Surface-Roughened Pt-Decorated Pd Nanoparticles as Efficient Electrocatalysts for Direct Alcohol Fuel Cells

Article abstract of DOI:10.1002/ejic.201800600

Platinum-decorated Pd nanoparticles (NPs) of Pd_100–xPt_x (x = 30, 40 and 50) are synthesised by galvanic replacement of Ag by Pt using a Pd@Ag core–shell structure as a template. The Pd@Ag core–shell material is prepared by chemical reduction of Ag⁺ over Pd NPs, with further Ag being galvanically replaced by Pt to form Pd_100–xPt_x NPs. The Pd_100–xPt_x NPs are characterised using optical absorption spectroscopy with surface plasmon resonance (SPR), transmission electron microscopy (TEM) and high resolution TEM (HRTEM). The NPs are adsorbed onto vulcan carbon (Pd_100–xPt_x/C) for further characterisation and electrochemical studies. The materials are characterised using powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV) studies. The CV studies indicate the absence of silver in the synthesised materials. The electrochemical oxidation of methanol and ethanol are carried out by cyclic voltammetry studies. The results show that Pd_100–xPt_x/C materials are good catalysts for methanol oxidation reactions (MORs) and ethanol oxidation reactions (EORs), with lower oxidation potentials and higher mass activities than commercial Pt/C catalysts. Among these materials, Pd₅₀Pt₅₀/C has a methanol oxidation potential of 0.18 V, with good mass activity, and the Pd₇₀Pt₃₀/C material is an efficient EOR catalyst, with higher mass activity than commercial Pt/C catalysts.

Full text of DOI:10.1002/ejic.201800600

A Journal of
Accepted Article
Title: Surface roughened Pt decorated Pd nanoparticle as efficient
electrocatalyst for direct alcohol fuel cell
Authors: N Prabu, D Jeyakumar, G Maduraiveeran, and M Sasidharan
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201800600
Link to VoR: http://dx.doi.org/10.1002/ejic.201800600

European Journal of Inorganic Chemistry
10.1002/ejic.201800600
Surface roughened Pt decorated Pd nanoparticle as efficient
electrocatalyst for direct alcohol fuel cell
[a],[b],*
[a]
[b]
Natarajan Prabu
, Duraisamy Jeyakumar , Govindhan Maduraiveeran ,
[b]
Manickam Sasidharan ,
Keywords:
Electrocatalyst, mass activity, electrochemical oxidation, galvanic replacement, cyclic
voltammetry.
Abstract: Platinum decorated Pd nanoparticles (NPs) of Pd100–xPt (‘x’ = 30, 40 and 50) were
x
synthesized by galvanic replacement of Ag by Pt using Pd@Ag core-shell as template.
+
Pd@Ag core-shell material was prepared by chemical-reduction of Ag over Pd NPs further
Ag was galvanically replaced by Pt to form Pd100–xPt NPs. Pd100–xPt NPs were characterized
x
x
using optical absorption spectroscopy with surface plasmon resonance (SPR), transmission
electron microscopy (TEM) and high resolution TEM (HR-TEM). The NPs were adsorbed on
vulcan carbon (Pd100–xPt /C) for further characterization and electrochemical studies. The
x
materials were characterized using Powder X-ray diffraction (XRD), X-ray photoelectron
spectroscopy (XPS) and cyclic voltammetry (CV) studies. The CV studies indicate the
absence of silver in the synthesized materials. The electrochemical oxidation of methanol and
ethanol were carried out by cyclic voltammetry studies. The results found that Pd100–xPt /C
x
materials are good catalyst for methanol oxidation reaction (MOR) and ethanol oxidation
reaction (EOR) with lower oxidation potential and higher mass activity than commercial Pt/C
catalyst. Among these materials Pd Pt /C was found to have methanol oxidation potential of
5
0
50
0
.18 V with good mass activity and Pd Pt /C material as efficient EOR catalyst with higher
70 30
mass activity than commercial Pt/C catalyst.
