SCR-LNT CATALYST COMBINATION FOR IMPROVED NOx CONTROL OF LEAN GASOLINE AND DIESEL ENGINES

Abstract

An aftertreatment system for use in a lean burn engine is disclosed. In one embodiment, the aftertreatment system includes a first catalyst in communication with an exhaust stream from the engine; a selective catalytic reduction catalyst in communication with the exhaust stream and positioned downstream of the first catalyst; and a lean NOx trap in communication with the exhaust stream and positioned downstream of the selective catalytic reduction catalyst; with the proviso that the first catalyst is not a selective catalytic reduction catalyst. A method for treating lean burn engine exhaust gases using the aftertreatment system is also disclosed.

Description

The present invention relates generally to a catalyst system to facilitate the reduction of nitrogen oxides (NO_x) and ammonia from an exhaust gas, and more particularly, to an aftertreatment system including a first lean NO_x trap; a selective catalytic reduction catalyst positioned downstream of the first lean NO_x trap; and a second lean NO_x trap positioned downstream of the selective catalytic reduction catalyst, and to methods of treating exhaust gas using the aftertreatment system.

BACKGROUND OF THE INVENTION

Aftertreatment systems using various catalyst systems have been used in the exhaust systems of vehicles to convert carbon monoxide, hydrocarbons, and nitrogen oxides (NO_x) produced during engine operation into non-polluting gases such as carbon dioxide, water, and nitrogen. Because of stricter fuel economy and emissions standards, it is increasingly desirable to operate engines under lean conditions to improve fuel efficiency and lower carbon dioxide emissions. Lean conditions have air/fuel ratios grater than the stoichiometric ratio (and air/fuel ratio of 14.6). Although operating under lean conditions improves fuel economy, it increases the difficulty of treating some polluting gases, such as NO_x.

Lean NO_x trap (LNT) catalysts have been used to reduce NO_x emissions. LNTs operate in a cycle of lean and rich conditions. When the engine is running under lean conditions, the LNT adsorbs NO_x until the LNT reaches its storage capacity. The NO_x is then reduced during the rich operation. Alternatively, NO_x reduction can be obtained by injecting a sufficient amount of a reductant into the exhaust independent of engine operation. Suitable reductants include, but are not limited to, hydrogen, hydrocarbons, carbon monoxide, diesel fuel, alcohols, and the like. The reductants reduce the NO_x adsorbed by the trap during the lean cycle, purging the LNT which is then ready for the next cycle. A system can include both rich/lean cycling and the addition of a reductant, if desired.

However, the NO_x conversion in LNTs is generally low. LNTs are known to have the problem of “NO_x slip,” which includes breakthrough of NO_x during the extended lean period of operation of the NO_x trap and also NO_x spikes generated during the transition from the lean to the rich cycle. The generation of ammonia in LNTs is another problem.

In order to solve the problems associated with LNTs in lean burn engines, some systems involve the combination of LNT catalysts followed by selective catalytic reduction (SCR) catalysts. In this arrangement, when the LNT undergoes NO_x reduction during rich operation, NH₃ can be generated. This can be used in the downstream SCR to reduce NO_x that slipped through the LNT. These systems can include the injection of a source of NH₃ such as urea or in situ generation of NH₃. U.S. Pat. No. 7,332,135 entitled “Catalyst System for the Reduction of NO_x and NH₃ Emissions,” which is incorporated herein by reference, describes one such system.

SCR systems are being developed that involve of the combination of diesel oxidation catalyst (DOC), SCR, and diesel particulate filter (DPF) catalysts. They can be arranged as DOC-SCR-DPF or DOC-DPF-SCR catalysts. Typically in these systems, urea is metered into the exhaust before the SCR, and is converted to NH₃ before and in the SCR catalyst. Test data indicates that high NO_x conversion, especially at lower temperatures, requires the presence of significant quantities of NH₃ on the SCR catalyst surface.

If the driver accelerates rapidly after operation at low temperatures (or if active DPF regeneration or similar active heating functions are initiated), the stored NH₃ can be released from the SCR catalyst. This can result in a large NH₃ spike, as much as about 1000 ppm. If the SCR is followed by other catalysts, such as DPF or NH₃ slip catalysts, the NH₃ may be converted to NO_x, N₂O, and/or N₂ depending on the catalyst and operating conditions.

However, if the SCR is the last catalyst in the system, the NH₃ will be emitted, which can form an aerosol in the atmosphere and result in an unpleasant smell. Although ammonia is not currently regulated, the Environmental Protection Agency is monitoring ammonia emissions. Consequently, it is important to develop methods to reduce ammonia emissions.

Therefore, there is a need for an aftertreatment system which provides high NO_x conversion and which reduces or eliminates emission of NH₃.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of one embodiment of the aftertreatment system of the present invention.

