Electrochemical characterization of platinum-ruthenium nanoparticles prepared by water-in-oil microemulsion

Article abstract of DOI:10.1016/j.electacta.2004.06.017

The synthesis, physical characterization, decontamination and some electrocatalytic properties of PtRu nanoparticles prepared using the microemulsion method are reported. The nanoparticles are synthesized by reduction with sodium borohydride of H₂PtCl₆ and RuCl ₃ in a water-in-oil microemulsion of water/polyethylenglycol- dodecylether (BRIJ 30)/n-heptane. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and energy dispersive analysis by X-rays (EDAX) experiments were carried out to characterize the single and bimetallic nanoparticles obtained. Cyclic voltammograms (CV) of clean nanoparticles were obtained after a controlled decontamination procedure of their surfaces. CO adsorption-oxidation and methanol electrooxidation were used as test reactions to check the electrocatalytic behaviour of the bimetallic nanoparticles. Pt ₈₀Ru₂₀ (nominal atomic composition) nanoparticles are the best electrocatalyst for both CO_ad and methanol oxidation. All these results show that the microemulsion method can be used to produce bimetallic nanoparticles in a very easy way. The method can be very easily scaled-up for industrial use.

Full text of DOI:10.1016/j.electacta.2004.06.017

Electrochimica Acta 49 (2004) 5079–5088
www.elsevier.com/locate/electacta
Electrochemical characterization of platinum–ruthenium
nanoparticles prepared by water-in-oil microemulsion
1
J. Solla-Gull o´ n , F.J. Vidal-Iglesias, V. Montiel , A. Aldaz
1
*
Departamento de Qu ´ı mica-F ´ı sica, Universidad de Alicante, Instituto Universitario de Electroqu ´ı mica, Apartado 99, 03080 Alicante, Spain
Received 3 March 2004; received in revised form 14 May 2004; accepted 10 June 2004
Available online 17 July 2004
Abstract
The synthesis, physical characterization, decontamination and some electrocatalytic properties of PtRu nanoparticles prepared using the
microemulsion method are reported. The nanoparticles are synthesized by reduction with sodium borohydride of H PtCl and RuCl in a
2
6
3
1
water-in-oil microemulsion of water/polyethylenglycol-dodecylether (BRIJ 30)/n-heptane. X-ray diffraction (XRD), X-ray photoelectron
spectroscopy (XPS), transmission electron microscopy (TEM) and energy dispersive analysis by X-rays (EDAX) experiments were carried
out to characterize the single and bimetallic nanoparticles obtained. Cyclic voltammograms (CV) of clean nanoparticles were obtained after a
controlled decontamination procedure of their surfaces. CO adsorption–oxidation and methanol electrooxidation were used as test reactions to
check the electrocatalytic behaviour of the bimetallic nanoparticles. Pt80Ru20 (nominal atomic composition) nanoparticles are the best
electrocatalyst for both COad and methanol oxidation. All these results show that the microemulsion method can be used to produce bimetallic
nanoparticles in a very easy way. The method can be very easily scaled-up for industrial use.
ß 2004 Elsevier Ltd. All rights reserved.
Keywords: Nanoparticles; Microemulsion; Pt-Ru catalyst; Methanol oxidation; Fuel cells
1
. Introduction
that the so-called ligand or electronic effect might also be
involved in the process [3,4]. In both cases, the catalyst
composition and specially its surface composition should
play a fundamental role on its electrocatalytic behaviour.
Thus, although this fact has been demonstrated for CO and
methanol oxidation in model studies on different PtRu
electrodes, —massive PtRu alloys, PtRu surface alloys,
Ru modified Pt electrodes and high surface area carbon
supported and unsupported PtRu catalysts [5–12]—, in the
case of high surface area catalysts (nanoparticles), systema-
tic studies of the influence of the atomic composition are still
in continuous progress.
Despite the search of new catalytic materials for fuel cell
systems, PtRu alloys still remain the best choice as anode
catalyst for low-temperature polymer electrolyte fuel cells
(
PEFC) both for PEFCs operated by H -rich reformate or for
2
PEFCs driven by direct oxidation of methanol (DMFC).
Anyway, and in both cases, the main problem is related to the
gradual poisoning of the catalyst by CO due to the presence
of CO trace impurities in the reformate or by CO formation
during the methanol dehydrogenation. The superior activity
of PtRu against other catalytic materials has been explained
by a bifunctional mechanism where the oxidation of
adsorbed CO is facilitated by the presence of –OHad species
formed at low anodic overpotentials on oxophylic surface
atoms such as Ru [1,2]. In addition, it should be pointed out
Nanoparticles are very adequate as electrocatalysts for
fuel cell and recent years have witnessed an increasing
interest on new synthesis methods that could be easily
translated to applied scale. Since the platinum-group metal
nanoparticles were synthesized in a water-in-oil microemul-
sion by Boutonnet et al. [13], the microemulsion technique
has been used to prepare a great number of nanomaterials
such as pure metals [13,14], alloys [14–16], metal borides
*
Corresponding author. Fax: +34 965 903537.
E-mail address: jose.solla@ua.es (J. Solla-Gull o´ n).
1
ISE member.
0013-4686/$ – see front matter ß 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2004.06.017

