Exhaust gas cleaning system for cleaning exhaust gases from gasoline engines

Description

[0001] The present invention relates to the purification of exhaust gases from an internal combustion engine that is predominantly operated with a stoichiometric fuel mixture. The exhaust system has, in particular, four purification functions in a specific sequence. A close-coupled TWC1 (three-way catalytic converter) is followed by a GPF (gasoline particulate filter) and a further TWC2 arranged downstream. The system also includes an ammonia storage function.

[0002] Exhaust gases from combustion engines that run predominantly (>50% of operating time) on a stoichiometric air / fuel mixture, such as gasoline or natural gas-powered spark-ignition engines, are cleaned using conventional methods with the aid of three-way catalytic converters (TWCs). These are capable of simultaneously converting the three main gaseous pollutants of the engine—hydrocarbons, carbon monoxide, and nitrogen oxides—into harmless components. Stoichiometric means that, on average, exactly as much air is available for the combustion of the fuel present in the cylinder as is required for complete combustion. The air-fuel ratio λ (air / fuel ratio) relates the actual mass of air available for combustion, mL,tats, to the stoichiometric mass of air, mL,st. λ = m L ,tats m L ,st

[0003] If λ < 1 (e.g., 0.9), this means "lack of air," and the exhaust mixture is described as rich. If λ > 1 (e.g., 1.1), this means "excess air," and the exhaust mixture is described as lean. The statement λ = 1.1 means that there is 10% more air present than would be necessary for a stoichiometric reaction.

[0004] Three-way catalysts typically use platinum group metals, especially platinum, palladium, and rhodium, as catalytically active materials, often mounted on γ-aluminum oxide as a support. These catalysts also contain oxygen storage materials, such as cerium / zirconium mixed oxides. In the latter, cerium oxide, a rare-earth metal oxide, is the key component for oxygen storage. Besides zirconium oxide and cerium oxide, these materials may contain additional components such as other rare-earth or alkaline-earth metal oxides. Oxygen storage materials are activated by the application of catalytically active materials like platinum group metals and thus also serve as a support for the platinum group metals.

[0005] Such catalytically active materials, along with their components, are applied to flow-through substrates via a coating process. After drying and calcining, the substrates can be installed in the exhaust system. Flow-through substrates are composed of ceramic materials such as silicon carbide, aluminum titanate, and cordierite, and have a long track record of proven effectiveness. They consist of numerous parallel channels formed by porous walls. The channels are open at both ends of the flow-through substrate. The exhaust gas flows from the inlet to the outlet, thereby contacting the catalytically active material applied to the walls.

[0006] In addition to gaseous pollutants, the exhaust gas from such combustion engines also contains very fine particles (PM), which result from the incomplete combustion of the fuel and consist primarily of soot. Unlike the particulate emissions from diesel engines, the particles in the exhaust gas of stoichiometric combustion engines, such as gasoline engines, are very small and have an average particle size of less than 1 µm. Typical particle sizes range from 10 nm to 200 nm. Furthermore, the amount of particles emitted is very low, ranging from 2 mg / km to 4 mg / km.

[0007] In the field of exhaust gas cleaning from lean-burn engines, particularly diesel engines, particulate filters made of ceramic materials such as silicon carbide, aluminum titanate, and cordierite have long proven their worth. These filters consist of numerous parallel channels formed by porous walls. The channels are alternately closed at one end of the filter, creating channels A, which are open on one side and closed on the other, and channels B, which are closed on the first side and open on the other. Exhaust gas flowing into channels A, for example, can only exit the filter via channels B and must therefore pass through the porous walls between channels A and B. As the exhaust gas passes through these walls, the particles are retained, and the exhaust gas is cleaned.These types of units are called wall flow filters.

[0008] The particles thus retained must subsequently be burned off or oxidized to prevent clogging of the filter or an unacceptable increase in the back pressure of the exhaust system. For this purpose, the wall flow filter can, for example, be provided with catalytically active coatings that lower the ignition temperature of soot. It is already known to apply such coatings to the porous walls between the channels (so-called on-wall coating) or to incorporate them into the porous walls (so-called in-wall coating). EP1657410A2 also describes a combination of both coating types, i.e., part of the catalytically active material is located within the porous walls and another part is on the surface of the porous walls.

[0009] The concept of removing particles from exhaust gas using wall flow filters has already been applied to the cleaning of exhaust gas from combustion engines operating with stoichiometric air / fuel mixtures; see, for example, EP2042226A2 (gasoline particle filter; GPF). According to this patent, a wall flow filter has two layers arranged one above the other, one of which can be located within the porous wall and the other on top of the porous wall.

