Comparison between Adsorption in Pores of a Simple Geometry and Realistic Models of Porous Materials
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Comparison between Adsorption in Pores of a Simple Geometry and Realistic Models of Porous Materials B. Coasne1, J. P. Pikunic1&, R. J. -M. Pellenq2*, K. E. Gubbins1 1
Department of Chemical Engineering, North Carolina State University, 113 Riddick Labs, Raleigh, North Carolina 27695-7905, USA. 2 Centre de Recherches en Matière Condensée et Nanosciences, CNRS, Campus de Luminy, 13288 Marseille cedex 09, France. * To whom correspondence should be sent: [email protected] ABSTRACT We study, by means of Grand Canonical Monte Carlo simulations, the effect of disorder in pore morphology and topology on adsorption of simple fluids at 77 K. We consider the case of microporous carbons and mesoporous silicas. For both systems, we found that morphological and topological disorder affects the adsorption isotherm. Consequently, we show that complex porous networks cannot be modeled as a collection of unconnected pores of simple geometry. In the case of microporous carbons, the isosteric heat of adsorption for disordered systems differs significantly from that obtained for an assembly of unconnected slit pores. In contrast, the isosteric heat of adsorption for the disordered mesoporous silica sample compares reasonably well with that observed for a silica material having regular cylindrical pores.
INTRODUCTION Confinement of fluids in porous media attracts a great deal of attention due to its numerous industrial applications in the field of catalysis, phase separation, etc. Adsorption measurements of simple fluids, such as argon and nitrogen, are also relevant to characterization of porous solids. Many experimental and theoretical studies have shown that confinement at the nanoscale strongly affects the thermodynamic properties of the fluid [1]. In particular, phase transitions (e.g., gas-liquid, solid-liquid) are shifted in pressure or temperature compared to their bulk counterparts. Most of the theoretical and simulation investigations of the behavior of confined fluids have been carried out in single pores with simple geometry and smooth walls. In contrast, the vast majority of real porous media exhibit complex pore networks that are disordered in terms of pore morphology (pore shape) and topology (the way the pore distribute and connect in space). Good examples of such systems are non-graphitizable microporous carbons and silica controlled mesoporous glasses. Even materials such as MCM-41, having unconnected (nearly) cylindrical mesopores, exhibit a certain degree of disorder: the roughness of their pore wall reflects in the behavior of confined fluids [2]. In this work, we discuss the effect of disorder on adsorption behavior in microporous and mesoporous materials. We performed Grand Canonical Monte Carlo simulations (GCMC) and compared adsorption isotherms and isosteric heats of adsorption of nitrogen at 77 K in microporous carbons (slit pore versus realistic saccharosebased porous carbon) and argon at 77 K in mesoporous silicas (cylindrical pore versus realistic Vycor porous glass).
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