Fast synthesis of Ag-Pd@reduced graphene oxide bimetallic nanoparticles and their applications as carbon-carbon coupling catalysts

Article abstract of DOI:10.1039/c4ra05186f

Ultrafine Ag-Pd bimetallic nanoparticles homogeneously distributed on reduced graphene oxide (rGO) were prepared by redox reactions between Pd ²⁺, Ag⁺ and GO. This method is simple, and the use of rGO as a support facilitated the separation and reuse of the Ag-Pd@rGO catalyst. The Ag-Pd@rGO bimetallic nanoparticles were characterized by ultraviolet-visible spectroscopy, Raman spectroscopy, thermogravimetric analysis, X-ray diffraction analysis, and X-ray photoelectron spectroscopy. The microstructure of the Ag-Pd@rGO was investigated with scanning electron microscopy and transmission electron microscopy. The Ag-Pd@rGO catalyst exhibited high catalytic activities in the Suzuki-Miyaura carbon coupling reaction and the Sonogashira carbon coupling (SCC) reaction. These reactions used mild, efficient, ligand-free and heterogeneous conditions. Interestingly, the presence or absence of oxygen in the SCC catalytic system resulted in two different products, in which oxygen can be used as "switch" to control different products. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra05186f

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Fast Synthesis of AgꢀPd@reduced graphene oxide bimetallic
nanoparticles and their applications as carbonꢀcarbon coupling
catalysts
Mingxi Chena Zhe Zhanga Lingzhi Lia Yu Liua* Wei Wangb Jianping Gaoa*
aSchool of Science, Tianjin University, Tianjin 300072, P R China
bSchool of Chemical Engineering, Tianjin University, Tianjin 300072, P R China
Abstract
Ultrafine AgꢀPd bimetallic nanoparticles homogeneously distributed on reduced graphene
oxide (rGO) were prepared by redox reactions between Pd2+, Ag+ and GO. This method is
simple and the use of rGO as a support facilitated the separation and reuse of the AgꢀPd@rGO
catalyst. The AgꢀPd@rGO bimetallic nanoparticles were characterized by ultravioletꢀvisible
spectroscopy, Raman spectroscopy, thermogravimetric analysis, Xꢀray diffraction analysis, and
Xꢀray photoelectron spectroscopy. The microstructure of the AgꢀPd@rGO was investigated
with scanning electron microscopy and transmission electron microscopy. The AgꢀPd@rGO
catalyst exhibited high catalytic activities in the SuzukiꢀMiyaura carbon coupling reaction and
the Sonogashira carbon coupling (SCC) reaction. These reactions used mild, efficient,
ligandꢀfree and heterogeneous conditions. Interestingly, the presence or absence of oxygen in
the SCC catalytic system resulted in two different products, which oxygen can be used as the
control products "switch".
Keywords: alloys; composite materials; nanostructures
*
Corresponding author. Tel.: +862227403475; Fax: (+86)22ꢀ2740ꢀ3475.
Eꢀmail address: jianpinggaols@126.com (J.P. Gao).
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1. Introduction
Noble metals with ultrafine sizes have attracted much interest because of their large
surface areas and large numbers of edge and corner atoms, which greatly improves their
catalytic activities and reduces the usage of the catalysts [1, 2]. However, nanoparticles usually
suffer from aggregation which reduces their total surface energy [3]. In order to prevent
aggregation, various stabilizers, such as surfactants and polymers, have been used to stabilize
the nanoparticles [4]. Unfortunately, stabilizers can severely reduce the catalytic activity of the
nanoparticles. Therefore, appropriate materials are needed to support nanoparticles in a manner
that maintains their high catalytic activity.
Carbon materials, including carbon black, carbon nanotubes (CNTs), graphene and other
derivatives have all been used as catalyst supports [5]. Among these carbon materials, graphene
has recently attracted the most intense attention, due to its unique atomꢀthick 2D structure
which has a high specific surface area, locally conjugated aromatic system, superior electron
mobility, and high chemical and thermal stability [6, 7]. There have been many reports on the
application of graphene as an ideal high performance support for metal, metal oxide and
enzyme catalysts [7ꢀ9]. These studies have demonstrated that graphene can significantly
enhance the catalytic activity of these catalysts [1, 10, 11].
