Catalysis for Chemical Engineers

A Brief History and Fundamental Catalytic Principles

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.

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

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

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.

Development of Important Industrial Catalytic Processes

Mittasch investigated over 2500 catalysts compositions!!!

Development of Important Industrial Catalytic Processes

It played a vital role as a feedstock for chemicals: 30 million tons per year in 2000

Development of Important Industrial Catalytic Processes

Production of Liquid Fuels!!!

Development of Important Industrial Catalytic Processes

NOCOCxHy

N2

CO2

H2O

O2

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

Chemical ReactionsFour 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

HI

Cl

Energy Intensive & Energy Intensive & damaging to equipment and damaging to equipment and materials & non-selectivematerials & non-selective

i (Ci)i

A. Active phase - metal that provides active sites where thechemical reaction takes place

B. Support or Carrier - high surface area oxide whichdisperses and stabilizes the active phase

(adds efficiency, physical strength, sometimes selectivity)

C. Promoter(s) - additive which improves catalyst properties, e.g. activity, selectivity, catalyst life

Components of a Typical Heterogeneous Catalyst

Pt Nanoparticles on Al2O3 Supports

macro-poresmeso-pores

Pt crystallites

high SA alumina

(10 nm)

(100-200 nm)

(1-5 nm)(a)

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

Components of a Typical Heterogeneous Catalyst

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, HCreforming, dehydrogenation, ammoniasynthesis, Fischer-Tropsch synthesis

oxides oxides of V, Mn, Fe,Cu, Mo, W, Al, Si,Sn, Pb, B

complete and partial oxidation ofhydrocarbons and CO, acid-catalyzedreactions (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

Support/Catalyst BET area (m2/g) Pore Vol. Pore Diam. (nm)

Activated Carbon 500-1500 0.6-0.8 0.6-2

Zeolites (Molecular Sieves) 500-1000 0.5-0.8 0.4-1.8

Silica Gels 200-600 0.40 3-20

Activated Clays 150-225 0.4-0.52 20

Activated Al2O3 100-300 0.4-0.5 6-40

Kieselguhr ("Celite 296") 4.2 1.14 2,200

Typical Physical Properties of Common Carrier (Supports)

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

Heterogeneous CatalysisSteps 1, 2, 6, & 7 are diffusional processes => Small dependences on tempSteps 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 >

Heterogeneous CatalysisSteps 1, 2, 6, & 7 are diffusional processes => Small dependences on tempSteps 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)

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

Internal Diffusion Effect on Reaction Rate

-rA = k η CAS

Where η = Effectiveness Factor

η = (CA)avg / CAS

CA

CAS

=coshcosh Φpore (1 - x/L)

( Φpore)cosh

η = (CA)avg / CAS = (tanh (Φpore) ) / Φpore

Φpore (Thiele Modulus) = L (k P / Deff)1/2

A Bk

L

x

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.

Elementary ReactionIt 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

Elementary ReactionIt 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

Elementary ReactionIt 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!!!

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

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 RateDirector of Vector Indicates Direction of RXN

Net RXN of AdsorptionNet RXN of Adsorption

Net RXN of Surface RXNNet RXN of Surface RXN

Net RXN of Desorption

Following Approximations Can Be Made:1. Adsorption of A is almost irreversible2. Both surface rxn and desoprtion steps are almost at equilibrium

Net RXN of Adsorption = Net RXN of Surface RXN = Net RXN of Desorption

What If Adsorption Is Rate Limiting Step?

Since it is an elementary reaction,

A + S A-Sk1

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

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 = CCSS + CCA-SA-S + CCB-SB-S = Const.

K2 = CCB-SB-S / CCA-SA-S

K3 = CB CCSS / CCB-SB-S

CS = Ctotal

1 + [ (1 + K2) CB / (K2 K3) ]

RT ln K = - G

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

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

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

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)

Structure Sensitive Reactions

CO Oxidation over Au/TiO2: Particle Size Effect

6 nm2.5 nm2 nmAu

TiO2

Particle Size Vs. Electronic Structure Change of Au