Surface Chemistry - 1

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Session
Surface Chemistry - 1
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Session Objectives

• Adsorption
• Adsorption versus absorption
• Types of adsorption physisorption and
  chemisorption
• Desorption
• Adsorption isotherms Freundlich and Langmuir
• Adsorption isobar
• Catalysis

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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
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Adsorbent Materials

• Activated Carbon
• Activated Alumina
• Silica Gel
• Molecular Sieves (Zeolites)
• Polar and Non-polar adsorbents

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Activated carbon

• 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.

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Adsorption vs absorption
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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.
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Types of adsorption

• Physical adsorption
• Chemical adsorption

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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 temperatureadsorption
decreases with increase in temperature. Effect
of pressure An increase in pressure causes an
increase in the magnitude of adsorption of an
adsorbent.
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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
Over a narrow range of p
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Freundlich Isotherm
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Langmuir isotherm
f fraction of surface area covered
p partial pressure of the adsorbate
f
m mass of adsorbate adsorbed per unit mass
of adsorbent
1-f
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Langmuir adsorption isotherm
Combining equations (1) and (2)
a ka x ka/kd b ka/kd
The values of constants a and b depend upon
the nature of adsorbate, nature of solid
adsorbent and temperature.
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Summary of adsorption isotherms
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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.
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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

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Need to make chemicals faster
Most Reactions are too slow to be useful...
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Ways to Make Chemicals Faster
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The Story of Catalysis
Why Catalysis ? What is a Catalyst ? How
Catalysts Work ?
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Role of a Catalyst
Catalysts speed up a chemical reaction
without being used up...
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Hydrogenation of alkene
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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.
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Catalysis
Positive catalysis
Negative catalysis
Auto catalysis
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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
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Catalysis
Enzyme catalysis
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
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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
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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
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