Methanol oxidation at platinized lead coatings prepared by a two-step electrodeposition-electroless deposition process on glassy carbon and platinum substrates

Article abstract of DOI:10.1016/j.electacta.2007.04.020

Platinized lead deposits, Pt(Pb), have been formed on glassy carbon (GC) and platinum electrodes by a two-step process, whereby a controlled amount of Pb was electrodeposited onto the substrates and was subsequently coated with a thin Pt layer upon immersion of the Pb/GC or Pb/Pt electrodes into a chloroplatinic acid solution. The spontaneous surface replacement of Pb by Pt resulted in Pt(Pb)/GC or Pt(Pb)/Pt electrodes which consisted of dispersed Pt(Pb) particles and displayed typical Pt surface electrochemistry in deaerated acid solutions. When tested as methanol oxidation anodes, these electrodes exhibited enhanced electrocatalytic activity both during voltammetric and constant potential experiments. This behaviour is attributed to an electronic effect of the underlying Pb onto the Pt surface layer.

Full text of DOI:10.1016/j.electacta.2007.04.020

Electrochimica Acta 52 (2007) 6254–6260
Methanol oxidation at platinized lead coatings prepared by a two-step
electrodeposition–electroless deposition process on
glassy carbon and platinum substrates
S. Papadimitrioua, A. Tegoua, E. Pavlidoub, G. Kokkinidisa, S. Sotiropoulosa,∗
a Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece
b Department of Physics, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece
Received 4 January 2007; received in revised form 5 March 2007; accepted 3 April 2007
Available online 7 April 2007
Abstract
Platinized lead deposits, Pt(Pb), have been formed on glassy carbon (GC) and platinum electrodes by a two-step process, whereby a controlled
amount of Pb was electrodeposited onto the substrates and was subsequently coated with a thin Pt layer upon immersion of the Pb/GC or Pb/Pt
electrodes into a chloroplatinic acid solution. The spontaneous surface replacement of Pb by Pt resulted in Pt(Pb)/GC or Pt(Pb)/Pt electrodes which
consisted of dispersed Pt(Pb) particles and displayed typical Pt surface electrochemistry in deaerated acid solutions. When tested as methanol
oxidation anodes, these electrodes exhibited enhanced electrocatalytic activity both during voltammetric and constant potential experiments. This
behaviour is attributed to an electronic effect of the underlying Pb onto the Pt surface layer.
© 2007 Elsevier Ltd. All rights reserved.
Keywords: Platinum catalysts; Electroless deposition; Lead electrodeposition; Methanol oxidation
1. Introduction
layer had been deposited on the electrode substrate [4,9] while
in that of Kokkinidis and co-workers the layer of the metal to
be exchanged (Cu or Pb) has been a few hundred monolayers
thick [8,10]. This method has an interesting potential for the
preparation of electrocatalysts both from a practical and a theo-
retical point of view. First, it minimises the amount of precious
metal used to that of a thin surface layer. Second, unlike other
bimetallic catalysts where the second component is exposed to
the reaction medium too (either in the form of an adatom or
as a surface alloy component), this type of catalyst allows the
study of the electronic effect alone of the second component
underlayer to the precious catalyst overlayer.
In all related studies the substrate, onto which the metal layers
to be exchanged (Cu or Pb) have been electrodeposited, has been
other than a carbon material (the standard support of commercial
electrocatalysts). Also, although the resulting electrodes have
been tested as electrocatalysts for oxygen reduction [10] and
hydrogen oxidation [9], they have not been tried in methanol
oxidation.
Most fuel cell electrocatalysts are formed by chemical meth-
ods, usually impregnation of high surface area carbons with the
metal ion, followed by reduction with an appropriate agent ([1]
and references therein). Electrodeposition of precious metals on
electrode substrates avoids the use of reducing agents and pro-
vides accurate control of catalyst loading but requires relatively
large solution volumes and high metal ion concentrations.
