Surface Chemistry

TERMS AND DEFINITIONS:

            1. Adsorption:  Concentration of liquid or gaseous molecules over the surface of a solid  material is known as adsorption. It is a surface phenomenon.

 (e.g.)  H₂ adsorption over nickel. 

 The solid material is known as adsorbent.  (e.g.) Nickel

 The gaseous or liquid molecules are adsorbate (e.g.) H₂  gas

2.If the concentration is of bulk phenomenon, it is absorption. (e.g) Absorption of Ink on the surface of a chalk .

TYPES OF ADSORPTION

1. If the adsorbent and adsorbate are bonded by weak Vanderwalls forces, it is known as ‘Physisorption’. (e.g) H₂ on charcoal.

2.If they are bonded by strong chemical covalent bonds, it is chemisorption. (e.g) H₂ on nickel.

No	PHYSISORPTION	CHEMISORPTION
1	Adsorption is due to weak Vanderwalls forces.	Due to strong covalent bonding.
2	They form multi layer.	Monolayer
3	Reversible	Irreversible
4	Equilibrium is easy	Not easy
5	Intermediate surface compound formation does not take place.	Takes place
6	Energy of activation (Ea) is low.	High
7	Heat of formation and enthalpy (∆H) is low	High
8	Inversely proportional to temperature	Direct proportion
9	Directly proportional to pressure, concentration	Inversely proportion
10	Non specific and non selective	Selective and specific
11	e.g - H2 on charcoal.	H2 on nickel.

FACTORS AFFECTING ADSORPTION

a)  Adsorption of gases on solids:  (e.g) H₂ on Charcoal (Physisorption).

                                                                   H₂ on nickel.  (Chemisorption)

i. Nature of gases

ii. Nature of solid

iii. Activation

iv. Reversible nature

v. Thermodynamic quantities. (∆H , E_a , T, P , C)

1. Nature of gases:  There are two types of gases.  Permanent (e.g. - N₂, H₂, O₂) and easily liquefiable (e.g.- HCl, NH₃, SO₂) gases. Easily liquefiable gases are more adsorbed than the permanent gases.  Because they are having very high critical temperature (T_c) and great Vander walls forces.

E.g – H₂  --  T_c  = 33K  Adsorption  = 4.5ml / 1g charcoal

        SO₂ --   T_c = 430K , Adsorption  =  380ml / 1g charcoal

2. Nature of solids:  For greater adsorption, the solid adsorbent must have

    i)  High porosity     ii) More surface area

(e.g) powdered alumina and charcoals are having greater adsorption capacity.

3. Activation:  For adsorbents, porosity is increased by rubbing, scratching and passing super heated steam.  Their surface area is increased by subdividing and powdering. The entire process is known as activation of adsorbents.

(e.g)  Charcoal     steam / ∆              Activated charcoal

4. Reversible nature:

Gas + Adsorbent solid  ↔  Gas – Solid

For Physisorption, reversibility is possible.

For Chemisorption, reversibility is very least possible.

5. a) Enthalpy, Heat of absorption , Energy of activations are low for Physisorption and high for chemisorption.

   b) Multilayer is favoured by Physisorption and monolayer is favoured by chemisorption.

   c) Pressure is directly proportional to Physisorption and inversely proportional to chemisorption.

   d) Temperature is inversely proportional to Physisorption.  In chemisorption, it is increasing with temperature, attains a maximum value and again it is coming down.

                             

                          Physisorption                                                                   

                           

 

 

b)Factors affecting  adsorption of solutes from solution:

1. Nature of solutes

2. Nature of adsorbent

1. Nature of solute:

   High molecular weight solutes are easily adsorbed.

2. Nature of adsorbent:

   Adsorbent should have high porosity and surface area.

ADSORPTION ISOTHERMS AND 5 MODELS

The relation between adsorption and pressure at constant temperature is known as adsorption isotherm.  It may be mathematical relationship or of graphical.

(x / m)  =    k  . P  ^1/n     (at constant T)

 

To convert the equation as   y = mx  + C form, taking log on both sides,

log   (x / m) =    log k  + (1/n) log P       (at constant T)                                                                                  

             Where   y =  log   (x / m)   :   m =  (1/n)  :       C = log k 

so, the plot will be a line with slope 1/n and intercept log k.

5 models of adsorption isotherms:  ( P_s   = Saturation pressure)

Model	Adsorbate	Adsorbent	Temp	Characteristics
I	Nitrogen	Charcoal	-1800C	Chemisorption (monolayer)
II	Nitrogen	Iron / Nickel	-1950C	Physisorption  (Multilayer)
III	Bromine	Silica	800C	Physisorption (Multilayer)
IV	Benzene	Silica gel	500C	Capillary Condensed state Physisorption
V	Water vapour	Carbon	1000C	Capillary Condensed state Physisorption

FREUNDLICH ADSORPTION ISOTHERM:

The relation between adsorption and pressure at constant temperarute is known as adsorption isotherm.  It may be mathematical relationship or of graphical.

