Surfacechemistry2009-10

8/8/2019 Surfacechemistry2009-10

1/54

Surface chemistry

SSNCE Dr.S.L


2/54

As we approach smaller dimensions, surface effects become muchmore dominant

Large particlesmall particle

High surface ratio

volume

Molecules (atoms) at a surface experience a net attractive forcetowards the interior

-This unbalanced force manifests itself

as surface tension in liquids


3/54

Rain drops - spherical

Systems attempt to minimize Surface to

Volume ratio


4/54

The unbalanced force on the surface of a solid results inADSORPTION

This force on the surface has tendency to attract and retain

molecules of other species

Residual force on the surface decreases after

adsorption


5/54

Absorption : involves passing of the substances

through the surface into the solid /liquid substance


6/54

Adsorption is the first step in the viral infection cycle.


7/54

Why is Adsorption Useful?

Used in many industrial processes: dehumidification

odour/colour/taste removal

gas pollutant removal (H2S)

water softening and deionisation

hydrocarbon fractionation

pharmaceutical purification


8/54

Terms involved

Absorbate

A substance that becomes absorbed into another

material, or absorbent. Absorbent

The substrate into which a substance is absorbed.

Absorption

The increase in quantity (transfer) of one materialinto another or of material from one phase into anotherphase.

AdsorbateA substance that becomes adsorbed at the interface orinto the interfacial layer of another material, oradsorbent.

Adsorbent


9/54

Sorption A process in which both of the processes of adsorption and

absorption takes place simultaneouslyDesorption release of adsorbed or absorbed substances into

surrounding medium

Sorbate A substance that becomes sorbed into an interface or another material

or both.

Sorbent

The substrate into which or onto which a substance is sorbed or both.

Monolayer adsorption Adsorption in which a first or only a layer of molecules becomes

adsorbed at an interface. In monolayer adsorption, all of the adsorbed molecules will be in

contact with the surface of the adsorbent. The adsorbed layer istermed a monolayer or monomolecular film


10/54

Difference between adsorption and

absorption Adsorption is a surface phenomenon due to surface tension

Absorption is a bulk phenomenon in which the substanceassimilated is uniformly distributed throughout the body of the solid

or liquid- due to capillary action

Adsorption is a fast process as compared to absorption, which is aslow process, because it involves the diffusion into the interior ofthe matter

Adsorption - Equilibrium between adsorbate & adsorbent is attainedrapidlyAbsorption uniformly distributed throughout the body & occursslowly

Adsorption depends on the surface area of the adsorbentAbsorption surface area independent


11/54

Adsorbent Materials

Activated Carbon Activated Alumina

Silica Gel

Molecular Sieves (zeolite)

Polar and Non-polar adsorbents


12/54

Characteristics of Adsorption

Spontaneous

Exothermic H = negative

Decrease in entropy & free energy

S = negative, G = negative

Occurs on the surface, specific & selective.

Rate of adsorption is T dependent


13/54

How do molecules bond the surface?

Two principal modes of adsorption of molecules onsurfaces

- based on distinction in the nature of the bondingbetween the molecule and the surface

Physical Adsorption ( Physisorption )

Chemical Adsorption ( Chemisorption )


14/54

Physisorption

Bonding is by weak Van der Waals -type forces / dipole forces.There is no significant redistribution ofelectron density in either the moleculeor at the substrate surface.

Less energy is released = enthalpy ofcondensation

Physisorbed molecule retains identity

Reversible process

Eg Adsorption of Hydrogen/oxygen oncharcoal

Physisorption can be multilayer


15/54

Chemisorption

Chemical bonding (using free valencies) occurs betweenadsorbent(surface) & adsorbate (between ionic & covalent bond)

Results in formation of surface compound of single layer

Irreversible

Eg : Adsorption of hydrogen on nickel

High heat of adsorption : - 200 KJ/mol

Activation energy is involved

Adsorbed molecule loses identity


16/54

ADSORPTION OF GASES ON SOLIDS (OCCLUSION)

Factors

(1) Nature of the gas

Easily liquefiable gases (like HCl, NH3, Cl2, etc.) areadsorbed more easily than the permanent gases (like

