ReaxFF Molecular Dynamics Simulations Research Articles

Reaction Dynamics of CO2 Hydrogenation on Iron Catalysts Using ReaxFF Molecular Dynamics Simulation.

The conversion of CO2 to hydrocarbons using catalysts is a promising route to utilize CO2 and produce more valuable chemicals in a sustainable manner. Recent studies have shown that iron-based catalysts perform well for the hydrogenation of CO2. While the hydrogenation reaction mechanism in the gas phase is straightforward, when catalyzed by iron it has been demonstrated to involve various chemical transformations, and the selectivity and conversion are strongly dependent on the particle size. To further investigate the dependence of the reactivity of iron catalysts on cluster size, we performed reactive molecular dynamics simulations using the ReaxFF force field (ReaxFF-MD) for iron nanoclusters of various sizes in a CO2 and H2-rich environment. We demonstrated that the homogeneous hydrogenation of CO2 was correctly described by this ReaxFF model. The dissociation mechanism of CO2 on the Fe4, Fe16 clusters, and the bcc(100) Fe slab agrees with previous DFT results. The ReaxFF-MD simulations suggest a strong dependence of reactivity on the cluster size, with the Fe4 cluster having the highest reactivity. We show that ReaxFF-MD provides a route to understand reaction mechanisms in these nonequilibrium reactive processes where fast processes and local minima are important.

Tribo-Chemical Reactions in DLC-Zirconia Sliding under Ethanol Gas Environment.

A reactive force field (ReaxFF) molecular dynamics simulation is performed for the sliding of diamond-like carbon (DLC) and yttria-stabilized zirconia (YSZ) under an ethanol gas environment, motivated by the previous experiment of ultralow friction phenomenon (friction fade-out). We observe (i) dissociation of ethanol into ethoxy and hydrogen, both of which simultaneously adsorb on the YSZ surface, and (ii) dissociation of ethanol into ethyl and hydroxy, the former of which forms a bond with another ethanol molecule and the latter of which adsorbs on the DLC surface. Reaction (i) is enhanced by the sliding motion, but occurs even without it, while reaction (ii) only occurs during sliding with a sufficiently high load pressure. The potentials of mean force for the two reactions are also calculated combining the steered MD and Jarzynski equality. It is shown that the activation energies of reactions (i) and (ii) are significantly lowered by the YSZ and DLC surfaces, respectively, as compared to those in a vacuum. The resultant activation energy is higher for reaction (ii) than for reaction (i).

Automated Skeleton Network Generation for ReaxFF Molecular Dynamics Simulations of Hydrocarbon Fuel Pyrolysis and Oxidation via a Rate-Based Algorithm.

In this study, we present an automated approach of rate-based skeleton network generation for ReaxFF MD simulation (RxMD-SN) for deriving the reaction kinetic mechanism of large hydrocarbon fuels in pyrolysis and oxidation from large-scale ReaxFF MD simulations. The approach contains the statistical calculation of reaction rate constants and the generation of skeleton reaction networks using a rate-based algorithm. The RxMD-SN method takes advantage of reaction flux ranking at a small time interval in terms of temporal reaction rate to extract the core reaction networks, which allows for keeping the rare reaction events that may be dominant in a certain period of the reaction network. The kinetic models derived from ReaxFF MD simulation in CH4 oxidation can reproduce what was obtained in the ReaxFF MD simulation, which demonstrates the capability of RxMD-SN in capturing the global reaction kinetics. An evaluation of reaction rate constants indicates that close kinetic parameters are shared for n-octane oxidation of similar reaction classes, shared oxidation reactions of CH4 against n-heptane, and shared pyrolysis reactions of the RP-3 surrogate fuel against n-heptane. This capability of RxMD-SN is particularly beneficial in meeting the challenges in characterizing the oxidation reaction kinetics of large hydrocarbon molecules. RxMD-SN approach is potentially a general approach in chemical kinetics modeling on the basis of ReaxFF MD simulations.

