EP1137486A2

EP1137486A2 - Catalyst body and method for the degradation of nitrogen oxides

Info

Publication number: EP1137486A2
Application number: EP99963228A
Authority: EP
Inventors: Stefan Fischer; Frank Witzel; Günther PAJONK
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1998-11-25
Filing date: 1999-11-12
Publication date: 2001-10-04
Also published as: TW546165B; NO20012588L; NO20012588D0; JP2002530190A; WO2000030746A2; DE19854502A1; US6569394B2; KR20010080539A; WO2000030746A3; US20020004446A1

Abstract

A gas stream (2) containing nitrogen oxides is fed via a catalyst body (4) which comprises an active material in the presence of a reducing agent. Said active material comprises a zeolite and titanium dioxide. According to the invention, the zeolite is an acid zeolite with exchanged hydrogen ions. Even at temperatures higher than 450 °C, the nitrogen oxides contained in said gas stream are effectively transformed into nitrogen and water. In the examples, the active mass also contains tungsten trioxide.

Description

description

Catalyst body and process for breaking down nitrogen oxides

The invention relates to a catalyst body for breaking down nitrogen oxides in the presence of a reducing agent, with an active composition which comprises a zeolite and titanium dioxide. The invention further relates to a process for the decomposition of nitrogen oxides in a gas stream, a gas stream containing nitrogen oxides being passed over the catalyst body. In particular, the nitrogen oxides on the catalyst body are converted to molecular nitrogen and water using the reducing agent, even in the presence of oxygen, in accordance with the selective catalytic reduction process.

A catalyst body of the type mentioned is known from GB 2 193 655 A. The active composition of the catalyst body specified there comprises a titanium dioxide with a low specific surface area and a zeolite containing copper obtained by ion exchange. The zeolite has an average pore diameter of 10 Å or less and a molar ratio of silicon oxide to aluminum oxide of 10 or more. The stated catalyst body should have a high mechanical strength and a good resistance of its catalytic activity to volatile catalyst poisons such as arsenic, selenium or tellurium. Mordenite, ZSM-5 and Ferπerit are given as preferred zeolites.

Furthermore, EP 0 393 917 A2 discloses a catalyst body for breaking down nitrogen oxides, the active composition of which comprises a zeolite which contains copper and / or iron after ion exchange. The zeolite has a molar ratio of silicon oxide to aluminum oxide of at least 10 and a pore structure, channels m in all three spatial directions having a diameter of at least 7 A. The catalyst body is to decompose the nitrogen oxides in a temperature range from 250 to 600 ° C. USY (Ultra Stabilized Y), Beta and ZSM-20 are stated as preferred zeolites.

In contrast, conventional catalyst bodies with an active composition which contains titanium dioxide and admixtures of vanadium oxide, tungsten oxide and / or molybdenum oxide are only suitable for decomposing nitrogen oxides up to a temperature of approximately 450 ° C. Since exhaust gases from a combustion plant containing nitrogen oxides, such as a power plant fired with fossil fuel, a gas turbine or an internal combustion engine, often have temperatures of up to 500 ° C. and above, the catalyst body specified in EP 0 393 917 A2 offers a large size Advantage.

From US 5,271,913 A a catalyst body for the degradation of nitrogen oxides is known, the active composition of which comprises a zeolite. The zeolite is impregnated with cerium oxide or iron oxide. The catalyst body is suitable for breaking down the nitrogen oxides according to the selective catalytic reduction process in a temperature range from 500 to

700 ° C. Furthermore, the specified catalyst body has a high resistance to sulfurous components contained in the exhaust gas. A zeolite of the ZSM-5 type is specified as the preferred zeolite, the molar ratio of silicon oxide to aluminum oxide being 20 or more.

