Composition Control Synthesis and Catalytic Properties in the Suzuki Reaction of Bimetallic PdSn Nanoparticles

Article abstract of DOI:10.1002/kin.20965

We have successfully prepared 6.5 nm palladium tin (PdSn) alloy nanoparticles (NPs) with tunable compositions by high-temperature reduction of tin acetate and palladium bromide in the presence of oleylamine and trioctylphosphine. The catalytic activities of PdSn NPs with different compositions were evaluated through Suzuki reactions. The PdSn nanocatalysts show better catalytic activity on Suzuki reactions than an equal amount of pure Pd NPs, and their catalytic activities are highly composition dependent. Among these NPs, Pd₆₃Sn₃₇/C NPs exhibited the highest catalytic performance with higher reaction activity, lower Pd leaching properties, and higher stability even after eight recycle reactions.

Full text of DOI:10.1002/kin.20965

Composition Control
Synthesis and Catalytic
Properties in the Suzuki
Reaction of Bimetallic PdSn
Nanoparticles
YONG LI,1 TIAN WANG, BO YANG, XI-KE TIAN
,2
1
1
1
1Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan, 430074, People’s Republic of China
2Engineering Research Center of Nano-Geo Materials of Ministry of Education China University of Geosciences, Wuhan,
4
30074, People’s Republic of China
Received 9 July 2015; revised 9 October 2015; accepted 9 October 2015
DOI 10.1002/kin.20965
Published online 23 November 2015 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: We have successfully prepared 6.5 nm palladium tin (PdSn) alloy nanoparticles
(NPs) with tunable compositions by high-temperature reduction of tin acetate and palladium
bromide in the presence of oleylamine and trioctylphosphine. The catalytic activities of PdSn
NPs with different compositions were evaluated through Suzuki reactions. The PdSn nanocat-
alysts show better catalytic activity on Suzuki reactions than an equal amount of pure Pd NPs,
and their catalytic activities are highly composition dependent. Among these NPs, Pd63Sn37/C
NPs exhibited the highest catalytic performance with higher reaction activity, lower Pd leaching
properties, and higher stability even after eight recycle reactions. ꢀC 2015 Wiley Periodicals,
Inc. Int J Chem Kinet 48: 3–10, 2016
INTRODUCTION
cesses, which are the first choice of chemists because of
their high activity and selectivity, however, show some
shortcomings, such as difficulty in separating the cat-
alyst from the final reaction product, high Pd-leaching
properties, low stabilities, and so on [2–4]. The re-
covery and reusability of the catalyst represents a key
issue for the sustainable development of any catalytic
process [5].
To solve above-mentioned problems, new cata-
lysts should be designed and applied. Bimetallic alloy
nanoparticles (NPs) are particularly important in the
field of catalysis because they often exhibit better cat-
alytic properties than their monometallic counterparts
Suzuki reactions are generally catalyzed by soluble
palladium (Pd) complexes with various ligands in or-
ganic solvents and, more recently, in aqueous media
as well [1]. Many of these homogeneous catalytic pro-
Correspondence to: Y. Li; e-mail: liyong07@126.com. Xike
Tian; e-mail: xktian@cug.edu.cn.
Contract grant sponsor: Fundamental Research Funds for the
Central Universities.
Contract grant number: CUG120802.
Contract grant sponsor: National Natural Science Foundation of
China.
Contract grant number: B070302.
ꢀ
2015 Wiley Periodicals, Inc.
C

4
LI ET AL.
[
6]. Pd-related NPs, such as Pd NPs, PdM (M = Cu,
(technical grade, 90%), and carbon black (Kejen EC
300J) were purchased from Sigma-Aldrich Strem
Chemical, Saint Louis, MO, USA. Acetic acid, K2CO3,
aryl iodide, aryl bromide, phenylboronic acid, and
other reagents were purchased from Sinopharm Chem-
ical Reagent (People’s Republic of China). All of the
reagents used herein were of analytical grade and used
as received without any further purification. Water used
in the experiments was deionized (DI) and doubly dis-
tilled prior to use.
