0.00

0.05

0.10

0.15

0.20

0.25

0.30

1.5 2.5 3.5 4.5 5.5 6.5

Cluster Diameter (nm)

TO

F (

1/si

te s

)

Structure Sensitivity of CO Oxidation over Au/TiO2

[Data for High SurfaceAre a Supported Catalysts from:Haruta, e t al., J. Catal.115 (1989) 301; see alsoHaruta, Cat. Today,36 (1997) 153.]

2CO + O2 2CO2

Nanoscience and Catalysis

Introduction to issues

Model catalyst preparation and characterization

Correlation among structural, chemical and electronic properties of supported metal clusters

Thermal stability of metal clusters

Grand challenges

D. Wayne GoodmanTexas A&M University

Department of Chemstry

Magnetic Moments Versus Metal Cluster Size

Ni

Co

Fe

From Gillas, Chatelain, and De Heer, Science (1994)

Ionization Potentials of Ni Clusters Versus Size

Knickelbein, Yang, Riley, JCP (1990)

~4.0 nm

~1.2 nm

0.4 eV

0.25 eV

Gold Supported on Titania

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

1.2 2.82.42.01.6

Au Particle Diameter (nm)

Propylene Oxide

Pro

du

ct Y

ield

(%

)

1

3

2 Propane

H2/O2/Propylene/Ar = 1:1:1:7

Pt = 1 atm

T = 350 K

CO2

Selective Oxidation of Propylene Over Au/TiO2

(Hayashi, Haruta, Shokubai, Catalysts and Catalysis, 1995)

Model Oxide-Supported Metal Catalysts

Single Crystal Oxide Support + Metal Clusters

Oxide Single Crystal

Oxide Single Crystal

e.g. MgO, TiO2

Metal Clusters1.0-50 nm

50 nm

TiO2(110)

TiO2(110)+

0.25 Au

50 nm

Morphology of the TiO2(110) Surface

50 nm

6.0 nm6.0 nm

[001]2.96Å

1

1

12

Oxygen Vacancies Top View

[110] 6.49Å

Side View

Bridging Oxygen

In-plane Oxygen

Titanium

Xu, Lai, Zajac, and Goodman, Phys. Rev. B (1997)

Au Cluster Growth on TiO2(110): Quasi 2-D & 3-D Au clusters

[001] [110]2-D (quasi 2-D) Au clusters can be observed at the initial stages (0.01 - 0.05 ML) of growth.

10 nm

10 nm

Xu, Lai, Zajac, and Goodman, Phys. Rev. B (1997)

STM: Au Clusters on TiO2(110)

30 nm

30 nm

Au/TiO2(110) : Cluster Size/Density Vs. Coverage

50 nm

50 nm

50 nm 50 nm 50 nm

Increasing Au coverage

Lai, St. Clair, Valden and Goodman, Prog. Surf. Sci (1998)

Model Oxide-Supported Metal Catalysts

Refractory Single Crystal

Thin Oxide Film Support + Metal Clusters

e.g. Mo, Re Ta, W

Refractory Single Crystal

Oxide Thin Filme.g. SiO2, Al2O3, MgO, TiO2

1-10 nm

Metal Clusters 1.0 - 50 nm Refractory Single Crystal

Oxide Thin Film

50 nm

1.0 ML Al2O3/

Re(0001)

400 nm

Re(0001)

0.5 ML Ag Al2O3/

Re(0001)50 nm

1.0 MLE Al2O3 on Re(0001)

200 nm 100 nm 16 nmSTM of 5.2 MLE Al2O3/ Re(0001)(5.0 V, 0.20 nA)

Low-energy electron diffraction8.7 MLE Al2O3/ Re(0001)

• long-range order• more diffuse spots indicate greater disorder as compared to thinner films

LEED

2.0 nm x 2.0 nm

3D surface image

• well-ordered, hexagonal, close-packed O2_

structure• spacings between indentations: 2.6 _ 2.7 Å

K. Luo, Q. Guo and D. W. Goodman, Chem. Phys. Letts., 330, 226-230 (2000).

Luo, Guo and Goodman, Chem. Phys. Letts.(2000)

