US20170226917A1

US20170226917A1 - System and method for cleaning exhaust gas while avoiding nitrous oxide

Info

Publication number: US20170226917A1
Application number: US15/423,667
Authority: US
Inventors: Frank Adam; Ruediger Hoyer
Original assignee: Umicore AG and Co KG
Current assignee: Umicore AG and Co KG
Priority date: 2016-02-04
Filing date: 2017-02-03
Publication date: 2017-08-10
Also published as: DE102017100518A1; CN107060959A

Abstract

The present invention relates to a method for cleaning exhaust gas, and a correspondingly designed exhaust gas system. The present method, or the corresponding system, serves to avoid the formation of nitrous oxide as a secondary exhaust gas, which may primarily be created during the loading of specific catalyst types with NH₃.

Description

The present invention relates to a method for cleaning exhaust gas, and a correspondingly designed exhaust gas system. The present method, or the corresponding system, serves to avoid the formation of nitrous oxide as a secondary exhaust gas, which may primarily be created during the loading of specific catalyst types with NH₃.
The exhaust gas of combustion engines in motor vehicles typically contains the harmful gases carbon monoxide (CO) and hydrocarbons (HC), nitrogen oxides (NO_x), and possibly sulfur oxides (SO_x), as well as particulates that mostly consist of soot residues and possibly adherent organic agglomerates. These are designated as primary emissions. CO, HC, and particulates are the products of the incomplete combustion of the fuel inside the combustion chamber of the engine. Nitrogen oxides form in the cylinder from nitrogen and oxygen in the intake air when the combustion temperatures locally exceed 1400° C. Sulfur oxides result from the combustion of organic sulfur compounds, small amounts of which are always present in non-synthetic fuels. For the removal of these emissions, which are harmful to health and environment, from the exhaust gases of motor vehicles, a variety of catalytic technologies for the purification of exhaust gases have been developed, the fundamental principle of which is usually based upon guiding the exhaust gas that needs purification over a catalyst consisting of a flow-through or wall-flow honeycomb-like body and a catalytically active coating applied to it. The catalyst facilitates the chemical reaction of different exhaust gas components while forming non-hazardous products, such as carbon dioxide and water.
The mode of operation and the composition of the catalysts that are used differ significantly in part according to the composition of the exhaust gas to be purified and the exhaust gas temperature level that is to be expected at the catalyst. A variety of compositions used as catalytically active coating in the catalyst contain components in which, under certain operating conditions, one or more exhaust gas components can be temporarily bound and intentionally released again when an appropriate change in operating conditions occurs. Components with such a capacity are generally referred to below as storage materials.
Nitrogen oxide storage catalysts (NSC's; LNT, NSR) are used to remove nitrogen oxides contained in lean exhaust gas of what are known as lean-mix engines (diesel, Lean-GDI). The cleaning effect is therein based upon the fact that the nitrogen oxides are stored by the storage material of the storage catalyst in the form of nitrates in a lean operating phase (storage phase, lean operation) of the engine. In a subsequent rich operating phase (regeneration phase, rich operation, DeNOx phase) of the engine, the previously formed nitrates are broken down, and the nitrogen oxides that are released again are converted—with the reducing, rich components of the exhaust gas—into nitrogen, carbon dioxide, and water at the storage catalyst during the rich operation. Among other things, hydrocarbons, carbon monoxide, ammonia, and hydrogen are designated as rich components of the exhaust gas.
The operating principle of nitrogen oxide storage catalysts is described in detail in the SAE document SAE 950809. The composition of nitrogen oxide storage catalysts is sufficiently known to the person skilled in the art. The nitrogen oxide storage materials are usually basic compounds of alkali metals or alkaline earth metals—for example, oxides, hydroxides, or carbonates of barium and strontium—which are deposited in finely distributed form on suitable substrate materials. Furthermore, a nitrogen oxide storage catalyst also has catalytically active precious metals of the platinum group and oxygen storage materials. The best-known materials for storing oxygen are cerium oxides and mixed oxides of cerium and zirconium that may be doped with further oxides—in particular, with rare-earth metal oxides such as lanthanum oxide, praseodymium oxide, neodymium oxide, or yttrium oxide (Autoabgaskatalysatoren, Grundlagen-Herstellung-Entwicklung-Recycling-Ökologie [Automobile Exhaust Gas Catalytic Converters, Foundations-Manufacture-Development-Recycling-Ecology], Christian Hagelüken, 2nd edition, 2005, p. 49; Catalytic Air Pollution Control, Commercial Technology, R. Heck et al., 1995, pp. 73-112). This composition lends a nitrogen oxide storage catalyst under stoichiometric operating conditions the functionality of a three-way catalyst (DE102010033689 and the literature cited therein).
The storage phase for nitrogen oxides (lean operation) usually lasts 100 to 2000 seconds and is dependent upon the storage capacity of the catalyst and the concentration of the nitrogen oxides in the exhaust gas. In the case of aged catalysts with reduced storage capacity, the duration of the storage phase can, however, also fall to 50 seconds or less. The regeneration phase (rich operation) is, in contrast, always significantly shorter and lasts only a few seconds (5 s-20 s). The exhaust gas escaping from the nitrogen oxide storage catalyst during regeneration essentially has no harmful substances and is composed approximately stoichiometrically. Its air ratio [lambda; λ] (lambda: indicates the ratio of fuel to air in the exhaust gas—see below) is nearly equal to 1 during this time period. At the end of the regeneration phase, the released nitrogen oxides and the oxygen bound to the oxygen storage components of the catalyst are no longer sufficient to oxidize all rich exhaust gas components. This therefore leads to a breakthrough of these components through the catalyst, and the air ratio decreases to a value below 1. The exhaust gas breaking through possibly includes greater quantities of ammonia (NH₃) that is formed from the over-reduction of nitrogen oxides. This breakthrough indicates the end of the regeneration and may be registered with the aid of a so-called lambda probe behind the storage catalyst (so-called sensor-controlled system).
Three-way catalysts (TWC's) have likewise long been sufficiently known to the person skilled in the art. They are used as exhaust gas cleaning components in gasoline engines operated with stoichiometric fuel. Three-way catalysts have been legally mandated since the eighties of the last century. The actual catalyst mass here consists for the most part of an oxidic substrate material having a high surface area, on which the catalytically active components are deposited with the finest distribution. The precious metals of the platinum group (platinum, palladium, rhodium, iridium, ruthenium, and osmium) are particularly suitable as catalytically active components for cleaning stoichiometrically composed exhaust gases. For example, aluminum oxide, silicon dioxide, titanium oxide, zirconium oxide and their mixed oxides, and zeolites are suitable as substrate materials. What are known as active aluminum oxides having a specific surface (BET surface, measured according to DIN 66132) of more than 10 m²/g are preferably used. Moreover, three-way catalysts include oxygen-storing components to improve the dynamic conversion. Among these are cerium oxide, praseodymium oxide, and cerium/zirconium mixed oxides (see above; EP1181970A1). Meanwhile, zoned and multi-layer systems having three-way activity have also become known (U.S. Pat. No. 8,557,204; U.S. Pat. No. 8,394,348).
