EP2640513A1

EP2640513A1 - Catalyst for removing nitrogen oxides from the exhaust gas of diesel engines

Info

Publication number: EP2640513A1
Application number: EP11785641.9A
Authority: EP
Inventors: Paul Spurk; Nicola Soeger; Elena Mueller; Stephan Malmberg
Original assignee: Umicore AG and Co KG
Current assignee: Umicore AG and Co KG
Priority date: 2010-11-16
Filing date: 2011-11-14
Publication date: 2013-09-25
Also published as: CN103180046A; WO2012065933A1; CN103180046B; US20130189172A1; US9095816B2

Abstract

The invention relates to a catalyst for removing nitrogen oxides from the exhaust gas of diesel engines, and to a method for reducing nitrogen oxides in the exhaust gas of diesel engines. The catalyst is made of a support body having a length L and a catalytically active coating that can consist in turn of one or more material zones. The material zones contain SCR catalytically active mixed oxides, comprising ceroxide, zirconium oxide, rare earth sesquioxide, and niobium oxide, and optionally tungsten oxide. The material zones further contain at least one compound selected from the group consisting of barium oxide, barium hydroxide, barium carbonate, strontium oxide, strontium hydroxide, strontium carbonate, praseodymium oxide, lanthanum oxide, magnesium oxide, magnesium/aluminum mixed oxide, alkali metal oxide, alkali metal hydroxide, alkali metal carbonate, and mixtures thereof. Noble metal can also optionally be present in the catalyst.

Description

Catalyst for the removal of nitrogen oxides from the exhaust gas of diesel engines

description

The invention relates to a catalyst for the removal of nitrogen oxides from the exhaust gas of diesel engines, and a method for reducing nitrogen oxides in the exhaust gas of diesel engines.

The exhaust gas from diesel engines contains in addition to the resulting from incomplete combustion of the fuel harmful gases carbon monoxide (CO) and hydrocarbons (HC) soot particles (PM) and nitrogen oxides (NO _x ). In addition, the exhaust gas from diesel engines contains up to 15% by volume of oxygen. It is known that the oxidizable noxious gases CO and HC can be converted by passing over a suitable oxidation catalyst into harmless carbon dioxide (C0 ₂ ), and particles can be removed by passing the exhaust gas through a suitable soot particle filter. The reductive removal of nitrogen oxides ("denitrification") is much more difficult due to the high oxygen content of the diesel exhaust A known denitrification process is the so-called SCR process (SCR = Selective Catalytic Reduction), ie the selective catalytic reduction of nitrogen oxides with the reducing agent ammonia In this case, ammonia as such or in the form of a precursor compound that is decomposable under ambient conditions to ammonia can be added to the exhaust gas stream, where "ambient conditions" is understood to be the environment of the compound decomposable to ammonia in the exhaust gas stream upstream of the SCR catalyst becomes. To carry out the SCR process, a source for providing the reducing agent, an injection device for the demand-based metering of the reducing agent into the exhaust gas and an SCR catalyst arranged in the flow path of the exhaust gas are necessary. The entirety of the reducing agent source, the SCR catalyst and the injector arranged upstream of the SCR catalytic converter is also referred to as the SCR system. Furthermore, nitrogen oxide storage catalysts can be used for denitrification of diesel exhaust gases. Their operation is described in detail in SAE SAE 950809. The cleaning effect of the nitrogen oxide storage catalysts is based on the fact that in a lean operating phase of the engine, the nitrogen oxides from the feed chermaterial of the storage catalyst are stored mainly in the form of nitrates. In a subsequent rich operating phase of the engine, the previously formed nitrates are decomposed and the released nitrogen oxides are reacted with the reducing exhaust gas components on the storage catalyst to nitrogen, carbon dioxide and water.

Combination processes for denitrification of internal combustion engine exhaust gases, in which SCR catalysts and nitrogen oxide storage catalysts are used, have also been described. In these combination systems, the nitrogen oxide storage catalyst is upstream of the SCR catalyst generally upstream and serves to generate ammonia during a rich operating phase, which is then stored in the downstream SCR catalyst. Nitrogen oxides that break through during the subsequent, lean phase of operation by the nitrogen oxide storage catalyst due to insufficient dynamic storage capacity can be reduced using the stored ammonia on the downstream SCR catalyst to nitrogen. Corresponding systems have been described, for example, in DE 10104160, DE 10308287 and WO 2008/0077602.

Reverse system configurations with a nitrogen oxide storage catalyst arranged downstream of the SCR catalytic converter have also already been proposed, as for example in EP 0 879 633 and US Pat. No. 7,160,832. Furthermore, catalysts are known in the prior art which both have nitrogen oxide-storing action and are capable of catalyzing the selective catalytic reduction of nitrogen with ammonia. EP 1 203 61 1 discloses an exhaust gas purification device for the selective catalytic reduction of nitrogen oxides under lean exhaust conditions, comprising at least one catalyst with a catalytically active component for the selective catalytic reduction (SCR component) and additionally at least one nitrogen oxide storage component (ΝΟχ component) , The catalyst is operated by the urea-SCR process, ie ammonia is used as the reducing agent for nitrogen oxides, which is generated from the lean exhaust gas added urea. DE 198 06 062 also discloses a reduction catalytic converter for reducing pollutants in diesel engine exhaust gases, which in its active composition contains, in addition to an SCR catalyst material based on the catalytically active oxides TiO ₂ , WO ₃ , MoO ₃ and V ₂ O _5, a ΝΟχ storage material. The NO _x storage material contains as active component at least one high-surface-area inorganic oxide, which is preferably selected from the group Al ₂ O ₃ , Si0 ₂ , Zr0 ₂ , zeolites and phyllosilicates.

