Supported Palladium-Gold Alloy Catalysts for Efficient and Selective Hydrosilylation under Mild Conditions with Isolated Single Palladium Atoms in Alloy Nanoparticles as the Main Active Site

Article abstract of DOI:10.1021/acscatal.6b02767

Supported Pd-Au alloy catalysts were developed for the highly efficient and selective hydrosilylation of α,β-unsaturated ketones and alkynes. The Pd/Au atomic ratio of the Pd-Au alloy and the supporting material affected the catalytic activity, and supported Pd-Au alloy nanoparticles with a low Pd/Au atomic ratio functioned as highly active heterogeneous catalysts under mild reaction conditions. Structural characterization of supported Pd-Au alloy catalysts by X-ray diffraction, X-ray absorption spectroscopy (XAS), and transmission electron microscopy revealed the formation of random Pd-Au alloy nanoparticles with a uniform size of around 3 nm on the support. Furthermore, XAS and X-ray photoelectron spectroscopy elucidated the charge transfer from Pd to Au and the formation of isolated single Pd atoms in random Pd-Au alloys with a low Pd/Au ratio, which enabled efficient hydrosilylation of a variety of substrates under mild reaction conditions.

Full text of DOI:10.1021/acscatal.6b02767

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Article
Supported Palladium–Gold Alloy Catalysts for Efficient and
Selective Hydrosilylation under Mild Conditions; Isolated Single
Palladium Atoms in Alloy Nanoparticles as the Main Active Site
Hiroki Miura, Keisuke Endo, Ryoichi Ogawa, and Tetsuya Shishido
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02767 • Publication Date (Web): 17 Jan 2017
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Supported Palladium–Gold Alloy Catalysts for
Efficient and Selective Hydrosilylation under Mild
Conditions; Isolated Single Palladium Atoms in
Alloy Nanoparticles as the Main Active Site
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Hiroki Miuraa,b,c, Keisuke Endoa, Ryoichi Ogawaa, Tetsuya Shishidoa,b,c*
a
Department of Applied Chemistry, Graduate School of Urban Environmental Sciences,
Tokyo Metropolitan University, 1ꢀ1 MinamiꢀOsawa, Hachioji, Tokyo 192ꢀ0397, Japan
b Research Center for Hydrogen Energyꢀbased Society, Tokyo Metropolitan University, 1ꢀ1
MinamiꢀOsawa, Hachioji, Tokyo 192ꢀ0397, Japan
c
Elements Strategy Initiative for Catalysts & Batteries, Kyoto University, Katsura,
Nishikyoꢀku, Kyoto 615ꢀ8520, Japan
* Corresponding author: Tel: +81ꢀ42ꢀ677ꢀ2850, Fax: +81ꢀ42ꢀ677ꢀ2850 (T. Shishido)
Eꢀmail address: shishidoꢀtetsuya@tmu.ac.jp (T. Shishido)
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Abstract
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Supported Pd–Au alloy catalysts were developed for the highly efficient and selective
hydrosilylation of α,βꢀunsaturated ketones and alkynes. The Pd/Au atomic ratio of the Pd–Au
alloy and the supporting material affected the catalytic activity, and supported Pd–Au alloy
nanoparticles with a low Pd/Au atomic ratio functioned as highly active heterogeneous
catalysts under mild reaction conditions. Structural characterization of supported Pd–Au alloy
catalysts by Xꢀray diffraction (XRD), Xꢀray absorption spectroscopy (XAS) and transmission
electron microscopy (TEM) revealed the formation of random Pd–Au alloy nanoparticles
with a uniform size of around 3 nm on the support. Furthermore, XAS and Xꢀray
photoelectron spectroscopy (XPS) elucidated the chargeꢀtransfer from Pd to Au and the
formation of isolated single Pd atoms in random Pd–Au alloys with a low Pd/Au ratio, which
enabled the efficient hydrosilylation of a variety of substrates under mild reaction conditions.
