Supported bimetallic AuPd clusters using activated Au25 clusters

Article abstract of DOI:10.1016/j.cattod.2016.07.016

Bimetallic AuPd nanoparticles on alumina supports were prepared using Au₂₅(SR)₁₈ precursors activated by mild calcination or LiBH₄ treatment, followed by selective deposition of Pd via ascorbic acid reduction. Comparison of their catalytic activity for the oxidation of crotyl alcohol showed that bimetallic structure had significantly improved catalysis compared to Pd/Al₂O₃. In particular, AuPd samples grown from LiBH₄-activated Au₂₅ clusters exhibit the highest catalytic activity as well as high selectivity towards crotonaldehyde formation, likely due to their smaller particle sizes as compared to AuPd samples grown from calcined Au₂₅ clusters. X-ray absorption spectroscopy (XAS) at the Au L₃-edge, Pd L₃-edge and Pd K-edges showed that the resulting bimetallic AuPd nanoparticles had Au-Pd core-shell structures with a 4d-electron poor Pd surface.

Full text of DOI:10.1016/j.cattod.2016.07.016

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Catalysis Today
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Supported bimetallic AuPd clusters using activated Au25 clusters
Kee Eun Leea,b, Atal Shivharea, Yongfeng Hub, Robert W.J. Scotta,∗
a Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, SK S7N 5C9, Canada
b Canadian Light Source Inc., 44 Innovation Boulevard, Saskatoon, SK S7N 2V3, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
Bimetallic AuPd nanoparticles on alumina supports were prepared using Au25(SR)18 precursors activated
by mild calcination or LiBH4 treatment, followed by selective deposition of Pd via ascorbic acid reduction.
Comparison of their catalytic activity for the oxidation of crotyl alcohol showed that bimetallic structure
had significantly improved catalysis compared to Pd/Al2O3. In particular, AuPd samples grown from
LiBH4-activated Au25 clusters exhibit the highest catalytic activity as well as high selectivity towards
crotonaldehyde formation, likely due to their smaller particle sizes as compared to AuPd samples grown
from calcined Au25 clusters. X-ray absorption spectroscopy (XAS) at the Au L3-edge, Pd L3-edge and Pd
K-edges showed that the resulting bimetallic AuPd nanoparticles had Au-Pd core-shell structures with a
4d-electron poor Pd surface.
Received 31 May 2016
Received in revised form 11 July 2016
Accepted 18 July 2016
Available online xxx
Keywords:
Au clusters
Bimetallic catalysts
AuPd catalysts
© 2016 Elsevier B.V. All rights reserved.
Alcohol oxidation
X-ray absorption spectroscopy
1. Introduction
raphy [22,23], which show that it exhibits unique geometrical and
electronic structures [24]. Because of the extremely small size of
Bimetallic nanoparticles (NPs) are promising catalysts due to
their unique catalytic, electronic and optical properties in compar-
ison to the monometallic NPs [1]. Adding a second metal allows
for the possibility of tailoring the electronic and geometric struc-
tures to enhance the activity and selectivity of NP catalysts, due
to changes in electron density and geometries of two different
atoms leading to synergistic effects [2,3]. Due to their superior
catalytic activities and selectivities, bimetallic NPs are used in
numerous reactions, including alcohol oxidations [4], oxidation of
other organic compounds [5,6], and catalytic reforming [7]. Par-
ticularly, AuPd catalysts have been renowned for their enhanced
reactivity and selectivity in several reactions such as the selective
oxidation of alcohols and alkenes [8] and CO oxidation [9]. For cat-
alytic reactions, AuPd NPs are typically deposited on oxide supports
such as Al2O3 [10], SiO2 [11] and CeO2 [12] to create highly active
heterogeneous catalysts [13]. As a part of the AuPd structure, Au NPs
hold great potential for homogeneous and heterogeneous catalysis
in various oxidation [14] and hydrogenation reactions [15]. Cur-
rent interest has been focused on Au clusters, such as Au25 [16],
Au38 [17] or Au144 [18] because they have been shown to have
unique catalytic behavior when their diameter is smaller than 2 nm
[19]. The structure of Au25(SR)18 clusters is well characterized by
theoretical calculations [20,21] and single-crystal X-ray crystallog-
Au25(SR)18 clusters, they show non-metallic behavior in the form of
HOMO-LUMO transitions, and have no surface plasmon resonance
band in the UV–vis spectrum. The non-metallic behavior of Au nan-
oclusters originates from the electron energy quantization effect as
a result of ultra-small size, which alters their catalytic properties
[25,26].
