The adsorption of Sn on Pt(1 1 1) and its influence on CO adsorption as studied by XPS and FTIR

Article abstract of DOI:10.1016/j.electacta.2003.07.009

The coverage of Sn on Pt(1 1 1) which is obtained by electrochemical deposition from 5 × 10^-5 M Sn²⁺ in 0.5 M H ₂SO₄ has been determined by XPS for different deposition times. Complete suppression of hydrogen adsorption corresponds to a coverage of ν_max = 0.35 (Sn to surface Pt atoms). Co-adsorption of CO with Sn on Pt(1 1 1) has been studied by FTIR spectroscopy. The IR spectra of the stretching vibration of CO can be interpreted in terms of the vibrational signature of the Pt(1 1 1)/CO system and no vibrational bands associated with CO on Sn are detected. At high Sn coverages, the 1840 cm^-1 band associated with bridge-bonded CO and the 2070 cm^-1 band assigned to on-top CO are present, however, no hollow site adsorption which is characterized by the 1780 cm^-1 band is revealed within the resolution of the experiment. This vibrational signature corresponds to a less compressed adlayer compared to the (2 × 2)-3CO saturation structure on Pt(1 1 1). At lower Sn coverages, signatures from both the compressed and the less compressed CO adlayer structures are seen in the spectra. From earlier structural and electrochemical studies it is known that Sn is adsorbed in 2D islands and influences CO molecules in its neighbourhood electronically. This leads to a disappearance of the IR band from CO adsorbed in the hollow site at high Sn coverages and to higher population of the weakly adsorbed state of CO for all Sn-modified surfaces, i.e. a relative increase of the amount of CO oxidised at low potentials. In addition to this electronic effect, Sn also exerts a co-catalytic effect at low Sn coverages on that part of CO which is adsorbed at a larger distance from Sn due to a bi-functional mechanism. The IR spectra shows for the Sn-modified Pt(1 1 1) surface that the transition from the compressed CO adlayer which is characterized by the hollow site adsorption of CO to the less compressed one which exhibits a characteristic band associated with bridge-bonded CO occurs already at 250 mV instead of 400 mV.

Full text of DOI:10.1016/j.electacta.2003.07.009

Electrochimica Acta 49 (2003) 73–83
The adsorption of Sn on Pt(1 1 1) and its influence
on CO adsorption as studied by XPS and FTIR
a
a
b,1
a,∗
S. Tillmann , G. Samjeské , K.A. Friedrich , H. Baltruschat
a
Institut für Physikalische und Theoretische Chemie, Universität Bonn, Römerstraße 164, D-53117 Bonn, Germany
Physik Department E19, Technische Universität München, James-Franck Str. 1, D-85748 Garching, Germany
b
Received 28 January 2003; accepted 11 July 2003
Abstract
The coverage of Sn on Pt(1 1 1) which is obtained by electrochemical deposition from 5×10−5 M Sn in 0.5 M H
2+
2
4
SO has been determined
by XPS for different deposition times. Complete suppression of hydrogen adsorption corresponds to a coverage of ϑmax = 0.35 (Sn to surface
Pt atoms).
Co-adsorption of CO with Sn on Pt(1 1 1) has been studied by FTIR spectroscopy. The IR spectra of the stretching vibration of CO can be
interpreted in terms of the vibrational signature of the Pt(1 1 1)/CO system and no vibrational bands associated with CO on Sn are detected.
−
1
−1
At high Sn coverages, the 1840 cm band associated with bridge-bonded CO and the 2070 cm band assigned to on-top CO are present,
−
1
however, no hollow site adsorption which is characterized by the 1780 cm band is revealed within the resolution of the experiment. This
vibrational signature corresponds to a less compressed adlayer compared to the (2 × 2)–3CO saturation structure on Pt(1 1 1). At lower Sn
coverages, signatures from both the compressed and the less compressed CO adlayer structures are seen in the spectra. From earlier structural
and electrochemical studies it is known that Sn is adsorbed in 2D islands and influences CO molecules in its neighbourhood electronically.
This leads to a disappearance of the IR band from CO adsorbed in the hollow site at high Sn coverages and to higher population of the weakly
adsorbed state of CO for all Sn-modified surfaces, i.e. a relative increase of the amount of CO oxidised at low potentials. In addition to this
electronic effect, Sn also exerts a co-catalytic effect at low Sn coverages on that part of CO which is adsorbed at a larger distance from Sn due
to a bi-functional mechanism. The IR spectra shows for the Sn-modified Pt(1 1 1) surface that the transition from the compressed CO adlayer
which is characterized by the hollow site adsorption of CO to the less compressed one which exhibits a characteristic band associated with
bridge-bonded CO occurs already at 250 mV instead of 400 mV.
