DE102018121503A1 - Exhaust gas purification with NO oxidation catalyst and SCR-active particle filter - Google Patents

Abstract

The present invention is directed to a system and method for pollutant reduction in the exhaust of predominantly lean-burn automotive engines having first and second catalytic converters. The first catalyst oxidizes NO to NO _2, and the second catalyst preferably reduces NOx in the lean exhaust by means of a reducing agent to N ₂ and H ₂ O.

Description

The present invention is directed to a system and method for pollutant reduction in the exhaust of predominantly lean-burn automotive engines having first and second catalytic converters. The first catalyst oxidizes NO to NO _2, and the second catalyst preferably reduces NOx in the lean exhaust by means of a reducing agent to N ₂ and H ₂ O.

Exhaust gases of motor vehicles with a predominantly lean-burn internal combustion engine contain, in addition to particulate emissions, in particular the primary emissions carbon monoxide CO, hydrocarbons HC and nitrogen oxides NOx. Due to the relatively high oxygen content of up to 15 vol.%, Carbon monoxide and hydrocarbons can be made relatively harmless by oxidation, but the reduction of nitrogen oxides to nitrogen is much more difficult.

One known method for removing nitrogen oxides from exhaust gases in the presence of oxygen is Selective Catalytic Reduction (SCR) using ammonia on a suitable catalyst. In this method, the nitrogen oxides to be removed from the exhaust gas are reacted with ammonia to nitrogen and water. The ammonia used as the reducing agent may be made available by metering an ammonia precursor compound such as urea, ammonium carbamate or ammonium formate into the exhaust line and subsequent hydrolysis. It is also possible to generate the ammonia in situ by over-reduction of NOx via a nitrogen oxide storage catalyst.

Particles can be removed very effectively with the help of particle filters from the exhaust gas. Wall flow filters made of ceramic materials have proven particularly useful. These are composed of a plurality of parallel channels formed by porous walls. The channels are mutually sealed gas-tight at one of the two ends of the filter, so that first channels are formed, which are open on the first side of the filter and closed on the second side of the filter, and second channels, which are on the first side of the filter closed and open on the second side of the filter. The exhaust gas flowing, for example, into the first channels can leave the filter again only via the second channels, and must for this purpose flow through the porous walls between the first and second channels. As the exhaust passes through the wall, the particles are retained.

It is also already known to coat wall-flow filters with SCR-active material and at the same time to remove particles and nitrogen oxides from the exhaust gas. Such products are commonly referred to as SDPF. However, if the required amount of SCR-active material is applied to the porous walls between the channels (so-called on-wall coating), this can lead to an unacceptable increase in the back pressure of the filter.

Against this background, for example, propose the JPH01-151706 and WO2005016497A1 to coat a wall flow filter with an SCR catalyst such that the latter penetrates the porous walls (so-called in-wall coating). It has already been suggested, see US2011274601A1 to introduce a first SCR catalyst into the porous wall, ie to coat the inner surfaces of the pores and to place a second SCR catalyst on the surface of the porous wall. The average particle size of the first SCR catalyst is smaller than that of the second SCR catalyst.

Particulate filters have to be regenerated at certain intervals, that is, the accumulated soot particles must be burned off in order to keep the exhaust back pressure in an acceptable range. For filter regeneration and the initiation of Rußabbrandes exhaust gas temperatures of about 600 ° C are required. Burning can cause very high temperatures, which can be> 800 ° C.

In the case of particulate filters, which are provided with SCR catalysts, the latter must withstand the high thermal loads during filter regeneration without serious loss of activity. However, there is still a considerable need for improvement in this regard. Currently, SCR catalyst coatings are available on filters that can withstand maximum temperatures of 800-850 ° C for some time. In exceptional cases, however, temperature peaks of up to 1000 ° C. or more can be achieved in the filter during soot regeneration if the soot burn-off occurs uncontrollably, as may occur in certain driving situations of the vehicle.

Transition-metal-ion-exchanged LEV-type SCR catalysts (http://europe.iza-structure.org/IZA-SC/ftc_fw.php?STC=LEV) are already in the US4867954 been described. Other LEV-based SCR catalysts are disclosed in US9242241BB, WO2008132452A2 or the WO2008118434 mentioned. Exhaust systems of a first oxidizing NO catalyst and a downstream SCR catalyst are also already known (eg WO2016024193A1 ; EP1054722B1 ; JP8103636A ; DE2832002A1 ; JP08-42329 ; US7005116 ). In this context, the disclosure of the WO2016024193A1 to a special multilayer oxidation catalyst in front of an SCR catalyst, wherein the oxidation catalyst is also capable of storing nitrogen oxides at low temperatures and releasing them at elevated temperatures. The SCR catalyst may have a zeolitic component (zeolite or zeotype such as an aluminum silicophosphate) of the LEV type. Optionally, the SCR catalyst may be located on a particulate filter.

From the cited prior art, it is further important to provide improved exhaust systems that are superior to the known ones. In this sense, the present invention has the object to develop an improved exhaust system for lean-burn car engines, which is particularly superior in terms of long-term stability of the known systems of the prior art. The system according to the invention should be better able to sufficiently reduce the nitrogen oxides in the exhaust gas of a lean-burn engine, even after long running times of the vehicle, without negatively influencing the reduction of other, harmful exhaust gas constituents via the charge.

These and other objects which are obvious from the state of the art are solved by a system and a method having the features of present claims 1 and 14. Preferred embodiments of the present system will become apparent from the depending from claim 1 dependent claims 2-13.

