Stabilization of Platinum
Using Gold Clusters
J. Zhang,1K. Sasaki,1E. Sutter,2R. R. Adzic1
We demonstrated that platinum (Pt) oxygen-reduction fuel-cell electrocatalysts can be stabilized
against dissolution under potential cycling regimes (a continuing problem in vehicle applications) by
modifying Pt nanoparticles with gold (Au) clusters. This behavior was observed under the oxidizing
conditions of the O2reduction reaction and potential cycling between 0.6 and 1.1 volts in over
30,000 cycles. There were insignificant changes in the activity and surface area of Au-modified Pt
over the course of cycling, in contrast to sizable losses observed with the pure Pt catalyst under the
same conditions. In situ x-ray absorption near-edge spectroscopy and voltammetry data suggest that
the Au clusters confer stability by raising the Pt oxidation potential.
on the effects of the oxide supports in facilitating
this activity (1–3). Other explanations of the
activity included the clusters’ distinct electronic
(4) or chemical properties (5). The mechanism of
oxygen adsorption and activation necessary for
rapid CO oxidation is controversial and opposite
to the observed lack of O2dissociation on Au
single crystals (6). Recently, Chen and Goodman
interface boundary sites are believed to be im-
portant for stabilizing oxygen-containing reaction
intermediates on Au clusters (8). Support effects
were reported on the nucleation, growth, and
morphology of Au nanoclusters for TiO2 and
As the underlying surface affects the Au
clusters, so the clusters can conversely be ex-
pected to alter the properties of the support sur-
faces. However, such effects have not yet been
studied, despite considerable scientific and tech-
have a stabilizing effect on an underlying Pt
metal surface under highly oxidizing conditions
and suppress Pt dissolution during the O2reduc-
tion reaction (ORR) during potential cycling,
Fuel cells are expected to become a major
source of clean energy (10, 11) with particularly
important applications in transportation. Despite
considerable recent advances, existing fuel-cell
technology still has drawbacks, including the in-
stability of the Pt electrocatalyst for the ORR at
the cathode (10). Recent work recorded a sub-
stantial loss of the Pt surface area over time in
(11) during the stop-and-go driving of an electric
car; this depletion exceeded the Pt dissolution
CO oxidation, there were several reports
rates observed upon holding at constant poten-
tials (12) for extended time spans. Our results
show promise toward resolving this impediment.
The Au clusters were deposited on a Pt cata-
lyst (carbon-supported Pt nanoparticles) through
galvanic displacement by Au of a Cu monolayer
on Pt (13). Underpotential deposition, which
involves a monolayer-limited process at poten-
tials above the thermodynamic values, was used
to coat a Pt surface with a monolayer of Cu. To
obtain some insights into a possible mechanism
of Au cluster formation on Pt nanoparticles, we
describe a more tractable model system: de-
positing Au onto a single-crystal Pt(111) sub-
form a surface oxide layer that inhibits ORR
activity and leads to its possible dissolution. It is
likely that Au clusters can affect this process,
which can provide information on the stabiliza-
tion effect. After undergoing several potential
sweeps to 1.2 V, the Au monolayer transformed
into three-dimensional clusters (Fig. 1). The
scanning tunneling microscopy (STM) image
shows clusterstwoto three monolayersthick and
2 to 3 nm in diameter. All potentials are given
with respect to a reversible hydrogen electrode
(RHE). The measurements were carried out at
room temperature, unless otherwise indicated.
The Au clusters on carbon-supported Pt
nanoparticles were generated using the same
method. Because the size of the Pt nanoparticles
is about 3 nm, the Au clusters on Au/Pt/C are
clearly much smaller than those on Au/Pt(111).
of CO to determine the Pt surface area blocked
by Au. A thin layer of catalyst was bonded by a
thin Nafion film to a glassy carbon disk of a
rotating disk electrode. The CO stripping mea-
surements on Pt/C and Au/Pt/C (fig. S1) re-
vealed that the Au clusters in Au/Pt/C covered
about 30 to 40% of the Pt surface. We assumed
inthiscalculationthat CO is notadsorbed onthe
Au surface under these conditions.