H2, N2, O2, etc.). The higher the critical temperature (Tc), the more easily

the gas is liquefied - more readily it is adsorbed


17/54

Gas SO2 NH3 CO2 CH4 CO N2 H2

Tc (K) 430 406 304 190 134 126 33

mL of gas 380 180 48 16.2 9.3 8.0 4.5

adsorbed

Ease of liquefaction and adsorption decreases --------------->

Adsorption of various gases by 1 g of activated charcoal


18/54

Nature of adsorbent

Different materials possess different extent of adsorption

greater the surface area of the adsorbent, greater is itsadsorption capacity

activated charcoal and silica gel excellent adsorbents high porosity & surface area

Surface activated to increase porosity


19/54

Methods of surface activation

Creation of rough surface by rubbing, by deposition

Dividing solid adsorbent to finer particles

By heating in superheated steam opens pores


20/54

Heat of adsorption

Physical adsorption heat evolved is low

Chemical adsorption heat evolved is high

Reversible character

Physical adsorption reversible adsorbate desorbed when Tis increased

Chemical adsorption irreversible chemical compound isformed at surface


21/54

Effect of Temperature

Adsorption isobar -

Temperature

surface

coverage

Temperature

surface

coverage

Physisorption

H -negativechemisorption


22/54

Effect of T -

Pressure

Vol.adsorbed

T1

T2 >T1

T3 >T2

T4 >T3

T5 >T4

Adsorption Isobar


23/54

Effect of P

Extent of adsorption (x/m) dependent on P Adsorptionisotherm

x/m

P

ab

Ps

Adsorption of a gas on a solid in a

closed vessel a reversible process

Free gas gas adsorbed on solid

Amount of gas adsorbed on a solid

depends on equilibrium P

From the graph it is clear that the extent ofadsorption (x/m) increases with increasingpressure (P) and becomes maximum at P

s,

called the saturation pressure.

x/m = KP 1/n


24/54

At low pressure, the graph is almost straight line (point a), thereby

indicating

x/m P or x/m = KP

At high pressure, the graph (beyond b) becomes independent of P

(parallel to X-axis,) thereby indicatingx/m = constant or x/m P0 or x/m = K .. (ii)

Point b saturation state

At intermediate pressure, x/m depends on 0 to 1 power of pressure(i.e., fractional power of pressure).

x/m P1/n or x/m = KP1/n Freundlichs adsorption isotherm


25/54

Log x/m = log K + 1/n log P

log x/m is plotted against log P, a straight line would be obtained

Slope=1/n

intercept+=log Klo

gx/m

log P

Deviates at high P


26/54


27/54

Langmuir Adsorption Isotherm

Theoretical Explanation forunimolecular layer formation

during physical & chemicaladsorption

Verifies Freundlich Isotherm

LimitationsFails at very high P owing tomultimolecular layer formation& condensation of gasmolecules in the pores of the

adsorbate-


28/54

Types of adsorption Isotherm

Ps refers to saturationpressure of the gas

Type I : Langmuir Isotherm:Monolayer formation

Eg: Adsorption of N2/ H2 oncharcoal at

180 C


29/54

Type II & III: Multi layerphysical adsorption onnon porous materials

The amount ofadsorption increases withincrease in pressure.

Additional layer formationdue to the extension ofvander waals force.

Adsorption of N2 on Pt at -195 C

Adsorption of Br2 on silica at 80 C


30/54

Type IV & V :

Formation of multimolecularlayer& also condensationof gas molecules withinnarrow capillary pores ofthe adsorbent

Adsorption of benzene

on silica gel at 50C

Adsorption of H2O vapour

on activated Carbon at 100C


31/54

Adsorption of Solutes from solution

Solid substances adsorb dissolved substances

(solutes) from solutionsActivated animal charcoal adsorbs colouring matter

present in sugar solution making the latter colourless.

An adsorbent adsorbs certain solute from solutionin preference to other solutes.

Charcoal adsorbs non-electrolytes more readily

whileAlumina adsorbs electrolytes preferentially from

solution


32/54

Types of adsorption of solutes from

solution

Positive adsorption

Negative adsorption


33/54

Applications of adsorption

In heterogenous catalysis : Contact Theory

Hydrogenation of Alkenes to Alkanes

Mechanism

Step 1. Hydrogen molecules react with the metal atoms at thecatalyst surface.

The relatively strong H-H sigma bond is broken and replaced

with two weak metal-H bonds.


34/54

Step 1 hydrogen first physisorbed& then becomes chemisorbed)


35/54

Step 2

pi bond of the alkeneinteracts with the metalcatalyst weakening thebond.

