Description of 20/30410823 DC 2020

This document describes a method for the evaluation of porosity and pore size distribution by physical adsorption (or physisorption). The method is limited to the determination of the quantity of a gas adsorbed per unit mass of sample as a function of pressure at a controlled, constant temperature [1-9]. Commonly used adsorptive gases for physical adsorption characterization include nitrogen, argon, krypton at the temperatures of liquid nitrogen and argon (77 K and 87 K respectively) as well as CO₂ (at 273K). Traditionally, nitrogen and argon adsorption at 77K and 87 K, respectively, allows one to assess pores in the approximate range of widths 0,45nm to 50 nm, although improvements in temperature control and pressure measurement now allow larger pore widths to be evaluated. CO₂ adsorption at 273K can be applied for the microporous carbon materials exhibiting so-called ultramicropores. Krypton adsorption at 77 K and 87 K is used to determine the surface area or porosity of materials with small surface area or for the analysis of thin porous films. The method described here is suitable for a wide range of porous materials. This standard focuses on the determination of pore size distribution from as low as 0,4 nm up to ca.100 nm, but will not focus on the assessment of surface area which is described in a dedicated standard (ISO 9277:2010).

The procedures which have been devised for the determination of the amount of gas adsorbed may be divided into two groups: (i) those which depend on the measurement of the amount of gas removed from the gas phase,i.e manometric (volumetric) methods; (ii) those which involve the measurement of the uptake of the gas by the adsorbent (i.e. direct determination of increase in mass by gravimetric methods).

In practice, static or dynamic techniques may be used to determine the amount of gas adsorbed. However, the manometric method is generally considered the most suitable technique for undertaking physisorption measurements with nitrogen, argon, and krypton at cryogenic temperatures (i.e., 77K and 87 K, the boiling temperature of nitrogen and argon, respectively) with the goal of obtain pore volume and pore size information. Hence, here we will focus only on the application of the manometric method.

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