Selective and efficient synthesis of decahedral palladium nanoparticles and its catalytic performance for Suzuki coupling reaction

Article abstract of DOI:10.1016/j.molcata.2012.08.023

A convenient, mild and cost-effective synthesis of decahedral palladium nanoparticles by exposing aqueous PdCl₂ solution to the sunlight in the presence of citric acid as a reducing agent and PVP as a capping agent was reported. Natural solar energy which acts as a driving force for the generation of palladium nanoparticles makes the process attractive and economical. The obtained nanoparticles were thoroughly characterized by using techniques like TEM, SAED, FEG-SEM, XRD and EDAX. It was observed that up to 70% of the palladium nanoparticles have decahedron shape, while remaining 30% consists of all other different shapes. The citric acid helps in shapes selective synthesis, whereas concentrated solar energy supplies dual energy which helps to speed up the reduction process. The aqueous reaction mixture of palladium nanoparticles was directly employed for the Suzuki coupling reaction and hence centrifuging efforts and cost of reagents required for the isolation of nanoparticles were avoided. The synthesized nanoparticles demonstrated excellent catalytic activity in Suzuki coupling reaction of aryl halides with phenyl boronic acid under mild reaction conditions. The methodology is applicable to diverse substrates providing good to excellent yields of desired products. Notably, the obtained yields with lowest catalytic loading resulting in highest TOF (0.05 mol% catalyst loading and TOF of 1960 h^-1) were among the best ever reported for the Suzuki coupling reaction. In addition, the catalyst could be reused for three more consecutive recycles. The effectiveness of present new protocol for the PdNPs synthesis was also compared with PdNPs prepared using conventional heating method.

Full text of DOI:10.1016/j.molcata.2012.08.023

Journal of Molecular Catalysis A: Chemical 365 (2012) 146–153
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Selective and efficient synthesis of decahedral palladium nanoparticles and its
catalytic performance for Suzuki coupling reaction
Aniruddha B. Patila, Dinkar S. Patilb, Bhalchandra M. Bhanagea,∗
a Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai 400019, India
b Laser and Plasma Technology Division, Bhabha Atomic Research Centre, Mumbai 400085, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 June 2012
Received in revised form 24 August 2012
Accepted 25 August 2012
Available online 4 September 2012
A convenient, mild and cost-effective synthesis of decahedral palladium nanoparticles by exposing aque-
ous PdCl2 solution to the sunlight in the presence of citric acid as a reducing agent and PVP as a capping
agent was reported. Natural solar energy which acts as a driving force for the generation of palladium
nanoparticles makes the process attractive and economical. The obtained nanoparticles were thoroughly
characterized by using techniques like TEM, SAED, FEG-SEM, XRD and EDAX. It was observed that up to
70% of the palladium nanoparticles have decahedron shape, while remaining 30% consists of all other
different shapes. The citric acid helps in shapes selective synthesis, whereas concentrated solar energy
supplies dual energy which helps to speed up the reduction process. The aqueous reaction mixture of
palladium nanoparticles was directly employed for the Suzuki coupling reaction and hence centrifuging
efforts and cost of reagents required for the isolation of nanoparticles were avoided. The synthesized
nanoparticles demonstrated excellent catalytic activity in Suzuki coupling reaction of aryl halides with
phenyl boronic acid under mild reaction conditions. The methodology is applicable to diverse substrates
providing good to excellent yields of desired products. Notably, the obtained yields with lowest cat-
alytic loading resulting in highest TOF (0.05 mol% catalyst loading and TOF of 1960 h−1) were among the
best ever reported for the Suzuki coupling reaction. In addition, the catalyst could be reused for three
more consecutive recycles. The effectiveness of present new protocol for the PdNPs synthesis was also
compared with PdNPs prepared using conventional heating method.
