Electrooxidation of ethanol on a Pt electrode in acid solutions: In situ ATR-SEIRAS study

Article abstract of DOI:10.1016/j.electacta.2004.10.063

The electrooxidation of ethanol was investigated on a Pt thin film electrode in a HClO₄ solution using surface enhanced infrared absorption spectroscopy (SEIRAS) with the attenuated total reflection (ATR) technique. The spectra indicate that during this reaction acetate and CO adsorbates are formed. The intensity of symmetric OCO stretching band of adsorbed acetate correlates well with voltammetry in the potential range between -0.1 and 0.85 V. The CO stretching band for adsorbed acetaldehyde and/or acetyl also was observed; these compounds are the reaction intermediates whose oxidation generates CO_ad and acetic acid. We also explored the oxidation behavior of adsorbed residues. The oxidation of acetaldehyde was studied for comparison.

Full text of DOI:10.1016/j.electacta.2004.10.063

Electrochimica Acta 50 (2005) 2415–2422
Electrooxidation of ethanol on a Pt electrode in acid solutions:
in situ ATR-SEIRAS study
M.H. Shao, R.R. Adzic∗
Department of Materials Science, Brookhaven National Laboratory, Upton, NY 11973, USA
Received 14 September 2004; received in revised form 12 October 2004; accepted 12 October 2004
Available online 21 November 2004
Abstract
The electrooxidation of ethanol was investigated on a Pt thin film electrode in a HClO4 solution using surface enhanced infrared absorption
spectroscopy (SEIRAS) with the attenuated total reflection (ATR) technique. The spectra indicate that during this reaction acetate and CO
adsorbates are formed. The intensity of symmetric OCO stretching band of adsorbed acetate correlates well with voltammetry in the potential
range between −0.1 and 0.85 V. The C=O stretching band for adsorbed acetaldehyde and/or acetyl also was observed; these compounds are
the reaction intermediates whose oxidation generates COad and acetic acid. We also explored the oxidation behavior of adsorbed residues.
The oxidation of acetaldehyde was studied for comparison.
© 2004 Elsevier Ltd. All rights reserved.
Keywords: Ethanol; Acetaldehyde; Acetic acid; Electrooxidation; Surface enhanced infrared absorption spectroscopy; Platinum
1. Introduction
electrodes during ethanol oxidation in acid solution. Iwa-
sita and coworkers observed a weak band around 1402 cm−1
and assigned it to adsorbed acetate [9]. A common adsor-
bate, CO, is formed from the dissociation of C C bonds
at low potentials, as seen in spectroscopic studies. From
their FT-IR results, Iwasita and Pastor [8] proposed that the
species Pt OCH2 CH3, (Pt)2=COH CH3, and Pt COCH3
were the possible adsorbed C2 residues of ethanol adsorp-
tion.
Surface enhanced infrared absorption spectroscopy
(SEIRAS) with attenuated total reflection (ATR) is a promis-
ing technique for studying solid–liquid interfaces. By opti-
mizing the thickness of the thin film electrode, interference
from the solution is minimized leaving only the vibration sig-
nals from the adsorbed species. Recently, Osawa and cowork-
ers [19–21] described useful findings on the oxidation of
methanol and other simple organic molecules using ATR-
SEIRAS.
Ethanol is a promising substitute for methanol as fuel
in direct alcohol fuel cells (DAFCs) due to its higher en-
ergy density and non-toxic properties [1]. The electrooxida-
tion of ethanol on platinum electrodes was investigated by
several spectroscopic methods, such as infrared reflection-
absorption spectroscopy (IRRAS) [2–13], on-line differential
electrochemical mass spectrometry (DEMS) [8,10,13–15],
and other techniques [13,16–18]. There is a number of ad-
sorbed species resulting from the oxidation of ethanol on
a Pt surface in acid solution [6,8,12,13,15]. Almost all re-
searchers agree that carbon dioxide, acetaldehyde, and acetic
acid are the main products of the oxidation of ethanol in
acid solution. Carbon dioxide and acetaldehyde were de-
tected by DEMS, while acetic acid could not be detected
directly by this method because of its very low volatility.
