Introduction to Surface Chemistry...Introduction to Surface Chemistry Reinhard Schomäcker Institut...

Institut für Chemie

IMPRS Block Course, March 2013

Introduction to Surface Chemistry

Reinhard Schomäcker


TU Berlin

Strasse des 17. Juni 124

10623 Berlin

[email protected]



Surface Chemistry

- Modification of surface

- Grafting

- Coating

- Chemical Vapor Deposition

- Atomic Layer Deposition

- Impregnation Techniques

- Etching

- Corrosion

- Electrochemical Resolution or Deposition

- Surface Analytics

- Microscopy, Scattering and Diffraction Methods

- Spectroscopy

- Catalysis

- heterogeneous oxidation catalysis

Wetting

Diffusion

Electrostatics

Adsorption

Structure

Geometry

= unknown local

concentration and

reactivity

Oxygen as a

„Probe molecule“



Outline

- Industrial examples

-Fundamentals

- Oxidations reactions with lattice oxygen

-the active component

-the support effects

-Oxidation reactions with adsorbed oxygen



Selective Oxidation of Hydrocarbons

20% of all industrial organic chemicals

O2 molecules:

triplet state

with 2 unpaired elctrons



Required material properties of oxidation catalysts



Oxides are

nonstochiometric

Nucleophilic and electrophilic oxidation



Nucleophilic lattice oxygen

Dynamic surface equilibrium

Electrophilic surface oxygen species

Radical reactions,

Total oxidation

C-H actication

Oxidative dehydrogenation

Insertion into activated C-H bonds

Vacancy formation

Bi2O3-MoO3 Co3O4

18O isotope exchange exp.



Lattice transport of O2- Redox mechanism

Oxidation reactions with nucleophilic lattice oxygen



2 HCl +1/2 O2 H2O + Cl2

-57,5 kJ/mol

Cu, Mn, Ni, Co, V, Mo, Hg, Ag, Nb, Ti, Zn, W, Ru, Ce

The Deacon process


IMPRS Block Course, March 2013 Teschner et al, J. Catal. 2012, 285, 273-284

DOI:10.1016/j.jcat.2011.09.039

pKpK

ppppK

kppk

OHOHClCl

OHCl

HClO

f

HCLOf

r

2222

1

2

1

25.15.0

2

5.01

2

1

1. O2 + 2* O2**

2. O2** 2 O*

3. O* + * + HCl OH* + Cl*

4. 2 Cl* 2 * + Cl2

5. 2 OH* O* + * + H2O

Fundamentals of the Deacon process

Micro kinetic model Energy profile for RuO2/SnO2

Nuria Lopez, ICIQ



0

0.075

0.15

0.225

0 2 4 6 8 10

t / s

pC

l2 / b

ar

400 °C

350 °C

325 °C

300 °C

pKpK

ppppK

kppk

OHOHClCl

OHCl

HClO

f

HCLOf

r

2222

1

2

1

25.15.0

2

5.01

2

1

EA,0 = 59.4 kJ/mol

DHAds,Cl2 = -104.3 kJ/mol

DHAds,H2O = -96.0 kJ/mol

0

0,075

0,15

0,225

0,3

0 0,075 0,15 0,225 0,3

pCl2, sim

pC

l2, e

xp

400 °C

350 °C

325 °C

300 °C

Experimental Investigation of the Deacon process

Kinetic parameter for RuO2/SnO2:



MO2 + 2 HCl MOCl2 + H2O DRHad

MOCl2 + 1/2 O2 MO2 + Cl2 DRHox

Comparison of Deacon catalysts

D. Teschner et al, J. Catal. 2012, 285, 273-284

J. Perez-Ramirez et al., J. Catal. 2012, 286, 287-297

J. Allen et al, J. Appl. Chem. 1962, 12, 406

M.W.M. Hisham, S.W. Benson, J. Phys. Chem. 1995,

99, 6194



Simplified energy profile of Deacon reaction



Typical Feature of Surface Chemistry: Vulcano Behaviour



Oxidations with lattice oxygen: Mars-van Krevelen - Mechanism

Lattice oxygen



Selectivity in hydrocarbon oxidation

Thermodynamics favours the ultimate formation of CO2 and H2O

Desired products of partial oxidation reactions are intermediates derived by

kinetic control

Competing parallel and consecutive reactions

C-H bonds of reactants usually stronger than those in intermediate products

All oxidation processes are highly exothermic

Expolsive regime in gas mixture



Oxides are

nonstochiometric




V

O

O OO V

O

O OO

O2

+5+4

+3 +5

V

O

O OO

V

O

O OO

H

CH

V

O

O OO

V

O

O OO

V

O

O OO

H

V

O

O OO

H

VO O

O

O

H H

V

O

O OO

+5+5

+4 +4

+5 +5

-H2O

X. Rozanska, R. Fortrie, J. Sauer, J. Phys. Chem. C, 111 (2007) 6041-6050.

OHHC

OHHC

n

nX

52

52

0

1,

j j

j

i

i

i n

n

S

Ethanol oxidation reaction network

ODH of

propane:



with n ≥ 1

k2/k5pO2 << 1

K1pC2H5OH > 1

2

5

12

1

12

21

2

1

521

52

1

52

1

k

pk

pKkpK

pKkr

O

OHHC

OHHC

OHHC

n

n

n

Mechanistic aspects

B. Kilos, A.T. Bell, E. Iglesia, J. Phys. Chem. C, 113 (2009) 2830-2836



Impact of vanadia dispersion on performance

- dispersion of vanadia influenced by support material

- different performance of monomers, oligomers and polymers

- different contribution of uncovered support surface

- no comparability of low loaded catalyst with different supports



- Preparation of near-monolayer catalysts by

thermal spreading of vanadyl acetylacetonate

- No over oxidation of the products under differential

conditions (X < 10%) and stoichiometric feed

support surface area

[m²/g]

surface density

[V/nm²]

wt% V

TiO2 17,1 3,5 0,6 %

Al2O3 200,9 3,1 4,9 %

ZrO2 52,1 3,1 1,4 %

CeO2 19,8 3,9 0,7 %

No crystalline V2O5 in all samples

Catalyst preparation



0

0,05

0,1

0,15

0,2

0,25

0,3

Al2O3 V2O5 ZrO2 CeO2 TiO2

TO

F (

20

0°C

) [m

ol/

mo

ls]

VOx/Support

10% Xat 42 V

OHC

n

nTOF

0

10

20

30

40

50

60

70

80

90

100

Al2O3 V2O5 ZrO2 CeO2 TiO2

EA

,ap

p [

kJ

/mo

l]

VOx/Support

T

TOFRE appA 1

ln,

Support effect



60

70

80

90

100

110

120

60 65 70 75 80 85 90 95

EA

,ap

p [

kJ

/mo

l]

EA,app ethanol [kJ/mol]

ODH of propane and methanol vs. ethanol

propane methanol*

VOx/TiO2

VOx/Al2O3

VOx/ZrO2

V2O5

VOx/CeO2

*L. J. Burchman, I. E. Wachs, Catal. Today, 49 (1999) 467-484

P.R. Shah, I. Baldychev, J.M. Vohs, R.J. Gorte, App. Cal. A, 361 (2009) 13-17

Support effect



TiO2 ZrO2 SiO2 V2O5 MgO a-Al2O3

0

50

100

150

200

250

300

350

DH

f [kJm

ol-1

]

DHf this work

DHf Allersma1967

DHf Todorova2007

DHf Sauer2004

2

ln

RT

ΔH

dT

Kd f

212

2OO pVeK

22

12 OeVO O

x

O

fmO ΔHΔEΔEV 31

Defect formation

Mass action law

Van’t Hoff’s law

500 450 400 350 300

-10,0

-9,5

-9,0

-8,5

-8,0

-7,5

-7,0

1

2

3

4

5

6

7

8

9

10

X(C

3H

8)

