Porous platinum mesoflowers with enhanced activity for methanol oxidation reaction

Article abstract of DOI:10.1016/j.jssc.2012.03.043

Porous Pt and Pt-Ag alloy mesoflowers (MFs) with about 2 μm in diameter and high porosity were synthesized using Ag mesoflowers as sacrificial template by galvanic reaction. The silver content in Pt-Ag alloys can be facilely controlled by nitric acid treatment. And the pure Pt MFs can be obtained by selective removal of silver element from Pt₇₂Ag₂₈ MFs electrochemically. Both Pt₄₅Ag₅₅, Pt₇₂Ag ₂₈ and pure Pt show a high catalytic performance in methanol oxidation reaction (MOR). Especially, pure Pt MFs exhibited a 2 to 3 times current density enhancement in MOR compared with the commercial used Pt black, which can be attributed to their porous nanostructure with 3-dimentional nature and small crystal sizes.

Full text of DOI:10.1016/j.jssc.2012.03.043

Journal of Solid State Chemistry 191 (2012) 239–245
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Porous platinum mesoflowers with enhanced activity for methanol
oxidation reaction
Lina Zhuang, Wenjin Wang, Feng Hong, Shengchun Yang n, Hongjun You n, Jixiang Fang, Bingjun Ding
School of Science, MOE Key Laboratory for Non-equilibrium Synthesis and Modulation of Condensed Matter, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Porous Pt and Pt–Ag alloy mesoflowers (MFs) with about 2 mm in diameter and high porosity were
Received 19 January 2012
Received in revised form
5 March 2012
synthesized using Ag mesoflowers as sacrificial template by galvanic reaction. The silver content in Pt–
Ag alloys can be facilely controlled by nitric acid treatment. And the pure Pt MFs can be obtained by
selective removal of silver element from Pt72Ag28 MFs electrochemically. Both Pt45Ag55, Pt72Ag28 and
pure Pt show a high catalytic performance in methanol oxidation reaction (MOR). Especially, pure Pt
MFs exhibited a 2 to 3 times current density enhancement in MOR compared with the commercial used
Pt black, which can be attributed to their porous nanostructure with 3-dimentional nature and small
crystal sizes.
Accepted 19 March 2012
Available online 27 March 2012
Keywords:
Noble metal
Platinum mesoflower
Methanol oxidation reaction
Porous nanostructure
& 2012 Elsevier Inc. All rights reserved.
1. Introduction
techniques for nanomaterials with multi-layer and porous nanos-
tructure constitute one of major challenges in research today.
Direct methanol fuel cells (DMFCs) are promising power
sources for portable electronics because of their attractive advan-
tages such as high energy density, low operation temperature,
and low pollution for environment [1]. Pt and Pt-based catalysts
are known to display the excellent catalytic behavior in DMFCs.
However, the expensive nature of platinum is a critical problem
and has limited its technological viability. Therefore, it is impor-
tant that platinum should be used efficiently, with every metal
atom functioning during a catalytic process. As a result, Pt
nanostructures with high porosity have become promising mate-
rials for DMFCs [2].
Recently, various intriguing Pt and Pt-based nanoparticles or
nanostructures with different morphologies and high porosity
have been successfully synthesized, such as flowerlike, concave,
porous, nanodendrites, nanowire network and urchinlike struc-
ture [3–7]. Among various nanostructures, the multi-layer and
porous mesostructures have attracted intensive research interest
because they have high surface area and supply enough adsorp-
tion sites for all involved molecules in a narrow space [8–11],
leading to the excellent electrocatalytic activity toward methanol
oxidation [12]. Therefore, the investigations into the preparation
Recently, biological molecules have been exploited to synthe-
size nanoparticles and nanofibers, because they could regulate the
unique shapes and the exact sizes of various nanocrystals [13].
Bovine serum albumin (BSA) is one of the most studied proteins,
and it has a strong affinity to a variety of inorganic molecules
binding to different sites, which makes possible utilization of
BSA-decorated nanomaterials in a variety of supramolecular
assemblies [14]. Good coverage of silver and gold nanoparticles
had been obtained by Ajay and coworkers using BSA as a template
because of its zwitterionic character at the protein isoelectric
point, which was used to bind to either cationic silver (Agþ) or
anionic gold (AuCl4ꢀ) ions in the foam [15]. These examples
indicate that BSA could probably be a promising template for
shape-controlled synthesis of noble metal nanoparticles and lead
to a special arrangement of noble metal atoms on the nanopar-
ticle surface, making them
a better candidate for catalytic
applications. However, to our best knowledge, no reports were
found to prepare the flower like Ag nanostructures with BAS as
soft template.