EP 0 666 099 describes a process for removing nitrogen oxides from oxidizing exhaust gases, which are passed through a special catalyst which stores the nitrogen oxides, wherein subsequently a reducing agent is added to the exhaust gas, whereby the nitrogen oxides adsorbed in the catalyst are reduced to nitrogen , The catalyst contains inorganic oxides and catalytically active components. The catalytically active components comprise on the one hand noble metals selected from platinum, palladium, rhodium and ruthenium, and on the other hand at least one alkali and / or alkaline earth metal. Furthermore, the catalyst may contain heavy metals selected from manganese, copper, cobalt, molybdenum, tungsten and vanadium or compounds thereof.

Mixed oxides which also contain niobium in addition to cerium, zirconium and rare earth metal are known from US Pat. No. 6,468,941 and are used there as oxygen-storing materials. Furthermore, WO2005 / 085137 also discloses cerium, zirconium and niobium-containing mixed oxides.

At present, the SCR process for denitrification of diesel exhaust gases for passenger car applications and commercial vehicle application is considered to be the most promising method of purifying nitrogen oxides. However, especially in the case of passenger cars, it can be observed that the temperatures of the exhaust gas to be cleaned, which occur in the New European Driving Cycle (NEDC), are shifting more and more into the colder range, since dosing of urea as the source of the reducing agent ammonia only takes place at temperatures 180 ° C is controlled, without accepting unwanted deposits of urea and secondary products in the exhaust systems, this development of exhaust temperatures to the fact that the SCR process in the so-called "inner city part" (ECE) of the NEDC no longer effective can be used. Nitrogen oxide breakthroughs during the ECE and thus exceeding the prescribed NO _x limits in the overall driving cycle NEDC are the result. The object of the present invention is to provide a catalyst and an exhaust gas purification process which, compared with the prior art systems, has an improved NO _x conversion performance throughout the NEDC-relevant Temperature range, but especially at lower temperatures, for example between 100 and 230 ° C, especially between 100 and 200 ° C, shows.

The object is achieved by a catalyst for removing nitrogen oxides from the exhaust gas of diesel engines comprising a support body of length L and a catalytically active coating comprising one or more material zones

a) a catalytically active mixed oxide consisting of cerium oxide, zirconium oxide,

Rare earth sesquioxide and niobium oxide and optionally tungsten oxide; and

b) at least one compound selected from the group consisting of

Barium oxide, barium hydroxide, barium carbonate, strontium oxide, strontium hydroxide, strontium carbonate, praseodymium oxide, lanthanum oxide, magnesium oxide, magnesium / aluminum mixed oxide, alkali metal oxide, alkali metal hydroxide, alkali metal carbonate and mixtures thereof, as well as by a process for removing nitrogen oxides from the exhaust gas of diesel engines cleaning exhaust gas has an air ratio λ, which is greater than 1 and is passed over a catalyst according to the invention.

The particular composition of the catalyst according to the invention has the effect that the nitrogen oxides present in the exhaust gas to be purified can be stored in the catalyst in the form of nitrates at temperatures which are less than or equal to 200.degree. As a result, nitrogen oxide breakthroughs in the catalyst in temperature ranges in which a urea dosage is not meaningfully possible, largely avoided. Exceed the exhaust gas temperatures 200 ° C, so that the need adapted metering of the reducing agent urea controlled, the stored at colder temperatures in the catalyst nitrogen oxides are released again and selectively reduced with ammonia to nitrogen. As a result of the synergistic interaction of the components contained in the catalyst, the NO conversion in the entire NEDC-relevant temperature range, but in particular at lower temperatures, e.g. between 100 and 230 ° C, especially between 100 and 200 ° C, compared to systems according to the prior art significantly increased.

Preferably, the catalytically active mixed oxide contained in the catalyst according to the invention consists of 15 to 50 wt .-% Ce0 ₂ , 3 to 25 wt .-% Nb ₂ 0 ₅ , 3 to 10 wt .-% rare earth sesquioxide SE ₂ 0 ₃ and zirconium oxide Zr0 ₂ , based on the total amount of this catalytically active mixed oxide. If the catalytically active mixed oxide contains tungsten oxide, it is preferably composed of 15 to 50 wt .-% Ce0 ₂ , 3 to 25 wt .-% Nb ₂ 0 ₅ , 3 to 10 wt .-% rare earth sesquioxide SE ₂ 0 ₃ , 3 bis 20 wt .-% W0 ₃ and zirconium oxide Zr0 ₂ , based on the total amount of this catalytically active mixed oxide. As preferred Seltenerdsesquioxide SE ₂ 0 _3, in particular lanthanum oxide La ₂ 0 _3, yttrium oxide Y ₂ 0 _3, and neodymium oxide Nd ₂ 0 ₂ are used.

Such a material is characterized by an excellent catalytic activity in the SCR reaction. In contrast to the usual SCR catalysts based on zeolite, this material has only a low ammonia storage capacity, which is very stable on the typical operating and aging conditions occurring in the exhaust system. This requires that the catalyst reacts very flexibly to the different reducing agent supply at a frequently required in automotive application highly dynamic dosing of urea and converts the offered reducing agent very quickly with the nitrogen oxides.

This improvement in the light-off and turnover behavior in the catalyst according to the invention is synergistically supported by the presence of a nitrogen oxide storage material selected from the group consisting of barium oxide, barium hydroxide, barium carbonate, strontium oxide, strontium hydroxide, strontium carbonate, praseodymium oxide, lanthanum oxide, magnesium oxide, magnesium / aluminum mixed oxide, Alkali metal oxide, alkali metal hydroxide, alkali metal carbonate and mixtures thereof.

The catalyst according to the invention preferably contains 0.1 to 25% by weight of a compound selected from the group consisting of barium oxide, barium hydroxide, barium carbonate, strontium oxide, strontium hydroxide, strontium carbonate, praseodymium oxide, lanthanum oxide, alkali metal oxide, alkali metal hydroxide, alkali metal carbonate and mixtures thereof on the total amount of the catalyst.

The catalyst according to the invention particularly preferably contains from 0.2 to 10% by weight of a compound selected from the group consisting of barium oxide, barium hydroxide, barium carbonate, strontium oxide, strontium hydroxide, strontium carbonate, praseodymium oxide, lanthanum oxide, alkali metal oxide, alkali metal hydroxide, alkali metal carbonate and mixtures thereof. based on the total amount of the catalyst. Most preferably, the catalyst of the invention contains 1 to 5 wt .-% barium oxide, based on the total amount of the catalyst.

