DE102023101763A1 - Exhaust system for predominantly stoichiometrically operated internal combustion engines, having a catalytic converter to reduce ammonia emissions - Google Patents

Abstract

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.

Description

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 predominantly (>50% of the operating time) with a 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 λ (A/F ratio; air/fuel ratio) relates the air mass m _L,tats actually available for combustion to the stoichiometric air mass m _L,st : $$\mathrm{\lambda }=\frac{m_{\text{L}\text{.acts}}}{m_{\text{L}\text{,st}}}$$

If λ < 1 (e.g. 0.9) this means "lack of air", one speaks of a rich exhaust gas mixture, λ > 1 (e.g. 1.1) means "excess air" and the exhaust gas mixture is referred to as lean. The statement λ = 1.1 means that 10% more air is present than would be necessary for the stoichiometric reaction. The same applies to the exhaust gases from internal combustion engines.

The catalytically active materials used in the known three-way catalysts are generally platinum group metals, in particular platinum, palladium and rhodium, which are present, for example, on 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 activated by applying catalytically active materials such as platinum group metals and thus also serve as a carrier material for the platinum group metals.

As part of the Euro 7 legislation, which will come into force in the mid-2020s, the emissions of ammonia (NH ₃ ) and nitrous oxide (N ₂ O) for stoichiometric combustion engines will be regulated for the first time. The toxic ammonia and the powerful greenhouse gas N ₂ O 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 for storing NH ₃ during the rich operating conditions of the internal combustion engine, especially for low and medium temperature ranges, since the ammonia is mainly formed under these exhaust gas conditions. The stored ammonia is then converted during lean operating points by oxidation on a layer containing precious metal and/or as part of an SCR reaction. The aim here is to achieve the lowest possible selectivity to N ₂ O. A special requirement for the catalysts considered here is the high aging stability of the materials used: In addition to the stability against lean gas conditions, their use in the exhaust system of stoichiometrically operated internal combustion engines 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 into nitrogen, has already been discussed, particularly in the diesel sector or for use in lean-burning DI petrol engines ( US5120695 ; EP1892395A1 ; EP1882832A2 ; EP1876331A2 ; WO12135871A1 ; US2011271664 AA; WO11110919A1 , EP3915679A1 ). The use of ammonia slip catalysts, or ASCs for short, has also already been described in the area of CNG engines (CNG = “Compressed Natural Gas”) ( EP2924258A1 ). These catalysts often consist of an SCR catalytically active component and a component that catalyzes the oxidation of ammonia. They are usually located in the underbody at the last point of the exhaust system tems. If there are not enough nitrogen oxides in the system to oxidize the stored ammonia, the ammonia can also be converted into nitrogen using the oxygen present via the ASC. As it turns out, the aging stability of the ASC catalysts also depends significantly on the materials used.

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 internal combustion engine, even under the new Euro 7 legislation. In particular, the corresponding limit values, especially for NH ₃ and N ₂ O, should be safely adhered to. In addition, the system should also be robust and agile in order to be able to withstand the working conditions in the exhaust system of a corresponding automobile for a sufficient period of time.

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 12, respectively. Claims 2 - 11 relate to preferred embodiments of the exhaust system and can accordingly also be applied to the method according to the invention.

• - eine erste Komponente mit einem übergangsmetallausgetauschten Zeolithen und/oder Zeotypen mit der Gerüststruktur eines CHA, LEV, ZFI, AFX oder AEI und einem jeweiligen SAR bzw. äquivalenten Verhältnisses des Zeotypen von > 32, besser >35;
• - eine zweite Komponente mit einem OSC-haltigen Edelmetallkatalysator, welcher Rhodium aufweist; und wobei die beiden Komponenten als übereinanderliegende Schichten auf einem Substrat aufgebracht sind, gelangt man relative einfach, dafür aber nicht minder überraschend zur Lösung der gestellten Aufgabe. Das erfindungsgemäße System zeichnet sich durch eine gute Performance sowohl hinsichtlich der angestammten Abgasbestandteile, als auch hinsichtlich der NH₃-und N₂O-Emissionen aus. Es reagiert gut unter den dynamischen Anforderungen im Abgasstrang eines Fremdzündungsmotors und ist insbesondere entsprechend robust, um auch eine ausreichende Dauer diesen Anforderungen gerecht zu werden.

