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Topical Workshop – MOF Catalysis

„Microkinetics in Heterogeneous Catalysis“

DFG Priority Program 1362

Roger Gläser Institut für Technische Chemie

Institut für Nichtklassische Chemie e.V.Universität Leipzig

12.04.2011

Outline

• Introduction

• Microkinetics in Catalysis• Fundamentals• Kinetics and Reaction Mechnisms:

Microkinetic Modelling• Examples

• Rate Procurement: Catalytic Testing• Testing Reactors and Set-ups• Transient Methods• Problems and Pitfalls

• Conclusions: MOFs and Microkinetics

References

• I. Chorkendorf, J.W. Niemantsverdiet: “Concepts of Modern Catalysis and Kinetics”, Wiley-VCH, Weinheim (2003).

• “Handbook of Heterogeneous Catalysis”, G. Ertl, H. Knözinger, F. Schüth, J. Weitkamp, eds., 2nd

edition, Wiley-VCH (2008), – Chapter 5.2, pp. 1445-1560.– F. Kapteijn, R.J. Berger, J.A. Moulijn, Chapter 6.1, pp.

1693-1714. – J. Weitkamp, R. Gläser, Chapter 9.2, pp. 2045-2053.

References

Steps in Heterogeneous Catalysis

gas phase

AB

A

B

Aads.

B

Bads.

Aboundary layer

pore diffusion

chemical reaction

adsorption/desorption

film diffusion

1

7

4

2

5

2

3

6

66666666666666

71

Micro- versus Macrokinetics

H2 or O2

catalyst+ solvent+ reactants

Microkinetics = kinetics of the chemical elmentary steps

Macrokinetics = kinetics of combined reaction and transport

transport - mass and heat transfer- within and across phases - all reactants and products

Why Do Microkinetics?• Catalyst and Reactor Design

• Nature and utilization of mass, surface area, porosity, active sites

• Kind and operating conditions of reactors• Reaction rate occurs in design equations

Heterogeneous Catalysis Engineering

• Elucidation of Reaction Mechanisms• Microkinetic Modelling• Falsification rather than proof• Requires experimental data of sufficient amount

and accuracy

TiO

O

O

RH Ti

O

O

O

RH

O

TiOH

OR

H

H2O2

H2O

Fundamentals

Fundamentals (II)

Fundamentals (III)Adsorption Models – Adsorption Isotherms

Fundamentals (IV)Langmuir Isotherms - Derivation

Fundamentals (V)Langmuir Isotherm – Derivation (II)

Microkinetic ModellingKinetic Models

Microkinetic Modelling (II)Hougen-Watson-Kinetics

Microkinetic Modelling (III)surface reaction is rate-determining

Microkinetic Modelling (IV)surface reaction is rate-determining

Microkinetic Modelling (V)surface reaction is rate-determining

Microkinetic Modelling (V)surface reaction is rate-determining

Microkinetic Modelling (VI)surface reaction is rate-determining

Microkinetic Modelling (VI)surface reaction is rate-determining

Microkinetic Modelling (VI)surface reaction is rate-determining

Terms in Kinetics

Example 1: N2O-Decomposition

steady-state assumption

elementary steps

Example 1: N2O-Decomposition (II)

assumption: step 2 is rate limiting, steps 1, 3 in quasi-equilibrium

site balance

Example 2: CO-Oxidationapplication: automotive off-gas treatment active component: noble metalsconversions:

J. Weitkamp, R. Gläser, "Katalyse", in: "Winnacker-Küchler: Chemische Technik", R. Dittmeyer, W. Keim, G. Kreysa, A. Oberholz, Eds., Vol. 1, Chapter 5, Wiley-VCH, Weinheim (2004), pp. 645-718.

a) CnHm + (n + m/4) O2 n CO2 + (m/2) H2O

b) 2 CO + O22 CO2

c) 2 NO + 2 CO N2 + 2 CO2

2 (n + m/4) NO + CnHm (n + m/4) N2 + (m/2) H2O+ n CO2

metalhousing

wivenet

catalystoff-gas

washcoat(primary particles)

washcoat(secundary particles)noble metal

particles

ceramic monolith

Example 2: CO-Oxidation (II)elementary steps

surface coverages

Example 2: CO-Oxidation (III)rate

low temperature high temperature

temperature / K temperature / K

Example 3: NH3-Synthesis

elementary steps

Example 3: NH3-Synthesis (II)

Figs. 7.21, 7.22

surface coverages

Example 3: NH3-Synthesis (III)rate

equilibrium condition

Example 3: NH3-Synthesis (IV)

