EP1727973A1

EP1727973A1 - Method for improving the efficiency of reducing nox in motor vehicles

Info

Publication number: EP1727973A1
Application number: EP05737907A
Authority: EP
Inventors: Stefan Hubig; Ronny Mönnig; Helmut Oswald; Peter Zima
Original assignee: Fiat Auto SpA; GM Global Technology Operations LLC
Current assignee: Fiat Auto SpA; GM Global Technology Operations LLC
Priority date: 2004-03-17
Filing date: 2005-03-12
Publication date: 2006-12-06
Also published as: DE102004013165A1; BRPI0508877A; US20070081934A1; CN100526618C; CN1977095A; WO2005088091A1; JP2007529299A

Abstract

The invention relates to a method for reducing NOX in exhaust gas flows of a motor vehicle, by means of a catalyst. Said method is characterised in that an NOX absorbing material is provided in the catalyst.

Description

Process for improving the effectiveness of NOx reduction in motor vehicles

Description

The invention relates to a method for improving the effectiveness of NOx reduction in motor vehicles.

The legislation provides for a drastic reduction in the pollutant limit values in the EU IV directive.

The so-called selective catalytic reduction method (SCR method) is known for reducing the NO _x content in the exhaust gas of an internal combustion engine operated with excess air. In this method, a selectively acting reducing agent, usually by injecting, is fed to the exhaust gas at a point upstream of a catalytic converter, by means of which the NO _x contained in the exhaust gas in a chemical reaction in the SCR catalytic converter becomes eco-neutral components (N ₂ , 0 ₂ , H ₂ 0) can be implemented. The known process is already used today in diesel engines in the heavy-duty (trucks) sector, with aqueous urea solutions (32.5% by mass; known under the name 'AdBlue') or solid urea in pelletized and powdered form frequently being used , Such reducing agents from which can release ammonia as an intermediate, for example

Urea, a urea-water solution, solid ammonium carbonate or gaseous ammonia itself. Even with non-stoichiometric metering, the reducing agents mentioned can reduce nitrogen oxides by more than 95% over (vanadium oxide-containing) catalysts. With ammonia (NH ₃ ) as a reducing agent, this process has been used successfully in power plants for nitrogen oxide reduction for decades.

Solid or liquid operating materials are better suited for mobile use. In contrast to toxic ammonia, they are harmless and eco-neutral, but allow the generation of the ammonia required for the catalytic reaction on board a motor vehicle. An example of such a substance is urea, from which ammonia can be obtained by thermal decomposition or, preferably, by hydrolytic processes. The problem is that, regardless of the catalytic converter and reducing agent, the exhaust gas temperatures may not be sufficient for selective catalytic reduction, for example in the cold start phase of the engine or when driving in the city with frequent idling phases. In particular, the targeted addition (metering) of the reducing agent then represents a complicated control problem that cannot always be solved satisfactorily. There is a risk of ammonia slipping (breakthrough of free NH ₃ through the catalyst), which must be avoided due to the toxicity of ammonia.

For this reason, the direct use of fuel as a reducing agent without any preparation appears promising. In diesel engines, for example, additional diesel fuel can be directly injected into the engine's push-out cycle using a conventional injection system. spray or provide an additional injection valve in front of the existing SCR catalytic converter through which diesel fuel or another suitable hydrocarbon is injected. In the case of gasoline engine internal combustion engines, the exhaust gas itself usually contains a sufficient amount of HC for NO _x reduction.

The catalysts known from the prior art (for example 3-way technology for gasoline and / or CNG-driven units) use porous ceramic or noble metal substrates with particularly large surface volumes on which catalytically active substances are present in a washcoat coating Precious metals such as platinum or rhodium are applied. However, these catalysts are complex to manufacture and therefore often very expensive. It has also been found that, over time, the environment becomes contaminated from heavy metal released from the catalyst. The automotive catalytic converters used today are also frequently extremely sensitive to sulfur and / or sulfates, which are catalyst poisons for these catalysts, as a result of which the catalyst is at least partially deactivated.