In another embodiment of the present invention, the catalyst preferably contains 0.1 to 50% by weight of a magnesium oxide or a magnesium / aluminum mixed oxide in addition to the compound selected from the group consisting of barium oxide, barium hydroxide, barium carbonate, strontium oxide, strontium hydroxide, strontium carbonate , Praseodymium oxide, lanthanum oxide, alkali metal oxide, alkali metal hydroxide and alkali metal carbonate and based on the total amount of the catalyst.

Particularly preferably, the catalyst of this embodiment of the invention contains 10 to 40 wt .-%, most preferably 15% to 25 wt .-% magnesium oxide or magnesium / aluminum mixed oxide in addition to the compound selected from the group consisting of barium oxide, barium hydroxide, barium carbonate , Strontium oxide, strontium hydroxide, strontium carbonate, praseodymium oxide, lanthanum oxide, alkali metal oxide, alkali metal hydroxide and alkali metal carbonate and based on the total amount of the catalyst. In colder operating points, in which a urea dosing is not yet possible, without taking unwanted deposits of urea and secondary products in the exhaust system in purchasing, the nitrogen oxides are incorporated with the formation of nitrates in the nitrogen oxide storage material. If the exhaust gas temperature upstream of the catalyst according to the invention exceeds a predetermined value, the metered addition of urea can take place such that the nitrogen oxides stored in the nitrogen oxide storage material are reduced in a very short time with ammonia from urea to nitrogen. The availability of the low but fast-working ammonia storage in the mixed oxide according to the invention makes it possible to guide this process in a particularly advantageous manner.

The catalytically active coating contains in preferred embodiments of the catalyst according to the invention, in addition to the catalytically active mixed oxide, a further oxide or further oxides, in particular a further cerium oxide and / or cerium / zirconium mixed oxide. Cerium oxides or cerium / zirconium mixed oxides - especially if they are cerium-rich, ie cerium oxide contents greater than 40 wt .-%, particularly preferably greater 60 wt .-%, each based on the total weight of the cerium / zirconium mixed oxide, have - promote the nitrogen oxide storage capacity in the low temperature range up to 200 ° C. In order to ensure particularly intimate contact with other materials storing nitrogen oxides in the catalyst according to the invention, this additional cerium oxide and / or cerium / zirconium mixed oxide is used as carrier oxide for the compound selected from the group consisting of barium oxide, barium hydroxide, barium carbonate, strontium oxide, strontium hydroxide, Strontium carbonate, praseodymium oxide, lanthanum oxide, magnesium oxide, magnesium / aluminum mixed oxide, alkali metal oxide, alkali metal hydroxide, alkali metal carbonate and mixtures. As a result, the ability of the catalyst according to the invention to dynamically store nitrogen oxides in the form of nitrates in the low-temperature range up to 200 ° C. and quickly release them again at relatively high temperatures is significantly improved.

The oxides mentioned, in particular cerium oxides and cerium / zirconium mixed oxides, are preferably doped or stabilized with other metals. Examples of the oxides mentioned are in particular lanthanum-doped aluminum / cerium mixed oxides, cerium / zirconium / praseodymium mixed oxides, cerium / zirconium / lanthanum mixed oxides, cerium oxide and cerium / zirconium mixed oxide.

Both the speed of the SCR reaction and the effectiveness of the storage of nitrogen oxides in the form of nitrates depend on the NO / N0 ₂ ratio in the exhaust gas to be cleaned. For example, the SCR reaction is fastest when the NO / N0 ₂ ratio is 1. The storage of nitrogen oxides in the form of nitrates proceeds most rapidly in the case of some storage materials such as, for example, barium oxide, if as much of the NO present in the exhaust gas has been previously oxidized to N0 ₂ . In both reactions, the adjustment of the NO / N0 ₂ ratio in situ on the catalyst surface can take place in a step upstream of the actual target reaction. For this purpose, preferred embodiments of the catalyst according to the invention in the catalytically active coating contain one or more noble metals which are selected from the group consisting of platinum, palladium, rhodium, iridium, ruthenium, gold, silver and mixtures and / or alloys thereof. Particularly preferred are the platinum group metals platinum, palladium, rhodium, ruthenium and mixtures and / or alloys thereof. The type and amount of precious metals to be used in the catalytic coating should be selected such that the resulting catalyst does not have any significant ammonia oxidation capability in the application-relevant temperature range. The preferable Selection of precious metals and their concentration is also determined by the overall composition of the catalyst and opens to the expert from the customary optimization experiments.

The components contained in the catalyst can be present in a homogeneous coating on the support body. In applications in which the catalyst according to the invention is exposed to moderate exhaust gas temperatures until the end of its intended use, good de-icing success can be achieved with such embodiments.

However, embodiments in which the catalytically active coating consists of two material zones are preferred, wherein the first material zone comprises the catalytically active mixed oxide consisting of cerium oxide, zirconium oxide, rare earth sesquioxide and niobium oxide and optionally tungsten oxide, while the compound is selected from the group consisting of barium oxide , Barium hydroxide, barium carbonate, strontium oxide, strontium hydroxide, strontium carbonate, praseodymium oxide, lanthanum oxide, magnesium oxide, magnesium / aluminum mixed oxide, alkali metal oxide, alkali metal hydroxide, alkali metal carbonate and mixtures thereof in the second material zone.

Of particular importance is the spatial separation of different, contained in the catalyst components in two material zones of which the catalytically active coating composed when the selected embodiment contains precious metal. In this case, embodiments are particularly preferred in which the first material zone comprises the catalytically active mixed oxide consisting of cerium oxide, zirconium oxide, rare earth sesquioxide and niobium oxide and optionally tungsten oxide, while the noble metal selected from the group consisting of platinum, palladium, rhodium, iridium , Ruthenium, gold, silver and mixtures and / or alloys thereof, contained in the second material zone.

