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

2. Adsorption versus Absorption Adsorption Absorption H H H H H H H HH H H H H H H H HH H 2 adsorption on palladium H H H H H H H H H H H H H H H H H H H 2 absorption  palladium hydride Surface processbulk process

3. Nomenclature Substrate or adsorbent: surface onto which adsorption can occur. example: catalyst surface, activated carbon, alumina Adsorbate: molecules or atoms that adsorb onto the substrate. example: nitrogen, hydrogen, carbon monoxide, water Adsorption: the process by which a molecule or atom adsorb onto a surface of substrate. Coverage: a measure of the extent of adsorption of a specie onto a surface Exposure: a measure of the amount of gas the surface had been exposed to ( 1 Langmuir = 10 -6 torr s) H H H H H H H HH HH H H H adsorbate adsorbent coverage  fraction of surface sites occupied

4. Types of Adsorption Modes Physical adsorption or physisorption Chemical adsorption or chemisorption Bonding between molecules and surface is by weak van der Waals forces. Chemical bond is formed between molecules and surface.

5. Characteristics of Chemi- and Physisorptions Chemisorption virtually unlimited range wide range (40-800 kJmol -1 ) marked difference for between crystal planes often dissociative and irreversible in many cases limited to a monolayer activated process Physisorption near or below T bp of adsorbate (Xe < 100 K, CO 2 < 200 K) heat of liquifaction (5-40 kJmol -1 ) independent of surface geometry non-dissociative and reversible multilayer occurs often fast, non-activated process Properties Adsorption temperature Adsorption enthalpy Crystallographic specificity Nature of adsorption Saturation Adsorption kinetic

6. Analytical Methods for Establishing Surface Bonds Infrared Spectroscopy Atoms vibrates in the I.R. range chemical analysis (molecular fingerprinting) structural information electronic information (optical conductivity) IR units: wavenumbers (cm-1), 10 micron wavelength = 1000 cm-1 Near-IR: 4000 – 14000 cm-1 Mid-IR: 500 – 4000 cm-1 Far-IR: 5 – 500 cm-1 http://infrared.als.lbl.gov/FTIRinfo.html

7. I.R. Measurement

8. I.R. Spectrum of CO 2 Symmetric Stretch Assymmetric Stretch Bending mode O C O A dipole moment = charge imbalance in the molecule

9. I.R. Spectrum of NO on Pt Temperature increases Adsorption decreases Molecular conformation changes

10. I.R. Spectrum of HCN on Pt 0.15 L HCN, 100 K weak chemisorption 1.5 L HCN, 100 K physisorption 30 L HCN, 200 K dissociative chemisorption H- C  N Pt (H-CN)  (HCN)  (HCN) Pt H- C  N (CN) C  N Pt (a)(b)(c)

11. Adsorption Rate R ads = k C x x - kinetic order k - rate constant C - gas phase concentration R ads = k’ P x x - kinetic order k’ - rate constant P - partial pressure of molecule R ads = A C x exp (-Ea/RT) Activation energyFrequency factor Temperature dependency of adsorption processes

12. Molecular level event Adsorption Rate R ads = S F = f(  ) P/(2  mkT) 0.5 exp(-Ea/RT) Sticking coefficient S = f(  ) exp(-Ea/RT) where 0 < S < 1 Flux (Hertz-Knudsen) F = P/(2  mkT) 0.5 where P = gas pressure (N m -2 ) m = mass of one molecule (Kg) T = temperature (K) (molecules m -2 s -1 ) Note: f(  ) is a function of surface coverage special case of Langmuir adsorption f(  ) = 1-  E(  ), the activation energy is also affected by surface coverage

13. Sticking Coefficient S = f(  ) exp(-Ea/RT) where 0 < S < 1 S also depends on crystal planes and may be influenced by surface reconstruction. Tungsten

