A green approach for the decoration of Pd nanoparticles on graphene nanosheets: An in situ process for the reduction of C-C double bonds and a reusable catalyst for the Suzuki cross-coupling reaction

Article abstract of DOI:10.1039/c5nj01221j

A new strategy for in situ synthesis of palladium nanoparticles (Pd NPs) decorated on reduced graphene oxide (rGO) nanosheets with controlled size and shape is reported. This strategy was designed as three processes in one pot, namely, (a) reduction of graphene oxide, (b) formation of Pd NPs on the rGO nanosheets and (c) simultaneous reduction of olefin. In this synthesis process, a hydrogen atmosphere was used to develop the Pd NPs-rGO nanocatalyst, which is reusable and easily separable. The influence of the size and morphology of the Pd-rGO-H₂ catalyst on the catalytic activity in the Suzuki cross-coupling reaction was investigated by comparing with other catalysts, Pd-rGO-As and Pd-rGO-Gl, and they were synthesized by different reducing agents, ascorbic acid and glucose, respectively. The catalysts were characterized by electron microscopy (HRTEM, SEM), FT-IR, XRD and XPS. The Pd-rGO-H₂ catalyst was found to possess excellent catalytic activity and recyclability in the Suzuki cross-coupling reaction under mild reaction conditions.

Full text of DOI:10.1039/c5nj01221j

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Graphical abstract
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Table of contents:
In situ synthesis of Pd nanoparticles with simultaneous reduction of alkene to alkane by using
hydrogen gas onto the reduced graphene oxide sheets and utilized as an efficient catalyst for the
Suzuki-cross coupling reaction.

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A green approach for decoration of Pd NPs on graphene nanosheets: An in-situ
process for the reduction of CꢀC double bond and reusable catalyst for Suzuki
crossꢀcoupling reaction
ad
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ad
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Najrul Hussain , Ashwini Borah , Gitashree Darabdhara , Pranjal Gogoi * Vedi Kuyil
C
cd
ad
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Azhagan , Manjusha V. Shelke and Manash R. Das *
a
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Materials Science Division, CSIRꢀNorth s Institute of Science and Technology, Jorhatꢀ785006,
Assam, India.
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Medicinal Chemistry Division, CSIRꢀNorth East Institute of Science and Technology, Jorhatꢀ
785006, Assam, India.
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Physical and Materials Chemistry Division, CSIRꢀNational Chemical Laboratory, Dr. Homi
Bhabha Road, Pune 411008, India
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Academy of Scientific and Innovative Research, Chennai– 600113, India
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*
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Corresponding author
Tel: +91ꢀ9957178399
Fax: +91ꢀ376ꢀ2370011
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Eꢀmail: mnshrdas@yahoo.com (M. R. Das)
gogoipranj@yahoo.co.uk (P. Gogoi)
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Abstract
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A new strategy for in-situ synthesis of palladium nanoparticles (Pd NPs) decorated on reduced
graphene oxide sheets (rGO) with controlled size and shape is reported. This strategy was
designed as three processes in oneꢀpot namely (a) reduction of graphene oxide (b) formation of
Pd NPs on the rGO nanosheets and (c) simultaneously reduction of olefin. In this synthesis
process hydrogen atmosphere was used to develop the Pd NPsꢀrGO nanocatalyst which is reꢀ
usable and easily separable. The influence of the size and morphology of the PdꢀrGOꢀH catalyst
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on the catalytic activity in the Suzuki crossꢀcoupling reaction was investigated by comparing
with other catalysts PdꢀrGOꢀAs and PdꢀrGOꢀGl synthesized by different reducing agent, ascorbic
acid and glucose, respectively. The catalysts were characterized by electron microscopy
(HRTEM, SEM), FTꢀIR, XRD and XPS. The PdꢀrGOꢀH catalyst was found to possess excellent
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catalytic activity and recyclability in the Suzuki crossꢀcoupling reaction under mild reaction
conditions.
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Keywords: Palladium nanoparticles, graphene, catalysis, carbonꢀcarbon double bond . Suzuki
cross coupling reaction
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Introduction
Palladium nanoparticles (Pd NPs) play an important role in wide range of research areas such as
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sensors and devices, energy storage materials and catalysis. In catalysis, they are particularly
used for the construction of carbonꢀcarbon bonds in some important reactions namely Heck,
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Suzuki, Stille and Sonogashira reaction. Also different types of palladium based bimetallic
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nanoparticles are important as catalyst for carbonꢀcarbon bond formation. Additionally they are
used as a catalyst for oxygen reduction reaction, formic acid oxidation and ethanol
electrooxidation. The use of Pd NPs as heterogeneous catalyst for the synthesis of fine
chemicals, pharmaceuticals, herbicides, polymers have advantages over homogeneous Pdꢀ
catalyst with respect to stability and selectivity. Additionally they are cheap, reusable and have
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multiple active sites.
In this regards, it is of immense interest to develop an efficient
heterogeneous catalyst in terms of stability, selectivity and reusability.
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In order to overcome some issues associated with homogenous palladium catalyst, enormous
efforts have been devoted to immobilize and stabilize the Pd NPs in various heterogeneous
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supports such as organic matrix, organicꢀinorganic fluorinated hybrid materials, polymers,
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glass polymer composites, ionic liquids. In addition to these, Pd NPs are also synthesized in
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different inorganic supports such as carbon,
carbon nanotubes, alumina, silica,
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zeolite,
clays, Zink ferrite,
etc. Among them carbon based supports has attracted
significant attention due to their chemical stability in various aggressive media, abundance, light
weight, inertness to supported metals, avoids the surface poisoning problem, easy to recover the
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precious metals by burning the support and its diverse structural allotropes.
Graphene is
particularly promising candidate as host material for the synthesis of Pd NPs among all carbon

