Polycyclic aromatic hydrocarbon formation mechanism in the “particle phase”. A theoretical study

The synthesis of polycyclic aromatic hydrocarbons (PAHs) and the formation of soot platelets occur both during combustion at relatively low [O2], or under pyrolysis conditions. When the PAH size grows beyond the number of three-four condensed cycles, the partitioning of PAHs between the gas and particle phases favours the latter (i.e. adsorption). This study aims to assess which role the soot particle plays during PAH synthesis, in particular if catalytic or template effects of some sort can be exerted by the soot platelet on the adsorbed growing PAH-like radical. Our theoretical calculations indicate that chain elongation by ethyne addition cannot compete with cyclization when both can take place in the growing PAH-like radical, already in the gas phase. When it is adsorbed, cyclization is found to become easier than in the gas phase (more so, in terms of Gibbs free energy barriers, at higher temperatures), hinting at some sort of template effect, while chain elongation by ethyne addition becomes somewhat more difficult. The underlying soot platelet can assist (at lower temperatures) the formation of a larger aromatic hydrocarbon, by a final hydrogen abstraction from that endocyclic saturated carbon the newly formed cycle still bears. As an alternative (at higher temperature), a spontaneous hydrogen atom loss can take place. Finally, at rather low temperatures, the addition of the growing radical to the underlying soot platelet might occur and cause some reticulation, form more disordered structures, i.e. soot precursors instead of PAHs.