October 20, 2006 - NSLS In-situ XAFS studies of fuel cell catalysts Carlo U. Segre Center for Synchrotron Radiation Research & Instrumentation and Department of Biological, Chemical & Physical Sciences, Illinois Institute of Technology Workshop on XAFS studies of nanoparticles and chemical transformations October 19-21, 2006 - NSLS
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In-situ XAFS studies of fuel cell catalysts · Develop methods for characterization of catalysts in fully operating fuel cells • Nanoparticle structure during operation • Surface
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October 20, 2006 - NSLS
In-situ XAFS studies of fuel cell catalysts
Carlo U. Segre
Center for Synchrotron Radiation Research & Instrumentation andDepartment of Biological, Chemical & Physical Sciences,
Illinois Institute of Technology
Workshop on XAFS studies of nanoparticles and chemical transformationsOctober 19-21, 2006 - NSLS
• In-situ x-ray absorption– XANES and EXAFS data: Separately taken at Ru K and Pt L3 edges. – Absorption edge jumps: ∆µx = 0.05 for Ru and ∆µx = 0.17 for Pt.– References: Pt foil, Ru metal, RuO2, RuO2-hydrate, as received PtRu– Monochromator: Double crystal Si (111) – Harmonic Rejection Mirror: Pt for Ru edge, Rh for Pt– Ion chamber detector gases: Incident beam; 80% He- 20% N2:
• Example of transitions in absorption due to density fluctuations (e.g. CO2)
• Note magnitude of Ru edge jump!
October 20, 2006 - NSLS
XANES fitting• Data were normalized and
aligned using Athena. • Least squares fitting of Ru edges
with Sixpack. • The standards for the least
squares fits were RuO2-hydrate and Ru powder.
11550 11560 11570 11580 11590 11600 116100.0
0.3
0.6
0.9
1.2
1.5
1.8
Nor
mal
ized
χ µ
(E)
Energy (eV)
250mV 300mV 350mV 400mV 450mV Pt Black Pt foil PtRu(66:34) as received catalyst
22080 22100 22120 22140 22160 22180 222000.0
0.2
0.4
0.6
0.8
1.0
1.2
250mV 300mV 350mV 400mV 450mV Ru metal PtRu(66:34) as
received catalyst Ru Oxide Ru oxide H2O
Norm
aliz
ed χ
µ (E
)
Energy (eV)22080 22100 22120 22140 22160 22180 22200
0.0
0.2
0.4
0.6
0.8
1.0
1.2
450mV (Potential vs DHE) Water 0.1M MeOH 2M MeOH
Nor
mal
ized
χ µ
(E)
Energy (eV)
Pt L3 edge
Ru K edge Ru K edge
As received
October 20, 2006 - NSLS
RuO2-hydrate fraction by XANES
200 250 300 350 400 450 500 550 6000.12
0.14
0.16
0.18
0.20 water 0.1M MeOH 2M MeOH
The
frac
tion
of R
u ox
ide
hydr
ate
Potential vs DHE (mV)
October 20, 2006 - NSLS
Pt foil XAFS Analysis
Remove background
Fit with simplemodel at multiplek-weightings
k-weight = 3 k-weight = 1
XANES region
EXAFS region
October 20, 2006 - NSLS
Pt EXAFS
• Potential dependent EXAFS at 0.1M. Pt EXAFS has excellent fit with a totally metallic environment. All data are nearly identical.
• FT range for k space is 2 Å to 13 Å.• Fit range for R space is 1.5 Å to 3 Å.
