DE102008010330A1 - SCR catalytic converter with ammonia storage function - Google Patents

Abstract

The present invention relates to an SCR catalytic converter, comprising a carrier permeable to exhaust gases and a layer of a catalytically active composition applied in regions on the carrier, wherein the SCR active component and the NH 3 storage component are different from one another. The invention further relates to the use of the inventive SCR catalyst for reducing nitrogen oxides of mobile or stationary combustion devices.

Description

The present invention relates to an SCR catalyst comprising an SCR active component and an NH ₃ storage component, wherein the NH ₃ storage component is preferably a zeolite. The invention further relates to the use of the SCR catalyst for reducing nitrogen oxides of mobile or stationary incinerators.

Selective catalytic reduction (SCR) refers to the selective catalytic reduction of nitrogen oxides from exhaust gases of combustion engines and also power plants. With an SCR catalyst, only the nitrogen oxides NO and NO ₂ (commonly referred to as NO _x ) are selectively reduced, with NH ₃ (ammonia) usually added to the reaction. Therefore, only the harmless substances water and nitrogen are formed as reaction products. For the use of motor vehicles carrying ammonia in compressed gas cylinders is a security risk. Therefore, precursor compounds of ammonia are usually used, which are decomposed in the exhaust line of the vehicles with formation of ammonia. Known in this context is, for example, the use of ^AdBlue® , which is an approximately 32.5 eutectic solution of urea in water. Other sources of ammonia include ammonium carbamate, ammonium formate or urea pellets.

Before the actual SCR reaction, ammonia must first be formed from urea. This is done in two reaction steps, collectively referred to as hydrolysis reaction. First, NH ₃ and isocyanic acid are formed in a thermolysis reaction. Isocyanic acid is then reacted with water to form ammonia and carbon dioxide in the actual hydrolysis reaction.

to Avoiding solid precipitates requires that the second reaction by choosing suitable catalysts and enough high temperatures (from 250 ° C) sufficiently fast. modern SCR reactors simultaneously assume the function of Hydrolysis catalyst.

The ammonia produced by the thermohydrolysis reacts on the SCR catalyst according to the following equations: 4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O (1) NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O (2) 6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O (3)

At low temperatures (<300 ° C), the conversion proceeds predominantly via reaction (2). For a good low-temperature conversion, it is therefore necessary to set a NO ₂ : NO ratio of about 1: 1. Under these circumstances, reaction (2) can take place at temperatures as low as 170-200 ° C.

The oxidation of NO to NO _x takes place in an upstream oxidation catalyst which is required for optimum efficiency.

If more reducing agent is metered than is reacted during the reduction with NO _x , it can lead to an undesirable NH ₃ slip. The removal of the NH ₃ can be achieved by an additional oxidation catalyst downstream of the SCR catalyst. This barrier catalyst oxidizes the ammonia, if any, to N ₂ and H ₂ O. In addition, careful application of the urea dosing is essential.

An important parameter for SCR catalysis is the so-called feed ratio α, defined as the molar ratio of metered NH ₃ to the NO _x present in the exhaust gas. Under ideal operating conditions (no NH ₃ slip, no side reactions, no NH ₃ oxidation), α is directly proportional to the NO _x reduction rate:
At α = 1 theoretically a one percent NO _x reduction is achieved. In practical use, with NH ₃ slip of <20 ppm, a NO _x reduction of 90% in stationary and transient operation can be achieved.

Due to the upstream hydrolysis reaction, in today's SCR catalysts an NO _x conversion> 50% is achieved only at temperatures above about 250 ° C, optimum conversion rates are achieved in the temperature window of 250-450 ° C.

The dosing strategy is of great importance in high NH ₃ storage capacity catalysts because the NH ₃ storage capability of prior art SCR catalysts typically decreases with increasing temperature.

Currently become predominant both in the power station sector and in the automotive sector Catalysts based on titanium dioxide, vanadium pentoxide and tungsten oxide used. Also, the use of SCR catalysts is based of zeolites known in the art.

adversely in the SCR catalysts used in the prior art, also in the zeolite-based SCR catalysts, it is relatively poor conversion of the exhaust gases in cold start operation. This means, the exhaust gases that occur when starting the engine flow over a still cold catalyst and are therefore not implemented. The urea dosing is done for the sake of a possible Condensation usually only above 250 ° C. The means that in the cold start phase almost no nitrogen oxide emissions be reduced.

