Electrochemical properties of palladium adlayers onPt(100) substrates

Article abstract of DOI:10.1016/j.susc.2004.03.074

The formation of submonolayers of palladium on well-defined Pt(100) electrodes is described. It has been found that the adsorption of NO at open circuit and its further reductive stripping enable the possibility to prepare Pt(100) electrodes fully covered by the first palladium layer, without contributions coming from palladium in the subsequent layers. This method enables a better characterization of the palladium islands formed in the submonolayer range. The CO displacement method points out that hydrogen and anion adsorption play a role in the charge transfer processes involved in the voltammetric profile. The analysis of the charge-potential curves is used to determine the values of the potentials of zero total charge (pztc) of the different adelectrodes. The pztc diminishes almost linearly with palladium coverage, this shift being related to increasing anion adsorption at low potentials. Adsorbed palladium does not electrocatalyze the oxidation of adsorbed CO.

Full text of DOI:10.1016/j.susc.2004.03.074

Surface Science 573 (2004) 32–46
www.elsevier.com/locate/susc
Electrochemical properties of palladium adlayers
on Pt(100) substrates
´
B. Alvarez, A. Bern a´ , A. Rodes, J.M. Feliu
*
Departamento de Qu ´ı mica F ı´ sica e Instituto Universitario de Electroqu ı´ mica, Universidad de Alicante, Apartado 99,
E-03080 Alicante, Spain
Received 25 August 2003; accepted for publication 2 March 2004
Available online 29 July 2004
Abstract
The formation of submonolayers of palladium on well-defined Pt(100) electrodes is described. It has been found that
the adsorption of NO at open circuit and its further reductive stripping enable the possibility to prepare Pt(100) elec-
trodes fully covered by the first palladium layer, without contributions coming from palladium in the subsequent layers.
This method enables a better characterization of the palladium islands formed in the submonolayer range. The CO dis-
placement method points out that hydrogen and anion adsorption play a role in the charge transfer processes involved
in the voltammetric profile. The analysis of the charge-potential curves is used to determine the values of the potentials
of zero total charge (pztc) of the different adelectrodes. The pztc diminishes almost linearly with palladium coverage,
this shift being related to increasing anion adsorption at low potentials. Adsorbed palladium does not electrocatalyze
the oxidation of adsorbed CO.
ꢀ
2004 Elsevier B.V. All rights reserved.
Keywords: Palladium; Platinum; Carbon monoxide; Electrochemical methods; Adatoms; Oxidation
1
. Introduction
mental research in Surface Electrochemistry [1].
Among the properties linked to the surface struc-
ture of the different single crystal metal electrodes
and their effects on the so-called structure sensitive
reactions, interest has been also focused in the
modification of the surface composition of well de-
fined metallic substrates by foreign adatom adsorp-
tion [2]. In this latter case interesting examples of
enhanced reactivity appear, mainly when the ada-
tom coverage is below the first monolayer amount.
The understanding of surface structure effects is
a key point in fundamental studies of electrochem-
ical reactivity. In this way, the use of well defined
surfaces has open a well established field of funda-
*
Corresponding author. Tel.: +34 96 590 9301; fax: +34 96
590 3537.
E-mail address: juan.feliu@ua.es (J.M. Feliu).
0
039-6028/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2004.03.074

