Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent

Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. This process differs from absorption, in which a fluid (the absorbate) is dissolved by or permeates a liquid or solid (the absorbent). While adsorption does often precede absorption, which involves the transfer of the absorbate into the volume of the absorbent material, alternatively, adsorption is distinctly a surface phenomenon, wherein the adsorbate does not penetrate through the material surface and into the bulk of the adsorbent. The term sorption encompasses both adsorption and absorption, and desorption is the reverse of sorption.

1. Surface Chemistry, Kinetics & Catalysis Reference books: Atkins' Physical Chemistry & Essentials of Physical Chemistry by Arun Bahl UNIT 3: Surface Chemistry, Kinetics & Catalysis Adsorption - Characteristics, Classification, Application, Adsorption isotherms - Freundlich , Langmuir & BET 1

2. Adsorption - Characteristics , Classification, Application, Adsorption isotherms - Freundlich , Langmuir & BET Definition: Adsorption is a process whereby a substance ( adsorbate or sorbate ) is accumulated on the surface of a solid (adsorbent, or sorbent). The adsorbate can be in a gas or liquid phase. Adsorption refers to the existence of a higher concentration of any particular component at the surface of a liquid or a solid phase. An example of an excellent adsorbent is the charcoal used in gas masks to remove poisons or impurities from a stream of air. INTRODUCTION TO ADSORPTION 2 Dr Busupalli

3. Adsorbent Adsorbate Solids that are used to adsorb gases or dissolved substances are called adsorbents ; examples: charcoal, alumina, silica gel The adsorbed molecules are usually referred to collectively as the adsorbate ; examples: gases like oxygen, nitrogen etc. Dr Busupalli 3

4. Difference between adsorption & absorption A b sorption is a b ulk phenomenon Adsorption is a surface phenomenon The word "adsorption" was coined in 1881 by German physicist Heinrich Kayser   Dr Busupalli 4

5. Surfaces for adsorption Source: Atkin’s Physical chemistry Dr Busupalli 5

6. Adsorption is accompanied by decrease in the free energy change of the system when, adsorption equilibrium is said to be established. If a material has a very high surface area, then lots of molecules ( adsorbate ) can stick/adsorb to the surface. Adsorption process is exothermic which means evolution of heat is accompanied in adsorption process i.e., ∆H (enthalpy) is negative. When adsorbate molecules are adsorbed on the surface, freedom of movement of molecules become restricted and this results in a decrease in entropy . Adsorption is a consequence of surface energy. In a bulk material, all the bonding requirements (be they  ionic,   covalent  or  metallic) of the constituent  atoms  of the material are fulfilled by other atoms in the material. But atoms on the surface of the adsorbent are not wholly surrounded by other adsorbent atoms and therefore can attract adsorbates . CHARACTERISTICS OF ADSORPTION Dr Busupalli 6

7. Factors affecting adsorption The extent of adsorption depends upon the following main factors: 1. Nature of adsorbate and adsorbent . Greater the surface area of the adsorbent, greater is the volume of the gas adsorbed. Different gases ( adsorbates ) are adsorbed differently by the same adsorbent at same temperature. 2. Activation of adsorbent . This is performed to increase the surface area of the adsorbent via making the surface of the adsorbent rough or subdividing the adsorbent to smaller grains. 3. Experimental conditions. ex: temperature , pressure, etc . Temperature - generally adsorption decreases with increase in temperature . Pressure – at constant temperature the adsorption increases with increase in pressure. Dr Busupalli 7

8. CLASSIFICATION OF ADSORPTION Types of adsorption Physical adsorption ( Physisorption ) Force of attraction existing between adsorbate and adsorbent are Van der Waal’s forces. 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. Chemical adsorption (Chemisorption ) Force of attraction existing between adsorbate and adsorbent are almost same strength as the chemical bonds, the adsorption is called chemical adsorption. In chemisorption the force of attraction is very strong, therefore adsorption cannot be easily reversed. Dr Busupalli 8

9. Differences between Physisorption & Chemisorption Physisorption Chemisorption 1. Force of attraction are Van der Waal's forces. 1. Forces of attraction are chemical bond forces (usually covalent bonds). 2. Low heat of adsorption usually in the range of 20-40 kJ/mol. 2. High heat of adsorption in the range of 40-400 kJ/mol. 3. It usually takes place at low temperature and decreases with increasing temperature. 3. It takes place at high temperature. 4. It is reversible. 4. It is irreversible. 5. It is related to the ease of liquefaction of the gas. 5. The extent of adsorption is generally not related to liquefaction of the gas. 6. It is not very specific. 6. It is highly specific. 7. It does not require any activation energy. 7. It requires activation energy. Dr Busupalli 9

10. APPLICATIONS OF ADSORPTION 1. Silica gel packets are kept in most products that we purchase, have you ever wondered why? Because of this application The silica gel packets keep the moisture out by allowing the moisture vapours to get adsorbed over the surface of silica or alumina gel particles. So next time, you may consider saving those tiny packets to use later as drying agents! Dr Busupalli 10

