Catalytic Converter Configuration and Method for Desulfurizing a NOx Storage Catalytic Converter

Abstract

A catalytic converter configuration for an internal combustion engine that is capable of running lean has an exhaust gas duct and a NOx storage catalytic converter arranged in the exhaust gas duct. A novel method allows desulfurizing the NOx storage catalytic converter. An H2S storage catalytic converter is provided. This catalytic converter is capable of storing hydrogen sulfide under a rich or stoichiometric exhaust gas atmosphere with lambda≦1 and oxidizing hydrogen sulfide under a lean exhaust gas atmosphere with lambda>1.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority, under 35 U.S.C. §119, of German patent application DE 10 2006 038 367.2, filed Aug. 16, 2006; the prior application is herewith incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to a catalytic converter configuration for internal combustion engines capable of running lean and having a NO_x storage catalytic converter arranged in their exhaust gas duct. The invention also relates to a method for desulfurizing the NO_x storage catalytic converter.

In internal combustion engines which are operated with a lean fuel-air mixture, traditional three-way catalytic converters are not sufficient for quantitative conversion of all exhaust gas constituents, in particular nitrogen oxides NO_x. Therefore, the arrangement of so-called NO_x storage catalytic converters in the exhaust gas ducts of such internal combustion engines is known. NO_x storage catalytic converters have a NO_x storage component which stores NO_x under a lean exhaust gas atmosphere. NO_x regeneration is performed through short rich intervals during which the stored NO_x is desorbed and converted to N₂ and O₂ by means of a catalytic component of the NO_x storage catalytic converter. The use of NO_x storage catalytic converters is known in both diesel engines and lean mix gasoline engines.

Due to the sulfur that is present in the fuel, the NO_x storage catalytic converter loses activity over its surface life. This is referred to as sulfur poisoning. The sulfur is deposited in the form of sulfate on the storage components of the catalytic converter, thereby blocking the NO_x storage sites of the storage catalytic converter so they are no longer available for storing NO_x. If a lower limit of NO_x storage capacity of the NO_x storage catalytic converter is reached, it must be desulfurized (desulfated). This is performed at high exhaust gas temperatures of at least 500° C. by exposing the storage catalytic converter to a rich exhaust gas atmosphere (lambda<1). At these temperatures, the stored sulfur is desorbed and reduced to SO₂ by the reducing exhaust gas atmosphere while also being reduced to hydrogen sulfide H₂S. The richer the exhaust gas conditions during desulfurization, the faster the desorption and reduction of sulfur take place, which has positive effects on thermal aging of the storage catalytic converter, but is associated with an increase in the unwanted formation of H₂S.

It is known that to perform desulfurization, the engine is operated alternately in rich operation with lambda<1 and in lean operation with lambda>1 for short intervals of time, i.e., in so-called wobble operation. This promotes the heating of the catalytic converter to the required desulfurization temperature and maintaining same. Furthermore, due to the alternating exposure of the storage catalytic converter to rich and lean exhaust gas, production of H₂S is decreased because in the lean intervals oxygen is incorporated into the catalytic converter, thereby ensuring oxidation of H₂S, which is formed as an intermediate, to SO₂. However, it has been found that complete desulfurization of a storage catalytic converter in wobble operation is also associated with a certain production of H₂S, the concentration of which may result in exceeding limit values.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a catalytic converter configuration and a method for desulfurizing a NO_x storage catalyst which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which allows desulfurizing the NO_x storage catalyst in such a way that it protects the catalytic converter and in which production of H₂S is minimized. It is a concomitant object to provide for a catalytic converter configuration that is suitable for implementing the novel method.

With the foregoing and other objects in view there is provided, in accordance with the invention, a catalytic converter configuration for an internal combustion engine that is capable of running lean, comprising:

an exhaust gas duct connected with the internal combustion engine;

a NO_x storage catalytic converter disposed in said exhaust gas duct; and

an H₂S storage catalytic converter configured to store hydrogen sulfide under a rich or stoichiometric exhaust gas atmosphere with lambda≦1 and to release the hydrogen sulfide under a lean exhaust gas atmosphere with lambda>1.

