DE102006011411B3 - catalyst - Google Patents

Abstract

The invention relates to a catalyst (1) for cleaning oxygen-containing exhaust gases (5) from a combustion system, in particular an internal combustion engine operated with excess air, comprising a reduction catalyst (2) for the selective catalytic reduction of nitrogen oxides contained in the exhaust gas (5) by means of a reducing agent and one of the reduction catalyst ( 2) oxidation catalyst (3) applied on the downstream side for the oxidation of the reducing agent. It is provided that the oxidation catalyst (3) is applied to the reduction catalyst (2) over an area on the outflow side, which is 1-19% of the total catalyst volume. The reduction catalyst (2) is composed of a number of individual catalysts (15, 16), the oxidation catalyst (3) being applied to the last individual catalyst (s) on the outflow side. With a wide range of uses, the catalyst (1) achieves a high degree of conversion of nitrogen oxides while avoiding slippage of reducing agents and undesirable side reactions.

Description

The The invention relates to a catalyst for the purification of oxygen-containing Exhaust gases of a combustion plant, in particular one with excess air operated internal combustion engine comprising a reduction catalyst for the selective catalytic reduction of nitrogen oxides contained in the exhaust gas by means of a reducing agent and a reduction catalyst downstream applied oxidation catalyst for the oxidation of the reducing agent, wherein the oxidation catalyst to the reduction catalyst via a downstream Range is applied, the 1-19% of the total catalyst volume.

Such a catalyst is from the EP 1 264 628 A1 is known and is used to purify the oxygen-containing exhaust gases of a combustion plant by means of selective catalytic reduction of nitrogen oxides, without in this case the reducing agent used is released into the environment. In this case, the reduction catalyst reduces nitrogen oxides to molecular nitrogen and water by means of a reducing agent, such as ammonia, in the presence of oxygen. This method of selective catalytic reduction is also known by the abbreviation SCR method. As the source of the reducing agent is optionally a reducing agent donating substance, such as urea, which releases ammonia in the exhaust gas added.

Next is from the EP 0 410 440 B1 a similar catalyst is known, wherein the provided with the oxidation catalyst downstream region of the reduction catalyst is 20-50% of the total volume.

The For example, to deliver the added reducing agent to the environment to avoid odor nuisance preferably be kept low. For this reason takes place in the applied SCR method a with respect to actual or the expected nitrogen oxide content of the exhaust gas substoichiometric Dosage of the reducing agent. Although this is a reductant emission certainly prevented, but is not on the reduction catalyst the maximum possible, but eliminates a smaller amount of nitrogen oxides. As a result the reduction catalyst must be designed sufficiently large and the control of Reducing agent dosage work exactly, especially during a transient engine operation, as is common in a motor vehicle, to achieve a significant nitrogen oxide reduction.

One Catalyst of the type mentioned circumvents this problem, by the reduction catalyst downstream of an oxidation catalyst for Oxidation of the reducing agent is applied. At this oxidation catalyst In the presence of oxygen, the reducing agent, in particular Ammonia, harmless Compounds, in particular molecular nitrogen and water, oxidized.

For this purpose, the reduction catalyst produced in one piece according to the EP 0 410 440 B1 coated downstream of the oxidation catalyst, wherein the coated area accounts for 20-50% of the total catalyst volume. The upstream reduction catalyst can now be used effectively. In particular, the reducing agent can be metered in stoichiometrically or superstoichiometrically in order to achieve maximum degradation of the nitrogen oxides.

Unfortunately, such catalyst has undesirable side reactions. So can again nitrogen oxides are formed. Also, for example, ammonia in the presence of excess air be converted to nitrous oxide or ammonium nitrate. Possibly can hydrocarbons present in the exhaust gas react to harmful nitro compounds.

According to the EP 1 264 628 A1 For example, the oxidation catalyst is supported on a honeycomb-type reduction catalyst over a downstream length of 1-20% of the total length.

task The invention is a catalyst of the type mentioned specify, while avoiding a Reduktionsmittelschlupfes and unwanted Side reactions as possible high degree of conversion for Allows nitrogen oxides, and the possible is versatile.

