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Surface Chemistry and Catalysis
Dr. Mothi Krishna Mohan
Assistant Professor
Dept. of Sciences and Humanities
Christ University, Bangalore
1
Surface Chemistry
Surface chemistry can be roughly defined as the study of chemical
reactions at interfaces.
Surface science is the study of chemical and physical phenomena that
occur at the interface of two phases.
It is closely related to surface engineering, which aims at modifying the
Chemical composition of a surface by incorporation of selected elements
or functional groups that produce various desired effects or improvements
in the properties of the surface or interface.
2
Objectives
 Adsorption
 Adsorption versus absorption
 Types of adsorption: physisorption and chemisorption
 Desorption
 Adsorption isotherms: Freundlich, Langmuir and BET
 Types of Adsorption Isotherm
 Adsorption isobar
3
Adsorption
The phenomenon of higher concentration of any molecular species at the
surface than in the bulk of the solid is known as Adsorption.
The major forces involved are intermolecular forces (Van der Waals forces)
Reasons behind Adsorption
 Unbalanced or residual forces,
 Extra valencies,
 Imperfections or defects,
Sometimes attracts more substances to the surface than in the bulk.
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Terms used in Surface Chemistry
 Adsorption : Higher concentration of substances on the surface.
 Absorption : Uniform distribution of substance throughout the
body of a solid or liquid.
 Sorption : Process in which both adsorption and absorption take
place simultaneously.
 Adsorbent : The solid that takes up gases, solute from solution are
called adsorbent.
 Adsorbate : The substance that gets adsorbed is known as adsorbate.
 Desorption : The process of removal of an adsorbed substance from
the surface on which it is adsorbed
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Adsorption Vs Absorption
 H2O on Activated carbon : Adsorption
 H2O in CaCl2 : Absorption
 NH3 on Charcoal : Adsoption
 NH3 in H2O : Absorption
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H
H
H
H
H
H
H
H
H
H2 adsorption on palladium
Surface Process
H
H
H
H
H HH H
H
H H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H2 absorption 
palladium hydride
Bulk Process
7
Adsorption is not necessarily a physical phenomenon always. It may
as well as be chemical process involving chemical interaction between
the surface atoms of the adsorbent and the atoms of the adsorbate.
Eg: Chemisorption of Oxygen by Carbon
Chemisorption of Hydrogen by Nickel
In both the cases suitable surface compound, frequently referred to as
Surface complex results.
8
Types of Adsorption
Positive Adsorption : Higher concentration of Adsorbate on the
surface than in the bulk.
Negative Adsorption : Lesser concentration of Adsorbate on the
surface than in the bulk.
Physical Adsorption : Physical forces are involved.
Chemical Adsorption : Chemical forces are involved.
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Characteristics of Adsorption
 Adsorption is accompanied by decrease in the ΔG (free energy) of
the system when ΔG = 0 adsorption equilibrium is said to be established.
 Adsorption is invariably accompanied by evolution of heat ie. It is
an exothermic process. In other words ΔH of adsorption is
always negative.
 When a gas is adsorbed, the freedom of movement of its molecules
become restricted. On account of this there is a decrease in
entropy after adsorption ie. ΔS is negative.
 According to Gibb’s Helmholtz Equation (ΔG = ΔH – TΔS),
Adsorption is a spontaneous reaction (Thermodynamically more
favourable).
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Molar Enthalpy of Adsorption
The amount of heat evolved when one mole of a gas or vapor is
adsorbed on a solid material is known as Molar Enthalpy of Adsorption
11
Chemisorption Vs Physisorption
Physisorption
Chemisorption
Physisorption occurs only
at very low temperatures
falling below the boiling
point of the adsorbate.
Physisorption decreases
with rise in temperature
Can occur at all the
temperatures.
The
magnitude
of
Chemisorption
increases
with rise in temperature.
This is just as the magnitude
of chemical reaction in a
given time increases with
rise in temperature
12
The
heat
evolved
in The
heat
evolved in
physisorption is quite low, chemisorption is very high,
varying generally between 4-40 varying generally between
KJ/mol.
40-400 KJ/mol.
