Patent Publication Number: US-2011064632-A1

Title: Staged Catalyst System and Method of Using the Same

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to an emission control system for reducing waste products from the exhaust of a combustion engine. 
     2. Background Art 
     Like gasoline engines, diesel engines have been widely used for transportation and many other stationary applications. A combustion exhaust from diesel engines often contains a variety of combustion waste products including unburned hydrocarbon (HC), carbon monoxide (CO), particulate matter (PM), nitric oxide (NO), and nitrogen dioxide (NO 2 ), with NO and NO 2  collectively referred to as nitrogen oxide or NO x . Removal of CO, HC, PM, and NO x  from the combustion exhaust is needed for cleaner emissions. The combustion exhaust treatment becomes increasing important in meeting certain emission requirements. 
     Conventional emission control systems often use separate devices for the reduction of NO x  and particulate matter. For example, a singular SCR (selective catalytic reduction) catalyst is used for converting NO x  to nitrogen (N 2 ) and a singular particulate filter (PF) is used for removing particulate matter. 
     However, conventional emission control systems have met with limited use as they lack, among other things, concurrent and balanced consideration for emission control efficiency and space conservation. 
     There is a continuing need to provide an emission control system with features more suitable for meeting increasingly stringent industry and environmental standards. 
     SUMMARY 
     One aspect of the present invention is directed to a staged catalyst system for reducing waste products in the exhaust from a combustion engine. In one embodiment, the staged catalyst system includes an integrated particulate filter block supporting thereupon a first selective catalytic reduction catalyst; and a flow-through catalyst block supporting thereupon a second selective catalytic reduction catalyst, wherein the integrated particulate filter block is disposed downstream of the combustion engine and upstream of the flow-through catalyst block. 
     In at least another embodiment, the integrated particulate filter block and the flow-through catalyst block are spaced of no more than 120 centimeters apart. 
     In at least yet another embodiment, the first selective catalytic reduction catalyst has a loading concentration from 0.5 to 3.0 grams per cubic inch of the integrated particulate filter block. In certain instances, the first selective catalytic reduction catalyst is catalytically active for converting 85 percent or more by volume of NO x  to nitrogen in a temperature range of 270 to 600 degrees Celsius. One example of the first selective catalytic reduction catalyst is an iron-containing zeolite. 
     In at least yet another embodiment, the second selective catalytic reduction catalyst has a loading concentration from 0.5 to 6.0 grams per cubic inch of the catalyst block. In certain instances, the second selective catalytic reduction catalyst is catalytically active for converting 85 percent or more by volume of NO x  to nitrogen in a temperature range of 170 to 450 degrees Celsius. One example of the second selective catalytic reduction catalyst is a copper-containing zeolite. 
     According to another aspect of the present invention, there is provided an emission control system containing the staged catalyst system described herein for reducing gases transported in an exhaust passage from a combustion engine and a reductant source for introducing reductant within the exhaust passage downstream of the combustion engine. 
     According to at least yet another aspect of the present invention, there is provided a method for reducing gases in the exhaust of a combustion engine. In at least one embodiment, the method includes contacting the exhaust with a reductant and an integrated particulate filter block to form a first treated exhaust, the integrated particulate filter block containing thereupon a first selective catalytic reduction catalyst; and contacting the first treated exhaust with a flow-through catalyst block containing thereupon a second selective catalytic reduction catalyst to form a second treated exhaust. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1B  schematically depict configurations of an emission control system according to various embodiments of the present invention; 
         FIG. 2  shows the percentage of NO x  removal as a function of catalytic temperature compared among catalyst configurations; 
         FIG. 3  shows the percentage of ammonia oxidation as a function of catalytic temperature compared among various catalyst configurations; 
         FIG. 4  shows ammonia slip, in ppm (parts per million), as a function of catalytic temperature compared among various catalyst configurations; 
         FIG. 5  shows the percentage of NO x  removal as a function of catalytic temperature compared among SCR 1 /PF catalyst configurations having various NO/NO 2  ratios; and 
         FIG. 6  shows the percentage of NO x  removal as a function of catalytic temperature compared among SCR 2  catalyst compositions having various NO/NO 2  ratios. 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale, some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for the claims and/or a representative basis for teaching one skilled in the art to variously employ the present invention. 
