Brunauer, Emmett and Teller (BET) Theory
by Dr. Aida Fuente Cuesta
The textural characteristics of porous solids are determined through the analysis of the N2 physisorption isotherm performed at 77K. As a first step, the shape of the adsorption-desorption isotherm provides information about the type of porosity present in the material (Type I to Type V isotherms of the IUPAC clasification).
The specific surface area is usually expressed as BET (Brunauer-Emmet-Teller) surface area, which is one of the key parameters to perform the textural characterisation of porous solid materials. It is determined by the analysis of the nitrogen adsorption isotherm in the relative pressure range 0.01 to 0.2. The principle of this method is the estimation of the surface area of the solid from a monolayer of adsorbed gas. Prior to the analysis it is necessary to carry out the degasification of the solid in such a way that the gases and vapours on the surface of the solid are removed but no modification (alteration of its structure, decomposition, etc.) of the sample occurs.
Besides, the quantity of nitrogen adsorbed at a relative pressure of ~0.9-0.95 allows determining the total pore volume of the sample. The total micropore volume can also be calculated through the application of the Dubinin-Radushkevich equation to the nitrogen isotherm at low relative pressures, or through comparison isotherms (t-plot or αs-plot). Consequently the quantity of mesopores can be estimated by the difference between the total pore volume and the micropore volume. The volume of narrow microporosity (pores < 0.7 nm) is normally determined through CO2 physisorption at 273K (from a relative pressure of 3×10-5 to 0.03), as kinetic restrictions exist when performing N2 physisorption at 77K.