Core-shell and alloy nanoparticles for oxygen reduction Graeme Henkelman - University of Texas at Austin
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Core-shell and alloy nanoparticles for
oxygen reduction
Graeme Henkelman
University of Texas at Austin
Using density functional
theory to characterize
and identify new
nanoparticle catalysts
Motivation • Pt is the best catalyst for the oxygen reduction reaction (ORR) but it is very expensive and there is not enough of it. • Fuel cells lose about 30% of the potential energy due to slow kinetics of the ORR at the Pt cathode. • Metal nanoparticles are attractive as catalysts because they show electronic properties that are qualitatively different from bulk metals. • Use theoretical methods to understand the factors the effect the catalytic activity of nanoparticles, and then design better catalysts.
Catalysis
Oxygen reduction: different catalysts change both the energy of saddle
points and the binding energy of products
noble thermodynamic
metal limit
2 H2 + O 2 2 H2O + (4 x 1.23) eV
reactive
metal
Optimal catalyst
• balance low barrier
with weak binding
O2 dissociative • approach the
adsorption thermodynamic limit
Volcano plots
optimal
Volcano plot:
A peak in catalytic reactive noble
activity corresponds metals metals
to the optimal
balance between
reactive and noble
metals
Pt has the highest
activity of any
single transition
metal catalyst for strong high
the O-reduction
binding barrier
reaction (ORR)
Bligaard, Nørskov, Dahl, Matthiesen, Christensen, and Schesten, J. Catal. 224, 206 (2004)
Brønsted-Evans-Polanyi relation
O2 dissociative adsorption
Similar catalysts: saddle
point energies are linearly
related to reaction energies
lower saddle
BEP
relation
weaker binding
Electronic structure:
Barriers and binding energies are
both determined by the energy of the
bonding electronic states (d-band)
Xu, Ruban, Mavrikakis, JACS.126, 4717 (2004)
Bligaard, Nørskov, et al., J. Catal. 224, 206 (2004)
ads to a broadening and shift of the troscopy (XAS), see figure 4. These spectroscopies
[14,15], see figure 3. There may be a provide an atom specific projection of the occupied
BEP relationship from d-band level
ved in this interaction, but since all the (XES) and unoccupied (XAS) electronic states [9,10].
have a half-filled s band in the metallic Since the decay process in XES and the excitation pro-
he band is broad, there will only be cess in XAS are dipole dominated and the transitions
in this interaction from one metal to are governed by the overlap with the N ls core orbital,
erences between the different transition only the N2p valence center of d-band
electrons
EF
contribute to the inten-
ssociatedd-band
primarily level:
with the d states. sity. Furthermore, using angle-resolved measurements
of the adsorbate states with a narrow the different px, py and pz components of the electronic
the center of the d-type
ates will give rise to the formation of structure can be separated. Figure 4 shows angular
density ofstates
and anti-bonding states
just relative
as in resolved XE and XA spectra on a common energy scale
bondingto and
theanti-bonding
Fermi-levelstates(EareF) with respect to the Fermi level of N adsorbed on Ni(100)
ngth of the bond will depend on the and Cu(100) with a coverage of half a monolayer [6–8].