Another problem known from the prior art catalyst technology, that for an optimal effect of the catalyst a certain threshold of N0 _X in the exhaust gas must be exceeded, but not this operating states in all areas of the engine is given. If you consider the combination of all these influences, there may be an insufficient NO _x reduction.

It is an object of the present invention to provide a method in which a reliable, highly effective and fast NO _x reduction in motor vehicles is achieved. The object is achieved by a method having the features of claim 1. Preferred developments of the method according to the invention are specified in dependent claims 2 to 10.

A method according to the invention for reducing NO _x in the exhaust gas flow of a motor vehicle by means of a catalytic converter or adsorbent is characterized in that an NO _x absorbing (or temporarily binding) material is present in the catalytic converter. Previously known methods for the reduction of N0 _X means catalyst are characterized in that _x exclusively of the reaction partners of the NO, it is NH _3, urea, or hydrocarbons are bound, after which a reaction of the bound and optionally transferred into a more reactive intermediate species reaction partner with the NO _{x of} the gas phase takes place, but it is very difficult to form a direct bond with the catalyst. However, according to the invention adsorbs the NO _x on the surface of the material, so therefore a local concentration of the N0 _X its subsequent reduction may be carried out first, which then can be carried out with greater efficiency.

Absorbing in the sense of the present invention means in particular that the NO _x -absorbing material preferably does not catalyze the reduction of the nitrogen oxides at lower temperatures, for example directly after an engine cold start. However, this material can also have a NO _x -reducing property or function when the temperature rises.

A preferred embodiment of the method according to the invention is characterized in that in the catalyst in addition to a NO _x -reducing catalyst material in a N0 _X - is available absorbent material. It should be noted that this can also be achieved by using a material as described above, which above all only has an absorbing effect at low temperatures and at the same time has a reducing effect at higher temperatures. However, at least two different materials can also be used, with one material being primarily absorbent, the other primarily reducing, but being able to complement one another in every conceivable way.

It is thus achieved by the method according to the invention that a sufficient reduction in NO _x can be achieved in all operating states of a lean-burn internal combustion engine. This happens first of all by the fact that NO _{x is} retained and enriched by the NO _x -absorbing material, which increases the effective local concentration. If the desorption temperature for the material is then clearly exceeded, the reduction with an increased concentration of NO _x can take place even more efficiently. This can be done either with the same material or with a special material.

In a preferred embodiment of the invention, the NO _x -absorbing and / or NO _x -reducing material is already at temperatures of <500 ° C, preferably <400 ° C, more preferably <300 ° C, still preferably <200 ° C, and most preferably <150 ° C and> 20 ° C NO _x absorbent. In this case, the NO _x will be absorbed at low exhaust gas temperatures, e.g. when the engine is started (starting the motor vehicle, especially in wintry conditions) (but not via nitrates, as is the case with a commercial NO _x trap ("NO _x - trap ") must first be formed before it is stored). Preferably, NO, but less NO ₂ (which is practically hardly formed by motor) is temporarily bound by the material and thus enriched. A preferred embodiment of the method according to the invention is characterized in that the NO _x absorbing material is selected from a group comprising natural, synthetic, ion-exchanging, non-ion-exchanging, modified, unmodified, "pillared", non-"pillared", Clay minerals, sepiolites, attapulgites, natural, synthetic, ion-exchanging, non-ion-exchanging, modified, unmodified, zeolites, Cu, Ba, K, Sr and Ag-laden, AI, Si and Ti "pilled" montmorillonites, hectorites doped with Fe, In, Mn, La, Ce, or Cu and mixtures thereof, Cu, Fe, Ag, Ce-loaded clinoptilolites and mixtures thereof.

