Process and apparatus for purifying exhaust gases from an internal combustion engine

Abstract

The present invention relates to a process for purifying NO x -containing exhaust gases formed in an internal combustion engine. In particular, the invention is directed at the purification of exhaust gases from an engine which produces a lean-burn exhaust gas, e.g. a diesel engine or a spark-ignition engine operated under lean conditions. Furthermore, the present invention relates to an appropriately equipped exhaust gas purification apparatus. The apparatus proposes a sequence of different catalysts for purifying primary axhaust gas of lean-burn engines, in which an oxidatively active particle filter ( 1 ) is followed by an oxidation catalyst ( 2 ) and this is in turn followed by an SCR catalyst, ( 3 c ) with the latter being arranged downstream of an addition of reducing agent ( 3 a ) located downstream of the oxidation catalyst ( 2 ).

1 . A process for purifying NO x -containing exhaust gases from a combustion process:
 characterized in that   the exhaust gases are treated by means of the following apparatuses in the order indicated:   a) by means of an oxidatively active particle filter ( 1 ),   b) by means of an oxidation catalyst ( 2 ) and   c) after addition of a sufficient amount of a reducing agent from a reducing agent source ( 3   a ) via a metering facility ( 3   b ), by means of an SCR catalyst ( 3   c ), with the oxidation catalyst ( 2 ) or its coating comprising the metals platinum or palladium or mixtures thereof.   
     
     
         2 . The process as claimed in  claim 1 ,
 characterized in that   the oxidation catalyst ( 2 ) does not contain any rhodium.   
     
     
         3 . The process as claimed in  claim 1 ,
 characterized in that   the oxidation catalyst ( 2 ) does not perform an NOx storage function.   
     
     
         4 . The process as claimed in  claim 1 ,
 characterized in that   the combustion process takes place in a lean-burn engine.   
     
     
         5 . The process as claimed in  claim 1 ,
 characterized in that   the particle filter ( 1 ) is coated with an oxidatively active catalyst.   
     
     
         6 . The process as claimed in  claim 1 ,
 characterized in that   the particle filter ( 1 ) or its coating comprises the metals platinum, palladium or rhodium or mixtures thereof.   
     
     
         7 . The process as claimed in  claim 6 ,
 characterized in that   the particle filter ( 1 ) or its coating has a molar ratio of platinum to palladium in the range from 15:1 to 1:3.   
     
     
         8 . The process as claimed in  claim 1 ,
 characterized in that   the particle filter ( 1 ) is coated with a catalyst having a 3-way function.   
     
     
         9 . The process as claimed in  claim 1 ,
 characterized in that   the oxidation catalyst ( 2 ) or its coating has a molar ratio of platinum to palladium in the range from 15:1 to 1:3.   
     
     
         10 . The process as claimed in  claim 1 ,
 characterized in that   the particle filter ( 1 ) has an NO x  storage function.   
     
     
         11 . The process as claimed in  claim 10 ,
 characterized in that   the particle filter ( 1 ) contains oxides of cerium, barium or strontium or mixtures thereof as NO x  storage components.   
     
     
         12 . The process as claimed in  claim 1 ,
 characterized in that   the molar NO 2 /NO x  ratio upstream of the SCR catalyst ( 3   c ) is set to values of from about 0.3 to 0.7 by regulating the amount of oxygen present in the exhaust gas.   
     
     
         13 . The process as claimed in  claim 1 ,
 characterized in that   the oxidation catalyst ( 2 ) is heat treated.   
     
     
         14 . An exhaust gas purification apparatus comprising, in the following order moving away from the combustion process:
 a) an oxidatively active particle filter ( 1 )   b) an oxidation catalyst ( 2 ) and   c) an SCR stage ( 3 ) comprising
 ( 3   a ) an external reducing agent source 
 ( 3   b ) a metering apparatus 
 ( 3   c ) a reduction catalyst (SCR catalyst), 
   
       with the oxidation catalyst ( 2 ) or its coating comprising the metals platinum or palladium or mixtures thereof. 
     
     
         15 . The exhaust gas purification apparatus as claimed in  claim 14 ,
 characterized in that   the configuration of the apparatus corresponds to  claim 2 .