Graphene oxide (GO) is a derivative of graphene which can be manufactured on a large
scale at a relatively low cost by using natural graphite as the raw material. It contains
oxygenꢀcontaining functional groups located mainly at the edges of the planes. This allows GO
to easily disperse in water to form stable GO suspensions [12, 13]. Therefore, metal or metal
oxides nanoparticles can be deposited onto the GO sheets. These graphene supported catalysts,
including Au, Pt, FeO(OH), SnO2, and TiO2 are highly active photoꢀ, electroꢀ and chemical
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catalysis [7,14ꢀ18].
Among the metallic catalysts, palladiumꢀbased catalysts have become a hot topic because
of their outstanding performance in carbon−carbon crossꢀcoupling [1, 19]. Three of the most
important processes, the Heck reaction, Suzuki−Miyaura coupling, and the Sonogashira
reaction have been widely studied with palladium catalysts in recent decades [20, 21]. However,
commercial palladiumꢀbased catalysts are relatively expensive, and they are easily poisoned
[22]. Moreover, they are sensitive to air and moisture, and require the addition of an organic
ligand, such as triphenyl phosphine, which not only pollutes the environment, but also causes
product separation issues. Heterogeneous catalysts can solve this problem [23]. For example,
palladiumꢀbased catalysts can be fastened onto a solid phase carrier, so that the catalyst can be
separated after the reaction and used repeatedly until it becomes inactivate. Recently, Jie et al.
reported that Ag can play an auxiliary role in the catalytic cycle [24]. Therefore, bimetallic
nanoparticles are expected to reduce the cost of catalysts and improve their resistance to
poisoning.
Here, a facile and environmentally friendly method was established to prepare reduced
graphene oxide (rGO) supported AgꢀPd@rGO bimetallic nanoparticles through a redox
reaction between GO, Ag and Pd precursors. In this method, GO plays the role of both
surfactant and support, and no additional surfactants were employed. The AgꢀPd@rGO
bimetallic nanoparticles were characterized and their ability to catalyze carbon−carbon
crossꢀcoupling reactions was investigated using the Suzuki−Miyaura carbon coupling reaction,
and the Sonogashira carbon coupling reaction. This is an efficient way to prepare AgꢀPd@rGO
with excellent carbon−carbon coupling performance.
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2. Experimental
2.1 Materials
Graphite was obtained from Huadong Graphite Factory. Palladium chloride, potassium
tetrachloropalladate, copper sulfate, silver nitrate, chloroauric acid, chloroplatinic acid,
iodobenzene, bromobenzene, phenylboronic acid, 1ꢀbromoꢀ4ꢀnitrobenzene, 4ꢀbromotoluene,
4ꢀbromochlorobenzene, 4ꢀbromoanisole, 4ꢀbromoaniline, 1ꢀbromoꢀ4ꢀiodobenzene and
phenylacetylene were all purchased from Aladdin Industrial Co. Potassium permanganate,
sodium nitrate, concentrated sulfuric acid, 30% hydrogen peroxide, sodium hydroxide,
hydrochloric acid, potassium carbonate, triethylamine and azabenzene were all purchased from
Tianjin Chemical Co. All chemicals were analytical grade and used as received.
2.2 Preparation of GO
GO was prepared from purified natural graphite by a modified Hummer’s method [25].
Briefly, concentrated H2SO4 was added to a 250ꢀmL flask filled with graphite, followed by the
addition of NaNO3, and then solid KMnO4 was gradually added with stirring while the
temperature was kept below 20 °C using a contact thermometer. Next the temperature was
increased to 30 °C and excess distilled water was added to the mixture. Then the temperature
was increased to 80 °C. Finally, 30% H2O2 was added until the color of mixture changed to
brilliant yellow. The mixture was filtered and washed several times with 5% aqueous HCl to
remove metal ions and then washed with distilled water to remove the acid. The resulting filter
cake was dried in air then reꢀdispersed into water. Suspended GO sheets were obtained after
ultrasonic treatment.