[0010] Since the introduction of the Euro 6c standard in September 2017, European emissions legislation has mandated emissions testing under real-world driving conditions (Real Driving Emissions; RDE). Depending on driving conditions, this can result in significantly higher demands on the catalytic converter, particularly regarding the dynamic conversion of carbon monoxide and nitrogen oxides. Under current emissions standards, pollutants such as hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), and particulate matter must comply with the Euro 6d limit values ​​under RDE conditions for all new vehicles. Furthermore, a certain fleet-wide CO₂ limit must not be exceeded. Future emissions standards in Europe and worldwide will undoubtedly place even more stringent demands on combustion engines and their exhaust aftertreatment systems.

[0011] It can be assumed that simple catalysts and / or filters will no longer be sufficient for such standards. Therefore, more complex exhaust systems will also have to be introduced in the area of ​​predominantly stoichiometrically operated combustion engines. For example, reference can be made to EP3639919A1, which proposes an exhaust system consisting of a first TWC followed by a GPF and another TWC downstream of the GPF. These systems must then ensure that sufficient catalytic activity and filtration efficiency are combined with the lowest possible exhaust backpressure. In particular, the exhaust backpressure of such a system leads to increased fuel consumption, which negatively impacts CO₂ performance. Furthermore, future emissions standards will increasingly focus on secondary emissions such as toxic NH₃ or climate-damaging N₂O.All these aspects should be addressed by a suitable future exhaust system.

[0012] Exhaust gas purification systems for internal combustion engines are described in the following publications: DE 10 2016 112657 A1, WO 2011 / 131324 A1, WO 2020 / 069548 A1, US 10 066 576 B2, US 10 914 217 B2, DE 10 2019 204744 A1 or US 2010 / 107606 A1.

[0013] Therefore, there remains a need for exhaust gas purification systems for combustion engines that predominantly run on stoichiometric fuel mixtures, which are capable of removing all necessary harmful components of the exhaust gas from these engines in the highest possible way, while resulting in the lowest possible increase in fuel consumption.

[0014] These and other problems arising from the prior art for those skilled in the art are solved by an exhaust gas system according to the present invention. By modifying an exhaust gas purification system for cleaning exhaust gases from a predominantly stoichiometrically operated internal combustion engine, comprising an engine-adjacent TWC1 on a flow substrate, a GPF arranged downstream of the TWC1 as a wall flow filter, and a further TWC2 on a flow substrate downstream of the GPF, such that it additionally includes materials for the temporary storage of ammonia, the stated problems are solved in a highly surprising and advantageous manner.

[0015] According to the invention, the term "temporary" means that the material can store ammonia under certain operating conditions of the exhaust system and, among other things, release it again or cease to absorb it. The storage material itself is not altered by this process. For example, at low temperatures, the material can store the ammonia formed in the system and release or convert it again at higher temperatures (see below). The storage material is then available for further ammonia storage. The establishment of a material for storing ammonia in the system described above represents an optimal combination of four functional exhaust gas purification components, enabling compliance with future emission limits and, moreover, offering the possibility of achieving a high degree of pollutant reduction in the system.

[0016] The additional storage function allows ammonia produced by the over-reduction of nitrogen oxides within the system to be captured and stored. This prevents it from being released into the environment. Provided the operating conditions of the exhaust gas purification system permit, the ammonia is desorbed and oxidized to nitrogen, for example, via a thermal toilet.

[0017] The ammonia storage function works by temporarily storing ammonia during operating conditions of the exhaust gas purification system where ammonia is produced but cannot be converted by conventional three-way catalytic converters, for example, due to a lack of oxygen or insufficient operating temperature. The stored ammonia can then be released when the operating conditions of the exhaust gas purification system change and subsequently or directly converted, for example, when sufficient oxygen is present.

[0018] The ammonia storage capacity of the catalyst substrate can be determined according to the procedure described below. The ammonia storage capacity addressed within the scope of this invention is expressed as the quotient of stored mass of ammonia per liter of catalyst substrate volume. The additional material for the temporary storage of ammonia should increase the ammonia storage capacity of the exhaust gas purification system to at least 0.25 g of ammonia per liter of substrate volume (measured in the fresh state). Overall, the storage capacity of the ammonia storage components used should be sufficient to allow the system to store between 0.25 and 10.0 g of NH₃ per liter of substrate volume, preferably between 0.5 and 8.0 g of NH₃ per liter of substrate volume, and particularly preferably between 0.5 and 5.0 g of NH₃ per liter of substrate volume (always based on the fresh state).These materials, which temporarily store ammonia, are present in sufficient quantity in the exhaust system. A preferred quantity has proven to be 50–350 g / L, preferably 75–300 g / L, and most preferably 100–250 g / L of substrate volume. The substrate volume is defined as the volume of the substrate on which the storage material is located.