These Task is for a catalyst according to the preamble of claim 1 according to the invention thereby solved, that the reduction catalyst from a number of individual catalysts is composed, wherein the oxidation catalyst or the downstream last single catalyst / is applied. Extensive investigations have shown that with a reduction catalyst on the downstream side an oxidation catalyst is applied, unwanted side reactions safely avoided be achieved and a high degree of conversion of nitrogen oxides can, if the oxidation catalyst applied downstream over a range is that 1-19% the total catalyst volume is.

Surprisingly, such a catalyst already allows a stoichiometric or superstoichiometric metered addition of the reducing agent, so that the reduction catalyst converts a maximum possible amount of nitrogen oxides, without that there is a reductant slip. At low nitrogen oxide concentrations and stoichiometric metered addition of the reducing agent, a volume of 1% provided with the oxidation catalyst is sufficient. If a volume of the entire catalyst of more than 19% is provided with the oxidation catalyst, then the mentioned undesired side reactions occur increasingly. In particular, nitrogen oxides are formed again on the oxidation catalyst in the interaction of nitrogen, oxygen and unconsumed reducing agent. A volume of oxidation catalyst which exceeds 19% thus does not serve to degrade the reducing agent, but undesirable reactions are then catalysed at free adsorption sites.

Of the specified catalyst leaves cost-effective because no two separate catalysts for the reduction or for the oxidation must be made. For the preparation of the reduction catalyst is known per se Made way. Subsequently is downstream of this reduction catalyst to create the oxidation catalyst changed the composition of the catalytically active surface. This can e.g. by re-stratifying or by introducing the oxidation the reducing agent catalysing compounds or elements happen. Alternative would be also replacing the already existing in terms of reduction catalytically active surface imaginable.

Of the Catalyst is placed in the exhaust duct of an incinerator and used in particular in the exhaust passage of an internal combustion engine. To different spatial Being able to meet conditions and exhaust gas compositions flexibly, is the reduction catalyst from a number of individual catalysts composed, wherein the oxidation catalyst or the downstream last single catalyst / is applied. This can be modular the desired Length of the Reduction catalyst over the number of individual catalysts can be adjusted. The respect the entire catalyst volume desired downstream side Area, which acts as an oxidation catalyst, can be over several the individual catalysts downstream extend. For a small volume range or for a relatively long one Single catalysts is only considered in the flow direction of the exhaust gas last single catalyst corresponding to the oxidation catalyst Mistake. The modular design of the catalyst as a whole offers a cost advantage, as this the variety of types to be produced Catalysts is significantly reduced. Furthermore, by the shorter individual modules achieved a lower thermal load of the monoliths.

Of the Reduction catalyst or the individual catalysts as such can as a supported catalyst be formed with an SCR-active coating, then what the oxidation catalyst is applied. In this embodiment variant is the catalytically active coating on a usually plate-shaped carrier material applied. The coated carrier plates are then used for Formation of the catalyst stacked, wherein for the exhaust gas flowing through Flow channels through in the carrier material introduced beads and / or waves are formed.

In In another advantageous embodiment, the reduction catalyst formed as an SCR-active Vollextrudat, whereupon the oxidation catalyst is applied. The Vollextrudat consists of a ceramic Mass acting as a slurry of metal oxides, in particular titanium dioxide, produced and extruded becomes. The molded body thus produced will follow dried and calcined to ceramic. The solid extrudate comprises a series of continuous Pores through which the exhaust gas flows and with the surface of the Catalyst in contact comes.

While accordingly the Vollextrudat is made of a ceramic mass, the SCR active is the catalytically active material which in particular the same composition as the mass of the Vollextrudats in the supported catalyst on the carrier material applied. This can be done by applying or dipping the substrate happen. Between the SCR-active material and the carrier material It is also possible to apply a layer of an aluminum oxide. As an alternative to a titanium dioxide ceramic can also be a zeolite as the catalytically active further components containing material be used.

