Surface chemistry

Surface Chemistry and Catalysis 2.1
SURFACE CHEMISTRY AND CATALYSIS
Introduction - Terminologies in surface chemistry - Difference between adsorption
and absorption - Types of adsorption - Adsorption isotherm- FreundlichAdsorption
Isotherms-LangmuirAdsorptionIsotherm-ContactTheory(or)Mechanismof
HeterogeneousCatalysis-KineticsofSurfaceReaction-KineticsofBimolecular
Reaction (Langmuir-Hinshelwood) - Types of Adsorption Isotherm - Application
of Adsorption- Terms - Mechanismof CatalyticReaction-Criteria (or) Characteristics
for Catalyst Types of Catalysis - Homogeneous Catalysis - Heterogeneous Catalysis-
CatalyticPoisoningandPromoters ApplicationofCatalysis- Biological Catalyst—
—Enzymes - Kinetics of Enzyme Catalysed Reaction Or Michaelis and Menten
equation - Factors Affecting Enzyme Activity
CHAPTER 4: Surface Chemistry
INTRODUCTION
SurfaceChemistryiscloselyrelatedto interfaceandcolloidalscience.Surfacechemistry
isimportantinmanycriticalchemicalprocesses,suchasenzymaticreactionsat biologicalinterfaces
found in cell walls and membranes, in electronics at the surfaces and interfaces of microchips
used in computers, and the heterogeneous catalysts found in the catalytic converter used for
cleaningemissionsinautomobileexhausts.
DEFINITION
Surface science is the study of chemical phenomena that occur at the interface
of two phases (solid–liquid interfaces, solid–gas interfaces, solid–vacuum interfaces,
and liquid-gas interfaces). (or)
It is defined as the study of chemical reactions at interfaces.
2.1

2.2
TERMINOLOGIES IN SURFACE CHEMISTRY
 Adsorbate: The substance whichgets adsorbed onanysurfaceiscalled adsorbate.
 Adsorbent: The substance on the surface ofwhichadsorptiontakes placeiscalled
adsorbent.
 Adsorption:Theprocess wherebymoleculesofgasesor liquidsadherechemicallyto
the surfaceofa solid.
(Eg.) Occlusionof Hydrogen gas on Palladiumwhere palladium is adsorbent and
hydrogen gas isadsorbate.
1Adsorbent - the gas adsorbed
on the surface of solids
Adsorbent - the solid where
adsorption takes place
Figure 2.1 : Adsorption Process
 Desorption: The removal of the adsorbed substance from a surface is
called desorption.
 Interface: Theplanewhichseparatesanytwo phaseisgenerallycalledaninterface.
 Absorption: Whenthemoleculesofa substanceareuniformlydistributedthroughout
the bodyofa solidor liquid.Thisphenomenon iscalled absorption.
 Sorption: Thephenomenoninwhichadsorptionandabsorptionoccursimultaneously
is calledsorption.
 Occlusion: When adsorption of gases occurs on the surface of metals it is
called occlusion.
Chemistry

Adsorption Desorption Absorption
Figure 2.2 : Occlusion Process
 Positive adsorption: When the concentration of solute adsorbed on the solid
adsorbent surface isgreater thaninthe bulk it iscalledpositiveadsorption.
(Eg.) Concentratedsolutionof KClisshakenwithbloodcharcoal, itshows positive
adsorption
 Negative adsorption: When the solvent from the solution maybe absorbed by the
adsorbent so that the concentration of the solute decreases and the concentration of
solution increases thanthe initialconcentration andit iscalled negativeadsorption.
(Eg.)DilutesolutionofKClisshakenwithbloodcharcoalitshowsnegativeadsorption.
Enthalpy or heat of adsorption
Amountofheat evolvedwhen1moleofanadsorbategetsadsorbed onthesurfaceofan
adsorbent iscalled Molar Heat or Molar Enthalpy ofAdsorption.
DIFFERENCE BETWEEN ADSORPTION AND ABSORPTION
Figure 2.3 : Illustration of Absorption and Adsorption
Adsorbant
Chemisorption
Physisorption
Absorption
Adsorption

2.4
TYPES OFADSORPTION
S.No. ABSORPTION ADSORPTION
1. It is a bulk phenomenon It is a surface phenomenon
2. It is a slow process It is a fast process
3. Substance uniformly distributed
throughout the surface
Higher concentration of molecular
species in the surface than in the bulk
4. Attainment of equilibrium takes
time
Equilibrium attained easily
5. Eg.: Ammonia adsorbed in
charcoal
Eg.: Ammonia adsorbed in water
d
e
rb
os
d
Physical Adsorption ChemicalAdsorption
Figure 2.4 : Illustration of Physical and Chemical Adsorption
e
dbr
os
Temperature
Temperature
Figure 2.5 : Amount of gas adsorbed vs temperature
Chemistry

Surface Chemistry and Catalysis
S.No Physical adsorption or
Physisorption
Chemical adsorption or Chemisorption
1. Caused by
vanderwaal’s forces
intermolecular Caused by chemical bond formation
2. Not specific in nature Highly specific in nature
3. Reversible in nature Irreversible in nature
4. Multimolecular layers are formed
on the adsorbed surface
Unimolecular layers are formmed on the
surface
5. Heaat of adsorption is less (20 to 40
KJ/mole)
Heat of adsorption is large
KJ/mole)
(80 to 240
6. No activation energy is required High activation energy is required
7. Depends on nature of gas. Easily
Liquefiable gases are adsorbed
easily
Depends
adsorbate
on nature of adsorbent and
8. Occurs at low temperature Increases with increase in temperature
9. Increase
adsorption
in pressure increase High pressure is favourable. Decrease in
pressure does not cause desorption
Adsorption of Gases on Solids
In adsorption ofgases onsolidsurface, the solidsurfaceiscalled the adsorbent andthe
gas adsorbediscalled adsorbate. Theextent ofadsorptiondepends on manyfactors.
Solid
Surface
M
Absorbed
Gas olecules
Solid
Figure 2.6 : Adsorption of gases on solids
2.5

2.6 Chemistry
2.2 FACTORS AFFECTING THE EXTENT OF ADSORPTION
(i) Nature ofAdsorbent
The adsorption depends on the type of adsorbents used. When the adsorbent is highly
porous the rate of adsorption increases. Activated carbon, metal oxides like aluminum oxide,
silicagel and clayare commonlyused adsorbents. The rate of adsorption can be increasedby
activation process. It helps in enhancingthe pores in the adsorbent Eg. charcoal adsorbs0.011
gms of CCl4 at 24°C and activated charcoal adsorbs1.48 gm of at 24°C.
Activation of adsorbent
Duringactivation,the adsorbentis heated insteam to about 1500°C.Heatingdrivesout
allimpurities andleads to a lagerfree surface for adsorption. It canbe done in3 givenways
 Bymakingthesurfaceofadsorbent rough.
 Byheating theadsorbent invacuumso that the water vapour present inpores leave
thosepores.
 Byincreasingthe surfacearea ofadsorbent
(ii) Surface area ofadsorbent
Increase in surface area of the adsorbent increases the adsorption of gases andthe
extent ofadsorptiondepends ontwo factors
 Greater thesurfaceareagreater theadsorption-Increaseinsurfaceareaincreases the
numberofadsorbing sites.
 Larger theporositygreater theadsorption-Finelydividedandhighlyporousmaterials
acts as good adsorbents.
Eg. Charcoal and silica gel (excellent adsorbents).
(iii) Nature of Gases
Theamountofgasadsorbedbyasoliddependsonthenatureofthegas.Easilyliquefiable
gases like HI, NH3, CI2, SO2 etc., are adsorbed more easily then the permanent gases like H2,
N2, and O2 etc. Physical adsorption is non-specificin nature, so any gas willbeadsorbed on the
surface under any given conditions of temperature and pressure. Chemisorption is specific in
nature so only those gases which forms chemical bonds will be adsorbed.