´
B. Alvarez et al. / Surface Science 573 (2004) 32–46
33
Palladium deposited on well-defined platinum
surfaces has received some attention since the dis-
covery of the possibility of preparation of these
adlayers in 1991 [3–5]. As it happens in most cases,
Pt(111) substrates have been much more investi-
gated than other orientations despite of that Pd
adsorbed on Pt(100) leads to a considerable
enhancement of formic acid oxidation rates at very
low potentials [6,7]. This situation may be related
to the difficulty to prepare well-defined Pt(100)
substrate electrodes as well as to the ordering of
the adlayers themselves, which have a greater ten-
dency to grow a second layer prior to the comple-
tion of the first one [5]. This tendency is much
more marked on Pt(100) substrates than on
Pt(111) substrates [8]. Fortunately, in the case of
palladium adsorbed on Pt(111) [9] and Pt(100)
practically the same. (ii) Further palladium deposi-
tion leads to a second voltammetric feature at 0.27
V, linked to adsorption on palladium atoms on
second, third and further layers [9]. (iii) The pztc
of the Pd–Pt(111) electrodes shifts to lower poten-
tials as the Pd covered surface increases in sulphu-
ric acid solutions, this fact being linked to the
adsorption of (bi)sulphate anions at lower poten-
tials [12]. (iv) Despite the apparently easier adsorp-
tion of oxygenated species, the palladium adlayers
are less electrocatalytic towards adsorbed CO oxi-
dation than the Pt(111) substrates [12].
Previous work shows that palladium deposited
on Pt(100) participates of similar characteristics
of those described above [5,8]. As on Pt(111) the
voltammetry of palladium on Pt(100) also leads
to two adsorption peaks and the same growth
mechanism has been proposed [10]. The present
paper is addressed to clarify some details in the
characterization of the main features of this bime-
tallic adelectrode, such as the ordered growth of
the first monolayer until its completion, a problem
that has not been solved yet. The coverage evalua-
tion through charge measurements and pztc deter-
mination of the submonolayer will be finally
addressed as well as its electrocatalytic ability to-
wards oxidation of adsorbed CO. Advantage is
made of the use of a new method of preparation
of these surfaces that enables to obtain more
reproducible results than other methods previously
used, particularly in the limit of the completion of
the first monolayer [8].
[
10] information from surface X-ray scattering is
available, especially for relatively large palladium
coverages. In both cases, the adlayer grows follow-
ing the Stranski–Krastanov mechanism. In this
way, the first palladium layer adsorbs well ordered
on the platinum substrate, as it would be expected
from the geometrical and chemical similarities be-
tween both metals.
Recent results [11] obtained in our laboratory
with Pt(100) and vicinal Pt(S)[n(100) · (111)]
electrodes allow us to better understand the behav-
iour of the Pt(100) substrate itself and extend the
work already done with palladium deposited on
Pt(111) and its vicinal surfaces. This earlier work
on these bimetallic noble metal-on-noble metal
electrodes has allowed us to determine the electro-
chemical characteristics of these Pd–Pt(111)
adlayers, such as the potentials of zero total charge
2. Experimental
(
pztc) in solutions containing different anions and
their electrochemical behaviour towards the CO
oxidation reaction [12,13]. Several general charac-
teristic features can be deduced from the voltam-
metric profile obtained for the Pd–Pt(111)
adlayer electrodes. The most relevant to the pre-
sent work are: (i) Adsorbed palladium defines a
characteristic hydrogen/anion adsorption state at
Working electrodes were prepared [16] from
small single crystal beads (2.2 and 4.5 mm dia-
meter for the samples used in the electrochemical
and spectroscopic experiments, respectively). These
beads resulted from the fusion and subsequent
careful cooling of 0.5 mm diameter platinum wire
(99.999% Alfa). The single crystal beads were ori-
ented by using the reflections of the flat (111) and
(100) facets spontaneously formed on the surface
of the beads. In the present case the reflection of
the topmost (100) facet was selected and the bead
fixed in the bench by an epoxy resin (Epofix from
0
.23 V vs. RHE in 0.5–0.1 M sulphuric acid, pro-
vided that the palladium coverage would be lim-
ited to the first monolayer, hPd 6 1. This first
layer grows epitaxially on the Pt(111) substrate
[
9,12,14,15], the atomic radii of Pt and Pd being

