EXHAUST GAS PURIFICATION SYSTEM FOR CLEANING EXHAUST GASOLINE ENGINES

Description

[0001] The present invention is directed to the purification of exhaust gases from an internal combustion engine operating predominantly with a stoichiometric fuel mixture. The exhaust system specifically comprises four purification functions in a specific sequence. A close-coupled TWC1 (three-way catalyst) is followed by a GPF (gasoline particulate filter) and another TWC2 arranged downstream. The system also features a hydrocarbon storage function.

[0002] Exhaust gases from internal combustion engines that predominantly (>50% of operating time) run on a stoichiometric air / fuel mixture, such as gasoline or natural gas-powered engines, are conventionally cleaned using three-way catalysts (TWCs). These catalysts are capable of simultaneously converting the engine's three main gaseous pollutants, namely 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 combustion air / fuel ratio λ (air / fuel ratio) relates the air mass m L,tats actually available for combustion to the stoichiometric air mass m L,st : λ = m L ,tats m L ,st

[0003] If λ < 1 (e.g., 0.9), this means "air deficiency," and the exhaust mixture is referred to as rich. λ > 1 (e.g., 1.1) means "excess air," and the exhaust mixture is referred to as lean. The statement λ = 1.1 means that 10% more air is present than required for stoichiometric reaction.

[0004] The catalytically active materials used in three-way catalysts are typically platinum group metals, particularly platinum, palladium, and rhodium, supported, for example, on γ-aluminum oxide. Three-way catalysts also contain oxygen storage materials, such as cerium / zirconium mixed oxides. In the latter case, cerium oxide, a rare earth metal oxide, is the fundamental component for oxygen storage. In addition to zirconium oxide and cerium oxide, these materials can contain additional components such as other rare earth metal oxides or alkaline earth metal oxides. Oxygen storage materials are activated by applying catalytically active materials such as platinum group metals and thus also serve as support materials for the platinum group metals.

[0005] Such catalytically active materials, including their constituents, are applied to flow-through substrates, for example, through a coating process. After drying and calcining the substrates, they can be installed in the exhaust system. Flow-through substrates are made of ceramic materials such as silicon carbide, aluminum titanate, and cordierite and have proven themselves for a long time. They consist of a multitude of parallel channels formed by porous walls. The channels are open at both ends of the flow-through substrate. The exhaust gas thus flows from the inlet to the outlet, contacting the catalytically active material applied to the walls.

[0006] In addition to gaseous pollutants, the exhaust gases from such combustion engines also contain extremely fine particulate matter (PM), which result from the incomplete combustion of the fuel and consist primarily of soot. In contrast to the particulate emissions from diesel engines, the particles in the exhaust gases of stoichiometrically operated combustion engines, such as gasoline engines, are very small, with 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 effective. These are constructed from a large number of parallel channels formed by porous walls. The channels are alternately closed at one of the two ends of the filter, so that channels A are formed which are open on the first side of the filter and closed on the second side of the filter, and channels B are formed which are closed on the first side of the filter and open on the second side of the filter. The exhaust gas flowing into channels A, for example, can only leave the filter again via channels B and to do this must flow through the porous walls between channels A and B. As the exhaust gas passes through the wall, the particles are retained and the exhaust gas is cleaned.Such units are called wall flow filters.

[0008] The particles thus retained must then be burned off or oxidized to prevent clogging of the filter or an unacceptable increase in the backpressure of the exhaust system. For this purpose, the wall-flow filter can, for example, be provided with catalytically active coatings that reduce 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 present in the porous walls and another part is present on the porous walls.

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

[0010] Since the Euro 6c standard came into force in September 2017, European emissions legislation has required exhaust gas measurements under real driving emissions (RDE). Depending on the driving conditions, this can place significantly higher demands on the catalytic converter, particularly with regard to the dynamic conversion of carbon monoxide and nitrogen oxides. The currently applicable emissions standards stipulate that all new vehicles must comply with the Euro 6d limit values ​​for pollutants such as HC, CO, NOx, and particulates under RDE conditions. In addition, a certain CO2 fleet limit should not be exceeded. Future emissions standards in Europe and around the world will certainly place even more stringent demands on combustion engines and their exhaust gas treatment, particularly with regard to secondary emissions such as N2O and NH3.

