WO2012034939A1

WO2012034939A1 - Catalyst for use in selective nox reduction

Info

Publication number: WO2012034939A1
Application number: PCT/EP2011/065631
Authority: WO
Inventors: Sven Kureti; Florian Schott; Melanie Leicht
Original assignee: Karlsruher Institut für Technologie
Priority date: 2010-09-15
Filing date: 2011-09-09
Publication date: 2012-03-22
Also published as: DE102010040808A1

Abstract

The invention relates to a catalyst for use in selective NO_X reduction in exhaust gases containing NO_X with hydrogen being supplied to the exhaust gases, wherein said catalyst comprises a basic body, a substrate made of zirconium oxide applied to the basic body, a promoter made of tungsten oxide applied to the substrate, and an active component applied to the substrate. The active component is made of palladium.

Description

Catalyst for use in selective NO _x reduction

The invention relates to a catalyst for use in a selective NO _x reduction in NO _x -containing exhaust gases while supplying hydrogen to the exhaust gases in the preamble of claim 1 in more detail defined. Furthermore, the invention relates to a method for producing a catalyst for selective NO _x reduction in NO _x -containing exhaust gases according to the preamble of claim 5 and a device for selective NO _x reduction in NO _x -containing exhaust gases and a method for selective NO _x reduction in NO _x -containing exhaust gases.

Due to the increasingly stringent emissions regulations, it will be necessary in the future, in particular in the case of motor vehicles powered by diesel engines, but also in industrial plants, where NO _x - containing exhaust gases are produced, to treat the exhaust gas to remove nitrogen oxides. Under the trade name of the nitrogen oxides are different oxidation states of nitrogen such as nitric oxide (NO) and nitrogen dioxide ^¬ (NO _2), summarized. The gases are very toxic to humans. In addition to sulfur dioxide, the nitrogen oxides contribute to acid rain, as formed by the reaction of nitrogen dioxide with water nitric acid and nitrous säu ^¬ re. This acid rain is partly responsible for the forest dying caused by the washing out of nutrients in the soil and for damage to buildings with acid-sensitive chen building materials. Furthermore, nitrogen oxides play a central role in the formation of ozone in layers close to the ground. Ozone can cause irritation and inflammation of the respiratory tract in humans and acts as a cytotoxin in plants. In the stratosphere, on the other hand, ozone is destroyed by nitrogen monoxide, which contributes to the formation of the ozone hole. The emission of nitrogen oxides can both natuer ^¬ union origin, for example from microbial processes and vegetation fires and anthropogenic origin, such as from power plants, industry, small furnaces in households and traffic, be.

In order to achieve a reduction of these nitrogen oxides, WO 2007/020035 A1 discloses a generic catalyst, a corresponding production method for the same and an apparatus and a method for selective NO _x reduction in NO _x -containing exhaust gases.

However, with the catalyst described there, it is still not possible to achieve any selective conversion of the nitrogen oxides which is sufficient for practice. In particular, a relatively large proportion of nitrous oxide, wherein the performed method therein is generated which is very disadvantageous as nitrous oxide approx more contributes 310 times to the greenhouse effect as Example ^¬ as CO ₂ · Another disadvantage of the described therein Kata ^¬ lysators is that the zirconia be ^¬ standing substrate only the tetragonal Kristallmodifi ^¬ cation can be used, which requires a very large manufacturing effort. Also, the platinum used for the active component provides a relatively large Cost factor and increases the overall cost of the catalyst significantly.

EP 1 475 149 A1 discloses a catalyst for the reduction of NO _x to N ₂ with hydrogen under O ₂ -rich conditions. This known catalyst is based on platinum, which is distributed in an amount between 0.1 and 2 percent by weight on a carrier material consisting of magnesium or cerium oxide or a precursor thereof. Although quite good results in the NO _x reduction are already achieved with this catalyst, with future pollutant limit values, however, this catalyst could also reach its limits.

A process for removing nitrogen oxides from an exhaust gas stream is described in EP 0 666 099 B1. The catalyst used to be adsorbed ^¬-sensitive in the exhaust nitrogen oxides, after which a gas is supplied with a specific content of a reducing substance at predetermined time intervals and for certain periods of time the catalyst. Such storage catalysts, in which basic components, such as lithium oxide, potassium oxide, sodium, Bari ^¬ oxide or similar oxides are used, but ever ^¬ require a relatively complicated control and usually have a high regeneration needs.