´
B. Alvarez et al. / Surface Science 573 (2004) 32–46
34
Struers). The crystals were cut with abrasive paper
Buehler) until the maximum diameter of the bead
trodes [18,19]. The preparation of the palladium
adlayers involves several specific steps and some
details and will be extensively described in the fol-
lowing section.
In all cases, large area platinum electrodes (spi-
ral wires of the same quality as that for the work-
ing electrodes) were used. All potentials were
measured against a reversible hydrogen electrode
immersed in a solution with the same pH as the
working solution, i.e. 0.1 M sulphuric acid (Merck
suprapur) in 18.2 MX ultrapure water (Elga Pur-
elab ultra).
(
and then the surface was polished with diamond
paste down to 0.25 lm. The cutting and polishing
machine was placed in the same bench used for the
orientation step in such a way that the single crys-
tal bead was immobilised in the whole preparation
process [16]. Once prepared, the crystal surface
was routinely annealed at 1100 ꢁC for 15 min, this
latter step being sufficient to repair the mechanical
damage of the topmost layers caused by the polish-
ing step to mirror finishing [17].
Prior to each experiment, the electrodes were
flame-annealed for 15 s in a Bunsen flame, cooled
down slowly in a reductive atmosphere (5–7:1
CO displacement experiments were described in
previous papers [12]. In these experiments, CO gas
(N50, LÕAir Liquide) was dosed at a constant
potential and the current transient recorded. NO
adlayers [20] were formed by means of open circuit
adsorption from a NO-saturated solution pre-
pared by bubbling NO gas (N20, LÕAir Liquide)
in water.
Ar + H N50, LÕAir Liquide) and finally quenched
2
with water in equilibrium with the same mixture of
gases. This step should be carefully handled since
it is crucial in the quality of the final surface.
The final voltammetric profile in the low potential
region was used as a fingerprint (Fig. 1). Among
other characteristics, the presence of a main
adsorption state at 0.39 V, jointly with the coinci-
dence of the current density values at 0.25 and 0.55
V is characteristic of well-ordered Pt(100) elec-
Spectroelectrochemical experiments were per-
formed with a Nicolet Magna 850 spectrometer
equipped with a MCT detector. The spectroelectr-
ochemical cell was provided with a prismatic CaF2
window bevelled at 60ꢁ. The spectra were usually
collected with p-polarized light with a resolution
ꢀ1
of 8 cm and are presented as the ratio ꢀlog(R/
R ), where R and R are the reflectance values cor-
0
0
responding to the single beam spectra recorded at
the sample and reference potentials, respectively.
3. Results and discussion
3.1. Adlayer preparation
After flame annealing and cooling in reductive
atmosphere, the Pt(100) substrate surface, pro-
tected with a droplet of water, was immersed at
ꢀ
.10 V in a 10 M PdSO dissolved in 0.1 M
4
5
0
H SO for just the time required (few seconds) to
2
4
attain the meniscus configuration. Then the poten-
tial was continuously cycled between 0.06 and 0.85
ꢀ
1
V at 50 mVs . Simultaneously, the voltammetric
profiles were continuously recorded. As shown in
Fig. 2, this is a convenient way to follow the dep-
osition of palladium [12]. In Fig. 2A it can be ob-
served that the characteristic adsorption peak of
Fig. 1. Cyclic voltammogram for Pt(100) in 0.1 M H
cooled down in H + Ar (charge on platinum free sites between
.25 and 0.55 V is = 259 lCcm ). Sweep rate: 50 mVs
2 4
SO
2
ꢀ2
ꢀ1
.
0

´
B. Alvarez et al. / Surface Science 573 (2004) 32–46
35
ꢀ
5
Fig. 2. Cyclic voltammograms for Pt(100) in 0.1 M H
2
SO
4
+ 10 M PdSO
4
, recorded in successive stages (A, B, C, D) during Pd
ꢀ1
deposition experiment. Sweep rate: 50 mVs
.
well ordered Pt(100) electrodes at 0.39 V progres-
sively diminishes whereas a new sharp adsorption
state appears at 0.17 V. The experimental result
can be interpreted as the progressive blocking of
the Pt(100) substrate sites by a first adlayer of pal-
ladium atoms directly deposited on the Pt(100)
substrate. Note that in the present procedure, the
palladium adatoms are not underpotentially
deposited. The deposition rate is quite slow as de-
duced from the evolution of the voltammetric pro-
file at the sweep rate used in these experiments. As
in the case of Pt(111), the peak at 0.17 V reflects
the existence of adsorption processes similar to
those taking place on the Pt(100) substrate, but
on the palladium covered part of the surface.
From the stability of the voltammetric curves re-
corded in the palladium-free solution it can be de-
duced that the deposited palladium atoms remain
stable on the surface in the whole potential range
explored (see below).
As the deposition continues a new feature ap-
pears at 0.28 V while the peak at 0.39 V has not
been completely suppressed (Fig. 2B). It is note-
worthy that the peak at 0.39 V, related to the

