Fullerenes and PAH in low-pressure premixed benzene/oxygen flames

The formation of fullerenes is thought to be a molecular weight growth process similar to the formation of polycyclic aromatic hydrocarbon (PAP) and soot in flames, although little is known about the specific mechanisms involved. The goal of this study is to investigate possible fullerenes formation pathways. This work measures for the First time concentration profiles of fullerenes C-60, C-70, C-76, C-78, and C-84, PAHs (up to 300 amu), and unidentified PAHs in the mass range between PAHs and soot in a heavily fullerene-forming premixed benzene/oxygen flame operated at the following conditions: fuel equivalence ratio, 2.4 (atomic C/O ratio, 0.96); cold gas velocity, 25 cm/s; pressure, 5.33 kPa; and fraction of argon in fuel mixture, 10 mol%. Two regions of fullerenes formation are identified in this flame. The First formation region occurs early in the flame simultaneously with PAH consumption. The rate of PAH consumption is more than large enough to account for the observed rate of fullerenes formation, so this formation region may involve reactions of PAH. The Second region, which accounts for most of the fullerenes mass produced in the flame, occurs later in the flame and is more consistent with stepwise acetylene addition to fullerene precursors. In both legions, fullerenes consumption that may involve reactions between fullerenes and soot is observed. In addition, measurements indicate that the different PAHs grow roughly simultaneously, reach a peak concentration, and decay together in a consumption process that occurs at the same time as a rapid rise in soot mass concentration early in the flame. This behavior is consistent with the major soot formation being from PAH and occurring along with the minor fullerenes formation in this flame.