In these NO _x storage catalysts, the mainly emitted NO on a platinum-containing catalyst is oxidized to NO ₂ , which is subsequently adsorbed to specific storage media, for example BaC0 ₃ . When the Speicherkapa ^¬ capacity of this catalyst is exhausted, a motorin- initiated reduced regeneration of the catalyst at wel ^¬ cher the nitrogen oxides are introduced into nitrogen over ^¬ leads. Another disadvantage of the known N0 _X storage catalysts is the risk of poisoning of the NO _x sorbents by the sulfur oxides SO ₂ and SO ₃ contained in the exhaust gas. To circumvent this problem, usually complex engine management strategies are required.

From EP 0960649 Bl an exhaust gas purifying catalyst is known in which the materials used, cerium oxide and / or zirconium mixed oxides having that, the ^¬ NEN, saturated hydrocarbons to entfer ^¬ nen from the exhaust gas. As a reducing agent for the nitrogen oxides contained in the exhaust gas, ammonia is used. The active component V ₂ O _{5 which} is frequently used in such SCR catalysts, however, is of toxicological concern and may also melt or vaporize at very high exhaust gas temperatures.

EP 0 763 380 A1 describes shell catalysts which consist of a core and at least one outer shell or of a support, at least one inner and at least one outer shell, the outer shells containing oxides of specific elements. Furthermore, a method for the catalytic removal of nitrogen oxides, carbon monoxide and hydrocarbons from the exhaust gases of internal combustion engines is described. Here, the exhaust gas has a temperature of 50 to 800 ° C and a pressure of 0.01 to 200 bar and it is again ammonia as ^{Reduktionsmit ¬} tel used. A disadvantage of the known solutions for NO _x removal from 0 ₂ -rich exhaust gases is in most cases also in that the nitrogen oxides are reacted we ^¬ kungsvoll only above 200 ° C. Because due to the con ^¬ tinuous optimization of the efficiency of internal combustion engines, the temperature of the exhaust gases is continuously reduced, resulting in the known solutions a big problem in terms of effectiveness. For example, in modern, diesel-powered internal combustion engines for passenger cars, the exhaust gas temperature in the relevant EU certification cycle is about 60% of the time below 150 ° C and about 75% of the time below 200 ° C.

DE 102 16 748 A1 describes modified catalysts for the selective hydrogenation of aromatic hydrocarbons having bulky or branched substituents. A NO _x reduction in NO _x -containing exhaust gases is not feasible with these catalysts.

In DE 44 36 890 AI a method for the simultaneous reduction of the contained in the exhaust gas of an internal combustion engine hydrocarbons, carbon monoxide and nitrogen oxides is described. The catalyst used in this case has an aluminum silicate as a high surface area support material.

It is an object of the present invention to provide a catalyst for use in selective NO _x reduction in NO _x -containing exhaust gases while supplying hydrogen to the exhaust gases and a process for producing the same, as well as an apparatus and method for selective NO _x reduction NO _x -containing exhaust gases with which nitrogen oxides can be reduced even at very low temperatures with high sales rates.

According to the invention, this object is achieved for the catalyst by the features mentioned in claim 1.

The inventors have found that, surprisingly, a very good conversion of nitrogen oxides can be achieved to nitrogen with the inserted for the active component palladium, although a combination of the carrier layer of zirconium oxide with the on the carrier layer placed on ^¬ promoter of tungsten oxide per se for platinum as active component was tailored. This achievable with palladium as the active component high selective reaction in conjunction with a very high activity is all the more surprising ^¬ , as the activity of the active component is very much dependent on the material of the carrier layer and the promoter used and the literature for this no reference can be found ,

A major advantage of the use of palladium as the active component of the catalyst according to the invention is that in contrast to a platinum as active component ^auf¬ pointing catalyst, the support layer of zirconium oxide does not have to be present in a particular crystal modification or crystal structure, but that any crystal modification used can be, which significantly reduces the manufacturing and cost compared to known solutions. The cost of the inventive ^¬ contemporary catalyst is thereby significantly reduced, that the raw material price of palladium is much lower than that of platinum.