The structure of the Au-modified Pt/C cat-
sion electron microscopy. Measurements were
performed using a high-resolution 300-kV field-
1Department of Chemistry, Brookhaven National Labora-
tory, Upton, NY 11973, USA.
Nanomaterials, Brookhaven National Laboratory, Upton,
NY 11973, USA.
2Center for Functional
Fig. 1. STM image (125 × 125 nm) of the Au
clusters on a Pt(111) surface, obtained by gal-
monolayer was deposited at underpotential on Pt
(111). The Au adlayer was subjected to 10 cycles
between 0.2 and 1.2 V versus RHE with a sweep
rate of 50 mV/s to obtain such clusters. The STM
image was acquired at the electrode potential of
0.8 V in 0.1 M HClO4at room temperature; the
tunneling current was 1.24 nA.
Fig. 2. Electronmicrographs ofa Au/Pt/C catalyst
made by displacement of a Cu monolayer by Au.
High-resolution images (A and B) show atomic
rows with spacings that are consistent with the
Pt(111) single-crystal structure. A different struc-
to the Au clusters.
12 JANUARY 2007VOL 315
on April 28, 2008
emission microscope (JEOL3000F) equipped
with an energy filter, an energy-dispersive x-ray
spectrometer, and an electron energy-loss spec-
trometer. Low-magnification images (fig. S2)
indicate the presence of metal particles averaging
3 to 5 nm in size on ~50-nm carbon spheres.
Figure 2 shows the morphology of two isolated
metal nanoparticles on the carbon support.
Energy-dispersive spectroscopy applied directly
to these particles showed the presence of 10 to
11% Au on Pt. The most frequently observed
lattice fringes fit well with the Pt(111) surface.
2 are ascribed to the Au clusters, which appear
amorphous rather than crystalline.
The in situ extended x-ray absorption fine
structure (EXAFS) spectra (fig. S3) of the Pt L3
and Au L3edges of the Au/Pt/C electrocatalyst
have an absorption intensity at the Au L3edge
(11,919 eV) that is ~28% of that at the Pt L3
edge (11,564 eV). The difference between the
absorption intensities approximates the compo-
sition of the Au/Pt electrocatalyst because of the
proximity of Pt and Au absorption coefficients.
The mole ratio of bulk atoms to surface atoms
for the 2- to 3-nm size of these nanoparticles is
~40 to 50%. If a 2/3 monolayer of Au is de-
posited on the surfaces of Pt nanoparticles, the
Pt:Au mole ratio must thus range from 1:0.26 to
1:0.33, which is in a very good agreement with
the above result of 28% (that is, 1:0.28) from
EXAFS spectra. Because Au L3absorption be-
gins only 355 eV after the onset of the Pt L3
edge, some fluctuations due to photoelectron
scattering in the Pt EXAFS spectrum must be
superimposed on the Au spectrum. The proxim-
ity of the Pt and Au L3edges makes the analysis
of such spectra questionable. Thus, the size of
the Au clusters could not be determined.
We found that the Pt nanoparticles retain
their ORR activity crucial for fuel-cell catalysts
after the deposition of Au clusters. On a rotating
disc-ring electrode, the activities of Au/Pt/C and
Pt/C differed by only 3 mV, expressed as the
half-wave potential of these two surfaces (fig.
S4). The small difference in the ring currents of
the two surfaces corroborates this conclusion. In
addition, the ring currents show a negligible
generation of H2O2, indicating a four-electron
reduction of O2to H2O on both surfaces.
The stabilizing effect of Au clusters on Pt was
determined in an accelerated stability test by
continuously applying linear potential sweeps
from 0.6 to 1.1 V, which caused surface
oxidation/reduction cycles of Pt. The surface
reaction involves the formation of PtOH and PtO
dissolution of Pt via the Pt2+oxidation state (12).
