WO2023198571A1

WO2023198571A1 - Exhaust gas system for predominantly stoichiometrically operated internal combustion engines, comprising a catalyst for reducing ammonia emissions

Info

Publication number: WO2023198571A1
Application number: PCT/EP2023/059080
Authority: WO
Inventors: Julius KOEGEL; Massimo Colombo; Sonja Buchberger; Marcus Schmidt
Original assignee: Umicore Ag & Co. Kg
Priority date: 2022-04-11
Filing date: 2023-04-06
Publication date: 2023-10-19
Also published as: WO2023198569A1; WO2023198574A1; WO2023198572A1; WO2023198570A1; WO2023198577A1; WO2023198575A1; WO2023198573A1

Abstract

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

Description

Exhaust system for predominantly stoichiometrically operated internal combustion engines, having a catalytic converter to reduce ammonia emissions

The present invention is directed to 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 in the cylinder as is needed for complete combustion. The combustion air ratio A (A/F ratio; air/fuel ratio) relates the air mass mL.tats actually available for combustion to the stoichiometric air mass mi_,st:

If A < 1 (e.g. 0.9) this means “lack of air”, one speaks of a rich exhaust gas mixture, A > 1 (e.g. 1.1) means “excess air” and the exhaust gas mixture is referred to as lean. The statement A = 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 γ-aluminum oxide as a support material. In addition, three-way catalysts contain oxygen storage materials, for example cerium/zirconium mixed oxides. In the latter, cerium oxide, a rare earth metal oxide, is the fundamental component for oxygen storage. In addition to zirconium oxide and cerium oxide, these materials can contain additional components such as other rare earth metal oxides or alkaline earth metal oxides. Oxygen storage materials are made by Application of catalytically active materials such as platinum group metals activates and thus also serves as a carrier material for the platinum group metals.

As part of the Euro 7 legislation, which will come into force in the mid-2020s, 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, particularly in the underbody of a gasoline-powered car, pose a major challenge.

Compliance with the strict emission values for ammonia requires the use of a storage material to store NH3 during the rich operating conditions of the internal combustion engine, especially for low and medium temperature ranges, 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 N2O. 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 requires that they also be stable in exhaust gas with a rich or stoichiometric composition under hydrothermal exhaust gas conditions.

The use of catalysts, which preferentially convert ammonia to 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; WO11110919A1, EP 3915679A1). The use of ammonia slip catalysts, or ASCs for short, has also already been described in the area of LNG gasoline engines (EP24258A1). These catalysts often consist of an SCR catalytically active component and a component that catalyzes the oxidation of ammonia. These catalytic converters are usually located in the underbody at the last point of the exhaust system. If for oxidation of the stored If there are not enough nitrogen oxides in the system, the ammonia can also be converted into nitrogen via the ASC with the oxygen present. As it has been found, a further disadvantage of the known systems for reducing ammonia emissions is that the transition metals in the SCR component, such as iron and/or copper, tend to dissolve in the exhaust system of a predominantly stoichiometrically operated internal combustion engine after a long period of use to diffuse the component for the oxidation of ammonia and poison it. The result is a lower activity of the SCR and the oxidative component.

Accordingly, it is an object of the present invention to present new exhaust systems which allow the operation of an internal combustion engine, in particular a predominantly stoichiometrically operated, spark-ignited internal combustion engine, even under the new Euro 7 legislation. In particular, the corresponding limit values for NH3 and N2O 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.

These and other tasks arising from the prior art for the person skilled in the art are solved by an exhaust system and a method for exhaust gas purification according to claims 1 and 9, respectively. Claims 2 - 8 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 the harmful exhaust gas components of internal combustion engines, in particular predominantly stoichiometrically operated internal combustion engines, such as spark-ignited gasoline engines, having a first three-way catalytic converter and, on the downstream side, a catalyst for reducing ammonia emissions, which has the following components:

- a first component with a metal-free zeolite and/or zeotype for storing ammonia;

- a second component with an OSC-free precious metal catalyst and/or an OSC-containing noble metal catalyst, the solution to the problem is achieved relatively easily, but no less surprisingly. The system according to the invention is characterized by an extremely good Performance in terms of reducing CO, HC and NOx emissions as well as NH3 and IX^O emissions. It reacts well to the dynamic requirements in the exhaust system of a gasoline engine and is sufficiently robust to meet these requirements for a sufficient period of time.

