On-line determination of soot oxidation reactivity

Soot particles in the atmosphere significantly affect human health as well as the climate. In order to avoid soot emissions, exhaust gases are filtered and the particles are trapped on a filter. The particles are eventually removed by catalytic oxidation to CO2, a process that requires elevated temperatures. The reactivity, i.e. the temperature required for this process, is currently determined after sample collection by techniques such as temperature-programmed oxidation (TPO), Raman microscopy or high-resolution transmission electron microscopy (HRTEM), which are all time-consuming and expensive. Furthermore, a significant amount of soot has to be collected prior to the analysis. This study aims for a straight-forward method to characterize the oxidation reactivity of exhaust soot on-line. A differential flow-through oven-system was built to thermally treat soot and determine the oxidation losses in comparison to a reference channel. The performance of the system was assessed by propane soot as well as by K2CO3-containing soot, which feature largely different reactivities in earlier studies. The results of the new system are compared to TPO measurements by using the same temperature program. K2CO3-containing soot shows oxidation at significantly lower temperatures than pure propane soot. Photoacoustic spectroscopy (PAS) revealed a considerable temperature-dependent decrease of soot mass concentrations, starting at lower temperatures for the K2CO3-containing soot. Beyond that, the new setup can derive the oxidation reactivity from isothermal measurements, thus enabling on-line monitoring of the soot oxidation reactivity in transient systems. Isothermal measurements on a real diesel engine were conducted and correlated to different engine and exhaust gas parameters.