Formation of incipient soot particles from polycyclic aromatic hydrocarbons: A ReaxFF molecular dynamics study

In this study, we present the results from a series of ReaxFF molecular dynamics (MD) simulations to uncover the underlying mechanisms behind the nucleation and growth of incipient soot particles from polycyclic aromatic hydrocarbons (PAHs). PAHs, namely, naphthalene, anthracene, pyrene, coronene, ovalene and circumcoronene, are selected for ReaxFF MD simulations over a range of temperatures from 400 to 2500 K. Distinctive mechanisms of incipient soot formation are identified with respect to PAH mass and temperature. At low temperatures (e.g., 400 K), all types of the above PAHs can nucleate into incipient soot particles in stacked structures due to physical interactions. With the increase of temperature, the possibility of physical nucleation decreases for each PAH. At moderate temperatures (e.g., 1600 K), it becomes difficult for these PAH monomers, except circumcoronene grows into incipient soot particles. When the temperature increases to 2500 K, all the PAHs become chemically active, which not only leads to the formation of incipient soot particles but also takes the graphitization with the increase of the carbon-to-hydrogen (C/H) ratios in the particles. In addition to the formation of fullerene-like soot particles, stacked particles connected by ‘carbon bridges’ are also observed for large PAHs like coronene, ovalene and circumcoronene

Investigation of methane oxidation by palladium-based catalyst via ReaxFF Molecular Dynamics simulation

Catalytic oxidations of methane over palladium-based nanoparticles, with and without oxygen coating, are investigated using ReaxFF Molecular Dynamics simulations. The simulation results show the complete dynamic process of the above catalytic reactions at the atomic level and help to reveal the underlying mechanisms both qualitatively and quantitatively. It is found that oxygen molecules are significantly easier to be adsorbed on both bare and oxygen-coated Pd surfaces compared with CH4. The presence of adsorbed O2 molecules on the surface blocks the active sites for CH4 adsorption on the oxygen-coated Pd surfaces. By comparing the adsorptive dissociation of CH4 over Pd nanoparticles with different levels of oxygen coverage, we find that it is much easier for the adsorptive dissociation of CH4 on oxygen-coated Pd nanoparticles than that on bare Pd nanoparticles at low temperatures. In contrast to the rapid dissociation of CH4 after adsorption, the dissociation of O2 requires much higher temperature than adsorption. Moreover, the CH4 dissociation rate increases with the rising temperature and is sensitive to the level of oxygen coverage on the surface. In addition, the activation energies for the adsorptive dissociation of CH4 are determined by fixed-temperature simulations from 400 to 1000 K through the changes of CH4 concentration and are found to be 3.27 and 2.28 kcal mol−1 on 0.3 and 0.7 ML oxygen-coated Pd nanoparticles, respectively, which are consistent with density functional theory calculations and experiments

Investigation of ethanol oxidation over aluminum nanoparticle using ReaxFF molecular dynamics simulation

Aluminum nanoparticles are an effective and economical additive for producing energetic fuels. In the present study, the state of the art ReaxFF molecular dynamics (MD) simulation has been used to uncover the detailed mechanisms of ethanol oxidation over aluminum nanoparticles with different oxidation states. The MD results reveal the dynamics process of ethanol oxidation reactions at nanoscales. The presence of aluminum nanoparticles is found to reduce the initial temperature of ethanol oxidation to 324 K. It is also found that compared to ethanol, oxygen molecules are more easily adsorbed on aluminum surfaces. Moreover, different oxidation states of aluminum nanoparticles influence the initial ethanol reactions on the nanoparticles’ surfaces. OH-abstraction is more commonly observed on pure aluminum nanoparticles while H-abstraction prevails on aluminum nanoparticles with oxide. The separated H atom from hydroxyl forms bonds with Al and O atom on aluminum nanoparticles surrounded by thin and thick oxide layers, respectively. Adsorptive dissociation of ethanol is hindered by the oxide layer surrounding the aluminum nanoparticle. Gas products like H2O and CO resulting from ethanol oxidation on aluminum nanoparticles with the thick oxide layer are observed while almost all the C, H and O atoms in ethanol diffuse into the nanoparticles without or with the thin oxide layer. For ethanol dissociation, a higher temperature is required than adsorption. In addition, the rate of ethanol dissociation increases with rising reaction temperatures. The activation energy for ethanol adsorptive dissociation is found to be 4.58 kcal/mol on the aluminum nanoparticle with the thin oxide layer, which is consistent with results from much more expensive DFT calculations

Atomistic insights into the dynamics of binary collisions between gaseous molecules and polycyclic aromatic hydrocarbon dimers

