Effects of surface structure on the molecular projection area. Adsorption of argon and nitrogen onto defective surfaces
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Effects of surface structure on the molecular projection area. Adsorption of argon and nitrogen onto defective surfaces A contribution on the occasion of 60th birthday of Professor Mietek Jaroniec L.F. Herrera · D.D. Do
Published online: 26 March 2009 © Springer Science+Business Media, LLC 2009
Abstract This paper studies the effects of surface structure (defective surfaces) on the molecular projection area of argon and nitrogen at 77 K. The determination of the molecular projection area is based on choice of the surface area of the structure studied and the adsorption data obtained from the GCMC simulation. Two methods were used to determine the surface area: the flat surface area that are commonly used in the literature and the geometrical surface area. The molecular projection areas of argon and nitrogen at 77 K vary with pressure over the recommended range for BET plot (reduced pressures from 0.05 to 0.3) and also they varies with the percentage of defects on the surface. Additionally, it is seen that the geometrical surface area method gives molecular projection area of defective surfaces values that are in accordance with the experimental value reported in the literature. Keywords Adsorption · Projection area · Simulation
argon and nitrogen are the most widely used, and the recommended values for the molecular projection area of nitrogen and argon are 0.162 and 0.138 nm2 , respectively (Gregg and Sing 1982). In a recent work of Do et al. (2007) they used the Monte Carlo simulation method to investigate in details the projection area of nitrogen and argon at 77 and 87.3 K on a homogeneous graphite surface, and they found that the area occupied by one molecule varies with loading and suggested a value of 0.155 nm2 for nitrogen and 0.128 nm2 for argon. These values are smaller than those recommended in the literature. The reason for the difference is that they used a homogeneous graphite surface, and here we argue that the heterogeneity of the surface could affect the molecular projection area. This is the objective of this paper, and we use a recently proposed defective surface model (Do and Do 2006) as the source of heterogeneity to investigate the molecular projection area of nitrogen and argon at 77 K.
1 Introduction
2 Theory
Traditionally, the determination of the surface area of a porous solid with adsorption methods requires the molecular projection area of the fluid molecule used (am ) as it is one of the most important information for solid characterization (Gregg and Sing 1982; Jaroniec et al. 1999, 2003; Kowalczyk et al. 2005; Kaneko et al. 1998; Cascarini de Torre et al. 1996). Among the various probe molecules used,
L.F. Herrera · D.D. Do () Department of Chemical Engineering, University of Queensland, St Lucia, Queensland 4072, Australia e-mail: [email protected]
2.1 Fluid–fluid potential model We model nitrogen as two Lennard Jones (LJ) dispersive sites and 3 partial charges to account for its quadrupole (Potoff and Siepmann 2001). The dispersive interaction energy between a site “a” on a mo
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