The object of the invention is to provide a catalyst body which, even in a temperature range from 400 to 750 ° C., is still suitable for breaking down nitrogen oxides in the presence of a reducing agent. For this purpose, the catalyst body should have both a sufficient mechanical and a sufficient catalytic stability. Furthermore, it is an object of the invention to provide a method for breaking down nitrogen oxides in a gas stream, with which an effective reduction in nitrogen oxides can be achieved even at gas temperatures between 400 and 750 ° C. The first-mentioned object is achieved according to the invention by a catalyst body having an active composition which comprises a zeolite and titanium dioxide in that the zeolite is an acidic zeolite exchanged with hydrogen ion.

An acidic zeolite exchanged for hydrogen ion is understood to mean a zeolite in which the exchangeable cations are predominantly exchanged for hydrogen ions. This can be done, for example, by thermal reaction of ammonium (NH ₄ ⁺ ) ions contained in synthetic zeolites, by hydrogen ion exchange or by hydrolysis of a zeolite containing multiply charged cations during dehydrogenation. In this regard, reference is made in particular to Kirk-Othmer, “Encyclopedia of Chemical Technology”, 3rd edition, volume 15, John Wiley & Sons, New York, 1981, pages 640 to 669.

In contrast to the prior art, it is not necessary for the catalyst body according to the invention that the zeolite of the active composition is exchanged for metal cations, i.e. that the exchangeable cations of the zeolite are replaced by metal cations, e.g. of copper or iron.

A zeolite is also understood to mean a framework aluminosilicate, the ratio of the oxygen atoms to the sum of the aluminum and silicon atoms being 2: 1. By exchanging some silicon atoms of oxidation level IV with aluminum atoms of oxidation level III, the framework or the framework structure as a whole receives a negative charge. This negative charge is compensated for by cations in the structure. These cations are so-called interchangeable cations, which can easily be replaced by other cations, in particular metal cations, by ion exchange.

A zeolite is further characterized by the fact that the framework structure has continuous pores with a characteristic pore size having. Zeolites are classified according to the molar ratio of - silicon oxide to aluminum oxide or according to the framework structure which is characteristic of this ratio. For the classification, reference is made to the article “Chemical Nominal and Formulation of Compositions of Synthetic and Natural Zeolites” by RM Barrer, Pure Appl. Che. 51 (1979), pages 1091 to 1100.

A natural zeolite is, for example, mordenite or chabazite. Synthetic zeolites are, for example, A, X and Y zeolites, which are synthetic forms of mordenite, a ZSM-5 (brand name of a synthetic zeolite manufactured by Mobil Oil Company Ltd.), a USY (Ultra Stabilized Y) or a Beta -Zeolite. The structure of the mordenite, the ZSM-5 and the Y zeolite will be discussed further on

Technical article "Acidity of the Lewis Centers m Zeolite Catalysts - NO as a Probe Molecule", Frank 0. Witzel, Progress Reports VDI, Series 3: Process Engineering, No. 292, 1992.

Extensive studies have shown that a catalyst body with an active composition which contains titanium dioxide and a hydrogen ion-exchanged acidic zeolite is suitable up to temperatures of 750 ° C. for a catalytic reduction of the nitrogen oxides according to the SCR process. Such a catalyst body is namely on the one hand catalytically active up to these high temperatures and on the other hand also has the necessary temperature stability. In addition, the catalyst body has a high stability to moisture and a high resistance to sulfur-containing components in an exhaust gas to be treated.

The catalyst body opens up the possibility of reducing nitrogen oxides in the exhaust gases of an internal combustion engine or a gas turbine, it being possible for very high temperatures of the exhaust gas to occur without additional measures being taken Lowering the temperature to protect the catalyst body "must be taken.

In a preferred embodiment, the active composition of the catalyst body has 40 to 60% by weight of zeolite. With this composition, particularly good temperature stability and particularly low deactivation of the catalytic activity at high temperatures are achieved.

It is also advantageous if the active composition 40 to 60

Wt .-% of an active component, each based on the weight of the active component 70 to 95 wt .-% titanium dioxide, 2 to 30 wt .-% tungsten trioxide, 0.1 to 10 wt .-% aluminum oxide and 0.1 to 10 % By weight of silicon dioxide. As a result, the catalyst body has a high catalytic activity with regard to the reduction of nitrogen oxides according to the SCR process, i.e. to break down nitrogen oxides in the presence of a reducing agent.

