Heterogeneous Catalysis: From Imagining to Imaging a ......Heterogeneous Catalysis: From Imagining...

Heterogeneous Catalysis: From Imagining to Imaging a Working Surface

Introduction to issues

Studies of model catalysts with surface analytical methods: special properties of metal nanoclusters

Thermal stability of metal nanoclusters

Strategies for designing sinter resistant catalysts

D. Wayne GoodmanTexas A&M University

Department of Chemistry

Heterogeneous Catalysis

Applications• Production of transportation fuels (440 oil refineries)

• Production of chemicals

• Cleanup of automotive/industrial exhaust gases

• “Green” chemistry (unwanted byproducts)

catalyst

catalyst

catalyst

catalyst

A

A

A

B

B

B

+ P

P

Ppote

ntia

l ene

rgy

adsorption reaction desorption

catalyst

catalyst

catalyst

catalyst

A

A

A

B

B

B

+ P

P

Ppote

ntia

l ene

rgy


catalyst

catalyst

catalyst

catalyst

A

A

A

B

B

B

+ P

P

Ppote

ntia

l ene

rgy


gas phase reaction

Heterogeneous Catalysis





catalyst

catalyst

catalyst

catalyst

A

A

B

B

P

Ppote

ntia

l ene

rgy


catalyst

catalyst

catalyst

catalyst

A

A

B

B

P

Ppote

ntia

l ene

rgy


catalyst

catalyst

catalyst

catalyst

A

A

B

B

P

Ppote

ntia

l ene

rgy


Ammonia Synthesis

catalyst

catalyst

catalyst

catalystA B

pote

ntia

l ene

rgy


catalyst

catalyst

catalyst

catalystA B

pote

ntia

l ene

rgy


catalyst

catalyst

catalyst

catalyst

AAN

N

BBH

H

pote

ntia

l ene

rgy


AANBBH

BBHAANBBH

BBH

BBHBBH

BBH

AAN





Carbon Monoxide and Nitric Oxide

catalyst

catalyst

catalystcatalyst

C Opo

tent

ial e

nerg

y


NO

CO

O

C OO

CO

O

N

NN

N

N

catalyst

catalyst

catalystcatalyst

C OC Opo

tent

ial e

nerg

y


NO

CO

CO

O

C OO C OC OO

CO

OC

OC

O

O

N

NN

N

N





2 CO + 2 NO 2 CO2 + N2

Carbon Monoxide and Nitric Oxide

catalyst

catalyst

catalystcatalyst

C Opo

tent

ial e

nerg

y


NO

CO

O

C OO

CO

O

N

NN

N

N

catalyst

catalyst

catalystcatalyst

C OC Opo

tent

ial e

nerg

y


NO

CO

CO

O

C OO C OC OO

CO

OC

OC

O

O

N

NN

N

N




• “Green” chemistry (eliminate unwanted byproducts)

2 CO + 2 NO 2 CO2 + N2

“Green” Chemistry with Catalysts

Catalytic route:Cl promoted Ag catalyst

C2H4 + 1/2O2 -----------------------------------------> C2H4O

C3H6 + ½ O2 C3H6O

Example: ethylene to ethylene epoxide

C2H4 + ½ O2 C2H4O

Old non-catalytic route (epichlorohydrine process):

Step 1: Cl2 + NaOH HOCl + NaClStep 2: C2H4 + HOCl CH2Cl-CH2OHStep 3: CH2Cl-CH2OH + 1/2Ca(OH)2 ½ CaCl2 + C2H4O + H2O----------------------------------------------------------------------------------------------------------------Total: Cl2 + NaOH + 1/2 Ca(OH)2 + C2H4 C2H4O + 1/2 CaCl2 + NaCl + H2O

Unique Catalytic Activity of Nanosized Gold Particles

1.2 2.82.42.01.6Au Particle Diameter (nm)

Propylene Oxide

Prod

uct Y

ield

(%)

1

3

2 Propane

H2/O2/Propylene/Ar = 1:1:1:7Pt = 1 atmT = 350 K

CO2

1.2 2.82.42.01.6Au Particle Diameter (nm)

