Improved electrocatalytic stability in ethanol oxidation by microwave-assisted selective deposition of SnO2 and Pt onto carbon

Article abstract of DOI:10.1039/c3ra40427g

Pt/SnO₂/C nanostructures with SnO₂/Pt molar ratios ranging from 2.5 to 0.6 were synthesized by simple and fast microwave-assisted routes. The materials are composed of 3-5 nm SnO₂ and Pt nanoparticles dispersed on the carbon support, with the morphology of the coating depending on the SnO₂/Pt ratio: a homogenous layer of nanoparticles coating the carbon surface is obtained for SnO₂/Pt of 2.5, whereas small Pt-SnO₂ clusters are formed for lower ratios. The electrocatalytic activity of the composites on the ethanol oxidation reaction (EOR) was studied by cyclic voltammetry and chronoamperometry. All the binary catalysts exhibited lower onset potentials for the EOR and slower decay of the current density with time than a commercial Pt/C catalyst. However, improved peak current densities were only observed for the composites with ratios 1.6, 1.0 and 0.6, indicating that the formation of metal and metal oxide nanoparticles clusters is favorable for the EOR. This morphology facilitates the hydroxyl groups transfer from the metal oxide to the platinum at low potentials and also the electron transfer between carbon and platinum. The best overall performance was found for the catalyst with SnO₂/Pt = 1, on which the number of three-phase boundaries is maximized. Moreover, the catalyst with SnO₂/Pt = 1 continued to exhibit significantly better catalytic performance on the EOR than the commercial catalyst after potential cycling.

Full text of DOI:10.1039/c3ra40427g

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Improved electrocatalytic stability in ethanol oxidation
by microwave-assisted selective deposition of SnO and
2
Cite this: RSC Advances, 2013, 3,
7
001
Pt onto carbon
a
b
c
abd
Patr ´ı cia A. Russo,3 Minjeh Ahn,3 Yung-Eun Sung* and Nicola Pinna*
Pt/SnO
and fast microwave-assisted routes. The materials are composed of 3–5 nm SnO
dispersed on the carbon support, with the morphology of the coating depending on the SnO
homogenous layer of nanoparticles coating the carbon surface is obtained for SnO /Pt of 2.5, whereas
small Pt–SnO clusters are formed for lower ratios. The electrocatalytic activity of the composites on the
2
/C nanostructures with SnO
2
/Pt molar ratios ranging from 2.5 to 0.6 were synthesized by simple
and Pt nanoparticles
/Pt ratio: a
2
2
2
2
ethanol oxidation reaction (EOR) was studied by cyclic voltammetry and chronoamperometry. All the
binary catalysts exhibited lower onset potentials for the EOR and slower decay of the current density with
time than a commercial Pt/C catalyst. However, improved peak current densities were only observed for
the composites with ratios 1.6, 1.0 and 0.6, indicating that the formation of metal and metal oxide
nanoparticles clusters is favorable for the EOR. This morphology facilitates the hydroxyl groups transfer
from the metal oxide to the platinum at low potentials and also the electron transfer between carbon and
Received 25th January 2013,
Accepted 28th February 2013
platinum. The best overall performance was found for the catalyst with SnO
of three-phase boundaries is maximized. Moreover, the catalyst with SnO
2
/Pt = 1, on which the number
/Pt = 1 continued to exhibit
DOI: 10.1039/c3ra40427g
2
www.rsc.org/advances
significantly better catalytic performance on the EOR than the commercial catalyst after potential cycling.
These intermediate species strongly adsorb on the platinum
surface, poisoning the catalyst by occupying the active sites,
which leads consequently to a decrease of the catalytic
Introduction
Polymer electrolyte membrane fuel cells (PEMFCs) are
environmental friendly electrochemical conversion devices
promising for transportation and small portable electronic
applications. Ethanol is a especially attractive fuel for PEMFCs
due to its high theoretical volumetric energy density (8 kWh
4,6
activity. The use of metal oxides such as RuO
ZrO , ATO or SnO either as platinum support or co-support,
has been proposed in order to improve the performance of Pt-
2 2 2
, CeO , TiO ,
2
2
7–19
based catalysts in the oxidation of ethanol.
