein OSC-haltiger Edelmetallkatalysator durch das Vorhandensein einer be- stimmten Menge an diesen Sauerstoffspeichermaterialien aus. Insbesondere weist diese Komponente Sauerstoffspeichermaterialien in einer Menge von mehr als 10 g/L bevorzugt mehr als 20 g /L und ganz bevorzugt mehr als 25 g/L Trägervolumen. Hierbei wird das gesamte Cer-Zirkonium-Mischoxid mit all seinen Bestandteilen eingerechnet. Entsprechende OSC-haltige Edelmetallkatalysatoren haben die Befähigung im schon leicht fetten Abgas eines überwiegend stöchiometrisch betriebenen Verbrennungsmo- tors oxidativ auf die vorhandenen Stoffe (NH3, HC, CO) zu wirken. Diese Komponente ist dabei bevorzugt so ausgelegt, dass sie bei entsprechend niedrigen Temperaturen schon aktiv wird. Der im Zeolith bzw. Zeotyp eingespeicherte Ammoniak wird hier über diese Komponente bevorzugt in nichtschädlichen Stickstoff umgewandelt. Die Oxidati- onswirkung sollte nicht zu groß sein, da ansonsten aus der Ammoniakoxidation ein ge- wisser Anteil an dem starken Treibhausgas N2O gebildet wird. Die Edelmetalle im OSC-haltigen Edelmetallkatalysator sind vorzugsweise ausgewählt aus der Gruppe bestehend aus Palladium oder Rhodium oder Platin, Platin und Rho- dium, Palladium und Rhodium bzw. Palladium und Rhodium und Platin zusammen. Be- vorzugt handelt es sich bei diesem Katalysator um eine mit dreiwegekatalytischer Fähig- keit ausgestattete Beschichtung. Diese weist besonders bevorzugt Edelmetalle ausge- wählt aus der Gruppe Platin und Rhodium, Palladium und Rhodium, vorzugsweise Rhodium alleine auf. In dem OSC-haltigen Edelmetallkatalysator können die Edelmetalle nur auf den temperaturstabilen, hochoberflächigen Trägermaterialien abgeschieden vor- liegen. Bevorzugt ist jedoch, wenn die Edelmetalle sowohl auf den genannten Träger- materialien als auch auf den Sauerstoffspeichermaterialien abgeschieden vorliegen. Sofern Rhodium in dieser Komponente vorhanden ist (ob alleine oder in Kombination mit den anderen vorher genannten Edelmetallen), sollte sich dieses vorzugsweise im Be- reich von 0,035 – 1,0 g/L, mehr bevorzugt 0,1 – 0,35 g/L Trägervolumen in der jeweiligen Komponente befinden. Sofern Palladium und/oder Platin ebenfalls in dieser Komponente vorhanden sind, gelten die oben für die OSC-freien Edelmetallkatalysatoren genannten Bereiche für diese Metalle. Geeignete dreiwegekatalytisch aktive Beschichtungen sind beispielsweise in DE102013210270A1, DE102020101876A1, EP3247493A1, EP3727655A1 beschrieben. Moderne Ottomotoren werden unter Bedingungen mit einem diskontinuierlichen Verlauf der Luftzahl ^ betrieben. Sie unterliegen in definierter Weise einem periodischen Wech- sel der Luftzahl ^ und somit einem periodischen Wechsel von oxidierenden und reduzie- renden Abgasbedingungen. Dieser Wechsel der Luftzahl ^ ist in beiden Fällen wesent- lich für das Abgasreinigungsergebnis. Hierzu wird der Lambdawert des Abgases mit sehr kurzer Zyklenzeit (ca.0,5 bis 5 Hertz) und einer Amplitude ^ ^ von 0,005 ^ ^ ^ ^ 0,05 um den Wert ^ = 1 (reduzierende und oxidierende Abgasbestandteile liegen in stöchiomet- rischem Verhältnis zueinander vor) geregelt. Aufgrund der dynamischen Betriebsweise des Motors im Fahrzeug treten zudem Abweichungen von diesem Zustand auf. Damit sich die genannten Abweichungen vom stöchiometrischen Punkt nicht nachteilig auf das Abgasreinigungsergebnis bei Überleiten des Abgases über den Dreiwegekatalysator auswirken, gleichen im Katalysator enthaltene Sauerstoffspeichermaterialien diese Ab- weichungen bis zu einem gewissen Grad aus, indem sie Sauerstoff nach Bedarf aus dem Abgas aufnehmen oder ins Abgas abgeben (Catalytic Air Pollution Control, Com- mercial Technology, R. Heck et al., 1995, S.90). In den OSC-haltigen Edelmetallkatalysatoren (wie in moderne Dreiwegekatalysatoren) befinden sich daher Sauerstoffspeichermaterialien, insbesondere Cer oder Ce/Zr- Mischoxide. Das Masseverhältnis von Ceroxid zu Zirkoniumoxid kann in diesen Mischoxiden in weiten Grenzen variieren. Es beträgt beispielsweise 0,1 bis 1,5, bevor- zugt 0,15 bis 1 oder 0,2 bis 0,9. Bevorzugte Cer/Zirkonium-Mischoxide umfassen ein oder mehrerer Seltenerdmetalloxide und können somit als Cer/Zirkonium/Seltenerdmetall-Mischoxide bezeichnet werden. Der Begriff „Cer/Zirko- nium/Seltenerdmetall-Mischoxid“ im Sinne der vorliegender Erfindung schließt physika- lische Mischungen aus Ceroxid, Zirkoniumoxid und Seltenerdoxid aus. Vielmehr sind „Cer/Zirkonium/Seltenerdmetall-Mischoxide“ durch eine weitgehend homogene, dreidi- mensionale Kristallstruktur gekennzeichnet, die idealerweise frei ist von Phasen aus rei- nem Ceroxid, Zirkoniumoxid bzw. Seltenerdoxid (sogenannte feste Lösung). Je nach Herstellungsverfahren können aber auch nicht vollständig homogene Produkte entste- hen, die in der Regel ohne Nachteil verwendet werden können. Analoges gilt für Cer/Zir- konium-Mischoxide, die kein Seltenerdmetalloxid enthalten. Im Übrigen umfasst der Be- griff Seltenerdmetall bzw. Seltenerdmetalloxid im Sinne vorliegender Erfindung kein Cer bzw. kein Ceroxid. Als Seltenerdmetalloxide in den Cer/Zirkonium/Seltenerdmetall- Mischoxiden kommen beispielsweise Lanthanoxid, Yttriumoxid, Praseodymoxid, Neody- moxid und/oder Samariumoxid in Betracht. Bevorzugt sind Lanthanoxid, Yttriumoxid und/oder Praseodymoxid. Besonders bevorzugt als Seltenerdmetalloxide sind Lan- thanoxid und/oder Yttriumoxid und ganz besonders bevorzugt ist das gemeinsame Vor- liegen von Lanthanoxid und Yttriumoxid, Yttriumoxid und Praseodymoxid, sowie Lan- thanoxid und Praseodymoxid im Cer/Zirkonium/Seltenerdmetall-Mischoxid. In einer be- vorzugten Ausführungsform weist dieser Edelmetallkatalysator zwei unterschiedliche Cer/Zirkonium/Seltenerdmetall-Mischoxide, vorzugsweise eines mit La und Y dotiertes und eines mit La und Pr dotiertes auf. In Ausführungsformen der vorliegenden Erfindung sind die Sauerstoffspeicherkomponenten vorzugsweise frei von Neodymoxid. Der Anteil an Seltenerdmetalloxid(en) in den Cer/Zirkonium/Seltenerdmetall-Mischoxi- den liegt vorteilhaft bei 3 bis 20 Gew.