EP3558494A1 - Scr catalyst device containing vanadium oxide and molecular sieve containing iron - Google Patents

Abstract

The invention relates to a catalyst device for purifying exhaust gases containing nitrogen oxide by means of selective catalytic reduction (SCR), comprising at least two catalytic regions, the first region containing vanadium oxide and cerium oxide, and the second region containing a molecular sieve containing iron. The invention also relates to uses, the catalyst device and methods for purifying exhaust gases.

Description

 SCR catalyst device containing vanadium oxide and iron-containing

 The invention relates to a catalytic device for purifying nitrogen oxide-containing exhaust gases by selective catalytic reduction (SCR), comprising at least two catalytic regions, wherein the first region contains vanadium oxide and cerium oxide, and wherein the second region contains iron-containing molecular sieve. The invention also relates to uses of the catalyst device and to methods of purifying exhaust gases.

State of the art

The selective catalytic reduction (SCR) process is used in the prior art to reduce nitrogen oxides in exhaust gases, for example from incinerators, gas turbines, industrial plants or internal combustion engines. The chemical reaction is carried out with selective catalysts, hereinafter referred to as "SCR catalysts", which selectively remove nitrogen oxides, especially NO and NO 2. However, undesirable side reactions are suppressed, in the reaction a nitrogen-containing reducing agent is fed, usually ammonia ( NH3) or a precursor compound, such as urea, which is mixed into the exhaust gas The reaction is a comproportionation Reaction products are essentially water and elemental nitrogen SCR catalysts often contain metal oxides, such as oxides of vanadium, titanium, tungsten, zirconium or Combinations thereof In addition, molecular sieves are often used as SCR catalysts, in particular zeolites exchanged with catalytically active metals.

An important application of SCR is the removal of nitrogen oxides from exhaust gases of combustion engines predominantly operated with lean air / fuel ratio. Such internal combustion engines are diesel engines and direct injection gasoline engines. They are collectively referred to as lean-burn engines. The exhaust gas from lean-burn engines contains, in addition to the noxious gases carbon monoxide CO, hydrocarbons HC and nitrogen oxides NOx, a relatively high oxygen content of up to 15% by volume. Carbon monoxide and hydrocarbons can be easily oxidized be made harmless. The reduction of nitrogen oxides to nitrogen is much more difficult because of the high oxygen content.

 Since internal combustion engines are operated transiently in the motor vehicle, the SCR catalytic converter must ensure the highest possible nitrogen oxide conversions with good selectivity even under strongly fluctuating operating conditions. In this case, a complete and selective conversion of nitrogen oxides at low temperatures must be ensured, as well as the selective and complete conversion of high amounts of nitrogen oxide in very hot exhaust gas, for example at full load. In addition, the highly fluctuating operating conditions cause difficulties in the exact dosage of ammonia, which should ideally be in stoichiometric ratio to the nitrogen oxides to be reduced. This results in high demands on the SCR catalyst, ie its ability to reduce nitrogen oxides in a wide temperature window with highly variable catalyst loads and fluctuating reducing agent supply with high conversion and selectivity to nitrogen.

EP 0 385 164 B1 describes so-called unsupported catalysts for the selective reduction of nitrogen oxides with ammonia, which in addition to titanium oxide and at least one oxide of tungsten, silicon, boron, aluminum, phosphorus, zirconium, barium, yttrium, lanthanum and cerium contain an additional component selected from the group of oxides of vanadium, niobium, molybdenum, iron and copper.

No. 4,961,917 relates to catalyst formulations for the reduction of nitrogen oxides with ammonia which, in addition to zeolites having a silica: alumina ratio of at least 10 and a pore structure linked in all spatial directions by pores having an average kinetic pore diameter of at least 7 angstroms, are iron and / or copper as promoters.

However, the catalyst devices described in the cited documents are in need of improvement, because only at relatively high temperatures above about 350 ° C or above about 450 ° C good nitrogen oxide conversion rates can be achieved. An optimal implementation is usually only in a relatively narrow temperature range. Such an optimum conversion is typical for SCR catalysts and conditioned by the mechanism of action. The reaction of nitrogen oxides on SCR catalysts with ammonia, which can be formed from a precursor compound such as urea, is carried out according to the following reaction equations:

4NO + 4NH ₃ + 0 ₂ -> 4N ₂ + 6H ₂ (1)

NO + NO ₂ + 2NH ₃ -> 2N ₂ + 3H ₂ 0 (2)

6N0 ₂ + 8NH ₃ ^ 7N ₂ + 12H ₂ 0 (3)

It is advantageous to increase the proportion of N0 ₂ and in particular to set a N0 ₂ : NO ratio of about 1: 1. Under these conditions, significantly higher conversion rates can be achieved even at low temperatures below 200 ° C, due to the significantly faster reaction (2) ("Fast SCR reaction") compared to reaction (1) ("Standard SCR reaction").

However, the nitrogen oxides NOx contained in the exhaust gas of lean-burn engines consist predominantly of NO and have only small proportions of N0 ₂ . The prior art therefore uses an upstream oxidation catalyst for the oxidation of NO to NO x.

Another problem with the removal of nitrogen oxides in the exhaust gas of lean-burn engines with SCR catalysts is that the ammonia due to the high oxygen content to low-valent nitrogen oxides, in particular nitrous oxide (N ₂ 0), is oxidized. As a result of this process, the reducing agent required for the SCR reaction is withdrawn from the process on the one hand, and on the other hand, the nitrous oxide escapes as undesirable secondary emission.

In order to be able to solve the described target conflicts and to be able to ensure the removal of the nitrogen oxides at all working temperatures which are often between 180.degree. C. and 600.degree. C., combinations of different SCR catalysts which combine advantageous properties are proposed in the prior art should.

US 2012/0275977 A1 relates to SCR catalysts in the form of molecular sieves. These are zeolites containing iron or copper. In order to remove nitrogen oxides as comprehensively as possible, different molecular sieves with different functionalities are preferably combined. US 2012/0058034 A1 proposes to combine zeolites with another SCR catalyst based on oxides of tungsten, vanadium, cerium, lanthanum and zirconium. The zeolites are mixed with the metal oxides and coated thereon with a suitable support, whereby a single catalyst layer having both functionalities is obtained.

Vanadium-based SCR catalysts have also been combined with iron-exchanged zeolites in the prior art. Thus, WO 2014/027207 A1 discloses SCR catalysts which contain as the first catalytic component an iron-exchanged molecular sieve and as second component a vanadium oxide which is applied to a metal oxide selected from aluminum, titanium, zirconium, cerium or silicon. The various catalysts are mixed and a single catalytic coating is formed on a suitable support. However, the efficiency of such a catalyst in the temperature range below 450 ° C and especially below 350 ° C is still in need of improvement.

