WO2007104382A1 - Catalyst for exhaust gas cleaning consisting of a number of individual catalysts - Google Patents

Abstract

A catalyst (1) is specified for the cleaning of oxygenous exhaust gases (5) of a combustion unit, especially of a combustion engine operated with an air excess, comprising a reduction catalyst (2) for the selective catalytic reduction of nitrogen oxides present in the exhaust gas (5) by means of a reducing agent, and an oxidation catalyst (3) installed on the downstream side of the reduction catalyst (2) for oxidizing the reducing agent. It is envisaged that the oxidation catalyst (3) is installed over a region upstream of the reduction catalyst (2) which is 1-19% of the total catalyst volume. The reduction catalyst (2) is composed of a number of individual catalysts (15, 16) together, the oxidation catalyst (3) being installed as the last individual catalyst(s) in the downstream direction. In various uses, the catalyst (1) achieves a high conversion of nitrogen oxides with avoidance of reducing agent slippage and undesired side reactions.

Description

 description

CATALYST FOR EXHAUST CLEANING COMPRISING A NUMBER OF

SINGLE CATALYSTS

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

Such a catalyst is known from EP 1 264 628 A1 and is used to purify the oxygen-containing exhaust gases of a combustion plant by means of selective catalytic reduction of nitrogen oxides, without the reductant used being released into the environment. The reduction catalyst hereby reduces by means of a reducing agent, such as e.g. Ammonia, in the presence of oxygen nitrogen oxides to molecular nitrogen and water. This method of selective catalytic reduction is also known by the abbreviation SCR method. As the source of the reducing agent, a reductant-donating substance, e.g. Urea, which liberates ammonia in the exhaust gas added.

Furthermore, a similar catalyst is known from EP 0410440 B1, wherein the downstream region of the reduction catalyst provided with the oxidation catalyst is 20-50% of the total volume.

The delivery of the added reducing agent to the environment must be kept as low as possible, for example, to avoid odor nuisance. For this reason, in the applied SCR method, a substoichiometric metering of the reducing agent takes place with respect to the actual or the expected nitrogen oxide content of the exhaust gas. Although this reliably prevents a reduction agent emission, the reduction catalyst does not have the maximum possible, but a smaller amount of nitrogen oxides eliminated. As a consequence, the reduction catalyst must be designed to be sufficiently large and the control of the reducing agent metering work exactly to achieve a significant reduction of nitrogen oxides, in particular during transient engine operation, as is usual in a motor vehicle.

A catalyst of the type mentioned circumvents this problem by the downstream of the reduction catalyst an oxidation catalyst for the oxidation of the reducing agent is applied. In this oxidation catalyst, in the presence of oxygen, the reducing agent, in particular ammonia, into harmless compounds, in particular molecular nitrogen and water, oxidized.

For this purpose, the reduction catalyst produced in one piece according to EP 0 410 440 B1 is coated downstream with the oxidation catalyst, the coated area making up 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.

Disadvantageously, such a catalyst has undesirable side reactions. So again nitrogen oxides can be formed. Also, for example, ammonia can be converted to nitrous oxide or ammonium nitrate in the presence of excess air. Optionally, hydrocarbons present in the exhaust may react to harmful nitro compounds. According to EP 1 264 628 A1, the oxidation catalyst is applied to a honeycomb-shaped reduction catalyst over a downstream length of 1 to 20% of the total length.

The object of the invention is to provide a catalyst of the type mentioned above, which allows avoiding a reducing agent slip and undesirable side reactions as high as possible degree of conversion of nitrogen oxides, and is as versatile as possible. This object is achieved according to the invention for a catalyst according to the preamble of claim 1 in that the reduction catalyst is composed of a number of individual catalysts, wherein the oxidation catalyst is applied to the last individual catalyst (s) on the outflow side. Extensive investigations have shown that with a Reduction catalyst, downstream of which an oxidation catalyst is applied, unwanted side reactions are reliably avoided and a high degree of conversion of nitrogen oxides can be achieved when the oxidation catalyst is applied downstream over a range which is 1 - 19% of the total catalyst volume.

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 causing a reduction agent slip. In the case of small 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 unreacted 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.

