US20120042631A1

US20120042631A1 - Catalyst materials for ammonia oxidation in lean-burn engine exhaust

Info

Publication number: US20120042631A1
Application number: US12/860,180
Authority: US
Inventors: Steven J. Schmieg; Wei Li; Chang H. Kim; Gongshin Qi
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2010-08-20
Filing date: 2010-08-20
Publication date: 2012-02-23
Also published as: DE102011109946A1; CN102400744A

Abstract

An exhaust aftertreatment system and a method for treating exhaust produced by a lean-burn engine are provided. The exhaust aftertreatment system may include an exhaust gas treatment subsystem and a clean-up oxidation catalyst located downstream of the exhaust gas treatment subsystem. The clean-up oxidation catalyst can selectively oxidize NH₃to N₂in the hot, oxygen-abundant exhaust flow emanated from the lean-burn engine and passed through the exhaust gas treatment subsystem to help prevent ammonia slip to the atmosphere. The clean-up oxidation catalyst comprises perovskite oxide particles and/or manganese-containing mixed metal oxide particles dispersed on a selective catalytic reduction (SCR) catalyst.

Description

TECHNICAL FIELD

The technical field relates generally to exhaust aftertreatment systems that treat the exhaust produced by a lean-burn engine and, more particularly, to catalyst materials that may be used to oxidize ammonia, among others, and help prevent ammonia slip to the atmosphere.

BACKGROUND

Lean-burn engines such as diesel engines and certain spark-ignition engines are supplied with, and combust, a lean mixture of air and fuel (oxygen-rich mixture) to achieve more efficient fuel economy. The exhaust emitted from such engines engine during periods of lean-burn operation may include a relatively high content of nitrogen (N₂) and oxygen (O₂), a relatively low content of carbon monoxide (CO) and unburned/partially-burned hydrocarbons (HC's), possibly some suspended particulate matter (i.e., in diesel engines), and small amounts of nitrogen oxides primarily comprised of NO and NO₂(collectively referred to as NO_X). The NO_Xconstituency of the exhaust may fluctuate between about 50 and about 1500 ppm and generally comprises far greater amounts NO than NO₂along with nominal amounts of N₂O. The hot engine exhaust, which can reach temperatures of up to about 900° C., often needs to be treated before it can be released to the atmosphere.
An exhaust aftertreatment system may be associated with the lean-burn engine to help remove unwanted gaseous emissions and particulate matter that may be present in the lean-burn engine exhaust. The exhaust aftertreatment system may be configured to receive an exhaust flow from the lean-burn engine and generally aspires to cooperatively (1) oxidize CO into carbon dioxide (CO₂), (2) oxidize HC's into CO₂and water (H₂O), (3) convert NO_Xgases into nitrogen (N₂) and O₂, and (4) filter off or otherwise destroy any suspended particulate matter. A variety of exhaust aftertreatment system architectures that employ specially-catalyzed components have been devised and are able to sufficiently facilitate these reactions so that the exhaust expelled to the environment contains a much more desirable chemical makeup.
The normal operation of many exhaust aftertreatment system designs can result in ammonia (NH₃) being introduced into the exhaust flow. Some exhaust aftertreatment system designs, for example, deliberately introduce controlled amounts of NH₃into the exhaust flow to provide a reductant species that reduces NO_Xto N₂over a selective catalytic reduction (SCR) catalyst. A metering system that directly injects NH₃or an NH₃precursor (i.e., urea) into the exhaust flow may be employed to supply NH₃to the SCR catalyst. Ammonia may also be passively generated from native NO_Xin the exhaust flow by periodically cycling a rich air/fuel mixture to the lean-burn engine and communicating the resultant engine exhaust over a close-coupled three-way-catalyst (TWC). The NH₃produced may then be stored on a downstream SCR catalyst and consumed when the lean-burn engine is combusting a lean air/fuel mixture. Other exhaust aftertreatment design systems, as another example, may include one or more lean NO_Xtraps (LNT) that occasionally have to be regenerated and desulfated which, in turn, may cause NH₃to be introduced into the exhaust flow.
A clean-up oxidation catalyst is oftentimes included at or near the end of the exhaust aftertreatment system to oxidize any residual NH₃and other gaseous emissions that might otherwise escape to the atmosphere. Ammonia that manages to pass through the exhaust aftertreatment system is often referred to as “ammonia slip.” The clean-up oxidation catalyst used at this particular juncture of the exhaust aftertreatment system is traditionally formulated with about 5 to 10 g/ft³of platinum group metals (PGM's) dispersed on a high-surface area support material such as alumina. Most or all of the clean-up oxidation catalyst's PGM loading is generally attributable to platinum, although smaller amounts of palladium and rhodium may also be included, if desired. A honeycomb flow-through monolith structure may carry the clean-up oxidation catalyst within several hundred to several thousand parallel flow-through cells to ensure sufficient contact between the exhaust flow and the clean-up oxidation catalyst.
The PGM's commonly used to make the clean-up oxidation catalyst, however, are quite expensive and have been shown, in some instances, to exhibit poor thermal durability when exposed to relatively high-temperature engine exhaust. The N₂selectivity of PGM-based clean-up oxidation catalysts can also be affected by thermal aging and fluctuations in the temperature and chemical composition of the exhaust flow. Such affects on N₂selectivity can cause the clean-up oxidation catalyst to oxidize NH₃into NO_X—instead of N₂—at an unacceptably high rate and thus lower the overall NO_Xconversion efficiency of the exhaust aftertreatment system.

SUMMARY OF EXEMPLARY EMBODIMENTS

A clean-up oxidation catalyst that comprises a selective catalytic reduction (SCR) catalyst and metal oxide particles selected from the group consisting of perovskite oxide particles and manganese-containing mixed metal oxide particles, and mixtures thereof, dispersed on the SCR catalyst may be employed at or near the end of an exhaust aftertreatment system to help prevent ammonia slip and the escape of unwanted gaseous emissions to the atmosphere. The clean-up oxidation catalyst can selectively oxidize NH₃to N₂in the hot, oxygen-abundant exhaust flow emanated from a lean-burn engine and communicated through the exhaust aftertreatment system. The inclusion of platinum group metals, such as platinum and palladium, in the clean-up oxidation catalyst may be avoided. Other exemplary and more detailed embodiments of the invention will become apparent from the detailed description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will become more fully understood from the detailed description and the accompanying drawings, wherein:

FIG. 1 is a generalized and schematic depiction of an exhaust aftertreatment system for a lean-burn engine that includes an exhaust gas treatment subsystem and a clean-up oxidation catalyst downstream from the exhaust gas treatment subsystem.

FIG. 2 is a generalized and schematic illustration of the relevant parts of one exemplary embodiment of the exhaust gas treatment subsystem.

FIG. 3 is a generalized and schematic illustration of the relevant parts of another exemplary embodiment of the exhaust gas treatment subsystem.

FIG. 4 is a graph that shows the oxidation capabilities of La_0.9Sr_0.1CoO₃particles for various reactions when exposed to simulated lean-burn engine exhaust.