WO 2009/103549 discloses combinations of zeolites and vanadium oxide in combination with other metal oxides. In order to improve the catalyst efficiency, it is proposed to divide the catalyst into zones. In this case, there is a zone with the zeolite at the exhaust gas inlet side, which serves as an N H3 storage component. Thereafter, at the outlet side, a zone with the SCR active component containing the vanadium-based SCR catalyst is included. The zeolite has only one storage function, while the following component catalyzes the SCR reaction with a vanadium oxide. WO 2008/006427 A1 relates to combinations of iron-exchanged zeolites with copper-exchanged zeolites. Specifically, it is proposed to first coat a ceramic support with the copper-exchanged zeolite, and to form a coating with the iron-exchanged zeolite. In this way, the different activities of the layers in different temperature ranges should be combined in an advantageous manner.

WO 2008/089957 A1 proposes to provide a ceramic support with a lower coating containing vanadium oxide and an upper coating containing iron-exchanged zeolites. Here, the upper coating with the Iron-exchanged zeolites prevent nitrous oxide from being formed at high operating temperatures.

EP 2 992 956 A1 describes an SCR catalyst having a "two-layer structure", wherein a layer containing V2O5 / T1O2 is disposed on a layer comprising a metal-exchanged zeolite.

SCR catalysts comprising vanadium-based formulations and Cu or Fe zeolites are also described in WO 2016/01 1366 A1, DE 10 2006 031 661 A1 and in Ind. Eng. Chem. 2008, 47, 8588-8593.

However, the described SCR catalysts are still in need of improvement in terms of their efficiency. There is a continuing need for catalysts that have high efficiency under various application conditions. In particular, there is a need for catalysts which are suitable both for the purification of NO-rich and NO2-rich exhaust gases, and which have a high efficiency over the entire temperature range of conventional applications, including at low and medium temperatures. Another problem of conventional catalysts is that nitrous oxide is formed in conventional combustion engine applications. This problem manifests itself, in particular, in the medium and low temperature ranges below about 450 ° C or below about 350 ° C. Exhaust gases from internal combustion engines often exhibit such temperatures in normal operation, which can lead to the undesirable formation and release of nitrous oxide.

In J.Phys. Chem. C2009, 1 13, 2, 1 177-21 184 it is reported that the reaction of nitric oxide (NO) with ammonia on V2O5-WO3 / T1O2 SCR catalysts can be improved by adding cerium oxide to said SCR catalysts. The measurements carried out, however, were made with SCR catalysts which contained only 0.1% by weight of vanadium and are therefore irrelevant in practice. The same applies to the data reported in Progress in Natural Science Materials International, 25 (2015), 342-352. Data concerning the conversion of nitrogen dioxide (NO2) are not included in the documents. Also, J.Fuel.Chem.Technol., 2008, 36 (5), 616-620, includes data on the influence of the content of ceria on V20s-CeO 2 TiO 2 SCR catalysts in the reaction of nitrogen monoxide (NO) with ammonia. Accordingly, positive effects of cerium oxide are observed only at levels of 20% by weight and higher. Data for the conversion of nitrogen dioxide (NO2) does not contain the document.

It is known that the NOx conversion with ammonia in the presence of V / T1O2-SCR catalysts very much depends on the NO2 / NOX molar ratio, see, for example, Environmental Engineering Science, Vol. 27, 10, 2010, 845-852, in particular Thus, results of the conversion of NO can not be used to predict results in the conversion of NO 2 or of NO x with a high NO 2 content. Our own investigations by the inventors of the present application show that the conversion of nitrogen oxides with ammonia is low Temperatures on vanadium and cerium-containing SCR catalysts depends in particular on the composition of the nitrogen oxide. In the case that the nitrogen oxide only consists of nitrogen monoxide (NO), the conversion decreases with increasing cerium oxide content. As the content of nitrogen dioxide (NO2) increases, however, the picture changes. Thus, for example, the conversion at a content of nitrogen dioxide of 75% increases with increasing proportion of cerium oxide, see FIGS. 1 and 2.

OBJECT OF THE INVENTION The object of the invention is to provide catalysts, processes and uses which overcome the disadvantages described above. In particular, SCR catalysts are to be provided which enable efficient removal of nitrogen oxides over a wide temperature range, and thereby also at low and medium temperatures. In this case, nitrogen oxides NO _x , in particular NO and NO 2, should be removed efficiently, while at the same time the formation of nitrous oxide N 2 O should be avoided. The catalysts should, especially in the temperature range of 180 ° C to 600 ° C, which is regularly in internal combustion engines of importance, have a high efficiency. In particular, the catalysts should be suitable for purifying exhaust gases with a relatively high proportion of NO 2, in particular if the ratio NO 2: NO is> 1: 1. The catalysts should also be efficient at low temperatures, where catalysts in the prior art are often less efficient, for example at <450 ° C or <350 ° C.

In particular, catalysts are to be provided which combine the following advantageous properties:

 a high efficiency with NO 2 rich exhaust gases in the temperature range of about 180 ° C to 600 ° C, and in particular at low temperatures and the avoidance of the formation of nitrous oxide.

Preferably, the catalysts should be effective both immediately after production and after prolonged service life and aging.

Disclosure of the invention

Surprisingly, the object underlying the invention is achieved by catalyst devices, uses and methods according to the claims.

The invention relates to a catalyst device for purifying nitrogen oxide-containing exhaust gases by selective catalytic reduction (SCR), comprising at least two catalytic regions, wherein the first region contains vanadium oxide and cerium oxide, and wherein the second region contains iron-containing molecular sieve.

The catalyst device serves to reduce nitrogen oxides ("NOx") in exhaust gases by the process of "selective catalytic reduction" (SCR). The exhaust gases may originate, for example, from internal combustion engines, incinerators, gas turbines or industrial plants. The SCR systematically reduces nitrogen oxides, especially NO and NO2. The reaction is carried out in the presence of a nitrogen-containing reducing agent, usually ammonia (NH3) or a precursor compound thereof, such as urea. The nitrogen-containing reducing agent is usually admixed with the exhaust gas. The catalyst contains at least two catalytic regions. In this case, "catalytically" means that each of the regions has catalytic activity in the SCR The first catalytic region is also referred to below as "vanadium-cerium catalyst".

In the context of this application, the term "metal oxide" generally refers to oxides of the metal, ie not only the metal monooxide with a stoichiometric ratio of 1: 1. The term "metal oxide" designates both concrete oxides and mixtures of different oxides of the metal ,

The first region contains vanadium oxide, which is preferably present as vanadium pentoxide V2O5. It is not excluded that a part of the vanadium has a different oxidation state and is present in another form. The vanadium oxide is preferably the essential catalytically active component of the first region, which is largely responsible for the reaction.

The first area contains additional ceria. In this case, the cerium oxide can be present in a defined oxidation state or as a mixture of different oxidation states. In one embodiment, the cerium oxide is wholly or partially present as CeO 2. Surprisingly, it has been found that by combining vanadium oxide with cerium oxide in a catalyst device which also has a catalytic region with iron-containing molecular sieve, the efficiency of the SCR can be significantly improved. By combining these catalysts and components, a very good depletion of NOx from exhaust gases with a high NO content or high NO2 content is achieved, and the formation of N2O is significantly suppressed. These advantageous effects are particularly evident with NO2-rich exhaust gases, and in a temperature range below 450 ° C, and especially in the range of 180 to 450 ° C, in which the efficiency of conventional catalysts is particularly in need of improvement and the undesirable formation of nitrous oxide represents a particularly relevant problem.

