Virgin and Extracted Soots in Premixed Methane Flames: A Comparison of Surface Functional Groups, Graphitization Degree, and Oxidation Reactivity

This study reports the similarities and differences in the physicochemical properties of virgin soot generated in premixed methane flames and the corresponding extracted soot following removal of its soluble organic fraction (SOF). In addition, the correlations between these physicochemical properties and the SOF content are investigated. Soot samples were acquired at varying heights above the burner using a probe sampling technique, and surface functional groups (SFGs) and degree of graphitization were analyzed by Fourier transform infrared and Raman scattering spectroscopies. The oxidation reactivities of both the virgin and extracted soots were evaluated in terms of the characteristic oxidation temperatures and apparent activation energies based on thermogravimetric analysis. Both materials undergo similar changes in the concentrations of aliphatic and aromatic C–H, AD1/AG and AD3/AG ratios, and apparent activation energy with increasing height above the burner but differ in their AD4/AG ratios. The presence of the SOF does increase the relative concentrations of aliphatic and aromatic C–H groups. Moreover, at the same sampling position, the AD1/AG, AD3/AG, and AD4/AG ratios of the virgin soot are greater than those of the extracted soot, while the apparent activation energy values of the virgin soot are lower. These data indicate that the virgin soot possesses less graphitic organization and lower resistance to oxidation relative to the extracted soot. In addition, there is a definite correlation between the oxidation reactivity and the SOF content.