Electro-oxidation of ethanol and ethylene glycol on carbon-supported nano-Pt and -PtRu catalyst in acid solution

Article abstract of DOI:10.1016/j.electacta.2009.07.054

Present paper reports kinetics of electro-oxidation of ethanol (EtOH) and ethylene glycol (EG) onto Pt and PtRu nanocatalysts of different compositions in the temperature range of 298-318 K. These catalysts have been characterized by SEM, EDX, XRD, CV and

Full text of DOI:10.1016/j.electacta.2009.07.054

Electrochimica Acta 54 (2009) 7299–7304
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electro-oxidation of ethanol and ethylene glycol on carbon-supported
nano-Pt and -PtRu catalyst in acid solution
Moitrayee Chatterjee, Abhik Chatterjee, Susanta Ghosh, I. Basumallick∗
Electrochemical Laboratory, Department of Chemistry, Visva-Bharati University, Santiniketan 731235, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Present paper reports kinetics of electro-oxidation of ethanol (EtOH) and ethylene glycol (EG) onto Pt
and PtRu nanocatalysts of different compositions in the temperature range of 298–318 K. These catalysts
have been characterized by SEM, EDX, XRD, CV and amperometry. It has been observed that apparent
activation energies for oxidation of EtOH and EG pass through a minimum at about 15–20 at.% of Ru in
the PtRu alloy catalysts. Anodic peak current vs. composition curve also shows a maximum around this
composition.
The results have been explained by a geometric model, which proposes requirement of an ensemble of
three Pt atoms with an adjacent Ru atom onto PtRu surface for an efficient electro-oxidation of EtOH or
EG. This is further supported from statistical data analysis of probability of occurrence of such ensembles
onto PtRu alloy surface.
Received 6 March 2009
Received in revised form 20 July 2009
Accepted 20 July 2009
Available online 28 July 2009
Keywords:
Electrocatalyst
Ethanol electro-oxidation
Ethylene glycol electro-oxidation
Bifunctional mechanism
Activation energy
Present results also suggest that electro-oxidation of EG onto nano-PtRu catalyst surfaces follows a
different path from that of EtOH at alloy composition less than 15 at.% of Ru.
© 2009 Elsevier Ltd. All rights reserved.
1. Introduction
ties of Pt in the presence of Ru or Ni in bimetallic alloy is explained
by (i) bifunctional mechanism [8,12] and (ii) electronic contribution
In the recent years, there is an increasing interest in the develop-
ment of catalyst for alcohol fuel cells. Among the alcohols, ethanol
(EtOH) [1–3] and ethylene glycol (EG) [4,5] are potentially attrac-
tive energy resource due to their non-toxicity and low volatility
together with high energy density. There is still a lack of cata-
lyst that can initiate complete oxidation of these alcohols at high
rate, which limits their potential uses in DAFCs. Electro-oxidation of
methanol has been studied exhaustively both in terms of its appli-
cation and theoretical analysis. These studies are mainly focused
on poisoning of costly electrocatalysts, like platinum, by CO formed
as an intermediate during alcohol oxidation [6–8]. Uses of binary
or more than two alloying metal catalysts are recommended as
effective method for removal of CO from the catalyst surface.
Recent reports on the preparation of nano-electrocatalysts of Pt
[9–11], PtRu [7,12–15], PtSn [2–4,16], PtNi [17], PtAu [18] PtRuMo
[19], PtRuNi [17,20], etc. and their application in alcohol oxidation
suggests that mechanisms of electro-oxidation of simple alcohols
onto these surfaces are complex and vary with alloy compositions.
Nanocatalysts are prepared either by chemical reduction with
a suitable reducing agent like sodium borohydride in presence
of a capping agent or by controlled electro-deposition techniques
[10,21–23]. The observed enhancement of electro-catalytic activi-
or electronic effect [8,18].
Carbon monoxide, which is produced as an intermediate during
electro-oxidation of alcohols, forms a strong bond with Pt, thus its
removal requires an adjacent Pt(OH) by H2O activation process:
H2O + Pt → Pt(OH)ads + H+ + e−
> 0.4vs.NHE
(1)
However, when PtRu is used, the water activation process occurs
at much lower potential.
H2O + Ru → Ru(OH)ads + H+ + e−
0.2–0.3 Vvs.NHE
(2)
Thus, overall alcohol oxidation including oxidation of adsorbed
CO can be achieved under energetically favorable condition in the
presence of Ru.
Pt(CO)ads + Ru(OH)ads → Pt + Ru + CO2 + H+ + e−
(3)
As shown in Eq. (3), in bifunctional mechanism Pt functions as
catalyst for deprotonation, de-electronation and Ru functions as
water molecule activator and finally these two functions act coop-
eratively [15].
In addition to this, due to the presence of Ru in PtRu alloy, elec-
tronic charge transfer from Ru to Pt occurs which makes a reduction
of density of states near the Fermi level of Pt and electron density
of 2ꢀ* MO of adsorbed CO is reduced and this makes Pt–CO bond
weaker. Because bonding energy of Ru–O is similar to that of Pt–C,
Ru easily oxidizes this loosely bound CO to CO2 [17].
∗
Corresponding author. Tel.: +91 3463 261526; fax: +91 3463 262672.
E-mail address: ibasumallick@yahoo.co.uk (I. Basumallick).
0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.07.054

