ReaxFF Molecular Dynamics Simulations Research Articles

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.

Process intensification of tar chemical looping reforming based on ReaxFF reaction kinetics investigation

Traditional chemical processes currently produce tar as a by-product with low efficiency. This paper proposes a novel tar chemical looping reforming (t-CLR) process based on reactive force field (ReaxFF) reaction kinetics investigation to enhance its utilization efficiency. Firstly, the t-CLR process is initially established to determine the optimum operating conditions. Through ReaxFF molecular dynamics (ReaxFF-MD) simulations conducted under above conditions, the activation energies of the fuel reactor, steam reactor, and air reactor are determined to be 63.25 kJ/kmol, 107.85 kJ/kmol, and 159.53 kJ/kmol respectively. By implementing micro-dynamic parameters filling, oxygen carrier recovery, and heat integration, the optimal t-CLR process achieves a 97.60 % recovery rate of the oxygen carrier and reduces external utility consumption by 71.39 %. The comprehensive framework with macro and micro dimensions is established based on the above studies to promote the cleaner production of traditional processes. Finally, the utilization efficiency of tar is efficiently improved through t-CLR technology, which allows for an adjustable C/H ratio in syngas. The t-CLR technology can be applied to the chemical and energy industries by adding value to by-products and optimizing energy utilization. Overall, this work provides important guidance for process intensification, energy conservation, environmental protection, and waste utilization. The study further promotes the development of industrial production in a cleaner and more efficient direction.

Experimental and reactive molecular dynamics simulation of municipal sludge extract pyrolysis

The sludge extract obtained from degradation solvent extraction exhibits high carbon content, low oxygen content, and ash-free characteristics, making it an ideal fuel due to its excellent performance. In this study, the pyrolysis process of the extract was systematically studied by combining GC-MS experiment with ReaxFF molecular dynamics simulation. The comparison between GC-MS experimental results and ReaxFF-MD simulation shows a high degree of similarity, thus confirming the accuracy of the simulation. This study identified 2000 K as the turning temperature between initial pyrolysis and secondary pyrolysis. In the temperature range of 1200–2000 K, the extract mainly undergoes primary pyrolysis, while in the temperature range of 2000–3000 K, the secondary reaction is remarkable, which is characterized by the further decomposition of tar free radicals and the condensation reaction involving tar free radicals or char. The analysis of typical gas content shows that H2, CO, NH3 and HCN are by-products of the secondary reaction of tar, and there are a lot of reducing gases at high temperature, such as H2, CO and CH4. Finally, the migration patterns of carbon, hydrogen, oxygen and nitrogen are discussed. It is worth noting that hydrogen and nitrogen mainly migrate to the pyrolysis gas phase. This not only laid a foundation for the separation of nitrogen and hydrogen in the future, but also triggered thinking about the potential of “low-carbon emission resource” in future research work. When compared to raw municipal sludge, the initial pyrolysis temperature of municipal sludge extract is much lower, and its pyrolysis process becomes faster. The advantages of reactive gas components indicate that it is more reactive. This variance results in a distinct product distribution and pyrolysis mechanism.

Detailed insights of polydimethylsiloxane (PDMS) degradation mechanism via ReaxFF MD and experiments

Polydimethylsiloxane (PDMS) serves as a widely used silicone polymer in ablative thermal protection material. Nevertheless, the intricate degradation mechanism governing PDMS at ultra-high temperatures remains unclear. Herein, PDMS pyrolysis was in-depth investigated by employing a combination of experimental methodologies and ReaxFF molecular dynamics (ReaxFF-MD) simulations. PDMS model was firstly validated by product distribution analysis and kinetics analysis. Molecular reaction tracking unveils that the pyrolysis pathway primarily is triggered by the cleavage of the Si-C bond, leading to cyclization reactions and fractures within the Si-O framework at elevated temperatures. The pathways leading to the formation of gaseous, liquid, and solid products during thermal cracking are systematically unveiled. Drawing from these insights, DFT calculations reveal Si-C bond dissociation energies (BDEs) of 98.8, 102.2, and 108.2 kcal/mol for Si-CH3, Si-C2H3, and Si-C6H5, respectively. MD simulations further indicate that the heightened Si-C BDEs across various silicone rubbers impede their decomposition, thereby augmenting the thermal stability of the material. This study augments our comprehension of the PDMS pyrolysis mechanism and offers theoretical insights for enhancing its thermal stability.

