DE102021121000A1 - Ammonia slip catalyst, exhaust aftertreatment device and motor vehicle - Google Patents

Abstract

The invention relates to an ammonia barrier catalyst (2), comprising a substrate which has a zeolite-containing coating, metals being embedded in the zeolite-containing coating as active centers for storing and converting ammonia and other chemical compounds, and the substrate being designed as a metal catalyst.

Description

The present invention relates to an ammonia blocking catalytic converter according to the preamble of claim 1 and an exhaust gas after-treatment device which has means for exhaust gas after-treatment which comprise an ammonia blocking catalytic converter.

Exhaust gas aftertreatment devices of the type mentioned above for the aftertreatment of the exhaust gases produced during the operation of a spark-ignition engine, in which the spark-ignition engine is always operated with a combustion air ratio λ very close to 1, are known from the prior art in various embodiments. These exhaust gas aftertreatment devices include a three-way catalytic converter system with at least one three-way catalytic converter that is set up to convert the air pollutants carbon monoxide, nitrogen oxides and hydrocarbons produced during the combustion process of a fuel-air mixture into carbon dioxide, nitrogen and water in a catalytic process to convert In addition, the three-way catalytic converter systems of modern exhaust gas aftertreatment devices often have at least one Otto particle filter, by means of which soot particles contained in the exhaust gas of the Otto engine can be at least partially filtered. In a lean-burn engine, which is a gasoline engine that is always operated with excess air (λ> 1), a three-way catalytic converter cannot be used because a lean-burn engine has a high excess of oxygen in the exhaust gas, so that by means of a Three-way catalytic converter no denitrification is possible.

1. a) während der Kaltstartphase des Ottomotors beim Durchlaufen der so genannten „Light-Off“-Temperatur des Drei-Wege-Katalysators,
2. b) während des Betriebs des Drei-Wege-Katalysators bei einer Betriebstemperatur >350° C durch Gemischabweichungen (unterstöchiometrisch, λ <1) einer Gemischvorsteuerung des Ottomotors und der Lambdaregelung.

NH ₃ (ammonia) emissions also occur during the operation of the Otto engine. These NH ₃ emissions are largely generated by the three-way catalytic converter. The development of the NH ₃ emissions in the three-way catalytic converter can be divided into two main paths:

1. a) during the cold start phase of the gasoline engine when passing through the so-called “light-off” temperature of the three-way catalytic converter,
2. b) during operation of the three-way catalytic converter at an operating temperature >350° C due to mixture deviations (sub-stoichiometric, λ <1) of a mixture pre-control of the gasoline engine and the lambda control.

In order to be able to temporarily store ammonia, it is possible to equip the exhaust aftertreatment device with an ammonia blocking catalytic converter (ASC for short), which is arranged downstream of the three-way catalytic converter. Such an ammonia blocking catalytic converter is distinguished by a zeolite-containing coating on a ceramic substrate, which is capable of temporarily storing the NH ₃ emissions occurring during operation of the Otto engine. The storage capacity of an ammonia blocking catalytic converter decreases significantly above the component temperature, especially in the aged state. At temperatures > 300° C, the memory function is almost no longer usable. The aging of the zeolite is also strongly temperature-dependent, with a component temperature of 800° C representing the permissible upper limit, which is difficult to achieve, especially in a gasoline engine due to the high exhaust gas temperatures.

Two cases must also be distinguished during operation and with regard to the mode of action of the ammonia blocking catalytic converter.

• Im Falle einer noch aktiven Speicherfunktion des Ammoniaksperrkatalysators können die anfallenden NH₃-Emissionen (insbesondere aus der Kaltstartphase des Ottomotors) im Zeolith des Ammoniaksperrkatalysators zwischengespeichert werden. Eine Umsetzung (Oxidation) des NH₃ in harmloses N₂ findet dabei zeitlich entprellt zu einem späteren Betriebszeitpunkt statt. Die Oxidation des gespeicherten NH₃ erfolgt dabei durch das im Motorbetrieb auftretende Stickstoffmonoxid (NO) und Sauerstoff (O₂). Die chemische Reaktion über NO ist vorteilhaft und daher zu bevorzugen, da hierbei weniger Lachgas (N₂O) als unerwünschtes Nebenprodukt entsteht als bei der Reaktion mit Sauerstoff. Stickstoffmonoxid (NO) und Sauerstoff (O₂) treten bei einem Ottomotor nur dann auf, wenn es zu einem (kurzfristigen) überstöchiometrischen Betrieb (λ > 1) des Katalysators kommt, wobei Sauerstoff alleine nur in Schubabschaltphasen entsteht. NO selbst gelangt dabei immer nur in geringen, limitierten Mengen in den Ammoniaksperrkatalysator. Wenn ausschließlich Sauerstoff als Oxidationsmittel vorhanden ist, kommt es neben der Lachgasbildung noch zu einem Anstieg von Stickstoffmonoxid (NO). Bei Erreichen einer Bauteiltemperatur von etwa 300° C im Ammoniaksperrkatalysator nimmt nicht nur dessen Speicherfähigkeit ab, sondern auch die Neigung der Reaktion mit Sauerstoff zu, was zu einer erhöhten, unerwünschten Lachgasbildung beziehungsweise der Bildung von NO führt.

