Rhodium in presence of platinum as a facilitator of carbon-carbon bond break: A composition study

Article abstract of DOI:10.1016/j.electacta.2010.10.006

In this work we investigate the acetaldehyde electrooxidation reaction on PtRh electrodeposits as a function of their composition. Potentiodynamic results show that the balance between CO₂ and acetic acid pathways depends on the PtRh compositio

Full text of DOI:10.1016/j.electacta.2010.10.006

Electrochimica Acta 56 (2011) 1337–1343
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Rhodium in presence of platinum as a facilitator of carbon–carbon bond break:
A composition study
∗
Edgard P. Leão, M. Janete Giz , Giuseppe A. Camara, Gilberto Maia
DQI/UFMS, C.P. 549, 79070-900 Campo Grande, MS, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 July 2010
Received in revised form
In this work we investigate the acetaldehyde electrooxidation reaction on PtRh electrodeposits as a
function of their composition. Potentiodynamic results show that the balance between CO2 and acetic
acid pathways depends on the PtRh composition. By using in situ FTIR we interpret this dependence in
terms of a dual role played by Rh during acetaldehyde electrooxidation. Rh seems to favor the break of
C–C bonds (favoring the production of CO2) whilst prevents the deliverance of oxygen-containing species,
thus inhibiting the oxidation steps.
3
0 September 2010
Accepted 2 October 2010
Available online 29 October 2010
©
2010 Elsevier Ltd. All rights reserved.
Keywords:
Electrocatalysis
Acetaldehyde oxidation reaction
Platinum–rhodium electrodeposits
In situ FTIR
1
. Introduction
species to CO requires oxygen species like Pt(OH)ad, therefore, the
2
region where oxidation occurs is at potentials above ∼0.6 V vs. RHE,
i.e., where the dissociation of water occurs [12]. Weaver and co-
workers have investigated the electrooxidation of acetaldehyde on
Pt (1 1 1) compared with disordered Pt (1 1 1) and polycrystalline Pt
[13]. Results show that there is a strong inhibition of acetaldehyde
electrooxidation caused by a smaller CO-coverage on this surface
if compared with polycrystalline and disordered Pt [13]. Accord-
ing to the authors, the markedly higher CO coverages formed on
disordered Pt (1 1 1) and polycrystalline Pt create a more favor-
able electrocatalytic situation in that the oxidation of CO produces
a “clean” substrate upon which the organic electrooxidation can
proceed [13].
Concerning electrodeposits, PtRh has been considered as a
suitable catalyst to enhance the break of C–C bonds during the
electrooxidation of ethanol [14] and 2-propanol [15], where is
demonstrated that PtRh surfaces increase C–C bond dissociation
when compared to Pt alone [14] and improves the selectiv-
ity towards CO2 production [15]. In recent works Adzic and
One of the major problems concerning ethanol electrooxidation
is the lack of catalysts which promote an efficient oxidation of the
alcohol to CO2 [1]. As a consequence, Pt-based catalysts show a
massive production of acetaldehyde under several experimental
conditions [1–4], which avoids the further development of fuel cells
feed with ethanol.
In this context, the understanding of the mechanism of acetalde-
hyde oxidation on Pt-based electrodes is of fundamental interest
in electrocatalysis, as a required step to improve the Faradaic effi-
ciency of the process [5]. In spite of this important matter, studies
involving the direct oxidation of acetaldehyde are still relatively
scarce in the literature [5–11]. Despite the lack of information
concerning the acetaldehyde/Pt system, some important struc-
tural effects are already consensual. For instance, it is known that
the adsorption and oxidation of acetaldehyde on Pt are strongly
affected by the surface structure. According to Iwasita and co-
workers Pt (1 0 0) produces greater amounts of CO2 and acetic acid
than Pt (1 1 1) [12]. Pt (1 0 0) also shows a greater capacity than Pt
co-workers [16,17] describe a new electrocatalyst, PtRhSnO /C,
2
(
1 1 1) for the breaking of C–C bonds, but at high potentials, the sur-
that is very effective in splitting the C–C bond in ethanol at
room temperature leading to a predominant production of CO2
at low overpotentials. According to the authors, the catalytic
property of this ternary electrocatalyst is attributed to a syner-
gistic effect between the constituents. Namely, SnO2 by strongly
adsorbing water and interacting with the Pt and Rh deposited
on its surface, apparently precludes the Rh and Pt sites from
reacting with H2O to form M–OH, making them available for
ethanol oxidation. Thus, SnO2 provides OH species to oxidize the
face of Pt (1 0 0) looses its activity. The authors argued that probably
the formation of acetic acid occurs via an Eley–Rideal mechanism,
while the adsorption step of the CO2 pathway requires free Pt sites
and occurs mainly at low potentials [12]. The oxidation of adsorbed
∗ Corresponding author. Tel.: +55 67 3345 7012; fax: +55 67 3345 3552.