In particular,
2
1
Kg ), easy storage and transportation, low toxicity (compared
to methanol for instance) and easy production from biomass
the ability of tin dioxide to promote the oxidation of
intermediate CO by providing oxygen species according to
1
–3
and agricultural products such as sugar cane.
Platinum is one of the most active catalysts for the ethanol
oxidation reaction (EOR). However, the oxidation of ethanol to
CO is a difficult and complex process that requires breaking
2
of C–C bonds and may involve several different mechanism
pathways with the formation of a high number of reaction
2,6,11,14
the bi-functional mechanism is well known.
materials containing SnO
promising electrocatalytic activities on the EOR.
example, high peak current densities have been reported for
the ethanol electrooxidation catalyzed by SnO @Pt/C core–
shell structures. The authors suggested that a change of the Pt
electronic structure induced by the particular Pt–SnO
In fact,
2
in their composition have shown
2,3,15,20
For
2
2
1
,3–5
x
intermediates such as CO, CH species or acetaldehyde.
2
structure could be responsible for the activity enhancement
observed.
a
Department of Chemistry, CICECO, University of Aveiro, 3810-193, Aveiro, Portugal
World Class University (WCU) program of Chemical Convergence for Energy &
b
Benzyl alcohol has proven to be a suitable medium for the
Environment (C2E2), School of Chemical and Biological Engineering, College of
Engineering, Seoul National University (SNU), Seoul, 151-744, Korea.
E-mail: nicola.pinna@hu-berlin.de
2
1–24
synthesis of a high variety of metal oxide nanoparticles
2
5
and hybrid materials. Non-aqueous sol–gel routes to metal
oxide nanoparticles have several advantages compared for
example with the more traditional aqueous sol–gel, including
a better control over the size and morphology of the particles,
due to the lower reactivity of the metal oxide precursors in
non-hydrolytic media, and higher crystallinity of the products.
c
Center for Nanoparticle Research, Institute for Basic Science, and School of
Chemical and Biological Engineering, Seoul National University, Seoul 151-742,
Korea. E-mail: ysung@snu.ac.kr
d
Humboldt-Universit a¨ t zu Berlin, Institut f u¨ r Chemie, Brook-Taylor-Straße 2, 12489,
Berlin
3
These authors equally participated to the work
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In the particular case of the ‘‘benzyl alcohol route’’, the solvent
acts as oxygen donor for the synthesis of the metal oxide and
as ligand, allowing the control over the nanoparticle size
without the need of additional stabilizing agents. This offers
further advantages over other non-hydrolytic routes, namely
Characterization
The X-ray powder diffraction patterns were recorded on a
PANalytical X’Pert MPD diffractometer at 45 kV and 40 mA,
using Cu-Ka radiation (l = 0.1541 nm). The carbon content of
the samples was determined with a TruSpec 630 elemental
analyzer. Nitrogen adsorption isotherms at 2196 uC were
measured on a Micromeritics Gemini. Prior to the measure-
ments the samples were outgassed at 150 uC overnight. The
micropore volume and external surface area of the uncoated
2
6
the high purity of the fabricated materials. The use of
microwave heating for the synthesis of nanomaterials is
becoming increasingly popular, as it considerably decreases
the reaction times and improves yields and reproducibility.
Microwave heating also brings great advantages to the
synthesis of nanoparticles in benzyl alcohol, allowing the
s
carbon were calculated by the a method using standard data
3
2
for nitrogen adsorption on non porous carbon. Scanning
electron microscopy (SEM) images were recorded with a
Hitachi SU-70 microscope operating at 15 kV. Transmission
electron microscopy (TEM) and high resolution TEM (HRTEM)
images were obtained on a Hitachi H9000 at an accelerating
voltage of 300 kV, and a Philips CM200 FEG operated at an
accelerating voltage of 200 kV. Energy dispersive X-ray
spectroscopy (EDS) analysis was performed with a Bruker
Quantax 400. The error associated with the determination of
the platinum and tin atomic percentages by EDS was below
2
7–29
preparation of metal oxides in a few minutes.