Ag, Ni, Co, Pt) alloy NPs, and Pd core/shell (Au/Pd,
Ag/Pd, Pd/Pt) [7,8] NPs were most widely applied to
many important reactions, such as carbon-coupling re-
actions, small molecule electrooxidation/reduction re-
actions, hydrocarbon hydrogenations, isomerization,
alkylation, hydrodealkylation, polymerization, so on
[
9,10]. For Pd alloy NPs, mixing a second nonprecious
element with the Pd metal is an effective way to reduce
the precious metal usage and may enhance its internal
properties due to synergistic effects and the rich diver-
sity of the compositions [11,12]. Among the Pd alloys,
the PdSn alloy nanostructures have attracted intense
attention in both synthesis and catalysis fields, such as
hydrogenation of butadiene, hydrogenation of nitrate
to nitrogen, and formic acid oxidation [13–15]. Previ-
ous studies show that the overall Pd/Sn composition
can be easily tuned with very limited morphology con-
trol. Pd1Sn1 nanostructures were synthesized from the
cubic β-Sn nanocubes via a simple solution-mediated
chemical conversion strategy when the Sn was partially
replaced by Pd when heated in Pd salt solution [16].
In the literature, PdSn NPs were synthesized by tra-
ditional coreduction of Pd and Sn salts directly on
different supporting materials such as amorphous car-
bon [17]. However, all the reported syntheses did not
provide sufficient control over either the morphology
or composition of the NPs to meet the high standard of
a catalyst.
In this paper, the uniform 6.5-nm PdSn alloy
NPs with tunable compositions on the Pd-rich end
were successfully synthesized via the controlled high-
temperature coreduction of palladium bromide and tin
acetate in the presence of oleylamine (OAm) and tri-
octylphosphine (TOP). PdSn NPs were loaded on car-
bon black for the evaluation of their catalytic abil-
ities in Suzuki reactions. The PdSn/C NPs showed
composition-dependent activity for Suzuki reactions
with an optimum composition of Pd63Sn37/C. For com-
parison, 6.5-nm Pd NPs were also prepared according
to the previous literature [18]. Even comparing to the
Pd/C NPs, Pd63Sn37/C NPs exhibited higher yield and
better stability and lower leaching rate of Pd in al-
loy. In addition, we also further evaluated the superior
Pd63Sn37 NPs/C catalyst for a group of common Suzuki
reactions.
Characterization Techniques
Transmission electron microscopy (TEM) images were
acquired on a Philips CM 20 (200 kV) transmis-
sion electron microscope. High-resolution transmis-
sion electron microscopy (HRTEM) images are ob-
tained with JEOL 2010 (200 kV) and JEOL 4000
EX transmission electron microscopes. X-ray pow-
der diffraction (XRD) patterns of the samples were
recorded on a Bruker AXS D8-Advanced diffractome-
˚
ter with Cu Kα radiation (λ = 1.5418 A). The com-
positions of the samples were measured by inductively
coupled plasma–atomic emission spectroscopy (ICP-
AES). The yields of Suzuki reactions were determined
by gas chromatography-mass spectroscopy (GC-MS)
analysis.
Preparation of PdSn NPs with Controlled
Compositions
PdSn and PdSn/C NPs with different compositions
were synthesized following the below procedure:
0.2 mmol PdBr2 and 0.1 mmol Sn(ac)2 were mixed
with 12 mL OAm in the reaction flask with magnetic
stirring and degassed at room temperature for 10 min
under N2 flow. 0.5 mL TOP was added into the de-
gassed reaction mixture; the mixture was degassed at
room temperature before being quickly heated up to
260°C, and the reaction was maintained at this tem-
perature for 1 h. After it was cooled down, Pd63Sn37
NPs were precipitated out through ethanol addition and
collected through centrifugation. Pd63Sn37 NPs were
redispersed in hexane and stored in hexane. The com-
position control was achieved simply by changing the
molar ration of PdBr2/Sn(ac)2 used in the synthesis. For
PdBr2/Sn(ac)2 (mmol/mmol) of 0.2/0.05, 0.2/0.15, and
0
5
.2/0.2, PdSn NPs with Pd/Sn atomic ratios of 77/23,
2/48, and 49/51 were synthesized, respectively.
