Reactive Force Field Molecular Dynamics Research Articles

Combustion characteristics and nitrogen conversion mechanism in ammonia/coal Co-firing process

The coupled combustion of coal with the carbon-free fuel NH3 is an effective technology for reducing CO2 emissions. Based on elemental analysis, proximate analysis, Fourier Transform infrared spectroscopy (FTIR), Carbon-13 Nuclear Magnetic Resonance (13C NMR), and X-ray photoelectron spectroscopy (XPS) characterizations, a coal macromolecular model was constructed. The combustion characteristics of coal/NH3 co-firing were investigated by reactive force field molecular dynamics simulations (ReaxFF MD). The transformation mechanism of elemental N was elucidated. The influence of temperature, oxygen concentration, and NH3 co-firing ratio (CR) on products was investigated. The results showed that during the non-isothermal combustion phase, the coal macromolecules decomposed more rapidly when the heating rate was low. In the isothermal phase, high temperatures promoted secondary reactions. The production of inorganic gases increased with increasing temperature. As the molar fraction of NH3 increased, the production of CO2 and CO was effectively reduced. There were three main pathways for NO formation. As NH3 decomposed to form NH2, NH2 was subsequently converted to NH, and NH reacted with O2 to form NO. The NH2 could also react with O radicals to form HNO, which was then converted to NO. Besides, both thermal NOx and prompt NOx were generated since the N2 was involved in the reaction. In addition, the production of NOx increased significantly at oxygen concentrations greater than 1.4.

Effect of H2O on macroscopic flame behaviors and combustion reaction mechanism of 1,1-difluoroethane (R152a)

1,1-difluoroethane (R152a) is one of the most prospective low GWP refrigerants but may trigger fires and explosions once it leaks. Understanding the flame propagation process and combustion mechanisms at different relative humidity conditions is essential to use flammable refrigerants safely. In this study, a high-speed camera has recorded the flame propagation of R152a combustion near the flammability limit. Combining with macroscopic experimental phenomena, we revealed the effect of H2O on the microscopic mechanism of R152a combustion by using reactive force field molecular dynamics simulations with a reliable force field. The promotion effects of H2O on the oxidation of R152a were revealed from an atomistic perspective. The differences of oxidation intermediates and products in different environments were analyzed for the first time. The H2O/O2 environment was the most potent promoter of R152a decomposition, followed by the O2 and the pyrolysis environment. The O2/H2O environment reduced the apparent activation energy of R152a, significantly enhanced the consumption rate of R152a, and enriched the number of species. O2 reacts with H2O or H to form OH to accelerate the reaction process. The H2O could provide more OH active fragments for the reaction. Moreover, it promotes the formation of HF and H2, H2O+F→HF+OH, H2O+H→H2+HO. This study aims to provide a guiding theory for the safe application and disaster prevention of flammable refrigerants.

Reactive Force Field Molecular Dynamics Investigation of NH3 Generation Mechanism during Protein Pyrolysis Process.

The mechanism of ammonia formation during the pyrolysis of proteins in biomass is currently unclear. To further investigate this issue, this study employed the AMS 2023.104 software to select proteins (actual proteins) as the model compounds and the amino acids contained within them (assembled amino acids) as the comparative models. ReaxFF molecular dynamics simulations were conducted to explore the nitrogen transformation and NH3 generation mechanisms in three-phase products (char, tar, and gas) during protein pyrolysis. The research results revealed several key findings. Regardless of whether the model compounds are actual proteins or assembled amino acids, NH3 is the primary nitrogen-containing product during pyrolysis. However, as the temperature rises to higher levels, such as 2000 K and 2500 K, the amount of NH3 decreases significantly in the later stages of pyrolysis, indicating that it is being converted into other nitrogen-bearing species, such as HCN and N2. Simultaneously, we also observed significant differences between the pyrolysis processes of actual proteins and assembled amino acids. Notably, at 2000 K, the amount of NH3 generated from the pyrolysis of assembled amino acids was twice that of actual proteins. This discrepancy mainly stems from the inherent structural differences between proteins and amino acids. In proteins, nitrogen is predominantly present in a network-like structure (NH-N), which shields it from direct external exposure, thus requiring more energy for nitrogen to participate in pyrolysis reactions, making it more difficult for NH3 to form. Conversely, assembled amino acids can release NH3 through a simpler deamination process, leading to a significant increase in NH3 production during their pyrolysis.