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J. Solla-Gull o´ n et al. / Electrochimica Acta 49 (2004) 5079–5088
[
17], metal oxides and hydroxides [18–23], organic poly-
(1253.6 eV) source. Spectra were obtained with a constant
pass energy of 50 eV. The pressure in the analysis chamber of
mers [24], etc. However and in spite of the fact that it is a
very easy and reproducible technique, very few works have
been devoted to the use of this method to synthesize
nanoparticles to be used as electrocatalyst [25–27]. From
an electrochemical point of view, the key problem of the
synthesis of nanoparticles in microemulsion is that the
nanoparticles obtained are coated with a film of surfactant
molecules that blocks the surface sites, modifying their
surface properties and particularly their electrocatalytic
properties. For that reason, in previous works, we have
developed some procedures to clean, without surface
damage or composition changes, Pt, Pd and Pt/Pd alloy
nanoparticles prepared with this methodology [25,28–30].
The aim of this paper is to show that the microemulsion
technique allows the fabrication of bimetallic nanoparticles
of Pt/Ru in a very easy way and with similar electrocatalytic
activity to that obtained using more elaborated methods.
Moreover, this method can be very easily scaled-up for
industrial applications. Thus, in this paper we present recent
results about the synthesis, physical characterization, decon-
tamination and some electrocatalytic properties of PtRu
nanoparticles prepared in microemulsion.
À10
the spectrometer was 5 Â 10
mbar during the measure-
ments. The BE values were obtained using a Peak-fit Program.
Transmission electron microscopy (TEM) experiments were
performed with a JEOL, JEM-2010 microscope working at
200 kV. The samplefor TEManalysiswas obtainedbyplacing
a drop of the dispersed solution onto a Formvar-covered
copper grid and evaporating it in air at room temperature.
The images were analysed with the analySIS 3.0 program. For
each sample, usually over more than 300 particles from
different parts of the grid were used to estimate the mean
diameter and size distribution of the nanoparticles. Energy
dispersive analysis by X-rays (EDAX) was obtained with a
OXFORD, INCA model system.
2.2. Preparation of the electrodes and electrochemical
measurements
The procedure used for the electrochemical study has
been previously reported [25,28–30]. Briefly, as a current
collector a polycrystalline gold disc electrode (5 mm dia-
meter) on which nanoparticles were deposited was used.
Metal nanoparticles were transferred to the gold collector by
depositing a drop (generally 5–10 mL) of the nanoparticle
water suspension on the surface of the gold disc. The counter
electrode was a gold wire. Potentials were measured against
a reversible hydrogen electrode (RHE) connected to the cell
through a Luggin capillary. Potentials are quoted versus this
reference electrode. All the electrochemical measurements
were performed in a 0.5 M H SO solution at room tem-
2
. Experimental
2
.1. Preparation and characterization of nanoparticles
Platinum and ruthenium nanoparticles were obtained by
reduction of either H PtCl or RuCl with sodium borohydride
2
6
3
2
4
1
using a w/o microemulsion of water/polyethylenglycol-dode-
1
perature. Electrolyte solutions were prepared with Milli-Q
cylether(BRIJ 30)/n-heptane. Thesizeofthemicroemulsion
droplet is determined by the molar ratio of water to surfactant
water and Merck ‘‘suprapur’’ sulphuric acid every day an
experiment was carried out. Solutions were de-oxygenated
by bubbling Ar during 20 min. The electrode potential was
controlled using a PGSTAT30 AUTOLAB system. CO
adsorption on the electrodes was carried out by bubbling
CO gas for 2 min at 0.03 V. After that, CO was removed
from the solution by bubbling argon for 10 min. Before each
experiment, the gold collector was mechanically polished
with alumina and rinsed with ultra-pure water to eliminate
the nanoparticles from previous experiments. Methanol
oxidation voltammograms and chronoamperometric experi-
ments were carried out in a 0.5 M H SO + 0.1 M MeOH
(v0). The concentration of surfactant, in volume, amounts in
this work to 16.5% of the total microemulsion volume. The
concentrations of H PtCl and RuCl solutions employed for
2
6
3
the synthesis were 0.1 M. For the preparation of alloyed
nanoparticles, an aqueous solution of H PtCl + RuCl with
2
6
3
the convenient atomic proportion of the two elements was
employed. In order to have micelles with the same size, i.e.
nanoparticles within the same size range, the molar ratiowater
to surfactant was maintained constant (v = 3.8). The reducing
0
agent (NaBH ) was added to the micellar solution as solid.
4
2
4
After complete reduction that takes place in a few minutes,
acetone was added to the solution to cause phase separation.
The precipitate formed by the metallic nanoparticles was
washed several times with acetone in order to eliminate
surfactant molecules and finally kept in ultra-pure water as
a suspension.
solution at room temperature. The methanol electrooxida-
tion currents were normalized to the total moles of Pt + Ru
used on the experiments. The methodology employed was
similar to that previously reported in reference [30]. Very
briefly, the metal loading was determined from the concen-
tration of the original solution containing the nanoparticles
which was calculated using ICP analysis. Moreover and in a
similar way that in reference [30], the stability of the
nanoparticles deposited was analyzed by plotting the
adsorption charge of the nanoparticles versus volume added.
The linearity found is the proof of this stability (see refer-
ence [30] for the case of PtPd nanoparticles).
X-ray diffraction (XRD) patterns were obtained with a
Phillips PW1710 using a Cu Ka source. The angular resolu-
tion in the 2u-scans was 0.058. The sample for X-ray analysis
was prepared after phase separation and washed with acetone.
X-ray photoelectron spectroscopy (XPS) measurements were
performed with a VG-Microtech Multilab, using a Mg Ka