     Moreover, except where otherwise expressly indicated, all numerical quantities in the description and in the claims are to be understood as modified by the word “about” in describing the broader scope of this invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary, the description of a group or class of material as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more members of the group or class may be equally suitable or preferred. 
     One aspect of the present invention is directed to a staged catalyst system, as generally shown at  106  in  FIG. 1A-1B , for use in reducing waste products in the exhaust of a combustion engine and in one particular embodiment, a diesel engine. In at least one embodiment, the staged catalyst system  106  comprises a SCR 1 /PF  108  and a SCR 2   110 . It has been found that the staged catalyst system  106  as contemplated herein provides a synergistically broadened catalytic temperature range and hence enhanced NO x  reduction efficiency in comparison to existing configurations. As illustrated in more detail below, the staged catalyst system  106  alleviates the occurrence of a sharp “fall-off” of NO x  removal efficiency at a given temperature, a phenomenon often observed with singular SCR/PF devices. In addition, the integrated catalyst system  106  also exhibits NO x  reduction in the 175 to 225 degrees Celsius temperature range, a range where SCR 1 /PF or SCR 2  alone is typically catalytically inactive. 
     While not intended to be limited to any particularly theory, the synergistic broadening effect may be explained by the following: a close coupling of the SCR 1 /PF  108  upstream of the SCR 2   110  induces at least a partial conversion of NO x  and hence a change in the ratio between various species of NO x . As such, the resulting ratio of NO 2 /NO is better suited for a downstream catalytic conversion by the SCR 2  catalyst  110 . In addition, it is believed that during an engine cold start, when the SCR 1 /PF catalyst  108  remains relatively less catalytically active, a reductant such as ammonia can slip through SCR 1 /PF  108  and SCR 2   110  helps to act on the ammonia as leaked through and reduces its release out into the air. Finally, it is believed that the broadened catalytic temperature range is due at least in part to the flexibility of having separate SCR catalyst loadings provided by the staged catalyst system  106  with the “SCR 1 /PF+SCR 2 ” configuration, wherein the first SCR catalyst “SCR 1 /PF” may be of a catalytic function that reacts well in a higher temperature range with a starting temperature of 250 degree Celsius, for instance. For a given amount of SCR catalyst loading needed for a particular application, the availability of a separate catalyst block, namely SCR 2   110 , to load share some portion of the required total SCR catalyst loading helps to relieve issues associated with back pressure build-up on the particulate filter block. 
     In practice, typically the closer the exhaust stream is to the engine  112 , the hotter the exhaust gets. As such, SCR 1 /PF  108  is in contact with a substantially hotter exhaust than the second SCR catalyst in the SCR 2   110  due to its relatively more downstream location. Accordingly, SCR 1 /PF  108  is designed to be catalytically active at a temperature range between 250 to 550 degrees Celsius—higher than the temperature range for SCR 2   110 . Likewise, the second SCR catalyst of the SCR 2   110  is designed to be catalytically active within a lower temperature window than SCR 1 /PF  108 , slated at a range of between 150 to 450 degrees Celsius. 
     The staged configuration of “SCR 1 /PF+SCR 2 ” provides a certain degree of flexibility to an emission control system wherein the SCR catalyst as supported on the SCR 1 /PF block  108  and the SCR 2  block  110  can be formulated to be of the same or different chemical composition such that the emission control system can be further optimized based on the catalyst selection and formulation thereof. For instance, Fe/zeolite is preferably used in the SCR 1 /PF block  108  and Cu/zeolite is preferably used in the SCR 2  block  110 . However, chemical compositions for the SCR 1 /PF block  108  and the SCR 2  block  110  do not necessarily have to be restricted to Fe/zeolite or Cu/zeolite alone. It is reasonable, and dependent upon application particulars at hand, the SCR 1 /PF  108  can contain a mixture of Fe and Cu with any suitable weight ratio, for instance, of from 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, to 10:1; the SCR 2  can contain a mixture of Fe and Cu with any suitable weight ratio, for instance, of from 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, or 1:10. 
     Furthermore, the staged catalyst system  106  “SCR 1 /PF+SCR 2 ” according to at least one embodiment of the present invention provides substantial space reduction in a range of 20 to 40 percent relative to convention systems. 
