Model Catalysts for Endothermic Fuels
Scott L. Anderson - University of Utah
Yang Dai and Tim Gorey
Eric Baxter, Matt Kane, Sloan Roberts
Tasks:
1. Prepare size-selected model catalysts to enable
detailed mechanistic studies of endothermic fuel
catalysis.
2. In situ studies at Utah of size effects/mechanism
3. Prepare samples for Argonne (GISAXS) and Stanford
(DESI)
Motivation: Mechanistic insight.
Excellent systems for theoretical studies.
First Year accomplishments:
• Construction of a new instrument to enable in situ
studies and transfer of stabilized samples to
collaborators in team.
• Initial studies of ethylene dehydrogenation over
Ptn/alumina
• Observation of support thickness as an activity
tuning mechanism
New instrument, constructed to allow
sample sharing.
In situ Techniques: XPS, UPS, ISS, TPD, ALD and
vapor deposited thin film growth, and size-selected
catalyst deposition, easy sample exchange.
Coming year:
1. Continue UHV size-selected model catalyst studies
• ALD and doping/poisoning effects
2. Samples sent regularly to Argonne and Stanford for in
operando and DESI/NAIMS studies
3. High pressure/temperature studies at Utah
Size-selected catalysts – why bother?
Real World Catalyst
Planar Model Catalysts
CO oxidation over Au/TiO2 catalysts
Haruta et al.
Measure size effects on activity and
on physical properties
• Look for size-dependent
correlations between activity
and physical properties
• Learn what factors control
activity
The Beamline and Endstation:
Pressure Drop:
3 Torr
Clustering
volume: 10
All
experiments
have
the
-10
Deposition
10 ofTorr
same
totalchamber:
number
Pd
or
atoms
ML),meters
IonPt
Guides
: (0.1 2.35
deposited
in the 0.06
form
of
Unguided sections:
meters
different size clusters
Transmission ~30% into 2mm
spot on target
•Cluster Beamline
•In situ analysis:
•XPS
•ISS
•UPS/INS
•Mass spectrometry
•TPD/TPR
•Pulsed reaction
•Electrochemistry
New Instrument
Tim
Gorey
Yang
Dai
Cluster source
Beam
forming/
differential
pumping
Mass selection
Deposition/Reactions
XPS/UPS/ISS
Rapid sample exchange WITH
wide range/fast T control
In situ sample prep, XPS, ISS, UPS,
AES, mass spectrometry, ALD
overcoating, evaporators.
Main manipulator
T = 100 – 2500 K
STATUS: Operational except for 16,000 amu mass
spec electronics – should be fixed this week.
Support Prep/”dirty” expts
Load/ALD
Chemistry on Size-Selected Ptn-based
catalysts
Initial model system: Ptn/alumina/Re(0001)
• Well characterized system – ordered film with controlled thickness
• Baseline for mixed silica/alumina support for acid-catalyzed chemistry
(later)
• Film thickness provides a potential activity control mechanism
• Alumina overcoating for cluster stabilization and activity modification
Initial reactions:
1. CO oxidation
• Familiar reaction, very common in UHV surface chemistry
• Spectroscopy comparisons – cluster physical properties
• Lots of literature data, so good for cluster-surface comparisons
2. Ethylene dehydrogenation/cracking/coking
• Easily studied under surface science conditions
• Shows range of behavior important in endothermic fuels
• Connects to mechanistic studies by Mavrikakis on Pt (stepped)
• Tractable for supported cluster theory by Alexandrova and Khanna
• Well studied on bulk Pt surfaces
Pt/alumina morphology
• Raw low flux ion scattering:
• Some adventitious CO
adsorbed
• Hard to avoid for small,
highly dispersed clusters
• Easily removed with He+
• Extrapolate to zero exposure
to get “as-deposited
Pt/Alumina ratio
Pt/alumina morphology
• Small clusters
depositing as
single layer
islands
• Transition to
multilayer
structures at Pt8
• Abrupt transition
is unusual
Binding sites (CO)
CO desorption
TD-ISS shows that there
are binding sites both
around the Ptn
periphery, and on top of
the clusters.
