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

Abstract

The synthesis of polycyclic aromatic hydrocarbons (PAHs) and the formation of soot platelets occur both during combustion at relatively low [O(2)], 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.

Chemically activated reactions on the C7H5 energy surface: propargyl + diacetylene, i-C5H3 + acetylene, and n-C5H3 + acetylene

“…a variety of environments, yet these reaction classes have not been explored in detail. Indarto et al 5 recently examined the possibility of diacetylene reacting with propargyl (as well as other free radicals) to form cyclic compounds with some triradical character. This so-called radical breeding mechanism 6 was found to be unfavored in terms of free energy for C 4 H 2 + C 3 H 3 .…”

“…Pathways to the seven-member (4) and six-member (6) triradicals were reported in ref. 5, and both reactions must surmount barriers above the entrance channel energy (although formation of a seven-member ring compound via TS2 occurs just above the entrance channel energy). The six-member ring triradical is close to being unstable, and will not be of any significance.…”

Heterogeneous reactions of HONO formation from NO2 and HNO3: a review

“…soot, has been reviewed several times [5][6][7][8]. In conclusion, the growth mechanisms can be divided into various mechanism schemes, such as hydrogen abstraction and acetylene addition (HACA mechanism, a model proposed by Franklach and Warnatz [9][10][11]), polyyne polymerization accompanied by radical breeding mechanism proposed by Krestinin [12][13][14], ionic model (proposed by Calcote [15]), or the combination of those mechanisms [16][17][18][19][20]. In this discussion, we will not fully address the carbonaceous (aggregate) particle formation by combustion/pyrolysis as the readers can easily access the above literatures to have a better understanding on the soot growth process at high temperatures.…”

First carbon ring closures started by the combustive radical addition of propargyl to butadiyne. A theoretical study

“…[17,18,19]. Now, the formation of the very first (possibly aromatic) ring molecule from small aliphatics has been often seen as the rate-determining step of soot growth, and appears in any case particularly interesting.…”

“…[33] Similar studies focused also on the formation of slightly larger systems (such as naphthalene [34,35], or indene [36,37]), or consider in general the mechanism of PAH growth. [38] Along this line, we have already considered [39] the first growth steps of aromatic systems through the ring closure-radical breeding polyyne-based mechanism proposed by Krestinin [40] (polyynes had already been considered at an earlier time by Homann and Wagner [41]). …”