[0019] Materials familiar to those skilled in the art can be used for the temporary storage of ammonia. Preferably, these are zeolites, such as mordenite (MOR), y-zeolites (FAU), ZSM-5 (MFI), ferrierite (FER), chabazite (CHA), AEI, LEV, and β-zeolites (BEA), as well as zeolite-like materials, such as aluminum phosphates (AIPO) and silicon aluminum phosphate (SAPO), or mixtures thereof (EP0324082A1). Materials that have proven particularly advantageous for the storage of NH3 are named in US2006 / 0010857AA and WO2004076829A1, respectively. ZSM-5 (MFI), chabazite (CHA), Levyn (LEV), ferrierite (FER), SAPO-34, and β-zeolites (BEA) are particularly preferred. CHA, LEV, BEA, and AIPO-34 or SAPO-34 are particularly preferred. CHA-type or BEA-type materials are highly preferred.To ensure improved storage activity, these materials are preferably equipped with transition metals from the group consisting of iron, copper, manganese, and silver. Copper and iron are particularly advantageous in this context. Those skilled in the art know how to equip the zeolites or zeolite-like material with the transition metals (PCT / EP2012 / 061382 and the literature cited therein) to provide good activity in the reduction of nitrogen oxides with ammonia.

[0020] The term "zeolite" refers to porous materials with a lattice structure of corner-sharing AlO₄ and SiO₄ tetrahedra according to the general formula (WM Meier, Pure & Appl. Chem., Vol. 58, No. 10, pp. 1323-1328, 1986): M m / z [m AlO₂ * n SiO₂ ] * q H₂O

[0021] The structure of a zeolite thus comprises a network made up of tetrahedra, enclosing channels and cavities. A distinction is made between naturally occurring and synthetically produced zeolites.

[0022] In this document, the term "zeolite-like compound" refers to a compound that exhibits the same structural type as a naturally occurring or synthetically produced zeolite compound, but differs in that the corresponding cage structure is not composed exclusively of aluminum and silicon framework atoms. In such compounds, the aluminum and / or silicon framework atoms are partially replaced by other trivalent, tetravalent, or pentavalent framework atoms, such as B(III), Ga(III), Ge(IV), Ti(IV), or P(V). In practice, the most common replacement of aluminum and / or silicon framework atoms with phosphorus atoms is found, for example, in silicon aluminum phosphates or in aluminum phosphates that crystallize in zeolite-like structures.Prominent examples are silicon aluminum phosphate SAPO-34, which crystallizes in a chabazite structure, and aluminum phosphate AlPO-34, which crystallizes in a chabazite structure.

[0023] The particular advantage of the use of transition-metal-exchanged zeolites or zeolite-like compounds according to the invention is the possibility that these substances are simultaneously capable of comproproportioning nitrogen oxides present in the exhaust gas and the stored ammonia to nitrogen. In this case, the ammonia storage acts as a catalyst for selective reduction (WO2008106518A2). Furthermore, it can be advantageous if a function for the oxidation of ammonia to nitrogen is added to this SCR catalyst (WO2008106519A1). It is therefore advantageous that the materials for the temporary storage of ammonia also have catalysts for the oxidation of NH3 to N2. As such, it is an ammonia oxidation catalyst (AMOX) or an ammonia barrier catalyst (US5120695; EP1892395A1; EP1882832A2; EP1876331A2; WO12135871A1; US2011271664AA; WO11110919A1).If there are insufficient nitrogen oxides present in the system to oxidize the stored ammonia, the ammonia can also be converted to nitrogen using available oxygen via the AMOX. In both cases – whether using an SCR catalyst or AMOX – no ammonia is released into the environment.

[0024] The material that temporarily stores ammonia can be distributed across one or more existing units within the exhaust gas aftertreatment system. This material can be combined with any existing catalytically active materials, or it can be located separately in zones or layers on one or more substrates. It is therefore advantageous to primarily locate the ammonia-storing material on the TWC2 unit. If ammonia is generated in the TWC1 or GPF units within the system, it can be stored or converted on the downstream TWC2 unit. Since the TWC2 unit is located further away from the engine exhaust, the temperature load is more moderate, which promotes better ammonia storage.

