Reactive Dynamics Simulations Research Articles

Deep Potential Molecular Dynamics Study of Propane Oxidative Dehydrogenation.

Oxidative dehydrogenation (ODH) of light alkanes is a key process in the oxidative conversion of alkanes to alkenes, oxygenated hydrocarbons, and COx (x = 1,2). Understanding the underlying mechanisms extensively is crucial to keep the ODH under control for target products, e.g., alkenes rather than COx, with minimal energy consumption, e.g., during the alkene production or maximal energy release, e.g., during combustion. In this work, deep potential (DP), a neural network atomic potential developed in recent years, was employed to conduct large-scale accurate reactive dynamic simulations. The model was trained on a sufficient data set obtained at the density functional theory level. The intricate reaction network was elucidated and organized in the form of a hierarchical network to demonstrate the key features of the ODH mechanisms, including the activation of propane and oxygen, the influence of propyl reaction pathways on the propene selectivity, and the role of rapid H2O2 decomposition for sustainable and efficient ODH reactions. The results indicate the more complex reaction mechanism of propane ODH than that of ethane ODH and are expected to provide insights in the ODH catalyst optimization. In addition, this work represents the first application of deep potential in the ODH mechanistic study and demonstrates the ample advantages of DP in the study of mechanism and dynamics of complex systems.

Combustion simulations of AlH3 and ethanol nanofluid by ReaxFF

To alleviate the growing energy crisis, ethanol has been developed and applied gradually as an ideal biofuel to replace gasoline. The AlH3 nanoparticle is an excellent fuel additive with broad applications. In this work, the ReaxFF-lg reactive dynamics simulations were proceeded to explore the detailed mechanisms of AlH3 and ethanol nanofluid fuel. At varying temperatures, AlH3 exhibits different morphologies: branch-like expansion at the lower temperatures; micro-explosion and reaggregation into larger clusters at high temperatures. Overall, AlH3 accelerates the decomposition of ethanol and shortens the ignition delay. AlH3 also reduces the activation energy for ethanol decomposition from 79.09 to 69.05 kJ/mol. The conversion of coordination number for Al-O bonds indicates that Al2O3 is the final product of Al element. The simulation results reveal a new mechanism: AlH3 promotes the dissociation of H in the hydroxyl, making the initial decomposition of ethanol to C2H5O (CH3CH2O, CH3CHOH and CH2CH2OH) selective. The released H forms more OH and HO2 radicals to promote the conversion of intermediates to stable products. The Al2O3 oxide layer delays the ethanol combustion kinetics as well. This study provides an atomic perspective on the combustion mechanisms of AlH3 and ethanol nanofluid fuel and is expected to guide the development and application of AlH3-based fuels.

The combustion-supporting mechanism of fluoropolymers on aluminum particles studied using reactive dynamics simulations

In order to clarify the combustion-supporting mechanism of fluoropolymers, this work focuses on the atomic details of the oxidation of Al nanoparticles with the fluoropolymers and O2, using reactive dynamics simulations.

Adsorption properties of hydrogen with iron oxides (FeO, Fe2O3): A ReaxFF molecular dynamics study

The reactive dynamics simulation with a reactive force field is a potentially effective method for investigating large systems (>1000 atoms) and has been employed in this study to analyze the hydrogen adsorption on FeO and Fe2O3. The adsorption behavior of hydrogen on FeO and Fe2O3 substrates have been compared by calculating the adsorption amount, adsorption rate and adsorption energy. The adsorption process at different temperatures (300 K and 1173 K) and pressures (200, 300 and 400 molecules) have been studied respectively, and the influences of these factors on the adsorption type and adsorption mechanism are compared, which are defined mainly by analyzing the radial distribution function (RDF) of different systems. The results show that Fe2O3-H2 system has a lower adsorption energy of −1.86 Kcal/mol, and a higher temperature promotes the chemical adsorption, increasing the hydrogen bond length. The results provide references and ideas for hydrogen adsorption on iron oxides.

