Microkinetic investigationsof heterogeneouslycatalyzedreactions · 2012. 10. 29. · Methanol...
Transcript of Microkinetic investigationsof heterogeneouslycatalyzedreactions · 2012. 10. 29. · Methanol...
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Microkinetic investigations of
heterogeneously catalyzed reactions(under industrially relevant conditions)
Jennifer Strunk
Junior Research Group Leader
Ruhr-University Bochum
Laboratory of Industrial Chemistry
Lecture series, Fritz-Haber-Institut, Berlin, Oct. 19, 2012.
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• Microkinetic investigations – what for?
• Elementary step kinetics
• Microcalorimetry
• Temperature-programmed desorption
• Examples: CO on Cu/ZnO/Al2O3
Methanol decomposition on ZnO
This lecture is based on the lecture „Modern Microkinetics“, Prof. Dr. M. Muhler, Industrial Chemistry, Ruhr-University Bochum.
ScopeScope of of thisthis lecturelecture
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1
2a
6b
2b
6a
7
3-5
outer surface
inner surface
boundary layer
2) Transport of the reactants to the surface
a) through the boundary layer to the outer
surface
b) from the outer surface to the inner surface
1) Transport of the reactants from the fluid bulk
to the boundary layer
7) Transport of the products from the boundary
layer to the fluid bulk
6) Transport of the products to the fluid bulk
a) from the inner surface to the outer surface
b) from the outer surface through the boundary
layer
3) Adsorption (chemisorption) of the reactant
4) Chemical reaction
5) Desorption of the product
Very frequently:
inner surface >> outer surface
TheThe sevenseven stepssteps in in heterogeneousheterogeneous gasgas--solid solid catalysiscatalysis
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k (T, c, p , p ...)1 2
Global kinetics (macroscopic) 1 Elementary step kinetics (microscopic) 1
k1 k2k3
k4
k7
k5
k6
Mean field approach (mesoscopic)
ComplexityComplexity of of HeterogeneouslyHeterogeneously CatalyzedCatalyzed ReactionsReactions
1 O. Hinrichsen, in: Catalysis from A to Z: A Concise Encyclopedia (ed. Herrmann, Cornils, Wong, Schlögl), 2nd edition, Wiley-VCH, Weinheim 2003.
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O. Hinrichsen, Catal.Today 53 (1999) 177-188.
Microkineticanalysis
Steady-stateexperiments
Transientexperiments
Characteri-zation
Spektros-copic
studies
Kinetic theories,thermodynamics
Single crystalsurfaces (UHV)
Real catalysts(high pressure)
PressureGap
MaterialGap
KnowledgeKnowledge--basebase approachapproach: Microkinetic : Microkinetic analysisanalysis
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Elementary step kinetics Global kinetics, X
Optimization of
Reaction parameters
(Choice of reactor)
Optimized Strategy for
catalyst preparation
Local approach
• Mechanistically proven model
• Physical interpretation of
kinetic parameters
Coverages of intermediates
Changes of the morphology
Process engineering/
Design, dimensioning
WhyWhy areare elementaryelementary stepstep kineticskinetics usefuluseful??
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( )
OHCO
HCO
g
totHCOOHCOa
PP
PP
K
PPPPPTR
EAr HCOOHCO
2
22
2
2
2
2
2
2
1
1exp
⋅⋅
⋅=
−⋅⋅⋅⋅⋅⋅
⋅−⋅=
β
βγαααα
αi apparent reaction order of component iγ fudge factor correcting the total pressure dependenceKg equilibrium constant for the WGS reaction
Power-law Model *1)
C.V. Ovesen, B.S. Clausen, B.S. Hammershøi, G. Steffensen, T. Askgaard, I. Chorkendorff,J.K. Nørskov, P.B. Rasmussen, P. Stoltze, P. Taylor, J. Catal. 158 (1996) 170-180.
