Patent Publication Number: US-2012042631-A1

Title: Catalyst materials for ammonia oxidation in lean-burn engine exhaust

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 (N 2 ) and oxygen (O 2 ), a relatively low content of carbon monoxide (CO) and unburned/partially-burned hydrocarbons (HC&#39;s), possibly some suspended particulate matter (i.e., in diesel engines), and small amounts of nitrogen oxides primarily comprised of NO and NO 2  (collectively referred to as NO X ). The NO X  constituency of the exhaust may fluctuate between about 50 and about 1500 ppm and generally comprises far greater amounts NO than NO 2  along with nominal amounts of N 2 O. 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 (CO 2 ), (2) oxidize HC&#39;s into CO 2  and water (H 2 O), (3) convert NO X  gases into nitrogen (N 2 ) and O 2 , 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 (NH 3 ) being introduced into the exhaust flow. Some exhaust aftertreatment system designs, for example, deliberately introduce controlled amounts of NH 3  into the exhaust flow to provide a reductant species that reduces NO X  to N 2  over a selective catalytic reduction (SCR) catalyst. A metering system that directly injects NH 3  or an NH 3  precursor (i.e., urea) into the exhaust flow may be employed to supply NH 3  to the SCR catalyst. Ammonia may also be passively generated from native NO X  in 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 NH 3  produced 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 X  traps (LNT) that occasionally have to be regenerated and desulfated which, in turn, may cause NH 3  to 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 NH 3  and 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/ft 3  of platinum group metals (PGM&#39;s) dispersed on a high-surface area support material such as alumina. Most or all of the clean-up oxidation catalyst&#39;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&#39;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 N 2  selectivity 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 N 2  selectivity can cause the clean-up oxidation catalyst to oxidize NH 3  into NO X —instead of N 2 —at an unacceptably high rate and thus lower the overall NO X  conversion 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 NH 3  to N 2  in 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 0.9 Sr 0.1 CoO 3  particles for various reactions when exposed to simulated lean-burn engine exhaust. 
         FIG. 5  is a graph that shows the NH 3  oxidation performance, when exposed to simulated lean-burn engine exhaust, of a degreened Fe/β-zeolite SCR catalyst, La 0.9 Sr 0.1 CoO 3  particles, and three degreened clean-up oxidation catalysts each with a different La 0.9 Sr 0.1 CoO 3  particle loading. 
         FIG. 6  is a graph that shows the NH 3  oxidation performance, when exposed to simulated lean-burn engine exhaust, of a high-temperature aged Fe/β-zeolite SCR catalyst, La 0.9 Sr 0.1 CoO 3  particles, and three high-temperature aged clean-up oxidation catalysts each with a different La 0.9 Sr 0.1 CoO 3  particle loading. 
         FIG. 7  is a graph that shows how much of the NH 3  oxidation shown in  FIG. 5  resulted in the formation of NO X . 
         FIG. 8  is a graph that shows how much of the NH 3  oxidation shown in  FIG. 6  resulted in the formation of NO X . 
         FIG. 9  is a graph that shows the NH 3  oxidation performance and the N 2  selectivity, 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 0.9 Sr 0.1 CoO 3  particles 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 NH 3  to N 2  in 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 NH 3  oxidation occurs and the oxidation reaction&#39;s N 2  selectivity 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&#39;s in the clean-up oxidation catalyst, although not prohibited, is not needed to achieve satisfactory NH 3  oxidation over a robust temperature range. The opportunity to reduce the amount of PGM&#39;s used in an exhaust aftertreatment system can contribute to significant cost savings and help counteract the durability and N 2  selectivity 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 NH 3  contained 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 N 2  and O 2 , 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&#39;s, and (3) a NO X  contingent primarily comprised of NO and NO 2 . The NO X  contingent of the exhaust flow  20  may fluctuate between about 50 and about 1500 ppm. The proportion of NO and NO 2  particles in the NO X  contingent usually ranges from approximately 80%-95% NO and approximately 5%-20% NO 2 . Such a NO/NO 2  particle distribution corresponds to a molar ratio of NO to NO 2  that 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 O 2  content, which is relatively high, and the CO and HC&#39;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&#39;s, NO X , and particulate matter, if present, from the exhaust flow  20 . A NO X  abatement component, such as a lean NO X  trap or a selective catalytic reduction (SCR) catalytic converter, may be included in the exhaust gas treatment subsystem  14  to reduce NO X  to N 2  in 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 X  abatement 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 N 2 , O 2 , H 2 O, and CO 2 . The intended operation of the exhaust gas treatment subsystem  14  and, in particular, the NO X  abatement component, may further result in NH 3  being present in the intermediate exhaust flow  22  as well as acceptable residual amounts of CO, HC&#39;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 NH 3  in 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 CO 2 ), HC&#39;s (to CO 2  and H 2 O) and NO (to NO 2 ). The oxidation of NO by the diesel oxidation catalyst typically does not decrease the NO X  content in the exhaust flow  20 ; instead, the