Pyrene dimerization in controlled temperature environment: An experimental study

Nucleation step in soot formation can be defined as the transition from gas phase to condensed phase state at flame temperature. This aspect of particle formation in flame is the least understood despite of the great effort of the last decades. Most of the models and of the experimental setups are not able to isolate the single step of nucleation. The formation of the first particles or species in condensed phase is studied in combustion environments in which many other processes - namely PAH growth from other small hydrocarbons and oxidation - are taking place. A tubular flow reactor working in the temperature range of 800-1100K with a stream containing pure PAH - pyrene in the current work - was used to isolate the pathways leading from gas to condensed phase. To avoid the formation of smaller compounds from the pyrene pyrolysis, temperature was kept lower than 1100K allowing a residence time long enough to favour mass growth. DMA measurements at the exit of the reactor showed the presence of particles. Particle formation was found to be sensitive to the temperature. In particular, the formation rate was higher at the higher temperature, suggesting the intervention of some chemical-driven pathway for the formation of the first condensed phase species. The gas and the particles exiting the reactor were trapped in ethanol and analyzed by UV-Visible and fluorescence spectroscopy, and size exclusion chromatography (SEC). The formed particles have lost molecular spectral features and have a molecular weight (MW)>=400: these characteristics allow to consider them the smallest species in condensed phase. Their MW evaluated by SEC and the analysis of their absorption spectra hinted to the predominance of oligomers formation, from dimers to tetramers, being trimers the most favored and stable species in the investigated conditions.