Recently, a new method for the preparation of precious metal
coatings has been proposed whereby the surface layer of a less
precious metal (Ru, Cu, Pb and Ti) is replaced by the precious
metal catalyst (Pt and Pd) by spontaneous electroless exchange
of the former by the latter, upon immersion into a complex solu-
tion of Pt or Pd ions [2–10]. In some cases the metal layer to be
replaced (mainly Cu or, in one case, Pb too) had been freshly
electrodeposited onto another electrode substrate, usually Au
[4,8–10]. In the work of Adzic and co-workers a Cu mono-
Pb-modified Pt electrodes (mostly of the Pb UPD type
[11–15] but also of the Pt–Pb alloy kind [16]) have long been
tested as anodes but the modification was found to have only a
∗
Corresponding author. Tel.: +30 2310 997742; fax: +30 2310 443922.
E-mail addresses: eczss@chem.auth.gr, eczss@otenet.gr (S. Sotiropoulos).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.04.020

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6255
small effect in Pt electrocatalytic activity towards methanol oxi-
dation. This is because Pb cannot provide adsorbed oxygenated
species for methanol oxidation [13], a disadvantage that off-
sets its “third-body effect” (intercepting the Pt sites arrangement
needed for poison adsorption [17]).
100 (EcoChimie) system, in small three-compartment cells
equipped with a luggin capillary at the end of the refer-
ence electrode (Ag/AgCl in 3 M NaCl, BAS Inc.; measured
as −0.040 V 5 mV versus SCE, i.e. +0.202 V 5 mV versus
SHE) chamber. A Pt coil served as the counter electrode, placed
in a compartment separated from that of the working electrode
by a glass frit. Three cells were used: one for Pb deposition,
one for preliminary Pt(Pb)/GC or Pt(Pb)/Pt activation-cleaning
in acid and one for studying Pt surface electrochemistry in acid
and methanol oxidation.
The aim of this work has been to prepare platinized lead,
Pt(Pb), deposits on glassy carbon (GC) supports and (as a con-
trol system) on Pt substrates too and test their electrocatalytic
activity for methanol oxidation. Specific objectives have been:
(i) The microscopic characterisation of Pt(Pb) deposits prepared
by surface replacement of pre-deposited thick Pb layers on GC
and Pt. (ii) The electrochemical characterisation of the resulting
Pt(Pb)/GC and Pt(Pb)/Pt electrodes by means of typical electro-
chemical reactions (hydrogen adsorption/desorption, hydrazine
and formic acid oxidation). (iii) The evaluation of these elec-
trodes as methanol oxidation anodes, by means of voltammetry
and constant potential amperometry, investigating the effect of
Pb on the catalytic properties of the Pt overlayer.
Following Pb deposition and immersion into the chloropla-
tinic acid exchange solution, as prepared Pt(Pb)/GC or Pt(Pb)/Pt
electrodes were scanned repeatedly (typically, more than 120
times) at 1 V s−1 in the “cleaning” 0.1 M HClO4 solution
between hydrogen and oxygen evolution, ensuring that any un-
reacted surface Pb was anodically dissolved during exposure to
positive potentials. The electrode was then transferred to a clean
deaerated 0.1 M HClO4 solution and scanned again for another
120 times until a steady state picture, typical of Pt surface elec-
trochemistry, was obtained. Finally, the solution was replaced by
theworking 0.5 MMeOH + 0.1 MHClO4 deaeratedsolutionand
the potential was swept at 5 mV s−1 between +0.1 and +0.8 V
for three times. Since the voltammetric picture of the second
and third run was similar, voltammograms presented here corre-
spond to the second run, deemed as representative of short-term
behaviour. Constant potential experiments lasting 300 s were
performed to obtain an estimate of medium-term catalytic activ-
ity towards methanol oxidation. Before the application of the
potential of interest all electrodes were subjected to the same
potential pulse protocol to ensure similar starting conditions:
they were brought in contact with the solution at −0.25 V ver-
sus Ag/AgCl for 2 s (just prior to hydrogen evolution, where
methanol oxidation is minimum), then stepped to +0.55 V versus
Ag/AgCl for 2 s (for oxidative desorption of adsorbed H during
the previous step and facile oxidation of CO formed by methanol
oxidation at this potential) and finally stepped to 0.05 V for 2 s
(double layer region); they were then stepped to the potential of
interest (+0.3, +0.4 and +0.5 V versus Ag/AgCl) for 300 s.
