EP3558493A1 - 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 using selective catalytic reduction (SCR), the catalyst device comprising at least two catalytic layers, the first layer containing vanadium oxide and a mixed oxide comprising titanium oxide and silicon oxide and the second layer containing a molecular sieve containing iron, wherein the first layer is applied onto the second layer. The invention also relates to uses of the catalyst device and a method for purifying exhaust gases.

Description

 SCR catalyst device containing vanadium oxide and iron-containing

 molecular sieve

The invention relates to a catalytic device for purifying nitrogen oxide-containing exhaust gases by selective catalytic reduction (SCR), comprising at least two catalytic layers, wherein the first layer contains vanadium oxide and the second layer contains iron-containing molecular sieve, wherein the first layer is applied to the second layer. 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 easily be neutralized by oxidation. 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 good nitrogen oxide conversion rates are achieved only at relatively high temperatures above about 350.degree. C. or above about 450.degree. 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, as compared to reaction (1) ("Standard SCR reaction") (2) ("Fast SCR reaction") Nitrogen oxides NOx, however, predominantly consist of NO and have only small proportions of N0 _2. The prior art therefore uses an upstream oxidation catalyst, for example platinum supported on alumina, for the oxidation of NO to NOx In the exhaust gas of lean-burn engines with SCR catalysts, the ammonia is oxidized to low-valent nitrogen oxides, in particular nitrous oxide (N ₂ O), as a result of which the process removes the reducing agent required for the SCR reaction, on the other hand the nitrous oxide escapes as unwanted 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 a suitable carrier therewith coated, whereby a single catalyst layer with 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 connected. The zeolite has only one storage function, while the following component catalyzes the SCR reaction with 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. In this case, the upper coating with the iron-exchanged zeolite should 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 lies on a layer which comprises a metal-exchanged zeolite. DE 10 2014 002 751 A1 also discloses an SCR catalyst While the lower layer may comprise iron or copper exchanged zeolites, the upper layer comprises vanadium pentoxide and titanium dioxide SCR catalysts comprising vanadium-based formulations and Cu or Fe zeolites also in WO 2016/01 1366 A1, DE 10 2006 031 661 A1 and in Ind.Eng.Chem.Res., 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 N02-rich exhaust gases, and which work efficiently 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 Progress reported in Natural Science Materials International, 25 (2015), 342-352 Dates. 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 FIG. 2. 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 at 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. 6 and 7. Object of the invention

The invention has for its object 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 NOx, especially NO and NO2, should be removed efficiently, while at the same time the formation of nitrous oxide N2O 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. 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. The catalysts should also be suitable for the purification of exhaust gases with a relatively high proportion of NO.

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

 a high efficiency with NC> 2-rich exhaust gases in the temperature range of about 180 ° C to 600 ° C, and in particular at low temperatures,

 a high efficiency with NO-rich exhaust gases, and

 - avoiding 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 catalytic device for purifying nitrogen oxide-containing exhaust gases by selective catalytic reduction (SCR), comprising at least two catalytic layers, wherein the first layer vanadium oxide and a mixed oxide comprising titanium oxide and silica and the second layer containing iron-containing molecular sieve the first layer is applied to the second layer. 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 takes place 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 layers, with a first, upper catalytic layer located on the second, lower catalytic layer. In this context, "catalytically" means that each of the layers has catalytic activity in the SCR It is particularly preferred that the first, upper layer directly contacts the exhaust gases, which means that the first layer is the outermost layer, over which no further layer However, it is also possible according to the invention for there to be at least one further functional layer above the first layer which is used, for example, for the pretreatment of the exhaust gases The pretreatment may be, for example, a catalytic pretreatment.

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 ,

In the context of this application, the term "mixed oxide" excludes physical mixtures of two of the more metal oxides, rather it stands for "solid solutions" with a uniform crystal lattice in which the individual metal oxides can no longer be distinguished or it stands for metal oxide agglomerates. which do not have a uniform crystal lattice and in which phases of the individual metal oxides can be distinguished. The first, upper layer 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 layer, which is largely responsible for the reaction. Therefore, the first catalytic layer is also referred to below as "vanadium catalyst".

The first, upper layer also contains a mixed oxide comprising titanium oxide and silicon oxide.

