Kinetics in heterogeneous catalysis

Christoph Sprung

Overview

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Basics: -  law of mass action -  conversion, yield, selectivity -  reaction rate, activtion energy -  kinetic orders, molecularity, rank -  power rate law

Second step: -  Adsorption (molecular, dissociative, competitive) -  Langmuir-Hinshelwood -  Eley-Rideal -  Mars-van-Krevelen -  rate determining step

Example: Steam Reforming

Example: Steam Reforming

Examples from Literature

Introduction

νaA + νbB ↔ νcC + νdD

Kinetics Thermodynamic

K = [C]vc[D]vd

[A]va[B]vb

r+ = k+[A]va[B]vb

r- = k-[C]vc[D]vd

2

K = [C]vc[D]vd

[A]va[B]vb

k+

k- = = const.

1. Order Reaction

3

A à Products

half live time: t½

t

ln [A

]

ln [A0]

[A0] t ½

2. Order Reaction

4

2 A à Products

half live time: t½

t

[A0] t ½

reversible

A 2 B C

irreversible

Complex Reaction Systems

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consecutive reactions parallel reactions

2 A E + F

B C

D 2 G

A 2 B C

Conversion-Yield-Selectivity

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A 2 B + C

Conversion (x)

Yield (y)

Selectivity (S)

Extend and Rate of Reaction

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extend of reaction rate of reaction

Turnover frequnency:

number of active sites

catalyst weight...

catalyst volume...

surface area...

Activation Energy

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temperature dependency of the kinetic constant

Arrhenius law:

preexponential factor temperature dependency small compared to exponential term

chemical kinetics

chem. kinetics + diffusion

diffusion

Order – Molecularity – Rank

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A B + C D

2 E

F G

2 H + I

Order: Molecularity:

Rank:

1st (A) unimolecular

--- 1st (B), 1st (C) bimolecular

primary (B,C) 1st (D), 2nd (E) trimolecular

primary (D of B+C) secondary (D of A)

1st (F) unimolecular

primary (F of D+2E) secondary (F of B+C)

tertiary (F of A)

Products Products Products

...in real world: Kinetic orders and molecularities cannot be predicted from stoichiometries, it has to be deduced from observations

do the experiment!

Si

xA

Steam Reforming

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CH4 COx/H2

H2O

CO2

O2

+

(synthesis gas)

CH4 + H2O D CO + 3 H2 ΔH298 K = +205.9 kJ/mol

CH4 + 2 H2O D CO2 + 4 H2 ΔH298 K = +164.7 kJ/mol

CO + H2O D CO2 + H2 ΔH298 K = -41.1 kJ/mol J. Rostrup-Nielsen and L. J. Christiansen. Catalytic science series: Concepts of syngas manufactore, volume 10.Imperial College Press, 2011

500 – 950 oC 20 – 30 bar

H2O/CH4 = S/C = 2.5

H2

NH3

CH3OH

Fischer-Tropsch products

Steam Reforming – Data Set

Influence of steam at constant methane partial pressure

Influence of methane at constant steam partial pressure

The constant partial pressure increases: black < red < green ...

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TOF vs. reactant partial pressure

Influence of steam at constant methane partial pressure

Influence of methane at constant steam partial pressure

r = k⋅p(CH4)n⋅p(H2O)m

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Pseudo kinetic modelling

r = k⋅p(CH4)n⋅p(H2O)m

r = 3.7*p(CH4)0.0

r = 9.8*p(CH4)0.2

r = 41*p(CH4)0.7

r = 26*p(CH4)0.5

r = 26.9 p(H2O)0.53

r = 5.2 p(H2O)0.0

r = 1.5 p(H2O)-0.17

r = 3.7 p(CH4)0.03

r = 9.8 p(CH4)0.2

r = 41.0 p(CH4)0.68

r = k’⋅p(H2O)m r = k’⋅p(CH4)n

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Summarised kinetic orders

r = k⋅p(CH4)n⋅p(H2O)m

Symbols

Background

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«Christmas tree»

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Kinetic orders – categorisation