EXPERIMENTAL
Materials
Preparation of PdSn/C NPs
Palladium(II) bromide (PdBr2, 99%), Tin(II) acetate
Ten milligrams of PdSn NPs was dispersed in 10 mL
hexane in a 20-mL vial, and the appropriate of Ketjen
(Sn(ac)2, 99%), OAm (technical grade, 70%), TOP
International Journal of Chemical Kinetics DOI 10.1002/kin.20965

SYNTHESIS AND CATALYTIC PROPERTIES OF BIMETALLIC NANOPARTICLES
5
Figure 1 (A) TEM image and SAED (inset) of the Pd77Sn23 NPs, (B) HRTEM image of the Pd77Sn23 NPs, and (C) TEM
image of Pd77Sn23/C NPs.
carbon support was carefully added to it. This mixture
was sonicated for 2 h, and PdSn/C NPs were separated
by adding 30 mL of ethanol and centrifugation. PdSn/C
nanocatalysts were dried and weighed. In addition, the
content of PdSn/C NPs was determined by ICP-AES
so as to ensure that the content of PdSn/C NPs was con-
sisted of 10 mg PdSn NPs and 10 mg carbon. Finally,
PdSn/C NPs were redispersed in a measured amount
of deionized water by sonication, resulting in water
dispersion.
mental Pd and Sn NPs. The composition of PdSn NPs
was easily controlled by simply changing the ratio of
PdBr2 and Sn(ac)2. With metal salts of Pd/Sn ratios
at 0.2/0.05, 0.2/0.07, 0.2/0.1, 0.2/0.15, and 0.2/0.2,
PdSn NPs with Pd/Sn atomic ratios of 77/23, 67/63,
63/37, 52/48, and 49/51 were obtained, respectively,
which was determined by the ICP-AES method, and
the atomic ratio of Pd in the precursors was carried over
to the final product. A further increase of the Sn(ac)2
to 0.25 mmol cannot lead to a further increase of Sn in
the NPs, and the excess Sn(ac)2 leads to a polydisperse
and ununiform product (Fig. 3f). So this synthesis has a
limited control over the composition with the Pd1Sn1 to
be the Sn-rich end. It looks that under current synthetic
conditions, Sn-rich PdSn alloy NPs could not be syn-
thesized. In Fig. 1A, the as-synthesized Pd77Sn23 NPs
exhibited spherical morphology as shown in the TEM
image with the size of about 6.5 nm. The Pd77Sn23 al-
loy NPs have a narrow size distribution with a standard
deviation of 8% with respect to the diameter of the
NPs (measured from Pd77Sn23 NPs). Crystal fringes
are observed from HRTEM (Fig. 1B), indicating the
polycrystalline structure of the as-prepared Pd77Sn23
NPs. X-ray diffraction (XRD) patterns were applied to
characterize the crystal phases of Pd77Sn23, Pd63Sn37,
and Pd52Sn48 NPs, which showed different XRD pat-
terns as the composition changes (Fig. 2). Furthermore,
we found that these NPs possess the similar structure
and the broad XRD peaks agree with the polycrys-
talline nature and sub-NPs size, which also validated
their polycrystalline structures. Figure 3 displays the
more representative TEM images of PdSn NPs with
different compositions (Pd77Sn23, Pd67Sn33, Pd63Sn37,
Pd52Sn48, and Pd49Sn51). We can see that the sizes of all
these prepared NPs are about 6.5 nm, which provides
a good platform to discuss the composition-dependent
catalysis activity. As seen from the XPS spectra of
Pd3d of Pd and PdSn NPs in Fig. 4, the binding en-
ergy of 335.4 and 340.7 eV corresponds to the 3d5/2
and 3d3 components of the metallic Pd(0) state, further
General Procedure for the Suzuki Reaction
Typically, the as-prepared 0.01 mmol catalyst was dis-
persed in 15 mL ethanol, and then 1 mmol aryl iodide,
1
.5 mmol phenylboronic acid, and 2 mmol K2CO3 were
added into a 25-mL round-bottom flask. The flask was
sealed and stirred in a water bath at 70°C for 1 h. Af-
ter cooling down to the room temperature, the catalyst
was recycled for the next reaction cycle by acetone
precipitation and centrifugation. And the filtrated so-
lution was collected and evaporated to get the product.
The presence of the product was determined by GC-
MS analysis. The experiments were repeated under the
same condition with the catalyst recycled. The palla-
dium content in the extractant solution was analyzed
using ICP-AES.