The spatial separation of the noble metal and the SCR reaction catalyzing mixed oxide of cerium oxide, zirconium oxide, rare earth sesquioxide and niobium oxide and optionally tungsten oxide ensures that the catalyst according to the invention has an excellent selectivity to nitrogen even at higher exhaust gas temperatures in the SCR reaction. As a result, under appropriate operating conditions, little NO _{x is formed} from the overoxidation of excess ammonia. Embodiments of the catalyst according to the invention in which two different material zones are present can in principle be configured as layered catalysts or as zone catalysts. To prepare such catalysts, two differently composed coating suspensions are used to provide a preferably used as a support body Durchflußwabenkörper of ceramic or metal with the corresponding catalytically active part coatings that form the material zones.

To produce a layered catalyst, first of all a catalytically active layer with a correspondingly composed coating suspension is applied over the entire length of the support body according to one of the conventional dipping, suction and / or pumping methods. After drying and optionally calcination of this first layer, the process is repeated with a second, differently composed coating suspension, so that a second catalytically active partial coating (material zone) is formed on the first catalytically active partial coating. In the finished layered catalyst, a material zone is thus applied directly to the support body and covers its entire length L. The other material zone is applied over it and completely covers that material zone on the exhaust side. FIG. 1 schematically shows the structure of such a layered catalyst, FIG. 1 a) showing an overview of the coated throughflow honeycomb body (1): FIG. 1 b shows schematically a single flow channel (2) as a section of the layered catalyst. Therein, the two superposed material zones (3a and 3b) are arranged on the flow channel limiting, gas-tight walls (4), from which the catalytically active coating of preferred embodiments of the catalyst according to the invention is composed. The arrows indicate the flow direction of the exhaust gas to be cleaned.

In a zone catalyst, the two material zones are arranged successively on the support body in the flow direction of the exhaust gas and form an inflow-side and an outflow-side zone. To produce the first zone, a coating suspension of suitable composition is introduced by means of one of the conventional dipping, suction and / or pumping methods, for example from the later upstream side of the catalyst into the throughflow honeycomb body of ceramic or metal which is preferably used as support body. However, the application ends after a defined distance in the support body, which is smaller than the length of the support body L. After drying and optionally calcination of the resulting part becomes Production of the second zone starting from the other side - for example, the later outflow side of the catalyst - introduced the second coating suspension. Their application also ends after a defined distance in the support body, which is smaller than the length L of the support body. Figure 2 shows schematically the structure of such a zone catalyst, wherein in Figure 2a) an overview view of the coated Durchflußwabenkörpers (1) is shown. FIGS. 2b to 2d schematically show a single flow channel (2) as a section of the zone catalyst and the partial coatings (material zones) arranged therein on the gas-tight walls (4) delimiting the flow channel. The length of the zones can be selected during the coating so that the material zones touch each other at a selected point on a partial length piece of the support body ("zones on impact", Figure 2b) 2d) Preferably, the zone lengths are selected such that a gap remains between the two material zones (Figure 2c) .The gap is preferably between 2 and 10 millimeters, more preferably between 3 and 6 millimeters long. This embodiment has particular advantages in noble metal-containing variants of the catalyst according to the invention, since herein the intimate contact between the precious metal contained in one material zones and the SCR catalytically active mixed oxide contained in the other material zone consisting of cerium oxide, zirconium oxide, rare earth sesquioxide and niobium oxide and optionally Tungsten oxide is completely suppressed d prevents the thermally diffusive transfer of the noble metal into the mixed oxide, resulting in a higher selectivity to nitrogen of the resulting catalyst as a result.

In the preferred embodiments of the layered catalyst of the present invention, the second material zone is the compound selected from the group consisting of barium oxide, barium hydroxide, barium carbonate, strontium oxide, strontium hydroxide, strontium carbonate, praseodymium oxide, lanthana, magnesia, magnesium / aluminum mixed oxide, alkali metal oxide, alkali metal hydroxide, alkali metal carbonate and mixtures thereof and / or precious metal, directly applied to the support body and covers it over its entire length L. The first material zone containing the catalytically active mixed oxide consisting of cerium oxide, zirconium oxide, rare earth sesquioxide and niobium oxide and optionally tungsten oxide is on the second material zone applied and covered over the entire length L completely. This arrangement of the material zones requires that nitrogen oxides from the second Material zone can be desorbed in the overlying SCR active material zone with ammonia to nitrogen.

In the preferred embodiments of the zone catalyst according to the invention, wherein both material zones are arranged successively on the support body in the flow direction of the exhaust gas, the second material zone covers 10 to 70% of the length L of the support body, calculated from the first end, while the first material zone 30 to 90% The length L of the support body covered, calculated from the second end.

The zone catalysts according to the invention are generally used so that the first end on the inflow side and the second end is downstream. This arrangement also has the advantage that nitrogen oxides which are desorbed from the second material zone storing the nitrogen oxides can be converted with ammonia to nitrogen at the downstream first material zone. In addition, a maximum possible temperature level is ensured in the catalyst by the upstream arrangement of the second material zone, which in total to optimal, i. compared to the inverse arrangement improved NOx storage rates of the second material zone leads.

However, it is also possible to arrange the zones in reverse so that the first material zone containing the SCR catalytically active mixed oxide of cerium oxide, zirconium oxide, rare earth sesquioxide and niobium oxide and optionally tungsten oxide, at the first end, that is arranged upstream, while the second material zone at the second end, that is arranged downstream. In these cases, the catalyst according to the invention is preferably followed by an additional SCR catalyst in the exhaust gas purification system. This results in a device which, in addition to a catalyst according to the invention comprises an SCR catalyst, which is arranged downstream of the catalyst according to the invention. Such a device is particularly preferably supplemented by an additional metering device for reducing agent, which is arranged between the catalyst according to the invention and the downstream SCR catalytic converter. In this embodiment, both nitrogen oxides, which are desorbed at higher exhaust gas temperatures from the second material zone of the upstream catalyst according to the invention, as well as nitrogen oxides which at higher temperatures optionally by overoxidation of ammonia over the arise second material zone, effectively be implemented over the downstream SCR catalyst to nitrogen.