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 over-reduction of nitrogen oxides via a three-way catalytic converter installed on the upstream side, but this cannot be converted by other conventional three-way catalytic converters, for example because of the Lack of oxygen or insufficient operating temperatures. The ammonia stored in this way can then be removed from storage 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 are not connected to each other. Zeolites with a two-dimensional channel system are characterized by the fact that their channels are connected to each other 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 preferably used in the present invention [Ch. Baerlocher, WM Meier and DH Olson, Atlas of Zeolite Framework Types, Elsevier, 2001], According to the invention, the term “zeolite” refers to porous materials with a lattice structure of corner-linked AIO4 and SiCU tetrahedra according to the general formula (WM Meier, Pure & Appl. Chem., Vol. 58, No. 10, pp. 1323-1328 , 1986):

Mm/z [m AIO2 * n SiC>2] * q H ₂ O

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). 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 AGO, AEI, AEN, AFN, AFT, AFX, ANA, APC, APD, ATT, BEA, BIK, CDO, CHA, DDR, DFT, EAB, EDI, EPI, ERI, ESV, ETL, 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. 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, KFI, LEV, UFI and the corresponding zeotypes of these structural types, such as: SAPO. Mixtures of the same can also be present. The use of CHA, FER, LEV and AEI is particularly preferred.

This first component is metal-free. In the context of the invention, metal-free means the absence of metals in ionic, oxidic or metallic form, which could catalyze the SCR reaction or ammonia oxidation. It therefore essentially contains no transition metals, in particular iron and/or copper, and also no precious metals. The need for prior contact with these metals is therefore eliminated. Due to the manufacturing process, the zeolite/zeotype may contain residues of metals such as sodium or potassium etc. in low concentrations. This is usually the case Zeolite/zeotype after calcination is preferably essentially in the H form (so-called white zeolite). Metals present during production, as well as transition metals and precious metals, are present in it in amounts of less than 500 ppm, more preferably less than 300 ppm and most preferably less than 100 ppm. Excluded from the exclusion are the actively added, non-catalytically active components mentioned below, in particular binders.

The aging stability of the zeolites or zeotypes used in the exhaust system of predominantly stoichiometrically burning engines is particularly in focus here, since higher temperatures generally prevail here than in a lean-burning engine. In this respect, materials are desired that can withstand the sometimes very high and rapidly changing hydrothermal conditions for as long as possible. On the other hand, the exhaust gas composition is also different compared to lean-burn engine exhaust. The concentration, in particular of hydrocarbons and carbon monoxide, which arrive at the catalyst according to the invention is, on the one hand, higher than in lean-burn engines and the composition also changes depending on the driving style around the stoichiometric range (rich/lean change). The hydrothermal temperature stability of zeolites and zeotypes depends heavily on the SAR value (silica-to-alumina ratio) of the zeolite or the ratio corresponding to this value for zeotypes. The amount of silicon atoms remaining in the framework is then related to the substitution atoms. It has proven to be advantageous if the zeolites have a SAR value of 10 - 50, preferably 12 - 35 and most preferably 13 - 30. The same applies to the zeotype with the corresponding ratio.

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 ZrÜ2, 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 should also not contain the transition metals specified above, in particular iron and/or copper, and no precious metals. Binders are suitable for ensuring stronger adhesion of the coating to a carrier or another coating. For this purpose, a certain particle size of the metal oxides in the binder is advantageous. This can be adjusted accordingly by a specialist. The ammonia storage ability or capacity addressed in the context of this invention is given as a quotient of the stored mass of ammonia per liter of catalyst support volume. The zeolites or zeotypes should increase the ammonia storage capacity of the exhaust gas purification system to at least 0.25 g of ammonia per liter of carrier volume (measured in the fresh state). Overall, the storage capacity of the ammonia storage components used should be sufficient so that the system contains between 0.25 and 10.0 g of NH3 per liter of carrier volume, 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 NH3/U-ter carrier volume of ammonia can be stored (always based on the fresh state). The zeolites or zeotypes are present in sufficient quantities in the catalyst to reduce ammonia emissions. The determination of the ammonia storage capacity is shown below.

The second component consists of an OSC-free precious metal catalyst and/or an OSC-containing noble metal catalyst. 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 selected from the group consisting of palladium, platinum, rhodium. OSC means Oxygen Storage Catalyst - oxygen storage catalyst. 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 20 g/L, preferably less than 10 g/L and most preferably less than 5 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 second component in the form of an OSC-free precious metal catalyst therefore has materials that have an oxidative effect on, among other things, ammonia. This component normally contains a temperature-stable, high-surface metal oxide and at least one noble metal selected from the group rhodium, platinum and palladium. The total noble metal content of this component is preferably from 0.015 - 5 g/L, more preferably from 0.035 - 1.8 g/L and particularly preferably from 0.07 - 1.2 g/L carrier volume. 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 in 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 La2Ü3 and based on the weight of the stabilized aluminum oxide, is used. Even in the case of aluminum oxide doped with barium oxide, the proportion of barium oxide is in particular 1 to 10% by weight, preferably 3 to 6% by weight, in each case calculated as BaO and based on the weight of the stabilized aluminum oxide. A 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 y-alumina with a surface area of 100 to 200 m ² /g is particularly advantageous in this context. Such active aluminum oxide has been widely described in the literature and is available on the market.