2.3 Synthesis of the AgꢀPd@rGO bimetallic nanoparticles
First, 5 mL of GO suspension (1 g/L), 5 mL of AgNO3 (1.23×10ꢀ3 mol/L) and 5 mL of
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PdCl2 (1.23×10ꢀ3 mol/L) were mixed in a beaker. Next dilute sodium hydroxide solution (0.1 M)
was added to adjust the pH to 8−9. Then the mixed solution was heated for 60 min at 85 °C
with vigorous stirring. After its color changed from brown to black, the solution was
centrifuged and washed several times with deionized water. Finally, the product was dried for
24 h at 60 °C in a vacuum oven to remove the water and then stored in a desiccator for further
analysis.
2.4 Characterization of the AgꢀPd@rGO bimetallic nanoparticles
2.4.1 Ultravioletꢀvisible spectroscopy analysis
The UVꢀVis absorption spectra of the GO and AgꢀPd@rGO suspensions were recorded
with a TUꢀ1901 UVꢀVis spectrophotometer.
2.4.2 Transmission electron microscopy analysis
The sample for transmission electron microscopy (TEM) observation was prepared by
placing drops of the diluted AgꢀPd@rGO aqueous suspension onto a carbon coated copper grid,
which was then dried under ambient conditions prior to being introduced into the TEM
chamber. TEM observation and energy dispersive Xꢀray spectroscopy (EDS) measurements
were performed using a Philips Tecnai G2F20 microscope at 200 kV.
2.4.3 Raman microscopy analysis
Raman measurements were performed with an in Via Raman Microscope (RENISHAW,
UK) high resolution Raman microscope, in a backscattered configuration.
2.4.4 Xꢀray diffraction analysis
The Xꢀray diffraction (XRD) diagrams of the samples were measured using an xꢀray
diffractometer (BDX3300) with reference target: Cu Ka radiation (λ = 1.54 Å), voltage: 30 kV
and current: 30 mA. The samples were measured from 10º to 40º (2θ) with steps of 4 º minꢀ1.
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2.4.5 Thermogravimetric analysis
The samples were first dried in a vacuum at 40 °C for 2 days before the thermogravimetric
analysis (TGA) was recorded with a RigakuꢀTDꢀTDA analyzer with a heating rate of
10 °C/min.
2.4.6 Xꢀray photoelectron spectroscopy (XPS) analysis
Elemental analysis of the AgꢀPd@rGO was conducted on an xꢀray photoelectron
spectrometer with an Mg Kα anode (PHI1600 ESCA System, PERKIN ELMER, US).
2.5 Chemical reaction catalyzed by AgꢀPd@rGO bimetallic nanoparticles
2.5.1 General procedure for the Suzuki−Miyaura carbon coupling (SMCC) reaction
Phenylboronic acid (0.5 mmol, 1 equiv), aryl bromide (0.6 mmol, 1.2 equiv), potassium
carbonate (1.0 mmol, 2 equiv) and AgꢀPd@rGO (1 mg, Pd 0.3% (atomic %), ~0.0003 equiv)
were added to a 25ꢀmL Schlenk flask that was then sealed with a Teflon screw cap. The flask
was connected to the Schlenk line and evacuated by a vacuum. The flask was then flushed with
nitrogen before nitrogenꢀpurged water (2.0 mL) was added via a syringe. The flask was
transferred to a preheated oil bath at 80 °C and the mixture was allowed to react for 10 h with
stirring. After cooling to room temperature, the product was extracted into ethyl acetate, dried
over Na2SO4 and then analyzed by Gas ChromatographyꢀMass Spectrometry (GCꢀMS). Finally,
the product was purified via flash chromatography (silica gel) using 10% ethyl acetate in
1
petroleum ether as the eluent and the product was analyzed by H nuclear magnetic resonance
(1HNMR).
2.5.2 Gas ChromatographyꢀMass Spectrometry
A Gas Chromatograph (TRACEDSQ, Thermo Finnigan, US), equipped with a capillary
column (30 m, 0.25 mm i.d., 0.25 ꢁm film thickness) and a mass spectrometer (TRACEDSQ,
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Thermo Finnigan, US), was used to analyze the products. The carrier gas was nitrogen, at a
flow rate of 1 mL/min. The column temperature was initially 100 °C for 2 min, then gradually
increased to 250 °C at 20 °C/min, and finally maintained at 250 °C for 2 min. For GCꢀMS
detection, an electron ionization system with an ionization energy of 70 eV was used. The
extracts were diluted 1:100 (v/v) with ethyl acetate, and 1.0 ꢁL of the diluted samples was
manually injected in the splitless mode.
