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Cite this: J. Mater. Chem. A, 2015, 3,
2050
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Nanoporous twinned PtPd with highly catalytic
activity and stability†
Xuefeng Zhang,a Pengfei Guan,b Lidija Malic,a Michel Trudeau,c Federico Roseid
and Teodor Veres*a
Growing needs for highly efficient energy storage devices have prompted increasing research efforts in
energy-efficient and sustainable materials. In this context, nanoporous noble metals have been studied
extensively because of their extraordinary properties. However, existing electrochemical/chemical
dealloying approaches for their synthesis largely lack the ability to optimize their structure/function
relationships. To overcome this limitation, we developed a thermal-decomposition strategy for the
synthesis of component-controllable nanoporous PtPd alloys composed of 2 nm sawtooth-like
ligaments induced by a high density of twinning boundaries (boundary spacing 1 nm). Such twinned
Received 17th November 2014
Accepted 27th November 2014
and ultrathin ligaments exhibit large curvatures between concave and convex regions, associated with
abundant low-coordination surface atomic steps and kinks. These low-coordination atoms are sites of
high catalytic activity, as confirmed by theoretical simulations. The optimized Pt25Pd75 sample exhibits
DOI: 10.1039/c4ta06250g
the best catalytic performance among all the currently reported catalysts, and has a mass activity of 1110
www.rsc.org/MaterialsA
mA mg
1
1
Pt
and high stability for the electro-oxidation of methanol.
Introduction
Direct methanol fuel cells (DMFC) that exhibit high energy
density have attracted considerable interest for portable energy
storage applications. However, during the electro-oxidation
process, methanol is dehydrogenated into adsorbed carbonaceous intermediates on Pt atoms,1 which heavily affect the
catalytic capacity by the formation of strong Pt–CO bonds that
lead to the degradation of catalytic activity. These issues have
been addressed by combining Pt with other transition metals in
various forms, known as the bi-functional catalytic mechanism,
which is responsible for accelerating the oxidation and removal
of carbonaceous molecules generated in the process.1–12 For
example, Pd can weaken the Pt–CO bond and promote the
oxidation process of CO to CO2 via water dehydrogenation, i.e.
the formation of Pd–OH. By adjusting the atomic ratio between
Pt and Pd, the degradation and dehydrogenation processes of
methanol can be coordinated effectively.7,8,12
a
National Research Council of Canada, 75 Boul. de Mortagne, Boucherville, Qu´ebec,
J4B 6Y4, Canada. E-mail: [email protected]
b
Department of Materials Science and Engineering, Johns Hopkins University,
Baltimore, MD 21218, USA
c
Materials Science, Hydro-Qu´ebec Research Institute, 1800 Boul. Lionel-Boulet,
Varennes, Qu´ebec, J3X 1S1, Canada
d
´ nergie, Mat´eriaux et T´el´ecommunications, 1650 Lionel Boulet,
INRS Centre E
Varennes, Qu´ebec, J3X 1S2, Canada
† Electronic supplementary information (ESI) available: Materials and methods,
Fig. S1 to S10, Tables S1 to S3, supplementary references (1–26). See DOI:
10.1039/c4ta06250g
2050 | J. Mater. Chem. A, 2015, 3, 2050–2056
Conventional routes for improving catalytic activity, either by
reducing particle size13,14 or by constructing high-index facets15
have proven effective, but they are structurally unstable when
downsized to 5 nm or smaller. Therefore, the agglomeration
and coarsening of catalysts cannot remain a desirable stability
during reaction processes. To enhance the catalytic stability,
porous structures, providing 3-dimentional skeleton support,
been achieved, which are usually synthesized by chemical/
electrochemical dealloying procedures.3,7,8,10,16–19
Results and discussion
Here we describe a thermal-decomposition strategy for the
synthesis of highly miscible PtPd nanostructures composed of
ultrathin ligaments with 2–5 nm diameter having a high density
(boundary spacing 1 nm) of twinning boundaries that induce
signicant surface steps and kinks. The composition of PtPd
nanoporous samples can be rationally adjusted by changing the
raw material ratio of palladium(II) chloride (PdCl2) and platinum(II) acetylacetonate [Pt(acac)2], as detailed in the ESI.† The
structures of Pt, PtPd, and Pd samples were characterized using
bright-eld scanning transmission electron microscopy (BFSTEM) and high angle annular dark eld STEM (HAADF-STEM).