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based supports like porous carbon, carbon nanotubes due to its availability, low cost, high
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corrosion resistance and their excellent dispersive nature.
Graphene with a twoꢀdimensional
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(2D) sp ꢀhybridized carbon structure and its derivatives has rapidly emerged as one of the most
exciting material due to its unique physical and chemical properties, such as high surface area,
high absorption coefficient, excellent conductivity, good thermal stability and strong mechanical
strength. Such properties of graphene and graphene based composites materials makes it as an
excellent candidate in the field of catalysis especially in photocatalysis, electrocatalysis, and
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organic reaction catalysis.
Recently several techniques including various reducing as well as capping agents have been
developed for the synthesis of Pd NPs over graphene/GO for catalysis. For example, Simaki and
his coꢀworkers have prepared Pd NPs on graphene substrate from palladium nitrate by
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microwave heating using hydrazine hydrate as reducing agent. CarreraꢀCerritos et al. have
synthesized Pd NPs on rGO as support using ethylene glycol, an alternative and environment
friendly reducing agent compared to hydrazine hydrate. However, such reducing agent requires
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long term thermal treatment which may easily affect the basic properties of graphene. For
oxygen reduction reaction, uniformly dispersed Pd NPs on rGO nanosheets was synthesized
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using oneꢀpot photoꢀassisted citrate reduction
and chemical synthesis via polyol reduction
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methods.
Fu and his coꢀworker reported a surfactantꢀfree synthesis of Pd NPs on rGO by
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controlling the reduction pathway as a catalyst for formic acid oxidation. Graphene ribbonꢀ
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supported Pd NPs was also reported as electro catalyst for formic acid electroꢀoxidation. Due to
the industrial and academic demand of carbonꢀcarbon bond forming reaction using Pdꢀcatalyst,
several methods have been developed for the synthesis of Pd NPs on rGO nanosheets. Mulhaupt
et al. reported an efficient method for the synthesis of Pd NPs by chemical reduction via

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immobilization of Pd on GO and used it for the Suzuki crossꢀcoupling reactions. Additionally
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laser synthesis of Pd NPs, microwaveꢀassisted chemical reduction of palladium salt, chemical
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reduction of palladium salt using NaBH4, sonication followed by heating of palladium acetate
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with GO, electrochemically coꢀdeposited Pd NPs and use of sodium dodecyl sulphate (SDS)
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as surfactant and reducing agent are the notable techniques for the synthesis of Pd NPsꢀrGO
composites and its catalytic application for carbonꢀcarbon bond forming reaction. Although, the
reported catalysts are easily recoverable and reusable, the major drawback is the use of toxic
reducing agent which could be ingredient of the catalyst as contaminant and also the reduction
technique which may create problem in controlling the reduction process. Therefore,
development of mild condition and preparation of heterogeneous catalyst adopting green
chemical route is a challenging task.
On the other hand, hydrogenation of unsaturated hydrocarbons in one of the most important
process in petrochemical industry for the conversion of alkenes and aromatics into saturated
alkanes and cycloalkanes and also in food industry for the processing of vegetable oil. This
carbonꢀcarbon double bonds reduction is normally accomplished by using hydrogen and
heterogeneous metal catalysts, e.g. Rh/C, Pd/C, Raney Nickel, or Adams catalyst (PtO ). In all
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the cases preꢀprepared or commercially available metal catalysts are directly used under
hydrogen atmosphere. To minimize the reaction steps, cost and reactionꢀpot, we have developed
an oneꢀpot reduction strategy for the synthesis of Pd NPs on rGO nanosheets (PdꢀrGOꢀH ) and
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reduction of olefin using palladium salt under hydrogen atmosphere. The synthesized Pd NPs on
rGO nanosheets were efficiently used for the Suzuki crossꢀcoupling reaction and the catalytic
activity of this catalyst was compared with other synthesized Pd NPs on rGO nanosheets using