Inhibition mechanism of CHF3 on hydrogen–oxygen combustion: Insights from reactive force field molecular dynamics simulations

To mitigate the risks linked to hydrogen and oxygen (H2–O2) combustion through CHF3 additives, Reactive Force Field Molecular Dynamics (ReaxFF MD) simulations are performed in this study. The primary objective is to investigate the inhibition mechanism of CHF3 on H2–O2 combustion from 2000 K to 2800 K. The simulation results demonstrate that the reaction pathways of hydrogen combustion are changed under the extended second explosion limit, and the main radicals involved in elementary reactions transform from H, OH, and O to H, OH, and HO2. CHF3 predominantly engages in reactions with H radicals to impede the continuation of chain reactions by forming stable HF molecules. OH radicals react with modest amounts of secondary fluorides such as CHF2OH, CH2F, and so on, while HO2 radicals are combined with even fewer intermediates like CHF2O and CHFOH to prevent chain propagations. Moreover, comprehensive and novel reaction pathways are proposed for the inhibition of H2–O2 combustion by CHF3. The parameters of the reaction kinetics indicate that the ignition delay is advanced and the activation energy for the combustion process is increased under the influence of CHF3. This study is expected to provide practical guidance on the inhibition of H2–O2 combustion by CHF3.

Investigation of lubrication mechanism of phosphate ionic liquid by ReaxFF molecular dynamics simulations

The frictional characteristics of the interface between ionic liquid (IL) and metal can be significantly improved by the formation of a tribo-film, while the lubrication mechanism remains unclear. This study utilized reactive force field molecular dynamics (ReaxFF MD) simulation to investigate the tribological properties, interfacial tribochemical reactions, and formation mechanism of the tributyltetradecyl phosphonium di(2-ethylhexyl) phosphate ([P44414] [DEHP]) tribo-film. The findings indicate that the friction coefficients of ILs decrease as the normal pressure increases. The combination of normal pressure and shear force promotes the dissociation of ionic liquid chains and enhances the binding of free radicals to the end faces. The friction force is primarily influenced by the number of Fe-C bonds at the interface, which demonstrates a positive correlation with the normal pressure.

Atomistic Insights into the Influence of High Concentration H2O2/H2O on Al Nanoparticles Combustion: ReaxFF Molecules Dynamics Simulation.

The combination of Al nanoparticles (ANPs) as fuel and H2O2 as oxidizer is a potential green space propellant. In this research, reactive force field molecular dynamics (ReaxFF-MD) simulations were used to study the influence of water addition on the combustion of Al/H2O2. The MD results showed that as the percentage of H2O increased from 0 to 30%, the number of Al-O bonds on the ANPs decreased, the number of Al-H bonds increased, and the adiabatic flame temperature of the system decreased from 4612 K to 4380 K. Since the Al-O bond is more stable, as the simulation proceeds, the number of Al-O bonds will be significantly higher than that of Al-H and Al-OH bonds, and the Al oxides (Al[O]x) will be transformed from low to high coordination. Subsequently, the combustion mechanism of the Al/H2O2/H2O system was elaborated from an atomic perspective. Both H2O2 and H2O were adsorbed and chemically activated on the surface of ANPs, resulting in molecular decomposition into free radicals, which were then captured by ANPs. H2 molecules could be released from the ANPs, while O2 could not be released through this pathway. Finally, it was found that the coverage of the oxide layer reduced the rate of H2O2 consumption and H2 production significantly, simultaneously preventing the deformation of the Al clusters' morphology.