STM:Model Oxide-supported Metal Catalysts

FromThin Oxide FilmsFrom Single Crystal Oxides

50 nm

50 nm

STM:Model Oxide-supported Metal Catalysts

FromThin Oxide FilmsFrom Single Crystal Oxides

50 nm

50 nm

Model Oxide-supported Metal Catalysts

Single Crystal Oxide Support Thin Film Oxide Support

50 nm

50 nm

50 nm

50 nm

TiO2(110)

+ +0.5 ML Au 0.5 ML Ag

1.0 ML Al2O3/

Re(0001)

REACTIONKINETICSCO/O2, CO/NO,

CO/H2, C2H6, C2H2 on:

Au, Pd, Ni/TiO2, SiO2, Al2O3

MIES

Au, Ag, Li/MgO,TiO2, SiO2

AES

Au, Cu, Ag, Pd,Ni/MgO, TiO2,

SiO2, Al2O3

LEED

MgO, TiO2, SiO2,Al2O3

Oxide-Supported Metal Clusters

IRASH2O, CO, NO,

CO/O2, CO/NO, CO/H2

on:Au, Cu, Pd,

Ni/MgO, TiO2, SiO2, Al2O3

HREELS,ELS

CH3OH, CO, NO, O2

onAu, Cu, Ag, Pd,

Ni/MgO, TiO2, SiO2, Al2O3

TPDH2O, CO, NO, O2

on:Au, Cu, Ag, Pd, Ni/MgO, TiO2,

SiO2, Al2O3

ISS

Au, Cu, Ag, Pd, Ni/MgO, TiO2,

SiO2, Al2O3

XPS, UPS

Au, Cu, Ag, Pd, Ni/MgO, TiO2,

SiO2, Al2O3

STM, STS

Au, Cu, Ag, Pd, Ni/MgO, TiO2,

SiO2, Al2O3

Elevated-Pressure Cell

Gate Valve

LEED

AESIonGun

Sample Manipulator

Sample PreparationChamber

XPS

Apparatus

STM

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

Cluster Diameter (nm)

TO

F (

1/si

te s

)

0.5

1

1.5

2

2.5

3

3.5

TO

F (

1/si

te s

)

*: The Au/TiO2 catalysts were prepared by a deposition-precipitation method, and the averaged cluster sizes were measured by TEM at 300 K. M. Haruta, et al., Catal. Lett. 44, 83 (1997).

*: The Au/TiO2 catalysts were prepared by vapor-depositing Au atoms onto a planar TiO2 film supported on Mo(100). A 1:5 CO:O2 mixture at a total pressure of 40 Torr and 350 K was used for reaction.

Au/TiO2(110) Model Catalyst*

Au/TiO2*

2CO + O2 2CO2

CO:O2 = 1:5PT = 40 Torr

350 K300 K

Unique Catalytic Activity of Nanosized Gold Particles

Valden, Pak, Lai and Goodman, Catal. Lett. (1998)

0.60

1.00

1.40

1.80

2.20

Act

ivit

y

0

15

30

45

60

0.0 2.0 4.0 6.0 8.0 10.0

Cluster Diameter (nm)P

opu

lati

on (

%)

Effect of Cluster Size and Morphology on Reactivity of Au/TiO2(110)

M. Valden, X. Lai, D.W. GoodmanScience 1998

30 nm

30 nm

10 nm

10 nm

CO + ½ O2 CO2

CO:O2 = 1:5PT = 40 Torr

BulkLarge

ParticleSmall

Particle

V / volts

I / n

A

V / volts

I / n

A

V / volts

I / n

A

+

-e -

e -

STM tipEf = 0. VEfEf

0 0 0

Ef EfEf

Eb Eb

EbEb

Scanning Tunneling Spectroscopy

BulkLarge

ParticleSmall

Particle

V / volts

I / n

A

V / volts

I / n

A

V / volts

I / n

A

+

-e -

e -

STM tipEf = 0. VEfEf

0 0 0

Ef EfEf

Eb Eb

EbEb

Scanning Tunneling Spectroscopy

bulk metal

large clusters

smallclusters

Scanning Tunneling Spectroscopy (STS) of Supported Metal Clusters: Finite Size Effects

CO Oxidation Activity vs. Electronic Structure

CO CatalyticActivity (CO2 moleculesper site per sec)

Cluster Diameter (nm)

• Ac tivi t y (to p) is e xp resse d as (pr od uct molecules) • (total Au a to ms)-1 • sec-1

[Catal . Lett. 44 (1997) 8 3].