Reversible
Irreversible
Favors multilayer adsorption
Favors monolayer adsorption
Forces of attraction are Vander
Wall’s forces
It does not require any Ea
Forces of attraction are
chemical forces
It requires high Ea
It is not very specific
It is very much specific
13
Applications of Adsorption
 One of the effective methods to create high vacuum with the help of
Charcoal.
 Activated charcoal is used in gas masks in which all the toxic gases
and vapors are adsorbed by the charcoal.
 Silica and Alumina gels are used as adsorbents for removing moisture
and for controlling humidities of rooms.
 Animal charcoal id used as a decoloriser in the manufacture of
cane sugar.
 It plays an important role in heterogeneous catalysis.
14
Factors Influencing Adsorption
 Surface Area
Adsorption being a surface phenomenon, Increase in the surface area of
the adsorbent, increases the total amount of gas adsorbed. Thus finely
divided metals (nickel, platinum) and porous substances (Charcoal, silica
gel) provides large surface area and are best solid adsorbents.
 Effect of Temperature and Pressure
The process of adsorption is an exothermic process. According to
Le-Chatlier’s principle, the magnitude of adsorption should increase
with decrease in temperature. Infact it is found to be so in case of
physical adsorption because vanderwaal’s forces are strong at low
temperatures. However, the chemisorption first increases with rise in
temperature and then starts decreasing. The initial increase shows that
like chemical reactions, chemisorption also needs activation energy.
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Adsorption Isobars
16
The magnitude of adsorption increases with increase in pressure.
17
 Nature of the Gas
More readly soluble and easily liquefiable gases such as Ammonia,
HCl, Cl2 and SO2 are adsorbed more effectively than the so called
permanent gases such as H2, N2 and O2. The Van der Walls or inter
molecular forces which are involved in adsorption are more predominant in the former category than in the latter category of gases.
18
Determination of Extend of Adsorption
The gas is contained in a vessel of known volume at a given temperature.
The pressure of the gas can be measured with the help of a manometer.
Then an adsorbent is introduced into the vessel by a suitable device. Adsorption takes place fairly quickly and the pressure of the gas falls. This is
Noted on the manometer. Knowing the fall of pressure, the quantity of gas
adsorbed by the solid can be calculated, assuming Boyle’s law to hold
good.
19
Desorption Activation Energy
Since the adsorbed molecule has a low binding energy, it may easily
shake off from the surface. That is the molecule will remain on the
surface for a very short time before it is desorbed. The rate constant
Follows Arrhenius-type relation
kdesorption = A𝒆−𝑬𝒂/𝑹𝑻
Hence 1/kdesorption is called the life time τ
τ = τ0 𝒆𝑬𝒂/𝑹𝑻
20
Freundlich Adsorption 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 graph is nearly a straight line.
x/m = kpn
x/m is the amount of gas adsorbed
k and n are constants
Here n is always less than one and therefore
x/m does not increase as rapidly as p
21
Langmuir Isotherm
Considered adsorption of Gases on the surface of a solid
Assumptions
1. The surface of solid to be made up of elementary sites each of
which could adsorb one gas molecule.
2. All the adsorption sites are equivalent and the ability of the gas
molecule to get bound to any one site is independent of whether
or not the neighboring sites are occupied.
3. There is a dynamic equilibrium exist between the adsorbed
molecule and free molecule.
Suppose if A is the gas molecule and M is the surface site , then
ka
A(g) + M(surface)
kd
AM
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Where ka and kd are the rate constants for adsorption and desorption
respectively. The rate of adsorption is proportional to pressure of A ie
PA and the number of vacant sites on the surface ie. N(1-θ) where N is
the total number of sites and θ is the fraction of total sites occupied by
The gas molecule.
Θ=
𝐍𝐮𝐦𝐛𝐞𝐫 𝐨𝐟 𝐚𝐝𝐬𝐨𝐫𝐩𝐭𝐢𝐨𝐧 𝐬𝐢𝐭𝐞𝐬 𝐨𝐜𝐜𝐮𝐩𝐢𝐞𝐝
𝐍𝐮𝐦𝐛𝐞𝐫 𝐨𝐟 𝐚𝐝𝐬𝐨𝐫𝐩𝐭𝐢𝐨𝐧 𝐬𝐢𝐭𝐞𝐬 𝐚𝐯𝐚𝐢𝐥𝐚𝐛𝐥𝐞
The rate of Adsorption = ka PA N(1-θ)……(a)
Rate of desorption is proportional to the number of adsorbed molecule,
Nθ.