´
B. Alvarez et al. / Surface Science 573 (2004) 32–46
36
remaining Pt(100) unblocked sites, maintains its
characteristic shape, suggesting the presence of
wide Pt(100) domains on these palladium-covered
electrodes. This in turn suggests that the growth of
palladium island growth is taken place.
without the appearance of new adsorption states
in the voltammogram.
The deposition procedure described above is
quite convenient because it is reasonably slow
and the electrode can be taken out from the depo-
sition cell at any time (e.g., palladium coverage),
rinsed with ultra pure water and immersed again
in a characterization cell containing only the sup-
porting electrolyte, 0.1 M sulphuric acid, free of
The appearance of a second adsorption state at
higher potential values than the first one has been
also reported for palladium deposited on Pt(100)
substrates by using the so-called forced deposition
method [5] as well as by others [10] and also occurs
on Pt(111) substrates. The new peak is associated
to the growth of palladium in second, third and
further layers [9,10] all of the newly deposited
atoms leading to the same voltammetric contribu-
tion. The main difference with Pt(111) substrates
is that this second adsorption state appears at ear-
lier deposition stages on Pt(100) substrates as
compared to Pt(111). Moreover, in this latter sub-
strate is possible to minimize the growth of the sec-
ond adsorption state by diminishing the palladium
deposition rate.
2
+
Pd cations. In this way, voltammograms of elec-
trodes containing a stable amount of adsorbed pal-
ladium are obtained, as shown in Fig. 3. These
profiles are similar to those reported previously
for palladium adlayers prepared by the forced dep-
osition method, i.e. using adsorbed hydrogen as a
reducing agent of the palladium cations reaching
the interface. It should be stated that the voltam-
metric profiles are completely stable if the palla-
dium contribution is small, i.e. when only the
first state at 0.17 V is observed. When the second
palladium adsorption states are present a slow
modification of the profiles is detected [5]. In this
way, continuous potential cycling after transfer
of the electrode to a pure sulphuric acid solution
leads to modifications in the relative intensity of
the adsorption states. The result of this experiment
is the progressive diminution of the peaks at 0.28 V
(related to the second stage of palladium deposi-
tion) and at 0.39 V (remaining free platinum sub-
strate sites) whereas the peak at 0.17 V increases
(palladium deposited directly over the Pt(100)
substrate).
In final deposition stages (Fig. 2C) the peak at
.28 V continues growing while the states at 0.17
0
and 0.39 V progressively decrease. This behaviour
supports the idea that the palladium deposition
may take place either on the unblocked Pt sites
or on the first palladium adlayer. Finally, Fig.
2
D, all the Pt(100) substrate sites are fully covered
by palladium and the peak at 0.28 V is the only
adsorption state that develops, the state at 0.17 de-
creases slowly. The process will continue until the
peak at 0.28 V is the only feature remarkable in the
voltammogram (not shown). The absence of the
peak at 0.17 V indicates that all the palladium
atoms in the first layer are finally covered by one
or more palladium atoms (multilayer). Some indi-
cation of hydrogen absorption in the multilayer is
given by the large currents recorded in the lower
potential range. As compared to the initial stages
of deposition in which palladium deposition is
linked to the decrease of the platinum free sites
Changes in the voltammetric peaks related to
palladium atoms deposited in the first and subse-
quent layers reflect a high mobility of the palla-
dium atoms deposited in the second stage, this
mobility being much higher than that observed
on Pt(111) substrates. It can be accepted that
potential cycling in the absence of palladium ions
in solution leads to a more ordered surface than
that obtained immediately after deposition [3,5].
This so-called electrochemical annealing of the
palladium layer, however, is not able to completely
remove the peak at 0.28 V in a reasonable time.
Moreover, the stable voltammetric profiles remain
unchanged after CO adsorption and stripping, see
below. In this way, it is difficult to obtain a surface
in which the only adsorption state would be that at
(Fig. 2A), the final evolution of the voltammetric
profile (from Fig. 2C and D) would mainly reflect
the progressive disappearance of the surface do-
mains uncovered and blocked by the first palla-
dium adlayer. This stage is more difficult to
monitor, because the second state in palladium
deposition leads to the growth of several layers

´
B. Alvarez et al. / Surface Science 573 (2004) 32–46
37
ꢀ2
Fig. 3. Cyclic voltammograms for different Pd–Pt(100) electrodes (A, charge on platinum free sites = 214 lCcm ; B, charge on
ꢀ
2
ꢀ2
platinum free sites = 105 lCcm ; C, charge on platinum free sites = 59 lCcm ; D, all platinum sites covered by palladium atoms) in
0
ꢀ1
.1 M H SO . Sweep rate: 50 mVs .
2 4
0
.17 V, i.e. that of a single palladium monolayer
after palladium deposition in pure 0.1 M sulphuric
acid is shown in panel A. This electrode shows a
relatively small adsorption state at 0.17 V and a
main peak at 0.28 V. Adsorption at the remaining
Pt(100) substrate sites free of palladium is ob-
served at potentials between 0.30 and 0.40 V.
The electrode is taken from the cell and immersed
in a NO-saturated solution in 0.1 M sulphuric acid
at open circuit. Then the electrode is rinsed with
water and immersed again in the electrochemical
cell at 0.8 V and the adsorbed NO is reductively
stripped (not shown here). The final voltammetric
profile is that shown in the panel B of Fig. 4. A
single peak at 0.17 V is recorded, the contribu-
tions assigned to the second stage of palladium
on Pt(100). It has been shown that Pt(111) elec-
trode substrates having more than one peak reflect
some disordering in the adlayer [21]. This is also at
the origin of the irreproducibilities observed in the
electrocatalytic behaviour of the Pd–Pt(100) elec-
trodes towards formic acid oxidation [6].
It is possible to overcome this difficulty by using
the adsorption and subsequent reductive stripping
of a NO layer on a reasonably covered palladium
surface as detailed in the following. This procedure
was accidentally discovered when studying the
reactivity of these adlayers towards NO reduction
and the transformation is described in Fig. 4. In
this experiment, the initial state of the electrode