[0025] According to the invention, the material temporarily storing the ammonia is arranged on a separate flow-through substrate. This flow-through substrate (CAT) can be arranged in the exhaust gas purification system according to principles known to those skilled in the art (see Fig. 2 According to the invention, the catalytic converter (CAT) is arranged downstream of the TWC2. The advantages are the same as those mentioned above for locating the corresponding material on the TWC2. Thus, the CAT – like the TWC2 – is preferably located in the underbody of the vehicle. Alternatively, however, the CAT can also be located upstream of the TWC2 and downstream of the TWC1, and upstream or downstream of the GPF. In particular, with this latter combination, it appears to be advantageous if the CAT is arranged downstream of the TWC1 outlet at a distance of 30–250 cm, preferably 40–200 cm, and most preferably 50–180 cm.

[0026] In general, it has proven advantageous if the individual substrates of the exhaust gas purification system according to the invention (TWC1, GPF, TWC2, CAT) occupy a specific volume ratio to one another. For example, it is advantageous if the TWC1 occupies a volume fraction of the total system that is between 20 and 50 vol.%, preferably 30–40 vol.%. The GPF should have a volume fraction of 20–60 vol.%, preferably 25–55 vol.%, relative to the total system. Likewise, the fraction of TWC2 in the total system should be 10–40 vol.%, preferably 15–35 vol.%. The catalytic converter, comprising the material for the temporary storage of nitrogen oxides, preferably 5–30 vol.%, more preferably 10–30 vol.%, of the total volume of the substrates in the exhaust gas purification system. The term "total system" here refers to the sum of the volumes calculated from the external dimensions of the substrates TWC1, GPF, TWC2 and KAT.

[0027] The catalytic converter (CAT) has a relatively high washcoat loading in g / L, ranging from 100 to 500 g / L, preferably 125 to 450 g / L, and most preferably 150 to 400 g / L. In one embodiment, the CAT's structure corresponds to that of an SCR catalyst or, preferably, to that of an AMOX catalyst as described above. The latter possesses an oxidation function to oxidize ammonia in the exhaust gas to nitrogen. For such catalysts, it has proven advantageous to have a zoned and / or layered structure. For example, it has proven beneficial if the CAT has at least a partially lower layer applied to the substrate with an oxidation function for the oxidation of NH₃ to N₂, and if this layer has at least a further layer containing the ammonia-storing material. The ammonia-storing layer can be designed in the form of an SCR catalyst as shown above.Particularly advantageous in this context are zeolites of CHA, BEA, or AEI exchanged with copper or iron transition metals. The oxidation coating essentially comprises the metals Rh, Pt, and / or Pd, which are preferably supported on high-surface-area active aluminum. The active aluminum contains up to 10 wt% of lanthanum and / or SiO₂ as an additive, based on the aluminum oxide. The loading ratio of the temporarily ammonia-storing coating to the oxidation coating should be between 10:1 and 1:10, more preferably 9:1 to 1:9, and most preferably between 8:1 and 1:8. This, along with the choice of noble metals, allows for a good balance between the oxidation effect of the coating and the reducing effect of the temporarily ammonia-storing layer, thus preferably generating nitrogen and preventing over-oxidation of the ammonia to climate-damaging nitrous oxide.

[0028] It can also be advantageous to add oxygen-storing materials to the ammonia-storing materials on the catalytic converter. The latter are the oxygen storage materials typically used in transhydrocarbon converters (TWCs). These provide a more oxygen-rich environment, which is beneficial for the oxidation of ammonia. Suitable materials are described, among others, in EP2007682A1, EP1921044A2, US6468941B1, US6585944B1, and US20050282698A1. Cerium oxides, cerium-zirconium mixed oxides, or cerium or cerium-zirconium mixed oxides doped with La, Y, Pr, or Nd are preferably suitable. The amount of oxygen-storing material can be determined by a person skilled in the art but should not be less than 0.05 g / g, preferably 0.1 g / g, based on the weight of the ammonia-storing material.

