Highly active PtAu nanowire networks for formic acid oxidation

Article abstract of DOI:10.1002/cplu.201402061

The PtAu alloy nanowire networks (NWNs) were synthesized directly in an aqueous solution using Triton X-114 as the structure-inducing agent. The NMNs formed based on the oriented-attachment growth mechanism, and they exhibited dramatically enhanced electr

Full text of DOI:10.1002/cplu.201402061

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DOI: 10.1002/cplu.201402061
Highly Active PtAu Nanowire Networks for Formic Acid
Oxidation
[a, c]
[d]
[a, c]
[a, c]
[b]
[a]
Meiling Xiao,
Songtao Li, Jianbing Zhu,
Kui Li,
Changpeng Liu,* and Wei Xing*
The PtAu alloy nanowire networks (NWNs) were synthesized di-
rectly in an aqueous solution using Triton X-114 as the struc-
ture-inducing agent. The NMNs formed based on the oriented-
attachment growth mechanism, and they exhibited dramatical-
ly enhanced electrocatalytic activity for formic acid electrooxi-
dation compared with commercial Pt black. Besides the ensem-
ble effect, the advantages of the unique structure, including
superior electrical conductivity and faster electron-transfer for
the electrochemical process, were regarded as the contributors
to the superior catalytic activity of the as-prepared PtAu
NWNs.
Introduction
Controllable synthesis of metal-containing nanomaterials with
tunable morphology and composition has attracted considera-
ble attention, because nanostructures with different crystal sur-
faces or sizes may possess different physical and chemical
were synthesized using seed-growth methodology and
[5]
showed high electrocatalytic activity. Nevertheless, a rapid
and facial synthesis of Pt-based alloy nanowires with controlla-
ble composition and morphology is highly desirable but not
realized so far.
[
1]
properties. Among various nanostructures, nanowires exhibit
distinct advantages in electrocatalysis because the one-dimen-
sional nanowires can act as self-support electrocatalysts, which
favor the mass-transfer and avoids support corrosion-induced
For formic acid electrooxidation (FAEO), Pt-based nanocrys-
tals have shown some shortcomings such as high cost and low
[6]
activity. Generally, FAEO on Pt-based catalysts proceeds
through a dual path mechanism that involves a direct dehy-
[
2]
activity loss. Particularly, a prominent example is Pt and Pt-
based nanowires, which have been extensively investigated
because of their unique catalytic performance towards organic
small molecules electrooxidation, such as formic acid and
[7]
drogenation pathway and an indirect dehydration pathway.
The strong adsorption of COads on Pt surface in the indirect
pathway will occupy its active sites and leads to poisoning Pt
catalysts. Therefore, it will be necessary to promote the direct
electrooxidation of formic acid and consequently enhance the
catalytic activity and stability for FAEO on the Pt surface. Inter-
rupting Pt neighboring sites by introducing foreign metal into
Pt is believed to be an effective approach to prevent the indi-
rect pathway because the dehydration pathway with the for-
mation of COads requires at least three contiguous Pt sites for
FAEO, whereas the direct pathway on the Pt surface requires
[
3]
methanol. Ultrathin Pt nanowires were prepared in the pres-
ence of Triton X-114, and exhibited enhanced activity and sta-
bility compared with commercial Pt/C for methanol electrooxi-
[
4]
[2b]
dation. Zhu et al. synthesized PdPt alloy nanowires using
Te nanowires as a sacrificial template and investigated their
electrocatalytic activity for ethanol and methanol oxidation re-
action (MOR) in alkaline medium. PtAu and PtNiAu nanowires
[8]
[
a] M. Xiao, J. Zhu, K. Li, Prof. Dr. W. Xing
State Key Laboratory of Electroanalytical Chemistry
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences, Changchun, Jilin 130022 (P. R. China)
Fax: (+86)431-85685653
only one Pt site. Among all these Pt-based alloys, PtAu has
received much attention owing to its good activity and stabili-
[8c,9]
[9e]
ty.