Triple Effects of the Physicochemical Interaction between Water and Copper and Their Influence on Microcutting.

Water has been recognized in promoting material removal, traditionally ascribed to friction reduction and thermal dissipation. However, the physicochemical interactions between water and the workpiece have often been overlooked. This work sheds light on how the physicochemical interactions that occur between water (H2O) and copper (Cu) workpiece influence material deformations during the cutting process. ReaxFF molecular dynamics simulations were employed as the primary method to study the atomistic physical and chemical interactions between the applied medium and the workpiece. Upon contact with the Cu surface, H2O dissociated into OH- ions, H+ ions, and traces of O2- ions. The OH- and O2- ions chemically reacted with Cu to form bonds that weakened the Cu-Cu bonds by elongation, while the H+ ions gained electrons and diffused into the Cu lattice as H- ions. The weakening of surface Cu bonds promoted plastic deformation and reduced the difficulty of material removal. Meanwhile, further addition of H2O molecules saw a plateau in hydrolysis and more dominance of H2O physical adsorption on Cu, which weakens the elongation of Cu-Cu bonds. While the ideal case for atomic-scale material removal was found with an optimal number of 240 H2O molecules, the presented Cu material state with more H2O molecules could account for the observations in microcutting. The constricted nature of physical adsorption and hydrogen ion diffusion in the surface layer prevented the propagation of dislocations through the surface, which subsequently caused pinning points to be closer together during chip formation as observed by smaller chip fold widths on the microscale. Theoretical and experimental analysis identified the importance of accounting for physicochemical interactions between surface media and the workpiece when considering material deformations at micronanoscale.

Revealing the mechanism of hydration on the CO2 kinetic adsorption of CaO surface via ReaxFF MD simulations with experiments

Hydration has been regarded as an effective method to improve the cyclic activity of CaO during the Calcium looping CO2 capture process, but its mechanism remains unclear. In this work, the hydration reaction of CaO surface and its underlying mechanism was studied by ReaxFF molecular dynamics simulation combing with TGA experiments. The effect of hydration on the initial CO2 adsorption rate showed a trend of promoting at low H2O degree and inhibiting at high H2O concentration. The CaO surface was easily passivated, and it can maintain its crystal structure and begin to distort at about 1.0 H2O monolayer contents. Hydration failed to promote the CO2 adsorption rate, inversely the CO2 adsorption process accelerated the diffusion of H ions inward the CaO lattice·H2O dissociation produced two hydroxyl groups: direct OwH and indirect OsH. Atomic density profiles showed that the direct hydroxyl pairs were always, and indirect OsH groups can consume surface oxygen active sites, reducing the initial rapid CO2 adsorption activity. On the pre-hydrated CaO surface, CO2 molecules were more likely to bind to solid oxygen active sites than OH groups. The H free radical diffused inside the lattice in the form of transition HO2 state, promoting the slow CO2 adsorption amount.

Enhancing Laser-Induced Graphene via Integration of Gold Nanoparticles and Titanium Dioxide for Sensing and Robotics Applications.

Laser-induced graphene (LIG) is a promising material for various applications due to its unique properties and facile fabrication. However, the electrochemical performance of LIG is significantly lower than that of pure graphene, limiting its practical use. Theoretically, integrating other conductive materials with LIG can enhance its performance. In this study, we investigated the effects of incorporating gold nanoparticles (AuNPs) and titanium dioxide (TiO2) into LIG on its electrochemical properties using ReaxFF molecular dynamics (MD) simulations and experimental validation. We found that both AuNPs and TiO2 improved the work function and surface potential of LIG, resulting in a remarkable increase in output voltage by up to 970.5% and output power density by 630% compared to that of pristine LIG. We demonstrated the practical utility of these performance-enhanced LIG by developing motion monitoring devices, self-powered sensing systems, and robotic hand platforms. Our work provides new insights into the design and optimization of LIG-based devices for wearable electronics and smart robotics, contributing to the advancement of sustainable technologies.

