Exact results for the reactivity of a single-file system

We derive analytical expressions for the reactivity of a Single-File System with fast diffusion and adsorption and desorption at one end. If the conversion reaction is fast, then the reactivity depends only very weakly on the system size, and the conversion is about 100%. If the reaction is slow, then the reactivity becomes proportional to the system size, the loading, and the reaction rate constant. If the system size increases the reactivity goes to the geometric mean of the reaction rate constant and the rate of adsorption and desorption. For large systems the number of nonconverted particles decreases exponentially with distance from the adsorption/desorption end.Comment: 4 pages, 2 figure

Parameterization of a reactive force field using a Monte Carlo algorithm

Abstract Parameterization of a Molecular Dynamics force field is essential in realistically modelling the physico-chemical processes involved in a molecular system. This step is often challenging when the equations involved in describing the force field are complicated as well as when the parameters are mostly empirical. ReaxFF is one such reactive force field which uses hundreds of parameters to describe the interactions between atoms. The optimization of the parameters in ReaxFF is done such that the the properties predicted by ReaxFF matches with a set of quantum chemical or experimental data. Usually, the optimization of the parameters is done by an inefficient single parameter parabolic-search algorithm. In this study, we use a robust Metropolis Monte-Carlo algorithm with Simulated Annealing (MMC-SA) to search for the optimum parameters for the ReaxFF force field in a high-dimensional parameter space. The optimization is done against a set of quantum chemical data for M gSO 4 hydrates. The optimized force field reproduced the chemical structures, the Equations of State and the water binding curves of M gSO 4 hydrates. The transferability test of the ReaxFF force field shows the extend of transferability for a particular molecular system. This study points out that the ReaxFF force field is not indefinitely transferable

Steady-State Properties of Single-File Systems with Conversion

We have used Monte-Carlo methods and analytical techniques to investigate the influence of the characteristic parameters, such as pipe length, diffusion, adsorption, desorption and reaction rate constants on the steady-state properties of Single-File Systems with a reaction. We looked at cases when all the sites are reactive and when only some of them are reactive. Comparisons between Mean-Field predictions and Monte-Carlo simulations for the occupancy profiles and reactivity are made. Substantial differences between Mean-Field and the simulations are found when rates of diffusion are high. Mean-Field results only include Single-File behavior by changing the diffusion rate constant, but it effectively allows passing of particles. Reactivity converges to a limit value if more reactive sites are added: sites in the middle of the system have little or no effect on the kinetics. Occupancy profiles show approximately exponential behavior from the ends to the middle of the system.Comment: 15 pages, 20 figure

Transient behavior in Single-File Systems

We have used Monte-Carlo methods and analytical techniques to investigate the influence of the characteristics, such as pipe length, diffusion, adsorption, desorption and reaction rates on the transient properties of Single-File Systems. The transient or the relaxation regime is the period in which the system is evolving to equilibrium. We have studied the system when all the sites are reactive and when only some of them are reactive. Comparisons between Mean-Field predictions, Cluster Approximation predictions, and Monte Carlo simulations for the relaxation time of the system are shown. We outline the cases where Mean-Field analysis gives good results compared to Dynamic Monte-Carlo results. For some specific cases we can analytically derive the relaxation time. Occupancy profiles for different distribution of the sites both for Mean-Field and simulations are compared. Different results for slow and fast reaction systems and different distribution of reactive sites are discussed.Comment: 18 pages, 19 figure

A DFT based equilibrium study on the hydrolysis and the dehydration reactions of MgCl 2

Magnesium chloride hydrates are characterized as promising energy storage materials in the builtenvironment. During the dehydration of these materials, there are chances for the release of harmful HCl gas, which can potentially damage the material as well as the equipment. Hydrolysis reactions in magnesium chloride hydrates are subject of study for industrial applications. However, the information about the possibility of hydrolysis reaction, and its preference over dehydration in energy storage systems is still ambiguous at the operating conditions in a seasonal heat storage system. A density functional theory level study is performed to determine molecular structures, charges, and harmonic frequencies in order to identify the formation of HCl at the operating temperatures in an energy storage system. The preference of hydrolysis over dehydration is quantified by applying thermodynamic equilibrium principles by calculating Gibbs free energies of the hydrated magnesium chloride molecules. The molecular structures of the hydrates (n = 0, 1, 2, 4, and 6) of MgCl2 are investigated to understand the stability and symmetry of these molecules. The structures are found to be noncomplex with almost no meta-stable isomers, which may be related to the faster kinetics observed in the hydration of chlorides compared to sulfates. Also, the frequency spectra of these molecules are calculated, which in turn are used to calculate the changes in Gibbs free energy of dehydration and hydrolysis reactions. From these calculations, it is found that the probability for hydrolysis to occur is larger for lower hydrates. Hydrolysis occurring from the hexa-, tetra-, and dihydrate is only possible when the temperature is increased too fast to a very high value. In the case of the mono-hydrate, hydrolysis may become favorable at high water vapor pressure and at low HCl pressure

Interplay between Anomalous Transport and Catalytic Reaction Kinetics in Single-File Nanoporous Systems

Functionalized nanoporous materials have broad utility for catalysis applications. However, the kinetics of catalytic reaction processes in these systems can be strongly impacted by the anomalous transport. The most extreme case corresponds to single-file diffusion for narrow pores in which species cannot pass each other. For conversion reactions with a single-file constraint, traditional mean-field-type reaction-diffusion equations fail to capture the initial evolution of concentration profiles, and they cannot describe the scaling behavior of steady-state reactivity. Hydrodynamic reaction-diffusion equations accounting for the single-file aspects of chemical diffusion can describe such initial evolution, but additional refinements are needed to incorporate fluctuation effects controlling, for example, steady-state reactivity localized near pore openings. For polymerization reactions with a single-file constraint, initial behavior depends strongly on system details such as catalytic site loading and reaction rate. However, long-time behavior often involves the formation of a dominant large polymer near each end of the pore, initially within the pore but subsequently partly extruding. In this partial extrusion regime, the kinetics is governed by the special features of the random walk describing the motion of the end of the partly extruded polymer, noting that this extruded end must return within the pore for further growth

Modeling thermochemical reactions in thermal energy storage systems

The focus of this chapter is mainly on molecular modeling techniques for the hydration and dehydration (sorption and desorption) processes occurring in salt hydrates at the nano-scale. Modeling techniques such as density function theory, molecular dynamics and monte carlo are briefly introduced. Some attention is also given to micro- and macro-scale modeling techniques used at larger length scales, such as Mampel's model and the continuum approach. Before introducing all the length (and time) scales involved when modeling a heat storage system, a qualitative description is given of the hydration and dehydration processes on the nano/micro-scale

Molecular dynamics simulation on rarefied gas flow in different nanochannel geometries

A three dimensional Molecular Dynamics simulation method was used to study the effect of different geometries for rarefied gas flows in nanochannels. Argon molecules have been used. The velocity profiles in the channel were obtained and analyzed with three different channel geometries: a circular, a rectangular (square), and a slit channel. We found that when using the same driving force, the maximum velocity of the flow increases when the geometry changes in the order from circular geometry to rectangular geometry to slit geometry, where the latter becomes 2∼2.5 times as large compared with either the rectangular or circular channel. Rectangular channels showed a similar maximum and slip velocity as the circular channel while the velocity profile was qualitatively similar to the slit channel for Kn higher than 1.0. We also investigated the effect of different Knudsen numbers on the velocity profiles. A channel width of 50nm is used for the simulation. We found that for Kn higher than 2∼3, the Knudsen number has a comparably small influence on the slip velocity for circular channel and rectangular channel