FIG. 5 is a graph that shows the NH₃oxidation performance, when exposed to simulated lean-burn engine exhaust, of a degreened Fe/β-zeolite SCR catalyst, La_0.9Sr_0.1CoO₃particles, and three degreened clean-up oxidation catalysts each with a different La_0.9Sr_0.1CoO₃particle loading.

FIG. 6 is a graph that shows the NH₃oxidation performance, when exposed to simulated lean-burn engine exhaust, of a high-temperature aged Fe/β-zeolite SCR catalyst, La_0.9Sr_0.1CoO₃particles, and three high-temperature aged clean-up oxidation catalysts each with a different La_0.9Sr_0.1CoO₃particle loading.

FIG. 7 is a graph that shows how much of the NH₃oxidation shown in FIG. 5 resulted in the formation of NO_X.

FIG. 8 is a graph that shows how much of the NH₃oxidation shown in FIG. 6 resulted in the formation of NO_X.

FIG. 9 is a graph that shows the NH₃oxidation performance and the N₂selectivity, when exposed to simulate lean-burn engine exhaust, of a degreened and a high-temperature aged clean-up oxidation catalyst material having 12 wt. % La_0.9Sr_0.1CoO₃particles loaded onto a Fe/β-zeolite SCR catalyst and, for comparison purposes, a conventional platinum-containing catalyst.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following description is merely exemplary in nature and is in no way intended to limit the claimed invention(s), its application, or its uses.
A clean-up oxidation catalyst that comprises metal oxide particles dispersed on a selective catalytic reduction (SCR) catalyst may be employed at or near the end of an exhaust aftertreatment system to help prevent ammonia slip and the escape of unwanted gaseous emissions to the atmosphere. The metal oxide particles may be selected from the group consisting of perovskite oxide particles, manganese-containing mixed metal oxide particles, and mixtures thereof. The clean-up oxidation catalyst can selectively oxidize NH₃to N₂in the hot, oxygen-abundant exhaust flow emanated from a lean-burn engine and communicated through an upstream portion of the exhaust aftertreatment system. The temperature at which optimum NH₃oxidation occurs and the oxidation reaction's N₂selectivity can be influenced by the loading of the metal oxide particles as well as the aging of the clean-up oxidation catalyst. The inclusion of PGM's in the clean-up oxidation catalyst, although not prohibited, is not needed to achieve satisfactory NH₃oxidation over a robust temperature range. The opportunity to reduce the amount of PGM's used in an exhaust aftertreatment system can contribute to significant cost savings and help counteract the durability and N₂selectivity issues sometimes observed in PGM-based catalysts.
FIG. 1 depicts a generalized and schematic illustration of an exhaust aftertreatment system 10 for managing an exhaust flow 20 produced by a lean-burn engine 12 that is combusting a lean air/fuel (A/F) mixture 18. The exhaust aftertreatment system 10 receives the exhaust flow 20 from the lean-burn engine 12 and communicates a treated exhaust flow 24 downstream for expulsion to the atmosphere. The exhaust aftertreatment system 10, as illustrated here, may include an exhaust gas treatment subsystem 14 and a clean-up oxidation catalyst 16. The exhaust gas treatment subsystem 14 decreases to acceptable levels the amount of unwanted gaseous emissions and, if present, particulate matter that are contained in the exhaust flow 20. The clean-up oxidation catalyst 16 oxidizes any NH₃contained in an intermediate exhaust flow 22 that flows from the exhaust gas treatment subsystem 14 to help prevent ammonia slip to the atmosphere. The treated exhaust flow 24 may flow through other mechanical equipment, such as a muffler, after engaging the clean-up oxidation catalyst 16 but generally does not interact with any additional catalysts before being released to the atmosphere.
The lean-burn engine 12 may be any engine that is constructed and designed to combust the lean A/F mixture 18 at least part of the time. The lean A/F mixture 18 supplied to the lean-burn engine 12 generally contains more air than is stoichiometrically needed to combust the associated fuel. Known mechanical or electronic mechanisms may be used to control the air to fuel mass ratio of the lean A/F mixture 18 which, in general, often ranges from about 15 to about 65 depending on the engine load, RPM, and the type of lean-burn engine 12 (i.e., diesel or spark-ignition) being operated. Examples of engines that may be employed as the lean-burn engine 12 include a charge compression-ignition (diesel) engine, a lean-burn spark-ignition (gasoline) engine such as spark-ignition direct injection (SIDI) engine, or a homogeneous charge compression ignition (HCCI) engine. The general construction and operating requirements of these types of engines are well known to skilled artisans and, as such, need not be described in further detail.
The combustion of the lean A/F mixture 18 in the lean-burn engine 12 generates mechanical power and the exhaust flow 20 that is supplied to the exhaust aftertreatment system 10. The exhaust flow 20 generally includes a relatively large amount of N₂and O₂, possibly some suspended particulate matter composed of uncombusted high-molecular weight hydrocarbons, and various unwanted gaseous emissions comprised of the following: (1) CO, (2) HC's, and (3) a NO_Xcontingent primarily comprised of NO and NO₂. The NO_Xcontingent of the exhaust flow 20 may fluctuate between about 50 and about 1500 ppm. The proportion of NO and NO₂particles in the NO_Xcontingent usually ranges from approximately 80%-95% NO and approximately 5%-20% NO₂. Such a NO/NO₂particle distribution corresponds to a molar ratio of NO to NO₂that ranges from about 4 to about 19. The exhaust flow 20 may reach temperatures of up to 900° C. depending the distance between the lean-burn engine 12 and the exhaust aftertreatment system 10 as well as the presence of any intervening components such as a turbocharger turbine and/or an EGR bleed line. The temperature of the exhaust flow 20 along with the O₂content, which is relatively high, and the CO and HC's content, which are relatively low, promote an oxidizing environment in the exhaust flow 20.
The exhaust gas treatment subsystem 14 encompasses a large variety of system architectures that operate to remove a substantial portion of the CO, HC's, NO_X, and particulate matter, if present, from the exhaust flow 20. A NO_Xabatement component, such as a lean NO_Xtrap or a selective catalytic reduction (SCR) catalytic converter, may be included in the exhaust gas treatment subsystem 14 to reduce NO_Xto N₂in the oxidative environment of the exhaust flow 20. An assortment of other components such as, but not limited to, diesel oxidation converters, three-way catalytic converters, diesel particulate filters, and reductant metering devices are available and can be assembled to operate with the NO_Xabatement component, either individually or in select combinations with one another, to constitute the exhaust gas treatment subsystem 14.
The intermediate exhaust flow produced by the exhaust gas treatment subsystem 14 primarily includes N₂, O₂, H₂O, and CO₂. The intended operation of the exhaust gas treatment subsystem 14 and, in particular, the NO_Xabatement component, may further result in NH₃being present in the intermediate exhaust flow 22 as well as acceptable residual amounts of CO, HC's, NO_X, and particulate matter. A number of factors and component operating requirements related to the lean-burn engine 12 and the exhaust gas treatment subsystem 14 may be responsible for the presence of NH₃in the intermediate exhaust flow 22 despite the sophisticated control strategies often associated with the exhaust aftertreatment system 10.
One exemplary embodiment of the exhaust gas treatment subsystem 14, which is identified as 14 a and depicted in FIG. 2, includes a diesel oxidation converter 30, an ammonia-SCR catalytic converter 32 located downstream of the diesel oxidation converter 30, a diesel particulate filter 34 located downstream of the ammonia-SCR catalytic converter 32, and a urea metering system 36. The exhaust gas treatment subsystem 14 a shown and described here may be used when the exhaust aftertreatment system 10 is coupled to a diesel engine.
The diesel oxidation converter 30 receives the exhaust flow 20 from the lean-burn engine 12. The diesel oxidation converter 30 houses a diesel oxidation catalyst that may comprise a combination of platinum and palladium or some other suitable oxidation catalyst formulation. The exhaust flow 20 traverses the diesel oxidation converter 30 and achieves intimate exposure with the diesel oxidation catalyst to promote the oxidation of CO (to CO₂), HC's (to CO₂and H₂O) and NO (to NO₂). The oxidation of NO by the diesel oxidation catalyst typically does not decrease the NO_Xcontent in the exhaust flow 20; instead, the molar ratio of NO to NO₂is merely decreased as NO is oxidized to NO₂. This downward adjustment to the NO:NO₂molar ratio may be desirable since the downstream ammonia-SCR catalyst 32 may convert NO_Xto N₂at lower temperatures more efficiently as the molar ratio of NO to NO₂decreases from that which is typically generated by the lean-burn engine 12.
