Combination of a Zeolite-Based SCR Catalyst with a Manganese-Based SCR Catalyst in the Bypass

Abstract

The invention relates to an exhaust-gas aftertreatment system for selective catalytic reduction with a plurality of SCR catalytic converters, which exhaust-gas aftertreatment system is able to reduce NOx in a large temperature range and can store SOx. The invention further relates to a method for treating an exhaust gas flow, in which method the exhaust-gas aftertreatment system according to the invention is used. The system comprises a high-temperature SCR catalyst for temperature ranges between 250° C. and 750° C. and a low-temperature SCR catalyst arranged downstream thereof for temperature ranges between 60° C. and less than 250° C. There is a reductant supply system directly upstream of the high-temperature SCR catalyst. The high-temperature SCR catalyst is designed to reduce NOx in exhaust gas that has a temperature above a temperature threshold value and to store SOx in the temperature range below the threshold value. The low-temperature SCR catalyst reduces NOx in the temperature range below the threshold value. In each case, the exhaust gas flows through the high-temperature SCR catalyst. An exhaust-gas bypass valve or flow control valve is arranged directly upstream of the low-temperature SCR catalyst. If the temperature of the exhaust gas is greater than or equal to the temperature threshold value, the exhaust gas is completely conducted past the low-temperature SCR catalyst. The high-temperature SCR catalyst advantageously contains a molecular sieve as a catalytically active layer, and the catalytically active layer of the low-temperature SCR catalyst is preferably a manganese-containing mixed oxide.

1 . An exhaust gas aftertreatment system coupled to an internal combustion engine in such a way that it receives the exhaust gas flow, comprising
 a selective catalytic reduction catalyst for medium to high temperature ranges (HT-SCR), wherein the medium temperature range comprises temperatures of 250° C. to less than 450° C. and the high temperature range comprises temperatures of 450° C. to 750° C., which catalyst is designed both
 to reduce NOx in an exhaust gas having a temperature above a temperature threshold value,
 and 
 
 to store the SOx contained in the exhaust gas across the temperature range of the exhaust gas, which is below a temperature threshold value, 
   a reductant supply system arranged directly upstream of the HT-SCR,   a low-temperature SCR (TT-SCR) arranged downstream of the HT-SCR, wherein the low temperature range comprises temperatures of 60° C. to less than 250° C., and wherein the TT-SCR is designed to reduce NOx in an exhaust gas having a temperature below a temperature threshold value,   a temperature sensor arranged directly downstream of the HT-SCR and measuring the temperature of the exhaust gas flow exiting such HT-SCR,   an exhaust gas bypass and/or flow control valve designed to conduct the exhaust gas flow in its entirety past the TT-SCR if such exhaust gas flow has a temperature greater than or equal to a temperature threshold value, wherein the exhaust gas bypass and/or flow control valve is arranged directly upstream of the TT-SCR.   
     
     
         2 . The exhaust gas aftertreatment system according to  claim 1 , wherein the HT-SCR is present in the form of a catalytically active layer on a carrier substrate, wherein the catalytically active layer is a molecular sieve selected from
 an aluminosilicate having a SAR of 5-50 and a silicon aluminum phosphate (SAPO) having an (Al+P)/Si value of 4-15 is selected,   wherein the molecular sieve contains 1-10% by weight of a transition metal selected from Fe, Cu, and mixtures thereof, calculated as Fe 2 O 3  and CuO respectively, based on the total weight of the molecular sieve,   and wherein the molecular sieve contains alkali metal and alkaline earth metal cations selected from Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, and mixtures thereof in a total amount of ≤1% by weight, calculated in the form of the pure metals and based on the total weight of the molecular sieve,   and wherein the molecular sieve contains the metals Co, Mn, Cr, Zr and Ni in a total amount of ≤1% by weight, calculated in the form of the pure metals and based on the total weight of the molecular sieve.   
     
     
         3 . The exhaust gas aftertreatment system according to  claim 2 , wherein the molecular sieve is selected from small-pore zeolites having a maximum pore size of eight tetrahedral atoms and beta zeolite. 
     
     
         4 . The exhaust gas aftertreatment system according to  claim 2 , wherein the molecular sieve is an Fe BEA having a SAR of 10 to 35, an alkali metal content of 0 to 0.7% by weight, calculated as pure metals, and an Fe content of 3 to 9% by weight, calculated as Fe 2 O 3 , wherein the alkali metal and Fe contents are each based on the total weight of the zeolite. 
     