14. Sticking Coefficient

15. Steering Effects

16. Surface Coverage (  ) Estimation based on gas exposure R ads = dN ads /dt = S F N ads  S F t Exposure time Molecules adsorbed per unit surface area Nearly independent of coverage for most situations

17. Adsorption Energetics d surface adsorbate Potential energy (E) for adsorption is only dependent on distance between molecule and surface P.E. is assumed to be independent of: angular orientation of molecule changes in internal bond angles and lengths position of the molecule along the surface

18. Physisorption versus chemisorption Adsorption Energetics surface  E(ads)  E(ads) <  E(ads) Physisorption Chemisorption small minima large minima weak Van der Waal formation of surface attraction force chemical bonds repulsive force attractive forces Chemisorption

19. Physical Adsorption d metal surface nitrogen Van der Waal forces E(d) 0.3 nm Note: there is no activation barrier for physisorption  fast process Applications: surface area measurement pore size and volume determination pore size distribution

20. The Brunauer-Emmett-Teller Isotherm BET isotherm where: n is the amount of gas adsorbed at P n m is the amount of gas in a monolayer P 0 is the saturation pressure n   at P = P 0 C is a constant defined as: H 1 and H L are the adsorption enthalpy of first and subsequent layers

21. BET Isotherm Assumptions adsorption takes place on the lattice and molecules stay put, first monolayer is adsorbed onto the solid surface and each layers can start before another is finished, except for the first layer, a molecule can be adsorbed on a given site in a layer (n) if the same site also exists in (n-1) layer, at saturation pressure (P 0 ), the number of adsorbed layers is infinite (i.e., condensation), except for the first layer, the adsorption enthalpy (H L ) is identical for each layers.

22. Activated Carbon Surface area ~ 1000 m 2 /g

23. Surface Area Determination BET surface area by N 2 physisorption - adsorption  - desorption Plot P/n(P 0 -P) versus P/P 0 calculate c and nm from the slope (c-1/ n m c) and intercept (1/n m c) of the isotherm measurements usually obtained for P/P 0 < 0.2 c = 69.25 n m = 4.2 x 10 -3 mol Area = 511 m 2 /g c = 87.09 n m = 3.9 x 10 -3 mol Area = 480 m 2 /g

24. BET Measurements Degassing Pure gas introduces into supply chamber  constant P 1 T 1 are recorded  V 1 Gas flows into adsorption cell P 2 and T 2 are recorded when equilibrium is reached  V 2 Volumetric Method

25. BET Measurements Dynamic Method Degassing Flow carrier gas (He) Pulse N 2 /He into adsorption cell at a given P N2 Record the amount of nitrogen adsorbed using TCD Calculate surface area (Rouquerol, 1999)

26. BET Measurements Gravimetric Method Degassing Record initial weight of adsorbent M 1 Introduce pure gas into adsorption cell Record the adsorbent equilibrium weight M 2 Record the equilibrium pressure (Rouquerol, 1999)

27. Adsorption Isotherm Adsorption Isotherm: –The equilibrium relationship between the amount adsorbed and the pressure or concentration at constant temperature (Rouquerol et al., 1999). Importance of Classification –Providing an efficient and systematic way for theoretical modeling of adsorption and adsorbent characteristics determination Rouqerol, F., J., Rouquerol and K., Sing, Adsorption by Powders and Porous Solids: Principles, Methodology and Applications, Academic Press, London (1999).

28. Adsorption Isotherm IUPAC Classification

29. Adsorption Isotherm IUPAC Classification

30. Adsorption Isotherm IUPAC Classification * Do, D. D., Adsorption Analysis: Equilibria and Kinetics, Imperial College Press, London (1998).