[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. As an example, reference is made to EP3639919A1, which proposes an exhaust system consisting of a first TWC followed by a GPF and a further 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 has a negative impact on the CO2 balance. In addition, future exhaust gas standards will increasingly focus on secondary emissions such as toxic NH3 or climate-damaging N2O.A corresponding future exhaust system should be able to address all of these aspects. Further exhaust gas purification systems are described in the following publications: EP 3 642 460 B1, DE 10 2019 219115 A1, US 2018 / 230882 A1, DE 10 2013 211387 A1, US 10 323 593 B2, or US 6 729 129 B2.

[0012] There is therefore still a need for exhaust gas purification systems for internal combustion engines predominantly operated with stoichiometric fuel mixtures, which are capable of removing all the necessary harmful components of the exhaust gases of these engines in the highest possible manner and at the same time result in the lowest possible additional fuel consumption.

[0013] These and other objects apparent to the person skilled in the art from the prior art are achieved by an exhaust system according to the present invention. By modifying the exhaust gas purification system for purifying exhaust gases from a predominantly stoichiometrically operated internal combustion engine, comprising a TWC1 close to the engine on a flow-through substrate, a GPF mounted downstream of the TWC1 as a wall-flow filter, and another TWC2 downstream of the GPF on a flow-through substrate, the system is modified to include additional materials for temporarily storing hydrocarbons, achieving an extremely surprising and advantageous solution to the stated objects.

[0014] The term "temporary" is understood in the invention to mean that the material can store hydrocarbons under certain operating conditions of the exhaust system and, under other conditions, release them again or no longer absorb them. The storage material itself is not altered by this. For example, the material can store the hydrocarbons present in the exhaust gas at low temperatures and release or convert them again at higher temperatures (see further below). This makes the storage material available for further storage of hydrocarbons.

[0015] The establishment of a material for storing hydrocarbons in the system described above represents an optimal combination of four functional exhaust gas purification components, allowing compliance with future exhaust gas limits and also opening up the possibility of achieving a high degree of pollutant reduction in the system. This additional storage function allows unconverted hydrocarbons in the exhaust gas to be captured and stored, particularly at low temperatures. They are thus prevented from entering the environment. If the operating conditions of the exhaust gas purification system permit, the hydrocarbons are desorbed again, particularly at higher exhaust gas temperatures, and oxidized, for example, via a TWC to water and carbon dioxide.

[0016] Materials for storing hydrocarbons are also called hydrocarbon traps (HCTs). The preferred HCTs absorb the hydrocarbons emitted from the exhaust gas at low temperatures of less than 300°C, preferably less than 350°C (at this temperature, net storage exceeds the amount released). The HCT can be combined with an oxidation catalyst. The hydrocarbon storage capacity is preferably determined in the fresh state. The hydrocarbon storage capacity of the materials used can be determined according to the procedure described in the cited literature. The hydrocarbon storage capacity or capacity addressed in this invention is given as the quotient of the stored mass of hydrocarbons per liter of substrate volume.Tests for determining hydrocarbon storage capacity are well known to those skilled in the art and are described in the literature, for example in Lupescu et al., "A New Catalyzed HC Trap Technology that Enhances the Conversion of Gasoline Fuel Cold-Start Emissions," SAE Int. J. Fuels Lubr 11(4): 411-425, 2018.

[0017] The additional material for temporarily storing hydrocarbons should increase the hydrocarbon storage capacity of the exhaust gas purification system to at least 0.02 g of hydrocarbons per liter of substrate volume (measured in the fresh state). Overall, the storage capacity of the hydrocarbon storage components used should be sufficient to allow the system to store between 0.02 and 7.0 g of hydrocarbons per liter of substrate volume, preferably between 0.05 and 6.0 g of hydrocarbons per liter of substrate volume, and particularly preferably between 0.08 and 5.0 g of hydrocarbons per liter of substrate volume (always based on the fresh state and the substrate on which the materials are located). These materials, which temporarily store hydrocarbons, are present in sufficient quantities in the exhaust gas system.A preferred amount 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 the volume of the substrate on which the storage material is located.