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 5 g/L, preferably more than 10 g/L and most preferably more than 20 g/L carrier volume. The entire cerium-zirconium mixed oxide with all its components is included.

Corresponding OSC-containing precious metal catalysts have the ability to have an oxidative effect on the substances present (NH3, HC, CO) in the already slightly rich exhaust gas of a predominantly stoichiometrically operated internal 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 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 precious metals can only be deposited on the temperature-stable, high-surface support materials present. 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 noble metals), this should preferably be in the range of 0.035 - 1.0 g/L, more preferably 0.1 - 0.35 g/L carrier volume 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, for example, in DE102013210270A1, DE102020101876A1, EP3247493A1,

EP3727655A1 described.

Modern gasoline engines are operated under conditions with a discontinuous course of the air ratio X. They are subject to a periodic change in the air ratio X in a defined manner and thus to a periodic change in oxidizing and reducing exhaust gas conditions. This change in the air ratio X is essential for the exhaust gas purification result in both cases. For this purpose, the lambda value of the exhaust gas is measured with a very short cycle time (approx. 0.5 to 5 Hertz) and an amplitude AÄ. from 0.005 <AÄ <0.05 around the value X = 1 (reducing and oxidizing exhaust gas components are present in a stoichiometric ratio to one another). Due to the dynamic operation of the engine in the vehicle, deviations from this state also occur. So that the deviations from the stoichiometric point mentioned do not have a negative impact on the exhaust gas purification result when the exhaust gas is passed over the three-way catalytic converter, oxygen storage materials contained in the catalytic converter compensate for these deviations to a certain extent by absorbing oxygen from the exhaust gas or releasing it into the exhaust gas as required ( Catalytic Air Pollution Control, Commercial Technology, R. Heck et al., 1995, p. 90).

The OSC-containing noble metal catalysts (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 thus be referred to as cerium/zirconium/rare earth mixed oxides. The term “cerium/zirconium/rare earth metal mixed oxide” in the sense of the present invention includes physical mixtures of cerium oxide and 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, products may not be completely homogeneous, which can generally be used without disadvantage. The same applies to cerium/zirconium mixed oxides that do not contain any rare earth metal oxide. Furthermore, the term rare earth metal or rare earth metal oxide in the sense of the present invention does not include cerium or cerium oxide. Suitable rare earth metal oxides in the cerium/zirconium/rare earth metal mixed oxides include, for example, lanthanum oxide, yttrium oxide, praseodymium oxide, neodymium oxide and/or samarium oxide. Lanthanum oxide, yttrium oxide and/or praseodymium oxide are preferred. Particularly preferred rare earth metal oxides are 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 metal 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 metal 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 The mass ratio of temperature-stable, high-surface carrier materials and oxygen storage components in this component is usually 0.25 to 1.5, for example 0.3 to 1.3. In an exemplary embodiment, the weight ratio of the sum of the masses of all support materials, such as aluminum oxides (including doped aluminum oxides) to the sum of the masses of all cerium/zirconium mixed oxides in the OSC-containing noble metal catalyst is 10:90 to 75:25, preferably 20 :80 to 65:35..

The first and second components preferably form an ammonia storage and a function for the oxidation of ammonia to nitrogen (e.g. as in WO2008106523A2), although the ammonia storage is metal-free within the scope of the invention. If there are not enough nitrogen oxides in the system to oxidize the stored ammonia, the ammonia can also be converted into nitrogen with the oxygen present via the second component. In both cases, if possible, no ammonia or N2O is released into the environment. In the broadest sense, the first component and the second component of the catalyst for reducing ammonia emissions can therefore preferably consist of an ammonia-storing coating paired with a second coating that has an oxidative effect on ammonia. As such, they are preferably present in separate coatings, more preferably at least partially overlapping, on the substrate. The length of the respective layer/component can be freely chosen by the person skilled in the art within the scope of the invention. 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. The components are in separate layers, separated from each other but lying on top of each other. It is particularly advantageous if both components are the same length and completely overlap each other. It is particularly preferred if the OSC-free and/or OSC-containing noble metal catalyst of component two is located as a lower layer under the first component made of zeolites and/or zeotypes for storing ammonia as an upper layer. The second component is preferably placed directly on the substrate as a coating. There is preferably no further layer between the first component and the second component (Fig. 2). However, the reverse orientation also provides good results and is also preferred (Fig. 3 - A2).