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ascorbic acid (PdꢀrGOꢀAs) and glucose (PdꢀrGOꢀGl). In our approach, the Pd NPs were
decorated on the rGO nanosheets by adopting simple solution chemistry under surfactantꢀfree
conditions.
Experimental Section
Materials
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Graphite powder (<20ꢁm, SigmaꢀAldrich, Germany), sulfuric acid (AR grade, Qualigens, India),
hydrochloric acid (AR grade, Qualigens, India), H O (30%, Qualigens, India), potassium
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permanganate (>99 %, EꢀMerck, India), glucose (EP standard, Sigma Aldrich, Germany),
Palladium(II) acetate (98 %, Sigma Aldrich, Germany), ascorbic acid (99%, Fisher Scientific,
India), ethanol (ACS reagent, Sigma Aldrich), Pd/C ( 10 wt%, Sigma Aldrich), Pd/Al O ( 5
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wt%, Sigma Aldrich) and all phenylboronic acids, alkenes and aryl halides were supplied from
Sigma Aldrich, USA and used as received.
Characterization techniques
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Xꢀray diffraction (XRD) analysis was performed with the scanning rate 3 deg·min by Rigaku
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Xꢀray diffractometer (model: ULTIMA IV, Rigaku, Japan) from 5ꢀ100 with a Cu Kα Xꢀray
source (λ = 1.54056 Å) at a generator voltage 40 kV and a generator current 40 mA.
Thermogravimetric analysis (TGA) of the samples were made by TAꢀSDT (model: Q600DT, TA
Instruments, USA) at a rate of 5 °C rise in temperature per minute. Transmission electron
microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images
were recorded on a JEOL JEMꢀ2011 electron microscope, Transmission Electron Microscope,
Japan operated at an accelerating voltage of 200 kV. Xꢀray photoelectron spectroscopy (XPS)

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measurements were carried out with VG Micro Tech ESCA 300 instrument at a pressure > 1 ×
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10 Torr (pass energy: 50 eV, electron take off angle: 60 and the overall resolution ~ 0.1 eV).
Reactions were monitored by thinꢀlayer chromatography (TLC) using aluminiumꢀbacked
MERCK 60 silica gel plates (0.2 mm thickness); the chromatograms were visualized first with
ultraviolet light (254 nm) and then by immersion in solutions of pꢀanisaldehyde followed by
heating. Flash column chromatography was performed with Merck silica gel 60 (230ꢀ400 mesh).
Reaction temperatures refer to external bath temperatures.
All NMR spectra were recorded on Bruker Advance DPX 300 or 500 MHz spectrometer.
Chemical shifts are reported on the δ scale (ppm) downfield from tetramethylsilane (δ=0.0 ppm)
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using the residual solvent signal at δ=7.26 ppm ( H) or δ=77 ppm ( C) as internal standard. Data
are reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, q =
quartet, m = multiplet, br = broad). Melting points were recorded in open capillary (pyrex) tubes
with a Buchiꢀ540 micro melting point apparatus and are uncorrected. FTIR spectra were
recorded on a PerkinꢀElmer 1640 FTꢀIR spectrophotometer on a thin film using CHCl in the
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range of 400ꢀ4000 cm . EI (GC) mass was recorded in a Trace DSQ GCꢀMS instrument
(Thermo Electron, Austria) through direct insertion probe (DIP).
Synthesis of the Pd NPsꢀrGO composite materials
The detailed synthesis method of GO and its characterization was reported in the previously
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published paper.
In-situ generation of Pd-rGO-H composite material and alkane
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For, synthesis of the Pd NPs onto the rGO nanosheets (PdꢀrGOꢀH ) and in-situ reduction of
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olefin under hydrogen atmosphere, 0.03 g of graphene oxide (GO) was dispersed in ethanol (15
mL). Palladium acetate (0.02 g, 0.09 mmol) followed by olefin (1 mmol) were added to the

