A green synthesis of "naked" Pt and PtPd catalysts for highly efficient methanol electrooxidation

Article abstract of DOI:10.1039/c6ra09516j

Up to now, green synthesis of "naked" nanoobjects with clean surfaces is still a great challenge since structure-directing agents such as surfactants are usually used to induce the anisotropic particle growth for interesting nanostructures in solution. Here, a green synthesis approach for preparing "naked" Pt nanoparticles (NPs) in acetic acid solution was reported. Interestingly, Pt NPs with different morphologies (well-defined polyhedrons, irregular polyhedron-constructed chain networks and nanoflower-constructed chain networks) could be successfully synthesized at different concentrations of acetic acid without adding additional structure-directing agents and reducing agents. The acetic acid itself works both as structure-directing agent and reducing agent for the formation of these interesting Pt NPs. Specifically; the synthesized NPs are "naked" with clean surfaces since the acetic acid is a weakly-capping agent which can be easily removed after the washing process. The versatility of this method is further demonstrated by preparation of "naked" PtPd alloy NPs. In particular, the as-prepared "naked" Pt_-5.20 and Pt₃Pd_1-5.20 NPs are demonstrated to be excellent catalysts with much higher electroactivity and better durability than the commercial Pt black catalyst (Hispec 1000) for the methanol oxidation reaction. The method reported in this work may pave the way for simple, low-cost, green and eco-friendly synthesis of "naked" metal catalysts for advanced electrocatalytic applications.

Full text of DOI:10.1039/c6ra09516j

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A green synthesis of “naked” Pt and PtPd catalysts
for highly efficient methanol electrooxidation†
Cite this: RSC Adv., 2016, 6, 56083
a
b
a
a
a
Baohua Zhang, Yiguo Xue, Hong Sun, Anning Jiang, Zhonghao Li*
a
and Jingcheng Hao
Up to now, green synthesis of “naked” nanoobjects with clean surfaces is still a great challenge since
structure-directing agents such as surfactants are usually used to induce the anisotropic particle growth
for interesting nanostructures in solution. Here, a green synthesis approach for preparing “naked” Pt
nanoparticles (NPs) in acetic acid solution was reported. Interestingly, Pt NPs with different
morphologies (well-defined polyhedrons, irregular polyhedron-constructed chain networks and
nanoflower-constructed chain networks) could be successfully synthesized at different concentrations
of acetic acid without adding additional structure-directing agents and reducing agents. The acetic acid
itself works both as structure-directing agent and reducing agent for the formation of these interesting
Pt NPs. Specifically; the synthesized NPs are “naked” with clean surfaces since the acetic acid is
a weakly-capping agent which can be easily removed after the washing process. The versatility of this
method is further demonstrated by preparation of “naked” PtPd alloy NPs. In particular, the as-prepared
Received 13th April 2016
Accepted 3rd June 2016
“
3
naked” Pt_5.20 and Pt Pd1_5.20 NPs are demonstrated to be excellent catalysts with much higher
electroactivity and better durability than the commercial Pt black catalyst (Hispec™ 1000) for the
methanol oxidation reaction. The method reported in this work may pave the way for simple, low-cost,
green and eco-friendly synthesis of “naked” metal catalysts for advanced electrocatalytic applications.
DOI: 10.1039/c6ra09516j
www.rsc.org/advances
17–20
fuel cells.
Among the Pt-based catalysts, it is reported that
Introduction
Pd can largely enhance the performance of Pt catalysts for
21–25
During the past decades, noble metal nanomaterials have various catalytic reactions.
Specically, Pt and Pd in PtPd
attracted considerable interest for a variety of research elds electrocatalysts show an obvious “synergistic effect” leading to
ranging from organic synthesis and fuel cells to environmental superior activity in comparison with individual Pt or Pd elec-
1
1,26
protection, especially for catalytic/electrocatalytic applications trocatalysts in the oxidation of methanol.
because of their specic physicochemical properties.
The “synergistic
In effect” of Pt and Pd in the alloys for methanol oxidation can be
general, the properties of metal nanoparticles (NPs) mostly attributed to the bifunctional methanol oxidation mecha-
1
–4
5–9
27,28
depend on their composition, size and morphology. There- nism.
fore, it is possible to tune the properties of metal NPs by while Pd mainly make water dehydrogenation form Pd–OH. The
tailoring their composition, size and morphology. For example, Pt–CO and Pd–OH react together to produce CO and therefore
the catalytic activity of reported metal NPs, such as nanosheet- the active metal surfaces refresh. However, the water dehydro-
Pt catalyzes methanol dehydrogenation to form Pt–CO
2
10
constructed Pd particles, three dimensional PtPd bimetallic genation on pure Pt occurs at a higher potential without Pd,
NPs and hollow PtPd alloyed nanocubes etc. is largely resulting in the overall oxidation process on pure Pt sluggish.
dependent on their morphologies.