[0018] Materials familiar to the person skilled in the art can be used for the temporary storage of hydrocarbons. These are preferably zeolites, such as mordenite (MOR), Y-zeolite (FAU), ZSM-5 (MFI), ferrierite (FER), chabazite (CHA), AEI, LEV, and β-zeolite (BEA), as well as zeolite-like materials, such as aluminum phosphate (AIPO) and silicon aluminum phosphate (SAPO), or mixtures thereof (EP0324082A1). Materials that have proven particularly suitable for use in the storage of NH3 are named in US2006 / 0010857AA and WO2004076829A1. ZSM-5 (MFI), chabazite (CHA), ferrierite (FER), SAPO-34, and β-zeolite (BEA) are particularly preferred. CHA, BEA, and AIPO-34 or SAPO-34 are particularly preferred. BEA-type materials are most preferred.To ensure improved storage activity, these materials are preferably coated with one or more metals from the group consisting of iron, copper, cesium, cerium, lanthanum, nickel, manganese, silver, and palladium. Iron is particularly advantageous in this context. Those skilled in the art know how to coat the zeolites or zeolite-like materials with the transition metals (PCT / EP2012 / 061382 and the literature cited therein) to ensure good activity for the temporary storage of hydrocarbons.

[0019] The term "zeolite" refers to porous materials with a lattice structure of corner-sharing AlO 4 and SiO 4 tetrahedra according to the general formula (WM Meier, Pure & Appl. Chem., Vol. 58, No. 10, pp. 1323-1328, 1986): M m / z [m AlO 2 * n SiO 2] * q H 2 O

[0020] The structure of a zeolite thus comprises a network of tetrahedra enclosing channels and cavities. A distinction is made between naturally occurring and synthetically produced zeolites.

[0021] In this document, the term "zeolite-like compound" refers to a compound that has the same structural type as a naturally occurring or synthetically produced zeolite compound, but differs from such compounds 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 used, for example, in silicon aluminum phosphates or in aluminum phosphates that crystallize in zeolite structural types.Prominent examples are the silicon aluminum phosphate SAPO-34, which crystallizes in the chabazite structure, and the aluminum phosphate AIPO-34, which crystallizes in the chabazite structure.

[0022] The particular advantage of the inventive use of transition metal-exchanged zeolites or zeolite-like compounds is the possibility that these substances, as already mentioned above, store the unburned hydrocarbons at lower temperatures, e.g., in the so-called cold-start mode, and then only release them again at elevated temperatures. At these temperatures, catalysts for oxidizing hydrocarbons are heated to such an extent that they can oxidize the desorbed hydrocarbons to water and carbon dioxide. Particularly when using the materials for temporarily storing hydrocarbons on a separate flow-through substrate, it is advantageous if these materials also contain catalysts for oxidizing hydrocarbons to H2O and CO2. Reference is made to what is said below on this point.

[0023] The material temporarily storing the hydrocarbons can be distributed on one or more of the existing units in the exhaust gas purification system. In relation to any catalytically active materials already present, the material can be associated with them or separated from them in zones or layers on one or more substrates. As such, it is advisable to locate the material temporarily storing the hydrocarbons predominantly on the TWC2. If, under certain operating conditions, hydrocarbons are present in increased quantities downstream of the TWC1, they can be stored or converted accordingly on the downstream TWC2. Since the TWC2 is further away from the engine exhaust, the temperature load is more moderate here, which promotes better storage.Once the TWC2, preferably located in the underbody, has warmed up to operating temperature, the stored hydrocarbons are desorbed and converted into CO2 and water above the TWC2. This also minimizes thermal damage to the material temporarily storing the hydrocarbons.

