Molecular Simulation of Argon Adsorption and Diffusion in a Microporous Carbon with Poroelastic Couplings.

Neglected for a long time in molecular simulations of fluid adsorption and transport in microporous carbons, adsorption-induced deformations of the matrix have recently been shown to have important effects on both sorption isotherms and diffusion coefficients. Here we investigate in detail the behavior of a recently proposed 3D-connected mature kerogen model, as a generic model of aromatic microporous carbon with atomic H/C ∼ 0.5, in both chemical and mechanical equilibrium with argon at 243 K over an extended pressure range.

Timescale prediction of complex multi-barrier pathways using flux sampling molecular dynamics and 1D kinetic integration: Application to cellulose dehydration.

Reactive molecular dynamics (MD) simulations, especially those employing acceleration techniques, can provide useful insights on the mechanism underlying the transformation of buried organic matter, yet, so far, it remains extremely difficult to predict the time scales associated with these processes at moderate temperatures (i.e., when such time scales are considerably larger than those accessible to MD).

Methane Diffusion in a Flexible Kerogen Matrix.

It has been recognized that the microporosity of shale organic matter, especially that of kerogen, strongly affects the hydrocarbon recovery process from unconventional reservoirs. So far, the numerical studies on hydrocarbon transport through the microporous phase of kerogen have neglected the effect of poromechanics, that is, the adsorption-induced deformations, by considering kerogen as a frozen, nondeformable, matrix. Here, we use molecular dynamics simulations to investigate methane diffusion in an immature (i.

Poroelasticity of Methane-Loaded Mature and Immature Kerogen from Molecular Simulations.

While hydrocarbon expulsion from kerogen is certainly the key step in shale oil/gas recovery, the poromechanical couplings governing this desorption process, taking place under a significant pressure gradient, are still poorly understood. Especially, most molecular simulation investigations of hydrocarbon adsorption and transport in kerogen have so far been performed under the rigid matrix approximation, implying that the pore space is independent of pressure, temperature, and fluid loading, or in other words, neglecting poromechanics. Here, using two hydrogenated porous carbon models as proxies for immature and overmature kerogen, that is, highly aliphatic hydrogen-rich vs highly aromatic hydrogen-poor models, we perform an extensive molecular-dynamics-based investigation of the evolution of the poroelastic properties of those matrices with respect to temperature, external pressure, and methane loading as a prototype alkane molecule.

From cellulose to kerogen: molecular simulation of a geological process.

The process by which organic matter decomposes deep underground to form petroleum and its underlying kerogen matrix has so far remained a no man's land to theoreticians, largely because of the geological (Myears) timescale associated with the process. Using reactive molecular dynamics and an accelerated simulation framework, the replica exchange molecular dynamics method, we simulate the full transformation of cellulose into kerogen and its associated fluid phase under prevailing geological conditions. We observe in sequence the fragmentation of the cellulose crystal and production of water, the development of an unsaturated aliphatic macromolecular phase and its aromatization.

Mechanism of strength reduction along the graphenization pathway.

Even though polycrystalline graphene has shown a surprisingly high tensile strength, the influence of inherent grain boundaries on such property remains unclear. We study the fracture properties of a series of polycrystalline graphene models of increasing thermodynamic stability, as obtained from a long molecular dynamics simulation at an elevated temperature. All of the models show the typical and well-documented brittle fracture behavior of polycrystalline graphene; however, a clear decrease in all fracture properties is observed with increasing annealing time.

Rippled nanocarbons from periodic arrangements of reordered bivacancies in graphene or nanotubes.

We report on various nanocarbons formed from a unique structural pattern containing two pentagons, three hexagons, and two heptagons, resulting from local rearrangements around a divacancy in pristine graphene, or nanotubes. This defect can be inserted in sheets or tubes either individually or as extended defect lines. Sheets or tubes containing only this defect as a pattern can also be obtained.

Reaction mechanism for the thermal decomposition of BCl3/CH4/H2 gas mixtures.

This paper presents an ab initio study of the B/C/Cl/H gas phase mechanism, featuring 10 addition-elimination reactions involving BH(i)Cl(j) (i + j ≤ 3) species and a first description of the chemical interaction between the carbon-containing and boron-containing subsystems through the three reactions BCl(3) + CH(4) ⇌ BCl(2)CH(3) + HCl, BHCl(2) + CH(4) ⇌ BCl(2)CH(3) + H(2), and BCl(2) + CH(4) ⇌ BHCl(2) + CH(3). A reaction mechanism is then proposed and used to perform some illustrative equilibrium and kinetic calculations in the context of chemical vapor deposition (CVD) of boron carbide. Our results show that the new addition-elimination reaction paths play a crucial role by lowering considerably the activation barrier with respect to previous theoretical evaluations; they also confirm that BCl(2)CH(3) is an important species in the mechanism.