It is known that three-way catalysts produce NH₃when they are acted upon by a rich exhaust gas mixture (SAE 2011010307). As a result, investigations were carried out in which, by means of a system comprising a three-way catalyst (TWC) located close to the engine (cc) and an SCR catalyst capable of storing NH₃located in the underfloor (uf), the exhaust gas situation can be further improved, even with combustion engines which are operated predominantly, on average, stoichiometrically, especially since, in this case, as indicated above, it works alternately in the light-rich to light-lean air/fuel range and, therefore, NH₃production and NOx slippage alternate (SAE 2011010307, DE102009056390A1, US20120247088A1, U.S. Pat. No. 8,522,536B2).
In gasoline engines that are lean-burning on average, it has also proven to be advantageous if three-way catalysts are able to likewise perform the functions of nitrogen oxide storage and nitrogen oxide reduction (what are known as TWNSC catalysts). Such catalysts are described in, for example, DE102009010711A1 or WO2010097146A1. The TWNSC has the task of functioning like a normal three-way catalyst in a stoichiometric air/fuel range. However, in a leaner range, a normal three-way catalyst cannot reduce nitrogen oxides. As a consequence of this, it is advisable to at least temporarily reduce the nitrogen oxide load of the exhaust gas by imparting a nitrogen oxide storage function (as described above for the NSC) to the three-way catalyst as described above. This is advantageous in particular for those TWNSC's that are located in a region near the motor in the exhaust gas tract. A significant widening of the storage and conversion window for nitrogen oxides may thereby be achieved (SAE 2013-01-1299).
In addition to the reduction of these primary emissions, more recent legislation governing exhaust gas emissions—in particular, in the US (GHG Emission Standard; SULEV)—not only mandates that these be drastically reduced, but also requires maximum suppression of the formation of the secondary emissions generated by the exhaust gas components themselves, such as methane (CH₄) and the potent greenhouse gas nitrous oxide (N₂O) (Federal Register, vol. 75, no. 88, 2010/Rules and Regulations, p. 25399; Federal Register, vol. 77, no. 199, 2012/Rules and Regulations, p. 62799; http://epa.gov/climatechange/ghgemissions/gases/n2o.htmt). Such guidelines for the discharge of the secondary emissions NH₃, CH₄, and N₂O are also currently under discussion in the European Union and should in any event be integrated into subsequent legislation.
Modern systems for preventing harmful exhaust gases in predominantly lean-operated combustion engines already often work with at least two nitrogen oxide storage catalysts, wherein one is positioned near the engine for faster heating, and the other is installed in the colder underbody region (WO06069652A1; WO10034452A1; JP2009150282A2). For example, given such an arrangement, a greater temperature range may be covered by the activity windows of the nitrogen oxide storage catalysts.
It is common to all catalyst types addressed above that, in an environment with rich exhaust gas mixture, they have the capability of reducing nitrogen oxides that are stored or present in the exhaust gas, not just into nitrogen, but also into ammonia (NH₃). Specifically, nitrogen oxide storage catalysts produce increased NH₃if they are loaded with a very rich exhaust gas mixture (SAE 2005-01-3876; DE102013218234A). In systems consisting of a nitrogen oxide storage catalyst near the engine (cc) and a nitrogen oxide storage catalyst located in the underbody (uf), the danger arises of an N₂O production of uf-NSC due to oxidation of the NH₃formed near the engine, since the regeneration of the cc-NSC's must be done with a rich air/fuel mixture (DE102014206455A). The bed temperature of this uf-NSC often moves continuously in the optimal nitrous oxide formation window 220-350° C. This cooler exhaust gas temperature at the nitrogen oxide storage catalyst placed in the underfloor region increases the danger of formation of the greenhouse gas N₂O from the NH₃formed during regeneration at the nitrogen oxide storage catalyst located close to the motor. This also applies, mutatis mutandis, to other combinations of the catalyst types described above (for example, TWC-NSC; NSC-TWC; TWNSC-NSC, etc.).
Furthermore, care must be taken that the direct injection of the fuel into the combustion chamber of gasoline engines, and the turbo charger arranged in turn downstream of the outlet valve, lead to a constant cooling of the exhaust gas temperature. The danger is therefore increased that the uf-NSC is more often within the nitrous oxide formation window. Moreover, above all in urban driving and with cross-country driving, these low temperatures are often no longer sufficient for operating an underfloor catalyst (for example, an underfloor catalyst operated behind a system made up of cc-NSC and uf-NSC) with sufficient conversion efficiency, especially for N₂O, which has the consequence that N₂O is released into the environment.
EP1536111B1 describes a method for the reduction of secondary emissions, such as methane or N₂O, in the exhaust gas of combustion engines that are fitted with NOx storage catalysts. In order to be able to reduce secondary emissions which, during the rich operation for regenerating the NOx storage catalyst, are formed via the latter, it is proposed that an additional catalyst be arranged downstream of the NOx storage catalyst. This catalyst is able to oxidize methane and N₂O and consists of two different catalytically active materials. A palladium-containing catalyst is proposed for the oxidation of methane, and an iron-zeolite catalyst is recommended for the reduction of N₂O. It is known that methane or N₂O can be effectively converted in lean atmospheres via palladium-containing or iron-zeolite catalysts. However, the conversion of N₂O in lean atmospheres via Pd catalysts is very small, and the conversion via iron-zeolite catalysts occurs only at higher temperatures in excess of approx. 400° C. To ensure that the operation of the catalyst for N₂O reduction is decidedly lean, which is certainly advisable for converting methane, EP1536111B1 additionally recommends a secondary air injection upstream of the catalyst positioned downstream. As described at the beginning, this does not, however, result in the desired N₂O reduction at low temperatures.
Exhaust gas systems which take further the concept of a three-way catalyst close to the engine and an SCR catalyst located downstream are described in DE102009054046A1. Among other things, a system is proposed in which a cc-TWC is followed by a uf-SCR and a further uf-TWC positioned downstream thereof. The uf-TWC arranged on the exhaust gas outlet side of the uf-SCR obviously has, according to this disclosure, the same design as the cc-TWC close to the engine. It is used, apparently without exception, to oxidize NH₃, which breaks through the uf-SCR catalyst.
The DE102011121848A1 also relates to an exhaust gas system consisting of a three-way catalyst followed by an ammonia SCR catalyst. The ammonia SCR catalyst has (1st) a base-metal-ion-substituted zeolite and/or a base-metal-ion-substituted silicon aluminum phosphate and (2nd) an oxygen-storing material which is selected from the group comprising a metal oxide or a mixed metal oxide that has an oxygen storage and release capacity. The arrangement in series of the TWC and the ammonia SCR catalyst increases the conversion of NOx to N₂in the exhaust gas flow with low oxygen content generated by the engine. The disclosure of this specification makes mention of the fact that the downstream ammonia SCR catalyst can fundamentally replace all or part of a second three-way catalyst in a system with two three-way catalysts. As already indicated, the ammonia SCR catalyst has an oxygen-storing material.
