On the mechanism of soot nucleation Frenklach, Michael; Mebel, Alexander M
Physical chemistry chemical physics : PCCP,
2020-Mar-04, Letnik:
22, Številka:
9
Journal Article
Recenzirano
Odprti dostop
The mechanism of carbon particulate (soot) inception has been a subject of numerous studies and debates. The article begins with a critical review of prior proposals, proceeds to the analysis of ...factors enabling the development of a meaningful nucleation flux, and then introduces new ideas that lead to the fulfillment of these requirements. In the new proposal, a rotationally-activated dimer is formed in the collision of an aromatic molecule and an aromatic radical; the two react during the lifetime of the dimer to form a stable, doubly-bonded bridge between them, with the reaction rooted in a five-member ring present on the molecule edge. Several such reactions were examined theoretically and the most promising one generated a measurable nucleation flux. The consistency of the proposed model with known aspects of soot particle nanostructure is discussed. The foundation of the new model is fundamentally the H-Abstraction-Carbon-Addition (HACA) mechanism with the reaction affinity enhanced by rotational excitation.
The present study undertakes a theoretical evaluation of thermal decomposition of aromatic-ring radicals. Potential energy surfaces and associated reaction rate coefficients were calculated for 1- ...and 2-naphthalenyl, acetanaphthylenyl, and pyrenyl radicals. Kinetic Monte-Carlo simulations were performed to examine the rates of ring cleavage in two sooting laminar premixed flames of ethylene. The simulations showed that the thermal aromatic-ring cleavage is comparable in rate to oxyradical decomposition in a heavier-sooting flame. The simulation also revealed, unexpectedly, fast internal ring radical migration, comparable in frequency to reaction events of aromatic growth.
Reaction-rate analysis of thermal ring cleavage in sooting flames shows that it can be comparable in rate to oxyradical decomposition and that fast internal ring radical migration is comparable in frequency to reaction events of aromatic growth.
The article addresses the formation mechanisms of naphthalene and indene, which represent prototype polycyclic aromatic hydrocarbons (PAH) carrying two six-membered and one five- plus a six-membered ...ring. Theoretical studies of the relevant chemical reactions are overviewed in terms of their potential energy surfaces, rate constants, and product branching ratios; these data are compared with experimental measurements in crossed molecular beams and the pyrolytic chemical reactor emulating the extreme conditions in the interstellar medium (ISM) and the combustion-like environment, respectively. The outcome of the reactions potentially producing naphthalene and indene is shown to critically depend on temperature and pressure or collision energy and hence the reaction mechanisms and their contributions to the PAH growth can be rather different in the ISM, planetary atmospheres, and in combustion flames at different temperatures and pressures. Specifically, this paradigm is illustrated with new theoretical results for rate constants and product branching ratios for the reaction of phenyl radical with vinylacetylene. The analysis of the formation mechanisms of naphthalene and its derivatives shows that in combustion they can be produced via hydrogen-abstraction-acetylene-addition (HACA) routes, recombination of cyclopentadienyl radical with itself and with cyclopentadiene, the reaction of benzyl radical with propargyl, methylation of indenyl radical, and the reactions of phenyl radical with vinylacetylene and 1,3-butadiene. In extreme astrochemical conditions, naphthalene and dihydronaphthalene can be formed in the C6H5 + vinylacetylene and C6H5 + 1,3-butadiene reactions, respectively. Ethynyl-substituted naphthalenes can be produced via the ethynyl addition mechanism beginning with benzene (in dehydrogenated forms) or with styrene. The formation mechanisms of indene in combustion include the reactions of the phenyl radical with C3H4 isomers allene and propyne, reaction of the benzyl radical with acetylene, and unimolecular decomposition of the 1-phenylallyl radical originating from 3-phenylpropene, a product of the C6H5 + propene reaction, or from C6H5 + C3H5.
Rotationally excited dimerization of aromatic moieties, a mechanism proposed recently to explain the initial steps of soot particle inception in combustion and pyrolysis of hydrocarbons, produces a ...molecular structure, termed E-bridge, combining the two aromatics via five-membered aromatic rings sharing a common bond. The present study investigates a hydrogen-mediated addition of acetylene to the fused five-membered ring part of the E-bridge forming a seven-membered ring. The carried out quantum-mechanical and rate theoretical calculations indicate the plausibility of such capping reactions, and kinetic Monte Carlo simulations demonstrate their frequent occurrence. The capping frequency, however, is limited by “splitting” the fused five-membered bridge due to five-membered ring migration. A similar migration of edge seven-membered rings is shown to be also rapid but short, as their encounter with five-membered rings converts them both into six-membered rings.
Polycyclic aromatic hydrocarbons (PAHs) represent key molecular building blocks leading to carbonaceous nanoparticles identified in combustion systems and extraterrestrial environments. However, the ...understanding of their formation and growth in these high temperature environments has remained elusive. We present a mechanism through laboratory experiments and computations revealing how the prototype PAH-naphthalene-can be efficiently formed via a rapid 1-indenyl radical-methyl radical reaction. This versatile route converts five- to six-membered rings and provides a detailed view of high temperature mass growth processes that can eventually lead to graphene-type PAHs and two-dimensional nanostructures providing a radical new view about the transformations of carbon in our universe.
