1. Catalysis and Mass Transfer

2. What is Catalysis? The science of catalysts and catalytic processes.
A developing science which plays a critically important role in the gas, petroleum, chemical, and emerging energy industries.
Combines principles from somewhat diverse disciplines of kinetics, chemistry, materials science, surface science, and chemical engineering. 2

3. What is Catalyst? N2 (Desired Reaction) rD NO rU (Undesired Reaction)
NH3 rD
Rate of formation of D
SD/U = = rU
Rate of formation of U Rh
SD/U Pt
SD/U
From

4. How Important Is Catalysis?
Fibers, Plastics, Food, Home Products, Pharmaceuticals
Chemicals
Raw Materials
Heating, Transportation, Power
Fuels
Four of the largest sectors of our world economy (i.e. the petroleum, power, chemicals, and food industries), which account for more than 10 trillion dollars of gross world product, are largely dependent on catalytic processes.

5. Development of Important Industrial Catalytic Processes
Mittasch investigated over 2500 catalysts compositions!!!

6. Development of Important Industrial Catalytic Processes
It played a vital role as a feedstock for chemicals: 30 million tons per year in 2000

7. Development of Important Industrial Catalytic Processes
Production of Liquid Fuels!!!

8. Development of Important Industrial Catalytic ProcessesNO CO
CxHy N2
CO2
H2O O2

9. Chemical Reactions Four Basic Variables to Control Chemical Reactions:
Temperature
Pressure
Conc
Contact time
Rate of Reaction = K(T) x F(Ci)
K(T) = A exp(-E/RT)
 i (Ci)i H I H C Cl H C H I Cl C H Cl I

10. Components of a Typical Heterogeneous Catalyst

11. Pt Nanoparticles on Al2O3 Supports

12. Components of a Typical Heterogeneous Catalyst

14. Active Catalytic Phases and Reactions They Typically Catalyze

15. Typical Physical Properties of Common Carrier (Supports)

16. Heterogeneous Catalysis
A (g)  B (g)
Support
(Al2O3)
Active Metals
(Pt, Co, MoO2)
support
Minimize P
Minimize Mass Transport Resistances
Maximize Activity
Minimize Poisoning and Fouling

17. Heterogeneous Catalysis
Steps 1, 2, 6, & 7 are diffusional processes => Small dependences on temp
Steps 3, 4, & 5 are chemical processes => Large dependences on temp
Given that the rates of the chemical steps are exponentially dependent on temperature and have relatively large activation energies compared to the diffusional process (20~200 kJ/mol Vs. 4-8 kJ/mol), they are generally the slow or rate-limiting processes at low reaction temperatures.
As the temperature increases, the rates of chemical steps with higher activation energies increase enormously relative to diffusional processes, and hence the rate limiting process shifts from chemical to diffusional.
Kapp(T) = Aapp exp(-Eapp/RT)

18. Film Mass Transfer Effect on Reaction Rate
If Boundary Layer is Too Thick,
Reaction Rate = Mass Transfer Rate k
A  B
-rA = kc (CAb – CAs)
where Kc = DAB / 
As the fluid velocity (U) increases and/or the particle size (Dp) decreases, the boundary layer thickness () decreases and the mass transfer coefficient (Kc) increases

19. Internal Diffusion Effect on Reaction Ratek
A  B
-rA = k η CAS
Where η = Effectiveness Factor
η = (CA)avg / CAS
cosh
cosh
Φpore (1 - x/L) CA =
CAS
cosh
( Φpore) L x
Φpore (Thiele Modulus) = L (k P / Deff)1/2
η = (CA)avg / CAS = (tanh (Φpore) ) / Φpore

22. Internal Diffusion Effect on Reaction Rate
While the equations above were derived for the simplified case of first-order reaction and a single pore, they are in general approximately valid for other reaction orders and geometry if L is defined as Vp/Sp (the volume to surface ratio of the catalyst particle). Hence, L = z/2, rc/2 and rs/3, respectively, for a flat plate of thickness z, a cylinder of radius rc, and a sphere of radius rs.

24. Fundamental Catalytic Phenomena and Principles
Chemical Properties
(Oxidation State, Acidity, Surface Composition)
Physical Properties
(Surface Area, Pore Structure, Pore Density)
Catalyst
Design
Catalytic Properties
(Activity and Selectivity)

25. Structure Sensitive Reactions

26. CO Oxidation over Au/TiO2: Particle Size Effect
2 nm
2.5 nm
6 nm Au
TiO2

27. Particle Size Vs. Electronic Structure Change of Au