J. Solla-Gull o´ n et al. / Electrochimica Acta 49 (2004) 5079–5088
5081
Fig. 1. XRD patterns of Pt/Ru nanoparticles.
3
. Results and dicussion
that only part of the Ru is present as PtRu alloy remaining the
other part in an amorphous state. No presence of hexagonal
structure has been detected. Despite this loss of crystallinity, a
linear relationship between lattice parameter, calculated with
Vegard’s law, and bulk composition is found with Ru con-
centrationsoflessthan60 at.%accordingtothephasediagram
for Pt–Ru alloys, Fig. 2. Watanabe et al. [32] and Gasteiger et
al. [33] have previously reported similar results.
3
.1. XRD, XPS and TEM/EDAX analyses
Cu Ka X-ray diffraction patterns of PtRu nanoparticles are
shownin Fig. 1. InthecaseofpurePt, thediffraction peaks ofa
face centered cubic structure can be observed. For PtRu
nanoparticles, a shift to higher 2u values and an increase of
peak broadening, with respect to the diffraction lines of pure
Pt, were observed as the nominal content of Ru increases,
indicating a decrease of both lattice parameter and mean
particle size [31]. However a gradual loss of signal is clearly
detected as the amount of Ru increases, which could indicate
To get information on surface composition and oxidation
states of Pt and Ru, a XPS analysis was carried out. As an
example, the Pt 4f spectrum obtained for Pt Ru nano-
5
0
50
particles is reported in Fig. 3a. The Pt 4f signal in all the
samples derives from the contribution of a single doublet
Fig. 2. Lattice parameter for Pt/Ru nanoparticles deduced from XRD.