[0024] Alternatively, but preferably, the material temporarily storing the hydrocarbons can also be arranged on a separate flow-through substrate. This flow-through substrate (CAT) can be arranged in the exhaust gas purification system according to aspects known to those skilled in the art (see Fig. 2 - 4). An arrangement in which the catalytic converter is arranged downstream of the TWC2 is preferred. The advantages are the same as those mentioned above for the location of the corresponding material on the TWC2. Thus, the catalytic converter - like the TWC2 - is preferably located in the underbody of the vehicle. According to the invention, the catalytic converter is located downstream of the TWC1 and behind the GPF. In particular, with this latter combination, it appears to be advantageous if the catalytic converter is arranged at a distance of 30 - 250 cm, preferably 40 - 200 cm and very preferably 50 - 180 cm downstream of the outlet of the TWC1. Very particular preference is given to locating the catalytic converter in the underbody of the vehicle.

[0025] In general, it has proven advantageous if the individual substrates of the exhaust gas purification system according to the invention (TWC1, GPF, TWC2, KAT) are in a specific ratio to one another with regard to the size of the volume they occupy. It is advantageous if TWC1 occupies a proportion of the volume of the overall system that lies between 20 and 50 vol.%, preferably 30-40 vol.%. The GPF should have a volume proportion of 20-60 vol.%, preferably 25-55 vol.%, based on the overall system. Likewise, the proportion of TWC2 in the overall system should be 10-40 vol.%, preferably 15-35 vol. The KAT comprising the material for temporarily storing nitrogen oxides has a proportion of preferably 5-30 vol.%, more preferably 10-30 vol.%, of the total volume of the substrates in the exhaust gas purification system. The total system is understood here as the sum of the volumes of the substrates of TWC1, GPF, TWC2 and KAT.

[0026] The catalytic converter has a relatively high washcoat loading in g / L, ranging from 100 - 500 g / L, preferably 125 - 450 g / L, and most preferably 150 - 400 g / L. In one embodiment, the structure of the catalytic converter corresponds to that of an oxidation catalyst. The latter has an oxidation function in order to be able to oxidize hydrocarbons in the exhaust gas. With such catalysts, it has proven advantageous if they are present in a zoned and / or layered structure. For example, the catalytic converter can have an upper layer applied to the substrate with an oxidation function for the oxidation of hydrocarbons and, at least partially beneath this layer, another layer containing the material that temporarily stores hydrocarbons. Alternatively, the catalytic converter can have a lower layer with an oxidation function, over which the material that temporarily stores the hydrocarbons is applied in an upper layer.In a further alternative embodiment, the temporary storage function and the oxidation function can also be present entirely or partially combined in the same layer and optionally have an additional layer with an oxidation function. Another possibility is to apply the temporary hydrocarbon storage function and the oxidation function to different substrates. Regarding the design and materials for the HCT, those from US20190351397AA, US20190351398AA, or US20190351393AA are particularly suitable.

[0027] The material temporarily storing the hydrocarbons can be designed as described above. Zeolites of CHA, BEA or AEI which have been transition metal-exchanged with copper or iron and / or palladium have proven particularly preferred here. BEA exchanged with iron or iron and palladium is particularly preferred. The oxidation function is essentially provided by the metals Rh, Pt and / or Pd, preferably Pt and / or Pd or Rh or Rh and Pd, which in one embodiment are supported on high-surface-area active aluminum. Active aluminum contains up to 10 wt.%, based on the aluminum oxide, of lanthanum and / or SiO2 as an admixture. In an alternative but preferred embodiment, the oxidation function is provided by Rh and / or Pd and / or Pt, which are supported on a mixture of high-surface-area aluminum oxide and common oxygen storage materials such as cerium oxides, cerium-zirconium mixed oxides or cerium or Pt doped with La, Y, Pr, Nd.Cerium-zirconium mixed oxides are supported. Through the design of the catalytic converter, the application, and the selection of the precious metals, the oxidative effect of the oxidative coating can be well balanced to exert a sufficient oxidative effect on the hydrocarbons.

[0028] It can also be advantageous if the materials on the catalytic converter that temporarily store hydrocarbons are also added to materials that are capable of storing oxygen. The latter are the oxygen storage materials typically used in TWCs. These provide an oxygen-rich environment, which is beneficial for the oxidation of hydrocarbons. 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 preferred. The amount of oxygen-storing material can be determined by a person skilled in the art, but should not be less than 0.1 g / g, preferably 0.20 g / g, based on the weight of the material that temporarily stores hydrocarbons.