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PtSn has also been recommended as a good bimetallic catalyst
for ethanol oxidation at high temperature [16], but as we are inter-
ested to low temperature fuel cells, our present study has been
confined to PtRu only.
The relative contribution of bifunctional and electronic effect
towards effective removal of CO is a subject of great controversy.
However, in a very recent paper Wieckowski et al. suggested that
for methanol electro-oxidation onto PtRu surface, bifunctional path
contributes about 80% and the rest is through electronic effect. It is
also interesting to note that in PtRu alloys, an alloy with 10 at.% of
Ru exhibits maximum catalytic efficiency towards methanol oxi-
dation. This has been explained nicely by a geometric model. It
is proposed that efficient adsorption of a methanol molecule onto
PtRu surface occurs when there is an ensemble of three Pt atoms
with an adjacent Ru atom and subsequently shown that probability
of occurrence of such ensembles on a hexagonal Pt (1 1 1) surface
is maximum at 10 at.% of Ru [6,8].
Fig. 1. Scanning electron microscope image of carbon felt in uncompressed state at
a magnification of 200×.
In our present study, we report preparation of nano-Pt and -
PtRu alloys with different atom compositions (Pt91Ru9, Pt79Ru21
,
Figs. 1 and 2 respectively. The felt has thickness ca. 2.4 mm, porosity
of 0.95, mean fibre diameter of ca. 20 ␮m and an electrical conduc-
tivity of 10 S/m. The EDX analysis was performed using an EXL II,
Oxford attached to the microscope.
X-ray diffraction pattern (XRD) was recorded by a Rigaku Mini-
flex diffractometer using a Cu K␣ source (ꢁ = 0.1542 nm) operating
at 30 kV and 15 mA. The diffractometer was operated in the contin-
uous mode with the scanning rate 1.00◦/min in the range of 30–90◦
(2ꢂ).
Electrochemical measurements were made using a three-
electrode cell by a potentiostat–galovanostat (PAR VersaStatTMII).
A Pt-foil (1 cm2) and a saturated calomel electrode were used as
counter and reference electrode respectively. Solutions of 0.5 M
H2SO4 and 1 M EtOH or 1 M EG in 0.5 M H2SO4 were purged with
nitrogen gas before each experiment. Cyclic voltammetry experi-
ments were carried out at four different scan rates: 0.01, 0.03, 0.05
and 0.07 V/s. Amperometric i–t experiments were done at 0.5 V at
298 K.
Pt67Ru33) by chemical method of nano-synthesis and their electro-
catalytic activities towards oxidation of EtOH and EG. These
catalysts are characterized by XRD, SEM and EDX. Cyclic voltam-
metric studies have been carried out at different temperatures
from 298 to 318 K with different scan rates. Durability of differ-
ent catalysts for electro-oxidation of EG and EtOH was tested with
amperometry experiments.
It is expected that present study will help understanding the
mechanism of electrochemical oxidation of EG and EtOH onto
nanobinary (PtRu) alloy surface at molecular level.
2. Experimental
2.1. Preparation of catalysts
Pt-based alloy nanoparticles were synthesized using a conven-
tional borohydride reduction method [17–20]. The carbon black
powder (S.A. 80 m2/g, Alfa Aaesar) was pretreated with hot aque-
ous nitric acid (1:1) for generating functional groups [24], washed
thoroughly with double distilled water, dried and used as sup-
port for the metal nanoparticles. The pretreated carbon powder
was then suspended in water–isopropanol mixture via sonication
for 30 min to make uniform carbon ink. Precursor salt solutions of
H2PtCl6·xH2O (Arrora Matthey) and RuCl6·xH2O (Aldrich) of desired
stoichiometry were added to this mixture ultrasonically for 20 min.
The pH of this mixture was adjusted to ∼11.0 by adding NaOH
and rose to a temperature to 353 K. Cold aqueous solution of 0.2 M
of sodium borohydride (Merck) was added drop wise (threefold
excess of calculated amount) under constant stirring condition.
The precipitate was then centrifuged and washed thoroughly with
double distilled water and dried in vacuum desiccator. The carbon-
supported catalyst so prepared contained about 20 wt.% of metal.
2.4. Surface area determination
The active surface area of the Pt and PtRu electrocatalysts
was determined by underpotential deposition of copper (Cu-UPD)
[22,25].
After electrochemical cleaning in 0.1 M H2SO4, the working elec-
trode was transferred into the solution containing 0.1 M H2SO4 and
2.2. Preparation of electrode
5 mg of the carbon-supported catalyst was mixed with 50 ␮l of
Nafion solution (5 wt.% of Nafion, Merck) and volume of the mixture
was made up to 1 ml by adding isopropanol. This mixture was then
ultra-sonicated for 30 min to make homogeneous ink. An aliquot of
0.2 ml of this ink was then sprayed over each sides of the carbon felt
(Alfa Aesar) support (1 cm × 1 cm) by a sprayer and dried at 338 K.
The metal loading was about 0.4 mg.
2.3. Characterizations
The SEM (Hitachi S-3000N equipment) characterization of
the carbon felt and catalyst-loaded carbon felt was shown in
Fig. 2. Scanning electron microscope image of Pt/C nanocatalyst onto carbon felt
support.