The nature of gas depends on two factors:
 Critical Temperature (maximum temperature above which the gas cannot be
liquefied). Liquefactions of gases depend on critical temperature. When the critical
temperature ismore the gaseswillbe liquefied andmore adsorption occurs.
 Van der Waal’s forces: Easilyliquefiable gases possess greater Vander Waal’s forces
than permanent gases, so theyare adsorbed morereadily.
(iv) Exothermic Nature
Heat of adsorption is definedas the energyliberated when 1 g mol of a gas is adsorbed
on a solidsurface. Increaseintemperatureincreasesthekineticenergyof the gas moleculesand
it resultsinmorenumberofcollisionsofgas moleculesovertheadsorbent surface.
(v) Pressure
When pressure is increased then the rate of adsorption increases initially.The extent of
adsorptionis expressedas x/m where‘x’is amount of adsorbate;‘m’is massof adsorbent when
the dynamicequilibrium is established between free gas and the adsorbed gas. But after some
time it reaches appoint where no more adsorption occurs and at this point adsorption is
independent ofpressure.
Figure 2.7 : Rate of adsorption
3. ADSORPTION OF SOLUTE FROMSOLUTIONS
The process of adsorption of solutes on solidsurface can take placefromsolutions.For
example the activated animal charcoal adsorbs colouring matter present in sugar solution and
clarifies the sugar solution. It also has the capacity to adsorb acetic acid and oxalic acid from
water therebyreducing the concentrationofacidsinwater.
Therearetwo(ormore)componentspresentina solutionnamelysoluteand solvent.The
solute maybe present inthe molecular or ionic form.The extent ofadsorption fromsolution
Low Pressure High

2.8 Chemistry
depends upon the concentration of the solute in the solution, and can be expressedby
the FreundlichIsotherm.
x  k c(1n)
m
(or) log
x
 log k 
1
log C
m n
where, x- is the mass of the solute adsorbed,
m -is the mass of the solid adsorbent,
c -is the concentration of the solute in the solution &
n -is a constant having value greater than one,
k -is the proportionality constant.
The value of k depends upon the nature of solid, its particle size, temperature, and the
natureof soluteand solventetc. It the graphisplot betweenx/m againstc whichgivesa straight
linewhichissimilarto Freundlichadsorption isotherm.
FACTORS AFFECTING ADSORPTIONOF SOLUTESFROM
SOLUTION
Nature ofadsorbent
Adsorptionofsolute fromsolutionishighlyspecific.Adsorptiondependsmainlyon nature
ofadsorbent.
Temperature
Adsorption from solution decreases with rise in temperature.
Concentration ofsolute
Adsorptionfromsolutiondecreasewithdecreaseinconcentrationofsolution.egcharcoal
adsorbs water from dilute KCl solution whereas charcoal adsorbs KCl from concentrated KCl
solution.
4. ADSORPTION ISOTHERM
Theprocessofadsorptionisusuallystudiedthroughgraphsknowasadsorptionisotherm.
It is the graph between the amounts of adsorbate (x) adsorbed on the surface of adsorbent (m)
andpressure (P)at constant temperature.

(or)
X/m
Adsorbent +Adsorbate Adsorption
p
x
1 
 
 k  pn
m
Adsorptionisothermshowstheamount ofmoleculesadsorbedon the solidsurfaceasa
functionoftheequilibriumpartialpressureat constant temperature.
Adsor tion Isotherm
Saturation Pressure
P PS
Figure 2.8 : Adsorption Isotherm
A plot of x/m vs P is plotted to obtain adsorption at constant temperature, Ps is called
the saturation pressure.
FREUNDLI H ADSORPTIONISOTHERMS
In 1909, Herbert Freundlich gave an expressionrepresentingthe isothermalvariationof
adsorptionofa quantityof gasadsorbedbyunitmassofsolidadsorbent withpressure.Freundlich
adsorption isotherm is an empiricalrelationbetween the concentration of a solute on thesurface
of an adsorbent to the concentration of the solute in the liquid with which it is in contact. The
isothermisgivenas:
m n
where x – is mass of adsorbate,
m – is mass of adsorbent,
P – istheequilibriumpressure ofadsorbate,
K & n – areconstants.
log
x
 log k 
1
log P
[ n > 1]
2.9

2.10 Chemistry
At low pressure, extent of adsorption is directly proportional to pressure:
x
 p1
m
At high pressure, extent of adsorption is independent of pressure:
x
 p0
m
At intermediate value of pressure, adsorption is directly proportional to pressure raised
to power 1
 n value is greater than 1.
n
x  p(1/ n)
m
To remove proportionality a proportionality constant ‘k’is used which is known as
adsorptionconstant andweget
1 
pn
m
The above equation is known as Freundlich Adsorption equation.
Plotting of Freundlich Adsorption Isotherm
1 
pn
m
Taking log both sides of equation, we get,
log
x
 log k 
1
log P
m n
Theequationiscomparablewithequationofstraightline,y =mx + c where,mrepresents
slope of the line and c represents intercept on y axis. Plotting a graph between log (x/m) and
logp, wewillget a straight linewithvalue ofslopeequal to 1/n andlogk as y-axisintercept.
x  k 
 
x  k 
 

log
Figure 2.10 : Equilibrium between tree molecule and adsorbed molecules
195 K
X
m
244 K
273 K
P
b
Q
X
m
a
1
Slope = n
log K (intercepted)
log p
Figure 2.9 : log (x/m) vs. log p graph
Limitation of Freundlich Adsorption Isotherm
1. Freundlichequation ispurelyempiricalandhasnotheoreticalbasis.
2. Theequation isvalidonlyupto acertain pressure andinvalidat higherpressure.
3. Theconstants k andn are nottemperature independent,theyvarywithtemperature.
4. Frendilich’sadsorptionisothermfailswhentheconcentrationofthe adsorbate isveryhigh.
LANGMUIR ADSORPTIONISOTHERM
In 1916, IrvingLangmuir proposed another adsorptionIsotherm whichexplained the
variationof adsorption withpressure
A
AB A
1 layer
st
Surface Surface

2.12 Chemistry
Assumptions of Langmuir Isotherm
Langmuir proposed his theory by making following assumptions.
(ii)
(iii)
(iv)
(i) Surfaceisenergeticallyuniform.Fixednumberofvacant oradsorptionsitesareavailable
on the surfaceofthesolid.
Allthe vacant sites are ofequalsizeandshape on the surface ofadsorbent.
Each sitecanholdmaximumofonegaseousmolecule anda constant amount ofheat
energy isreleased.
Heat ofadsorption isconstant throughout thesurfaceand it ranges from0 to 1.
Dynamicequilibriumexistsbetweenadsorbedgaseousmoleculesandthefreegaseous
molecules.
(v) Adsorptionismonolayeror unilayer.
Derivation
LangmuirEquationdepictstherelationshipbetweentheextentofadsorptionandpressure.
Langmuir proposed that dynamic equilibrium exists between adsorbed gaseous molecules and
thefreegaseousmolecules.Usingtheequilibriumequation,equilibriumconstantcanbecalculated.
Adsorption
A(g)B(s) □ AB
Desorption
where A(g )  is unadsorbed gaseous molecule
B(s)  is unoccupied metal surface and
AB  is adsorbed gaseous molecule
According to Kinetic theory,
Rate of forward reaction = Ka [A][B]
Rate of backward reaction = Kd[AB]
At equilibrium, Rate of forward reaction is equal to Rate of backward reaction
Ka [A] [B] = Kd [AB]
A new parameter ‘’ is introduced.