We conducted the test by applying potential
sweeps at the rate of 50 mV/s to a thin-layer
rotating disk electrode in an O2-saturated 0.1 M
ison, a Pt/C catalyst with the same Pt loading as
cycling conditions. After 30,000 cycles, changes
in the Pt surface area and electrocatalytic activity
of the ORR were determined.
The catalytic activity of Au/Pt/C, measured as
the currents of O2reduction obtained before and
dation in half-wave potential over the cycling
period (Fig. 3A); in contrast, the corresponding
change for Pt/C amounts to a loss of 39 mV (Fig.
3C). The same experiment with Au/Pt/C at 60°C
showed no loss of activity (fig. S5), affording ad-
ditional evidence for the stabilizing effect of Au
clusters on the underlying Pt.
Voltammetry was used to determine the Pt
measuring H adsorption before and after potential
cycling. Integrating the charge between 0 and
0.36 Vassociated with H adsorption for Au/Pt/C
shows no change, indicating no recordable loss of
Pt surface area (Fig. 3B). However, for Pt/C, only
~55% of the original Pt surface area remained
surface-area measurements are in good agree-
ment with the measured ORR activities.
For the Pt/C catalyst (11.9 mgPt/cm2), the mea-
sured degradation of the half-wave potential (E1/2)
after 30,000 cycles (at room temperature) was
39 mV. If the Pt specific activity does not vary
significantly with the potential cycling, and
assuming a constant Tafel slope b of –120 mV,
the remaining Pt surface area after potential cy-
cling can be estimated by using the expression
(see the supporting online material) DE1/2= –b ×
log(SPt/SPt0), where SPtis the Pt surface area after
cycling and SPt0is the initial Pt surface area. For
the loss in E1/2of 39 mV, the calculated value for
initial one. This is less than the observed 55%,
but the difference is not surprising given a pos-
sible change of the interfacial conditions during
Fig. 3. Polarization curves
on Au/Pt/C (A) and Pt/C (C)
catalysts on a rotating disk
electrode, before and after
30,000 potential cycles.
Sweep rate, 10 mV/s; rota-
tion rate, 1600 rpm. Vol-
tammetry curves for Au/Pt/C
(B) and Pt/C (D) catalysts
before and after 30,000
cycles; sweep rate, 50 and
20 mV/s, respectively. The
0.6 to 1.1 V in an O2-
saturated 0.1 M HClO4so-
lution at room temperature.
For all electrodes, the Pt
loading was 1.95 mg (or 10
nmol) of Pt on a 0.164 cm2
glassy carbon rotating-disk
electrode. The shaded area in (D) indicates the lost Pt area.
j / mAcm-2
E / V RHE
0.0 0.40.8 1.2
j / mAcm-2
E / V RHE
E / V RHE
j / mAcm
After 30,000 cycles
E / V RHE
After 30,000 cycles
j / mAcm-2
Fig. 4. (A) XANES spec-
tra obtained with the
Au/Pt/C catalyst at the Pt
L3edge at four different
potentials. (B) A compar-
ison of the change of the
absorption edge peaks of
the XANES spectra for
Au/Pt/C and Pt/C as a
function of potential, ob-
tained with the electro-
catalysts at four different
potentials in 1 M HClO4.
normalized absorption µ
energy / eV
A uP t/C
∆(µ − µ
E / V R H E
VOL 315 12 JANUARY 2007
on April 28, 2008
30,000 cycles and the approximations involved
in the calculation. A similar expression for the
cell can be found in reference (11).
of ORR inhibition, despite blockage of approx-
imately one-third of the Pt sites on Au/Pt/C by
Au, are intriguing phenomena that may have
additional applications beyond fuel cells. To
elucidate the origin of the observed stabilization
effect of Au clusters, we determined by in situ
the oxidation state of Pt as a function of potential
for the Au/Pt/C and Pt/C surfaces. The data offer
strong evidence of decreased oxidation of Pt
nanoparticles covered by Au in comparison with
the oxidation of Pt nanoparticles lacking such
coverage. In the XANES spectra for Au/Pt/C at
the Pt L3edge (Fig. 4A), the intensity of the
absorption bands reflects the depletion of the d
band caused by the oxidation of Pt; a very small
potential dependence indicates such oxidation.
