Exhaust gas purifying catalyst and exhaust gas purifying system

Abstract

An exhaust gas purifying catalyst comprises a substrate, an inner catalytic layer coated on the substrate, an intermediate catalytic layer containing Pd laid over said inner catalytic layer, and an outer catalytic layer containing a NOx adsorption material laid over said intermediate catalytic layer. The Pd isolated from the NOx adsorption material is prevented from lowering its low temperature activity.

What is claimed is:  
     
         1 . An exhaust gas purifying catalyst comprising at least a metal selected from a group of alkaline metals and alkaline earth metals; and Pd, wherein said Pd is partly isolated from said metal so as to be neither electrically nor chemically affected by said metal.  
     
     
         2 . An exhaust gas purifying catalyst as defined in  claim 1 , comprising two different catalytic layers formed on a substrate so as to be laid over each other, said two different catalytic layers containing said Pd and said metal separately.  
     
     
         3 . An exhaust gas purifying catalyst as defined in  claim 2 , and further comprising a layer of zeolite laid under said two different catalytic layers, said layer of zeolite absorbing HC in an exhaust gas while said exhaust gas purifying catalyst remains low in temperature and releasing HC into said exhaust gas when said exhaust gas purifying catalyst falls in temperature.  
     
     
         4 . An exhaust gas purifying catalyst as defined in  claim 3 , wherein outer one of said two different catalytic layer contains said metal and inner one of said two different catalytic layer contains said Pd.  
     
     
         5 . An exhaust gas purifying catalyst as defined in  claim 1 , wherein exhaust gas purifying catalyst contains said metal more than 15 g per one liter of said substrate.  
     
     
         6 . An exhaust gas purifying system for purifying exhaust gases discharged into an exhaust line from an engine, said exhaust gas purifying system comprising: 
 a catalyst disposed in said exhaust line, said catalyst comprising at least a metal selected from a group of alkaline metals and alkaline earth metals, a NOx adsorption material operative to adsorb NOx in said exhaust gas while said exhaust gas has a comparatively high oxygen concentration and release said NOx adsorbed therein and adsorbs SOx as said exhaust gas lowers its oxygen concentration, and Pd; and    temperature control means for raising a temperature of said catalyst so as thereby to cause said NOx adsorption material to release said SOx adsorbed therein;    wherein said Pd is partly isolated from said NOx adsorption material so as to be neither electrically nor chemically affected by said NOx adsorption material.