Keywords:
Solar energy
Decahedral palladium nanoparticles
Phosphine-free
Suzuki coupling
Aryl halides
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
ultrasound-assisted membrane reduction [14], solvothermal
method [15], reduction and annealing method [16]. Though
The synthesis of metal nanoparticles with a variety of geome-
tries is an especially important issue because of their electronic,
optical, magnetic and biomedical properties can be tuned by
controlling their size and shape [1–4]. The activity of metal
nanoparticles in chemical reaction depends on their surface proper-
ties which are influenced by the shape of synthesized nanoparticles
[5]. Consequently, several face-centered cubic (fcc) structured
noble metal nanoparticles have been synthesized, for example,
shape selective synthesis has been achieved for a various metals
such as Au, Ag, Pd, PT, Rh and Ir [6–11].
Particle having five fold twinning structure is considered
as a decahedral particle which shows better optical electronic
and catalytic applicability in organic synthesis. In this regards,
various techniques have been developed for the decahedral
nanoparticles synthesis such as reflux condensation [12], ICP [13],
above mentioned techniques have proven their application in the
nanoparticles synthesis, most of these methods suffer from one or
more drawbacks such as use of as higher temperature, higher pres-
sure, longer reaction time, use of conventional energy source and
sophisticated equipment setup. To overcome these drawbacks, the
attention has been paid on free and greener energy sources like
solar energy for the synthesis of nanoparticles. Recently, Chien et al.
reported the synthesis of decahedral gold nanoparticles using solar
energy, in which the reaction mixture was exposed to normal solar
radiations where the temperature was up to 35 ◦C [17]. Hence the
development of newer, economic and eco-friendly techniques for
the synthesis of nanoparticles is very much needed.
The metal nanoparticles, like PdNPs synthesis required higher
driving force. The energy obtained by normal sunlight is not suf-
ficient for the synthesis of PdNPs and to overcome this problem,
we have recently reported concentrated solar energy concept for
the synthesis of PdNPs [18]. The concentrated solar energy acts as
a dual effect energy source (thermal and rotational) which helps
to speed up the reaction, compared to the conventional meth-
ods like reflux. This new protocol uses naturally available solar
∗
Corresponding author. Tel.: +91 22 3361 1111/2222; fax: +91 3361 1020.
E-mail addresses: bm.bhanage@ictmumbai.edu.in, bm.bhanage@gmail.com,
bhalchandra bhanage@yahoo.com (B.M. Bhanage).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.08.023

A.B. Patil et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 146–153
147
energy, which is greener, non-polluting and cost effective energy
source.
In literature Lim et al. reported decahedral palladium nanopar-
ticles using citric acid as a reducing agent by reflux condensation
method, in which author reported up to 90% decahedral PdNPs after
26 h [12]. In a continuation of our studies on the noble metallic
nanoparticles herein we report shape selective synthesis of deca-
hedral PdNPs (up to 70% yield) by the application of concentrated
solar energy for 6 h in the aqueous medium by overcoming the
drawback of mixed morphology (triangular, octahedral, decahe-
dral and icosahedra). An aqueous solution containing PdCl2, citric
acid and PVP (poly vinyl pyrrolidone) was exposed to sunlight
from 10:00 am to 4:00 pm in the absence of any operations that
required electricity, such as stirring the solution. We found that
the existing reactant concentrations locked down the formation
of a thermodynamically-favourable five fold twinning structure,
whereas dual energy effect obtained by the concentrated solar
energy is responsible to speed up the reaction. Present results were
also compared with the results obtained by the conventional heat-
ing method showing the excellence of our new protocol.
For the selective construction of carbon–carbon bonds,
palladium-catalysed Suzuki reaction of arylboronic acids with aryl
halides is one of the most influential and versatile method. It
has been widely used in the synthesis of pharmaceuticals, herbi-
cides, natural products, and advanced materials [19–22]. Till date
various phosphine-based as well as phosphine-free palladium com-
plexes were used as catalysts for these coupling reactions [23–25].
As we know phosphine ligands are very much toxic, expensive
and moisture-sensitive this makes the ‘ligand-free’ palladium cat-
alysts prominent tool for both the economic and environmental
concerns [26]. Recently, several colloidal palladium nanoparticles
are reported for the Suzuki-coupling reactions [27–30]. However,
despite of their potential utility, they suffer from one or more
drawbacks such as the use of hazardous organic solvents, use of
expensive ligands, difficulties in isolation of nanoparticles, high-
est catalyst loading, longer reaction time and difficulties associated
with catalyst–product separation thereby, limiting their general
applications. Hence, development of an efficient, environmental
benign, thermo-stable, the catalyst which will efficiently catalyse
the Suzuki C C coupling reaction under milder reaction condition
is a subject of our current interests.
