DE102006038367A1 - Desulfurization of a NOx storage catalyst - Google Patents

Abstract

The invention relates to a catalyst arrangement (12) for a lean burn internal combustion engine (10) with an exhaust gas duct (14) and an NO <SUB> x </ SUB> storage catalytic converter (18) arranged in the exhaust gas duct (14). The invention further relates to a method for desulphurizing a NO <SUB> x </ SUB> storage catalytic converter (18) arranged in an exhaust gas duct (14) of a lean-burnable internal combustion engine (10). According to the invention, an H <SUB> 2 </ SUB> S storage catalytic converter (20) is provided. This is suitable to store under rich or stoichiometric exhaust gas atmosphere with lambda <= 1 hydrogen sulfide (H <SUB> 2 </ SUB> S) and to oxidize it under lean exhaust gas atmosphere with lambda> 1.

Description

The invention relates to a catalyst arrangement for lean-burn internal combustion engines, in the exhaust passage of a NO _x storage catalyst is arranged. The inventor further relates to a method of desulfurizing the NO _x storage catalyst.

In internal combustion engines, which are operated with a lean air-fuel mixture, conventional 3-way catalysts for the quantitative conversion of all exhaust gas constituents, in particular of nitrogen oxides NO _x , not sufficient. Therefore, the arrangement of so-called NO _x storage catalysts in the exhaust ducts of such internal combustion engines is known. NO _x storage catalytic converters have an NO _x storage component which stores NO _x under a lean exhaust gas atmosphere. The NO _x regeneration takes place by means of short fat intervals in which the stored NO _{x is} desorbed and converted to N ₂ and O ₂ by means of a catalytic component of the NO _x storage catalytic converter. The use of NO _x storage catalytic converters is known both in diesel engines and in lean-burn gasoline engines.

Due to sulfur contained in the fuel, the NO _x storage catalyst loses its duration of activity (sulfur poisoning). The sulfur accumulates in the form of sulfate on the storage components of the catalyst, whereby the NO _x storage locations of the storage catalyst are blocked and are no longer available for storage of NO _x . If a lower limit of the NO _x storage capacity of the NO _x storage catalytic converter is reached, it must be desulfurized (desulfated). This occurs at high exhaust gas temperatures of at least 500 ° C and exposure of the storage catalytic converter with a rich exhaust gas atmosphere (lambda <1). At these temperatures, a desorption of the stored sulfur, which is reduced by the reducing exhaust gas atmosphere to SO ₂ but also to hydrogen sulfide H ₂ S. The richer the exhaust conditions during the desulfurization, the faster the desorption and reduction of sulfur, which has a favorable effect on the thermal aging of the storage catalyst, but is associated with an increase in the undesirable H ₂ S formation.

To carry out the desulfurization is known to drive the engine alternately in short time intervals in rich operation with lambda <1 and in lean operation with lambda> 1, that is in the so-called wobble operation. This promotes the heating of the catalyst to the required desulfurization temperature and maintenance thereof. In addition, the H ₂ S formation is reduced by the alternation of the storage catalyst with lean and rich exhaust gas, since in the lean intervals oxygen is incorporated into the catalyst, which provides for oxidation of the intermediately formed H ₂ S to SO ₂ . It turns out, however, that a complete desulfurization of the storage catalyst is also connected in the wobble operation with a certain formation of H ₂ S, the concentration of which may lead to a limit exceeding.

The invention is therefore based on the object to provide a catalyst-friendly process for the desulfurization of a NO _x storage catalyst available, in which the H ₂ S formation is minimized. It is further intended to provide a catalyst arrangement suitable for carrying out the method.