[a] Dr. N. Prabu, Dr. D. Jeyakumar
Functional Materials Division, CSIR-Central Electro Chemical Research Institute,
Karaikudi, Tamil Nadu-630 006 India
Dr. N. Prabu (Corresponding author) E-mail: prabuak5@gmail.com
prabu.n@res.srmuniv.ac.in www.cecri.res.in
[b] Dr. G. Maduraiveeran, Dr. M. Sasidharan
Research Institute & Department of Chemistry, SRM Institute of Science and
Technology, Chennai, Tamil Nadu- 603 203, India. www.srmuniv.ac.in
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European Journal of Inorganic Chemistry
10.1002/ejic.201800600
Introduction
Direct alcohol fuel cell (DAFC) is one of the promising power source technology for
[
1–3]
portable electronic devices and electric vehicles.
Precious metal based catalysts have been
received much attention on the direct-electrocatalytic oxidation of methanol and ethanol
[4]
under acid medium in the energy related applications. Among the all the materials
developed for methanol oxidation reaction (MOR) and ethanol oxidation reaction (EOR),
[
5–10]
platinum (Pt) based materials are still the best electrocatalyst.
However, abundance of Pt
on earth’s crust is scarce and costly. Thus it is essential to reduce the quantity of Pt utility in
fuel cells. It is already reported that palladium based nanostructured materials offered
[11]
moderate catalytic activity for various electrochemical reactions in fuel cells.
There are
few reports on the usage of Pd NPs for direct methanol and ethanol fuel cells (DMFCs and
[
12–14]
DEFCs).
Srabanti Ghosh and co-workers have reported Pd nanowires and porous Pd nanoballs
connected with three-dimensional Pd nanowires as the electrocatalysts for EOR and they
[9,10]
appeared as promising candidates for fuel cell applications.
In spite of these successful
demonstrations, the high mass activity with low electrochemical oxidation potential for
[
12,13]
methanol and ethanol oxidation on Pd surface is still not satisfactory.
Therefore, the
efforts have been devoted to control the morphology and composition of Pd-Pt based
bimetallic nanostructures to offer the enhanced catalytic performance and lower utilization of
[15–21]
amount of Pt.
Hence, it is highly desirable to develop the efficient electrocatalyst of Pd-
Pt NPs for MOR and EOR under acid electrolyte with high activity and durability.
The structural morphology of Pd-Pt nanomaterials may be engineered as the core-shell
structures (Pd@Pt), Pt decorated Pd nanoparticles, Pd-Pt nanoporous materials, etc. The Pt-
based bimetallic catalysts not only inherit the properties of Pt constituent but can also display
superior catalytic performance when compared with their counterparts due to its synergistic
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European Journal of Inorganic Chemistry
10.1002/ejic.201800600
[
22–28]
effect.
Among various bimetallic nanostructures, Pt-M NPs (M = Au, Pd, Ag, Cu, Mn,
Fe and Ni), Pt-Pd systems have received considerable attention because of their high catalytic
[
29–33]
activity and durability.
Recently, Pt-based core/shell nanocrystals have been found to be
[
26,34]
promising catalysts owing to their improved catalytic activity and durability.
The
core/shell structure with thin Pt shell, not only maximizes Pt exposure and minimizes the
amount of Pt in catalysis, but also offers the desired core-shell interactions to tune both
[
27,29]
electronic and surface strain effects.
One of the problems of using bimetallic catalyst was high cathodic over potential for
[
35]
MOR that reduces the performance of DMFC.
To resolve the problem of high alcohol
oxidation overpotential, it is necessary to develop a special catalyst that will minimize the
alcohol oxidation overpotential and also to improve the performance of the catalytic activity.
Literature survey has shown that the Pd-Pt nanostructures demonstrated better activity in
[
29]
DMFC and DEFC. The formation and growth of Pd-Pt nanostructures plays a crucial role
in the electrochemical performance of the catalysts. Hence, this investigation describes the
facile synthesis of Pt decorated Pd NPs (Pd100-xPt NPs, where ‘x’ = 30, 40, 50) in the average
x
particle size of 3 to 7 nm with Pt excess in the surface. The as-synthesized Pd100-xPt NPs are
x
effectively used for the potential MOR and EOR under acid electrolyte.
