ReaxFF Molecular Dynamics Simulations Research Articles

A study on the abnormal thermal behaviors of barkinite by ReaxFF molecular dynamics simulation

A study on the abnormal thermal behaviors of barkinite by ReaxFF molecular dynamics simulation

A ReaxFF and DFT study of effect and mechanism of an electric field on JP-10 fuel pyrolysis

To investigate the sustainable use of electric fields in improving aviation fuel combustion efficiency, a ReaxFF molecular dynamics simulation was conducted using exo-TCD (exo-tetrahydrocyclopentadiene) as the major component of JP-10 fuel. The study aimed to investigate the pyrolysis efficiency and the mechanism of electric field modulation at different temperatures. It elucidates that the pyrolysis reaction activation energy and molecular motion are the key factors affected by the co-effect of temperature and electric field. A filed strength of 10−5 V/Å was shown to increase both the reaction barriers and the probability of collision for JP-10 molecule pyrolysis, which promotes a higher reaction rate in weak electric fields with high-temperature pyrolysis. The DFT calculation was used to analyze the electronic structure and reaction potential of the JP-10 molecule under the electric field, which further discloses the effect mechanism of electric field on the pyrolysis reaction pathway. This study provides a detailed explanation of combined effects of temperature and electric field effect on aviation fuel pyrolysis to enable effective combustion of aviation fuels.

Effect of methanol on the pyrolysis behaviour of kerogen by ReaxFF molecular dynamics simulations

ABSTRACTA series of molecular dynamics simulations were performed using the ReaxFF reactive force field to investigate the pyrolysis of Longkou oil shale kerogen in the presence of methanol. The roles of methanol in kerogen pyrolysis were deeply explored to reveal the microscopic mechanism of methanolysis by analyzing the distribution of products, the consumption of methanol, and the formation processes of typical products at different temperatures. The results demonstrate that the yield of shale oil in the presence of methanol is consistently higher than that of kerogen direct pyrolysis at the same temperature, which is consistent with the trend of oil yield with temperature reported by previous methanolysis experiments. Methanol supplies a large number of small radicals under the pyrolysis condition, which can facilitate the breakage of weak bonds and take part in the formation of products through combining with the radical fragments generated by the pyrolysis of kerogen. This study will provide theoretical guidance for understanding the methanolysis process of oil shale.

Atomic insights into hydrogen peroxide decomposition on the surface of pure and pre-treated silver: A reactive molecular dynamics simulation study

Based on ReaxFF molecular dynamics simulation, H2O2 decomposition on pure and pre-treated silver catalyst is studied at a microscopic level. The pretreatment undergoes a surface annealing with redundant H2O2 on top of catalyst. The simulation results indicate that H2O2 decomposition can be drastically accelerated by elevating temperature. The extent of decomposition is closely related to bond, hydrogen bond, and potential energy of simulation system. It is found that H2O2 decomposition follows a four-stage HOOH bond break mechanism while it needs three times longer for pre-treated silver catalyst than pure silver catalyst. The activation energy of pre-treated silver catalyst is about 50 % lower than pure silver catalyst. However, the overall reaction rate of pre-treated silver catalyst is about an order of magnitude slower than pure silver catalyst, which owes to the much smaller pre-factor. It is believed that although the pretreatment causes silver oxide to be formed on the surface, it leads to more hydrogen and oxygen atoms deposited so that most active sites for catalytic reaction are blocked for further decomposition. This may be helpful to understand the inherent mechanism of H2O2 decomposition on silver-based catalyst and provide instructional advice for further catalyst modification design.

A detailed reaction mechanism for hexamethyldisiloxane combustion via experiments and ReaxFF molecular dynamics simulations