2.5.3 General procedure for the Sonogashira carbon coupling (SCC) reaction
Phenylacetylene (0.5 mmol, 1 equiv), iodobenzene (0.6 mmol, 1.2 equiv), potassium
carbonate (1.0 mmol, 2 equiv) and AgꢀPd@rGO (1 mg, Pd 0.3% (atomic %), ~0.0003 equiv)
were added to a 25ꢀmL Schlenk flask that was then sealed with a Teflon screw cap. The flask
was connected to the Schlenk line and evacuated by a vacuum. The flask was then flushed with
nitrogen before nitrogenꢀpurged water (2.0 mL) was added via a syringe. Then, the flask was
transferred to a preheated oil bath at 85 °C and allowed to react for 10 h with stirring. After
cooling to room temperature, the product was extracted into ethyl acetate, dried over Na2SO4
and subjected to GCꢀMS analysis. The product was purified via flash chromatography (silica
gel) using 10% ethyl acetate in petroleum ether as the eluent.
3. Results and Discussion
3.1 The transformation of GO during the reaction
The AgꢀPd@rGO bimetallic nanoparticles were prepared by direct reactions between the
GO and the metallic precursors. In order to explore the transformation of GO during the
reaction, the products were characterized with UVꢀVis, Raman spectroscopy, TGA, XRD and
XPS. Ultravioletꢀvisible spectroscopy is a useful tool to characterize the structural changes of
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graphene oxide before and after reactions. As shown in Fig. 1a, the maximum absorption peak
(λmax) of the GO suspension is at 232.6 nm, but after the reaction, the peak redꢀshifted to 268.4
nm. This demonstrates that the GO was reduced and the electronic conjugation within the
graphene nanosheets was restored during the reaction [26]. Since GO can be reduced in NaOH
solution, a control experiment was also conducted. GO was added to a solution of NaOH and
heated in the same conditions for 60 min at 85 °C. Under these conditions, the maximum
absorption peak red shifted, but only to 251.3 nm. Therefore, the reduction of GO in
AgꢀPd@rGO may be attributed to a cooperative action between the NaOH and the metallic
precursors.
Raman spectroscopy is also widely used to analyze carbon materials and can provide
information about defect density and structures, disorder and doping levels [27]. Generally, the
Raman spectrum of graphene is characterized by two main features, the G mode arising from
the first order scattering of the E2g phonons of sp2 C atoms (usually observed at ~1575 cmꢀ1)
and the D mode due to the breathing mode of κꢀpoint phonons with A1g symmetry (at ~1350
cmꢀ1). Changes in the relative intensities of the D and G bands (D/G) indicate changes in the
electronic conjugation state of the GO. The Raman spectra of GO and AgꢀPd@rGO are shown
in Fig. 1b. The D/G ratio of GO increased from 0.99 to 1.21 after the reaction. This increase
suggests that sp2 domains were formed during the reaction and GO was reduced [28].
Figure 1c shows the XRD patterns of pristine graphite, GO and RGO. Graphite has a
narrow and strong diffraction peak at around 2θ = 26.5° (dꢀspacing is 0.34 nm), but GO has a
broad peak at about 11.4° (the interlayer spacing is 0.78 nm). This peak is due to the
oxygenꢀcontaining functional groups. The larger interlayer distance can be attributed to the
formation of hydroxyl, epoxy, and carboxyl groups which increases the distance between the
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layers. After the reaction, this peak disappeared and there is a decrease in the interlayer spacing
which indicates that the oxygenꢀcontaining functional groups have been removed. These results
are consistent with the results for rGO reduced chemically (with NaBH4) or thermally and thus
indicate that GO was reduced here [29].