(...)r(CH4) for methane conversion rate (...)r(H2O) for steam conversion rate

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Steady state approach

𝜃𝐴→ B 𝐴+  𝜃𝑉⇌𝜃𝐴

𝐴→ B

high pressure limit

low pressure limit

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Steady state approach

𝜃𝐴→ B 𝐴+  𝜃𝑉⇌𝜃𝐴

𝐴→ B

adsorption faster compared to reaction

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Bodenstein

A B C k1 k2

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active site

10 nm

10 nm

plane

step

kink

MgAl2O4

Ni-particle

Si O

H

Al

Zeolite ZSM-5

Brønsted-acid

corner

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Adsorption A

A* A* A* B*

dissociation competitive

A

A* A*

A B A2

molecular

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LH-HW mechanism

kinetic factor driving force

adsorption term

A

A*

B

B* A* B*

C

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Langmuir-Hinshelwood

Hougen-Watson

Eley-Rideal

A

A*

C B

A* C*

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Mars-van-Krevelen

O O O O O O O O O O O O

O O O O O O O O O O O O

O O O

O O O

O O O O O O O O O O

O O O O O O

O O O O O O

O O O2

A

AO

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Adsorption Enthalpy

T

Sabatier’s principle – Volcano plot

r

weak adsorption à  low concentration of

reactants on the catalyst surface

optimum

strong adsorption à  reactants block surface

sites and hinder the reaction

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van’t Hoff

Rate determining step

G. Jones et al. J. Catal. 259 (2008) 147

r

quasi-equilibrium

rate determining step

Dissociative CH4 adsorption

Oxidation of C*/CH*/CH2*

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Mechanism Steam Reforming CH4 + H2O D CO + 3 H2 CH4 + 2 H2O D CO2 + 4 H2 CO + H2O D CO2 + H2

Reactants: Oxidation: Products:

...you may only prove a mechanism wrong (not right)!

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L-H mechanisms

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LH-HW mechanisms

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Model – A

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Model – B

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Model – C

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Model – D

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Model – E

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Prenatal and post-mortem investigation

Starting material Material after reaction

?2 wt-% NiO/NiAl2O4 After treatment at 873 K under steam reforming conditions

Reaction

C Detailed analysis due to optimal investigation conditions

DLimited relation of post-mortem results to characteristics under reaction conditions

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Surface during reaction Surfaces under working conditions

High pressure STM •  Pt(111) at 350 K •  different gas atmospheres

Somorjai et al. J. Am. Chem. Soc. 131 (2009) 16589 36

Metal Nanoparticles Surface composition of alloy particles

Somorjai et al. J. Am. Chem. Soc. 131 (2009) 16589; Tao et al. Science 322 (2008) 932 37

In-situ analysis Methanol synthesis over Cu/ZnAl2O4

Le Peltier et al. J. Mol. Catal. A Today 122 (1997) 131

Methoxy: CH3O-* Formate: OCHO* Carbonyl: CO* CO + 2 H2 D CH3OH

Surface

Reaction pathway elucidation:

•  Desorption of methoxy considered rate determining

rejected

Gas phase

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Distinguishing a Mechanism Methanol synthesis over Cu/ZnAl2O4

Le Peltier et al. J. Mol. Catal. A Today 122 (1997) 131

Methoxy: CH3O-* Formate: OCHO* Carbonyl: CO* CO + 2 H2 D CH3OH

Reaction pathway elucidation:

•  Pathway through carbonyl species

Langmuir-Hinshelwood expression

Cu Cu

H2 CO

Non-competitive Cu

H2 CO

competitive

competitive

non-competitive

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Selective activity on LDH crystals: Transesterification and hydrolysis

Roeffaers et al. Nature 439 (2006) 572

488 nm Ar+ laser

Concept: fluorescent product photo-bleaches (~1 s)

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Transesterification and hydrolysis

Roeffaers et al. Nature 439 (2006) 572

Initial rates

q Transesterification q

{1010} - {0001}

q Hydrolysis q

Initial rates

5 µm

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Reduction of resazurin to resorufin

Xu et al. Nat. Mater. 7 (2008) 992

Detection of fluorescence signal of resorufin (the reactant is not fluorescent)

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Resazurin to resorufin: kinetic

Xu et al. Nat. Mater. 7 (2008) 992

k2: substrate-assisted

k3: direct dissociation

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Resazurin to resorufin: surface reconstruction

Chen et al. Chem. Soc. Rev. 39 (2010) 4560; Zhou et al. J. Am. Chem. Soc. 132 (2010) 138

Surface reconstruction

TOF

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Bezemer et al. J. Am. Chem. Soc. 128 (2006) 3956

Fischer-Tropsch Synthesis H2/CO = 2, 1 bar, 220 oC

Particle size Pyrrole hydrogenation

140 oC

Kuhn et al. J. Am. Chem. Soc. 130 (2008) 14026 45

Summary

t

ln [A

] ln [A0] 2 A

E + F B

C D

2 G

A A

A* A*

kinetic factor driving force

adsorption term

Rate expressions Kinetic orders

Activation energy Conversion, Selectivity, Yield, Rate

Adsorption Langmuir-Hinshelwood

Eley-Rideal Mars-van-Krevelen

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Literature

“Kinetics of multisteip reactions” F.G. Helferich in Comprehensive chemical kinetics, Vol. 40, by N. J. B. Green, Elsevier, 2004

“Chemical Kinetics and Reaction Dynamics” S. K. Upadhyay, Springer, 2006

“Kinetics of Catalytic Reactions” M. A. Vannice, Springer, 2005

“Engineering Catalysis” D. Y. Murzin , De Gruyter, 2013

“Concepts of modern Catalysis and Kinetics” I. Chorkendorff and J. W. Niemantsverdriet , Wiley-VCH, 2003

“Chemical Kinetics and Catalysis” R. A. van Santen and J. W. Niemantsverdriet , Springer, 1995

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