A preferred embodiment of the method according to the invention is characterized in that the NO _x -reducing material is selected from a group comprising natural, synthetic, ion-exchanging, non-ion-exchanging, modified, unmodified, “pillared”, non-“pillared” ^λ , clay minerals, sepiolites, attapulgites, natural, synthetic, ion-exchanging, non-ion-exchanging, modified, unmodified, zeolites, Cu, Ba, K, Sr or Ag-laden, as well as AI, Si or Ti "pilled" Montmorillonite, hectorite doped with Fe, In, Mn, La, Ce, or Cu as well as mixtures thereof, Cu, Fe, Ce, Ag-loaded clinoptilolite and mixtures thereof.

A preferred catalyst or a preferred absorbent in the context of the present invention is characterized in that it is based on clay minerals and synthetic or naturally occurring zeolites. For the purposes of the present invention, based on clay mineral means in particular that the catalyst is> 30% (% by weight), preferably> 60% (% by weight) and most preferably> 80% (% by weight) Clay minerals exist. The actual reducing agent used for this purpose is the hydrocarbons available in the motor vehicle (directly or first “reformed”) and / or CO and / or H ₂ .

A preferred embodiment of a catalyst according to the present invention is characterized in that it additionally contains zeolites (in the same phase, as mixed crystals or also as mechanical mixing). The proportion of zeolites is preferably> 10% (% by weight), more preferably> 20% (% by weight) and most preferably> 30% (% by weight).

A preferred embodiment of a catalyst according to the present invention is characterized in that it contains oxidative and reductive regions, which depend on

Embodiment were realized both on one and the same or on different minerals (clay mineral, zeolite). A particularly efficient reduction of NO can always take place if part of the NO is first oxidized to NO ₂ and another part of NO is reduced to NH ₃ using the hydrocarbons. Then a recombination of several species adsorbed on the catalyst to N ₂ and water takes place.

In the presence of regions within the catalyst which have an oxidizing effect and those which together with the hydrocarbons have a reducing effect, the efficiency of the NO _x reduction can thus be proven to be considerably increased.

Clay minerals include in particular phyllosilicates, but also band silicates [eg palygorskite (attapulgite) u. Sepiolite (meerschaum) understood. A preferred embodiment of a Catalyst according to the present invention is characterized in that the clay mineral is selected from the group containing kaolinite, ilerite, kanemite, magadiite, smectite, montmorillonite, bentonite, hectorite, palygorskite and sepiolite and mixtures thereof. Bentonite, sepiolite, hectorite and montmorillonite are particularly preferred.

A preferred embodiment of a catalyst according to the present invention is characterized in that the clay mineral of the catalyst in particular has a basic action

Cations, preferably selected from the group containing Ba, Na, Sr, Ca and Mg and mixtures thereof. Ba ²⁺ ions in particular are known to bind hydrocarbons together with suitable clay minerals and to convert them into more reactive species, such as aldehydes, which then enable NO _x reduction.

A preferred embodiment of a catalyst according to the present invention is characterized in that the catalyst has oxidative metal ions, preferably selected from the group comprising Ag, Ce, Fe, Cu, La, Pr, Th, Nd, In, Cr, Mn, Co and Ni and mixtures thereof. An oxidation of NO to NO ₂ can thus be brought about by the mechanism described above.

A preferred embodiment of a catalyst according to the present invention is characterized in that the catalyst is based on modified bentonite. Other particularly preferred catalysts are characterized in that they contain modified clay minerals selected from the group comprising bentonites, smectites, hectorites and mixtures thereof pilled with aluminum, silicon or titanium (oxides). Another embodiment of the catalyst which is particularly preferred in the context of this invention contains at least one oxidative region which contains, for example, zeolites and a reductive region which can be formed by clay minerals. Because of the known shape selectivity of the zeolites, they are particularly suitable for oxidizing only the NO, while the hydrocarbons, due to their size, can reach the reactive centers of the zeolite much more slowly and thus are practically not oxidized. Clay minerals, on the other hand, are particularly suitable for absorbing suitable hydrocarbons due to their essentially two-dimensional pore systems.

