Amorphous CuPt Alloy Nanotubes Induced by Na2S2O3 as Efficient Catalysts for the Methanol Oxidation Reaction

Article abstract of DOI:10.1021/acscatal.6b00540

Here we propose amorphous CuPt alloy hollow nanotubes as efficient catalysts for the methanol oxidation reaction (MOR) prepared by Na₂S₂O₃-assisted galvanic replacement reaction. The formation mechanism can be explained by the nanoscale Kirkendall-effect-induced hollowing process of the galvanic replacement reaction. The electrochemical tests suggest that the amorphous CuPt alloy exhibits better MOR activity and stability than the crystalline CuPt and commercial Pt/C catalysts, which can be ascribed to the enhanced CO tolerance ability of amorphous alloy. XPS measurements demonstrate that the enhanced anti-CO poison characteristic of amorphous CuPt alloy originates from the strong interaction between Pt and Cu atoms as a result of a unique crystallization state. This research not only provides a facile approach to synthesize amorphous alloy but also opens up an interesting way for amorphous Pt-based alloy to apply to the MOR.

Full text of DOI:10.1021/acscatal.6b00540

Research Article
pubs.acs.org/acscatalysis
Amorphous CuPt Alloy Nanotubes Induced by Na2S2O3 as Efficient
Catalysts for the Methanol Oxidation Reaction
Yige Zhao, Jingjun Liu,* Chenguang Liu, Feng Wang,* and Ye Song
State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for
Materials, Beijing University of Chemical Technology, Beijing 100029 China
S
* Supporting Information
ABSTRACT: Here we propose amorphous CuPt alloy hollow
nanotubes as efficient catalysts for the methanol oxidation
reaction (MOR) prepared by Na2S2O3-assisted galvanic
replacement reaction. The formation mechanism can be
explained by the nanoscale Kirkendall-effect-induced hollow-
ing process of the galvanic replacement reaction. The
electrochemical tests suggest that the amorphous CuPt alloy
exhibits better MOR activity and stability than the crystalline
CuPt and commercial Pt/C catalysts, which can be ascribed to
the enhanced CO tolerance ability of amorphous alloy. XPS
measurements demonstrate that the enhanced anti-CO poison
characteristic of amorphous CuPt alloy originates from the
strong interaction between Pt and Cu atoms as a result of a
unique crystallization state. This research not only provides a facile approach to synthesize amorphous alloy but also opens up an
interesting way for amorphous Pt-based alloy to apply to the MOR.
KEYWORDS: amorphous alloy, galvanic replacement reaction, methanol oxidation reaction, CO tolerance, bifunctional mechanism
range, which allows the fine-tuning of electronic properties to
meet the demands of highly catalytic performance.26 It has been
demonstrated that electron transfer between Pt and another
metal (M) for the Pt-based alloy is one of the major effects to
modify the adsorption energy of CO adsorbed on the Pt surface
during MOR.27 The amorphous alloys are likely to form a unique
electron structure to promote the electron transfer between Pt
and M atoms, leading to a lower electron density of Pt 5d orbital.
The lowered electron density will lessen the electron back-
donation from Pt 5d orbital to the CO 2π* orbital and thus
destabilize the Pt-CO bond, resulting in the enhanced anti-CO
poison capacity and improved MOR activity. Considering their
special properties, amorphous alloys may open up an interesting
way to gain efficient Pt-based catalysts for MOR. To our best
knowledge, amorphous Pt-based alloys applied for MOR have
rarely been researched.