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suspension and the resulting mixture was stirred overꢀnight under hydrogen atmosphere. The
composite material was separated by filtration and repeatedly washed with ethanol and finally
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dried in an air oven at 50 C to get PdꢀrGOꢀH . The filtrate was concentrated and was purified by
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chromatography using silica gel (60ꢀ120 mesh) with EtOAc/hexanes as eluent to obtain the
desired organic product 4.
Pd-rGO-Gl composite material
In the synthesis of the Pd NPs on the rGO nanosheets using glucose (PdꢀrGOꢀGl), Palladium
acetate (0.02 g, 0.09 mmol) was dispersed in water (10 mL). Ammonia solution (25%; 0.06 mL)
was added and then ultrasonicated for 15 min. A glucose solution in water (1g in 10 mL) was
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added and the resulting reaction mixture was stirred at 120 C for 1 h. A suspension of GO
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(0.012 gL in water, 30 mL) was added to the reaction mixture and continued to stir for another
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1 h at 120 C. Finally, black precipitate of the composite materials was obtained. The composite
material was separated by filtration and repeatedly washed with acetone followed by water and
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finally dried in an air oven at 50 C for overnight.
Pd-rGO-As composite material
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Palladium acetate (0.02 g, 0.09 mmol) was dispersed in water (10 mL). A suspension of GO
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(0.012 gL in water, 30 mL) was added and stirred for 15 min. Finally, ascorbic acid (0.1M, 6
mL) was added dropꢀwise to the reaction mixture and ultrasonicated for 2 h. The color of the
reaction mixture changed from brown to black due to the formation of Pd NPs on rGO sheets.
The product was treated with H O to remove excess amount of ascorbic acid and finally the
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solid product was separated by centrifuge and washed with ethanol followed by water and then
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General procedure for the Suzuki cross coupling reaction
To a suspension of PdꢀrGOꢀH (0.025 g) in ethanol (5 mL) were added aryl halide (1 mmol), aryl
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boronic acid (1.2 mmol) and a solution of sodium carbonate (2 mmol in 5 mL water). The
resulting mixture was vigorously stirred at room temperature for 2 h. Upon the completion of the
reaction (monitored by thin layer chromatography), the catalyst was recovered by simple
filtration and the filtrate was poured into water. The organic product was extracted with ethyl
acetate (3x10 mL). The combined organic fractions were dried over Na SO and concentrated.
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The residue was purified by chromatography using silica gel (60ꢀ120 mesh) with EtOAc/hexanes
as eluent to obtain the desired coupling product 5.
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Characterization of Pd NPs on to the rGO nanosheets.
The particle size and surface morphology of the synthesized PdꢀrGOꢀH composites was studied
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by transmission electron microscope (TEM) and high resolution transmission electron
microscope (HRTEM). The TEM images as shown in Fig.1(aꢀe) clearly displays the uniform
distribution of Pd NPs onto the surface of rGO nanosheets and most of the particles are spherical
in nature. The crystal lattice of Pd NPs and fringes of rGO nanosheets are clearly resolved in
HRTEM image (Fig. 1f) and it is found to be 0.16 nm. The average size of the particle is
determined from the particle size distribution curve and was found to be around 6 nm. The
diffraction dots observed in the SAED images as shown in Fig. 1g proves the crystalline nature
of the Pd NPs and the crystal lattices of Pd NPs corresponding to the (111), (200) and (220)
plane are clearly visible.

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Fig. 1 TEM images of PdꢀrGOꢀH : (a) TEM images along with particle size distribution; (bꢀe)
TEM images; (f) HRTEM image along with fringe spacing; (g) SAED image of the NPs
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However, the Pd NPs in the PdꢀrGOꢀAs composites synthesized by ascorbic acid posses average
size of around 18 nm which is slightly greater than PdꢀrGOꢀH (Fig. 2(aꢀc)). The average size of
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the Pd NPs in PdꢀrGOꢀGl synthesized by using glucose as reducing agent is about 47 nm which
is much more larger than the Pd NPs synthesized by hydrogen and also most of the particles are
agglomerated (Fig. 2(dꢀf)). This is due to the slow deposition of Pd NPs onto the rGO nanosheets
by weak reducing agent such as glucose, which lower the nucleation rates and results larger
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particle size. The morphology and microstructure of Pd NPs as well as its support i.e rGO
sheets were investigated by Scanning electron microscope (SEM) (shown in Fig. S2 in
supporting information). The crumpled and rippled structure of GO resulting from deformation
upon the exfoliation and restacking processes is partially destroyed in rGO sheets due to the
reduction of large amount of oxygen containing functional groups. However, the rGO nanosheets
were layered in structure, it is irregular and folded where the spherical Pd NPs are uniformly
distributed
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Fig. 2 TEM images of PdꢀrGOꢀAs (aꢀc) and PdꢀrGOꢀGl (dꢀf)
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2θ Value
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Fig. 3 XRD pattern of (a) PdꢀrGOꢀH , (b) PdꢀrGOꢀAs and (c) PdꢀrGOꢀGl.
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The Fig. 3 shows the XRD pattern of a) PdꢀrGOꢀH b) PdꢀrGOꢀAs and c) PdꢀrGOꢀGl. As shown
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in Fig. 3a for PdꢀrGOꢀH the prominent peaks at 2θ values of about 40.11, 46.66, 68.12, 82.10
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and 86.61 are assigned to the (111), (200), (220), (311) and (222) crystallographic planes of Pd
NPs with corresponding dꢀspacing value of 2.24, 1.95, 1.38, 1.17 and 1.23, respectively. All the
diffraction peak values along with the dꢀspacing values are matching with JCPDS card No. 01ꢀ
071ꢀ3757. The average crystallite size of the Pd nanoparticle was determined by using PDXL
software from XRD and found to be 7.6 nm which is very close to the particle size analyzed by
TEM images. Similar crystallographic planes of Pd NPs for PdꢀrGOꢀAs and PdꢀrGOꢀGl are
shown in Fig.3b and Fig. 3c. The broad diffraction peak found at 2θ value ~24.8 for all this
composite is attributed to the rGO nanosheets where the Pd NPs are uniformly deposited.
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Thermogravimetric analysis (TGA) of the PdꢀrGOꢀH composite materials shows that the major
2
2
2
2
2
2
2
2
2
2
2
2
2
81
82
83
84
85
86
87
88
89
90
91
92
weight loss was observed in two steps. In the first step, the weight loss of GO was found to be
37.72 % which is decreases upto 13.62 % in case of PdꢀrGOꢀH composite materials in the
2
o
o
temperature range 120 C to 300 C due to reduction of the oxygen containing functional groups
such as carbonyl, hydroxyl, epoxy and carboxyl groups. Moreover, another weight loss is
o
o
observed in the temperatuer range of 400 C to 650 C for PdꢀrGOꢀH composites material and it
2
5
5,56
is similar to GO which results from the pyrolysis of the carbon skeleton (Fig. 4).
The
synthesized catalysts (PdꢀrGOꢀH , PdꢀrGOꢀAs, PdꢀrGOꢀGl) were further characterized by FTIR
2
spectroscopy (details provided in supporting information).

