Changes in Structure and Reactivity of Soot during Oxidation and Gasification by Oxygen, Studied by Micro-Raman Spectroscopy and Temperature Programmed Oxidation

Micro-Raman spectroscopy (micro-RS) and Temperature Programmed Oxidation (TPO) combined with FTIR gas analysis have been used to determine structural changes and oxidation behavior in samples of spark discharge (GfG) and heavy duty engine (EURO IV) soot upon oxidation by oxygen in a temperature range between 293 K and 873 K. Raman spectra of soot and FTIR spectra of oxidation products have been recorded before and during the oxidation process. For micro-RS analysis spectral parameters have been determined by a five band curve fitting procedure (G, D1-D4). For GfG soot the relative intensity of D3 band is decreasing and the two observed Raman peaks are getting more separated during the TPO. This suggests a rapid preferential oxidation of highly reactive amorphous carbon. The decrease of the D1 band width indicates a decrease of chemical heterogeneity and an increase of structural order upon oxidation. Changes in Raman spectroscopic parameters are in good agreement with the behavior of soot during oxidation determined by CO2 emission with FTIR. In contrast to GfG soot the spectral parameters of EURO IV soot remained mostly unchanged during the oxidation process, so that EURO IV soot shows just minor changes in structure upon oxidation. Overall Raman spectroscopic parameters provide information about changes in structural order of graphitic and amorphous carbon fractions during oxidation and can be used to analyze oxidation readiness of soot. Thus micro-Raman spectroscopy may become a rapid analytical tool for the determination of soot reactivity by analysis of the structure.