Scheme 1. Synthesis of palladium nanoparticles by using concentrated solar energy
in aqueous medium.
a typical procedure for the synthesis of PdNPs, PVP (126 mg) and
citric acid (161 mg) were added to palladium (II) chloride (31 mg)
dissolved in deionized water (10 mL (Scheme 1). The obtained mix-
ture was exposed for 6 h under solar radiations concentrated by
‘Fresnel lens (1 × 1 ft)’ at noon in summer (temperature range was
85–95 ◦C). As the solar energy effect is special and temporal, the
experiment was carried out at a specific location at the time from
10:00 am to 4:00 pm. After completion of the reaction the nanopar-
ticle solution was diluted up to 100 mL with deionized water and
used as a stock solution for the Suzuki cross coupling reaction. The
author’s laboratory is located at Matunga, Mumbai (Geographical
location: Greater Bombay, Maharashtra, India, Asia. Geographical
Coordinates: 19◦2ꢀ0ꢀꢀ North, 72◦51ꢀ0ꢀꢀ East). The energy input for
the reaction is measured by the pyranometer DWR 8101 which
was recorded as 575 W ft−2
.
2.3. Characterization of prepared palladium (0) nanoparticles
The Pd (0) nanoparticles were characterized by various analyti-
cal techniques. The X-ray crystallographic pattern was obtained by
˚
the XRD miniflex Rigaku model using Cu K␣ = 1.54 A with a scanning
rate of 2◦/min from 0◦ to 80◦. Transmission electron microscopy
(TEM) and selected area electron diffraction (SAED) was recorded
with a Philips model CM 200. Field emission gun-scanning electron
microscopy (FEG-SEM) was done by JEOL JSM-7600F and Electron
dispersive X-ray spectral (EDAX) analysis done by a JEOL-JFC 1600.
2.4. Typical procedure for Suzuki cross coupling reaction
The reaction was carried out in a sealed tube with spin
bar consisting phenylboronic acid (0.146 g, 1.2 mmol), potassium
hydroxide (0.112 g, 2 mmol), iodobenzene (0.206 g, 1 mmol) and
PdNPs solution (50 ␮L, 0.0005 mmol) in aqueous medium at 100 ◦C
for 1 h with constant stirring (Scheme 2). The progress of the reac-
tion was monitored using GC analysis (Perkin-Elmer, Clarus 400).
After completion of the reaction, the reaction mixture was cooled
to room temperature and the reaction mixture was extracted with
ethyl acetate for three times, dried over Na2SO4 and the solvent
was evaporated under vacuum. The obtained crude product was
then purified by column chromatography using silica gel, (100–200
mesh size) with petroleum ether/ethyl acetate (PE–EtOAc, 95:05)
as eluent to give pure product. All the products are well known and
were confirmed by GC–MS (Shimadzu GC–MS QP 2010).
We herein first time reporting shape selective synthesis of deca-
hedral PdNPs by concentrated solar energy and its catalytic activity
for the Suzuki C C coupling reaction. The catalyst exhibited a
remarkable activity and the system tolerates a wide variety of sub-
strates with good to excellent yields of desired products. As per
our knowledge, the results show better yields with lowest catalyst
loading (0.0005 mmol) and high TOF value (1960 h−1) for Suzuki
reaction.
For the recycling experiments, we have carried out the reaction
of iodobenzene and phenyl boronic acid (4 mmol each). The PdNPs
could be recycled and reused by separating them from the reaction
mixture through centrifugation at 10,000 rpm at 25 ◦C for 15 min.
Supernatant solution was removed and additional ethyl acetate
was added to wash settled PdNPs. Again supernatant solution was
removed and catalyst was taken for further recycle studies. The
catalyst showed good performance up to three recycles.