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Keywords
Hydrosilylation, heterogeneous catalysts, Pd–Au alloy, silyl enol ether, vinylsilane, Xꢀray
absorption spectroscopy
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1. Introduction
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Hydrosilylation of unsaturated organic compounds is the most straightforward and
atomꢀeconomical method for synthesizing organosilicon molecules,1–3 which are widely used
as important synthetic intermediates in the fields of organic chemistry and medicinal
chemistry as well as in polymer science. To achieve efficient and economical hydrosilylation,
highly active catalytic systems that use transitionꢀmetals are needed.4–9 Furthermore, the
current demand for green transformations of organic molecules enhances the importance of
heterogeneous catalysts that can facilitate the recycling of precious metals and help to prevent
contamination of the products by toxic metals.10–20 Among a series of heterogeneous catalysts,
metal nanoparticles (NPs) and their immobilized variants have been extensively investigated
and are considered to be promising tools for the realization of environmentally benign
organic transformations.21–23 To date, monometallic NPs composed of Pt,24–28 Pd,29–32 Rh,33–34
Ni,35and Au36–40 have been used as efficient catalysts for the hydrosilylation of unsaturated
organic compounds. However, the reactions have often been carried out at elevated
temperatures since they show lower activities than homogeneous catalysts. Thus, the
development of a novel catalytic system that can operate at ambient temperature would be
very important with regard to both synthetic and green chemistry. On the other hand,
considerable attention has recently been focused on the catalysis of alloy NPs in the field of
synthetic chemistry.41–44 Alloy NPs, which are composed of two or more kinds of metals, may
exhibit unique catalytic functions due to their completely different electronic states and
structural conformations compared to the original monometallic NPs (ligand and ensemble
effects). For instance, Pd–Au alloy NPs can provide certain catalytic functions in organic
syntheses,45–48 and Pd–Au alloy catalysts have been shown to offer superior catalytic
activities in the oxidation of alcohols,49–51 polyols52–55 and hydrocarbons.56,57 Sakurai et al.
developed the Pd–Au bimetallic nanoclusterꢀcatalyzed efficient Ullman coupling of aryl
chlorides under mild reaction conditions, whereas monometallic Pd or Au NPs showed no
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activity.58–61 Very recently, Yamamoto and Jin et al. reported the 1,4ꢀhydrosilylation of cyclic
α,βꢀunsaturated ketones with the use of nanoporous Pd–Au alloy as a heterogeneous catalyst
to provide the corresponding silyl enol ethers.62,63 However, elevated temperatures were
required to obtain the products in satisfactory yields, and the successful substrates were
limited to cyclic enones. Hence, novel catalytic systems that can be applied to the reactions of
both cyclic and acyclic α,βꢀunsaturated ketones under mild reaction conditions are still
needed. Furthermore, the correlations of the structure and electronic states with the activity of
Pd–Au alloy catalysts have not been fully clarified.
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Herein, we describe the development of highly active supported Pd–Au alloy catalysts for
the hydrosilylation of α,βꢀunsaturated ketones as well as internal alkynes. The atomic ratio of
Pd/Au and the supports for the catalysts remarkably affected their catalytic activities. The
detailed characterization of supported Pd–Au catalysts by a series of spectroscopic techniques
revealed the formation of isolated single Pd atoms surrounded by Au atoms in a random Pd–
Au alloy with a low Pd/Au atomic ratio, which realized the highly efficient selective
hydrosilylation of various unsaturated compounds under mild reaction conditions.
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2. Results and discussion
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2.1 Catalytic performance of supported Pd–Au alloy catalysts for the hydrosilylation of
an α,β-unsaturated ketone.
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The hydrosilylation of α,βꢀunsaturated carbonyl compounds provides the corresponding
silyl enol ethers, which are widely used as important substrates in organic synthetic reactions
such as Mukaiyamaꢀaldol condensations.64–76 The reactions of an acyclic α,βꢀunsaturated
ketone (1a) with triethylsilane (2a) in the presence of SiO2ꢀsupported Pd or Au monometallic
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catalysts at 75 C gave the corresponding silyl enol ether 3a in very low yields (Table 1,
entries 1 and 2). In contrast, supported Pd–Au alloy catalysts gave 3a (Table 1, entries 3–8),
and supported 1Pd3Au/SiO2 showed the highest activity among the SiO2ꢀsupported catalysts
(Table 1, entry 6). Notably, the product was obtained with high Zꢀselectivity. In reactions with
homogeneous metal complex catalysts, the stereoselectivity of the products is generally
controlled with the aid of phosphine ligands.75 In contrast, the present supported Pd–Au
catalysts did not require the addition of organic ligands to realize highly stereoselective
reactions. Furthermore, the reactions proceeded chemoselectively, and products resulting
from the 1,2ꢀhydrosilylation of 1a were not obtained at all. Subsequently, the effect of the
support on the catalytic activity of 1Pd3Au alloy nanoparticles was investigated. The reaction
at room temperature with catalysts supported on SiO2, Al2O3 and CeO2 resulted in low yields
of 3a (Table 1, entries 9–11). On the other hand, ZrO2ꢀ and TiO2ꢀ supported catalysts showed
moderate to good activities (Table 1, entries 12 and 13), and the product 3a was obtained in
the highest yield with Nb2O5ꢀsupported catalysts. With these results in mind, we again
surveyed the optimal Pd/Au ratio for Nb2O5ꢀsupported catalysts (Table 1, entries 14–19). As a
result, Nb2O5ꢀsupported 1Pd5Au alloy showed the highest activity, and 3a was obtained in a
total yield of 85% after the reaction had proceeded for 1 h at room temperature. Neither the
support nor the Pd/Au atomic ratio influenced the E/Z selectivities of the products.
