Carbon-supported binary PtSn/C and ternary PtSnNi/C catalysts were prepared for the electro-oxidation of ethanol. The carbon-supported nanoparticles were synthesised by employing a modified polyol methodology and characterised in terms of structure, morphology and composition by using XRD, EDX and TEM techniques. Their electrocatalytic behaviour for ethanol oxidation (EO) was investigated by employing a disccomposite electrode covered by a thin layer of catalyst imbedded in a Nafion polymer electrolyte film. An enhancement of the EO through a negative shift of the onset oxidation potential and higher current density at constant potentials were observed on the PtSnNi/C electrodes as compared to those for PtSn/C electrodes. The temperature was changed in the range 40 °C ≤ t ≤ 80 °C, and the variation of the activation energy calculated by linear regression of the Arrhenius plots.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Bonesi, Alejandro Roberto

Castro Luna Berenguer, Ana María del Carmen

Triaca, Walter Enrique

English

SEDICI - Repositorio de la UNLP

Portugaliae Electrochimica Acta 2009, 27(3), 193-201
DOI: 10.4152/pea.200903193
PORTUGALIAE 
ELECTROCHIMICA 
ACTA
ISSN 1647-1571
Nanocatalysts for Ethanol Oxidation.
Synthesis and Characterisation
A. Bonesi, W.E. Triaca, AM. Castro Luna *
INIFTA, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Suc. 4, CC 161900,
La Plata, Argentina
Received 25 April 2008; Accepted 10 December 2008
Abstract
Carbon-supported binary PtSn/C and ternary PtSnNi/C catalysts were prepared for the 
electro-oxidation of ethanol. The carbon-supported nanoparticles were synthesised by 
employing a modified polyol methodology and characterised in terms of structure, 
morphology and composition by using XRD, EDX and TEM techniques. Their electro­
catalytic behaviour for ethanol oxidation (EO) was investigated by employing a disc­
composite electrode covered by a thin layer of catalyst imbedded in a Nafion polymer 
electrolyte film. An enhancement of the EO through a negative shift of the onset 
oxidation potential and higher current density at constant potentials were observed on 
the PtSnNi/C electrodes as compared to those for PtSn/C electrodes. The temperature 
was changed in the range 40 °C < t < 80 °C, and the variation of the activation energy 
calculated by linear regression of the Arrhenius plots.
Keywords: tin, nickel, platinum, ethanol, polyol, catalyst.
Introduction
Fuel cells are attractive devices to easily convert chemical energy into electricity. 
Especially those that employ PEM technology and work at low temperatures are 
potential tools for portable systems.
Many studies have been conducted using methanol as anode fuel in the DMFC 
[1], Unfortunately, there are serious limitations to its utilisation, such as its slow 
oxidation kinetics, the catalyst poisoning by strongly adsorbed intermediates and 
the methanol crossover, that disturb the cathodic reaction [2, 3], Moreover, due 
to its toxicity, methanol is an environmentally unfriendly compound. In the last
Corresponding author. E-mail address: castrolu@inifta.unlp.edu.ar 
A. Bonesi et al. /Port. Electrochim. Acta 27 (2009) 193-201
years, ethanol from biomass fermentation has been proposed as an alternative 
ecological fuel.
The ethanol oxidation mechanism has been deeply studied on Pt [4-6], and it has 
been established that only a small amount of adsorbed ethanol goes to CO2, 
whereas a great proportion of ethanol goes to acetaldehyde and acetic acid, 
highlighting that the C-C bond is difficult to break [5,6]. Experimental 
techniques such as differential electrochemical mass and in situ infrared 
reflectance spectroscopy have identified the surface intermediates on Pt as 
CH3COH, CH3CO, Cl l\ and CO [6-8].
The aim of the DEFC researchers is to synthesise a suitable catalyst able to get 
the maximum conversion efficiency of ethanol into CO2. Recently, a theoretical 
study on the influence of the modification of the Pt surface structure on EO has 
been reported taking into account that EO is a structure-sensitive reaction. It has 
been found that the open surface Pt(lOO) is the best facet to fully oxidise ethanol 
at low coverage [9].