Microkinetic Model *1)
H2O(g) + * H2O* (1)
H2O* + * OH* + H* (2)
2 OH* H2O* + O* (3)
OH* + * O* + H* (4)
2 H* H2(g) + 2* (5)
CO(g) + * CO* (6)
CO* +O* CO2* + * (7)
CO2* CO2 (g) + * (8)
CO2* + H* HCOO* + * (9)
HCOO* + H* H2COO* + * (10)
H2COO* + 4 H* CH3OH(g) + H2O(g) + 5* (11)
ExampleExample 1: Water gas 1: Water gas shiftshift reactionreaction
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CO + 2 H2 = CH3OH ∆RH = - 92 kJ/mol CO2 + 3 H2 = CH3OH + H2O ∆RH = - 50 kJ/mol CO + H2O = CO2 + H2 (WGSR) ∆RH = - 42 kJ/mol Cu/ZnO/Al2O3 5.0 – 10.0 MPa, 500 K
dioxomethylene
OC
O
H H
Cumethoxy
Cu
OCH3
OC
O
H
formate
Cu
Steps Surface reactions
1 H O(g) + * 2 H O* + * 3 2OH* 4 OH* + * 5 2H* 6 CO(g) + * 7 CO* + O* 8 CO * 9 CO * + H*10 HCOO* + H*11 H COO* + H*12 H CO* + H*13 CH OH*14 H COO* + *15 HCHO*16 H COO* + H*
2
2
2
2
2
3
3
2
2
H O* OH* + H*H O* + O*O* + H*H + 2*CO*CO * + *CO (g) + *
+ * + *
+ O*CH OH* + *CH OH(g) + *HCHO* + O*HCHO(g) + *HCHO* + OH*
2
2
2
2
2
3
3
HCOO*H COO*2H CO*3
* : free surface siteX* : adsorbed molecule or atom X
WG
SR
T.S. Askgaard, J.K. Nørskov, C.V. Ovesen, P. Stoltze, J. Catal. 156 (1995) 229-242.
WGSR: water gas shift reaction
ExampleExample 2: 2: BridgingBridging pressure/materialpressure/material gapsgaps forfor methanolmethanol synthesissynthesis
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adsorption and desorption
MechanisticMechanistic stepssteps of of methanolmethanol synthesissynthesis on on coppercopper
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adsorption and desorption
H2, CO, CO2
MechanisticMechanistic stepssteps of of methanolmethanol synthesissynthesis on on coppercopper
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Dissociation and surface reaction
formate HCO2
MechanisticMechanistic stepssteps of of methanolmethanol synthesissynthesis on on coppercopper
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surface reaction: hydrogenation
dioxomethylene H2CO2
MechanisticMechanistic stepssteps of of methanolmethanol synthesissynthesis on on coppercopper
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surface reaction
methoxy CH3O
MechanisticMechanistic stepssteps of of methanolmethanol synthesissynthesis on on coppercopper
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desorption
methanol CH3OH
MechanisticMechanistic stepssteps of of methanolmethanol synthesissynthesis on on coppercopperMechanisticMechanistic stepssteps of of methanolmethanol synthesissynthesis on on coppercopper
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hydroxyl OH, H2O
surface reaction, desorption
MechanisticMechanistic stepssteps of of methanolmethanol synthesissynthesis on on coppercopper
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P.L. Hansen, J.B. Wagner, S. Helvig, J.R. Rostrup-Nielsen, B.S. Clausen, H. Topsøe, Science 295 (2002) 2053-2055.
In situ TEM images (A, C, and E) of a Cu/ZnO catalyst in various gas environments together with thecorresponding Wulff constructions of the Cu nanocrystals (B, D, and F). (A) The image was recorded at a pressure of 1.5 mbar of H2 at 220 oC. The electron beam is parallel to the [011] zone axis of copper. (C) Obtained in a gas mixture of H2 and H2O, H2: H2O = 3:1 at a total pressure of 1.5 mbar at 220 oC. (E) Obtained in a gas mixture of H2 (95%) and CO (5%) at at total pressure of 5 mbar at 220 oC.