By providing an exhaust system for reducing harmful exhaust gas components from predominantly stoichiometrically operated internal combustion engines, in particular 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 transition metal-exchanged zeolite and/or zeotype with the framework structure of a CHA, LEV, ZFI, AFX or AEI and a respective SAR or equivalent ratio of the zeotype of >32, better >35;
• - a second component with an OSC-containing noble metal catalyst which has rhodium; and where the two components are applied as layers on top of one another on a substrate, the solution to the problem is achieved relatively easily, but no less surprisingly. The system according to the invention is characterized by good performance both in terms of the original exhaust gas components and in terms of the NH ₃ and N ₂ O emissions. It reacts well to the dynamic requirements in the exhaust system of a spark ignition engine and is particularly robust in order to meet these requirements for a sufficient period of time.

The components of the catalyst for reducing ammonia emissions are applied to a support, preferably to a flow-through substrate, using a coating step familiar to those skilled in the art ( 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 are made of metal, for example WO17153239A1 , WO16057285A1 , WO15121910A1 and literature cited therein) or ceramic materials. “Corrugated substrates” can also be viewed as flow-through substrates. These are known to those skilled in the art as carriers made of corrugated sheets made of inert materials. Suitable inert materials are, for example, fibrous materials with an average fiber diameter of 50 to 250 μm and an average fiber length of 2 to 30 mm. Fibrous heat-resistant materials made of silicon dioxide, especially glass fibers, are preferred. However, refractory ceramics such as cordierite, silicon carbite or aluminum titanate etc. are preferably used as honeycomb carriers. The number of carrier channels per area is characterized by the cell density, which is usually between 300 and 900 cells per square inch (cells per square inch, cpsi). The wall thickness of the channel walls for ceramics is between 0.5 - 0.05 mm.

The total amount of 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.

According to the invention, the components are present as separate coatings one above the other on the substrate. The first component is in the first layer, the second component in the second. It is preferred if the second layer lies completely above the first and completely covers it. The reverse arrangement is also possible. The first layer does not protrude beyond the second layer at any end. It is particularly preferred if the two coatings with the respective components are of the same length ( 2 ). The length of the layers can be chosen by the specialist. They are preferably located on a flow-through substrate and here take up a length of at least 10% and a maximum of 100%, more preferably 20% - 90%, extremely preferably 30% - 80% of the substrate length. In this case, a coating that lies above another comes into contact with the exhaust gas first before the latter.

As already indicated above, a first component of the catalyst for reducing ammonia emissions consists of zeolites and/or zeotypes for storing ammonia. In principle, those skilled in the art are familiar with the zeolites and zeotypes available for this purpose from the diesel sector. The way the zeolites or zeotypes work is based on the fact that they can temporarily store ammonia in operating states of the exhaust gas purification system in which ammonia is produced, for example, by 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. According to the invention, three-dimensional zeolites or zeotypes are used in the present invention [Ch. Baerlocher, W.M. Meier and D.H. 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-connected AlO ₄ and SiO ₄ tetrahedra according to the general formula (WM Meier, Pure & Appl. Chem., Vol. 58, No. 10, pp. 1323 -1328, 1986): M _m/z [m AlO ₂ * n SiO ₂ ] * q H ₂ O

The structure of a zeolite 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.

Zeolites or zeotypes can also be classified according to their pore structure. A distinction is made between small-pore, medium-pore and large-pore zeolites. The extra-large pore zeolites are of academic interest. Small-pore zeolites are those with a largest ring size of 8 tetrahedral atoms. Large-pore zeolites have a top ring size of 12 tetrahedral atoms (https://en.wikipedia.orq/w/index.php?title=Zeolite&oldid=1103217432 and the literature cited there). Small-pore zeolites or zeotypes for storing ammonia are those with the framework structure of a CHA, LEV, ZFI, AFX or AEI. CHA is extremely preferred in this context.

The aging stability of the zeolites or zeotypes used in the exhaust system of predominantly stoichiometrically burning engines is particularly in focus here, since higher temperatures generally prevail here than in a lean-burning engine. In this respect, such materials are desired which can withstand the sometimes very high and rapidly changing hydrothermal conditions for as long as possible. On the other hand, the exhaust gas composition is also different compared to lean-burn engine exhaust. The concentration, in particular of hydrocarbons and carbon monoxide, which arrive at the catalyst according to the invention is, on the one hand, higher than in lean-burn engines and the composition also changes depending on the driving style around the stoichiometric range (rich/lean change). The hydrothermal temperature stability of zeolites and zeotypes is therefore particularly in demand. The zeolites preferably have a SAR value (silica-to-alumina ratio) or the zeotypes have a ratio corresponding to this value of > 32 to < 45, most preferably from > 35 to < 40. The SAR value is calculated as follows Zeolites relate the amount of silicon atoms remaining in the framework to the substitution atoms (Al ions). This results in the number of negative charges in the base body and thus a measure of the number of counterions to be absorbed until electroneutrality is achieved. A corresponding ratio can be determined for Zeotype.