Furthermore, it is advantageous that the inventive Ka ^¬ talysator already at very low temperatures shows a high activity so that it can be used without any problems even in future internal combustion engines in which much lower exhaust temperatures are expected. In particular, this property is also advantageous in the on ^¬ rate of the inventive catalyst in industrial investments, because there the air to be cleaned electricity it needs to be warmed ^¬ often in order to achieve _NOx reduction.

To the high activity at low temperatures in particular, the use of hydrogen as Redukti ^¬ onsmittel contributes, thereby effectively implemented in the inventive catalyst _NOx at temperatures above 120 ° C. The direct conversion of NO _x with H ₂ to N ₂ and H ₂ O avoids the disadvantages associated with NO _x storage catalysts. The use of hydrogen as Redukti ^¬ onsmittel is toxicologically completely harmless, since the reaction product is water.

In a very advantageous development of the invention it can be provided that the zirconium oxide of the carrier layer has a monoclinic crystal modification.

Alternatively, it can be provided that the zirconium oxide of the carrier layer exhibits a cubic crystal ^modification . Both of the mentioned crystal modification, monoclinic or cubic, are relatively easy to prepare and therefore bring considerable cost advantages over a tetragonal crystal modification, as required, for example, in WO 2007/020035 Al.

A particularly good selective reaction and a high Akti ^¬ vity of the catalyst could be observed when the mass ratio of the active component from palladium to the promoter of tungsten oxide 1: 150 to 1:10, especially 1: 100 to 1:25, is.

A process for the preparation of the catalyst according to the invention results from the features of claim 5.

With the aid of this process, the catalyst according to the invention can be produced particularly easily.

Claim 8 results in a device for the selective reduction of NO _x in NO _x -containing exhaust gases with a catalyst arranged in an exhaust gas line according to the invention.

The device according to the invention enables an optimal use of the catalyst for selective NO _x reduction of exhaust gases, wherein the hydrogen can be introduced in a simple manner by means of egg ^¬ ner hydrogen supply into the exhaust pipe. By combining the use of zirconia for the carrier layer, tungsten oxide for the promoter and palladium for the active component, a very high DeNO _x activity and a very high selectivity can be achieved in comparison to known solutions, to which especially the supply of hydrogen contributes to the exhaust gases. The use of tungsten oxide as Promo ^¬ ter, which increases both the activity of the active component of palladium and the selectivity in the reduction of NO _x to N ₂ , is particularly advantageous, since this is a toxicologically harmless substance that neither prepares in the production even with a possible later disposal problems.

A method for selective NO _x reduction in NO _x -containing exhaust gases with such a device is given in claim 9. Using this method, the above ^¬ he läuterten advantages of the device according to the invention are particularly well used, resulting in a very high conversion of NO _x. Moreover, the process can be carried out with simple means and leads to a very high N ₂ selectivity.

This method can be used particularly well, the temperature of the exhaust gases is 120-200 ° C.

Further advantageous embodiments and modifications of the invention will become apparent from the remaining dependent claims. Embodiments of the invention are shown in principle with reference to the drawings.

It shows:

1 is a schematic representation of a first embodiment of the structure of the catalyst of the device according to the invention; Figure 2 is a schematic representation of a second embodiment of the structure of the catalyst of OF INVENTION ^¬ to the invention apparatus.

Fig. 3 is a schematic representation of an imple mentation of the catalyst as a pellet;

Fig. 4 is a schematic representation of an imple mentation of the catalyst as a honeycomb body;

Figure 5 is a schematic representation of an imple mentation of the catalyst as an extrudate.

Fig. 6 is a schematic representation of the invention

Apparatus for selective NO _x reduction in an internal combustion engine;

Fig. 7 is a schematic representation of the invention

Apparatus for selective NO _x reduction in an industrial application;

Fig. 8 is a graph showing the conversion and the selectivity of a first composition of the catalyst according to the invention; and

9 is a graph showing the conversion and selectivity of a second composition of the catalyst of the present invention. Fig. 1 shows in a very schematic, in particular not to scale, and greatly enlarged representation of a first embodiment of a catalyst 1, which for selective NO _x reduction in the exhaust gases of a shown in Fig. 6, preferably operating on the diesel principle Internal combustion engine 2 is used. The catalyst 1 has a base body 3 which generates the same rigidity and which may be formed, for example, as a honeycomb body and may consist of a ceramic material. The main body 3 can also be designed as a soot filter to filter out any soot particles contained in the exhaust gases. Alternatively, the base body 3 may be formed, for example, from a metal substrate.