• High est Au /T iO2 ac tivi ty f or C O o xidatio n occ urs at th e metal -to-n on metal ele ctro n ic transi tio n.

C orre la tion o f CO o xi da t ion a ct ivi ty w ith e le ct ron ic s tru ctu re

Ac

tivi

ty

0 .8

1 .2

1 .6

2 .0

Ban

d G

ap (

V)

0 .0

0 .3

0 .6

0 .9

1 .2

1 .5

C lu ste r D iam e te r (n m )0 2 4 6 8 1 0

Po

pul

atio

n (%

)

0

1 5

3 0

4 5

6 0Au cl ust ers with aband gap of 0 .2-0.6 Vmeasured by STS

Au/TiO2( 110)

Au/TiO2

3D, 3M L

2D/3D, 2ML

2D, 1M L

0.8

2.0

1.6

1.2

0.0

0.3

0.6

0.9

1.2

1.5

0 108642

2CO + O2 2CO2

CO:O2 = 1:5

PT = 40 TorrT = 350 K

Cluster BandGap (Volts) as Measured by STS

Au/Ti(110)-(1x1)

Au/Ti(110)-(1x1)

M. Valden, X. Lai, D.W. GoodmanScience 1998

1.5 2.0 2.5 3.0 4.03.5 4.5

Particle Diameter (nm)

Clausius-Clapeyron

Redhead AppoximationCO Heat ofAdsorption (kcal/mol)

10

20

18

16

14

12

Isosteric Heats of CO Adsorption Vs. Au Cluster Size

20 30 40 5050

55

60

65

70

75

Desor

ption

Energ

y / kJ

mol

-1

Average Cluster Size / Å

CO Desorption Energies onAu/Titania Versus Cluster Size

Bulk AuBULK GOLD

Meier, Bukhtiyarov, and Goodman, J. Phys. Chem, (2002)

E(eV)

DO

S (

stat

es/e

V)

Au/TiO2(110)

Au(001)

FLAPW Calculations for Au and Au/TiO2(110)

Yang, Wu, Goodman, PRB (2000)

FLAPW calculated chargedensity difference obtainedby substracting the superposition of the charge densities of a Au monolayerand TiO2(110) from that ofAu/TiO2(110)

Yang, Wu, Goodman, PRB (2000)

Theory: FLAPW Calculations of Au/TiO2(110)

Predicts an initial statecore level shift for Au on TiO2 to a lower binding energy of ~1.2 eV, consistent with the experimental value of 1.0 eV

Au

Ti

0 4 8 12 16 20 24

84.0

84.4

84.8

85.2

85.6

Bulk Au

Bulk Au

Bin

din

g E

ne

rgy

(eV

)

Au coverage (ML)

0 1 2 3 4 5 6 7

84.0

84.2

84.4

84.6

84.8

85.0

~ 1.6 eV

~ 0.8 eV

Au/SiO 2

Bin

din

g E

ne

rgy

(eV

)

Au coverage (ML)

Au/SiO 2

Core Level ShiftsAu/TiO2

0

.5

1.0

1.5

- .5

-1.0

2.0

Final State Initial State TotalContributions

BE

1.61.8

-0.2

1.8

-1.0

0.8

(eV)

XPS Core Level Shifts: Au/TiO2(110)

~3.0 nm

St.Clair and Goodman, Topics in Catal (2000)

Au Cluster Height vs. STM Tip Bias

Atomicsteps

1 2

A

B

2

1

+

-

Atomicsteps

1 2

A

B

2

1

+

-

+1 0 -1Au Au AuAu

Au Au Au AuAu

AuAu AuAuAuAu

+0.03 eV

-0.11 eV

Charge on Au Clusters vs. CO Binding EnergyP. Bagus, unpublished data

TPD: Au from SiO2 as a Function of Coverage

Au Coverage

Heat of sublimation

Decreasing Cluster

Size

Luo, Kim, and Goodman, J. Mol. Catal (2001)