RESULTS AND DISCUSSION
The as-synthesized PdSn NPs were prepared by high-
temperature reduction of PdBr2 and Sn(ac)2 in the pres-
ence of TOP and OAm. In the reaction, TOP served as
the cosurfactant and is likely to form a Pd- and Sn-TOP
co-complex and promoted the coreduction of Pd and
Sn salts. In addition, OAm acted as the solvent, surfac-
tant, and reductant in the synthetic procedures. In the
absence of OAm and TOP, the quality of PdSn alloy
NPs is quite poor and often contaminated by the ele-
International Journal of Chemical Kinetics DOI 10.1002/kin.20965

6
LI ET AL.
Figure 4 XPS spectra of Pd 3d of Pd and PdSn NPs.
Figure 2 XRD patterns of the as-prepared Pd77Sn23 NPs,
Pd63Sn37 NPs, and Pd Sn48 NPs.
52
and PdSn NPs were conducted in ethanol solution at
70°C for 1 h with the addition of K2CO3 to neutralize
the acid generated. And the yields of Suzuki reactions
are listed in Fig. 5. We can see that mixing Sn to synthe-
size PdSn NPs is quite vital for the catalytic ability of
the Suzuki reaction, and the approximate existence of
tin would greatly improve the catalytic ability. The en-
hancement probably results from the electronic effect
and the synergistic effect in the PdSn NPs. The addi-
tion of Sn in our study would lead to a decrease of Pd
(3d) binding energy intensity and weaken the energy of
carbon and substituted atoms bond, as shown in Fig. 4,
testifying to the presence of metallic Pd in the either
Pd and PdSn NPs.
When PdSn NPs alone were used as nanocatalysts in
the Suzuki reaction, the NPs would aggregate with each
other greatly, thus decreasing the specific surface areas
and catalytic ability. So PdSn NPs should be loaded on
carbon (PdSn/C) to catalyze the Suzuki reaction, and
its TEM image is shown in Fig. 1C. Suzuki reactions of
iodobenzene and phenylboronic acid catalyzed by Pd
Figure 3 TEM images of PdSn NPs with different compositions: (a) Pd77Sn23 NPs, (b) Pd67Sn33 NPs, (c) Pd63Sn37 NPs,
d) Pd Sn48 NPs, (e) Pd49Sn NPs, and (f) Pd49Sn51 NPs.
(
52
51
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SYNTHESIS AND CATALYTIC PROPERTIES OF BIMETALLIC NANOPARTICLES
7
Figure 5 The yields of the Suzuki reaction of iodobenzene
and phenylboronic acid catalyzed by Pd/C NPs and PdSn/C
NPs with different compositions (Pd77Sn23 NPs, Pd63Sn37
Figure 6 The yields for eight cycles of the Pd63Sn37/C
NPs catalyzed Suzuki reaction of iodobenzene and phenyl-
boronic acid. Reaction conditions: 0.01 mmol Pd63Sn37/C
NPs as nanocatalysts; 1 mmol iodobenzene, 1.5 mmol
phenylboronic acid, and 2 mmol K2CO3 in ethanol solution
at 70°C for 1 h.
NPs, and Pd Sn48 NPs). Reaction conditions: 0.01 mmol
52
Pd/C or PdSn/C NPs as nanocatalysts; 1 mmol iodobenzene,
.5 mmol phenylboronic acid, and 2 mmol K2CO3 in ethanol
1
solution at 70°C for 1 h.
Lifetime is an important factor to influence the
catalytic ability. For that reason, the lifetime of the
Suzuki reaction of iodobenzene and phenylboronic
acid catalyzed by Pd63Sn37/C NPs was investigated.
As seen in Fig. 6, the yield of the reaction is de-
creasing from almost 100% to about 93% through
the recycling experiments due to the subtle size in-
crease of NPs, and the morphologies of Pd63Sn37/C
NPs have almost no changes (Figs. 7A and 7B), which
indicated that Pd63Sn37/C NPs are efficient nanocata-
lysts for this kind of Suzuki reactions. The Pd-leaching
property is another important parameter affecting the
thus enhancing the catalytic ability [19–22]. In the al-
loy NPs, although Sn itself has no catalytic activity and
only functions as a co-catalyst, the synergistic effect
greatly affected the catalytic ability. Among these al-
loy NPs, Pd63Sn37/C catalyst has the highest catalytic
performance, resulting from an obvious decrease of
Pd (3d) intensity and the most optimal synergistic ef-
fect between and Pd and Sn [23–25]. Therefore, the
amount of the additive Sn is a pivotal factor influ-
encing the catalytic performance of Pd for the Suzuki
reaction.