Study on OH radical oxidation of 4H-SiC in plasma based on ReaxFF molecular dynamics simulation

The application of hydroxyl (OH) radicals to promote chemistry during the polishing of the SiC substrates has received increasing attention from researchers. Compared to catalyzing OH radicals in solution, catalyzing OH radicals in the gaseous state have faster speed and more environmentally friendly generation. In this study, a stable OH radical plasma jet was generated for the oxidation modification of 4H-SiC surface, by excitation and ionizing water molecules in plasma. The deposition of OH radicals on the 4H-SiC surface was simulated with ReaxFF molecular dynamics to study the nanoscale oxidation mechanism. The experimental results demonstrate that the OH radical can achieve surface oxidation of 4H-SiC, thus effectively reducing the hardness. The simulation results revealed that the deposition of OH radicals on the 4H-SiC surface generated varieties of different reaction processes, such as adsorption, dissociation, decomposition, rebound, collision, and injection. The OH radical deposition rate and substrate temperature determine the reaction process. The results of this study provide theoretical support for further revealing the mechanism of action of OH radicals on 4H-SiC and for realizing efficient plasma-assisted polishing.

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.

Insights into the atomic-scale removal mechanism of single crystal diamond in plasma-assisted polishing with quartz glass

The atomic removal mechanism in plasma-assisted polishing (PAP) of single crystal diamond (SCD) substrate is investigated using ReaxFF molecular dynamics simulation. For comparison, another two simulations, polishing without O radical irradiation, polishing with the O atoms’ oxidation action omitted, are conducted. C atoms in PAP are removed by attachment to quartz glass via C-O-Si bonds or transformation into non-diamond. Such removals are achieved by the synergistic action of O atoms’ oxidation and Si-O-C bonds’ dragging force provided by plasma and quartz glass. As a result, material removal rate in PAP is much higher than that in another two cases. Thus, promoting the oxidizing properties of polishing plates and atmospheres is proposed as a route to accelerate diamond polishing efficiency.

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.

Understanding the thermal activation behavior of montmorillonite and kaolinite at atomic level by ReaxFF molecular dynamics simulations

The origin of the difference in activity between calcined kaolinite and calcined montmorillonite is not well understood. Herein, reactive force field molecular dynamic (ReaxFF MD) simulations were adopted to study the thermal activation behavior of montmorillonite and kaolinite. It was found that during the calcination process, the removal of hydroxyl groups from kaolinite and montmorillonite not only leads to the break of Al (Mg)-O bonds, but also results in the formation of Si[5] and the break of Si-O bonds. The octahedral alumina sheet in kaolinite is more easily destroyed than that of montmorillonite. Most of the hydroxyl groups in kaolinite can be removed after calcination at 800 °C. While only 32.5% of the hydroxyl groups in montmorillonite are removed after calcination at 900 °C. The combination of free hydroxyl groups with Si[4] mainly causes the break of Si-O bonds in Si-O-Al(Mg) structure and the destruction of connection between the tetrahedral silica sheet and octahedral alumina sheet. While the Si-O-Si networks in both kaolinite and montmorillonite are difficult to disrupt. Disrupting the network structures of Si-O-Si sheet is the key to improving the activity of montmorillonite. This work provides an insight into the thermal activation of montmorillonite and kaolinite at atomic level, and is beneficial for exploring suitable pathways for simultaneous activation of kaolin and montmorillonite.