In other words, the objects of the invention are achieved by a catalytic converter configuration that includes an H₂S storage catalytic converter that is suitable for storing hydrogen sulfide H₂S under rich or stoichiometric exhaust gas atmosphere, i.e., with an air/fuel ratio of lambda≦1 and then releasing it in a lean exhaust gas atmosphere, with lambda>1, in particular essentially as sulfur dioxide SO₂. Through the inventive arrangement of the H₂S storage catalytic converter it is possible to desulfurize the NO_x storage catalytic converter under constantly rich exhaust gas conditions and thus at relatively low temperatures, thereby delaying its thermal aging. At the same time unwanted H₂S emissions are greatly reduced or even prevented entirely.

According to an advantageous embodiment of the invention, the H₂S storage catalytic converter is connected downstream from the NO_x storage catalytic converter in the direction of flow of the exhaust gas, the H2S storage catalytic converter being arranged directly adjacent to the NO_x storage catalytic converter or at a distance therefrom. As an alternative the H₂S storage catalytic converter may also be integrated into the NO_x storage catalytic converter, i.e., there may be a mixed coating of H₂S and NO_x storage catalytic converter components on a common support for the catalytic converter. All these requirements ensure that the H₂S released from the NO_x storage catalytic converter is stored and converted in the H₂S storage catalytic converter.

As the H₂S storage component of the H₂S storage catalytic converter, it preferably contains a metal component which is suitable for binding H₂S as metal sulfide in particular under rich or stoichiometric exhaust gas atmosphere. The prerequisite for selecting the suitable metal component is its suitability for entering into a metal-sulfur compound in the rich exhaust gas and releasing the sulfur, preferably in the form of SO₂, under lean conditions. Metals of subgroups I, II and/or VIII of the periodic system of elements are especially suitable for this purpose, preferably using silver (Ag), zinc (Zn), cadmium (Cd), iron (Fe), cobalt (Co) and/or nickel (Ni). The loading of the H₂S storage catalytic converter with the metal component depends on the desired H₂S storage capacity of the H₂S storage catalytic converter, in particular after typical sulfur loading of the NO_x storage catalytic converter to be removed at the time of its desulfurization. In typical cases, a specific metal loading of at least 0.2 g/l has proven advantageous, in particular at least 0.3 g/L, especially preferably at least 0.5 g/l.

Furthermore it is preferably also provided that the H₂S storage catalytic converter may additionally have an oxidation catalytic component or a three-way catalytic component under a lean exhaust gas atmosphere, i.e., it catalyzes oxidation of carbon moNOxide CO and/or hydrocarbons HC and—in the case of a three-way catalytic effect—additionally supports reduction of nitrogen oxides NO_x. To do so, the catalytic converter may additionally contain at least one noble metal, e.g., platinum (Pt), palladium (Pd) and/or rhodium (Rh).

Another advantageous embodiment according to the invention, the catalytic converter configuration also has an oxygen-storing component downstream from the NO_x storage catalytic converter and/or the H₂S storage catalytic converter, this oxygen-storing component being capable of storing oxygen in a lean exhaust gas atmosphere and releasing oxygen under a rich exhaust gas atmosphere, so that H₂S desorbed from the NO_x storage catalytic converter is oxidized to SO₂. Such oxygen-storing components (OSC for oxygen storage component) are known in the state of the art and therefore will not be explained further here. In addition with the help of the oxygen stored in the OSC, the components CO and HC can also be converted and thus emissions in rich operation can be reduced.

The present invention also relates to a method for desulfurizing a NO_x storage catalytic converter in which hydrogen sulfide coming from the NO_x storage catalytic converter is stored in an H₂S storage catalytic converter and is released again, preferably as SO₂ under a lean exhaust gas atmosphere having a lambda>1 in particular above a certain catalytic converter temperature. The release temperature in the H₂S storage catalytic converter may be induced by an upstream particle filter reaction, for example (by way of PF regeneration).

