CN118632737A - Exhaust system for internal combustion engines operating predominantly in stoichiometric mode, containing a catalyst for reducing ammonia emissions - Google Patents

Abstract

The present invention relates to an exhaust system for reducing exhaust emissions, and in particular ammonia emissions, in the exhaust system of a spark-ignition engine operating predominantly in a stoichiometric manner.

Description

Exhaust systems for internal combustion engines operating predominantly in stoichiometric mode, containing catalysts for reducing ammonia emissions

The present invention relates to an exhaust system for reducing exhaust emissions, and in particular ammonia emissions, in the exhaust system of a spark-ignition engine operating predominantly in a stoichiometric manner.

Exhaust gases from internal combustion engines (i.e. spark-ignition engines or Otto engines powered by gasoline or natural gas) which are operated predominantly (operating time>50%) with a stoichiometric air/fuel mixture are purified in a conventional method with the aid of a three-way catalyst (TWC). Such a catalyst is able to simultaneously convert the three main gaseous pollutants of the engine (i.e. hydrocarbons, carbon monoxide and nitrogen oxides) into harmless components. "Stoichiometric" means that on average exactly as much air is available for the combustion of the fuel present in the cylinder as is required for complete combustion. The combustion air ratio λ (A/F ratio; air/fuel ratio) sets the relationship between the mass of air actually available for combustion, m _{L, actual} and the stoichiometric air mass, m _{L, stoichiometric} :

If λ<1 (e.g. 0.9), this means "not enough air" and we speak of a rich exhaust gas mixture; λ>1 (e.g. 1.1) means "excess air" and the exhaust gas mixture is called lean. The expression λ=1.1 means that there is 10% more air than required for the stoichiometric reaction. The same applies to the exhaust gas from an internal combustion engine.

The catalytically active materials used in known three-way catalysts are usually platinum group metals, in particular platinum, palladium and rhodium, which are present, for example, on gamma-aluminum oxide as a support material. In addition, three-way catalytic converters contain oxygen storage materials, for example cerium/zirconium mixed oxides. In the latter case, cerium oxide, a rare earth metal oxide, constitutes the basic component for oxygen storage. In addition to zirconium oxide and cerium oxide, these materials may also contain additional components, such as further rare earth metal oxides or alkaline earth metal oxides. The oxygen storage material is activated by applying a catalytically active material, such as a platinum group metal, and therefore also serves as a support material for the platinum group metal.

The Euro 7 regulation, which will come into force in the mid-2020s, will for the first time regulate the emissions of ammonia (NH ₃ ) and nitrous oxide (N ₂ O) for internal combustion engines operating in a stoichiometric manner. Toxic ammonia and the potent greenhouse gas N ₂ O are known as secondary emissions, and their emissions cannot be reduced sufficiently by current exhaust gas aftertreatment systems. Complying with the stringent limits for secondary emissions in a wide range of driving situations requires the development of robust technical solutions in the form of new catalysts for gasoline exhaust systems. Extremely dynamic ambient conditions, especially in the underbody of gasoline vehicles, represent a major challenge.

Compliance with strict emission values for ammonia, in particular for the medium and low temperature ranges, requires the use of storage materials for storing NH ₃ during the rich operating conditions of the internal combustion engine, since ammonia is mainly formed under these exhaust gas conditions. The conversion of the stored ammonia then takes place during the lean operating point by oxidation on the layer containing the noble metal and/or as part of the SCR reaction. In this case, the aim is to achieve the lowest possible selectivity for N ₂ O. A special requirement for the catalysts considered here is a high aging stability of the materials used: In addition to the stability under lean gas conditions, their use in the exhaust system of internal combustion engines operating in a stoichiometric manner requires that they are stable even under hydrothermal exhaust gas conditions in exhaust gases with a rich or stoichiometric composition.

In particular, in the diesel sector or for use in lean-burn DI gasoline engines, the use of catalysts that preferentially convert ammonia into nitrogen has been discussed (US5120695; EP1892395A1; EP1882832A2; EP1876331A2; WO12135871A1; US2011271664AA; WO11110919A1, EP3915679A1). Even in the field of LNG gasoline engines (EP24258A1), the use of ammonia slip catalysts (or ASC for short) has been described. These catalysts usually consist of an SCR catalytically active component and an ammonia oxidation catalytic component. These catalysts are usually located in the underbody at the last point in the exhaust system. If there are not enough nitrogen oxides in the system to oxidize the stored ammonia, the ammonia can also be converted into nitrogen via the ASC with the oxygen present. It has been shown that the aging stability of ASC catalysts also depends significantly on their design.

It is therefore an object of the present invention to provide a novel exhaust system which allows the operation of internal combustion engines, in particular internal combustion engines which are mainly operated in a stoichiometric manner, even under the new Euro 7 regulation. In particular, the relevant limit values should be reliably observed, in particular for NH ₃ and N ₂ O. Furthermore, the system should also be correspondingly robust and flexible in order to be able to withstand the operating conditions in the exhaust system of the corresponding motor vehicle for a sufficiently long time.

These objects and further objects which are obvious to the person skilled in the art from the prior art are achieved by an exhaust gas system and a method for exhaust gas purification according to claim 1 and claim 11. Claims 2 to 10 relate to preferred embodiments of the exhaust gas system and therefore also apply to the method according to the invention.