Insight into energy release characteristics of TiH2 dust explosion through ignition experiments and molecular dynamic simulations

TiH2 is an efficient hydrogen storage material with high energy density and relatively stable chemical quality, which is commonly used in metal sponge production, hydrogen storage, battery technology, rocket propellants, etc. In this study, TiH2 dust explosion is systematically investigated by integrating ignition experiments with reactive force field molecular dynamics simulation. Findings show that when the TiH2 dust concentration is 750 g·m−3, both the explosion pressure and explosion pressure rise rate reach their maximum values under airtight and venting conditions. There are three stages in the whole combustion process: the combustion development stage, the stable combustion stage, and the decay stage. The flame length (L) and flame propagation velocity (v) grow with the increase of dust concentration. TiH2 dust explosion with different concentrations in macroscopic tests is approximated by simulating the combustion behaviors of the TiH2 system with different densities. The combustion reaction rate of TiH2 grows with an increase in system density, and the simulation results are consistent with experimental findings.

Experimental and ReaxFF MD study on the release characteristics of gaseous nitrogen during pressurized bituminous coal gasification

Coal gasification is the leading technology of clean coal technology such as Integrated Gasification Combined Cycle (IGCC). The release pattern of gaseous nitrogen during coal gasification is crucial for clean coal technology. This paper investigated the gaseous nitrogen emission characteristics of pressurized gasification through experiments and molecular dynamics studies. The experimental results show that the elevated temperatures promoted the release of NO and accelerated the conversion of nitrogen-containing precursors (HCN, NH3). However, high pressure weakened the effect of temperature and resulted in the decreased emissions of HCN, NH3, and NO. Because high pressure facilitated the conversion of fuel nitrogen to nitrogen. The emissions of HCN and NH3 decreased as the CO2 blending ratio increased, while NO emissions increased. The observed difference was primarily attributed to the amounts of free radicals and was also related to gas-phase reactions involving H2O and CO2. Furthermore, as the primary nitrogen-containing structure in bituminous coal, the pyrrole was used in Reactive force field molecular dynamics (ReaxFF MD) to study the release of gas-phase nitrogen during gasification process. The simulation results show that both temperature and pressure promote the migration of pyrrole nitrogen and also provided the main nitrogen migration paths during gasification which can be used to (indirectly) confirm the experimental analysis. The O2/N2/CO2 gasification system supplied additional O and OH radicals to enhance NO emissions, while H2O directly participated in reaction paths such as CN, NCO → NHi or provided H radicals to increase NH3 emissions.

Effect of Fe and its oxides on steam gasification mechanism of lignin using ReaxFF molecular dynamics simulations

Lignin is an important biomass component and its gasification process has been extensively studied. Iron is widely found in nature, and the corresponding iron-based catalysts have been heavily used in the study of various systems. The advantages are that it is inexpensive and not easy to pollute the environment, and it can also be extracted from the system and reused. However, studies on the effect of iron and its oxides on the steam gasification of lignin are limited, most studies have focused on supercritical water gasification. In this study, the effects of Fe and its oxides on the steam gasification process of lignin dimers were simulated using reactive force field (Reaxff) molecular dynamics (MD). By analyzing the gas products and the evolution of the structures. It was found that low valence Fe greatly improved the efficiency of lignin steam gasification, while higher valence iron oxides were significantly less effective. There were also significant differences in the distribution of gas products for different valence states of Fe. Low-valent Fe greatly improves the yield of combustible gases such as H2 and CO, while high-valent Fe is better at generating CO2. Carbon accumulation occurs on the surface of the catalyst, which reduces the catalytic effect. By comparing the effects of different temperatures on the steam gasification of lignin, it was found that increasing the temperature accelerates the decomposition process of lignin, and that the steam gasification of lignin has a critical temperature at which the yield of the main gaseous products of lignin is the highest.

ReaxFF molecular dynamics simulations of methane clathrate combustion.

