near-surface alloys for improved catalysis - aps physics | aps home
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Near-Surface Alloys forImproved Catalysis
Manos Mavrikakis
Department of Chemical and Biological EngineeringUniversity of Wisconsin – Madison
Madison, Wisconsin 53706
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Acknowledgements
Ratko Adzic (BNL)
J. Chen (U of Delaware)
Jeff Greeley, Ye Xu, Anand Nilekar
Jens Nørskov and colleagues @ CAMP – Denmark
DoE (BES-Chemical Sciences)
NSF - CTS
NPACI, DOE-NERSC, PNNL (supercomputing resources)
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Outline
• H2 catalytic chemistry:– Identifying Promising Catalysts from 1st Principles:
H2 and H on Bimetallic Near Surface Alloys (NSAs)
• Oxygen Reduction Reaction (ORR)– Improved catalysts with ML structures– Further improvements with mixed-metal ML structures
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1. J. Kitchen, N. Khan, M. Barteau, J. Chen, B.
Yashhinskiy, and T. Madey, Surf. Sci. 544 (2003) 295
1Ni/Pt(111)
H2/Rh(111)
2. R. Schennach, G. Krenn, B. Klötzer, K. Rendulic, Surf. Sci. 540 (2003) 237
Hydrogen on NSA’s
2V/Rh(111 )
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MethodsDensity Functional Theory – DACAPO total energy code 1,2
Periodic self-consistent PW91-GGA 3
Ultra-soft Vanderbilt pseudo-potentials 4
Plane wave basis sets with 25-Ry kinetic energy cut-off
Spin polarization as needed
Four-metal-layer slabs; (2x2) unit cell; top two layers relaxed
First Brillouin zone sampled at 18 k-points
Nudged Elastic Band method for reaction paths 5
1. B. Hammer, L. B. Hansen, J. K. Nørskov, Phys. Rev. B 59, 1999, 7413.2. J. Greeley, J. K. Nørskov, M. Mavrikakis, Annu. Rev. Phys. Chem. 53, 2002, 319.3. J. P. Perdew et al., Phys. Rev. B 46, 1992, 6671.4. D. H. Vanderbilt, Phys. Rev. B 41, 1990, 7892.5. G. Henkelman, H. Jónsson, J. Chem. Phys. 113, 2000, 9978.
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Ideal Bimetallic Near Surface Alloys • Segregation properties of two metals are
critical• Consider two special classes:
– Overlayers– Subsurface Alloys
Overlayers* Subsurface Alloys
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Eseg
-1.25 -1.00 -0.75 -0.50 -0.25 0.00 0.25 0.50 0.75 1.00 1.25
|B.E
. Hso
l | - |B
.E. H
host|
-1.25
-1.00
-0.75
-0.50
-0.25
0.00
0.25
0.50
0.75
1.00
1.25
Au*/Pt
Cu/Pt
Ir/Pt
Ni/Pt
Ru/Pt
Co/Pt
Rh/Pt
Mo/PtFe/Pt
Ta/PtV/Pt
W/Pt
Re/Pt
Mo/Pd
Fe/Pd
Ir/Pd
Ru/Pd
V/PdTa/Pd
Mo/Ru
W/Ru Ru/Rh
Ir/Rh
Mo/Rh Re/Rh
W/RhTa/Rh
Pt*/Ru
Pd*/Ru
Cu*/Ru
Ni*/RuCo*/Ru
Fe*/Ru
Cu*/Re
Mo*/Re
Rh*/ReNi*/Re
Pd*/Re Pt*/Re
Pt subsurface alloysPd subsurface alloysIr subsurface alloysRu subsurface alloysRh subsurface alloysRu-based overlayersRe-based overlayers
H-induced segregation
Stability of NSA’s with Respect to Hydrogen-induced Segregation
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B.E.H (eV)
-3.3 -3.2 -3.1 -3.0 -2.9 -2.8 -2.7 -2.6 -2.5 -2.4 -2.3 -2.2 -2.1
AuIr CuNiCoMo FeWTaV Pt
Cu/
Pt
Ir/P
tR
h/P
tN
i/Pt
Ru/
Pt
Re/
Pt
Co/
Pt
Fe/P
t
W/P
t
Mo/
Pt
V/P
tTa
/Pt
Pd
Ir/P
d
Ru/
Pd
Re/
Pd
Fe/P
d
Mo/
Pd W
/Pd
V/P
d
Ta/P
d
Ru/
Rh
Rh
Ir/R
h
Mo/
Rh
Re/
Rh
Re
Ni*/
Re
Rh*
/Re
Cu*
/Re
Pt*
/Re
Pd*
/Re
Ni*/
Ru
Pd*
/Ru
Cu*
/Ru
Pt*
/Ru
Ru
B.E.H on Close-Packed Metal SurfacesOverlayers* Subsurface Alloys
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Correlation of B.E.H with Clean Surface Properties
d ε (eV)
-3.4 -3.2 -3.0 -2.8 -2.6 -2.4 -2.2 -2.0
B.E
. H(e
V)
-3.2
-3.0
-2.8
-2.6
-2.4
-2.2
-2.0
-1.8
Pt subsurface alloysPd subsurface alloys
Rh subsurface alloys
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-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
-6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0.0
EbFS (eV, PW91)
Eb
TS (e
V, P
W91
)
O2 dissociation: Does EbTS follow Eb
FS?
