Electrooxidation of methanol on upd-Ru and upd-Sn modified Pt electrodes

Article abstract of DOI:10.1021/jp0651891

The electrochemical oxidation of methanol has been investigated on underpotentially deposited-ruthenium-modified platinum electrode (upd-Ru/Pt) and on underpotentially deposited-tin-modified platinum electrode (upd-Sn/Pt). The submonolayers of upd-Ru and upd-Sn on a Pt electrode increased the rate of methanol electrooxidation several times as large as that on a pure Pt electrode. The best performance for methanol electrooxidation was obtained on a ternary platinum based catalyst modified by upd-Ru and upd-Sn simultaneously. The influence of the submonolayers of upd-Ru adatoms and upd-Sn adatoms on the oxidation of methanol in acid has been investigated. The effect of Ru on methanol electrooxidation lies on the distribution of Ru adatoms on a Pt surface. It has been shown that as long as the amount of upd-Ru deposits were controlled in a proper range, upd-Ru deposits would enhance the methanol oxidation obtained on a Pt electrode at whichever deposition potential the upd-Ru deposits were obtained. The effects of tin are sensible to the potential range. The enhancement effect of upd-Sn adatoms for the oxidation of methanol will disappear as the electrode potential is beyond a certain value. It is speculated that there exists a synergetic effect on the Pt electrode as adatoms Ru and Sn participate simultaneously in the methanol oxidation.

Full text of DOI:10.1021/jp0651891

J. Phys. Chem. B 2006, 110, 26055-26061
26055
Electrooxidation of Methanol on upd-Ru and upd-Sn Modified Pt Electrodes
Z. D. Wei,*,†,‡ L. L. Li,‡,| Y. H. Luo,‡ C. Yan,‡ C. X. Sun,† G. Z. Yin,§ and P. K. Shen
Key Laboratory of High Voltage Engineering and Electrical New Technology, Chongqing UniVersity,
Chongqing, 400044; China; School of Chemistry and Chemical Engineering, Chongqing UniVersity,
Chongqing, 400044; China; Key Laboratory of Exploitation of Southwestern Resources & the EnVironmental
Disaster Control Engineering, Chongqing UniVersity, Chongqing, 400044; China; School of Material Science
and Engineering, Chongqing UniVersity, Chongqing, 400044; China; and The State Key Laboratory of
Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-Sen UniVersity,
Guangzhou 510275, China
ReceiVed: August 11, 2006; In Final Form: October 4, 2006
The electrochemical oxidation of methanol has been investigated on underpotentially deposited-ruthenium-
modified platinum electrode (upd-Ru/Pt) and on underpotentially deposited-tin-modified platinum electrode
(upd-Sn/Pt). The submonolayers of upd-Ru and upd-Sn on a Pt electrode increased the rate of methanol
electrooxidation several times as large as that on a pure Pt electrode. The best performance for methanol
electrooxidation was obtained on a ternary platinum based catalyst modified by upd-Ru and upd-Sn
simultaneously. The influence of the submonolayers of upd-Ru adatoms and upd-Sn adatoms on the oxidation
of methanol in acid has been investigated. The effect of Ru on methanol electrooxidation lies on the distribution
of Ru adatoms on a Pt surface. It has been shown that as long as the amount of upd-Ru deposits were controlled
in a proper range, upd-Ru deposits would enhance the methanol oxidation obtained on a Pt electrode at
whichever deposition potential the upd-Ru deposits were obtained. The effects of tin are sensible to the potential
range. The enhancement effect of upd-Sn adatoms for the oxidation of methanol will disappear as the electrode
potential is beyond a certain value. It is speculated that there exists a synergetic effect on the Pt electrode as
adatoms Ru and Sn participate simultaneously in the methanol oxidation.
