Exhaust Gas Purifying Apparatus

Abstract

Disclosed is an exhaust gas purifying apparatus, which includes an upstream catalyst ( 3 ) having a straight flow structure supported with a first NOx occluding material for occluding NOx in a low temperature range, and with a precious metal, a midstream catalyst ( 4 ) having a straight flow structure supported with a second NOx occluding material for efficiently occluding NOx in an intermediate temperature range, and with a precious metal, and a downstream catalyst ( 5 ) having a wall flow structure supported with a third NOx occluding material for efficiently occluding NOx in a high temperature range, and with a precious metal. Three types of catalyst corresponding to the temperature of the exhaust gas are capable of sufficiently occluding NOx, thus improving both NOx purification performance and PM purification performance.

1 . An exhaust gas purifying apparatus for purifying an exhaust gas from a diesel engine, comprising:
 an upstream catalyst, disposed upstream with respect to an exhaust gas flow direction, and having a straight flow structure supported with a first NOx occluding material for occluding NOx in a low temperature range, and with a precious metal;   a midstream catalyst, disposed downstream of the upstream catalyst with respect to the exhaust gas flow direction, and having a straight flow structure or a wall flow structure supported with a second NOx occluding material, which occludes more NOx than does the first NOx occluding material in an intermediate temperature range, which is higher than the low temperature range, and with a precious metal; and   a downstream catalyst, disposed downstream of the midstream catalyst with respect to the exhaust gas flow direction, and having a straight flow structure or a wall flow structure supported with a third NOx occluding material, which occludes more NOx than do the first NOx occluding material and the second NOx occluding material in a high temperature range, which is higher than the intermediate temperature range, and with a precious metal.   
     
     
         2 . The apparatus according to  claim 1 , wherein the first NOx occluding material comprises at least one selected from among Li and Mg. 
     
     
         3 . The apparatus according to  claim 1 , wherein the second NOx occluding material comprises at least one selected from among Ba, Ca, and Sr. 
     
     
         4 . The apparatus according to  claim 1 , wherein the third NOx occluding material comprises at least one selected from among K, Na, Cs, and Rb. 
     
     
         5 . The apparatus according to  claim 1 , wherein the upstream catalyst and the midstream catalyst have a straight flow structure, and the downstream catalyst has a wall flow structure.