On the base body 3, a carrier layer 4 is applied, which consists of zirconium oxide (Zr0 ₂ ). The carrier layer 4 is preferably extremely porous, may consist of a hydrophilic material and has a surface area of at least 50 m ² / g, preferably of at least 100 m ² / g, of the carrier layer 4. The zirconium oxide may have a monoclinic or a cubic, but optionally also a tetragonal, crystal modification.

On the side facing away from the base body 3 of the carrier layer 4, the same is provided with a coating 5, which has a promoter 5a and an active component 5b on ^¬ . The active component 5b is palladium, and the promoter 5a is tungsten oxide. The coating 5 of the carrier layer 4 is constructed so that at the base ^¬ body 3 facing away, so on the outer surface of the la Catalyst 1 both palladium and tungsten oxide vorlie ^¬ conditions.

In the embodiment of the catalyst 1 illustrated in FIG. 1, the active component 5b and the promoter 5a are applied together to the carrier layer 4 for the production thereof, which results in a certain mixing of the active component 5b and the promoter 5a, ie Palladium is in the form of single particles in the tungsten oxide. The tungsten oxide and the palladium are present as tungsten salt or palladium salt in an aqueous solution, which is applied to the carrier layer 4 and is ^subsequently dried. In addition, a chemical or electrochemical deposition is also suitable for the preparation of the catalyst 1.

In the disclosed embodiment the catalyst 1 shown in FIG. 2, the promoter 5a and then the active compo nent ^¬ 5b is initially applied to the carrier layer 4. The tungsten oxide is present as tungsten salt in an aqueous solution which is applied to the carrier layer 4. Specifically, a solution consisting of 0.776 g of ammonium metatungstate (Fluka, W content: 67%) and 0.32 g of distilled water can be prepared and applied per gram of zirconium oxide forming the carrier layer 4. After impregnation of the carrier with the ammonium metatungstate solution, it is dried at 100.degree. C. for 5 hours. After drying of the support layer 4, Ge ^¬ optionally followed by calcination and reduction in a reducing agent containing gas stream, which is also present in an aqueous solution of palladium salt is applied to the coated carrier with the tungsten oxide layer 4 and then dried again. To activate the palladium component, the substance is heated, for example, at a heating rate of 100 K / h in a volume flow (500 ml / min consisting of 10 vol .-% hydrogen and 90 vol .-% nitrogen to 300 ° C and 30 min at this temperature maintained. in this case, the palladium salt is simply reacted with the metal. tungsten oxide and zirconium, however, are not reduced. Subsequently, the material is 5 calcined at 500 ° C in stati ^¬ shear atmosphere, wherein the heating rate in turn is 100 K / min. This final conditioning The purpose of the catalyst is to expose the catalytic converter to typical exhaust gas temperatures, which can be used to exclude temperature-dependent catalyst changes and associated artefacts in the subsequent activity studies.The nitrates used are completely converted into the corresponding oxides during NO _x -leaching. 2 illustrated procedure therefore requires z white drying steps, whereas in the preparation of the catalyst 1 shown in FIG. 1, only one drying step is required.

In Figures 3, 4 and 5, various forms of the catalyst 1 are shown, in addition to which, however, other forms from management forms are conceivable.

Thus, Fig. 3 shows an imple mentation of the catalyst 1 in the form of a pellet. Such pellets, which can be formed as shown ku ^¬ gel, but also rod-shaped or in a similar form, can be advantageously used in a suitable number, especially in stationary applications, since there the inflow surface of the catalyst 1 annä- can be chosen freely. The size of the pellets may vary depending on the application. In this embodiment, the carrier layer 4 can assume the function of the main body 3, ie it is possible to dispense with the actual main body 3, since this is formed by the carrier layer 4. This can make it possible to introduce a greater proportion of the active component 5b in a volumenmä ^¬ SSIG equal Catalyst 1, whereby higher nitrogen oxide conversions can be achieved.

