Bikau Shukla

Role of Phenyl Radicals in the Growth of Polycyclic Aromatic Hydrocarbons

To investigate the role of phenyl radical in the growth of PAHs (polycyclic aromatic hydrocarbons), pyrolysis of toluene with and without benzene has been studied by using a heatable tubular reactor couple with an in-situ sampling vacuum ultraviolet (VUV) single photon ionization (SPI) time-of-flight mass spectrometer (TOFMS) at temperatures 1155-1467 K and a pressure of 10.02 Torr with 0.56 s residence time. When benzene was added, a significant increase of phenyl addition products (biphenyl, terphenyl, and triphenylene) was observed and the mass spectra showed a clear regular sequence with an interval of approximately 74 mass number, corresponding to the phenyl addition (+C6H5) followed by H-elimination (-H) and cyclization (-H2). The analysis showed that the PAC (phenyl addition/cylization) mechanism is efficient for the growth of PAHs without a triple fusing site, for which the HACA (hydrogen abstraction/C2H2 addition) step is inefficient, and produces PAHs with five-membered rings. The PAC process was also suggested to be efficient in the subsequent growth of PAHs with five-membered rings. The role of the PAC mechanism in combustion conditions is discussed in relation to the importance of disordered five-membered ring structure in fullerene or soot core.

Role of methyl radicals in the growth of PAHs

The role of methyl radicals in the networking of sp2 carbons has been explored through kinetic analysis of mass spectra of the gas-phase products of the pyrolysis of toluene and toluene/acetone mixtures. Pyrolytic reactions were performed in a flow tube reactor at temperatures of 1140–1320 K and a constant total pressure of 10.38 Torr with a residence time of 0.585 s. On addition of acetone, methyl substituted products and their derivatives were enhanced. Mass peaks were observed in several sequences at an interval of 14 mass units; these ions correspond to methyl substituted products formed as a result of hydrogen abstraction (−H) followed by methyl radical addition (+CH3). Each major peak was usually preceded by a peak at two mass units lower, which was likely produced through dehydrogenation/dehydrocyclization (−H2) of methyl substituted products. Detected species include a large number of alkyl, cyclotetrafused (CT), cyclopentafused (CP) mono-, di-, and polycyclic aromatic hydrocarbons (PAHs) along with primary PAHs. The analysis showed that MAC (methyl addition/cyclization) has a unique capacity to induce the sequential growth of hexagonal networks of sp2 carbons from all fusing sites [1] of a PAH. Moreover, MAC was found capable of answering an important question in PAH growth, which is expansion of the CT → CP → hexagonal network for which other reported mechanisms are inefficient.

Novel Products from C6H5 + C6H6/C6H5 Reactions

To date only one product, biphenyl, has been reported to be produced from C(6)H(5) + C(6)H(6)/C(6)H(5) reactions. In this study, we have investigated some unique products of C(6)H(5) + C(6)H(6)/C(6)H(5) reactions via both experimental observation and theoretical modeling. In the experimental study, gas-phase reaction products produced from the pyrolysis of selected aromatics and aromatic/acetylene mixtures were detected by an in situ technique, vacuum ultraviolet (VUV) single photon ionization (SPI) time-of-flight mass spectrometry (TOFMS). The mass spectra revealed a remarkable correlation in mass peaks at m/z = 154 {C(12)H(10) (biphenyl)} and m/z = 152 {C(12)H(8) (?)}. It also demonstrated an unexpected correlation among the HACA (hydrogen abstraction and acetylene addition) products at m/z = 78, 102, 128, 152, and 176. The analysis of formation routes of products suggested the contribution of some other isomers in addition to a well-known candidate, acenaphthylene, in the mass peak at m/z = 152 (C(12)H(8)). Considering the difficulties of identifying the contributing isomers from an observed mass number peak, quantum chemical calculations for the above-mentioned reactions were performed. As a result, cyclopenta[a]indene, as-indacene, s-indacene, biphenylene, acenaphthylene, and naphthalene appeared as novel products, produced from the possible channels of C(6)H(5) + C(6)H(6)/C(6)H(5) reactions rather than from their previously reported formation pathways. The most notable point is the production of acenaphthylene and naphthalene from C(6)H(5) + C(6)H(6)/C(6)H(5) reactions via the PAC (phenyl addition-cyclization) mechanism because, until now, both of them have been thought to be formed via the HACA routes. In this way, this study has paved the way for exploring alternative paths for other inefficient HACA routes using the PAC mechanism.