(x / m)  =    k  . P  ^1/n     (at constant T)

                                                                                                       

To convert the equation as   y = mx  + C form,  taking log on both sides,

         log   (x / m) =    log k  + (1/n) log P       (at constant T)

                                                                                  

             where   y =  log   (x / m)            m =  (1/n)                   C = log k 

so, the plot will be a line with slope 1/n and intercept log k.

From the above graph, there may be three cases:

Case 1- At low pressure, adsorption is proportional to pressure.

               (x/m) ∞  P   (or)  (x/m)  =  k . P¹

Case – 2 - At high pressure, adsorption is almost constant.

               (x/m)  = k   (or)  (x/m)  =  k . P⁰

Case – 3 - At Intermediate  pressure.

               (x/m)  = k . P  ^{0  --  1}  (or)  (x/m)  =  k . P ^1/n, where n is an integer.

This is known as Freundlich adsorption isotherm.

Limitations of Freundlich isotherm:

1. It is only empirical formula and no theoretical evidences.

2. It is deviated at high pressure.

3. It is not good at high concentration.

4. n and k are temperature dependent.

LANGMUIR ADSORPTION ISOTHERM

Postulates:

1. In adsorbent, surface valencies are not fulfilled.

2. Adsorbates are of mono layer thickness.

3. Adsorbates are of uniform distribution.

4. No interaction between adjacent gas molecules.

5. The gas molecules do not move around on the surface.

Derivation:

1. As per dynamic equilibrium,

                          k_a

Gas  +  Solid                   G – S      where, k_a  =  Adsorption rate constant

                            k_d                                                      k_d  =  Desorption rate constant

 2.  Let us consider,

           Total area of adsorbent = 1

            Gas covered area           =θ             

      Then, uncovered area         =(1-θ)         

  

3.   Rate of adsorption, R_a = K_a (1-θ) P

       Rate of desorption, R_d = k_d . θ                 

                                                                                            

                                                                                              

4. At equilibrium,

            R_d       =       R_a

            k_d . θ = Ka (1-θ) P

            k_d . θ  = K_a  P  - K_a θ P 

             k_d . θ  +  K_a θ P  = K_a  P

            θ  ( k_d  + k_a P ) = k_a P

             θ   =   k_a P _____           (dividing by k_d)                  

                       ( k_d  + k_a P )

             θ   =    ( k_a / k_d) .P________

                          (k_d / k_d) + (k_a / k_d).P

AS, ( k_a / k_d) = K, another constant, known as Adsorption co-efficient,

                 θ  =     K . P__

                             1 + K.P

5.But, amount of gas adsorbed (x) is proportional to θ.

              x ∞ θ   and   x  =  K¹ θ

               x         =      K¹ K P

                                      1 + KP

               1 + KP    =      K¹ K P

                                            x

                                     

                1 + KP     =      P

                   K.K¹              x

              P                 =     1 __     +      K.P

              x                        KK¹              KK¹     ,    which is in the form of

               y                =        C       +      m . x,

 

       where,  intercept C  =  1 / KK¹  and  slope  m = K / KK¹                                                      

                                            

                                 

              P                 =     1 __     +      K.P

              x                        KK¹              KK¹     ,

                                         

Case – 1  -, At low pressure, P is negligible.

 So,            1 __    >>      K.P

                 KK¹                KK¹

       Hence,  P       =     1 __       (or)   x = KK¹.P = K^” P¹

                     x             KK¹                 

Case – 2 - At high  pressure, P is very big. 

So,            1 __    <<        K.P

                 KK¹                 KK¹

Hence, P         =        K.P    (or)   x = constant (or)  x=K”P⁰

             x                    KK¹             ,

Case – 3, At normal pressure,   x  = K” P ^{0 – 1}  (or)  x = K” Pⁿ , where n= 0 – 1

This proves that at normal pressure, Langmuir adsorption resembles Freundlich isotherm. But still, Langmuir adsorption holds good at low pressure but fails at high pressure.

ROLE OF ADSORBENTS IN CATALYSIS

A substance used to alter the rate of a reaction is called catalyst.  But it should not involve in the reaction.  The process is known as Catalysis.

If it increases the rate of reaction, it is positive catalyst. If it decreases the rate of a reaction, it is negative catalyst.

If reactants and catalyst are in same phase it is homogeneous catalysis. If they are in different phases, it is heterogeneous catalysis.