A preferred embodiment of a catalyst according to the present invention is characterized in that the catalyst is a zeolite selected from the group comprising naturally occurring, ion-exchanged and / or synthesized zeolite A, zeolite X, zeolite Y, heulandite, clinoptilolite, chabasite, erionite, Contains mordenite, ferrierite, MFI (ZSM-5), zeolite beta faujasite, mordenite or mixtures thereof.

Zeolites which can be used in the context of the present invention can also be selected from the group comprising zeolite A, zeolite X, Y and / or Heulandite. The use of clinoptilolite, chabasite, erionite, mordenite, ferrierite, MFI (ZSM-5) and zeolite beta is preferred. The latter zeolite structures are characterized by a lower Al content, which on the one hand reduces the ion exchange capacity but on the other hand has the advantage of high temperature resistance (up to 550 ° C continuous operation). Faujasite, Heulandite and are particularly suitable zeolites

To call mordenite. Together with the zeolites X and Y, the mineral faujasite belongs to the faujasite types within the zeolite structure group 4, which are characterized by the double six-ring subunit DβR (compare Donald W.

Breck: "Zeolite Molecular Sieves", John Wiley & Sons, New York,

London, Sydney, Toronto, 1974, page 92). In addition to the faujasite types mentioned, the zeolite structure group 4 also includes the naturally occurring minerals chabazite and gmelinite and other synthetically obtainable zeolites.

Heulandite have in particular the general formula (Na, K) Ca [Al ₉ Si ₂₇ 0 ₇₂ ] -24H ₂ 0 or Ca ₄ [Al ₈ Si ₂₈ 0 ₇₂ ] ^• 24H ₂ 0). Together with the SiO ₂ -rich clinoptilolite, they are crystal. monoclinic in the Krist. -Class 2 / m-C2h and form flaky to tabular crystals, often individually or grown in subparallel aggregates, also peeled, flaky or late aggregates with perfect cleavage with pearlescent-like sheen on the gap surfaces (see also Gottardi-Galli, Natural Zeolites, pp. 256-284).

Mordents have the general structure Na ₃ KCa ₂ [Al ₈ Si ₄ oθ9 ₆ ] -28H ₂ 0. The units of the crystal structure are five-membered rings of tetrahedra that form chains one above the other. Quad rings are formed by common corners of two tetrahedra of five rings; Quadruple and Five rings together enclose twelve rings, see p. Illustration. Mordenite forms tiny prismatic, acicular or fine-fiber white to colorless crystals, often as cotton-like aggregates, and the like. massive porcelain-like masses (see also Gottardi-Galli, Natural Zeolites, pp. 223-233, Berlin-Heidelberg: Springer 1985). Faujasite-type zeolites are composed of ß-cages which are tetrahedral linked by D6R subunits, the ß-cages being arranged in the diamond similar to the carbon atoms. The three-dimensional network of the zeolites of the faujasite type suitable according to the invention has pores of 2.2 and 7.4

Ä on, the unit cell also contains 8 cavities (supercages) with a diameter of approx. 13 Ä and can be separated by the

Describe formula Na ₈₆ [(A10 ₂ ) ₈₆ (Si0 ₂ ) ι ₀₆ ] 'n H ₂ 0 (n is preferred

264). (All data from: Donald W. Breck: "Zeolite Molecular Sieves", John Wiley & Sons, New York, London, Sydney, Toronto,

1974, pages 145, 176, 177).

Mixtures, mixed crystals and / or co-crystals of zeolites of the faujasite type in addition to other zeolite structures, which do not necessarily have to belong to the zeolite structure group 4 (according to Breck's classification), are also according to the invention (also in the form of mechanical mixtures) suitable, preferably containing at least 70% by weight of the zeolites of the faujasite type, mordenites and / or heulandites.

The zeolites used in the context of this invention preferably have pore sizes of 2.8-8.0 Å. In general, the pore radius mentioned is partly considerable with the Al content of the zeolites and the type u. The amount of co-cations for the charge balance (alkali, alkaline earth metals, subgroup elements) varies.

