1. Isotherms and surface reaction mechanisms

2. 2 Heterogeneous Catalytic Reaction Process  Journey for reactant molecules:  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/neighbor 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

3. 3  Catalyst composition Active phase  mostly metal/metal oxide Promoter  Reaction promoter (e.g. Al - Fe for NH 3 production)  Electronic 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 Catalyst Active phase Support t Promoter

4. 4  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  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

5. 5  Preparation of catalysts Precipitation To form non-soluble precipitate by desired reactions at certain pH and temperature Adsorption & ion-exchange Cationic: S-OH + + C +  SOC + + H + Anionic: S-OH - + A -  SA - + OH - I-exch. S-Na + + Ni 2+  S-Ni 2+ + Na + Impregnation Fill the pores of support with a metal salt solution of sufficient concentration to give the correct loading. Dry mixing Physically mixed, grind, and fired Solid Catalysts precipitate or deposit precipitation filter & wash the resulting precipitate Drying & firing precursor solution Support add acid/base with pH control Support Drying & firing Pore saturated pellets Sol n. of metal precursor Amount adsorbed Concentration Support Drying & firing

6. 6  Preparation of catalysts Catalysts need to be calcined (fired) in order to decompose the precursor and to receive desired thermal stability. The effects of calcination temperature and time are shown in the figures on the right.  Commonly used Pre-treatments Reduction  if elemental metal is the active phase Sulphidation  if a metal sulphide is the active phase Activation  Some catalysts require certain activation steps in order to receive the best performance.  Even when the oxide itself is the active phase it may be necessary to pre-treat the catalyst prior to the reaction  Typical catalyst life span Can be many years or a few mins. Solid Catalysts 0 25 50 75 100 500600700800900 Temperature °C BET S.A. m 2 /g 0 40 0 10 Time / hours BET S.A. Activity Time Normal use Induction period dead

7. 7 Very important step in solid catalysed reaction processes 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  Usually a process dealing with small capacity  Operation is usually batch type and requires 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

8. 8  Characterization of adsorption system Adsorption isotherm - most commonly used, especially to catalytic reaction system and separations, T=const. Adsorption isobar - (usage related to industrial applications) Adsorption Isostere - (usage related to industrial applications) Pressure Vol. adsorbed T1 T2 >T1 T3 >T2 T4 >T3 T5 >T4 Vol. adsorbed Temperature P1 P2>P1 P3>P2 P4>P3 Pressure Temperature V2>V1 V1 V3>V2 V4>V3 Adsorption Isotherm Adsorption Isobar Adsorption Isostere

9. 9  The Langmuir adsorption isotherm Basic assumptions  surface uniform (  H ads does not vary with coverage)  monolayer adsorption  no interaction between adsorbed molecules  adsorbed molecules immobile

10. 10  The Langmuir adsorption isotherm Case I - single molecule adsorption when adsorption is in a dynamic equilibrium A (g) + M (surface site)  AM rate of adsorptionr ads = k ads (1-  ) P rate of desorption r des = k des  at equilibrium r ads = r des  k ads (1-  ) P = k des  rearrange it for  let  B 0 is adsorption coefficient case I A

11. 11  The Langmuir adsorption isotherm (cont’d) Case II - single molecule adsorbed dissociatively A-B(g) + 2 M(surface site)  A-M +B-M rate of A-B adsorption r ads =k ads (1   )   )P AB =k ads (1   ) 2 P AB rate of A-B desorption r des =k des     =k des  2 at equilibrium r ads = r des  k ads (1   ) 2 P AB = k des  2 rearrange it for  Let.  case II A B B A ====

12. 12  The Langmuir adsorption isotherm (cont’d) Case III - two molecules adsorbed on two sites A (g) + B (g) + 2 M (surface site)  A-M + B-M rate of A adsorptionr ads,A = k ads,A (1     ) P A rate of B adsorptionr ads,B = k ads,B (1     ) P B rate of A desorptionr des,A = k des,A   rate of B desorptionr des,B = k des,B   at equilibrium r ads,A = r des,A and  r ads,B = r des,B  k ads,A (1     )P A =k des,A   and k ads,B (1     )P B =k des,B   rearrange it for  where are adsorption coefficients of A & B. case III A B

13. 13  The Langmuir adsorption isotherm (cont’d) Adsorption Strong k ads >> k des k ads >> k des B 0 >>1B 0 >>1 Weak k ads << k des k ads << k des B 0 <<1B 0 <<1 case II A B case I A

14. 14  The Langmuir adsorption isotherm (cont’d) Adsorption A, B both strong A strong, B weak A weak, B weak case III A B

15. 15  Langmuir adsorption isotherm case I case II Case III  Langmuir adsorption isotherm : logical picture of adsorption process  It fits many adsorption systems but not all of them  Assumptions made by Langmuir do not hold in all situations, causing errors  Solid surface is heterogeneous thus the heat of adsorption is not a constant at different   Physisorption of gas molecules on a solid surface can be more than one layer large B 0 (strong adsorp.) small B 0 (weak adsorp.) moderate B 0 Pressure Amount adsorbed mono-layer Strong adsorption k ads >> k des Weak adsorption k ads << k des

16. 16 Many other isotherms are proposed in order to explain the observations  The Temkin (or Slygin-Frumkin) isotherm Assuming adsorption enthalpy  H decreases linearly with surface coverage From ads-des equilibrium, ads. rate  des. rate r ads =k ads (1-  )P  r des =k des  Other adsorption isotherms  H of ads  Langmuir Temkin

