BET surface area measurements

K. Gopalakrishnan

MS Engg. Student

CPMU

Adsorption is the adhesion of atoms, ions or molecules of gas, liquid, or dissolved solids to a surface.

This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid.

Adsorption vs Absorption

1. The surface of the adsorbent is uniform, that is, all the adsorption sites are equivalent.

2. Adsorbed molecules do not interact.3. All adsorption occurs through the same

mechanism.4. At the maximum adsorption, only a monolayer

is formed: molecules of adsorbate do not deposit on other, already adsorbed, molecules of adsorbate, only on the free surface of the adsorbent.

Langmuir equation

θ is the fractional coverage of the surface, P is the gas pressure or concentration, α is a constant.

S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc., 1938, 60, 309.

Brunauer, Emmett and Teller Theory

Often molecules do form multilayers, that is, some are adsorbed on already adsorbed molecules and the Langmuir isotherm is not valid.

Basic assumptions1. the same assumptions as that of Langmuir but allow multi-layer

adsorption2. the heat of ads. of additional layer equals to the latent heat of

condensation3. based on the rate of adsorption=the rate of desorption for each

layer of ads. the following BET equation was derived

Where P - equilibrium pressureP0 -saturate vapour pressure of the adsorbed gas at the

temperatureP/P0 is called relative pressure

V - volume of adsorbed gas per kg adsorbentVm - volume of monolayer adsorbed gas per kg adsorbentc - constant associated with adsorption heat and

condensation heat

)(11

1 00

0 P/PcV

c

cV)P/P(V

P/P

mm

Applications of BET surface area measurements:

1. Calculation of surface areaof a solid material

2. Pore Size of Porous materials

3. Pore Volume of Porous materials

Its main application is in catalysis

Choice of Gas and Temperature

• Gases– Nitrogen– Argon– Krypton– Carbon dioxide– Others

Temperatures– Liquid Nitrogen– Liquid Argon– Dry ice/acetone– Water/ice– Others

Sorption isotherm measurement –volumetric methods

“Dead-space”

Physical volume

that the gas occupies

in the sample cell.

Vtube-VsampleDead space

Particle size Determination using BET surface area analysis

The particle size, D is given by

D=6000/(SBET X ρ)

Where, SBET is BET surface area

ρ is the true density of the sample

e.g., ρ of titania is 4.2 g/mL

• Five types of physisorption isotherms are found over all solids

– Type I is found for porous materials with small pores e.g. charcoal. It is clearly Langmuir monolayer type, but the other 4 are not

– Type II for non-porous materials

– Type III porous materials with cohesive force between adsorbate molecules greater than the adhesive force between adsorbate molecules and adsorbent

– Type IV staged adsorption (first monolayer then build up of additional layers)

– Type V porous materials with cohesive force between adsorbate molecules and adsorbent being greater than that between adsorbate molecules

I

II

III

IV

V

relative pres. P/P0

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Nitrogen adsorption-desorption isotherm of SBA-15

Thank You