E.g.

N₂  + 3 H₂   Fe       2 NH₃  .  Fe is Positive / Heterogeneous catalyst

.

H₂O₂    Dil. H₂SO₄   H₂O  +  ½ O₂  ,. Dil. H₂SO₄  is negative / Homogenous catalyst.

 

Adsorption is mainly used in Heterogeneous catalysis.

Steps involved in Heterogeneous Catalysis – Contact theory.

1. Adsorption - Here the reactant molecules (A, B) are adsorbed on the solid adsorbent(C).by strong chemical bond or weak Vander walls bond.

2. Activated complex formation - The adjacent adsorbate molecules are forming weak bond (A-B) and the activated complex is formed.

3. Decomposition – The bond between A and B is getting strengthen while the bond between A-B and C is getting weaken. This is called decomposition.

4. Desorption – From the decomposed activated complex, the final product is released. This is known as desorption.

 A             B             A-----------B                     A               B                               A      B

  

 

                                                                                                                                  +

 C             C            C               C                     C              C                               C        C                

Step –I                        Step-II                 step – III                                     Step -IV

Adsorption             Activated complex              Decomposition                     Desorption

E.g

CH₂ = CH₂   +  H₂     Nickel    CH₃  - CH₃  (Ethylene  to Ehtene) 

 

Where   A =  Ethylene   B = Hydrogen   C= Nickel and A-B = Ethene

Factors affecting heterogeneous catalysis:

1.  Finely divided catalyst  

2. Rough surface

3. Promoters

 

1.  In finely divided catalysts, active centre increases. So, adsorption increases.

 

E.g -                                                                                        

                                                                                              

                                                             Finely divided       6  

 

Free valencies in combined state =  10                   Divided state valencies = (6 x 4)=24

2. Rough surfaces may have cracks, peaks and corners.  So, number of active centre increases and adsorption increases.

3.  Promoters are the substances increasing the activity of catalyst.  (e.g) Mo increases Ni activity in Haber’s process.  They increase the peaks and cracks and also  the spaces between the catalyst, so adsorbed molecules bonds are further weaken and cleaved.

ADSORPTION AND ION EXCHANGE PROCESS

 Ion exchange method  ( Demineralisation)

Ion exchange process depends on the concept of adsorption of Ca and Mg ions on resins.

Working:

Here all the cations and anions are completely removed.  It uses two cylinders of cation exchange cylinder and anion exchange cylinder filled with resins.Resins are linear, insoluble, cross linked, organic polymers. There are 2 types.

i)                    Cation exchange resins – RH₂   (e.g)  Sulphonated coals , RSO₃H

ii)                  Anion exchange resins. R’(OH)₂ (e.g)Ureaformaldehyde,Amines R-NH₂

The water is fed into cylinder –I  where all the cations are replaced by RH₂  

Resins. The cation free water is fed to cylinder II, where all the anions are    replaced. So, the resultant water is free from all types of ions.

             RH₂  + CaCl₂  à  R Ca  +  2 HCl

R’(OH)₂  +  2 HCl  à  R’Cl₂ + 2H₂O

Regeneration:            On prolonged use, as  all the resins are exhausted, there will be no H⁺ orOH ^– ions to exchange the unwanted ions. So, they have to be regenerated.

Cation resins are regenerated by HCl and anion resins by NaOH.

R Ca  +  2 HCl  à  RH₂  + CaCl₂

R’Cl₂ + 2NaOH  à    R’(OH)₂  +  2NaCl

APPLICATION IN POLLUTION ABATEMENT:

1.When wood or coconut shell is heated in absence of air, charcoal is obtained. This is called carbonization. The charcoal is activated by passing steam and heated with Phosphoric acid as dehydrating agent at 500⁰C to make it more porous.

2. There are two types of activated charcoal. GAC(Granular activated Charcoal)  and PAC( Powdered Activated charcoal.

No	GAC	PAC
1	For long term use	Short term use
2	Dia is greater than 0.1mm	Less than 0.1mm
3	Used for gases and liquids	Mainly for liquids
4	Regeneration is possible	Not possible

WATER POLLUTION TREATMENT ( USING GAC)

1. Down flow contactors:

The polluted water is fed at the top of the columns containing GAC.  The impurities are adsorbed on GAC.  If higher degree of purity is needed, then numbers of columns are increased. The clogging in GAC is removed by backwashing. There are two types of down flow contactors.  A) Parallel type  b) Series type

No	Series model	Parallel model
1	High degree of purity	Low degree of purity
2	Low volume only purified	High volume can be purified
3	Backwash is frequently needed	Not frequently needed.
4	Low life time of GAC	High life time
5	Out put of one column will be in put for another column.	Single source and single terminus.