DE102010014468A1 relates to a method for the aftertreatment of exhaust gas of essentially lean-burn combustion engines, as well as an appropriately advantageous exhaust gas aftertreatment system. In particular, this invention relates to the reduction in the proportion of the greenhouse gas N₂O in the total exhaust gas of a corresponding combustion system having an NOx storage catalyst as an exhaust gas cleaning element. The objective of the invention is to operate the N₂O reduction catalyst arranged downstream of the NOx storage catalyst under λ≦1 conditions, as soon as the N₂O formed by the NOx storage catalyst reaches the N₂O reduction catalyst.
DE102013218234 is directed toward a use of different regeneration strategies of nitrogen oxide storage catalysts as a function of the exhaust gas temperatures. In particular, this application relates to the reduction in the proportion of the greenhouse gas N₂O (created as a secondary emission during the regeneration of the storage catalyst) in the total exhaust gas of an exhaust gas aftertreatment system with at least one NOx storage catalyst as exhaust gas cleaning element.
It is the aim of the present invention to specify a possibility with which the emission of N₂O is optimally suppressed under regeneration conditions or given rich exhaust gas conditions for gasoline engines that predominantly run lean on average, having an exhaust gas system made up of at least two catalysts of the TWC, TWNSC, or NSC types. The method applied for this, or the corresponding system, should thereby be optimally robust and, from an economic and ecological standpoint, at least equivalent—but advantageously superior—to systems of the prior art.
These and other aims evident from the prior art to the person skilled in the art are achieved by a method having the features of the objective claim 1. Preferred embodiments of this method are addressed in the subclaims that are dependent upon claim 1. Claim 13 relates to a correspondingly designed exhaust gas aftertreatment system, and claim 16 relates to a corresponding use.
The given aim is achieved in a robust and very advantageous manner in that, in a method for reduction of harmful automobile exhaust gas components with the aid of an exhaust gas system having at least two catalysts selected from the group comprising NSC, TWC, and TWNSC, wherein the exhaust gas is diverted around the downstream catalyst of the at least two catalysts if this is within a temperature window in which it is capable of forming N₂O from NH₃, and the upstream catalyst of the at least two catalysts produces NH₃. Via the circumvention of the downstream catalyst, the NH₃formed by the first catalyst during the regeneration is directed past said downstream catalyst and is provided to possible following catalysts as a reducing agent, or may be oxidized to form nitrogen via what is known as an ammonia oxidation catalyst (AMOX) or ammonia slip catalyst (ASC). It is, consequently, surprising that, in spite of this measure according to the invention, no other impairment of the exhaust gas reduction is detectable.
The present invention may be considered for all vehicles in which more than one of the cited catalysts are used in combination. These are preferably gasoline engines predominantly operating on average with a leaner A/F ratio (air/fuel ratio). The expression “predominantly on average” thereby takes into consideration the fact that modern gasoline engines are not statically operated with a fixed air/fuel ratio (A/F ratio; λ value). Three-way catalysts that contain oxygen-storing material are loaded with exhaust gas by gasoline engines under conditions involving a discontinuous progression of the air ratio λ. They involve a periodic change of the air ratio λ in a defined manner, and thus a periodic change in oxidizing and reducing exhaust gas conditions. This change in the air ratio λ is in both cases significant for the exhaust gas purification result. To this end, the λ value of the exhaust gas is regulated with a very short cycle time (ca. 0.5 to 5 Hertz) and an amplitude Δλ of 0.005≦Δλ≦0.07 at the value λ=1 (reducing and oxidizing exhaust gas components are present in a stoichiometric relationship to each other). Therefore, on average, the exhaust gas under such operating conditions should be described as “on average” stoichiometric. In order for these deviations to not have a negative effect on the purification results of exhaust gas when the exhaust gas flows over the three-way catalytic converter, the oxygen-storing materials contained in the catalyst balance out these deviations to a certain degree by absorbing oxygen from the exhaust gas and releasing it into the exhaust gas as needed (Catalytic Air Pollution Control, Commercial Technology, R. Heck et al., 1995, p. 90). However, due to the dynamic manner of operation of the engine in the vehicle, further deviations from this condition also occur intermittently. For example, under extreme acceleration or during braking while coasting, the operating conditions of the engine, and thus of the exhaust gas, can be adjusted and can, on average, be hypostoichiometric or hyperstoichiometric. However, the gasoline engines used according to the invention have an exhaust gas which is predominantly—i.e., for the majority of the duration of the combustion operation—operated with an air/fuel ratio that is leaner on average. In a preferred embodiment, the downstream catalyst of the at least two catalysts is selected from the group consisting of TWNSC and NSC.
Furthermore, a variant is preferred in which the exhaust gas system downstream of the at least two catalysts has at least one NOx reduction catalyst. Both HC and CO and NOx breakthroughs through the at least two upstream TWC, NSC, or TWNSC catalysts may be reduced by, for example, one or more nitrogen oxide storage catalyst (NSC's) placed downstream. However, the embodiment is especially preferred in which the NOx reduction catalyst is designed as one or more SCR catalysts, or a combination of one or more SCR catalysts and one or more NSC's. As already indicated above, ammonia is easily formed in the regeneration of the NSC's or TWNSC's, or also in a TWC under rich exhaust gas conditions. In an SCR catalyst that has a storage capacity for ammonia, this ammonia may then be stored and—as a result—might be made available for reduction of the nitrogen oxides breaking through the upstream catalysts. Ammonia-storing SCR catalysts are sufficiently known to the person skilled in the art and are described further below. Particularly preferred is an arrangement of at least one SCR catalyst, followed by at least one nitrogen oxide storage catalyst as a NOx reduction catalyst.
As already indicated above, a temperature window exists in which the catalyst types considered here (NSC, TWC, TWNSC) may oxidize ammonia to form nitrous oxide, which is predominantly the case at a catalyst temperature of <350° C. This temperature window extends approximately from 220° C. to 350° C., which is why a detour of the exhaust gas around the downstream catalyst of the at least two catalysts preferably takes place in this range. More preferable is a detour when the catalyst in question is within a temperature window of 250° C.-350° C., and especially preferably within a temperature window of 270° C.-330° C.
It is likewise advantageous if the at least two catalysts are positioned as close to the engine as possible. For the reason already cited above—that the exhaust gases of the vehicles considered here always get colder—an advantage results when the catalysts are in contact with optimally hot exhaust gas. This is the case if the at least two catalysts are preferably located in the first half of the exhaust gas tract, as measured from the motor output to the end of the exhaust pipe (FIG. 1). Moreover, the catalysts positioned in such a manner accordingly achieve their activity range sooner than units positioned further toward the end of the exhaust gas tract, which further contributes to decreasing the pollutant emissions. The at least two catalysts are preferably positioned not more than 1.5 m, and more preferably not more than 70 cm, from the exit of the motor.