     An additional benefit is that the staged catalyst system, “SCR 1 /PF+SCR 2 ” enhances ammonia oxidation and further reduces ammonia slip past the emission control system. For mobile diesel engine applications, overdosing of ammonia is sometimes inevitable in practice. Any ammonia in excess of what can be consumed during the NO x  conversion reactions should be eliminated or the excess ammonia can slip through the system and be released into to the air causing pollution. One way of eliminating the excess ammonia is via oxidization reactions wherein the noxious ammonia is oxidized into N 2  that is relatively safer to be released into the air. 
     As used herein and unless otherwise noted, the term DPF or DF refers to the particulate filter employed to remove particulate matter or the like. 
     The present invention is capable of reducing waste products from the exhaust of an internal combustion engine such as a diesel engine or a gasoline engine. The staged catalyst system  106 , as described in more detail below, is believed to provide at least one of the following advantages including—a broadened catalytically active temperature range; more adequate mixing between the exhaust and the catalysts; and a reasonable reduction in the overall system complexity and space required. 
     In at least one embodiment, an emission control system uses a staged catalyst system illustrated as “SCR 1 /PF+SCR 2 ”, which includes a first SCR catalyst (SCR 1 /PF) followed by a particulate filter with a second SCR catalyst (SCR 2 ). The second SCR catalyst can be attached to a flow-through monolith and therefore, the exhaust flows through the monolith while being treated by the second SCR catalyst. The first SCR catalyst can be coated on a wall-flow PM filter and the resulting SCR 1 /PF block performs concurrently at least two functions, namely reduction of the NO x  and removal of the particulate matter. 
     With respect to the Figures that will be described in detail below, like numerals are used to designate like structures throughout the Figures. An emission control system in accordance with at least one embodiment of the present invention is generally shown at  100  in  FIG. 1A . The emission control system  100  includes an exhaust passage  102  and the staged catalyst system  106  described herein. In the illustrated embodiment, a reductant  119  is disposed within the exhaust passage  102  downstream of an engine  112 . An aperture  118  is optionally located on the exhaust passage  102  and disposed between the engine  112  and the staged catalyst system  106  to facilitate the introduction of the reductant  119  into the exhaust passage  102 . The reductant  119 , capable of reducing NO x  to nitrogen N 2 , is injected into the exhaust passage  102  optionally through a nozzle (not shown). The injection of the reductant  119  is optionally achieved through the use of a valve  120  which can be employed to meter needed amounts of the reductant  119  into an exhaust  117  from a reductant source  104 . The exhaust  117  with the reductant  119  is then conveyed further downstream to the staged catalyst system  106  for the reduction of NO x  and the removal of the particulate matter. 
     In at least one embodiment, the aperture  118  is disposed no more than 140 centimeters upstream of the SCR 1 /PF block  108 . In at least another embodiment, the aperture  118  is disposed no more than 100 centimeters upstream of the SCR 1 /PF block  108 . 
     In at least another embodiment, the range of the distance between the aperture  118  and the SCR 1 /PF block  108  may be independently selected from a range of no less than 0.5 centimeters, 10 centimeters, 20 centimeters, 30 centimeters, 40 centimeters, 50 centimeters, 60 centimeters, or 70 centimeters, to no greater than 140 centimeters, 130 centimeters, 120 centimeters, 110 centimeters, 100 centimeters, 90 centimeters, or 80 centimeters. 
     The reductant  119  may be of any material suitable for reducing NO x  to a harmless, releasable substance such as nitrogen N 2 . Exemplary reducing agents are hydrocarbon (HC), ammonia (NH 3 ), an ammonia precursor such as liquid urea, or any combination thereof. As is known, when exposed to a warm or hot exhaust, urea readily decomposes to ammonia. In certain embodiments, a molar ratio NH 3 /NO x  is typically kept at a value predesignated so as to minimize NH 3  slip past the catalysts and out into the air. An exemplary molar ratio of NH 3 /NO x  is at or near one (1). Decomposition of urea and subsequent reduction of NO x  typically occurs according to the following scheme: 
     Urea decomposition: 
     
       
         
         
             
             
         
       
     
     NO x  reduction: 
       4NO+4NH 3 +O 2 →4N 2 +6H 2 O
 
       6NO 2 +8NH 3 →7N 2 +12H 2 O
 
       2NH 3 +NO+NO 2 →2N 2 +3H 2 O
 
     Any suitable SCR catalyst compositions can be used in the SCR 1 /PF  108  and the SCR 2   110  to catalyze the reduction of NO x . In certain instances, the SCR catalysts are capable of converting at least 50% of NO x  to nitrogen (N 2 ), depending on the amount of the reductant  119  supplied. Useful SCR catalysts should have thermal resistance to temperatures greater than 650 degree Celsius so that the SCR catalysts remain structurally integral throughout an exhaust treatment process. 