This object is achieved by a catalyst arrangement having the features of claim 1. The catalyst arrangement according to the invention is characterized by an H ₂ S storage catalytic converter which is suitable for storing under high or stoichiometric exhaust gas atmosphere, ie with an air / fuel ratio of lambda ≦ 1, hydrogen sulfide H ₂ S and under a lean exhaust gas atmosphere, the means with lambda> 1, in particular substantially as sulfur dioxide SO ₂ release again. The inventive arrangement of the H ₂ S storage catalytic converter, it is possible to desulfurize the NO _x storage at permanently rich exhaust conditions and thus at relatively low temperatures, whereby its thermal aging is delayed. At the same time, unwanted H ₂ S emissions can be greatly reduced or even completely prevented.

According to an advantageous embodiment of the invention, the H ₂ S storage catalytic converter is connected downstream of the NO _x storage catalytic converter in the exhaust gas flow direction, wherein it can be arranged directly adjacent to the NO _x storage catalytic converter or at a distance therefrom. Alternatively, the H ₂ S storage catalyst can also be integrated in the NO _x storage catalytic converter, that is to say a mixed coating of H ₂ S and NO _x storage catalytic converter components can be present on a common catalyst support. All of these arrangements ensure that the H ₂ S released from the NO _x storage catalyst is stored and reacted in the H ₂ S storage catalyst.

As H ₂ S storage component of the H ₂ S storage catalyst, this preferably contains a metal component which is suitable to bind H ₂ S in particular as a metal sulfide under rich or stoichiometric exhaust gas atmosphere. Prerequisite for the selection of the suitable metal component is their suitability to enter into the rich exhaust gas, a metal-sulfur compound and under lean Be conditions to release the sulfur preferably in the form of SO ₂ . Particularly suitable metals for this purpose are metals of groups I, II and / or VIII of the Periodic Table of the Elements, where preferably silver (Ag), zinc (Zn), cadmium (Cd), iron (Fe), cobalt (Co) and / or nickel (Ni) can be used. In this case, the loading of the H ₂ S storage catalyst with the metal component depends on the desired H ₂ S storage capacity of the H ₂ S storage catalyst, in particular for a typical sulfur to be removed from the NO _x storage catalyst at the time of desulfurization. In typical cases, a specific metal loading of at least 0.2 g / l has proven to be advantageous, in particular of at least 0.3 g / l, more preferably of at least 0.5 g / l.

Preferably, it is further provided that the H ₂ S storage catalyst under lean exhaust gas atmosphere may additionally comprise an oxidation catalytic or 3-way catalytic component, that is, an O _x idation of carbon monoxide CO and / or hydrocarbons HC catalysed and - in the case of a 3- Pathway catalytic effect - additionally supports the reduction of nitrogen oxides NO _x . For this purpose, the catalyst may additionally contain at least one noble metal, for example platinum (Pt), palladium (Pd) and / or rhodium (Rh).

A further advantageous embodiment of the invention provides that the catalytic converter arrangement further comprises a oxygen-storing component downstream of the NO _x storage catalytic converter and / or the H ₂ S storage catalyst, which under lean exhaust gas atmosphere stores and releases oxygen under rich exhaust gas atmosphere, whereby the NO _x - Storage catalyst desorbed H ₂ S is oxidized to SO ₂ . Such oxygen storage components (OSC for oxygen storage component) are known in the art and will not be explained in detail here. Furthermore, with the help of the oxygen stored in the OSC, the components CO and HC can be converted and thus the emissions are reduced in rich operation.

The invention further relates to a process for the desulfurization of a NO _x storage catalyst, stored in the emerging from the NO _x storage hydrogen sulfide in a H ₂ S storage catalyst and under a lean exhaust gas atmosphere with lambda> 1, especially above a certain catalyst temperature, preferably as SO ₂ is released again. The release temperature in the H ₂ S storage catalyst can be caused, for example, by an upstream particle filter reaction (by way of PF regeneration).

In this case, it is particularly advantageous to apply the lambda ≦ 1 alternately to the NO _x storage catalyst during the desulphurization in the so-called wobble mode and to lambda> 1 in lean intervals with lambda> 1. In principle, however, the desulfurization with the catalyst arrangement described above can also be carried out in continuous rich operation of the internal combustion engine.

Further advantageous embodiments of the invention are the subject of the other dependent claims.