The catalyst according to the invention is suitable for removing nitrogen oxides from the exhaust gas of diesel engines. Although the catalyst has the ability to store nitrogen oxides, it is not operated cyclically in alternately rich and lean exhaust gas. The exhaust gas to be cleaned has an air ratio λ, which is greater than 1, and is passed to the denitration over the catalyst of the invention. Preference is given to the exhaust gas to be purified before it enters the catalyst ammonia or a compound decomposable to ammonia as a reducing agent from a source independent of the engine supplied. Particularly preferred is the use of urea as decomposable to ammonia compound, which is supplied to the exhaust gas to be cleaned only when the temperatures are higher than or equal to 180 ° C. At temperatures which are less than or equal to 200 ° C, the catalyst according to the invention, via which the exhaust gas to be purified is passed, stores nitrogen oxides in the form of nitrates. At temperatures higher than 200 ° C, these nitrogen oxides are released again. Still on the catalyst according to the invention their selective catalytic reduction takes place with ammonia to nitrogen.

Depending on the application, it may be advantageous to additionally direct the exhaust gas to be purified via a catalyst which predominantly accelerates the selective catalytic reduction of nitrogen oxides with ammonia. The supplementary aftertreatment over an SCR catalyst is useful, for example, if there is a risk that nitrogen oxides break through the catalyst according to the invention in specific operating points or are released from the nitrogen oxide storage in the catalyst according to the invention, without sufficient catalyst being present on the catalyst according to the invention Reduction of nitrogen oxides to nitrogen can take place. Thus, a need for a corresponding system design could arise if the SCR catalytically active mixed oxide consisting of cerium oxide, zirconium oxide, rare earth sesquioxide and niobium oxide and optionally tungsten oxide is present in a zone which is dimensioned comparatively short. In such cases, the integration of the catalyst according to the invention in an exhaust system is required, which may - in addition to other emission control units such as diesel oxidation catalyst and / or diesel particulate filter - also contain other Entstickungskatalysatoren, preferably SCR catalysts. The invention will be explained in more detail below with reference to some figures and an embodiment. Show it:

Figure 1: Schematic representation of a layered catalyst according to the invention comprising a Durchflußwabenkörper (1) and the catalytically active coating (3), which is composed of two superimposed material zones (3a and 3b).

Material zone (3a) contains a catalytically active ischoxide consisting of cerium oxide, zirconium oxide, rare earth sesquioxide and niobium oxide and optionally tungsten oxide.

Material zone (3b) contains at least one compound selected from among barium oxide, barium hydroxide, barium carbonate, strontium oxide, strontium hydroxide, strontium carbonate, praseodymium oxide, lanthanum oxide, magnesium oxide, magnesium / aluminum mixed oxide, alkali metal oxide, alkali metal hydroxide, alkali metal carbonate and

Mixtures thereof.

Figure 1 b) shows a section of the coated Durchflußwabenkörper comprise a single flow channel (2), on the gas-tight walls (4), the coating is applied.

Figure 2: Schematic representation of a zone catalyst according to the invention comprising a Durchflußwabenkörper (1) and the catalytically active

Coating (3), which is composed of two superposed material zones (3a and 3b).

Material zone (3a) contains a catalytically active mixed oxide consisting of cerium oxide, zirconium oxide, rare earth sesquioxide and niobium oxide and optionally tungsten oxide.

Mixtures thereof.

FIGS. 2 b) to 2 d) show a single flow channel (2) as a section of the zone catalyst and the material zones arranged therein on the gas-tight walls (4) delimiting the flow channel. in various embodiments:

Figure 2b): "zones on impact";

Figure 2c): Zone coating with remaining free gap; Figure 2d): coating with overlapping zones.

In the following examples,% unless stated otherwise, in each case% by weight.

The flow-through honeycomb bodies used are, unless stated otherwise, those made of cordierite, which at a diameter of 38.1 mm have a length of 76.2 mm, a cell density of 62 cells / cm ² and a wall thickness of 0.165 millimeters.

example 1

96 wt .-% of a mixed oxide of the composition Zr ₀ , 64Ce ₀ , 2Yo _, o75Nb _0, o7502 (according to the teaching of US 6,468,941) was impregnated by the Incipient Wetness method with 4 wt .-% barium oxide, dried and in the usual manner calcined.

From the material thus obtained, a coating suspension was prepared and thus a Durchflußwabenkörper with an amount of 320 g / L (248.64 g / L of the mixed oxide and 71, 36 g / L barium oxide) occupied. Drying and calcining resulted in a catalyst according to the invention.

Example 2

A layered catalyst according to the invention according to FIG. 1 was produced as follows: a) To produce the second material zone (3b) to be applied directly to the throughflow honeycomb body, a coating suspension of the following composition was prepared:

20.5% of a lanthanum-doped aluminum / cerium mixed oxide

32.4% of a ceria-doped aluminum / magnesium mixed oxide

30.6% cerium oxide

16.4% of a 16.7 wt .-% BaO occupied commercial cerium / zirconium mixed oxide With this suspension a Durchflußwabenkörper was coated in the usual manner, dried and calcined. The applied amount was 220 g / L. (b) For the production of the first material zone 3a) was prepared a coating suspension of a mixed oxide of the composition Zr _0.6 4Ce _0, _2iYo, o75Nb _0, o7502 (according to the teaching of US 6,468,941) and thus the a) obtained according to, single coated Flow honeycomb body once again coated. The applied amount was 100 g / L. Drying and calcination in a known manner resulted in a layered catalyst according to the invention.

