A detailed insight of key reactive events related to oxidation and pyrolysis of hydrocarbon fuels further enhances our understanding of combustion chemistry. Though comprehensive kinetic models are ...available for smaller hydrocarbons (typically C3 or lower), developing and validating reaction mechanisms for larger hydrocarbons is a daunting task, due to the complexity of their reaction networks. The ReaxFF method provides an attractive computational method to obtain reaction kinetics for complex fuel and fuel mixtures, providing an accuracy approaching ab-initio-based methods but with a significantly lower computational expense. The development of the first ReaxFF combustion force field by Chenoweth et al. (CHO-2008 parameter set) in 2008 has opened new avenues for researchers to investigate combustion chemistry from the atomistic level. In this article, we seek to address two issues with the CHO-2008 ReaxFF description. While the CHO-2008 description has achieved significant popularity for studying large hydrocarbon combustion, it fails to accurately describe the chemistry of small hydrocarbon oxidation, especially conversion of CO2 from CO, which is highly relevant to syngas combustion. Additionally, the CHO-2008 description was obtained faster than expected H abstraction by O2 from hydrocarbons, thus underestimating the oxidation initiation temperature. In this study, we seek to systemically improve the CHO-2008 description and validate it for these cases. Additionally, our aim was to retain the accuracy of the 2008 description for larger hydrocarbons and provide similar quality results. Thus, we expanded the ReaxFF CHO-2008 DFT-based training set by including reactions and transition state structures relevant to the syngas and oxidation initiation pathways and retrained the parameters. To validate the quality of our force field, we performed high-temperature NVT-MD simulations to study oxidation and pyrolysis of four different hydrocarbon fuels, namely, syngas, methane, JP-10, and n-butylbenzene. Results obtained from syngas and methane oxidation simulation indicated that our redeveloped parameters (named as the CHO-2016 parameter set) has significantly improved the C1 chemistry predicted by ReaxFF and has solved the low-temperature oxidation initiation problem. Moreover, Arrhenius parameters of JP-10 decomposition and initiation mechanism pathways of n-butylbenzene pyrolysis obtained using the CHO-2016 parameter set are also in good agreement with both experimental and CHO-2008 simulation results. This demonstrated the transferability of the CHO-2016 description for a wide range of hydrocarbon chemistry.
In this work, the authors have developed a reactive force field (ReaxFF) to investigate the effect of water molecules on the interfacial interactions with vacancy defective hexagonal boron nitride ...(h-BN) nanosheets by introducing parameters suitable for the B/N/O/H chemistry. Initially, molecular dynamics simulations were performed to validate the structural stability and hydrophobic nature of h-BN nanosheets. The water molecule dissociation mechanism in the vicinity of vacancy defective h-BN nanosheets was investigated, and it was shown that the terminal nitrogen and boron atoms bond with a hydrogen atom and hydroxyl group, respectively. Furthermore, it is predicted that the water molecules arrange themselves in layers when compressed in between two h-BN nanosheets, and the h-BN nanosheet fracture nucleates from the vacancy defect site. Simulations at elevated temperatures were carried out to explore the water molecule trajectory near the functionalized h-BN pores, and it was observed that the intermolecular hydrogen bonds lead to agglomeration of water molecules near these pores when the temperature was lowered to room temperature. The study was extended to observe the effect of pore sizes and temperatures on the contact angle made by a water nanodroplet on h-BN nanosheets, and it was concluded that the contact angle would be less at higher temperatures and larger pore sizes. This study provides important information for the use of h-BN nanosheets in nanodevices for water desalination and underwater applications, as these h-BN nanosheets possess the desired adsorption capability and structural stability.
In this work, the authors have developed a reactive force field (ReaxFF) and performed molecular dynamics simulations to investigate the effect of water molecules on the interfacial interactions with vacancy defective hexagonal boron nitride (h-BN) nanosheets.
The ReaxFF protein reactive force field (protein-2013) has been successfully employed to simulate the biomolecules and membrane fuel cells, but it inaccurately describes the weak interaction of ...functionalized hydrocarbon/water molecules in condensed phase, especially for the density. In this article, the development of a ReaxFF force field (CHON-2017_weak) on the basis of protein-2013 is presented that improves the weak interaction description for atom pairs of carbon, hydrogen, oxygen, and nitrogen. To examine the quality of the force field, we performed a series of molecular dynamics simulations with model systems. These simulations, describing density trends for pure and mixture compound systems, demonstrate that CHON-2017_weak force field predictions are in good agreement with experimental data. Furthermore, ReaxFF can also describe the phase separation in hexane–water mixture and dissolution of ethanol or tetramethylammonium (TMA) in liquid water. To validate it in the application of membrane fuel cells, we studied structural property and degradation mechanism of TMA in alkaline aqueous solution, as well as some typical chemical reactions for small compounds. On the basis of our results, an additional reaction pathway is proposed for the degradation of TMA, which seems to be more energetically favorable compared to the main mechanism predicted from quantum mechanics calculations.