AbstractHexamethyldisiloxane (HMDSO) is one of the main impurities in the syngas produced from sewage and landfill plants. In order to utilize this syngas or control the characteristics of the generated silica particles, it is crucial to understand the chemical kinetics of HMDSO combustion. This study investigated the process of HMDSO combustion using synchrotron radiation mass spectrometry (SRMS), gas chromatography (GC), and ReaxFF molecular dynamics simulations. First, the force field used for ReaxFF simulation was validated by comparing the energies of different bond lengths, bond angles, and dihedral angles with the ones from DFT calculations. Good agreements were found. Then, ReaxFF simulations of HMDSO combustion with this force field were conducted under various conditions, which include different equivalence ratios (0.67, 1.0, and 1.5) and temperatures ranging from 2000 to 3500 K. The oxidation characteristics of HMDSO were analyzed, including the evolution of gas products and particle formation. Finally, based on the results from experiments and ReaxFF simulations, the reaction pathways, reaction lists, and reaction kinetics data during HMDSO combustion were obtained. A detailed reaction mechanism was proposed and validated by applying it in modeling the H2/HMDSO/O2 combustion systems. The temperature and part of the gas products such as CO and CO2 as well as SiO could be well predicted.

Decomposition mechanism of clean air based on ReaxFF molecular dynamics simulation and quantum chemical calculation

SF6 is widely used in gas switchgear but has a strong greenhouse effect. The development of an environment-friendly switchgear that can replace SF6 is a current research hotspot. As a SF6 alternative technology, vacuum arc-extinguishing chamber plus clean air insulation has shown high application prospects. The clean air inside the switchgear decomposes under the effect of high temperature and discharge; however, only a few studies focused on its decomposition mechanism. In this work, the decomposition mechanism of clean air and the effect of temperature on the decomposition are simulated at the atomic level based on ReaxFF (Reactive force field) molecular dynamics and quantum chemistry theory. Results showed that the decomposition of clean air mainly generates NO, NO2, and N2O. NO is the main product at high temperatures and thus can be the characteristic decomposition product of clean air. The clean air has good self-recovery characteristics, and its decomposition can be substantially promoted by increasing the temperature. The decomposition rates of N2 and O2 under 3000 K can reach 7.00% and 8.00%, respectively, which are twice and four times those under 2000 K. These results can provide theoretical basis and engineering guidance for the development of environment-friendly switchgear with vacuum arc-extinguishing chamber plus clean air insulation.

Catalytic hydropyrolysis of lignin: Insights into the effect of Ni catalyst and hydrogen using ReaxFF molecular dynamics simulation

Catalytic hydropyrolysis (CHP) of lignin as an emerging technology provides an efficient way of utilizing waste resources and promoting the production of alternative fuels and valuable bio-based products. The reinforcement of selective catalysts and hydrogen facilitates the overall conversion process but the mechanism remains difficult to clarify due to the complex nature of the system. This work employs a ReaxFF reactive molecular dynamics simulation method to explore the CHP of lignin dimer DMPD and the reaction mechanism at the molecular level. The results suggest that the hydropyrolysis (HP) system achieves a deoxygenation degree of 12.0 % for liquid products and the incorporation of hydrogen into pyrolysis suppresses the production of CO while promoting H2O and CH4 generation. The introduction of the Ni catalyst in the CHP system results in a liquid product yield of 68.9 wt % at 2000 K with a higher deoxygenation degree of 14.6 % due to the participation of a large amount of •H radicals species dissociated on the catalyst surface. The CHP reactions remarkably promote the production of phenols such as guaiacol and catechol and catechol is more likely to generate from the dynamic evolution of reaction intermediates. The integrated system also improves the production of H2O and light hydrocarbons like CH4 and C2H6. The first-order kinetic model reveals that the average activation energies of C-C, C-H, C-O bonds, and DMPD in the CHP system are significantly reduced to 113.12, 55.79, 59.99, and 132.17 kJ/mol in contrast to HP. The results indicate that the CHP system is more favorable for producing desirable liquid products with lower energy barriers.

Comparison of H2O Adsorption and Dissociation Behaviors on Rutile (110) and Anatase (101) Surfaces Based on ReaxFF Molecular Dynamics Simulation.