In a preferred embodiment, the first layer contains at least one further component selected from oxides of tungsten and aluminum. Especially Preferably, the first layer contains oxides of vanadium, silicon, tungsten and titanium, preferably in the form of V2O5, S1O2, 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 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 inner surface of the catalyst 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, upper layer is titanium dioxide, which constitutes, for example, more than 50% by weight, more than 80% by weight or more than 90% by weight of the layer. For example, a catalyst layer based on oxides of vanadium, silicon, titanium and tungsten contains substantially T1O2 in the anatase modification. T1O2 can be stabilized by WO3 to achieve thermal durability improvement. 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-based catalytic component is the high activity in SCR 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 molecular sieves to provide a catalyst having excellent cold-start properties.

In a preferred embodiment, the first layer additionally contains ceria. Surprisingly, it has been found that by combining vanadium oxide with cerium oxide, the efficiency of the SCR can be significantly improved. The effect is particularly pronounced with prolonged service life of the catalyst, associated with aging of the catalyst. Thus it was found that after aging of the catalyst both the Removal of nitrogen oxides and the prevention of the formation of N2O is particularly effective when the vanadium catalyst in the first layer additionally contains ceria. In general, the advantageous effect of cerium oxide is manifested above all in the low temperature range, in particular below 400 ° C., and especially in the range from 180 to 400 ° C. This is of particular advantage, since the optimal removal of nitrogen oxides and the avoidance of the formation of nitrous oxide are particularly problematic, especially at low temperatures. These effects are particularly pronounced in NO ₂ -rich exhaust gas, that is, at NO ₂ : NO x> 0.5. The catalyst 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 layer. The catalyst preferably contains from 0.5 to 15% by weight, in particular from 1 to 7% by weight, of silicon dioxide, calculated as S1O2 and based on the weight of the first layer. 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 layer. The catalyst preferably contains from 0.2 to 10% by weight, in particular from 0.5 to 5% by weight, or from 0.5 to 3% by weight of cerium oxide, calculated as CeO 2 and based on the weight of the first layer. 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 layer 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 to 17 wt% tungsten oxide, calculated as WO3,

 (c) from 0 to 10% by weight of cerium oxide, calculated as CeC> 2,

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

 (e) from 0.5 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 layer.

Most preferably, the first layer contains vanadium dioxide, ceria, titania and silica, but no tungsten oxide. Also particularly preferably, the first layer contains 0.5 to 10 wt.% Vanadium oxide; 2 to 17% by weight of tungsten oxide; 0 to 7 wt.% Cerium oxide, and 25 to 98 wt.% Titanium dioxide, based on the weight of the first layer. Also particularly preferably, the first layer contains vanadium oxide, tungsten oxide, cerium oxide, titanium dioxide and silicon dioxide,

 In this case, the first layer preferably has the following composition:

 (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 0.2 to 5% by weight of cerium oxide, calculated as CeC> 2,

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

 (e) from 0.5 to 25% by weight of silica,

wherein the sum of the proportions of cerium oxide + tungsten oxide <17 wt., In each case based on the weight of the first layer.

The catalyst contains a second catalytic layer underlying the first layer of the vanadium catalyst. This second layer contains 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 from S1O4 AIO4 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 made according to their Topology divided into different structures. 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 in the second, lower layer 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 inventively preferred S1O2 / Al2O3 ratio is in the range of 5: 1 to 50: 1 and particularly preferably in the range of 10: 1 to 30: 1.

The molecular sieve in the second layer contains iron and is preferably an iron-containing zeolite. It has been found that iron-containing zeolites in combination with the first layer containing vanadium oxide catalyze a particularly efficient SCR in the layer arrangement according to the invention. Preference is given to using a zeolite which is exchanged with iron ions ("iron-exchanged zeolite") or in which at least a part of the aluminum atoms of the aluminosilicate framework is isomorphously substituted by iron and thus forms a ferrosilicate framework.

Particularly preferably, the zeolite is exchanged with iron. The zeolite is preferably one of the type BEA. Very particular preference is given to an iron-exchanged zeolite of the BEA type with an SAR 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 layer 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.

Instead of an iron-containing zeolite, the second layer may also contain a ferrosilicate or ferrosilicates. The second, lower layer can contain, in addition to the iron-containing molecular sieve, 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 layer is, for example, up to 15%.

The thickness of the first and second layers and the amount of the catalysts in the first and second layers are matched to one another to obtain a desired removal of nitrogen oxides. In a preferred embodiment, the catalyst device additionally comprises a carrier in addition to the first and second coatings. The carrier is a device to which the catalytic coatings are applied. 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 which find particular use 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 particular a wall-flow filter, in which the exhaust gases on the inlet side flow into outlet-closed channels, flow through the channel walls, and close the carrier through inlet and outlet sides Leave the channels open on the outlet side.

In a further embodiment, the second, lower layer simultaneously serves as a carrier. This is especially possible when the second, lower catalytic layer as extrudate provided. In this case, the second, lower layer itself forms a carrier device, on which the first, upper layer is applied. 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.