The HAADF-STEM image in Fig. 1a represents the mass
contrast, whereby regions with higher-density and/or heavier
elements are imaged with brighter contrast. From Fig. 1a, a
typical nanoporous Pt25Pd75 sample structure has an average
size of 50–100 nm in the form of a cluster with a mean ligament
diameter of 2.3 0.6 nm. The three-dimensional Pt25Pd75
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Fig. 2 Comparisons of Pt and Pd compositions in products and
precursors. (a) Atomic ratio. (b) Mass ratio.
Nanoporous PtPd structures. (a) HAADF-STEM image of a
Pt25Pd75 cluster with the three-dimensional nanoporous architecture
and ligaments 2–5 nm in diameter. Scale bar, 20 nm. The inset in a
indicates that the initial growth occurs along with [111] direction. Scale
bar, 5 nm. (b and c) HAADF-STEM images and the corresponding EDX
maps of a representative Pt25Pd75 cluster and a sub-branch of Pt25Pd75
grown at the tip. Scale bars, 20 nm and 3 nm. (d) XRD patterns of Pt, Pd
and PtPd samples with various compositions. The different half-height
width and positions of the (111) plane diffraction peaks show a
composition-dependent grain size and lattice parameter. (e) Ligament
diameter (from STEM statistics) and grain size (calculated by the (111)
plane based on Scherrer's equation), indicating that the smallest grain/
ligament size is in composition range of 20–60 at% Pt.
Fig. 1
structure is composed of randomly oriented branches, in which
the central brighter regions correspond to the overlapped ligaments. The yield of these nanostructures in the nal product
reaches 100% by statistical analysis using STEM.
The composition distribution of Pt and Pd was visualized
through a sequence of annular dark-eld (ADF) images and Xray energy-dispersive (EDX) maps, which presents the microstructural bimetallic interfaces. As shown in Fig. 1b, the
distribution of Pd and Pt of the Pt25Pd75 sample throughout the
entire domain of a single cluster is clearly observed and shows a
slight uctuation of the composition ratio. Further magnication and focusing of the EDX analysis on the growth tip of a
branch (Fig. 1c) demonstrates that Pd and Pt are highly
miscible.
By adjusting the raw material ratio of PdCl2 and Pt(acac)2, we
synthesized the following compositions of nanoporous structures: Pt8Pd92, Pt14Pd86, Pt25Pd75, Pt43Pd57, Pt58Pd42, as well as
polyhedral Pt nanoparticles. The composition ratios of Pt versus
Pd for all PtPd samples were determined by elemental analysis
using energy-dispersive X-ray spectroscopy (ESI Table S1, and
Fig. S1–S6†). Comparisons of Pt and Pd compositions in products and precursors shown in Fig. 2a and b conrm that there is
almost no loss of metal components during the preparation
processes. The X-ray diffraction (XRD) patterns (Fig. 1d and ESI
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Fig. S7 and Table S2†) show ve broad diffraction peaks corresponding to the face-center-cubic (FCC) metal structure. The
average grain size estimated using Scherrer's formula exhibits
an initial decrease with increasing Pd component and reaches a
minimum value of 2.3 0.6 nm for Pt25Pd75 composition
(Fig. 1e). The value of the computed grain size is slightly lower
than the ligament diameter estimated by statistical analysis of
STEM images, indicating the presence of polycrystalline
domains conned within a ligament.