The present invention preferably takes place with temperature control. The temperature control may be determined via calculation in the vehicle electronics (Van Basshuysen/Schäfer, Handbuch Verbrennungsmotor [Manual of Combustion Engines] 2nd edition (2002), Vieweg Verlag; Kanemitsu Nishio, The Fundamentals of Automotive Engine Control Sensors, 1st Edition (2001), p. 122 ff. and p. 128 ff., Fontis Media SA, and Bosch Ottomotormanagement [Gasoline Engine Management], 1st edition (1998), p. 182 ff. and 254 ff., Vieweg Verlag). The determination of the temperature of the downstream catalyst of the at least two catalysts is likewise possible with the aid of a temperature sensor. This is preferably located behind the just-mentioned catalyst in the exhaust gas tract. It is furthermore preferred to position this temperature sensor as close as possible to the corresponding catalyst in order to be able to assess the temperature of said catalyst from the exhaust gas temperature directly and without large error. In an especially preferred embodiment, this temperature sensor is therefore located downstream of the corresponding catalyst, but upstream of the merger of the diversion of the exhaust gas around the corresponding catalyst and the main exhaust gas tract. An additional sensor may preferably additionally be located downstream of the merger of the exhaust gas tracts.
In order to be able to determine the temperature of the exhaust gas as exactly as possible, it may furthermore be advantageous to position a 2nd or 3rd temperature sensor upstream of the corresponding downstream catalyst of the at least two catalysts. This 2nd temperature sensor may be arranged between the at least two catalysts, and preferably upstream of the branching of the diversion of the exhaust gas around the catalyst located downstream. Furthermore, it is preferred that a 3rd temperature sensor be likewise present upstream of the first catalyst of the at least two catalysts.
The diversion of the exhaust gas around the downstream catalyst of the at least two catalysts is preferably produced by means of a device for activating and deactivating the diversion, which device is positioned at the merger of the diversion and the main exhaust gas tract. The diversion of the exhaust gas around the downstream catalyst of the at least two catalysts takes place via a branching in the exhaust gas tract between the at least two catalysts, and a merger of the exhaust gas lines after the downstream catalyst of the at least two catalysts. The diversion of the exhaust gas around the downstream catalyst of the at least two catalysts is preferably produced by means of a device for activating and deactivating the diversion, in the form of a valve or an exhaust gas flap, wherein the device is positioned at the merger of the diversion and the main exhaust gas tract (FIG. 1). Such controls are familiar to the person skilled in the art (http://www.pierburg-service.de/ximages/pgpi1004-adeweb.pdf, http://www.tiaisport.com/index.php/tial-products/wastegates/43-v60; EP2556223B1, DE102011101079A1).
The present invention also deals with a correspondingly designed exhaust gas system for exhaust gas aftertreatment, comprising at least two catalysts from the group consisting of NSC, TWC, and TWNSC, wherein the system is designed so that the exhaust gas may be diverted around the downstream catalyst of the at least two catalysts if this is in a situation in which it is capable of forming N₂O from NH₃. The preferred system embodiments described above for the method accordingly apply here to the exhaust gas system being considered. Likewise addressed is the use of the system for cleaning exhaust gas of a gasoline engine that is predominantly operated with an A/F mixture that is lean on average.
The regulation of the method described here may take place according to the measures known to the person skilled in the art. As described above, the most varied sensors (temperature, NOx, A sensors) that at any time measure the state of the exhaust gas with regard to specific components and transmit these values to the engine control unit (ECU) may be used to support the regulation and adjustment of the exhaust gas system. Due to cost considerations, however, an embodiment in which the regulation and adjustment of the exhaust gas system takes place partially or exclusively via data values (so-called maps) stored in the ECU seems to be particularly preferred.
NOx Storage Catalyst:
As stated, NOx storage catalysts are comprised of materials that may remove nitrogen oxides from the exhaust gas flow under lean-operating exhaust gas conditions and may desorb and convert the nitrogen oxides under lambda=1 or rich exhaust gas conditions.
The nitrogen oxide storage catalysts that are to be used here are sufficiently known to the person skilled in the art (EP0982066A2, EP1317953A1, WO2005/092481A1). Furthermore, the statements in EP1911506A1, as well as EP1101528A2 and the literature cited therein, are referenced with regard to the design and composition of nitrogen oxide storage catalysts (NSC's). The catalyst materials that are used are applied, jointly or separately from one another, according to the methods known to the person skilled in the art, in the form of a coating onto monolithic, inert bodies made of ceramic (for example, cordierite) or metal exhibiting 4- or 6-sided honeycombs. The honeycomb bodies possess flow channels, arranged in a narrow grid across their cross-sections, parallel to the longitudinal axis of the honeycomb bodies, for the exhaust gas to be cleaned. The catalytically active coating is deposited on or in the wall surfaces of the dividing walls bounding the flow channels, in concentrations of 50 to 450 grams per liter (g/l) of volume of the honeycomb bodies, preferably 200-400 g/l, and especially preferably 250-350 g/l. The catalyst material includes the nitrogen oxide storage material and a catalytically active component. The nitrogen oxide storage material in turn consists of the actual nitrogen oxide storage component that is deposited in a highly-dispersed form on a substrate material. The basic oxides of alkali metals, alkaline earth metals—in particular, however, barium oxide and the rare-earth metals—especially, cerium oxide—are predominantly used as storage components, which react with nitrogen dioxide to form the corresponding nitrates. Preferred storage materials are compounds containing Mg, Ba, Sr, La, Ce, Mn, and K. The precious metals of the platinum group (for example, Pt, Pd, Rh), which are normally deposited together with the storage component onto the substrate material, are conventionally used as catalytically active components. For the most part, active aluminum oxide with a large surface area is used as the carrier material.
TWC:
Three-way catalysts (TWC) are able to simultaneously remove the three pollutant components HC, CO, and NOx from a stoichiometric exhaust gas mixture (λ=1 conditions). They may also convert the nitrogen oxides under rich exhaust gas conditions. They for the most part include platinum group metals—such as Pt, Pd, and Rh, wherein Pd and Rh are particularly preferred—as catalytically active components. The catalytically active metals are often deposited with high dispersion on oxides of aluminum, zirconium, and titanium, or mixtures thereof, which have a large surface area and which may be stabilized by additional transition elements, such as La, Y, Pr, etc. Three-way catalysts also include oxygen storage materials (for example, Ce/Zr mixed oxides; see below). For example, a suitable three-way catalytic coating is described in EP181970B1, WO2008-113445A1, WO2008-000449A2 by the applicant, which are referenced here.
