Reactivity of soot emitted from different hydrocarbon fuels: Effect of nanostructure on oxidation kinetics

Soot oxidation kinetics is relevant for the exhaust pollutant reduction through processing in abatement systems based on filtering and oxidation as diesel particulate filters. Soot nanostructure impact on oxidation reactivity was studied by thermogravimetric analysis of soot produced in premixed laminar flames burning different hydrocarbon fuels, namely methane, ethylene and benzene, and characterized by spectroscopic and microscopic analysis. Soot oxidation kinetics was multistep and two activation energies, 180 and 240 kJ/mol, could be evaluated. Preexponential factors obtained upon data fitting of the Arrhenius plots with the two activation energies, showed an increasing trend going from aliphatic (methane, ethylene) toward benzene soot, consistently with the rise of the maximum oxidation temperature. The higher and lower activation energies were correlated, respectively, to amorphous carbon, coalesced on soot particles and present in form of tortuous and not-stacked lamellae inside the particle, and to the rest of the particle, characterized by a better nanoscale organization. Through a detailed chemico-physical characterization, the different propensity to be oxidized of soot samples was correlated to the nanostructural features of the most ordered part burning at 240 kJ/mol.