[0029] TWC1 and TWC2 are modern three-way catalysts. A person skilled in the art knows which one to use for the present purpose (see, for example, WO2019121994A1, WO2019121995A1, WO9535152A1, WO2008000449A2, EP0885650A2, EP1046423A2, EP1726359A1, EP1541220A1, EP1900416B1, EP3045226A1, WO2009012348A1, and EP1974809B1). Three-way catalysts consist essentially of the components noble metal, a high-surface-area support oxide, and oxygen storage material. The oxygen storage materials are, in particular, those containing mixed cerium / zirconium / rare-earth metal oxides. Suitable rare earth metal oxides include, for example, lanthanum oxide, yttrium oxide, praseodymium oxide, neodymium oxide, samarium oxide, and mixtures of one or more of these metal oxides. Lanthanum oxide, yttrium oxide, neodymium oxide, and mixtures of one or more of these metal oxides are preferred.Particularly preferred are lanthanum oxide, yttrium oxide and a mixture of lanthanum oxide and yttrium oxide in this context.

[0030] For the support oxide for catalytically active metals, those skilled in the art consider highly surface-surface, temperature-stable oxides. These are typically aluminum oxides, silicon oxides, zirconium oxides, or titanium oxides, or mixtures thereof. Active aluminum oxide, in particular, is well known to those skilled in the art in this context. It specifically refers to γ-aluminum oxide with a surface area of ​​100 to 200 m² / g. Active aluminum oxide is widely described in the literature and is commercially available. It generally contains silicon oxide or lanthanum oxide as a stabilizer in an amount of up to 10 wt% based on the aluminum oxide. Three-way catalysts mostly contain platinum group metals, such as Pt, Pd, and Rh, as catalytically active components, with Pd and Rh being particularly preferred. The catalytically active metals are often highly dispersed and deposited on the high-surface-surface oxides and the oxygen storage materials.It is particularly preferred if the precious metals are pre-fixed to the oxygen storage material before this is mixed with the other components in the coating mixture. Zoned or layered embodiments are now the norm for TWCs. In a preferred embodiment, at least the TWC1 has a two-layer structure with two different three-way coatings, preferably as described in EP3247493A1.

[0031] The three-way catalysts are preferably applied to a flow-through substrate by a coating step familiar to those skilled in the art. Flow-through substrates are catalyst carriers commonly used in the prior art, which can consist of metal (corrugated carrier, e.g., WO17153239A1, WO16057285A1, WO15121910A1 and the literature cited therein) or ceramic materials. Refractory ceramics such as cordierite, silicon carbide, or aluminum titanate, etc., are preferably used. The number of channels per area is characterized by the cell density, which is usually between 300 and 900 cells per square inch (cpsi). The wall thickness of the channels is between 0.05 and 0.5 mm for ceramics.

[0032] According to the invention, the TWC1 is preferably installed close to the engine. This means that the distance between the turbocharger and the intake-side end face of the TWC1 is only 1–40 cm. Preferably, the catalyst TWC1 is installed 2–30 cm and, more preferably, 3–20 cm from the turbocharger. The GPF can be installed at some distance from the TWC1, e.g., in the underbody of the vehicle. However, the embodiment in which the GPF is installed quite close to the outlet-side end of the TWC1 is preferred. The distance between the outlet-side end face of the TWC1 and the GPF is preferably 1–60 cm, more preferably 2–50 cm, and most preferably 3–40 cm.

[0033] The TWC2 is either installed directly after the filter in a position close to the engine, or preferably in the vehicle's underbody within the exhaust system. This results in a different temperature load for this TWC compared to the TWC1. Consequently, the two TWCs differ in several characteristics. Preferably, the TWC2 has a lower washcoat loading than the TWC1, particularly by a factor of 1.25 to 4, more preferably 1.5 to 3. Preferably, the TWC1 consists of at least 2 to 4 different three-way catalyst layers or zones, while the TWC2 preferably consists of at least 1 to 2 three-way catalyst layers or zones. The TWCs have a typical washcoat loading, preferably 100 to 400 g / L, more preferably 125 to 375 g / L, and most preferably 150 to 325 g / L.In a further embodiment, it has proven advantageous if the washcoat loading in g / L of TWC1 is greater than that of TWC2, in particular by a factor of between 1.25 and 4, more preferably 1.5 and 3. Furthermore, the precious metal concentration in g / L on TWC1 is preferably greater than on TWC2, in particular by a factor of 1.25 to 20, more preferably 1.5 to 10.