During the carbonization process of raw polymer precursors, graphitic structures can evolve. The presence of these graphitic structures affects mechanical properties of the carbonized carbon fibers. ...To gain a better understanding of the chemistry behind the evolution of these structures, we performed atomistic-scale simulations using the ReaxFF reactive force field. Three different polymers were considered as a precursor: idealized ladder PAN (polyacrylonitrile), a proposed oxidized PAN, and poly(p-phenylene-2,6-benzobisoxazole). We determined the underlying molecular details of polymer conversion into a carbon fiber structure. Because these are C/H/O/N-based polymers, we first developed an improved force field for C/H/O/N chemistry based on the density functional theory data with a particular focus on N2 formation kinetics and its interactions with polymer-associated radicals formed during the carbonization process. Then, using this improved force field, we performed atomistic-scale simulations of the initial stage of the carbonization process for the considered polymers. On the basis of our simulation data, the molecular pathways for the formation of low-molecular-weight gas species and all-carbon ring formation were determined. We also examined the possible alignment of the developed all-carbon 6-membered ring clusters, which is crucial for the further graphitic structure evolution.
The oxidation of two-dimensional Ti
3
C
2
MXene structures has been recognized as a promising method for the formation of hybrid structures of carbon supported nanotitania. Here, we studied the ...oxidation of Ti
3
C
2
MXene structures using reactive force field (ReaxFF) based molecular dynamics simulations. To investigate the effect of oxidizing agent, we used three different environments including dry air (oxygen molecules), wet air (oxygen and water molecules) and hydrogen peroxide (H
2
O
2
molecules). Oxidation simulations were performed at different temperatures of 1000, 1500, 2000, 2500 and 3000 K. We found that by controlling the temperature, carbon supported titania can be formed by diffusion of Ti atoms to the surface of MXene structures during the oxidation. These results were confirmed by the increase in the average bond orders of C-C and Ti-O and decrease in the average bond orders of Ti-C. Moreover, it was revealed that by increasing the temperature, the oxidation rate increases, and the rate depends on the oxidant according to the following order: H
2
O
2
> wet air > dry air. This was validated by heating the MXene in wet air and dry air followed by examining the products using X-ray diffraction and Raman spectroscopy. To investigate the effect of the presence of the oxidant, the MXene structure was also heated under vacuum, and it was found that in this case the MXene structure was converted to cubic TiC. Furthermore, it was indicated that during the heating process of the MXene structure in a vacuum, the average bond orders for C-C, Ti-C and Ti-O do not change significantly due to the lack of oxidants in the environment, leaving room for only topotactic transformation of atoms during the heating. The formation of the TiC structure by heating the MXene in an inert environment was confirmed experimentally as well.
The effect of different environments on the oxidation of Ti
3
C
2
MXene structures is investigated using ReaxFF molecular dynamics simulations and experimental techniques.
Nanostructured birnessite exhibits high specific capacitance and nearly ideal capacitive behaviour in aqueous electrolytes, rendering it an important electrode material for low-cost, high-power ...energy storage devices. The mechanism of electrochemical capacitance in birnessite has been described as both Faradaic (involving redox) and non-Faradaic (involving only electrostatic interactions). To clarify the capacitive mechanism, we characterized birnessite's response to applied potential using ex situ X-ray diffraction, electrochemical quartz crystal microbalance, in situ Raman spectroscopy and operando atomic force microscope dilatometry to provide a holistic understanding of its structural, gravimetric and mechanical responses. These observations are supported by atomic-scale simulations using density functional theory for the cation-intercalated structure of birnessite, ReaxFF reactive force field-based molecular dynamics and ReaxFF-based grand canonical Monte Carlo simulations on the dynamics at the birnessite-water-electrolyte interface. We show that capacitive charge storage in birnessite is governed by interlayer cation intercalation. We conclude that the intercalation appears capacitive due to the presence of nanoconfined interlayer structural water, which mediates the interaction between the intercalated cation and the birnessite host and leads to minimal structural changes.
