Method for coating a catalysed particulate filter and a particulate filter

Abstract

Method for the preparation of a wall flow particulate filter catalysed at its inlet side with a first catalyst having activity in the removal of residual hydrocarbons and carbon monoxide and catalysing at rich burn engine operation conditions the reaction of nitrogen oxides with hydrogen and/or carbon monoxide to ammonia and catalysed at its outlet side with a second catalyst having activity in the selective reduction of NOx by reaction with ammonia being formed in the inlet side. The method involves the provision of a first catalyst having a particle size smaller than the filter wall mean pore size, and a second catalyst having a particle size larger than the filter wall mean pore size, and mixing the first and second catalyst into one suspension, which is used for wash-coating from the outlet end. The first catalyst thereby diffuses into the partition wall.

1 . Method of preparation a catalysed wall flow filter, comprising the steps of
 a) providing a wall flow filter body with a plurality longitudinal inlet flow channels and outlet flow channels separated by gas permeable porous partition walls;   b) providing a catalyst washcoat comprising a first catalyst composition being active in reaction of nitrogen oxides with carbon monoxide and hydrogen to ammonia and a second catalyst composition being active in selective reduction of nitrogen oxides by reaction with ammonia to nitrogen, the first catalyst composition has a mode particle size being smaller than average pore diameter of the porous partition walls and the second catalyst composition has a mode particle size being larger than the average pore diameter of the porous partition walls;   c) coating the filter body with the catalyst washcoat by introduction of the washcoat into outlet end of the outlet channels; and   d) drying and heat treating the coated filter body to obtain the catalysed particulate filter.   
     
     
         2 . The method of  claim 1 , wherein the catalyst being active in conversion of nitrogen oxides to ammonia includes palladium, platinum, a mixture of palladium and rhodium and a mixture of palladium, platinum and rhodium. 
     
     
         3 . The method of  claim 1 , wherein the catalyst being active in conversion of nitrogen oxides to ammonia consists of palladium. 
     
     
         4 . The method according to  claim 1 , wherein the catalyst being active in the selective reduction of nitrogen oxides comprises at least one of a zeolite, a silica aluminum phosphate, an ion exchanged zeolite, silica aluminum phosphate promoted with iron and/or copper, one or more base metal oxides. 
     
     
         5 . The method according to  claim 1 , further comprising the steps of
 providing a second washcoat containing a catalyst composition being active in the oxidation of ammonia; and   coating a part of outlet channels at region at the outlet end with the second washcoat.   
     
     
         6 . A catalysed wall flow filter being prepared in accordance with  claim 1 .