surface chemistry and catalysis surface chemistry and catalysis 2.3 adsorption desorption absorption

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  • Surface Chemistry and Catalysis 2.1

    Unit�II

    CHAPTER 2: Surface Chemistry

    INTRODUCTION

    Surface Chemistry is closely related to interface and colloidal science. Surface chemistry is important in many criticalchemical processes, such as enzymatic reactions at biological interfaces 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 cleaning emissions in automobile exhausts.

    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

    SURFACE CHEMISTRY AND CATALYSIS

    Introduction - Terminologies in surface chemistry - Difference between adsorption and absorption - Types of adsorption - Adsorption isotherm- Freundlich Adsorption Isotherms- Langmuir Adsorption Isotherm - Contact Theory (or) Mechanism of Heterogeneous Catalysis - Kinetics of Surface Reaction - Kinetics of Bimolecular Reaction (Langmuir-Hinshelwood) - Types of Adsorption Isotherm - Application of Adsorption- Terms - Mechanism of Catalytic Reaction- Criteria (or) Characteristics for Catalyst Types of Catalysis - Homogeneous Catalysis - Heterogeneous Catalysis- Catalytic Poisoning and Promoters Application of Catalysis- Biological Catalyst— —Enzymes - Kinetics of Enzyme Catalysed Reaction Or Michaelis and Menten equation - Factors Affecting Enzyme Activity

  • 2.2

    TE

    ERMINOLO

    Adsor

    Adsor

    adsorb

    Adsor

    the sur

    (Eg.) O

    hydrog

    Desor

    called

    Interfa

    Absor

    the bod

    Sorpti

    is calle

    Occlu

    called

    OGIES IN

    rbate: The su

    rbent: The su

    bent.

    rption: The pr

    rface of a soli

    Occlusion of

    gen gas is ads

    rption: The

    desorption.

    ace: The plan

    rption: When

    dy of a solid o

    ion: The phen

    ed sorption.

    usion: When

    occlusion.

    N SURFACE

    ubstance whic

    ubstance on th

    rocess whereb

    id.

    f Hydrogen g

    sorbate.

    Figure 2.1

    removal of

    ne which sepa

    n the molecule

    or liquid. Thi

    nomenon in w

    adsorption o

    1

    E CHEMIS

    ch gets adsorb

    he surface of

    by molecules

    gas on Palladi

    Ad on

    Adsorbent - adsorption ta

    : Adsorption Pr

    f the adsorbe

    arates any two

    es of a substan

    is phenomeno

    which adsorpti

    of gases occ

    STRY

    rbed on any su

    f which adsor

    s of gases or li

    ium where pa

    dsorbent - the n the surface of

    the solid wher akes place

    rocess

    ed substance

    o phase is gen

    nce are uniform

    on is called ab

    ion and absorp

    curs on the su

    urface is calle

    rption takes p

    iquids adhere

    alladium is ad

    gas adsorbed f solids

    re

    e from a sur

    nerally called a

    mly distribute

    bsorption.

    ption occur si

    urface of me

    Chemistry

    ed adsorbate.

    place is called

    chemically to

    dsorbent and

    rface is

    an interface.

    ed throughout

    imultaneously

    etals it is

    y

    .

    d

    o

    d

    t

    y

  • Surface Chemistry and Catalysis 2.3

    Adsorption Desorption Absorption

    Figure 2.2 : Occlusion Process

    Positive adsorption: When the concentration of solute adsorbed on the solid

    adsorbent surface is greater than in the bulk it is called positive adsorption.

    (Eg.) Concentrated solution of KCl is shaken with blood charcoal, it shows positive

    adsorption

    Negative adsorption: When the solvent from the solution may be absorbed by the

    adsorbent so that the concentration of the solute decreases and the concentration of

    solution increases than the initial concentration and it is called negative adsorption.

    (Eg.) Dilute solution of KCl is shaken with blood charcoal it shows negative adsorption.

    Enthalpy or heat of adsorption

    Amount of heat evolved when 1 mole of an adsorbate gets adsorbed on the surface of an adsorbent is called Molar Heat or Molar Enthalpy of Adsorption.

