Chemistry

Surface Chemistry - 1

Session

Session Objectives

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

Adsorption

The phenomenon of higher concentration of any molecular species at the surface than in the bulk

AdsorbentThe substance on the surface of which adsorption takes place is called adsorbent

AdsorbateThe substance which is being adsorbed on the surface of another substance.

DesorptionThe process of removal of an adsorbed substance from the surface on which it is absorbed

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

Adsorbent Materials

• Made from nutshells, wood, and petroleum, bituminous coal by heating in the absence of oxygen to dehydrate and carbonize (remove volatile components),

• "Activation" is the process that produces the porous structure essential for effective adsorption by oxidation of carbon with water vapor or CO2.

• Activated carbon attracts non-polar molecules such as hydrocarbons.

• Typical surface areas are 300 to 1500 m2/g.

Activated carbon

Adsorption vs absorption

Types of Adsorption

Positive adsorption occurs when the concentration of adsorbate is higher on the surface of adsorbent than in the bulk.

Negative adsorption occurs when the concentration of adsorbate is less on the surface of adsorbent than in the bulk.

Types of adsorption

S. No. Physical adsorption Chemical adsorption1 Caused by intermolecular van der

Waals' forcesCaused by chemical bond formation

2 It is not specific It is highly specific3 It is reversible It is irreversible4 Heat of adsorption is low (20-40 kJ/mol) High heat of adsorption (80-240 kJ/mol)5 Low temperature is favourable Increases with high temperature6 Results multilayer adsorption Results unimolecular layer

Comparison between physisorption and chemisorption

1. Physical adsorption

2. Chemical adsorption

Factors affecting adsorptionEffect of adsorbate: The easily liquifiable gases like NH3, HCl, CO2 etc. are adsorbed to a greater extent than the permanent gases such as H2 ,O2, N2, etc.

Effect of specific area of the absorbent: The greater the specific area of the solid, the greater would be its adsorbing capacity.

Effect of temperature:adsorption decreases with increase in temperature.

Effect of pressure: An increase in pressure causes an increase in the magnitude of adsorption of an adsorbent.

1/n xKpm

Over a narrow range of p

Freundlich IsothermA graph between the amount (x/m) adsorbed by an adsorbent and the equilibrium pressure of the adsorbate at constant temperature is called adsorption isothermAt low pressure the graph is nearly straight line

At high pressure x/m becomesindependent of p

0x pm

1x pm

Freundlich Isotherm

X1 loglogKlogpMn

Rate of adsorption a ar =k p(1- f)

Rate of desorption d dr =k f

At equilibrium, ra = rd; a

a d

k pf = .........(i)k p+k

Mono-layer coverage 'a

x =k f .........(2)m

m: mass of adsorbate adsorbed per unit mass of adsorbent

f: fraction of surface area covered

f

1-fp: partial pressure of the adsorbate

Langmuir isotherm

Langmuir adsorption isotherm:

apx /m1 bp

The values of constants ‘a’ and ‘b’ depend upon the nature of adsorbate, nature of solid adsorbent and temperature.

a = ka x ka’/kdb = ka/kd

Combining equations (1) and (2):

Summary of adsorption isotherms

Easy to fit adsorption data

Chemisorptions and physisorption

Freundlich

Useful in analysis of reaction mechanism

Chemisorption and physisorption

Langmuir

NoteApplicationIsotherm equation

Name

1/nx =Kpm

x ap=m 1+bp

Adsorption isobar Graph between the amount adsorbed(x/m) and temperature at a constant equilibrium pressure of adsorbate gas is known as adsorption isobar

Chemisorption isobar shows an initial increase with temperature andthen expected decrease .The initial increase is because of the fact that the heat supplied acts as activation energy required in chemisorption.

Application of Adsorption

In clarification of sugar

In gas masks

In catalysis

In adsorption indicators

In chromatographic analysis

In softening of hard water

In preserving vacuum

In paint industry

In removing moisture from air in the storage of delicate instruments

Need to make chemicals faster

Most Reactions are too slow to be useful...

Ways to Make Chemicals FasterWays to Make Chemicals Faster

Pressure

Disadvantage--Cause Explosions

Catalysts!!!!

Disadvantage--Costly

Disadvantage--Too hot!

Temperature

Add other Chemicals

Disadvantage--Separate chemicals

The Story of Catalysis

Why Catalysis ?What is a Catalyst ?How Catalysts Work ?

Role of a CatalystCatalysts speed up a chemical reaction

without being used up...

Catalyst + Reactants

Catalyst-Reactants

Catalyst + Products

Hydrogenation of alkene

Important properties of catalyst

Activity: A reasonable rate of reaction is needed.Selectivity: Byproducts should be minimized.Cost: The acceptable cost depends upon the catalyst lifetime and product value.

Catalysis

Positive catalysis

Negative catalysis

Auto catalysis

2MnO

3 2270 C

2KClO 2KCl 3O

Alcohol3 2 22CHCl O 2COCl 2HCl

3 2 5 2 3 2 5CH COOC H H O CH COOH C H OH

CatalysisTypes of catalysis

Homogeneous catalysis

Process Products Catalyst1. Hydrolysis of an ester. Acid and Alcohol H+

2. Oxidation of SO2 to SO3 SO3 NO in lead chamber process

Heterogeneous catalysis

Process Products Catalyst1. Ammonia synthesis NH3 Fe with Al2O3

2. Methanol synthesis MeOH ZnO/Cr2O3

Catalysis

Enzyme catalysis

Reaction Catalyst1. Invertase

2. Zymase

12 22 11 2 6 12 6 6 12 6Sucrose Glucose Fructose

C H O H O C H O C H O

6 12 6 2 5 2Glucose Ethanol

C H O 2C H OH CO

A coenzyme is an organic non-protein molecule that is a functional part of an enzyme. Coenzymes are not used up in the reactions in which they assist

Do Catalysts Live Forever?

NO!!!

They can die from:

-- poisons which contaminate the catalyst-- large molecules which cover the catalyst -- over heating, over pressurizing-- crumbling/crushing

Automotive Emission ControlAutomobile catalytic converters need to catalyze several reactions– CHx + O2 CO2 + H2O– CO + O2 CO2

– NOx +CHx N2 + H2O + CO2

– This is achieved by the use of a supported precious metal catalyst like platinum, palladium etc.– Catalyst needs O2 to operate, CeO2 acts as a temporary regenerable source of O2. CeO2 Ce2O3 + 1/2O2

Thank you