The specified catalyst can be produced inexpensively, since no two separate catalysts for reduction or for oxidation must be made. For the preparation of the reduction catalyst is prepared in a conventional manner. Subsequently, the composition of the catalytically active surface is changed downstream of this reduction catalyst to create the oxidation catalytic converter. This can be done, for example, by re-coating or by introducing compounds or elements catalyzing the oxidation of the reducing agent. Alternatively, it would also be conceivable to replace the existing catalytically active surface in terms of reduction. The catalyst is included in the - A -

Gas channel of a combustion system and in particular used in the exhaust passage of an internal combustion engine. In order to be able to respond flexibly to different spatial conditions and exhaust gas compositions, the reduction catalytic converter is composed of a number of individual catalytic converters, wherein the oxidation catalytic converter is applied to the last single catalytic converter or catalytic converters on the outflow side. In this case, the desired length of the reduction catalyst can be set in a modular manner via the number of individual catalysts. The desired downstream of the entire catalyst volume outflow area, which acts as an oxidation catalyst, may extend over several of the individual catalysts downstream. With a small volume range or with relatively long individual catalytic converters, only the last individual catalytic converter considered in the flow direction of the exhaust gas is correspondingly provided with the oxidation catalytic converter. The modular design of the catalyst as a whole offers a cost advantage, as this significantly reduces the variety of types of catalysts to be produced. Furthermore, a lower thermal load of the monoliths is achieved by the shorter individual modules.

As such, the reduction catalyst or catalysts may be formed as a supported catalyst having an SCR-active coating, and then the oxidation catalyst is applied. In this embodiment variant, the catalytically active coating is applied to a usually plate-shaped carrier material. The coated carrier plates are then stacked to form the catalyst, wherein for the flowing exhaust gas flow channels are formed by introduced into the substrate corrugations and / or waves.

In another advantageous embodiment, the reduction catalyst is formed as an SCR active Vollextrudat, whereupon the oxidation catalyst is applied. The bulk extrudate consists of a ceramic mass which is produced and extruded as a slurry of metal oxides, in particular titanium dioxide. The shaped body thus produced is then dried and calcined to ceramic. The bulk extrudate comprises a series of continuous pores through which the exhaust gas flows and contacts the surface of the catalyst. Accordingly, while the bulk extrudate is made of a ceramic mass which is SCR active overall, the catalytically active material, which may in particular have the same composition as the mass of the bulk extrudate, is applied to the support material in the supported catalyst. This can be done by applying or dipping the substrate. A layer of an aluminum oxide can also be applied between the SCR-active material and the carrier material. As an alternative to a titanium dioxide ceramic, it is also possible to use a zeolite as material containing the catalytically active further components.

In an advantageous embodiment of the invention, the oxidation catalyst is applied to the reduction catalyst as an impregnation or impregnation. Here, the later acting as an oxidation catalyst part of the reduction catalyst is immersed, for example, in a solution containing an oxidation catalyzing substances or their reactive precursors. These substances precipitate on the open surface of the catalytically active material of the reduction catalyst or penetrate into the volume of the material. By optionally required aftertreatment, such as a temperature treatment in a corresponding atmosphere, the impregnated material of the reduction catalyst is then converted to the catalytically active material of the oxidation catalyst.

The catalytically active material of the reduction catalyst, which is used both as Besen ichtung 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. With regard to a large catalytically active surface, titanium dioxide is used in the anatase structure. Such a titanium dioxide may be prepared, for example, by flame hydrolysis or by precipitation.

The catalytically active material of the oxidation catalyst preferably also comprises predominantly titanium dioxide and, as additives, platinum, rhodium and / or palladium. The noble metals can be introduced by impregnation of the catalytically active material of the reduction catalyst with an aqueous solution of hexachloroplatinic acid, palladium chloride and / or rhodium chloride. Alternatively, a γ-aluminum oxide with additions of cerium oxide and zirconium oxide can be applied to the catalytically active material of the reduction catalyst as the oxidation coating. The noble metals are then introduced into the alumina. As the alumina, a cordierite, ie, a magnesium-aluminum silicate, can also be used. Incidentally, the γ-aluminum oxide can also be applied as a coating on the plate catalyst and be impregnated with the corresponding substances to form the catalytically active material of the reduction catalyst.

Embodiments of the invention will be explained in more detail with reference to a drawing. Showing:

FIG. 1 schematically shows a reduction catalyst with downstream oxidation catalyst and

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

1 shows schematically a catalyst 1 designed as a vol-extruded honeycomb body, as it could be used as the last single catalyst downstream. The catalyst 1 comprises a reduction catalyst 2 and a downstream oxidation catalyst 3. The material used for the honeycomb body of the catalyst 1 is a titanium dioxide ceramic which comprises catalytically active additives vanadium and molybdenum and tungsten compounds. The downstream applied oxidation catalyst 3 is prepared by impregnation of the reduction catalyst 2 with platinum.