This object is achieved in a relatively simple and relatively unsurprising manner by proposing an exhaust system for reducing harmful exhaust gas components from an internal combustion engine, in particular an engine which is mainly operated in a stoichiometric manner, such as a spark-ignition gasoline engine, the exhaust system comprising a first three-way catalyst and, downstream thereof, a catalyst for reducing ammonia emissions, the exhaust system having the following components:

- a first component comprising a transition metal exchanged zeolite and/or zeotype having a three-dimensional framework structure;

- a second component, the second component comprising an OSC-containing precious metal catalyst, the OSC-containing precious metal catalyst comprising rhodium; and

The two components are applied as superimposed layers on a substrate. The system according to the invention is characterized by extremely good performance both with regard to conventional exhaust gas components and with regard to NH ₃ and N ₂ O emissions. It responds well to the dynamic requirements of the exhaust gas system of a spark-ignition engine and is correspondingly robust so as to also meet these requirements within a sufficient period of time.

The components of the catalyst for reducing ammonia emissions are applied to a carrier, preferably to a flow-through substrate, by a coating step familiar to those skilled in the art (DE102019100099A1 and the literature cited therein). In this case, filter substrates such as wall-flow filters are also possible. The flow-through substrate is a catalyst carrier commonly used in the prior art, which may be composed of a metal (e.g., WO17153239A1, WO16057285A1, WO15121910A1 and the literature cited therein) or a ceramic material. "Corrugated substrates" can also be considered as flow-through substrates. These are known to those skilled in the art as carriers, which are made of corrugated sheets composed of inert materials. Suitable inert materials are, for example, fiber materials having an average fiber diameter of 50µm to 250µm and an average fiber length of 2mm to 30mm. Preferably, a fiber heat-resistant material made of silicon dioxide, specifically glass fiber, is used. However, refractory ceramics such as cordierite, silicon carbide or aluminum titanate are preferably used as honeycomb carriers. The number of channels per surface area is characterized by the pore density, which typically ranges between 300 and 900 pores per square inch (cpsi). The channel walls in the ceramic have a wall thickness between 0.5 mm and 0.05 mm.

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

According to the invention, the components are present on the substrate as separate coatings stacked on top of each other. It is preferred that the second component is completely located above the first component and completely covers the first component. This means that the first component does not protrude beyond the second component at any end. It is particularly preferred that the lengths of the two coatings with their respective components are equal (Fig. 2). The length of the layers can be selected by a person skilled in the art. They are preferably located on the flow-through substrate and occupy at least 10% and a maximum of 100%, more preferably 20% to 90%, and extremely preferably 30% to 80% of the length of the substrate.

As already mentioned above, the first component of the catalyst for reducing ammonia emissions consists of a zeolite and/or zeotype for storing ammonia. In principle, those skilled in the art are familiar with zeolites and zeotypes from the diesel field that can be used for this purpose. In this case, the working mode of the zeolite or zeotype is based on the fact that they can temporarily store ammonia in the operating state of the exhaust gas purification system, where ammonia is produced, for example, by excessive reduction of nitrogen oxides via a three-way catalyst installed upstream, but it cannot be converted by other conventional three-way catalysts, for example due to lack of oxygen or insufficient operating temperature. The ammonia stored in this way can then be removed and subsequently or directly converted when the operating state of the exhaust gas purification system changes, for example when there is enough oxygen or nitrogen oxides.

According to the present invention, zeolites and zeolites are present in the first component of the catalyst for reducing ammonia emissions. According to the classification of IZA (https://europe.iza-structure.org/IZA-SC/ftc_table.php), the International Zeolite Association, zeolites or zeolites can be divided into different categories. Zeolites are then classified, for example, according to their channel systems and their framework structures. For example, halogenite and mordenite are classified as zeolites with one-dimensional channel systems. Their channels are not connected to each other. Zeolites containing two-dimensional channel systems are characterized in that their channels are connected to each other in a type of layered system. The third group has a three-dimensional framework structure with cross-layer connections between channels. In the present invention, three-dimensional zeolites or zeolites are used according to the present invention [Ch. Baerlocher, W.M.Meier and D.H. Olson, Atlas of Zeolite Framework Types, Elsevier, 2001].

According to the present invention, the term "zeolite" refers to a porous material having a lattice structure of corner-connected AlO ₄ and SiO ₄ tetrahedra according to the following general formula (WM Meier, Pure & Appl. Chem., Vol. 58, No. 10, pp. 1323-1328, 1986):

Therefore, the structure of zeolite is made up of a grid, and this grid is made of tetrahedron and surrounds passage and cavity.There is a difference between naturally occurring zeolite and the zeolite of synthetic production.The term "zeolite-like" is understood to mean a zeolite-like compound with the same structural type as the naturally occurring or synthetically produced zeolite compound, but the difference from this type of compound is that the corresponding cage structure is not only composed of aluminum and silicon framework atoms.In this type of compound, aluminum and/or silicon structural atoms are replaced by other trivalent, tetravalent or pentavalent structural atoms, such as B (III), Ga (III), Ge (IV), Ti (IV) or P (V).The modal method used in implementation is to replace aluminum and/or silicon framework atoms with phosphorus atoms, for example, in silicoaluminophosphate or aluminum phosphate that are crystallized into zeolite structure type.