HAADF-STEM imaging was also employed to gain insight on
the growth mechanism of the nanoporous structure. The
Pt25Pd75 sample image (inset of Fig. 1a) reveals a detailed
structure of the primary branch consisting of multiple subbranches in the early stage of growth. The image conrms that
all the branches grow initially along the [111] direction. TEM
and STEM observations provide the detailed microstructures of
PtPd catalysts, conrming the product is porous structure with
continuous growth at [111] crystalline plane directions, which is
absolutely different from agglomerated nanoparticles with
random crystalline planes. In addition, the presence of a
monolayer of oleylamine (OAm) molecules adsorbed on the
surface of the Pt25Pd75 ligaments was conrmed by combined
SEM/STEM analysis (Fig. 3) and Fourier transform infrared
(FTIR) spectroscopy (Fig. 4). These results suggest that channelmediated growth previously reported for ultrathin noble metal
nanowires is the dominant mechanism in the case of PdPt
nanoporous structures.6,20–22 A high density of twin boundaries
appears for all the compositions of PtPd samples (ESI Fig. S1–
6†). The formation of twinning boundaries plays a critical role
in exponential reduction of the surface energy for FCC metals at
the nanoscale.23 Such structural features result in abundant
atomic steps and low-coordinated surface atoms, associated
with multiple exposed {111} and {100} facets, which provide the
energetically favorable conditions for multi-branched growth.
Subsequently, PtPd branches could be “welded” at connecting
junctions by mechanical and thermal activation, similarly to the
welding of ultrathin nanowires (Au and Ag) by mechanical
contact24,25 or low-energy light irradiation.26
In the absence of the Pd precursor, however, the nal
product consists solely of polyhedral Pt nanoparticles (ESI
Fig. S1†). This may be a consequence of the relatively higher
redox potential of Pt+2/Pt (+1.188 V) that results in a nucleation
burst leading to the formation of a very large number of small Pt
particles followed by their rapid agglomeration, which thus
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Fig. 3 Surface chemistry of nanoporous PtPd structures. (a–c) TE, ZC and SE-mode images of a Pt25Pd75 cluster, indicating a monolayer
oleylamine (OAm) on the surface of ligaments with a mean thickness of 1 nm.
Fig. 5 Scheme of formation mechanism of nanoporous PtPd structures. Formation mechanism of PtPd nanoporous structure under
assistance of oleylamine molecules.
Fig. 4 Surface chemistry of nanoporous PtPd structures. (a) Fast
Fourier transform infrared (FTIR) spectroscopy, confirming the
appearance of OAm molecules in the Pt25Pd75 sample; (b) molecular
structure of an oleylamine molecule.
limits the growth of Pt branches with insufficient feedstock
[Pt(acac)2]. In comparison, the lower redox potential of Pd+2/Pd
(+0.915 V) together with the slower decomposition rate of PdCl2,
favor the growth of Pd branches along the energetically preferential [111] directions.27 The scheme of growth process is shown
in Fig. 5. In addition to Pd+2 and Pd nuclei, the supersaturated
Pt nucleus provides a stable feedstock for the growth of PtPd
branches by Ostwald ripening.28 Under thermal assistance, Pt
and Pd diffuse and nally form a highly miscible composition.