As for the synthetic routes of amorphous alloys, the two
currently used methods are the rapid quenching and the chemical
reduction methods.25,28 The rapid quenching method is
technologically difficult and expensive for large-scale production
of nanomaterials because it must guarantee a cooling rate of at
least 105 to ∼106 K/s in vacuum or inert atmosphere. In terms of
the chemical reduction method, the strong and exothermic
reduction reaction probably results in serious aggregation, thus
INTRODUCTION
■
Direct methanol fuel cells (DMFCs), as important alternative
energy sources for transportation and portable electronics, have
attracted extensive interest because of their low operation
temperature, convenient storage of liquid fuel, and high energy
density.1−6 Electrocatalysts are essential components in DMFCs
because of the sluggish anode kinetics of the methanol oxidation
reaction (MOR). Although platinum has been considered as the
most common and efficient catalyst, the high cost and easy
poisoning by intermediate CO during MOR confine its practical
application.7−9 Up to now, Pt alloyed with a non-noble metal
(M) is deemed to be the most effective way to protect Pt from
CO poisoning,10−13 accompanied by reducing the costs. It is
well-documented that catalytic performances of Pt−M alloy
catalysts can be influenced by their morphology,14,15 chemical
composition,16,17 and surface structure.18,19 These factors are
strongly dependent on the atomic arrangement and coordination
related to low-coordinated atoms, steps, kinks, and vacancies,20
which are easily able to interact with reactants and act as active
sites to break chemical bonds, resulting in the excellent catalytic
performances.21,22
Amorphous alloys, with long-range disordered but short-range
ordered atomic arrangement, have more prominent catalytic
properties relative to their crystalline counterparts.23 On the one
hand, they possess abundantly unsaturated sites with high surface
energy,24 which are crucial for the activation of reactants in
catalysis to enhance the catalytic properties.25 On the other hand,
the compositions of amorphous alloys can be regulated in a wide
Received: February 22, 2016
Revised: April 24, 2016
© XXXX American Chemical Society
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Figure 1. (A) SEM image of amorphous CuPt nanotubes (Inset: TEM image of amorphous CuPt); (B) XRD patterns of amorphous CuPt, crystalline
CuPt, and Cu nanowire. The green vertical lines represent the peak positions of pure Pt (PDF no. 04-0802). The inset shows the SAED image of
amorphous CuPt; (C) STEM image of amorphous CuPt; (D−F) EDX mapping of Pt (D), Cu (E), and the composite Pt versus Cu (F). (G) Schematic
illustration for the formation of amorphous CuPt alloy NTs.
decreasing the catalytic activity. In this regard, galvanic
replacement reaction exhibits a fairly facile and mild route to
the fabrication of bimetallic nanostructures.29,30 Over the past
decades, the galvanic replacement reaction is usually applied to
synthesize nanocrystalline materials.31−33 As demonstrated in
our previous work,34 for the system involving Cu nanowires
(NWs) and a H2PtCl6 precursor without any additive, the
resulting product is crystalline CuPt alloy nanotubes (NTs). It
seems difficult to generate amorphous CuPt NTs by employing
the conventional galvanic replacement reaction.
Herein, we report the Na2S2O3-induced formation of
amorphous CuPt alloy NTs by galvanic replacement reaction
between H2PtCl6 and Cu nanowires, where Na2S2O3 serves as
the inhibitor of disproportionation reaction of CuCl. The
formation mechanism can be explained by the nanoscale
Kirkendall-effect-induced hollowing process of the galvanic
replacement reaction. The amorphous CuPt alloy exhibits better
MOR activity and stability than the crystalline CuPt and
commercial Pt/C catalysts, which can be ascribed to the
enhanced CO tolerance ability of amorphous alloy. XPS
measurements suggest that the enhanced anti-CO poison
characteristic of amorphous CuPt alloy results from the strong
interaction between Pt and Cu atoms.
