Christopher J. Pope

Fullerenes synthesis in combustion

Abstract The early suggestion in fullerenes research that fullerenes might be produced in flames was soon supported by the observation of polyhedral carbon ions in flames and in 1991 was confirmed by the recovery and identification of fullerenes C60 and C70 from benzene/oxygen flames. Recent research has determined the effects of pressure, carbon/oxygen ratio, temperature and the type and concentration of diluent gas, on the yields of C60 and C70 in subatmospheric pressure premixed laminar flames of benzene and oxygen. Similar flames but with acetylene as fuel have also been found to produce fullerenes, but in smaller yields than with benzene fuel. The largest observed yields of C60 + C70 from benzene/oxygen flames are substantial, being 20% of the soot produced and 0.5% of the carbon fed. The largest rate of production of C60 + C70 was observed at a pressure of 69 Torr, a C/O ratio of 0.989 and a dilution of 25% helium. Several striking differences between fullerenes formation in flames as compared to the widely used graphite vaporization method include, in the case of flames, an ability to vary the C70/C60 ratio from 0.26 to 8.8 (cf., 0.02 to 0.18 for graphite vaporization) by adjustment of flame conditions and production of several isomers each of fullerenes C 60, C70, C60O and C70O. Many of the apparent isomers are thermally metastable, one C60 converting to the most stable form with a half-life of 1h at 111°C. The structures of the apparent C60 and C70 isomers necessarily must include abutting five-membered rings, previously assumed to be disallowed because of their high strain energy. The chemistry of fullerenes formation in flames is in some ways similar to that of soot formation, but important differences are seen and assumed to reflect the effects of the curved, strained structures of fullerenes and their precursors.

Effects of PAH isomerizations on mutagenicity of combustion products

Abstract Most of the mutagenicity of mixtures of polycyclic aromatic hydrocarbons (PAH) mixtures found in combustion exhaust gases is contributed by a relatively small number of the many PAH present. Since PAH mutagenicity is structure and hence isomer sensitive, changes in the distribution of isomers can change the mutagenicity of the mixture. Whether isomerization reactions in combustion play a significant role in determining the distributions of PAH isomers and the mutagenicity of product mixtures is assessed here for the following pairs of isomers: 1. (1) fluoranthene-pyrene 2. (2) fluoranthene-acephenanthrylene 3. (3) cyclopenta[ cd ]pyrene-benzo[ ghi ]fluoranthene 4. (4) benzo[ k ]fluoranthene-benzo[ a ]pyrene Concentration ratios of the isomer pairs were measured in ethylene combustion with naphthalene injection using a plug flow reactor at equivalence ratios of 1.2 and 2.2 and temperatures of 1520, 1620, and 1705 K, and compared with equilibrium ratios based on properties computed from molecular mechanics and semiempirical quantum mechanical programs [MM3; MNDO, AM1, and PM3 in both restricted- and unrestricted-Hartree-Fock forms]. Bacterial mutagenicity was measured by a forward mutation assay using Salmonella in the presence of rat liver supernatant, and found to vary significantly among the above compounds. The measured concentration ratios for isomer pairs (2) and (3) are near the equilibrium values and becoming more so as temperature increases, but the measured ratios for isomer pairs (1) and (4) are far from the equilibrium values at all the temperatures. From kinetics estimations, the characteristic isomerization time for isomer pairs (2) and (3) at 1705 K and perhaps at 1620 K is less than the experimental residence times, while the only isomerization mechanisms envisioned for isomer pairs (1) and (4) would not be kinetically viable at these temperatures. These results indicate that isomerizations affecting mutagenicity are significant under these conditions for isomer pairs (2) and (3), which involve only compounds containing both five- and six-membered rings, but not for isomer pairs (1) and (4), each of which includes a compound containing only six- membered rings.

