Reverse Monte Carlo modeling in confined systems

Abstract

An extension of the well established Reverse Monte Carlo (RMC) method for modeling systems under close confinement has been developed. The method overcomes limitations induced by close confinement in systems such as fluids adsorbed in microporous materials. As a test of the method, we investigate a model system of 36 Ar adsorbed into two zeolites with significantly different pore sizes: Silicalite-I (a pure silica form of ZSM-5 zeolite, characterized by relatively narrow channels forming a 3D network) at partial and full loadings and siliceous Faujasite (which exhibits relatively wide channels and large cavities). The model systems are simulated using Grand Canonical Monte Carlo and, in each case, its structure factor is used as input for the proposed method, which shows a rapid convergence and yields an adsorbate microscopic structure in good agreement with that of the model system, even to the level of three body correlations, when these are induced by the confining media. The application to experimental systems is straightforward incorporating factors such as the experimental resolution and appropriate q-sampling, along the lines of previous experiences of RMC modeling of powder diffraction data including Bragg and diffuse scattering.

Reverse Monte Carlo modeling: The two distinct routes of calculating the experimental structure factor

“…Out of the presently available software, the RMCPOW algorithm seems to be the most general choice; RMCProfile [16] may also be applicable of experimental data over extremely wide Q-range are available. This conclusion is also supported by a previous RMC study that showed that the 'crystallography route' provided an appropriate description of simple adsorbed fluids in zeolites, although in that case simulated target structure factors were used [23].…”

“…Since it is obvious that the simple route, via the calculation of the RDF, cannot be applicable for crystals, what needs to be tested is whether the more time consuming 'crystallographic' approach [15] can be used for isotropic disordered systems, such as liquids. Beyond the 'per se' interest, the timeliness of such a study lies in that a very important class of 'mixed' systems, 'fluids in pores' would require a method that can handle both perfect crystals (like zeolites) and liquids (like water) [21,22,23]. Note that the 'crystallographic' approach has already been proven to reproduce the atomic structure of simple adsorbed fluids (up to the level of three body correlations) in zeolites of varying pore sizes using the N-RMC method in which the number of particles is an additional adjustable parameter [23].…”

“…Beyond the 'per se' interest, the timeliness of such a study lies in that a very important class of 'mixed' systems, 'fluids in pores' would require a method that can handle both perfect crystals (like zeolites) and liquids (like water) [21,22,23]. Note that the 'crystallographic' approach has already been proven to reproduce the atomic structure of simple adsorbed fluids (up to the level of three body correlations) in zeolites of varying pore sizes using the N-RMC method in which the number of particles is an additional adjustable parameter [23]. In that work the target structure factor was obtained by simulation rather than from experiments and the study was restricted to simple fluids.…”

Experimental and Simulation Studies of the Stepped Adsorption of Toluene on Pure-Silica MEL Zeolite

“…The N -RMC method is a variant of RMC modeling specifically designed to deal with tightly confined media. As the method has already been detailed elsewhere, , only a brief description is provided here. In short, the N -RMC method consists in performing, in addition to particle/molecule displacement and rotation attempts, insertion and deletion trials for adsorbate molecules.…”

“…Instead, here we use the reverse Monte Carlo method . Introduced in 1988, this technique has since then been widely used to elucidate the structure of liquids and amorphous materials, as well as that of crystalline materials with partial disorder. − Some of us have recently proposed an extension of the method, the N -RMC, designed to avoid the diffusion problems that appear when studying the structure of fluids in tightly confining media . The usefulness of this approach has been proven by studying the structure of argon on MEL at liquid nitrogen temperature in conjunction with experimental time-of-flight (TOF) neutron diffraction patterns .…”