Case 1 (T< approx. 300° C, memory function active):

• If the storage function of the ammonia slip catalyst is still active, the NH ₃ emissions occurring (in particular from the cold start phase of the gasoline engine) can be temporarily stored in the zeolite of the ammonia slip catalyst. A conversion (oxidation) of the NH ₃ into harmless N ₂ takes place debounced in time at a later operating time. The stored NH ₃ is oxidized by the nitrogen monoxide (NO) and oxygen (O ₂ ) that occur during engine operation. The chemical reaction via NO is advantageous and therefore preferable, since less nitrous oxide (N ₂ O) is produced as an undesirable by-product than in the reaction with oxygen. Nitrogen monoxide (NO) and oxygen (O ₂ ) only occur in a gasoline engine when the catalytic converter is (briefly) over-stoichiometric (λ > 1), with oxygen alone only being produced in overrun cut-off phases. NO itself ever gets into the ammonia slip catalyst only in small, limited amounts. If only oxygen is present as an oxidizing agent, there is an increase in nitrogen monoxide (NO) in addition to the formation of nitrous oxide. When a component temperature of around 300°C is reached in the ammonia slip catalyst, not only does its storage capacity decrease, but also the tendency to react with oxygen increases, which leads to increased, undesirable nitrous oxide formation or the formation of NO.

• Bei Ottomotoren treten die Reaktanten NH₃, NO und O₂ in der Regel nie zeitgleich auf. Bei höheren Temperaturen und damit im Falle einer inaktiven bzw. nur noch begrenzt verfügbaren Speicherfunktion gewinnt die instantane Umwandlung des angebotenen Ammoniaks im Ammoniaksperrkatalysator an Bedeutung. Eine Einspeicherung (Zwischenspeicherung) und erst ein späterer Umsatz sind dann nicht mehr möglich. Mangels simultan nicht verfügbarem Oxidationsmittel (NO oder O₂) ist die Wirksamkeit des Ammoniaksperrkatalysators in diesem Temperaturbereich sehr stark eingeschränkt.

Case 2 (T> approx. 300° C, memory function inactive):

• In Otto engines, the reactants NH ₃ , NO and O ₂ generally never occur at the same time. At higher temperatures and thus in the case of an inactive or only limited memory cher function, the instantaneous conversion of the available ammonia in the ammonia slip catalyst gains in importance. A storage (intermediate storage) and only a later turnover are then no longer possible. In the absence of simultaneously unavailable oxidizing agent (NO or O ₂ ), the effectiveness of the ammonia slip catalyst is very limited in this temperature range.

For example, ammonia that is formed in the three-way catalytic converter when driving due to rich mixture deviations (sub-stoichiometric, λ <1) can no longer be converted. A possible remedy here is the introduction of additional air upstream of the ammonia blocking catalytic converter. This can produce a deliberate, permanent excess of oxygen in the ammonia blocking catalytic converter, so that the NH ₃ previously formed in the three-way catalytic converter can then continue to be post-treated and converted in the ammonia blocking catalytic converter. However, the previously described disadvantages of oxidation with oxygen and the increased formation of NO and N ₂ O come into play in this operation and will therefore in turn severely limit such use.

A three-way catalytic converter cannot be used for exhaust gas aftertreatment in a diesel engine or a lean-burn engine. In these engines, nitrogen oxides (NOx) can be reduced, for example, by means of a selective catalytic reaction (SCR). In this process, ammonia or urea, for example, is added to the exhaust gas as a reducing agent before the exhaust gas enters a catalytic converter and the NOx is converted into N ₂ and water. It can happen that there is an excess of ammonia that should not get into the environment and could also damage parts of the exhaust system of the motor vehicle. For this reason, exhaust gas aftertreatment devices of diesel engines or lean-burn engines also use ammonia barrier catalysts, which are arranged downstream of the SCR catalyst in the direction of flow of the exhaust gases. Using the ammonia slip catalysts, it is possible to remove ammonia from the exhaust gas by converting it into nitrogen.