Further problems arise with fast load changes. With a load change, a sudden change in the exhaust gas concentration is connected and the urea dosing can be adjusted only offset in time due to the inertia of the dosing system. In particular, the prior art catalysts have problems in the occurrence of NH ₃ slip. This leads to unnecessary emission of NO _x .

The The object of the present invention was therefore an SCR catalyst with improved cold start properties and improved performance to provide during load changes.

The object is achieved by an SCR catalyst comprising a catalytically active composition comprising an SCR-active component and an NH ₃ storage component, wherein the SCR-active component and the NH ₃ storage component are different from one another.

It has surprisingly been found that by the presence of the NH ₃ storage component no impregnation with a noble metal such as palladium, platinum, etc. on the SCR catalyst is needed. It has also been found that the presence of the ammonia storage component in the SCR catalyst eliminates the need for another oxidation catalyst downstream of the SCR catalyst in an exhaust system. This blocking catalyst used hitherto in the prior art for the removal of NH ₃ oxidizes the possibly occurring ammonia to N ₂ and H ₂ O. Due to the storage of the excess NH _3, it is therefore surprisingly possible to dispense with the blocking catalyst.

Consequently The subject of the present invention also includes an exhaust system containing an oxidation catalyst and an SCR catalyst.

Of the Term "SCR active component" refers to a material which catalyzes the above-mentioned SCR reaction.

The term "NH ₃ storage component" is understood as meaning a material which is capable of reversible adsorption of NH _3. It is preferably a porous or particularly preferably a microporous material.

The storage component is preferably a zeolite. The term "zeolite" is used in the context of the present invention as defined by the International Mineralogical Association ( DS Coombs et al., Can. Mineralogist, 35, 1997, 1571 ) a crystalline substance from the group of aluminum silicates with spatial network structure of the general formula M ^{n +} [(AlO ₂ ) x (SiO ₂ ) y] x H ₂ O understood that consist of SiO ₄ / AlO ₄ tetrahedra, which are linked by common oxygen atoms to a regular three-dimensional network. The ratio of Si / Al = y / x is always ≥ 1 according to the so-called "Löwenstein rule", which prohibits the adjacent occurrence of two adjacent negatively charged AlO ₄ tetrahedra. Although there are more exchange sites for metals available at a low Si / Al ratio, the zeolite is becoming increasingly thermally unstable.

The zeolite structure contains voids and channels characteristic of each zeolite. The zeolites are classified according to their topology into different structures (see above). The zeolite framework contains open cavities in the form of channels and cages that are normally occupied by water molecules and extra framework cations that can be exchanged. An aluminum atom has an excess negative charge which is compensated by these cations. The interior of the pore system represents the catalytically active surface. The more aluminum and the less silicon a zeolite contains, the denser the negative charge in its lattice and the more polar its internal surface. The pore size and structure are determined by the Si / Al ratio, which determines most of the catalytic character of a zeolite, in addition to the parameters of manufacture (use or type of template, pH, pressure, temperature, presence of seed crystals).

By the presence of z. B. 3-valent atoms (eg., Al or Ga) receives the zeolite has a negative lattice charge in the form of so-called Anion sites, in the vicinity of which the corresponding cation positions are located. The negative charge is due to the incorporation of cations compensated in the pores of the zeolite material. The zeolites are different mainly according to the geometry of the cavities, which are formed by the rigid network of SiO4 / AlO4 tetrahedra. The entrances to the cavities are from 8, 10 or 12 "rings" formed (narrow, medium and large pore zeolites). Certain zeolites show a uniform structure structure (eg ZSM-5 with MFI topology) with linear or zigzag shape running channels, in others close behind the pore openings larger cavities on, for. B. in the Y and A zeolites, with the topologies FAU and LTA.

in principle In the context of the present invention, any zeolite, In particular, each 10 and 12 "ring" zeolite can be used. According to the invention, preference is given to zeolites with the Topologies AEL, BEA, CHA, EUO, FAO, FAU, FER, KFI, LTA, LTL, MAZ, MOR, MEL, MTW, LEV, OFF, TONE and MFI. Very particularly preferred zeolites the topological structures FAU, MOR, BEA, MFI and MEL.