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1
4
2
2
2
2
2
2
2
3
3
93
94
95
96
97
98
99
00
01
37.72 %
GO
Pd-rGO-H2
1
3.62 %
1
50
300
450
600
o
750
900
Tem perature ( C )
Fig. 4 TGA curve of the GO and PdꢀrGOꢀH composite materials
2
3
3
3
3
3
3
3
3
3
3
3
3
3
02
03
04
05
06
07
08
09
10
11
12
13
14
The formation of different sizes of Pd NPs synthesized by different reducing agents onto the rGO
surfaces is reflected by XPS analysis. The C1s XPS core level spectrum of the PdꢀrGOꢀH2
composites (Fig. 5A) shows the binding energy of C−C/C−H, C−C, C−O and C=O species at
5
7
284.35, 285.3, 286.35 and 287.68 eV, respectively. The intensity of these peaks is lower in
comparison to GO nanosheets due to the reduction of oxygen containing functional groups. The
5
3
C1s XPS core level spectrum of the GO has been already reported in our previous publication.
The Pd 3d core level XPS spectrum of the PdꢀrGOꢀH composites shows the binding energy of
2
3d5/2 and Pd 3d3/2 of Pd(0) nanoparticles at 335.9 and 341.05 eV, respectively, which confirms
5
2
the formation of Pd NPs in zero oxidation state onto the surface of rGO nanosheets (Fig. 5B).
The XPS spectrum of PdꢀrGOꢀGl (shown in Fig. 5C) provides evidence that a shift of 1.53 eV
towards the lower binding energy implies the formation of larger Pd NPs (47 nm) as compared to
52
the PdꢀrGOꢀH having particle size around 6 nm. This observation is also supported by
2

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1
5
3
3
3
3
3
3
3
15
16
17
18
19
20
21
HRTEM analysis of Pd NPs synthesized on rGO nanosheets using H , ascorbic acid and glucose
2
C1s
Pd 3d3/2
341.05 eV
Pd 3d5/2
335.9 eV
A
B
CꢀC sp2
284.35 eV
CꢀC sp3
285.3 eV
CꢀO
86.35 eV C=O
87.68 eV
2
2
282
284
286
288
290
292
332
334
336
338
340
342
344
346
Binding Energy (eV)
Binding Energy (eV)
3
3
3
3
3
3
3
3
22
23
24
25
26
27
28
29
Pd 3d5/2
C
334.37 eV
Pd 3d3/2
39.6 eV
3
3
32
336
340
344
Binding Energy (eV)
3
3
30
31
Fig. 5 (A) C1s and (B) Pd 3d XPS core level spectra of PdꢀrGOꢀH composite material (C) Pd 3d
XPS core level spectra of PdꢀrGOꢀGl composite material
2
3
3
32
33
Catalytic activity
3
3
34
35
The oneꢀpot reduction process for the synthesis of Pd NPs and reduction of a variety of alkenes
were investigated (Table 1). Various terminal, exocyclic, endocyclic and strained olefins were

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6
3
3
3
3
3
3
3
3
3
3
3
3
3
3
36
37
38
39
40
41
42
43
44
45
46
47
48
49
efficiently hydrogenated to their corresponding alkanes with excellent yields. The reaction
conditions notably showed a great tolerance to a range of functional groups including ether,
ester, carboxylic and hydroxide groups. Most importantly, couplingꢀprone functional groups
such as boronic acid and chloro group remain intact during our catalytic hydrogenation process
(4b and 4m, Table 1). Various exocyclic and endocyclic double bonds were examined and
efficiently hydrogenated to their corresponding single bond with excellent yields. Exocyclic
double bond of the (ꢀ)ꢀIsopulegol was hydrogenated to achieve (ꢀ) Menthol (4g, Table 1). Next, a
number of conjugated double bonds were also examined. In this regards, trans cinnamic acid and
it methyl, ethyl and isopropyl ester derivatives were converted to their corresponding reduced
products (4iꢀ4l, Table 1). To illustrate the versatility of the present catalyst system, strained
alkenes 3βꢀchloroꢀ5ꢀcholestene 1m and 3βꢀacetoxyꢀ5ꢀcholestene 1n were considered and gave
the corresponding hydrogenated products 3βꢀchloroꢀ5αꢀcholestane 4m and 3βꢀacetoxyꢀ5αꢀ
cholestane 4n respectively. The specific rotation of the product 4m was compared with the
2
3
58
reported value (found [α] = +5.3 (c=0.3, CH Cl ); lit. rep. [α] = +5.0 (c =0.3, CH Cl ).
D
2
2
D
2
2
3
3
50
51
Suzuki crossꢀcoupling reaction is one of the most important and versatile method for the carbonꢀ
carbon bond formation. Various phenylboronic acid and aryl halides were considered as coupling
partners for this reaction to investigate the catalytic activity of Pd NPs obtained by using
different reducing agents such as hydrogen, ascorbic acid and glucose. The performances of the
catalytic activities of the catalysts are summarized in Table 2.
3
3
3
3
3
3
3
3
52
53
54
55
56
57
58
59

