Aromatic structures of carbonaceous materials and soot inferred by spectroscopic analysis (original) (raw)
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Carbon, 2008
A B S T R A C T Large polycyclic aromatic hydrocarbons were identified in carbon particulate sampled in a fuel-rich premixed ethylene flame. The particulate was extracted with dichloromethane (DCM) in order to separate the soluble organic species (DCM-extract) from the solid carbon (soot). After DCM extraction soot was re-extracted with N-methyl pyrrolidone (NMP) obtaining the NMP-extract. Both the DCM-extract and NMP-extract were further fractionated by size exclusion chromatography in selected molecular weight (MW) ranges. Large polycyclic aromatic hydrocarbons obtained by regular incorporation of C 2 and/or C 2 H 2 unit (24/26 rule) occurring in both odd and even series of carbon atom number of polycyclic aromatic hydrocarbons, were identified by laser desorption ionization-mass spectrometry (LDI-MS) analysis of the lighter MW fractions of both the DCM-extract and NMP-extract (100-400 u MW of the DCM-extract and 200-600 u fractions of the NMP-extract). The LDI-MS spectra of the heaviest MW fractions of DCM-extract and NMP-extract (600-2000 u and 600-5000 u fraction) showed a continuous spectrum of masses typical of polymeric structures. The UV-visible absorption and emission spectral analysis corroborated the assignment of lighter and of heavier fractions of DCM-extract and NMP-extract to PAH and to polymeric aromatic structures, respectively.
The Molecular Composition of Soot
Angewandte Chemie International Edition, 2020
Soot (sometimes referred to as black carbon) is produced when hydrocarbon fuels are burned. Our hypothesis is that polynuclear aromatic hydrocarbon (PAH) molecules are the dominant component of soot, with individual PAH molecules forming ordered stacks that agglomerate into primary particles (PP). Here we show that the PAH composition of soot can be exactly determined and spatially resolved by low‐fluence laser desorption ionization, coupled with high‐resolution mass spectrometry imaging. This analysis revealed that PAHs of 239–838 Da, containing few oxygenated species, comprise the soot observed in an ethylene diffusion flame. As informed by chemical graph theory (CGT), the vast majority of species observed in the sampled particulate matter may be described as benzenoids, consisting of only fused 6‐membered rings. Within that limit, there is clear evidence for the presence of radical PAH in the particulate samples. Further, for benzenoid structures the observed empirical formulae l...
Proceedings of the Combustion Institute, 2013
A reconstruction method of UV-Visible spectra, recently developed for discriminating the contribution of different molecular weight (MW) species to the spectral properties of carbonaceous species formed in a premixed ethylene flame, was applied to a benzene flame. The reliability and sensitivity of the technique in detecting specific structural features of the carbonaceous species was demonstrated. The flame-formed carbon species were deposited on quartz plates inserted in the soot formation region of a premixed fuel-rich benzene flame. The dichloromethane-soluble (DCM-soluble) and dry soot fractions were separated in a very wide MW range, from 200 to 1E11 u, by Size Exclusion Chromatography (SEC) coupled with on-line UV-Visible spectroscopy. The optical band gap (E g) and the UV peak position of each MW-segregated fraction were evaluated by means of a spectral reconstruction procedure, to get details on the carbon network structure in terms of sp 2 and sp 3 sites and size and stacking of the aromatic units. Moreover, the MW-segregated fractions of the species soluble in DCM were also analyzed by fluorescence emission spectroscopy and by atmospheric pressure laser desorption ionization-mass spectrometry for determining their aromatic character. The output of the spectral reconstruction procedure provided insights in the chemical structure and evolution of carbonaceous species from Polycyclic Aromatic Hydrocarbons (PAH) to soot nuclei and finally to soot aggregates. The comparison with data obtained in a premixed ethylene flame burning in similar temperature conditions showed some structural differences in flame-formed carbonaceous species due to the effect of the different chemical environment in which soot precursor are formed.
Aromatic site description of soot particles
Combustion and Flame, 2008
A new, advanced soot particle model is developed that describes soot particles by their aromatic structure, including functional site descriptions and a detailed surface chemistry mechanism. A methodology is presented for the description of polyaromatic hydrocarbon (PAH) structures by their functional sites. The model is based on statistics that describe aromatic structural information in the form of easily computed correlations, which were generated using a kinetic Monte Carlo algorithm to study the growth of single PAH molecules. A comprehensive surface reaction mechanism is presented to describe the growth and desorption of aromatic rings on PAHs. The model is capable of simulating whole particle ensembles which allows bulk properties such as soot volume fraction and number density to be found, as well as joint particle size and surface area distributions. The model is compared to the literature-standard soot model [J. Appel, H. Bockhorn, M. Frenklach, Combust. Flame 121 (2000) 122-136] in a plug-flow reactor and is shown to predict well the experimental results of soot mass, average particle size, and particle size distributions at different flow times. Finally, the carbon/hydrogen ratio and the distribution of average PAH sizes in the ensemble, as predicted by the model, are discussed.