Examples of suitable three-dimensional zeolites are of the structural types CHA, AEI, BEA, AFX. It is particularly preferred that the zeolite or zeotype in the automotive exhaust gas catalyst according to the invention is selected from three-dimensional zeolites such as CHA, AEI and the corresponding zeotypes of these structural types, such as SAPO. Mixtures thereof may also be present. The use of CHA is very particularly preferred.

Here, the aging stability of zeolites or zeolites used in the exhaust system of engines that burn mainly in a stoichiometric manner is of particular concern, because the temperatures here are generally higher than in lean-burn engines. In this regard, it is desirable that those materials can withstand the sometimes very high and strongly changing hydrothermal conditions for as long as possible. In addition, the exhaust gas composition is also different compared to the exhaust gas of a lean-burn engine. In particular, the concentrations of hydrocarbons and carbon monoxide that reach the catalyst according to the present invention are higher than those in a lean-burn engine, and depending on the driving mode, the composition also varies around the stoichiometric range (rich/lean variation). In this case, the hydrothermal temperature stability of zeolites and zeolites depends greatly on the SAR value (silicon dioxide to aluminum oxide ratio) of the zeolite or the ratio corresponding to this value in the zeolite. The amount of silicon atoms remaining in the framework is then compared with the substituted atoms. It has proven to be advantageous that the zeolite has a SAR value of 10 to 50, preferably 12 to 35, and most preferably 13 to 30. The same applies to zeolites with corresponding ratios.

According to the present invention, the zeolite or zeolite-like used is ion-exchanged with transition metal ions. The latter is preferably selected from iron and/or copper. Iron is particularly preferred because it has a smaller oxidizing effect on ammonia than copper. These compounds have the ability to react nitrogen oxides present in the exhaust gas and stored ammonia under lean conditions to form nitrogen. In this case, the zeolite or zeolite-like is used as a catalyst for selective catalytic reduction (SCR) (see WO2008106518A2, WO2017187344A1, US2015290632AA, US2015231617AA, WO2014062949A1, US2015231617AA). In this case, SCR capability is understood to mean the ability to selectively convert _NOx and _NH3 in lean exhaust gas into nitrogen.

Advantageously present in the catalyst for reducing ammonia emissions, metals such as iron and/or copper, are present in the first component in a specific proportion. This is 0.4% to 10% by weight of the first component, more preferably 0.8% to 6% by weight, and very preferably 1.5% to 4.8% by weight. The ratio of iron and/or copper to aluminum of the zeolite is between 0.15 and 0.8, preferably between 0.2 and 0.5, and most preferably between 0.3 and 0.5. For zeolites, the corresponding ratio is applicable to the exchange sites available there. In this case, the metal is at least partially present in the zeolite or zeolite in the form of ion exchange. Preferably, the zeolite or zeolite of ion exchange has been introduced into the first component. However, the zeolite or zeolite can also be mixed with, for example, a binder and a solution of metal ions in a liquid (preferably water) and then dried (preferably by spray drying). Here, a specific amount of metal can also be present on the binder in the form of an oxide. Both methods are possible.

In addition to the zeolite or zeolite-like, the first component may preferably contain other components, in particular catalytically inactive components, such as a binder. For example, active temperature-stable metal oxides with little or no catalytic activity, such as SiO ₂ , Al ₂ O ₃ and ZrO ₂ , are suitable as binders. The skilled person is aware of the materials that can be used here. The proportion of such binders in the first component may, for example, reach up to 15% by weight of the component, preferably up to 10% by weight. As mentioned, the binder may also contain the metals listed above. The binder is suitable for ensuring a stronger adhesion of the coating to the carrier. For this reason, a specific particle size of the metal oxide in the binder is advantageous. This can be adjusted accordingly by the skilled person.

The ammonia storage capacity discussed in the context of the present invention is specified as the quotient of the mass of ammonia stored per liter of catalyst support volume. The zeolite or zeotype should increase the ammonia storage capacity of the exhaust gas purification system to at least 0.25 g of ammonia per liter of support volume (measured in the fresh state). In general, the storage capacity of the ammonia storage component used should be sufficient so that between 0.25 g and 10.0 g of NH ₃ / liter of support volume, preferably between 0.5 g and 8.0 g of NH ₃ / liter of support volume, and particularly preferably between 0.5 g and 5.0 g of NH ₃ / liter of support volume of ammonia can be stored in the system (always relative to fresh conditions). The zeolite or zeotype is already present in the catalyst in sufficient quantity to reduce ammonia emissions. The determination of the ammonia storage capacity is shown further below.

The second component consists of an OSC-containing noble metal catalyst containing rhodium. Specifically, the noble metals include platinum group metals platinum, palladium and rhodium. OSC stands for oxygen storage catalyst. The OSC-containing noble metal catalyst therefore contains oxygen storage material.

OSC noble metal-containing catalysts have the function of storing oxygen in the exhaust gas of an internal combustion engine. Cerium or a cerium-zirconium mixed oxide (see below) is always used as an oxygen storage material. Therefore, OSC noble metal-containing catalysts are characterized by the presence of a specific amount of these oxygen storage materials. Specifically, the component contains an oxygen storage material in an amount greater than 5 g/L of the support volume, preferably greater than 10 g/L of the support volume, and most preferably greater than 20 g/L of the support volume. In this case, the entire cerium-zirconium mixed oxide with all its components is included.