There are usually three different centers in zeolites, referred to as α, β and γ positions, which define the position of exchange sites (also referred to as "interchangeable locations"). All of these three positions are accessible to reactants during the NH ₃ -SCR reaction, particularly when using MFI, BEA, FAU, MOR, MTW, and MEL zeolites.

Surprisingly, it has been found that the improvement in cold start emissions is dependent on the NH ₃ storage capability of the SCR catalyst. However, the NH ₃ storage capacity depends on the (Brønstedt) acidic surface centers of the zeolite preferably used according to the invention. Different types of zeolites have different strong acid sites and thus a different adsorption and desorption behavior. Strongly acidic centers form stronger bonds to the adsorbed NH ₃ molecules than weakly acidic centers.

It has been found that the desorption temperature is significantly higher for a zeolite with predominantly strongly acidic centers than for weakly acidic centers. Thus, according to the invention, the NH ₃ storage function, ie the amount of adsorbing NH ₃ or the NH ₃ to be desorbed, can be adjusted in a targeted manner by the appropriate selection of the zeolite type. Since the number of acidic centers can be influenced by the molar ratio of SiO ₂ to Al ₂ O ₃ (the so-called "module"), the NH ₃ storage capacity of an SCR catalyst can thus be precisely adjusted by varying this ratio.

The inventively preferred SiO ₂ / Al ₂ O ₃ modulus (molar ratio) is in the range of 5: 1 to 150: 1, more preferably in the range of 5: 1 to 50: 1 and most preferably in the range of 10: 1 to 30: 1st

The NH ₃ storage zeolite is preferably present in an amount of from 2 to 30% by weight, more preferably in an amount of from 5 to 15% by weight, in the catalytically active composition.

In addition to the amount of zeolite, as already explained above, the type of zeolite used is important since not all zeolites have an optimum NH ₃ storage capacity. The zeolite is therefore preferably selected from the group comprising FAU (faujasite), MOR (mordenite), BEA (beta zeolite), MFI (Mobil Five, ZSM-5, Zeolite Secondary Mobile No. 5) and MEL, combinations of which The above zeolites can be used. Preferably, all said zeolites are used in the H form. The cited catalysts and processes for their preparation are known in the art.

In preferred developments of the invention, the NH ₃ storage zeolite is a metal-exchanged zeolite, for example an iron, copper or cobalt-exchanged zeolite. Preferably, the metal-exchanged zeolite is also selected from the group comprising FAU (faujasite), MOR (mordenite), BEA (beta zeolite), MFI (Mobil Five, ZSM-5, Zeolite Secondary Mobile No. 5) and MEL, including combinations of the mentioned zeolites can be used.

at Metal exchanged or doped zeolites becomes the metal content or the degree of exchange of a zeolite decisively determines the metal species present in the zeolite. This can the zeolite both with a single metal or with different ones Be doped metals. The preferred metals for the Exchange and doping are catalytically active metals such as Fe, Ce, Co, Ni, Ag, V, Rh, Pd, Pt, Ir. According to the invention Zeolites, most preferably the iron, copper or cobalt species contain. The production process for metal exchanged Zeolites, for example via solid or liquid phase exchange, are known in the art.

In addition to the NH ₃ storage component, the SCR catalyst also requires an SCR active component. This may be, for example, a conventional vanadium / titanium / tungsten based component. Typically, the vanadium / titanium / tungsten based component is an oxide coating that substantially contains TiO ₂ in the anatase modification. TiO ₂ is usually stabilized by WO _{3 in} order to achieve an improvement in its thermal stability. The proportion of WO ₃ is typically about 10 wt .-%. The actually active component forms the V ₂ O ₅ , which is typically present as a monolayer on the TiO ₂ particles. It is also possible to add a further NH ₃ storage component as proposed below.