Propylene Oxide

Prod

uct Y

ield

(%)

1

3

2 Propane

H2/O2/Propylene/Ar = 1:1:1:7Pt = 1 atmT = 350 K

CO2

from Haruta, et al., Shokubai,Catalysts and Catalysis (1995)

Au/TiO2

Cluster Diameter (nm)

Propylene Oxidation

2CO + O2 2CO2

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.0 2.0 4.0 6.0 8.0 10.0


TO

F (1

/site

s)

300 K

Au/TiO2

CO:O2 = 1:5PT = 40 Torr

2CO + O2 2CO2

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.0 2.0 4.0 6.0 8.0 10.0


TO

F (1

/site

s)

300 K

Au/TiO2

CO:O2 = 1:5PT = 40 Torr

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.0 2.0 4.0 6.0 8.0 10.0


TO

F (1

/site

s)

300 K

Au/TiO2

CO:O2 = 1:5PT = 40 TorrCO:O2 = 1:5

PT = 40 Torr

from Haruta, et al., Catalysis Letters (1997)

Au/TiO2


CO Oxidation

Unique Catalytic Activity of Nanosized Gold Particles

2CO + O2 2CO2

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.0 2.0 4.0 6.0 8.0 10.0


TO

F (1

/site

s)

300 K

Au/TiO2

CO:O2 = 1:5PT = 40 Torr

2CO + O2 2CO2

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.0 2.0 4.0 6.0 8.0 10.0


TO

F (1

/site

s)

300 K

Au/TiO2

CO:O2 = 1:5PT = 40 Torr

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.0 2.0 4.0 6.0 8.0 10.0


TO

F (1

/site

s)

300 K

Au/TiO2

CO:O2 = 1:5PT = 40 TorrCO:O2 = 1:5

PT = 40 Torr

from Haruta, et al., Catalysis Letters (1997)

Au/TiO2


CO Oxidation

TEM Image of Gold Supported onTitania (from M. Date, ONRI)

Model Oxide-Supported Metal Catalysts

Metal Clusters1.0-50 nm

Oxide Single Crystal

Oxide Single Crystal

e.g. TiO2

50 nm

TiO2(110)

TiO2(110)+

0.25 Au

50 nm

S. Shaikhutdinov, R. Meyer, D. Lahav, M. Baeumer, T. Kluener, H.-J. FreundPhys. Rev. Lett. (2003)

Pd on FeO(111)/Pt(111)

Elevated-Pressure Cell

Gate Valve

LEED

AES IonGun

Sample Manipulator

Sample PreparationChamber

XPS

Apparatus

STM

M. Valden, X. Lai, and D. W. Goodman, Science 281, 1647 (1998)

Au/TiO2(110):1D→2D→3D2D 3D1D

0.6

1.0

1.4

1.8

2.2

Act

ivity

0

15

30

45

60

0 2 4 6 8 10Cluster size (nm)

Popu

latio

n (%

)

CO + 1/2O2 CO2

CO:O2 = 1:5PT = 40 Torr

M. Valden, X. Lai, and D. W. Goodman, Science 281 (1998) 1647.30nm x 30nm

Unique catalytic activity of Au/TiO2(110)

0.6

1.0

1.4

1.8

2.2

Act

ivity

0

15

30

45

60

0 2 4 6 8 10Cluster size (nm)

Popu

latio

n (%

)

CO + 1/2O2 CO2

CO:O2 = 1:5PT = 40 Torr

M. Valden, X. Lai, and D. W. Goodman, Science 281 (1998) 1647.1.5 2.0 2.5 3.0 4.03.5 4.5

Particle size (nm)

CO

Hea

t of A

dsor

ptio

n(k

cal/m

ol)

10

20

18

16

14

12

Bulk AuBULK GOLD

D. C. Meier, D. W. Goodman, J. Am. Chem. Soc. 126 (2004) 1892.

Unique properties of Au/TiO2(110)

6.0 nm6.0 nm

Surface defects on TiO2(110)