For example, Zhang and co-workers prepared PtAu
alloy nanoparticles by the coreduction of metal precursors and
the obtained catalysts exhibited high electrocatalytic activity
E-mail: xingwei@ciac.jl.cn
[10]
[b] Prof. Dr. C. Liu
owing to the ensemble effect for FAEO. Yin et al. successfully
improved direct electrooxidation of formic acid on PtAu alloy
catalyst by heat treatment. Using graphene as the support,
PtAu alloy nanoparticles were synthesized with a maximum
Laboratory of Advanced Power Sources
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences, Changchun, Jilin 130022 (P. R. China)
Fax: (+86)431 856856
ꢀ
2
E-mail: liuchp@ciac.jl.cn
power density of 185 mWcm , which is 3.5 times more than
[9d]
[c] M. Xiao, J. Zhu, K. Li
that of commercial Pt/C at 333 K. Obviously, it is reasonable
to expect high catalytic performance of PtAu alloy nanowires
for FAEO because of the advantage in both composition and
nanostructure.
Graduate University of Chinese Academy of Sciences
Beijing 100039 (P. R. China)
[d] S. Li
Institute of Mathematics
Herein, we developed a facile and effective wet chemical
route for the controllable fabrication of PtAu nanowire net-
works (NWNs), in which the atomic ration of Pt/Au was desira-
Jilin University, Changchun, Jilin 130022 (P. R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201402061.
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bly tuned by variation in the feeding solution. The PtAu NWNs,
having different compositions, exhibited obviously enhanced
electrocatalytic performance compared to commercial Pt black
catalyst. The promotion effect increased with the increase of
Au atom fraction as a result of the inhibition of indirect path-
way for FAEO. The possible reasons for the enhanced electro-
catalytic behavior of the PtAu NWNs were investigated system-
atically.
Results and Discussion
Figure 1 shows TEM and HRTEM images of the as-prepared
PtAu NWNs. From Figure 1A–C, one can clearly see that high-
quality NWNs have been formed throughout the whole area of
observation. This was achieved without the presence of sepa-
rated nanoparticles. The Pt/Au atomic ratio of Pt Au NWNs
1
1
was further determined by inductively coupled plasma (ICP)
analysis. The ratio was also 1.01:1, thus confirming nearly
Figure 2. TEM images of Pt
A) 1 s, B) 1 min, C) 10 min, and D) 30 min.
1
Au
6
nanostructures formed at different reaction:
1
00% yield of Pt Au NWNs, the same was for the other PtAu
1 1
particles (Figure 2B). As the reac-
tion time increased to 10 min,
nanowires were fully formed,
and the nanoparticles could be
no longer observed (Figure 2C).
Increasing the reaction time to
30 min, no further change in the
morphology can be observed,
thus indicating that the steady
state has been reached with the
formation of nanowires. The
morphological change from
sphere to nanowire with the re-
action time and the existence of
many kinks and grains strongly
suggest that the PtAu NWNs
form through an oriented at-
[4]
tachment mechanism.
The XRD patterns are shown
in Figure 3. All the PtAu NWNs
show a typical fcc structure, fur-
1 1 1 3 1 6
Figure 1. A–C) TEM and D–F) HRTEM images for Pt Au , Pt Au , and Pt Au NWNs, respectively.
NWNs. With the increase in Au content, the diameter of PtAu
NWNs increased from 3.4 nm to 4.9 nm for Pt Au NWNs and
1
1
Pt Au NWNs, respectively. Figure 1D depicts the HRTEM image
1
6
of Pt Au , and shows clear lattice fringes with an interfringe
1
1
distance of approximately 0.231 nm, which corresponds to the
crystal lattice of the (111) planes of face-centered cubic (fcc)
PtAu alloy. Similarly, clear lattice fringes can be observed in Fig-
ure 1E, F with a slightly greater distance. The TEM images of
Pt Au NPs are presented in Figure S1 in the Supporting Infor-
1
6
mation.