©
2003 Elsevier Ltd. All rights reserved.
Keywords: CO adsorption; CO oxidation; Sn adsorption; Pt(111); IR spectroscopy; XPS; FTIR
◦
1
. Introduction
23.4 structure based on STM results [1]. It is generally ac-
cepted that the oxidation of the adlayer proceeds according
to a Langmuir–Hinshelwood mechanism. The exact iden-
tity of the oxygen species, however, is not really clear.
Also, there are contradictory results described in the litera-
ture concerning the surface diffusion of CO. Whereas, fast
diffusion should lead to the classical rate equation for a
Langmuir–Hinshelwood mechanism usually treated in text-
books (the rate is proportional to Θ(1 − Θ), where Θ is
the ratio between coverage and maximum coverage), slow
diffusion leads to an oxidation behaviour determined by a
nucleation and growth mechanism [2–6].
Despite its importance for low temperature fuel cells, the
exact mechanism of CO oxidation on Pt and the role of
co-catalysts is still far from being understood. On Pt(1 1 1),
the saturation coverage of CO corresponds to a (2×2)–3CO
adlayer with the coverage of ϑ = 0.75 at low potentials ac-
cording to STM and FTIR results [1]. Here, the coverage ϑ
is the ratio of CO molecules to Pt-surface atoms. At a po-
tential around 0.4 V a part of the adsorbate is oxidised, and
the adlayer transforms into a lower coverage adlayer with
√
√
ϑ = 0.68; Villegas et al. have suggested a ( 19 × 19)R
The transition between the high coverage phase and low
coverage phase leads to the so-called pre-peak in cyclic
voltammetry, which was also observed on polycrystalline Pt
and other surfaces (like vicinally stepped Pt(1 1 1) [7–10]),
∗
Corresponding author. Fax: +49-228-734540.
E-mail address: baltruschat@uni-bonn.de (H. Baltruschat).
1
Present address: ZSW, Helmholtzstraße 8, D 89081 Ulm, Germany.
0
013-4686/$ – see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2003.07.009

7
4
S. Tillmann et al. / Electrochimica Acta 49 (2003) 73–83
and which is often referred to as weakly adsorbed state. By
recording potential transients during oxidation of the ad-
sorbed CO under galvanostatic conditions, we were able to
show that the oxidation of that amount of CO which des-
orbs during the transition between the two adsorbate states
does not follow any of the Langmuir–Hinshelwood type
rate equations [11]. The transients rather suggest an Eley
Rideal mechanism. Probably, CO is oxidised at defect sites
on the surface, the number of which is constant and to which
CO diffuses quickly at high coverages. In contrast, the gal-
vanostatic potential transients during further complete oxi-
dation show the potential maximum typical for both types of
Langmuir–Hinshelwood mechanisms (i.e. fast or slow dif-
fusion) only after oxidation of approximately one tenth of
the adsorbate at nearly constant potential.
CO to Pt. On a Pt(1 1 1) electrode, Sn is much less active,
and this is due to the formation of 2D islands, which, how-
ever, are very mobile in the absence of co-adsorbed CO as
shown in a recent STM study [24]. For the maximum cov-
erage ϑ (ratio of Sn atoms to surface Pt atoms) of Sn on
Pt(1 1 1), we postulated a value of 0.33. This was only based
on a correlation of the oxidation peak charge (presumably
OH adsorption on or within the Sn adlayer on Pt(1 1 1)) to
the suppression of hydrogen adsorption.
It is the aim of this work to get a further insight into the
interaction of Sn and CO. CO adsorbed on Sn decorated Pt
single crystal surfaces is investigated. The absolute Sn cov-
erage which is of paramount importance for the interpreta-
tion of the results was determined by XPS. Furthermore, the
structural properties of the electrode surfaces were investi-
gated by studying the influence of co-adsorbed Sn on the
vibrational spectra of CO.
Concerning the action of co-catalysts, there is also an on-
going debate. It is generally accepted that Ru acts according
to the bi-functional mechanism, see, e.g. Watanabe and Mo-
too [12]. On a Pt(1 1 1)-surface covered by 2D Ru islands,
CO adsorbed in between on Pt areas is able to diffuse to
the Ru sites, where it reacts with oxygen species adsorbed
on Ru [13–18]. However, we have recently shown that for
very low Ru coverages and large distances between Ru sites
CO is oxidised at two different potentials [7]. This has not
only been observed in cyclic voltammetry, but has also been
verified in galvanostatic potential transients, where two po-
tential plateaus are found [11]. This may either be due to
the fact that the diffusion rate depends on CO coverage and
is influenced by Ru [7,18] or signify that Ru influences the
adsorption enthalpy in its vicinity [11], pointing towards an
electronic effect. An argument for the latter assumption is
that even the positive of the two peaks is considerably lower
than that found on pure Pt. These findings have been sub-
stantiated by theoretical calculations [6,19]. It has also been
shown that neither of these two effects alone is able to ex-
plain the co-catalytic effect of Ru in fuel cell anodes [20].