The Invention will be described below in embodiments with reference to FIG associated Drawings explained. Show it:

1 an internal combustion engine with a downstream catalytic converter arrangement according to a first embodiment of the invention,

2 an internal combustion engine with a downstream catalytic converter arrangement according to a second embodiment of the invention,

3 an internal combustion engine with a downstream catalytic converter arrangement according to a third embodiment of the invention,

4 Time courses of the lambda value before and after NO _x storage catalytic converter, and the SO ₂ - and H ₂ S concentration after NO _x storage catalytic converter during the desulfurization of the NO _x storage catalytic converter in the catalyst arrangement according to 1 , and

5 Time courses of the SO ₂ and H ₂ S concentration after NO _x storage catalyst according to the present invention and prior art.

1 shows an internal combustion engine 10 , which can be operated with a lean air-fuel mixture. This may be a diesel engine or a lean-running gasoline engine.

The internal combustion engine 10 is a total with 12 downstream catalytic converter arrangement according to a first advantageous embodiment of the present invention, in which the exhaust gas of the internal combustion engine 10 is passed and post-treated. The catalyst arrangement 12 includes an exhaust passage 14 in which various catalysts are arranged. According to the illustrated embodiment is a pre-catalyst 16 acting oxidation or 3-way catalyst at a near-engine position of the internal combustion engine 10 arranged. Alternatively or in addition to the precatalyst 16 can be arranged in the engine-near position a particulate filter, not shown here for the storage of soot particles, the optionally also with the oxidation or 3-way pre-catalyst 16 may be combined on a common carrier.

Downstream of the precatalyst 16 (or the alternatively or additionally provided particulate filter) is an NO _x storage catalyst 18 in the exhaust duct 14 intended. The NO _x storage catalytic converter 18 has an NO _x storage component which stores NO _x under a lean exhaust gas atmosphere and at intermediate regeneration intervals in which the internal combustion engine 10 is operated with a stoichiometric or rich air-fuel mixture releases again. One in the NO _x storage catalytic converter 18 integrated oxidation or 3-way component catalyzes the conversion of the desorbed nitrogen oxides into nitrogen N ₂ and oxygen O ₂ during the NO _x regeneration.

Undesirably, in addition to the NO _x storage and storage of sulfur contained in the fuel in the form of sulfate in the NO _x storage catalyst 18 instead of. Since the sulfur during the usual NO _x regenerations of the NO _x storage catalyst 18 is not removed from this, he accumulates in the storage catalytic converter 18 and leads to an increasing blockage of NO _x storage locations. In order to ensure a sufficient NO _x storage activity, desulfurizations are carried out at longer intervals. This is the NO _x storage catalyst 18 heated by motorized measures to a desulfurizing temperature of, for example 550 ° C or more and at least temporarily applied with a rich or stoichiometric exhaust gas atmosphere with lambda ≤ 1. Under these conditions, desorption of sulfur and its release takes place mainly in the form of sulfur dioxide SO ₂ . However, some of the sulfur is released during desulfurization in the form of hydrogen sulfide H ₂ S, which is undesirable because of the toxicity and odor nuisance of H ₂ S. The H ₂ S formation is the more intense, the fatter air-fuel mixture and the longer the NO _x storage catalyst 18 is acted upon with rich exhaust gas. On the other hand, with a mildly rich air-fuel mixture, complete desulfurization of the NO _x storage catalyst can not 18 be achieved or desulphurization must be carried out at higher catalyst temperatures, which in turn with a thermal load of the NO _x storage catalytic converter 18 and increased fuel consumption.