The synthesis of morphology-controlled NPs including Pt
11
12
Until now, Pt has always been considered as the best elec- and PtPd NPs is generally based on the classical solution
trocatalyst for methanol oxidation in the direct methanol fuel synthesis method, in which NPs are synthesized with the help of
1
3–16
cell.
Moreover, the Pt-based materials are also developed as additives such as surfactants, polymers, fatty acids and fatty
29
efficient electrocatalysts for methanol oxidation reactions in amine etc. These additives are oen dened as structure-
directing agents. However, it was reported that these agents
oen strongly adsorb and cap the surfaces of NPs, blocking the
exposure of the surface active sites. Specically, they are very
a
Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of
Education, Jinan 250100, China. E-mail: zhonghaoli@sdu.edu.cn; Fax: +86 531
difficult to remove, leading to a subsequent reduction in cata-
8
b
8564750; Tel: +86 531 88363821
30–32
Geotechnical and Structural Engineering Research Center of Shandong University, lytic activity.
Therefore, partly loss of activity of these
Jinan 250061, China
synthesized catalysts is unavoidable during the catalytic or
electrocatalytic reactions. Moreover, the introduction of
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c6ra09516j
1
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3 1
additional structure-directing agents such as surfactants and synthesis of pure Pt NPs. The synthesized Pt and Pt Pd NPs
polymers will make the synthesis risk toxicity since some with different concentrations of acetic acid were labeled as Pt_x
surfactants or polymers themselves are not low toxic chem- and Pt Pd (x presents the concentration of acetic acid).
3
1_x
10
icals. In principle, to ensure the successful formation of metal
NPs from metal salts, the synthesis system usually contains
additional reducing agents. The reducing agents like hydrazine
Characterization
X-ray diffraction (XRD) was performed using a Rigaku Dmax-rc
X-ray diffractometer with Ni-ltered Cu Ka (l ¼ 1.5418 A) radi-
hydrate, NaBH
4
, and CO are oen used, which certainly
˚
10,33–36
increases the toxic/unsafe risk from these chemicals.
ation. Transmission electron microscopy (TEM) was performed
using a JEM 1400 TEM operating at 120 kV. HAADF-STEM-EDS
mapping were performed with a JEM-2100F. For the measure-
ment of particle size, over 100 particles were measured using
photoshop. Fourier transform infrared spectroscopy (FTIR)
spectra were performed on Nicolet 6700 Fourier transform
infrared spectrometer. The composition of the bimetallic
particles was determined by inductively coupled plasma atomic
emission spectroscopy (ICP-AES, VISTAMPX).
Therefore, a facile green method for the preparation of “naked”
metal nanocatalysts with clean surfaces is still of great chal-
lenge. To achieve this goal, a simple synthesis system without
additional surfactants, polymers and toxic chemicals will be
highly demanded.
Herein we describe a simple, low-cost, green and eco-friendly
method to synthesize the “naked” Pt NPs with clean surfaces in
acetic acid solution without using additional structure-directing
agent or reducing agent. The acetic acid itself works both as
structure-directing agent and reducing agent for the formation
of the Pt NPs. Specically, the synthesized NPs have clean
Electrocatalytic activity
surface since the acetic acid is a weakly-capping agent which Electrochemical measurements were performed on an electro-
can be removed aer the washing process. Various interesting chemical workstation (CHI model 660C). The conventional
Pt morphologies such as well-dened polyhedrons, irregular three electrode test cell contained a glassy carbon electrode
polyhedron-constructed chain networks and nanoower- (GCE, diameter: 3 mm) as working electrode, a Pt wire as
constructed chain networks can be successfully synthesized. counter electrode and a saturated calomel electrode (SCE) as
This method can be further extended for the synthesis of reference electrode. The GCE was rst polished carefully with
“
naked” PtPd alloy NPs. Furthermore, the as-prepared “naked” alumina powder, and then washed by sonication in ethanol and
ꢁ
1
Pt_5.20 and Pt Pd
show superior electrocatalytic perfor- water respectively. 5 mL aqueous solution (2 mg mL ) of as-
3
1_5.20
3 1
mance in comparison with the commercial Pt black catalyst for prepared Pt, Pt Pd particles or the commercial Pt black was
the electrooxidation of methanol.