In this work, we synthesized the Ag mesoflowers (MFs)
composed of multi-layer Ag nanosheets with a thickness about
10 nm, using BSA as capping agent and ascorbic acid as reductant.
And these 3-dimensional Ag nanostructures were further used as
sacrificial template for preparation of Pt nanostructures. In the
previous research, most of the products after the galvanic reaction
with Ag NPs as template were hollow structures, and the excessive
amount of Pt precursor could destroy the final structure of the
samples [16,17]. However, in current case, the morphologies of
n Correspondence to: School of Science, MOE Key Laboratory for Non-equili-
brium Synthesis and Modulation of Condensed Matter, State Key Laboratory for
Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, People’s
Republic of China. Fax: þ86 29 82665995.
E-mail addresses: ysch1209@mail.xjtu.edu.cn (S. Yang),
hjyou@mail.xjtu.edu.cn (H. You).
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jssc.2012.03.043

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L. Zhuang et al. / Journal of Solid State Chemistry 191 (2012) 239–245
template can be well inherited by the final product, and a novel
flower-like Pt and PtAg alloy mesoparticles with porous structure
can be prepared. The measurement in electrochemical catalytic
performance shows that these prepared Pt MFs exhibit a predo-
minant enhancement in electrocatalytic activity toward electro-
chemical oxidation of methanol, which can be potentially used to
design eletrodes for DMFCs.
solution (diluted from 5 wt% Nafion, Ion Power Inc.) was pipeted
on the electrode surface in order to attach the catalyst particles
onto the glassy carbon and dried in a stream of air, yielding a
Nafions film thickness of ca. 0.1
mm. The electrochemical mea-
surement used a CHI 760 dual channel electrochemical working
station (CH instrument, Inc) with a three-electrode system, which
consists of a glass carbon electrode with the sample on it, a Pt
network counter electrode and a double junction Ag/AgCl refer-
ence electrode (Pine, Phychemi Company Limited, 3 M KCl) which
have a salt bridge (aqueous KNO3 solution with a concentration of
10%) between electrode and the solution. The temperature during
the measurement was kept at about 25 1C. A 0.1 M perchloric acid
(HClO4) aqueous solution was used as the supporting electrolyte
in the removal of Ag. The potential for removing silver was
monitored based on cyclic voltammetry (CV). Before each experi-
ment, the solution was purged with N2 for 30 min to remove
dissolved oxygen gas. The potential was cycled between ꢀ0.2 and
2. Experimental details
2.1. Synthesis of Ag mesoflowers (MFs)
Ag MFs were synthesized by reducing AgNO3 (Aldrich, 99þ %)
with ascorbic acid in the presence of bovine serum albumin (BSA)
working as the template. In a typical synthesis, 0.2 g BSA and
0.169 g AgNO3 were added to 50 mL of deionized water in a
100 mL beaker. The mixture was stirred at room temperature for
30 min until the aqueous solution was transparent. After that, the
solution was put in the water-bath with the temperature of 80 1C
for 10 min. Then 0.17 g ascorbic acid was added into the solution
and the reaction mixture turned light green instantaneously. The
mixture was kept at 80 1C for another 10 min and the stirring was
maintained through the whole process. The obtained product was
collected by centrifugation and washed with water and ethanol to
remove most of the BSA. During the washing process, the
suspension was centrifuged at 6000 rpm for either 10 or 20 min
(depending on whether ethanol or water was used) to remove the
excess BSA. Finally, the precipitate was re-dispersed in the
deionized water for further use.
1.1 V for 20 times at a scanning rate of 50 mV sꢀ1
.
2.4. Characterization
The morphologies of the Ag, Pt–Ag alloy and pure Pt MFs were
characterized using Scanning electron microscopy (SEM) and
transmission electron microscopy (TEM). SEM was conducted on
a JSM-7000F at 10 KV. TEM samples were prepared by drop
casting a dispersion of the MFs onto carbon-coated copper grids.
