Molecular Dynamics Simulation of the Low-Temperature Partial Oxidation of CH4

Detailed Reaction Kinetics for Hydrocarbon Fuels: The Development and Application of the ReaxFFCHO-S22 Force Field for C/H/O Systems with Enhanced Accuracy

Efficient and accurate reactive force fields (e.g., ReaxFF) are pivotal for large-scale atomistic simulations to comprehend microscopic combustion processes. ReaxFF has been extensively utilized to describe chemical reactions in condensed phases, but most existing ReaxFF models rely on quantum mechanical (QM) data nearly two decades old, particularly in hydrocarbon systems, constraining their accuracy and applicability. Addressing this gap, we introduce a reparametrized ReaxFFCHO-S22 for C/H/O systems, tailored for studying the pyrolysis and combustion of hydrocarbon fuel. Our approach involves high-level QM benchmarks and large database construction for C/H/O systems, global ReaxFF parameter optimization, and molecular dynamics simulations of typical hydrocarbon fuels. Density functional theory (DFT) computations utilized the M06-2X functional at the meta-generalized gradient approximation (meta-GGA) level with a large basis set (6-311++G**). Our new ReaxFFCHO-S22 model exhibits a minimum 10% enhancement in accuracy compared to the previous ReaxFF models for a large variety of hydrocarbon molecules. This advanced ReaxFFCHO-S22 not only enables efficient large-scale studies on the microscopic chemical reactions of more complex hydrocarbon fuel but also can extend to biofuels, energetic materials, polymers, and other pertinent systems, thus serving as a valuable tool for studying chemical reaction dynamics of the large-scale hydrocarbon condensed phase at an atomistic level.

High-Temperature Pyrolysis of N-Tetracosane Based on ReaxFF Molecular Dynamics Simulation

In order to further understand the high-temperature reaction process and pyrolysis mechanism of hydrocarbon fuels, the high-temperature pyrolysis behavior of n-tetracosane (C₂₄H₅₀) was investigated in this paper via the reaction force field (ReaxFF) method-based molecular dynamics approach. There are two main types of initial reaction channels for n-heptane pyrolysis, C-C and C-H bond fission. At low temperatures, there is little difference in the percentage of the two reaction channels. With the temperature increase, C-C bond fission dominates, and a small amount of n-tetracosane is decomposed by reaction with intermediates. It is found that H radicals and CH₃ radicals are widely present throughout the pyrolysis process, but the amount is little at the end of the pyrolysis. In addition, the distribution of the main products H₂, CH₄, and C₂H₄, and related reactions are investigated. The pyrolysis mechanism was constructed based on the generation of major products. The activation energy of C₂₄H₅₀ pyrolysis is 277.19 kJ/mol, obtained by kinetic analysis in the temperature range of 2400-3600 K.

Reactive Molecular Dynamics Study of Effects of Small-Molecule Organic Acids on PMIA Thermal Decomposition

Reactive molecular dynamics was used to investigate the atomic-level mechanism of formic acid-accelerated deterioration of meta-aramid (PMIA) fibers. The simulation results showed that formic acid promoted PMIA decomposition. The activation energy of a composite system (PF) consisting of formic acid and PMIA was 106.94 kJ/mol at 2000-3000 K, which is 11.95% lower than that of pure PMIA. The main small-molecule products of the PF system were H/C/O-containing molecules (H₂O, CO, and CO₂), hydrocarbon molecules (e.g., CH₄, ^•C₂H, C₂H₄, and C₃H₄), N-containing molecules (N₂, NH₃, and HCN), H₂, and various free radicals. Formic acid can promote the production of small molecules such as CO, CO₂, and H₂O. The N-H bonds, C-N bonds and the amide C═O double bond of PMIA were vulnerable to CO, H ions, and free radicals produced by formic acid decomposition, and this decreased the PMIA stability. Temperature is an important factor in the thermal decomposition of PMIA and can accelerate reactions in the PF system. The initial reaction rate of PMIA at 3000 K was 8.1 times that at 2000 K, and the intermediate reaction rate was 6.2 times that at 2200 K; temperature also affects the types of pyrolysis products, for example, hydrocarbons are high-temperature products.

