Theoretical Investigation of Soot Nanoparticle Inception via Polycyclic Aromatic Hydrocarbon Coagulation (Condensation): Energetic, Structural, and Electronic Features

Abstract

Carbon nanoparticles, generated during combustion at relatively low [O 2 ], or under pyrolysis conditions, can be seen both as soot precursors and as primary pollutants in themselves, since they are also directly emitted in the troposphere by vehicles. Soot particle inception (transition of relatively low-mass molecular systems from the gaseous phase to a solid nature) occurs at least in part via polycyclic aromatic hydrocarbon (PAH) coagulation/condensation. Complexes of different PAH systems, bound only by dispersion/multipolar forces, are investigated here by density functional theory, and their structural and energetic features discussed. The energetic features of the complexes allow to define an interplane interaction energy per C atom which compares satisfactorily with published experimental data on graphite exfoliation (i.e., removal of a single layer from the top of its bulk). The temperature dependence of the equilibrium K for these systems is then calculated to estimate the importance of PAH coagulation (condensation) in carbon nanoparticle generation. Energy alone would suggest that the larger interacting systems will be better stabilized by dispersion forces, but the trends in free energies are affected also by the entropy factor. This implies that beyond some temperatures the components of the largest systems will be more prone to fly apart than those of smaller systems, thus limiting the size of crystallites beyond some temperature. On the basis of our computational results, at high temperatures sheer stacking via van der Waals interaction can hardly be a major factor in causing soot nanoparticle inception.

Thermal evaporation of pyrene clusters

“…However this result has been challenged both by further modeling studies [5][6][7] and experimental results [8]. This has motivated many modeling studies in which the clusterization of PAH-like species has been evaluated as a function of different parameters such as PAH size or the presence of aliphatic chains but also by considering chemical dimerization as a way to increase the stability of these species at high temperature [9][10][11][12][13][14][15][16][17]. Soot nucleation might therefore be induced by three-dimensional crosslinked structures rather than by stack clusters [5,18].…”

Structure of incipiently sooting ethylene–nitrogen counterflow diffusion flames at high pressures

“…http://dx.doi.org/10.1016/j.combustflame.2013.11.023 particles of nanometric size. There is general agreement that aggregation of PAH molecules is a likely first step of soot particle nucleation [2][3][4][5][6][7]. Therefore, accurate knowledge of the kinetics of one and two ring aromatic chemistry predating PAH formation is needed.…”

Carbonaceous Nanoparticle Molecular Inception from Radical Addition and van der Waals Coagulation of Polycyclic Aromatic Hydrocarbon-Based Systems. A Theoretical Study

“…Figure a exemplifies a complex, shown only for system 1 (those for the other systems are similar). The parallel arrangement of the two moieties reflects vdW interactions, which in relatively large systems favor it with respect to a perpendicular layout, in which one molecule points with some C–H bonds toward the π system of the other. , The Δ E ZPE values with respect to the reactants will now be examined (Table ). For the complexes of systems 1, 2, 4, and 5, we start from the not unexpectedly rather close values of ca.…”

“…49 We have recently studied by quantum mechanical methods the growth of an aromatic system adsorbed on a model soot platelet, 55 and the feasibility of association of PAHs via vdW interaction. 56,57 In the latter study, it was concluded that, at high temperature, pure vdW stacking can hardly be a major factor in triggering soot nanoparticle inception. In fact, although energetic effects would suggest that larger interacting systems will be better stabilized, entropy changes work in the opposite direction.…”