• - einen Binder auf Basis eines hochoberflächigen Metalloxids aufweist,
• - ein ursprüngliches Si/Al- bzw. (Al+P)/Si-Verhältnis von 5 bis 40 im zeolithischen Gerüst hat, und
• - mit Cu- und/oder Fe-Ionen behandelt vorliegt, wobei der Gesamtgehalt an Cu- und/oder Fe-Ionen im Bereich von 2 Gew.-% bis 10 Gew.-% bezogen auf das Gewicht des Materials beträgt, und

im Material ein Verhältnis von im LEV ionenausgetauschten zu nicht-ionenausgetauschten Cu- und/oder Fe-Ionen im Bereich von 20:80 bis 90:10 vorliegt,
gelangt man überraschend, dafür aber nicht minder vorteilhaft zur Lösung der gestellten Aufgabe. Durch die Verortung eines SCR-Katalysators aus einem entsprechenden Material auf einen Partikelfilter, der abstromseitig zu einem oxidativ auf NO wirkenden ersten Katalysator angeordnet ist, erhält man ein Abgasnachbehandlungssystem, welches bezogen auf den Einsatzort des SCR-Katalysators über das Lebensalter des Systems betrachtet eine wesentlich bessere Performance in der Stickoxidreduktion zu leisten im Stande ist. Selbst nach einer sehr starken Alterung behält der SCR-Katalysator mit diesem Material entgegen dem momentan am häufigsten verwendeten zeolithischen Material des Standes der Technik - CHA - eine ansprechende Leistung in der Stickoxidreduktion (4). Dies ist besonders beim Einsatz auf oder in einem Partikelfilter wichtig, da hier - wie gesagt - zeitlich begrenzt während der Regeneration des Filters extrem hohe Temperaturen entstehen können. Ein solches System bietet daher über die Gesamtlaufzeit des Automobils eine bessere Reduktionsleistung in Bezug auf die Stickoxide im Abgas als beispielsweise ein vergleichbarer Katalysator auf Basis eines Zeolithen vom CHA-Typ. Des Weiteren ist zu vermerken, dass der SCR-Katalysator auf Basis zeolithischer Materialien vom LEV-Typ verglichen mit einem Zeolithen vom CHA-Typ verbesserte Stickoxidreduktionswerte dann imstande ist zu liefern, wenn ein erhöhtes Angebot an NO₂ im Abgas vor dem SCR-Katalysator vorliegt. Dieses wird durch den oxidativ auf NO wirkenden ersten Katalysator im Abgassystem bewerkstelligt, der so ausgelegt sein sollte, dass entsprechend hohe NO₂-Werte (z.B. ≥30% im NOx) über möglichst viele Betriebspunkte des Magermotors (z.B. > 50%, bevorzugt > 60%, ganz bevorzugt >70% der Betriebszeit) erzielt werden. Dies war umso mehr überraschend, als dass für SCR-Katalysatoren auf Basis von kleinporigen Zeolithen (zu denen auch der LEV-Typ gehört) vom CHA-Typ gefunden wurde, dass deren Stickoxidreduktionswerte unter 50% NO₂-Anteil (Vol.-%) im NOx eher unabhängig vom NO₂-Angebot im mageren Abgas sind und darüber (>50 Vol.-%) eher negativ durch erhöhte NO₂-Werte beeinflusst werden (1). CHA und LEV sind darüber hinaus vom Aufbau her eng verwandt und gehören beide der sogenannten ABC-6-Familie an (https://de.wikipedia.org/wiki/Zeolithgruppe). Dass sie sich trotzdem so unterschiedlich verhalten, war vor der vorliegenden Erfindung nicht zu erwarten gewesen. Es sei weiterhin erwähnt, dass ein Katalysator mit einer zeolithischen Komponente vom LEV-Typ wesentlich weniger N₂O-Bildungstendenz (Treibhausgas) zeigt als ein entsprechender Katalysator mit einer CHA-Typ-Komponente.Characterized in that a system for exhaust aftertreatment of exhaust gases of a lean-burn engine seen in the exhaust line in the flow direction oxidatively acting on NO first catalyst and the downstream side a reductively with the aid of a reducing agent acting on NO x (NO ₂ and NO and N ₂ O) acting second catalyst the first catalyst is on a flow monolith and the second catalyst is on and / or in a wall flow filter and the latter contains material comprising an aluminosilicate or an aluminum silicophosphate zeolitic LEV type zeolite framework, the material comprising:

• comprises a binder based on a high surface area metal oxide,
• has an original Si / Al or (Al + P) / Si ratio of 5 to 40 in the zeolitic framework, and
• - Is treated with Cu and / or Fe ions, wherein the total content of Cu and / or Fe ions in the range of 2 wt .-% to 10 wt .-% based on the weight of the material, and

the material has a ratio of LEV ion-exchanged to non-ion-exchanged Cu and / or Fe ions in the range from 20:80 to 90:10,
one arrives surprisingly, but not less advantageous to solve the task. By locating an SCR catalyst of a corresponding material on a particulate filter, which is arranged downstream of an oxidative acting on NO first catalyst, to obtain an exhaust aftertreatment system, which regards the place of use of the SCR catalyst over the age of the system considered a significant better performance in nitrogen oxide reduction is able to afford. Even after a very high aging, the SCR catalyst with this material retains an attractive nitrogen oxide reduction performance (CHA), contrary to the currently most commonly used zeolitic material (CHA) ( 4 ). This is particularly important when used on or in a particulate filter, as here - as stated - for a limited time during the regeneration of the filter extremely high temperatures can occur. Such a system therefore offers a better reduction performance with respect to the nitrogen oxides in the exhaust gas over the entire running time of the automobile than, for example, a comparable catalyst based on a CHA-type zeolite. It is further noted that the SCR catalyst based on LEV type zeolitic materials is able to deliver improved nitrogen oxide reduction values compared to a CHA type zeolite when there is an increased supply of NO ₂ in the exhaust gas upstream of the SCR catalyst , This is accomplished by the oxidizing NO acting first catalyst in the exhaust system, which should be designed so that correspondingly high NO ₂ values (eg ≥30% in NOx) over as many operating points of the lean-burn engine (eg> 50%, preferably> 60 %, more preferably> 70% of the operating time) can be achieved. This was all the more surprising as it was found that for SCR catalysts based on small-pore zeolites (of which the LEV type also belongs) of the CHA type, their nitrogen oxide reduction values were below 50% NO ₂ content (% by volume). in NOx are rather independent of the NO ₂ supply in the lean exhaust gas and above (> 50% by volume) are influenced rather negatively by increased NO ₂ values ( 1 ). CHA and LEV are also closely related in structure and both belong to the so-called ABC-6 family (https://de.wikipedia.org/wiki/Zeolithgruppe). That they behave nevertheless so differently, was not to be expected before the present invention. It should also be noted that a LEV-type zeolitic-type catalyst exhibits significantly less N ₂ O-forming tendency (greenhouse gas) than a corresponding catalyst having a CHA-type component.