M. Chatterjee et al. / Electrochimica Acta 54 (2009) 7299–7304
7301
Table 1
Surface compositions of different catalyst from EDX analysis, electroactive surface area from Cu-UPD experiment and mean particle size calculated from XRD.
Catalyst
At.% Ru
Surface area (cm2/mg)
Cu stripping
Size from XRD (nm)
H adsorptiona
Pt
0
9
21
33
106
109
115
107.32
117
–
–
10.4
13.3
12.4
11.3
Pt9Ru91
Pt79Ru21
Pt67Ru33
–
a
Calculated assuming that to form a monolayer of adsorbed hydrogen per cm−2 of smooth Pt is necessary a charge of 210 ␮C.
2 mM CuSO4 solution. It was then polarized at 0.8 V for 120 s and
stepped to 0.059 V for 300 s duration to deposit Cu monolayer onto
catalyst surface. A linear voltammetric scan was performed with
0.01 V/s from 0.059 to 0.8 V to remove the adsorbed Cu monolayer.
The charge associated with this copper stripping was calculated by
subtracting the charge associated for background process. The elec-
troactive surface area was then calculated with the assumption of
an adsorption ratio of a single Cu atom to each surface metal atom
and a monolayer charge of 420 ␮C/cm2.
clusters of nano-Pt/-C particles are dispersed almost uniformly onto
carbon felt surface.
Results of EDX analysis for PtRu catalysts are shown in Table 1.
These surface compositions are expected from the stoichiometry of
precursor salts used during preparation of the catalysts.
Fig. 3a shows XRD characterization of Pt, Pt91Ru9, Pt79Ru21 and
Pt67Ru33. The broad peak at 2ꢂ around 40◦ clearly indicates the
formation of nanocrystal of Pt (1 1 1). The presence of Ru (1 0 1)
planes in catalyst Pt79Ru21 is also indicated. On the basis of half-
peak width of the Pt (1 1 1) peak, the size of the nanoparticles is
estimated via Scherrer’s formula [19] and given in Table 1.
A close look of Fig. 3b shows that the Pt (1 1 1) peaks are slightly
shifted with smaller peak height in the presence of Ru, which indi-
cates good alloy formation.
3. Results and discussion
3.1. SEM, EDX and XRD analysis of Pt/C and PtRu/C nanoparticles
SEM pictures of carbon felt and Pt/C nanoparticles loaded carbon
felt are shown in Figs. 1 and 2 respectively. It is seen that small
3.2. Cyclic voltammetry experiments
Arrhenius plots are drawn for ethanol (EtOH) and ethylene
glycol (EG) electro-oxidations using oxidative current values for
forward scan at three different potentials 0.45, 0.5 and 0.6 V onto
different catalysts surfaces in the temperature range of 298–318 K.
Typical Arrhenius plots for EG and EtOH are shown in Fig. 4. Respec-
Fig. 4. Arrhenius plots for (a) ethylene glycol and (b) ethanol electro-oxidation onto
Pt/C catalyst with scan rate 0.05 V/s at two different potentials: (ꢀ) 0.5 V and (ꢀ)
0.6 V.
Fig. 3. X-ray diffraction analysis of pure Pt and Pt–Ru nanoparticles: (a) wide scan-
ning of X-ray diffraction and (b) fine scanning of (1 1 1) peak.