1. Introduction
into Pt catalysts has been known to be significantly effective
for CO removal at a relatively negative potential.1-4 The
The direct methanol fuel cell (DMFC) is a variant of the
proton exchange membrane (PEM) fuel cell, which uses aqueous
methanol directly without prior reforming. Operating fuel cells
with liquid fuel is essential for transport applications. In addition,
since DMFC can operate at temperature below 100 °C without
a fuel processor, the complexity and cost of the system is
considerably reduced. These advantages allow DMFC to be
suitable for use in portable electronic devices. However, the
performance of DMFC is obviously deteriorated by poor
electrochemical activity and the kinetic loss of methanol
oxidation at the anode, which account for a reduction in cell
voltage of more than 0.3 V at 500 mA/cm2 (at 90 °C). To
increase the anode and cathode activities, the electrocatalysts
employed in DMFC are usually unsupported and the platinum
loading is typically as high as 5-10 mg/cm2 in contrast with
carbon-supported electrocatalysts in H2-fueled PEM fuel cell,
loaded with 0.2-0.5 mg/cm2 of platinum. This platinum loading
is too high to be practical for commercial exploitation of DMFC.
Additionally, Pt is easily subject to CO poison, an intermediate
of methanol oxidation although platinum is the most active
catalyst of all metallic catalysts for methanol oxidation. So, it
is necessary to remove CO from the surface of platinum,
especially at a relatively negative potential. Adding ruthenium
catalytic action of PtRu proceeds mainly by the so-called
“bifunctional mechanism”, that is, Ru sites adsorb oxygen-
containing species at a potential of 0.2-0.3 V lower than the
pure Pt sites, and the adsorbed carbonaceous species are
preferentially oxidized by the oxygen-containing species.5 The
whole mechanism can be shown as follows:
Pt + CH3OH f Pt-CO + 4H+ + 4e-
Ru + H2O f Ru-OH + H+ + e-
(1)
(2)
Ru-OH + Pt-CO f Ru + Pt + CO2 + H+ + e- (3)
The electrocatalytic activity of PtRu depends on several
factors, among which the composition and electronic state
density get principal attention.6,7 According to the so-called
bifunctional mechanism, Pt and the second atoms, that is, Ru,
independently play their own roles in the catalysis of methanol
oxidation. This suggests that it would not be critical whether
Pt-Ru is present in forms of alloy, bimetallic compound,
adatoms on substrate, or just simple mixture. The most important
is in the geometric distribution and composition of the PtRu.
Wasmus and Kuver discussed this in a review paper.5 Two
optimum atomic percentages, 50 atom % and 10 atom %, of
ruthenium in PtRu were suggested on the basis of Pt-COads
and 3Pt-CH3OH, respectively.
* Author to whom correspondence should be addressed. E-mail: zdwei@
cqu.edu.cn.
† Key Laboratory of High Voltage Engineering and Electrical New
Technology, Chongqing University.
‡ School of Chemistry and Chemical Engineering, Chongqing University.
§ Key Laboratory of Exploitation of Southwestern Resources & the
Environmental Disaster Control Engineering, Chongqing University.
| School of Material Science and Engineering, Chongqing University.
Sun Yat-Sen University.
PtSn is another catalyst system which has been widely
discussed for enhancing methanol oxidation on Pt. The catalytic
activity of Sn atoms in PtSn catalysts on methanol oxidation is
usually interpreted in terms of a bifunctional mechanism.8
10.1021/jp0651891 CCC: $33.50 © 2006 American Chemical Society
Published on Web 12/05/2006

26056 J. Phys. Chem. B, Vol. 110, No. 51, 2006
Wei et al.
Figure 1. CV in 0.5 M H2SO4 at a sweep rate of 50 mV s-1 on a Pt electrode (solid line) and upd-Ru/Pt electrodes (dashed line) formed in 0.5
M H2SO4 containing 5 mM RuCl3 at Eupd ) 0.58 V, Qupd ) 10 mC cm-2 (a); Eupd ) 0.68 V, Qupd ) 15 mC cm-2 (b); Eupd ) 0.78 V, Qupd ) 15mC
cm-2 (c); and Eupd ) 0.78 V, Qupd ) 25 mC cm-2 (d).
