Continuous soot surface growth over carbene active site through spin density migration and spontaneous dehydrogenation: A DFT study

Abstract

Surface growth process contributes the majority of soot mass, but the detailed mechanism remains vague, especially at post-flame zone where reactive radicals like H· are insufficient to continuously generate active sites. In this work, the reactivity of carbene active center on soot surface towards different categories of hydrocarbons was evaluated through DFT calculation, and the self-sustaining mechanisms for continuous further growth were proposed. Acetylene, aromatic molecules (benzene and naphthalene) and resonance stabilized radicals (RSRs, cyclopentadienyl and indenyl) were selected as representative reactants. Potential energy surfaces were obtained at the M06-2X/cc-pVTZ level of theory, and spin density distributions were presented to analyze the evolution of active sites. The surface growth process was divided into two stages, i.e., initial capture and further growth. During initial capture process, carbene active site exhibits high reactivity towards all kinds of representative reactants, and the energy barriers are lower than 10 kJ/mol except for benzene (26.17 kJ/mol). Meanwhile, the initial capture processes are exothermic and capturing RSRs shows the largest exothermicity. After the carbene site was occupied through initial capture process, the graphene flake still tends to retain the radical-like behavior and shows open-shell character, providing possibilities for further surface growth. During further growth process, spin density migration mechanism combined with spontaneous dehydrogenation step could achieve continuous surface capture without involving exogenous radicals. For acetylene, the unpaired electrons will preferentially locate at the terminal carbon of the growing chain to continuously refresh the active site. Extra dehydrogenation steps are required for the further growth involving aromatics and RSRs, otherwise the surface growth steps would be less exothermic and more reversible. Covalent bonding of these sp2-hybridized hydrocarbon species and the carbene site will create a vulnerable tertiary CH bond. Spontaneous dehydrogenation through the breakage of the tertiary CH bond can reproduce active site with localized spin density, after which the further growth process will be energetically and kinetically more feasible.