AtomicAtomic resolutionresolution in in situsitu Transmission Transmission ElectronElectron MicroscopyMicroscopy
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J.-D. Grunwaldt, A.M. Molenbroek, N.-Y. Topsøe, H. Topsøe, B.S. Clausen, J. Catal. 194 (2000) 452-460.
a)
Oxidizedatmosphere
Reducedatmosphere
Oxygen vacanciesReduced Zn
Cu
Zn
a) Round-shaped particles under oxidizing syngas co nditions
a)
Oxidizedatmo sphere
Reducedatmosphere
Cu
Zn
b)
a) Round-shaped particles under oxidizing syngas co nditionsb) Disc-l ike particles under more reducing conditio ns
Oxygen vacanciesReduced Zn
a)
c)
Oxidizedatmo sphere
Reducedatmosphere
Cu
Zn
b)
a) Round-shaped particles under oxidizing syngas co nditionsb) Disc-l ike particles under more reducing conditio nsc) Surface Cu-Zn alloying due to stronger reducing conditions
Oxygen vacanciesReduced Zn
a) d)
c)
Oxidizedatmosphere
Reducedatmosphere
Oxygen vacanciesReduced Zn
Cu
Zn
b)
a) Round-shaped particles under oxidizing syngas co nditionsb) Disc-like particles under more reducing conditio nsc) Surface Cu-Zn alloying due to stronger reducing conditionsd) brass alloy formation due to severe reducing con ditionsoxidizing
atmosphere
reducing
atmosphere
DynamicDynamic BehaviorBehavior of of Cu/ZnOCu/ZnO: : AlloyAlloy ModelModel
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Tentative Surface
Reaction Mechanism
crucial kinetic
parametersTransient
ExperimentsReactor Model
Surface Science
Studies
TST, collision
theory
Ab initio
calculation
Analogies
Comparison of Expe-
riment and Simulation
Steady-state
kinetics
optimization of the
not-constrained
kinetic parameters,
sensitivity analysis
Microkinetic Model
DevelopmentDevelopment of a of a microkineticmicrokinetic modelmodel
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Microkinetic Modeling
Arrhenius form for elementary steps
k = A ·exp (-Eact /(R ·T))
Microkinetic Microkinetic ModelingModeling
⇒ 4 kinetic parameters(2 preexponential factors, 2 activation energies) per reversible elementary step!
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J.A. Dumesic et al., The Microkinetics of Heterogeneous Catalysis, ACS Professional Ref. Book, Washington, DC 1993.
KineticKinetic Parameters Parameters basedbased on TSTon TST
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KineticKinetic Parameters Parameters basedbased on TSTon TST
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HeatHeat of Adsorptionof Adsorption
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• Thermal Desorption Spectroscopy
yields Ea,des
• Isosteric Heat of Adsorption
using the Clausius-Clapeyron Equation
• Calorimetry
direct measurement of Q
RoutesRoutes to to thethe HeatHeat of Adsorptionof Adsorption
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Thermal desorption spectra can be easily analysed by application of the Redhead
formula. Only valid if no readsorption occurs.
( )[ ]3.64/βTlnRTE mm −ν=• E: Activation energy of the desorption
• Tm: Peak maximum
• β: Heating rate• ν: Preexponential factor (Arrhenius)
QMS
turbo-
molecular
pump
Thermal Thermal DesorptionDesorption SpectroscopySpectroscopy
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Rads
θ
∆H
T1lnp =
∂∂
Clausius-Clapeyron equation
IsostericIsosteric HeatsHeats of Adsorptionof Adsorption
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The adsorption microcalorimetry set-up is based on the work of B. E. Spiewakand J. A. Dumesic.B. E. Spiewak, J. A. Dumesic, Thermochimica Acta 290 (1996) 43-53
Small doses of adsorptivegas are expanded into thecalorimeter.The Tian-Calvetcalorimeter measures theresulting heatflow(isothermal mode, 300 K). The amount of adsorbedmolecules is measuredvolumetrically.The complete set-up ismetal-tightened and thermostated.
PI
He
M
adsorptive gas
calorimeter
turbopump
hotbox
Microcalorimetry: Experimental Microcalorimetry: Experimental setset--upup
System originally set up at LTC by Dr. Raoul Naumann d‘Alnoncourt
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• Pretreatment of the sample (up to 3 days)
• Sealing of the sample in a pyrex capsule filled with He
• Transfer of the capsule into the calorimeter
• Degassing of the complete set-up at 418 K (2 days)
• Reaching thermal equilibrium at RT (over night)
• Crushing the capsule and reducing the He pressure
• Waiting for a stable baseline (2 hours)
• Starting the automatic dosing sequence (50 cycles, 2 days)
Microcalorimetry: Microcalorimetry: NecessaryNecessary experimental experimental stepssteps
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Dosing cycle:
1. Evacuating the dosing section
2. Filling the dosing section withadsorptive gas (100 Pa = 1 µmol)
3. Opening the measuring cell
4. Heat flow measurement (60 min)
Microcalorimetry: Microcalorimetry: MeasurementMeasurement procedureprocedure
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Calorimetric data and pressure data are collected simultaneously during themeasurement.Processing of the calorimetric data (integration of the heat flow for each singlepulse) yields the evolved heat.
