Production and characterization of soot aerosols internally-mixed with salts

Diesel soot is one of the major pollutants in the world and is classified as carcinogenic by the World Health Organisation's International Agency for Research on Cancer. Soot/black carbon also has the second largest impact on global warming after CO₂. In North America and Europe, soot is mainly emitted by diesel engines.

To minimize emissions, particulate filters are used. Regeneration of the filter is done by oxidation (combustion) of the soot. Uncatalyzed oxidation requires temperatures > 600 °C in O₂, resulting in poor fuel efficiencies. Additives enhance the soot reactivity during soot formation, leading to internally-mixed soot and possible oxidation temperatures in the range of 300 °C - 400 °C. Previous results showed that besides oxides, also salts were able to lower the temperatures for soot oxidation significantly (Bladt et al., 2014), which means that there must be some kind of interaction. Until then, only small amounts of soot were sampled, which were not enough for BET sorption measurements. Therefore, the propane/air diffusion burner was optimized for the controlled production of higher amounts of soot by introducing mass flow controllers, best possible conditions for maximum soot production and a thermophoretic precipitator for soot collection.

Ion Chromatography (IC) was used to characterize the salt content in the soot samples, Raman-Microspectroscopy (RM) was applied to analyze the graphitic soot structure and the presence of salts/minerals or hydrocarbons. It was found that there is no significant difference in the soot structure with and without salts, confirming previous results (Bladt et al., 2014).

To simulate combustion in a Diesel Particulate Filter, Temperature-Programmed Oxidation (TPO) was used. Soot samples were combusted in a defined atmosphere and temperature range. Combustion products were detected in an FTIR spectrometer. The temperature of maximum CO and CO₂ emission (T_max) was used as a benchmark for soot oxidation reactivity (see figure). Increased salt-contents lead to lower T_max and lower ratios of CO/CO₂ at T_max. Both findings indicate that salts promote complete soot oxidation.

Another way to gain insight into structural changes of solids is BET analysis, which is a method to determine specific surface areas. If there is an interaction between the salts and the primary soot particles and/or a change in the inter-layer spacing between the graphene planes, there might also be a change in the specific surface areas of the internally-mixed soots.