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Transcript of 0.00 0.05 0.10 0.15 0.20 0.25 0.30 1.52.53.54.55.56.5 Cluster Diameter (nm) TOF (1/site s) Structure...
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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
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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