Carbon-addition-hydrogen-migration reactions in soot surface growth

In the past several decades, substantial understanding has been gained on the soot formation mechanisms [1]. Among all the processes, soot surface growth plays an essential role in controlling the soot mass growth rate [2-4] and is usually described by the surface hydrogen-abstraction-carbon-addtion (HACA) mechanism at the molecular level, [5, 6] in which hydrogen abstraction takes place first to form an aryl radical site, followed by acetylene addition to accomplish mass growth. Consequently, the HACA mechanism requires that the hydrogen atoms should be sufficiently abundant to form active sites. As a result, soot mass growth is not likely to occur at temperatures below 1500 K in the postflame region based on this mechanism because of the low concentration of hydrogen atoms [1]. However, recent experiments, including morphological observations, chemical composition analysis, soot particle size statistics and soot volume observations, [7-13], unexpectedly showed that in an ethylene-oxygen-argon laminar premixed flame (φ=2.5) at 1450 K, soot volume fraction increases with the height above the burner and the nascent soot is wax-like and rich of aliphatics with unity C/H ratio [1, 10]. These observations indicate that the nascent soot formed in laminar premixed flames has an aromatic-core-alphatic-shell structure, in which the aromatic core is first formed in the early stages of the flame when the temperature is high, followed by the formation of the aliphatic shell when the gas temperature becomes lower.[1] Furthermore, Raman spectroscopic evidence suggests that the aliphatic groups in the nascent soot are alkyl or alkenyl functionalities.[10] None of these experimental evidences can be explained by the surface HACA mechanism. Consequently, as a supplementary mechanism to surface HACA, a new mechanism of soot mass growth without hydrogen atoms, and hence without radicals, needs to be identified to explain these experimental results.