11. 2. Pollution Masks Amid this pandemic, masks have become an extended part of our body. If you ever try to cut open a pollution mask, you shall find it consists of two or more layers of fabric, and between the layers exists activated carbon granules or a filter sheet, which serves as a purifier. It allows clean air to reach our nostrils by adsorbing the dust and smoke particles. The activated carbon element or the filter sheet is the adsorbent, while the dust and smoke particles are adsorbates . Dr Busupalli 11

12. 3. In curing the diseases It should not be surprising at all to tell that adsorption is used in curing many diseases. The disease-causing germs get deposited on the surface of the drug taken against it, which later gets ejected from the body through natural processes. Dr Busupalli 12

13. 4. Charcoal gas masks The masks used in mining or other related activities are required to be designed with the utmost efficiency and care because the personnel working there have to deal with toxic and poisonous gases. In such applications, the manufacturers make use of adsorption properties of the substance like charcoal to filter out the unwanted gases or hazardous fumes to avoid any dangerous outcomes. Dr Busupalli 13

14. 5. Purification of water Another common example of adsorption is when you purify water using alum. During this process, a small piece of alum is inserted in water and is allowed to rest. A little later, you can observe the impurities getting combined to form a large cluster. Now, one can easily remove it by various separation methods, such as sedimentation, sieving, etc . Dr Busupalli 14

15. 6. Removing hardness from water Removing hardness from water signifies removing elements like calcium and magnesium for they make water hard. Usually, these elements can easily be removed by the process of ion exchange in which the cation /anion resins are added to water, the hardness causing elements get stuck to the resins and can be filtered out easily. Finally, providing us with softened water. Dr Busupalli 15

16. 7. Misty windows Every night a layer of water vapour gets deposited on the surface of windows, which is more prominently visible during cold weather. This is yet another example of adsorption from daily life. Therefore, the next time you write your name on the fog deposited on your windowpane, give thanks to the wonderful phenomenon of adsorption . And, this phenomenon can be applied in cleaning delicate surfaces such as glass etc. Dr Busupalli 16

17. 8. Decoloring of matter A lot of entities get contaminated due to the presence of impurities around them. This contamination leads to a color change that may be compensated by placing them in a decolorizing solution, such as fuller’s earth (clay material) or charcoal solution. The impurities get deposited or dissolved in the solution, leaving the entities decolored . Dr Busupalli 17

18. 9. In heterogeneous catalysis Adsorption plays a very important role in the heterogeneous catalysis processes. Reactants get adsorbed onto the surface of the catalyst in the first step. After the reaction is completed on the catalysts surface the products are then desorbed from the catalyst surface. As the entire process of adsorption and reaction and desorption occurs on the surface of the heterogeneous catalyst and not in the bulk of the catalyst, this signifies the role of adsorption in heterogeneous catalysis. Dr Busupalli 18

19. 10. Other applications of adsorption Adsorption in metallurgy : during the process of concentration of ore, for example in froth floatation process, adsorption of particles onto the froth occurs facilitating the concentration of the ore. Adsorption in chromatography : to separate pigments. Adsorption in virology : viruses first adsorb onto the hosts and then colonize the host machinery to replicate and to cause disease. Adsorption in polymer science : to produce polymer products such as non-stick coatings and biomedical devices. Polyelectrolytes adsorption (non-covalent layer-by-layer adsorption) onto solid surfaces is extremely useful in industries dedicated to oil recovery, nutrition, concrete etc . Dr Busupalli 19

20. ADSORPTION ISOTHERMS The free gas and the adsorbed gas are in dynamic equilibrium, and the fractional coverage (θ) of the surface depends on the pressure of the overlying gas. The variation of θ with pressure at a chosen temperature is called the adsorption isotherm . In simpler terms Adsorption isotherm describes the amount of adsorbate on the adsorbent as a function of its pressure (if gas) or concentration (for liquid phase solutes) at constant temperature. Different adsorption isotherms: 1) Langmuir adsorption isotherm 2) Freundlich adsorption isotherm 3) BET adsorption isotherm Dr Busupalli 20

21. Langmuir adsorption isotherm This simplest physically plausible isotherm is based on three assumptions : 1. Adsorption cannot proceed beyond monolayer coverage . 2. All sites are equivalent and the surface is uniform (that is, the surface is perfectly flat on a microscopic scale ). 3. The ability of a molecule to adsorb at a given site is independent of the occupation of neighbouring sites (that is, there are no interactions between adsorbed molecules). Dr Busupalli 21

22. The dynamic equilibrium is given as: A(g) + M(surface ) AM(surface) A(g) is gaseous adsorbate M(surface) is solid adsorbent AM (surface) is the material where the gaseous adsobate is adsorbed onto the solid adsorbent surface. with rate constants k a for adsorption and k d for desorption. The rate of change of surface coverage due to adsorption is proportional to the partial pressure p of A and the number of vacant sites N (1 − θ ), where N is the total number of sites : Dr Busupalli 22