0 2 4 6 8 10 12
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3 250mV 300mV 350mV 400mV 450mV
k χ,
k
k, Å-1
2 4 6 8 10 12
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3 Exp. data at 350mV Fit with model
k χ,
k
k, Å-1
0 2 4 6
0
2
4
6
8 Exp. data at 350mV Fit with model
|χ(R
)|, Å
-4
R, Å
October 20, 2006 - NSLS
Ru EXAFS Fitting
• Addition of Ru-O neighbors improves the EXAFS fit.• The peak at about 1.3Å is ascribed to oxygen bound to Ru.• The asymmetric distribution of the Ru-O peak is consistent with disorder
0 2 4 6
0
1
2
3
4
5 Exp. data at 350mV Fit with Ru
|χ(R
)|, Å
-4
R, Å0 2 4 6
0
1
2
3
4
5 Exp. data at 350mV Fit with Ru and Ru-O
|χ(R
)|, Å
-4R, Å
October 20, 2006 - NSLS
Ru EXAFS
• Potential dependent EXAFS at 0.1M MeOH
• [MeOH] dependent EXAFS • Model fit at 350mV
0 2 4 6 8 10
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3 250mV 300mV 350mV 400mV 450mV
k χ,
k
k, Å-12 4 6 8 10
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3 Exp. data at 350mV Fit with model
k χ,
k
k, Å-1
2 4 6 8 10-0.2
-0.1
0.0
0.1
0.2
0.3 Water 0.1M MeOH 2M MeOH
kχ, k
k, Å-1
October 20, 2006 - NSLS
Metal cluster structural model
FCC structure, count first shell neighbors only from Pt and Ru edgesThis only “sees” atoms in the metallic cluster
Fractional Coordination #’sY = Ru around PtX = Pt around Ru
Frenkel 1998, Shibata 2003
October 20, 2006 - NSLS
[Pt]nPtN
BPt-RuY =
[Ru]nRuN
BRu-PtX =
BPt-Ru = BRu-Pt
October 20, 2006 - NSLS
Metallic nanoparticle structure• First shell analysis
• Fit Pt and Ru EXAFS simultaneously at each potential. No potential dependence observed.
• Simultaneously fit Pt and Ru data at all potentials. Identical overall average coordination was observed.
• Use fractional coordination numbers, X (Pt around Ru) and Y (Ru around Pt) and total coordination number about each atom, n (Frenkel 1998, Shibata 2003)
• Bond lengths and Debye-Waller factors are consistent with literature values for C supported Pt-Ru catalyst (Russel 2001, Camara 2002)
0.27 ± 0.02Y0.54 ± 0.02X8.2 ± 0.2n
= 0.5=XY
[Pt][Ru]
October 20, 2006 - NSLS
Apply model to as-received catalyst
Pt not simply metallic but has oxygen near neighbors
Ru shows large increase in number of oxygen near neighbors
• Ru oxidation ~15%• N = 8.2• [Ru]/[Pt] = 0.50• No Pt-O bonds• Ru-O bonds ~0.24 avg
S. Stoupin et al., J.Phys Chem B, 110, 9932 (2006).
October 20, 2006 - NSLS
[Ru]/[Pt] = 35/65 = 0.54
October 20, 2006 - NSLS
Possible Structural Model
FCC alloy phase with an amorphous Ru ghost phase
As received catalyst In situ catalyst
•Higher surface area may be critical in catalyst performance•Incorporated O eases CO oxidation
October 20, 2006 - NSLS
Conclusions
Metallic cluster of the catalyst nanoparticleComposition is about 2:1 Pt:Ru
Model fit suggests that the alloy is not totally randomized(i.e. X ≠ .65 & Y ≠ .35)
Pt is metallic within the potential window (250mV and 450mV) in water or aqueous methanol.
Ru–O bonds are not potential or [MeOH] dependent (Rolison)On the surface?
In a separate phase?
The potential transition point is not accompanied by ensemble changes at the surface.
Lots more to do!
October 20, 2006 - NSLS
Extra bonus material
Another example of the kind of experiment which is well-suited to an undulator beamline. Where the sample is damaged rapidly by the x-ray beam!