Hydronium (H3O+) and hydroxide (OH–) ions have anomalously large diffusion constants in aqueous solutions due to their combination of vehicular and Grotthuss hopping diffusion mechanisms. An ...improvement of the ReaxFF reactive water force field on the basis of our first-generation water force field (water-2010) is presented to describe the proton transfer (PT) mechanisms of H3O+ and OH– in water. Molecular dynamics simulation studies with the water-2017 force field support the Eigen–Zundel–Eigen mechanism for PT in acidic aqueous solution and reproduce the hypercoordinated solvation structure of the OH– in a basic environment. In particular, it predicts the correct order of the diffusion constants of H2O, H3O+, and OH– and their values are in agreement with the experimental data. Another interesting observation is that the diffusion constants of H3O+ and OH– are close to each other at high concentration due to the strong correlation between OH– ions in basic aqueous solution. On the basis of our results, it is shown that ReaxFF provides a novel approach to study the complex acid–base chemical reactions in aqueous solution with any pH value.
In this article, we report the development of a ReaxFF reactive potential that can accurately describe the chemistry and dynamics of carbon condensed phases. Density functional theory (DFT)-based ...calculations were performed to obtain the equation of state for graphite and diamond and the formation energies of defects in graphene and amorphous phases from fullerenes. The DFT data were used to reparametrize ReaxFFCHO, resulting in a new potential called ReaxFFC‑2013. ReaxFFC‑2013 accurately predicts the atomization energy of graphite and closely reproduces the DFT-based energy difference between graphite and diamond, and the barrier for transition from graphite to diamond. ReaxFFC‑2013 also accurately predicts the DFT-based energy barrier for Stone–Wales transformation in a C60(I h ) fullerene through the concerted rotation of a C2 unit. Later, MD simulations of a C180 fullerene using ReaxFFC‑2013 suggested that the thermal fragmentation of these giant fullerenes is an exponential function of time. An Arrhenius-type equation was fit to the decay rate, giving an activation energy of 7.66 eV for the loss of carbon atoms from the fullerene. Although the decay of the molecule occurs primarily via the loss of C2 units, we observed that, with an increase in temperature, the probability of loss of larger fragments increases. The ReaxFFC‑2013 potential developed in this work, and the results obtained on fullerene fragmentation, provide an important step toward the full computational chemical modeling of coal pyrolysis, soot incandescence, high temperature erosion of graphitic rocket nozzles, and ablation of carbon-based spacecraft materials during atmospheric reentry.
Atomically thin two-dimensional (2D) metals may be key ingredients in next-generation quantum and optoelectronic devices. However, 2D metals must be stabilized against environmental degradation and ...integrated into heterostructure devices at the wafer scale. The high-energy interface between silicon carbide and epitaxial graphene provides an intriguing framework for stabilizing a diverse range of 2D metals. Here we demonstrate large-area, environmentally stable, single-crystal 2D gallium, indium and tin that are stabilized at the interface of epitaxial graphene and silicon carbide. The 2D metals are covalently bonded to SiC below but present a non-bonded interface to the graphene overlayer; that is, they are 'half van der Waals' metals with strong internal gradients in bonding character. These non-centrosymmetric 2D metals offer compelling opportunities for superconducting devices, topological phenomena and advanced optoelectronic properties. For example, the reported 2D Ga is a superconductor that combines six strongly coupled Ga-derived electron pockets with a large nearly free-electron Fermi surface that closely approaches the Dirac points of the graphene overlayer.
The cold sintering process (CSP) densifies ceramics at much lower temperatures than conventional sintering processes. Several ceramics and composite systems have been successfully densified under ...cold sintering. For the grain growth kinetics of zinc oxide, reduced activation energies are shown, and yet the mechanism behind this growth is unknown. Herein, we investigate these mechanisms in more detail with experiments and ReaxFF molecular dynamics simulations. We investigated the recrystallization of zinc cations under various acidic conditions and found that their adsorption to the surface can be a rate‐limiting factor for cold sintering. Our studies show that surface hydroxylation in CSP does not inhibit crystallization; in contrast, by creating a surface complex, it creates an orders of magnitude acceleration in surface diffusion, and in turn, accelerates recrystallization.
In the cold sintering process, the presence of water enhances the surface diffusion of ions at low temperatures and results in extraordinary grain growth. This accelerated surface diffusion mechanism has been revealed using ReaxFF simulations to model the ZnO/water/acetic acid interface.
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