Results and discussion
x
Physico-chemical characterisation of Pd100-xPt nanoparticles.
The formation of Pd NPs was primarily confirmed by observing the colour change to
pale brown from pale yellow in colour of Pd(NO ) in sodium citrate upon reduction with
3
2
[36]
NaBH4.
As shown in Figure 1A,B(a), there was no specific surface plasmon resonance
[37]
(
SPR) absorption band for Pd NPs. Silver shell was formed over Pd NPs using AgNO as a
3
precursor which is reduced by NaBH under similar experimental conditions. The colour of
4
(
Pd@Ag) Pd100-xAg (‘x’ = 30, 40 & 50) NPs was found to be dark brown in colour and the
2x
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European Journal of Inorganic Chemistry
10.1002/ejic.201800600
colour was increased with increasing the Ag content. The Ag shell formed over Pd NPs
exhibited a broad SPR peak in the range of 350 to 400 nm (Figure 1A (b)), showing the
successful formation of Pd@Ag core-shell NPs (Pd100-xAg ). With increasing ‘x’ values, the
2x
SPR improved marginally as displayed in (Figure 1B (b)). It is reported in the literature that
[38]
mono-Ag NPs or bi-metallic Pd-Ag alloy NPs exhibited a strong and sharp SPR peak.
The Ag in the Pd-Ag NPs was galvanically replaced by Pt using K PtCl solution and the
2
4
colloidal NPs solution colour was changed into black in colour, revealed the formation of Pt
NPs on Pd NPs. Pt NPs with size less than 20 nm do not exhibit any specific SPR absorption
[
39]
band in the visible region.
The Pd100–xPt NPs exhibited no significant SPR peak in the
x
range of ~300 – 400 nm was shown in Figure 1A (c) and 1B (c) due to the replacement of Ag
by Pt. However, the optical density (OD) of the optical absorption spectra was increased,
suggested the enhanced scattering core-shell nanoparticles by forming the surface roughness
of the sample which was caused by the galvanic replacement of two Ag atoms by single Pt
atom as shown in the Eqn. (1)
2
+
+
Pd100-xAg + xPt → Pd
Pt + 2Ag
---- (1)
2x
100-x
x
Figure 1. Optical absorption spectra of Pd100-xPt NPs with different ‘x’ values of (A) 30 &
x
(
B) 40, in that (a) Pd NPs, (b) Pd100-xAg NPs, (c) Pd
Pt NPs.
100-x x
2x
The absorption peak for K PtCl at ~262 nm was almost diminished due to the ascription
2
4
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European Journal of Inorganic Chemistry
10.1002/ejic.201800600
of successful galvanic replacement of Ag by Pt completely in Pd100-xAg NPs, indicating the
2x
2
+
absence of unreduced Pt ions in the solution.
Figure 2. TEM images of (A) Pd Ag NPs and (B) Pd Pt NPs.
5
0
100
50 50
Figure 2 shows the TEM image of Pd Ag and Pd Pt NPs. The Pd-Ag nanoparticles
5
0
100
50 50
presented the worm-like elongated morphology and some definite spherical shaped particles
were randomly dispersed in the dimension range of 3.0-5.0 nm as presented in Figure 2A.
The morphology of NPs was similar for all values of ‘x’ when Ag was replaced by Pt. Figure
2
B displays the TEM image of Pd Pt NPs, which showed spherical in shape and the
50 50
extensive worm-like structure was destroyed after replacement of Ag. These Pd100-xAg and
2x
Pd100-xPt NPs were evaluated further using HR-TEM analysis for calculating the‘d’ values
x
from lattice fringes.