It appears that an improvement in electro-catalytic ethanol oxidation (EO) is 
possible with multifunctional catalysts. Binary or ternary Pt-based alloys are 
proposed because each component makes a contribution by: i) providing an 
adequate Pt-Pt distance to favour adsorption and breaking of the ethanol 
molecule, ii) providing oxygenated species to facilitate the CO-like adsorbate 
oxidation, or iii) modifying the electronic structure of Pt to weaken the Pt­
adsorbate bond [10].
The addition of Sn to Pt enhances the activity for EO, particularly at lower 
potentials, as compared to that obtained with pure Pt/C anode catalysts. Through 
DEMS, it has been established that EO at lower potential goes to CO2 and 
acetaldehyde, whereas at higher potentials the production of CH3COOH 
increases [7]. Lamy et al. studied the electro-oxidation of ethanol on PtSn 
catalysts with Pt:Sn atomic ratios varying from 90:10 to 50:50 at room 
temperature. A great enhancement of the electro-catalytic activity was observed 
for non-alloyed PtSn with 10 atom percent (at%) Sn [11], Jiang et al. compared 
the catalytic activity of a partially alloyed Pt-Sn catalyst with that of a quasi-non­
alloyed Pt-SnO2 catalyst. The PtSnO2 catalyst showed higher catalytic activity 
for ethanol electro-oxidation than the PtSn alloy [12], Colmati et al. found that 
thermally treated (200 °C) PtSn catalysts have a better performance for EO 
because the thermal treatment gave rise to the formation of a predominant phase 
of cubic Pt3Sn and to a lesser extent, a hexagonal PtSn phase, the latter showing a 
poor activity for ethanol oxidation[13].
From a practical point of view, the fuel cell catalysts have to fulfil the 
requirements of being nanosized particles with uniform composition, high alloy 
content and high dispersion on the selected support, i.e., Vulcan XC-72R. At 
present, there is a wide interest in new approaches to obtain nanoparticles of 
supported catalysts to be used for EO. The methodology usually includes a 
chemical step to form oxidised colloidal particles, followed by another step to 
reduce and deposit the catalysts on the carbon particles [12, 14-15],
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In this work, new binary and ternary nanoparticle catalysts, synthesised by a 
modified polyol methodology, were physically and electrochemically 
characterised for EO.
Experimental
Supported combined catalysts such as PtSn/C and PtSnNi/C were synthesised by 
employing the polyvynilpyrrolidone (PVP), ethylene glycol (EG) and water 
methodology. [14], The sequential steps involved in the preparation were: i) a 
predetermined amount of Vulcan XC-72R was added to the calculated volume of 
an ethylene glycol, water and PVP solution, the whole mixture was stirred for 5 
min; ii) the required precursors, i.e., PtCl6H2, SnCl2 and NiCl2, in the required 
stoichiometric amount, were introduced into the composite mixture giving a 
colloidal combination of metal oxide-hydroxide, mixed for 15 min to obtain a 
well dispersed intermediate colloidal solution; iii) the temperature was increased 
up to 140 °C and kept constant for 1 h to allow the nucleation and growth of the 
metals on the Vulcan support, next the suspension was cooled up and filtered by 
suction, the solid obtained was washed repeatedly with distilled water, and the 
product dried at 60 °C for 24 h in a vacuum stove.
All catalysts were characterised according to their particle size, morphology, and 
composition and dispersion degree by powder X-ray diffraction (XRD), scanning 
electron microscopy (SEM) and energy dispersive X-ray analysis (EDX).
To study the activity of the catalysts for EO, a glassy carbon disc (0.071 cm2 
geometric area) covered by a thin layer of catalyst with a metal loading of 28 
pg/crn2 imbedded in a Nafion polymer electrolyte film of 0.1 pm was employed. 