In An advantageous embodiment of the invention is the oxidation catalyst the reduction catalyst as a saturation or impregnation applied. This is the later acting as an oxidation catalyst part of the reduction catalyst for example, in a solution submerged, the oxidation catalyzing substances or their reactive precursors contains. These substances are thereby impacted on the open surface of the catalytic active material of the reduction catalyst down or penetrate into the volume of the material. By any necessary after-treatment, how a temperature treatment in appropriate atmosphere, will then the impregnated Material of the reduction catalyst to the catalytically active material transformed the oxidation catalyst.

The catalytically active material of Redukti onskatalysators, which is used both as a coating for the support material of the plate catalyst and as a material of the bulk extrudate advantageously comprises predominantly titanium dioxide and as additives vanadium, molybdenum, tungsten and / or their oxides.

Regarding a big one catalytically active surface Titanium dioxide is used in the anatase structure. Such a thing Titanium dioxide can be produced, for example, by flame hydrolysis or by precipitation be.

The Catalytically active material of the oxidation catalyst preferably comprises also predominantly Titanium dioxide and as additives Platinum, rhodium and / or palladium. The precious metals can through impregnation of the catalytically active material of the reduction catalyst an aqueous solution of hexachloroplatinic acid, Palladium chloride and / or rhodium chloride are introduced.

alternative can on the catalytically active material of the reduction catalyst as oxidation coating a γ-alumina with additives of ceria and zirconia are applied. The precious metals will be then introduced into the alumina. As alumina can also a cordierite, i. a magnesium-aluminum silicate used. Furthermore may be the γ-alumina also be applied as a coating on the plate catalyst and for the formation of the catalytically active material of the reduction catalyst be impregnated with the appropriate substances.

embodiments The invention will be explained in more detail with reference to a drawing. Showing:

1 schematically a reduction catalyst with downstream oxidation catalyst and

2 a multi-part reduction catalyst with downstream oxidation catalyst, used for the exhaust gas treatment of an internal combustion engine.

1 schematically shows a trained as a fully extruded honeycomb body catalyst 1 how it could be used as downstream last single catalyst. The catalyst 1 includes a reduction catalyst 2 and downstream of an oxidation catalyst 3 , As a material for the honeycomb body of the catalyst 1 is a titanium dioxide ceramic used, which includes as catalytically active additives Vana dium and molybdenum and tungsten compounds. The downstream applied oxidation catalyst 3 is by impregnation of the reduction catalyst 2 made with platinum.

The illustrated catalyst 1 is traversed by the exhaust gas of an incinerator in the illustrated arrow direction. The exhaust 5 contains on the inlet side nitrogen oxides and oxygen and as additionally introduced reducing agent ammonia. To achieve an effective reduction of nitrogen oxides in the exhaust gas 5 Ammonia is slightly more than stoichiometrically metered. At the reduction catalyst 2 be in the exhaust 5 contained nitrogen oxides with ammonia in the presence of oxygen to form molecular nitrogen and water. Excess ammonia then flows through the downstream deposited oxidation catalyst 3 , There, ammonia is oxidized with oxygen to form molecular nitrogen and water. An ammonia slip into the environment is certainly avoided.

The volume fraction of the oxidation catalyst 3 on the total volume of the catalyst 1 is 18%. This ensures that the residual ammonia is oxidized to nitrogen, with undesirable side reactions, such as the formation of nitrous oxide or of new nitrogen oxides are avoided.

In 2 is an emission control system 6 for a diesel engine 8th shown as an internal combustion engine. The diesel engine 8th works superstoichiometrically; so that the resulting exhaust gas 5 Contains oxygen.

For cleaning, the exhaust gas flows 6 via an exhaust manifold 10 in an exhaust pipe 12 , then becomes via a catalyst 1 passed and enters the exhaust cleaned 14 in the area.

The in the exhaust pipe 12 arranged catalyst 1 consists of a total of four individual modules. Each of these individual modules is formed as a Vollextrudat from a titanium dioxide ceramic and SCR-active by additions of vanadium pentoxide, tungsten trioxide and molybdenum trioxide. The inlet side arranged three individual catalysts 15 are made entirely as reduction catalysts. The downstream last single catalyst 16 is made as a reduction catalyst whose last third is active by impregnating the SCR-active material with platinum and palladium as the oxidation catalyst. The volume fraction of the oxidation catalyst 3 to the total catalyst volume is 10%.

