Catalyst system and use thereof

0001The present invention relates to a catalyst system and its use for the purification of the exhaust gases of a combustion engine which comprises, in a common converter housing, at least two catalytic converters arranged one behind the other and at a distance from one another in the direction of flow of the exhaust gas and catalytically reducing hydrocarbons and carbon monoxide in a lean exhaust gas Can burn. The catalyst system is used in the exhaust system of a diesel engine for heating the exhaust gas to a soot ignition temperature for the active regeneration of a downstream soot filter.

0002The soot generated by diesel engines must be removed from the exhaust gas for air purification by soot filters. As the soot build up in the filter increases, the exhaust gas pressure generated by the filter increases and decreases the power of the engine. The filter must therefore be regularly regenerated by burning the soot.

0003Regeneration is a problem, especially in modern diesel engines, since the exhaust gas of these engines is too cold in normal operation to ignite the soot on the filter. For this purpose, exhaust gas temperatures of 500 to 700 ° C. would be necessary.

0004Passive and active processes have become known for the regeneration of soot filters. In the case of the passive methods, for example, according to the<patcit id="pcit0001" dnum="EP0341832A2"><text>EP 0 341 832 A2</text></patcit> A soot filter is continuously oxidized by nitrogen dioxide as oxidizing agent at exhaust gas temperatures below 400 ° C. For this purpose, an oxidation catalytic converter is arranged upstream of the soot filter, which converts the nitric oxide present in the exhaust gas into nitrogen dioxide. This process assumes that the exhaust gas contains sufficient nitric oxide. It is disadvantageous that the presence of hydrocarbons in the exhaust gas impedes the oxidation of nitric oxide to nitrogen dioxide. This defect may be determined in accordance with the<patcit id="pcit0002" dnum="US6877313B1"><text>US 6,877,313 B1</text></patcit> In that two oxidation catalysts are arranged upstream of the soot filter, the first catalyst essentially burning the hydrocarbons in the exhaust gas and the second catalyst oxidizing the nitrogen oxides remaining in the exhaust gas to form nitrogen dioxide.

0005Passive methods can not ensure a fail-safe operation of the soot filter. At least from time to time, the filter must be actively regenerated. For this purpose, fuel is injected into the exhaust stream upstream of the oxidation catalytic converter and burned on the oxidation catalytic converter, or the concentration of unburnt hydrocarbons in the exhaust gas is increased by engine measures. The catalytic combustion in the oxidation catalytic converter has to heat the exhaust gas by 200 to 400 ° C. to the soot ignition temperature, depending on the instantaneous operating state of the engine. High temperature peaks occur in the interior of the oxidation catalyst which can lead to premature aging of the catalyst. As a result, the reaction of the hydrocarbons and of carbon monoxide on the oxidation catalyst deteriorates with increasing operating time.

0006The <patcit id="pcit0003" dnum="DE102005017378A1"><text>*** "" DE 10 2005 017 378 A1. *** "</text></patcit> Describes an exhaust gas purification device with a catalyst body which, in the flow direction of the exhaust gas, consists of a plurality of catalyst disks disposed one behind the other. The catalyst disks consist of a sintered metal powder for converting the hydrocarbons, carbon monoxide and nitrogen oxides contained in the exhaust gas to carbon dioxide, water and nitrogen. The distance between the catalyst disks is about 3 mm.

0007The <patcit id="pcit0004" dnum="US20030099583A1"><text>US 2003/0099583 A1</text></patcit> Describes an exhaust gas purification apparatus which comprises a three-way catalyst A for reacting hydrocarbons, carbon monoxide and oxides of nitrogen in the stoichiometric exhaust gas, and a downstream catalyst B comprising zeolites for the absorption of hydrocarbons.

