Raman microspectroscopic analysis of changes in the chemical structure and reactivity of soot in a diesel exhaust aftertreatment model system

Raman microspectroscopy has been applied to follow structural changes in spark discharge (GfG) soot and light-duty diesel vehicle (LDV) soot upon oxidation and gasification by nitrogen oxides and oxygen in a diesel exhaust aftertreatment model system at 523 and 573 K. Raman spectra have been recorded before and during the oxidation process, and spectral parameters have been determined by curve fitting with five bands (G, D1-D4). For GfG soot, a steep initial decrease of the relative intensity of the D3 band suggested rapid preferential oxidation of a highly reactive amorphous carbon fraction, while a less steep but also substantial decrease of band widths (in particular, the D1 band) indicated a slower overall increase of chemical homogeneity and structural order in the partially oxidized material. The spectroscopic changes are in agreement with a strong decrease of chemical reactivity at increasing mass conversion of GfG soot. In contrast, the spectral parameters and reactivity of partially oxidized LDV soot remained largely unchanged throughout the oxidation process. Overall, the spectroscopic and kinetic findings suggest that Raman spectroscopic parameters provide information about the relative abundance and structural order of graphitelike and amorphous carbon and can be used as proxies for the chemical reactivity of soot undergoing oxidation and gasification. Thus, Raman spectroscopy promises to become an efficient tool for further investigation and optimization of diesel exhaust aftertreat ment in continuously regenerating traps and particle filters.