The rate of Desorption = kd N θ……(b)
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Since at equilibrium, the rate of adsorption is equal to rate of desorption,
Then we can write from equations (a) and (b),
Ka pA N(1-θ) = kd N θ…..(c)
Where K =
𝐤𝐚
𝐤𝐝
Substitute the above equation in equation (c) , then
K pA (1-θ) = θ……(d)
24
Equation (d) may rewritten as
𝟏−𝜽
𝜽
𝟏
𝜽
𝟏
𝜽
=
𝟏
𝑲𝒑𝑨
-1=
𝟏
𝑲𝒑𝑨
=
𝟏
𝑲𝒑𝑨
𝑲𝒑𝑨
𝟏+𝑲𝒑𝑨
+1=
𝟏+𝑲𝒑𝑨
𝑲𝒑𝑨
= θ…………(e)
Equation (e) is called Langmuir adsorption isotherm
25
Langmuir Plot
26
Assumptions for Langmuir Adsorption Isotherm
 The adsorbed gas behaves ideally in the vapor phase.
 Only a monolayer is formed by the adsorbed Gas.
 The surface of the solid is homogeneous so that each binding site
has the same affinity for the gas molecule.
 There is no lateral interaction between the adsorbed molecules.
 The adsorbed gas molecules are localized ie. They do not move
around on the surface.
27
Drawbacks of Langmuir Assumptions
 The first assumption holds at low pressure.
 The second assumption breaks down when the pressure of the gas
is increased.
 Third assumption is not true because the actual surfaces are
heterogeneous.
 Fourth and fifth assumptions are also not valid.
28
Special features of Langmuir Isotherm
Consider equation (e) , At low pressure KpA << 1, so that
θ = K pA
The fraction of the surface covered is directly proportional to the
Partial pressure of the gas molecule. The behavior corresponds to
First-order reaction and is depicted by the initial steep rise of the
Isotherm.
At high pressure, KpA ˃˃ 1, so the equation (e) reduces to θ = 1.
29
The entire surface gets covered by a monolayer of the gas there by
making the reaction independent of pressure. That is the reaction
becomes zero order.
Since at low pressure, adsorption is proportional to pressure
θ = K (pA)1
At high pressure θ = K (pA)0
It follows that at intermediate pressure, the following expression
should hold good
θ = K (pA)n ( Freundlich Isotherm)
30
BET theory of Multilayer Adsorption
Assumptions
Here we are considering physical adsorption resulting in the multi
layer adsorption. In BET it is assumed that the solid surface
possesses uniform, localized sites and adsorption at one site does not
affect adsorption at neighboring sites .
It is further assumed that the molecule can be adsorbed in second,
third…and nth layer, the surface area available for the nth layer being
equal to the coverage of (n-1)th layer.
The energy of adsorption in the first layer is E1, assumed to be
constant and energy of adsorption in succeeding layers is assumed to
Be the same as EL, the energy of liquefaction of gases.
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Equation
𝒑
𝒗𝐭𝐨𝐭𝐚𝐥 (𝒑𝟎−𝒑)
=
𝟏
𝐯𝐦𝐨𝐧𝐨𝐂
+
𝒄−𝟏
𝒑
( )
𝒗𝐦𝐨𝐧𝐨𝐂 𝒑𝟎
Vtotal is the volume of the gas adsorbed, vmono is the volume adsorbed
when the surface of the solid is covered completely with a monolayer
of the adsorbed gases and c is a constant depending upon the nature
of the gas. Its numerical value is given approximately by the expression
c = 𝑒 𝐸1−𝐸𝐿/𝑅𝑇 in which E1 is the heat of adsorption in the first layer
And EL is the heat of liquefaction of the gas
32
BET Plot
33
Types of Adsorption Isotherm
Type I
Type II
34
35
Type I isotherms (e.g. ammonia on charcoal at 273 K) show a
fairly rapid rise in the amount of adsorption with increasing
pressure up to a limiting value. They are referred to as Langmuirtype isotherms and are obtained when adsorption is restricted to a
monolayer. Type I isotherms have also been found for physical
adsorption on solids containing a very fine pore structure (e.g.
nitrogen on microporous carbon at 77 K).