The corresponding OSC-containing noble metal catalyst has the ability to oxidize substances (NH ₃ , HC, CO) present in the already slightly rich exhaust gas of an internal combustion engine that is operated predominantly in a stoichiometric manner. In this case, the component is preferably designed in such a way that it becomes active at correspondingly low temperatures. Ammonia stored in the zeolite or zeolite-like is preferably converted into harmless nitrogen via this component. The oxidation should not be too great, otherwise certain amounts of the potent greenhouse gas N ₂ O would be formed by oxidation of the ammonia.

Therefore, the second component in the form of an OSC noble metal catalyst contains a material that has an oxidizing effect on ammonia in particular. Preferably, the component contains a temperature-stable, high surface area metal oxide, an oxygen storage material and at least the noble metal rhodium. Platinum and/or palladium may also be present. However, most preferably, only rhodium is present in the second component. The total noble metal content of the component is preferably 0.015 g/L to 5 g/L carrier volume, more preferably 0.035 g/L to 1.8 g/L carrier volume, and particularly preferably 0.07 g/L to 1.2 g/L carrier volume. If platinum and/or palladium are used, the former should be in the range of 0.015 g/L to 1.42 g/L carrier volume, more preferably 0.035 g/L to 0.35 g/L carrier volume in the coating. When present in the coating, palladium can be present in a carrier volume between 0.015 g/L and 1.42 g/L, preferably 0.035 g/L to 0.35 g/L carrier volume.

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

The noble metal in the OSC-containing second component is usually fixed on one or more temperature-stable, high-surface-area metal oxides as support materials. All materials familiar to the person skilled in the art for this purpose are considered to be supported materials. Such materials are specifically metal oxides with a BET surface area of 30 m ² /g to 250 m ² /g, preferably 100 m ² /g to 200 m ² /g (determined according to the latest version of DIN 66132 on the date of application). Particularly suitable support materials for noble metals are selected from the series consisting of the following: aluminum oxide, doped aluminum oxide, silicon oxide, titanium dioxide and mixed oxides of one or more of them. Doped aluminum oxide is, for example, aluminum oxide doped with lanthanum oxide, zirconium oxide, barium oxide and/or titanium oxide. Aluminum oxide or lanthanum-stabilized aluminum oxide is advantageously used, wherein the amount of lanthanum used is 1% to 10% by weight, preferably 3% to 6% by weight, calculated in each case as La ₂ O ₃ and based on the weight of the stabilized aluminum oxide. Likewise in the case of aluminum oxide doped with barium oxide, the proportion of barium oxide, calculated in each case as BaO and based on the weight of the stabilized aluminum oxide, is in particular 1% to 10% by weight, preferably 3% to 6% by weight. Another suitable support material is a 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. In this context, lanthanum-stabilized gamma aluminum oxide having a surface area of 100 m ² /g to 200 m ² /g is particularly advantageous. Activated aluminum oxides of this type are frequently described in the literature and are commercially available.

Modern Otto engines operate under conditions of a discontinuous process with an air ratio λ. They are subject to periodic changes in the air ratio λ and thus to periodic changes in the exhaust gas redox conditions in a defined manner. In both cases, such changes in the air ratio λ are significant for the exhaust gas purification results. For this purpose, the λ value of the exhaust gas is adjusted to have a very short cycle time (approximately 0.5 Hz to 5 Hz) and an amplitude Δλ of 0.005 ≤ Δλ ≤ 0.05 at the value λ=1 (the reduced and oxidized exhaust gas components are present in a stoichiometric relationship with each other). Deviations from this state also occur due to the dynamic engine operating mode of the vehicle. In order not to have the mentioned deviations have a negative impact on the exhaust gas purification results when the exhaust gas flows through the three-way catalyst, the oxygen storage material contained in the catalyst offsets these deviations to a certain extent by absorbing oxygen from the exhaust gas or releasing oxygen into the exhaust gas as required (Catalytic Air Pollution Control, Commercial Technology, R. Heck et al., 1995, p. 90).

The OSC noble metal catalyst (modern three-way catalyst) of the second component therefore contains an oxygen storage material, specifically cerium or Ce/Zr mixed oxide. The mass ratio of cerium oxide to zirconium oxide in these mixed oxides can vary over a wide range. This ratio is, for example, 0.1 to 1.5, preferably 0.15 to 1.5 or 0.2 to 0.9. The preferred cerium/zirconium mixed oxide contains one or more rare earth metal oxides, and can therefore be referred to as cerium/zirconium/rare earth metal mixed oxides. Within the meaning of the present invention, the term "cerium/zirconium/rare earth metal mixed oxide" does not include a physical mixture of cerium oxide, zirconium oxide and rare earth oxide. On the contrary, "cerium/zirconium/rare earth metal mixed oxide" is characterized by a substantially uniform three-dimensional crystal structure of a phase (called a fixed liquid) that is ideally free of pure cerium oxide, zirconium oxide or rare earth oxide. However, depending on the manufacturing process, an incompletely uniform product may be produced, which can generally be used without disadvantages. The same applies to cerium/zirconium mixed oxides that do not contain any rare earth metal oxide. In all other aspects, the term "rare earth metal" or "rare earth metal oxide" within the meaning of the present invention does not include cerium or cerium oxide. Lanthanum oxide, yttrium oxide, praseodymium oxide, neodymium oxide and/or samarium oxide can be considered, for example, as rare earth metal oxides in a cerium-zirconium-rare earth metal mixed 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 preferably lanthanum oxide and yttrium oxide, yttrium oxide and praseodymium oxide, as well as lanthanum oxide and praseodymium oxide are present simultaneously in a cerium/zirconium/rare earth metal mixed oxide. In a preferred embodiment, this noble metal catalyst comprises two different cerium/zirconium/rare earth metal mixed oxides, preferably one doped with La and Y, and one doped with La and Pr. In an embodiment of the invention, the oxygen storage component preferably does not contain neodymium oxide.