While Au has interesting catalytic activity on its own, many
groups, including our own, have previously shown that AuPd
bimetallic NPs are extremely efficient catalysts for low tempera-
ture oxidation reactions, including the direct formation of hydrogen
peroxide from hydrogen and the room temperature oxidation of
allylic alcohols [8,10,27–29]. Our group and others have also shown
that the most active species for catalytic activity is typically Au-Pd
core-shell particles, in which the Au core withdraws 4d electron
density from the Pd shell, thus stabilizing the Pd towards further
oxidation [30,31]. Bimetallic Au24Pd clusters have been fabricated
by several groups; however, yields of specific AuPd clusters can
be very low [32,33]. Nevertheless, they have been shown to be
promising model catalysts. For example, Xie and coworkers pre-
pared single Pd atom doped Au25 (PdAu24) clusters and deposited
them on multi-walled carbon nanotubes (CNT) substrate, and found
that they had over three times the catalytic activity of Au25/CNT
materials for the oxidation of benzyl alcohol [34].
As an alternative strategy to make model AuPd catalysts using
Au25 clusters, we grew Pd shells onto pre-activated Au25 clusters
to form well-defined Au-Pd core-shell particles. Here, we prepared
AuPd bimetallic NPs using Au25(SC2H4Ph)18 precursors to form Au
∗
Corresponding author.
E-mail address: robert.scott@usask.ca (R.W.J. Scott).
http://dx.doi.org/10.1016/j.cattod.2016.07.016
0920-5861/© 2016 Elsevier B.V. All rights reserved.
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cores, followed by the deposition of Pd shells. The bimetallic NPs
were prepared by a sequential reduction method, using two dif-
ferent methods to activate the Au cores (i.e. in order to remove
the thiolate stabilizers), followed by deposition of Pd onto the
activated Au cores. The first method involves using mild heat treat-
ment at 250 ◦C for 2 h to remove the thiolate ligands, while the
second involves using a moderate reducing agent, lithium borohy-
dride (LiBH4) to reduce thiolate ligands off the Au25(SR)18 clusters.
Our previous studies indicated that Au25 clusters grew slightly to
ca. 2 nm upon calcination at 250 ◦C [35], while higher calcination
temperatures of 500 ◦C showed significant nanoparticle sintering.
EXAFS and XANES analyses were also conducted to elucidate the
geometric and electronic structure of our bimetallic NPs. TEM anal-
ysis shows that the resulting particle size of bimetallic NPs are
2–3 nm or 4–5 nm depending upon the activation method for Au25
precursor, with calcination of Au25 clusters leading to larger AuPd
NP sizes. EXAFS results indicate that the bimetallic AuPd NPs have
core-shell structures and XANES data shows the presence of 4d-
electron deficient Pd on the surface of the AuPd NPs. AuPd NPs
showed higher catalytic oxidation activities and selectivity toward
crotonaldehyde than pure Pd systems.
with methanol to remove excess reducing agent. After washing, the
wet powder was dried in air.
2.4. Preparation of AuPd nanoparticles
Activated Au25/Al2O3 materials from above were used for the
preparation of sequentially reduced AuPd NPs in various mole
ratios; 3:1, 1:1 and 1:3. For the 1:1 AuPd sample, 4.1 × 10−5
moles Au (12.0 mg Au25(SC2H4Ph)18 clusters before reduction) was
deposited on 200 mg of Al2O3 substrate. After the activation (ther-
mal or LiBH4 treatment), 8.2 × 10−5 moles (14 mg) of ascorbic acid
was added followed by 4.1 × 10−5 moles of Pd(II) acetate (9.1 mg
Pd acetate). The 3:1 AuPd and 1:3 AuPd samples were prepared in
the same way with different molar ratios.
2.5. Catalytic measurements
The oxidation of crotyl alcohol was conducted at 40 ◦C for 4 h.
The crotyl alcohol was added along with 50 equivalents of cata-
lyst in 3 mL H2O. The mixture was purged under O2 during the
reaction. A Pd-on-alumina substrate was used as a reference mate-
rial. To extract the products, 1 mL of CDCl3 was added into the
resulting mixture and the mixture was vigorously shaken to trans-
fer the products into the organic phase. The yield, selectivity and
conversion were analyzed by 1H NMR. All the catalytic reactions
were repeated 2 times and the results show a good reproducibility.
