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.
3. What is Catalyst?
A catalyst is a material that enhances the rate and selectivity of a chemical
reactions and in the process is cyclically regenerated.
Fe2+ + Ce4+ Fe3+ + Ce3+ (Slow Reaction)
2Fe2+ + Mn4+ 2Fe3+ + Mn2+
Mn2+ + 2Ce4+ Mn4+ + 2Ce3+
Fe2+ + Ce4+ Fe3+ + Ce3+
(Fast Reaction)
Homogeneous Catalysis
CO + H2O CO2 + H2 @ low temperature (Slow Reaction)
S* + H2O H2 + O-S*
O-S* + CO CO2 + S*
CO + H2O CO2 + H2
(Faster Reaction)
Heterogeneous Catalysis
4. What is Catalyst?
From http://www.automotivecatalysts.umicore.com
NO
N2
NH3
(Desired Reaction)
(Undesired Reaction)
SD/U =
rD
rU
rD
rU
Rate of formation of D
Rate of formation of U
=
Rh SD/U
Pt SD/U
5. How Important Is Catalysis?
Raw Materials
Chemicals
Fuels
Fibers, Plastics, Food,
Home Products,
Pharmaceuticals
Heating,
Transportation, Power
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.
10. How to Define Reaction Rate??
Reaction Rate (r) =
1
i * Q
dni
dt
Q = V, W or S.A. of catalyst
i = Stoichiometric Coefficient i iMi = 0 involving species Mi
(i is negative for reactants and
positive for products)
e.g. 2NH3 = N2 + 3H2 2 x (NH3) -1 x (N2) -3 x (H2) =
2N + 6H – 2N – 6H = 0
ni = # of moles of species Mi
11. Chemical Reactions
Four Basic Variables to Control Chemical Reactions:
(1)Temperature
(2)Pressure
(3)Conc
(4)Contact time
Rate of Reaction = K(T) x F(Ci)
K(T) = A exp(-E/RT)
C
H
H
H
I
Cl
C
H
H
H
Cl
I
C
H
H
H
I
Cl
Energy Intensive &
damaging to equipment and
materials & non-selective
i (Ci)i
12. A. Active phase - metal that provides active sites where the
chemical reaction takes place
B. Support or Carrier - high surface area oxide which
disperses and stabilizes the active phase
(adds efficiency, physicalstrength, sometimes selectivity)
C. Promoter(s) - additive which improves catalyst properties,
e.g. activity, selectivity, catalyst life
Components of a Typical
Heterogeneous Catalyst
14. Heterogeneous Catalysis
A (g) B (g)
•Minimize P
•Minimize Mass Transport
Resistances
•Maximize Activity
•Minimize Poisoning and
Fouling
Support
(Al2O3)
Active Metals
(Pt, Co, MoO2)
support
15. Components of a
Typical Heterogeneous Catalyst
Component Material Types Examples
Active Phase: metals noble metals: Pt, Pd; base metals: Ni, Fea
metal oxides transition metal oxides: MoO2, CuO
metal sulfides transition metal sulfides: MoS2, Ni3S2
Promoter:
textural metal oxides Al2O3, SiO2, MgO, BaO, TiO2, ZrO2
chemical metal oxides alkali or alkaline earth: K2O, PbO
Carrier or
Supportb
stable, high surface area
metal oxides, carbons
Group IIIA, alkaline earth and transition
metal oxides, e.g. Al2O3, SiO2, TiO2,
MgO, zeolites, and Carbon
16.
17. Active Catalytic Phases and Reactions
They Typically Catalyze
Active Phase Elements/Compounds Reactions Catalyzed
metals Fe, Co, Ni, Cu,
Ru, Pt,
Pd, Ir,Rh, Au
hydrogenation, steam reforming, HC
reforming, dehydrogenation, ammonia
synthesis, Fischer-
Tropsch synthesis
oxides oxides of V,Mn, Fe,
Cu, Mo, W, Al,
Si,
Sn, Pb, B
complete and partial oxidation of
hydrocarbons and CO, acid-catalyzed
reactions (e.g. cracking,
isomerization,
alkylation), methanol synthesis
sulfides sulfides of Co, Mo,
W, Ni
hydrotreating (
hydrodesulfurization,
hydrodenitrogenation,hydrodemetallation),
hydrogenation
carbides carbides of Fe, Mo, W hydrogenation, FT synthesis
19. Heterogeneous Catalysis
A (g) B (g)
•Minimize P
•Minimize Mass Transport
Resistances
•Maximize Activity
•Minimize Poisoning and
Fouling
Support
(Al2O3)
Active Metals
(Pt, Co, MoO2)
support
20. 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
T2
T1
1.75
Phase
Order of Magnitude
cm2/s m2/s Temp and Pressure Dependences
From Elements of Chemical Reaction Engineering, S. Fogler
d
d
For Knudsen Diffusion
For Bulk, Molecular or
Fick’s Diffusion
d <
d >
21. 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)
22. Film Mass Transfer Effect on
Reaction Rate
If Boundary Layer is Too Thick,
Reaction Rate = Mass Transfer Rate
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
k
23. Internal Diffusion Effect on
Reaction Rate
-rA = k η CAS
Where η = Effectiveness Factor
η = (CA)avg / CAS
CA
CAS
=
cosh
cosh Φpore (1 - x/L)
( Φpore)
cosh
η = (CA)avg / CAS = (tanh (Φpore) ) / Φpore
Φpore (Thiele Modulus) = L (k P / Deff)1/2
A B
k
L
x
24. 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.