64800 75600 86400
0.0
0.1
dQ/d
t / m
W
t / s
Microcalorimetry: Data Microcalorimetry: Data processingprocessing
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0 20 40 60 800
20
40
60
80
0 20 40 60 800
20
40
60
80
Q /
kJ/m
ol
Coverage / µmol/gcat
Pressure / Pa
Cov
erag
e / µ
mol
/gca
t
Differential Differential heatheat of of adsorptionadsorption and and adsorptionadsorption isothermisotherm: CO on Cu: CO on Cu
Adsorption of CO on a copper catalyst
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Why measure TPD?
• Adsorption kinetics of single molecules are probed:Important for improving catalystsFaster adsorption may increase overall rate
• Heterogeneous surfaces:Are there multiple adsorption sites for my reactant?
• 1st vs. 2nd order desorption:Does my reactant molecule dissociate on my catalyst?
TemperatureTemperature--programmedprogrammed desorptiondesorption
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Four general steps
• Catalyst pretreatmente. g. oxidation, reduction, reaction conditions
• Adsorptionoften small amount of probe molecule in inert gas
• Purgein inert gas at a fixed temperature
• Temperature-programmed desorptionin inert gas flow, increase temperature (linearly) with time
TPD: Experimental TPD: Experimental procedureprocedure
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Variation of:
• Heating rate- heating-rate-variation method, e. g. 5 K/min, 2 K/min, 1 K/min
- At constant initial coverage: usually θ0 = 1
• Initial coverage- with constant heating rate- different coverages can be
obtained by a variation of dosing time & temperature
- non-activated adsorption(CO on Cu):initial coverage variation bypartial desorption
TPD: Experimental TPD: Experimental procedureprocedure –– advancedadvanced
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TPD: Experimental TPD: Experimental setset--upup
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First order:The TPD peaks do not shift as a function of coverage; asymmetric peak shape
FirstFirst--orderorder desorptiondesorption kineticskinetics
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Second order:The TPD peaks do shift as a function of coverage; more symmetric peak shape
SecondSecond--orderorder desorptiondesorption kineticskinetics
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ExampleExample: : HydrogenHydrogen desorptiondesorption fromfrom Cu(111) Cu(111) singlesingle crystalcrystal (TDS)(TDS)
Please note: Due to surface reconstruction, much more complicateddesorption traces are obtained in case of Cu(100) and Cu(110).
G. Anger et al. Surface Science 220 (1989) 1-17.
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HH22 TPD TPD fromfrom Cu/ZnO/AlCu/ZnO/Al22OO33: Variation of : Variation of thethe HeatingHeating RateRate