0008document <patcit id="pcit0005" dnum="JP2006329020A"><text>JP 2006 329020 A</text></patcit> Discloses a catalyst system for the purification of the exhaust gases of a combustion engine comprising, in a common converter housing, at least two catalytic converters which are arranged one behind the other in the direction of flow of the exhaust gas and at a distance d from one another and can catalytically burn hydrocarbons and carbon monoxide in a lean exhaust gas followed by a second Converter housing with a soot filter.

0009It is an object of the present invention to provide a catalyst system which has a lower aging in the catalytic combustion of hydrocarbons in the exhaust gas and can therefore advantageously be used in the active regeneration of soot filters.

0010This object is achieved by a catalyst system which comprises, in a common converter housing, at least two catalytic converters which are arranged one behind the other and at a distance from one another in the flow direction of the exhaust gas and which can catalytically combust hydrocarbons and carbon monoxide in a lean exhaust gas. The catalyst system is characterized in that the distance between two adjacent catalysts is between 2 and 30 mm and that the catalysts are oxidation catalysts or a combination of oxidation catalysts and three-way catalysts.

0011It has been found that the aging of a catalyst can be slowed down during catalytic combustion when it is divided into at least two individual catalysts and these are arranged at a narrow distance of only a few millimeters from one another. This result is surprising because the small distance of only a few millimeters between the successive catalysts can not cool the exhaust gas. Obviously, the temperature distribution in the catalysts is positively influenced by the separation of the catalysts. This prevents temperature peaks which would lead to rapid aging of the catalyst.

0012The hydrocarbons required for the catalytic combustion can be sprayed into the exhaust stream either with the aid of a metering device arranged in front of the first catalytic converter or the fraction of unburnt hydrocarbons in the exhaust gas of the engine can be increased by engine measures, for example by post-injection of fuel into the cylinders of the engine.

0013Suitable catalysts for the catalyst system are those which can oxidize hydrocarbons in the lean exhaust gas. In particular, these catalysts are oxidation catalysts and three-way catalysts, which can also be used in combination.

0014The catalysts preferably contain an oxidation-active catalyst layer which is applied to a ceramic or metallic honeycomb body and which comprises at least one noble metal from the group consisting of platinum, palladium and rhodium on carrier materials from the group consisting of active alumina, silicon oxide, titanium oxide, zirconium oxide, cerium oxide and mixtures or mixed oxides thereof. For storage of hydrocarbons, the catalyst layer may additionally comprise zeolites. The two catalysts may have the same or different compositions

0015The catalyst system is preferably used in the exhaust gas tract of a diesel engine for heating the lean exhaust gas to a temperature between 500 and 700 ° C. behind the last catalyst by burning fuel on the catalysts. The fuel required for this purpose can be introduced into the exhaust stream with the aid of the metering device upstream of the first catalytic converter, whereby it must be ensured that the exhaust gas composition remains net oxidizing. Alternatively, the proportion of unburnt hydrocarbons in the exhaust gas can be increased by engine measures, for example by post-injection of fuel into the cylinders of the diesel engine, it also being necessary to ensure that the exhaust gas composition remains net oxidizing. With the exhaust gas heated in this way, a soot filter arranged behind the catalytic converter system can be actively regenerated.

0016The purpose of the proposed catalyst system is to heat the exhaust gas preferably to a temperature of more than 500 ° C. and up to 700 ° C. by burning combustible components contained in the exhaust gas or separately supplied. Combustible components contained in the exhaust gas are, for example, unburned or incompletely combusted hydrocarbons from diesel fuel and carbon monoxide.

0017The distance between the two catalysts surprisingly leads to a slower aging of the catalyst system, which can be seen in a better conversion of carbon monoxide and hydrocarbons after aging. The catalyst system according to the invention therefore has, after aging, lower emissions of carbon monoxide and hydrocarbons than a corresponding one-piece catalyst or two catalysts arranged one after the other without any spacing.

