To investigate the initial chemical events associated with high-temperature gas-phase oxidation of hydrocarbons, we have expanded the ReaxFF reactive force field training set to include additional transition states and chemical reactivity of systems relevant to these reactions and optimized the force field parameters against a quantum mechanics (QM)-based training set. To validate the ReaxFF potential obtained after parameter optimization, we performed a range of NVT−MD simulations on various hydrocarbon/O2 systems. From simulations on methane/O2, o-xylene/O2, propene/O2, and benzene/O2 mixtures, we found that ReaxFF obtains the correct reactivity trend (propene > o-xylene > methane > benzene), following the trend in the C−H bond strength in these hydrocarbons. We also tracked in detail the reactions during a complete oxidation of isolated methane, propene, and o-xylene to a CO/CO2/H2O mixture and found that the pathways predicted by ReaxFF are in agreement with chemical intuition and our QM results. We o...

ReaxFF Reactive Force Field for Molecular Dynamics Simulations of Hydrocarbon Oxidation

The reactive force-field (ReaxFF) interatomic potential is a powerful computational tool for exploring, developing and optimizing material properties. Methods based on the principles of quantum mechanics (QM), while offering valuable theoretical guidance at the electronic level, are often too computationally intense for simulations that consider the full dynamic evolution of a system. Alternatively, empirical interatomic potentials that are based on classical principles require significantly fewer computational resources, which enables simulations to better describe dynamic processes over longer timeframes and on larger scales. Such methods, however, typically require a predefined connectivity between atoms, precluding simulations that involve reactive events. The ReaxFF method was developed to help bridge this gap. Approaching the gap from the classical side, ReaxFF casts the empirical interatomic potential within a bond-order formalism, thus implicitly describing chemical bonding without expensive QM calculations. This article provides an overview of the development, application, and future directions of the ReaxFF method.

https://biblio.ugent.be/publication/8514189/file/8514192.pdf

The ReaxFF reactive force-field: development, applications and future directions

This critical review covers the application of computer simulations, including quantum calculations (ab initio and DFT), grand canonical Monte-Carlo simulations, and molecular dynamics simulations, to the burgeoning area of the hydrogen storage by metal–organic frameworks and covalent-organic frameworks. This review begins with an overview of the theoretical methods obtained from previous studies. Then strategies for the improvement of hydrogen storage in the porous materials are discussed in detail. The strategies include appropriate pore size, impregnation, catenation, open metal sites in metal oxide parts and within organic linker parts, doping of alkali elements onto organic linkers, substitution of metal oxide with lighter metals, functionalized organic linkers, and hydrogen spillover (186 references).

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Recent advances on simulation and theory of hydrogen storage in metal–organic frameworks and covalent organic frameworks

Atomistic modeling of interfaces and their impact on microstructure and properties

With the aim of developing a computationally inexpensive method for modeling the high-temperature reaction dynamics of transition metal catalyzed reactions we have developed a ReaxFF reactive force field in which the parameters are fitted to a substantial quantum mechanics (QM) training set, containing full reaction pathways for relevant reactions. In this paper we apply this approach to reactions involving carbon materials plus Co, Ni, and Cu atoms. We find that ReaxFF reproduces the QM reaction data with good accuracy while also reproducing the binding characteristics of Co, Ni, and Cu atoms to hydrocarbon fragments. To demonstrate the applicability of ReaxFF we performed high-temperature (1500 K) molecular dynamics simulations on a nonbranched all-carbon feedstock in the presence and absence of Co, Ni, and Cu atoms. We find that the presence of Co and Ni leads to substantial amounts of branched carbon atoms, leading eventually to the formation of carbon-nanotube-like species. In contrast, we find that under the same simulation conditions Cu leads to very little branching and leads to products with no nanotube character. In the absence of metals no branching is observed at all. These results suggest that Ni and Co catalyze the production of nanotube-like species whereas Cu does not. This is in excellent agreement with experimental observations, demonstrating that ReaxFF can provide a useful and computational tractable tool for studying the dynamics of transition metal catalytic chemistry.