Oxygen-storing materials have redox properties and can react with oxidizing components, such as oxygen or nitrogen oxides in oxidizing atmosphere, or with reducing components, such as hydrogen or carbon monoxide, in reducing atmosphere. The embodiment of the exhaust gas aftertreatment of a combustion engine operating essentially in the stoichiometric range is described in EP1911506A1. In this case, a particulate filter provided with an oxygen storage material is used. Advantageously, such an oxygen-storing material consists of a cerium/zirconium mixed oxide. Additional oxides—of rare-earth metals in particular—can be present. Preferred embodiments of the particulate filter according to the invention thus additionally include lanthanum oxide or neodymium oxide. Cerium oxide, which can be present as Ce₂O₃as well as CeO₂, is used most frequently. In this regard, reference is also made to the disclosure of U.S. Pat. No. 6,605,264BB and U.S. Pat. No. 6,468,941BA.
Additional examples of oxygen-storing materials comprise cerium and praseodymium or corresponding mixed oxides, which may additionally include following components selected from the group of zirconium, neodymium, yttrium, and lanthanum. These oxygen-storing materials are often doped with precious metals, such as Pd, Rh, and/or Pt, whereby the storage capacity and storage characteristics can be modified. As stated, these substances are able to remove oxygen from the exhaust gas in lean operation and to release it again under rich exhaust gas conditions. This prevents the NOx conversion via the TWC from decreasing and NOx breakthroughs from occurring during a short-term deviation of the fuel-air ratio from lambda=1 into lean operation. Furthermore, a filled oxygen storage prevents HC and CO breakthroughs when the exhaust gas temporarily passes into the rich range, since, under rich exhaust gas conditions, the stored oxygen reacts first with the excess HC and CO before a breakthrough occurs. In this case, the oxygen storage serves as a buffer against fluctuations around lambda=1. A half-filled oxygen storage exhibits the best performance in terms of being able to absorb short-term deviations from lambda=1. Lambda sensors are used in order to be able to determine the fill level of the oxygen storage during operation.
The oxygen-storage capacity correlates with the age condition of the entire three-way catalyst. As part of the OBD (on-board diagnosis), the determination of the storage capacity serves to identify the current activity—and therefore the age condition—of the catalyst. The oxygen-storing materials that are described in the publications are advantageously those that permit a change to their oxidation state. Other such storage materials and three-way catalysts are described in WO05113126A1, U.S. Pat. No. 6,387,338BA, U.S. Pat. No. 7,041,622BB, EP2042225A1, for example.
TWNSC:
As stated above, these catalysts are comprised of materials that—under stoichiometric exhaust gas conditions—impart to the catalyst the function of a three-way catalyst, and that have a function for the storage of nitrogen oxides under lean exhaust gas conditions. The manufacturing of a corresponding TWNSC preferably takes place via the assembly of materials that are used for the construction of a three-way catalyst and a nitrogen oxide storage catalyst. The two functions of the TWNSC that are described here may thereby be present on the carrier, blended with or separate from one another in different layers or zones. A particularly preferred embodiment of such a catalyst is described in WO2010097146A1 or WO2015143191A1, for example.
SCR Catalysts:
The NH₃-storing SCR catalyst located in the underfloor (uf) can be designed in accordance with the types known to the person skilled in the art. As a rule, this is a supporting body provided with a catalytically active material for the SCR reaction, or a supporting body which was extruded from a catalytically active material. In the first case, catalytically active material is commonly understood to be the “washcoat,” with which the supporting body is provided. However, along with the—in the proper sense of the word—‘catalytically active’ component, it can also contain other materials, such as binders from transition metal oxides, and large-surface carrier oxides, such as titanium oxide, aluminum oxide—especially, gamma-Al₂O₃, zirconium oxide, or cerium oxide. Also suitable as SCR catalysts are those that are made up of one of the materials listed below. However, zoned, layered arrangements or multi-brick arrangements (preferably 2- or 3-brick arrangements) with the same or different materials may also be used as SCR components. Mixtures of different materials on one brick are also conceivable.
The actual catalytically active material used according to the invention is preferably selected from the group of transition-metal-exchanged zeolites or zeolite-like materials. These types of compounds are sufficiently known to the person skilled in the art. In this regard, the materials are preferably selected from the group comprising levynite, AEI, KFI, chabazite, SAPO-34, ALPO-34, zeolite β, and ZSM-5. Particularly preferably used are zeolites or zeolite-like materials of the chabazite type (in particular, CHA or SAPO-34), as well as LEV. In order to ensure sufficient activity, these materials are preferably provided with transition metals from the group consisting of iron, copper, manganese, and silver. It should be mentioned that copper is especially advantageous in this respect. The metal-to-aluminum-frame ratio (or, with SAPO-34, the metal-to-silicon-frame ratio) is normally between 0.3 and 0.6, and preferably at 0.4-0.5. The person skilled in the art knows how one has to furnish the zeolites or zeolite-like materials with the transition metals (EP324082A1; WO1309270711A1, PCT/EP2012/061382, and the literature therein cited), to be able to provide good activity with respect to the reduction of nitrogen oxides with ammonia. Furthermore, vanadium compounds, cerium oxides, cerium/zirconium mixed oxides, titanium oxide and tungsten-containing compounds, and mixtures thereof can also be used as catalytically active material.
Materials, which in addition have proven themselves to be advantageous for the application of storing NH₃, are known to the person skilled in the art (US20060010857AA; WO2004076829A1). In particular, microporous solid materials, such as so-called molecular sieves, are used as storage materials. Such compounds, selected from the group comprising zeolites, such as mordenite (MOR), Y-zeolites (FAU), ZSM-5 (MFI), ferrierite (FER), chabazite (CHA) and other “small pore zeolites” such as LEV, AEI or KFI, and β-zeolites (BEA), as well as zeolite-like materials, such as aluminum phosphate (AIPO) and silicon aluminum phosphate SAPO or mixtures thereof, can be used (EP0324082A1). Particularly preferably used are ZSM-5 (MFI), chabazite (CHA), ferrierite (FER), ALPO- or SAPO-34, and 3-zeolites (BEA). Especially preferably used are CHA, BEA, and AIPO-34 or SAPO-34. Extremely preferably used are materials of the LEV or CHA type, and here maximally preferably CHA or LEV. Insofar as a zeolite or a zeolite-like compound as just mentioned above is used as catalytically active material in the SCR catalyst, the addition of further NH₃-storing materials can, advantageously, naturally be dispensed with. Overall, the storage capacity of the ammonia-storing components used can, in a fresh state at a measuring temperature of 200° C., amount to more than 0.9 g NH₃per liter of catalyst volume—preferably between 0.9 and 2.5 g NH₃per liter of catalyst volume, and particularly preferably between 1.2 and 2.0 g NH₃/liter of catalyst volume, and most particularly preferably between 1.5 and 1.8 g NH₃/liter of catalyst volume. The ammonia-storing capacity can be determined using synthesis gas equipment. To this end, the catalyst is first conditioned at 600° C. with NO-containing synthesis gas to fully remove ammonia residues in the drilling core. After the gas has been cooled to 200° C., ammonia is dosed into the synthesis gas at a space velocity of, for example, 30,000 h⁻¹until the ammonia storage in the drilling core is completely filled, and the ammonia concentration measured downstream of the drilling core corresponds to the starting concentration. The ammonia-storing capacity results from the difference in the amount of ammonia dosed overall from the amount of ammonia measured on the downstream side in relation to the catalyst volume. The synthesis gas is typically composed of 450 ppm NH₃, 5% oxygen, 5% water, and nitrogen. In a further preferred embodiment, the SCR catalyst does not have any capacity for storing oxygen. In this case, the uf-SCR catalyst does not have any material with the capacity for storing oxygen.