[0029] TWC1 and TWC2 are modern three-way catalysts. Those skilled in the art will know which ones 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 essentially consist of the components noble metal, a high-surface-area support oxide, and oxygen storage material. The oxygen storage materials are particularly those containing cerium / zirconium / rare earth metal mixed oxides. Examples of suitable rare earth metal oxides include 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 very particularly preferred is a mixture of lanthanum oxide and yttrium oxide in this context.

[0030] High-surface-area, temperature-stable oxides are considered by those skilled in the art as support oxides for catalytically active metals. These are generally aluminum oxides, silicon oxides, zirconium oxides, or titanium oxides, or mixtures thereof. Active aluminum oxide, in particular, is known to those skilled in the art in this context. It refers, in particular, 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.

[0031] Three-way catalysts usually 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 deposited in highly dispersed form on the high-surface-area oxides and the oxygen storage materials. It is particularly preferred if the precious metals are pre-fixed to the oxygen storage material before it is mixed with the other components in the coating mixture. Zoned or layered designs 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.

[0032] 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 supports commonly used in the art, which can be made of metal (corrugated carriers, e.g., WO17153239A1, WO16057285A1, WO15121910A1 and literature cited therein) or ceramic materials. Refractory ceramics such as cordierite, silicon carbide, or aluminum titanate, etc., are preferred. The number of channels per unit area is characterized by the cell density, which is typically between 300 and 900 cells per square inch (cpsi). The wall thickness of the channel walls for ceramics is between 0.5 and 0.05 mm.

[0033] According to the invention, the TWC1 is preferably installed close to the engine. This means that there is a distance of only 1 - 40 cm between the turbocharger and the inlet-side end face of the TWC1. Preferably, the TWC1 catalytic converter is installed 2 - 30 cm and very 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 preferred embodiment is one in which the GPF is installed quite close to the outlet end of the TWC1. Preferably, the distance between the outlet-side end face of the TWC1 and the GPF is 1 - 60 cm, more preferably 2 - 50 cm and very preferably 3 - 40 cm. The TWC2 is either installed directly after the filter in a position close to the engine, or is preferably used in the underbody of the vehicle in the exhaust system. As a result, the temperature load of this TWC is different from that of the TWC1. For this reason, the two TWCs differ from each other in some characteristics.TWC2 preferably has a lower washcoat loading than TWC1. TWC1 preferably consists of at least 2-4 different three-way catalyst layers or zones, while TWC2 preferably consists of at least 1-2 three-way catalyst layers or zones. 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 between 1.25-4, more preferably 1.5-3. In addition, the precious metal concentration on TWC1 is preferably greater than on TWC2, in particular by a factor of 1.25-20, more preferably 1.5-10. The TWCs have a conventional washcoat loading. This is preferably 100-400 g / L, more preferably 125-375 g / L, and very preferably 150-325 g / L.

