Micro-FTIR study of soot chemical composition—evidence of aliphatic hydrocarbons on nascent soot surfaces

Abstract

Previous studies suggest that soot formed in premixed flat flames can contain a substantial amount of aliphatic compounds. Presence of these compounds may affect the kinetics of soot mass growth and oxidation in a way that is currently not understood. Using an infrared spectrometer coupled to a microscope (micro-FTIR), we examined the composition of soot sampled from a set of ethylene-argon-oxygen flames recently characterized (A. D. Abid, et al. Combust. Flame, 2008, 154, 775-788), all with an equivalence ratio Φ=2.07 but varying in maximum flame temperatures. Soot was sampled at three distances above the burner surface using a probe sampling technique and deposited on silicon nitride thin film substrates using a cascade impactor. Spectra were taken and analyses performed for samples collected on the lowest five impactor stages with the cut-off sizes of D(50)=10, 18, 32, 56 and 100 nm. The micro-FTIR spectra revealed the presence of aliphatic C–H, aromatic C–H and various oxygenated functional groups, including carbonyl (C=O), C–O–C and C–OH groups. Spectral analyses were made to examine variations of these functional groups with flame temperature, sampling position and particle size. Results indicate that increases in flame temperature leads to higher contents of non-aromatic functionalities. Functional group concentrations were found to be ordered as follows: [C=O]<[C–O]<[aliphatic C–H]. Aliphatic C–H was found to exist in significant quantities, with very little oxygenated groups present. The ratio of these chemical functionalities to aromatic C–H remains constant for particle sizes spanning 10-100 nm. The results confirm a previous experimental finding: a significant amount of aliphatic compounds is present in nascent soot formed in the flames studied, especially towards larger distances above the burner surface.

Influence of Combustion Conditions on Hydrophilic Properties and Microstructure of Flame Soot

“…13 carbonyl group, assigned to ketone species (1710 cm −1 ), was observed for decane soot. 13,20,21,40,41 As seen in Figure 7, compared to the functional groups on the rich toluene flame soot, no significant changes were observed on the lean toluene flame soot. For n-hexane and decane soot, the lean flame soot exhibited lower intensities in the peaks of alkynesC−H, aromatic C−H, unsaturated CH 2 , and highly substituted aromatic compounds, while the carbonyl (CO) group bound to an aromatic ring showed a larger intensity than that of the rich flame soot.…”

“…The two peaks at 2918 and 2846 cm −1 for toluene and decane flame soot were related to the CH 2 and CH 3 stretch. 20 An absorption band at 1440 cm −1 for n-hexane soot corresponded to the scissor vibration of unsaturated CH 2 . 20,32,41 Three bands in the range of 900−700 cm −1 for the three fuels' flame soot were attributed to the substitution modes of aromatic compounds.…”

“…[15][16][17]19,22,23 Recent studies have shown that residence time in the flame and flame temperature can induce modifications in the microstructures and functional groups of soot. 15,20 Thus, this demonstrates that the structure and properties of soots should greatly depend on their formation conditions. On the other hand, the CCN and ice nuclei (IN) abilities of soot particles significantly depend on their hydrophilicity.…”

“…18 Functional groups, including aliphatic or aromatic C−H, carbonyl CO, and ethers C−O, have been confirmed through infrared spectroscopy. 13,20,21 Microstructures including graphitic carbon, disordered carbon, and amorphous carbon have also been detected using Raman spectroscopy. [15][16][17]19,22,23 Recent studies have shown that residence time in the flame and flame temperature can induce modifications in the microstructures and functional groups of soot.…”

Croissance des HAP et des suies et transition phase gaz-phase solide dans les processus de combustion

“…Une méthode complémentaire, d'autant plus efficace que la masse des échantillons de suies est très faible, est la technique de désorption laser/ionisation laser/spectrométrie de masse [10,32]. Elle peut être complétée par des analyses par FTIR ou Raman [36].…”