The components of the catalyst for reducing ammonia emissions are applied to a carrier using a coating step familiar to those skilled in the art. preferably applied to a flow-through substrate (DE102019100099A1 and 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 pm 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 channels of these carriers per area is characterized by the cell density, which is usually between 300 and 900 cells per square inch (cells per square inch, cpsi). The wall thickness of the channel walls for ceramics is between 0.5 - 0.05 mm.

The total amount of coatings in the catalyst to reduce ammonia emissions is selected so that the catalyst according to the invention is used as efficiently as possible overall. In the case of one or more flow-through substrates, for example, the total amount of coatings (solids content) per carrier volume (total volume of the carrier) can be between 100 and 600 g/L, in particular between 150 and 400 g/L. The first component is preferably used in an amount of 50 to 350 g/L, in particular between 120 and 250 g/L, particularly preferably about 145 - 230 g/L of carrier volume. The second component is preferably used from 50 to 350 g/L, in particular between 120 and 250 g/L, particularly preferably from about 145 - 230 g/L carrier volume.

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. An exhaust system is therefore preferred that additionally has a possibly catalytically coated GPF between the first three-way catalytic converter and the catalytic converter to reduce ammonia emissions (FIG. 6). GPF are gasoline particle filters and are well known to those skilled in the art (EP3737491A1, EP3601755A1). An exhaust gas design in which the first three-way catalytic converter and the GPF are installed in a position close to the engine is particularly preferred.

Close to the engine in the sense of the invention refers to an area in the exhaust system that is in a position close to the engine, i.e. approx. 10 - 80 cm, preferably 20 - 60 cm away from the engine outlet. It has proven to be advantageous if the catalytic converter is installed last in the exhaust direction in the underbody of a vehicle to reduce ammonia emissions, so that the exhaust gas is then released into the ambient air. The exhaust system can also have additional exhaust units such as additional three-way catalytic converters or hydrocarbon storage (HC traps) or nitrogen oxide storage (LNT). The underbody is the area below the driver's cab.

In a further preferred embodiment, at least a second three-way catalytic converter (TWC) is located 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 located 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, if necessary, a 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 WO2019219816).

In a further aspect, the present invention relates to a method for reducing harmful exhaust gas components from predominantly stoichiometrically operated internal combustion engines, in particular spark-ignited gasoline engines, in which the exhaust gas is 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 stoichiometric 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 over-oxidation to N2O does not occur. Even after intensive aging, the exhaust system is robust enough to fully meet Euro 7 requirements. In particular, it may come as a surprise that the catalysts advocated here for reducing ammonia emissions work without transition metal-exchanged zeolites and/or zeotypes. This has the advantage that the second component for ammonia oxidation is prevented from being poisoned with the transition metals, which means that its high activity remains still well preserved under harsh aging conditions. This promises a long active lifespan for the targeted exhaust system.

Characters:

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: Emission values for catalysts without TWC coating in comparison

Fig. 5: Emission values for catalysts with TWC coating with and without zeolite coating in comparison

Fig. 6: Exhaust system according to the invention with a three-way catalytic converter close to the engine (A), GPF close to the engine (B) and the following catalyst to reduce ammonia emissions (C).

Examples:

A. Determination of 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 ol.-% oxygen, 5 vol.-% water and the rest nitrogen with a space velocity of 30,000 h ^-1 for 10 minutes conditioned. The measuring temperature of 200 °C is then approached 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 NHs storage phase is initiated by switching on a gas mixture of 450 ppm ammonia, 0 vol.% oxygen, 5 vol.% water and the rest nitrogen with a space velocity of 30,000 IT ¹ . This gas mixture remains switched on until a steady ammonia breakthrough concentration is recorded downstream from 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 NHs storage phase until stationarity is reached, taking into account the measured stationary NHs breakthrough concentration and the known volume flow (hatched area in Figure 1). The ammonia storage capacity is calculated as the quotient of the stored mass of ammonia divided by the volume of the tested core.