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Fig. 5. Cyclic voltammograms of 1 M ethylene glycol and ethanol in 0.5 M H2SO4 with scan rate 50 mV/s at 298 K. (a) Ethylene glycol and (b) ethanol oxidation onto different
catalyst surfaces; oxidation of both ethylene glycol and ethanol onto (c) pure Pt and (d) Pt79Ru21 catalysts.
tive CVs and representative CVs for EG and EtOH at 298 K with
0.05 V/s scan rate are shown in Fig. 5. All the current values are
normalized with respect to effective electroactive area obtained
from Cu-UPD experiment.
The linear relationship between log i and 1/T for all the plots
indicates that the reaction mechanism is not changed with tem-
perature within the temperature range studied here. Slopes of the
Arrhenius plots in Kelvin, are the measure of apparent activation
Fig. 6. Slopes of Arrhenius plots vs. at.% Ru at three different potentials, at (a) 0.45 V, (b) 0.5 V and at (c) 0.6 V for electro-oxidation of (ꢀ) ethylene glycol and (ꢀ) ethanol at
298 K.

M. Chatterjee et al. / Electrochimica Acta 54 (2009) 7299–7304
7303
be due to the difference in their adsorption behavior on the catalyst
surfaces.
Anodic peak current vs. composition curve as shown in Fig. 7
also indicates a maximum at about 15–20 at.% of Ru. This may be
considered as an additional support of the preceding conclusion.
The observed minimum at the activation energy (or maximum
at anodic peak current) vs. at.% of Ru composition profile may
be explained assuming adsorption of EtOH and EG, also requires
an ensemble of three Pt atoms with an adjacent Ru atom simi-
lar to methanol adsorption (Fig. 8). It is found from the reported
probability distribution vs. at.% of Ru curve [12], the probability of
occurrence of such an ensemble is quite high (∼84% of the maxi-
mum probability) at 15–20 at.% of Ru in the PtRu alloys.
It is well reported that methanol oxidation is most effective at
10 at.% of Ru [12] in PtRu alloy. This has been explained by a theo-
retical model based on statistical interpretation, which requires an
ensemble of three Pt atoms with an adjacent Ru atom for effective
oxidative removal of CO at low potential and probability of occur-
rence of such an ensemble is maximum at that atom composition.
From the present study, it is observed that PtRu shows maxi-
mum catalytic activity for EtOH and EG oxidations at ∼15–20 at.%
of Ru. This suggests, both of these alcohols require an ensemble
of Pt atoms and adjacent Ru atom, similar to that of methanol, for
effective adsorption and electro-oxidation.
Fig. 7. Variation of forward peak current density, ipa with at.% of Ru for electro-
oxidation of (ꢀ) ethylene glycol and (ꢀ) ethanol with 30 mV/s scan rate at 298 K.
3.3. Catalyst deactivation studies for ethylene glycol and ethanol
electro-oxidations
Fig. 8. Pictorial representation of adsorbed ethanol and the different adsorbed
residues showing the three Pt with one adjacent Ru ensemble.
The durability of the catalysts in the reaction conditions for alco-
hol oxidations is one of the major requirements in DAFCs. This can
be roughly judged by steady current densities on the amperomet-
ric i–t curves. Fig. 9 shows such curves for EG (Fig. 9a) and EtOH