Turnover numbers (TON) were calculated by dividing the moles of
product formed by the moles of Pd in the catalyst.
2. Experimental
2.1. Materials
All chemicals are commercially available and used as received.
Tetraoctylammonium bromide (TOAB, 98%), lithium borohydride
(LiBH4, 2.0 M in THF), hydrogen tetrachloroaurate (III) trihy-
drate (HAuCl4·3H2O, 99.9% on metal basis) and porous aluminum
oxide (Al2O3, 58 Å, ∼150 mesh) were purchased from Sigma-
Aldrich. Phenylethanethiol (C8H9SH, 99%) was purchased from
Acros Organics. Palladium (II) acetate was purchased from Alfa
Aesar. High purity tetrahydrofuran (THF) was purchased from EMD
(HPLC grade). High purity acetonitrile and 100% ethanol were pur-
chased from Sigma-Aldrich. The water used in all experiments
was produced with a Milli-Q NANO pure water system (resistivity
18.2 Mꢀ cm).
2.6. Characterization
Absorption spectra were recorded on a Varian Cary 50 Bio
UV–vis spectrometer with an optical path length of 1 cm and a scan
range of 200–1100 nm. Transmission electron microscopy (TEM)
analyses of the NPs were conducted using a HT7700 TEM (Hitachi)
operating at 200 kV. The Au25 and Pd NPs on alumina substrates
were thoroughly ground with a small amount of ethanol in a mortar.
TEM samples were prepared by drop-casting NPs onto a graphene-
coated lacey carbon TEM grid (Electron Microscopy Sciences). Pd
K-edge and Au L3-edge X-ray absorption spectroscopy were con-
ducted on the HXMA (Hard X-ray Micro-Analysis) beamline at
Canadian Light Source (CLS) in transmission mode. Pd L3-edge
XANES (X-ray Absorption Near Edge Structure) spectroscopy was
also performed at the CLS on the SXRMB (Soft X-ray Microcharac-
terization Beamline). Pd L3-edge data was collected in fluorescence
mode under a helium atmosphere to reduce beam loss from scat-
tering. The software package IFEFFIT was used for data processing.
The EXAFS fitting at the Pd K-edge was performed in the R-space
2.2. Synthesis of Au25(SC2H4Ph)18 clusters
The synthesis of Au25(SC2H4Ph)18 clusters was carried as
described previously [36], 50 mL of THF and 500 mg of HAuCl4.3H2O
were mixed with 1.2 equiv. of TOAB and the solution was slowly
stirred for 10 min. Then 5 equivalents of phenylethanethiol was
added drop-wise and the solution was stirred until it became
transparent. 10 equivalents of NaBH4 in 2 mL ice cold water was
added all at once and the final solution was left stirring for 4 days
in air. After the reaction was over, the solvent was evaporated
using a rotary evaporator and the reaction residue were sequen-
tially washed with copious amounts of 75/25, 85/15 and 90/10
mixtures of ethanol/H2O. After washing, Au25(SC2H4Ph)18 clusters
were extracted with acetonitrile and the solution was filtered. The
filtrate was evaporated using a rotary evaporator and the final clus-
ters dissolved in THF. The yield of the synthesis was 24%.
between 1.4–3.0 Å, and bulk Pd fcc bulk lattice parameters were
2
used as a model for fitting. The amplitude reduction factor, So
,
was found to be 0.93 for Pd foil, and that value was used for fitting
the bimetallic samples.
3. Result and discussion
Fig.
1 shows the UV–vis spectrum of the as-synthesized
2.3. Reduction of Au25(SC2H4Ph)18/Al2O3
Au25(SC2H4Ph)18 clusters. The spectral features confirm that the
Au25 sample is nearly monodisperse. The lowest energy band at
685 nm corresponds to the HOMO−LUMO transition due to the
Au13 core in the Au25 structure, and the other two featured bands
(associated with the exterior Au12 shell in Au25 structure) at 445
and 400 nm are assigned to mixed intra-band (sp ←− sp) and inter-
band (sp ←− d) transitions, and an inter-band transition (sp ←− d),
respectively [23]. We have previously shown that the clusters are
nearly monodisperse with sizes of ca. 1 nm by TEM analysis and
MALDI mass spectrometry [36].