Understanding the ignition and dynamic processes for the combustion of hydrate is crucial for efficient energy utilization. Through reactive force field molecular dynamics simulations, we studied the high-temperature decomposition and combustion processes of methane hydrates in a pure oxygen environment. We found that at an ignition temperature of 2800K, hydrates decomposed from the interface to the interior, but the layer-by-layer manner was no longer strictly satisfied. At the beginning of combustion, water molecules reacted first to generate OH•, followed by methane oxidation. The combustion pathway of methane is CH4→CH3•→CH3O•→CH2O→HC•O→HCOO•→CO(CO2). During the combustion process, a liquid water layer was formed between melted methane and oxygen, which hindered the reaction's progress. When there is no heat resistance, oxygen will transform into radicals such as OH• and O•, which have faster diffusion rates, allowing oxygen to conveniently cross the mass transfer barrier of the liquid water layer and participate in the combustion process. Increasing the amount of OH• may cause a surge in the reaction. On the other hand, when significant heat resistance exists, OH• is difficult to react with low-temperature hydrate components, but it can transform into O• to trigger the oxidation of methane. The H• generated has a sufficient lifetime to contact high-temperature oxygen molecules, converting oxygen into radicals that easily cross the water layer to achieve mass transfer. Therefore, finding ways to convert oxygen into various radicals is the key to solving the incomplete combustion of hydrates. Finally, the reaction pathways and microscopic reaction mechanisms of each species are proposed.

Carbon structural evolution and interfacial reaction mechanism investigation of the green anode in kneading and baking via ReaxFF-MD and force-biased Monte Carlo

ABSTRACT Baking is the most costly and critical process in anode preparation, and the study from the microscopic scale is conducive to the optimisation of anode baking process. In this research, a hybrid simulation method of reactive force field molecular dynamics (ReaxFF-MD) and force-biased Monte Carlo (fbMC) was proposed to understand the interaction mechanism between coal tar pitch (CTP) and calcined petroleum coke (CPC) during kneading and baking processes, and to explore the optimal ratio of CTP from the molecular scale. Results showed that the CTP content of 14 wt.% was beneficial for anode production. The mechanism of anode coking behaviour at high temperatures was also analyzed. The carbonisation of CTP during high-temperature baking broke the carbon rings and formed straight chains, exposing active carbon atoms that can bond with the remaining layers. As the high-temperature simulation progresses, the carbon chains at the bonding sites became increasingly cyclized and aromatised, which made the layered structure of the baked block similar to that of graphite. The generation of various volatile hydrocarbons during the baking process was observed, namely CH4, C2H4, C2H2, C2H6, and C3H6. This work provides theoretical basis for process optimisation.

Computational and experimental approaches into molecular structure mechanism of ZQV coal and the COx gas releases during pyrolysis

Understanding the structural features of coal and its reactive behaviors in thermal processing is the fundamental to achieve high-efficiency utilization of coal. However, the molecular structural features and the evolution characteristics at atomic scales in thermal processing remain unclear in the microscopic views, particularly for the low-rank coals, which usually exhibits high oxygen content with a variety of oxygen-containing organic functional groups and gets lots of CO/CO2 release during pyrolysis. This research provides a comprehensive analysis of the ZaoQuan vitrinite (ZQV) bituminous coal's structural attributes and its pyrolysis-induced reactivity, which is crucial for optimizing the thermal conversion of low-rank coals. The structural parameters characteristics and molecular structures of ZQV have been revealed based on series of materials characterizations and molecular modeling calculations. The single molecular description of ZQV was conceptualized to be C209H132O41N2 under the average molecular approximation, and the particle models with size about 10 nm was also been obtained. Based on the built molecular models, reactive force field molecular dynamics (ReaxFF MD) simulations then provided insights into the dynamic behavior of molecular structures under pyrolysis conditions. Experimental validation corroborates the computational predictions, revealing a strong coherence between observed gas products and those derived from simulation data. Crucially, this study vividly captures the dynamic migration patterns of oxygen-containing groups during coal's thermal decomposition, thus contributing to the understanding of gasification processes. The release mechanisms for CO and CO2 are systematically dissected, proposing three and two distinct pathways respectively. These findings not only consolidate the molecular narrative of coal's structure and pyrolysis reactivity but also serve as a reliable foundation for interpreting the COx evolution of low-rank coals. Our integrated approach propels the scientific comprehension of coal pyrolysis and is anticipated to inform the development of more effective coal utilization pathways and thermal treatment technologies.

