Effects of molecular O2 and NO2 on particle size distribution, morphology and nanostructure of diffusion flame soot oxidized in a flow reactor

The oxidation process of soot particles in O2 and NO2 atmospheres is investigated in a flow reactor at various temperatures. The particles are generated using an ethylene diffusion flame and sampled at the tip of the flame using an ejector diluter sampling system. Soot is oxidized by three kinds of diluted gas (pure N2, 10000 ppm O2 with N2 and 10000 ppm NO2 with N2) in the flow reactor and measured with DMS500, transmission electron microscopy (TEM), high-resolution TEM, Raman microscopy and thermogravimetric analysis (TGA). The results show that the particle size, primary particle diameter, agglomeration degree and concentration of soot particles decrease as the oxidation temperature increases. The overall oxidation capacity of NO2 is stronger than that of O2 at low temperatures (200 and 400 °C), whereas at high temperatures (600 and 800 °C) O2 is stronger. Both internal oxidation and external oxidation exist during the oxidation process of soot with O2 and NO2. The average fringe length of the lattice increases and the tortuosity of the lattice and the Id/Ig ratios of the soot particles decrease as temperature increases, meaning that the nanostructure of the soot particles becomes more ordered. The results of the multi-diagnostics above indicate three main forms of oxidation during the overall oxidation process: internal oxidation, external oxidation and internal collapse. The oxidation of O2 is dominated by internal oxidation and internal collapse at high temperatures (600 and 800 °C), whereas the oxidation of NO2 is dominated by external oxidation throughout the oxidation temperature range.