In Fig. 4, the catalyst 1 is shown in an already mentioned above form of a honeycomb body 17, which can be advantageously used in particular in exhaust gases with high flow rates, since they produce a very low exhaust back pressure. In the honeycomb body 17 is a so-called wash coat 18 is applied, may be resorted to the application in principle to most ^¬ te techniques. In a first embodiment, the washcoat 18 can only be the zirconium oxide and, in this case, tungsten oxide and palladium are applied to the washcoat 18. Furthermore, it is possible that the washcoat 18 consists of zirconium oxide and tungsten oxide and palladium is applied to the washcoat 18. A further possibility is that the washcoat 18 consists of Zirkoni ^¬ oxide, tungsten oxide and palladium, which is then preferably applied as a powder before ^¬. The washcoat 18 can therefore be constructed as shown in FIG. 1 or according to FIG. 2, or as a powder. For applying the washcoat 18 in addition known binders can be used.

Finally, in FIG. 5, the catalyst 1 is represented as an extrudate 19, that is to say as a product produced by extrusion. provides. In this case, the carrier layer 4 again assumes the function of the main body 3, ie it can be dispensed with as in the embodiment of Fig. 3 on the actual base body 3. In principle, the waiver is on the ground ^¬ body 3 even when all other forms of leadership from possible if it is possible that the supporting layer 4 assumes the radio ^¬ tion of the body. 3 The extrudate consists fully ^¬ constantly of the "finished" catalyst material, the Grundkör ^¬ per 3 or the material of the support layer 4 is thus the Ka ^¬ talysatorpulver itself of the extrudate 19 may additionally be used per se known binders.

Fig. 6 shows a device 6 for selective NO _x reduction in the exhaust gases of the internal combustion engine 2, with which it is possible to perform a method for selective NO _x reduction in the exhaust gases of the internal combustion engine 2, wherein the exhaust gases hydrogen is supplied as a reducing agent , By supplying hydrogen to the exhaust gases of a direct conversion of NO results at the process described in Figures 1 to 5 in various embodiments, catalyst 1 _x with H ₂ to N ₂ and H ₂ O. In order to be able to exhaust directly re ^¬ duce , the catalyst 1 is in an outgoing from the internal combustion engine 2 exhaust pipe 7 is arranged ^¬ . The device 6 further comprises a hydrogen supply device 8, which has a container 9, for example in the form of a printing cylinder, which is provided with a feed opening 10. To the feed opening 10, a supply line 11 is connected, which leads to the exhaust pipe 7. The feed opening 10 is provided with a Provided closure 12, the opening state by means of a control device 13 can be changed. The control device 13 is in turn connected to a arranged in the exhaust pipe 7 NO _x sensor 14 and a sensor 15, which is arranged in a leading to the internal combustion engine 2 on ^¬ suction line 16. The sensor 15 is formed in the present case as known per se air mass sensor and serves to measure the flowing to the internal combustion engine 2 air mass flow. Alternatively, the sensor 15 could also be arranged in the exhaust pipe 7. Furthermore, it would be possible to provide a temperature sensor in the exhaust pipe 7. Optionally, one of the two sensors 14 or 15 could be dispensed with.

By the two sensors 14 and 15, it is possible to determine a certain state in the exhaust pipe 7 or in the intake pipe 16 and forward it to the control device 13. The control device 13 is thereby able to direct the hydrogen is present in the container 9 in dependence of the NO _x concentration in the exhaust gas line 7, or a function of an air mass flow through the Ver ^¬ internal-combustion engine 2 in the exhaust pipe. 7 In this way a larger quantity of hydrogen in the exhaust gas line 7 into ^¬ passes may be, for example, at an acceleration ^¬ process in which a higher NO _x emissions to be expected. It is also possible to deposit the required data in a map in the control device 13 in order to supply an increased amount of hydrogen at certain times. Of course, the hydrogen can also be passed continuously into the exhaust pipe 7. Another possibility is a specific one To conduct basic amount of hydrogen in the 7Abgasleitung 7 and the introduced amount of hydrogen to be raised stabili ^¬ hen in an established by one of the sensors 14 or 15 needed.

