Objective:

 

1.  To demonstrate the adsorption phenomena.

2.  To show students how to determine the adsorption parameters.

 

 

 Theory:

 

Adhesion of atoms, ions, bimolecules or molecules of gas, liquid or dissolved solids to a surface is called adsorption. This process creates a film of the adsorbate –the molecules or atoms being accumulated, on the surface of the adsorbent.

 

  Examples:

 

• Activated charcoal adsorbs gases like CO₂ , SO₂, Cl₂ etc.
• Pt or Ni metal kept in contact with a gas adsorbs the gas - Hydrogenation of oils.
• Animal charcoal, when added to acetic acid solution and shaken vigorously, adsorbs acetic acid.
• Molasses is decolourised by activated charcoal.

 

The molecules of gases or liquids or the solutes in solutions adher to the surface of the solids. In adsorption process, two substances are involved. One is the solid or the liquid on which adsorption occurs and it is called adsorbent. The second is the adsorbate, which is the gas or liquid or the solute from a solution which gets adsorbed on the surface.

Adsorbent: The substance on whose surface the adsorption occurs is known as adsorbent.

Adsorbate: The substance whose molecules get adsorbed on the surface of the adsorbent ( i.e. solid or liquid ) is known as adsorbate. 

Adsorption is different from absorption. In absorption, the molecules of a substance are uniformly distributed in the bulk of the other, whereas in adsorption molecules of one substance are present in higher concentration on the surface of the other substance.

Types of adsorption:

 

Depending upon the nature of forces existing between adsorbate molecules and adsorbent, the adsorption can be classified into two types:

1. Physical adsorption (physisorption): If the force of attraction existing between adsorbate and adsorbent are Vander Waal’s forces, the adsorption is called physical adsorption. It is also known as Vander Waal’s adsorption. In physical adsorption the force of attraction between the adsorbate and adsorbent are very weak, therefore this type of adsorption can be easily reversed by heating or by decreasing the pressure.

 

2. Chemical adsorption (chemisorption): If the force of attraction existing between adsorbate and adsorbent are almost same strength as chemical bonds, the adsorption is called chemical adsorption. It is also known as Langmuir adsorption. In chemisorption the force of attraction is very strong, therefore adsorption cannot be easily reversed. 

 

Comparison between Physisorption and Chemisorption

  

Physisorption	Chemisorption
Low heat of adsorption usually in the range of 20-40 kJ mol-1	High heat of adsorption in the range of 40-400 kJ mol-1
Force of attraction are Van der Waal's forces	Forces of attraction are chemical bond forces
It usually takes place at low temperature and decreases with increasing temperature	It takes place at high temperature
It is reversible	It is irreversible
It is related to the ease of liquefaction of the gas	The extent of adsorption is generally not related to liquefaction of the gas
It is not very specific	It is highly specific
It forms multi-molecular layers	It forms monomolecular layers
It does not require any activation energy	It requires activation energy

Factors affecting adsorption:

 

The extent of adsorption depends upon the following factors:

1. Nature of adsorbate and adsorbent.
2. The surface area of adsorbent.
3. Activation of adsorbent.
4. Experimental conditions. E.g., temperature, pressure, etc.

 

Adsorption Isotherm:

 

 

Adsorption process is usually studied through graphs known as adsorption isotherm. That is the amount of adsorbate on the adsorbent as a function if its pressure or concentration at constant temperature .The quantity adsorbed is nearly always normalized by the mass of the adsorbent to allow comparison of different materials.

  

Basic Adsorption Isotherm:

 

From the above we can predict that after saturation pressure P_s, adsorption does not occur anymore, that is there are limited numbers of vacancies on the surface of the adsorbent. At high pressure a stage is reached when all the sites are occupied and further increase in pressure does not cause any difference in adsorption process. At high pressure, Adsorption is independent of pressure.

 

Type of Adsorption Isotherm:
 

 

Five different types of adsorption isotherm and their characteristics are explained below.

 

Type I Adsorption Isotherm:

 

 

 

Type I Adsorption Isotherm
 

• The above graph depicts Monolayer adsorption.
• This graph can be easily explained using Langmuir Adsorption Isotherm.
•  If BET equation, when P/P₀<<1 and c>>1, then it leads to monolayer formation and Type I Adsorption Isotherm is obtained.
•  Examples of Type-I adsorption are Adsorption of Nitrogen (N₂) or Hydrogen (H) on charcoal at temperature near to -1800°C.