An advantage of a z. B. vanadium-based component as SCR-active component is the excellent low-temperature activity of such a system. The inventive combination of vanadium-based catalysts with zeolites can thus make use of the low-temperature activity of these catalysts and at the same time the NH ₃ storage capacity of the zeolites and thus to provide an SCR catalyst with excellent cold-start properties.

alternative to the vanadium-based SCR-active component according to the invention also a SCR active zeolite (eg, a metal doped or exchanged Zeolite such as a Cu zeolite or Fe zeolite can be used.

In further advantageous embodiments of the invention, this provides many possible combinations for an SCR catalyst. For example, according to the invention, an SCR-active zeolite which is active at low temperatures can be combined with an SCR-active zeolite which is active at higher temperatures and an NH ₃ storage zeolite. A corresponding example of such a combination is, for example, a mixture of a copper-exchanged zeolite such as Cu-ZSM-5 with a modulus of 150, which has a low temperature activity, an iron-exchanged zeolite, such. B. Fe-BEA with a modulus of 35 exhibiting high temperature activity and a non-metal exchanged zeolite such as H-BEA with a modulus of 20 serving as the NH ₃ storage zeolite.

In order to is a good catalytic reaction from low to high temperatures to a catalyst comprising the inventive Combination as a catalytic composition is given.

Has according to the invention it also turned out to be an advantage if the catalytically active Composition of the SCR catalyst in the form of an at least partially Coating on a preferably monolithic carrier is present. As monolithic carrier come for example metallic or ceramic carrier z. B. in honeycomb or Foam form used.

It has proved to be particularly advantageous if the NH ₃ storage component is applied only at the inlet side and the SCR-active component only at the outlet side of such a coated monolithic carrier. It would also be possible for all of the components, such as the NH ₃ storage component and the SCR active component (s), to be applied as a homogeneous coating to the entire surface of the monolith, thus greatly simplifying the preparation of such catalysts.

The Application of the SCR catalyst takes place in the state of Technique known methods, for example by applying a Washcoats, by coating in an immersion bath or by spray coating be made.

Of the Accordingly, SCR catalyst according to the invention is outstandingly suitable for reducing nitrogen oxide emissions from mobile or stationary Incinerators.

mobile Combustion devices in the context of this invention are, for example Internal combustion engines of motor vehicles, in particular diesel engines, Generating sets based on internal combustion engines, or other units based on internal combustion engines.

at the stationary combustion devices are usually around power plants, combustion plants, waste incineration plants and also to heating systems of private households.

Consequently is also the subject of the invention, a method for reducing nitrogen oxide emissions in mobile or stationary incinerators, the method being characterized in that an exhaust gas flow over passed an SCR catalyst according to the invention becomes.

The Invention will now be described with reference to the following examples and figures, but not to be construed as limiting, closer explained.

It demonstrate

1 the NH ₃ storage capacity of zeolites as a function of the SiO ₂ / Al ₂ O ₃ modulus,

2 a TPD diagram of NH ₃ desorption in a Fe doped zeolite of type MFI with a modulus of 25.

1 shows the NH ₃ storage capacity of zeolites as a function of the SiO ₂ / Al ₂ O ₃ modulus.

How out 1 As can be seen, the optimum modulus is in the range of 5 to 50, most preferably in the range of 10 to 30.

This especially in the automotive sector Question coming zeolite types with the topology FAU, MOR, BEA, MFI, MEL too. These are used individually or as mixtures.

however The selection of the module will also be through the analysis of the later Intended use and conditions. The application area of the catalyst is determined by several factors. For example The temperature range during operation varies from car to truck considerably. Typical work areas of internal combustion engines in trucks lie between 180 to 430 ° C, in cars are often Temperatures up to 600 ° C reached. in principle is true that the smaller the engines are (in terms of their cubic capacity) the more intensively a full load operation occurs and thus the temperatures rise. Next plays a role, whether in the exhaust system the SCR catalytic converter before or after the diesel particulate filter (DPF) is built in, as during the regeneration of the diesel particulate filter by burning off the collected soot in the filter above of about 600 ° C can form high temperature peaks. It follows that, especially at longer thermal Strains of the SCR catalyst high temperature stable zeolites be used with as high a module as possible as non-limiting examples Fe-BEA with a module of 50 or Fe-ZSM-5 with a modulus of 150.