y of these states. If only the bonding In the XE spectra of N adsorbed on Cu, both the pxy
here will be a strong bond, whereas if and pz components exhibit two strong peaks, repre-
states are also, filled the bond becomes senting the bonding and antibonding states. In the XA
ker. spectra, on the other hand, no strong peaks are
, the occupancy of the anti-bonding observed. For N adsorbed on Ni we only observe one
the number of electrons in the system. strong peak at high binding energy in the XE spectra,
at a metal surface. Here there is an due to occupied bonding pxy and pz states. The
EF Binding energy is largely
O(2p) determined by the level of
the metal d-band with
respect to the Fermi level
llustration of the formation of a chemical bond between an adsorbate valence level and the s and dHammer
states of aand Nørskov, Adv. Catal. 45, 71 (2000)
transition
Rh/Pt Fe/Pd
Ir/Rh Ir/Pt Ir/Pd
Cu/Pt
orption and Dissoc
Near surface alloys for tuning catalysts
iation of O2 on Pt!M Alloys ARTICLES
ble 5. Several Characteristics of the Electronic and Geometric
uctures of the Four Clean Surfacesa
Overlayers* Subsurface alloys
min vertical
separation b/w
!d f max corrugation Pt atoms in top
(eV) σ (%) of top layer (Å) two layers (Å)
111) -2.52 5.93 93.3 0.00 2.33
-compressed Pt(111) -2.63 6.20 93.2 0.00 2.39
kin on Pt3Co(111) Pt-I -2.58 6.14 93.4 0.10 2.32
Pt-III -2.79 6.13 93.3
Co(111) Pt -2.69 5.88 93.5 0.06 2.28
Co -1.45 5.56 79.0
0001) -1.48 5.57 81.3 0.00 n/a
0.00 0.25 0.50 0.75 1.00 1.25
Co(0001) is included as a reference. See the text for definitions of the
E
ameters. n/a stands for not applicable.
CsegL E S Overlayers: Xu et al.
gregate toAlloy the surfacemetal and can
a purewet the surface,
Pt overlayer forms, which Figure 7. Binding energies of atomic O vs d-band centers (!d) of the four
-I point in Figure 7 by calculating a V2-weighted !d62 for clean surfaces. Labels identify the adsorption sites. The best linear fit is
ees withor theform
experimentala subsurface
finding that Pt-only alloy layers form
-Iaresite:
denoted as solute/host (Edrawn.
3Co alloyspairs. uponThe x axis indicates
3,11 Thisthe is energy
Pt3Co or near-Pt annealing. so seg) for a
enotesPtthe
cause difference
atoms segregatebetween
strongly the magnitudes
to the surface of the hydrogen binding
of Pt-Co energies
Esp comes from the coupling of the adsorbate levels to the metal
oys (the 2V
segregation
d metal surfaces.
2
!energy +of VPt is
2
!
about
Regions in which hydrogen-induced -0.5 eV), but the
-2.71 segregation is expected
Pt-III d,Pt-III Pt-I d,Pt-I
) ) sp states and is usually the main stabilizing contribution. The
dering
_effective
matics) are Subsurface
energy, which
present. 2VSee text+alloys:
could2 help Co2
inVconnection atoms incorporate into
to the Au*/Pt system. The colour
next twocode
terms arise from the coupling of the renormalized
surface layer of this Pt-III Pt-I 54
particular facet, is not strong enough.
ne ordering Change
subsequent figures.
energy the suppressed
is further d-band bylevel the ferromagnetism
adsorbate states to the metal d states. Ed_orth is the cost of
same orthogonalization, which is proportional to V2, the d-band
the Pt3problem
alloy.exists
Co(and Our for
MC the Pt 3
simulationsCo(111)
reactivity) of the surface showsurface:
that, if We
the are
alloy coupling matrix element. Ed_hyb is the gain from hybridization,
include the fcc-P
confined in the ferromagnetic point in Figure state,7,the butcritical
at the Co center
temperature
eorder
the OTatom interacts with disparate Co and Pt d states which is proportional to V2/|!d - !a|, where !d and !a are the
disorder transition is reduced to ∼500 K, signifi- centers of the metal d band and the renormalized adsorbate
)where )the
aneously
BE
ntly H | |
sol
eo effective
–
lower (rBE
than host
the
Pt(111)/Cov
!
the paramagnetic
d
H
cannot | ,∼1000
be
1.64;KrPt(111)/CoV
BE
if
calculated
the
H is
alloy 2.32;
is
according
see reftochange
energy
allowed
to the
state.from the gas phase onto the metal al.,
61),
cross
spirit
when
states,atomic
respectively.58 According to the analysis of Hammer et
drogen
d-band
is
Besenbacher,
adsorbed
model. Chorkendorff,
Nevertheless, Clausen,
considering Hammer, the Molenbroek,
clear
surface;
59 if the see
change is small in the adsorbate-surface interaction,
This picture is valid, however, only in the case of a clean
g.nce 1).ofIf,
Nørskov, for theexample,
and Stensgaard, EonsegScience
Ptis positive, then in vacuumone thecansolute
expect a linear correlation over a small range of !d:
Co(111) O surface.
