II.2.2. Specific surface area (BET)

Brunauer-Emmet-Teller specific surface area is an important parameter characterizing solid materials.

Specific surface area is determined by physical adsorption of inert-gas molecules that interact with the solid only by van der Waals forces. Before sorption, the samples were pre-treated at 200 °C for 2 h under vacuum in order to remove H₂O and CO₂, which could be adsorbed on the material, and then small amounts of the inert gas (N₂) under various pressures were admitted, in steps, into the evacuated sample chamber. Gas molecules stick to the surface of the solid and form a multilayer. The surface areas were determined from N₂ adsorption isotherms measured from p/p₀ = 0.1 to 0.3, using BET equation [15]:

where P and P_o – the equilibrium and the saturation pressure of adsorbate at the temperature of adsorption; v – the adsorbed gas quantity; v_m – adsorbed gas quantity for a densely packed monolayer; c – the BET constant, which represents the heat of molecules adsorption.

Surface area BET (A_BET) in m­²/g of solid was calculated using the following formules:

where a_m – area occupied by a molecule of adsorbed gas (a_mN₂ = 16.2 A, T = 77.4 K);

n_m – the number of moles of adsorbed gas; N_A – Avogadro’s number; ρ – gas density;

v_m – molar volume of adsorbate; M_m – molecular weight of N₂; m – mass of the sample.

The isotherms catalysts were measured for calcined samples. The measurements were performed on an Tristar-II Micrometrics apparatus at -196 °C. Prior to adsorption the samples were outgasses at 150 °C for 3 h.