A hydrogen atom istransferred from the catalystsurface to one of the

carbons of the double bond.


36/54

Step 3

The pi bond of the alkeneinteracts with the metalcatalyst weakening the

bond.

A second hydrogen atom istransferred from the catalyst

surface forming the alkane


37/54

Step 4

The alkane is released fromthe catalyst's surfaceallowing the catalyst toaccept additional hydrogen

and alkene molecules.


38/54

Finely divided catalyst

& Rough catalystsurface possess high

Activity

Catalytic poisons &

inhibitors reduce the

activity of catalyst


39/54

Promotersincrease the rate of the reaction


40/54

In Adsorption Chromatography

closely related substances

with almost similar andphysical and chemicalproperties which cannot beseparated from one another

by ordinary means areadsorbed to differentextents on the surface ofadsorbent.

This facilitates separationand purification ofcomponents in a mixture.


41/54

Method

Stationary phase : Some common adsorbents

used are : aluminium oxide, silica gel, MgO, MgCO3,CaCO3, Charcoal, cellulose, Fullers earth.

Mobile Phase : Solvents used are: benzene,cyclohexane, chloroform, ethylalcohol, water , CCl4,

pyridine etc.


42/54

Step 1 :Separation of mixture into

different components

A number of horizontal bands of differentcolors are produced in the column.

Only a partial separation of various

constituents


43/54

Different stages of column

chromatography


44/54

Step 2 : Development of chromatogram

To improve the separation of constituents in amixture

Using either the original or some othersuitable solvent slowly through the column.

The solvent used is called Eluent


45/54

Step 3 : Isolation & Estimation of components

Colored compounds produce colored rings, zones or bands

Colorless compounds are observed by either exposing UVlight on the column or by spraying suitable chemical reagent

in order to make them colored.

Colored components are dissolved separately in suitablesolvents and estimated quantitatively


46/54

Applications of column chromatography:

Quantitative separation of two or more components

of a mixture (separation of metal ions, amino acids,

proteins)

Purification of substances from their contaminants

Concentration of solutes from dilute solutions

Identification of products.(eg : isomers of organic

compounds


47/54

Role of adsorption in softening of water

Demineralisation process

Electrical demineralisation process Zeolite process


48/54

Deionisation of water

El i l d i li i f


49/54

Electrical demineralisation of water

Ion exchange resins are supported on a paper or a

fibre Used as membranes, ion-selective membranes

Cation selective membrane permits only cations not

anions

Anion selective membrane permits only anion

The rate of diffusion of ion is increased by passing

electric current through the electrode placed near

the membrane

El i l d i li i f


50/54

Electrical demineralisation of water

-

+

+

+

+

+

+

--

-

-

-

-

Cation exchangemembrane

anion exchangemembraneSoft water

Harde water

Z li (h d d di l i


51/54

Zeolite process (hydrated sodium alumino

silicates Na2O. Al2O3. xSiO2. yH2O)h


52/54

zeolite

Water softening processZeolite regeneration process

Spent zeolite

Zeolites insoluble in water & are cation Exchangers


53/54

Zeolites insoluble in water & are cation Exchangers

Ca(HCO3)2 + Na2 Z 2NaHCO3 + CaZ

Regeneration CaZ + NaCl Na2Z + CaCl2

Disadvantages

Cannot be used for turbid water clogs the bed Cannot be used for highly acidic or alkaline water as it attacks the bed Water should be free from iron / manganese salts. When adsorbed in

the bed , they cannot be regenerated Hot water should not be used

Advantages - removes hardness completely - occupying small space

S r


54/54

Summary

Adsorptionphenomenon of concentration of a gas/ liquid (adsorbate) at thesurface of a solid (adsorbent) with which it is in contact.

Adsorption on a surface can be through vanderwaals force(physisorption) or

through a chemical bond formation (chemisorption)

Increase in P upto saturation pressure increase the extent of adsorption on theadsorbent.

Freundlich & Langmuir adsorption Isotherm explain the effect of P onadsorption.

Adsorption of solutes from solution also obey Freundlich isotherm

Phenomenon of adsorption finds wide use in chromatography, in heterogenouscatalysis, in softening of water, in pollution abatement etc

Surfacechemistry2009-10

Documents

Transcript of Surfacechemistry2009-10