´
B. Alvarez et al. / Surface Science 573 (2004) 32–46
38
2 4
Fig. 4. Stable cyclic voltammograms for: (A) Pd–Pt(100) electrode (hPd > 1) in 0.1 M H SO before NO adsorption and reductively
stripping; (B) same Pd–Pt(100) electrode (hPd ꢁ 1) in 0.1 M H
2
SO
4
after NO adsorption and reductively stripping. Sweep rate: 50
ꢀ1
mVs
.
deposition and uncovered platinum have been
eliminated. The resulting voltammetric profile is
stable upon successive cycles, thus suggesting that
the final adlayer does not undergo further modifi-
cation. It should be stressed that this experiment
also reflects the different reactivity of the palla-
dium atoms deposited in the first and second
stages towards NO adsorption and may be used
to obtain a Pt(100) substrate fully covered by pal-
ladium atoms. In this way the palladium adatoms
directly deposited over the platinum substrate
appear to be particularly stable. The NO proce-
dure essentially enhances the mobility observed
in the so-called electrochemical annealing experi-
ment [3,5]. It is known that adsorbates could con-
tribute to the ordering of palladium surfaces [22].
The easiest way to obtain a Pt(100) electrode
fully covered by a single palladium monolayer is
to deposit palladium until all the Pt(100) sites
are blocked and then remove the excess by the
NO treatment described above. Some NO adsorp-
tion/stripping steps may be required if the amount
of deposited palladium is exceedingly large. This
sort of back-titration has been proven useful to
achieve the completion of the first palladium
adlayer.
Although the effect of NO has not been investi-
gated in detail and probably the open circuit step
would involve different nitrogen and oxygen spe-
cies [23], the results described in Fig. 4 may be
summarized by the following scheme:
Ptð100Þ þ Ptð100Þ–Pd –Pd þ NO
1
n
! Ptð100Þ–Pd –NO þ Pd
1
sol
ð1Þ
In which Pd
layer, Pd in second and further layers and Pdsol
means palladium adatoms in the first
1
n
represents any stable palladium species in solution,
such as neutral or cationic complexes [24].
3.2. Spectroelectrochemical characterization
The modification of the surface structure after
NO adsorption and reduction can be followed also

´
B. Alvarez et al. / Surface Science 573 (2004) 32–46
39
spectroscopically through the changes in the infra-
red spectra obtained in the C–O stretching region
for CO-saturated palladium films. These spectra
were collected both for a surface giving rise to
the voltammetric profile shown in Fig. 4A (before
NO adsorption and reduction) and for the surface
obtained after the reduction of the NO adlayer
(
Fig. 4B). In the corresponding experiments, CO
was dosed for 2 min at 0.10 V and then removed
from the solution by bubbling Ar for 5 min. Satu-
rated CO adlayers are expected to be formed under
these conditions. Spectra were collected at various
sample potentials between 0.10 and 0.50 V. These
spectra were referred to the single beam spectrum
collected at 1.0 V just after the oxidative removal
of the CO adlayer. It has to be remarked here that
the adsorption and subsequent oxidation of the
CO adlayer do not produce significant changes in
the voltammetric profile of the palladium-covered
Pt(100) electrode, irrespective of the palladium
coverage and the observation of one or two
palladium peaks. Spectrum a (lower) in Fig. 5
corresponds to the initial state of the palladium-
covered Pt(100) electrode giving rise to the
voltammogram in Fig. 4A, i.e. before NO adsorp-
tion. This spectrum shows a main band at 1947
ꢀ
1
cm that can be ascribed to the C–O stretching
of CO molecules adsorbed on the palladium atoms
Fig. 5. In situ infrared spectra collected at 0.10 V for a
saturated CO adlayer formed on a palladium film electrode-
posited on a Pt(100) electrode (a) stable profile after deposition
and (b) after the adsorption and subsequent reduction of a NO
adlayer. Reference potential: 1.0 V. A total of 100 interfero-
grams were collected at each potential. Test solution: 0.1 M
[
25,26]. In addition, a small band can be observed
ꢀ
1
at 2047 cm . This feature, which can be related to
the CO molecules adsorbed on the remaining plat-
inum sites (not covered by palladium atoms), is
not observed in the spectrum obtained for the pal-
ladium-covered Pt(100) electrode once it has been
subjected to the NO adsorption/reduction treat-
ment (Fig. 5, spectrum b). It has to be noted that
the band at ꢁ1945 cmꢀ is also the only feature
in the spectra obtained for the palladium multilay-
ers for which, as seen in Fig. 3D, only the voltam-
metric peak at 0.28 V is observed (see below). This
2 4
H SO .
Pt(100) electrodes initially covered by different
amounts of palladium (different relative intensities
of the voltammetric peaks at 0.17 and 0.28 V), to
prepare palladium submonolayers for which the
palladium covered areas do not have palladium
atoms in a second layer. This is shown in Fig. 6
for different palladium adlayers at different extent
of substrate blockage. It is noteworthy that, as in
Fig. 3, the voltammetric profile of the remaining
platinum free sites closely resembles that of the
clean Pt(100) in the base electrolyte. This suggests
that wide Pt(100) bidimensional domains are
present on the surface and conversely that the
1
ꢀ
1
result seems to indicate that the band at 2047 cm
is related to platinum and not to palladium sites.
The use of the NO adsorption step not only
enables the preparation of palladium adlayers on
highly blocked Pt(100) electrodes (very close to
the palladium monolayer) that show a single
adsorption state at 0.17 V as unique palladium
contribution. It is also possible, by starting from