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Table 1. Optimization of the catalyst
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yield (%)a
entry
catalyst
Pd/SiO2
temp. / oC
time
3a
Z/E
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r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
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1Pd1Au/SiO2
1Pd2Au/SiO2
1Pd3Au/SiO2
1Pd5Au/SiO2
1Pd10Au/SiO2
1Pd3Au/SiO2
0
3
89 : 11
92 : 2
94 : 6
94 : 6
93 : 7
93 : 7
ꢀ
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11 1Pd3Au/CeO2
12 1Pd3Au/ZrO2
13 1Pd3Au/TiO2
14 1Pd3Au/Nb2O5
15 1Pd5Au/Nb2O5
16 1Pd10Au/Nb2O5
17 Au/Nb2O5
3 h
1
3 h
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94 : 6
93 : 7
93 : 7
92 : 8
93 : 7
93 : 7
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85
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3 h
1 h
3 h
3 h
18 Pd/Nb2O5
aDetermined by GLC.
3 h
0
ꢀ
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A series of silyl enol ethers were synthesized through the hydrosilylation of unsaturated
ketones with 1Pd5Au/Nb2O5 catalysts under mild conditions (Table 2). The reactions of a
wide range of substituted chalcones with triethylsilane proceeded smoothly to give the
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corresponding silyl enol ethers (3b
silyl enol ether bearing a furan ring (3j) was obtained in satisfactory yield. Alkylꢀsubstituted
ꢀunsaturated ketone could also be used in the present catalytic system to furnish 3k in
good yield. The reactions of an ꢀunsaturated aldehyde and methyl ketones provided the
corresponding products (3l 3n) in high yields, while a low regioselective reaction and
–3i) in good to high yields with excellent Zꢀselectivity. A
α,β
α,β
–
Z/E
1,2ꢀhydrosilylation were observed as undesirable side reactions. The reaction of cyclic
ketones gave the corresponding silyl enolates (3o and 3p) in good yield. Although longer
reaction times and elevated temperatures were required, the reactions of chalcone with
hydrosilanes other than triethylsilane gave the corresponding products (3q–3s) in high yields.
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Table 2. Scope of substratesa
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OSiEt3
Ph
OMe
3b 67% (94 : 6)
3c 65% (94 : 6)
3d 80% (96 : 4)
3e 72% (96 : 4)
3h 59% (94 : 6)
3f 80%b (95 : 5)
3g 64%c (99 : 1)
3i 69% (95 : 5)
3j 54%c (90 : 10)
3l 75%b,d (0 : 100)
3m 80%b,e (36 : 64)
3k 68% (93 : 7)
3n 77% (32 : 68)
3q 76%f (95 : 5)
3o 68%
3p 66%
3r 81%g (92 : 8)
3s 46%b,g (99 : 1)
a Isolated yields. Values in parentheses show the Z:E ratio determined by 1H
NMR. b Yield was determined by H NMR using mesitylene as an internal
1
standard. c Reaction for 3 h. d 1,4ꢀaddition/1,2ꢀaddition=33/67. e 1,4ꢀaddition
/1,2ꢀaddition =65/35. Reaction for 5 h. g Reaction at 75 oC, for 8 h.
f
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The supported Pd–Au alloy catalysts were also useful in the hydrosilylation of alkynes to
give vinylsilanes, which are versatile and valuable organosilicon compounds in organic
synthesis. Careful screening of the supported catalysts also demonstrated that 1Pd5Au/Nb2O5
was a useful catalyst for the hydrosilylation of alkynes.77 Table 3 shows the results of the
reactions of a variety of internal alkynes with hydrosilanes under mild conditions; the
corresponding vinylsilanes were obtained in high yields. Several examples of the
heterogeneous Ptꢀ, Pdꢀ, Rhꢀ or Auꢀcatalyzed hydrosilylation of internal alkynes have been
developed so far. We describe here the first example of such reactions at ambient temperature.
All of the products in the present alkyne hydrosilylation were obtained with complete trans
configurations.