The exhaust flow 20 exiting the diesel oxidation converter 30 is then combined with NH₃to form an exhaust mixture 44. The NH₃may be supplied by the urea metering device 36 which includes an on-board and refillable urea storage tank 38 fluidly connected to a urea injector 40. Urea, which is stored in the urea storage tank 38, may be injected into the exhaust flow 20 exiting the diesel oxidation converter 30 through the urea injector 40. The urea then quickly evaporates and undergoes thermolysis and hydrolysis reactions in the hot and oxygen-abundant exhaust flow 20 to generate NH₃and form the exhaust mixture 44. The amount of urea injected into the exhaust flow 20 may be monitored and controlled by known control techniques that attempt to regulate the amount of NH₃present in the exhaust mixture 20 despite fluctuations in the temperature, chemical composition, and flow rate of the exhaust flow 20. A mixer 42 or other suitable device may be provided upstream of the ammonia-SCR catalytic converter 32 to help evaporate the injected urea and homogeneously distribute small particles of NH₃throughout the exhaust mixture 44.
The ammonia-SCR catalytic converter 32 receives the exhaust mixture 44 and discharges a NO_X-treated exhaust flow 46. The ammonia-SCR catalytic converter 32 houses an appropriate ammonia-SCR catalyst that can absorb NH₃and facilitate the reduction of NO_Xin the oxidative environment fostered by the exhaust mixture 44. The exhaust mixture 44 traverses the ammonia-SCR catalytic converter 32 and achieves intimate exposure with the ammonia-SCR catalyst to enable the reduction of NO_X, largely to N₂and H₂O, in the presence of NH₃and O₂. The newly-generated N₂is then communicated from the ammonia-SCR catalytic converter 32 in the NO_X-treated exhaust flow 46. The ammonia-SCR catalyst may comprise, for example, an ion-exchanged base-metal zeolite such as a Cu/zeolite or a Fe/zeolite. Several different zeolite crystal structures including β-zeolites and MFI-type zeolites are commonly used to make the ammonia-SCR catalyst. Supported base metal oxides, such as V₂O₅-WO₃/TiO₂and V₂O₅/TiO₂, may also be employed to formulate the ammonia-SCR catalyst.
A number of factors may influence the dynamic operating conditions of the ammonia-SCR catalytic converter 32 and result in NH₃being present in the NO_X-treated exhaust flow 46 and ultimately the intermediate exhaust flow 22. Fast increases in the temperature or flow rate of the exhaust mixture 46 and/or the over-injection of urea into the exhaust flow 20, for instance, may trigger the release of significant amounts of NH₃from the ammonia-SCR catalyst or simply allow NH₃to pass through the ammonia-SCR catalytic converter 32 unabsorbed and unreacted.
The NO_X-treated exhaust mixture 46 discharged from the ammonia-SCR catalytic converter 32 is then supplied to the diesel particulate filter 34 to remove any suspended particulate matter. The diesel particulate filter 34 may be constructed according to any known design. The intermediate exhaust flow 22 emerges from the diesel particulate filter 34.
Skilled artisans will appreciate that many modifications can be made to the exhaust gas treatment subsystem 14 a. For example, ammonia may be directly injected into the exhaust flow 20 instead of urea to form the exhaust mixture 44. As another example, the diesel particulate filter 34 may be placed between the diesel oxidation converter 30 and the ammonia-SCR catalytic converter 32 or combined with the diesel oxidation converter 30. As yet another example, the exhaust gas treatment subsystem 14 a may be altered to operate with a spark-ignition engine by substituting a catalytic converter that houses a three-way-catalyst (TWC) for the diesel oxidation converter 30 and removing the diesel particulate filter 34. The TWC may comprise a combination of platinum, palladium, and rhodium, or it may comprise some other suitable catalyst formulation.
Another exemplary embodiment of the exhaust gas treatment subsystem 14, which is identified as 14 b and depicted in FIG. 3, includes a catalytic converter 50 and a lean-NO_Xtrap (LNT) 52 located downstream of the catalytic converter 50. The exhaust gas treatment system 14 b shown and described here may be used when the exhaust aftertreatment system 10 is coupled to a spark-ignition engine such as a SIDI engine.
The catalytic converter 50 receives the exhaust flow 20 from the lean-burn engine 12. The catalytic converter 50 houses a TWC catalyst that may comprise a combination of platinum, palladium, and rhodium. Other suitable TWC formulations may of course be employed. The exhaust flow 20 traverses the catalytic converter 50 and achieves intimate exposure with the TWC catalyst to promote the oxidation of CO (to CO₂) and HC's (to CO₂and H₂O). The TWC catalyst generally does not include a high enough proportional platinum loading to oxidize NO and significantly change the NO to NO₂molar ratio in the exhaust flow 20. The TWC catalyst can also oxidize CO and HC's and simultaneously reduce NO_Xto N₂during momentary periods when the lean-burn engine 12 is supplied with a rich A/F mixture to, for example, regenerate or desulfate the LNT 52.
The LNT 52 receives the exhaust flow 20 exiting the catalytic converter 50. The exhaust flow 20 traverses the LNT 52 and achieves intimate contact with a LNT catalyst housed in the LNT 52. The LNT catalyst exhibits NO₂trapping and NO_Xconversion capabilities and generally comprises an oxidation catalyst, a NO_Xstorage catalyst, and a NO_Xreduction catalyst. The oxidation catalyst oxidizes NO to NO₂and the NO_Xstorage catalyst traps or “stores” NO₂as a nitrate species as the exhaust flow 20 traverses the LNT 52. The oxidation catalyst may also oxidize other gaseous emissions such as CO and HC's, if present. The NO_Xreduction catalyst, as described below, catalyzes the reduction of NO and NO₂into N₂during regeneration of the LNT 52. A typical LNT catalyst formulation may comprise, for example, an alumina washcoat appropriately loaded with platinum, rhodium, and barium carbonate (BaCO₃).
The NO_Xstorage capacity of the LNT catalyst is not unlimited and at some point may need to be regenerated or purged of NO_X-derived nitrate compounds. The LNT catalyst may be regenerated by introducing reductants—such as CO, HC's and H₂—into the exhaust flow 20. This may be accomplished by decreasing the air to fuel mass ratio of the lean A/F mixture 18 so that the lean-burn engine 12 combusts a stoichiometric or rich A/F mixture. The resultant delivery of rich-burn exhaust effluents to the LNT 52 by way of the exhaust flow 20 causes the NO_X-derived nitrate compounds stored in the LNT catalyst to become thermodynamically unstable which, in turn, triggers the release of NO_Xand the regeneration of future NO_Xstorage sites. The liberated NO_Xis then reduced to N₂by excess reductants. The newly-generated N₂is communicated from the LNT 52 in the intermediate exhaust flow 22. Some NH₃may also be introduced into the intermediate exhaust flow 22 since it is possible for the NO_Xreduction catalyst to reduce NO_Xto NH₃—instead of N₂—during regeneration.
Skilled artisans will appreciate, much like before, that many modifications can be made to the exhaust gas treatment subsystem 14 b. For example, an oxidation catalyst, such as a diesel oxidation catalyst or a two-way-catalyst, may be provided upstream of the LNT 52 in addition to the catalytic converter 50 to lower the NO to NO₂molar ratio in the exhaust flow 20. This downward adjustment to the NO:NO₂molar ratio may be desirable since the downstream LNT 52 may convert NO_Xto N₂at lower temperatures more efficiently as the molar ratio of NO to NO₂decreases from that which is typically generated by the lean-burn engine 12. As another example, the exhaust gas treatment subsystem 14 b may be altered to operate with a diesel engine by substituting a diesel oxidation converter that houses a diesel oxidation catalyst for the catalytic converter 50 and adding a diesel particulate filter. The combustion of diesel fuel in the lean-burn engine 12, however, may additionally require periodic desulfation of the LNT catalyst to remove accumulate sulfur oxides (SO_X). The LNT catalyst may be desulfated, for example, by introducing reductants into the exhaust flow 20 and heating the LNT catalyst to elevated temperatures of about 600° C. The desulfation of the LNT catalyst may cause NH₃to be introduced into the intermediate exhaust flow 22 in much the same way that regeneration of the LNT catalyst does. While the problem of accumulated SO_Xin the LNT catalyst is more prevalent when the lean-burn engine 12 is a diesel engine, desulfaton of the LNT catalyst or desulfation-like procedures may also be practiced under certain circumstances when the lean-burn engine is a spark-ignition engine.
Referring back to FIG. 1, the clean-up oxidation catalyst 16 receives the intermediate exhaust flow 22 and oxidizes any residual NH₃to N₂. Other unwanted gaseous emissions that may have slipped through the exhaust gas treatment subsystem 14 may also be oxidized. The clean-up oxidation catalyst 16 may, in one embodiment, be housed in a separate catalytic converter device that is fluidly connected to the end of the exhaust gas treatment subsystem 14. The clean-up oxidation catalyst 16 may, for example, be carried on a support body contained within a canister. The canister may be constructed to communicate the intermediate exhaust flow 22 across or through the substrate body to induce intimate exposure between the intermediate exhaust flow 22 and the clean-up oxidation catalyst 16. Various constructions of the substrate body are possible. The substrate body may be a monolithic honeycomb structure that includes several hundred to several thousand parallel flow-through cells per square inch. Each of the flow-through cells may be defined by a wall surface to which the clean-up oxidation catalyst 16 is washcoated. The monolithic honeycomb structure may be formed from a material capable of withstanding the temperatures and chemical environment associated with the intermediate exhaust flow 22. Some specific examples of materials that may be used include ceramics such as extruded cordierite, α-alumina, silicon carbide, silicon nitride, zirconia, mullite, spodumene, alumina-silica-magnesia, zirconium silicate, sillimanite, petalite, or a heat and corrosion resistant metal such as titanium or stainless steel. The clean-up oxidation catalyst 16 may also, in another embodiment, be zone coated onto the trailing end of the most-downstream component or components included in the exhaust gas treatment subsystem 14 that communicate the intermediate flow 22 such as, for example, and with reference to FIG. 2, the diesel particulate filter 34.
