1 Example Heterogeneous Catalytic Reaction Process  The long journey for reactant molecules to .travel within gas phase . cross gas-liquid phase boundary. travel within liquid phase/stagnant layer . cross liquid-solid phase boundary. reach outer surface of solid . diffuse within pore . arrive at reaction site . be adsorbed on the site and activated . react with other reactant molecules, either being adsorbed on the same/neighbour sites or approaching from surface above  Product molecules must follow the same track in the reverse direction to return to gas phase  Heat transfer follows similar track   gas phase pore porous solid liquid phase / stagnant layer      gas phase reactant molecule Catalysis & Catalysts

2  Some common solid support / carrier materials  Alumina  Inexpensive  Surface area: 1 ~ 700 m 2 /g  Acidic  Silica  Inexpensive  Surface area: 100 ~ 800 m 2 /g  Acidic  Zeolite  mixture of alumina and silica,  often exchanged metal ion present  shape selective  acidic Solid Catalysts Catalysis & Catalysts  Other supports  Active carbon (S.A. up to 1000 m 2 /g)  Titania (S.A. 10 ~ 50 m 2 /g)  Zirconia (S.A. 10 ~ 100 m 2 /g)  Magnesia (S.A. 10 m 2 /g)  Lanthana (S.A. 10 m 2 /g) pore porous solid Active site

3  Adsorption  Adsorption is a process in which molecules from gas (or liquid) phase land on, interact with and attach to solid surfaces.  The reverse process of adsorption, i.e. the process in which adsorbed molecules escape from solid surfaces, is called Desorption.  Molecules can attach to surfaces in two different ways because of the different forces involved. These are Physisorption (Physical adsorption) & Chemisorption (Chemical adsorption) PhysisorptionChemisorption forcevan der Waalschemical bond number of adsorbed layersmultionly one layer adsorption heatlow (10-40 kJ/mol)high ( > 40 kJ/mol) selectivitylowhigh temperature to occurlowhigh Adsorption on Solid Surface Catalysis & Catalysts

4  Adsorption process Adsorbent and adsorbate  Adsorbent (also called substrate) - The solid that provides surface for adsorption  high surface area with proper pore structure and size distribution is essential  good mechanical strength and thermal stability are necessary  Adsorbate - The gas or liquid substances which are to be adsorbed on solid Surface coverage,  The solid surface may be completely or partially covered by adsorbed molecules Adsorption heat  Adsorption is usually exothermic (in special cases dissociated adsorption can be endothermic)  The heat of chemisorption is in the same order of magnitude of reaction heat; the heat of physisorption is in the same order of magnitude of condensation heat. Adsorption on Solid Surface Catalysis & Catalysts define  =  = 0~1 number of adsorption sites occupied number of adsorption sites available

5  Applications of adsorption process  Adsorption is a very important step in solid catalysed reaction processes  Adsorption in itself is a common process used in industry for various purposes  Purification (removing impurities from a gas / liquid stream)  De-pollution, de-colour, de-odour  Solvent recovery, trace compound enrichment  etc…  Usually adsorption is only applied for a process dealing with small capacity  The operation is usually batch type and required regeneration of saturated adsorbent Common adsorbents: molecular sieve, active carbon, silica gel, activated alumina.  Physisorption is an useful technique for determining the surface area, the pore shape, pore sizes and size distribution of porous solid materials (BET surface area) Adsorption on Solid Surface Catalysis & Catalysts

6 Activated Carbon Surface area ~ 1000 m 2 /g

7  Catalyst composition  Active phase  Where the reaction occurs (mostly metal/metal oxide)  Promoter  Textual promoter (e.g. Al - Fe for NH 3 production)  Electric or Structural modifier  Poison resistant promoters  Support / carrier  Increase mechanical strength  Increase surface area (98% surface area is supplied within the porous structure)  may or may not be catalytically active Solid Catalysts Catalysis & Catalysts Catalyst Active phase Support Promoter

8 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

9 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 = torr s) H H H H H H H HH HH H H H adsorbate adsorbent coverage  fraction of surface sites occupied

10 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.

11 Characteristics of Chemi- and Physisorptions Chemisorption virtually unlimited range wide range ( 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 liquefaction (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

12 Analytical Methods for Establishing Surface Bonds Infrared Spectroscopy Atoms vibrate 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 – cm-1 Mid-IR: 500 – 4000 cm-1 Far-IR: 5 – 500 cm-1

13 I.R. Measurement

14 I.R. Spectrum of CO 2 A dipole moment = charge imbalance in the molecule Asymmetric stretch Vertical and horizontal bend

15 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

16 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

17 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

18 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

19 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

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

21 Physical Adsorption d metal surface nitrogen Van der Waals 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

22 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 Rouquerol, F., J., Rouquerol and K., Sing, Adsorption by Powders and Porous Solids: Principles, Methodology and Applications, Academic Press, London (1999).

23 Adsorption Isotherm IUPAC Classification

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

25 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

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

27 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-A] [S - *] [A] K =  S-A] is proportional to  [S-*] is proportional to 1-  [A] is proportional to partial pressure of A

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

29 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)  bP 1+ bP  =

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

31 Adsorption Isotherms

32 Henry’s Adsorption Isotherm Special case of Langmuir isotherm bP << 1  = bP

33 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 ln  = lnP + B kP 1/n  = Valid for low partial pressure most frequently used for describing pollutant adsorption on activated carbons

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

35 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

36 BET Isotherm Assumptions adsorption takes place on the lattice and molecules stay put, first monolayer is adsorbed onto the solid surface and each layer 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.

37 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)

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

39 Chemical Adsorption Processes Physisorption + dissociative chemisorption d E(d) dissociation chemisorption H2H2 H 2  2 H physisorption atomic chemisorption

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

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

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

43 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