Catalyst system and use thereof

0001The present invention relates to a catalytic converter system and its use for cleaning the exhaust gases of an internal combustion engine, which contains at least two catalytic converters in a common converter housing, which are arranged one behind the other and at a distance from one another in the flow direction of the exhaust gas and which catalytically convert hydrocarbons and carbon monoxide into a lean exhaust gas can burn. The catalytic converter system is used in the exhaust system of a diesel engine to heat the exhaust gas to a soot ignition temperature for active regeneration of a downstream soot filter.

0002The soot produced by diesel engines must be removed from the exhaust gas by soot filters to keep the air clean. As soot accumulates in the filter, the exhaust back pressure generated by the filter increases and reduces engine performance. The filter must therefore be regenerated regularly by burning the soot.

0003Regeneration is a particular problem for modern diesel engines, since the exhaust gas from these engines is too cold during normal operation to ignite the combustion of the soot on the filter. This would require exhaust gas temperatures of 500 to 700 °C.

0004Passive and active methods have become known for the regeneration of soot filters. In the passive method, 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 an oxidizing agent at exhaust gas temperatures below 400 °C. For this purpose, an oxidation catalytic converter is arranged in front of the soot filter, which converts the nitrogen monoxide present in the exhaust gas into nitrogen dioxide. This procedure assumes that the exhaust gas contains sufficient nitrogen monoxide. The disadvantage is that the presence of hydrocarbons in the exhaust gas prevents the oxidation of nitrogen monoxide to form nitrogen dioxide. This defect can, according to the<patcit id="pcit0002" dnum="US6877313B1"><text>US 6,877,313 B1</text></patcit> be remedied in that two oxidation catalysts are arranged in front of the soot filter, the first catalyst essentially combusting 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 cannot guarantee 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 gas flow upstream of the oxidation catalytic converter and burned on the oxidation catalytic converter, or the concentration of unburned hydrocarbons in the exhaust gas is increased by engine measures. The catalytic combustion in the oxidation catalytic converter must heat up the exhaust gas by 200 to 400 °C to the soot ignition temperature, depending on the current operating status of the engine. High temperature peaks occur inside the oxidation catalytic converter, which can lead to premature aging of the catalytic converter. As a result, the conversion of hydrocarbons and carbon monoxide at the oxidation catalytic converter 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 consists of a plurality of catalyst disks arranged one behind the other in the flow direction of the exhaust gas. The catalytic converter disks consist of a sintered metal powder to convert the hydrocarbons, carbon monoxide and nitrogen oxides contained in the exhaust gas into 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 device which contains a three-way catalytic converter A for converting hydrocarbons, carbon monoxide and nitrogen oxides in the stoichiometric exhaust gas and a downstream catalytic converter B which has zeolites for absorbing hydrocarbons.

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

0009It is the object of the present invention to specify a catalytic converter system which has less aging during the catalytic combustion of hydrocarbons in the exhaust gas and can therefore be used advantageously in the active regeneration of soot filters.

0010This object is achieved by a catalyst system according to claim 1.

0011It has been shown that the aging of a catalytic converter in catalytic combustion can be slowed down if it is divided into at least two individual catalytic converters and these are arranged at a close distance of only a few millimeters from one another. This result is surprising, since the small distance of only a few millimeters between the consecutive catalytic converters cannot cool down the exhaust gas. Obviously, the temperature distribution in the catalysts is positively influenced by the separation of the catalysts. This avoids temperature peaks that would lead to rapid aging of the catalytic converter.

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

0013All catalysts that can oxidize hydrocarbons in the lean exhaust gas are suitable for the catalyst system. These are oxidation catalysts and three-way catalysts, which can also be used in combination.

0014The catalysts contain an oxidation-active catalyst layer applied to a ceramic or metallic honeycomb body and containing at least one noble metal from the group of platinum, palladium and rhodium on support materials from the group of active aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, cerium oxide and mixtures or mixed oxides thereof. In order to store hydrocarbons, the catalyst layer can additionally have zeolites. The two catalysts have different compositions.

