Abstract:
Particles of mixed oxides of cerium, zirconium, and copper (CeZrCuO x ) may be prepared as catalysts and used to preferentially catalyze the oxidation of CO in exhaust streams containing CO and NH 3 . In one practice, this CeZrCuO x  catalyst may be used in combination with a close-coupled PGM catalyst which promotes the formation of NH 3  in the exhaust during fuel-rich operation, and at least one under-floor NH 3 -SCR catalyst, which catalyzes the reduction of NO x  in the exhaust stream during fuel-lean operation using NH 3  as a reductant. During fuel-rich engine operation, the exhaust stream may be doped with oxygen downstream of the PGM catalyst and passed in contact with particles of the CeZrCuO x  catalyst so that residual CO in the exhaust may be oxidized to CO 2 , without oxidation or other conversion of the NH 3 .

Description:
TECHNICAL FIELD 
       [0001]    This disclosure is related to catalyst materials for the selective oxidation of carbon monoxide in a flowing gas stream comprising carbon monoxide, ammonia, and oxygen. For example, such catalysts may be useful in the treatment of exhaust gas from a lean burn, gasoline-fueled engine that is operated to passively provide ammonia for the reduction of nitrogen oxides in the exhaust. 
       BACKGROUND 
       [0002]    Gasoline engines on automotive vehicles have been controlled to operate close to a stoichiometric air-to-fuel ratio (AFR) so that carbon monoxide (CO), unburned or partially burned hydrocarbons (HC), and nitrogen oxides (NO x ) in the exhaust stream can simultaneously be converted to carbon dioxide (CO 2 ), nitrogen (N 2 ), and water when passed in contact with a suitable platinum group metal (PGM) catalyst. In this mode of engine operation, the catalyst is characterized as a three-way-catalyst (TWC). With an ever increasing need for higher fuel economy there is current interest in operating gasoline engines, such as spark-ignition direct-injection (SIDI) engines, at AFR values that are relatively lean of the stoichiometric ratio (i.e., fuel-lean). The direct injection of gasoline into each engine cylinder allows a combustible air-fuel mixture to be formed near the spark plug for initiating combustion with leaner mixtures elsewhere in the combustion chamber. In some limited periods of engine operation, the AFR may be slightly rich of the stoichiometric ratio, but for most periods of engine operation the engine is operated fuel-lean to maximize fuel efficiency. 
         [0003]    In comparison to stoichiometric AFR engine operation, fuel-lean engine operation reduces the amount of CO and HC in the exhaust, but increases the amount of NO x , which must be removed. The exhaust from such lean burn engines is still passed through a PGM catalyst-containing flow-through reactor to oxidize much of the CO and HC to CO 2  and water. But then a reductant material is added to the exhaust stream that is reactive with NO x  to convert it to N 2  and water. The reductant material-containing exhaust is further passed through a reduction catalyst-containing flow-through reactor to promote the reduction of NO x  to N 2  and water. Since the added reductant material and the reduction catalyst must work together in treatment of the exhaust, this practice is called a selective catalytic reduction (SCR) with respect to the removal of NO x . 
         [0004]    In a known SCR practice (especially for diesel engines), it is common to store an aqueous solution of urea on the vehicle and to inject a controlled amount of urea solution, as needed, into the exhaust stream. The urea quickly decomposes into ammonia (and carbon dioxide), and the ammonia serves as the reductant material for the NO x  reduction reaction. The flowing stream is then passed over a suitable reduction catalyst, such as a particulate copper-exchanged zeolite and/or an iron exchanged zeolite material. This method of NO x  reduction is commonly referred to as an NH 3 -selective catalytic reduction (SCR) of NO x . 
         [0005]    The on-vehicle storage of a reductant material, such as urea, and its managed injection into the flowing exhaust from the engine permits flexibility in the management of lean-burn engine operation. The NO x  content of the exhaust may be measured with a NO x  sensor, and the addition of the reductant material may be computer-controlled in response to the output of the NO x  sensor. But the vehicle operator must continually replenish the supply of urea solution, and must also keep the solution from freezing. Ammonia is a suitable reductant for NO x  in a vehicle exhaust stream, but there would be a benefit if the reductant or its precursor did not have to be separately stored on the vehicle. 