[0034] The GPF is in the form of a wall-flow filter. Any ceramic material commonly used in the state of the art can be used as a wall-flow filter. 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 downstream ends of the inlet channels and the upstream ends of the outlet channels sealed with gas-tight plugs, offset from each other. 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 an excellent particle filtration effect. The filtration properties for particles can be tailored to the porosity, pore / radius distribution, and wall thickness.The porosity of the wall-flow filters is typically more than 40%, generally from 40% to 75%, especially from 45% to 70% [measured according to DIN 66133 - latest version on the filing date]. The average pore size (diameter) is at least 3 µm, e.g., from 3 µm to 34 µm, preferably more than 5 µm, in particular from 5 µm to 28 µm or from 7 µm to 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 optionally with an additional, wet-processed coating (EP3595796A1, WO2020200394A1, WO2020200397A1). Corresponding wet-processed filters with improved filtration efficiency are presented in WO2019121375A1, WO2020200398A1, and PCT / EP2019 / 057995. Alternatively or cumulatively, coating with a dry powder is also considered (see, for example, DE102018108346A1, US8277880B2). Such coatings primarily improve the filtration efficiency of the filter without excessively impairing the exhaust backpressure of the filter. However, the GPF can also accommodate 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 inlet and / or outlet channels.In this case, this will generally be a coating with a three-way catalyst. The composition corresponds to the above-mentioned explanations. Thus, all components in the exhaust gas purification system according to the invention can contain catalytically active precious metals of the above-mentioned origin. In an advantageous embodiment, the GPF has a higher precious metal concentration in g / L substrate volume than the TWC2. If present, it is advantageous if the catalytic converter has a higher precious metal concentration in g / L substrate volume than the TWC2. However, both of these embodiments can also occur reciprocally depending on the application. In a special case, the TWC2 can be designed such that it only has 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 temporary ammonia storage (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 a power source. The advantage of such systems is that they avoid operating conditions of the exhaust gas purification system in which the catalysts are not active or are no longer sufficiently active due to low temperatures. Such operating conditions primarily occur when the vehicle has only recently been started. By rapidly electrically heating the catalyst substrates, the catalysts located thereon can be brought to conversion temperature, thus reducing the emission of unreacted pollutants. In this case, it is particularly advantageous if at least one of the catalyst substrates present in the system (TWC1, GPF, TWC2, or KAT) is an electrically heatable substrate. This is particularly preferred if this is TWC1. Due to its position in the system, this is the first to reach operating temperature.Therefore, the power consumption is lowest here, which is particularly advantageous for hybrid applications, as it can conserve battery life. 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, a so-called ammonia storage catalyst can be located in the exhaust gas purification system according to the invention, preferably upstream, downstream, or in the TWC2 in the direction of exhaust flow. This can help reduce any ammonia formed in the system before it is released into the environment. The ammonia is temporarily stored and can be oxidized with either available oxygen or nitrogen oxide. This produces nitrogen and water. Materials familiar to those skilled in the art can be used for the temporary storage of ammonia. Preferably, these are zeolites such as mordenites (MOR), Y-zeolites (FAU), ZSM-5 (MFI), ferrierites (FER), chabazites (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 suitable for use in storing NH 3 are named in US2006 / 0010857AA and WO2004076829A1. ZSM-5 (MFI), chabazite (CHA), ferrierite (FER), SAPO-34, and β-zeolite (BEA) are particularly preferred. CHA, BEA, and AIPO-34 or SAPO-34 are very particularly preferred. CHA-type materials, and in this case SSZ-13, are extremely preferred. To ensure improved storage activity, these materials are preferably provided with transition metals from the group consisting of iron, copper, manganese, and silver. Copper is particularly advantageous in this context. The person skilled in the art knows how to add the transition metals to the zeolites or zeolite-like material (PCT / EP2012 / 061382 and the literature cited therein) in order to provide good activity towards the reduction of nitrogen oxides with ammonia.

[0039] The ammonia storage capacity of the substrate can be determined according to the following procedure. The ammonia storage capacity referred to in this invention is given as the quotient of stored mass of ammonia per liter of catalyst volume. It is determined experimentally in a flow tube reactor. To avoid undesirable ammonia oxidation on the reactor material, a quartz glass reactor is used. A core sample is taken as a test specimen from the area of ​​the catalyst whose ammonia storage capacity is to be determined. A core sample with a diameter of 1 inch and a length of 3 inches is preferably taken as the test specimen. 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 nitrogen monoxide, 5 vol% oxygen, 5 vol% water, and the remainder nitrogen with a space velocity of 30,000 h -1<.The test is then carried out in a gas mixture of 0 vol.% oxygen, 5 vol.% water, and the remainder nitrogen at a space velocity of 30,000 h -1< to a measuring temperature of 200 °C. After the temperature has stabilized, the NH 3 storage phase is initiated by applying a gas mixture of 450 ppm ammonia, 0 vol.% oxygen, 5 vol.% water, and the remainder nitrogen at a space velocity of 30,000 h -1<. This gas mixture remains applied until a steady-state ammonia breakthrough concentration is recorded downstream of the test specimen. The NH 3 concentration measured after the catalyst at steady-state may also be below the metered NH 3 concentration.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 the . Fig. 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 additional material for storing 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. Overall, the storage capacity of the ammonia storage components used should be sufficient to allow the system to store 0.25 to 3.5 g of NH 3 per liter of substrate volume, preferably between 0.5 and 2.2 g of NH 3 per liter of substrate volume, and most preferably between 0.5 and 2.0 g of NH 3 per liter of substrate volume.