Moreover,
our group has recently reported the one-pot microwave-
assisted synthesis of metal oxides/reduced graphene oxide
3
0
(
RGO) nanocomposites in benzyl alcohol. Tin oxide/RGO and
magnetite/RGO composites were produced in 5–10 min and a
more homogeneous coating of the RGO sheets was attained
when microwave heating was applied instead of the traditional
heating methods. Here, we use this approach to prepare binary
Pt–SnO2 electroctalysts supported on carbon black, for the
oxidation of ethanol in acidic medium at room temperature.
2
0% for all the samples investigated (ICP-AES analysis was
performed for some of the samples and the corresponding
SnO /Pt ratios were found to be in agreement with the EDS
results). High-resolution X-ray photoelectron spectroscopy
HR-XPS) measurements were performed on the soft X-ray
2 2
Pt/SnO /C composites with different SnO /Pt ratios and coat-
2
ing morphologies were produced in this manner. The activity
of the catalysts on the EOR was studied by cyclic voltammetry
and chronoamperometry and their electrochemical stability
was assessed by potential cycling.
(
beam line (8A1) connected to an undulator (U7) at the Pohang
Accelerator Laboratory (PAL). The end station was composed
of a high performance electron analyzer (SCIENTA-200) with
energy and angular resolution of 5 meV and 0.5u, respectively.
The experiment was carried out in an ultrahigh vacuum (UHV)
Experimental
2
10
chamber with a base pressure ¡5 6 10
were measured using 630 eV of incident photon energy.
Torr. All spectra
Synthesis of the composites
Carbon black Vulcan XC72 was used as support of the tin oxide
and platinum nanoparticles. The carbon (1 g) was previously
Electrochemical experiments
oxidized by refluxing in 5 M HNO
3
at 80 uC for 4 h, washed
The electrochemical measurements were performed in a three-
electrode electrochemical cell using a glassy carbon electrode
as a working electrode, platinum wire as a counter-electrode
and a saturated calomel electrode (SCE) as a reference
electrode. All electrochemical data were recorded against the
reversible hydrogen electrode (RHE). Catalyst inks were
prepared by mixing 0.01 g of the composite material with 50
mL of DI water, 600 mL of 2-propanol and 15 mL of a 5 wt%
Nafion solution (Aldrich), after which it was dropped onto the
glassy carbon electrode with a micropipette and dried. The
activity of the materials for the ethanol oxidation reaction
(EOR) was studied by cyclic voltammetry conducted at room
with water and dried. Deposition of the SnO2 and Pt
nanoparticles was performed by microwave-assisted synthesis
3
0,31
routes previously reported.
First, SnO /C composites with
2
amounts of tin oxide ranging from 10 to 50 wt% were prepared
by changing the quantities of carbon support and metal oxide
precursor used in the synthesis. Briefly, suitable quantities of
carbon and tin(IV) chloride (99.995%, Aldrich) were placed in a
microwave vial containing 20 mL of benzyl alcohol, under
argon. The mixture was sonicated to disperse the carbon
material and then heated in a CEM Discover SP microwave at
185 uC for 10 min. The solid was collected by centrifugation,
2
1
washed with tetrahydrofuran and dried at 65 uC. Platinum NPs
temperature and at a scan rate of 20 mV s , in an oxygen-free
solution of 0.1 M HClO and 0.1 M ethanol purged with argon.
The chronoamperometric measurements were performed in a
0.1 M HClO and 0.1 M ethanol solution at a constant
were then deposited onto the SnO /C materials by a micro-
4
2
wave-assisted ‘‘polyol’’ method. In all cases the amount of Pt
deposited was 20 wt%: 0.10 g of the SnO
2
/C nanocomposite
4
was added to a solution of 0.053 g of H PtCl (99.995%,
potential of 0.5 V for 1000 s. In order to assess the
electrochemical stability, accelerated degradation tests (ADT)
were performed: the composites were submitted to 1000
2
6
Aldrich) in 20 mL of ethylene glycol, under argon. A solution of
NaOH in ethylene glycol was added to raise the pH to 11–12.
The suspension was sonicated for 10 min and submitted to
microwave irradiation for 5 min at 140 uC. The final solid was
recovered by centrifugation, washed with ethanol and ultra-
pure water and dried at 65 uC.
2
1
potential cycles in 0.1 M HClO
4
at a scan rate of 100 mV s
.