         [0006]    U.S. Patent Application Publication No. 2010/0107605 (the “&#39;605 application”), titled “Passive Ammonia-Selective Catalytic Reduction for NO x  Control in Internal Combustion Engines,” is assigned to the assignee of this invention and discloses a method of passively generating NH 3  in an exhaust stream of a multi-cylinder, spark ignition, direct fuel injection, four stroke, gasoline engine that is primarily operated in a fuel-lean mode. But there are periods during which the “lean-burn” engine operates close to a stoichiometric AFR, or slightly fuel-rich of the stoichiometric ratio. Such periods may include, for example, engine-idle modes and vehicle acceleration modes of operation. The inventors in the &#39;605 application recognized that oxygen-depleted engine exhaust contained NO x , CO, and hydrogen in sufficient quantities and proportions to form ammonia as the exhaust flowed through the PGM flow-through reactor close-coupled to the exhaust manifold of the engine. The inventors further recognized and disclosed practices for utilization of this passively-generated ammonia to reduce NO x  in a NH 3 -SCR catalyst-containing flow-through reactor located downstream of the PGM reactor in the flow of the exhaust. The inventors recognized that, rather than adding urea solution to the exhaust, the passively-generated ammonia may eliminate the need for urea storage and injection. 
         [0007]    The downstream NH 3 -SCR catalyst serves to temporarily store the passively-generated NH 3  during fuel-rich operation. During subsequent periods of fuel-lean engine operation, the NH 3 -SCR catalyst effectively converts NO x  in the exhaust stream to N 2  and water using the stored NH 3 . But, this passive NH 3  generation method requires the engine&#39;s modes of operation to be efficiently managed to provide enough NH 3  to the NH 3 -SCR catalyst during fuel-rich operation so that a suitable supply of NH 3  is available on the NH 3 -SCR catalyst during fuel-lean operation to remove NO x  from the exhaust. 
         [0008]    It is recognized that NH 3  generation on a PGM catalyst is enhanced when the AFR of the engine&#39;s combustible mixture is about 14 to 14.2 during fuel-rich operation. However, when the engine is operating in this fuel-rich range, the PGM catalyst may not then serve to effectively oxidize the CO in the exhaust due to the temporarily diminished supply of oxygen. And the CO is not likely to be oxidized in the NH 3  selective reduction reactor either. 
         [0009]    One method of removing CO from the exhaust stream resulting from these fuel-rich periods includes passing the exhaust stream in contact with a second oxidation catalyst and with an auxiliary air injection, downstream of the PGM catalyst to promote the oxidation of residual CO in the exhaust to CO 2  and water. However, this oxidation catalyst must be able to oxidize the residual CO without also oxidizing the NH 3  which is to be used by the downstream reduction catalyst-containing flow-through reactor. 
       SUMMARY OF THE DISCLOSURE 
       [0010]    This disclosure describes an oxidation catalyst that can be used to preferentially catalyze the oxidation of CO in a gas stream comprising at least carbon monoxide, ammonia, and oxygen. As will be described further, the oxidation catalyst is usually used in the form of small particles of mixed oxides of cerium, zirconium, and copper (CeZrCuO x ) coated on wall surfaces of a flow-through reactor. When the temperature of the gas stream is in a suitable range, the catalyst effectively oxidizes carbon monoxide to carbon dioxide with minimal effect on the ammonia content of the gas stream. While the disclosed catalyst is useful generally for selective treatment of a stream of CO and ammonia, it is particularly useful for selective oxidation of CO when ammonia is generated in a CO-containing fuel-rich engine exhaust then flowing over a PGM catalyst. In this important embodiment of the invention, carbon monoxide is converted to carbon dioxide, but the ammonia content of the exhaust stream is preserved for use in an NH 3 -SCR reactor for NO x . Accordingly, this oxidation catalyst may be referred to herein as a CO-selective oxidation catalyst. 