The relationship between structure and reactivity plays a dominant role in water dissociation on the various TiO2 crystallines. To observe the adsorption and dissociation behavior of H2O, the reaction force field (ReaxFF) is used to investigate the dynamic behavior of H2O on rutile (110) and anatase (101) surfaces in an aqueous environment. Simulation results show that there is a direct proton transfer between the adsorbed H2O (H2Oad) and the bridging oxygen (Obr) on the rutile (110) surface. Compared with that on the rutile (110) surface, an indirect proton transfer occurs on the anatase (101) surface along the H-bond network from the second layer of water. This different mechanism of water dissociation is determined by the distance between the 5-fold coordinated Ti (Ti5c) and Obr of the rutile and anatase TiO2 surfaces, resulting in the direct or indirect proton transfer. Additionally, the hydrogen bond (H-bond) network plays a crucial role in the adsorption and dissociation of H2O on the TiO2 surface. To describe interfacial water structures between TiO2 and bulk water, the double-layer model is proposed. The first layer is the dissociated H2O on the rutile (110) and anatase (101) surfaces. The second layer forms an ordered water structure adsorbed to the surface Obr or terminal OH group through strong hydrogen bonding (H-bonding). Affected by the H-bond network, the H2O dissociation on the rutile (110) surface is inhibited but that on the anatase (101) surface is promoted.

A study on the pyrolysis of n-hexane initiated by 1-nitropropane: Molecular dynamics simulations and SVUV-PIMS experiments

To investigate the influence of 1-nitropropane (1-NP) initiator on the pyrolysis of n-hexane (n-C6H14), molecular dynamics (MD) simulations and synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) experiments were used to analyze the pyrolysis processes of n-C6H14 and n-C6H14 with 10 % 1-NP added. ReaxFF MD simulations were used to analyze the pyrolysis rates and product distributions of n-C6H14 and 1-NP/n-C6H14 at different temperatures. The mechanism of action of n-C6H14 pyrolysis initiated by 1-NP was investigated. Additionally, the pyrolysis products of n-C6H14 and 1-NP/n-C6H14 were identified and quantitatively analyzed using the SVUV-PIMS technique. These experimental results were compared with the ReaxFF MD simulation results to validate the reliability of the simulations and to complement each other. The results indicate that the presence of 1-NP promotes the pyrolysis of n-C6H14 and significantly improves the chemical heat sink of the fuel. The chemical heat sink in the 1-NP/n-C6H14 system differs from that in the n-C6H14 system by 28.78 kJ/kg at 2103 K. A significant increase in C2H4 production in the pyrolysis products of n-C6H14 with the addition of 1-NP. The pyrolysis of 1-NP/n-C6H14 yielded approximately twice as much C2H4 as n-C6H14 at 2103 K. The pyrolysis reactions were described by first-order kinetics, and the activation energy for the pyrolysis of 1-NP/n-C6H14 was determined to be 140 kJ/mol, approximately 48 % lower than that of n-C6H14. The composition of the pyrolysis products was confirmed by SVUV-PIMS experiments, which were in good agreement with the ReaxFF MD simulations. The mechanism of pyrolysis of n-C6H14 initiated by 1-NP was elucidated: the NO2 radicals generated by 1-NP react with H radicals of the system to form OH radicals. The primary reaction of n-C6H14 involves the abstraction of H by OH, leading to the formation of C6H13. Subsequently, C6H13 is further decomposed to form 1-C3H7, 1-C4H8, 1-C5H10 and 2-C6H12.

Mechanistic study of the influence of aluminum nanoparticles on the pressure sensitivity of 1,3,5-trinitro1,3,5-triazinane (RDX) thermal decomposition

The incorporation of Al nanoparticles (Al NPs) into 1,3,5-trinitro1,3,5-triazinane (RDX) can catalyze its thermal decomposition and reduce the pressure exponent (n) of solid propellants. However, the underlying reaction mechanism at a deeper level remains unclear. In this study, the reaction processes of Al NPs catalyzing the thermal decomposition of RDX under different initial pressures were investigated by utilizing the ReaxFF molecular dynamics (ReaxFF-MD) simulations combined with DFT calculations, and the influence mechanism of Al NPs on the pressure sensitivity of RDX thermal decomposition was revealed from various perspectives. In the initial stage of the reaction, unlike the nitro group dissociation observed in pure RDX, the incorporation of Al NPs alters the initial decomposition pathway of RDX, making it more easily adsorbed on the surface of Al and enhancing the early-stage decomposition rate. Furthermore, Al NPs exhibit higher catalytic activity in low-pressure systems. In the later stage, because of the massive adsorption of free radicals generated from RDX decomposition on the surface of Al NPs, the decomposition rate of RDX decreases significantly, especially in high-pressure systems. Consequently, Al NPs synergistically reduce the pressure sensitivity of the RDX thermal decomposition from two different perspectives. Compared to traditional experimental research or single RDX simulation studies, this work further explores the interaction between Al and RDX from a microscopic perspective, addressing the gap in the field of the influence mechanisms of Al NPs on the pressure sensitivity of RDX thermal decomposition. It also establishes a theoretical foundation for the practical application of Al@RDX fuels.