Creating a Reaction Data Set Labeled with Reaction Class for Automated Reaction Classification for ReaxFF Molecular Dynamics Simulations of Realistic Fuel Pyrolysis

Creating a Reaction Data Set Labeled with Reaction Class for Automated Reaction Classification for ReaxFF Molecular Dynamics Simulations of Realistic Fuel Pyrolysis

The pyrolysis of 4,4,8,10-tetramethyl decalin and the influence of molecular structures on oil thermal cracking: A ReaxFF molecular dynamics simulation and DFT study

The molecular structure of different hydrocarbons in crude oil has a significant effect on the thermal cracking mechanism. This study delves into the thermal cracking mechanism, pyrolysis process, and kinetics of 4,4,8,10-tetramethyl decalin (a representative bicyclic sesquiterpane in crude oil) using reactive molecular dynamics simulation (ReaxFF–MD) at the temperature range from 2000 K to 3000 K. Interestingly, the major intermediate products from thermal pyrolysis simulation align closely with the main ion fragments in the mass spectrometry of 4,4,8,10-tetramethyl decalin. The results can be directly used to analyze the pyrolysis mechanism. It reveals five reaction pathways implicated in the initial pyrolysis of 4,4,8,10-tetramethyl decalin: three demethylation reactions (more likely to occur) and two ring-opening reactions. By employing first-order kinetic analysis based on ReaxFF–MD simulations within the temperature range of 2000 K to 3000 K, the apparent activation energies for 4,4,8,10-tetramethyl decalin are calculated and compared with our prior simulation results for n-tetradecane. The initial pyrolysis of 4,4,8,10-tetramethyl decalin predominantly entails the cleavage of C–C bonds in both branched chain and carbon ring systems, resulting in the generation of methyl and high molecular weight ring-structured radicals. Conversely, the initial pyrolysis of n-tetradecane yields low molecular weight straight–chain radicals by cleaving C–C bonds except for the terminal position. This implies that the bicyclic sesquiterpane 4,4,8,10-tetramethyl decalin serves as the favorable precursor for producing methane (CH4) and liquid hydrocarbons (C6–C13) during the primary formation stage of light oil. With an equivalent number of carbon atoms, n-tetradecane serves as the preferred precursor for generating wet gases (C2–C5) at the initial formation stage of wet gases. The insights derived from the ReaxFF–MD simulation offer valuable perspectives for comprehending the process of thermal cracking in crude oil under geological conditions.

ReaxFF molecular dynamics simulation on the combustion mechanism of toluene/ethanol/n-heptane mixed fuel

The addition of ethanol and toluene to diesel fuel serves as an effective approach to mitigate engine knock. In this work, we employed ReaxFF molecular dynamics simulations to investigate the oxidation of toluene, ethanol, and n-heptane mixed fuels. Results show that toluene, ethanol, and 60.12 % of n-heptane molecules initiate dehydrogenation via active species such as O2, OH, HO2, etc. The initial decomposition mechanisms of the remaining n-heptane molecules involve the fission of CC bonds and direct dehydrogenation reactions. The formation of intermediate C2H4 primarily occurs via β-cleavage of saturated alkyl radicals with higher carbon atom numbers. The products CO and CO2 molecules strongly depend on the concentrations of C2H4 and CH3. Notably, the incorporation of ethanol/toluene in the system diminishes the presence of OH, thereby decreasing the consumption of C2H4 by OH. The activation energy values (43.36 ∼ 45.19 kcal mol−1) align well with experimental data.

