Oxidation of isotopically-labeled ethanol on platinum-tin-rhodium surfaces: Enhancing the production of CO2from methyl groups

Article abstract of DOI:10.1016/j.elecom.2014.09.009

In this paper, we investigate the electro-oxidation of isotopically labeled ethanol on PtSnRh electrodeposits. By monitoring the oxidation products by in situ FTIR we show that the high catalytic activity of PtSnRh catalysts is due to their ability to promote the C-C cleavage, as well as the oxidation of methyl groups. Also, the use of Sn in proper ratio enhances the global performance of PtSnRh catalysts towards the oxidation of ethanol, since it seems to prevent the electro-reduction of -CH₃groups to methane, making them available to produce CO₂in the same potential range where the alcoholic group is oxidized. As a consequence, important amounts of CO₂are produced from both extremities of ethanol.

Full text of DOI:10.1016/j.elecom.2014.09.009

Electrochemistry Communications 48 (2014) 160–163
Contents lists available at ScienceDirect
Electrochemistry Communications
journal homepage: www.elsevier.com/locate/elecom
Short communication
Oxidation of isotopically-labeled ethanol on platinum–tin–rhodium
surfaces: Enhancing the production of CO from methyl groups
2
Gisele A.B. Mello, Manuel J.S. Farias, M. Janete Giz, Giuseppe A. Camara ⁎
Instituto de Química, Universidade Federal de Mato Grosso do Sul, C.P. 549, 79070–900 Campo Grande, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 July 2014
Received in revised form 20 August 2014
Accepted 8 September 2014
Available online 17 September 2014
In this paper, we investigate the electro-oxidation of isotopically labeled ethanol on PtSnRh electrodeposits. By
monitoring the oxidation products by in situ FTIR we show that the high catalytic activity of PtSnRh catalysts is
due to their ability to promote the C–C cleavage, as well as the oxidation of methyl groups. Also, the use of Sn
in proper ratio enhances the global performance of PtSnRh catalysts towards the oxidation of ethanol, since it
3 2
seems to prevent the electro-reduction of –CH groups to methane, making them available to produce CO in
the same potential range where the alcoholic group is oxidized. As a consequence, important amounts of CO
are produced from both extremities of ethanol.
2
Keywords:
Ethanol electro-oxidation
Platinum–tin–rhodium
Isotopically-labeled ethanol
© 2014 Elsevier B.V. All rights reserved.
Selectivity towards CO
2
1
. Introduction
Ru [7,8], Rh [9] and Sn [10] catalyze the electro-oxidation of ethanol (and
related species) compared to Pt alone. Ad-atoms can accelerate reactions
(i) by supplying oxygen donor species (coming from water molecules) at
lower potentials than Pt [11]; (ii) via electronic effects, in which ad-atoms
likely induce changes in the energies of the adsorbent sites [12]; (iii) or
via blocking of those sites required for the formation of poisoning
species, as adsorbed CO and molecular fragments (third-body effect)
[13].
In this context, some research groups have demonstrated that ternary
PtSnRh electrodes present higher catalytic activity for ethanol electro-
oxidation than their binary counterparts [14–17]. For instance, by using
Density Functional Theory and in situ FTIR Adzic et al. showed that
The electro-oxidation of ethanol on Pt-based electrodes proceeds
through parallel pathways [1]. For the oxidation of ethanol, the route
that produces CO releases 12 electrons per molecule, but the pathways
generating acetic acid and acetaldehyde predominate in acidic medium
and correspond to 4 and 2 electrons, respectively [1]. By using
isotopically-labeled ethanol on Pt-based catalysts, it has been shown
that the C\C bond cleavage in the hydrogen region (E b 0.3 V) is
followed by the partial electroreduction of the methyl group (–CH
methane/ethane [2,3]. If we consider the pathways through which
ethanol is oxidized exclusively to CO , the non-oxidation of –CH groups
2
3
) to
2
3
−
−
impacts in an energetic loss of about 58% [≡(7e /12e ) × 100] of the
total charge per oxidized molecule. Indeed, only a small fraction of –
CH
PtRhSnO
These authors interpreted the gain in the catalytic activity as a result of
the strong interaction of SnO with water, which precludes Rh and Pt
2
enhance C–C splitting during ethanol electro-oxidation [14].
3
is converted to CO
2
(b20%) during the electro-oxidation of ethanol
2
on Pt-based surfaces [4,5].
from reacting with water and make them available for steps of adsorption
and oxidation [14]. More recently, Strasser et al. investigated PtSnRh
catalysts emphasizing the three-dimensional atomic arrangement of
these materials [17]. The authors found that the combination of these
elements produces a Niggliite structure that presents a remarkable
performance towards ethanol electro-oxidation [17].
Despite the interest illustrated in these works, there is a lack of
knowledge on how Sn and Rh co-catalysts influence the activity of
Pt-based catalysts towards ethanol oxidation, specifically concerning
The literature suggests that the interaction between ethanol and the
surface of a catalyst is of paramount importance for the improvement of
its electro-oxidation on Pt-based electrodes [4–6]. For polycrystalline Pt
it has been noted that the extent to which –CH
depends on the potential of adsorption of ethanol [5]. Inside the
hydrogen-region, little CO is formed from –CH , while this pathway
3 2
is converted to CO
2
3
is favored when the polarization is performed at more positive
potentials [5].
Due to the difficulty to fully electro-oxidize ethanol, the investigation
of metals that can act as co-catalysts to Pt has attracted increasing atten-
tion in the last years. It is already well-established that ad-atoms, such as
3 2
the conversion of –CH groups to CO . Indeed, at least to our knowledge,
recent works of Feliu's group with isotopically labeled ethanol represent
the only attempts to understand the role of a co-catalyst in molecular
terms, but they are limited to binary PtSn surfaces [10], or Pt decorated
by Ru [4]. Hence, it is important to use isotopically labeled ethanol to
discriminate the contributions of both carbons and to gain some
⁎
Corresponding author. Tel.: +55 67 3345 3576; fax: +55 67 3345 3552.
E-mail address: giuseppe.silva@ufms.br (G.A. Camara).
http://dx.doi.org/10.1016/j.elecom.2014.09.009
388-2481/© 2014 Elsevier B.V. All rights reserved.
1

