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!