It is particularly advantageous if the active component comprises 8 to 12% by weight of tungsten trioxide.

It is also advantageous if a USY, a beta or a ZSM-5 zeolite is used as the zeolite. Such a zeolite is particularly well suited for the desired catalytic use due to its framework structure.

For the catalytic activity it is further advantageous if the active mass has a BET surface area of 30 to 150 m ² / g and a pore volume, measured by the mercury penetration method, of 100 to 1000 ml / g.

After the hydrogen-ion-exchanged, acidic zeolite has been provided, the active composition of the catalyst body can be produced as follows in a manner known per se.

Including the zeolite, the individual components or their components are mixed, ground and / or kneaded Precursor compounds (for the metal oxides specified, for example, water-soluble salts) and, if appropriate, with the addition of customary ceramic auxiliaries and fillers and / or glass fibers, a starting material. The starting mass is then either further processed into full extrudates or applied as a coating to a ceramic or metallic carrier in honeycomb or plate form. Then the starting mass is dried at a temperature of 20 to 100 ° C. After the drying process, the starting mass is calcined to the active mass by calcining at temperatures between 400 and 700 ° C.

In addition, the calcined active composition can be pre-aged after the calcining process by a final heat treatment at a temperature higher than the calcining temperature. For pre-aging, a temperature is selected which is approximately 50 ° C. above the later maximum operating temperature of the catalyst body. The final heat treatment is carried out over a period of 20 to 80 hours.

In this way, the catalyst body has an improved temperature resistance.

The object with regard to a process for the decomposition of nitrogen oxides in a gas stream is achieved according to the invention in that a gas stream containing nitrogen oxides is passed over the specified catalyst body in the presence of a reducing agent, the nitrogen oxides being converted to nitrogen and water.

It is advantageous for the process and particularly cost-effective if ammonia or an aqueous urea solution is added to the gas stream as a reducing agent.

The gas stream is advantageously passed over the catalyst body at a temperature of 250 to 750 ° C. Within this specified temperature range there is a effective conversion of nitrogen oxides to nitrogen and water instead. A deactivation of the active mass of the catalyst body is not to be expected.

Exemplary embodiments of the invention are explained in more detail with reference to a drawing and examples.

Show:

1 shows a honeycomb-shaped catalyst body in an exhaust gas cleaning system of a diesel engine,

2 shows, in a first diagram for catalyst bodies of different composition, the course of the conversion from NO to N ₂ as a function of the temperature of the exhaust gas stream;

3 shows, in a second diagram, the course of the conversion from NO to N ₂ as for a new catalyst body exposed to a temperature load

Function of the temperature of the exhaust gas flow.

Figure 1 shows an exhaust gas purification system for the catalytic removal of nitrogen oxides according to the SCR method from the exhaust gas of a diesel engine 1 not shown. In this case, the exhaust gas of the diesel engine 1 flows as a gas stream 2 through an exhaust pipe 3 and a catalyst body 4 arranged in the exhaust pipe 3 Catalyst body 4 is designed as a honeycomb body through which flow can pass and has a number of parallel channels 5 through which flow can pass. After flowing through the catalyst body 4, the gas stream 2 freed from the nitrogen oxides is released into the environment via an outlet 6.

The catalyst body 4 is produced as a full extrudate from the active composition. The active composition comprises 50% by weight of ZSM-5 zeolite and 50% by weight of an active component which 90% by weight of titanium dioxide and 10% by weight of tungsten trioxide. The proportions by weight of the usual auxiliaries and fillers are not included.

The catalyst body 4 was produced by mixing a titanium dioxide / tungsten oxide coprecipitate with an acidic, hydrogen ion-exchanged ZSM-5 zeolite. Such a zeolite is available as a so-called ZSM-5 zeolite m H form from Alsi-Penta. With the addition of water, a kneadable mass is produced from the mixture, and this is further processed by extrusion to form the honeycomb body. The honeycomb body is dried at 80 ° C and finally calcined at a temperature of 600 ° C.