G.A.B. Mello et al. / Electrochemistry Communications 48 (2014) 160–163
161
knowledge about the influence of foreign atoms (like Rh and Sn) on the
ethanol electro-oxidation mechanism. In this context, despite the
unique characteristics ascribed to Pt–Sn–Rh combinations in the
literature, until now there are no works showing specifically how
The atomic compositions of the PtSnRh ternary electrodeposits were
estimated by energy dispersive X-ray analysis (EDX) using a Zeiss-
Leica/440 instrument provided with a SiLi detector. The mean values
were obtained from three points to each sample. Two samples were
prepared with PtSnRh compositions of (79:17:04) and (80:07:13). It is
worth noting that due to the penetration depth of the electron beam,
EDX analysis provides a bulk composition that does not necessarily
correspond to the surface atomic composition.
2
these catalysts act on the conversion of methyl groups to CO . This
issue is the main concern of the present work. Here we perform an in
situ FTIR study with isotopically labeled ethanol on ternary Pt–Sn–Rh
catalysts and investigate the ability of these surfaces to oxidize both
2 3 2
groups (–CH OH and –CH ) to CO .
3. Results and discussion
2
. Experimental
3.1. Analysis of polarization curves
4
+
Working electrodes were prepared by electro-reduction of Pt
Fig. 1 shows polarization curves obtained for both PtSnRh
compositions in 0.05 M ethanol solutions. Both curves show an
inflection in the current values at E ~ 0.25 V, which suggests that both
ternary compositions are active to electro-oxidize ethanol at very low
potentials. For both compositions, the oxidation profile is very similar
at low potentials, but the current densities become progressively higher
for that composition richer in Sn as the potential is increased. Such
behavior suggests that Sn plays a central role in the electro-oxidation
of ethanol and anticipates that PtSnRh (79:17:04) favors that pathway
responsible for the rising of currents at high potentials.
(
(
(
H
2
PtCl
SnCl
Sigma-Aldrich, ACS reagent, 70%) at 0.08 V vs. RHE for 10 min. A
6
—Sigma-Aldrich), Rh3 (RhCl
+
3
—Sigma-Aldrich) and Sn
2+
2
∙ 2H O—Merck) on a polished gold disk in 1.0 M HClO
2
4
platinum sheet of large area was used as a counter-electrode, and the
potentials were measured against a reversible hydrogen electrode
(
RHE) in a 0.1 M HClO
4
solution. For the spectroelectrochemical
experiments the solutions were prepared with Milli-Q water
13
13
(
—
18.2 MΩ cm) and 0.05 M ethanol (12CH
Isotec).
For the estimation of the surface areas we used the CO-stripping
procedure in a 0.1 M HClO solution, as described elsewhere [8]. The
3
–
CH
2
OH, 98 atom%
C
4
3.2. In situ FTIR characterization of isotopically labeled ethanol oxidation
charge involved in the oxidation of a monolayer of adsorbed CO was
used to estimate the surface areas, assuming a charge density of
Spectra of Fig. 2A and B were collected during the oxidation of
isotopically labeled ethanol on PtSnRh 80:07:13 and 79:17:04,
respectively. In these experiments, spectra were recorded from 0.05 V
up to 0.80 V with steps of 0.05 V. In Fig. 2A, at ~0.50 V two bands at
−
2
4
20 μC cm
.
For the electro-oxidation of ethanol the potential was kept at E =
0
.05 V and ethanol was admitted into the cell to reach the concentration
−
1
−1
of 0.05 M. Next, chronoamperometric experiments were performed by
monitoring the current vs. time response after the application of a