Contrary to this general belief, Tillmann et al.9 hold that Sn
mainly has an electronic influence on neighboring Pt atoms and
thus changes the binding energy of CO to Pt.
The underpotential deposition (upd) of foreign atoms on a
substrate metal has received much attention in electrocatalysis.
It seems that the submonolayer characteristic of upd-Ru and
upd-Sn deposits on Pt electrode would be very favorable to the
methanol oxidation according to the so-called bifunctional
mechanism. Although PtRu and PtSn catalysts have been
extensively used for methanol electrooxidation, comparably little
work on upd-Ru on Pt electrode for methanol oxidation can be
found in the literature.10 In this communication, we present a
upd-Ru and a upd-Sn on Pt surface to get upd-Ru and upd-Sn
Figure 2. Methanol electrooxidation in 0.5 mol/L H2SO4 and 2 mol/L
modified Pt electrodes for methanol oxidation. The effects and
CH3OH at a sweep rate of 1 mV s-1 on a Pt electrode (line 1), on a
role of upd-Ru and upd-Sn on methanol oxidation are examined
opd-Ru/Pt made in 0.5 M H2SO4 containing 5 mM RuCl3 at potential
using electrochemical methods.
of 0.08 (line 2), and on a upd-Ru/Pt at 0.48 (line 3) and 0.58 V (line
4). For all Ru-modified Pt electrodes, the deposition maintains 5 min
at the particular potential for all Ru.
2. Experimental
2.1. Preparation of upd-Ru/Pt and upd-Sn/Pt Electrodes.
Pt disk electrode was polished on a polishing cloth with a
suspension of 0.05 µm alumina and water and then was cleaned
by ultrapure water from Mill-Q water treatment system. Then,
the electrode was immersed in nitrogen-purged 0.5 M H2SO4
and was voltammetrically scanned from 0 to 1.8 V (vs Ag/
AgCl) at a rate of 100 mV s-1 to clean the surface. The
cleanliness of the surface was checked by cyclic voltammetry
from -0.2 to 1.2 V (vs Ag/AgCl). The geometric surface area
of the Pt disk electrode is 0.071 cm2 (diameter 3 mm).
The underpotential deposition was performed using CHI 660b
electrochemical station (CH Instrument, Inc) in the cell consist-
ing of a Pt disk electrode with a roughness factor of 1.57, a Pt
sheet counter electrode, and a saturated-potassium-chloride silver
chloride electrode (SSCE) reference electrode. All potentials
in this study were quoted with reference to SSCE (i.e., 0.20 V
vs SHE) unless otherwise stated. As for preparation of upd-
Ru/Pt, the cleaned Pt disk electrode was immersed in a solution
of 0.5 M H2SO4 containing 5 mM RuCl3. The upd-Ru deposition
was performed by stepping the electrode potential to a potential
Eupd, at which the Ru ad-atoms were underpotentally deposited.
In this work, potentials, such as 0.48 V, 0.58 V, 0.68V, and
0.78V, were chosen to carry out the upd-Ru building on the Pt
electrode. The deposition at the above potentials is an under-
potential deposition for Ru in comparison with the equivalence
potential, 0.40 V, of the reaction
RuCl52- + 3e- ) Ru + 5Cl-
The amount of deposited ruthenium was controlled by
(4)

Electrooxidation of Methanol on Pt Electrodes
J. Phys. Chem. B, Vol. 110, No. 51, 2006 26057
Figure 3. Methanol electrooxidation in 0.5 mol/L H2SO4 and 2 mol/L
CH3OH at a sweep rate of 1 mV s-1 on upd-Ru/Pt made in 0.5 M
H2SO4 containing 5 mM RuCl3 at potentials of 0.48 and 0.58 V with
different deposition times. From lines 1 to 4, corresponding deposition
time is 5, 10, 15, and 20 min, respectively.
deposition charge (Qupd), for instance, 10, 15, 20, and 25 mC
cm-2 and so forth.