dropped onto the surface of the GCE, respectively. Aer the
solution was dried, 5 mL of Naon (0.5 wt%) was added to the
surface of the modied electrode. The GCE was dried naturally
before using. The cyclic voltammetry (CV) proles were rst
Experimental section
Materials
ꢁ
1
recorded at a scan rate of 50 mV s within the potential range
from ꢁ0.25 V to 1.0 V vs. SCE in a 0.5 M HClO solution. For
methanol oxidation, the CV proles were measured in 0.5 M
4
H PtCl $6H O and K PdCl were purchased from Aladdin
2
6
2
2
4
Industrial Corporation. Naon solution was purchased from
Sigma-Aldrich. Johnson-Matthey platinum black catalyst
ꢁ
1
4 3
HClO + 1.0 M CH OH solution at a scan rate of 50 mV s
within the potential range from ꢁ0.2 V to 1.0 V vs. SCE. The
accelerated cycling tests for methanol oxidation were conducted
for 3000 cycles. All CVs were obtained at room temperature and
(Hispec™1000) was purchased from Alfa Aesar. All the regents
were used without further purication.
the electrolyte solutions were purged with high purity N gas
prior to the test.
2
Synthesis
2 6
In a typical synthesis of Pt particles, 245 mL of 0.1 M H PtCl $
2
6H O (0.0245 mmol) was injected into 16 mL aqueous solution
with the acetic acid concentration of 0.10 M, 0.21 M, 0.52 M,
Results and discussion
1.04 M, 3.11 M, 4.16 M, 5.20 M, 8.33 M, 10.41 M, 14.57 M, Here, the synthesis of Pt NPs was performed in a wide range of
respectively. Then the solution was sonicated for a few minutes acetic acid concentrations. The resultant Pt NPs at different
to form a uniform solution. Subsequently, the resultant solu- concentrations of acetic acid were labeled as Pt_x (x presents the
tion was put into a Teon-lined autoclave and the autoclave was concentration of acetic acid). The typical synthesis process was
ꢀ
heated at 180 C for 2 h. The autoclave was then cooled to room illustrated in Fig. 1A. Fig. 1B shows the XRD patterns of the
temperature naturally. Finally, the resultant products were selected synthesized Pt particles with typical morphologies at
recovered by repeated centrifugation and washed with ethanol the acetic acid concentration of 0.21 M, 3.11 M and 5.20 M,
and water, respectively. The synthesized product was then dried respectively. All diffraction peaks can be indexed as the face-
ꢀ
at 60 C for 5 h. The procedure for synthesizing PtPd (Pt Pd was centered cubic (fcc) Pt (JCPDS card, 04-0802). The XRD result
3
1
synthesized as the typical samples) bimetallic NPs was similar conrms that the formation of pure Pt powers with highly
as that described above with the molar ratio H PtCl $6H - crystallization. The intensity of diffraction peaks in XRD
2
6
2
O : K
2 4
PdCl equal to 3 : 1 (0.0184 mmol : 0.0061 mmol) while patterns is closely related to the crystallinity of prepared
the total molar number of metal salt was same as that for the nanomaterials. In general, the higher intensity of diffraction
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Fig. 1 Typical schematic illustration (A) for the preparation of Pt NPs at
various concentration of acetic acid. XRD patterns (B) of the as-
prepared Pt_0.21, Pt_3.11 and Pt_5.20 particles synthesized at the acetic Fig. 2 Low and high magnification TEM images of Pt_0.21, Pt_3.11 and
acid concentration of 0.21 M (a), 3.11 M (b) and 5.20 M (c), respectively. Pt_5.20 particles synthesized at the acetic acid concentration of 0.21 M
(a, b), 3.11 M (c, d) and 5.20 M (e, f).