October 20, 2006 - NSLS
Dilute magnetic semiconductors
• Cations replaced by Mn, Co, Fe, etc. • Typical examples are: ZnO-Mn, CdS-Mn, ZnS-Mn, etc.• Host s-p band ⇔ Mn2+ d electron exchange interactions• Unusual magnetotransport and magnetooptical phenomena
Carrier induced ferromagnetism in InAs-Mn and GaAs-Mn
• DMS nanocrystals are unique systemssemiconductor confinement effectsmagnetic properties
October 20, 2006 - NSLS
Acknowledgements
Graduate studentsMehdi Ali – IIT PhysicsRanjani Viswanatha - IIS
CollaboratorsDipankar Das Sarma - IISSoma Chattopadhyay – MRCATTomohiro Shibata – MRCATMali Balasubramamian – XOR20Shelly Kelly – ANL BioSciences
FundingMRCAT is supported by contributions from MRCAT member institutions.
The APS is funded by the U. S. Department of Energy, Office of Basic Energy Sciences under Contract number W-31-109-Eng-38.
October 20, 2006 - NSLS
Sample preparation and characterization
• Wet chemical synthesis starting with Mn-acetate and Zn-acetate.
• Capping with polyvinylpyrollidone (PVP) results in smaller sized particles (5 nm or less) with uniform size distribution
• Bulk sample synthesized by annealing the powders at 1200°C for 12 hours in air. The size of the bulk particles is ~1.5 microns.
• Size calculated using Scherrer’s equation and verified by TEM.
• The percentage of Mn doping in the samples was estimated byEDAX and ICP-AES.
• Bandgap was measured by UV-VIS Absorption spectroscopy.
• Magnetic properties were measured by Electron Paramagnetic Resonance (EPR).
October 20, 2006 - NSLS
XRD , UV-VIS and EPR results
3 0 4 0 5 0 6 0 7 0
( i i ) B u l k x = 0 . 0 1
( v i i ) x = 0 . 0 2 3
( v i ) x = 0 . 0 1(1
02)
(110
)
(103
)
(200
)(1
12)
(201
)
(101
)(0
02)
(100
)
( i v ) x = 0 . 0
( v i i i ) x = 0 . 0 5
( i i i ) S i m u l a t e d f o r 4 . 7 n m d i a m e t e r
( v ) x = 0 . 0 0 5
2 θ ( d e g )
Inte
nsity
(arb
. uni
ts)
( i ) B u l k x = 0 . 0 3 0 0 3 2 0 3 4 0 3 6 0 3 8 0 4 0 0
( i i ) x = 0 . 0 0 5Inte
nsity
(a. u
.)
λ ( n m )
( v ) x = 0 . 0 5
( i v ) x = 0 . 0 2 3
( i i i ) x = 0 . 0 1
( i ) x = 0 . 0
B u l k B a n d g a p
C a p p e d Z n 1 - x M n x O : U V - a b s o r p t i o n
2900 3000 3100 3200 3300 3400 3500 3600
Magnetic Field (Gauss)
Inte
nsity
(arb
. uni
ts)
(ii) 4.7 nm
(i) Bulk
Signal [II]
Signal [I]
XRD shows formation of wurtzite nanocrystals
Increase in the bandgap compared to the bulk, some variation with Mn concentration
EPR spectra from the doped samples exhibit well resolved hyperfine splitting of isolated Mn2+ ions, suggesting that Mn-Mn interactions are rather weak.
October 20, 2006 - NSLS
Sample degradation of ZnO-Mn
Clear evidence of reduction of Mn with time exposed to x-raysObserved with bending magnet beam tooUndulator quick scans can help!
October 20, 2006 - NSLS
ZnO-Mn XANES
• Average valence state changes from bulk to nanoparticle sample
• Mn(II) dominates bulk sample
• Mn(III)-Mn(IV) dominates nanoparticle samples
6532 6536 6540 6544 65480.0
0.2
0.4
6530 6540 6550 6560 65700.0
0.4
0.8
1.2
1.6
2.0
x m
u
Energy (eV)
xmu
Energy (eV)
BULK NANO-Mn:1.0% NANO-Mn:5.0% MnO Mn3O4
Mn2O3
MnO2
October 20, 2006 - NSLS
Mn replaces Zn in the bulk sample but not in the nanoparticle