For comparison, the activity of commercial Pt/C catalyst
(Johnson Matthey) with 20 wt% Pt on the EOR was also tested
in the same conditions.
7
002 | RSC Adv., 2013, 3, 7001–7008
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Table 1 Quantities of metal oxide precursor and carbon used in the synthesis, and composition and textural characteristics of the prepared Pt/SnO
2
/C catalysts
21c
a
b
2
21
3
Sample
Carbon/g
Metal oxide precursor/mmol
C wt%
SnO
2
/Pt
S
BET/m g
Vp(0.95)/cm g
Pt/SnO
Pt/SnO
Pt/SnO
Pt/SnO
2
/C-1
/C-2
/C-3
/C-4
0.30
0.40
0.48
0.52
2.00
1.14
0.80
0.39
42
56
66
69
2.5
1.6
1.0
0.6
108
134
154
142
0.16
0.18
0.24
0.24
2
2
2
a
b
c
0
Carbon weight percentage determined by CHNS elemental analysis. Molar SnO
2
/Pt ratio estimated by EDS. Pore volume obtained at p/p =
0
.95.
Fig. 2 shows the X-ray diffraction patterns of the composites
with different SnO /Pt ratios. All diffractograms exhibit peaks
of the cassiterite structure of SnO (JCPDS file no. 41-1445) and
of the face-centered cubic platinum lattice (JCPDS file no. 87-
640). Specifically, diffraction by the (111), (200) and (220)
Results and discussion
2
Catalysts characterization
2
2
To prepare the Pt/SnO /C nanocomposites, several amounts of
0
tin dioxide were first deposited on the surface of the carbon
black particles, by reaction of tin(IV) chloride in benzyl alcohol
in the presence of the carbon and under microwave irradia-
tion. This approach was recently reported by our group to
prepare tin dioxide/reduced graphene oxide nanocomposites
in short times and with high yields. It has been extended
here for the coating of a common carbon support used in
electrocatalysis (Carbon black Vulcan XC72) in order to
produce catalysts for ethanol electrooxidation. Identical
amounts of platinum (20 wt%) were then deposited on all
lattice planes of platinum are observed at approximately 39, 46
and 67u (2h), while the diffraction peaks associated with the
(110), (101) and (211) planes of the tin oxide structure are seen
at approximately 26, 34 and 52u (2h). The change in the SnO2/
3
0
Pt ratio of the samples is noticeable on the variation of the
intensity of the corresponding reflections: as the amount of tin
oxide nanoparticles decreases, the corresponding diffraction
peaks become progressively less intense. The average size of
2
the SnO /C composites.
The quantity of metal oxide precursor and carbon black
used in the synthesis, and the chemical composition and
textural characteristics of the obtained materials are registered
2
in Table 1. The carbon content and SnO /Pt molar ratio of the
composites were determined by carbon elemental analysis and
EDS (Fig. 1b), respectively, and the results obtained are in
close agreement with the nominal values. Since the platinum
2
wt% is the same in all samples, the decrease of the SnO /Pt
ratio reflects the decrease of the metal oxide content of the
materials. The proximity between the nominal and experi-
mental chemical composition of the fabricated materials,
reflects another advantage of the microwave heating: owing to
the higher microwave absorption ability of the carbon particles
compared to the solvent, the nanoparticles selectively grow on
the carbon surface, in a process promoted by the surface
functional groups. Therefore, the formation of free or weakly
attached particles is minimized, which leads to a more precise
control over the composition of the materials and higher
reproducibility. Table 1 also shows the apparent BET surface
areas and pore volumes of the samples. The uncoated carbon
2
21
support, i.e., the oxidized carbon black, has SBET of 253 m g
,
3
21
micropore volume of 0.045 cm g and surface area external
2
21
to the micropores of 146 m g . The deposition of tin oxide
and platinum NPs on the carbon surface leads to a consider-
able decrease of the surface area and pore volume, probably
caused by partial or complete blockage of the microporosity of
the support. As expected, in general the surface area and pore
volume decreases as the amount of tin oxide deposited on the
carbon surface increases, with the samples containing the
lowest amounts of metal oxide NPs having similar textural
characteristics.
Fig. 1 SEM image of Pt/SnO
Pt/SnO /C-1 (c), Pt/SnO /C-2 (d), Pt/SnO
composites.