Let  bethenumberofsitesofthesurfacewhicharecoveredwithgaseousmoleculeand
(1–è) be the fraction of surface unoccupied by gaseous molecule. Rate of forward direction
depends upon two factors, number of sites available on the surface of adsorbent, (1 – ) and
pressure, P.
Rate of forward reaction  P (1  )
Rate of adsorption  P (1  ) or
Rate of adsorption  Ka P (1  )
Rateofbackwardreactionor rateofdesorption depends uponnumberofsitesoccupied
bythe gaseous molecules on the surface ofadsorbent.
Rate of desorption   (or)
Rate of desorption = Kd 
At equilibrium, rate of adsorption is equal to rate of desorption.
Ka P (1  )  Kd 
The above equation can be written in terms .
Ka P  Ka P  Kd 
Ka P  Ka P  Kd 
Ka P  ( Kd  Ka P)

Ka P
(Kd  Ka P)
Divide numerator and denominator on RHS by Kd , we get
Ka P
 
But K 
Ka
Kd
Kd
Kd 
Ka P
Kd Kd

2.14 Chemistry
KP
Substituting in the above equation we get
 
1  KP
This is known as Langmuir Adsorption Equation.
Alternate form of Langmuir Adsorption Equation
Langmuir adsorptionequationcanbe writteninanalternateformintermsofvolumeof
gas adsorbed. Let V be volume of gas adsorbed under given sets of conditions of temperature
and pressure and Vmono be the adsorbed volumeof gas at high pressure conditionsso as to cover
the surface with a unilayer of gaseous molecule.
 
V
Vmono
Substituting the value of  in Langmuir equation
V KP
Vmono 1  kP
(or) Vmono
1
1
kP
in terms of pressure P we get, Langmuir Adsorption Equation in alternate form.
Thus, if we plot a graph between P/V vs P, we will obtain a straight line with
Slope = 1/Vmono and intercept =1/ KVmono.
Limitations of Langmuir Adsorption Equation
(ii)
(iii)
(i) The adsorbed gas has to behave ideally in the vapour phase. Langmuir equation is
validunderlowpressure only.
LangmuirEquation assumes that adsorption is monolayer.But, monolayer formation
is possible only under low pressure condition. Under high pressure condition the
assumptionbreaks down as gas moleculesattract moreand moremoleculestowards
eachother.
Anotherassumptionisthesurfaceofsolidishomogeneousbutinrealsolidsurfacesis
heterogeneous.


5. CONTACT THEORY (OR) MECHANISM OF HETEROGENEOUS
CATALYSIS (OR) ADSORPTIONTHEORY
This theory postulated by Faraday in 1883. It explains the action of heterogeneous
catalysis. Heterogeneous catalysis has fivesteps.
(iv) Langmuir equationassumed that molecules do not interact with each other. This is
impossibleasweak force ofattractionexistseven between molecules ofsametype.
(v) Inadsorptionliquefactionofgasestakingplace,whichresultsindecreaseinrandomness
but thevalueisnotzero.
(ii)
(iii)
(iv)
Figure 2.11 : Mechanism of heterogeneous catalysis
(i) Diffusion of Reactant(s) to the Surface: The reactants diffuse to the surface of the
catalysts.Someof the reactant cross thebarrierand enterthe interiorexposedsurface
that includespathsandcracks onthe externalsurface.
Adsorption of reactants: Bonds are formed as the reactant(s) are adsorbed onto
the surface of the catalyst. The abilityfor an atom or moleculeto stick to the surface
isknownastheStickingCo-efficient.
Reaction: The reactants, when bound to the surface have a higher probability of
reactingwitheachother,andafterthereaction, theyformanintermediatecompound.
Desorptionofproducts: Theintermediatecompound gets desorbedfromthe surface,
whichagainbecomes availableforadsorptionfor othermolecules
(v) DiffusionofProduct(s):The intermediate compound thendisintegrates to formthe
finalproductsandthe productsarethendesorbed fromthe surfaceofthe catalyst.

2.16
For example,
1. Conversion of ethylene toethane
Ethylene
Ni surface
1.
2.
3.
Ethylene absorbed on
surface breaking  bonds
N2 (g)  N2 (adsorbed)
N2 (adsorbed)  2 N(adsorbed)
H2 (g)  H2 (adsorbed)
A ( g) B( s )
N2(g) 3H2(g)   2N3(g)
A.  A( ads )
B. ( g)  B( ads )
A( ads)B(ads)  r dsAB( ads)  f ast AB(g)
k  
A B
b P
Figure 2.12 : Conversion of ethylene to ethane
2. Haber–Bosch reaction
The original Haber–Bosch reaction chambers used osmium as the catalyst, less
expensive iron-based catalyst, which is still used today.
N2 (g)3H2 (g)    2 NH3 (g)
The reactionmechanism,involvingtheheterogeneous catalyst, isbelievedto involvethe
followingsteps:
Chemistry

4.
5.
H2 (adsorbed)  2 H (adsorbed)
N (adsorbed) + 3 H (adsorbed)  NH3 (adsorbed)
NH3 (adsorbed)  NH3 (g)6.
6. KINETICS OF SURFACE REACTION
Thekineticsofheterogeneously-catalyzedreactionsmightvarywiththepartialpressures
of the reactant gases above the catalyst surface which can be predicted by usingthe Langmuir
isotherm.
1. KINETICS OF UNIMOLECULAR DECOMPOSITION
Examples of unimolecular decomposition
1. Decomposition ofNH3 to N2 and H2onmetal surfaces,
2. DecompositionofPhosphineonglass,
3.Decomposition of Formic acid on glass, Pt, Ag, Au, or TiO2.
Consider thesurface decompositionofa moleculeA, i.e. the process
A(g)    A (adsorbed)    Pr oduct
Assumption
1. Thedecompositionreactionoccurs uniformlyacrossthesurfacesites.Molecule ‘A’maybe
adsorbed andisnot restricted to a limitednumberofspecific sites.
2. Theproducts are veryweaklybound to the surface and, theycanbe easilydeformed.
3. Therate determiningstep isthesurfacedecomposition step.
According to Langmuir adsorption isothermmolecule ‘A’adsorbed on the surface is in
equilibriumwiththe gas phase andthesurfaceconcentrationisrepresented as:
  b  P
(1bP)

2.18 Chemistry
k
Rate 
Therateofthe surfacedecompositionisgiven byanexpression:
Rate  k 
Substituting, θ , in the rate expression we get:
k b P
(1 b P)
The reaction is expressed within two limits:
a. Lowpressurelimit: bP 1(First orderreactionwitha firstorderconstant k=k b )
Rate  k  P
(1 bP)
Rate  K P  K
b
K
as b isconstant.So Rate □k b P
Under low pressure ‘è’ is very small and rate is directly proportional to pressure
b. High pressure limit: bP 1 (Zero order reaction)
Rate  k b  P
(1 bP)
Rate 
(1 bP)
Therefore
(1 + bP) ~ b P and
Under high pressure θ is almost unity.
Rate ~k

Rate ~ k
(zero order)
Rate
Rate ~ kbP
(first order)
Figure 2.13 : Graphical representation of unimolecular surface decomposition
6.2 KINETICS OF BIMOLECULAR REACTION(Langmuir-Hinshelwood)
Between molecularadsorbates.
Consider the reaction:
A(g)A(ads)
B(g)B(ads)
A(ads)  B(ads) rds
AB(ads) fast
AB(g)
Assumption:
Thesurface reactionbetweenthetwo adsorbedspeciesistheratedeterminingstep.The
rate ofthereaction ofthetwo adsorbed moleculesforbiomolecular surface willbegivenby:
Rate = k A B
According to Langmuir adsorption isotherm:
  b P
(1bP)
where two molecules (A & B) are competing for the same adsorption sites then
A 
bA PA
1 bAPA bBPB
and B 
bBPB
1 bAPA bBPB
Substituting these into the rate expression gives:
k bA PA bB PBRate  k A B 
1 b P  b P2
A A B B