In order to make an additional refueling of hydrogen superfluous, the hydrogen supply device 8 may be connected to a device, not shown, for reforming hydrocarbon or derivatives to hydrogen from the fuel provided for the internal combustion engine 2, ie a reformer or the like. The necessary hydrogen may also be prepared by means of electrolysis ei ^¬ ner suitable substance, for example water. These two possibilities are summarized herein by the term "H2 generator." In such a case, the container 9 can either be omitted or replaced by a smaller intermediate storage.

If the main body 3 is not simultaneously designed as a soot filter, as described briefly above, so in the flow direction of the exhaust gas in the 7Abgasleitung 7 an unillustrated particulate filter upstream or downstream of the catalyst 1. The main body 3 thus serves only for holding the carrier layer 4.

In Fig. 7, the device 6 in an industrial application, ie for selective NO _x reduction in the exhaust gases of an only schematically indicated industrial plant 17, for example, a power plant shown. The device 6, with the exception of the omission of the sensor 15 and the suction line 16 at ^¬ similar to that shown in FIG. 6 trained det, works in contrast to the application in the internal combustion engine 2 in a stationary operation. The most important difference to the transient operation of the catalyst 1 in the internal combustion engine 2 is that in stationary operation a much faster equilibrium is established, resulting in very good working conditions for the catalyst 1.

In the following table some exemplary are ^¬ indicated sammensetzungen of catalyst 1:

In principle, the palladium content between 0.1 - 1 Mas ^¬ transmitter%, especially between 0.35 - are 0.55% by mass, wherein a lower palladium content tends to require a higher exhaust gas temperature, the cost of the respective catalyst 1, however, depending on the size the same partially significantly reduced. Furthermore, a mass ratio of the active component 5b of palladium to the promoter 5a of tungsten oxide of 1: 150 to 1:10, in particular 1: 100 to 1:25 has proven to be particularly advantageous. In the present case, the mass% data of tungsten and palladium is calculated to be 100% of the support material 4, ie it results with the carrier material 4 total more than 100%. The width ^¬ ren should be noted that the aforementioned 8.7% by mass of tungsten corresponds to approximately 11% by mass of tungsten oxide.

The temperature-programmed reaction (TPR) method was used to determine the activity or selectivity ^{behavior of} the various catalyst materials. To carry out the measurements, the catalyst mass was first pressed at 5 bar and then brought to a particle size of 125-250 μm. With the aid of two geeig ^¬ neter sieves particles, the μπι greater than 250 and less than 125 were μπι, sorted out. Subsequently, 500 mg of the material was baked for 15 minutes at 500 ° C in an argon stream (500 ml / min) to adsorb adsorbed species such. As water, hydrocarbons or carbon dioxide to remove.

This ensured reproducible experimental conditions. Subsequently, synthetic diesel model exhaust gas, the composition of which was based on typical values of a real exhaust gas, and hydrogen were added. To ensure li ^¬-linear temperature variation, the measurement with a cooling ramp of 2 K / min in the range between 50 ° C and 500 ° C was performed. The measurement conditions, which can vary in ge ^¬ know, indicated in brackets, are listed in the following table:

Temperature range 50 - 500 ° C

The experiments with the catalyst compositions given above led to the following result:

When increasing the Pd loading from 0.14 to 0.27 mass% on the catalyst system Pd / 8, 7W / Zr02 the integral N ₂ - selectivity increases dramatically. However, as the Pd loading is further increased to 0.82 mass%, there is no longer any significant change in nitrogen selectivity in the H ₂ -deNO _x reaction. While at a Pd loading of 0.55% by mass of palladium, the maximum possible NO (62%) is achieved _x conversion, the integral NO _x takes -Sales with increasing Pd loading. At 0.41 mass% palladium, the maximum of the integral NO _x conversion is achieved. In view of the reproducibility of the measurements indicate the Katalysato ^¬ ren with Pd loadings of 0.27; 0.41; 0.55; and 0.68 very similar X (NO _x ) i _n t and S (N ₂ ) i _n t. In the following ^table , the most important criteria for assessing the individual catalysts are summarized with regard to their activity or selectivity behavior. Optimal conditions exist when the integral NO _x conversion and the integral N ₂ selectivity are as high as possible.