peaks represents the higher crystallinity. From Fig. 1, it shows
that the intensity of diffraction peaks Pt NPs decreases with the chain is composed of irregular polyhedrons. These irregular
increasing of acetic acid concentration. This indicates that the polyhedrons are linearly interconnected and further self-
crystallinity of Pt NPs decreases with the increasing of acetic assemble to chains. For the Pt_5.20 particles synthesized at
acid concentration. The ratio between the intensities of (111) 5.20 M acetic acid, Pt nanoower-constructed chain networks
and (200) peaks for Pt_0.21, Pt_3.11 and Pt_5.20 is 2.49, 2.29, and form, as shown in Fig. 2e and f. The size of the nanoowers is
2
.20, respectively. Compared with the reported value (JCPDS 22.9 ꢂ 3.9 nm. The enlarged TEM image in Fig. 2f further
card, 04-0802) for the conventional Pt powder sample (1.89), it reveals that the Pt owerlike nanostructures are composed of
indicates that the diffraction from {111} planes is enhanced for small nanoparticle subunits, in which the small NPs are with
the synthesized Pt_0.21, Pt_3.11 and Pt_5.20 particles, reecting the size of 4.3 ꢂ 1.1 nm. Such three-dimensional hierarchical
the predominant growth of the (111) planes in Pt_0.21, Pt_3.11 structure could be very useful in the catalytic application since
and Pt_5.20 NPs.
each nanoower structure shows a bumpy surface which
TEM characterization is used to understand the morphol- benets for the active sites such as high density of atomic steps,
37,38
ogies of the typical synthesized Pt_0.21, Pt_3.11 and Pt_5.20 NPs. ledges and kinks.
From Fig. 2, it also shows that the particle
Fig. 2a and b shows the low and high magnication TEM size of the synthesized Pt increases with the acetic acid
images of the as-prepared Pt_0.21 NPs synthesized at 0.21 M concentration. With the increase of acetic acid concentration,
acetic acid. From the TEM images in Fig. 2a and b, it can be seen the formation of Pt becomes faster as the higher acetic acid
that well-dened polyhedron particles consisting of cube, concentration can reduce the platinum salt faster. In this case,
octahedra, tetrahedra and pentahedra form. Most of the parti- the Pt crystal grows faster. Therefore larger Pt particles form at
cles are cubes, whereas spherical particles are virtually absent. higher acetic acid concentration.
The average size of the Pt_0.21 particles is 11.7 ꢂ 2.5 nm. Fig. 2c
We also performed the synthesis in a wide range of acetic
and d shows the TEM images of Pt_3.11 NPs synthesized at acid concentrations from 0.10 M to 14.57 M. The morphology
3
1
.11 M acetic acid. The average size of the Pt_3.11 particles is and size of these synthesized Pt NPs were summarized in Table
6.1 ꢂ 2.8 nm. However, the morphology of Pt NPs is 1. The related TEM images are shown in Fig. S1 (ESI†).
_
3.11
obviously different from the Pt
NPs. Clearly, there are a lot
0.21
We have succeeded in synthesizing the Pt particles according
_
of interconnected chains across the whole section (Fig. 2c). to the procedure above without using any additional reducing
Each chain is fused with the adjacent ones to form Pt chain agents. This indicates that the successful formation of Pt
networks. High magnication TEM image (Fig. 2d) reveals each particles is attributed to the acetic acid which works as the
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Table 1 Summary of morphology and size of the Pt NPs synthesized in result in rapid nucleation to form numerous Pt NPs. The initial
a wide range of acetic acid concentrations
formed Pt NPs are more energetically favorable to grow into
a owerlike shape since such aggregation can reduce the
Pt NPs
Morphology
Size (nm)
44
surface energy. The as-prepared Pt nanoowers can further
have a growth through an attachment growth mode, resulting in
nanoower-constructed chain networks.
Pt_0.10
Pt_0.21
Pt_0.52
Pt_1.04
Pt_3.11
Well-dened polyhedrons
Well-dened polyhedrons
Well-dened polyhedrons
Well-dened polyhedrons
Irregular polyhedron-constructed
chain networks
Irregular polyhedron-constructed
chain networks
Nanoower-constructed
chain networks
Nanoower-constructed
chain networks
Nanoower-constructed
chain networks
Nanoower-constructed
chain networks
19.2 ꢂ 3.4
11.7 ꢂ 2.5
12.2 ꢂ 2.4
27.1 ꢂ 4.3
16.1 ꢂ 2.8
We further extended the present method to synthesize
Pt-based bimetallic NPs. We selected PtPd as the studied system
since PtPd bimetallic nanomaterials are among the most
11,45–47
Pt_4.16
Pt_5.20
Pt_8.33
Pt_10.41
Pt_14.57
13.2 ꢂ 2.9
22.9 ꢂ 3.9
113.1 ꢂ 8.9
interesting bimetallic nanomaterials
with excellent cata-
lytic performance. Here, we studied on the synthesis of PtPd
NPs with the concentrations of acetic acid at 0.21 M, 3.11 M and
5.20 M since typical interesting Pt particles can be obtained at
these acetic acid concentrations. As we mainly focus on the
345.2 ꢂ 14.3 green, simple and eco-friendly synthesis of “naked” PtPd NPs
with clean surfaces but not on the study of composition-
309.6 ꢂ 18.2
3 1
dependent PtPd particles, we choose Pt Pd as the model PtPd
bimetallic particles for the synthesis. The synthesis process of
Pd Pt NPs was similar to that of Pt NPs. The only difference was
3
1
that the metal salt of H PtCl $6H O was replaced by the
2
6
2
reducing agent. To further investigate the reduction mecha- combination of H
2
PtCl
6
$6H
2
O and K
2
4
PdCl with the molar ratio
nism, we took the supernatant aer the reaction for FTIR of H PtCl $6H O : K
2
6
2
2
4
PdCl equal to 3 : 1 while the total amount
characterization, as shown in Fig. 3. The FTIR spectrum of the of metal salt was the same as that for pure Pt NPs. The resultant
supernatant exhibits typical absorption peaks, which is similar Pt
3
Pd
Pd1_x (x presents the concentration of acetic acid). The
Pd NPs was characterized by ICP-AES.