     As used herein and unless otherwise identified, a SCR catalyst is “catalytically functional” in a given temperature when at that temperature, the SCR catalyst is able to convert 50 percent or more by volume of NO x  to nitrogen. 
     As used herein and unless otherwise identified, a SCR catalyst is “catalytically active” in a given temperature when at that temperature, the SCR catalyst is able to convert 85 percent or more by volume of NO x  to nitrogen. 
     In at least one embodiment, the first SCR catalyst  108  is catalytically functional in a temperature range of about 150 to 650 degrees Celsius and in at least another embodiment is catalytically active for converting 85 percent or more by volume of NO x  to nitrogen in a temperature range of 270 to 600 degrees Celsius. 
     In yet at least one embodiment, the second SCR catalyst of the SCR 2  block  110  is catalytically functional in a temperature range of about 150 to 650 degrees Celsius and in at least another embodiment is catalytically active for converting 85 percent or more by volume of NO x  to nitrogen in a temperature range of 170 to 450 degrees Celsius. The catalytically active temperature range for the second SCR catalyst of the SCR 2  block  110  is generally lower in contrast with the range for the first SCR catalyst of the SCR 1 /PF block  108 . This is beneficial at least in that the staged catalyst distribution provided by the staged catalyst system  106  responds to a similarly staged operating temperature profile, e.g., in a gradually cooling fashion along the exhaust passage  102  as the exhaust  117  proceeds downstream from the engine  112 . 
     Suitable SCR catalysts are described in U.S. Pat. No. 4,961,917 to Byrne, the entire content of which is incorporated by reference herein. Some suitable compositions include one or both of an iron and a copper metal atom present in a zeolite in an amount of from about 0.1 to 30 percent by weight of the total weight of the metal atoms plus zeolite. Zeolites are resistant to sulfur poisoning and remain active during a SCR catalytic reaction. Zeolites typically have pore sizes large enough to permit adequate movement of NO x , ammonia, and product molecules N 2  and H 2 O. The crystalline structure of zeolites exhibits a complex pore structure having more or less regularly recurring connections, intersections, and the like. By way of example, suitable zeolites are made of crystalline aluminum silicate, with a silica to alumina ratio in the range of 5 to 400 and a mean pore size from 3 to 20 Angstroms. 
     Suitable SCR catalyst to be used in the staged catalyst system  106  may be a physical mixture of two or more catalysts in any suitable ratio. By way of example, the first SCR catalyst  108  of the staged catalyst “SCR 1 /PF+SCR 2 ” system  106  can be an iron-containing zeolite combined with one or more other metals selected from the group consisting of vanadium, chromium, molybdenum, tungsten, or any combinations thereof. Similarly, the SCR 2  catalyst of the staged catalyst “SCR 1 /PF+SCR 2 ” system  106  can be a copper-containing zeolite combined with one or more other metals selected from the group consisting of vanadium, chromium, molybdenum, tungsten, or any combinations thereof. 
     Monoliths are well known but are generally described as a ceramic block made of a number of parallel tubes. The monolith may be made of ceramic materials such as cordierite, mullite, and silicon carbide or metallic materials such as iron cromium alloy, stainless steel, and Inconel®. The individual tubes of the monolith may be of any suitable size, and in certain embodiments are of a size of 0.5 to 10 millimeters in diameter. 
     Because of the number of the passages, the contact area between an exhaust and the first SCR catalyst is relatively high. Further, the tubes are substantially straight, hollow, and parallel to the flow of the exhaust, therefore flow obstruction to the exhaust is effectively minimized. 