Probing the tribological behaviors of a-C films in CO2 environment based on ReaxFF molecular dynamics simulation

In this paper, ReaxFF simulation is used to study the friction characteristics of amorphous carbon (a-C) films in CO2 environment by considering the influences of CO2 molecular number and the normal load. The results manifest that the decrease of friction at the beginning of sliding stage is mainly due to the physical adsorption of CO2 molecules, while in the stable stage, the low and stable friction is caused by the combination of chemical adsorption and physical adsorption. The large amount of CO2 molecules greatly reduce the friction force by generating more C-O terminal groups, which are not easily damaged by the increased normal load. In addition, due to the presence of CO2 molecules, graphitization has little effect on the friction behaviors of a-C films in CO2 environment.

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.

Molecular level structure development of Indian coal using experimental, ML and DFT techniques

The nature of interaction between coal surface and reagent affects fine coal flotation yield. Hence, it is crucial to understand the nature of coal surface to improve flotation yield. Therefore, determination of coal surface at the molecular level is important to optimize the flotation performance. But coal is a complex material, and its molecular presentations depend highly on its geographical origin. In this work, various experimental techniques such as ultimate analysis, proximate analysis, TGA, FTIR and solid state 13C NMR were used to characterize the structure of low-rank Indian coal. The data extracted from these experiments were then used to develop molecular level presentation of coal using machine learning (ML) and DFT. Five stable molecular level structures were proposed for this Indian coal. DFT calculated FTIR spectra matches reasonably with the experimental FTIR data. Finally, a 3D model of the coal sample was developed, and reactive force field (ReaxFF) molecular dynamics simulations were performed for thermal decomposition analysis.

Thermal decomposition mechanism study of 1-acetyl-3,5-dinitro-1,3,5-triazacyclohexane (TAX)

ABSTRACT 1-Acetyl-3,5-dinitro-1,3,5-triazacyclohexane, codenamed TAX, is a by-product of the RDX/HMX production process. The thermal decomposition process of TAX has not been studied at present. Understanding the decomposition mechanism of TAX can improve the understanding of its performance, which is helpful for the treatment of RDX/HMX production wastewater. The thermal decomposition process of 1-acetyl-3,5-dinitro-1,3,5-triazacyclohexane (TAX) was studied by TG-DSC and ReaxFF molecular dynamics (ReaxFF MD) simulation. The results showed that the activation energy of TAX calculated by the Kissinger and Flynn-Wall-Ozawa methods is 140 kJ·mol−1, and the activation energy obtained by the Friedman method is 154 kJ·mol−1; The thermal decomposition of TAX may generate the main intermediate products C5H9N4O3, C5H9N3O, C3H5NO, C2H3O, HNO2, N2O2 and NO, etc. and the intermediate products continued to decompose to generate the final products CH2O, CO2, CO, H2O and N2; The reaction rate constants of each stage of TAX were calculated, and the possible thermal decomposition pathways of TAX were obtained: TAX → C5H9N4O3 + NO2 → C5H9N3O + C3H5NO + C2H4N3O2 + NO2 → C2H3O + CH2N + HNO2 + N2O2 + NO2 + NO + H + OH + … → CH2O + CO2 + CO + H2O + N2 + … This study contributes to a deeper understanding of the decomposition process of TAX after heating and has specific significance for reducing the thermal hazard of the TAX pyrolysis process.

Initial reaction mechanism of lignin and polyethylene steam co-gasification based on ReaxFF molecular dynamics simulation

Initial reaction mechanism of lignin and polyethylene steam co-gasification based on ReaxFF molecular dynamics simulation

ReaxFF molecular dynamics simulation of single-crystalline silicon plasma polishing and subsurface damage removal

Recently, plasma etching has become one of the most promising polishing methods due to its low cost and environmental friendliness. In this study, the mechanism and parametric influence of plasma etching were investigated using ReaxFF-based molecular dynamics (RBMD) simulation, allowing the characterization of the process of F atom etching of monocrystalline silicon and the removal of subsurface defects from an atomic perspective. We simulated the process of F-atom etching and the mechanism of F-atom removal of the subsurface damage of single-crystalline silicon. The simulation results indicate that SiF4 generated by monocrystalline silicon under the action of F atoms gradually generated SiF5- groups and detached from the substrate as the reaction time increases. Simultaneously, under the action of F atoms, Si atoms gradually filled the subsurface defects, achieving damage removal. In addition, the incident angle can be increased in the plasma polishing process to reduce the thickness of the amorphous deformation layer and improve the surface quality.