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7
a
3
60
Table 1. Oneꢀpot reduction of palladium salt and various olefins under hydrogen atmosphere
b
Entry
1
Substrate
(aꢀn)
Product
4(aꢀn)
Yield
(%)
81
1
2
3
4
79
82
85
5
52
6
7
49
92
8
9
37
80
82
1
1
0
1
87
95
95
1
1
2
3
1
4
98
a
b
3
61
Reaction conditions: Olefin (1 mmol), GO (0.03 g), Pd(OAc) (0.02 g), H (balloon), RT, overꢀnight; isolated yields
2 2

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8
a
3
3
62
63
Table 2: Comparison of catalytic activity of different synthesized Pd NPsꢀrGO composites
Entry
Catalyst
Elementabl
Contents
Particle
Diameter
RꢀX
R´ꢀB(OH)2
Yield
(%)
c
d
C
H
O
1
2
3
4
5
6
7
8
9
R= Phꢀ; X= Br
R= p(OMe)Phꢀ; X=Br R´= Phꢀ
R= Phꢀ; X=Br
R= Phꢀ; X= Cl
R= Phꢀ; X= Br
R´= Phꢀ
96
95
92
PdꢀrGOꢀH2
PdꢀrGOꢀAs
PdꢀrGOꢀGl
55.03 2.38 42.51 6 ± 2
45.25 2.43 52.28 18 ± 9
44.20 1.97 53.81 47 ± 5
R´= p(PhCH O)Phꢀ
R´= p(OMe)Phꢀ
R´= Phꢀ
2
e
80
81
76
85
R= p(OMe)Phꢀ; X=Br R´= Phꢀ
R= Phꢀ; X=Br
R= Ph; X= Cl
R= Phꢀ; X= Br
R= p(OMe)Phꢀ; X=Br R´= Phꢀ
R= Phꢀ; X=Br
R´= p(PhCH O)Phꢀ
R´= p(OMe)Phꢀ
R´= Phꢀ
2
e
72
72
65
70
1
1
1
0
1
2
R´= p(PhCH O)Phꢀ
R´= p(OMe)Phꢀ
2
e
R= Phꢀ; X= Cl
50
a
3
3
3
64
65
66
Reaction conditions: RꢀX (1 mmol), R′ꢀB(OH) (1.2 mmol), catalyst (0.025 g), Na CO (2 mmol), H O/EtOH (10 mL, 1:1,
2
2
3
2
b
c
d
v/v), RT, 2 h; Calculated from CHN analysis, Average size with standard deviation as measured by TEM analysis; Isolated
e
o
Yield, The reactions were performed at 80 C for 7h
3
3
3
3
3
3
3
3
3
3
3
3
67
68
69
70
71
72
73
74
75
76
77
78
As shown in Table 2, the catalysts PdꢀrGOꢀH was efficient with respect to yields of the product
2
in comparison to other two catalysts PdꢀrGOꢀAs and PdꢀrGOꢀGl. The Pd NPs obtained by using
hydrogen as reducing agent controlled the particle size with uniform distribution, which
enhances the reactive sites for the Suzuki crossꢀcoupling reaction in comparison to PdꢀrGOꢀAs
and PdꢀrGOꢀGl which has large sized agglomerated nanoparticles. After optimizing PdꢀrGOꢀH2
as suitable catalyst for Suzuki crossꢀcoupling reaction, bromobenzene 2c and phenylboronic acid
3a were considered as coupling partners for further optimization studies. Results are summarized
in Table 3. Initially, the coupling reaction was carried out in pure water, which results low yield
(~30%) of product 5a (Table 3, entry 1). To improve the yield, several organic coꢀsolvents
including THF, acetone, ethanol, methanol, DMF, and acetonitrile were screened. Among them,
ethanol was found to be the best coꢀsolvent for the Suzuki cross coupling reaction. Additionally