In the case of less noble co-catalysts, it is also often as-
sumed that they act according to the bi-functional mecha-
nism. In the case of Mo, this effect seems to be limited to
the oxidation of the weakly adsorbed CO [21]. In the case
of the Pt3Sn(h k l) alloys, a high activity for bulk CO ox-
idation has been shown that is correlated to a weakly ad-
sorbed state of CO with an onset potential with is lower by
approximately 300 mV [22]. Moreover, it was also demon-
strated with Sn modifications of Pt single crystal surfaces
that a major promoting effect is the shift of the onset poten-
tial of the oxidation of the weakly adsorbed state of CO to
much lower values and, in addition, that the population of
the weakly adsorbed state is drastically increased [7,8,23].
A prerequisite for the high CO oxidation activity of Sn/Pt
surfaces is a uniform distribution of co-adsorbed Sn, e.g. by
step decoration of the Pt(3 3 2) electrode. The oxidation po-
tential of the “strongly adsorbed state”, on the other hand,
is hardly shifted. We therefore assume that contrary to what
is generally believed, Sn mainly has an electronic influence
on neighbouring Pt atoms, changing the binding energy of
2
. Experimental
The single crystals were prepared following Clavilier
et al.’s flame annealing method [25]. They were about 1 cm
in diameter and 3–4 mm thick. After cleaning by cycling
in supporting electrolyte they were heated in a hydro-
gen flame (hydrogen was from Messer Griesheim, purity
5.0) to slightly red colour for 30 s. Afterwards, they were
transferred into the electrochemical cell still glowing to
cool down in a straight stream of argon (argon was from
Messer Griesheim, purity 5.0). This normally takes about
4
min. After this procedure, they were carefully contacted
with the electrolyte using the hanging meniscus technique.
The preparation was then checked by recording a cyclic
voltammogramm.
All solutions were made from supra pure sulphuric acid
from Merck and suprapure (Millipore) water. The adsorption
of tin was performed in a second cell, in order to avoid
contamination of the first cell by tin. During the transfers
between the two cells a drop of electrolyte protected the
surface. Tin solutions were made with 0.5 M H2SO4 and
Sn SO4 from Merck; they had to be freshly prepared because
of an autocatalytic oxidation of Sn to Sn by oxygen which
leads to the build-up of a SnO2 layer on the glass surface.
Tin was adsorbed at 250 mV for 3 min under potential
control. After thoroughly rinsing with ‘Millipore’ water the
crystal was transferred back to the tin-free cell. The crystals
were cleaned in HNO3 and fluoric acid after all experiments
involving Sn deposition.
II
IV
2
.1. IR measurement
The FTIR spectrometer was a Bruker IFS 66 V with an
evacuated chamber to exclude water and CO2 from the
IR beam. The IR light source of the spectrometer is a
silicon-carbide spiral (globar ) and the detector used is a
Hg–Cd–Te semiconductor crystal which was cooled with
liquid nitrogen.
TM

S. Tillmann et al. / Electrochimica Acta 49 (2003) 73–83
75
The measurements were performed in an external reflec-
tion geometry and IR absorption experiments with Pt sin-
gle crystals require a thin-film electrolyte configuration to
minimise IR absorption in the aqueous electrolyte. Prior to
preparation the crystal was mounted on a special holder,
which allows to adjust its height in the cell for normal CVs
Preparation of the Pt(1 1 1) consisted of sputtering
−
5
(8 × 10 Torr Ar 5.0) and subsequent annealing using a
custom-built electron-beam gun. Orientation and cleanliness
were checked by LEED and AES. A CV (0.5 M H2SO4,
−
1
scan speed 50 mV s ) was recorded prior to Sn depo-
sition to have additional information on the condition of
the surface. As the reference electrode a saturated calomel
electrode (with a potential of +0.64 V versus NHE) was
used, connected to the working electrode compartment by
a Teflon tube. Sn deposition was then carried out by cy-
cling the electrode in a 0.5 M H2SO4 solution containing
(“up”) and for IR measurements (“down”; the crystal is lo-
cated on the CaF2 window). In the ‘down’ position, the thin
layer of electrolytes between the crystal and the window
has a thickness of about 3 ␮m. In ‘up’ position, the elec-
trolyte contact was made in the hanging meniscus configu-
ration, whereas in the position ‘down’ the crystal was fully
covered by the electrolyte. In the IR cell, the concentration
of sulphuric acid was restricted to 0.1 M H2SO4 to prevent
dissolution of the CaF2 window (the solubility of CaF2 in-
creases with decreasing pH of the solution). CO adsorption
was made with the crystal in the ‘up’ position. The potential
was held at 50 mV while CO was led over the electrolyte.