DESCRIPTION OF THE INVENTION

The present invention meets this need by providing an aftertreatment system which improves NO_x conversion and reduces ammonia emissions in lean burn engines. In one embodiment, the system includes a first catalyst in communication with an exhaust stream from the engine; a selective catalytic reduction catalyst in communication with the exhaust stream and positioned downstream of the first catalyst; and a lean NO_x trap in communication with the exhaust stream and positioned downstream of the selective catalytic reduction catalyst; with the proviso that the first catalyst is not a selective catalytic reduction catalyst. The ammonia produced in the first catalyst or added to the SCR is reacted with NO_x from the first catalyst. Any excess ammonia from the SCR can reduce the NO_x stored on the LNT to N₂.

In another embodiment, the system consists essentially of a an optional first catalyst in communication with an exhaust stream from the engine; a selective catalytic reduction catalyst in communication with the exhaust stream and positioned downstream of the optional first catalyst; an optional second catalyst in communication with the exhaust stream and positioned downstream of the selective catalytic reduction catalyst; a lean NO_x trap in communication with the exhaust stream and positioned downstream of the optional second catalyst and the selective catalytic reduction catalyst.

Another aspect of the invention relates to a method for treating lean burn exhaust gases. In one embodiment, the method includes providing an aftertreatment system in an exhaust gas passage of a lean burn engine, the system comprising a first catalyst in communication an exhaust stream from the engine; a selective catalytic reduction catalyst in communication an exhaust stream and positioned downstream of the first catalyst; and a lean NO_x trap in communication the exhaust stream and positioned downstream of the selective catalytic reduction catalyst; with the proviso that the first catalyst is not a selective catalytic reduction catalyst; and exposing the aftertreatment system to engine exhaust gas containing NO_x such that at least a portion of said NO_x contained in said exhaust gas is converted to N₂, and at least a portion of the NH₃ supplied to or generated by the selective catalytic reduction catalyst converts NO_x stored on the LNT to N₂.

FIG. 1 shows a schematic illustration of one embodiment of an aftertreatment system 5. The engine 10 can be controlled by a controller 15 to generate lean and rich conditions in the engine.

Exhaust gas from the engine 10 flows to an first catalyst 20. The first catalyst is not required, but can be included, if desired. The system can include one or more first catalysts. If a first catalyst 20 is included, it is not an SCR. Suitable first catalysts include, but are not limited to, LNTs, DOCs, DPFs, three-way catalysts (TWCs), hydrocarbon traps, and combinations thereof.

The LNT is capable of adsorbing NO_x from the exhaust gas and storing it during fuel-lean operation of the engine, then releasing and converting it to nitrogen gas when the engine operation changes to stoichiometric or rich (λ<1). Alternatively, the engine temperature can be raised so that NO_x is released.

The LNT can be any type of conventional LNT, which are well known in the art. An LNT typically includes a catalyst comprising one or more precious metals, including, but not limited to, platinum, palladium, rhodium, and combinations thereof. The LNT can also include an NO_x absorbing material or NO_x storage component/material, for example an alkali or alkaline earth metal, such as barium, or cesium, or a rare earth metal such as cerium, and/or a composite of cerium and zirconium. A catalyst material that does not contain a NO_x storage material can also be used. These materials are supported on binder materials such as alumina. Alternatively, other LNTs, including, but not limited to, alumina-based LNTs, can also be used. LNTs using alumina as the NO_x absorbent material have been shown to be desirable for use in lean burn engines because they effectively store and convert NO_x at the low temperatures encountered in such engines, maintain their activity with extended use, and undergo efficient desulphurization. Alumina-based LNTs generally include a catalyst, a NO_x adsorbent material comprising alumina, and optionally, from 0 to about 4 wt % of an alkaline earth metal oxide. Alumina-based LNTs are described in U.S. application Ser. No. 11/298,805, filed Dec. 9, 2005, entitled “Alumina-Based Lean NO_x Trap System And Method Of Use,” which is incorporated herein by reference.

Another type of first catalyst is a DOC, which is capable on converting NO to NO₂. It will also oxidize and/or store gaseous hydrocarbon species and some heavier hydrocarbon species that would otherwise end up adsorbed on the downstream LNT, SCR, and particulate filter. Conventional and other types of DOCs can be used. Conventional DOCs are well known in the art and typically include a precious metal-based catalyst on a support. The precious metal is generally one or more of Pt, Pd, Rh, Ag, Au, and the like. The support is typically alumina, silica, zirconia, titania, and the like, or combinations thereof. One or more promoters, such as, Mn, Mg, Ce, Ba, and the like, can be included, if desired. One example of a suitable DOC using a silica-zirconia support is U.S. Pat. No. 6,813,884, issued Nov. 9, 2004, entitled “Method Of Treating Diesel Exhaust Gases”, which is incorporated herein by reference.