In a further preferred embodiment, the second layer is applied directly to the carrier. Alternatively, at least one further functional layer, for example a further catalytic layer, which has other catalyst compounds and thereby introduces a further activity, may be present between the support and the second layer.

Preferably, the entire interior of the catalyst device according to the invention is coated. This means that all areas that contact the exhaust gases are coated. It is preferred that the entire interior is completely coated with the first and second layers. In this embodiment, a particularly efficient conversion of nitrogen oxides is obtained because the entire inner surface of the device is used catalytically.

Generally, it is highly preferred that the exhaust gases first contact the first upper layer having the vanadium catalyst in the flow direction. Without wishing to be bound by theory, it is believed that an off-gas fraction pretreated with the vanadium catalyst then advantageously reacts with the iron-containing zeolite to achieve particularly efficient depletion of nitrogen oxides while avoiding the formation of nitrous oxide becomes. In particular, it has been found that the arrangement of the catalysts in this order leads to a particularly efficient purification of NOx, even at relatively low temperatures, and to avoid the formation of nitrous oxide, especially at high levels of NO2 in NOx.

In a preferred embodiment, the second layer is completely applied to the carrier. In this case, the first layer can be present completely or only in regions (in one or more zones) on the second layer. In a preferred embodiment, the first layer is completely applied to the second layer. This means that there is no region on the inner surface of the catalyst device in which the lower second layer directly contacts the exhaust gases. It is advantageous that only exhaust gases that have been pretreated in the first, upper layer, get into the lower second layer.

In a further preferred embodiment, the first layer is applied in regions on the second layer. In this embodiment, there are regions in which the lower, second layer comes into direct contact with the exhaust gases. It is highly preferred that the exhaust gases first contact the first, upper layer. Thus, it is particularly preferred that in the flow direction first a region is present which has a first, upper layer. Also in this embodiment, it is ensured that the exhaust gases first come into contact with the upper layer containing the vanadium catalyst, and thereby pretreated before they reach the second layer.

In a preferred embodiment, the exhaust gases when leaving the catalyst device last contact the first, upper layer. Thus, a reaction with the vanadium oxide takes place last. Preferably, the second, lower layer is present at the outlet of the catalyst device below the first, upper layer. In such an embodiment of the device, the SCR is particularly efficient.

For the above reasons, it is generally particularly preferred that the interior of the catalyst device is completely equipped with the second, lower layer on which the first, upper layer is completely present.

In a further preferred embodiment, the first layer or the second layer consists 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. For example, the first upper layer may consist of an upper sublayer containing vanadium oxide and a lower sublayer containing vanadium oxide and, in addition, ceria. The lower, second layer can consist, for example, of a first and second partial layer containing iron-containing zeolites with different activities. In these embodiments advantageous that suitable properties can be combined by suitable selection and combinations of different catalysts of the same type, for example, to obtain a reactivity in a wide temperature range. In a further embodiment, the catalyst device has various regions (zones) which follow one another in the exhaust gas flow direction. Different catalysts of the first and / or second layer can be combined in different zones in order to obtain advantageous properties of the catalyst device. Thus, for example, it is conceivable that a zone with a first and second catalyst are present on the inlet side, which is particularly efficient at relatively high temperatures, while at the outlet side there is a zone having an activity optimum at a lower temperature. The catalyst device may have a plurality of successive zones, for example 2, 3, 4 or 5 zones. In a preferred embodiment, the catalyst device has no further layers apart from the first and the second layer, in particular no further catalytic layers, and above all no further layers which contain vanadium oxide or iron-containing zeolites. The catalyst device preferably has no noble metal. In particular, the first and second layers do not contain precious 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 application of coating suspensions (so-called "washcoats"), by coating in a dipping bath or by spray coating The lower layer can also be an extrudate Coatings with washcoats As is generally customary, washcoats are coating suspensions in which the solids or precursor compounds are suspended and / or dissolved in order to produce the catalytic layers Such washcoats are provided in very homogeneous form with finely divided ingredients, so that the carriers are as possible evenly coated can. After application of the washcoats follow usual post-treatment steps, such as drying, calcination and annealing.

In a preferred embodiment, in the catalyst device, the ratio of the weight (per catalyst volume) of the first to the second layer is greater than 0.2, in particular between 0.2 and 15, and particularly preferably between 1 and 6.

The total amount of the coating 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. Depending on various influencing factors (eg engine calibration, operating condition, type and design of upstream catalytic converters), the NO 2 content in NOx can 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, inter alia, to increase the exhaust gas of lean-burn engines, especially diesel engines, the proportion of NO2.