In order to synthesize clusters supported on alumina, the
Au25(SC2H4Ph)18 clusters were deposited on the Al2O3 substrate
via a wetness impregnation method, to give a final Au loading of
ca. 4 wt%. Two different methods were used to remove the sta-
bilizer; the first involved heating at 250 ◦C for 2 h under flowing
air. The second involved adding 0.02 mL of LiBH4 (2.0 M in THF) to
Au25(SC2H4Ph)18/Al2O3 in hexane, and then stirring vigorously for
20 min. The resulting dispersed powder was washed several times
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AuPd nanoparticles prepared from thermally- and LiBH4-activated
Au25 are 5 nm and 2.5 nm, respectively. The particle sizes seen for
the bimetallic samples are all slightly higher than that seen for
the core materials (ca. 3.5 nm and 1 nm, respectively), which is a
good indication that bimetallic nanoparticles were indeed formed.
However, we note that at such small NP sizes, one is not able to dif-
ferentiate between core-shell structures or alloys via bright-field
TEM since both Au and Pd have similar contrast. Thus, in order to
understand the arrangement of the two different elements in the
final AuPd NPs, EXAFS analysis of the samples at the Pd K edge
was done in order to understand the coordination environment of
the Pd. To get additional information on the electronic interactions
between the metals, the Au L3 and Pd L3-edge XANES spectra for
the AuPd nanoparticles were also measured.
The XANES spectra of the Au L3-edge of AuPd NPs are shown in
Fig. 5. The L3-edge probes the transition of 2p electrons to the 5d
bands. The peak at 11925 eV is known as white line and its inten-
sity is related to the presence of unoccupied 5d states (d-holes).
Two physical phenomena can affect the intensity of the white line;
one is intrinsic and the other is extrinsic. The size of the cluster
can be considered as an intrinsic effect and charge transfer can be
considered as an extrinsic effect [39]. For example, Au NPs have a
lower white line intensity compared to the bulk Au metal because
Au NPs have a reduced number of Au Au bonds due to their size,
resulting in an increase in the 5d level occupancy and hence in a
decrease in the white line intensity. In Fig. 5, thermally-activated
Au25 NPs have higher white line intensity than the bimetallic sam-
ples and the same general trend is observed in the LiBH4 treated
samples. This suggests that the AuPd samples have less vacancies
in the 5d band due to electron transfer from Pd, which implies that
Pd and Au are in close proximity to each other [40]. This conclusion
is further supported by XANES and EXAFS data shown later. Further
reduction in the intensity of white line is seen for 3:1 AuPd sam-
ples, which is probably due to a larger amount of contact, and thus
stronger interaction, between the Au and Pd in this sample (pre-
sumably, higher Pd loadings give thicker Pd shells, and thus have
more Pd atoms that are not in contact with Au). One other signif-
icant difference between the chemically and thermally activated
Au25 NPs is that the chemically activated Au25 NPs show a negli-
gible multiple-scattering peak at 11950 eV, which is a sign of their
small size [38]. However, the bimetallic samples grown from this
precursor show a pronounced peak at this energy, which implies
that there has been growth of Pd onto the small Au seeds, leading
to a higher number of scattering neighbours.
Fig. 1. UV–vis spectrum of Au25(SC2H4Ph)18 clusters in THF.
In order to have a further insight into their morphology and
dimensions, all catalysts were investigated by high resolution TEM.
For the catalytic reaction, Au25 clusters were deposited on the alu-
mina substrate and post-treated by LiBH4 or by heating at 250 ◦C for
2 h to remove the thiol stabilizer. The TEM of thermally activated
Au25 NPs have a significant distribution in sizes compared to the
LiBH4 activated Au25 NPs, in which only ca. 1 nm particles can be
observed (Fig. 2). It should be noted that metal NPs on the support
are relatively hard to distinguish in TEM images; Au or Pd NPs are
observed as dark dots on the alumina substrate. Previous attempts
to image similar materials by STEM have been unsuccessful due to
beam damage of clusters. We have previously shown that thermal
treatments successfully remove all thiolates from the Au25 clusters,
albeit with a significant increase in size due to sintering [35]. Thio-
late reduction via BH4 addition as a method of cluster activation has
previously shown by Asefa and coworkers [37], and verified by our-
selves as a way of activating Au clusters with minor changes in the
Au cluster size, which is consistent with results shown here [38]. It
is interesting to note that Au25(SC2H4Ph)18 clusters are more stable
−
to thiolate reduction in solution in the presence of BH4 reducing
agents than on alumina surfaces; this may be due to the presence
of acidic groups on the support surface [38].