According to the invention, the zeolite or zeotype used is ion-exchanged with transition metal ions. The latter are preferably selected from the group consisting of iron and/or copper. Iron is particularly preferred because it has a less oxidizing effect on ammonia compared to copper. These compounds have the possibility of comproportioning nitrogen oxides present in the exhaust gas and the stored ammonia into nitrogen when lean. In this case, the zeolite or zeotype described acts as a catalyst for selective catalytic reduction (SCR) (see WO2008106518A2 , WO2017187344A1 , US2015290632 AA, US2015231617 AA, WO2014062949A1 , US2015231617 AA). In the present case, SCR capability is understood to mean the ability to selectively convert NO _x and NH ₃ in the lean exhaust gas into nitrogen.

The metals, such as iron and/or copper, which advantageously occur in the catalyst to reduce ammonia emissions, are present in a certain proportion in the first component. This is 0.4 - 5, more preferably 0.8 - 4 and very preferably 1.5 - 3.5% by weight of the first component. The iron and/or copper to aluminum ratio is between 0.1 - 0.8, preferably between 0.2 - 0.5 and most preferably between 0.3 - 0.5 for zeolites. For the zeotype, a corresponding ratio applies to the exchange places available there. The metals are at least partially present in ion-exchanged form in the zeolites or zeotypes. Preferably, ion-exchanged zeolites or zeotypes are already introduced into the first component. However, it can also be the case that the zeolites or zeotypes are brought together with, for example, a binder and a solution of the metal ions in a liquid, preferably water, and then dried (preferably by spraying). Here, certain proportions of the metals can also be found on the binder in the form of oxides. Both approaches are possible.

However, ion exchange poses a challenge for small-pore zeolites. While ion exchange for large-pore zeolites can be carried out as part of the washcoat process, since the ions readily penetrate into the large zeolite pores and occupy the ion exchange positions of the material, the occupation of small-pore zeolites requires a separate one Process step. This can be, for example, an incipient wetness process or an ion exchange process with, if necessary, subsequent spray drying. The expert knows how to proceed here.

In addition to the zeolites or zeotypes, the first component can preferably have other non-catalytically active components, such as binders. Suitable binders are, for example, non-catalytically active or only slightly catalytically active, temperature-stable metal oxides, such as SiO ₂ , Al ₂ O ₃ and ZrO ₂ . The expert knows which materials come into question here. The proportion of such binders in the first coating can, for example, be up to 15% by weight, preferably up to 10% by weight, of the coating. The binder can also contain the above-mentioned transition metals, in particular iron and/or copper. Binders are suitable for ensuring stronger adhesion of the coating to a carrier or another coating. For this purpose, a certain particle size of the metal oxides in the binder is advantageous. This can be adjusted accordingly by a specialist.

The ammonia storage capacity or capacity addressed in the context of this invention is given as a quotient of the stored mass of ammonia per liter of catalyst support volume. The first component should increase the ammonia storage capacity of the exhaust gas purification system to at least 0.25 g of ammonia per L of carrier volume (measured in the fresh state). Overall, the storage capacity of the ammonia storage components used in the form of the zeolites or zeotypes should be sufficient so that in the system between 0.25 and 10.0 g of NH ₃ per liter of carrier volume, preferably between 0.5 and 8.0 g of NH ₃ per liter of carrier volume and especially preferably between 0.5 and 5.0 g NH ₃ /liter carrier volume 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-containing noble metal catalyst containing rhodium. Precious metal refers in particular to the platinum group metals platinum, palladium and rhodium. OSC means Oxygen Storage Component. An OSC-containing noble metal catalyst therefore has oxygen storage materials.

