THERMOGRAVIMETRIC AND CHEMICOPHYSICAL ANALYSES OF SOOT OXIDATION

Carbon particles are widely distributed in the atmospheric aerosol as they mainly originate from a large variety of combustion sources emitting from micrometer sized soot aggregates down to ultrafine nanosized particles. Carbon particulate affects Earth's energy balance by scattering and absorbing radiation and by modifying the amount and properties of clouds, consequently causing climate changes. Moreover, nanometric particles have harmful impact on human health, thus the control of their emission is fundamental to limit their detrimental effects. Internal combustion engines are a significant emission source of carbon particles, especially as regards road vehicles for which particle filters are required to meet the limit of number and mass particles established by environmental regulations. Soot oxidation is the process at the basis of the regeneration of the particulate filters. This work aims at getting insights into the structural evolution of soot during its oxidation through an experimental study combining thermal analysis and chemico-physical characterization. Soot was collected from a laminar premixed flame and subjected to thermogravimetric analysis to extract its kinetic parameters. The activation energy of soot sample was found to be within the range of values typically found for carbon black and soot. A programmed thermogravimetric analysis was performed and stopped at different values of mass loss in order to obtain samples, which are representative of different advancements of soot oxidation, starting from the raw unoxidized soot: at the beginning (M70, 70% mass remaining after oxidation), in the middle (M50, 50% mass remaining) and almost at the end of soot oxidation (M20, 20% mass remaining). Raman spectroscopy allowed studying the nanostructure of partially oxidized soot samples. The Raman analysis showed that the more amorphous soot fraction burns first, thereafter soot does not undergo any significant change in the aromatic layers composing its turbostratic structure. The specific surface area was evaluated through the Brunauer-Emmett-Teller (BET) method for raw soot and partially oxidized soot (M70) showing a remarkable increase of the surface area after the oxidation process. This indicates that oxidation, after having preferentially removed the organic part, behaves in a non-preferential way towards the soot particle, suggesting the occurrence of internal burning of the particles.