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 oxidizing agent oxygen impedes the reductive removal 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 be carried out at relatively low temperatures, in particular at lower temperatures Temperature the formation of nitrous oxide can be avoided. 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 for preventing the formation of nitrous oxide (N2O), in particular in the purification of NC> 2-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. It was therefore surprising that the inventive arrangement of the first and second layer is an efficient SCR and at the same time only relatively small amounts of nitrous oxide arise. 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 in the method with further Combined exhaust gas purification equipment in series or in parallel, such as other 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 cleaning exhaust gases in a wide temperature range. It is 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. The effect according to the invention can even be improved if cerium oxide is added to the vanadium catalyst, as a result of which, inter alia, a further reduction of nitrous oxide, in particular at low temperature, can be achieved. 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.

FIGS. 1 to 5 show in graphical form results of the SCR reaction according to exemplary embodiment 3 with model exhaust gases with catalyst devices according to the invention and comparison devices which were produced according to example 2. In all figures measured values are given at different temperatures of the model exhaust gases. FIGS. A and b each show the proportion of NOx removed from the model exhaust gas by the catalyst device. In the figures c is in each case indicated, which concentration N2O was measured after the catalyst device.

Figs. 6 and 7 show the dependence of the conversion of the content of the exhaust gas to NO 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 as the tungsten or vanadium precursor in an amount such that a catalyst of the composition is 87.4% by weight TiO ₂ , 4.6% by weight SiO ₂ , 5.0% by weight WO 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% Ceria)

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 SCR catalyst based on iron-exchanged beta zeolite. 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 SCR catalyst based on iron-exchanged beta zeolite. 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 10 in Suspended water, so a catalyst of the composition results in 90% by weight of β-zeolite and an iron content, calculated as Fe 2 C> 3, of 4.5% by weight.

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 (flow substrates). 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 VK1 and VK3 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. applied from the outlet side to the length of the area Z2S1 according to 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 range Z1 S2 with one of the suspensions A - 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-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 zones Z1 S2 and Z2S2, one of the dispersions A - D was applied to the entire length of the component of 76.2 mm by 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, calcining and heat treatment and the length of the zones in% relative to the total length of the catalyst device are indicated in each case. The catalysts VK1 to VK4 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 entire length of the carrier of length Z2 (range Z2S1), dry at 120 ° C, then at 15 minutes 350 ° C, then calcined at 600 ° C for a period of 3 hours. If desired, then apply one of the dispersions A to D over the entire length of the component thus obtained (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 SCR Measurement Method 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 and TP2 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 . 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): U NOx [%] - (1 - Xoutput (NOx) / Xput (NOx)) ^* 100 [%]

With

XInput (NOx) = XInput (NO) + XInput (N0 ₂ )

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 measurement series TP1, in which the model exhaust gas as nitrogen oxide contained NO only, are summarized in Table 2. The results of the test series TP2, in which the model exhaust gas as nitrogen oxides contained 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. In FIGS. 1 to 5, results for comparison are also shown graphically. Tables 4 and 5 summarize the conditions and results of the tests with catalyst devices after aging. 1 a, b show that the catalysts K1 and K2 according to the invention, in which a first top layer with a vanadium catalyst is located above a second, lower layer with an iron-exchanged zeolite, remove nitrogen oxides much more efficiently than a comparative catalyst VK1 a layer of iron-exchanged zeolite over a vanadium oxide layer. The effect is particularly pronounced at temperatures below about 400 ° C. The effect is also obtained for a model exhaust with a high NO2 content of 66.7% (Figure 1 b). In Fig. 1 c it is shown that with the inventive catalysts K1 and K2 compared to catalyst VK1 significantly less N2O is formed. This effect is particularly pronounced at temperatures below 400 ° C.

The catalysts K1 and K2 differ in that the catalyst K2 in addition to the vanadium oxide additionally contains a cerium oxide in the first, upper layer. The results show that, especially in the temperature range below 350 ° C by adding a cerium oxide, a further improvement is achieved, both the removal of NOx further improved and the formation of N2O is further reduced.

FIG. 2 graphically shows further comparisons of the conventional catalyst VK1 with catalysts K1, K3, K4 and K5 according to the invention, the catalysts according to the invention differing in the amount of the catalysts used in the upper and lower layers. The results show that a significant effect is achieved even with considerable variation of the amounts of catalyst. Catalyst K1 shows the most significant effect in preventing the formation of N2O (Figure 2c).

