Equilibrium properties of the reaction H2 ⇌ 2H by classical molecular dynamics simulations (original) (raw)
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Physical Chemistry Chemical Physics, 2014
We show how we can find the enthalpy of a chemical reaction under non-ideal conditions using the Small System Method to sample molecular dynamics simulation data for fluctuating variables. This method, created with Hill's thermodynamic analysis, is used to find properties in the thermodynamic limit, such as thermodynamic correction factors, partial enthalpies, volumes, heat capacities and compressibility. The values in the thermodynamic limit at (T,V, m j ) are then easily transformed into other ensembles, (T,V,N j ) and (T,P,N j ), where the last ensemble gives the partial molar properties which are of interest to chemists. The dissociation of hydrogen from molecules to atoms was used as a convenient model system. Molecular dynamics simulations were performed with three densities; r = 0.0052 g cm À3 (gas), r = 0.0191 g cm À3 (compressed gas) and r = 0.0695 g cm À3 (liquid), and temperatures in the range; T = 3640-20 800 K.
The mechanism of energy transfer in H2O–H2O collisions – a molecular dynamics simulation
Chemical Physics, 1998
Earlier work on the activation-deactivation mechanism of gas phase unimolecular reactions is extended to the study of the detailed energy transfer mechanism in collisions of water molecules. Molecular dynamics simulations of binary collisions between a reactant water molecule at high internal energy with medium molecules at various selected initial temperatures are Ž. Ž. compared with results from approximate statistical theory. Energy transfer is related to i interaction strength, ii hard Ž. Ž. Ž. atom-atom encounters, iii multiple minima in the center of mass separation, iv collision lifetime and v anharmonicity of the intramolecular potential function. The observed trends are interpreted within the framework of the partially ergodic Ž. multiple encounter theory PEMET of collisional energy transfer. By comparison with typical stable molecule collisions the water-water collisions are more efficient as a reflection of the strong hydrogen bonding interactions. A good agreement between PEMET and molecular dynamics simulations over a wide range of interaction strengths and initial reactant energies is shown, indicating the possibility of a priori use of the PEMET model.
Dissociative Water Potential for Molecular Dynamics Simulations
The Journal of Physical Chemistry B, 2007
A new interatomic potential for dissociative water was developed for use in molecular dynamics simulations. The simulations use a multibody potential, with both pair and three-body terms, and the Wolf summation method for the long-range Coulomb interactions. A major feature in the potential is the change in the shortrange O-H repulsive interaction as a function of temperature and/or pressure in order to reproduce the densitytemperature curve between 273 K and 373 at 1 atm, as well as high-pressure data at various temperatures. Using only the change in this one parameter, the simulations also reproduce room-temperature properties of water, such as the structure, cohesive energy, diffusion constant, and vibrational spectrum, as well as the liquid-vapor coexistence curve. Although the water molecules could dissociate, no dissociation is observed at room temperature. However, behavior of the hydronium ion was studied by introduction of an extra H + into a cluster of water molecules. Both Eigen and Zundel configurations, as well as more complex configurations, are observed in the migration of the hydronium.
Computing properties of the hydrogen dissociation reaction in and away from equilibrium
Molecular Simulation, 2016
We study the dissociation of hydrogen from molecule to atoms and show how we can compute thermodynamic and transport properties of both species in a mixture under non-ideal conditions. The small system method can be used to sample fluctuations of a few atoms or molecules in a small volume element, and gives fast access to accurate thermodynamic data of mixtures that are non-ideal. From the results of equilibrium and non-equilibrium molecular dynamics simulations of the dissociation of hydrogen in a thermal field, we compute coefficients for transport of heat and mass for the gas mixture (0.0052 g cm −3) at average temperature 10400 K. We show that the interdiffusion coefficient, the thermal conductivity and the Dufour effect are significantly affected by the presence of the reaction.