All three products were detected by in situ FT-IR. Shin et
al. [11] noted adsorbed acetic acid on Pt(1 1 1) and Pt(3 3 5)
In this article, we discuss results from the study of the
electrooxidation of ethanol on a Pt thin film electrode by in
situ ATR-SEIRAS that describes the behaviors of adsorbed
acetate ions and acetaldehyde/acetyl resulting from the reac-
∗
Corresponding author. Tel.: +1 631 344 4522; fax: +1 631 344 5815.
E-mail address: adzic@bnl.gov (R.R. Adzic).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2004.10.063

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M.H. Shao, R.R. Adzic / Electrochimica Acta 50 (2005) 2415–2422
tion. The oxidation of adsorbed residues formed at −0.1 V
3. Results and discussion
also was investigated.
3.1. Electrooxidation of ethanol
Fig. 1 shows the cyclic voltammogram for the Pt thin film
electrodein0.1 MHClO4 solution. Thecurveissimilartothat
of polycrystalline Pt. The roughness factor of the electrode
is about 7, obtained by integrating the hydrogen adsorption
peaks and assuming that the monolayer charge density for
hydrogen adsorption–desorption is 210 ␮C cm−2 [19].
Fig. 2a depicts the first and second voltammograms for Pt
in 0.1 M HClO4 + 0.1 M ethanol as a solid and dotted line, re-
spectively. During the positive scan, the oxidation of ethanol
2. Experimental
The prism for the ATR-SEIRAS measurements was a
hemispherical Si crystal 2.54 cm in diameter (ISP Optics,
New York). Before chemical deposition, the surface for IR
reflection was polished with alumina powder, using 0.05 ␮m
grade for the final polishing. After sonicating the crystal in
acetone and water, the polished surface was treated with 4%
HF for 1 min to remove the oxide film. Then, 2 ml of plating
solution (0.01 M K2PtCl6 + 10% HF) was repeatedly dropped
on the HF-treated surface for 30 min at room temperature.
The plating solution was refreshed after 15 min to maintain
an adequate source of Pt. The deposition process, which in-
2−
2−
volves a discharge of PtCl6 and formation of SiF6 in
the anodic process, was described in Ref. [22]. However, the
detailed deposition mechanism is still not clear.
The Pt thin film was rinsed with a plentiful amount of
Milli-Q UV plus water, and then attached to a three-electrode
spectro-electrochemical cell with a platinum foil connecting
the Pt film electrode to a CHI604A electrochemical analyzer.
A Pt ring and an Ag/AgCl, Cl− electrode were used as the
counter- and reference-electrode, respectively. The potentials
are expressed with respect to this reference electrode. Before
beginning the experiments, the thin film electrode was cycled
in a 0.1 M HClO4 solution in the potential range between
hydrogen adsorption and oxygen evolution until stable CVs
were obtained.
Fig. 1. Cyclic voltammogram of the Pt thin film electrode in 0.1 M HClO4
solution; sweep rate 20 mV s−1
.
Rapid scan FT-IR experiments were performed with a
Nicolet Nexus 670 FT-IR spectrometer equipped with a
MCT detector cooled with liquid nitrogen. An unpolarized
light beam was used. The spectral resolution was set to
4 cm−1 and 64 interferograms were together added to each
spectrum. Spectra are given in absorbance units defined as
A = −log(R/R0), where R and R0 represent the reflected IR
intensities corresponding to the sample- and reference-single
beam spectrum, respectively. The reference spectrum was
collected at −0.1 V in HClO4 solution without organic com-
pounds, except where otherwise stated. During the exper-
iments, the electrochemical cell was purged with nitrogen
gas.
