Adsorption

Adsorption is the phenomenon of accumulation of large number of molecular species at the surface of liquid or solid phase in comparison to the bulk.

How does adsorption occur?

The process of adsorption arises due to the presence of unbalanced or residual forces at the surface of liquid or solid phase. These unbalanced residual forces have tendency to attract and retain the molecular species with which it comes in contact with the surface.

Adsorption is essentially a surface phenomenon.

Adsorption is is completely different from Absorption. Absorption means uniform distribution of the substance throughout the bulk. Adsorption essentially happens at the surface of the substance. When both Adsorption and Absorption processes take place simultaneously, the process is called sorption.

Adsorption process involves two components Adsorbent and Adsorbate.

Adsorbent is the substance on the surface of which adsorption takes place. Adsorbate is the substance which is being adsorbed on the surface of adsorbent. Adsorbate gets adsorbed.

Adsorbate + Adsorbent give rise to Adsorption

Some modern techniques have been used to study surface.

1. Low energy electron diffraction (LEED).2. Photo electron spectroscopy (PES).3. Scanning Tunneling microscopy (STM).

Adsorption in liquids

Adsorption on liquid surfaces can be understood by considering a simple example. In case of a liquid like water, the molecules present on the surface are attracted inwards by the molecules present in the bulk. This gives rise to surface tension. But the water molecules

present within the bulk are equally attracted from all the sides and the net force experienced a molecule in bulk is zero.

This clearly shows that particles at surface and particles at the bulk are in different environment.

Water molecules on surface are experiencing unbalanced forces as compared to molecule inside which experiences forces from all direction.

Adsorption in solids

In case of solids, these residual forces arise because a of unbalanced valence forces of atoms at the surface. The generation of these forces on solid surface can be explained diagrammatically as follows:

Cleavage of a big crystal into smaller units

Due to cleavage of a big crystal into smaller unit, residual forces or vacancies gets generated on the surface of the solid. Occupancy of these vacancies by some other molecular species results into Adsorption.

Adsorption is a spontaneous process

For reaction or process to be spontaneous, there must be a decrease in free energy of the system, i.e. ΔG of the system must have negative value.

Also we know, ΔG = ΔH – TΔS

And during this process of adsorption, randomness of the system decreases and hence ΔS is negative. We can rewrite above equation as

Therefore for a reaction to be spontaneous ΔH has to be negative and

Thus adsorption is an exothermic process.

Types of Adsorption

Forces of attraction exist between adsorbate and adsorbent. These forces of attraction can be due to Vanderwaal forces of attraction or due to chemical bonding. On the basis of type of forces of attraction adsorption can be classified into two types: Physical Adsorption or Chemical Adsorption.

PHYSICAL ADSORPTION

CHEMISORPTIONS

The forces operating in these are weak     van der Waal’s forces.

The forces operating in these cases are similar to those of a chemical bond.

The heat of adsorption are low i.e. about 20 – 40 kJ mol-1

The heat of adsorption are high i.e. about 40 – 400 kJ mol-1

No compound formation takes place in these cases.

Surface compounds are formed.

The process is reversible i.e. desorption of the gas occurs by increasing the temperature or decreasing the pressure.

The process is irreversible. Efforts to free the adsorbed gas give some definite compound.

It does not require any activation energy.

It requires activation energy.

This type of adsorption decreases with increase of temperature.

This type of adsorption first increases with increase of temperature. The effect is called activated adsorption.

It is not specific in nature i.e. all gases are adsorbed on all solids to some extent.

It is specific in nature and occurs only when there is some possibility of compound formation between the gas being adsorbed and the solid adsorbent.

The amount of the gas adsorbed is related to the ease of liquefaction of the gas.

There is no such correlation exists.

It forms multimolecular layer.

It forms unimolecular layer.

Physical Adsorption vs. Temperature graph

Chemical Adsorption vs. Temperature Graph

Adsorption Isotherm

The process of Adsorption is usually studied through graphs called as adsorption isotherm. It is the graph between the amounts of adsorbate (x) adsorbed on the surface of adsorbent (m) and pressure at constant temperature.

Factors on which Adsorption Depends

1. Temperature

Adsorption increases at low temperature conditions.

Adsorption process is exothermic in nature. According to Le Chatleir principle, low temperature conditions would favour the forward direction.

2. Pressure

With the increases in pressure, adsorption increases up to a certain extent till saturation level is achieved. After that no more adsorption takes place.

3. Surface Area

Adsorption is a surface phenomenon therefore it increases with increase in surface area.

4. Activation of Adsorbent

Activation of adsorbent surface is done so as to provide more number of vacant sites for adsorption. This can be done by breaking solid crystal in small pieces, heating charcoal at high temperature, breaking lump of solid into powder or other methods suitable for particular adsorbent.

Applications of Adsorption

1. Charcoal is used as a decoloriser as it adsorbs the coloring matter from the coloured solution of sugar.

2. Silica gel adsorbs moisture from the desiccators.

3. Silica and alumina gels are used as adsorbents for removing moisture and for controlling humidity of rooms.

4. Activated charcoal is used in gas masks as it adsorbs all the toxic gases and vapours and purifies the air for breathing.

5 .Adsorption processes are useful in carrying out heterogeneous catalysis.

Adsorption Isotherms

The adsorption isotherm is a graph between the amounts of adsorbate (x) adsorbed on the surface of adsorbent (m) and pressure at constant temperature. Different adsorption isotherms have been Freundlich, Langmuir and BET isotherms.

Basic Adsorption Isotherm

In the process of adsorption, adsorbate gets adsorbed on adsorbent.

According to Le-Chatelier principle, the equilibrium would shift in that direction where the stress can be relieved.

Thus with the increases in pressure, the equilibrium is shifted in the forward direction because it leads to a decrease in the number of molecules and releaves the stress.

