The study of Pt@Au electrocatalyst based on Cu underpotential deposition and Pt redox replacement

Article abstract of DOI:10.1016/j.electacta.2008.12.004

Electrochemical study of the decorated Pt@Au catalyst synthesized by Cu underpotential deposition (UPD)-Pt redox replacement technique has been conducted in this work. The parameters affecting the Cu UPD on Au/C nanoparticles in sulfuric acid electrolyte,

Full text of DOI:10.1016/j.electacta.2008.12.004

Electrochimica Acta 54 (2009) 3092–3097
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
The study of Pt@Au electrocatalyst based on Cu underpotential deposition and
Pt redox replacement
a
a
b
b,∗
a,∗
Yaolun Yu , Yueping Hu , Xuewei Liu , Weiqiao Deng , Xin Wang
a
School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore, 639798, Singapore
School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, 639798, Singapore
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 September 2008
Received in revised form
Electrochemical study of the decorated Pt@Au catalyst synthesized by Cu underpotential deposition
UPD)-Pt redox replacement technique has been conducted in this work. The parameters affecting the Cu
(
UPD on Au/C nanoparticles in sulfuric acid electrolyte, including the UPD potential, deposition time and
potential sweep rate, were investigated in detail. Anode stripping method was used to calculate the charge
of the deposited Cu adlayers. Results showed that Pt@Au catalyst prepared by this UPD-redox replacement
approach is not a core-shell structure but a decorated structure. A series of decorated Pt@Au/C catalysts
with various Pt coverages were synthesized and examined for formic acid oxidation (FAO). It is found that
the specific activity of Pt atoms increases with the decrease of Pt surface coverage on Au. Life test showed
that better stability was pertained for this decorated Pt@Au/C catalyst compared to Pt/C towards FAO.
© 2008 Elsevier Ltd. All rights reserved.
2
2 November 2008
Accepted 2 December 2008
Available online 6 December 2008
Keywords:
Underpotential deposition
Pt redox replacement
Fuel cell electrocatalyst
Poly-crystalline gold
Formic acid oxidation
1. Introduction
different reactions [13–16]. Among them, of particular interest is
its application to synthesize Pt based core-shell catalyst for fuel
The phenomenon of underpotential deposition (UPD) has been
discovered and studied for decades. Due to the stronger bond
between the foreign metal adatoms with the metal substrate as
compared to the same type substrate, the UPD foreign metals
would deposit at a potential more positive to the Nernst potential,
where bulk deposition takes place, forming one monolayer or sub-
monolayers onto the substrate surface [1,2]. Such phenomena allow
precise and reproducible control of the amount of the foreign metal
adatoms on the substrate and are suitable for the study of surface
coverage dependence of various systems [3]. As a result, it is very
important in the surface chemistry and would make the researchers
have better understandings on the initial electrodeposition process.
In recent years, Brankovic et al. [4] demonstrated a sponta-
neous irreversible redox replacement process, in which the UPD
cell reactions, with an objective to reduce the usage of Pt. Tang et
al. [15] prepared Pt overlayer/Au/graphite catalyst and showed high
electrocatalytic activity and good long-term stability for methanol
oxidation. Zhai et al. [16] prepared multiple Pt layers on Au sub-
strate by controlling the number of UPD-redox replacement cycles
and studied the influence of the Pt shell thickness to the oxygen
reduction activity.