The as-formed Pd Pt NPs was analysed by HR-TEM elemental mapping to confirm
5
0
50
the surface availability of Pd and Pt atoms that was shown in supporting information figure
S1. The elemental mapping confirms that surface has maximum of Pt than Pd and some
places are empty(white in color) in the Pd-Pt image that confirms the roughened place or
voids created during galvanic replacement reaction of Ag by Pt. Figure 3 displays the HR-
TEM images of Pd Ag and Pd Pt NPs, showed the crystalline nature. The d-spacing of
5
0
100
50 50
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European Journal of Inorganic Chemistry
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the Pd100-xAg NPs are both twinned and individual particles and indexed as (111) plane of
2x
Ag. The Pd100-xPt NPs was calculated and indexed as (111) plane of Pt and the‘d’ values are
x
2
.36 and 2.26 Å for Pd100-xAg and Pt
Pt respectively. The value 2.36 Å corresponds to
100-x x
2x
[
35,40]
the (111) plane of Ag (2.359 Å) and 2.26 Å to that of Pt.
Hence, it is clear that the Pd100-
Ag NPs possessed Ag on the surface and while Ag was replaced by Pt in Pd100-xAg , the
x
2x
2x
surface was enriched with Pt.
Figure 3. HR-TEM images of (A) Pd Ag NPs and (B) Pd Pt NPs.
5
0
100
50 50
Figure 4 depicts the XRD pattern of Pd100-xAg /C materials. The diffraction lines at
2x
around 40.11, 46.66 and 68.08 correspond to (111), (200) and (220) lines of Pd (ICDD
No. 00-05-0681). The obtained XRD peaks were found to shift towards the Ag lines (ICDD
No. 00-04-0783) with increasing ‘x’ values and the XRD lines for Ag appear at around 38.11
(111), 44.27 (200) and 64.42 (220) degrees. Generally, this type of shift in the XRD peaks
[
41]
was attributed to the formation of nano-alloys. However, recent observation of Kummel et.
[
35]
al. and our studies indicate that nanomaterials do not obey Vegard’s law. It is understood
that for nano-alloys, core-shell structures, decorated metal NPs, the shift of the XRD lines
was towards the second component composition. There is no split up of XRD lines, unless
there is distinct phase segregation. Hence, in the present case, the shift in the XRD lines
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European Journal of Inorganic Chemistry
10.1002/ejic.201800600
towards the Ag with increasing Ag concentration, indicating the formation of Ag shell on Pd
core to form Pd100-xAg NPs.
2x
Figure 4. Powder XRD patterns of Pd100-xAg /C materials with different ‘x’ values of (a) 30,
2x
(b) 40, (c) 50, respectively (# XRD lines from carbon material).
Figure 5 represents the XRD patterns of the Pd100-xPt /C materials. The Pd
Pt /C
100-x x
x
materials exhibited the lattice constant ‘a’ values of 3.889 and 3.923 Å (ICDD No.
5-0681 and 04-802 for fcc of Pd and Pt, respectively. These materials demonstrated the
0
broad XRD patterns with FWHM values more than 2.0 degrees. Generally, Pt NPs exhibited
the XRD lines at around 39.76 (111) and 44.63 (200), and Pd NPs showed at around 40.11
(
111) and 46.65 (200) peaks. The other peaks observed at around 68.08, 82.09, and 86.59
are minor peaks, corresponded to (220), (311) and (222) planes, respectively. As in the case
of Pd100-xAg NPs and Pd Pt NPs, XRD lines are between that of Pd and Pt materials.
2x
100-x
x
With increase in ‘x’ values the lines shifted towards that of Pt, confirmed the Pt decoration on
Pd surface. The XRD lines of carbon material in Pd100-xAg /C and Pd Pt /C remained at
2x
100-x
x
2
4.61 at the same place.
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European Journal of Inorganic Chemistry
10.1002/ejic.201800600
Figure 5. Powder XRD patterns of Pd100-xPt /C materials with different ‘x’ values of (a) 30,
x
(b) 40, (c) 50, respectively (# XRD lines of carbon material).
The crystallite size of the Pd100-xPt /C material was calculated to be 2.5, 3.0 and 3.8 nm
x
for ‘x’ = 30, 40 and 50 respectively, based on the XRD patterns using Debye-Scherer’s
formula. These values are well-agreed with the values obtained from HR-TEM images. The
lattice parameter was also calculated. It can be seen that ‘a’ value increases with an increase
in the ‘x’ value, but the lattice parameter values does not matches with Pd or Pt lattice
parameter values. The values are 3.9416, 3.9674 and 3.9979 of ‘x’ = 30, 40 and 50,
respectively and the reason for the higher ‘a’ value was lattice contraction in the materials
due to Pt decoration on Pd surface.