Type II isotherms (e.g. nitrogen on silica gel at 77 K) are
frequently encountered, and represent multilayer physical
adsorption on non-porous solids. They are often referred to as
sigmoid isotherms. For such solids, point B represents the
formation of an adsorbed monolayer.
36
Type IV isotherms (e.g. benzene on iron(III) oxide gel at 320 K)
level off near the saturation vapour pressure and are considered to
reflect capillary condensation in porous solids, the effective pore
diameters usually being between 2 nm and 20 nm. The upper limit of
adsorption is mainly governed by the total pore volume.
Types III (e.g. bromine on silica gel at 352 K) and V (e.g. water
vapour on charcoal at 373 K) show no rapid initial uptake of gas,
and occur when the forces of adsorption in the first monolayer are
relatively small. These isotherms are rare.
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Catalysis
38
Catalyst is a substance that alters the rate of
the reaction at which a chemical system
approaches
equilibrium
,
without
being
substantially consumed in the process.
Catalyst affects only the rate of the reaction,i.e.
Kinetics.
It changes neither the
thermodynamics of the reaction nor the equilibrium
composition.
39
Thermodynamics says NOTHING about the
rate of a reaction.
Thermodynamics : Will a reaction occur ?
Kinetics
: If so, how fast ?
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Kinetic Vs. Thermodynamic
A reaction may have a large, negative ΔGrxn, but the rate may
be so slow that there is no evidence of it occurring.
Conversion of graphite to diamonds is a thermodynamic
favor process (ΔG -ve ).
C (graphite)  C (diamond)
Kinetics makes this reaction nearly impossible (Requires a
very high pressure and temperature over long time)
41
Reaction path for conversion of A + B into AB
42
Catalyst lowers the activation energy for both forward and
reverse reactions.
43
Why are Catalysts Important?
Catalysts can:
• make a reaction possible under achievable conditions
• reduce the necessity of expensive & dangerous conditions
• generate high yields and high product purity
• reduce the amount of side-product and waste created
• generate non-racemic mixtures of enantiomers
• make a chemical process “greener”
44
Homogeneous and Heterogeneous catalysis
45
Homogeneous Catalysis
Action • catalyst and reactants are in the same phase
• reaction proceeds through an intermediate species with
lower energy
• there is usually more than one reaction step
• transition metal ions are often involved - oxidation state
changes
• Homogeneous
catalysts: efficiency expressed as turnover freq., N
• Slectivity : a “selective” catalyst yields a high proportion of
desired product with a marginal amount of side-products.
46
Homogeneous catalysts are more attractive for study in research labs
because:
a) the mechanism is accessible to detailed study
(i.e., the rate data is easier to interpret)
b) the species in solution are more easily characterized
Advantages of homogeneous catalysis on an industrial scale:
1. High selectivity
2. Ease of heat dissipation from exothermic reactions
Disadvantages of homogeneous catalysis on an industrial scale:
1. Scale-up can be costly, difficult, and dangerous
2. Separation is required
47
Examples:
1. Oxo process: Hydroformylation of alkenes
2. Wacker-Hoechst process: Oxidation of alkenes
O
RHC CH2 + O2
Pd(II) + Cu(II)
H3C
R
R = H, aldehyde
R = CnHn+2, ketone
48
5 Important Steps
1. Ligand Coordination and Dissociation
2. Insertion and Elimination
3. Nucleophilic attack on coordinated ligands
4. Oxidation and Reduction
5. Oxidative addition and Reductive
elimination
49
Heterogeneous Catalysis
Heterogeneous catalysts are more widely used in industry because:
1) Solid catalysts are robust at high T, P.
2) No solvents are required.
3) No separation of the product from the catalyst is required.
50
Classes of Heterogeneous Catalyst
There are 2 classes of Heterogeneous catalyst:
1. Uniform catalyst is a high surface area material.
e.g., ZSM-5
Na3(AlSi31O64)3•16H2O
2. Multiphasic catalyst is deposited on a high surface area material.
Typical inert supports
Silica gel, SiO2
microcrystalline γ-alumina, Al2O3
51
Heterogeneous Catalysts: Active Sites
The active sites of heterogeneous catalysts are NOT uniform.
Many types of sites are exposed on the surface of a material.
