Colloid chemistry chapter 5 adsorption on gas solid surface

• Type I arises when only one type of site: • Initially surface fills randomly • Eventually saturates when surface filled or pores filled with a porous material 24 6... • Type III ari

Dr Ngo Thanh An

COLLOID CHEMISTRY

Chapter 5 – Adsorption on G-S surface

1 Concept of adsorption

1 Concept of adsorption

1 Concept of adsorption

1 Concept of adsorption

2 Application of adsorption

2 Application of adsorption

Zeolite structureSilica structure

Adsorbent

3 Physical and chemical adsoprtion

3 Physical and chemical adsoprtion

3 Physical and chemical adsoprtion

4 Thermodynamic consideration

P RT

2

T

H T

o g

T

P R

T

H T

2

ln

RT

H RT

H

H RT

H

H T

RT

H const

ln

4 Thermodynamic consideration

5 Equilibrium of adsoprtion

6 Adsoprtion isotherm

Ⅰ Microporous (Active carbon, Zeolite)

Ⅱ Non-porous （ Metal powder)

Ⅲ Non porous and weak adsorption interaction

Ⅳ Mesoporous （ Silica gel （

Ⅵ Energetically uniform surface

（ 2nm

Micropore

2nm （50nm

6 Adsoprtion isotherm

Why 5 Types Of Adsorption Isotherms?

• Type I arises when only one type

of site:

• Initially surface fills randomly

• Eventually saturates when surface

filled (or pores filled with a porous

material)

24

6 Adsoprtion isotherm

• Type III arises when there are

strong attractive interactions

leading to condensation

• Initially, no adsorption

• Pressure increases lead to

nucleation and growth of islands

• Eventually liquids condense on the

• Type II arises when the is more

than one adsorption site

• Initial rapid adsorption

• Saturates when first site filled

• Second rise when second site fills

• Second site could be a second

monolayer, a second site on the

surface In porous materials, it

can also be a second type of pore.

26

6 Adsoprtion isotherm

• Type V is another case for

attractive interactions

• Initially no adsorption

• Next nucleation and growth of

islands or liquid drops

• Coverage saturates when no more

space to hold adsorbates

• Type IV occurs when there are

multiple phase transitions due to

a mixture of attractive and

repulsive interactions

• Can also arise in multilayer

adsorption where adsorption on

second layer starts before first

layer saturates

28

6 Adsoprtion isotherm

Mechanism of Adsorption on Non-porous Surfaces

0.548 1

t s

p/p 0

n

1 0

t s

/nm



n

0.548 1 0

6 Adsoprtion isotherm

Adsorption on Mesoporous Samples: Capillary Condensation

6 Adsoprtion isotherm

Capillary condensation is the "process by which multilayer

adsorption from the vapor [phase] into a porous medium proceeds to the point at which pore spaces become filled with condensed liquid from the vapor [phase] The unique aspect of capillary condensation is that vapor condensation occurs below

RTr

V P

Pv: equilibrium vapor pressure; Psat: saturation vapor pressure;

r: radius of capillary; : liquid/vapor surface tension; Vm: liquid molar volume;

R: ideal gas constant; T: temperature

6 Adsoprtion isotherm

Capillary condensation

System A → Pv=0, no vapor is present in the system

System B → Pv=P1<Psat, capillary condensation occurs and liquid/vapor equilibrium is reached

System C → Pv=P2<Psat, P1<P2, as vapor pressure is increased condensation continues in order

to satisfy the Kelvin equation

System D → Pv=Pmax<Psat, vapor pressure is increased to its maximum allowed value and the pore is filled completely

This figure is used to demonstrate the concept that by increasing the vapor pressure in a given system, more condensation will occur In a porous medium, capillary condensation will always occur if Pv ≠ 0.

The relation of equilibrium vapor pressure to the saturation vapor pressure can be thought of as a relative humidity measurement for the atmosphere As Pv/Psat increases, vapor will continue to condense inside a given capillary If Pv/Psat decreases, liquid will begin to evaporate into the atmosphere as vapor molecules

6 Adsoprtion isotherm

Capillary condensation

p/p 0

n

0.25 0

p/p 0

n

1 0

p/p 0

n

1 0

6 Adsoprtion isotherm

6.1 Freundlich adsorption isotherm

6.2 Langmuir theory and adsorption isotherm

Figure 4.5 Langmuir’s model of the structure of the adsorbed layer The black dots

represent possible adsorption sites, while the white and mauve ovals represent

adsorbed molecules

6.2 Langmuir theory and adsorption isotherm

Extent of adsorption usually given by fractional coverage

coverage monolayer

have we

1 when

) (N sites surface

of number total

) (N occupied sites

surface of

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

6.2 Langmuir theory and adsorption isotherm

• Molecules in gas and surface are in dynamic equilibrium

G (g) +  (surface) ↔ G-

• Isotherm describes pressure dependence of equilibrium

• Langmuir isotherm proposed by Irving Langmuir

• (1932 Noble Prize)

• Adsorption saturates at 1 monolayer

• All sites are equivalent

• Adsorption is independent of coverage

Site conservation

θA + θ* = 1 + rateEquilibriumads = ratedes A A a d

A

, 1

KP K k k KP

G2 (g) + 2 (surface) ↔ 2G-

1

KP KP



1 1

a a a

a a b b

b b b

a a a

a a b b

b b b

（ Surface is energetically homogenious.

（ There is no lateral interface between adsorbed molecules.

（ The adsorption energies in the second and all higher layers are equal to condensation energy of adsorptive.

Assumptions

6.3 BET adsorption isotherm

6.3 BET adsorption isotherm

2 The data given below are for the adsorption of nitrogen on alumina at 77.3 K Show that they fit in a BET isotherm in the range of adsorption and find Vm and hence surface area of alumina (m2 /g) At 77.3 K, saturation pressure, Po = 733.59 torr The volumes are corrected to STP and refer to 1g of alumina Given: contact area of one N2 molecule =16.2x10-20m2)