Revealing the molecular structure of soot precursors

Abstract

The earliest stages of soot formation in flames are believed to involve the formation of small, nanoscale clusters of polycyclic aromatic hydrocarbon molecules. The structure of these clusters is still highly uncertain, however, impeding the construction of quantitative models of soot inception and growth. To provide insight into the structure of incipient soot, we produced nanoclusters of hydrocarbon molecules by annealing coronene films deposited on Pt(111), and examined them with scanning tunneling microcopy. We find that clusters containing~20e100 molecules, are disordered agglomerations of stacks that are~5e6 molecules tall. These structures are quite distinct from crystalline coronene, but bear a striking resemblance to recently proposed models for the equilibrium structure of similarly-sized clusters that are assumed to initiate soot formation. In contrast to mature soot, the surfaces of these clusters contain very few molecules with graphitic planes oriented parallel to the surface.

Thermal evaporation of pyrene clusters

“…Experimentally, the most stable structures of clusters of large PAH were often reported to present stack patterns, sometime suggesting parallel displaced features, the presence of a molecule on a stack side [21,[37][38][39]. This is in line with theoretical calculations which investigated dimers at the ab inito level [30,[39][40][41][42][43][44][45][46][47][48] and larger clusters using force field or coarse grained models [9,19,30,[47][48][49][50][51] often combined with global exploration schemes.…”

“…Soot nucleation might therefore be induced by three-dimensional crosslinked structures rather than by stack clusters [5,18]. Still PAH clusters are considered as part of the structure of soot particles [19][20][21] but the lack of detailed diagnosis for incipient particles in flames makes it difficult to conclude [22].…”

Partitioning of polycyclic aromatic hydrocarbons in heterogeneous clusters

“…stacked morphologies, predicted by global optimisation methods and MD studies using the isoPAHAP force field [22,10,27], have been confirmed recently using scanning tunnelling microscopy of coronene clusters [36].…”

“…where E pos is the position potential energy for a single atom i in kJ mol strengths [59] and has been observed experimentally in PAH clusters [36]. As predicted by quantum calculations of PAH dimers, the slipped-parallel and sandwich molecule arrangements are dominant.…”

Dynamical simulation of collision-induced dissociation of pyrene dimer cation

“…Clusters of polycyclic aromatic hydrocarbons (PAH) are relevant systems in several scientific fields, including astrophysics and combustion sciences. The role of PAH clusters in combustion processes is still under debate, in particular as they might or not be intermediate systems in the soot particles growth [1][2][3][4][5][6][7][8][9][10]. In astrophysics, PAHs have been proposed in the mid-eighties to explain a series of infrared emission bands ubiquitous in the Universe [11,12].…”