         [0011]    In one embodiment, this CO-selective oxidation catalyst may be used in an exhaust treatment system for a lean-burn engine along with: (1) a PGM catalyst, preferably close-coupled to the exhaust manifold, and (2) at least one zeolite-based NH 3 -SCR catalyst, which is positioned in the exhaust flow downstream of the PGM catalyst. In this system, the CO-selective oxidation catalyst is located downstream of the PGM catalyst and upstream of at least one NH 3 -SCR catalyst reactors. In this arrangement, the CO-selective oxidation catalyst minimizes oxidation of the NH 3  generated by the PGM catalyst in the exhaust stream during fuel-rich engine operation. Thus, the NH 3  will be available to one or more NH 3 -SCR catalyst reactors to participate in NO x  reduction reactions during fuel-lean operation. 
         [0012]    In another embodiment, the exhaust treatment system may include an O 2  dosing or injection device, located upstream of the CO-selective oxidation catalyst to provide a suitable amount of oxygen in the exhaust stream for selective oxidation of CO over the mixed oxide catalyst. The O 2  dosing device should be placed downstream of the PGM catalyst so as not to interfere with the PGM catalyst&#39;s NH 3 -generation performance. 
         [0013]    In accordance with disclosed embodiments, the particulate mixed oxide CeZrCuO x  catalyst may be used to effectively and selectively catalyze the oxidation of CO in NH 3  and O 2 -containing exhaust streams which are at a temperature in the range of about 200-400° C.. In this temperature range the mixed oxide particles have minimal tendency to oxidize ammonia. 
         [0014]    The CeZrCuO x  mixed oxide particles may be prepared for use as the CO-selective oxidation catalyst using a co-precipitation method. In some preferred embodiments, the molar ratios of Ce, Zr, and Cu in the CO-selective oxidation catalyst may be controlled to fall within the following ranges: Ce/(Ce—Zr—Cu)=0.50-0.70, Zr/(Ce—Zr—Cu)=0.10-0.20, and Cu/(Ce—Zr—Cu)=0.15-0.35, with the sum of the molar ratios of Ce, Zr, and Cu equal to 1. The value of x depends on the proportions and oxidation state of the metal elements but typically is in the range of about one to 2 +  or three. These compositions may be formed by dissolving suitable precursor compounds of cerium, zirconium, and copper in desired molar proportions in a common solvent. Preferably the precursor compounds contain oxygen. For example, nitrates of cerium, zirconium, and copper may be dissolved in water. 
         [0015]    In the example of an aqueous solution of the respective nitrates, a precipitating agent, such as ammonia or sodium hydroxide, may be added to increase the pH of the solution and trigger co-precipitation of Ce, Zr, and Cu mixed hydroxides. The dried precipitate may be calcined in air at a temperature of about 400° C.. to 500° C.. for 4 to 6 hours to dehydrate the Ce, Zr, and Cu hydroxides and form mixed oxides of the respective base metals. The term “mixed oxide” is used in this specification to mean oxides that may contain cations of more than one base metal element or that the cations may be present in more than one oxidation state. 
         [0016]    In many embodiments of the invention the mixed oxide catalyst will be used in combination with other catalytic reactors to dynamically treat the exhaust constituents from a vehicle engine that operates primarily in a fuel-lean mode, but with intervening periods of fuel-rich mode operation. Indeed, the mixed oxide catalyst will be particularly useful when engine operation is managed to provide a passive supply of ammonia for the catalytic reduction of NO x  produced in lean-burn engine operation. In general, the fuel-lean or fuel-rich engine exhaust is passed first through a PGM catalyst-containing reactor that is close-coupled to the engine exhaust manifold (for higher exhaust gas temperatures), and then through one or more downstream NH 3 -SCR reactors. Thus, the mixed oxide catalyst of this invention will typically be used immediately after the exhaust gas has passed through the PGM reactor. 
         [0017]    The mixture of carbon monoxide and ammonia will be formed, as described above in this specification, when exhaust constituents from a vehicle&#39;s engine fuel-rich operation is passed in contact with a PGM catalyst. The PGM catalyst may include particles of one or more platinum group metals (PGMs), such as platinum (Pt), palladium (Pd), and rhodium (Rh), supported on particles of alumina (Al 2 O 3 ) and/or ceria (CeO 2 ). However, it may be preferred to use PGM materials that do not store oxygen during fuel-lean operation for oxidation during fuel-rich operation so as to minimize oxidation of ammonia. For example, the PGM catalyst may comprise Al 2 O 3 -supported particles of Pt and Rh, wherein the molar ratio of Pd to Rh is 6:1. The downstream NH 3 -SCR catalyst may comprise particles of a copper (Cu) or iron (Fe) ion-exchanged suitable zeolite or silicoaluminophosphate composition. 