2. Experimental
2.1. Materials and methods
Palladium (II) chloride (Parekh Platinum Ltd.), 99% citric acid
(Polypharm), poly vinyl pyrrolidone (PVP, MW-40000) were
sourced from CDH and used without further purification. Deionised
water distilled by water purification system (Milli-Q). Glassware
and Teflon coated magnetic stir bars were cleaned with acetone,
followed by drying at 130 ◦C in an oven.
2.2. Preparation of palladium (0) nanoparticles
The synthesis of palladium nanoparticles is carried out in the
25 mL round bottom flask fitted with a water condenser (Fig. 1). In
Scheme 2. Pd nanoparticles-catalysed Suzuki cross coupling reaction of aryl halides
and phenyl boronic acid.

148
A.B. Patil et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 146–153
Fig. 1. Schematic representation of reaction setup for the synthesis of PdNPs.
2.5. GC analysis
GC–MS (EI) m/z (%) = 184(100) [M]+, 183(20.7), 169(52.3),
168(11.9), 141(34.8), 139(12.2), 115(31.7).
6. 1-([1,1ꢀ-Biphenyl]-4-yl)ethanone (Table 2, entry 6)
GC–MS (EI) m/z (%) = 196(59) [M]+, 181(100), 153(40.5),
152(51.7).
The reaction mixture was analysed by gas chromatography
equipped with a flame ionization detector (FID) and a capillary col-
umn (Perkin-Elmer, Clarus 400, 30 m × 0.32 mm × 0.25 ␮m). The GC
parameters were as follows: initial temperature 80 ◦C; initial time
1 min; solvent delay 3 min; temperature ramp 1 for 10 ◦C/min; final
temperature 250 ◦C; final time 25 min; injector port temperature
250 ◦C; detector temperature 250 ◦C and injection volume 0.6 ␮L.
7. [1,1ꢀ-Biphenyl]-2-amine (Table 2, entry 7)
GC–MS (EI) m/z (%) = 169(100) [M]+, 168(89.6), 167(42.8),
83(19.7).
8. 1-Phenylnaphthalene (Table 2, entry 8)
GC–MS (EI) m/z (%) = 204(100) [M]+, 203(95), 202(58.5),
101(34.6).
2.6. Spectral data of selected products
2.6.1. Characterization of biphenyls
3. Results and discussion
1. 1,1ꢀ-Biphenyl (Table 2, entries 1, 12)
GC–MS (EI) m/z (%) = 154(100) [M]+, 153(38.3), 152(26.4),
77(8), 76(17.1).
3.1. Preparation and characterization PdNPs
2. 4-Methyl-1,1ꢀ-biphenyl (Table 2, entries 2, 9)
GC–MS (EI) m/z (%) = 168(100) [M]+, 167(65.6), 165(26),
153(17), 152(21.1), 83(10).
Reduction kinetics play a key role in controlling the nucleation
and growth of nanoparticles. In this work, we have employed citric
acid as a reducing agent. Citric acid works as a reducing agent in a
manner similar to the mechanism of a conventional citrate-based
synthesis of noble metal nanoparticles. Citric acid can effectively
block oxidative etching by competing with oxygen adsorption onto
the Pd surface or exhausting the adsorbed oxygen atoms [8,31,32].
Earlier, Xiong et al. have shown the influence of temperature
on the morphology of nanoparticle formation [33]. The shape and
size of the product depended on temperature in three different
3. 4-Methoxy-1,1ꢀ-biphenyl (Table 2, entries 3, 10, 13)
GC–MS (EI) m/z (%) = 184(100) [M]+, 169(50.5), 141(39.7),
115(25.5).
4. 2-Methyl-1,1ꢀ-biphenyl (Table 2, entries 4, 11)
GC–MS (EI) m/z (%) = 168(100) [M]+, 167(85.9), 166(13.3),
165(39.3), 153(37.1), 152(26.6), 115(15.3), 83(16.8), 82(11.2).
5. 2-Methoxy-1,1ꢀ-biphenyl (Table 2, entry 5)

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149
ways. First, slow reduction at low temperatures greatly reduced the
level of super saturation and hence the number of seeds formed
in the nucleation step. At the same concentration of Pd precur-
sor, a decrease in the number of seeds resulted in the formation
of Pd nanoparticles with larger sizes. Second, the reduction rate,
depending on the temperature, was critical to the control of reduc-
tion kinetics and hence the formation of triangular nanoplates.