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Table 3. Hydrosilylation of internal alkynes by PdAu/Nb2O5
5a 88%
5b 85%
5c 75%
5d 73%b
5f 88%
5e 93% (47 : 53)c
o
a Isolated yield. b Reaction at 40 C for 3 h. c Regioselectivity was determined by 1H NMR.
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2.2 Characterization of supported Pd–Au alloy catalysts
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To understand the remarkable effects of the Pd/Au ratio on the catalytic activities in the
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hydrosilylation of α,βꢀunsaturated ketones, the structural and electronic states of supported
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Pd–Au alloy catalysts were characterized by a series of spectroscopic techniques. Although
Nb2O5ꢀsupported catalysts showed the highest activity, the following discussions on the effect
of the Pd/Au ratio on catalytic activity are based on the characterization results for
SiO2ꢀsupported catalysts. As described later, this is because some of the characterization data
for Nb2O5ꢀsupported catalysts could not provide sufficient information about the structure
and electronic state of Pd–Au NPs due to the detection limits.
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Figure 1 shows the XRD patterns of SiO2ꢀsupported Pd–Au alloy catalysts with different
Pd/Au ratios. In the patterns of SiO2ꢀsupported Pd or Au monometallic catalysts, the
diffraction peaks due to Au(111) and Pd(111) appeared at 2θ = 38.3 and 40.1 (ICDD PDFꢀ2;
Au: No.00ꢀ004ꢀ0784 and Pd: No.00ꢀ005ꢀ0681), respectively. In the spectra of Pd–Au alloy
catalysts, the peaks appeared between the two original peaks, and shifted to a lower angle
with a decrease in the Pd/Au atomic ratio. Figure 2 shows a plot of the lattice parameters of
Pd–Au alloy NPs estimated from the XRD diffraction pattern of (111) phases as a function of
the amount of Au in Pd–Au NPs as determined by atomic absorption analysis.78 The dashed
line in Figure 2 shows the theoretical relationship between the lattice constant and the Pd/Au
ratio given by Vegard’s law.79 The lattice parameter increased as the Au content in the alloy
NPs increased, and the plot closely follow the dashed line. These results indicate that the two
different metals in the NPs are homogeneously mixed to form random alloys. On the other
hand, in the patterns of Al2O3ꢀ, CeO2ꢀ, ZrO2ꢀ, TiO2ꢀ and Nb2O5ꢀsupported catalysts, distinct
peaks attributed to 1Pd3Au NPs and Pd–Au NPs with different Pd/Au ratios were not
identified due to the formation of small alloy NPs (Figures S1 and S2 in SI).
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Au(111)
Pd(111)
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200
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(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
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2θ / degree
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Figure 1. XRD patterns of SiO2ꢀsupported PdAu alloy catalysts with
different Pd/Au ratios (a)Pd only, (b)3Pd1Au, (c)1Pd1Au, (d)1Pd2Au,
(e)1Pd3Au, (f)1Pd5Au, (g)1Pd10Au, (h)Au only
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Au content (%)
Figure 2. Dependence of the lattice parameters of PdAu alloys on the
relative amount of Au.
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The morphologies of Pd–Au alloy NPs were identified by TEM observation.
HAADFꢀSTEM images and the size distribution histograms for SiO2ꢀ or Nb2O5ꢀsupported
Pd–Au alloy catalysts with a series of Pd/Au ratios are shown in Figures 3 and 4. Figure 5
shows those of 1Pd3Au alloy catalysts on various supports. Although a small fraction of
PdAu NPs with large diameter (>10 nm) in the NPs with low Pd/Au ratios were observed,
wellꢀdispersed Pu–Au alloy NPs with a diameter of ca. 3 nm were found to be mainly present
on the surface of each support and noteworthy differences in the mean diameter were not
observed. This strongly suggests that remarkable differences in the catalytic activities for
hydrosilylation were mainly dominated by the factor other than the differences in the particle
size of the alloy NPs.
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To evaluate the electronic state of supported Pd–Au alloy catalysts, XPS measurements
were carried out. Figure 6 shows the XP spectra of SiO2ꢀsupported Pd–Au catalysts with a
series of Pd/Au atomic ratios. In the spectrum of 1Pd10Au/SiO2, the peaks due to Au 4f7/2 and
4f5/2 appeared at 84.1 eV and 87.6 eV, respectively. On the other hand, the peaks of other Pd–
Au alloy catalysts were shifted to a lower binding energy with an increase in the Pd/Au ratio.