The reduction of GO was also confirmed by TGA and the results are shown in Fig. 1d. The
GO has two main weight losses. The first rapid weight loss (~15%) from 30 to 150 ºC is due to
the dehydration of the GO. The second weight loss (~25%) between 200 and 250 ºC represents
the decomposition of the oxygenꢀcontaining functional groups [27]. In AgꢀPd@rGO, the
second weight loss is obviously much lower than that in GO, indicating that most of the
oxygenꢀcontaining functional groups were removed during the reaction.
(a)
(b)
A: GO
D
B: GO+Ag+/Pd2+
C: GO+NaOH
G
D: AgꢀPd@rGO
AgꢀPd@rGO
D
C
B
GO
A
220
240
260
280
300
320
340
500
1000
1500
2000
2500
3000
3500
Raman shift (cmꢀ1)
Wavelength (nm)
100
(c)
(d)
90
80
70
60
50
40
30
AgꢀPd@rGO
GO
AgꢀPd@rGO
GO
Graphite
15
100
200
300
400
500
600
700
10
20
25
30
2θ (degree)
Temperature (°C)
Figure 1. (a) UVꢀVis absorption spectra of the GO, GO+Ag+/Pd2+, GO+NaOH and
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AgꢀPd@rGO; (b) Raman spectra of GO and AgꢀPd@rGO; (c) XRD patterns of graphite, GO
and AgꢀPd@rGO; and (d) TGA curves of GO and AgꢀPd@rGO
In addition to the above qualitative analyses, a quantitative analysis of the GO and
AgꢀPd@rGO was carried out by XPS and the results are shown in Fig. 2 and Table 1. The
relative oxygen content of the AgꢀPd@rGO bimetallic nanoparticles is obviously lower than
that of pristine GO (Fig. 2a) and the elemental ratio of C/O increased from 2.9 to 6.1 (Table 1),
which is higher than that in rGO reduced by NaBH4 (4.78) [30].
The C1s peaks for GO and the AgꢀPd@rGO bimetallic nanoparticles are shown in Figs. 2b
and 2c. The spectrum of C1s can be deconvoluted into three peaks at 284.5, 286.4, and 288.0
eV which are attributed to C=C double bonds, CꢀO single bonds of the epoxy and/or the alkoxy
groups, and C=O double bonds, respectively [27]. After the reaction, the relative intensity of
the peaks at 286.4 eV and 288.0 eV decreased whereas the relative intensity of the peak at
284.5 eV increased significantly. These changes clearly demonstrate the removal of the
oxygenꢀcontaining groups and the formation of sp2 domains in GO. The XPS results further
prove that GO was reduced. In addition to the C and O peaks, the XPS spectrum of
AgꢀPd@rGO also shows Ag and Pd peaks. This indicates that rGO contained Ag and Pd (Figs.
2d and 2e) which will be discussed in the next section.
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(a)
GO
AgꢀPd@rGO
800
600
400
200
0
Binding Energy (eV)
(b)
(c)
C=C
C=C
CꢀO
O=C
288
CꢀO
O=C
288
292
284
280
292
284
280
Binding Energy (eV)
Binding Energy (eV)
(d)
(e)
344
340
336
332
376
372
368
364
Binding Energy (eV)
Binding Energy (eV)
Figure 2. (a) XPS spectra of GO and AgꢀPd@rGO, the inset is the magnified spectra of the
Ag3d peaks and Pd3d peaks of AgꢀPd@rGO; (b) curve fit for C1s of GO; (c) curve fit for C1s of
AgꢀPd@rGO; (d) curve fit for Ag3d of AgꢀPd@rGO; and (e) curve fit for Pd3d of AgꢀPd@rGO.
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Table 1. Percentage of carbon, oxygen, palladium and silver in GO and AgꢀPd@rGO
Sample
Analysis
Silver
0
Carbon
75.2
Oxygen
24.8
Palladium
C/O ratio
0
2.9
GO
C=C
61.9
CꢀO
C=O
15.2
22.9
Carbon
85.4
Oxygen
13.9
Silver
0.3
Palladium
C/O ratio
0.3
6.1
AgꢀPd@rGO
C=C
80.0
CꢀO
C=O
9.9
10.0
3.2 The transformation of the metal ions during the reaction
XPS can not only characterize the transformation of GO during the reaction, but also those
of AgNO3 and PdCl2. The Ag3d spectrum of AgꢀPd@rGO (Fig. 2d) has two peaks. The one at
367.7 eV is attributed to Ag3d5/2 and the other at 373.8 eV is due to Ag3d3/2. Similarly, the Pd3d
spectrum also has two peaks (Fig. 2e); the peaks at 335.6 and 340.9 eV correspond to Pd3d5/2
and Pd3d3/2 respectively. These results indicate that the Ag+ and Pd2 + ions were transformed to
Pd0 and Ag0 and attached to the rGO nanosheets.