The first catalyst which acts oxidatively on NO and which is used in the present exhaust aftertreatment system should be such that, whenever possible, there is sufficient NO ₂ in the exhaust gas before the second catalyst. Normally, SCR catalysts reduce NOx in the lean exhaust gas by means of ammonia or an ammonia-forming precursor compound particularly well whenever the ratio of NO: NO ₂ in the NOx is 1: 1 - so-called rapid SCR reaction ( Tünter et al., Ind. Eng. Chem. Prod. Res. Dev. 1985, 25, 633-636 ; Kato et al., J. Phys. Chem. 1981, 85, 4099-4102 ; DE2832002A1 ; Kasaoka et al., Nippon Kagaku Kaishi, 6 (1978), p. 874-881 ). The EP1147801B1 Proposes that for transition-metal-exchanged zeolites as SCR catalysts, the exhaust gas mixture should first be passed over an oxidation catalyst before being passed together with ammonia via the SCR catalyst. The NO ₂ : NOx content should be between 30 and 70 vol.% Here. In the EP1054722B1 is required for transition metal exchanged zeolites as SCR catalysts in this context that as far as possible all NO is to be oxidized to NO ₂ .

Small pore zeolites such as CHA show a completely different behavior here. Metkar et al. (Chemical Engineering Science 87 , (2013), 51-66) have found that the dependence of the nitrogen oxide reduction performance on the NO: NO ₂ ratio is more or less absent in the case of copper-exchanged CHA zeolites or merely above 50% by volume NO ₂ strongly decreases at low temperatures ( 1 ). It has now been found that, surprisingly, the likewise small-pore LEV-based SCR catalysts show a different behavior on the other hand. Accordingly, it is particularly advantageous if the oxidizing NO-acting first catalyst in this particular system is such that the NO ₂ : NOx volume ratio after this first catalyst measured in the WLTC in the cycle average is between 20% by volume and 60%. Vol .-%, preferably at 30 vol.% - 50 vol.% And most preferably at 30 vol.% - 40 vol.% Is. Thus, the LEV-based SCR catalyst, with appropriate aging, exhibits more than double the improvement in nitrogen oxide reduction performance when transitioning from 16 to 36% by volume of NO ₂ in total cycle-average NOx (WLTC), like the very related CHA zeolite type (low vs high NO ₂ : NOx in 4 ).

Preferably, the first catalyst is selected from the group consisting of diesel oxidation catalyst (DOC), nitric oxide storage function (DNC) diesel oxidation catalyst, nitrogen oxide storage and reduction catalyst (NSC). The first catalyst should be such that the NO ₂ : NOx molar ratio after the first catalyst measured in the WLTC in the cycle average is between 10% by volume and 60% by volume, preferably 20% by volume and 55% by weight. Vol .-% and particularly preferably 25 vol .-% and 50 vol .-% is.

The first catalyst may contain a noble metal content of 50-180 g / ft ³ . It preferably has a noble metal content of 60-150 g / ft ³ , more preferably 70-140 g / ft ³ . Particularly suitable noble metals are rhodium, platinum, palladium and iridium, preferably rhodium, platinum, palladium. Very particularly preferred is the use of platinum as the best NO-oxidizing catalyst in this context. A corresponding catalyst should therefore definitely have platinum as the catalyst component. This can be done in a simple diesel oxidation catalyst. Such catalysts are well known to the person skilled in the art ( WO2017093720A1 ; WO201603253A1 ; US2016341091AA ; DE202015004981 ; WO2015150805A1 ; WO2015110817A1 as well as cited literature).

In addition, the first catalyst may preferably be capable of storing nitrogen oxides. In particular, he should be able to store nitrogen oxides at the lowest possible temperatures can. In such an embodiment, the nitrogen oxides would then be stored at lower temperatures (cold start, city driving) and released at higher temperatures than NO ₂ again, which contributes to the fact that the NO ₂ : NOx ratio over a very wide temperature range in for the SCR catalyst on LEV-based optimal range can be maintained. Such catalysts are for example in the WO14164876A1 . WO16141140A1 . WO20150853030A1 . WO2015143225A1 . WO2017001828A1 . US2016341091 . WO2016151296A1 or WO2016135465 respectively. GB2538877A1 or WO2016079507 called. In a preferred embodiment, the catalyst described here is capable of storing nitrogen oxides even at below 100 ° C. (at least 30% by volume, preferably 40% by volume, particularly preferably at least 50% by volume of the incoming NOx in the empty state measured) and at> 250 ° C, more preferably at> 300 ° C and most preferably at> 350 ° C to give them back to the exhaust gas.

