Microkinetic investigationsof …...• Reaching thermal equilibrium at RT (over night) • Crushing the capsule and reducing the He pressure • Waiting for a stable baseline (2 hours)

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.

• 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

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

k (T, c, p , p ...)1 2

Global kinetics (macroscopic) 1 Elementary step kinetics (microscopic) 1

k1 k2

k3

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.

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

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??

( )

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

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

adsorption and desorption

MechanisticMechanistic stepssteps of of methanolmethanol synthesissynthesis on on coppercopper

adsorption and desorption

H2, CO, CO2


Dissociation and surface reaction

formate HCO2


surface reaction: hydrogenation

dioxomethylene H2CO2


surface reaction

methoxy CH3O


desorption

methanol CH3OH

MechanisticMechanistic stepssteps of of methanolmethanol synthesissynthesis on on coppercopperMechanisticMechanistic stepssteps of of methanolmethanol synthesissynthesis on on coppercopper

hydroxyl OH, H2O

surface reaction, desorption


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

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


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


a) d)

c)

Oxidizedatmosphere

Reducedatmosphere


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

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

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!

J.A. Dumesic et al., The Microkinetics of Heterogeneous Catalysis, ACS Professional Ref. Book, Washington, DC 1993.

KineticKinetic Parameters Parameters basedbased on TSTon TST

KineticKinetic Parameters Parameters basedbased on TSTon TST

HeatHeat of Adsorptionof Adsorption

• 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

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

Rads

θ

∆H

T1lnp =

∂∂

Clausius-Clapeyron equation

IsostericIsosteric HeatsHeats of Adsorptionof Adsorption

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

• 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

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

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

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

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

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

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

TPD: Experimental TPD: Experimental setset--upup

First order:The TPD peaks do not shift as a function of coverage; asymmetric peak shape

FirstFirst--orderorder desorptiondesorption kineticskinetics

Second order:The TPD peaks do shift as a function of coverage; more symmetric peak shape

SecondSecond--orderorder desorptiondesorption kineticskinetics

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.

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.

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 = 3·1011 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 −

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

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

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

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

** COCO ↔+

( )COCOads

freiCOadsads

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 iA

ii,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

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

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

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.

• 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

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.

Documents

Microkinetic investigationsof …...• Reaching thermal equilibrium at RT (over night) • Crushing the capsule and reducing the He pressure • Waiting for a stable baseline (2 hours)