Decreasing Cluster Size

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

Valden, Lai & Goodman, Science (1998)

2CO + O2 2CO2

CO:O2 = 1:5

PT = 40 Torr

T = 350 K

Au/TiO2(110)

ReactionRate, CO2

moleculesper site persecond

Reaction Time (minutes)

Morphological Changes of Au/TiO2(110)

Fresh 0.25 ML Au/TiO2(110)

After 120 min exposure to 10 Torr CO-O2 (2:1)

A B

C D

A B

C D

50 nm

Before Rx After Rx

50 nm

Lai and Goodman, J. Mol. Catalysis (2000)

Microscope: RHK VT - UHV300Variable Temperature: 100 - 600 KPressure Range: 10-10 - 103 Torr

eB

IP

loadlock

TMP

MS

S

Metal Doserse-Beam

AES

SampleStorage

SP SP

10-10 TorrS

TM

Gas

UHV-Elevated Pressure STM Apparatus

10-10 - 103 Torr 10-10 Torr

UHV 5.4 Torr CO:O2

Surface regrowth around Au clusters

Cluster size reduction

Adhesion of cluster to tip

Cluster size increase

Surface roughening

STM of Au Clusters on TiO2 at 400K

Kolmakov and Goodman, Catal. Letts (2000)

STM Tip as a Mask for Metal Deposition

sample

evaporator

tip

d

shadow

sample

evaporator

tip

d

shadow

Kolmakov and Goodman, Physical Review B, submitted

Large Scale STM Image of Tip Silhouette for Au Deposition

RT Au Deposition Same area after 950K anneal

250 nm 250 nm

a ba b

Kolmakov and Goodman, Physical Review B, submitted

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, Physical Review B, submitted

sample

tip

d

shadow

Au Evaporation onto

Ag/TiO2 at RT

Ag

Au + Ag

Comparative Stability of Ag and Au-Ag Clusters to 2 x 104 Pa Air

Ag particles

Au and Au-Ag mixedclusters

TiO

2 (110)

Kolmakov and Goodman, Physical Review B, submitted

IRAS: CO/Cu-Pd/Al2O3 ISS: Cu-Pd/Mo(110)

Pd

Cu

IRAS: Site Differentiation of Mixed-Metal Catalysts

Rainer, Xu, Holmblad, and Goodman, J. Vac. Sci. Technol. A (1997)

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

What do we need to know about supported nanoclusters?

• Properties as a function of metal-to-nonmetal transition for a range of transition metals physical structure (bond lengths, angles, etc.) electronic properties (valence band, core levels, etc.) optical properties (plasmon, HOMO-LUMO gap, etc.) melting temp., sublimation temp., etc. adsorbate bonding energies, vibrational freq., etc. catalytic properties for several probe reactions

• Methods for the preparation of mono-dispersed clusters with selected sizes

Precise relationship between physical and chemical properties theory ------- experiment

• Detailed sintering kinetics and role of support in altering catalytic activity

• Quantitative thermochemical information about metal wetting, nucleation, and particle sintering

Coworkers

Current:

Dr. Ashok SantraByoun Koun Min

Dr. Young Dok KimJeff Stultz

Emrah OzensoyDr. Christian Hess

Cheol-Woo YiDr. Changmin Kim

Dr. Paul BagusTushar Choudhary

Fan YangTao Wei

Dheeraj KumarDr. Sivadinarayana Chinta

Past:

Dr. Andrei Kolmakov Dr. Charles Chusuei

Dr. Micha ValdenDr. Xiaofeng Lai Dr. Doug Meier

Dr. Todd St. ClairDr. Gerry Zajac

Dr. Jens GuensterDr. Qinlin Guo

Dr. Valeri BukhtiyarovDr. Kent Davis

Kai Luo

Support

Department of Energy, the Office of Basic Energy Sciences, Division of Chemical Sciences

The Robert A. Welch Foundation

Dow Chemical Company