GAS ADSORPTION TECHNIQUE

Gas adsorption

• This technique is based on BET theory which aims to explain physical adsorption of gas molecules on solid surface.

• Adsorption is a consequence of surface energy

LangmuirProvides a means of determining surface area based on manolayer coverage of the solid surface by the adsorptiveBETThe method of Brunauer–Emmett–Teller  is employed to determineSurface area on a model of adsorption which incorporates multilayer coverage.

Gas Adsorption Can measure only open pores

Pore size : 0.4 nm – 50 nm

Easy

Established technique

Method • Solid pretreated by applying heat, vacuum or

flowing gas to remove adsorbed contaminants (water, CO2)

• Then solid cooled under vacuum usually to cryogenic temperature(77k or -195 deg C)

• Specific dose of adsorptive gas( such as nitrogen, argon, CO2 etc.) is passed on solid surface on regular increments.

• After each dose of adsorbate , pressure is allowed to equilibrate & quantity adsorbed is calculated.

• The quantity adsorbed at each pressure defines adsorption/sorption isotherm

• From this isotherm quantity of gas required for monolayer formation is determined

• Then area covered by each adsorbed gas molecule is known

• Finally surface area is determined.

• Using gas adsorption technique porosity and pore size can also be calculated.