A Role of NO ₂ on Soot Oxidation in DPFs and Effect of Soot Cake Thickness in Catalyzed DPFs Using Temperature-Programmed Oxidation and Electron Microscopic Visualization

Development of the diesel particulate filter (DPF) aims to attain fast oxidation of accumulated soot at low temperature. Numerous researchers have explored the characteristics of soot oxidation under ambient conditions of simulated exhaust gas using thermogravimetric analysis or a flow reactor. In this study, temperature programmed oxidation (TPO) experiments were carried out for soot entrapped in miniaturized DPFs, cut-out from practical particulate filters, yielding wall-flow features typically encountered in real-world DPFs. Furthermore, when using the miniaturized samples, highly accurate lab-scale measurements and investigations can be facilitated. Examining different temperature ramping rates used for the TPO experiments, we propose a rate of 10°C/min as the most effective in analyzing soot oxidation in the practical filter substrates. Then, wash-coated catalyzed filters (CDPFs) were benchmarked with bare-type DPFs to clarify their effects on soot oxidation in a practical wall-flow system. According to the Arrhenius expression, differences in soot cake thickness in CDPFs reflect various values of estimated activation energy. This is due to the soot-catalyst proximity. With presence of 450 ppm nitrogen dioxide (NO₂) in a reactant gas mixture, the soot oxidation range was extent to a lower temperature. Moreover, a reduction in the estimated activation energy was achieved, even in the case of using bare-type DPFs. The thick soot cake layers in bare-type DPF result in a significant amount of soot mass remaining after treatment at 600°C, a typical active regeneration temperature. Subsequently, soot residuals were traced and characterized after a complete active regeneration process. For these reasons, thickness of a soot cake layer was proposed to be a new factor to define an updated regeneration strategy.