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9
3
3
3
79
80
81
different bases including Na CO , K CO , Cs CO , and K PO were examined and results almost
2 3 2 3 2 3 3 4
identical product yield, whereas strong base like NaOH and KOH gave low yield of the product
5a.
a
3
3
82
83
Table 3. Effect of solvent and base on the Suzuki crossꢀcoupling reactions.
PdꢀrGOꢀH2
Reaction Conditions
Br
B(OH)2
+
3
3
84
85
2c
3a
5a
3
86
87
88
Coꢀsolvent
Base
Reaction
Yield
3
b
conditions
(%)3
o
1
2
3
4
5
6
7
8
9
without
MeOH
MeOH
EtOH
EtOH
EtOH
EtOH
EtOH
DMF
THF
Na CO
100 C, 6h
30389
70390
2
3
Na CO
r.t, 3h
r.t, 3h
r.t, 2h
r.t, 2h
r.t, 2h
r.t, 3h
r.t, 3h
r.t, 6h
r.t, 12h
2
3
3
3
3
3
3
3
3
3
91
92
93
94
95
96
97
98
K CO
72
96
95
95
51
44
60
2
3
Na CO
2
3
K CO
2
3
K PO
3
4
NaOH
KOH
K CO
2
3
10
K PO
53399
77400
3
4
o
11
THF
Na CO
60 C, 3h
2
3
4
4
4
4
01
02
03
04
a
Reaction conditions: 2c (1 mmol), 3a (1.2 mmol), PdꢀrGOꢀH (0.025 g), base (2 mmol) in 5 ml H O,
2
2
b
solvent (5ml) Isolated Yield
4
4
4
4
4
4
4
4
4
05
06
07
08
09
10
11
12
13
After the optimization of the reaction conditions for Suzuki crossꢀcoupling reaction, a variety of
aryl halides including, bromides and iodides were efficiently coupled with various arylboronic
acids. As shown in Table 4, it is evident that the coupling reactions proceeded smoothly under
mild conditions affording excellent yields of the coupling products. The mild reaction conditions
notably showed a great tolerance of various ether linkage, nitro, fluoro and methyl groups.
Importantly, other boronic acid surrogate, phenylboronic acid neopentylglycol ester was also
used as coupling partner for the Suzuki crossꢀcoupling reaction with bromobenzene to obtain 5a
with 94% yield under this catalytic system. To further extend the scope of our Suzuki crossꢀ
coupling reaction, aryl chloride was also investigated as coupling partners (Table 4, entries 14

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0
4
4
4
4
4
4
4
4
4
4
4
14
15
16
17
18
19
20
21
22
23
24
and 15). However, the coupling of cholorobenzene 2f with aryl boronic acids required long
reaction time and high reaction temperature. The catalytic performance of our catalyst PdꢀrGOꢀ
H for the Suzuki crossꢀcoupling reaction is compared with other supported palladium catalyst.
2
Commercially available Pd/C and Pd/Al O were examined for the Suzuki cross coupling
2
3
reaction of bromobenzene with 4ꢀmethoxyphenylboronic acid. Among all the catalyst, PdꢀrGOꢀ
H showed superior results in terms of yield and reaction time, which might be due to the
2
uniform distribution of small Pd NPs onto the rGO nanosheet. Furthermore we have compared
our synthesized catalyst (PdꢀrGOꢀH ) with previously reported Pd catalyst for the Suzuki crossꢀ
2
coupling reaction. The comparison results are summarized in Table 5. From these comparison
results, we can conclude that more impressively our synthesized catalyst PdꢀrGOꢀH presented
2
an obvious advantage over other reported catalyst.
4
4
4
4
4
4
4
4
25
26
27
28
29
30
31
32

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1
a
4
33
Table 4. Suzuki crossꢀcoupling reaction of aryl halides with arylboronic acids
Entry
1
Aryl halide
Boronic acid/ester
Product
5(aꢀk)
Yield
(%)
b
2
(aꢀf)
3(aꢀi)
96
(HO)2B
3
a
9
9
9
9
5
6
6
4
2
3
4
5
2a
3a
2b
2b
9
9
6
3
3
a
5a
6
7
2c
2c
9
5
8
9
9
9
2
6
2c
2c
1
0
5a
9
9
9
5
4
6
1
1
1
2
3a
Me
1
1
3
4
2e
2f
3a
3a
5k
5a
5i
7
3c
C
1
5
3d
80
a
4
4
4
34
35
36
Reaction conditions: Aryl halide (1 mmol), arylylboronic acid (1.2 mmol), catalyst (25 mg), Na CO (2 mmol),
2 3
b
C
o
H O/EtOH (10 mL, 1:1, v/v), r.t, 2 h. Isolated Yield. The reaction was performed at 80 C for 7h
2