100 200 300 400 500
0.00
0.02
0.04
0.06
0.08
0.10
wcat
= 200 mgQ
He = 100 Nml/min
2 K/min, Tmax
= 288 K
6 K/min, Tmax
= 297 K
E
fflu
ent m
ole
frac
tion
H2
/ %
Temperature / K
100 200 300 400 500
0.00
0.02
0.04
0.06
0.08
0.10
wcat
= 200 mgQ
He = 100 Nml/min
2 K/min, Tmax
= 288 K
6 K/min, Tmax
= 297 K
10 K/min, Tmax
= 303 K
E
fflu
ent m
ole
frac
tion
H2
/ %
Temperature / K
second-order desorption from Cu surface sites
no readsorption
100 200 300 400 500
0.00
0.02
0.04
0.06
0.08
0.10
wcat
= 200 mgQ
He = 100 Nml/min
2 K/min, Tmax
= 288 K
6 K/min, Tmax
= 297 K
10 K/min, Tmax
= 303 K
15 K/min, Tmax
= 306 K
E
fflu
ent m
ole
frac
tion
H2
/ %
Temperature / K
Pretreatment
Methanol Synthesis
Flushing in He at 493 K
for 0.5 h
Cooling in He to 240 K
Dosing 100% H2 for
0.5 h at 240 K and at
p = 15 bar
Cooling in H2 to 78 K
Flushing in He at 78 K
T. Genger, O. Hinrichsen, M. Muhler, Proc. Europacat IV, Rimini 1999, p. 175.
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3.25 3.30 3.35 3.40 3.45 3.5012.5
13.0
13.5
14.0
14.5
15.0
15.5
H2 TPD, β variation
2lnT
max
-lnβ
1000 K / Tmax
Cu/ZnO/Al2O
3Ades = 3·10
11 s-1
Edes = 78 kJ mol-1 §
no readsorption,
no diffusion limitation
T. Genger, O. Hinrichsen, M. Muhler, Europacat IV, Rimini 1999.§ T. Genger, O. Hinrichsen, M. Muhler, Catal. Lett. 59 (1999) 137-141.
2nd Order Plot 2nd Order Plot AccordingAccording to to PolanyiPolanyi--WignerWigner EquationEquation
11
2ln ln
nndes m desm
m des m
R A n En
T E RT
β − − = Θ −
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T. Genger, O. Hinrichsen, M. Muhler, Proc. Europacat IV, Rimini 1999.§ T. Genger, O. Hinrichsen, M. Muhler, Catal. Lett. 59 (1999) 137-141.
Ades = 3·1011 s-1
Edes = 78 kJ mol-1 §
HH22 TPD TPD fromfrom Cu/ZnO/AlCu/ZnO/Al22OO33: Variation of : Variation of thethe HeatingHeating RateRate
0.00
0.02
0.04
0.06
0.08
0.10
E
fflue
nt m
ole
frac
tion
H2
/ %
Cu/ZnO/Al2O
3
100 200 300 400 500
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
Effl
uent
mol
e fr
actio
n H
2 / %
Temperature / K
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Pretreatment
Methanol Synthesis
Flushing in H2 and He at
493 K for 0.5 h
Cooling in He to 78 K
H. Wilmer, T. Genger, O. Hinrichsen, J. Catal. 215 (2003) 188-198.
100 200 300 400 5001.8
1.9
2.0
2.1
2.2
2.3
Tmin
= 270 K2 K/min
2.1% H2 in He
Q = 20 Nml/min
E
fflue
nt m
ole
frac
tion
H2
/ %
Temperature / K
Tmin
= 299 K15 K/min
Tmin
= 291 K10 K/min
Tmin
= 283 K6 K/min
dissociative adsorption on Cu surface sites
high activation barrier
TemperatureTemperature--programmedprogrammed adsorptionadsorption of Hof H22 on Cu/ZnO/Alon Cu/ZnO/Al22OO33
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H. Wilmer, T. Genger, O. Hinrichsen, J. Catal. 215 (2003) 188-198.§ T. Genger, O. Hinrichsen, M. Muhler, Stud. Surf. Sci. Catal. 130 (2000) 3825-3830.