         [0018]    Particles of the PGM catalyst, the CeZrCuO x  mixed oxide catalyst, and the NH 3 -SCR catalyst may be dispersed on one or more, high surface area, support bodies in the form of a thin washcoat layer or layers. These catalyst-bearing support bodies may be housed within flow-through reactors and positioned along the flow path of the exhaust stream from the engine. The specific location of each of these reactors in the exhaust treatment system may be dependent upon the operating temperature range of each of the catalyst materials. In general, particles of the PGM catalyst may be housed within a flow-through reactor that is close-coupled to the engine, and particles of the NH 3 -SCR catalyst may be housed downstream in the exhaust flow path in one or more flow-through reactors that are located under the vehicle floor. The CO-selective oxidation catalyst may be positioned in the exhaust system at locations downstream of the PGM reactor and suitable for oxidizing CO and preserving ammonia for an NH 3 -SCR reactor. 
         [0019]    It is generally found that a vehicle engine can be managed to operate fuel rich during engine idling, vehicle acceleration, and other modes of operation even when the engine is managed to operate fuel-lean during most of its operating modes. So long as the NH 3 -SCR catalyst particles have an adequate supply of NH 3 , NO x  may be continuously removed from the exhaust stream, even during prolonged periods of fuel-lean engine operation. To determine when such a fuel-rich period or cycle should be initiated, the exhaust system may include at least one NO x  sensor located downstream of the one or more NH 3 -SCR reactors. If a NO x  sensor indicates that NO x  is escaping from the one or more NH 3 -SCR reactors, a fuel-rich engine operation cycle may be triggered to replenish NH 3  storages sites on the NH 3 -SCR catalyst particles. In addition, each NH 3 -SCR reactor may be equipped with a NH 3  sensor to signal when the NH 3  storages sites on the NH 3 -SCR catalyst are saturated with NH 3  so that fuel-lean operation can quickly resume. 
         [0020]    Furthermore, the passive NH 3 -SCR system may replenish NH 3  storages sites on the NH 3 -SCR catalyst by taking advantage of the fuel-rich periods that inevitably and repeatedly occur during normal, every-day driving conditions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a schematic illustration of an automotive vehicle&#39;s internal combustion engine, fuel supply system, air intake system, and exhaust system. In this illustration, the exhaust system includes three serially-arranged catalyzed flow-through reactors which are formulated to cooperatively and successfully treat the exhaust stream from the engine before discharge to the atmosphere. 
           [0022]      FIG. 2  is a cutaway view of one of the schematically illustrated catalyzed flow-through reactors depicted in  FIG. 1 . This catalyzed flow-through reactor includes a suitable high temperature capability and oxidation resistant metal container, which houses a catalyst-bearing support body that is held in place by a mounting material. In this illustration, the support body is an extruded ceramic honeycomb-shaped monolith and includes several small, square, parallel flow-through channels which are defined by walls that extend longitudinally from an inlet to an outlet face of the support body. A portion of the container, the mounting material, and the support body have been cut-way in this illustration to better reveal the internal structure of the support body. 
           [0023]      FIG. 2A  is an enlarged view of a portion of the inlet face of the extruded ceramic honeycomb-shaped support body depicted in  FIG. 2 , which better reveals the internal flow-through channels and walls of the support body. 
           [0024]      FIG. 3  is a graph of Conversion (%) vs. Temperature (° C..) showing the CO (solid line curve) and NH 3  (dashed line curve) conversion performance of a conventional Pd/Rh three-way catalyst as a function of temperature in a gas stream comprising 5000 ppmv CO, 1500 ppmv NH 3 , 5 vol. % CO 2 , 0.5 vol. % O 2 , 5 vol. % H 2 O, with N 2  as balance; SV=30,000 h -1 . The percent CO and NH 3  conversion values were calculated by comparing the concentration of CO and NH 3  in the gas stream at both the inlet ([CO] inlet  or [NH 3 ] inlet ) and the outlet ([CO] outlet  or [NH 3 ] outlet ) of a quartz tubular reactor. 