To obtain the adsorbed residues, the potential was held at
−0.1 V for 10 min in 0.1 M ethanol or acetaldehyde solution
in 0.1 M HClO4. The solution was replaced with 0.1 M per-
chloric acid while keeping the potential at −0.1 V. The FT-IR
experiments were then carried out without organics in the so-
lution phase. A full coverage of CO was obtained by keeping
the electrode potential at −0.1 V for 10 min in CO-saturated
0.1 M HClO4 solution, followed by purging the solution with
nitrogen gas for 30 min to remove the dissolved CO. All ex-
periments were carried out at room temperature (∼20 ◦C).
Fig. 2. The first (solid line) and second (dotted line) potential sweep of the
Pt electrode in 0.1 M HClO4 + 0.1 M C2H5OH; sweep rate 5 mV s−1 (a),
and the integrated band intensities of COL and adsorbed acetate taken from
Fig. 3 (b).

M.H. Shao, R.R. Adzic / Electrochimica Acta 50 (2005) 2415–2422
2417
cated 1396–1410 cm−1 appears above 0.3 V in the positive
scan. Fig. 2b plots the intensity changes of COL and this band
against potential. The intensity of COL begins to decrease at
0.2 V, coincident with the emergence of the anodic current in
voltammetry. The band that appears at 0.35 V continues to
increase rapidly, passing through a maximal intensity around
0.65 V where the coverage of CO is very small. During the
reverse scan, the intensity of this band stays around half of the
maximum down to 0.65 V where it increases again and passes
through another maximum at 0.5 V. Interestingly, these two
peaks in intensity curve coincide well with the peaks I and
III in the voltammogram.
We assigned the band at 1396–1410 cm−1 to the symmet-
ric OCO stretching mode (νsOCO) of the adsorbed acetate
ions [9,11]. Several researchers have described the adsorp-
tion behavior of acetic acid [25–29]. Acetate ions, rather than
undissociated acetic acid, adsorb perpendicularly on a Pt sur-
face with two O atoms bonding to Pt with bridge geometry
[28]. There is a tail or shoulder (∼1350 cm−1) at the low fre-
quency side of the νsOCO band. This feature can be assigned
to CH3 in plane bending of the adsorbed acetate (δCH3) [30].
Anti-symmetric OCO stretching (∼1550 cm−1) is not ob-
served because of the surface selection rule of SEIRAS that
permits only those vibrations with dipole changes perpendic-
ular to the surface to be observed [31].
To confirm our assignment of the symmetric OCO stretch-
ing mode (νsOCO) to the adsorbed acetate ions, the spectrum
taken in the solution of 0.1 M HClO4 + 0.1 M CH3COOH at
0.6 V is shown for comparison at the bottom of Fig. 3 (dot-
ted line). The reference spectrum was taken at −0.1 V in
the same solution. All the bands from solution phase were
subtracted completely. Both the shape and position of the
band are identical to those recorded during the oxidation of
ethanol, confirming this assignment. Shin et al. [11] also ob-
served adsorbed acetic acid on Pt(1 1 1) and Pt(3 3 5) elec-
trodes during ethanol oxidation. Their results indicate that its
adsorption correlates with the drop in the ethanol oxidation
current, which is not seen in our data.
Fig. 3. SEIRA spectra recorded during ethanol oxidation in the first potential
sweep in Fig. 2a.The dotted line at the bottom is the spectrum of the Pt
electrode in 0.1 M HClO4 + 0.1 M CH3COOH at 0.6 V normalized to the
spectrum taken at −0.1 V in the same solution.
gives an anodic current peak (peak I) at 0.65 V. Above 0.85 V,
the current raises again. In the negative scan, there is one an-
odic peak (peak III) at 0.5 V with a shoulder around 0.4 V.
Fig. 2b is discussed later.