The electrochemical catalysis of methanol oxidation was
evaluated systematically in an Ag/AgCl three-electrode reaction
cell at room temperature. The PtPd samples were deposited on
Vulcan XC-72 carbon support with 20 wt% of metals, following
an ultra-sonication dispersion of 1 hour and a treatment of 99%
acetic acid overnight. The cyclic voltammetry (CV) curves for
each catalyst were recorded at room temperature in an N2purged solution of 0.5 M H2SO4 and 1 M methanol by scanning
the voltage from 0 to 1 V at 50 mV s 1 sweep rate. The PtPd
samples exhibit signicant improvement for catalytic performances of methanol electro-oxidation, as shown in Fig. 6 (ESI
Fig. S8–11 and Table S3†). Because Pd has no electrocatalytic
2052 | J. Mater. Chem. A, 2015, 3, 2050–2056
activity for methanol oxidation in sulfuric acid medium,29 we
therefore counted the mass activity of bimetallic PtPd catalysts
only based on the Pt fraction. The mass current densities and
normalized residual currents of Pt-based catalysts reported in
the literatures were compared with the PtPd samples presented
herein, as shown in Fig. 6a. It should be noted that Pt25Pd75
exhibits the largest mass current of 1110 mA mg 1 Pt 1, which is
higher than Pt43Pd57 and Pt58Pd42 although they own relatively
larger electrochemical surface areas (ECSAs). In the methanol
oxidation, Pd and Pt are responsible for the water dehydrogenation (Pd–OH formation) and methanol dehydrogenation (Pt–
CO formation), respectively. The reaction between Pd–OH and
Pt–CO produces CO2 and regains the active metal surface,
which is signicantly co-affected by both the Pt/Pd ratio and
their reaction dynamics. Therefore, although Pt43Pd57 catalyst
presents the maximized ECSAs among all the samples, Pt25Pd75
catalyst has the best catalytic performance due to its suitable Pt/
Pd match related to the electrochemical dynamics. In comparison to polyhedral Pt nanoparticles, the current densities of
Pt58Pd42, Pt43Pd57 and Pt25Pd75 samples curves exhibit signicantly higher and more stable values with enhancement ratios
ranging from 140% to 390% despite the lower Pt content.
Additionally, the optimized current density is higher than those
of Pt-on-Pd nanodendrites (490 mA mg 1 Pt 1 (ref. 7) and 647
mA mg 1 Pt 1 (ref. 8)) which have a spatial phase-separation of
the Pt and Pd domains with limited Pd/Pt interfaces. The
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Fig. 6 Catalytic performance of nanoporous PtPd with controllable compositions in the electro-oxidation of methanol. (a) Electro-oxidation
peak current of methanol as a function of Pt/Pd composition (upper panel); residual current extracted from chronoamperometric curves
recorded at 0.6 V for 2000 s, exhibiting better durability of nanoporous PtPd structures than that of polyhedral Pt nanoparticles (lower panel). (b)
Redox potentials and their ratio in the forward and backward sweeps (IF/IB), showing an increase from 0.75 to 1.93 as increasing the Pd content.
(c) Comparison of catalytic activity and stability of Pt and Pt-based catalysts in various forms. The normalized residual currents are recorded at
2000 s for our Pt and nanoporous PtPd samples whose compositions are marked, or at shorter time for some cited references. Detailed
descriptions and cited references are provided in ESI Table S3,† in which several catalysts having higher mass activities but quite lower stabilities
are not integrated into Fig. 2c.
chronoamperometric curves of Pt25Pd75 sample indicate the
presence of 75.9% residual current aer 2000 s reaction (Fig. 6b
and 7). The enhancement for the stability is evidently ascribed
to the bi-functional catalytic mechanism from the synergistic
effect of Pd and Pt,1–12,30–32 in which the intermediate carbonaceous residues can be effectively oxidized and removed, resulting in a sustainable electro-oxidation process. Electrocatalytic
cycling stability has also been measured by using CV cycling, as
shown in Fig. 8, indicating only a 21% loss of activity aer 500
cycles. The microstructures analyzed by TEM (Fig. 9), for the asmade Pt25Pd75 and aer 500 CV cycles, further conrm high
structural stability at electrochemical reactions.
The tolerance towards intermediate carbonaceous species is
also evaluated by the ratio of the peak current values in the
forward and backward sweeps (IF/IB). The low IF/IB ratio is an
indication of poor methanol electro-oxidation and excessive
accumulation of carbonaceous residues on the catalyst surface
in the forward sweep.30–32 As shown in Fig. 6b, the IF/IB ratio
exhibits a signicant increase from 0.75 to 1.93 when the Pd
content is increased. This conrms the enhanced ability to
remove carbonaceous species formed on the catalyst during the
forward sweep. In addition, with the increasing Pd content, the
Fig. 7 Catalytic stability of nanoporous PtPd. (a and b) Chronoamperometric and normalized chronoamperometric curves
recorded at 0.6 V for 2000 s of nanoporous PtPd structures than that
of polyhedral Pt nanoparticles; (c) column pattern of residual current
density (normalized) after 2000 s measurement.