Figs. 3 and 4 show the TEM images of differently produced
AuPd bimetallic nanoparticles with various molar ratios of Au and
Pd prepared from the activated Au25/Al2O3 samples via selective
reduction of Pd onto the Au seeds. The average particles sizes of
Fig. 2. TEM images of (a) thermally treated (250 ◦C) Au25/Al2O3, (b) LiBH4 treated Au25/Al2O3. Arrows point to the individual NPs/clusters on the alumina.
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Fig. 3. TEM images of bimetallic NPs produced from thermally activated (250 ◦C) Au25 precursors (a) 3:1 AuPd, (b) 1:1 AuPd and (c) 1:3 AuPd.
Fig. 4. TEM images of bimetallic NPs produced from LiBH4 activated Au25 precursors (a) 3:1 AuPd, (b) 1:1 AuPd and (c) 1:3 AuPd. Arrows point to the individual nanoparti-
cles/clusters on the alumina.
Fig. 5. Au L3 edge XANES spectra of AuPd structures with different molar ratios using (a) thermally activated Au25 NPs, and (b) LiBH4 activated Au25 NPs.
The corresponding Pd-L3 XANES spectra are shown in Fig. 6
for all the bimetallic AuPd samples studied. The most prominent
feature of the Pd L3-edge is the white-line at 3173 eV, caused by
excitation of electrons from the 2p to the 4d band [41]. A sig-
nificant decrease in the d-occupancy of Pd is as indicated by the
more intense white line seen for the low Pd loading samples (3:1
AuPd and 1:1 AuPd) compared to the high Pd loading sample (1:3
AuPd). This was observed for bimetallic structures prepared from
both thermally activated Au25 samples and LiBH4 activated Au25
samples. This suggests that there are strong electronic interactions
between Pd and Au atoms, which is in agreement with the results
of the Au L3-edge XANES. However, the resulting change in the
Pd L3-edge is much larger, suggesting significant electron 4d with-
drawal from Pd in these systems. Marx et al. have noted that the
occupancy of all the s, p, and d valence orbitals needs to be consid-
ered when examining electronic effects, which may suggest that
6s/6p occupancy of Au also increases [42]. The other possibility for
such an intense white line is surface oxidation of Pd in the sam-
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Fig. 6. Pd L3 edge XANES spectra of AuPd structures with different molar ratio using (a) thermally activated Au25 NPs (b) LiBH4 activated Au25 NPs.
Table 1
Summary of EXAFS fits at the Pd K-edge.
Sample
Shell
N
R(Å)
␴2/Å2
ꢁE0(eV)
−11(1)
R-factor
0.016
Au:Pd = 3:1 thermal
Pd-Pd
Pd-Au
Pd-Pd
Pd-Au
Pd-Pd
Pd-Au
Pd-Pd
Pd-Au
Pd-S
Pd-Pd
Pd-Au
Pd-S
Pd-Pd
Pd-Au
Pd-S
3.5(1.0)
3.3(1.2)
4.4(4)
2.68(1)
2.71(1)
2.71(1)
2.71(2)
2.71(1)
2.72(1)
2.72(1)
2.74(2)
2.19(2)
2.73(1)
2.76(1)
2.25(1)
2.74(1)
2.76(1)
2.28(1)
2.745(8)
0.005
0.002
0.005
0.003
0.006
0.001
0.003
0.004
0.009
0.009
0.004
0.011
0.007
0.006
0.003
0.008
Au:Pd = 1:1 thermal
Au:Pd = 1:3 thermal
Au:Pd = 3:1 LiBH4
−10(1)
−9 (1)
−11(1)
0.017
0.018
0.033
3.5(5)
4.7(1.2)
2.6(1.2)
1.1(1.1)
2.3(4.3)
2.6(1.6)
4.8(5)
1.6(9)
1.2(2)
3.9(3)
1.0(7)
Au:Pd = 1:1 LiBH4
Au:Pd = 1:3 LiBH4
Pd NPs
−5.1(0.5)
−7(1)
0.014
0.006
0.014
0.9(2)
10.4(7)
Pd-Pd
−5.9(0.5)
ples (to PdO) or Pd-thiolate formation at the surface. Indeed, EXAFS
data from the LiBH4-activated samples does show some Pd-S con-
tributions, which could lead to increased white line intensities (see
below). However, Pd K-edge EXAFS for PdAu/Al2O3 samples pre-
pared from thermally activated Au25 precursors in R-space shows
that the degree of oxidation and/or Pd-thiolate interactions is low,
as evidenced by little-to-no Pd-O or Pd-S contributions (see below).