0018The distance between the catalysts, on the one hand, must not be too small to ensure the observed effect; On the other hand, a too great distance is to be avoided for spatial and thermal reasons since the exhaust gas is cooled between the catalysts with increasing distance. For common cross-sectional dimensions of car exhaust gas catalysts with diameters between 10 and 20 centimeters, a distance between the two catalysts of 2 to 30 mm has proven to be successful. A distance between 5 and 20 mm is preferably selected.

0019However, the distance between the two catalysts has only an influence on the aging of the rear catalyst which is related to the exhaust gas flow. In order to increase the aging stability of the front catalyst, a catalyst system which is as stable as possible should therefore be selected. Experience has shown that a catalyst containing platinum in combination with palladium is more resistant to high temperatures than a pure platinum catalyst. Preferably the platinum / palladium ratio of the first or front catalyst should be between 4: 1 and 1: 1. The second catalyst has the task of burning as completely as possible the carbon monoxide, which is not completely converted by the first catalyst, and the hydrocarbons. For this purpose, a platinum / palladium catalyst with a lower palladium content than in the first catalyst or a pure platinum catalyst is suitable.

0020The catalyst layers of the two catalysts can therefore have different compositions in order to optimally meet their respective function in the catalyst system. Preferably, however, both catalyst layers are selected identically.

0021The raw emissions of a diesel engine during the cold start phase consist essentially of unburned hydrocarbons. During the cold start phase these components can not be converted by the still cold catalysts. To reduce these emissions, the front and / or rear of the two catalysts of the catalyst system according to the invention may contain zeolites which temporarily store the hydrocarbons. As the temperature of the exhaust gas increases, the stored hydrocarbons are desorbed again and can then be converted by the catalysts active at the higher temperatures.

0022The invention is explained in more detail below with reference to the example and the two figures. It shows:<dl id="dl0001"><dt><b><u>FIG</u>:</b></dt><dd>Construction of an exhaust system with soot filter and catalyst system according to the invention for the active regeneration of the filter</dd><dt><b><u>FIG</u>:</b></dt><dd>Comparison of emissions from three different catalyst systems</dd></dl>

0023The FIGURE shows an exhaust system (1) of a diesel engine. The exhaust system includes a first converter housing (2) in which two oxidation-active catalysts (3) and (4) are located, which are spaced apart from one another. The distance d of the two catalysts is in the range between 2 and 30 mm. The two catalysts can be identical or different in length. Behind the catalyst system is a second converter housing (5) with a soot filter (6). By burning additional fuel in the catalyst system, the downstream soot filter can be actively regenerated.

<u>example</u>

0024Three different catalyst systems were compared on a 2.2 liter diesel engine with regard to their residual emissions in the NEDC (New European Driving Cycle). As catalyst carriers were in each case honeycomb bodies made of cordierite with a cell density of 62 cm^-2 And a thickness of the channel walls of 0.17 mm, which were coated with a conventional diesel oxidation catalyst with 3.18 g of platinum per liter of honeycomb body volume.<dl id="dl0002"><dt>System 1:</dt><dd>One-piece honeycomb body Diameter 14.4 cm; Length 8.9 cm</dd><dt>System 2:</dt><dd>Two honeycomb bodies Diameter 14.4 cm; Length of each honeycomb body 4.45 cm; Distance d between the honeycomb bodies: 0.00 cm</dd><dt>System 3:</dt><dd>Two honeycomb bodies according to the invention Diameter 14.4 cm; Length of each honeycomb body 4.45 cm; Distance d between the honeycomb bodies: 1 cm</dd></dl>

0025The catalyst systems were aged for a period of real-time by post-injection for 21 hours. After that, their residual emissions were measured on a diesel vehicle in the NEDC. The results are in<figref idrefs="f0001">FIG</figref> Respectively.

0026The catalyst system 3 according to the invention shows significantly less CO and HC emissions during the test than the conventional one-piece system 1 and the system 2 with two honeycomb bodies without a space between the honeycomb bodies.