Although ideally UPD can precisely control the amount of for-
eign metal adlayers on the substrate, there is still something unclear
about the formation of “monolayer” structure on poly-crystalline
nanoparticle substrate by UPD-redox replacement technique. First,
it comes from the difficulty to in-situ observe the morphology of
the UPD as well as the redox replaced adlayer using microscopy. So
far almost all the coupled in-situ researches with microscopy were
carried out using (h, k, l) mono-crystalline surfaces as the substrate
[2,4,6,10,17]. Second, the potential and time which is selected to
form a monolayer in the UPD region is arbitrary. Many researchers
would select an empirical potential just above that where bulk
deposition commences and they claimed that they have made
monolayer core-shell catalyst. Since the UPD can occur in a wide
potential range, the exact UPD potential can have big effect on the
resulting Cu adlayer coverage. To our experience, only several mil-
livolts in the region close to redox potential would greatly change
the resulting Cu adlayer coverage, and no complete monolayer
on nanoparticle substrate can be formed before the overpotential
4+
2−
Cu adlayer is oxidized by more noble metal cations, e.g. Pt , Pd
+
and Ag , and these noble metal cations are reduced and deposited
simultaneously. Finally, they could get a morphology controlled
core-shell structure for catalysis application [5–12]. Since then,
many researchers have applied the UPD-redox replacement tech-
nique to the synthesis of various kinds of catalysts targeting at
∗ Corresponding authors. Tel.: +65 6316 8866; fax: +65 6794 7553.
E-mail addresses: WQDeng@ntu.edu.sg (W. Deng), WangXin@ntu.edu.sg
(
X. Wang).
0
013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2008.12.004

Y. Yu et al. / Electrochimica Acta 54 (2009) 3092–3097
3093
deposition (OPD) occurs. Consequently, this will affect the structure
and catalytic activity of the Pt adlayer resulting from the subse-
quent redox replacement. Therefore, the deposition potential and
time must be chosen carefully. However, to our knowledge, there
are rarely published papers addressing this point by direct electro-
chemical methods and evidences [18].
Here we focus on the UPD behavior of Cu deposited on the as-
prepared poly-crystalline Au/C substrate in sulfuric acid electrolyte
and investigated the parameters affecting the Cu UPD, including
the UPD potential, deposition time and potential sweeping rate.
We use anode stripping method to calculate the oxidation charge
associated with the UPD Cu. By comparing the UPD oxidation
charge with the reduction charge of the monolayer Au oxide of
the Au/C substrate, we can get the direct information between the
applied potential and the surface coverage. Furthermore, a nom-
inal Pt@Au/C “core-shell” structured catalyst was synthesized by
redox replacement of the UPD Cu with K PtCl . Since our pre-
2
4
vious research showed that Pt-decorated Au/C catalyst possessed
different electrochemical characteristics than core-shell structured
Pt@Au/C [17], we verified that the as-prepared Pt@Au/C was not
a perfect core-shell structure catalyst but a decorated one by
formic acid oxidation (FAO). Based on that, by simply varying the
UPD potential of Cu on Au/C, a series of decorated Pt@Au/C cat-
alyst with different Pt coverage on Au/C were synthesized and
their reactivity towards FAO were tested. It is noteworthy that
our Pt@Au/C with 3% Pt coverage has a comparative activity with
the 75% coverage sample. Bearing in mind the principle of min-
imizing Pt usage, the activity of the decorated Pt@Au/C with 3%
Pt coverage was 20 folds more active compared to Pt/C towards
FAO if the data were to be normalized by Pt usage. Finally, life
test were conducted, results showed that the decorated Pt@Au/C
catalyst had better stability towards FAO compared to Pt/C. The
cause of the declining activity during life test was also briefly dis-
cussed.
Fig. 1. Voltammetry curve for the underpotential deposition of Cu on the Au/C
substrate in 0.1 M sulfuric acid with 1 mM CuSO4, sweep rate 10 mV/s. Inset: Voltam-
metry curve for the as-prepared 20 wt%. Au/C substrate in the absence of Cu2+ in
0.5 M sulfuric acid, sweep rate 100 mV/s.
electrode was mounted and immersed into the Argon (ultrapure,
SOXAL) deaerated electrolyte for the electrochemical tests.
