Physisorption in Porous Materials

Abstract

Physisorption or physical adsorption is the mechanism by which hydrogen is stored in the molecular form, that is, without dissociating, on the surface of a solid material. Responsible for the molecular adsorption of H 2 are weak dispersive forces, called van der Waals forces, between the gas molecules and the atoms on the surface of the solid. These intermolecular forces derive from the interaction between temporary dipoles which are formed due to the fluctuations in the charge distribution in molecules and atoms. The combination of attractive van der Waals forces and short range repulsive interactions between a gas molecule and an atom on the surface of the adsorbent results in a potential energy curve which can be well described by the Lennard-Jones Eq. (2.1).where e is the depth of the energy well, r i the distance between an H 2 molecule and a generic atom i on the surface and s the value of r i for which the potential is zero [1]. The minimum of the potential curve occurs at a distance from the surface which is approximately the sum of the van der Waals radii of the adsorbent atom and the adsorbate molecule. For microporous materials (r pore < 1 nm) [2] where the pore dimensions are comparable to the diameter of the adsorbate molecules this interaction potential is also influenced by the pore shape and the pore dimensions. Adsorption is an exothermic process, that is, heat is released during the adsorption of gas molecules on a surface. Owing to the low polarizability of the H 2 molecule this type of interaction is very weak and leads to low values of the heat of adsorption, typically in the range of 1-10 kJ mol À1 [3]. Compared to chemisorption which involves a change in the chemical nature of the H 2 molecule by, for example, dissociation, physisorption has approximately a ten times smaller heat of adsorption. Chemical hydrides and complex hydrides, which store hydrogen chemically, both have the disadvantage of producing an excessive amount of heat during fast Handbook of Hydrogen Storage. Edited by Michael Hirscher

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