Patent Publication Number: US-2010115930-A1

Title: Exhaust after treatment system

Description:
BACKGROUND OF THE INVENTION 
     Emission treatment systems for internal combustion engines may include an oxidation catalyst upstream of a Selective Catalytic Reduction system that is useful for remediation of the nitrogen oxides (NO X ) in the exhaust stream. In diesel engines, a soot filter that is commonly referred to as a diesel particulate trap may also be included in the system for the removal of particulates from the exhaust gas. 
     Diesel engine exhaust is a heterogeneous mixture which contains not only gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NO X ”), but also condensed phase materials (liquids and solids) which constitute the particulate matter. Catalyst compositions, and substrates on which the catalysts are disposed may be provided in diesel engine exhaust systems to convert certain, or all of these exhaust constituents to non-regulated components. For example, diesel exhaust systems may include one or more of a diesel oxidation catalyst, a diesel particulate filter and a catalyst for the reduction of NO X . 
     One after treatment technology in use for high particulate matter reduction is the diesel particulate filter (“DPF”). There are several known filter structures that are effective in removing the particulate matter from diesel exhaust such as honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal fibers, etc. The ceramic wall flow filters have experienced significant acceptance in automotive applications. The filter is a physical structure for removing particles from exhaust and, as such, accumulating particles will have the effect of increasing the backpressure on the engine. To address backpressure increases caused by the particulate accumulation the DPF is periodically regenerated. Regeneration involves the burning of accumulated particulates in what is typically a high temperature (&gt;600 C), oxygen rich (lean) environment that may result in an increase in the levels of NO X  components in the exhaust gas stream. Similarly, in gasoline engines that employ lean burn technologies for increased fuel efficiency, a similar oxygen rich environment may also result in an increase in the levels of NO X  components in the exhaust gas. 
     A NO X  abatement technology that is being developed for automotive applications is Selective Catalytic Reduction (“SCR”) in which NO X  is reduced with ammonia (“NH 3 ”) to nitrogen (“N2”) over a catalyst that is typically comprised of base metals. For automotive applications, urea (typically present in an aqueous solution) is used as the source of the ammonia. SCR provides efficient conversion of NO X  as long as the exhaust temperature is within the active temperature range of the catalyst. An issue with known SCR catalysts is that high exhaust temperatures, such as are experienced during the DPF regeneration event in a diesel system or high load operation in a gasoline engine, may render many SCR catalyst compositions less catalytically effective while cooler, low load temperatures of engine exhaust may have a similar effect on other catalyst compositions. 
     Discrete substrates each containing catalysts to address specific components of the exhaust are available. However, it is desirable to reduce the overall size, complexity and cost of complete systems. One approach to achieve this goal is to coat the DPF with a catalyst composition which is effective for the conversion of the NO X  component of the exhaust stream and which is capable of efficient conversion at high and at low temperatures, across the entire range of operation of the DPF. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In an exemplary embodiment, a selective catalytic reduction system for reducing oxides of nitrogen (“NO X ”) in the exhaust gas of an internal combustion engine comprises a ceramic monolith disposed within the exhaust gas and having longitudinally extending exhaust flow passages. A high temperature catalyst composition selected for high temperature catalytic reduction is applied to an inlet portion of the exhaust flow passages and a low temperature catalyst composition selected for low temperature catalytic reduction is applied to an outlet portion of the exhaust flow passages. The high temperature and the low temperature catalytic reduction catalysts operate to reduce oxides of nitrogen at high load and low load operation of the internal combustion engine. 
     These and other features and advantages of the invention will become more apparent to those skilled in the art from the detailed description of exemplary embodiments. The drawings that accompany the detailed description are described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an exhaust treatment system for an internal combustion engine; 
         FIG. 2  is an axial sectional view which schematically shows a ceramic wall flow monolith; 
         FIG. 3  is a NO X  reduction efficiency curve for the exhaust treatment system of  FIG. 1 ; 
         FIG. 4  is an NH 3  oxidation curve for the exhaust treatment system of  FIG. 1 ; 
         FIG. 5  is an embodiment of a catalyst loading curve for the exhaust treatment system of  FIG. 1 ; and 
         FIG. 6  is another embodiment of a catalyst loading curve for the exhaust treatment system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , an exemplary embodiment of the invention is directed to an exhaust treatment system  10  for the reduction of regulated exhaust constituents of an internal combustion engine, such as diesel engine  12 . The treatment system  10  includes an exhaust conduit  14  that transports the exhaust gas from the diesel engine  12  to the various exhaust treatment components of the exhaust treatment system. The exhaust components may include an oxidation catalyst  16  that is useful in treating unburned gaseous and non-volatile hydrocarbons and carbon monoxide, which are combusted to form carbon dioxide and water. 