hyde during the reaction. This agrees with the previous reported The atomic ratios of Pt to Pd for Pt
1
NPs at various acetic acid concentrations are labeled as
to the controlled acetaldehyde solution. Thus, it can be Pt
3
conclude that the acetic acid is partly changed into acetalde- composition of the Pt
3
1
Pd1_0.21, Pt
that acetic acid can decompose into hydrogen, acet- Pt Pd1_5.20 synthesized with the concentrations of acetic acid at
3
Pd1_3.11, and
3
3
39,40
work,
aldehyde and carbon dioxide at heating condition. Therefore, 0.21 M, 3.11 M and 5.20 M were determined to be 3.52 : 1,
the actual reducing agent in the present synthesis could be 3.29 : 1 and 3.41 : 1, respectively, which is well consistent with
attributed to the generated H upon the decomposing of acetic the ratio of precursors supplied. This indicates the complete
2
acid. The FTIR spectrum of the as-prepared Pt NPs aer the reduction of the reaction due to the addition of acetic acid.
reduction by using acetic acid is shown in Fig. S2 (ESI†), which
demonstrates the negligible organic agent surrounding the a detailed TEM study of these NPs was undertaken. TEM images
surface of the as prepared Pt NPs.
of the as-prepared Pt Pd1_3.11, and Pt
The observed different morphologies at various acetic acid presented in Fig. 4a–f. The morphologies of Pt
concentrations in Table 1 can be explained as follows. As the Pt Pd1_3.11, and Pt Pd1_5.20 NPs are almost similar to the
acetic acid concentration is low, the reduction rate of platinum as-prepared Pt_0.21, Pt_3.11 and Pt_5.20 NPs, respectively. The
ions to platinum particles is low. Specically, since (111) facets average size of Pt Pd1_0.21, Pt Pd1_3.11, and Pt Pd1_5.20 NPs is
has the lowest surface energy for a face-centered cubic struc- 13.1 ꢂ 1.8 nm, 18.9 ꢂ 4.1 nm and 18.3 ꢂ 4.7 nm, respectively.
3 1
To visualize the morphology of the synthesized Pt Pd NPs,
Pd1_0.21, Pt
Pd1_5.20 are
Pd1_0.21
3
3
3
,
3
3
3
3
3
3
4
1
ture, the formation of polyhedrons such as cubes with maxi- The size difference between the Pt
mized expression of (111) facets is thermodynamically favorable
to reduce the total surface energy. Therefore, the morphologies
of as-prepared Pt NPs at low concentration of acetic acid are
well-dened polyhedrons, which mostly are cubes (agree with
the XRD results). As the concentration of acetic acid increases,
the morphology of resultant Pt particles is irregular polyhedron-
constructed chain network, in which the adjacent Pt NPs are
oen in close contact within the chains. In this case, the
primary Pt particles are rstly formed via the reduction of
platinum ions by the acetic acid and there is enough acetic acid
for selectively tuning the Pt growth into irregular polyhedron
particles based on that small molecule can also act as a struc-
3 1
Pd particles and the
4
2
ture-directing agent for crystal growth. The formed irregular
polyhedron Pt NPs can further follow an attachment growth
mode
4
1,43
to form chain networks. As the concentration of acetic
Fig. 3 FTIR spectra of the supernatant after the reaction (a) and the
O) solution (b).
acid further increases, the abundant acetic acid molecules can controlled acetaldehyde (C
2 4
H
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corresponding Pt particles could be attributed to the addition of
K PdCl .