for Co
In center
the presence of O, 279,
3Co(111), 1913
oneifhaswe (1998).
calculate
to take into
llthmetic
be found
Greeley
nsideration and
mean inof the
an Mavrikakis, the subsurface
additional Nature
!dgainvalues of layers
of Materials
the 3,of
the three
energy 810
due the host.
(2004)
atoms
to the Further, if H 2 TS
Dendrimer encapsulated nanoparticles
Dendrimer encapsulation:
make reproducible alloy or core/
shell nanoparticles
Core/shell:
use core metal to
tune the reactivity
of the shell
R. W. J. Scott, O. M. Wilson, S.-K. Oh, E. A. Kenik, and R. M. Crooks, J. Am. Chem. Soc. 126, 15583 (2004).
O. M. Wilson, R. W. J. Scott, J. C. Garcia-Martinez, and R. M. Crooks, J. Am. Chem. Soc. 127, 1015 (2005).
Structural information from X-ray scattering
1 truncated-octahedron 2 cubo-octohedron 3 icosahedron
PDF X-ray Data: Valeri Petkov disordered
ï
ï
match disordered
theory
(QHUJ\SHU$WRP H9
ï
ï 2
3
ï
ï
1
ï match Truncated octahedron (1) fits
ï
ordered experiment the experimental data best and
has the lowest DFT energy
3')(UURU
O2 dissociation on Pd-shell nanoparticle
Choose Pd shell because it is
close to Pt
See how the core metal changes
the ORR on the shell
E=0 is O2 in gas phase
A truncated octahedral structure
has the lowest energy in vacuum
Reaction are assumed to take place
on the (111) facet; this is the lowest
energy, and most noble surface
Tang and Henkelman, J. Chem. Phys. 130, 194504 (2009).BEP relationship for nanoparticles
Pd-shell nanoparticles:
follow a BEP relationship as
the core metal is changed
BEP
relation d-band center of the shell:
is a good measure of the barrier
and binding for the ORR
Tune the Pd shell to
be like Pt by choosing
a non-noble core metalActivity is not intermediate to the core and shell
A Pd shell
particle, combined
with a less nobel
metal core, results
in a particle with a Mo@Pd
shell that is more Co@Pd
noble than Pd Core Shell
Pd
Co
Mo
Possibility: can a core-shell particle be constructed from non-noble metals that
reacts like a noble metal?Does strain makes the d-band shift?
Surface strain: has been shown to correlate with
the d-band center and binding energetics
Nanoparticle shells have less surface strain so
that it does not (alone) set the d-band center
Correlation between O
and CO energetics and
the surface lattice
constant on Ru(0001)
Mavrikakis, Hammer, and Nørskov,
Phys. Rev. Lett. 81, 2819 (1998)Ligand effect: the rectangular d-band model
uced segregation
Assuming constant band filling, fd,
and a rectangular d-band shape: w
V/Pt
Ta/Pt W/Pt
V/Pd
Ta/Rh
W/Rh Mo/Pt Ta/Pd Re/Pt
W/Pd
Ta/Pt Ru/Pt
h Mo/Rh Re/Rh Mo/Pd Re/Pd
Ni/Pt
Co/Pt Ru/Pd E
Pd
Ir/Pt Ir/Pd
�d EF
Pt
� � �
1 1
w= �d
erlayers* Subsurface alloys 12 0.5 − fd
Pt Slope for unstrained near surface
M alloys of Pt gives a d-band filling:
Pt fd = 0.93
0.50 0.75 1.00 1.25 Kitchin, Nørskov, Barteau, and Chen, Phys. Rev. Lett. 93, 156801 (2004)
Kitchin, Nørskov, Barteau, and Chen, J. Chem. Phys. 120, 10240 (2004)Rectangular d-band model for nanoparticles?