2. Experimental
2.1. Pb/GC, Pt(Pb)/GC, Pb/Pt and Pt(Pb)/Pt coatings
Electrodeposition of Pb on GC and Pt was carried out from
0.1 M HClO4 + 0.01 M PbCO3 deaerated solutions in the mass
transfer control region (typically −0.55 V versus Ag/AgCl) as
established by preliminary deposition voltammetry. The total
charge density passed was 187 mC cm−2, corresponding to the
equivalent of 950 flat Pb monolayers (taking the atomic radius of
Pb as 0.175 nm and assuming the FCC crystal structure for the Pb
deposit [18]). Initial experiments established that, due to deposit
non-uniformity, this was the minimum quantity of deposited Pb
on GC that was required for eventual clear Pt surface electro-
chemistry (unaffected by the substrate) and significant methanol
oxidation rates.
In this way, the amount of Pb deposited on GC has been
controlled by the charge that passed during lead electrodepo-
sition under constant potential; this charge corresponds to ca.
0.19 mg cm−2, based on Faradays law and assuming a 100% cur-
rent efficiency (preliminary gravimetric experiments confirmed
current efficiencies higher than 96%).
2.3. Electrode materials and chemicals
Glassy carbon from Alfa Aesar (1 mm thick) was cut into
3 mm diameter discs and sealed into glass tubes with epoxyresin
glue. A Pt disc electrode (1.5 mm diameter) from BAS Inc.
was used as an alternative substrate. Electrodes to be used for
SEM/EDS experiments were prepared by connecting the car-
bon disc or a piece of a Pt foil (Alfa Aesar) to a glass tube via a
bridge made of a shrinkable thermoplastic tube. The latter was
filled with mercury to ensure electrical contact between the disc
and a commercial wire inserted from the open end of the glass
tube. In the few indicative experiments of hydrazine oxidation,
a glassy carbon (3 mm diameter) RDE controlled by a Taccusel
EDI101T motor was used as the substrate electrode.
Pb/GC and Pb/Pt electrodes were subsequently immersed
in a 0.1 M HCl + 10−3 M K2PtCl6 solution for 15 min so that
spontaneous Pb replacement by Pt occurred:
2Pb/GC + PtCl62− → Pt(Pb)/GC + 2Pb2+ + 6Cl−
(1)
The Pt(Pb) deposits were etched off from the Pt(Pb)/GC elec-
trodesbyimmersioninaknownvolume(5 ml)ofaquearegiaand
the solution was analysed by AA; Pt loadings in the 3–5 ␮g cm−2
range were found from a series of experiments and these very
low values are indicative of a very thin Pt skin.
2.2. Electrochemical characterisation of coatings
HClO4 from Riedel (puriss p.a., ACS reagent, ≥70%)
and PbCO3 from Sigma–Aldrich (ACS reagent) were used
in the preparation of Pb deposition solutions. H2PtCl6 hex-
ahydrate from Sigma–Aldrich (ACS reagent, ≥37.50% as Pt)
Voltammetric and constant potential deposition or chronoam-
perometry experiments were carried out with the Autolab

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was employed for the Pt exchange solution. MeOH was from
Riedel (Chromasolv®, for HPLC, gradient grade, ≥99.9%).
Hydrazinium sulphate pro analysi was from Merck and Formic
acid 98–100%, Analytical Reagent, Reag. ACS, Reag. Ph.Eur.
was from Riedel-de Haen.
mass transfer control potential range. A thick and dense partic-
ulate deposit can be seen which, upon closer inspection reveals
the existence of large pyramidal particles (rather monodisperse,
1–2 ␮m in size), sub-micron nuclei (polydisperse) that coalesce
at locations and a few sub-micron needles.
Fig. 1(C) and (D) shows SEM micrographs of the Pt(Pb)
deposits resulting from the immersion of the Pb/GC electrodes
into the chloroplatinic acid exchange solution and the subse-
quent potential cycling in acid. It can be seen that the deposit
is now sparser (with distinct uncovered GC spots, confirmed by
EDS)andthinner, madeupofsmaller(<1 ␮m)andmorerounded
polyhedra particles as well as isolated nano-sized nuclei. EDS
analysis confirmed the existence of Pt in all deposit locations.
