1. Mechanisms of Catalytic Reactions and Characterization of Catalysts

2. Catalyst affects only the rate of the reaction,i.e.Kinetics.
What is a Catalyst ?
Catalyst is a substance that increases the rate of the reaction at which a chemical system approaches equilibrium , without being substantially consumed in the process.
Catalyst affects only the rate of the reaction,i.e.Kinetics.
It changes neither the thermodynamics of the reaction nor the equilibrium composition.

3. Chemical Reaction Thermodynamic
Thermodynamics says NOTHING about the rate of a reaction.
Thermodynamics : Will a reaction occur ?
Kinetics : If so, how fast ?

4. C (graphite)  C (diamond)
Kinetic Vs. Thermodynamic
A reaction may have a large, negative DGrxn, but the rate may be so slow that there is no evidence of it occurring.
Conversion of graphite to diamonds is a thermodynamic favor process (G -ve ).
C (graphite)  C (diamond)
Kinetics makes this reaction nearly impossible (Requires a very high pressure and temperature over long time)

5. Kinetic Vs. Thermodynamic
Reaction Profile
Reaction path for conversion of A + B into AB

6. Activation Energy Definition
Activation Energy : The energy required to overcome the reaction barrier. Usually given a symbol Ea or ∆G≠
The Activation Energy (Ea) determines how fast a reaction occurs, the higher Activation barrier, the slower the reaction rate. The lower the Activation barrier, the faster the reaction

7. Activation Energy Catalyst Affect
Catalyst lowers the activation energy for both forward and reverse reactions.

8. Activation Energy
Catalyst Affect
This means , the catalyst changes the reaction path by lowering its activation energy and consequently the catalyst increases the rate of reaction.

9. How a Heterogeneous Catalyst works ?
Substrate has to be adsorbed on the active sites of the catalyst

10. Surface process bulk process Absorption and Adsorption H2 absorption 
palladium hydride H
H2 adsorption on
palladium
Surface process
bulk process

11. Adsorption In physisorption The bond is a van der Waals interaction
adsorption energy is typically 5-10 kJ/mol. ( much weaker than a typical chemical bond )
many layers of adsorbed molecules may be formed.

12. Adsorption For Chemisorption
The adsorption energy is comparable to the energy of a chemical bond.
The molecule may chemisorp intact (left) or it may dissociate (right).
The chemisorption energy is kJ/mol for molecules and kJ/mol for atoms.

13. Characteristics of Chemi- and Physisorptions
E(d)
physisorption
atomic chemisorption
DE(ads) < DE(ads)
Physisorption Chemisorption
small minima large minima
weak Van der Waal formation of surface
attraction forces chemical bonds
physisorption/
desorption
chemisorption CO

14. Adsorption and Catalysis
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 H
adsorbate
adsorbent H
coverage q = fraction of surface sites occupied

15. Adsorption Mechanisms
Langmuir-Hinshelwood mechanisms:
1. Adsorption from the gas-phase
2. Desorption to the gas-phase
3. Dissociation of molecules at the surface
4. Reactions between adsorbed molecules
Two Questions:
Is the reaction has a Langmuir-Hinshelwood mechanism?
What is the precise nature of the reaction steps?
Cannot be solved
without experimental or computational studies

16. Example The Reaction A2 + 2B = 2AB
Langmuir-Hinshelwood mechanisms
Example The Reaction A2 + 2B = 2AB
may have the following mechanism
A2 + * = A2* A2* + * = 2A* B + * = B* A* + B* = AB* + * AB* = AB + *

17. The last step cannot occur in a Langmuir-Hinshelwood mechanism
Adsorption Mechanisms
Eley-Rideal mechanism:
Adsorption from the gas-phase
Desorption to the gas-phase
Dissociation of molecules at the surface
Reactions between adsorbed molecules
Reactions between gas and adsorbed molecules
The last step cannot occur in a Langmuir-Hinshelwood mechanism

18. Eley-Rideal mechanism
Example
The reaction
A2 + 2B = 2AB
may have the following Eley-Rideal mechanism
A2 + * = A2* A2* + * = 2A* A* + B = AB + *
where the last step is the direct reaction between the adsorbed molecule A* and the gas-molecule B.

19. Eley-Rideal or Langmuir-Hinshelwood?
For the Eley-Rideal mechanism:
the rate will increase with increasing coverage until the surface is completely covered by A*.
For the Langmuir-Hinshelwood mechanism:
the rate will go through a maximum and end up at zero, when the surface is completely covered by A*.
This happens because the step B + * = B*
cannot proceed when A* blocks all sites.
The trick is that the step B + * = B*
requires a free site.