TEM and high-resolution TEM images were acquired using a JEM-
2100. X-ray diffraction (XRD) patterns were obtained on a Bruker
D8 Advance Diffractometer (Bruker AXS) using a Cu Ka radiation.
It’s noteworthy that the TEM and SEM sample of pure Pt MFs were
prepared by separating the sample from the electrode through
ultrasonic process. Typically, after silver dissolution, the electrode
was slightly dipped into a glass vial containing 0.5 mL ethanol,
while with the ultrasonic treatment, the sample could be sepa-
rated from the electrode and dispersed in ethanol. The solution
was further condensed by evaporating the ethanol in air. And
then the dispersion of the MFs was drop casting onto TEM and
SEM substrates followed by the UV irradiation for 30 min to
remove the Nafion film. The energy dispersive X-ray (EDX)
analysis, which was obtained with an Oxford INCA detector
installed on the JEOL JSM-7000F instrument, was used to deter-
mine the compositions of the samples.
2.2. Galvanic replacement reaction
For the synthesis of Pt MFs, chloroplatinic acid was utilized in
the galvanic displacement of the Ag MFs. Cleaned Ag MFs (10 mg)
were dispersed in water (50 mL) and refluxed under argon in a
three-neck round bottom flask (100 mL) equipped with conden-
ser, addition funnel, thermocouple, and stir bar. Upon reaching
reflux temperature at 80 1C, chloroplatinic acid (10 mL, 5.0 mM)
was added dropwise through the addition funnel over a period of
20 min. The resulting mixture was maintained at the reflux
temperature for 20 min until its color became stable and subse-
quently quenched in an ice bath. After quenching, the product
was washed in a saturated sodium chloride (Fisher Scientific)
solution to remove the silver chloride precipitate, followed by
washing in water and ethanol. Acid treatment was performed in
2 M HNO3 solution for 60 h and 120 h with stirring, leading to the
products of Pt45Ag55 and Pt72Ag28, respectively.
2.5. Electrochemical measurements
All the electrocatalytic samples were prepared in the same
way as the Pt72Ag28 sample, which was prepared for removing the
residual Ag. Electrochemical properties were measured on the
same electrochemical workstation described above. The total
2.3. Preparation of pure Pt MFs
amount of the metals used in each test was fixed at 5 mg. The
commercial used Pt black (JM) was used for comparison. Hydro-
gen adsorption-desorption cyclic voltammograms (CVs) were
recorded in a nitrogen-protected 0.1 M perchloric acid aqueous
solution with a scanning rate of 50 mV sꢀ1. The solution was
purged by N2 for 30 min to deplete dissolved O2. The region for
hydrogen adsorption (ꢀ0.144–0.206 V on the backward potential
scan) was used to estimate the electrochemical active surface
areas (EASAs). For methanol oxidation reaction, an air-free aqu-
eous solution containing 0.5 M methanol and 0.1 M perchloric
acid was used.
Pure Pt MFs were obtained after the selective removal of Ag
from Pt72Ag28 MFs electrochemically because Ag has a much
lower standard electrode potential than Pt. In a typical procedure,
one milligram of nitric acid-treated Pt72Ag28 MFs were dispersed
in
a mixture of deionized water and isopropanol (Vwater/
V2ꢀisopropanol¼4/1) at a concentration of 1.0 mg metal catalyst/
mL solvent, followed by sonication for 10 min. The final weight
percent of metallic catalysts was determined by thermo-gravi-
metric analysis (TGA) using an STA449C simultaneous TGA/DSC
system from NETZSCH Instruments at
a
ramping rate of
The current densities in cyclic voltammograms and chron-
oamperograms are evaluated with respect to the electrochemi-
cally active Pt area estimated from the hydrogen underpotential
deposition charge, assuming that oxidation of a monolayer of
HUPD consumes 210
conducted at room temperature (25 1C).
10 1C minꢀ1 under an air flow of 50 mL minꢀ1. The mixture was
deposited on a glass carbon electrode (3 mm in diameter) and
dried under a stream of air. The amount of the metal used in this
experiment was 5
mg unless stated otherwise. After evaporation
m
C cmꢀ2 charge. All of the experiments were
of the water and isopropanol in air, 10
mL of a 0.025 wt% Nafion

L. Zhuang et al. / Journal of Solid State Chemistry 191 (2012) 239–245
241
3. Results and discussions
Pt(0)þAg (0)-PtAg (alloy)
(2)
Fig. 1(a) and (b) illustrates a typical morphology of the as-
prepared Ag MFs using BSA as template, which is characterized by
a multi-layer structure with the average diameter of about 2 mm.