Theoretical rate constant of methane oxidation from the conventional transition-state theory

The potential energy surface for the first step of the methane oxidation CH₄ + O₂➔CH₃ + HO₂ was studied using the London-Eyring-Polanyi-Sato equation (LEPS) and the conventional transition-state theory (CTST). The calculated activation energy and rate constant values were in good agreement with the experimental and theoretical values reported in the literature using the shock tube technique and coupled cluster method respectively. The rate equation from CTST, although simple, provides good results to study the H-shift between methane and the oxygen molecules.

Molecular Dynamics Simulations on the Thermal Decomposition of Meta-Aramid Fibers

The thermal decomposition mechanism of a meta-aramid fiber was simulated at the atomic level using the ReaxFF reactive force field. The simulation results indicated that the main initial decomposition positions of the meta-aramid fiber elements were Caromatic ring⁻N and C=O, which could be used as targets for the modification of meta-aramid fibers. The meta-aramid fiber elements first decomposed into C6⁻C13 and then into smaller segments and micromolecular gases. The temperature was shown to be the key factor affecting the thermal decomposition of the meta-aramid fibers. More complex compositions and stable gases were produced at high temperatures than at lower temperatures. HCN was a decomposition product at high temperature, suggesting that its presence could be used for detecting thermal faults in meta-aramid fibers. Generation path tracing of the thermal decomposition products NH₃ and H₂O was also performed. NH₃ was produced when the NH₂ group captured an H atom adjacent to the system. H₂O was formed after a carbonyl group captured an H atom, became a hydroxyl group, with subsequent intramolecular dehydration or intermolecular hydrogen abstraction.

Quantum Chemical Molecular Dynamics Simulations of 1,3-Dichloropropene Combustion

Oxidative decomposition of 1,3-dichloropropene was investigated using quantum chemical molecular dynamics (QM/MD) at 1500 and 3000 K. Thermal oxidation of 1,3-dichloropropene was initiated by (1) abstraction of allylic H/Cl by O2 and (2) intra-annular C-Cl bond scission and elimination of allylic Cl. A kinetic analysis shows that (2) is the more dominant initiation pathway, in agreement with QM/MD results. These QM/MD simulations reveal new routes to the formation of major products (H2O, CO, HCl, CO2), which are propagated primarily by the chloroperoxy (ClO2), OH, and 1,3-dichloropropene derived radicals. In particular, intra-annular C-C/C-H bond dissociation reactions of intermediate aldehydes/ketones are shown to play a dominant role in the formation of CO and CO2. Our simulations demonstrate that both combustion temperature and radical concentration can influence the product yield, however not the combustion mechanism.

Mechanical properties and defect sensitivity of diamond nanothreads

One of the newest carbon allotropes synthesized are diamond nanothreads. Using molecular dynamics, we determine the stiffness (850 GPa), strength (26.4 nN), extension (14.9%), and bending rigidity (5.35 × 10(-28) N·m(2)). The 1D nature of the nanothread results in a tenacity of 4.1 × 10(7) N·m/kg, exceeding nanotubes and graphene. As the thread consists of repeating Stone-Wales defects, through steered molecular dynamics (SMD), we explore the effect of defect density on the strength, stiffness, and extension of the system.

Imaging the C black formation by acetylene pyrolysis with molecular reactive force field simulations

Formation of C black undergoes four main stages: chain elongation, chain branching, cyclization and cycle-condensation, and folding of condensed cycles.

MOLECULAR DYNAMIC SIMULATIONS OF SOLID NITROMETHANE UNDER HIGH PRESSURES

We report ReaxFF molecular dynamic simulations of structure change of crystalline nitromethane and the formation of hydrogen bond under high pressure. Under high pressure, the angles between C–N bonds and X, Y and Z axes have changed. Through the calculation of g(r) of O and H atoms, we found a new peak near 1.6 Å, which indicates the formation of the hydrogen bond between O and H atoms. We calculated the distribution of the angles of the C–N bonds orientations, the distribution of the dihedral angle of CNOO , and the charge distribution of nitromethane molecules under various pressures, and made a comparison between low and high pressures. The effects of hydrogen bonding in high explosive materials are discussed.