The OSC-containing precious metal catalyst therefore has a function of storing oxygen in the exhaust gas of the internal combustion engine. In addition to the precious metals and possibly temperature-stable, high-surface metal oxides, oxygen storage materials are also present in the noble metal catalyst (containing OSC). Cerium or cerium-zirconium mixed oxides (see below) are consistently used as oxygen storage materials. Accordingly, an OSC-containing noble metal catalyst is characterized by the presence of a certain amount of these oxygen storage materials. In particular, this component has oxygen storage materials in an amount of more than 10 g/L, preferably more than 15 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 (NH ₃ , 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 N ₂ O will be formed from ammonia oxidation.

The second component in the form of the OSC-containing noble metal catalyst therefore has materials that have an oxidative effect on, among other things, ammonia. This component preferably contains a temperature-stable, high-surface metal oxide, oxygen storage material and at least the noble metal rhodium. Platinum and/or palladium may also be present. However, only rhodium is most preferably present in the second component. 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. If platinum and/or palladium is used, the former should be in the range of 0.015 - 1.42 g/L, more preferably 0.035 - 0.35 g/L carrier volume in the coating. Palladium can be present in the coating between 0.015 - 1.42 g/L, preferably 0.035 - 0.35 g/L carrier volume.

Rhodium is present in the second component according to the invention (alone or in combination with the other aforementioned noble metals). The carrier volume in this component should preferably be in the range of 0.035 - 1.0 g/L, more preferably 0.1 - 0.35 g/L. If palladium and/or platinum are also present in this component, the above ranges apply to these metals. Equipped in this way, this component has three-way activity. Suitable three-way catalytically active coatings (TWC) are, for example, in DE102013210270A1 , DE102020101876A1 , EP3247493A1 , EP3727655A1 described.

The noble metals in the OSC-containing second component are usually also 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 La ₂ O ₃ 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. 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 includes preferably at least one aluminum oxide or doped aluminum oxide. La-stabilized γ-aluminum oxide 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.

Modern gasoline engines are operated under conditions with a discontinuous course of the air ratio λ. They are subject to a periodic change in the air ratio λ in a defined manner and thus to a periodic change in oxidizing and reducing exhaust gas conditions. This change in the air ratio λ is essential for the exhaust gas purification result in both cases. For this purpose, the lambda value of the exhaust gas is adjusted by the value λ = 1 with a very short cycle time (approx. 0.5 to 5 Hertz) and an amplitude Δλ of 0.005 ≤ Δλ ≤ 0.05 (reducing and oxidizing exhaust gas components are present in a stoichiometric ratio to one another) regulated. 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) of the second component 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 excludes physical mixtures of cerium oxide, zirconium oxide and rare earth oxide. Rather, “cerium/zirconium/rare earth metal mixed oxides” are characterized by a largely homogeneous, three-dimensional crystal structure, which is ideally free of phases made of pure cerium oxide, zirconium oxide or rare earth oxide (so-called solid solution). Depending on the manufacturing process, 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 therefore preferably have the temperature-stable, high-surface support materials mentioned and, in addition to these, the oxygen-storing materials just explained. 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. In the OSC-containing noble metal catalyst, the noble metals can only be deposited on the temperature-stable, high-surface support materials. However, it is preferred if the noble metals are deposited both on the carrier materials mentioned and on the oxygen storage materials.

The first and second components preferably form an ammonia storage with SCR functionality and a function for oxidizing ammonia to nitrogen (e.g. as in WO2008106523A2 ). If there are not enough nitrogen oxides in the system to oxidize the stored ammonia, the ammonia can also be converted into nitrogen with the oxygen present via the second component. In both cases, if possible, no ammonia or N ₂ O is released into the environment. In the broadest sense, the first component and the second component of the catalyst for reducing ammonia emissions can therefore preferably consist of an ammonia-storing coating paired with a second coating that has an oxidative effect on ammonia. As such, according to the invention, they are present in separate coatings one above the other on the substrate. It is particularly preferred if both coatings are of the same length. It is particularly preferred if the OSC-containing noble metal catalyst of component two is located as a top layer over the first component made of zeolites and / or zeotypes for storing ammonia as a bottom layer. Most preferably, no further layers are present below or above these two coatings on the substrate.