The Pd0 and Ag0 were produced through redox reactions between the GO and the metallic
precursors. It is speculated that the standard electrode potential is the key factor in determining
the redox reaction. Table 2 shows the standard electrode potentials of Au(Ⅲ), Pt(Ⅱ), Ag(Ⅰ),
Pd(Ⅱ) and Cu(Ⅱ) under alkaline condition. The electrode potential of GO is +0.48 (relative to
the standard hydrogen electrode).1 Therefore, when the standard electrode potential of an ion is
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greater than +0.48, it can be reduced by GO. The standard electrode potentials of Ag + and Pd 2+
are both lower than that of GO; however, adjusting the pH and concentration of the precursors
changes the potential of Ag+ and Pd2+ so they can be reduced.
In order to confirm this conjecture, five different salt solutions (shown in Table 2) were
reacted with GO under identical conditions, and the products were characterized by XRD (Fig.
3). In the XRD pattern of Ag/rGO, the peaks at 38.1°, 44.3°, 64.4° and 77.4°, correspond to the
faceꢀcentered cubic (fcc) structure of Ag (111), (200), (220) and (311), respectively (Joint
Committee on Powder Diffraction Standards (ICSD, 87ꢀ0597). Similarly, the other four spectra
confirm the existence of Au (ICSD, 04ꢀ0784), CuO (ICSD, 80ꢀ1916), Pt (ICSD, 01ꢀ1190), and
Pd (ICSD, 46ꢀ1043), which coincides with the products predicted in Table 2. These results
support the conjecture that Pd0 and Ag0 were produced through redox reaction between the GO
and the metallic precursors.
Table 2. Standard electrode potentials of some ions at 298K
Standard Electrode Potential (Eθ
Reactants Products
Electrode Reaction
(V))
[AuCl4]－＋3e－＝Au＋4Cl－
HAuCl4
K2PtCl4
AgNO3
PdCl2
Au
Pt
+1.002
+0.755
+0.342
+0.07
－
[PtCl4]2 ＋2e－＝Pt＋4Cl－
Ag2O＋H2O＋2e－＝2Ag＋2OH－
Pd(OH)2＋2e－＝Pd＋2OH－
Cu(OH)2＋2e－＝Cu＋2OH－
Ag
Pd
CuSO4
CuO
ꢀ0.222
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GO+AgNO
3
470
376
282
188
GO+HAuCl
4
GO+CuSO
4
GO+K PtCl
2
4
GO+PdCl
2
94
Intensity
Intensity
Intensity
Intensity
Intensity
30
40
50
60
70
80
2θ(degree)
Figure 3. XRD patterns of Ag/rGO, Au/rGO, CuO/rGO, Pt/rGO and Pd/rGO
The TEM photos in Fig. 4 give a more direct view of the morphology of the AgꢀPd@rGO.
The AgꢀPd@rGO was dispersed in ethanol, transferred to a 200ꢀmesh, carbonꢀcoated copper
grid and then observed by TEM. Nanoparticles with diameters of ~10 nm are uniformly
distributed on the GO nanosheets (Fig. 4a). In the high magnification view (Fig. 4b), different
lattice fringe structures are observed on the surfaces of most of the nanoparticles which
indicates that each nanoparticle contains different crystal structures.
Figure 4c is the EDX spectrum of a single nanoparticle, and it clearly shows that the
nanoparticle is composed of Pd and Ag. Other particles were also analyzed by EDX and the
results were similar. Figure 4d shows the crossꢀsectional compositional line profiles measured
over the nanoparticle. Both the Ag and Pd elements appear to be quite evenly distributed along
the line. This is clear evidence that AgꢀPd bimetallic nanoparticles are loaded onto the rGO.