EXPERIMENTAL SECTION
■
Preparation of CuPt Alloy Nanotubes. The amorphous
CuPt nanotubes were fabricated by a Na2S2O3-aided galvanic
replacement reaction in aqueous solution composed of H2PtCl6,
copper nanowires, where Na2S2O3 served as the inhibitor of
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disproportionated reaction of CuCl generated by the replace
reaction between the Cu and H2PtCl6. First, copper nanowires
with about 100 nm in diameter and several micrometers in length
were prepared through a simple chemical reduction as follows:
20 mL of copper nitrate (0.1 M) was dissolved into NaOH (10
M) aqueous solution in a round-bottom flask with stirring, and
then ethylenediamine (4 mL) and 1 mL of hydrazine (85%) were
added. When all the reactants mixed uniformly, the flask was put
in a water bath at 80 °C for 1 h. Subsequently, the sedimentary
products were filtered and washed using deionized water and
ethanol to gain the copper nanowires. Second, the copper
nanowires (64 mg) and Na2S2O3 (79 mg) were dissolved into
100 mL of N2-saturated deionized water in a three-necked flask
and dispersed by ultrasonic agitation. Afterward, the H2PtCl6
solution was added dropwise to the flask to promote the galvanic
reaction, which lasted for half an hour, producing the amorphous
CuPt nanotubes. Finally, the as-prepared CuPt nanotubes were
filtered and washed with deionized water and ethanol. For
comparison, the crystalline CuPt nanotube was also prepared by
the same method, except for no addition of Na2S2O3 in the
galvanic replacement reaction.
Physical Characterizations. The morphologies of the CuPt
alloy nanotubes were observed by scanning electron microscopy
(SEM, JEOL 6701F) and transmission electron microscopy
(TEM). The TEM and selected area electron diffraction (SAED)
characterization were performed with a JEOL TEM 2010
microscope. The structure analysis was carried out using X-ray
diffraction (XRD) with Cu Kα radiation at λ = 1.5406 Å. The
atom dispersion was explored by an aberration-corrected
scanning transmission electron microscope (STEM, JEOL
ARM 200F) and energy dispersive X-ray spectroscopy (EDX).
X-ray photoelectron spectra (XPS, ESCALAB 250) were
obtained from a monochromator (Al KR source) calibrated
compared to the C (1s) peak at 284.6 eV. The alloy compositions
were researched by inductively coupled plasma mass spectrom-
etry (ICP−MS) measurements.
Electrochemical Measurements. The MOR activities and
stabilities of the CuPt nanotubes were evaluated through cyclic
voltammetry (CV) measurements in N2-saturated 0.1 M HClO4
and 0.5 M CH3OH solution in a conventional three-electrode
system by Autolab PGSTAT30. For the electrochemical cell, a
glassy carbon (GC) electrode was used as the working electrode.
The Pt foil and saturated calomel electrode (SCE) with a salt
bridge acted as the counter electrode and reference electrode,
respectively. All potentials are calibrated to RHE. To prepare the
working electrode, 10 mg of catalysts were dispersed ultrasoni-
cally in alcohol (2 mL) and Nafion (100 μL). Afterward, 10 μL of
this above catalyst ink was pipetted onto a GC substrate (0.071
cm2). For the CO stripping tests, CO was preadsorbed on the
GC electrode coated with electro-catalyst by bubbling in the
electrolytes for 30 min while keeping the electrode potential at
−0.15 V (vs SCE) in 0.1 M HClO4. After ventilation with N2 for
another 30 min to remove any dissolved CO in the electrolyte,
the adsorbed CO was oxidized by CV at a sweep rate of 50 mV/s,
obtaining the CO stripping curve. Two consecutive CV curves
were also recorded to confirm the complete oxidation of the
adsorbed CO gas. All electrochemical measurements were
performed at room temperature and at ambient pressure.
of the Cu35Pt65 (atom %, determined by ICP−MS measurement)
prepared with the addition of Na2S2O3. We can observe a hollow
structure of the alloy with about 70−105 nm diameter, 15 nm
wall thickness. Besides, their lengths reach to several micro-
meters (Figure S1), the value of which relies on the length of
copper nanowire templates (Figure S2). As displayed in the XRD
spectrum (Figure 1B), no obvious peaks can be seen for the
sample fabricated in the presence of Na2S2O3, indicating that its
structure is amorphous. Apart from that, the selected area
electron diffraction (SAED) pattern of the amorphous specimen
in Figure 1B inset exhibits a continuous hollow ring that is a
characteristic diffraction pattern of an amorphous material,
further confirming the amorphous structure. Moreover, a
contrastive sample of crystalline CuPt alloy (Cu35Pt65, atom %)
was also prepared without Na2S2O3 to study the effect of the
additive on the crystallization state of Cu−Pt alloy. We can see
that the appearance of the sample prepared without Na2S2O3
(Figures S3 and S4A) is similar to that of the amorphous sample.