molar ratio of NO to NO 2  is merely decreased as NO is oxidized to NO 2 . This downward adjustment to the NO:NO 2  molar ratio may be desirable since the downstream ammonia-SCR catalyst  32  may convert NO X  to N 2  at lower temperatures more efficiently as the molar ratio of NO to NO 2  decreases 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 NH 3  to form an exhaust mixture  44 . The NH 3  may 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 NH 3  and 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 NH 3  present 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 NH 3  throughout 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 NH 3  and facilitate the reduction of NO X  in 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 N 2  and H 2 O, in the presence of NH 3  and O 2 . The newly-generated N 2  is 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 V 2 O 5 -WO 3 /TiO 2  and V 2 O 5 /TiO 2 , 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 NH 3  being 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 NH 3  from the ammonia-SCR catalyst or simply allow NH 3  to 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 X  trap (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 CO 2 ) and HC&#39;s (to CO 2  and H 2 O). The TWC catalyst generally does not include a high enough proportional platinum loading to oxidize NO and significantly change the NO to NO 2  molar ratio in the exhaust flow  20 . The TWC catalyst can also oxidize CO and HC&#39;s and simultaneously reduce NO X  to N 2  during 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 NO 2  trapping and NO X  conversion capabilities and generally comprises an oxidation catalyst, a NO X  storage catalyst, and a NO X  reduction catalyst. The oxidation catalyst oxidizes NO to NO 2  and the NO X  storage catalyst traps or “stores” NO 2  as 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&#39;s, if present. The NO X  reduction catalyst, as described below, catalyzes the reduction of NO and NO 2  into N 2  during 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 (BaCO 3 ). 
     The NO X  storage 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&#39;s and H 2 —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 X  and the regeneration of future NO X  storage sites. The liberated NO X  is then reduced to N 2  by excess reductants. The newly-generated N 2  is communicated from the LNT  52  in the intermediate exhaust flow  22 . Some NH 3  may also be introduced into the intermediate exhaust flow  22  since it is possible for the NO X  reduction catalyst to reduce NO X  to NH 3 —instead of N 2 —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 NO 2  molar ratio in the exhaust flow  20 . This downward adjustment to the NO:NO 2  molar ratio may be desirable since the downstream LNT  52  may convert NO X  to N 2  at lower temperatures more efficiently as the molar ratio of NO to NO 2  decreases 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 NH 3  to 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 X  in 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 NH 3  to N 2 . 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 NH 3  contained in the intermediate exhaust flow  22  into N 2  and H 2 O. 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 NH 3  oxidation occurs and the NH 3  oxidation reaction&#39;s N 2  selectivity. The clean-up oxidation catalyst  16  may also, in a related and coupled NH 3  reaction, catalytically reduce the amount of any residual NO X  contained in the intermediate exhaust flow  22  if both NO X  and NH 3  are 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 ABO 3 . The “A” and “B” atoms may be complimentary cations of different sizes that coordinate with oxygen anions. A unit cell of the ABO 3  crystal 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 ABO 3  crystal 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 ABO 3  crystal structure and designated by the general formula A 1-X A1 X B 1-Y B1 Y O 3 , 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 LaCoO 3 , La 0.9 Sr 0.1 CoO 3  , LaMnO 3 , La 0.9 Sr 0.1 MnO 3 , LaFeO 3 , and LaSr 0.1 Fe 0.9 O 3 . 
     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 X Ce Y O Z , manganese-zirconium oxides, Mn X Zr W O Z , or manganese-cerium-zirconium oxides, Mn X Ce Y Zr W O 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 -0.5CeO 2  where V ranges from 2 to 3, 0.3MnO V -0.7CeO 2  where V ranges from 2 to 3, 0.1MnO V -0.9CeO 2  where V ranges from 2 to 3, Mn 0.1 Ce 0.9 O 2 , Mn 0.2 Ce 0.8 O 1.9 , and Mn 0.5 Ce 0.5 O 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 NH 3  to N 2  just 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 NH 3  molecules 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 NH 3  molecules. The ability of the perovskite oxide particles and the manganese-containing mixed metal oxide particles to efficiently oxidize NH 3  may significantly diminish or altogether eliminate the need to include PGM&#39;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&#39;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 NH 3  to N 2  quite efficiently at lower exhaust temperatures (up to about 350° C.) although, at higher temperatures (above about 350° C.), it begins to oxidize some NH 3  to NO X  instead of N 2 . 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 NH 3  to N 2  more consistently with complete or almost complete N 2  selectivity at both lower temperatures (up to about 350° C.) and higher temperatures (above about 350° C.). High-temperature aging can further affect the NH 3  oxidation efficiency and the N 2  selectivity of the clean-up oxidation catalyst  16 . Such aging, in general, improves the N 2  selectivity of the clean-up oxidation catalyst  16  at higher temperatures at the expense of NH 3  oxidation efficiency at lower temperatures. 