Resultsfromtheelectrodeareadepicted in Fig. 1(C), showedrel-
3. Results and discussion
3.1. Microscopic characterisation of Pb and Pt(Pb)
deposits
Fig. 1(A) and (B) shows SEM micrographs of a Pb deposit
formed by the electrodeposition of 187 mC cm−2 (correspond-
ing to ca. 0.19 mg cm−2 of Pb, see Section 2) onto GC in the
Fig. 1. SEM micrographs of (A) and (B) electrodeposited Pb on GC (at −0.55 V vs. Ag/AgCl; total charge density 187 mC cm−2, equivalent of 950 Pb monolayers)
and (C) and (D) Pt(Pb) prepared by electroless platinization of the deposits of (A) and (B) (15 min immersion in a 0.1 M HCl + 10−3 M K2PtCl6 solution). SEM
micrographs of (E) and (F) show Pt(Pb) deposits prepared as those of (A) and (D) on Pt.

S. Papadimitriou et al. / Electrochimica Acta 52 (2007) 6254–6260
6257
ative Pt and Pb precentage composition of 72.62% and 27.38%,
respectively (it should be stressed that EDS analysis only pro-
vides some information on the composition of the deposit layer
since it is neither a surface sensitive tool, such as XPS or AES,
or can provide a depth profile). The reduction in particle num-
ber and size is mainly due to the competition of Pb dissolution
in the acid solution and its exchange by Pt which protects the
underlying Pb. Once a protective Pt surface layer is formed, the
Pb particles are stabilised and no further dissolution takes place
(the further reduction in size of very small features such as the
needles of Fig. 1(B), results in their collapse). Exposure to pos-
itive potentials during potential cycling had a relatively small
effect, mainly on particle number, as confirmed by SEM.
Although the main aim of this work has been the study of
Pt(Pb) onto a carbon material support (GC), to exclude any pos-
sible GC substrate effects, we prepared similar (187 mC cm−2
of initial Pb) deposits on a Pt electrode substrate. Fig. 1(E) and
(F) shows Pt(Pb) deposits formed by the two-step technique on
Pt. In contrast to Fig. 1(C), Fig. 1(E) shows an almost complete
coverage of the surface by a skin of coalescent Pt(Pb) nuclei
(confirmed by the presence of Pb at all locations by means of
EDS), on top of which larger particles lie. A close compari-
son of Fig. 1(D) and (F) reveals that the large Pt(Pb) particles
on Pt are smaller (less than 1 ␮m in diameter) than those on
GC. The complete coverage of the Pt surface by the Pt(Pb)
deposit may be attributed to smaller Pb particle size which per-
mits their fast and complete coverage/protection by Pt before
leaching of Pb occurs, as well as to better adherence to the
substrate.
Fig. 2. (A) Voltammograms (at 1 V s−1 potential scan rate) of a Pt(Pb)/GC
(r = 1.3) and the GC substrate electrode, in a deaerated 0.1 M HClO4 solution.
(B) Same as in (A) but for a Pt(Pb)/Pt (r = 6.8) electrode and a bulk Pt disc
(r = 1.0) electrode.
3.2. Electrochemical characterisation of Pt(Pb)/GC and
Pt(Pb)/Pt electrodes
value of 1.3 whereas the polished Pt disc of Fig. 2(B) had r = 1.0.
This relatively small electroactive area indicates that incomplete
coverage offsets the particulate form of the deposit. To the con-
trary, the surface electrochemistry of Pt(Pb)/Pt electrodes (see
for example Fig. 2(B)) shows a perfectly flat double layer region
and, from the hydrogen adsorption charge, r values in the 5–7
range were obtained in line with the higher surface coverage on
Pt (confirmed by the SEM micrographs of the previous section).