Also possible is the use of nitrogen oxide storage catalysts. Their design and operation is sufficiently familiar to the skilled person (eg EP2985068A1 . EP2982434A1 ). With regard to the use of nitrogen oxide storage catalysts in the system according to the invention, it should be noted that these can be used very particularly preferably in this regard, because they are capable, under regeneration conditions (exposure to reductive Components from the exhaust gas (HC, CO, H ₂ )) to form a sufficient amount of NH ₃ . This NH ₃ can be stored in the downstream filter with the LEV-based SCR catalyst and used for the SCR reaction. Accordingly, injection of additional reducing agent for the SCR reaction as described below can be dispensed with or limited to a necessary extent ( WO10114873A2 ). In addition, due to their noble metal content, these catalysts cause the NOx, which possibly could not be stored due to excessive temperatures and thus leave the nitrogen oxide storage catalyst again in the lean, to have a correspondingly high NO ₂ content. Thus, this exhaust is then again particularly well suited for the SCR reaction on the following second catalyst.

However, an injection device for the addition of ammonia or an ammonia precursor compound, such as urea, may also be present between the first and second catalyst ( WO08106518A2 ).

As already indicated several times, the material of the second catalyst consists of at least one binder and a zeolitic aluminum silicate or aluminum silicophosphate of the structural type LEV (http://www.iza-structure.org/databases/) as well as certain transition metal ions. The material is prepared, for example, so that the LEV zeolite in its example NH ₄ ⁺ , Na ⁺ or H ⁺ form is contacted with a solution of copper and / or iron ions. After separation of the zeolite from the solution, the latter is preferably brought together with the binder without being washed and applied as a suspension and / or introduced into a corresponding wall-flow filter (coating). Alternatively, however, the zeolite may also be contacted with the binder together with the solution of copper and / or iron ions. If appropriate, such a suspension can be applied directly to the wall and / or into the wall-flow filter without having to separate it once again from the solution of the copper and / or iron ions. Another alternative option is the spray-drying. Again, the LEV zeolite and the solution of copper and / or iron ions can optionally be spray-dried together with the binder. The material produced by these exemplary processes always contains ion-exchanged and non-ion-exchanged iron or copper ions. The term "ion-exchanged" is understood according to the invention as meaning that the cations sit in so-called exchange sites in the cavities and pores of the LEV zeolite, where they are capable of compensating for one or even two negative charges of the zeolitic framework. The metal to framework aluminum or, in the case of the corresponding SAPO, the framework silicon ratio is generally between 0.3 and 0.5, preferably 0.4 to 0.48 for the ion-exchanged metal ions. The person skilled in the art knows how to provide the zeolitic (zeolite or zeolite-like materials) materials with the transition metals ( EP324082A1 ; WO1309270711A1 PCT / EP2012 / 061382 and literature cited therein) in order to be able to provide a good activity towards the reduction of nitrogen oxides with ammonia. Such exchange ion-sitting ions are visible in electron spin resonance analysis and can be quantified (Quantitative EPR, Gareth R. Eaton, Sandra S. Eaton, David P. Barr, Ralph T. Weber, Springer Science & Business Media, 2010). All non-ion exchanged cations are located elsewhere in the material inside or outside the zeolite. The latter do not compensate for negative charge of the zeolitic framework. They are invisible in the EPR and can thus be calculated from the difference between the total metal loading (eg determined by ICP) and the value determined in the EPR. It has been found that it is precisely a reasonable proportion of unexchanged ions that can impart sufficient stability and / or activity to the material used according to the invention. This was very surprising against the background of the prior art. The ratio of ion-exchanged to non-ion-exchanged Cu and / or Fe ions is preferably 50:50 to 90:10 and most preferably 70:30 to 85:15.

Such a catalyst produced shows special properties. Preferably, such a catalyst is used as the second catalyst in the system according to the invention, which after hydrothermal aging at 900 ° C for 16 h compared to a hydrothermal aging at 800 ° C for 16 h only a decrease in the total NOx conversion in the WLTC of less than 15% at a mean NO ₂ : NOx molar ratio of 36% in cycle average measured in WLTC. Aging is described in the example section.

Furthermore, the use of a catalyst as second catalyst in the system according to the invention is preferred, which after hydrothermal aging at 900 ° C for 16 h compared to a correspondingly aged catalyst with a material comprising a CHA-type zeolite skeleton at least 3 times higher overall NOx conversion in WLTC at a mean NO ₂ : NOx molar ratio of 36% in cycle average measured in WLTC.

In addition, a second catalyst prepared as described above after hydrothermal aging at 900 ° C for 16 h, a total NOx conversion in the WLTC of> 60% at a mean NO ₂ : NOx molar ratio of 36% in the cycle average measured in WLTC.

The zeolitic LEV substance present in the material of the second catalyst can be constructed according to the invention from an aluminosilicate or an aluminum silicophosphate type. The second catalyst preferably has exclusively an LEV-type zeolite zeolitic aluminum silicate framework. Preferably, synthetic LEV zeolites are used. These are often denoted by the names 3 , ZK-20, LZ 132 or ZSM-45 in the literature. Very particularly preferred is the use of ZSM-45 in this context. In the aluminosilicate zeolites of the LEV type, the Si / Al value in the original zeolite is preferably 7 to 30, more preferably 10 to 20. However, the same values can also be used for the aluminosilicate-containing LEV grades (for example rubble). 1 , Sapo 35 ) are used for the (Al + P) / Si ratio. Originally, in this context means the value of the zeolitic material used.

The particulate filter preferably used in the present exhaust system is of the wall-flow filter type. Such filters are well known to those skilled in the art. The coating of such a wall-flow filter can take place on the wall or in the wall, in the pores of the walls. Such coating techniques are also familiar to the person skilled in the art.

However, the wall-flow filter may also be an extrudate based on the catalytically active material having the second catalyst ( WO2008 / 132452 A2 ).