For upd-Sn/Pt electrode, tin was underpotentially deposited
according to a previous study11 just at -0.17 V from freshly
prepared deposition solution of 10-5 M SnSO4 in 0.5 M H2SO4
with a deposition time of 10 s, 50 s, 100 s, and 300 s. The
solution had to be freshly prepared to avoid the oxidation of
Sn2+ to Sn4+ by dissolved oxygen.
The upd-RuSn/Pt electrode was prepared by construction of
upd-Ru/Pt at 0.58 V first and then by construction of upd-Sn at
-0.17 V sequentially.
2.2. Electrochemical Measurement. After deposition, upd-
Ru and upd-Sn modified Pt electrodes were washed with
ultrapure water and then were transferred to a cell containing
0.5 M H2SO4 with or sometimes without 0.5 M CH3OH for
electrochemical measurement.
Figure 4. Methanol electrooxidation in 0.5 M CH3OH + 0.5 M H2-
SO4 at a sweep rate of 5 mV s-1 on upd-Ru/Pt electrodes obtained at
different underpotentials with different deposition charges compared
with pure Pt electrode.
testify to the presence of Ru adatoms on the Pt electrode as
shown in Figure 1. Although widely used,13,14 it has been proved
not to be good to estimate the coverage of Ru adatoms according
to the difference in area of the hydrogen desorption in the CV
from -0.20 to 0.15 V before and after upd-Ru in Figure 1
because of the overlap of the hydrogen desorption and ruthenium
oxidation currents, with the latter commencing at around 0.05
V. In addition, seemingly more than one monolayer of hydrogen
can be established at a ruthenium surface because of absorption
into the oxide lattice as a result of the formation of ruthenium
bronzes and also dissolution of atomic hydrogen into the metallic
ruthenium.12,15
3. Results
3.1. Methanol Oxidation on upd-Ru/Pt Electrode. The CV
(i.e., cyclic voltammetry) of Pt disk electrode and upd-Ru
modified Pt electrode (upd-Ru/Pt) were recorded in 0.5 M
H2SO4 as shown in Figure 1. To avoid the oxidation of the
ruthenium to a state that either dissolves in the aqueous phase
and is lost or produces a form of oxidized ruthenium that is in
a state that cannot be electrochemically reduced,12 the vertex
potential in the CV of upd-Ru/Pt electrode was kept below 0.9
V. The suppressed area of hydrogen desorption in a potential
range from -0.20 to 0.15 V and the Pt-O electrochemical
reduction from 0.60 to 0.20 V in the CV of upd-Ru/Pt electrode
Ru + nH+ + ne- f RuHn
(5)
The coverage of Ru adatoms was controlled by the length of
the deposition time at a certain deposition potential, in other

26058 J. Phys. Chem. B, Vol. 110, No. 51, 2006
Wei et al.
Figure 5. Methanol oxidation in 0.5 M CH3OH + 0.5 M H2SO4 at a
sweep rate of 5 mV s-1 on upd-Ru/Pt electrodes made in 0.5 M H2SO4
containing 5 mM RuCl3 at different potentials for different deposition
charges.
TABLE 1: Performance of upd-Ru/Pt Electrode Prepared in
Different Potentials
Pt
upd-Ru/Pt upd-Ru/Pt upd-Ru/Pt
0.58 0.68 0.78
10 15 15
E
Q
upd/V
upd/mC cm-2
E @ 0.5 mA cm-2/V 0.37
j @ 0.3 V/mA cm-2
0.13
0.27
0.97
0.29
0.69
0.27
1.02
words, by the deposition charge, Qupd, as illustrated in Figure
1. Because of the presence of other reactions in a potential range
of upd-Ru deposition, such as formation of surface Pt-O, partial
reduction of RuCl52-, and so forth, the deposition charge Qupd
is certainly not totally consumed in upd-Ru deposition. Con-
sequently, the coverage of Ru adatoms cannot be estimated
according to Qupd. In effect, only a small portion of Qupd was
used for a submonolayer of upd-Ru deposits, but most Qupd was
consumed in side reactions such as eq 5 in the potential range
of underpotential implement in consideration of that the
roughness factor of the Pt disk working electrode is about 1.57.
