Oxidation-derived maturing process of soot, dependent on O2-NO2 mixtures and temperatures

Oxidizer- and temperature-dependent soot properties were investigated to better understand soot oxidation process. For this work, partially oxidized Printex-U samples as surrogate soot, under three different O-2-NO2 mixtures, were analyzed by using the high resolution-transmission electron microscope, Raman microscope and Fourier transform infrared spectroscopy-attenuated total reflectance. The results show that the maturing process was much dependent on NO2 content in the O-2-NO2 mixture: with no NO2 added, soot oxidized through the internal-burning out process, whereas with increased NO2 in the mixture, soot tended to oxidize through the external burning process. As the NO2 content increased, the preferential oxidation of less-ordered carbon crystallites decreased and as a result the maturing process was delayed. Internal burning-out process by O-2 only was also verified at various temperatures and with actual engine soot such as diesel soot and gasoline direct-injection (GDI) soot by TEM observations. Despite similar internal burning-out process, however, it was shown that oxidation at increased temperature resulted in relatively less-ordered soot, implying that soot maturing process was delayed with temperature. Since soot oxidation rate increased with temperature, the increase in oxidation temperature seems to diminish preference on short-ranged crystallites like more efficient O-2-NO2 cases. (C) 2015 Elsevier Ltd. All rights reserved.