The first catalyst can be a DPF, which captures carbonaceous particulate material and converts NO_x compounds. Any type of conventional DPF can be used.

A TWC could be used as the first catalyst in a gasoline engine. The TWC is capable of oxidizing CO and hydrocarbons to CO₂ and H₂O. The TWC can also convert NO_x to N₂ or optionally convert NO_x to NH₃. Any type of conventional TWC can be used. Conventional TWCs are well known in the art, and typically include one or more of Pt, Pd, Rh, Ag, Au, Ir, and the like. The support is typically alumina, silica, zirconia, titania, and the like, or combinations thereof. One or more promoters, such as Mn, Mg, Ce, Ba, and the like, can be included, if desired.

From the first catalyst 20, the exhaust flows to the SCR 25. The SCR 25 is capable of storing NH₃ and reducing nitrogen oxides such as NO and NO₂ to nitrogen (N₂) or nitrous oxide (N₂O). Reductants such as ammonia, urea, hydrogen, alcohols, hydrocarbons, and/or diesel fuel can be injected at 30 in front of the SCR 20 to aid in reduction, if desired. Alternatively, ammonia can be generated in situ, for example on an LNT. Conventional SCRs are well known in the art. They typically include a base metal catalyst on a high surface area support, such as alumina, silica, titania, zirconia, SiC, cordierite, zeolite, or a combination of these. For example, it can be a base metal including, but not limited to, Cu, Fe, Ce, Mn, or combinations thereof. Alternatively, vanadium-based SCR catalysts that typically contain Ti and W can be used. Base metals are generally able to effectuate NO_x conversion using ammonia while both the base metals and the high surface area support material serve to store NH₃.

The exhaust gas flows from the SCR 25 to one or more second catalysts 35. The second catalyst is not required, but can be included, if desired. The system can include one or more second catalysts. Suitable second catalysts include, but are not limited to, DOCs, DPFs, LNTs, NH₃ slip catalysts, hydrocarbon traps, and combinations thereof.

In one embodiment, the first catalyst is a DOC, which is followed by an SCR, which is followed by the second catalyst which is a DPF. In another embodiment, there could be two first catalysts, which could be a DOC and a DPF. They would both be positioned upstream of the SCR 25.

From the second catalyst 35, the exhaust flows to an LNT 40. The LNT functions as an ammonia slip catalyst, which is selective to nitrogen. During normal operation, the LNT 40 traps any NO_x slipping through the SCR 25. In addition, on a rapid transient when NH₃ slip occurs from the SCR 25, the NH₃ can reduce the LNT-stored NO_x to N₂. This both improves NO_x conversion and reduces or eliminates the NH₃ emission. Any conventional or non-conventional LNT (as discussed above) can be used. Desirably, the LNT (first and/or second) is one that stores NO_x at very low temperatures (e.g., down to about room temperature). Stored sulfur is often easier to remove with this type of LNT than with other types.

It may not be necessary for the LNT to undergo NO_x reduction by rich operation, because the purpose is to store NO_x until an NH₃ release. However, enrichment by any means conventionally used for LNT systems can also be used, if desired.

One issue with LNTs in general is sulfur poisoning. In this system, sulfur will be at least partially trapped in the upstream DOC and SCR. Both catalysts will release sulfur when heated, which can occur under conditions such as full load operation or active DPF regeneration. The LNT storage material can be chosen such that it desorbs sulfur at the DPF regeneration temperature even in lean exhaust (e.g., Ce or alumina). This is possible because the LNT NO_x storage function is only required to be usable under low temperature operation. The NH₃ release occurs on transition from cold to hot conditions. Alternatively, a hot/rich SO_x desorption mode as used in conventional LNT operations could also be employed if desired.

The aftertreatment system is not required to include the first catalyst. In that case, the SCR, is positioned before the LNT. Any NH₃ stored or generated in the SCR can reduce the NO_x stored on the LNT to N₂, thus reducing or eliminating the emission of NH₃, and improving the conversion of NO_x.

The catalysts can be made by any known method, including, but not limited to, washcoating the catalyst onto the support, or extruding the catalyst.

The various catalysts can be separate or combined. For example, the SCR and LNT could be placed on separate catalyst substrates, or they could be incorporated into a single substrate and/or a single converter can. The catalysts can be combined by slicing the SCR and LNT substrates to create separate zones, Alternatively, the catalyst compositions can be incorporated into separate layers or a combined layer on the same substrate. These combinations can be made within the pores of the monolithic substrate, which would allow SCR and LNT catalysts to be combined with a DPF. Suitable combined catalysts are described in U.S. Pat. Nos. 7,332,135, and 7,225,613, which are incorporated herein by reference. Other types of combinations are possible as is well known in the art.