The influence of the potential held in Ru deposition on methanol
oxidation is shown in Figure 2. Either upd-Ru (curves 3 and 4)
or overpotentially deposited Ru, i.e., opd-Ru (curve 2) shows a
good enhancement for methanol oxidation on Pt electrode in
0.5 M CH3OH + 0.5 M H2SO4. As far as the deposition
conditions illustrated in Figure 2 are concerned, upd-Ru
made at 0.58 V exhibits the best enhancement for methanol
oxidation. Figure 3 shows the influence of Ru deposition time
at potentials of 0.48 and 0.58 V on methanol oxidation in
0.5 mol/L H2SO4 and 2 mol/L CH3OH. Figure 3 tells us that
too much Ru deposits on a Pt electrode are not so good for
Figure 7. Potential vs time on Pt and upd-Ru/Pt at 5 mA cm-2 (a)
and 10 mA cm-2 (b) in 0.5 M H2SO4 + 0.5 M CH3OH.
Figure 8. CV of a pure Pt in 0.1 M HClO4 (solid line) and in 0.1 M
HClO4 with 10-6 M Sn2+ (dashed line) at 50 mV s-1
.
methanol oxidation. This is in good agreement with the
bifunctional mechanism of methanol oxidation. The number
of exposed Pt sites is necessary for the initial dehydogenation
of methanol molecules.16 Figure 4 shows methanol oxidation
on upd-Ru/Pt electrodes obtained at three different underpo-
tentials with different deposition charges in comparison with a
pure Pt electrode in 0.5 M CH3OH + 0.5 M H2SO4. The best
results obtained at potentials 0.58, 0.68, and 0.78 V, respectively,
were overlapped together in Figure 5. It gives a sign that as
long as the amount of upd-Ru deposits were controlled in a
proper range, upd-Ru deposits would enhance the methanol
oxidation obtained on a Pt electrode at whichever deposition
Figure 6. Current vs time as potential is kept at 0.28 V in 0.5 M
H2SO4 + 0.5 M CH3OH on a pure Pt electrode and a upd-Ru/Pt
electrode.

Electrooxidation of Methanol on Pt Electrodes
J. Phys. Chem. B, Vol. 110, No. 51, 2006 26059
Figure 11. 3-D plot of IR spectra corresponding to the first (1) and
the fourth (2) continuous cyclic voltammograms of methanol oxidation
on a Pt electrode with modulation of a potential sweeping at a rate of
50 mV s-1 in 1 M CH3OH and 0.1 M HClO4.
Figure 9. Methanol electrooxidation in 0.5 M H2SO4 + 0.5 M CH3-
OH at a sweep rate of 5 mV s-1 on a pure Pt electrode (a), a upd-Ru/
Pt electrode formed in 0.5 M H2SO4 containing 5 mM RuCl3 at Eupd
)
0.58 V with Qupd ) 10 mC cm-2 (b), and upd-Sn/Pt electrodes formed
in 0.5 M H2SO4 containing 10-5 M SnSO4 at Eupd ) 0.172 V hold for
10 s (c) and for 100 s (d). (2) The potential window of 1 from 0 to
0.25 V.
Figure 10. Current vs time as potential is kept at 0.38 V in 0.5 M
H2SO4 + 0.5 M CH3OH on a pure Pt electrode (a) and upd-Sn/Pt
electrodes formed in 0.5 M H2SO4 containing 10-5 M SnSO4 at Eupd
)
Figure 12. Methanol electrooxidation in 0.5 M H2SO4 + 0.5 M CH3-
OH at a sweep rate of 5 mV s-1 on a pure Pt electrode (a), a upd-Ru/
Pt electrode formed at Eupd ) 0.58 V with Qupd ) 10 mC cm-2 (b), a
upd-Sn/Pt electrode formed in 0.5 M H2SO4 containing 10-5 M SnSO4
at Eupd ) 0.172 V hold for 10 s (c), and a upd-RuSn/Pt electrode formed
0.172 V hold for 10 s (b), 50 s (c), and 100 s (d).
potential the upd-Ru deposits were obtained. The reason why
more deposition charge is consumed for upd-Ru deposition at
a more positive deposition potential is because the more positive
the deposition potential is, the lower the current efficiency is
for Ru deposition.