-% bezogen auf das Cer/Zirkonium/Seltenerdme- tall-Mischoxid. Sofern die Cer/Zirkonium/Seltenerdmetall-Mischoxide als Seltenerdme- tall Yttriumoxid enthalten, so ist dessen Anteil bevorzugt 4 bis 15 Gew.-% bezogen auf das Cer/Zirkonium/Seltenerdmetall-Mischoxid. Sofern die Cer/Zirkonium/Seltenerdme- tallmischoxide als Seltenerdmetall Praseodymoxid enthalten, so ist dessen Anteil bevor- zugt 2 bis 10 Gew.-% bezogen auf das Cer/Zirkonium/Seltenerdmetall-Mischoxid. Sofern die Cer/Zirkonium/Seltenerdmetall-Mischoxide als Seltenerdmetall Lanthanoxid und ein weiteres Seltenerdoxid enthalten, wie zum Beispiel Yttriumoxid oder Praseodymoxid, so ist deren Massenverhältnis insbesondere 0,1 bis 1,25, bevorzugt 0,1 bis 1. Üblicherweise enthält dieser Edelmetallkatalysator Sauerstoffspeichermaterialien in Mengen von 15 bis 120 g/l, bezogen auf das Volumen des Trägers bzw. Substrates. Die OSC-haltigen Edelmetallkatalysatoren weisen ebenfalls die für die OSC-freien Edel- metallkatalysatoren genannten temperaturstabilen, hochoberflächigen Trägermateria- lien und zusätzlich zu diesen den Sauerstoff speichernde Materialien auf. Das Masse- verhältnis von temperaturstabilen, hochoberflächigen Trägermaterialien und Sauer- stoffspeicherkomponenten in dieser Komponente beträgt üblicherweise 0,25 bis 1,5, bei- spielsweise 0,3 bis 1,3. In einer beispielhaften Ausführungsform beträgt das Gewichts- verhältnis der Summe der Massen aller Trägermaterialien, wie z.B. Aluminiumoxide (ein- schließlich dotierter Aluminiumoxide) zur Summe der Massen aller Cer/Zirkonium- Mischoxide im OSC-haltigen Edelmetallkatalysator beträgt 10:90 bis 75:25, bevorzugt 20:80 bis 65:35.. Die erste und die zweite Komponente bilden vorzugsweise einen Ammoniakspeicher und eine Funktion zur Oxidation von Ammoniak zu Stickstoff ab (z.B. wie in WO2008106523A2). Sofern zur Oxidation des eingespeicherten Ammoniaks nicht ge- nügend Stickoxide im System vorhanden sein sollten, kann der Ammoniak über der zwei- ten Komponente auch mit vorhandenem Sauerstoff zu Stickstoff umgesetzt werden. In beiden Fällen erfolgt möglichst keine Abgabe von Ammoniak oder N2O an die Umwelt. Im weitesten Sinne können daher die erste Komponente und die zweite Komponente des Katalysator zur Verminderung der Ammoniak-Emissionen bevorzugt aus einer am- moniakspeichernden Beschichtung gepaart mit einer oxidativ auf Ammoniak wirkenden zweiten Beschichtung bestehen. Als solche liegen sie erfindungsgemäß in separaten Beschichtung übereinander auf dem Substrat vor. Besonders bevorzugt ist es, wenn beide Beschichtungen gleich lang sind. Dabei ist es ganz besonders bevorzugt, wenn für eine weiterhin verbesserte Dreiwegeaktivität der OSC-haltige Edelmetallkatalysator der Komponente zwei als Oberschicht über der erste Komponente aus Zeolithen und/oder Zeotypen zur Speicherung von Ammoniak als Unterschicht lokalisiert ist. Äu- ßerst bevorzugt sind keine weiteren Schichten unter oder über diesen zwei Beschich- tungen auf dem Substrat vorhanden. In einer weiteren bevorzugten Ausführungsform hat es sich als vorteilhaft erwiesen, wenn zwischen den beiden eben genannten Schichten/Komponenten eine dünne wei- tere separate Schicht aus inerten temperaturstabilen, hochoberflächigen Metalloxiden vorhanden ist. Der Fachmann orientiert sich an den weiter vorne erwähnten Beschich- tungsmethoden für deren Herstellung. Diese dünne zwischen 5 µm und 200 µm, bevor- zugt zwischen 10 µm und 150 µm hohe Schicht hilft, die Alterungsstabilität des Kataly- sators zur Verminderung der Ammoniak-Emissionen weiter zu steigern. Wie sich herausgestellt hat, kann ein Nachteil der bekannten Systeme zur Minderung der Ammo- niakemissionen der sein, dass die in der ersten Komponente befindlichen Übergangs- metalle wie z.B. Eisen und/oder Kupfer nach längerer Dauer des Gebrauchs im Abgas- strang eines überwiegend stöchiometrisch betriebenen Verbrennungsmotors dazu nei- gen, in die Komponente zur Oxidation von Ammoniak zu diffundieren und diese zu ver- giften. Eine geringere Aktivität der Ammoniak speichernden als auch der oxidativen Komponente ist die Folge. Als Materialien dieser Schicht kommen insbesondere solche ausgewählt aus der Gruppe bestehend aus Aluminiumoxid, Siliziumoxid, Titanoxid, Zir- konoxid, Zeolithe oder Mischungen derselben in Frage. Ganz besonders bevorzugt ist in diesem Zusammenhang eine Schicht aus Aluminiumoxid bzw. Siliziumoxid, die sich vor- zugsweise in gleicher Länge über der Unterschicht und unter der Oberschicht auf dem Substrat befindet. Das vorliegende Abgassystem weist einen ersten Dreiwegekatalysator und einen abstromseitig positionierten Katalysator zur Verminderung der Ammoniak-Emissionen auf. Der erste Dreiwegekatalysator kann dabei die gleichen Bestandteile aufweisen wie der OSC-haltige Edelmetallkatalysator der zweiten Komponente. Bevorzugt ist er wie in DE102013210270A1, DE102020101876A1, EP3247493A1, EP3727655A1, vorzugs- weise wie in EP3247493A1 beschrieben aufgebaut. Abstromseitig bezeichnet die Tat- sache, dass der Abgasfluss zuerst den anstromseitigen Katalysator trifft und anschlie- ßend erst den abstromseitig positionierten. Für