Apparatus and method for purifying exhaust gas of internal combustion engine

Abstract

In an exhaust gas purifying apparatus provided with a lean NOx catalyst supporting a catalyst layer, which contains a NOx trapping component, on a honeycomb substrate formed so as not to cause an alkali attack, the invention prevents trapped NOx from being dissociated and exhausted during the time of a rich spike. A NOx reducing catalyst with the function of reducing NOx by a reductant in a rich or stoichiometric condition, e.g., a three-way catalyst, is disposed downstream of the lean NOx catalyst. In the case of increasing the amount of the NOx trapping component in the lean NOx catalyst to enhance a NOx trapping capability, even if a part of trapped NOx is dissociated during the time of the rich spike, the dissociated NOx can be reduced by the NOx reducing catalyst disposed on the downstream side.

1 . An exhaust gas purifying apparatus for an internal combustion engine, comprising: 
 a lean NOx catalyst disposed in an exhaust passage of said internal combustion engine, having the function of trapping NOx in a lean atmosphere under coexistence of oxygen, and including a catalyst layer that has the function of reducing the trapped NOx by a reductant in a rich or stoichiometric atmosphere and is supported on a honeycomb substrate having a low affinity with components of said catalyst layer; and    a NOx reducing catalyst disposed downstream of said lean NOx catalyst and having the function of reducing NOx by a reductant in the rich or stoichiometric atmosphere,    wherein said NOx reducing catalyst reduces NOx that is dissociated from said lean NOx catalyst when a condition of exhaust gas in said exhaust passage is changed over from a lean condition to a rich or stoichiometric condition.    
   
   
       2 . An exhaust gas purifying apparatus for an internal combustion engine according to  claim 1 , wherein said lean NOx catalyst contains a NOx trapping component and a NOx reducing component in said catalyst layer.  
   
   
       3 . An exhaust gas purifying apparatus for an internal combustion engine according to  claim 1 , wherein said lean NOx catalyst is a three-way catalyst.  
   
   
       4 . An exhaust gas purifying apparatus for an internal combustion engine according to  claim 1 , wherein said lean NOx catalyst contains, as said NOx trapping component, at least one element selected from among alkali metals and alkaline-earth metals, and said honeycomb substrate contains no Si as a component thereof.  
   
   
       5 . An exhaust gas purifying apparatus for an internal combustion engine according to  claim 4 , wherein the NOx trapping component in said lean NOx catalyst contains at least one element selected from among Li, Na, K, Cs, Sr and Ba, and an amount of the NOx trapping component supported on said honeycomb substrate is not smaller than 20 g per bull volume of said honeycomb substrate.  
   
   
       6 . An exhaust gas purifying apparatus for an internal combustion engine according to  claim 4 , wherein said lean NOx catalyst contains a noble metal as the NOx reducing component.  
   
   
       7 . An exhaust gas purifying apparatus for an internal combustion engine according to  claim 1 , wherein said lean NOx catalyst and said NOx reducing catalyst are set to have a relative relationship therebetween such that a linear speed of the exhaust gas flowing through said NOx reducing catalyst is comparable to or higher than a linear speed of the exhaust gas flowing through said lean NOx catalyst.  
   
   
       8 . An exhaust gas purifying apparatus for an internal combustion engine according to  claim 7 , wherein said NOx reducing catalyst is constituted by a honeycomb substrate having a catalyst layer supported thereon, and said honeycomb substrate of said NOx reducing catalyst has a smaller porosity than said honeycomb substrate of said lean NOx catalyst.  
   
   
       9 . An exhaust gas purifying apparatus for an internal combustion engine according to  claim 1 , further comprising a three-way catalyst disposed in said exhaust passage upstream of said lean NOx catalyst.  
   