in 0.5 M H2SO4 containing 5 mM RuCl3 at Eupd ) 0.58 V with Qupd
)
10 mC cm-2 and then in 0.5 M H2SO4 containing 10-5 M SnSO4 at
Eupd ) 0.172 V hold for 10 s (d). (2) The potential window of 1 from
0 to 0.2 V.
On a upd-Ru/Pt electrode, there are two methanol oxidation
peaks. One at a relatively positive potential corresponds to the
oxidation of methanol on the pure Pt island, and one at a
relatively negative potential corresponds to on the upd-Ru/Pt
island. By contrast, there is only one methanol oxidation peak
either on an opd-Ru/Pt electrode or on a pure Pt electrode as
shown in Figure 2. Obviously, it is because there is only one
type of electrode surface in the case of either an opd-Ru/Pt
electrode or a pure Pt electrode. The position of methanol
oxidation peak corresponding to the Pt island at the potential
scale stays at almost the same potential from the upd-Ru/Pt
electrode to a pure Pt electrode. However, the position of
methanol oxidation peak corresponding to upd-Ru/Pt island at
the potential scale changes with difference in preparation of
the upd-Ru/Pt electrode as shown in Figures 2-4. This suggests

26060 J. Phys. Chem. B, Vol. 110, No. 51, 2006
Wei et al.
TABLE 2: Comparison among Pt, upd-Ru/Pt, upd-Sn/Pt, and upd-RuSn/Pt
electrode
V0/Va
V0 -/Vb
j @ 0.20 V/mA cm-2
j @ 0.30 V/mA cm-2
j @ 0.32 V/mA cm-2
j @ 0.40 V/mA cm-2
Pt
0.35
0.20
0.10
0.10
-0.067
-0.086
-0.115
-0.124
-0.135
-0.503
-0.369
-0.636
-0.172
-0.790
-0.476
-0.790
-0.873
-2.214
-0.873
-1.526
upd-Ru/Pt
upd-Sn/Pt
upd-RuSn/Pt
0.15
0.25
0.25
a V0: onset potential of methanol oxidation. b V0 -: potential of V0 shift to negative direction relative to Pt.
that the electrochemical performance of the upd-Ru/Pt electrode
with different Ru adatoms is different from one another. It may
be ascribed to the difference in electronic state density of the
upd-Ru/Pt electrode resulting from the difference in the amount
of Ru adatoms.6
whether a binary system is an alloy or just a mixture of two
metals, and Sn may leach out under acid conditions being in
turn readsorbed electrolytically at Pt sites. Xia19 investigated
the influence of upd-Sn on the electrooxidation of formic acid
on platinum in an acidic solution and proposed that the
+
electrooxidation is accelerated by formation of Sn[(OH)]ad
.
The data taken from Figure 5 are summarized in Table 1. It
clearly demonstrates that the potential at methanol oxidation
current of 0.5 mA cm-2 shifts toward the negative direction,
and the methanol oxidation current at potential of 0.3 V increases
several times with the modification of upd-Ru to the Pt electrode.
The long-run tests for methanol oxidation on a pure Pt
electrode and on a upd-Ru/Pt electrode are illustrated in Figure
6 and Figure 7. The enhancement of upd-Ru deposits in
electrocatalysis of methanol oxidation on a Pt electrode is
apparent in every respect of the low oxidation potential at a
certain oxidation current (see Figure 6) and the large oxidation
current at a certain oxidation potential (see Figure 7). It is
interesting why the methanol oxidation on a pure Pt electrode
and on a upd-Ru/Pt electrode oscillates only at a large oxidation
current, for example, 10 mA cm-2, but not at a small oxidation
current, for example, 5 mA cm-2. It is beyond the scope of this
communication to discuss this phenomenon thoroughly. The
detailed investigation about this issue has been under way.