E-mail addresses: mjgiz@nin.ufms.br, janetegiz@gmail.com (M.J. Giz).
0
013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.10.006

1
338
E.P. Leão et al. / Electrochimica Acta 56 (2011) 1337–1343
dissociated CO at Rh sites, and Pt facilitates ethanol dehydro-
genation. Moreover, the electronic structure of Rh is modified by
the presence of Pt, thus weakening the bonding of ethanol and
intermediates with the surface and facilitating C–C bond breaking
2.3. In situ FTIR
Once obtained, PtRh electrocatalysts were characterized in
−
3
0.1 mol dm
HClO4 and transferred to a spectroelectrochemical
[
16,17].
However, at least to our knowledge, there are no studies
cell in order to perform the following experiments:
involving the use of PtRh as catalysts for the electrooxidation of
acetaldehyde. This issue is the main concern of the present work.
In order to accomplish this, we investigate the influence of the
atomic composition of PtRh electrodeposits (ranging 0–100 at.% Rh,
estimated from energy dispersive X-ray analysis) on acetaldehyde
electrooxidation in acidic media. PtRh electrodeposits were inves-
tigated by electrochemical and in situ FTIR techniques. The catalytic
activity was correlated with yields of products. Results show that
the parallel reactions are affected by the composition of the cata-
lyst, being the production of acetic acid inhibited by the presence
of Rhodium.
(
i) Pure CO was adsorbed for 5 min at 0.05 V. After that, CO was
eliminated from the solution by bubbling pure N2 for 10 min
−1
and a cyclic voltammogram was recorded at 1 mV s in the
potential range of 0.05–0.8 V while FTIR spectra were collected.
This initial procedure provides a CO signal corresponding to the
oxidation of a monolayer of CO and it was used as a standard to
estimate the CO2 formation from acetaldehyde.
(ii) For the investigation of acetaldehyde electrooxidation,
−3
0
.1 mol dm
acetaldehyde was admitted to the cell and the
FTIR spectra were collected during a linear sweep voltam-
−1
mogram (ꢀ = 1 mV s ). The scan rate and the number of
interferograms were chosen in order to allow that the spectra
were collected at intervals of ca. 0.05 V in the potential range
of 0.05–1.45 V.
2
. Experimental
2.1. Solutions, electrodes, chemicals
In situ FTIR measurements were carried out by using a MB-100
Solutions were prepared from Milli-Q® water and analytical
spectrometer (Bomem) with a MCT detector. Reflectance spectra
were collected as the ratio (R/Ro) where R represents a spectrum at
a given potential and Ro is the spectrum collected at 0.05 V. Posi-
tive and negative bands represent the consumption and production
of substances, respectively [18]. Spectra were computed from the
average of 50 interferograms. The spectral resolution was set to
grade reagents: CH CHO (Merck, p.a.), HClO4 (Suprapur, Merck),
3
RuCl3 hydrate (Aldrich) and H PtCl6 (Alfa Aesar).
2
The electrodeposits were prepared by electroreduction of
Rh3+ and Pt (from RuCl3 and H PtCl6 aqueous solutions) in
4+
2
−
.1 mol dm HClO and 0.2 V vs. RHE, during a time period of 1 min.
4
3
0
−
1
8
cm . The electrochemical IR cell was fitted with a CaF2 planar
PtRh catalysts were electrodeposited whether on a polished gold
2
window for the collection of bands corresponding to the formation
of CO2 and acetic acid.
disk (0.78 cm of geometric area) for FTIR experiments or on a gold
2
foil (0.32 cm ) for cyclic voltammetry. The nominal composition of
the electrodeposits was varied from 10 to 80 at.% of Rh. The atomic
compositions reported here represent the average of six different
measurements on the same sample and were determined by energy
dispersive X-ray analysis (EDX) using a Zeiss-Leica/440 instrument
provided with a SiLi detector. The mean values obtained for the
different compositions show no significant differences from the
corresponding nominal compositions.
