1. Adsorption and Catalysis
Dr. King Lun Yeung
Department of Chemical Engineering
Hong Kong University of Science and Technology
CENG 511
Lecture 3

2. Physical Adsorption Texture and morphology
specific surface area of catalyst
pore size
pore shape
pore-size distribution (same size or various sizes?)
pore volume

3. Pore Size and Shape Pore Diameter Pore Shape micropores (< 2 nm)
mesopores (2 – 50 nm)
macropores (> 50 nm)
Pore Shape
cylinder
slit
ink-bottle
wedge

4. Pore Size and Shape
Pore Structure
Silica
Carbon
Zeolite

5. Pore Size and Shape Why is it important?
it dictates the diffusion process through the material.

6. Pore Size and Shape Why is it important?
directly affect the selectivity of the catalytic reaction.

7. Pore Size and Shape Measurement Techniques 1 10 100 1000 10000
Pore diameter (nm)
Micro
Meso
Macro N 2
capillary condensation Hg
porosimetry

8. N2 Physisorption Adsorption and Desorption Isotherms Desorption

9. N2 Physisorption Adsorption and Desorption Isotherms III n p / VI V IVI V I II B IV

10. Isotherms Type I Langmuir Adsorption Isotherm Assumptions:/
Assumptions:
homogeneous surface
(all adsorption sites energetically identical)
monolayer adsorption (so no multilayer adsorption)
no interaction between adsorbed molecules

11. Isotherms Type II Type IV Multilayer adsorption (starting at B)/ n ad B
Multilayer adsorption (starting at B)
Common for pore-free materials
Type IV n ad p / B
Similar to II at low p
Pore condensation at high p

12. Isotherms Type III Type IVn ad p /
Strong cohesion force between adsorbed molecules, e.g. when water adsorbs on hydrophobic activated carbon
Type IV n ad p /
Similar to III at low p
Pore condensation at high p

13. Physisorption
Surface area measurement

14. Physisorption
Different Adsorbates Used in Physisorption Studies

15. N2 Physisorption Adsorption and Desorption Isotherms
Langmuir Adsorption?
No:
strong adsorption at low p due to condensation in micropores
at higher p saturation due to finite (micro)pore volume

16. BET Isotherm

17. BET Isotherm

18. BET Isotherm Nonporous Silica and Alumina Low p/p0:
filling of micropores
favoured adsorption at most reactive sites (heterogeneity)
High p/p0:
capillary condensation
BET equation
Range 0.05 < p/p0 < 0.3 is used to determine SBET

19. Pore Size and Surface Area

20. Pore Size Distribution
Kelvin Equation

21. Pore Size Distribution
Kelvin Equation
Cylindrical pore
Ink-bottle pore
Pore with shape of interstice between close-packed particles
Adsorbed layer t dp dm

22. Kelvin Equation

23. Kelvin Equation
Pore filling Model
Cylindrical Pore Channel

24. Hysteresis Loop
Information on pore shape

25. Pore Size Distribution
t-Method
nad t
Proportional to St
Note:
nad is experimental result
t is calculated from correlation t versus p

26. Kelvin Equation
t-Method

27. Kelvin Equation
Shape of t-plots

28. Kelvin Equation
Interpretation of t-Plot
-alumina

29. Kelvin Equation
Pore Size Distribution
-alumina
r = t + 2V
RTIn P0 P

30. Mercury Porosimetry
Pore Size Distribution

31. Mercury Porosimetry
Pore Size Distribution
-alumina

32. N2 Physisorption versus Hg Porosimetry
Hg cannot penetrate small (micro)pores, N2 can
Uncertainty of contact angle and surface tension values
Cracking or deforming of samples

33. Texture Data on Common Catalysts

34. N2 Adsorption Isotherms & Pore Volume Distributions
wide-pore silica
-alumina

35. N2 Adsorption Isotherms & Pore Volume Distributions
-alumina
activated carbon

36. N2 Adsorption Isotherms & Pore Volume Distributions
Raney Ni
ZSM-5

37. Hg Intrusion Curves & Pore Volume Distributions
wide-pore silica
-alumina

38. Hg Intrusion Curves & Pore Volume Distributions
-alumina
activated carbon

39. Hg Intrusion Curves & Pore Volume Distributions
Raney Ni
ZSM-5

40. BET- & t-plots
wide-pore silica
-alumina

41. BET- & t-plots
-alumina
activated carbon

42. BET- & t-plots
Raney Ni
ZSM-5

43. Chemisorption Surface Characterization Specific surface area of phases
Types of active sites
Number of active sites
Reactivity of active sites
Stability of active sites

44. Chemisorption
Metal Dispersion

45. Adsorption Mode

46. Adsorption Stoichiometry

47. Particle Size and Dispersion

48. Supported Metal Particles

49. Number of Surface Atoms

50. Pulse Chemisorption

51. Monolayer capacity: 0.06 mmol / g Pt
Pulse Chemisorption
On-line Thermoconductivity Detector
Monolayer capacity: mmol / g Pt

52. Step Chemisorption
On-line Mass Spectrometer

53. Temperature Programmed Desorption
Adsorption Site Differentiation
NH3 desorption from HZSM-5

54. Temperature Programmed Desorption
Adsorption Energetics
After ammonia saturation the sample is degassed at 120 °C for 60 minutes
Heating Rate of 5, 10, 15 and 20 °C/min

55. Temperature Programmed Desorption
Adsorption Energetics
12.49
A factor
24.51
Ed (kJ/mole)
5.4639
Intercept
Slope
Beta = heating rate [K / min]
Tp = maximum desorption peak
temperature
Ed = Desorption energy [Kj / mole]
A = Arrhenius factor
R = [J / mol K]

56. Temperature Programmed Reduction
characterisation of oxidic catalysts and other reducible catalysts
qualitative information on oxidation state
quantitative kinetic data
optimisation of catalyst pretreatment

57. Temperature Programmed Reduction
Fe2O3

58. Temperature Programmed Reduction
Fe2O3
Dry H2/Ar

59. Temperature Programmed Reduction
Fe2O3
Wet H2/Ar (3% H2O)

60. Temperature Programmed Reduction
Fe2O3

61. Kinetic Models for Reduction

62. Infrared Spectroscopy

63. Infrared Spectroscopy
Reactor Cell
Transmittance
DRIFTS

64. Analysis of Catalyst Preparation
Surface Hydroxyl Groups
NH4ReO4 Alumina
Dry impregnation
Drying 383 K, 16 h
Calcination 323 K, 2 h
Re2O7/ Alumina

65. Analysis of Catalyst Preparation

66. IR Probe Molecule
Acidity Measurement

67. IR Probe Molecule
Acidity Measurement

68. Kelvin Equation
Pore Size Distribution

69. Kelvin Equation
Pore Size Distribution

70. In-Situ Reaction Study
TCE Photocatalytic Oxidation

71. In-Situ Reaction Study
PCO of Ethylene

72. In-Situ Reaction Study
PCO of 1,1-DCE

73. In-Situ Reaction Study
PCO of cis-1,2-DCE

74. In-Situ Reaction Study
PCO of trans-1,2-DCE

75. In-Situ Reaction Study
PCO of TCE

76. In-Situ Reaction Study
PCO of Tetrachloroethylene