anstromseitig gilt das Umgekehrte. Es hat sich im Hinblick auf die Euro 7-Gesetzgebung als vorteilhaft erwiesen, wenn ein Abgassystem für einen überwiegend stöchiometrisch verbrennenden Motor ein Aggregat zum Filtern kleiner Ruß- und Aschepartikel aufweist. Bevorzugt ist demnach ein Abgas- system, dass zusätzlich einen ggf. katalytisch beschichteten GPF zwischen dem ersten Dreiwegekatalysator und dem Katalysator zur Verminderung der Ammoniak-Emissionen aufweist (Fig.6). GPF sind Gasoline Partikel Filter und sind dem Fachmann hinlänglich bekannt (EP3737491A1, EP3601755A1). Besonders bevorzugt ist ein Abgasdesign, bei dem der erste Dreiwegekatalysator und der GPF in motornaher Position verbaut sind. Motornah im Sinne der Erfindung bezeichnet einen Bereich im Abgasstrang, der sich in motornaher Position befinden, also ca.10 – 80 cm, vorzugsweise 20 – 60 cm vom Mo- torausgang entfernt. Es hat sich als vorteilhaft herausgestellt, wenn der Katalysator zur Verminderung der Ammoniak-Emissionen im Unterboden eines Fahrzeugs in Abgasrich- tung an letzter Stelle verbaut ist, so dass danach das Abgas dann an die Umgebungsluft abgegeben wird. Gleichfalls kann das Abgassystem noch weitere Abgasaggregate wie weitere Dreiwegekatalysatoren oder Kohlenwasserstoffspeicher (HC-Traps) oder Stick- oxidspeicher (LNT) aufweisen. Der Unterboden ist der Bereich unterhalb der Fahrerka- bine. In einer weiterhin bevorzugten Ausführungsform befindet sich zwischen dem ersten Drei- wegekatalysator und vor dem Katalysator zur Verminderung der Ammoniak-Emissionen im erfindungsgemäßen Autoabgassystem mindestens ein zweiter Dreiwegekatalysator (TWC). Die Dreiwegeaktivität ist weiter vorne schon beschrieben worden. Es wird explizit auf das dortige Bezug genommen, insbesondere was die Art und Menge der einzelnen Bestandteile anbelangt. Bei diesem Dreiwegekatalysator handelt es sich vorzugsweise um einen wie er im Stand der Technik beschrieben ist (DE102013210270A1, DE102020101876A1, EP3247493A1, EP3727655A1). Bei den TWCs sind gezonte oder gelayerte Ausführungsformen mittlerweile der Normalfall. In einer weiter bevorzugten Ausführungsform besitzt im erfindungsgemäßen Autoabgassystem zumindest einer der zusätzlichen Katalysatoren mit Dreiwegeaktivität einen 2-Schichtaufbau mit zwei unter- schiedlichen Dreiwegebeschichtungen, vorzugsweise wie in EP3247493A1 beschrie- ben. Der eben beschriebene zumindest zweite Dreiwegekatalysator im erfindungsgemä- ßen Abgassystem kann im Unterboden des Fahrzeugs verbaut sein, er kann sich jedoch auch in motornaher Position befinden. Die Fülle an möglichen Euro 7 Systemen ist groß. So können pro Abgasstrang bis zu 4 Dreiwegekatalysatoren vor dem Katalysator zur Verminderung der Ammoniak-Emissionen vorhanden sein. In einer alternativen Ausführungsform befindet sich vor dem Katalysator zur Verminde- rung der Ammoniak-Emissionen mindestens ein Dreiwegekatalysator und ein ggf. kata- lytisch beschichteter Wandflussfilter (GPF). Der Katalysator zur Verminderung der Am- moniak-Emissionen befindet sich dabei im Unterboden bevorzugt an letzter Stelle und in fluider Kommunikation mit dem oder den weiteren Katalysatoren bzw. dem Filter des Autoabgassystems. Bevorzugt weist das Autoabgassystem dabei keine zusätzliche Ein- spritzeinrichtung für Ammoniak oder eine Vorläuferverbindung für Ammoniak auf. Mög- lich ist allerdings, dass sich im Abgasstrang aufstromseitig zum Katalysator zur Vermin- derung der Ammoniak-Emissionen oder aufstromseitig zum Wandflussfilter eine Zuga- beeinheit für Sekundärluft befindet (analog WO2019219816A1). In einem weiteren Aspekt bezieht sich die vorliegende Erfindung auf ein Verfahren zur Verminderung schädlichen Abgasbestandteile von überwiegend stöchiometrisch betriebenen Verbrennungsmotoren, insbesondere fremdgezündeten Benzinmotoren, bei dem das Abgas über ein erfindungsgemäßes Abgassystem geleitet wird. Es sei da- rauf hingewiesen, dass die bevorzugten Ausführungsformen des Autoabgassystems mutatis mutandis auch für das vorliegende Verfahren gelten. Die vorliegende Erfindung richtet sich auf ein Abgasreinigungssystem insbesondere für stöchiometrisch betriebene Verbrennungsmotoren. Es gibt Betriebspunkte eines stöchi- ometrisch verbrennenden Motors, bei dem ein fettes Abgas innerhalb eines bestimmten Temperaturintervalls erzeugt wird. Das kann dazu führen, dass über einem Dreiwege- katalysator ankommende Stickoxide überreduziert werden zu Ammoniak. Dieser Ammo- niak sollte nicht an die Umwelt abgegeben werden. Der Ammoniak wird daher über dem Katalysator zur Verminderung der Ammoniak-Emissionen eingespeichert und anschlie- ßend bei leicht oxidierenden Bedingungen zu Stickstoff oxidiert werden. Dabei muss auch hier darauf geachtet werden, dass möglichst keine Überoxidation zu N2O stattfin- det. Gleichfalls ist der Ammoniakspeicherkatalysator leichter herzustellen, da nur ein vereinfachter Ionenaustauschschritt bei der Herstellung der ersten Komponente von Nö- ten ist. Trotzdem zeigt sich das System selbst nach intensiver Alterung robust genug, um die Euro 7-Anforderungen vollständig zu erfüllen. 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 :
Figure imgf000003_0001