UPD tests were conducted in 1 mM CuSO + 0.1 M H SO elec-
4
2
4
trolyte. A typical procedure of the anode stripping test was as
follows: First, the electrode was held at 0.65 V for 20 s to ensure
that there was no copper on the surface, and then the potential
was swept at 400 mV/s to the desired UPD potential. After certain
minutes of holding time, the potential of the electrode was swept
back to 0.65 V and maintained there for 60 s. Various UPD potential,
deposition time and sweep rate were used to identify their effect
on the resulting UPD charge.
When doing the redox replacement reaction, after holding the
electrode at UPD potential in CuSO + H SO electrolyte solution for
2. Experimental
4
2
4
3
min, the electrode was quickly taken out of the copper electrolyte
with potential control (same as the deposition potential to avoid
oxidation of deposited Cu) and put into the Argon deaerated 5 mM
2.1. Preparation of nanocrystalline Au/C
K PtCl + 0.05 M H SO solution for an arbitrary time of 10 min.
Then the electrode was washed with copious ultrapure water for
further tests.
The method to prepare Au/C substrate can be found elsewhere
2
4
2
4
[
19,20]. HAuCl ·3H O and NaBH was used as the gold precursor
4
2
4
and reducing agent, respectively. As-received XC-72R carbon black
was used as the support.
FAO tests were conducted in deaerated 0.5 M HCOOH + 0.5 M
H SO electrolyte at 1000 rpm rotation speed using Princeton Ring-
The metal loading was 20 wt%. The particle size was around
.7 nm by XRD and verified by TEM (results not shown here).
2
4
Disk Electrode System (Model 636).
3
2.2. Electrochemical test
3. Results and discussion
Glassy carbon disk (PINE, USA, 5 mm diameter) was used as
3.1. UPD studies
the working electrode; Ag/AgCl electrode (+0.197 V vs. NHE) and
Pt foil were used as the reference electrode and counter electrode,
respectively. All the electrochemical tests were conducted at room
temperature using Autolab PGSTAT30 potentiostat (Eco Chemie,
The Netherlands). The electrode potential was reported against
Ag/AgCl electrode. CuSO ·5H O (99.995%) and K PtCl (99.99%) are
A typical CV curve for poly-crystalline Au could be clearly seen
by the inset of Fig. 1. Burshtein method was adopted to calculate
the surface area of Au/C substrate [22,23]. Normally, 1.35–1.40 V
was selected as the Burshtein Minimum potential. By integrating
the reduction peak at 940 mV (normally from 600 to 1150 mV), the
reduction charge of Au oxide monolayer was calculated. By using
the suggested value of 400 ␮C/cm2 for poly-crystalline Au, the real
electrochemical surface area of Au/C could then be calculated.
The cyclic voltammogram for Cu UPD on poly-Au (Fig. 1) was
similar to those in the literatures for Cu UPD on Au(1 1 1) [13,24].
4
2
2
4
procured from Sigma–Aldrich; the electrolytes were prepared from
as-received sulfuric acid (98%, Merck) and ultrapure water from
Millipore MilliQ Plus system (∼18.1 Mꢀ).
Porous film method [21] was adopted to prepare the elec-
trode. The catalyst slurry was prepared by supersonically dispersing
ꢀ
2.0 mg catalyst in 1.0 mL ethanol for 10 min, followed by the addi-
Three peak pairs were observed. Peak pair A and A around 0.25 V
ꢀ
tion of 0.5 mL Nafion solution (GasHub, 5 wt% isopropanol/water
suspension) with another 20 min of supersonication. Then 15 ␮L
of the homogenous suspension was dropped onto glassy carbon
disk using microsyringe (Hamilton, USA). After drying in air, the
and C and C around 0 V stood for the UPD and OPD main peak
ꢀ
pair, respectively. The small peak pair B and B around 0.1 V was
probably due to the surface structure transferring process for Cu
UPD on Au(1 1 1) in sulfuric acid electrolyte [25].

