Alumina-based lean NOx trap system and method of use

Abstract

An alumina-based lean NO x trap system for use in a lean burn engine such as a diesel engine 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. The system preferably includes at least a first alumina-based lean NO x trap, and a second lean NO x trap which is positioned downstream from the first lean NO x trap in an engine exhaust. The second lean NO x trap may comprise an alumina-based trap or a conventional lean NO x trap. The lean NO x trap system 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 500° C. The alumina-based lean NO x trap system also undergoes efficient desulphurization and maintains its activity with extended use.

1 . A lean NO x  trap system for use in a lean burn engine exhaust comprising: 
 a first lean NO x  trap comprising a catalyst, a NO x  absorbent material comprising alumina, and optionally, an alkaline earth metal oxide; and    a second lean NO x  trap; wherein said system 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 500° C.    
   
   
       2 . The system of  claim 1  wherein said first lean NO x  trap comprises from 0 to about 4.0 wt % of said alkaline earth metal oxide.  
   
   
       3 . The system of  claim 1  wherein said second lean NO x  trap comprises a catalyst, a NO x  absorbent material comprising alumina, and optionally, from 0 to about 4 wt % an alkaline earth metal oxide.  
   
   
       4 . The system of  claim 1  wherein said second lean NO x  trap comprises a catalyst and from about 20 to 35 wt % of a metal oxide on an alumina or silica support.  
   
   
       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  having a conversion efficiency of at least 50% at a temperature between about 150° C. to about 500° C.  
   
   
       7 . The system of  claim 1  wherein said first 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 alkaline earth metal is selected from Mg, Sr, Ba, and Ca.  
   
   
       9 . The system of  claim 1  further including a third 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.  
   
   
       10 . A lean NO x  trap system for use in a lean burn engine exhaust comprising: 
 a first lean NO x  trap comprising a catalyst, a NO x  absorbent material comprising alumina, and from 0 to about 4.0 wt % of an alkaline earth metal oxide; and    a second lean NO x  trap comprising a catalyst, a NO x  absorbent material comprising alumina, and from 0 to about 4.0 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 first lean NO x  trap.  
   
   
       12 . The system of  claim 10  further including a diesel particulate filter.  
   
   
       13 . The system of  claim 10  further including a third lean NO x  trap comprising a catalyst, a NO x  absorbent material comprising alumina, and from 0 to about 4.0 wt % of an alkaline earth metal oxide.  
   
   
       14 . The system of  claim 13  wherein said third lean NO x  trap is positioned downstream from said second lean NO x  trap.  
   
   
       15 . A method for treating lean burn engine exhaust gases comprising: 
 providing a lean NO x  trap system in an exhaust gas passage of a lean burn engine, said system comprising at least first and second lean NO x  traps; wherein said first lean NO x  trap comprises a catalyst, a NO x  absorbent material comprising alumina, and optionally, from 0 to about 4 wt % of an alkaline earth metal oxide; and    exposing said lean NO x  trap 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  at a temperature between about 150° C. to 500° C.    
   
   
       16 . The method of  claim 15  wherein said system provides a conversion efficiency of at least 50% at a temperature between about 150° C. to about 500° C.  
   
   
       17 . The method of  claim 16  wherein said engine exhaust gas is subjected to a fixed lean time/rich time ratio (x/y=R), and wherein the NO x  conversion efficiency is increased by reducing x and y at a temperature of about 150 to 200° C.  
   
   
       18 . The method of  claim 17  including sensing the concentration of NO x  in said exhaust gas and adjusting the lean/rich cycle of said engine to increase said NO x  conversion efficiency.  
   
   
       19 . The method of  claim 15  wherein said NO x  in said first lean NO x  trap is converted at a temperature of between about 150° C. to about 350° C.  
   
   
       20 . The method of  claim 15  wherein said NO x  in said second lean NO x  trap is converted at a temperature of between about 150° C. to about 350° C.  
   
   
       21 . The method of  claim 15  wherein said second lean NO x  trap is positioned downstream from said first lean NO x  trap.  
   
   
       22 . The method of  claim 15  wherein said first lean NO x  trap is positioned downstream from said second lean NO x  trap.  
   
   
       23 . The method of  claim 15  wherein said first lean NO x  trap and said second lean NO x  trap are combined.  
   
   
       24 . The method of  claim 15  wherein said second lean NO x  trap comprises a catalyst, a NO x  absorbent material comprising alumina, and optionally, from 0 to about 4 wt % of an alkaline earth metal oxide.  
   
   
       25 . The method of  claim 15  wherein said second lean NO x  trap comprises a catalyst and from about 20 to 35 wt % of a metal oxide on an alumina or silica support.  
   
   
       26 . A method for desulphurization of a lean NO x  trap system in a lean burn 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, an alkaline earth metal oxide; and a second lean NO x  trap positioned downstream from said first lean NO x  trap; said system being positioned in the exhaust passage of said engine such that it is exposed to exhaust gases therein; and    heating said exhaust gases in said first lean NO x  trap to a temperature of between about 550° C. and about 650° C. for at least 40 seconds.    
   
   
       27 . The method of  claim 26  wherein said first lean NO x  trap comprises from 0 to about 4 wt % of said alkaline earth metal oxide.  
   
   
       28 . The method of  claim 26  wherein said second lean NO x  trap comprises a catalyst, a NO x  absorbent material comprising alumina, and optionally, from 0 to 4 wt % of an alkaline earth metal oxide.  
   
   
       29 . The method of  claim 26  wherein said second lean NO x  trap comprises a catalyst and from about 20 to 35 wt % of a metal oxide on an alumina or silica support.  
   
   
       30 . The method of  claim 26  wherein said system further includes a bypass exhaust line for directing exhaust gas through said second lean NO x  trap during normal vehicle operation but which diverts exhaust gas around said second lean NO x  trap during desulphurization of said first lean NO x  trap.  
   
   
       31 . The method of  claim 26  wherein during desulphurization, λ is between about 0.98 to about 0.99.  
   
   
       32 . The method of  claim 26  wherein said exhaust gases are heated for about 40 to 120 seconds.  
   
   
       33 . The method of  claim 26  wherein said desulphurization of said lean NO x  trap system produces a sulfur product comprising at least 90% SO 2 .