Pt
Cu
Ir
Ag
Au +10%
Pt3Co
Pt3Co skin
Pt -2%
Pt3Fe
Pt3Fe skin
Y. Xu, A. V. Ruban, M. Mavrikakis, JACS 126, 4717 (2004).
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Hydrogen B.E. (eV)
-0.3 -0.2 -0.1 0.0 0.1 0.2
ETS
(eV
)
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Au
Cu
Noble metals
V/Pd
Pd*/Re
Re/Pd
Ta/PdPd-terminated alloys
Ta/Pt
Mo/Pt
Re/Pt
Ru/PtPt-terminated alloys
BEP Plot for H2 Dissociation onNSA’sJ. Greeley, M. Mavrikakis, Nature Materials 3, 810 (2004)
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Pt2+
Pd Pd
CuUPD/Pd + Pt 2+ Pt/Pd + Cu2+
Cu2+
Cu Pt
Pt
Ru
Pt
Ru
Metal monolayer deposition by galvanic displacement of a less noble
metal monolayer deposited at underpotentials
Electroless (spontaneous) deposition of one metal on
another metal
Brankovic, S. R.; Wang, J. X.; Adzic, R. R. Surf. Sci. 2001, 474, L173
Brankovic, S. R.; McBreen, J.; Adzic, R. R. J. Electroanal. Chem. 2001, 503, 99
Sasaki, K.; Wang, J. X.; Balasubramanian, M.; McBreen, J.; Uribe, F.; Adzic, R. R. Electrochim. Acta 2004, 49, 3873
Zhang, J.; Vukmirovic, M.; Xu, Y.; Mavrikakis, M.; Adzic, R. R. Angew. Chem. Int. Ed. 2005, 44, 2132
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A new kind of Minority/Defect site for TM Catalysis
Isolated metal hetero-atoms near metal surfaces can be viewed as generating an alternative type of “defect/minority” surface sites, associated with a different kind of near-surface “impurity”.
These sites could be more poison-resistant and possess better catalytic kinetics than the rest of the surface sites.
Pt
V
J. Greeley, M. Mavrikakis, Catalysis Today 111, 52 (2006)
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NSA’s - Summary• First-Principles Methods can help with identifying promising
bimetallic NSAs with interesting catalytic properties
• Example: 1. H and H2 on NSA’s: Fine-tuning BEH is possible2. Some NSA’s: Activate H2 easily AND bind atomic H weakly useful for highly selective low T H-transfer reactions
• Developing Catalyst Preparation Techniques with Layer-by-Layer control of metal deposition (ALD-like) is critical for making the desired NSAs
• New type of “NSA” - defect site
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Low Temperature Fuel cells Proton Exchange Membrane
(PEM) fuel cellCathodeAnode
Representative Catalysis
Anode:
2H2 ↔ 4 H+ + 4 e-
Cathode:
O2 ↔ 2 O-
2O- +2H+ +2e-↔ 2OH-
2OH- + 2H+ ↔ 2H2O
Oxygen Reduction Reaction (ORR)- Very slow kinetics
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Pt monolayers on transition metals
• Pt monolayers on
– Ru(0001), Ir(111), Rh(111), Au(111) and Pd(111)
– Ru(0001), Ir(111) and Rh(111) lead to compression of Pt overlayer
– Au(111) leads to expansion of Pt overlayer
– Pd(111) has almost same lattice constant as Pt(111)
Pt monolayer
Substrate(Ru, Ir, Rh, Pd, Au)
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O2 Dissociation Barrier
0.30
0.50
0.70
0.90
1.10
1.30
-4.5 -4.0 -3.5 -3.0
E bO/eV
E bTS
/eV
PtML/Ir(111)
PtML/Au(111)
Pt(111) PtML/Pd(111)/
TSbEeV
/ObE eV
Compressive
strain
Expansive
strain
J Zhang, MB Vukmirovic, Y Xu, M Mavrikakis, RR Adzic, Angew. Chemie, Int. Ed., 44, 2132 (2005)
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OH Formation Barrier
0.30
0.50
0.70
0.90
1.10
1.30
-4.5 -4.0 -3.5 -3.0
E bO/eV
E bTS
/eV
PtML/Ir(111)
PtML/Au(111)
Pt(111)
PtML/Pd(111)
/
TSbEeV
/ObE eV
Expansive
strain
Compressive
strain
J Zhang, MB Vukmirovic, Y Xu, M Mavrikakis, RR Adzic, Angew. Chemie, Int. Ed., 44, 2132 (2005)
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The best catalyst performs a Balancing Act
0-4.5 -4.0 -3.5 -3.0
Eb /eV
0.3