Similar results for all
cluster sizes examined.
Ptn/alumina electronic properties
Can’t do XPS due to interference from Re support – use UPS to look at valence structure
Example data for Pt7/alumina/Re(0001)
• Pt valence band
easily seen in
band gap region
of alumina
spectrum
• Subtract alumina
signal, and fit to
obtain the Ptn
spectrum
Examine dependence of valence band top on
size and oxidation, and CO exposure
• O2 exposure has little
effect on band top
• CO exposure has a
large effect
• Counterintuitive!
What does this tell
us about electronic
properties?
• Khanna’s calculations
explain the effect
Examine dependence of valence band maximum
on size and oxidation, and CO exposure
• Properties converge
with size – still far
from bulk like.
• CO shift become
negligible for larger
clusters
Ptn/Alumina/Re(0001):
Size dependent CO oxidation
7
Residual CO 1st TPR, 300K oxidation
CO2 production 1st TPR, 300K oxidation
30
Pt 1
Pt 1
Pt 2
6
Pt 2
Pt 4
25
Pt 4
Pt 7
5
Pt 7
Pt 8
Pt 8
Pt 10
Pt 14
3
Pt 18
2
Counts (x102)
4
20
13C16O
13C16O18O
Counts (x102)
Pt 9
1
Pt 9
Pt 10
Pt 14
15
Pt 18
10
5
0
-1
100
200
300
400
Temperature (K)
500
600
0
100
200
300
400
500
600
Temperature (K)
•1st TPR shows significant size dependence
•Residual CO desorption also shows dependence on size – ratios of low temperature and
high temperature peaks change with cluster size
Ptn/Alumina/Re(0001):Effect of oxidation temperature
Integrated CO2 Production, 300K oxidation
Integrated CO2 Production, 180K oxidation
• Activity converges with repeated TPRs, with the small clusters increasing in activity
• For 180K oxidation, the large clusters decrease in activity, while the small clusters still
increase
NOTE: Inverted Scale
What about catalyst electronic structure?
•When Pt is deposited, since it is a metal, it is located near the Fermi level in the band gap
region of the alumina, making it easy to distinguish between signal from the support and the
clusters.
•The location of the Pt signal in the UP spectrum shifts depending on what cluster size is
deposited – and this shift correlates with how much CO2 each cluster can produce.
Thermal Decomposition of Ethylene on
Pt (111)
0.75 L C2D4 Dose
Zhou, X-L., X-Y. Zhu, and J. M. White. "A TPD, SIMS
and Δφ study of the influence of coadsorbed
potassium on the adsorption and decomposition of
ethylene on Pt (111)." Surface science 193.3 (1988):
387-416.
• Ethylene, hydrogen, and ethane were the only
thermal desorption products observed
• Ethylene absorbed on Pt(111) at 100 K forms a di-σ
bond.
• β1 – dehydrogenation of ethylene to form ethylidyne
(Pt3≡C-CH3).
• β3 – dehydrogenation of ethylidyne to form fragments
CxH (x=1,2)
• β4 – Total dehydrogenation leaving Carbon on the
surface
Experimental Procedure
• Cool sample to ≈ 160 K
• Anneal sample @ 2100 K for 5 min
• Grow alumina film, 5 x 10-6 torr O2, sample held @ 970 K
• Re-doped alumina (~2-3%) (n-doped)
• Re sites are potentially reactive – need to characterize
•
•
•
•
Characterize by XPS
Flash sample ≈ 800 K
Deposit Ptn Clusters (0.1 ML equivalent)
Study chemistry by Temperature-Programmed Desorption
– 5 L dose of C2D4 @ 150 K
– Heat sample from 135 to 900 K @ 3 K/s
• Characterize carbon deposition by XPS
• Examine reaction with O2 and H2O for carbon removal
Mass 32 Desorption from Al2O3/Re(0001)
8000
L C2D4
@150 K
• Adsorption (5 L) was done at5 150
K. Enough
to saturate surface if all sticks
5 L C2D4 @150 K
– Above the desorption temperature for
5 L C2D4
@120
K
ethylene on perfect alumina
terrace
sites.
