Au-Pd core-shell nanoparticles as alcohol oxidation catalysts: Effect of shape and composition

Article abstract of DOI:10.1002/cssc.201300483

The right combination: Shape and composition are two important factors governing the catalytic properties of bimetallic nanocatalysts. Combining gold and palladium in a Au-core-Pd-shell structure coupled with an icosahedral morphology is shown to maximize catalytic performance for the selective oxidation of the biorenewable sources glycerol and 1,2-propanediol to glyceric and lactic acids, respectively. Copyright

Full text of DOI:10.1002/cssc.201300483

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DOI: 10.1002/cssc.201300483
Au–Pd Core–Shell Nanoparticles as Alcohol Oxidation
Catalysts: Effect of Shape and Composition
[a]
[a]
[b]
[a]
[a]
Soshan Cheong, Leah Graham, Gemma L. Brett, Anna M. Henning, John Watt,
[
b]
[c]
[c]
[b]
Peter J. Miedziak, Minghui Song, Yoshihiko Takeda, Stuart H. Taylor, and
Richard D. Tilley*
[a]
[
10]
Gold and palladium nanoparticles (NPs) have attracted much
ly either larger than 20 nm, at which NP catalytic properties
[1]
[11]
[12]
attention because of their unique catalytic properties. Owing
to synergistic effects between these two metals, bimetallic Au–
Pd NPs have shown much improved activity and selectivity for
a number of industrially important reactions compared to their
may be reduced, or smaller than 5 nm, where shape con-
trol is usually difficult to achieve. As a result, comparative stud-
ies on the catalytic properties of faceted bimetallic NPs in the
[13]
critical size range of 10–15 nm have been limited.
[
2]
monometallic counterparts. For this reason, Au–Pd NPs, espe-
cially those with a Au core and Pd shell, where all the surface
atoms are of the more catalytically active metal, have emerged
as a new system of study for tuning the composition of NPs to
tailor their catalytic properties. Different research groups have
demonstrated such optimization of catalytic properties in reac-
Herein we report a comparative study of the effect of shape
and composition on NP catalytic activity, and selectivity for in-
dustrially important oxidation reactions in the presence of Au-
core–Pd-shell NPs. Such a study of using faceted NPs of less
than 15 nm is unique and vitally important for catalysis. Our re-
sults show that optimization of both shape and composition
are critical for maximized catalytic performance. It is demon-
strated that both Au–Pd composition and icosahedral shape
are essential for high conversion and selectivity for glyceric
acid and lactic acid from the oxidation of glycerol and 1,2-pro-
panediol, respectively. The highly faceted icosahedral struc-
tures are also shown to be a key feature for improving the
conversion of glycerol and 1,2-propanediol by a factor of 3–4
compared to those enclosed by multiple facets. Glycerol is an
excess byproduct of biodiesel production and 1,2-propanediol
[
3]
[4]
tions such as alcohol oxidation, Suzuki coupling, and
[
5]
oxygen reduction reactions.
Shape control of catalytically active NPs is one of the best
methods for improving catalytic performance, because catalyt-
ic properties are highly dependent on the crystallographic
[
6]
planes that terminate the NP surfaces. Typically, faceted NPs
such as those with cubic, tetrahedral, and icosahedral struc-
[7]
tures are more active than NPs with spherical shapes. In par-
ticular, icosahedral NPs that are multiply twinned and bound
by twenty {111} facets have been shown to be more catalytical-
[14]
is often synthesized from glycerol. The selective oxidation
yielding glyceric acid and lactic acid, both being important bio-
[8]
ly active than those with different polyhedral structures. Fac-
eted NPs with sizes below 15 nm can exhibit much improved
catalytic performance, however, they have also proved chal-
[15]
chemical reagents, has great potential for a more effective
utilization of biorenewable sources and further expanding the
usage of their derivatives.
[
9]
lenging to synthesize. Moreover, bimetallic NPs with well-de-
fined core–shell structures reported in the literature are typical-
The icosahedral Au–Pd core–shell NPs were synthesized ac-
cording to a seeded-growth method, where an icosahedral Au
[3a]
seed was slowly coated by layers of Pd atoms. For compari-
son of catalytic properties, Au–Pd core–shell NPs enclosed by
multiple facets and Pd icosahedra of similar sizes were pre-
pared by a modification of the synthesis. Au seeds without Pd
coating were also included in the study as a control.
