Method for decreasing nitrogen oxides in hydrocarbon-containing exhaust gases using an scr catalyst based on a molecular sieve

Abstract

The invention relates to a process for treating diesel engine exhaust gases comprising nitrogen oxides (NO x ) and hydrocarbons (HC) by selective catalytic reduction of the nitrogen oxides with ammonia or a compound decomposable to ammonia as a reducing agent over an SCR catalyst based on a molecular sieve. The properties of the catalyst used are such that the hydrocarbons present in the exhaust gas are kept away from the catalytically active sites in the catalyst over which the reactions take place by the molecular sieve-like action of the zeolite present in the catalyst. This prevents HC-related degradation and aging effects of the SCR catalyst and achieves a considerable improvement in nitrogen oxide conversions in HC-containing exhaust gas.

1 . A process for treating diesel engine exhaust gases comprising nitrogen oxides (NO x ) and hydrocarbons (HC), comprising:
 a) the addition of ammonia (NH 3 ) as such or in the form of a compound which gives rise to ammonia under ambient conditions from a source which does not form part of the exhaust gas line to the exhaust gas stream comprising nitrogen oxides and hydrocarbons; and   b) the selective reaction of NO x  with the NH 3  added to the exhaust gas stream over an SCR catalyst comprising a zeolite exchanged with copper (Cu) and/or iron (Fe), characterized in that   the hydrocarbons present in the exhaust gas are kept away from the active sites in the catalyst over which the reactions take place by the molecular sieve-like action of the zeolite.   
     
     
         2 . The process as claimed in  claim 1 , wherein
 the zeolite present in the SCR catalyst is selected from the group consisting of ferrierite, chabazite and erionite.   
     
     
         3 . The process as claimed in  claim 2 , wherein
 the zeolite is ferrierite.   
     
     
         4 . The process as claimed in  claim 1 , wherein
 the hydrocarbon content in the exhaust gas before the reaction of NO x  over the SCR catalyst in step b) is at least 50 ppm.   
     
     
         5 . The process as claimed in  claim 4 , wherein
 the hydrocarbon content in the exhaust gas before the reaction of NO x  over the SCR catalyst in step b) has transient peak values of at least 300 ppm.   
     
     
         6 . The process as claimed in  claim 1 , wherein
 the hydrocarbon content in the exhaust gas is not significantly reduced after the reaction of NO x  over the SCR catalyst in step b) at an exhaust gas temperature at the inlet of the SCR catalyst of up to 180° C., said SCR catalyst comprising a zeolite exchanged with copper (Cu).   
     
     
         7 . The process as claimed in  claim 1 , wherein
 the hydrocarbon content in the exhaust gas is not significantly reduced after the reaction of NO x  over the SCR catalyst in step b) at an exhaust gas temperature at the inlet of the SCR catalyst of up to 280° C., said SCR catalyst comprising a zeolite exchanged with iron (Fe).