3
094
Y. Yu et al. / Electrochimica Acta 54 (2009) 3092–3097
was clean and free of deposited Cu. It is reasonable to assume that
the effect of the pseudo double layer from the Au substrate was
the same throughout the sweeping process and the current at the
end of the sweeping process would be the current from the pseudo
double layer capacitance only. The oxidation charge of Cu adlayer
was then calculated by integrating the current after subtract the
current from the double layer capacitance, as the hatched area in
Fig. 2(a).
There was no doubt that the deposition potential and time were
the key factors for the UPD process, while the oxidation sweep rate
would also affect the calculation of the anode stripping charge of
the deposited adlayers. Fig. 2(b) shows the calculated oxidization
charge from the deposited Cu adlayer vs. the potential sweep rate
at different deposition potentials ranging from the pure UPD region
(e.g. 0.20 V) to the OPD dominated region (e.g. 0 V). It can be noticed
that the oxidation charge from Cu adlayer depends on the sweep
rate. From 200 to 800 mV/s, the calculated charge was stable, while
when the scan rate went outside this range, the calculated charge
would become smaller, especially when the scan rate was lower
than 100 mV/s. When the sweep rate was too slow, due to the pro-
longed sweep period, more proportion of oxidation charge could
not be counted in when the transient current was smaller than the
double layer current. On the contrary, if the sweep rate was too fast,
the time for the transient current to decay to the real current from
the pseudo double layer capacitance might be too short. Both of
the situations would lead to the underestimation of the calculated
charge.
In order to prove our assumption that the current at the end
of sweeping process would be the current from the pseudo double
layer capacitance only, the charging current at the end of the poten-
tial sweeping process for the case of UPD at 0.10 V is also shown in
Fig. 2(b) as a dashed line. The current vs. sweep rate was a straight
line with a linear regression coefficient of 0.99862, indicating that
our assumption was well applicable.
Fig. 2(c) shows the effect of the deposition time on the oxidation
charges at different deposition potentials. After about 60 s when the
UPD process got saturated, there were no big differences in the oxi-
dation charges as time progressed when the deposition potential
was within UPD region (0.20–0.05 V). On the other hand, depo-
sition time would affect the oxidation charge greatly in the OPD
range, because Cu OPD is not a self-terminating process and allows
multilayer deposition.
Throughout this work, we took 400 mV/s and 180 s as the opti-
mized oxidation sweep rate and deposition time.
The IV curves during the oxidation process are shown in Fig. 3(a).
Only one oxidation peak could be seen when the deposition poten-
tial was higher than 0.23 V. There gradually formed a second
shoulder peak when the potential was lowered. Since at this poten-
tial, OPD could not occur yet; then the shoulder peak might only
reflect the structural change of the Cu UPD adlayer [2]. With the
further decrease of potential, the UPD shoulder peak overlapped
with the OPD oxidation peak and finally exceeded the main UPD
peak (as for the case of −0.002 V) in Fig. 3(a)).
Fig. 2. (a) Current transient during the anodic stripping process; deposition poten-
tial 0.25 V, sweep rate 400 mV/s. (b) The effect of the sweep rate on the oxidation
charge of the Cu adlayer under different deposition potentials. The deposition time
is fixed at 180 s. The current at the end of the oxidation sweep process for the case
of 0.10 V was also shown as the dashed line. (c) Oxidation charge of the Cu adlayer
vs. deposition time at different deposition potentials.
Fig. 3(b) shows the oxidation charge of Cu deposited on Au under
different potentials. The monotonic increase of the oxidation charge
with the decrease of deposition potential meant that there was
no distinct point which separated UPD and OPD regions apart;
instead, a combined region (from 0.005 to 0.015 V) set between
the UPD dominant region (above 0.02 V) and the OPD dominant
region (below 0 V). We could also get the same conclusion from
charge calculation. In this particular case, the reduction charge of
the Au surface oxide monolayer was 6.873e-4 C, while the oxida-
tion charge of the UPD dominant region at the lowest potential of
0.02 V was 4.71827e-4 C, only about 70% surface coverage at this
deposition potential. (Theoretically, the charge of a Cu monolayer
on Au electrode should be the same as the reduction charge of
In this paper, we adopted the Steps & Sweep technique and used
anode stripping method to calculate UPD adlayer charge.
