SCR-LNT CATALYST COMBINATION FOR IMPROVED NOx CONTROL OF LEAN GASOLINE AND DIESEL ENGINES

Abstract

An aftertreatment system for use in a lean burn engine is disclosed. In one embodiment, the aftertreatment system includes a first catalyst in communication with an exhaust stream from the engine; a selective catalytic reduction catalyst in communication with the exhaust stream and positioned downstream of the first catalyst; and a lean NO x trap in communication with the exhaust stream and positioned downstream of the selective catalytic reduction catalyst; with the proviso that the first catalyst is not a selective catalytic reduction catalyst. A method for treating lean burn engine exhaust gases using the aftertreatment system is also disclosed.

1 . An aftertreatment system for use in a lean burn engine comprising:
 a first catalyst in communication with an exhaust stream from the engine;   a selective catalytic reduction catalyst in communication with the exhaust stream and positioned downstream of the first catalyst; and   a lean NO x  trap in communication with the exhaust stream and positioned downstream of the selective catalytic reduction catalyst;   with the proviso that the first catalyst is not a selective catalytic reduction catalyst.   
   
   
       2 . The system of  claim 1 , wherein the first catalyst is selected from lean NO x  traps, diesel oxidation catalysts, diesel particulate filters, three-way catalysts, hydrocarbon traps, and combinations thereof. 
   
   
       3 . The system of  claim 1  further comprising a second catalyst in communication with the exhaust stream and positioned downstream of the selective catalytic reduction catalyst and upstream of the lean NO x  trap. 
   
   
       4 . The system of  claim 3  wherein the second catalyst is selected from lean NO x  traps, diesel oxidation catalysts, diesel particulate filters, three-way catalysts, hydrocarbon traps, NH 3  slip catalysts, and combinations thereof. 
   
   
       5 . The system of  claim 3 , wherein the first catalyst is a diesel particulate filter, and wherein the second catalyst is a diesel oxidation catalyst. 
   
   
       6 . The system of  claim 2 , wherein the first catalyst is a diesel particulate filter and a diesel oxidation catalyst 
   
   
       7 . The system of  claim 1  wherein the lean NO x  trap comprises a support, a precious metal catalyst on the support, and optionally one or more alkali metal oxides or alkaline earth metal oxides on the support. 
   
   
       8 . The system of  claim 1  wherein the selective catalytic reduction catalyst comprises a support, and a catalyst comprising an oxide of a base metal or a zeolite on the support. 
   
   
       9 . A method for treating lean burn engine exhaust gases comprising:
 providing an aftertreatment system in an exhaust gas passage of a lean burn engine, the system comprising a first catalyst in communication an exhaust stream from the engine; a selective catalytic reduction catalyst in communication an exhaust stream and positioned downstream of the first catalyst; and a lean NO x  trap in communication the exhaust stream and positioned downstream of the selective catalytic reduction catalyst; with the proviso that the first catalyst is not a selective catalytic reduction catalyst; and   exposing the aftertreatment system to engine exhaust gas containing NO x  such that at least a portion of said NO x  contained in said exhaust gas is converted to N 2 , and at least a portion of the NH 3  supplied to or generated by the selective catalytic reduction catalyst converts NO x  stored on the LNT to N 2 .   
   
   
       10 . The method of  claim 9  further comprising operating the engine in a cycle of lean and rich operation. 
   
   
       11 . The method of  claim 9  further comprising heating the exhaust gases for desorption of sulfur. 
   
   
       12 . The method of  claim 9 , wherein the first catalyst is selected from lean NO x  traps, diesel oxidation catalysts, diesel particulate filters, three-way catalysts, hydrocarbon traps, and combinations thereof. 
   
   
       13 . The method of  claim 9  further comprising a second catalyst in communication with the exhaust stream and positioned downstream of the selective catalytic reduction catalyst and upstream of the lean NO x  trap. 
   
   
       14 . The method of  claim 13 , wherein the second catalyst is selected from lean NO x  traps, diesel oxidation catalysts, diesel particulate filters, three-way catalysts, NH 3  slip catalysts, hydrocarbon traps, and combinations thereof 
   
   
       15 . The method of  claim 13 , wherein the first catalyst is a diesel particulate filter, and wherein the second catalyst is a diesel oxidation catalyst. 
   
   
       16 . The method of  claim 9 , wherein the first catalyst is a diesel particulate filter and a diesel oxidation catalyst. 
   
   
       17 . The method of  claim 11  wherein the lean NO x  trap comprises a support, a precious metal catalyst on the support, and optionally one or more alkali metal oxides or alkaline earth metal oxides on the support. 
   
   
       18 . The method of  claim 11  wherein the selective catalytic reduction catalyst comprises a support, and a catalyst comprising an oxide of a base metal or a zeolite on the support. 
   
   
       19 . An aftertreatment system for use in a lean burn engine consisting essentially of:
 an optional first catalyst in communication with an exhaust stream from the engine;   a selective catalytic reduction catalyst in communication with the exhaust stream and positioned downstream of the optional first catalyst;   an optional second catalyst in communication with the exhaust stream and positioned downstream of the selective catalytic reduction catalyst;   a lean NO x  trap in communication with the exhaust stream and positioned downstream of the optional second catalyst and the selective catalytic reduction catalyst.   
   
   
       20 . The system of  claim 17  wherein the optional first catalyst is selected from lean NO x  traps, diesel oxidation catalysts, diesel particulate filters, three-way catalysts, hydrocarbon traps, and combinations thereof; and wherein the optional second catalyst is selected from lean NO x  traps, diesel oxidation catalysts, diesel particulate filters, three-way catalysts, NH 3  slip catalysts, hydrocarbon traps, and combinations thereof.