Portraits of Soot Molecules Reveal Pathways to Large Aromatics, Five-/Seven-Membered Rings, and Inception through π-Radical Localization

Abstract

Incipient soot early in the flame was studied by high-resolution atomic force microscopy and scanning tunneling microscopy to resolve the atomic structure and orbital densities of single soot molecules prepared on bilayer NaCl on Cu(111). We resolved extended catacondensed and pentagonalring linked (pentalinked) species indicating how small aromatics cross-link and cyclodehydrogenate to form moderately sized aromatics. In addition, we resolved embedded pentagonal and heptagonal rings in flame aromatics. These nonhexagonal rings suggest simultaneous growth through aromatic cross-linking/cyclodehydrogenation and hydrogen abstraction acetylene addition. Moreover, we observed three classes of open-shell π-radical species. First, radicals with an unpaired π-electron delocalized along the molecule's perimeter. Second, molecules with partially localized π-electrons at zigzag edges of a π-radical. Third, molecules with strong localization of a π-electron at pentagonal-and methylene-type sites. The third class consists of π-radicals localized enough to enable thermally stable bonds, as well as multiradical species such as diradicals in the open-shell triplet state. These π-diradicals can rapidly cluster through barrierless chain reactions enhanced by van der Waals interactions. These results improve our understanding of soot formation and the products formed by combustion and could provide insights for cleaner combustion and the production of hydrogen without CO 2 emissions.

Phenalenyl growth reactions and implications for prenucleation chemistry of aromatics in flames

“…88 The other proposition pursues the premise of PAH p-dimerization. 12,18,23,[89][90][91][92][93] It has been established, 15,16,22,90,94 though, that p-monoradicals, such as phenalenyl, preferentially form edge s-bonded dimers rather than p-p stacked ones, both bonds being relatively weak. As a resolution to this, Martin and co-workers 12,90,92 suggested dimerization of p-biradicals.…”

“…96 In fact, the latter processes, on their own, can create molecular structures, as illustrated in Fig. 10, having five-membered-ring features attributed 23 to PAH-PAH cross-linking whereas no such PAH reactions were included in the model.…”

“…20,21 Of interest to soot formation in flames is the phenalenyl propensity to dimerize, 15,22 as a possible initiating step of particle inception, 18,19 and its further growth into larger PAH structures enabling such inception. 14,23 Our prior kinetic study 14 included growth of phenalenyl but in a rather generic form, treating reactions of phenalenyl-type structures as those of ''regular'' PAHs; in other words, all PAHedge growth reactions were assigned to the same reaction-rate classes. Given the abundance of phenalenyl revealed in the prior numerical observations, here we examine these reactions with further scrutiny.…”

“…20,21 Of interest to soot formation in flames is the phenalenyl propensity to dimerize, 15,22 as a possible initiating step of particle inception, 18,19 and its further growth into larger PAH structures enabling such inception. 14,23…”

Machine Learning Suggests Possible Bias in Molecular Simulations for Asphaltene Systems

“…The structures identified in petroleum samples are largely consistent with the continental asphaltene configuration, although a few archipelago geometries were also observed. Additionally, Schulz et al, 18 Commodo et al, 19 and Lieske et al 20 studied the early stages of soot formation using AFM. The structures imaged show a significant presence of penta-rings and aliphatic side chains.…”