Mesoporous silica encapsulated core-shell NiRh@NiO nanocatalyst for performance-enhanced ethanol steam reforming

A novel mesoporous SiO2 encapsulated core-shell NiRh@NiO nanostructure was constructed to enhance the activity and stability of ethanol steam reforming (ESR). At 550 °C and ethanol-WHSV = 21 h−1, the specific activity and conversion loss (17 h) of NiRh@NiO@m-SiO2 were 0.42 molethanol/(gcat.·h) and 10.9 %. The alloying of NiRh core, dominated NiO in shell, and abundant mesopore of SiO2 were confirmed by systematic characterization techniques. The in situ DRIFTS results indicated that alloying Ni and Rh boosted ethoxide dehydrogenation to acetaldehyde and acetate demethanation to carbonate. ReaxFF molecular dynamics (MD) simulations suggested that NiO shell was conducive to water activation, which, in turn, promoted the conversion of CHxOy species. The SiO2 encapsulation derived confinement effect inhibited both metal core sintering and leaching caused by the filamentous carbon. The abundant mesopores of SiO2 ensured the facile in-diffusion of water and out-diffusion of carbonaceous products, suppressing carbon deposition within the SiO2 encapsulation and the destruction of core-shell structure.

Enhancing electrochemical performance of colorless polyimide-derived laser-induced graphene via gold nanoparticles integration: Towards advanced wearable and energy harvesting applications

In addressing the limitations of traditional laser-induced graphene (LIG) derived from colored polymers, our research pivots to the utilization of colorless polyimide (CPI) for LIG fabrication, aiming to leverage CPI’s optical transparency for enhanced applicability in advanced technologies. Despite CPI-based LIG (C-LIG)’s potential, its electrochemical performance has lagged behind that of pure graphene, prompting a need for performance enhancement. Our study introduces novel oxygen plasma treatment and multi-step laser process- ing techniques, underpinned by ReaxFF molecular dynamics simulations, to integrate gold nanoparticles (AuNPs) into the C-LIG matrix, creating Au-C-LIG. These methodologies yielded electrochemical improvements, with surface potential and work function increases from 4.70 to 4.94 eV, enhancing the triboelectric effect. Validation through SEM, XPS, XRD, and TEM analyses confirmed these enhancements. Furthermore, the Au-C-LIG-based pressure sensor demonstrated exceptional sensitivities of 27.56 kPa−1 (0–3 kPa), 6.97 kPa−1 (3–7 kPa), and 0.65 kPa−1 (7–25 kPa), alongside rapid response/recovery times of 38/35 ms, surpassing existing graphene-based sensors. The introduction of these novel fabricating techniques enhances the potential of C-LIG and improves its applicability in the realms of wearable devices and energy harvesting.

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.

Study on mechanism of supercritical water gasification of hemicellulose for hydrogen-rich syngas based on experiments and reaction molecular dynamics simulations

Hydrogen-rich syngas from supercritical water gasification (SCWG) of renewable biomass is a promising technology. However, the mechanistic studies related to experiments and simulations on hemicellulose, an important component of biomass, are still limited. Therefore, in this study, the SCWG mechanism of hemicellulose was investigated using a batch reactor and via ReaxFF molecular dynamics simulations. The results indicated that the long chains of xylan broke to short chains during the heating process, which was accompanied by the occurrence of ring-opening reactions. SCWG occurred more completely with increasing temperature or decreasing concentration, when hydrogen was more selective and methane was produced mainly by gas reforming reactions. At 600 °C and 5 g/100 g water, the total gas yield of xylan SCWG was 38.65 mmol g−1, and the hydrogen yield could reach 14.48 mmol g−1. The liquid products mainly included furfural and phenol. The solid products consisted of a high C content, which could reach more than 90 % at 500 °C, and were dominated by polycyclic aromatic hydrocarbons, and at 600 °C by benzene and naphthalene. Xylan formed carbon balls on the surface of the solid products, while hemicellulose exhibited the formation of a dense porous structure.

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