In order to ensure a corresponding activity in the catalytic reaction of the exhaust gas components, in particular the reduction of the nitrogen oxide on the second catalyst, a certain loading of the filter with the catalytically active material of the second catalyst is necessary. In this case, the material in addition to the metal ion-exchanged with Fe ions or Cu ions aluminosilicate of the structural type LEV further components, in particular a binder based on a high surface area, heat stable metal oxide. Such binders commonly consist of high surface area, heat stable oxides selected from the group of alumina, silica, zirconia, titania or mixtures thereof. Preferably, a high surface area, heat stable alumina is used in this regard. The binder is preferably present in an amount of less than 20% by weight, preferably from 15% by weight to 8% by weight and very particularly preferably from 14% by weight to 10% by weight, in the catalytically active material , It should be noted that the binder can be added to the actual catalytically active material (the ion-exchanged aluminosilicate) both before the actual ion exchange and thereafter. As such, the wall flow filter is coated with a suspension of the catalytically active material containing the binder. The wall-flow filter advantageously exhibits a total charge of 60-200 g / l, preferably 70-160 g / l, and particularly preferably 80-150 g / l with the second catalyst. Very particular preference is given to an embodiment in which the second catalyst is located in the wall pores and on the walls of the wall-flow filter. The distribution between the expense and inner wall coating can be less than 50:50, preferably less than 40:60 and most preferably less than 30:70 based on the weight of the overall coating (with the effort material is meant the material which on the Channel wall of the filter is located and therefore the channel in cross-section reduced, the Inwandbeschichtung is in the pores of the wall flow filter before). Most preferably, the amount of material in the wall pores is 80-160 g / l, preferably 90-160 g / l, and more preferably 100-150 g / l. It is also possible to apply another SCR-active substance to the wall than to the wall and vice versa. Likewise, a mixture of different SCR active components may be present in the filter. Also possible is the zoned arrangement of different substances for the SCR reaction in a filter. Preferred in this context is an embodiment in which the second catalyst is located in a range of ≦ 60%, preferably ≦ 50% of the length of the filter calculated from the output, since the greatest heat is produced during the regeneration of the filter. The remainder of the filter may then have another SCR-active coating, eg one based on a CuCHA zeolite. It is only important that the filter used in some way have the corresponding LEV-type SCR-active components in an amount that achieves the above-defined objects of the present invention. In particular, the filter according to the invention after a hydrothermal aging of 16 hours at 900 ° C should still show at least 85% of the NOx conversion measured over the WLTC, compared with a material hydrothermally aged for 16 hours at 800 ° C. Here, the exhaust gas mixture to be treated is designed to contain between 30 vol.% And 40 vol.% NO ₂ in NOx in the WLTC cycle average.

Other preferred system designs include, for example, ammonia barrier catalyst (ASC, AMOX) after the second catalyst in the exhaust line. Ammonia barrier catalysts have an oxidizing effect on ammonia, so that it is preferably oxidized to nitrogen. Such catalysts have been described in the prior art in the form of platinum-containing oxidation catalysts and / or copper-containing zeolites ( EP2117707A1 ; WO201603249A1 ; WO2011140251A2 ; WO2012132095A1 ; WO2010062730A2 ). Preferably, a such catalyst on the downstream end of the outlet channels of the wall flow filter with the second catalyst in a length of at most 50%, preferably less than 30%, more preferably less than 25% of the total length of the wall flow filter be present, preferably only in the outlet channels.

In a further preferred alternative, a further SCR catalyst can be present downstream of the wall-flow filter and, if appropriate, on the upstream side of an ammonia barrier catalytic converter. The type of this SCR catalyst can be chosen freely by the person skilled in the art. Catalysts whose catalytically active material is selected from the group of transition-metal-exchanged zeolites or zeolite-like materials are preferably used in this regard. Such compounds are well known to those skilled in the art. Preferred in this regard are materials from the group consisting of levynite, AEI, KFI, chabazite, SAPO-34, SAPO-35, zeolite β and ZSM-5. Particular preference is given to using zeolites or zeolite-like materials of the chabazite type, in particular CHA or SAPO-34, and also LEV. These materials, to ensure sufficient activity, are preferably provided with transition metals selected from the group consisting of iron, copper, cerium, manganese and silver, or mixtures thereof. Very particularly advantageous is copper and / or iron to call in this context. The metal to framework aluminum or SAPO-34 framework silicon ratio is typically between 0.3 and 0.5, preferably 0.4 to 0.48 for the ion-exchanged metal ions. One skilled in the art will know how to provide the zeolitic (zeolite or zeolite-like materials) materials with the transition metals ( EP324082A1 ; WO1309270711A1 PCT / EP2012 / 061382 and literature cited therein) in order to be able to provide a good activity towards the reduction of nitrogen oxides with ammonia. Furthermore, vanadium compounds, cerium oxides, cerium / zirconium mixed oxides, titanium dioxide and tungsten- and manganese-containing compounds and mixtures thereof can also be used as the catalytically active material. Very particular preference is given to a catalyst based on a CuOHA-type aluminosilicate in this connection, which has the above features and has an SAR of 5-20, preferably 8-15, and most preferably 9-12.

The ammonia storage capacity of the SCR-active material used in the catalysts mentioned, in particular the zeolitic material of the SCR catalysts used in the fresh state at a measurement temperature of 200 ° C more than 0.9 g NH ₃ per liter of catalyst volume, preferably between 0.9 and 2.5 g NH ₃ per liter catalyst volume and more preferably between 1.2 and 2.0 g NH ₃ / liter of catalyst volume, and most preferably between 1.5 and 1.8 g NH ₃ / liter of catalyst volume. The ammonia storage capacity will be determined by means of a synthesis gas plant. For this purpose, the catalyst is first conditioned at 600 ° C with NO-containing synthesis gas to completely remove ammonia residues in the core. After the gas has been cooled to 200 ° C., ammonia is then metered into the synthesis gas at a space velocity of, for example, 30000 h ^-1 until the ammonia storage of the drill core is completely filled and the measured ammonia concentration after the drill core corresponds to the initial concentration. The ammonia storage capacity results from the difference between the total metered and the downstream measured ammonia amount based on the catalyst volume. The synthesis gas is typically composed of 450 ppm NH ₃ , 5% oxygen, 5% water and the remainder nitrogen.