http://www.wag.caltech.edu/home/weiqiao/ReactiveFFcarbon.pdf

Development of the ReaxFF Reactive Force Field for Describing Transition Metal Catalyzed Reactions, with Application to the Initial Stages of the Catalytic Formation of Carbon Nanotubes

Using a reactive force field (ReaxFF), we investigated the structural, energetic, and adhesion properties, of both solid and liquid Al/alpha-Al2O3 interfaces. The ReaxFF was developed solely with ab initio calculations on various phases of Al and Al2O3 and Al-O-H clusters. Our computed lattice constants, elastic constants, surface energies, and calculated work of separation for the solid-solid interface agree well with earlier first-principles calculations and experiments. For the liquid-solid system, we also investigated the nonwetting-wetting transition of liquid Al on alpha-Al2O3(0001). Our results revealed that the evaporation of Al atoms and diffusion of O atoms in alpha-Al2O3 lead to the wetting of liquid Al on the oxide surface. The driving force for this process is a decrease in interfacial energy. The nonwetting-wetting transition was found to lie in the 1000–1100 K range, which is in good agreement with sessile drop experiments.

Adhesion and nonwetting-wetting transition in the Al/α-Al 2 O 3 interface

Sugar, water and free volume networks in concentrated sucrose solutions

Molecular dynamics modeling of stishovite

Carbon has diverse structures, especially (0-dimensional) fullerenes, (1-dimensional) tubes have generated great interest among scientists and technologists. Studies of the structures and properties of these low dimensional carbon molecules show tremendous potential for use in nanoscale device applications. Motivated by these exciting possibilities in finding new forms of carbon materials and their potential applications, we have designed hypothetical single wall carbon nano-toroids. Carbon toroids are also ideal in studying the elastic and plastic deformation behavior of nanotubes under bending loads. We can accurately correlate the behavior of the tubes to its uniform curvature. In this particular work we focused on the rings of (10,10) tubes and studied their energetics, structure and mechanical properties as a function of their toroidal radius. Especially important, we have observed specific strain release paths for the toroids, i.e. the toroids going through plastic deformations and nucleating a number of kinks. We have studied phase stability diagram of toroids with kinks as a function of ring radius.

https://journals.tubitak.gov.tr/physics/issues/fiz-06-30-4/fiz-30-4-3-0607-9.pdf

Computational Studies on Mechanical Properties of Carbon Nanotori

Introduction Nafion is a polyelectrolyte consisting of nonpolar N=(CF2-CF2) and polar-ionizable P=(O-CF2-CF(CF3))2-CF2-CF2-SO3H) segment. Hydrated Nafion 117 with an average of 6.5 hydrophobic tetrafluoroethylene units per vinyl ether sulfonate unit is widely used for membranes in fuel cell operations due to its relatively high proton conductivity, mechanical and electrochemical stability. Although the ratio of hydrophobic to hydrophilic monomers is well known, the monomeric sequence in the Nafion polymer is not well characterized and its affect has not been included as a design variable to improve fuel cell performance. There is a general consensus supported by experiments and simulations that hydrated Nafion has a nanosegregated structure consisting of hydrophobic and hydrophilic domains. Notwithstanding, the details of the segregated morphology remain unclear and highly debated. In this work we explore in particular the effect of monomer sequence on the structure and transport in hydrated Nafion 117 by Molecular Dynamics (MD) simulations. Our goal is to elucidate the effect of the monomeric sequence of polar (P) and nonpolar (N) monomeric units in Nafion on the phase-segregated morphology and water/hydronium transport.

Tahir Çaǧin

Papers

Adhesion and nonwetting-wetting transition in the Al/α-Al 2 O 3 interface

Sugar, water and free volume networks in concentrated sucrose solutions

Molecular dynamics modeling of stishovite

Computational Studies on Mechanical Properties of Carbon Nanotori

Effect of monomeric sequence on nanophase-segregated structure and water transport in Nafion 117