After the UPD process, anode stripping was realized by sweeping
the electrode potential at constant rate to the terminal potential of
0.65 V. A typical anode stripping result is shown in Fig. 2(a). It can
be seen that the transient current reached the maximum at the very
beginning of the oxidation process; it decayed quickly and formed
a flat roof at the end of the sweeping process. The holding potential
at 0.65 V was high enough to ensure that the electrode substrate

Y. Yu et al. / Electrochimica Acta 54 (2009) 3092–3097
3095
decorated catalysts with different surface coverages by fine tuning
the deposition potential during the UPD process.
Here we focused on the decorated Pt@Au/C catalyst for FAO
reaction. Recently, we reported that Pt-decorated Au/C catalyst syn-
thesized by a two-step chemical approach had enhanced activity
towards FAO [19]. This enhanced activity is believed to result from
the ensemble effect that a single Pt atom would have much higher
activity compared to the bulk Pt clusters [27]. In this work, we
adopted the UPD-redox replacement technique, to get the deco-
rated Pt@Au/C catalysts with controlled Pt coverages and to test
their activities towards FAO. The nominal Pt coverage was calcu-
lated as the ratio of the charge from Cu adlayer oxidation and that
of the Au oxide reduction. Here, we assume that the efficiency of
the redox replacement reaction of Cu atoms by Pt ions was 100%.
During these experiments, the current density could not be nor-
malized by the hydrogen desorption peak charge from the CV in
the sulfuric acid electrolyte, because it is not accurate anymore to
calculate the electrode surface area by Pt alone for these decorated
bi-metal catalysts. Instead, the current density was normalized by
the Au electrode surface areas.
Fig. 4 shows the forward voltammograms of the FAO on a
series of decorated Pt@Au/C with different surface coverages.
The inset of Fig. 4 shows the CV curves in blank 0.5 M H SO .
2
4
A gradually decreased Pt surface area with the decrease of the
nominal Pt coverage could be clearly seen through the hydrogen
adsorption/desorption area, which indicates the success on con-
trolling the amount of Pt on Au/C by using UPD-redox replacement
technique.
As could be seen in Fig. 4, the second anodic dehydration peak
around 0.75 V decreased with the lowering of the total Pt amount,
and finally diminished when the Pt coverage was below 34%, sug-
gesting that oxidation of the formic acid would be shifted to the
dehydrogenation branch to form CO while CO poisoning was grad-
2
ually suppressed with the lowered Pt coverage. One thing should be
noticed that even for the minimum coverage of Pt (3%) on Au in this
study, the forward scan peak current density reaches 68% of that
of the 75% coverage sample (the sample with the highest activity).
This meant that much higher activity per Pt atom towards formic
acid oxidation could be achieved with only a little amount of Pt. To
quantify the specific activity of Pt atoms towards formic acid oxi-
dation, the measured CV peak current at 0.3 V was also normalized
by the total number of Pt atoms (Au itself has negligible activity)
and the results are shown in Table 1. It was found that the specific
activity of Pt atoms increased with the decreased of Pt surface cov-
erage. The 3% coverage Pt@Au/C sample showed 17.0 and 23.9 folds
Fig. 3. (a) IV curves for anode stripping process at different deposition potentials. (b)
Oxidation charge of the Cu adlayer obtained under different deposition potentials.
Au surface oxide monolayer, if both of the reactions were ideally
two electron transfer reactions.) This meant that no ideal mono-
layer of Cu can be formed on Au nanoparticle substrate by UPD
only.
There were several possible reasons for the partial covered struc-
ture. First, the atomic size difference between Cu and Au may
prevent the formation of a complete monolayer. Dimitrov and co-
workers [18,26] pointed out that it was theoretically impossible to
deposit a real monolayer of Cu per one “building block” reaction.