The exhaust aftertreatment system according to the invention consists, as described in detail above, at least of a first and a second catalyst. The catalysts can be installed under the car according to the expert. A preferred embodiment assumes that the first catalyst is installed in a near-engine (cc) position, while the second and optionally further catalyst (s) is located in the underbody of the vehicle (uf area).

In a further aspect, the present invention also relates to a method for purifying the exhaust gas of lean-burn car engines. The method is characterized by passing the exhaust gas emitted from the engine via an exhaust aftertreatment system as just described before it is released into the ambient air. In the process, external ammonia or an ammonia precursor compound may be added between the first catalyst and the second catalyst. However, if the first catalyst is a nitrogen oxide storage and reduction catalyst (NSC, NSR), it is also possible to generate the ammonia in situ by over-reduction of the nitrogen oxides present in the nitrogen oxide storage catalyst and to store it in the second catalyst for the further reduction of nitrogen oxides. Such methods are well known to those skilled in the art (H.Noack, F. Welsch, J. Cross, Baron, Bremm; NSC and SCR as components in a modular system for sustainable solutions of passenger car exhaust aftertreatment; 24th Aachen Colloquium Automobile and Engine Technology, 2015).

Flow-through monoliths are conventional catalyst carriers in the art which, as in the case of the abovementioned filter materials, can consist of metal or ceramic materials. Preference is given to refractory ceramics such Example cordierite used. The ceramic flow-through monoliths usually have a honeycomb structure of rectangular, square, triangular or hexagonal geometry consisting 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 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, which is usually between 300 and 900 cells per square inch (cpsi). The wall thickness of the channel walls is between 0.5 and 0.05 mm for ceramics.

As particulate filter, it is possible to use all filter bodies customary in the state of the art made of preferably ceramic substrates of the wall flow type. Porous wall flow filter substrates of cordierite, silicon carbide or aluminum titanate are preferably used. These wall flow filter substrates have inflow and outflow channels, with the outflow-side ends of the inflow channels and the inflow-side ends of the outflow channels being closed relative to one another with gas-tight "plugs". Here, the exhaust gas to be cleaned, which flows through the filter substrate, forced to pass through the porous wall between the inlet and outlet, which causes an excellent particle filter effect. Due to the porosity, pore / radius distribution, and thickness of the wall, the filtration property can be designed for particles. The catalyst material may be in the form of coatings in and / or on the porous walls between the inlet and outlet channels. In the present case it is also possible to use filters which have been extruded directly or with the aid of binders from the corresponding catalyst materials, that is to say that the porous walls consist directly of the catalyst material, as is the case, for example, in the case of zeolite or vanadium-based SCR catalysts Case can be. This embodiment is included here. Such extruded SCR filters may additionally have an SCR coating as described above in and / or on the porous walls. Preferably to be used filter substrates, the EP1309775A1 . EP2042225A1 or EP1663458A1 be removed. The porosity of these filters is usually more than 40%, generally from 40% to 75%, especially from 45% to 70% [measured according to DIN 66133 - latest version on filing date]. The average pore size is at least 7 .mu.m, for example from 7 .mu.m to 34 .mu.m, preferably more than 10 .mu.m, in particular from 10 .mu.m to 20 .mu.m or from 21 .mu.m to 33 .mu.m [measured according to DIN 66134 latest version on the filing date]. The finished and coated filters have a pore size of usually 10 microns to 33 microns and a porosity of 50% to 65% and are particularly preferred (each measured according to the DIN shown above).

The term coating is understood to mean the application of catalytically active materials and / or storage components to a largely inert support body, which can be constructed like a previously described wall-flow filter or flow-through monolith. The coating takes on the actual catalytic function and contains storage materials and / or catalytically active metals, which are usually deposited in highly disperse form on temperature-stable, high-surface area metal oxides. The coating is usually carried out by applying an aqueous suspension of the storage materials and catalytically active components - also called washcoat - on or in the wall of the inert support body. After application of the suspension, the support is dried and optionally calcined at elevated temperature. The coating may consist of a layer or be composed of several layers, which are applied one above the other (multi-layered) and / or staggered (zoned) to a support body.

In addition to the actual catalytically active coating, the washcoat can additionally contain other materials which support the catalytic function of the catalytically active material and / or the storage components but do not actively intervene in the reaction. Materials used here are further additives known to the person skilled in the art, in particular so-called binders. Among other things, the latter ensure that the materials and components involved in the reaction can adhere sufficiently firmly to the corresponding substrate. In the present case, preferably optionally doped high surface area aluminas are used as such materials. The binder is used in a specific amount of up to 20 wt .-% in the respective catalytically active material.

If subsoil (uf) is mentioned, this refers in the context of the present invention to an area in the vehicle in which the catalyst is at a distance of 0.2-3.5 m, more preferably 0.5-2 m and most preferably 0.7 to 1.5 m after the end of the first close-coupled catalyst of at least two catalysts, preferably mounted under the driver's cab.

As close to the engine (cc) is referred to in the context of this invention, an arrangement of the catalyst at a distance from the exhaust outlet of the cylinder of the engine of less than 120 cm, preferably less than 100 cm and most preferably less than 50 cm. Preference is given to the close-coupled catalyst arranged directly after the merger of the exhaust manifold in the exhaust pipe.