31. Adsorption Isotherm Capillary Condensation Mesopores  Capillary condensation  Hysteresis occurs Different hysteresis  Different network structure Narrow distribution of uniform pores  Type IVa Complex structure made up of interconnected networks of different pore sizes and shapes  Type IVb

32. Adsorption Isotherm Type VI Isotherm Highly uniform surface  Layer by layer adsorption  Stepped isotherm Example: Adsorption of simple non-porous molecules on uniform surfaces (e.g. basal plane of graphite)

33. Adsorption Isotherm Composite Isotherm N 2 adsorption in (a) micropores and (c) micropores and mesopores Type I Type I & IV (Rouquerol, 1999)

34. Chemical Adsorption d Pt surface CO E(d) rere Note: there is no activation barrier for adsorption  fast process, there us an activation barrier for desorption  slow process. Applications: active surface area measurements surface site energetics catalytic site determination = strength of surface bonding = equilibrium bond distance =  H(ads) Ea(ads) = 0 Ea(des) = -  H(ads)

35. Chemical Adsorption Processes Physisorption + molecular chemisorption d E(d) physisorption chemisorption CO

36. Chemical Adsorption Processes Physisorption + dissociative chemisorption d E(d) dissociation chemisorption H2H2 H 2  2 H physisorption atomic chemisorption Note: this is an energy prohibitive process

37. Chemical Adsorption Processes Physisorption + molecular chemisorption physisorption/ desorption chemisorption CO d E(d) physisorption atomic chemisorption

38. Chemical Adsorption Processes Physisorption + molecular chemisorption direct chemisorption CO d E(d) physisorption atomic chemisorption

39. Chemical Adsorption Processes Energy barrier Ea(ads) ~ 0 Ea(ads) > 0

40. Chemical Adsorption Processes Energy barrier ~ -  H(ads) - E a des = -  E(ads) Chemical Adsorption is usually an energy activated process.

41. Formation of Ordered Adlayer Ea(surface diffusion) < kT activated carbon CH 4 Krypton

42. Formation of Ordered Adlayer Chlorine on chromium surface

43. Adsorbate Geometries on Metals Hydrogen and halogens Hydrogen 1-H atom per 1-metal atom H-H HH HH 2-D atomic gas Halogens high electronegativity  dissociative chemisorption Halogen atom tend to occupy high co-ordination sites: X-X XX XX ionic bonding (111) (100) XX XX compound

44. Adsorbate Geometries on Metals Oxygen and Nitrogen (111) (100) Oxygen both molecular and dissociative chemisorption occurs. molecular chemisorption   -donor or  -acceptor interactions. dissociative chemisorption  occupy highest co-ordinated surface sites, also causes surface distorsion. O=O OO   Nitrogen molecular chemisorption   -donor or  -acceptor interactions. NNNN NNNN

45. Adsorbate Geometries on Metals Carbon monoxide forms metal carbides with metals located at the left-hand side of the periodic table. molecular chemisorption occurs on d-block metals (e.g., Cu, Ag) and transition metals COCO COCO Terminal (Linear) all surface Bridging (2f site) all surface Bridging (3f hollow) (111) surface CC CC metal carbide

46. Adsorbate Geometries on Metals Ammonia and unsaturated hydrocarbons Ammonia NH3NH3 NH 2 (ads) + H (ads)  NH (ads) + 2 H (ads)  N (ads) + 3 H (ads) Ethene 2 HC=CH 2

47. Active Surface Area Measurement  ost common chemisorption gases: hydrogen, oxygen and carbon monoxide Pulse H 2, O 2 or CO gases exhaust carrier gas helium or argon thermal conductivity cell (TCD) furnace catalyst

48. Catalyst Surface Area and Dispersion Calculation Pulse H 2 then titrate with O 2 exhaust carrier gas helium or argon thermal conductivity cell (TCD) furnace 1 g 0.10 wt. % Pt/  -Al 2 O 3 T = 423 K, P = 1 bar (STP) 3.75 peaks (H 2 ) 4.50 peaks (O 2 ) 100  l Avogrado’s number: 6.022 x 10 23 Pt lattice constant: a = 3.92 (FCC) Calculate surface area of Pt and its dispersion.