2
4
Fig. 5 shows the XRD patterns of the as-prepared various
Pt Pd particles. All the particles show four diffraction peaks
3
1
corresponding to (111), (200), (220) and (311) of the typical face-
centred cubic (fcc) phase, respectively. The corresponding
diffraction peaks of the Pd Pt NPs are located between that of
3 1
pure Pt (JCPDS. no. 04-0802) and Pd (JCPDS. no. 65-2867), which
demonstrates their alloyed structure. The XRD result conrms
the formation of pure PtPd powders with highly crystallization
and alloyed structure. From Fig. 5, it shows that the intensity of
diffraction peaks Pt Pd NPs increases with the increasing
3
1
Fig. 5 XRD patterns of the as-prepared Pt
3
Pd
1
particles synthesized
of acetic acid concentration. This indicates that the crystallinity
of Pt Pd NPs increases with the increasing of acetic acid
with various acetic acid concentrations: 0.21 M (a), 3.11 M (b) and
.20 M (c).
3
1
5
concentration. Comparing the result of Fig. 1 with that of Fig. 5,
it is obviously the change of the crystallinity with the increase of
acetic acid concentration is completely different between Pt NPs
48
2ꢁ
lattice match (99.23%) and similar redox potential ([PdCl
4
]
/
and Pt Pd1 NPs. Such difference could be attributed to the
2ꢁ
49
3
Pd: 0.755 V versus SHE, [PtCl
6
]
/Pt: 0.591 V versus SHE),
addition of K PdCl for the synthesis of Pt Pd NPs.
2
4
3
1
therefore the successful formation of alloy structure could be
easily understood.
To further analyze the structure and elemental composition
of the synthesized Pt Pd , the high-angle annular dark-eld
scanning TEM (HAADF-STEM) and the element mapping of
the typical Pt Pd1_5.20 sample are characterized (Fig. 6). The
3
1
The electrocatalytic property of the typical synthesized Pt
and Pt
understand their potential application in direct methanol fuel
cells (Fig. 7). Here the typical Pt_0.21, Pt_3.11, Pt_5.20, Pt
Pt Pd1_3.11 and Pt Pd1_5.20 NPs are selected for the electro-
3 1
Pd alloy NPs towards methanol oxidation was tested to
3
uniform color distribution veries that both Pt and Pd are
homogeneously distributed throughout the NPs, indicating that
the as-prepared particle is an alloy structure which is consistent
with the result of the XRD. The Pt and Pd have extremely high
3
Pd1_0.21,
3
3
catalytic performance evaluation since these particles are
unique and representative. The commercial state-of-the-art
Johnson-Matthey platinum black catalyst (Hispec™1000) is
used as a reference for comparison. Fig. 7a and b shows the
cyclic voltammograms (CVs) of the different catalysts at a scan
ꢁ1
rate of 50 mV s in 0.5 M HClO
4
solution. The current densities
are normalized to the electrochemically active surface areas
Fig. 4 TEM images of Pt
3 3 3
Pd1_0.21, Pt Pd1_3.11 and Pt Pd1_5.20 particles
synthesized with various acetic acid concentrations: 0.21 M (a, b), Fig. 6 HAADF-STEM images (a–b), and EDS mapping images (c–e) of
3
.11 M (c, d) and 5.20 M (e, f).
3
Pt Pd1_5.20 bimetallic particles ((c) overlap, (d) Pt green, (e) Pd red).
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(
ECSA). The semi-reversible peaks appeared at around ꢁ0.1 V as-prepared Pt_0.21, Pt_3.11, Pt_5.20, Pt
3 3
Pd1_0.21, Pt Pd1_3.11,
correspond to the hydrogen adsorption and desorption while Pt Pd NPs and the commercial Pt black, respectively. The
3
1_5.20
irreversible peaks at around +0.6 V are due to the platinum ECSA of the commercial Pt black is slightly higher than the as-
5
0
oxidation and reduction. From the CVs in HClO solution, the prepared Pt and Pt Pd NPs because the size of the commercial
4
3
1
55
3 1
ECSA of the as-prepared Pt and PtPd electrocatalysts can be Pt black (5.6 nm ) is smaller than as-prepared Pt and Pt Pd
estimated by integrating the charge passed during the hydrogen NPs. To test the electrocatalytic activity of various catalysts
2
8,29,51–54
adsorption–desorption from the electrode surface.