25. Elementary Reaction
It is one that proceeds on a molecular level exactly as written in the balanced
stoichiometric equation
A + B C
If it is an elementary reaction,
A B C
-rA = k [A]1 [B]1
26. Elementary Reaction
It is one that proceeds on a molecular level exactly as written in the balanced
stoichiometric equation
O3 O2 + O
Is this an elementary reaction?
If it is an elementary reaction,
-rO3 = k [O3]1
27. Elementary Reaction
It is one that proceeds on a molecular level exactly as written in the balanced
stoichiometric equation
O3 O2 + O
On molecular level, what really is really happening is:
O2 + O3 O2 +O2 + O
-rO3 = k [O3]1 [O2]1
We never really know for sure if we have an elementary reaction based on
the balanced stoichiometric equation!!!
28. Heterogeneous Catalysis
A (g) B (g)
Active Metals
(Pt, Co, MoO2)
support
A + S A-S
A-S B-S
B-S B + S
k1
k-1
k2
k-2
k3
k-3
Proposed Reaction Mechanism
29. What If Adsorption Is Rate
Limiting Step?
Adsorption
of A
Surface RXN
of A to B
Desorption
of B
Length of Vector Is Proportional to RXN Rate
Director of Vector Indicates Direction of RXN
Net RXN of Adsorption
Net RXN of Adsorption
Net RXN of Surface RXN
Net RXN of Surface RXN
Net RXN of Desorption
Following Approximations Can Be Made:
1. Adsorption of A is almost irreversible
2. Both surface rxn and desoprtion steps are almost at equilibrium
Net RXN of Adsorption = Net RXN of Surface RXN = Net RXN of Desorption
30. What If Adsorption Is Rate
Limiting Step?
Since it is an elementary reaction,
A + S A-S
k1
Where S is a free surface site and A-S is a chemisorbed complex
-rA = k1 CA CS
v = CS / Ctotal
v = the fractional coverage of vacant site
How can we experimentally measure Cs ???
Cs = functions of parameters that one can experimentally
measure or easily obtain
31. What If Adsorption Is Rate
Limiting Step?
Since both surface rxn and desorption steps are in near equilibrium,
A-S B-S
B-S B + S
k2
k-2
k3
k-3
rnet = k2 CA-S –k-2 CB-S 0 k2 / k-2 = K2 = CB-S / CA-S
rnet = k3 CB-S –k-3 CB CS 0 k3 / k-3 = K3 = CB CS / CB-S
Both K2 and K3 are equilibrium constants which one can obtain:
Let us do the site balance,
Ctotal = CS + CA-S + CB-S =
Const.
K2 = CB-S / CA-S
K3 = CB CS / CB-S
CS =
Ctotal
1 + [ (1 + K2) CB / (K2 K3) ]
RT ln K = - G
32. What If Adsorption Is Rate
Limiting Step?
CS =
Ctotal
1 + [ (1 + K2) CB / (K2 K3) ]
From the site balance and quasi-equilibrium approximation,
-rA = k1 CA CS
From the rate limiting step,
Ctotal
1 + [ (1 + K2) CB / (K2 K3) ]
=
k1 Ctotal
1 + K’ CB
k1
=
Where K’ = (1 + K2) / (K2 K3)
CA = PA / RT
If A and B behave according to the ideal gas law,
CB = PB / RT
CA CA
33. What If Surface Reaction Is
Rate Limiting Step?
K1
1 + K1 PA
k2 PA
-rA =
A + S A-S
A-S B-S
B-S B + S
k1
k-1
k2
k-2
k3
k-3
Rate Limiting Step
Figure 1.16 from Fundamentals of Industrial
Catalytic Processes
34. What If Desoprtion Is Rate
Limiting Step?
K1
1 + (K1 + K1 K2) PA
k3 PA
-rA =
A + S A-S
A-S B-S
B-S B + S
k1
k-1
k2
k-2
k3
k-3
Rate Limiting Step
K2
35. Fundamental Catalytic Phenomena
and Principles
Catalyst
Design
Catalytic Properties
(Activity and Selectivity)
Chemical Properties
(Oxidation State, Acidity,
Surface Composition)
Physical Properties
(Surface Area, Pore
Structure, Pore Density)