To study the growth process of the nanowire networks, in-
termediates were collected at different time intervals. As
shown in Figure 2A, at the reaction time of 1 s, only nanoparti-
ꢀ
cles were formed owing to the coreduction of AuCl and
4
2ꢀ
PtCl6 by NaBH . By extending the reaction time to 1 min,
4
nanowires began to appear together with the elongated nano-
Figure 3. XRD patterns of PtAu NWNs with different Pt/Au atomic ratios.
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ther indicating a high crystallinity of these nanocrystals. It is
found that each diffraction peak of the samples is located in
the position between the corresponding Au and Pt peaks, indi-
cating the formation of PtAu alloy structure, and the peak po-
sitions shift towards lower 2q angles with the increase of Au
fraction.
Then XPS measurements were carried out to probe the elec-
tron effect of Au on Pt and the corresponding spectra are pre-
sented in Figure S2. For Pt Au NWNs, the binding energies of
1
1
Pt 4f7/2 and 4f5/2 peaks were almost invariable compared with
that for monometallic Pt. As Au content was further increased,
the Pt 4f7/2 peaks negatively shift by 0.4 eV for Pt Au NWNs
1
3
and 0.45 eV for Pt Au NWNs, respectively, as shown in Fig-
1
6
ure S2B and Figure S2C. This shows the electronic modification
of Au on Pt, which originated from the electron transfer from
[
11]
Au to Pt. The atomic ratios of Pt/Au on surface calculated
from the XPS spectra are 0.98:1, 1:3.07, and 1:6.11 for Pt Au ,
1
1
Pt Au , and Pt Au NWNs, respectively. These ratios are nearly
1
3
1
6
consistent with the ratios of the precursors. The atomic ratios
of Pt/Au were also estimated on the basis of ICP analysis to be
1
.01:1, 1:3.09 ,and 5.85:1 for Pt Au , Pt Au , and Pt Au NWNs,
1 1 1 3 1 6
respectively, and these values are very close to the XPS analy-
sis. The results demonstrate that Pt and Au can be reduced si-
multaneously by the strong reducing agent NaBH to form uni-
4
form alloy phase. Similar phenomenon can be found in other
[
12]
reports in the literature.
Cyclic voltammograms of different electrocatalysts in 0.5m
Figure 5. Forward voltammetric curves of A) mass activity and B) specific ac-
H SO are presented in Figure 4. Typical hydrogen adsorption/
2
4
ꢀ
1
tivity for each catalyst for FAEO at a scan rate of 50 mVs
.
desorption behavior can be clearly observed on these catalysts.
The characteristic reduction peaks of Pt and Au oxides are de-
ꢀ
1
5
0 mVs . Figure 5A,B compares the anodic current densities
of the PtAu NWNs and commercial Pt black catalyst, which was
normalized to Pt loading on the electrode and the electro-
chemical active specific surface area (ECSA) of Pt, respectively.
The first peak (doted p1), located between 0.2 and 0.3 V, is at-
tributed to the direct oxidation of HCOOH to CO via the dehy-
2
drogenation pathway, whereas the second peak (doted p2) lo-
cated between 0.6 and 0.7 V, is associated with the oxidation
of the COads generated from the dissociative adsorption step
[9f]
via the indirect pathway. The mass activities of the p1 on the
PtAu NWNs are much higher than those of the commercial Pt
black. Especially, Pt Au NWNs, possess the highest mass activi-
1
6
ꢀ
1
ty of 1886 mAmg
which is about 59 times higher than
Pt,
Figure 4. CVs of different catalysts in 0.5m H
2
SO
4
solution purged with N
2
at
commercial Pt black. Moreover, the peak potential of HCOOH
ꢀ
1
a scan rate of 50 mVs
.