A preferred embodiment of a catalyst according to the present invention is characterized in that the percentage by weight of copper and / or iron in the catalyst, measured on the weight of the entire catalyst, is between> 0% by weight and <25% by weight, preferably between> 0.01 wt% and <20 wt%, more preferably between> 0.05 wt% and <15 wt%, and most is preferably between> 0.1% by weight and <10% by weight. Iron and

Due to their catalytic activity, copper has another efficiency-increasing effect. Other suitable metals include Silver,

Cerium, manganese, indium and / or platinum, the latter being less preferred.

With the exception of copper, iron and possibly titanium, the catalyst is preferably free of heavy metals, wherein free of heavy metals in the context of the present invention means that the catalyst is less than l 1% by weight, preferably less than 0 0.8% by weight, still preferably less as ≤0.6% by weight, more preferably less than ≤0.4% by weight, and most preferably less than <0.1% by weight of heavy metals. Heavy metals in the sense of the present invention are understood to mean in particular the platinum group elements.

A preferred embodiment of a catalyst according to the present invention is characterized in that the catalyst additionally carries metal oxides, the metal of the metal oxide being, apart from optionally copper, iron, indium, molybdenum or titanium, not a heavy metal.

The catalyst particularly preferably also contains aluminum oxide. This has a large surface-increasing effect due to the pillar process, in which the interlayer spacing of the minerals can be widened permanently by the nano-oxides formed, which in turn enables the generation of a permanent pore system within the catalyst. For this, reference is made to ND Hudson et al, Microporous and Mesoporous Materials, 1999, pp. 447-459. Another preferred oxide is titanium oxide or silicon dioxide, which can also be used to increase the surface area and to establish the pillared clays. A preferred embodiment of a catalyst according to the present invention is characterized in that the

Proportion of metal oxide in mmol per g catalyst <100 mmol

Metal / g, still preferably <50 mmol metal / g, still <20 mmol metal / g, ≤IO mmol metal / g and most preferably from <6 mmol metal / g to> 0 mmol metal / g, preferably> 1 mmol metal

/ g is.

In a preferred embodiment of the catalyst, copper can be used as the catalytically active component. The copper presumably plays the crucial role of an active center in the complex catalytic process of NO _x reduction. This role can obviously also take on iron, manganese, indium, molybdenum and to a certain extent also titanium, which are therefore also preferred in the context of the present invention. These co-cations as promoters are believed to further improve the effectiveness of the copper.

As mentioned above, copper-laden zeolites (such as Cu / ZSM-5) are in principle already known as active catalysts in DeNOx processes, but so far it has not been possible to produce sufficiently stable forms for real exhaust gas conditions (up to 800 ° C, up to 20 vol -% water, sulfur compounds). The clay minerals may have the crucial function of stabilizing co-cations. Modified clay minerals (ion-exchanged pillared clays, so-called PILC) or naturally occurring zeolites, such as clinotilolite and / or mordenite, are particularly suitable for this.

The percentage by weight of (elemental) copper in the catalyst, measured on the weight of the entire catalyst, is preferably between> 0.01% and <25%, preferably between> 0.1% and <20%, more preferably between ≥ 1% and <15%, and most preferably between> 2% and <10%. This information also applies to the active metal or iron acting as a co-cation, although mixtures of both metals also tested positive. Activity improvements could be achieved by adding Ti and / or Ag, Ce additives and / or La additives and / or Ca, Co, Ni, In, Cr and Mn as trace amounts, which are therefore also preferred additives. In the case of clay minerals, samples which have been pillared and ion-exchanged with Al, Si and / or with Ti and / or Cu, Fe are particularly effective and are preferred to this extent.

A preferred embodiment of a catalyst according to the present invention is characterized in that the microporous mean pore size is between> 0 nm and <2 nm, preferably between> 0.1 nm and <1.0 nm, more preferably between> 0.2 nm and <0.8 nm, and most preferably between> 0.21 nm and <0.6 nm.