The catalysts were electrochemically characterised employing electrochemical 
techniques such as cyclic voltammetry, and chronoamperometry. A Pt foil of 1 
cm2 geometric area was used as counter electrode and a saturated calomel 
electrode as reference electrode. In this work, the potentials are referred to that of 
the reversible hydrogen electrode (RHE). The working electrolyte was 1 M 
C2H5OH + 0.5 M H2SO4. The electrochemical experiments were conducted at 
temperatures varying between 40 °C and 80 °C. Before the EO runs, the disc­
composite electrode was cycled at 0.10 V s’1 between 0.05 and 0.65 V in a N2- 
purged sulphuric electrolyte solution for 5 min to clean its surface. Real areas 
were determined by considering the anodic charge involved in the CO-stripping 
peak by assuming that CO is linearly adsorbed on one Pt site and taking into 
account that 420 pC are equivalent to 1 cm2. The CO-stripping voltammogram 
was run at 0.01 Vs-1, after CO adsorption at 0.05 V from a CO-saturated 0.5 M 
H2SO4 solution for 10 min, and subsequent bubbling of N2 for 10 min. Then, 
ethanol was added at 0.05 V and the current-potential curves for EO were 
recorded at 0.01 Vs’1. Chronoamperometric measurements of EO at 0.5 V were 
also recorded. The current densities are referred to the CO-calculated real area.
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Results and discussion
Physical characterisation
For both catalysts, the XRD patterns of synthesised PtSn/C and PtSnNi/C, Fig. 1, 
show the characteristic peaks of the fee crystalline Pt. The diffraction peaks at ca. 
40°, 45°, 70° and 80° are due to diffraction at the (111), (200), (220) and (311) Pt 
planes, respectively. The diffraction peaks for PtSn are slightly shifted to lower 2 
0 values with respect to the corresponding peaks for the Pt catalyst [16]. The shift 
of the peaks to lower angles reveals the alloy formation between Pt and Sn, 
which is caused by the incorporation of Sn in the fee structure of Pt. For PtSnNi, 
as observed in other ternary catalysts, the presence of a third non-alloyed metal 
can cause a decrease of the degree of alloying of the binary base alloy with 
respect to that of the binary catalyst [17].
Figure 1. XRD patterns of PtSn/C and PtSnNi/C catalysts.
There is also a small diffraction peak at around 52° attributed to SnO2 (211), as 
reported by Jiang et al. [18]. The presence of a small amount of SnO2 in the 
vicinity of Pt nanoparticles can supply oxygen-containing species for the 
oxidation of adsorbed species from ethanol adsorption on neighbouring Pt sites, 
enhancing their catalytic activity for EO at low potentials.
Table 1. EDX results for the synthesized catalysts.
catalyst at% Pt at% Sn at% Ni
Pt-Sn/C 57 43
PtSnNi/C 65 29 6
The average crystallite size of the synthesised nanoparticles was estimated from 
Scherrer’s equation and the (220) diffraction Pt peak, resulting to be about 4 nm 
for PtSn and 3 nm for PtSnNi.
The EDX metal composition in atom percent (at%) for PtSn/C and PtSnNi/C 
catalysts is shown in Table 1. It can be observed that the ternary catalyst has a 
lower amount of Sn.
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1.4
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
BV(RHE)
Figure 2. a) Cyclic voltammogram of EO in 1 M C2H5OH + 0.5 M H2SO4 at v = 0.01 
Vs“1 t = 60 °C. (b) Chronoamperometrie curves of EO in 1 M C2H5OH + 0.5 M H2SO4 
on PtSn/C and PtSnNi/C electrodes at E = 0.5 V t = 60 °C.
Electrochemical characterisation
The electrochemical equipment consisted of a bipotentiostat (Taccussel, type Bi­
Pad), a waveform generator (PARC 173) and an XYt recorder (Houston 2000).
Cyclic voltammograms for EO in 1 M C2H5OH + 0.5 M H2SO4 at 60 °C, 
employing binary PtSn/C and ternary PtSnNi/C catalysts, are shown in Fig 2a. 
The PtSnNi/C catalyst lowers the onset potential of ethanol oxidation and 
increases the current densities at potentials higher than 0.4 V, as compared to the 
PtSn/C catalyst. The transient currents for EO at 0.5 V are shown in Fig 2b. For 
both current-time curves an initial current drop in the first 5 min is observed, 
followed by a slower decay, but the current values obtained for PtSnNi/C were 
always higher than those obtained for PtSn/C.