These advantages by the addition of cerium oxide can be particularly pronounced after aging of the catalyst. Thus, it has been found that after aging of the catalyst both the removal of nitrogen oxides and the avoidance of the formation of N2O is particularly effective when a vanadium catalyst additionally contains cerium oxide. This is an advantage since such catalysts in conventional applications, in particular for the purification of exhaust gases from internal combustion engines, must have a long service life and are exposed to strongly changing external conditions (temperature, humidity, exhaust gas composition) which favor such aging processes.

In a preferred embodiment, the first region contains at least one further component selected from oxides of tungsten, titanium, silicon and aluminum. It is preferred that the first region additionally contains at least one, preferably at least two, metal oxides which are selected from T1O2, S1O2, WO3 and Al2O3. Particularly preferably, the first region contains oxides of vanadium, cerium, tungsten and titanium, wherein the oxides of vanadium, tungsten and titanium are preferably present as V2O5, WO3 and T1O2.

The metal oxides may have catalytic activity in the SCR or contribute to the catalytic activity. For example, according to the invention, vanadium oxide, cerium oxide and tungsten oxide have catalytic activity.

The metal oxides can also have no or only low catalytic activity and serve, for example, as a carrier material. Such non-catalytic components serve, for example, to increase the internal surface and / or to produce a porous structure. For example, titanium oxide is preferably used as a carrier material. This may contain amounts of other non-reactive or only slightly reactive metal oxides, such as silica or aluminum trioxide. The support material is generally in excess, with the catalytic component generally being applied to the surface of the inert component.

In a preferred embodiment, the major constituent of the first region is titanium dioxide, which is, for example, greater than 50 wt%, greater than 80 wt%, or greater than 90 wt% of the range. For example, a catalyst based on oxides of vanadium, cerium, titanium and tungsten contains essentially T1O2 in the anatase modification. T1O2 can be stabilized by WO3 in order to achieve an improvement of the thermal durability. The proportion of WO3 is typically from 5 to 15% by weight, for example from 7 to 13% by weight. An advantage of the vanadium oxide and ceria-based catalytic component is the high SCR activity at low temperatures. According to the present invention, the low-temperature activity of the vanadium-based catalysts is advantageously combined with the specific activity of the iron-containing zeolites to provide a catalyst having excellent cold-start properties.

The first range preferably contains from 0.5 to 10% by weight, in particular from 1 to 5% by weight, vanadium oxide, calculated as V2O5 and based on the weight of the first range. The first range preferably contains from 0.2 to 10% by weight, in particular from 0.5 to 7% by weight, from 0.5 to 5% by weight or from 0.5 to 3% by weight of cerium oxide, calculated as CeÜ2 and based on the weight of the first area. The catalyst preferably contains from 1 to 17% by weight, in particular from 2 to 10% by weight, of tungsten oxide, calculated as WO 3 and based on the weight of the first range. The statement "calculated as" takes into account that in the technical field elemental analysis generally determines the quantities of metals.

In a preferred embodiment, the first region preferably contains or consists of the following oxides of metals:

 (a) from 0.5 to 10% by weight of vanadium oxide, calculated as V2O5.

 (b) from 0.2 to 10% by weight of cerium oxide, calculated as CeC> 2,

 (c) from 0 to 17 wt% tungsten oxide, calculated as WO3,

 (d) from 25 to 98% by weight of titanium oxide, calculated as T1O2,

 (e) from 0 to 15% by weight of silica, calculated as S1O2,

 (f) from 0 to 15% by weight of alumina, calculated as Al 2 O 3,

each based on the weight of the first area.

The first range particularly preferably contains 0.5 to 10% by weight of vanadium oxide; 0.5 to 7% by weight of cerium oxide; 2 to 17 wt.% Tungsten oxide, and 25 to 98 wt.% Titanium dioxide, based on the weight of the first range.

Particularly preferably, the first region contains the following oxides of metals, or consists thereof:

 (a) from 1 to 5% by weight of vanadium oxide, calculated as V2O5,

(b) from 1 to 15 wt% tungsten oxide, calculated as WO3, (c) from 0.5 to 5% by weight of cerium oxide, calculated as CeC> 2,

 (d) from 73 to 98% by weight of titanium dioxide, and

 (e) from 0 to 25% by weight of silica,

the sum of the proportions being cerium oxide + tungsten oxide <17% by weight,

in each case based on the weight of the first area ..

The catalyst contains a second catalytic region containing iron-containing molecular sieve. The term "molecular sieve" refers to natural and synthetic compounds, in particular zeolites, which have a high adsorption capacity for gases, vapors and solutes with specific molecular sizes, By appropriate selection of the molecular sieve it is possible to separate molecules of different sizes Molecular sieves generally have uniform pore diameters on the order of the diameter of molecules and a large internal surface area (600-700 m ² / g).

In a particularly preferred embodiment, the molecular sieve is a zeolite. The term "zeolite" generally in accordance with the definition of the International Mineralogical Association (DS Coombs et al., Can. Mineralogist, 35, 1997, 1571) is a crystalline substance from the group of aluminum silicates with spatial network structure of the general formula M ^{n +} [(AI02 ) X (Si0 ₂ ) Y] xH ₂ 0. The basic structure is formed of Si0 ₄ / Al0 ₄ tetrahedra, which are linked by common oxygen atoms to a regular three-dimensional network.

The zeolite structure contains voids and channels characteristic of each zeolite. The zeolites are classified into different structures according to their topology. The zeolite framework contains open cavities in the form of channels and cages that are normally occupied by water molecules and special framework cations that can be exchanged. The entrances to the cavities are formed by 8, 10 or 12 "rings" (narrow, medium and large pore zeolites). In a preferred embodiment, the zeolite has a structure whose maximum ring size is defined by more than 8 tetrahedrons. Preferred zeolites according to the invention are those having the topologies AEL, AFI, AFO, AFR, ATO, BEA, GME, HEU, MFI, MWW, EUO, FAU, FER, LTL, MAZ, MOR, MEL, MTW, OFF and TON. Particular preference is given to zeolites of the topologies FAU, MOR, BEA, MFI and MEL.

For the purposes of the present invention, preference is given to using a zeolite, in particular each 10- and 12-ring zeolite, which has an SiO 2 / Al 2 O 3 molar ratio (molar ratio, SAR ratio) of 5: 1 to 150: 1. The preferred SiO 2 Al 2 O 3 module according to the invention is in the range from 5: 1 to 50: 1 and particularly preferably in the range from 10: 1 to 30: 1.

The molecular sieve in the second region contains iron, and is preferably a zeolite containing iron. It has been found that iron-containing zeolites in combination with the first range, containing vanadium oxide and cerium oxide, catalyze a particularly efficient SCR. Preference is given to using a zeolite which has been exchanged with iron ions ("iron-exchanged zeolite") or in which at least a part of the aluminum atoms of the ferrosilicate skeleton is isomorphously substituted by iron.