           [0025]      FIG. 4  is a graph of Conversion (%) vs. Temperature (° C..) showing the CO (solid line curve) and NH 3  (dashed line curve) conversion performance of a Ce 0.6 Zr 0.15 Cu 0.25 O 2  catalyst as a function of temperature in a gas stream comprising 5000 ppmv CO, 1500 ppmv NH 3 , 5 vol. % CO 2 , 0.5 vol. % O 2 , 5 vol. % H 2 O, with N 2  as balance; SV=30,000 h -1 . The percent CO and NH 3  conversion values were calculated by comparing the concentration of CO and NH 3  in the gas stream at both the inlet ([CO] inlet  or [NH 3 ] inlet ) and the outlet ([CO] outlet  or [NH 3 ] outlet ) of a quartz tubular reactor. The region between darkened vertical lines at 200° C. and 400° C., also indicated by the horizontal line with arrow heads, represents the temperature range at which the CeZrCuO x  catalyst can be effectively used to selectively oxidize CO, instead of NH 3  in exhaust streams. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]      FIG. 1  is a schematic illustration of the engine and exhaust passage system of an automotive vehicle  10  that includes an internal combustion engine  12 , a fuel supply system  14 , an air intake system  16 , and an exhaust system  18 . In modern engines, the operation and coordination of each of these systems is electronic computer-controlled as part of an overall engine management system. The multi-cylinder, reciprocating piston, internal combustion engine  12  may be a compression ignition engine (i.e., diesel engine), a spark ignition engine (i.e., gasoline engine), or a combination. During its operation, engine  12  inducts or draws in air flow  20  by the air intake system  16 , which includes an air filter  22 , a throttle-body valve  24 , and an intake manifold  26  coupled to the engine  12 . The engine  12  is supplied with fuel by the fuel supply system  14 , which includes a fuel tank  28 , a fuel pump  30 , and a fuel filter  32  that deliver fuel through a fuel line to the intake manifold region  34  of the engine overlying the cylinders  36  of the engine. 
         [0027]    In  FIG. 1 , the intake manifold region  34  is the area of the engine in which a plurality of fuel injectors or nozzles (not illustrated) may be located. The fuel injectors or nozzles of the fuel injection system are configured and controlled to open for an amount of time to dispense or spray a desired amount of pressurized fuel near intake valve openings (not illustrated) or directly into combustion chambers of the engine&#39;s cylinders  36 . Alternatively, the fuel injection system may include a single fuel injector that is located at a suitable position within the intake manifold region  34 . This single injector may be controlled to dispense a predetermined amount of fuel into the incoming air flow  20  so that the desired amount of fuel is sequentially supplied to each of the cylinders  36  of the engine  12  as their respective intake valves open to receive the carefully controlled charge of air and fuel. 
         [0028]    The management of airflow and fuel injection and amount and timing to the engine in response to driver throttle or brake actuation is provided by at least one electronic control module (ECM)  38 . The ECM  38  monitors certain engine operating parameters by receiving input data through a plurality of signal leads which are attached to various sensors on the engine and the engine&#39;s related systems (not shown). The ECM  38  processes the input data and generates output data which is sent via another set of signal leads to actuators on various engine and vehicle components. In  FIG. 1 , no leads are shown from any engine systems to the ECM  38  and no leads are shown to any actuators on the engine or on vehicle components. However, a plurality of leads would normally be present as part of the vehicle&#39;s electronic management system. 
         [0029]    In particular, the ECM is responsible for sending signals to actuators that operate the fuel injectors and/or the throttle-body valve  24  of the vehicle  10  so that the combustible mixture supplied to the engine exhibits the instantly-desired air-to-fuel mass ratio (AFR). In this way, the ECM is able to continuously manage the AFR of the mixture of air and fuel that is supplied to the engine  12  in accordance with an engine control strategy. The quantity of fuel injected into the cylinders or into the incoming air flow may be controlled to maintain a predetermined lean AFR or a predetermined rich AFR, or to switch between the two modes of engine operation. Or, the amount of injected fuel may be controlled so that the AFR of the combustible mixture fluctuates in a narrow range, such as above and below the stoichiometric AFR. 