A Novel Method for Characterizing the Diameter of Single-Wall Carbon Nanotubes by Optical Absorption Spectra

The potentiality of optical absorption spectroscopy (OAS) for the estimation of mean diameter of single-wall carbon nanotubes (SWCNTs) from electronic transition energies has been explored. The observed dependence of electronic transition energies of both metallic and semiconducting SWCNTs on their mean diameters clearly showed that transition energies scale inversely with the tube diameter. In the present study, the applicability of this estimation method has been experimentally confirmed for the diameter range of 1–2 nm and is expected to be useful for the characterization of wide range of diameters of SWNCTs.

Fractionation of single wall carbon nanotubes by length using cross flow filtration method.

A novel system for fractionating single wall carbon nanotubes (SWCNTs) by length via a three-step cross-flow filtration has been developed in which three membrane filters of different pore sizes, 1.0, 0.45, and 0.2 microm, were used. SWCNTs dispersed in water with the help of sodium carboxymethylcellulose (CMC) detergents were successfully sorted into four samples, and the atomic force microscopy (AFM) observation of those samples confirmed that their length distribution peaks are within the expected ranges from pore sizes of used filters. However, the result of the similar filtration process using a different detergent, sodium dodecylbenzenesulfonate (SDBS), showed no pronounced correlation between the length distribution of SWCNTs and the pore size. The observed difference in the sorting phenomena caused by the detergent type suggests that the permeation property depends on the complex structure resulting from the dispersed SWCNTs and detergent molecules.

Importance of fundamental sp, sp2, and sp3 hydrocarbon radicals in the growth of polycyclic aromatic hydrocarbons.

The most basic chemistry of products formation in hydrocarbons pyrolysis has been explored via a comparative experimental study on the roles of fundamental sp, sp(2), and sp(3) hydrocarbon radicals/intermediates such as ethyne/ethynyl (C(2)H(2)/C(2)H), ethene/ethenyl (C(2)H(4)/C(2)H(3)), and methane/methyl (CH(4)/CH(3)) in products formations. By using an in situ time-of-flight mass spectrometry technique, gas-phase products of pyrolysis of acetylene (ethyne, C(2)H(2)), ethylene (ethene, C(2)H(4)), and acetone (propanone, CH(3)COCH(3)) were detected and found to include small aliphatic products to large polycyclic aromatic hydrocarbons (PAHs) of mass 324 amu. Observed products mass spectra showed a remarkable sequence of mass peaks at regular mass number intervals of 24, 26, or 14 indicating the role of the particular corresponding radicals, ethynyl (C(2)H), ethenyl (C(2)H(3)), or methyl (CH(3)), in products formation. The analysis of results revealed the following: (a) product formation in hydrocarbon pyrolysis is dominated by hydrogen abstraction and a vinyl (ethenyl, C(2)H(3)) radical addition (HAVA) mechanism, (b) contrary to the existing concept of termination of products mass growth at cyclopenta fused species like acenaphthylene, novel pathways forming large PAHs were found succeeding beyond such cyclopenta fused species by the further addition of C(2)H(x) or CH(3) radicals, (c) production of cyclopenta ring-fused PAHs (CP-PAHs) such as fluoranthene/corannulene appeared as a preferred route over benzenoid species like pyrene/coronene, (d) because of the high reactivity of the CH(3) radical, it readily converts unbranched products into products with aliphatic chains (branched product), and (e) some interesting novel products such as dicarbon monoxide (C(2)O), tricarbon monoxide (C(3)O), and cyclic ketones were detected especially in acetone pyrolysis. These results finally suggest that existing kinetic models of product formation should be modified to include the reported novel species and their formation pathways. It is expected that outcomes of this study will be useful to understand the products formation from reactors to interstellar atmospheres as well as the growth mechanism of carbon nanomaterials.

Wall-Number Selectivity in Single/Double-Wall Carbon Nanotube Production by Enhanced Direct Injection Pyrolytic Synthesis

The effect of methane (CH4) gas as a secondary carbon source in addition to liquid feedstock, on the number of walls and the diameter of carbon nanotubes (CNTs) produced by floating-catalyst CVD under different conditions has been investigated in this study. Transmission electron microscopy reveals that the products mainly contain single-wall (SW) and double-wall (DW) CNTs with the relative abundance of DWCNTs being 9.2–50.7%, which increases linearly with increasing CH4 gas flow rate. In contrast, the distributions of the tube diameters of SW- and DWCNTs are almost constant and are much less dependent on the CH4 gas flow rate. These results suggest that the carbon generated from the pyrolysis of CH4 mainly contributes to the formation of additional walls during the CNT growth in the present CVD system.