Example 3

A flow honeycomb body was in a known manner with 320g / L of a mixture of the composition described in Example 2a) (68.75%) with a mixed oxide composition Zr _0.6 4Ce _0, 2Yo _, o75Nb _0, o7502 (corresponding to the teaching of US 6,468,941 ) (31, 25%) coated. Drying and calcining in a known manner resulted in a catalyst according to the invention. Example 4

20.5% of a lanthanum-doped aluminum / cerium mixed oxide

32.4% of a ceria-doped aluminum / magnesium mixed oxide

30, 1% cerium oxide

16.4% of a 16.7 wt .-% BaO occupied commercial cerium / zirconium mixed oxide 0.5% platinum

With this suspension, a Durchflußwabenkörper was coated in the usual manner, dried and calcined. The applied amount was 220 g / L. b) For the preparation of the first material zone (3a) was from a mixed oxide of composition Zr _0.6 4Ce _0, 2iYo _, o75Nb _0, o7502 (according to the teaching of US

No. 6,468,941) produced a coating suspension and thus once more coated the simply coated flow-through honeycomb body obtained according to a). The applied amount was 100 g / L. Drying and calcination in a known manner resulted in a layered catalyst according to the invention. Example 5

A zone catalyst according to the invention according to FIG. (2b) was produced as follows: a) For the production of the front, upstream material zone (3b), a flow honeycomb body with a length of 76.2 mm over a length of 50.8 mm from a first end with the coated in Example 4a) coating. The applied amount was 320 g / L. It was then dried and calcined. b) For the production of the rear, outflow-side material zone (3a), the flow-through honeycomb body obtained according to a) was coated starting from the second end over a length of 25.4 mm with a coating suspension comprising a mixed oxide of composition Zr ₀ , 49Ce ₀ , 3iYo _, o4 ₃ Nb ₀ , i502. The applied amount was 200 g / L. It was then dried and calcined.

The resulting catalyst is hereinafter referred to as K5.

Example 6

A zone catalyst according to the invention, in the use of which the material zone (3a) is arranged on the inflow side and the material zone (3b) downstream, was produced as follows: a) For the production of the rear, downstream material zone, a flow honeycomb body with a length of 76.2 mm was placed on a Length of 50.8 mm coated from a first end with a coating suspension containing the following ingredients:

16.8% of a lanthanum-doped aluminum / cerium mixed oxide

29.1% of a ceria-doped aluminum / magnesium mixed oxide

23.2% cerium / zirconium / praseodymium mixed oxides

22.0% of a 9.2 wt .-% BaO occupied commercial cerium / zirconium / lanthanum mixed oxide

8.2% of a 13.6 wt .-% SrO occupied cerium / zirconium / praseodymium mixed oxide 0.2% platinum

0.5% palladium

The applied amount was 320 g / L. It was then dried and calcined. b) To produce the front, upstream material zone of the flow honeycomb body obtained according to a) was coated starting from the second end over a length of 25.4 mm with the coating suspension described in Example 5b). The applied amount was 200 g / L. It was then dried and calcined. c) During use according to the invention, a separate SCR catalytic converter is connected downstream of the flow honeycomb body thus obtained. This was obtained by coating a flow honeycomb body of length 76.22 mm with the coating suspension described in Example 5b). The applied amount was 200 g / L. It was then dried and calcined.

Example 7

A layered catalyst according to the invention according to FIG. 1 was produced as follows:

The single-coated flow honeycomb body produced in accordance with Example 2a) was used to prepare the first material zone (3a) with a mixed oxide having the composition Zr ₀ .5 ₉ Ce _0. ₂ iYo _, i Nb ₀ , i0 ₂ (corresponding to the teaching of US Pat. No. 6,468,941). coated coating suspension coated once more. The applied amount was 100 g / L. Drying and calcination in a known manner resulted in a layered catalyst according to the invention, which is referred to below as K7. Example 8

The single-coated flow honeycomb body produced according to Example 4a) was used to prepare the first material zone (3a) with a mixed oxide having the composition Zr _0, 5 ₉ Ce ₀ , ₂ iYo _, i Nb ₀ , i0 ₂ (corresponding to the teaching of US Pat. No. 6,468,941). coated coating suspension coated once more. The applied amount was 100 g / L. Drying and calcination in a known manner resulted in a layered catalyst according to the invention, which is referred to below as K8. Example 9

A layered catalyst according to the invention according to FIG. 1 was produced as follows: a) A coating suspension of the following was used to produce the second material zone (3b) to be applied directly to the throughflow honeycomb body

Composition made:

19, 1% of a lanthanum-doped aluminum / cerium mixed oxide

31.4% of a cerium oxide-doped aluminum / magnesium mixed oxide

23.6% cerium oxide

16.7% of a 15.7 wt .-% BaO occupied commercial cerium / zirconium / lanthanum mixed oxide

7.6% of a 5 wt .-% K ₂ 0 occupied cerium / zirconium / praseodymium mixed oxide

0.2% platinum

1, 4% gold

With this suspension, a Durchflußwabenkörper was coated in the usual manner, dried and calcined. The applied amount was 220 g / L. b) To prepare the first material zone (3a), a coating suspension was prepared from a mixed oxide of the composition Zr _0.5 Ce ₀ , ₂ Yo, o6Nbo, o6Wo, i70 ₂ , and thus the once coated flow-through honeycomb body obtained according to a) was coated once more , The applied amount was 100 g / L. Drying and calcination in a known manner resulted in a layered catalyst according to the invention. Example 10

77.7% by weight of a mixed oxide having the composition Zr ₀ , 59Ce ₀ , ₂ iYo _, i Nb ₀ , 10 ₂ (corresponding to the teaching of US Pat. No. 6,468,941) was impregnated with 22.3% by weight barium oxide by the Incipient Wetness method, dried and calcined in the usual way. From the material thus obtained, a coating suspension was prepared and thus a Durchflußwabenkörper with an amount of 320 g / L (248.64 g / L of the mixed oxide and 71, 36 g / L barium oxide) occupied. Drying and calcining resulted in a catalyst according to the invention, hereinafter referred to as K10. Example 1 1

A flow honeycomb body was coated in a known manner with 400 g / L of a mixture of 20 wt .-% cerium oxide, 10 wt .-% of a lanthanum-doped aluminum / cerium mixed oxide and 20 wt .-% of a lanthanum-doped cerium / zirconium / praseodymium mixed oxide and 50 wt .-% of a mixed oxide of the composition Zr _0, 5Ce _0, 2Yo, o6Nbo, o6Wo _, i502 (according to the teaching of US 6,468,941) (200 g / L) coated. Drying and calcination in a known manner resulted in a catalyst according to the invention, which is referred to below as K11. Example 12

20.5% of a lanthanum-doped aluminum / cerium mixed oxide

32.5% of a ceria-doped aluminum / magnesium mixed oxide

30, 1% cerium oxide

The applied amount was 320 g / L. It was then dried and calcined. b) To produce the front, upstream material zone of the flow honeycomb body obtained according to a) was coated starting from the second end over a length of 25.4 mm with the coating suspension described in Example 5b). The applied amount was 200 g / L. It was then dried and calcined.