+
[
a] Dr. S. Cheong, L. Graham, A. M. Henning, Dr. J. Watt, Prof. R. D. Tilley
School of Chemical and Physical Sciences
and
The MacDiarmid Institute for Advanced Materials and Nanotechnology
Victoria University of Wellington
Gate 7 Kelburn Parade, Wellington 6012 (New Zealand)
E-mail: richard.tilley@vuw.ac.nz
Figure 1a shows a transmission electron microscopy (TEM)
image of the as-synthesized icosahedral Au–Pd core–shell NPs,
which were 11.0Æ0.6 nm in size and monodisperse. The NPs
formed a regular hexagonal close packed monolayer on the
TEM grid due to the well-defined faceting of the icosahedral
morphology. The high-angle annular dark-field scanning TEM
[
b] Dr. G. L. Brett, Dr. P. J. Miedziak, Dr. S. H. Taylor
Cardiff Catalysis Institute, School of Chemistry
Cardiff University, Main Building
Park Place, Cardiff CF10 3AT (UK)
[
c] Dr. M. Song, Dr. Y. Takeda
Advanced Key Technologies Division
National Institute for Materials Science
(
HAADF-STEM) image in Figure 1b shows a clear contrast dif-
3
-13 Sakura, Tsukuba, Ibaraki 305-0003, Tsukuba (Japan)
ference between the heavier-atom Au core and the lighter-
atom Pd shell. Figure 1c shows a TEM image of the multifacet-
ed Au–Pd NPs of 14.0Æ1.2 nm in size. A HAADF-STEM image
of the multifaceted Au–Pd core–shell NPs is shown in Fig-
ure 1d, in which the contrast between Au and Pd clearly
shows the uneven coating of the Pd shell, which results in a rel-
atively broader size distribution. Figure 1e and f show TEM
+
[
] Present address:
Center of Integrated Nanotechnologies
Sandia National Laboratories
Albuquerque, New Mexico 87185 (USA)
This manuscript is part of a Special Issue on the synthesis and use of
shape-controlled particles. The Table of Contents will appear here. This
text will be updated once the Special Issue has been assembled.
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Figure 1. a) TEM, and b) HAADF-STEM images of icosahedral Au–Pd core–
shell nanoparticles. c) TEM, and d) HAADF-STEM images of multifaceted Au–
Pd core–shell nanoparticles. TEM images of e) icosahedral Pd, and f) Au seed
nanoparticles.
images of the icosahedral Pd NPs 11.1Æ1.0 nm in size and of
the icosahedral Au seeds 7.3Æ0.5 nm in size, respectively. Both
the Pd NPs and Au seeds tend to arrange in a regular hexago-
nal packing on the TEM grids.
Figure 2. a) High-resolution TEM image of an icosahedral Au–Pd core–shell
nanoparticle viewed along a <111 > zone axis as indicated by the corre-
sponding FFT, shown in (b). c) High-resolution TEM image of a multifaceted
Au–Pd core–shell nanoparticle, and d) the corresponding FFT of the image.
e) High-resolution TEM image of an icosahedral Pd nanoparticle with the
corresponding FFT shown in (f). g) High-resolution TEM image of an icosahe-
dral Au nanoparticle viewed down a <111 > zone axis as indicated by the
corresponding FFT shown in (h).
The high-resolution TEM (HRTEM) image of an icosahedral
Au–Pd core–shell NP in Figure 2a shows that the surface of
the NP consists of multiply twinned structures. The image also
shows a threefold axis oriented parallel to the electron beam.
The spots of the corresponding fast Fourier transform (FFT) of
the HRTEM image, as seen in Figure 2b, are shown to have
a hexagonal symmetry and were indexed to {220} reflections,
indicating that the NP observed is bound by {111} lattice
planes. In comparison, a HRTEM image of a multifaceted Au–
Pd core–shell NP is shown in Figure 2c. The FFT of the image
in Figure 2d indicates that the NP is polycrystalline and con-
sists of twinned planes. Figure 2e and g show the typical
HRTEM images of a Pd NP and a Au seed NP, respectively. Simi-
lar to what is observed in Figure 2a, each of these images cor-
responds to a multiply twinned icosahedral morphology
viewed along a <111 > zone axis, with only {111} surfaces ex-
posed, which is characteristic of a face-centered cubic icosahe-
The catalytic performance of the NPs was assessed for key
reactions that convert the biorenewable sources glycerol and
1,2-propanediol the into industrially important chemicals glyc-
eric and lactic acid, respectively. All catalysts were prepared
with a loading of 1 wt% metal NPs on activated carbon.