(%)

ln

T [°C]

ΔE*

ΔEm

En

erg

y

X

ΔEm

ΔHf

Theorie of defect formation

T. Allersma, R. Hakim, T. N. Kennedy, J. D. Mackenzie, J. Chem. Phys., 46 (1967) 154-160

J. Sauer, J. Doebler, Dalton Transactions, 19 (2004) 3116-3121

T. K. Todorova, M. V. Ganduglia-Pirovano, J. Sauer, J. Phys. Chem. C., 111, (2007), 5141-5153



Energy profile of ethanol partial oxidation

VOx/Al2O3

Ea,app

ΔHdef

VVV

VO

2 +

C2H

5O

H(g

)

+ ½

O2(g

)

VVV

V(O

H) 2

-C2H

4O

+ ½

O2(g

)

VIV

VIV

O+

H2O

(g)

+

C2H

4O

(g)

+ ½

O2(g

)

VVV

VO

2+

H2O

(g)

+

C2H

4O

(g)

ΔHR

TS 1

reaction cordinateE

[k

J/m

ol]

reaction coordinate



40

50

60

70

80

90

100

110

120

30 50 70 90 110 130

EA

,ap

p [

kJ

/mo

l]

∆Hf [kJ/mol]

propane

ethanol

methanol*

VOx/TiO2

VOx/ZrO2

VOx/Al2O3

V2O5

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0,45

40 60 80 100 120

TO

F [

mo

l/m

ols

]

∆Hf [kJ/mol]

ethanol (200°C)

methanol **

propane (400°C)

VOx/TiO2

VOx/ZrO2 VOx/Al2O3

V2O5

**I. E. Wachs, Catal. Today, 100 (2005) 79-94

*L. J. Burchman, I. E. Wachs, Catal. Today, 49 (1999) 467-484

P.R. Shah, I. Baldychev, J.M. Vohs, R.J. Gorte, App. Cal. A, 361 (2009) 13-17

BEP- Correlations with defect formation enthalpy



4V/13Ti/SBA-15 catalyts structure based on spectroscopical

and reactivity results.

Preparation of a high performance catalyst for ODP


IMPRS Block Course, March 2013 G. Deo, I.E.Wachs, J. Catal, 146 (1994) 323-334

Activity scaling with H2-TPR peak temperatures



Catalytic cycle of propene oxidation



21

2

1

1

1

5

121

12

1

O

CHCH

CH

pk

pKkpK

pKkr

n

n

n

Mars-van-Krevelen Rate Law

Rate of reduction

Rate of reoxidation

Steady state assumption

Surface oxygen coverage

Overall rate

+



Oxidation reactions with adsorbed Oxygen

- Total Oxydation reactions of VOC

- Partial Oxidation of Methane

- Epoxydation of Ethene

- Oxidative Coupling of Methane



Epoxidation of Ethene

Singularities: Ag as selective catalyst,

oxygen as oxidant,

only working with ethene

Proposed Pathways

Of selective and unselective

reaction

Oxidation reactions with electrophilic oxygen



Oxygen is more

electrophilic at high

coverage

Kinetics:

Eley-Rideal



Possible intermediate

DFT calculations of activation barriers for model discrimination



Proposed structures

For DFT calculations



Vulcano Plot for

EO-TOF



Oxidative Coupling of Methane (OCM) to Ethylene

o Worldwide research efforts to convert CH4 into chemicals (C2H4, CH3OH…)

o Oxidative Coupling of Methane (OCM) is a promising direct conversion route

of methane to ethylene but has not reached industrial practice yet

43

Desired:

Undesired: 2 CH4 + 3 O2 DG (800°C) = - 1222 kJ mol-12 CO + 4 H2O

2 CH4 + 4 O2 DG (800°C) = - 1602 kJ mol-12 CO2 + 4 H2O

kinetic control by means of a catalytic process needed

2 CH4 + O2 C CH

H

H

H+ 2 H2O DG (800°C) = - 306 kJ mol-1



Pioneering work

0

20

40

60

80

100

120

140

160

1985 1990 1995 2000 2005 2010

Reihe1

G.F. Keller, M.M. Bhasin, J. Catal. 1982, 73, 9 catalyst

M. Hinsen, M. Baerns, Chem. Z. 1983, 107, 223 „

A.C. Jones, J.J. Leonardo, A.J. Sofranko, periodic

US patent 4 443644, 1984 feed strategy

U. Zavyalova, M. Holen, R. Schlögl, M. Baerms, ChemCatChem, 2011, 3, 1935

year

papers



Ethylene Production by Oxidative Coupling of Methane

Model Studies for Understanding Complexity

* table adopted from E. V. Kondratenko, M. Baerns

Handbook of Heterogeneous Catalysis, Vol. 6, Ch. 13.17, 2008



Mn-doped Na2WO4/SiO2

*U. Simon, O. Görke, A. Bertold, S. Arndt, R. Schomäcker,

H. Schubert, Chem. Eng. J.168 (2011) 1352-1359

200 g batches with spray drying technology

X.Fang et al, J. Mol. Catal. (China), 6 (1992) 427



Very low specific surface < 5 m2/g

Similar activation energies EA,app = 150 kJ/mol

Inactive < 600 °C, no lattice oxygen involved

Common features of Catalysts for Oxidative Coupling of Methane

0 390 400 410 420 430 440

100

120

140

160

180

200

220

240

260

280 VOx/TiO

2

VOx/CeO

2

VOx/ZrO

2

EA

,ap

p +

DH

va

p [kJ/m

ol]

D0 (C-H) [kJ/mol]

Eth

anol

Cyc

lohe

xane

Pro

pane

Eth

ane

Met

hane

EA,0

380 390 400 410 420 430 4400

20

40

60

80

100

120

140

160

180 Mn/Na

2WO

4/SiO

2

Gasphase with O*

EA

,ap

p

D0 (C-H) [kJ/mol]

Eth

anol

Pro

pane

Eth

ane M

etha

ne

Hypothesis: adsorbed atomic oxygen



Experiments in a TAP reactor

with J. Perez-Ramires, ICIQ, Taragona, now ETH Zurich



J. Lunsford, Angew. Chemie, Int. Ed, 1995, 34, 970

Defects ? TMn+ TM(n+1)+ + e-

2 10-11 1/cm

Li+0-.

Stability ?

Selectivity ?

Operation conditions ?

Mechanism of Oxidative Coupling of Methane

Arndt et al., Catalysis Reviews, 53, 2011, 424-514



OCM is possible in the gas phase

but not at 1bar and with poor selectivities

CH3 + CH3

+ M C2H6 + M* (trimolecular)

Catalyst = M ?

R. Horn, S. Mavlyankariev

Mechanism of Oxidative Coupling of Methane

C H 4

*

O 2

O O O

+ C H 3

+ C H 3 C 2 H 6

2 10-11 1/cm3

3 2 6 2 5 C H C H C H C 2 H 4

C O x

+ O 2

+ O 2

+ O 2

2 10-13 1/cm3



0

10

20

30

40

50

60

70

0 3 6 9 12 15

C2 S

ele

cti

vit

y [

%]

CH4 Conversion [%]

8:1 4:1 2:1

Mn-Na2WO4/SiO2, 100 mg, 750°C, Feed Gas:

Catalyst Testing and kinetic studies



Product selectivity of Catalysts




The surface is,

What matters !



Selectivity in hydrocarbon oxidation

Thermodynamics favours the ultimate formation of CO2 and H2O

Desired products of partial oxidation reactions are intermediates derived by

kinetic control

Competing parallel and consecutive reactions

C-H bonds of reactants usually stronger than those in intermediate products

Surface structure and reactivity is the essential control parameter

- Definded sites of selective reactivity are required

- Stability of sites against reaction conditions and product essential

for processes

Introduction to Surface Chemistry...Introduction to Surface Chemistry Reinhard Schomäcker Institut...

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