However, the XRD result of the contrastive sample (Figure 1B)
shows that it presents a typically crystalline state with a face-
centered-cubic structure, as proven by its SAED image (Figure
S4B). In addition, according to the previous report,35 a high
alloying degree contributes to the catalytic performance for the
alloy catalysts. Therefore, we studied the alloying degree of the
amorphous CuPt alloy by evaluating atomic-level scattering of Pt
and Cu atoms in the NTs using energy-dispersive X-ray
spectroscopy (EDX) in an aberration-corrected scanning
transmission electron microscope (STEM), as shown in Figure
1(C−F). The Pt versus Cu image (Figure 1F) exhibits the
uniform distribution of Pt and Cu atoms, suggesting that the
amorphous alloy NTs have a high alloying degree.
The possible formation mechanism of the amorphous CuPt
alloy NTs can be explained by the nanoscale Kirkendall-effect-
induced hollowing process of the galvanic replacement reaction
2−
between Cu NWs and PtCl6 ions with the help of Na2S2O3.
The reactions are displayed in the following equations:36
Cathode:
PtCl62− + 2e− → PtCl42− + 2Cl− (0.68 V vs SHE)
PtCl42− + 2e− → Pt + 4Cl−
(0.73 V vs SHE)
(1)
Anode:
Cu + Cl− → CuCl + e−
CuCl + xS2O3 → [CuCl(S2O3)x]2x−
(0.137 V vs SHE)
2−
(2)
Overall:
4Cu + 4xS2O3 + PtCl6
→ Pt + 4[CuCl(S2O3)x]2x− + 2Cl−
2−
2−
(3)
It is well-known that the disproportionated reaction of CuCl
(2CuCl → Cu + Cu2+ + 2Cl−) easily happens at room
temperature. However, Na2S2O3, as a complexing agent for
CuCl, can restrain its disproportionation, making the anode
reaction (eq 2) occur. Since the standard reduction potential of
PtCl62−/Pt pairs (0.68 V) is significantly more positive than that
of Cu/CuCl (0.137 V), the overall reaction (eq 3) will take place
spontaneously. On the basis of the stoichiometric relationship in
eq 3, the deposition of one Pt atom will consume four Cu atoms,
which leads to redundant lattice vacancies on the surface of Cu
RESULTS AND DISCUSSION
■
Characterizations of Amorphous CuPt NTs. Figure 1A
and its inset show the scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) images, respectively,
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Figure 2. (A) CV curves in N2-saturated 0.1 M HClO4 and 0.5 M CH3OH solution at a scan rate of 10 mV/s at room temperature; (B) onset potentials;
(C−E) long-term stability measurements: CV curves of (C) amorphous CuPt, (D) crystalline CuPt, and (E) commercial Pt/C before and after 1000
cycles; (F) The loss values of Jf after 1000 cycles.
NWs template because less Pt atoms cannot compensate the
vacancies left by Cu atoms. However, for the system without
Na2S2O3, the overall reaction is shown below34.
vacancies to balance the diffusivity difference.38 As the reaction
time increases, the vacancies inside the NWs will reach saturation
and condense to form Kirkendall voids.39 Subsequently, these
voids tend to grow and collapse in the center, until the interior
material disappears, eventually leaving amorphous Pt−Cu alloy
NTs behind (Figure 1G).