After 3 min of adsorption argon was bubbled through the
electrolyte for 10 min to remove the residual CO.
After positioning the crystal in the “down” position with
an orientation of maximum reflection signal, the spectra
were recorded at constant potential. Three hundred spec-
tra were averaged for proper signal-to-noise ratio. In the
potential range between 50 and 850 mV versus RHE, the
spectra were acquired in 100 mV intervals. The spectrum
at 850 mV was selected as a reference spectrum and the
spectra al lower potentials were normalised with respect
to the reference spectrum which was chosen because no
CO is adsorbed and no significant oxide is formed at this
potential.
−
5
3 × 10 M SnSO4.
From the XPS data, the coverage of the electrode with Sn
(ϑSn) was calculated using an internal standard as described
in a previous article [21].
3. Results
An absolute value for the coverage of Sn can only
be obtained using a spectroscopic method such as XPS.
Fig. 1 shows a cyclic voltamogram of Sn-modified Pt(1 1 1)
recorded in the ante chamber of our UHV system prior to
recording the XPS and Auger electron spectra. In the inset
of Fig. 1, a CV of the surface in sulphuric acid is shown.
We have demonstrated before that the anodic peak at 0.55 V
is due to a surface redox process and not due to desorp-
tion of Sn [8,24]. An Auger electron spectrum obtained
after emersion (Fig. 2) shows that the surface is free from
contaminations within the resolution of the measurement.
With LEED, only slightly diffuse (1 × 1) spots could be
observed.
For the IR spectra and CVs measurements with lower CO
initial coverages the following procedure was applied: 50 ␮l
of a CO saturated solution were injected with a syringe into
the IR cell while bubbling with argon and the crystal being
in the ‘up’ position at 50 mV. The bubbling was continued
for 10 min to remove the residual CO.
Fig. 3 shows an example of the XP spectrum. The dou-
blet of the Sn 3d peak, which was used for the quantitative
evaluation, is clearly visible. The data are summarised in Ta-
ble 1. Interestingly, the position of the Sn 3d peak does not
depend on the potential at which the electrode is emersed
from the electrolyte. The binding energy of 486 eV corre-
sponds to SnO2. Therefore, Sn is probably oxidised by water
after loss of potential control, or, more probably, by sulphate
ions staying on the surface, because of its unnoble charac-
ter. Please note that the ratio of the surface concentrations
of O to that of S was 4:1 in all experiments. Other origins
than sulphate for the oxygen in the SnO2 would result in
higher oxygen coverages (O to S ratio). A similar oxidation
by sulphate was suggested for Bi adsorbed on Pt(1 1 0) [26].
In order to obtain the coverage which corresponds to
full blockage of the hydrogen adsorption, we plotted the
hydrogen adsorption charge (as obtained by integration be-
tween 0.07 and 0.35 V) versus the coverage as determined
by XPS and extrapolated the coverage to full suppression
of hydrogen (Fig. 4). This linear extrapolation is justified
by the linear relation between residual hydrogen adsorp-
tion charge and the charge for the peak at 0.55 V found
previously [8,24]. Please note that the charge at zero Sn
2
.2. UHV
UHV experiments were carried out in a modified
Perkin-Elmer 548 system, which is described in more de-
tail in ref. [21]. The base pressure after bake-out is about
−
10
−9
1
× 10
Torr and rises up to 3 × 10 Torr after a few
electrochemical experiments. A gate valve detachable ante
chamber with a horizontal moveable glass cell allows to per-
form the electrochemical experiments without any exposure
of the cell to the laboratory ambience. Prior to an experi-
ment the ante chamber was back-filled with highly purified
−
7
Ar 5.0 to ambient pressure and evacuated to 1 × 10 Torr
after the experiment.
The disk-shaped Pt(1 1 1) single crystal (∅ = 10 mm,
◦
oriented within an accuracy of 0.5 ) was attached through
two Pt wires to the sample holder. This sample holder allows
an hanging meniscus configuration in the electrochemical
experiments.
−
2
coverage of 160 ␮C cm
corresponds to the maximum

7
6
S. Tillmann et al. / Electrochimica Acta 49 (2003) 73–83
0.06
0.04
0.02
0.00
0
0
0
.04
.02
.00
-
0.02
-0.04
-
0.06
0
.0
0.2
0.4
0.6
0.8
1.0
E / V vs RHE
-
-
-
0.02
0.04
0.06
0
.0
0.2
0.4
0.6
0.8
E / V vs RHE
−
5
−1
.
