Comparative NEXAFS Study on Soot Obtained from an Ethylene/Air Flame, a Diesel Engine, and Graphite

Abstract

Microstructure and molecular structure properties of different carbonaceous byproducts from combustion are fundamental to evaluate the radiative properties of such materials when combustion aerosols interact with solar radiation in atmosphere. Carbon near edge X-ray absorption fine structure (NEXAFS) spectroscopy is presented as an analytical technique for the carbon-specific characterization of various soot species. Ethylene soot, diesel soot, and carbon black NEXAFS spectra are discussed in relation to graphite as the most genuine model compounds of soot. Characteristic resonances in the spectra allow for the direct molecular speciation of the graphite-like solid core in soot, surface functional groups, and aromatic and aliphatic components, depending on the origin of the soot. Moreover, in the experimental case of ethylene soot samples probed at increasing distances from the burner nozzle, namely, at increasing residence times, NEXAFS experiments lead to the evidence that the soot collected near the nozzle appears to be more graphitized than soot sampled at larger heights-above-burner, in qualitative agreement with laser light scattering depolarization measurements reported here and previous findings obtained by electron microscopy characterization. The possible implications of the presented results on soot formation mechanism, combustion source attribution of carbon particulates, and UV-vis sunlight absorption by black carbon aerosols are briefly discussed.

Formation and emission of large furans and oxygenated hydrocarbons from flames

“…The majority of the species predicted by SNAPS experienced molecular growth as they evolved in the flame, and furans constituted the largest group of oxygenated species at large distance from the fuel outlet (DFFO), where the flame temperature was around 1,750 K. An important furan-destruction pathway identified in the simulations is CO reactions: CO can open furan rings and abstract the oxygen atom to form CO 2 , leaving the original furan ring as an aliphatic radical side chain. This mechanism may explain the observed aliphatics on the surface of soot particles under some conditions (25,27,31,39).…”

“…Previous work has suggested that oxygenated species can be attached to surfaces of soot particles of varying maturity emitted from flames and diesel engines, even before atmospheric processing (24)(25)(26)(27)(28)(29)(30)(31)(32). Functional groups that have been identified include alcohols/enols, carbonyls, peroxies, and ethers.…”

Dynamic Molecular Structure of Plant Biomass-Derived Black Carbon (Biochar)

“…The 1s-π* transitions at 285 eV are characteristic of aromatic double bonds, but lack information on the degree of condensation of these aromatic components. The 1s-σ* resonance, however, has been used to infer the degree of condensation and the long-range order of other carbonaceous materials (28,34). Sharp 1s-σ* exciton peaks absorbing between 291 and 292 eV (see Fig.…”

Chemical bonding and structure of black carbon reference materials and individual carbonaceous atmospheric aerosols

“…n-hexane soot, produced from a diffusion flame under quiescent conditions was supplied by ISCBCRM. The diesel soot NEXAFS spectrum was supplied by A. Braun [Braun, et al, 2004;di Stasio and Braun, 2006] and was acquired from NIST SRM 1650 purchased from NIST in Gaithersburg, MD, USA [Huggins, et al, 2000]. HULIS samples, including Suwannee…”

“…This offers advantages over techniques previously used to study chemical bonding in atmospheric particles that examine bulk samples [Mertes, et al, 2004;Sze, et al, 2001]. An extensive guide to NEXAFS spectroscopy is in existence [Stöhr, 2003] and also a review of NEXAFS spectroscopy related to environmental studies [Myneni, 2002], in addition to the application of the technique to carbonaceous particulate matter [Braun, et al, 2004;di Stasio and Braun, 2006] and atmospheric particles [Maria, et al, 2004;Russell, et al, 2002], thus only a brief outline of the technique is presented here. In NEXAFS spectroscopy, X-rays incident upon a sample are absorbed through excitations of core electrons into an unoccupied molecular orbital, resulting in considerable fine structure above the absorption edge.…”