2Cu + PtCl62− → Pt + 2Cu2+ + 6Cl−
(4)
We can see that the generation of one Pt atom only requires two
Cu atoms. Therefore, for the reaction system with Na2S2O3,
there will be more lattice vacancies, resulting in higher surface
free energy. In this case, the atoms are inclined to rearrange
randomly because the energy of amorphous structure is lower
than that of crystalline structure with lots of defeats.37 In other
words, the amorphous structure is more favorable than the
crystalline structure in this case. As the reaction proceeds
continuously, vast Cu atoms are consumed and the high surface
energy will trigger the accelerated outward diffusion of Cu atoms
from the interior to the surface. Meanwhile, Pt atoms will keep on
depositing on the surface and diffuse inward to form the Pt−Cu
alloy. As a consequence, a total outward Cu atoms flux from the
interior to the surface occurs, accompanying the inward flux of
Electrocatalytic Measurements. The catalytic activity of
amorphous CuPt alloy (Cu35Pt65, at%) toward MOR was studied
by a three-electrode system in a N2-purged 0.1 M HClO4 and 0.5
M CH3OH solution at a scan rate of 10 mV/s, and the samples of
crystalline CuPt alloy (Cu35Pt65, at%) and a commercial Pt/C (E-
TEK, 40 wt %) were also tested for comparison. The current
densities are normalized by the geometric area of the electrode
(0.071 cm2). From the resultant cyclic voltammetry (CV) curves
shown in Figure 2A, it can be seen that the onset potential
decreased in the sequence: amorphous CuPt > crystalline CuPt >
Pt/C. The amorphous CuPt exhibits a marked negative shift of
21 mV and 25 mV with respect to the crystalline CuPt and
commercial Pt/C (Figure 2B), respectively. Moreover, the
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Figure 3. (A−C) CO stripping voltammograms; (D) onset potentials for CO oxidation; (E) schematic illustration of the CO tolerance abilities for the
amorphous CuPt, crystalline CuPt, and commercial Pt/C catalysts.
amorphous CuPt has the maximum forward peak current density
(Jf) values (7.8 mA.cm−2, Figure S5), higher than the
corresponding values of the crystalline CuPt (7.1 mA.cm−2)
and Pt/C (6.7 mA·cm−2). If only considering the mass of Pt, the
mass activities (Jf values normalized to Pt loading amount, Figure
S6) of the amorphous alloy (373.7 mA.mg−1Pt) is still higher than
that of the crystalline CuPt (340.1 mA·mg−1Pt) and Pt/C (281.8
mA·mg−1Pt). These outcomes suggest that the amorphous CuPt
catalyst exhibits remarkably enhanced activity toward MOR,
compared with the crystalline CuPt and Pt/C catalysts. Besides,
the stabilities of the above catalysts were estimated by potential
cycling at a scan rate of 10 mV/s before and after 1000 cycles,
while the intermediate process (1−1000 cycles) was performed
at a scan rate of 200 mV/s to accelerate the reaction. The
obtained CV curves before and after 1000 cycles are shown in
Figure 2(C−E). It can be observed that the amorphous CuPt
shows a least loss of Jf (9.6%) and exhibits the highest stability
among these catalysts (Figure 2F). To compare our catalysts with
the commercial Pt−Ru catalyst, we further performed the MOR
measurement for the commercial Pt−Ru/C catalyst (Johnson
Matthey), as shown in Figure S7. We can observe that our
amorphous CuPt catalyst has relatively poor MOR activity
compared with the Pt−Ru alloy. The reason why the amorphous
catalyst does not exhibit more excellent activity than the
commercial Pt−Ru alloy may be that the present alloy
composition (Cu35Pt65) is not optimal. It will be our further
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Figure 4. (A) XPS spectra showing Pt 4f peaks; (B) fitted Pt(0) 4f7/2 core-level binding energies; (C) schematic illustration of the mechanism of
enhanced anti-CO poison property.
investigation work to further improve the MOR activity of the
amorphous CuPt catalyst by adjusting the atomic ratio of Pt and
Cu atoms. To eliminate the carbon effect, we also performed the
MOR measurement for the Pt black catalyst, as shown in Figure
S8. We can observe that our amorphous CuPt catalyst has higher
forward peak current than the Pt black catalyst, indicating that
our amorphous CuPt catalyst still has better MOR activity than
the Pt black catalyst.