It has surprisingly proven to be advantageous if there is a thin, separate layer between the two layers just mentioned. The expert is guided by the coating methods mentioned above for their production. This thin layer, between 5 µm and 200 µm, preferably between 10 µm and 150 µm thick, helps to further increase the aging stability of the catalyst to reduce ammonia emissions. As has been shown, a disadvantage of the known systems for reducing ammonia emissions can be that the transition metals in the first component, such as iron and/or copper, tend to enter the exhaust system of a predominantly stoichiometrically operated internal combustion engine after a long period of use Component for the oxidation of ammonia to diffuse and poison it. The result is a lower activity of the ammonia-storing and oxidative components. Suitable materials for this layer are, in particular, those selected from the group consisting of aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, zeolites or mixtures of one or more of these. A layer made of aluminum oxide is particularly preferred in this context. The inert layer is preferably located at the same length above the first layer and below the second layer on the substrate.

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. Preferably it is as in DE102013210270A1 , DE102020101876A1 , EP3247493A1 or EP3727655A1 constructed as described. Zoned or layered versions are now the norm for TWCs. In the car exhaust system according to the invention, the first catalyst with three-way activity has, in a further preferred embodiment, a 2-layer structure with two different three-way coatings, preferably as in EP3247493A1 described. Downstream refers to the fact that the exhaust gas flow first hits the upstream catalytic converter and then the downstream catalytic converter. The reverse applies to the upstream side.

With regard to the Euro 7 legislation, it has proven to be advantageous if an exhaust system for a predominantly stoichiometric engine has a unit for filtering small soot and ash particles. Preference is therefore given to an exhaust system that additionally has a possibly catalytically coated GPF between the first three-way catalytic converter and the catalytic converter to reduce ammonia emissions ( 5 ). GPF are gasoline particle filters and are well known to those skilled in the art ( EP3737491A1 , EP3601755A1 ). Particularly preferred is an exhaust gas design in which the first three-way catalytic converter and the GPF on the downstream side are installed in a housing close to the engine, if necessary.

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). tig to the catalyst to reduce ammonia emissions. 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 catalytic converter 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 in EP3247493A1 described. 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 a second three-way catalyst and a possibly catalytically coated wall flow filter (GPF) in front of the catalyst to reduce ammonia emissions. The catalyst for reducing ammonia emissions is preferably located last in the underbody and in fluid communication with the further catalyst or catalysts or the filter of the car exhaust system.

The car exhaust system preferably has no additional injection device for ammonia or a precursor compound for ammonia. However, it is possible that there is an addition unit for secondary air in the exhaust system upstream of the catalytic converter to reduce ammonia emissions or upstream of the wall flow filter (analog WO2019219816A1 ).

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 N ₂ O does not occur. Even after intensive aging, the present exhaust system is robust enough to fully meet the Euro 7 requirements. The exhaust system shown here leads to a significantly improved suppression of ammonia emissions and the formation of nitrous oxide and promises a long active service life.

Characters:

• 1 : Chart explaining the measurement of ammonia storage capacity.
• 2 : Catalyst for reducing ammonia emissions (1), coating with an OSC-containing noble metal catalyst containing rhodium (2), coating with transition metal-exchanged zeolites or zeotypes for storing ammonia (3).
• 3 : Schematic representation of the catalysts tested in the underbody position, where TWC and SCR coatings can be combined in different ways.
• 4 : Emission values for the in 3 catalysts shown in comparison.
• 5 : Preferred exhaust system with TWC close to the engine, followed by a possibly catalytically coated GPF and a catalytic converter to reduce ammonia emissions in the underbody area.

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 vol.% oxygen, 5 vol.% water and the rest nitrogen with a space velocity of 30000 h ^-1 for 10 minutes conditioned. The measuring temperature of 200 °C is then 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 NH ₃ storage phase is initiated by switching on a gas mixture of 450 ppm ammonia, 0 vol.% oxygen, 5 vol.% water and the rest nitrogen at a space velocity of 30,000 h ^-1 . This gas mixture remains switched on until a 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 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 1 ). The ammonia storage capacity is calculated as the quotient of the stored mass of ammonia divided by the volume of the tested core.

B. Production of the ammonia-storing SCR layers

B1. Preparation of the Fe-loaded zeolite

The zeolite was coated with iron using an incipient wetness process with an iron(III) nitrate solution in a solids mixer. This was followed by treatment in the oven for 8 h at 120 °C and for 2 h at 550 °C in air. For the zeolite of the structural type chabazite (CHA) with a SAR value of 38, a composition with 2.5 wt% Fe ₂ O ₃ based on the total mass of zeolite and Fe ₂ O ₃ was prepared. For the reference catalyst with chabazite (CHA) with a SAR value of 24, a composition with 4.0 wt% Fe ₂ O ₃ based on the total mass of zeolite and Fe ₂ O ₃ was prepared

B2. Production of an iron-containing zeolite coating

The coating with an Fe-loaded zeolite was carried out after co-grinding with Nyacol ^® -AL20 binder on a cordierite substrate with 164.8 g/L 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 600 °C for 2 h. A layer containing precious metal 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.