Particularly preferably, the zeolite is exchanged with iron. The zeolite is preferably one of the type BEA. Particularly preferred is an iron-exchanged zeolite of the BEA type with a SAR ratio of 5: 1 to 50: 1. Preparation processes for iron-containing zeolites, for example via solid or liquid phase exchange, are known to the person skilled in the art. The proportion of iron in the iron-containing zeolite is, for example, up to 10% or up to 15%, calculated as Fe 2 O 3 and based on the total amount of the iron-containing zeolite. Preferred iron-containing and iron-exchanged zeolites according to the invention are described, for example, in US2012 / 0275977A1. In a preferred embodiment, the second region contains at least two different iron-containing zeolites. It is advantageous that various desired properties can be combined. For example, an iron-containing zeolite that is active at low temperatures can be combined with an iron-containing zeolite that is active at higher temperatures. The second region can contain, in addition to the iron-containing zeolite, further components, in particular non-catalytically active components, such as binders. Examples of suitable binders are non-catalytically active or only slightly catalytically active metal oxides, such as S1O2, Al2O3 and Zr02. The proportion of such binders in the second region is, for example, up to 15% by weight.

In a preferred embodiment, the catalyst device comprises a carrier in addition to the first and second catalytic regions. The carrier is a device to which the catalytic regions are applied, usually by coating. The support is catalytically not active, that is inert for the reaction. The carrier may be a metallic or ceramic carrier. The carrier may have a honeycomb structure of parallel exhaust gas flow channels or be a foam. In a preferred embodiment, the carrier is a monolith. Monoliths are integral ceramic carriers used particularly in the automotive industry and have the parallel channels from the inlet to the outlet side through which the exhaust gases flow. The structure is also referred to as a "honeycomb." Alternatively, the carrier may be a filter in which the exhaust gases on the inlet side flow into outlet-closed channels through which channel walls diffuse and exit the carrier through channels closed on the outlet side and opened on the outlet side.

In a further embodiment of the invention, a catalytic region simultaneously serves as a carrier. This is especially possible when the first or the second catalytic region is provided as an extrudate. In this case, the extruded area itself forms the carrier device to which the other area is applied by coating. Such a carrier device is obtainable, for example, when the iron-containing zeolite is incorporated into the walls of the exhaust ducts. This embodiment has the advantage that no additional inert support is required, so that the interior of the catalyst device can be made particularly compact.

The catalyst device according to the invention contains at least one "first" and one "second" region with different catalyst activities. The catalyst device according to the invention can exactly a first area and a second area but also more than a first and second area, for example 2, 3 or 4 first and / or second areas. The terms "first region" (with a vanadium oxide and a cerium oxide) and "second region" (with an iron-containing molecular sieve) are used in the context of this application for the sole purpose of distinction. They contain no statement about the spatial arrangement of the areas. Consequently, they do not mean that the first area is located in front of the second area.

The first region containing vanadium oxide and cerium oxide and the second region containing iron-containing molecular sieve are spatially separated from each other. The regions may be consecutive zones in the exhaust gas flow direction. Alternatively, the regions may be in the form of superimposed layers, so that, for example, the second region forms a lower layer and the first region forms an overlying layer.

In a preferred embodiment, the entire interior of the catalyst device according to the invention is coated with the first and / or second region. Preferably, the entire interior is completely coated with the first and second areas. In this embodiment, a particularly efficient conversion of nitrogen oxides is obtained because the entire inner surface of the device is used catalytically. The size and thickness of the first and second regions and the amount of the catalysts in the first and second regions are coordinated so that a desired depletion of nitrogen oxides is obtained.

Generally, it has been found that selective catalytic reduction (SCR) is advantageous when, in accordance with the invention, two catalytic regions, the first region containing vanadium oxide and cerium oxide and the second region containing iron-containing molecular sieve, are combined in a catalyst device. By this combination of catalysts, nitrogen oxides NOx can be efficiently removed under various conditions while suppressing the formation of nitrous oxide.

In a particularly preferred embodiment, the catalyst device is configured such that the exhaust gases first contact the first region containing the vanadium-cerium catalyst. Without being bound by theory, it is believed that an exhaust fraction that reacts with the vanadium-cerium catalyst, then reacts advantageously with the iron-containing zeolite, so that a particularly efficient depletion of nitrogen oxides while avoiding the formation of nitrous oxide is achieved. The arrangement of the catalysts in this order can also lead to a particularly efficient purification of NO 2, even at relatively low temperatures, while avoiding the formation of nitrous oxide.

In a further preferred embodiment, the catalytic converter device is configured such that the exhaust gases first contact the second region containing the iron-containing molecular sieve.

In a preferred embodiment, the first and the second region are arranged one after the other in the exhaust gas flow direction. In the context of this application, such adjacent regions are referred to as "zones." In this case, the first region can be a first zone and the second region can be a second zone or vice versa The exhaust gases which pass in the direction of flow of the catalytic converter contact the various zones one after the other In one preferred embodiment, the catalyst device may have exactly one first and one second zone Alternatively, the catalyst device may have a plurality of consecutive zones, for example 2, 3 or 4. The length of the zones will be in thickness and desired catalytic efficiency For example, a zone may be between 20 and 80%, or between 30 and 70%, of the length of the device, in particular, the zones may have equal lengths, In a particularly preferred embodiment, the first zone is a first zone and the second zone a second zone, the first zone in the exhaust gas flow direction before the second zone is located. Thus, the first zone which first contacts the exhaust gases contains the vanadium-cerium catalyst. The first zone is arranged on the inlet side. The second region containing the iron-containing molecular sieve forms the second zone, which lies downstream in the flow direction and on the outlet side. It has been found that a catalyst device with such a spatial configuration catalyzes the SCR particularly efficiently and almost completely suppresses the formation of nitrous oxide. In a particularly preferred embodiment, the first region is a first zone and the second region is a second zone, wherein the second zone lies in the exhaust gas flow direction before the first zone. Thus, the first zone which first contacts the exhaust gases contains the iron-containing molecular sieve. The second zone is arranged on the inlet side. The first zone with the vanadium-cerium catalyst forms the second zone, which lies downstream in the flow direction and on the outlet side. Also, a catalyst device having such a spatial configuration efficiently catalyzes the SCR efficiently and reduces the formation of nitrous oxide. In a preferred embodiment, the first and second regions form superimposed layers. In this case, the first region with the vanadium-cerium catalyst particularly preferably forms the upper layer. Among other things, it has been found that such a catalyst device has advantages after aging, which may indicate high durability and durability. In a further preferred embodiment, the second region forms the upper layer, so that the first region lies below.

In a preferred embodiment, the second region containing the iron-containing molecular sieve is completely applied to the support as a lower layer. In this case, the first region containing the vanadium-cerium catalyst may be formed completely or only in zones on the second layer in the form of a first layer.

In a preferred embodiment, the first region is applied as an upper layer completely on the second region as a lower layer. That is, there is no region on the inner surface of the catalyst device in which the lower second region containing the iron-containing molecular sieve directly contacts the exhaust gases. It is advantageous that only exhaust gases that have been pretreated in the upper layer containing the vanadium-cerium catalyst, get into the lower layer. In a further preferred embodiment, the first region is not continuous as the upper layer, but applied in zones on the second region as the lower layer. In this embodiment, subregions are present in which the lower layer, which contains the iron-containing molecular sieve, comes into direct contact with the exhaust gases. It is preferred that the exhaust gases first with the upper layer, the vanadium-cerium Contains catalyst, get in contact. Thus, it is particularly preferred that in the flow direction first a region is present which has an upper layer. Also in this embodiment, it is achieved that the exhaust gases first come into contact with the upper layer containing the vanadium-cerium catalyst, and thereby pretreated before they reach the second layer.