0015The catalytic converter system is preferably used in the exhaust tract of a diesel engine to heat the lean exhaust gas to a temperature between 500 and 700° C. downstream of the last catalytic converter by burning fuel on the catalytic converters. The fuel required for this can be injected into the exhaust gas stream upstream of the first catalytic converter with the aid of the metering device, care being taken to ensure that the exhaust gas composition remains net oxidizing. Alternatively, the proportion of unburned 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, care also being taken 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 preferably heat the exhaust gas to a temperature of more than 500° C. and up to 700° C. by burning combustible components contained in the exhaust gas or supplied separately. Combustible components contained in the exhaust gas are, for example, unburned or incompletely burned hydrocarbons from the diesel fuel and carbon monoxide.

0017Surprisingly, the distance between the two catalysts leads to slower aging of the catalyst system, which can be seen from better conversion of carbon monoxide and hydrocarbons after aging. After aging, the catalytic converter system according to the invention therefore has lower emissions of carbon monoxide and hydrocarbons than a corresponding one-piece catalytic converter or than two catalytic converters arranged one behind the other without a gap.

0018On the one hand, the distance between the catalysts must not be too small in order to ensure the observed effect; on the other hand, too great a distance should be avoided for spatial and thermal reasons, since the exhaust gas between the catalytic converters is cooled as the distance increases. A distance of 2 to 30 mm between the two catalytic converters has proven useful for common cross-sectional dimensions of automotive exhaust gas catalytic converters with diameters between 10 and 20 centimeters. A distance between 5 and 20 mm is preferably chosen.

0019However, the distance between the two catalytic converters only has an effect on the aging of the rear catalytic converter in relation to the exhaust gas flow. In order to increase the aging stability of the front catalytic converter, a catalytic converter system that is as temperature-stable as possible should therefore be selected. Experience has shown that a catalyst containing platinum in combination with palladium withstands high temperatures better than a pure platinum catalyst. The platinum/palladium ratio of the first or front catalyst should preferably be between 4:1 and 1:1. The second catalytic converter has the task of burning as completely as possible the carbon monoxide that has not been completely converted by the first catalytic converter and the hydrocarbons. A platinum/palladium catalyst with a lower palladium content than in the first catalyst is suitable for this task.

0020The catalyst layers of the two catalysts therefore have different compositions in order to optimally do justice to their respective function in the catalyst system.

0021The raw emissions of a diesel engine during the cold start phase consist mainly of unburned hydrocarbons. During the cold start phase, these components cannot be converted by the still cold catalytic converters. In order to reduce these emissions, the front and/or the rear of the two catalytic converters of the catalytic converter system according to the invention can 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 catalytic converters, which are active at 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>figure 1</u>:</b></dt><dd>Structure of an exhaust system with a soot filter and a catalyst system according to the invention for active regeneration of the filter</dd><dt><b><u>figure 2</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 contains a first converter housing (2) in which two spaced apart oxidation-active catalytic converters (3) and (4) are located. The distance d between the two catalytic converters is in the range between 2 and 30 mm. The two catalysts can be of the same or different lengths. A second converter housing (5) with a soot filter (6) is located behind the catalytic converter system. The downstream soot filter can be actively regenerated by burning additional fuel in the catalytic converter system.

<u>example</u>

0024Three different catalytic converter systems on a diesel vehicle with a displacement of 2.2 liters were compared in terms of their residual emissions in the NEDC (New European Driving Cycle). In each case, honeycomb bodies made of cordierite with a cell density of 62 cm were used as catalyst carriers^-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 volume.<tables id="tabl0001" num="0001"><table frame="none"><tgroup cols="2"><colspec colnum="1" colname="col1" colwidth="22mm" /><colspec colnum="2" colname="col2" colwidth="144mm" /><tbody><row><entry colsep="0" rowsep="0">System 1:</entry><entry rowsep="0">One-piece honeycomb diameter 14.4 cm; Length 8.9cm</entry></row><row><entry colsep="0" rowsep="0">System 2:</entry><entry rowsep="0">Two honeycomb diameter 14.4 cm; length of each honeycomb 4.45 cm; Distance d between the honeycomb bodies: 0.00 cm</entry></row><row><entry colsep="0">System 3:</entry><entry>Two honeycomb bodies according to the invention diameter 14.4 cm; length of each honeycomb 4.45 cm; Distance d between the honeycomb bodies: 1 cm</entry></row></tbody></tgroup></table></tables>

0025The catalyst systems were realistically aged for 21 hours by post-injection. After that, their residual emissions were measured on a diesel vehicle in the NEDC. The results are in<figref idref="f0001">figure 2</figref> shown.

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 with no gap between the honeycomb bodies.