Moreover, we performed the CO stripping cyclic voltammetry
(CV) for the three catalysts in 0.1 M HClO4 solution with a
sweep rate of 50 mV/s, as shown in Figure 3A−C). For the three
catalysts, the hydrogen desorption peaks were completely
suppressed in the first positive scan, which was due to the
saturated coverage of COads on the active surface sites. A broad
peak corresponding to CO anodic oxidation appeared in the
potential range from 0.7 to 1.1 V. In the reverse sweep, the
hydrogen adsorption peak arose since the active sites were freed
after the oxidation removal of CO. Along with the oxidation
removal of adsorbed CO, the hydrogen adsorption/desorption
peaks reappeared in the second scan.40 Significantly, we can see
that the CO anodic oxidation on the amorphous CuPt alloy (0.72
V) starts much earlier than that on the crystalline CuPt (0.76 V)
and Pt/C catalysts (0.79 V), as shown in Figure 3D. This result
indicates that the amorphous CuPt catalyst has the weakest CO
adsorption and best CO tolerance ability among the three
catalysts (Figure 3E), leading to the highest MOR activity and
stability. To further investigate the CO tolerance ability of the
amorphous CuPt alloy, we also performed the CO stripping
cyclic voltammetry (CV) for the commercial Pt−Ru/C catalyst
(Johnson Matthey) to make a comparison with the amorphous
alloy, as shown in Figure S9. We can see that the CO anodic
oxidation on commercial Pt−Ru/C catalyst starts earlier (0.50
V) than that on the amorphous CuPt alloy (0.72 V), indicating
that the commercial Pt−Ru/C catalyst has a stronger CO
oxidation and better CO tolerance abilities than our amorphous
CuPt alloy. As shown in Figure S7, the Pt−Ru catalyst displays
better MOR activity compared to the amorphous CuPt catalyst.
It reveals that the avoidance for CO poisoning is mainly
responsible for the MOR activity for the Pt-based catalysts. This
conclusion has been verified by previous published papers.4,7,8,41
As well-known, the activity of alloy catalyst strongly depends on
their compositions. So, if the CO tolerance ability of the alloy is
further improved by adjusting the atomic ratio of Pt and Cu
atoms, the MOR activity of the amorphous would be further
improved. If so, the amorphous CuPt alloy will have promising
commercial value.
Origin of Enhanced CO Tolerance. To gain further insight
into the mechanism of enhanced anti-CO ability for the
amorphous CuPt alloy, X-ray photoelectron spectra (XPS)
experiments were carried out to give detailed analysis of the
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Pt − OH2 → Pt − OH + H+ + e−
Pt − CO + Pt − OH → 2Pt + CO2 + H+ + e−
surface electronic state. The obtained Pt 4f core-level binding
energy is shown in Figure 4A. It can be seen that Pt has diverse
valence states such as Pt0 (metallic Pt) and Pt2 (PtO and
Pt(OH)2) in these catalysts. By comparing their relative
intensities, we conclude that Pt0 dominates among the
components because it can offer more active sites for MOR
than Pt.3,42 Thus, the Pt0 binding energies represented by Pt0
4f7/2 are compared and depicted in Figure 4B. We can observe
that the Pt0 4f7/2 peak of the amorphous CuPt shifts to a higher
binding energy (71.6 eV) in contrast to that of crystalline CuPt
(71.3 eV) and Pt/C (70.9 eV) catalysts. Owing to the higher
electronegativity of Pt (2.28) than that of Cu (1.90), Pt atoms
will withdraw the electrons from Cu in their alloy.43 In general,
the increased electron density around Pt would cause downshift
of Pt 4f binding energy in its alloys.8 However, in our case, the
observed Pt 4f binding energy of the amorphous alloy is
upshifted as compared to that of the other Pt-based catalysts. The
upshift of Pt 4f binding energy in the amorphous CuPt alloy can
be explained by the difference in work function of Pt and CuPt
alloy and the rehybridization of both d-band and sp-band in the
CuPt alloy, according to the previous report.44 Because of Pt