To reduce the nitrogen oxides according to the SCR process, an introduction device 7 for a reducing agent is arranged on the exhaust line 3 m in the flow direction in front of the catalyst body 4. The introduction device 7 in this case comprises a reducing agent container 8 with a reducing agent line 9 connected to the exhaust line 3. The reducing agent line 9 m m in the exhaust line 3 m an injection nozzle 10. To introduce the reducing agent m the gas stream 2, an aqueous urea solution 11 by means of a Compressor 12 introduced via the controllable valve 13, depending on requirements, the exhaust line 3. In the hot gas stream 2 is

Urea reacted by pyro- and / or thermolysis m the reducing agent ammonia. The nitrogen oxides contained in the gas stream 2 in the presence of ammonia are then converted to molecular nitrogen and water on the catalyst body 4 in accordance with the SCR method.

The high-temperature activity and high-temperature stability of the catalyst body according to the invention are demonstrated below using exemplary embodiments. Example A

For the preparation, a titanium dioxide / tungsten trioxide coprecipitate composed of 90% by weight of titanium dioxide and 10% by weight of tungsten trioxide is mixed with a ZSM-5 zeolite of the H form with the addition of customary ceramic auxiliaries and fillers, mixed and added processed from water to a so-called slip, ie a liquid ceramic mass. The slip is then applied as a coating to a honeycomb-shaped carrier made of cordierite (a magnesium-alumino-silicate with the composition Mg ₂ Al ₄ Si5θi8 with a rhombic-halohedral structure). With an inflow area of 150 x 150 mm ^{2, the} cordierite carrier has ² 1225 continuous channels. By drying at a temperature of 90 ° C and then calcining at a temperature of 600 ° C, the coated cordierite support is further processed into the catalyst body with the active composition applied thereon.

The proportions of the starting materials are chosen so that the active composition of the finished catalyst body has equal proportions of the active component, including titanium dioxide and tungsten trioxide, and of the zeolite.

Example B In the same way as in Example A, a coated cordierite carrier is produced in such a way that the active composition of the finished catalyst body has a weight ratio of the active component, comprising titanium dioxide and tungsten trioxide, to the zeolite of 75 to 25.

Example C

According to Example A, a coated cordierite support is produced in such a way that the active composition of the finished catalyst body has a weight ratio of the active component, comprising titanium dioxide and tungsten trioxide, to the zeolite of 25 to 75. Example D

The coprecipitate of titanium dioxide and tungsten trioxide given in Example A is mixed with a ZSM-5 zeolite in H form with the addition of ceramic auxiliaries and fillers, ground and processed to a kneadable, plastic mass with the addition of water. The kneadable mass is then extruded into a honeycomb-shaped catalyst body. The honeycomb-shaped catalyst body produced as a full extrudate again has 1225 parallel flow channels with an inflow area of 150 x 150 mm ² . The catalyst body is dried at 90 ° C and then calcined at 600 ° C. This final process gives the catalyst body its catalytic activity.

Example E

The honeycomb-shaped catalyst body produced as a full extrudate according to Example D is subjected to a constant temperature load of 700 ° C. for a period of 500 hours.

attempt

A model exhaust gas is passed over the catalyst body according to Examples A to E at a space velocity of 15500 / h. The model exhaust gas is nitrogen and comprises 200 ppm nitrogen monoxide NO, 200 ppm ammonia NH ₃ as a reducing agent, 11% by volume oxygen 0 ₂ and 10% by volume water H ₂ 0.

At the temperatures of the model exhaust gas of 450 ° C, 500 ° C, 540 ° C, 570 ° C, 600 ° C, 650 ° C and 750 ° C, the catalytic conversion of nitrogen monoxide NO to molecular nitrogen N is successively carried out on the catalyst body ₂ measured. For this purpose, the content of nitrogen monoxide NO before and after the catalyst body and the content of nitrogen N ₂ after the catalyst body in the model exhaust gas are measured.