2.20 Chemistry
Condition
1. Reactant Aand B in first order then
bA PA 1 and bB PB 1
A & B are very low.
Hence, Rate  k bAPAbBPB kPAPB
2. Firstorder inA,but negativefirst order inBthen
bA PA 1 bB PB
A  0, B 1 so Rate 
k bAPA
bBPB

k PA
PB
Figure 2.14 : Illustration of different types of Adsorption Isotherm
7. TYPES OF ADSORPTION ISOTHERM
Adsorption process is usually studied through graphs known as adsorptionisotherm.
Aftersaturation pressure Ps, adsorptiondoesnot occuranymore, asthere are limitednumbersof
vacancies onthe surface ofthe adsorbent.At highpressure whenallthe sitesare occupied and
further increaseinpressuredoesnot causeanydifferenceinadsorption process.At highpressure,
adsorption is independent of pressure. There are 5 different types of adsorption isotherms and
eachofthemhasspecificcharacteristics.
Monolayer
I
III II
Multilayer
IV
V
Condensation in
pores/capillaries

Type I Adsorption isotherm is for very small pores or microporous adsorbents.
Adsorption occursbyfillingof microporesandit mainlydepictsMonolayer adsorption.
Eg.AdsorptionofNitrogen or Hydrogen oncharcoal around 1800°C.
2
2HK2(Cg)Ol3H2Mn((Ogo2))S 22KCHl (33Og)
3r37oo0mCtemp.2
H2O2Noreaction2H2O2Ptblack
2H2O
RCOORHO2RCOOHROH
CH610O5n+Hn2O nC1H22O211
2CHO HnOnCHO
6 105n 2 122211
nCH122O211nHO2nCH122O211 VVma[Sx
][ES]K[E][xS]
0Km[S] 1(K2K)3
[Et][xS][E][xS]
[ES](K2K3)/K[1ES]([S]Km
X[S]
(V)Vmax[S]KmVax[S]1
(V)[S][S]2Vmax
1 K [S]
Rate(v)Vm[S]V[S]maxmax
[ES][Et]x[S]/([S]Km
[Et][S]
[ES]K
1[S]/Km
Km(K2K)3K1
[ES][E][S]/Km
C2HC5lACll3 CH6C52H5HCl
RCOORHOHor OH RCOOHROHester 2
2KMnO+45HC2O24+3H2SO42McaSntalyOst4+KS2O48HO2+10CO2
lA23O
AOFes23
N23H22HS2NH3
/////
2H2+O2
CO
tP2HO2
C2H5OHCH2ethenCe2O2S O2O2
Pt2SO3
CC2HH5OOHHCH+HC3ehtHCanoOHl+HO2CHO
themonolayer formationofthe adsorbed molec u lesare complete, multilayerformationstartstoc1a2nesug22ar112 6g1luco2se66fruc12tos6e
CHC3OOCH25H+CHC3OOH C2HO5H
ethylacetate orOHaceticacid ethanol
2NaS2O3O2Alcohol2NaS2O4 4NO+4
NH4N6HO
3 2 2 2
2NO24NH323N26HO2
+
NH2NO2HNO2HO2
NH2NO2OHHO2NHNO2CIntoemrmpleedixate
NHNO2N2OOH
Fe
□oM2NH3
2AsH32As3H2 catalyst
take place corresponding to the ‘sharp knee’of the isotherms.
Eg: Iron (Fe) catalyst and Nitrogen (N (g)) adsorbed at 1950°C on silica gel.f
TypeIII Adsorption Isotherm also shows large deviation from Langmuir model. This
isotherm explains the formation of multilayer. They are characterized principallyby heats of
adsorption whichare lessthantheadsorbate heat ofliquefaction.
X
m
Ps
P
Figure 2.15 : Type I
TypeIIAdsorptionisothermshowslargedeviationfromLangmuirmodelofadsorption.
They are most frequentlyencountered when adsorption occurs on nonporous powders or
macroporousadsorbentswithunrestrictedmonolayer-multilayeradsorption.
Theintermediateflat regionintheisothe rmcorresponds to monolayerformation.When
X
m
P
Figure 2.16 : TypeII
Ps

2.22 Chemistry
Eg: Bromine (Br2) at 790°C on silica gel or Iodine (I2) at 790°C on silica gel.

Type IV Adsorption Isotherm occur on porous adsorbents possessing pores in the
range of approximately 15-1000 angstroms (A). At lower pressure region of graph is quite
similartoTypeII. Thisexplainsformationofmonolayerfollowedbymultilayer.
The intermediate flat region in the isotherm corresponds to monolayer formation.
The saturation level reaches at a pressurebelow the saturation vapor pressure. This can
beexplainedon thebasisofapossibilityof gasesgettingcondensedinthetinycapillarypores of
adsorbent at pressure below the saturation pressure (PS) ofthe gas.
Eg.AdsorptionofBenzeneonIronOxide(Fe2O3)at500°Candadsorptionof Benzeneon silica
gel at500°C.
Figure 2.18 : Type IV
Type V Adsorption Isotherm results from small adsorbate-adsorbent interaction
potentials similarto the Type III isotherms.However, TypeV isotherms are also associated with
pores inthe samerange as those oftheTypeIV isotherms.
Eg: Adsorption of Water (vapors) at 1000°C on charcoal.
X
m
P
Figure 2.17 : TypeIII
Ps
X
m
P Ps

2.24
Figure 2.20 : Different types of adsorption isotherms
8. APPLICATION OFADSORPTION
Extensive application of adsorption is been listed below
(i) Production of high vacuum
In Dewarflasksactivated charcoalisplaced between thewallsofthe flasksothat gas
enteringinto theannular spacegets adsorbed.
(ii) In Gasmasks
Activated carbonisusedingasmasksto adsorb poisonousgases (e.g. oxideofsulphur,
NOx etc.) andpurifies air forbreathing.
Volume of
gasadsorbed
I
P0
X
m
II
P
III
P0
IV
P0
Pressure
Ps
V
P0
Figure 2.19 : Type V
TypeIIIandTypeV isotherms donot havethe ‘sharpknee’shapeimplyingstronger
adsorbate – adsorbate interactions thanadsorbate-adsorbent interaction.
Type IV and V shows phenomenon of capillary condensation of gas.
Chemistry
P0

Figure 2.21 : Adsorption of poisonous gases using activated charcoal
(iii) In desiccation ordehumidification
Certain substances can be used to remove water vapour or moisture present in theair.
Silica gel and alumina are used for dehumidification in electronic equipment.
(iv) In clarification ofsugar
Sugarisdecolorizedbytreatingsugarsolutionwithanimalcharcoalpowderwhichremoves
thecolourproducing substances.
(v) In paintindustry
Thepaintshouldnot containdissolvedgasesas it inhibitstheadherencecapacityofpaint
to the surface to be coated. The dissolved gases are therefore, removed bysuitable adsorbents.
Thisisdonebyaddingsuitableliquidswhich adsorbsthesefilms.Suchliquidsarecalledwetting
agents.Eg.Useofspiritaswettingagentinfurniturepainting..
(vi) Adsorption chromatography
Analytical method, in which molecules are separated according to their
adsorptive properties, where a mobile fluid phase is passed over an immobile solid adsorptive
stationaryphase.
(vii) In adsorptionindicators
Various dyes which possess adsorption property have been introduced as indicators
mainlyin precipitation titrations. For example KBr is titrated with AgNO3 using eosin as an
indicator
gas molecules
Activated Charcoal

