Quan-De Wang

ReaxFF molecular dynamics simulations of oxidation of toluene at high temperatures.

Aromatic hydrocarbon fuels, such as toluene, are important components in real jet fuels. In this work, reactive molecular dynamics (MD) simulations employing the ReaxFF reactive force field have been performed to study the high-temperature oxidation mechanisms of toluene at different temperatures and densities with equivalence ratios ranging from 0.5 to 2.0. From the ReaxFF MD simulations, we have found that the initiation consumption of toluene is mainly through three ways, (1) the hydrogen abstraction reactions by oxygen molecules or other small radicals to form the benzyl radical, (2) the cleavage of the C-H bond to form benzyl and hydrogen radicals, and (3) the cleavage of the C-C bond to form phenyl and methyl radicals. These basic reaction mechanisms are in good agreement with available chemical kinetic models. The temperatures and densities have composite effects on toluene oxidation; concerning the effect of the equivalence ratio, the oxidation reaction rate is found to decrease with the increasing of equivalence ratio. The analysis of the initiation reaction of toluene shows that the hydrogen abstraction reaction dominates the initial reaction stage at low equivalence ratio (0.5-1.0), while the contribution from the pyrolysis reaction increases significantly as the equivalence ratio increases to 2.0. The apparent activation energies, E(a), for combustion of toluene extracted from ReaxFF MD simulations are consistent with experimental results.

Novel silicon-doped, silicon and nitrogen-codoped carbon nanomaterials with high activity for the oxygen reduction reaction in alkaline medium

Novel silicon-doped, silicon and nitrogen-codoped carbon nanomaterials were prepared by chemical vapor deposition and evaluated by electrochemical tests. The Si-doped CNSs and Si, N-codoped CNTs exhibited high electrocatalytic activity and long-term stability for the oxygen reduction reaction in alkaline medium due to changes in the charge density and the energetic characteristics of the carbon framework, as evidenced by density functional theory calculations. This may be of great importance for the design of low-cost and efficient metal-free cathode electrocatalysts in future alkaline fuel cells.

Effects of fuel additives on the thermal cracking of n-decane from reactive molecular dynamics.

Thermal cracking of n-decane and n-decane in the presence of several fuel additives are studied in order to improve the rate of thermal cracking by using reactive molecular dynamics (MD) simulations employing the ReaxFF reactive force field. From MD simulations, we find the initiation mechanisms of pyrolysis of n-decane are mainly through two pathways: (1) the cleavage of a C-C bond to form smaller hydrocarbon radicals, and (2) the dehydrogenation reaction to form an H radical and the corresponding decyl radical. Another pathway is the H-abstraction reactions by small radicals including H, CH(3), and C(2)H(5). The basic reaction mechanisms are in good agreement with existing chemical kinetic models of thermal decomposition of n-decane. Quantum mechanical calculations of reaction enthalpies demonstrate that the H-abstraction channel is easier compared with the direct C-C or C-H bond-breaking in n-decane. The thermal cracking of n-decane with several additives is further investigated. ReaxFF MD simulations lead to reasonable Arrhenius parameters compared with experimental results based on first-order kinetic analysis. The different chemical structures of the fuel additives greatly affect the apparent activation energy and pre-exponential factors. The presence of diethyl ether (DEE), methyl tert-butyl ether (MTBE), 1-nitropropane (NP), 3,6,9-triethyl-3,6,9-trimethyl-1,2,4,5,7,8-hexaoxonane (TEMPO), triethylamine (TEA), and diacetonediperodixe (DADP) exhibit remarkable promoting effect on the thermal cracking rates, compared with that of pure n-decane, in the following order: NP > TEMPO > DADP > DEE (∼MTBE) > TEA, which coincides with experimental results. These results demonstrate that reactive MD simulations can be used to screen for fuel additives and provide useful information for more comprehensive chemical kinetic model studies at the molecular level.

Computational Study of the Reaction Mechanism of the Methylperoxy Self-Reaction

To provide insight on the reaction mechanism of the methyperoxy (CH(3)O(2)•) self-reaction, stationary points on both the spin-singlet and the spin-triplet potential energy surfaces of 2(CH(3)O(2)•) have been searched at the B3LYP/6-311++G(2df,2p) level. The relative energies, enthalpies, and free energies of these stationary points are calculated using CCSD(T)/cc-pVTZ. Our theoretical results indicate that reactions on a spin-triplet potential energy surface are kinetically unfavorable due to high free energy barriers, while they are more complicated on the spin-singlet surface. CH(3)OOCH(3) + O(2)(1) can be produced directly from 2(CH(3)O(2)•), while in other channels, three spin-singlet chain-structure intermediates are first formed and subsequently dissociated to produce different products. Besides the dominant channels producing 2CH(3)O• + O(2) and CH(3)OH + CH(2)O + O(2) as determined before, the channels leading to CH(3)OOOH + CH(2)O and CH(3)O• + CH(2)O + HO(2)• are also energetically favorable in the self-reaction of CH(3)O(2)• especially at low temperature according to our results.

