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
AdsorbentThe substance on the surface of which adsorption takes place is called adsorbent
AdsorbateThe substance which is being adsorbed on the surface of another substance.
DesorptionThe 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 adsorption1 Caused by intermolecular van der
Waals' forcesCaused by chemical bond formation
2 It is not specific It is highly specific3 It is reversible It is irreversible4 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 temperature6 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.
1/n xKp
m
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 becomesindependent of p
0xp
m
1xp
m
Freundlich Isotherm
X1loglogKlogp
Mn
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
NoteApplicationIsotherm 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 andthen 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 FasterWays 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
2MnO
3 2270 C
2KClO 2KCl 3O
Alcohol3 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 Catalyst1. 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