3. Results and discussion
The composition of the PtRh electrodeposits was determined
by EDX analysis and the results are shown in Table 1. Usually this
technique provides bulk compositions and due to the penetration
of the X-rays the results can be considered as representing only
approximately the surface composition. The data in Table 1 rep-
resent the average of six different points in each sample. All the
experiments for the same sample present virtually the same atomic
composition. It can be observed that the values determined by EDX
analysis are very close from the corresponding nominal compo-
sitions indicating that the electrodeposition of the metals follows
their respective bath concentration even at high concentrations of
Rh. These results are in contrast to that observed by Gupta and Datta
for the electrodeposition of PtRh on graphite [19]. According to the
authors the electrodeposition of Rh is hindered on increasing the
concentration of RhCl3 in the deposition bath due to the formation
2.2. Electrochemical experiments
After the electrodeposition the electrodes were thoroughly
washed with Milli-Q water and transferred to an electrochemi-
−3
cal cell containing 0.1 mol dm HClO . The state of the electrode
4
surface was checked by cyclic voltammetry among 0.05 and
−1
1
.45 V with a scan rate of 0.02 V s in the supporting electrolyte.
Then, the potential was kept at 0.05 V and acetaldehyde was
−
3
admitted in the cell to reach the concentration of 0.1 mol dm
,
being a new voltammetric cycle obtained in the presence of
acetaldehyde. All the experiments were performed at room
of the complex [RhCl6]3 at higher RhCl3 concentrations [19]. This
behavior was not observed for the electrodeposits prepared in this
work as can be seen in Table 1. One possible explanation is the low
−
◦
temperature (25.0 ± 1.0 C). Prior to the measurements, the solu-
tions were deaerated with N . All potentials were measured
2
against a reversible hydrogen electrode (RHE) in the same elec-
trolyte.
Table 1
For the estimation of the real surface area of PtRh electrode-
posits the electrodes were saturated with carbon monoxide by
bubbling the gas during 10 min at 0.05 V vs. RHE in the solution
containing the supporting electrolyte. The excess of CO was then
eliminated from the solution by bubbling pure N2 for 10 min and
EDX results of the various surface compositions of Pt:Rh catalysts prepared by
electrodeposition.
Catalyst
Pt:Rh
Nominalcontent(at.%)
Determined byEDX(at.%)
Pt
Rh
Pt
Rh
−
1
90
10
20
30
40
50
60
80
91
80
67
61
55
41
16
9
20
33
39
45
59
84
cyclic voltammograms were recorded at 0.02 mV s in the poten-
tial range of 0.05–0.80 V. The first cycle provided the total charge of
CO oxidation, a second one was taken to check the recovering of the
original voltammetric profile. It was assumed that the oxidation of
8
7
0
0
60
50
40
−
2
one CO-monolayer generates a charge density of 420 ␮C cm of
20
real area.

E.P. Leão et al. / Electrochimica Acta 56 (2011) 1337–1343
1339
−
3
−3
Fig. 1. Electrochemical behavior of 0.1 mol dm acetaldehyde in 0.1 mol dm HClO4 at: (A) Pt; (B) Pt91Rh9; (C) Pt80Rh20; (D) Pt67Rh33; (E) Pt61Rh39; (F) Pt55Rh45; (G) Pt41Rh59;
(
◦ −1
H) Pt16Rh84; and (I) Rh electrodeposits. CVs (—) for the acetaldehyde-containing solution and (- - -) for the supporting electrolyte. T = 25 C; ꢀ = 20 mV s . Current densities
refer to the estimated real surface area.
concentration of RhCl3 that was used in the preparation of these
electrodeposits. For all the electrodeposits the total concentration
3.1. Acetaldehyde electrooxidation – electrochemical behavior
−3
of the metals in the bath was 0.01 mol dm . We believe that this
low concentration prevents the formation of complex and allows
the full deposition of Rh on the gold surface. Another important
point which must be considered here are the different substrates
that were used, graphite by Gupta and Datta [19] and gold surfaces
in our case. It is known that metallic surfaces like gold show higher
conductance than graphite which probably facilitate the deposition
of Rh on this kind of surface.
An aspect which must be taken into account is the influence of
the roughness of the surfaces. For electrodeposits is already known
that an increase of the roughness can influence the balance of the
oxidation pathways of organic molecules [20]. As a way to prevent
this effect we decided to apply the reduction potential for 1 min.