This clearly indicated that electron transfer from Pd atoms to Au atoms occurs when these
elements form an alloy. This electron transfer is consistent with the Pauling electronegativity
of Au (2.54) relative to Pd (2.20). On the other hand, the peaks ascribed to Pd 3d5/2 and 3d3/2
generally appear at around 335 eV and 340 eV, respectively.80 In the present cases, however,
these were too difficult to analyze due to their low intensity and overlapping of the peaks
attributed to Au 4d5/2 (Figure S3 in SI). Similar phenomena were observed in the spectra of
Nb2O5ꢀsupported Pd–Au alloy catalysts with a variety of Pd/Au atomic ratios (Figures S4 and
S5 in SI).
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(a)
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Binding energy / eV
Figure 6. XP spectra around the Au 4f states of SiO2ꢀ
supported PdAu alloy catalysts with different Pd/Au ratios
(a)3Pd1Au, (b)1Pd1Au, (c)1Pd2Au, (d)1Pd3Au, (e)1Pd5Au,
(f) 1Pd10Au.
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To gain further insight into the local structures and electronic states of Pd–Au alloy NPs on
supports, a series of catalysts were characterized by XAS. Normalized Au L3ꢀedge XANES
spectra of SiO2ꢀsupported Pd–Au alloy catalysts and Au foil are shown in Figure 7. The shape
and absorption edge energy of the spectra of supported Pd–Au alloy catalysts with different
Pd/Au ratios are very close to those of Au foil, which suggests that the Au in alloy NPs is
present as metallic states. On the other hand, the whiteꢀline intensities of supported Pd–Au
alloy catalysts appeared at 11921 eV, which is lower than that for Au foil. Furthermore, the
height of the whiteꢀline decreased with an increase in the Pd/Au ratio in alloy NPs. The Au
L3ꢀedge Xꢀray absorption whiteꢀlines correspond to electronic transitions from the 2p3/2
coreꢀlevel state and directly reflect the electronic states of the vacant d orbitals of Au.81 The
intensity of the Au L3 Xꢀray absorption spectrum is related to the dꢀelectron valences, and the
low intensity of the whiteꢀline indicates the presence of electronꢀrich Au species. These
results clearly indicate that chargeꢀtransfer from Pd to Au occurs by alloying of Pd and Au.
These results are completely consistent with the results of XPS analysis. Pd Kꢀedge XANES
spectra of SiO2ꢀsupported Pd–Au alloy catalysts are shown in Figure 8. Regardless of the
Pd/Au molar ratio, Pd Kꢀedge XANES spectra shows the presence of metallic Pd species.
Generally, the electronic states of Pd species can be evaluated by the E0 values in XANES
spectra. However, energy shifts in E0 values of a series of PdAu catalysts was unclear. This is
probably due to lower energy resolution at Pd Kꢀedge (24 .4 keV) than that at Au L3ꢀegde
(11.9 keV). We are now investigating the PdꢀL3 edge XANES spectra of a series of PdAu
catalysts with different PdAu ratio to evaluate their electronic states of the vacant d orbitals of
Pd species in PdAu alloys.
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0.85
Order of intensity at 11921 eV
0.5
Au foil
1Pd10Au
1Pd5Au
1Pd3Au
1Pd2Au
1Pd1Au
3Pd1Au
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(h)
(A)
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Photon energy / eV
Photon energy / eV
Figure 7. Au L3ꢀedge XANES spectra (A) and their magnification around the
whiteꢀline (B) of SiO2ꢀsupported PdAu alloy catalysts with different Pd/Au
ratios (a)3Pd1Au, (b)1Pd1Au, (c)1Pd2Au, (d)1Pd3Au, (e)1Pd5Au, (f) 1Pd10Au,
(g)Au only, and (h) Au foil.
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(a)
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(e)
(f)
(g)
(h)
24320
24360
Photon energy / eV
24400
Figure 8. Pd Kꢀedge XANES spectra of (a) Pd foil and
SiO2ꢀsupported PdAu alloy catalysts with different Pd/Au
ratio (b)Pd only, (c)3Pd1Au, (d)1Pd1Au, (e)1Pd2Au,
(f)1Pd3Au, (g)1Pd5Au, (h) 1Pd10Au
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We also performed curveꢀfitting analyses of AuꢀL3 and PdꢀK edge EXAFS spectra of
supported Pd–Au catalysts.82 Figure 9 shows the relationship between the coordination
number of supported Pd–Au alloy catalysts and the Au content in Pd–Au alloy catalysts as
estimated by atomic absorption spectroscopy. The coordination numbers of PdꢀPd and AuꢀPd
gradually decreased with a decrease in the Pd/Au atomic ratio in the Pd–Au alloy NPs. In
contrast, the coordination numbers of PdꢀAu and AuꢀAu increased with a decrease in the
Pd/Au ratio. Based on these results, possible structures of Pd–Au alloys with different Pd/Au
ratios are illustrated in Scheme 1. The configuration of Pd and Au atoms on the surface of
Pd–Au alloy NPs varies with changes in the Pd/Au ratio, and a number of isolated single Pd
atoms surrounded by Au atoms are present in Pd–Au alloys with a low Pd/Au atomic ratio.