To avoid this problem according to the invention comprises the catalyst arrangement 12 a H ₂ S storage catalyst 20 on, which in the example of the 1 downstream of the NO _x storage catalytic converter 18 and immediately adjacent to it. The H ₂ S storage catalyst 20 is designed to store under heavy exhaust conditions H ₂ S, in particular in the form of a metal sulfide and release under lean exhaust conditions and oxidize to SO ₂ . For this purpose, for example, a coating with a transition metal, for example nickel, which binds H ₂ S as nickel sulfide is suitable. The H ₂ S storage catalyst 20 Advantageously, in addition to a 3-way catalytic component, which in lean operation of the internal combustion engine 10 the conversion of HC, CO and NO _x takes over, so that the H ₂ S storage catalyst 20 the precatalyst 16 support or even replace it. Compared to conventional 3-way catalysts, the coating of the H ₂ S storage catalyst 20 a significantly increased loading with the transition metal. In this case, the transition metal loading of the H ₂ S storage catalyst depends 20 typically during desulphurisation of the NO _x storage catalyst 18 present sulfur loading and is for example at least 0.2 g per liter of catalyst volume. The arrangement of the H ₂ S storage catalyst 20 allows the desulfurization of the NO _x storage catalytic converter 18 at relatively low temperatures and low lambda values without accepting unwanted H ₂ S emissions. Due to the low desulfurization temperatures, a thermal load of the NO _x storage catalytic converter 18 and thus a shortening of its life avoided. According to the in 1 Example shown, the NO _x - and the H ₂ S storage catalyst 18 . 20 be arranged as spatially separated zones on one and the same catalyst support or each directly on a separate catalyst carrier adjacent to each other.

The exhaust duct 14 also houses various gas sensors. Upstream of the precatalyst 16 (or particulate filter) is an oxygen-sensitive gas sensor 22 , In particular arranged a lambda probe, which in a known manner, the lambda control of the air-fuel mixture of the internal combustion engine 10 regulates. Another oxygen-sensitive gas sensor 24 is downstream of the H ₂ S storage catalyst 20 arranged. This can also be a lambda probe or a NO _x sensor that provides an oxygen-dependent signal. Additional gas sensors and / or temperature sensors can additionally in the exhaust duct 14 be installed.

The internal combustion engine 10 is via an air intake duct 26 , in which there is an adjustable throttle 28 is located, supplied with air. For controlling the internal combustion engine 10 is an electronic engine control unit 30 present, in which the signals of the gas sensors 22 and 24 , possibly existing further gas and / or Tem temperature sensors and various operating parameters of the internal combustion engine 10 , For example, speed, desired torque of the driver, among others, received. In dependence of these data using stored characteristic curves or characteristic maps, the engine control unit leads 30 the controller, for which it affects parameters such as fuel quantity, injection timing, ignition angle (in gasoline engines), EGR rate, air mass, among others. In particular, in the engine control unit 30 also a program algorithm for operating the exhaust system 12 , in particular for carrying out the desulfurization of the NO _x storage catalytic converter 18 deposited.

The 2 and 3 show further embodiments of the catalyst arrangement according to the invention 12 , wherein corresponding elements with the same reference numerals as in 1 are designated. For the sake of clarity are in the 2 and 3 the signal lines and the engine control unit not shown.

In the 2 shown plant is different from 1 in that the NO _x storage catalyst 18 and the H ₂ S storage catalyst 20 not adjacent to each other, but spaced from each other in the exhaust passage 14 are installed. In this constellation is the oxygen-sensitive gas sensor 24 preferably between the storage catalysts 18 . 20 arranged.

In the in 3 illustrated embodiment of the NO _x storage catalytic converter 18 and he H ₂ S storage catalyst 20 as a mixed coating on the same catalyst support present, that is, a local separation of both functions is not provided here.

Carrying out a method for desulfurizing a NO _x storage catalytic converter in a system according to 1 but without H ₂ S storage catalyst 20 is in a typical embodiment based on the time course of various emission parameters in

4 explained. An NO _x storage catalyst was used which had a total noble metal loading of Pt, Pd, and Rh of 110 g / ft ³ and barium (Ba) and cerium (Ce) as NO _x storage components. In 4 shows the curve 100 with the lambda probe 22 measured lambda value upstream of the NO _x storage catalytic converter 18 during its desulfurization while the bend 102 with the lambda probe 24 measured lambda value downstream of the NO _x storage catalytic converter 18 represents. The courses 104 and 106 show the measured at the exhaust outlet downstream of the NO _x storage catalytic emissions of sulfur dioxide SO ₂ or hydrogen sulfide H ₂ S. The implementation of the method according to the invention for the desulfurization of the NO _x storage catalytic converter 18 That is, using an H ₂ S storage catalyst 20 , corresponds with the exception of the H ₂ S curve (curve 106 ) in principle the in 4 Shown.