Catalyst loading Txmax X (NO _x ) _max X (NO _x ) int S (N ₂ ) int

Short name [Pd-eq] [_C] [%] [%] [%]

0, 14Pd / 8, 7W / Zr0 ₂ 1 180 59 40 86 0, 27Pd / 8, 7W / Zr0 ₂ 2 155 53 36 95

0, 41Pd / 8, 7W / Zr0 ₂ 3 160 55 38 95

0, 55Pd / 8, 7W / Zr0 ₂ 4 150 62 34 97

0, 68Pd / 8, 7W / Zr0 ₂ 5 150 56 34 95

0, 82Pd / 8, 7W / Zr0 ₂ 6 150 43 30 94

For the catalyst composition with the catalyst abbreviation 0, 41Pd / 8, 7W / Zr0 ₂ , respective graphs are shown in FIGS. 8 and 9, in which the conversion and the selectivity are plotted against the exhaust gas temperature. In FIG. 8, the NO _x conversion X and the N ₂ and N ₂ O selectivity S in the temperature-programmed H ₂ -deNO _x reaction under standard reaction conditions are given in the table above and in FIG NO _x conversion X and the N ₂ - and N ₂ 0 selectivity S in the stationary H ₂ -deNO _x - reaction under standard reaction conditions as shown in the table above.

Comparing the TPR examination according to FIG. 8 with the stationary measurement according to FIG. 9, it can be seen that the NO _x conversion and the integral NO _x conversion are significantly higher for stationary reaction control. In terms of measurement accuracy, however, the deviations in terms of integral N ₂ selectivity and N ₂ selectivity in the NO _x conversion maximum are only marginal. This result shows that the tempera ^¬ turprogrammierte reaction with a ramp of 2 K / min is too fast so that the stationary conditions are not given in full.

Claims

Patent claims

1. A catalyst for use in a selective NO _x - reduction in NO _x -containing exhaust gases while supplying hydrogen to the exhaust gases, comprising a base body, a carrier applied to the base body of zirconium oxide, an applied on the support layer promoter of tungsten oxide and on the carrier layer applied active component,

d a d u r c h e c e n c i n e s that

the active component (5b) consists of palladium.

2. Catalyst according to claim 1,

d a d u r c h e c e n c i n e s that

the zirconium oxide of the carrier layer (4) is a monoclinic

Having crystal modification.

3. Catalyst according to claim 1,

d a d u r c h e c e n c i n e s that

the zirconia of the carrier layer (4) is cubic

Having crystal modification.

4. Catalyst according to claim 1, 2 or 3,

d a d u r c h e c e n c i n e s that

the mass ratio of the active component (5b) of Pal ^¬ ladium to the promoter (5a) of tungsten oxide 1: 150 to 1:10, especially 1: 100 to 1:25, is.

5. A process for preparing a catalyst for selective NO _x reduction in NO _x -containing exhaust gases after a of claims 1 to 4, wherein on a base body a

Carrier layer of zirconium oxide is applied, wherein a promoter of tungsten oxide and an active component is applied to the carrier layer,

d a d u r c h e c e n c i n e s that

as the active component (5b) palladium is used.

6. The method according to claim 5,

d a d u r c h e c e n c i n e s that

for the carrier layer (4) zirconium oxide having a monoclonal crystal modification is used.

7. The method according to claim 5,

d a d u r c h e c e n c i n e s that

for the support layer (4), zirconium oxide is used with a kubi ^¬ rule crystal modification.

8. Device (6) for the selective reduction of NO _x in NO _x -containing exhaust gases, with a in an exhaust line (7) arranged catalyst (1) according to one of claims 1 to 4, and with a hydrogen supply means (8) for introducing Hydrogen in the exhaust pipe (7).

9. A method for selective NO _x reduction in NO _x -containing exhaust gases by means of a device according to claim 8, wherein in the exhaust gas line (7), which is traversed by the exhaust gases, hydrogen is introduced.

10. The method according to claim 9,

d a d u r c h e c e n c i n e s that

the temperature of the exhaust gases is 120-200 ° C.