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J. Solla-Gull o´ n et al. / Electrochimica Acta 49 (2004) 5079–5088
Fig. 3. XPS spectra for Pt50Ru50 nanoparticles.
except for the case of Pt Ru (nominal) where a small
8
sensitivity factor of Pt and Ru. As it can be observed, the
calculated atomic compositions are very close to the nom-
inal composition of the nanoparticles. These results show the
effective control on the nanoparticle composition. In this
point, it is important to note that XPS measurements are
generally used to determine the surface composition. How-
ever, in electrocatalyst this XPS surface concentration cor-
responds to a thicker layer than that that takes part in the
electrochemical reaction. Thus, these XPS experiments
could only give us some indications about the surface
composition.
0
20
contribution of a second doublet is also observed. The Pt 4f7/
line occurs at a binding energy (BE) of 71.4 Æ 0.3 eV
2
assigned to Pt(0). In the case of Pt Ru (nominal), the
8
0
20
second doublet occurs at BE of 72.7 Æ 0.3 eV assigned to
Pt(II) species. The relative concentration of this PtO is
around 18%. Very small differences are observed in terms
of BE as a function of Ru concentration, suggesting that
there is no strong modification of the electronic environment
of Pt surface site with the change of the composition of the
nanoparticles.
Despite the interference of C1s signal, the Ru species
were studied by analysing the Ru 3d5/2 line. The spectrum
corresponding to Pt Ru nanoparticles is shown in Fig.
TEM/EDAX analyses were performed to measure both
particle size and atomic composition. Fig. 4 shows the mean
particle size in the whole range of Pt and Ru concentrations.
In all cases an average particle size of around 4 Æ 1 nm was
5
0
50
3
b. In all the samples, Ru 3d5/2 derives from the contribu-
tion of two doublets at different binding energies. The
highest BE peak (282.3 Æ 0.3 eV) is related to oxidized
Ru(IV) species, whereas the lowest BE species (280.8 Æ
found. These results are in agreement with the fact that v
0
was maintained constant for the synthesis of the nanopar-
ticles. EDAX experiments were carried out in three different
regions of the sample and the results are summarized in
Table 2. As it was previously shown for XPS experiments,
the calculated atomic compositions of the nanoparticles
correspond with the nominal ones.
0
.3 eV) are mainly due to metallic and/or lower oxidation
state of Ru species [34–36]. For PtRu alloys, the high BE
species have a relative concentration of 30 Æ 5%, whereas
in the case of pure Ru, the relative concentration of these
high BE species is around 60%. The other two peaks are
associated with two different C species. One signal is due to
the presence of residual surfactant on the surface of the
nanoparticles (this residual surfactant was removed using
our electrochemical cleaning protocol [29,30]) and the
second one is associated with the atmospheric contamina-
tion of the sample.
Table 1
Quantitative XPS data for Pt/Ru nanoparticles
Nominal atomic
Band observed
Calculated atomic
composition (Pt/Ru)
composition (Pt/Ru)
8
0/20
0/40
Pt 4f7/2 Ru 3d5/2
Pt 4f7/2 Ru 3d5/2
Pt 4f7/2 Ru 3d5/2
Pt 4f7/2 Ru 3d5/2
Pt 4f7/2 Ru 3d5/2
76.8/23.2
52.4/47.6
45.9/54.1
35.5/64.5
18.7/81.3
6
XPS experiments were also used to calculate the atomic
composition of PtRu nanoparticles (Table 1) by integration
of the Pt and Ru deconvoluted signal and the atomic
50/50
4
2
0/60
0/80