ReaxFF molecular dynamics simulation and experimental validation about chemical reactions of water and alcohols on SiC surface

The high chemical inertness of silicon carbide (SiC) makes it difficult for surface chemical reactions to occur. Surface oxidation reactions are crucial for improving the processing efficiency and surface quality of SiC wafers. Extensive research has been conducted on conventional aqueous polishing slurries, while investigations on non-aqueous solvent-based polishing slurries have emerged as alternative options for enhancing SiC surface processing, as evidenced by recent scholarly reports. However, the surface chemical reaction mechanism of silicon carbide in either water or other hydroxyl groups containing solvents remains still unclear and is not well understood. In this work, ReaxFF molecular dynamics simulation was used to study the dynamic reaction processes between solvents and 6H SiC (001) surfaces. The oxidation of SiC surface in water and alcohol solvent is found to go through three stages: in the first stage, the solvent molecules move towards the SiC surface and react with the uncoordinated Si atoms and C atoms on the surface; in the second stage, the solvent molecules adsorbed on the surface of SiC reduces the Si–C bonding energy and promotes the breaking of Si–C bonds; in the third final stage, the migration of OH and H on the SiC surface induces the formation of Si–O–Si bonds, which promotes the continuous oxidation of the SiC surface. The order of simulated chemical reactivity between solvent and silicon carbide surface is consistent with experimental polishing removal rates. The reaction mechanism of solvent on silicon carbide surface was proposed by combining ReaxFF simulation results with polishing experiments, AFM, and XPS data analysis, shedding new insights on better understanding the mechanism of SiC chemical mechanical polishing.

The mechanism of H2O in the superheated steam affecting pyrolysis of the kaolinite-associated kerogen

In this work, ReaxFF molecular dynamics (MD) simulation was adopted to investigate the effect of superheated steam on the conversion of the kaolinite-associated Barkol kerogen (BLK) and reveal the corresponding mechanism. The ReaxFF simulated weight loss rate (DTG) and release tendency of H2O, H2 and CO2 for BLK and the kaolinite-associated BLK agreed well with the results of Py-MS experiments. H-rich rate, double bond equivalents (DBEs), and hydrocarbon content were adopted to assess the quality of C5-C40, and configurations of C40+ were extracted to investigate the characteristics of residues. And the H2Osteam- and kaolinite-involved (steam refers to the superheated steam) reactions were analyzed. The results indicate that B- and L-acid sites of kaolinite co-catalyzed decomposition of BLK into heavy oil and shale gas in the kaolinite-pyrolysis system, and in steam/kaolinite-pyrolysis system, H2Osteam further promoted decomposition of BLK into higher quality shale oil, especially for C5-C13 components, remaining higher aromatic and porous residues. And this enhanced effect of H2Osteam is attributed to kaolinite and the induced decomposition of H2Osteam molecules and their participation as reactants in reactions in two aspects: i) interaction between kaolinite and H2Osteam, on one hand, inhibited formation of L-acids and facilitated generation of B-acids to catalyze carbocation ion reactions process, further weakened dehydrogenation of organics catalyzed by L-acid sites, and enhanced cracking of residues catalyzed by B-acaid sites, on the other hand, promoted decomposition of H2Osteam molecules to form H-rich environment and further weakened dehydrogenation of organics; ii) attacking Car directly, H2Osteam promoted shedding of alkyl side chains and ring-opening of aromatics to increase the –CH2– content in shale oil. This paper provides theoretical guidance for further understanding mechanism of H2Osteam on pyrolysis of kaolinite-associated kerogen and corresponding catalyst development and preparation.