(Fig. 9b) oxidations onto the Pt and PtRu alloys of different alloy
compositions at 0.5 V. It is seen that catalyst deactivation for Pt is
reduced significantly in the PtRu alloy for both the alcohols and the
rate of deactivation is slightly higher for EtOH oxidation.
This may be due to the fact that EG oxidation is relatively slow
onto PtRu surface because of negative role of Ru on dissociative
adsorption of EG products like glycolic and oxalic acids are formed
via non-poisoning route [5].
energy, higher value of the slope indicates higher activation energy
of the reaction at a given potential. Fig. 6 shows the variation of
these slopes (average values over four scan rates) with at.% of Ru.
It is interesting to note that variation of the apparent activation
energy (indicated in the slope value for log i vs. 1/T plot) with at.%
of Ru in PtRu alloy passes through a minimum at around 15–20 at.%
of Ru for both EG and EtOH oxidations. This indicates clearly that
the activation energy of the oxidation reaction of these two alco-
hols is a function of at.% of Ru. The minimum activation energy
for EG oxidation at all these potentials is lower than that of EtOH
oxidation. Again, while activation energy vs. at.% of Ru profile for
EG oxidation shows a clear minimum at these potentials, similar
curve for EtOH shows a different behavior. At the pre-minimum
composition, the variation of activation energy with at.% of Ru
is relatively small whereas for post-minimum composition sharp
inhibition of activation is observed. The different behavior of EG
and EtOH oxidation at the pre-minimum catalyst composition may
4. Conclusions
The work presents an interesting finding on activation energies
of electrochemical oxidation of EtOH and EG onto nano-Pt, -PtRu
surface. The apparent activation energies vs. at.% of Ru strongly
suggest that oxidation of EtOH and EG onto PtRu surface is most
effective with 15–20 at.% of Ru in these nanocatalysts. Admittedly,
Fig. 9. Amperometric i–t curves of (a) EG and (b) EtOH electro-oxidation onto Pt and PtRu catalysts in N2-saturated solution of 1 M EG and EtOH in 0.5 M H2SO4 at 0.5 V.

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M. Chatterjee et al. / Electrochimica Acta 54 (2009) 7299–7304
this finding is based on our study within the temperature range of
298–318 K and may not be applicable at higher temperature range.
The results are explained in terms of bifunctional mechanism
and requirement of specific geometric arrangement of Pt–Ru for
effective adsorption and oxidation. The ensemble model of oxida-
tion, which was proposed earlier for methanol oxidation, seems to
hold good for electro-oxidation of ethanol and ethylene glycol onto
PtRu surface.
Statistical analysis of probability of occurrence of such ensemble
onto Pt–Ru surface is also very high at this composition (15–20 at.%
of Ru). Thus, present finding establishes an important relationship
between experimental data and statistical analysis.
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Acknowledgement
The Authors thank the Department of Science and Technology
(DST), India for financial support in the form of a project.
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