Exhaust gas purifying system and catalyst

Abstract

An exhaust gas purifying system for an automotive internal combustion engine which discharges oxygen-excessive exhaust gas under lean-burn operation. The exhaust gas purifying system comprises a NOx reducing catalyst for reducing NOx in the presence of at least hydrogen serving as reducing agent. The NOx reducing catalyst is disposed in an exhaust gas passageway of the engine. The NOx reducing catalyst includes a porous substrate, and platinum and cesium carried on the porous substrate. A catalyst is disposed in the exhaust gas passageway in order to produce hydrogen so as to enrich the oxygen-excessive exhaust gas with hydrogen before the oxygen-excessive exhaust gas reaches the NOx reducing catalyst. The oxygen-excessive exhaust gas enriched with hydrogen is introduced through the exhaust gas passageway to the NOx reducing catalyst. Here, the oxygen-excessive exhaust gas enriched with hydrogen is brought into contact with the NOx reducing catalyst under a condition in which temperature of at least one of the NOx reducing catalyst and the oxygen-excessive exhaust gas enriched with hydrogen is within a range of from 250 to 600° C.

What is claimed is:  
     
         1 . An exhaust gas purifying system comprising: 
 a NOx reducing catalyst for reducing NOx in presence of at least hydrogen serving as reducing agent, said NOx reducing catalyst being disposed in an exhaust gas passageway of a combustion device, said NOx reducing catalyst including a porous substrate, and platinum and cesium carried on said porous i substrate; and    a device for enriching oxygen-excessive exhaust gas with hydrogen before the oxygen-excessive exhaust gas reaches said NOx reducing catalyst, the oxygen-excessive exhaust gas enriched with hydrogen being introduced through the exhaust gas passageway to said NOx reducing catalyst;    wherein the oxygen-excessive exhaust gas enriched with hydrogen is brought into contact with said NOx reducing catalyst under a condition in which temperature of at least one of said NOx reducing catalyst and the oxygen-excessive exhaust gas enriched with hydrogen is within a range of from 250 to 600° C.    
     
     
         2 . An exhaust gas purifying system as claimed in    claim 1   , wherein said enriching device includes a device for reforming HC and CO, arranged to produce partial oxide of HC and to reform CO in the oxygen-excessive exhaust gas.  
     
     
         3 . An exhaust gas purifying system as claimed in    claim 2   , wherein said HC and CO reforming device includes a device for controlling temperature and pressure of the oxygen-excessive exhaust gas and concentration of oxygen, water, HC and CO in the oxygen-excessive exhaust gas.  
     
     
         4 . An exhaust gas purifying system as claimed in    claim 2   , wherein said HCO and CO reforming device includes a catalyst disposed in the exhaust gas passageway, said catalyst being arranged to produce hydrogen from at least one of HC and CO in the oxygen-excessive exhaust gas.  
     
     
         5 . An exhaust gas purifying system as claimed in    claim 1   , further comprising a device for controlling the oxygen-excessive exhaust gas enriched with hydrogen in a manner that composition of the oxygen-excessive exhaust gas enriched with hydrogen, to be brought into contact with the NOx reducing catalyst, meets a relation represented by a formula ((a concentration of hydrogen/a concentration of total reducing components in the oxygen-excessive exhaust gas)≦0.81.  
     
     
         6 . An exhaust gas purifying system comprising: 
 a NOx reducing catalyst for reducing NOx in presence of at least hydrogen serving as reducing agent, said NOx reducing catalyst being disposed in an exhaust gas passageway of a combustion device, said NOx reducing catalyst including a porous substrate, and platinum and cesium carried on said porous substrate;    means for enriching oxygen-excessive exhaust gas with hydrogen before the oxygen-excessive exhaust gas reaches said NOx reducing catalyst, the oxygen-excessive exhaust gas enriched with hydrogen being introduced to said NOx reducing catalyst; and    means for causing the oxygen-excessive exhaust gas enriched with hydrogen to be brought into contact with said NOx reducing catalyst under a condition in which temperature of at least one of said NOx reducing catalyst and the oxygen-excessive exhaust gas enriched with hydrogen is within a range of from 250 to 600° C.    
     
     
         7 . An exhaust gas purifying system for an internal combustion engine, comprising: 
 a NOx reducing catalyst for reducing NOx in presence of at least hydrogen serving as reducing agent, said NOx reducing catalyst being disposed in an exhaust gas passageway of the engine, said NOx reducing catalyst including a porous substrate, and platinum-and cesium carried on said porous substrate; and    at least one of a control unit for controlling engine operating parameters of the engine and a catalyst disposed in the exhaust gas passageway upstream of said NOx reducing catalyst, said control unit and catalyst being arranged to enrich oxygen-excessive exhaust gas with hydrogen before the oxygen-excessive exhaust gas reaches said NOx reducing catalyst, the oxygen-excessive exhaust gas enriched with hydrogen being introduced through the exhaust gas passageway to said NOx reducing catalyst;    wherein the oxygen-excessive exhaust gas enriched with hydrogen is brought into contact with said NOx reducing catalyst under a condition in which temperature of at least one of said NOx reducing catalyst and the oxygen-excessive exhaust gas enriched with hydrogen is within a range of from 250 to 600° C.    
     
     
         8 . A catalyst for reducing NOx in an exhaust gas purifying system including a device for enriching oxygen-excessive exhaust gas with hydrogen, the oxygen-excessive exhaust gas flowing through an exhaust gas passageway of a combustion device, 
 said NOx reducing catalyst functioning in the presence of at least hydrogen, serving as reducing agent, which is in the oxygen-excessive exhaust gas enriched with hydrogen, said NOx reducing catalyst being disposed in the exhaust gas passageway and including a porous substrate, and platinum and cesium carried on said porous substrate in a mol ratio of cesium/platinum≦0.5/1,    wherein the oxygen-excessive exhaust gas enriched with hydrogen is brought into contact with said NOx reducing catalyst under a condition in which temperature of at least one of said NOx reducing catalyst and the oxygen-excessive exhaust gas enriched with hydrogen is within a range of from 250 to 600° C.    
     
     
         9 . A process of purifying exhaust gas, comprising: 
 providing a NOx reducing catalyst for reducing NOx in presence of at least hydrogen serving as reducing agent, said NOx reducing catalyst being disposed in an exhaust gas passageway of a combustion device, said NOx reducing catalyst including a porous substrate, and platinum and cesium carried on said porous substrate;    enriching oxygen-excessive exhaust gas with hydrogen before the oxygen-excessive exhaust gas reaches said NOx reducing catalyst, the oxygen-excessive exhaust gas enriched with hydrogen being introduced through the exhaust gas passageway to said NOx reducing catalyst; and    causing the oxygen-excessive exhaust gas enriched with hydrogen to be brought into contact with said NOx reducing catalyst under a condition in which temperature of at least one of said NOx reducing catalyst and the oxygen-excessive exhaust gas enriched with hydrogen is within a range of from 260 to 600° C.