The in situ SEIRA spectra recorded during ethanol ox-
idation in the first potential sweep (Fig. 2a) are shown in
Fig. 3. The frequencies and assignments of the observed
bands are listed in Table 1. The bands for the linear and
bridge-bonded CO at 2030–2065 and 1800–1840 cm−1 [23],
respectively, are clearly observed at all potentials. The dis-
torted shape of the COL bands, with negative-going peak at
the high-wavenumber side, reflects the optical behavior of
the Pt thin film [24]. Besides the CO bands, a new band lo-
The negative going band at ∼1598 cm−1 is assigned to
HOH bending mode (δHOH) of adsorbed water [32]. A pos-
itive going band at 1620–1635 cm−1 appears and grows be-
low 0.5 V in the negative potential scan; there are two pos-
sible assignments for it. One is the up-shifted δHOH band
of adsorbed water. However, the corresponding νOH band of
adsorbed water was not observed. Moreover, this band is not
observed during the oxidation of CO, methanol, and other C1
molecules except acetaldehyde, which is discussed in Section
3.3. Thus, it is unlikely that it originates from adsorbed water,
but rather from the adsorbed C C species.
Table 1
ATR-SEIRAS bands assignments
Wavenumber (cm−1
)
Assignment
2030–2065
1800–1840
1705
Linear CO [23]
Bridged CO [23]
C=O stretching of acetaldehyde and acetic acid
in solution [9,42]
To analyze this band further, we show, in Fig. 4, selected
SEIRA spectra obtained in the second potential sweep (dotted
line in Fig. 2a). With increasing potential the band becomes
broader and its frequency increases. It disappears at about
0.6 V during the positive scan. We assign it to νCO of ad-
sorbed acetaldehyde and/or acetyl [33–35]. The first step in
adsorption of ethanol involves the displacement of adsorbed
1620–1635
C=O stretching of adsorbed acetaldehyde and
acetyl [34,35]
HOH deformation of adsorbed water ([32] and
references cited therein)
OCO stretching of adsorbed acetate [9,11,28]
CH3 in plane bending of adsorbed acetate [30]
CO stretching of acetic acid in solution [2,9,26]
∼1598
1396–1410
∼1350
1280

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M.H. Shao, R.R. Adzic / Electrochimica Acta 50 (2005) 2415–2422
acetaldehyde and η1 (C)-acetyl cannot be distinguished in
the present case. The band of νCO broadens with increas-
ing potential (Fig. 4) possibly implying that more than one
species was adsorbed in that potential region.
When Pt is exposed to an ethanol solution, a consider-
able amount of COad is produced and that can happen only
through dissociation of the C C bond of the adsorbed ac-
etaldehyde/acetyl:
Pt(CH3CHO) + Pt → Pt(CO) + Pt(CH3) + H+ + e− (2)
Alternatively, the oxidative removal of one hydrogen from
acetaldehyde can form acetyl, which can dissociate to form
CO, viz.,
Pt(CH3CHO) → Pt(CH3CO) + H+ + e−
Pt(CH3CO) + Pt → Pt(CO) + Pt(CH3)
(3)
(4)
The competition between CO and acetaldehyde/acetyl for ad-
sorption sites results in most of the Pt sites being covered by
CO molecules because of their higher adsorption strength.
In the positive scan, most COad is oxidized to CO2 leaving,
in the negative scan, many unoccupied Pt sites for acetalde-
hyde/acetyl adsorption. Thus, COad can be generated again.
This CO adsorption dynamics determines at what low CO
coverage the νCO band of η1 (O)-acetaldehyde/η1 (C)-acetyl
can be observed. At low CO coverage, the adsorption of η2
(O,C)-acetaldehyde also is possible but it cannot be deter-
mined by this technique, as indicated above. In the hydrogen
adsorption region, CH3ad, generated in the reaction 2 and
4, can be reduced to CH4 or combine with the other CH3ad
species to produce CH3CH3 [41].
