NOx abatement and N2O formation over urea-SCR systems with zeolite supported Fe and Cu catalysts in a nonroad diesel engine

Abstract

NO and NO2 emissions over urea-based selective catalytic reduction (SCR) systems were investigated in a nonroad diesel engine. For catalysts of the SCR systems, two types of metal ion-exchanged zeolites were used; one is Fe, the other Cu. The Fe/zeolite was composed of Fe only, however, the Cu/zeolite consisted of Cu (80%) and Fe (20%). Nitrogen oxides (NOx) conversion efficiency of the Fe/ and Cu/zeolite catalysts remained greater than 80% in the temperature range of approximately 300 and 450 °C. The Cu/zeolite catalyst retained high efficiency above approximately 450 °C, however, the Fe/zeolite exhibited a drastic decrease resulting from the deterioration of NO conversion efficiency due to the ammonia storage capability of Fe/zeolite, which was inferior to that of Cu/zeolite. It was also found that NO was oxidized to NO2 in the diesel oxidation catalyst and diesel particulate filter, and NO2/NOx ratio at the SCR inlet consistently maintained thermodynamic equilibrium irrespective of the SCR catalyst. The ratio was higher than 50% between 200 and 410 °C. N2O emission that forms in SCR systems through side reactions during NOx reduction was also studied because N2O is a greenhouse gas with approximately 300 times greater global warming potential than carbon dioxide. The ratio of N2O at the SCR outlet to NOx at the SCR inlet for both catalysts was higher between 200 and 410 °C than at other temperatures, which implies that N2O formation is considerably involved in the NO2/NOx ratio. The Cu/zeolite catalyst in the urea-SCR system exhibited higher NO and NO2 conversion efficiencies while maintaining a lower amount of N2O formation compared to the Fe/zeolite catalyst.