A molecular dynamics study of the reaction H2+OH→H2O+H
The Journal of Chemical Physics, 1985
Classical trajectory calculations• have been •performed to determine the influence of trans.latiomi.l. temperature, 'H 2 vibrational energy, H 2 rotational en~rgy, OH vibrational energy, and OH rotational' energy on the reaction, H 2 + OH. _. H 2 0 + H. The potential energy surface was a modification of the Schatz-Elgersma analytical' fit to the Walsh-Dunning• surface. Reactivity increases with translational t'emperature, and is most strongly influenced by. it. • Rotational • excitation of either or• both molecules••suppresses reactivity. Vibrational excitation of i! 2. ~nharices reactivity, and vibfational excitation of OH has no effect. A thermal rate coefficient was computed for the reaction at 1200 and 2000 K. The computed value compares favorably with the experimen~ at 2000 K, while the agreement at 1200 K is less satisfactory. The a~reement between theory and experirnent :at. both temperatures indicates that the potential surface is a reasoriable representation of the HHOH pote.ntial energy surface~ •;,.
Molecular Dynamics Simulation of Dissociation Kinetics
Journal of Thermophysics and Heat Transfer, 2001
The vibrational energy distribution and the degree of dissociation within a system of hydrogen and oxygen molecules was modeled using molecular dynamics (MD). The first step in this process was to model the atomic and molecular interactions. Since hydrogen and
Competition between exchange and dissociation processes in He+H2+ collisions
Chemical Physics, 1989
The dynamics of (He, Hz) collisions on an accurate ab initio potential-energy surface (PES) has been investigated using the three-dimensional (3D) quasiclassical trajectory (QCT) approach for u&-S and j=O of H: over a wide range of relative translational energies (E,,,,) of the reactants. In addition to the substantial agreement between theory and experiment for the exchange reaction as was reported earlier, we find that the computed collision-induced dissociation (CID) cross section (a") values and their dependence on v and E,,,, also are in nearquantitative accord with the available experimental results. The dominance of CID over exchange at high energies and the increase in the branching ratio F'=aD/aE with u are also nearly quantitatively reproduced by our computations, thus lending credence to the accuracy of the PES and the reliability of the trajectory approach.
Solubility of Water in Hydrogen at High Pressures: A Molecular Simulation Study
Journal of Chemical & Engineering Data, 2019
Hydrogen is one of the most popular alternatives for energy storage. Because of its low volumetric energy density, hydrogen should be compressed for practical storage and transportation purposes. Recently, electrochemical hydrogen compressors (EHCs) have been developed that are capable of compressing hydrogen up to P = 1000 bar, and have the potential of reducing compression costs to 3 kWh/kg. As EHC compressed hydrogen is saturated with water, the maximum water content in gaseous hydrogen should meet the fuel requirements issued by the International Organization for Standardization (ISO) when refuelling fuel cell electric vehicles. The ISO 14687−2:2012 standard has limited the water concentration in hydrogen gas to 5 μmol water per mol hydrogen fuel mixture. Knowledge on the vapor liquid equilibrium of H 2 O−H 2 mixtures is crucial for designing a method to remove H 2 O from compressed H 2. To the best of our knowledge, the only experimental high pressure data (P > 300 bar) for the H 2 O−H 2 phase coexistence is from 1927 [J. Am. Chem. Soc., 1927, 49, 65−78]. In this paper, we have used molecular simulation and thermodynamic modeling to study the phase coexistence of the H 2 O−H 2 system for temperatures between T = 283 K and T = 423 K and pressures between P = 10 bar and P = 1000 bar. It is shown that the Peng-Robinson equation of state and the Soave Redlich-Kwong equation of state with van der Waals mixing rules fail to accurately predict the equilibrium coexistence compositions of the liquid and gas phase, with or without fitted binary interaction parameters. We have shown that the solubility of water in compressed hydrogen is adequately predicted using force-field-based molecular simulations. The modeling of phase coexistence of H 2 O−H 2 mixtures will be improved by using polarizable models for water. In the Supporting Information, we present a detailed overview of available experimental vapor−liquid equilibrium and solubility data for the H 2 O−H 2 system at high pressures.