Polycyclic aromatic hydrocarbon (PAH) dimers are important intermediates in combustion and soot formation. The scattering dynamics of gaseous molecules colliding with PAH dimers and the subsequent PAH dimer stability are investigated by performing molecular dynamics (MD) simulations. Effects of properties of the surrounding gaseous molecules and PAH dimers as well as temperature are investigated in this study. Depending on the residence time of N2 molecules trapped by the PAH dimers, two scattering types, that is, specular scattering and inelastic scattering, have been observed, which is correlated to the temperature and the type of the PAH dimer. Specifically, specular scattering preferentially takes place at high temperatures on small PAH dimers, while inelastic scattering tends to happen at low temperatures on large PAH dimers. During collision, energy transfer between the gaseous molecule and the PAH dimer changes the thermodynamic stability of the PAH dimer. Statistical analysis indicates that the decomposition rate of a PAH dimer to PAH monomers is sensitive to temperature and the PAH dimer type. Furthermore, effects of the gaseous molecule type on the PAH dimer stability are considered. The molecular mass of the colliding gaseous molecule is a key factor in determining the PAH dimer stability, as heavier gaseous molecules are more effective in promoting the PAH dimer decomposition. Results from this study indicate that collisions with gaseous molecules decrease the PAH dimer stability, while increasing the PAH dimer size and decreasing the collision temperature both decrease the decomposition rate of the PAH dimer

Atomistic insight into the effects of electrostatic fields on hydrocarbon reaction kinetics

Reactive Molecular Dynamics (MD) and Density Functional Theory (DFT) computations are performed to provide insight into the effects of external electrostatic fields on hydrocarbon reaction kinetics. By comparing the results from MD and DFT, the suitability of the MD method in modeling electrodynamics is first assessed. Results show that the electric field-induced polarization predicted by the MD charge equilibration method is in good agreement with various DFT charge partitioning schemes. Then, the effects of oriented external electric fields on the transition pathways of non-redox reactions are investigated. Results on the minimum energy path suggest that electric fields can cause catalysis or inhibition of oxidation reactions, whereas pyrolysis reactions are not affected due to the weaker electronegativity of the hydrogen and carbon atoms. MD simulations of isolated reactions show that the reaction kinetics is also affected by applied external Lorentz forces and interatomic Coulomb forces since they can increase or decrease the energy of collision depending on the molecular conformation. In addition, electric fields can affect the kinetics of polar species and force them to align in the direction of field lines. These effects are attributed to energy transfer via intermolecular collisions and stabilization under the external Lorentz force. The kinetics of apolar species is not significantly affected mainly due to the weak induced dipole moment even under strong electric fields. The dynamics and reaction rates of species are studied by means of large-scale combustion simulations of n-dodecane and oxygen mixtures. Results show that under strong electric fields, the fuel, oxidizer, and most product molecules experience translational and rotational acceleration mainly due to close charge transfer along with a reduction in their vibrational energy due to stabilization. This study will serve as a basis to improve the current methods used in MD and to develop novel methodologies for the modeling of macroscale reacting flows under external electrostatic fields

Structural hierarchies define toughness and defect-tolerance despite simple and mechanically inferior brittle building blocks

Mineralized biological materials such as bone, sea sponges or diatoms provide load-bearing and armor functions and universally feature structural hierarchies from nano to macro. Here we report a systematic investigation of the effect of hierarchical structures on toughness and defect-tolerance based on a single and mechanically inferior brittle base material, silica, using a bottom-up approach rooted in atomistic modeling. Our analysis reveals drastic changes in the material crack-propagation resistance (R-curve) solely due to the introduction of hierarchical structures that also result in a vastly increased toughness and defect-tolerance, enabling stable crack propagation over an extensive range of crack sizes. Over a range of up to four hierarchy levels, we find an exponential increase in the defect-tolerance approaching hundred micrometers without introducing additional mechanisms or materials. This presents a significant departure from the defect-tolerance of the base material, silica, which is brittle and highly sensitive even to extremely small nanometer-scale defects

Hydrodynamic slip can align thin nanoplatelets in shear flow

The large-scale processing of nanomaterials such as graphene and MoS2 relies on understanding the flow behaviour of nanometrically-thin platelets suspended in liquids. Here we show, by combining non-equilibrium molecular dynamics and continuum simulations, that rigid nanoplatelets can attain a stable orientation for sufficiently strong flows. Such a stable orientation is in contradiction with the rotational motion predicted by classical colloidal hydrodynamics. This surprising effect is due to hydrodynamic slip at the liquid-solid interface and occurs when the slip length is larger than the platelet thickness; a slip length of a few nanometers may be sufficient to observe alignment. The predictions we developed by examining pure and surface-modified graphene is applicable to different solvent/2D material combinations. The emergence of a fixed orientation in a direction nearly parallel to the flow implies a slip-dependent change in several macroscopic transport properties, with potential impact on applications ranging from functional inks to nanocomposites.Energy Technolog