     The SCR catalyst may be any material that can help facilitate the oxidation of NH 3  to N 2  when 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/Al 2 O 3 ). 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 0.9 Sr 0.1 CoO 3  particles washcoated onto a Fe/β-zeolite and was either degreened or subjected to high-temperature aging. The weight percent loading of the La 0.9 Sr 0.1 CoO 3  particles 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 0.9 Sr 0.1 CoO 3  particles and the SCR catalyst). While this Example evaluates different loadings of La 0.9 Sr 0.1 CoO 3  particles (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 0.9 Sr 0.1 CoO 3  particles. First, appropriate amounts of La(NO 3 ) 3 .6H 2 O, Co(NO 3 ) 2 .6H 2 O, and Sr(NO 3 ) 2  were dissolved in distilled water with citric acid monohydrate. The amount of water used was 46.2 mL per gram of La(NO 3 ) 3 .6H 2 O, 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/NO 2  gases. 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 ZrO 2  balls to prevent such powder displacement yet still allow for gas mobility. The prepared La 0.9 Sr 0.1 CoO 3  particles were characterized by N 2  physisorption for surface area measurements and X-ray diffraction for their bulk structure measurements. 
     The La 0.9 Sr 0.1 CoO 3  particles were then ball milled with 6.33 mL of water per gram of the La 0.9 Sr 0.1 CoO 3  particles for 18 hours. Afterwards, the slurry was stirred continuously and 0.33 mL HNO 3  (0.1M) per gram of the La 0.9 Sr 0.1 CoO 3  particles and 5 mL of water per gram of the La 0.9 Sr 0.1 CoO 3  particles was added. The resulting washcoat solution had a concentration of 0.114 grams of La 0.9 Sr 0.1 CoO 3  particles 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 0.9 Sr 0.1 CoO 3  particles, 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 0.9 Sr 0.1 CoO 3  particle loading of either 1.0, 5.5, or 12 wt. %. Two core samples were prepared for each wt % loading of La 0.9 Sr 0.1 CoO 3  particles. One clean-up catalyst at each La 0.9 Sr 0.1 CoO 3  particle loading was degreened and the other was high-temperature aged. The degreened clean-up oxidation catalysts were hydrothermally aged in air+10% H 2 O for 5 hours at 550° C. The high-temperature aged clean-up oxidation catalysts were hydrothermally aged in air+10% H 2 O for 48 hours at 700° C. 
     Monolithic core samples were also prepared that included only a Fe/β-zeolite SCR catalyst, only La 0.9 Sr 0.1 CoO 3  particles, 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 0.9 Sr 0.1 CoO 3  particles alone were aged slightly different as indicated below. 
       FIG. 4  shows the catalytic performance of the La 0.9 Sr 0.1 CoO 3  particles 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+O 2 ) is identified by numeral  60 , the conversion of NH 3  (NH 3 +O 2 ) is identified by numeral  62 , and the conversion of NO X  (NO+NH 3 +O 2 ) is identified by numeral  64 . The La 0.9 Sr 0.1 CoO 3  particles were hydrothermally aged in air+10% H 2 O for 5 hours at 700° C. The simulated exhaust feedstream passed over the La 0.9 Sr 0.1 CoO 3  particles to determine NO conversion ( 60 ) had a space velocity of about 30,000 h −1  and comprised approximately 10% O 2 , 400 ppm NO, and the balance N 2 . The simulated exhaust feedstream passed over the La 0.9 Sr 0.1 CoO 3  particles to determine NH 3  conversion ( 62 ) had a space velocity of about 30,000 h −1  and comprised approximately 10% O 2 , 5% H 2 O, 5% CO 2 , 200 ppm NH 3 , and the balance N 2 . The simulated exhaust feedstream passed over the La 0.9 Sr 0.1 CoO 3  particles to determine NO X  conversion ( 64 ) had a space velocity of about 30,000 h −1  and comprised approximately 10% O 2 , 5% H 2 O, 5% CO 2 , 200 ppm NO, 200 ppm NH 3 , and the balance N 2 . 
     As shown in  FIG. 4 , the La 0.9 Sr 0.1 CoO 3  particles oxidized NO and NH 3  rather well at temperatures above about 225° C. and 350° C., respectively. The undesirable oxidation of NH 3  into 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 NH 3  to N 2  was much preferred over the oxidation of NO X  to N 2 . This oxidation selectivity resulted in the overall NO X  conversion 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 0.9 Sr 0.1 CoO 3  particles are also shown for comparison purposes. 