     Downstream of the oxidation catalyst, a reductant may be injected as a spray via injector nozzle  18 , into the exhaust gas flow  20 , in exhaust conduit  14 . Aqueous urea may be used as the ammonia precursor that may be mixed with air in the injector nozzle  18  to aid in dispersion of the injected spray. The exhaust stream containing the added ammonia is conveyed to a Selective Catalyst Reduction (“SCR”) device; in this case, Diesel Particulate Filter (“DPF”)  22 . The DPF is operable to filter the exhaust gas to remove carbon and other particulates, and to reduce the oxides of nitrogen (“NO X ”) resident in the exhaust stream through the use of multiple SCR catalysts. 
     The DPF  22  may be constructed with a ceramic wall flow monolith  23 ,  FIG. 2 , which has a plurality of longitudinally extending passages  24  formed by longitudinally extending walls  26 . The passages  24  include inlet passages  28  that have an open inlet end  30  and a closed outlet end  32 , and outlet passages  4  that have a closed inlet end  36  and an open outlet end  38 . Exhaust gas entering the DPF through the inlet end  30  of the inlet passages  28  is forced to migrate through the longitudinally extending walls  26  to the outlet passages  34 . It is through this wall flow mechanism that the exhaust gas is filtered of carbon and other particulates. The filtered particulates  40  are collected on the walls  26  of the inlet passages  28 . The accumulating particulates will have the effect of increasing the backpressure on the diesel engine  12 . To address backpressure increases caused by the particulate accumulation the DPF is periodically regenerated. Regeneration involves burning of the accumulated particulates  40  in what is typically a high temperature (&gt;600 C), oxygen rich (lean) environment that may result in an increase in the levels of the NO X  component in the exhaust gas stream. 
     In an exemplary embodiment of the emission treatment system  10 , a first SCR catalyst composition  42  preferably contains a zeolite and base metal component such as Iron (“Fe”) which can operate efficiently to convert NO X  constituents in the exhaust gas flow  20  at the high temperatures experienced in the DPF  22  during regeneration (i.e. &gt;600 C). Other suitable high temperature metals may include Cobalt (“Co”). The high temperature SCR catalyst composition  42  is applied to the walls of the inlet passages  28  of the ceramic wall flow monolith  23 . A second SCR catalyst composition  44 , also preferably containing a zeolite and base metal component such as Copper (“Cu”) which can operate efficiently to convert NO X  constituents in the exhaust gas flow  20  at low temperatures experienced in the DPF  22  during low load operation (i.e. &lt;600 C), is similarly applied to the walls of the outlet passages  34  of the ceramic wall flow monolith  23 . Other suitable low temperature metals may include Vanadium (“V”) and the like.  FIGS. 3 and 4  illustrate the performance of the two SCR catalyst compositions  42  and  44  across the operating temperature range of the DPF  22 . The NO X  reduction efficiency of the high temperature SCR catalyst composition  42  extends the conversion range of the device into the temperature range experienced during high load operation and regeneration of the DPF. The NO X  reduction efficiency of the low temperature SCR catalyst composition  44  extends the conversion range of the device into the temperature range experienced during low load or start up operation of the engine. In addition, and as illustrated in FIG.  4 , during low temperature operation, the low temperature SCR catalyst composition  44  receives NH 3  that moves past the high temperature SCR catalyst composition  42 . The low temperature SCR catalyst composition  44  utilizes the NH 3  to insure effective NO X  reduction. As such, the dual SCR catalyst combination allows the DPF to operate as an effective SCR system which is useful for remediation of the NO X  in the engine exhaust stream during high load operation or under DPF regeneration cycles as well as during low temperature, light load operation. 
     In an exemplary embodiment illustrated by the catalyst loading charts of  FIGS. 5 and 6 , application or loading of the high temperature and the low temperature SCR catalysts  42 ,  44  may be varied along the axial length of the filter resulting in a relatively uniform total catalyst loading  46  along the length of the ceramic monolith  23 . In the embodiment illustrated in  FIG. 5 , the concentration of each catalyst varies in an axial direction so as to gradually increase or decrease as the case may be. The result is a uniform total loading of catalysts  42 ,  44  respectively. In the embodiment illustrated in  FIG. 6 , catalyst loading is constant in the axial direction but each catalyst primarily occupies a particular axial portion of the monolith  23 , again resulting in a uniform total loading of catalysts  42 ,  44  respectively. 
     While the invention has been described with application to a ceramic wall flow monolith for the purpose of combining the DPF and the SCR catalyst devices, thereby eliminating a separate device from the exhaust system, it is contemplated that, in some circumstances separate devices may be dictated by the application. As indicated earlier, in gasoline engines that employ lean burn technologies for increased fuel efficiency, a similar oxygen rich environment may also result in an increase in the levels of NO X  components in the exhaust gas. While a DPF is typically not required with gasoline engines, the treatment of the exhaust gas flow from a lean burn gasoline engine may well benefit from a high temperature catalytic reduction catalyst composition applied to an inlet portion of a the exhaust flow passages of a flow-through (i.e. non-wall flow) monolith and a low temperature catalytic reduction catalyst composition applied to an outlet portion of the exhaust flow passages. As such, it is contemplated that the invention may also have application to straight flow ceramic monolith devices without straying from the scope of the invention. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and the scope of the appended claims.