In current case, Pt–Ag alloy and pure Pt mesoflowers were well
prepared by using the as-prepared Ag MFs as template. As one can
see from the SEM images in Fig. 2(a) and (d), the resulting
products was still keep a morphology similar with that of Ag
MFs. As we have described in experimental part, Pt–Ag alloy MFs
with different atomic ratios can be syntheiszed by acid treatment
of the products for 60 h and 120 h after galvanic reaction, which
can be detected by EDX analysis. As shown in Fig. 2(b) and (e), the
EDX results shows that the acid treatment performed in 2 M
HNO3 solution for 60 h and 120 h leading to the products with
atomic ratios between Pt and Ag were 1:1.2 and 26:1, from which
the samples can be defined as Pt45Ag55 and Pt72Ag28, respectively.
The representative TEM images (Fig. 2(c) and (g)) of single
Pt45Ag55 and Pt72Ag28 alloy MFs show a similar profile with that
of Ag MFs in which the petals stretch out from the center.
However, compared with the silver MFs, the alloy MFs shows a
broadening of SAED spots, especially for that of the high-index
crystal planes, suggest a common orientation with a small angle
lattice mismatch between the primary NP building units. The
HRTEM images shown in Fig. 2(d) and (h) recorded from the
cycled area in Fig. 2(c) and (g) further support this suggestion. As
typically indicated by the black lines in Fig. 2(h), the lattice
orientation of two attached grains has a misorientation less than
51. Meanwhile, the HRTEM studies reveal that the petals of alloy
MFs are porous structure and composed of nanoparticles with
diameter less than 5 nm, as labeled by the arrows and dashed
lines in Fig. 2(d).
It can be clearly found that the petals on the Ag MFs surface
stretching to different directions which have a thickness about
10 nm (Fig. 1(b)). The EDX result shown in the insert of Fig. 1a
indicates that the products are exclusively composed of silver.
TEM image (Fig. 1(c)) provides more details for this structure, and
the clear contrast between the dark center and outer bright
regions further indicates that the petals radially grow from the
center of the particle. The insert in Fig. 1(c) is the corresponding
selected-area electron diffraction (SAED) patterns obtained from
one of the petals, revealing they are single-crystalline and can be
indexed to face-centered cubic (fcc) Ag. The HRTEM image in
Fig. 1(d) provides further insight into its structure. It is observed
that the individual petals exhibit good crystalline and highly
ordered, continuous fringe patterns. The measured interplane
spacing for the lattice fringe is 0.24 nm, which corresponds to
the {1 1 1} lattice planes of fcc Ag.
Ag nanoparticles with various morphologies have been widely
used as sacrifical template to synthesize other noble metal particles
such as Au, Pt and Pd, with hollow and porous structures [16–19].
Since the standard reduction potential of PtCl26ꢀ/Pt pair (0.73 V vs
standard hydrogen electrode, SHE) is higher than that of the Agþ/Ag
pair (0.80 V vs SHE), silver would be oxidized into Agþ when silver
nanostructures and H2PtCl6 are mixed in an aqueous solution:
4Ag(s)þPtCl26ꢀ-Pt(0)þ4Ag(þaq)þ6Cl(ꢀaq)
(1)
Fig. 1. (a) Overview SEM image of the Ag MFs, the inset showing the corresponding EDX pattern; (b) enlarged SEM image of several Ag MFs with uniform size; (c) TEM
image of a single Ag MF, the inset is the corresponding SAED pattern; (d) HRTEM image of the thin layer in Ag MF. The silicon signals came from the silicon substrate on
which the sample is supported.

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L. Zhuang et al. / Journal of Solid State Chemistry 191 (2012) 239–245
Fig. 2. Representative (a) SEM, (b) TEM and (c) HRTEM images of Pt45Ag55; (d) SEM, (e) TEM and (f) HRTEM images of Pt72Ag28. The insets in (a) and (d) are the enlarged
SEM images for Pt45Ag55 and Pt72Ag28, respectively. The insets in (b) and (e) are the corresponding SAED patterns.