It has been well established that in binary Pt catalysts, the addition of Sn lowers 
the onset potential for EO and leads to a slight increase in the activity of the 
PtSn/C catalyst [7]. However, there is not improvement in the selectivity for 
complete oxidation to CO2, and ethanol oxidation to acetaldehyde and acetic acid 
prevails.
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Effect of temperature for the EO on PtSn/C and PtSnNi/C
The effect of different temperatures on the EO was followed by cyclic 
voltammetry varying the temperature in the 40-80 °C temperature range. The 
voltammograms for EO on PtSn/C and PtSnNi/C electrodes are shown in Figs. 3a 
and 3b, respectively. As can be observed the current density increases as the 
temperature is increased. At potentials higher than 0.4 V, for PtSn the current 
density increases less than that of the Ni-containing catalysts when the potential 
is increased, so at high potentials better performances are obtained with the 
ternary PtSnNi catalysts.
For the PtSnNi/C electrode, it can also be observed that the EO onset potentials 
are lower than those for PtSn/C catalysts, and at constant potential the current 
densities are higher in the whole temperature range.
The apparent activation energies for EO on PtSn/C and PtSnNi/C catalysts were 
obtained at different potentials, Fig. 4, by linear regression of the Arrhenius 
plots, which are the insets of the figure. On both catalysts the apparent activation 
energy increases with increasing potentials; this fact reveals that the rate-limiting 
step is one related to ethanol adsorption [19]. Furthermore, lower activation 
energies are obtained with PtSnNi/C.
It is important to bear in mind that a good electrocatalyst towards the complete 
oxidation of ethanol to CO2 must facilitate the C-C bond breaking, because EO 
proceeds via: i) the formation of adsorbed CO and CHx species, leading to 
electrode poisoning, and ii) the formation of an adsorbed species with two 
carbons, i.e., acetyl species, that can be dissociated on the electrode surface and 
converted in carbon dioxide after reaction with oxygenated species, or desorbed 
and transferred to the bulk as acetaldehyde and/or acetic acid [7].
In this respect, it must be noted that the adsorption and dehydrogenation of 
ethanol need Pt surface ensembles, which are less abundant at the alloy surfaces. 
Therefore, the fact that PtSnNi/C shows a better performance for EO in acidic 
media than the PtSn/C catalyst can be attributed to a partial Ni dissolution, which 
could change the morphology of the catalyst clusters, increasing the presence of 
Pt ensembles for ethanol dehydrogenation and C-C bond splitting, while Ni 
oxide and Sn oxide species provide the oxygenated species for the subsequent 
oxidation.
Another possible explanation for the better catalytic behaviour of PtSnNi/C 
arises after considering that, in agreement with the electro-negativity series, an 
electron transfer from Ni and Sn to Pt can be possible. In consequence, it could 
be inferred that these electron transfers can also contribute to weakening the 
bond of PtCO-like species, assisting the conversion to CO2 [20]. Furthermore, it 
must be noticed that for PtSnNi/C the existence of a surface layer containing 
Ni(OH)2 and/or NiOOH can be possible. Such a Ni-hydroxide layer on the 
PtSnNi catalyst may display high activity with respect to ethanol electro­
oxidation due to oxygen-containing species formed on the catalyst [21-23],
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A. Bonesi et al. /Port. Electrochim. Acta 27 (2009) 193-201
E'V(RHE)
E/V(RHE)
Figure 3. Cyclic voltammograms for EO 1 M C2H5OH + 0.5 M H2SO4 at different 
temperatures on (a) PtSnNi/C and (b) PtSn/C.
Figure 4. Arrhenius plots for EO on PtSn/C and PtSnNi/C at various potentials.
EV(RHE)
-0- PtSnNi/C
-
PtSn C B
□
■
□
■
- □
- □
Conclusions
PtSn/C and PtSnNi/C catalysts synthesised by the polyol methodology were 
employed to determine their catalytic activity for ethanol electro-oxidation. The 
results indicate that the PtSnNi/C catalyst performs better than the binary PtSn/C 
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A. Bonesi et al. /Port. Electrochim. Acta 27 (2009) 193-201
catalyst due to the promoting effect of Ni. Ni dissolution could change the 
morphology and accessibility of the catalyst clusters by increasing the number of 
Pt ensembles for ethanol dehydrogenation and C-C bond splitting. Moreover, it 
is possible that Ni oxide species such as NiO, Ni(OH) 2 and NiOOH, contribute 
to the oxidative removal of CO and CH3CO species adsorbed on adjacent Pt 
active sites, thus enhancing the activity for the electro-oxidation of ethanol at low 
potentials.
