| dc.description.abstract | Fullerenes are molecules comprised entirely of sp2-bonded carbon atoms arranged in pentagonal and hexagonal rings to form a hollow, closed-cage structure. Fullerenes, such as C60 and C70, are single-shell molecules, while carbon nanostructures--a larger class of structures that includes fullerenes as a subset--typically contain many shells and hundreds or thousands of carbon atoms. C60 and C70, first discovered in 1985, were isolated macroscopically in 1991 from soot produced in laminar low pressure premixed benzene/oxygen/argon flames operated at fuel-rich conditions. Studies of these flames indicated that fullerene yields depend on adjustable parameters like temperature, pressure, atomic carbon/oxygen ratio, and residence time. In addition, high resolution transmission electron microscopy (HRTEM) showed that benzene flame soot also contains carbon nanostructures, including fullerene onions and nanotubes. Although some conditions under which fullerenes form in flames have been identified, little is known about the formation mechanisms of either fullerenes or carbon nanostructures. One possible mechanism involves a molecular weight growth process analogous to soot formation including 1) the stepwise addition of acetylene to curved precursor molecules and 2) the coagulation of aromatic precursor molecules, followed by bond rearrangement to form the closed-cage structure. Polycyclic aromatic hydrocarbons (P AH), which participate in soot nucleation and growth, are potential precursors in these mechanisms. Carbon nanostructures may form by a similar molecular weight growth process or by the rearrangement of carbon material in the condensed soot. Understanding these mechanisms and modeling the formation kinetics is important if combustion is to be used as a process for the synthesis of fullerenes and carbon nanostructures. Therefore, this work focuses on 1) developing a detailed understanding of the fullerenes formation mechanisms in premixed benzene/oxygen/argon flames by measuring concentration profiles for fullerenes (C60, C70, C76, C7s, and C84)., PAH, and light gas species and using the data to evaluate kinetic models consistent with proposed mechanisms and 2) understanding how carbon nanostructures form and evolve in premixed benzene/oxygen/ argon flames by using HRTEM to observe changes in soot and nanostructures with residence time in the flame. A laminar premixed benzene/oxygen/argon flat flame was operated at the following conditions: fuel equivalence ratio, 2.4 (atomic C/0 ratio, 0.96); cold gas velocity at the burner, 25 emfs; pressure, 40 torr; and fraction of argon in fuel mixture, 10 mol%. Concentrations of C6o, C10, C16, C1s, and C84 and 14 P AH were measured at different axial distances (residence times) in the flame, and an additional 16 PAH were identified without quantitation, by sampling condensible flame material through a quartz probe and analyzing the samples by high performance liquid chromatography (HPLC) and gas chromatography/mass spectrometry (GC/MS). The fullerenes concentration profiles show two regions of fullerenes formation and conswnption. The first region, at short residence times in the flame, coincides with the onset of soot formation and immediately follows the maximwn P AH concentration in this flame. The rate of conswnption of P AH is more than sufficient to account for the rate of formation offullerenes in this region of the flame, consistent with the view that reactive coagulation of P AH could be the dominant pathway for fullerenes formation. The second region, at longer residence times, shows significantly higher fullerenes concentrations and occurs in a part of the flame where the concentration of P AH is below the detection limit but where the concentration of acetylene remains high enough for acetylene to be the main reactant in this region of fullerenes formation. The observed rate of conswnption of acetylene through this region is more than sufficient to account for the observed rate of formation of fullerenes. The decrease in fullerenes concentration in the downstream part of both regions appears to be a result of competition between formation and conswnption reactions. Calculations show that neither oxidation nor pyrolysis alone can account for the observed conswnption of fullerenes, but reactions with soot may explain the observed conswnption. The fullerenes may be incorporated into the soot as surface growth species, and conswnption dominates when the concentration of PAH o; acetylene, which are the reactants for fullerenes formation, is lowered sufficiently. A study of changes in soot and carbon nanostructures with residence time in the same premixed benzene/oxygen/argon flame was conducted. Samples were taken by three different methods: scraping solids from surfaces inside the combustion chamber, collecting material from different vertical positions in the flame through a quartz probe, and collecting condensible material on an electron microscope grid at different vertical positions in the flame with a thermophoretic sampler. HRTEM imaging of all samples showed soot particles composed to some extent of amorphous and fullerenic carbon (i.e., curved layers, spiral shells, ,and fullerene molecule-sized closed-shell structures). Qualitative and quantitative analyses ofresidence time-resolved samples showed that the carbon layers increase in length and decrease in radius with increasing residence time in the flame and that the nwnber of closed-shell structures, possibly fullerene molecules, increases with residence time in the flame. This observation is consistent with fullerenes concentration increasing with residence time and with a consumption pathway in which fullerenes react with soot. Overall, the data suggest that the formation of amorphous and fullerenic carbon occurs in milliseconds, with the fullerenic carbon becoming more curved as a soot particle traverses the length of the flame. This formation process is consistent with the heterogeneous reaction of gas phase P AH or light hydrocarbons with carbon layers in the solid phase soot. Conversely, the formation of carbon nanostructures, such as nanotubes and fullerene onions, appears to require much longer residence times, perhaps seconds or minutes. This is consistent with the internal rearrangement of carbon layers in the solid phase which appears to occur while the soot is exposed to the high temperature flame environment for extended periods of time. | en_US |
