Kinetics and visualization of soot oxidation using transmission electron microscopy

Visualization of the oxidation of individual soot particles using transmission electron microscopy (TEM) was performed and quantitative oxidation rates were determined. Soot particles were sampled from an ethylene diffusion flame burner and collected on a TEM grid. Individual particles were then tracked by creating maps of TEM images at different magnifications, and images were compared before and after the oxidation process in a separate furnace at low temperatures (< 800 K). Mono-area soot particles were sampled on TEM grids using a differential mobility analyzer (DMA) and a low-pressure impactor. It was found that the mobility equivalent diameter determined by the DMA is equivalent to the projected area equivalent diameter determined by the TEM for particles below 150 nm. Groups of mono-area particles were visualized before and after the oxidation process and oxidation rates were determined from the change in projected area equivalent diameter by image-processing a series of TEM images. For these low temperatures (< 800 K), an activation energy of 148 kJ/mol was found for the oxidation of soot particles. The overall oxidation rate was found to be significantly higher than in prior studies, and some possible reasons for the discrepancy discussed. (C) 2003 Published by Elsevier Inc. on behalf of The Combustion Institute.