From the U.S. 9,453,443 B2 an exhaust gas aftertreatment device for a diesel engine or a lean-burn engine is known, which has an ammonia blocking catalytic converter and an NO _x storage catalytic converter arranged in front of it in the direction of flow of the exhaust gases.

Ammonia slip catalysts, like catalysts for selective catalytic reduction (SCR), use a zeolite-containing coating on a ceramic substrate, with metals such as copper or iron being embedded in the zeolite-containing coating as active centers for storing and converting ammonia and other chemical compounds. The structure and storage capacity of the zeolite is largely determined by its aging condition. It has been shown that the zeolite-containing coatings of ammonia slip catalysts are highly sensitive to thermal aging processes. Maximum temperatures of > approx. 800° C should therefore be avoided as far as possible with regard to the resulting thermal aging. This upper temperature limit poses major challenges, in particular for the integration of ammonia slip catalysts in vehicles with petrol engines due to the higher exhaust gas temperatures compared to diesel engines. Furthermore, an operating window that is as wide as possible at lower temperatures (<approx. 300° C.) would also help to improve the effectiveness of an ammonia blocking catalyst.

The invention therefore sets itself the task of providing an ammonia blocking catalyst and an exhaust gas aftertreatment device of the type mentioned at the outset, in which thermal aging processes of the ammonia blocking catalyst at high exhaust gas temperatures, as occur in particular in gasoline engines, can be effectively reduced and the operating window of the ammonia blocking catalyst at lower operating temperatures can be increased.

This problem is solved by a generic ammonia barrier catalyst with the features of the characterizing part of claim 1 and a generic exhaust gas aftertreatment device with the features of the characterizing part of claim 2. The dependent claims relate to advantageous developments of the invention.

An ammonia barrier catalyst according to the invention is characterized in that the substrate is designed as a metal catalyst, and an exhaust gas aftertreatment device according to the invention is characterized in that the ammonia barrier catalyst is designed according to claim 1 . In order to circumvent the limitations regarding the aging of the ammonia blocking catalytic converter at high component temperatures, which occur in particular in the exhaust aftertreatment of a gasoline engine, the ammonia blocking catalytic converter must be heated up as slowly as possible and the maximum component temperature must be lowered. In order to achieve this, the zeolite-containing coating of the ammonia barrier catalyst is not - as is customary in the prior art - on a ceramic substrate, but on applied to a metal catalyst. A metal catalyst of this type is distinguished--particularly with an appropriate structural design--by a very high thermal heat capacity and high radial and axial heat losses due to the high thermal conductivity of the metal structure. As a result, the ammonia blocking catalytic converter can be kept colder for a longer period of time through deliberate use of heat loss, and the maximum temperature of the ammonia blocking catalytic converter can be effectively limited in a simple manner.

In an advantageous development, it is proposed that the exhaust gas after-treatment device includes an exhaust gas cooling device which is arranged upstream of the ammonia blocking catalytic converter. This exhaust gas cooling device advantageously enables additional cooling of the hot exhaust gases.

Under certain circumstances, conventional heat exchanger concepts can lead to increased additional pressure losses, which are undesirable. In a particularly advantageous embodiment, it is therefore proposed that the exhaust gas cooling device has a number of gyroid structures. Preferably, gyroid structures that are specially flow-optimized and in particular produced by means of additive manufacturing are used, by means of which good heat transfer can be achieved on the one hand and on the other hand the additional pressure loss can at least be reduced or avoided entirely if possible. An additional heat dissipation due to a resulting heat loss can thus also be implemented in the exhaust gas aftertreatment device, so that the operating range of the ammonia blocking catalytic converter can also be further optimized as a result.

Preferably, the exhaust gas after-treatment device can have at least one three-way catalytic converter, which is arranged upstream of the exhaust gas heating device.

In a further embodiment, there is the possibility that the exhaust gas aftertreatment device has at least one particle filter, which is arranged upstream of the exhaust gas cooling device. This makes it possible to filter the soot particles contained in the exhaust gases.

According to a further aspect, the present invention relates to a motor vehicle, comprising a spark-ignition engine and an exhaust gas after-treatment device which is connected to the spark-ignition engine. According to the invention, the motor vehicle is characterized in that the exhaust gas aftertreatment device is designed according to one of claims 2 to 6 and that the Otto engine is designed for operation with a combustion air ratio λ very close to 1.

Further features and advantages of the present invention become clear from the following description of preferred exemplary embodiments with reference to the enclosed 1 , which shows a highly simplified representation of part of an exhaust gas aftertreatment device 1 of an internal combustion engine.