         [0030]    The combustible mixture of air and fuel is supplied to cylinders  36  (usually four to eight) of the engine  12  and the various combustion products are expelled from the cylinders  36  of the engine  12  through an exhaust manifold  40  as an effluent exhaust stream  42 . The exhaust system  18  of the vehicle  10  comprises an enclosed and contained exhaust duct and exhaust treatment components that cooperate to receive the exhaust stream  42  from the engine  12  and to discharge a treated exhaust stream  44  from the tailpipe  46  to the ambient atmosphere. Treatment of the combustion products is accomplished by passing the exhaust stream  42  through various flow-through devices. For example, a typical exhaust system  18  of an automotive vehicle  10  includes a muffler  48  and a resonator  50  for reducing the amount of noise emitted by the exhaust system  18 . The exhaust system  18  of most engines also includes at least one catalyzed flow-through reactor to promote (1) the oxidation of CO to CO 2 , (2) the oxidation of HC to CO 2  and water, and (3) the reduction of NO x  to N 2  and water in the effluent exhaust stream  42  from the engine  12 . 
         [0031]    The exhaust system  18  shown in  FIG. 1  illustrates three serially-arranged catalyzed flow-through reactors  52 ,  54 ,  56  located within the path of the exhaust stream  42  from the engine  12 . However, a variety of locations and arrangements are possible for the at least one catalyzed flow-through reactor of the exhaust system  18 . For example, each of the catalyst materials of the passive NH 3 -SCR exhaust treatment system may be located in separate flow-through reactors, or more than one of the catalyst materials may be housed within a single flow-through reactor. In addition, the flow-through reactors need not be arranged in a progressive series flow path. For example, the flow-through reactors may be arranged in parallel and a portion of the exhaust stream from the engine may be controlled to pass through one flow-through reactor, which the remaining portion of the exhaust passes through another flow-through reactor. Alternatively, the exhaust stream from the engine may be controlled to pass though some, but not all, of the flow-through reactors at a given time. 
         [0032]    In the embodiment illustrated in  FIG. 1 , the PGM catalyst powder may be washcoated within a first flow-through monolith reactor  52 , which is close-coupled to the exhaust manifold  40  of engine  12 . The CO-selective oxidation catalyst powder may be washcoated within a second flow-through monolith reactor  54 , which is positioned downstream of the first flow-through reactor  52 , relative to a flow direction of the exhaust stream  42  from the engine  12 . And the NH 3 -SCR catalyst powder may be washcoated within a third flow-through monolith reactor  56 , which is positioned downstream of both the first and second reactors  52 ,  54  in an under-floor position. In this embodiment, an oxygen dosing or injection device  58  may be located upstream of the second flow-through reactor  54 , but downstream of the first flow through (PGM) reactor  52 . The oxygen dosing device  58  is preferably configured to inject an amount of oxygen into the flowing exhaust stream  42  before the exhaust passes over particles of the CO-selective oxidation catalyst. 
         [0033]    In another embodiment, particles of a first NH 3 -SCR catalyst may be washcoated within a second flow-through monolith reactor  54 , which is positioned downstream of the first flow-through monolith reactor  52  in an under-floor position. And particles of the CO-selective oxidation catalyst and additional NH 3 -SCR catalyst may be washcoated within a third flow-through monolith reactor  56 , which is positioned downstream of both the first and second monolithic reactors  52 ,  54  in an under-floor position. In this embodiment, the oxygen dosing device  58  may be located upstream of the third flow-through monolith reactor  56 , but downstream of the first and second flow-through monolith reactors  52 ,  54 . 
         [0034]    In yet another embodiment, particles of a first NH 3 -SCR catalyst may be washcoated within a second flow-through monolith reactor  54 , which is positioned downstream of the first flow-through monolithic reactor  52  in an under-floor position. And particles of the CO-selective oxidation catalyst and additional NH 3 -SCR catalyst may be washcoated within two flow-through reactors that are arranged in parallel and positioned downstream of both the first and second monolith reactors  52 , 54  in an under-floor position (not shown). 