2.26 Chemistry
(viii) HeterogeneousCatalysis
In heterogeneous catalytic reactions adsorption of gaseous reactants on solidcatalyst
occurs. The adsorption mechanism is responsiblefor the greater efficiencyof the catalyst inthe
finelydividedstateandhelpsusto understandthe actionofcatalyst promotersandpoisons.eg,
1. Finelypowdered nickelisused forthehydrogenation ofoils.
2. In manufactureofsulphuric acid finelydivided vanadiumpentaoxide (V2O5) isused inthe
contact process.
(i) diffusion of reactants tosurface;
(ii) adsorptionofreactants to surface;
(iii) reaction on thesurface;
(iv) desorptionofproducts from surface;
(v) diffusion of products away fromthe
surface.
Figure 2.22 : Process of Heterogeneous Catalysis
(ix) In adsorptionindicators
In manyprecipitationtitrations manydyes are used as indicators which work on the
principleofadsorption.
(x) In curingdiseases
Somepharmaceuticaldrugshavethecapacityto adsorbthegermsonthemandkillthem
andprotect usfromdiseases.
(xi) Lake test foraluminium
It is based on adsorption of litmus paper byAl(OH)3 precipitate
diffusion
adsorption
reaction
diffusion

(xii) Separation of inertgases
Due to the difference in degree of adsorption of gases by charcoal, a mixture of inert
gasescanbeseparated byadsorptionon coconut charcoalat different lowtemperatures.
(xiii) In softening of hardwater
The use of ion exchangers for softening of hard water is based upon the principle of
adsorption chromatography. The ion exchange resins helps to remove hardness causing ions
fromwater andmakeit usefulforindustrialanddomesticapplications.
(xiv) Arsenic Poisoning
Colloidal ferric hydroxide is administered which adsorbs arsenic and removes it from
bodybyvomiting
(xv) Formationof stable emulsions in cosmeticsand syrupsetc.
(xvi) Froth floatationmethod
Used for concentration of sulphide ores is based on adsorption.
(xvii) In cleaning action of soaps and detergents
Figure 2.23 : Cleaning actions of soaps and detergents
Fat or Oil Stain Add
Surfactant
Adsorbed Surfactant
lowers the interfacial
tension between the
fabric and the stain
fabric substrate fabric substrate
Mechanical
agitation
fabric substrate
Stain does not desorb
spontaneously
Mechanical
agitation
Fat or Oil
Globule
Fat or Oil
Globule
fabric substrate
Clean Fabric + adsorbed surfactant
prevents re-adsorption of fat globule
fabric substrate

2.28 Chemistry
(xviii) Applicationofadsorbentson pollutionabatement
o Manypollutants, both natural and synthetic, are gaseous in nature and it need to be
effectivelyremovedfromtheexhaust.
o Gaseous industrial pollutants include HCl, H2SO4, H2S, SOx, NOx, NH3, Ethylene,
Benzene,Ethanol,andHAP’s.Adsorptionisamasstransferprocessinwhicha porous
solidcomesincontactwitha liquidor gaseousstreamto selectivelyremovepollutants
or contaminatesbyadsorbingthemonto the solid.
o Themostcommonadsorbentsusedinindustryareactivatedcarbon,silicagel,activated
alumina(aluminaoxide),andzeolite.Activatedcarbonisthemostcommonnon-polar
adsorbent. Polaradsorbents havea great attractionto absorb moisture.
o Most industrial exhaust streams contain moisture the use of polar adsorbents is
significantly limited for air pollution control systems. There are two main types of
adsorption systems;fixedbedor continuous.
[1] Fixed Bed or Packed BedSystems
Thesearequitesimpledevices.Thefixedbedor packedbedreactorsaremostcommonly
used forstudyof solidcatalyst.Afixed bedreactorusuallyconsistsof a cylindricalvesselpacked
withtheadsorbentmaterial(eg.activatedcarbon)anditcontainsmoresurfaceareaforadsorption.
The contaminated or the polluted air enters the fixed bed system at the side, where there is an
exhaust distributor. The exhausted air exits the fixed bed adsorber clean of pollutants or
contaminates. Once the adsorbent is fullysaturated with adsorbate the system requires change-
out ofthespent materials,whichisthenpackedwithnewadsorbentmaterial.Thespentadsorbent
willbethermallycleaned.
Advantages
1. Idealplugflowbehavior
2. Lower maintenancecost
Disadvantage
1. Plugging ofbed due to cokedepositionwhichresultsinhighpressure drop.

Figure 2.25 : Schematic representation of fixed bed reactor
[2] Continuous Flow ReactorSystems
Continuous-flow reactors are almost always operated at steady state. These are more
complex and provide continuous operations. These systems provide in-situ desorption of the
adsorbates from the adsorbent. This can be done with superheated or saturated steam. The
adsorbate can be condensed, collected, and re-used inthe process. There are two adsorber
eChlorin
Laden
Gas
Scrubbing
Tower
Recirculating
CausticLiquor
Cooler
Recirculating
Pump
a
Blowdown
Stream and
Optional
Preheater
catalyst
on support
diffuser
reactants
Figure 2.24 : Fixed Bed Reactor Device
Vent
gas
Caustic M ke-up
products and
unreacted materials
to separation
“End of Pipe”
Fixed BedReactor
Liquid
Effluent
(to drain
orrecycle)
2.29

Input Rate
Figure 2.26 : Continuous Flow Reactor
[3] SpecialTypeofContinuousSystem(ZeoliteConcentrator,orRotary Concentrator)
Ahydrophobic zeolite is designed in a monolithic rotor in which the contaminated air
flows. An integrated thermal oxidizer is used to provide desorption of the solvents from the
zeolite.Theyhavewideacceptance inindustrialairpollutioncontrolapplications.
CHAPTER 3: Catalysis
A catalyst is defined as a substance which alters the rate of a chemical reaction,
itself remaining chemically unchanged at the end of the reaction. The process is
called catalysis.
1. INTRODUCTION
The word “catalyst” was introduced into science by the great Swedish chemist Jons
JakobBerzelius(1779-1848)whoalsodeterminedtheatomicandmolecularweightsofthousands
of substances,discoveredseveralelementsincludingselenium,first isolatedsiliconand titanium,
andcreated thepresent systemofwriting chemicalsymbolsandreactions.
Catalystsareofimmenseimportanceinchemistryandbiology.Allenzymesare catalysts
that expeditethe biochemicalreactions necessaryfor life.(eg) Enzymes insalivaacceleratethe
conversionofstarchto glucose.
1. DEFINITION
The process by which a substance speeds up a chemical reaction without being
consumed or altered in the process.
uO tput Rate
Liquid Surface
V
2.30 Chemistry
units inthe system.The gases are beingadsorbed inone unit as the other unit is beingdesorbed
with steam. The exhaust from the desorbed bed can be condensed for solvent reuse or other
beneficialpurpose.
Stirrer

2.30
A & B = Reactants
P = Product
Figure 3.1 : Catalysis Cycle
1. TERMS
1. Catalyst
A catalyst is a substance that decreases the activation energy of a chemical reaction
without itself being changed at the end of the chemical reaction
(or)
It is the small amount of substance which alters the velocity of reaction without
altering in any change in mass and composition at the end of the reaction.
2. Positive and negativecatalyst
Catalysthelpsin alteringthe velocityofthe reaction and thecatalyst is calledas positive
catalyst.eg.MnO2 acts as catalyst in decomposition of KClO3 into KCl and O2.Some catalyst
have the capacity to retard the chemical reaction and they are called as negative catalyst .eg
Alcoholretards theconversionofchloroformto phosgene.
3. AutoCatalysis
Iftheproductsofa reactionactasacatalystforthereaction,itisreferredto asautocatalysis
Eg. RCOOR H2O    RCOOH  ROH
A
Reactants
B
Catalyst
Bonding
B
x
A B
Catalyst
Catalysis
Cycle
Reaction
a
positive
feedback
P
egin
Ne t Cycle
Catalyst
P
C talyst
Separation
Product + Catalysts
Chemistry

