1. Catalysis and Heterogeneous Catalysis

2. Catalysis and Heterogeneous Catalysis : objectives
Develop understanding of catalysts, reaction mechanisms and catalytic reactor design
Define the catalyst and its properties
Descript the steps of catalytic reactions
Develop a rate law and determine the rate law parameters

3. Definition
The catalyst is a substance that affect the rate of reaction but emerges from the process unchanged.
The catalyst usually changes a reaction rate by promoting a different molecular path (mechanism) for the reaction. E

4. types of catalysts: Homogeneous
Heterogeneous Solid (Porous and nonporous catalysts
Honeycomb
Supported and unsupported
Molecular Sieve

5. types of catalysts: Selectivity Catalyst deactivation and active sites
Physical adsorption and chemisorption
Monolithic catalysts: are normally encountered in processes where pressure drop and heat removal are major consideration.

7. Classification of catalysts
Reaction Catalyst
Halogenation-dehalogenation CuCl2, AgCl, Pd
Hydration-dehydration Al2O3, MgO
Alkylation-dealkylation AlCl3, Pd, Zeolite
Hydrogenation-dehydrogenation Co, Pt, Cr2O3, Ni
Oxidation Cu, Ag, Ni, V2O5
Isomerization AlCl3, Pt/Al2O3, Zeolite

8. Steps in catalytic reaction
Mass transfer (diffusion) of the reactants from bulk fluid to the external surface of the catalyst.
Diffusion from pore mouth through the pores to the internal catalyst surface.
Adsorption onto the catalyst surface.
Reaction on the catalyst surface.
Desorption of the products
Diffusion of products from the interior to the pore mouth.
Mass transfer of products from external pellet surface to the bulk fluid.

9. Catalytic Reaction

10. Rate Law
The rate law in hetrogeneous catalysis seldom follow the power law discussed in Ch.5 for homogeneous reactions.
To form reaction rate law we need to formulate or propose catalytic mechanism then derive the rate law.
Adsorption step
Surface reaction
Desorption step
One is the rate limiting

11. Step 1: diffusion from the bulk to the external transport
Assume this step is the slowest step. In this step the reactant A at a bulk concentration CAb must travel through the boundary layer of thickness of δ to the external surface of the pellet where concentration is CAs.
Hence, reaction rate = rate of transfer
–r/A= kC(CAb – CAs)
where kC is mass transfer coefficient which is a function
(1)particle size and
(2) fluid velocity.

12. Step 1: diffusion from the bulk to the external transport
kC=DAB/ δ
As the velocity across the pellet is increased, the boundary layer become smaller and the mass transfer rate is increased.
The external resistant decreased as the particle size is decreased

13. Step 2: Internal diffusion
At high fluid velocity where no longer the external diffusion is the limiting step and internal diffusion is the slowest step.
As A diffuses into the interior of the pellet, it reacts with the catalyst deposited on the sides of the pore wall.
The rate of reaction is = krCAs
The rate is dependent on the particle size of the catalyst

14. Adsorption Isotherm
chemisorptions is important step in any catalytic reaction.
If S represent the active site, and A•S represent the combination of A and S forming from the reaction
A + S  A•S
Consider first Adsorption Isotherm of the components

15. Consider it as elementary reaction
Adsorption Isotherm
For example for CO on the metal surface
CO + S   CO•S (molecular or nondissociated adsorption) (Nickel catalyst) or
CO + 2S   C•S+O•S (dissociated adsorption) (Iron catalyst)
Consider it as elementary reaction

16. Rate of attachment = kAPCOCv
the rate of detachment of molecules from the surface is 1st order
Rate of detachment =k-ACCO●S
The net rate of adsorption rAD= kAPCOCv- k-ACCO●S
The ratio kA/k-A = KA is the adsorption equilibrium constant

17. The ratio kA/k-A = KA is the adsorption equilibrium constant
kA: independent of T
k-A: increase exponentially with T
KA: decrease exponentially with T

18. The site balance : Ct = Cv + CCO●S
PCO
CCO.S
Langmuir Isotherm

19. Dissociative adsorption

20. Surface reaction

21. Single Site mechanism N
Al2O3
N=n-pentene I
I=i-pentene
Each step of the reaction mechanism is elementary, the surface reaction rate law is

22. Dual Site mechanism The surface reaction rate law is
Type I
C4H9OH
C4H8
H2O
The surface reaction rate law is
Langmuir-Hinshelwood Kinetics

23. Dual Site mechanism The surface reaction rate law is Type IICO O
CO2
Pt Pt
Pt Pt
The surface reaction rate law is
Langmuir-Hinshelwood Kinetics

24. Dual Site mechanism The surface reaction rate law is
Type III CO O
CO2
Pt Fe
Pt Fe
The surface reaction rate law is
Langmuir-Hinshelwood Kinetics

25. Eley-Rideal mechanism
C6H6- C3H6
The surface reaction rate law is

26. Desorption
The reacting material will desorb into the gas phase

27. The rate limiting step
When hetrogeneous reactions are carried out at steady state, the rate of each of the 3 reaction steps in series (adsorption-surface reaction-desorption) are equal
- r’A=rAD=rS=rD
Rate limiting or rate controlling. That is if we could make this step go faster , the reaction would proceed at an accelerated rate.

28. The rate limiting step
The approach in determining the catalytic and hetrogeneous mechanisms is usually termed the Langmuir-Hinshelwood approach since it is derived from ideas proposed by Hinshelwood based on Langmuir principles for adsorption

29. Langmuir-Hinshelwood approach
It consist of
1st assuming a sequence of steps in the reaction. In writing the sequence, one must choose among such mechanisms as molecular or atomic adsorption, and single or dual site reaction.
2nd, rate laws are written for the individual steps. Assuming that all steps are reversible.
3rd Finally , a rate – limiting step is postulated , and steps that are not rate limiting are used to eliminate all coverage dependent terms.

30. Example
(rapid)
(Rate limiting)
(Rapid)

31. Example: Octane Rating
K.I. t

32. Example: Octane Rating

33. Example: Catalytic Reaction to Improve the Octane Number of Gasoline

34. Formulating mechanism, rate limiting and rate law
The limiting step is n-pentene to i-pentene over alumina
Select a mechanism
Adsorption N+SN●S
Surface reaction N●S+S I●S+S
Desorption I●S I+S
Treat each step as elementary reaction

35. Formulating mechanism, rate limiting and rate law
2. Assume a rate limiting step
Choose the surface reaction first. The rate law is for the surface reaction is
3. Find the expression for concentration of the adsorbed species CI●S
Write a site balance : Ct = Cv + CN●S + CI●S

36. 5. Drive the rate law by combining steps 2,3, and 4
6. Compare the derived rate law with the experimental data
If the model dose not agree with the data:
Assume a different rate limiting (repeat 2 – 6)
If not, then change the mechanism (e.g. single site mechanism)
Adsorption N+SN●S
Surface reaction N●S I●S
Desorption I●S I+S

37. If the single site mechanism is correct then the rate law is
Note : if more than one mechanism agrees with the experimental data then we need to compare there residuals

38. Different Surface Mechanisms

39. Micro-electronic Fabrication

40. Chemical Vapor DepositionGe
Cl2 H H Ge Ge Ge Ge Ge Ge
2HCl (g) Ge Ge Ge Ge Ge Ge Ge

45. Model Discrimination H2 + C2H4  C2H6
Is carried over molybdenum catalyst
run
Rxn rate PE
PEA PH 1 2
3.13 3
5.12 5 4
3.82
4.2 6
2.4
0.5 7
3.8 8
2.2 9
0.75