As the modulus increases, so does the hydrothermal stability and thus the high temperature stability. The NH ₃ absorption capacity runs like the 1 it is exactly the opposite. This results in the above-mentioned preferred range according to the invention.

However, not all areas of the module are accessible with any type of zeolite which can be used according to the invention. For example, with a BEA zeolite only SiO ₂ : Al ₂ O ₃ ratios of above 19 can be achieved. In order to achieve a certain storage capacity of NH ₃ , therefore, in the case of using such zeolites, the amount must be increased. In the zeolites preferred according to the invention, the lower and upper limits for the SiO ₂ : Al ₂ O ₃ ratios (modules) are as follows:
FAU: 3-10, MOR: 10-400, BEA: 19-1000, MFI: 19-1000, MEL: 19-1000.

2 shows the result of the temperature programmed desorption (TPD) of NH ₃ in an iron exchanged zeolite with topology MFI and a SiO ₂ / Al ₂ O ₃ modulus of 25.

Out 2 It can be seen that the highest desorption occurs in the range of 200-290 ° C followed by another maximum between 310-400 ° C.

The procedure was as follows:
The zeolite with the module according to the invention was saturated with NH ₃ . That is, the maximum possible NH ₃ concentration was adsorbed on the surface and then desorbed by increasing the temperature. The amount of NH ₃ was detected via a mass spectrometer and correlated simultaneously with the dispensing temperature. Thus, both the amount and the temperature at which the desorption was carried out, whereupon indications of the strength of the bond between NH ₃ and the zeolite were obtained.

The test conditions were as follows: NH ₃ loading: Gas: 0.473% NH ₃ in He Loading temperature [° C]: 110 NH ₃ desorption: Gas: helium Heating rate [° C / min]: 5 Final temperature [° C]: 750 Holding period [h]: 1 NH3 detection with mass spectrometer, mass number 16: Peak area calibration (FE / μmol NH ₃ ]: 1,16E-08 Peak area desorption [FE]: 1.13e-06

embodiments

Example 1:

Preparation of the catalyst 1

The carrier structure used was a ceramic carrier from NGK with a honeycomb density of 400 cpsi. To prepare the washcout with the catalytically active composition for the SCR catalyst, 90 g of TiO ₂ powder, 10 g of WO ₃ , 20 g of ZSM-5, 20 g of SiO ₂ sol, 15 g of TiO ₂ sol, 20 ml of 10 % aqueous Vanadyloxalat solution and 80 g of distilled water mixed together. The ZSM-5 zeolite had a modulus of 25.

Of the thus prepared washcoat was on the ceramic monoliths Vibration applied, the volume of the washcoat being 50-120% the carrier volume corresponded. By means of known per se Procedure, the monolith was emptied. The washcoat residues at the Outlet side of the monolith were sucked off. The SCR catalyst thus obtained was then dried at 80 ° C and at about 500 ° C. calcined for 5 hours.

analog applies when using, for example, metallic monoliths, instead of the ceramic monoliths used in Example 1 can also be used.

Example 2:

Preparation of the catalyst 2

As the support structure, the same structure as in Example 1 was used. To prepare the washcoat, 90 g of TiO ₂ powder, 10 g of WO ₃ , 10 g of ZSM-5, 10 g of BEA, 20 g of SiO ₂ sol, 15 g of TiO ₂ sol, 20 ml of 10% aqueous vanadyl oxalate solution and 80 g of distilled water mixed together. The ZSM-5 zeolite had a modulus of 25, the BEA zeolite a modulus of 150.

Of the thus prepared washcoat was as in Example 1 on the monoliths applied and then calcined.

Example 3:

Preparation of the comparative catalyst, Catalyst 3

When Carrier structure was the same structure as in Example 1 used.