     In at least one embodiment, the SCR 1 /PF block  108  is provided with a SCR catalyst loading concentration in grams per cubic inch of a loading volume, generally shown at “A” in  FIG. 1A . In certain instances, the SCR 1 /PF block  108  has a SCR catalyst loading concentration in a range independently selected from no less than 0.5 g/in 3 , 1.0 g/in 3 , or 1.5 g/in 3 , to no greater than 2.0 g/in 3 , 2.5 g/in 3 , or 3.0 g/in 3 . 
     In at least another embodiment, the SCR 2  block  110  is provided with a SCR catalyst loading concentration in grams per cubic inch of a loading volume, generally shown at “B” in  FIG. 1A . In certain instances, the SCR 2  block  110  has a SCR catalyst loading concentration in a range independently selected from no less than 0.5 g/in 3 , 1.0 g/in 3 , 1.5 g/in 3 , 2.0 g/in 3 , to no greater than 6.0 g/in 3 , 5.0 g/in 3 , 4.0 g/in 3 , or 3.0 g/in 3 . 
     In at least another embodiment, the ratio of the SCR catalyst loading concentration of the SCR 1 /PF  108  relative to the loading concentration of the SCR 2   110  is 0.1 to 3.0, in another embodiment of 0.5 to 2.8, and in another embodiment of 1.0 to 2.5. 
     In at least another embodiment, the distance between the SCR 1 /PF  108  and the SCR 2   110  may be independently selected from no less than 0.5, 10 centimeters, 20 centimeters, 30 centimeters, 40 centimeters, 50 centimeters, or 60 centimeters, to no greater than 70 centimeters, 80 centimeters, 90 centimeters, 100 centimeters, 110 centimeters, or 120 centimeters. 
     In at least one embodiment, the first SCR catalyst is supported on a wall-flow particulate filter to form the SCR 1 /PF block having a plurality of substantially parallel tubes extending along the longitudinal axis of the particulate filter. Typically, each tube is blocked at one end of the particulate filter, with alternate passages blocked at opposite ends. An exemplary wall-flow particulate filter is composed of ceramic-like materials such as cordierite, α-alumina, silicon carbide, silicon nitride, zirconia, mullite, spodumene, alumina-silica-magnesia, or zirconium silicate. The pore sizes and level of porosity are selected to allow flow exchange through the pores while ensuring that all back pressure on the particulate filter to be at a minimum. 
     The SCR catalyst can be coated onto the SCR 1 /PF block  108  or the SCR 2  block using any suitable method. One exemplary method of such a coating is illustrated in U.S. Pat. No. 7,229,597 to Patchett et al., the entire contents of which are incorporated herein by reference. In essence, the particulate filter with a desired porosity is immersed in a catalyst slurry which is then allowed to dry under compressed air. This dipping-drying process may be repeated till the desired level of coating is achieved. After coating, the particulate filter may be dried at a temperature of 100 degrees Celsius and subsequently calcined at a temperature in the range of 300 to 500 degrees Celsius. 
     In at least one embodiment, and as shown in  FIG. 1B , an oxidation catalyst  114  can be disposed within the exhaust passage  102  between the engine  112  and the aperture  118 . Oxidation catalysts that contain platinum group metals, base metals and combinations thereof promotes the conversion of both HC and CO gaseous pollutants and at least some portion of the particulate matter through oxidation of these pollutants to carbon dioxide and water. The oxidation catalyst  114  generally helps to break down pollutants in the exhaust to less harmful components. In particular, the oxidation catalyst  114  utilizes palladium and platinum catalysts to mainly help reduce the unburned hydrocarbon and carbon monoxide according to the following reaction formula: CO+O2→CO2. 
     When the oxidation catalyst  114  is used as illustrated in  FIG. 1B , an exhaust containing unburned hydrocarbons (HC), carbon monoxide (CO), nitrogen oxide (NO x ), and particulate matter (PM) is emitted from the engine  112  through exhaust passage  102  to the oxidation catalyst  114 . In the oxidation catalyst  114 , unburned hydrocarbon and carbon monoxide are combusted to form carbon dioxide and water. Removal of the HC and CO using the oxidation catalyst  114  helps to relieve some burden on the downstream staged catalyst system  106  in remediating the exhaust. 