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 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 should be safely adhered to in addition to the traditional ones for CO, HC and NOx. 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 as possible to produce. 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, on the downstream side, a catalytic converter for reducing ammonia emissions, which has the following components: - a first component comprising 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 component has a mixture of small-pore and large-pore zeolites or zeotypes, 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 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 sufficiently robust to meet these requirements over a sufficient period of time become. In addition, it is more advantageous to produce in terms of production technology. In addition, the large-pore zeolites or zeotypes can serve as hydrocarbon traps and thus help to reduce HC emissions during, for example, cold start conditions. 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 as separate coatings one above the other on the substrate. It is preferred if the second component is completely over 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. 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. In this case, a coating that lies above another comes into contact with the exhaust gas first before the latter. 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. Two- and/or three-dimensional zeolites or zeotypes are preferred in the present invention for use [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 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 units. Large-pore zeolites have a top ring size of 12 tetrahedral units (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 zeol types for storing ammonia are preferably selected from the group consisting of BEA, FAU or MOR. BEA is particularly preferred in this context. 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 small-pore zeolites used preferably have a SAR value (silica-to-alumina ratio) or the zeotypes have a ratio corresponding to this value of >15 to <45, very preferably of >20 to <35. Extremely preferred is a Range from 20 – 30 in this context. The large-pore zeolites used preferably have a SAR value (silica-to-alumina ratio) or the zeotypes have a ratio corresponding to this value of >10 to <50, very preferably from >10 to <40. Extremely preferred is a range of 10 – 30 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 connections have the ability to nitrogen oxides present in the exhaust gas and the stored ammonia in the lean state to be proportioned into nitrogen. 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 NOx and NH3 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 0.05 - 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. However, ion exchange represents a challenge for small-pore zeolites. While ion exchange for large-pore zeolites can be carried out as part of the washcoat process, since the ions readily penetrate into the large zeolite pores and occupy the ion exchange positions of the material, the occupation of small-pore zeolites requires porous zeolites require a separate process step. This can be an incipient wetness process or an ion exchange process with, if necessary, subsequent spray drying. One approach to saving this additional step and the associated costs and production time is to coat with a mixture of a small-pore zeolite, a large-pore zeolite and a soluble salt of the ions to be exchanged (e.g. Fe(NO3)3 x 9 H2O, Cu acetate) with, if necessary, a common binder system onto a substrate. The dissolved ions are first absorbed by the large-pore zeolite. A distribution of the ions between large and small pore zeolite then occurs during the calcination process. In addition to saving one process step, it is expected that in this way The resulting catalyst benefits from the high stability of the ammonia storage function of the small-pore zeolite as well as from the usually high SCR activity of the large-pore exchanged zeolite and thus shows good performance and selectivity in the application described. 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 via the then active three-way catalysts or oxidation catalysts. The first component has large and small pore zeolites. These can be deposited in separate layers on the substrate. The order of the layers of large and small pore zeolites and the corresponding loading amounts can be varied. In addition, it is possible to mix the different types of zeolite and apply them to the support as part of a homogeneous coating. The quantitative ratio of the different types of zeolites to one another can be varied. The small-pore zeolite can be used in a transition metal-free form or coated with transition metals, preferably iron or copper. The large and small pore zeolites are preferably used in a weight ratio of 5:1 to 1:5, more preferably 2:1 to 1:2. A balance between the ability to store HC and the ability to store ammonia can be specifically adjusted by the ratio of large-pore and small-pore zeolites or zeotypes in the mixture or layers and adapted to the application at hand. In addition to the zeolites or zeotypes, the first component can preferably have other 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 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 first component should increase the ammonia storage capacity of the exhaust gas purification system to at least 0.25 g of ammonia per L of carrier volume (measured in the fresh state). Overall, the storage capacity of the ammonia storage components used in the form of zeolites or zeotypes should be sufficient to ensure that there is between 0.25 and 10.0 g of NH3 per liter of carrier volume in the system, preferably between 0.5 and 8.0 g of NH3 per liter of carrier volume and particularly preferably between 0.5 and 5.0 g of NH 3 /liter of carrier volume of ammonia can be stored (always based on the fresh state). The zeolites or zeotypes are present in a sufficient amount 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-free precious metal catalyst and/or an OSC-containing noble metal catalyst. 