The clean-up oxidation catalyst 16 comprises metal oxide particles selected from the group consisting of perovskite oxide particles and manganese-containing mixed metal oxide particles dispersed on a selective catalytic reduction (SCR) catalyst. The clean-up oxidation catalyst 16 can catalyze the oxidation of appreciable amounts of any NH₃contained in the intermediate exhaust flow 22 into N₂and H₂O. The metal oxide particle loading and the aging of the clean-up oxidation catalyst 16 can influence the catalytic activity of the clean-up oxidation catalyst 16 including the temperature at which optimum NH₃oxidation occurs and the NH₃oxidation reaction's N₂selectivity. The clean-up oxidation catalyst 16 may also, in a related and coupled NH₃reaction, catalytically reduce the amount of any residual NO_Xcontained in the intermediate exhaust flow 22 if both NO_Xand NH₃are present. Any appropriate technique may be used to disperse the metal oxide particles onto the SCR catalyst including washcoating and incipient wet impregnation.
The perovskite oxide particles that may be dispersed on the SCR catalyst encompass a class of compounds defined by the general formula ABO₃. The “A” and “B” atoms may be complimentary cations of different sizes that coordinate with oxygen anions. A unit cell of the ABO₃crystal structure may feature a cubic closest packing arrangement with the “A” cation, which is generally the larger of the two cations, centrally located and surrounded by eight “B” cations situated in the octahedral voids of the packing arrangement. The “A” and “B” cations in such a packing arrangement respectively coordinate with twelve and six oxygen anions. The unit cell of the ABO₃crystal structure, however, is not necessarily limited to a cubic closest packing arrangement. Certain combinations of the “A” and “B” cations may indeed deviate from the cubic closest packing arrangement and assume, for instance, an orthorhombic, rhombohedral, or monoclinic packing structure. Small amounts of the “A” and/or “B” cations, moreover, may be substituted with different yet similarly sized “A1” and “B1” promoter cations to give a supercell crystal structure derived from the general ABO₃crystal structure and designated by the general formula A_1-XA1_XB_1-YB1_YO₃, where both X and Y range from 0 to 1.
The perovskite oxide particles may comprise the same perovskite oxide or a mixture of two or more perovskite oxides. A great many combinations of perovskite oxides are available for use in the clean-up oxidation catalyst 16 since no fewer than 27 cations may be employed as the “A” cation and no fewer than 36 cations may be employed as the “B” cation. A listing of the cations most frequently employed as the “A” cation includes those of calcium (Ca), strontium (Sr), barium (Ba), bismuth (Bi), cadmium (Cd), cerium (Ce), lead (Pb), yttrium (Y), and lanthanum (La) while a listing of the cations most commonly employed as the “B” cation includes those of cobalt (Co), titanium (Ti), zirconium (Zr), niobium (Nb), tin (Sn), cerium (Ce), aluminum (Al), nickel (Ni), chromium (Cr), manganese (Mn), copper (Cu), and iron (Fe). Some specific and exemplary perovskite oxides that may constitute all or part of the perovskite oxide particles include LaCoO₃, La_0.9Sr_0.1CoO₃, LaMnO₃, La_0.9Sr_0.1MnO₃, LaFeO₃, and LaSr_0.1Fe_0.9O₃.
The manganese-containing mixed metal oxide particles that may be dispersed on the SCR catalyst may include at least one of manganese-cerium oxides, Mn_XCe_YO_Z, manganese-zirconium oxides, Mn_XZr_WO_Z, or manganese-cerium-zirconium oxides, Mn_XCe_YZr_WO_Z, with X ranging from 0.02 to 0.98, Y ranging from 0.02 to 0.98, W ranging from 0.02 to 0.98, and Z ranging from 1.0 to 3.0. Some specific examples of suitable manganese-containing mixed metal oxides include, but are not limited to, 0.5MnO_V-0.5CeO₂where V ranges from 2 to 3, 0.3MnO_V-0.7CeO₂where V ranges from 2 to 3, 0.1MnO_V-0.9CeO₂where V ranges from 2 to 3, Mn_0.1Ce_0.9O₂, Mn_0.2Ce_0.8O_1.9, and Mn_0.5Ce_0.5O_1.75.
The perovskite oxide particles and the manganese-containing mixed metal oxide particles, either alone or in combination, can help the clean-up oxidation catalyst 16 catalytically oxidize NH₃to N₂just as efficiently as platinum when exposed to the hot and oxygen-abundant intermediate exhaust flow 22. While not wishing to be bound by theory, it is believed that the perovskite oxide particles and the manganese-containing mixed metal oxide particles have the ability to donate oxygen anions to NH₃molecules while temporarily forming oxygen vacancies in their crystal structures. Readily available oxygen contained in the engine exhaust then disassociates to fill those oxygen anion vacancies and possibly react with additional NH₃molecules. The ability of the perovskite oxide particles and the manganese-containing mixed metal oxide particles to efficiently oxidize NH₃may significantly diminish or altogether eliminate the need to include PGM's such as platinum in the clean-up oxidation catalyst 16. The clean-up oxidation catalyst 16 may, as a result, provide the exhaust aftertreatment system 10 with a smaller amount of PGM's than a comparable exhaust aftertreatment system that incorporates a conventional PGM-based clean-up oxidation catalyst to prevent ammonia slip and the escape of other unwanted gaseous emissions.
The amount of the metal oxide particles present in the clean-up oxidation catalyst 16 may range from about 0.1 wt. % to about 20 wt. %, more specifically from about 0.5 wt. % to about 15 wt. %, and even more specifically from about 1.0 wt. % to about 12 wt. %, based on the weight of the clean-up oxidation catalyst 16. The specific metal oxide particle loading may be chosen, if desired, based on the normal expected operating temperature window of the exhaust flow 20 and the aging of the clean-up oxidation catalyst 16. A degreened (lightly-aged) clean-up oxidation catalyst 16 with a higher metal oxide particle loading (greater than about 5 wt. %), for example, tends to oxidize NH₃to N₂quite efficiently at lower exhaust temperatures (up to about 350° C.) although, at higher temperatures (above about 350° C.), it begins to oxidize some NH₃to NO_Xinstead of N₂. As another example, a degreened clean-up oxidation catalyst 16 with a lower metal oxide particle loading (less than about 2 wt. %) tends to oxidize NH₃to N₂more consistently with complete or almost complete N₂selectivity at both lower temperatures (up to about 350° C.) and higher temperatures (above about 350° C.). High-temperature aging can further affect the NH₃oxidation efficiency and the N₂selectivity of the clean-up oxidation catalyst 16. Such aging, in general, improves the N₂selectivity of the clean-up oxidation catalyst 16 at higher temperatures at the expense of NH₃oxidation efficiency at lower temperatures.
The SCR catalyst may be any material that can help facilitate the oxidation of NH₃to N₂when exposed to the intermediate exhaust 22. The SCR catalyst is generally a porous and high-surface area material—a wide variety of which are commercially available. The specific SCR catalyst used to formulate the clean-up oxidation catalyst 16 may be an ion-exchanged base metal zeolite or silver-supported alumina (Ag/Al₂O₃). The zeolite may be a β-type zeolite, a Y-type zeolite, or a MFI-type zeolite. Some specific examples of suitable ion-exchanged base metal zeolites that may be used include, but are not limited to, a β-zeolite that is ion-exchanged with Cu or Fe, a MFI-type zeolite that is ion exchanged with Cu or Fe, and a Y-type zeolite that is ion-exchanged with Na, Ba, Cu, or CuCo.
The particular composition of the clean-up oxidation catalyst 16 may be formulated based on a number of factors including the type and normal expected operating parameters of the lean-burn engine 12 and the design and construction of the exhaust gas treatment system 10. The clean-up oxidation catalyst 16 may, for example, comprise about 10-15 wt. % metal oxide particles washcoated onto a copper exchanged or iron exchanged β-zeolite. This particular catalyst composition may be employed if the lean-burn engine 12 is expected to generally operate at low speeds and/or with a low load demand. The clean-up oxidation catalyst 16 may also, as another example, comprise about 0.5-2.0 wt. % metal oxide particles washcoated onto a copper exchanged or iron exchanged β-zeolite. This particular catalyst composition may be employed if the lean-burn engine 12 is expected to generally operate at high speeds and/or with a high load demand. Other compositions of the clean-up oxidation catalyst 16 may of course be formulated and utilized by skilled artisans who are familiar with lean-burn engine exhaust aftertreatment technology.