Aads = 1·103 (Pa s)-1
Eads = 51 kJ mol-1 §
100 200 300 400 5001.7
1.8
1.9
2.0
2.1
2.2
2.3
2.1% H2 in He
Q = 20 Nml/min
Effl
uent
mol
e fr
actio
n H
2 / %
Temperature / K
1.7
1.8
1.9
2.0
2.1
2.2
2.3
Effl
uent
mol
e fr
actio
n H
2 / %
TemperatureTemperature--programmedprogrammed adsorptionadsorption of Hof H22 on Cu/ZnO/Alon Cu/ZnO/Al22OO33
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TPD
• transient method• temperature dependence• processing of data yields
information about adsorption kinetics and thermodynamics
Microcalorimetry
• thermodynamic equilibrium• isothermal conditions• processing of data yields
adsorption heats and isotherms (equilibrium data)
Combination by microkineticmodelling
TPD under the influence of readsorption differential heats from microcalorimetry
CombiningCombining TPD and Microcalorimetry: CO on Cu/ZnO/AlTPD and Microcalorimetry: CO on Cu/ZnO/Al22OO33
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** COCO ↔+
( )COCOadsfreiCOadsads
pk
pkr
θθ
−⋅⋅=⋅⋅=1
COdesdes kr θ⋅=
adsorption of a single component,
readsorption occurring freely (non-activated)
⇒ rate expression for the
forward reaction
⇒ rate expression for the
reverse reaction
with: rate constants expressed by the Arrhenius-equation
−⋅=RT
EAk iAii
,exp
Microkinetic Microkinetic ModelingModeling of TPD of CO on Cu/ZnO/Alof TPD of CO on Cu/ZnO/Al22OO33
with EA,ads = 0 ⇒ EA,des = ∆Hads
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TPD-experiment: reflected adsorption enthalpy is the mean value of enthalpies of all molecules desorbing in a certain coverage range
⇒ Mean values can be calculated from microcalorimetry data in the same coverage intervals
55,50,000 - 0,167
58,50,000 - 0,102
62,10,000 - 0,054
[kJ mol-1][-]
∆HadsCoverage interval
⇒ Calculation of Aads from adsorption entropy (Method by Scholten& Konvalinka)
Microkinetic Microkinetic ModelingModeling of TPD of CO on Cu/ZnO/Alof TPD of CO on Cu/ZnO/Al22OO33
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250 300 350 4000.0
0.1
0.2
0.3
E
fflue
nt m
ole
fract
ion
of C
O /
%
Temperature / K
dashed lines: exp.
bold lines: theor.
CO TPD spectra and adsorption isotherms can be modeled in good agreement using calorimetric data.
J. Strunk et al., Phys. Chem. Chem. Phys. 8 (2006) 1556-1565.
0 25 50 75 1000.00
0.05
0.10
0.15
experimental Temkin model
Frac
tiona
l cov
erag
e
Pressure / Pa
350 K
325 K
300 K
Microkinetic Microkinetic modelingmodeling of TPD of CO on Cu/ZnO/Alof TPD of CO on Cu/ZnO/Al22OO33
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Kähler et al., ChemPhysChem. 11 (2010) 2521-2529.
Methanol Methanol decompositiondecomposition on on ZnOZnO NanoTekNanoTek®
400 450 500 550 600 650 700
0.00
0.02
0.04
0.06
0.08
CO2
H2
H2O
CO CH
3OH
567 K
517 K
491 K
ef
fluen
t mol
e fr
actio
n / %
temperature / K
-H2
lattice O
• Dissociative adsorption of methanol as methoxy species(and OH or bulk H) on ZnO (at least 58 % of the Zn2+
adsorption sites occupied).• For pure ZnO the mass balance is closed.• At 491 and 517 K two different methoxy species are
decomposed to form formate and H2.
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• EA for the conversion of methoxy to formate species are 109 kJ mol-1 and
127 kJ mol-1.
• Desorption at higher temperatures is ascribedto the decomposition of formates.
0.00185 0.00190 0.00195 0.00200 0.00205-15.6-15.4-15.2-15.0-14.8-14.6-14.4-14.2-14.0-13.8
EA=127 kJ/mole
EA=109 kJ/mole
1. decomp. peak 2. decomp. peak
ln(β
/Tm
ax
2 )
1/Tmax
400 450 500 550 600 650 7000.00
0.05
0.10
0.15
0.20
0.25
0.30
515 K 499 K 491 K
540 K 525 K 517 K
efflu
ent m
ole
frac
tion
/ %
temperature / K
Methanol Methanol decompositiondecomposition on on ZnOZnO NanoTekNanoTek®
Kähler et al., ChemPhysChem. 11 (2010) 2521-2529.
Variation of heating rate Arrhenius plot
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SummarySummary
• Knowledge of the elementary steps from model experiments undercontrolled conditions can be used to understand catalytic reactionunder industrially relevant conditions.
- Example: Cu catalyst in methanol synthesis – Cu(111) in UHV
• Microcalorimetry can be used to obtain heats of adsorption of reactants on industrial catalysts.
• TPD is a viable method to probe adsorption/desorption kinetics orthe decomposition of intermediates. For the modeling, single crystal data (e.g. Cu(111)) or heats of adsorption from microcalorimetry can be used (e.g. CO on Cu/ZnO/Al2O3)
• Microkinetic modeling can bridge pressure and material gaps in studies of heterogeneously catalyzed reactions.