3.2. Methanol Oxidation on upd-Sn/Pt Electrode. The CV
of a pure Pt electrode in absence (solid line) and presence
(dashed line) of 10-6 M Sn2+ are shown in Figure 8. The
suppressed hydrogen desorption area in the CV indicates the
deposits of Sn adatoms on the Pt surface.
The electrooxidation of methanol on the upd-Sn modified Pt
electrodes is shown in Figure 9 together with that on a Pt
electrode and a upd-Ru/Pt electrode for comparison. Methanol
oxidation on the upd-Sn/Pt electrode shifts toward an even more
negative potential than that on upd-Ru/Pt. In a potential range
from 0 to 0.22 V, the current of methanol oxidation on upd-
Sn/Pt with underpotential deposition of 100 s is larger than that
on upd-Ru/Pt. It suggests that the upd-Sn adatoms are more
favorable to enhance the methanol electrooxidation in the lower
potential range than the upd-Ru adatoms. Figure 10 reveals that
the current of methanol electrooxidation increases with coverage
of upd-Sn deopists within the potential range of 0.22 V. It does
not mean that the higher the coverage of the upd-Sn deposits,
the better it will be for methanol oxidation. Over the poten-
tial of 0.22 V, Figure 9 shows that the low coverage of upd-
Sn deposits on the Pt surface is favorable for methanol
electrooxidation.
Bittins-Cattaneo and Iwasita reported that the species of upd-
Sn adatoms on platinum surface in the potential range 0.2-
0.45 V (vs RHE) exists as an adsorbed Snad .
2+ 20 However, the
detailed mechanism of tin for methanol oxidation would still
be a subject for debate.
To find something new about the mechanism of methanol
oxidation, we investigated the surface states of the Pt electrode
in methanol electrooxidation by use of surface-enhanced infrared
absorption spectroscopy (SAIRAS). Figure 11 shows the 3-D
plot of IR spectra corresponding to the first (1) and the fourth
(2) continuous cyclic voltammograms of methanol oxidation
on a Pt electrode with modulation of a potential sweeping at a
rate of 50 mV s-1 in 1 M CH3OH and 0.1 M HClO4. The bands
at 2000-2100 and 1800-1900 cm-1 correspond to the absor-
bance of linear-CO (COl) and bridge-CO (COb), respectively.
The band intensities of linear- and bridge-CO for the first CV
cycle are small at low potentials below 0.35 V, but the band
intensity of linear-CO increases remarkably after 0.35 V, at
which the electrooxidation of CH3OH starts to occur signifi-
cantly. The intensity of linear-CO band stays nearly constant
in the cathodic direction sweeping. It is supposed that the amount
of CO generated by eq 1 is offset by the consumption of eq 2.
In the successive potential sweeping, the intensities of linear-
CO bands increase gradually, while the intensities of bridge-
CO bands remain almost constant and are independent of
potentials. The intensities of bridge-CO bands are smaller than
that of linear-CO since the first cycle. This may imply that only
a few sites on the Pt surface are suitable for the bridge-CO
selective absorption. In addition, according to Zhou et al.,21 the
bridge-CO is unstable and can transform to a linear-CO with
the increase of potential. The increased potential decreases the
electron density of Pt surface and thus blocks the electron flow
from the Pt atoms to the σ-π bond of Pt-CO and finally
weakens the Pt-COb bond. The fact that the significant increase
in the intensities of linear-CO bands accompanying the obvious
decrease in current of the CVs discloses that the poisons to the
Pt electrode come from the linear-CO absorbance. The 3-D plot
of IR spectra with potential sweeping on a upd-Sn/Pt electrode
displays the similar behavior as on the Pt electrode shown in
Figure 11. This suggests that CO poison to the upd-Sn/Pt
electrode is still a key problem. The mechanism of methanol
oxidation despite on the Pt electrode or on a upd-Sn/Pt electrode
is no different in the light of CO poisons.