FIG. 3 shows a comparison of the conventional catalyst VK1 with catalysts K3 and K6 according to the invention. FIG. 3 also shows that the catalysts according to the invention remove NO.sub.x much more efficiently, while significantly reducing the formation of N.sub.2O.

FIG. 4 shows results for catalyst devices subjected to an artificial aging process as described above. 4 shows a comparison of the conventional catalyst VK1 with the catalysts K1 and K2 according to the invention. The results show that even after aging the catalyst according to the invention significantly depletes NOx more efficiently and reduces the formation of N 2 O than the catalyst Comparative Catalyst. The experiments also show that especially after aging, the advantages of cerium oxide in the vanadium catalyst are particularly pronounced. Both in the depletion of NOx and in the prevention of nitrous oxide shows a more significant improvement of the catalyst K2 compared to the comparative catalyst, but also with respect to the catalyst K1 with the addition of cerium oxide.

FIG. 5 shows further results which were obtained with catalysts after the aging process, for the catalysts K1, K4 and K5 according to the invention and comparative catalyst VK1. Also Fig. 5 shows after aging a significant improvement of the catalysts of the invention in terms of NOx depletion and prevention of nitrous oxide formation.

In the case of comparative catalyst VK3, the two catalysts are not superimposed in layers, but the exhaust gases first enter the inlet side into a zone containing the iron-containing zeolite and then enter an outlet-side zone with the vanadium catalyst. The results summarized in Tables 2 and 3 show that such a comparative catalyst not only exhibits comparatively low NOx conversion with both NO and NC> 2 rich exhaust gases, but causes higher nitrous oxide generation.

Overall, the experiments show that the inventive SCR catalysts in which a vanadium oxide layer is disposed on an iron-containing zeolite layer, achieve significant improvements in the removal of NOx and the prevention of the formation of nitrous oxide. The catalyst devices according to the invention are suitable not only for the reaction with NO-rich exhaust gases, but also for the treatment of NO2-rich exhaust gases. The advantages with NO2-rich exhaust gases are particularly pronounced in the temperature range below 450 ° C or below 350 ° C. The catalyst devices according to the invention thus combine several advantageous properties with each other, namely a high efficiency with NO2 rich exhaust gases in the temperature range of about 180 ° C to 500 ° C, and in particular at low temperatures, high efficiency with NO rich exhaust gases, and avoiding the formation from nitrous oxide. The effects are evident both with freshly prepared catalysts and after an aging process. The effect can even be improved if a ceria is added to the vanadium catalyst, which has particular advantages in aged catalysts. Table 2: Conditions and results of the reduction of NO with various catalyst devices (TP1 test) at different temperatures actually measured 1 to 8. Indicated is the depletion of NO _x at Katalysatorvorrichtungsauslass in% with respect to the initial amount used.

Table 3: Test conditions and results of the reduction of NO 2: NO in a ratio of 3: 1 (experiment TP2) at various 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 Different Catalyst Devices After Aging (Run TP1) at Various Actually Measured Temperatures 1-8. Specified is the depletion of NO _x at the catalyst device outlet in% relative to the initially used amount.

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, relative to the initially used amount, and the measurements of 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 layers, wherein the first layer contains vanadium oxide and a mixed oxide comprising titanium oxide and silica and the second layer comprises iron-containing molecular sieve, the first layer on the second layer is applied.

2. The catalyst device according to claim 1, wherein the first layer contains at least one further component selected from oxides of tungsten and

Aluminum.

3. Catalyst device according to at least one of the preceding claims, wherein the first layer additionally contains cerium oxide.

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

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

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

 (c) from 0 to 10% by weight of cerium oxide, calculated as CeC> 2,

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

 (e) from 0.5 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 layer.

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

A catalyst device according to claim 5, wherein the zeolite is an iron-exchanged zeolite.

7. A catalyst device according to any one of claims 5 or 6, wherein the zeolite has a structure whose maximum ring size has more than 8 tetrahedrons.

The catalyst device of at least one of claims 5 to 7, 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.

The catalyst device according to any one of claims 1 to 8, wherein the first layer is fully coated on the second layer.

10. Catalyst device according to at least one of claims 1 to 8, wherein the first layer is applied in regions on the second layer.

1 1. Catalyst device according to at least one of the preceding claims, wherein the second layer is applied to an inert support, which is preferably a ceramic monolith.

12. 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).

13. Use according to claim 12 for preventing the formation of nitrous oxide.

14. Use according to claim 12 and / or 13, wherein the exhaust gases have a ratio NO2 NOX of> 0.5 and / or a temperature of 180 ° C to 450 ° C.

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

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

 (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).