For supported Pd–Au catalysts with Pd/Au ratios of 1/3 and 1/5, which showed high catalytic
activity in the hydrosilylation of α,βꢀunsaturated ketone and internal alkynes, the coordination
numbers of PdꢀPd and AuꢀPd linkages are quite small (< 1.0). These results suggest that Pd–
Au alloy with isolated single Pd atoms surrounded by Au atoms showed high activity in
hydrosilylation. Moreover, Figure 10 shows the relationship between the ratio of
PdꢀAu/PdꢀPd coordination numbers and the turnover frequency (TOF: product mmol• Pd
mmolꢀ1• hꢀ1) based on a Pd atom toward the alkyne hydrosilylation of PdAu/SiO2 catalysts
with various Pd/Au ratios. The ratio of PdꢀAu/PdꢀPd coordination numbers reflects the
degree of isolation of Pd atoms in alloy particles and a high value indicates the presence of a
large number of isolated single Pd atoms surrounded by Au atoms on the surface of Pd–Au
alloy NPs. The close correlation between these two factors clearly suggests that such single
Pd atoms in Pd–Au NPs act as the main active site for the efficient and selective
hydrosilylation of α,βꢀunsaturated ketones and internal alkynes, and the activity of each
catalyst completely depends on the number of isolated single Pd atoms on the surface of Pd–
Au NPs. Similar catalysis have been reported by Zhang and coworkers, in which isolated
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single Pd atom in Au NPs worked as main active site for Ullmann coupling of aryl
chlorides.61
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(B)
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Au content (%)
Figure 9. Relationship between Au content in PdAu/SiO2
and (A) PdꢀPd(●) and PdꢀAu(○) and (B)AuꢀPd(■) and
AuꢀAu(□) coordination numbers estimated by Pd Kꢀ and
Au L3ꢀedge XAFS analysis.
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3Pd1Au 1Pd1Au 1Pd2Au 1Pd3Au 1Pd5Au 1Pd10Au
Figure 10. Relationship between PdꢀAu/PdꢀPd coordination numbers
ratio with the TOF based on a Pd in PdAu/SiO2 toward alkyne
hydrosilylation at 40 oC.
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In the present catalytic system, the supports also remarkably affected the catalytic activity
for hydrosilylation,83 and catalysts supported on acidic metal oxides such as Nb2O5 and TiO2
showed activities superior to those supported on neutral or basic metal oxides. In the XANES
and EXAFS spectra of Nb2O5ꢀsupported Pd–Au catalysts, the overall trend was similar to
those of SiO2ꢀsupported catalysts (Figures S10ꢀS14 and Table S4). The exact role of the
support in the enhanced catalytic activity is not fully understood. It is likely that the Lewis
acidic property of the support decreases the electron density of Pd–Au alloy NPs, which
accelerates the reductive elimination of intermediate C to realize the rapid formation of
products 3. Likewise, the isolated Pd species on the surface of PdAu alloy NPs with low
Pd/Au ratio become electron deficient due to high electronegativity of Au atom, which may
accelerate the catalytic reaction. Detailed kinetic and computational investigations on the
effect of alloying Pd and Au and their supports are currently underway in our laboratory.
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We developed highly active supported Pd–Au alloy catalysts for the hydrosilylation of
α,βꢀunsaturated ketones and alkynes. While supported monometallic Au or Pd NP catalysts
were totally ineffective, the reactions with Pd–Au alloy catalysts proceeded efficiently under
mild reaction conditions to give the corresponding silyl enol ethers and vinylsilanes in high
yields with excellent selectivities. Particularly, supported Pd–Au alloy catalysts with low
Pd/Au atomic ratios showed high activity. The detailed characterization of supported Pd–Au
alloy catalysts by a series of spectroscopic techniques revealed the formation of a random
Pd–Au alloy with a uniform size of around 3 nm. Furthermore, the electronic state and
configuration of Au and Pd atoms in Pd–Au alloy NPs could be varied by changing the Pd/Au
atomic ratio. For catalysts with a low Pd/Au atomic ratio, isolated single Pd atoms were
present in the random Pd–Au alloy, which acted as the main catalytic active site for the
efficient hydrosilylation of unsaturated compounds under mild reaction conditions. Further
applications of supported Pd–Au alloy catalysts in other synthetic reactions as well as
elucidation of the effect of alloying Pd and Au and their supports are currently under
investigation in our laboratory.