Figure 7 TEM images of (A) Pd63Sn37/C NPs after one cycle and (B) Pd63Sn37/C NPs after eight cycles of the Suzuki reaction
of iodobenzene and phenylboronic acid.
International Journal of Chemical Kinetics DOI 10.1002/kin.20965

8
LI ET AL.
Figure 9 Yields of the Suzuki reaction of iodobenzene and
phenylboronic acid catalyzed by Pd63Sn37/C nanocatalysts
as a function of time under standard conditions and removed
Pd63Sn37/C after 15 min.
Figure 8 Pd-leaching contents of Pd63Sn37/C NPs deter-
mined by the ICP-AES method in the cycled Suzuki re-
actions of iodobenzene and phenylboronic acid. Reaction
conditions: 0.01 mmol Pd63Sn37/C NPs as nanocatalysts;
1
mmol iodobenzene, 1.5 mmol phenylboronic acid, and 2
mmol K2CO3 in ethanol solution at 70°C for 1 h.
action mixture solution was still heated and refluxed
for further reacting. We found that the Suzuki reaction
of iodobenzene and phenylboronic acid still contin-
ued until the reaction yield reached at 91% after about
another 30 min. Inductively coupled plasma analysis
indicated that leached Pd species existed in the filtrate
after catalyst removal. The experimental results indi-
cated that a small amount of Pd was leached into the
reaction mixture solution, and the leached Pd species
can indeed catalyze the Suzuki reaction. Furthermore,
this implies that leached Pd species play an impor-
tant role in the high activity of Pd63Sn37/C for Suzuki
couplings.
To find out what parameters to influence the Pd-
leaching properties, after literature survey, we know
that one or more of the reactants, i.e., the aryl-halide,
the aryl-borate, or the base, are the factors causing
Pd leaching [32]. Especially, oxidative addition of the
aryl bromides is the main factor causing Pd leach-
ing during Suzuki couplings. For the Suzuki reaction
of iodobenzene and phenylboronic acid catalyzed by
PdSn/C nanocatalysts, the catalytic efficiency of the
present NPs is due to Pd leaching, which is triggered
by the synergistic action of PhB(OH)2 and K2CO3,
and that the leached Pd species survive long enough in
the reaction medium to allow the next catalytic cycle.
The corresponding catalytic efficiencies are remark-
able since the amounts of leached Pd species are very
small [33,34]. For carbon-coupling reactions, Reetz
and Westermann indicated that leaching is likely to
occur from the nanoparticle surfaces [35] because the
surface atom of a metal is the highly active catalytic
site [36,37]. Lai and co-workers assumed that the NPs
were subjected to a chemical dealloying process [38].
catalytic ability. So Pd-leaching properties of
Pd63Sn37/C NPs in every recycle Suzuki reaction were
determined by Pd content determination in the extrac-
tant solution by using the ICP-AES method (Fig. 8). As
seen, for the Suzuki reactions, the Pd-leaching amount
is only increasing from 0.023% for the first recycle
to 0.069% for the eighth recycle reaction, which may
be due to the increase of nanosizes [26,27]. To fig-
ure out whether the catalytic ability stems from the
NPs themselves, in other words heterogeneous cataly-
sis [28,29], or results from traces of Pd leaching into
solution and acting homogeneously [30]. We carried
out a detailed investigation of the mechanism of Pd
leaching from Pd63Sn37/C during Suzuki coupling re-
actions of iodobenzene and phenylboronic acid. First,
the existence of Pd leaching during our model reac-
tion was verified using the filtration test, which is a
straightforward method to distinguish homogeneous
and heterogeneous catalytic activity, based on the com-
parison of the reaction progress before and after re-
moval of the solid phase [31]. If the reaction proceeds
after the removal of the solid catalyst—in this case
Pd63Sn37/C—by filtration, this is clear evidence that
leaching forms homogeneous Pd in the filtrate that cat-
alyzes the Suzuki reaction. As shown in Fig. 9, the
reaction yields of about 99% for the Suzuki reaction
after reacting for 60 min. In another experiment, the
Suzuki reaction was conducted under the standard con-
ditions. After about 15 min, the catalyst Pd63Sn37/C
was removed from the reaction mixture solution with
the reaction yield for about 27 min. After that, the re-
International Journal of Chemical Kinetics DOI 10.1002/kin.20965

SYNTHESIS AND CATALYTIC PROPERTIES OF BIMETALLIC NANOPARTICLES
9
Table I The Yields of Suzuki Reactions of Different Aryl
Derivatives with Phenylboronic Acid Using Pd63Sn37/C
a
NPs as Nanocatalysts
Entry
Aryl Halides
Product
Yield (%)
1
2
95
99
3
4
42
98
5
6
23
99
7
78
91
Figure 10 Schematic presumption of the catalytic mech-
anism for the Suzuki reaction of iodobenzene and phenyl-
boronic acid catalyzed by PdSn/C nanocatalysts.