Exploring carbon dioxide generation in combustion of long-flame coal in Huating mining area by using ReaxFF MD

Coal combustion contributes to 44% of global fuel CO2 emissions. Until now, there has less effective means to study the generation mechanism of carbon dioxide. To approach the generation mechanism of CO2 in coal combustion process at atomic level, the structure information of long-flame coal from Huating mining area, Gansu, China was characterized by ultimate analysis, 13C NMR and XPS. According to these information, one new average structure model was established, its parameters are consistent with the experimental results. Then reactive force field (ReaxFF) molecular dynamics (MD) simulations in 2000–3500 K were performed. Four types of intermediates (Rn–O, Rn–CHO, Rn–CO2, and C2O2) that correspond to four generation routes of CO2 were obtained from the ReaxFF MD simulations. The intermediates can generate CO2 through attacks from oxygen-containing radicals or cleavage of the C–C bond. Rn–O and Rn–CHO can generate CO as intermediates. The oxidation route of acetylene, which first oxidised into C2O2 and then further oxidised into CO2, was analysed using density functional theory (DFT) calculations at the M062X/6–311** level. The DFT results demonstrate the precision of ReaxFF MD calculations. The results can improve the understanding of CO2 generation during coal combustion.

Atomic-scale insight into arc plasma radiation-induced gassing materials ablation: photothermal decomposition behavior

In this study, we present a novel computational atomistic study of the photothermal decomposition behavior of arc plasma on radiation-induced gassing materials ablation, studying a polyamide 66 (PA66) system using reactive force field (ReaxFF) molecular dynamics (MD). We determine the infrared (IR) vibrational frequency of the PA66 permanent molecular dipole using MD and then computationally impose an electric field at the same frequency to simulate photothermal decomposition by IR, verifying our observations with gas chromatography-mass spectrometry (GCMS) of experimental decomposition. MD indicates that photothermal decomposition reaction is dominated by either cleavage at low temperature or cyclization at high temperature. At low temperature, initial chain scission takes place at the two amide C–N, and the remaining chains break down into a variety of molecular fragments and free radicals. Further increasing the temperature stabilizes a variety of branched chain structures via cyclization, debranching and polymerization, with further cleavage forming hydrocarbons and volatile small molecule gases. Overall, H2, CO, H2O, alkanes and alkenes are the main gaseous products and cyclic structures (especially nitrogen-containing three-membered ring) are the main solid products during the photothermal decomposition of PA66, and their formation results from a variety of complex chemical reactions. The results of MD cover the experimental observations of GCMS, demonstrating that this computational methodology helps us understand the molecular breakdown mechanisms of arc plasma radiation-induced gassing materials. We also discuss the physical mechanism by which the main gas can accelerate arc quenching, and the importance and necessity of using electric fields to simulate IR photothermal decomposition of arc-induced ablation.

Low-temperature degradation of waste epoxy resin polymer improved by swelling-assisted pyrolysis

The disposal of waste epoxy resin matrix composite becomes an urgent issue, and pyrolysis is an effective method to decompose the epoxy resin polymer, but the high operating temperature hinders its application. In this study, a swelling-assisted pyrolysis method was proposed for the low-temperature decomposition of epoxy resin. It was found that the degradation ratio of epoxy resin increased from 59.13 % to 79.10 % at 300 °C after the swelling treatment with acetic acid. Compared with traditional pyrolysis, the cracking of epoxy resin was promoted in the swelling-assisted pyrolysis, increasing the yields of pyrolysis gas and oil, as well as enriching the light components in pyrolysis oil and raising the graphite degree of pyrolysis char. Following the characterizations and reactive force field molecular dynamics (ReaxFF-MD) simulation, the key mechanisms of swelling-assisted pyrolysis were determined as the interaction of the enhanced heat transfer and the accelerated ring-open reactions. After the swelling treatment, epoxy resin became porous, improving the heat transfer in the pyrolysis process. Meanwhile, abundant O radicals were generated with the decomposition of acetic acid, accelerating the ring-open reactions of benzene rings. The results could provide promising guidance for the pyrolysis recovery of epoxy resin matrix composites towards the circular economy.