EXAMPLE

This Example demonstrates the catalytic activity of several exemplary clean-up oxidation catalysts that were evaluated in a laboratory reactor configured to flow a simulated lean-burn engine exhaust feedstream. Each of the exemplary clean-up oxidation catalysts evaluated had a different weight percent loading of La_0.9Sr_0.1CoO₃particles washcoated onto a Fe/β-zeolite and was either degreened or subjected to high-temperature aging. The weight percent loading of the La_0.9Sr_0.1CoO₃particles for each exemplary clean-up oxidation catalyst is based on the weight of the clean-up oxidation catalyst (i.e., the total weight of the La_0.9Sr_0.1CoO₃particles and the SCR catalyst). While this Example evaluates different loadings of La_0.9Sr_0.1CoO₃particles (perovskite oxide particles) on a Fe/β-zeolite SCR catalyst, it is expected that the same general results and data would be achieved by either mixing the perovskite oxide particles with manganese-containing mixed metal oxide particles or completely substituting the perovskite oxide particles for manganese-containing mixed metal oxide particles.
A citric acid method was used to prepare a quantity of La_0.9Sr_0.1CoO₃particles. First, appropriate amounts of La(NO₃)₃.6H₂O, Co(NO₃)₂.6H₂O, and Sr(NO₃)₂were dissolved in distilled water with citric acid monohydrate. The amount of water used was 46.2 mL per gram of La(NO₃)₃.6H₂O, and the citric acid was added to the distilled water in a 10 wt. % excess to ensure complete complexation of the metal ions. The solution was set on a stirring and heating plate and stirred for 1 hour at room temperature. The solution was then heated to 80° C. under continuous stirring to slowly evaporate the water until the solution became a viscous gel and started evolving NO/NO₂gases. The resulting spongy material was crushed and calcined at 700° C. for about 5 hours in static air. The temperature was then ramped down at a rate of 10° C. per minute. When the temperature reached just below 300° C., the citrate ions combusted vigorously and caused a large spike in temperature and powder displacement. The powder was thus covered with several layers of ZrO₂balls to prevent such powder displacement yet still allow for gas mobility. The prepared La_0.9Sr_0.1CoO₃particles were characterized by N₂physisorption for surface area measurements and X-ray diffraction for their bulk structure measurements.
The La_0.9Sr_0.1CoO₃particles were then ball milled with 6.33 mL of water per gram of the La_0.9Sr_0.1CoO₃particles for 18 hours. Afterwards, the slurry was stirred continuously and 0.33 mL HNO₃(0.1M) per gram of the La_0.9Sr_0.1CoO₃particles and 5 mL of water per gram of the La_0.9Sr_0.1CoO₃particles was added. The resulting washcoat solution had a concentration of 0.114 grams of La_0.9Sr_0.1CoO₃particles per mL. The slurry was washcoated onto a monolithic honeycomb core sample (¾ inch diameter by 1 inch length with a flow-through cell density of 400 per square inch) that had already been washcoated with the Fe/β-zeolite. Next, after washcoating of the La_0.9Sr_0.1CoO₃particles, the monolithic honeycomb core sample was dried and calcined at 550° C. for 5 hours in static air.
This procedure was repeated several times to prepare monolithic honeycomb core samples that had a La_0.9Sr_0.1CoO₃particle loading of either 1.0, 5.5, or 12 wt. %. Two core samples were prepared for each wt % loading of La_0.9Sr_0.1CoO₃particles. One clean-up catalyst at each La_0.9Sr_0.1CoO₃particle loading was degreened and the other was high-temperature aged. The degreened clean-up oxidation catalysts were hydrothermally aged in air+10% H₂O for 5 hours at 550° C. The high-temperature aged clean-up oxidation catalysts were hydrothermally aged in air+10% H₂O for 48 hours at 700° C.
Monolithic core samples were also prepared that included only a Fe/β-zeolite SCR catalyst, only La_0.9Sr_0.1CoO₃particles, and a conventional platinum-containing catalyst, all for comparative evaluation purposes. The Fe/β-zeolite SCR catalysts were either degreened or high-temperature aged similar to the clean-up oxidation catalysts. The La_0.9Sr_0.1CoO₃particles alone were aged slightly different as indicated below.
FIG. 4 shows the catalytic performance of the La_0.9Sr_0.1CoO₃particles for various reactions at temperatures ranging from 150° C. to 550° C. Temperature (° C.) is plotted on the X-axis and conversion (%) is plotted on the Y-axis. The conversion of NO (NO+O₂) is identified by numeral 60, the conversion of NH₃(NH₃+O₂) is identified by numeral 62, and the conversion of NO_X(NO+NH₃+O₂) is identified by numeral 64. The La_0.9Sr_0.1CoO₃particles were hydrothermally aged in air+10% H₂O for 5 hours at 700° C. The simulated exhaust feedstream passed over the La_0.9Sr_0.1CoO₃particles to determine NO conversion (60) had a space velocity of about 30,000 h⁻¹and comprised approximately 10% O₂, 400 ppm NO, and the balance N₂. The simulated exhaust feedstream passed over the La_0.9Sr_0.1CoO₃particles to determine NH₃conversion (62) had a space velocity of about 30,000 h⁻¹and comprised approximately 10% O₂, 5% H₂O, 5% CO₂, 200 ppm NH₃, and the balance N₂. The simulated exhaust feedstream passed over the La_0.9Sr_0.1CoO₃particles to determine NO_Xconversion (64) had a space velocity of about 30,000 h⁻¹and comprised approximately 10% O₂, 5% H₂O, 5% CO₂, 200 ppm NO, 200 ppm NH₃, and the balance N₂.
As shown in FIG. 4, the La_0.9Sr_0.1CoO₃particles oxidized NO and NH₃rather well at temperatures above about 225° C. and 350° C., respectively. The undesirable oxidation of NH₃into NO_X, however, occurred at temperatures of about 250° C. to about 450° C. At temperatures above about 450° C., as shown, the oxidation of NH₃to N₂was much preferred over the oxidation of NO_Xto N₂. This oxidation selectivity resulted in the overall NO_Xconversion being negative.
FIGS. 5-8 depict some catalytic performance data of the exemplary clean-up oxidation catalysts. The same catalytic performance data for the Fe/zeolite SCR catalyst and the La_0.9Sr_0.1CoO₃particles are also shown for comparison purposes.
