Fullerene ions and their relation to PAH and soot in low‐pressure hydrocarbon flames

Positive and negative fullerene ions from C32 to over C600 have been analysed in low‐pressure premixed flames of acetylene and benzene with oxygen. Mass spectra were taken through a molecular beam sampling system by a linear time‐of‐flight mass spectrometer (TOF‐MS) and a reflectron TOF‐MS. Fullerenes are mainly formed during the beginning of soot formation, and less in the oxidation zone or under nonsooting conditions in benzene flames. When soot is present, the whole spectrum of fullerene ions is formed, whereas in the oxidation zone mainly smaller fullerenes, preferentially C60, C70 and C50 occur. — Polycyclic aromatic hydrocarbons (PAH) are the precursors of fullerenes, both in the presence of soot and under homogeneous conditions. However, hydrogenated fractions of carbon cages as intermediates could not be detected. This is attributed to fast unimolecular reactions and to formation reactions of fullerenes in young soot particles. On account of correlations with concentration profiles of large PAH ions, on the one hand, and with the soot particle mass distributions, on the other, possible mechanisms of fullerene formation are discussed. Fullerenes do not grow further, as do the soot particles, but they are oxidized at high temperature. Thermal decomposition was not observed. From comparison of fullerene formation with the two fuels and under different burning conditions it is concluded that negative fullerene ions are formed by electron attachment to neutral fullerenes and that for positive fullerene ions there is hardly any ionisation equilibrium.