The proportion of rare earth metal oxide in the cerium/zirconium/rare earth metal mixed oxide 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 oxide contains yttrium oxide as 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 oxide contains praseodymium oxide as 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 oxide contains lanthanum oxide and another rare earth oxide as rare earth metal, such as yttrium oxide or praseodymium oxide, their mass ratio is specifically 0.1 to 1.25, preferably 0.1 to 1. Typically, such a noble metal catalyst contains an oxygen storage material in an amount of 15 g/L to 120 g/L, based on the volume of the carrier or substrate.

Therefore, the OSC noble metal-containing catalyst has the above-mentioned temperature-stable, high-surface-area support material and the oxygen storage material just explained in addition to these. The mass ratio of the temperature-stable high-surface-area support material and the oxygen storage component in this component is generally 0.25 to 1.5, such as 0.3 to 1.3. In an exemplary embodiment, the weight ratio of the sum of the mass of all support materials, such as alumina (including doped alumina) in the OSC noble metal-containing catalyst to the sum of the mass of all cerium/zirconium mixed oxides is 10:90 to 75:25, preferably 20:80 to 65:35. In the OSC noble metal-containing catalyst, the noble metal can only be deposited on the temperature-stable high-surface-area support material. However, it is preferred that the noble metal is deposited on both the support material and the oxygen storage material.

The first component and the second component preferably form an ammonia storage material having an SCR function 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, ammonia can also be converted into nitrogen with the oxygen present. In both cases, as little ammonia or _N2O as possible is released into the environment. Therefore, in the broadest sense, the first component and the second component of the catalyst for reducing ammonia emissions may preferably consist of an ammonia storage coating paired with a second coating having an oxidizing effect on ammonia. Therefore, according to the present invention, they are present in separate coatings overlapping each other on a substrate. It is particularly preferred that the lengths of the two coatings are the same. It is particularly preferred that the OSC noble metal catalyst of component two is located as an upper layer above the first component made of zeolite and/or zeolite for storing ammonia as a lower layer. Most preferably, there is no additional layer on the substrate below or above these two coatings.

In another preferred embodiment, it has proven to be advantageous if there is a thin additional separate layer of an inert, temperature-stable, high-surface-area metal oxide as mentioned above between the two layers just mentioned. Those skilled in the art will produce them using the coating methods mentioned above. This thin layer between 5µm and 200µm, preferably between 10µm and 150µm0, is highly helpful in further increasing the aging stability of the catalyst for reducing ammonia emissions. The results show that the disadvantage of the known systems for reducing ammonia emissions may be that transition metals (such as iron and/or copper) in the SCR component tend to diffuse into the ammonia oxidation component and poison it after long-term use in the exhaust system of an internal combustion engine that mainly operates in a stoichiometric manner. The result is a lower activity of the SCR and the oxidation component. In particular, those materials selected from aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, zeolite or mixtures thereof can be used as the material of this layer. In this case, a layer of aluminum oxide or silicon oxide is very particularly preferred, which is preferably located on the substrate within the same length range above the lower layer and below the upper layer.

The present exhaust system has a first three-way catalyst and a downstream catalyst for reducing ammonia emissions. In this case, the first three-way catalyst may have the same components as the OSC precious metal catalyst containing the second component. Preferably, it is structured as described in DE102013210270A1, DE102020101876A1, EP3247493A1, EP3727655A1. Partitioned or layered embodiments are now the standard for TWC. In an automobile exhaust system according to the present invention, in a further preferred embodiment, the first catalyst having three-way activity has a 2-layer structure comprising two different three-way coatings, preferably as described in EP3247493A1. Downstream refers to the fact that the exhaust gas flow first hits the upstream catalyst and then only hits the catalyst positioned downstream. The opposite applies to the upstream side.

With regard to the Euro 7 regulation, it has proven advantageous that the exhaust system for engines that burn predominantly in a stoichiometric manner has a unit for filtering small soot and ash particles. Therefore, preference is given to an exhaust system that also has a GPF, which may be catalytically coated, between the first three-way catalyst and the catalyst for reducing ammonia emissions ( FIG. 8 ). GPFs are gasoline particulate filters and are well known to those skilled in the art ( EP3737491A1 , EP3601755A1 ). Particularly preferred is an exhaust design in which the first three-way catalyst and the downstream GPF are installed in a housing close to the engine.

Close to the engine in the sense of the present invention is understood to mean an area of the exhaust system which is located close to the engine, i.e. approximately 10 cm to 80 cm, preferably 20 cm to 60 cm from the engine outlet. It has proven to be advantageous if the catalyst for reducing ammonia emissions is installed last in the direction of the exhaust gases in the underbody of the vehicle, so that the exhaust gases are subsequently released into the ambient air. The exhaust system may also contain additional exhaust units, such as an additional three-way catalyst or a hydrocarbon storage unit (HC trap) or a nitrogen oxide storage unit (LNT). The underbody is the area below the cab.