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4. Experimental
4.1. Materials
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PdCl2 and HAuCl4·3H2O were purchased from FURUYA METAL Co. Ltd and Wako
Chemicals, respectively. Al2O3 (Sumitomo Chemical Co., Ltd, AKPꢀG015; JRCꢀALOꢀ8
equivalent), TiO2 (JRCꢀTIOꢀ4), ZrO2 (JRCꢀZROꢀ3), CeO2 (JRCꢀCEOꢀ2), were obtained from
the Catalysis Society of Japan. SiO2 (CARiACT Qꢀ10) and Nb2O5·nH2O were kindly
provided by Fuji Silysia Chemical Ltd and CBMM, respectively. Nb2O5 was obtained by the
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calcination of Nb2O5·nH2O at 550 C for 3 h under air flow. Poly(Nꢀvinylpyrrolidone) (PVP,
K30) was purchased from Wako Chemicals. Au(en)2Cl3 (en = ethylenediamine) was
synthesized by the method reported in the literature.84 Other chemicals were of analytical
grade and used as received without further purification.
4.2. Preparation of supported catalysts
4.2.1. Preparation of supported Pd–Au alloy NPs catalysts by a sol-immobilization
method
Supported Pd–Au alloy catalysts were prepared through a colloid immobilization method.85
To an aqueous solution (40 mL) containing the desired molar ratio of PdCl2 to HAuCl4·3H2O
was added PVP (K30, 36 mg), and the solution was cooled to 273 K with an ice bath.
Subsequently, 0.1 M aqueous solution of NaBH4 (8.0 mL, NaBH4/metal (mol/mol) = 5) was
added rapidly under vigorous stirring. After 0.5 h of colloid generation, 0.97 g of inorganic
support was added to the colloidal solution, which was acidified to pH 1–3 with 0.1 M
hydrochloric acid. After vigorous stirring overnight at room temperature, the resulting gray
powder was separated from the suspension by centrifugation, thoroughly washed with
distilled water, and dried overnight at 80 oC. The obtained catalysts were denoted
xPdyAu/support, where x and y indicate the molar ratio of Pd to Au. The total loading
amount of metal was set at 3wt%. Supported monometallic Pd and Rh NPs catalysts were
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prepared by a method similar to that used for supported Pd–Au catalysts, but with the use of
PdCl2 or RhCl3 as a metal precursor.
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4.2.2. Preparation of Au/SiO2 and Au/Nb2O5 catalysts by a deposition-precipitation
method
SiO2ꢀsupported Au catalysts were prepared by a depositionꢀprecipitation method with the
use of Au(en)2Cl3 as a Au precursor.84 The pH value of an aqueous solution containing 50 mg
of Au(en)2Cl3 was adjusted to 10 by 1M NaOH solution. After 0.97 g of SiO2 or Nb2O5 was
added to the solution, the pH value was reꢀadjusted to 10 with 1M NaOH solution. The
suspension was stirred at 343 K for 2 h, separated by centrifugation, and then washed three
times with methanol. The resulting powder was dried in vacuo at 343 K for 5 h. Prior to their
use in catalytic reactions, the catalysts were reduced in a hydrogen atmosphere at 423 K for 1
h and calcined in air at 773 K for 1h. The total loading amount of metal was set at 3wt%.
4.2.3. Preparation of Nb2O5-supported metal NPs catalysts prepared by an
impregnation method
Nb2O5ꢀsupported monometallic (Pt or Ru) catalysts were prepared by an impregnation
method with the use of H2PtCl6 or RuCl3 as a metal precursor. Nb2O5 was added to an
aqueous solution of the metal precursor, and the suspension was stirred at 353 K for 2 h. After
the evaporation of water, the resulting powder was calcined in air at 673 K for 1 h, and then
reduced in a hydrogen atmosphere at 673 K for 1 h to give Nb2O5ꢀsupported Pt and Ru
catalysts. The total loading amount of metal was set at 3wt%.
4.3. Physical and analytical measurements
The products of the catalytic runs were analyzed by GCꢀMS (Shimadzu GCMSꢀQP2010,
o
CBPꢀ10 capillary column, i.d. 0.25 mm, length 30 m, 50
−
250 C) and gas chromatography
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(Shimadzu GCꢀ2014, CBPꢀ10 capillary column, i.d. 0.25 mm, length 30 m, 50−250 C).