3.2.1. Surface electrochemistry in acid
Fig. 2(A) shows fast potential sweep voltammograms (at
1 V s−1) recorded in a deaerated 0.1 M HClO4 solution at a
Pt(Pb)/GC and the GC electrode substrate; Fig. 2(B) shows,
similar voltammograms at a bulk Pt disc and at a Pt(Pb)/Pt
disc electrode. The characteristic features of Pt surface elec-
trochemistry (hydrogen adsorption/desorption peaks and oxide
formation/stripping wave/peak) are clearly observed at the
Pt(Pb)/GC electrode, indicating that full platinization of the
surface of the Pb particles has been achieved and that the
contribution of uncovered areas of the GC substrate did not
obscure to any significant extent the voltammetric picture. (Note
that the higher double layer currents and an oxidative “hump”
recorded for the Pt(Pb)/GC should be attributed to GC due to
the higher carbon capacitance and surface group electrochem-
istry [19,20]—see also the voltammogram for GC in the same
figure.) The clearly resolved hydrogen adsorption/desorption
region permitted an estimate of Pt electroactive surface area,
Ae, as calculated from the corresponding cathodic charge over
the capacitive current and the charge associated to the forma-
tion or stripping of a H monolayer (210 ␮C cm−2 [21]). This
can be quantified by a “roughness factor” r = Ae/Ags where Ags
is the substrate geometric area. Electrodes with r values in the
1.2–2 range were obtained; the one depicted in Fig. 2(A) had a
3.2.2. Hydrazine and formic acid oxidation
Despite the fact that any defective Pt shell–Pb core particles
should have dissolved away during the excursion into the anodic
potentials included in the pre-treatment voltammetry, the possi-
bility that some additional Pb might dissolve during methanol
oxidation experiments and re-deposit as a UPD layer on the Pt
skin, does not seem remote at first glance. The clear picture
of Pt surface electrochemistry obtained at the Pt(Pb) electrodes
(Fig. 2(A)and(B))isafirstevidenceagainstthispossibilitysince
it can be contrasted to the distortion caused by even fractions of a
Pb UPD, formed at Pb(II) concentrations as low as 10−6 M [11].
Nevertheless, to completely exclude such an effect we have also
studied two typical electrochemical reactions for Pt electrodes:
hydrazine oxidation which is strongly hindered by the presence
of Pb UPD [22] and formic acid oxidation which is significantly
enhanced by Pb UPD [23–27].

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Fig. 3. Voltammograms (at 10 mV s−1 potential scan rate) of a Pt(Pb)/GC RDE
and a bulk Pt RDE in a deaerated 0.001 M hydrazine + 0.1 M HClO4 solution,
at a 500 rpm rotation rate. Current density is per substrate geometric area.
Fig. 5. Voltammograms (second positive-going scan at 5 mV s−1 potential scan
rate) of a Pt(Pb)/GC (r = 1.3), a Pt(Pb)/Pt (r = 6.8) and a bulk Pt disk (r = 1.0)
electrode in a deaerated 0.5 M MeOH + 0.1 M HClO4 solution. Current density
is per substrate geometric area.
Fig. 3 presents slow potential sweep voltammograms (at
10 mV s−1) in a 0.001 M hydrazine + 0.1 M HClO4 solution at a
RDE(500 rpm)Pt(Pb)/GCand, forcomparison, atabulk PtRDE
(500 rpm). It can readily be seen that the half-wave potentials
of the two voltammograms are similar and no noticeable shift
to more positive potentials (characteristic of the presence of Pb
UPD [22]) is observed. Furthermore, the limiting current densi-
ties for hydrazine oxidation at the Pt and Pt(Pb)/GC electrodes
are similar, indicating a complete overlapping of the diffusion
fields of the Pt(Pb) particles formed on the GC electrode, result-
ing in a coincidence of the planar diffusion field cross-section
with the electrode geometric area (the current “well” observed in
the limiting current region has been observed before at RDEs and
attributed to adsorption/desorption of impurities, e.g. chloride
ion traces, in the double layer region [22]).
metric picture at the Pt(Pb)/GC in the absence of deliberately
added Pb(II) is qualitatively the same with that of bulk Pt, with
a strongly hindered oxidation process during the positive-going
scan (consisting of two waves, whose relative height depends
on impurities, potential limits, Pt crystalline form, etc.) and a
more facile oxidation during the reverse scan [23–27]. To the
contrary, the presence of Pb(II) ions in the solutions that leads
to the formation of a Pb UPD in the region of formic acid oxi-
dation, strongly catalyses the process resulting in much higher
currents and a smaller hysterisis between the forward and reverse
peaks.