XPS spectra corresponding to the 3d levels of Pd and 4f levels of Pt in Pd100-xPt /C
x
(‘x’= 30, 40 and 50) were measured. The peaks were deconvulated using Gaussian-
2
Lorentzian curve fitting and the χ values were <3.0. In the case of Pd 3d levels (Figure 6A),
there are two sets of peaks for Pd 3d levels (335.96 and 341.36 eV) and (337.12 and 342.52
[
37,42]
eV)
. The bulk Pd 3d levels exhibited at around (335.40 and 341.10 eV) and (336.90 and
[
38,42]
0
2+
3
42.41eV)
. These values correspond to Pd and Pd-O (Pd ) in Pd
Pt /C materials,
100-x x
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European Journal of Inorganic Chemistry
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0
the first set of peak corresponds to the 3d3/2 and 3d5/2 level of Pd and the second set of peak
correspond to 3d3/2 and 3d5/2 level of Pd-O. With increasing ‘x’ value, peak position was
shifted towards lower binding energy values. The peak area ratios of these two peaks were
calculated for different ‘x’ values. The increase in ‘x’ value, the amount of Pd in Pd100-xPt /C
x
decreases as the surface is enriched with Pt which may be due to the relatively higher
2
+
0
abundance of Pd for ‘x’ = 50. The peak area ratio of Pd and Pd-O remarkably changed with
change in ‘x’ values. The Pt-O peak area was found to increase with increasing ‘x’ values.
0
For ‘x’= 50 Pd and Pd-O peak areas were almost equal, showed that the surface has equal
percentage of Pt and Pt-O as clearly shown in Figure 6.
Figure 6. XPS of (A) Pd100-xAg /C and (B) Pd
Pt /C with different ‘x’ values of (a) 30,
100-x x
2x
(b) 40 and (c) 50.
In the case of Pt 4f levels (Fig. 6B), there were three sets of peaks for Pt 4f levels
0
2+
(
72.21 and 75.81 eV), (73.08 and 76.68 eV) and (74.53 and 78.13), corresponding to Pt , Pt
4+
[43–46]
and Pt in Pd
Pt /C material
. The first set of peaks corresponds to 4f7/2 and 4f5/2
1
00-x
x
0
level of Pt and second set of peaks corresponds to 4f and 4f level of Pt-O and third set of
7
/2
5/2
4
+
0
peaks corresponds to 4f7/2 and 4f level of Pt . The peak area for Pt was increased (‘x’ =
5/2
4
4
0) and it was decreased (‘x’ = 50). Also the peak area of Pt(II) and Pt(IV) increased for ‘x’ =
0 and then decreased for ‘x’= 50 composition. The Pt 4f levels showed that the surface has
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European Journal of Inorganic Chemistry
10.1002/ejic.201800600
more oxides when compared to metallic state of Pt where ‘x’ value increases both Pd and Pt
surface have more percentage of its oxides than its metal content.
Electrochemical characterization of Pd100-xPt /C materials
x
The cyclic voltammetric curves (CVs) of the as-formed Pd100–xPt /C materials (‘x’ = 30, 40
x
and 50) coated on a GC electrode recorded in 1.0 N H SO and are presented in Figure 7.
2
4
Figure 7. Cyclic Voltammogram of Pd Pt /C recorded in 1.0 N H SO .
5
0
50
2
4
As presented in Figure 7, the Pd Pt /C electrode exhibited the surface oxidation of Pd
5
0
50
and Pt which were found at ~0.1 V vs. MMS onwards as a wave and the surface oxides of Pd
[47]
and Pt. These peaks were reduced during the reverse scan at ~-0.045V vs MMS.
surface Pd-O and Pt-O during anodic scan were reduced almost around the same potential,
with in 10 mV) and the peaks are merged together to form a broad peak. In addition, Pt
exhibited hydrogen adsorption-desorption peaks at around -0.5 V vs MMS as shown in Figure
. It is observed that the cathodic processes of Pd and Pt were inseparable and it was difficult
The
(
7
to evaluate the electrochemically active surface area (ECASA) of these two individual
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European Journal of Inorganic Chemistry
10.1002/ejic.201800600
elements. Hence, the ECSA was calculated from the total peak area of the reverse scan that
is Pt-O and Pd-O reduction peak from the bare graph. With increase in ‘x’ value the reduction
peak current was also increased subsequently as shown in Figure 8.