Influence of the double bond on the hydrogen abstraction reactions of methyl esters with hydrogen radical: an ab initio and chemical kinetic study

This work reports a systematic ab initio and chemical kinetic study of the rate constants for hydrogen atom abstraction reactions by hydrogen radical on the isomers of unsaturated C6 methyl esters. Geometry optimizations and a frequency calculations of all of the species involved, as well as the hindrance potential descriptions for reactants and transition states have been performed at the B3LYP/6-311G(2d,d,p) level of theory implemented in the composite CBS-QB3 method. The intrinsic reaction coordinate (IRC) calculations are performed to verify that the transition states are the right minima connecting the reactants and the products. The hindered rotor approximation has been used for the low frequency torsional modes in both reactants and transition states. The high-pressure limit rate constants for every reaction channel in certain methyl ester fuel molecules are calculated via conventional transition-state theory with the asymmetric Eckart method for quantum tunneling effect by using the accurate potential energy information obtained with the CBS-QB3 method. The individual rate constants at different reaction sites for all the methyl esters in the temperature range from 500 to 2500 K are calculated and fitted to the modified three parameters Arrhenius expression using least-squares regression. Further, a branching ratio analysis for each reaction site has also been investigated for all of the methyl esters. To the best our knowledge, it is the first systematic theoretical studies to investigate the influence of the double bond on the elementary reaction kinetics of methyl esters. This work not only provides accurate reaction rate coefficients for combustion chemical kinetic modeling, but also helps to gain further insight into the combustion chemistry of biodiesel in future investigations.

An updated detailed reaction mechanism for syngas combustion

Detailed reaction mechanisms for syngas combustion are analyzed in the present work. The exhaustive list of reactions within different reaction mechanisms is summarized and the differences among these mechanisms are presented. Detailed comparisons of the contemporary choices of the reaction rate coefficients are analyzed in detail, and an updated reaction mechanism for syngas combustion together with the recommended rate coefficients with minimal uncertainties have been proposed. The importance of the listed reactions and their corresponding rate constant uncertainties are analyzed according to a detailed survey of the sources of the rate constants together with sensitivity analysis and rate-of-production (ROP) analysis. It is found that with the development of advanced experimental and computational methods, the rate constant uncertainties of a large number key reactions are minimized. However, large uncertainties still exist for hydrogen peroxide (H2O2) and hydroperoxyl radical (HO2) related reaction sequences, which play a key role for syngas combustion at high-pressure and low-to-intermediate temperature conditions. The updated detailed mechanism is validated against standard targets, including ignition delay time and laminar flame speeds over a wide range of conditions. The present work not only provides an updated detailed mechanism for syngas combustion, but also provides fundamental information to further develop a universal core reaction mechanism which is the foundation to the development of combustion mechanisms of all the other hydrocarbon fuels.

Continuum Model for Electronic Polarization Based on a Novel Dielectric Response Function

A generalized response function based on the use of dielectric spectra for dielectric relaxation process is derived. We apply the general response function to the special case in order to examine how special dielectric relaxation functions developed by other authors for solvent relaxation can be derived based on our formulations. Three typical solvents, water, methanol, and acetonitrile are used to investigate the electronic polarization processes of polar solvents. The solvent electronic polarization process is shown after a linear variation with the external electric field imposed on the solvent. The results show a conclusion that the electronic polarization of the solvents will accompany the electronic transition synchronously, without time lag.

Reaction Kinetics of Hydrogen Atom Abstraction from C4–C6 Alkenes by the Hydrogen Atom and Methyl Radical

Alkenes are important ingredients of realistic fuels and are also critical intermediates during the combustion of a series of other fuels including alkanes, cycloalkanes, and biofuels. To provide insights into the combustion behavior of alkenes, detailed quantum chemical studies for crucial reactions are desired. Hydrogen abstractions of alkenes play a very important role in determining the reactivity of fuel molecules. This work is motivated by previous experimental and modeling evidence that current literature rate coefficients for the abstraction reactions of alkenes are still in need of refinement and/or redetermination. In light of this, this work reports a theoretical and kinetic study of hydrogen atom abstraction reactions from C4-C6 alkenes by the hydrogen (H) atom and methyl (CH3) radical. A series of C4-C6 alkene molecules with enough structural diversity are taken into consideration. Geometry and vibrational properties are determined at the B3LYP/6-31G(2df,p) level implemented in the Gaussian-4 (G4) composite method. The G4 level of theory is used to calculate the electronic single point energies for all species to determine the energy barriers. Conventional transition state theory with Eckart tunneling corrections is used to determine the high-pressure-limit rate constants for 47 elementary reaction rate coefficients. To faciliate their applications in kinetic modeling, the obtained rate constants are given in the Arrhenius expression and rate coefficients for typical reaction classes are recommended. The overall rate coefficients for the reaction of H atom and CH3 radical with all the studied alkenes are also compared. Branching ratios of these reaction channels for certain alkenes have also been analyzed.

Germanium-doped and germanium/nitrogen-codoped carbon nanotubes with highly enhanced activity for oxygen reduction in alkaline medium

To develop effective cathode electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells, germanium (Ge)-doped and Ge/N-co-doped carbon nanotubes (CNTs) were synthesized by chemical vapor deposition in this work. Electrochemical tests demonstrated that the as-prepared Ge-doped and GeN-codoped CNTs exhibited obviously enhanced ORR activity in alkaline medium, showing that the doping of Ge into the carbon matrix could also improve the ORR activities of the CNTs and N-CNTs as in the cases of other reported heteroatoms and would be of great importance for designing more effective ORR electrocatalysts in future alkaline fuel cells.

Time-Dependent Stokes Shift from Solvent Dielectric Relaxation

The Stokes shift response function, which is related to the time dependent solvation energy, is calculated with the dielectric response function and a novel expression of nonequilibrium solvation energy. In the derivation, relationship between the polarization and the dielectric response function is used. With the dipole-in-a-sphere model applied to the system coumarin 343 and water as the solvent, encouraging agreement with the experimental data from Jimenez et al. is obtained [Nature 369, 471 (1994)].