         [0035]    The operating temperature range of each of these catalysts during typical vehicle driving conditions can be controlled by adjusting the distance each catalyst-washcoated converter is located from the engine in the exhaust system. In general, the closer each catalyst-bearing support body is to the hot exhaust outlet of engine, the higher its operating temperature will be. In one embodiment, the PGM catalyst may be held within a reactor that is close-coupled to the engine so that, during normal driving conditions, the average temperature of the PGM catalyst is in the range of about 350-550° C. On the other hand, the CO-selective oxidation catalyst and the NH 3 -SCR catalyst may be held within at least one reactor that is positioned downstream of the PGM catalyst and under the vehicle floor, so that, during normal driving conditions, the average temperature of these catalysts is in the range of about 200-400° C. 
         [0036]    In addition, particles of any one of these catalyst materials may be housed within multiple serially-arranged reactors (with some reactors being closer to the engine that others) so that some of the catalyst particles experience a relatively high-temperature operating range and the other catalyst particles experience a relatively low-temperature operating range. In this way, the architecture of the exhaust treatment system can be configured to cover a wide range of driving conditions. 
         [0037]    By way of illustration, a suitable catalyzed flow-through reactor  60  for the passive NH 3 -SCR exhaust treatment system is shown in  FIG. 2 . The catalyzed flow-through reactor  60  comprises an alloy steel container  62  shaped with an upstream opening  64  and a downstream opening  66 . The upstream opening  64  is configured to receive the exhaust stream  42  and the downstream opening  66  is configured to discharge the exhaust stream  42 . The body of the container  62  is often round or elliptical in cross-section and is sized to hold a catalyst-bearing support body  68 . The support body has an inlet face  70  and an outlet face  72 , which are transverse to the flow direction of the exhaust stream  42 . The support body  68  is held in place within the container  62  by a thermally insulating and physically durable mounting material  74 . 
         [0038]    The support body  68  shown in  FIG. 2  is an extruded ceramic, honeycomb-shaped monolith. However, other thermally stable materials, such as stainless steel, may be used to form other suitable high surface area support bodies. As shown in  FIG. 2 , the extruded ceramic support body  68  includes several small, square, parallel flow-through channels  76  that are defined by walls  78  which extend longitudinally from the inlet face  70  to the outlet face  72  of the support body  68 . The inlet face  70  of the support body  68  is sized to provide a suitable number of channels  76  (preferably, at least  400  per square inch) to collectively accommodate a desired flow rate for the exhaust stream  42 , and, thus, a desired residence time of the exhaust gases within the support body  68 . In  FIG. 2 , a portion of the container  62 , mounting material  74  and support body  68  have been cut-away to better reveal the many internal channel openings  76  and the channel walls  78  extending from the inlet to the outlet of the support body. 
         [0039]    In practices of this invention, fine catalyst particles are deposited onto the walls  78  of the small flow-through channels  76  in the form of a thin washcoat layer or layers. The high total surface area of the many channel walls  78  provides sufficient contact surface area between the exhaust flow  42  and the catalyst particles for the desired oxidation, reduction and storage reactions to occur. 
         [0040]    The exhaust flow may be exposed to other devices or mechanical equipment not expressly shown in  FIG. 1  that may or may not help treat the exhaust gas constituents. These devices include, for example, a diesel particulate filter, a three-way-catalyst, a lean NO x  trap, an exhaust gas recirculation line, and/or a turbocharger turbine. Skilled artisans will undoubtedly know of, and understand, these and the many other devices that the exhaust flow could be exposed to. 
         [0041]    The above discussion with respect to  FIGS. 1 and 2  describes locations for the subject selective carbon monoxide oxidation catalyst in an automotive vehicle in which the engine is being operated fuel-lean overall but with periodic fuel-rich cycles to generate sufficient ammonia for NO x  reduction using an ammonia selective catalytic reduction reactor. The following disclosure pertains to the preparation of the mixed oxide catalyst and to its use in oxidation of CO in a gas stream also containing ammonia and oxygen. 
       EXAMPLE 
       [0042]    In this example, particles of CeZrCuO x  mixed oxides were prepared via a co-precipitation method. The CO and NH 3  oxidation activity of the as-prepared CeZrCuO x  particles was then compared to that of a conventional Pd/Rh TWC catalyst. The Pd/Rh catalyst was obtained from BASF and had a Pd to Rh molar ratio of 6:1. 