As temperature was increased, the yield of nanoplates decreased
distinctly. Third, oxidative etching of twinned particles could be
partially eliminated at low temperatures. In general, the morphol-
ogy of nanoparticles is strongly dependent on temperature, due to
the influence of temperature on both reduction kinetic and oxida-
tive etching of multiple twinned species.
Similarly, Lim et al. obtained decahedral PdNPs using citric acid,
by 26 h reflux condition [12], it clears that thermal effect helps in the
synthesis of decahedral PdNPs along with citric acid; but it will take
longer time; whereas the concentrated solar energy because of dual
energy effect works in a same way in shorter reaction time. It helps
to speed up the reduction and hence gradually increases the level
of super saturation which results the nanoparticles of lower size.
As mentioned above the yield of nanoplates formation is decreased
whereas decahedral nanoparticle formation is increased due to the
higher temperature effect obtained by dual energy source. The dual
energy effect also helps to speed up the oxidative etching process,
helps in the decahedral nanoparticle formation, which is less at
low temperature. ‘Fresnel lens’ is often used as solar concentrator.
The reaction rate increased sharply with temperature, as indicated
by a quick change in colour for the solution (from yellow-brown
to dark brown). In the PdNPs synthesis, this dual energy source is
responsible to speed up the synthesis process and decreases the
time required for synthesis.
Fig. 2. TEM image of PdNPs synthesized by solar irradiation after 6 h.
Herein, continue with our previous work we focus on shape con-
trolled palladium (0) nanoparticles synthesis and their application
in the C C bond formation. The PdNPs were synthesized by PdCl2
using citric acid as reducing and PVP as a capping agent under the
concentrated solar radiations in aqueous media. We observed that
the prepared PdNPs solution was steady for several months with-
out any agglomeration signifying that PVP serve as very efficient
capping agent, along with the citric acid acting as a reducing agent.
The synthesized Pd nanoparticles are characterized by using
techniques like TEM, FEG-SEM and XRD. The Transmission electron
microscopy (TEM) image demonstrated that the obtained prod-
uct is in the nano region with uniform size ranging from 30 nm to
45 nm containing higher (up to 70%) concentration of decahedral
particles (Fig. 2). The SAED pattern shows the crystalline nature of
obtained palladium nanoparticles and the three diffused rings indi-
cating (1 1 1), (2 0 0), and (2 2 0) reflections of fcc palladium (Fig. 3).
This result was supported by the FEG-SEM image which gave an
idea about three dimensional structures (Fig. 4). The XRD pattern
of the PdNPs in which peaks observed at 2ꢀ of 40, 47 and 69, which
readily assigned to the (1 1 1), (2 0 0) and (2 2 0) replicate the fcc
phase of metallic palladium (JCPDS card no. 5-681) appreciate the
crystalline nature of the nanoclusters (Fig. 5). The 100% metallic
palladium was shown by the EDAX pattern (Fig. 6). To show the
effectiveness of our methodology we carried out the same PdNPs
synthesis under same reaction parameters on a hot plate for 6 h,
wherein we obtained PdNPs of various morphologies (decahedral
nanoparticles where up to 15% (Fig. 7)). Fig. 8 gives the clear idea
about the PdNPs synthesised under concentrated solar energy and
under normal heating condition.
Fig. 3. SAED pattern of PdNPs synthesized by solar irradiation after 6 h.
reaction temperature and time for a model coupling reaction
of iodobenzene with phenylboronic acid were carried out as
illustrated in Table 1. Initial studies were conducted using the
mixture of PVP stabilized PdNPs which was directly employed
as a catalyst for Suzuki cross coupling reactions of iodobenzene
3.2. Catalytic activity of PdNPs in the Suzuki cross-coupling
reactions
To optimize the reaction conditions, a series of experiments
under varied conditions in terms of catalyst loading, base, solvent,
Fig. 4. FEG-SEM image of PdNPs synthesized by solar irradiation after 6 h.