oxidation via the direct pathway on Pt Au6 NWNs demon-
1
strates a negative shift of 65 mV in contrast with that on Pt
black. These results indicate that all the PtAu NWNs are more
active for HCOOH oxidation than Pt black. In addition, the rela-
tected clearly at around 0.4 V and 0.9 V for the PtAu alloy
NWNs. The relative surface Au content of each catalyst can be
[11]
compared by the area of the reduction peak of Au oxides. It
can be seen that a larger Au fraction in the feeding solution
will result in a higher Au content on the surface of the PtAu
NWNs. Meanwhile, the reduction peak of Pt oxide for PtAu
NWNs shifts to a negative position compared with commercial
Pt black, thus further reflecting the electronic modification
effect of Au on Pt.
tive ratio of current densities from the two peaks (I /I ) deter-
p1 p2
mines which pathway is dominant during FAEO and conse-
quently indicates the degree of tolerance with respect to CO
[13]
poisoning. The much higher values of Ip1/Ip2 observed at the
PtAu NWNs compared with Pt black demonstrate that the de-
hydrogenation path is promoted significantly on these PtAu
NWNs and the direct electrooxidation of HCOOH can be fur-
ther improved by increasing the Au fraction. Detailed electro-
chemical data for these catalysts were listed in Table S1.
The electrocatalytic activities of the catalysts were examined
in a solution of 0.5m H SO +0.5m HCOOH at a scan rate of
2
4
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The enhancement of formic acid electrooxidation on these
PtAu NWNs could be attributed to their unique structure,
which favors the mass transfer and charge transfer therefore
improving the elctrocatalytic activity. To confirm this proposal,
Pt Au NPs were tested for comparison. As shown in Figure 6,
1
6
the Pt Au6 NWNs still exhibit 1.99-fold higher mass activity
1
Figure 6. Forward voltammetric curves of mass activity for different catalysts
ꢀ
1
2 4
in a solution of 0.5m H SO +0.5m HCOOH at a scan rate of 50 mVs .
than Pt Au NPs. The value of I /I for Pt Au NWNs is also
1
6
p1 p2
1
6
higher than that for Pt Au NPs, indicating that nanowire struc-
1
6
ture can promote the direct electrooxidation of HCOOH more
effectively. Figure S3 shows the Nyquist spectra of Pt Au6
Figure 7. LSVs of each catalyst: A) Pt Au NWNs; B) Pt Au NPs in 0.5m
1
1
6
1
6
NWNs and Pt Au NPs. It can be observed that compared with
H
2
SO
4 3 2
+1m CH OH solution purged with N with different scan rates of 50,
1
6
ꢀ
1
75, 100, 150, 200, and 250 mVs . The inset shows the linear relationship be-
PtAu NPs, PtAu alloy with the network structure demonstrates
tween peak current and the square root of the scan rate.
[
14]
superior electrical conductivity. To further explore the merits
of the unique structure, the linear sweep voltammetric (LSV)
experiments of catalysts for the FAEO are shown in Figure 7
with different scan rates of 50, 75, 100, 150, 200, and
a contributor to the higher activity of PtAu NWNs relative to Pt
black.
ꢀ
1
250 mVs , respectively, to characterize the electron-transfer
Besides the unique 3D structure, other possible reasons for
the enhanced electrocatalytic activity for FAEO were also inves-
tigated. Generally, ensemble effects and electronic effects are
applied for the improvement in the catalytic performance for
FAEO. For ensemble effects, foreign metal introduced into Pt
can interrupt Pt neighboring sites, thus preventing the dehy-
dration pathway for FEAO. The above electrochemical results
reflect that the enhanced electrochemical properties of these
PtAu NWNs are most likely due to the ensemble effect. For
electronic effects, the modified electronic structure of Pt by
foreign metal will lower the CO adsorption energy on Pt sur-
face, thus facilitating the removal of COads. However, the nega-
tive shift of Pt 4f core level for PtAu catalysts from the XPS re-
sults suggests the increase of the Fermi level, which leads to
a larger contribution of back donation of Pt 5d electrons to
process on the two catalysts. The curves of the peak current
versus the square root of scan rates are inserted in the corre-
sponding figures. The obtained linear relationship is attributed
to a diffusion-controlled process during the electrooxidation.