ReaxFF simulations on the combustion of Al and n-butanol nanofluid

The renewable biofuel n-butanol is an alternative to gasoline and diesel in internal combustion engines. The addition of aluminum nanoparticles (ANPs) would improve the performance of these nanofluid fuels. The reactive dynamics simulation of the ignition and combustion process of ANP/n-butanol nanofluid fuel was carried out. The combustion mechanisms of n-butanol fuel with and without ANPs, as well as the aggregation of nanodroplets and the sintering degree of nanoparticles were explored. Simulation results show that the combustion of aluminum-based n-butanol fluid fuel undergoes a three-stage: chain expansion, micro-explosion, and steady reaction. In general, the addition of ANPs improves combustion completeness and reduces the ignition delay of n-butanol. The kinetic calculations show that the activation energy of n-butanol combustion is reduced by 39.9%. A new reaction mechanism was proposed from the results: n-butanol reacts with oxygen free radicals generated by Al rather than O2. On the other hand, n-butanol prevents the aggregation and sintering of aluminum nanoparticles. Therefore, the ANP additives and n-butanol nanofluid promote mutually in the combustion. This work is expected to guide the practical application of aluminum-based alcohol fuels.

A study of the effect of H2O on char oxidation during O2/H2O combustion using reactive dynamic simulation

In present work, the effect of H2O on the oxidation of char during O2/H2O combustion is investigated by reactive molecular dynamics simulation (ReaxFF MD). The simulation results show that H2O promotes the overall carbon conversion of char, which is consistent with the experimental results. By analyzing the distribution of H2 yield, the contributions of the char-H2O gasification reaction to the overall carbon conversion at different temperatures are quantitatively determined. The activation energy of char-H2O gasification reaction obtained from ReaxFF MD simulations is 199 kJ/mol. The contributions of the oxidation reactions to the overall carbon conversion in O2/H2O combustion are also obtained. The results show that the carbon oxidation is enhanced by the reactivity of H2O. The effect mechanism of H2O on char oxidation is revealed by analyzing the distributions of OH radicals·H2O significantly increases the concentration of OH radicals through the reaction of H2O with O radicals. Then, the contribution of H2O to the oxidation rate of char at different char conversions in O2/H2O combustion is quantitatively determined based on the assumption that char oxidation and char gasification reactions occur at separate active carbon sites. Finally, the effect of H2O concentration on the contribution of H2O to the oxidation rate of char is investigated. The results show that the oxidation rate contributed by H2O is enhanced with increasing H2O concentration.

Role of Dissolved Oxygen in Iron Oxidation in Supercritical Water: Insights from Reactive Dynamics Simulations

Oxygenated treatment is considered to be an effective chemical water treatment method and is widely used in supercritical power plants. Previous isotope tracer experiments reported that dissolved o...

Desensitizing Effect of Graphene Oxide on Thermolysis Mechanisms of 4,4'-Azo-1,2,4-triazole Studied by Reactive Molecular Dynamics Simulations.

Graphene oxide (GO) has obvious desensitizing effect on the thermal decomposition of energetic materials such as HMX, CL-20, etc. 4,4'-Azo-1,2,4-triazole (ATRZ) is known as a new type of energetic material with high N content; the underlying thermal decomposition mechanism of graphene oxide-ATRZ (GO-ATRZ) complex with low sensitivity has not been studied. The present work studies the thermal decomposition mechanisms of GO, ATRZ,and the GO-ATRZ complex (the number of carboxyl groups on GO:ATRZ = 2:1) by the ReaxFF molecular reactive dynamic simulations and kinetics calculations. As a result, it has been found that the main decomposition pathway of GO is the exfoliation of hydroxyl and carboxyl groups on the graphene sheet, whereas ATRZ breaks its five-membered ring as the main decomposition path, and the ring further decomposes into small molecules, such as CHN, N2, HN2, H2N2, etc. The major effect of GO on ATRZ is probably derived from the stable graphene sheet, which has a space effect on ATRZ, and the strong oxidizing hydroxyl groups produced during GO decomposition, which results in the formation of CON and CHON. By calculating the activation energy of N2 generation in the reactions, it can be concluded that the addition of GO can increase the decomposition activation energy of ATRZ (41.1 kJ·mol-1) in comparison with that of its pure substance (25.0 kJ·mol-1). Therefore, GO can be combined with ATRZ as a desensitizer where GO can improve the molecular stability of ATRZ.