       FIGS. 5 and 6  show the NH 3  oxidation performance, in the absence of NO, of the Fe/β-zeolite SCR catalyst alone, the La 0.9 Sr 0.1 CoO 3  particles 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 NH 3  conversion (%) 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 0.9 Sr 0.1 CoO 3  particles 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 −1  and comprised approximately 10% O 2 , 5% H 2 O, 5% CO 2 , 200 ppm NH 3 , and the balance N 2 . 
     The NH 3  conversion of the Fe/β-zeolite SCR catalyst is identified by numeral  70  in  FIG. 5  and numeral  70 ′ in  FIG. 6 , the NH 3  conversion of the La 0.9 Sr 0.1 CoO 3  particles is identified by numeral  72  in  FIG. 5  and numeral  72 ′ in  FIG. 6 , the NH 3  conversion 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 NH 3  conversion 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 NH 3  conversion 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 0.9 Sr 0.1 CoO 3  particle loading ( FIG. 5 ) oxidized NH 3  more effectively than the La 0.9 Sr 0.1 CoO 3  particles alone while the high-temperature aged clean-up oxidation catalysts with the same La 0.9 Sr 0.1 CoO 3  particles loadings ( FIG. 6 ) oxidized NH 3  quite comparably to the La 0.9 Sr 0.1 CoO 3  particles alone. The clean-up oxidation catalysts having a 1.0 wt. % La 0.9 Sr 0.1 CoO 3  particle loading—both degreened and high-temperature aged—and the Fe/β-zeolite SCR catalyst alone oxidized NH 3  to a lesser extent. 
       FIGS. 7 and 8  are related to  FIGS. 5 and 6 , respectively, and show how much of the oxidized NH 3  formed NO X . Temperature (° C.) is plotted on the X-axis and NO X  selectivity (%)—i.e., the conversion of NH 3  to 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 0.9 Sr 0.1 CoO 3  particles in both of  FIGS. 7 and 8  were hydrothermally aged for 5 hours at 700° C. 
     The NO X  selectivity of the Fe/β-zeolite SCR catalyst is identified by numeral  80  in  FIG. 7  and numeral  80 ′ in  FIG. 8 , the NO X  selectivity of the La 0.9 Sr 0.1 CoO 3  particles is identified by numeral  82  in  FIG. 7  and numeral  82 ′ in  FIG. 8 , the NO X  selectivity 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 X  conversion 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 X  conversion 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 0.9 Sr 0.1 CoO 3  particles alone began to oxidize NH 3  to NO X  quite 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 0.9 Sr 0.1 CoO 3  particle 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 0.9 Sr 0.1 CoO 3  particle loadings, however, showed an uptick in NO X  selectivity once temperatures eclipsed about 400° C. This increase in NO X  selectivity was less pronounced for the high-temperature aged 5.5 wt. % and 12 wt. % La 0.9 Sr 0.1 CoO 3  particle loaded clean-up oxidation catalysts than for the corresponding degreened clean-up oxidation catalysts. 
       FIG. 9  relates to  FIGS. 5-8  and compares the NH 3  oxidation performance and the N 2  selectivity of the degreened and high-temperature aged 12 wt. % La 0.9 Sr 0.1 CoO 3  particle loaded clean-up oxidation catalysts with that of the conventional platinum-containing catalyst. Temperature (° C.) is plotted on the X-axis and both NH 3  conversion (%) and N 2  selectivity (%) are plotted on the Y-axis. The conventional platinum-containing catalyst included 5 g/ft 3  of platinum supported on alumina. The 12 wt. % La 0.9 Sr 0.1 CoO 3  particle 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 −1  that comprised approximately 12% O 2 , 4.5% H 2 O, 4.5% CO 2 , 200 ppm NH 3 , and the balance N 2 . 
     The NH 3  conversion of the degreened 12 wt. % La 0.9 Sr 0.1 CoO 3  particle loaded clean-up oxidation catalyst is identified by numeral  90 , the N 2  selectivity of the degreened 12 wt. % La 0.9 Sr 0.1 CoO 3  particle loaded clean-up oxidation catalyst is identified by numeral  92 , the NH 3  conversion of the high-temperature aged 12 wt. % La 0.9 Sr 0.1 CoO 3  particle loaded clean-up oxidation catalyst is identified by numeral  94 , the N 2  selectivity of the high-temperature aged 12 wt. % La 0.9 Sr 0.1 CoO 3  particle loaded clean-up oxidation catalyst is identified by numeral  96 , the NH 3  conversion of the conventional platinum-containing catalyst is identified by numeral  98 , and the N 2  selectivity 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 0.9 Sr 0.1 CoO 3  particle loaded clean-up oxidation catalysts demonstrated at least comparable and, in many respects superior, N 2  selectivity and NH 3  oxidation 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.