´
B. Alvarez et al. / Surface Science 573 (2004) 32–46
40
Fig. 6. Stable cyclic voltammograms for Pd–Pt(100) electrodes with different palladium coverages (A = 0.05, B = 0.15, C = 0.49,
ꢀ1
2 4
D = 1) in 0.1 M H SO after NO adsorption and reductively stripping. Sweep rate: 50 mVs .

´
B. Alvarez et al. / Surface Science 573 (2004) 32–46
41
palladium adatoms are not randomly spread after
deposition as suggested by the voltammetric re-
sponse. In this way, as in the case of Pt(111) sub-
strates, island growth of palladium may be also
anticipated in the present case [12].
band for CO adsorbed on palladium sites. The
intensity of this feature increases as the palladium
coverage increases (see below for the determina-
tion of the palladium coverage). At the same time,
the intensity of the characteristic bands at 2044
ꢀ
1
The infrared spectra of the CO-saturated Pd–
Pt(100) electrodes with palladium coverage below
unity give additional support to the conclusion de-
rived above about an island growth for the palla-
dium films. Some of these spectra are shown in
Fig. 7 where the spectra corresponding to the
Pt(100) electrode and to a palladium multilayer
are also given for comparison. The deposition of
palladium atoms at subcoverage gives rise to the
and 1862 cm for the Pt(100) surface steadily de-
creases. It has to be noted that the frequency of
both the palladium and platinum features is nearly
the same in all the spectra in Fig. 7 irrespective of
the palladium coverage. This fact strongly indi-
cates that there is not intermixing between palla-
dium and platinum surface sites and thus
supports the island growth of the palladium films
[14]. It has to be remarked that the spectra ob-
tained for the palladium monolayer and for a
multilayer of palladium atoms deposited on the
Pt(100) substrate are quite similar. As it can be
clearly appreciated in Fig. 7, there are no bands
ꢀ
1
appearance at 1944 cm of the C–O stretching
ꢀ
1
above 2000 cm that can be ascribed to CO ad-
sorbed at atop palladium sites.
3.3. Electrochemical characterization
The stable voltammetric profiles of these adlay-
ers enable the measurement of the electric charge
densities corresponding to the deposited palladium
and the remaining free platinum sites. These
charges may be compared and plotted as done pre-
viously with adlayers prepared by means of the
forced adsorption technique [5]. Fig. 8 shows that
this plot is linear, with a slope of ꢀ1, following the
equation:
0
qPt ¼ q ꢀ q ;
ð2Þ
where qPt stands for the charge density under the
Pt
Pd
adsorption state linked to the free platinum sites
0
Pt
(
main peak at 0.39 V), q corresponds to the
charge density of the initial Pt(100) substrate
and qPd represents the charge density under the
palladium adsorption state at 0.17 V. This equa-
tion suggests epitaxial growth of the palladium ad-
layer and is the same as reported when working
with less ordered adlayers [5] indicating that palla-
dium in the second deposition state has similar
adsorption/blockage properties as that in the first
one. From charge measurements the determina-
tion of the palladium coverage is straightforward
and hPd may be defined as
Fig. 7. In situ FTIRspectra for CO-saturated palladium-
covered Pt(100) electrodes in the submonolayer range. 100
interferograms were collected for each sample potential at 0.10
V and referred to a single beam spectrum collected at 1.0 V. The
spectrum collected for CO adsorbed on a palladium multilayer
is also given for comparison.