A preferred embodiment of a catalyst according to the present invention is characterized in that the mesoporous mean pore size is between> 0 nm and <10 nm, preferably between ≥ 1 nm and <9 nm, more preferably between> 2 nm and ≤ 8 nm, and am most preferably between> 2.5 nm and <7 nm.

A preferred embodiment of a catalyst according to the present invention is characterized in that the surface (measured by the BET method or in the multi-point method) of the clay mineral and / or zeolite, which forms the base of the catalyst, in the catalyst product between> 0 m ² / g and <1000 m ² / g, preferably between> 20 m ² / g and ≤ 800 m ² / g, more preferably between> 50 m ² / g and <600 m ² / g, and most preferably between> 90 m ² / g and ≤450 m ² / g.

A preferred embodiment of a catalyst according to the present invention is characterized in that the micropore volume of the clay mineral and / or zeolite, which forms the base of the catalyst, in the catalyst product is preferably between> 0 cm ³ / g and <0.4 cm ³ / g between> 0.02 cm ³ / g and ≤ 0.25 cm ³ / g, more preferably between> 0.04 cm ³ / g and ≤0.2 cm ³ / g, and most preferably between> 0.05 cm ³ / g and ≤0.18 cm ³ / g.

A preferred embodiment of a catalyst according to the present invention is characterized in that the mesopore volume of the clay mineral and / or zeolite, which forms the base of the catalyst, in the catalyst product is preferably between> 0 cm ³ / g and ≤1.0 cm ³ / g between> 0.01 cm ³ / g and ≤ 0.80 cm ³ / g, more preferably between> 0.015 cm ³ / g and ≤0, 60 cm ³ / g, and most preferably between> 0.020 cm ³ / g and ≤0.51 cm ³ / g.

A preferred embodiment of a catalyst according to the present invention is characterized in that the interlayer distance between two layers of the clay mineral and / or zeolite-like mineral which forms the base of the catalyst in the catalyst product is between> 0 nm and ≤5 nm, preferably between> 0.5 nm and ≤3 nm, more preferably between> 1.0 nm and <2.5 nm, and most preferably between ≥1.4 nm and ≤2.1 nm.

A preferred embodiment of a catalyst according to the present invention is characterized in that the Catalyst a thermal load in the permanent state of

> 300 ° C, preferably from> 400 ° C, more preferably from> 500 ° C, still preferably from> 600 ° C, and most preferably from> 650 ° C and ≤700 ° C.

The binder required for the formation of monolith can likewise be produced on the basis of the material already described, in which the doping with the active element is dispensed with here. Thus, full extrudates come from a clay mineral / zeolite composite as a catalyst and / or

Adsorbent in question. If you want to use metal foils as a substrate, the active material can also be applied using a wash-coat technology (coating). The range of modification options and / or manufacturing processes is therefore not exhausted; also plasma-based

Processes for coating or CVD (chemical vapor deposition), impregnation, impregnation (wet precipitation) and other methods of catalyst preparation that are frequently used can be used successfully.

On the one hand, it is possible to use directly occurring minerals (zeolites and clay minerals) according to the invention, and on the other hand synthetically producible aluminosilicates with the structure mentioned can be used. They can usually be produced very inexpensively, due to low synthesis temperatures (<100 ° C, no autoclave technology), short synthesis times and the saving or omitting of expensive, organic template molecules (mostly alkylammonium salts, such as TPABr / TPAOH) for the production. The advantages of natural minerals come into their own, particularly with clay minerals, since a synthesis and / or purification process in preparation for delamination, pillarn / ion exchange is very time-consuming and costly. The method according to the invention mainly offers the following advantages: Significant reduction in NO _x cold start emissions, since NO _{x is} removed from the exhaust gas mainly by enrichment instead of reduction. An effective reduction only occurs when the temperature rises. The periodic regeneration of a NO _x trap (NO _x trap) otherwise known from the prior art is not required. In particular, the extra fatlifting times to be observed do not apply. The method can save more than 5% -7% fuel on average, since the engine is operated with a λ of 1.1 (ie with excess air), especially when starting the engine and in a very wide map area can. Already in the warm-up phase, the NO _x emissions compared to engines according to the state of the art sometimes drop considerably and experience an average reduction (reduction) of at least 52%. The sulfur content of the fuel and / or the engine oils are only of minor importance for the reduction performance and / or the absorption processes, so that in some cases considerable local differences in the fuel qualities cannot permanently damage the catalytic converter unit.

