Computer Simulations of Diffusion in Monolayers Physisorbed in Model Pores
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COMPUTER SIMULATIONS OF DIFFUSION IN MONOLAYERS PHYSISORBED IN MODEL PORES Mary S. IBojan and William Steele Department of Chemistry, 152 Davey Laboratory, Penn State University, University Park, PA 16802, USA ABSTRACT Two types of model pore have been considered as sorbents of methane at 300 K: the first is a straight cylinder with atomically rough walls characteristic of an amorphous solid. Three cylinder radii plus the reference flat surface were taken for study. The second is a parallel-walled slit pore made up of graphite basal planes that had been modified by the insertion of sulfide atoms. Here, two values of sulfur loading were taken together with the reference pure graphite pore. Two values of the wall spacing were assumed for each sulfur loading. Thermodynamic and structural properties are presented elsewhere; here, the dependence of diffusion constants as a function of the monolayer density is presented for methane in the various model pores. It is shown that these diffusivities depend primarily on the methane-methane collisions in the monolayer films. There is a slight dependence upon the nature of the solid surface which is interpreted in terms of the hindrances produced by soft obstacles to molecular motion parallel to the surfaces. 1.
INTRODUCTION
Computer simulation has recently been proven to be a powerful method for study of the complex thermodynamic and transport phenomena occurring in physisorbed films and in the closely related systems of fluids condensed in porous solids. In both cases, the molecular picture usually is based on the assumption that the solid adsorbent is a rigid, unreactive source of external potential energy that attracts adsorbate molecules to the surface while providing limits to the geometric space- available to the fluid. For obvious reasons, most of these studies have employed, simple models of the pore geometry: straight-walled cylinders1 or parallel-walled slit pores 2 with featureless surfaces are the two most popular. In both cases, there is one parameter that is needed to characterize the small size of the system: the pore diameter or slit wall spacing, to be specific. However, more realistic pores can be modeled and studied. The present paper describes molecular dynamics simulation studies of two such systems. In both cases, the atomic structure of the pore walls is explicitly accounted for in the gas-solid interaction potentials used. Since the pore size parameters are taken to be in the micropore range of 2-5 molecular diameters, the atomic structure of the pore walls significantly affects the thermodynamic and transport properties of an adsorbed gas. The sorbate studied is methane at 300 K. The specific model pores studied are of two types: straight cylinders with walls whose atomic structure is that of an amorphous solid; and second, parallel slit pores whose walls are graphite basal planes that have had sulfur atoms added as chemical impurities. Thermodynamics and structural data have been reported elsewhere for these systems;(3A) here, the emphasis is on the
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