Figure 8. CV curves of Pd100-xPt /C coated on GC electrode in 1.0 N H SO using MMS as
x
2
4
the reference electrode for different ‘x’ values (a) 30, (b) 40, (c) 50.
Electrochemical oxidation of methanol on Pd100-xPt /C
x
The electrocatalytic activities of the Pd100-xPt materials are evaluated towards the
x
methanol and ethanol oxidation reactions. Figure 9 shows the CV curves of the Pd100-xPt /C
x
electrode recorded for 0.5 M methanol in 1.0 N H SO . The methanol oxidation reaction
2
4
(
MOR) takes place at around 0.180 V during the forward scan and during reverse scan
oxidation of carbonaceous species takes place at around 0.080 V vs MMS reference electrode
at the Pd100-xPt /C electrode. In Pd Pt /C materials, the ratio of forward peak current (i )
x
100-x
x
f
and reverse peak current (i ) ratio was found to be ~1.0. The peak current was normalised
b
with total active surface area of Pd and Pt calculated from the respective Pd-O and Pt-O
redox peak area before methanol addition. The maximum peak current of 820 mA was
achieved at Pd Pt /C material at 0.180 V for 0.5 M methanol at room temperature. The
5
0
50
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European Journal of Inorganic Chemistry
10.1002/ejic.201800600
Pd100-xPt /C material showed methanol oxidation potential more cathodic than commercial
x
[48]
catalyst of Pt/C and Pt-Ru/C
for MOR, steering its efficiency towards MOR.
Figure 9. CV curves of the Pd100-xPt /C coated GC electrode recorded for 0.5 M methanol in
x
1
.0 N H SO with different ‘x’ values (a) 30, (b) 40, & (c) 50 normalised with total active
2 4
surface area of Pd and Pt.
The CV curves of the Pd Pt /C coated GC electrode recorded for MOR with different
5
0
50
concentration of methanol varied from 165 mM to 984 mM was shown in Figure 10. The
MOR peak current at around 0.2 V vs. MMS was increased with increasing the concentration
of methanol in the electrolyte solution, suggested the steady-state catalytic current. The peak
position of MOR for 0.1 M methanol concentration was around 0.18 V and it was slightly
shifted to higher potentials with increasing concentration of methanol. The catalyst stability
for methanol oxidation reaction was evaluated using chronoamperometry studies @ 0.186 V
vs. MMS reference electrode with 0.5 M ethanol and 1.0 N H SO as background electrolyte
2
4
in room temperature. The chrono studies also support that Pd Pt /C has maximum stability
50
50
than Pd Pt /C and Pd Pt /C materials as shown in supporting information figure S2.
70
30
60 40
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European Journal of Inorganic Chemistry
10.1002/ejic.201800600
Figure 10. MOR CV curves of Pd Pt /C coated GC electrode in 1.0 N H SO with different
5
0
50
2
4
concentration of methanol (a) 165 mM, (b) 328 mM, (c) 492mM, (d) 656 mM, (e) 820 mM,
f) 984 mM (peak current was normalized with Pd & Pt ECSA).
(
Figure 11. MOR CV curves of the Pd100-xPt /C coated GC electrode recorded for
x
0
.5 M methanol in 1.0 N H SO with different ‘x’ values (a) 30, (b) 40 & (c) 50.
2 4
The MOR mass activity of Pd100-xPt /C was calculated based on the CV results of Figure
x
1
1. The methanol oxidation peak current was normalised with total metal weight present in
the material. The mass activity was increased with increasing ‘x’ values. The Pd Pt /C
5
0
50
material was achieved the maximum mass activity of ~47 A/mg of metal. The reverse peak
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European Journal of Inorganic Chemistry
10.1002/ejic.201800600
current was increased and the if/ib ratio was less than one, showed that reverse peak current
was more in Pd Pt /C material.