potential step of 0.05 V, from E = 0.05 to 0.80 V. For each step the
potential was kept constant for 50 s and the last current value before
the next step was taken to build the curves depicted in Fig. 1.
2343 cm
assigned to 12CO
bands shows that on PtSnRh catalysts, both methyl and alcohol carbon
and 2277 cm
rise simultaneously. These bands are
13
2
and CO , respectively [6]. The coexistence of both
2
groups are converted into CO
shows a band at ~1974 cm
2
1
in the same potential range. Fig. 2B
at 0.45 V (the exact wavenumber is
−
In situ FTIR spectra were acquired by using a FTIR spectrometer
equipped with a MCT detector. The electrochemical IR cell was fitted
potential-dependent) that is attributed to the stretching of linearly-
bonded CO [6]. Once both groups form CO it is expected that the
2
12
13
12
to a CaF
2
planar window. Further details of the spectroelectrochemical
dissociative adsorption of CH
COads. However, the band at low frequencies is not visualized in the
3
–
CH
2
OH yields both COads and
13
cell and setup can be consulted in [18]. Spectra were computed from
12
13
3
R
2 interferograms. Reflectance spectra were collected as the ratio (R/
) where R represents a spectrum at a given potential and R is the
present conditions, probably because COads and COads oscillators
are coupled and maximize that band located at higher frequencies
0
0
12
13
“
background” spectrum collected at E = 0.05 V. All the experiments
( COads signal) at the expenses of the lower frequency band ( COads)
were performed at room temperature. The same procedure of potential
steps described in the previous paragraph was adopted here.
[19]. Moreover, as previously noted by Vielstich et al., the use of
electrodeposited surfaces reduces the band intensities for adsorbed
species [20]. According to the authors, the reason for this effect probably
originates from the rough structure of such surfaces that imposes
important restrictions in the angles of incidence that can be perceived
by the detector [20]. Still, as the amount of COads coming from methyl
groups is lower [21], these effects combined can help to explain that a
single COads band is visualized in the present spectra. Other bands
0.04
0.03
0.02
0.01
0.00
PtSnRh (80:07:13)
PtSnRh (79:17:04)
−
1
at ~1378/1266 cm
can be attributed to the coupling of the
νC–OH + δO–H of \COOH groups of acetic acid [22]. Interestingly,
both bands can be visualized at 0.30 V (Fig. 2A–B) which indicates that
the formation of acetic acid is probably the responsible for the rising of
−
1
currents in Fig. 1. Finally, a large band at ~ 1108 cm corresponds to
C–H wagging vibrations [22] superimposed to the Cl–O stretching of
perchlorate [18]. Also, it is important noting that the magnitudes of the
bands cannot be compared between both series, once they were not
normalized by the corresponding areas of the electrodes.
3.3. PtSnRh facilitates the electro-oxidation of methyl groups
0.0
0.2
0.4
0.6
0.8
Potential vs. RHE/ V
2
New information on the selectivity towards the CO pathways
Fig. 1. Polarization curves for the oxidation of 12CH
–13CH
during the electro-oxidation of ethanol was afforded by taking the ratio
3
2
OH on PtSnRh electrodeposits
+ 0.05 M
12
(compositions indicated in the figure) between 0.05 and 0.80 V in 0.1 M HClO
4
of integrated band intensities extracted from Fig. 2, A( CO2, methyl)/
13
ethanol.
A( CO2, alcohol), as a function of the potential for both catalysts (Fig. 3).