2
/C-3 (a) and EDS spectrum (b); TEM images of the
2
2
2
/C-3 (e) and Pt/SnO /C-4 (f)
2
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2 2
Fig. 2 X-ray powder diffraction patterns of Pt/SnO /C-1 (a), Pt/SnO /C-2 (b), Pt/
SnO
2
/C-3 (c) and Pt/SnO
2
/C-4 (d) nanocomposites (X and * indicate the most
and Pt structures, respectively).
intense reflections of the SnO
2
the tin oxide nanoparticles, estimated with the Scherrer
equation from the XRD data of the Pt/SnO /C composites
and SnO /C samples prior to Pt deposition, is ca. 5, 4, 4 and 4
2
2
nm for SnO /Pt ratios of 2.5, 1.6, 1.0 and 0.6, respectively. The
2
slight decrease of the average particle size with the decrease of
the SnO /Pt ratio can be attributed to the decrease of the
2
amount of metal oxide precursor added to the synthesis in
order to produce composites with low tin oxide weight%. On
the other hand, the average size of the Pt nanoparticles
estimated from the (200) reflection is ca. 3, 4, 4 and 4 nm for
SnO
calculated average size of the Pt nanoparticles is slightly lower
for the highest SnO /Pt ratio, which can be explained by
2
/Pt ratios of 2.5, 1.6, 1.0 and 0.6, respectively. The
Fig. 3 Pt 4f (a) and Sn 3d (b) XPS spectra of Pt/SnO /C-1.
2
2
differences in the dispersion of the platinum nanoparticles on
surfaces with distinct metal oxide coverage (the dispersion is
power spectrum (PS) of the whole image in Fig. 4a, showing
reflections associated to the 111 and 200 lattice planes of
platinum and 110 of the tin oxide cassiterite structure. Fig. 4 c
better on the carbon with the highest amount of SnO
Fig. 3 shows the Pt 4f and Sn 3d XPS spectra of the Pt/SnO
C-1 sample. The binding energy of Pt 4f7/2 is 70.9 eV (Fig. 3a),
2
).
2
/
6
,8
corresponding to platinum in the zero-valent state.
The
binding energy of the Sn 3d5/2 peak (487.6 eV) indicates the
presence of tin oxide on the material (Fig. 3b). The value is
shifted to higher binding energies with respect to the binding
4
+
33
energy corresponding to the Sn oxidation state (486.4 eV).
Shifts in the binding energies can be attributed to interactions
between the various components of the sample (SnO , Pt), as
2
6
,8
reported by other authors.
The TEM images of the as-prepared nanocomposites
Fig. 1c–f) clearly show an increase in the density of the
(
particles deposited on the surface of the carbon support with
the increase of the SnO /Pt ratio. Moreover, the nanoparticles
2
homogeneously coat the entire surface of the carbon particles
in sample Pt/SnO /C-1. However, there seems to be a tendency
2
towards the formation of nanoparticles agglomerates as the
SnO /Pt ratio decreases, which consequently results in
2
portions of the carbon surface being left uncoated. A
representative HRTEM image is presented in Fig. 4. It shows
an edge of a carbon particle containing an agglomerate of
metal oxide and metal nanoparticles. Fig. 4b displays the
2
Fig. 4 High resolution TEM image of Pt/SnO /C-2 (a). Power spectra of the
whole image (b) and of the delimited regions (c, d). Indices in grey for the SnO
cassiterite and in black for the Pt fcc structure, respectively.
2
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and d are the PSs of the delimited regions in Fig. 4a and are
due to SnO
are not oriented along a particular zone axis but the 3.35 Å and
.27 Å reflections of the 110 and 111 lattice planes of the
cassiterite and fcc Pt structures, respectively, are clearly visible.
The size of the SnO and Pt nanoparticles estimated from the
2
and Pt nanoparticles, respectively. The particles
2
2
TEM images is 3–5 nm in both cases, which is in close
agreement with those calculated from the XRD data. From the
study of many HRTEM images, it was found that the Pt
nanoparticles preferentially nucleate, and are therefore depos-
2
ited, at the SnO sites. This is attributed to the fact that the
platinum–metal oxide interactions are stronger than plati-
8
,34
num–carbon interactions.