It is especially advantageous to alternately treat the NO_x storage catalytic converter in so-called wobble operation during desulfurization, i.e., with rich intervals with lambda≦1 and lean intervals with lambda>1. However, the desulfurization may also essentially be performed in continuous rich operation of the internal combustion engine using the catalytic converter configuration described above.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in catalytic converter configuration and method for desulfurizing a NO_x storage catalytic converter, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 shows an internal combustion engine having a downstream catalytic converter configuration according to a first embodiment of the invention;

FIG. 2 shows an internal combustion engine having a downstream catalytic converter configuration according to a second embodiment of the invention;

FIG. 3 shows an internal combustion engine having a downstream catalytic converter configuration according to a third embodiment of the invention;

FIG. 4 shows curves of the lambda value as a function of time, measured upstream and downstream from the NO_x storage catalytic converter as well as the SO₂ and H₂S concentrations after the NO_x storage catalytic converter during desulfurization of the NO_x storage catalytic converter in the catalytic converter configuration according to FIG. 1; and

FIG. 5 shows curves of the SO₂ and H₂S concentrations downstream from the NO_x storage catalytic converter according to the present invention and the state of the art as a function of time.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is illustrated an internal combustion engine 10 which can be operated with a lean air-fuel mixture. This may be a diesel engine or a gasoline engine capable of running lean.

According to a first advantageous embodiment of the present invention, a catalytic converter configuration 12 is connected downstream from the internal combustion engine 10. Exhaust gas emanating from the internal combustion engine 10 is directed into the catalytic converter configuration, where it is after-treated. The catalytic converter configuration 12 comprises an exhaust gas duct 14 in which various catalytic converters are arranged. According to the embodiment depicted here, an oxidation catalytic converter or a three-way catalytic converter that acts as a precatalytic converter 16 is arranged in a position near the internal combustion engine 10. Alternatively or in addition to the precatalytic converter 16 a particulate filter (not shown here) may also be arranged near the engine for storing soot (carbon black) particles and may optionally also be combined with the oxidative or three-way precatalytic converter 16 on a common support.

Downstream from the precatalytic converter 16 (and/or the particle filter provided as an alternative or in addition), a NO_x storage catalytic converter 18 is provided in the exhaust gas duct 14. The NO_x storage catalytic converter 18 has a NO_x storage component which stores NO_x under a lean exhaust gas atmosphere and releases it again in the intermediate regeneration intervals during which the internal combustion engine 10 is operated with a stoichiometric or rich air-fuel mixture. An oxidation component or three-way component integrated into the NO_x storage catalytic converter 18 catalyzes the reaction of the desorbed nitrogen oxides N₂ and oxygen O₂ during the NO_x regeneration.

In addition to NO_x storage, incorporation of sulfur present in the fuel also takes place in the form of sulfate in the NO_x storage catalytic converter 18 in an unwanted manner. Since the sulfur is not removed from the NO_x storage catalytic converter 18 during the usual NO_x regeneration thereof, it therefore accumulates in the storage catalytic converter 18 and leads to increasing blockage of the NO_x storage sites. To ensure an adequate NO_x storage activity, desulfurization is performed at greater intervals. To do so, the NO_x storage catalytic converter 18 is heated to a desulfurization temperature of 550° C. or more, for example, through engine-related measures, and is at least temporarily exposed to a rich or stoichiometric exhaust gas atmosphere having a lambda<1. Under these conditions, sulfur is desorbed and released mainly in the form of sulfur dioxide SO₂. However, a portion of the sulfur is released in the form of hydrogen sulfide H₂S during the desulfurization, which is also unwanted because of the toxicity of H₂S and its odor problem. The formation of H₂S is even more intense, the richer the air-fuel mixture and the longer the NO_x storage catalytic converter 18 is exposed to the rich exhaust gas. On the other hand, complete desulfurization of the NO_x storage catalytic converter 18 cannot be achieved with an air-fuel mixture that is only slightly rich and/or desulfurization must be performed at higher catalytic converter temperatures, which is associated with a thermal burden on the NO_x storage catalytic converter 18 and an increased fuel consumption.