In one embodiment, the exhaust gases upon leaving the catalyst device last contact the first region containing the vanadium-cerium catalyst. Thus, a reaction with the vanadium oxide takes place last. A lower layer with the iron-containing molecular sieve is preferably present below the upper layer at the outlet of the catalyst device.

In a further preferred embodiment, the first region and / or the second region consist of two or more superimposed partial layers. The sub-layers may differ, for example, in terms of physical properties, such as density or porosity, or the chemical properties, such as the composition of the individual components.

In a preferred embodiment, the catalyst device contains no further layers. Therefore, at each location of the device, the exhaust gases contact the first or second area immediately. Particularly preferably contacted in the device, the first region containing the vanadium-cerium catalyst, the exhaust gases directly. The first area forms an outer layer over which no further layer is applied.

It is also possible according to the invention for there to be at least one additional additional functional area which serves, for example, for the pre- or post-treatment of the exhaust gases. The pre- or post-treatment can be, for example, a catalytic pretreatment. In a preferred embodiment, the catalyst device has at least one further third region which comprises a vanadium oxide catalyst without cerium oxide. The third region may be selected as described above for the first region, with the difference that no ceria is included. For example, a zone may be formed of a sub-layer containing a vanadium oxide without cerium oxide and a sub-layer containing a vanadium oxide with cerium oxide. consist. In this case, in addition to the first, second and third regions, there are preferably no further, different regions thereof.

For example, the second region may consist of first and second sublayers containing iron-containing molecular sieves having various activities. In such partial layer embodiments, it is generally advantageous that, by suitable selection and combinations of different catalysts of the same type, their properties can be combined in a targeted manner in order, for example, to obtain a reactivity in a broad temperature range.

In a preferred embodiment, the catalyst device has no further regions except the first and the second region, in particular no further catalytic regions. The catalyst device preferably has no noble metal. In particular, the first and second regions do not contain noble metals such as platinum, gold, palladium and / or silver. In accordance with the present invention, a high efficiency catalyst device is provided without requiring the use of expensive and non-abundant precious metals.

The application of the SCR catalyst is carried out by methods known in the art, for example by applying coating suspensions (so-called "washcoats"), by coating in an immersion bath or by spray coating As washcoats, coating coats in which the solids or precursor compounds are suspended and / or dissolved to form the catalytic regions are commonly used, such as washcoats being provided in very homogeneous form with finely divided ingredients to allow the washcoats to be coated After application of the washcoats, usual post-treatment steps, such as drying, calcination and tempering, follow. In a preferred embodiment, in the catalyst device, the ratio of the weight (per catalyst volume) of the first to the second region is greater than 0.2, in particular between 0.2 and 15, and particularly preferably between 1 and 6. In another preferred embodiment, the first Range a zone extending from the first end of the beam over 20 to 80% of its total length. In this case, the second region is a zone that extends from the second end of the carrier over 20 to 80% of its total length. The total amount of areas is selected so that the device as a whole is used as efficiently as possible. In the case of a flow-through substrate, for example, the total amount of coatings (solids content) per carrier volume (total volume of the catalyst device) can be between 100 and 600 g / l, in particular between 100 and 500 g / l. The second, lower layer is preferably used in an amount of 50 to 200 g / l, in particular between 50 and 150 g / l, particularly preferably of about 100 g / l. The first, upper layer is preferably used in an amount of 100 to 400 g / l, in particular between 200 and 350 g / l, particularly preferably of about 280 g / l. In the case of a filter substrate, substantially less washcoat is generally used, for example in a total amount of 10 to 150 g / l.

The invention also provides the use of a catalyst device according to the invention for the purification of nitrogen oxide-containing exhaust gases by selective catalytic reduction (SCR). The present invention likewise provides a process for removing nitrogen oxides from the exhaust gas of combustion engines operated with a lean air-fuel fuel, which is characterized in that the exhaust gas is passed through a catalyst device according to the invention. The process according to the invention is particularly advantageous if the NO.sub.2 content in the nitrogen oxide exceeds 50% (NO.sub.xO.sub.x> 0.5), that is to say, for example, is 75%.

The exhaust gases are preferably those from incinerators. The incinerators can be mobile or stationary. Mobile combustion devices in the context of this invention are, for example, internal combustion engines of motor vehicles, in particular Diesel engines. The stationary combustion facilities are usually power plants, combustion plants or heating systems of private households. The exhaust gases preferably originate from lean-burn engines, that is to say combustion engines which are operated predominantly with a lean air / fuel ratio. Lean engines are especially diesel engines and direct injection gasoline engines.

The invention also relates to a system or a device comprising a catalyst device according to the invention and an upstream device which releases exhaust gases, in particular an internal combustion engine. The system may include other devices, such as exhaust pipes or other catalyst devices. The invention also relates to an automobile comprising the exhaust device according to the invention or the system.

Depending on various factors (e.g., engine calibration, operating condition, type and design of upstream catalysts), the NO 2 content in NOx may exceed 50%. According to the invention, it has been found that the catalyst device catalyzes the SCR of exhaust gases with high NO 2 contents (NO 2 / NO x> 0.5) particularly efficiently, even in the problematic medium to low temperature range below about 450 ° C., in particular below 350 ° C. In particular, it has been found that with such exhaust gases an efficient removal of the nitrogen oxides while avoiding the formation of nitrous oxide is achieved. These results were surprising since it was known in the art that vanadium catalysts are relatively inefficient in SCR with NO 2 -rich exhaust gases, especially at low temperatures, while the more efficient iron-zeolite catalysts produce a high proportion of N 2 O.

In a preferred embodiment, the exhaust gases originate from an upstream oxidation catalyst. Such upstream oxidation catalysts are used in the prior art in order to increase the proportion of NO 2 in the exhaust gases of lean-burn engines, in particular diesel engines.

Preferably, the exhaust gases at the introduction into the catalyst device to a relatively high oxygen content, for example, at least 5 vol.%, At least 10 vol.% Or at least 15 vol.% Is. Exhaust gases from lean-burn engines regularly have such high oxygen contents. The oxidant oxygen makes the reductive removal difficult of nitrogen oxides by means of SCR. Surprisingly, it has been found that the catalyst devices according to the invention also efficiently remove nitrogen oxides from exhaust gases with a high oxygen content and at the same time prevent the formation of nitrous oxide.

In the SCR reaction of exhaust gases with the catalyst device according to the invention, more than 90%, preferably more than 95%, of NO _x and / or NO 2 are preferably removed.

According to the invention, it is advantageous that an efficient purification of exhaust gases by SCR can also take place at relatively low temperatures, it being possible to avoid the formation of nitrous oxide, especially at low temperatures. The use of the catalyst device is particularly advantageous at temperatures in the range of below 450 ° C, especially from 180 to 450 ° C, and more preferably between 200 and 350 ° C.