Stepwise reaction on catalyst surface
Figure 3.2 : Mechanism of catalytic reaction
3. CRITERIA (OR) CHARACTERISTICS FOR CATALYST
(ii)
(i) A catalyst remains unchanged in mass and chemical composition at the end of the
reaction qualitative and quantitative analysis show that a catalyst undergoes no change
inmassofchemicalnature. However,itmayundergoa physicalchange.Thusgranular
manganese dioxide(MnO2) used as a catalyst in the thermaldecomposing of potassium
chlorate is left as a fine powder at the end to the reaction.
A small quantity of catalyst is generally need d to produc almost unlimited reaction
Sometimes a trace of a metal catalyst is required to affect very large amounts of
reactants. For example, oneten-millionthofitsmass offinelydivided platinumisall
that is needed to catalyse the decomposition of hydrogen peroxide.
On the other hand, there are catalysts which need to be present inrelatively large
amount to beeffective.ThusinFriedel-Craft reaction,
anhydrous aluminium chloride functions as a catalyst effectively when present to the
extent of 30 per cent of the mass of benzene. For the acid and alkaline hydrolysisof
anester
d
d
Adsorption of gas molecules on to catalyst
active sites. The molecules are chemically
bonde to the catalyst
Adsorption as separate atoms, individually
chemically bon ed to catalyst active sites
AlCl3
6 6 2 5 6 5 2 5C H  C H Cl     C H C H HCl
2. MECHANISM OF CATALYTIC REACTION
increasing energy
gas molecule
gas molecule
desorption of
product
adsorbed product
2.31

2.32 Chemistry
H2  O2      No reactionroom temp.
2H2  O2 P
t bla
ck
 2H2O
Thus it is now considered that the catalyst can initiate a reaction. According to this
view, the reacting molecules (in the absence of catalyst) do not possess minimum
kineticenergiesforsuccessfulcollisions.Themoleculesreboundfromcollisionwithout
reacting atall
Acatalystshouldremainunchangedinmassandchemicalcompositionduringendof
thereaction.
 or OH
RCOOR  H O  H
    RCOOH  ROH
ester
2
the rate of reaction is proportional to the concentration of the catalyst (H+ or OH–)
(iii) Acatalystis more effectivewhen finelydivided In heterogeneous catalysis,the solid
catalystismoreeffectivewhenina stateoffinesubdivisionthanitisusedinbulk.Thus
a lumpof platinum will have muchless catalytic activitythan colloidal or platinised
asbestos. Finelydivided nickelisa better catalyst thanlumpsofsolidnickel.
Acatalystis specificinits action Whilea particularcatalystworks for onereaction, it
willnot necessarilywork for anotherreaction.Differentcatalysts,moreover, can bring
about completelydifferent reactions for the samesubstance. For example, ethanol
(C2H5OH) gives ethene (C2H4) when passed over hot aluminium oxide,
C H OH  A
l2O
3
 CH C  O
(iv)
2 5 2 2 2
ethene
but with hot copper it gives ethanal (CH3CHO)
C H OH Cu
 CH CHO +H
(Dehydration)
2 5 3 2
ethanol
(Dehydrogenation)
(v) A catalyst cannot, in general, initiate a reaction In most cases a catalyst speeds up a
reactionalreadyinprogressand does not initiate(or start) the reaction.But thereare
certain reactions where the reactants do not combine for very long period (perhaps
years). For example, a mixture of hydrogen and oxygen, which remains unchanged
almost indefinitelyat roomtemperature, canbebrought to reaction bythe catalyst
platinum black in a few seconds.
(vi)

(vii) Catalystcanalteronlythespeedofthereactionbutitshouldnot affecttheequilibrium
ofthe reaction.
Catalysts are more active at its optimum temperature. Change of temperature alters
therateofa catalyticreactionasit woulddoforthe samereactionwithout a catalyst.
The catalytic activity can be altered by adding a small amount of foreign substance.
Suchsubstanceswhichcatalysethecatalystarecalledaspromotersandthesubstance
whichinhibitsthe reaction are called as catalytic poisons oranti-catalyst.
(viii)
(ix)
4. TYPES OFCATALYSIS
Catalytic reactions can be broadly divided into the following types,
HOMOGENEOUS CATALYSIS
Whenthe reactants and the catalyst areinthe samephase(i.e. solid,liquidor gas) it is
said to behomogeneous.
Examples of Homogenous Catalysis
1. The depletion of ozone (O3) in the ozone layer of the Earth’s atmosphere by chlorine free
radicals (Cl ) is a an example where the reactant and product exist in gaseous phase.Slow
breakdown of manmade chlorofluorohydrocarbons (CFCs),release chlorine free radical
into the atmosphere, which converts gaseous ozone to gaseous oxygen (O2).
2. Fischer esterification: Reaction of carboxylic acid with an alcohol involves the use of
sulfuricacidasthecatalystandisanexamplewhereeverythingiscontainedina liquidphase.
HETEROGENEOUS CATALYSIS
The catalytic process inwhichthe reactants and the catalyst are indifferent phases is
known as heterogeneouscatalysis.
Examples of Heterogeneous Catalysis
1. The catalytic converters in automobiles convert exhaust gasessuchas carbon monoxide (CO)
and nitrogen oxides (NOx) into more harmless gases like carbon dioxide (CO2) and
nitrogen (N2). Metals (solids) like platinum (Pt), palladium (Pd) and rhodium (Rh) are
used as the catalyst.

2 KClO
2.34 Chemistry
2. Manufacturing ofsulfuricacid (H2SO4)involve solid vanadiumpentoxide (V2O5)as the
catalyst to convert gaseous sulfurdioxide(SO2)intogaseous sulfurtrioxide (SO3).
3. Catalytic hydrogenation of liquid Unsaturated hydrocarbons (alkenes) reacts with
gaseous hydrogen (H2) to produce liquid saturated hydrocarbons (alkanes) where metals
like platinum (Pt)and palladium(Pd)asthecatalyst..
4. HaberProcess
H2 (g)3H2 (g)    2H3 (g)
Thecatalyst isporousironprepared byreducing magnetite,Fe3O4,withpotassiumhydroxide
(KOH) added as a promoter.
Positive Catalysis: When the rate of the reaction is accelerated bythe foreign substance, it is
said to be a positive catalyst and phenomenon as positive catalysis. Examples of positive
catalysis aregivenbelow.
(i) Decomposition ofKClO3
MnO2 (S)
o
370 C
3     2 KCl 3O 2
Negative Catalysis: There are certain, substance which, when added to the reaction mixture,
ret ard the reaction rat e instead of increasing it. These are called negative
catalyst or inhibitors and the phenomenon isknownas negative catalysis.
Some examples are as follows.
(i) Oxidationofsodiumsulphite
2 Na2SO3  O2     2 Na SOAlcohol
2 4
Tetra EthylLead (TEL) isaddedto petrolto retardthe ignitionofpetrolvapours on
compressioninaninternalcombustionengineandthusminimizethe knockingeffect.
(ii)

CH3COOC2 H5  H2O  CH3COO  C2 H5OH
catalyst
(2) OxidationofOxalicacid.Whenoxalicacidisoxidisedbyacidifiedpotassiumpermanganate,
manganous sulphateproduced duringthereactionacts as a catalyst forthe reaction.
2 KMnO4 + 5H2C2O4 + 3H2SO4    2 MnSO4 + K2SO4  8 H2O +10 CO2
catalyst
(3) DecompositionofArsine.Thefreearsenicproduced bythedecompositionofarsine(AsH3)
autocatalyses thereaction.
   2As
catalyst
2 AsH3  3H2
5. AUTOCATALYSIS
When one of the products of reaction itself acts as a catalyst for that reaction
the phenomenon is called Autocatalysis.
In autocatalysis the initial rate of the reaction rises as the catalytic product is formed,
insteadof decreasingsteadily(Figure). The curveplotted between reaction rate andtimeshows
a maximumwhenthereaction iscomplete
Completion of reaction
Sigmoid Curve
Time
Figure 3.3 : Rate of autocatalytic reaction
Achemical reaction is said to haveundergone autocatalysis, or be autocatalytic, ifthe
reaction product isitselfthecatalyst forthat reaction.
Examples of Autocatalysis
(1) Hydrolysisof an Ester.The hydrolysis of ethyl acetate forms acetic acid (CH3COOH)
andethanol. Ofthese products, acetic acidacts as a catalyst for the reaction.
Percentagereaction