In the case of the materials with
lower SnO /Pt ratios (Pt/SnO /C-2, Pt/SnO /C-3 and Pt/SnO /C-
2
2
2
2
4), in which the quantity of tin dioxide is not enough to cover
most of the carbon surface, as occurs with Pt/SnO /C-1, the
2
higher affinity of the platinum for the metal oxide leads to the
formation of small Pt–SnO clusters, as shown in Fig. 4. As it
2
will be shown below, this particular type of morphology is
especially favorable for the ethanol oxidation reaction.
Electrocatalytic oxidation of ethanol
The electrochemical activity of the Pt/SnO /C nanocomposites
2
toward the ethanol oxidation reaction was investigated by
cyclic voltammetry and the results are presented in Fig. 5a. For
comparison, the voltammogram of a commercial Pt/C catalyst
with 20 wt% Pt is also presented. The curves show multiple
oxidation peaks, similarly to results reported in the litera-
2
,14,35
ture.
The multiple oxidation peaks are caused by
intermediate species formed after the ethanol oxidation
reaction, especially unstable carbon species that react rapidly
on metal surfaces. Intermediate species that may be produced
during ethanol oxidation include acetaldehyde, acetate or
1
,3,36
acetic acid, in addition to CH
x
fragments.
The presence of
many unstable intermediates during the electrochemical
reactions also induces current perturbations during potential
cycling, which can be minimized by controlling the scan rate,
but not fully eradicated. The onset potential for the ethanol
oxidation reaction is similar for all Pt/SnO /C catalysts (0.21 V),
2
and 0.2 V lower than that of the Pt/C. This means that tin oxide
decreases the activation energy of the reaction, so the
oxidation starts at lower potential. Moreover, the onset
potential is not influenced by the quantity of SnO
2
present
Fig. 5 a) Forward scans of the cyclic voltammograms of the Pt/SnO
and Pt/C measured at room temperature in 0.1 M HClO /0.1 M ethanol solution
with a scan rate of 20 mV s ; b) chronoamperometry curves of the Pt/SnO /C
materials and Pt/C measured at room temperature and 0.5 V in 0.1 M HClO /0.1
M ethanol solution; c) forward peak current densities (bars) and current at 0.5 V
2
/C materials
4
on the sample. Additionally, the current density of the binary
systems at low potential (0.4 V) is in all cases higher than that
of Pt/C (between 2.5 and 4.6 times higher), with the lowest
increase corresponding to the material with higher amount of
metal oxide NPs and the maximum being observed for Pt/
21
2
4
after 1000 s (circles) as a function of the amount of tin dioxide in the Pt/SnO
materials.
2
/C
2 2
SnO /C-3. With the exception of Pt/SnO /C-1, the composites
also produce higher peak currents densities than Pt/C (Fig. 5a
and c). In particular, the anodic peak current density of
ethanol oxidation on Pt/SnO /C-3 is approximately 33% higher
2
than that of Pt/C. Therefore, all three catalysts with lower
amounts of tin oxide show better performance for the
electrocatalyic oxidation of ethanol at room temperature than
the commercial catalyst, with the optimal performance being
advantage of the materials described here is the simplicity and
quickness of the procedure by which they are synthesized.
Moreover, the characteristics of microwave heating lead to a
selective growth of the nanoparticles on the carbon surface,
therefore allowing a relatively precise control over the
materials’ composition.
achieved with a SnO
2
/Pt ratio of 1. It is worth noticing that
The electrochemical active surface area (ECSA) is an
important parameter affecting the performance of the cata-
improvement of the electrocatalytic activity of platinum by tin
2
,3,14,15,20
oxide has been reported by several other authors.
The
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lysts. However, the determination of ECSA by hydrogen
desorption for materials containing metal oxides does not
lead to highly exact values due to the presence of a high
amount of functional groups on the metal oxide. Nevertheless,
considering that (i) the size of the platinum nanoparticles is
decreases for lower SnO
are in contact with the metal oxide, which limits the OH
transfer between metal oxide and metal.