The density of states of
the Pd shell shifts down
as the band widens
Rectangular d-band model:
� � �
1 1
w= �d
12 0.5 − fd
For the Pd shell, fd ≈ 0.91
which predicts a w vs εd
slope of -0.70
This is larger than the
observed slope of -0.32,
indicating that another factor
is lowering the d-bandd-band shift in Co@Pd core-shell particles
Higher energy
electrons from the
Co core are
moving to the Pd
shell causing the
d-band to lower
with respect to the
Fermi level
ΔEd ≡ d-band
shiftCharge transfer causing the d-band shift?
core shell shell core
Co
Fermi level e Pd
Pd
e Au
d-band
d-band d-band
good
d-band
When the Pd shell accepts electrons from the core, the d-band level of
the shell is lowered with respect to the Fermi-level
Test this theory: do a Bader population analysis of the charge density
from DFT calculationsAtomic charges with a robust Bader algorithm
Find Bader volumes
(a) Follow ascent trajectories
between grid points in the
charge density.
(b) Bader volumes are the
collection of points which all
lead to the same charge
density maximum
For solids and complex systems
Can be used with delocalized (e.g. plane
wave) basis sets.
H H
Is robust for complex bonding geometries.
Cost scales linearly with system size.
O
Wenjie Tang and G Henkelman, “A grid-based
Bader analysis algorithm without lattice bias”
J. Phys.: Condens. Matter 21, 084204 (2009).Bader analysis to quantify charge transfer
Charge transfer
From core to shell is correlated
with the d-band center
A charge transfer of 0.15 e/atom
shifts the d-band by 0.12 eV based
upon the DOS at the Fermi Level
d-band (1.25 states/eV/atom)
center of Pt
Contributions to d-band shift
The observed d-band shift (0.5 eV)
can be decomposed into ligand
induced band widening (0.24 eV)
and charge transfer induced
band filling (0.12 eV) -- a significant
contribution for nanoparticles.What about random alloys?
Strong Weaker
binding binding
Pd Pt
?
What is the
Alloy binding strength
of O on Pd/Pt
alloysO-binding on Pt/Pd alloys
O-binding on Pt/Pd alloys
First guess
PtPd Alloy
0
Binding energy
Pt
PdO-binding on Pt/Pd alloys
First guess DFT: O-binding on PtPd alloys
-1.5
slab
Binding energy (eV)
-1.6
PtPd Alloy
0 -1.7
Binding energy
-1.8
-1.9
Pt
-2.0
0 0.2 0.4 0.6 0.8 1
Pd
Pd Pt percentage PtO-binding on Pt/Pd alloys
First guess DFT: O-binding on PtPd alloys
-1.5
slab
Binding energy (eV)
-1.6
PtPd Alloy
0 -1.7
Binding energy
-1.8
-1.9
Pt
-2.0
0 0.2 0.4 0.6 0.8 1
Pd
Pd Pt percentage Pt
experiment
Ye and Crooks, JACS, 129, 3627 (2007)O-binding on Pt/Pd alloys
First guess DFT: O-binding on PtPd alloys
-1.5
slab
Binding energy (eV)
-1.6
PtPd Alloy
0 -1.7
Binding energy
-1.8
-1.9
Pt
-2.0
0 0.2 0.4 0.6 0.8 1
Pd
Pd Pt percentage Pt
Peaked at 1:1
-1.60
79-atom
Binding energy (eV)
-1.65
particle
-1.70
-1.75
experiment -1.80
-1.85
0 0.2 0.4 0.6 0.8 1
Pt percentage
Ye and Crooks, JACS, 129, 3627 (2007)Peak in activity is geometry related
-1.0
No peak !