Fig. 1. Cyclic voltammogramm (after five cycles) of the Pt(1 1 1) (—) in a 0.5 M H2SO4 solution containing 3 × 10 M SnSO4. Scan speed: 50 mV s
−
1
Inset: For comparison the CV obtained in pure 0.05 M H2SO4 in a conventional glass cell is also show, scan speed 50 mV s
.
hydrogen adsorption charge at the clean Pt(1 1 1). Similar
to ref. [24], no background subtraction was used for the
data shown in Fig. 4. Since even at the highest achievable
Sn coverages a residual limiting charge of 25–30 ␮C cm
due to double layer charging is obtained in the hydro-
gen adsorption region (cf. [8], where in a plot versus the
anodic peak charge a corresponding correction was used for
the high Sn coverages), this is the charge corresponding to
full blockage of hydrogen adsorption. The value of ϑmax =
0.35 is, within experimental error, identical to the value of
ϑmax = 0.32 postulated previously. Nevertheless, in the fol-
lowing, all coverage values are given as relative coverages
−
2
0
0
0
0
,3
,2
,1
,0
Pt
Pt
-0,1
-0,2
-0,3
-0,4
-0,5
Sn
O
100
200
300
400
500
600
Kinetic energy
Fig. 2. AES spectrum of the Pt(1 1 1) before (- - -) and after (—) Sn deposition. Prior to the back transfer, the electrode was two times immersed into a
−
4
1
× 10 M H2SO4 solution at +0.21 V vs. RHE. Eprim = 3 kV, Isample = 10 ␮A.

S. Tillmann et al. / Electrochimica Acta 49 (2003) 73–83
77
0.4
0.3
0.2
0.1
O
Sn
Pt
Pt
Pt
0
.9
0
0
0
0
0
0
0
0
0
.8
.7
.6
.5
.4
.3
.2
.1
.0
O
Pt
Pt
Sn 3d3
/2 /2
5
-
0.1
540 520 500 480 460
binding energy / eV
5
00
400
300
200
100
0
binding energy / eV
Fig. 3. XPS overview spectra after Sn deposition. Inset: Detail showing the Sn 3d3/2, Sn 3d5/2 peak, that was used for quantification (hatched area).
X-ray power: 14 kV, 14 mA.
as determined from the suppression of the hydrogen adsorp-
tion. Absolute coverages are obtained according to Fig. 4,
Fig. 5 for different potentials. Spectra for CO on the Sn-free
surface are included for comparison and are similar to those
reported before [1]. In the latter case, CO occupies three-fold
H
from these using the relation ϑSn = 1/3 × Θ .
Sn
H
Sn
−1
The influence of UPD-Sn (Θ = 0.74; ϑSn = 0.25) on
hollow sites (band maximum at 1780 cm ) at low potential
−
1
the IR spectra of co-adsorbed CO on Pt(1 1 1) is shown in
in addition to on-top sites (2067 cm ). This was ascribed
180
160
140
120
100
8
6
4
2
0
0
0
0
0
0
.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50
Sn
ϑXPS
Fig. 4. Plot of the hydrogen charge after Sn deposition QH obtained from the CV vs. coverage determined by XPS ϑSn. The extrapolation is done
Sn
−
2
such that the straight line intersects with the y-axis at 160 ␮C cm . Horizontal line (- - -): correction of the background charge of the fully Sn-covered
Pt(1 1 1), obtained by integration of the CV in the potential range between 70 and 350 mV.

7
8
S. Tillmann et al. / Electrochimica Acta 49 (2003) 73–83
Fig. 5. IR spectra of a saturated CO adlayer on Pt(1 1 1) in 0.1 H2SO4, with varying potential at ΘSn = 0 (—) and ΘSn = 0.74 (- - -).
to a compressed (2×2)–3CO structure which is transformed
of a small amount of CO adsorbed in hollow sites which
are transferred to the bridge site at this potential as on clean
Pt(1 1 1). Another possible explanation is the increase of the
population of the bridge sites due to an expanded adlayer as
a consequence of CO electro-oxidation. Note in this respect
that the on-top band intensity is decreased significantly.
Complete oxidation starts at lower potentials compared to
the system without Sn, as seen from the disappearance of
the bands at 650 mV.
into a lower coverage adlayer above 400 mV which was pro-
√
√
◦
posed to correspond to a ( 19 × 19)R 23.4 structure. In
−
1
the new CO adlayer, CO occupies twofold (≈1840 cm
)
−
1
and on-top sites (≈2070 cm ). The spectrum obtained at
4
50 mV shows that both structures still coexist at this poten-
−
1
tial, giving rise to the double peak at 1780 and 1840 cm
.