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 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 oxidation power can be adjusted, among other things, by the amount of platinum and the Pt:Pd and/or Pt: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 coating layer. 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 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 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 La2O3 and based on the weight of the stabilized Aluminum oxide is used. Also in the case of aluminum oxide doped with barium oxide, the proportion of barium oxide, 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 (NH 3 , 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 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 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 carrier 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 effect 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 or 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 used as Cerium/zirconium/rare earth metal mixed oxides are referred to. 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 that are not completely homogeneous may not be created, 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. Examples of rare earth metal oxides in the cerium/zirconium/rare earth metal mixed oxides are 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 lanthanum oxide and/or yttrium oxide and very particularly preferred is the joint presence of lanthanum oxide and yttrium oxide, yttrium oxide and praseodymium oxide, as well as lanthanum oxide 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 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 the oxidation of 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, 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, for further improved three-way activity, 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 there is a further thin, separate layer of inert, temperature-stable, high-surface metal oxides between the two layers/components just mentioned. The expert is guided by the coating methods mentioned above for their production. This thin layer, which is between 5 µm and 200 µm, preferably between 10 µm and 150 µm high, helps to further increase the aging stability of the catalyst to reduce ammonia emissions. Like yourself As has been pointed out, a disadvantage of the known systems for reducing ammonia emissions can be that the transition metals in the first component, such as iron and/or copper, become part of the exhaust system of a predominantly stoichiometrically operated internal combustion engine after a long period of use tend to diffuse into the ammonia oxidation component and poison it. The result is a lower activity of the ammonia-storing and oxidative components. 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 in 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, 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 is delivered. 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 one 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 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 WO2019219816A1). In a further aspect, the present invention relates to a method for reducing harmful exhaust gas components from predominantly stoichiometric operated internal combustion engines, in particular spark-ignited gasoline engines, in which the exhaust gas is passed through an exhaust system according to the invention. It should be noted that the preferred embodiments of the automobile 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. The ammonia storage catalyst is also easier to produce, since only a simplified ion exchange step is required in the production of the first component. Nevertheless, even after intensive aging, the system is robust enough to fully meet the Euro 7 requirements.
Figuren: Fig.1: Chart zur Erläuterung der Messung der Ammoniakspeicherfähigkeit. Fig.2: Katalysator zur Verminderung der Ammoniak-Emissionen (1), Beschichtung mit metallfreien Zeolithen oder Zeotypen zur Speicherung von Ammoniak (2) und einer Be- schichtung mit einem OSC-freien Edelmetallkatalysator und/oder einem OSC-haltigen Edelmetallkatalysator (3). Fig.3: Schematische Darstellung der in Unterboden-Position getesteten Katalysatoren. Fig.4: Verringerung der Ammoniak-Emissionen durch die Katalysatoren A und B gegen- über einem System ohne Unterbodenkatalysator. Fig.5: Verringerung der Kohlenwasserstoff-Emissionen durch die Katalysatoren A und B gegenüber einem System ohne Unterbodenkatalysator. Fig.6: Erfindungsgemäßes Abgassystem mit motornahem Dreiwegekatalysator (A), mo- tornahem GPF (B) und folgendem Katalysator zur Verminderung der Ammoniak-Emis- sionen (C). Figures: Fig.1: Chart to explain the measurement of ammonia storage capacity. Fig.2: Catalyst for reducing ammonia emissions (1), coating with metal-free zeolites or zeotypes for storing ammonia (2) and a coating with an OSC-free noble 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: Reduction in ammonia emissions through catalytic converters A and B compared to a system without an underbody catalytic converter. Fig.5: Reduction in hydrocarbon emissions through catalytic converters A and B compared to a system without an underbody catalytic converter. 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).