In a preferred embodiment, the use is to avoid the formation of nitrous oxide (N2O), especially in the purification of N02-rich exhaust gases and in particular at temperatures below 450 ° C or below 350 ° C. According to the invention, it has been found that an efficient depletion of nitrogen oxides can take place, at the same time avoiding the formation of nitrous oxide, or relatively little nitrous oxide being produced. According to the prior art, in the SCR reaction with vanadium catalysts and iron-containing zeolites, especially in the purification of NC> 2-rich exhaust gases at low or medium temperatures, relatively high amounts of nitrous oxide are formed. Therefore, it was surprising that with a vanadium catalyst, which additionally contains a cerium oxide, an efficient SCR takes place and at the same time only relatively small amounts of nitrous oxide are formed. Preferably, the concentration of nitrous oxide after SCR with the catalyst device of the present invention is not higher than 50 ppm, 20 ppm or 10 ppm. In particular, such concentrations at temperatures in the range of 180 ° to 450 ° C, in particular from 200 ° C to 350 ° C, should not be exceeded.

The invention also provides a process for purifying exhaust gases, comprising the steps:

(i) providing a catalyst device according to the invention, (ii) introducing exhaust gases which receive nitrogen oxides into the catalyst device,

(iii) introducing a nitrogen-containing reducing agent into the catalyst device, and

 (iv) reducing the nitrogen oxides in the catalyst device by Selective Catalytic Reduction (SCR).

The process according to the invention is particularly advantageous if the NO.sub.2 content in the nitrogen oxide exceeds 50% (NO.sub.2 / NO.sub.x> 0.5), that is to say, for example, is 75%. The exhaust gases introduced in step (ii) preferably originate from lean-burn engines, in particular from an oxidation catalytic converter connected downstream of the engine. The catalyst device according to the invention can be combined in the process with further devices for purifying exhaust gases in series or in parallel, such as further catalysts or filters.

In the SCR reaction, a nitrogen-containing reducing agent, preferably ammonia (NH 3) or a precursor compound thereof, such as urea, is supplied. The nitrogen-containing reducing agent is preferably added to the exhaust gas prior to entry into the catalyst device, but it can also be introduced separately into the catalyst device.

The catalyst device according to the invention solves the problem underlying the invention. A catalytic device for purifying exhaust gases by SCR is provided, which efficiently removes nitrogen oxides while preventing the formation of nitrous oxide. The device is suitable for purifying exhaust gases in a wide temperature range, and in particular also at low temperature. It is particularly suitable for the purification of N02-rich exhaust gases, which in the operation of diesel engines z. B. arise in conjunction with an oxidation catalyst. The catalyst device exhibits a high catalytic activity even after aging and prevents or minimizes the formation of nitrous oxide. Because of the high efficiency of the SCR under various application conditions, even at low temperature and at both low and high NO 2 levels, the catalyst devices are highly suitable for automotive applications. embodiments

preliminary tests

FIGS. 1 and 2 show the dependence of the conversion of nitrogen monoxide, see FIG. 1, or nitrogen dioxide (NO ₂ / NO _x = 75%), see FIG. 2, on the cerium content of the SCR catalyst.

In FIGS. 1 and 2,

Reference = SCR catalyst of 3% V ₂ 0 ₅ , 4.3% W0 ₃ and balance Ti0 ₂ with 5% Si0 ₂ 1% cerium oxide = as reference, but Ti0 ₂ / Si0 ₂ by ceria to a cerium oxide content of Catalyst replaced by 1%

5% cerium oxide = as reference, but Ti0 ₂ / Si0 ₂ replaced by cerium oxide to a cerium oxide content of the catalyst of 5%

10% cerium oxide = as reference, but Ti0 ₂ / Si0 ₂ replaced by cerium oxide to a cerium oxide content of the catalyst of 10%

The catalysts were coated in the usual way to commercial flow substrates with a washcoat loading of 160 g / l and measured at GHSV = 60000 1 / h, the NOx conversion at the following test gas composition: NOx: 1000 ppm N0 ₂ / NOx: 0% (FIG 1) or 75% (Figure 2)

NH ₃ : 1 100 ppm (FIG. 1) or 1350 ppm (FIG. 2)

0 ₂ : 10%

H ₂ 0: 5%

N ₂ : rest

As shown in FIG. 1, the conversion of NO with increasing content of cerium oxide deteriorates, whereas, as shown in FIG. 2, the conversion at NO ₂ / NO _x = 75% improves with increasing content of cerium oxide. Example 1 Production of the Coating Suspensions (Washcoats)

Preparation of Coating Suspension A (Vanadium SCR) A commercially available 5 wt% silica doped anatase titanium dioxide was dispersed in water. Subsequently, an aqueous solution of ammonium metatungstate and ammonium metavanadate dissolved in oxalic acid was added as the tungsten or vanadium precursor in an amount such that a catalyst of the composition 87.4% by weight of TiO ₂ , 4.6% by weight of SiO ₂ , 5, 0 wt .-% W0 ₃ and 3.0% by weight V2O5 results. The mixture was stirred vigorously and finally homogenized in a commercially available stirred ball mill and ground to d90 <2 μηι.

Preparation of coating suspension B (vanadium SCR with 1% of a cerium oxide)

A commercially available 5 wt% silica doped anatase titanium dioxide was dispersed in water. Subsequently, an aqueous solution of ammonium metatungstate as a tungsten precursor, ammonium metavanadate dissolved in oxalic acid as a vanadium precursor and an aqueous solution of cerium acetate as a cerium precursor were added in an amount to yield a catalyst of a composition containing 86.4% by weight of T1O2, 4.6 Wt .-% S1O2, 5.0 wt .-% W0 ₃ , 3.0 wt .-% V ₂ 0 ₅ and 1% Ce0 _{2 is} calculated. The mixture was stirred vigorously and finally homogenized in a commercially available stirred ball mill and ground to d90 <2 μηι. Preparation of Coating Suspension C (Fe-SCR, SAR = 25)

A coating suspension was prepared for a commercially available iron-exchanged beta zeolite based SCR catalyst. For this purpose, a commercial Si0 ₂ binder, a commercial boehmite binder (as coating aids), iron (III) nitrate nonahydrate and commercially available beta zeolite having a molar Si0 ₂ / Al ₂ O ₃ ratio (SAR) of 25 in Water so that a catalyst of the composition results in 90% by weight of β-zeolite and an iron content, calculated as Fe ₂ O ₃ , of 4.5% by weight. Preparation of coating suspension D (Fe-SCR, SAR = 10)

A coating suspension was prepared for a commercially available iron-exchanged beta zeolite based SCR catalyst. For this purpose, a commercial Si0 ₂ binder, a commercial boehmite binder (as Coating aids), iron (III) nitrate nonahydrate and commercially available beta zeolite having a SiO 2 molar ratio (SAR) of 10 in water such that a catalyst of the composition comprises 90% by weight of β-zeolite and a Iron content, calculated as Fe 2 C> 3, of 4.5 wt .-% results.