Oxygen vacancies

Ultraviolet Photoelectron Spectroscopy (UPS): Defects on TiO2(110)

Krischok, Guenster, Goodman, Hoefft, and Kempter, submitted for publication

Ti+3(3d)Ti+3(3d)

TiO2(110) + O2 400 K

TiO2(110) + O2 750 K

[001]2.96Å

1

1

12

Oxygen Vacancies Top View

[110] 6.49Å

Side View

[001]2.96Å

1

1

12

Oxygen Vacancies Top View

[110] 6.49Å

Side View

oxygen vacancies

E. Wahlstroem, N. Lopez, R. Schaub, P. Thostrup, A. Ronnau, C. Africh, E. Laegsgaard, J. K. Norskov, and F. Besenbacher, Phys. Rev. Lett. 90, 101 (2003)

Bridging oxygen vacancies are the active nucleation sites for Au clusters

a) STM image of a small Au clusters on TiO2 . Vacancies are marked with squares. b) Simulated STM image of a single Au atom trapped in a oxygen vacancy. c) Line profiles comparing DFT theoretical results and experiment.

Role of Oxygen Defects in Metal Cluster Nucleation and Growth on TiO2(110)

30 nm

Au on TiO2(110): Cluster Anchored via Reduced Titania

E. Wahlstroem, N. Lopez, R. Schaub, P. Thostrup, A. Ronnau, C. Africh, E. Laegsgaard, J. K. Norskov, and F. Besenbacher, Phys. Rev. Lett. 90, 101 (2003)

Single oxygen vacancy can bind 3 Au atoms on average

DFT shows that Au nanoparticles promote the exchange of oxygen vacancies between the surface and bulk of titaniaRodriguez et al, J. Am. Chem. Soc., 124 (2002) 5242

and

Au bilayer

50 x 50 nm

(1 0)

(0 1)

(1 1)

[110

]-

[111]

MoMoTiO

OO

(8x2)

Reduced Titania Surface: TiOx/Mo(112)

[111

]

[110]

STM

LEED

Chen et al., Surf. Sci. Lett. (2005), & Science 306 (2004) 252.Full-1ML-Ti3+!!

1.0 monolayers Au:(1x1)-Au/TiOx/Mo(112)

M. S. Chen and D. W. Goodman, Science 306 (2004) 252;STM: Chen et al. (2006)

3x3nm2

1.33 monolayers Au:(1x3)-Au/TiOx/Mo(112)

(1 0)(0 1)(1 1)

8x8nm2

Relative Catalytic Activity of Mono- and Bi-layer Au/TiOx

0

1

2

3

4

0 1 2 3 4 5 6

Au coverage (ML)

0

1

2

3

4

0 1 2 3 4 5 6

Au coverage (ML)

CO + ½ O2 CO2CO:O2 = 1:5

PT = 10 Torr

TOF:molecules CO2

produced per Au atom per sec

MoTiMo

Au(1x1)

MoTi

iMo

Au(1x1)

Mo TiMo

AuAu

(1x3)

Mo iMo

AuAu

(1x3) Ti

Chen, et al., Science, 306, 252 (2004)

45 times higher than that found for the most active Au/TiO2high surface area supported catalyst

Similarity of Au nanoparticles & the (1x3) well-ordered bilayer

Both form 1D-like chain for the topmost Au atoms!

[001

]

[110]

Summary: Properties of Supported Au Nanoclusters

● Core-level shifts are markedly non-bulk-like at <ca. 3.0 nm.● Surface plasmon not observed for clusters <ca. 3.0 nm.

● Adsorbate binding energies, e.g. CO and O2, changesignificantly from the bulk values for clusters < 3.0 nm.

● Sublimation energies of clusters < 3.0 nm are markedly lower than the corresponding bulk value.

● Nanoclusters are generally unstable to reaction conditions, i.e., understanding and maintainingstability is a key to technological break-throughs.

● DFT calculations show center of Au d-band significantlydestabilized for Au/TiO2 compared to Au.