This short time of electrodeposition generates electrodeposits with
relative low roughness, as demonstrated for PtRu deposits elec-
trodeposited in the same gold substrates used here [20]. In the
present case all the electrodeposits presented roughness factors
Fig. 1 shows the first scan of cyclic voltammograms recorded
−
1
at 20 mV s
tions in solutions containing 0.1 mol dm
for PtRh electrodeposits with different composi-
−
3
HClO4 (dashed line)
−
3
−3
and 0.1 mol dm HClO + 0.1 mol dm acetaldehyde (solid line).
4
For comparison, voltammograms for Pt and Rh electrodeposits
are included. For all the compositions a partial blockage of H-
adsorption in the hydrogen region is appreciated indicating a strong
adsorption of acetaldehyde at the initial potential of 0.05 V. The
onset for oxidation of acetaldehyde occurs at 0.7 V, developing two
well-defined peaks centered at ca. 0.9 V and 1.25 V during the posi-
tive scan, except for the catalyst with high content of Rh (Pt16Rh84
)
in which the acetaldehyde oxidation is very similar to the pure Rh
electrodeposit [21]. After reversing the potential at 1.45 V, the reac-
tivation of the surface for the electrooxidation (anodic current at
0.6 V) is apparent as soon as the platinum oxide is reduced (absence
of cathodic current), for the catalysts containing 9–33 at.% of Rh.
Increasing the Rh content this reactivation peak is inhibited and a
cathodic current appears, mainly for the Pt16Rh84 and Rh catalysts.
An interesting point is the difference in the behavior of the two oxi-
dation peaks when the catalysts have different contents of Pt and
Rh. Fig. 1B–G shows that as the catalyst surface is enriched with Rh,
(
i.e., the ratio between the electrochemical and geometric areas) in
the range comprised among 11 and 16. These relatively close values
suggest that the roughness is not a determinant parameter for the
attribution of the corresponding catalytic activities.

1
340
E.P. Leão et al. / Electrochimica Acta 56 (2011) 1337–1343
the second oxidation peak is inhibited while the first peak grows
up progressively.
The ratios between the current densities referring to peak I and
peak II were plotted as a function of Rh content and are shown in
Fig. 2. It can be observed that as Rh is added to the electrodeposit
the ratio jpeak I/jpeak II increases almost proportionally. When the
Rh content is less than 40 at.% the ratio of jpeak I/jpeak II < 1, i.e., the
second peak prevails. For electrodeposits with Rh concentrations
high than 50 at.% the first oxidation peak is favored and the second
one is inhibited.
A similar behavior was already observed by Gupta and Datta for
the electrooxidation of ethanol on PtRh electrodeposits on graphite
[
19]. The authors attributed this effect to the strong bond between
Rh–O which limits Rh as source of oxygen. Consequently, with the
enrichment of the catalyst by Rh, the ability to oxidize ethanolic
residues at high potentials decreases, which explains the absence
of the second oxidation peak [19].
In our case the results show that the presence of Rh associated
to Pt exerts a strong influence on the acetaldehyde electrooxida-
tion. It is already known from a previous work that, for pure Pt,
the regions corresponding to peak I and II are mainly associated
with the production of CO2 and acetic acid, respectively [5]. If one
assumes that the acetaldehyde electrooxidation on PtRh electro-
catalysts follows the same pathways that for pure Pt electrodes, it
can be inferred that the presence of Rh would be responsible for
one of the following effects:
Fig. 2. jpeak I/jpeak II ratio as a function of the amount of Rh in the electrodeposit. Data
were calculated from voltammograms in Fig. 1.
(1) Rh aid to catalyze the break of the C–C bond in acetalde-
hyde. This break would produce preferentially species with
one carbon which could explain the progressive diminution
of the peak II as the Rh content increases since species con-
−
3
−3
Fig. 3. In situ FTIR spectra for PtRh electrodeposits in 0.1 mol dm HClO4 in the presence of 0.1 mol dm acetaldehyde at different potentials as indicated. Reference spectra
taken at 0.05 V. Sample spectra measured after each 0.05 V. Spectra averaged from 50 scans. Compositions indicated in the figure.

E.P. Leão et al. / Electrochimica Acta 56 (2011) 1337–1343
1341
taining 2-C are being progressively consumed during the C–
C scission, or;
(
2) Rh anticipates the potentials where the formation of acetic acid
occurs so that they became coincident with those ones corre-
sponding to the production of CO2.