2
/Pt ratios, i.e., not all Pt nanoparticles
Chronoamperometric measurements performed at 0.5 V for
1000 s are shown in Fig. 5c and are consistent with the CV data
reported in Fig. 5a. The current densities of all the binary
composite catalysts are higher than that of Pt/C during the
measurement time. The decay of the current density with time
during ethanol oxidation on Pt-based catalysts is attributed to
the poisoning of the metal active sites by reaction intermedi-
ates like CO. The slower decrease of the current density
similar in the materials with SnO
(
(
2
/Pt ratio of 1.6, 1.0 and 0.6;
ii) the amount of platinum is the same in all the samples; and
iii) the platinum was reduced after tin dioxide deposition, the
ECSA of the samples should be similar, as reported by other
1
9
authors.
As mentioned before, the oxidation of ethanol at platinum
surfaces involves a complex set of reactions and the formation
2
observed for the Pt/SnO /C materials compared to the Pt/C
x
of a high number of intermediate products, including CH ,
catalyst reflects the promoting effect of tin oxide on CO
oxidation (i.e., its anti-poisoning effect) and the consequent
higher electrochemical stability and efficiency of the former
catalysts.
The electrochemical stability of the Pt/SnO /C-3 binary
2
catalyst was further investigated by performing ADT (acceler-
ated degradation test), which consisted of potential cycling
1
,4,15
CH O or CO species, which can be further oxidized to CO .
x
2
Intermediate species such as CO adsorb strongly on the
surface of the Pt catalyst, poisoning it by making the active
sites unavailable for further reaction. The effect of tin oxide on
the electrocatalytic activity of Pt has been extensively studied
and is attributed to an increase of the CO-tolerance of the
metal catalyst by the metal oxide (i.e., to an anti-poisoning
effect of the metal oxide), through a bi-functional mechan-
between 0.5 and 1.0 V in 0.1 M HClO at a scan rate of 100 mV
4
21
s
2
. The CV and chronoamperometry curves of the Pt/SnO /C-3
1
1,37
2,15
ism
or an electronic effect,
which can also occur
catalyst before and after 1000 potential cycles are compared in
simultaneously. On the one hand, it has been reported that the
metal oxide in contact with metal particles can change the
electronic structure of the metal catalyst. These changes in the
electronic structure can lead to a more efficient oxidation of
the intermediates or to weaker Pt–CO interactions with a
2
,15,38
similar outcome.
On the other hand, the increase of the
CO-tolerance of Pt through the bi-functional mechanism arises
from the ability of tin oxide to dissociate water molecules at
lower potentials than platinum, forming (OH)ad species that
2
provide the oxygen required for the oxidation of CO to CO at
1
1,37
the metal surface and freeing the active sites.
Furthermore, it has also been suggested that saturation of
SnO sites by H O/OH weakens the interaction of water with
2
2
Pt, making the metal active sites available for ethanol
3
oxidation. The decrease of the onset potential observed for
all binary catalysts studied here and the peak current densities
observed for some of them can be partially explained by these
effects. However, the electrocatalytic performance of the
composites clearly also depends on the morphology and
quantity of the metal oxide nanoparticles present on the
sample. When the carbon surface is homogeneously and
almost entirely coated with tin dioxide and platinum
nanoparticles, low oxidation peak current densities are
2 2
observed (Pt/SnO /C-1). SnO has low electrical conductivity,
therefore, a high density of tin oxide nanoparticles dispersed
on the carbon surface generates an isolating barrier that
hinders the electron transfer between platinum and support.
The electrocatalytic performance is significantly improved
2
when small clusters of Pt and SnO nanoparticles are formed.
This morphology facilitates the hydroxyl groups transfer from
the metal oxide to the platinum at low potentials and also the
electron transfer between carbon and platinum. The optimal
2
Fig. 6 a) Forward scans of the cyclic voltammograms of the Pt/SnO /C-3 catalyst
performance is obtained for a SnO /Pt ratio of 1.0, suggesting
2
before and after ADT and of Pt/C measured at room temperature in 0.1 M
that the size and amount of the clusters also play an important
role on the electrocatalytic activity, with the number of three-
phase boundaries being maximized in this case. The number
of interfacial regions between tin dioxide and platinum
21
4
HClO /0.1 M ethanol solution with a scan rate of 20 mV s ; b) chronoam-
perometry curves of the Pt/SnO /C-3 catalyst before and after ADT and of Pt/C
2
measured at room temperature and 0.5 V in 0.1 M HClO
solution.