In conclusion, the electrochemical characterisation of our
Pt(Pb) electrodes presented in Sections 3.2.1 and 3.2.2 confirms
the complete coverage of the Pb particles by a protective Pt skin
and the lack of any Pb anodic dissolution at potentials relevant
to organics oxidation.
Fig. 4 presents slow potential sweep voltammograms (at
10 mV s−1) in a 0.2 M formic acid + 0.1 M HClO4 solution at
a stationary Pt(Pb)/GC electrode in the presence and absence of
Pb(II) in the solution. The Inset shows a similar voltammogram
in the absence of Pb(II) for a bulk Pt electrode. The voltam-
3.3. Methanol oxidation in acid
Fig. 5 presents slow potential sweep voltammograms (at
5 mV s−1; positive going scan) in a 0.5 M MeOH + 0.1 M HClO4
solution at a Pt(Pb)/GC, a Pt(Pb)/Pt and, for comparison, at a
bulk Pt disc electrode (the ones corresponding to the surface
voltammetry of Fig. 2).
First of all, it is seen that the peak of methanol oxidation
appears at the same potential for all electrodes. It has been well-
documented [11–16] that this peak results from an increase in the
current as the potential becomes positive enough for high rates of
methanol oxidation to occur (and the poisonous CO intermediate
to be oxidized-removed), followed by a current fall beyond that
peak as full coverage of the Pt layer with surface oxides and a
decrease in the number of catalytic sites take place. Since the
oxidative removal of CO at high potentials is not expected to
depend on electrode catalytic activity and the potential region
of Pt oxide formation does not seem to depend either on whether
Pt(Pb) or Pt is used (see surface electrochemistry in Fig. 2(A)
and (B)), this peak should appear at the same potential for all
electrodes.
Fig. 4. Cyclic voltammograms (at 50 mV s−1 potential scan rate) of a Pt(Pb)/GC
electrode in a deaerated 0.2 M formic acid + 0.1 M HClO4 solution, in the pres-
ence and absence of 0.001 M Pb(II), as indicated on the graph. Inset: same as in
the main graph but for a bulk Pt disk electrode in the absence of 0.001 M Pb(II).
Current density is per substrate geometric area.

S. Papadimitriou et al. / Electrochimica Acta 52 (2007) 6254–6260
6259
Table 1
Current densities per electroactive Pt area, ie, at three applied potential values, for methanol electrooxidation at Pt(Pb)/GC, Pt(Pb)/Pt and Pt electrodes, during
voltammetric and chronoamperometric experiments in 0.5 M MeOH + 0.1 M HClO4
+0.3 V vs. Ag/AgCl
+0.4 V vs. Ag/AgCl
+0.5 V vs. Ag/AgCl
ie (␮A cm−2) from voltammetry at 5 mV s−1
Pt (Pb)/GC (r = 1.3)
Pt(Pb)/Pt (r = 6.8)
14
24
8
192
243
111
609
722
391
Pt (r = 1.0)
ie (␮A cm−2) after 300 s at constant potential
Pt (Pb)/GC (r = 1.7)
Pt(Pb)/Pt (r = 5.5)
21
55
0
61
123
1
149
249
140
Pt (r = 1.0)
Second, and most important, it is seen that the platinized
lead electrodes exhibit higher currents per substrate geometric
area than the surface area increase would explain. This is con-
firmed by the results of Table 1 where the current densities per
electroactive area are tabulated for the three electrodes and for
three indicative potentials of +0.3, +0.4 V (foot of the wave)
and +0.5 V (close to the peak of the wave). Thus, an enhanced
catalytic activity of the Pt(Pb)-modified electrodes towards
methanol electrooxidation is already suggested by voltammetry.
However, with respect to the prospective use of such catalysts
in direct methanol fuel cells and in connection with their per-
formance deterioration with time due to electrode poisoning, it
is well-established that catalysts for methanol electrooxidation
should be evaluated under constant potential conditions [28–30].
Fig. 6 shows the current–time response following a potential step
to +0.4 V versus Ag/AgCl at a Pt(Pb)/GC, a Pt(Pb)/Pt electrode
and a bulk Pt disc, in a solution of 0.5 M MeOH + 0.1 M HClO4.