     
         5 . The exhaust gas aftertreatment system according to  claim 2 , wherein the molecular sieve is a Cu CHA having a SAR of 10 to 35, an alkali metal content of 0 to 0.7% by weight, calculated as pure metals, and a Cu content of 1 to 7% by weight, calculated as CuO, wherein the alkali metal and Cu contents are each based on the total weight of the zeolite. 
     
     
         6 . The exhaust gas aftertreatment system according to  claim 1 , wherein the TT-SCR is a monolithic flow-through substrate with a manganese-containing coating as catalytically active layer. 
     
     
         7 . The exhaust gas aftertreatment system according to  claim 6 , wherein the manganese-containing coating is a manganese-containing mixed oxide selected from
 mixed oxides of the general formula Mn a Me 1-a O b , where Me is one or more elements from the group Fe, Co, Ni, Cu, Zr, Nb, Mo, W, Ag, Sn, Ce, Pr, La, Nd, Ti and Y and where a=0.02-0.98 and b=1.0-2.5,   mixed oxides of the general formula Mn w Ce x Me 1-w-x O y , where Me is one or more elements from the group Fe, Co, Ni, Cu, Zr, Nb, Mo, W, Ag, Sn, Pr, La, Nd, Ti and Y and where w=0.02-0.98, x=0.02-0.98 and y=1.0-2.5,   spinels of the general formula MnMe 2 O 4  or MeMn 2 O 4 , where Me is Fe, Al, Cr, Co, Cu or Ti.   
     
     
         8 . The exhaust gas aftertreatment system according to  claim 1 , and further comprising a second reductant supply system arranged directly upstream of the TT-SCR. 
     
     
         9 . The exhaust gas aftertreatment system according to  claim 1 , wherein the reductant supply system contains a) a reductant source, b) a reductant pump, and c) a reductant dispenser or a reductant injector. 
     
     
         10 . The exhaust gas aftertreatment system according to  claim 1 , wherein an oxidation catalyst is located directly upstream of the reductant supply system located directly upstream of the HT-SCR. 
     
     
         11 . The exhaust gas aftertreatment system according to  claim 9 , wherein a second HT-SCR is located between the HT-SCR and the temperature sensor. 
     
     
         12 . The exhaust gas aftertreatment system according to  claim 9 , wherein a catalytically coated diesel particulate filter (CDPF) is located between the oxidation catalyst and the reductant supply system. 
     
     
         13 . The exhaust gas aftertreatment system according to  claim 9 , wherein one or more HT-SCRs are arranged upstream of the oxidation catalyst and the reductant supply system is located directly upstream of the the HT-SCR that is closest to the internal combustion engine. 
     
     
         14 . The exhaust gas aftertreatment system according to  claim 1 , wherein an ammonia slip catalyst is arranged downstream of the TT-SCR. 
     
     
         15 . A method for treating an exhaust gas flow, comprising:
 providing an exhaust gas aftertreatment system comprising an SCR catalyst for medium to high temperatures (HT-SCR), a low-temperature SCR (TT-SCR) arranged downstream of the SCR for medium to high temperatures, a temperature sensor arranged directly downstream of the HT-SCR and an exhaust gas bypass and/or flow control valve arranged directly upstream of the low-temperature SCR,
 wherein the SCR catalyst for medium to high temperatures, wherein the medium temperature range comprises temperatures of 250° C. to less than 450° C. and the high temperature range comprising temperatures of 450° C. to 750° C., is designed both
 a) to reduce NOx in an exhaust gas having a temperature above a temperature threshold value,
 and 
 
 b) to store the SOx contained in the exhaust gas across the temperature range of the exhaust gas which is below a temperature threshold value, 
 
 and the low-temperature SCR is designed to reduce NOx in an exhaust gas having a temperature below a temperature threshold value, wherein the low temperature range comprises temperatures of 60° C. to less than 250° C., 
 and wherein the temperature sensor measures the temperature of the exhaust gas flow exiting the HT-SCR, 
 and wherein the exhaust gas bypass and/or flow control valve is designed to conduct the exhaust gas flow in its entirety past the low-temperature SCR, if such exhaust gas flow has a temperature greater than or equal to a temperature threshold value, 
   storing the SOx contained in the exhaust gas in the HT-SCR,   reducing NOx in an exhaust gas flow with the low-temperature SCR catalyst, if a temperature of the exhaust gas flow is within a low temperature range;   conducting the exhaust gas stream in its entirety past the low-temperature SCR, if such exhaust gas flow has a temperature greater than or equal to a temperature threshold value, and   reducing NOx in an exhaust gas flow with the SCR catalyst for normal to high temperatures, if the temperature of the exhaust gas flow is within a normal to high temperature range.