Theoretical study of the decomposition of BCl3 induced by a H radical.

We report on a theoretical study of the gas-phase decomposition of boron trichloride in the presence of hydrogen radicals using ab initio energetic calculations coupled to TST, RRKM, and VTST-VRC kinetic calculations. In particular, we present an addition-elimination mechanism (BCl(3) + H → BHCl(2) + Cl) allowing for a much more rapid consumption of BCl(3) than the direct abstraction reaction (BCl(3) + H → BCl(2) + HCl) considered up to now. At low temperatures, T ≤ 800 K, our results show that a weakly stabilized complex BHCl(3) is formed with a kinetic law compatible with the consumption rate measured in the former experiments.

Hindered rotor models with variable kinetic functions for accurate thermodynamic and kinetic predictions.

We present an extension of some popular hindered rotor (HR) models, namely, the one-dimensional HR (1DHR) and the degenerated two-dimensional HR (d2DHR) models, allowing for a simple and accurate treatment of internal rotations. This extension, based on the use of a variable kinetic function in the Hamiltonian instead of a constant reduced moment of inertia, is extremely suitable in the case of rocking/wagging motions involved in dissociation or atom transfer reactions. The variable kinetic function is first introduced in the framework of a classical 1DHR model.

Molecular dynamics of carbon dioxide, methane and their mixtures in a zeolite possessing two independent pore networks as revealed by computer simulations.

The molecular motion of methane (CH(4)) and carbon dioxide (CO(2)) sorbed in the two independent pore networks, being termed hereafter as large cavity (LC) and sinusoidal channel (SC) regions of the siliceous MWW-framework-type zeolite ITQ-1, is studied by means of atomistic computer simulation. Equilibrium molecular dynamics predicts different loading dependences for the two gases, for both the self and the collective (Maxwell-Stefan) diffusion coefficients; in particular, the transport coefficients of CH(4) go through a maximum as its loading in the zeolite increases, whereas CO(2) dynamics exhibits the decreasing trend with loading usually observed in nanoporous materials. The different loading dependence of the self-diffusivity for the two sorbates is attributed to their different probability density distribution within the supercages in the LC system of the ITQ-1 unit cell.

The self-referential method for linear rigid bodies: application to hard and Lennard-Jones dumbbells.

The self-referential (SR) method incorporating thermodynamic integration (TI) [Sweatman et al., J. Chem.

The self-referential method combined with thermodynamic integration.

The self-referential method [M. B. Sweatman, Phys.

Hit and miss of classical nucleation theory as revealed by a molecular simulation study of crystal nucleation in supercooled sulfur hexafluoride.

Classical nucleation theory pictures the homogeneous nucleation of a crystal as the formation of a spherical crystalline embryo, possessing the properties of the macroscopic crystal, inside a parent supercooled liquid. In this work we study crystal nucleation in moderately supercooled sulfur hexafluoride by umbrella sampling simulations. The nucleation free energy evolves from 5.

Sorption thermodynamics of CO2, CH4, and their mixtures in the ITQ-1 zeolite as revealed by molecular simulations.

The thermodynamic properties and siting of carbon dioxide and methane sorbed in the siliceous form of zeolite MCM-22, ITQ-1, were studied by means of grand canonical Monte Carlo simulation. ITQ-1 comprises two independent pore systems of different geometry. It was found to be CO(2)-selective toward CO(2)/CH(4) gas mixtures, its equilibrium selectivity being distinctly higher in its sinusoidal channel pore system than in the large cavity system over a wide range of pressures starting from the Henry law regime, at the three temperatures considered.

Molecular simulation of the homogeneous crystal nucleation of carbon dioxide.

We report on a molecular simulation study of the homogeneous nucleation of CO2 in the supercooled liquid at low pressure (P = 5 MPa) and for degrees of supercooling ranging from 32% to 60%. In all cases, regardless of the degree of supercooling, the structure of the crystal nuclei is that of the Pa3 phase, the thermodynamically stable phase. For the more moderate degree of supercooling of 32%, the nucleation is an activated process and requires a method to sample states of high free energy.

Atomistic simulation of the homogeneous nucleation and of the growth of N2 crystallites.

We report on a computer simulation study of the early stages of the crystallization of molecular nitrogen. First, we study how homogeneous nucleation takes place in supercooled liquid N(2) for a moderate degree of supercooling. Using the umbrella sampling technique, we determine the free energy barrier of formation for a critical nucleus of N(2).

Reorganization and growth of metastable alpha-N2 critical nuclei into stable beta-N2 crystals.

We report on results on the crystal nucleation and growth of nitrogen. Using molecular dynamics simulations, we show that while nucleation proceeds into the metastable alpha-phase (i.e.