In this sense, it is important to verify which of these two hypoth-
esis apply to the PtRh/acetaldehyde system and to establish to
which extent the pathways involved are affected by the presence
of Rh. This task can be accomplished by following the formation of
CO2 and acetic acid using in situ FTIR spectroscopy. The results are
presented and discussed in the next section.
3.2. Acetaldehyde electrooxidation – spectroscopic data
Fig. 3 presents FTIR spectra obtained during acetaldehyde elec-
trooxidation for all the electrodeposits investigated. For purposes
of clarity, only some selected potentials are shown. Bands corre-
−
1
sponding to the production of CO (2343 cm ) and acetic acid
(
2
−1
1280 cm ) are present for all the compositions, which evidences
that both pathways are taking place on PtRh surfaces. CO2 appears
around 0.55 V and remains being formed at least until 0.85 V (not
shown). For higher potentials, there is a slow decreasing of the
CO2 band intensities, which means that their production has been
ended. The persistence of the band at higher potentials suggests
that CO2 remains “trapped” in the thin layer due to the low diffu-
sion.
On the other hand, the corresponding band for acetic acid
−1
(
1280 cm ) starts to develop at 0.7 V and, conversely to CO2,
remains growing until 1.45 V.
Fig. 4. Integrated absorbances for (A) CO2 and (B) acetic acid as a function of poten-
One important aspect concerning the foregoing discussion is the
−3
−3
tial, from spectra measured in 0.1 mol dm acetaldehyde + 0.1 mol dm HClO4 for:
(ꢀ) Pt91Rh9; (᭹) Pt80Rh20; (ꢁ) Pt67Rh33; (ꢁ) Pt61Rh39; (ꢂ) Pt55Rh45; (ꢀ) Pt41Rh59;
(ꢁ) Pt16Rh84. Experimental conditions as in Fig. 3.
appearance of CO band in a region of potential where no oxidation
2
currents are apparent (see Fig. 1). However, a closer inspection of
the voltammograms (not shown here) attests the presence of small
oxidation currents already at 0.55 V for intermediate compositions,
which justify the presence of CO2 at those potentials.
Although qualitatively important the bands cannot be directly
correlated with the intrinsic activity of the PtRh deposits at this
point. In order to do that the electroactive areas and roughness
should be considered, once we are dealing with different surfaces.
The procedure which allows such correlation will be presented in
details later.
and the catalytic activity observed by electrochemical measure-
ments the individual yields of CO2 and acetic acid were calculated
after evaluation of the respective integrated band intensities. Val-
−
1
−1
ues of the effective absorption coefficient, εeff (mol cm ), were
calculated based on the procedure proposed by Weaver and co-
workers [13,22]. Shortly, a monolayer of CO was adsorbed on the
electrodes, following the same experimental setup adopted for the
CO-stripping procedure. Next, the electrodes were scanned among
−
1
0
.05–1.45 V at 1 mV s whilst the development of the band cor-
3.3. Accompanying the soluble products
responding to the production of CO2 was followed by FTIR in situ.
The collected spectra allowed the determination of the area cor-
Fig. 4A and B shows the integrated absorbances of the bands cor-
responding to the maximum absorbance for CO by integration of
2
−
1
responding to the production of CO2 and acetic acid, respectively,
as a function of the potential. The absorbance values were obtained
by the integration of the absorption bands and normalization by the
electrochemical area of each electrodeposit. Fig. 4A shows that the
production of CO2 starts at ca. 0.5–0.55 V and the further develop-
ment of the band depends on the composition of the electrode. For
those materials containing 10 and 20 at.% of Rh CO2 is produced
until 1.1 V. When the Rh content is increased there is a dislocation
the band located at 2343 cm . Then, this area was correlated with
the amount of CO2 generated from CO (obtained by integration of
the CO-electrooxidation charge). This procedure was repeated for
each electrodeposit, because each electrode presents a specific sur-
face area. As the εeff values for acetic acid cannot be obtained by
a similar approach we assume that εeff values for CO2 and acetic
acid follow the relationship calculated by Weaver and co-workers
[13,22] and by comparison with the values obtained for CO2 in the
present case the corresponding acetic acid absorption coefficients
were estimated.
of the CO production for lower potentials. Also, for electrodeposits
2
richer in Rh the production of CO2 reaches its maximum at lower
potentials than those observed for surfaces with lower content of
this metal. Despite the multiplicity of curves shown in Fig. 4A it
is feasible to say that the production of CO2 is favored at compo-
sitions around Pt67Rh33, being inhibited for those electrodeposits
with higher contents of Rh.