By using ellipsometric technique, Frelink et al.17,18 found that
Sn-oxide is formed at a very lower potential than Pt-oxide and
that the Sn-oxide disappeared when methanol is present in the
solution because of the reaction between Sn-oxide and the
poisoning intermediate formed from the oxidation of methanol.
The mechanism of the effect of Sn on the oxidation of methanol
is extraordinarily complex. The puzzlement is that the bifunc-
tional mechanism cannot explain why electrosorbed or elec-
trodeposited Sn on the Pt surface is a good catalyst for methanol
oxidation, while PtSn alloys are not. Moreover, as Wasmus and
Kuver discussed in a review paper,5 it is sometimes unclear
3.3. Methanol Oxidation on upd-RuSn/Pt Electrode. Ac-
cording to the foregoing discussion, ruthenium and tin adatoms
on the Pt surface play an enhancement role in methanol
oxidation in a different potential range. There may be a
possibility to get an electrode with a good catalysis in a wide
potential range if Ru and Sn adtoms present on platinum surfaces
simultaneously. It is supposed that ruthenium and tin would not

Electrooxidation of Methanol on Pt Electrodes
J. Phys. Chem. B, Vol. 110, No. 51, 2006 26061
compete but would complement each other, yielding an even
higher synergetic electrocatalytic activity for methanol oxidation.
With this belief, we prepared upd-RuSn/Pt electrode and
examined its catalytic activity in 0.5 M CH3OH + 0.5 M H2-
SO4.
was observed on the upd-Ru/Pt electrode compared with a pure
Pt electrode, indicates that some synergetic effects work as
adatoms Ru and Sn participate simultaneously in the methanol
oxidation.
Acknowledgment. This work was financially supported by
NSFC of China under Grant Nos. 20476108, 20476109, and
20676156 by the Ministry of Education, China, under Grant
No. NCET-04-0850.
Figure 12 shows the methanol electrooxidation on a pure Pt
electrode, a upd-Ru/Pt electrode, a upd-Sn/Pt electrode, and a
upd-RuSn/Pt electrode. The key data have been summarized in
Table 2. As listed in Table 2, the methanol electrooxidation on
a upd-RuSn/Pt starts as early as that on a upd-Sn/Pt but starts
much earlier than that on a upd-Ru/Pt and, needless to say, on
a pure Pt electrode. Compared with a upd-Ru/Pt electrode, the
introduction of upd Sn atoms into the upd-Ru/Pt electrode leads
to a high current of methanol oxidation in a potential range
lower than 0.32 V. Meanwhile, the current of methanol oxidation
on a upd-RuSn/Pt electrode is even higher than that on a upd-
Sn/Pt in a low potential range. It indicates that some synergetic
effects work as adatoms Ru and Sn participate simultaneously
in the methanol oxidation. Over a potential of 0.32 V, adatoms
Sn would bring a detrimental effect on methanol oxidation on
the upd-Ru/Pt electrode. It is not difficult to understand that at
a quite higher potential it is possible for formation of oxygen-
containing species, such as Ru-OH, which is a better CO
remover than Sn-OH.
References and Notes
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4. Conclusions
The influence of the submonolayers of upd ruthenium
adatoms and upd tin adatoms on the oxidation of methanol in
acid has been investigated.
On the basis of the facts, Ru and Sn adatoms on the Pt surface
play an enhancement role in methanol oxidation in a different
potential range. A ternary catalyst containing Ru, Sn, and Pt
elements was prepared. It was shown that the Pt electrode
modified by upd-Ru and upd-Sn adatoms exhibits the best
catalysis for methanol electrooxidation in every respect of the
onset oxidation potential and the oxidation current in a certain
potential range. The larger current of methanol electrooxidation
on the upd-RuSn/Pt electrode, that is, a upd-Ru/Pt electrode
further modified by upd-Sn adatoms, than that on the upd-Ru/
Pt electrode in a potential range, in which no enhancement effect
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