4
/0.1 M ethanol
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006 | RSC Adv., 2013, 3, 7001–7008
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Fig. 7 TEM images of the Pt/SnO
2
/C-3 catalyst (a) before and (b) after ADT; (c) particle size distribution before and after ADT obtained by analysis of TEM data of the
samples.
Fig. 6 with the initial curves of Pt/C. TEM images of the
material before and after ADT are also shown in Fig. 7. After
potential cycling, a slight increase in the onset potential of
ethanol oxidation (from 0.21 to 0.24 V) is observed, in addition
to a decrease of the peak current density (ca. 13% for the
anodic oxidation peak). Moreover, a faster decay of the current
density with time is also seen (Fig. 6b). Nevertheless, even after
SnO /Pt of 2.5, while for lower ratios the nanoparticles tend to
form agglomerates leaving high portion of the carbon surface
uncoated. Moreover, the Pt nanoparticles preferentially nucle-
2
ate at the SnO
contact, which results in the formation of small Pt–SnO2
clusters on the materials with lower SnO /Pt ratios. The latter
2
sites and so they are deposited in their close
2
type of morphology was found to be more favorable for the
electrocatalytic oxidation of ethanol. All the materials showed
enhanced performance at low potential on the oxidation of
ethanol at room temperature compared to a commercial Pt/C
catalyst. However, the peak current densities were found to
depend on the amount and morphology of the nanoparticles
1
000 cycles, the performance of the Pt/SnO /C-3 catalyst on the
2
EOR is quite superior to that of the commercial one (Pt/C),
indicating a significant stability of the former.
Continuous potential cycling in acidic media can lead to the
growth and agglomeration of the platinum nanoparticles,
either by redeposition of previously dissolved platinum or
migration on the support surface. These processes, in turn,
cause ECSA loss and consequently decrease the catalyst
activity. As can be seen in Fig. 7, no drastic physical changes
of the material are observed from the TEM images taken after
ADT, in agreement with the catalytic results. Moreover,
platinum particle size distributions before and after ADT
2
coating. Materials consisting of small Pt–SnO clusters showed
improved overall performances, with optimal results being
obtained for SnO /Pt = 1. This morphology facilitates the
2
hydroxyl groups transfer from the metal oxide to the platinum
at low potentials and also the electron transfer between carbon
and platinum, with the number of three-phase boundaries
2
being maximized when SnO /Pt = 1. Furthermore, this
(Fig. 7c), calculated by analysis of the TEM data, confirm this
composite continued to show significantly better catalytic
activity on the EOR after 1000 potential cycles compared with
the commercial catalyst.
observation. A median particle size of 4.2 nm was found after
ADT, against 3.9 nm obtained for the pristine material, which
may account for the loss of performance observed. It has been
shown before that metal oxides can have a stabilizing effect
over platinum nanoparticles, because the interactions of the
metal with the metal oxide are stronger than with the carbon
Acknowledgements
8
,34
surface,
which leads to lower rates of migration, agglom-
This work was financially supported by the WCU (World Class
University) program through the National Research
Foundation (NRF) of Korea funded by the Ministry of
Education, Science and Technology (R31-10013) and
Fundaç ˜a o para a Ci ˆe ncia e a Tecnologia (FCT) project PTDC/
CTM/098361/2008 and grant SFRH/BPD/79910/2011. Dr M.-G.
eration and growth of the metal particles during potential
cycling.
Conclusions
Willinger and Prof. R. Scholgl from the Fritz Haber Institute of
¨
SnO
2
/C composites with several amounts of tin oxide
the Max Planck Society are acknowledged for the use of the
CM200 FEG electron microscope and Catherine Marichy from
the University of Aveiro for the measurements. YES acknowl-
edges the support by the Research Center Program of IBS
nanoparticles were prepared by a simple and fast microwave-
assisted synthesis in benzyl alcohol. Pt NPs were subsequently
deposited by microwave-assisted ‘‘polyol’’ method. The mor-
phology of the coating was found to depend on the amount of
tin dioxide present on the material: a homogeneous coating of
the carbon surface by a layer of nanoparticles was obtained for
(Institute for Basic Science) in Korea.
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