Fig. 4. SEIRA spectra recorded during ethanol oxidation in the second po-
tential sweep shown in Fig. 2a.
water molecules on the platinum surface, followed by the ␣-
C H dissociation leading to formation of adsorbed acetalde-
hyde [8]:
C2H5OH + Pt(H2O)
→ Pt(CH3CHO) + H2O + 2H+ + 2e−
(1)
The CO coverage begins to decrease above 0.2 V, while the
oxidation of adsorbed C2 species to acetate starts at 0.35 V
(Figs. 3 and 4). Both reactions require PtOH as a source of
oxygen. The reactions of adsorbed C2 species can be written
in the following way, which is supported by the IR data:
The acetaldehyde, generated in this reaction, can be adsorbed
on a metal surface via two possible bonding configurations
called η1 (O) and η2 (O,C) [33–40]. In the η1 (O) adsorp-
tion geometry, electron donation from the lone pair orbital
of O only to an empty σ orbital of Pt forms a weak bond. Pt
*
Pt(Ch3CO) + Pt(OH)
back-donates electrons to a carbonyl ␲ orbital. Zhao et al.
[34] in UHV observed the νCO band of η1 (O)-acetaldehyde
adsorbed on Pt(1 1 1) at 1667 cm−1 by high-resolution elec-
tron energy loss spectroscopy (HREELS). In the η2 (O,C)
geometry, both the C and O atoms interact with the Pt sur-
face through the carbonyl ␲ orbital. The skeletal plane of
acetaldehyde is parallel to the Pt surface, with the Pt dxz or-
→ Pt(CH3COO) + Pt + H+ + e−
(5)
Pt(CH3CHO) + Pt(OH)
→ Pt(CH3COO) + Pt + 2H+ + 2e−
(6)
Two weak bands at 1705 cm−1 (C=O stretching) [9,42] and
1280 cm−1 (C O stretching) [2,9,26], which appear in the
potential range above 0.5 V, both come from the acetic acid
produced close to the Pt surface.
*
bital overlapping the carbonyl ␲ orbital [33,39–40] allow-
*
ing for sufficient electron back-donation from Pt to the ␲
orbital. There is no SEIRA active mode in the η2 (O,C) con-
figuration because of this parallel bonding. Thus, the band
at 1620–1635 cm−1 results from the νCO of η1 (O)-, rather
than η2 (O,C)-acetaldehyde. On the other hand, η1 (C)-acetyl
was reported as one of the adsorbed intermediates in the
reaction of acetaldehyde on Pt (S)-[6(1 1 1) × (1 0 0)] [33]
and Pt(1 1 1) [34] with νCO at 1650 and 1647 cm−1, respec-
tively. Given the angle of 120◦ for sp2 hybridization band of
carbon atom, the Pt C O angle should be non-zero yield-
ing a perpendicular dipole component. Therefore, η1 (O)-
CO2 and acetaldehyde both reach their maximum in peak
I [8,13,15] at 0.65 V in Fig. 2a. The SEIRA results show that
acetate also has the highest coverage on Pt surface at 0.65 V.
Note that adsorbed acetate is an inhibitor in ethanol oxidation
since it is strongly bonded to the surface and it is difficult to
oxidize it to CO2. Such a strong adsorption prevents acetate
from desorbing even in a high potential range (Fig. 5). The
intensity of νsOCO band does not change much between 0.9
and 1.3 V even though PtOH forms in this range. At potentials

M.H. Shao, R.R. Adzic / Electrochimica Acta 50 (2005) 2415–2422
2419
Fig. 5. The integrated band intensities (solid line) of the adsorbed acetate
taken from the SEIRA recorded in the potential sweep of 5 mV s−1 (dotted
line) for the Pt electrode in 1.0 M HClO4 + 0.1 M C2H5OH.
above 1.35 V, there is a large decrease in the intensity of
νsOCO. This may result from the desorption of acetate caused
by the formation of PtO, or from oxidation of the adsorbed
acetate in this potential range.
3.2. Electrooxidation of acetaldehyde
Fig. 7. The first (solid line) and second (dotted line) potential sweep of the
Pt electrode in 0.1 M HClO4 + 0.1 M CH3CHO; sweep rate 5 mV s−1 (a),
and the integrated band intensities of COL and adsorbed acetate taken from
Fig. 6 (b).