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forward and backward peaks gradually shi to the lower
potential values, indicating an enhanced kinetics of methanol
electro-oxidation. From the comparisons summarized in
Fig. 6c, the Pt25Pd75 sample exhibits an overall better performance than Pt and Pt-based catalysts previously reported, possessing both high activity and high stability. Catalytic activity
and stability are correlated with the microstructure of the
catalysts. The HAADF-STEM image in Fig. 10a reveals a threedimensional porous architecture consisting of sawtooth-like
ligaments with characteristic diameters of 2–5 nm, associated
with a complex arrangement of convex and concave surfaces.
The stepped surface facets shown in the magnied image
(Fig. 10b) give rise to a curvature that change along the surface
line where the twinning boundaries terminate, as indicated by
arrows. The kinks induced by twinning boundaries result in an
inward surface curvature, a phenomenon that was not observed
in any other nanoporous metal or alloy reported to
date.3,7,8,10,16–19 Adjacent to the twinning boundaries, a column of
surface atoms protruding from the regular surface was
observed. This complex structure is associated with a high
density of atomic steps where the atoms are thought to be lowcoordination. These low-coordination atoms play an important
role that is responsible for the high catalytic activity of nanoscale catalysts.33–35 Specically, low-coordination metal atoms
interact more strongly with molecules due to a local up-shi
and narrowing of the d-band, which weakens chemical bonding
Fig. 8 Catalytic stability of nanoporous PtPd. Cyclic voltammograms
(CV) of Pt25Pd75 sample for the first and the 500th cycles in N2-saturated 0.5 M H2SO4 containing 1 M methanol (scan rate: 50 mV s 1).
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Fig. 9 Structural stability of nanoporous PtPd catalysts during electrochemical reactions. TEM images of Pt25Pd75/Vulcan XC-72 carbon catalyst
(a) before and (b) after 500 cycles of cyclic voltammograms in N2-saturated 0.5 M H2SO4 containing 1 M methanol (scan rate: 50 mV s 1).
Fig. 10 Enhanced surface strain by twinning boundaries. (a) High-resolution HAADF-STEM images near center region of a Pt25Pd75 cluster,
showing the microstructure of nanopores and high lattice coherence on junctions. Scale bar, 5 nm. (b) Sawtooth-like ligaments induced by a high
density of twinning boundaries with a twinning space of 1 nm were further observed. Careful identification reveals that most of the exposed
facets on the PtPd ligaments were {111} and {110} planes with a large number of low-coordinated surface stepped atoms arising from the
concave and convex architectures. Scale bar, 2 nm. (c) High-resolution HAADF-STEM image of a Pt25Pd75 ligament containing two twining
boundaries, revealing the atomic-scale lattice structure. Scale bar, 1 nm. (d) Strain maps of 3xx and 3zz corresponding to the atomic structure as
shown in (c) were obtained by first principle calculations. Color represents variation of strain from 10.0 to 10.0%.
2054 | J. Mater. Chem. A, 2015, 3, 2050–2056
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of molecules and reduces reaction barriers relative to close
packed metal surfaces.33
The surface strain of nano-scale catalysts is directly related to
the density and catalytic activity of low-coordination surface
atoms.33,35,36 While large positive out-of-plane strain can be
induced by the intrinsic nature of the curved ligaments of
nanoporous structures,35 the exact role that the twinning
boundaries play in the surface strain is still unclear. A
comprehensive study of lattice distortion near the twinning
boundary for nanoporous PtPd is crucial for understanding the
origin of high-performance electro-oxidation of methanol.
From Fig. 10c, a representative region is examined to uncover
two crossed twinning boundaries that are conned within a
ligament having a 3 nm diameter. These twinning boundaries
induce sawtooth-like surface steps along the highly active {111}
and {110} facets. This structure was employed as the basis for a
model constructed to further study the effect of the twinning
boundary on the surface strain using ab initio calculations
(Fig. 10d). For the strain mapping analysis, predened directions (x and z) were chosen parallel and perpendicular to the
{111} surface plane, respectively. Aer structural relaxation, the
calculated strains were obtained by comparison with the {111}
lattice spacing of bulk Pt metal. Strain mapping analysis
provides intrinsic insights into this experimental conguration,
showing that the strain occurs only slightly in the in-plane
direction, while it becomes more pronounced out-of-plane. The
local strains, particularly those in the regions of twinning
boundary and the induced convex corner, are enhanced up to
10%, and more than 5% at the normal ligament surface.