Mechanism of initial activation of carbon–oxygen bonds for deoxidation of acetic acid

Producing valuable fuel-ranged hydrocarbons from cellulosic biomass is an effective approach to addressing potential energy issues. However, the cellulose-derived bio-oil may contain carboxylic acid components that contribute to the unfavorable acidic and corrosive nature, which necessitates upgrading through the hydrodeoxygenation (HDO) treatment using heterogeneous catalysis. Here, we performed reactive force-field molecular dynamics (ReaxFF-MD) to study the hydrogenolysis, dehydrogenation and decarboxylation mechanisms of the acetic acid model compound on the Ni-based catalyst. Comparative analysis of the activation of C–O and C=O bonds was presented, followed by an examination of the associative desorption of H2 and the dynamic evolution of main intermediates. Furthermore, the conversion pathways of key intermediates were determined and four main pathways for ethanol generation and the temperature-response mechanism for coking formation were proposed. This mechanism diagram explains the strong effects of a hydrogen environment in biomass-related reactions, highlighting the dynamic mechanism of selective hydrogenation and deoxygenation of carbon–oxygen bonds.

Computational insight of lithium adsorption and intercalation in bilayer TiC3

Lithium-ion batteries (LIBs) have gained significant attention owing to their long lifespan. However, these batteries offer unmatched energy storage capacity and suffer from restricted lithium-ion mobility within the electrodes. Here, we employ first-principles calculation to investigate the two-dimensional TiC3 bilayer material. The results exhibit a remarkably high specific capacity of 1277 mAh/g and a low diffusion energy barrier of 0.12 eV. The TiC3 bilayer is anticipated to show high electrical conductivity, maintaining its metallicity due to strong bonding with four Li atoms. Additionally, its high thermal and dynamic stabilities are expected to significantly enhance the battery performance. Notably, the AB stacking bilayer TiC3 experiences a mere 6.01 % increase in volume, considerably smaller compared to the 28 % increase observed in the SiC bilayer. This suggests that TiC3 bilayers remain intact even at the highest concentration of lithium adsorptions. We also explored the solid-electrolyte interface (SEI) formation at the outset of battery operation using reactive force field molecular dynamics simulation. The reactive products of SEI are nicely matched with previous experimental and theoretical findings. All these intriguing properties position the TiC3 bilayer as an exceptionally promising material for use in LIBs.

Atomic surface achieved through a novel cross-scale model from macroscale to nanoscale.

Chemical mechanical polishing (CMP) is widely used to achieve an atomic surface globally, yet its cross-scale polishing mechanisms are elusive. Moreover, traditional CMP normally employs toxic and corrosive slurries, resulting in potential pollution to the environment. To overcome these challenges, a novel cross-scale model from the millimeter to nanometer scale is proposed, which was confirmed by a newly developed green CMP process. The developed CMP slurry consisted of hydrogen peroxide, sodium carbonate, sodium hydroxycellulose, and silica. Prior to CMP, fused silica was polished by a ceria slurry. After CMP, the surface roughness (Sa) was 0.126 nm, the material-removal rate was 88.3 nm min-1, and the thickness of the damaged layer was 8.8 nm. The proposed model was built by fibers, through integrating Eulerian and Lagrangian models and reactive force field-molecular dynamics. The results predicted by the model were in good agreement with those of CMP experimentally. A model for large-sized fibers revealed that a direct contact area of 11.12% was obtained for a non-woven polishing pad during the CMP experiments. Another model constructed via combining Eulerian and Lagrangian functions showed that the stress at the intersections of the fibers varied mainly from 0.1 to 0.01 MPa and was higher than the stress at other parts. An increase in viscosity led to a decrease in the areas with low stress, demonstrating that viscosity enhanced the stress and facilitated the removal of material. At the microscale and nanoscale, the stress of the abrasive surface exposed to the workpiece changed from 2.21 to 6.43 GPa. Stress at the interface contributed to the formation of bridging bonds, further promoting the removal of material. With increasing the compressive stress, the material-removal form was transformed from a single atom to molecular chains. The proposed model and developed green CMP offer new insights to understand the cross-scale polishing mechanism, as well as for designing and manufacturing novel polishing slurries, pads, and setups.