Second, the co-adsorbed ions from the supporting electrolyte or
−
the trace amount of Cl ions contamination from the reference elec-
trode might influence the structure of the Cu UPD adlayer, because
halogen ions strongly absorbed on Au surface and weakened the
Cu-Au bond [17]. Recently, Martinez-Ruiz et al. [24] reported that
on Iodine modified Au(1 1 1) substrate in sulfuric acid electrolyte,
Cu could not form a full monolayer before OPD took place. Third,
the morphology effect of the porous Au/C substrate might also play
a role. From the reduction charge of the Au oxide and the reported
2
value of 400 ␮C/cm , the surface roughness factor of the Au/C elec-
trode prepared by the porous film method was calculated at the
range of 6–10. Some of the Au atoms on the rough surface might
not be accessible by the Cu ions.
3.2. Decorated Pt@Au/C
Fig. 4. Voltammogram of as-prepared Pt@Au/C with different Pt coverages in 0.5 M
formic acid and 0.5 M H2SO4, sweep rate 100 mV/s, rotation speed 1000 rpm. The
current density is normalized with the Au electrode surface area.
Although we cannot make a real monolayer of Cu on Au, the
study of Cu UPD reminded us that we could synthesize a series of

3
096
Y. Yu et al. / Electrochimica Acta 54 (2009) 3092–3097
Table 1
Normalized peak current for formic acid oxidation on Pt atoms.
Nominal Pt coverage (%)
Peak current per Pt atom
100
0.4335
75
0.611
34
1.225
14
2.189
3
10.373
−
18
(
10
A/atom)
more active towards FAO compared to the 75 and 100% coverage
samples, respectively.
Since the decorated Pt@Au/C catalyst has much lower tendency
towards CO poisoning, it may have better stability during the FAO
life test. However, on the other hand, it is well known that Pt/Au
system is strongly segregated and Pt is easily covered by Au, which
may mark the end of the catalyst. The overall durability of the dec-
orated catalyst depends on both factors. To get more information
about the durability, the short-term stability of the catalyst towards
FAO was investigated by two approaches and the results were com-
pared with 20 wt%. Pt/C sample made by EG method (particle size
around 4 nm, the synthesis method could be found elsewhere [28]).
First, a prolonged CV tests up to 2000 cycles were conducted. As
could be seen in Fig. 5, the activity towards FAO of the Pt/C catalyst
always decreased as the test continued; while for the 3% coverage
Pt@Au/C sample, before 200 cycles of CV, the activity did not change
too much (it actually slightly increased). After 200 cycles, the activ-
ity started to decrease with a very fast declining rate. Nevertheless,
the activity of 3% Pt@Au/C at the end of 2000 cycles is still bet-
ter than 20% Pt/C. The CV results in blank 0.5 M H SO4 electrolyte
2
Fig. 6. CV results of (a) 3% Pt@Au/C (b) Pt/C in 0.5 M H2SO4 before and after prolonged
CV test. Sweep rate 100 mV/s, rotation speed 1000 rpm.
before and after the prolonged CV tests are shown in Fig. 6. The
active surface area of both catalysts showed a continuous loss. The
activity decline of the catalysts was mainly due to the active sur-
face area loss of Pt. For the decorated catalyst, it is surprising that
the activity did not decline although the Pt surface area dropped
significantly from 0 to 200 cycles. As evidenced by the negligible
hydrogen desorption area, most of the active surface area was lost
after 500 cycles, probably caused by the strong segregation of Pt/Au
system or the dissolution of Pt. However, at such low Pt loading, it
might not be accurate to quantify the Pt amount using the hydro-
gen desorption area. Indeed, the Pt oxide reduction peak was still
detectable after 500 cycles and showed a negative shift in peak
position. This negative shift could be explained by the upper shift
in the Pt d-band center, which leads to a higher adsorption energy
of adsorbate, oxygen containing species in this case [29].