Simultaneous Particle and Molecule Modeling (SPAMM): An Approach for Combining Sectional Aerosol Equations and Elementary Gas-Phase Reactions

ABSTRACT A method for writing the sectional aerosol equations in a form suitable for combination with a detailed kinetic model for gas-phase reactions has been developed. The sectional equations are given for arbitrary values of d, the ratio of molecular weights of adjacent bin boundaries, and are solved for d ≥ 2 for three different cases of the intra-bin distribution: (1) constant mass density w.r.t. ln(v), where v = molecular weight; (2) constant number density w.r.t. ln(v), or alternately, constant mass density w.r.t. v; (3) constant number density w.r.t. v. All the solutions given conserve mass; the extent of deviation from conservation of particle number is evaluated. An example of the approach is given for soot formation in combustion. The aerosol sections describe mass above 400 amu, with d = 2. Aerosol reactions are: bin-bin coagulation; addition of C2H2; addition of polycyclic aromatic hydrocarbons (PAH) with 2 to 10 benzenoid rings; oxidation via OH attack. The set of aerosol reactions is appen...

Thermodynamic limitations for fullerene formation in flames

Thermodynamic driving forces for fullerene formation in flames are considered from three different perspectives: 1) global equilibrium, 2) free energy changes for individual reactions leading to fullerene formation, and 3) relative stabilities of C30Hx polycyclic aromatic hydrocarbons (PAH). The ranges of conditions which promote formation of fullerenes and their precursors are determined.

Variation of equivalence ratio and element ratios with distance from burner in premixed one-dimensional flames

Differential diffusion effects in a low-pressure premixed, laminar, one-dimensional benzene/oxygen/argon flame are shown to lead to changes in the elemental mass fractions. The effect is caused by unequal diffusion of reactants away from the burner and products towards the burner. Analysis of data previously collected from this flame shows the local equivalence ratio to vary by as much as 25% from its inlet value, with similar variations found for the atomic C/O ratio (up to 25%) and atomic C/H ratio (up to 10%). Variations of comparable magnitude are predicted by detailed kinetic modeling of the same flame using a published mechanism. The variations predicted for a similar flame but at atmospheric pressure are slightly smaller than those at low-pressure, which is consistent with the constitutive equations. Similar effects would be expected for any flame in which the mass average flame velocity is comparable to the diffusion velocities of the major flame species. The results have implications for consideration of molecular-weight growth and soot formation in such systems, since these processes are strongly dependent upon the C/O and equivalence ratios.

Further testing of the fullerene formation mechanism with predictions of temperature and pressure trends

A previously published detailed kinetic mechanism for C 60 and C 70 formation in flames is explored beyond the previously performed preliminary test of kinetic plausibility. Consideration of the sensitivity of model predictions to rate coefficients for the specific reaction types shows that the only remaining area of uncertainty is in how the forward rates of the individual C 2 H 2 addition/ring-formation reactions are affected by the varying amounts of curvature-induced strain introduced. An overall test of the mechanisms sensitivity to the thermochemical properties of the species reinforces the kinetic plausibility of the mechanism by confirming that none of the reactions in the mechanism has an insuperable thermodynamic barrier. The mechanisms ability to predict trends with temperature and pressure is limited by the simplified modeling approach used. Existing data are consistent with maxima in fullerence formation with respect to both temperature and pressure. The mechanism predicts a maximum fullerence formation rate near 2100 K, reflecting competition between processes accelerated by higher temperatures (H abstraction and intramolecular rearrangements) and processes thermodynamically disfavored by higher temperatures (C 2 H 2 -addition/ring-formation and intramolecular rearrangements). Predictions of a monotonic increase in fullerene formation rate with increasing pressure are explained in terms of the chemical processes not yet included in the fullerene formation mechanism, most notably reactive coagulation of PAH with each other and with soot.

Synthesis of Fullerenes (C 60 AND C 70 ) by Combustion of Hydrocarbons in A Flat Flame Burner

Fullerene (C 60 and C 70 ) synthesis by combustion of ethylene and benzene in a flat flame burner was investigated. This method of fullerene synthesis is particularly attractive because of its potential of scale up. Also the ability to change the flame conditions andcontrol the yield of C 60 and C 70 makes this method versatile. No fullerenes were found in soot samples collected from the ethylene flame. However, fullerenes were formed in a benzene flame with C/O = 0.88 and operated at 40 torr, with cold gas velocity of 25.3 cm/s (273 K) and containing 10% argon. The concentration of fullerenes in this flame was found to depend strongly on the height above the burner surface. It exhibited a strong maxima at about 1.0 cm above the burner height suggesting the presence of both growth and destruction mechanisms.