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（a）
（b）
（c）
（d）
Figure 4. TEM images of the AgꢀPd@rGO with the scale of (a) 100 nm and (b) 5 nm; (c) EDX
of AgꢀPd@rGO; and (d) Crossꢀsectional compositional line profiles for AgꢀPd@rGO
UVꢀVis analysis was also conducted and the spectra are shown in Fig. 5. The UVꢀVis
spectrum of Ag/rGO has a peak at 410 nm which is due to the absorption of the Ag
nanoparticles. The Pd/rGO spectrum does not have a peak at this wavelength and the
AgꢀPd@rGO spectrum has a relatively weak peak. This confirms the existence of AgꢀPd
bimetallic nanoparticles.
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A
B
A: Ag/rGO
B: Alloy/rGO
C: PdO/rGO
C
300
400
500
Wavelength (nm)
Figure 5. UVꢀVis absorption spectra of the Ag/rGO, AgꢀPd@rGO and Pd/rGO
3.3 Catalytic activity of AgꢀPd@rGO in Sonogashira carbon coupling (SCC) reaction
The catalytic activity of AgꢀPd@rGO in the Sonogashira reaction was also explored and
the results are shown in Table 3 and Fig. S1. The expected product is the crossꢀcoupling
product, 1,2ꢀdiphenylethyne (2a), but interestingly, 1,4ꢀdiphenylbutaꢀ1,3ꢀdiyne (2b) which is a
homocoupling byꢀproduct, was dominant under some conditions [31]. The reaction was
performed with different alkali salts and in different gas environments to investigate their
effects on the reaction.
The Ag catalyst was inactive under a nitrogen atmosphere. However, the activity of the
AgꢀPd@rGO catalyst was relatively high in the SCC reaction with a yield of 92% of 2a which
was a little higher than that for the Pd catalyst (86%). The solvent and the reactants were
sparged with nitrogen gas before the reaction to remove any dissolved oxygen and the reaction
was carried out in a Schlenk tube which prevented the introduction of any oxygen into the
reaction system. With no oxygen present, the products contain almost no 2b. (Table 3: entries
1ꢀ3)
When the gas environment was changed, the yields of 2a and 2b also changed. For the
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reaction under air, the yield of 2a was much less and the yield of 2b increased. When oxygen
was introduced during the reaction, the product contained only 2b. Therefore, the atmosphere
controls which type of reaction occurs, crossꢀcoupling or homocoupling. By adjusting the
oxygen content, the ratio of 2a to 2b can be adjusted. (Table 3: entries 3, 7, 9)
The catalytic efficacy was further evaluated by adding different alkali salts. The yield
dropped significantly in the presence of a strong base like NaOH. The catalytic activity of
AgꢀPd@rGO in the presence of triethylamine was better than that with NaOH (Table 3: entries
4, 5). However the addition of triethylamine, causes the AgꢀPd@rGO catalyst to crossꢀlink,
which can create difficulties in separating and recycling the catalyst. Therefore, K2CO3 is the
most effective alkali in terms of activity, selectivity and recyclability (Table 3: entries 3ꢀ9).
Table 3. Sonogashira carbon coupling reaction of iodobenzene and phenylacetylene
Entry
Atmosphere
Alkali
K2CO3
K2CO3
Catalyst
Ag
Yield of 2a
Trace
86%
Yield of 2b
Trace
1
2
3
4
5
6
7
N2
N2
N2
N2
N2
Air
Air
Pd
Trace
K2CO3 AgꢀPd@rGO
Et3N AgꢀPd@rGO
92%
Trace
81%
Trace
NaOH AgꢀPd@rGO
NaOH AgꢀPd@rGO
K2CO3 AgꢀPd@rGO
74%
Trace
12%
Trace
7%
72%
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8
9
Air
O2
Et3N
AgꢀPd@rGO
Trace
Trace
34%
85%
K2CO3 AgꢀPd@rGO
3.4 Catalytic activity of AgꢀPd@rGO in Suzuki−Miyaura carbon coupling (SMCC)
reaction
The SMCC reaction was selected as a model reaction to investigate the catalytic activity of
AgꢀPd@rGO. The SMCC reaction is most efficient under basic conditions and it has been
reported that the presence of K2CO3 makes the reaction more efficient [32]. Kim et. al have
reported that rGO can be used as a phase transfer agent, and that it tends to be present between
the organic and aqueous phases [33]. The catalytic activity of Ag, Pd and AgꢀPd@rGO were
tested for the SMCC reaction of 4ꢀmethylphenylboronic acid and different bromobenzenes
under heterogeneous conditions with K2CO3 as an acid binding agent. The results are shown in
Table 4 and according to the GC results (Fig. S2ꢀS9) the product was pure.