In a further preferred embodiment, in the automobile exhaust system according to the present invention, at least one second three-way catalyst (TWC) is located between the first three-way catalyst and the upstream of the catalyst for reducing ammonia emissions. The three-way activity has been described above. This is explicitly mentioned, specifically about the type and amount of the individual components. The three-way catalyst is preferably one as described in the prior art (DE102013210270A1, DE102020101876A1, EP3247493A1, EP3727655A1). Partitioned or layered embodiments are now the standard for TWC. In a further preferred embodiment, in the automobile exhaust system according to the present invention, at least one of the additional catalysts with three-way activity has a 2-layer structure including two different three-way coatings, preferably as described in EP3247493A1. The above-mentioned at least second three-way catalyst in the exhaust system according to the present invention can be installed in the underbody of the vehicle, but it can also be located in a position close to the engine. The range of possible European 7 systems is large. For example, up to four three-way catalysts may be installed per exhaust system upstream of the catalyst for reducing ammonia emissions.

In an alternative embodiment, at least one second three-way catalyst and a possibly catalytically coated wall-flow filter (GPF) are located upstream of the catalyst for reducing ammonia emissions. In this case, the catalyst for reducing ammonia emissions is preferably located at the rearmost point of the vehicle underbody and is in fluid communication with other catalysts or filters of the vehicle exhaust system. Preferably, in this case, the vehicle exhaust system does not have additional injection devices for ammonia or ammonia precursor compounds. However, the addition unit for secondary air may be located in the exhaust system upstream of the catalyst for reducing ammonia emissions or upstream of the wall-flow filter (similar to WO2019219816).

In another aspect, the invention relates to a method for reducing harmful exhaust gas components from an internal combustion engine, in particular a spark-ignition gasoline engine, which is operated predominantly in a stoichiometric manner, wherein 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 relates to an exhaust gas purification system, in particular an exhaust gas purification system for an internal combustion engine that operates in a stoichiometric manner. There are operating points of the engine that burns in a stoichiometric manner, in which rich exhaust gas is produced within a specific temperature range. This can lead to excessive reduction of nitrogen oxides arriving via a three-way catalyst to ammonia. The ammonia should not be released into the environment. Therefore, ammonia is stored via a catalyst for reducing ammonia emissions and then oxidized to nitrogen under slightly oxidizing conditions. Attention must also be paid here to ensure that excessive oxidation to _N2O does not occur as much as possible. Even after intensive aging, the exhaust system is robust enough to fully meet the European 7 requirements. The coating design proposed here leads to significantly improved suppression of ammonia emissions and nitrous oxide formation. This ensures a long effective service life of the envisaged exhaust system.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1: Graph to explain the measurement results of ammonia storage capacity.

Figure 2: shows a catalyst for reducing ammonia emissions (1), having a coating of an OSC noble metal catalyst comprising rhodium (2), having a coating of a transition metal exchanged zeolite or zeotype for storing ammonia (3).

Figure 3: Schematic representation of the catalysts tested in the underbody position, TWC and SCR coatings in different combinations with one another.

FIG. 4 : Schematic representation of the catalysts tested in an underbody location with different arrangements of a rhodium-containing TWC coating or a platinum-containing oxide layer.

FIG. 5 : shows the comparison results of the emission values of the catalysts shown in FIG. 3 .

FIG. 6 : shows the comparison results of the emission values of the catalysts shown in FIG. 4 .

FIG. 7 : shows the comparison results of N ₂ O selectivity of the catalysts shown in FIG. 4 .

Figure 8: shows a preferred exhaust system with a TWC close to the engine followed by a possibly catalytically coated GPF and a catalyst for reducing ammonia emissions in the underbody area.

Example

A. Determination of Ammonia Storage Capacity

It is determined experimentally in a flow tube reactor. In order to avoid undesirable ammonia oxidation to the reactor material, a reactor made of quartz glass is used. From the area of the catalyst whose ammonia storage capacity is to be determined, a core is taken as a sample. Preferably, a core with a diameter of 1 inch and a length of 3 inches is taken as a sample. The core is inserted into the flow tube reactor and at a temperature of 600° C., in a gas atmosphere of 500ppm nitric oxide, 5% by volume oxygen, 5% by volume water and the rest being nitrogen, it is conditioned at a space velocity of 30,000 h ^-1 for 10 minutes. Subsequently, at a space velocity of 30,000 h ^-1 in a gas mixture of 0% by volume oxygen, 5% by volume water and the rest being nitrogen, a measurement temperature of 200° C. is approached. After temperature stabilization, the NH ₃ storage stage is initiated by accessing a gas mixture of 450ppm ammonia, 0% by volume oxygen, 5% by volume water and the rest being nitrogen at a space velocity of 30,000 h ^-1 . The gas mixture is added until a fixed ammonia permeation concentration is recorded downstream of the sample. The mass of ammonia stored on the sample was calculated from the recorded ammonia breakthrough curve by integrating from the start of the NH ₃ storage phase until stabilization, taking into account the measured steady-state NH ₃ breakthrough concentration and the known volumetric flow rate (shaded area in Figure 1). The ammonia storage capacity was calculated as the quotient of the mass of ammonia stored divided by the tested core volume.

B. Preparation of Ammonia Storage SCR Layer

B1. Preparation of Cu-loaded zeolite

The zeolite was coated with copper using the incipient wetness method with a copper (II) nitrate solution in a solid mixer. The samples were then treated in an oven at 120°C for 8 hours and at 600°C for 5 hours in air. For zeolites of the Levyn (LEV) structure type, a composition with 3.5 wt% CuO, based on the total mass of zeolite and CuO, was prepared.