FIGS. 5 and 6 show the NH₃oxidation performance, in the absence of NO, of the Fe/β-zeolite SCR catalyst alone, the La_0.9Sr_0.1CoO₃particles alone, and the clean-up oxidation catalysts at temperatures ranging from 150° C. to 550° C. Temperature (° C.) is plotted on the X-axis and NH₃conversion (%) is plotted on the Y-axis. The Fe/β-zeolite SCR catalyst and the clean-up catalysts represented in FIG. 5 were degreened while the Fe/β-zeolite SCR catalyst and the clean-up oxidation catalysts represented in FIG. 6 were high-temperature aged. The La_0.9Sr_0.1CoO₃particles in both of FIGS. 5 and 6 were hydrothermally aged for 5 hours at 700° C. The simulated exhaust feedstream passed over the Fe/β-zeolite SCR catalyst and the clean-up oxidation catalysts had a space velocity of about 30,000 h⁻¹and comprised approximately 10% O₂, 5% H₂O, 5% CO₂, 200 ppm NH₃, and the balance N₂.
The NH₃conversion of the Fe/β-zeolite SCR catalyst is identified by numeral 70 in FIG. 5 and numeral 70′ in FIG. 6, the NH₃conversion of the La_0.9Sr_0.1CoO₃particles is identified by numeral 72 in FIG. 5 and numeral 72′ in FIG. 6, the NH₃conversion of the clean-up oxidation catalyst having 1.0 wt. % perovskite oxide particles is identified as numeral 74 in FIG. 5 and numeral 74′ in FIG. 6, the NH₃conversion of the clean-up oxidation catalyst having 5.5 wt. % perovskite oxide particles is identified as numeral 76 in FIG. 5 and numeral 76′ in FIG. 6, and the NH₃conversion of the clean-up oxidation catalyst having 12.0 wt. % perovskite oxide particles is identified as numeral 78 in FIG. 5 and numeral 78′ in FIG. 6.
As shown in FIGS. 5 and 6, the degreened clean-up oxidation catalysts having a 5.5 wt. % and 12 wt. % La_0.9Sr_0.1CoO₃particle loading (FIG. 5) oxidized NH₃more effectively than the La_0.9Sr_0.1CoO₃particles alone while the high-temperature aged clean-up oxidation catalysts with the same La_0.9Sr_0.1CoO₃particles loadings (FIG. 6) oxidized NH₃quite comparably to the La_0.9Sr_0.1CoO₃particles alone. The clean-up oxidation catalysts having a 1.0 wt. % La_0.9Sr_0.1CoO₃particle loading—both degreened and high-temperature aged—and the Fe/β-zeolite SCR catalyst alone oxidized NH₃to a lesser extent.
FIGS. 7 and 8 are related to FIGS. 5 and 6, respectively, and show how much of the oxidized NH₃formed NO_X. Temperature (° C.) is plotted on the X-axis and NO_Xselectivity (%)—i.e., the conversion of NH₃to NO_X—is plotted on the Y-axis. The Fe/β-zeolite SCR catalyst and the clean-up oxidation catalysts represented in FIG. 7 were degreened while the Fe/β-zeolite SCR catalyst and the clean-up oxidation catalysts represented in FIG. 8 were high-temperature aged. The La_0.9Sr_0.1CoO₃particles in both of FIGS. 7 and 8 were hydrothermally aged for 5 hours at 700° C.
The NO_Xselectivity of the Fe/β-zeolite SCR catalyst is identified by numeral 80 in FIG. 7 and numeral 80′ in FIG. 8, the NO_Xselectivity of the La_0.9Sr_0.1CoO₃particles is identified by numeral 82 in FIG. 7 and numeral 82′ in FIG. 8, the NO_Xselectivity of the clean-up oxidation catalyst having 1.0 wt. % perovskite oxide particles is identified as numeral 84 in FIG. 7 and numeral 84′ in FIG. 8, the NO_Xconversion of the clean-up oxidation catalyst having 5.5 wt. % perovskite oxide particles is identified as numeral 86 in FIG. 7 and numeral 86′ in FIG. 8, and the NO_Xconversion of the clean-up oxidation catalyst having 12.0 wt. % perovskite oxide particles is identified as numeral 88 in FIG. 7 and numeral 88′ in FIG. 8.
As can be seen, the La_0.9Sr_0.1CoO₃particles alone began to oxidize NH₃to NO_Xquite readily at temperatures above about 300° C. while the Fe/β-zeolite SCR catalyst—both degreened and high-temperature aged—produced essentially no NO_X. The degreened and the high-temperature aged clean-up oxidation catalysts having a 1.0 wt. % La_0.9Sr_0.1CoO₃particle loading also generated very little NO_X, if any. The degreened and the high-temperature aged clean-up oxidation catalysts with 5.5 wt. % and 12 wt. % La_0.9Sr_0.1CoO₃particle loadings, however, showed an uptick in NO_Xselectivity once temperatures eclipsed about 400° C. This increase in NO_Xselectivity was less pronounced for the high-temperature aged 5.5 wt. % and 12 wt. % La_0.9Sr_0.1CoO₃particle loaded clean-up oxidation catalysts than for the corresponding degreened clean-up oxidation catalysts.
FIG. 9 relates to FIGS. 5-8 and compares the NH₃oxidation performance and the N₂selectivity of the degreened and high-temperature aged 12 wt. % La_0.9Sr_0.1CoO₃particle loaded clean-up oxidation catalysts with that of the conventional platinum-containing catalyst. Temperature (° C.) is plotted on the X-axis and both NH₃conversion (%) and N₂selectivity (%) are plotted on the Y-axis. The conventional platinum-containing catalyst included 5 g/ft³of platinum supported on alumina. The 12 wt. % La_0.9Sr_0.1CoO₃particle loaded clean-up oxidation catalysts were exposed to the same simulated exhaust feedstream as recited in the description of FIGS. 5-6. The conventional platinum-containing catalyst was exposed to a simulated exhaust feedstream at a space velocity of about 60,000 h⁻¹that comprised approximately 12% O₂, 4.5% H₂O, 4.5% CO₂, 200 ppm NH₃, and the balance N₂.
The NH₃conversion of the degreened 12 wt. % La_0.9Sr_0.1CoO₃particle loaded clean-up oxidation catalyst is identified by numeral 90, the N₂selectivity of the degreened 12 wt. % La_0.9Sr_0.1CoO₃particle loaded clean-up oxidation catalyst is identified by numeral 92, the NH₃conversion of the high-temperature aged 12 wt. % La_0.9Sr_0.1CoO₃particle loaded clean-up oxidation catalyst is identified by numeral 94, the N₂selectivity of the high-temperature aged 12 wt. % La_0.9Sr_0.1CoO₃particle loaded clean-up oxidation catalyst is identified by numeral 96, the NH₃conversion of the conventional platinum-containing catalyst is identified by numeral 98, and the N₂selectivity of the conventional platinum-containing catalyst is identified by numeral 100.
As shown, both of the degreened and the high-temperature aged 12 wt. % La_0.9Sr_0.1CoO₃particle loaded clean-up oxidation catalysts demonstrated at least comparable and, in many respects superior, N₂selectivity and NH₃oxidation performance to that of the conventional platinum-containing catalyst.
The above description of embodiments is merely exemplary in nature and, thus, variations thereof are not to be regarded as a departure from the spirit and scope of the invention.

