Method for the on-board reactivation of thermally aged nitrogen oxide storage catalysts in motor vehicles having an internal combustion engine operated predominantly under lean conditions

Abstract

Nitrogen oxide storage catalysts are used to remove the nitrogen oxides present in the lean exhaust gas of lean-burn engines. Storage catalysts are thermally aged by high temperatures. Ageing is due to sintering of the catalytically active noble metal components of the catalyst and to formation of compounds of the storage components with the support materials. According to the invention, the formation of compounds of the storage materials can be largely reversed by treatment of the storage material with a gas mixture containing carbon dioxide, optionally water vapor and optionally nitrogen oxides at temperatures in the range from 200° C. to 950° C., preferably from 300° C. to 700° C. The reactivation can be carried out under emission-neutral conditions directly in the vehicle during driving operation by setting of suitable exhaust gas conditions and regulating the air/fuel ratio.

1 . A method for the on-board reactivation of a thermally aged nitrogen oxide storage catalyst which comprises basic strontium or barium compounds or strontium and barium compounds on a support material containing cerium oxide and additionally contains strontium and/or barium compounds with the support material formed by thermal ageing and which is used for purification of exhaust gases of a motor vehicle having a lean-burn engine,
 said method comprising wherein the compounds of strontium and/or barium with the support material are decomposed during normal driving operation by treatment with a gas mixture which contains carbon dioxide, optionally water vapor and optionally nitrogen oxides and is generated by the engine of the vehicle, with the oxygen content of the gas mixture during the decomposition process not exceeding the air index λ=1.   
     
     
         2 . The method as claimed in  claim 1 , wherein the nitrogen oxide storage catalyst is a constituent of an exhaust gas purification unit on a vehicle having a lean-burn engine and the generation of the gas mixture required for reactivation of the catalyst is carried out in operating states of the lean-burn engine having exhaust gas temperatures in the range from 200° C. to 950° C. 
     
     
         3 . The method as claimed in  claim 2 , wherein a stoichiometric air/fuel ratio is set in the operating state of the lean-burn engine used for reactivation of the catalyst so that the exhaust gas used for reactivation has an air index of λ=1 and the duration of the reactivation is at least a few minutes. 
     
     
         4 . The method as claimed in  claim 2 , wherein a substoichiometric air/fuel ratio is set in the operating state of the lean-burn engine used for reactivation of the catalyst so that the exhaust gas used for reactivation has an air index of λ<1 and the duration of the reactivation is at least a few minutes. 
     
     
         5 . The method as claimed in  claim 3 , wherein the exhaust gas used for reactivation of the catalyst contains 5-20% of carbon dioxide, 5-15% by volume of water vapor, 0-5% by volume of nitrogen dioxide and essentially nitrogen as balance. 
     
     
         6 . The method as claimed in  claim 3 , wherein the exhaust gas purification unit contains at least one catalytic converter. 
     
     
         7 . The method as claimed in  claim 6 , wherein the exhaust gas purification unit contains at least one catalytic converter close to the engine and an underbody converter. 
     
     
         8 . The method as claimed in  claim 7 , wherein both converters contain at least one nitrogen oxide storage catalyst. 
     
     
         9 . The method as claimed in  claim 7 , wherein the converter close to the engine contains a three-way catalyst and the underbody converter contains a nitrogen oxide storage catalyst. 
     
     
         10 . The method as claimed in  claim 7 , wherein the converter close to the engine contains a diesel oxidation catalyst and the underbody converter contains a nitrogen oxide storage catalyst. 
     
     
         11 . The method as claimed in  claim 9 , wherein the temperature upstream of the underbody converter is from 300° C. to 850° C. 
     
     
         12 . The method as claimed in  claim 1 , wherein a reducing agent is introduced directly into the exhaust gas train upstream of the first nitrogen oxide storage catalyst present in the exhaust gas unit in order to adjust the air index of the gas mixture generated by the engine.