The catalyst according to the invention thus obtained is referred to below as K12. Example 13

91% by weight of a mixed oxide of the composition Zr ₀ , 64Ce _0, 2Yo, o75Nb _0, o7502 (corresponding to the teaching of US Pat. No. 6,468,941) was used after the incipient wetness Process impregnated with 9 wt .-% barium oxide, dried and calcined in the usual way.

Example 14

An inventive catalyst in the form of a layered catalyst according to FIG. 1 was produced.

To prepare the second material zone (3b) to be applied directly to the flow through honeycomb body, a coating suspension having the following composition was prepared (the amounts refer to the volume L of the resulting catalyst): 45 g / L of a lanthanum-doped aluminum / cerium mixed oxide;

71 g / L of a ceria-doped aluminum / magnesium mixed oxide;

1.06 g / L palladium as palladium nitrate solution;

36 g / L with 16.67 wt .-% BaO occupied cerium / zirconium mixed oxide;

66 g / L of ceria;

All raw materials used are commercially available. Only in the with

Barium oxide coated cerium / zirconium mixed oxide is a self-produced powder component. For their preparation, a commercially available cerium / zirconium mixed oxide was slurried in an aqueous barium acetate solution. The suspension thus obtained was dried at 120 ° C over the period of 10 h and then calcined at 500 ° C for 2 hours. The powder thus obtained was ground and used to prepare the coating suspension.

With the coating suspension thus obtained, a Durchflußwabenkörper was coated with 62 cells per square centimeter, a cell wall thickness of 0, 165 millimeters and a length of 76.2 mm in a conventional and known in the art dip coating process. The component was dried and calcined at 500 ° C for 2 hours. For the production of the first material zone (3a) was a commercially available cerium-zirconium mixed oxide having a weight ratio of Ce0 ₂ : Zr0 ₂ of 1: 1, 1 and a Nd ₂ 0 ₃ content of 5.3 wt .-% with an aqueous solution of ammonium niobium oxalate and calcined at 500 ° C for a period of 2 hours. The composite oxide composition according to the invention thus obtained consisted of 38% by weight of CeO ₂ , 14.5% by weight of Nb ₂ O ₅ , 4.5% by weight of Nd ₂ O ₃ and 43% by weight of ZrO ₂ . From the mixed oxide, a coating suspension was prepared, with which the Durchflußwabenkörper already simply coated as described above was coated again. The resulting component was calcined after drying at 500 ° C for a period of 2 hours.

The catalyst according to the invention exhibits significantly improved nitrogen oxide conversions in the NEDC-relevant temperature range and in particular in the low-temperature range up to 230 ° C., above all up to 200 ° C., compared with prior art catalysts and thus has reduced NO in the NEDC compared to conventional emissions _x emissions.

Comparative Examples 1 to 3

A flow honeycomb body with a length of 76.2 mm was coated in a known manner with a coating suspension containing a mixed oxide composition Zr _0.59 Ce _0.2 iYo _, iNb _0, i0 ₂ (according to the teaching of US 6,468,941). The amounts applied were:

Comparative Example 1: 250 g / L

Comparative Example 2: 320 g / L

Comparative Example 3: 100 g / L

After the coating was dried and calcined in a known manner. The resulting catalysts are hereinafter referred to as VK1, VK2 and VK3, respectively.

Comparative Example 4

A flow honeycomb body with a length of 76.2 mm was coated in a known manner with a coating suspension containing a mixed oxide of composition Zr _0.49 Ce ₀ , 3iYo, o43Nb _0, i ₅ 0 ₂ . The applied amount was 200 g / L. After the coating was dried and calcined in a known manner. The resulting catalyst is hereinafter referred to as VK4. Comparative Example 5

A flow honeycomb body with a length of 76.2 mm was coated in a known manner with a coating suspension containing a mixed oxide composition Zr _0.5 Ce ₀ , 2Yo, o6Nbo, o6Wo, i702. The applied amount was 200 g / L. After the coating was dried and calcined in a known manner. The obtained catalyst is hereinafter referred to as VK5.

Comparative tests

The ΝΟχ-conversion of the inventive catalysts K7, K8 and K10 were compared with the catalysts VK1, VK2 and VK3 in a test gas plant. In an analogous manner, the NO _x conversion of the inventive catalysts K5 and K12 were compared with VK4 and the NO _x conversion of the inventive catalyst K1 1 with VK5. The catalysts according to the invention and comparison catalysts each compared contain in each case the identical mixed oxide consisting of cerium oxide, zirconium oxide, rare earth sesquioxide, niobium oxide and optionally tungsten oxide.

For the comparisons, the test method described below was used:

Before the start of the test phase, conditioning takes place at 550 ° C, so that the

Catalyst sample at the beginning of the test phase is free of adhering NO _x and NH ₃ .