Claims

What is claimed is:

1. An exhaust aftertreatment system for removing gaseous emissions and suspended particulate matter, if present, contained in exhaust produced by a lean-burn engine that is combusting a lean mixture of air and fuel, the exhaust aftertreatment system comprising:

an exhaust gas treatment subsystem that receives an exhaust flow from the lean-burn engine and communicates an intermediate exhaust flow, the exhaust gas treatment subsystem comprising a NO_Xabatement component and operating to oxidize carbon monoxide and unburned and/or partially burned hydrocarbons, remove suspended particulate matter if present, and reduce NO_Xso that the intermediate exhaust flow comprises a lesser amount of carbon monoxide, unburned and/or partially burned hydrocarbons, suspended particulate matter if present, and NO_Xthan the exhaust flow; and

a clean-up oxidation catalyst that receives the intermediate exhaust flow from the exhaust gas treatment subsystem and communicates a treated exhaust flow, the treated exhaust flow being communicated to the atmosphere without being exposed to another catalyst material after the clean-up oxidation catalyst, the clean-up oxidation catalyst comprising (1) a selective catalytic reduction catalyst and (2) metal oxide particles dispersed on the selective catalytic reduction catalyst, wherein the metal oxide particles are selected from the group consisting of perovskite oxide particles, manganese-containing mixed metal oxide particles, and mixtures thereof, and wherein the metal oxide particles are present in an amount that ranges from about 0.1 wt. % to about 20 wt. % based on the weight of the clean-up oxidation catalyst.