EXAFS structural analysis was performed at the Pd K-edge only;
Au L3 edge data was of poor quality and could not be success-
fully modeled. Models for the 3:1, 1:1 and 1:3 AuPd NPs were
prepared based on model PdAu alloy fcc structures. Quality fits
for all the samples were obtained with the first shell fits for the
AuPd NPs shown in Figs. 7 and 8. The values of coordination num-
bers (N), nearest-neighbours distances (R) and Debye Waller factors
(␴2) determined by EXAFS fitting are reported in Table 1. For the
bimetallic systems, the total coordination numbers (CNs) were
obtained from the summation of two coordination numbers as fol-
lows: NPd(total) = NPd-Pd + NPd-Au. In the case of Pd reference sample
(Pd/Al2O3), the first shell is composed of only Pd–Pd contributions,
with a NPd-Pd of 10.4(7), which is lower than the bulk fcc CN of 12,
thus indicating the presence of a fair number of under-coordinated
atoms on the surface of the Pd NPs. Coordination numbers typically
decrease with decreasing particle size due to fewer metal metal
bonds for surface atoms. For the bimetallic NPs using thermally
activated Au25 precursors, the NPd(total) coordination numbers are
in the 6.8–7.9 range. Smaller values of NPd(total) are obtained, indi-
cating that a large amount of Pd atoms are located on the surface of
the particles [43]. Random alloys show a ratio between Pd–Pd and
Pd-Au coordination numbers which is equivalent to the molar Au
to Pd ratio of these elements in the cluster [44]. Our data show that
the ratio between NPd–Pd and NPd–Au is not equal to the molar Au
to Pd ratio in the sample which indicates that the samples are not
random alloys. The low CNs relative to the Pd NPs is a strong indi-
cation of a core-shell structure. In the case of chemically activated
Au25 precursors for AuPd NPs, the EXAFS spectra in R space (Fig. 8)
are significantly different than AuPd NPs from thermally activated
Au25 precursors (Fig. 7), in that there is a strong Pd-S contribution.
Nevertheless the NPd(total) (now including the Pd-S CN) are also in
the 5.8–7.6 range. Benfield et al. showed the coordination number
varies between approximately 7 and 9 for particles in the range of
1.5 and 3 nm; as our values are lower than this, there are a large
number of ca. 2 nm particles [44].
Significant progress has been made in developing AuPd bimetal-
lic NPs that are able to selectively oxidize allylic alcohols (allyl
alcohol and crotyl alcohol) to their aldehyde and ketone coun-
terparts [10,30,45]. Here, the oxidation reaction studied is the
low-temperature, base free oxidation of crotyl alcohol in water. It
is reported that decomposition of crotyl alcohol can occur over Pd
surfaces at higher temperatures to form CO and propene [30,45],
resulting in lower turnover frequencies of the substrate and the
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Fig. 7. EXAFS single-shell fits in R-space for the AuPd NPs formed from thermally activated Au25 clusters with different molar ratios (a) 3:1 AuPd, (b) 1:1 AuPd and (c) 1:3
AuPd.
Fig. 8. EXAFS single-shell fits in R-space for the AuPd nanoparticles formed from LiBH4-activated Au25 clusters with different molar ratios (a) 3:1 AuPd, (b) 1:1 AuPd and (c)
1:3 AuPd.