Catalysts were used analogously to known processes as in 3 a and b shown schematically.

E. Aging and testing of the catalysts

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 NH ₃ emissions” refers to the NH ₃ emissions of a system with one of the catalytic converters shown in the underbody position over the entire driving cycle in relation to the emissions of the corresponding system in the absence of a catalytic converter in the underbody position . The value “Selectivity to N ₂ O” relates the additional N ₂ O molecules formed by the ASC in the bottom position to the NH ₃ molecules converted by the ASC according to the formula: $$\begin{array}{l}{\text{selectivity}}_{\ddot{\text{a}}}\text{O}=100\cdot 2\cdot \left[\left({\text{Amount of substance N}}_2\text{O to ASC}\right)-\left({\text{Amount of substance N}}_2\text{O beforeASC}\right)\right]\\ \text{/}\left[\left({\text{Amount of <figure-callout id="3" label="substance N" filenames="DE102023101763A1\_0003.png" state="{{state}}">substance N</figure-callout>}}_3\text{before ASC}\right)-\left({\text{Amount of <figure-callout id="3" label="substance NH" filenames="DE102023101763A1\_0003.png" state="{{state}}">substance NH</figure-callout>}}_3\text{according to ASC}\right)\right].\end{array}$$

F. Results

Comparison of catalysts with different zeolite materials: See Fig. 4

All catalysts contain a TWC layer of 5 g/ft ³ Rh.

Aging: Fuel-cut aging, 115 h, 800 °C inlet temperature for the catalytic converters in the underbody position

Volume of the underbody catalytic converter: 0.83 L

A catalyst containing an SCR layer with a chabazite structural type iron-containing zeolite with a SAR value of 38 and combined in a layered design with a 5 g/ft ³ Rh TWC layer shows improved catalytic performance in comparison to a corresponding catalyst in which an iron-containing chabazite with a SAR value of 24 is used. The catalyst according to the invention is also superior to corresponding catalysts based on commercially available iron-containing zeolites of the BEA structural type, since the latter no longer have any catalytic performance after the aging conditions used here.

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

Table 1 shows the temperatures at which the catalysts A and B show 50% conversion for hydrocarbons, carbon monoxide and nitrogen oxides in a light-off test after fuel-cut aging in the underbody position. Both catalysts therefore have additional three-way catalytic activity. Table 1: T ₅₀ LO HC T ₅₀ LO CO T _{50 NO} A 348 343 339 b 327 321 318

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

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Claims (12)

Exhaust system for reducing harmful exhaust gas components from predominantly stoichiometrically operated internal combustion engines, in particular spark-ignited gasoline engines, comprising a first three-way catalytic converter and, downstream of this, a catalyst for reducing ammonia emissions, characterized in that it has the following components: - a first component with a transition metal-exchanged zeolite and /or zeotypes with the framework structure of a CHA, LEV, ZFI, AFX or AEI and a SAR or equivalent ratio of the zeotype of >32; - a second component with an OSC-containing noble metal catalyst which has rhodium; and wherein the two components are applied as superimposed layers on a substrate. exhaust system Claim 1 , characterized in that the second component lies completely above the first and completely covers it. Exhaust system according to one of the preceding claims, characterized in that the two layers are the same length. Exhaust system according to one of the Claims 1 - 3 , characterized in that the zeolite or zeotype used is those selected from the group consisting of CHA and AEI. Exhaust system according to one of the preceding claims, characterized in that the transition metal-exchanged zeolite and/or zeotype has a SAR or an equivalent ratio of the zeotype of >35 and <45. Exhaust system according to one of the preceding claims, characterized in that iron and/or copper are present as transition metals. Exhaust system according to one of the preceding claims, characterized in that the first component has an ammonia storage capacity of between 0.25 and 10.0 g NH ₃ per liter of carrier volume. Exhaust system according to one of the preceding claims, characterized in that the noble metals in the noble metal catalyst are deposited both on temperature-stable, high-surface area support materials and on oxygen storage materials. 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 for reducing ammonia emissions. exhaust system Claim 9 , characterized in that the first three-way catalytic converter and the GPF are installed in a position close to the engine. 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. 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.

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