alloying with Cu, the change of its work function leads to the
upshift of the reference level, that is, the Fermi energy (EF), in
photoelectron measurements.45,46 This will result in the upshift
of Pt 4f binding energy in the amorphous alloy. In addition,
Mukerjee et al. revealed that the total electron number per Pt
atom increased, but the Pt 5d electron decreased, when Pt was
alloyed with a second metal like Cu.47 Therefore, in our
amorphous alloy, although Pt withdraws electron from Cu, Pt 5d
electron still decreased. That is, Pt atom in the amorphous alloy
still has more 5d vacancies. According to the reported
literature,48 the core-levels and valence bands of the metal are
more sensitive to the variations of the d electron occupancy than
to those of the s, p orbitals. Therefore, the loss of Pt 5d electrons
in amorphous alloy may also lead to the positive shifts of Pt 4f
binding energies. Like an Au−Al alloy system with different
electronegativity,48 the loss of 5d electrons at the Pt site is
perhaps due to the valence electron redistribution caused by
intra-[Pt(5d) → Pt(6s, 6p)] orbital hybridization, as Pt is alloyed
with Cu. As reported, the formation of the Pt−CO bond depends
on the electron donation from the 5σ orbital of CO to the empty
Pt d orbital and the back-donation from the filled Pt d orbital to
the 2π* orbital of CO.49 In our work, although the amorphous
CuPt has more 5d vacancies, the Pt atoms in the amorphous Pt−
Cu alloy have withdrawn some electrons from Cu, making the
total electrons increase, which possibly makes the adsorption of
CO as electron-donating species more difficult. Moreover, it has
been proposed that the back-donation will strengthen the Pt-CO
bond by removing the Pt excess negative charge.50 As for the
amorphous CuPt, it has more 5d orbital vacancies and a less filled
5d orbital, which reduces the back-donation and weakens the
stability of Pt-CO bond (Figure 4C). More importantly,
according to the previous report,49 the Pt atom with less d
electron density may be highly active for the oxidation of the
adsorbed water described by eq 5. Considering the so-called “bi-
functional mechanism” (eq 6) proposed by Watanabe and
Motoo,51 the formative Pt−OH can contribute to the removal of
the CO intermediate (Figure 4C). On the basis of the above
reasons, we can conclude that the unique interaction between Pt
and Cu atoms in the amorphous CuPt alloy contributes to the
enhanced ability of CO removal, improving the MOR activity
and stability.
(5)
(6)
CONCLUSION
In summary, we have fabricated amorphous CuPt alloy NTs by
Na2S2O3-assisted galvanic replacement reaction between PtCl6
■
2−
and Cu NWs. The possible formation mechanism of the
amorphous CuPt alloy can be explained by the nanoscale
Kirkendall-effect-induced hollowing process of the galvanic
replacement reaction. As a result, the amorphous CuPt alloy
catalyst exhibits better MOR activity and stability than the
crystalline CuPt and commercial Pt/C. The outstanding
performance of amorphous CuPt catalyst is due to its enhanced
anti-CO poison characteristic, which can be ascribed to the
strong interaction between Pt and Cu atoms as a result of special
electronic state. This work not only provides a facile approach to
fabricate amorphous CuPt alloy with improved electrochemical
performance but also opens a new direction for amorphous Pt-
based alloy to apply to MOR.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.6b00540.
■
S
SEM, TEM, SAED images; CV curves for MOR; CO
stripping test (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: liujingjun@mail.buct.edu.cn. Tel: +86-10-64411301.
*E-mail: wangf@mail.buct.edu.cn. Tel: +86-10-64451996.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was financially supported by the National Natural
Science Funds of China (Grant No. 50972003, 51125007).
■
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