LEAN NOx TRAP/CONVERSION CATALYST

Abstract

A process and composition for converting nitrogen oxides present in exhaust gases produced by an internal combustion engine utilize cobalt and at least one alkali metal or alkaline earth metal as a component of an adsorbent. The process involves contacting the exhaust gas with an adsorbent which adsorbs the nitrogen oxides in lean-burn conditions and recurrently reducing the oxygen concentration of the exhaust gas. During such periods of reduced oxygen concentration, the nitrogen oxides are then desorbed and reduced to nitrogen, thereby reducing the concentration of the nitrogen oxides in the exhaust gas. The composition of the adsorbent comprises an oxide support and at least two components loaded on the support and containing cobalt and at least one alkali metal or alkaline earth metal. The composition demonstrates improved activity at higher temperatures and improved thermal stability.

1 . A composition which adsorbs nitrogen oxides present in exhaust gases produced by an engine during lean-burn conditions, said composition comprising:
 an oxide support; and   at least two components loaded on said support and comprising cobalt and at least one alkali metal or alkaline earth metal.   
   
   
       2 . The composition of  claim 1 , wherein said at least one alkali metal or alkaline earth metal comprises at least one of cesium and barium. 
   
   
       3 . The composition of  claim 1 , wherein said cobalt and at least one alkali metal or alkaline earth metal are a ternary mixture. 
   
   
       4 . The composition of  claim 3 , wherein said at least one alkali metal or alkaline earth metal is selected from the group consisting of potassium and cesium, potassium and barium, and cesium and barium. 
   
   
       5 . The composition of  claim 1 , wherein said at least two components further comprise a platinum group metal. 
   
   
       6 . A process for converting nitrogen oxides present in exhaust gas produced by an engine, said process comprising:
 contacting said exhaust gas containing nitrogen oxides with an adsorbent comprising:   an oxide support; and   at least two components loaded on said support and comprising cobalt and at least one alkali metal or alkaline earth metal, wherein said nitrogen oxides are adsorbed by said adsorbent in lean-burn conditions; and   recurrently reducing oxygen concentration of said exhaust gas to cause desorption of said nitrogen oxides from said adsorbent and reduction of said nitrogen oxides to nitrogen.   
   
   
       7 . The process of  claim 6 , wherein said at least one alkali metal or alkaline earth metal comprises potassium. 
   
   
       8 . The process of  claim 6 , wherein said at least two components further comprise a platinum group metal. 
   
   
       9 . An exhaust system for converting nitrogen oxides present in exhaust gas produced by an engine, said exhaust system comprising:
 an upstream catalyst capable of reducing nitrogen oxides; and   a downstream lean NOx trap composition comprising an oxide support and at least two components comprising cobalt and at least one alkali metal or alkaline earth metal, said components loaded on said oxide support.   
   
   
       10 . The exhaust system of  claim 9 , wherein said upstream catalyst is selected from the group consisting of a three way catalyst, a lean NOx catalyst, and an oxidation catalyst. 
   
   
       11 . The exhaust system of  claim 9 , wherein said at least one alkali or alkaline earth metal is selected from the group consisting of potassium, cesium, and barium. 
   
   
       12 . The exhaust system of  claim 9 , wherein said at least two components comprise said cobalt and said at least one alkali or alkaline earth metal in a mole ratio of between 5:1 to 1:5. 
   
   
       13 . The exhaust system of  claim 9 , wherein said oxide support comprises ceria. 
   
   
       14 . The exhaust system of  claim 9 , wherein said lean NOx trap composition further comprises a platinum group metal. 
   
   
       15 . The exhaust system of  claim 9 , wherein said lean NOx trap composition further comprises rhodium. 
   
   
       16 . The exhaust system of  claim 9 , wherein said oxide support consists of ceria. 
   
   
       17 . The exhaust system of  claim 9 , wherein said at least one alkali metal or alkaline earth metal comprises potassium. 
   
   
       18 . The exhaust system of  claim 17 , wherein said cobalt is in the form of cobalt oxide and said potassium is in the form of potassium oxide. 
   
   
       19 . The exhaust system of  claim 18 , wherein said cobalt oxide and said potassium oxide are in the form of simple oxides.