1. Adsorption at solids Solid: Adsorbent Gas/Solute: Adsorbate
Surfaces chemistry
Adsorption at solids
Solid: Adsorbent
Gas/Solute: Adsorbate

2. Outline Adsorption Comparison of physisorption and chemisorption
The Langmuir treatment of adsorption
Adsorption Kinetics
Analytical Aspects Of Adsorption
Other isotherms (BET, Freundlich)
Texts: Introduction to colloids and Surface Chemistry – D.J. Shaw
Physical Chemistry - Atkins

3. Applications
Central importance to many areas of pure and applied research:
Electronic device manufacture
- Heterogeneous catalysis
(e.g., Hydrogenation of alkenes, cracking of crude oil over silica- alumina : zeolites)

4. Applications Wastewater treatment - Environmental chemistry
(e.g. Leaching of pesticides in soil, chelation of metal ions in humic acids)
- Chromatography

5. Adsorption
Is the withdrawal of a substance from a bulk phase (aqueous or solution) and its accumulation at an interface.
Strictly a surface phenomenon
It is sometimes accompanied by deeper penetration of the adsorbed substance into the body (bulk) of a solid adsorbent, akin to the formation of a solid solution – absorption
The term Sorption covers both phenomena.

6. Non-Dissociative adsorption is said to occur when a molecule adsorbs on to the surface from the gas phase without fragmentation.
When fragmentation does occur, the adsorption process is termed dissociative.
The free gas and the adsorbed gas are in dyanamic equilibrium.
Fractional coverage (ϴ )or extent of adsorption depends on : T, P (gas) or conc. (solute) and on effective surface area.
The variation of ϴ with pressure at a chosen temperature is called the adsorption isotherm.

7. Finely divided solids possess a very high SPECIFIC
SURFACE AREA (SSA) / m2g-1
(activated C : ~ 1000 ; Si gel : ~ 500)
Adsorption is spontaneous process, therefore
(adsorption equilibrium if )
On the other hand, the adsorbed state is more “ordered” (2D vs 3D),
hence :
(non-dissociative adsorption)(translational freedom reduced)
non-dissociative adsorption exothermic

8. Exception: Dissociative adsorption (e.g., H2 on glass 2 H(ads) )
, ,(endothermic adsorption), such that

9. Adsorption Physical adsorption (physisorption)
The bonding interaction between adsorbate and adsorbent is long range but weak and is associated with van der Waals-type interactions.
The small DHads is insufficient to lead to bond breaking so the physiosorbed molecule retains its identity, though it might be distorted by the surface.
Chemical adsorption (chemisorption)
chemical bonds are formed between the molecules (atoms) and the surface.
Note : both types of adsorption are exothermic

10. Comparison of physisorption and chemisorption
Cause
non-specific, long range (dispersion, forces) Van der Waals forces
No electron Transfer (redistribution of e- density)
Covalent/electrostatistic forces, electron transfer
Adsorbents
All solid
Some solids
Adsorbates
All gases below the critical point, intact molecules
Some chemically reactive gases, dissociation into atoms, ions, radicals
Temperature range
Low temperatures
Generally high temperatures
Heat of adsorption
Low,~heat of condensation (typical DHads ≈ - 20kJmol-1), always exothermic
High, ~heat of reaction
(DHads ≈ kJmol-1)Exothermic

11. Comparison of physisorption and chemisorption
Rate
Very fast
Strongly temp. dependent
Activation energy
no barrier activation barrier; Low
Generally high (unactivated: low)
Surface Coverage
multilayer
monolayer
reversibility
Highly reversible (adsorbate layer is always in eqm with molecules of gas phase)
Often irrreversible
(C + O2(ads) CO, CO2 at high T)
Applications
Determination of surface area and pore size
Determination of surface concentrations and kinetics, rates of adsorption and desorption, determination of active centres
Example
N2 on C
C6H6 on Pd

12. Adsorption
Physisorption
Chemisorption
Activated
- temperature sensitive
- varies according to a finite activation energy
Non-activated
rapid adsorption and near zero activation energy

13. ENERGETICS OF ADSORPTION
Adsorbate is diatom X2
X - X
X - X d

14. Physisorption Pure physisorption (e.g. Ar / metals ):
the only attraction between the adsorbing species and the surface arises from weak, van der Waals forces.
these forces give rise to a shallow minimum in the PE curve at a relatively large distance from the surface (typically d > 0.3 nm) before the strong repulsive forces arising from electron density overlap cause a rapid increase in the total energy.
there is no barrier to prevent the atom or molecule which is approaching the surface from entering this physisorption well, i.e. the process is not activated and the kinetics of physisorption are invariably fast.
P.E. d
representation of the variation of the (PE or E ) of the system as a function of the distance (d) of an adsorbate from a surface.
E p
d- distance from surface

15. Dissociative (chemical) adsorption
- D(X2) X2 X X EP
D(X2) – EC
EC – EP
X - X X X
Metal

16. Kinetics of Desorption/ Adsorptionkd
Xads Xdes
kd / s-1 : desorption rate constant
Arrhenius: kd = A exp(- Ed/RT)
A ~ vibrational frequency
Residence time ~ half-life ;
For Ed / kJ mol-1 = t1/2 ~ 10-8 s (physisorption)
Ed / kJ mol-1 = ~ 1 hr (chemisorption)
: average time between two successive attempts to escape from surface:

17. Analytical Aspects Of Adsorption
Quantitative measures of adsorption
# moles of adsorbate per gram of adsorbent : X / m (in mol g-1)
or, in the case of adsorption from the gas phase,
adsorption volume (V) per gram of adsorbent, where : V = (nadsRT/P)/m
evaluated at STP (25oC, 1 atm), i.e. V = (nads x 22.4 dm3)/m 
V = V(T,P) gas solid
X/m = X/m(T,c) solution solid
X = Vsoln(cini – cfin) where; (cfin = c = equilibrium conc.)
T constant: V(P) or X/m(c) : Recall:
relationship between the amount adsorbed (X) and the concentration (c) is known as adsorption isotherm.

19. TYPE I ISOTHERMS: THE LANGMUIR MODEL
Monolayer adsorption (Chemi/Physisorption)
V/cm3g-1 Vm
p/atm
Model assumptions:
1.Uniform surface with N equivalent adsorption sites per cm2
2. No interference of adsorbed particles with an adjacently adsorbed molecule
3. One molecule per site
4. Molar heat of adsorption is the same for all sites and independent of
fractional coverage θ
5. No dissociation

21. Y(g) + S(surface site) Y - S (associative adsorption) p 1 – θ θ
Fractional coverage θ = Ns/N
= # sites occupied by adsorbate per cm2
total number of available adsorption sites
Can also be defined in terms of relative volumes and relative masses
θ = V = X
Vm Xm
(gas/solid) (solution/solid)
Kinetic scheme:
Y(g) + S(surface site) Y - S   (associative adsorption)
p – θ θ
Equilibrium : rate of adsorption = rate of desorption
ka p (1 – θ) = kd θ 
(Adsorption from solution: replace p by c)
K p = θ / (1 – θ)  adsorption constant (in atm-1 or M-1) 
K(T) = ka / kd ka kd

22. when Kp << 1(low p) ; θ ~ Kp when p ∞ : θ 1
Langmuir isotherm (T const.) 
As p ; θ = 0
when Kp << 1(low p) ; θ ~ Kp
when p ∞ : θ  
In terms of adsorption volume:
p = gaseous partial pressure
c = aqueous concentration
K= Langmuir equilibrium constant
1/V
Vm = 1 / intercept
K = intercept / slope
1/VmK
1/Vm
1/p

23. Alternatively: - Graph of p/V vs p has slope = 1/Vm, intercept = 1/VmK
mm = 1 / intercept
K = intercept / slope
corresponding mass
Vm and mm : total number of sites corresponding to a monolayer

24. Dissociative adsorptionY Y
Y S Y – S  
p 1 – θ θ
 Adsorption equilibrium
ka p (1 – θ)2 = kd θ2
Thus:
from which we obtain: (K = K(T)) Y Y Y Y S S S S S S S S S S ka kd

25. with θ = V/Vm (V = adsorption volume of Y2 at STP) this can be reorganised to:
where:
1/V
Vm = 1 / intercept;
K = (intercept / slope)2
1/Vm√K
1/Vm
1/√p
Vm = 1 / slope;
K = (slope / intercept)2

26. Limitations to Langmuir Model
Does not explain multi-layer adsorption and limited to low pressure studies.
∆Hads is not independent of coverage:
also on a real surface some sites are better so gas molecules search for these first and ∆Hads is greater for better sites.
as molecules of adsorbate pack closer on the surface with increasing coverage, inevitably some lateral interactions will result, which will change ∆Hads.

27. MULTILAYER ADSORPTION – BET Model
Model assumptions:
(1) smooth, uniform surface
same number of adsorbate molecules in each layer
when full
(3) no lateral interactions
(4) (heat of adsorption is same for each layer except layer 1)
(5) dynamic equilibrium between adjacent layers
non-dissociative adsorption 5 4 3 2 1

28. Coverage : θ = V / Vm
Vm = adsorption volume (STP) occupied by molecules
covering a monolayer (so θ may now become > 1!) 
2 equilibrium constants:
K2(T) defined analogously for layers 2, 3,…
Define :
C is BET constant

29. Chemisorption in layer # 1 (type II isotherms)
so that :
Chemisorption in layer # 1
(type II isotherms)
c(T) decreases with increasing T
Define : z = p / p0 ( p and p0 = equilibrium and saturated vapour pressure of adsorbate at temperature T respectively)

30. contains 2 parameters : Vm and c(T)
Brunauer-Emmett-Teller (BET) isotherm
contains 2 parameters : Vm and c(T)
 Linearised form (multiply both sides by and invert): T z

31. Vm = 1 / (slope + intercept) # adsorption sites
c(T) = 1 + (slope / intercept)
Vm allows us to calculate an effective surface area of substrate.
Determination of specific surface area
SSA = adsorbent area / adsorbent mass
 = NA nmax a / m
= NA Vm a / (22.4 dm3 m)
NA = 6.0 x 1023 mol-1 ;
a = area of one adsorbate molecule
Vm = volume corresponding to one monolayer
nmax total number of moles corresponding to one
monolayer