Exploring the impact of backbone architecture on the thermal decomposition of silicon-containing arylacetylene resins

Silicon-containing arylacetylene (PSA) resins are promising heat-resistant polymers. However, little is known about their decomposition mechanisms. A molecular dynamics (MD) study based on reactive force field (ReaxFF) was employed to investigate the thermal decomposition mechanism of two representative PSA resins containing benzene or naphthalene in the backbone. The effect of heating rate, temperature, and crosslinking degree on decomposition behavior was examined, and the temperatures of three decomposition stages were disclosed. The predicted 5% decomposition temperatures (Td5) and decomposition products are consistent with experimental findings. The ReaxFF MD simulation reveals that the Td5 is dominated by the secondary decomposition and final decomposition, in which the high molecular weight and the stable CC bond of naphthyl play a dominant role in determining high-temperature resistance of the naphthalene-containing PSA resin. We also disclose the decomposition pathways of PSA-type resin. The results gained from the present work can be a helpful guideline for designing resins with excellent heat resistance.

Exploring the Interaction of Water with Open Metal Sites in MIL-101(Cr) by 1H NMR Relaxometry and ReaxFF Molecular Dynamics Simulations

Exploring the Interaction of Water with Open Metal Sites in MIL-101(Cr) by ¹H NMR Relaxometry and ReaxFF Molecular Dynamics Simulations

Reactive Force Field (ReaxFF) and Universal Force Field Molecular Dynamic Simulation of Solid Electrolyte Interphase Components in Lithium-Ion Batteries

Abstract Understanding solid electrolyte interphase (SEI) is essential for the diagnosis of lithium-ion batteries because many aspects of battery performance such as safety and efficiency depend on these characteristics. LiF, Li2O, and Li2CO3 are important inorganic components of SEI. This electrode–electrolyte surface forms during the battery’s first charging/discharging cycle, preventing electrons’ movement through the electrolyte and stabilizing the lithium-ion battery. However, the concern is inorganic SEI components cause rate limitation of lithium-ion diffusivity through the SEI layer. Lithium-ion diffusivity through the SEI layer depends on many factors such as temperature, the width of the SEI layer, and the concentration/density of the layer. Lithium-ion diffusivity dependence on temperature, at working temperatures of lithium-ion batteries was observed at temperatures from 250 K to 400 K and diffusion coefficient data at higher temperatures have also been observed. Lithium-ion diffusivity at varying concentration/density was also observed in this paper using the reactive force field (ReaxFF) molecular dynamic simulation. To improve the lithium-ion diffusivity, vacancy defects were created in the inorganic components of the SEI layer LiF, Li2O, and Li2CO3 and the diffusion coefficient was obtained using the ReaxFF molecular dynamic simulations. Another approach to improve the lithium-ion diffusivity is doping alkali metal ions such Na, Ca, K, and Mg in the inorganic components of SEI layers of LiF, Li2O, and Li2CO3 and simulated using the universal force field (UFF), and the diffusion coefficient was observed.

