Nanoporous materials are found in many research and industrial applications, including controlled drug delivery, energy conversion and storage, etc. A comprehensive characterization of nanoporous materials with regard to pore size, surface area, and pore size distribution is required in order to select and optimize the performance of these materials.
Pore size distribution is a key factor when characterizing porous materials. For example, surface area and porosity are important properties in the field of catalyst design. Total surface area is vital for the performance of solid catalysts since it dictates the number of active sites on the surface of the catalyst. The pore size, pore size distribution, and pore volume control transport phenomena, which in turn dictate selectivity in catalyzed reactions.
Mesopores are pores of internal width between 2 and 50 nm, while micropores are defined as pores with internal diameters of less than 2 nm. Characterization of the micropores involves the use of physisorptive gases that can penetrate into the pores under investigation. Gases used are those which are physically bound at the solid surface, a process referred to as physisorption; for example, N2 at 77K, Ar at 87K, and CO2 at 273K. Micropores are filled at very low relative pressure (P/P0), therefore, specialized instrumentation is required to measure these low pressures.
Due to its excellent adsorption behavior, argon is an ideal probe gas for characterizing the pore structure of absorbent substrates. At 87K, argon fills micropores at a much higher relative pressure versus nitrogen at 77K, thus enabling a more efficient analysis. In order to access the narrowest micropores, the use of CO2 can be utilized. Near room temperature, CO2 molecules can easily reach micropores less than 0.7 nm in size. As such, CO2 has become the standard choice of gas for the characterization of microporous carbons.
On an adsorption isotherm plot, the micropore region is indicated by a large and steep increase of the isotherm near its origin and a subsequent leveling off to a plateau. Extracting the pore size information from the experimental adsorption isotherm requires the use of a realistic pore-filling model. Advances in physical adsorption characterization have led to the development and application of the density functional theory (DFT). Other, simpler macroscopic methods that predate DFT, such as Horvath-Kawazoe (HK) and Dubinin, have been historically used as the pore-filling models.
Through the appropriate materials characterization techniques, the non-local density function theory (NLDFT) pore-size distribution can be calculated, along with pore volume information. The NLDFT pore-filling model correctly describes the adsorption on a molecular level and is applicable over the entire range of micro- and mesopores. NLDFT models for various adsorbent/adsorptive systems are commercially available.