[0040] The material for temporarily storing ammonia can also be capable of comparably converting nitrogen oxides present in the exhaust gas and the stored ammonia into nitrogen. In this case, the ammonia storage device acts like a catalyst for selective catalytic reduction (SCR; WO2008106518A2). Furthermore, it can be advantageous if this SCR catalyst is also added a function for oxidizing ammonia to nitrogen (WO2008106519A1). It is therefore advantageous for the materials for temporarily storing ammonia to also contain catalysts for oxidizing NH3 to N2. As such, the present case is an ammonia oxidation catalyst (AMOX) or an ammonia barrier catalyst (US5120695; EP1892395A1; EP1882832A2; EP1876331A2; WO12135871A1; US2011271664AA; WO11110919 A1).If there aren't enough nitrogen oxides in the system to oxidize the stored ammonia, the ammonia can also be converted to nitrogen with available oxygen via the AMOX. In both cases—as an SCR catalyst or AMOX—no ammonia is released into the environment.

[0041] It is also advantageous if the exhaust gas purification system according to the invention includes a function for temporarily storing nitrogen oxides. This prevents excessive amounts of unreacted nitrogen oxides from being emitted during so-called cold-running phases of the vehicle. These are then stored and later, when sufficient temperature and reducing agent are present in the exhaust system, reduced 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 preferred materials, reference is made to the relevant literature (WO2020058265A1; EP3695902A1; US2019120109AA; US2018318763AA; US2015266002AA; WO2019134958A1).

[0042] The present invention also relates to a method for purifying exhaust gases from a predominantly stoichiometrically operated internal combustion engine, in which the exhaust gas is passed through an exhaust gas purification system according to one of the preceding claims. The preferred and alternative embodiments mentioned for the system also apply mutatis mutandis to the method.

[0043] With the exhaust system and the method presented according to the present invention, it is possible to comply with the exhaust gas limits of future, even stricter exhaust gas standards. In addition to the standard values ​​such as HC, CO, NOx and soot, the system according to the invention, at least in its advantageous embodiments, also allows the reduction of so-called secondary pollutants, such as NH 3 , N 2 O and others. In particular, the close-coupled arrangement of the TWC1 enables very high conversion rates for the emission-relevant pollutants CO, HC and NOx. The additional TWC2 can have a supporting effect if necessary and help to ensure the high conversion rates of CO, HC and NOx, especially at operating points with high load and exhaust gas mass flows. The use of the particulate filter, on the other hand, leads to significant soot separation rates, so that the given emission limits can be reliably complied with.It is well known to those skilled in the art that conventional three-way catalysts cannot completely oxidize certain amounts of hydrocarbons under the corresponding temperature regimes and engine operating points. Finally, the use of catalytic converters ensures that hydrocarbons formed, especially during cold starts, can be significantly reduced. This makes such a system ideal for use in automobiles that will have to comply with future strict emission limits for approval. Fig. 1 : The typical concentration curve of an ammonia absorption measurement is shown here. Fig. 2 : Shows an embodiment of a system according to the invention with catalytic converter downstream of the TWC2. Fig. 3 : Shows an embodiment of a system according to the invention with KAT between GPF and TWC2. Fig. 4 : Shows an embodiment of a system according to the invention with catalytic converter upstream of 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+KAT in comparison. Fig. 6 : Averaged cumulative modal curves of HC, CO and NOx during the first 200s of the driving cycle. Examples: Experimental data