The combustion air ratio (A / F ratio, air / fuel ratio) sets the actual air mass m _{L, tats} available for combustion in relation to the minimum necessary stoichiometric air mass m _{L, st} required for complete combustion: $$\lambda =\frac{m_{\text{L}\text{tats}}}{m_{\text{L}\text{st}}}$$

If λ = 1, then the ratio is considered to be a stoichiometric combustion air ratio with m _{L, tats} = m _{L, st} ; this is the case if theoretically all fuel molecules can completely react with atmospheric oxygen without any oxygen missing or unburned oxygen left over.

• λ < 1 (z. B. 0,9) bedeutet „Luftmangel“: fettes oder auch reiches Abgasgemisch
• λ > 1 (z. B. 1,1) bedeutet „Luftüberschuss“: mageres oder auch armes Abgasgemisch

For internal combustion engines:

• λ <1 (eg 0.9) means "lack of air": rich or rich exhaust gas mixture
• λ> 1 (eg 1.1) means "excess air": lean or poor exhaust gas mixture

Statement: λ = 1.1 means that 10% more air is present than would be necessary for the stoichiometric reaction. This is also called excess air. Preferably, however, an air-fuel mixture is 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, very particularly preferably between 0.95 and 0.99.

If in the present text of lean-burn car engines is mentioned, it is hereby referred mainly to diesel engines and mainly on average lean-burn gasoline engines. The latter are mainly on average with lean A / F ratio (air / fuel ratio) powered gasoline engines. The term "predominantly on average" takes account of the fact that modern gasoline engines are not operated statically at a fixed air / fuel ratio (A / F ratio, λ value). For example, Are three-way catalysts containing oxygen storage material acted upon by gasoline engines with exhaust gas with a discontinuous course of the air ratio λ. They undergo a periodic change in the air ratio λ and thus a periodic change of oxidizing and reducing exhaust conditions in a defined manner. This change in the air ratio λ is essential for the exhaust gas purification result in both cases. For this purpose, the λ value of the exhaust gas with a very short cycle time (about 0.5 to 5 hertz) and an amplitude Δλ of 0.005 ≤ Δλ ≤ 0.07 to the value λ = 1 (reducing and oxidizing exhaust gas constituents are in stoichiometric relationship before). On average, therefore, in such operating conditions, the exhaust gas is to be described as "on average" stoichiometric. So that these deviations do not adversely affect the exhaust gas purification result when passing the exhaust gas over the three-way catalyst, the oxygen storage materials contained in the catalyst compensate for these deviations to a certain extent by absorbing oxygen from the exhaust gas as required or releasing it into the exhaust gas (Catalytic Air Pollution Control, Commercial Technology, R. Heck et al., 1995, p. 90). Due to the dynamic operation of the engine in the vehicle, however, other deviations from this condition occur at times. For example, during extreme accelerations or when braking in overrun operating states of the engine and thus the exhaust gas can be adjusted, which may be over-or substoichiometric on average. However, the gasoline engines used in accordance with this invention have an exhaust gas which predominantly, i. burns in the majority of the combustion operation, an average lean air / fuel ratio.

list of figures

• 1 : Conversion of NOx via a CuCHA catalyst as a function of the NO: NO x ratio for different temperatures; Feed: 500 ppm NOx, 500 ppm NH ₃ , 5% O ₂ , 2% H ₂ O; the N ₂ -concentration after the catalyst is shown ( Metkar et al., Chemical Engineering Science 87, (2013), 51-66 ).
• 2 : Schematic test setup for the determination of NOX sales in the WLTC. with diesel oxidation catalyst (DOC), diesel particulate filter with SCR functionality (SDPF), exhaust gas sampling points and dosing unit for the aqueous urea solution.
• 3 : Velocity profile of the WLTC of the WLTC used to validate the catalysts (bottom), time history of the inlet temperature of the measured SDPFs (center) and NO ₂ : NOx molar ratios (low and high) over the course of the WLTC.
• 4 : NOx conversion in the WLTC of the example catalyst SDPF 1 and the catalyst SDPF according to the invention 2 after hydrothermal aging at 800 ° C and 900 ° C respectively for 16h at a low mean NO ₂ : NOx molar ratio (16%) and high mean NO ₂ : NO x molar ratio (36%) in the WLTC.

Examples:

Preparation of the catalysts

Example 1: Preparation of a CHA-based catalyst (SDPF 1) on a filter substrate

An aqueous suspension of a powder consisting of 92.6% copper exchanged commercial chabazite and 7.4% Al ₂ O _{3 is} prepared. In this case, the chalcazite-type zeolite has an SiO ₂ / Al ₂ O ₃ ratio of 30. The Cu content, based on the zeolite, calculated as CuO is 4.0% by weight. The suspension is applied to a commercial filter substrate such that its loading after drying at 90 ° C and calcination at 550 ° C with dried material is 120 g / L substrate volume.

Example 2: Preparation of a LEV-based catalyst (SDPF 2) used according to the invention on a filter substrate

It is an aqueous suspension of copper-exchanged commercial Levyne (Cu-LEV, 2h calcined at 850 ° C for 2h) with a SiO ₂ / Al ₂ O ₃ ratio of 31 and a Cu content of 3.5 wt .-% calculated as CuO based on the zeolite and a boehmite sol having a content of 20 weight percent Al ₂ O ₃ prepared so that the weight percentage of the copper-exchanged Levyne (LEV) 88% and the weight percentage of Al ₂ O ₃ 12% in the dried material. The ratio of LEV ion-exchanged to non-ion-exchanged Cu is approximately 80:20. The suspension is applied in a commercial filter substrate so that its loading in the pores after drying at 90 ° C and calcination at 550 ° C with dried material 120 g / L substrate volume.