150
A.B. Patil et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 146–153
Table 1
Effect of reaction parameters on Suzuki coupling reaction of iodobenzene with phenyl boronic acid.a
Entry
Catalyst Conc. (mmol)
Base
Temperature (◦C)
Time (min)
Yield (%)b
Effect of catalyst loading
1
2
3
4
5
0.001
KOH
KOH
KOH
KOH
KOH
100
100
100
100
100
60
60
60
60
60
98
97
46
20
NR
0.0005
0.0002
0.0001
0.0000
Effect of base
6
7
8
0.0005
0.0005
0.0005
NaOH
K2CO3
Na2CO3
100
100
100
60
60
60
95
88
86
Effect of temperature
9
0.0005
0.0005
0.0005
0.0005
0.0005
KOH
KOH
KOH
KOH
KOH
60
80
100
120
27
60
60
60
60
1440
75
94
97
97
34
10
11
12
13c
Effect of time
14
15
16
0.0005
0.0005
0.0005
KOH
KOH
KOH
100
100
100
30
45
120
64
90
98
a
Reaction conditions: iodobenzene (1 mmol), phenyl boronic acid (1.2 mmol), base (2 mmol), water (4 mL).
GC yield.
Reaction time (24 h).
b
c
Fig. 5. X-ray diffraction pattern of PdNPs synthesized by solar irradiation after 6 h.
Fig. 7. FEG-SEM image of PdNPs synthesized by heating at 90 ◦C for 6 h.
Fig. 8. Percentage decahedral PdNPs synthesized under conventional heating and
concentrated solar energy.
Fig. 6. EDAX pattern of PdNPs synthesized by solar irradiation after 6 h.

A.B. Patil et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 146–153
151
Table 2
Pd nanoparticles-catalysed Suzuki cross coupling reaction of aryl halides and phenyl boronic acida
.
Aryl boronic acid
Entry
1
Aryl iodide
Product
Yield (%)b
97
2
3
4
97
98
91
5
6
7
92
91
90
8
96
9
98
98
98
10
11

152
A.B. Patil et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 146–153
Table 2 (Continued)
Entry
Aryl iodide
Aryl boronic acid
Product
Yield (%)b
82
12
13
84
a
Reaction conditions: aryl halide (1 mmol), arylboronic acid (1.2 mmol), Pd (0) nanoparticles (0.0005 mmol), KOH (2 mmol), water (4 mL), 100 ◦C, 60 min.
GC yield.
b
with phenylboronic acid under aqueous medium. The obtained
120 ◦C (Table 1, entries 9–13) were studied addressing 100 ◦C as
a best temperature condition (Table 1, entry 11). In addition to
this 34% product yield was observed at room temperature even
after 24 h. To optimize the time we carried out the model reaction
for different time intervals in the range of 30–120 min (Table 1,
entries 14–16), and it was observed that desired product could
be achieved with highest yield in 60 min. The Final optimized
parameters for the Suzuki C C coupling reaction are iodobenzene
(1 mmol), phenyl boronic acid (1.2 mmol), PVP stabilized deca-
hedral PdNPs (0.0005 mmol), KOH (2 mmol), at 100 ◦C for 1 h in
aqueous medium (Table 1, entry 2). Herein we also focused on the
catalytic activity of PdNPs synthesized by the conventional heating
method (90 ◦C for 6 h); wherein we obtained 93% conversion under
optimized conditions.
These optimized conditions were applied to the C C coupling of
various aryl halides with phenyl boronic acids and its derivatives,
the results are summarized in Table 2. Various electron donating
and withdrawing groups like CH3, OCH3, COCH3, NH2 on both
aryl iodide and boronic acid smoothly undergoes Suzuki coupling
reaction providing desired diaryls (Table 2) and show high TOF
values in the presence of PVP stabilized palladium (0) nanoclus-
ters in water. It is clearly seen from Table 2 that the Suzuki
coupling reaction of phenyl boronic acid and its derivatives with
aryl halides bearing either electron donating or electron with-
drawing substituent were afforded the corresponding biaryls in
excellent yields (Table 2, entries 1–11). The reaction was also
studied for aryl bromides in which the reaction of bromoben-
zene with phenyl boronic acid and o-methoxy bromobenzene
with phenyl boronic acid gave a good yield of the desired prod-
uct under the optimized reaction conditions (Table 2, entries 12
and 13).
results proved that synthesized PdNPs are a highly active catalyst
in comparatively mild reaction conditions. The probable reason for
high catalytic activity is might be due to the nanoparticles having
better numbers of decahedral particles. A decahedral particle
having a number of sharp and smooth faces which accountable to
increase the available surface area in the reaction. Along with this,
well dispersion and stability of PdNPs in aqueous medium may
contribute to the better performance for Suzuki reaction.
