Reactive Dynamics Research Articles

Graphitization of low-density amorphous carbon for electrocatalysis electrodes from ReaxFF reactive dynamics

We predict the three-dimensional structure of amorphous carbon generated by heating diamond superlattice at 6000 K with rapid quenching from the liquid phase for densities ranging from 2.0 to 3.5 g/cm3, in comparison with 2.26 and 3.54 g/cm3 for bulk graphite and bulk diamond, respectively. These predictions are based on reactive dynamics (RD) simulations using the ReaxFF reactive force field. Here, we simulate the graphitization of amorphous carbon at high temperature to calculate physical properties relevant to conductive carbon supports useful for electrocatalysts. The low-density graphitic materials mostly oriented in the (002) plane with a main X-ray diffraction (XRD) peak between 26 and 28°, as observed experimentally. For low density carbon (2.0–2.5 g/cm3), we find >90% sp2 character with ∼2-1% sp and <8% sp3. While for higher density carbon, the amount of sp2 fraction decreases with density and find 70.0% sp3 with 29.7% sp2 and 0.3% sp for 3.4 g/cm3 density, which can be compared to DLC of 3.24 g/cm3 density resulting good agreement with XPS experiments. Based on the simulated 3D structure, we create 2D surface slab consisting of various defective sites within the surface. The 2D surface dominates with hexagonal carbon ring along with few pentagon and heptagon rings in the graphitic structure that may be useful as electrocatalysts for different energy conversion reactions.

The READY program: Building a global potential energy surface and reactive dynamic simulations for the hydrogen combustion.

READY (REActive DYnamics) is a program for studying reactive dynamic systems using a global potential energy surface (PES) built from previously existing PESs corresponding to each of the most important elementary reactions present in the system. We present an application to the combustion dynamics of a mixture of hydrogen and oxygen using accurate PESs for all the systems involving up to four oxygen and hydrogen atoms. Results at the temperature of 4000 K and pressure of 2 atm are presented and compared with model based on rate constants. Drawbacks and advantages of this approach are discussed and future directions of research are pointed out.

Toward a Process-Based Molecular Model of SiC Membranes. 2. Reactive Dynamics Simulation of the Pyrolysis of Polymer Precursor To Form Amorphous SiC

In part 1 of this series we developed the reactive force field ReaxFF, choosing the parameters adjusted to fit quantum mechanics description of prototypical reactions. In the present paper we use ReaxFF for reactive dynamics (RD) simulation of thermal decomposition of a silicon-containing polymer, hydridopolycarbosilane (HPCS) over a wide range of temperature. Many properties of the material during its pyrolysis were computed and were found to be in good agreement with the experimental data. We then simulated pyrolysis of the HPCS under conditions that closely mimic the conditions of the fabrication of nanoporous SiC membranes to produce amorphous SiC material. Again, the computed properties of the SiC ceramic were found to be in good agreement with the experimental data. In particular, the ReaxFF RD simulations generate ceramic SiC that contains mostly Si–C bonds with some Si–Si and C–C bonds, consistent with experimental data. This indicates again the accuracy of the parameters of the ReaxFF determined in Part 1.

Mechanism and Kinetics for the Initial Steps of Pyrolysis and Combustion of 1,6-Dicyclopropane-2,4-hexyne from ReaxFF Reactive Dynamics

We report the kinetic analysis and mechanism for the initial steps of pyrolysis and combustion of a new fuel material, 1,6-dicyclopropane-2,4-hexyne, that has enormous heats of pyrolysis and combustion, making it a potential high-energy fuel or fuel additive. These studies employ the ReaxFF force field for reactive dynamics (RD) simulations of both pyrolysis and combustion processes for both unimolecular and multimolecular systems. We find that both pyrolysis and combustion initiate from unimolecular reactions, with entropy-driven reactions being most important in both processes. Pyrolysis initiates with extrusion of an ethylene molecule from the fuel molecule and is followed quickly by isomerization of the fuel molecule, which induces additional radicals that accelerate the pyrolysis process. In the combustion process, we find three distinct mechanisms for the O(2) attack on the fuel molecule: (1) attack on the cyclopropane, ring expanding to form the cyclic peroxide which then decomposes; (2) attack onto the central single bond of the diyne which then fissions to form two C(5)H(5)O radicals; (3) attack on the alkyne-cyclopropane moiety to form a seven-membered ring peroxide which then decomposes. Each of these unimolecular combustion processes releases energy that induces additional radicals to accelerate the combustion process. Here oxygen has major effects both as the radical acceptor and as the radical producer. We extract both the effective activation energy and the effective pre-exponential factor by kinetic analysis of pyrolysis and combustion from these ReaxFF simulations. The low value of the derived effective activation energy (26.18 kcal/mol for pyrolysis and 16.40 kcal/mol for combustion) reveals the high activity of this fuel molecule.