The size, shape, material selection and technical conception of the above-mentioned components as well as the components to be used according to the invention described in the exemplary embodiments are not subject to any special exceptional conditions, so that the selection criteria known in the field of application can be used without restriction. Further details, features and advantages of the subject of

Invention result from the subclaims and from the following description of the associated drawings, in which - by way of example - several exemplary embodiments for carrying out the method according to the invention are shown. In the drawings:

Figure 1 is a - very schematic - cross section through a reactor with a catalyst with a serial arrangement of NO _x absorbing and NO _x reducing material.

Figure 2 is a - very schematic - cross section through a reactor with a catalyst with an alternating arrangement of NO _x absorbing and NO _x reducing material. and FIG. 3 shows a - very schematic - cross section through a reactor with a catalyst with a (homogeneous) distribution of NO _x -absorbing and NO _x -reducing material within the coating or directly in the catalyst (in the case of full extrudates).

Fig. 1 shows a - very schematic - cross section through a reactor 10 with a catalyst with a serial arrangement of NO _x absorbing and NO _x reducing material 20 and 30. The exhaust gas enters the reactor 10 through the inlet 12 approximately in Direction of the arrow and first meets a material 20 which contains a NO _x absorbing material. Thus, NO _x is initially taken up from the gas phase; this applies in particular after a cold engine start, since the exhaust gas temperature for the reduction of nitrogen oxides according to conventional are too low and the NO _x concentration is particularly high. The amount of NO _x absorbent introduced

Material is the expected raw emissions of the engine as well as the

Exhaust gas temperature level adjusted. The NO _x absorbing material can be introduced into the reactor 10 in all ways known from the prior art, in particular as pellets

Wash-coat or on a carrier material (preferred here

Metals and / or ceramics).

In the flow direction of the exhaust gas, a material 30 is then again connected, which contains NO _x -reducing material or consists entirely of it. Here, too, all shaping processes known from the prior art can be taken into account, in particular pellets, washcoat or carrier materials (metals and / or ceramics are preferred here), but also monolithic bodies (for example full extrudates).

Increases during driving the engine temperature and hence delayed the exhaust gas temperature is calculated from the material 20 _x contains a NO absorbing material slowly NOx desorbed and now travels in a higher concentration to the material 30 which _x contains NO -reducing material. Here, a reduction of the NO _x is then catalyzed by reducing agents such as hydrocarbons, ammonia or CO / H ₂ present in the exhaust gas. Because of the high selectivity of the NO _x -reducing material and / or the NO _x -absorbing material, the nitrogen oxides are generally and preferably converted with sufficient conversion rates without additional injection of fuel or engine-controlled post-injection. However, for special driving conditions, additional devices such as evaporators for metering a suitable reducing agent under controlled characteristic and / or control devices for additional motor measures can also be provided. FIG. 2 shows a - very schematic - cross section through a reactor 10 with a catalyst with an alternating arrangement of NO _x -absorbing and NO _x -reducing material 20 and 30, respectively. In this reactor 10 there are several absorption processes as described above - and reduction steps instead. The layers and the amounts of NO _x -absorbing or reducing material are preferably adapted to the engine and exhaust gas profile and therefore do not need to be identical to one another. This applies in particular if the catalytic activity of the NO _x -reducing material 30 decreases due to the decreasing exhaust gas temperature along the reactor. In this case, individual sections or layers 30 can then be made wider and / or larger, or the concentration of NO _x -reducing material 30 can be increased. On the other hand, the layers of NO _x -absorbing material 20 can also be designed in such a way that, for example, a larger amount of NO _x -reducing material 20 is initially present in order to initially achieve the most complete possible absorption of the NOx. This also ensures that NOx breakthroughs (for reasons of too high space velocity, ie short dwell time) on the reduction catalytic converter cannot escape into the atmosphere untreated, but can be detoxified in subsequent reaction compartments.