   
       10 . An exhaust gas purifying apparatus for an internal combustion engine, comprising an exhaust gas purifying catalyst and a filter trapping and removing particulates which are disposed in an exhaust passage of said internal combustion engine, 
 wherein said exhaust gas purifying catalyst comprises a lean NOx catalyst having the function of trapping NOx in a lean atmosphere under coexistence of oxygen and including a catalyst layer that has the function of reducing the trapped NOx by a reductant in a rich or stoichiometric atmosphere and is supported on a honeycomb substrate having a low affinity with components of said catalyst layer, and a NOx reducing catalyst having the function of reducing NOx by a reductant in the rich or stoichiometric atmosphere;    said lean NOx catalyst is disposed upstream or downstream of said filter, and said NOx reducing catalyst is disposed downstream of said lean NOx catalyst and said filter; and    said NOx reducing catalyst reduces NOx that is dissociated from said lean NOx catalyst when a condition of exhaust gas in said exhaust passage is changed over from the lean atmosphere to the rich or stoichiometric atmosphere.    
   
   
       11 . An exhaust gas purifying apparatus for an internal combustion engine according to  claim 10 , wherein said lean NOx catalyst is disposed downstream of said filter, and said NOx reducing catalyst is disposed downstream of said lean NOx catalyst.  
   
   
       12 . An exhaust gas purifying apparatus for an internal combustion engine according to  claim 10 , wherein said lean NOx catalyst is disposed upstream of said filter, and said NOx reducing catalyst is disposed downstream of said filter.  
   
   
       13 . An exhaust gas purifying apparatus for an internal combustion engine according to  claim 10 , wherein said lean NOx catalyst contains, as said NOx trapping component, at least one element selected from among alkali metals and alkaline-earth metals, and said honeycomb substrate contains no Si as a component thereof.  
   
   
       14 . An exhaust gas purifying apparatus for an internal combustion engine according to  claim 13 , wherein said lean NOx catalyst contains a noble metal as the NOx reducing component.  
   
   
       15 . An exhaust gas purifying apparatus for an internal combustion engine according to  claim 10 , wherein said NOx reducing catalyst is a three-way.  
   
   
       16 . An exhaust gas purifying apparatus for an internal combustion engine according to  claim 10 , wherein said lean NOx catalyst and said NOx reducing catalyst are set to have a relative relationship therebetween such that a linear speed of the exhaust gas flowing through said NOx reducing catalyst is comparable to or higher than a linear speed of the exhaust gas flowing through said lean NOx catalyst.  
   
   
       17 . An exhaust gas purifying apparatus for an internal combustion engine according to  claim 16 , wherein said NOx reducing catalyst is constituted by a honeycomb substrate having a catalyst layer supported thereon, and said honeycomb substrate of said NOx reducing catalyst has a smaller porosity than said honeycomb substrate of said lean NOx catalyst.  
   
   
       18 . An exhaust gas purifying method for use in an internal combustion engine to purify exhaust gas exhausted from said internal combustion engine by an exhaust gas purifying catalyst disposed in an exhaust passage, the method comprising the steps of: 
 trapping NOx contained in the exhaust gas from said internal combustion engine, which is operated in a lean atmosphere under coexistence of oxygen, and removing the NOx from the exhaust gas by a lean NOx catalyst having the function of trapping the NOx in a lean atmosphere and including a catalyst layer that has the function of reducing the trapped NOx by a reductant in a rich or stoichiometric atmosphere and is supported on a honeycomb substrate having a low affinity with components of said catalyst layer; and    temporarily changing over an operation mode of said internal combustion engine to operation in the rich or stoichiometric atmosphere, and reducing NOx dissociated from said lean NOx catalyst with the changeover of the operation mode by a NOx reducing catalyst having the function of reducing NOx by a reductant in the rich or stoichiometric atmosphere.    
   
   
       19 . An exhaust gas purifying method for an internal combustion engine according to  claim 18 , wherein a linear speed of the exhaust gas flowing through said NOx reducing catalyst is comparable to or higher than a linear speed of the exhaust gas flowing through said lean NOx catalyst.