PM and NOx reduction characteristics of LNT/DPF+SCR/DPF hybrid system

The purpose of study is to investigate effects and differences when applying a hybrid system of LNT/DPF+SCR/DPF catalysts and LNT/DPF in diesel engine. A lean NOx trap (LNT) catalyst was prepared using an impregnation method, and a selective catalytic reduction (SCR) catalyst was prepared through an ion-exchange method. Each catalyst was dip-coated in an advanced cordierite-diesel particulate filter (AC-DPF). Its de-NOx and de-PM characteristics were analyzed using a model gas test-rig. As a result, NOx conversion of the hybrid system was higher than that of single LNT/DPF system. Because of NO2 and NH3, which is formed in a LNT/DPF in the rich air-fuel ratio duration is used a reductant for SCR/DPF catalyst of the hybrid system. Along with SCR/DPF, an increase in NOx conversion occurs through HC-SCR reaction because fuel as a dosing reductant is used not only for LNT/DPF but also for SCR/DPF. For de-PM characteristics, the light-off temperature 50, which is 50% of the oxidizing temperature of the PM, is 598 °C in the hybrid system and 627 °C in LNT/DPF. Moreover, the rate of PM oxidation over the hybrid system is higher than that of LNT/DPF because the oxidizing power of SCR/DPF was additionally added into the hybrid system.