Graphene oxide coated silicon carbide films under projectile impacts

The quest for hybrid materials to withstand extreme loading conditions has been a canonical field of research over the past decades. As an emerging hybrid material, graphene oxide (GO)-silicon carbide (SiC) offers remarkable thermo-chemo-mechanical properties with applications in defense, energy, and aerospace engineering. Researches to unravel the various properties of the aforementioned material subjected to different loading conditions have been on the rise to further promote its potential for the pertinent industries. Nonetheless, impact resistance characterization of such composite received less attention due to experimental and computational difficulties. Here, the aforementioned problem is investigated by leveraging ReaxFF molecular dynamics simulations. Accordingly, the response of 4H-SiC thin films coated by GO samples under indentation and high-velocity projectile impact is assessed. It is found that (a) composite films containing GO samples with higher oxidation degree demonstrate softer behavior under indentation, and (b) fracture pattern and penetration-resistance under high-velocity impact are contingent on the oxidation degree of the coating layers. In essence, impact-induced complete perforation is more localized to the impacted zone as oxidation degree of the coating layers increases. Therefore, oxidation content can be considered as a novel tuning factor for the fracture behavior of the GO-SiC thin films under projectile impacts. The influence of oxygen functional groups on the interaction energy between GO and SiC layers are also discussed. The findings of this study reveal some insights into the bottom-up design pathways for developing novel protective barriers in which GO is used as a coating layer or reinforcement for SiC ceramics.

Tuning the reactivity of Ni/MoS2 membrane for efficient methane pyrolysis and hydrogen production: A multi-scale study

This study investigates the performance of a Ni-coated MoS2 membrane for catalysis and separation in the methane pyrolysis process. Bifunctional membranes provide a low-energy and low-emission hydrogen generation and purification production process. To elucidate the reaction mechanism of the prepared membrane, a combined experimental and computational approach was employed using ReaxFF molecular dynamics simulations. The results demonstrate excellent catalytic and separation performance of the membrane, which was achieved through the increased specific surface area and reduced pore size of the MoS2 flower cluster. The Ni coating of MoS2 was found to attract methane, and transfer electrons to destabilize the CH bond, thus enhancing methane deep crack. In addition, the prepared membrane successfully blocked methane through Mo-edge defects, while promoting methane pyrolysis and driving H proton transportation for the H2 formation. The permeation test verified the excellent CH4/H2 separation performance of the ZSM-5 membrane coated with Ni and MoS2. This work provides theoretical and experimental guidance for the development of bifunctional membranes for methane pyrolysis to hydrogen generation, which has potential to reduce energy cost and greenhouse gas emissions.

Sintering and oxidation characteristics of aluminum nanoparticles coated with hydrocarbons: A ReaxFF molecular dynamics simulation study

Aluminum nanoparticles (ANPs) have attracted significant attention for combustion applications owing to their high energy density and reactivity. Surface coating has been proposed to overcome the limitations of ANPs, which are sintering and oxidizing at moderate temperatures; however, the thermal behavior of surface-coated ANPs remains unclear. In this study, reactive molecular dynamics simulations of ANP sintering and oxidation were performed to investigate the effectiveness of hydrocarbon surface coatings at the molecular level. Our results will aid the experimental design of novel energetic materials.

Energy evolution mechanism of a PVDF activated nano-aluminum based metastable intermixed composites

The preparation and energy release mechanism of highly reactive n-Al/F-polymer metastable intermixed composites are both desirable and challenging. In this study, we employed an optimized electrospray sampling strategy to significantly enhance the reaction activity of aluminum-based MICs, as well as their combustion performance. It is evident that the optimization of the sampling strategy yielded remarkable results. Compared to the physical mixture, MICs-1 exhibited a 7.2-fold and 9.9-fold increase in Pmax and Pmax/Δt, respectively. The combustion rate experienced an 8.5-fold improvement, and the energy release efficiency rose from 52.18 % to 68.11 %. An analysis of the energy evolution mechanism reveals a mitigation of the reactive sintering phenomenon observed in MICs-1. The presence of PVDF facilitates the reactivity of aluminum powders by selectively corroding the alumina shell. The reaction kinetics parameters indicate that MICs-1 demonstrates characteristic three-step reaction behavior and exhibits a lower reaction energy barrier. Furthermore, ReaxFF molecular dynamics simulations demonstrate the adsorption and dissociation of PVDF on the surface of aluminum powders, leading to erosion defects in the alumina crystal structure. These defects, in turn, induce the ignition reaction.