Initial Decay Mechanism of the Heated CL-20/HMX Cocrystal: A Case of the Cocrystal Mediating the Thermal Stability of the Two Pure Components

Energetic cocrystallization, by combining existing molecules together, is thought to be new strategy for creating energetic materials. Nevertheless, the underlying mechanism of its influences on properties and performances in comparison with their pure components remains unclear. The present work reveals the cocrystallization influence of a typical energetic cocrystal of CL-20/HMX on thermal stability, by ReaxFF molecular reactive dynamic simulations and kinetics calculations on the pure and cocrystals. As a result, we find that the cocrystal mediates the thermal stability of pure crystalsand this is in agreement with experimental observations. The initial decay steps in pure crystals remain still in the cocrystal, that is, the independent and intramolecular reactions of N–N bond cleavage governing the initial decay of the pure CL-20 and HMX crystals also dominate in the cocrystal of CL-20/HMX. Meanwhile, during the thermal decomposition of the cocrystal, CL-20 releases heat faster than HMX, thus the heat...

(Invited) Computational Design of Coatings, Interfaces, and Nano-Structures for Si Based Electrodes

The chemical composition and mechanical properties change of coating materials, interfaces, and Si itself during lithiation and delithiation process imposed a grand challenge to design coating/Si nanostructure as an integrated electrode system. In our work, reactive force field (ReaxFF) parameters for Li-Si-Al-O-F were developed along with a new algorithm to simulate lithiation and delithation processes with different rates. With the reactive dynamics simulations, we were able to simultaneously track and correlate the lithiation-delithiation rate, compositional change, mechanical property evolution, stress distributions, and fracture. Interesting insights were obtained from these studies. For example, it was found that the lithiation rate during the initial mixing stage increases exponentially with vacancy concentrations in Si; and it was discovered the self-accelerating Li diffusion in Al2O3 coating forms a well-defined Li concentration gradient, leading to an elastic modulus gradient, which effectively avoids local stress concentration and mitigates crack propagation. A new mechanics model based on these varying properties was developed to determine how to stabilize the coating with a critical size ratio.

Self-generated concentration and modulus gradient coating design to protect Si nano-wire electrodes during lithiation.

Surface coatings as artificial solid electrolyte interphases have been actively pursued as an effective way to improve the cycle efficiency of nanostructured Si electrodes for high energy density lithium ion batteries, where the mechanical stability of the surface coatings on Si is as critical as Si itself. However, the chemical composition and mechanical property change of coating materials during the lithiation and delithiation process imposed a grand challenge to design coating/Si nanostructure as an integrated electrode system. In our work, we first developed reactive force field (ReaxFF) parameters for Li-Si-Al-O materials to simulate the lithiation process of Si-core/Al2O3-shell and Si-core/SiO2-shell nanostructures. With reactive dynamics simulations, we were able to simultaneously track and correlate the lithiation rate, compositional change, mechanical property evolution, stress distributions, and fracture. A new mechanics model based on these varying properties was developed to determine how to stabilize the coating with a critical size ratio. Furthermore, we discovered that the self-accelerating Li diffusion in Al2O3 coating forms a well-defined Li concentration gradient, leading to an elastic modulus gradient, which effectively avoids local stress concentration and mitigates crack propagation. Based on these results, we propose a modulus gradient coating, softer outside, harder inside, as the most efficient coating to protect the Si electrode surface and improve its current efficiency.

Atomistic Origin of Brittle Failure of Boron Carbide from Large-Scale Reactive Dynamics Simulations: Suggestions toward Improved Ductility.

Ceramics are strong, but their low fracture toughness prevents extended engineering applications. In particular, boron carbide (B(4)C), the third hardest material in nature, has not been incorporated into many commercial applications because it exhibits anomalous failure when subjected to hypervelocity impact. To determine the atomistic origin of this brittle failure, we performed large-scale (∼200,000 atoms/cell) reactive-molecular-dynamics simulations of shear deformations of B(4)C, using the quantum-mechanics-derived reactive force field simulation. We examined the (0001)/⟨101̅0⟩ slip system related to deformation twinning and the (011̅1̅)/⟨1̅101⟩ slip system related to amorphous band formation. We find that brittle failure in B(4)C arises from formation of higher density amorphous bands due to fracture of the icosahedra, a unique feature of these boron based materials. This leads to negative pressure and cavitation resulting in crack opening. Thus, to design ductile materials based on B(4)C we propose alloying aimed at promoting shear relaxation through intericosahedral slip that avoids icosahedral fracture.

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.