Measurement of Reaction Rates

Reactors for Catalytic Testing

Batch Continuous flow: PFR

spinning basket reactor

Reactors for Kinetic Measurements

Gradientless reactorwith internal recycling loop

Set-Ups for Kinetic Measurements

Set-Ups for Kinetic Measurements (II)

Set-Ups for Kinetic Measurements (III)

H2

N2

CO2

CH4

Ar

O2

H2OControlledEvaporator

Mixer

TCV1

TE

TCV1

TCV1

Purge gasInfrared

gas analyser

FE4

FE4

FE4

FE4

FE4

FE4

FE4

GC

Set-Ups for Kinetic Measurements (IV)

Transient Methods

TAP = Temporal Analysis of Products

R.J. Berger, F. Kapteijn, J.A. Moulijn, G. B Marin, J. De Wilde, M. Olea, D. Chen, A. Holmen, L. Lietti, E. Tronconi, Y. Schuurman, Appl. Catal. A: General 342 (2008) 3.

Transient Methods (II)

TAP = Temporal Analysis of Products

R.J. Berger, F. Kapteijn, J.A. Moulijn, G. B Marin, J. De Wilde, M. Olea, D. Chen, A. Holmen, L. Lietti, E. Tronconi, Y. Schuurman, Appl. Catal. A: General 342 (2008) 3.

Transient Methods (III)SSITKA = Steady-State Isotope Transient Kinetic Analysis

O2

O2

PC

MS

GSSIR-GA

Abgas

MFC

LFC

MFC

MFC

CEM

18

He od.N2

(Isotop)H2O od. ZP/H2O

C4-HC

MK

KF

R

TIRC

Mass Flow ControlerLiquid Flow ControlerControlled Evaporator MixerMischkammerMassenspektrometerGas Stream SelectorInfrarot-GasanalysatorReaktor mit HeizmantelComputerSteuereinheit zur Regelung undKontrolle der TemperaturKühlfalle

MFCLFCCEMMKMSGSSIR-GARPCTIRC

KF

Legende:

Bypass

Transient Methods (IV)SSITKA = Steady-State Isotope Transient Kinetic Analysis

0 200 400 600 800 1000 1200

0 200 400 600 800 1000 1200

Edukt: Flow-Profil

T5T4T3

T2T2

TOS* 10-1 (s)

MS-

Inte

nsitä

t (w.

E.)

me: 74

me: 60

me: 46

me: 58

me: 94

me: 112

0 200 400 600

70

80

90

1000

10

20

300

1

2

3

Isotop-Umschaltung

16O - 16O

stea

dy s

tate

TOS (s)

16O - 18O

F (i O

-n O) i

n C

H 3CO

OH

(%)

18O-18O

260°C 220°C 200°C 180°C 160°C

Transient Methods (V)SSITKA = Steady-State Isotope Transient Kinetic Analysis

Testing: Batch or Continuous

• Batch Easy to handleCommonly available and cost efficient Simple sampling− Reaction and deactivation kinetics coupled− Data analysis difficult

• Continuous-flow− Elaborate handling− Dedicated equipment, partly costly− Sampling often difficult (online analysis)Reaction and deactivation kinetics uncoupledData analysis straight forward

PitfallsB

ATC

H• High initial rates

• Fast deactivation

• Low conversions

• Strong endo- or exothermic reactions

• Low catalyst amounts

• Varying or unsteady reactant flow

• Changing bed heights

• Changing pressure dropCO

NTI

NU

OU

S

Ten Commandments for Testing Catalysts1. Specify objectives2. Use efficient strategy3. Chose right reactor type4. Establish ideal flow patterns5. Ensure isothermal conditions6. Minimize transport effects

Small particles Low conversionsModerate temperatures

7. Obtain meaningful dataRate, TOF, space time yield

8. Determine the stability9. GLP: reproducibility, blank runs, cleanliness10.Report unambiguously

MOFs and Microkinetics?

K. Leus, I. Muylaert, M. Vandichel, G.B. Marin, M. Waroquier, V. Van Speybroeck, P. Van der Voort, Chem. Commun. 46 (2010) 5085.

Conclusions

• Microkinetics• are needed for reactor design• can help to elucidate reaction mechanisms• require knowledge on elementary steps and active sites• might be mathematically challenging• are complementary to experimental kinetic data

• … and MOFs• only few discussions on catalytic mechanisms• further characterization of active sites needed• continuous-flow testing essentially absent

… an attractive field for future research!