     In addition, the oxidation catalyst  114  also converts a certain portion of the nitric oxide NO to nitrogen dioxide NO 2  such that the NO/NO 2  ratio is more suitable for downstream SCR catalytic reactions. An increased proportion of NO 2  in the NO x , due to the catalytic action of the upstream oxidation catalyst  114 , enhances the reduction of NO x  as compared to exhaust streams containing smaller proportions of NO 2  in the NO x  component. Furthermore, the oxidation catalyst  114  helps to regenerate particulate filter  110  for continuous engine operation. During diesel engine operation, soot typically will accumulate on the particulate filter  110  over time and cause back pressure elevation which diminishes the full operating efficiency of the engine. One solution is to generate a sufficiently high temperature in the range of about 600 to 700 degrees Celsius to induce the combustion of the soot by injecting fuel onto the oxidation catalyst  114 . 
     The staged catalyst system  106  may be further altered in its configuration without materially changing its intended function. 
     As shown in  FIG. 1B , a second oxidation catalyst  124  can be disposed downstream of the staged catalyst system  106 . When used in concert with the first oxidation catalyst  114 , the second oxidation catalyst  124  mainly serves to oxidize ammonia molecules that may have slipped through the exhaust passage  102  and to convert the slipped ammonia molecules to N 2 . In addition, any unburned hydrocarbon that is left untreated may be oxidized at this point before final release into the air. 
     One or more embodiments of the present invention are further illustrated by the following non-limiting examples. 
     EXAMPLE  
     Example 1  
     Catalytic efficiency testing is carried out in a steady state whereas the catalysts used are in a non-aged or fresh condition; whereas ammonia is supplied at a level of 350 ppm (parts per million); whereas a simulated exhaust stream is provided to have 350 ppm NO x ; and whereas other testing parameters are set as follows: 14% of oxygen, 5% of carbon dioxide, 4.5% of water, and nitrogen to balance. The term “SCR 1 /PF+SCR 2 ” represents an integrated particulate filter and SCR system whereas the first SCR catalyst as denoted “SCR 1 /PF” is an iron-containing zeolite coated on a diesel particulate filter and the SCR 2  stands for the second SCR catalyst, with the second SCR catalyst being a copper-containing zeolite. The integrated particulate filter and SCR system is tested at a space velocity of 30,000 per hour or 30 K/h. 
     Space velocity is defined as v/V whereas v is the flow rate of an exhaust expressed in the unit of liter per hour and V is the volume of catalysts within a portion of the exhaust passage through which the exhaust passes. In this experiment, an exhaust travels in a flow rate of 6.44 liters per minute and the SCR 1 /PF is provided in a volume of 1 cubic inch, then the resulting space velocity is (6.44 L/min)(60 min)/(0.01287 L) which equals approximately 30K per hour. The testing is conducted in parallel comparison to SCR 1 /PF alone or to SCR 2  alone. It is noted that in the “SCR 1 /PF+SCR 2 ” configuration that the exhaust flow rate doubled to 12.88 liters per minute to maintain a space velocity of 30K per hour. When the amount of exhaust and hence the exhaust flow rate remains the same, for example, 6.44 liters per minute, “SCR 1 /PF+SCR 2 ” at 15K per hour, space velocity is comparable to SCR 1 /PF alone of 30K per hour space velocity or SCR 2  alone of 30K per hour space velocity. For the purpose of the experiments disclosed herein, a less than maximum amount of NO x  is supplied through the exhaust; or put in a different way, the SCR 1 /PF, the SCR 2 , and the SCR 1 /PF+SCR 2  at the space velocity of 30 K/h are each capable of converting the entire amount of the NO x  under a suitable operating temperature. 
       FIG. 2  depicts NO x  efficiency as a function of operating temperatures in degree Celsius. At the space velocity of 30 K/h and to remove at least 90 percent by weight of the total NO x  supplied, the SCR 1 /PF is catalytically active within a temperature range of about 285 to about 540 degree Celsius. A comparable range for the SCR 2  catalyst is about 200 to about 420 degree Celsius. 
     In contrast, the integrated particulate filter and SCR system shown at “SCR 1 /PF+SCR 2 ” has a temperature range from about 210 to about 540 degrees Celsius spanning 330 degrees—a range at least 110 degrees broader than the range for SCR 2  alone and at least 75 degrees broader than the range for SCR 1 /PF alone. 