49. Isotherms Langmuir isotherm S - * + A (g)  S-A surface sites Adsorbed molecules  H(ads) is independent of  the process is reversible and is at equilibrium [S-M] [S - *] [A] K =  S-M] is proportional to  [S-*] is proportional to 1-  [A] is proportional to partial pressure of A

50. Isotherms Langmuir isotherm  (1-  ) P b = Where b depends only on the temperature bP 1+ bP  = Molecular chemisorption Where b depends only on the temperature (bP) 0.5 1+ (bP) 0.5  = Dissociative chemisorption

51. Variation of  as function of T and P   bP at low pressure   1 at high pressure   P P b T b  when T  b  when  H(ads) 

52. Determination of  H(ads)  P InP T T ii 1/T (P 1, T 1 ) (P 2, T 2 )  InP (   ads  R  1/T )  =const =

53. Adsorption Isotherms

54. Henry’s Adsorption Isotherm Special case of Langmuir isotherm bP << 1  = bP V = k’P where k’ = bV 

55. The Freundlich Isotherm Adsorption sites are distributed exponentially with  H(ads)  H(ads)   i (1-  i ) b i P =   i N i  N i  = R  A In  = InP + B kP 1/n  = Valid for low partial pressure most frequently used for describing pollutant adsorption on activated carbons

56. The Temkin Isotherm  H(ads) decreases with  A InBP  =  H(ads)  Valid at low to medium coverage gas chemisorption on clean metal surfaces

57. Thermal Desorption Spectroscopy Thermal desorption spectra of CO on Pd(100) after successive exposure to CO gases 0.2 - 50 L

58. Chemical Adsorption d Pt surface CO E(d) rere Note: there is no activation barrier for adsorption  fast process, there us an activation barrier for desorption  slow process. Applications: active surface area measurements surface site energetics catalytic site determination = strength of surface bonding = equilibrium bond distance =  H(ads) Ea(ads) = 0 Ea(des) = -  H(ads)

59. Thermal Desorption Spectroscopy Thermal desorption spectra of CO on Pd(100) after successive exposure to CO gases Desorption Rate { -dNa dT dt } = N a m k exp ( -E d RT ) Linear heating rate T = T 0 +  t dT dt =  Assuming k and E d are independent of coverage and m = 1 (i.e., first order desorption) 0.2 - 50 L -dNa dT d -dNa dT [ ] E d RT p 2 = exp ( -E d RT ) k 

60. Thermal Desorption Spectroscopy Determination of E des using different heating rates (  ) E d RT p 2 = exp ( -E d RT ) k  slope, m  Ea TPD provides important information on adsorption/desorption energetics and adsorbate-surface interactions.

61. Thermal Desorption Spectroscopy Thermal desorption spectra of CO on Ni(100) after successive exposure to CO gases 0.2 - 50 L Assuming k and E d are independent of coverage and m = 2 (i.e., first order desorption) -dNa dT d -dNa dT [ ] Second order desorption E d RT p 2 = exp ( -E d RT ) k  2(N a ) p Characterized by a shift in the peak maxima toward lower temperature as the coverage increases

62. Activation Energies for CO Desorption

63. Influence of Surface Overlayer Catalyst poison, strong adsorbates and coke Sulfur-treated catalyst Clean catalyst CO desorption

64. Ordered Adsorbate layer H 2 /Rh(110) O 2 /Rh(110)

65.    TPD from Rh(110) Thermal Desorption Spectroscopy

66. Ordered Adsorbate layer benzene/ZnO(1010)

67. Kelvin Probe Measures the change in work function (  ) Typical Kelvin probe for adsorption studies Scanning Kelvin probe for surface work function (i.e., elemental and compositional) imaging also known as scanning electrical field microscopy

68. Kelvin Probe Basic principle Vibrating capacitor measures   is the least amount of energy needed for an electron to escape from metal to vacuum.  is sensitive optical, electrical and mechanical properties of materials