A quantitative study of the clustering of polycyclic aromatic hydrocarbons at high temperatures

Abstract

The clustering of polycyclic aromatic hydrocarbon (PAH) molecules is investigated in the context of soot particle inception and growth using an isotropic potential developed from the benchmark PAHAP potential. This potential is used to estimate equilibrium constants of dimerisation for five representative PAH molecules based on a statistical mechanics model. Molecular dynamics simulations are also performed to study the clustering of homomolecular systems at a range of temperatures. The results from both sets of calculations demonstrate that at flame temperatures pyrene (C(16)H(10)) dimerisation cannot be a key step in soot particle formation and that much larger molecules (e.g. circumcoronene, C(54)H(18)) are required to form small clusters at flame temperatures. The importance of using accurate descriptions of the intermolecular interactions is demonstrated by comparing results to those calculated with a popular literature potential with an order of magnitude variation in the level of clustering observed. By using an accurate intermolecular potential we are able to show that physical binding of PAH molecules based on van der Waals interactions alone can only be a viable soot inception mechanism if concentrations of large PAH molecules are significantly higher than currently thought.

Radical–radical reactions, pyrene nucleation, and incipient soot formation in combustion

“…They concluded that the equilibrium of the dimerization favors dissociation at high temperatures and that chemical growth to PAHs much larger than pyrene is required before homogeneous condensation can lead to significant soot formation under many combustion conditions. Kraft and co-workers [19] came to the same conclusion in their calculations of PAH clustering for particle inception. However, these two studies focused on pyrene homodimers and equilibrium considerations.…”

Using real particulate matter to evaluate combustion catalysts for direct regeneration of diesel soot filters

“…This means that about 25% of the soot mass is made up of condensed or adsorbed polyaromatic molecules, which are the precursors of the elemental carbon within the particulate [29], and yet these are rarely taken into consideration during the screening of combustion catalysts for vehicle applications. Unlike both the current regeneration strategies, which require high temperatures and therefore can destroy the organic fraction at the same time as the elemental carbon, the release of polyaromatics during direct catalytic combustion of soot poses an emission problem in its own right [31].…”

“…This information was provided by HPLC analysis of the organic fraction dissolved in the toluene during Soxhlet extraction, which allowed us to separate and quantify 11 polyaromatic molecules. These were predominantly the following 3-and 4-ring compounds, which are intermediates in the formation of solid carbon particulate by the dehydrogenation and cyclisation of C 2 and C 3 hydrocarbons [29]: phenanthrene (0.505 mmol per g of soot), fluoranthene (0.431 mmol) and pyrene (0.396 mmol). Together the polyaromatic hydrocarbons accounted for essentially all of the organic fraction of the soot.…”

Size-dependent melting of polycyclic aromatic hydrocarbon nano-clusters: A molecular dynamics study

“…Recently [66] we have used molecular dynamics simulations of homogeneous PAH systems to determine the size of PAH molecule required for inception of PAH dimers and larger clusters at flame temperatures. The results shed light on the soot inception mechanism and were based on an intermolecular potential fitted to high-level ab initio SAPT (DFT) calculations [63,64,67].…”

“…[63]. The parameters of isoPAHAP potential were fitted to the interaction energies of 2500 pseudo-random coronene dimer configurations calculated by the PAHAP potential [66]. The resulting parameters for the isoPAHAP potential are shown in Table 1.…”