B. Production of the ammonia-storing SCR layers

B1 Preparation of the Cu-loaded zeolite

The zeolite was coated with copper using an incipient wetness process with a copper(II) nitrate solution in a solids mixer. This was followed by treatment in the oven for 8 h at 120°C and for 2 h at 600°C in air. For a zeolite of the chabazite structural type, a composition with 3.8 wt% CuO based on the total mass of zeolite and CuO was prepared. B2. Production of the copper-containing SCR coating

The coating with a Cu-loaded zeolite was carried out after joint grinding with Nyacol®-AL20 binder on a cordierite substrate with the desired washcoat loading (88% zeolite, 12% binder). The coated catalyst thus obtained was dried at 90 °C and then calcined at 350 °C for 15 min and annealed in air at 550 °C for 2 h. If necessary, a layer containing precious metal can be applied as a top layer to the now coated carrier.

B3. Production of copper-free zeolite coatings

The coating with white 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 loading (88% zeolite, 12% binder). The coated catalyst thus obtained was dried at 90 °C, annealed at 350 °C for 15 min and then calcined 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 precious metal-containing coatings with TWC activity

Lanthanum oxide-stabilized alumina was suspended in water along with an oxygen storage component comprising 24 wt% ceria, 60 wt% zirconia, 3.5 wt% lanthana, and 12.5 wt% yttria, and lanthanum acetate as an additional source of lanthana. The weight ratio of aluminum oxide to oxygen storage component to additional lanthanum oxide was 43.6:55.7:0.7. A rhodium nitrate solution was then added to the suspension thus obtained with constant stirring. The resulting coating suspension was used directly to coat a commercially available substrate, with the coating taking place over 100% of the substrate length. The total loading of this washcoat on the catalyst was 122 g/L, the precious metal loading was 0.177 g/L (5 g/ft ³ ). The coated catalyst thus obtained was dried and then calcined. If necessary, a layer free of precious metals can be applied as a top layer to the now coated carrier. D. Preparation of the platinum-containing SiC^/AhCh layer without TWC activity

A silicon-aluminum mixed oxide consisting 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. If necessary, further layers can be applied as a top layer to the now coated carrier.

Catalysts were prepared as shown schematically in Figures 4 and 5.

E. Aging and testing of 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 close to the engine (maximum bed temperature 1030 °C). The aging time 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 NHs emissions” refers to the NH3 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 the underbody position. F. Results

Comparison of an ASC with a white zeolite with an ASC with a copper-containing SCR coating:

See Fig. 4

All catalysts with oxidation layer contain 1 g/ft ³ pt.

Aging: Fuel-cut aging, 76 h, 800 °C inlet temperature for the catalytic converters in the underbody position

Volume of the underbody catalytic converter: 1 L

A catalyst in which a copper-free zeolite layer is combined in a layered design with a 1 g/ft ³ Pt oxidation layer shows improved catalytic performance compared to a corresponding catalyst in which a copper-containing SCR in place of the white zeolite layer is used.

Comparison of a TWC with a TWC combined with a white zeolite coating:

See Fig. 5

Both catalysts with TWC layer contain 5 g/ft ³ Rh.

Aging: Fuel-cut aging, 38 h, 800 °C inlet temperature for the catalytic converters in the underbody position

Volume of the underbody catalytic converter: 0.83 L

A catalyst in which a copper-free zeolite layer is combined in a layered design with a TWC layer at 5 g/ft ³ Rh shows improved catalytic performance compared to a pure TWC coating without zeolite.

Light-off behavior of TWC catalysts with different precious metal loadings (see Table 1):

Catalysts with a TWC layer with different precious metal contents are compared with each other. Table 1 shows the temperatures at which the catalysts show 50% conversion for hydrocarbons, carbon monoxide and nitrogen oxides in a light-off test after fuel-cut aging in the underbody position. A lower one Tso value corresponds to a higher catalytic activity. A catalytic converter containing only rhodium shows the best starting behavior in this test.

Table 1:

Claims

Patent 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, on the downstream side, a catalyst for reducing ammonia emissions, characterized in that it has the following components:

- a first component with a metal-free zeolite or zeotype for storing ammonia;

- a second component with an OSC-free precious metal catalyst and/or an OSC-containing noble metal catalyst.

2. Exhaust system according to claim 1, characterized in that the precious metals in the OSC-free or OSC-containing noble metal catalyst are selected from the group consisting of palladium, platinum, rhodium.

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

4. Exhaust system according to one of the preceding claims, characterized in that the metal-free zeolites or zeotypes for storing ammonia and the OSC-free or OSC-containing noble metal catalyst are present in separate coatings.

5. 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.

6. Exhaust system according to one of the preceding claims, characterized in that it additionally has a GPF between the first three-way catalyst and the catalyst to reduce ammonia emissions.

7. 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.

8. 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.

9. 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.