Heterogeneous Catalysis: From Imagining to Imaging a ......Heterogeneous Catalysis: From Imagining...
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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
adsorption reaction desorption
catalyst
catalyst
catalyst
catalyst
A
A
A
B
B
B
+ P
P
Ppote
ntia
l ene
rgy
adsorption reaction desorption
gas phase reaction
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
B
B
P
Ppote
ntia
l ene
rgy
adsorption reaction desorption
catalyst
catalyst
catalyst
catalyst
A
A
B
B
P
Ppote
ntia
l ene
rgy
adsorption reaction desorption
catalyst
catalyst
catalyst
catalyst
A
A
B
B
P
Ppote
ntia
l ene
rgy
adsorption reaction desorption
Ammonia Synthesis
catalyst
catalyst
catalyst
catalystA B
pote
ntia
l ene
rgy
adsorption reaction desorption
catalyst
catalyst
catalyst
catalystA B
pote
ntia
l ene
rgy
adsorption reaction desorption
catalyst
catalyst
catalyst
catalyst
AAN
N
BBH
H
pote
ntia
l ene
rgy
adsorption reaction desorption
AANBBH
BBHAANBBH
BBH
BBHBBH
BBH
AAN
Applications• Production of transportation fuels (440 oil refineries)
• Production of chemicals
• Cleanup of automotive/industrial exhaust gases
• “Green” chemistry (unwanted byproducts)
Carbon Monoxide and Nitric Oxide
catalyst
catalyst
catalystcatalyst
C Opo
tent
ial e
nerg
y
adsorption reaction desorption
NO
CO
O
C OO
CO
O
N
NN
N
N
catalyst
catalyst
catalystcatalyst
C OC Opo
tent
ial e
nerg
y
adsorption reaction desorption
NO
CO
CO
O
C OO C OC OO
CO
OC
OC
O
O
N
NN
N
N
Applications• Production of transportation fuels (440 oil refineries)
• Production of chemicals
• Cleanup of automotive/industrial exhaust gases
• “Green” chemistry (unwanted byproducts)
2 CO + 2 NO 2 CO2 + N2
Carbon Monoxide and Nitric Oxide
catalyst
catalyst
catalystcatalyst
C Opo
tent
ial e
nerg
y
adsorption reaction desorption
NO
CO
O
C OO
CO
O
N
NN
N
N
catalyst
catalyst
catalystcatalyst
C OC Opo
tent
ial e
nerg
y
adsorption reaction desorption
NO
CO
CO
O
C OO C OC OO
CO
OC
OC
O
O
N
NN
N
N
Applications• Production of transportation fuels (440 oil refineries)
• Production of chemicals
• Cleanup of automotive/industrial exhaust gases
• “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
Cluster Diameter (nm)
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
Cluster Diameter (nm)
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
Cluster Diameter (nm)
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
Cluster Diameter (nm)
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
Cluster Diameter (nm)
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
Cluster Diameter (nm)
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
Cluster Diameter (nm)
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
Cluster Diameter (nm)
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)