Chronoamperometric experiments were also conducted at the
potential of 0.15 V and the results are shown in Fig. 7. The grad-
ual current decay was due to both the Pt surface area loss and CO
poisoning effect. The activity loss due to surface area loss is perma-
nent and could not be resumed anymore. To quantify the effect due
to CO poisoning, an electrode cleaning procedure was introduced
during the chronoamperometric experiment, where the electrode
was first held at 0.9 V for 10 s, then at −0.2 V and 0.75 V for 1 s to
clean the catalyst surface. After that, the electrode was held at the
life test potential of 0.15 V for another 20 min. The short life test
duty cycle was repeated twice to see whether the reactivity could
be resumed. As could be seen, the current decreased quite fast in
the very beginning of the CA test, while it could be resumed after
the surface cleaning procedure for both catalysts. However, the 3%
Fig. 5. Prolonged CV results of (a) 3% Pt@Au/C (b) Pt/C in 0.5 M formic acid and 0.5 M
H2SO4, sweep rate 100 mV/s, rotation speed 1000 rpm. Only the forward scans were
shown.

Y. Yu et al. / Electrochimica Acta 54 (2009) 3092–3097
3097
the surface, electrochemically available and possessing high spe-
cific activity, this catalyst is promising to be used in formic acid fuel
cell applications.
Acknowledgements
This work is supported by Academic research fund AcRF tier
1
(RG40/05) and AcRF tier 2 (ARC11/06), Ministry of Education, Sin-
gapore.
References
[
[
1] D.M. Kolb, Advances in Electrochemistry and Electrochemcial Engineering,
Wiley-Interscience, 1978.
2] O.M. Magnussen, J. Hotlos, G. Bettel, D.M. Kolb, R.J. Behm, Journal of Vacuum
Science & Technology B 9 (1991) 969.
[
[
[
3] E. Herrero, L.J. Buller, H.D. Abruna, Chemical Reviews 101 (2001) 1897.
4] S.R. Brankovic, J.X. Wang, R.R. Adzic, Surface Science 474 (2001) L173.
5] J.L. Zhang, M.B. Vukmirovic, Y. Xu, M. Mavrikakis, R.R. Adzic, Angewandte
Chemie – International Edition 44 (2005) 2132.
6] J. Zhang, Y. Mo, M.B. Vukmirovic, R. Klie, K. Sasaki, R.R. Adzic, Journal of Physical
Chemistry B 108 (2004) 10955.
7] J. Zhang, F.H.B. Lima, M.H. Shao, K. Sasaki, J.X. Wang, J. Hanson, R.R. Adzic, Journal
of Physical Chemistry B 109 (2005) 22701.
8] J.L. Zhang, M.B. Vukmirovic, K. Sasaki, A.U. Nilekar, M. Mavrikakis, R.R. Adzic,
Journal of the American Chemical Society 127 (2005) 12480.
Fig. 7. Life test of 3% Pt@Au/C and Pt/C in 0.5 M formic acid and 0.5 M H2SO4 under-
potential of 0.15 V, rotation speed 1000 rpm. Current densities are normalized by the
calculated electrochemical surface area of Pt.
[
[
[
coverage of Pt@Au/C sample had much lowered declining rate com-
pared to the Pt/C catalyst. For Pt/C catalyst, it would suffer from the
CO poisoning and cause activity decline from the very beginning
of the test. While for the decorated Pt@Au/C sample, it had much
less CO-like species formed in the beginning and the accumulation
of CO is also much slower. The much alleviated CO poisoning effect
makes the decorated Pt@Au/C sample more capable to catalyze FAO
compared to Pt/C catalyst.
[9] J. Zhang, K. Sasaki, E. Sutter, R.R. Adzic, Science 315 (2007) 220.
10] M.B. Vukmirovic, J. Zhang, K. Sasaki, A.U. Nilekar, F. Uribe, M. Mavrikakis, R.R.
Adzic, Electrochimica Acta 52 (2007) 2257.