 

Type II Adsorption Isotherm:

 

 

Type II Adsorption Isotherm
 

• Type II Adsorption Isotherm shows large deviation from Langmuir model of adsorption.
• The intermediate flat region in the isotherm corresponds to monolayer formation.
• In BET equation, value of C has to be very large in comparison to 1.
• Examples of Type-II adsorption are Nitrogen (N₂ (g)) adsorbed at -1950°C on Iron (Fe) catalyst and Nitrogen (N₂ (g)) adsorbed at -1950°C on silica gel.

 

Type III Adsorption Isotherm:

 

 

Type III Adsorption Isotherm
 

• Type III Adsorption Isotherm also shows large deviation from Langmuir model.
•  In BET equation value if C <<< 1 Type III Adsorption Isotherm obtained.
• This isotherm explains the formation of multilayer.
• There is no flattish portion in the curve which indicates that monolayer formation is missing.
• Examples of Type III Adsorption Isotherm are Bromine (Br₂) at 790°C on silica gel or Iodine (I₂) at 790°C on silica gel.

 

Type IV Adsorption Isotherm:

 

 

Type IV Adsorption Isotherm
 

• At lower pressure region of graph is quite similar to Type II. This explains formation of monolayer followed by multilayer.
• The intermediate flat region in the isotherm corresponds to monolayer formation.
• The saturation level reaches at a pressure below the saturation vapor pressure. This can be explained on the basis of a possibility of  gases getting condensed in the tiny capillary pores of adsorbent at pressure below the saturation pressure (P_S) of the gas.
• Examples of Type IV Adsorption Isotherm are of adsorption of Benzene on Iron Oxide (Fe₂O₃) at 500°C and adsorption of Benzene on    silica gel at 500°C.

 

Type V Adsorption Isotherm:

 

Type V Adsorption Isotherm
 

• Explanation of Type V graph is similar to Type IV.
• Example of Type V Adsorption Isotherm is adsorption of Water (vapors) at 1000°C on charcoal.
• Type IV and V shows phenomenon of capillary condensation of gas.

 

Freundlich Adsorption Isotherm:

 

In 1909, Freundlich expressed an empirical equation for  representing the isothermal variation of adsorption of a quantity of gas adsorbed by unit mass of solid adsorbent with pressure. This equation is known as Freundlich Adsorption Isotherm or Freundlich Adsorption equation or simply Freundlich Isotherm.

 

Where ,

x/m = adsorption per gram of adsorbent which is obtained be dividing the amount of adsorbate (x) by the weight of the adsorbent (m).

P is Pressure, k and n are constants whose values depend upon adsorbent and gas at particular temperature .

Though Freundlich Isotherm correctly established the relationship of adsorption with pressure at lower values, it failed to predict value of adsorption at higher pressure. This relation is called as the freundlich adsorption isotherm. As see the following diagram. The value of x/m is increasing with increase in p but as n>1 it does not increase suddenly. This curve is also called the freundlich isotherm curve.

 

Taking the logarithms of a first equation.

 

Hence, if a graph of log x/m is plotted against log p, it will be a straight line in the following diagram.

 

From this the value of slope equal to 1/n and the value of intercept equal to log k can be obtained. Over and above, it the graph of log x/m against log p comes out to be a straight line, it can be assured that the freundlich adsorption isotherm is satisfied for this system.

 

Langmuir Adsorption Isotherm:

 

In 1916, Irving Langmuir published a new model isotherm for gases adsorbed to solids, which retained his name. It is a semi-empirical isotherm derived from a proposed kinetic mechanism. This isotherm was based on different assumptions one of which is that dynamic equilibrium exists between adsorbed gaseous molecules and the free gaseous molecules.

It is based on four assumptions:

1. The surface of the adsorbent is uniform, that is, all the adsorption sites are equivalent.
2. Adsorbed molecules do not interact.
3. All adsorption occurs through the same mechanism.
4. At the maximum adsorption, only a monolayer is formed: molecules of adsorbate do not deposit on other, already adsorbed, molecules of adsorbate, only on the free surface of the adsorbent.