Beispiele: A. Bestimmung der Ammoniakspeicherkapazität Diese wird experimentell in einem Strömungsrohrreaktor bestimmt. Zur Vermeidung von unerwünschter Ammoniakoxidation am Reaktormaterial wird ein Reaktor aus Quarzglas verwendet. Aus dem Bereich des Katalysators, dessen Ammoniakspeicherkapazität be- stimmt werden soll, wird ein Bohrkern als Prüfling entnommen. Bevorzugt wird ein Bohr- kern mit 1 Zoll Durchmesser und 3 Zoll Länge als Prüfling entnommen. Der Bohrkern wird in den Strömungsrohrreaktor eingesetzt und bei einer Temperatur von 600 °C in einer Gasatmosphäre aus 500 ppm Stickstoffmonoxid, 5 Vol.-% Sauerstoff, 5 Vol.-% Wasser und Rest Stickstoff mit einer Raumgeschwindigkeit von 30000 h-1 für 10 Minuten konditioniert. Anschließend wird in einem Gasgemisch aus 0 Vol.-% Sauerstoff, 5 Vol.-% Wasser und Rest Stickstoff mit einer Raumgeschwindig- keit von 30000 h-1 die Messtemperatur von 200 °C angefahren. Nach Stabilisierung der Temperatur wird die NH3-Speicherphase durch Aufschalten eines Gasgemisches aus 450 ppm Ammoniak, 0 Vol.-% Sauerstoff, 5 Vol.-% Wasser und Rest Stickstoff mit einer Raumgeschwindigkeit von 30000 h-1 eingeleitet. Dieses Gasgemisch bleibt so lange auf- geschaltet, bis abströmseitig vom Prüfling eine stationäre Ammoniakdurchbruchskon- zentration verzeichnet wird. Die auf dem Prüfling eingespeicherte Masse an Ammoniak wird aus der aufgezeichneten Ammoniak-Durchbruchskurve durch Integration vom Start der NH3-Speicherphase bis zum Erreichen der Stationarität unter Einbeziehung der ge- messenen stationären NH3-Durchbruchskonzentration sowie dem bekannten Volumen- fluss berechnet (schraffierte Fläche in der Figur 1). Die Ammoniakspeicherkapazität wird berechnet als Quotient aus der eingespeicherten Masse an Ammoniak, geteilt durch das Volumen des getesteten Bohrkerns. B. Herstellung der ammoniakspeichernden SCR-Schichten B1. Herstellung der eisenhaltigen Beschichtung auf Basis eines großporigen Ze- oliths Die Herstellung der eisenhaltigen Zeolithbeschichtung erfolgte mit Hilfe eines Washcoats bestehend aus einem großporigen Zeolith vom Strukturtyp BEA, Eisen(III)- nitrat-Lösung und einer geeigneten Menge eines Bindersystems bestehend aus einer Al2O3- und einer SiO2-Komponente. Die Beschichtung mit der angestrebten Washcoat- Menge erfolgte in einem Schritt über 100% der Substratlänge. Der so erhaltene be- schichtete Katalysator wurde bei 90 °C getrocknet und anschließend für 15 min bei 350 °C kalziniert und für 2 h bei 550 °C in Luft getempert. Auf den nun beschichteten Träger können gegebenenfalls weitere Schichten als Top-Layer aufgebracht werden. B2. Herstellung der Beschichtung auf Basis eines kleinporigen Zeoliths Eine Beschichtung mit weißem (=übergangsmetallfreiem) Zeolith vom Typ Chabasit er- folgte nach gemeinsamer Mahlung des in Wasser suspendierten Zeolithmaterials mit Nyacol®-AL20-Binder auf einem Cordierit-Substrat mit der gewünschten Washcoat-Be- ladung (88% Zeolith, 12% Binder). Der so erhaltene beschichtete Katalysator wurde bei 90 °C getrocknet, für 15 min bei 350 °C kalziniert und anschließend für 2 h bei 600 °C in Luft getempert. Auf den nun beschichteten Träger können gegebenenfalls weitere Schichten als Top-Layer aufgebracht werden. C. Herstellung der platinhaltigen SiO2/Al2O3-Schicht ohne TWC-Aktivität Ein Silizium-Aluminium-Mischoxid, das aus 95 Gew.-% Aluminiumoxid und 5% Silizi- umoxid besteht, wurde in Wasser suspendiert. Die so erhaltene Suspension wurde nach Einstellung des pH-Wertes auf 7.6 ±0.4 unter ständigem Rühren mit einer EA-Platin- Lösung versetzt. Die resultierende Suspension wurde gemahlen und nach Stabilisierung mit Ammoniumacetat zur Beschichtung eines handelsüblichen Trägers eingesetzt, wo- bei die Beschichtung über 100% der Trägerlänge erfolgte. Die Gesamtbeladung dieses Washcoats auf dem Katalysator betrug 25 g/L, die Edelmetallbeladung 0.106 g/L (3 g/ft3). Der so erhaltene beschichtete Katalysator wurde getrocknet und anschließend kal- ziniert und getempert. Auf den nun beschichteten Träger können gegebenenfalls weitere Schichten als Top-Layer aufgebracht werden. D. Herstellung der edelmetallhaltigen Beschichtungen mit TWC-Aktivität Mit Lanthanoxid stabilisiertes Aluminiumoxid wurde zusammen mit einer Sauerstoffspei- cherkomponente, die 24 Gew.-% Ceroxid, 60 Gew.-% Zirkoniumoxid, 3.5 Gew.-% Lan- thanoxid und 12.5 Gew.