Since acetaldehyde is an intermediate in the oxidation of
ethanol, it is interesting to study its oxidation separately. The
in situ SEIRA spectra of acetaldehyde oxidation recorded in
the first potential sweep (solid line in Fig. 7a) are shown in
Fig. 6. The observed bands are due to the adsorbed CO and
acetate, and their potential dependences are similar to those
observed in the oxidation of ethanol. Fig. 7b shows the inten-
sity changes of COL and νsOCO against potential. The in-
tensity of COL begins to decrease at ca. 0.2 V and disappears
completely at 0.9 V. The band of νsOCO of adsorbed acetate
appears at 0.35 V, where the intensity of COL is reduced by
approximately 50%. The intensity of this band continues to
increase rapidly, reaching its maximum at 0.65–0.7 V where
the coverage of CO is very small. During the reverse scan, the
intensity of this band is around half of its maximum until at
0.7 V where it increases again forming a peak at 0.5 V. These
two peaks in intensity curve coincide well with the peaks II
and IV in the voltammogram.
The band at 1620–1635 cm−1 is observed between −0.1
and 0.4 V in the positive scan of the first potential sweep. That
indicates that η1 (O)-acetaldehyde/acetyl is adsorbed on the
Pt surface, even though the CO coverage is large in the low
potential range. This feature is different from that observed
in the ethanol oxidation (Fig. 3) in which the νCO band of η1
(O)-acetaldehyde/acetyl does not appear until in the negative
potential scan. Fig. 8 shows SEIRA spectra obtained in the
second potential sweep (dotted line in Fig. 7a), in which the
behavior of adsorbed η1 (O)-acetaldehyde/acetyl is identical
to that in Fig. 4.
In Figs. 6 and 8, the weak band at 1705 cm−1 is observed at
all potentials. It is tentatively assigned to the C=O stretching
of acetaldehyde in bulk solution [9,42]. When more acetic
acid is produced (at high positive potential), this band be-
comes stronger due to the contribution of the C=O stretch of
acetic acid near Pt surface.
Fig. 6. SEIRA spectra recorded during acetaldehyde oxidation in the first
potential sweep in Fig. 7a.

2420
M.H. Shao, R.R. Adzic / Electrochimica Acta 50 (2005) 2415–2422
Fig. 8. SEIRA spectra recorded during acetaldehyde oxidation in the second
potential sweep shown in Fig. 7a.
3.3. Electrooxidation of adsorbed residues
Fig. 9a shows the spectra for the oxidation of adsorbed
residues formed on a Pt electrode in ethanol solution. The
negative going band around 1598 cm−1 indicated the loss
of adsorbed water. Besides the bands for adsorbed CO, the
band for adsorbed acetate is clearly observed at potentials
higher than 0.5 V. This finding is not quite in agreement with
the view that all adsorbed residues are oxidized to CO2 [8].
The existence of this band undoubtedly shows that, at least
partially, acetate is generated from adsorbates containing an
intact C C bond. One possible source for it is adsorbed η2
(O,C)-acetaldehyde that is SEIRA-inactive. This assumption
is consistent with the SEIRA results since no other bands re-
lated to the C C species were recorded. When the CO cover-
age is high, the dissociation of the C C bond stops, leaving
a small amount of adsorbed η2 (O,C)-acetaldehyde on the
surface that is oxidized to acetic acid at high potential. η1
(O)-acetaldehyde was not observed, probably because of its
weaker adsorption bond and small content.
Fig. 9. SEIRA spectra obtained during the oxidation of adsorbed residues
of ethanol (a) and acetaldehyde (b) in 0.1 M HClO4 solution; Ead = −0.1 V.
In this pathway, the strongly adsorbed intermediates are not
involved. This means that most of adsorbed acetate is pro-
duced through the direct oxidation of ethanol, rather than
from adsorbed species. This conclusion is consistent with
the fact that the intensity of νsOCO is not obviously different
in Figs. 3 and 4, even though there is much more adsorbed
acetaldehyde and/or acetyl in the latter.