To avoid this problem, the catalytic converter configuration 12 according to this invention has an H₂S storage catalytic converter 20 which is arranged downstream from the NO_x storage catalytic converter 18 and directly adjacent thereto in the example shown in FIG. 1. The H₂S storage catalytic converter 20 is designed to store H₂S in the form of a metal sulfide in particular under rich exhaust gas conditions and to release and oxidize it to SO₂ under lean exhaust gas conditions. For example, a coating comprising a transition metal, e.g., nickel that binds H₂S as nickel sulfide is suitable for this. The H₂S storage catalytic converter 20 advantageously also has a three-way catalytic component that is responsible for the reaction of HC, CO and NO_x during lean operation of the internal combustion engine 10, so that the H₂S storage catalytic converter 20 supports the precatalytic converter 16 or may even replace it. In comparison with conventional three-way catalytic converters, the coating of the H₂S storage catalytic converter 20 has a much greater loading with the transition metal. The transition metal loading of the H₂S storage catalytic converter 20 depends on the sulfur load typically preventing in desulfurization of the NO_x storage catalytic converter 18, amounting to at least 0.2 g per liter catalytic converter volume, for example. The arrangement of the H₂S storage catalytic converter 20 allows desulfurization of the NO_x storage catalytic converter 18 to be performed at relatively low temperatures and low lambda values without having to accept undesirable H₂S emissions. Due to the low desulfurization temperatures, a thermal burden on the NO_x storage catalytic converter 18 and thus a shortening of its lifetime are prevented. According to the example illustrated in FIG. 1, the NO_x and the H₂S storage catalytic converters 18, 20 may be designed as spatially separate zones on one and the same catalytic converter support or on a separate catalytic converter support directly adjacent to one another.

The exhaust gas duct also contains various gas sensors. Upstream from the precatalytic converter 16 (and/or the particulate filter) there is an oxygen-sensitive gas sensor 22, in particular a lambda probe which regulates the lambda regulation [sic; enables lambda regulation] of the air-fuel mixture of the internal combustion engine 10 in a known way. Another oxygen-sensitive gas sensor 24 is arranged downstream from the H₂S storage catalytic converter 20. This may also be a lambda probe or a NO_x sensor, which supplies an oxygen-dependent signal. Additional gas sensors and/or temperature sensors may also be additionally installed in the exhaust gas duct 14.

The internal combustion engine 10 is supplied with air through an air intake duct 26 in which there is an adjustable throttle valve 28. An electronic engine control unit 30 which is provided for controlling the internal combustion engine 10 receives the signals of the gas sensors 22 and 24, any other gas and/or temperature sensors that might be present as well as various operating parameters of the internal combustion engine 10, e.g., rotational speed, the driver's intended torque, etc. Depending on this data, the engine control unit 30 executes the control using stored characteristic lines and/or engine characteristics maps, to which end the control unit influences parameters, e.g., the quantity the fuel, the injection point in time, the firing angle (in gasoline engines), the exhaust gas recirculation EGR rate, air mass, etc. The engine control unit 30 in particular also contains a program algorithm for operating the exhaust gas system 12, in particular for performing the desulfurization of the NO_x storage catalytic converter 18.

FIGS. 2 and 3 show other embodiments of the inventive catalytic converter configuration 12, whereby corresponding elements are labeled with the same reference numerals as those shown in FIG. 1. For the sake of simplicity, the signal lines and the engine control unit are not shown in FIGS. 2 and 3.

The system shown in FIG. 2 differs from that in FIG. 1 in that the NO_x storage catalytic converter 18 and the H₂S storage catalytic converter 20 are not installed adjacent to one another but instead are installed with a distance between them in the exhaust gas duct 14. In this constellation, the oxygen-sensitive gas sensor 24 is preferably arranged between the storage catalytic converters 18 and 20.

In the embodiment shown in FIG. 3, the NO_x storage catalytic converter 18 and the H₂S storage catalytic converter 20 are present as a mixed coating on the same catalytic converter support, i.e., no local separation of the two functions is provided here.

