Exhaust-gas purification system for the selective catalytic reduction of nitrogen oxides in the lean exhaust gas of internal combustion engines and method of exhaust-gas purification

Abstract

The present invention relates to an exhaust-gas purification system for the selective catalytic reduction of nitrogen oxides. The system includes at least one catalyst having catalytically active components for the selective catalytic reduction (SCR components). An NOx storage catalyst ( 5 ) is arranged upstream of the SCR catalyst ( 3 ) in the exhaust-gas purification system. For performing the selective catalytic reduction, metering means ( 8 ) for supplying a compound decomposable into ammonia is provided between the NOx storage catalyst and the SCR catalyst ( 3 ). At low exhaust-gas temperatures, the NOx storage catalyst ( 5 ) adsorbs the nitrogen oxides contained in the exhaust gas and desorbs them only at rising exhaust-gas temperatures, so that they can afterwards be converted by the SCR catalyst ( 3 ) which is active then. This results in an altogether improved conversion rate for the nitrogen oxides.

1 . An exhaust-gas purification system for the selective catalytic reduction of nitrogen oxides, which includes at least one catalyst having catalytically active components for the selective catalytic reduction (SCR components) and through which the lean exhaust gas of an internal combustion engine flows, 
 wherein an NOx storage catalyst is arranged upstream of the SCR catalyst and metering means for supplying a precursor compound of ammonia to the exhaust gas is located between the NOx storage catalyst and the SCR catalyst.    
   
   
       2 . The exhaust-gas purification system according to  claim 1 , wherein the NOx storage catalyst is applied onto a diesel particulate filter.  
   
   
       3 . The exhaust-gas purification system according to  claim 2 , wherein the diesel particulate filter is a wall flow filter, a foamed ceramic filter, a ceramic fiber filter or a wire-mesh filter.  
   
   
       4 . The exhaust-gas purification system according to  claim 1 , wherein the NOx storage catalyst includes at least one compound of elements from the group consisting of alkali metals, alkaline-earth metals or rare earths and is activated with at least one of the platinum group metals platinum, palladium, rhodium or iridium.  
   
   
       5 . The exhaust-gas purification system according to  claim 4 , wherein the NOx storage catalyst additionally includes catalytically active components on the basis of support oxides from the group consisting of aluminum oxide, silicon dioxide, cerium oxide, zirconium oxide, titanium oxide or mixed oxides thereof which are coated with at least one of the platinum group metals platinum, palladium, rhodium and iridium.  
   
   
       6 . The exhaust-gas purification system according to  claim 5 , wherein for storing nitrogen oxides, the NOx storage catalyst includes storage components on the basis of cerium oxide, which are coated with platinum, and additionally an oxidizing catalyst on the basis of aluminum oxide, which is also coated with platinum.  
   
   
       7 . The exhaust-gas purification system according to  claim 1 , wherein the SCR components include a solid acid system of titanium dioxide and vanadium oxide.  
   
   
       8 . The exhaust-gas purification system according to  claim 7 , wherein the solid acid system includes at least one of the components from the group consisting of tungsten oxide, molybdenum oxide, silicon dioxide, sulfate and zeolites, wherein the zeolites may be present in the acid H-form or be exchanged with metal ions within their exchange capacity.  
   
   
       9 . The exhaust-gas purification system according to  claim 8 , wherein the SCR components include at least one zeolite, wherein the zeolites are present in the acid H-form or are exchanged with metal ions within their exchange capacity.  
   
   
       10 . The exhaust-gas purification system according to  claim 3 , wherein the diesel particulate filter, in addition to the NOx storage catalyst, also includes catalytically active components for lowering the ignition temperature of diesel soot.  
   
   
       11 . The exhaust-gas purification system according to  claim 1 , wherein an oxidizing catalyst for the oxidation of surplus ammonia is arranged downstream of the SCR catalyst.  
   
   
       12 . The exhaust-gas purification system according to  claim 11 , wherein a hydrolysis catalyst for the hydrolysis of the precursor compound decomposable into ammonia is inserted between the metering means for ammonia and the SCR catalyst.  
   
   
       13 . A method of removing nitrogen oxides from the lean exhaust gas of an internal combustion engine by selective catalytic reduction using ammonia, wherein the internal combustion engine is operated continuously with a lean air/fuel mixture and the resulting lean exhaust gas is routed first over a NOx storage catalyst and subsequently over an SCR catalyst for the selective catalytic reduction, wherein a compound decomposable into ammonia is supplied to the exhaust gas between the NOx storage catalyst and the SCR catalyst.  
   
   
       14 . The method according to  claim 13 , wherein the NOx storage catalyst is based on cerium oxide or a mixed oxide of cerium oxide and zirconium oxide.  
   
   
       15 . The method according to  claim 14 , wherein the exhaust system contains in addition a diesel particulate filter which is regenerated from time to time by increasing the exhaust gas temperature to the ignition temperature of the diesel soot collected on the filter and at the same time the NOx storage catalyst is automatically desulfated.  
   
   
       16 . The method according to  claim 15 , wherein the NOx storage catalyst is coated on a diesel particulate filter which is regenerated from time to time by increasing the exhaust gas temperature to the ignition temperature of the diesel soot collected on the filter and at the same time the NOx storage catalyst is automatically desulfated.