It was hard to remove silver element from the alloys com-
pletely. Our additional experiment reveals that there were still
18% (in weight) Ag remained even after immersing in 2 M nitric
acid for 5 day. This phenomenon may be due to the formation of
Pt–Ag solid solution during the galvanic reaction occurring
between metallic silver and Pt (IV) ions, which leads to the
dissolution of Ag in nitric acid difficult. Therefore, in order to
completely remove the residual Ag from Pt–Ag alloy MFs, the
electrochemical method was employed, as described in experi-
mental part. As shown in Fig. 3, the peak located at around 1.0 V
(vs. Ag/AgCl) can be ascribed to the silver electro-oxidation
reaction presented a continuous decrease during the CV cycling,
which nearly disappeared after 10 CV cycles, indicating the silver
was removed from the electrode after the CV cycling through
follow Eq. (3) [20].
Ag(s)þe-Ag(þaq)
(3)
The SEM images of the final products were displayed in
Fig. 4(a) and (b), indicating that the morphology of the mother
Ag MFs shape were kept very well after the galvanic reactions and
electrochemical dissolution of Ag from PtAg alloys. Further EDX
analysis (inset in Fig. 4(a)) was used to determine the elemental
compositions for the final products. The result shows that no
silver signals can be detected, indicating the final product is
exclusively composed of platinum. SAED pattern in Fig. 4(d)
indicates that Pt MF structure is polycrystalline in nature with
certain degrees of preferential orientation, which is similar with
that of their alloy counterparts. TEM images (Fig. 4(c)–(e))
provide more details for this structure that each sheet on the Pt
MFs consists of a three-dimensinal continuous nanoporous struc-
ture with a pore size around 2 nm (Fig. 4(e)). The pores uniformly
dispersed on above petals are smaller and denser than those
(ꢁ5 nm) shown in the porous metals prepared through galvanic
reaction or dealloy treatment reported by others [16–19]. A large
number of crystal defects can be observed in the HRTEM image, as
marked in Fig. 4(f) by black arrows, which may potentially act as
the active site during the catalysis.
Fig. 3. CV curves of ten continuous cycles for Ag dissolution from Pt72Ag28 MFs
with a scan range between ꢀ0.2 and 1.1 V in 0.1 M HClO4 solution at room
temperature.
were phase pure. The observed diffraction angles for both
Pt45Ag55 and Pt72Ag28 nanostructures fell between those of the
two pure metals and the diffraction peak positions of alloy
samples linearly shifted and moved to high angles with the
decrease of Ag/Pt ratio (dashed lines in Fig. 5), suggesting the
formation of alloy nanoparticles [21]. It is well known that Ag–Pt
bimetallics have a large miscibility gap at the temperature below
about 1190 1C in bulk [22], and they form alloys only at very high
atomic content of either Ag or Pt [23]. However, both theoretical
studies and experimental data have shown that the small-size-
effect can greatly promote the miscibility between the metal
elements with the decrease in particle size [21,24–27]. In current
case, as labeled in Fig. 2(d), the average cluster sizes were about
5 nm, which was much smaller than the threshold value of the
size (20 nm) of alloying nanoparticles for the unusual solid
solubility [26]. Therefore, with the progress of reaction, platinum
ions became reduced to metal leading to the formation of Pt–Ag
solid solution with the residual silver.
XRD was further used to analyze the crystalline structure of
Ag, Pt and Pt–Ag samples, respectively. As shown in Fig. 5, both
four kinds of samples show diffractions that could be indexed to
(1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes of a fcc lattice. No
other diffraction peak was observed, indicating these products
One potentially very important application area of such
3-dimentional Pt nanostructures is to prepare the fuel cell

L. Zhuang et al. / Journal of Solid State Chemistry 191 (2012) 239–245
243
Fig. 4. SEM images of Pt MFs at low magnification (a) and high magnification (b), the inset in (a) showing the corresponding EDX pattern; TEM image of a single Pt MFs
(c) and a single layer of the MF (d), the inset shows the corresponding SAED pattern (the circled part); (e) and (f) show the enlarged TEM and HRTEM images of the circled
part in Fig. 2(d).