8
a
Reaction conditions: 0.01 mmol Pd63Sn37/C NPs as nanocata-
lysts; 1 mmol aryl derivatives, 1.5 mmol phenylboronic acid, and
In performing the Suzuki reaction of iodobenzene and
phenylboronic acid catalyzed by PdSn NPs, the het-
erogeneous reaction occurs only on the surface of the
PdSn NPs adsorbed on the carbon support. In this path-
way, the surface atoms of the palladium nanoclusters
participate in the first step of the Suzuki couplings, the
oxidative addition of aryl bromides. Palladium then
becomes Pd(II) and catalyzes the rest of the cycle. It
is assumed that the carbon support and the adjacent
Pd(0) firmly bond the Pd(II) species and prevent it
from leaching during the course of the reaction. In
the homogeneous pathway, palladium leaches from
the support to the solution, after becoming a soluble
Pd(II) species [39], and then catalyzes the remaining
steps of the reaction. At last, the reaction ends at the
reformation of the NPs. The speculation mechanism of
the Suzuki reaction catalyzed by the PdSn/C nanocat-
alyst is shown in Fig. 10.
2
2 3
mmol K CO in ethanol solution at 70°C for 1 h.
yield of m-iodotoluene and 1-iodo-2-methoxybenzene
with phenylboronic acid is 78% and 91%, respec-
tively (Table I, entries 7 and 8), showing that electron-
withdrawing substituents enhance the coupling prod-
uct formation whereas electron-donating groups have
a negative influence on the reaction process. From the
catalytic ability investigations of Pd63Sn37/C NPs, we
can come to a conclusion that these NPs are efficient
nanocatalysts for Suzuki reactions.
CONCLUSIONS
In summary, we have successfully synthesized a series
of PdSn NPs with different compositions by tuning
the ratios of precursors. The NPs were loaded onto
the surfaces of carbon for the catalytic abilities in-
vestigations in Suzuki reactions. Among the nanocat-
alysts, Pd63Sn37/C NPs are better nanocatalysts than
the NPs with other different compositions. We also in-
vestigated the reusability of Pd63Sn37/C NPs in Suzuki
reactions, and the NPs can be recycled for at least
eight times with few changes in NPs morphology and
yields. Pd63Sn37/C NPs exhibited quite low Pd leach-
ing with no more than 0.069% even after eight recycle
reactions. In summary, Pd63Sn37/C NPs are very good
An expanded analysis of the catalyst reactivity on
aryl halides and phenylboronic acid was further stud-
ied, as demonstrated in Table I. The yields of the reac-
tions were found to be between 23% and 99% depend-
ing on the nature of aryl bromo and iodo derivatives.
For the reactions of bromobenzene, p-bromophenol,
4
-bromobenzene carboxylic acid, iodobenzene, p-
iodophenol, 4-iodobenzene carboxylic acid, and aryl-
boric acid, the yields of bromobenzene derivatives
reactions are lower than those of iodobenzene deriva-
tives (Table I, entries 1–6). It is due to stronger
electronic negative properties of iododerivatives. The
International Journal of Chemical Kinetics DOI 10.1002/kin.20965

1
0
LI ET AL.
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International Journal of Chemical Kinetics DOI 10.1002/kin.20965