Substrates:
The catalysts (NSC, TWC, TWNSC) may be arranged on a monolithic channel flow supporting body (flow-through) or a wall-flow substrate (wall-flow) or a particulate filter.
In the prior art, flow-through monoliths are typical catalyst substrates that may consist of metal or ceramic materials, as in the case of the aforementioned filter materials. Fire-proof ceramics, such as cordierite, are preferably used. The flow-through monoliths made of ceramic mostly have a honeycomb structure that consists of continuous channels, which is why flow-through monoliths are also referred to as channel-flow monoliths. The exhaust gas can flow through the channels and, in doing so, comes into contact with the channel walls, which are coated with a catalytically active substance and possibly a storage material. The number of channels per area is characterized by the cell density that typically ranges between 300 and 900 cells per square inch (cpsi). The wall thickness of the channel walls in ceramics is between 0.5-0.05 mm.
All filter bodies made of metal and/or ceramic materials that are typical in the prior art may be used as particulate filters. These include, for example, metallic fabric and knitted filter bodies, sintered metal bodies, and foam structures made of ceramic materials. Porous wall-flow filter substrates made of cordierite, silicon carbide, or aluminum titanate are preferably used. These wall-flow filter substrates have inlet and outlet channels, wherein the respective downstream ends of the inlet channels and the upstream ends of the outlet channels are offset against each other and closed with gas-tight “plugs.” In this case, the exhaust gas that is to be purified and which flows through the filter substrate is forced to pass through the porous wall between the inlet and outlet channels, which induces an excellent particulate filter effect. The filtration property for particulates can be designed by means of porosity, pore/radii distribution, and thickness of the wall. The catalyst material may be present in form of coatings in and/or on the porous walls between the inlet and outlet channels. Filters may also be used that have been extruded directly or with the aid of binders from the corresponding catalyst materials, meaning that the porous walls directly consist of the catalyst material—for example, as can be the case in the case of SCR catalysts based upon zeolite or vanadium. Such extruded SCR filters may additionally have an SCR coating (as described above) in and/or on the porous walls. Preferably used filter substrates may be learned from EP1309775A1, EP2042225A1, or EP1663458A1.
Coatings:
The term “coating” refers to the application of catalytically active materials and/or storage components onto a largely inert supporting body, which may be designed like a previously described wall-flow filter or flow-through monolith. The coating takes on the actual catalytic function and includes storage materials and/or catalytically active metals that are for the most part deposited in a highly-dispersed form onto temperature-stable metal oxides with a large surface area. The coating for the most part takes place via the application of an aqueous suspension of the storage materials and catalytically active components—also called a washcoat—onto or into the wall of the inert supporting body. After the application of the suspension, the substrate is dried and, if applicable, calcined at increased temperature. The coating may consist of one layer or be made up of multiple layers that are applied atop one another (multi-layered) and/or offset relative to one another (zoned) onto a supporting body.
Vehicles operating with gasoline engines produce soot particles which are likewise regulated by corresponding legislation—or soon will be. The emission of soot particles is especially to be monitored—for example, in light of the particulate load in inner cities. In light of this, it is advantageous to use particulate filters in exhaust gas systems of vehicles which are operated with such engines. The person skilled in the art knows how he is to advantageously position such particulate filters in corresponding exhaust gas systems. For example, the Introduction of a diesel particulate filter in the forward region of the exhaust gas system, which naturally experiences more heat than is available to the exhaust gas system in the underbody of the vehicle, is suitable for diesel vehicles that produce a relatively cold exhaust gas. The high heat is particularly advantageous for a sufficient regeneration of the diesel particulate filter. In contrast, vehicles with lean-burning gasoline engines produce a relatively hot exhaust gas. Here, it is therefore likewise conceivable to preferably arrange the particulate filter in the underbody of the vehicle. These may thereby be advantageously mounted in front of or behind the uf-NOx reduction catalyst, or the uf-NOx reduction catalyst is placed on a corresponding filter. In a particularly preferred embodiment of the present invention, however, the particulate filter is combined with one of the two nitrogen oxide storage catalysts such that the nitrogen oxide storage catalyst is present as a coating on or in the particulate filter. This embodiment is most preferable for vehicles with lean-burning gasoline engines.
Insofar as underbody (uf) is discussed in the text, in connection with the present invention, this relates to a region in the vehicle in which the catalyst is installed at a distance of 0.2-3.5 m, more preferably 0.5-2 m, and especially preferably 0.7-1.5 m after the end of the first catalyst of the at least 2 catalysts near the engine—preferably, below the driver cabin (FIG. 1).
What is designated as near the engine (cc) within the scope of this invention is an arrangement of the catalyst at a distance of less than 120 cm, preferably less than 100 cm, and especially preferably less than 50 cm from the exhaust gas outlet of the cylinder of the engine. The catalyst near the engine is preferably arranged directly after the merger of the exhaust gas manifold into the exhaust gas tract.
The combustion air ratio (A/F ratio; air/fuel ratio) sets the air mass m_L,tatswhich is actually available for combustion in relation to the minimum required stoichiometric air mass m_L,st, which is required for complete combustion:
$λ = \frac{m_{L, tats}}{m_{L, st}}$
If λ=1, the ratio applies as a stoichiometric air-fuel ratio where m_L,tats=m_L,st; this is the case if all fuel molecules can theoretically react fully with the atmospheric oxygen, without the absence of oxygen or without unburned oxygen being left over.
The following applies to combustion engines:
λ<1 (for example, 0.9) means “lack of air”: rich exhaust gas mixture
λ>1 (for example, 1.1) means “surplus air”: lean exhaust gas mixture
Statement: λ=1.1 means that 10% more air is present than would be necessary for the stoichiometric reaction. This is, at the same time, designated as surplus air. However, an air-fuel mixture is preferably maintained during the regeneration, which corresponds to a lambda value of 0.8 to 1. This value is particularly preferably between 0.85 and 0.99, and especially preferably between 0.95 and 0.99.