0 50 100 150 2000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

()

CO Oxidation Over Au/TiO2 as a Function of Reaction Time

2CO + O2 2CO2

CO:O2 = 1:5

PT = 40 Torr

T = 350 K

Au/TiO2(110)

ReactionRate, CO2moleculesper site persecond

Reaction Time (minutes)

ABefore Rx DAfter Rx

Sintering Mechanisms

Gibbs-Thompson relationship

μ – chemical potential; σ – surface free energyM/ρ – atom volume; r – cluster radius

interparticle transportAtom Migration (Ostwald Ripening) : atoms/atom ensembles migrate to adjacent clusters to form larger clusters

P. Wynblatt and N. A. Gjostein, Acta Metallurgica, 24, 1165 (1976)

A

Cluster size increaseCluster size decrease

Cluster disappears

630A x 630A 630A x 630A

STM: 0.5 MLE Au/TiO2 (110), CO/O2 (1:1), 4.2 Torr @ 420K

+ 1 hour

1

2

3

4

5

6 7

8

910

1

2

3

4

5

6 7

8

910

0 10 20 30 40 50 60 70Yang and Goodman, 2004 0

2

4

6

8

10

12

14

16

18

20

22

24

Clu

ster

vol

ume

(nm

3 )

time (minute)

12345678910

Sintering Mechanisms

Gibbs-Thompson relationship

μ – chemical potential; σ – surface free energyM/ρ – atom volume; r – cluster radius

interparticle transportAtom Migration (Ostwald Ripening) : atoms/atom ensembles migrate to adjacent clusters to form larger clusters

P. Wynblatt and N. A. Gjostein, Acta Metallurgica, 24, 1165 (1976)

A

particle migration/coalescence

Cluster Migration : Clusters migrate along the surface, collide with others and coalescence

B

Au/TiO2(110) Before and After Annealing to 950K

As deposited After a 950K x 30 min. anneal

100 nm 100 nm

a b

Kolmakov and Goodman, Chem. Rev., 2003

Role of support in metal activation & cluster sintering:

SiO2 versus TiO2?

Model Oxide-Supported Metal Catalysts

Refractory Single Crystal

Thin Oxide Film Support + Metal Clusters

e.g. Mo, ReTa, W

Refractory Single Crystal

Oxide Thin Filme.g. SiO2, Al2O3, MgO, TiO21-10 nm

Metal Clusters1.0 - 50 nm Refractory Single Crystal

Oxide Thin Film

SiO2/Mo(112)

50 nm

400 nmMo(112)

0.5 ML Au SiO2/

Mo(112)50 nm

0.7 nm thick, sharp hexagonal LEED with a band gap ~8.9 eV (STS)

1. Si @RT2. O2 @ 800K3. Anneal @1200 K

Preparation & Characterization of Ultra-thin, Well-ordered SiO2/Mo(112)

400 nm

Schroeder, Adelt, Richter, Naschitzki, Baumer, and Freund. Surf. Rev. Lett. 7 (2000)

10 nm

15 10 5 0

oxygenvacancies

coun

t rat

e (a

rb. u

nits

)

binding energy (eV)

Defects on SiO2 Surfaces Studied by Metastable Electron Impact Spectroscopy: MIES

Low defect density

High defect density

0.40 ML of Au 0.033 ML/min

300 K

200 nm 200 nm

0.40 ML of Au 0.033 ML/min

300 K

Au Cluster Nucleation on Low-Defect Versus High-Defect SiO2

“Au + Low Defect SiO2” “Au + High Defect SiO2”

0.7 ML Au at room T

0.7 ML Au after a 850 K anneal

Au Cluster Nucleation on Defective SiO2

0

50

100

150

200

250

300

Num

ber D

ensi

ty o

f Au

Clu

ster

s (x

1010

/ cm

2 )

@ room Tanneal to 850K

A comparison of Au cluster density after deposition at RT and a subsequent anneal to 850K

0.2ML 0.7ML 1.3ML

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4160

180

200

220

240

260

280

300

Coverage (ML)

10/

Num

ber D

ensi

ty o

f Au

Clu

ster

(x10/

/cm2 )

