Chemistry

Surface Chemistry - 1

Session

Session Objectives

� Adsorption

� Adsorption versus absorption

� Types of adsorption: physisorption and chemisorption

� Desorption

� Adsorption isotherms: Freundlich and Langmuir

� Adsorption isobar

� Catalysis

Adsorption

The phenomenon of higher concentration of any molecular species at the surface than in the bulk

Adsorbent The substance on the surface of which adsorption takes

place is called adsorbent

Adsorbate The substance which is being adsorbed on the

surface of another substance.

Desorption The process of removal of an adsorbed substance from

the surface on which it is absorbed

• Activated Carbon

• Activated Alumina

• Silica Gel

• Molecular Sieves (Zeolites)

• Polar and Non-polar adsorbents

Adsorbent Materials

• Made from nutshells, wood, and petroleum, bituminous coal by heating in the absence of oxygen to dehydrate

and carbonize (remove volatile components),

• "Activation" is the process that produces the porous

structure essential for effective adsorption by oxidation of carbon with water vapor or CO2.

• Activated carbon attracts non-polar molecules such as

hydrocarbons.

• Typical surface areas are 300 to 1500 m2/g.

Activated carbon

Adsorption vs absorption

Types of Adsorption

Positive adsorption occurs when the concentration of adsorbate is higher on the

surface of adsorbent than in the bulk.

Negative adsorption occurs when the concentration of adsorbate is less on the surface of adsorbent than in the bulk.

Types of adsorption

S. No. Physical adsorption Chemical adsorption

1 Caused by intermolecular van der

Waals' forces

Caused by chemical bond formation

2 It is not specific It is highly specific

3 It is reversible It is irreversible

4 Heat of adsorption is low (20-40 kJ/mol) High heat of adsorption (80-240 kJ/mol)

5 Low temperature is favourable Increases with high temperature

6 Results multilayer adsorption Results unimolecular layer

Comparison between physisorption and chemisorption

1. Physical adsorption

2. Chemical adsorption

Factors affecting adsorption

Effect of adsorbate: The easily liquifiable gases like NH3, HCl, CO2 etc. are adsorbed to a greater extent than the

permanent gases such as H2 ,O2, N2, etc. Effect of specific area of the absorbent: The greater

the specific area of the solid, the greater would be its adsorbing capacity.

Effect of temperature:adsorption decreases with increase in temperature.

Effect of pressure: An increase in pressure causes an

increase in the magnitude of adsorption of an adsorbent.

Over a narrow range of p

Freundlich Isotherm

A graph between the amount (x/m) adsorbed by an adsorbent and the equilibrium pressure of the adsorbate

at constant temperature is called adsorption isotherm

At low pressure the graph is nearly straight line

At high pressure x/m becomes independent of p

0xp

mα

1xp

mα

Freundlich Isotherm

Rate of adsorption a ar =k p(1- f)

Rate of desorption d dr =k f

At equilibrium, ra = rd; a

a d

k pf = .........(i)

k p+k

Mono-layer coverage '

a

x=k f .........(2)

m

m: mass of adsorbate adsorbed per unit mass of adsorbent

f: fraction of surface area covered

f

1-f

p: partial pressure of the adsorbate

Langmuir isotherm

Langmuir adsorption isotherm:

apx /m

1 bp=

+

The values of constants ‘a’ and ‘b’ depend upon the nature of adsorbate, nature of solid

adsorbent and temperature.

a = ka x ka’/kd

b = ka/kd

Combining equations (1) and (2):

Summary of adsorption isotherms

Easy to fit adsorption data

Chemisorptions and physisorption

Freundlich

Useful in analysis of reaction mechanism

Chemisorption and physisorption

Langmuir

Note Application Isotherm equation

Name

1/nx=Kp

m

x ap=

m 1+bp

Adsorption isobar

Graph between the amount adsorbed(x/m) and temperature at a constant equilibrium pressure of adsorbate gas is known

as adsorption isobar

Chemisorption isobar shows an initial increase with temperature and then expected decrease .The initial increase is because of the fact that

the heat supplied acts as activation energy required in chemisorption.

Application of Adsorption

� In clarification of sugar

� In gas masks

� In catalysis

� In adsorption indicators

� In chromatographic analysis

� In softening of hard water

� In preserving vacuum

� In paint industry

� In removing moisture from air in the storage of

delicate instruments

Need to make chemicals faster

Most Reactions are too slow to be useful...

Ways to Make Chemicals Faster

Pressure

Disadvantage--Cause Explosions

Catalysts!!!!

Disadvantage--Costly

Disadvantage--Too hot!

Temperature

Add other Chemicals

Disadvantage--Separate chemicals

The Story of Catalysis

Why Catalysis ? What is a Catalyst ? How Catalysts Work ?

Role of a Catalyst

Catalysts speed up a chemical reaction without being used up...

Catalyst + Reactants

Catalyst-Reactants

Catalyst + Products

Hydrogenation of alkene

Important properties of catalyst

Activity: A reasonable rate of reaction is needed.

Selectivity: Byproducts should be minimized.

Cost: The acceptable cost depends upon the catalyst lifetime and product value.

Catalysis

Positive catalysis

Negative catalysis

Auto catalysis

°

→ +2MnO3 2

270 C2KClO 2KCl 3O

+ → +Alcohol

3 2 22CHCl O 2COCl 2HCl

+ → +3 2 5 2 3 2 5CH COOC H H O CH COOH C H OH

Catalysis

Types of catalysis

Homogeneous catalysis

Process Products Catalyst 1. Hydrolysis of an ester. Acid and Alcohol H+

2. Oxidation of SO2 to SO3 SO3 NO in lead chamber process

Heterogeneous catalysis

Process Products Catalyst

1. Ammonia synthesis NH3 Fe with Al2O3

2. Methanol synthesis MeOH ZnO/Cr2O3

Catalysis

Enzyme catalysis

Reaction Catalyst

1. Invertase

2. Zymase

12 22 11 2 6 12 6 6 12 6Sucrose Glucose Fructose

C H O H O C H O C H O+ → +

6 12 6 2 5 2Glucose Ethanol

C H O 2C H OH CO→ +

A coenzyme is an organic non-protein molecule that is a functional part of an enzyme. Coenzymes are not

used up in the reactions in which they assist

Do Catalysts Live Forever?

NO!!!

They can die from:

-- poisons which contaminate the catalyst -- large molecules which cover the catalyst -- over heating, over pressurizing

-- crumbling/crushing

Automotive Emission Control

Automobile catalytic converters need to catalyze several reactions

– CHx + O2 �CO2 + H2O

– CO + O2 � CO2

– NOx +CHx � N2 + H2O + CO2

– This is achieved by the use of a supported precious metal catalyst like platinum, palladium etc.

– Catalyst needs O2 to operate, CeO2 acts as a temporary regenerable source of O2.

CeO2 � Ce2O3 + 1/2O2

Thank you