The relationship between the peak current and the square
[
15]
root of scan rates complies with Equation (1):
5
0
1=2
1=2 1=2
i ¼ 2:99 ꢁ 10 nðan Þ AC D0
v
ð1Þ
n
1
Where i is the peak current, n the electron-transfer number
p
for the total reaction, n’ the electron transfer number in the
rate-determining step, a the electron transfer coefficient of the
rate-determining step, A the electrode surface area, C the
1
bulk concentration of the reactant, D the diffusion coefficient,
o
[17]
n the potential scan rate. In the same electrolyte and reaction,
the 2p* orbital of CO, and an increased CO coverage. The
result indicated that the incorporation of Au in Pt will make
the removal of CO more difficult and decrease the CO toler-
ance of the Pt catalyst. To clearly demonstrate the ability of
COads removal on PtAu NWNs, CO-stripping experiments were
carried out and the results are presented in Figure S4. It can
be observed that both the onset and peak potentials for CO
oxidation on the PtAu NWNs are much higher than those on
the parameters n, C , and D are the same. Thus, the slope of
1
o
[16]
ip versus the square root of scan rates is decided by an’. The
corresponding slopes for Pt Au NWNs and Pt Au NPs are 69.1
1
6
1
6
and 22.7, respectively. This result implies that the unique struc-
ture favors electron transfer during electrooxidation process,
resulting in the improved electrocatalytic activity. The above
results strongly confirm that the nanowire network structure is
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the Pt black. The result confirms that the modified electronic
structure of Pt with Au could not facilitate the removal of
COads. Based on the above results, it can be stated that the su-
perior activity of PtAu NWNs is due to the suppressed indirect
pathway and the promoted direct pathway of FAEO caused by
ensemble effect rather than electronic effect.
cess of the PtAu NWNs, products were collected at 1 s, 1 min,
1
0 min, and 30 min. The procedure was under ambient tempera-
ture. For comparison, Pt Au nanoparticles (NPs) were synthesized
1
6
by using polyvinyl pyrrolidone (PVP) as a capping agent. Then
.143 mm H PtCl , 0.857 mm HAuCl and 50 mg PVP were dissolved
0
2
6
4
in 50 mL deionized water. After stirring for 1 h, a freshly prepared
solution of NaBH was added dropwise, followed by stirring for an-
4
Figure 8 shows the ECSAs of different catalysts estimated
using the electric charges accumulated during the CO strip-
ping. All the PtAu NWNs exhibit higher ECSA than Pt black and
other 1 h. The black nanocrystal dispersions were then centrifuged,
washed several times with ethanol, and dispersed in ethanol for
use in further experiments.
2
ꢀ1
the Pt Au6 NWNs possess the highest ECSA of 74.8 m g ,
1
Transmission electron microscopy (TEM) and high-resolution
[
12a]
which is comparable with that of commercial Pt/C.
A higher
(HRTEM) images were obtained on a Philips TECNAI G2 electron
ECSA may be another reason for the improved catalytic activity
for FAEO.
microscope operating at 200 kV. The bulk composition of catalyst
was evaluated by both inductively coupled plasma optical emis-
sion spectrometer (X Series 2, Thermo Scientific USA). X-ray diffrac-
tion (XRD) measurements were performed with a PW-1700 diffrac-
tometer using a Cu K (l=1.5405 ꢁ) radiation source (Philips Co.).
a
X-ray photoelectron spectroscopy (XPS) measurements were car-
ried out on MgKa radiation source (Kratos XSAM-800 spectrome-
ter).