850 Kanneal

200 nm 200 nm

Sintering of Au Clusters on SiO2

• Sintering of Au on SiO2 more facile than on TiO2i.e, Au binds less strongly to SiO2 than to TiO2

Strategies for a Sinter-Resistant Support: TiO2 Dispersed onto and into SiO2

Mo(112)

SiO2: 1 – 10 nm

Mo(112)

SiO2: 1 – 10 nm

Mo(112)

SiO2: 1 – 10 nm

TiO2islands

Auclusters

Mo(112)

SubstitutionalTiO2

Mo(112)

or

100 nm 100 nm

1.0 ML SiO2/Mo(112) 0.2 ML TiOx/SiO2/Mo(112)

TiOx Islands Dispersed on SiO2

100 nm 100 nm

0.2 ML TiOx/SiO2/Mo(112)

Au Particles Deposited onto TiOx Islands Dispersed on SiO2

0.4 ML Au/TiOx/SiO2/Mo(112)

TiOx/SiO2

+0.4 ML Au

SiO2

Au/SiO2 versus Au/TiOx/SiO2: 850 K Anneal

100 nm

+0.4 ML Au

100 nm

“before” “after”

Ti Point Defects on SiO2

15 nm

15 nm

~0.1nm

0 2 4 6 80.00

0.05

0.10

0.15

Hei

ght (

nm)

Length (nm)

STM: TiOx-SiO2 Thin Film with 8% Ti

Scan across Ti defects

TiSi O

Mo (112)

TiSi O

Mo (112)

TiSi O

Mo (112)

2.4 nm

Au/Ti defectTi defect

Ti/SiO2/Mo(112) Au/Ti/SiO2/Mo(112)

Decoration of Ti Point Defects with Gold

+ 0.04 ML Au

Catalytic reactivity and selectivity are markedly different for clusters < ~3.0 nm.

Nanoclusters are generally unstable to reaction conditions, i.e., understanding and maintaining stability are the keys to technological break-throughs.

Core-level shifts, valence band structure, sublimation energies, and adsorbate binding energies are unique for clusters < ~3.0 nm.

Conclusions

Coworkers

ISSKai Luo

Stepahnus Axnanda

Lo-T IRASCheol-Woo Yi

IRASTao Wei

Matt LundwallSTMMingshu Chen

Hi-press STMFan Yang

Patrick Han

Rx-XPSDheeraj KumarMingshu Chen

HREELSZhen Yan

Ming-shu Chen

PM-IRASYun Cai

MIESSungsik Lee

Hi-SA Supported CatalystsZhen YanBo Wang

Au/SiO2

Au/TiO2(8%)-SiO2

Au/TiO2(17%)-SiO2

After Anneal 850 K

After ReactionCO:O2 = 2:1,

60 Torr,370 K,

120 minutes

0.0

0.5

1.0N

orm

aliz

ed A

u cl

uste

r den

sity Au/TiO2(17%)-SiO2

Au Cluster Density After The Indicated Treatment Normalized To The Cluster Density After Nucleation At Room Temperature

XPS Core Level Shifts: Au/SiO2 vs. Au/TiO2

Core Level Shift: Bulk – Small Cluster LimitAu/SiO2 Au/TiO2

0

.5

1.0

1.5

- .5

-1.0

2.0

Final State Initial State TotalContributions

BindingEnergy(ev)

1.61.8

-0.2

1.8

-1.0

0.8

Core Level Shift: Bulk – Small Cluster LimitAu/SiO2 Au/TiO2

0

.5

1.0

1.5

- .5

-1.0

2.0

Final State Initial State TotalContributions

BindingEnergy(ev)

1.61.8

-0.2

1.8

-1.0

0.8

XPS Core Level Shifts

Implications: electron-rich Au on TiO2!

E(eV)

Au/TiO2(110)Au(001)

DO

S (s

tate

s/eV

)DFT Calculations for Au and Au/TiO2(110)

Yang, Wu, Goodman, PRB (2000)

1 ML Au on Au(001) 1 ML Au on TiO2(110)

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