The measurement results are summarized in FIGS. 2 and 3. FIG. 2 shows the measured dependency of the NO / N ₂ conversion in percent on the temperature of the model exhaust gas for the catalyst bodies according to Examples A, B and C.

FIG. 3 shows the dependence of the measured NO / N ₂ conversion in percent on the temperature for the catalyst body according to Examples D and E.

As can be seen from FIG. 2, the catalyst bodies according to Examples A, B and C show a catalytic conversion of between 40 and 60% in the range of high temperatures between 450 and 650 ° C. This means that between 40 and 60% of the NO contained in the model exhaust gas was converted to N ₂ . The catalyst body according to Example A shows a catalytic conversion of 50% and above over the entire temperature range from 450 to 650 ° C. The measured conversion of the catalyst body according to Example C even increases with higher temperatures.

Only at temperatures above 650 ° C does the catalytic activity of the catalyst body decrease. Even at a temperature of 700 ° C, however, the catalysts according to Examples A and C still show a conversion of about 43%.

This result clearly demonstrates the suitability of the catalyst bodies according to Examples A to C for the decomposition of nitrogen oxides according to the SCR process in the high temperature range from 450 to over 700 ° C. A comparable catalyst body with an active material based on titanium dioxide with admixtures of tungsten trioxide and / or vanadium pentoxide, on the other hand, only shows negligible conversion for nitrogen oxides according to the SCR process from a temperature of 450 ° C.

The temperature resistance of the catalyst body is clear from Figure 3. As indicated above, the model exhaust gas indicated is also passed over the catalyst bodies according to Examples D and E. For the temperatures 450 ° C, 500 ° C, 540 ° C, 570 ° C, 600 ° C, 650 ° C and 700 ° C, the catalytic conversion of nitrogen monoxide NO to molecular nitrogen N _{2 is} determined in succession. As a result, the measured NO / N ₂ conversion in percent as a function of the temperature is shown in FIG. 3. It can clearly be seen that the catalyst body according to Example E, which was exposed to a high temperature load, also experienced only a loss of its high catalytic activity of about 10% even after this load.

Claims

claims

1. catalyst body (4) for the decomposition of nitrogen oxides in the presence of a reducing agent, with an active composition which comprises a zeolite and titanium dioxide, so that the zeolite is a hydrogen ion-exchanged, acidic zeolite.

2. Catalyst body (4) according to claim 1, d a d u r c h g e k e n n z e i c h n e t that the active composition contains 40 - 60 wt .-% of zeolite.

3. catalyst body (4) according to claim 1 or 2, characterized in that the active composition contains 40-60 wt .-% of an active component, each based on the weight of the active component 70-95 wt .-% titanium dioxide, 2-30 % By weight of tungsten trioxide, 0.1-10% by weight of aluminum oxide and 0.1-10% by weight of silicon dioxide.

4. A catalyst body (4) according to claim 3, d a d u r c h g e k e n n z e i c h n e t that the active component comprises 8 - 12 wt .-% of tungsten trioxide.

5. Catalyst body (4) according to one of claims 1 to 4, that the zeolite is a USY, a beta or a ZSM5 zeolite.

6. Catalyst body (4) according to one of claims 1 to 5, characterized by a BET surface area of 30-150 m ² / g and a pore volume, measured by the mercury penetration method, of 100-1000 ml / g.

7. Process for the decomposition of nitrogen oxides in a gas stream (1), wherein a gas stream (1) containing nitrogen oxides in the presence of a reducing agent over a catalyst body (4). measured is passed to one of claims 1 to 6, and wherein the nitrogen oxides are converted to nitrogen and water.

8. The method according to claim 7, wherein the gas stream (1) as a reducing agent, an aqueous urea solution (11) is added.

9. The method according to claim 7 or 8, wherein the gas stream (1) at a temperature of 250-750 ° C is passed over the catalyst body (4).