Probing structures of soot formed in premixed flames of methane, ethylene and benzene flames

HR-TEM analysis was used in conjunction with elemental analysis and UV-Visible and FT-IR spectroscopy to infer the structural features of soot formed along the axis of premixed flames of methane, ethylene and benzene flames burning at different temperatures (1600-1800 K) and rather similar sooting conditions. The order degree and the main structural parameters of soot particles (layer length, number of stacked layers, interspatial distance, etc.) were investigated by HR-TEM analysis. The aromatic and aliphatic character of soot was instead inquired mainly by FT-IR and UV-Visible spectroscopic analysis. HR-TEM image analysis showed small differences in the soot structure indicating a slight increase of order extent for effect of the higher flame temperature, longer residence time and passing from methane to benzene. The dehydrogenation and graphitization processes occurring during soot formation were successfully followed by measuring the H/C ratio of soot and considering peculiar spectroscopic parameters as the intensity ratio of the isolated/adjacent aromatic hydrogens, the specific absorption coefficient and the optical band gap. The very low and quasi-invariant hydrogen content and optical band gap of benzene soot were found in contrast with the strong variations of the absorption mass coefficient. On the opposite, methane soot showed very high H/C atomic ratios and optical band gaps, both largely decreasing during soot formation, in contradiction with the small variation of the absorption coefficient. Generally, by relating the H/C ratio to the specific absorption, it was found that the specific absorption slightly increases for H/C variations from 0.3 down to 0.1, whereas the strong increase of the absorption coefficient occurred below the H/C value of 0.1. Some insights on the soot structure and chemical environment effect on soot inception, dehydrogenation and graphitization processes were obtained on the basis of gathered structural data.