After conditioning, it is cooled to the test temperature of 150 ° C. under protective gas (N ₂ ). The test temperature is held constant at 150 ° C for 5 minutes. During the test phase, the following gas composition is metered:

0 ₂ / Vol%: 8

NO / Vppm: 250

N0 ₂ / Vppm: 250

H ₂ 0 / Vol%: 10

Residual gas N ₂

The space velocity during the entire test was: GHSV = 30,000 h ^-1

From the known, metered nitrogen oxide contents, which were verified during conditioning at the beginning of the respective test run with a pre-catalyst exhaust gas analysis, and the measured nitrogen oxide contents after catalyst, the nitrogen oxide conversion over the catalyst was calculated as follows:

Output NO _x )

u _N0 \ ° (i) · 100

Input (NO _x ) with CEn / Output (NO _x ) - CEn / Off (NO) + CEin / Off (NÖ2) ■■■

In order to demonstrate the advantages of the catalysts according to the invention in comparison with the comparative catalysts in the range of lower temperatures, the mean nitrogen oxide conversion in the test phase was determined over the entire period of 5 minutes.

The following results were obtained:

It thus becomes clear that the catalysts according to the invention in comparison to the comparative catalysts in the test phase, which depicts the range of lower temperatures before the addition of NH ₃ , are distinguished by substantially better NOx conversions.

Claims

claims

Catalyst for removing nitrogen oxides from the exhaust gas of diesel engines comprising a support body of length L and a catalytically active coating of one or more material zones containing

Rare earth sesquioxide and niobium oxide and optionally tungsten oxide; and

b) at least one compound selected from the group consisting of barium oxide, barium hydroxide, barium carbonate, strontium oxide, strontium hydroxide, strontium carbonate, praseodymium oxide, lanthanum oxide, magnesium oxide, magnesium / aluminum mixed oxide, alkali metal oxide, alkali metal hydroxide, alkali metal carbonate and mixtures thereof.

Catalyst according to Claim 1, characterized in that the catalytically active mixed oxide, based on its total amount, is composed as follows: Ce0 ₂ : 15-50% by weight

Nb ₂ 0 ₅ : 3 - 25% by weight

SE ₂ 0 ₃ : 3 - 10% by weight

Zr0 ₂ : rest

Catalyst according to claim 1, characterized in that the catalytically active mixed oxide, based on its total amount, is composed as follows:

CeO ₂ : 15-50% by weight

Nb ₂ 0 ₅ : 3 - 25% by weight

SE ₂ 0 ₃ : 3 - 10% by weight

W0 ₃ : 3 - 20% by weight

Zr0 ₂ : rest

Catalyst according to one or more of claims 1 to 3, characterized in that the catalytically active coating contains a cerium oxide and / or cerium / zirconium mixed oxide, which is used as a carrier oxide for the compound selected from the group consisting of barium oxide, barium hydroxide, barium carbonate, strontium oxide , Strontium hydroxide, strontium carbonate,

Praseodymium oxide, lanthanum oxide, magnesium oxide, magnesium / aluminum Mixed oxide, alkali metal oxide, alkali metal hydroxide, alkali metal carbonate and mixtures thereof.

Catalyst according to one of the preceding claims, characterized in that the catalytically active coating contains one or more noble metals which are selected from the group consisting of platinum, palladium, rhodium, iridium, ruthenium, gold, silver and mixtures and / or alloys thereof.

Catalyst according to one of claims 1 to 4, characterized in that the catalytically active coating consists of two material zones, wherein the first material zone comprises the catalytically active mixed oxide consisting of cerium oxide, zirconium oxide, rare earth sesquioxide and niobium oxide and optionally tungsten oxide, while the compound selected from the A group consisting of barium oxide, barium hydroxide, barium carbonate, strontium oxide, strontium hydroxide, strontium carbonate, praseodymium oxide, lanthanum oxide, magnesium oxide, magnesium / aluminum mixed oxide, alkali metal oxide, alkali metal hydroxide, alkali metal carbonate and mixtures thereof is contained in the second material zone.

Catalyst according to claim 5, characterized in that the catalytically active coating consists of two material zones, wherein the first

Material zone the catalytically active mixed oxide consisting of cerium oxide,

Zirconia, rare earth sesquioxide and niobium oxide, and optionally tungsten oxide, while the noble metal selected from the group consisting of platinum, palladium, rhodium, iridium, ruthenium, gold, silver and mixtures and / or alloys thereof is contained in the second material zone.

Catalyst according to claim 6 or 7, characterized in that the second material zone is applied directly to the support body and covers it over its entire length L, while the first material zone is applied to the second material zone and this completely over the entire length L.

Catalyst according to claim 6 or 7, characterized in that both material zones in the flow direction of the exhaust gas successively on the

Support body are arranged, wherein the second material zone 10 to 70% of the length L of the support body, calculated from the first end of the support body, covered, while the first material zone 30 - 90% of the length L of

Supporting body, calculated from the second end of the support body, covered.

10. A catalyst according to claim 6 or 7, characterized in that both

Material zones in the flow direction of the exhaust gas successively on the

Supporting bodies are arranged, wherein the first material zone 10 to 70% of the length

L of the support body, calculated from the first end of the support body, covered, while the second material zone 30 - 90% of the length L of the support body, calculated from the second end of the support body, covered.

1 1. A method for the removal of nitrogen oxides from the exhaust gas of diesel engines, characterized in that the exhaust gas to be cleaned has an air ratio λ, which is greater than 1 and is passed through a catalyst according to one of claims 1 to 10.

12. The method according to claim 11, characterized in that the exhaust gas to be purified before entering the catalyst ammonia or a compound decomposable to ammonia as a reducing agent from a motor independent

Source is supplied.

13. The method according to claim 12, characterized in that the catalyst, via which the exhaust gas to be purified is passed, stores at temperatures which are less than or equal to 200 ° C, nitrogen oxides in the form of nitrates, these at temperatures higher than 200 ° C are released again and catalyzed at the same time their selective catalytic reduction with ammonia.

14. The method according to any one of claims 11 to 13, characterized in that the exhaust gas to be purified is additionally passed over a catalyst which predominantly accelerates the selective catalytic reduction of nitrogen oxides with ammonia.

15. Apparatus for carrying out the method according to claim 14 containing

a catalyst according to any one of claims 1 to 8 or a catalyst according to claim 10, and a downstream arranged SCR catalyst.