´
B. Alvarez et al. / Surface Science 573 (2004) 32–46
42
pected when CO adsorption takes place at 0.1 V,
following the displacement reaction:
þ
M ꢀ H þ CO ! M ꢀ CO þ H þ e
ð4Þ
where M would represent either Pt or Pd adsorp-
tion sites on the electrode surface. Thus the dis-
placement process supplies mainly the charge
transferred by the desorption of the previously ad-
sorbed hydrogen atoms, which were produced in
the corresponding reductive adsorption of protons
during the negative-going sweep of the voltammo-
gram. It should be stated that the displaced charge
also would involve the free charge at the displace-
ment potential and that some free charge would re-
main on the CO covered electrode at the potential
of the experiment, but both quantities may be con-
sidered negligible as a first approximation [27].
The combination of the displaced charge densities
at 0.1 V, qdis (0.1 V), and the integration of the vol-
tammetric profile enables to the determination of
the charge density in the whole potential range:
Z E
Fig. 8. Plot of charge corresponding to adsorption processes
occurring onto Pt(100) free sites (qPt) vs. charge associated to
adsorption processes taking place onto palladium adsorbed
atoms (qPd), at different palladium coverages of the Pt(100)
jjðEÞj
qðEÞ ¼
dE ꢀ q ð0:1 VÞ;
dis
ð5Þ
r
0:1 V
where j(E) is the voltammetric current density
(either in the positive or negative-going sweeps)
and r is the sweep rate.
2 4
electrode. 0.1 M H SO .
Fig. 9 shows the corresponding charge density-
potential curves, as obtained from Eq (5), as well
as the corresponding qdis (0.1 V) values, for
Pt(100) electrodes with different palladium cover-
ages. The charge at the lower potentials can be as-
signed to reductively adsorbed hydrogen
(oxidatively desorbed by CO). The plot in Fig. 9
shows that the charge densities displaced at 0.10
V, which are close to the maximum hydrogen cov-
erage, are close to the same experimental value
irrespectively of the palladium coverage. This
means that hydrogen adsorption remains constant
in all the electrodes, including that adsorbed on
surface defects [11] yet present on the Pt(100) sub-
strate. Charge density curves in Fig. 9 show that
the amount of hydrogen on the surface diminishes
as the potential increases and finally vanishes at
higher potentials. Conversely, the charge density
on the electrode at high potentials becomes posi-
tive and would correspond to the adsorption of
anionic species:
qPd
max
Pd
h
Pd ¼ q
;
ð3Þ
max
0
Pt
where qPd (’q ) would correspond to the maxi-
mal charge density of the fully covered Pt(100)
surface, i.e. to the completion of the first palla-
dium monolayer (Fig. 6D) in which the contribu-
tion of the free platinum sites becomes negligible
as the state at 0.39 V is no longer observed in
the voltammetric profile.
The understanding of the adsorption processes
responsible for the appearance of the voltammetric
profiles can be gained from the comparison of the
voltammetric charges and the data supplied by CO
charge displacement experiments [11,12]. In the
latter experiments, charge flowing through the
external circuit when CO is adsorbed on the elec-
trode surface at constant potential can be deter-
mined from the integration of the corresponding
current-time transient curve. Previously studied
cases suggest that oxidation currents would be ex-

´
B. Alvarez et al. / Surface Science 573 (2004) 32–46
43
Fig. 9. Plot total charge vs. electrode potential of Pd–Pt(100)
electrode in 0.1 M H SO at different palladium coverages; (h)
corresponds to the experimental value of the charge displaced at
.10 V.
2
4
Fig. 10. Plot of the potential of zero total charge (pztc) of Pd–
Pt(100) electrodes vs. palladium coverage, in 0.1 M H SO
2
4
.
0
when the monolayer is completed. The negative
shift of the pztc when the adlayer is completed is
considerably larger than that observed on
Pt(111) substrates for the same system [12].
ꢀ
M þ A ! M ꢀ A þ e
ð6Þ
In this way, the voltammetric profiles would
involve the contribution of at least two charge
transfer processes: anion desorption and hydro-
gen adsorption in the negative-going sweep
and the opposite when the potential sweep is
reversed at the lower potential limit of the vol-
tammogram.
In the potential range explored there should be a
particular potential value at which the total charge
density at the electrode surface (including the free
charges at this particular potential) vanishes. This
potential has been identified with the potential of
zero total charge (pztc) of the electrode. The values
of the pztcÕs are plotted as a function of the palla-
dium coverage in Fig. 10. It can be seen that the
pztc diminishes almost linearly with the palladium
coverage and finally attains a nearly constant value
The negative shift of the pztc after palladium
deposition indicates a shift of the onset of anion
adsorption towards less positive potentials. This
behaviour can be confirmed from the in situ infra-
red spectra reported in Fig. 11A and B, which were
obtained in the sulphuric acid solution for the
Pt(100) and the palladium monolayer deposited
on this substrate, respectively. These spectra corre-
spond to different sample potentials and were re-
ferred to the single beam spectrum obtained at
0.10 V where the adsorption of anions is expected
to be negligible. We are aware that the absence of
an excess of a test electrolyte other than sulphuric
acid gives rise to the overlapping of the (bi)sul-
phate adsorbate bands and the bisulphate