– Desorption is probing defects and dopant
sites
• No decomposition products are observed
• Defect and dopant sites bind ethylene, but do
not catalyze decomposition
7000
6000
Counts
5000
4000
3000
2000
1000
0
-1000
100
200
300
400
500
Temperature (K)
600
700
800
β = 3 K/s
900
Desorption of intact ethylene from Pt
nanoparticles grown on alumina surface
Mass 32 Desorption from 1 ML
Pt1/Al2O3/Re(0001)
14000
12000
1st TPD
2nd TPD
10000
Leak in 30
L 18O2 at
600 K
8000
4th TPD
Counts
6000
3rd TPD
4000
2000
0
-2000
-4000
100
200
300
400
500
600
Temperature (K)
700
800
β=3
900
• In 1st run, ethylene
sticks preferentially
to Pt sites
• After 1st run, looks
like clean alumina
• Pt surface is
passivated.
• No significant
recovery from O2
exposure
• Dose and
temperature
were low
Desorption of hydrogen from decomposition
4500
D2 desorption from 1 ML Pt/alumina/Re(0001)
Leak in 30 L
18O at 600 K
2
to see if
passivating
carbon can
be burned off
4000
3500
3000
Counts
2500
2000
1500
1000
500
0
-500
100
200
300
400
500
600
Temperature (K)
700
• 1st TPD run shows
1st TPD
multistep hydrogen
desorption,
2nd TPD
consistent with
3rd TPD
sequential
4th TPD
dehydrogenation
on Pt.
• After 1st TPD run,
dehydrogenation is
greatly reduced
• Pt surface is
passivated.
• No significant
recovery from O2
exposure
• but dose and
temperature
were low
800
900
β=3
Significant carbon deposition observed by XPS
Can be burned off
45
10
40
9
35
8
7
30
6
25
5
20
4
15
3
10
2
5
1
0
0
150
350
Time (seconds)
550
CO Production (x104)
Oxygen Counts (x104)
Note: XPS sensitivity to carbon is low
Will install Auger gun next vent
Ethylene on size-selected clusters/alumina
450
1st TPD Mass 4 Desorption from 0.1ML Ptx/Al2O3/Re(0001)
Qualitative
structure is
similar to that for
larger Pt NPs, but
the details are
quite sizedependent. See
multistep
800
900 dehydrogenation
β = 3 K/s
even on Pd2
Pt 14
Pt 10
Pt 7
Pt 4
Pt 2
Pt 1
Clean Alumina
Counts
350
250
150
50
-50
100
6800
200
300
400
500
600
700
Temperature (K)
1st TPD Mass 32 Desorption from 0.1ML Ptx/Al2O3/Re(0001)
5800
4800
3800
Counts
Most of the
parent desorption
is from alumina,
but only 10% of
the surface is Ptcovered.
Pt 14
Pt 10
Pt 7
Pt 4
Pt 2
Pt 1
Clean Alumina
2800
1800
800
-200
100
200
300
400
500
600
Temperature (K)
700
800
β = 3 K/s
900
Ethylene on size-selected clusters/alumina
2nd TPD Mass 4 Desorption from 0.1ML
Ptx/Al2O3/Re(0001)
Pt 14
350
250
Counts
Clusters are almost
completely
deactivated after one
TPD run. Carbon
formation poisons the
active binding sites
Pt 10
Pt 7
Pt 4
Pt 2
Pt 1
150
50
-50
100
7800
200
300
400
500
600
Temperature (K)
700
800
900
β = 3 K/s
2nd TPD Mass 32 Desorption from 0.1ML Ptx/Al2O3/Re(0001)
Little sign of ethylene
sticking to Pt sites.
Some combination of
sintering and
poisoning.
Pt 14
Pt 10
Pt 7
Pt 4
Pt 2
Pt 1
5800
Counts
3800
1800
-200
100
200
300
400
500
600
Temperature (K)
700
800
900
β = 3 K/s
Size-dependent initial dehydrogenation activity
140000
120000
Mass 4 Integrated Area
100000
80000
60000
40000
20000
0
0
5
10
Cluster Size
β=3
• Activity is quite high
• ~30% decomposition in a
single ethylene-Pt
interaction
• Size dependence for highly
active catalysts is expected to
be weak
• Surprisingly small clusters are
able to bind and decompose
ethylene
• Something special about Pt7?