To remove the in the exhaust 5 contained nitrogen oxides is the exhaust gas 5 before reaching the catalyst 1 supplied as a reducing agent ammonia. This is in a storage container 18 stored an aqueous urea solution, which via a feed line 19 the exhaust pipe 12 is forwarded. About one in the supply line 19 arranged control valve 20 is the amount of aqueous urea solution to be added by the diesel engine 8th produced sticko adjusted to xidkonzentration. For this purpose, a control unit, not shown, accesses an implemented characteristic curve which predicts a nitrogen oxide concentration on the basis of current engine codes.

The supplied aqueous urea solution eventually becomes in the exhaust pipe 12 by means of a spray nozzle 21 finely atomised. By the in the exhaust 5 In this case urea is pyrolyzed or hydrolyzed in ammonia.

The ammonia added in approximately stoichiometric amounts in accordance with the nitrogen oxide concentration becomes the reduction catalyst 2 formed from the single catalysts 15 and 16 , reacted with the nitrogen oxides to form molecular nitrogen and water. Due to a low catalyst temperature or due to adsorption or desorption effects excess ammonia is subsequently to the oxidation catalyst 3 oxidized.

That of nitrogen oxides with optimal utilization of the reduction catalyst 2 cleaned exhaust gas 5 finally flows over the exhaust 14 in the nearby areas. The 10% volume fraction of the catalyst 1 as the oxidation catalyst 2 is used to safely prevent ammonia slip. Undesirable side reactions due to oxidative processes are certainly avoided due to the short oxidation range.

1

catalyst

2

reduction catalyst

3

oxidation catalyst

5

exhaust

6

emission control system

8th

diesel engine

10

exhaust

12

exhaust pipe

14

Exhaust

15

single catalyst

16

single catalyst

18

reservoir

19

feed

20

control valve

21

atomizer

Claims (7)

Catalyst ( 1 ) for the purification of oxygen-containing exhaust gases of a combustion plant, in particular an internal combustion engine operated with excess air, comprising a reduction catalyst ( 2 ) for the selective catalytic reduction of in the exhaust gas ( 5 ) contained nitrogen oxides by means of a reducing agent and a reduction catalyst ( 2 ) downstream of the deposited oxidation catalyst ( 3 ) for the oxidation of the reducing agent, wherein the oxidation catalyst ( 3 ) the reduction catalyst ( 2 ) is applied over a downstream region, which amounts to 1 to 19% of the total catalyst volume, characterized in that the reduction catalyst ( 2 ) from a number of individual catalysts ( 15 . 16 ), wherein the oxidation catalyst ( 3 ) the one or more downstream single catalyst (s) ( 15 . 16 ) is applied. Catalyst ( 1 ) according to claim 1, characterized in that the reduction catalyst ( 2 ) is formed as a supported catalyst with an SCR-active coating, whereupon the oxidation catalyst ( 3 ) is applied. Catalyst ( 1 ) according to claim 1, characterized in that the reduction catalyst ( 2 ) is formed as an SCR-active Vollextrudat, whereupon the oxidation catalyst ( 3 ) is applied. Catalyst ( 1 ) according to one of the preceding claims, characterized in that the oxidation catalyst ( 3 ) the reduction catalyst ( 2 ) is applied as a coating, in particular using a cordierite. Catalyst ( 1 ) according to one of the preceding claims, characterized in that the oxidation catalyst ( 3 ) the reduction catalyst ( 2 ) is applied as a saturation or impregnation. Catalyst ( 1 ) according to one of the preceding claims, characterized in that the catalytically active material of the reduction catalyst ( 2 ) predominantly comprises titanium dioxide and, as additives, vanadium, molybdenum, tungsten trioxide and / or their oxides. Catalyst ( 1 ) according to one of the preceding claims, characterized in that the catalytically active material of the oxidation catalyst ( 3 ) predominantly comprises titanium dioxide and, as additives, platinum, rhodium and / or palladium.