First-Principles-Based Reaction Kinetics for Decomposition of Hot, Dense Liquid TNT from ReaxFF Multiscale Reactive Dynamics Simulations

The reaction kinetics of the thermal decomposition of hot, dense liquid TNT was studied from first-principles-based ReaxFF multiscale reactive dynamics simulation strategy. The decomposition process was followed starting from the initial liquid phase, decomposition to radicals, continuing through formation of carbon-clusters products, and finally to formation of the stable gaseous products. The activation energy of the initial endothermic decomposition rate and the subsequent exothermic reactions were determined as a function of density. Analysis of fragments production in different densities and temperatures is presented. We find that unimolecular C–N bond scission dominates at the lower densities (producing NO2), whereas dimer formation and decomposition to TNT derivatives and smaller gaseous fragments prevails at higher compressions. At higher densities, enhanced carbon-clustering is observed, while the initial gaseous fragments formation is suppressed. Increasing the temperature speeds up the producti...

Reactive Dynamics Simulation of Monolayer and Multilayer Adsorption of Glycine on Cu(110)

The process of mono- and multilayer adsorption of glycine on copper surface Cu(110) and the preferred binding modes of the molecule were studied theoretically by means of classical reactive (ReaxFF) molecular dynamics simulations. Starting from glycines in gas phase in the neutral nonzwitterionic form, the most stably adsorbed structures are found to be the molecules which release their carboxyl protons (anionic form) to molecules in the second layer and place both the carboxyl oxygens and the nitrogen atom on top of copper sites, at an average distance of about 2 Å. The surface binding mechanism consists of different phases during which major conformational rearrangements and several intermediate adsorption configurations are observed. The overall stability of the glycine adlayers is essentially due to the combination of different intermolecular forces, namely chemical bonds with the copper top layer and intermolecular hydrogen bonds within the adsorbed molecular units. At low coverage the molecules are prevalently attached to the substrate in a bidentate fashion, i.e., through the nitrogen atom and one oxygen atom of the carboxyl group, whereas at higher coverage the molecules tend to diffuse on the surface and pack in long-range ordered heterochiral domains where tridentate geometries are most likely observed. The picture that emerges from the present reactive dynamics simulations satisfactorily agrees with the experimental data and theoretical results, based on geometry optimization, reported to date.

Anisotropic shock sensitivity for β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine energetic material under compressive-shear loading from ReaxFF-lg reactive dynamics simulations

We report here the predictions on anisotropy of shock sensitivity and of chemical process initiation in single crystal β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (β-HMX) using compressive shear reactive dynamics (CS-RD) model with ReaxFF-lg reactive force field. Analysis of resolved shear stress induced by uniaxial compression along three shock directions normal to (110), (011), and (010) planes leads to identify eight slip systems as candidates for shear deformation. For each of the eight slip systems, non-equilibrium reactive dynamics simulations were carried out to determine thermal, mechanical, and chemical responses to shear deformation. Shock direction normal to (010) plane exhibits large shear stress barriers arising from steric hindrance between molecules of adjacent layers leading to local dramatic energy and temperature increases under shear flow that in turn accelerate chemical bond breaking and initial product formation processes, promoting further molecular decomposition and eventually transition to detonation. This suggests that single crystal β-HMX is sensitive to shocks in direction normal to (010) plane. Shock directions normal to (110) and (011) planes reveal significantly less steric hindrance, leading to more modest energy and temperature increases followed by slower chemical reaction initiation. Thus, shock directions normal to (110) and (011) planes are less sensitive than shock direction normal to (010) plane, which agree with interpretations from currently available plate impact experiments on HMX. This validation of CS-RD and ReaxFF for characterizing sensitivity of single crystal energetic materials indicates that these methods can be applied to study sensitivity for more complex polymer bonded explosives and solid composite propellants having complex microstructures, corrugated interfaces, as well as defects.

Development of a ReaxFF Reactive Force Field for Ettringite and Study of its Mechanical Failure Modes from Reactive Dynamics Simulations

Ettringite is a hexacalcium aluminate trisulfate hydrate mineral that forms during Portland cement hydration. Its presence plays an important role in controlling the setting rate of the highly reactive aluminate phases in cement paste and has also been associated with severe cracking in cured hardened cement. To understand how it forms and how its properties influence those of hardened cement and concrete, we have developed a first-principles-based ReaxFF reactive force field for Ca/Al/H/O/S. Here, we report on the development of this ReaxFF force field and on its validation and application using reactive molecular dynamics (RMD) simulations to characterize and understand the elastic, plastic, and failure response of ettringite at the atomic scale. The ReaxFF force field was validated by comparing the lattice parameters, pairwise distribution functions, and elastic constants of an ettringite crystal model obtained from RMD simulations with those from experiments. The predicted results are in close agreement with published experimental data. To characterize the atomistic failure modes of ettringite, we performed stress-strain simulations to find that Ca-O bonds are responsible for failure of the calcium sulfate and tricalcium aluminate (C3A) column in ettringite during uniaxial compression and tension and that hydrogen bond re-formation during compression induces an increase in plastic strain beyond the material's stress-strain proportionality limit. These results provide essential insight into understanding the mechanistic role of this mineral in cement and concrete degradation, and the ReaxFF potential developed in this work serves as a fundamental tool to further study the kinetics of hydration in cement and concrete.