Table 2
relates to the spectroscopic results of Au L3 and Pd L3-edge XANES,
indicating that stronger electronic interaction between Au and Pd
Catalytic results for the oxidation of crotyl alcohol.
leads to enhanced catalytic activity. The ideal molar ratio of AuPd
was found to be 3:1; the main reason for the suppressed catalytic
activity at higher loadings of Pd is likely reduced Au-Pd interactions,
as evidenced by higher Pd–Pd coordination numbers for 1:3 AuPd
samples. These results agree with those of Xie et al. who noted that
Pd doped Au25 clusters (PdAu24) exhibit a significant improvement
in catalysis for benzyl alcohol oxidation [34], with the conversion
TON
Selectivity for
Crotonaldehyde (%)
Pd
3.8
51.4
54.4
70.3
69.9
70.3
69.8
74.8
3:1 AuPd thermal
1:1 AuPd thermal
1:3 AuPd thermal
3: 1 AuPd LiBH4
1:1 AuPd LiBH4
1:3 AuPd LiBH4
43.7
27.0
11.9
66.4
45.7
30.2
of PdAu24 is almost three times higher than that of Au25
.
deactivation of the catalyst, thus catalysts that can be effective at
low temperatures are needed. The reaction temperature was opti-
mized at 40 ◦C using 3 mL aqueous solution containing the catalyst
with a continuous flow of oxygen. Pure Au catalysts are catalytically
inert towards crotyl alcohol under these conditions. The catalytic
activity of the AuPd nanoparticles toward the oxidation of crotyl
alcohol was measured and is summarized in Table 2; the selectivity
towards the crotonaldehyde product is also listed. While croton-
aldehyde was the majority product, 3-buten-1-ol, 1-butanol, and
1-butanal also appeared as hydrogenation and isomerzation prod-
ucts. The TON of Pd/Al2O3 catalysts is very low at 3.8, showing that
the monometallic Pd nanoparticles have low catalytic activity and
selectivity toward crotyl alcohol oxidation. However, the bimetallic
NPs show an increase in TON by a factor of 10–20. Both thermally-
and LiBH4-activated AuPd samples show the same trend in that
the catalytic activity is increased by decreasing the amount of Pd.
The 3:1 AuPd samples (both thermally and LiBH4 activated) exhibit
the highest turnover numbers. In general, AuPd NPs produced from
LiBH4 activated Au25 clusters showed stronger catalytic results in
comparison to the thermally activated samples, owing to smaller
particle size and higher surface area. The catalytic activity also cor-
4. Conclusion
AuPd NPs were prepared using Au25(C2H4Ph)18 cluster precur-
sors in this study. To remove the thiol stabilizer in the gold cluster,
two methods were used to activate the clusters; the first was a
moderate (250 ◦C) thermal treatment and the other was a LiBH4
treatment. AuPd NPs generated from thermally-activated Au25
clusters are significantly larger than their LiBH4 activated counter-
parts. Au and Pd L3-edge XANES show that the 3:1 AuPd samples
have the strongest electronic interactions between Au and Pd. Pd
K-edge EXAFS fitting results indicated that higher Pd-Au coordina-
tion numbers were obtained in 3:1 AuPd samples in comparison to
1:3 AuPd samples. Our catalytic results for crotyl alcohol oxidation
shows that addition of Pd onto activated Au25 clusters improves
the activity, with the highest TON observed in the 3:1 AuPd sample.
Furthermore, the catalysts prepared by LiBH4 activated Au25/Al2O3
were much more effective catalysts for the oxidation reaction. This
reduction method maintains the original size of the Au25 clusters
after activation and exhibits great potential for the development of
effective oxidation catalysts.
Please cite this article in press as: K.E. Lee, et al., Supported bimetallic AuPd clusters using activated Au25 clusters, Catal. Today (2016),
http://dx.doi.org/10.1016/j.cattod.2016.07.016

G Model
CATTOD-10325; No. of Pages7
ARTICLE IN PRESS
K.E. Lee et al. / Catalysis Today xxx (2016) xxx–xxx
7
Acknowledgements
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We acknowledge financial support from the National Sciences
and Engineering Research Council of Canada (NSERC). XAS exper-
iments described in this paper were performed at the Canadian
Light Source, which is supported by the Natural Sciences and Engi-
neering Research Council of Canada, the National Research Council
Canada, the Canadian Institutes of Health Research, the Province of
Saskatchewan, Western Economic Diversification Canada, and the
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Please cite this article in press as: K.E. Lee, et al., Supported bimetallic AuPd clusters using activated Au25 clusters, Catal. Today (2016),
http://dx.doi.org/10.1016/j.cattod.2016.07.016