FIGURES

FIG. 1: The complete layout of a corresponding exhaust gas system is described

FIG. 2: Partial region of the complete exhaust gas system that is situated in the area near the motor, given flow through the cat BOX 2 (bypass closed).

FIG. 3: Partial region of the complete exhaust gas system that is situated in the area near the motor, given circumvention of the cat BOX 2 (bypass open).

FIG. 4: Complete exhaust gas system, given flow through the cat BOX 2 (bypass closed).

FIG. 5: Complete exhaust gas system, given circumvention of the cat BOX 2 (bypass open).

FIG. 6: NOx conversion in the load range of below 300° C. and relative N₂O formation

FIG. 7: NOx conversion in the load range of below 350° C. and relative N₂O formation; relation to FIG. 6 with regard to the N₂O formation

FIG. 8: NOx conversion in the load range of above 400° C. and relative N₂O formation; relation to FIG. 6 with regard to the N₂O formation

FIG. 9: NOx conversion and N₂O formation in the system of FIG. 1 in NEDC, given bypass control according to the claim.

EXAMPLE OF THE MODE OF OPERATION OF THE INVENTION

Complete Layout of Exhaust Gas System (See FIG. 1)
Mode of Operation:

- 1) For lambda greater than 1, the bypass is closed (FIG. 2 with the depiction of cat BOX 1 and cat BOX 2)

For motor operation given lambda greater than 1, the bypass is closed (FIG. 2). NOx from the motor exhaust gas is stored in the catalysts so that the concentration NOx 1 in the exhaust gas is greater than the concentration NOx 2, and the concentration of NOx 2 in the exhaust gas is greater than the concentration of NOx 3. This applies to the following temperatures:

- Temperature (Temp 2) cat BOX 1 is less than 350° C., and temperature (Temp 3) cat BOX 2 is less than 350° C.
- Temperature (Temp 2) cat BOX 1 is greater than 350° C., and temperature (Temp 3) cat BOX 2 is less than 350° C.
- Temperature (Temp 2) cat BOX 1 is greater than 350° C., and temperature (Temp 3) cat BOX 2 is greater than 350° C.
- 2) For lambda less than 1, the bypass is open; the minimum temperature for cat BOX 1 is greater than 350° C., measured at Temp 2.

For motor operation given lambda less than 1, the bypass is open if the temperature (Temp 2) of cat BOX 1 is greater than 350° C., and the temperature (Temp 3) of cat BOX 2 is less than 350° C. (FIG. 3). The termination of the operation below lambda 1 takes place via the NOx sensor 1 or via a model/map stored in the ECU.