The aftertreatment system can be used in lean burn engines of all types, including gasoline engines, diesel engines, and hydrogen engines.

While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes in the methods and apparatus disclosed herein may be made without departing from the scope of the invention, which is defined in the appended claims.

Claims

1. An aftertreatment system for use in a lean burn engine comprising:

a first catalyst in communication with an exhaust stream from the engine;

a selective catalytic reduction catalyst in communication with the exhaust stream and positioned downstream of the first catalyst; and

a lean NOx trap in communication with the exhaust stream and positioned downstream of the selective catalytic reduction catalyst;

with the proviso that the first catalyst is not a selective catalytic reduction catalyst.

2. The system of claim 1, wherein the first catalyst is selected from lean NOx traps, diesel oxidation catalysts, diesel particulate filters, three-way catalysts, hydrocarbon traps, and combinations thereof.

3. The system of claim 1 further comprising a second catalyst in communication with the exhaust stream and positioned downstream of the selective catalytic reduction catalyst and upstream of the lean NOx trap.

4. The system of claim 3 wherein the second catalyst is selected from lean NOx traps, diesel oxidation catalysts, diesel particulate filters, three-way catalysts, hydrocarbon traps, NH3 slip catalysts, and combinations thereof.

5. The system of claim 3, wherein the first catalyst is a diesel particulate filter, and wherein the second catalyst is a diesel oxidation catalyst.

6. The system of claim 2, wherein the first catalyst is a diesel particulate filter and a diesel oxidation catalyst

7. The system of claim 1 wherein the lean NOx trap comprises a support, a precious metal catalyst on the support, and optionally one or more alkali metal oxides or alkaline earth metal oxides on the support.

8. The system of claim 1 wherein the selective catalytic reduction catalyst comprises a support, and a catalyst comprising an oxide of a base metal or a zeolite on the support.

9. A method for treating lean burn engine exhaust gases comprising:

providing an aftertreatment system in an exhaust gas passage of a lean burn engine, the system comprising a first catalyst in communication an exhaust stream from the engine; a selective catalytic reduction catalyst in communication an exhaust stream and positioned downstream of the first catalyst; and a lean NOx trap in communication the exhaust stream and positioned downstream of the selective catalytic reduction catalyst; with the proviso that the first catalyst is not a selective catalytic reduction catalyst; and

exposing the aftertreatment system to engine exhaust gas containing NOx such that at least a portion of said NOx contained in said exhaust gas is converted to N2, and at least a portion of the NH3 supplied to or generated by the selective catalytic reduction catalyst converts NOx stored on the LNT to N2.

10. The method of claim 9 further comprising operating the engine in a cycle of lean and rich operation.

11. The method of claim 9 further comprising heating the exhaust gases for desorption of sulfur.

12. The method of claim 9, wherein the first catalyst is selected from lean NOx traps, diesel oxidation catalysts, diesel particulate filters, three-way catalysts, hydrocarbon traps, and combinations thereof.

13. The method of claim 9 further comprising a second catalyst in communication with the exhaust stream and positioned downstream of the selective catalytic reduction catalyst and upstream of the lean NOx trap.

14. The method of claim 13, wherein the second catalyst is selected from lean NOx traps, diesel oxidation catalysts, diesel particulate filters, three-way catalysts, NH3 slip catalysts, hydrocarbon traps, and combinations thereof

15. The method of claim 13, wherein the first catalyst is a diesel particulate filter, and wherein the second catalyst is a diesel oxidation catalyst.

16. The method of claim 9, wherein the first catalyst is a diesel particulate filter and a diesel oxidation catalyst.

17. The method of claim 11 wherein the lean NOx trap comprises a support, a precious metal catalyst on the support, and optionally one or more alkali metal oxides or alkaline earth metal oxides on the support.

18. The method of claim 11 wherein the selective catalytic reduction catalyst comprises a support, and a catalyst comprising an oxide of a base metal or a zeolite on the support.

19. An aftertreatment system for use in a lean burn engine consisting essentially of:

an optional first catalyst in communication with an exhaust stream from the engine;

a selective catalytic reduction catalyst in communication with the exhaust stream and positioned downstream of the optional first catalyst;

an optional second catalyst in communication with the exhaust stream and positioned downstream of the selective catalytic reduction catalyst;

a lean NOx trap in communication with the exhaust stream and positioned downstream of the optional second catalyst and the selective catalytic reduction catalyst.

20. The system of claim 17 wherein the optional first catalyst is selected from lean NOx traps, diesel oxidation catalysts, diesel particulate filters, three-way catalysts, hydrocarbon traps, and combinations thereof; and wherein the optional second catalyst is selected from lean NOx traps, diesel oxidation catalysts, diesel particulate filters, three-way catalysts, NH3 slip catalysts, hydrocarbon traps, and combinations thereof.