RRKM-Master Equation calculations have been performed to evaluate temperature- and pressure-dependent rate coefficients for acetylene addition reactions to the C6H5, C6H4C2H, C6H5C2H2, and C6H4C2H3 ...radicals. These calculations indicate a strong pressure dependence for the role of various Hydrogen-Abstraction-C2H2-Addition (HACA) sequences for the formation of naphthalene from benzene. At atmospheric and lower pressures the C8H7 radicals, C6H4C2H3 and C6H5C2H2, cannot be stabilized above 1650K. As a result, both the Bittner–Howard HACA route, in which a second acetylene molecule adds to C6H5C2H2, and the modified Frenklach route, where a second C2H2 adds to the aromatic ring of C6H4C2H3 obtained by internal hydrogen abstraction, are unrealistic under low pressure flame conditions. At the higher pressures of some practical combustion devices (e.g., 100atm) these routes may be operative. Naphthalene is predicted to be the main product of the C6H5C2H2+C2H2 and C6H4C2H3+C2H2 reactions in the entire 500–2500K temperature range independent of pressure (ignoring the issues related to the instability of C8H7 species). Frenklach's original HACA route, where the second C2H2 molecule adds to the aromatic ring activated by intermolecular H abstraction from C8H6, involves the C6H4C2H+C2H2 reaction, which is shown to predominantly form dehydrogenated species with a naphthalene core (naphthyl radicals or naphthynes) at T <2000K and diethynylbenzene at higher temperatures. The temperature and pressure dependence of rate coefficients for the various reaction channels has been analyzed and the results clearly demonstrate the importance of pressure for the reaction outcome. Thus, one must use caution when using low-pressure flame studies to validate PAH mechanisms for use in broader ranges of pressure.
An analytical protocol based on surface-enhanced Raman spectroscopy (SERS) and aimed at the detection of toxicologically relevant concentrations of JWH-018 in oral fluid is presented for the first ...time. A DFT-supported in-depth vibrational characterization of the drug in the solid state and in solution was also performed, providing a body of literature for future spectroscopic work on the compound. A Langmuir adsorption model was used to derive quantitative parameters such as the affinity of JWH-018 for citrate-capped gold nanospheres as well as the LOD. The application of the implemented method to the analysis of extracts from fortified oral fluid samples demonstrates the feasibility of SERS as an alternative to current immunoassays as a screening tool for use in emergency room settings.
In previous studies, AuAg colloidal nanostar formulations were developed with the two-fold aim of producing optimized surface-enhanced Raman spectroscopy (SERS) substrates and investigating the ...nature of the capping process itself. Findings demonstrated that the nanoparticle metals are alloyed and neutral, and capping by stabilizers occurs via chemisorption. This study utilizes citrate as the model stabilizer and investigates the mechanistic aspects of its interaction with mono- (Au20) and bimetallic (Au19Ag) surfaces by density functional theory (DFT) calculations. Citrate was modeled according to the colloid’s pH and surrounded by a water and sodium first solvation shell. A population of stable cluster–citrate structures was obtained, and energies were refined at the uB3LYP//LANL2TZ(f)/cc-pVTZ level of theory. Solvation was accounted for both explicitly and implicitly by the application of the continuum model SMD. Results indicate that both direct binding and binding by water proxy through the charge-transfer complex formation are thermodynamically favorable. Water participation in citrate adsorption is supported by the adsorption behavior observed experimentally and the comparison between experimental and DFT-simulated IR spectra. Vibrational mode analysis suggests the possible presence of water within a crystal in dried nanostar residues. All ΔG ads(aq) indicate a weak chemisorptive process, leading to the hypothesis that citrate could be displaced by analytes during SERS measurements.
The energetics and kinetics of phenalene and phenalenyl growth reactions were studied theoretically. Rate constants of phenalene and phenalenyl H-abstraction and C
2
H
2
addition to the formed ...radicals were evaluated through quantum-chemical and rate-theory calculations. The obtained values, assigned to all π radicals, were tested in deterministic and kinetic Monte Carlo simulations of aromatics growth under conditions of laminar premixed flames. Kekulé and non-Kekulé structures of the polycyclic aromatic hydrocarbons (PAHs) evolving in the stochastic simulations were identified by on-the-fly constrained optimization. The numerical results demonstrated an increased PAH growth and qualitatively reproduced experimental observations of Homann and co-workers of non-decaying PAH concentrations with nearly equal abundances of even and odd carbon-atom PAHs. The analysis revealed that the PAH growth proceeds
via
alternating and sterically diverse acetylene and methyl HACA additions. The rapid and diverse spreading in the PAH population supports a nucleation model as PAH dimerization, assisted by the non-equilibrium phenomena, forming planar aromatics first and then transitioning to the PAH-PAH stacking with size.
Phenalenyl growth reactions were studied theoretically. Application of the computed rate constants to all π radicals revealed that the PAH growth proceeds
via
alternating and sterically diverse acetylene and methyl HACA additions.
Nonequilibrium precursor mediated kinetics has been discovered for reactions of gaseous molecules at high temperatures. A theoretical analysis was carried out on dimerization of midsize polycyclic ...aromatic hydrocarbons (PAH), the presumed critical step in formation of carbonaceous particles in terrestrial and extraterrestrial environments. The nonequilibrium precursor state originates from inelastic collisional dynamics of two PAH monomers, with low-frequency modes acting as a sink for translational energy in the reaction coordinate. Owing to the prolonged lifetime of the nonequilibrium physical dimer, the probability of chemical dimerization increases by an order of magnitude. This phenomenon brings us closer to a solution for the carbon-particle inception puzzle and should prove useful for the fundamental understanding of gas-phase chemical reactions involving large molecules.
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