J. Solla-Gull o´ n et al. / Electrochimica Acta 49 (2004) 5079–5088
5083
Fig. 4. Mean particle size of Pt/Ru nanoparticles vs. atomic composition. TEM image of Pt50Ru50 nanoparticles.
3
3
.2. Electrochemical characterization
potential to 0.8 V. In the case of Pt nanoparticles the
voltammetric profile is characteristic of clean surfaces,
showing the very well-known adsorption states. Increasing
the Ru composition, the so-called hydrogen region appears
structureless and an increase of the charge in the double
layer region is also observed. This increase in the double
layer region current has been previously described and it
has been even used to estimate the amount of Ru for
platinum single crystals decorated by spontaneous deposi-
tion with Ru [39,40]. Moreover, the voltammetric profile of
the different PtRu nanoparticles is similar to those pre-
viously reported [5,8,11,41–44]. Only in the case of pure
Ru it is necessary to point out that in our voltammogram no
signal of adsorbed hydrogen is observed. This result is
different to those from other works where a clear hydrogen
adsorption region can be seen [5,7,8,42,45]. The absence
of this region could be explained taking into account the
amorphous state of our Ru nanoparticles as it was shown in
XRD experiments.
.2.1. Voltammetry and COad oxidation on PtRu
nanoparticles
The decontamination procedure used to remove the
surfactant layer from the surface of the nanoparticles was
described in previous papers [29,30]. Very briefly, after the
deposition of the nanoparticles on the gold support, the
electrode was immersed into the sulphuric acid solution
and a constant potential of 0.03 V was applied for 3 min.
After that, the potential was stepped to 0.05 Vand to observe
the surface condition several voltammograms were recorded
between 0.05 and 0.4 V. This procedure was repeated at least
three times. After that and to improve the surface cleanliness
of the electrode, CO was adsorbed on the nanoparticles
holding the electrode at a potential of 0.03 V and bubbling
CO into the solution during 2 min. After that, the CO was
electrochemically stripped from the surface.
The voltammograms of clean Pt, Ru and Pt/Ru nano-
particles can be observed in Fig. 5. Since at potentials
above 0.9–0.95 V corrosion and loss of Ru have been
reported [37,38], we have limited the positive upper
CO adsorption–oxidation may be quantitatively used as a
composition and structure sensitive surface probe [46]. In
this way, Figs. 6 and 7 show the CO stripping on PtRu
nanoparticles and the correlation between atomic composi-
tion and CO stripping peak potential, respectively.
Table 2
CO stripping on pure Pt (Fig. 6a) leads to a shoulder at
approximately 0.5 Vand two main peaks at 0.72 and 0.76 V.
Whereas the shoulder has been correlated with the oxidation
of bridge-bonded CO [47], the multiplicity in the main
oxidation current has been attributed to contributions from
different crystalline orientations [46,48]. Similar peaks have
been also found on low-index Pt single crystal surfaces, with
slightly different peak potentials for the different orienta-
tions [49–51].
Atomic composition of Pt/Ru nanoparticles from EDAX experiments
Sample-1
Sample-2
Sample-3
Pt
Pt100
Pt100
Pt100
Pt80Ru20
Pt60Ru40
Pt50Ru50
Pt40Ru60
Pt20Ru80
Ru
Pt78.5Ru21.5
Pt60.5Ru39.5
Pt47.9Ru52.1
Pt38.3Ru61.7
Pt20.6Ru79.4
Ru100
Pt77.1Ru22.9
Pt60.5Ru39.5
Pt47.7Ru52.3
Pt39.9Ru60.1
Pt18.7Ru81.3
Ru100
Pt80.8Ru19.2
Pt59.7Ru40.3
Pt43.5Ru56.5
Pt42.2Ru57.8
Pt20.0Ru80.0
Ru100

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J. Solla-Gull o´ n et al. / Electrochimica Acta 49 (2004) 5079–5088
Fig. 5. Voltammograms of (a) Pt, (b) Pt80Ru20, (c) Pt60Ru40, (d) Pt50Ru50, (e) Pt40Ru60, (f) Pt20Ru80 and (g) Ru nanoparticles. Test solution, 0.5 M H
À1
2
SO
4
;
sweep rate, 20 mV s
.
Fig. 6. CO stripping on (a) Pt, (b) Pt80Ru20, (c) Pt60Ru40, (d) Pt50Ru50, (e) Pt40Ru60, (f) Pt20Ru80 and (g) Ru nanoparticles. Test solution, 0.5 M H
À1
2
SO
4
; sweep
rate, 20 mV s
.