-% Yttriumoxid umfasste, und Lanthanacetat als zusätzlicher Lanthanoxidquelle in Wasser suspendiert. Das Gewichtsverhältnis von Aluminiumoxid zu Sauerstoffspeicherkomponente zu zusätzlichem Lanthanoxid betrug 43.6:55.7:0.7. Die so erhaltene Suspension wurde anschließend unter ständigem Rühren mit einer Rhodiumnitrat-Lösung versetzt. Die resultierende Beschichtungssuspension wurde di- rekt zur Beschichtung eines handelsüblichen Substrats eingesetzt, wobei die Beschich- tung über 100% der Substratlänge erfolgte. Die Gesamtbeladung dieses Washcoats auf dem Katalysator kann beispielsweise 122 g/L betragen, die Edelmetallbeladung 0.177 g/L (5 g/ft3). Der so erhaltene beschichtete Katalysator wurde getrocknet und anschlie- ßend kalziniert und getempert. Auf den nun beschichteten Träger kann gegebenenfalls eine edelmetallfreie Schicht als Top-Layer aufgebracht werden. Es wurden Katalysatoren wie in Fig.3 schematisch gezeigt hergestellt. E. Alterung und Testung der ASCs Alterungsbedingungen: Zur Bestimmung der katalytischen Eigenschaften der erfindungsgemäßen Katalysatoren wurden diese zunächst in einer Motorprüfstandsalterung hinter einem motornahen TWC in Unterboden-Position gealtert („Fuel-Cut-Alterung“). Die Alterung besteht aus einer Schubabschaltungsalterung mit 950 °C Abgastemperatur vor dem Eingang des motor- nahen TWC (Maximale Betttemperatur 1030 °C). Die Alterungsdauer und die Einlass- temperatur für den Katalysator in Unterboden-Position sind jeweils individuell für jeden Test angegeben. Testbedingungen: Die unterschiedlichen Katalysatoren wurden in Unterboden-Position an einem hochdy- namischen Motorprüfstand in einem WLTC-Fahrzyklus getestet. Hierbei wurde ein in Serie produzierter Pd/Rh-haltiger TWC im gealterten Zustand in motornaher Position platziert. Der Wert „Verringerung der NH3-Emissionen“ bezieht sich jeweils auf die NH3- Emissionen eines Systems mit einem der gezeigten Katalysatoren in Unterboden-Posi- tion über den gesamten Fahrzyklus im Verhältnis zu den Emissionen des entsprechen- den Systems in Abwesenheit eines Katalysators in Unterboden-Position. F. Ergebnisse Vergleich eines ASCs mit einem großporigen Zeolith mit einem ASC mit einem großporigen in Kombination mit einem kleinporigen Zeolith: Siehe Fig.4 und 5 Alle Katalysatoren mit Oxidationsschicht enthalten 3 g/ft3 Pt. Alterung: Fuel-Cut-Alterung, 19 h, 830 °C Einlasstemperatur für die Katalysatoren in Un- terboden-Position Volumen des Unterboden-Katalysators: 0,83 L Ein Katalysator, in dem eine Beschichtung bestehend aus einem kleinporigen Zeolith in einem geschichteten Design mit einer Beschichtung bestehend aus einem eisenhaltigen großporigen Zeolith und einer Oxidationsschicht mit 3 g/ft3 Pt kombiniert ist, zeigt eine verbesserte katalytische Performance bezüglich NH3-Umsatz im Vergleich zu einem ent- sprechenden Katalysator, der keine Beschichtung mit einem kleinporigen Zeolith enthält. Beide Katalysatoren zeigen zudem die Einspeicherung und den Umsatz von Kohlenwas- serstoffen. 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. Production of the iron-containing coating based on a large-pore zeolite The production of the iron-containing zeolite coating was carried out using a washcoat consisting of a large-pore zeolite of the BEA structural type, iron (III) nitrate solution and a suitable amount of a binder system consisting of a Al2O3 and a SiO2 component. The desired amount of washcoat was coated in one step over 100% of the substrate length. 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, further layers can be applied as a top layer to the now coated carrier. B2. Production of the coating based on a small-pore zeolite A coating with white (=transition metal-free) zeolite of the Chabazite type was carried out after joint grinding of the zeolite material suspended in water with Nyacol ® -AL20 binder on a cordierite substrate with the desired washcoat finish. charge (88% zeolite, 12% binder). The coated catalyst thus obtained was dried at 90 °C, calcined at 350 °C for 15 min and then annealed in air at 600 °C for 2 h. If necessary, further layers can be applied as a top layer to the now coated carrier. C. 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 aluminum oxide and 5% 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 and tempered. If necessary, further layers can be applied as a top layer to the now coated carrier. D. 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 wt.% cerium oxide, 60 wt.% zirconium oxide, 3.5 wt.% lanthanum oxide and 12.5 wt. % 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. The suspension thus obtained was then mixed with constant stirring Rhodium nitrate solution added. 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 can be, for example, 122 g/L, the precious metal loading 0.177 g/L (5 g/ft 3 ). The coated catalyst thus obtained was dried and then calcined and tempered. If necessary, a layer free of precious metals can be applied as a top layer to the now coated carrier. Catalysts were prepared as shown schematically in Figure 3. 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 with a large pore zeolite with an ASC with a large pore in combination with a small pore zeolite: See Figures 4 and 5 All catalysts with an oxidation layer contain 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 A catalyst in which a coating consisting of a small-pore zeolite in a layered design combined with a coating consisting of an iron-containing large-pore zeolite and an oxidation layer with 3 g/ft 3 Pt, shows improved catalytic performance in terms of NH 3 conversion compared to a corresponding catalyst that does not contain a coating with a small-pore zeolite. Both catalysts also show the storage and conversion of hydrocarbons.