[0034] The GPF (Ground Flow Filter) is in the form of a wall flow filter. All conventional ceramic materials can be used as wall flow filters. Porous wall flow filter substrates made of cordierite, silicon carbide, or aluminum titanate are preferred. These wall flow filter substrates have inlet and outlet channels, with the outlet ends of the inlet channels and the outlet channels being offset from each other and sealed with gas-tight plugs. This forces the exhaust gas to be cleaned, which flows through the filter substrate, to pass through the porous wall between the inlet and outlet channels, resulting in excellent particle filtration. The filtration performance for particles can be tailored by adjusting the porosity, pore / radius distribution, and wall thickness.The porosity of the wall flow filters is generally greater than 40%, typically between 40% and 75%, and particularly between 45% and 70% [measured according to DIN 66133 - latest version on the filing date]. The average pore size (diameter) is at least 3 µm, e.g., between 3 µm and 34 µm, preferably greater than 5 µm, in particular between 5 µm and 28 µm or between 7 µm and 22 µm [measured according to DIN 66134, latest version on the filing date].

[0035] The GPF can be used in the system according to the invention uncoated, with a dry powder coating, or with an optional additional wet-process coating (EP3595796A1, WO2020200394A1, WO2020200397A1). Corresponding wet-process filters with improved filtration efficiency are presented in WO2019121375A1 and WO2020200398A1, respectively. Alternatively or cumulatively, coating with a dry powder is also possible (see, for example, DE102018108346A1, US8277880B2). Such coatings primarily improve the filter's filtration efficiency without excessively increasing the filter's exhaust backpressure. The GPF can also incorporate a catalytic coating. A catalyst material can be applied in the form of a coating suspension onto and / or into the porous walls of the inflow and / or outflow channels.In this case, the system will typically consist of a coating with a three-way catalyst. The composition corresponds to the descriptions given above. Thus, all components in the exhaust gas purification system according to the invention can contain catalytically active precious metals of the origin specified above. In an advantageous embodiment, the GPF has a higher precious metal concentration in g / L substrate volume than the TWC2. If present, the CAT has a lower precious metal concentration in g / L substrate volume than the TWC2. However, depending on the application, both of these embodiments can also occur in reverse. In a particular case, the TWC2 can be designed to contain only rhodium as the precious metal.

[0036] If the GPF is coated, the washcoat loading is 10–200 g / L, preferably 20–175 g / L, and most preferably 25–150 g / L. It is advantageous if the substrate containing the materials for the temporary storage of ammonia (CAT) has a higher washcoat loading in g / L than the GPF.

[0037] Electrically heated catalysts (EHCs) are special catalyst bodies that can be heated by applying an electric current. The advantage of such systems is that they prevent operating conditions of the exhaust aftertreatment system in which the catalysts are not active, or no longer sufficiently active, due to low temperatures. Such operating conditions primarily occur when the vehicle has just been started. By rapidly heating the catalyst substrates electrically, the catalysts on them can be brought up to their operating temperature, thus reducing the emission of unreacted pollutants. In this case, it is particularly advantageous if at least one of the catalyst substrates in the system (TWC1, GPF, TWC2, or CAT) is an electrically heated substrate. It is most preferred if this is TWC1, as it is the first to reach operating temperature due to its position in the system.Therefore, electricity consumption is minimal here, which is highly advantageous, especially for hybrid applications, as it conserves battery power. Corresponding EHC systems in hybrid vehicles are known to those skilled in the art (US8776500BB).

[0038] To further reduce the pollutants emitted by the vehicle during cold-start conditions, a so-called hydrocarbon trap (HCT) can be incorporated into the exhaust gas purification system according to the invention, preferably in the TWC1. The HCT captures the emitted hydrocarbons from the exhaust gas at low temperatures of less than 200°C, preferably less than 250°C (net capture). The HCT can be combined with an oxidation catalyst. Platinum and / or rhodium and / or palladium are suitable for this purpose. Particularly suitable configurations and materials for the HCT include those from US20190351397AA, US20190351398AA, or US20190351393AA. Zeolites such as Beta, replaced with a transition metal such as iron or copper, are most preferably used as the storage material.

[0039] It is also advantageous if the exhaust gas purification system according to the invention includes a functionality for the temporary storage of nitrogen oxides. This prevents the emission of excessive amounts of unreacted nitrogen oxides during the vehicle's so-called cold-start phases. These are then stored and subsequently reduced, when sufficient temperature or reducing agent is present in the exhaust system, either by reductive regeneration (so-called NOx storage catalysts; NSC) or by thermal desorption (passive NOx storage; PNA) and reduction via a three-way catalyst. Regarding the mode of operation and the materials preferably used, reference is made to the relevant literature (WO2020058265A1; EP3695902A1; US2019120109AA; US2018318763AA; US2015266002AA; WO2019134958A1).