The catalytic activity of Ag was very low in the SMCC reaction and the catalytic ability of
AgꢀPd@rGO was similar to that of Pd. If the functional group on the pꢀarylbromide was a
strong electron withdrawing group, the yield of the SMCC reaction was very high (> than 99%)
due to electronic effects; however, if the functional group was an electron donor group, the
yield of the SMCC reaction was slightly lower.
Since the nitroꢀgroup is an electron withdrawing group and the yield of this reaction
was >99% for both Pd and AgꢀPd@rGO, the reaction between phenylboronic acid and
1ꢀbromoꢀ4ꢀnitrobenzene was selected to explore the reusability of the catalysts. The results are
shown in Fig. 6. The Pd catalyst quickly lost its activity upon reuse which demonstrates the
poisoning of catalyst. However, the AgꢀPd@rGO catalyst could be recycled 10 times with a
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negligible drop in activity and achieved a total turnover number of more than 30,000.
Meanwhile, after catalytic reaction, the morphology of the catalyst did not change much (Fig.
S10). The good recyclability of AgꢀPd@rGO provides a way to greatly reduce the cost of the
catalyst.
Table 4. Suzuki−Miyaura carbon coupling reaction of phenylboronic acid and arylbromide
Entry
1
Arylbromide
Product
Catalyst
Ag
Yield*
Trace
TON*
ꢀꢀꢀꢀ
2
AgꢀPd@rGO >99%
3333
2000
ꢀꢀꢀꢀ
1a
3
Pd
>99%
Trace
4
Ag
5
AgꢀPd@rGO 90%
3000
1780
ꢀꢀꢀꢀ
1b
1c
1d
1f
6
Pd
89%
7
Ag
Trace
8
AgꢀPd@rGO 98%
3267
1960
ꢀꢀꢀꢀ
9
Pd
98%
10
11
12
13
14
15
Ag
Trace
AgꢀPd@rGO 91%
3033
1840
ꢀꢀꢀꢀ
Pd
92%
Ag
Trace
AgꢀPd@rGO >99%
Pd 92%
3333
1840
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16
17
18
19
20
21
22
23
24
Ag
Trace
ꢀꢀꢀꢀ
3333
2000
ꢀꢀꢀꢀ
AgꢀPd@rGO >99%
1g
1h
1i
Pd
>99%
Trace
Ag
AgꢀPd@rGO >99%
3333
1940
ꢀꢀꢀꢀ
Pd
97%
Ag
Trace
AgꢀPd@rGO >99%
Pd >99%
3333
2000
*TON is defined as mole substrate reacted per mole catalyst
*Yield was determined by GC and TON was calculated from GC yield.
100
80
60
40
20
Pd
AgꢀPd@rGO
0
2
4
6
8
10
No. of cycle
Figure 6. Recyclability of the AgꢀPd@rGO catalyst in the reaction of phenylboronic acid and
1ꢀbromoꢀ4ꢀnitrobenzene
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4. Conclusions
In summary, a facile method for synthesizing AgꢀPd@rGO bimetallic nanoparticles with a
homogeneous distribution using a redox reaction between Pd2+, Ag+ and and GO has been
developed. AgꢀPd@rGO is a highly active catalyst for SMCC and SCC reactions because the
whole synthesis process needs no dispersants. Meanwhile, these catalytic reactions employ
mild, efficient, ligandꢀfree and heterogeneous conditions. More interestingly, oxygen can be
used as the "switch" to control products in the SCC reaction. This simple, straightforward and
general method is useful for the facile preparation of supported metal nanocatalysts.
Acknowledge
Funding from the National Science Foundation of China (21202115, 21276181, and 21074089)
is gratefully acknowledged.
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