UV-visible spectroscopy of organic carbon particulate sampled from ethylene/air flames
Chemosphere, 2001
A systematic comparison of spectra obtained with extra and in situ diagnostics in the soot preinception region of rich, premixed ethylene air¯ames suggests that combustion generated organic carbon (OC) particulate can be extracted from¯ames and isolated from other¯ame material for further chemical analysis. Both the trend with height above the burner and the form of UV¯uorescence and absorption spectra from extra situ sampled material captured in water agree with those measured in situ. These results show that the OC particulate formed in¯ames is partially water soluble. However, the collection eciency can be increased using less polar solvents, like acetonitrile and dichloromethane. Thē uorescence spectra from the water samples are comprised both a naphthalene-like component and a broad band UV uorescence component similar to that observed in situ which is attributed to¯ame generated OC particulate. The broad band UV¯uorescence centered around 320 nm is also observed very early in¯ames and does not change considerably with increasing¯ame residence time. These results support previous hypotheses that the UV broad band uorescence is from carbonaceous material comprised two-ring aromatics, formed earlier than soot in the¯ame, and is still present along with soot at higher heights or¯ame residence times. Ó
Combustion Science and Technology, 2012
The optical properties of carbonaceous species are object of research across different scientific fields as astrophysics, solid-state physics, atmospheric chemistry and physics, materials, petrology and combustion. Indeed, they are important to the aim of: identifying the composition of interstellar matter, evaluating the air quality and the relative impact on human health and climate change, determining properties of petroleum macromolecules, controlling the production of amorphous carbon-based materials, measuring soot loading and heat transfer in combustion aerosols. In this work a method of analysis of UV-Visible spectra of combustion-generated carbonaceous species was developed for investigating the relationship between the spectral features and the complex composition of carbonaceous species. The UV-Visible spectra of carbonaceous species, caught on quartz plates inserted along the axis of ethylene fuel-rich premixed flame, were measured. The contribution of the main components (dry soot and condensed organic species adsorbed on it) was firstly evaluated by measuring the UV-Visible spectra of the deposited particulate matter before and after treatment with dichloromethane (DCM). A deeper investigation of the contribution of components having different molecular weight (MW) was performed by Size Exclusion Chromatography (SEC) of the dry soot and DCMsoluble fractions and on-line measurements of UV-Visible spectra of MW-segregated fractions. These spectra were used for evaluating: i) the optical band gap of each MWsegregated fractions and ii) the UV peak position of the components of particulate matter by means of a deconvolution procedure of the dry soot and DCM-soluble species spectra. Structural information on the absorbers were obtained associating each band gap value with the relative UV peak position, very sensitive to the sp 2 and sp 3 sites and to the size and stacking of the aromatic units of the carbon network. Overall, this procedure has shown to be able to discriminate the contribution of the different components of particulate matter giving information on their molecular form.
Analytical and Bioanalytical Chemistry, 2008
The analysis of organic compounds in combustion exhaust particles and the chemical transformation of soot by nitrogen oxides are key aspects of assessment and mitigation of the climate and health effects of aerosol emissions from fossil fuel combustion and biomass burning. In this study we present experimental and analytical techniques for efficient investigation of oxygenated and nitrated derivatives of large polycyclic aromatic hydrocarbons (PAHs), which can be regarded as well-defined soot model substances. For coronene and hexabenzocoronene exposed to nitrogen dioxide under simulated diesel exhaust conditions, several reaction products with high molecular mass could be characterized by liquid chromatographyatmospheric pressure chemical (and photo) ionization-mass spectrometry (LC-APCI-MS and LC-APPI-MS). The main products of coronene contained odd numbers of nitrogen atoms (m/z 282, 256, 338), whereas one of the main products of hexabenzocoronene exhibited an even number of nitrogen atoms (m/z 391). Various reaction products containing carbonyl and nitro groups could be tentatively identified by combining chromatographic and mass spectrometric information, and changes of their relative abundance were observed to depend on the reaction conditions. This analytical strategy should highlight a relatively young technique for the characterization of various soot-contained, semi-volatile, and semi-polar reaction products of large PAHs.