Glycerol or 1,2-propanediol was converted into glyceric acid or
lactic acid, respectively, in a glass reactor under 3 bar oxygen
at 608C for 4 h. The substrate-to-metal ratio was 1000.
The icosahedral Au–Pd core–shell NPs showed the highest
conversion of glycerol and 1,2-propanediol among all the NPs
tested, as can be clearly seen in Figure 3. In terms of NP com-
position, the icosahedral Au–Pd NPs showed a much higher
conversion rate of 5–16 times than that of the Au seeds, and
[
16]
dron. The icosahedral shape of the NPs can be confirmed
from the respective FFTs of the images in Figure 2g and h,
which are consistent with the FFT seen in Figure 2b.
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As for the oxidation of 1,2-propanediol, whilst all the NPs
show high selectivity of >80% to lactic acid, an increase in the
selectivity is typically observed for the icosahedral structures,
as shown in Table 2. In previous studies, the reaction rate and
products obtained have been shown to be sensitive to the NP
[19]
size, catalyst support, and reaction conditions. In our study,
where the NPs are similar in sizes, and the support for all the
catalysts and reaction conditions is identical, the high selectivi-
ty observed indicates that the icosahedral shape may play
a unique role in the selective formation of lactic acid.
The results show that the icosahedral structure has a signifi-
cant effect on the catalytic activity and selectivity of the NPs
studied. Icosahedral NPs are bound by {111} facets exclusively,
a characteristic that is critically important in catalysis where
surface atomic structures dictate the catalyst performance. Par-
ticularly for Au–Pd, it has been shown that a decrease in the
proportion of {111}-type facets in a Au–Pd nanoalloy catalyst
resulted in a lowering of catalytic activity in the oxidation of
Figure 3. Comparison of conversion rate (C) for the oxidation of
&
glycerol,
and
&
1,2-propanediol by the nanoparticles tested.
[20]
an enhancement of 1.6–3 times compared to icosahedral Pd
NPs of similar sizes. The enhancement in the activity suggests
that coating the Au seeds with Pd shells gives rise to synergis-
tic effects between the metals, in comparison to Au and Pd
benzyl alcohol. This is indeed reflected by the lower activity
observed for the multifaceted NPs, which are bound by a mix-
ture of poorly defined facets, when compared to the Au–Pd
and Pd NPs with icosahedral morphologies.
[
17]
single-metal NPs. In terms of NP shape, the icosahedral Au–
Pd NPs achieved a higher conversion of glycerol and 1,2-pro-
panediol, by a factor of 3 and 4, respectively, compared to the
multifaceted Au–Pd NPs. This indicates that the {111}-bound
icosahedral structures are catalytically active and important for
both of these oxidation processes.
A unique feature of face-centered cubic icosahedral struc-
tures is that they are considered to be an assembly of 20 {111}-
[21]
terminated, apex-sharing tetrahedral building blocks.
However, these tetrahedral subunits are highly strained in
order to fill 3D space, and as a result, there are 30 twin boun-
[22]
daries among the 20 {111} facets of an icosahedral structure.
The selectivity of the catalysts for the oxidation of glycerol
and 1,2-propanediol are shown in Tables 1 and 2, respectively.
As shown in Table 1, the catalysts all achieve a selectivity of
It is established that catalytic activity of a metal NP can be en-
hanced by surface atoms located at the corners and edges of
[7a,23]
the NP.
The much-enhanced conversions achieved by the
7
7–85% to glyceric acid. The slight difference in the selectivity
use of icosahedral structures can, therefore, be explained in
terms of a high fraction of surface atoms on the {111} facets
coupled with a high density of twins and sharp edges on the
surfaces of the NPs.
between the Au-based NPs and Pd NPs could be due to
a higher resistance to overoxidation exhibited by Au than by
[
18]
Pd catalysts under the glycerol oxidation conditions.
Along with NP shape, the composition of the NPs also plays
an important role in catalysis. Oxidation of glycerol and 1,2-
propanediol in the presence of Au and Pd monometallic and
alloy NPs has been studied by different research groups as an
Table 1. Catalysis of glycerol oxidation.
