Exhaust system for a diesel engine comprising a nox- trap

Abstract

An exhaust system for a diesel engine includes a NO x -trap having at least one first NO x absorbent and platinum. The exhaust system also has at least one second NO x absorbent disposed upstream of the NO x trap. The at least one second NO x absorbent is alumina, ceria, or both.

1 . An exhaust system for a diesel engine, which system comprises a NO x -trap comprising at least one first NO x  absorbent and platinum; and 
 at least one second NO x  absorbent consisting essentially of at least one of alumina and ceria disposed upstream of the NO x -trap.    
   
   
       2 . An exhaust system according to  claim 1 , wherein the first NO x -absorbent is selected from the group consisting of alkaline earth metal compounds, alkali metal compounds, rare earth metal compounds and mixtures of any two or more thereof.  
   
   
       3 . An exhaust system according to  claim 2 , wherein the or each alkaline earth metal compounds is selected from the group consisting of barium, magnesium, strontium and calcium.  
   
   
       4 . An exhaust system according to  claim 2 , wherein the or each alkali metal compounds is selected from the group consisting of potassium and caesium.  
   
   
       5 . An exhaust system according to  claim 2 , wherein the or each rare earth metal compounds is selected from the group consisting of cerium, yttrium, lanthanum and praseodymium.  
   
   
       6 . An exhaust system according to  claim 2 , wherein the or each alkaline earth metal compounds, the or each alkali metal compounds or the or each rare earth metal compounds is supported on a support material.  
   
   
       7 . An exhaust system according to  claim 6 , wherein the support is selected from the group consisting of alumina, silica, titania, zirconia, ceria and mixtures or a composite oxide of any two or more thereof.  
   
   
       8 . An exhaust system according to  claim 2 , wherein the first NO x  absorbent is a support for the platinum.  
   
   
       9 . An exhaust system according to  claim 1 , further comprising a catalyst for oxidising NO to NO 2  disposed between the at least one second NO x  absorbent and the NO x -trap.  
   
   
       10 . An exhaust system according to  claim 9 , wherein the NO oxidation catalyst is platinum on an alumina support.  
   
   
       11 . An exhaust system according to  claim 9 , further comprising a particulate filter disposed between the oxidation catalyst and the NO x -trap.  
   
   
       12 . An exhaust system according to  claim 1 , wherein the NO x -trap further comprises a particulate filter.  
   
   
       13 . An exhaust system according to claim,  1  further comprising a catalyst comprising a catalyst component for oxidising hydrocarbon and carbon monoxide to water and carbon dioxide and an oxygen storage component, wherein the catalyst is disposed downstream of the NO x -trap.  
   
   
       14 . An exhaust system according to  claim 13 , wherein the oxidation catalyst comprises platinum or palladium supported on a bulk ceria-zirconia mixed oxide oxygenation storage component.  
   
   
       15 . A diesel engine comprising an exhaust system according to  claim 1 .  
   
   
       16 . A light-duty diesel engine according to  claim 15 .  
   
   
       17 . An engine according to  claim 15 , comprising an engine control unit, when in use, intermittently to adjust the exhaust gas composition to the rich side for regenerating the at least one first NO x  absorbent.  
   
   
       18 . A flow-through substrate comprising: a NO x -trap comprising a first zone coated with a composition comprising at least one first NO x  absorbent and platinum, and a second zone coated with a composition comprising at least one second NO x -absorbent, which at least one second NO x  absorbent consists essentially of alumina and/or ceria.  
   
   
       19 . A method of treating NO x  in the exhaust gas of a diesel engine, which method comprising: 
 (i) absorbing NO x  from lean exhaust gas in at least one second NO x  absorbent consisting essentially of alumina and ceria, when a downstream NO x -trap comprising at least one first NO x  absorbent and platinum is inactive for reducing NO x ;    (ii) thermally desorbing NO x  from the at least one second NO x  absorbent; and    (iii) reducing NO x  on the downstream NO x -trap using a suitable reductant.    
   
   
       20 . A method according to  claim 19 , further comprising the step between steps (i) and (ii) of adsorbing NO x  on the at least one second NO x  absorbent of the downstream NO x -trap.