Each type of site may serve a unique function in the catalysis.
e.g., crystalline solids
Irregularities (e.g., steps)
Many exposed planes
52
Principle of Active sites
The Sabatier principle proposes the existence of an unstable
intermediate compound formed between the catalyst surface and at
least one of the reactants. The Sabatier principle of unstable surface
Intermediate requires chemical bonding of reactants to the catalyst
surface, most likely between atoms or functional groups of reactant
and surface atoms. This leads to the principle of active sites.
An active site is a point on the surface that can form strong
chemical bonds with an adsorbed atom or molecule.
53
Heterogeneous Catalysis
Adsorption (STEP 1)
Incoming species lands on an active site and forms bonds with the catalyst. It may use
some of the bonding electrons in the molecules thus weakening them and making a
subsequent reaction easier.
Reaction (STEPS 2 and 3)
Adsorbed gases may be held on the surface in just the right orientation for a reaction to
occur. This increases the chances of favourable collisions taking place.
Desorption (STEP 4)
There is a re-arrangement of electrons and the products are then released from54 the
active sites
Examples
Copper Dehydrogenation (oxidation)
C2H5OH ——>
CH3CHO + H2
Alumina Dehydration
C2H5OH ——>
C2H4 + H2O
55
The Surface Reaction Step
56
57
Another type of dual -site mechanism is the reaction between two
adsorbed species, such as reaction of CO and O
A third dual -site mechanism is the reaction of two species adsorbed
on different types of sites S and S’ such as the reaction of CO and O
58
Reaction involving either single or dual-site mechanism, which were
discussed sometimes referred to as following
Langmuir-Hinshelwood Kinetics
A third type mechanism is the reaction between an adsorbed molecule
and a molecule in the gaseous phase , such as the reaction of benzene
and propelene – Eley- Rideal mechanism.
59
Types of Catalysis
 Electrocatalysis
(Oxidation or reduction by transfer of electrons)
 Photocatalysis
(Catalysis by means of Light)
Biocatalysis
(Glucoammylase immobilized on SiO2 for the isomerization of
glucose to fructose)
Environmental Catalysis
(Rh-Pt-CeO2-Al2O3 deposited on Ceramics)
Greener Catalysis
(High
yield, Selective, marginal side products, No solvents)
60
Classification of Solid Catalysts
 Unsupported Catalysts
Supported Catalysts
Confined Catalysts
Hybrid Catalysts
Polymerization Catalysts
61
Unsupported Catalysts
Metal Oxides
Metal oxides are usually solids. Their bulk character mainly depend on
the bonding character between metal and oxygen.
Metal oxides have widely varying electronic properties and include
Insulators (Al2O3, SiO2), Semiconductors (TiO2, ZnO, NiO), metallic
Conductors (TiO, NbO), superconductors
These are catalytically active substances where their surface properties
and chemistry being determined by structure and composition, bonding
character, coordination of surface atoms and hydroxyl groups in
crystallographic phases. They can develop acid base and redox properties
62
Usually very active catalyst that do not require high surface
area
e.g., Iron catalyst for ammonia production (Haber process)
Acidic and basic sites and acid-base pair sites have been identified
on the surfaces of aluminas (Al2O3)
As a result of this complexity, alumina surfaces develop a rich
surface chemistry and specific catalytic properties.
SiO2 is weakly acidic and MgO is basic solid.
63
64
Molecularly Imprinted Catalysts
Molecular imprinting permits the heterogeneous supramolecular
catalysis to be performed on surfaces of organic or inorganic material
with substrate recognition.
Imprinted catalysts can be prepared on the basis of Al3+ doped silica gel
and of crosslinked polymers.
When transition-state or reaction-intermediate analogues are
used as templates for molecular imprinting, specific adsorption
sites are created.
65
66
Metal-Organic Frameworks
MOF’s are highly porous, crystalline solids consisting of a three
dimensional network of metal ions attached to multidentate organic
molecules.
Similar to Zeolites, the spatial organization of the structural units
give rise to a system of channels and cavities on the nanometer
length scale.
Due to their extremely high surface areas and their tunable pore
structure with respect to size, shape, and function, MOFs are
highly interesting materials for various applications.
Eg: Catalytic reactions, Gas storage etc.