5
0
50
Electrochemical oxidation of ethanol on Pd100-xPt /C
x
The ethanol oxidation reaction (The catalyst stability for methanol oxidation reaction
was evaluated using chronoamperometry studies @ 0.183 V vs. MMS reference electrode
with 0.5 M methanol and 1.0 N H SO as background electrolyte in room temperature. EOR
2
4
of Pd100-xPt /C coated GC electrode was evaluated using 0.5 M ethanol in 1.0 N H SO as
x
2
4
background electrolyte and are shown in Figure 12.
Figure 12. CV curves of the Pd100-xPt /C coated GC electrode recorded for
x
0
.5 M ethanol in 1.0 N H SO with different ‘x’ values (a) 30, (b) 40, (c) 50.
2 4
The ethanol oxidation first peak obtained at around 0.270 V vs. MMS corresponded to
the conversion of ethanol to acetic acid and the second peak at around 0.629 V vs. MMS was
due to the further oxidation of ethanol to carbonaceous species. The total of four electrons is
involved in this reaction. The maximum mass activity of 35 A/mg was attained for Pd Pt /C
7
0
30
material. The anodic peak current was found to decrease with increasing ‘x’ values. The
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European Journal of Inorganic Chemistry
10.1002/ejic.201800600
decrease in magnitude of peak current revealed the excess Pt on the surface of Pd Pt /C and
60
40
Pd Pt /C was reduced the EOR activity. It is pertinent to note that Au
Pt /C exhibited
100-x x
50
50
similar EOR activity. The conversion of C H OH to complete mineralization ca., CO and
2
5
2
H O does not take place as evidenced by the electrochemical oxidation of acetaldehyde and
2
acetic acid on their materials as well. The CV data indicates that Pd Pt /C material exhibits
7
0
30
good activity for EOR. The catalyst stability for ethanol oxidation reaction was evaluated
using chronoamperometry studies @ 0.253 V vs. MMS reference electrode with 0.5 M
ethanol and 1.0 N H SO as background electrolyte in room temperature. The chrono studies
2
4
also support that Pd Pt /C has maximum stability than Pd Pt /C and Pd Pt /C materials
7
0
30
60 40
50 50
as shown in supporting information figure S3.
Conclusion
In summary, we have synthesized a series of Pd decorated Pt nanoparticles (Pd100–xPtx
NPs, where ‘x’ = 30, 40 and 50) using galvanic replacement reaction by H PtCl using Pd
100-
2
4
Ag as the template and application in alcohol oxidation reactions. The Ag decorated on Pd
x
2x
NPs (Pd100-xAg NPs) was synthesized using facile chemical reduction approach. The optical
2x
spectral studies of Pd100–xPt NPs primarily confirmed the Pt decoration on the surface of Pd
x
which was not physical mixture of Pd and Pt NPs. The attained surface roughened Pd100-xPtx
NPs were further confirmed by HRTEM images due to the galvanically replaced two Ag
atoms with one Pt atom. The XPS studies also revealed that the higher abundance of Pd (II),
Pt (II) and Pt (IV) states was found in the Pd100–xPt NPs. It is found that the surface
x
availability of Pt (II) states immensely supported the enhancement of the electrochemical
activity towards the methanol and ethanol oxidation reactions. The Pd Pt /C exhibited an
5
0
50
improved MOR activity with higher mass activity and low overpotential than that of the
commercial Hi-spec Pt/C material. In the EOR study, the Pd Pt /C demonstrated a better
70
30
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European Journal of Inorganic Chemistry
10.1002/ejic.201800600
activity when compared to commercial Pt/C and other catalysts investigated in the present
study.
Experimental
Materials and characterization
Potassium tetrachloroplatinate(II) (99.99%), potassium tetrachloropaladate(II) (99.99%),
palladium nitrate dihydrate (40% Pd basis), sodium borohydride (NaBH -98%), Nafion 117
4
solution in low aliphatic alcohols (5%) were purchased from Sigma-Aldrich India Pvt. Ltd.,
India and used as received. Trisodium citrate (AR grade), silver nitrate (AR grade), sulphuric
acid (AR grade), methanol (AR grade), ethanol (AR grade), were procured from Sisco
Research Laboratories Pvt. Ltd., India. Millipore water was used for all the preparative work.