         [0043]    A precursor solution was prepared by dissolving metal nitrates of (NH 4 ) 2 Ce(NO 3 ) 6 , Zr(NO 3 ) 4 , and Cu(NO 3 ) 2  with molar ratios of 0.6:0.15:0.25 in deionized water at room temperature. Once the metal nitrates were dissolved in solution, the precursor solution was held with stirring for about 0.5 hours. 
         [0044]    Next, a one molar sodium hydroxide solution was added to the precursor solution with vigorous stirring until its pH value reached 10 and a suspended co-precipitate of the base metals formed. The suspension was left at room temperature for 18 hours with mild stirring. Thereafter, the suspension was heated to 80° C. for 2 hours, and the co-precipitate was filtered from the liquid phase. The filtered precipitate was washed with hot deionized water, dried overnight at 110° C., and then calcined at 400° C. for 4 hours. 
         [0045]    The CO and NH 3  oxidation activity of the as-prepared CeZrCuO x  mixed oxide particles and the Pd/Rh particles was measured by placing 0.1056 cc of the respective catalyst powder in separate packed bed quartz tubular reactors, each having an outer diameter of ⅜ inch. A gas stream comprising 5000 ppmv CO, 1500 ppmv NH 3 , 5 vol. % CO 2 , 0.5 vol. % O 2 , 5 vol. % H 2 O, with N 2  as balance was fed in separate tests to each of the oxidation catalyst-filled tubular reactors through heated stainless steel lines at an hourly gas space velocity of SV=30,000 h -1  in each test. 
         [0046]    The CO and NH 3  conversion performance of the CeZrCuO x  catalyst and the Pd/Rh catalyst were separately measured, each over a temperature range of 100° to 550° C. The percent CO and NH 3  conversion values were calculated by comparing the concentration of CO and NH 3  in the gas stream at both the inlet ([CO] inlet  or [NH 3 ] inlet ) and the outlet ([CO] outlet  or [NH 3 ] outlet ) of each tubular reactor. Thus, the conversion (%) of CO or NH 3  is equal to [CO] inlet /[CO] outlet  or [NH 3 ] inlet /[NH 3 ] outlet . 
         [0047]    The percent conversion values of CO (solid line curve) and NH 3  dashed line curve) versus gas stream temperature for the Pd/Rh catalyst (for a conventional three-way catalyst) is presented graphically in  FIG. 3 . Like data for the CeZrCuO x  catalyst is presented in  FIG. 4 . As shown in  FIGS. 3 and 4 , both the Pd/Rh catalyst and the CeZrCuO x  catalyst began oxidizing CO (solid line, both figures) at around 150° C. and reacted &gt;90% CO conversion at about 225° C. The Pd/Rh catalyst began oxidizing NH 3  (dashed line, both figures) at around 210° C. and reached &gt;90% NH 3  conversion at 225° C. But, the CeZrCuO x  catalyst did not begin oxidizing NH 3  until around 375° C., and did not reach &gt;90% NH 3  conversion until about 475° C. 
         [0048]    It is found that similar CeZrCuO x  catalyst compositions prepared in a like manner are likewise affective for the selective oxidation carbon monoxide in gas streams also containing ammonia and oxygen over temperature ranges of about 200° C. to about 400° C. In general, it is preferred that the molar ratios of Ce, Zr, and Cu in the particles of co-precipitated mixed oxides be controlled to fall within the following ranges: Ce a Zr b Cu c Ox where a has a value in the range of 0.50 to 0.70, b has a value in the range of 0.10-0.20, and c has a value in the range of 0.15-0.35, with the sum of these molar ratios of Ce, Zr, and Cu equal to 1. The value of x depends on the proportions and oxidation state of the metal elements but typically is in the range of about one to three. 
         [0049]    Therefore, particles of CeZrCuO x  mixed oxides can effectively be used to selectively oxidize CO in exhaust streams comprising carbon monoxide, ammonia, and oxygen, and having a temperature in the range of about 200° C. to 400° C., which is shown in  FIG. 4  as the “CO Selective Oxidation Regime.” The gas stream may contain other non-interfering constituents such as water, carbon dioxide, and nitrogen. Under these conditions much of the carbon monoxide is oxidized to carbon dioxide and much of the ammonia is unaffected. 
         [0050]    Practices of the invention have been described using illustrative examples which are not intended to limit the scope of the claimed invention.