Composition profiles and reaction mechanisms in a near-sooting premixed benzene/oxygen/argon flame

Bittner, J.D.; Howard, Jack B.

doi:10.1016/s0082-0784(81)80115-4

Cited by 306 publications

(284 citation statements)

References 48 publications

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“…The elimination of CO from oxidized PAH is thought to be a source of fivemembered rings [115] in the structure of combustion-generated PAH, which are precursors to fullerenes in flames [32,92,116]. However, the highest concentration of fullerenes is in the region of the flame where the precursor concentration is decreasing due to oxidation.…”

Section: Fullerenesmentioning

confidence: 99%

Combustion synthesis of fullerenes and fullerenic nanostructures

Goel

Hebgen

Sande

et al. 2002

Carbon

117

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Fullerenes are molecules comprised entirely of sp 2 -bonded carbon atoms arranged in pentagonal and hexagonal rings to form a hollow, closed-cage structure. Buckyballs, a subset which contains C 60 and C 70 , are single-shell molecules while fullerenic nanostructures can contain many shells and over 300 carbon atoms. Both fullerenes and nanostructures have an array of applications in a wide variety of fields, including medical and consumer products. Fullerenes were discovered in 1985 and were first isolated from the products of a laminar low-pressure premixed benzene/oxygen/argon flame operating at fuel-rich conditions in 1991. Flame studies indicated that fullerene yields depend on operating parameters such as temperature, pressure, residence time, and equivalence ratio. High-resolution transmission electron microscopy (HRTEM) showed that the soot contains nanostructures, including onions and nanotubes.Although flame conditions for forming fullerenes have been identified, the process has not been optimized and many flame environments of potential interest are unstudied. Mechanistic characteristics of fullerene formation remain poorly understood and cost estimation of large-scale production has not been performed. Accordingly, this work focused on: 1) studying fullerene formation in diffusion and premixed flames under new conditions to identify optimal parameters; 2) investigating the reaction of fullerenes with soot; 3) positively identifying C 60 molecules in HRTEM by tethering them to carbon black; and 4) providing a cost estimation for industrial fullerenic soot production.Samples of condensable material from laminar low-pressure benzene/argon/oxygen diffusion flames were collected and analyzed by high-performance liquid chromatography (HPLC) and HRTEM. The highest concentration of fullerenes in a flame was always detected just above the height where the fuel is consumed. The percentage of fullerenes in condensable material increases with decreasing pressure and the fullerene content of flames with similar cold gas velocities shows a strong dependence on length. A shorter flame, resulting from higher dilution or lower pressure, favors the formation of fullerenes rather than soot, exhibited by the lower amount of soot and precursors in such flames. This indicates a stronger correlation of fullerene consumption to soot levels than of fullerene formation to precursor concentration. The maximum flame temperature seems to be of minor importance in formation. The overall highest amount of fullerenes was found for a surprisingly high dilution of fuel with argon. The HRTEM analysis showed an increase of the curvature of the carbon layers, and hence increased fullerenic character, with increasing distance from the burner up to the point of maximum fullerene concentration, after which it decreases, consistent with the HPLC analysis. The soot shows highly ordered regions that appear to have been cells of fullerenic nanostructure formation. The samples also included fullerenic nanostructures such as tubes and sp...

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Section: Fullerenesmentioning

confidence: 99%

Combustion synthesis of fullerenes and fullerenic nanostructures

Goel

Hebgen

Sande

et al. 2002

Carbon

117

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“…Here, PAHs are suggested to be formed via "polymerization" of acetylene via the HACA mechanism (hydrogen abstraction acetylene addition) starting with the addition of a phenyl radical to acetylene 27 followed by acetylene additions eventually closing the secondary ring. 31 However, recent crossed beam studies 32,33 and electronic structure calculations 34 showed that the reaction of phenyl radicals (C 6 H 5 ) with acetylene (C 2 H 2 ), which leads to the synthesis of phenylacetylene, has an entrance barrier of 16 kJ mol -1 . This barrier can certainly be overcome in high temperature combustion flames, but not in the low temperature, hydrocarbon-rich atmosphere of Titan (80-160 K).…”