Fig. 6(d), the specific area and mass current densities of the pure
Pt MFs sample are of 3.42 mA cmꢀ2 and 455 mA mgꢀ1, respec-
tively, about 2 to 3 times higher than the values of Pt black
1.81 mA cmꢀ2 and 157 mA mgꢀ1, respectively. And such values
were also higher than that of porous Pt nanotubes in specific area
value (1.62 mA cmꢀ2) [31] and the porous single-crystalline Pt
nanoparticles in mass current density (205 mA mgꢀ1) [12]. This
fact indicates the pure Pt MFs exhibit much improved perfor-
mance toward MOR.
The peaks of methanol oxidation with high current density are
clearly observed at around 0.7 V (vs. Ag/AgCl) in anodic sweep
and at around 0.55 V (vs. Ag/AgCl) in cathodic sweep. Generally,
as described by others, the following conventional elementary
reaction processes are considered [32–34]:
Fig. 5. The XRD patterns for Ag (a), Pt45Ag55 (b), Pt72Ag28 (c) and Pt (d) MFs.
PtþCH3OH-Pt–COþ4Hþ þ4eꢀ
PtþH2O-Pt–OHþHþ þeꢀ
Pt–COþPt–OH-2PtþCO2 þHþ þeꢀ
(4)
(5)
(6)
electrode. Herein, the electrochemical catalytic activity in metha-
nol oxidation was examined, as this reaction was sensitive to the
surface electronic structure and atomic arrangement of Pt [28,29].
The ECSAs of the Pt and Pt–Ag MFs were calculated by measuring
While the total electrooxidation of methanol is expressed as:
CH3OHþH2O-CO2þ6Hþ6eꢀ
(7)
the hydrogen adsorption on Pt (Fig. 6(a)). The ECSAs for Pt45Ag55
,
Pt72Ag28, Pt MFs and Pt black (JM) were 10.3, 14.1, 13.9 and
12.96 m2 g–1, respectively. The ECSAs of Pt72Ag28 and Pt MFs were
comparable to that of Pt black, which should be due to that Pt or
PtAg MFs were composed by the cluster with an average size less
than 5 nm, while the crystal size in Pt black was ranging from 5–
10 nm (Fig. S1 in supplemental data).
The cyclic voltametric (CV) measurements of methanol oxida-
tion were carried out in solution of 0.1 M HClO4þ0.5 M CH3OH at
room temperature at a scan rate of 50 mV sꢀ1 (Fig. 6(b) and (c)),
from which we observed that the onset potential at first anodic
scanning for methanol oxidation is about 0.38 V, 0.37 V, 0.39 V
and 0.40 V for Pt45Ag55, Pt72Ag28, pure Pt MFs and Pt black,
respectively, indicating that both pure Pt–Ag and Pt MFs have
an enhancement in the kinetics of methanol oxidation reaction
compared with the Pt black. Herein the onset potential being
defined as the potential at which 10% of the current value at the
peak potential was reached [30]. More importantly, as shown in
First, when the potential arrives at around 0.50 V in the anodic
sweep, methanol molecule adsorbs on the Pt surface and gen-
erates Pt–CO (Eq. (4)). Meanwhile, water molecule oxidation
produces Pt–OH at more anodic potential, as illustrated by
Eq. (5). With the increase of potential, the adsorption and
oxidation will be enhanced gradually, which lead to a continuous
increase in current density until the potential reaches to around
0.7 V, as shown in Fig. 6(b). As the potential increases, more and
more reaction products will prevent the deeper oxidation. As a
result, the peak will then decrease. Under this potential region,
the redox of two adsorbed species will produce Pt and CO2 for
desorption (Eq. (6)). While when the potential arrives at around
0.65 V in the cathodic sweep, methanol reoxidation is generated
again and the peak climbs to a new high value of around 0.55 V.
The ratio between the forward (If) and reverse (Ib) anodic peak
current densities, If/Ib, has been used to describe the tolerance of a

244
L. Zhuang et al. / Journal of Solid State Chemistry 191 (2012) 239–245
Fig. 6. (a) CV profiles of Pt and Pt–Ag MFs in 0.1 M HClO4 solution at room temperature; (b) and (c) CVs of MOR on Pt and Pt–Ag MFs catalysts in 0.1 M HClO4 and 0.5 M
CH3OH for specific area current and specific mass current, respectively; (d) Comparison of peak specific area current and peak specific mass current of Pt MFs, Pt–Ag MFs
and Pt black. All the scanning rate was 50 mv sꢀ1
.
catalyst to the accumulation of carbonaceous species [35–40].