- 3) For lambda less than 1, the bypass is closed if the minimum temperature for cat BOX 2 is greater than 350° C., measured at Temp 3.

For motor operation given lambda less than 1, the bypass is closed if the temperature (Temp 2) of cat BOX 1 is greater than 350° C., and the temperature (Temp 3) of cat BOX 2 is greater than 350° C. (FIG. 2). The termination of the motor operation given lambda less than 1 takes place via the NOx sensor 2 or via a model/map stored in the ECU.

- 4) For lambda greater than 1 and concentration NOx 2 is equal to concentration NOx 3; this means that no storage of NOx takes place via cat BOX 2.

For motor operation given lambda greater than 1 and a concentration NOx 2 that is equal to a concentration NOx 3 (meaning that no storage of NOx takes place in cat BOX 2), the bypass is open (FIG. 3).

- 5) For lambda less than 1, if the minimum temperature (Temp 5) for cat BOX 3 is greater than 350° C.

For motor operation given lambda less than 1, the bypass is closed if the temperature (Temp 2) of cat BOX 1 is greater than 350° C., the temperature (Temp 3) of cat BOX 2 is greater than 350° C., and the temperature (Temp 5) of cat BOX 3 is likewise greater than 350° C. (FIG. 4). The termination for the operation at lambda less than 1 takes place via NOx sensor 3 or via a model/map stored in the ECU.

- 6) For lambda less than 1, if the minimum temperature (Temp 2) for cat BOX 1 is greater than 350° C.

For motor operation given lambda less than 1, the bypass is open if the temperature (Temp 2) of cat BOX 1 is greater than 350° C., and the temperature (Temp 3) of cat BOX 2 is less than 350° C. The termination for the operation at lambda less than 1 takes place via NOx sensor 1 or via a model/map stored in the ECU. A combination of SCR and NSC catalysts is hereby preferred for cat BOX 3, wherein the SCR catalyst is arranged upstream of the NSC catalyst (FIG. 5).
Additional Examples and Exhaust Gas Measurement:
Stationary Tests at a Highly Dynamic Motor Test Stand for Obtaining the Results of FIGS. 6, 7, and 8:
In the stationary test on a system of FIG. 1, 10 rich/lean cycles were run in succession. Wherein the termination criteria for lean operation at 50 ppm NOx slip was

- a) position NOX Sensor 1 with open bypass; and
- b) position NOX Sensor 2 without bypass.

The regeneration of the NOx storage catalysts takes place via the rich operation of the motor test stand, over an established unit of time. The time unit is selected so that all catalysts are sufficiently regenerated.
Of the 10 rich/lean cycles, the last 5 are used to calculate the NOx conversion. This ensures that the system is in equilibrium. The person skilled in the art also knows this as a steady state.
The respective target temperatures at the catalyst are generated by the variation of the load at the motor test stand. In the test, 3 different load points were hit in order to generate the temperatures<300° C., <350° C., and >400° C. at cat BOX 1. Corresponding probe analysis is used to measure the secondary emissions—for example, of N₂O.
Dynamic Run Cycle at a Highly Dynamic Test Stand to Obtain the Results of FIG. 9:
In the application of the run cycle NEDC at a highly dynamic test stand, motor data are read out from a production vehicle which is in testing operation and transferred to the controller of the highly dynamic test stand. It is hereby especially to be noted that the reproducibility of the applied tests reaches the highest degree of precision.
If the test conditions are applied as described above, the exhaust gas system is tested with and without bypass in the NEDC cycle.
It hereby applies that the termination for the respective lean or rich phases normally occurs via lambda sensor 3 or NOX sensor 3.

Claims

1. Method for reduction of harmful automobile exhaust gas components with the aid of an exhaust gas system having at least two catalysts selected from the group consisting of NSC, TWC, and TWNSC, wherein the exhaust gas is diverted around the downstream catalyst of the at least two catalysts if this is within a temperature window in which it is capable of forming N₂O from NH₃, and the upstream catalyst of the at least two catalysts produces NH₃.

2. Method according to claim 1,

characterized in that

the exhaust gas is produced by a gasoline engine that is operated predominantly with an A/F mixture that is lean on average.

3. Method according to claim 1,

characterized in that

the downstream catalyst of the at least two catalysts is selected from the group consisting of TWNSC and NSC.

4. Method according to claim 1,

characterized in that

the exhaust gas system downstream of the at least two catalysts has at least one NOx reduction catalyst.

5. Method according to claim 4,

characterized in that

the NOx reduction catalyst is made up of at least one SCR and/or NSC catalyst.

6. Method according to claim 5,

characterized in that

the at least one SCR catalyst is arranged upstream of the at least one NSC catalyst.

7. Method according to claim 1,

characterized in that

the diversion of the exhaust gas takes place when the downstream catalyst of the least two catalysts has a temperature of less than 350° C.

8. Method according to claim 1,

characterized in that

the at least two catalysts are located in the first half of the exhaust gas tract, as measured from the motor output to the end of the exhaust pipe.

9. Method according to claim 1,

characterized in that

at least one temperature sensor is located in the flow direction of the exhaust gas after the downstream catalyst of the at least two catalysts.

10. Method according to claim 1,

characterized in that

at least one temperature sensor is located between the at least two catalysts.

11. Method according to claim 1,

characterized in that

the diversion of the exhaust gas around the downstream catalyst of the at least two catalysts is effected by means of a device for activating and deactivating the diversion, which device is positioned at the merger of the diversion and the main exhaust gas tract.

12. Method according to claim 11,

characterized in that

the device for activating and deactivating the diversion is a valve or an exhaust gas flap.

13. System for exhaust gas aftertreatment, having at least two catalysts from the group consisting of NSC, TWC, and TWNSC, wherein the system is designed so that the exhaust gas may be diverted around the downstream catalyst of the at least two catalysts if this is capable of forming N₂O from NH₃.

14. System according to claim 13,

characterized in that

15. System according to claim 13,

characterized in that

16. A method for the aftertreatment of the exhaust gas of a gasoline engine that is operated predominantly with an A/F mixture that is lean on average, comprising passing exhaust gas within the system of claim 13.