Definition of hydrothermal aging

The aging of the above catalysts was carried out in a commercial furnace in a hydrothermal atmosphere (10% H ₂ O, 10% O ₂ , balance N ₂ ) for a holding time of 16 h at a temperature of 800 ° C and 900 ° C, respectively.

test method

The test was carried out on a diesel engine test bench with a standard 3.0 L diesel engine. 2 schematically shows the structure of the exhaust system used, the catalyst volumes used and the position of the dosing unit for the aqueous urea solution and the position of the exhaust gas sampling points for calculating the NOx conversion of the commercial, CHA-based catalyst SDPF 1 and the catalyst used according to the invention SDPF 2 in the WLTC (Worldwide Harmonized Light-Duty Vehicles Test Cycle). 3 shows the velocity profile, the inlet temperature of the measured catalysts and the different NO ₂ : NOx molar ratios.
The different NO ₂ : NOx molar ratios in the WLTC were adjusted by using different aged diesel oxidation catalysts.

results

4 shows the NOx conversion in the WLTC of the example catalyst SDPF 1 and the catalyst used according to the invention SDPF 2 after hydrothermal aging at 800 ° C and 900 ° C respectively for 16h at a low mean NO ₂ : NOx molar ratio (16%) and high mean NO ₂ : NO x molar ratio (36%) in the WLTC.

After a hydrothermal aging at 800 ° C. for 16 h, the NO x conversions of the catalyst SDPF used according to the invention differ 2 at low, average NO ₂ : NOx molar ratio of only 5% of the NOx conversion of the commercial catalyst SDPF 1 , An increase in the average NO ₂ : NOx molar ratio merely leads to an increase in NOx of 5% for SDPF 1 or 6% for SDPF2 in the WLTC.

After a hydrothermal aging at 900 ° C for 16 h, the NOx conversions of the commercially available catalyst SDPF 1 below 20%. Increasing the NO ₂ : NOx molar ratio increases NOx conversion by 4%. The inventively used catalyst SDPF 2 shows at low NO _2: NOx molar ratio with 50% significantly higher NOx conversion than the SDPF 1 , In addition, the NOx conversion increases by 11% at the transition to the high NO ₂ : NOx ratio.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been 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.

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Claims (14)

System for the exhaust aftertreatment of exhaust gases of a lean-burn engine comprising a first catalyst in the exhaust line in the flow direction oxidatively acting on NO and on the downstream side a reductively with the aid of a reducing agent on NO x (NO ₂ and NO and N ₂ O) acting second catalyst, wherein the first catalyst a flow monolith and the second catalyst is on and / or in a wall flow filter and the latter contains material comprising an LEV-type aluminosilicate or aluminum silicophosphate zeolite framework, the material comprising: - a high surface area refractory metal oxide based binder , - has an original Si / Al or (Al + P) / Si ratio of 5 to 40 in the zeolitic skeleton, - is treated with Cu and / or Fe ions, wherein the total content of Cu and / or Fe ions (calculated as CuO) in the range of 2 wt .-% to 10 wt .-% based on the weight of the material beträg t, characterized in that in the material a ratio of LEV ion-exchanged to non-ion-exchanged Cu and / or Fe ions in the range of 20:80 to 90:10 is present. System according to one of Claims 1 characterized in that the first catalyst is selected from the group consisting of Diesel Oxidation Catalyst (DOC), Nitrogen Oxyd Function (DNC) Diesel Oxidation Catalyst, Nitrogen Oxide Storage and Reduction Catalyst (NSC). System after Claim 1 and / or 2, characterized in that the first catalyst is such that the NO ₂ : NOx molar ratio after the first catalyst measured in the WLTC in the cycle average is between 10 vol% and 60 vol%. System according to one of Claims 1 - 3 , characterized in that the first catalyst has a noble metal content of 50-180 g / ft ³ . System according to one of Claims 1 - 4 , characterized in that the second catalyst after hydrothermal aging at 900 ° C for 16 h compared to a hydrothermal aging at 800 ° C for 16 h only a decrease in total NOx conversion in the WLTC of less than 15% with a mean NO ₂ : NOx molar ratio of 36% in cycle average measured in WLTC. System according to one of Claims 1 - 5 , characterized in that the second catalyst after hydrothermal aging at 900 ° C for 16 h compared with a correspondingly aged catalyst with a material comprising a zeolite CHA-type framework at a total of at least 3 times higher total NOx conversion in the WLTC at Mean NO ₂ : NOx mole ratio of 36% in cycle average measured in WLTC achieved. System according to one of Claims 5 or 6 , characterized in that the second catalyst after hydrothermal aging at 900 ° C for 16 h, a total NOx conversion in the WLTC of> 60% with a mean NO ₂ : NOx molar ratio of 36% in the cycle average measured in the WLTC due. System according to one of Claims 1 - 7 , characterized in that the second catalyst comprises exclusively an LEV-type zeolite zeolite containing aluminum silicate. System according to one of Claims 1 - 8th , characterized in that the binder is a high-surface metal oxide selected from the group Al ₂ O ₃ , SiO ₂ , ZrO ₂ , TiO ₂ , or mixtures thereof. System according to one of Claims 1 - 9 , characterized in that the wall flow filter in the pores of the walls has a loading of the second catalyst of 80 - 160 g / ft ³ . System according to one of Claims 1 - 10 , characterized in that the second catalyst is located in the wall pores and on the walls of the wall flow filter. System according to one of Claims 1 - 11 , characterized in that the wall flow filter on the downstream end of the outlet channels carries a Ammoniaksperrkatalysator. System according to one of Claims 1 - 11 , characterized in that downstream of the wall flow filter, a further SCR catalyst is present. Process for purifying the exhaust gas of lean-burn car engines; characterized in that the exhaust gas emitted from the engine via a system according to one of Claims 1 - 13 conducts before it is released into the ambient air.

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