Sintering of calcium oxide (CaO) during CO2 chemisorption: a reactive molecular dynamics study

Reactive dynamics simulations with the reactive force field (ReaxFF) were performed in NVE ensembles to study the sintering of two solid calcium oxide (CaO) particles with and without CO(2) chemisorption. The simulated sintering conditions included starting adsorption temperatures at 1000 K and 1500 K and particle separation distances of 0.3 and 0.5 nm. The results revealed that the expansion of sorbent particles during CO(2) chemisorption was attributed to the sintering of two CaO-CaO particles. Increasing the adsorption temperature resulted in more particle expansion and sintering. The shorter the distance between two particles, the faster the rate of sintering during CO(2) adsorption. A detailed analysis on atom spatial variations revealed that the sorbent particles with a larger separation distance had a larger CO(2) uptake because of less sintering incurred. The chemisorptions of CO(2) on CaO particles sintered at high adsorption temperatures were also simulated to mimic the process of sorbent regeneration. It was found that regeneration would be more difficult for sintered particles than for fresh particles. In addition, a possible sintering barrier, magnesium oxide (MgO), was introduced to prevent CaO particles from sintering during CO(2) chemisorption. It was found that the MgO particles could reduce the sintering of CaO particles during CO(2) chemisorption. Simulation results from this study provided some guidelines on synthesizing or selecting sorbents with less sintering effect for multiple CO(2) adsorption-regeneration cycles.

Development and validation of a ReaxFF reactive force field for Cu cation/water interactions and copper metal/metal oxide/metal hydroxide condensed phases.

To enable large-scale reactive dynamic simulations of copper oxide/water and copper ion/water interactions we have extended the ReaxFF reactive force field framework to Cu/O/H interactions. To this end, we employed a multistage force field development strategy, where the initial training set (containing metal/metal oxide/metal hydroxide condensed phase data and [Cu(H(2)O)(n)](2+) cluster structures and energies) is augmented by single-point quantum mechanices (QM) energies from [Cu(H(2)O)(n)](2+) clusters abstracted from a ReaxFF molecular dynamics simulation. This provides a convenient strategy to both enrich the training set and to validate the final force field. To further validate the force field description we performed molecular dynamics simulations on Cu(2+)/water systems. We found good agreement between our results and earlier experimental and QM-based molecular dynamics work for the average Cu/water coordination, Jahn-Teller distortion, and inversion in [Cu(H(2)O)(6)](2+) clusters and first- and second-shell O-Cu-O angular distributions, indicating that this force field gives a satisfactory description of the Cu-cation/water interactions. We believe that this force field provides a computationally convenient method for studying the solution and surface chemistry of metal cations and metal oxides and, as such, has applications for studying protein/metal cation complexes, pH-dependent crystal growth/dissolution, and surface catalysis.

Early maturation processes in coal. Part 2: Reactive dynamics simulations using the ReaxFF reactive force field on Morwell Brown coal structures

This paper reports reactive dynamics (RD) simulations of a macro-model of Morwell Brown coal using the ReaxFF reactive force field. We find that these reactive MD simulations successfully reproduce thermal decomposition processes of defunctionalization, depolymerization and rearrangement of the residual structure observed in various experimental studies. For example, our simulations indicate that the decarboxylation and dehydroxylation of the lignin side chain of the Morwell model involves the formation of double bonds conjugated with the aromatic rings. The process of defunctionalization of the methoxy functions involving the formation of phenolic structures in the residue has been confirmed. We also observe that gaseous hydrocarbons are generated by cleavage of C–C bonds of the lignin side chain. The success in using ReaxFF RD to describe the molecular processes underlying the kinetics in pyrolysis of this model of coal plus the success of a similar previous study on the algaenan of Botryococcus braunii race L biopolymer model of kerogens suggests that such computation can be useful in providing molecular based kinetic models for other pyrolysis processes underlying the organic transformations in sedimentary materials.