The toward the oxidation of methanol, the ECSA normalized initial
2
ꢁ1
2
ꢁ1
2
ꢁ1
calculated ECSAs are 11.4 m g , 13.7 m g , 13.5 m g
,
4
CVs of 1 M methanol in a 0.5 M HClO were recorded, as shown
2
ꢁ1
2
ꢁ1
2
ꢁ1
2
ꢁ1
9.1 m
g
, 9.0 m
g
, 11.8 m
g
, 17.5 m
g
for the in Fig. 7c and d. Characteristic anodic peaks in the forward and
Fig. 7 (a and b) ECSA-normalized CVs of Pt_0.21, Pt_3.11, Pt_5.20, Pt
HClO , (c and d) ECSA-normalized and (e and f) mass-normalized initial CVs of Pt_0.21, Pt_3.11, Pt_5.20, Pt
the commercial Pt black in 0.5 M HClO and 1 M CH OH solution, (g) typical ECSA-normalized CVs and (h) mass-normalized of Pt_5.20
Pt Pd1_5.20 and the commercial Pt black in 0.5 M HClO
3
Pd1_0.21, Pt
3
Pd1_3.11, Pt
3
Pd1_5.20 NPs and the commercial Pt black in 0.5 M
4
3
Pd1_0.21, Pt Pd1_3.11, Pt Pd1_5.20 NPs and
3
3
4
3
,
ꢁ1
3
4 3
and 1 M CH OH solution at different cycle number. The scan rate is 50 mV s .
56088 | RSC Adv., 2016, 6, 56083–56090
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reverse sweeps associated with methanol oxidation are black in the corresponding cycles. For example, the current
ꢁ
2
observed. The peak current densities are 0.83, 0.59, 1.01, 0.94, densities of Pt_5.20 and Pt Pd
are 0.61 and 0.82 mA cm ,
3
1_5.20
ꢁ2
0.99, 1.37 and 0.42 mA cm for as-prepared Pt_0.21, Pt_3.11
,
respectively at 3000 cycles, while the corresponding value of
Pt_5.20, Pt Pd , Pt Pd , Pt Pd NPs and commercial commercial Pt black is only 0.30 mA cm . Comparing the
ꢁ
2
3
1_0.21
3
1_3.11
3
1_5.20
Pt black, respectively. Thus, the peak current densities of all as- current density at 3000 cycles with the maximum current
prepared Pt and Pt Pd are higher than the commercial Pt density for each catalyst, the loss of activity of commercial Pt
black. Moreover, the Pt_5.20 exhibits the highest peak current black is 67.6%, which might be caused by Pt nanoparticle
3
1
56
density among the studied Pt catalysts while Pt
the highest peak current density among the studied PtPd Pt
bimetallic catalysts. Among all of the studied catalysts, the peak results evidently show that the Pt_5.20 and Pt Pd
3
Pd1_5.20 exhibits ripening and aggregation. In contrast, the Pt_5.20 and
Pd1_5.20 NPs lose 53.7% and 47.1%, respectively. The above
have much
3
3
1_5.20
current density of Pt Pd
NPs is the highest, about better durability in comparison with the commercial Pt black
3
1_5.20
3
.26 times higher than commercial Pt black. Following for the methanol oxidation, thanks to their structural advan-
Pt Pd1_5.20 is the as-prepared Pt_5.20 NPs, about 2.40 times tages. Fig. 7h shows the corresponding mass-normalized
higher than the commercial Pt black. The mass-normalized current densities of Pt_5.20, Pt Pd1_5.20 and Pt black with
current densities of as-prepared Pt_0.21 (0.095 A mg ), Pt_3.11 increasing cycle number up to 3000 cycles. The mass-
3
3
ꢁ
1
ꢁ
1
ꢁ1
(
0.082 A mg ), Pt_5.20 (0.137 A mg ), Pt
3
Pd1_0.21 (0.102 normalized current densities of Pt_5.20 and Pt
3
Pd1_5.20 are all
ꢁ
1
ꢁ1
ꢁ1
A mg ), Pt
3
Pd1_3.11 (0.106 A mg ), Pt
3
Pd1_5.20 (0.189 A mg
)
much higher than commercial Pt black in the corresponding
ꢁ
1
NPs and commercial Pt black (0.075 A mg ) are also presented cycles, demonstrating the advantage of the as-prepared Pt_5.20
,
(
Fig. 7e and f). The peaks of mass-normalized current density of and Pt Pd1_5.20 catalysts in the long-term use. Thus, the
3
Pt Pd
3
and Pt_5.20 are 2.52 and 1.83 times higher than the as-prepared Pt_5.20 and Pt Pd
have superior cycling dura-
1_5.20
3
1_5.20
commercial Pt black, respectively. Both Pt_5.20 and Pt
are with similar morphology (nanoower-constructed chain commercial Pt black catalyst. The onset potential (Eonset) of the
networks), therefore one can imagine that such morphology is Pt_5.20, Pt Pd1_5.20 and the commercial Pt black for the oxidation
3
Pd1_5.20 bility for methanol electrooxidation in comparison with
3
superior to other morphologies towards methanol oxidation in of methanol is shown in Table S1 (ESI†). Generally speaking, the