2.36 Chemistry
6. CATALYTIC POISONING ANDPROMOTERS
Promoters
The activity of a catalyst can often be increased by addition of a small quantity of a
second material.This second substanceis eithernot a catalyst itselffor the reaction or it maybe
a feeblecatalyst.
A substance which, though itself not a catalyst, promotes the activity of a catalyst
is called a promoter. They are substances when added in small concentration can increase the
activityofa catalyst.
Example of Promoters
Molybdenum(Mo) or aluminiumoxide (Al2O3)promotes the activityof iron catalyst in
theHabersynthesisforthemanufactureofammonia.
Fe
N2  3H2 □ 2NH3
Mo
Insomereactions,mixturesofcatalystsareusedto obtainthemaximumcatalyticefficiency.
For example, in the synthesis of methanol (CH3OH) from carbon monoxide and hydrogen, a
mixtureofzincand chromiumoxideisusedas a catalyst.
CO + 2H  Z
nO
CH OH
Explanation of PromotionAction
The theory of promotion of a catalyst is not clearly understood. Presumably:
(1) Change of Lattice Spacing. The lattice spacingof the catalyst is changed thus enhancing
the spaces between the catalyst particles.Theabsorbed moleculesof the reactant (say H2)
are further weakened andcleaved.Thismakesare reaction go faster.
(2) Increase of Peaks and Cracks. The presence of the promoter increases the peaks and
cracks on the catalyst surface. This increases the concentration of the reactant molecules
andhence therate ofreaction.
The phenomenon of promotion is a common feature of heterogeneous catalysis.
Cr2O3
2 3

Distance between
catalyst particles
Figure 3.4 : How the change of crystal lattice spacing of catalyst makes the reaction go faster.
Covalent bond
much weakened
and cleavesreadily
CATALYTIC POISONS
Smallamountsofsubstancescanreducetheactivityofcatalyst.Ifthereductioninactivity
is reversible, the substances are called inhibitors. Inhibitors are sometimes used to increasethe
selectivityofa catalyst byretardingundesirablereactions.
A substance which destroys the activity of the catalyst to accelerate a reaction is
called a poison and the process is called Catalytic Poisoning.
Examples of Catalytic Poisoning
(1) Theplatinumcatalystusedintheoxidationofsulphurdioxide(Contact Process), ispoisoned
by arsenicoxide (As2O3)
SO O2 2
 P
t

As2O3
2SO3
(2) The ironcatalyst used inthe synthesisofammonia(Haber Process) ispoisoned byH2S.
N2  3H2
 F
e

H2S
2NH3
(3) Theplatinumcatalyst usedinthe oxidationofhydrogenispoisonedbycarbon monoxide
2H +O P
t
 2H O
2 2
CO
2

2.38 Chemistry
Explanation of Catalytic Poisoning
(1) The poison is adsorbed on the catalyst surface in preference to the reactants. Even a
monomolecular layer renders the surface unavailable for further adsorption of the
reactants.Thepoisoning byAs2O3 or CO appears to beof thiskind.
O O O
C C C
Types of Catalytic Poison
(i) TemporaryPoisoning
Catalyst regains its activity when the poison is removed from the reaction
(ii) PermanentPoisoning
a. Catalyst cannotregainitsactivityevenifthe catalyticpoisonisremoved.
b. Eg.AS2O3 poisonscatalyst Pt permanentlyinmanufacturingofSO3.
Figure 3.5 : Poisoning of platinum catalyst by carbon monoxide
(2) Thecatalystmaycombinechemicallywiththeimpurity.Thepoisoningofironcatalyst
byH2S fallsinthisclass
Fe  H2S    FeS +H2
7. ACID AND BASE CATALYSIS
Anumber of homogeneous catalytic reactions are known which are catalysed by
acids orbases, orboth acids and bases.Theseare often referred to asAcid-Base catalysts.
Arrhenius pointed out that acid catalysis was, in fact, brought about by H+ ions supplied by
strong acids, whilebase catalysis was caused byOH– ionssuppliedbystrong bases.

(2) Keto-Enol Tautomerism ofAcetone:
(3) DecompositionofNitramide:
H+
NH2 NO2    N2O  H2O
(4) HydrolysisofanEster :
MechanismofAcid-BaseCatalysis
(a) In acid catalysis, the H+ (or a proton donated by Bronsted acid) forms an intermediate
complex with the reactant, which then reacts to give back the proton. For example, the
mechanismofketo-enoltautomerismofacetone is:
12 22 11 2 6 12 6 6 12 6
cane sugar glucose fructose
Many reactions are catalyzed byboth acids and bases. Typical reactions catalysed by
proton transfer are esterification and aldol reaction. Catalysis by either acid or base can be in
two different ways(Specificcatalysisandgeneralcatalysis).
Example
 Acid-specific(acid catalysis)—Decompositionof Sucroseintoglucoseandfructose
occurs inpresenceofsulfuricac d
 Base-specific (base catalysis) —Addition of hydrogen cyanide to aldehydes and
ketones inthepresence ofsodiumhydroxide
Examples of Acid-Base Catalysis
(1) Inversion of CaneSugar:
C H O  H  H
+
 C H O C H O
CH COOC H
3
ethylacetate
2 5 or OH 3
aceticacid
+
 H
 CH COOH  C HOH
2 5
ethanol

2.40
(b) In base catalysis, the OH– ion (or any Bronsted base) accepts a proton from the reactant
to form an intermediate complex which then reacts or decomposes to regenerate the OH–
(or Bronsted base). For example, the decomposition of nitramide by OH– ions and
3
CH COO– ionsmaybeexplained as follows: By OH– ions:
NH NO  OH
   H O  NHNO
2 2 2 2
Intermediate
Complex
NHNO
   N O  OH
2 2
8. APPLICATION OFCATALYSIS
1. Catalytic Converters
Adeviceincorporated inthe exhaustsystemof a motor vehicle,containinga catalystfor
convertingpollutant gasesintolessharmfulones.
Catalytic converters are used for mitigating automobile exhaust emissions. Catalytic
convertersareusedwithininternalcombustionenginesfueledbyeither petrol (gasoline)or diesel.
It converts threeharmfulsubstancesinto harmlessones carbonmonoxide(a poisonousgas)into
carbon dioxide, nitrogen oxides (cause acid rain and smog) into nitrogen and oxygen, and
hydrocarbons (cause smogandrespiratoryproblems)into carbondioxide andwater.
Catalyticconvertersconsistofa stainlesssteelbox attachedto themufflerandcontaining
ceramicbeads or honeycombcoated with catalysts (Expensivemetals likeplatinum,Palladium,
Rhodiumare used.Alumina,Ceria canalsobeused whencombinedwithexpensive metals.)
Eg: Consider the reaction
2 CO + 2     2 CO2  N2
Carbon monoxide and Nitrogen monoxide wil be adsorbed on the surface of catalyst
wheretheyget adsorbedto catalystandresult informationofcarbondioxideandnitrogenwhich
gets desorbed.
Catalytic converters can be affected by catalytic poison.
Chemistry

