Adsorption isotherms

Adsorption, types of adsorption, physisorption, chemisorption, mechanism of adsorption, Difference between adsorption and absorption, Factors affecting adsorption, applications of adsorption-
Gas masks
Adsorption indicators
Chromatographic separation
Removal of coloring matter
Heterogeneous catalysis
Controlling humidity
Curing diseases
Froth flotation process
Production of high vacuum
Purification,
adsorption equilibrium, adsorption isotherms, Langmuir isotherm- assumptions, Langmuir equation, limitations of Langmuir isotherm, equation, Freundlich isotherm- Assumptions of Freundlich Isotherm,Limitations of Freundlich Isotherm,Differences between Freundlich and Langmuir adsorption isotherms, BET isotherm-Drawbacks of BET adsorption theory, Types of BET adsorption isotherms, Differences between Langmuir and BET adsorption isotherm, Applications of BET isotherm, Why is Langmuir surface area always higher than BET surface area?
Temkin isotherm, D-R isotherms, Drawbacks of D-R Isotherm, Drawbacks of Temkin Isotherm, Uses of D-R isotherms, applications of adsorption isotherms -Spontaneity,
Exothermicity,
Percentage removal of adsorbate,
Langmuir parameters- maximum adsorbent uptake and affinity between adsorbent and adsorbate,Freundlich parameters- adsorption capacity of adsobents.
BET isotherms- specific surface area, pore size distribution curves
D-R parameters- adsorption mechanism
Temkin parameters- adsorbent-adsobate interactions

1. Adsorption isotherms Dr. K.K. Chahal Jaskirat Kaur (L-2016-BS-265-M) 1

2. C ontents 2

3. Adsorption 3

4. Some I mportant Terms 4

5. Some commonly used adsorbents 5

6. Examples of adsorption 6

7. Difference between adsorption and absorption 7 It is the phenomenon in which the higher conc. of particles of gas or liquid on surface than in bulk. Surface phenomenon Rapid in beginning but slowly decreases. Silica gel absorbs water vapour . It is the phenomenon in which the particles of gas or liquid uniformly distributed throughout. Bulk phenomenon Occurs at uniform rate Anhydrous CaCl 2 absorbs water. Adsorption Absorption

8. continued…… 8 Absorption Adsorption

9. Mechanism of Adsorption 9 Adsorption is surface phenomenon. It arises because of unbalanced forces on the surface of solids and liquids. The surface is under tension due to unbalanced forces. The surface of the solid or liquid tends to satisfy their residual forces by attracting and retaining the molecules of other species when brought in contact with them.

10. Continued….. 10

11. Types of Adsorption 11

12. continued…… PROPERTIES PHYSISORPTION CHEMISORPTION Bonding Weak, long range forces, van der Waals interactions. Strong, short range forces, Chemical bonding involving orbital overlap and charge transfer. Enthalpy 20-40 kJ mol -1 80-240 kJ mol -1 Saturation Multi-layer Mono-layer Nature reversible irreversible Effect of temperature Occurs at low temperature and decreases with increase in temperature. Occurs at high temperature and increases with increase in temperature. Effect of pressure Increases with increase in pressure of adsorbate. Decreases with increase of pressure Activation energy Not needed High activation energy is needed. Surface specificity No Yes 12

13. continued …… 13 Lenard-Jones model for physisorption and chemisorption of molecules.

14. Factors affecting adsorption 14

15. Applications of Adsorption 15

16. Adsorption equilibria 16

17. Continued…. 17

18. Adsorption isotherm 18

19. Continued… In adsorption , adsorbate gets adsorbed on adsorbent . The direction of equilibrium would shift in that direction where the tension can be relieved. In excess of pressure to the equilibrium system, the equilibrium will shift in the direction where the number of molecules decreases. Number of molecules decreases in forward direction . Therefore, forward direction of equilibrium will be favored . 19

20. Basic A dsorption isotherm 20

21. Types of isotherms 21

22. Langmuir Adsorption isotherm 22

23. Basic assumptions of Langmuir Model 23

24. Langmuir Adsorption isotherm The linear form of the Langmuir isotherm Where q e = Quantity of adsorbate adsorbed per unit weight of adsorbent at equilibrium (mgg -1 ) C e = Concentration of adsorbate at equilibrium in solution after adsorption (mgl -1 ) q max = Maximum adsorption capacity(mgg -1 ) b = Langmuir adsorption equilibrium constant (lmg -1 )   24

25. Continued…. Plot of original equation 25

26. Continued…. The rearranged form of the Langmuir isotherm Where q e = Quantity of adsorbate adsorbed per unit weight of adsorbent at equilibrium (mgg -1 ) C e = Concentration of adsorbate at equilibrium in solution after adsorption (mgl -1 ) q max = Maximum adsorption capacity(mgg -1 ) b = Langmuir adsorption equilibrium constant ( lmg -1 )     26

27. Continued…. Isotherm is plotted between 1/q e versus 1/C e and the slope of the graph gives the value of q max and intercept gives the value of b. 27

28. Limitations of Langmuir isotherm 28

29. Freundlich Adsorption Isotherm 29

30. Assumptions of Freundlich Isotherm 30

31. Freundlich Adsorption Isotherm 31 The linear equation x/m = k.P 1/n (n > 1) Or x/m = k.C 1/n (n > 1) where x is the mass of the gas adsorbed on mass m of the adsorbent P is pressure, C is equilibrium  concentration of adsorbate in solution, k and n are constants.