Example 2: Preparation of the Catalyst Devices

Various catalyst devices were prepared by coating ceramic supports with the coating suspensions A to D. As support conventional ceramic monoliths were used with parallel, open on both sides flow channels. In this case, a first and a second layer (S1, S2) were applied to each carrier, wherein each layer was divided into two adjacent zones (Z1, Z2). The exhaust gases to be purified flow in the direction of flow into the catalyst device, ie via the upper layer 2 and from zone 1 to zone 2. In Scheme 1, the structure of the catalyst devices with four catalytic regions lying in two layers and two zones is shown.

Scheme 1: Schematic structure of the catalyst devices produced according to the embodiments

Flow direction ->

The compositions and the amounts of coating suspensions A to D used are summarized in Table 1 below. The table also shows which catalytic layers S1 and S2 and zones Z1 and Z2 were applied. The catalysts E1 to E5 are according to the invention and V1 to V4 are comparative catalysts. First, one of the dispersions A to D was applied by a conventional dipping method from the inlet side over the length of the region Z1 S1 of a commercial flow substrate of 62 cells per square centimeter, a cell wall thickness of 0.17 millimeters and a length of 76.2 mm. The partially coated component was first dried at 120 ° C. Subsequently, one of the dispersions A to D was applied to the length of the region Z2S1 from the outlet side by the same procedure. The coated component was then dried at 120 ° C, calcined at 350 ° C for 15 minutes, then calcined at 600 ° C for 3 hours. When dispersion and washcoat loading were identical in the zones Z1 S1 and Z2S1, one of the dispersions A-D was applied to a commercially available flow-through substrate with 62 cells per square centimeter and a cell wall thickness of 0.17 millimeters over the entire length of 76.2 mm applied. It was then dried at 120 ° C, calcined at 350 ° C for 15 minutes, then calcined at 600 ° C for 3 hours.

The thus calcined component was then coated according to the method described above, starting from the inlet side over the length of the region Z1 S2 with one of the suspensions A to D and dried at 120 ° C. The previously described step was skipped if no coating was provided for the area Z1 S2. Subsequently, starting from the outlet side, the coating was carried out over the length of the Z2S2 region with one of the suspensions A to D. The mixture was then dried at 120.degree. The previously described step was skipped if no coating was provided for the area Z2S2. It was then calcined for 15 minutes at 350 ° C, then for 3 hours at 600 ° C. If dispersion and washcoat loading were identical in the areas Z1 S2 and Z2S2, one of the dispersions A to D was applied to the entire length of the component of 76.2 mm according to the method described above. It was then dried at 120 ° C, calcined at 350 ° C for 15 minutes, then calcined at 600 ° C for 3 hours.

Table 1: Preparation of the catalyst devices with coating suspensions A to D in the first and second layers (S1, S2) and the first and second zones (Z1, Z2). The total amount in each of the four ranges (S1Z1 to S2Z2) in g / l after drying, calcination and heat treatment and the length of the zones in% are given on the total length of the catalyst device. The catalysts V1 to V4 are comparative catalysts.

As an alternative to the described method, it would also be possible to produce two catalysts (Z1, Z2) which correspond to zones Z1 and Z2 described above, and to test both catalysts one after the other (Z1 before Z2). Catalyst Z1: First apply one of the dispersions A to D over the entire length of the support with the length Z1 (range Z1 S1), dry at 120 ° C, then at 350 ° C for 15 minutes, then for a period of 3 hours Calcine 600 ° C. If desired, then apply one of the dispersions A to D over the entire length of the component thus obtained (area Z1 S2), then calcine at 350 ° C for 15 minutes, then calcine at 600 ° C for 3 hours.

Catalyst Z2: First apply one of the dispersions A to D over the full length of the support of length Z2 (range Z2S1), dry at 120 ° C, then at 350 ° C for 15 minutes, then at 600 for 3 hours Calcine ° C. If provided, then one of the dispersions A to D over the entire length of the sun apply (area Z2S2), then calcine at 350 ° C for 15 minutes, then calcine at 600 ° C for 3 hours.

Example 3: Reduction of nitrogen oxides by means of SCR

measurement methods

The catalyst devices prepared according to Example 2 were tested for their activity and selectivity in the selective catalytic reduction of nitrogen oxides. At various defined temperatures (measured at the inlet side of the catalyst), the nitrogen oxide conversion was measured as a measure of the SCR activity and the formation of nitrous oxide. On the inlet side model exhausts were introduced which contained preset proportions of, among others, NO, NH ₃ , NO ₂ and O ₂ . The measurement of the nitrogen oxide conversions took place in a reactor made of quartz glass. Drilling cores with L = 3 "and D = 1" were tested between 190 and 550 ° C under steady state conditions. The measurements were carried out under the following summarized test conditions. GHSV is the gas volume flow (gas hourly space velocity, gas flow rate: catalyst volume). The conditions of the measurement series TP1 to TP4 are summarized below:

Test parameter set TP1:

Gas hourly space velocity GHSV = 60000 1 / h with the synthesis gas composition: 1000 vppm NO, 1 100 vppm NH ₃ , 0 vppm N ₂ 0

xNOx = xNO + XNO2 + XN2O, where x is concentration (vppm)

10% by volume 0 ₂ , 5% by volume H ₂ O, balance N ₂ .

Test parameter set TP2

 GHSV = 60000 1 / h with the synthesis gas composition:

250 vppm NO, 750 vppm N0 ₂ , 1350 vppm NH ₃ , 0 vppm N ₂ 0

xNOx = xNO + XNO2 + XN2O, where x is concentration (vppm)

10 vol.% Q ₂ , 5 vol.% H ₂ O, balance N ₂ . Test parameter set TP3

 GHSV = 30000 1 / h with the synthesis gas composition:

500 vppm NO, 550 vppm NH ₃ , 0 vppm N ₂ 0

xNOx = xNO + XNO2 + XN2O, where x is concentration (ppm)

10% by volume 0 ₂ , 5% by volume H ₂ O, balance N ₂ .

Test parameter set TP4

 GHSV = 30000 1 / h with the synthesis gas composition:

125 vppm NO, 375 vppm N0 ₂ , 675 vppm NH3, 0 vppm N ₂ 0

a = xNH3 / XNO _x = 1, 35

xNOx = xNO + XNO2 + XN2O, where x is concentration (ppm)

10% by volume 0 ₂ , 5% by volume H ₂ O, balance N ₂ . The nitrogen oxide concentrations (nitrogen monoxide, nitrogen dioxide, nitrous oxide) after the catalyst device were measured. From the nitrogen oxide contents set in the model exhaust gas, which were verified during the conditioning at the beginning of the respective test run with a pre-catalyst exhaust gas analysis, and the measured nitrogen oxide contents after the catalyst device, the nitrogen oxide conversion over the catalyst device for each temperature measurement point was as follows calculated (x is the concentration in vppm):

UNOX [%] = (1 - Xoutput (NOx) / Xinput (NOx)) ^* 100 [%]

With

XInput (ΝΟχ) = XInput (NO) + XInput (NO2)

XAusgang (NOx) = XAusgang (NO) + XAusgang (N02) 2 * X + _O utput (N ₂ 0).