At 650 mV band, intensities are decreased because oxida-
tion also of the less compressed structure occurs slowly at
this potential.
The spectra of the CO stretching vibrations on Sn-modified
Pt(1 1 1) are best discussed in terms of the band of the
system Pt(1 1 1)/CO: No additional vibrational bands of
CO are present because CO is not adsorbed on tin [27].
However, marked changes compared to Pt(1 1 1)/CO are
detected when CO is co-adsorbed with Sn. The band sig-
nal for CO in threefold hollow sites is negligible even at
Spectra for lower Sn coverages (ΘH = 0.2; ϑSn = 0.07)
Sn
are shown in Fig. 6 (A and C). When discussing for instance
the spectrum at 250 mV it is apparent that all vibrational
features discussed before are present. A broad band below
−
1
1800 cm corresponds to CO in hollow sites and the band
−
1
above 1800 cm is assigned to the bridge site adsorption
of CO. In addition, the on-top band shows a shoulder at
−
1
2070 cm which can be discerned clearly in the spectrum
at 250 mV. This frequency coincides with the band position
for on-top bonded CO on the Sn-free surface. As a conse-
quence, these spectra can be interpreted as being due to a su-
perposition of spectra for the Sn-free surface and that of the
higher Sn-covered surface. Therefore, the most obvious in-
terpretation is that the undisturbed, compressed (2 × 2)–CO
structure (“type I CO”) and an adlayer influenced by tin
(“type II CO”) coexists in separate patches on the surface.
The IR-band frequencies for Pt(1 1 1) surfaces with dif-
ferent Sn coverages are summarised in Fig. 7. At low poten-
tials, the change of band frequencies with potential (Stark
shift) is hardly influenced by Sn and the stretching vibra-
tion bands for on-top CO exhibit a potential dependence
5
0 mV. Interestingly, the spectrum at 50 mV resembles that
of CO on the clean Pt(1 1 1) surface at 650 mV and the
CO layer consists of CO adsorbed in two-fold and on-top
sites. Except for changes of the band positions due to the
potential dependence of the peak frequency with potential
the spectra do not exhibit much variance. One important
aspect is the larger band width of the on-top CO band on
−
1
Sn-modified Pt(1 1 1). Values in the range of 15–17 cm
−
1
for the FWHM is found compared to 9–10 cm for the
neat Pt(1 1 1) surface. This is a consequence of inhomo-
geneous broadening and indicates that the CO adsorbates
experience larger variations in their interaction with either
the surface or the neighbouring molecules (dipole–dipole
interactions). The potential-induced shift in band frequency
can be described with equal validity as a change in the of
−
1
−1
in the range of about 20 cm
V . Furthermore, also the
absolute band frequency is only slightly decreased in the
case of Sn modifications which is probably due to a reduced
dipole–dipole interaction. However, the change of frequency
in the potential range of CO oxidation is considerable. This
is both due to the start of oxidation at lower potentials as
∗
the d␲ –CO back-bonding or as a shift due to the static
electric field in the double layer, the so-called electrochem-
ical Stark effect. A slight increase of the intensity of the
−
1
band at 1840 cm at 450 mV may be due to the presence

S. Tillmann et al. / Electrochimica Acta 49 (2003) 73–83
79
alloy electrode leads to a segregation of small CO patches
on the surface and to extremely compressed CO islands due
to the repulsive interaction of CO and Sn.
4. Discussion
The presence of Sn islands is already obvious from the
cyclic voltammogramm for the Pt(1 1 1) electrode covered
by Sn with a coverage ϑSn = 0.07, shown as insert in
Fig. 6(B): The spike at 0.45 V indicates the adsorption of
sulphate on Pt(1 1 1). If the Sn was distributed homoge-
neously over the surface no sulphate adsorption current spike
would be expected. The Sn islands, however, seem to be
very mobile and are only visible by STM when CO is
co-adsorbed [24]. For the patches on the surface with type
II CO, which is influenced by Sn, there are two possibilities
(cf. Fig. 8):
(
a) Either these islands influence CO molecules in their
vicinity, giving rise to an adlayer of CO with on-top and
bridge site positions; this influence may be electronic in
nature, as in the case of Ru [11], or be due to the distur-
bance of the ordered CO (2 × 2) adlayer structure close
to the islands. Such a disturbance due to steps leading
to a disappearance of CO in three-fold hollow sites and
appearance of CO in bridge sites was observed in ref.
[28,29].
(
b) Or within the islands a mixed adsorbate structure of Sn
and CO is formed, similar to that found for the Bi–CO
system on Pt(1 1 1) [8]. These two possibilities will be
discussed below.