This effect is more evident in the relative change
edge spectra for Au/Pt/C and Pt/C as a function
of potential (Fig. 4B). The increase in the inten-
sity of the absorption edge peak for the Au/Pt/C
electrocatalyst commences at considerably high-
er potentials than does that for the Pt/C catalyst;
thus, the oxidation of Pt nanoparticles modified
by Au clusters requires much higher potentials
than are necessary for unmodified Pt nanopar-
ticles. The high Pt oxidation potential of the
Au/Pt/C electrocatalyst (that is, the lower extent
of Pt oxidation) is clearly the major mechanism
for the stabilization effect of Au clusters. A
decreased Pt oxidation can also be discerned
from a comparison of voltammetry curves for
Au/Pt/C and Pt/C, as well as for Au/Pt(111) and
Pt(111).The lower chargeassociated withthe Pt
oxidation at the potential region between 0.7
and 1.1 V clearly reveals the reduced oxidation
of Au-modified surfaces (fig. S6). Table 1 gives
a summary of the observed changes in surface
area and catalytic activity data caused by po-
Nørskov and co-workers recently proposed a
model describing the activity of metal adlayers
(14, 15), according to which the characteristics of
center (ed), play a decisive role in determining
surface reactivity. Density functional theory (DFT)
calculations showed that the binding energies and
reactivity of small adsorbates correlate well with
the position of edon strained surfaces and metal
overlayers (16), in accordance with data from
numerous experimentalstudies (17–20).The small
Au clusters have more low-coordinate Au atoms
with low coordination numbers have higher-lying
d states, which are more reactive and interact more
strongly with the adsorbate states (21). Au clusters
on oxide supports can thereby activate molecular
oxygen at room temperature (22–24).
When Au clusters are bound on a metallic,
rather than oxide, substrate, the electronic inter-
actions differ. Roudgar and Grob (25) used DFT
calculations to demonstrate a significant coupling
of d orbitals of small Pd clusters to the Au(111)
substrate. An equivalent type of interaction
between Au and Pt can account for the observed
stabilizationof Pt.When clusters of the softer Au
metal are placed on the surface of considerably
harder Pt, there is practically no mixing between
them. Del Pópolo et al. (26) previously reached a
similar conclusion regarding Pd on an Au
system. The surface alloying of Au with Pt,
although unlikely, also wouldmodify thePtelec-
tronic structure toward a lower Pt surface energy,
or lower-lying Pt d-band states.
The high ORR activity of the Au clusters on a
modified Pt/C electrocatalyst is a counterintuitive
observation. Au is not an active catalyst for the
ORR to H2O; instead, H2O2 is quantitatively
generated in a two-electron reduction at most
surfaces [except for Au(100) and its vicinal
surfaces in alkaline solutions (27)]. Because Pt
reduces O2to H2O in a four-electron process, a
decreaseofthe reduction currentfor the Pt surface
that is one-third covered by a monolayer of Au
would be expected. Without this decrease, it
appears that Au clusters have a very high activity,
in stark contrast to the behavior of the bulk Au or
of carbon-supported Au nanoparticles. As dis-
cussed above, the mechanism of oxygen acti-
vation by Au clusters is controversial. Several
researchers ascribed the activity of Au clusters to
their interactions with oxides, resulting in charged
Au particles (28). In view of our preceding dis-
cussion (25), such a process is not likely to occur
with Au clusters at metal supports. To explain the
observed activity, we might consider an efficient
spillover of H2O2from Au clusters to the sur-
could account for the negligible loss of activity of
the Pt surface toward ORR.
Our studies raise promising possibilities for
synthesizing improved ORR Pt-based catalysts
and for stabilizing Pt and other Pt-group metals
under oxidizing conditions.