our case. This can be attributed that the electrocatalytic activity much negative Eonset is more benecial for the oxidation of
of Pt depends not only on the surface-to-volume ratio, but also methanol. From the Table S1,† the Eonset of Pt Pd1_5.20 at 3000
3
on the structure or arrangement of the surface atoms. Here the cycles is lower than commercial Pt black while the Eonset of
synthesized Pt_5.20 and Pt Pd
NPs have a three-dimension Pt_5.20 is similar to the commercial Pt black. Therefore, it is clear
1_5.20
3
3
(3D) nanoower-constructed chain network structure. On one that the Pt_5.20 and Pt Pd1_5.20 have superior electrocatalytic
hand, the present 3D structure is helpful for the transport of the properties toward methanol oxidation in comparison with
reactant due to the porous characteristic in the structure. On commercial Pt black.
the other hand, each nanoower structure shows a bumpy
surface providing more catalytically active sites such as high
Conclusions
35,36
density of atomic steps, ledges and kinks.
the electrocatalytic activity of Pt Pd
The reason that
bimetallic NPs is better In this work, we proposed a very simple, green and eco-friendly
1_5.20
3
than monometallic Pt_5.20 NPs can be explained as follows. For route to synthesize “naked” Pt and PtPd alloy NPs with clean
Pt Pd1_5.20, Pt catalyzes methanol dehydrogenation into Pt–CO surfaces in acetic acid solution. The synthesis does not need any
while Pd makes water dehydrogenation into Pd–OH.
reaction between Pt–CO and Pd–OH produces CO and there- surfactants. The acetic acid itself acted as the reducing agent and
3
27,28
The additional reducing agents and structure-directing agents such as
2
fore the active metal surfaces refresh, resulting in higher elec- the structure-directing agent for the formation of the Pt and PtPd
trocatalytic activity. However, the water dehydrogenation on alloy particles. Pt and PtPd alloy NPs with various interesting
pure Pt occurs at a higher potential without Pd, which makes morphologies (well-dened polyhedrons, irregular polyhedron-
methanol oxidation process sluggish in this case. Therefore the constructed chain networks and nanoower-constructed chain
electrocatalytic activity of the synthesized Pt Pd
bimetallic networks) could be controllable synthesized by tuning the
1_5.20
3
NPs is better than the synthesized monometallic Pt_5.20 NPs. We concentration of acetic acid. Particularly, the electrochemical
further used accelerated durability tests to characterize the results showed that as-prepared Pt_5.20 and Pt Pd1_5.20 exhibit
cycling durability of the as-prepared catalysts. Here, the Pt_5.20 much higher activity and better durability than the commercial Pt
and Pt Pd1_5.20 NPs are selected for the study of catalytic dura- black for the methanol oxidation reaction, verifying their prom-
bility since Pt_5.20 and Pt Pd1_5.20 represent the best Pt catalyst ising application in fuel cell areas. The present synthesis strategy
3
3
3
among the studied Pt catalysts and the best PtPd alloy among thus provides a promising way for the synthesis of “naked” metal
the studied PtPd alloy catalysts, respectively. Fig. 7g shows the catalysts for advanced electrocatalytic applications.
ECSA-normalized current densities of the of Pt_5.20, Pt Pd
3
1_5.20
NPs with increasing cycle number up to 3000 cycles in the
solution of 1 M CH OH and 0.5 M HClO solution. For
3 4
Acknowledgements
comparison, the corresponding result of the commercial Pt This work is supported by National Natural Science Foundation
black is also included. It is clear that all the current densities of of China (Grant No. 21173127) and the Fundamental Research
Pt_5.20 and Pt Pd1_5.20 NPs are much higher than commercial Pt Funds of Shandong University (Grant No. 2015JC003).
3
This journal is © The Royal Society of Chemistry 2016
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2
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