0.5
0.7
0.9
1.1
1.3
Ea
/eVPd
Au Pt Ir- 2- jK
/mA cm
3
6
9
12
15
18 Pd
Ru
Rh
Ea for O2dissociation (filled red circles)
Ea for OH formation (open circles)
Kinetic currents (jK; blue squares)jk value for Pt(111) (black square) obtained from Markovic et.al., J. Electroanal. Chem. 1999, 467, 157
1--PtML/Ru(0001)
2--PtML/Ir(111)
3--PtML/Rh(111)
4--PtML/Au(111)
5--Pt(111)
6--PtML/Pd(111)
Pt(111)
PtML/Pd(111)
O
J Zhang, MB Vukmirovic, Y Xu, M Mavrikakis, RR Adzic, Angew. Chemie, Int. Ed., 44, 2132 (2005)
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Further Decrease of Pt -loading:The role of “OH poisoning” in ORR
• Pt binds OH strongly OH blocks active sites on Pt
• Replace part of the PtML on Pd(111) with another transition metal M
HypothesisM will attract initial OH and induce repulsion on neighbouring Pt-OH decrease OH binding and OH coverage on Pt
OHPt
Pd
OM
Pd
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ORR Experiments on (MxPt1-x)ML/Pd(111): Current .vs. Surface Composition
0 20 40 60 80 100
2
4
6
8
10(IrxPt1-x)ML / Pd (111)
(RuxPt1-x)ML / Pd (111)
Pt molar percentage %020406080100
-j k/ m
Acm
-2 a
t 0.8
5V R
HE
M (Ru or Ir) molar percentage %
• Effect of addition of different amount of M: Ir & Ru in PtML/Pd(111)
– Around 20% M gives highest ORR activity
J Zhang, MB Vukmirovic, K. Sasaki, A. U. Nilekar, M Mavrikakis, RR Adzic, JACS, 127, 12480 (2005)
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Mixed metal Pt monolayer• Modeling these systems with
DFT:– (Pt3M)ML/Pd(111) to get
θM = 0.25 ML
• Calculate: OH+OH or O+OHrepulsion with M being: Pt monolayer
Pd Substrate
Other metal (M)
Au Pd Pt RhRu
Os Re
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OH+OH or O+OH optimal structures
M with much higher reactivity than Pt in PtML/Pd(111): Os, Re (break the OH bond to form H2O and O)
O on M-top and 1st OH on Pt-Pt-bridge site
M with reactivity higher than Pt in PtML/Pd(111): Rh, Ru, Ir
1st OH on M-top and 2nd OH on Pt-top
M with reactivity less or equal compared to Pt in PtML/Pd(111): Au, Pd, Pt
1st OH on Pt-top and 2nd OH on Pt-Pt-bridge site
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R2 = 0.99
12
17
22
27
32
37
42
47
52
-0.2 0 0.2 0.4 0.6 0.8 1
OH-OH or OH-O repulsion relative to OH-OH repulsion on PtML/Pd(111) /eV
-j k a
t 0.8
V / m
A.c
m-2
Au
Ru
Ir Os
Re
Pd
Pt
Rh
Comparison of theory with experiment
(Pt3M)ML/Pd(111)
400% increase in ORR activity for (Pt3Re)ML/Pd(111) compared to Pt(111)
M
J Zhang, MB Vukmirovic, K. Sasaki, A. U. Nilekar, M Mavrikakis, RR Adzic,
JACS, 127, 12480 (2005)
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ORR - Summary:• A combination of first-principles studies with experiments can identify key
ORR reactivity descriptors:
• Kinetics of O-O Bond-breaking
• Kinetics of O-H Bond-making
• OH-poisoning
• Use less Pt: PtML/Pd(111) 30% more current [compared to Pt(111)]
• Use even less Pt: Pt3M/Pd(111) up to 400% increase in current [compared to Pt(111)]
• First-principles can help with further improvements of ORR catalysts, by identifying materials which are:
– Cheaper (low Pt loading)
– More active
– Increased stability of Pt against oxidation
BEO
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Anand Nilekar
University of Wisconsin-Madison
Department of Chemical and Biological Engineering
Dr. Ye XuDr. Jeff Greeley
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Acknowledgements
Ratko Adzic (BNL)
J. Chen (U of Delaware)
Jeff Greeley, Ye Xu, Anand Nilekar
Jens Nørskov and colleagues @ CAMP – Denmark
DoE (BES-Chemical Sciences)
NSF - CTS
NPACI, DOE-NERSC, PNNL (supercomputing resources)