Process
Exhaust
Eg. Lead is a verygood examplefor catalyticpoisoning. It gets adsorbed to the honeycombof
expensive metals and inhibits the function of catalyst. Catalytic converter has also forced the
removalofleadfrompetrol.
Catalyst
N2
O2
H2ONOx
O2
Exhaust to
Atmosphere
NH3
4 NO + 4 NH3  2    4 N2  6H2O
2 NO2  4 NH3  2    3 N2  6H2O
Figure 3.6 : Basic Catalytic Converter
2. PetroleumRefining
(i)
(ii)
(iii)
Fluid catalytic cracking: Breaking large hydrocarbon into smallerhydrocarbons.
Catalyticreforming:Reformingcrudeoiltoproducehighqualitygasoline component
Hydrodesulfurization: Removing sulfur compounds from refinery intermediate
products
(iv) Hydrocracking:Breakinglarge hydrocarbon moleculesinto smallerones
(v) Alkylation: Converting isobutane and butylenes into a high-quality gasoline
component
(vi) Isomerization:Converting pentane into ahigh-qualitygasolinecomponent
3. Chemicals andpetrochemicals
(i)
(ii)
(iii)
Haber process for ammoniaproduction
Styrene and Butadiene synthesisforuseinproducing syntheticrubber
Contact process forproductionof sulfuricacid
(iv) Ostwald process forproduction of nitricacid

2.42 Chemistry
2C6H10O5   nH2O
n
(v) Methanolsynthesis
(vi) Production ofdifferentplasticsandsyntheticfabrics
4. Other
(i) Fischer-Tropsch and Coalgasification processes for producing synthetic fuelgases
and liquidfuels
(ii) Variousprocessesforproducingmanydifferentmedicine
9. BIOLOGICAL CATALYST——ENZYMES
Numerousorganicreactionsaretakingplaceinthebodyofanimalsandplantsto maintain
the life process. These reactions being slow remarkably catalysed by the organic compounds
known as Enzymes. All enzymes have been found to be complex protein molecules. Thus:
Enzymesareproteinmoleculeswhichact as catalyststo speeduporganicreactionsinlivingcells.
Thecatalysisbrought about byenzymes isknownas EnzymeCatalysis.
Eachenzymeisproducedina particularlivingcellto catalysea reactionoccurringinthat
cell.Manyenzymeshavebeen identifiedand obtained in pure crystallinestate fromthe cellsto
whichtheybelong. Howeverthefirstenzymeasprepared bysynthesisinthe laboratoryin1969.
Enzymes are substances found in biological systems that act as catalyst for specific
biochemical process. Enzymes are usuallyprotein or steroid which is synthesized in the living
cells of animals and plants. Enzymes catalyze reactions inside organism. Enzymes possess a
incrediblecapacityto carryout complexchemicalreactionslikehydrolysis,oxidation,reduction
etc.
Eg.(i) Amylase isanenzyme whichbreaks downstarch intoglucose.
C6H10O5  + nH2O
n
   nC12H22O11
(ii) Diastaseconvertsstarchto maltoseandmaltase convertsmaltoseto glucose
   nC12 H22O11
nC12H22O11  nH2O    nC12 H22O11

Figure 3.7 : Rate of free energy vs course of reaction
Enzymes do not
 Changetheequilibriumconstantforareaction
 ChangeΔGfor a reaction
 Convert anonspontaneous reactionintoa spontaneousreaction.
KINETICS OF ENZYME CATALYSED REACTION OR MICHAELIS
AND MENTENEQUATION
Consider the enzyme catalyzed reaction:
Freeenergy
barrier
reactants
E2
G
course of reaction
products
energy
barrier
reactants
Catalyst
Reaction
E2
G
products
course ofreaction
Enzymesspeedupreactionsbyloweringactivationenergy. Manyenzymeschangeshape
when substrates bind. This is termed as “induced fit”. Enzymes have active sites. The enzyme
activesiteis the locationon the enzymesurfacewheresubstrates bind,and where the chemical
reactioncatalyzed
Uncatalyst
Reaction
energy
2.43

2.44
Step 1: Formation of enzyme substrate complex
Step 2: Decomposition of enzyme substrate complex
where the terms V0, Vmax and [S] and there is a constant Km, which is known as Michaelis
constant.
Rate of formation of ES = k1 [E]×[S]
Rate of breakdown of ES = (k2 + k3) × [ES]
At steadystate, the formationand the breakdownare equal. Thissteadystatewould
onlybe temporary.
k1 [E]×[S] = (k2 + k3) × [ES]
Rearranging the above equation :
(or)
m 
0
K
1
But K (K2  K3 ) and substituting in above equation and we get
m
It can be expressed as follows:
V  Vmax [S]
K [S]
[ES]  K [E] x[S]
1
(K  K )
[ES] 
[E] x[S]
(K2  K3 ) / K1
[ES]  [E] [S] / Km
2 3
Chemistry

T
The total amount of enzyme equals the free and that bound to substrate
[ET] = [E] + [ES]
[E] = [E ]  [ES]
Substitute the value of E
[Et][S]
Then [ES] 
K
1[S] / Km
This simplifies to:
[ES]  [Et] x[S] / ([S] Km 
Multiplyingboth sidesbythe kineticconstant k3 givesthevelocityofthereaction
V = k3 × [ES] = k3×[ET] ×(([S] ([S] + KM )
and substituting Vmax for k3×[ET] leads to the familiar form of the Michaelis Menten Equation:
[ES] 
[E]x[S]
([S]  Km
The above equation is calledMichaelis –Menton equation.This equationis applicable
to enzymecatalysedreactionhavinga singlesubstrate.Aquantitative estimationof initialrateof
reaction, maximumvelocity and substrate concentration is combined through a constant called
Michaelisconstant.
Case 1: First order reaction-When concentration of substrate is low.
If Km >> S then s is neglected and then the equation becomes
Rate (v) = Vmax ×[S] | (KM)
Case 2: Zero order reaction-When concentration of substrate is high.
V V
X[S]
max
[S] K

2.46 Chemistry
0.10
0.05
1  Km
 [S]
Rate(v) Vmax[S] Vmax[S]
Rearranging the above equation:
1  Km
 [S]
Rate(v) Vmax[S] Vmax[S]
Theaboveequationissimilarto anequationofstraight liney=mx+ c.Agraphisplotted
between 1/rate and1/[S]we get a straight line.
0.15
If Km << S then Kmisneglected and equationbecomes
Rate (v) = Vmax ×[S] | [S] so Vmax =Constant
Case 3: If Km = [S] then
Rate (V)  Vmax [S]  1 V
[S][S] 2
Reciprocating the above equation
max
0.1 0.0 0.2 0.30.1
1/[s]
Figure3.8
where slope = Km /Vmax and Intercept =1/Vmax
1/V

FACTORS AFFECTING ENZYME ACTIVITY
a. EnzymeConcentration
 If we keepthe concentration of the substrate constant andincreasethe concentration
of the enzyme,the rate of reaction increases linearly.(That is if the concentrationof
enzymeisdoubled,the rate doubles.)
 Thisisbecauseinpracticallyallenzymereactionsthemolarconcentrationoftheenzyme
isalmost alwayslower thanthatofthe substrate.
b. SubstrateConcentration
 If we keep the concentration of the enzyme constant and increasethe concentration
ofthesubstrate,initially,therateincreaseswithsubstrateconcentration,butat a certain
concentration, therate levelsout andremainsconstant.
 So at somepoint, increasingthe substrate concentration does not increasethe rateof
reaction, because the excess substrate cannot find any active sites to attach to.
c. Temperature
 For enzyme-catalyzed reactions, like all chemical reactions, rate increases with
temperature. However, enzymes are proteins, and at higher temperatures proteins
become denatured andinactive. Thus,everyenzymehasanoptimumtemperature.
 Optimum temperature - the temperature at which enzymeactivityis highest-usually
about 25oC40oC.
d. Effect ofpH
 SmallchangesinpHcanresultinenzymedenaturationand lossofcatalyticactivity.
 Because the charge on acidic and basic amino acid residues located at the active site
dependson pH.Most enzymesonlyexhibitmaximumactivityover a verynarrowpH
range.
 Most enzymeshave an optimumpH that fallswithin the physiological rangeof7.0-
7.5.
 Notable exceptionsarethe digestive enzymespepsinandtrysin.
 pepsin(active inthe stomach) - optimumpH of1.5
 trypsin(active inthe smallintestine) - optimumpHof8.0
  

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