A comprehensive study of pyrolysis characteristics of silicone-modified phenolic aerogel matrix Nanocomposites: Kinetic Analysis, ReaxFF MD Simulations, and ANN prediction

The pyrolysis characteristics of the polymeric nanocomposites are dominantly regulated by the pyrolyzable components to accommodate a specific thermodynamic environment. The pyrolysis mechanisms of the pyrolyzable system including silicone and phenolic aerogels (SiOC/PR) of silicone-modified phenolic aerogel matrix nanocomposites are carried out by kinetic analysis, ReaxFF molecular dynamics (MD) simulations, and artificial neural network (ANN). The experimental results show that the SiOC/PR systems exhibit better thermal stability when the mass ratio of SiOC to PR is 1:3. A distributed activation energy model with five pseudo-components is employed to calculate the kinetic parameters of each decomposition stage of SiOC/PR systems for describing the decomposition characteristics. The significant pyrolysis products detected by TG-FTIR are examined by ReaxFF MD simulations. MD results reveal that the methanol evolution of SiOC/PR systems is distinct in a high-temperature oxygen-free environment, where a mass ratio 1:3 of SiOC to PR is more likely to release methanol and forms a silicon oxide-rich phase with high bond energy. Two reaction mechanisms for methanol identified by the MD trajectory are the extraction reactions of methyl intermediates as well as hydroxymethyl intermediates. The mixture ratio of the pyrolyzable system of the SiOC/PR nanocomposites with optimal thermal resistance equaling 0.22 is predicted by ANN. This work can help understand the system’s chemical features and guide the pyrolyzable system design of the SiOC/PR nanocomposites.

New insights into the carbon chain structure of alcohol on the combustion of diesel surrogates using ReaxFF molecular dynamics simulations

Achieving clean and efficient combustion of diesel fuel is the main trend in the development of diesel engines in the future. Adding oxygen-containing alcohol to diesel fuel is an effective way to improve diesel combustion efficiency and reduce carbon smoke emissions, which is of great significance for reducing the consumption of traditional fossil fuels and reducing environmental pollution. In this work, the reactive molecular dynamics (RMD) simulations are conducted to compare the combustion of diesel without/with various alcohols, focusing on the intermediates and product distribution, oxygen consumption, ignition delay time, initial combustion pathways, and the related kinetics. It can be found that the addition of alcohols can significantly improve the combustion of diesel with the promoting order being n-pentanol > n-butanol > ethanol > cyclopentanol. The longer ignition delay of the diesel/ethanol is related to the lower cetane number of ethanol compared to other alcohols. The promoting combustion effect of alcohols can be attributed to higher reaction rate of their unimolecular bond dissociations and hydrogen abstraction reactions. In addition, the straight-chain alcohols have higher oxygen consumption and perform better in reducing soot than cyclic alcohols with the same number of carbon atoms. A significant decrease in activation energy is observed in the diesel/n-pentanol combustion system compared to the pure diesel combustion based on the RMD Arrhenius parameters from the first-order kinetic analysis. From the RMD simulations on the diesel/n-pentanol with different oxygen concentrations, it can lead to the conclusion that when the oxygen amount in the system reaches stoichiometric combustion, adding more oxygen does not produce a significant promoting effect on the diesel combustion process.

Understanding CO2 mineralization and associated storage space changes in illite using molecular dynamics simulation and experiments

Clay minerals have the potential to capture anthropogenic CO2 emissions permanently and safely. Understanding the kinetics of cation leaching and carbonate formation, as well as changes in clay structure, has resource and environmental implications. However, metadynamics mechanism and relevant structural changes in representative clay minerals exposed to supercritical carbon dioxide (scCO2) are rarely studied in current research. In this work, ReaxFF molecular dynamics simulation in combination with scCO2‒H2O‒illite experiments at 10 MPa and 333 K were carried out to investigate the mechanisms of mineralization and structure alteration under geological conditions. The results show that interlayer K+ cations were leached out due to surface non-bridging oxygen protonation, subsequently bonding with HCO3− and forming K2CO3 molecules at the surface. Upon analyzing the chemical and structural results of experiments, carbonate precipitation and accumulation reduce storage space and modify the composition of illite, but the octahedral and tetrahedral sheets of the illite are structurally stable. The efficiency of mineralization is typically dominated by the exposed surface, where sufficient cations can be provided to enhance interactions at the illite/liquid interface. In comparison to a decrease in plane porosity of 27.3%, the mineralization degree with values between 15.22% and 33.12% is comparable and acceptable. These findings present a non-structural mechanism in clay minerals that might have critical influence on CO2 geo-sequestration in shale gas reservoirs.