First, starting from a lean exhaust gas atmosphere with lambda ≈ 1.01 for heating the NO _x storage catalytic converter 18 and initiate its desulfurization a rich exhaust gas atmosphere with lambda ≈ 0.98 set. Once the rich exhaust the NO _x storage catalytic converter 18 achieved, this leads to an intense release of SO ₂ (curve 104 , Time 176000). However, SO ₂ release drops off again very rapidly in favor of an increase in H ₂ S emissions (curve 106 ). From about the time 192000 is started with the so-called wobble operation in which the internal combustion engine 10 alternately with a lean air-fuel mixture with λ ≈ 1.01 and a rich air-fuel mixture with λ ≈ 0.98 is rubbed. The downstream of the NO _x storage catalytic converter 18 measured lambda curve 102 follows this change with a certain delay and attenuated amplitudes, which is caused on the one hand by the exhaust gas flow time and on the other hand by the consumption of the oxygen of the exhaust gas to reduce the sulfur. The changeover between the lean and rich intervals is made by the lambda probe 24 downstream of the NO _x storage catalytic converter 18 regulated. The switchover from lean to rich is made as soon as it detects a lean exhaust gas. Conversely, the switch from rich to lean is made as soon as the lambda probe 24 registered the breakthrough of fats exhaust. In every fat interval there is an increase in SO ₂ emissions 104 after the NO _x storage catalyst 18 observed towards the end of the fat interval - with increasing consumption of the oxygen stored in the lean intervals - in favor of H ₂ S emission 106 drops.

In 5 are comparative H ₂ S emissions according to the prior art (see. 4 ) and according to the present invention for the total desulfurization time. This shows curve 108 the measured H ₂ S endemission at the exhaust outlet of an exhaust system according to the prior art without downstream H ₂ S storage catalytic converter. Curve 110 however, shows the corresponding course in an exhaust system with H ₂ S storage catalyst 20 according to 1 , In both cases, an NO _x storage catalyst was used which had a total noble metal loading of Pt, Pd, and Rh of 110 g / ft ³ and barium (Ba) and cerium (Ce) as NO _x storage components. In the course according to the invention according to curve 110 was also an H ₂ S storage catalyst 20 used, which had a nickel loading of> 0.5 g / l. It can be clearly seen that the total H ₂ S emissions is reduced to a fraction of the prior art according to the present invention.

At the end of the desulfurization, a regeneration of the H ₂ S storage catalytic converter 20 to achieve and thus restore the H ₂ S storage function at the beginning of the next desulfurization, this is acted upon at a high exhaust gas temperature with a lean exhaust gas atmosphere. The sulfur is oxidized to SO ₂ and from the surface of the catalyst 20 away. For the regeneration of the H ₂ S storage catalytic converter standard operating situations of the internal combustion engine 10 be used. For example, the regeneration during the heating phase for the desulfurization of the NO _x storage catalytic converter 18 or - in the diesel vehicle - during a heating phase of a particle filter regeneration done. In addition, it can also be done in driving situations with medium loads in which a lean exhaust gas atmosphere, a temperature increase of the exhaust gas takes place. In all the above modes of the internal combustion engine 10 the formation of SO ₂ is favored by the high exhaust gas temperature and the oxygen excess of the exhaust gas.