Claims

Patentansprüche 1. Abgassystem zur Verminderung von schädlichen Abgasbestandteile von Ver- brennungsmotoren, insbesondere überwiegend stöchiometrisch betriebenen Benzinmotoren, aufweisend einen ersten Dreiwegekatalysator und abstromseitig hierzu einen Katalysator zur Verminderung der Ammoniak-Emissionen, dadurch gekennzeichnet, dass dieser folgende Bestandteile aufweist: - eine erste Komponente mit einem übergangsmetallausgetauschten Zeolithen und/oder Zeotypen zur Speicherung von Ammoniak; - eine zweite Komponente mit einem OSC-freien Edelmetallkatalysator und/oder einem OSC-haltigen Edelmetallkatalysator, wobei die erste Komponente eine Mischung aus kleinporigen und großporigen Zeolithen bzw. Zeotypen aufweist. 2. Abgassystem nach Anspruch 1, dadurch gekennzeichnet, dass die kleinporigen Zeolithe oder Zeotype zur Speicherung von Ammoniak ausge- wählt sind aus der Gruppe bestehend aus CHA, AEI, AFX, BIK, DDR, ERI, LEV oder LTA. 3. Abgassystem nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, dass die großporigen Zeolithe oder Zeotype zur Speicherung von Ammoniak ausge- wählt sind aus der Gruppe bestehend aus BEA, FAU oder MOR. 4. Abgassystem nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, dass als Übergangsmetalle Eisen und/oder Kupfer vorliegen. 5. Abgassystem nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, dass die erste Komponente eine Ammoniakspeicherfähigkeit von zwischen 0,25 und 10,0 g NH3 pro Liter Trägervolumen aufweist. 6. Abgassystem nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, dass die Edelmetalle im OSC-freien bzw. OSC-haltigen Edelmetallkatalysator ausge- wählt sind aus der Gruppe bestehend aus Palladium, Platin, Rhodium. 7. Abgassystem nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, dass im Falle des Vorhandenseins von OSC-haltigen Edelmetallkatalysatoren die Edelmetalle dabei sowohl auf temperaturstabilen, hochoberflächigen Trägerma- terialien als auch auf den Sauerstoffspeichermaterialien abgeschieden vorliegen. 8. Abgassystem nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, dass es zusätzlich einen GPF zwischen dem ersten Dreiwegekatalysator und dem Ka- talysator zur Verminderung der Ammoniak-Emissionen aufweist. 9. Abgassystem nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, dass der erste Dreiwegekatalysator und der GPF in motornaher Position verbaut sind. 10. Abgassystem nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, dass der Katalysator zur Verminderung der Ammoniak-Emissionen im Unterboden ei- nes Fahrzeugs in Abgasrichtung an letzter Stelle verbaut ist. 11. Verfahren zur Verminderung schädlichen Abgasbestandteile von überwiegend stöchiometrisch betriebenen Verbrennungsmotoren, insbesondere fremdgezün- deten Benzinmotoren,‘ dadurch gekennzeichnet, dass das Abgas über ein Abgassystem gemäß einem der vorhergehenden Ansprüche geleitet wird. 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 component having a mixture of small-pore and large-pore zeolites or zeotypes. 2. Exhaust system according to claim 1, characterized in that the small-pore zeolites or zeotypes for storing ammonia are selected from the group consisting of CHA, AEI, AFX, BIK, DDR, ERI, LEV or LTA. 3. Exhaust system according to one of the preceding claims, characterized in that the large-pore zeolites or zeotypes for storing ammonia are selected from the group consisting of BEA, FAU or MOR. 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 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. 6. Exhaust system according to one of the preceding claims, characterized in that The precious metals in the OSC-free or OSC-containing precious metal catalyst are selected from the group consisting of palladium, platinum, rhodium. 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 passed through an exhaust system according to one of the preceding claims.
PCT/EP2023/059084 2022-04-11 2023-04-06 Exhaust gas system for predominantly stoichiometrically operated internal combustion engines, comprising a catalyst for reducing ammonia emissions WO2023198575A1 (en)

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DE102023101768.3A DE102023101768A1 (en) 2022-04-11 2023-01-25 Exhaust system for predominantly stoichiometrically operated internal combustion engines, having a catalytic converter to reduce ammonia emissions
DE102023101772.1A DE102023101772A1 (en) 2022-04-11 2023-01-25 Exhaust system for predominantly stoichiometrically operated internal combustion engines, having a catalytic converter to reduce ammonia emissions
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PCT/EP2023/059082 WO2023198573A1 (en) 2022-04-11 2023-04-06 Exhaust gas system for predominantly stoichiometrically operated internal combustion engines, comprising a catalyst for reducing ammonia emissions
PCT/EP2023/059079 WO2023198570A1 (en) 2022-04-11 2023-04-06 Exhaust gas system for predominantly stoichiometrically operated internal combustion engines, comprising a catalyst for reducing ammonia emissions
PCT/EP2023/059083 WO2023198574A1 (en) 2022-04-11 2023-04-06 Exhaust gas system for predominantly stoichiometrically operated internal combustion engines, comprising a catalyst for reducing ammonia emissions
PCT/EP2023/059087 WO2023198577A1 (en) 2022-04-11 2023-04-06 Exhaust gas system for predominantly stoichiometrically operated internal combustion engines, comprising a catalyst for reducing ammonia emissions
PCT/EP2023/059078 WO2023198569A1 (en) 2022-04-11 2023-04-06 Ammonia-blocking catalyst for stoichiometric internal combustion engines
PCT/EP2023/059080 WO2023198571A1 (en) 2022-04-11 2023-04-06 Exhaust gas system for predominantly stoichiometrically operated internal combustion engines, comprising a catalyst for reducing ammonia emissions
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PCT/EP2023/059083 WO2023198574A1 (en) 2022-04-11 2023-04-06 Exhaust gas system for predominantly stoichiometrically operated internal combustion engines, comprising a catalyst for reducing ammonia emissions
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