Adsorption and Catalysis Dr. King Lun Yeung Department of Chemical Engineering Hong Kong University of Science and Techn

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

2. H H H H H H H H H Adsorption versus Absorption H2 adsorption on palladium Adsorption Absorption Surface process bulk process H H H H H H H H H H H H H H H H H H H H H H H H H H H H2 absorption  palladium hydride

3. H H H H H H H H H H H H H H 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) coverage q = fraction of surface sites occupied adsorbate adsorbent

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

5. Properties Adsorption temperature Adsorption enthalpy Crystallographic specificity Nature of adsorption Saturation Adsorption kinetic 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 Tbp of adsorbate (Xe < 100 K, CO2 < 200 K) heat of liquifaction (5-40 kJmol-1) independent of surface geometry non-dissociative and reversible multilayer occurs often fast, non-activated process Characteristics of Chemi- and Physisorptions

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 CO2 O C O Symmetric Stretch Assymmetric Stretch A dipole moment = charge imbalance in the molecule Bending mode

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

10. H- C  N 0.15 L HCN, 100 K weak chemisorption H- C  N Pt 1.5 L HCN, 100 K physisorption H- C  N C  N 30 L HCN, 200 K dissociative chemisorption Pt Pt I.R. Spectrum of HCN on Pt (a) (b) (c) d(HCN) 2d(HCN) n(H-CN) n(CN)

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

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

13. Sticking Coefficient S = f(q) 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. Sticking Coefficient Steering Effects

16. Surface Coverage (q) Estimation based on gas exposure Rads = dNads/dt = S • F Nads S • F • t Nearly independent of coverage for most situations Exposure time Molecules adsorbed per unit surface area

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

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

20. The Brunauer-Emmett-Teller Isotherm BET isotherm where: n is the amount of gas adsorbed at P nm is the amount of gas in a monolayer P0 is the saturation pressure n   at P = P0 C is a constant defined as: H1 and HL 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 (P0), the number of adsorbed layers is infinite (i.e., condensation), • except for the first layer, the adsorption enthalpy (HL) is identical for each layers.

22. Activated Carbon Surface area ~ 1000 m2/g

23. Surface Area Determination BET surface area by N2 physisorption  - adsorption - desorption c = 69.25 nm = 4.2 x 10-3 mol Area = 511 m2/g Plot P/n(P0-P) versus P/P0 calculate c and nm from the slope (c-1/ nmc) and intercept (1/nmc) of the isotherm measurements usually obtained for P/P0 < 0.2 c = 87.09 nm = 3.9 x 10-3 mol Area = 480 m2/g

24. BET Measurements Volumetric Method • Degassing • Pure gas introduces into supply chamber  constant P1 T1 are recorded  V1 • Gas flows into adsorption cell • P2 and T2 are recorded when equilibrium is reached  V2

25. BET Measurements Dynamic Method • Degassing • Flow carrier gas (He) • Pulse N2/He into adsorption cell at a given PN2 • Record the amount of nitrogen adsorbed using TCD • Calculate surface area (Rouquerol, 1999)

26. BET Measurements Gravimetric Method • Degassing • Record initial weight of adsorbent M1 • Introduce pure gas into adsorption cell • Record the adsorbent equilibrium weight M2 • 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 Type I & IV Type I N2 adsorption in (a) micropores and (c) micropores and mesopores (Rouquerol, 1999)

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

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

36. Chemical Adsorption Processes Physisorption + dissociative chemisorption H2 H2  2 H E(d) dissociation chemisorption physisorption d atomic chemisorption Note: this is an energy prohibitive process

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

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

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

40. Chemical Adsorption Processes Energy barrier Chemical Adsorption is usually an energy activated process. - Eades= -DE(ads) ~ -DH(ads)

41. Formation of Ordered Adlayer Ea(surface diffusion) < kT activated carbon CH4 Krypton

42. Formation of Ordered Adlayer Chlorine on chromium surface

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

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

45. C  C  CO Adsorbate Geometries on Metals Carbon monoxide 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 Terminal (Linear) all surface CO metal carbide Bridging (2f site) all surface Bridging (3f hollow) (111) surface

46. Adsorbate Geometries on Metals Ammonia and unsaturated hydrocarbons Ammonia NH3 NH2 (ads) + H (ads)  NH (ads) + 2 H (ads)  N (ads) + 3 H (ads) Ethene 2HC=CH2

47. Pulse H2, O2 or CO gases Active Surface Area Measurement Most common chemisorption gases: hydrogen, oxygen and carbon monoxide furnace catalyst thermal conductivity cell (TCD) exhaust carrier gas helium or argon

48. furnace 1 g 0.10 wt. % Pt/g-Al2O3 T = 423 K, P = 1 bar Pulse H2 then titrate with O2 thermal conductivity cell (TCD) exhaust carrier gas helium or argon Catalyst Surface Area and Dispersion Calculation (STP) 100 ml 3.75 peaks (H2) 4.50 peaks (O2) Avogrado’s number: 6.022 x 1023 Pt lattice constant: a = 3.92 (FCC) Calculate surface area of Pt and its dispersion.

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

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