These results indicate that the enrichment of twinning
Journal of Materials Chemistry A
boundaries provides more active sites for catalysis that are the
origin of high-performance electro-oxidation of methanol.
To understand the role of highly miscible Pt/Pd interfaces in
suppressing the excessive accumulation of carbonaceous residues on the catalyst surface, CO molecules adsorbed onto four
sites with different distances to the interface were characterized
using spin-polarized rst-principles calculations (Fig. 11a).
From Fig. 11b, on the site closest to the interface (denoted as
Pt1), the CO adsorption energy associated with triple Pt–C
bonding corresponds to 1.756 eV, which is signicantly lower
than those at more distant sites (Pt2, Pt3 and Pt4). Density
functional theory (DFT) calculations provide the charge density
distributions of the four congurations. As shown in Fig. 11c,
the charge density increase located at Pt1 is evidently reduced in
comparison to those at sites Pt2, Pt3 and Pt4. It suggests that the
interactions between Pt1 and CO molecules are weakened,
which is consistent with the result of adsorption energy calculation. This is further demonstrated by quantitative DOS analysis for four different sites of Pt atoms without CO absorption,
as shown in Fig. 11d. Pt1 atoms exhibit anomalous DOS in
comparison with the other three sites, which implies that the
local electronic structure of Pt atoms at Pt/Pd interfaces is
affected by coordinated Pd atoms. These atomic-scale insights
conrm that Pt1 is the most favourable site for the bi-functional
catalytic mechanism.1 At this location, Pt atoms coordinate with
Pd atoms resulting in the weakest Pt–CO bonding, which
promotes the oxidation process of CO to CO2. With high
miscibility of Pt and Pd, the exposed Pt/Pd interfaces conned
within ultrathin ligaments thus provide the optimal conguration. In relation with the three-dimensional open porous
Fig. 11 Theoretical simulation for the effect of atomic-scale Pt/Pd interfaces of nanoporous PtPd alloy on the methanol electro-oxidation. (a)
Four configurations of CO adsorption on various sites around Pt/Pd interfaces. (b) Dependence of absorbance energy and C–O bond length on
the four indexed sites in (a). (c) Charge density distribution of Pt1 and Pt4 sites, indicating the weaker Pt–C bond at Pt1 site than Pt4. (d) Quantitative
DOS in various configurations, indicating the origin of electronic structures for the weakened Pt–C bond at Pt1.
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structure, these Pt/Pd interfaces can accelerate the catalytic
process of carbonaceous residues and effective renewal of ions
and molecules.
Conclusions
In summary, a simple thermal-decomposition strategy was
employed to synthesize component-controllable PtPd nanoporous catalysts including an optimized Pt25Pd75 composition
which exhibits the smallest ligament size, a high density of
twinning boundaries, as well as sub-nanometre scale Pt/Pd
interfaces. These nanoporous PtPd catalysts are both highly
active and stable, as demonstrated by their performance in
methanol electro-oxidation. Additionally, the synthesis strategy
we employed holds great potential for large-scale industrial
production due to the processing simplicity and low cost, which
is a promising avenue for many energy-related applications.
Acknowledgements
This work was jointly supported by the National Research
Council of Canada (NRC). F.R. acknowledges partial salary
support from the Canada Research Chairs program and is
grateful to NSERC for an EWR Steacie Memorial Fellowship.
P.F.G. was funded by the U.S. NSF under Grant no. DMR
1005398, and would like to thank the Center for Computational
Materials Science, Institute for Materials Research, Tohoku
University, for affording time on the Hitachi SR11000 (model
K2) supercomputing system.
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