32. Continued… The linear form of Freundlich isotherm is or Where q e = Extent of adsorbate adsorbed per unit weight of adsorbent at equilibrium (mgg -1 ) P e = Equilibrium pressure of adsorbent in solution after adsorption C e = Equilibrium concentration of adsorbent in solution after adsorption (mgl -1 ) K f = Freundlich constant indicating adsorption capacity n = Empirical constant   32

33. Continued…. 33

34. Continued… The parameters of the Freundlich isotherm can be determined by plotting logq e versus logC e . 34

35. Limitations of Freundlich Isotherm 35

36. Differences between Freundlich and Langmuir adsorption isotherms 36

37. Brunauer –Emmett–Teller isotherm 37

38. Continued… 38

39. Continued…. 39 The concept of the theory is an extension of the Langmuir theory, which is a theory for monolayer molecular adsorption, to multilayer adsorption with the following hypotheses G as molecules physically adsorb on a solid in layers infinitely Gas molecules only interact with adjacent layers Langmuir theory can be applied to each layer

40. Continued… The BET equation (P/P o )/Q ((P/P o )-1) = 1/kQ m + ( k-1 /Q m k )(P/P o ) Where P and P o are the equilibrium and the saturation pressure of adsorbates at the temperature of adsorption,  Q (in moles)  is the amount adsorbed on 1g of adsorbent and   Q m  is the  monolayer adsorption capacity .   k  is the  BET constant . 40

41. Continued… Plot of BET isotherm 41

42. Drawbacks of BET adsorption theory 42

43. Applications of BET isotherm 43

44. Types of BET adsorption isotherms 44

45. Type I BET Adsorption Isotherm 45

46. Type II Adsorption Isotherm 46

47. Type III Adsorption Isotherm 47

48. Type IV BET Adsorption Isotherm 48

49. Type V BET Adsorption Isotherm 49

50. Type VI BET Adsorption Isotherm 50

51. Differences between Langmuir and BET adsorption isotherm 51

52. BET adsorption isotherm 52

53. Dubinin Raduhkevitch (D-R) adsorption isotherm : 53

54. Continued……. The linear form of D-R model Where Q e =amount of adsorbate adsorbed per unit weight of adsorbent at equilibrium (mgg -1 ) Q m = Maximum adsorption capacity of adsorbent ( mgg 1 ) β = constant related to adsorption energy  = polanyi potential (kJ 2 mol -2 )   54

55. Continued…….  can be acquired from the following relation Where, R is gas constant T is the temperature in (K) C e is the concentration of adsorbate at equilibrium in solution after adsorption(mgl -1 ). The experimental data can be evaluated by plotting lnQ e against  2 . The value of Q m and β are estimated from the intercept and slope respectively. 55

56. Continued……. 56 The experimental data is evaluated by plotting lnq e against  2 .

57. Continued……. 57

58. Continued……. 58

59. Uses of D-R isotherms 59

60. Drawbacks of D-R Isotherm 60

61. Temkin Adsorption Isotherm 61

62. Continued……. Tempkin model was: Where K T = equilibrium binding constant (lmg -1 ) B T = heat of adsorption ( Tempkin constant) (Jmol -1 ) b T = Tempkin isotherm constant related to variation of adsorption energy (Jmol -1 ), B T = R is the gas constant (8.314 JK -1 mol -1 ), T = temperature (K)   62

63. Continued……. 63 Plot of amount adsorbed q e against lnC e

64. Drawbacks of Temkin Isotherm 64

65. Applications of Adsorption Isotherms 65

66. Continued……. 66

67. Summary Adsorption-The phenomenon of higher concentration of any species of solid or liquid or gas at the surface than in the bulk of a material is known as adsorption. Desorption-The process of removal of adsorbed substances from the surface on which it is absorbed. Some commonly used adsorbents- Silica , Activated charcoal , Alumina , Bentonite clay, Zeolites, Bagasse Absorption is the phenomenon in which the particles of gas or liquid uniformly distributed throughout the bulk and it occurs at uniform rate. While, adsorption is surface phenomenon. It arises because of unbalanced forces on the surface of solids and liquids. 67

68. Continued……. The extent of adsorption increases with the increase of surface area per unit mass of adsorbent at given temperature and pressure. Types of adsorption physisorption and chemisorption Factors affecting adsorption temperature, pressure, nature of adsorbent and adsorbate Adsorption isotherm- It is the graph between the amounts of adsorbate (x) adsorbed on the surface of adsorbent (m) and pressure (P) at constant temperature. Types of adsorption isotherms-Langmuir isotherm, Freundlich isotherm, BET isotherm, DR isotherm, Temkin isotherm 68

69. Continued……. The Langmuir adsorption assumed that fix number of accessible sites are available on the adsorbent surface, with same energy. Surface is homogeneous and monolayer adsorption takes place. Fruendlich isotherm- This isotherm was an empirical expression that accounts for surface heterogeneity by multilayer adsorption, exponential distribution of active sites of adsorbent and their energies towards adsorbate . Freundlich adsorption isotherm failed at higher pressure. The BET theory applies to systems of multilayer adsorption and usually utilizes probing gases that do not chemically react with material surfaces as adsorbates to quantify specific surface area. 69

70. Continued……. The concept of the theory is an extension of the Langmuir theory, which is a theory for  monolayer molecular adsorption, to multilayer adsorption. Surface is assumed to be homogenous which the case is not always. Lateral interaction between the adsorbed molecules in neglected. The D-R equation is an adaptation of the earlier Polanyi potential theory of adsorption. D-R isotherm helps to determine the adsorption mechanism i.e. helps in distinguishing between the chemical adsorption and physical adsorption The D-R isotherm does not reduce to Henry's law at low pressures, which is a necessity for a thermodynamically consistent. 70

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