´
B. Alvarez et al. / Surface Science 573 (2004) 32–46
44
Fig. 11. In situ FTIRspectra obtained at various sample potentials for (A) bare and (B) a palladium monolayer formed on a Pt(100)
electrode. Test solution: 0.1 M H SO . Reference potential: 0.10 V. 1000 interferograms were collected at each potential.
2
4
consumption bands which appears in the same
spectral range [28,29]. Despite of this overlapping,
the occurrence of (bi)sulphate adsorption gives rise
to a net positive-going S–O stretching band at
ladium adsorbed on Pt(100) upon the oxidation of
adsorbed CO. This is the last step in the charge
displacement experiments used to determine the
pztcÕs as described in the previous section. Fig.
12 shows that the presence of palladium on the
electrode surface has a negative effect in CO oxida-
tion. As a general rule, the main stripping peak ap-
pears at higher potentials (see the insight of Fig.
ꢀ
1
ꢁ1180 cm . This band, which appears at poten-
tial higher than 0.35 V for the Pt(100) [30], is read-
ily observed at 0.25
monolayer.
V for the palladium
12) as the palladium coverage increases. At inter-
3
.4. CO oxidation
mediate palladium coverages a small pre-oxidation
step around 0.85 V is observed although this lower
potential stage appears at higher potentials than
the main peak recorded in absence of adsorbed
palladium.
Despite of that cyclic voltammetry is not the
best technique to study electrochemical kinetics it
can be used to qualitatively show the effect of pal-

´
B. Alvarez et al. / Surface Science 573 (2004) 32–46
45
Fig. 12. Cyclic voltammograms recorded during the electro-oxidation of CO adlayers in 0.1 M H
2
SO
4
for Pd–Pt(100) electrodes at
different coverages (solid line hPd = 0.05, dashed line hPd = 0.49, dotted line hPd = 0.80), as comparison, curve corresponding to bare
Pt(100) is showed (dashed dotted line). Inset shows the shift CO adlayer oxidation peak potential vs. palladium coverage. Sweep rate:
ꢀ
1
5
0 mVs .
This inhibition of CO oxidation is not related to
contribution from subsequent palladium deposited
in further layers. The method is a sort of back-
titration that uses NO adsorption at open circuit
and its reductive stripping. The spectroelectro-
chemical characteristics of this first monolayer
suggest epitaxial growth of the palladium islands.
The charge transfer processes on these elec-
trodes have been characterized by CO displace-
ment. Hydrogen and anion adsorption have been
evidenced as responsible of the voltammetric
charge transfer. The pztc diminishes as the palla-
dium coverage increases.
the concentration of adsorbed CO. In fact, the CO
coverage, once corrected from the anion readsorp-
tion charge [31], is maintained around 0.75, inde-
pendently of the palladium coverage. The
inhibition of CO oxidation on palladium covered
electrodes has been also observed on Pt(111) sub-
strates [12]. This negative effect is just the opposite
to that observed when the platinum electrodes are
modified by adsorbed rhodium [32].
It is interesting to point out that the palladium
covered Pt(100) electrodes are among the best
electrocatalizers described for formic acid oxida-
tion [6]. The lack of activity for CO oxidation sug-
gests that the active intermediate reaction path in
formic acid oxidation does not involve any partic-
ipation of adsorbed CO.
Unlike for the direct oxidation of formic acid [6]
the presence of palladium on the surface has a neg-
ative effect on the oxidation of adsorbed CO.
Acknowledgments
4
. Conclusions
This work has been carried out in the
framework of the project Grupos03/208 from
the Generalitat Valenciana. One if us (A.B.)
acknowledge the MCYT for the award of a doc-
toral grant.
In this paper it has been shown a procedure to
prepare Pt(100) electrodes covered by palladium
up to a full monolayer, in absence of any other

´
B. Alvarez et al. / Surface Science 573 (2004) 32–46
46
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