67
Despite their higher metal content compared to zeolites, the use of
MOFs in heterogeneous catalysis is restricted due to their relatively
low stability at elevated temperature and in the presence of water
vapor or chemical reagents. In addition, the metal ions in MOFs are
often blocked by the organic linker molecules and are therefore not
accessible for catalytic reactions. However, successful applications of
especially stable Pd MOFs in alcohol oxidation, Suzuki C–C coupling
and olefin hydrogenation have been reported.
68
Hybrid catalysts
Hybrid catalysts combine the properties of Homogeneous and heterogeneous catalysis.
The goal of approach is to combine the positive aspects of Homogeneous
catalysts in terms of activity, selectivity and the variability of steric and
electronic factors by the appropriate choice of ligands with advantages
of Heterogeneous catalysts such as ease of preparation and recovery of
catalyst.
This can be achieved by immobilization of active metal complexes, Organometallic compounds, enzymes on solid support.
Eg: Anchoring of catalytically active species via covalent bonding on the
surface of inorganic supports such as SiO2, MCM-41, Zeolites etc. 69
Chemical fixation by ionic bonding using ion exchange
Deposition of species on the surface of the material through chemisorption or through CVD.
Ship-in-a-Bottle Catalysts
Metal complexes which are physically entrapped in the confined
spaces of Zeolite cages are known as Ship-in-a-Bottle or tea-bag
catalysts. The entrapped complexes retain many of the solution
properties. Here catalytic performance can be modified by shape
selectivity, electrostatic environment and acid-base properties of
Zeolite host. These are known as Host – guest supramolecular
Compounds.
70
Zeolites
What is ZSM-5 Catalyst ?
 It is an abbreviation for (Zeolite Scony Mobile Number 5 )
 First synthesized by Mobil Company in 1972
 It replaces many Homogeneous Catalysts were
used in
many petrochemical processes
 ZSM-5 has two diameters for its pores : d1= 5.6 Å , d2= 5.4 Å
Where as, Zeolite Y has a diameter = 7.4 Å
71
Properties ZSM-5
The ZSM-5 zeolite catalyst is used in the petroleum industry
for hydrocarbon interconversion.
ZSM-5 zeolite is a highly porous aluminosilicate with a high
silica/alumina ratio.

It has an intersecting two-dimensional pore structure.

The aluminum sites are very acidic.

The acidity of the zeolite is very high.

The reaction and catalysis chemistry of the ZSM-5 is due
to this acidity.
72
Shape selective catalysts
Zeolites and related materials contain regular microporous, the
diameter of which are determined by structure of the molecule.
The pore sizes are well defined and have dimensions similar to those
of small organic molecules.
The geometric constraints may act on sorption of reactants, on the
Transition state of the catalyzed reaction, or on the desorption of
Products.
Based on that shape selective effects have been classified as
a) Reactant shape selectivity b) restricted transition state shape
selectivity c) product shape selectivity
73
Reactant Shape Selectivity
Cracking of n-heptane and 1-methylhexane – Kinetic diameter of
n-heptane is smaller than that of 1-methylhexane, so that the latter
is not able to enter micropores so that shape selective cracking of
n-heptane takes place when both hydrocarbons are present in the feed.
Transition state shape selectivity
Transalkylation of m-Xylene – The reaction is bimolecular and formation
of 1,3,4-trimethylbenzene has a less bulky transition state than the
formation of 1,3,5-trymethylbenzene.
74
Product shape selectivity
Alkylation of toluene – p-Xylene can be selectively formed by methylation of toluene with methanol and zeolites whose pore openings only
Allow p-Xylene to be released. The o and m isomers either accumulate
in zeolite cages or are isomerized to p-Xylene.
75
Classification of Shape selective effects
76
Photocatalysis
Generally catalysts are the substances that
alters the rate of a chemical reaction.
Photocatalysis is the acceleration of photoreaction
in the presence of a catalyst.
In heterogeneous photocatalysis almost all the
reactions take place at the surface of a catalyst.
77
Photocatalysis can be divided into two classes of
Processes
 Catalyzed photoreaction
 Sensitized photoreaction
Semiconductor materials can be act as good
Photocatalysts.
Chem. Rev., 1995, 95, 735-758.