X output (N20) was weighted by a factor of 2 to account for stoichiometry. In order to determine how the aging of the catalysts affects the result, the catalyst devices were subjected to hydrothermal aging for 100 hours at 580 ° C in a gas atmosphere (10% O 2, 10% H 2 O, balance N 2). Subsequently, the conversions of nitrogen oxides were determined according to the method described above. Results

The results of the test series TP1 and TP3, in which the model exhaust gas contained only NO as nitrogen oxide, are summarized in Table 2. The results of the test series TP2 and TP4, in which the model exhaust gas contained nitrogen oxides NO and NO2 in the ratio 1: 3, are summarized in Table 3. In each case, the tables indicate which catalyst was used in accordance with Example 2 (Table 1). For each defined temperature value, it is indicated what percentage of the initial concentration of NOx was removed. Table 3 also shows, for each temperature value 2 to 7, the absolute amount of N2O measured at each temperature value after the catalyst. Tables 4 and 5 summarize the conditions and results of the tests with catalyst devices after aging.

The experiments show that the SCR catalysts according to the invention with two catalytic regions, the first region containing a vanadium oxide and a cerium oxide, and the second region containing an iron-containing molecular sieve, a high efficiency in the reductive removal of nitrogen oxides (NOx) reach through SCR. They are not only suitable for the reaction with NO-rich exhaust gases, but also for the treatment of N02-rich exhaust gases. The reduction of nitrogen oxides from N02-rich exhaust gases is particularly efficient in the temperature range below 400 ° C or below 350 ° C. In addition, in the case of the SCR with catalysts according to the invention, the formation of nitrous oxide can be suppressed. Thus, the catalysts eliminate E3, E4 and E5 nitrogen oxides NOx in both NO-rich exhaust gases (Table 2) and in N02-rich exhaust gases (Table 3) almost completely, the formation of nitrous oxide is almost completely suppressed. Catalyst E2 also effectively enriches nitrogen oxides NOx from both NO and NO 2 rich exhaust gases and reduces the amount of nitrous oxide produced. The advantages of catalyst E2 after aging are particularly pronounced (Tables 4, 5), which is particularly important for practical applications in connection with internal combustion engines. Also, the catalyst E1 shows a high efficiency with reduced nitrous oxide formation. The catalyst devices according to the invention thus combine several advantageous properties with each other, namely a high efficiency with N02-rich exhaust gases in the temperature range of about 180 ° C to 500 ° C, and especially at low temperatures, high efficiency with NO-rich exhaust gases, and avoidance the formation of nitrous oxide. The effects show up with freshly prepared catalysts and after an aging process.

Table 2: Conditions and Results of the Reduction of NO with Various Catalyst Devices (Experiments TP1 and TP3) at Various Actually Measured Temperatures 1-8. Specified is the depletion of NO _x at the catalyst device outlet in% relative to the initial charge.

Table 3: Test conditions and results of the reduction of NO 2: NO in the ratio 3: 1 (experiments TP2 and TP4) at different actually measured temperatures. For measurements 2 through 7, the depletion of NO _x at the catalyst device outlet is indicated in% with respect to the initial charge used, as well as the measurements of N 2 O in ppm at the catalyst device outlet.

Table 4: Conditions and Results of the Reduction of NO with Various Catalyst Devices After Aging (Experiments TP1 at Various Actually Measured Temperatures 1 to 8). Specified is the depletion of NO _x a catalyst device outlet in% relative to the initial charge.

Table 5: Test conditions and results of the reduction of NO 2: NO in the ratio 3: 1 (experiment TP2) with different catalyst devices after aging at various actually measured temperatures. For measurements 2 through 7, the depletion of NO _x in% at the catalyst device outlet, in relation to the initially used amount, and the measurements for N 2 O in ppm at the catalyst device outlet are shown.

Claims

1 . A catalytic device for purifying nitrogen oxide-containing exhaust gases by selective catalytic reduction (SCR) comprising at least two catalytic regions, the first region containing vanadium oxide and cerium oxide, and the second region containing iron-containing molecular sieve.

2. A catalyst device according to claim 1, wherein the first region contains at least one further component selected from oxides of tungsten, titanium, silicon and aluminum.

A catalyst device according to any one of the preceding claims, wherein the first range comprises from 0.2 to 10% by weight of ceria, calculated as CeO 2 and based on the weight of the first range.

The catalyst device according to any one of the preceding claims, wherein the first region has from 0.5 to 7% by weight of ceria, calculated as CeO 2 and based on the weight of the first region. 5. Catalyst device according to at least one of the preceding claims, wherein the first range of 0,

5 to 3 wt .-% of cerium oxide, calculated as CeO 2 and based on the weight of the first region having.

6. Catalyst device according to at least one of the preceding claims, wherein the first region comprises the following components:

 (a) from 0.5 to 10% by weight of vanadium oxide, calculated as V2O5.

 (b) from 0.2 to 10% by weight of cerium oxide, calculated as CeO 2,

 (c) from 0 to 17 wt% tungsten oxide, calculated as WO3,

 (d) from 25 to 98% by weight of titanium oxide, calculated as T1O2,

 (e) from 0 to 15% by weight of silica, calculated as S1O2,

 (f) from 0 to 15% by weight of alumina calculated as Al 2 O 3

 and in each case based on the weight of the first area.

A catalyst device according to any one of the preceding claims, wherein the molecular sieve is a zeolite.

A catalyst device according to claim 7, wherein the zeolite is an iron-exchanged zeolite which preferably has a structure whose maximum ring size has more than 8 tetrahedrons.

The catalyst device of at least one of claims 7 or 8, wherein the zeolite has a structure selected from AEL, AFI, AFO, AFR, ATO, BEA, GME, HEU, MFI, MWW, EUO, FAU, FER, LTL, MAZ, MOR, MEL, MTW, OFF and TONE.

A catalyst device according to any one of the preceding claims, which is configured such that the exhaust gases first contact the first region.

1 1. The catalyst device of claim 10, wherein the first and second regions form adjacent zones, the first region being a first zone and the second region being a second zone, the first zone being upstream of the second zone in the exhaust gas flow direction.

The catalyst device according to claim 10, wherein the first and second regions form superimposed layers, the first region being the upper layer.

A catalyst device according to any of the preceding claims, wherein the first and second regions are supported on an inert support, which is preferably a ceramic monolith.

14. System or device comprising a catalyst device according to the invention and an upstream device that releases exhaust gases, in particular an internal combustion engine.

15. Use of a catalyst device according to at least one of the preceding claims for the purification of nitrogen oxide-containing exhaust gases by selective catalytic reduction (SCR).

16. Use according to claim 15 for preventing the formation of nitrous oxide.

17. Use according to claim 15 and / or 16, wherein the exhaust gases have a ratio NO2 NO _X of> 0.5 and / or a temperature of 180 ° C to 450 ° C.

18. A process for purifying exhaust gases comprising the steps of:

 (i) providing a catalyst device or system according to at least one of claims 1 to 14,

 (ii) introducing exhaust gases which receive nitrogen oxides into the catalyst device,

(iii) introducing a nitrogen-containing reducing agent into the catalyst device, and

 (iv) reducing the nitrogen oxides in the catalyst device by Selective Catalytic Reduction (SCR).