    DIFFERENCE BETWEEN ADSORPTION AND ABSORPTION

    Figure 2.3 : Illustration of Absorption and Adsorption

    Adsorbant

    Chemisorption Physisorption

    Absorption Adsorption

  • 2.4

    TY

    debrosd

    S.No.

    1. It

    2. It

    3. Su

    th

    4. A

    tim

    5. Eg

    ch

    YPES OF A

    P

    d e b r o s d

    ABSOR

    is a bulk phen

    is a slow proc

    ubstance unifo

    hroughout the

    Attainment of e

    me

    g.: Ammoni

    harcoal

    ADSORPTI

    Physical Ad Figure 2.4

    Temp Figure

    RPTION

    nomenon

    cess

    ormly distribu

    surface

    equilibrium tak

    ia adsorbed

    ION

    dsorption : Illustration of

    perature 2.5 : Amount o

    e bdrosd

    It is a su

    It is a fa

    uted Higher

    species

    kes Equilib

    in Eg.: Am

    Che f Physical and C

    of gas adsorbed

    e bd r o s d

    ADSOR

    urface phenom

    ast process

    concentrati

    in the surface

    rium attained

    mmonia adsorb

    emical Adso Chemical Adso

    Tem

    d vs temperatu

    RPTION

    menon

    on of mo

    e than in the bu

    easily

    bed in water

    orption orption

    perature

    re

    Chemistry

    lecular

    ulk

    y

  • Surf

    S.N

    Ads

    gas a

    face Chemis

    No

    1. Caus

    vand

    2. Not s

    3. Reve

    4. Mult

    on th

    5. Heaa

    KJ/m

    6. No a

    7. Depe

    Liqu

    easil

    8. Occu

    9. Incre

    adso

    orption of

    In adsorp adsorbed is c

    stry and Cat

    Physical ad

    Physiso

    sed by

    derwaal’s force

    specific in nat

    ersible in natur

    timolecular lay

    he adsorbed su

    at of adsorptio

    mole)

    activation ener

    ends on nature

    uefiable gase

    y

    urs at low temp

    ease

    rption

    in pr

    Gases on S

    ption of gases alled adsorb

    Solid Surface

    F

    talysis

    dsorption or

    orption

    es

    intermolec

    ture

    re

    yers are forme

    urface

    on is less (20 to

    rgy is required

    e of gas. Easily

    es are adso

    mperature

    ressure incr

    olids

    on solid surf bate. The exte

    igure 2.6 : Adso

    Chem

    cular Caused

    Highly

    Irrever

    ed Unimo

    surface

    o 40 Heat o

    KJ/mo

    d High a

    y

    orbed

    Depen

    adsorb

    Increas

    rease High p

    pressur

    face, the solid ent of adsorp

    orption of gase

    mical adsorpt

    d by chemical

    y specific in na

    rsible in nature

    olecular layers

    e

    f adsorption is

    ole)

    activation ener

    ds

    bate

    on natu

    ses with increa

    pressure is favo

    re does not ca

    d surface is ca ption depends

    Abs Gas M

    es on solids

    tion or Chem

    bond formati

    ature

    e

    s are formmed

    s large (8

    rgy is required

    ure of adso

    ase in tempera

    ourable. Decre

    ause desorption

    alled the adsor s on many fac

    sorbed Molecules

    Solid

    2.

    misorption

    on

    d on the

    0 to 240

    d

    orbent and

    ature

    ease in

    n

    rbent and the ctors.

    5

    e

  • 2.6 Chemistry

    2.2 FACTORS AFFECTING THE EXTENT OF ADSORPTION

    (i) Nature of Adsorbent

    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, silica gel and clay are commonly used adsorbents. The rate of adsorption can be increased by activation process. It helps in enhancing the pores in the adsorbent Eg. charcoal adsorbs 0.011 gms of CCl4 at 24°C and activated charcoal adsorbs 1.48 gm of at 24°C.

    Activation of adsorbent

    During activation, the adsorbent is heated in steam to about 1500°C. Heating drives out all impurities and leads to a lager free surface for adsorption. It can be done in 3 given ways

    By making the surface of adsorbent rough.

    By heating the adsorbent in vacuum so that the water vapour present in pores leave those pores.

    By increasing the surface area of adsorbent

    (ii) Surface area of adsorbent

    Increase

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