Modeling High Rate Impact Sensitivity of Perfect RDX and HMX Crystals by ReaxFF Reactive Dynamics

We report a methodology for rapid assessment of impact sensitivity of energetic materials which uses the ReaxFF reactive force field in reactive dynamics (RD) simulations of the high rate compression/expansion of a perfect energetic crystal. This approach is validated here to study the high rate impact sensitivity of 1,3,5-trinitrohexahydro-s-triazine (RDX) crystal and octahydro-1,3,5,7-tetrazocine (HMX) crystal at different phases (α, β, γ, and δ). These simulations found that for a compression rate of ∼8.76 km/s along the [100] direction, RDX crystals at lower volume compression ratios (x = 30, 35, 38%) led only to a few RDX molecules decomposing and only into primary products. However, at higher compression ratios (x ≥ 40%), all RDX molecules in the crystal decompose very quickly, leading to both primary and secondary decomposition reactions, including various intermediates such as NO2, NO, HONO, and OH and final products such as H2O, N2, CO, and CO2. For the various phases of HMX, these ReaxFF RD simulations found noticeably higher impact sensitivity for the δ-phase than for other three phases (α, β, and γ). At the same compression ratio x = 40%, all HMX molecules in δ-phase decompose leading to both primary and secondary reaction. However, at 40%, only few HMX molecules in α-, β-, and γ-phases decompose. For a higher compression ratio (x = 42%), increased HMX decomposition is observed for all four phases. These simulation results for both RDX and HMX crystals agree qualitatively with experiment observations. We also observe a variation of the strain energy in different HMX phases induced by the high rate compression, which could be related to the sensitivity difference of HMX phases. These simulations typically took less than 18 h to run on a single 3.0-GHz processor, demonstrating that the fast compression approach by MD simulations with the ReaxFF force field can be used for a quick evaluation of the sensitivity of energetic materials.

Application of the ReaxFF Reactive Force Field to Reactive Dynamics of Hydrocarbon Chemisorption and Decomposition

We report here reactive dynamics (RD) simulations of the adsorption and decomposition of a gas of 20−120 methane, ethyne, ethene, benzene, cyclohexane, or propene molecules interacting with a 21 Å diameter nickel nanoparticle (468 atoms). These RD simulations use the recently developed ReaxFF reactive force field to describe decomposition, reactivity, and desorption of hydrocarbons as they interact with nickel surfaces. We carried out 100 ps of RD as the temperature is ramped at a constant rate from 500 to 2500 K (temperature programmed reactions). We find that all four unsaturated hydrocarbon species chemisorb to the catalyst particle with essentially no activation energy (attaching to the surface through π electrons) and then proceed to decompose by breaking C−H bonds to form partially dehydrogenated species prior to decomposition to lower order hydrocarbons. The eventual breaking of C−C bonds usually involves a surface Ni atom inserting into the C−C bond to produce an atomic C that simultaneously with C−C cleavage moves into the subsurface layer of the particle. The greater stability of this subsurface atomic C (forming up to four Ni−C bonds) over adatom C on the particle surface (forming at most three Ni−C bonds) is critical for favorable cleaving of C−C bonds. For the two saturated hydrocarbon species (methane and cyclohexane), we observe an activation energy associated with dissociative chemisorption. These results are consistent with available experimental reactivity data and quantum mechanics (QM) energy surfaces, validating the accuracy of ReaxFF for studying hydrocarbon decomposition on nickel clusters.