11] R.R. Adzic, J. Zhang, K. Sasaki, M.B. Vukmirovic, M. Shao, J.X. Wang, A.U. Nilekar,
M. Mavrikakis, J.A. Valerio, F. Uribe, Topics in Catalysis 46 (2007) 249.
12] K. Sasaki, Y. Mo, J.X. Wang, M. Balasubramanian, F. Uribe, J. McBreen, R.R. Adzic,
Electrochimica Acta 48 (2003) 3841.
13] M.F. Mrozek, Y. Xie, M.J. Weaver, Analytical Chemistry 73 (2001) 5953.
[
[
[
[
[
[
14] Y.D. Jin, Y. Shen, S.J. Dong, Journal of Physical Chemistry B 108 (2004) 8142.
15] H. Tang, J.H. Chen, M.Y. Wang, L.H. Nie, Y.F. Kuang, S.Z. Yao, Applied Catalysis A
4
. Conclusions
–
General 275 (2004) 43.
[
16] J.F. Zhai, M.H. Huang, S.J. Dong, Electroanalysis 19 (2007) 506.
The charge of the Cu adlayer deposited on the as-prepared poly-
[17] S. Manne, P.K. Hansma, J. Massie, V.B. Elings, A.A. Gewirth, Science 251 (1991)
83.
18] L.T. Viyannalage, R. Vasilic, N. Dimitrov, Journal of Physical Chemistry C 111
2007) 4036.
1
crystalline Au/C nanoparticles was calculated by anode stripping
method. Results showed that under the current experiment condi-
tion, i.e. in sulfuric acid supporting electrolyte and using the porous
film electrode, we could not get an ideal fully covered monolayer
of Cu adlayer underpotentially deposited on Au/C nanoparticles
before OPD started.
Based on that, various decorated Pt@Au/C catalyst with tun-
able Pt coverage were synthesized by using UPD-redox replacement
technique. The decorated Pt@Au/C catalyst showed much higher
activity towards formic acid oxidation than that of Pt/C. Particu-
larly, the highest specific activity was obtained in the case of the
minimum Pt coverage (3% in this study). Moreover, the decorated
Pt@Au/C catalyst showed better stability than the Pt/C towards FAO
due to the suppressed CO poisoning species formed on Pt. The
apparent advantage of the catalyst prepared by this approach is
the significant reduced Pt usage. With all the Pt atoms existing on
[
(
[19] N. Kristian, Y.S. Yan, X. Wang, Chemical Communications (2008) 353.
[20] N. Kristian, X. Wang, Electrochemistry Communications 10 (2008) 12.
[
21] G. Tamizhmani, J.P. Dodelet, D. Guay, Journal of the Electrochemical Society 143
1996) 18.
(
[
[
[
[
[
22] H. Angersteinkozlowska, B.E. Conway, A. Hamelin, L. Stoicoviciu, Journal of Elec-
troanalytical Chemistry 228 (1987) 429.
23] A.A. Michri, A.G. Pshchenichnikov, R.K. Burshtein, Soviet Electrochemistry 8
(
1972) 351.
24] A. Martinez-Ruiz, M. Palomar-Pardave, N. Batina, Electrochimica Acta 53 (2008)
2115.
25] M.H. Holzle, U. Retter, D.M. Kolb, Journal of Electroanalytical Chemistry 371
(
1994) 101.
26] R. Vasilic, N. Vasiljevic, N. Dimitrov, Journal of Electroanalytical Chemistry 580
(2005) 203.
[
27] S. Park, Y. Xie, M.J. Weaver, Langmuir 18 (2002) 5792.
[
28] Z.H. Zhou, S.L. Wang, W.J. Zhou, G.X. Wang, L.H. Jiang, W.Z. Li, S.Q. Song, J.G. Liu,
G.Q. Sun, Q. Xin, Chemical Communications (2003) 394.
[29] B. Hammer, J.K. Nørskov, Advances in Catalysis 45 (2000) 71.