References and Notes
1. Y. Iizuka et al., Catal. Today 36, 115 (1997).
2. M. Comotti, W. C. Li, B. Spliethoff, F. Schuth,
J. Am. Chem. Soc. 128, 917 (2006).
3. M. Valden, X. Lai, D. W. Goodman, Science 281, 1647
4. C. C. Chusuei, X. Lai, K. Luo, D. W. Goodman, Top. Catal.
14, 71 (2001).
5. D. C. Meier, D. W. Goodman, J. Am. Chem. Soc. 126,
6. R. Meyer, C. Lemire, S. K. Shaikhutdinov, H. Freund,
Gold Bull. 37, 72 (2004).
7. M. S. Chen, D. W. Goodman, Catal. Today 111, 22 (2006).
8. B. Hammer, Top. Catal. 37, 3 (2006).
9. B. K. Min, W. T. Wallace, D. W. Goodman, Surf. Sci. 600,
10. H. A. Gasteiger, S. S. Kocha, B. Sompalli, F. T. Wagner,
Appl. Catal. B Environ. 56, 9 (2005).
11. P. J. Ferreira et al., J. Electrochem. Soc. 152, A2256
12. R. Woods, in Electroanalytical Chemistry, A. J. Bard, Ed.
(Marcel Dekker, New York, 1976), vol. 9.
13. S. R. Brankovic, J. X. Wang, R. R. Adzic, J. Serb. Chem.
Soc. 66, 887 (2001).
14. J. Greeley, J. K. Nørskov, M. Mavrikakis, Annu. Rev. Phys.
Chem. 53, 319 (2002).
15. B. Hammer, J. K. Nørskov, in Advances in Catalysis
(Elsevier, Amsterdam, 2000), vol. 45, pp. 71–129.
16. Y. Xu, A. V. Ruban, M. Mavrikakis, J. Am. Chem. Soc. 126,
17. E. Christoffersen, P. Liu, A. Ruban, H. L. Skriver,
J. K. Nørskov, J. Catal. 199, 123 (2001).
18. F. B. de Mongeot, M. Scherer, B. Gleich, E. Kopatzki,
R. J. Behm, Surf. Sci. 411, 249 (1998).
19. J. A. Rodriguez, D. W. Goodman, Science 257, 897
20. J. Zhang, M. B. Vukmirovic, Y. Xu, M. Mavrikakis,
R. R. Adzic, Angew. Chem. Int. Ed. 44, 2132 (2005).
22. Y. D. Kim, M. Fischer, G. Gantefor, Chem. Phys. Lett. 377,
23. D. Stolcic et al., J. Am. Chem. Soc. 125, 2848 (2003).
24. B. E. Salisbury, W. T. Wallace, R. L. Whetten, Chem. Phys.
262, 131 (2000).
25. A. Roudgar, A. Groß, Surf. Sci. 559, L180 (2004).
26. M. G. Del Pópolo, E. P. M. Leiva, M. Mariscal,
W. Schmickler, Surf. Sci. 597, 133 (2005).
27. R. Adzic, in Electrocatalysis, J. Lipkowski, P. N. Ross, Eds.
(Wiley-VCH, New York, 1998), pp. 197–242.
28. B. Yoon, H. Hakkinen, U. Landman, J. Phys. Chem. A 107,
29. S. Strbac, R. R. Adzic, J. Electroanal. Chem. 403, 169
30. This work is supported by the U.S. Department of Energy,
Divisions of Chemical and Material Sciences, under
contract no. DE-AC02- 98CH10886. We thank H. Isaacs
for helpful discussions.
Supporting Online Material
Figs. S1 to S6
31 August 2006; accepted 15 November 2006
Table 1. A comparison of surface area and the catalytic activity data for Pt/C and Au/Pt/C before
and after 30,000 potential cycles from 0.6 to 1.1 V under the oxidizing conditions of the O2
reduction reaction. Data were obtained from Fig. 3.
at 1600 rpm (V)
Kinetic current density
at 0.85 V (mA/cm2)
at 0.85 V (A/m2Pt)
12 JANUARY 2007VOL 315
on April 28, 2008