10

Internal combustion engine

12

catalyst assembly

14

exhaust duct

16

precatalyzer

18

NO _x storage catalyst

20

H ₂ S storage catalyst

22

oxygen-sensitive Gas sensor / oxygen sensor

24

oxygen-sensitive gas sensor / lambda or NO _x probe

26

air intake duct

28

throttle

30

Engine control unit

100

Lambda before NO _x storage catalytic converter

102

Lambda after NO _x storage catalytic converter

104

Concentration SO ₂

106

Concentration H ₂ S

108

Concentration H ₂ S without H ₂ S storage catalyst

110

Concentration H ₂ S with H ₂ S storage catalyst

Claims (11)

Catalyst arrangement ( 12 ) for a lean-running internal combustion engine ( 10 ) with an exhaust duct ( 14 ) and one in the exhaust duct ( 14 ) arranged NO _x storage catalytic converter ( 18 ), characterized by an H ₂ S storage catalyst ( 20 ), which is suitable to store under rich or stoichiometric exhaust gas atmosphere with lambda ≤ 1 hydrogen sulfide (H ₂ S) and release under lean exhaust gas atmosphere with lambda> 1. Catalyst arrangement ( 12 ) according to claim 1, characterized in that the H ₂ S storage catalyst ( 20 ) the NO _x storage catalytic converter ( 18 ) is spaced or adjacently downstream or in the NO _x storage catalytic converter ( 18 ) is integrated. Catalyst arrangement ( 12 ) according to one of the preceding claims, characterized in that the H ₂ S storage catalyst ( 20 ) contains at least one metal component which is suitable to bind under fat or stoichiometric exhaust gas atmosphere with lambda ≤ H ₂ S as metal sulfide, in particular at least one metal of the I., II. and / or VIII. subgroup, preferably at least one component of the group Ag , Zn, Cd, Fe, Co and Ni. Catalyst arrangement ( 12 ) according to claim 3, characterized in that a specific loading of the H ₂ S storage catalyst ( 20 ) with the at least one metal component is at least 0.2 g / l, in particular at least 0.5 g / l. Catalyst arrangement ( 12 ) according to one of the preceding claims, characterized in that the H ₂ S storage catalyst ( 20 ) additionally has an oxidation catalytic or 3-way catalytic component. Catalyst arrangement ( 12 ) according to claim 5, characterized in that the H ₂ S storage catalyst ( 20 ) has at least one noble metal component, in particular selected from the group of platinum, palladium and rhodium. Catalyst arrangement ( 12 ) according to one of the preceding claims, characterized in that an oxygen-storing component downstream of the NO _x storage catalytic converter ( 18 ) and / or the H ₂ S storage catalyst ( 20 ) is provided. Catalyst arrangement ( 12 ) according to one of the preceding claims, characterized in that downstream of the NO _x storage catalytic converter ( 18 ) and / or the H ₂ S storage catalyst ( 20 ) an oxygen-sensitive gas sensor ( 22 . 24 ), in particular a lambda probe and / or a NO _x sensor is arranged. Method for desulphurizing an exhaust gas channel ( 14 ) of a lean-running internal combustion engine ( 10 ) arranged NO _x storage catalytic converter ( 18 ), wherein at a desulphurisation temperature of the NO _x storage catalytic converter ( 18 ) this is acted upon at least temporarily with a rich or stoichiometric exhaust gas atmosphere with lambda ≦ 1, characterized in that from the NO _x storage catalytic converter ( 18 ) leaking hydrogen sulfide (H ₂ S) in a H ₂ S-Speicherka talysator ( 20 ) is stored and released under a lean exhaust gas atmosphere with lambda> 1. A method according to claim 9, characterized in that the NO _x storage catalytic converter ( 18 ) during the desulfurization alternately in fat intervals with a rich or stoichiometric exhaust gas atmosphere with lambda ≤ and in lean intervals with a lean exhaust gas atmosphere with lambda> 1 is applied. A method according to claim 9 or 10, characterized in that a regeneration of the H ₂ S storage catalyst ( 20 ) of the oxidation product of H ₂ S, in particular a regeneration of SO ₂ , by charging the H ₂ S storage catalyst ( 20 ) takes place with a lean exhaust gas atmosphere and a minimum temperature.