For comparison, the spectra for the oxidation of ad-
sorbed residues formed in acetaldehyde solution are shown in
Fig. 9b. The band for adsorbed acetate is also clearly observed
at potentials higher than 0.4 V. It appears that the adsorbed
acetate is an oxidation product of the same adsorbed residue,
η2 (O,C)-acetaldehyde, as that one originating from ethanol.
η1 (O)-acetaldehyde/η1 (C)-acetyl presented at the beginning
of the potential sweep in Fig. 6 is not observed here. It is pos-
sible that it formed COad or desorbed from the Pt. The inten-
sity of the νsOCO band is also considerably smaller than that
of bulk acetaldehyde oxidation because the adsorbed acetate
is produced by direct oxidation of acetaldehyde in the bulk
solution [35].
The intensity of the νsOCO band is much smaller than
those in Fig. 3. The likely explanation of this difference is the
acetate coverage that is greater in the presence of ethanol in
solution. Ethanol can react with adsorbed water or hydroxyl
species directly to produce acetic acid via a four-electron
oxidation pathway when more free Pt sites become available
as a consequence of the oxidation of COad [6]:
C2H5OH + Pt(OH) → Pt(CH3COO) + 4H+ + 4e− (7)
CH3CHO + Pt(OH) → Pt(CH3COO) + 2H+ + 2e− (8)

M.H. Shao, R.R. Adzic / Electrochimica Acta 50 (2005) 2415–2422
2421
action. The adsorption behavior of acetic acid, one of the
main soluble products of ethanol oxidation, was followed for
the first time during ethanol oxidation. The band intensity of
adsorbed acetate correlates well with the anodic current in
both potential scan directions (except current peak II). Even
at high potentials, desorption of acetate is not obvious due
to its strong adsorption bond. This strength is detrimental for
ethanol oxidation since the adsorbed acetate blocks Pt sites
for that reaction.
Most of adsorbed acetate comes from the direct oxida-
tion of ethanol. However, at least part of it comes from ad-
sorbed acetaldehyde that forms at low adsorption potentials.
We consider the adsorbed acetaldehyde and/or acetyl to be
reaction intermediates generating COad and acetic acid dur-
ing the oxidation of ethanol. The oxidation of acetaldehyde
also generates adsorbed CO and acetate with the acetate cov-
erage correlating well with the current in voltammetry. For
both ethanol and acetaldehyde, adsorbed residues consist of
a large quantity of CO and a small amount of acetaldehyde
at −0.1 V.
Acknowledgments
This work is supported by US Department of Energy, Di-
visions of Chemical and Material Sciences, under the Con-
tract No. DE-AC02-98CH10886. The authors appreciate the
valuable discussions with P.W. Faguy and N.S. Marinkovic.
M.H. Shao acknowledges partial support from Department of
Materials Science and Engineering, State University of New
York at Stony Brook.
Fig. 10. Cyclic voltammograms for the oxidation of adsorbed residues of
ethanol (solid line) and pre-adsorbed CO (dotted line) at the Pt electrode in
0.1 M HClO4 (a), and the integrated band intensities of COL taken from the
SEIRA spectra in Fig. 9a (b).
Fig. 10 shows a comparison between the oxidation of ad-
sorbed residues and pre-adsorbed CO adlayer. In Fig. 10a,
the solid and dotted lines represent the oxidation of adsorbed
residues of ethanol and the CO adlayer, respectively. The lat-
ter was formed at −0.1 V and the oxidation measured after
removing dissolved CO. The CO adlayer is completely oxi-
dized in a sharp peak at 0.4 V. The oxidation of ethanol ab-
sorption residues occurs at slightly less positive potential. Its
peak current Ip is only about half of that of the pre-adsorbed
CO adlayer. The oxidation of COL from the residue is very
slow above 0.35 V, and it is not completed until at 0.75 V
(Fig. 10b). The difference in the oxidation peaks indicates
that there are other adsorbed residues besides CO on the Pt
surface. It appears that the COL oxidation is affected by the
oxidation of adsorbed C2 residues. A shift of COL oxidation
to more negative potentials may be caused by interaction be-
tween COad and adsorbed species.
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