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NMR spectra were recorded on a JMNꢀECS400 (FT, 400 MHz (1H), 100 MHz (13C))
) of 1H and 13C{1H} NMR spectra are referenced to SiMe4.
instrument. Chemical shifts (
δ
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The supported catalysts were analyzed by nitrogen gas adsorption, TEM, XRD, XPS and
XAFS. High angle annular dark fieldꢀscanning transmission electron microscope
(HADDFꢀSTEM) images were recorded using a JEOL JEMꢀ3200FS transmission electron
microscope. The samples were prepared by depositing drops of ethanol suspensions
containing small amounts of the powders onto carbonꢀcoated copper grids (JEOL Ltd.)
followed by evaporation of the ethanol in air. Xꢀray powder diffraction analyses were
performed using Cu K
α
radiation and a oneꢀdimensional Xꢀray detector (XRD: SmartLab,
=36° to 42° at a scanning rate of 0.067 sꢀ1 and
RIGAKU). The samples were scanned from 2
Ө
a resolution of 0.01°. Xꢀray photoelectron spectroscopy (XPS) of the catalysts was performed
using a JPSꢀ9010 MX instrument. The spectra were measured using MgKα radiation (15 kV,
400 W) in a chamber at a base pressure of ~10ꢀ7 Pa. All spectra were calibrated using C1s
(284.6 eV) as a reference. The Brunauer–Emmett–Teller (BET) specific surface area was
estimated from N2 isotherms obtained using a BELSORPꢀmini II (BEL Japan, Osaka, Japan)
at 77 K. The analyzed samples were evacuated at 573 K for 2 h prior to the measurement. Pd
Kꢀedge and Au L3ꢀedge XAFS measurements were performed at the BL01B1 beam line at
SPringꢀ8 operated at 8 GeV using a Si(311) twoꢀcrystal monochromator. XAFS spectra were
obtained at room temperature. XAFS data were processed to isolate EXAFS spectra from the
background using Athena software. Replicate scans were averaged to increase the
signalꢀtoꢀnoise ratios. The resulting EXAFS spectra (k space) were Fourier transformed (Pd
K, k range/10 nm−1 3.0 < k< 15.0; Au L3, k range/10 nm−1 3.0 < k< 15.0) and fitted in R space
(Pd K, ꢀR = 1.8− 3.2 /10ꢀ1 nm; Au L3, ꢀR= 1.8−3.3/10ꢀ1 nm) using Artemis software. Pd−Pd,
Pd−Au, Au−Au, and Au−Pd single scattering paths were generated using FEFF. Amplitude
2
reduction factors, for the Pd−Pd and Au−Au paths, S0 = 0.87 and 0.82, respectively, were
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determined by fitting the EXAFS of Pd and Au foils, respectively. The reported EXAFS
parameters C.N. (coordination number), R (interatomic distance), σ2 (Debye− Waller factor),
and ꢀE0 (inner potential shift) are based on simultaneous fits of k3 ꢀweighted spectra.86, 87
XANES were analyzed using the REX2000 version 2.5 (Rigaku). The actual contents of
palladium and gold species immobilized on the supports were determined by atomic emission
spectroscopic analysis with a SHIMADZU AAꢀ6200.
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4.4 General procedure for catalytic hydrosilylation
A typical reaction procedure is as follows: α,βꢀunsaturated ketones (1.0 mmol) and CH3CN
(2.0 mL) were added to a Schlenk tube containing the supported Pd–Au catalyst (2 mol% as
metal) under an Ar atmosphere. The amount of Pd–Au catalyst used in the catalytic reaction
was calculated based on the total amount of the two metals. The reaction was initiated by the
injection of 1.0 mmol of hydrosilane at room temperature. The progress of the reaction was
monitored by thin layer chromatography (TLC) and GC analysis. After the ketones were
completely comsumed, the products were quantified by GC using biphenyl as an internal
standard. For isolation of the products, the solid catalyst was removed by passing the mixture
through a 0.45 ꢁm polytetrafluoroethylene (PTFE) filter (Millipore Millex LH) The
remaining solution was concentrated under reduced pressure and purified through silica gel
column chromatography (hexane : NEt3 = 100 : 1, v/v) to give the product. The isomeric ratio
and E/Z configuration of the products were determined by 1H NMR and NOESY,
respectively.
4.5 Characterization data of the products
Products 3a75, 3l88, 3m75, 3n75, 3o62 and 3p62 are known compounds and were identified by
comparison of their NMR features with the respective reported data.
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