Summing up, the improved performance for EO obtained with PtSnNi/C 
catalysts can be attributed to a cooperative effect between the numerous active Pt 
ensembles for ethanol dehydrogenation and C-C bond rupture, and the larger OH 
availability.
Acknowledgements
The authors are grateful to Comisión de Investigaciones de la Provincia de Bs As (CIC) 
project C 2006-2007 and ANPCyT project PICT 10-13572 and CONICET for financial 
support. AMCL is member of the research career at CIC.
References
1. A.S. Arico, S. Srinivasan, V. Antonucci, Fuel Cells 1 (2001) 133.
2. A.K. Shukla, R.K. Raman, Annual Review of Materials Research 33 (2004) 
155.
3. A. Stassi, C.D’Urso, A. DiBlassi, V. Baglio, V. Antonucci, A.S. Arico, 
A.M. Castro Luna, A. Bonesi, W.E. Triaca, J. Appl. Electrochem. 36 ( 
2006)1143.
4. T. Iwasita, E. Pastor, Electrochim. Acta 39 (1994) 531.
5. C. Lamy, E.M. Belgsir, J.M. Leger, J. Appl. Electrochem. 31 (2001) 799.
6. G.A. Camara, T. Iwasita, J. Electroanal. Chem. 578 (2005) 315.
7. H. Wang, Z. Jusys, R.J. Behm, J. Power Sources 154 (2006) 351.
8. J.P.I. de Souza, S.L. Queiroz, K. Bergamaski, E.R. Gonzalez, F.C. Nart, J.
Phys. Chem. B 106 (2002) 9825.
9. Hui-Fang Wang, Zhi-Pan Liu, J. Am. Chem. Soc.
10. E. Antolini, J. Power Sources 2007 (170) 1.
11. C. Lamy, S. Rousseau, E.M. Belgsir, C. Coutanceau, J.-M. Léger, 
Electrochim. Acta 49 (2004) 3901.
12. L Jiang, G. Sun, S. Sun, J. Liu, S. Tang, H. Li, B. Zhou, Q. Xin, 
Electrochim. Acta 50 (2005) 5384.
13. F. Colmati, E. Antolini, E.R. Gonzalez, Appl. Catalysis B: Environmental 
73 (2007) 106.
14. M. Chen, Y. Xing, Langmuir 21 (2005) 9334.
15. A. Bonesi, G. Gara ven ta, W.E. Triaca, A.M. Castro Luna, Int. J. Hydrogen 
Energy 33 (2008) 3499.
16. A. Bonesi, G. Gara venta, W.E. Triaca, A.M. Castro Luna, Actas Hyfusen 
(2007) paper 13-03.
17. F. Colmati, E. Antolini, E.R. Gonzalez, J. Alloys and Compounds 456 
(2008) 264.
200
A. Bonesi et al. /Port. Electrochim. Acta 27 (2009) 193-201
18. L. Jiang, L. Colmenares, Z. Jusys, G.Q. Sun, R.J. Behm, Electrochim. Acta 
53 (2007) 377.
19. F. Ficiouglu, F. Kadirgan, J. Electroanal. Chem. 430 (1997) 179.
20. J.-H. Choi, K.-W. Park, B.-K. Kwon, Y.-E. Sung, J. Electrochem. Soc. 150 
(2003) A973.
21. Z-B. Wang, G-P. Yin, J. Zhang, Y-Ch. Sun, P-F. Shi, J. Power Sources 160 
(2006)37.
22. E.V. Spinace, M. Linardi, A.O. Neto, Electrochem. Commun. 7 (2005) 365.
23. E. Ribadeneira, B.A. Hoyos, J. Power Sources 180 (2008) 238.
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