B2. Preparation of Fe-loaded zeolite

The zeolite was coated with iron using an incipient wetness method with an iron (III) nitrate solution in a solid mixer. Subsequently, the sample was treated in an oven at 120°C for 8 hours and at 600°C for 2 hours in air. For zeolites of the chabazite structure type (CHA), a composition with 4.0 wt% _Fe2O3 , based on the total mass of _zeolite and _{Fe2O3 ,} was prepared.

B3. Preparation of copper-containing zeolite coating

After co-grinding with Nyacol ^® AL20 binder on a cordierite substrate, coating with Cu-loaded zeolite (88% zeolite, 12% binder) was carried out at a washcoat loading of 150 g/L. The coated catalyst thus obtained was dried at 90°C, then calcined at 350°C for 15 minutes and annealed at 550°C in air for 2 hours. If desired, a layer containing noble metals can be applied as a top layer to the now coated support.

B4. Preparation of iron-containing zeolite coating

After co-grinding with Nyacol ^® AL20 binder on a cordierite substrate, coating with Fe-loaded zeolite was carried out at a washcoat loading of 164.8 g/L (88% zeolite, 12% binder). The coated catalyst thus obtained was dried at 90°C, then calcined at 350°C for 15 minutes and annealed at 550°C for 2 hours in air. If desired, a layer containing a noble metal can be applied as a top layer to the now coated support.

C. Preparation of TWC-Active Precious Metal-Containing Coatings

Alumina stabilized with lanthanum oxide is suspended in water together with an oxygen storage component containing 24% by weight of cerium oxide, 60% by weight of zirconium oxide, 3.5% by weight of lanthanum oxide and 12.5% by weight of yttrium oxide. The weight ratio of aluminum oxide to oxygen storage component to additional lanthanum oxide is 43.6:55.7:0.7. The suspension thus obtained is subsequently mixed with a rhodium nitrate solution under constant stirring. The resulting coating suspension is used directly for coating a commercially available substrate, the coating being applied over 100% of the length of the substrate. The total loading of the repair substrate coating on the catalyst is 122 g/L, the loading of precious metals is 0.177 g/L (5 g/ft ³ ). The coated catalytic converter thus obtained is dried and then calcined. If desired, a layer free of precious metals can be applied as a top layer to the now coated support.

D. Preparation of platinum-containing SiO ₂ /Al ₂ O ₃ layer without TWC activity :

A silicon-aluminium mixed oxide consisting of 95% by weight of aluminium oxide and 5% by weight of silicon oxide is suspended in water. The suspension thus obtained is adjusted to pH 7.6±0.4 and then treated with an EA platinum solution under constant stirring. The resulting suspension is ground and, after stabilisation with ammonium acetate, used to coat a commercially available support, the coating being applied over 100% of the support length. The total loading of the repair substrate coating on the catalyst is 25 g/L and the loading of the noble metal is 0.106 g (3 g/ft ³ ). The coated catalytic converter thus obtained is dried and then calcined. If desired, further layers can be applied as top layers to the now coated support.

The catalysts were prepared or assembled as schematically shown in FIGS. 3 and 4 .

E. Aging and testing of ASC

Aging conditions :

In order to determine the catalytic properties of the catalysts according to the invention, these were first aged in an engine test bench downstream of the TWC close to the engine in the undercarriage position ("fuel cut-off aging"). The aging process consisted of an overrun cut-off aging process with an exhaust gas temperature of 950° C. (maximum bed temperature 1030° C.) before the TWC input close to the engine. The aging time and the inlet temperature of the catalyst in the undercarriage position were specified individually for each test.

Test conditions :

Different catalysts were tested in the underbody position on a highly dynamic engine test bench in the WLTC driving cycle. Here, a Pd/Rh-containing TWC produced in the aged state was placed in a position close to the engine. The "reduction of NH ₃ emissions" values refer in each case to the emissions of the corresponding system without the presence of a catalyst in the underbody position, the NH ₃ emissions of the system with one of the catalysts in the underbody position being shown over the entire driving cycle. The "selectivity to N ₂ O" values relate to the additional N ₂ O molecules formed by the ASC in the underbody position relative to the NH ₃ molecules converted by the ASC according to the following formula:

F. Results

Comparison results of different combinations of SCR and TWC layers :

See Figure 3 and Figure 5

All catalysts with TWC layer contained 5 g/ft ³ Rh.

Aging: Fuel cut-off aging, 38 hours, catalyst inlet temperature in the underbody position is 800°C

Underbody catalyst volume: 0.83L

The catalyst in which the SCR layer and the TWC layer containing 5 g/ft ³ of Rh were combined in a layered design improved catalytic performance compared to the catalyst in which the SCR layer and the TWC layer were arranged one layer after another.

Comparison results of a catalyst with SCR and a rhodium-containing TWC coating with a catalyst with SCR and a platinum-containing oxide coating :

See Figure 4, Figure 6 and Figure 7

The catalyst comprising the TWC layer contained 5 g/ft ³ Rh and the catalyst comprising the oxide layer contained 3 g/ft ³ Pt.