17. 17 The Temkin (or Slygin-Frumkin) isotherm Q s is the heat of adsorption. When Q s is a linear function of  i. Q s =Q 0 -iS (Q 0 is a constant, i is the number of covered sites and S represents the surface site), the overall coverage When b 1 P >>1 and b 1 Pexp(-i/RT) <<1, we have  =c 1 ln(c 2 P), where c 1 & c 2 are constants  Valid for some adsorption systems. Other adsorption isotherms  H of ads  Langmuir Temkin

18. 18  The Freundlich isotherm assuming logarithmic change of adsorption enthalpy  H with surface coverage From ads-des equilibrium, ads. rate  des. rate r ads =k ads (1-  )P  r des =k des   H of ads  Langmuir Freundlich

19. 19  The Freundlich isotherm Q i : heat of adsorption. a function of  i. If there are N i types of surface sites, each expressed as N i =aexp(-Q/Q 0 ) (a and Q 0 are constants), corresponding to a fractional coverage  i, the overall coverage the solution for this integration expression at small  is: ln  =(RT/Q 0 )lnP + constant, or as is the Freundlich equation normally written, where c 1 =constant, 1/c 2 =RT/Q 0  Freundlich isotherm fits, not all, but many adsorption systems.  H of ads  Langmuir Freundlich

20. 20  BET (Brunauer-Emmett-Teller) isotherm Multilayer adsorption Basic assumptions  Same assumptions as that of Langmuir but allows multi-layer adsorption  Heat of ads. of additional layer equals to the latent heat of condensation  Based on the rate of adsorption=the rate of desorption for each layer of ads. the following BET equation was derived WhereP - equilibrium pressure P 0 -saturate vapour pressure of the adsorbed gas at the temperature P/P 0 is called relative pressure V -volume of adsorbed gas per kg adsorbent V m -volume of monolayer adsorbed gas per kg adsorbent c - constant associated with adsorption heat and condensation heat Note: for many adsorption systems c=exp[(H 1 -H L )/RT], where H 1 is adsorption heat of 1st layer & H L is liquefaction heat, so that the adsorption heat can be determined from constant c.

21. 21  Remarks on the BET isotherm Fits reasonably well all known adsorption isotherms observed so far for various types of solid, although there are fundamental defects in the theory (no interaction between adsorbed molecules, surface homogeneity and liquefaction heat for all subsequent layers being equal). Gives accurate account of adsorption isotherm only within restricted pressure range. At very low (P/P 0 0.35) it becomes less applicable. The most significant contribution of BET isotherm to the surface science is that the theory provided the first applicable means of accurate determination of the surface area of a solid (since 1945).

22. 22  Summary of adsorption isotherms NameIsotherm equationApplicationNote Langmuir Temkin  =c 1 ln(c 2 P) Freundlich BET Adsorption on Solid Surface Chemisorption and physisorption Chemisorption Chemisorption and physisorption Multilayer physisorption Useful in analysis of reaction mechanism Chemisorption Easy to fit adsorption data Useful in surface area determination

23. Adsorption Isotherm IUPAC Classification 23 micropores macropores mesopores

24. Adsorption Isotherms Calculated with classical DFT 24

25. 25  Langmuir-Hinshelwood mechanism Surface-catalyzed reaction in which 2 or more reactants adsorb on surface without dissociation A (g) + B (g)  A (ads) + B (ads)  P (the desorption of P is not r.d.s.) Rate of reaction r i =k[A][B]=k  A  B From Langmuir adsorption isotherm (the case III) we know A B + +  P

26. 26  Langmuir-Hinshelwood mechanism When both A & B are weakly adsorbed (B 0,A P A <<1, B 0,B P B <<1), 2nd order reaction A B + +  P When A is strongly adsorbed (B 0,A P A >>1) & B weakly adsorbed (B 0,B P B A inhibits the reaction

27. 27  Eley-Rideal mechanism This mechanism deals with the surface-catalyzed reaction in which one reactant, A, adsorbs on surface without dissociation and other reactant, B, approaching from gas phase reacts with A A(g)  A(ads) P (the desorption of P is not r.d.s.) The rate of reaction r i =k[A][B]=k  A P B From Langmuir adsorption isotherm (the case I) we know A  PB + B (g)

28. 28  Eley-Rideal mechanism When both A is weakly adsorbed or the partial pressure of A is very low (B 0,A P A <<1), 2nd order reaction When A is strongly adsorbed or the partial pressure of A is very high (B 0,A P A >>1) 1st order w.r.t. B A  PB

29. 29  Mechanism of surface-catalysed reaction with dissociative adsorption The mechanism of the surface-catalysed reaction in which one reactant, AD, dissociatively adsorbed AD (g)  A (ads) + D (ads) P (desorption of P is not r.d.s.) The rate of reaction r i =k[A][B]=k  AD P B From Langmuir adsorption isotherm (case II): + B (g)  P B A D

30. 30  Mechanism of surface-catalysed reaction with dissociative adsorption When both AD is weakly adsorbed or the partial pressure of AD is very low (B 0,AD P AD <<1), The reaction orders, 0.5 w.r.t. AD and 1 w.r.t. B When A is strongly adsorbed or the partial pressure of AD is very high (B 0,A P AD >>1) 1st order w.r.t. B  P B A D

31. 31  Mechanisms of surface-catalysed rxns involving dissociative adsorption In a similar way one can derive mechanisms of other surface-catalysed reactions, in which  dissociatively adsorbed one reactant, AD, reacts with another associatively adsorbed reactant B on a separate surface site  The use of these mechanism equations Determining which mechanism applies by fitting experimental data to each. Helping in analysing complex reaction network Providing a guideline for catalyst development (formulation, structure,…). Designing / running experiments under extreme conditions for a better control …