78
Irradiated n-type Semiconductors
Generation of “electron- hole pair” intermediate.
79
General mechanism of photodegradation
Complete Mineralization
80
Activity Studies
10 ml of 10-4 M aqueous solution of Herbicide.
Light Source – 150W ozone free Xe lamp with 420-630 nm
dichoric mirror as the filter.
The concentration of the Herbicide was monitored using HPLC
(on a 25/15cm, 5μm ODS-2 C-18 column -UV/PDA detector).
Percentage Degradation = {Co – C} X 100 / Co, Where C0 and C are
the initial and final concentration of the Herbicides.
81
Photo-oxidation
82
Preparation of Catalysts
Sol-Gel Method
83
84
Advantages of Sol-Gel Technique
• Preparation normally carried out at room temp.
• Chemical conditions are mild
• Gives better surface area
• Gives better pore sized particles
• Gives better nano scaled particles
85
Hydrothermal and Solvothermal Synthesis
• Synthesis includes various techniques of crystallizing
substances from high-temperature aqueous solutions at high
vapor pressures.
86
Steps involved:
 Reactants are dissolved in water and taken in a
bomb
 Reaction mixture is heated above the BP of water
for long time periods
 The catalyst is extracted and separated.
 Same procedure is followed for SOLVO-THERMAL
process where a suitable solvent is used instead of
water.
87
Solution Combustion Synthesis
Oxidizer + Fuel
oxide catalyst
88
Mix
Stir
Heat
Steps involved
Cool
Extrac
t
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Advanatages of Solution Combustion method
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Characterization of Catalysts
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X-ray Photoelectron Spectroscopy (XPS)
Theory – Photoelectric Effect
• XPS spectra are obtained by irradiating a material with a beam of X-rays
while simultaneously measuring the kinetic energy and number of electrons
that escape from the top 0 to 10 nm of the material being analyzed.
•
It is known as ESCA.
•
XPS detects all elements except Hydrogen and Helium.
•
Detection limits for most of the elements (on a modern instrument) are in
the parts per thousand range. Detection limits of parts per million
(ppm) are possible, but require special conditions: concentration at top
surface or very long collection time (overnight).
• Al Kα X-rays or Mg Kα X-rays (monochromatic)
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Einstein’s Photoelectric Equation
• In solid-state physics, the work function (sometimes spelled
workfunction) is the minimum thermodynamic work (i.e.
energy) needed to remove an electron from a solid to a point
in the vacuum immediately outside the solid surface.
• Why UHV - Remove adsorbed gases from the sample.
Eliminate adsorption of contaminants on the sample. Prevent
arcing and high voltage breakdown. Increase the mean free
path for electrons, ions and photons. Electron counting
detectors in XPS instruments are typically one meter away
from the material irradiated with X-rays. This long path length
for detection requires such low pressures.
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• A typical XPS spectrum is a plot of the number of electrons
detected (sometimes per unit time) (Y-axis, ordinate) versus the
binding energy of the electrons detected (X-axis, abscissa). Each
element produces a characteristic set of XPS peaks at
characteristic binding energy values that directly identify each
element that exists in or on the surface of the material being
analyzed.
•
The number of detected electrons in each of the characteristic
peaks is directly related to the amount of element within the
XPS sampling volume. To generate atomic percentage values,
each raw XPS signal must be corrected by dividing its signal
intensity (number of electrons detected) by a "relative
sensitivity factor" (RSF), and normalized over all of the
elements detected.
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• Signals detected from analytes at the surface much stronger than
the signals detected from analytes deeper below the sample
surface. Thus, the signal measured by XPS is an exponentially
surface-weighted signal.
•
1–20 minutes for a broad survey scan that measures the amount
of all detectable elements, typically 1–15 minutes for high
resolution scan that reveal chemical state differences (for a high
signal/noise ratio for count area result often requires multiple
sweeps of the region of interest), 1–4 hours for a depth profile that
measures 4–5 elements as a function of etched depth (this process
time can vary the most as many factors will play a role).
• The minimum analysis area ranges from 10 to 200 micrometres.
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Spin-Orbit Splitting
Spin-orbit splitting is
an initial state effect.
