NOx reduction and N2O emissions in a diesel engine exhaust using Fe-zeolite and vanadium based SCR catalysts

Among various approaches used to comply with strict diesel engine exhaust regulations, there is increasing interest in urea based selective catalytic reduction (SCR) as a NOx reduction technology, due to its high reduction and excellent fuel efficiencies. NOx reduction by SCR catalysts is affected by variations in the NO2/NOx ratio, caused by oxidation catalysts such as the diesel oxidation catalyst (DOC) and diesel particulate filter (DPF) installed in diesel engines. Recently, it has been reported that the greenhouse gas (GHG) variant N2O, which is a by-product of the NOx conversion process in the after-treatment system, will be subject to regulation. Using a real diesel engine installed with DOC and DPF, the NOx reduction and N2O emission performances of commonly used Fe-zeolite and V2O5-WO3/TiO2 catalysts were investigated under various operating conditions. The exhaust of the diesel engine used in this study had a NO2/NOx ratio of over 50% for temperatures below 400 degrees C due to the oxidation catalysts, while the NO2/NOx ratio was significantly lower for temperatures above 400 degrees C. Under such conditions, it was found that the Fe-zeolite and V2O5-WO3/TiO2 catalysts were noticeably affected by the NO2/NOx ratio and exhaust temperature. Although both catalysts showed satisfactory NO conversions, the V2O5-WO3/TiO2 catalyst showed decreasing NO2 conversion rates between 250 degrees C and 320 degrees C. The V2O5-WO3/TiO2 catalyst exhibited NH3 slip relatively frequently because of its low NH3 storage capacity. For the Fe-zeolite catalyst, a significant increase in the amount of generated N2O was observed for high NOx conversion conditions due to side reactions with ammonia, NO2, and oxygen, while that phenomenon was not observed for the V2O5-WO3/TiO2 catalyst. (C) 2016 Elsevier Ltd. All rights reserved.