Whereas at 50 mV, the bands for both CO adsorbed at
three-fold hollow sites and at bridge sites are of equal height,
−
1
that at 1840 cm for bridge site CO starts to increase al-
ready at 250 mV. Therefore, the transition of the compressed
adlayer to the less compressed one is facilitated by Sn. This
is in accordance with the lower oxidation potential of the
weakly adsorbed CO on Sn-modified Pt surfaces.
It might be explained by a bi-functional effect of Sn,
acting on the oxidation of those CO molecules which are
oxidised during the transition from the compressed to the
less compressed structure. Since CO does not adsorb on
Sn the adsorption of the oxygen species responsible for
the oxidation is not hindered by the CO adlayer. We have
shown previously, that the main effect of Sn modifying a
Pt(1 1 1) surface consists in decreasing this onset potential.
However, the amount of CO being oxidised in this pre-peak
is not largely increased and the potential of the main oxi-
dation peak also is only slightly decreased [8]. Therefore,
if Sn acts according to the bi-functional mechanism, it is
only active for the weakly adsorbed state (pre-peak). The
IR spectra show that not only for type II CO the oxidation
potential is decreased. The transfer from three-fold hollow
sites to bridge sites rather occurs in that part of the adsorbate
which is not in close proximity to Sn and therefore forming
Fig. 6. (A) IR spectra of Pt(1 1 1)/ΘSn = 0.2 in 0.1 M H2SO4. (B)
Cyclic voltammogramm in IR cell corresponding to measurements in (A)
−1
dU/dt = 50 mV s , ꢀU = 50–650 mV vs. RHE. (C) Comparison of IR
spectra of Pt(1 1 1) in 0.1 M H2SO4 at 0.25 V vs. RHE, ΘSn = 0 and 0.2.
well as the somewhat increased amount of CO oxidised in
the “pre-peak”. The potential dependence of the band fre-
quencies is at variance with a recent publication by Sta-
menkovic et al. on Pt3Sn(1 1 1) electrodes in which a strong
−
1
−1
V is reported [27]. Also a
Stark tuning slope of 45 cm
−
1
CO on-top band with frequencies of about 2090 cm are
shown. This difference is probably related to the distribution
of Sn atoms on the surface. Following the interpretation of
Stamenkovic et al., the homogenous distribution of Sn in the

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0
S. Tillmann et al. / Electrochimica Acta 49 (2003) 73–83
2075
(A)
2070
2065
2060
Pt(111) without Sn
Pt/Sn θSn=0.2
2055
Pt/Sn θSn=0.34
'Pt/Sn θSn=0.74
1860
(B)
1840
1820
1800
(C)
Pt(111) without Sn
Pt/Sn θSn=0.2
Pt/Sn θSn=0.34
Pt/Sn θSn=0.74
1780
100
200
300
400
500
600
700
E / V vs. RHE
Fig. 7. Wavenumbers of stretching vibrations of CO on Pt(1 1 1) for different tin coverages as a function of the oxidation potential. (᭹) Neat Pt(1 1 1)/CO;
(
᭺) Pt/SnΘSn = 0.2; (ᮀ) Pt/SnΘSn = 0.34; (ꢀ) Pt/SnΘSn = 0.74. (A) Potential dependence of band frequencies of on-top CO: at low potentials
the on-top stretching band is deconvoluted into two maxima for ΘSn = 0.2. (B) Potential dependence of band frequencies of bridge-bonded CO: three
different tin coverages are compared to neat Pt(1 1 1). (C) Potential dependence of band frequencies of the hollow site of CO: three different tin coverages
are compared to neat Pt(1 1 1).
the compressed layer (type I CO). Probably the repulsive
interaction of Sn leads to a lower stable CO coverage on
the surface and favouring expanded CO adlayers.
The situation changes drastically when Sn is decorating
the steps of a Pt(3 3 2) or a Pt(7 5 5) surface and forming
monoatomic rows: up to 50% of the adsorbed CO is then
oxidised in the “pre-peak”. As we have outlined before [8],
the only probable explanation for this is, that Sn exerts an
electronic, repulsive influence on CO molecules adsorbed in
its vicinity. This allows us to distinguish between the two
possibilities mentioned above for the influence of Sn on the
IR spectra of type II CO on Pt(1 1 1). Is this part of the
CO adsorbate influenced by the neighbourhood of pure Sn
islands or is it a mixed adsorbate layer with Sn forming
islands? It is likely that this part of the adsorbed CO for
which the IR spectrum is changed is also the part of the CO
which is most easily oxidised due to the action of Sn. The
number of molecules thus influenced is larger when, due
to step decoration, Sn is forced into a more homogeneous
distribution over the surface instead of being adsorbed in

S. Tillmann et al. / Electrochimica Acta 49 (2003) 73–83
81
Fig. 8. Schematic drawing of the two types of CO domains. Two possibilities for Sn–CO interaction: (a) effect of pure Sn islands on CO in its
neighbourhood; (b) CO–Sn co-adsorbate. (c) Model of the CO distribution at a Sn decorated stepped Pt single crystal electrode. ( ) Undisturbed CO
(type I); ( ) region of CO influenced by Sn (type II); ( ) pure Sn regions.
