In situ Raman microspectroscopic analysis of soot samples with different organic carbon content: Structural changes during heating

Raman microspectroscopy (RM), thermo-optical analysis and Fourier transform infrared (FTIR) spectroscopy were used to characterize soot with different organic carbon (OC) content (4%, 47% and 87%), generated with a CAST burner at various propane-to-air ratios. While the FTIR analysis revealed the organic composition of the samples, the RM analysis gave an increased fluorescence background and an additional shoulder in the spectrum with increasing OC content. According to RM data and in agreement with high-resolution transmission electronic microscopy (HRTEM) analysis, the nanostructural order was high for the soot with 4% of OC and low for the soot with 87% of OC. Additionally, a heating stage was used to perform in situ RM analysis during the soot oxidation at temperatures up to 600 °C in air. The (fluorescent) organic components were evaporated/transformed or oxidized with increasing temperature (up to 500 °C), and the actual soot nanostructure changed significantly. At 600 °C the chemical heterogeneity vanished and the structural order increased, since the organic components as well as amorphous carbon were oxidized by that time. These results can help in understanding the relation between the OC content in the soot and its structure, reactivity and impact on the environment.