Basic Adsorption Isotherm

From the graph, we can predict that after saturation pressure Ps, adsorption does not occur anymore. This can be explained by the fact that 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.

Freundlich Adsorption Isotherm

In 1909, Freundlich gave an empirical expression for 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.

Where x is the mass of the gas adsorbed onto mass m of the adsorbent at pressure p and k, n are constants whose values depend upon the adsorbent and the gas at particular temperature.

A plot of log (x/m) Vs log P is a straight line with slope = 1/n and intercept = log k. Hence their values can be determined experimentally.

Though Freundlich Isotherm correctly established the relationship of adsorption with pressure at lower values, it failed to predict value of adsorption at higher pressure.

Langmuir Adsorption Isotherm

In 1916, Irving Langmuir proposed another Adsorption Isotherm which explained the variation of Adsorption with pressure.

Assumptions of Langmuir Isotherm

Langmuir proposed his theory by making the following assumptions.

1. Fixed number of vacant or adsorption sites are available on the surface of solids.2. All the vacant sites are of equal size and shape on the surface of adsorbent.

3. Each site can hold maximum of one gaseous molecule and a constant amount of heat energy is released during this process.

4. Dynamic equilibrium exists between adsorbed gaseous molecules and the free gaseous molecules.

Where A(g) is unadsorbed gaseous molecule, B(s) is unoccupied metal surface and AB is Adsorbed gaseous molecule.

5. Adsorption is monolayer or unilayer.

Derivation

Langmuir Equation depicts a relationship between the number of active sites of the surface undergoing adsorption (i.e. extent of adsorption) and pressure.

Let θ be the number of sites of the surface which are already covered with gaseous molecules. Therefore, the fraction of surface which are unoccupied by gaseous molecules will be (1 – θ).

Now, rate of forward reaction depends upon two factors: Number of sited available on the surface of adsorbent, (1 – θ) and Pressure, P. Therefore rate of forward reaction is directly proportional to both mentioned factors.

Similarly, Rate of reverse reaction or rate of desorption depends upon the number of sites occupied by the gaseous molecules on the surface of adsorbent.

At equilibrium, rate of adsorption is equal to rate of desorption.

Ka P (1 – θ) = Kd θ

We can solve the above equation to write it in terms of θ.

KaP – KaP θ = Kd θ

KaP = KaP θ + Kd θ

KaP = (Kd + KaP) θ

Divide numerator and denominator on RHS by Kd, we get

But

Therefore we get

Langmuir Adsorption Equation

This is known as Langmuir Adsorption Equation.

Freundlich Adsorption Equation: A Special Case of Langmuir Equation

We consider Langmuir Equation

At low pressure, the value of KP<<1. Therefore,

The above equation shows linear variation between extent of adsorption of gas and pressure.

At high pressure value of KP>>1

The extent of adsorption, θ is independent of pressure at high pressure conditions. The reaction at this stage becomes zero order

Combining the results of the above equations we can conclude that

Equation (3) is in agreement with Freundlich adsorption equation.

We can say that Freundlich adsorption equation is a special case of Langmuir equation.

Limitations of Langmuir Adsorption Equation

1. The adsorbed gas has to behave ideally in the vapor phase. This condition can be fulfilled at low pressure conditions only. Thus Langmuir Equation is valid under low pressure only.

2. Langmuir Equation assumes that adsorption is monolayer. But, monolayer formation is possible only under low pressure condition. Under high pressure

condition the assumption breaks down as gas molecules attract more and more molecules towards each other. BET theory proposed by Brunauer, Emmett and Teller explained more realistic multilayer adsorption process.

3. Another assumption was that all the sites on the solid surface are equal in size and shape and have equal affinity for adsorbate molecules i.e. the surface of solid if homogeneous. But we all know that in real solid surfaces are heterogeneous.

4. Langmuir Equation assumed that molecules do not interact with each other. This is impossible as weak force of attraction exists even between molecules of same type.

5. The adsorbed molecules has to be localized i.e. decrease in randomness is zero (ΔS = 0).This is not possible because on adsorption liquefaction of gases taking place, which results into decrease in randomness but the value is not zero.

From above facts we can conclude that, Langmuir equation is valid under low pressure conditions.

BET adsorption Isotherm

BET Theory put forward by Brunauer, Emmett and Teller explained that multilayer formation is the true picture of physical Adsorption.

One of the basic assumptions of Langmuir Adsorption Isotherm was that adsorption is monolayer in nature. Langmuir adsorption equation is applicable under the conditions of low pressure. Under these conditions, gaseous molecules would possess high thermal energy and high escape velocity. As a result of this less number of gaseous molecules would be available near the surface of adsorbent.

Under the condition of high pressure and low temperature, thermal energy of gaseous molecules decreases and more and more gaseous molecules would be available per unit surface area. Due to this multilayer adsorption would occur. The multilayer formation was explained by BET Theory. The BET equation is given as

The another form of BET equation is

Where Vmono be the adsorbed volume of gas at high pressure conditions so as to cover the surface with a unilayer of gaseous molecules,

the ratio is designated C. K1 is the equilibrium constant when single molecule adsorbed per vacant site and KL is the equilibrium constant to the saturated vapor liquid equilibrium.

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 -1800C.

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 -1950C on Iron (Fe) catalyst and Nitrogen (N2 (g)) adsorbed at -1950C 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 790C on silica gel or Iodine (I2) at 790C 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 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 (PS) of the gas.

Examples of Type IV Adsorption Isotherm are of adsorption of Benzene on Iron Oxide (Fe2O3) at 500C and adsorption of Benzene on silica gel at 500C.

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 1000C on charcoal.

Type IV and V shows phenomenon of capillary condensation of gas.