Fig. 3 shows a - very schematic - cross section through a reactor 10 with a catalyst 20 'with a homogeneous distribution of NO _x absorbing and NO _x reducing material in the catalyst. The catalyst 20 'is preferably over one

Pelleting process produced. This catalyst 20 ′ ensures that there is a consistently high NO _x concentration in the desorption phase in the entire reactor bed established. This in turn has a favorable effect on the conversion rate and thus the efficiency of the entire process, since otherwise concentration gradients can be expected along the flow direction from the inlet 12 to the outlet 14, which in turn can have a negative effect on the conversion rate (known dependence of the reaction rate on the reactant concentrations, kinetics ).

A method as described above and / or a suitable catalyst according to the present invention can be used in all motor vehicles and motor vehicle types; it does not matter whether it is e.g. is about cars or trucks or whether petrol, diesel or CNG engines are used. Engines equipped with the latest combustion processes such as HCCI (homogeneous charge compression ignition) or CAI (controlled auto ignition) can also benefit from this process.

Claims

Patent claims

1. A method for NO _x reduction in the exhaust gas stream of a motor vehicle by means of a catalyst, characterized in that an N0 _X - absorbing material is present in the catalyst.

2. The method according to claim 1, characterized in that an NO _x -reducing material is additionally present in the catalyst.

3. The method according to claim 1 or 2, characterized in that the NO _x absorbing and / or NO _x reducing material already at temperatures of ≤500 ° C, preferably <400 ° C, more preferably ≤300 ° C, is still preferably <200 ° C, and most preferably ≤150 ° C and> 20 ° C NO _x - absorbent.

4. The method according to claim 2 or 3, characterized in that the NO _x absorbing material is arranged in front of the NO _x reducing material in the exhaust gas stream.

5. The method according to any one of claims 2 to 4, characterized in that in the exhaust gas stream at least one section containing the NO _x -absorbing material and at least one section containing the NO _x -reducing material are alternately arranged one after the other.

6. The method according to claim 2 or 3, characterized in that the NO _x - absorbing material and the NO _x -reducing material are distributed approximately homogeneously in the catalyst zone.

7. The method according to any one of claims 1 to 6, characterized in that the NO _x - absorbent material is selected from a group comprising natural, synthetic, ion-exchanging, non-ion-exchanging, modified, non-modified, "pillared", non- "pillared ^λ clay minerals, sepiolites, attapulgites, natural, synthetic, ion-exchanging, non-ion-exchanging, modified, unmodified, zeolites, Cu, Ba, K, Sr and Ag-laden, AI, Si and Ti - “pilled ^λ montmorillonites, Hectorite doped with Fe, In, Mn, La, Ce, or Cu as well as mixtures thereof, Cu, Fe, Ag, Ce-loaded clinoptilolites and mixtures thereof.

8. The method according to any one of claims 2 to 7, characterized in that the NO _x -reducing material is selected from a group comprising natural, synthetic, ^" ion-exchanging, non-ion-exchanging, modified, non-modified," pillared ", not - "pillared ^λ , clay minerals, sepiolites, attapulgites, natural, synthetic, ion-exchanging, non-ion-exchanging, modified, unmodified, zeolites, Cu, Ba, K, Sr or Ag-laden as well as Al, Si or Ti "Pilled" montmorillonite, hectorite doped with Fe, In, Mn, La, Ce, or Cu and mixtures thereof, Cu, Fe, Ce, Ag-loaded clinoptilolites and mixtures thereof.

9. Catalyst suitable for carrying out the process according to one of claims 1 to 8.

10. Motor vehicle comprising a catalyst and / or a method according to any one of the preceding claims.