23. The rate of change of θ due to desorption is proportional to the number of adsorbed species , N θ : At equilibrium there is no net change (that is, the sum of these two rates is zero), and solving (via kinetic derivation) for θ gives the Langmuir isotherm : p Dr Busupalli 23

24. In other words, at equilibrium, the rate of desorption is equal to the rate of adsorption. So , K 1 (1-θ)P = K 2 θ K 1 P – K 1 θP = K 2 θ K 1 P = θ( K 2  + K 1 P) Where, the rate of adsorption depends on available sites on the adsorbent for adsorption i.e., (1-θ) at partial pressure of the adsorbate P R 1  = K 1 (1-θ)P ………………….(i ) And the rate of desorption depends on the fraction covered θ R 2  = K 2 θ ……………………..( ii) where K 1  and K 2  are adsorptions and desorption constants respectively. ……………………..( iii ) Dr Busupalli 24

25. Dr Busupalli 25

26. Limitations of Langmuir adsorption isotherm 1) It assumes monomolecular adsorption onto the surface of the adsorbent, but in reality many layers of molecules could get adsorbed onto adsorbent. 2) It assumes no interaction between the adsorbed molecules, whereas in certain reactions there are noticeable interaction observed. 3) The Langmuir equation works reasonably well at low pressure but fails to provide adsorption information at high pressures. 4) The effect of temperature is not well considered. 5) Relation between heat of adsorption and surface area not explained. Thus these limitations of the Langmuir adsorption isotherm led to proposal of Several other isotherms such as Freundlich and BET among others. Dr Busupalli 26

27. Freundlich adsorption isotherm Dr Busupalli 27

28. The Langmuir isotherm is applied to monolayer adsorption on homogeneous sites, whereas the Freundlich isotherm suites are applied to multilayer adsorption on heterogeneous sites . Dr Busupalli 28

29. Limitations of Freundlich adsorption isotherm Freundlich adsorption isotherm is based on assumption that every adsorption site is equivalent. It is applicable to physical adsorption . Dr Busupalli 29

30. BET adsorption isotherm If the initial adsorbed layer can act as a substrate for further (for example, physical) adsorption , then, instead of the isotherm levelling off to some saturated value at high pressures , it can be expected to rise indefinitely. The most widely used isotherm dealing with multilayer adsorption was derived by Stephen Brunauer , Paul Emmett, and Edward Teller, and is called the BET isotherm : Salient features of BET isotherm: 1) Gas molecules physically adsorb on a solid in layers infinitely 2) gas molecules only interact with adjacent layers; and the Langmuir theory can be applied to each layer . 3) the enthalpy of adsorption for the first layer is constant and greater than the second (and higher). 4) the enthalpy of adsorption for the second (and higher) layers is the same as the enthalpy of liquefaction . Dr Busupalli 30

31. a BET isotherm plots the amount of gas adsorbed as a function of the relative pressure. There are five types of adsorption isotherms possible from the surface adsorption analyses of materials using BET. The BET Equation uses the information from the isotherm to determine the surface area of the sample, where X is the weight of nitrogen adsorbed at a given relative pressure ( P/Po ), X m  is monolayer capacity, which is the volume of gas adsorbed at standard temperature and pressure (STP), and C is constant. ( STP is defined as 273 K and 1 atm ) Dr Busupalli 31

32. BET Type I isotherm When P/P o  < 1 and C > 1  Type I is a pseudo-Langmuir isotherm because it depicts monolayer adsorption. A type I isotherm is obtained when P/P o  < 1 and C > 1 in the BET equation, where P/P o  is the partial pressure value and c is the BET constant, which is related to the adsorption energy of the first monolayer and varies from solid to solid. The characterization of microporous materials, those with pore diameters less than 2 nm, gives this type of isotherm. This isotherm shows that the extent of adsorption increases with pressure until it reaches saturation, at which point no further adsorption occurs. Volume or Dr Busupalli 32

33. BET Type II isotherm When C > 1 A type II isotherm is very different than the Langmuir model. The flatter region in the middle represents the formation of a monolayer. A type II isotherm is obtained when C > 1 in the BET equation. This is the most common isotherm obtained when using the BET technique. It is obtained when the bilayer is formed only after the monolayer has been fully formed, and the trilayer is formed only after the bilayer has been fully formed. Dr Busupalli 33

34. BET Type III isotherm When C < 1 A type III isotherm is obtained when the C < 1 and shows the formation of a multilayer. It is obtained when the formation of monolayers, bilayers, trilayers , and other layers all take place at the same time, resulting in an almost exponential increase in the amount of adsorption. Dr Busupalli 34

35. BET Type IV isotherm At the lower pressure regions, it shows the formation of a monolayer followed by a formation of multilayers. BET surface area characterization of mesoporous materials, which are materials with pore diameters between 2 - 50 nm, gives this type of isotherm. Dr Busupalli 35

36. BET Type V isotherm This type of adsorption isotherm is obtained only when intermolecular attraction effects are large, and adsorption takes place in pores and capillaries . Type V isotherms are very similar to type IV isotherms. Dr Busupalli 36