     Example 2  
     The experiment is carried out under the same conditions set forth in Example 1 above. In this example, ammonia oxidation is being monitored. Ammonia oxidation is an alternative indication of how much ammonia is being consumed in the process of both reducing NO x  and being oxidized by the oxygen in the exhaust. The ammonia that is not oxidized or consumed typically slips past the catalysts and is released into the air. 
     As depicted in  FIG. 3 , and within a temperature range of 150 to 550 degrees Celsius, ammonia (NH 3 ) usage through ammonia oxidation is the lowest in the SCR 1 /PF alone (and in this example, Fe/zeolite on DPF) and highest in the SCR 2  alone (and in this example, Cu/zeolite) at a space velocity of 30 K/h. Fe/zeolite on DPF catalyst has comparably the lowest NH 3  oxidation activity even up to 500 degree Celsius while Cu/zeolite catalyst has higher NH 3  oxidation activity. At 30 K/h space velocity, 100% NH 3  removal efficiency is achieved at 440 degree Celsius. With the staged configuration, the NH 3  oxidation efficiency is shifted to high temperature but the 100% NH 3  removal efficiency is still achieved at 440 degree Celsius. The staged configuration, SCR 1 /PF+SCR 2 , affords relatively lower ammonia oxidation at cooler temperatures to favor rapid NO x  conversion with NH 3  and at warmer temperatures, such as from 440 degrees Celsius and above, to favor ammonia slip control such that excess ammonia is oxidized to form less harmful N 2 . 
     Example 3  
     The experiment is carried out under the same conditions illustrated in Example 1 above. In this example, ammonia slip is being monitored. Amongst all four configurations tested and as shown in  FIG. 4 , the SCR 1 /PF alone configuration elicits the highest ammonia slip at temperatures in the range of 150 to 300 degrees Celsius. Ammonia slip in the SCR 1 /PF configuration quickly decreases to below 10 ppm at above 350 degree Celsius. 
     Since ammonia oxidation is generally reciprocal to ammonia slip in a given catalyst environment, the ammonia slip profile of the SCR  1  configuration shown in  FIG. 4  is observed to be reciprocally consistent with the ammonia oxidation of the same SCR 1 /PF configuration as reported in  FIG. 3 . This is consistent with the activity of ammonia oxidation reported in  FIG. 3  for SCR 1 /PF. All tested configurations exhibit minimum ammonia slip at a level at or below 50 ppm, when the catalyst temperature is at or above 250 degrees Celsius. 
     Example 4  
     This experiment is carried out under the same conditions illustrated in Example 1 above. In this example, the NO x  removal efficiency of the SCR 1 /PF block alone is examined as a function of operating temperature in degrees Celsius compared among various NO/NO2 composition ratios in the simulated exhaust stream. As shown in  FIG. 5 , when provided with a 50/50 stoichiometric mixture of NO/NO 2  in the simulated exhaust stream, the SCR 1 /PF alone elicits a 90% NO x  removal at a temperature of 160 degree Celsius, in comparison to a temperature of 300 degree Celsius for the less stoichiometric comparables having NO/NO 2  ratios of 100%/0% and 80%/20%. The results demonstrated here are consistent with the finding that a partial conversion of NO to NO 2  to bring a NO/NO 2  ratio closer to a stoichiometry is beneficial for NO x  conversion efficiency. 
     Example 5  
     This experiment is carried out under the same conditions illustrated in Example 1 above. In this example, the NO x  removal efficiency of the SCR 2  block alone is examined as a function of operating temperature in degrees Celsius compared among various NO/NO 2  composition ratios in the simulated exhaust stream. As shown in  FIG. 6 , when provided with a 50/50 stoichiometric mixture of NO/NO 2  in the simulated exhaust stream, the SCR 2  alone elicits generally higher NO x  removal at a temperature of from 150 to 250° C., in comparison to the less stoichiometric comparables having NO/NO 2  ratios of 100%/0% and 80%/20%. The results demonstrated here are consistent with the finding that a partial conversion of NO to NO 2  to bring a NO/NO 2  ratio closer to a stoichiometry is beneficial for NO x  conversion efficiency. 
     While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.