[0040] The present invention also relates to a method for cleaning exhaust gases from a predominantly stoichiometrically operated internal combustion engine, in which the exhaust gas is passed through an exhaust gas cleaning system according to one of the preceding claims. The preferred and alternative embodiments named for the system also apply mutatis mutandis to the method.

[0041] With the exhaust system and the presented method according to the present invention, it is possible to comply with the emission limits of future, even stricter emission standards. In addition to standard values ​​such as HC, CO, NOx, and soot, the system according to the invention also allows the reduction of so-called secondary pollutants, such as NH3, N2O, and others. In particular, the close proximity of the TWC1 to the engine enables very high conversion rates for the emission-relevant pollutants CO, HC, and NOx. The additional TWC2 can provide support, if necessary, and especially at operating points with high load and exhaust mass flows, help to ensure the high conversion rates of CO, HC, and NOx. The use of the particulate filter, on the other hand, leads to significant soot removal rates, so that the specified emission limits can be reliably met.It is well known to experts that conventional three-way catalytic converters can produce certain amounts of ammonia under the relevant temperature conditions and engine operating points. However, the use of materials that temporarily store ammonia, for example in a separate catalyst substrate (CAT), ensures that secondary pollutants such as ammonia can be significantly reduced. This makes such a system ideally suited for use in automobiles that will have to comply with future stringent emissions limits for type approval. Figures:

[0042] Fig. 1 : Here, the typical concentration curve of an ammonia absorption measurement is shown. Fig. 2 : Shows an embodiment of a system according to the invention with CAT downstream of TWC2. Fig. 3 : Shows an embodiment of a system according to the invention with CAT between GPF and TWC2. Fig. 4: Shows an embodiment of a system according to the invention with a catalytic converter on the upstream side to the GPF. Fig. 5 : Average bag emissions for THC / NMHC / CO / NOx of the two exhaust aftertreatment systems TWC-GPF-TWC and TWC-GPF-TWC+CAT in comparison. Fig. 6 : Average emissions for NH 3 and N 2 O of the two exhaust aftertreatment systems TWC-GPF-TWC and TWC-GPF-TWC+KAT in comparison. Fig. 7 : Average cumulative modal profiles of emissions for NH 3 and N 2 O of the two exhaust aftertreatment systems TWC-GPF-TWC (black) and TWC-GPF-TWC+KAT (blue) in comparison. Examples: Determination of ammonia storage capacity

[0043] This is determined experimentally in a flow-tube reactor. To prevent unwanted ammonia oxidation of the reactor material, a quartz glass reactor is used. A core sample is taken from the area of ​​the catalyst whose ammonia storage capacity is to be determined. Preferably, a core sample with a diameter of 1 inch and a length of 3 inches is taken. The core sample is inserted into the flow-tube reactor and conditioned for 10 minutes at a temperature of 600 °C in a gas atmosphere consisting of 500 ppm nitric oxide, 5 vol% oxygen, 5 vol% water, and the remainder nitrogen at a space velocity of 30,000 h⁻¹. Subsequently, the measurement temperature of 200 °C is reached in a gas mixture consisting of 0 vol% oxygen, 5 vol% water, and the remainder nitrogen at a space velocity of 30,000 h⁻¹. After the temperature has stabilized, the NH₃ storage phase is initiated by introducing a gas mixture consisting of 450 ppm ammonia, 0 vol% oxygen, 5 vol% water, and the remainder nitrogen at a space velocity of 30,000 h⁻¹.A mixture of 5% oxygen, 5% water, and the remainder nitrogen is introduced at a space velocity of 30,000 h⁻¹. This gas mixture remains connected until a steady-state ammonia breakthrough concentration is recorded downstream of the test specimen. The mass of ammonia stored on the test specimen is calculated from the recorded ammonia breakthrough curve by integrating from the start of the NH₃ storage phase until steady state is reached, taking into account the measured steady-state NH₃ breakthrough concentration and the known volumetric flow rate (hatched area in the diagram). Figure 1 The ammonia storage capacity is calculated as the quotient of the stored mass of ammonia divided by the volume of the tested drill core. Experimental data

[0044] A Euro 6 gasoline vehicle with a 1.5L DI engine was fitted with an artificially end-of-life aged exhaust system consisting of a first close-coupled TWC with a 1.26L catalyst volume (substrate dimensions 118.4mm x 114.3mm) and a conventional three-way coating with 1.77 g / L precious metal (0 / 92 / 8 Pt / Pd / Rh), a downstream uncoated GPF with a 1.39L catalyst volume (substrate dimensions 132.1mm x 101.6mm), and a second TWC located in the underbody with a 1.26L catalyst volume (substrate dimensions 118.4mm x 114.3mm) and a conventional three-way coating with 0.83 g / L precious metal (0 / 80 / 20 Pt / Pd / Rh). The system was tested on a chassis dynamometer. RTS performed an aggressive driving cycle. This system is referred to as the TWC-GPF-TWC reference system and has a total substrate volume of 3.9 L. Emissions of THC, NMHC, CO, NOx, NH3, and N2O were measured; the measurement technique used is known to those skilled in the art.The values ​​shown are the average of several measurements.

