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Seminar on Adsorption : Specific Surface Area

BET Theory

The specific surface area is usually calculated by using BET theory from the gas adsorption isotherm data. The model of BET theory is that the adsorptive starts to adsorb on the strong energy sites on the surface first, and then, as the pressure is increased, the adsorptive adsorbs on the next energy level sites. The model also takes account of the 2nd, 3rd and higher layer adsorption.

BET Theory

This BET theory for multi-layer adsorption is the expansion of Langmuir equation, which is for the single layer adsorption.

This theory well fits to type II and IV isotherms at the relative pressure between 0.05 and 0.35. The monolayer adsorption amount (Vm) and C parameter are calculated from the slope and intercept of BET-plot by using the least square fitting. C parameter represents the surface interaction energy, so it must take a positive value.

Type IV isotherm and BET-plot

The specific surface area is calculated from this Vm and cross sectional area (σ).


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