Method for producing catalytically active wall flow filters

Abstract

The present invention relates to a wall flow filter, to a method for the production and the use of the filter for reducing harmful exhaust gases of an internal combustion engine. Particle filters are commonly used for filtering exhaust gases from a combustion process. Also disclosed are novel filter substrates and their specific use in exhaust gas aftertreatment.

1 . A method for producing a coated ceramic wall flow filter having at least two catalytically active zones, the wall flow filter having a first end face, a second end face and a length L and a porosity of at least 50% to at most 80% and a mean pore diameter of 5 - 50 µm, the method comprising the following steps in sequence:
 i) an excess of a first coating suspension is introduced into the first end face by applying a pressure difference via the wall flow filter; 
 ii) with a pressure difference reversal, an excess of the first coating suspension is removed from the wall flow filter; 
 iii) following a pause period after ii), a second coating suspension without excess is introduced into the wall flow filter via the second end face by applying a pressure difference via the wall flow filter. 
 
     
     
         2 . Method according to  claim 1 , wherein in step i), the first coating suspension is introduced into the vertically locked wall flow filter from the lower, first end face into the wall flow filter, and in step iii) the second coating suspension is introduced from the upper, second end face into the wall flow filter. 
     
     
         3 . (canceled) 
     
     
         4 . Method according to  claim 1 , wherein the pressure pulse for the pressure difference reversal is at least 150 mbar and at most 400 mbar. 
     
     
         5 . Method according to  claim 1 , wherein a pressureless holding time before the pressure difference reversal of up to 10 seconds is maintained. 
     
     
         6 . Method according to  claim 1 , wherein one zone has a positive gradient for the amount of catalytically active material in the coating direction. 
     
     
         7 . Catalytically coated ceramic wall flow filter for the treatment of exhaust gases of a combustion process produced according to  claim 1 . 
     
     
         8 . Catalytically active wall flow filter according to  claim 7 , wherein the catalytically active coatings of the filter are selected from the group consisting of three-way catalyst, SCR catalyst, nitrogen oxide storage catalyst, oxidation catalyst, soot-ignition coating, hydrocarbon storage. 
     
     
         9 . Catalytically active wall flow filter according to  claim 8 , wherein the catalytically active coatings are located in the pores and/or on the surfaces of the channel walls of the filter. 
     
     
         10 . A method of oxidizing hydrocarbons and/or carbon monoxide and/or reducing nitrogen oxide by contacting an exhaust gas with the wall flow filter of  claim 7 . 
     
     
         11 . The method of  claim 1 , wherein the pause period includes drying the first coating suspension on the wall flow filter. 
     
     
         12 . The method of  claim 11 , wherein the pause period includes both drying and calcining the first coating suspension on the wall flow filter. 
     
     
         13 . The method of  claim 1 , wherein in step i) the first coating suspension travels up from the lower, first end face into the wall flow filter, and each of steps ii) and iii) include a suction draw at the lower end of the wall flow filter. 
     
     
         14 . The method of  claim 13 , wherein the suction draw level at step ii) is stronger than that at step iii). 
     
     
         15 . The method of  claim 14 , wherein the suction pulse time at step ii) is shorter in duration than that at step iii). 
     
     
         16 . The method of  claim 14 , wherein the first coating suspension travels up from the lower, first end face into the wall flow filter due to an upward directed pressure application at the lower end face of the wall flow filter. 
     
     
         17 . The method of  claim 1 , wherein the first coating suspension has a lesser solid content than the second coating suspension. 
     
     
         18 . The method of  claim 1 , wherein the first and second coating suspsensions have different particle sizes such that one of the two coating suspension is an in-wall coating while the other is an on-wall coating. 
     
     
         19 . The method of  claim 1 , wherein the first coating suspension is applied as to define a postivie gradiant that results in the first coating applied having less catallyticallly active material at the first face end of the wall flow filter as compared to downstream relative to a direction of first coating suspension application. 
     
     
         20 . The method of  claim 1 , wherein the wall flow filter remains in a common orientation for each of steps i), ii) and iii). 
     
     
         21 . The method of  claim 1 , wherein the pause includes a drying, or drying and calcining, treatment after ii) and before the second coating is introduced in iii). 
     
     
         22 . The method of  claim 21 , wherein, following step iii), the second coating suspension is subjected to another drying, or drying and calcining, step.