1. Gas-Solid Interactions
Light bulbs, Three way catalysts, Cracking, Corrosion, Electronic Devices etc.

2. Adsorption-the key step
Extent of adsorption usually given by fractional coverage
N is often equivalent to number of surface atoms of the substrate
Associative (or non-dissociative) adsorption is when a molecule adsorbs without fragmentation
Dissociative adsorption is when fragmentation occurs during the adsorption process

3. Adsorption Isotherms
Models describing equilibrium between the gaseous and the adsorbed phases at a given fixed temperature
Simplest is that based on Irving Langmuir’s proposals
Born in Brooklyn January 31, 1881
Graduated from Columbia University in 1903
Postgraduate under Nernst in Göttingen
Post at Stevens Institute of Technology New Jersey
1909 hired by General Electric Company
Studies embraced chemistry, physics, and engineering
Investigated properties of adsorbed films and the nature of electric discharges in high vacuum and in certain gases at low pressures.

4. Langmuir Isotherm Simplifying assumptions
Adsorption proceeds to monolayer formation only
All sites are equivalent and the surface is uniform
Molecule adsorption is independent of occupation of neighbouring sites

5. Langmuir Isotherms

6. SA/(mass of substrate)
Using the isotherm
SA = N x Am
Specific surface area
SA/(mass of substrate)
Atkins & dePaula 8th p. 918
Attard & Barnes p. 4

7. Dissociative Adsorption

8. Typical Langmuir isotherms
Associative adsorption isotherms
Dissociative adsorption isotherms

9. Gas adsorption to a solid is exothermic.
Heats of Adsorption
Gas adsorption to a solid is exothermic.
The magnitude and variation as a function of coverage may reveal information concerning the bonding to the surface.
Calorimetric methods determine heat, Q evolved.
qi = integral heat of adsorption
qD = differential heat of adsorption

10. Enthalpy of Adsorption
Heats of adsorption change as a function of surface coverage
differentiate
Van’t Hoff equation

11. Isosteric enthalpy of adsorption
Re-arranging Langmuir
Differentiate & re-arrange
Use van’t Hoff

12. Measuring isosteric enthalpies
Attard & Barnes p. 83
Isosteric HEATS of adsorption sometimes used instead of enthalpies

13. Measuring isosteric enthalpies
Atkins & de Paula, 8th p
Note

14. BET Isotherm
When adsorption of a gas can occur over a previously adsorbed monolayer of the gas
Brunauer, Emmett & Teller extends the Langmuir isotherm model to multilayer adsorption
Assumptions:
Adsorption of 1st layer takes place on a surface of uniform energy
2nd layer only adsorbs on 1st, 3rd on 2nd, etc. When p=p*, infinite layers form.
At equilibrium, rates of condensation & evapouration are same for each individual layer
For layers ≥ 2, ΔH0AD = -Δ H0VAP

15. BET
As before, we can replace N with masses or volumes.

16. BET “knee” in some isotherms represents monolayer coverage
BET underestimates adsorption at low p and overestimates adsorption at high p

17. Using the BET Atkins & dePaula 8th p. 921
Principle behind the surface area and pore size analyzers on the market. Use nitrogen at 77K as adsorbate. Knowing size of molecule, the surface area and/or pore size can be determined from the isotherm.

18. IUPAC Classification

19. Other isotherms
When adsorption sites are not equivalent, enthalpy of adsorption changes as a function of coverage
Temkin: Assumes enthalpy changes linearly with pressure
Freundlich: Assumes enthalpy changes logarithmically with pressure
Try example in Attard & Barnes, p.83