Section: Introductionmentioning

confidence: 99%

Crossed Molecular Beam Study on the Formation of Phenylacetylene and Its Relevance to Titan’s Atmosphere

et al. 2010

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The crossed molecular beam experiment of the deuterated ethynyl radical (C 2 D; X 2 Σ + ) with benzene [C 6 H 6 (X 1 A 1g )] and its fully deuterated analog [C 6 D 6 (X 1 A 1g )] was conducted at a collision energy of 58.1 kJ mol -1 . Our experimental data suggest the formation of the phenylacetylene-d 6 via indirect reactive scattering dynamics through a long-lived reaction intermediate; the reaction is initiated by a barrierless addition of the ethynyl-d 1 radical to benzene-d 6 . This initial collision complex was found to decompose via a tight exit transition state located about 42 kJ mol -1 above the separated products; here, the deuterium atom is ejected almost perpendicularly to the rotational plane of the decomposing intermediate and almost parallel to the total angular momentum vector. The overall experimental exoergicity of the reaction is shown to be 121 ( 10 kJ mol -1 ; this compares nicely with the computed reaction energy of -111 kJ mol -1 . Even though the experiment was conducted at a collisional energy higher than equivalent temperatures typically found in the atmosphere of Titan (94 K and higher), the reaction may proceed in Titan's atmosphere as it involves no entrance barrier, all transition states involved are below the energy of the separated reactants, and the reaction is exoergic. Further, the phenylacetylene was found to be the sole reaction product.

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“…The soot particle sizes are 2-8 nm for the C 6 H 6 flame [5], and 5-15 nm for the C 2 H 2 flame [2] for the ranges in height above burner corresponding to the peak of k Harris to the last points plotted. Also shown are a [H] profile [21] for the C 6 H 6 flame and an experimental [H] profile for a similar C 6 H 6 flame with ‫ס‬ 1.8 [22]. As mentioned before, the radicals increase in the region of interest, obviating the radical-based interpretation of declining soot reactivity from explaining the decline in k Harris An analysis similar to that above was done for the two PFR conditions shown before.…”

Section: Analysis Of Soot Growth Ratementioning

confidence: 97%

“…However, in the PFR, [H] decays only toward equilibrium, as seen in both measurements and simulations [13], in contrast to the oscillations and net increase that would be required to give the experimental soot concentration profiles. In premixed one-dimensional aromatics flames, soot inception and the decline in apparent reactivity [5] occur while primary oxidation is still taking place and [H], [OH], and T are increasing [21][22][23], in conflict with the radical-based hypothesis. Even for premixed aliphatic flames, in which soot formation occurs in the burned gas region, where [H] and [OH] are decaying toward equilibrium [24,25], Colket and Hall [26] found that the decay of the apparent C 2 H 2 -soot growth rate constant, experimentally determined for C 2 H 4 flames [3] and C 2 H 2 flames [4], does not match the decay predicted by the radical-based hypothesis.…”

Section: Analysis Of Soot Growth Ratementioning

confidence: 99%

Analysis of soot surface growth pathways using published plug-flow reactor data with new particle size distribution measurements and published premixed flame data

Kronholm

Howard

2000

Proceedings of the Combustion Institute

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Soot particle size distributions were measured using thermophoretic sampling, followed by electron microscopy, at different residence times during soot formation in a plug-flow reactor (PFR) under two sets of premixed C 2 H 4 /air combustion conditions (1 atm, equivalence ratio 2.2, and 1520 K and 1620 K) for which published concentration profiles of gas species and soot mass are available. The data were used to calculate the Harris global rate coefficient, k Harris , for C 2 H 2 addition to soot. The C 2 H 2 -soot reactivity as indicated by k Harris is found to oscillate significantly and to show a net increase with residence time in the PFR, neither of which behaviors would be expected if C 2 H 2 were actually the growth reactant. In contrast, use of the data to compute the global collision efficiency for dichloromethane (DCM) soluble polycyclic aromatic hydrocarbon (PAH) addition to soot, c PAH-soot , gives a soot reactivity with no net increase with residence time, and smaller oscillations explainable by the observation that higher molecular weight PAH fluctuate more than the total DCM soluble fraction. Interpreting the Harris C 2 H 2 addition mechanism as a globalization of the PAH addition mechanism with constant soot reactivity indicates that the oscillations and net increase of the Harris C 2 H 2 -soot reactivity are consistent with the variations in the concentration of the PAH reactants and particle size. Also consistent with these results are previous observations that the Harris C 2 H 2 -soot reactivity in premixed flames decreases sharply with residence time. Analysis here of published data from premixed one-dimensional C 2 H 2 /O 2 and C 6 H 6 /O 2 /Ar flames shows that the declining C 2 H 2 -soot reactivity agrees with the decline in the concentration of PAH reactants and increase in soot particle size, assuming a constant PAH-soot reactivity.

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Composition profiles and reaction mechanisms in a near-sooting premixed benzene/oxygen/argon flame

Cited by 306 publications

References 48 publications

Combustion synthesis of fullerenes and fullerenic nanostructures

Combustion synthesis of fullerenes and fullerenic nanostructures

Crossed Molecular Beam Study on the Formation of Phenylacetylene and Its Relevance to Titan’s Atmosphere

Analysis of soot surface growth pathways using published plug-flow reactor data with new particle size distribution measurements and published premixed flame data

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