20. Catalyst Preparation (1) Unsupported Catalyst
Usually very active catalyst that do not require high
surface area
e.g., Iron catalyst for ammonia production (Haber process)
(2) Supported Catalyst
requires a high surface area support to disperse the primary catalyst
the support may also act as a co-catalyst (bi-functional)
or secondary catalyst for the reaction (promoter)

21. Supported Catalyst Highly dispersed metal on metal oxide
Nickel clusters
SiO2

22. Molecules in Zeolite Cages and Frameworks
+ p-xylene
ZSM-5
Y-zeolite
Paraffins

23. What is ZSM-5 Catalyst ?
It is an abbreviation for (Zeolite Scony Mobile Number 5 )
First synthesized by Mobil Company in 1972
It replaces many Homogeneous Catalysts were used in
many petrochemical processes
ZSM-5 has two diameters for its pores : d1= 5.6 Å , d2= 5.4 Å
Where as, Zeolite Y has a diameter = 7.4 Å

24. Properties ZSM-5
The ZSM-5 zeolite catalyst is used in the petroleum industry for hydrocarbon interconversion.
ZSM-5 zeolite is a highly porous aluminosilicate with a high silica/alumina ratio.
It has an intersecting two-dimensional pore structure.
The aluminum sites are very acidic.
The acidity of the zeolite is very high.
The reaction and catalysis chemistry of the ZSM-5 is due to this acidity.

25. Structure of ZSM-5

26. Aromatic Isomerization
Observed by Haag and Co-workers P-Selectivity is achieved due to the high diffusion coefficient of P-Substituted Molecules Relative to that of Ortho or Meta.
A side reaction in homogenous solution phase isomerization is the Bimolecular disproportionation of Xylene.
This Can not occur when using zeolite such
as ZSM-5 since the pore size will not allow
for the bulky bimolecular transition state
necessary for disproportionation.

27. Relative Diffusion Coefficient
+ p-xylene
ZSM-5 1
m-Xylene
o-Xylene
p-Xylene
Relative Diffusion Coefficient
Compound
> 10000

28. Effective Diameter of intracrystalline Cavity (nm)
15000
0.70
Mordenite
> 1.2
0.80
Effective Diameter of intracrystalline Cavity (nm)
> 45000 HY
10000
ZSM-4
ZSM-5
Kdis/Kiso
Zeolite Catalyst
Type
1000
0.60
Selectivities of acidic zeolite for disproportionation and isomerization of Xylene. The HZSM-5 Catalyst is preferred, minimizing the bimolecular disproportionation reaction by virtue of restricted transition state selectivity.

29. Schematic diagram of product shape selectivity: Para-xylene diffuses preferentially out of the zeolite channels
p-xylene
m-xylene

30. Acidity of the Catalyst Vs. Acid Sites
Theoretically,
As the Acid Sites increase the Acidity increases
Experimentally,
As the Acid Sites increase the Acidity decreases
How we can understand this behavior?

31. Acidity of the Catalyst Vs. Acid Sites
Al is the Acidic Site
But,
Si is more electronegative than Al B C N O F Al Si P S Cl
Electronegativity Effect
O O
Al Si Al H+ Si
Si Makes the release of the H+ faster since it attracts Electrons toward it which make the proton away of it

32. Acidity of the Catalyst Vs. Acid Sites
It is possible to conclude that:
the acid strength of a site will increase when there is a decrease in the number of Al atoms in the Next Nearest Neighbor position of the Al atom.
So, the strongest type of framework Bronsted site is
A completely isolated Al tetrahedron which have zero NNN or (0NNN)

33. Acidity Characterization of a Catalyst
Acidity of the catalysts can be assess by:
I. Temperature Programmed Desorption (TPD)
II. Fourier Transformation Infrared spectroscopy ( FTIR)
III. Induced Laser spectroscopy (IL)

34. I. Temperature Programmed Desorption (TPD)
Pure carrier gas (typically helium) flows over the sample as the temperature is raised to desorb the previously adsorbed gas e.g. NH3
2. This characteristic "fingerprint" for each catalyst,used to determine:
the distribution of acid-site strength if ammonia is the sorbed gas,
or the distribution of basic sites if carbon dioxide is the sorbed gas.

35. I. Temperature Programmed Desorption (TPD)
Time(min)

36. II. Fourier Transformation Infrared spectroscopy ( FTIR)
By this method we assess acidic site:
Bronsted acidic site or Lewis acidic site
For example: if pyridine is the probe molecule, It will give peaks at:
cm-1 for Bronsted site
cm-1 for Lewis site

37. II. Fourier Transformation Infrared spectroscopy ( FTIR)

38. II. Fourier Transformation Infrared spectroscopy ( FTIR)

39. III. Induced Laser spectroscopy (IL)
To assess the acidity by IL:
1. Probed molecule has to be prepared in different acidic concentration solutions
2. Life time measurements has to run onto each concentration
3. Calibration curve between life time Vs. concentration of the acid

40. III. Induced Laser spectroscopy (IL)

41. Conclusion
There is no single method that can be used to determine all aspects of acidity for a solid i.e. nature, strength and the number of acid sites
Each method measures only certain aspects and,therefore, the application of many methods is desirable.
THE END

42. THANK YOU End of the Catalysis and Catalysts References
A.Corma,Inorganic solid acids and their use in acid-catalyzed hydrocarbon reactions. Chem. Rev.1995, 95,
Bruce C. gates, Catalytic Chemistry.1992.
Zaki Seddigi, Characterization of the acidic properties of zeolite and their catalytic behavior in the synthesis of MTBE
Ali El-Rayyes, Study of the photochemical properties of some aromatic compound on molecular sieves using a picosecond pulse laser system
Keith Laidler, Chemical Kinetics, 1987.