[0045] In contrast, a system according to the patent claims mentioned here was presented. For this purpose, the same Euro 6 gasoline vehicle with a 1.5L DI engine was used, equipped with an exhaust system artificially aged to end-of-life. This system consisted of a first engine-mounted TWC with a 1.26L catalyst volume (substrate dimensions 118.4mm x 114.3mm) and a conventional three-way coating with 1.77 g / L precious metal (0 / 92 / 8 Pt / Pd / Rh), a downstream uncoated GPF with a 1.39L catalyst volume (substrate dimensions 132.1mm x 101.6mm), a second TWC located in the underbody with a 0.63L catalyst volume (substrate dimensions 118.4mm x 57.2mm) and a conventional three-way coating with 0.83 g / L precious metal (0 / 80 / 20 Pt / Pd / Rh), and a catalytic converter located downstream of this second TWC. with 0.63L catalyst volume (substrate dimensions 118.4mmx57.2mm) and a coating that can also temporarily store ammonia, with 0.11 g / L precious metal (100 / 0 / 0 Pt / Pd / Rh).The system was tested on a chassis dynamometer using an aggressive RTS driving cycle. This system, designated TWC-GPF-TWC+KAT, has a total substrate volume of 3.9 L. Emissions of THC, NMHC, CO, NOx, NH3, and N2O were measured using methods known to those skilled in the art. The values ​​shown represent the average of several measurements.

[0046] Figure 5 shows comparable performance of the TWC-GPF-TWC+KAT system compared to the TWC-GPF-TWC reference system. Figure 6 The so-called secondary emissions of NH3 and N2O are shown, whereby the TWC-GPF-TWC+CAT system can reduce NH3 emissions by more than half, while N2O emissions only increase slightly. This behavior becomes even clearer in Figure 7, in which the averaged cumulative modal emissions for NH3 and N2O are depicted over the entire driving cycle. This effect is achieved with the same total substrate volume in both systems being compared, i.e., with half the TWC2 of the TWC-GPF-TWC+CAT system compared to that of the TWC-GPF-TWC reference system. Therefore, with the same TWC2 volume in both systems, an even greater advantage for the TWC-GPF-TWC+CAT system can be expected.

Claims

1. Exhaust gas purification system for purifying exhaust gases of a predominantly stoichiometrically operated internal combustion engine having a TWC1 near the engine on a through-flow substrate, a GPF attached downstream of the TWC1 as a wall-flow filter, and a further TWC2 on a through-flow substrate downstream of the GPF, the system also having materials for temporarily storing ammonia, characterized in that the materials for temporarily storing ammonia are arranged on a separate through-flow substrate and the latter is arranged downstream of the TWC2.

2. System according to claim 1, characterized in that this additional material increases the ammonia storage capability to at least 0.25 g of ammonia per L of substrate volume.

3. System according to claim 1 and / or 2, characterized in that the materials for temporarily storing ammonia are present in the system in an amount of 50 - 350 g / L substrate volume.

4. System according to any one of the preceding claims, characterized in that the materials for temporarily storing ammonia have materials selected from the group consisting of zeolites or zeolite-like materials.

5. System according to any one of the preceding claims, characterized in that the materials for temporarily storing ammonia also have catalysts for oxidizing NH3 to form N2.

6. System according to any one of the preceding claims, characterized in that the substrate having the materials for temporarily storing ammonia accounts for a fraction of 5 - 30 vol.% of the total volume of substrates in the exhaust gas purification system.

7. System according to claim 6, characterized in that the substrate having the materials for temporarily storing ammonia has a greater washcoat loading in g / L than the GPF.

8. System according to any one of the preceding claims, characterized in that at least one substrate can be electrically heated.

9. Method for purifying exhaust gases of a predominantly stoichiometrically operated internal combustion engine, in which the exhaust gas is passed through an exhaust gas purification system according to any one of the preceding claims.