Aging: Fuel cut-off aging, 19 hours, catalyst inlet temperature in the underbody position is 830°C

Underbody catalyst volume: 0.83L

The catalyst in which the SCR layer was combined with a TWC layer containing 5 g/ft ³ Rh in a layered design showed improved catalytic performance and reduced selectivity to N ₂ O compared to the catalyst in which the SCR layer was combined with a typical oxidation washcoat containing 3 g/ft ³ Pt.

Furthermore, the coating in which the TWC coating is on the SCR coating shows better performance with respect to NH ₃ and N ₂ O. The design of the SCR catalyst having a three-dimensional framework structure is also preferred.

Start-up behavior of TWC catalysts with different noble metal loadings (see Table 1) :

Catalysts containing TWC layers with different noble metal contents were compared. Table 1 shows the temperatures at which the catalysts show 50% conversion of hydrocarbons, carbon monoxide and nitrogen oxides in a light-off test after fuel cut-off aging in the underbody position. Lower T ₅₀ values correspond to higher catalytic activity. In this test, the catalyst containing only rhodium shows the best start-up behavior.

Table 1 :

Claims (as amended under Article 19)

1. An exhaust system for reducing harmful exhaust components, the exhaust system comprising an internal combustion engine operating mainly in a stoichiometric manner, and a first three-way catalyst and a catalyst downstream thereof for reducing ammonia emissions,

Its characteristics are:

The system has the following components:

- A first component, the first component comprising a transition metal exchanged zeolite and/or zeotype having a three-dimensional framework structure;

- a second component, the second component comprising an OSC-containing precious metal catalyst, the OSC-containing precious metal catalyst comprising rhodium; and

The two components are applied as superimposed layers on a substrate, and a zeolite or zeotype selected from CHA, AEI and AFX can be used as the zeolite or zeotype having a three-dimensional framework structure.

2. The exhaust system according to claim 1,

Its characteristics are:

The second component is completely located above the first component and completely covers the first component.

3. An exhaust system according to any one of the preceding claims,

Its characteristics are:

The two layers have the same length.

4. An exhaust system according to any one of the preceding claims,

Its characteristics are:

Iron and/or copper are present as transition metals.

5. An exhaust system according to any one of the preceding claims,

Its characteristics are:

The zeolite or zeotype has an ammonia storage capacity of between 0.25 g and 10.0 g NH ₃ per litre of carrier volume.

6. An exhaust system according to any one of the preceding claims,

Its characteristics are:

The precious metal in the precious metal catalyst is deposited on a temperature-stable high surface area support material and an oxygen storage material.

7. An exhaust system according to any one of the preceding claims,

Its characteristics are:

The exhaust system also has a GPF between the first three-way catalyst and the catalyst for reducing ammonia emissions.

8. The exhaust system according to claim 7,

Its characteristics are:

The first three-way catalyst and the GPF are installed close to the engine.

9. An exhaust system according to any one of the preceding claims,

Its characteristics are:

The catalyst for reducing ammonia emissions is installed at the rearmost point in the exhaust gas direction under the vehicle.

10. A method for reducing harmful exhaust gas components from an internal combustion engine, in particular a spark-ignition gasoline engine, operating primarily in a stoichiometric manner,

Its characteristics are:

Passing the exhaust gas through an exhaust system according to any one of the preceding claims.

Claims (11)

1. An exhaust system for reducing harmful exhaust components from an internal combustion engine, in particular a spark-ignition gasoline engine, which is mainly operated in a stoichiometric manner, comprising a first three-way catalyst and a catalyst downstream thereof for reducing ammonia emissions, It is characterized in that The system has the following components: - a first component comprising a transition metal exchanged zeolite and/or zeotype having a three-dimensional framework structure; - a second component, the second component comprising an OSC-containing precious metal catalyst, the OSC-containing precious metal catalyst comprising rhodium; and The two components are applied as superimposed layers to the substrate. 2. The exhaust system according to claim 1, It is characterized in that The second component is completely located above and completely covers the first component. 3. An exhaust system according to any one of the preceding claims, It is characterized in that Both layers are the same length. 4. An exhaust system according to any one of the preceding claims, It is characterized in that Iron and/or copper are present as transition metals. 5. The exhaust system according to claim 1 or claim 2, It is characterized in that As the zeolite or zeotype having a three-dimensional framework structure, a zeolite or zeotype selected from the group consisting of CHA, AEI, BEA and AFX may be used. 6. An exhaust system according to any one of the preceding claims, It is characterized in that The zeolite or zeotype has an ammonia storage capacity of between 0.25 g and 10.0 g NH ₃ per litre of carrier volume. 7. An exhaust system according to any one of the preceding claims, It is characterized in that The noble metal in the noble metal catalyst is deposited on a temperature stable, high surface area support material and on an oxygen storage material. 8. An exhaust system according to any one of the preceding claims, It is characterized in that The exhaust system further has a GPF interposed between the first three-way catalyst and the catalyst for reducing ammonia emission. 9. The exhaust system according to claim 8, It is characterized in that The first three-way catalyst and the GPF are installed close to the engine. 10. An exhaust system according to any one of the preceding claims, It is characterized in that The catalyst for reducing ammonia emissions is installed at a rearmost point in the exhaust gas direction under the vehicle. 11. A method for reducing harmful exhaust gas components from an internal combustion engine, in particular a spark-ignition gasoline engine, operating predominantly in a stoichiometric manner, It is characterized in that The exhaust gas is passed through an exhaust system according to any one of the preceding claims.