Alumina-based lean NOx trap system and method of use in dual-mode HCCI engines

Abstract

An alumina-based lean NO x trap system for use in a HCCI engine exhaust is provided which includes at least one alumina-based lean NO x trap comprising a catalyst, an alumina NO x absorbent material, and optionally, from 0 to about 4 wt % of an alkaline earth metal oxide; and a conventional three-way catalyst. The lean NO x trap system substantially oxidizes HC and CO and converts at least a portion of NO x contained in the exhaust gas to N 2 at a temperature between about 150° C. to about 250° C. in HCCI mode. The system also effectively removes HC, CO and NO x at high temperature when the engine is in SI mode (stoichiometric conditions). The alumina-based lean NO x trap in the system also undergoes efficient desulphurization and maintains its activity with extended use.

1 . A lean NO x  trap system for use in a HCCI engine exhaust comprising: 
 a first lean NO x  trap comprising a catalyst, a NO x  absorbent material comprising alumina, and optionally, from 0 to about 4 wt % of an alkaline earth metal oxide; and a three-way catalyst; wherein said system oxidizes HC and CO and converts at least a portion of NO x  contained in exhaust gas from said engine to N 2  at a temperature range of between about 150° C. to about 250° C.    
   
   
       2 . The system of  claim 1  wherein said catalyst in said lean NO x  trap is selected from platinum, rhodium, and combinations thereof.  
   
   
       3 . The system of  claim 1  having a NO x  conversion efficiency of at least 50% at a temperature between about 150° C. to about 250° C.  
   
   
       4 . The system of  claim 1  having a HC and CO conversion efficiency of at least 70% at a temperature between about 150° C. to about 250° C.  
   
   
       5 . The system of  claim 1  wherein said catalyst in said first lean NO x  trap is selected from platinum, rhodium, and combinations thereof.  
   
   
       6 . The system of  claim 1  wherein said alkaline earth metal in said metal oxide is selected from Mg, Sr, Ba, and Ca.  
   
   
       7 . The system of  claim 1  wherein said lean NO x  trap further includes less than about 4 wt % of a stabilizing metal selected from La, Ce, and Ba.  
   
   
       8 . The system of  claim 1  wherein said three-way catalyst is positioned downstream from said lean NO x  trap.  
   
   
       9 . The system of  claim 1  wherein said lean NO x  trap and said three-way catalyst are combined.  
   
   
       10 . The system of  claim 1  further including a second lean NO x  trap comprising a catalyst, a NO x  absorbent material comprising alumina, and optionally, from 0 to 4 wt % of an alkaline earth metal oxide.  
   
   
       11 . The system of  claim 10  wherein said second lean NO x  trap is positioned downstream from said three-way catalyst.  
   
   
       12 . A method for treating HCCI engine exhaust gases comprising: 
 providing a lean NO x  trap system in an exhaust gas passage of a HCCI engine said system comprising a first lean NO x  trap comprising a catalyst, a NO x  absorbent material comprising alumina, and optionally, from 0 to about 4 wt % of an alkaline earth metal oxide; and a three-way catalyst;    exposing said lean NO x  trap system to HCCI engine exhaust gas containing NO x  such that HC and CO are substantially oxidized, and at least a portion of said NO x  contained in said exhaust gas is converted to N 2  at a temperature between about 150° C. to 250° C.    
   
   
       13 . The method of  claim 12  wherein said three-way catalyst is positioned downstream from said first lean NO x  trap.  
   
   
       14 . The method of  claim 12  wherein said first lean NO x  trap is positioned downstream from said three-way catalyst.  
   
   
       15 . The method of  claim 12  wherein said first lean NO x  trap and said three-way catalyst are combined.  
   
   
       16 . The method of  claim 12  wherein said system includes a second lean NO x  trap positioned downstream from said three-way catalyst, said second lean NO x  trap comprising a catalyst, a NO x  absorbent material comprising alumina, and optionally, from 0 to about 4 wt % of an alkaline earth metal oxide.  
   
   
       17 . A method for desulphurization of a lean NO x  trap system in a HCCI engine comprising: 
 providing a lean NO x  trap system including a first lean NO x  trap comprising a catalyst, a NO x  absorbent material comprising alumina, and optionally, from 0 to about 4 wt % of an alkaline earth metal oxide; and a three-way catalyst positioned downstream from said first lean NO x  trap; said system being positioned in the exhaust passage of said HCCI engine such that it is exposed to exhaust gases therein; and    heating said exhaust gases to a temperature of between about 550 and 650° C. for at least 60 seconds by operating said engine in SI mode.    
   
   
       18 . The method of  claim 17  wherein said desulphurization of said lean NO x  trap system produces a sulfur product comprising at least 90% SO 2 .  
   
   
       19 . The method of  claim 17  wherein during said desulphurization, x is between 0.98 and 0.99.