Desorption of Polycyclic Aromatic Hydrocarbons from Soot Surface: Pyrene and Fluoranthene
Journal of Physical Chemistry A, 2008
The kinetics of the thermal desorption of a set of three-to five-ring polycyclic aromatic hydrocarbons (PAHs) from a laboratory-generated kerosene soot surface was studied over the temperature range 250-355 K in a low-pressure flow reactor combined with an electron-impact mass spectrometer. Two methods were used to measure the desorption rate constants: monitoring of the surface-bound PAH decays due to desorption using off-line HPLC measurements of their concentrations in soot samples and monitoring of the desorbed molecules (anthracene and phenanthtrene) in the gas phase using in situ mass spectrometric detection. The Arrhenius parameters (A factors and activation energies) for the desorption rate constants of 10 soot-bound PAHs were determined. The PAH-soot binding energies were found to be similar for PAHs with the same number of carbon atoms and to increase with increasing number of PAH carbon atoms. The experimental data are discussed in the frame of the existing theoretical gas to particle partitioning model.
Aromatics oxidation and soot formation in flames
1993
Scope This project is concerned with the kinetics and mechanisms of aromatics oxidation and the growth process to polycyclic aromatic hydrocarbons (PAH) of increasing size, soot and fullerenes formation in flames. The overall objective of the experimental aromatics oxidation work is to extend the set of available data by measuring concentration profiles for decomposition intermediates such as phenyl, cyclopentadienyl, phenoxy or indenyl radicals which could not be measured with molecular-beam mass spectrometry to permit further refinement and testing of benzene oxidation mechanisms. The focus includes PAH radicals which are thought to play a major role in the soot formation process while their concentrations are in many cases too low to permit measurement with conventional mass spectrometry. The radical species measurements are used in critical testing and improvement of a kinetic model describing benzene oxidation and PAH growth. Thermodynamic property data of selected species are determined computationally, for instance using density functional theory (DFT). Potential energy surfaces are explored in order to identify additional reaction pathways. The ultimate goal is to understand the conversion of high molecular weight compounds to nascent soot particles, to assess the roles of planar and curved PAH and relationships between soot and fullerenes formation. The specific aims are to characterize both the high molecular weight compounds involved in the nucleation of soot particles and the structure of soot including internal nanoscale features indicative of contributions of planar and/or curved PAH to particle inception. Recent Progress A) Experimental Experimental work assessing PAH reaction pathways has been finished. Mole fraction profiles of PAH radicals and the corresponding parent molecules were determined using nozzle beam sampling followed by radical scavenging reactions with dimethyl disulfide. Scavening products, i.e., methylthio compounds (R-SCH 3) and stable PAH were analysed by gas chromatography coupled with mass spectrometry (GC-MS). PAH isomers including specific radical sites were unambiguously identified using authentic standards, commercially available or synthesized in the present work. Two benzene/oxygen/argon flames (30% argon, cold gas velocity v= 50 cm s-1 , 20 torr) which were nearly sooting or slightly sooting with equivalence ratios of φ= 1.8 and 2.0, respectively, were investigated. All PAH profiles in the φ= 2.0 flame were shifted by ≈ 2 mm towards the burned gases compared to the φ= 1.8 flame. No increase of peak mole fractions was observed for one-and two-ring aromatics while an at least 2-fold increase was found for larger PAH. Experimental uncertainties were assessed by means of control experiments using equipment developed and built by Homann and coworkers, including combustion chamber, sampling and scavenging equipment. Shape and location of all measured compounds were very similar in both experiments while peak mole fractions were mostly lower in the control experiments. The results of the comparison ranged from a nearly perfect match such as for indene to ≈ 2-5-fold discrepancies. Larger discrepancies, observed in a few cases such as for 5ethynylacenaphthylene, were attributed to possible experimental errors, e.g., incomplete peak separation in the GC analysis. As a general trend, partial equilibrium of (8) PAH + H PAH• + H 2 is more and more closely approached with increasing size of the molecule. The experimental 2-naphthyl : phenyl mole fraction ratio was found to exceed the partial equilibrium value. Therefore, decomposition of PAH radicals forming butadiyne, e.g., the reverse of reaction (10) phenyl + C 4 H 2 2-naphthyl is suggested as contributing to the consumption of some PAH radicals. Thermodynamic considerations were found to be important but not sufficient for a complete understanding of PAH formation and consumption. Background, data, their analysis and conclusion are reported below.