For any electron in
orbital with orbital
angular momentum,
coupling between
magnetic fields of
spin (s) and angular
momentum (l) occurs
Lower BE
Higher BE
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Photoelectrıct effect
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X-ray Fluorescence
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Auger Spectroscopy
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• X-rays and gamma rays can be energetic enough to expel tightly
held electrons from the inner orbitals of the atom. The removal
of an electron in this way makes the electronic structure of the
atom unstable, and electrons in higher orbitals "fall" into the
lower orbital to fill the hole left behind. In falling, energy is
released in the form of a photon, the energy of which is equal to
the energy difference of the two orbitals involved. Thus, the
material emits radiation, which has energy characteristic of the
atoms present.
• When a core electron is removed, leaving a vacancy, an electron
from a higher energy level may fall into the vacancy, resulting in
a release of energy. Although most of the time this energy is
released in the form of an emitted photon, the energy can also be
transferred to another electron, which is ejected from the atom.
This second ejected electron is called an Auger electron.
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• Satellites arise when a core electron is removed by a
photoionization. There is a sudden change in the effective charge
due to the loss of shielding electrons.
• Shake-up: The outgoing electron interacts with a valence
electron and excites it (shakes it up) to a higher energy level. As
a consequence the energy core electron is reduced and a satellite
structure appears a few eV below (KE scale) the core level
position.
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Shake-off: The valence electron is ejected from the ion completely
(to the continuum). Appears as a broadening of the core level peak
or contribute to the inelastic background.
Emission from non-monochromatic x-ray sources produces satellite
peaks in XPS spectrum at lower BE.
Ghost peaks are due to contamination of the x-ray source, which
produces x-ray emission at different wavelength and it can also
due to contamination of the sample holder etc.
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Quantification of XPS
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Scanning Electron Microscopy
• A scanning electron microscope (SEM) is a type of electron
microscope that produces images of a sample by scanning it with a
focused beam of electrons.
• The types of signals produced by a SEM include secondary
electrons (SE), back-scattered electrons (BSE), characteristic Xrays.
• The signals result from interactions of the electron beam with
atoms at or near the surface of the sample.
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•
In the most common or standard detection mode, secondary
electron imaging or SEI, the SEM can produce very highresolution images of a sample surface, revealing details less than
1 nm in size. Due to the very narrow electron beam, SEM
micrographs have a large depth of field yielding a characteristic
three-dimensional appearance useful for understanding the
surface structure of a sample.
• Back-scattered electrons (BSE) are beam electrons that are
reflected from the sample by elastic scattering. BSE signal is
strongly related to the atomic number (Z) of the specimen. BSE
images can provide information about the distribution of
different elements in the sample.
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• Characteristic X-rays are emitted when the electron beam
removes an inner shell electron from the sample, causing a
higher-energy electron to fill the shell and release energy. These
characteristic X-rays are used to identify the composition and
measure the abundance of elements in the sample.
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Powder X-ray Diffraction
X-rays are partially scattered by atoms when they strike the
surface of a crystal. The part of the X-ray that is not
scattered passes through to the next layer of atoms, where
again part of the X-ray is scattered and part passes through
to the next layer. This causes an overall diffraction pattern,
similar to how a grating diffracts a beam of light. In order
for an X-ray to diffract the sample must be crystalline and
the spacing between atom layers must be close to the
radiation wavelength. If beams diffracted by two different
layers are in phase, constructive interference occurs and the
diffraction pattern shows a peak, however if they are out of
phase, destructive interference occurs appear and there is
no peak. Diffraction peaks only occur if
sinθ=nλ/2d
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4000
Intensity
3000
2000
1000
0
10
20
30
40
50
60
70
80
Angle (2)
Amorphous Pattern
Crystalline Pattern
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Crystallite Size of a material can be determined by Scherrer equation
D = Kλ / βcosθ
where K is a dimensionless constant, 2θ is the diffraction angle, λ
is the wavelength of the X-ray radiation, and β is the full width at
half-maximum (FWHM) of the diffraction peak . Crystallite size
is inversely related to the FWHM of an individual peak. That is
narrower the peak, larger the particle size. Ultimately, the size of
the nanomaterials purely depends on the FWHM of the
diffraction peak. Phase identification is another important task of
XRD. In powder X-ray diffraction, the phase corresponding to a
particular plane can be identified by comparing it with reference
spectra from JCPDS.
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Thank You
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