Vulcan XC-72 was used as a supporting matrix for the Pd100-xPt catalyst.
x
The formation of the NPs was confirmed by optical absorption spectroscopy of Agilent
diode array spectrometer. The size and shape of the NPs are characterized by HR-TEM
2
analysis of FEI make with Technai 30 G model. The colloidal NPs were adsorbed on the
carbon (Vulcan XC-72) in the ratio of 1:4 of metal and carbon. The carbon supported
materials were further characterized by X-ray diffraction (XRD) of Bruker D8 advanced
Diffractometer with Cu K (1.5406 Å) as source and X-ray Photoelectron Spectroscopy
(
XPS) of Thermo Fisher (MultiLab 2000) spectrometer, fitted with twin anode X-ray source
using Mg Kα radiation (1253.6 eV).
Synthesis of Pd100-xPt nanoparticles
x
Palladium NPs were initially prepared using chemical reduction method. In typical, 9.2
mg (28.0 µM) of Pd(NO ) precursor was prepared in 100 ml pure water. 1.5 ml of 1% tri-
3
2
sodium citrate was slowly added to the reaction precursor. The reaction mixture was stirred
vigorously and 1.0 ml of freshly prepared 0.15% of NaBH was added by drop wise. The
4
resulting solution was turned to light brown in colour from pale yellow, which was kept for 3
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European Journal of Inorganic Chemistry
10.1002/ejic.201800600
h under vigorous stirring. The as-prepared colloidal Pd NPs was further used for the
preparation of Pd@Ag NPs.
A 3.48 mg (20.52 µmole) of AgNO was dissolved in 1.0 ml of pure water and was
3
added to the Pd NPs. 1.5 ml of 1% tri-sodium citrate was added to the Pd NPs colloidal
solution mixture followed by 1.0 ml of 0.05% freshly prepared NaBH was slowly added to
4
the reaction mixture under vigorous stirring. The reaction mixture was turned from pale
brown to dark brown in colour, and it was continuously stirred for another 30 min. The
formation of Ag NPs was initially confirmed by a change in colour of the solution. The
prepared Pd@Ag NPs was kept overnight at room temperature. Subsequently, 4.25 mg (10.26
µmole) of K PtCl dissolved in 1 ml of water was added to the colloidal solution of Pd@Ag
2
4
NPs and the resulting solution was continuously stirred for 3 h to form Pd@Pt NPs. The
Pd@Pt NPs was supported on carbon (Vulcan XC-72) matrix for the metal loading of 20
wt%, these carbon supported materials are used for further characterisation. The Pd100-xPtx
NPs were dissolved in aqua regia. After evaporating the excess acid, the contents were
dissolved in dilute HNO and analysed by inductively coupled plasma mass spectrometry
3
(
ICP-MS) and the ‘x’ values were calculated from the data.
Electrochemical Characterization of Pd@Pt/C
The electrochemical experiments were done by glassy carbon (GC) modified electrodes
using Pd@Pt/C materials. The carbon supported catalyst was dispersed in 250 µl of water and
sonicated for 5 min, after that 50 µl of Nafion® (5 wt%) solution was added and sonicated for
1
0 min to form homogenous ink solution. This solution (5 µl) was dropcasted on the glassy
2
carbon (GC) surface (3 mm dia/0.07 cm geometric area) and dried at room temperature for 1
h. This Pd100-xPt /C material coated GC electrode was used for the electrochemical
x
characterizations and the alcohol oxidation reactions.
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European Journal of Inorganic Chemistry
10.1002/ejic.201800600
Acknowledgements:
One of the authors N. Prabu thank Council of Scientific and Industrial Research (CSIR), New
Delhi, for the financial support of Senior Research Fellowship to carry out this work and N. P
thanks to Central Electro Chemical Research Institute (CECRI) for giving opportunity to
carry out the research work.
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