Electrochemical measurements were carried out with an EG&G
mode 273 potentiostat/galvanostat and a conventional three-elec-
trode test cell. Working electrodes were prepared by mixing the
catalyst with Nafion and ethanol. The mixture was sonicated and
about 10.0 mL was applied onto a glassy carbon disk (diameter=
4
mm). A Pt foil and a saturated calomel electrode (SCE) were used
as the counter and the reference electrodes, respectively. All of the
potentials are relative to the SCE electrode, unless otherwise
noted. To activate and clean the catalyst surface, the working elec-
trodes were potentially cycled from ꢀ0.2 V and 1.2 V at a scan rate
ꢀ
1
of 50 mVs in a 0.5m H SO solution until a stable response was
2
4
Figure 8. Specific ECSAs for different catalysts.
obtained. The electrochemical impedance spectra (EIS) were re-
corded at 10 points per decade over the frequency range from
1
00 kHz to 10 mHz in 0.5m H SO solution at 0 V. The amplitude of
2 4
the sinusoidal potential signal was 5 mV. To evaluate the activity of
the catalysts for formic acid electrooxidation, the cyclic voltamme-
Conclusion
In summary, a series of composition-controllable PtAu NWNs
with dramatically enhanced catalytic activity for FAEO were
synthesized by a facile, one-pot synthetic approach. The
growth process was investigated, revealing the nanowire is
formed through an oriented attachment mechanism. It was
discovered that the Au atom ratio played an important role in
the promotion process. Ensemble effects and the unique struc-
ture were the main reasons resulted in the superior activity for
FAEO. This study provides prospects for rational design of
nanostructured electrocatalysts for commercial application.
try (CV) experiments were carried out in 0.5m H SO +0.5m
2 4
HCOOH solution at a scan rate of 50 mVs . For CO stripping vol-
ꢀ1
tammetric experiments, CO was absorbed at 0.02 V in 0.5m H SO
2
4
solution for 10 min, excess CO in the electrolyte was then purged
out with N2 for 10 min, and then two first cycles recorded at
ꢀ1
5
0 mVs . The electrochemical active specific surface area (ECSA)
of Pt was estimated assuming that the coulombic charge necessary
for oxidation of a monolayer of linearly adsorbed CO was
ꢀ
2 [18]
420 mCcm
.
To investigate the electron transfer on the catalysts,
the linear sweep voltammetric (LSV) experiments at different scan
rates were conducted.
Experimental Section
PtAu alloy NWNs with different Pt/Au atomic ratios (1:1; 1:3; 1:6)
were fabricated in this study, denoted as Pt Au , Pt Au , and Pt Au
6
NWNs, respectively. For a typical synthesis of PtAu NWNs with an
Acknowledgements
1
1
1
3
1
atomic ratio of Pt/Au=1:1, an aqueous solution that contained
This study was supported by the High Technology Research Pro-
gram (863 program, no. 2012AA053401) of the Science and Tech-
nology Ministry of China, National Basic Research Program of
China (973 Program, nos. 2012CB932800 and 2012CB215500),
National Natural Science Foundation of China (21073180,
ꢀ1
H PtCl (0.5 mm), HAuCl (0.5 mm), and Triton X-114 (1 gL ) were
2
6
4
added into a three-necked, round-bottomed flask (100 mL), and
the total volume was adjusted to 50 mL. Then the mixture was
stirred for 1 h. Subsequently, 5 mL of freshly-prepared solution
containing 185 mg NaBH was quickly injected into the flask. After
4
21011130027), the Science & Technology Research Programs of
stirring for 10 min, the black nanocrystal dispersions were then
centrifuged, washed several times with ethanol and dispersed in
ethanol for use in further experiments. To study the growth pro-
Jilin Province (20100420), and the Recruitment Program of For-
eign Experts (WQ20122200077).
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Sun, Chem. Commun. 2010, 46, 4252–4254; c) N. Kristian, Y. S. Yan, X.
Wang, Chem. Commun. 2008, 353–355.
Keywords: ensemble effects · formic acid · nanostructures ·
oxidation · PtAu
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