2. The exhaust aftertreatment system set forth in claim 1, wherein the perovskite oxide particles comprise at least one of LaCoO₃, La_0.9Sr_0.1CoO₃, LaMnO₃, La_0.9Sr_0.1MnO₃, LaFeO₃, or LaSr_0.1Fe_0.9O₃, and wherein the manganese-containing mixed metal oxide particles comprise at least one of Mn_XCe_YO_Z, Mn_XZr_WO_Z, or Mn_XCe_YZr_WO_Zin which X ranges from 0.02 to 0.98, Y ranges from 0.02 to 0.98, W ranges from 0.02 to 0.98, and Z ranges from 1.0 to 3.0.

3. The exhaust aftertreatment system set forth in claim 1, wherein the selective catalytic reduction catalyst comprises silver-supported alumina, an ion-exchanged base-metal zeolite, or a base metal oxide selected from the group consisting of V₂O₅-WO₃/TiO₂, V₂O₅/TiO₂, and mixtures thereof.

4. The exhaust aftertreatment system set forth in claim 3, wherein the ion-exchanged base-metal zeolite comprises at least one of a β-zeolite that is ion-exchanged with at least one of a Cu or Fe, a MFI-type zeolite that is ion-exchanged with at least one of a Cu or Fe, or a Y-type zeolite that is ion-exchanged with at least one of a Na, Ba, Cu, Co, or CuCo.

5. The exhaust aftertreatment system set forth in claim 1, wherein the amount of metal oxide particles dispersed on the selective catalytic reduction catalyst ranges from about 0.5 wt. % to about 15 wt. % based on the weight of the clean-up oxidation catalyst.

6. The exhaust aftertreatment system set forth in claim 1, wherein the amount of metal oxide particles dispersed on the selective catalytic reduction catalyst ranges from about 1.0 wt. % to about 12 wt. % based on the weight of the clean-up oxidation catalyst.

7. The exhaust aftertreatment system set forth in claim 1, wherein the NO_Xabatement component is a lean NO_Xtrap that comprises a LNT catalyst.

8. The exhaust aftertreatment system set forth in claim 7, wherein the exhaust gas treatment subsystem further comprises at least one of a diesel oxidation converter or a catalytic converter located upstream of the lean-NO_Xtrap, wherein the diesel oxidation converter comprises a diesel oxidation catalyst and the catalytic converter comprises a three-way-catalyst.

9. The exhaust aftertreatment system set forth in claim 1, wherein the NO_Xabatement component is an ammonia-SCR catalytic converter that comprises an ammonia-SCR catalyst.

10. The exhaust aftertreatment system set forth in claim 9, wherein the exhaust gas treatment subsystem further comprises at least one of a diesel oxidation converter or a catalytic converter located upstream of the ammonia-SCR catalytic converter, wherein the diesel oxidation converter comprises a diesel oxidation catalyst and the catalytic converter comprises a three-way-catalyst.

11. The exhaust aftertreatment system set forth in claim 9, wherein the exhaust gas treatment subsystem further comprises a urea-metering device that introduces urea into the exhaust flow to form an exhaust mixture that comprises ammonia and the exhaust flow, and wherein the ammonia-SCR catalytic converter receives the exhaust mixture.

12. The exhaust aftertreatment system set forth in claim 1, wherein the clean-up oxidation catalyst is carried on a support body and housed in a canister that is fluidly connected to the exhaust gas treatment subsystem.

13. A method for removing gaseous emissions and suspended particulate matter, if present, contained in exhaust produced by a lean-burn engine that is combusting a lean mixture of air and fuel, the method comprising:

supplying a lean-burn engine with a lean mixture of air and fuel;

combusting the lean mixture of air and fuel in the lean-burn engine to produce an exhaust flow;

delivering the exhaust flow to an exhaust gas treatment subsystem that comprises a NO_Xabatement component that can catalytically reduce NO_Xto N₂;

operating the exhaust gas treatment subsystem to produce an intermediate exhaust flow that comprises a lesser amount of carbon monoxide, unburned and/or partially burned hydrocarbons, suspended particulate matter if present, and NO_Xthan the exhaust flow; and

delivering the intermediate exhaust flow to a clean-up oxidation catalyst to oxidize ammonia, if present, and produce a treated exhaust flow, the clean-up oxidation catalyst comprising (1) a selective catalytic reduction catalyst and (2) metal oxide particles dispersed on the selective catalystic reduction catalyst, wherein the metal oxide particles are selected from the group consisting of perovskite oxide particles, manganese-containing mixed metal oxide particles, and mixtures thereof.

14. The method set forth in claim 13, further comprising communicating the treated exhaust flow to the atmosphere without exposing the treated exhaust flow to another catalyst material after the clean-up oxidation catalyst.

15. The method set forth in claim 13, wherein operating the exhaust gas treatment subsystem comprises:

delivering the exhaust flow to a diesel oxidation converter or a catalytic converter, the diesel oxidation converter comprising a diesel oxidation catalyst and the catalytic converter comprising a three-way-catalyst; and

delivering the exhaust flow to a lean-NO_Xtrap that includes a LNT catalyst that includes an oxidation catalyst, a NO_Xstorage catalyst, and a NO_Xreduction catalyst.

16. The method set forth in claim 13, wherein operating the exhaust gas treatment subsystem comprises:

delivering the exhaust flow to a diesel oxidation converter or a catalytic converter, the diesel oxidation converter comprising a diesel oxidation catalyst and the catalytic converter comprising a three-way-catalyst;

introducing ammonia into the exhaust flow to form an exhaust mixture; and

delivering the exhaust mixture to an ammonia-SCR catalytic converter that comprises an ammonia-SCR catalyst.

17. The method set forth in claim 13, wherein the metal oxide particles comprise at least one of LaCoO₃, La_0.9Sr_0.1CoO₃, LaMnO₃, La_0.9Sr_0.1MnO₃, LaFeO₃, LaSr_0.1Fe_0.9O₃, Mn_XCe_YO_Z, Mn_XZr_WO_Z, or Mn_XCe_YZr_WO_Zin which X ranges from 0.02 to 0.98 Y ranges from 0.02 to 0.98, W ranges from 0.02 to 0.98, and Z ranges from 1.0 to 3.0.

18. The method set forth in claim 13, wherein the selective catalytic reduction catalyst comprises at least one of silver-supported alumina, an ion-exchanged base-metal zeolite, or a base metal oxide selected from the group consisting of V₂O₅-WO₃/TiO₂, V₂O₅/TiO₂, and mixtures thereof.

19. The method set forth in claim 18, wherein the ion-exchanged base-metal zeolite comprises at least one of a β-zeolite that is ion-exchanged with at least one of a Cu or Fe, a MFI-type zeolite that is ion-exchanged with at least one of a Cu or Fe, or a Y-type zeolite that is ion-exchanged with at least one of a Na, Ba, Cu, Co, or CuCo.

20. The method set forth in claim 13, wherein the metal oxide particles are dispersed on the selective catalytic reduction catalyst in an amount that ranges from about 0.1 wt. % to about 20 wt. % based on the weight of the clean-up oxidation catalyst.