• Pt7 is found to be inactive
for carbon
electrooxidation
• Pt7 has an anomalously
high 4f binding energy
15
(on carbon)
• Pt7 also has anomalously
high valence band top
Effects of support film thickness
CO oxidation over Pd20/alumina/Re(0001)
8
6
1st TPR
2nd TPR
3rd TPR
Avg TPR
4
0
1
2
3
4
5
Al2O3 Film Thickness (nm)
6
Discovered under prior
AFOSR MURI support
Pd20 is active when
deposited directly on ReOx
and on thick alumina films,
but between 1 – 2 nm
alumina thickness, the
activity is substantially
reduced
10
13
C16O18O Production (x104)
12
7
8
Is support thickness a
possible catalyst tuning
strategy?
12
10
8
1st TPR
2nd TPR
3rd TPR
Avg TPR
6
4
1
2
3
4
5
6
7
8
1.2
1.0
Al2O3 Film Thickness (nm)
Al 2s (alumina)
O 1s (alumina)
O 2p (alumina)
Al 2s (Pd/alumina)
O 1s (Pd/alumina)
O 2p (Pd/alumina)
0.8
0.6
0.4
0.2
0.0
5
1
2
10
3
4
5
6
7
8
Al2O3 Film Thickness (nm)
4
3
8
Avg TPR
Re Intensity
2
6
1
4
0
0
B.E. shift (eV)
1.8
1.6 0
1.4
1.2
1.0
B.E.(eV)
0.8
0.6
0.4
0.2
0.0
12
0
Activity
Activity minimum occurs
in a region where both
electronic and dopant
properties of the catalyst
are changing rapidly.
1
2
3
4
5
Al2O3 Film Thickness (nm)
6
7
8
Re concentration
13 16 18
C O O Production (x104)
What happens as we vary thickness?
Theory by Reber/Khanna
to help understand dipole
effects on spectroscopy
and chemistry
Thickness-dependent fields due to band-bending
χ
Δχ
Δχ
EC
ΦRe
VBB
ΦSB
VBB
Φalumina
EC
ΦSB
Φalumina
EC
EV
Re(0001)
Re(0001)
Re(0001)
EV
EV
Al
2s
O 1s
Al
2s
Al
2s
O 1s
Al2O3
O 1s
Al2O3
Al2O3
(a) Bulk Re and alumina before
contact
(b) Thin Al2O3 film on
Re(0001)
(c) Pd on thin
Al2O3/Re(0001)
Addition of Pd decreases the dipole strength (Δχ) and the amount of band bending (VBB)
Summary of work to date:
1.
2.
3.
Completed construction of new instrument
• ALD overcoating
• Dosers for support/cluster doping
• Transfer/load-lock system for samples for Winans and Zare
• Short-term hold up: mass spec not correctly loaded, at
factory for reloading .
Characterization of Pt/alumina/Re(0001)
• XPS, UPS, ISS spectroscopy
• Adsorbate sites and energetics for CO and ethylene
• Initial reactions – ethylene dehydrogenation, coke
formation
Film thickness effects on activity (underway)
• Pt/ReOx extremely active for coking – no intact ethylene
desorption
Future plans
1.
2.
3.
4.
ALD overcoating to modify adsorption/coking behavior
A. Alumina to poison highly reactive sites
B. Ring-fence/encapsulate clusters to stabilize
Continued surface chemistry work on hydrocarbon –model
catalyst interactions
A. Binding energies of molecules of interest
B. Dehydrogenation/cracking chemistry
C. Effects of ALD on sinter stability and chemical behavior
Samples provided to Zare and Winans for NAIMS analysis and in
operando GISAXS/Raman study.
With and without ALD overcoating.
Construct copy of GISAXS cell (minus the diamond windows) for
Utah high P/T work.
A. Quick/local studies to complement studies at APS.
B. Mass spec sampling detection