162
G.A.B. Mello et al. / Electrochemistry Communications 48 (2014) 160–163
E/ V
0.05
0.20
1974
0
0
0
.40
.60
.80
1
%
2
343
1266
A
B
1378
1500
2277
2500
2000
1500
1000
2500
2000
1000
-1
Wavenumber/ cm
Fig. 2. In situ FTIR spectra obtained for (A) PtSnRh (80:07:13) and (B) PtSnRh (79:17:04) in the presence of 0.05 M 12CH
–13CH
OH + 0.1 M HClO . Spectra were obtained after steps of
2 4
3
0.05 V. Background spectrum collected at 0.05 V. The wavenumbers corresponding to the major features in the spectra are indicated.
Here is important noting that the potential range depicted in Fig. 3 is
limited by those potentials where both signals are present. It can be
observed that throughout the potential range comprised between
the onset potential of ethanol oxidation, but does not enhance the
performance of Pt towards the oxidation of methyl groups to CO
[4]. Furthermore, in a previous work Iwasita and Pastor [2] showed
that –CH groups from ethanol are reduced to methane and ethane
on polycrystalline Pt. Once reduced, these molecules apparently detach
from the surface and do not regenerate the original methyl groups
when the potential is increased, which implies that more than half of
the energetic content of the ethanol molecule is lost.
2
0
.55 V and 0.80 V the catalyst richer in Sn (17%) produces proportionally
more CO coming from methyl groups compared with that composition
possessing 7% of Sn. The A( CO
3
2
12
13
2
2
)/A( CO ) ratio for PtSnRh (79:17:04)
is about 0.57 at 0.60 V and reaches 0.72 at 0.80 V. This result contrasts
with the poor ability of Pt surfaces to electro-oxidize methyl groups to
CO
2
. For instance, previous data showed a A(CO2, methyl)/A(CO2, alcohol
)
Based on these data, our results suggest that on PtSnRh surfaces the
ratio of only 0.10 at 0.70 V on Pt [5]. Also, by using BB-SFG (Broad Band
Sum Frequency Generation) for the study of CH – CH OH oxidation
3 2
production of Sn oxides at low potentials prevents the reduction of –CH
groups, making them available for steps of oxidation. As a consequence,
the production of CO from methyl groups is particularly favored for
PtSnRh (79:17:04), which partially explains the high currents in Fig. 1
(the production of acetic acid and acetaldehyde also contribute for
3
12
13
on polycrystalline Pt, Kutz et al. concluded that the coverage of COads
coming from methyl group represents only 17% of the coverage of COads
coming from the alcoholic group [21]. In another study with Pt(332)
surfaces decorated by Ru ad-atoms Souza-Garcia et al. found a A(CO2,
methyl)/A(CO2, alcohol) ratio of about 0.15 at 0.65 V [4] (in this case, ethanol
was adsorbed at 0.1 V). In this work the authors argue that Ru diminishes
2
those currents). At the same time, the presence of adsorbed –CH
3
seems to be favored by minor amounts of Rh and corroborate the
observations made by Adzic et al. [14], who stated that the presence
of Rh in ternary PtSnRh catalysts favor the C–C splitting required for
the full oxidation of ethanol. Finally, although the catalysts investigated
2
are not entirely selective towards the production of CO (acetic acid is
also formed at low potentials), the present study brings important
practical implications because it shows that PtSnRh catalysts are able
to promote the C–C cleavage, as well as the oxidation of methyl groups.
Complementary studies must be performed in order to estimate the
amounts of each oxidation product (including acetic acid and
acetaldehyde) and build a more complete picture of the influence of
each metal on the electro-oxidation of ethanol.
4. Conclusions
-PtSnRh catalysts show a good catalytic activity towards the
electro-oxidation of ethanol, with onset potentials of oxidation around
0
.3 V (vs. RHE).
coming from methyl (12CO
2 2
Fig. 3. Integrated band intensities ratio for CO ) and alcoholic
-The monitoring of 12CO
2
and 13CO
bands shows that PtSnRh of
2
1
3
12
13
groups ( CO
2
) obtained during the electro-oxidation of 0.05 M CH
3
–
CH
2
OH on
atomic composition (79:17:04) produces proportionally more CO
coming from methyl groups compared with PtSnRh (80:07:13).
2
PtSnRh electrodeposits. Data extracted from Fig. 2.

G.A.B. Mello et al. / Electrochemistry Communications 48 (2014) 160–163
163
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The authors thank CNPq (grants # 405695/2013-6, # 313402/2013-2,
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