The amount (W/mol cm 2) of a given species inside the thin
−
layer cavity follows the relationship W = A /ε , where A is the inte-
i
eff
i
−
1
grated absorbance, expressed in cm . Values of εeff for both the
species are presented in Table 2.
From the values presented in Table 2, and considering that
appreciable differences of charge are involved in the formation
of CO2 and acetic acid, the charges referring to the production of
both substances were estimated, assuming that each mol of CO2
produced from acetaldehyde generates a charge of 10 F, whilst pro-
Conversely to the behavior observed for CO , Fig. 4B shows
2
that acetic acid presents a maximum production for the elec-
trode with less Rh, indicating that acetic acid is inhibited by the
enrichment of the surface by this metal. As an attempt to estab-
lish a direct correlation between the amounts of both products
−
1
duction of acetic acid involves the transfer of 2 F mol . Since the

1
342
E.P. Leão et al. / Electrochimica Acta 56 (2011) 1337–1343
Table 2
oxidation charge. Another remarkable aspect is that the maximum
qCO2 increases up to Pt67Rh33 and remains virtually constant for
the others compositions except for Pt20Rh80 where the excess of
Rh inhibits the production of any species that depend on activated
oxygen to be formed.
Effective absorption coefficients obtained from the maximum absorbance for CO2
for PtRh electrodeposits.
Pt:Rh
εeff/mol 1 cm
− −1
CO2
Acetic acid
In contrast, qHAc decreases progressively when the catalysts are
richer in Rh. The present results can be understood if we assume
that surfaces containing Rh play a dual role during acetaldehyde
electrooxidation. Rh can help to break C–C bonds necessary to pro-
duce CO2 (anticipating its production, as shown in Figs. 3 and 4),
but, at the same time, its presence can inhibit the activation of water
necessary to promote the oxidation steps, as already argued. This
interpretation explains why the production of CO2 is favored at
intermediate PtRh compositions whilst for Pt91Rh9, Pt80Rh20 and
Pt16Rh84 it is sensibly decreased. It is also valid to interpret the
progressive deactivation of acetic acid pathway. In that case, the
enrichment of the surface by Rh is only prejudicial, because both the
effects – the enhanced ability to break C–C bonds and the increas-
ing difficulty to donor oxygen-containing species – are unfavorable
to the production of acetic acid.
7
7
7
7
7
7
7
6
9
8
6
6
5
4
1
1:09
2.9 × 10
2.7 × 10
3.8 × 10
3.3 × 10
2.7 × 10
3.0 × 10
2.6 × 10
4.8 × 10
6
0:20
7:33
1:39
5:45
1:59
6:84
4.5 × 10
6
6
6
6
6
6.4 × 10
5.4 × 10
4.5 × 10
5.0 × 10
4.3 × 10
amounts (W) were founded, the corresponding charge densities
were calculated using the considerations above.
Fig. 5 shows the charge densities corresponding to both path-
ways and the global charge density as a function of the potential
imposed to the work electrode, here after, designed qCO2 , qHAc and
qtotal. It can be observed that qCO2 are sensibly lower than qHAc, i.e.,
the production of CO represents a minor contribution for the global
2
Fig. 5. Charge densities for (᭹) CO2, (ꢀ) acetic acid and (ꢁ) total charge density calculated from the amounts of CO2 and acetic acid as a function of the potential. Compositions
indicated in the figure.

E.P. Leão et al. / Electrochimica Acta 56 (2011) 1337–1343
1343
4
. Conclusions
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Rh plays a dual role during acetaldehyde electrooxidation,
because it helps to break C–C bonds whilst inhibits the activation
of water necessary to promote the oxidation steps.
As a consequence, the production of CO2 is favored at inter-
mediate PtRh compositions and the production of acetic acid is
inhibited for all compositions.
[
[
-
-
[
[
[
When the surfaces are enriched with Rh the prevention of acetic
acid production makes the CO pathway more and more relevant
2
[
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for the global activity.
[
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Acknowledgements
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The authors acknowledge financial assistance from CNPq, FUN-
DECT (Process 23/200.065/2008) and FINEP (ProCaC, Institutional
Process 01.06.0939.00). M.J. Giz and E.P. Leão are indebted to CNPq
for a fellowship.
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[
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