Low If/Ib ratio is an indication of poor oxidation of methanol to
carbon dioxide during an anodic scan and accumulation of
carbonaceous residues on the catalyst surface. Catalyst with low
If/Ib ratio is prompt to extended CO poisoning [41,42], while a
higher ratio indicates more effective removal of the poisoning
species on the catalyst surface [39]. The If/Ib ratios calculated from
the CV curves for Pt45Ag55, Pt72Ag28, pure Pt MFs and Pt black
were 1.13, 1.36, 0.988 and 0.982, respectively. It appears that
although the remained Ag produces a negative effect on anodic
scanning current density, the Pt–Ag alloys presents a better CO
tolerance than pure Pt catalyst. This result was also proved by He
and Peng in their recent works [20,43].
As reported by others, the porous substrate, such as highly
ordered mesoporous carbon, can significantly improve the spe-
cific activity of platinum-based catalysts because of the facile
transport of methanol and the oxidation products in the pores
[44–45]. In current case, The noteworthy electrochemical cataly-
tic performance of Pt MFs can be attributed to their unique
3-dimensional architecture, where the interconnected interstices
and channels extending in all three dimensions allow the rapid
diffusion and mass transport of external reagents directly to
the surface of Pt catalysts, as well as unblocked transport
of electrons, which contributes greatly to the enhancement of
catalytic properties [46–56]. As for the Pt–Ag alloys, it is well
known that methanol electrooxidation occurs preferentially on
electrode surfaces with continuous neighboring Pt sites [50].
Therefore, when the Ag content is higher in the Pt–Ag MFs, a
number of Pt atoms will exist in the form of monomers (single
atoms), dimers, or clusters of Pt atoms which do not provide
effective dissociative adsorption of methanol molecules [51].
Meanwhile, Ag atom itself is much less active to MOR in acidic
Fig. 7. Chronoamperometric response of oxidation of 0.5 M methanol in 0.1 M
HClO4 solution under a constant potential of 0.65 V (vs. Ag/AgCl).
conditions, so the decline of activity can be correlated to the
accumulation of more Ag atoms on the surface [20]. Therefore,
after electrochemically removing Ag atoms, the pure Pt MFs
exhibit distinct enhancement for MOR. However, the improved
performance in tolerating poisoning species for Pt–Ag alloy,
especially for Pt78Ag22 may be due to the electronic structure of
Pt being changed by forming alloy with Ag [38], but the mechan-
ism was still not clear yet, which need to be further investigated.
The long-term performance of Pt and Pt–Ag MFs catalysts
compared with commercial used Pt black (TKK) was evaluated by
chronoamperometric measurement under a constant potential of
0.65 V versus Ag/AgCl for 4000 s, as shown in Fig. 7. The
polarization currents of all the present catalysts decrease rapidly
at the initial stage. This may be due to the formation of
intermediate species, such as COads, COOHads, and CHOads, during
the methanol oxidation reaction [52–54]. And the following

L. Zhuang et al. / Journal of Solid State Chemistry 191 (2012) 239–245
245
degradation in the current can be attributed to the accumulation
of COads poisioning species on the surface of Pt electrode during
the MOR process [55,56]. Pt MFs produced an initial specific
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a final specific activity of
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comparison, Pt black produced an initial specific asctivity of
1.75 mA cmꢀ2 and a final specific activity of 0.15 mA cmꢀ2
,
indicating the lost activity was more than 92%. Similar with the
observations from Fig. 6, the samples of Pt45Ag55 and Pt72Ag28
,
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In summary, we synthesized 3-D Ag MFs using BSA as capping
agent, which can be further used as sacrificial template for the
preparation of porous Pt and Pt–Ag alloy nano-catalysts. The
morphology of Ag MFs can be kept very well by their Pt and Pt–Ag
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applied in DMFCs in comparison with the commercial used Pt
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facile approach for the development of high performance Pt or
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This work was supported by the National Natural Science
Foundation of China (No. 50901056), Doctoral Fund for New
Teachers (No.20090201120053) and the Fundamental Research
Funds for the Central Universities. J.X.F. is supported by Tengfei
Talent Project of Xi’an Jiaotong University.
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Appendix A. Supporting information
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Supplementary data associated with this article can be found in
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