To prepare the washcoat, 110 g of TiO ₂ powder, 10 g of WO ₃ , 20 g of SiO ₂ sol, 15 g of TiO ₂ sol, 20 ml of 10% aqueous vanadyl oxalate solution and 80 g of distilled water were mixed together.

Of the thus prepared washcoat was as in Example 1 on the monoliths applied and calcined.

With The catalysts of Examples 1 to 3 thus obtained were laboratory test bench investigations with simulated cold start and simulated load changes on one dynamic test bench for rapid change of temperature, Exhaust and throughput performed.

The Evaluation was as a correlation of the invention Catalysts 1 and 2 with respect to the comparative catalyst Example 3 performed. Criteria were in particular the Cold start behavior and behavior during load changes.

The cold start was simulated by a temperature increase from room temperature to 400 ° C with a heating rate of 50 K / min. Rapid change of throughput and temperature simulates a load change from full load to idle and vice versa. The space velocity of 100,000 h ^-1 and the temperature of 400 ° C (full load) were changed to a space velocity of 30,000 h ^-1 and a temperature of 180 ° C (idle).

It was found that by the adsorption of NH ₃ by an NH ₃ storage component according to the invention in the catalytic composition already in the cold start phase significant increases in sales were achieved (see Table 1). The (preconditioned) catalyst had already adsorbed NH ₃ and therefore exhibited this effect. The same was obtained with a sudden increase in the NO _x concentration (change idle to full load), which also here on the NH ₃ reservoir could be accessed. In the event of a sudden reduction in NO _x (change from full load to idling), overdosed NH ₃ is stored in the storage component, thus reducing NH ₃ slip (see Table 1).

The Exhaust composition corresponded to a typical diesel car and it a load change between full load and idle was simulated.

The results are shown in Table 1. Table 1: Results of the catalysts according to the invention in relation to the comparative catalyst 3 Cold start phase fresh Cold start phase conditioned Last change phase NO _x conversion NO _x conversion NO _x conversion NH ₃ slip Cat 1 +/- 0% + 31% + 9% -32% Cat 2 +/- 0% + 28% + 14% -34%

Both catalysts according to the invention showed a significant improvement with respect to the NO _x conversion and a reduced NH ₃ slip at the catalyst outlet.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited non-patent literature

• DS Coombs et al., Can. Mineralogist, 35, 1997, 1571 [0021]

Claims (12)

An SCR catalyst comprising a catalytically active composition containing an SCR active component and an NH ₃ storage component, wherein the SCR active component and the NH ₃ storage component are different from each other. The SCR catalyst of claim 1, wherein the NH ₃ storage component comprises a zeolite having a SiO ₂ / Al ₂ O ₃ molar ratio of 5: 1 to 150: 1. The SCR catalyst of claim 2, wherein the zeolite in an amount of 2 to 30% by weight in the catalytically active composition is included. The SCR catalyst of claim 3, wherein the zeolite is selected from the group comprising zeolites of topology FAU, MOR, BEA, MFI and MEL, or combinations thereof. The SCR catalyst of claim 4, wherein the zeolite is a metal-exchanged zeolite. SCR catalyst according to one of the preceding claims, characterized in that the SCR active component on Vanadium-based component and / or an SCR-active zeolite is. The SCR catalyst of claim 6, wherein the SCR active Component of a low-temperature SCR-active and / or a high-temperature SCR-active Zeolites. SCR catalyst according to one of the preceding claims, characterized in that the SCR catalyst coating as Be is present on a monolithic carrier, wherein the carrier a honeycomb body, a monolith or a metal or ceramic foam is. The SCR catalyst of claim 8, wherein the NH ₃ storage component is on the entrance side and the SCR active component is on the exit side of the support. Emission control system for cleaning of diesel engine exhaust containing a group of consecutively arranged catalyst devices consisting of an oxidation catalyst and an SCR catalyst according to any one of claims 1 to 9th Use of the SCR catalyst according to one of the claims 1 to 8 for the reduction of nitrogen oxide emissions mobile or stationary Incinerators. Method for reducing nitrogen oxide emissions in mobile or stationary incinerators, wherein an emerging during combustion exhaust gas flow over An SCR catalyst according to any one of claims 1 to 8 is directed.