The illustrated catalyst 1 is flowed through by the exhaust gas of an incinerator 5 in the illustrated arrow direction. The exhaust gas 5 contains on the inlet side nitrogen oxides and oxygen and as additionally introduced reducing agent ammonia. To achieve an effective reduction of the nitrogen oxides in the exhaust gas 5, ammonia is added slightly more than stoichiometrically. At the reduction catalyst 2 are 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 molecular nitrogen and water. An ammonia slip into the environment is certainly avoided.

The volume fraction of the oxidation catalyst 3 in 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 Fig. 2, an emission control system 6 for a diesel engine 8 is shown as an internal combustion engine. The diesel engine 8 works more than stoichiometrically, so that the resulting exhaust gas 6 contains oxygen.

For purification, the exhaust gas 6 flows via an exhaust manifold 10 into an exhaust pipe 12, is then passed through a catalytic converter 1, and cleans out via the exhaust 14 into the environment.

The arranged in the exhaust pipe 12 catalyst 1 is composed of a total of four individual modules. Each of these individual modules is formed as a bulk extrudate of titanium dioxide ceramic and SCR-active by additions of vanadium pentoxide, tungsten trioxide and molybdenum trioxide. The three individual catalytic converters 15 arranged on the inflow side are manufactured entirely as reduction catalysts. The last single catalyst 16 on the outflow side is manufactured 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 nitrogen oxides contained in the exhaust gas 5, the exhaust gas 5 is supplied before reaching the catalyst 1 as a reducing agent ammonia. For this purpose, an aqueous urea solution is stored in a reservoir 18, which is fed via a feed line 19 to the exhaust pipe 12. About a arranged in the supply line 19 Control valve 20 is adapted to be metered amount of aqueous urea solution of the nitrogen oxide concentration generated by the diesel engine 8. 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 fed aqueous urea solution is finally finely atomized in the exhaust pipe 12 by means of a spray nozzle 21. As a result of the temperature prevailing in the exhaust gas 5, urea is pyrolyzed or hydrolyzed in ammonia.

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

The purified by nitrogen oxides under optimal utilization of the reduction catalyst 2 exhaust gas 5 finally flows through the exhaust 14 into the environment. The 10% by volume of the catalyst 1, which is used as the oxidation catalyst 2, is sufficient to safely prevent ammonia slip. Undesirable side reactions due to oxidative processes are certainly avoided due to the short oxidation range.

LIST OF REFERENCE NUMBERS

catalyst

reduction catalyst

oxidation catalyst

exhaust

emission control system

diesel engine

exhaust

exhaust pipe

Exhaust

single catalyst

single catalyst

reservoir

feed

control valve

atomizer

Claims

claims 1. Catalyst (1) for purifying oxygen-containing exhaust gases of an incinerator, in particular a 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 a reduction catalyst (2) oxidation catalyst (3) applied downstream, for oxidation of the reducing agent, wherein the oxidation catalyst (3) is applied to the reduction catalyst (2) via an outflow-side region which amounts to 1 to 19% of the total catalyst volume, characterized in that the reduction catalytic converter (2) is composed of a number of individual catalytic converters (15, 16), wherein the oxidation catalytic converter (3) is applied to the last individual catalytic converter (s) (15, 16) downstream of the latter. 2. Catalyst (1) according to claim 1, characterized in that the reduction catalyst (2) is designed as a supported catalyst with an SCR active coating, whereupon the oxidation catalyst (3) is applied. 3. Catalyst (1) according to claim 1, characterized in that the reduction catalyst (2) is formed as a SCR-active Vollextrudat, whereupon the oxidation catalyst (3) is applied. 4. Catalyst (1) according to one of the preceding claims, characterized in that the oxidation catalyst (3) the reduction catalyst (2) as a coating, in particular using a cordierite, is applied. 5. Catalyst (1) according to one of the preceding claims, characterized in that the oxidation catalyst (3) is applied to the reduction catalyst (2) kung or impregnation kung or impregnation. 6. Catalyst (1) according to one of the preceding claims, characterized in that the catalytically active material of the reduction catalyst (2) comprises predominantly o titanium dioxide and as additives vanadium, molybdenum, tungsten trioxide and / or their oxides. 7. Catalyst (1) according to one of the preceding claims, characterized in that the catalytically active material of the oxidation catalyst (3) predominantly Titanium dioxide and platinum, rhodium and / or palladium as additives.