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Published May 2013 | metadata_only
Book Section - Chapter

Investigation into the Mechanism of Soot Deposition from Gaseous Polycyclic Aromatic Hydrocarbons


We conducted an ab-initio quantum computational study to address certain issues present in current models of soot nucleation. Most models base soot nucleation upon the dimerization of gas phase polycyclic aromatic hydrocarbons (PAHs) that arises through collisions between these gaseous molecules. These models contain three major flaws and ultimately violate the second law of thermodynamics. Firstly, soot particles are formed at all temperatures, including room temperature, a phenomenon that is not observed experimentally. Secondly, these models predict that collisions between aromatic molecules of any size, including benzene, will form a soot particle. Thirdly, the dimers produced by these collisions are predicted to be infinitely stable. In an attempt to correct for the first two issues, we hypothesized that only collisions that included at least one excited state PAH, which would not be found at low temperatures, could form a stable dimer. The calculations of the excitation energy difference between excited and ground states were performed at the B3LYP level with the Dunnings Correlation Consistent basis sets. The cc-PVDZ basis set proved itself sufficient, as its excitation energy calculations differed from those of higher order Dunning's sets by only a few percent. The results suggest that, while the excitation energy negatively correlated with molecular weight, it was strongly dependent upon the structure of the given PAH. PAHs that more closely resembled the n-acenes in structure had lower excitation barriers than other PAHs of similar mass. Using the calculated excitation energies, we evaluated the population of excited states at a given temperature assuming a Boltzmann equilibrium distribution. We found that only higher mass PAHs, particularly anthracene and tetracene, form a sufficiently large population of excited molecules at common sooting flame temperatures. Then, to tackle the third issue presented by current models, we used benzene and naphthalene as test cases to determine the stability of any dimers formed from a successful collision. Even though we found such dimers unlikely to form, they provided computationally efficient results that should generalize to higher order PAHs. These calculations were carried out using the MP2/cc-PVDZ level of theory. We compared the energy of two molecules, one in its ground and one in its excited state, an "infinite distance" apart to two in close proximity (3-4 angstroms) and found that, for benzene, dimerization provides significant stabilization to the two molecules ( 30 kJ/mol). The present results suggest that a collision based model involving one ground state and one excited state PAH might adequately capture the physics of soot nucleation.

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© 2013 Curran Associates, Inc. Paper # 070EN-0337 The authors gratefully acknowledge funding from the U.S. Department of Energy-Basic Energy Sciences (DE-SC006591).

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August 19, 2023
August 19, 2023