Microkinetic Modeling of Catalytic Reactions

Modern history of catalysis

1910’s 1940’s 1990’s

Michaelis-Menten

Langmuir-Hinschelwood

Hougen-Watson

Dumesic-Rudd Froment

ki is extracted from fitting particular kinetic equations

ki is either calculated or measured

Catalytic Reactors

Development of Important Industrial Catalytic Processes

Production of Liquid Fuels!!!

Development of Important Industrial Catalytic Processes

NO CO CxHy

N2 CO2 H2O

O2

Transport and Reaction Processes in a Catalytic Reaction

Heterogeneous Catalysis

A (g) B (g)

• Minimize ΔP • Minimize Mass Transport

Resistances • Maximize Activity • Minimize Poisoning and

Fouling

Support (Al2O3)

Active Metals (Pt, Co, MoO2)

support

Surface reaction

Metal

1.  Adsorption 2.  (Diffusion on the surface) 3.  Surface reaction 4.  Desorption

Composition of conventional model

CH4

CO CO2

2 H2O

4 H2

H2 H2O

H2O

H2

Syngas production

I. II.

III.

rCO = -r1+r3 rCH4 = r1-r2 rCO2 = r2-r3 rH2 = r1+4r2-r3 rH2O = -r1-2r2+r3

Why do we need microkinetic modeling n  Experimental kinetic study used to determine

details in the mechanism - Problem: Different models may fit data equally well

n  Deduction of kinetics from a proposed reaction mechanism

n  Historically macroscopic descriptions of the reaction kinetics were used

n  Today, detailed scientific information available n  Guidance for catalytic reaction synthesis at

various levels of detail

What is microkinetics about?

Definition of microkinetic analysis examination of catalytic reactions in terms of elementary chemical reactions that occur on the catalytic surface and their relation with each other and with the surface during a catalytic cycle

J. A. Dumesic et al., Ind. Eng. Chem. Res. 1987, 26 (1399)

It means that the subject of investigation is not the overall reaction but each particular elementary reaction.

Use of Microkinetic Modeling

n  Use of kinetic model for description of – Reaction kinetic data

n  Spectroscopic observations n  Microcalorimetry and TPD n  Reduction of large kinetic mechanisms

Composition of MK model

Overall reactions A+B ↔ AB A+2C ↔ AC2

Elementary reactions A + * ↔ A* B + * ↔ B* C + * ↔ C* A* + B* ↔ C* + * A* + C* ↔ AC* + * AC* + C* ↔ AC2* + * AB* ↔ AB + * AC2* ↔ AC2 + *

2 Independent reactions 12 Species 5 gaseous A, B, AB, C, AC2 G 7 surface *, A*, B*, C*, AC*, AB*, AC2* Sur 12 Equations (G-1) reactor equations

depends whether reactor is CSTR or plug flow

(Sur-1) Steady state dΘ/dt=0

1 mass conversion Σyi=1

1 site conversion ΣΘi=1

Parameters for Microkinetic Analysis

n  Sticking coefficients n  Surface bond energies n  Pre-exponential factors for surface

reactions n  Activation energies for surface reactions n  Surface bonding geometries n  Active site densities and ensemble sizes

Elementary Reaction It is one that proceeds on a molecular level exactly as written in the balanced stoichiometric equation

A + B à C If it is an elementary reaction,

A B C

-rA = k [A]1 [B]1

Elementary reactions

n  elementary reaction is such a reaction in which one or more of the species react directly to form products

n  molecularity is the number of colliding molecules in a single reaction step

n  different types Dissociation AB = A + B Combination A + B = AB Disproportionation AB + C = A + BC

Kinetic variables in MK

n  Preexponential factor n  Transition State theory n  Collision theory

n  Activation energy n  Unity Bond Index – Quadratic Exponential

Potential (UBI-QEP) n  DFT

n  Energy barrier n  UBI-QEP

Collision theory

is used to determine rate constants for adsorption processes in terms of number of gas-phase molecules colliding with a surface per unit are per unit time

demanded inputs

n  sticking coefficient (as a function of temperature) n  pressure

Composition of MK model

Overall reactions A+B ↔ AB A+2C ↔ AC2

Elementary reactions A + * ↔ A* B + * ↔ B* C + * ↔ C* A* + B* ↔ C* + * A* + C* ↔ AC* + * AC* + C* ↔ AC2* + * AB* ↔ AB + * AC2* ↔ AC2 + *

2 Independent reactions 12 Species 5 gaseous A, B, AB, C, AC2 G 7 surface *, A*, B*, C*, AC*, AB*, AC2* Sur 12 Equations (G-1) reactor equations

depends whether reactor is CSTR or plug flow

(Sur-1) Steady state dΘ/dt=0

1 mass conversion Σyi=1

1 site conversion ΣΘi=1

Example of Microkinetic Model – NH3 Decomposition over a Ruthenium Catalyst

Deshmukh et al, Microreactor Modeling for Hydrogen Production from Ammonia Decomposition on Ruthenium, Ind. Eng. Chem. Res. 2004, 43, 2986-2999

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Axial Lenth, (m)

Mas

s Fr

actio

n of

Sur

face

Spe

cies

Surface Phase Concentration Profiles in Modeling of NH3 Decomposition using Microkinetic Analysis

H(s)N(s)NH(s)NH2(s)NH3(s)Ru(s)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Axial Lenth, (m)

Mas

s Fr

actio

n of

gas

pha

se s

peci

esGas Phase Concentration Profiles in Modeling of NH3 Decomposition using Microkinetic Analysis

NH3H2N2Ar

MK model - example

3 CO + 2 H2O = 2 CO2 + CH3OH

rCO* = 3r1-r5-2r11 r* = -3r1-r2+2r3+r4+r5+r6

+r7+r8-2r9-2r10+2r11 rH2O* = 2r2-2r9 rCO2* = 2r3-2r11 rCH3OH* = -r4+r8 rH* = -r5-r6-r7-r8+2r9+2r10 rHCO* = r5-r6 rH2CO* = r6-r7 rH3CO* = r7-r8 rOH* = 2r9-2r10 rO* = 2r10-2r11

CO H2 CO2 H2O

a) T = 623K ptot = 2,1 MPa H2/CO = 3

b)  T = 573K ptot = 1,1 MPa H2/CO = 3 c) T = 553K ptot = 2,1 MPa H2/CO = 3 d) T = 523K ptot = 2,1 MPa H2/CO = 3

Examples

G. Lozano-Blanco et al., Ind. Eng. Chem. Res. 2008, 47 (5879)

Compare of micro and conventional models

Micro Conventional

Detail

Very complex

Limited to the terminal species and

r.d.s.

Information

Trends, what’s going on on the surface

Accurate information the selectivity,

temperature profile

Interval of application

Wide

Narrow, determined by experimental

conditions

Application Catalyst and process design

Reactor and process design