[0044] A Euro 6 gasoline vehicle with a 1.5L DI engine was tested with an artificially aged exhaust system to end-of-life consisting of a first close-coupled TWC with 1.26L catalyst volume (substrate dimensions 118.4mmx114.3mm) and a conventional three-way coating with 1.77 g / L precious metal (0 / 92 / 8 Pt / Pd / Rh), a downstream, uncoated GPF with 1.39L catalyst volume (substrate dimensions 132.1mmx101.6mm) and a second TWC located in the underbody with 1.26L catalyst volume (substrate dimensions 118.4mmx114.3mm) and a conventional three-way coating with 0.83 g / L precious metal (0 / 80 / 20 Pt / Pd / Rh) and on a The chassis dynamometer was driven in an aggressive RTS driving cycle. This system is referred to as the TWC-GPF-TWC reference system and has a total substrate volume of 3.9L. Emissions of THC, NMHC, CO, NOx, NH3, and N2O were measured; the measurement technology used is familiar to those skilled in the art.The mean value of several measurements is shown.

[0045] This was contrasted with a system according to the patent claims mentioned here.For this purpose, the same Euro 6 petrol vehicle with a 1.5L DI engine was tested with an artificially aged exhaust system to end-of-life, consisting of a first close-coupled TWC with a catalyst volume of 1.26L (substrate dimensions 118.4mmx57.2mm) and a conventional three-way coating with 1.77 g / L precious metal (0 / 92 / 8 Pt / Pd / Rh), an uncoated GPF with a catalyst volume of 1.39L (substrate dimensions 132.1mmx101.6mm) arranged downstream, a second TWC with a catalyst volume of 0.63L (substrate dimensions 118.4mmx57.2mm) arranged in the underbody and a conventional three-way coating with 0.83 g / L precious metal (0 / 80 / 20 Pt / Pd / Rh) and a KAT with 0.63L catalyst volume (substrate dimensions 118.4mmx57.2mm) and a coating that can also temporarily store hydrocarbons, with 0.83 g / L precious metal (0 / 80 / 20 Pt / Pd / Rh) driven on a roller dynamometer in an RTS aggressive driving cycle.This system is called TWC-GPF-TWC+KAT and has a total substrate volume of 3.9L. The emissions of THC, NMHC, CO, NOx, NH3, and N2O were measured; the measurement technique used is familiar to those skilled in the art. The average values ​​shown are those of multiple measurements.

[0046] Figure 5 shows the advantages of the TWC-GPF-TWC+KAT system compared to the TWC_GPF_TWC reference system in a reduction of THC and NMHC emissions by 15%, without negatively affecting CO or NOx emissions. The advantages for HC emissions are even more evident in Figure 6, where the averaged cumulative modal curves of HC, CO and NOx during the first 200s of the driving cycle are shown. Here, HC emissions can be reduced by more than a third using the TWC-GPF-TWC+KAT system in blue compared to the TWC-GPF-TWC reference system in black, with no negative impact on CO or NOx performance. This effect is achieved with the same total substrate volume for the two compared systems, i.e. with half the TWC2 of the TWC-GPF-TWC+KAT system compared to that of the TWC-GPF-TWC reference system. With the same volume of TWC2 in both systems, an even greater advantage can be expected for the TWC-GPF-TWC+KAT system.

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 another TWC2 on a through-flow substrate downstream of the GPF, the system additionally comprising materials for temporarily storing hydrocarbons, characterized in that these are located downstream of the TWC1 and behind the GPF.

2. System according to claim 1, characterized in that by means of this additional material, the hydrocarbon storage capacity is increased to at least 0.020 g of hydrocarbons per L substrate volume.

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

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

5. System according to any of the preceding claims, characterized in that the materials for temporarily storing hydrocarbons likewise comprise catalysts for oxidizing hydrocarbons to H2O and CO2.

6. System according to any of the preceding claims, characterized in that the materials for temporarily storing hydrocarbons are arranged on a separate through-flow substrate.

7. System according to claim 6, characterized in that the substrate having the materials for temporarily storing hydrocarbons makes up a proportion of 5-30 vol.% of the total volume of the substrates in the exhaust gas purification system.

8. System according to any of claims 6 to 7, characterized in that the substrate having the materials for temporarily storing hydrocarbons has a greater washcoat loading in g / L than the GPF.

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

10. 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 of the preceding claims.