Performance of a method for desulfurizing a NO_x storage catalytic converter in a system according to FIG. 1 but without the H₂S storage catalytic converter 20 is illustrated in a typical embodiment on the basis of the curve of various exhaust gas parameters as a function of time in FIG. 4. The NO_x storage catalytic converter used here has a total noble metal load of Pt, Pd and Rh amounting to 110 g/ft³ as well as barium (Ba) and cerium (Ce) as NO_x storage components. In FIG. 4, curve 100 shows the lambda value measured with the lambda probe 22 upstream from the NO_x storage catalytic converter 18 during its desulfurization, while curve 102 shows the lambda value measured with the lambda probe 24 downstream from the NO_x storage catalytic converter 18. Curves 104 and 106 show the emissions of sulfur dioxide SO₂ and/or hydrogen sulfide H₂S measured at the exhaust outlet downstream from the NO_x storage catalytic converter. Except for the H₂S curve (curve 106), the execution of the inventive method for desulfurizing the NO_x storage catalytic converter 18, i.e., using an H₂S storage catalytic converter 20, corresponds in principle to that illustrated in FIG. 4.

First, starting from a leaner exhaust gas atmosphere with lambda≈1.01 a rich exhaust gas atmosphere with lambda≈0.98 is adjusted for heated the NO_x storage catalytic converter 18 and for initiating its desulfurization. As soon as the rich exhaust gas reaches the NO_x storage catalytic converter 18, it leads to an intense release of SO₂ (curve 104, point in time 176000). However, release of SO₂ drops again very rapidly in favor of an increase in the H₂S emissions (curve 106). Starting approximately at the point in time 192000, so-called wobble operation is begun, in which the internal combustion 10 is operated alternately with a lean air-fuel mixture with lambda≈1.01 and a rich air-fuel mixture with lambda≈0.98. The lambda curve 102 measured downstream from the NO_x storage catalytic converter 18 follows this change with a certain delay and with reduced amplitudes, which is caused first by the exhaust gas running time and secondly by the consumption of oxygen in the exhaust gas to reduce the sulfur. Switching between lean and rich intervals is regulated by the lambda probe 24 downstream from the NO_x storage catalytic converter 18. The switch from lean to rich is made as soon as the probe detects a lean exhaust gas. Conversely, the switch from rich to lean is made as soon as the lambda probe 24 records the breakthrough of rich exhaust gas. In each rich interval, an increase in SO₂ emission 104 downstream from the NO_x storage catalytic converter 18 can be observed, declining toward the end of the rich interval—with increasing consumption of the oxygen incorporated in the lean intervals—in favor of the H₂S emission 106.

FIG. 5 shows comparatively the H₂S emissions according to the state of the art (see FIG. 4) and according to the present invention for the entire duration of desulfurization. Curve 108 shows the measured final H₂S emission at the exhaust gas outlet of an exhaust gas system according to the state of the art without a downstream H₂S storage catalytic converter. Curve 110 however shows the corresponding curve with an exhaust gas arrangement having an H₂S storage catalytic converter 20 according to FIG. 1. In both cases a NO_x storage catalytic converter having a total noble metal load of 110 g/ft³ Pt, Pd and Rh plus barium (Ba) and cerium (Ce) as NO_x storage components was used. In the inventive curve as represented by curve 110, an H₂S storage catalytic converter 20 having a nickel load of >0.5 g/L was additionally used. It can be seen clearly here that the total H₂S emissions according to the present invention are reduced to a fraction in comparison with the state of the art. To achieve a regeneration of the H₂S storage catalytic converter 20 following the desulfurization and thus to restore the H₂S storage function at the start of the next desulfurization process, the catalytic converter is exposed to a lean exhaust gas atmosphere at an elevated exhaust gas temperature. In doing so, the sulfur is oxidized to SO₂ and removed from the surface of the catalytic converter 20. For regeneration of the H₂S storage catalytic converter, standard operating situations of the internal combustion engine 10 may be utilized. For example, regeneration may be performed during the heating phase for desulfurization of the NO_x storage catalytic converter 18 or—in a diesel vehicle—during a heating phase of particulate filter regeneration. In addition, it may also be performed in driving situations of moderate load, in which there is an increase in temperature of the exhaust gas in a lean exhaust gas atmosphere. With all the aforementioned modes of operation of the internal combustion engine 10, formation of SO₂ is facilitated by the high exhaust gas temperature and the oxygen excess of the exhaust gas.