-1.2
binding energy(eV)
Frozen
-1.4
Relaxed
-1.6
-1.8
-2.0
0 0.2 0.4 0.6 0.8 1
Pt percentage
Shape change is important when oxygen binds to nanoparticlesDecomposition of the binding energy
ΔE
chemical Eb dE geometric
bonding relaxation
0 0
-1.25 -1.25
-2.50 -2.50
softest
-3.75 -3.75
dE
Eb Stronger binding
-5.00 -5.00
Ag Pd Pt Cu Ir Rh Ag Pd Pt Cu Ir RhElectronic and geometric effects
0.0
-0.5
dE: geometric
relaxation
Energy
-1.0
-1.5
Eb: chemical
bonding
-2.0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Pd Pt percentage Pt
-1.6
O binding energy
-1.7
-1.8
E: binding
chemical geometry
-1.9 bonding, Eb relaxation, dE energy
dominates dominates
-2.0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Pd prevents the large O-Pt relaxation
1
Slab
0.8
ΔR / ΔR( Pt )
0.6
140
0.4
79
0.2
ΔR : distance change due to oxygen binding
0
0 0.2 0.4 0.6 0.8 1
Pt PercentangeGenetic algorithm to identify new catalysts
Use a genetic algorithm to optimize Parent Parent
the properties of nanoparticles, e.g.
the fitness: Pd Pt
� �
� Pt(111) �
Φ ≡ ��shell
d − �d � Co Ni
Co6@Pd32 Ni6@Pt32
Ti Cr Fe Ni Zn Zr Mo Ru Pd Cd Hf W Os Pt Hg ???
Sc V Mn Co Cu Y Nb Tc Rh Ag La Ta Re Ir Au
Hg
Au
Pt
Ir
Os
5d Re
W
Ta Co6@Pd32 Ni6@Pt32
Hf
La Co6@Pt32 Ni6@Pd32
Cd
Ag Possible offspring
Shell metal
Pd
Rh
Ru
4d Tc
Mo
Nb
Zr Best possible candidates can
Y
Cu
Zn be explored experimentally
Ni
Co
Fe
3d Mn
Cr
V N Froemming and G Henkelman “Optimizing core-
Ti
Sc shell nanoparticle catalysts with a genetic algorithm”
3d 4d 5d J. Chem. Phys. 131 234103 (2009).
Core metalAlchemical derivatives
O
bind
ѥ
n,I H9
0.1
NI 45 46 47
Alchemical changes change the 5K3G$J
WRZHDNHQELQGLQJ
electrostatic potential, to first order
initial: O final: A B
45 46 47 They are predictive for integer
Rh Pd Ag changes in atomic numbers
C D
2 H
H9
F D ( F
1 J
(¨Ѥ
0 B so they could be used for gradient
bind
A G H based material optimization
(
Ѥ
-1 I
C
-2 G I J
D Sheppard, G Henkelman, and O. A. von Lilienfeld
-2 -1 0 1 2 “Alchemical derivatives of reaction energetics” J.
bind
у( H9 Chem. Phys. 133 084104 (2010).Conclusions The reactivity of core/shell nanoparticle catalysts can be tuned by systematically varying the core metal. Pd shelled particles with Mo and Co cores are similar to Pt in how they bind oxygen. Strain is much less important for 1nm particles as it is for bulk near surface alloys. Charge transfer is important for core/shell nanoparticles; rigidity for alloy particles.
Research Group
Matt
Welborn
Albert
Lu
Wenjie
TangAcknowledgments
Funding Research Group
NSF - CAREER Wenjie Tang Dan Sheppard
Welch Foundation Matt Welborn Sam Chill
Advanced Research Program Chun-Yaung Lu Pheghao Xiao
DOE - SISGR, EFRC Liang Zhang Rye Terrell
Nathan Froemming Phani Dathar
Computer Time
EMSL at the Pacific Northwest Collaborators
National Lab Crooks Group Valeri Petkov
Texas Advanced Computing Center Anatoly Frenkel
Software tools
http://theory.cm.utexas.edu/vtsttools/! aKMC, Dimer, NEB, and dynamical matrix
! methods implemented in the VASP code
http://theory.cm.utexas.edu/bader/! Bader charge density analysis
http://eon.cm.utexas.edu/! The EON distributed computing projectYou can also read