Molecular analysis of hydrogen-bond structures in polymer electrolyte membrane in polymer electrolyte fuel cells below freezing temperatures

Fuel cells that generate electricity by using hydrogen and oxygen have been attracted to achieve carbon neutralization in the world because fuel cells don’t produce the greenhouse gases like carbon dioxide. The Polymer Electrolyte Fuel Cell (PEFC) has been used for many applications, such as cars and home stationary power supplies. Protons are transported in the polymer electrolyte membrane (PEM) and the transportation efficiency affects power generation efficiency. The internal state of the PEM has a great impact on the proton transport; however, the state is still not unclear below freezing temperatures because contained water might be frozen. Below freezing temperatures, proton-hopping, which is the mediation of proton conduction by water molecules, is important for proton transport. Proton-hopping is the mediation of proton conduction by water molecules. Therefore, we aim to understand the internal state of PEM below freezing temperatures. We performed reactive force-field molecular dynamics (ReaxFF MD) simulations to incorporate the proton-hopping. Our simulations consisted of two parts. First, bulk water was simulated at different temperatures to find the melting point of water by ReaxFF MD simulation. Radial distribution function (RDF) was used to find out whether the water is frozen or not. RDF of bulk water have a peak. When water is frozen, RDF of water have two distinct peaks and water molecules do not have translational or rotational motion. The size of the simulation box was 30.0 Å×30.0 Å×30.0 Å and the number of water molecules is 865. The temperature of the system was set at 1 K, 100 K, 200 K and 300 K. The time step was set at 0.50 fs and the sampling interval was 0.50 fs and the sampling interval was 100 steps. Second, the water condition in Nafion membrane was simulated. We used 4 Nafion polymer chains, 40 hydronium ions and 120 water molecules (water content: λ=4). The time step was set at 0.25 fs and the sampling interval was 100 steps. System of hydronium ions, water and Nafion changes faster than bulk water system and requires shorter timesteps. The temperature of the system was set at 300 K and 200 K. The annealing process was applied in order to equilibrate the system. After annealing, the production run was 250 ps using NVT ensemble. We performed the same analysis of the first simulation, and we analyzed the condition of water in the Nafion membrane. The RDF from the first simulation is shown in left figure. This figure shows that the two peaks were clearly visible at 1 K, 100 K and 200 K, although the second peak was barely visible at 300 K. Based on these results, the melting point of water in the ReaxFF potential was expected between 200 K and 300 K. The results of the second simulation are shown on the right figure. The results show that the heights of the two peaks are higher at 200 K than at 300 K. From these results, we could not find out whether the water in Nafion membrane was frozen or not only by these analysis methods. In this study, the melting point of water in the ReaxFF potential was found, and the state of water in polymer below freezing temperatures was analyzed. It was discussed in a paper by Qiangu Yan et al. According to them, water inside the Nafion membrane can be divided into three states. These are free water, bound water, and antifreeze water. While free water freezes, bound water has a lower melting point than normal water, and antifreeze water is not freeze. The specific heat of water in polymers should be analyzed in the future. Proton diffusion considering proton-hopping should also be analyzed in the future. Figure 1

Co-pyrolysis behavior and synergistic mechanism of sub-bituminous coal and polypropylene: A ReaxFF molecular dynamics simulation

The co-pyrolysis behavior of coal and polypropylene (PP) was investigated by reactive force field molecular dynamics (ReaxFF MD) simulation in the present study. In addition to analyzing the co-pyrolysis behavior at the atomic scale, the trajectories of different elements from different material sources were first observed. The weight loss of coal and PP is consistent with previous experimental studies, demonstrating the reliability of the ReaxFF MD simulation. The analysis of pyrolysis products showed that all blended systems contributed to tar and H2 yields at high temperatures compared to the calculated values. Among them, the coal/PP (6:4) system had the most significant effect on improving tar yield. H2 can be divided into three types according to different sources of H·, and with the increase of PP proportion, H· from PP is more willing to produce HPP2 at 3000 K. Then, the kinetics of the co-pyrolysis systems with multiple heating rates were studied, and only the activation energy of the coal/PP (4:6) system was reduced. Subsequently, we gave the mechanism by combining the pyrolysis process and the change in the number of free radicals. The blended systems promoted the production of more H· through PP, which made heavy compounds in coal further decomposition.