Langmuir suggested that adsorption takes place through this mechanism:

 

Where ,

A(g) = unadsorbed gaseous molecule

B(s) = unoccupied metal surface

AB = Adsorbed gaseous molecule.

The direct and inverse rate constants are k and k^-1

Based on his theory, Langmuir derived an equation which explained the relationship between the number of active sites of the surface undergoing adsorption and pressure. This equation is called Langmuir Equation.

 

Where,

θ= the number of sites of the surface which are covered with gaseous molecule,

P= pressure 

K =is the equilibrium constant for distribution of adsorbate between the surface and the gas phase .

The basic limitation of Langmuir adsorption equation is that it is valid at low pressure only.

At lower pressure, KP is so small, that factor (1+KP) in denominator can almost be ignored. So Langmuir equation reduces to

 

At high pressure KP is so large, that factor (1+KP) in denominator is nearly equal to KP. So Langmuir equation reduces to

 

Adsorbents:

 

 

 

The material upon whose surface the adsorption takes place is called an adsorbent .Activated carbon is used as an adsorbent

• Adsorbents are used usually in the form of spherical pellets, rods, moldings, or monoliths with hydrodynamic diameters between 0.5 and 10 mm.
• They must have high abrasion resistance, high thermal stability and small pore diameters, which results in higher exposed surface area and hence high surface capacity for adsorption.
• The adsorbents must also have a distinct pore structure which enables fast transport of the gaseous vapors.

Most industrial adsorbents fall into one of three classes:

• Oxygen-containing compounds - Are typically hydrophilic and polar, including materials such as silica gel and zeolites.
• Carbon-based compounds - Are typically hydrophobic and non-polar, including materials such as activated carbon and graphite.
• Polymer-based compounds - Are polar or non-polar functional groups in a porous polymer matrix.

Activated carbon is used for adsorption of organic substances and non-polar adsorbates and it is also usually used for waste gas (and waste water) treatment. It is the most widely used adsorbent since most of its chemical (eg. surface groups) and physical properties (eg. pore size distribution and surface area) can be tuned according to what is needed. Its usefulness also derives from its large micropore (and sometimes mesopore) volume and the resulting high surface area.
 

 

Mechanism of Adsorption Using Adsorbent:

 

 

 

Applications of adsorption:

The principle of adsorption is employed,
 

1.  in heterogeneous catalysis.
2.  in gas masks where activated charcoal adsorbs poisonous gases.
3.  in the refining of petroleum and decolouring cane juice.
4.  in creating vacuum by adsorbing gases on activated charcoal.
5.  in chromatography to separate the constituents' of a mixture.
6.  to control humidity by the adsorption of moisture on silica gel.
7.  in certain titrations to determinate the end point using an adsorbent as indicator (Example: Flouroscein).

 

Procedure of adsorption:

500 ml of 0.5N oxalic acid solution is prepared. Five well cleaned, dried, reaction bottles (250 ml) are taken and are labeled. About 2 g of the activated animal charcoal are accurately weighed and transferred carefully into each of the bottles. By means of a burette 50, 40, 30, 20 and 10 ml of 0.5N oxalic acid are added followed by 0, 10, 20, 30 and 40 ml of distilled water so that the total volume (50 ml) remains constant in each bottle. These bottles were shaken thoroughly nearly for an hour by means of a mechanical shaker and they are set aside in a trough containing water to reach equilibrium.

The supernatant liquid of each of the bottles are filtered through a small dry filter paper and the filtrate is collected in properly labeled conical flasks.(The initial 5 ml or 10 ml of the filtrate is rejected) 10 ml of the filtrate is pipetted out into a clean conical flask. It is titrated against standardized KMnO4 solution until a pink colour appears. The titration is repeated to get concordant values. From the titre values, the concentration of oxalic acid remaining and hence the amount of oxalic acid adsorbed are calculated.

In order to test the validity of Freundlich adsorption isotherm plot log (x/m) against log Ce. The slopes and intercepts of the plot will give 1/n and log k respectively and hence n and K can be calculated.

 

                                                                                                                                                                                  

    
 

Validity of Langmuir adsorption equation can be tested by plotting Ce/(x/m) Vs Ce. A linear plot obtained shoe the applicability of the isotherm. Calculate the constants "a" and "b" from the slope and intercept on the ordinate axis.