Claims

1. A catalytic converter configuration for an internal combustion engine that is capable of running lean, comprising:

an exhaust gas duct connected with the internal combustion engine;

a NOx storage catalytic converter disposed in said exhaust gas duct; and

an H2S storage catalytic converter configured to store hydrogen sulfide under a rich or stoichiometric exhaust gas atmosphere with lambda≦1 and to release the hydrogen sulfide under a lean exhaust gas atmosphere with lambda>1.

2. The catalytic converter configuration according to claim 1, wherein said H2S storage catalytic converter is disposed downstream and at a spacing distance from said NOx storage catalytic converter.

3. The catalytic converter configuration according to claim 1, wherein said H2S storage catalytic converter is disposed downstream and adjoining said NOx storage catalytic converter.

4. The catalytic converter configuration according to claim 1, wherein said H2S storage catalytic converter is integrated in said NOx storage catalytic converter.

5. The catalytic converter configuration according to claim 1, wherein said H2S storage catalytic converter contains at least one metal component suitable for binding H2S as a metal sulfide under a rich or stoichiometric exhaust gas atmosphere with lambda≦1.

6. The catalytic converter configuration according to claim 5, wherein said metal component includes at least one metal of subgroups I, II and/or VIII of the table of elements.

7. The catalytic converter configuration according to claim 5, wherein said metal component includes at least one component selected from the group consisting of Ag, Zn, Cd, Fe, Co, and Ni.

8. The catalytic converter configuration according to claim 5, wherein a specific loading of said H2S catalytic converter with said at least one metal component amounts to at least 0.2 grams per liter.

9. The catalytic converter configuration according to claim 5, wherein a specific loading of said H2S catalytic converter with said at least one metal component amounts to at least 0.5 grams per liter.

10. The catalytic converter configuration according to claim 1, wherein said H2S storage catalytic converter additionally has an oxidation catalytic component or a three-way catalytic component.

11. The catalytic converter configuration according to claim 5, wherein said H2S storage catalytic converter contains at least one noble metal component.

12. The catalytic converter configuration according to claim 11, wherein said noble metal component is selected from the group consisting of platinum, palladium, and rhodium.

13. The catalytic converter configuration according to claim 1, which further comprises a component for storing oxygen disposed downstream of said NOx storage catalytic converter and/or of said H2S storage catalytic converter.

14. The catalytic converter configuration according to claim 1, which comprises an oxygen-sensitive gas sensor disposed downstream of said NOx storage catalytic converter and/or of said H2S storage catalytic converter.

15. The catalytic converter configuration according to claim 14, wherein said oxygen-sensitive gas sensor is a lambda probe or a NOx sensor.

16. A method of desulfurizing a NOx storage catalytic converter disposed in an exhaust gas duct of an internal combustion engine that is capable of running lean, which comprises:

exposing the NOx storage catalytic converter at least temporarily to a rich or stoichiometric exhaust gas atmosphere with lambda≦1 at a desulfurization temperature of the NOx storage catalytic converter; and

storing hydrogen sulfide released from the NOx storage catalytic converter in an H2S storage catalytic converter; and

releasing the hydrogen sulfide in a lean exhaust gas atmosphere with lambda>1.

17. The method according to claim 16, which comprises, during desulfurization, alternately exposing the NOx storage catalytic converter to rich intervals with a rich or stoichiometric exhaust gas atmosphere with lambda≦1 and in lean intervals to a lean exhaust gas atmosphere with lambda>1.

18. The method according to claim 16, which comprises regenerating the H2S storage catalytic converter by an oxidation product of H2S.

19. The method according to claim 16, which comprises regenerating the H2S storage catalytic converter by regeneration of SO2 by exposure of the H2S storage catalytic converter to a lean exhaust gas atmosphere and a minimum temperature.