In transition metal catalysed reactions, the amount of catalyst
employed proves to be an important aspect, considering which
efforts were made to determine the optimum concentration of the
catalyst (Table 1, entries 1–4). Initially the reaction was carried out
by 0.001 mmol catalyst loading where we obtained 98% yield, then
the catalyst loading was decreased to half (0.0005 mmol) and it was
observed that reaction gave yield (97%). A further decrease in the
catalyst loading (0.0002 and 0.0001 mmol), we observed the prod-
uct yield of 46% and 20%, respectively. To check the necessitate of
PdNPs in Suzuki cross coupling reaction the control experiment
was carried out in the absence of catalyst keeping other reaction
parameters constant however no yield of the desired product was
obtained even after completion of 24 h (Table 1, entry 5) signifying
that PdNPs was responsible for the respective transformation.
Considering the importance of neutralizing base in the reac-
tion, we screened a variety of inorganic bases (Table 1, entries 6–8)
among those screened inorganic bases such as KOH, NaOH, Na2CO3
and K2CO3 we found that KOH gave an excellent yield (97%) of the
desired product. Accordingly further studies were carried out with
KOH as a base.
In order to examine the effect of temperature on reaction
outcome, different temperature conditions ranging from 60 ◦C to
Table 3
Activities of various Pd (0) nanoparticles catalyst in terms of TOF values have been tested for Suzuki cross-coupling of aryl bromides or iodides and phenylboronic acid for
same amount of reactants in water (1.0 mmol aryl bromide and 1.2 mmol phenylboronic acid)
.
Entry
R
X
Catalyst
Temp. (◦C)
100
100
100
25
100
50
90
100
100
100
50
Time
Yield (%)
TOF (h−1
1680
576
456
490
72
12
38
10
1960
48
)
Ref.
1
2
3
4
5
6
7
8
9
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
Br
Br
Br
Br
Br
Br
Br
Br
I
Pd-PVP (MTPs)
PSSA-co-MA-Pd0
Pd-SDS
Pd-PEG
Pd-1/FSG
60 min
10 min
5 min
120 min
12 h
12 h
9 h
5 h
84
96
95
98
86
90
68
99
98
95
86
This study
[34]
[35]
[36]
[37]
[38]
[39]
[40]
This study
[41]
Fe3O4-Pd
pEVPBr-Pd
Pd-polystyrene
Pd-PVP (MTPs)
Pd-CD
60 min
120 min
4 h
10
11
I
I
Pd-PS-PEO
22
[42]

A.B. Patil et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 146–153
153
Acknowledgement
The authors express their gratitude towards DAE-ICT (Depart-
ment of Atomic Energy and Institute of Chemical Technology,
Mumbai, India) for providing financial assistance.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.molcata.2012.08.023.
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4. Conclusion
In summary, herein we demonstrated a simple, rapid and high
yielding synthetic route to prepare decahedral palladium nanopar-
ticles by concentrated solar energy using PVP as a capping agent
and citric acid as reducing agent. The aqueous solution of PdNPs
was directly employed in the Suzuki coupling reaction, and hence
save time and cost of isolation. To the best of our knowledge, the
prepared nanoparticles demonstrated excellent catalytic activity in
C
C coupling reaction with lowest catalyst loading ever reported.
The significant advantages of this protocol are simple operation and
being an environmentally gracious process in excellent yields with
the highest TOF values for the Suzuki coupling reaction catalysed
by palladium nanoparticles. The new PdNPs synthesis methodol-
ogy showed excellent performance compared to the conventional
heating method. The catalyst showed best performance up to three
recycles.