islands. We assign the band of 1840 cm 1 in the spectrum
at 50 mV in Fig. 6 to bridge-bonded CO in these electroni-
cally influenced CO adlayer regions. Since on Pt(1 1 1), the
absolute amount of CO oxidised in the pre-peak does not
depend much on the Sn coverage [8], this adlayer is not
a mixed Sn–CO adlayer. If there was a mixed adlayer (in
addition to CO which is not influenced by Sn), one would
expect a direct relation or even a proportionality of the
amount of CO in the weakly adsorbed state on the Sn cover-
age. Also, the absence of the high frequency band at about
−
seems that not all of the CO thus influenced is oxidised in
the pre-peak. Only the relative population of the weakly ad-
sorbed state is largely increased within these electronically
influenced parts of the surface, but the strongly adsorbed
state also persists in these parts of the surface. This is cor-
roborated by the finding that even on a Pt3Sn(1 1 0) alloy,
where each Pt atom should be in direct neighbourhood to
a Sn atom and therefore all CO molecules should be influ-
enced, only about 1/3 of all CO is oxidised in the pre-peak
[22].
Another possibility has to be taken into account. It has
been found by us and others that CO on stepped Pt(3 3 2)
and Pt(7 5 5) surfaces at potentials below the occurrence of
the pre-peak adsorbed CO is only adsorbed on top and on
bridge-bonded positions, [28,29]. Obviously a certain min-
imal size of CO domains (corresponding to more than 5 Pt
atoms) is necessary for a (2×2)–3CO adlayer to form. Then,
the effect of Sn on the IR spectra simply could be a structural
disturbance which is not directly connected to the electronic
effect of Sn on the adsorption strength of CO. We could then
assume that in the electronically influenced regions all CO
molecules are oxidised in the pre-peak region. Our data in
ref. [8] where we varied the Sn coverage on a Pt(3 3 2) sur-
face, do not allow a clear distinction of these two possibili-
ties. However, if we assume this latter possibility to be true,
we can estimate from these data the range over which this
effect extends. The Pt(3 3 2) was annealed in ref. [8], which
is now known to lead to facetting, involving a doubling the
−1
2
090 cm reported by Stamenkovic et al. [27] supports this
assumption. However, the results obtained with step dec-
oration on Pt(3 3 2) suggest that the distribution of the Sn
is decisive for the amount of Sn in the weakly adsorbed
state.
In the case of pure Sn islands formed on Pt(1 1 1), the
amount of CO thus influenced should be proportional to
the perimeter of the islands. Assuming a disk shape of the
islands, the amount of type II CO should be proportional to
the square root of the Sn coverage as long as they do not
overlap. The lengthy structure of the islands [24] suggests
that the islands overlap at a low coverage already; then the
increase of the perimeter becomes slower and the amount
of type II CO stays nearly constant and even decreases in
accordance with the results given in ref. [8].
Even at a high Sn coverage, where according to the IR
spectra all CO is influenced by Sn (cf. Fig. 5), the main ox-
idation peak is still larger than the pre-peak. Therefore, it

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S. Tillmann et al. / Electrochimica Acta 49 (2003) 73–83
terrace width to 10 rows of Pt atoms [11,30,31]. At a relative
Sn coverage of 50%, where CO can adsorb on five rows of
Pt atoms, (assuming ideal step decoration, cf. ref. [24]) 60%
of CO was found to be oxidised in the pre-peak. Since Sn
is adsorbed on both sides of this “band” of Pt, 40% of these
Pt atoms are in direct neighbourhood of Sn, meaning that
the electronic effect of Sn on Pt only extends over one row
of Pt atoms. The pre-peak on Sn covered Pt(1 1 1) is larger
than one would expect in this case from the size and distri-
bution of the Sn islands, but this may also be due to the mo-
bility of the Sn atoms when some CO starts being oxidised
In addition, there also seems to be an effect according to
the bi-functional mechanism, which is active only for the
weakly adsorbed state but extends over all the surface in-
cluding that part of the adsorbate, which is not electronically
influenced by Sn. Similar to the case of Mo, this effect does
not work for the strongly adsorbed CO.
Acknowledgements
Financial support by the DFG is gratefully acknowledged.
[24].
We therefore can exclude the formation of a mixed Sn–CO
adlattice, but we cannot clearly distinguish between the fol-
lowing two possibilities at this stage:
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