NOx and N2O emissions over a Urea-SCR system containing both V2O5-WO3/TiO2 and Cu-zeolite catalysts in a diesel engine

Abstract To develop a novel selective catalytic reduction (SCR) system, V 2 O 5 -WO 3 /TiO 2 and Cu-zeolite catalysts were combined within one canister with the former upstream and the latter downstream. The nitrogen oxides (NOx) and N 2 O emissions for the combined catalyst were investigated under operating conditions of engine loads and speeds in a 3.4 L diesel engine. The results from the combined catalyst were compared with those from the vanadium-based and Cu-zeolite catalysts alone. The NO conversion efficiencies for all the catalysts were higher than approximately 80% at temperatures ranging from 200 to 410 °C, which corresponds to the NO 2 excess range. In other words, the NO 2 /NOx ratio was higher than 50% for the temperature range between 200 and 410 °C, although the maximum NO 2 /NOx ratio was obtained at 270 °C. Beyond 410 °C, the efficiency of the Cu-zeolite catalyst remained high; however, the vanadium-based catalyst started to deteriorate at 410 °C. The deterioration of the combined catalyst was not much larger than that of the vanadium-based catalyst owing to the superior capability of the Cu-zeolite catalyst. For the NO 2 conversion efficiency, the combined catalyst mostly followed the Cu-zeolite catalyst. The vanadium-based catalyst exhibited an efficiency drop at approximately 270 °C, which is the temperature of the highest NO 2 /NOx ratio; however, this efficiency drop was avoided in the combined catalyst. For N 2 O produced over the catalysts, the Cu-zeolite catalyst produced the most N 2 O, while the vanadium-based catalyst produced the least. Because the amount of N 2 O produced behind the SCR was at most five times that in front of the SCR, efforts to inhibit N 2 O production from SCR are required when considering the global warming potential of N 2 O. The combined catalyst produced intermediate levels of N 2 O.