It is seen that, apart from the fact that the current density is higher
at the Pt(Pb)-modified electrodes (as it is also the case for the
corresponding current densities per electroactive Pt area—see
Table 1 again), the signal at the latter is falling at a much smaller
rate than that at the pure Pt electrode (similar extremely low cur-
rents have been reported after long periods for pure Pt in [28]
too), indicating a diminished contamination rate at the modified
electrodes. This is further supported by noting in Table 1 that
methanol oxidation currents after 300 s are more enhanced at
+0.3 and +0.4 V for Pt(Pb) electrodes (where the poison oxida-
tion rates are expected to be low) than at the higher potential of
+0.5 V (where the oxidative removal of the poison is expected
to be more efficient).
A comparison of our results with those of the seminal works
of Refs. [28,29] for smooth Pt–Ru electrodes (the state-of-art
material for methanol anodes), reveals that the current den-
sities obtained at our Pt(Pb) electrodes after 300 s at +0.3 V
versus Ag/AgCl, 3 M NaCl (i.e. at ca. +0.5 V versus SHE) in
a 0.5 M MeOH solution are, depending on electrode, in the
21–55 ␮A cm−2 range (Table 1) i.e. in the lower end of the
30–200 ␮A cm−2 range (depending on Ru content) reported
under similar conditions in [28,29]. Nevertheless, the advantage
of our electrodes lies on the fact that Pt modification is based
on a cheap Pb substrate rather than on an expensive Ru second
metal component. At this point we would like to stress that the
reason our results should be compared with those of [28,29] is
two-fold: first, the potential and methanol concentration condi-
tions are identical to ours; second, and more importantly, Pt–Ru
electrodes used therein were smooth, unsupported electrodes, a
situation more similar to our large particle and low roughness
factor (ca. 1.2–7) Pt(Pb) electrodes than that of commercial pre-
cious metal/carbon powder catalysts. Finally, we should also
point out that potentials as high as +0.5 V versus SHE (+0.3 V
versus Ag/AgCl) have been used in basic research studies util-
ising smooth electrodes to characterise the catalytic activity
of materials towards methanol oxidation (as in this work and
[28,29]). This is because current densities (especially during
long term experiments) at smooth electrodes are very small (and
thus prone to errors) as opposed to the situation of high surface
arearealcatalystelectrodes(whichareusuallyevaluatedatlower
potentials [30]).
The increased tolerance of Pt(Pb) electrodes to poisoning
during methanol oxidation at first sight contradicts the gener-
ally accepted view that Pb has a very small effect on MeOH
oxidation at Pt as an adatom [11–15] (see also the discus-
sion in Section 1). However, molecular orbital calculations
have predicted that a “ligand effect” of Pb (and Sn) on Pt
should be present too, promoting CO desorption from Pt [31].
This electronic-“ligand” effect of Pb on Pt should be the only
effect operative when a Pb underlayer is fully coated with Pt,
hence it could be responsible for the high catalytic activity and
medium term stability of Pt(Pb) electrodes towards methanol
oxidation.
Fig. 6. Chronoamperometric curves following the application of +0.4 V vs.
Ag/AgCl at Pt(Pb)/GC (r = 1.7), Pt(Pb)/Pt (r = 5.5) and bulk Pt disk (r = 1.0)
electrodes in a deaerated 0.5 M MeOH + 0.1 M HClO4 solution. Current density
is per substrate geometric area.

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[3] G. Kokkinidis, D. Stoychev, V. Lazarov, A. Papoutsis, A. Milchev, J. Elec-
4. Conclusions
troanal. Chem. 511 (2001) 20.
[4] S.R. Brankovic, J.X. Wang, R.R. Adzic, Surf. Sci. 474 (2001) L173.
[5] S.R. Brankovic, J. McBreen, R.R. Adzic, J. Electroanal. Chem. 503 (2001)
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[6] S.R. Brankovic, J. McBreen, R.R. Adzic, Surf. Sci. 479 (2001) L363.
[7] S.R. Brankovic, J.X. Wang, Y. Zhu, R. Sabatini, J. McBreen, R.R. Adzic,
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(i) Adherent platinized lead deposits with particle size in the
few microns and sub-micron ranges, have been prepared
on glassy carbon and platinum substrates by a two-step
process that involves lead electrodeposition and sponta-
neous platinum deposition on lead from its chloro-complex
solutions.
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