The internal combustion engine is preferably an Otto engine, which is always operated with a combustion air ratio λ very close to 1. Very high exhaust gas temperatures, typically around 800° C., can occur during the operation of the Otto engine. If the exhaust gas after-treatment device 1 is designed for the after-treatment of exhaust gases from such an Otto engine, the exhaust gas after-treatment device 1 has, in a manner known per se, a three-way catalytic converter which is not explicitly shown here and which is set up to Mixture to convert the resulting air pollutants carbon monoxide, nitrogen oxides and hydrocarbons in a catalytic process into carbon dioxide, nitrogen and water. In addition, the exhaust gas aftertreatment device 1 preferably has a gasoline particle filter, which is also not explicitly shown here, by means of which soot particles contained in the exhaust gas of the gasoline engine can be at least partially filtered.

Furthermore, the exhaust gas aftertreatment device 1 has an ammonia blocking catalytic converter 2 which is accommodated in an exhaust gas line 3 . This ammonia blocking catalytic converter 2 comprises a substrate which has a zeolite-containing coating, with metals, in particular copper or iron, being embedded in the zeolite-containing coating as active centers for storing and converting ammonia and other chemical compounds. In order to circumvent the limitations with regard to aging of the ammonia blocking catalytic converter 2 at high component temperatures, as they occur in particular in the exhaust gas aftertreatment device 1 of a gasoline engine, the ammonia blocking catalytic converter 2 must be heated up as slowly as possible and the maximum component temperature must be lowered. In order to achieve this, the zeolite-containing coating is not applied to a ceramic substrate, as is usual in the prior art, but to a metal catalyst. A metal catalytic converter of this type is distinguished--particularly with an appropriate structural design--by a very high thermal heat capacity and by high radial and axial heat losses due to the high thermal conductivity strength of the metal structure. As a result, the ammonia blocking catalytic converter 2 can be kept colder for longer due to the intentionally used heat loss Q2 and the maximum temperature of this component can be effectively limited in a simple manner.

In addition, the exhaust gas aftertreatment device 1 in this exemplary embodiment has an exhaust gas cooling device 4, which is arranged in the exhaust pipe 3 upstream of the ammonia blocking catalytic converter 2 - i.e. upstream of the ammonia blocking catalytic converter 2 in the direction of flow of the hot exhaust gases - and is designed to additionally cool the hot exhaust gases. Under certain circumstances, conventional heat exchanger concepts can tend to increased additional pressure losses, which are undesirable. Preferably, the exhaust gas cooling device 4 therefore has gyroid structures that are specially flow-optimized and, in particular, produced by means of additive manufacturing, by means of which good heat transfer can be achieved on the one hand and on the other hand the additional (undesirable) pressure loss can be at least reduced or, if possible, avoided entirely. An additional heat dissipation through a heat loss Q1 can thus also be implemented by means of the exhaust gas heat dissipation device 4 in the exhaust gas aftertreatment device 1, so that the operating range of the ammonia blocking catalytic converter 2 can thereby also be optimized.

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited 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.

Patent Literature Cited

• US9453443B2 [0010]

Claims (7)

Ammonia barrier catalyst (2) comprising a substrate which has a zeolite-containing coating, metals being embedded in the zeolite-containing coating as active centers for storing and converting ammonia and other chemical compounds, characterized in that the substrate is designed as a metal catalyst. Exhaust gas aftertreatment device (1) of an internal combustion engine, comprising means for exhaust gas aftertreatment which have an ammonia blocking catalyst (2), characterized in that the ammonia blocking catalyst (2) after claim 1 is executed. Exhaust aftertreatment device (1). claim 2 , characterized in that the exhaust gas treatment device (1) comprises an exhaust gas cooling device (4) which is arranged upstream of the ammonia blocking catalytic converter (2). Exhaust aftertreatment device (1). claim 3 , characterized in that the exhaust gas cooling device (4) has a number of gyroid structures. Exhaust aftertreatment device (1) according to one of claims 2 until 4 , characterized in that the exhaust gas aftertreatment device (1) has at least one three-way catalytic converter which is arranged upstream of the exhaust gas cooling device (4). Exhaust aftertreatment device (1) according to one of claims 2 until 5 , characterized in that the exhaust gas aftertreatment device (1) has at least one particle filter which is arranged upstream of the exhaust gas cooling device (4). Motor vehicle comprising an Otto engine and an exhaust gas aftertreatment device (1) which is connected to the Otto engine, characterized in that the exhaust gas aftertreatment device (1) according to one of claims 2 until 6 is designed and that the gasoline engine is designed for operation with a combustion air ratio λ very close to 1.

Images