[
a]
Catalyst
Conversion [%] Selectivity [%]
GA TA GlycoA OA FA
[18,19,24]
important application of Au- and Pd-based catalysts.
Nevertheless, these reactions have not been reported for Au–
Pd core–shell NPs, which are shown to be highly selective to
glyceric and lactic acids in the respective reactions. It is consid-
ered that in the core–shell structure, the improved catalytic ac-
tivity and selectivity are likely due to electronic effects, as the
Pd surface is influenced by the underlying Au core of the
icosahedral Au–Pd
multifaceted Au–Pd 13.5
icosahedral Pd
icosahedral Au
43.0
84.7 3.6 11.0
0.8 0.1
1.3 5.0
2.8 5.6
0.3 0.2
82.7 4.7
77.2 7.6
6.4
6.7
25.8
8.5
85.4 1.1 13.0
[a] GA=glyceric acid. TA=tartronic acid. GlycoA=glycolic acid. OA=
oxalic acid. FA=formic acid.
[25]
NPs. Such synergistic effects are most prominent when the
Pd shell is less than 3 nm, as NPs with thicker shell behave
[3a,9b,26]
more Pd-like, resulting in lower activities.
Table 2. Catalysis of 1,2-propanediol oxidation.
In conclusion, our results show that the use of Au–Pd core–
shell NPs improves the conversion of glycerol and 1,2-propane-
diol by factors of up to 16 and 3 compared to Au and Pd NPs,
respectively. Additionally, an enhancement in the activity is fur-
ther promoted by the icosahedral structures of the NPs, as ico-
sahedral Au–Pd NPs are 3–4 times more active than those
bound by multiple facets. The comparison between the icosa-
hedral and multifaceted Au–Pd core–shell, and Au and Pd
[
a]
Catalyst
Conversion [%]
Selectivity [%]
LA
PA
AA
FA
icosahedral Au–Pd
multifaceted Au–Pd
icosahedral Pd
53.5
12.6
17.3
3.2
87.6
81.7
86.3
85.5
11.5
18.3
13.7
14.5
0.8
0
0
0
0
0
0
icosahedral Au
0
[
a] LA=lactic acid. PA=pyruvic acid. AA=acetic acid. FA=formic acid.
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single metal NPs offers an interesting example for investigating
the morphological and compositional effect on the catalytic
activity and selectivity of metal catalysts.
PROP-29690-HVMSSI. S.C., L.G., and R.D.T. also thank the Mac-
Diarmid Institute for funding.
Keywords: gold · heterogeneous catalysis · nanocatalysts ·
oxidation · palladium
Experimental Section
Nanoparticle synthesis: The Au–Pd core–shell NPs were synthe-
[3a]
sized according to a previously reported method. Typically, gold
chloride trihydrate (19.7 mg; Aldrich, 99.9%) dissolved in toluene
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2
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2
Aldrich, 99%] dissolved in toluene (0.5 mL) in the presence of hex-
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2
added to give a Pd/Au molar ratio of 2:1. The multifaceted Au–Pd
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Catalytic testing: A Colover glass reactor was charged with glycerol
or 1,2-propanediol (0.3m), sodium hydroxide (2 equiv mol/mol)
and catalyst (substrate/metal ratio 1000:1). The reactor was heated
to 608C and pressurized with oxygen (3 bar) and purged 3 times.
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Characterization: Low- and high-resolution TEM images were taken
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at 200 keV. Scanning TEM images were recorded using a JEOL
2
redispersing the dried samples in toluene and dropping onto
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6
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8
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Published online on && &&, 0000
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2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 5
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5
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These are not the final page numbers!

COMMUNICATIONS
S. Cheong, L. Graham, G. L. Brett,
A. M. Henning, J. Watt, P. J. Miedziak,
M. Song, Y. Takeda, S. H. Taylor,
R. D. Tilley*
The right combination: Shape and
composition are two important factors
governing the catalytic properties of bi-
metallic nanocatalysts. Combining gold
and palladium in a Au-core–Pd-shell
structure coupled with an icosahedral
morphology is shown to maximize cata-
lytic performance for the selective oxi-
dation of the biorenewable sources
glycerol and 1,2-propanediol to glyceric
and lactic acids, respectively.
&& – &&
Au–Pd Core–Shell Nanoparticles as
Alcohol Oxidation Catalysts: Effect of
Shape and Composition
ꢀ
2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 5
&
6
&
ÞÞ
These are not the final page numbers!