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2
2
4
4
4
37
38
39
Table 5. Comparison of efficiency of various supported palladium catalysts in Suzuki crossꢀ
coupling reaction
Catalyst
Conditions
Time
Yield
Ref
This
(
%)
94
76
68
PdꢀrGOꢀH2
Pd/C
Na CO , EtOHꢀH O, r.t
2 h
work
2
3
2
Pd/Al O
2
3
o
o
NHCꢀPd/GOꢀIL
Pd/Fe O /sꢀG
K CO , EtOH–H O (1 : 1), 60 Cꢀ90 C
2.5ꢀ12 h
45 min
1ꢀ2.5 h
15ꢀ20 min 13ꢀ73
3ꢀ24 h
1 h
42ꢀ98
79ꢀ95
86ꢀ94
59
60
61
62
63
64
65
66
2
3
2
o
K CO , EtOH–H O (1 : 1), 80 C
3
4
2
3
2
EPROꢀPd
K CO , EtOH, Reflux
2 3
o
G/MWCNTs/Pd
Pd–Ni(20)/RGO
Pd/CNTꢀSiC
PdꢀCNTꢀEDꢀOH Na CO , DMF, 110, C
PdꢀslGOꢀ60
K CO , EtOH–H O (1 : 1), 60 C
2
3
2
o
K CO , EtOH–H O (1 : 1), 30ꢀ60 C
31ꢀ99.3
98
94
44ꢀ99
2
3
2
o
K PO EtOH–H O (4 : 1), 60 C
3
4
2
o
24 h
1ꢀ24 h
2
3
K CO , Aqueous 50% EtOH, reflux
2
3
4
4
40
41
Reusability of the catalyst
4
4
4
4
4
4
4
4
4
4
4
4
4
42
43
44
45
46
47
48
49
50
51
52
53
54
Since the reusability of the catalyst is an important factor in the field of catalysis, we have
studied the reusability of the PdꢀrGOꢀH catalyst for the Suzuki crossꢀcoupling reaction. After
2
performing the catalytic reaction, the catalyst was recovered by simple filtration and washed with
water (2 to 3 times) followed by acetone and dried in air oven. The recovered catalyst was
consecutively used for four times without loss of its significant activity for Suzuki crossꢀ
coupling reaction (Shown in Fig. 6). The recovered catalyst was characterized by XRD, HRTEM
and XPS (shown in Fig. 7). The XRD pattern and XPS analysis of the catalyst PdꢀrGOꢀH shows
2
that the Pd remains in same oxidation state as before the reaction. It is noticed that the most of
the Pd NPs are uniformly distributed onto the surface of rGO nanosheets before used it as a
catalyst in the reaction. However, a fraction of NPs are agglomerated and accumulated in very
few portion of the support (shown in red mark). We presume that the catalytic efficiency was
th
dropped to 62% in the 5 run of the catalyst due the decrease in the surface area and thereby the
4
1
coordination sites.

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3
4
4
4
4
4
4
4
55
56
57
58
59
60
61
Moreover we have performed hot filtration test to examine whether palladium was being to
leached or not. For this experiment bromobenzene and phenylboronic acid were considered
Suzuki cross coupling reaction. After continuing the reaction for 1 h, the catalyst was removed
by filtration and the conversion was determined by GC and was found 58% yield. To check the
progress of the reaction, the filtrate part was further heated for another 3h. From the results
obtained by GC it was clear that no further progress of the reaction was observed after separation
of the catalyst. This clearly revealed that no palladium was leached from the catalyst.
4
4
4
4
62
63
64
65
1
00
96 %
96%
94%
88%
8
6
4
2
0
0
0
0
0
62%
4
4
66
67
1st
2nd
3rd
4th
5th
Run
4
4
68
69
Fig. 6 Recycling of the Pd NPsꢀrGOꢀH catalyst for Suzuki crossꢀcoupling reaction
2
4
4
4
70
71
72

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2
4
Pd 3d3/2
40.96 eV
Pd 3d5/2
35.43 eV
3
3
(111)
(200)
(220)
(311)
332
336
340
344
15
30
45
60
75
2θ value
Binding Energy (eV)
4
4
4
4
73 Fig. 7 Characterization of the PdꢀrGOꢀH catalyst after performing the reaction (a) XRD pattern
2
(b) Pd 3d XPS core level XPS spectra (c) TEM images
74
75
76

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2
5
4
77 Conclusion
4
4
4
4
4
4
4
4
4
4
4
4
78
79
80
81
82
83
84
85
86
87
88
89
In conclusion, we have developed a reduction strategy for the oneꢀpot synthesis of Pd
nanoparticles on reduced graphene oxide and reduction of olefin using palladium salt under
hydrogen atmosphere. The synthesized Pd nanoparticles were efficiently used as a reusable
catalyst for Suzuki crossꢀcoupling reaction and were compared with other Pd nanoparticles on
reduced graphene oxide catalyst using ascorbic acid and glucose as reducing agents. The catalyst
is efficient, easily recoverable and reusable for another four times without losing its activity and
was characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, Xꢀray
diffraction, high resolution transmission electron microscope, Xꢀray photoelectron spectroscopy
analysis before and after its catalytic activity. This study open up a new door for the in-situ
generation of the Pd nanoparticles on the reduced graphene oxide nanosheets under hydrogen
atmosphere which is a simple and ecoꢀfriendly synthesis method.
4
90 Acknowledgements
4
4
4
4
4
4
91
92
93
94
95
96
The authors thank the Department of Science and Technology, New Delhi for financial support
[SR/FT/CSꢀ136/2011 and SB/FT/CSꢀ036/2014 (CSIRꢀNEIST project No. GPP 269 and GPP
293)] and Director, CSIRꢀNorth East Institute of Science and Technology, Jorhat, India for the
interest in this work and facilities
. NH acknowledges to UGC, New Delhi for JRF grant. GD
acknowledges DST, New Delhi, India for DSTꢀINSPIRE Fellowship grant.
4
4
97
98

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6
4
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