GCMC Simulations of Gas Adsorption in Carbon Pore Structures

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Abstract

environmental protection, mixture separation, water purification and gas storage (Ruthven, 1984).Moreover, gas adsorption measurements are widely used in materials science as a reliable method for the characterization of porous materials (Gregg & Sing, 1982;Lowell & Shields, 1991).The better understanding of the detailed mechanism that takes place during these operations is essential for designing improved processes.The materials that are commonly used in these fields are crystalline, ordered, and amorphous porous materials, such as zeolites, amorphous silica and alumina, activated carbons, metal-organic frameworks (MOFs) and other advanced materials.The large surface areas of these materials and the confinement offered by their extended pore network enhance their catalytic, sorptive and separation activity.Porous materials are classified by the International Union of Pure and Applied Chemistry (IUPAC) in micropores with pore diameters less than 2 nm, mesopores having pore widths between 2 and 50 nm and macropores with pore diameters greater than 50 nm (Everett, 1972).The pore width is defined as the diameter (D) in the case of cylindrical pores or as the distance between opposite walls (H) in the case of slit-shaped pores.Modern industrial and technological needs has led materials science (and vice versa) towards the development of novel materials having extremely small pore widths.The size of these pores is approaching few molecular diameters and materials with such pore systems reveal a wide range of properties, that differ significantly from mesopores and big micropores.Adsorption depends strongly on the structural properties of the adsorbent material, e.g. the specific surface area, the porosity and the pore dimensions.In general, the existence of a large specific surface area and of an extensive number of readily accessible small sized pores is desirable as in pores of molecular dimensions the adsorbent field is further intensified due to the overlapping solid wall potentials, resulting in enhanced adsorption capacity.The pore filling mechanism is sufficiently described for the case of mesopores and macropores.The Kelvin equation is a purely thermodynamic model that applies at subcritical temperatures and relates the relative pressure (P/P 0 ) at which capillary condensation occurs to the pore width (Gregg & Sing, 1982).However, the equation fails to apply in micropores, mainly because, a "real" adsorbate phase of molecular dimensions cannot be defined.

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