J. Solla-Gull o´ n et al. / Electrochimica Acta 49 (2004) 5079–5088
5085
À1
2 4
Fig. 7. CO oxidation peak potential vs. atomic Pt percentage in the nanoparticles. Test solution, 0.5 M H SO ; sweep rate, 20 mV s .
With only a 20% of Ru in the catalyst, a very clear
negative shift (approximately 0.2 V compared with pure Pt)
on the CO stripping peak can be observed. This shift has also
been observed in previous studies [5,11]. Increasing the Ru
concentration, the voltammetric profile of the CO oxidation
remains quite similar but the CO stripping peak potential
shifts to more positive values (Fig. 7). This tendency is
different to that reported in other studies. For example, while
with sputter-cleaned PtRu alloys the lowest potential for the
oxidation of CO has been obtained for Pt Ru [5], with
Fig. 8 shows the methanol oxidation voltammograms for
PtRu nanoparticles prepared in microemulsion. For a better
comparison of the electrocatalytic activity of the different
PtRu nanoparticles, it is necessary to rationalize their activ-
ities. In this aspect, several groups have used the ‘‘so-called’’
hydrogen adsorption/desorption process to calculate the
surface area of the electrode. This method, which works
well for Pt surfaces, is not suitable for PtRu due to the poorly
separated underpotential and overpotential hydrogen evolu-
tion regions and specially by the presence of an undefined
double layer contribution. On the other hand, CO stripping
has been also used as a method to determine the surface area
of PtRu nanoparticles [11]. We have used the number of
moles (Pt + Ru) of the particle, previously employed for
PtPd nanoparticles [30], to rationalize the electrocatalytic
activity. One of the interests in this way of presenting the
experimental data is to cancel the difference in atomic mass
between Pt and Ru. This way of rationalizing is obviously
different to that typically used in fuel cell electrode char-
acterization (activity per 1 mg metal of catalyst).
5
0
50
PtRu alloy nanoparticles, the CO oxidation peak potential
remains practically constant from 20 to 60% of Ru [11]. The
differences found could be associated with a different sur-
face atomic composition as a result of the different methods
of synthesis. Moreover, it should be pointed out that CO
oxidation is a structure sensitive reaction and nanoparticles
synthesized by different means may have different surface
structure [52,53].
In the case of pure Ru, a single broad oxidation peak at
.63 V is found. It is important to note that, although the
0
voltammetric profile in absence of adsorbed CO (Fig. 6g)
was different to that previously published [5,7,8,42,45], the
voltammetric profile of CO stripping is very similar to that
reported [5,7,8,33]. The absence of this ‘‘so-called’’ hydro-
gen adsorption desorption region could be related to the
existence of Ru species in an amorphous state, according to
the XRD results.
For pure Pt the methanol oxidation voltammetric profile
is very well-known and it has been reported for massive and
high surface area Pt electrodes [7,55]. In our case, the upper
potential of the cycle was limited to 0.9 V to avoid the re-
organization of the surface Pt atoms of the nanoparticle,
what will be described in more detail in a forthcoming
publication [56]. When the methanol oxidation on pure Pt
is compared with the oxidation on PtRu nanoparticles of
different compositions (Fig. 8), some important features
should be mentioned. The ‘‘molar oxidation current’’
decreases continuously as the Ru composition increases.
This decay seems to be due to the presence of higher
amounts of Ru on the surface, which is inert for methanol
dehydrogenation. However, the onset of the methanol oxi-
dation shifts to lower potentials than for pure platinum. For
3
.2.2. Methanol oxidation on PtRu nanoparticles
It is widely accepted that PtRu is the most promising
binary electrocatalyst for methanol oxidation. Its superior
activity has been observed for different PtRu materials, such
as PtRu alloys [3,6,7], PtRu electrodeposits [42], Ru elec-
trodeposits on Pt [39] and Ru adsorbed on Pt single-crystal
[
10] and on Pt carbon supported electrodes [54].

5086
J. Solla-Gull o´ n et al. / Electrochimica Acta 49 (2004) 5079–5088
Fig. 8. Methanol electrooxidation on (a) Pt, (b) Pt80Ru20, (c) Pt60Ru40, (d) Pt50Ru50, (e) Pt40Ru60, (f) Pt20Ru80 and (g) Ru nanoparticles. Test solution, 0.5 M
À1
H
2
SO
4
+ 0.1 M MeOH; sweep rate, 20 mV s
.
the alloys, the peak (positive scan) potential remains prac-
tically constant (approximately 0.73 V) in the whole com-
position range. According to all these data, it is very difficult
to choose the most active electrocatalyst for methanol
oxidation. For this reason, chronoamperometric experiments
with different potential steps were performed in a 0.5 M
H SO + 0.1 M MeOH solution at room temperature and at
‘‘molar oxidation currents’’ were obtained with Pt Ru
80 20
(nominal) independently of the applied potential and spe-
cially at potentials useful for a DMFC. PtRu nanoparticles
with Pt ranging between 80 and 40% show higher oxidation
currents than pure platinum. A similar tendency was
obtained by Jusys et al. [11] where the oxidation current
was normalised to the surface area calculated by DEMS. The
nominal composition Pt Ru also coincides with that given
2
4
different electrode potentials. Fig. 9 resumes the results
obtained after 200 s at room temperature. Thus, the highest
8
0
20
by Gasteiger et al. [7] (experimentally and by statistical
2 4
Fig. 9. ‘‘Molar current current’’ for methanol electrooxidation vs. alloy composition. Test solution, 0.5 M H SO + 0.1 M MeOH solution.

J. Solla-Gull o´ n et al. / Electrochimica Acta 49 (2004) 5079–5088
5087
calculus) for bulk Pt/Ru alloys. Finally it is important to
recall that these experiments were performed at room tem-
perature, which implies that similar experiments should be
carried out at more elevated temperatures to evaluate the
optimal atomic composition in these new conditions.
to the ‘‘Ministerio de Ciencia y Tecnolog ´ı a’’, Spain, for their
research grant.
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[
[
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